Effect of sintering temperature on electrical properties

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temperature on the structural and electrical properties of. SrBi4TI4O15 (SBT) ..... paraelectric phase transition (Tm) temperature in the sam- ple can be analyzed ...
J Mater Sci: Mater Electron DOI 10.1007/s10854-015-2777-x

Effect of sintering temperature on electrical properties of SrBi4Ti4O15 ceramics P. Nayak • T. Badapanda • S. Panigrahi

Received: 18 November 2014 / Accepted: 29 January 2015 Ó Springer Science+Business Media New York 2015

Abstract The paper reports the influence of sintering temperature on the structural and electrical properties of SrBi4TI4O15 (SBT) ceramic synthesized by the solid state reaction method. The ceramic powders were calcined at 850 °C (Sample A), 900 °C (Sample B) for 3h and sintered at three different temperatures i.e. 1,050, 1,100 and 1,150 °C for 1 h. The XRD analysis confirms that all the samples exhibit single phase orthorhombic structure, excluding of any secondary phases. Scanning Electron Micrograph shows randomly oriented plate-like microstructure. A sharp phase transition from ferroelectric to paraelectric is observed in the temperature dependent dielectric studies of all SBT ceramics. The AC conductivity analysis of all the samples is carried out as a function of frequency at various temperatures. The ferreoelctric and piezoelectric properties of the all the samples were studied. It is observed that the samples calcined at 900 °C for 3 h and sintered at 1,100 °C for 1 h shows high dielectric and piezoelectric behavior due to large orthorhombic distortion.

1 Introduction Layered Perovskite ferroelectric materials, belonging to well known Aurivillius family have much attracted in recent years due to their low dielectric constant, high tran-

P. Nayak (&)  S. Panigrahi Department of Physics, National Institute of Technology, Rourkela 769008, Odisha, India e-mail: [email protected] T. Badapanda Department of Physics, C.V. Raman College of Engineering, Bhubaneswar 752054, Odisha, India

sition temperature, low coercive field, large anisotropy in the electro mechanical coupling factor K and good fatigue endurance [1, 2]. These characteristics makes the system to be important for potential application in various devices. The chemical formula of the Aurivillius family is (Bi2O2)2? (Am-1BmO3m?1)2-, where A represents mono-, di- or tri- valent ions such as K?, Na?, Ba2?, Pb2?, Sr2?, Ca2?, Bi3? and rare earth elements, B denotes tetra, penta or hexa valent ions such as Ti4?, Ta5?, Nb5?, W6?, Mo6?, etc.; and m = 1,2, 3, 4, 5 refers to the number of BO6 octahedra between neighbouring (Bi2O2)2? layers [3–6]. The ferroelectric properties in the bismuth layer structure ferroelectric compounds (BLSF) is due to only the BO6 octahedra in the Perovskite block along the c-axis. Among the different compounds of bismuth layer structure ferroelectrics (BLSFs), the three layer (m = 3) Bi4Ti3O12 is widely used as capacitor, transducer and memory devices [7, 8]. Its ferroelectric to paraelectric phase transition temperature is near about 675 °C. Due to the high transition temperature, it is not only used for non-volatile ferroelectric random access memories (FeRAM), but also for the piezoelectric device for high frequencies and high temperature applications. The major disadvantages associated with the system are that it has high electrical conductivity and loss which prevents the material from polling. To improve its polling behavior various pervoskites have been added. Stronsium Titanate (SrTiO3) is added in Bi4 Ti3O12 give rise to a new material SrBi4Ti4O15 (SBT) which is considerably easy to pole and therefore, expected to be useful for special applications at relatively high temperature [9]. SBT as m = 4 member of Aurivillius family has Sr and Bi ion at A-site and Ti at B-site of the perovskite (An-1BnO3n?3)2- block ((Bi2O2)2? (SrBi2)Ti4 O13)2-). The phase formation, dielectric, ferroelectric and piezoelectric properties of the polycrystalline SBT have

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been randomly investigated [10–12]. SBT shows anisotropic ferroelectricity, which is strongly associated with the crystal structure. Because of even layer (m = 4) no polarization along c-axis, as there is a mirror plane perpendicular to the axis results polarization along a-, b-axis [13]. The novel electric properties (e.g., high transition temperature, low dielectric loss, ferroelectric and piezoelectric properties) have made this material to more attractive in the current field of science and technology. The properties of SBT ceramic are largely dependent on the elemental composition, route of material synthesis, synthesis condition (sintering temperature, time, etc.), grain size surface, morphology, and heterogeneity in the materials. Various techniques also have been adopted for the preparation of SBT ceramic. Some of the well-known methods are: (a) Rout et al. [14] synthesized SBT ceramic using soft chemical route (b) Ferrer [15] et al. prepared SBT by mechanochemical activation route (c) Hoa Hua [16] adopted the molten salt synthesis method (d) Simo˜es [17] prepared by polymer precursor techniques and found the remanent polarization is 5.4lC/cm2 (e) Venkata Ramana in 2014 adopted sol gel techniques to prepare SBT [18]. The major problems associated with these methods are either the process duration or the difficulties in achieving the desired product phase composition. Also, all these techniques require special chemicals and equipments. So it is advisable to prepare the SBT ceramic by conventional solid state reaction due to its low cost and easy processing. But preparation of SBT compound by conventional solid state reaction method demands careful optimization in calcination and sintering characteristics because loss of Bi2O3 during sintering is a measurement problem which results non-stochiometry in the compound. It has been reported that addition of excess Bi2O3 is also acting as a sintering agent which seem to control the loss of volatilisation of Bi2O3, but produces intermediate Bi4Ti3O12 phase [19]. Previous studies on SBT ceramic prepared by solid state reaction method reveals that variation in the reported electrical properties and the reported processing time period and temperature vary over a wide range during calcination (800–1,000 °C) and sintering (1,050–1,250 °C). Chen et al. [20] prepared SBT ceramic sintered for 1,200 °C—2 h with the Pr value 13.39 lC/cm2. Recently Sarah in 2011 used in solid state reaction method to sinter the SBT ceramic for a longer time, which results promising piezoelectric properties [21]. Hoa Hua prepared SBT ceramic sintered 1,000–1,200 °C for 2–3 h which gives saturated P–E loop [22]. This suggested that the sintering temperature had significant influences on the densification, microstructure and electrical properties. These variations give sufficient motivation to investigate the influence of calcinations and sintering temperature on the structural as well as the electrical properties of SBT ceramic. In this paper, we optimize the sintering condition to prepare single

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phase SBT ceramic. The stable dielectric, ferroelectric and piezoelectric properties have also been investigated with a wide range of experimental condition.

2 Experimental procedures The SBT ceramics were synthesized using the solid state reaction technique. The starting raw materials are oxide or carbonate powders of SrCO3, Bi2O3, and TiO2 with 99.9 % purity. To obtain a homogeneous mixture, the powders were weighed and mixed by ball milling for 24 h with ZrO2 ball using acetone as mixing media. After drying at 80 °C, they were calcined at two different temperatures 850 °C for 3 h (Sample-A) and 900 °C for 3 h (Sample-B). The calcined powders were ground thoroughly and milled again. The milled powders were mixed with a binder (3 %PVA) and were pressed into circular disks using a hydraulic pressure of P = 300 Mpa. Finally the pellets were sintered in three different temperatures, such as 1,050, 1,100, and 1,150 °C for 1 h with a heating rate 5 °C per minute using a conventional furnace in order to choose the appropriate sintering temperature. Phase formation and crystallinity of the sintered samples were characterized by means of X-ray diffraction (XRD) at room temperature in the range 2h = 20–80 °C using Cu–Ka radiation with a step size of scan 0.05. The density is measured using a density kit based on the Archimedes principle. The microstructure is observed using scanning electron microscopy (SEM; JEOL-T 330). For electrical characterization the sintered pellets were polished with fine emery paper and coated with silver paint. The electrical parameters (dielectric and impedance) were measured using a computer-controlled Wayner–Kerr high frequency LCR meter in the temperature range RT to 700 °C. For the piezoelectric measurements the samples were polled by DC electric field of 10 kV/cm for 30 min at temperature 200 °C. The piezoelectric constant (d33) was measured directly by using YE2730A d33 meter. The polarization verses electric field (P–E loop) was obtained by the automated P–E loop tracer using a maximum field of 60 kV/cm at room temperature.

3 Results and discussion 3.1 X-ray diffraction and micro structural analysis Figure 1 shows the XRD pattern of SBT ceramics obtained from various sintering conditions. All the peaks obtained from the X-ray diffraction patterns of the ceramics were indexed based on orthorhombic cell associated with the A21am space group, which is matched with the JCPDS file

J Mater Sci: Mater Electron

observed that the densities of the samples increase with increase in sintering temperature for Sample A. Among the three specimens of Sample B, density increases with an increase in sintering temperature up to 1,100 °C and then decreases at the higher sintering temperature of 1,150 °C. The reduction in the density in the higher sintering sample is observed due to stochiometric loss of Bi2O3. Also, it has been observed that the density of the sample A is less that Sample B. Highest density is observed in the sample sintered at 1,100°C of sample B approaching 94.89 % of the theoretical value. 3.2 Dielectric studies

Fig. 1 XRD patterns of SBT ceramic for Sample A and B at various sintering temperature

number 43-0973 for SBT ceramic. No additional peaks or any impurity peaks are observed, results different calcination and sintering temperature does not affect the crystal structure. The lattice parameters were calculated by using the CHEKCELL software, which are listed in Table 1. In samples A (calcined 850 °C for 3 h), as the sintering temperature increases lattice constant a and b slightly increases where as the lattice constant c decreases. Further an increase in orthorhombic distortion also observed with increase in sintering temperature. But in Sample B (calcined at 900 °C) as sintering temperature increases lattice constant a, b slightly increases and c decreases up to 1,100 °C. In comparing all the six samples a relatively large value of orthorhombic distortion (value 1.0022) is obtained in the samples sintered at 1,100 °C for Sample B. The SEM micrographs of Sample A and Sample B ceramics are shown in Fig. 2. It can be seen that all the ceramics have a plate-like morphology. This plate-like morphology of the grain is a characteristic feature of bismuth layer compounds. The measured density values at different synthesis conditions are presented in Table 1. It is

Figure 3(a) shows temperature dependent dielectric measurements from room temperature to 700 °C at a selected frequency (1 MHz) of sample A and sample B with various sintering temperatures. A sharp dielectric peak corresponding to the Curie temperature can be detected for all the samples in the temperature range 500–550 °C. It can be noticed that with rising sintering temperature the maximum dielectric constant gradually increases in both the samples. The dielectric constant is low for sample of low sintering temperature may due to the small grain size of the specimen. In low sintered ceramics the number of domain variant is limited, results a strong coupling between the grain boundary and domain wall which reduce the domain wall motion. These effects lower the value of dielectric constant. On the other hand with increasing sintering temperature grain growth occurs results domain wall motion easier which cause the higher dielectric constant in the materials. It is also observed from the figure that there is very small variation in the transition temperature with respect to sintering temperature in Sample A. In Sample B the transition temperature decreases with increase in sintering temperature. The transition temperature decreases due to decrease in orthorhombicity of the ceramic as shown in Table 1. The curie point of sintered at 1,100 °C ceramic of Sample B is 527 °C which is nearly equal to SBT single crystal with a moderate dielectric constant [23].

Table 1 Comparison of structural properties for SBT ceramic sintered at various temperatures: a, b, c—lattice parameters, b/a-orthorhombic distortion Sintering temperature °C

a (A°)

b (A°)

c (A°)

b/a

Measured density (g/ cm3)

Percentage of theoretical density

Orthorhombicity [2(a-b)/(a ? b)]

A

B

A

B

A

B

A

B

A

B

A

B

A

B

1,050/1 h

5.4265

5.4266

5.4362

5.4376

40.9432

40.9440

1.0017

1.0020

6.623

7.369

82.89

92.22

1.7

2.0

1,100/1 h

5.4271

5.4272

5.4375

5.4394

40.9425

40.9435

1.0019

1.0022

6.845

7.582

85.66

94.89

1.9

2.2

1,150/1 h

5.4272

5.4260

5.4382

5.4362

40.9414

40.9465

1.0020

1.0018

7.279

6.972

91.1

87.25

2.0

1.8

A—sample A, B—sample B

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J Mater Sci: Mater Electron Sample A -1050°C

Sample B -1100°C

Sample A -1100°C

Sample A -1150°C

Sample B -1050°C

Fig. 2 SEM micrographs of SBT ceramic for Sample A and B at various sintering temperature

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Sample B -1150°C

Fig. 2 continued

Figure 3(b) depicts the tan d verses temperature curve at 1 MHz frequency of Sample A and Sample B. A change in slope or a sharp peak has been observed in all the samples at a particular temperature near the TC value. This loss peak indicates that it is due to the movement of ferroelectric domain walls. The displacement of domain wall contributes to the dielectric loss in that material. Therefore up to the TC value the loss is minimum and beyond the TC the loss tand begins to increase once again, which is common feature observed in bismuth layer structure ferroelectrics. Figure 4a, b shows the frequency dependence of dielectric constant (er) and loss (tand) at room temperatures. It is clear from the figure that, in all the cases er and dielectric loss (tand) decreases as a function of frequency and stabilize in the high frequency region. This is very much consistent with a normal behavior of a dielectric material. At lower frequency, er is expected to be higher due to the presence of all the different types of polarizations (i.e., interface, dipole, ionic, atomic, electronic etc.). However in the high -frequency region (greater than 104 Hz), it can be seen that the dielectric constant quickly decreases as the frequency increases. This may due to, when the frequency is increased the variation in the field become too rapid for

J Mater Sci: Mater Electron

(a)

1050-1H 1100-1H 1150-1H

1200

850-3h-1050-1h

230

SAMPLE B

850-3h-1100-1h 850-3h-1150-1h

220

900-3h-1050-1h 900-3h-1100-1h

800 400 1200

1050-1H 1100-1H 1150-1H

800

SAMPLE A

400

Dielectric constane(εεr)

Dielectric constant(εεr)

(a) 1600

900-3h-1150-1h

210 200 190 180 170

0 100

200

300

400

500

600

160

700

Temperature(°C) 2

10

(b)

3

10

4

5

10

6

10

10

Frequency(Hz)

4.5 850-3h-1050-1h 850-3h-1100-1h 850-3h-1150-1h 900-3h-1050-1h 900-3h-1100-1h 900-3h-1150-1h

4.0 3.5 3.0

(b) 0.28

850-3h-1050-1h 850-3h-1100-1h 850-3h-1150-1h

0.24

900-3h-1050-1h

2.5

tan(δ)

900-3h-1100-1h

2.0

0.20

900-3h-1150-1h

1.5

0.16

tanδ

1.0 0.5

0.12

0.0

0.08

-0.5 100

200

300

400

500

600

700

Temperature°C

0.04

Fig. 3 a Temperature dependence of dielectric constant (er) at 1 MHz frequency for Sample A and B sintered at various temperature. b Temperature dependence of dielectric loss (tand) at 1 MHz frequency for Sample A and B sintered at various temperatures

the molecular dipoles to follow, so that their contribution to polarization becomes less with a measurable lag because of internal frictional forces. The highest value of dielectric constant at room temperature is observed in sample B which is sintered at 1,150 °C, but loss is also high. Among all the samples, the sample sintered at 1,100 °C of sample B has relatively high dielectric constant and low dielectric loss. The order of diffusivity corresponds to ferroelectric– paraelectric phase transition (Tm) temperature in the sample can be analyzed by modified Curie–Weiss law: 1 1  ¼ ðT  Tm Þc =C 0 e0 em

ð1Þ

where c is the diffusivity, C0 is the Curie–Weiss constant, e0 is dielectric constant at a given temperature T and em is dielectric constant at Tm. A plot of ln (1/e0 - 1/em) as a function of ln(T - Tm) is shown in Fig. 5. The degree of the diffuseness (c) of the phase transition is obtained from

0.00 2

10

3

10

4

10

5

10

6

10

Frequency(Hz)

Fig. 4 a Frequency dependence of dielectric constant (er) for Sample A and B at various sintering temperature. b Frequency dependence dielectric loss (tand) for Sample A and B at various sintering temperature

the slope of ln (1/e0 - 1/em) vs ln (T - Tm) plot. The value of c generally lies between 1 and 2. The c values are presented in Fig. 5 which shows that the transitions are normal phase transition in all samples. 3.3 DC and AC conductivity analysis The variation of ac conductivity with frequency for all the samples at 600 °C is given in Fig. 6. The ac conductivity (rac) was calculated from the dielectric data using the relation rac ¼ eo er xr tand

ð2Þ

where eo is the permittivity of free space, xr is the angular frequency, er is the dielectric constant and tand (D) is the dielectric loss. The pattern of conductivity in all samples

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rac ðxÞ ¼ rdc þ Axn

-6.0 850-3h-1050-1h 850-3h-1100-1h 850-3h-1150-1h 900-3h-1050-1h 900-3h-1100-1h 900-3h-1150-1h

-6.5 -7.0 -7.5

1/ε-1/εm

-8.0 -8.5 -9.0 -9.5 -10.0

γ =1.03 γ =1.07 γ =1.11 γ =1.16 γ =1.10 γ =1.13

-10.5 -11.0 -11.5 1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

ln(T-Tm)

Fig. 5 ln(1/e - 1/em) as a function of ln(T - Tm) at frequency of 1 MHz for Sample A and B at various sintering temperature

0.0 850-3h-1050-1h 850-3h-1100-1h 850-3h-1150-1h 900-3h-1050-1h 900-3h-1100-1h 900-3h-1150-1h

-0.5

logσ σac(S/m)

-1.0

ð3Þ

where n is the frequency exponent in the range of 0 \ n \ 1; A, and n are thermally activated quantities. The rac was found to decrease slightly with an increase in sintering temperature up to 1,100 °C, where as conductivity rapidly increases for samples sintered at 1,150 °C. At high sintering temperature, there is a possibility for loss of oxygen, thus creating vacancies as per thermal equilibrium conditions. When the samples are cooled to room temperature, oxygen re-enters the lattice, but may not totally compensate for the loss of oxygen during sintering. The entry of oxygen is compensated only on the surface while the interior of the sample has oxygen vacancy. So the conductivity of the samples can be associated with the mobility of oxygen vacancies and increase in high sintering temperature. The variation in the Dc conductivity as a function of temperature of Sample A and Sample B ceramics sintered at different temperatures are compared in Fig. 7. The nature of variation suggests that the electrical conduction in the materials is a thermally activated process and follows the Arrhenius relation [27]:   Ea rdc ¼ r0 exp ð4Þ kB T

-1.5 -2.0 -2.5 -3.0 0

10

1

10

2

10

3

10

4

10

5

10

6

10

7

10

Frequency(Hz)

where the symbols have their usual meanings. The activation energy for conduction (Eg) could be calculated from the slope of the straight line obtained from log rdcvs 1000/T plot. The observed activation energy of the sample A and Sample B in the ferroelectric as well as the paraelectric region is given in Table 2. The activation energy obtained in our study is less than the reported values [27–30]. It was found that, the activation energy, decrease in 1,150 °C sintered ceramic

Fig. 6 Frequency dependence of ac conductivity of SBT ceramic for Sample A and B at 600 °C 850-3h-1050-1h 850-3h-1100-1h 850-3h-1150-1h 900-3h-1050-1h 900-3h-1100-1h 900-3h-1150-1h

-1.5

123

-2.0 -2.5

logσdc

shows some typical features such as (1) dispersion in the low frequency range of investigation and (2) conductivity independent in the low frequency (up to 104 Hz) and merging at high frequency with the change of slope. The nearly frequency independent region (at low frequency) indicates the long range movement of mobile charge carriers [24]. The frequency at which the change in slope occurs is known as hopping frequency [25]. Hopping frequency is observed to shift towards the high frequency side with the increase of temperature. In the high frequency region, the increase in conductivity is due to the hopping of charge carrier infinite clusters. The conductivity increases with increasing of temperature. This behavior suggests that electrical conduction in the material governed by the Jonscher’s universal power law [26]:

-3.0 -3.5 -4.0 -4.5 0.9

1.0

1.1

1.2

1.3

1.4

1.5

1000/T(°C)

Fig. 7 Temperature dependence of dc conductivity for Sample A and B at different sintering temperature

J Mater Sci: Mater Electron

compared to 1,050 °C and 1,100 °C sintered ceramic of sample B. The break in the slope in all samples represents the transition temperature, which agrees the dielectric study. This behavior is almost similar for the prepared ceramic by Raghavender [31].

(a)

2.0

1050°C

1150°C

1100°C

1.5

2

Polarization(μC/cm )

1.0

3.4 Ferroelectric and piezoelectric measurement Figure 8a, b shows the electrical polarization versus electric field (P–E) hysteresis loop at 50 Hz frequency under a maximum electric field of 60 kV/cm measured for sample A and sample B respectively. The variation of remnant polarization (Pr) on sintering temperature (Ts) is listed in Table 2. It is noted that Sample B showed good ferroelectric properties compared to sample A. The Pr values increased at first (up to 1,100 °C) and then decreased with further increase in sintering temperature of sample B. The increase in Pr with sintering temperature can be attributed to the decrease in the defect mobility which leads to low dc conductivity or the decrease in concentration of oxygen vacancies in the system. As a result, the domain pinning effect gets decreased, enhancing 2Pr. In general, the Bi2O2 layers of BLSF plays the role of an insulating layer by compensating the space charge effect and also refrains the oxygen vacancies from accumulating at the domain walls, thus pinning the domains [32]. With increase in sintering temperature, due to Bismuth loss, Bi2O2 layer is weakened and it is unable to prevent the collection of oxygen vacancies at domain walls, thus resulting in a decreased 2Pr [33]. The Pr reached a maximum value of 0.61 lC/cm2 of sample B when the sintering temperature was 1,100 °C. Piezoelectric property of SBT ceramics were measured as a function of sintering temperature. The value of piezoelectric coefficient d33 for all the samples measured at room temperature is presented in Table 2. A high d33 coefficient (11 pC/N) was measured on ceramics sintered at 1,100 °C of sample B as they could be electrically poled effectively due to their lower dielectric loss and high resistivity. In comparison to lead-based piezo ceramics, d33 values for Aurivillius family of oxides are generally very

0.5 0.0 -0.5 -1.0 -1.5

SAMPLE A

-2.0

-80 -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60 80

Electric field(kV/cm))

(b)

1050°C

2.0

1150°C

1100°C

2

Polarization(μ μC/cm )

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0

Sample-B -80 -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60 80

Electric field(kV/cm)

Fig. 8 Room temperature P–E hysteresis loops at various sintering temperature. a Sample A. b Sample B

low. This is partly attributed to the low values of dielectric constants and high anisotropy in electrical properties associated with these compounds [34].

Table 2 Comparison of electrical properties for SBT ceramics sintered at various temperatures: Tc—transition temperature, Activation energy (Ea), Pr—remnant polarization, d33-piezoelectric constant respectively Sintering C temperature °C

TC (1 MHz)

Dielectric permittivity (em) at 1 MHz

Activation energy (Ea) in eV 450–500 °C

550–800 °C

Pr(lC/cm2)

d33 (pC/N)

A

B

A

B

A

B

A

B

A

B

A

B

1,050/1 h

534

536

1,104

1,359

0.31

0.34

0.46

0.53

0.37

0.53

7

8

1,100/1 h

535

525

1,116

1,403

0.37

0.42

0.48

0.56

0.38

0.61

8

11

1,150/1 h

530

503

1,123

1,526

0.41

0.36

0.51

0.47

0.52

0.34

9

9

A—Sample A, B—Sample B

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4 Conclusion In this study SrBi4Ti4O15 ceramic has been synthesized using different sintering conditions. All the samples show single phase materials with plate like grains. The temperature dependent dielectric study shows that all samples show a sharp phase transition. The dielectric constant increases with increase in sintering temperature. The sintering temperature had strong effects on structural and electrical properties of the SBT ceramics. The sample calcined at 900 °C with sintering temperature 1,100 °C shows a dense microstructure with the presence of large orthorhombic distortion. The same sample also shows a transition temperature at about 525 °C temperature, which is close to the single crystals. Even the same sample shows highest dielectric constant with low dielectric loss. Ferroelectric hysteresis with high remnant polarization and a high piezoelectric charge coefficient d33 = 11 pC/N is also observed in this sample. So to summarize, SBT ceramic calcined at 900 °C and sintered at 1,100 °C exhibits good dielectric, ferroelectric, and piezoelectric properties, which will be suitable material for various industrial application.

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