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Recent advances in pyroelectric materials and their applications:

People Counting, Cooling and Energy Harvesting

Roger WhatmoreTyndall National Institute

University College Cork, Lee Maltings, Cork, Ireland

Visiting Professor, Cranfield University, Cranfield, Beds MK43 0AL, UK

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Acknowledgements• Engineering and Physical Sciences Research Council

– For funding under various projects• Cranfield University

– Chris Shaw– Jeff Alcock– Qi Zhang– Zhaorong Huang

• Cambridge University– Alex Mischenko– Neil Mathur– Jim Scott

• IRISYS Ltd

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Talk Synopsis• Background to pyroelectrics• Pyroelectrics – Applications in IR sensor arrays

– Pyroelectrics as movement sensors– Example of an array-based “people sensor” system– Imaging Radiometry

• Pyroelectric ceramic materials for arrays• Structured pyroelectric materials and MEMS devices

– Functionally gradient pyroelectric ceramics– Radiation collection structures using thin films

• Electrocaloric effect in PZT thin films• Pyroelectrics for Energy Harvesting

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Pyroelectric and Electrocaloric Effects

TTDP

E

ii

,

iT

i EESS

,

Specific HeatDielectric

Permittivity

Pyroelectric Effect

Electrocaloric Effect

T E

S D

Entropy Electric Displacement

Electric Field

Temperature

Electrothermal Effects

Pyroelectric Coefficient (p) ,, TiE

ii E

STDp

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Pyroelectric Infrared Detectors

Electrode (Area A)

Pyroelectric material

Ip

Ps

P

T T

p=dPs/dT

Pyroelectric Coefficient

Rate of change of element temperature with time

Pyroelectric Current = Ip = A.p.dT/dt

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RGipd

Incident radiation(Power W(t))

Electrode(Area A, Emissivity )

Thermal Conductance

CE

VS

RL

CA

Vo

0V

Polar axis of element

IR Temperaturechange

Pyroelectriccurrent

Voltage Output

Schematic diagram ofpyroelectric infra-red detector

Devices are “AC” coupled to radiation flux

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Silicon amplifier/multiplexer chip

Pyroelectric Thermal Sensor Arrays

To signal processing/display

In absence of chopper blade, a constant IR flux will lead to a constant pyroelectric temperature and no signal is produced. Non-chopped arrays “see” only moving warm (or cold) objects.

Conductive bumps

If imaging of static objects is required, a rotating chopper blade is included to modulate the IR onto the pyroelectric material.

Array elements

Pyroelectric ceramic

Rotating chopper blade (Optional)

Electric motor

Germanium lens

Incident IR radiation

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Packaged Pyroelectric Array

www.irisys.org

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People Sensing

What the Array ‘Sees’Field of View

10cm

www.irisys.org

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Supermarket checkout application (e.g. Tesco)

Allows managers a rapid and real-time summary of queue lengths and the ability to efficiently meet the “one in front” requirement.

“As a result, nearly a quarter of a million more customers every week don't have to queue.” – Sir Terry Leahy CEO Tesco plc - 6th October 2006

www.irisys.org

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• Chopped operation• Gives thermal reading• Low cost• Applications to machine

monitoring and process control

Imaging Radiometer

www.irisys.org

16x16

16x16 Interpolated to 128x128

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Pyroelectric Ceramics

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We also require:• Good mechanical & chemical

characteristics• For focal plane preparation

• Lapping/polishing• Metallisation• Photolithography• Reticulation• Hybridization technology

• Thin film devices• Require determination of

properties in thin films

Important Properties Relevant to Pyroelectric Arrays

• Pyroelectric coefficient• Dielectric properties

• Dielectric constant• Dielectric loss

• Thermal properties• Specific heat• Density• Thermal conductivity

• Electrical Resistivity• Piezoelectric properties

• Determines microphonic noise, secondary & tertiary pyro effects

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Functionally Gradient Pyroelectric Ceramics• Design a ceramic structure to give a higher performance

figure of merit.

Dense ceramic

Porous ceramic

Dense ceramic

Porous layer:

• Reduces average dielectric constant

• Reduces volume specific heat

• Introduces thermal barrier – reduces thermal diffusivity

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Functionally Gradient Pyroelectric Ceramics -Manufacture

Roll of celluloseacetate sheetas carrier tape

Flat glass support

Slip Hopper

Ceramic Slip

Doctor Blade

Infra-red lamps

Drying zoneMotion ofcarrier tape

Tape cast with latex binder

PZT plus 50 micron starch granules

Standard Tape (55% solids)

Laminate

1:1:1 1:2:1

Sinter

A. Navarro, R.W. Whatmore and J.R. Alcock (2004) “Preparation of functionally gradient PZT ceramics using tape casting” J. Electroceramics 13 (1-3) 413-416

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dense

porous

dense

Functionally graded tri-layer structure

Fabricated by lamination of tape cast layers

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Theoretical Analysis

)231( LDL P

Permittivity:Bruggeman Model1:

100

120

140

160

180

200

220

240

260

280

0 5 10 15 20 25 30 35Porosity of Layer (%)

Perm

ittiv

ity

2.50

2.60

2.70

2.80

2.90

3.00

3.10

3.20

0 5 10 15Average Porosity (%)

Aver

age

Pyro

elec

tric

Coe

ffici

ent

(10-4

Cm

-2K-1

)

Pyroelectric effect:Assume pyroelectric effect is proportional to the volume of pyroelectric material between the electrodes

pA=pD(1-PA)

1. Bruggeman D.A.G. Ann. Phys. Lpz, (1935) 24, 636

D, pD = permittivity, pyro coefficient of fully-dense ceramic

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0.04000.04200.0440

0.04600.04800.05000.05200.0540

0.05600.05800.0600

0 2 4 6 8 10 12 14Average Porosity (%)

Volta

ge F

igur

e of

Mer

it (V

m2 J-1

)

Functionally graded tri-layer structure - properties

Variation of FV with porosity in tri-layer structure

oV c

pF'

Theoretical model

1:1:1

1:2:1

Shaw CP, Whatmore RW, Alcock JR (2007) “Porous, functionally gradient pyroelectric ceramics” Journal of the American Ceramic Society 90 (1) 137-142 (Jan 2007)

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Pyroelectric Thin Films & MEMS Structures

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Integrated Arrays using Ferroelectric Thin Films

Top electrode

Electrical connection

Contact 'foot'

Ferroelectriclayer

Silicon substrateVacuum gap

Bridge 'leg'

Requires high quality ferroelectric thin films at low deposition temperatures (<550ºC for survival of Al/Si metallisation)

• Low cost

• Thin films (low thermal mass)

• Excellent isolation

• High performance

IR absorption in element

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20 30 40 50

2Theta(Deg)

Inte

nsity

(a.u

)

[111][Pt]

[100][200]

PMZT

PZT

PtSi

100nm

Five layers of Mn (1%) -doped PZT (30/70), thickness=400nm processed on hot plate at 530 to 560ºC

Pt (Protection)

PMZT

Pt electrode

XRD pattern showing high degree of 111 orientation of PZT and PMZT films on 111 Pt electrode

FIB / TEM image showing the epitaxial growth of the (111) PMZT film on the (111) electrode

Sol gel PZT and PMZT thin films

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Dielectric (33Hz) and Pyroelectric Properties of PZT and PMZT (M=1%Mn)Films

Pyroelectric coefficient 2.11 3.52 x10-4 (C/m2K)

Figure of Merit FD (33Hz) 1.15 3.85 x10-5 (Pa-0.5)

PZT3070 PMZT3070

tan' oD c

pF

33-100Hz

Mn doping leads to a significant reduction in dielectric constant and loss and hence a large improvement in the pyroelectric FD. Best FD is equivalent to bulk pyroelectric ceramics

PZT3070 PMZT3070

Dielectric Constant 375 260Dielectric Loss (%) 1.61 0.06

NB: All dielectric properties measured at low frequencies

Q. Zhang and R.W. Whatmore (2003) “Improved ferroelectric and pyroelectric properties in Mn-doped lead zirconate titanate thin films” J. Appl. Phys. 94 (8) 5228-5233

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Pyro IR Detectors with Integrated Radiation Collectors• It is not possible for the active area of the thermal detector to fill the

available space in the pixel because of the need for good thermal isolation (long legs)

• Exploit the principle of the non-imaging radiation collector in order to collect the radiation from an area close to that of the full pixel down onto the active area of the detector.

• Smaller active areas will give higher specific detectivity, if the radiation is collected from a larger area into a detector with small thermal mass

Conductive Bump

Pyroelectric element

HARM micromachined silicon wafer

Radiation collector cavities

Si Readout IC

Incident IR

Schematic of Concept

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CPC Cavity Device Structures

Optical micrographs

SEM cross section of detector structure with isotropically-etched cavity

Optical micrographs showing the detector structures and contact areas. Note the thermal isolation structure defined for the sensitive area

Contact

Sensitive area

100m

a

b100m

Collector cavity

c

R.W. Whatmore, S. Landi, C.P. Shaw and P.B. Kirby “Pyroelectric Arrays using Ceramics and Thin Films Integrated Radiation Collectors: Design Fabrication and Testing” Ferroelectrics 31811-22 (2005)

Collection efficiency of x2 demonstrated

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Giant Electrocaloric Effects in Ferroelectric Thin Films

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PZT Phase Diagram –Near PbZrO3

100

200

00 2 4 6 8 10

AO

FR(LT)

FR(HT)

= FR(HT) → FR(HT) Transition

T (ºC)

At % Ti

PC

= AO → FR(HT) Transition (Heating)

= FR(HT) → AO Transition (Cooling)

= FR(HT) → PC Transition (Ceramics)

= FR(LT) → AO Transition

= FR(HT) → PC Transition (Crystals)

PbZrO3

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Electro-caloric Effects in PZT95/5 Films

dETPT

cT

E

E

E ,

2

1'

1

T(ºC)

Temperature change can be calculated from:

Previous highest T=2.5K in

Pb0.99Nb0.02(Zr0.75Sn0.20Ti0.05)O3

ceramics at 750V (30kVcm-1)

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Why is Effect so big?• Electric fields that can be applied to a thin film

are much greater than can be obtained in the bulk (480kV/cm cf 30kV/cm)

• PZT95/05 sits at a very interesting point in the phase diagram– Tricritical behaviour in the FR(HT) to PC Transition1

– Change from AO to FR(LT) phase at room temperature

1. R.W.Whatmore, R. Clarke, A.M. Glazer: "Tricritical Behaviour of PbZrxTi1-xO3 Solid Solutions", J. Phys. C.: Solid State Physics 11 3089-3102 (1978)

Pyroelectric Energy Harvesting• Depends upon cycling clockwise around the P-E loop.

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The Ericsson or “Olsen” cycle (shown here for a Pb0.99Nb0.02(Zr0.68Sn0.25Ti0.07)0.98O3(PNZST) Ceramic)1

1. R. B. Olsen and D. Evans, "PYROELECTRIC ENERGY-CONVERSION - HYSTERESIS LOSS AND TEMPERATURE SENSITIVITY OF A FERROELECTRIC MATERIAL," J. Appl. Phys. 54 (10), 5941-5944 (1983)

A-B: Isothermal (low T) ↑ in E & PB-C: ↑ in T at constant E, ↓ in PC-D: ↓ in E & PD-C: ↓ in T at constant E, ↑ in P

T=130°C

T=190°C

A

B

C

D

Field (E)

Polarisation (P)

Pyroelectric Energy Harvesting

• Other cycle types

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Dis

plac

emen

t (D

)Electric Field (E)

Stirling cycle

Dis

plac

emen

t (D

)

Electric Field (E)

CoolIsotherm

HotIsotherm

OpenCircuit

OpenCircuit

A

B C

D

Resistive cycle

Pyroelectric Energy Harvesting

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Observed Performances (Ceramics)Material Cycle 

TypeTmin

°CTmax

°CEmin

MVm‐1

Emax

MVm‐1

Max Energy per cyclemJcm‐3

Ref

PNZST Ceramic Ericsson 158 170 0.4 2.8 95‡ 1PNZST Ceramic Stirling 158 170 0.4 2.8 67 1PNZST Ceramic Resistive 158 170 26 1PNZST Ceramic Ericsson 145 175 0.8 3.2 300 2PMN/PT 90/10 Ceramic Ericsson 35 85 0.5 3.5 186 3PLZT 8/65/35 Ceramic * Ericsson 25 160 0.2 7.5 888 4

Refs:1: R. B. Olsen and D. Evans, "PYROELECTRIC ENERGY-CONVERSION - HYSTERESIS LOSS AND TEMPERATURE SENSITIVITY OF A FERROELECTRIC MATERIAL," J. Appl. Phys. 54 (10), 5941-5944 (1983)2: R. B. Olsen, D. A. Bruno, and J. M. Briscoe, "PYROELECTRIC CONVERSION CYCLES," J. Appl. Phys. 58 (12), 4709-4716 (1985)3: G. Sebald, S. Pruvost, and D. Guyomar, "Energy harvesting based on Ericsson pyroelectric cycles in a relaxor ferroelectric ceramic," Smart Materials & Structures 17 (1) (2008)4: F. Y. Lee, S. Goljahi, I. M. McKinley, C. S. Lynch, and L. Pilon, "Pyroelectric waste heat energy harvesting using relaxor ferroelectric 8/65/35 PLZT and the Olsen cycle," Smart Materials & Structures 21 (2) (2012)

‡At an efficiency of 15% of Carnot*Produced a power output of 15.8mWcm-3 at 0.02Hz

Pyroelectric Energy Harvesting

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Predicted Performances (Thin Films)

Material Cycle Type T°C

EMVm‐1

Max Energy per cyclemJcm‐3

η /ηCarnot

PMN/PT 90/10 Ericsson 75 90 432 34PZT95/05  Ericsson 220 78 596 54

G. Sebald, S. Pruvost, and D. Guyomar, "Energy harvesting based on Ericsson pyroelectric cycles in a relaxor ferroelectric ceramic," Smart Materials & Structures 17 (1) (2008)A. S. Mischenko, Q. Zhang, R. W. Whatmore, J. F. Scott, and N. D. Mathur, "Giant electrocaloric effect in the thin film relaxor ferroelectric 0.9 PbMg(1/3)Nb(2/3)O(3)-0.1 PbTiO(3) near room temperature," Appl. Phys. Lett. 89 (24) (2006)A. Mischenko, Q. Zhang, J.F. Scott, R.W. Whatmore and N.D. Mathur “Giant electrocaloric effect in thin film PbZr0.95Ti0.05O3” Science 311 1270-1271 (3rd March 2006)

Pyroelectric Energy Harvesting• 100’s mJcm-3 per cycle can be extracted for

temperature variations of a few 10’s °C• Operational range RT to 100’s °C• Efficiencies (15 to 50% of Carnot) significantly

higher than thermoelectrics• Oxide ceramics, (crystals), polymers & thin films all

possible candidates• Issues: cracking (oxides), breakdown, high field

requirements (polymers and thin films), need a mechanism to convert a T difference into a T variation 33

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Conclusions• Pyroelectric arrays offer excellent capabilities, especially for

people sensing.• The versatility of pyroelectric ceramics, with control of their

properties through chemical doping, offers great advantages in array fabrication

• There is little prospect for radical improvement in pyroelectric figures of merit with conventional pyroelectric materials, but new ideas such as using functionally-gradient materials may offer a way around this impasse. Significant improvements in FV have been demonstrated.

• PMZT3070 thin films with excellent pyroelectric properties (FD=38.5Pa-1/2 – equivalent to the bulk) have been demonstrated. Controlled introduction of porosity can give figure-of-merit improvements, as with bulk materials

• The giant electrocaloric effect in thin films is an interesting new direction with possibilities for cooling and energy harvesting