Effect of sintering parameters on the dynamic

Integrated Ferroelectrics An International Journal

ISSN: 1058-4587 (Print) 1607-8489 (Online) Journal homepage: http://www.tandfonline.com/loi/ginf20

Effect of sintering parameters on the dynamic hysteresis scaling behavior of Ba0.85Sr0.15Zr0.1Ti0.9O3 ceramics Satyanarayan Patel, K. S. Srikanth, Arif Ali Baig Moghal, Niyaz Ahamad Madhar & Rahul Vaish To cite this article: Satyanarayan Patel, K. S. Srikanth, Arif Ali Baig Moghal, Niyaz Ahamad Madhar & Rahul Vaish (2016) Effect of sintering parameters on the dynamic hysteresis scaling behavior of Ba0.85Sr0.15Zr0.1Ti0.9O3 ceramics, Integrated Ferroelectrics, 176:1, 95-108 To link to this article: http://dx.doi.org/10.1080/10584587.2016.1249269

Published online: 14 Dec 2016.

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Date: 16 December 2016, At: 00:03

INTEGRATED FERROELECTRICS , VOL. , – http://dx.doi.org/./..

Effect of sintering parameters on the dynamic hysteresis scaling behavior of Ba. Sr. Zr. Ti. O ceramics Satyanarayan Patela , K. S. Srikantha , Arif Ali Baig Moghalb , Niyaz Ahamad Madharc , and Rahul Vaisha a School of Engineering, Indian Institute of Technology Mandi, India; b Department of Civil Engineering, College of Engineering, King Saud University, Riyadh, KSA; c Department of Physics and Astronomy, College of Sciences, King Saud University, Riyadh, KSA

ABSTRACT

ARTICLE HISTORY

This article presents the effect of processing parameters on the ferroelectric hysteresis behavior of Ba0.85 Sr0.15 Zr0.1 Ti0.9 O3 (BSZT) ceramics. The ferroelectric hysteresis scaling relations for coercive field (Ec ) and remnant polarization (Pr ) as a function of temperature have been proposed. The power law temperature exponents based on scaling were systematically established for all the hysteresis parameters. The temperature dependent scaling of Ec and Pr at sintering temperature of 1400, 1425, 1450 and 1475°C yields Ec αT0.43 , Ec αT0.84 , Ec αT0.50 , Ec αT0.37 and Pr αT−1.73 , Pr αT−1.55 , Pr αT−1.73 , Pr αT−1.69 respectively. Additionally, the scaling relations for the samples sintered at 1450°C at different time intervals of 3, 4, 5 and 6 hrs were also established. Finally, to understand the domain dynamics, back switching polarization (Pbc ) as a function of temperature (T) was also estimated by Arrhenius law and the average activation energy was evaluated.

Accepted  May  KEYWORDS

Hysteresis scaling; lead-free; ferroelectric

1. Introduction Over the last few decades, lead-based ferroelectric materials have been widely studied mainly due to their excellent ferroelectric, pyroelectric and piezoelectric properties which are desirable for sensors, transducers and energy storage applications [1, 2]. Researchers are now focused on lead-free functional materials due to environmental issues. In this context, various lead-free materials have emerged over the years e.g. BaTiO3 , Na0.5 Bi0.5 TiO3 and K0.5 Na0.5 NbO3 [3–7]. These ferroelectric materials have been fabricated at the morphotropic phase boundary and have demonstrated enhanced ferroelectric properties [8–11]. Lead-free ceramics (Ba1−x Cax )(Zry Ti1−y )O3 (BCT-BZT) have promising dielectric, piezoelectric, pyroelectric and ferroelectric properties [5, 8–14]. It has significant piezoelectric and pyroelectric coefficients of 630pC/N and 5.84–17.14 × 10−4 C/m2 .K (at 300 K), respectively [5, 15]. Additionally, it is reported for promising piezoelectric CONTACT Rahul Vaish [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ginf. ©  Taylor & Francis Group, LLC

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coupling coefficient (kp ) of 0.56 [5, 15]. Further, BCT-BZT ceramics have electric field tunable dielectric constant. It can be tuned upto 82% at 40 kV/cm (DC field) with low dielectric loss of 0.01 [16]. Additionally, it consists morphotropic phase boundary (MPB) of two phases rhombohedral-tetragonal which alter piezoelectric, pyroelectric, ferroelectric and dielectric properties. Therefore, researchers have extensively studied BCT-BZT-based MPB compositions [8, 11, 14, 17–19]. Further, (Ba1−x Cax )(Zry Ti1−y )O3 (BCT-BZT) has received attention in the field of solid state refrigeration because it shows large electrocaloric strength [20–23]. A number of researchers have investigated that the dielectric, piezoelectric, pyroelectric and ferroelectric performance are dependent on the microstructure. In this direction, Mishra et al. studied BCT-BZT compositions for dielectric, ferroelectric and piezoelectric properties sintered at different temperatures ranging from 1300 to 1400°C [24]. In another study, Wu et al. investigated effect of the sintering temperature (varied from 1300 to 1500°C) on the microstructure and electrical properties of (Ba0.85 Ca0.15 )(Zr0.1 Ti0.9 )O3 (BCZT) ceramics [10]. It is reported that relative density, grain size, dielectric and piezoelectric constants vary from 86 to 94%, ∼5.2 to ∼20.1μm, 1750 to 3000 and 50 to 442pC/N, respectively with increase in sintering temperature from 1300 to 1500°C [10]. Similarly, Cai et al. studied grain size effect on domain structure and ferroelectric properties [25]. Additionally, sintering temperature effect on grain size, density, dielectric, piezoelectric and ferroelectric properties also investigated for (Ba0.93 Ca0.07 )(Zr0.05 Ti0.95 )O3 , (Ba0.98 Ca0.02 )(Zr0.02 Ti0.98 )O3 , Ba1−x Zrx Ti1−x O3 , and BCZT ceramics [9, 26–29]. It is evident from literature that dielectric, piezoelectric and pyroelectric properties of BCZT can be tuned by sintering temperature and dwell time [9, 26–30]. However, ferroelectric hysteresis loop parameters relations with sintering temperature have not been explored. Therefore, this work presents temperature dependent ferroelectric hysteresis scaling relations of coercive field (Ec ), remnant polarization (Pr ) and back switched polarization (Pbc ) as a function of applied temperature for varying sintering temperature and time. In this direction, we have fabricated (Ba0.85 Sr0.15 )(Zr0.1 Ti0.9 )O3 (BSZT) ceramics (where Ca has been replaced with Sr in BCZT) which are sintered at various temperatures and time intervals. 2. Experimental Conventional solid-state fabrication route was employed to prepare (Ba0.85 Sr0.15 )(Zr0.1 Ti0.9 )O3 (BSZT) ceramics. The pure powders of BaCO3 , SrCO3 , TiO2 and ZrO2 were used as initial precursors. Then, powders were weighed and mixed according to their desired stoichiometric ratio. This mixture was subsequently milled in acetone to have homogeneity. The homogeneous composition of BSZT mixture was kept in an alumina crucible and subjected to calcinations twice at 1300°C and 1350°C for 6 hr in air. Polyvinyl alcohol binder 2% by weight, was added to the calcined powder and the resultant mixture was used to prepare the pellets of 12 mm × 0.8 mm (diameter × thickness) dimension using cold pressing at 3–4 ton/cm2 . Then pellets were sintered at various temperatures (1400, 1425,

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Figure . X-ray diffraction patterns for BSZT samples. Inset shows the density as a function of sintering temperature and time for BSZT.

1450 1 nd 1475°C) and time intervals (3, 4, 5 and 6 hrs). The prepared samples were finally coated with silver paste to make electrodes. Powder X-ray diffraction technique was employed to confirm the phase formation. Archimedes principle was used to measure the density of samples in order to check densification. The polarization versus electric field hysteresis loops were recorded at different magnitude of electric field (50 Hz) and temperatures using a modified Sawyer Tower circuit. 3. Results and discussion Figure 1 shows the X-ray diffraction patterns for Ba0.85 Sr0.15 Zr0.1 Ti0.9 O3 (BSZT) ceramics at room temperature for calcined powder which confirm the presence of single phase. The sharpness of X-ray diffraction peaks indicates microcrystallinity of samples. Further it is observed that XRD peaks have same positions for BSZT ceramics without any extra phase. Therefore it can be concluded that the calcined sample contains pure perovskite phase as there were no secondary phase obtained. ˚ and 4.002A, ˚ Further, lattice parameters a and c have already been reported as 4.003A respectively for BSZT ceramics [31]. Hence, this article portrays the X-ray diffraction analysis only to confirm the phase formation. The inset of Fig. 1 shows the density of the sintered samples at different sintering temperature and time. The density of sintered samples which was measured by Archimedes principle was found to be 5.36, 5.42, 5.53 and 5.55 g/cm3 when samples are sintered at 1400, 1425, 1450

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Figure . (a) Polarization-electric field (P-E) hysteresis loops for BCZT and BSZT at room temperature and (b) dielectric constant at  MHz for BSZT ceramics, inset shows corresponding dielectric loss plot.

and 1475°C, respectively for 4 hr. The density of the samples sintered at 1450°C for different time intervals (3–6 hrs) is also shown in the inset of Fig. 1. The measured density was found to be 93–96% of the theoretical density which confirms the highly dense structure of the samples under study. Further Fig. 2 (a) shows polarization versus electric field hysteresis loops at room temperature for BCZT and BSZT whereas Fig. 2 (b) shows dielectric constant at 1 MHz for BSZT. Inset of Fig. 2(b) shows dielectric loss at 1 MHz. It clearly indicates (Fig. 2a) that replacing the Ca2+ with Sr2+ improves ferroelectric. Sr2+ addition significantly increases saturation polarization from 15 to 25μC/cm2 corresponding dielectric constant can also increases. Dielectric relaxation study has not been performed here although such studies are important to understand role of Ca/Sr in dielectric properties. For further study, BSZT composition is focused in this article. To understand and explore the sintering temperature effect on the ferroelectric properties, polarization-electric field (P-E) loops were recorded for sample sintered at 1400, 1425, 1450 and 1475°C for 4 hr (Fig. 3(a)) at constant electric field and temperature. It is observed that the polarization increases with the increase in sintering temperature and attains a maximum value at 1450°C. Further, sintering duration

Figure . Polarization-electric field (P-E) hysteresis loops for BSZT under (a) different sintering temperatures (at constant sintering duration  hr) and (b) varying sintering duration (at constant sintering temperature °C) under constant electric field.


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Figure . P-E loops (at different temperatures) of samples sintered for  hr at (a) °C, (b) °C, (c) °C and (d) °C.

also varied at 1450°C and corresponding P-E loops are depicted in Fig. 3(b). The maximum polarization was observed for sample sintered for 5 hr at 1450°C. It is also evident from inset of Fig. 1 that maximum density of 5.63 g/cm3 for sample sintered at 1450°C/5 hr. The polarization decreases for the sample sintered beyond 1450°C and 5 hr. It can be said that it has least porosity and large grain size which can increase the polarization. Therefore, with increase in grain size leads to increase in domain size which enhances polarization due to the increment in sintering temperature and time duration [25, 32, 33]. Further, dependency of electrical properties can be attributed to the changes of domain structure in coarse and fine grained samples. For ferroelectrics, the proportion of grains which contributes in polarization reverse can 3 ad be expressed as, f = f0 [1 − e −G ] where d is the grain size, Ga is a constant KT and represents the grain anisotropy energy density [33]. Hence, as the number of grains contributing in polarization reverse increases, it enhanced ferroelectric properties. Additionally, the grain size effect on piezoelectric in perovskite type ferroelectrics is known to be strongly influenced by the movement of the 90° domain walls [33]. The 90° domain size increased with grain sizes in the range of 0–10 μm and was found to be stable with further increase in grain size from 10 to 100 μm in as reported by Hao et al. in BCT-BZT ceramics [33]. Hence, it can be said that the Pr and Ec have (observed for BSZT ceramic samples sintered at different

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temperatures and durations) different behavior. Further, Cai et al. provide a detail discussion for grain size effect on domain structure and ferroelectric properties of BaZr0.2 Ti0.8 O3 ceramics [25]. Similarly a number of researchers have shown grain size effect on polarization in BCT-BZT-based compositions [8–10, 24, 26, 29]. Therefore, present work does not deal with grain size analysis, hence scanning electron microscopy (SEM) is not shown here. Additionally, we are restricted only on ferroelectric hysteresis analysis. Therefore, we have studied remnant polarization (Pr ) and coercive field (Ec ) as a function of temperature under different electric field. To explore ferroelectric hysteresis characteristics for BSZT ceramic samples sintered at different temperatures of 1400, 1425, 1450 and 1475°C at 4 hr, P-E loops were recorded which are depicted in figures 4(a), (b), (c) and (d), respectively. It is demonstrated that the P-E loops are well saturated which give a clear picture of the ferroelectric nature of the BSZT samples. Similarly, P-E loops for different sintering duration 3, 5 and 6 hrs at 1450°C were also recorded (not shown here). It was observed that the polarization increases with increase in time and attains a maximum for sample sintered for 5 hrs then decay further increase in sintering duration. In order to understand the dynamic hysteresis behavior in BSZT ceramics; the temperature (T) dependent scaling relations were observed for Pr and Ec . The graphs were plotted for logarithmic form of Pr and Ec versus T at different magnitudes of electric field as depicted in Figs. 5 and 6. Linear least square fitting method with R2

Figure . Plots of ln Ec as a function of ln T for samples sintered (a) °C, (b) °C, (c) °C and (d) °C for  hr. Inset shows slope as a function of applied electric field (E) for under study sintering duration and temperature.


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Figure . Logarithmic plots of ln Pr with ln T for samples sintered at (a) °C, (b) °C, (c) °C and (d) °C for  hr corresponding slope versus electric field plots are depicted in inset.

= 0.95–0.99 was used to study the dependence of ln Ec on ln T as shown in Figs. 5(a) (b), (c) and (d). The scaling relation between Ec and T can be expressed as Ec αTμ , where μ is determined from the slope of ln Ec versus ln T which is given as (sample sintered for 1400°C at 4 hr corresponds to Fig. 4(a)): ln Ec = μ ln T + YEc or ln Ec = (0.001E + 0.43) ln T + YEc

(1)

where YEc is Y intercept. The dependence of slope (μ) on E is shown in the inset of Fig. 5(d) where μ = 0.001E+0.43 for sample sintered at 1400°C at 4 hr. Slope is almost similar at different electric field. When E is constant the contribution of E is very negligible due to small multiplication factors [21, 34, 35]. Then μ = 0.001E+0.43 can be written as μ = 0.43. This assumption is used in present study. Hence, for a constant magnitude of electric field, Eq. 1 can be written as: ln Ec = 0.43 ln T + YEc

(2)

Here, YEc indicates the value of Ec as T reaches zero. However, there is a small deviation from practical approach [21, 34, 35]. The present work does not focus on Ec as the T reaches zero. YEc is term to fulfill the relation only which has physically no meaning. Hence, YEc can be safely neglected. Thus for a constant value of E, we get the following relationship: Ec αT 0.495

(3)

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Figure . Ps −Pr versus temperature (T) plots for the samples sintered at (a) °C, (b) °C, (c) °C and (d) °C for  hr under various electric fields.

Similar calculations are done for the dependence of Ec on T for the sample sintered at different temperature as shown in Fig. 5(b) 1425, (c) 1450 and (d) 1475°C. The scaling exponents of Ec on T for the sample sintered at 1400, 1425, 1450 and 1475°C for 4 hr are listed in table 1. Correspondingly, slope as a function of applied electric field (E) are also shown in Fig. 5(d). Similarly, table 1 provides the dependence of ln Ec on ln T under different magnitudes of E for the samples sintered at 1450°C at varying time interval of 3, 5 and 6 hrs, respectively. Corresponding, temperature dependent dynamic scaling relations are found as Ec αT0.56 , Ec αT0.50 , Ec αT0.53 , and Ec αT0.43 as given in table 1. It is observed that μ increases (0.43 to Table . Temperature dependent scaling exponents of coercive electric field (Ec ), remnant polarization (Pr ) and activation energy (EA ) for BSZT sintered at different temperature and time interval. EA (eV)

°C/ hr °C/ hr °C/ hr °C/ hr °C/ hr °C/ hr °C/ hr °C/ hr

μ (Ec ∝ T μ )

δ (Pr ∝ T δ )

 kV/cm

 kV/cm

 kV/cm

 kV/cm

 kV/cm

. . . . . . . .

− . − . − . − . − . − . − . − .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .


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0.84) as increase in sintering temperature (1400 to 1425°C) then decreases to 0.37 at 1475°C. On the other hand, as the sintering time increases, μ decreases from 0.56 to 0.43 (3 to 6 hr). However, for sintering duration 3, 4 and 5 hr, Ec scaling exponent shows almost same behavior. It can be said that grain size can significantly affect coercive electric field. Similar methodology is adopted for temperature dependent scaling of Pr on T. In this direction, figs. 6(a), (b), (c) and (d) show ln Pr versus ln T plots for the sample sintered at different temperature 1400, 1425, 1450 and 1475°C, respectively. The relation between Pr and T can be written as ln Pr = ln T + YPr . Further the dependence of slope on E is shown in the inset of Fig. 6(d) and the least square fit for slope (δ) yields δ = −1.73 for the sample sintered at 1450°C for 4 hr as given in Eq. 4. Pr αT −1.73

(4)

Similar calculations are done to obtain the scaling relations for Pr on T for the samples sintered at various temperatures as presented in table 1. The temperature dependent scaling of Pr yields Pr αT−1.73 , Pr αT−1.55 , Pr αT−1.73 and Pr αT−1.69 when the samples were sintered at temperature range of 1400, 1425, 1450 and 1475°C, respectively for 4 hr as depicted in figs. 6(a), (b), (c) and (d), respectively. Further, the scaling relations for the samples sintered at 1450°C at different time intervals of 3, 4, 5 and 6 hr were also established as listed in table 1. The relations calculated based on power law scaling methodology are given by Eqs. 3 and 4 can also be explained in terms of domain switching of applied electric field and temperature. Many researchers across the globe have successfully reported the temperature and electric field dependent domain rotation [36, 37]. It is well known that at small electric field, depolarization can be occurred at lower temperature because of smaller polarization. The loop remains unsaturated for lower magnitudes of electric field at a particular frequency and temperature and forms a minor loop. This region is called as unsaturated loop. However, as strong electric field (larger magnitude) applied minor loop transforms into a major loop which is the region of saturated loop. This hysteresis loop is well saturated. In ferroelectrics, this phenomenon always holds good at a finite temperature and frequency range. This domain switching phenomenon is applicable for all the ferroelectric materials known so far [21, 36, 37]. Hence, we are interested only in the saturated regime of P-E loops to determine the hysteresis characteristics of the sample under study. Additionally, mechanical stress and frequency also affects the hysteresis parameters as discussed by a number of researchers [35, 38, 39]. Therefore, considering mechanical stress and frequency the scaling characteristics yields a different set of exponents as reported by many researchers across the globe [35, 38, 39]. However, presents work is limited to study the temperature effect on hysteresis parameters. Hence frequency effect on hysteresis parameter has not been considered in the present work whereas it is also an important parameter which is previously discussed by many researchers [35, 38, 39].

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Further, it is known that temperature also affects domain wall motion in ferroelectric materials [36]. Therefore Ec , Pr and saturation polarization decreases with increase in temperature. This is due to fact that as the temperature increases lattice vibrations also increases which decreases the domain switching and oxygen trapping possibility [34, 35, 38–40]. Additionally, at higher temperature stability of the polarization in each domain is reduced. Hence, hysteresis loop area reduces and hysteresis parameter decreases with increase in temperature [34–36]. This temperature induced domain switching phenomenon is valid for all ferroelectric materials. However, domain switching can be tuned by suitable composition selection at MPB, doping, domain states, crystal structure and defects [8, 13, 24]. Further, polarization behavior can be explained in terms of domain back-switching [36]. Generally, domain polarization consists of two parts, switchable polarization in domain which is known as Ps and back-switched polarization. At a constant temperature when a suitable electric field is applied to the ferroelectrics material, the polarization reaches a saturation polarization Ps and on removal of electric field the polarization reaches a stable polarization of Pr [36]. This relaxation process leads to back switching and the corresponding polarization is known as polarization back switching denoted as Pbc . It can be expressed as [36]: Pbc = Ps − Pr

(5)

Figures 7 show the Ps −Pr versus temperature plots under different electric field for the samples sintered at various temperatures (1400, 1425, 1450 and 1475°C for 4 hr). Similarly, Ps −Pr versus temperature plots were also plotted for sample sintered at time intervals of 3, 5 and 6 hr (1450°C) which are not shown here. It shows that as the temperature increases the back-switching polarization also increases. This back switching phenomenon also depends on the applied electric field. According to the classical theory of nucleation by random fluctuation, it is noted that the rate of nucleation (γ ) increases with increase in temperature [4, 36, 41]. Therefore, the temperature dependent dynamic relaxation response time (τ ) of domain nucleation can be determined by [36, 42, 43]: 1 U0 τ = = exp (6) γ kB T where Uo and kB are the energy barrier for domain nucleation/growth and Boltzmann constant, respectively. Equation (6) indicates that as the temperature increases relaxation response time decreases. Further, equations (5) and (6) can be considered as similar dynamics for domain nucleation and growth. The relationship of Ps −Pr obeys the Arrhenius law in the high field regime which can be express as [36] : EA (7) ps − pr = p0 exp − kB T where EA and Po are average activation energy of the trapped charge defects and a constant, respectively. The EA can be calculated from the slope of ln (Ps −Pr ) versus 1/T. Hence, ln (Ps −Pr ) versus 1000/T plots are shown in Fig. 8 for the samples


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Figure . ln (Ps −Pr ) versus ln (/T) plots for sample sintered at (a) °C, (b) °C, (c) °C and (d) °C for  hr. The inset shows activation energy (EA ) versus electric field plots for under study sintering temperature.

kept at 1400, 1425, 1450 and 1475°C for 4 hr. The inset of Fig. 8(d) shows the average activation energy. The activation energy decays from 0.119 eV to 0.084 eV at 1450°C as the electric field is increased from 18 to 33 kV/cm as given in table 1. The activation energy for the samples sintered at 1400, 1425, 1450 and 1475°C for 4 hr is found out as 0.101, 0.103, 0.119 and 0.153 eV, respectively under the electric field of 18 kV/cm as presented in table 1. The activation energy decays with increase in electric field at a given sintering temperature. Further a maximum EA of 0.153 eV found at 1475°C/4 hr under the application of 18 kV/cm. Similarly, activation energy was estimated for samples sintered at 1450°C for time interval of 3, 4, 5 and 6 hrs as given in table 1. The EA yields 0.107, 0.119, 0.124 and 0.118 eV samples sintered at 1450°C for 3, 4, 5 and 6 hrs, respectively under the electric field of 18 kV/cm as listed in table 1. The low value of activation energy may be attributed to the fact that enhancement of charge carriers helps to jump between localized states.

4. Conclusions In the present work, the effect of sintering parameters on the hysteresis characteristics of Ba0.85 Sr0.15 Zr0.1 Ti0.9 O3 (BSZT) ferroelectric bulk ceramics was explored in detail. The samples were sintered at 1400, 1425, 1450 and 1475°C for varying time

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interval (3, 4, 5 and 6 hrs) to study the hysteresis properties. The density of the sample increases with increase in sintering temperature and time. It varies from 5.37 to 5.64 g/cm3 . The temperature dependent dynamic scaling was investigated and power law was obtained. The temperature dependent scaling of Ec and Pr at different sintering temperatures of 1400, 1425, 1450 and 1475°C yields Ec αT0.43 , Ec αT0.84 , Ec αT0.50 , Ec αT0.37 and Pr αT−1.73 , Pr αT−1.55 , Pr αT−1.73 , Pr αT−1.69 respectively. Similar, scaling relations of Ec and Pr for different sintering time intervals 3, 4, 5 and 6 hrs at 1450°C was also systematically estimated. It is observed that back-switching polarization and temperature relations follow the Arrhenius law. The average activation energy for domain switching is found to be ∼0.084–0.142 eV when sintering temperature increases 1400 to 1475°C for 4 hr. Similarly, sintering time dependent activation energy is also estimated with applied electric field.

Funding The project was financially supported by King Saud University, Vice Deanship of Research Chairs.

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