Energy Storage Properties And Electrocaloric Effect Of Bismuth Ferrite-Barium Titanate-Based Ceramics

Electrocaloric effect(ECE)is the isothermal entropy change and adiabatic temperature change in a polar material when an external electric field is applied or removed.As a new type of solid-state refrigeration technology,ECE materials derived devices with a large?T,high efficiency,large electrocaloric strength and broad operational temperature range are considered to have the potential to replace the traditional vapor-compression technology in refrigeration field because of its high efficiency(>60%Carnot Cycle efficiency),miniaturization and environmental friendliness.In addition,although the actual charge/discharge time of energy storage devices may be affected by the many factors,such as load resistance,the dielectric energy storage capacitors offer higher power density and higher charge/discharge rates compared to batteries and supercapacitors.However,the biggest problem is its low energy density.Currently,the dielectric energy storage capacitors have been used in various power equipment and electronic systems,especially in the field that requires high pulse power.The xBiFeO₃-(1-x)BaTiO₃based ferroelectric materials hold the advantage of large polarizations.In this dissertation,a doping modification method is adopted to achieve higher energy storage density in xBiFeO₃-(1-x)BaTiO₃materials while maintaining the advantages of large polarization,reduced leakage current and remnant polarization.Although energy storage properties of xBiFeO₃-(1-x)BaTiO₃materials have been reported in recent years,there are very few studies on the pyroelectric effect and ECE for this material.Thus,the ECE of xBiFeO₃-(1-x)BaTiO₃-based ceramics were studied and good results were obtained.The main contents of this dissertation are as follows,(1)xBiFeO₃-(1-x)BaTiO₃relaxor ferroelectric ceramics were synthesized via the high-temperature solid state reaction process.The X-ray diffraction(XRD)patterns showed that the structure of samples transforms from a tetragonal phase to a pseudo-cubic phase when x?0.025.The fitted universal Curie-Weiss exponents and slim shapes of polarization-electric field(P-E)hysteresis loops indicated that xBiFeO₃-(1-x)BaTiO₃(x?0.025)ceramics transform from a normal ferroelectric to a relaxor ferroelectric.The energy-storage property of 0.1BiFeO₃-0.9BaTiO₃was also measured at room temperature.The maximum discharged energy density and efficiency procured were 1.22 J·cm^-3and85.98%at 160 k V·cm^-1,respectively.It was worth noting that the positive electrocaloric effect was observed in xBiFeO₃-(1-x)BaTiO₃bulk ceramics,no matter directly measured using a thermocouple,or indirectly calculated using the Maxwell relation.And a(35)T_maxof1.55 K was indirectly calculated from the temperature-dependent P-E hysteresis loops for0.05BiFeO₃-0.95BaTiO₃;while a(35)T_maxof 2.20 K was directly obtained using the thermocouple.The positive electrocaloric effect means that the xBiFeO₃-(1-x)BaTiO₃is a conventional ferroelectric or relaxor ferroelectric.(2)Due to the evaporation of Bi₂O₃during the sintering process at high temperatures,and the variation of chemical valence of iron ions,there are many point defects and also a large leakage current existed in BFO,which make the ferroelectricity of BFO difficult to be presented and measured.Although the forming of solid solution with BaTiO₃(BTO)or other oxide ferroelectrics may mitigate the leakage current,high loss is still existed in BFO-based materials.In this dissertation,this problem is attempted to be solved by adding Mn⁴⁺ions into the BFO-BTO solid solution.0.7(BFO)-0.3(BTO)+xwt%Mn O₂ceramics were prepared using the conventional solid-state reaction at high temperatures.The microstructure,dielectric and ferroelectric characteristics were investigated via doping with different Mn⁴⁺ions.Results indicated that the crystallographic structure was rhombohedral and pseudocubic phase coexistence.It was observed that certain Mn⁴⁺ionic content may lead to the reduction of loss tangent and leakage current for BFO-BTO ceramics,which were accounted for the compensation of Mn⁴⁺ions with the oxygen vacancies.In addition,it was procured that the0.7BFO-0.3BTO+0.5wt%Mn O₂ceramic offered a maximum polarization of 50.53?C/cm~2at100 k V/cm.Finally,the direct approach was used to measure the ECE.It was found that using Lu et al.proposed polarization flip method,the ECE temperature change was observed to increase almost 8 times when changing the electric field from 0 to-30 k V/m with respect to the changing of electric field from 30 k V/cm to 0.This verified that the Lu et al's method was also applicable to polycrystalline first-order phase transition ferroelectrics.(3)0.7Bi_(1-x)Nd_xFe O₃-0.3BaTiO₃(BNFO-BTO,x=0.005,0.010,0.020 and 0.050)ceramics were prepared via the high-temperature solid-state reaction process.The XRD peaks were recorded that the main phase was pseudocubic in 0.7Bi_(1-x)Nd_xFe O₃-0.3BaTiO₃ceramics.The Scanning Electron Microscopy(SEM)micrographs demonstrated that all BNFO-BTO ceramics showed dense microstructures.In addition,the temperature dependence of dielectric behaviors and ferroelectric hysteresis loop shapes indicated relaxor ferroelectric type phase transition and frequency dispersion behaviors and remnant polarization were enhanced in BNFO-BTO ceramics with more Bi ions replaced by Nd ions.Conducted impedance analysis on 0.7Bi_(1-x)Nd_xFe O₃-0.3BaTiO₃ceramics and two dielectric responses of the grain at high frequency and grain boundary at low frequency were illustrated,respectively.Using calculations based on Curie-Weiss Law,the relaxation activation energy(E_a)and conductance activation energy(E_c)were explored,and it is concluded that doping Nd³⁺successfully reduces the concentration of oxygen vacancies(OVs)and the formation of oxygen vacancy clusters,and suppresses the conductivity in BNFO-BTO ceramics.(4)Dielectric capacitors with a high power density and fast charging/discharging rate are regarded as alternatives for energy storage applications.However,lower energy storage density and efficiency are critical issues that have to be addressed for applications as energy storage capacitors.(0.7-x)BiFeO₃-0.3BaTiO₃-x Ba Zn_1/3Nb_2/3O₃+0.1wt%Mn O₂(BFO-BTO-BZNO)ceramics were prepared by the solid-state reaction approach.Both the temperature dependence of dielectric constant and slim P-E hysteresis loops confirm that(BFO-BTO-BZNO)ceramics were relaxor ferroelectrics.Furthermore,the energy storage properties of(BFO-BTO-BZNO)were calculated based on the hysteresis loops and direct measurements of the discharged pulse currents measured at room temperature.The results indicate that the doping of BZNO can adjust the maximum and remnant polarizations of BFO-BTO based bulk ceramics,thereby affecting the energy storage properties.And the maximum energy storage density obtained was 1.61 J/cm~3at 180 k V/cm and room temperature.(5)0.58BiFeO₃-0.3BaTiO₃-0.12Na Nb O₃+x wt%Ca/B/Si/K_0.01/Na_0.02(CBSKN)ceramics were prepared using the traditional high temperature solid-state reaction method.XRD results show that 0.58BFO-0.3BTO-0.12NNO+x wt%CBSKN is a pseudocubic structure,classified to the Pm^<sub>3m space group.The broad peak of dielectric constant as a function of temperature and the slim P-E hysteresis loops of 0.58BFO-0.3BTO-0.12NNO+x wt%CBSKN ceramics indicate that the samples are relaxor ferroelectrics.In addition,the addition of CBSKN glass phase reduced the?P,and the appropriate amount of CBSKN glass can improve the breakdown electric fields of ceramics.The calculation results of energy storage characteristics showed that the maximum energy storage density of CBSKN05 ceramic at 190 k V/cm is 2.13 J/cm~3.Finally,the energy storage stability of the samples was investigated by testing the P-E hysteresis loops at different temperatures and frequencies at 100 k V/cm.The W_recoand?of CBSKN05 ceramics were at 0.65 J/cm~3and77%,with the temperature range of 293 K?363 K.The W_recoand?of CBSKN05 ceramics are in the range of 0.56 J/cm~3?0.64 J/cm~3and 73.28%?78.87%,respectively,with the frequency range of 1 Hz?100 Hz.The results indicated that the CBSKN05 ceramics have better frequency and temperature stabilities.