hydrogen induced damage of lead zirconate titanate (pzt)
TRANSCRIPT
Hydrogen‐Induced Damage of Lead‐Zirconate‐Titanate (PZT)
by
ALI SHAFIEI MOHAMMADABADI
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
in
THE FACULTY OF GRADUATE STUDIES
(Materials Engineering)
THE UNIVERSITY OF BRITISH COLUMBIA
(Vancouver)
April 2013
© Ali Shafiei Mohammadabadi, 2013
ii
Abstract
Lead-Zirconate-Titanate Pb(Zr,Ti)O3 (PZT) based actuators are evaluated by
automotive industry for advanced fuel-injection systems, including hydrogen injection.
However, hydrogen can have deleterious effect on the PZT's functionality and properties.
The general objective of this work is to study the interactions between PZT and hydrogen.
The results of long-term (200-1200 hours) high-pressure (10 MPa) hydrogen exposure on the
PZT microstructure show that hydrogen has only superficial effects on the microstructure of
bare PZT. However, when an electrode is attached to PZT, the hydrogen damage increased; a
porous layer developed immediately adjacent to the electrodes on the PZT surface due to
hydrogen spillover. The kinetics of the PZT structural modifications due to hydrogen was
investigated by online monitoring of the electrical properties of PZT above the Curie
temperature, up to 650C. The results show that the structural changes can be described by
the classical nucleation and growth theory. The growth of the new structure appears to be
limited by the diffusion of protons into PZT, with a calculated activation energy of 0.440.09
eV, at 450-650C. Two relaxation peaks were observed in the dissipation factor curves of the
hydrogen-treated PZT. While the kinetics of one of the relaxation peaks obeys the classical
Arrhenius law with the activation energy of 0.66 eV, the other peak shows an unusual
relaxation kinetic. The mechanisms for the formation of these relaxation peaks are
determined. Low temperature (20C) diffusion of hydrogen into the PZT was also studied,
using the water electrolysis technique. Based on the microstructural observations, the
iii
diffusion coefficient of hydrogen in PZT was calculated as 9×10-11 cm2/sec. The Maxwell-
Wagner polarization mechanism is determined to be responsible for the changes in the
hydrogen-affected PZT capacitance. In the last part of the project, alumina coatings were
applied to PZT plates using the sol-gel technique, to explore the possibilities of decreasing H2
damage to PZT. The functionality of the coating against hydrogen damage was evaluated by
the water electrolysis technique. Significant decrease of hydrogen damage was observed even
for highly porous coatings. The mechanisms by which the alumina coating decreases the
hydrogen damage were tentatively proposed.
iv
Preface
This research work was conducted as part of a NSERC Strategic Project Grant
awarded to The University of British Columbia, Simon Fraser University and Westport
Innovations Inc. of Vancouver. The journal papers listed below have been prepared from the
work presented in the dissertation. I am the primary contributor to all of them, and the co-
authors contributions are as follows: my supervisor, Dr. Tom Troczynski, extensively
commented on the experimental and analysis methods, and the results interpretation in all
three papers. C. Oprea edited all three papers, and provided the SEM pictures in all three
papers. T. Nickchi contributed in the second paper by commenting on the experimental
setup. Dr. A. Alfantazi contributed in the second and third papers by providing and
commenting on the experimental setup.
1- A. Shafiei, C. Oprea, T. Troczynski, Investigation of the effects of high-pressure hydrogen
on Pb(Zr,Ti)O3 (PZT) ceramics, Journal of the American Ceramic Society, 2012, 95(2), 782–
787
2- A. Shafiei, T. Nickchi, C. Oprea, A. Alfantazi, T. Troczynski, Investigation of hydrogen
effects on the properties of Pb(Zr,Ti)O3 in tetragonal phase using water electrolysis
technique, Applied Physics Letters, 2011, 99 (21), 212903-212906
3- A. Shafiei, C. Oprea, A. Alfantazi, and T. Troczynski, In situ monitoring of the effects of
hydrogen on Pb(Zr,Ti)O3 structure, Journal of Applied Physics, 2011, 109 (11), 114108-
114116
Chapter 5-1 is based on paper “1”. Chapter 5-2 is based on paper “3”. Chapter 5-3 is
based on paper “2”. Please check the first pages of these chapters to see footnotes with similar
information.
v
Table of Contents
Table of Contents
Abstract .......................................................................................................................................... ii
Preface ........................................................................................................................................... iv
Table of Contents ........................................................................................................................... v
List of Tables ................................................................................................................................ vii
List of Figures .............................................................................................................................. viii
Nomenclature .............................................................................................................................. xiii
Acknowledgements .................................................................................................................... xvi
Dedication .................................................................................................................................. xvii
1 Introduction ................................................................................................................................ 1
2 Literature Review ....................................................................................................................... 4
2.1 Lead Zirconate Titanate (Pb(Zr,Ti)O3) ............................................................................... 4
2.2 Hydrogen damage of PZT .................................................................................................... 9
2.3 Mechanisms of hydrogen damage of PZT ........................................................................ 12
2.3.1 Hydrogen incorporation into PZT ............................................................................. 12
2.3.2 Stable forms of hydrogen in PZT................................................................................ 22
2.3.3 Stable sites of H+ in PZT .............................................................................................. 24
2.3.4 Effect of hydrogen on PZT ......................................................................................... 26
2.4 Dielectric spectroscopy of PZT ......................................................................................... 31
2.5 High pressure hydrogen compatibility of PZT ................................................................. 33
2.6 Methods of decreasing the hydrogen damage to PZT ...................................................... 35
3 Scope and Objectives ................................................................................................................ 37
4 Materials and Methods ............................................................................................................. 40
4.1 Samples ............................................................................................................................... 40
4.2 Gas hydrogen treatment .................................................................................................... 42
4.3 Water electrolysis treatment of PZT ................................................................................ 47
4.4 Alumina sol-gel coating ..................................................................................................... 51
4.5 Characterization techniques .............................................................................................. 55
4.5.1 X-ray diffraction analysis (XRD) ................................................................................ 55
4.5.2 Scanning electron microscopy (SEM) ........................................................................ 55
4.5.3 Electrical properties measurements ............................................................................ 55
5 Results and Discussion .............................................................................................................. 57
vi
5.1 High-Pressure conditions (T=100C, p=10 MPa, t=200-1200 hours)* ............................. 57
5.1.1 H2 effects on PZT microstructure ............................................................................... 57
5.1.2 H2 effects on the electrical properties of PZT ............................................................ 68
5.2 High-Temperature conditions (T=450-600C, p=0.013 MPa) * ....................................... 72
5.2.1 H2 effects on PZT microstructure ............................................................................... 72
5.2.2 H2 effects on PZT electrical properties ....................................................................... 75
5.3 Water-electrolysis treatment of PZT* ............................................................................... 96
5.3.1 Microstructure ............................................................................................................. 96
5.3.2 Electrical properties of PZT exposed to water electrolysis ..................................... 102
5.4 Ceramic Coatings for PZT Damage Protection .............................................................. 112
5.4.1 Alumina coatings microstructure ............................................................................. 112
5.4.2 Hydrogen resistivity of alumina-coated PZT .......................................................... 120
6 Conclusions ............................................................................................................................. 129
7 Future Work ............................................................................................................................ 133
References .................................................................................................................................. 136
vii
List of Tables
Table 1- The heat and entropy of dissolution of hydrogen in different metals [22] ............... 17
Table 2- The EDX analysis for the bright particles in Figure 44b ............................................ 73
Table 3- Different values of exponents for the equation (28) [67] ............................................ 80
Table 4- The fitting values obtained for the equation (30) ....................................................... 82
Table 5- The fitting values obtained for the HN equation ........................................................ 91
Table 6- Microstructural charachteristics of alumina coatings a a function of TC [91]....... 117
Table 7- The EDX analysis for the -alumina coating (high concentration of Au is due to the
gold coating on the sample for SEM analysis) .......................................................................... 120
viii
List of Figures
Figure 1- A typical polarization versus electric field (P-E) hysteresis for ferroelectrical
materials ......................................................................................................................................... 2
Figure 2- A schematic of an electronic fuel injector based on PZT actuators [3] ...................... 3
Figure 3- Binary phase diagram of PbZrO3-PbTiO3 system (refer to text for explanation of the
symbols) [5] .................................................................................................................................... 5
Figure 4- Unit cell for PZT (a) above and (b) below Curie temperature .................................... 6
Figure 5- TEM image showing domains in PZT [8] and a schematic of the 180 (1-1) and 90
(2-2) domain walls in PZT ............................................................................................................. 8
Figure 6- A ferroelectric ceramic with differently oriented domains inside each grain (a)
before and (b) after polarization, with remnant strain ................................................................ 9
Figure 7- Initiation and growth of hydrogen fissures during charging at 50 mA/cm2 for 2h (b)
and 4h (c); the black bar is 50 µm [12] ....................................................................................... 12
Figure 8- Different paths for the incorporation of hydrogen into PZT: (1) through the surface
of bare PZT (only at high temperatures) and (2) through the surface of electrodes (at low
temperatures). .............................................................................................................................. 14
Figure 9- Switching charge as a function of H2 annealing temperature with and without
upper Pt electrode [17] ................................................................................................................ 14
Figure 10- Remnant polarization as a function of the H2 annealing temperature for capacitors
with Pt, Pd, Au or Ag electrodes (applied voltage of 5V) [19] .................................................. 15
Figure 11- Solubility (cc of hydrogen per 100 g of metal) of hydrogen inside metals at 1
atmosphere pressure of hydrogen [24] ....................................................................................... 18
Figure 12- Schematic cross-section of the electrode/PZT assembly in H2 gas (it is assumed
that hydrogen diffuses only in the z direction).......................................................................... 19
Figure 13- Hydrogen diffusion coefficient in different metals [25] .......................................... 21
Figure 14- The stable positions of protons according to the reference [32] in a) perovskites
with large lattice constants, and b) perovskites with short lattice constants ........................... 25
Figure 15- PZT crystal structure showing the possible location of protons in the lattice of
PZT [31]; b) stable lattice site of protons for tetragonal PbTiO3 and c) cubic phase of PbTiO3
supposed by Park and Chadi [30] ................................................................................................ 26
Figure 16- Proposed PZT damage mechanisms can be categorized by the place where damage
occurs ............................................................................................................................................ 27
Figure 17- The microstructure of bare PZT plates .................................................................... 40
Figure 18- The microstructure of the PZT samples ................................................................... 41
Figure 19- Micrographs of the surface of the electrodes: silver (a) and silver-palladium (b) . 42
Figure 20- The time-temperature schedule of the ‘High-Pressure’ hydrogen treatment used
in this work .................................................................................................................................. 43
Figure 21- Schematic of an actuator made from PZT plates stacked together, b) the
equivalent electrical circuit of an actuator ................................................................................. 43
ix
Figure 22- The schematic of the setup used in this work for online monitoring the electrical
properties ...................................................................................................................................... 44
Figure 23- The ‘High-Temperature’ hydrogen treatment used in this work ........................... 45
Figure 24- A typical Nyquist plot for the PZT at 500°C in air (the frequency is swiped
between 106 and 10-3 Hz) ............................................................................................................. 47
Figure 25- Schematic of the setup for the water electrolysis experiment ................................ 49
Figure 26- Schematic of the steps used for the preparation of the alumina coating ................ 52
Figure 27- Micrographs of PZT surface: a) the as-received sample; b) after 1200 h hydrogen
treatment ...................................................................................................................................... 58
Figure 28- Surfaces of PZT plates at higher magnifications: (a) before and (b) after 1200 h
hydrogen treatment ..................................................................................................................... 58
Figure 29- XRD results of bare PZT for as-received and after 1200 h hydrogen treatment ... 59
Figure 30- Cross-section of the as-received sample (a) in comparison to the cross section of
the sample after 1200 h hydrogen treatment (b) ....................................................................... 60
Figure 31- Cross section of the hydrogen treated sample for 1200 hours, close to the surface
....................................................................................................................................................... 60
Figure 32- Low magnification (a) and high magnification (b) images of damaged layer on the
PZT surface next to the Ag electrode after 400 hours hydrogen-treatment ............................ 61
Figure 33- The interface of the Ag electrode with PZT after grinding and polishing, for the
as-received sample (a) and for the sample hydrogen-treated for 400 hours (b); no detachment
of the electrode from the PZT and no damaged layer are visible ............................................. 63
Figure 34- The spillover mechanism of hydrogen atoms from the surface of the Ag electrode
to the surface of the PZT ............................................................................................................. 63
Figure 35- The detachment of the Ag electrode from the PZT for the sample treated for 600 h
....................................................................................................................................................... 64
Figure 36- Detachment of the Ag electrode from PZT; some cracks are present on the surface
of the electrode for the sample heat-treated for 1200 hours ..................................................... 64
Figure 37- Micrographs of the sample with Ag/Pd electrodes after hydrogen-treated for 200h:
the 1x10 mm side face (a) and its cross-section (b) .................................................................... 65
Figure 38- Surface of the side face of the sample with Ag/Pd electrode: (a) as-received, (b)
hydrogen-treated for 400 h; noticeable corroded area next to the electrode (c) ..................... 67
Figure 39- Micrograph of the cross-section of the sample shown in Figure 38 ....................... 68
Figure 40- Capacitance of PZT sample with Ag/Pd electrode in high-pressure hydrogen
atmosphere (a); at point ‘1’ the heater is on, and at point ‘3’ the heater is off. (b): capacitance
variation with temperature in hydrogen atmosphere ............................................................... 69
Figure 41- Capacitance of PZT sample with Ag electrode in high-pressure argon atmosphere
....................................................................................................................................................... 70
Figure 42- Capacitance of PZT sample with Ag electrode in high-pressure hydrogen
atmosphere ................................................................................................................................... 71
Figure 43- Image from the side surface of the PZT plate with Ag electrode for (a) as-received
and (b) after hydrogen treatment (for 2 h / 400C / p= 0.013 MPa) ......................................... 73
x
Figure 44- Metallic lead in hydrogen treated PZT samples (for 2 h / 600C / p= 0.013 MPa) 73
Figure 45- The XRD pattern for as-received and hydrogen treated PZT with Ag electrodes 74
Figure 46- The XRD pattern for hydrogen treated PZT with Ag electrodes at 500C and
600C ............................................................................................................................................ 75
Figure 47- (a) The general trend of PZT capacitance variation with time in hydrogen
atmosphere at 500°C; (b) measurements for the real part of the impedance (ZRe) and the
calculated values of R according to equation (8) (the data is obtained at the constant
frequency of 1 kHz) ..................................................................................................................... 76
Figure 48- The ZRe -ZIm plot for PZT plate heat treated at 550C and the resistance
determined using the ZRe -ZIm plots for PZT; the noise in the ZRe-ZIm plots corresponds to the
times when the heater was on. ................................................................................................... 77
Figure 49- The variation of PZT capacitance at 530°C, 550°C and 600°C ................................ 78
Figure 50- The general trend for the isothermal α - time plots having different time steps,
time equal to zero shows the start of the reaction [68] ............................................................. 79
Figure 51- The results of fitting the capacitance data to equation (30) for the temperatures of
550C (a) and 600C (b) ............................................................................................................... 81
Figure 52- (a) The results of fitting the capacitance data to equation (30); (b) the activation
energy of hydrogen diffusion, obtained from the fit ................................................................. 82
Figure 53- The Grotthuss mechanism for diffusion of protons in PZT, including the
reorientation and hopping of protons between oxygen onions ................................................ 83
Figure 54- Hypothetical schematic of the different modes which can be assumed for the
dissolution of hydrogen in PZT; (a) where the diffusion of protons into PZT occur uniformly
from the surface; in this case the diffusion equation with proper initial and boundary
equation could be used for determining the total amount of protons in PZT; (b) where the
diffusion of protons can occur from limited places in the PZT; in this case the nucleation and
growth models can be used to describe the total amount of protons in PZT ........................... 84
Figure 55- Changes of capacitance C and dissipation factor DF of hydrogen-treated sample
(for 24 hrs / 550C / p= 0.013 MPa) versus temperature (The thick grey line shows the
changes of capacitance for as-received sample) ......................................................................... 86
Figure 56- Schematics of the dipolar polarization mechanisms, wherein direction of the
dipoles changes with changing the direction of applied voltage .............................................. 87
Figure 57- Schematics of the Maxwell-Wagner polarization mechanism, wherein differences
in the electrical properties of different regions cause charge accumulation at the interfaces
between the different regions, leading to the increase of capacitance ..................................... 88
Figure 58- Variations of ε’ and ε’’ for hydrogen-treated samples with the frequency of applied
voltage in the temperature range of 200-325C, with 25C increments .................................. 89
Figure 59- The results of fitting the ε´ and ε’’ data to the Debye equation for at T= 325C, for
PZT hydrogen-treated samples (for 24 hrs / 550C / p= 0.013 MPa) ........................................ 90
Figure 60- The results of fitting the ε´ and ε’’ data to the Havriliak–Negami equation for at
T= 325C, for PZT hydrogen-treated samples (for 24 hrs / 550C / p= 0.013 MPa) ................ 91
xi
Figure 61- The activation energy for the ion jumping, obtained from the fits, for PZT
hydrogen-treated samples (for 24 hrs / 550C / p= 0.013 MPa) ................................................ 92
Figure 62- Variation of DF with frequency in the temperature range 22-42C, for PZT
hydrogen-treated samples (for 24 hrs / 550C / p= 0.013 MPa) ................................................ 94
Figure 63- Micrographs of the cross-section through PZT plate after water electrolysis: a)
low-magnification image (after 48 hours water electrolysis); b) microstructure of the
corroded layer (close to the electrode); c) microstructure in a region far from the corroded
layer .............................................................................................................................................. 97
Figure 64- The microstructure of PZT after water electrolysis just beneath the electrode
(after removing the electrode) .................................................................................................... 99
Figure 65- XRD pattern of the as-received PZT sample versus the water electrolyzed PZT
sample, using the following parameters: I=100 mA/cm2, t=48 hours ..................................... 100
Figure 66- The thickness of the corroded layer versus the square root of time of water
electrolysis .................................................................................................................................. 102
Figure 67- The changes of capacitance (C) and dissipation factor (DF) versus the duration of
water electrolysis at the frequency of 1 kHz ............................................................................ 103
Figure 68- Variations of electrical properties after 6 hours water electrolysis and subsequent
aging in air: a) capacitance (C); b) dissipation factor (DF) (: as-received,: after water
electrolysis, : after aging) ........................................................................................................ 104
Figure 69- Variations of electrical properties after 10 hours water electrolysis and subsequent
aging in air: a) capacitance (C); b) dissipation factor (DF) (: as-received,: after water
electrolysis, : after aging) ........................................................................................................ 104
Figure 70- Variations of capacitance (C) and dissipation factor (DF) after water electrolysis
for 48 hrs, and subsequent aging at room temperature in air (■: as-received, : after water
electrolysis, ▲:after 10 hours aging, : after 24 hours aging) ................................................ 105
Figure 71- The results of fitting the ε´´ (ε´´=DFε´) data to Debye and Havriliak–Negami
equation. An iterative MATLAB code was developed and used for the fitting procedure. .. 109
Figure 72- The changes of the capacitance (C) and dissipation factor (DF) for (a) a leaky
capacitor with electronic conduction, (b) for a capacitor with hopping charge carriers
adapted from [38] ....................................................................................................................... 109
Figure 73- Low magnification image of the coating on the surface of PZT (a) after dip coating
with pure boehmite sol (b) before and (c) after heat treatment of the coating in the furnace,
in some places on the surface of the coating, detachment of the coating was observed ....... 113
Figure 74- Low magnification image of the coating on the surface of PZT after dip coating
before the heat treatment of the coating in the furnace (comparing with Figure 73a, a smooth
uniform coating has formed on the surface of PZT with the addition of PVA to sol) .......... 114
Figure 75- Low magnification (a) and high magnification (b) images of the coating on the
surface of PZT after dip coating and after heat treatment of the coating in the furnace
(comparing with Figure 73b and c, a smooth uniform coating has formed on the surface of
PZT with the addition of PVA to sol) ...................................................................................... 115
xii
Figure 76- Low magnification (a) and high magnification (b) images of the cross section of
the alumina coating. As it can be seen from (b), the coating had enough fluidity to fill out the
pores on the surface of PZT ...................................................................................................... 116
Figure 77- High resolution image of the cross section through the alumina coating processed
at 450C in air for 5 hours ......................................................................................................... 118
Figure 78- Transformation sequence of the different aluminum hydroxides with temperature
(adapted from [93]). ................................................................................................................... 119
Figure 79- XRD results for the as-received boehmite powder and after heat treatment at
450C for 5 hr ............................................................................................................................. 119
Figure 80- The cross section of the sample with Au-Pd electrodes and after 24 hours water
electrolysis. The thickness of the corroded layer is about 100 microns ................................. 121
Figure 81- The cross section of the sample with Au-Pd electrodes and alumina coating and
after 48 hours water electrolysis ............................................................................................... 122
Figure 82- Schematic for the reaction of hydrogen atoms with -alumina particles 1)
transformation of hydrogen atoms to hydrogen molecules which leave the system away from
the coating (i.e. as hydrogen bubbles during water electrolysis), 2) diffusion of hydrogen
atoms through the electrode and attachment to -alumina particles, followed by surface and
bulk diffusion through -alumina towards PZT ...................................................................... 123
Figure 83- An image of the cross section of PZT with alumina coating on top ..................... 124
Figure 84- The cross section of the sample with Au-Pd electrodes and thin alumina coating
and after 144 hours water electrolysis ...................................................................................... 125
Figure 85- Schematic image for the combination of hydrogen atoms at the interface of
metallic electrode with - alumina ........................................................................................... 125
Figure 86-Equivalent electrical circuit for PZT and PZT with coatings ................................ 128
xiii
Nomenclature
Latin Symbols A Constant
a Area of electrodes
Hydrogen concentration
C Capacitance
C0 Concentration of hydrogen
D Diffusion coefficient
d Distance between electrodes
DF Dissipation factor
DPb Lead diffusion coefficient in PbTiO3
DM Diffusion coefficient of hydrogen in metallic electrode
DPZT Diffusion coefficient of hydrogen in PZT
E Electric Field
Ec Coercive Field
f Frequency
fMax Frequency at which the maximum occurs
fugacity of H2
g Gravity
G Conductance
h Coating thickness
hH Partial enthalpy
ΔH Activation Energy
ΔHM Heat of dissolution of hydrogen inside metal
i Imaginary number
ic Charging current density
JH Flux of hydrogen atoms
k Coefficient in equation (10)
KIC Fracture toughness of the un-affected PZT
KIH Fracture toughness of hydrogen-treated PZT
m Constant
n Constant
nis Number of interstitial sites per metal atom
P Polarization
p Pressure
P0 Constant
p0 Pressure at standard conditions
PH2 Partial pressure of hydrogen
Pr Remnant Polarization
Ps Spontaneous Polarization
Q Activation energy
R Ohmic resistance
SH Non-configurational part of entropy
ΔSM Dissolution of hydrogen in different metals
xiv
t Time
T Temperature
Tc Curie temperature
Y Admittance
x Thickness of corroded layer
Z Impedance
ZIm Imaginary part of impedance
ZRe Real part of impedance
Greek Symbols θ Constant
α Fraction of the volume converted to the product of reaction
LV Sol-vapor surface energy
β Constant
ε Dielectric constant
ε0 Vacuum permittivity
τ Relaxation time
0 Constant
ε´ Real part of dielectric constant
ε´´ Imaginary part of dielectric constant
εs Dielectric constant when 0
ε Dielectric constant when
Viscosity of the sol
Density of the sol
Withdraw speed
( ) Chemical potential of gaseous hydrogen per molecule
( ) Chemical potential of hydrogen atom dissolved in the metallic electrode per atom
( ) Chemical potential at a given standard state
( ) Chemical potential at a given standard state
Angular frequency
Abbreviation
AO Orthorhombic phase
EDX Energy-dispersive X-ray spectroscopy
FCC Face-centered cubic lattice
FM Monoclinic phase
FR Rhombohedral phase
FT Tetragonal phase
KTN Potassium tantalate niobate
MPB Morphotropic phase boundary
MW Maxwell-Wagner polarization mechanism
PVA Polyvinyl alcohol
PZT Lead zirconate titanate
xv
Abbreviation (continue)
Pc Cubic perovskite structure
SEM Scanning electron microscope
TEM Transmission electron microscope
TC Curie temperature
XRD X-ray diffraction
xvi
Acknowledgements
I would like to take this opportunity to express my utmost gratitude toward my
supervisor, Dr. Tom Troczynski for his continuous trust, patience, and guidance throughout
the course of this work. I also acknowledge Prof. Akram Alfantazi, Prof. Guangrui Xia, and
Prof. Steve Cockcroft for their valuable comments.
My sincere thanks go to Carmen Oprea, who has been a valuable and amazing
colleague and friend, and has been continuously supporting and helping me throughout the
years I have been at UBC. She taught me to write “in comparison to” instead of “in compare
to”. She taught me to write “reach” instead of “reach to”. Thank you so much Carmen!
I would like to thank all staff members in the Department of Materials Engineering at
The University of British Columbia for their assistance with my research work. My special
thanks to all colleagues and officemates for providing a friendly environment that I was
always pleased to work in. Natural Sciences and Engineering Research Council of Canada
(NSERC) and Westport are greatly acknowledged for financial support.
Special thanks are owed to my parents, who have supported me throughout my years of
education.
xvii
Dedication
To my patient parents
1
1 Introduction
Lead Zirconate Titanate (Pb(Zr,Ti)O3) or PZT is the general name for the perovskite
solid solutions between PbZrO3 and PbTiO3. PZT is well known because of its unique
electrical properties such as high dielectric, piezoelectric and electro-optic coefficients. The
main reason for having such interesting electrical properties is the unique crystal structure
and the arrangement of ions inside unit-cell of PZT. In the unit-cell of PZT, the titanium or
zirconium ions reside off-center in the octahedral interstitial positions surrounded by six
oxygen ions. Therefore, the center of negative charge of oxygen ions will not coincide with
the center of positive charge of Ti or Zr ions and this arrangement results in permanent
dipoles inside the unit-cell of PZT. These built-in permanent dipoles in the unit-cell of the
crystal structure of PZT are the origin of the superior electrical properties of PZT.
Application of an electric field of sufficient magnitude will cause these built-in
dipoles inside the PZT to switch to a different, stable direction in accordance to the direction
of the applied electric field. Moreover, by removal of the electric field, the dipoles will not
return to their original direction. This brings up one of the very interesting properties of
PZT, which is the switchable Polarization (P)-Electric Field (E) hysteresis, as schematically
shown in Figure 1. This property of PZT is generally known as ferroelectricity, and it has
also been seen in other oxides like BaTiO3. This property of PZT has enabled the extensive
use of these materials in applications such as ferroelectric random access memories (FeRAM).
It is known that hydrogen treated PZT may not show this hysteresis anymore. Although the
2
main reason for this phenomena is not known very well, this has been attributed to the
formation of [OH]– dipoles, which inhibits the switching of the spontaneous dipoles in PZT.
This effect of hydrogen on the polarization hysteresis of PZT has been the main reason for
investigating the effect of hydrogen on PZT since 1995 [1-2].
Figure 1- A typical polarization versus electric field (P-E) hysteresis for ferroelectrical materials
In addition to the ferroelectrical properties, a “poled” PZT ceramic (i.e. PZT with
oriented dipoles) can also show superior piezoelectric properties, which have caused the
extensive use of PZT in other applications, such as actuators and sensors. Recently, attention
has been paid to the effect of hydrogen on the piezoelectric properties of PZT as well, as it
has been suggested that hydrogen might also have deleterious effects on these properties [3].
For example, Figure 2 schematically shows a modern electronic fuel injector that uses PZT
E(V/m)
P (C/cm2)
Pr
Ps
Ec
Ps :spontaneous polarizationPr :remnant polarizationEc :coercive field
Ec
3
actuators for valve opening, instead of the conventional solenoid technology. These fuel
injectors have been introduced by the leading engine manufacturers in recent years. One of
the issues of using such fuel injectors in a hydrogen atmosphere is the possible deleterious
effects which hydrogen may have on the functionality of the PZT actuators [3].
Figure 2- A schematic of an electronic fuel injector based on PZT actuators [3]
The above points provide rationale for the investigation of the interactions of
hydrogen with PZT, which is important topic both from practical and scientific points of
view. The objective of the present work is to address some of the issues regarding the
interaction of hydrogen and PZT. More specifically, we focused on the kinetics of
degradation of PZT properties by hydrogen. Attempts have also been made to propose
techniques to inhibit or decrease the hydrogen damage to PZT. In this regard, the sol-gel
technique was used to develop hydrogen barrier coatings on the surface of PZT.
4
2 Literature Review
2.1 Lead Zirconate Titanate (Pb(Zr,Ti)O3)
PZT is the general name for the perovskite solid solutions between PbZrO3 and
PbTiO3, as shown in the binary phase diagram in Figure 3. Starting at PbTiO3 part of the
diagram with a ferroelectric tetragonal (FT) phase, by increasing the amount of PbZrO3 in the
solution, the composition Pb(Zr0.53Ti0.47)O3 is reached, where the ferroelectric tetragonal
phase starts to transforms into another ferroelectric phase, however, with a different crystal
structure (rhombohedral phase (FR)) [4]. The composition where the tetragonal phase (FT)
transforms to the rhombohedral phase (FR) is considered to be the morphotropic phase
boundary (MPB) in the PZT binary phase diagram (Figure 3). Instead of a sharp boundary,
the MPB is often observed in real systems as a region of phase coexistence whose width
depends on the compositional homogeneity and on the sample processing conditions [5].
Therefore, the location of this boundary has not been determined exactly, as its positions
changed from report to report. However, the recent work by Noheda et al. [5] has changed
our understanding of the MPB in PZT. Their studies show that instead of a boundary, a low-
symmetry monoclinic phase (FM) exists between the tetragonal and rhombohedral phases and
the superior electrical properties of PZT around MPB are actually due to the very high
polarization in this phase (Figure 3). In other words, the monoclinic phase acts as a bridge for
the phase transformation from tetragonal to rombohedral and vice versa.
5
The rhombohedral part of the diagram itself consists of two other phases: a high-
temperature phase (FR(HT)) and a low-temperature ferroelectric rhombohedral (FR(LT)) phase.
The PZT structure close to the PbZrO3 part of the diagram up to about 10 %mole of PbTiO3
(Figure 3) has an antiferroelectric, orthorhombic (AO) structure. This phase of PZT has a very
complex structure, in the sense that the displacement of cations along the [110] direction is
coupled with octahedral tilts [4]. According to Figure 3, we can see that depending on its
composition, PZT can have different crystal structures, and consequently PZT can show
different piezoelectric, pyroelectric and electro-optic coefficients. The great technological
and commercial importance of PZT is actually due to such variations in the electrical
properties, which can be obtained by changing the PZT composition. Especially when the
PZT is doped with other secondary ions, it can show even more interesting electrical
properties [4]. The most widely used PZT ceramics today have the compositions near the
MPB composition. For example, Pb(Zr0.53Ti0.47)O3 is the composition for the PZT ceramics
used in this work.
Figure 3- Binary phase diagram of PbZrO3-PbTiO3 system (refer to text for explanation of the symbols) [5]
Tem
per
atu
re (
°C)
100
200
300
400
500
4020 60 80 1000
PbZrO3 PbTiO3
FT
PC
FR(HT)
FR(LT)
AO
MPB
Mole % PbTiO3
FM
6
The cubic perovskite structure of PZT (Pc) above Curie temperature is shown in
Figure 4a, wherein each lead ion is surrounded by 12 oxygen ions. The oxygen ions plus the
lead ions form a face-centered cubic (FCC) lattice. The titanium or zirconium ions reside in
the octahedral interstitial positions surrounded by six oxygen ions. Temperature has a strong
effect on the cubic structure shown in Figure 4a. When the temperature decreases to about
375C, the structure changes to the tetragonal shown in Figure 4b. The octahedral site is now
distorted, with the Ti or Zr in off-center positions, resulting in a permanent dipole. This
"built-in" permanent dipole in the unit cell of the crystal structure of PZT is the origin of the
superior dielectrical and piezoelectrical properties of PZT. This is sometimes called a
spontaneous polarization. The temperature of transformation from the cubic to the
tetragonal phase is called the Curie temperature (Tc).
Figure 4- Unit cell for PZT (a) above and (b) below Curie temperature
Pb
O
Ti or Zr
Above TC
(a)Below TC
Dipole direction
(b)
7
During the transformation from the cubic to the tetragonal phase, permanent dipoles
form inside the PZT crystal; however, the direction of these dipoles is not the same
throughout the whole crystal or inside each PZT grain. The regions inside PZT crystals
where the dipoles are aligned in the same direction are called ferroelectric domains. As soon
as spontaneous dipoles start to form during the phase transformation from the cubic to the
tetragonal, surface charges start to appear on the surface of PZT. Such surface charges
produce very high electric fields in the order of MV m−1 [7]. Therefore, the electrostatic
energy of the system increases due to the existence of such electric fields. The electrostatic
energy associated with these electric fields can be minimized if the PZT crystal can split into
separate ferroelectric domains, and this is one of the reasons why ferroelectric domains form
inside the PZT crystals [7].
Another reason for the formation of ferroelectric domains inside the PZT crystals is to
minimize the elastic energy associated with the mechanical constraints to which the PZT
crystal is subjected as it is cooled, through the paraelectric–ferroelectric phase transition [7].
To better understand this, assume that a part of the PZT crystal is subjected to compression
forces while cooling down from the cubic phase to the tetragonal phase. At the phase
transformation temperature, spontaneous dipoles start to form inside PZT; however, they
will form in the directions perpendicular to the compression forces, in order to minimize the
elastic energy of the system. Therefore, the mechanical forces, and the elastic energy of the
system are minimized by the fragmentation of the grains into separate domains [7]. As a
result, a complex domain structure develops in each grain, according to the allowed
8
directions of the polarization in each domain. In the tetragonal phase, the so-called separated
180 and 90 domain walls form (Figure 5) [7].
Figure 5- TEM image showing domains in PZT [8] and a schematic of the 180 (1-1) and 90 (2-2) domain walls
in PZT
In each ferroelectric domain inside a grain of a ferroelectric ceramic, a uniform
orientation of the dipoles exists. It should be noted that since the grains and the domains
contained in them are randomly oriented, the properties of the ferroelectric ceramics are
isotropic both after the synthesis and after the cooling below the Curie temperature [7]. By
applying an electric field with sufficient magnitude, the spontaneous polarization inside the
ferroelectric ceramic can switch to a different, stable direction. By stable direction we mean
that the spontaneous polarizations will not return to its original direction and magnitude
when the electric field is removed, as shown in Figure 6. This brings up the most important
characteristic property of PZT, which is the switchable Polarization (P)-Electric Field (E)
hysteresis, as schematically shown in Figure 1.
1
1
2
2
9
Figure 6- A ferroelectric ceramic with differently oriented domains inside each grain (a) before and (b) after
polarization, with remnant strain
2.2 Hydrogen damage of PZT
Four different kinds of degradation of the electrical properties after H2 treatment have
been reported for PZT: 1) loss of polarization hysteresis [9], 2) increase in leakage current [9],
3) drop in resistivity [10], and 4) decrease in dielectric constant [9]. To confirm that this
degradation is not just due to the reducing nature of the atmosphere (usually a mixture of
nitrogen and hydrogen), the ferroelectric capacitor characteristics were also measured after
the heat treatment in nitrogen and they were unchanged. Therefore, the degradation is due
to the hydrogen in the atmosphere entering PZT and altering atomic structure of PZT.
The decrease in resistivity after H2 treatment has also been reported: the resistivity of
as-grown PZT drops from 5×1011 Ωcm to 2×107 Ωcm after annealing in forming gas (a
mixture of up to 5.7% hydrogen and nitrogen) at 400°C for 30 min [10]. Although strong
changes have been observed in the polarization hysteresis characteristics and leakage current
(a) (b)
remnant strain
10
of the hydrogen-treated Pt/PZT/Pt ferroelectric capacitors, their relative dielectric constant
measured for a low-voltage signal was not greatly affected; it was about half of the as-
received sample, i.e. 800. This may indicate that the PZT film was not damaged completely
through its thickness [9]. The degradation mechanisms will be discussed in the next section,
and it will be shown that the deterioration occurs mostly at the interface between PZT and
electrode.
There are just a few papers regarding the degradation of the mechanical properties of
the PZT after hydrogen treatment [11-13]. The general conclusions drawn from these papers
are the following:
-The cohesive strength of PZT decreases due to the presence of H or H+ in the lattice.
-Recombination of H or H+ at grain boundaries and in micro-voids may form molecular
hydrogen; when the internal pressure of H2 exceeds the cohesive strength of the grain
boundaries, fissures or micro-cracks appear.
Delayed hydrogen-induced failure has been reported for PZT ceramics during charging by
hydrogen under a constant load [11]. Therefore, it can be concluded that hydrogen atoms
incorporated into the structure of PZT can decrease the cohesive strength of PZT, although
they also have a considerable effect on ferroelectric properties. Wang et al. have studied
hydrogen induced delayed fracture of PZT [11]. Their results show that the strength of PZT
decreases with increasing the hydrogen concentration inside the specimen. Moreover, they
have also found that the KIH/KIC decreases with the hydrogen concentration in the specimen,
C0, in the form of KIH/KIC = 0.400-0.155ln(C0) where C0 is the concentration of hydrogen
11
which can diffuse out from the samples after charging, KIH is the fracture toughness of
hydrogen-treated sample and KIC is the fracture toughness of the un-affected sample. They
have reported that the fracture mostly occurred in inter-granular mode, which shows that
cracks were mainly initiated at the grain boundaries.
Peng et al. have investigated the initiation and propagation of hydrogen fissures in a
PZT ferroelectric ceramic during charging by hydrogen without loading [12]. Their results
show that when hydrogen concentration in PZT exceeds a certain value (about 260 ppm),
hydrogen fissures or micro-cracks form within PZT. Figure 7, reproduced from [12], shows
the initiation of such micro-cracks at grain boundaries. Usually a typical sintered PZT
ceramic is not 100% dense, and there are many voids and porosities at the grain boundaries.
The recombination of hydrogen atoms at such porosity or voids results in increasing pressure
of the molecular hydrogen (H2) inside such holes, and when the hydrogen pressure inside
these voids or porosities becomes equal to the strength of PZT at the grain boundary, which
has been decreased by the presence of atomic hydrogen, hydrogen fissures or microcracks
form [12].
12
Figure 7- Initiation and growth of hydrogen fissures during charging at 50 mA/cm2 for 2h (b) and 4h (c); the
black bar is 50 µm [12]
2.3 Mechanisms of hydrogen damage of PZT
2.3.1 Hydrogen incorporation into PZT
Hydrogen damage has been linked to structural modifications of PZT, which lead to
the changes in the properties of PZT [1-2]. This could occur due to chemical reactions
between hydrogen and constituents of PZT, or simply due to the presence of hydrogen (in
different forms of ions, atoms, or molecules) within PZT. Before discussing these
mechanisms, we first need to address two important issues: 1) the paths for the incorporation
of hydrogen in PZT and 2) the stable forms of hydrogen inside the lattice of PZT (H or H+).
13
There are many uncertainties regarding the second issue, and in the following sections we
will review the results reported in other studies.
2.3.1.1 Mechanism
In general two different scenarios can be considered. The first mechanism is that
hydrogen molecules dissociate at the PZT surface, and as a result H atoms diffuse into the
structure (Figure 8 ).
Generally, such mechanism can be active in bare (no electrodes) PZT crystals, as
observed in a few experiments [9, 14-15]. However, most oxides have limited hydrogen
diffusivity [16], and the results show that hydrogen can be incorporated into the PZT
structure only at temperatures higher than 400°C [17].
Another possible mechanism, especially at temperatures as low as 200°C, is the
incorporation of H into the structure from the metallic electrode. Hydrogen molecule
dissociates at the surface of the electrode, and thus produced H atoms diffuse to the
electrode/PZT interface, then continue into the PZT crystal and modify the PZT structure
(Figure 8). Indeed, this is the most probable mechanism, and it has been reported in many
studies [9, 14, 18-21] as the degradation of PZT properties can be correlated with the
properties of electrodes. For example, the PZT-electrode assembly with In2O3 electrode
showed the least amount of degradation when subjected to hydrogen atmosphere, whereas Pt
is the worst electrode [21], due to the highly catalytic nature of Pt helping in dissociating the
hydrogen molecules into atoms.
14
H2HH
HH
H2
H
H
Electrode
PZT
2
1
Figure 8- Different paths for the incorporation of hydrogen into PZT: (1) through the surface of bare PZT (only
at high temperatures) and (2) through the surface of electrodes (at low temperatures).
Figure 9 [17] shows that degradation of PZT with Pt electrodes is more severe in
comparison to the bare PZT, which clearly confirms the catalytic behavior of Pt. Figure 10
[19] shows the effects of different electrodes on the remnant polarization (Pr). It is clear that
capacitors with Au or Ag electrodes are much more stable during H2 annealing than those
using Pt or Pd. The difference in the level of H2 damage corresponds to the difference in the
catalytic activity in the hydrogenation reaction and the adsorptive properties of hydrogen by
metallic electrodes [19].
Figure 9- Switching charge as a function of H2 annealing temperature with and without upper Pt electrode [17]
15
Figure 10- Remnant polarization as a function of the H2 annealing temperature for capacitors with Pt, Pd, Au or
Ag electrodes (applied voltage of 5V) [19]
2.3.1.1 Interactions between hydrogen and metallic electrodes
The incorporation of hydrogen in PZT through the metallic electrode includes the
steps of (i) hydrogen absorption into the metallic electrode and dissociation of hydrogen
molecules on the surface of the metallic electrode, and (ii) diffusion of hydrogen atoms
through the metallic electrode and into PZT (Figure 8). The absorption of hydrogen
molecules (H2) into metallic electrode via the gas phase can be described by the following
chemical reaction [22]:
( ) (1)
where [H] refers to hydrogen atoms dissolved in the metallic electrode. At equilibrium, the
chemical potential of hydrogen in the gas phase is equal to the chemical potential of
hydrogen dissolved in the metallic electrode, therefore:
( ) ( ) (2)
16
where ( ) is the chemical potential of gaseous hydrogen per molecule and ( ) is the
chemical potential of hydrogen atom dissolved in the metallic electrode per atom.
The chemical potential of gaseous hydrogen can be written as follows:
( )
( ) (
) (3)
where ( ) refers to the chemical potential at a given standard state, is the Boltzmann
constant, is the fugacity of H2 and is the pressure at standard conditions (1 bar);
is
defined as the activity of hydrogen ( ). At pressure of hydrogen below 10 MPa [22-23]
hydrogen can be considered as an ideal gas; therefore, will be equal to
, the partial
pressure of hydrogen. Therefore, the chemical potential of gaseous hydrogen can be written
as follows:
( )
( ) (
) (4)
On the other hand, the chemical potential of hydrogen in the metallic electrode can be
written as [22]:
( ) ( ) ( ( )) (5)
where ( ) is the chemical potential at a given standard state, and ( ) is the activity of
hydrogen in the metal. The chemical potential of atomic hydrogen in metals can also be
written as [23]:
( ) ( (
)) (6)
where is the partial enthalpy, is the non-configurational part of entropy, is the
number of interstitial sites per metal atom, and is the number of hydrogen atoms per
17
metal atom (i.e. hydrogen solubility, expressed as atomic fraction). Taking into account
equations (2), (4) and (6), the following relations can be obtained for the hydrogen solubility
into the metallic electrode:
(
)
( ) (7)
or
(
)
(8)
where is the heat of dissolution of hydrogen inside the metal, and is the entropy of
mixing. The values of and for the dissolution of hydrogen in different metals can be
obtained experimentally, and the literature data are reported in Table 1. The value of
depends on the electronic structure of the metal in which hydrogen is being dissolved, and
the is predicted by theoretical studies to be around -7.8 [22].
Table 1- The heat and entropy of dissolution of hydrogen in different metals [22]
Metal ([eV per atom]) / T(C)
Fe (bcc) 0.25 -6 <900
Al 0.70 −6 500
Ni 0.17 −6 350–1400
Pd 0.1 −7 −78–75
Pt 0.48 −7 −
Cu 0.44 −6 <1080
Ag 0.71 −5 550–961
Au 0.37 −9 700–900
U(α) 0.1 −6 <668
18
For small hydrogen concentrations ( ), equation (7) can be written as:
(
) (
) (9)
or
(10)
This relation is known as Sieverts’ law [22-23], which states that the solubility of
hydrogen in metals is proportional to the square root of the partial pressure of the hydrogen
in equilibrium with the metal. is known as the solubility coefficient. Sieverts’ law is also
applicable for other diatomic gases (eg. N2, O2) [22]. Figure 11 shows the experimental data
on hydrogen solubility in various metals as a function of temperature [24].
Figure 11- Solubility (cc of hydrogen per 100 g of metal) of hydrogen inside metals at 1
atmosphere pressure of hydrogen [24]
Let us consider the role of the metallic electrode on the level of hydrogen absorption
by PZT. The case under consideration is the metallic electrode attached to PZT, and both are
4 8 12 16 20 24
10000/T K-1
-8
-4
0
4
8
Ln
cH
T (C)
Cu
Fe
Ag
Pt
Ni
Pd
Al
2004006001000
19
in the hydrogen atmosphere (Figure 12). In this case, the hydrogen adsorption in PZT
includes the steps of (i) the hydrogen absorption into the metallic electrode and its
dissociation, and (ii) diffusion of hydrogen atoms through the metallic electrode and into
PZT (it is assumed that hydrogen diffusion is just in the z direction). The chemical potential
of hydrogen in the metallic electrode ( ) and PZT ( ( )) can be described by [25]:
( ) (
) (11)
where is the hydrogen concentration at the partial enthalpy of , is the gas constant
and stands for (metal) or .
Figure 12- Schematic cross-section of the electrode/PZT assembly in H2 gas (it is assumed that hydrogen diffuses
only in the z direction)
During hydrogen diffusion, the following boundary equilibrium conditions can be
considered [25]:
at interface A :
( ) ( ) (12)
at interface B : ( ) ( ) (13)
It should be noted that both ( ) and ( ) are functions of time and space; that is
because during the hydrogen diffusion, when the system is not in equilibrium, is changing
H2 (g)
electrode
PZT
A
Bz
20
with time and space, and therefore both ( ) and ( ) are changing with time and
space [25]. Comparing equations (11) and (13) the following relation can be written:
(
) (
) (14)
By adding the term
( ) to both sides of equation (14), the following relation is obtained
[25]:
(
) (
) (15)
where and are the heat of hydrogen dissolution in metallic electrode and PZT,
respectively. Equation (15) shows that the , the hydrogen concentration inside PZT,
depends on , and . According to this equation, we expect that with changing
the metallic electrode, different amounts of hydrogen would dissolve in PZT. The different
level of hydrogen dissolution in PZT due to the different electrodes is therefore the reason
for the dependency of the damage affected by hydrogen on the metallic electrode (Figure
10). Another important boundary condition which must be satisfied in order to conserve the
H atoms is the equality of the flux of hydrogen atoms which leave the metallic electrode to
the flux of hydrogen atoms which enter the PZT at the interface B ( ) [25]. In other words,
at the interface B, the following equation must be satisfied:
( )
( )
(16)
where is the diffusion coefficient of hydrogen in the metallic electrode, and is the
diffusion coefficient of hydrogen in PZT. According to equation (16), and are other
parameters which control the hydrogen entry to PZT. Therefore, based on equations (15)
21
and (16), one can conclude that the hydrogen incorporation, hence the damage to PZT,
indeed depends on , and , i.e. on the type of the electrode used.
According to equation (16), the rate of hydrogen atoms which leave the metallic
electrode increases with . Moreover, the time needed for the hydrogen atoms to reach the
interface between the electrode and PZT from the surface of the electrode depends also on
. However, often the electrodes used in making the PZT capacitors, e.g. for FeRAMs, are
very thin (about 10 nm). Therefore, we expect that these diffusion times (<1 sec) are much
smaller than the typical duration (30 min) of hydrogen treatments (the depth of hydrogen
diffusion ( ) from the surface of the electrode can be estimated by √ . If = 10
nm, and if we assume to be equal to 10-5 cm2/sec at 150C Figure 13, then the time for
hydrogen atoms to reach the interface between the electrode and PZT will be on the order of
microseconds). Figure 13 shows the hydrogen diffusion coefficient for different metals as a
function of temperature [25].
Figure 13- Hydrogen diffusion coefficient in different metals [25]
4 8 12 16 20 24 28
10000/T K-1
0
0.2
0.4
0.6
0.8
D (
cm2 /
sec)
10
-4
T (C)
Cu
Au
Ag
Pt
Ni
Pd
2004006001000
22
In summary, the role of metallic electrodes on the damaging effect of hydrogen on
PZT can be evaluated by considering the system parameters such as hydrogen diffusivity ,
heat of hydrogen dissolution in the metal , and hydrogen concentration in the metal
. If we want to predict the amount of hydrogen damage with different electrodes, not
only the above parameters, but their interplay should be considered as well; moreover, the
respective PZT parameters ( , , ) should be considered. For example, by
considering Figure 11 and Figure 13, one can see that the hydrogen diffusion and hydrogen
solubility are higher in Pd than in Pt. Therefore, one may expect that Pd may cause more
degradation to PZT in comparison to Pt. However, greater damage has been observed in the
samples with Pt electrode (Figure 10), possibly due to the higher heat of hydrogen solubility
in Pt than in Pd.
2.3.2 Stable forms of hydrogen in PZT
In general there are three possible forms of hydrogen in oxides; it can exist as a
hydrogen atom (H), as a hydrogen ion (H+ or H-) or as a hydrogen molecule (H2). As an atom
H, it will just fill the interstitial sites of the lattice, with no interaction with other elements
of the structure, especially with oxygen anions. These hydrogen atoms can diffuse freely
through the structure without changing the electrical properties of the oxide [27]. On the
other hand, hydrogen atom can ionize to H+ and release one electron in the lattice. The
produced proton (H+) cannot exist as such, because it is very unstable in this form [28], so it
23
will react with the oxygen anions of the lattice and form an O-H bond. This bond is
directional, and depending on the interatomic distances, the coordination number of the
proton would be one or two [28]. When there are large interatomic distances, the
coordination number of the proton is one. On the other hand, a proton can be shared
between two oxygen anions when the distance between the oxygen anions is short enough
[28] (the interatomic distances in the crystalline lattice strictly depend on the radii of cations
and anions which formed the structure). Finally, hydrogen can also exist as a molecule
within the structure of oxides. In this form, hydrogen might only exist at grain boundaries,
voids, or pores, where there is enough space for a hydrogen molecule. It has been suggested
that hydrogen atoms may form hydrogen molecules at grain boundaries, and this may cause
the formation of cracks in these regions [12].
The important question is “what is the stable form of hydrogen in the crystalline
lattice of PZT?” Xiong and Robertson [29] have investigated stable forms of hydrogen in the
structure of PbTiO3 and PbZrO3 using the first-principle of quantum mechanics or ab-initio
calculations. Their results show that a hydrogen atom will form a donor state in these
structures, i.e. the stable form of hydrogen was the proton (H+) [29]. This result can also be
applicable for PZT, as the formation of [OH]– bonds in PbTiO3 has also been confirmed by
Park and Chadi [30]. Using first-principles calculations, they have shown that hydrogen
impurities will act as shallow donors in the structure of PbTiO3. Raman spectroscopy results
of hydrogen treated PZT samples also confirmed the existence of O-H bonds in the structure
of PZT [31]. Therefore, it is reasonable to presume that the stable form of hydrogen in the
24
crystalline PZT is H+. However, based on the above results, one cannot conclude that
hydrogen cannot exist in other forms (H or H2), but it is certain that some of the hydrogen
atoms will be ionized inside the PZT lattice.
2.3.3 Stable sites of H+ in PZT
Determining the stable sites of protons in PZT matters, as protons form directional
bonding with oxygen ions, and this could affect the built-in dipoles in PZT. Generally the
stable sites of H+ in the crystalline lattice of perovskite oxides depend on the binding
interactions between the H+ and oxygen anions [32]. In other words, since H+ will form an
OH- bond in the crystalline lattice, it is reasonable to expect that the position of H+ will
depend on the oxygen sites in the structure. Furthermore, one can say that “protons are
localized within the valence electron density of the oxygen” [32]. The positions of OH- bond
also will be determined by its interaction with cations existing in the structure [32]. Kreuer
has suggested two stable sites for protons [32], depending on the lattice constants of the
perovskite oxides. For the perovskites with large lattice constants, the stable sites of protons
will be the edges of octahedrals (Figure 14a). On the other hand, for the perovskites with
short lattice constants, the protons also could be shared between two oxygen ions of two
adjacent octahedrals (Figure 14b) [32].
25
(a)
(b)
Figure 14- The stable positions of protons according to the reference [32] in a) perovskites with large lattice
constants, and b) perovskites with short lattice constants
Aggarwal et al. have also investigated the possible sites of protons in the structure of
PZT [31]. They considered four possible sites for protons, and based on the Raman spectra
obtained for hydrogen treated PZT samples, concluded that the more probable site for H+ is
between the apical oxygen ions and Ti (Figure 15a) [31]. They concluded that just one of the
apical oxygen ions will react with a proton [26]. Park and Chadi [30] have investigated the
stable sites of protons in the crystalline lattice of PbTiO3 using first-principles calculations.
For the tetragonal phase of PbTiO3 they have suggested different stable positions for protons,
as depicted in Figure 15b. They have also predicted that depending on the position of
protons, the [OH]– will either destroy, or enhance the polarization of the spontaneous
dipoles in PZT [30]. However, they have predicted that in the presence of the [OH]– dipoles,
the spontaneous dipoles cannot be switched by applying the electric field. On the other
26
hand, for a cubic PbTiO3 they have found the most stable site to be the site shown in Figure
15c.
(a)
(b)
(c)
Figure 15- PZT crystal structure showing the possible location of protons in the lattice of PZT [31]; b) stable
lattice site of protons for tetragonal PbTiO3 and c) cubic phase of PbTiO3 supposed by Park and Chadi [30]
2.3.4 Effect of hydrogen on PZT
We can now focus on the possible mechanisms responsible for the degradation of PZT
by hydrogen. As seen in Figure 16, the structural degradation can be categorized by the place
27
where it occurs: a) at the grain boundaries and at the interfaces between PZT and electrodes,
and b) in the crystalline lattice of PZT.
O and Pb vacancies(mostly at grain boundaries)
degraded layer due to [OH]–
(inside the crystalline lattice)
Figure 16- Proposed PZT damage mechanisms can be categorized by the place where damage occurs
Damage at the grain boundaries and interfaces of PZT and electrodes
When H atoms diffuse from the electrode and reach the interface between the
electrode and PZT, some may diffuse fast through the grain boundaries. Such hydrogen
atoms may affect the structure and properties of the PZT in different ways. The first
mechanism is that hydrogen atoms may undergo the following reaction [33]:
2H + O2- H2O + (17)
Where is the oxygen vacancy with two electrons. Oxygen vacancies produced by the
reaction (17) can further ionize and produce two free electrons; this can decrease resistivity
of PZT. However, because H2O cannot exist inside the PZT ceramic body, reaction (17) can
only occur on the surface of the PZT ceramic grains [11]. Therefore, for such a reaction to
continue at the interfaces, oxygen atoms must diffuse from the bulk of the grains to the
interfaces, and water molecules should also diffuse out from the sample along the grain
28
boundaries. Generally, such diffusion, especially the oxygen diffusion in PZT, is slow. As
such, the above damage by H can only occur at the grain boundaries of PZT and the damage
is limited to the grain boundaries [11, 34]. Therefore, the damage that occurs by reaction (17)
is only at the surface of the ceramic particles, which may include electrode/PZT and grain
boundaries (Figure 16).
It should be noted that the oxygen vacancies might also be produced by the following
reaction, without a direct reaction with hydrogen:
+ ½ O2 (g) (18)
The above reaction occurs in reducing atmospheres. Experiments show that treatment of
PZT under a hydrogen deficient atmosphere (such as pure N2) has no noticeable effects on
the properties of PZT, while similar treatment in hydrogen has considerable effects on the
properties of PZT [16, 34]. It appears that the rate of the reaction (17) is too low to have
noticeable effects on the properties of PZT. Shimakawa and Kubo have proposed that PZT
color change (after exposure to H2) from white to black is due to the formation of such
oxygen vacancies [35]. It was also supposed that oxygen defects would produce donor levels
within the PZT band gap, which would account for the change in color and would increase
the leakage current in capacitors [35].
Another mechanism, by which H atoms could change the PZT structure, is the formation
of Pb vacancies. Indeed, the presence of metallic Pb at the electrode/PZT interface and at
grain boundaries after the hydrogen treatment was previously reported [34-36]. The
electrical properties of PZT may change due to the formation of Pb vacancies in PZT,
29
because they can produce free holes in the valence band of PZT. The formation of lead
vacancies is limited by the diffusion of lead, which is very slow in PZT at temperatures less
than 600C (DPb in PbTiO3 =7.210-4exp(-181800/RT) (cm2/sec), equivalent to 9.9910-15 cm2/sec
at 600C and to 2.710-11 cm2/sec at 1000C [37]) and is therefore likely limited only to the
interfaces [35]. As the amount of metallic Pb is low in the H2-treated PZT (less than 0.3%
[35]), Pb vacancies cannot be considered the responsible mechanism for the alteration of
ferroelectric properties and color change from white to black [35]. On the other hand,
Ikarashi [36] has concluded that the reduction of PbO in PZT to metallic lead is the main
reason for the degradation of the ferroelectric properties of PZT [36]. The author has
suggested that the degradation of the properties of PZT by hydrogen annealing could be
avoided by using electrode materials that prevent the Pb diffusion from PZT [36]. It should
be noted that the reduction of PbO in PZT to metallic lead does not occur only due to
reducing atmosphere. H can diffuse into the PZT structure and change its atomic bonding
[34], leading to reduction of PbO in PZT to metallic lead. No reduction to metallic lead was
observed in the bare PZT plates up to 600C, while for PZT samples with Pt electrode the
reduction to lead was observed at temperatures as low as 320C [34]. The changes in the
atomic bonding of PZT can be due to the existence of H+ within the lattice and its bonding
with O2-. This is discussed in the next section.
Damage of the crystalline lattice of PZT
Another mechanism proposed for the changes of the electrical properties of PZT is
the ionization of hydrogen atoms inside the lattice of PZT [31]:
30
H H+ + e- (19)
Upon ionization, hydrogen releases an electron, which decreases the resistivity of the films,
and the produced hydrogen ion will interact with an oxygen ion to form a polar hydroxyl
bond [OH]- [31]:
H+ + O2- [OH]- (20)
Aggarwal et al. presumed that the above mechanism is the main reason for the changes in
the ferroelectric properties of PZT [31]. They have concluded this from the fact that oxygen
diffusion coefficient in PZT at 200°C is not high enough to cause significant damage. Another
reason to support this idea is that the annealing of PZT films in oxygen deficient atmospheres
does not affect their ferroelectric properties [31]. It was concluded that the reactions (3) and
(4) during PZT annealing with forming gas are the primary avenues for the degradation of
ferroelectric properties [31]. The [OH]− ion acts as a fixed dipole, which does not allow the
switching of the ferroelectric domains [17]; this idea has also been supported by theoretical
studies [29-30]. As said before, it is supposed that the bonding of hydrogen with oxygen
anions may change the atomic bonding of oxygen with the other elements of PZT (Pb, Zr,
Ti) and as a result, it might cause some changes in the atomic bonding of PZT elements [35].
Among the reactions mentioned above, probably the reactions (19) and (20), occurring in the
crystalline lattice of PZT, are most likely responsible for the changes in the electrical
properties of PZT. That is because they affect the crystalline lattice of PZT, while the other
reactions just modify the interfaces.
31
2.4 Dielectric spectroscopy of PZT
Dielectric spectroscopy has been used for studying the properties of a wide range of
materials, such as glasses, polymers, and ceramics [38]. This technique measures the
polarization response of a dielectric medium to an applied electric field; when an electric
field is applied to a dielectric medium, charges, (including ions and electrons), molecules, and
dipoles are displaced according to the applied electric field, which causes polarization in the
dielectric medium. By measuring the polarization of the dielectric medium, different
parameters of the dielectric medium, such as impedance (Z), admittance (Y), and dielectric
constant (ε) can be obtained. By analyzing these parameters, useful information can be
obtained regarding the charges, dipoles, and molecules inside the dielectric medium. The
parameters commonly used for analyzing insulators include dissipation factor DF and
dielectric constant ε, which are measured in a wide range of frequencies (from mHz to MHz)
and temperature, and are then used to analyze the data by fitting the results to one of the
available mathematical models [38].
The classical model of dielectric relaxation of a dielectric medium containing dipoles
is the Debye model. According to this model, the polarization ( ) of a dielectric medium
changes in accordance to following equation:
( ) (
) (21)
32
where is a constant, t is time, and τ shows the relaxation time of the dipoles. Based on the
equation (21) for polarization, the complex dielectric constant ε (where ε= ε´+iε´´) can be
expressed as
( )
(22)
where ε is the dielectric constant when , εs is the dielectric constant when 0, is
the angular frequency, and i is the imaginary number [38]. A more flexible model,
commonly used for modeling the dielectric constant data, is the Havriliak–Negami equation
[38-39]. According to this model, the complex dielectric constants ε of a dielectric can be
evaluated by:
( )
( ( ) ) (23)
where θ and β are constants between 0 and 1 [39]. By fitting the dielectric data to one of the
above equations, one can obtain different information about the dipoles inside the dielectric
medium, such as the number of dipoles and the activation energy for moving (or hopping) of
ions. For example, Kamishima et al. [40] investigated the dielectric properties of proton
conductor Yb-doped SrZrO3 after hydration in water, and using this technique they were
able to anticipate the position of the [OH] bonds inside the SrZrO3. In this work, we have
attempted (for the first time according to our knowledge) to use this technique to assess the
effect of hydrogen on the properties of PZT.
33
2.5 High pressure hydrogen compatibility of PZT
The high pressure hydrogen compatibility of PZT is an issue which has recently been
raised due to the possible application of piezoelectric actuators in the hydrogen fuel injectors.
Modern electronic fuel injectors that use lead-zirconate-titanate (PZT) based actuators for
valve opening, instead of the conventional solenoid technology, have been introduced by the
leading engine manufacturers in recent years (Figure 2). Since the valve is actuated quicker
(i.e. about 5 times faster [41]) by the piezoelectric actuators than the conventional solenoid
technology, very precise injection intervals become possible between the pre- and main
injection. Consequently, fuel consumption and emissions are noticeably reduced (by up to 15
percent) [42]. Piezoelectric actuators also facilitate an increase in the injection pressure, up
to 250 MPa; the higher the pressure and the more accurate the dosing and timing of the
injection, the more efficient (and therefore less polluting) the combustion event becomes
[42]. It should be noted that whereas the valve needle stroke was fixed in the previous
electromagnetic injection systems, in injectors where the piezoelectric actuator acts directly
on the needle, the needle stroke can be varied by changing the magnitude of the applied
voltage, thus enabling better control over the valve opening.
It was reported previously that PZT thin films lose their ferroelectrical properties
after hydrogen treatment [2]. However, the hydrogen environment used in the published
studies [2, 9-10] is not comparable to the hydrogen environment conditions in an engine,
where the pressure of the gas is up to 30 MPa, and the maximum operating temperature of
34
the PZT ceramic is about 100C. Alvine et al., have investigated the effect of high pressure
hydrogen on the properties of PZT films [43]. They studied the structural and compositional
changes of PZT thin films (50 nm) after 24 hours of high pressure hydrogen treatment
(p=13.8 MPa, T=100C). The most important structural changes which they observed was the
hydrogen induced blistering on the surface of bare PZT films and PZT film with Pd
electrodes [38]. They have also observed “significant mixing of the Pd layer into the PZT film
along with migration of Pb into the Pd layer” [43]. The hydrogen absorption for bare PZT
films was about 10 at%. The Pd layer on the surface of PZT films had a considerable effect on
the amount of hydrogen absorption: due to the presence of the Pd layer the hydrogen
concentration in the PZT ceramic is increased to nearly 20 at% [43]. Other results from
their experiments are as followings:
(a) Piezo actuators can degrade in high-pressure hydrogen environments due to the
hydrogen uptake in the PZT plates.
(b) The amount of hydrogen absorption is a function of the metallic electrode and it increases
for different electrodes in the following order: Pd> Al> W> Ti> Cu.
(c) Lead migrates into all the above mentioned electrodes, with the possible exception of Ti.
One of the main issues not addressed in the work of Alvine et al. is the possible effects
of the high pressure hydrogen environment on the electrical properties of PZT. Therefore,
in the present work, in addition to investigation the effect of high-pressure hydrogen
environment on the microstructure of PZT, we also investigated the effect of hydrogen on
the electrical properties of PZT.
35
2.6 Methods of decreasing the hydrogen damage to PZT
One such method is based on the experimental observations that the degradation of
properties of PZT is related to the hydrogen reactivity and absorption of the metallic
electrodes. For example, PZT-electrode assembly with Au electrodes showed the least
amount of degradation when subjected to an hydrogen atmosphere, whereas Pt was the
worst electrode [20], due to the catalytic nature of Pt in dissociating the hydrogen molecules
into atoms. Therefore it has been suggested to use electrodes with less catalytic activity such
as Au, instead of electrodes such as Pt [9]. Moreover, it was suggested that oxygen plasma
treatment of the Pt electrode can be used to reduce its catalytic activity [44], as this
procedure modifies the surface of the electrode. Conductive metal oxide electrodes like IrO2,
LaNiO3 have been tried, and it has been found that annealing in a hydrogen containing
atmosphere will not degrade the properties of PZT with such electrodes [20]. Abdolghafar
et. al. proposed IrO2 as the top electrode to prevent the hydrogen damage to the PZT during
hydrogen treatment [20]. Their results show that PZT capacitors with IrO2 electrodes have
poor hydrogen resistivity because of the reduction of the IrO2 to metallic Ir during hydrogen
treatment; however, after oxygen pre-annealing at 600C, the PZT capacitors with IrO2
electrode showed excellent hydrogen damage resistivity. The hydrogen damage resistivity
after oxygen annealing is attributed to the enhancement of the IrO2 structure by oxidation.
Another technique designed to improve the hydrogen resistivity of the PZT
capacitors is using a hydrogen diffusion-barrier layer on top of the capacitor. This technique
36
prohibits hydrogen diffusion by encapsulating the whole capacitor (electrode/PZT/electrode)
in hydrogen barrier layer(s), sometimes called a hard mask. It has even been shown that even
sidewall diffusion barriers can dramatically enhance the hydrogen resistivity [40]. Both
conductive (e.g. TiAlN) and non-conductive layers (e.g. Al2O3, SiO2) can be used as a hard
masks. Saito et al. have investigated different oxides (Al2O3, HfO2, Bi3Ti4O12, ZrO2 and SiO2)
as encapsulation layers [46]. They suggested Al2O3 and SiO2 as promising hydrogen diffusion
barrier layers.
While the above techniques try to prohibit hydrogen damage during the hydrogen
treatment process, another idea is to recover the properties of PZT capacitors after hydrogen
annealing [21]. This could be done by high temperature (600-700C) annealing of the PZT
capacitor in atmospheres like N2 or air after the hydrogen gas treatment. It has been reported
that the Pt/PZT/Pt assembly will recover its properties after treatment in O2 gas [17 and 21].
The recovery of the properties has been suggested to be due to the diffusion of hydrogen
atoms out of the sample during the post-annealing treatment. These results confirm the
recently published theoretical and experimental results by Bjorheim et al., who investigated
the hydration thermodynamics of PbZrO3 and concluded that protons which are absorbed
inside PZT from the hydrogen gas can be removed from PZT by heat treatment in air at
temperatures higher than 700C, or at lower temperatures in dry oxidizing or inert
atmospheres [47].
37
3 Scope and Objectives
Scope
The investigation of the interactions of hydrogen with PZT is an important topic both
from practical and scientific points of view and still many issues need to be addressed in this
regard. While a lot of attention has been paid to the effects of hydrogen on the ferroelectrical
properties of PZT, there are very few publications on the effects of hydrogen on the
piezoelectrical properties of PZT. This is an important issue for using the piezo-actuators in
the advanced internal clean-combustion engines, and indeed there is very limited
understanding of the performance and durability of such piezo-actuators in hydrogen
environments. The broad scope of this project is to examine the key research issues related to
the performance of the PZT-based piezo actuators exposed to hydrogen, and to develop
methods for preventing or limiting the damaging effects due to hydrogen, with the aim of
extending the lifetime of the actuators. Specifically, the scope of this work involves the
following activities:
(a) Bare PZT plates and similar plates including silver electrodes (Ag/PZT/Ag) are
exposed to hydrogen atmosphere at 10 MPa pressure for 200-1200 hours. The changes in the
microstructure of PZT plates are investigated using scanning electron microscopy combined
with energy-dispersive X-ray spectroscopy (SEM/EDX), and X-ray diffraction (XRD). The
effect of hydrogen on the electrical properties is investigated by measuring the changes in
the capacitance of Ag/PZT/Ag capacitors online during the hydrogen treatment.
38
(b) The kinetics of PZT structural modification due to hydrogen exposure is
investigated using online monitoring of the electrical properties of PZT. Specifically, the
changes in the capacitance of the Ag/PZT/Ag capacitors are measured online during the
hydrogen treatment at temperatures higher than the Curie temperature. Furthermore, we
measure the dielectric constant and dissipation factor of PZT after the hydrogen treatment in
a wide range of frequency (from 12 Hz to 200 kHz) and temperatures (25 to 400C); these
data are then analyzed by fitting the results to one of the existing mathematical models.
(c) The effect of hydrogen on the microstructure and electrical properties of PZT in
the tetragonal phase (i.e. at room temperature) is investigated using the water electrolysis
technique, wherein PZT plates are exposed to atomic hydrogen generated in water
electrolysis for 6 and 48 hours. SEM is used for evaluating the changes in the microstructure.
The capacitance of Ag/PZT/Ag capacitors is measured on-line during the water electrolysis
and after finishing the hydrogen exposure (i.e. during PZT aging in air), to evaluate the
effects of hydrogen on the electrical properties of PZT.
(d) Alumina coatings are applied to PZT plates using sol-gel technique, to explore the
possibilities of decreasing H2 damage to PZT. The functionality of the coating against
hydrogen damage is evaluated by water electrolysis technique.
It is anticipated that the findings of this project will contribute to the fundamental
understanding of PZT-hydrogen interaction, and will also provide Canadian clean-engine
technology developers with strategies for improving hydrogen technology, thus becoming
more competitive in the global market.
39
Objectives
The broad objective of the present work is to evaluate the interactions between the
PZT-based piezoelectric materials used in piezo actuators, and hydrogen. Based on the
improved understanding of such interactions and the resulting damage to PZT, we aim to
propose methods to decrease the negative effects of H2 on PZT. Within this broad goal of the
project, we have the following specific objectives:
1. To evaluate and quantify the effects of long-term high-pressure gaseous hydrogen
exposure on electrical properties and microstructure of PZT
2. To determine the parameters describing the kinetics of the interactions between
hydrogen and PZT, in particular the incorporation of hydrogen into PZT and the resulting
changes in the PZT properties
3. To evaluate and quantify the effects of hydrogen on the electrical properties and
microstructure of PZT below the Curie temperature, i.e. in the temperature range of the
actuator's use in the engine
4. To propose and develop methods to decrease and prevent the hydrogen damage to
PZT, such as through the deposition of protective coatings.
40
4 Materials and Methods
4.1 Samples
“PZT” plates, 10×10×1 mm, (PIC 255, PI Ceramic GmbH, Lindenstrasse, 07589
Lederhose - Germany) of the composition Pb(Zr0.53Ti0.47)O3 with 1% Nb2O5 dopant were used
in this work. The effect of hydrogen on PZT microstructure was investigated using poled
PZT plates with electrodes, as well as bare, not poled plates. Two types of electrodes were
used: silver (which contained small additions of Bi) and silver-palladium alloy (76 wt% Ag +
24 wt% Pd, as given by SEM/EDX); they were screen-printed on the large faces of the plates,
in 20 µm layers. Silver and their alloys are usually used in making the actuators, and
therefore, we considered this type of electrodes. The microstructure of the bare PZT samples
is shown in Figure 17.
Figure 17- The microstructure of bare PZT plates
bare PZT
41
The interface between the electrode and PZT is shown in Figure 18. As it can be seen
in this figure, there is a good adhesion between the electrodes and the PZT for inside section
(Figure 18a) and poorer on the edge section (Figure 18b). Later it will be shown how
hydrogen affects this interface.
Ag or Ag/Pd electrode
PZT
Figure 18- The microstructure of the PZT samples
Figure 19 shows micrographs of as-received electrodes' surfaces; the Ag-Pd electrodes
were much more porous than the Ag electrodes.
Ag or Ag/Pd electrode
PZT
42
Figure 19- Micrographs of the surface of the electrodes: silver (a) and silver-palladium (b)
4.2 Gas hydrogen treatment
Two hydrogen gas treatment procedures were used in this work. The one which we
call “High-Pressure” hydrogen treatment is schematically shown in Figure 20. For this
condition of hydrogen treatment different exposure times (200-1200 hours) were used, while
the pressure and temperature were kept constant at 10 MPa (pure hydrogen), and 100C,
respectively. Before point (a), the chamber was purged several times with argon to remove
air. The temperature and pressure of the gas were chosen according to the practical condition
of the actuators in the engine [43]. Therefore, the results of this experiment can be used to
evaluate the possible effects of high-pressure hydrogen environment on the microstructure
of PZT plates used in fuel injectors. After finishing the experiments the samples were taken
Ag or Ag/Pd electrode
PZT
43
out from the chamber and microstructural analysis was done using XRD and SEM
techniques.
Tem
per
atu
re
a bc
d
e
Time
Vacuum On
Cooling in HydrogenHydrogen
On
Heater on
Figure 20- The time-temperature schedule of the ‘High-Pressure’ hydrogen treatment used in this work
Fuel injection actuators are made of many (>50) single PZT plates stacked together, and
the electrical properties of each single layer determine the performance of the whole
actuator (Figure 21).
+-
electrodes
PZT plate
+-
R
C
(a) (b)
Figure 21- Schematic of an actuator made from PZT plates stacked together, b) the equivalent electrical circuit
of an actuator
44
Therefore, measuring the capacitance of a single PZT plate is a suitable way to
investigate the effect of hydrogen on the performance of the actuator assembly in an
hydrogen atmosphere. To this end, two thin copper wires were attached to the electrodes,
and a GW Instek LCR meter (LCR-821) was used to measure online the capacitance of PZT
plates during hydrogen exposure. The capacitance measurements were performed with an
internal voltage of 0.125 V at a constant frequency of 1000 Hz. Figure 22 schematically
shows the experimental setup for measuring the electrical properties.
P= up to 10 MPaT= up to 700 °C
H2
LCR meteror
Potentiostat
Silver Electrodes
Copper wires connected to LCR meter
High-Pressure Vessel
PZT
PZT microstructure
(10×10×1 mm)
Connected to the vacuum pump and
hydrogen
Figure 22- The schematic of the setup used in this work for online monitoring the electrical properties
The other hydrogen treatment procedure, which we would call “High-Temperature”
hydrogen treatment, is schematically shown in Figure 23. From point “a” to “b” the sample
was heated in air up to the desired temperature in 450C-650C range. The chamber was then
vacuumed (700 mm Hg (0.09 MPa)) at point “b”, and at point “c” pressurized with 0.13 MPa
45
of 90% Ar/10% H2 gas mix. A drop of about 5C was observed, but the temperature
recovered after 15 minutes.
Tem
per
atu
re
a
b c d
e
Time
Heating in Air
Vacuum On
Hydrogen On
Cooling in Hydrogen
Figure 23- The ‘High-Temperature’ hydrogen treatment used in this work
To measure the resistance and capacitance of PZT plates, two thin copper wires were
attached to the silver electrodes using high temperature silver paste. The same LCR meter
(LCR-821) was again used to measure the capacitance and the ‘real’ part of the impedance
(ZRe) of PZT plates (Figure 22). The frequency was 1000 Hz for all measurements. The
electrical properties were monitored online during the hydrogen treatment: capacitance and
ZRe values were collected every 0.896 second.
Knowing the relation between the ZRe and the capacitance, the total resistance of the
PZT plates can be estimated. Figure 24 shows a typical ZRe-ZIm curve measured for this type
of PZT plates in air. Curves with the same trend were obtained for other temperatures, and
also in the hydrogen atmosphere.
46
According to Figure 24, the PZT plate can be considered as a parallel R-C circuit.
Therefore, the relation between the ZRe, R and C is as follows [49]:
(24)
Using equation (24) the ohmic resistance (R) of PZT can be calculated, but this method is not
without errors, especially at low temperatures. The reason is that with decreasing the
temperature, the Nyquist diagrams at high frequencies show a depressed semicircle; in other
words, equation (24) is no further valid [49]. Therefore, we used the R values obtained by the
equation (24) to just qualify the trend for the changes of R during the hydrogen treatment.
We cannot say that the R values obtained from the equation (24) are the exact values for R
during the hydrogen treatment; however, they can show the general trend for the changes of
the resistance. In order to measure the exact values for R, we used another technique,
detailed in the next paragraph. .
In order to determine the electrical resistance of PZT, the variation of resistance
from the ZRe-ZIm curves was also calculated. The ZRe-ZIm curves were recorded every 5
minutes during the hydrogen treatment; all ZRe-ZIm curves were semi-circles where the
diameter of the whole circle can be considered as the ohmic resistance of PZT. For this set of
experiments we used smaller PZT plates (4×4×1 mm), but with the same composition and
electrodes; the estimation of R for both sets of sample showed the same trend.
It should be noted again that we used the R values obtained either from equation (28)
or from the ZRe-ZIm curves just to identify the general trend for the changes of R during the
47
hydrogen treatment. The electrical property used to monitor the structural changes was the
capacitance C, which we were able to read directly from the LCR meter with reasonable
accuracy. The accuracy of the LCR meter used in this work was 0.05% for measuring the
capacitance.
Figure 24- A typical Nyquist plot for the PZT at 500°C in air (the frequency is swiped between 106 and 10-3 Hz)
4.3 Water electrolysis treatment of PZT
The water electrolysis technique was used to charge the PZT samples with hydrogen,
following a previously reported methodology [33]. In comparison to the treatment in
hydrogen gas, which needs temperatures higher than room temperature, this technique has
the advantage that it can be done at room temperature; this inhibits the de-polarization of
PZT samples due to elevated temperatures. The basic idea for this technique is that one of
the silver electrodes attached to PZT is used as the cathode during water electrolysis. In this
0 100 200 300 400
ZRek
0
100
200
300
400
Im (k
)
R
C
48
way, the hydrogen atoms which became released on the surface of the silver electrode can
further diffuse into the electrode and into the PZT attached to the electrodes. The water
electrolysis was performed in a 0.1 M NaOH solution, with a current density of 100 mA/cm2;
the current density was kept constant by varying the voltage, to ensure the same rate of
hydrogen release. The leakage of the solution to the interface between the electrode and PZT
and the release of hydrogen at the interface caused the detachment of the electrode from
PZT after very short experimental time (around 5 minutes). In order to restrict the leakage of
the solution to the interface between the Ag electrodes and PZT, the edges of the specimens
were encased in epoxy. After the samples were encased in epoxy, the electrodes did not
detach from the sample during the water electrolysis.
At different times after the beginning of the water electrolysis (ie. t= 10, 60, 120, 300,
600, 1200 minutes) the samples were taken out, dried, and the capacitance was measured. To
measure the capacitance of PZT plates, the same LCR meter (GW Instek-LCR-821) was used.
An internal bias of 0.125 V was used for measuring the capacitance, and the frequency was
kept constant at 1000 Hz during all measurements. Figure 25 schematically shows the setup
which is used for the water electrolysis experiment. After water electrolysis, the samples
were prepared for the microstructural analysis. In this order, they were grinded, polished
and then the cross section was studied by SEM.
49
Figure 25- Schematic of the setup for the water electrolysis experiment
In order to estimate the equivalent pressure of the hydrogen above the electrode
during the water electrolysis, and to compare the results with the gas hydrogen treatment,
we must first quantify the hydrogen absorption into the Ag layer during the water
electrolysis. The electrolysis is actually a common and efficient technique for charging
metals with hydrogen, wherein very high equivalent pressures of hydrogen can be produced
above the metallic electrode [50]. It has been reported that very high fugacity of hydrogen
on the order of 106 atmospheres (corresponding to pressures of about 104 atmospheres) can be
obtained with cathodic charging [50]. Because of this high efficiency in introducing the
hydrogen atoms inside metals, this technique has being used for producing metal hydrides,
and for studying of hydrogen embrittlement in metals [51]. Wu measured the hydrogen
solubility in iron by hydrogen cathodic charging [52]. At a potential of 0.25 V and in 1M
PZTAg
H+
H+
H
H
H
H
H
H
H+
0.1 M NaOH
anode (+) cathode (-)
50
H2SO4 solution at room temperature, he has found that the solubility of hydrogen in iron can
be described by the following relation:
( ) (25)
where is the charging current density. Therefore, for a typical value for (0.1A/cm2) at
room temperature, 0.53 ppm of hydrogen would dissolve in iron. On the other hand, the
solubility of gaseous hydrogen of pressure PH2 in Fe can be obtained from the following
relation: [53]
( ) (
)
(26)
Therefore, at 1 atmosphere partial pressure of hydrogen and at room temperature, the
solubility of hydrogen in Fe would be about 0.00076 ppm, much lower than the predicted
solubility of hydrogen in Fe during cathodic charging. Therefore, we can conclude that the
fugacity of hydrogen gas above the Fe electrode during water electrolysis is much higher
than the fugacity of gaseous hydrogen. This simple comparison confirms that high solubility
of hydrogen in metals can be obtained with cathodic charging. Danielson found that the
solubility of hydrogen in AA5083 aluminum alloy in cathodic charging (3.410-7 g-atoms
H/cm3) is higher than the solubility of hydrogen in gas atmosphere (110-11 g-atoms H/cm3),
by about 4 orders of magnitude [54]. He concluded that the cathodic charging of hydrogen
has a major effect on increasing the hydrogen solubility in Al.
There is no published data on the absorption of hydrogen atoms inside silver during
the cathodic charging. Therefore, it is impossible to estimate the equivalent pressure of the
51
hydrogen above the electrode during the water electrolysis. However, it will be shown later
(Section 5-3) that noticeable damage occurred to PZT after water electrolysis treatment. We
therefore propose that this high degree of degradation is due to the higher hydrogen
solubility in silver during the water electrolysis, in comparison to the gas atmosphere.
4.4 Alumina sol-gel coating
Sol preparation
Boehmite (ALOOH) sol was used as precursor sol for the alumina (Al2O3) coatings.
Boehmite sol was prepared by dissolving 10 gr boehmite powder (Dispersal Sol P2, Condea
Chemie GmbH, Germany) into 100 ml distilled water. The solution was mixed for 20
minutes at room temperature using magnetic stirrer. To increase the viscosity of the sol, PVA
solution (10 wt% PVA) was added to the Boehmite sol (for the total amount of PVA equal
4.5 wt% of sol) and then solution was again stirred for 10 minutes to homogenize the sol
before using it for dip coating. Figure 26 schematically shows the procedure used for the
preparation of the sol.
52
Figure 26- Schematic of the steps used for the preparation of the alumina coating
The coating process
The dimension of PZT plates used for coating was 10×10×1 mm. Before coating, the
surface of the samples was ground, and then polished with 1 micron diamond paste. The
samples were washed with acetone before coating. To apply the boehmite sol to the surface
of PZT plates dip coating technique is used. “Dip coating" is a common coating technique
100 ml Deionized water
Stir at room temperature
for 20 minutes
10 gr AlOOH
100 ml PVA (10 wt%)
Stir at room temperature
for 20 minutes
Stable sol
Final sol
Dip coating with withdraw speed
of 3 cm/min
Firing at 450°C for 5 hours
53
used in applications such as optical coatings and membranes. During the dip coating process,
the substrate is submerged into the sol and carefully withdrawn out of the sol [55]. We
selected this technique for the coating as it is relatively simple, and the samples were
relatively small. The dip coating was done using the SCS PL3201 Dip Coater (manufactured
by SCS, 7645 Woodland Drive, Indianapolis, Indiana, 46278, USA) with a withdraw speed of
3 cm/minutes. The thickness of the coating can be evaluated by the Landau-Levich equation
[56]:
( )
( ) (27)
where is the coating thickness, is the viscosity of the sol, is the withdraw speed, is
the sol-vapor surface energy, is the density of the sol, and is the gravity. Considering the
above equation and using the and values for water, and considering the viscosity of the
sol equal to 20 mPa.s [57], the thickness of the single-layer of as-deposited coating (i.e. before
heat treatment) was calculated to be about 7 µm.
Firing treatment
Heat treatment is always needed for the densification and crystallization of the sol
layer which is applied on the surface on the PZT plates. In this way, a relatively dense film
can be obtained on the surface, depending on the temperature and time of the heat
treatment. The heat treatment used in this work was at 450C in air for 5 hours [58]. When
doing high temperature processing of the coating layer, we have to consider its interaction
54
with the PZT (i.e. change of properties because of the changing chemistry). The temperature
that Boehmite starts to transforms to alumina is reported to be about 450C [58]. Therefore,
we selected this temperature for our heat treatment procedure. Furthermore, the lower the
heat treatment temperature, the lower the risk for cracking is during the cooling due to
thermal expansion. Based on the literature data [59-60] it is expected that after this treatment
the coating still contained about 40 vol% of porosity. Therefore, the presence of the coating
will not prevent access of molecular hydrogen to PZT surface, but it will prevent direct
contact between the metallic electrode and PZT. Thus it is expected that there will be no
access of atomic hydrogen to the coated PZT surface. The coating and firing steps were
repeated 3 times in order to achieve > 5 m thick coatings. As shown later in Section 5-4-1,
the actual thickness of the coatings after heat treatment, as determined by SEM, varied
between 5 and 10 µm.
Assessment of coatings effects
To assess the effects of the coatings on hydrogen penetration into PZT, water
electrolysis technique was used. In this regard, thin Au-Pd electrodes were sputtered on the
faces of the PZT plates coated with alumina. Au-Pd electrodes were also deposited on the
faces of a PZT plate without alumina coating and that sample was used as a reference sample.
The water electrolysis was performed in a 0.1 M NaOH solution, with a constant current
density of 100 mA/cm2 and with the voltage between 7-10 V. After water electrolysis, the
cross section of the sample was studied with SEM and the amount of hydrogen damage was
studied.
55
4.5 Characterization techniques
4.5.1 X-ray diffraction analysis (XRD)
X-ray diffraction analysis (XRD) was used in this work for the phase analysis of PZT
plates after hydrogen treatment, using Rigaku Multifles diffractometer operated at 40kV, and
30 mA (X-ray:CuKα). To perform the XRD analysis, the metallic electrodes were detached
from the PZT plates, and the remaining bare PZT plate was used for the analysis. The
dimensions of the PZT plates were about 10101 mm. XRD spectrums were collected over
diffraction angles between 20 and 80 at a speed of 2 /min.
4.5.2 Scanning electron microscopy (SEM)
The microstructural investigation was performed with a Hitachi S 3000-N Scanning
Electron Microscope with an Electron Dispersive X-Ray Spectroscopy detector and Quartz
X-One X-ray post-processing software. The SEM/EDS investigations were performed under
low vacuum (20 kPa) - variable pressure mode, using the back scattered electron method,
which offers very good element contrast and allows the study of non-conducting specimens
without applying any conductive coating to avoid charging and contamination and surface
alterations of samples through additional processing and handling.
4.5.3 Electrical properties measurements
To measure the capacitance of the PZT plates, a GW Instek LCR meter (LCR-821) was
used. In online measurements, capacitance measurements were performed with an internal
56
voltage of 0.125 V at a constant frequency of 1000 Hz. The data were collected every 0.896
second. Two modes were used for measuring: C-DF mode for the capacitance (C) and the
dissipation factor (DF) of samples, and Z-theta mode for the real part of the impedance (ZRe)
of samples. The dielectric constant () of the samples calculated through the equation of the
capacitance for parallel-plate capacitors: = (dC)/( 0a) where d is the distance between the
electrodes (1 mm in this work), C is the capacitance of parallel plate capacitors measured by
LCR meter, 0 is the permittivity of vacuum, and a is the area of the electrodes (1 cm2 in this
work). To measure the capacitance and the dissipation factor of PZT capacitors after water
electrolysis, the same equipment and internal voltage was used; however the frequency was
not constant, and it was changed between 12 and 200 kHz.
To measure the resistance of PZT capacitors, the ZRe-ZIm curves were recorded every 5
minutes during the hydrogen treatment; all ZRe-ZIm curves were semi-circles where the
diameter of the whole circle can be considered as the ohmic resistance of PZT. To obtain the
ZRe-ZIm curves (or Nyquist plot), a potentiostat VersaSTAT 4 was used, and the frequency of
the applied voltage (1 V) was swiped between 106 and 10-3 Hz.
57
5 Results and Discussion
5.1 High-Pressure conditions (T=100C, p=10 MPa, t=200-1200 hours)
5.1.1 H2 effects on PZT microstructure
To address the high pressure hydrogen compatibility of PZT plates, a high-pressure
hydrogen treatment was considered as is described in Section 4-2. The results of this study
can be used to evaluate the possible effects of high-pressure hydrogen environment on the
microstructure of PZT plates used in fuel injectors. The results of this experiment are
brought in Sections 5-1-1 (microstructure) and 5-1-2 (electrical properties).
Bare PZT Plates
Figure 27 shows micrographs of the top surface (10x10 mm) of bare PZT plates before
(as-received) and after the exposure to the hydrogen atmosphere for 1,200 hrs at 10 MPa and
at 100C; no sample preparation was done before taking the pictures. No noticeable
structural changes were observed on the bare PZT plates in Figure 27; however, the higher
magnification images of the surfaces show that the grain boundaries of the samples treated
for 600 and 1200 hours form clear discontinuities, approximately 100 nm wide. As shown in
Figure 28b, the PZT grains in the sample treated for 1,200 hours are separated, suggesting
that the grain boundaries are affected by the hydrogen. The XRD results of the as-received
and hydrogen-treated bare PZT up to 1200 hours at 100°C did not indicate any new
diffraction peaks, Figure 29. However, it was observed that the XRD pattern of the hydrogen
treated samples has shifted to lower two-theta angles by about 0.4 degree.
58
bare PZT
Figure 27- Micrographs of PZT surface: a) the as-received sample; b) after 1200 h hydrogen treatment
bare PZT
Figure 28- Surfaces of PZT plates at higher magnifications: (a) before and (b) after 1200 h hydrogen treatment
One of the reasons for such systematic shift could be change of internal stresses
during the treatment [61], particularly in the regions close to the surface of the PZT plates.
59
As it can be seen in Figure 27, the grain boundaries on the surface of PZT plates are corroded
after the hydrogen treatment, which may affect the internal stress. Furthermore, changes of
the lattice parameters due to the dissolution of hydrogen atoms in PZT could also cause shifts
in the diffraction pattern [9, 18, 35]. While we were not able to measure the hydrogen
content of our samples after hydrogen treatment, it has been reported that high-pressure
hydrogen dissolves in PZT to a level of 4-10 at% [43].
Figure 29- XRD results of bare PZT for as-received and after 1200 h hydrogen treatment
The microstructure of the hydrogen-treated samples was also investigated in the 1x10
mm cross-sections of the plates. Figure 30 shows the cross-sections of the as-received sample
and the sample which was hydrogen treated for 1200 hours.
10 20 30 40 50 60 70 80 90
2
as-received
hydrogen treated
(00
1)
(10
0)
(11
0)
(11
1)
(00
2)
(20
0)
(10
2)
(21
0)
(11
2)
(21
1)
(02
2)
(22
0)
60
bare PZT
Figure 30- Cross-section of the as-received sample (a) in comparison to the cross section of the sample after
1200 h hydrogen treatment (b)
According to this figure, one can conclude that no changes have occurred in the
regions far from the surface (below 10 µm depth). In other words, the hydrogen damage
appears to be limited to the regions close to the surface, while the center of the sample was
not affected (as observed under SEM). This is better shown in Figure 31.
Figure 31- Cross section of the hydrogen treated sample for 1200 hours, close to the surface
61
The general conclusion from the SEM microstructural observations of the bare PZT
plates is that the high pressure hydrogen atmosphere would affect the microstructure of the
PZT only close to the surface (to 10 µm depth), and no differences were observed in the
microstructure of the samples farther from the surface.
PZT Plates with Ag Electrodes
An image of the unprocessed side face (10x1mm) of a PZT plate with silver electrode
after 400 hours of hydrogen treatment is shown in Figure 32, clearly showing a degraded
layer formed just adjacent to the electrode. Similar microstructures were also observed for
the 200 and 1200 hours hydrogen-treated samples; the degree of damage was proportional to
the duration of exposure. The higher magnification image of the degraded layer, Figure 32b,
shows a porous structure, where voids are left behind by individual PZT grains detached
from the surface of the sample.
Ag electrode
PZT
Figure 32- Low magnification (a) and high magnification (b) images of damaged layer on the PZT surface next
to the Ag electrode after 400 hours hydrogen-treatment
62
Such a damaged layer was not observed in the cross-section of any of the treated
samples with Ag electrodes, as shown for example in Figure 33. The hydrogen treatment
conditions for the samples shown in Figure 32 and Figure 33 are the same; however, Figure
32 shows an image from the surface of the sample while Figure 33 shows an image from the
cross section of the sample. As this porous layer formed only on the surface of the PZT plates
and only next to the electrodes, we conclude that this damaged layer is probably due to the
spillover of hydrogen atoms from the surface of silver electrodes to the surface of PZT. By
hydrogen spillover we mean the formation of hydrogen atoms on the surface of the metallic
electrode attached to PZT and diffusion of those hydrogen atoms to the surface of the PZT.
Therefore hydrogen spillover is just limited to the surface and regions of PZT close to the
electrode. This is in accordance to our microstructural observations. This is schematically
shown in Figure 34. The spillover of hydrogen from the surface of metals to the surface of
oxides has been reported previously in systems such as Pd/SiO2 [62], Pt/Al2O3 [63], Pt/WO3
[64].
Another feature observed on these samples was the detachment of the electrodes
from the PZT surface after prolonged hydrogen treatment of 600 hours and 1200 hours, as
shown in Figure 35.
63
Ag electrode
PZT
Figure 33- The interface of the Ag electrode with PZT after grinding and polishing, for the as-received sample
(a) and for the sample hydrogen-treated for 400 hours (b); no detachment of the electrode from the PZT and no
damaged layer are visible
Figure 34- The spillover mechanism of hydrogen atoms from the surface of the Ag electrode to the surface of
the PZT
H2HHH2 H
H
H
H
H
Electrode
PZTHydrogen spillover
64
Ag electrode
PZT
Figure 35- The detachment of the Ag electrode from the PZT for the sample treated for 600 h
This could be due to the accumulation of hydrogen molecules at the interface of silver
electrode and PZT. For the sample treated for 1200 hours, the electrode was detached from
the PZT on almost half of the interface. The electrode itself was damaged at the edges, and
cracks in the silver electrode were also observed for the sample heat-treated for 1200 hours,
Figure 36.
Ag electrode
PZT
Figure 36- Detachment of the Ag electrode from PZT; some cracks are present on the surface of the electrode
for the sample heat-treated for 1200 hours
65
PZT Plates with Ag/Pd Electrodes
Degradation of the properties of PZT after hydrogen treatment has been reported to
correlate with the type of electrode in contact with PZT [9, 21]. It was proposed [21] that H2
dissociates at the surface of the electrode, and H atoms diffuse to the electrode/PZT interface;
hydrogen atoms that reached the electrode/PZT interface could further diffuse into the PZT,
or re-combine to form molecular H2, leading to local blisters [21]. To investigate the effects
of electrode on the degradation of PZT, plates with Ag/Pd electrodes were also evaluated in
this work. A micrograph of the unprocessed side face of a PZT plate with Ag/Pd electrodes
after hydrogen treatment for 200 hours is shown in Figure 37a; a damaged layer similar to
that seen in Figure 37b formed in the vicinity of the electrode.
Ag/Pd electrode
PZT
Ag/Pd electrode
PZT
Figure 37- Micrographs of the sample with Ag/Pd electrodes after hydrogen-treated for 200h: the 1x10 mm side
face (a) and its cross-section (b)
66
However, the layer was also limited only to the surface of the sample; no damaged
layer was visible in the cross-section (Figure 37b), but the electrode itself was degraded –
either detached or weakened to the extent that it was destroyed during the sample
preparation.
The surface of the side of the sample (1x10mm) hydrogen-treated for 400 hours is
shown in Figure 38b. Evidence of extensive corrosion is seen, especially in the immediate
vicinity of the electrodes (Figure 38b, c). Such a structure is an indication of damage to the
grain boundaries, probably caused by the diffusion of hydrogen, possibly involving
recombination of atomic to molecular H2 at the grain boundaries. The micrograph of the
cross-section of the sample hydrogen-treated for 400 hours is shown in Figure 39 wherein in
some parts of the cross section a noticeable portion (about 100 µm depth below the electrode)
was corroded after hydrogen treatment. We have not investigated the effect of hydrogen on
the PZT plates with Ag/Pd electrodes for 600 and 1200 hours. However, we expect that
increasing the time of hydrogen exposure will cause more severe damages to these samples as
well.
67
Ag/Pd electrode
PZT
(b)
(b)
Ag/Pd electrode
PZT
(b)
(b)
Ag/Pd electrode
PZT
(b)
(b)
Figure 38- Surface of the side face of the sample with Ag/Pd electrode: (a) as-received, (b) hydrogen-treated for
400 h; noticeable corroded area next to the electrode (c)
When compared to the not-damaged sample with Ag electrodes (Figure 33b), the
depth of the hydrogen damage in the cross section of the sample with Ag/Pd electrodes
68
indicates that the samples with Ag electrodes are more resistant to hydrogen damage. As
palladium is well-known for its ability to absorb atomic hydrogen [65], from a practical point
of view this means that it is better to use Ag electrodes instead of Ag/Pd electrodes. If the
replacement of the electrodes is not possible, the amount of palladium in Ag/Pd electrodes
should be decreased as much as possible or other electrodes with less hydrogen reactivity
should be used in the manufacturing of the actuators.
Ag/Pd electrode
PZT
Figure 39- Micrograph of the cross-section of the sample shown in Figure 38
5.1.2 H2 effects on the electrical properties of PZT
A typical curve showing changes of capacitance of PZT plates with Ag/Pd electrodes
in high pressure (10 MPa) hydrogen atmosphere is shown in Figure 40a. With increasing the
temperature from 20C (point 1) to 100C (point 2), the sample capacitance also increases
69
from 1.67 nF (point 1) to 1.98 nF (point 2). This is most likely due to the fact that the
switching of the dipoles becomes easier at higher temperature [66]. Figure 40b shows the
variations in the capacitance and temperature; the increase of the capacitance follows the
increase of the temperature. Figure 41 shows the variation of capacitance of PZT plates with
Ag electrodes in argon atmosphere at the same pressure of 10 MPa. The changes of PZT
capacitance are similar to those in H2 atmosphere (the same trend for the capacitance
variation was also observed for the heat-treatment in air).
1 10 100
Time (hour)
1.6
1.7
1.8
1.9
2
2.1
C(n
F)
1
3
4
(a)2
0 4 8 12
Time (hour)
0
20
40
60
80
100
120
Tem
per
ture
(°C
)
1.92
1.94
1.96
1.98
2
2.02
C(n
F)
(b)
Figure 40- Capacitance of PZT sample with Ag/Pd electrode in high-pressure hydrogen atmosphere (a); at point
‘1’ the heater is on, and at point ‘3’ the heater is off. (b): capacitance variation with temperature in hydrogen
atmosphere
It seems that hydrogen has no noticeable effects on the capacitance of PZT plates
under the conditions of this work (T=100C, p=10 MPa, time=200 hours). The reason why
the capacitance drops after reaching the maximum value (point 2 in Figure 40a) could be the
70
increase in the temperature of the samples. The dielectric constant of ferroelectric ceramics
shows an aging effect after any abrupt thermal changes, or application of strong mechanical
stress [66], due to the rearrangement of the ferroelectric domains. The experiment was done
just once for the samples with Ag/Pd electrodes. The error for measured capacitance values is
within 0.05% according to the specification of the LCR meter used. Similar results were also
seen for the sample with Ag electrodes (Figure 42), in repeated experiments. The general
conclusion from the above results is that the dielectric constant of PZT plates will not change
in high-pressure hydrogen condition (p=10 MPa, T=100C) after 200 hours. However, in the
next section we will see that considerable changes occur in PZT microstructure and electrical
properties in high temperature conditions.
1 10 100
Time (hour)
1.6
1.7
1.8
1.9
2
2.1
C(n
F)
Figure 41- Capacitance of PZT sample with Ag electrode in high-pressure argon atmosphere
71
1 10 100
Time (hour)
1.6
1.7
1.8
1.9
2
2.1
C(n
F)
Figure 42- Capacitance of PZT sample with Ag electrode in high-pressure hydrogen atmosphere
72
5.2 High-Temperature conditions (T=450-600C, p=0.013 MPa)
5.2.1 H2 effects on PZT microstructure
To address the kinetics of interactions of hydrogen with PZT plates, a high-temperature
hydrogen treatment was considered as it is described in Section 4-2. While for the samples
hydrogen-treated in the low-temperature / high-pressure conditions we did not observe
noticeable changes in the microstructure of PZT (except for the sample with Ag/Pd
electrodes hydrogen treated for 400 h, Figure 39), we observed considerable changes in the
microstructure of the samples hydrogen-treated at high temperatures. Figure 43 compares
the microstructure of the side surface of the PZT plate with silver electrode in the as-
received condition and after heat treatment at 400 C for 2 hours. According to this figure,
noticeable damage has occurred on the surface of the sample hydrogen-treated at high
temperatures. Another aspect which was observed in some of the samples was the lead
reduction on the surface of some of the samples, in the regions adjacent to the electrodes.
This is shown in Figure 44 for the sample hydrogen-treated at 600C for 2 hours, wherein
lead particles on the surface of PZT can be observed as the bright-contrasted sub-micron
particles. It was considered that these particles were metallic lead based on the much higher
ratio Pb/O (14.0), compared to the surrounding PZT particles (2.8) (Table 2). However, no
lead reduction was observed in the cross- section of the samples.
73
Table 2- The EDX analysis for the bright particles in Figure 44b
Element O Ti Zr Pb
Concentration (wt%)
Bright particles 22 6 10 62
Concentration (wt%)
PZT grains 6 5 5 84
Ag electrode
PZT
Figure 43- Image from the side surface of the PZT plate with Ag electrode for (a) as-received and (b) after
hydrogen treatment (for 2 h / 400C / p= 0.013 MPa)
Ag electrode
PZT
Figure 44- Metallic lead in hydrogen treated PZT samples (for 2 h / 600C / p= 0.013 MPa)
74
XRD analysis was also done on the samples to study the possible phase changes in the
PZT after hydrogen treatment. To do this analysis, the silver electrodes were removed from
the plates after the hydrogen treatment. The results are shown in Figure 45. Comparing the
XRD results, one can say that no new peaks have formed and PZT still has maintained its
tetragonal structure after hydrogen treatment.
Figure 45- The XRD pattern for as-received and hydrogen treated PZT with Ag electrodes
(for 24 h / 550°C /p=0.013 MPa)
According to Figure 45, a small shift can be observed in the XRD pattern of PZT after
hydrogen treatment. Moreover, it can be seen that the relative intensity of some peaks
changed after hydrogen treatment. The reason for these changes could be due to the
hydrogen dissolution and the formation of lattice defects such as lead and oxygen vacancies
[35]. Nevertheless, the important point is that no new peaks have formed in the XRD pattern
of PZT samples after hydrogen treatment, indicating that no new phases, detectable by XRD,
have formed inside PZT. We also investigated the XRD patterns for other samples treated at
10 20 30 40 50 60
2
as-received
after treatment
(00
1)
(10
0)
(11
1)
(00
2)
(20
0)
(10
2)
(21
0)
(11
2)
(21
1)
(11
0)
75
500°C and 600°C, and again we did not observed any new peaks after hydrogen treatment
(Figure 46). Indeed, other studies about the structural changes in PZT after hydrogen
treatment also have not reported the formation of any new XRD peaks [9, 18, 35]. However,
changes in the lattice parameters of PZT after hydrogen treatment due to hydrogen
dissolution and formation of lattice defects such as lead and oxygen vacancies have been
reported [9, 18, 35]. These changes also could be the reasons for the observed small shifts in
the XRD pattern after hydrogen treatment (Figure 45).
Figure 46- The XRD pattern for hydrogen treated PZT with Ag electrodes at 500C and 600C
5.2.2 H2 effects on PZT electrical properties
Figure 47a shows a typical variation of the capacitance with the temperature and
duration of H2 exposure. The hydrogen treatment begins at t=0 in Figure 47 (point c in
Figure 23) and, as seen in the inset in Figure 47a (magnifying the effects in the first 40 min of
the exposure), the sample capacitance starts changing immediately after the exposure to H2
10 20 30 40 50 60
2
600C
500C
(00
1)
(10
0)
(11
1)
(00
2)
(20
0)
(10
2)
(21
0)
(11
2)
(21
1)
(11
0)
76
began. The reference test of 1 hour heat treatment in Ar atmosphere did not result in
noticeable changes in the PZT resistance or capacitance.
0 400 800 1200Time (min)
0
10
20
30
40
C (
nF
)
(a)
0 20 401.6
2
2.4
2.8
0 400 800 1200
Time (min)
20
24
28
32
36
40
ZR
e (k
)0
100
200
300
R (
k
)
(b)
Figure 47- (a) The general trend of PZT capacitance variation with time in hydrogen atmosphere at 500°C; (b)
measurements for the real part of the impedance (ZRe) and the calculated values of R according to equation (8)
(the data is obtained at the constant frequency of 1 kHz)
Figure 47b shows that the variation of the real part of the impedance ZRe (as measured
by the LCR meter) corresponds to the variation of capacitance observed in Figure 47a; the
secondary Y axis shows the resistance calculated from equation (8). While resistance
variation with time shows the same trend as the variation of capacitance, the relative amount
of the change of resistance is different from the relative change of capacitance. For example,
while after 20 mins the capacitance increases from 2 nF to 2.75 nF, there is a decrease in
resistivity to 60% of the initial value (i.e. from 250 k to 100 k ). To examine the trend of
resistance decrease, ZRe vs. ZIm curves were re-plotted and a typical result is shown in Figure
48a for the sample hydrogen treated at 550C. The intersection of the semi-circles in Figure
77
48a with the ZRe axis shows the ohmic resistance of the PZT plates [49]. In this way, we
measured the ohmic resistance of the PZT plates for different durations of hydrogen
exposure and the results are shown in Figure 48b. According to this figure, the resistance
decreases at almost the same rate as in Figure 47b, showing the general trend for the
resistance variation. The curves in Figure 48 also show the relatively fast decrease in
resistance in the first 20 min of H2 exposure. For the sample hydrogen-treated at 550C the
resistance decreases from 500 k to 200 k, and for the sample hydrogen-treated at 600C
the resistance decreases from 220 k to 100 k after 20 min of hydrogen exposure. The
decrease in resistivity after hydrogen treatment is also reported elsewhere [33].
0 200 400 600
ZRe(k)
0
200
400
600
ZIm
(k
)
0 min
5 mins
30 mins
Figure 48- The ZRe -ZIm plot for PZT plate heat treated at 550C and the resistance determined using the ZRe -ZIm
plots for PZT; the noise in the ZRe-ZIm plots corresponds to the times when the heater was on.
Figure 49 illustrates capacitance change for 530, 550 and 600C, showing a similar
trend, which suggests similar structural changes in PZT, although the time until the
capacitance reaches the maximum value decreases with increasing the temperature (600 min
0 50 100 150 200 250
Time (min)
0
100
200
300
400
500
R (
k
)
550 oC
600 oC
78
at 530C, 250 min at 550C, and 40 min at 600C). Therefore, we may conclude that the
structural changes affected by hydrogen in PZT are thermally activated processes, as it is
proposed in [14].
0 200 400 600 800 1000
Time (min)
0
20
40
60
C (
nF
)
530oC
0 100 200 300
Time (min)
0
20
40
60
80
C (
nF
)
550°C
0 20 40 60Time (min)
0
200
400
600
800
1000
C (
nF
)
600°C
Figure 49- The variation of PZT capacitance at 530°C, 550°C and 600°C
Based on the above data, it is hypothesized that the variations observed in the
capacitance and resistance of PZT are due to the structural modifications caused by the
presence of hydrogen in PZT. These structural changes seem to conform to “Isothermal
79
Solid-State Reactions” where the process occurs by the nucleation and the growth of the
product nuclei [68]. A typical plot α-time (t) for the solid state chemical reactions is shown
in Figure 50 [68], where α is the fraction of the volume converted to the product of reaction.
Before reaching point A, the short progress of the chemical reaction happens in the less
stable sites of the media in which reaction is occurring; (A-B) step shows the incubation time
needed for the development of growth nuclei; (B-C) is the much longer acceleratory period
related to the development of the stable nuclei formed in the previous step; in this period
new stable nuclei may also form; (C-D) is the step where the further expansion of the nuclei
is not possible due to the impingement and consumption of reactant and this leads to the
deceleratory or decay period [68]. Kinetics of such reactions can be written in the form of
f(α)=kt where k depends exponentially on temperature, k=A exp (-Q/RT), where Q is the
activation energy for the process [68-69], and A is a constant.
Time
0
0.2
0.4
0.6
0.8
1
1.2
A B
CD
start of reaction
completion of reaction
Figure 50- The general trend for the isothermal α - time plots having different time steps, time equal to zero
shows the start of the reaction [68]
80
“The rate-determining step can be either (i) diffusion, i.e. the transportation of
participants to, or from, a zone of preferred reaction, or (ii) a chemical reaction, i.e. one or
more bond redistribution steps, generally occurring at a reaction interface” [68]. The model
which is frequently used to describe the sigmoid isothermal α – time plots is the Avrami-
Erofeev (A-E) relation, also known as Johnson-Mehl-Avrami-Kolmogorov, or JMAK,
equation [68-69]:
[–ln (1– α)](1/n)=kt (28)
or
α = 1 – exp (–ktn) (28a)
where n is a constant. Depending on the nucleation and growth conditions, n can have
different values, as summarized in
Table 3.
Table 3- Different values of exponents for the equation (28) [68]
Model Phase Boundary control (n) Diffusion control (m)
Three-dimensional growth (Spherical particles of reactant)
Nucleation rate
1. Constant
2. Zero (instantaneous)
3. Deceleratory
4
3
3-4
2.5
1.5
1.5-2.5
Two-dimensional growth (Laminar particles of reactant)
Nucleation rate
1. Constant
2. Zero (instantaneous)
3. Deceleratory
3
2
2-3
2
1
1-2
One-dimensional growth (Lath-shaped particles of reactant)
Nucleation rate
1. Constant
2. Zero (instantaneous)
3. Deceleratory
2
1
1-2
1.5
0.5
0.5-1.5
81
The above equations are applicable to our condition if we define α as follows:
α= [C(t) – C()]/[Cmax – C()] (29)
where C(t) shows the capacitance at time t, Cmax shows the maximum capacitance, and is
the incubation time (up to point B in Figure 50). Considering the equations (28) and (29) we
have to fit the data to the following equation:
α= [C(t) – C()]/[Cmax – C()]=1 – exp (–k(t-)n) (30)
Therefore, to fit our data to the equation (30) we need to find the values for , n, and k. The
best fits were obtained with the values reported in Table 4. The results of the fitting for the
two temperatures of the 550C and 600C are shown in Figure 51 shows the results of the
fitting for all temperatures investigated in this work.
0 100 200 300
t- (min)
0
0.2
0.4
0.6
0.8
1
1.2
(a)
0 20 40 60
t-(min)
0
0.2
0.4
0.6
0.8
1
1.2
(b)
Figure 51- The results of fitting the capacitance data to equation (30) for the temperatures of 550C (a) and
600C (b)
82
ln(t-)
-15
-10
-5
0
5
ln(-
ln(1
-))
500
550
600
(a)
530
0.001 0.0011 0.0012 0.0013 0.0014
1/T (K-1)
-20
-15
-10
-5
ln(k
)
Q=42,433 ± 8087 J/mol(b)
Figure 52- (a) The results of fitting the capacitance data to equation (30); (b) the activation energy of
hydrogen diffusion, obtained from the fit
Table 4- The fitting values obtained for the equation (30) Temperature n (sec) ln(k) R-squared
500 C 1.690.2 9000500 -17.480.72 0.9813
530 C 1.360.2 3700250 -12.910.5 0.9861
550 C 1.810.2 2100250 -15.760.46 0.9863
600 C 1.480.2 45050 -9.990.24 0.9850
Based on the obtained values for k, the activation energy for the limiting process was
obtained to be 0.440.09 eV, Figure 52b. This is close to the reported activation energy for
diffusion of H+ in zirconate and titanate perovskite oxides, i.e. 0.44-0.50 eV for BaZrO3-(2-
10)%Y [70], 0.83 0.62 eV for BaZrO3 [71], 0.50 0.22 eV for SrTiO3 [71], 0.420.30 eV for
CaTiO3 [71]. Therefore we can hypothesis that the rate-limiting phenomenon for the
structural changes observed in PZT is the diffusion of protons. As activation energy for the
oxygen diffusion is about 1 eV [71], it appears that protons have the main contribution to the
structural degradation of PZT.
Comparing the average value of n (1.56) determined in this work with the data in
83
Table 3, one may hypothesize about the nucleation and growth mechanism by which
the structural changes occur in PZT (for example, three-dimensional, two-dimensional or
one-dimensional growth). During the growth, the coalescence of the developed nuclei and
the ingestion of undeveloped nucleation sites may occur, as the coalescence and ingestion are
characteristics of the sigmoid isothermal α – time plots [68]. It should be noted that the value
of n itself is not enough to confirm the specific nucleation and growth mechanism; some
independent confirmation, such as structural observations, are also needed. However, the
value of n suggests the rate-limiting phenomenon is hydrogen diffusion, and not its chemical
reaction with oxygen. The mechanism responsible for diffusion (or conduction) of protons in
oxides, especially in perovskite oxides, is believed to be the Grotthuss mechanism [72],
schematically shown in Figure 53.
Figure 53- The Grotthuss mechanism for diffusion of protons in PZT, including the reorientation and hopping
of protons between oxygen onions
According to this model, the diffusion of protons consists of (i) transfer from one
stable position to another stable position in the structure (hopping) and then (ii)
reorientation of the proton for transferring to another site. Therefore, it is probably the
proton reorientation and hopping between the different oxygen atoms which leads to the
84
expansion of the new structure formed in PZT. These protons penetrate into PZT and form a
solid solution within PZT. Figure 54 schematically illustrates this process.
Another explanation for the variation of the electrical properties would be that a new
“phase”, with a new crystal structure is forming in PZT, where protons become a part of its
chemical composition. Formation of new phases has been reported after H2 exposure for
other oxides, such as Sr6Ta2O11, and Ba2In2O5 [73-76]. The formation of the new hydrate
composition was reported to occur by a disorder-order phase transition leading to saturated
solid solution of the oxide and protons [73]. We however did not see any differences in the
XRD spectrum of the as-received and hydrogen treated samples (Figure 45), either because
no phase transition has occurred or due to the relatively small volume of such phase, e.g. <0.5
vol% [75].
Figure 54- Hypothetical schematic of the different modes which can be assumed for the dissolution of hydrogen
in PZT; (a) where the diffusion of protons into PZT occur uniformly from the surface; in this case the diffusion
equation with proper initial and boundary equation could be used for determining the total amount of protons
in PZT; (b) where the diffusion of protons can occur from limited places in the PZT; in this case the nucleation
and growth models can be used to describe the total amount of protons in PZT
85
To further study the effect of hydrogen on the electrical properties of PZT, we used
the dielectric spectroscopy technique. The technique measures the dissipation factor DF and
dielectric constant ε of the material in a wide range of frequency (from mHz to MHz) and
temperature, which are then used to analyze the data by fitting the results to one of the
physical or mathematical models [38] (the relevant relationships between dielectric
parameters of materials are listed in Chapter 2-3, equations (21-23)). We investigated the
dielectric constant of PZT after hydrogen treatment in the frequency range of 12-200 kHz
and in the temperature range of 25-400°C. The data obtained from these experiments were
analyzed to determine the effect of hydrogen on the dielectric properties of PZT. We have
also used this data to correlate the changes in dielectric properties with the effects of
hydrogen on the microstructure of the PZT plates.
Figure 55 shows the variations in capacitance (C) and dissipation factor (DF) after
hydrogen gas treatment versus temperature at the frequency of 1 kHz. A capacitance peak is
observed at 375C, which could be attributed to the phase transition of PZT from cubic to
tetragonal phase at the Curie temperature. Because the high dielectric constant of PZT at the
Curie temperature is due to the dipoles, it appears that these dipoles are still present in PZT
after the hydrogen treatment. However, the capacitance at the peak (20 nF) was only about
half of the capacitance before the hydrogen treatment (40 nF), therefore the dipoles inside
the PZT are probably affected by hydrogen. The decrease in dielectric constant after
hydrogen treatment has been reported before [31]. What should be pointed out is that the
existence of dipoles even after hydrogen treatment does not necessarily mean that their
86
directions could be switched with changing the direction of electric field as it proposed by
Aggarwal et al. [31]. They suggested that the [OH]− group acts as a fixed dipole, which does
not allow switching of the ferroelectric dipoles and domains inside the PZT [31], and
therefore PZT may not show polarization hysteresis after hydrogen treatment even if it has a
tetragonal structure [31]. Accordingly, the lower value of the capacitance may be attributed
to the interaction of [OH]- dipoles with the dipoles inside the PZT.
Figure 55- Changes of capacitance C and dissipation factor DF of hydrogen-treated sample (for 24 hrs / 550C /
p= 0.013 MPa) versus temperature (The thick grey line shows the changes of capacitance for as-received
sample)
Figure 55 also shows two relaxation peaks (R1 and R2) in the dissipation factor, while
no such peaks were observed for the as-received sample. Generally, there are two conditions
which can lead to such relaxation peaks in the dissipation factor. First, when there are
dipoles in the dielectric medium and therefore, the dipolar polarization mechanism is active
[38] (Figure 56). For this mechanism, the relaxation peak occurs when the natural vibrational
100 200 300 400 500
Temp (°C)
0
10
20
30
40
50
C (
nF
)
0
1
2
3
4
5
DF
R1
R2
87
frequency of such dipoles coincides with the frequency of the applied voltage [38].
Therefore, one might conclude that the relaxation peaks in Figure 55 may be due to the
dipoles formed in PZT after the hydrogen treatment.
a
+
-
+
- +
-
+
-
b
+
-
+
-
+
-
+
-
+
-
+
-
c +
-
+
-
+
-
+
-
+
-
+ + + + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + + + +
_ _ _ _ _ _ _ _
Figure 56- Schematics of the dipolar polarization mechanisms, wherein direction of the dipoles changes with
changing the direction of applied voltage
Another reason for the occurrence of the relaxation peaks could be the Maxwell-
Wagner (MW) polarization mechanism, which is active in inhomogeneous systems, where
the dielectric material consists of regions with different electrical properties (Figure 57). This
"extrinsic" polarization mechanism can be explained by considering the heterogeneity of the
system without any microscopic polarization process inside the sample [38, 77]. When a
voltage is applied across the dielectric medium, due to the differences in electrical
conduction in different regions of a heterogeneous material, charge accommodation occurs at
88
the interfaces of the different regions, leading to the increase in the capacitance of the
sample. The frequency response of such a system is similar to the frequency response of a
Debye relaxor [77], leading to a Debye type relaxation peak. The relaxation time in such
heterogenic systems depends on dielectric constant and conductivity of the different regions
in that medium. To further understand the physics behind such relaxation peaks, we have
changed the frequency of the voltage applied to the samples under the hydrogen atmosphere
at different temperatures, ranging from 200C to 325C for the first relaxation peak and from
22C to 42C for the second relaxation peak.
a
b
c
+ + + + + + + + + +
_ _ _ _ _ _ _ _
+ + + + + + + + + +
_ _ _ _ _ _ _ _
_ _ _ _ ___
_
+ + + + ++ + +
____
+++++
_+
++
___
Figure 57- Schematics of the Maxwell-Wagner polarization mechanism, wherein differences in the electrical
properties of different regions cause charge accumulation at the interfaces between the different regions,
leading to the increase of capacitance
89
Relaxation Peak #1 (R1)
Figure 58 shows the changes of ε´ and ε´´ (where ε´´=DF ε´) versus frequency in the
temperature range of 200-325C, with 25C increments.
1 100 10000 1000000
f(Hz)
0
2000
4000
6000
8000
'
(a)
325C
200C
1 100 10000 1000000
f(Hz)
0
1000
2000
3000
''
(b)
325C
200C
Figure 58- Variations of ε’ and ε’’ for hydrogen-treated samples with the frequency of applied voltage in the
temperature range of 200-325C, with 25C increments
While decreasing the temperature, the frequency at which the relaxation peak occurs
also decreases. This may be because the polarization process which has led to such relaxation
becomes slower at lower temperatures, as the relaxation time is inversely proportional to the
frequency at which the maximum occurs (fMax=1/τ, where τ is the relaxation time). This is
typical condition for the dipolar polarization, where dipoles change their positions with
changing the direction of the applied electric field [38]. At higher temperatures, such dipoles
can change their direction faster and as such, the relaxation peak moves to higher
frequencies [38]. Therefore, one may conclude that the first peak observed in Figure 55 is
due to the dipoles formed in PZT after the hydrogen treatment. To further investigate the
90
nature of such dipoles (such as the activation energy for the relaxation process), the results
should be fitted to one of the physical models of this polarization mechanism.
The model most frequently used for the description of such relaxation peaks is the
Debye model (Chapter 2-3, pages 32-33, equations (21-22)). We tried to fit our data to the
Debye equation, but the fit was poor (Figure 59). This is likely because the Debye equation
assumes that the dipoles do not interact with each other, but this is not usually valid for
dipoles inside a dielectric medium.
1 100 10000 1000000
f(Hz)
0
1000
2000
3000
'' Debye model
Figure 59- The results of fitting the ε´ and ε’’ data to the Debye equation for at T= 325C, for PZT hydrogen-
treated samples (for 24 hrs / 550C / p= 0.013 MPa)
A more flexible model, commonly used for modeling dielectric constant data, is the
Havriliak–Negami equation [38-39] (Chapter 2-3, page 33, equation (23)). The fit based on
the Havriliak–Negami equation is much better than the fitting results based on the Debye
equation, Figure 60 (Table 5 compiles the best fit parameters). An iterative MATLAB code
was developed and used for fitting procedure.
91
1 100 10000 1000000
f(Hz)
0
2000
4000
6000
8000
'(a)
HN model
1 100 10000 1000000
f(Hz)
0
1000
2000
3000
''
(b)
HN model
Figure 60- The results of fitting the ε´ and ε’’ data to the Havriliak–Negami equation for at T= 325C, for PZT
hydrogen-treated samples (for 24 hrs / 550C / p= 0.013 MPa)
Table 5- The fitting values obtained for the HN equation
Temperature Δε=εs-ε (sec) θ β
325 C 7300 0.0019 0.8 1
300 C 4900 0.0029 0.8 1
275 C 4100 0.004 0.7 1
250C 3200 0.0078 0.8 1
225C 2700 0.025 0.8 0.8
200C 2000 0.05 1 0.6
The activation energy for the relaxation process behind the relaxation peak #1 can be
evaluated from the values obtained for . As is the average residence time of an ion at any
given site, it changes with temperature according to =0×exp(-ΔH/RT), where ΔH is the
activation energy for ions jumping from one position to another [38]. This equation shows
that the kinetics of the relaxation of the system follows the Arrhenius law, i.e. the relaxation
time of the system decreases with increasing the temperature (as vibration frequency of ions
increases, the probability of ions jumping from one position to another position increases,
hence the average residence time or decreases). The relaxation time fit to the above
equation (Figure 61) yields the activation energy of about 0.66 eV.
92
0.0016 0.0018 0.002 0.00221/T (K-1)
-10
-5
0
ln(
)
Q=63,760 J/mol
Figure 61- The activation energy for the ion jumping, obtained from the fits, for PZT hydrogen-treated samples
(for 24 hrs / 550C / p= 0.013 MPa)
One of the microstructural changes reported for PZT after hydrogen treatment is the
lead reduction and owing to that, the presence of lead vacancies in PZT [35]. Indeed we also
observed lead reduction in our samples after hydrogen treatment (Figure 44). Therefore, the
relaxation peak #1 may be tentatively assumed to be due to the hopping of lead cations in the
PZT lattice. However, the reorientation of such dipoles should be relatively slow below
300C, where the diffusion of lead cations is very slow [37]. The activation energy for the
diffusion of lead ions in PbTiO3 has been reported to be about 1.89 eV [37], which is
significantly higher than the energy obtained here for the reorientation of the dipoles.
Therefore it can be concluded that the first relaxation peak is not due to the hopping of lead
cations. Another structural change proposed for PZT after the hydrogen treatment is the
existence of oxygen vacancies in the lattice of PZT [29, 35], so the relaxation peak could be
due to the hopping of oxygen anions in the PZT lattice. However, we believe that the
temperature is not high enough for the oxygen ions to have enough mobility in the PZT
93
lattice. Therefore, the dipole reorientation or the relaxation peak cannot be due to the
hopping of oxygen anions. Kamishima et al. [40] investigated the dielectric properties of the
Yb-doped SrZrO3 after hydration in water atmosphere. While they did not observe any
relaxation peaks for the pure SrZrO3, they did observe a relaxation peak for the sample with
1 wt% Yb. For samples with more than 1 wt% Yb, they observed yet another relaxation
peak. The activation energy obtained for the first relaxation peak was about 0.58 eV, and
they attributed this activation energy to the Yb-OH dipoles. They also attributed the second
relaxation peak, observed in samples with more than 1%Yb, to Yb-OH dipoles in the Yb-
clusters. Therefore, we might tentatively conclude that the relaxation peak which we also
observed is due to the dipoles formed by the protons with the dopant (Nb) in the PZT. We
realize that more experimentation and analysis is needed to fully confirm this hypothesis.
Relaxation Peak #2
Figure 62 shows the variation of dissipation factor (DF=ε´´/ε´) with frequency in the
temperature range 22-47C. When temperature increases, the frequency at which the
maximum occurs moves to lower values. If we assume that this relaxation peak is due to the
reorientation of dipoles inside the PZT under the applied electric field, then the movement
of the peak to the higher frequency values with decreasing temperature means that the
hopping of ions becomes slower with increasing temperature. However, this cannot be true,
as the vibration of the ions increases at higher temperatures, hence the possibility for their
hopping increases. Therefore, we can conclude that this peak is not due to the dipoles inside
94
the PZT. This unusual direct dependency of the relaxation time with the temperature was
also observed in other systems. For example, it has been reported for
Na58(AlO2)58(SiO2)136mH2O zeolite (NaY) of the faujasite type, and it was assigned to the
relaxation of the water molecules confined inside the molecular cages of NaY [78-79]. A
similar unusual relaxation process has also been observed for potassium tantalate niobate
(KTN) crystal doped with copper, where relaxation occurred below the ferroelectric phase
transition [80]. This relaxation process has been attributed to “the reorientation of virtual
dipoles provided by the Cu ions hopping between different states of local equilibrium” [81].
Figure 62- Variation of DF with frequency in the temperature range 22-42C, for PZT hydrogen-treated
samples (for 24 hrs / 550C / p= 0.013 MPa)
Different explanations could be considered for this non-monotonic relaxation
kinetics, e.g. [81] suggests that “this situation usually occurs for ‘small’ systems where
relaxing particles become able to participate in the relaxation due to the formation of some
1 100 10000 1000000
f(Hz)
0
0.1
0.2
0.3
0.4
0.5
DF
42C
32C
27C
22C
95
‘defects’ in ordered structure”. Therefore, we hypothesise that such a relaxation peak could
indicate the formation of structural defects which hydrogen produces in PZT. Indeed,
different structural defects have been proposed for PZT after hydrogen treatment, such as
oxygen and lead vacancies, and the formation of [OH]- dipoles [31]. Therefore, the
interaction between such defects and [OH]- dipoles in PZT might be the reason for the
formation of this relaxation peak. Another explanation for the second relaxation peak and its
unusual kinetics behavior could be due to the Maxwell-Wagner polarization mechanism, as
first proposed in [81-82]. As mentioned before, this polarization mechanism is active in non-
homogenous systems. In the previous section, we showed that a new structure forms inside
the PZT during the treatment in hydrogen atmosphere. Therefore, it can be assumed that the
second relaxation peak is due to the formation of a new phase in PZT, with different
electrical properties. In other words, this relaxation peak confirms our idea that a new
structure with new electrical properties forms inside the PZT during the hydrogen
treatment. It is very difficult to pinpoint the exact reason for the formation of the second
relaxation peak in the dissipation factor curve of PZT after hydrogen treatment; further
study is needed to understand the physics behind the second relaxation peak and the details
of the structural changes induced by hydrogen in PZT.
96
5.3 Water-electolysis treatment of PZT
5.3.1 Microstructure
The water electrolysis technique was used to charge the PZT samples with hydrogen,
following previously reported methodology [33]. In comparison to the treatment in
hydrogen gas (which needs elevated temperatures), water electrolysis technique can be
completed even at room temperature; this inhibits the de-polarization of PZT samples due to
the effects of temperature.
Figure 63 shows SEM images of the cross-sections through PZT plates in the region
adjacent to the electrode which functioned as the cathode. As seen in Figure 63a, the
structure of PZT immediately below the electrode is different from the structure in the
center, far from the electrode. Moreover, comparing Figure 63b and c, showing the same
magnification images, one can see that the grain boundaries of PZT after water electrolysis
are extensively corroded in comparison to the microstructure of the as-received samples.
97
Figure 63- Micrographs of the cross-section through PZT plate after water electrolysis: a) low-magnification
image (after 48 hours water electrolysis); b) microstructure of the corroded layer (close to the electrode); c)
microstructure in a region far from the corroded layer
According to the Figure 63, a corroded layer was formed just beneath the electrode
after the water electrolysis. To see the possible changes in the crystal structure of PZT in the
98
corroded layer, the silver electrode was detached from the surface of the PZT and the XRD
test was done on the top surface of as-received sample and on the corroded layer following
the treatment. Figure 64 shows the microstructure of the PZT surface right below the
electrode (after the electrode removal). As it can be seen, the grain boundaries are
extensively corroded, which is in accordance to Figure 63b. According to Figure 65, no new
peaks have formed in the XRD pattern of samples after the water electrolysis treatment;
therefore, we can conclude that no new phase has formed inside PZT after the water
electrolysis, and PZT still has its tetragonal structure after this treatment. However, a
systematic shift to lower two-theta values in the XRD pattern of samples after water
electrolysis is observed. Moreover, the tetragonal splitting of (100), (200) peaks became more
significant after the treatment. Huang et al. have investigated the changes in PZT structure
after water electrolysis using the XRD technique, and they observed a very small increase in
the lattice parameter of PZT after this treatment [83]. Therefore, the changes observed in
the XRD pattern of PZT after water electrolysis in this study can also be due to the
dissolution of hydrogen inside the PZT and the changes of the lattice parameters of PZT. It
should be noted that Huang et al. did not observe corrosion in grain boundaries of PZT after
water electrolysis, as we did in this work. Therefore, the corrosion of the grain boundaries
and changes in the microstructure of PZT could cause the difference in the XRD pattern
after water electrolysis.
Other parameter which should be considered here is the roughness of the surface of
samples. After the water electrolysis, the interface between the electrode and PZT was
99
extensively degraded, and the electrode was easily detached from the PZT surface. After
removing the electrode, we did not further polished the surface because the corroded layer
was very thin, so the surface of the sample which was used for XRD was very rough, as it can
be seen in Figure 63. Therefore, the roughness of the surface could be another reason for the
changes in the XRD pattern after the water electrolysis [61]. The conclusion from the above
discussion is that the hydrogen has diffused trough the crystal lattice and/or grain boundaries
of PZT during water electrolysis, without the formation of any new phase detectable by
XRD, and PZT still maintained its tetragonal structure after the water electrolysis treatment;
however, the hydrogen presence inside the crystalline lattice of PZT could have caused some
modifications in the lattice parameters of PZT.
Figure 64- The microstructure of PZT after water electrolysis just beneath the electrode (after removing the
electrode)
100
Figure 65- XRD pattern of the as-received PZT sample versus the water electrolyzed PZT sample, using the
following parameters: I=100 mA/cm2, t=48 hours
The damage to the grain boundaries in the corroded layer can be due to the diffusion
of hydrogen atoms from the silver electrode to the grains, or preferably grain boundaries
(with more open structure) of PZT, followed by the formation of hydrogen molecules at the
grain boundaries [12]. The formation of hydrogen molecules would be accompanied by local
increase of pressure, thus stresses along the grain boundaries, which leads to cracks. If it is
assumed that the damage in grain boundaries is due to the diffusion of atomic hydrogen
predominantly along the grains boundaries, the thickness of the corroded layer can be used
for estimating the diffusion coefficient of hydrogen atoms along the grain boundaries of PZT.
The thickness of the corroded layer ( ) is proportional to the diffusion coefficient of atomic
hydrogen (D) by the relation of √ where t is the duration of water electrolysis [83].
Because the thickness of the corroded layer was not the same along the cross section of the
sample, the area of the corrosion layer was first measured and then the average thickness of
the corroded layer was calculated by dividing the area by the width of the cross section (10
10 20 30 40 50 60 70 80 90
2
as-received
after treatment
(00
1)
(10
0)
(11
1)
(00
2)
(200
)
(10
2)
(21
0)
(11
2)
(21
1)
(02
2)
(22
0)
(11
0)
101
mm). The results, Figure 66, indicate that the value for the diffusion coefficient of hydrogen
in PZT at room temperature is about 9×10-11 (cm2/sec). It should again be emphasized that we
believe that this value is related to the diffusion of atomic hydrogen along the grain
boundaries of PZT, and not necessarily inside the crystalline lattice of PZT. This is lower
than the values obtained by other researchers using the same technique: 4.9×10-8 cm2/sec
[83]. This discrepancy in data might be related to the slightly different composition of the
samples, or it might be related to the recombination of hydrogen atoms into hydrogen
molecules along the grain boundaries. Moreover, the value of D= 4.9×10-8 cm2/sec is obtained
based on the advancement of a layer of a different color (yellowish to grey) in the PZT
charged in NaOH solutions at room temperatures with a current of 50 mA/cm. In our work,
we did not observe any change in color in our samples and we measured the above value
based on the thickness of the corroded layer. Therefore, this discrepancy in data might be
related to the different interactions between hydrogen and PZT, as proposed by Alvine et al
[84]. They recently investigated the hydrogen diffusion inside PZT using the proton nuclear
magnetic resonance (1HNMR) and quasi-elastic neutron scattering (QENS) techniques after
charging the samples with high-pressure gaseous hydrogen (T=100C, p=32 MPa for 1HNMR
analysis, and T=100C, p= 17 MPa for QENS analysis). They obtained different values for the
hydrogen diffusion coefficient at room temperature using these techniques; using 1HNMR
they obtained D = 6×10-14 cm2/sec, and using the QENS technique they obtained D = 3×10-6
cm2/sec [84]. Because the diffusion results were several orders of magnitude different, they
concluded that there were different diffusive processes for hydrogen inside the PZT [84].
102
Therefore, the discrepancies between different hydrogen diffusion coefficient values could
be due to the different interactions occurring between the hydrogen and PZT.
0 200 400 600
time1/2 (sec1/2)
0
0.1
0.2
0.3
x (m
m)
25°C
Figure 66- The thickness of the corroded layer versus the square root of time of water electrolysis
5.3.2 Electrical properties of PZT exposed to water electrolysis
It is reasonable to assume that the changes which occur in the microstructure of PZT
will affect the dielectric properties of PZT as well. Figure 67 shows the variation of the
capacitance and dissipation factor of PZT capacitors during water electrolyzes at the
frequency of 1 kHz, showing that both parameters increase with time of water electrolysis.
The increase in capacitance and dissipation factor after water electrolysis has also been
reported in other studies [85-89].
103
Figure 67- The changes of capacitance (C) and dissipation factor (DF) versus the duration of water electrolysis at
the frequency of 1 kHz
To further study the effect of water electrolysis on the electrical properties of PZT,
the capacitance and dissipation factor of the treated samples were measured versus the
frequency of applied voltage. Figure 68a shows the variation of capacitance (C) and
dissipation factor (DF) versus frequency for the PZT samples before and immediately after
water electrolysis, for 6 hours. The capacitance right after water electrolysis was higher than
the initial value. The same trend is also observed for the dissipation factor above 200Hz.
After aging for 24 hours in air however, the capacitance decreased significantly below the
initial values (e.g. at 103 Hz, the capacitance values were 1.7 nF, 1.8 nF and 0.2 nF for the as-
received, hydrogen-treated for 6 hrs, and 24 hrs aged samples respectively), as also shown in
other researchers’ studies [33]. The dissipation factor was however higher for the aged
sample versus the hydrogen-treated sample, for frequencies below 20 kHz. While for the as-
0 20 40 60
time(hours)
1.4
1.5
1.6
1.7
1.8
C(n
F)
0.012
0.016
0.02
0.024
DF
104
received sample the dissipation factor was almost constant, after hydrogen treatment and
aging, it increases with decreasing frequency, as seen in Figure 68b.
1 100 10000 1000000
f(Hz)
0
0.5
1
1.5
2
2.5
C(n
F)
(a)
Figure 68- Variations of electrical properties after 6 hours water electrolysis and subsequent aging in air: a)
capacitance (C); b) dissipation factor (DF) (: as-received,: after water electrolysis, : after aging)
Another interesting finding is the relaxation peak observed in the sample after water
electrolysis for 10 hours, and after 24 hours aging in air (Figure 69).
1 100 10000 1000000
f(Hz)
0
0.5
1
1.5
2
2.5
C(n
F)
(a)
1 100 10000 1000000
f(Hz)
0
0.1
0.2
0.3
DF
(b)
Figure 69- Variations of electrical properties after 10 hours water electrolysis and subsequent aging in air: a)
capacitance (C); b) dissipation factor (DF) (: as-received,: after water electrolysis, : after aging)
1 100 10000 1000000
f(Hz)
0
0.1
0.2
0.3
DF
(b)
105
The formation of the relaxation peak is better seen in Figure 70, which shows the
changes of dissipation factor and capacitance for the sample which was water-electrolyzed
for 48 hours. According to Figure 70, no relaxation peak in DF was observed for the samples
tested immediately after water electrolysis; however after aging for 10 hrs, a relaxation peak
starts to form (at about 105 Hz), and can be clearly observed at about 103 Hz after ageing the
samples for 24 hrs. The relaxation peaks in the dissipation factor have also been reported for
other oxides [85-88], but not for PZT.
Figure 70- Variations of capacitance (C) and dissipation factor (DF) after water electrolysis for 48 hrs, and
subsequent aging at room temperature in air (■: as-received, : after water electrolysis, ▲:after 10 hours aging,
: after 24 hours aging)
Considering the previously published results, a few issues need to be addressed: the
first issue is why does the dielectric constant increase during water electrolysis, and then
decreases after aging in air. It should be noted that the increase in the dielectric constant
during water electrolysis and further changes in capacitance during aging were also reported
1 100 10000 1000000
f(Hz)
0
0.5
1
1.5
2
2.5
C(n
F)
(a)
1 100 10000 1000000
f(Hz)
0
0.1
0.2
0.3
0.4
0.5D
(b)F
106
for oxides such as TiO2, SrTiO3, BaTiO3, CaCu3Ti4O12, BiFeO3, and WO3 [85, 88]. Chen et al.
proposed that the increase in dielectric constant could result from the dipoles formed by the
protons inside the oxide [86], related to the complexes formed by protons with structural
defects such as oxygen vacancies. It was assumed in [86] that during the aging in air, such
protons leave the oxide and this leads to the recovery of the electrical properties, or to
decrease of the dielectric constant, as it was observed for BaTiO3. Another mechanism which
could be considered for the increase of the dielectric constant after water electrolysis is in
accordance to that proposed in the work of Park and Chadi [30]; they investigated stable sites
of protons in PbTiO3 using first-principles calculations. For the tetragonal phase of PbTiO3,
their results show that “the direction of the [OH]– dipole is favorably aligned with the host
polarization” [30]. Thus [OH]– should enhance polarization of the spontaneous dipoles in
PZT, and therefore this could be the reason for the increase in the dielectric constant of PZT
after hydrogen treatment. As the bond between proton and oxygen is strong, protons
attached to oxygen atoms cannot easily diffuse out from the PZT bulk. Therefore, it might be
concluded that the [OH]– dipoles could not be the reason for the increase in the dielectric
constant right after water electrolysis.
The mechanism which we propose here for the increase of the dielectric constant, not
mentioned in the previous studies [86-89], is the Maxwell-Wagner (MW) polarization, active
in inhomogeneous systems where the dielectric material consists of regions with different
electrical properties [77]. As mentioned before (Chapter 5-2-2), this "extrinsic" polarization
mechanism can be explained by considering the heterogeneity of the system without any
107
microscopic polarization (dipolar polarization) process inside the sample [38, 77]. Due to the
differences in electrical conduction in different regions of such heterogeneous material,
charge accommodation occurs at the interfaces, leading to increase of capacitance of the
sample. According to Figure 63a, although a corroded layer formed beneath the electrode,
most of the sample was unaffected. Therefore, the assumption of an inhomogeneous
dielectric medium after water electrolysis is reasonable for our samples. As such, we believe
that the increase in capacitance right after the water electrolysis is due to the formation of
the corroded layer, and to the difference in the electrical properties of this layer and the
unaffected layer of PZT. Furthermore, during the aging, hydrogen atoms diffuse out from the
corroded layer, and this leads to further changes in capacitance.
The formation of the relaxation peaks could also be explained by the MW
polarization mechanism. The microstructure of PZT after water electrolysis can be
considered as a dielectric made of two different layers perpendicular to the electric field. It
can be shown that the frequency response of such layered system is similar to the frequency
response of a Debye relaxor [77], leading to a Debye type relaxation peak. In our opinion,
this is the main reason for the observation of the relaxation peak in the dissipation factor.
The relaxation time () can be evaluated as = (C1+C2)/(G1+G2) where C1, C2 are the
capacitance, and G1, G2 are the conductance of corroded and un-affected layers, respectively
[77]. Accordingly, the frequency at which the relaxation occurs (f=1/) depends on the
electrical properties of the different regions, and variations in these electrical properties will
change the relaxation time and the frequency at which relaxation occurs [38, 77]. We
108
propose that the changes in the capacitance and the movement of the relaxation peak to
lower frequencies with sample aging could be explained by the MW polarization mechanism.
When hydrogen atoms diffuse out of the PZT bulk during aging, the electrical properties of
the corroded layer change, and this changes the frequency at which the relaxation occurs.
The relaxation peak was however not observed for the samples exposed for <10 hours to
water electrolysis, likely because of the insufficient thickness of the corroded layer.
Chen et al [86] have proposed that the relaxation peak is due to dipoles related to the
complexes which protons form with structural defects in the oxide. If this is the case, the
intensity of the relaxation peaks should decrease with aging, when the protons diffuse out of
the sample and the number of dipoles inside the oxide decreases. However, the intensity of
the relaxation peaks increases as more hydrogen diffuses out from the sample while aging
continues (Figure 70), suggesting that the relaxation peak is not due to the dipoles, but rather
is due to the MW polarization.
Figure 71 shows the poor fit of the Figure 70 data to the Debye equation, possibly
because the thickness of the corroded layer was not uniform, leading to a distribution of
relaxation times (as it depends on the thickness for the corroded layer). Figure 71 shows that
the Havriliak–Negami equation (Chapter 2-3, equation 23) fits well our experimental results,
for ε = 450, εs = 2500, = 0.012 sec-1, θ = 0.9, and β = 0.4.
109
f (Hz)
0
200
400
600
800
1000
''
10310 105
Debye
Havriliak-Negami
Figure 71- The results of fitting the ε´´ (ε´´=DFε´) data to Debye and Havriliak–Negami equation. An
iterative MATLAB code was developed and used for the fitting procedure.
Typical trends for the changes of the dielectric constant and dissipation factor of a
leaky capacitor (due to the electronic conduction) are shown in Figure 72a. Figure 72b also
shows the typical trend for the changes of the dielectric constant of a capacitor with mobile
charges which can move by hopping.
Figure 72- The changes of the capacitance (C) and dissipation factor (DF) for (a) a leaky capacitor with
electronic conduction, (b) for a capacitor with hopping charge carriers adapted from [38]
110
The trends shown in Figure 72 are similar to the trends observed for our samples,
aged in room conditions after water electrolysis (i.e. compare Figure 68 and Figure 72). This
trend is not obvious in Figure 70 because of the existence of the relaxation peak, and because
of the limits on the frequency that can be used. Therefore, it can be concluded that the
ohmic resistance of the PZT plates decreases after being charged with hydrogen. The
increase in the electrical conduction of PZT after water electrolysis is also reported in other
studies [33, 83].
Different mechanisms could be considered for the decrease in the resistivity of the
samples. The first mechanism relates it to the formation of oxygen vacancies (2H + O2- →
H2O + ); ionization of the vacancies (
) will contribute up to two electrons available for
conduction [89]. Another possible mechanism includes the ionization of hydrogen atoms
inside the lattice of PZT (H → H+ + e-) [31], with the electrons produced available for
conduction through hopping [31]. While these mechanisms can explain the increase in the
dissipation factor, they cannot explain why the dielectric constant decreases after water
electrolysis. The reason could be damage to the grain boundaries of PZT, or reaction of
protons with PZT and [OH]– dipoles formation. These dipoles could hinder the switching of
the dipoles inside PZT, and therefore affect the movement of the domain walls, and
consequently decrease the dielectric constant of PZT [82]. The disappearance of switchable
polarization hysteresis of PZT after hydrogen treatment has been also attributed to the
formation of [OH]– dipoles, which inhibits the switching of the spontaneous dipoles in PZT
[88]. Protons bonded with oxygen ions cannot easily leave the PZT bulk during aging, i.e.
111
high temperature treatment (>700C) is needed [47]. Thus the decrease in capacitance
demonstrated in our experiments is likely due to the persistence of [OH] – dipoles within the
PZT structure, obstructing the movement of the PZT domains, and the formation of oxygen
vacancies. The decrease of dielectric constant of BaTiO3 at high frequency after water
electrolysis was previously linked to interstitial hydrogen on the domain walls of BaTiO3
[88].
112
5.4 Ceramic Coatings for PZT Damage Protection
5.4.1 Alumina coatings microstructure
Attempts have been made in the past few years to solve the issue of hydrogen damage
in PZT during the forming gas annealing [55]. In this section, we investigated the possibility
of coating the PZT with alumina using the sol gel technique. Alumina has been proposed
before as a hydrogen barrier layer for PZT, and it has been shown that the alumina layer can
successfully act as a hydrogen barrier layer [55]. However, the method which we propose
here is the simple sol-gel method (as described in Chapter 4-4), which is different from the
method used in the former works for alumina deposition. In this section we show that while
the developed coating is porous, it can still significantly decrease the amount of hydrogen
damage to PZT.
Figure 73a shows the surface of the coating obtained by dip coating with pure
Boehmite sol, before drying; some excessive sol accumulation occurred on the surface of
PZT, probably because of the surface porosity on the PZT. According to Figure 73b, c, cracks
and coating detachment were observed in the places where sol accumulation occurred after
firing. Generally, the quality of the coating obtained with pure Boehmite sol was not
sufficient to demonstrate its effect on H2 damage protection of PZT.
113
Figure 73- Low magnification image of the coating on the surface of PZT (a) after dip coating with pure
boehmite sol (b) before and (c) after heat treatment of the coating in the furnace, in some places on the surface
of the coating, detachment of the coating was observed
114
One of the ways to improve the quality of the coating is the addition of PVA to the
sol, which increases viscosity of the sol [90]. The microstructures of the coating processed
with 10wt% PVA are shown in Figure 74. Figure 74 shows the surface of the coating directly
after dip coating (before drying it in the furnace); it can be observed that now a smooth
uniform coating has formed on the surface of PZT. Comparing the Figure 73 with Figure 74,
one can see that the quality of the coating was noticeably enhanced. The small pits which
can be seen on the coating are due to the pores on the surface of the PZT. As it will be shown
later, the sol had enough fluidity to fill out the pores on the surface of the PZT, but not
completely, so small dimples formed the surface of the coating.
Figure 74- Low magnification image of the coating on the surface of PZT after dip coating before the heat
treatment of the coating in the furnace (comparing with Figure 73a, a smooth uniform coating has formed on
the surface of PZT with the addition of PVA to sol)
Figure 75 shows the surface of PZT plates after firing the coating in the furnace; a
smooth crack-free coating developed on the surface of the sample. It should be mentioned
that during the dip coating process, excessive accumulation of the sol occurred at the bottom
115
edge of the sample, and because of this, some cracks were observed at the very end of the
sample; however, the center of the sample was crack-free. The darker areas seen in Figure
75b are pores on the surface of PZT, just covered by the very thin (1-2 m) coating.
Figure 75- Low magnification (a) and high magnification (b) images of the coating on the surface of PZT after
dip coating and after heat treatment of the coating in the furnace (comparing with Figures 73b and c, a smooth
uniform coating has formed on the surface of PZT with the addition of PVA to sol)
Figure 76 shows the cross section of a sample with alumina coating after three
consecutive depositions: there is an evidence of good adhesion between the PZT and the
coating. Furthermore, no cracks were observed in the coating, and there were no pores or
cracks at the interface between the alumina layer and PZT. It should be noted that there
were some cracks in the coating at the bottom end of the sample, where sol accumulation
occurred. As it can be seen from Figure 76b, the sol had good fluidity and surface wettability,
so it was able to fill the holes and grooves on the surface of the PZT plates.
116
Figure 76- Low magnification (a) and high magnification (b) images of the cross section of the alumina coating.
As it can be seen from (b), the coating had enough fluidity to fill out the pores on the surface of PZT
Leenaars et al. have measured the amount of porosity of Boehmite coatings after
treatment at different temperatures [91]. The porosity and the size of pores are presented in
Table 6, which shows that even after firing as high as 1000C, the coating still contains a
noticeable amount (>40 vol%) of porosity. They also have found that the prolonged heat
treatment for 850 hours at 400C did not change the amount of porosity. Similar results have
also been reported in other works [92], therefore, we also expect the coating to be porous
although we did not measure porosity of the coatings processed in this work.
117
Table 6- Microstructural charachteristics of alumina coatings a a function of TC [91]
As seen from the high magnification image of the cross-section shown in Figure 77,
the alumina coating after firing is a conglomerate of seemingly separate agglomerates in the
range of about 20-50 nm, with <10 nm particles within the agglomerates. According to the
information provided by the manufacturer of the commercial Boehmite powder used in this
work, each of these particles are actually agglomerations of a few individual crystalline
particles (the average size for agglomerated Boehmite particles is 25 nm and for individual
crystallites is 4.5 nm [57] ). The important point which can be taken from Figure 77 is that
the coating is porous, with inter-agglomerate pore sizes < 100 nm. As the alumina coating is
a meso-porous medium, we expect that it will not prevent the access of molecular hydrogen
(H2) to the PZT surface; however it may prohibit or decrease the diffusion of atomic
118
hydrogen (H). To further understand the microstructure of the coating after the heat
treatment, XRD analysis was performed to identify what phases were present.
Figure 77- High resolution image of the cross section through the alumina coating processed at 450C in air for
5 hours
Figure 78 shows the transformation sequences of Boehmite with the firing
temperature [93], suggesting that after heat treatment at 450C, the structure of the coating is
-alumina. However, XRD analysis on PZT plates with thin alumina coatings (< 5 um), did
not detected any alumina, and only PZT peaks were identified. Consequently, to understand
the crystal structure of the coatings, 10 g bulk sample of Boehmite powder was heat-treated
at 450C in air for 5 hours (i.e. following the same heat treatment procedure used for the
coatings) and then the crystal structure was investigated by XRD, Figure 79. The peaks are
119
identified as -alumina [94] and they are not very sharp, which indicates a low degree of
crystallinity and a fine particle size distribution.
Figure 78- Transformation sequence of the different aluminum hydroxides with temperature (adapted from
[93]).
Figure 79- XRD results for the as-received boehmite powder and after heat treatment at 450C for 5 hr
It should be pointed out here that the -alumina has a spinel structure, but the
chemical formula for -alumina containing hydrogen (H) has been the subject of debate [95].
10 20 30 40 50 60 70 80 90
2
as-received
after treatment(111)
(220)
(311)(222)
(400)
(511)
(440)
(444)
120
A recent theoretical study by Sohlberg et al. has shown that the -alumina can exist over a
range of H content and the chemical formula for -alumina can be presented as H3mAl2-mO3
[95]. This theoretical chemical formula for -alumina has been confirmed by the available
experimental data for the -alumina structure [95]. It should be pointed out that there are
different types of hydrogen atoms positions in the structure of the -alumina particles [95].
H atoms present in the bulk of the -alumina particles occupy octahedral and tetrahedral
sites. Additionally hydrogen atoms can be present on the surface of the -alumina particles,
without specific preference towards lattice positions [95]. These hydrogen atoms have
different mobilities and thermal stabilities in the structure of -alumina, and therefore they
play different roles in the properties of -alumina [95]. According to the elemental
compositions by EDX shown in Table 7, the Al/O ratio (0.75) in the coating is lower than the
theoretical ratio Al/O in Al2O3 (1.12). The lower value of Al/O ratio could be because of the
OH bands in the structure of the -alumina.
Table 7- The EDX analysis for the -alumina coating (high concentration of Au is due to the gold coating on the
sample for SEM analysis)
Element O Na Al Cl Ti Au
Concentration (wt%) 43.59 1.76 32.86 0.96 1.83 21.27
5.4.2 Hydrogen resistivity of alumina-coated PZT
To assess the hydrogen resistivity the coated PZT, water electrolysis technique was
used. Thin (10 nm) Au-Pd electrodes were sputtered over the alumina-coated PZT plates, as
well as on as-received PZT, for reference tests. Figure 80 shows the cross section of the
reference sample, without alumina coating, after water electrolysis at room temperature and
121
the following test parameters: I= 100 mA/cm2, V= 6-10 V, time= 24 hours, 0.1 M NaOH
solution; a corroded layer has formed close to the top surface of the sample (the metallic
electrode cannot be seen in this figure because it is only about 10 nm thick). It can be also
seen that the grain boundaries of PZT close to the electrode are extensively corroded, which
is not the case in the center of the section far from the electrode.
Figure 80- The cross section of the sample with Au-Pd electrodes and after 24 hours water electrolysis. The
thickness of the corroded layer is about 100 microns
Figure 81 shows the cross section of the sputtered sample with alumina coating after
48 hours of water electrolysis (and all other parameters same as these used to produce the
sample in Figure 77). The effect is quite dramatic - the corrosion beneath the electrode is
limited to < 5 m surface film, which suggests that the porous alumina layer deposited by the
sol-gel technique can act as an effective hydrogen barrier layer.
Au-Pd electrode
PZT
122
Figure 81- The cross section of the sample with Au-Pd electrodes and alumina coating and after 48 hours water
electrolysis
Because the damage to PZT is due to the diffusion of hydrogen atoms (H) into PZT [9,
14, 18-21], we can conclude that the alumina coating has blocked or decreased the diffusion
of hydrogen atoms from the metallic electrode to the surface of PZT. However, as it was
observed in Figure 77 (and confirmed through [91]), the coating is 40 vol% porous, and
thus molecular hydrogen could easily pass through it. Therefore, the question which may
arise is how this coating decreases the damage to PZT, despite allowing H2 access to PZT
surface. Several hypotheses could be formulated in this regard. First consider the possibilities
of the reaction of hydrogen atoms with the coating (refer to Figure 82 for the schematic
illustration of this possibility). Joubert et al. have investigated the reaction of - alumina
dehydrated at 500C with hydrogen and concluded that hydrogen can react and be absorbed
Au-Pd electrode
PZT
coating
123
on the surface of the - alumina particles at temperatures as low as 25C [96], due to the
defective surface structure of -alumina [96]. On the other hand, Yu et al. have investigated
the surface diffusion of hydrogen atoms on the surface of - alumina and they observed
noticeable surface diffusion at temperatures higher than 250C [97]. Therefore, one
mechanism by which the coating could decrease the amount of hydrogen damage could be
reaction of the -alumina coating with the hydrogen atoms. In other words, - alumina
coating could act like a “sponge” absorbing hydrogen atoms before they reach the surface of
the PZT (Figure 82). It should be noted that the hydrogen atoms absorbed on the surface of
- alumina particles can diffuse along their surfaces and it is anticipated that sooner or later
damage should start to PZT when hydrogen atoms reach the surface of PZT. If this scenario
is true, then we expect that after a prolonged time of water electrolysis, damage to PZT
should occur.
Figure 82- Schematic for the reaction of hydrogen atoms with -alumina particles 1) transformation of
hydrogen atoms to hydrogen molecules which leave the system away from the coating (i.e. as hydrogen bubbles
during water electrolysis), 2) diffusion of hydrogen atoms through the electrode and attachment to -alumina
particles, followed by surface and bulk diffusion through -alumina towards PZT
Au-Pd electrode
PZT
coating
H
H2
HH
H2
HH
H
H
H
H1
2
ɣ-Al2O3
H
electrode
coating
124
To examine this theory, we made a thinner alumina coating (1 m thick, produced
through single-dip-coating process, Figure 83), and extended the time of water electrolysis to
144 hours (6 days), at conditions same as before (I= 100 mA/cm2, V= 6-10 V, room
temperature, 0.1 M NaOH solution).
Figure 83- An image of the cross section of PZT with alumina coating on top
The microstructure of PZT after water electrolysis for these conditions (Figure 84)
shows that the PZT is somewhat damaged in regions close to the electrode (to the depth of
about 10-20 m). Therefore, it may be concluded that the reaction of the alumina coating
with the hydrogen is a possible mechanism by which the presence of the coating decreases
and delays the hydrogen damage. However, another explanation to be considered is the
transformation of hydrogen atoms to hydrogen molecules after leaving the metallic
electrode, schematically shown in Figure 85. As palladium is well-known for its hydrogen
catalytic activity, hydrogen atoms (H) could easily transform into hydrogen molecules (H2)
on the surface of the electrode at the interface between the electrode and coating (Figure 85).
125
Figure 84- The cross section of the sample with Au-Pd electrodes and thin alumina coating and after 144 hours
water electrolysis
Figure 85- Schematic image for the combination of hydrogen atoms at the interface of metallic electrode with
- alumina
Au-Pd electrode
PZT
coating
Au-Pd electrode
PZT
coating
H H
H H
H2
HH HH
H2
HH
H2
1
2
ɣ-Al2O3
electrode
coating
126
Such hydrogen molecules released on the interface of the alumina coating with the
electrode can further diffuse through the pores of the coating and reach the surface of PZT.
Therefore, it may seem that damage could occur to PZT as alumina coating cannot block the
hydrogen molecules. However, as proposed in [9], and confirmed in this work (Chapter 5-1-
1), hydrogen molecule itself cannot damage the PZT below 400C [9]. That is because
hydrogen molecules cannot dissociate on the surface of PZT at low temperatures [9].
It might be concluded therefore that the transformation of hydrogen atoms to
hydrogen molecule on Pd surface, and physical separation of the PZT surface from the
electrode surface (by the porous alumina) is likely another mechanism by which the alumina
coating (despite its porosity) decreases the amount of hydrogen damage of PZT.
An additional point which should be mentioned here is that the metallic electrode
could be porous. If this is the case, then water could diffuse inside the alumina coating
during water electrolysis and thus fraction of the hydrogen molecules released on the
electrode surface dissolve in the water (the solubility of hydrogen gas in the water at room
temperature is 0.8 mole/liter [99]). Therefore hydrogen molecules would be in contact with
the surface of PZT in water, which is also the case in the samples without alumina coating.
However, because the damage was demonstrated to be considerably lower in the coated PZT
samples, we conclude that the degradation during the water electrolysis is due to the
hydrogen atoms that diffuse from the metallic electrode into PZT, and not to the presence of
the molecular hydrogen in water.
127
The conclusion that can be drawn from the above experiment is that as far as there is
no interaction between atomic hydrogen (H) and PZT, no damage would occur to PZT. If the
direct contact between the metallic electrode and PZT can be diminished or decreased, such
that no hydrogen atoms will be in contact with PZT, then and no damage would occur to
PZT. Through such mechanism, even a porous coating between the electrode and PZT can
noticeably decrease the amount of damage. It is possible that by applying this method in the
manufacturing of PZT actuators, even with other types of porous coatings, the deleterious
effect of hydrogen would be greatly diminished.
It should be however remembered that a thin insulating surface layer might degrade
functionality and performance of electrode/PZT/electrode assembly [100]. Such layer would
act as a capacitor in series with the PZT (Figure 86), and as a result the externally applied
voltage would be distributed across the sample inversely proportionally to the capacitance of
each layer. Therefore, higher voltages would be needed to drive the PZT actuator. To solve
these issues two solutions could be considered: (1) make the coating layer as thin as possible,
and/or (2) make the coating layer from materials with dielectric constant higher than PZT.
In both these cases, the capacitance of the coating layer will be higher than the capacitance
of the PZT layer, and therefore the voltage across the PZT layer would be almost the same as
the applied voltage to the whole assembly.
128
Figure 86-Equivalent electrical circuit for PZT and PZT with coatings
There is very recent research supporting the idea of introducing a coating between
the electrode and PZT [100]; this work showed that a thin alumina layer between the PZT
and electrode will not affect the functionality of the PZT capacitors. The reason for this was
not clear; however it was suggested that a thin alumina layer between the electrode and PZT
will not act as an insulator and it may act as a resistor [100]. That is because when a voltage is
applied across the sample with alumina coating, a considerable fraction of the applied voltage
would be across the thin alumina layer, whereas the layer would become conductive under
high voltages by either Schottky emission or thermionic field emission [100].
PZT PZT coating
C PZT C PZTC coat C coat
129
6 Conclusions
High-pressure hydrogen treatment
The microstructural and capacitance changes in the PZT ceramics exposed to high-pressure
(10 MPa) hydrogen atmosphere at 100C were investigated in this part of the work. For bare
PZT, no noticeable damage was observed to the PZT structure after hydrogen treatment for
up to 1200 hours. The grain boundaries of PZT were corroded only in the regions just below
the surface (about 10 µm deep) of the samples. In samples with Ag electrodes, the presence of
metallic electrodes greatly increases the damaging effects of hydrogen on PZT. The structural
degradation observed in the samples with Ag electrodes consisted of the development of a
very porous layer adjacent to the electrodes on the surface, and the detachment of the
electrodes from PZT. It was proposed that the hydrogen spillover is the responsible
mechanism for the formation of the porous electrode on the surface of the samples. In PZT
samples with Ag/Pd electrodes, the PZT damage noticeably increased compared to Ag-only
electrode. It is therefore suggested to decrease the amount of Pd in the Ag/Pd electrodes to
increase the resistance of the actuators to hydrogen damage. No considerable changes were
observed in the dielectric constant of PZT after 200 hours hydrogen treatment at 100°C.
High-temperature hydrogen treatment
The kinetics of the PZT structural modifications due to hydrogen exposure was
investigated by online monitoring of the electrical properties of PZT above Curie
130
temperature, up to 650C. Considerable changes were observed in the microstructure of the
PZT samples hydrogen-treated at high temperatures, including the detachment of single PZT
grains from the surface, as well as reduction to metallic lead on the surface of the samples. It
was found that the changes in PZT exposed to high-temperature H2 can be described by a
simple nucleation and growth model. Assuming that the changes are controlled by protons
diffusion, the resulting activation energy for the diffusion of protons in cubic PZT was
determined to be 0.440.09 eV.
The dielectric spectroscopy study of PZT samples shows that even after the high
temperature hydrogen treatment, Ti-O and Zr-O dipoles are still present inside the PZT. The
results show two relaxation peaks in the dissipation factor curve of the hydrogen-treated
PZT. While the first peak indicates that the kinetics obeys the classical Arrhenius law, with
the activation energy of 0.664 eV, the second peak indicates the presence of unusual
relaxation kinetics: the relaxation time increases with increasing temperature. This non-
monotonic relaxation kinetics can be attributed to the defects that hydrogen has produced
inside the PZT, or it can be due to the Maxwell-Wagner polarization mechanism.
Water electrolysis treatment
The interaction of hydrogen with PZT in the tetragonal phase was investigated using
the water electrolysis technique. Development of a hydrogen-affected (corroded) layer
adjacent to the electrode functioning as the cathode was observed during water electrolysis.
131
The thickness of the corroded layer was used to calculate the diffusion coefficient of
hydrogen atoms in PZT, and the value obtained was 9×10-11 cm2/sec. A composite model was
proposed for the microstructure of PZT affected by hydrogen generated during water
electrolysis, and changes of the electrical properties of PZT are linked to the model. The
Maxwell-Wagner polarization mechanism was proposed to be responsible for the changes in
the dielectric properties of PZT after hydrogen charging. Although this polarization
mechanism has been ignored in previous works by other researchers, we believe it is
responsible for the variation in electrical properties of other oxide ceramics during water
electrolysis as well. Furthermore, the results indicate that after aging, the resistivity and
high-frequency dielectric constant of PZT decrease. The decrease in capacitance is expected
to be due to [OH]– dipoles hindering the movement of PZT domains.
Alumina coatings for PZT protection from hydrogen
In this part of the research we have investigated the possibility of protecting PZT
from hydrogen damage by coating it with alumina using the sol gel technique; subsequently
we have assessed the hydrogen resistance of the coated PZT. The results show that the
quality of the alumina coatings obtained with pure boehmite sol was not very good, i.e.
cracks and coating detachment were observed. However, the addition of the poly-vinyl
alcohol (PVA) to the boehmite sol considerably enhanced the quality of the final coating;
neither cracks nor detachment were observed. The hydrogen resistance of the alumina
coated PZT was investigated using the water electrolysis technique, and the results have
132
shown that the alumina coatings noticeably decrease the level of hydrogen damage to PZT.
It is anticipated that the main contributor to the decreased PZT damage is the physical
separation of the metallic electrode from PZT by the coatings. The insulation of the PZT
from the electrode leads to the re-combination of hydrogen atoms into molecules on the
electrode surface and within the pores of the coating, which effectively prevents access of
the damaging atomic hydrogen to the surface of PZT.
Impacts of the work
The results of the study of long-term high-pressure gaseous hydrogen exposure of
PZT could be beneficial to the design of the modern electronic fuel injectors that use PZT
actuators for valve opening, instead of the conventional solenoid technology. The results
show that the metallic electrode has considerable effects on the level of hydrogen damage,
and it was suggested that Ag/Pd electrodes should be replaced with pure Ag electrodes in
making such actuators. Kinetics of the hydrogen damage to PZT was also investigated in the
present work. The results could be used for the prediction of the degradation caused by the
hydrogen treatment. The results could be used for the re-design of the hydrogen treatment
process of FeRAMs. We also have shown that even a porous separation layer between an
electrode and PZT acts towards decreasing the PZT hydrogen damage. The results of this
part of the research could be further used to better understand the mechanism of PZT
degradation by hydrogen, and to design new methods for decreasing the hydrogen damage to
PZT and other ceramics.
133
7 Future Work
We suggest that the technique we used in this work for studying the kinetics of
interactions between hydrogen and PZT can be also used for studying the kinetics of the
interaction between hydrogen and other oxides, in particular oxides designated as the
possible fast proton conductors replacing polymeric proton conductors in fuel cells. These
oxides include BaCeO3, BaZrO3, and SrCeO3. The proposed nucleation and growth model for
the structural changes in PZT could be also valid for other oxides, so a general model could
be evaluated for studying the kinetics of interactions between hydrogen and oxides in
hydrogen atmosphere.
The results of this work show that during the exposure of PZT to hydrogen, the
capacitance of PZT capacitors changed, and these changes were due to the diffusion of
protons into PZT. Further work should be done to build upon this observation to develop a
quantitative relationship between the concentration of protons inside PZT and the
capacitance increase. Such quantitative relationship could be generalized for other proton
conductor oxides as well. The quantitative relationship between the capacitance and protons
content inside oxides could be further developed into a method to evaluate the amount of
protons inside the oxides, based on capacitance changes measurements.
134
Unusually large capacitance values were observed in PZT capacitors in hydrogen
atmosphere at temperatures of 600-650C (unfortunately due to the limitations of the
instruments, the maximum number we were able to read was approximately 100 F).
Further investigation is therefore needed to determine the polarization mechanisms
responsible for this high capacitance observed in the hydrogen-treated PZT capacitors. The
research in this area might lead to the development of “high-temperature” super-capacitors.
The very high capacitance in PZT capacitors could be then used for many practical
applications, such as energy storage.
According to the dielectric spectroscopy results, hydrogen forms dipoles inside PZT.
The nature of such dipoles should be better understood. In this regard, PZT with different
amounts of dopants (Nb) should be prepared, and it should be investigated whether there is
any correlation between the amount of dopants inside PZT and the intensity of the
relaxation peaks. The important point about dipoles formed with Nb is that they may
prohibit the switching of Ti-O or Zr-O dipoles. Therefore, the interactions between the
dipoles that form with Nb and the Ti-O or Zr-O dipoles should be further investigated,
maybe using computer modeling techniques.
We have proposed that a porous layer separating metallic electrode from PZT should
be sufficient to substantially decrease the hydrogen damage of PZT. This theory should be
further tested with various porous coatings other than -alumina, and the mechanisms
135
responsible for decreased hydrogen damage of PZT should be identified. Depending on the
electrical properties of the insulator layer between PZT and electrode, the functionality and
performance of electrode/PZT/electrode assembly could be degraded. The role of the coating
between the electrode and PZT on the performance of electrode/PZT/electrode assembly
should be evaluated. Insulators with dielectric constant higher or equal to the dielectric
constant of PZT could be explored as the coating materials, to preserve the performance of
electrode/PZT/electrode assembly.
136
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