Effect of sintering temperature on phase structure,microstructure, and electrical properties of (K0.5Na0.5)NbO3–(Ba0.6Sr0.4)0.7Bi0.2TiO3 lead-free ceramics

Hualei Cheng • Wancheng Zhou • Hongliang Du •

Fa Luo • Dongmei Zhu • Boxi Xu

Received: 6 July 2013 / Accepted: 8 November 2013 / Published online: 19 November 2013

� Springer Science+Business Media New York 2013

Abstract Lead-free 0.98(K0.5Na0.5)NbO3–0.02(Ba0.6Sr0.4)0.7

Bi0.2TiO3 (abbreviated as 0.98KNN–0.02BSBT) ceramics

were prepared by the conventional solid-state sintering

method. Effect of sintering temperature on 0.98KNN–

0.02BSBT ceramics was systematically investigated. The

frequency dependent dielectric permittivities show that the

ceramics sintered at different temperatures are indeed ‘‘relax-

or-like’’ ferroelectric ceramics, which possess a diffuse phase

transition without a strong frequency dispersion of dielectric

permittivity. The diffuseness parameter c, the comparison of

the relaxor behavior based on empirical parameters (DTdiffuse)

and the slimmer P–E hysteresis loops confirm that the ‘‘re-

laxor-like’’ characteristics of the ceramics are strengthened

with increasing sintering temperature. At the optimum sinter-

ing temperature, the dielectric permittivity maximum (emax)

has a value of approximately 2795 (at 1 KHz), tan d is lower

than 2.5 % and the diffuseness parameter c = 1.68 at a broad

usage temperature range (150–350 �C), which indicate its

potential application in high temperature multilayer ceramics

capacitor field.

Introduction

Recently, high temperature relaxor ferroelectrics with

complex perovskite structure have received much attention,

as these materials can be widely used in fabrication of

multilayer ceramic capacitors, electrostrictive actuators,

and electromechanical transducers [1–6]. However, most

high temperature relaxor ferroelectrics based on the

PbTiO3–Bi(Me0,Me00)O3 ferroelectric perovskites, have the

obvious disadvantages associated with the volatility and

toxicity of PbO [1–6].

The search for lead-free high temperature relaxor fer-

roelectric materials has become a very hot topic because of

the environmental protection, human health, and the sus-

tainable development of the world. At present, many

studies about lead-free high temperature relaxor ferro-

electrics are focused on (Na0.5Bi0.5)TiO3, BiScO3–BaTiO3

and (K0.5Na0.5)NbO3–(KNN) based ceramics because of

the relatively high temperature of the maximum dielectric

permittivity (Tm) [7–22]. The KNN-based lead-free relaxor

ferroelectric ceramics, such as, KNN–(Ba0.5Sr0.5)TiO3,

KNN–Bi(Mg2/3Nb1/3)O3, and KNN–Bi(Zn2/3Nb1/3)O3

show high relative permittivity at a broad temperature

usage range [17–22]. In order to further improve the

dielectric temperature stability of KNN ceramics, the

environment-friendly relaxor ferroelectrics (Ba0.6Sr0.4)0.7-

Bi0.2TiO3 (BSBT) were introduced into KNN ceramics to

form a new solid solution (K0.5Na0.5)NbO3–

(Ba0.6Sr0.4)0.7Bi0.2TiO3 (KNN–BSBT) [23]. The results

indicate that KNN–BSBT ceramics show excellent

dielectric properties and relaxor behavior. In addition,

0.98KNN–0.02BSBT ceramics show a mixed phase of

orthorhombic and tetragonal phases.

The relationship of composition-structure-properties-

processing has become the core of materials science. The

H. Cheng (&) � W. Zhou � H. Du � F. Luo � D. Zhu

State Key Laboratory of Solidification Processing, Northwestern

Polytechnical University, Xi’an 710072, Shaanxi, China

e-mail: hualeicheng@163.com

H. Du

e-mail: duhongliang@126.com

B. Xu

School of Materials Science and Engineering, Nanyang

Technological University, 50 Nanyang Avenue,

Singapore 639798, Singapore

123

J Mater Sci (2014) 49:1824–1831

DOI 10.1007/s10853-013-7870-z

ceramics with the same composition and preparation

technique, but different processes and processing parame-

ters, exhibit absolutely different microstructure, and then

their properties may differ greatly. So, a careful investi-

gation and optimization of sintering parameter is needed to

obtain reproducible and high quality materials. In addition,

it is well-known that the properties of KNN ceramics are

extremely sensitive to the processing conditions, especially

the sintering temperature [24–31]. So far, KNN ceramics

are mostly prepared within a narrow processing window in

terms of sintering temperature [26, 27]. The effects of

sintering condition on the microstructure and electrical

properties of KNN ceramics have been investigated in prior

studies. Slight fluctuation of sintering temperature may

result in deteriorated performance [28–31]. However, little

is known so far on the effect of sintering temperature on the

microstructure and dielectric relaxor properties of KNN–

BSBT ceramics. Therefore, as an extension to the research

on the KNN–BSBT ceramics, the main purpose of this

study is to determine the temperature and frequency de-

pendences of the dielectric relaxation of ceramics in the

0.98KNN–0.02BSBT ceramics fabricated at different sin-

tering temperatures. Meanwhile, the microstructure evolu-

tion induced by sintering temperature was closely

investigated in terms of scanning electron microscopy

(SEM) and X-ray diffraction (XRD) experiments.

Experimental procedure

0.98(K0.5Na0.5)NbO3–0.02(Ba0.6Sr0.4)0.7Bi0.2TiO3 ceramics

were prepared using the conventional solid-state sintering

method. Reagent-grade oxide and carbonate powders of

K2CO3 (99.0 %), Na2CO3 (99.8 %), Nb2O5 (99.9 %), Bi2O3

(99.0 %), BaCO3 (99.9 %), SrCO3 (99.8 %), and TiO2

(99.9 %) were used as the starting materials. Before being

weighed, these powders were separately dried in an oven at

110 �C for 5 h. They were milled for 24 h using planetary

milling with zirconia ball media and alcohol, then dried and

calcined at 950 �C for 5 h. After the calcinations, these

powders were ball-milled again for 12 h, dried, and pressed

into disks of 12 mm in diameter and 1 mm in thickness under

300 MPa using polyvinyl alcohol (PVA) as a binder. After

burning off PVA, the pellets were sintered at 1100–1140 �C

for 2 h. The obtained samples were polished. Silver paste was

fired on both sides of the samples at 810 �C for 20 min as the

electrodes for the sake of measurements.

The phase structures of the sintered ceramics were exam-

ined using X-ray powder diffraction analysis with a Cu Karadiation (Philips X-Pert ProDiffractometer, Almelo, and The

Netherlands) at room temperatures. The microstructure evo-

lution was observed using scanning electron microscopy

(SEM) (TESCAN VEGA 3 SBH, Tayasaf, Brno, and

Czechia). The dielectric spectrum measurements were per-

formed using the LCR meter (Agilent E4980, USA) with a

heat rate of 3 �C/min in a temperature range of 25–460 �C and

a frequency range of 1–1000 kHz. The polarization versus

electric field (P–E) hysteresis loops were observed using a

Radiant Precision Workstation.

Results and discussion

Phase and microstructure analysis

Previous report [23] showed that the phase structure of the

0.98KNN–0.02BSBT ceramics sintered at 1100 �C is a

mixed phase of orthorhombic and tetragonal phases. In

order to investigate the nature of the mixed phase, XRD

patterns of the 0.98KNN–0.02BSBT ceramics sintered at

1100 �C were measured at different temperatures. Fig. 1a

shows the phase structures of the 0.98KNN–0.02BSBT

ceramics sintered at 1100 �C, which are measured at dif-

ferent temperatures. The orthorhombic phase and tetrago-

nal phase are characterized by (202)/(020) peak and (002)/

(200) peak splitting at about 45 Æ , respectively. According

Fig. 1 XRD patterns of the 0.98KNN–0.02BSBT ceramics sintered

at 1100 �C and measured at different temperatures in the 2h ranges of

a 20�–60� and b 40�–50�

J Mater Sci (2014) 49:1824–1831 1825

123

to Fig. 1a, it can be concluded that the phase structure of

the 0.98KNN–0.02BSBT ceramics sintered at 1100 �C

transforms from the mixed phase to tetragonal phase and

then to cubic phase with increasing measurement temper-

ature. The XRD patterns of specimens show the mixed

phase of orthorhombic and tetragonal phases at 27, 50, 80,

and 100 �C, tetragonal phase at 120, 150, 200, and 300 �C

and cubic phase at 400 �C. This result indicates that the

mixed phase of orthorhombic and tetragonal phases,

namely, the so called morphotropic phase boundary (MPB)

in 0.98KNN–0.02BSBT ceramics is strongly temperature

dependent. Figure 1b shows the enlarged XRD patterns of

the 0.98KNN–0.02BSBT ceramics sintered at 1100 �C and

measured at different temperatures in the 2h ranges of

40–50�. The splitting of peaks between 45� and 46� con-

firms the MPB nature of the ceramics.

Figure 2a shows the XRD patterns of the 0.98KNN–

0.02BSBT ceramics sintered at different temperatures. As

can be seen from these patterns, a complete perovskite

structure is formed with sintering temperature in the range

of 1100–1140 �C. With increasing sintering temperature,

however, the (200) peaks change, indicating the develop-

ment of phase structure. Therefore, the corresponding

expanded XRD patterns for specimens are given in the 2hranges of 40–50�, as shown in Fig. 2b. For the 0.98KNN–

0.02BSBT ceramics, sintered at 1100 �C, the XRD patterns

show the mixed phase of orthorhombic and tetragonal

phases [23]. As the sintering temperature increased, the

patterns of the ceramics changed from tetragonal to

pseudo-cubic structure. This observation is obviously

associated with lattice parameters, as shown in Fig. 2c. In

previous studies [29–31], this kind of temperature depen-

dence of phase structure transition behavior was also

found, and this behavior was attributed to the different

extents of the volatilization of alkali metal ions during

sintering at different temperatures. In this study, both Na

and K undergo more severe losses with increasing sintering

temperature. The weight changes of the samples are

recorded before and after sintering, and the resultant weight

loss (the percentage of mass decrease relative to the mass

before sintering) values are 0.36, 0.40, and 0.75 % for

ceramics sintered at 1120, 1125, and 1140 �C, respectively.

So, the real composition of the 0.98KNN–0.02BSBT

ceramics with different sintering temperatures should be

different from the nominal one and the volatilization of

alkali metal ions should be related to the phase structural

transition behavior.

Figure 3 shows the SEM micrographs of the polished

and thermally etched surface of the 0.98KNN–0.02BSBT

ceramics sintered at different temperatures. As shown in

Fig. 3a, the ceramics sintered at 1100 �C show a charac-

teristic quasi-cubic morphology, and the average grain size

is about 1.5 lm. When the sintering temperature is

increased to 1110 �C, the microstructure becomes denser

and the grains grow much larger, the average grain size is

about 3.0 lm, as shown in Fig. 3b. Details of the average

grain size G and bulk density of the specimens studied are

summarized in Table 1. However, the abnormal grain

growth (AGG) behavior appears in the 0.98KNN–

0.02BSBT ceramics sintered at 1120 �C (Fig. 3c). It is

difficult to distinguish large grains which grow signifi-

cantly and result in a bimodal distribution containing

coarse and faceted grains 5 lm in length; many fine grains

are distributed at the boundaries of coarse grains. The AGG

Fig. 2 a XRD patterns of the 0.98KNN–0.02BSBT ceramics sintered

at different temperatures and b the expanded XRD patterns in the 2hrange of 40�–50� and c the variation of the lattice parameters

1826 J Mater Sci (2014) 49:1824–1831

123

Fig. 3 SEM micrographs of the polished and thermally etched surface of the 0.98KNN–0.02BSBT ceramics sintered at different temperatures:

a 1100, b 1110, c 1120, d 1125, and e 1140 �C

J Mater Sci (2014) 49:1824–1831 1827

123

behavior is intensified in the specimens sintered at

1125 �C, as shown in Fig. 3d. It is considered that the

AGG behavior should be attributed to the presence of a

liquid phase as described in previous reports [29]. How-

ever, it is very difficult to find liquid phase from the

micrographs for the 0.98KNN–0.02BSBT ceramics with

sintering temperature in the range of 1100–1125 �C,

implying that the liquid phase formed during the sintering

could easily dissolve in the KNN ceramics that led to its

eventual disappearance with sintering time [32]. Further

increasing sintering temperature to 1140 �C, many distinct

pores exist in the grain boundary of 0.98KNN–0.02BSBT

ceramics, and the liquid phase can be observed in Fig. 3e,

which may be ascribed to the solidus temperature of KNN

which is near 1140 �C [33]. The optimum sintering tem-

perature is determined as the sintering temperature at

which the ceramic has the largest density [28]. From

Table 1, it is found that the maximum density of 4.58 g/cm3

is obtained for the 0.98KNN–0.02BSBT ceramics sintered at

1120 �C, which indicates that the optimum sintering tem-

perature for 0.98KNN–0.02BSBT ceramics is 1120 �C.

Electrical properties

Figure 4 shows the temperature dependence of dielectric

permittivity at various frequencies for the 0.98KNN–

0.02BSBT ceramics with different sintering temperatures. In

Fig. 4a, the samples sintered at 1100 �C, similar to pure

KNN ceramics [33], have two phase transitions above room

temperature, corresponding to the ferroelectric orthorhom-

bic-tetragonal polymorphic phase transition (TO–T) and the

tetragonal-cubic transition (TC). However, as the sintering

temperature increased, the polymorphic phase transition

(PPT, at TO–T) disappears and only the cubic-tetragonal

phase transition is observed above room temperature

(Fig. 4b–e). In addition, it is found that the maximum

dielectric permittivity (emax) decreases steadily with

increasing sintering temperature. When the sintering tem-

perature is 1140 �C, the emax (at 1 kHz) decreases rapidly

and has a low value of 1462, which lied in the lower density

and poor microstructure. More interesting, it is also found

that the er-T curves of the ceramics sintered at different

temperatures exhibit strong diffuse phase transition charac-

teristics with broad dielectric peaks. Particularly, the

0.98KNN–0.02BSBT ceramics sintered at 1120 �C show a

broad and stable permittivity maximum (emax) approxi-

mately 2795 (at 1 kHz) at a broad usage temperature range

(150–350 �C), which indicates the potential application in

high temperature multilayer ceramics capacitor.

In general, the relaxor ferroelectrics have two typical

characteristics: the diffuse phase transition and the frequency

dispersion of dielectric permittivity. From Fig. 4, the fre-

quency dispersion of the dielectric permittivity is not evident

through all the measured temperature range. We only found

that the 0.98KNN–0.02BSBT ceramics sintered at different

temperatures show the diffuse phase transition. Therefore, we

think that 0.98KNN–0.02BSBT ceramics sintered at different

temperatures only show a ‘‘relaxor-like’’ characteristic,

namely, only possessing the diffuse phase transition without a

strong frequency dispersion of dielectric permittivity.

A modified Curie–Weiss law is used to explain the

dielectric behavior of complex ferroelectrics with diffuse

phase transition, which is described as follows [34]:

1

e� 1

em

¼ ðT � TmÞc

C

where C is the Curie constant and c is a diffuseness

parameter ranging from one (a normal ferroelectric) to two

(an ideal relaxor ferroelectric). Figure 5 shows the plot of

log (1/e - 1/em) as a function of log (T - Tm) at 1 kHz for

the 0.98KNN–0.02BSBT ceramics with different sintering

temperatures. It is found that a linear relationship is

obtained in all specimens and the c value varies from 1.2 to

1.8, indicating that the ‘‘relaxor-like’’ behavior of the

0.98KNN–0.02BSBT ceramics is strengthened with

increasing sintering temperature. In addition, the

parameter of DTdiffuse (1 kHz) is introduced to investigate

the ‘‘relaxor-like’’ behavior of the 0.98KNN–0.02BSBT

ceramics sintered at different temperatures. The specific

symbols of the diffuseness degree are defined as [35]:

DTdiffuseð1 kHzÞ ¼ T0:9emð1 kHzÞ � Temð1 kHzÞ

Table 1 shows the maximum dielectric constant (em),

DTdiffuse(1 kHz) and the diffusion coefficient c at 1 kHz for

the 0.98KNN–0.02BSBT ceramics sintered at different

temperatures. As shown in Table 1, the diffuseness degree

DTdiffuse(1 kHz) displays an increasing trend throughout the

sintering temperature range between 1100 and 1125 �C,

which is in good agreement with the relaxation variation in cvalues. These results suggest that the ‘‘relaxor-like’’ char-

acteristic is strengthened with increasing sintering temper-

ature. It is generally accepted that the properties of relaxors

are closely related to their unique polar structure, namely to

the existence of polar nanoregions (PNRs) and their response

Table 1 Summary of parameters of the 0.98KNN–0.02BSBT

ceramics sintered at different temperatures

Tsintering (�C) q (g/cm3) G (lm) em

(1 kHz)

c DTdiffuse

(1 kHz) (�C)

1100 4.50 *1.5 3285.1 1.20 12.8

1110 4.56 *3.0 3107.4 1.42 44.7

1120 4.58 1.0–5.0 2795.1 1.68 64.2

1125 4.57 [5.0 2435.1 1.80 94.3

1140 4.37 *2.5 1462.7 – –

1828 J Mater Sci (2014) 49:1824–1831

123

to external stimuli [36]. In 0.98KNN–0.02BSBT ceramics

sintered at different temperatures, heterovalent substitution,

such as Ti4? (0.074 nm, CN = 6) enter into the six fold

coordinated B-site to substitute for Nb5? (0.064 nm,

CN = 6), and the Bi3? (0.13 nm, CN = 12), Ba2?

(0.161 nm, CN = 12) and Sr2?(0.144 nm, CN = 12) enter

the A-site of the perovskite structure to substitute for Na?

(0.139 nm, CN = 12) and K? (0.164 nm, CN = 12) break

the translational symmetry of a crystal structure, the long-

range ferroelectric state is destroyed, and polar clusters or

micro-domains are formed. On the other hand, the increasing

sintering temperature gives rise to compositional fluctuation;

meanwhile, more macro-domains (long-range ordered

regions) breakup into micro-domains with increasing sin-

tering temperature and ultimately form polar micro-regions

[37]. Hence, the ‘‘relaxor-like’’ behavior of the 0.98KNN–

0.02BSBT ceramics sintered at different temperatures is

strengthened with increasing sintering temperature.

Figure 6 shows the room temperature polarization–

electric field (P–E) hysteresis loops (at 1 Hz) of the

0.98KNN–0.02BSBT ceramics sintered at different tem-

peratures. It can be seen that the P–E hysteresis loops

Fig. 4 Temperature dependence of dielectric permittivity at various frequencies for the 0.98KNN–0.02BSBT ceramics with different sintering

temperatures

J Mater Sci (2014) 49:1824–1831 1829

123

become slimmer with increasing sintering temperature. It is

well known that another typical characteristic of relaxor

ferroelectrics is a slim P–E loop [36]. So, the slimmer

hysteresis loops further confirm the ‘‘relaxor-like’’ behav-

ior in 0.98KNN–0.02BSBT ceramics is strengthened with

increasing sintering temperature.

Figure 7 shows the temperature dependence of dielectric

loss (tan d) for the 0.98KNN–0.02BSBT ceramics sintered

at different temperatures, which are measured at 1 kHz. All

the ceramics sintered between 1100 and 1125 �C have low

dielectric loss (\3 %) in the temperature range of

25–300 �C. While at the temperature above 300 �C, the

tan d value becomes very large at high temperature, indi-

cating space charge polarization and associated ionic

conductivity. The tan d value of the 0.98KNN–0.02BSBT

ceramics sintered at 1120 �C is lower than 2.5 % at the

high temperature from 25 to 350 �C.

Conclusion

Effect of sintering temperature on phase structure, micro-

structure, and dielectric properties of 0.98KNN–0.02BSBT

ceramics were investigated. The results are summarized as

follows:

1. Sintering temperature plays an important role in the

phase structure transition of the 0.98KNN–0.02BSBT

ceramics. The phase structure of the ceramics changed

from the mixed phase of orthorhombic and tetragonal

phases to pseudo-cubic with increasing sintering

temperature from 1100 to 1140 �C. The optimum

sintering temperature for 0.98KNN–0.02BSBT ceram-

ics should be 1120 �C.

2. The ‘‘relaxor-like’’ behavior of the ceramics is

strengthened with increasing sintering temperature.

The comparison of the relaxor behavior based on

empirical parameters (DTdiffuse), and the slimmer P–E

hysteresis loops further confirm the enhanced dielec-

tric ‘‘relaxor-like’’ behavior.

3. Dielectric properties of the 0.98KNN–0.02BSBT

ceramics under the optimum sintering temperature

are the maximum permittivity (emax) of approximately

2795 (at 1 kHz) and the tan d is lower than 2.5 % at a

broad usage temperature range (150–350 �C), indicat-

ing the potential for applications in high temperature

multilayer ceramic capacitors.

Acknowledgements This work was supported by the Doctorate

Foundation of Northwestern Polytechnical University (No.

CX201108), the National Natural Science Foundation of China (Grant

No. 51072165), and the fund of State Key Laboratory of Solidification

Processing in NWPU (No. KP200901).

Fig. 5 Plot of log (1/e - 1/em) as a function of log (T - Tm) at

1 kHz for the 0.98KNN–0.02BSBT ceramics sintered at different

temperatures. (The symbols experimental data, the solid line fitting to

modify Curie–Weiss law)

Fig. 6 The room temperature polarization–electric field (P–E) hys-

teresis loops (at 1 Hz) of the 0.98KNN–0.02BSBT ceramics sintered

at different temperatures

Fig. 7 Temperature dependence of dielectric loss for the 0.98KNN–

0.02BSBT ceramics sintered at different temperatures and measured

at 1 kHz

1830 J Mater Sci (2014) 49:1824–1831

123

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