Electrocaloric Effect in Ba(Zr,Ti)O₃–(Ba,Ca)TiO₃ Ceramics Measured Directly

Abstract

In this paper, we report on studies of the electrocaloric (EC) effect in lead‐free (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 ceramics with compositions range between 0.32 ≤ x ≤ 0.45. The EC effect was measured directly using a modified differential scanning calorimeter. The maximum EC temperature change, ΔTdirect = 0.33 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.32 at ~63°C. We found that the EC effect peaks not only around the Curie temperature but also at the transition between the ferroelectric phases with different symmetries. A strong discrepancy observed between the results of the direct measurements and indirect estimations points out that using Maxwell's equations is invalid for the thermodynamic nonequilibrium conditions that accompany only partial (incomplete) poling of ceramics. We also observe a nonlinearity of the EC effect above the Curie temperature and in the temperature range corresponding to the tetragonal ferroelectric phase.

Large Electrocaloric Effect in Lead-Free (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 Ceramics Prepared via Citrate Route

“…Due to the small density and heat capacity of the BCZT-Li ceramics, their electrocaloric responsivity, being 0.164 K·mm/kV, can be comparable to the largest values reported of the lead-free piezoelectric ceramics [12,32]. The electrocaloric effect and responsivity values would be even larger if measured by the direct method, in which the discrepancy can be attributed to different polarization switching time needed measured by different methods and disequilibrium unsaturation polarized state for polycrystalline ceramics [32]. …”

“…Therefore, the electrocaloric figure of merit depends mainly on the external applied electric field, which also induces the change of the ferroelectric phase transition type, i.e., from nearly first-order phase transition to almost second-order one, leading to the decrease of the ξ value [4,12]. Due to the small density and heat capacity of the BCZT-Li ceramics, their electrocaloric responsivity, being 0.164 K·mm/kV, can be comparable to the largest values reported of the lead-free piezoelectric ceramics [12,32]. The electrocaloric effect and responsivity values would be even larger if measured by the direct method, in which the discrepancy can be attributed to different polarization switching time needed measured by different methods and disequilibrium unsaturation polarized state for polycrystalline ceramics [32].…”

Origins of the Inverse Electrocaloric Effect

“…While in (1− x )Ba(Zr 0.2 Ti 0.8 )O 3 – x (Ba 0.7 Ca 0.3 )TiO 3 and (Ba 0.9 Ca 0.1 )(Ti 0.5 Zr 0.5 )O 3 ceramics the inverse ECE was related to the tetragonal‐rhombohedral and orthorhombic–tetragonal transition, respectively, it was actually observed at temperatures several tens of degree above the corresponding transition temperatures. For some similar materials the comparison between the directly and indirectly measured ECE shows that the negative sign of the latter is an artifact . Therefore, for NBT‐18KBT (0.92 Na 0.5 Bi 0.5 TiO 3 –0.18 K 0.5 Bi 0.5 O 3 ) ceramics, Le Goupil et al .…”

“…While in (1Àx)Ba(Zr 0.2 Ti 0.8 )O 3 -x-(Ba 0.7 Ca 0.3 )TiO 3 [72] and (Ba 0.9 Ca 0.1 )(Ti 0.5 Zr 0.5 )O 3 [73] ceramics the inverse ECE was related to the tetragonal-rhombohedral and orthorhombic-tetragonal transition, respectively,i tw as actually observed at temperaturess everalt ens of degree above the corresponding transition temperatures.F or some similar materials the comparison between the directly and indirectly measured ECE shows that the negative sign of the latter is an artifact. [31,66,74] Therefore,f or NBT-18KBT (0.92 Na 0.5 Bi 0.5 TiO 3 -0.18 K 0.5 Bi 0.5 O 3 )c eramics, Le Goupil et al [66] found that direct measurements yield ap ositive ECE in the full studied temperature range while indirect estimations result in an inverse ECEa tl ow temperatures.T he authors assigned this discrepancy to be ar esult of the combination of the hysteresis loops not being fully saturated at the maximal field used (22 kV cm À1 )a nd the decrease of the coercive field with increasing temperature,w hich leads to an increaseo ft he induced polarization with temperature.S imilarly,B irks et al [31] found that for unpoled NBT ceramics the indirectly estimated ECE changes the sign from negative to positive on heating whereas the directly measured effect remains positive in the full temperature range.T hey attributed this discrepancyt oa ni ncreased resistance of polarizationr eorientationt ot he change in the electricf ield when the temperaturei sl owered.T he low-temperature state of unpoled NBT has relaxor character, with the polar structure consisting of polar nanoregions( PNR) with sizes of af ew nanometers,w ith dipole moments frozen below the freezing temperature.A tl ow temperatures these PNRs are difficult to reorient using an applied electric field. Upon heating, their reorientationb ecomes easier andt he polarization measured from hysteresis loops grows.…”

Enhanced piezoelectric properties and electrocaloric effect in novel lead‐free (Bi0.5K0.5)TiO3‐La(Mg0.5Ti0.5)O3 ceramics

“…Regarding the ECE measurement for relaxor ferroelectric ceramics, the directly measured ECE method is essential because the Maxwell relations can be only applied to equilibrium states . For example, in Ba(Zr,Ti)O 3 ‐(Ba,Ca)TiO 3 and Aurivillius relaxor ferroelectric ceramics, large discrepancies have been observed between the direct and indirect ECE measurements . Therefore, the direct measurement of ECE is essential to disclose the cooling ability of the BKT‐based relaxor ferroelectric ceramics.…”