Study On Electrocaloric Effect Of Ba_1-xSr_xTiO₃-Based Pb-free Ferroelectric Ceramics

Traditional vapor compression refrigeration technology has low energy conversion efficiency,serious environmental pollution,and cannot be applied to microelectronic devices.In the face of the growing demand for refrigeration in various fields,such as frozen storage,refrigerated transport,smart home,special equipment,micro electronic circuit heat dissipation,etc.,the traditional vapor compression refrigeration technology is gradually unable to meet the needs.The development of energy-saving,environmentally friendly,miniaturizable new refrigeration technology is imminent.With many advantages such as high efficiency,low cost,energy saving and environmental protection,easy miniaturization,mature large-scale production process and direct electric drive,electrocaloric refrigeration technology has become a strong candidate for new refrigeration technology.At present,the research based on electrocaloric refrigeration is still in the development stage,many problems need to be solved.The core challenge is to obtain electrocaloric materials with large electrocaloric response,wide working temperature span and operating temperatures that match the application scenario.Barium titanate（Ba TiO₃,BT）ceramics,as a common material for electrocaloric research,have the advantages of high dielectric constant,low dielectric loss,good ferroelectric properties and excellent pyroelectric properties.However,the electrocaloric performance of pure BT differs greatly from the actual application.In this paper,the comprehensive electrocaloric performance of BT ceramics is improved by ion doping modification as follows:The Ba_1-xSr_xTiO₃ series ceramics were firstly prepared by A-site substitution of BT by Sr²⁺.The experimental results show that the introduction of Sr²⁺can effectively reduce the Curie temperature（T_C）of BT ceramics and affect the electrocaloric performance of BT ceramics.Ba_0.75Sr_0.25TiO₃（BST-25）ceramics with T_C close to room temperature（59℃）and excellent electrocaloric properties,obtained an adiabatic temperature change（?T）of 0.82 K at 30 k V/cm,have the potential to be used as electrocaloric refrigeration device materials.Therefore,in this paper,BST-25 ceramics were selected as a subsequent study substrate to further improve the electrocaloric properties of the ceramics by B-site doping.Zr⁴⁺was successfully introduced into BST-25 ceramics by B-site substitution strategy.It was found that the introduction of Zr⁴⁺could gradually close the rhombohedral-orthorhombic,orthorhombic-tetragonal and tetragonal-cubic phase transitions of the ceramics and obtain continuous phase transitions in a certain temperature range,finally obtained the large electrocaloric effect in a wide temperature range.When the Zr⁴⁺content was 6%,the ceramics obtained a?T of 1.07 K at 50 k V/cm,and the working temperature span(T_span)reached 44℃.The effect of Hf⁴⁺,which has different periods in the same main group as Zr⁴⁺,on the electrocaloric properties of BST-25 ceramics was also investigated in this paper.Results show that Hf⁴⁺doping can effectively regulate the T_span of BST-25 ceramics to room temperature,and the prepared Ba_0.75Sr_0.25Ti_0.925Hf_0.075O₃ ceramics obtained a?T of 1.03 K at 50 k V/cm（36℃）with a T_span of 45℃.Finally,to further improve the?T of the ceramics near room temperature,Sn⁴⁺was introduced into the BST-25 ceramics in this paper.It was found that Sn⁴⁺can enhance the dielectric properties while reducing T_C.The T_C of Ba_0.75Sr_0.25Ti_0.95Sn_0.05O₃ceramic is only 17℃ and the dielectric constant is as high as 43570.Ba_0.75Sr_0.25Ti_0.95Sn_0.05O₃ ceramics obtained a?T of 1.17 K at 40 k V/cm（30℃）,successfully obtaining a large electrocaloric effect at room temperature.In this paper,BT is is selected as the substrate.Firstly,the T_C of BT was regulated by A-site ion doping strategy,so that the working temperature of the modified ceramics was close to room temperature.On this basis,the comprehensive electrocaloric properties of the ceramics are further improved by B-site ion substitution,and finally lead-free ferroelectric ceramics with large electrocaloric effect in the wide temperature span are obtained near room temperature,which can provide solid theoretical basis for the development of lead-free electrocaloric refrigeration ceramics.