Electrocaloric Effect In Na_0.5Bi_0.5TiO₃-based Ferroelectrics With Wide Operating Temperature Range

Refrigeration technology plays an important role in the construction of national economy,the modernization of national defense and military,and the development of science and technology.Solid-state refrigeration based on the electrocaloric effect（ECE）has important potential applications in microelectromechanical systems because of high efficiency,eco-friendly,easy miniaturization,high energy conversion efficiency,and fast working response.The key to ECE is to seek new materials with excellent properties.This thesis selects environmentally friendly lead-free Na_0.5Bi_0.5Ti O₃materials as the research object.The performance of ECE is optimized by the method of component doping and constructing multilayer ceramic structures.It is hoped to obtain a large adiabatic temperature change（ΔT）at room temperature with a wide working temperature range of ECE.Firstly,Na_0.5Bi_0.5Ti O₃-x Sr_0.85Bi_0.10Ti O₃（NBT-x SBT）ceramics（where x=0.15,0.20,0.25 and 0.30）were prepared by a solid-state method.TheΔT was optimized by adjusting the content of Sr_0.85Bi_0.10Ti O₃to build the ferroelectric（FE）-relaxation ferroelectric（RFE）phase transition at room temperature.The results show that the maximum polarization of the ceramic increases slightly with the increase of SBT content to 0.25 mol,but the residual polarization decreases sharply.The temperature of FE-RFE phase transition shifts to room temperature at which the coexistence of FE and RFE phases are observed.When the SBT content is 0.20 mol,theΔT_max=1.19 K was obtained at 60 k V/cm at 105°C using an indirect test method,andΔT/ΔE is 0.216 K mm/k V.When the SBT content increases to0.25 mol,theΔT_maxobtained at 60 k V/cm was 0.87 K,and the operating temperature range is from 48℃to 81℃,achieving effective cooling near room temperature.Secondly,the ferroelectric-paraelectric phase transition（FE-PE）temperature was adjusted to room temperature in the Na_0.5Bi_0.5Ti O₃-y Sr_0.85Bi_0.10Ti O₃（NBT-y SBT）ceramics（y=0.35,0.38,0.41,0.44,0.47 and 0.50）by further increasing SBT content,which is in favor of enhancingΔT and broadening the operating temperature span of ECE.The results show that the FE-PE phase transition temperature moves to room temperature when the SBT content is 0.50 mol.TheΔT_maxof NBT-0.50SBT is 0.306 K at 80 k V/cm,and temperature span is 65 K near room temperature.In order to improve the breakdown strength,the multilayer ceramic capacitor（MLCC）of NBT-0.50SBT was prepared by a tape-casting method to further enhance the ECE.TheΔT_maxof NBT-0.50SBT MLCC is2.59 K at 300 k V/cm,ΔT/ΔE is 0.130 K mm/k V,and temperature range is from 30℃ to90℃.Finally,the（0.75-z）Na_0.5Bi_0.5Ti O₃-0.25Sr_0.85Bi_0.10Ti O₃-z Ca Zr O₃（NBT-SBT-z CZ）system was used to obtain the negative ECE in a wide temperature range by adjusting the sintering temperature and Ca Zr O₃content.The results show that the stability of dielectric properties of 2CZ ceramics is enhanced as the sintering temperature increases,and the working temperature span of the negative ECE is extended from 10 K to 70 K.With the increases of Ca Zr O₃content,theΔT of ECE decreases slightly and the working temperature span increases significantly.The maximum value of negative ECE for 4CZ ceramic is-0.450 K at room temperature.The negativeΔT of 8CZ ceramic is-0.28 K,which corresponds to a working temperature range of 30℃-110℃.