Microstructure Effects And Its Mechanism Study On The Energy Storage Properties For Ca_0.6Sr_0.4TiO₃ Ceramics

With the rapid development of pulse power technique in hybrid electric vehicle,aerospace exploration and deep oil/gas extraction,energy storage capacitors with high working temperature,high energy density and high reliability have been actively studied.However,the performance of energy storage capacitors is mainly decided by the dielectrics applied.Linear dielectrics Ca_0.6Sr_0.4TiO₃?CST?,with medium dielectric constant,wide band gap,and low temperature capacitance coefficient?TCC?,are considered to be suitable in exploration for high temperature and high energy density dielectrics.However,due to the existence of interfacial polarization,the broaden of P-E loops at high electric field is inevitable,even for linear dielectrics,leading to decreased energy storage efficiency,and inhibiting the achievement of high energy density.Additionally,the migration of charge carriers?like oxygen vacancies?at high temperature can be accelerated,resulting in increased ionic conduction and reduced energy storage efficiency,also limiting the achievement of high energy density.Consequently,the modification of microstructure,including interfacial polarization and oxygen vacancies,is an effective way to improve the energy storage behavior at high temperature.Based on this,CST was selected to be the research object,on the one hand,energy storage behavior?from 25-150??was improved by the regulation of microstructure,like grain boundary interfacial polarization,grain boundary barrier heights and oxygen vacancies,which was modified by structure design,composition doping and preparation technic modulation.On the other hand,the relation between modified microstructure and improved energy storage behavior has been investigated,comprehensively and deeply.Finally conclusions can be drawn as following:Effect of grain boundary modification on the energy storage properties of CST:firstly highly insulating HfO₂ was introduced into CST ceramics as intergranular phase,in this way the interfacial polarization was modified and the breakdown strength?BDS?and energy storage efficiency???was enhanced.CST ceramics with4 wt%HfO₂ additives showed improved average BDS from 23.6kV/mm to 28.9kV/mm,and increased?from 85%to 95%at 24kV/mm,when compared with pure CST.In addition to that,interfacial polarization was quantified and found to demonstrate an inverse relation with energy storage efficiency,leading to the conclusion that the suppressed interfacial polarization should be responsible for improved energy storage behavior in this material system.Secondly,O₂ annealing,N₂ annealing and H₂O₂ dipping were applied on as-sintered CST ceramics,to modify the grain boundary barrier heights,and improve the BDS and?.By this approach,the BDS and?increased to 27.4kV/mm and 92%for CST ceramics with O₂annealing treatments,while decreased to 20.8kV/mm and 83%for CST ceramics with O₂ annealing treatments.In addition to that,grain boundary barrier heights were quantified and found to share the same evolution trend with energy storage efficiency,leading to the conclusion that the increased grain boundary barrier heights should be responsible for improved energy storage behavior in this material system.Effect of Mn doping on the energy storage properties of CST:Mn doping was used to effectively decrease the conductivity of CST ceramics,and improve the energy storage properties,not only at room temperature,but also at 150?.CST ceramics with 0.5 mol%Mn addition showed the optimum energy storage behavior,with high BDS of 37.4kV/mm and 34.4kV/mm,high energy storage density?Wr?of1.82 J/cm³ and 1.09 J/cm³,and high?of 83%and 62%,at 25? and 150?,respectively.Additionally,the defect dipole formation mechanism was proved to be responsible for enhanced energy storage properties,by the study of electron paramagnetic resonance?EPR?,dielectric-frequency spectra,conduction relaxation,and thermally stimulated depolarization current?TSDC?.To be specific,the migration of oxygen vacancies were inhibited by the formation of defect dipoles[Mn_Ti?-V_O?],thus leading to reduced ionic conductivity.Effect of oxygen vacancy modification on the electrical properties of Mn doped CST ceramics:firstly O2 annealing and N2 annealing were applied on as-sintered Mn doped CST ceramics,to modify the grain boundary barrier heights,and improve the energy storage properties.In this research,O₂ annealing was found to result in enhanced resistivity and energy storage efficiency of bulk ceramics,while N₂annealing lead to the opposite results.CST ceramics with 0.5 mol%Mn addition and O₂ annealing treatments showed the optimum energy storage behavior,with high BDS of 39.4kV/mm and 36.4kV/mm,high energy storage density?Wr?of 1.92J/cm³ and 1.19 J/cm³,and high?of 88%and 65%,at 25? and 150?,respectively.Secondly,pure and Mn doped CST were sintered in air,O₂ and N₂,respectively,in this way,not only grain boundaries,but also grains were affected.Ceramics sintered in air were found to demonstrate higher resistivity than those sintered in O₂ and N₂,both for pure and Mn doped CST.Additionally,Mn doped CST ceramics always showed better insulating properties than pure CST,which was a sintering atmosphere-independent phenomenon.Study on the oxygen vacancy-related defect mechanism in Mn doped CST ceramics:the totally different evolution trend of insulating properties resulting from atmosphere annealing and atmosphere sintering was noticed,and clarified,by studying the oxygen vacancy-related defect mechanism in this Mn doped CST system.For the condition of atmosphere annealing,the increased bulk resistivity should be ascribed to increased grain boundary barrier heights,which was modified by the concentration of oxygen vacancies in grain boundary region.However,for the condition of atmosphere sintering,the variation of insulating properties should be attributed to the transition of conduction mechanism,which was modified by oxygen partial pressure?PO₂?,i.e.,the concentration of oxygen vacancies in the lattice.To be specific,the P_O2 for N₂ sintering is in the region of n-type electronic conduction,and the P_O22 for O₂ sintering is in the region of p-type electronic conduction,while the P_O2for air sintering is in the turning point from n-type electronic conduction to p-type electronic conduction,consequently,ceramics sintered in air demonstrated higher resistivity than those sintered in O₂ and N₂.Additionally,holes were trapped by the increase of Mn valence state in O₂ atmosphere,and electrons were trapped by the decrease of Mn valence state in N₂ atmosphere,while the migration of oxygen vacancies were inhibited by the formation of defect dipoles[Mn_Ti?-V_O?]in air atmosphere,which can be reasonable explanation for better insulating properties of Mn doped CST ceramics,no matter sintered in what kind of atmosphere.