Coprecipitation Preparation And Electrocaloric Effects Of Barium Strontium Niobate

The electrocaloric effect(ECE)refrigeration is a new solid state refrigeration technology which is environmentally friendly and efficient.The unfilled tungsten bronze ceramic SrxBa1-xNb2O6(x = 0.25?0.75,SBN for short)has excellent pyroelectric properties at room temperature,furthermore,it's free of lead or other elements those are harmful to the human body and the environment,all these advantages make it an ideal ECE material.In this paper,the synthesis mechanism of SBN powders prepared by coprecipitation method was discussed,and the effects of crystal structure,composition and microstructure in SBN on dielectric,ferroelectric properties as well as ECE were systematically investigaged.The SBN precursor powders(x = 0.4,0.5,0.6 and 0.75)were synthesized by coprecipitation method and calcined at different temperatures according to their compositional differences.The composition and structure of calcined powders at different temperatures were analyzed by X-ray powder diffraction,Raman and infrared spectroscopy.It was found that the synthesis mechanism of coprecipitation method was similar to that of the conventional sinterning(CS)method.However,the powder prepared by the coprecipitation method have smaller particle size and more uniform components,and the formation temperatures of the respective phases are lower than those prepared by CS method.SBN ceramics(SBN40,SBN50,SBN60 and SBN75)were sintered by using the corresponding ceramic powders which were prepared by coprecipitation method.With the increasing of Sr content,the sintering temperature increased,while the optimum density decreased.The ceramic properties depended strongly on composition,with the increasing of Sr content,the Curie temperature gradually decreased from 110 ? to near room temperature,and the ceramic changed from normal ferroelectric to relaxation ferroelectric.Under a electric field of 90 kV/cm,SBN60 obtained the largest ?T and?S at 90 ?,0.51 ? and 0.65 J/(kg-K),respectively.Compared with the SBN60 ceramics produced by CS,the temperature of maximum ECE value was lowered by 15 ?,and ?T and ?S were increased by 24.4%and 71.1%,respectively.Sr0.7Ba0.3CeyNb2O6+? precursor powders(y= 0,0.02,0.04 and 0.06)were prepared by coprecipitation method and sintered to corresponding ceramics(SBN70,SBN70-Ce2,SBN70-Ce4 and SBN70-Ce6).With the increasing of Ce content in SBN ceramic,the grain size and cell volume decreased,while the columnar crystals still existed,and the ceramic porosity increased,as a result,the density decreased.The Ce content in the SBN ceramics also influced their physical properties,as the Ce content increased,the ceramic Curie temperature gradually decreased,the relaxation became stronger,and the dielectric strength at room temperature became larger.Proper amount of Ce enhanced the ECE of SBN70 near room temperature.At 20 ? and under a electric field of 100 kV/cm,SBN70-Ce2 obtained the ?T and ?S of 0.33 ? and 0.46 J/(kg�K),respectively,which were 32%and 9.5%higher than SBN70 without adding Ce.