| In order to replace the traditional compression refrigeration, it has been highly demanded to research new type eco-friendly cooling technology with low power consumption and high energy efficiency. Among various alternative cooling technologies, electro-caloric effect (ECE) of ferroelectric material attracts many concerns. Although the lead-based compounds show excellent ECE, their harm to environment restrains the commercial application. So it is an inevitable trend to explore high performance lead-free electro-caloric (EC) materials. Exploiting lead-free ferroelectric material with high EC performance over a broad temperature region around room temperature (RT) has already become one of the research focuses.BaTiO3 is the most important parent materials in the research of lead-free EC materials, and its performance can be improved by ion substitution and doping. Among them, BaSnxTi1-xO3(BST) system shows excellent dielectric, ferroelectric and pyroelectric properties. Moreover, its phase diagram exhibits multi-phase critical point and morphotropic phase boundaries(MPB), the temperature of ferroelectric-paraelectric phase transition is quite low as well, which indicate that BST has the potential to be the high EC material. We have systematically studied the relationships between the EC property, components, dielectric property, ferroelectric property and microstructure, respectively, and find an effective way to improve the ECE of lead-free materials.The BST ceramics (x=0.02,0.06,0.08,0.10,0.12,0.15,0.18) are fabricated by using the conventional solid state reaction technique. The microstructure is analyzed by using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), and it shows that Sn4+-doping inhibits the grain growth. The dielectric properties of BST system show that the temperature region of ferroelectric-paraelectric (F-P) phase transition becomes broader with the increase of Sn4+-composition. The P-E hysteresis loops at room temperature indicate the contribution of critical point and MPB to ferroelectric property.The ECE is calculated from the temperature dependence of the saturated polarization by using Maxwell’s relations. The connections between the EC properties and the ferroelectric phase transition of each component are compared, and the following conclusions can be obtained:1. The four-phase-coexisting point which contains a large number of polarized-states with similar energy levels can induce large entropy change under the change of applied electric field and cause a large EC effect. Moreover, if the energy needed for switching the different polarized-states are very small, then a giant EC effect can be induced under a low electric field. Consequently, the sample (x=0.12) near quasi-quadruple critical point has the best EC performance with △T/△E=0.027K·cm/kV and △S/△E=0.0345 J·cm kg-1K-1kV-1 under a △E=10kV/cm.2. Diffused phase transition will broaden the region of the working temperature for EC ceramics. As compared with the narrow EC peak of pure BaTiO3, the EC property of BST (x=0.12) ceramic maintains over a broader temperature range from 20℃ to 75℃.The BST(x=0.18) ceramic shows relatively high ECE and a wide EC peak (70K) around RT. Such a broad peak suggests its stable EC performance around RT.3. The MPB can enhance the EC effect of ceramics around RT. BST ceramics with a composition x=0.02 near MPB shows an enhanced EC performance (△T/△E=0.037 Kcm/kV). Moreover, as the transition temperatures of ferroelectric- ferroelectric and F-P approach, both phase transitions contribute to ECE, resulting in a broad working-temperature region of EC materials. Thus x=0.08 exhibits large EC output in the temperature region of near 100 K. |
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