Doping behaviors of yttrium, zinc and gallium in BaTiO3 ceramicsfor AC capacitor application

Min-Jia Wang • Hui Yang • Qi-Long Zhang •

Liang Hu • Dan Yu • Zhi-Sheng Lin •

Zi-Shan Zhang

Received: 16 February 2014 / Accepted: 17 April 2014 / Published online: 26 April 2014

� Springer Science+Business Media New York 2014

Abstract The doping behaviors of yttrium, zinc and gal-

lium and their effects on the dielectric properties and

microstructures of BaTiO3 were investigated. Y3? dissolved

in the lattice of BaTiO3, replacing both Ba2? site and Ti4?

site; while Zn2? and Ga3? tended to occupy Ti4? site.

Compared with Y2O3 and Ga2O3, ZnO suppressed grain

growth of BaTiO3 more effectively and promoted greater

uniformity of grains, thus reducing the dielectric loss. The

addition of Ga2O3 inhibited the appearance of second phase

Y2Ti2O7 and enhanced the sinterability, which was ascribed

to the compensation mechanism and synergistic effect.

Proper amount of Y2O3, Ga2O3 and ZnO significantly

improved the dielectric temperature characteristics due to

the formation of the core–shell structure in the codoped

BaTiO3 ceramics. High performance dielectrics with er of

2,690, tand of 1.0 % (at 1 kHz), and alternating current

breakdown voltage E [ 3.73 kV/mm, were achieved.

1 Introduction

BaTiO3 (ABO3)-based ceramics have attracted plenty of

interest and attention in the electronic fields such as filter

circuit, rectification circuit, coupling circuit, bypass of

mobile phones and personal computers [1, 2]. Large

quantities of electron components that meet the market

requirements are produced by chemically and physically

modification. MLCC and relevant materials working on

strong alternating electric field are one of these components

that have not been extensively studied, and the compre-

hensive performance of many reported products can’t meet

the application requirements [3–8]. As we know, it will

generate instant alternating current in a charge and dis-

charge circuit. In the strong alternating electric field,

electrostriction and the inner electric field stress is formed

within MLCC and causes heat to accumulate, which results

in cracking and breaking down of dielectric. So an ideal

commercialized AC MLCC needs to have suitable per-

mittivity, stable temperature coefficient of capacitance, low

dielectric loss and high AC breakdown voltage, which

are decided by its grain size, uniformity and degree of

densification [9, 10].

BaTiO3–MgO-based composite ceramics system is used

widely in the product of high stable capacitance tempera-

ture characteristics due to low value of DC/C25 between a

desired temperature range and the low dielectric loss [11–

14]. Since ZnO and MgO have the similar ionic radius and

valence shell structure, BaTiO3–ZnO-based composite

ceramics system also has attracted attentions, but the exist

of second phase Y2Ti2O7 is proposed detrimental to the

reliability of ceramic capacitors [15]. It is known that both

Y2O3 and SiO2 are effective additives for improving the

sintering and dielectric properties of BaTiO3 ceramics [16,

17]. In addition, Ga2O3 can improve the electrical prop-

erties, making the transition more prominent and restore

long range ordering [18, 19]. Moreover, the Y/Zn/Ga/Si

codoped BaTiO3 materials have not been researched until

now. In this paper, in order to meet the AC MLCC

M.-J. Wang � H. Yang � Q.-L. Zhang (&) � L. Hu � D. Yu

Department of Materials Science and Engineering,

Zhejiang University, Hangzhou 310027, China

e-mail: mse237@zju.edu.cn

H. Yang

Zhejiang California International NanoSystems Institute,

Hangzhou 310029, China

Z.-S. Lin � Z.-S. Zhang

Fujian Torch Electron Technology Co., LTD.,

Quanzhou 362000, China

123

J Mater Sci: Mater Electron (2014) 25:2905–2912

DOI 10.1007/s10854-014-1958-3

performance requirements, we prepared the core–shell

structure in BaTiO3 ceramics by codoping Y2O3, Ga2O3,

ZnO and SiO2, and studied the doping effect on the phase

constitution, microstructure and dielectric properties of

BaTiO3 ceramics.

2 Experimental

2.1 Synthesis

BaTiO3 powder doped with rare earth and metal elements

was prepared by traditional solid phase method. Com-

mercial BaTiO3 powder (Ba/Ti = 0.999) via hydrother-

mally synthesis method with an average particle size about

0.5 lm was used as starting powder. Doped materials Y2O3

(99.99 %), ZnO (99 %), Ga2O3 (99.99 %), and SiO2 (AR)

were mixed together with BaTiO3 powder according to the

composition in Table 1. The mixed powders were ball-

milled in a polyurethane mill bottle with zirconia balls,

using ethanol as the grinding medium for 6 h. After being

dried and sieved, the powders were granulated using 8 wt%

polyvinyl alcohol (PVA) as binder and pressed into pellets

about 10 mm in diameter and 1 mm in thickness in a

stainless steel die under a uniaxial pressure of 300 MPa.

Then the pellets were baked at 500 �C for binder burn out,

and then sintered at various temperatures for 3 h

(1,260–1,420 �C).

2.2 Characterization

The crystalline phase of the sintered samples was charac-

terized by X-ray diffraction (XRD, EMPYREAN, PAN

Analytical Co., Netherlands, Cu–Ka radiation in the 2hrange 10–80� with a step of 0.01�) after ground into fine

powder. The bulk density of the sintered samples was

measured using Archimedes method. The fracture surface

morphology of sintered samples was determined using a

scanning electron microscope (FSEM, SIRION-100, and

FEI, USA). For a further study of microstructure, TEM

samples were prepared by mechanical polishing and ion

beam thinning (Gatan PIPS), and then the bright field

image, high-resolution image, etc. were observed using

transmission electron microscopy (TEM, JEM-1200EX,

JEOL) by controlling the directions of incident electron

beam. Energy dispersive spectroscopy (EDS) was used to

analyze the composition profiles of the dopants in the

core–shell structure.

2.3 Dielectric measurements

For dielectric properties measurements, silver-paste was

printed on both sides of the well-polished samples, fol-

lowed by heat treating at 650 �C for 30 min to serve as

electrodes. The permittivity and dielectric loss were mea-

sured in a range from -55 to 125 �C using an HP 4278

LCR meter at 1 kHz with 1Vrms. The AC breakdown

voltage (BDV) was determined by an AC voltage tester

7120 at 50 Hz with a rate of 200 V/s (Maximum 5 kV).

3 Results and discussion

3.1 Crystallization behavior

The XRD patterns of BaTiO3 ceramics with different dopant

sintered at respective optimal temperature for 3 h are shown

in Fig. 1. Table 2 illustrates the results of XRD analyses. B0

(undoped), B2 (Zn single-doped), B3 (Ga single-doped) and

B5 (Zn/Ga codoped) samples exhibit tetragonal structure. B1

(Y single-doped) and B4 (Y/Ga codoped) samples show

orthorhombic structure. However, pseudocubic structure is

obtained for the B6 (Y/Zn codoped) and B7 (Y/Zn/Ga cod-

oped) samples. The peaks shift is also observed from the

enlarged view of (110) peaks at diffraction angle (2h)

31–32�, which is attributed to the lattice distortion resulted

from the incorporation of dopants. The (110) peak of B1

shifts to lower 2h degrees, due to the substitution of Y3? for

A- and B-site. Y3? ionic radius of A- and B-site are 1.223 and

0.9 A, respectively [20], between Ba2? (1.61 A, 12 coordi-

nate) and Ti4? (0.605 A, 6 coordinate), so Y3? can occupy

A-site and B-site in BaTiO3 lattice simultaneously. How-

ever, the solid solubility limit of Y3? substitution for Ba2?

site (*1.5 mol%) is lower than that of Y3? for Ti4? site

(*12.2 mol%) [21]. The more contribution of Y3? substi-

tution for Ti4? site than Ba2? site makes the (110) peak shift

to lower angle. Contrary to the peak shift influenced by Y3?,

the (110) peaks of B2 and B3 shift to higher 2h degrees. It is

considered that similar to Mg2? effect, Zn2? and Ga3?

dopant generates numerous oxygen vacancies, leading to the

lattices shrinking [22]. Although the ionic radius of Zn2?

(0.74 A, 6 coordinate) and Ga3? (0.62 A, 6 coordinate) are

Table 1 Composition of specimens

Specimen BaTiO3

(mol%)

Y2O3

(mol%)

ZnO

(mol%)

Ga2O3

(mol%)

SiO2

(mol%)

B0 100 0 0 0 1

B1 100 2.5 0 0 1

B2 100 0 2.5 0 1

B3 100 0 0 1 1

B4 100 2.5 0 1 1

B5 100 0 2.5 1 1

B6 100 2.5 2.5 0 1

B7 100 2.5 2.5 1 1

2906 J Mater Sci: Mater Electron (2014) 25:2905–2912

123

larger than Ti4?, the (110) peaks shift towards higher angle.

Similarly, the (110) peaks of B4–B7 tend to shift to higher 2hdegrees due to the effect of oxygen vacancies. The above

phenomenon of peaks shift is also observed at diffraction

angle (2h) 44–46�, thus it is concluded that Y, Zn and Ga

have been incorporated into the lattice. Peak splitting of

(200) and (002) planes at diffraction angle (2h) 44–46�occurs in B0–B5 samples, which illustrates obviously that

Ga3? dopant can increase tetragonality while Zn2? dopant

decreases tetragonality. The tetragonality is the relative ratio

of the peak intensity of the (200) plane to that of the (002)

plane [23]. It can be seen from Fig. 1c that sample B7 shows

a pseudocubic structure at the whole sintering temperatures

from 1,260 to 1,420 �C, but the (110) peak shifts toward

lower 2h degrees at 1,380 �C. The reason is that at low sin-

tering temperatures, the only cation sites available to be

occupied by Y3? will be those belonging to Ba2?, but at high

sintering temperatures, Y3? will occupy preferentially both

sites that lead the system to thermodynamic equilibrium

[24]. A small amount of second phase Y2Ti2O7 occurs in the

sample B1, B4 and B6, which can be seen clearly in the

enlarged view near (110) peaks at diffraction angle (2h)

30.5–32� (Fig. 1b). It indicates that Y3? is difficult to dis-

solve into the lattice completely as its oxides, but the second

phase can be eliminated by codoping Ga3? and Zn2?, maybe

due to the increase of solid solution caused by valence

compensation mechanism.

3.2 Densification and surface morphology

Bulk densities of samples B0–B7 sintered at respective

optimal temperature for 3 h and sample B7 sintered at

various temperatures are listed in Tables 3 and 4. Bulk

densities-temperatures curves of the samples contained

Fig. 1 a X-ray diffraction patterns of the samples B0–B7 sintered at

respective optimal temperature for 3 h (line profiles of (110), (200)

and (002) peaks shown in the inset); b The enlarged view near (110)

peaks; c X-ray diffraction patterns of the sample B7 sintered at

various temperatures

J Mater Sci: Mater Electron (2014) 25:2905–2912 2907

123

Y3? are illustrated in Fig. 2. It can be seen from Tables 3,

4 and Fig. 2 that Y2O3 dopant lead higher densification

temperature of BaTiO3 ceramics, the densification tem-

perature of B1 and B4 sample is 1,380 �C, while the

densification temperature of B6 and B7 sample is 1,340 �C.

However, bulk densities of Zn2? (B2) and Ga3? (B3)

doped samples can reach saturation at a lower temperature

(1,260 �C). It indicates that ZnO and Ga2O3 can improve

densification and decrease sintering temperature of doped

BaTiO3, which can be attributed to valence compensation

mechanism and changes in the mass caused by atomic

replacement. According to Kr}oger–Vink notation, the

possible defect equations are provided as follows:

Y2O3 ! 2Y�Ba þ 1=2V00Ti þ 3OO ð1Þ

Ga2O3 ! 2Ga0Ti þ V�O þ 3OO ð2Þ

ZnO ! Zn00Ti þ V��O þ OO ð3Þ

Y2O3 þ Ga2O3 ! 2Y�Ba þ 2Ga0Ti þ 6OO ð4Þ

Y2O3 þ ZnO ! 2Y�Ba þ Zn00Ti þ 4OO ð5Þ

2Y2O3 ! 2Y�Ba þ 2Y0Ti þ 6OO ð6Þ

The above valence compensation reaction (4)–(6) can

reduce excessive oxygen vacancies and improve sintering.

Densification can also be reflected from SEM images of the

sample fracture surface in Fig. 3. Morphology of

inhomogeneous grains with wide grain size distributions is

observed for B0 sample illustrated in Fig. 3a. The Y3?

doped sample (B1) sintered at 1,380 �C has an obvious

abnormal grain growth phenomenon shown in Fig. 3b,

maybe due to the high sintering temperature. It can be seen

from Fig. 3c, d, f that Zn2? and Ga3? can suppress the

grain growth in different degrees. It is known that the

incorporation of Zn2? and Ga3? will bring about plenty of

oxygen vacancies. The formation of oxygen vacancies can

lead to lattice distortion and ions located in the grain

boundary, which baffles motion of the grain boundary and

accordingly inhibits grain growth [25]. Compared with

single Y3? doped sample, Y3? and Ga3? codoped sample

shows a much smaller grain size distribution (Fig. 3e). The

Y/Zn codoped sample (B6) and Y/Zn/Ga codoped sample

(B7) sintered at 1,340 �C have similar micromorphology

with reasonably good close packing of grains and high

uniformity. Porous structure occurred in higher sintering

temperature for B7 sample (Fig. 3h, j).

3.3 Core–shell microstructures

Figure 4 shows the TEM micrographs of BaTiO3 codoped

with Y/Zn/Ga sintered at 1,340 �C for 3 h. Core–shell

structure is clearly revealed by the bright field image of

Fig. 4a and the evident interface between core and shell

Table 2 The results of XRD analyses from Fig. 1

Specimen Dopants (002) (200) Peaks Crystal structure (110) Peak shift toward Main mechanism

B0 / Splitting Tetragonal Lower 2h angle Ionic substitution

B1 Y Splitting Orthorhombic Higher 2h angle Oxygen vacancies

B2 Zn Splitting Tetragonal Higher 2h angle Oxygen vacancies

B3 Ga Splitting Tetragonal Higher 2h angle Oxygen vacancies

B4 Y/Ga Splitting Orthorhombic Higher 2h angle Oxygen vacancies

B5 Zn/Ga Splitting Tetragonal Higher 2h angle Oxygen vacancies

B6 Y/Zn No splitting Pseudocubic Higher 2h angle Oxygen vacancies

B7 Y/Zn/Ga No splitting Pseudocubic Higher 2h angle Oxygen vacancies

Table 3 Bulk densities and dielectric properties of specimen sintered at respective optimal temperature

Specimen Bulk density (g/cm3) Permittivity Dielectric loss Max|DC/C25| (%) BDV (kv/mm)

B0 1,260 �C 5.70 3,765 0.019 47.89 3.02

B1 1,380 �C 5.60 2,627 0.017 25.19 3.63

B2 1,260 �C 5.73 2,367 0.004 25.94 4.21

B3 1,260 �C 5.71 1,359 0.013 382.76 0.73

B4 1,340 �C 5.49 2,179 0.013 21.10 4.03

B5 1,260 �C 5.69 2,502 0.004 21.94 4.40

B6 1,340 �C 5.70 2,851 0.010 14.98 3.47

B7 1,340 �C 5.73 2,690 0.010 14.90 3.73

2908 J Mater Sci: Mater Electron (2014) 25:2905–2912

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under HRTEM (High-Resolution TEM) in Fig. 4b. The

selected area electron-diffraction patterns from the core

shown in Fig. 4c indicate that an orthorhombic structure

(lattice parameter a = *4.0 A, c = *5.6 A) is formed.

The HRTEM lattice spacing measured in Fig. 4b also

confirms this. It can be seen from the results of EDS line

profile analysis in Fig. 4d that the concentrations of Y, Zn

and Ga are preserved to a certain depth from the grain

boundary and decrease toward the core region. So it is

apparent that Y3?, Zn2? and Ga3? can be dissolved into the

grains together, segregated at grain boundaries and play a

significant role in the formation of core–shell structure in

BaTiO3 ceramics.

3.4 Dielectric properties

Dielectric properties of samples sintered at respective

optimal temperature and sample B7 sintered at various

temperatures are listed in Tables 3 and 4, respectively. B0

sample possesses maximum room temperature permittivity

(*3,765) due to tetragonal structure and the contribution

of 90� domain walls determined by the grain size [26].

However, B3 sample with small grain size has low room

temperature permittivity in spite of tetragonal structure.

Other samples have the permittivity between 2,000 and

3,000, owe to the decrease in ionic polariability of the

dipole moment. According to Shannon’s conclusion about

ion polarizabilities [27], the values of a (Y3?) and a (Zn2?)

are 3.84 and 2.09 A3, respectively, smaller than a (Ba2?)

(6.4 A3) and a (Ti4?) (2.94 A3). In addition, permittivity

depends on the number of ion polarization in unit volume

[28], which can be decided by the bulk densities in a

degree. Thus the sample B7 sintered at 1,340 �C has the

max permittivity in the range of temperatures from 1,260 to

1,420 �C.

It is noticed that the Zn2? doped sample has the lowest

dielectric loss among all the samples. Sua et al. reported

the probable mechanism of dielectric loss influenced by

Mg2?. Mg2? can replace Ti4? as an acceptor for BaTiO3

with a double ionized oxygen vacancy (VO••) formed

simultaneously. Charged oxygen vacancies, in combination

with Mg2?, obviously lead to the local deformation of

perovskite unit cells. Thus, both electric dipoles formed

from MgTi00-VO

•• complex and elastic dipoles due to dis-

tortions caused by Vo•• will be present in the BaTiO3

ceramics. Oxygen vacancies in BaTiO3 reside at the cor-

ners of octahedral, are well interconnected, and can

therefore be regarded as relatively ‘‘mobile’’. Mobile defect

complexes migrate to domain boundaries. Orientation of

the electric and elastic dipoles results in domain-wall

pinning and thus a reduction of the dissipation in the fer-

roelectric state [29]. Since Zn2? and Mg2? have the similar

ionic radius and valence shell structure, thus we take it for

granted that the above mechanism can also be applied to

Zn2?. Moreover, Zn2? doped sample with low tand can be

partly attributed to the well-densified structure without any

visible pores and cracks. It can be seen from the Table 3

that Y3? and Ga3? have no obvious effect on the dielectric

loss.

Figure 5 shows temperature coefficient of capacitance

(TCC) curves of the samples sintered at respective optimal

temperature for 3 h. The curves of samples with tetragonal

phase exhibit a sharp peak at Curie temperature

(Tc & 125 �C), which is caused by tetragonal-to-cubic

phase transition. The Curie peak increases as the tetrago-

nality increases. The other doped groups exhibit relatively

Table 4 Bulk densities and

dielectric properties of

specimen B7 sintered at various

temperatures

Sintering

temperature

Bulk density

(g/cm3)

Permittivity Dielectric loss Max|DC/C25| (%) BDV

(kv/mm)

1,260 5.51 2,323 0.013 14.89 3.79

1,300 5.64 2,521 0.012 17.44 3.46

1,340 5.73 2,690 0.010 14.90 3.73

1,380 5.69 2,608 0.011 19.83 3.27

1,420 5.69 2,677 0.017 25.33 2.40

Fig. 2 Bulk densities-temperatures curves of samples B1, B4, B6 and

B7

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123

Fig. 3 SEM of the fracture

surface of BaTiO3 samples:

a B0 sintered at 1,260 �C, b B1

at 1,380 �C, c B2 at 1,260 �C,

d B3 at 1,260 �C, e B4 at

1,340 �C, f B5 at 1,260 �C,

g B6 at 1,340 �C, h B7 at

1,380 �C, i B7 at 1,340 �C, j B7

at 1,420 �C

2910 J Mater Sci: Mater Electron (2014) 25:2905–2912

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stable characteristics through the entire temperatures from

-55 to 125 �C. Among these samples, B6 and B7 samples

exhibit the best TCC characterization, meet the EIA-X7R

requirements that keep the max|DC/C25|under 15 %. The

capacitance–temperature stability characteristics are asso-

ciated with the core–shell structure. The dopants Y3?, Zn2?

and Ga3? can diffuse into BaTiO3 grains to a certain depth

collectively to form the shell of pseudocubic phase, and

lead to the volume fraction of grain core decreasing. Thus,

the sharp dielectric peak at Tc is depressed distinctly and

becomes widely broad.

The AC voltage resistance characteristics show no

obvious regularity among these samples. All samples can

improve the AC voltage resistance characteristics com-

pared with the undoped one except for Ga3? single-doped

sample. It indicates that the AC resistance characteristics

decrease with the increase in tetragonality. In addition, it is

believed that AC Breakdown Voltage (BDV) is related to

doped elements, amount of dopants, sintering temperature,

and etc. It is also believed that dielectric loss, grain size, as

well as frequency of closed pores, are the factors to affect

AC breakdown field strength [30].

4 Conclusions

In the paper, the doping behaviors of yttrium, zinc and

gallium and their effects on the dielectric properties and

microstructures of BaTiO3 were investigated. Y3? dis-

solved into the lattice of BaTiO3, and replaced both Ba2?

site and Ti4? site; while Zn2? and Ga3? tended to occupy

Ti4? site. Compared with Y2O3 and Ga2O3, ZnO sup-

pressed grain growth of BaTiO3 more effectively and

Fig. 4 a TEM bright field image, b HRTEM (High-Resolution TEM), c selected area electron-diffraction patterns from the core of core–shell

structure, and d EDS line profile for the core–shell grain of B7 sample sintered at 1,340 �C

Fig. 5 Temperature coefficient of capacitance (C - C25)/C25 of the

samples B0–B7 sintered at each own optimal temperature for 3 h

J Mater Sci: Mater Electron (2014) 25:2905–2912 2911

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promoted greater uniformity of grains, thus reducing the

dielectric loss. Ga2O3 was conducive to the formation of

pure phase and improved sintering, which was ascribed to

the compensation mechanism and synergistic effect. The

formation of core–shell structure led to the dielectric

temperature characteristics improving by codoping proper

amount of Y2O3, Ga2O3 and ZnO. BaTiO3-0.025Y2O3-

0.025ZnO-0.01Ga2O3-0.01.

SiO2 ceramics sintered at 1,340 �C exhibit the best

dielectric properties: er = 2,690, tand = 1.0 % (at 1 kHz),

DC/C25 \*15 % (from -55 to 125 �C) and AC break-

down voltage E [ 3.73 kV/mm. This material has a

potential application in AC multilayer ceramic capacitor.

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