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Structural, dielectric and electrocaloric properties in lead-free Zr-doped Ba0.8Ca0.2TiO3 solid solution
Solid State Communications ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Structural, dielectric and electrocaloric properties in lead-free Zr-doped Ba0.8Ca0.2TiO3 solid solution B. Asbani a,b, Y. Gagou a,n, J.-L. Dellis a, A. Lahmar a, M. Amjoud b, D. Mezzane b, Z. Kutnjak c, M. El Marssi a a
LPMC, Université de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cédex, France LMCN, F.S.T.G. Université Cadi Ayyad, BP 549, Marrakech, Morocco c Jozef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 February 2016 Received in revised form 23 March 2016 Accepted 3 April 2016 By Prof. E.V. Sampathkumaran
We investigate in the present work the additional data points (x ¼0.02, 0.06 and 0.08) to the previous reported paper (Asbani et al., 2015) [21] that sufficiently enrich to understand the EC properties and underlying phase diagram of the lead-free Ba0.8Ca0.2Ti(1 x)ZrxO3 (xBCTZ) system. X-ray diffraction analysis performed at room temperature, confirms a continuous tetragonal solid solution with P4mm (No. 99) space group that evolved to pseudo-cubic symmetry for the high Zr-content compounds. Ferroelectric and paraelectric behaviors were highlighted using P–E hysteresis data versus temperature. Phase transition was confirmed by dielectric permittivity measurements versus temperature showing a decrease of the Curie temperature when Zr-content increases. From P–E hysteresis recording the electrocaloric temperature change (ΔT) was calculated that ranged in between 0.12 and 0.27 K and the electrocaloric responsivity (ξ) in 0.15 to 0.34 10 6 km/V under 7.95 kV/cm applied electric field. The zero-field entropy is compared to electrocaloric isothermal entropy to estimate the extent the EC is being under-driven. & 2016 Published by Elsevier Ltd.
Keywords: A. Ferroelectrics B. BCTZ ceramics C. Perovskite oxide D. Electrocaloric E. X-ray diffraction
1. Introduction Nowadays, the cooling technology based on an electrocaloric effect (ECE) undergoes a renewal, since the discovery of a giant electrocaloric effect in some organic and inorganic ferroelectric materials [1–4]. The main mechanism underlying the ECE in ferroelectric material revolves around the change of the entropy (S) that is governed by the change of ferroelectric dipolar due to relative movement of the ions in the structure under the applied electric field. If the polarization changes rapidly, in the case of adiabatic thermodynamic changes, the entropy remains stable but the internal temperature varies [5]. Significant electrocaloric effect was reported in numerous lead (Pb) based materials [6–9]. However, they are no more desirable in the technological systems because Pb is known to be harmful for human health and environmental damage. Due to the stringent environmental directives, research is devoted to environmentallyfriendly materials for electrocaloric cooling devices in theoretical as well as in experimental works during the last decade. Some of them can be recalled here, for instance, large ECE was reported in n
Corresponding author. E-mail address: [email protected] (Y. Gagou).
various organic [10–13] and in lead free inorganic materials [14– 17]. Giant temperature variation ΔT up to 40 K has been reported for thin-film oxides or polymers whereas only ΔT up to a few Kelvin are reported for the bulk oxides [18] that is sufficient for applications. This is supported by numerous parameters influencing the ECE like, the nature of phase transition, the dipoles order, chemical composition, temperature rate and poling electric field history [19] that could control the magnitude of the ECE. To compare the ECE performance in different materials a new coefficient has been proposed, called as the electrocaloric responsivity ΔT/ΔE, taking into account both the temperature and the electrical applied electric field variation in the system [20]. High electrocaloric temperature changes could be achieved in bulk materials as reviewed by Valant et al. [18]. Thus, bulk materials with high ECE could have enough cooling capacity for mean- and large-scale cooling applications. The novelty of the present work being to fill out the phase diagram with three additional values of x, the optimal EC result of 0.34 10 6 km/V is obtained for x¼ 0.04, as determined in the previous APL paper and thus not novel [21]. The influence of Zr-content on structural, dielectric, ferroelectric and electrocaloric properties basing on BCTZ phase diagram was investigated. Emphasis is made on the global influence on ECE in lead free Ba0.8Ca0.2Ti(1 x)ZrxO3 (xBCTZ) solid solution when Zr-content varies in the range 0 rx r0.1.
http://dx.doi.org/10.1016/j.ssc.2016.04.001 0038-1098/& 2016 Published by Elsevier Ltd.
Please cite this article as: B. Asbani, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.001i
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2. Experimental Polycrystalline ceramics were prepared using a conventional solid-state reaction technique starting from raw materials CaCO3(98.5%), BaCO3(99%), TiO2(99.8%) and ZrO2(99.99%). Stoichiometric amounts of oxides and carbonates were thoroughly mixed with ethanol in an agate mortar for 2 h, then dried, and calcined for 16 h at 1523 K. Single-phase formation were confirmed by x-ray diffraction using a Bruker D4 Endeavor Diffractometer equipped with a Lynkeye detector with copper radiation (λKα1 ¼ 1.54056 Å and λKα2 ¼1.54439 Å). The grounded powders were pressed into disks of 6 mm in diameter and 1 mm in thickness under a uniaxial pressure of 55.7 bars. The pellets were sintered at 1673 K for 4 h, in air. The grain morphology was carried out using a scanning electron microscopy Philips XL30 apparatus. The dielectric properties were studied by using a Solartron SI-1260 spectrometer in the temperature and frequency ranges between 303 K–473 K and 100 Hz–100 kHz, respectively, after deposited silver electrodes on the pellets circular faces. The ferroelectric hysteresis loops were measured at the driving frequency of 5 Hz using a ferroelectric test system (AiXACCT, TF Analyzer 1000) at different temperatures on cooling, to minimize degradation in polarization due to fatigue [1] related to the fact that the dynamics of variation of the polarization is softer on cooling. Heat capacity was measured by a differential scanning calorimeter Netzsch DSC 204F1 with a heating rate of 10 K/min under a constant argon flow of 200 mL/min. The temperature was controlled using a Linkam TMS600 hot stage allowing 7 0.1 °C temperature stability.
3. Structural properties Structural properties were determined by x-ray diffraction (XRD) patterns analysis. All the peak positions are consistent with the standard x-ray data of the pure tetragonal perovskite structure. No other crystalline impurity phases were detected in the samples within the detection limits of the instrument. A detailed structural analysis by profile adjustment evidenced tetragonal symmetry phases for all Zr-content compounds, those were chosen based on reported phase diagrams of BCT-BTZ systems [22,23]. The x-ray patterns performed on xBCZT ceramics have been shown in Ref. [21]. The symmetry are refined in P4mm (No. 99) space group and led to rapid convergence for all the compositions. We report in Fig. 1a as example, the obtained result on the composition x ¼0.02 showing the experimental, calculated and their difference using fullprof software [24]. The variation of a and c lattice parameters versus Zr-content was plotted in Fig. 1b which shows their merging toward the same value (a parameter increases
where c parameter slightly decreases), characteristic of progressive changing from tetragonal to pseudo-cubic symmetry for high Zr-content compounds. However, the global structure remained tetragonal and ferroelectric. This non neglected variation of a-lattice parameter implies lattice dilatation corroborated by ionic radius, attributed to the replacing of smaller Ti þ 4 (rTi ¼ 0.605 Å) by a larger Zr þ 4 (rZr ¼0.72 Å) [25]. The lattice distortion can be also described in term of its tetragonality (described by c/a coefficient) that decreases to approach 1 (minimal value), with the increasing Zr-content. The refined lattice parameters (a and c), the lattice tetragonality coefficient, the volume of unit cell and the reliability factors (chi² and RF) for all the compositions are summarized in Table 1. Fig. 2 shows SEM micrographs of all the synthesized. The micrographs corresponding to x ¼0.0 and x ¼0.04 are those reported in Ref. [21] for better understanding of the evolution of ceramic density as function of Zr-content. These images present microstructures showing an increasingly compactness of the ceramics with zirconium doping confirming that Zr4 þ ion affects strongly the microstructure and consequently a ferroelastic displacement in the octahedral sites. Furthermore, a welding effect between the grains is observed leading to better density of the ceramics that increased with Zr-content.
4. Dielectric analysis Temperature dependence of the real part of dielectric permittivity for all xBCZT (x¼0.02, 0.06 and 0.08) ceramics in the frequency range 100 Hz–100 kHz, is shown in Fig. 3a, b and c, respectively. The insets represent the heat capacity variation as function of temperature. The ε'(T) curves exhibit λ-shape behavior, evidencing ferroelectric-paraelectric phase transition. These curves show also a broader phase transition anomaly for high Zrcontent compounds. This could be supported by atomic disorder on octahedral sites. However, for each ceramic, the Curie temperature does not change with the frequency sweep, confirming a classical ferroelectric type material without any relaxor behavior. Moreover, the ferroelectric transition temperature TC shifts towards lower temperatures when Zr-content increases as in Fig. 3d, in good agreement with the literature [26]. The maximum value of dielectric constant (at 100 Hz) increases with Zr-content up to ε' ¼7000 for x ¼0.03 and then, decreases for x 40.03 to a lower value (ε'¼ 5000). In Fig. 3d we plot also the variation of TC showing the decrease of TC as function of Zr-content. This result is supported by the better compactness of ceramics with increasing Zr-content leading to better ferroelectric ordering with less defects which could be in favor of earlier phase transition at heating.
Fig. 1. (a) Result of profile adjustment of x-ray diagram performed on 0.02BCTZ ceramics showing experimental, calculated and their difference. (b) a and c lattice parameters as function of Zr-content x.
Please cite this article as: B. Asbani, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.001i
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5. P–E hysteresis loops analysis P–E hysteresis loops at different temperatures are presented in the Fig. 4 and show ferroelectric character of the ceramics. At low temperature, the saturated polarization is easily reached under relatively low electric field. When temperature increases the hysteresis loop becomes slimmer, then transforms to a linear curve Table 1 Lattice parameters and reliability factors deduced from profile adjustment of xBCTZ as function of Zr-content. x (%)
a(Å)
c(Å)
c/a
V(Å3)
Chi2
RF-factor
0 2 4 6 8 10
3.966 3.9766 3.9874 3.9924 3.997 4.0031
4.0077 4.0094 4.0132 4.0157 4.0067 4.0058
1.0105 1.0082 1.0064 1.0058 1.0024 1.0006
63.0383 63.404 63.8097 64.0087 64.0118 64.1933
1.02 1.3 1.05 1.32 1.18 1.93
1.8 2.3 2.44 3.6 1.49 1.81
3
around TC, signature of ferroelectric-to-paraelectric phase transition. The room temperature variation of the polarization with composition shows a maximum value around x ¼0.04 (reported in reference [21]) with the slimmest P–E hysteresis loop. The coercive field seems to have a particular low value for x ¼0.02 and 0.04 (see Fig. 4d) where the remnant polarization exhibits high values. These two compositions are particular in this system for interesting ferroelectric applications. For x4 0.04 the width of the hysteresis (2EC) seems to increases with Zr-content while spontaneous polarization decreases toward a minimal value of 3 mC/cm². This corroborates the previous observations on structural and SEM analysis.
6. Electrocaloric investigations The electrocaloric temperature variation ΔT was deduced from the electric polarization variation as a function of temperature and
Fig. 2. SEM micrographs of xBCTZ ceramics for various Zr-contents (scale bar is 10 mm). Image corresponding to x¼ 0 and x¼ 0.04 are taken from Ref. [21].
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Fig. 3. Temperature dependence of dielectric permittivity and losses versus temperature for the composition (a) x ¼ 0.02, (b) x ¼0.06, (c) x¼ 0.08. The insets show specific heat of corresponding xBCTZ ceramics. (d) Ferroelectric transition temperature TC and maximum dielectric permittivity (at 100 Hz) versus Zr-content.
Fig. 4. P–E hysteresis loops of xBCTZ ceramics measured at different temperatures for (a) x ¼0.02, (b) x ¼ 0.06, (c) x¼ 0.08. (d) P–E hysteresis loops recorded at 303 K for all studied composition showing a slimmer curve at x¼ 0.04 (low EC).
electric field. Therefore, the electrocaloric temperature change ΔT as function of applied electric field varies from initial value E1 to final value E2 and can be deduced from the following equation [1]: ΔT ¼
1
ρ
Z
E2
E1
T ∂P dE; cp ∂T E
ð1Þ
where ρ is the density of the material and cp is the specific heat at constant pressure. The electrocaloric temperature change (ΔT) as a function of temperature, under different electric field E is presented in Fig. 5 (a–c) those show ΔT value ranging between 0.12 and 0.27 K, under
7.95 kV/cm. It should be pointed out that the ECE peaks are remarkably broad that is highly desirable for cooling applications. The magnitude of ECE peaks is due to the rapid decrease of ∂P polarization (high ∂T ) versus temperature in phase transition region resulting from large variation in dipolar entropy that occurs near TC and leads to ΔT enhancement. The most important parameter that has to be considered during development of EC materials is the electrocaloric responsivity ΔT/ΔE which is as suitable parameter to characterize the ECE performance in different materials. In the present study, xBCTZ exhibits a large value of electrocaloric responsivity ranging between 0.17 and 0.34 10 6 km/V under 7.95 kV/cm applied
Please cite this article as: B. Asbani, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.001i
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5
Fig. 5. Electrocaloric temperature change for BCTZ ceramics (a) x¼ 0.02, (b) x¼ 0.06, (c) x ¼0.08 as function of temperature. The insets show polarization as function of temperature that evidences field-driven shift in transition temperature. (d) Electrocaloric temperature change for all studied compositions as function of Zr-content at 7.95 kV/cm².
Fig. 6. (a) Zero-field entropy change (calculated from cp(T)) as function of temperature. The inset shows cp(T) (baselines are black) for all the compositions. (b) Electrocaloric entropy change calculated from the adiabatic temperature change for x¼ 0.04. The inset shows EC entropy ender E¼ 7.95 kV/cm for all the compositions.
electric field, where the maximal value is obtained for x¼ 0.04. This result is very interesting and makes the xBCTZ system one of the promising candidates for electrocaloric cooling devices in a large temperature range. To shed more information about the EC behavior in this solid solution, we calculate the zero-field entropy near the phase transition temperature. The entropy is calculated by the integration of cp(T)/T in the experimental temperature range [27]. Fig. 6(a) shows a jump of zero-field entropy that represents its variation (noted ΔS0), at the phase transition. The higher value is obtained for x¼ 0 and is 2.15 J/kg K. The inset of Fig. 6(a) shows the specific heat cp(T) for all the composition with black baseline. The electrocaloric entropy variations (ΔSE) was also determined using the Maxwell RE relation ΔSðT Þ ¼ ρ 1 0 ð∂P=∂TÞE dE [27] and reported in figure Fig. 6(b) for 4% Zr-content which increases with the applied electric field. The inset of Fig. 6(b) shows the EC entropy variation as function Zr-content, at E¼ 7.95 kV/cm and the maximum value is obtained for x ¼0.04 and is 0.31 J/kg K. The discrepancy between
both values (ΔSE and electric field.
ΔS0) can be attributed to the small applied
7. Conclusion Zirconium insertion effects on structural, dielectric, ferroelectric and electrocaloric properties in lead-free Ba0.8Ca0.2Ti(1 x)ZrxO3 (0rxr0.1) solid solution had been studied. x-ray diffraction analysis shows a tetragonal symmetry refined in P4mm (No. 99) space group that change progressively to a pseudo-cubic structure for high Zr-content, supported by the increasing of a-axis parameter. All studied compounds remained tetragonal and ferroelectric. Curie temperature decreases while Zr-content increases without any relaxor behavior. Electrocaloric effect deduced from P–E hysteresis indirect method pointed out electrocaloric temperature change (ΔT) ranging in 0.12–0.27 K and electrocaloric responsivity (ΔT/ΔE) in 0.17–0.34 10 6 km/V under 7.95 kV/cm with a higher values for
Please cite this article as: B. Asbani, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.001i
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x¼0.04. It was seen that ECE extends in a large temperature region around phase transition; interesting for cooling/heating applications. The zero-field entropy is compared to electrocaloric isothermal entropy that show a discrepancy attributive to insufficient of applied electric field.
Acknowledgments The authors thank the “Program Hubert Curie-Maghreb” (PHCMaghreb no. 27958YF) for the financial support.
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Please cite this article as: B. Asbani, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.001i
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Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Structural, dielectric and electrocaloric properties in lead-free Zr-doped Ba0.8Ca0.2TiO3 solid solution B. Asbani a,b, Y. Gagou a,n, J.-L. Dellis a, A. Lahmar a, M. Amjoud b, D. Mezzane b, Z. Kutnjak c, M. El Marssi a a
LPMC, Université de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cédex, France LMCN, F.S.T.G. Université Cadi Ayyad, BP 549, Marrakech, Morocco c Jozef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 February 2016 Received in revised form 23 March 2016 Accepted 3 April 2016 By Prof. E.V. Sampathkumaran
We investigate in the present work the additional data points (x ¼0.02, 0.06 and 0.08) to the previous reported paper (Asbani et al., 2015) [21] that sufficiently enrich to understand the EC properties and underlying phase diagram of the lead-free Ba0.8Ca0.2Ti(1 x)ZrxO3 (xBCTZ) system. X-ray diffraction analysis performed at room temperature, confirms a continuous tetragonal solid solution with P4mm (No. 99) space group that evolved to pseudo-cubic symmetry for the high Zr-content compounds. Ferroelectric and paraelectric behaviors were highlighted using P–E hysteresis data versus temperature. Phase transition was confirmed by dielectric permittivity measurements versus temperature showing a decrease of the Curie temperature when Zr-content increases. From P–E hysteresis recording the electrocaloric temperature change (ΔT) was calculated that ranged in between 0.12 and 0.27 K and the electrocaloric responsivity (ξ) in 0.15 to 0.34 10 6 km/V under 7.95 kV/cm applied electric field. The zero-field entropy is compared to electrocaloric isothermal entropy to estimate the extent the EC is being under-driven. & 2016 Published by Elsevier Ltd.
Keywords: A. Ferroelectrics B. BCTZ ceramics C. Perovskite oxide D. Electrocaloric E. X-ray diffraction
1. Introduction Nowadays, the cooling technology based on an electrocaloric effect (ECE) undergoes a renewal, since the discovery of a giant electrocaloric effect in some organic and inorganic ferroelectric materials [1–4]. The main mechanism underlying the ECE in ferroelectric material revolves around the change of the entropy (S) that is governed by the change of ferroelectric dipolar due to relative movement of the ions in the structure under the applied electric field. If the polarization changes rapidly, in the case of adiabatic thermodynamic changes, the entropy remains stable but the internal temperature varies [5]. Significant electrocaloric effect was reported in numerous lead (Pb) based materials [6–9]. However, they are no more desirable in the technological systems because Pb is known to be harmful for human health and environmental damage. Due to the stringent environmental directives, research is devoted to environmentallyfriendly materials for electrocaloric cooling devices in theoretical as well as in experimental works during the last decade. Some of them can be recalled here, for instance, large ECE was reported in n
Corresponding author. E-mail address: [email protected] (Y. Gagou).
various organic [10–13] and in lead free inorganic materials [14– 17]. Giant temperature variation ΔT up to 40 K has been reported for thin-film oxides or polymers whereas only ΔT up to a few Kelvin are reported for the bulk oxides [18] that is sufficient for applications. This is supported by numerous parameters influencing the ECE like, the nature of phase transition, the dipoles order, chemical composition, temperature rate and poling electric field history [19] that could control the magnitude of the ECE. To compare the ECE performance in different materials a new coefficient has been proposed, called as the electrocaloric responsivity ΔT/ΔE, taking into account both the temperature and the electrical applied electric field variation in the system [20]. High electrocaloric temperature changes could be achieved in bulk materials as reviewed by Valant et al. [18]. Thus, bulk materials with high ECE could have enough cooling capacity for mean- and large-scale cooling applications. The novelty of the present work being to fill out the phase diagram with three additional values of x, the optimal EC result of 0.34 10 6 km/V is obtained for x¼ 0.04, as determined in the previous APL paper and thus not novel [21]. The influence of Zr-content on structural, dielectric, ferroelectric and electrocaloric properties basing on BCTZ phase diagram was investigated. Emphasis is made on the global influence on ECE in lead free Ba0.8Ca0.2Ti(1 x)ZrxO3 (xBCTZ) solid solution when Zr-content varies in the range 0 rx r0.1.
http://dx.doi.org/10.1016/j.ssc.2016.04.001 0038-1098/& 2016 Published by Elsevier Ltd.
Please cite this article as: B. Asbani, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.001i
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2. Experimental Polycrystalline ceramics were prepared using a conventional solid-state reaction technique starting from raw materials CaCO3(98.5%), BaCO3(99%), TiO2(99.8%) and ZrO2(99.99%). Stoichiometric amounts of oxides and carbonates were thoroughly mixed with ethanol in an agate mortar for 2 h, then dried, and calcined for 16 h at 1523 K. Single-phase formation were confirmed by x-ray diffraction using a Bruker D4 Endeavor Diffractometer equipped with a Lynkeye detector with copper radiation (λKα1 ¼ 1.54056 Å and λKα2 ¼1.54439 Å). The grounded powders were pressed into disks of 6 mm in diameter and 1 mm in thickness under a uniaxial pressure of 55.7 bars. The pellets were sintered at 1673 K for 4 h, in air. The grain morphology was carried out using a scanning electron microscopy Philips XL30 apparatus. The dielectric properties were studied by using a Solartron SI-1260 spectrometer in the temperature and frequency ranges between 303 K–473 K and 100 Hz–100 kHz, respectively, after deposited silver electrodes on the pellets circular faces. The ferroelectric hysteresis loops were measured at the driving frequency of 5 Hz using a ferroelectric test system (AiXACCT, TF Analyzer 1000) at different temperatures on cooling, to minimize degradation in polarization due to fatigue [1] related to the fact that the dynamics of variation of the polarization is softer on cooling. Heat capacity was measured by a differential scanning calorimeter Netzsch DSC 204F1 with a heating rate of 10 K/min under a constant argon flow of 200 mL/min. The temperature was controlled using a Linkam TMS600 hot stage allowing 7 0.1 °C temperature stability.
3. Structural properties Structural properties were determined by x-ray diffraction (XRD) patterns analysis. All the peak positions are consistent with the standard x-ray data of the pure tetragonal perovskite structure. No other crystalline impurity phases were detected in the samples within the detection limits of the instrument. A detailed structural analysis by profile adjustment evidenced tetragonal symmetry phases for all Zr-content compounds, those were chosen based on reported phase diagrams of BCT-BTZ systems [22,23]. The x-ray patterns performed on xBCZT ceramics have been shown in Ref. [21]. The symmetry are refined in P4mm (No. 99) space group and led to rapid convergence for all the compositions. We report in Fig. 1a as example, the obtained result on the composition x ¼0.02 showing the experimental, calculated and their difference using fullprof software [24]. The variation of a and c lattice parameters versus Zr-content was plotted in Fig. 1b which shows their merging toward the same value (a parameter increases
where c parameter slightly decreases), characteristic of progressive changing from tetragonal to pseudo-cubic symmetry for high Zr-content compounds. However, the global structure remained tetragonal and ferroelectric. This non neglected variation of a-lattice parameter implies lattice dilatation corroborated by ionic radius, attributed to the replacing of smaller Ti þ 4 (rTi ¼ 0.605 Å) by a larger Zr þ 4 (rZr ¼0.72 Å) [25]. The lattice distortion can be also described in term of its tetragonality (described by c/a coefficient) that decreases to approach 1 (minimal value), with the increasing Zr-content. The refined lattice parameters (a and c), the lattice tetragonality coefficient, the volume of unit cell and the reliability factors (chi² and RF) for all the compositions are summarized in Table 1. Fig. 2 shows SEM micrographs of all the synthesized. The micrographs corresponding to x ¼0.0 and x ¼0.04 are those reported in Ref. [21] for better understanding of the evolution of ceramic density as function of Zr-content. These images present microstructures showing an increasingly compactness of the ceramics with zirconium doping confirming that Zr4 þ ion affects strongly the microstructure and consequently a ferroelastic displacement in the octahedral sites. Furthermore, a welding effect between the grains is observed leading to better density of the ceramics that increased with Zr-content.
4. Dielectric analysis Temperature dependence of the real part of dielectric permittivity for all xBCZT (x¼0.02, 0.06 and 0.08) ceramics in the frequency range 100 Hz–100 kHz, is shown in Fig. 3a, b and c, respectively. The insets represent the heat capacity variation as function of temperature. The ε'(T) curves exhibit λ-shape behavior, evidencing ferroelectric-paraelectric phase transition. These curves show also a broader phase transition anomaly for high Zrcontent compounds. This could be supported by atomic disorder on octahedral sites. However, for each ceramic, the Curie temperature does not change with the frequency sweep, confirming a classical ferroelectric type material without any relaxor behavior. Moreover, the ferroelectric transition temperature TC shifts towards lower temperatures when Zr-content increases as in Fig. 3d, in good agreement with the literature [26]. The maximum value of dielectric constant (at 100 Hz) increases with Zr-content up to ε' ¼7000 for x ¼0.03 and then, decreases for x 40.03 to a lower value (ε'¼ 5000). In Fig. 3d we plot also the variation of TC showing the decrease of TC as function of Zr-content. This result is supported by the better compactness of ceramics with increasing Zr-content leading to better ferroelectric ordering with less defects which could be in favor of earlier phase transition at heating.
Fig. 1. (a) Result of profile adjustment of x-ray diagram performed on 0.02BCTZ ceramics showing experimental, calculated and their difference. (b) a and c lattice parameters as function of Zr-content x.
Please cite this article as: B. Asbani, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.001i
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5. P–E hysteresis loops analysis P–E hysteresis loops at different temperatures are presented in the Fig. 4 and show ferroelectric character of the ceramics. At low temperature, the saturated polarization is easily reached under relatively low electric field. When temperature increases the hysteresis loop becomes slimmer, then transforms to a linear curve Table 1 Lattice parameters and reliability factors deduced from profile adjustment of xBCTZ as function of Zr-content. x (%)
a(Å)
c(Å)
c/a
V(Å3)
Chi2
RF-factor
0 2 4 6 8 10
3.966 3.9766 3.9874 3.9924 3.997 4.0031
4.0077 4.0094 4.0132 4.0157 4.0067 4.0058
1.0105 1.0082 1.0064 1.0058 1.0024 1.0006
63.0383 63.404 63.8097 64.0087 64.0118 64.1933
1.02 1.3 1.05 1.32 1.18 1.93
1.8 2.3 2.44 3.6 1.49 1.81
3
around TC, signature of ferroelectric-to-paraelectric phase transition. The room temperature variation of the polarization with composition shows a maximum value around x ¼0.04 (reported in reference [21]) with the slimmest P–E hysteresis loop. The coercive field seems to have a particular low value for x ¼0.02 and 0.04 (see Fig. 4d) where the remnant polarization exhibits high values. These two compositions are particular in this system for interesting ferroelectric applications. For x4 0.04 the width of the hysteresis (2EC) seems to increases with Zr-content while spontaneous polarization decreases toward a minimal value of 3 mC/cm². This corroborates the previous observations on structural and SEM analysis.
6. Electrocaloric investigations The electrocaloric temperature variation ΔT was deduced from the electric polarization variation as a function of temperature and
Fig. 2. SEM micrographs of xBCTZ ceramics for various Zr-contents (scale bar is 10 mm). Image corresponding to x¼ 0 and x¼ 0.04 are taken from Ref. [21].
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Fig. 3. Temperature dependence of dielectric permittivity and losses versus temperature for the composition (a) x ¼ 0.02, (b) x ¼0.06, (c) x¼ 0.08. The insets show specific heat of corresponding xBCTZ ceramics. (d) Ferroelectric transition temperature TC and maximum dielectric permittivity (at 100 Hz) versus Zr-content.
Fig. 4. P–E hysteresis loops of xBCTZ ceramics measured at different temperatures for (a) x ¼0.02, (b) x ¼ 0.06, (c) x¼ 0.08. (d) P–E hysteresis loops recorded at 303 K for all studied composition showing a slimmer curve at x¼ 0.04 (low EC).
electric field. Therefore, the electrocaloric temperature change ΔT as function of applied electric field varies from initial value E1 to final value E2 and can be deduced from the following equation [1]: ΔT ¼
1
ρ
Z
E2
E1
T ∂P dE; cp ∂T E
ð1Þ
where ρ is the density of the material and cp is the specific heat at constant pressure. The electrocaloric temperature change (ΔT) as a function of temperature, under different electric field E is presented in Fig. 5 (a–c) those show ΔT value ranging between 0.12 and 0.27 K, under
7.95 kV/cm. It should be pointed out that the ECE peaks are remarkably broad that is highly desirable for cooling applications. The magnitude of ECE peaks is due to the rapid decrease of ∂P polarization (high ∂T ) versus temperature in phase transition region resulting from large variation in dipolar entropy that occurs near TC and leads to ΔT enhancement. The most important parameter that has to be considered during development of EC materials is the electrocaloric responsivity ΔT/ΔE which is as suitable parameter to characterize the ECE performance in different materials. In the present study, xBCTZ exhibits a large value of electrocaloric responsivity ranging between 0.17 and 0.34 10 6 km/V under 7.95 kV/cm applied
Please cite this article as: B. Asbani, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.001i
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Fig. 5. Electrocaloric temperature change for BCTZ ceramics (a) x¼ 0.02, (b) x¼ 0.06, (c) x ¼0.08 as function of temperature. The insets show polarization as function of temperature that evidences field-driven shift in transition temperature. (d) Electrocaloric temperature change for all studied compositions as function of Zr-content at 7.95 kV/cm².
Fig. 6. (a) Zero-field entropy change (calculated from cp(T)) as function of temperature. The inset shows cp(T) (baselines are black) for all the compositions. (b) Electrocaloric entropy change calculated from the adiabatic temperature change for x¼ 0.04. The inset shows EC entropy ender E¼ 7.95 kV/cm for all the compositions.
electric field, where the maximal value is obtained for x¼ 0.04. This result is very interesting and makes the xBCTZ system one of the promising candidates for electrocaloric cooling devices in a large temperature range. To shed more information about the EC behavior in this solid solution, we calculate the zero-field entropy near the phase transition temperature. The entropy is calculated by the integration of cp(T)/T in the experimental temperature range [27]. Fig. 6(a) shows a jump of zero-field entropy that represents its variation (noted ΔS0), at the phase transition. The higher value is obtained for x¼ 0 and is 2.15 J/kg K. The inset of Fig. 6(a) shows the specific heat cp(T) for all the composition with black baseline. The electrocaloric entropy variations (ΔSE) was also determined using the Maxwell RE relation ΔSðT Þ ¼ ρ 1 0 ð∂P=∂TÞE dE [27] and reported in figure Fig. 6(b) for 4% Zr-content which increases with the applied electric field. The inset of Fig. 6(b) shows the EC entropy variation as function Zr-content, at E¼ 7.95 kV/cm and the maximum value is obtained for x ¼0.04 and is 0.31 J/kg K. The discrepancy between
both values (ΔSE and electric field.
ΔS0) can be attributed to the small applied
7. Conclusion Zirconium insertion effects on structural, dielectric, ferroelectric and electrocaloric properties in lead-free Ba0.8Ca0.2Ti(1 x)ZrxO3 (0rxr0.1) solid solution had been studied. x-ray diffraction analysis shows a tetragonal symmetry refined in P4mm (No. 99) space group that change progressively to a pseudo-cubic structure for high Zr-content, supported by the increasing of a-axis parameter. All studied compounds remained tetragonal and ferroelectric. Curie temperature decreases while Zr-content increases without any relaxor behavior. Electrocaloric effect deduced from P–E hysteresis indirect method pointed out electrocaloric temperature change (ΔT) ranging in 0.12–0.27 K and electrocaloric responsivity (ΔT/ΔE) in 0.17–0.34 10 6 km/V under 7.95 kV/cm with a higher values for
Please cite this article as: B. Asbani, et al., Solid State Commun (2016), http://dx.doi.org/10.1016/j.ssc.2016.04.001i
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x¼0.04. It was seen that ECE extends in a large temperature region around phase transition; interesting for cooling/heating applications. The zero-field entropy is compared to electrocaloric isothermal entropy that show a discrepancy attributive to insufficient of applied electric field.
Acknowledgments The authors thank the “Program Hubert Curie-Maghreb” (PHCMaghreb no. 27958YF) for the financial support.
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