Study On Energy Storage Performance Of Sodium Titanate-Based Lead-Free Ceramics

With the rapid development of pulse power technology in the fields of aerospace,hybrid vehicles and ultra-deep oil drilling,the energy storage dielectric capacitors with high energy density,high temperature stability and high reliability are highly demanded.The energy storage performance of dielectric capacitors depends largely on the dielectric materials employed.Therefore,designing and fabrication of dielectric materials with excellent integrated energy storage performance are important in order to meet this requirement.Bi_0.5Na_0.5TiO₃?abbreviated as BNT?is a kind of ferroelectric material with ABO₃-type perovskite structure.The temperature dependent permittivity curve shows double dielectric peaks,which represent the temperatures of T_d and T_m,respectively.The flat dielectric constant plateau forms between the two dielectric peaks,will greatly benefit the temperature stability over a broad application temperature range.In addition,BNT also exhibits a high dielectric constant?⁶00?and saturated polarization(³8?C cm^-2),which has been actively studied for the energy storage applications.In this thesis,Bi_0.51Na_0.47TiO₃ is the main research object.The ceramic end member Ba?Zr0.3Ti0.7?O3 with strong relaxor characteristics was selected to form a solid solution with BNT.On one hand,it introduces relaxor features into the BNT matrix.On the other hand,the breakdown strength of the ceramic system can be improved due to the larger bandgap of Zr⁴⁺when compared to Ti⁴⁺.The synergistic contributions from both features will lead to high energy storage density and high energy efficiency in BNT-BZT100x ceramics.As the BZT content is 35 mol%,BNT-BZT35 ceramic?with the thickness of 0.12 mm?exhibits an average breakdown strength of 250 kV cm^-1,which is much higher than that of BNT matrix(¹80 kV cm^-1);the maximum storage density of 2.8 J cm^-3 and high energy storage efficiency of91%are simultaneously achieved at the electric field of 260 kV cm^-1;in addition,the BNT based ceramics possess fast discharge speed,good temperature stability and fatigue endurance.In order to further improve the maximum polarization of BNT-BZT35 ceramics(29?C cm^-2 at the electric field of 260 kV cm^-1),rare earth element La was selected to dope with the BNT-BZT35 ceramic to prepare a series of BB35-100yLa ceramics.The dielectric constant of the rare-earth doped ceramic is increased,resulting in a higher maximum polarization(31.8?C cm^-2).As the doping content of La³⁺is 4mol%,the maximum storage density of 3.2 J cm^-3 and high energy storage efficiency of 86%are simultaneously achieved at the electric field of 260 kV cm^-1 in BB35-4La ceramic?with the thickness of 0.12 mm?;it also possesses fast discharge speed,good temperature stability and fatigue endurance.In order to improve the energy storage efficiency of BNT-BZT35 ceramic while maintaining the high energy efficiency,the element Nb was selected to dope with the BNT-BZT35 ceramic to prepare a series of BB35-100zNb ceramics.The depolarization temperature is further lowered to near room temperature,thereby realizing the stabilization of the antiferroelectric phase at lower temperature.As the doping content of Nb⁵⁺is 1 mol%,BB35-1Nb ceramic?with the thickness of 0.12mm?exhibits a lower depolarization temperature?T_d⁷3°C?;the maximum storage density of 3.2 J cm^-3 and ideal energy storage efficiency of 93%are simultaneously achieved at the electric field of 280 kV cm^-1;as expected,it also possesses fast discharge speed,good temperature stability and fatigue endurance.