microstructure, dielectric and ferroelectric properties of barium zirconate titanate ceramics...

Microstructure, dielectric and ferroelectric properties of bariumzirconate titanate ceramics prepared by microwave sintering

Wei Cai • Chunlin Fu • Gang Chen •

Xiaoling Deng • Kaihua Liu • Rongli Gao

Received: 11 June 2014 / Accepted: 7 August 2014 / Published online: 17 August 2014

� Springer Science+Business Media New York 2014

Abstract Barium zirconate titanate ceramics were fabri-

cated by microwave sintering. Effects of microwave sin-

tering time on microstructure, dielectric and ferroelectric

properties of barium zirconate titanate ceramics have been

investigated. The result shows that the ceramic samples

sintered at 2.5 kW for 15–30 min are single phase perov-

skite structure and there is no secondary phase observed.

As the microwave sintering time extends, barium zirconate

titanate ceramics become more uniform and the grain size

increases. The data of dielectric properties indicate that the

samples prepared by microwave sintering for 15–30 min

are the ferroelectrics with diffuse phase transition and the

diffuseness of phase transition weakens with the extending

of microwave sintering time. As microwave sintering time

increases, the remnant polarization increases initially and

then decreases. Moreover, the remnant polarization and the

coercive field of the samples sintered for 15 and 20 min

decrease as measuring frequency increases, but the mea-

suring frequency has little effect on ferroelectricity of the

sample sintered for 30 min. The temperature dependences

of hysteresis loops further prove that the samples are fer-

roelectrics with diffuse phase transition.

1 Introduction

Barium zirconate titanate (BaZrxTi1-xO3, short for BZT) has

attracted great attention for its potential applications for

dynamic random access memories, tunable microwave

devices and capacitors, due to its high dielectric constant,

low dielectric loss and large tunability [1–3]. Most studies

are focused on preparation, microstructure, dielectric and

ferroelectric properties of BZT ceramics [4–7]. BZT

ceramics can be fabricated by a conventional solid state

reaction [8, 9]. But the solid state reaction has some disad-

vantages such as long processing time, low purity and

inhomogeneous grain size, which results in poor dielectric

properties. Therefore, the other alternative methods have

been attempted to prepare BZT ceramics, including sol–gel

method [10], hydrothermal synthesis [11], combustion route

[12], spark-plasma sintering [13], hot pressing [14]. How-

ever, these methods are complex and expensive, which

makes them difficult in industrial application. Microwave

sintering for electronic ceramics is superior to conventional

sintering owing to its unique characteristics, such as rapid

heating, enhanced densification rate and improved micro-

structure [15]. Microwave heating differs significantly from

conventional heating. In the microwave sintering process,

the heat is generated internally within the material instead of

originating from external sources, and hence there is an

inverse heating profile. The heating is very rapid as the

material is heated by energy conversion rather than by

energy transfer, which occurs in conventional techniques.

Microwave sintering ensures considerable time and energy

saving, and therefore is considered as one of the most pro-

spective sintering techniques in material processing. The

method has been widely applied in the fabrication of elec-

tronic ceramics [16–21]. Among them, the microwave sin-

tering of BZT ceramics has been studied. Mahajan et al.

W. Cai (&) � C. Fu � G. Chen � X. Deng � K. Liu � R. Gao

School of Metallurgy and Materials Engineering, Chongqing

University of Science and Technology, University Town,

Shapingba District, Chongqing 401331, People’s Republic

of China

e-mail: [email protected]

W. Cai � C. Fu � G. Chen � X. Deng � R. Gao

Chongqing Key Laboratory of Nano-Micro Composite Materials

and Devices, Chongqing 401331, People’s Republic of China

123

J Mater Sci: Mater Electron (2014) 25:4841–4850

DOI 10.1007/s10854-014-2242-2

fabricated the BaZr0.1Ti0.9O3 ceramics by microwave sin-

tering and found that at room temperature the microwave

sintered samples exhibit improved electrical properties

exhibiting higher resistivity, higher dielectric constant and

lower dielectric loss compared with the sample prepared by

conventional sintering [22]. Sun et al. [23, 24] used the

powder prepared by hydrothermal method as raw materials

and fabricated single phase and dense BaZr0.05Ti0.95O3

ceramics at 1,300 �C by microwave sintering and studied the

effects of soaking time on dielectric and ferroelectric prop-

erties. As Zr content increases, the phase-transition tem-

peratures of BaZrxTi1-xO3 approach each other and only one

phase transition exists at x = 0.20 [25]. And the ferroelectric

phase transition temperature of BaZr0.2Ti0.8O3 is near the

room temperature and the relatively mild temperature-

dependence is observed [26]. However, microwave sintering

process of BaZr0.2Ti0.8O3 ceramics has not yet been repor-

ted. In this work, BaZr0.2Ti0.8O3 ceramics with relative high

density have been successfully fabricated by short time

microwave sintering and effects of microwave sintering time

on microstructure, dielectric and ferroelectric properties

have been investigated.

2 Experimental

2.1 Ceramics preparation

BaZr0.2Ti0.8O3 ceramics were prepared by microwave

sintering process. The starting raw chemicals were high

purity BaCO3 (C99.9 %, Sinopharm Group Co. Ltd), TiO2

(C99.9 %, Sinopharm Group Co. Ltd) and ZrO2 (C99.9 %,

Sinopharm Group Co. Ltd) powders. BaCO3, TiO2 and

ZrO2 were carefully weighed in stoichiometric proportion

and were added into ball milling jar, then milled for 4 h in

distilled water and zirconia media. After the slurry was

dried, the mixture was calcined in an alumina crucible at

1,100 �C for 4 h in muffle furnace. The calcined powders

were remilled for 4 h and then dried. The powders added

with 7 wt% binder were compacted into disk-shaped pel-

lets with a diameter of 10.0 mm and thickness of 1.0 mm at

20 MPa pressure. The green pellets were sintered using a

microwave furnace (WLD3S-09, Nanjing Sanle, China,

4 kW, 2.45 GHz, single mode). Figure 1 is the schematic

illustration of the system. The green pellets were placed

into the Al2O3 crucible. And then the crucible was placed

into the other Al2O3 crucible filled with tetra-needle like

ZnO whiskers (Chengdu Crystrealm Co. Ltd). The Al2O3

crucible was placed into the mullite sagger filled with

Al2O3 fiber cotton. The temperature was measured with

infrared thermometer. The microwave sintering tempera-

ture was controlled by adjusting microwave power. The

BZT ceramics were sintered at 2.5 kW for different

microwave sintering time (10–40 min) (The time is the

interval from open-microwave to close-microwave). But

the result shows that the sample sintered for 10 min at

2.5 kW can not form the ceramics and the sample sintered

for 40 min at 2.5 kW sticks together with the Al2O3 cru-

cible. Therefore, the BZT ceramics were sintered at

2.5 kW for 15, 20 and 30 min, respectively.

2.2 Ceramics characterization

The crystal structure of ceramic sample was confirmed by

X-ray diffractometer (XRD, DX-2700, Dandong Fangyuan,

China) with Cu Ka (k = 0.15418 nm) radiation in a wide

range of 2h (20� B 2h B 80�). Surface morphology of the

sintered samples was examined by scanning electron

microscope (SEM, S-3700N, Hitachi, Japan). The density

of the BZT ceramics was determined by Archimede’s

method in distilled water at room temperature.

Fig. 1 Schematic diagram of microwave sintering

Fig. 2 Room temperature XRD patterns of BZT ceramics sintered

for different times

4842 J Mater Sci: Mater Electron (2014) 25:4841–4850

123

In order to measure the dielectric and ferroelectric

properties, silver paste was painted on the polished sintered

samples as the electrodes and fired at 500 �C for 15 min.

The capacitances of the ceramics were determined by an

impedance analyzer (HP 4980A, Agilent, USA) at 1 V/mm

from -50 to 160 �C. The dielectric constant was calculated

from the capacitance using the following equation:

e ¼ Cd=e0A ð1Þ

where C is the capacitance (F), e0 the free space dielectric

constant value (8.85 9 10-12 F/m), A represents the

capacitor area (m2) and d represents the thickness (m) of

the ceramics. The ferroelectric and leakage measurements

were performed out using a ferroelectric test system

(TF2000e, aixACCT, Germany).

3 Results and discussion

3.1 Crystal structure

Figure 2 shows room temperature XRD patterns and cor-

responding refinement patterns (the refinement patterns are

obtained by Jade software) of BZT ceramics sintered for

15–30 min. The lattice parameters of BZT ceramics sin-

tered for different times are shown in Table 1. Firstly, the

XRD patterns are virtually the same and show only single

phase perovskite structure without the evidence of the

second phase. XRD patterns of BZT ceramics are in

agreement with the respective joint committee on powder

diffraction standards (Cubic, Pm-3m, JCPDS file no.

75-0461). It indicates that the samples with cubic structure

at room temperature should be paraelectric phase. Sec-

ondly, the 2h of the sample sintered at 2.5 kW for 30 min

is obviously lower than that of the samples sintered at

2.5 kW for 15 and 20 min, which indicates that the lattice

constant of the sample sintered for 30 min is more than that

of the samples sintered for 15 and 20 min according to

Bragg equation (shown in Table 1).

3.2 Surface morphology

Figure 3 shows the surface morphologies of BZT ceramics

sintered for different microwave sintering times. Firstly,

Table 1 Lattice parameters of BZT ceramics sintered for different

times

Microwave sintering time (min) Lattice constant

a (nm) b (nm) c (nm)

15 0.4019 0.4019 0.4019

20 0.4012 0.4012 0.4012

30 0.4052 0.4052 0.4052

Fig. 3 Surface morphologies of the BZT ceramics sintered at 2.5 kW for different microwave sintering times. a 15 min, b 20 min, c 30 min

J Mater Sci: Mater Electron (2014) 25:4841–4850 4843

123

the average grain size of BZT ceramics increases with the

extending of microwave sintering time. The grain size

(15–40 lm) of the sample sintered for 30 min is much

more than that of the samples sintered for 15 and 20 min,

which indicates that it is difficult to obtain the smaller grain

size for BZT ceramics sintered for relatively long micro-

wave sintering time. Secondly, there are some small grains

and pores in the samples sintered for 15 and 20 min, and

the number of small grain and pore decreases with the

extending of sintering time. It indicates that the sample

gradually becomes denser as sintering time extends.

Although the uniformity of grain of BZT ceramics is

inferior to the ceramic sample prepared by Mahajan et al.

[15] (the preparation condition is 1.1 kW for 4 h), the

sintering time (15–30 min) is much shorter. Moreover, the

grain of BZT ceramics by microwave sintering is smaller

than that of the sample by conventional sintering [27].

Table 2 shows the density of BZT ceramics sintered for

different times. It is seen that the relative density of BZT

ceramics sintered for 30 min is the maximum (95.4 %) and

the density increases with the extending of sintering time,

which indicates that the short time microwave sintering for

BZT ceramics with higher density is feasible.

3.3 Dielectric properties

Figure 4 shows the temperature dependences of dielectric

properties of the BZT ceramics sintered for different sin-

tering times. Firstly, it can be found that the temperature at

which the maximum in the dielectric constant appears (Tm)

of the BZT ceramics sintered for 15, 20 and 30 min is -20,

-10 and -15 �C, respectively. It indicates that the samples

sintered for 15–30 min are paraelectric phase at room

temperature. The Tm of BaZr0.2Ti0.8O3 ceramics by

microwave sintering is lower than that of the ceramics

prepared by conventional sintering (Tm is about 20–50 �C

[27–29]). It may be due to the grain size. The result is

consistent with the XRD patterns. Secondly, the dielectric

constant of the sample decreases initially and then

increases as the sintering time extends. Furthermore, the

dielectric losses of the samples sintered for 20 and 30 min

are lower than that of the sample sintered for 15 min. It

relates to densification of the samples. Thirdly, there are

obvious broad dielectric constant peaks in the samples

sintered for 15–30 min, which indicates that there is diffuse

phase transition in BZT ceramics.

To further investigate the diffuse phase transition of

ferroelectrics, a modified Curie–Weiss law was proposed to

describe the diffuseness of the ferroelectric phase transition

as [30]:

1

e� 1

em

¼ ðT � TmÞc

C0 ð2Þ

where em and Tm represent the dielectric constant maxi-

mum and the corresponding temperature of dielectric

constant maximum, c and C’are constant. The diffuseness

constant c gives information on the character of the phase

transition: for c = 1, a normal Curie–Weiss law is fol-

lowed, whereas c = 2 describe a complete diffuse phase

transition. The plots of ln(1/e - 1/em) as a function of

ln(T - Tm) at 1 kHz for BZT ceramics sintered for

15–30 min are shown in Fig. 5. A linear relationship is

observed for BZT ceramics. The slope of the fitting curves

Table 2 Density of BZT ceramics sintered for different times

Microwave sintering

time (min)

Density

(g/cm3)

Relative

density (%)

15 5.45 91.5

20 5.61 94.2

30 5.69 95.4

Theoretical density of BaZr0.2Ti0.8O3 is 5.96 g/cm3Fig. 4 Temperature dependences of dielectric constant and dielectric

loss for BZT ceramics measured at 1 kHz


123

using Eq. (2) is used to determine the c value. It is found

that the diffuseness constant c of the samples sintered for

15, 20 and 30 min is 2.05, 1.51 and 1.48, respectively. It is

evident that the diffuseness of the phase transition of BZT

ceramics decreases with the increasing of microwave sin-

tering time. It is because that the grain size of BZT

ceramics increases as microwave sintering time increases

and the diffuseness of phase transition in BaTiO3-based

ceramics decreases with the increase of grain size [31].

Figure 6 shows dielectric constant as a function of tem-

perature at 100, 300 and 500 Hz for BZT ceramics sintered

for different microwave sintering times. It is seen that there

is no obvious frequency dispersion around the dielectric

constant peaks for BZT ceramics sintered for 15–30 min.

Tang et al. [32] reported that the peak temperature Tm of

dielectric constant would not shift with frequency when

x in BaZrxTi1-xO3 is lower than 0.25. The results of Fig. 6

are basically in agreement with the above reports. Com-

bined with the diffuseness constant c of BZT ceramics

mentioned above, it is concluded that the BaZr0.2Ti0.8O3

ceramics prepared by microwave sintering for 15–30 min

is not relaxor ferroelectrics but merely the ferroelectrics

with diffuse phase transition or relaxor-like ferroelectrics.

3.4 Ferroelectric properties

Figure 7 shows the room temperature hysteresis loops of

BZT ceramics sintered for 15–30 min. The remnant

polarization (2Pr) and coercive field (2EC) of the samples

are shown in Table 3. Firstly, although the measuring

temperature (25 �C) is above Tm of BZT ceramics sintered

for 15–30 min (see Figs. 4a and 6), the hysteresis loops of

the samples indicates that there is ferroelectric character.

This further proves that there is diffuse phase transition in

BZT ceramics sintered for 15–30 min. The ferroelectric

characteristics at room temperature result from the coex-

istence of ferroelectric phase and paraelectric phase in the

samples. Secondly, the remnant polarization of BZT

ceramics increases initially and then decreases with the

increasing of microwave sintering time (see Table 3). It is

worthwhile to note that the remnant polarization of the

sample with medium grain size (sintered for 20 min) is

much more than that of the samples with the minimum

(sintered for 15 min) and maximum grain size (sintered for

30 min). It suggests that the remnant polarization depends

on not only grain size of the sample but also the other

factors such as the amount of ferroelectric phase in the

Fig. 5 Plot of ln(1/e - 1/em) as a function of ln(T - Tm) of BZT ceramics sintered for different times. a 15 min, b 20 min, c 30 min


123

sample with diffuse phase transition. On the one hand,

grain boundary is a low permittivity region. That means the

grain boundary has poor ferroelectricity. Polarization of

grain boundary may be little. Space charges in grain

boundary exclude polarization charge on grain surface, and

depletion layer on grain surface can be formed [33]. It

results in polarization discontinuity on grain surface to

form depolarization field so that polarization decreases.

The remnant polarization of the sample with large grain

size is more than that of the sample with small grain size.

On the other hand, the amount of ferroelectric phase in

BZT ceramics with diffuse phase transition has an impor-

tant effect on the remnant polarization. The remnant

polarization increases with the increase of the amount of

ferroelectric phase. As mentioned above, the diffuseness

constant c of the sample sintered for 30 min is the mini-

mum, which indicates that the amount of ferroelectric

phase at room temperature is the minimum. Although the

sample sintered for 30 min has the maximum grain size,

the amount of ferroelectric phase in the sample is the

Table 3 The remnant polarization and coercive field of BZT

ceramics sintered for different sintering times

Sintering time (min) 2Pr (lC/cm2) 2EC (kV/cm)

15 2.22 5.6

20 8.88 4.2

30 0.70 4

Fig. 6 Dielectric constant as a function of temperature measured at different frequencies for BZT ceramics sintered for different microwave

sintering times. a 15 min, b 20 min, c 30 min

Fig. 7 Room temperature hysteresis loops of BZT ceramics sintered

for different microwave sintering times and measured at 500 Hz


123

lowest and the effect of ferroelectric phase content on the

remnant polarization is more than that of grain size so that

the remnant polarization of BZT ceramics sintered for

30 min is the minimum. Thirdly, the coercive electric field

of the BZT ceramics decreases with the increase of sin-

tering time. It could be attributed to effect of grain size.

Energy barrier for switching ferroelectric domain must be

broken through and energy barrier increases as grain size

decreases. So reversal polarization process of a ferroelec-

tric domain is easier inside a large grain than in a small

grain [34]. As mentioned above, the grain size of the BZT

ceramics increases with the increase of sintering time so

that the coercive field decreases.

Figure 8 shows room temperature hysteresis loops of

BZT ceramics sintered for different times and measured at

various frequencies. It is seen that the hysteresis loops of

BZT ceramics sintered for 15–20 min become slimmer with

the increasing of frequency (shown in Fig. 8a, b), which

indicates that the remnant polarization (Pr), the spontaneous

polarization (PS) and the coercive field (EC) decrease as

frequency increases. It may be attributed to the difference of

polarization mechanism. There are electron and ion dis-

placement polarization, turning-direction polarization and

space charge polarization caused by oxygen vacancy in

BZT ceramics sintered for 15 and 20 min. When the fre-

quency increases from 500 to 1,000 Hz, the space charge

polarization cannot catch up with change of electric field so

that the remnant polarization, the spontaneous polarization

and the coercive electric field decrease. Nevertheless, the

hysteresis loop of BZT ceramics sintered for 30 min has no

evident change as frequency increases from 500 to

1,000 Hz. For BZT ceramics sintered for 30 min, there may

be just electron and ion displacement polarization, turning-

direction polarization. These polarizations can keep up with

the change of electric field so that the remnant polarization

and coercive field don’t change with frequency.

Figure 9 shows room temperature hysteresis loops of

BZT ceramics measured at various voltages. Firstly, it is

seen that the remnant polarization, spontaneous polariza-

tion and coercive field of BZT ceramics sintered for

15–30 min increase with the increasing of electric field. It

is because that the increased electric field energy caused by

increased electric field makes more ferroelectric domain

reverse to gain higher polarization [35]. Secondly, the

hysteresis loops of BZT ceramics sintered for 15 and

30 min are still not saturated when the applied electric

Fig. 8 Room temperature hysteresis loops of BZT ceramics sintered for different times and measured at various frequencies. a 15 min,

b 20 min, c 30 min


123

voltage increases from 500 to 1,500 V. But for BZT

ceramics sintered for 20 min, the hysteresis loop gradually

becomes relative saturated as electric voltage increases.

Figure 10 shows the Hysteresis loops of BZT ceramics

sintered for 20 and 30 min measured at different temper-

atures. Firstly, the hysteresis loop of BZT ceramics sintered

for 20 min measured at 30 �C shows obvious ferroelectric

characteristics (see Fig. 10a). As mentioned above, the Tm

of BZT ceramics sintered for 20 min is about -10 �C (see

Figs. 4a and 6b). It suggests that BZT ceramics still have

ferroelectricity when temperature is above its Tm, which

indicates that the sample sintered for 20 min is

Fig. 9 Room temperature hysteresis loops of BZT ceramics sintered for different times and measured at various voltages and 500 Hz. a 15 min,

b 20 min, c 30 min

Fig. 10 Hysteresis loops of BZT ceramics sintered for 20 min. a and 30 min b measured at different temperatures and 1 kHz


123

ferroelectrics with diffuse phase transition, i.e. there is

coexistence of ferroelectric phase and paraelectric phase in

the sample at 30 �C, implying that micropolar clusters exist

above Tm which is a typical ferroelectric-relaxor charac-

teristic [36]. But when temperature is above 60 �C, it is

found that the P–E curve of BZT ceramics sintered for

20 min is basically linear relation, which indicates that its

ferroelectric characteristics disappear. Secondly, the P–E

curve of BZT ceramics sintered for 30 min measured at

30 �C shows certain ferroelectric characteristics. But the

remnant polarization, spontaneous polarization and the

coercive fields decrease simultaneously with the rise of

temperature, and when the temperature is 120 �C, the P–

E curve of the sample is close to linear relation. Although

the BZT ceramics sintered for 30 min still have ferro-

electric characteristics when measuring temperature is

above Tm because of its diffuse phase transition, the higher

temperature brings thermal disturbance for orderliness of

polarization and decreases the interaction of dipoles and

weakens the ferroelectricity [37]. Moreover, the higher

temperature makes switching of ferroelectric domain easier

and decreases the coercive field [38]. The above results

further prove that there is diffuse phase transition and re-

laxor-like behavior in BZT ceramics prepared by micro-

wave sintering.

4 Conclusions

Barium zirconate titanate ceramics with relative high den-

sity were successfully prepared by microwave sintering for

short time. The effects of microwave sintering time on the

crystal structure, surface morphologies, dielectric and fer-

roelectric properties of BZT ceramics have been investi-

gated systematically. The samples sintered at 2.5 kW for

15–30 min are single phase cubic perovskite structure. As

microwave sintering time extends, the grain size and den-

sity of BZT ceramics increases and the sample gradually

becomes denser. There are obvious broad dielectric con-

stant peaks in the samples sintered for 15–30 min. The

diffuseness of the phase transition of BZT ceramics weak-

ens with the increasing of microwave sintering time. It is

seen that there is no obvious frequency dispersion around

the dielectric constant peaks for BZT ceramics sintered for

15–30 min. It is concluded that BaZr0.2Ti0.8O3 ceramics

prepared by microwave sintering for 15–30 min is the fer-

roelectrics with diffuse phase transition or relaxor-like

ferroelectrics. The remnant polarization increases initially

and then decreases with the increase of microwave sintering

time. It results from a combination of grain size and the

amount of ferroelectric phase in the sample with diffuse

phase transition. Moreover, the remnant polarization and

the coercive field of BZT ceramics sintered for 15 and

20 min decrease as frequency increases, but the frequency

has little effect on ferroelectricity of the sample sintered for

30 min. It is seen that the remnant polarization, spontaneous

polarization and coercive field of BZT ceramics sintered for

15–30 min increase with the increasing of electric field. The

temperature dependences of hysteresis loop further prove

that the sample sintered for 20 and 30 min is ferroelectrics

with diffuse phase transition.

Acknowledgments This work was supported by the National Natural

Science Foundation of China (51102288, 51372283), the Scientific and

Technological Research Program of Chongqing Municipal Education

Commission (KJ131402), Natural Science Foundation of Chongqing

(CSTC2012jjA50017), the Research Foundation of Chongqing Uni-

versity of Science and Technology (CK2013B08) and the Cooperative

Project of Academician Workstation of Chongqing University of Sci-

ence and Technology (CKYS2014Z01, CKYS2014Y04).

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