| With the development trend of electronic products becoming miniaturized and multi-functional,the integration degree of electronic devices in equipment is getting higher and higher,which makes thermal failure become the key to seriously restrict the reliable operation of electronic products.Therefore,the research and development of efficient miniature refrigerators is the essential research direction in the field of electronic technology at present.Ferroelectric refrigeration technology based on electrocaloric effect is considered as one of the most potential alternatives to traditional gas compression technology due to its high efficiency,environmental protection and easy miniaturization.In recent years,a relaxor ferroelectric material,bismuth sodium titanate(Na0.5Bi0.5TiO3,NBT),has attracted extensive attention of researchers due to its excellent dielectric,piezoelectric and ferroelectric properties.Many studies have shown that ferroelectric phase transition plays a decisive role in the electrocaloric effect,especially around the phase transition temperature,which often produces a significant peak of the electrocaloric effect,and NBT relaxor ferroelectric materials have various phase transition behavior under the applied of electric field and temperature,which is a potential electrocaloric material.In this paper,several sodium bismuth titanate-based relaxor ferroelectric ceramics with different phase transition behaviors were designed and developed,and the electrocaloric effect was characterized by direct isothermal heat flow measurement.The electric field-temperature phase diagram was established through a comprehensive analysis of the microstructure and macroscopic properties,and the influence law of different phase transformation behaviors on the electrocaloric effect under the joint applied of electric field and temperature is clarified.At the same time,the physical origin of the deviation of the indirect method is clarified.The main research results of this paper are as follows:(1)The phase transition behavior of(0.95-x)(Na0.5Bi0.5)TiO3-0.05SrTiO3x(K0.5Na0.5)NbO3(NBT-ST-xKNN)ceramics and its physical correlation with the electrocaloric effect have been systematically studied.In this system,a morphotropic phase boundary region(x=0.05-0.08)was constructed for the coexistence of rhombohedral and tetragonal phases at room temperature.Moreover,the addition of KNN enhanced the relaxor properties of the material,and the depolarization temperature(Td)gradually decreased from 120℃ to below 0℃.The x=0.05 component in the morphotropic phase boundary near the rhombohedral phase boundary has the largest adiabatic temperature variation Δ Tmax=0.88 K(@40 kV/cm)and the widest operating temperature region Tspan=79℃.The x=0.07 component,which is at the critical point between ergodic and non-ergodic states,exhibits the optimal room temperature electrocaloric performance ΔT30℃=0.71 K(@40 kV/cm).Based on the electric-temperature phase diagram analysis of x=0.05 component,the electric-induced phase transition from the short-range ordered polar nanoscale region to the long-range ordered macroscopic ferroelectric domain can produce excellent electrocaloric effect,which is much higher than that caused by the polarization shift of macroscopic ferroelectric domain or polar nanoscale regions.(2)The influence of phase transition behavior of(1-x)(Na0.5Bi0.5)TiO3xCaTiO3(NBT-xCT)system on the electrocaloric effect is systematically studied.In this system,a morphotropic phase boundary with rhombohedral and orthorhombic phases coexistence is constructed,and the relaxor property is enhanced by adding CT.The peak value of electrocaloric effect appeared near freezing temperature(Tf)and moved to low temperature with the increase of CT content.The x=0.10 component in the morphotropic phase boundary region has the optimal electrocaloric effect ΔTmax=1.28 K(@60 kV/cm)and a wide operating temperature region Tspan=65℃.The excellent electrocaloric effect is mainly due to the transition between various polar nanoscale regions and the transition between them and the long-range ferroelectric domain induced by the electric field.For the more relaxed x=0.12 component in the morphotropic phase boundary region,the decrease trend of the electrocaloric strength with the electric field enhancement is slow,which is conducive to obtain excellent electrocaloric performance at high electric field.In addition,because the Maxwell relation neglects the difference of order entropy between the relaxor phase and the paraelectric phase,the value of the electrocaloric effect calculated by the indirect method is more than once as high as that measured by the direct method.(3)The phase transition behavior of the rhombohedral and pseudocubic phase coexistence region in(1-x)(Na0.5Bi0.5)TiO3-xNaNbO3(NBT-xNN)ceramic system and its influence on the electrocaloric effect have been systematically studied.The addition of NN enhances the breakdown field strength and improves the relaxor characteristics,and the phase transition temperature of ergodic and non-ergodic states gradually moves to the low temperature direction.The x=0.10 component of the morphotropic phase boundary region has excellent adiabatic temperature variation ΔTmax=1.34 K(@80 kV/cm、60℃).By analyzing the hysteresis loop of variable temperature under different electric fields,it is found that the negative electrocaloric effect of NBT relaxor ferroelectric ceramics calculated by indirect method is false,which is caused by polarization unsaturation.(4)Based on the results of the above systems,a morphotropic phase boundary for the coexistence of orthorhombic and pseudocubic phases in the ternary system(0.9-x)(Na0.5Bi0.5)TiO3-0.1CaTiO3-xNaNbO3(NBT-CT-xNN)is established,and the physical correlation between the phase transition characteristics and the electrocaloric effect is studied.With the increase of NN content,the saturation electric field and breakdown field strength are significantly increased,the dielectric relaxor characteristics are more obvious,and the dielectric maximum temperature(Tm)and freezing temperature(Tf)are slightly decreased.The x=0.01 component has the highest electrocaloric effect ΔTmax=2.02K(@100 kV/cm),while the x=0.03 component in the morphotropic phase boundary region near the orthotropic phase boundary has the optimal room temperature electrocaloric effect.ΔTmax=1.91 K(@100 kV/cm)and excellent temperature stability. |
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