Scholarly article on topic 'Enhanced thermal energy conversion and dynamic hysteresis behavior of Sr-doping Ba0.85Ca0.15Ti0.9Zr0.1O3 ferroelectric ceramics'

Enhanced thermal energy conversion and dynamic hysteresis behavior of Sr-doping Ba0.85Ca0.15Ti0.9Zr0.1O3 ferroelectric ceramics Academic research paper on "Nano-technology"

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{"Energy harvesting" / "Olsen cycle" / "Dynamic hysteresis scaling" / Ferroelectric}

Abstract of research paper on Nano-technology, author of scientific article — Satyanarayan Patel, Deepakshi Sharma, Anupinder Singh, Rahul Vaish

Abstract This paper deals with a study of Ba0.85Ca0.15−xSrxTi0.9Zr0.1O3 (BCT-BZT-Sr) (x = 0%, 5%, 10% and 15%) polycrystalline ceramics for thermal energy conversion applications using the Olsen cycle. A maximum energy conversion density of 108 kJ/m3 was observed in pristine BCT-BZT for cycle operating at 30–90 °C and 0–20 kV/cm. Moreover, the samples with Sr contents (x) of 5%, 10% and 15% display an energy conversion density of 179 kJ/m3, 149 kJ/m3 and 200 kJ/m3, respectively. The Sr addition almost double the energy conversion over the pristine sample under the similar conditions. Also, the temperature (T)-dependent dynamic hysteresis scaling relations for coercive field (E C) and remnant polarization (P r) were systematically investigated. The power-law scaling exponents are obtained for E C versus T as E C ∝ T −0.65, E C ∝ T −0.61, E C ∝ T −0.56 and E C ∝ T −0.63 for the samples with Sr contents of 0%, 5%, 10% and 15%, respectively. Similarly, the scaling relations for P r are P r ∝ T −1.59, P r ∝ T −1.59, P r ∝ T −1.36 and P r ∝ T −1.32 for the samples with Sr contents of 0%, 5%, 10% and 15%, respectively.

Academic research paper on topic "Enhanced thermal energy conversion and dynamic hysteresis behavior of Sr-doping Ba0.85Ca0.15Ti0.9Zr0.1O3 ferroelectric ceramics"

Accepted Manuscript

Satyanarayan Patel, Deepakshi Sharma, Anupinder Singh, Rahul Vaish

Enhanced thermal energy conversion and dynamic hysteresis behavior of Sr-doping Ba0.85Ca0.15Ti0.9Zr0.1O3 ferroelectric ceramics

Reference:

S2352-8478(15)30005-8 10.1016/j.jmat.2016.01.002 JMAT 45

To appear in: Journal of Materiomics

Received Date: 12 October 2015 Accepted Date: 26 January 2016

Please cite this article as: Patel S, Sharma D, Singh A, Vaish R, Enhanced thermal energy conversion and dynamic hysteresis behavior of Sr-doping Ba0.85Ca0.15Ti0.9Zr0.1O3 ferroelectric ceramics, Journal of Materiomics (2016), doi: 10.1016/j.jmat.2016.01.002.

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Graphical Abstract

Enhanced thermal energy conversion and dynamic hysteresis behavior of Sr-added Ba0.85Ca0.15Ti09Zr01O3 (BCT-BZT) ferroelectric ceramics

Satyanarayan Patel, Deepakshi Sharma, Aditya Chauhan, Anupinder Singh and Rahul Vaish

Olsen cycle is used to find out energy harvesting potential. Power-law scaling exponents are obtained for coercive field (EC) and remnant polarization (Pr) versus temperature (T). Temperature dependence back-switching kinetics of ferroelectric domain is explained by Arrhenius law.

■ 14.5kV/cm • 18.0kV/cm ▲ 21.5kV/cm ▼ 26.0kV/cm 4 29.0kV/cm — Linear fit

4: >il >

3.6 3.8 4.0 4.2 4.4 4.6 InT(lrTC)

■ 14.5 kV/cm • 18.0 kV/cm A 21.5 kV/cm ▼ 26.0 kV/cm < 29.0 kV/cm — Linear fit

3.6 3.8 4.0 4.2 In T(ln°C)

2.9 3.0 3.1 3.2 3.3 1000/T(K 1)

Enhanced thermal energy conversion and dynamic hysteresis behavior of Sr-doping Bao.85Cao.15Tio.9Zro.1O3 ferroelectric ceramics

Satyanarayan Patelat, Deepakshi Sharmabt, Anupinder Singhb and Rahul Vaisha*

aSchool of Engineering, Indian Institute of Technology Mandi, 175 001, Himachal Pradesh,

India.

bDepartment of Physics, Guru Nanak Dev University, Amritsar, 143005, Punjab, India.

*Email: rahul@iitmandi.ac.in, Phone: +91-1905-237921, Fax: +91-1905-237945 ''These authors contributed equally to this work

Abstract

This paper deals with a study of Ba0.85Ca0.15-xSrxTi0.9Zr01O3 (BCT-BZT-Sr) (x=0%, 5%, 10% and 15%) polycrystalline ceramics for thermal energy conversion applications using the Olsen cycle. A maximum energy conversion density of 108 kJ/m3 was observed in pristine BCT-BZT for cycle operating at 30-90 °C and 0-20 kV/cm. Moreover, the samples with Sr contents (x) of

5%, 10% and 15% display an energy conversion density of 179 kJ/m , 149 kJ/m and 200 kJ/m , respectively. The Sr addition almost double the energy conversion over the pristine sample under the similar conditions. Also, the temperature (T)-dependent dynamic hysteresis scaling relations for coercive field (EC) and remnant polarization (Pr) were systematically investigated. The power-law scaling exponents are obtained for EC versus T as EC 00 j"0-65, EC 00 T"0'61, EC 00 T"0'56 and EC oc T"0-63 for the samples with Sr contents of 0%, 5%, 10% and 15%, respectively. Similarly, the scaling relations for Pr are Pr o rL59, Pr o rL59, Pr o rL36 and Pr o rL32 for the samples with Sr contents of 0%, 5%, 10% and 15%, respectively.

Keywords: Energy harvesting; Olsen cycle; dynamic hysteresis scaling; ferroelectric.

1. Introduction

In last few decades, ferroelectric materials have been used in a wide range of scientific and technological applications. All of these applications can be credited to their versatile properties, i.e., piezoelectric/pyroelectric/ferroelectric effects[1-5]. These are used in a variety of smart applications such as sensors, actuators and transducers. They also possess a high energy conversion potential, which can be used to power micro-electromechanical systems (MEMS) and other nano/micro devices[1-3,5-8]. Technologically and commercially important ferroelectric materials include Pb(ZrxTii-x)Ü3 (PZT) and Pb(Mgi/3Nb2/3)Ü3 (PMN) ceramics[5,9,10]. In recent years, significant efforts have been directed towards the design of morphotropic phase boundary (MPB)-based new solid solutions such as xPb(Mgi/3Nb2/3)Ü3-(1-x)PbTiÜ3, >Pb(Mgi/3Nb2/3)Ü3-(1-j)PbxZni-xTiÜ3, 8/65/35 PLZT and yPb(ZrxTi1-x)Ü3-(1-j)PbZm/3Nb2/3Ü3[11-15]. Ferroelectric materials fabricated near their MPB are capable of displaying remarkably enhanced piezo/pyro/ferroelectric properties. However, as serious environmental and health problems of Pb toxicity, researchers have focused on development of competing lead-free ferroelectric/piezoelectric materials. Hence recent studies are focused on compositions based on BaTiÜ3, Bi05Na05TiÜ3, K05Na05NbÜ3 and their MPB compositions for ferroelectric applications[9,16-18].

In this case, lead-free Ba0.85Ca015Ti0.9Zr01Ü3 (BCT-BZT) ceramics have been recently studied due to its promising piezoelectric and pyroelectric properties as compared to other lead-free and lead-based materials[19-24]. A colossal piezoelectric coefficient (d33) of 630 pC/N and high coupling coefficient (kp) of 0.56 were reported. These values are comparable with Pb-based existing materials[19]. It possesses a rather high pyroelectric coefficient of 5.84-17.14 *10-4 °C/m2-K at 300 K[20]. Also, BCT-BZT exhibits electric field-dependent tunable dielectric

properties, which can be tuned up to82% at an applied DC field of 40 kV/cm[21]. Also, it exhibits a low dielectric loss of 0.01. These potential properties were credited to the coexistence of two phases (i.e., rhombohedral-tetragonal) with an intermediate (unstable) orthorhombic phase, which minimizes the switching energy barrier. The effects of composition and MPB on the ferroelectric properties of BCT-BZT were studied extensively[23,25-29]. It was indicated that BCT-BZT could be a potential material for substitution of current lead-based materials for many technologically important applications.

Some studies were also carried out on the caloric application of BCT-BZT ceramics[30,31]. Gurvinderjit Singh et al. reported a high electrocaloric coefficient of ~0.38 K mm/kV in single crystal of 0.45BaZr0.2Ti0.8O3-0.55Ba0.7Ca03TiO3 near the tetragonal-to-cubic phase transition[32]. A peak electrocaloric effect of 0.45 K (347 K) was predicted for 0-3 kV/mm electric field in Fe-doped bulk (Ba0.865Ca0.135Zr0.1089Ti0.8811Fe0.01)O3[30]. However, BCT-BZT shows a large electric and stress induced strain. Hence, a recent work showed a rather high elastocaloric effect of 1.55 K at an initial material temperature of 340 K and applied compressive stress of 0-250 MPa (under a constant electric field of 2 MV/m) in this compound[33]. However, the most befitting application of BCT-BZT could be that of high-field thermal energy harvesting. In this case, Vats et al. reported the Olsen cycle based energy conversion in BCT-BZT[34]. They found the maximum harness-able energy density of ~87 kJ/m3, when cycle was operated at 20120 oC and 1-10 kV/cm. This work will deal with Sr added Ba0 85Ca0.15-xTi0.9SrxZr01O3 (BCT-BZT-Sr) (x=0%, 5%, 10% and15%) for thermal energy conversion using the Olsen cycle. In addition, the dynamic hysteresis scaling was also performed to investigate the effect of Sr addition on the BCT-BZT ferroelectric properties. Furthermore, the domain dynamic behavior was analyzed using the activation energy variation.

2. Experimental

Ba0.85Ca0.15-xSrxTi0.9Zr0.1O3 (BCT-BZT-Sr) polycrystalline ceramics were prepared via a solid state reaction route. Four different samples were made by differing the composition of strontium (x=0%, 5%, 10%, 15%). For this purpose, reagent grade powders of barium carbonate (BaCO3), calcium oxide (CaO), titanium oxide (TiO2), zirconium oxide (ZrO2) and strontium carbonate (SrCO3) were mixed in a stoichiometric ratio. This mixture was ground in acetone to obtain the physical homogeneity using mortar and pestle. Afterward, the mixture was calcined twice at 1350 oC and 1400 oC for 6 h with intermediate grinding to ensure physical and chemical homogeneity. PVA was added in calcined powder and the powder was cold-pressed to form green pellets with the size of 12mmximm (diameterxthickness). Finally, sintering was carried out at 1450oC for 4 h with a heating and cooling rate of 6 oC/min. The phase formation of the calcined and sintered samples was determined by powder X-ray diffraction (XRD). The density of sintered pellets was determined according to the Archimedes principle. The surface profile and morphology of the sintered pellets were analyzed by scanning electron microscopy (SEM). Silver paste was coated on the flat surfaces to make electrical contacts for polarization versus electric field (P-E) hysteresis measurement. The P-E hysteresis loops were measured at different temperatures using P-E loop tracer (Marine India).

3. Results and Discussion

Figure 1 shows the XRD patterns of the BCT-BZT-Sr ceramics at room temperature. The absence of extra peaks indicates the formation of desired phase. All the major peaks can be indexed based on the standard XRD pattern of polycrystalline tetragonal BaTiO3/CaTiO3. Figures 2(a) (b), (c) and (d) show the SEM micrographs of sintered samples with Sr contents of

0%, 5%, 10% and 15%, respectively. Clearly, all the samples appear to be dense and relatively free of defects. This notion is also confirmed by the density measurement, revealing that all the sintered samples possess ~94% of the theoretical density. Figures 3(a), (b), (c) and (d) show the P-E loops for the samples with Sr contents of 0%, 5%, 10% and 15%, respectively at various electric fields and a constant temperature (30 oC). All the P-E loops are recorded at constant a/c frequency of 50Hz. The results indicate that as the electric field increases, minor unsaturated P-E loops start moving towards saturation and finally form saturated loops. This can be attributed to an increase in coercive field (EC), saturation polarization (PS) and remnant polarization (Pr). The hysteresis parameters variations appear due to the domain growth and domain rotation/alignment, which take place in the direction of applied electric field. In addition, it shows that the saturation polarization increases with the increase of Sr content. This is due to a fact that BCT-BZT-Sr

ceramics coexist the rhombohedral and tetragonal phases whereas Sr doping can induces the

lattice distortion and changes the phase composition. It is since the ionic radius of Sr is greater than that of Ca2+.

Figures 4(a), (b), (c) and (d) show the P-E loops for the BCT-BZT samples doped with Sr contents of 0%, 5%, 10% and 15%, respectively, as a function of applied temperature and constant electric field. At a given electric field, the corresponding coercive field (EC), saturation polarization (PS) and remnant polarization (Pr) decrease as the temperature increases. Therefore, it opposes the electric-field induced ferroelectric switching of dipoles as the temperature increases, hence decreasing the polarization. As a result, the EC, PS and Pr decrease with the increase of temperature.

High-field thermal energy conversion can be attempted using ferroelectric loops using the Olsen cycle. The Olsen cycle, proposed by Olsen et al. is an analogy of the Ericson cycle, which can be

utilized to produce electricity using a high-field pyroelectric effect[7,35-38]. Olsen proposed that since a traditional P-E loop operates in an anti-clockwise manner to turn electricity into heat; it would be possible to produce electricity using waste heat if the cycle can be operated in a clockwise manner. The Olsen cycle allows for rather high conversion efficiencies up to 37.5% of the Carnot cycle (measured experimentally)[7,35-38], and consists of two isothermal and two isoelectric field processes (see Figure 4(c)). The area under the cycle shows the energy conversion density of the material. The Olsen cycle is completed in four processes[7,35-38], i.e., process 1-2 is performed at a constant lower temperature of TL as electric field increases form lower limit (EL) to higher limit (EH), correspondingly, polarization of the material increases; In process 2-3, heat (QS) is supplied to the material, which increases the temperature and produces lattice vibration. Thus material depolarizes at a constant electrical field. This generates a large depolarization current, which can be stored or used for powering electronic equipment using suitable circuit. Temperature of the material increases from a lower temperature (TL) to a higher temperature (TH); In process 3-4, lowering of the electric field from EH to EL is done at a constant temperature, TH. The polarization also decreases due to the absence of electric field, which generates a weak depolarization current. Finally, in process 4-1, the extraction of heat from the system at a constant electric field (EL) is done so that the material reaches to its initial state and completes the cycle. A detailed discussion on the Olsen cycle is available in a number of articles [7,37,38]. The energy conversion density can be estimated by[7,37,38]:

W = f^axEdP (1)

where W represents the energy conversion density in the material, E is the applied external field and Pr and Pmax are the remnant and maximum polarization, respectively. This cycle was

employed on the BCT-BZT-Sr compositions. Figures 5(a), (b), (c) and (d) show the results of the samples with Sr contents of 0%, 5%, 10% and 15%, respectively. For the energy conversion, EL and TL are kept constant at 0 kV/cm and 30 oC, respectively. However, Figure 5 shows the effect of TH as a function of EH. It is indicated that the energy density increases with the increase of the temperature and electric field span. This is due to a fact that the polarization increases as the electric field increases. Hence, the amount of energy conversion scales up. Similarly, the energy density also increases as the TH increases due to the decrease of maximum polarization. It is also interesting to note that the slope of energy conversion differs from each other in the compositions. This effect can be a result of shift in the Curie temperature of the materials due to the addition of Sr. Figure 6 shows the energy density for all the compositions when EL=0 kV/cm, TH=90 oC and TL=30 oC. The maximum energy conversion of 108 kJ/m3 appears in pristine BCT-BZT ceramics. However, for the samples with Sr contents of 5%, 10% and 15%, the energy density of 179 kJ/m3, 149 kJ/m3 and 200 kJ/m3 could be obtained, respectively. The cycle parameters are 30-90 °C and 0-20 kV/cm. Therefore, the addition of Sr improves the energy storage density by ~200% as compare to the parent compound (BCT-BZT). Moreover, a maximum energy conversion of 300 kJ/m3 is obtained for 15% Sr BCT-BZT (see Table 1). Table 1 shows a comparison of the Olsen cycle-based energy conversion density in bulk ferroelectric materials. Some researchers applied the Olsen cycle on thin film ferroelectrics and reported a great amount of energy conversion potential [37-40]. However, thin films possess a small volume, hence it cannot fulfill the energy requirements for practical application. In Table 1, there are only bulk ferroelectric materials for the comparison. However, most of the materials exhibit a great converted energy density in a large temperature range, above room temperature and under a high electric field. In order to compare the performance of the composition, we

selected some materials, which have TL of <100 C or AE (EL-EH) of < 100 kV/cm (see Table 1). It is due to a fact that materials having TL of >100 C or AE (EL-EH) of >100 kV/cm may not be used in the practical applications. Clearly, the present composition has a relatively large energy conversion potential in small temperature and electric field ranges. Vats et al. [34] reported the waste energy conversion using BCT-BZT, which was found to be 87 kJ/m3 (see Table 1). However, compared to the previously reported energy conversion, the energy conversion obtained in this study can be increased by 200% due to the addition of 15% Sr content. It can be concluded that for small electric field and temperature change BCT-BZT-Sr is a fairly promising material for thermal energy harvesting.

Table 1 Comparison of energy conversion potentials in bulk ferroelectric materials.

Energy AT (oC) El (kV/cm) EH (kV/cm) AE

Materials conversion (kJ/m3) TL (oC) Th (oC) (kV/c m) Reference

PZST 100 146 159 13 0 29 29 [35]

PZST 131 157 177 20 4 32 28 [41]

PMN-10PT 186 30 80 50 0 35 35 [3]

PMN-32PT 100 80 170 90 2 9 7 [42]

PZT (hard) 189 25 160 135 1 20 19 [8]

PZN-4.5PT 217 100 160 60 0 20 20 [6]

8/65/35 PLZT (thick films) 888 25 160 135 2 75 73 [38]

PZT (soft) 92 25 160 135 1 20 19 [8]

PZN-5.5PT 150 100 190 90 0 12 12 [7]

60/40 P(VDF-TrFE) 521 25 110 85 200 500 300 [37]

61.3/29.7/9 P(VDF-TrFE-CFE) 900 25 120 95 200 600 400 [36]

K[(Nb0.90Ta<).10)0.99Mn0.01] O3

Ba0 .85 Sr0.15Ti0.9Zr0.1O3

Bi0.5Na0.38 K0.12TiO3

Bi0.5Na0 .35 L0.15TiO3

0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba07Ca03)TiO3

0.88Bi05Na05TiO3-

0.02SrTiO3-

0.1Bi05Li05TiO3

(Bi0.5Na0.5)0.915(Bi0.5K0.5)0.05 Ba0.02Sr0.015TiO3

300 1986 1146

87 2130 1523

30 25 25

20 20 20

90 110 120

60 85 95

140 120

160 140

30 52 112

10 60 40

30 51 111

Present work [43] [43]

[34] [40]

The temperature-dependent dynamic hysteresis behavior was analyzed for all the understudy compositions. Figures 7 and 8 show the coercive field (EC) and remnant polarization (Pr) as functions of temperature at various electric fields. In Figure 7(a), there are the coercive field (EC) versus temperature (T) logarithmic plots, shows a similar decay pattern of EC with T at all the applied electric field. The dependence of ln EC on ln T was determined by the linear least square-fitting method (with R2~0.96-0.98). The scaling relation between EC and T can be determined from the slope of ln EC versus ln T. An inset of Figure 7(a) shows an electric field dependence slope (a). Hence, the dependence of EC and T can be written as

ln EC=a ln T+Yec (2)

where a and YEc are slope and Y-intercept. For BCT-BZT, it can be written as ln EC=ailn T+YEc = (0.0013E-0.65) ln T + YEc (3)

where a1=(0.0013E-0.65) and E is the applied electric field. In Eq. (3), YEc can be only calculated when T approaches to zero. The ferroelectric dipoles freeze at absolute zero temperature, and this behavior is totally different from the dipoles at high temperature. The dipole wall motion is entirely different at T=0. Therefore, we can neglect the effect of Y-intercept because we do not deal with the effect on EC when T=0. Moreover, YEc is only used to fulfill the validity of linear fit. Eq. (3) can be rewritten as

lnEc = (0.0013E - 0.65) lnT (4)

For a constant electric field, contribution of E is negligible as compared to the other factors. Hence, we can safely neglect this factor and these assumptions are used in this work. Therefore, for a constant electric field, Eq. (4) can be expressed as

ln Ec =-0.65 ln T (5)

The scaling relation between Ec and T can be written as ECo T1 or ECo T0'65, where a1 is the dimensionless constant. Similarly, the scaling relation between EC and T can be written as ECo T*2, Ec k T*3 and Ec k T4 for the samples with Sr contents of 5%, 10% and 15%, which are shown in Figures 7(b), (c) and (d), respectively. The corresponding inset of Figures shows the variation of slopes with applied electric field. The scaling relations for the samples with Sr contents of 5%, 10% and 15% are EC ocT061, EC oT0'56 and EC ocT0 63, respectively.

It is assumed that a slight difference in the scaling exponent should appear, which could be the

effect of difference in switching behavior due to applied electric field. Hence, the estimated

scaling relation can be explained based on the domain switching as functions of E and T. The

phenomenon in which the ferroelectric material changes from one spontaneously polarized state

to another is due to electrical or mechanical loads known as domain switching. The reversal of

the spontaneous polarization in a ferroelectric crystal is governed by two mechanisms, i.e., the nucleation of new domains and the growth of these domains by domain wall motion[31,44]. At low electrical fields, nucleation is the slower mechanism and hence dominates the switching process. At high electrical fields, domain wall motion determines the rate of switching[31,44]. During this process, a hysteresis loop increases. Above a particular electrical field, further polarization stops, which may lead to its dielectric breakdown[45]. Therefore, we only considered the saturated P-E loops to avoid the effect of electric field.

The similar calculations were performed to obtain a temperature-dependent relation for Pr. Figure 8 shows ln Pr versus ln T at different electric fields. Pr versus T plots for BCT-BZT are shown in Figure 8(a) and can be written as

ln Pr= fit ln T + YPr (6)

where 9 represents the slope and YPr represents the Y-intercept. It can be thus expressed as ln Pr = ln T + YPr = (0.0052E - 1.59) ln T + YPr (7)

Taking the similar assumption, we can write the above equation as

Pr x T$% or Pr x T"159 (8)

The similar scaling relation was established for BCT-BZT-Sr having Sr contents of 5%, 10% and 15%, as shown in Figures 8(b), 8(c) and (d), respectively. The insets of the corresponding figures show the electric field dependent slope. The scaling relation for Pr versus T can be written as Pr x T2 or Pr x TL59, Pr x T93 or Pr x TL36 and Pr x T94 or Pr x TL32 for the samples with Sr contents of 5%, 10% and 15%, respectively.

Also, this polarization domain wall motion depends upon T. Temperature is inversely proportional to the parameters EC, Pr and PS. This is because the lattice vibration increases with the increase in temperature, enhancing the rotation of domains and decreasing the stability of the polarization. This behavior can be explained by back-switching dependency of polarization on the temperature[44, 46]. In general, domain polarization movement takes place in two steps, firstly, switchable polarization in domain, which is known as PS and another is back-switched polarization, which is known as P. Polarization reaches to PS when the external electrical field is applied and, it again comes to stable polarization Pr in the absence of electrical field. Such a phenomenon is known as back-switching polarization. Back-switching polarization (Pbc) can be expressed by[46]

Pbc= Ps - Pr (9)

Figures 9(a), (b), (c) and (d) show the Pbc versus T plots at various electric fields for the samples with Sr contents of 0%, 5%, 10% and 15%, respectively. In Figure 9(a), back switching polarization firstly increases with increase in temperature up to 315 K, and then decreases with temperature. The similar behaviors are also shown in Figures 9(b), (c) and (d). It is indicated that at 315 K, there should exist a phase transition. However, it possesses a diffuse phase transition in this region. In the high-field ranges, the relationship between PS-Pr and T obeys the Arrhenius law[44,46]

Ps-Pr=P0 exp(-EA)/(fe T)) (10)

where EA is the average activation energy of trapped charge defect such as oxygen vacancy, and P0 is a constant. The activation energy of the domain switching can be estimated by the slope of ln (PS-Pr) versus 1/T. Therefore, Figures 10(a), (b), (c) and (d) show ln (PS-Pr) versus 1000/T

plots for the samples with Sr contents of 0%, 5%, 10% and 15%, respectively. A slope of applied electric field dependence of the activation energy can be fitted well to a simple exponential decay function. Figure 11 shows the activation energy (EA) dependence on the electric field. It decreases with increase in electric field. This can be an effect of a large number of space charge carriers in oxidation-reduction processes due to the lower activation energy.

4. Conclusions

High-field thermal energy conversion was explored using the Olsen cycle for Ba0 85Ca015-xSrxTi0.9Zr0.1O3 (BCT-BZT-Sr) compositions. The maximum energy conversion density of 108 kJ/m3 was observed in the sample without Sr content when the cycle was operated at 30-90 °C and 0-20 kV/cm. However, the energy conversion densities of 179, 149 and 200 kJ/m were obtained for the samples with Sr contents of 5%, 10% and 15%, respectively, showing almost 200% improvements in the energy conversion density due to the addition of Sr. Also, the maximum energy conversion potential was 300 kJ/m for the sample with Sr content of 15% at 30-90 °C and 0-30 kV/cm. The temperature-dependent hysteresis characteristics of BCT-BZT-Sr (when x=0%, 5%, 10% and 15%) were studied. Ferroelectric scaling relations for coercive field (EC) and remnant polarization (Pr) as functions of temperature (T) were estimated. The scaling relations for EC versus T were EC x t065, Ec x T061, EC x T056 and EC x T063 for the samples with Sr contents of 0%, 5%, 10% and 15%, respectively. The scaling relations for Pr versus T were Pr x TL59, Pr x TL59, Pv x T136 and Pv x T132 for the samples with Sr contents of 0%, 5%, 10% and 15%, respectively. The temperature-dependent back-switching kinetics of ferroelectric domain was clarified by the Arrhenius law to analysis the hysteresis characteristics with respect to the effect of temperature in an external electric field.

Acknowledgments

One of the authors (Rahul Vaish) acknowledges the support from the Indian National Science Academy (INSA), New Delhi, India, through a grant by the Department of Science and Technology (DST), New Delhi, under INSPIRE faculty award-2011 (ENG-01) and INSA Young Scientists Medal-2013.

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Figure captions:

Figure 1: X-Ray diffraction patterns of Bao.85Cao.15-xSrxTio.9Zro.1O3 (BCT-BZT-Sr) when x=0%, 5%, 10% and 15%.

Figure 2: Scanning electron micrographs (SEM) of sintered pellets for the samples with Sr contents of (a) 0%, (b) 5%, (c) 10% and (d) 15%.

Figure 3: Polarisation versus electric field (P-E) hysteresis loops as a function of applied electric field at a constant temperature of 30oC for Ba0.85Ca015-xSrxTi0.9Zr01O3 samples when x=0%, (b) x=5%, (c) x=10% and (d) x=15%.

Figure 4: Plot of (P-E) hysteresis loops under different temperature at a constant electric field the samples with Sr contents of (a) 0%, (b) 5%, (c) 10% and (d) 15%.

Figure 5: Energy conversion density as a function of applied higher electric field (EH) at different temperatures for Ba0.85Ca0.15-xSrxTi0.9Zr01O3 when (a) x=0%, (b) x=5%, (c) x=10% and (d) x=15% when lower electric field (EL) and temperature (TL) are 0 kV/cm and 30 oC, respectively.

Figure 6: A comparison plot of energy conversion density versus higher electric field (EH) for all the compositions when cycle is operated between 30-90oC at a lower electric field (EL) of 0 kV/cm.

Figure 7: Logarithmic plots of coercive field (EC) versus temperature (T) under different electric fields for Ba0.85Ca0.15-xSrxTi0.9Zr0.1O3 when (a) x=0%, (b) x=5%, (c) x=10% and (d) x= 15%. The inset indicates the corresponding plots for slope versus electric field.

Figure 8: Logarithmic plots of remnant polarization (Pr) versus temperature (T) at different electric fields for Ba0.85Ca0.15-xTi0.9SrxZr01O3 when (a) x=0%, (b) x=5%, (c) x=10% and (d) x=15%. The inset indicates the corresponding plots for slope versus electric field.

Figure 9: Plots for back switching polarisation (PS-Pr) versus temperature (T) under different magnitude of electric field for Ba0.85Ca0.15-xSrxTi0.9Zr01O3 (a) x=0%, (b) x=5%, (c) x=10% and (d) x=15% Sr content.

Figure 10: Plots of ln (PS-Pr) versus 1000/T under different magnitude of electric field for Ba0.85Ca0.15-xSrxTi0.9Zr01O3 samples when (a) x=0%, (b) x=5%, (c) x=10% and (d) x=15%.

Figure 11: Activation energy versus electric field for all the Sr added compositions of BCT-BZT.

PhD Scholar, Indian Institute of Technology Mandi, India Email: satyanarayan_patel@students.iitmandi.ac.in

Satyanarayan Patel received a Bachelor degree in Mechanical Engineering with honors from Rajasthan Technical University Kota, India in 2010 and Master degree in Energy Engineering from Malaviya National Institute of Technology Jaipur, India in 2012. From 2012 onwards he has been enrolled as a PhD. research scholar at Indian Institute of Technology Mandi, India. He has published more than 40 research papers in international peered reviewed SCI Journals. His present research interests include ferroelectric materials for energy storage and solid state refrigeration. Satyanarayan is also interested in FEM modeling, energy harvesting from mechanical vibration and heat, ferroelectric scaling laws and piezoelectric devices.

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