Study On Structure,polarization Behavior And Energy Storage Characteristics Of（Bi,A）（Ti,Me）O₃ Perovskite Materials

Relaxor ferroelectrics can be divided into several states with temperature:paraelectric state,ergodic state,and ferroelectric state（or nonergodic state）.In different states,relaxor ferroelectrics exhibit unique electrical properties and can be used in the fields of energy storage dielectrics and piezoelectrics.At present,the widely studied relaxor ferroelectrics are mainly concentrated on Ba Ti O₃ based,Pb Ti O₃ based and Bi_0.5Na_0.5Ti O₃ based solid solution systems.Bi_0.5K_0.5Ti O₃（BKT）has a tetragonal phase at room temperature,and its depolarization temperature is higher than Bi_0.5Na_0.5Ti O₃.The dielectric,ferroelectric,energy storage and piezoelectric properties of BKT-based relaxor ferroelectrics should be paid attention.In this study,Bi_0.5K_0.5Ti O₃（BKT）was used as the parent phase,and several BKT-based relaxor ferroelectric solid solution ceramics were studied.In this article,Bi_0.5K_0.5Ti O₃-Ca Ti O₃（BKT-CT）solid solution ceramics were successfully synthesized,and their structure and electrical properties were studied.When the CT content is 0-18 mol%,the compositions show an obvious tetragonal distortion;when the CT content is 18mol%-21 mol%,the tetragonal distortion is small;when the CT content is 25 mol%,the composition shows an average cubic structure The relationship between the dielectric constant and temperature shows that there is a spontaneous ferroelectric-relaxor phase transition when the CT content is below 18 mol%;there is only one dielectric anomaly when the CT content is above18 mol%.The piezoelectric coefficient decreases with increasing CT content.When the CT content reaches 25 mol%,the piezoelectric coefficient is only 3 p C/N.The electrostrain curve changes from a butterfly shape to a bud shape.The finding suggests a ferroelectric-relaxor phase transition.The electrostrain shows a maximum value（S_m）when the CT content is 15%.The S_mvalue is 0.08%,and the corresponding large-signal piezoelectric coefficient(9^*₃₃ is 133 pm/V.In addition,we also observed double ferroelectric hysteresis loops in 0.79BKT-0.21CT during the ferroelectric-relaxor transition,which may be related to the field-induced phase transition.The single-site disorder relaxor ferroelectric 0.5Bi_0.5K_0.5Ti O₃-0.5Sr Ti O₃（BKT-ST）were modified with four low-melting materials,Bi(Mg_1/2Ti_1/2)O₃（BMT）,Bi(Mg_1/2Zr_1/2)O₃（BMZ）,Bi(Zn_1/2Ti_1/2)O₃（BZT）and Bi(Mg_1/3Nb_2/3)O₃（BMN）.The structural,dielectric and ferroelectric properties were studied.All the compositions show a pesudocubic phase structure.The dielectric constant remains stable in a wide temperature range,and the dielectric loss（（27）0.2）is small in the temperature range,but there is a large difference in effective energy storage density.Among them,0.95(0.5Bi_0.5K_0.5Ti O₃-0.5Sr Ti O₃)-0.05Bi(Zn_1/2Ti_1/2)O₃ has the best energy storage characteristics.Based on the above results,the structure,dielectric,AC impedance,band gap and energy storage characteristics of（1-x）(0.5Bi_0.5K_0.5Ti O₃-0.5Sr Ti O₃)-x Bi(Zn_1/2Ti_1/2)O₃ were studied.The compositions exhibits a macroscopic cubic phase when the BZT content is between 0 and 0.125mol%.The impure phase appears when the BZT content reaches 0.125 mol%.Scanning electron microscopy shows that the number of pore decreases significantly with the BZT content increases from 0 to 0.05%.The room-temperature dielectric constant（1 k Hz）at of all the compositions is between 2200 and 2700,the dielectric loss is about 0.03.All the compositions shows a relatively diffuse relaxor-type dielectric anomalous peak.Further analysis shows that the freezing temperature T_f is around-80 ^oC,and the Burns temperature T_B is in the range from 266 ^oC to 390^oC.The AC impedance spectrum analysis reveals that the carrier activation energy is 1.89 e V,which is about 60%of the band gap 3.28 e V.Therefore,the conduction of BKT-ST-BZT ceramics is contributed by the intrinsic carrier.BKT-ST-0.05BZT has a maximum effective energy storage density of 3.06 J/cm³ under 260 k V/cm.The energy storage efficiency is 88.2%.The energy storage density changes little in the temperature range of 30-150 ^oC.The thermal stability of energy storage properties might be related to a wide-temperature-range ergodic relaxor state.