Electrocaloric Effect And Energy Storage Performance Of (1-x) NBT-xST Lead-free Ceramics

Theelectrocaloric effet of ferroelectric materials can be used for refrigeration.This new method has the advantage of high efficiency, environmental protection and lower cost.Its variable size can satisfy the cooling requirements of different devices. There exist some problems of Lead-free ferroelectric ceramics refrigeration:The refrigeration temperature is less than 1 k; refrigeration temperature range is narrow; refrigeration working temperature iseither too high or too low.Sodium bismuth titanate(Na0.5Bi0.5Ti O3, referred to NBT) is considered the most promising material to replace lead-based, as well as one of the most promising lead-free piezoelectric materials. Near depolarization temperature, the material shows ferroelectric / anti-ferroelectric phase transition, whichbrings a larger changing in electric heating temperature. In this paper, strontium titanate(ST) is adopted to regulation the depolarization temperature of NBT ceramics, so that the ceramicscan refrigerate near room temperature. In addition, when the depolarization temperature is reduced to below room temperature, the ceramics have a large energy density,because the NBT-ST ceramics are antiferroelectric phases at room temperature.(1-x) NBT-x ST ceramics is preparedby solid preparation method. X-ray diffraction(XRD) and scanning electron microscope are used to study the effects of ST content on the(1-x)NBT-x ST ceramics microstructure and surface topography.Study the leakage behavior combined with dielectric properties at room temperature.Electric hysteresis loops tested at room temperature and dielectric spectrums tested at different temperature are used to study phase transformation of(1-x)NBT-x ST ceramics.In order to divide the ferroelectric phase, electric hysteresis loops of ferroelectric phase tested at different temperatureare used in the calculation of electrocaloric effect.The results show that ST can reduce the grain size of ferroelectric ceramics. When ST content is 10% and 15%, it is ferroelectric phase;while ST content is 25% and 26%, ferroelectric and anti-ferroelectric are coexistence; when ST content is 30% and 40%, only anti-ferroelectric phase can be found. Phase transformationsat different temperature are relaxed. When Electric field is E=50 k V/cm, ST content is 25%,the maximum refrigerating temperature difference is ΔT = 1.634 K, ΔT/ΔE = 0.03268 m K·cm/V;When the content of ST is26% and ΔT =1.52 K, ΔT/ΔE =0.0304 m K·cm/V. Refrigeration working temperature range is wide and near room temperature.Electric hysteresis loops under different temperature and different electric fields are used to calculate energy storage properties of the(1-x) NBT-x ST ceramics.At room temperature, when electric field E=65 k V/cm, the maximumeffective energy storage density of ceramics which contain 30% ST is 0.6506 J/cm3.When ST content is 26% at 100℃, the maximum effective energy storage density is0.699 J/cm3. Mn Substituting enhanced breakdown strength. At room temperature the maximum energy storage density of 0.5% Mn substitution NBT 0.7-0.3 ST ceramics is 0.965 J/cm3 and the electric field is 95 k V/cm.