Designing Antiferroelctric Ceramics Used For Pulse Power Capacitors And Investigation On The Pulse Discharge Process

Pulse power technology has extensive applications in many engineering areas.It is now very urgent to develop pulse capacitor that with high energy density,being able to release large discharge current rapidly and withstanding several thousand times of charge-discharge cycling.Antiferroelectric（AFE）ceramics is a promising candidate for pulse capacitor due to their high energy density and rapid discharge process.Up to now,only companies like NOVOCAP and TRS in US and TDK in Japan can yield the antiferroelectric capacitor products,but they are blocked in China’s technology.Therefore it is of great importance to investigate and understand more about the antiferroelectric ceramics,and to push the development of antiferroelectric ceramic capacitors in China.Based on the above background,this work focused on the PLZST-based AFE ceramics and AgNbO₃-based lead free AFE ceramics,investigated systematically on the energy storage properties and charge-discharge process of those materials and discussed deeply with some scientific problems.The main content and results of this thesis are as follows:As for PLZST-based antiferroelectric ceramics:（1）Designing through the phase diagram for the system Pb_0.98La_0.02（Zr,Sn,Ti）_0.995O₃,AFE ceramic composition with high energy storage density and high energy efficiency was obtained as Pb_0.98La_0.02(Zr_0.35Sn_0.55Ti_0.10)_0.995O₃,which has an excellent properties within PLZST system:the energy storage density W_re is 1.39J/cm³,and the energy efficiencyηis 90%.During the temperature range of 25℃-85℃,a good temperature stability of W_re is obtained:varying 0.2%per degree.As for AgNbO₃-based antiferroelectric ceramics:（2）For the first time,we found that the antiferroelectric perovskite with typical double hysteresis loops locates in a certain area of the diagram of average electronegativity difference-tolerance factor.This has some guiding significance for designing new AFE ceramic composition with high energy storage density.（3）By using the average electronegativity difference-tolerance factor diagram,new ceramic compositions of Ag_1-3xLa_xNb O₃ with enhanced antiferroelectricity and large energy density were obtained.It was found that La³⁺ion in the A-site of AgNbO₃ can effectively reduce the tolerance factor and maitain the average electronegativity in a certain range.Thus the antiferroelectricity of AgNbO₃ was enhanced:the forward switching field and backward switching field both increase with increment of La content,and W_ree was improved.Particularly,the energy density of Ag_0.88La_0.04NbO₃reaches 4.6J/cm³,which is the largest value in lead-free ceramics in present publications.（4）To reduce the remanent polarization P_r and further enhance the energy density of Ag_0.97La_0.01NbO₃,a small amount of Mn was added.It was found that the coexistence of Mn²⁺/Mn³⁺at A-site disrupts the randomly disordered Nb⁵⁺at high temperature,leading to the disappearance of freezing temperature T_f.This results in a stable antiferroelectric phase M₂ over a broad temperature（from-5℃to 247℃）and a good temperature stability of energy density:varying less than 1.1%per 10℃during 25℃¹45℃.However,the pulse discharge performance of ceramic samples under actual load is the fundamental basis to evaluate their usability:（5）An approximate expression of maximum power density p_maxax was obtained through calculating,as p_max（28）EI_max/2S.The calculated result is very close to the measured value,but lager than the measured value above the forwarding switching field E_F.This results from the decreasing of the capacitance of AFE ceramics above E_F.（6）For Pb_0.98La_0.02(Zr_0.35Sn_0.55Ti_0.10)_0.995O₃ ceramic sample,When the working electric field is 8.2kV/mm,the maximum power density reaches 18MW/cm³,and the pulse power properties does not show degradation until 1500 times of charge-discharge cycling;For Mn-doped Ag_0.97La_0.01NbO₃ ceramic sample,a maximum power density of390 MW/cm³ was obtained,which is about 8 times larger than previously reported maximum value（50 MW/cm³）.This reveals two advantages of AgNbO₃-based antiferroelectric ceramics for power electronic application:a relatively high antiferroelectric-ferroelectric phase transition field and a large saturation polarization.（7）Dielectric anomalies was found after pulse discharge process of Pb_0.98La_0.02(Zr_0.35Sn_0.55Ti_0.10)_0.995O₃ ceramics:The dielectric constant rises up rapidly after the pulse discharge process,and it increases with the increment of electric field.But when the working electric field reaches a certain value above E_F,the dielectric constant does not increase anymore.Furthermore,when the working electric filed is above a certain value,the temperature dependant dielectric constant curves show some anomalies:the dielectric peak value increases and the curie temperature T_C decrease.However,the temperature dependant dielectric constant curves could recover to the original state after heating above the phase transition temperature of multicell cubic phase-paraelectric phase.These dielectric anomalies are caused by the polarization effect of electric field.When the working electric field is large enough to induce or partially induce the AFE-FE phase transiton,the intermediate FE state causes elastic strain,which induces prefer orientation of the final AFE state and the observed dielectric anomalies.