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Abstract
The electrocaloric effect (ECE) in lead free (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) and (Ba1-xSrx)(Zr0.1Ti0.9)O3 (BSZTx) ceramics over a wide composition range (x=0.3-0.7 for BZT-xBCT and x=0-0.35 for BSZTx) were investigated using an indirect method based on the thermodynamics Maxwell relations.
In BZT-xBCT ceramics, the maximum adiabatic temperature change Δ𝑇 was found to shift from the proximity of Curie temperature to higher temperatures, owing to a synergistic effect of field induced change in the phase transition temperature and alignment and growth of polar nano-regions. Coexistence of positive and negative ECEs was confirmed in BCT-rich compositions (x ≥ 0.5). The abnormal negative ECE is presumably attributed to the rhombohedral to tetragonal transition occurred at relatively low temperatures. The most promising ECE was found in BZT-0.4BCT ceramics, which exhibited a uniform ECE in the temperature range of interest, with a promising Δ𝑇 of 0.58 K at ~ 125 ℃ under a moderate electric field of 28 kV/cm. A weakened ECE was shown in the unique composition at morphotropic phase boundary (x = 0.5) due to the possible electric field induced second order phase transition.
In BSZTx ceramics, the maxima ECE occurred at temperatures very close to their Curie points. The maximum adiabatic temperature changes Δ𝑇 and the Curie temperatures of BSZTx decrease with the increasing Sr content. The electrocaloric coefficient (𝜉=Δ𝑇Δ𝐸) in BSZTx is among the highest reported ECE for lead free materials in the literature up to this date. The most promising EC materials are BSZT0 (Δ𝑇 = 0.38 K, 𝜉=0.253 10-6K*m/V) and BSZT0.35 (Δ𝑇 = 0.31 K, 𝜉=0.207 10-6K*m/V), which make BSZTx attractive for both room temperature and high temperature (~100 ℃) cooling application.