The Defect Chemistry And Conductance Mechanism Of SrTiO₃-based Ceramics With Colossal Permittivity And Low Dielectric Loss

Since modern electronic industry has increasingly required the miniaturization of electronic devices,it is urgent to explore the colossal permittivity and low dielectric loss materials with excellent stability in a wider frequency and temperature range.It is of great significance to the development of the miniaturization,multifunctions of electronic equipments and novel energy storage devices.However,there are still many issues in the current research field of colossal permittivity materials:large dielectric loss(>0.05),strong temperature/frequency/electric field bias,lead containing colossal permittivity materials are harmful to the environment,some colossal permittivity materials have poor reliability and short service life due to resistance degradation.Moreover,the interpretation mechanism of its dielectric property source is also controversial.All of above have limited the industrial application process of ceramics with colossal permittivity.The concentration and type of internal defects of ceramics are adjusted by doping with rare earth element,introducing high insulation additives and regulating the heat treatment atmosphere to ST,aiming to improve the dielectric properties of optimized ST-based ceramics.Monitoring the resistance degradation behavior of ST-based ceramics,analyzing the influence of defect chemistry state on dielectric properties and conductance mechanism during heat treatment,and finally it is obtained ST-based ceramics with high insulation resistivity,long service life,colossal permittivity and low dielectric loss,which will promote the industrial application process of this kind of ceramics.The main research work is as follows:Sr1-1.5xYxTiO3 ceramics with colossal permittivity and low dielectric loss were prepared by traditional solid-state method in different sintering atmospheres.The influence of the microscopic defect of Sr1-1.5xYxTiO3 ceramics on the dielectric properties was comprehensively analyzed by XRD,SEM,XPS,EPR,and dielectric property test etc.Sr0.979Y0.014TiO3(SYT14)ceramics sintered in nitrogen obtained excellent dielectric properties with frequency stability:colossal permittivity(CP,εr:～28084@1 kHz,27685@2 MHz),Ultra-low tanδ(～0.007@1 kHz,0.003@2 MHz).Due to the Y3+ions could enter both the A and B-sites,defect dipoles were formed,including[VSr"-Vo¨],[Ti4+·e-Vo¨-e·Ti4+],[YSr·-e·Ti4+],[2Y·Sr-VSr″］,[YSr·-YTi′]and[2YTi′-Vo¨],which mainly contributes to a colossal permittivity and low dielectric loss of the SYT14 ceramics.The ceramics were prepared according to Sr1-1.5.xYxTiO3(valence equilibrium,SYT-V)and Sr1-xYxTiO3(position equilibrium,SYT-S).SYT14-V ceramics(valence equilibrium,Sr0.979Y0.014TiO3)and SYT14-S ceramics(position equilibrium,Sr0.986Y0.014TiO3)were selected for comparative study.It was found that different stoichiometric ratios had influence on the crystal structure and micro morphology of SYT14-V and SYT14-S ceramics.Both SYT14-V and SYT14-S ceramics have obtained CP and lower tanδ.Within the test range of 20 Hz～104 Hz εr is greater than 20000.SYT14-V and SYT14-S ceramics show different charge compensation mechanisms.SYT14-V ceramics have relatively more VSr"and Vo¨,which are associated with other point defects and form more abundant defect dipoles,so the dielectric properties of SYT14-V ceramics show more excellent frequency stability.The redox-active Ce3-/Ce4+ions were selected to be doped into ST ceramics(valence equilibrium,Sr1-1.5xCerTiO3,SCT),and the ceramics were treated by reduction and re-oxidation.The Sr0.985Ce0.01TiO3 ceramics after reduction and reoxidation(SCT10-N-O)have an excellent comprehensive property.In detail,if CP is 12406,tanδ is 0.03(1 kHz),resistivity(ρ)is higher than 1010 Ω·cm(>1 kV/cm,DC),and low resistance degradation rate within 8 hours under a high temperature of 200℃ and a DC electric field of 2～4.5 kV/cm.The redox process of amphotropic Ce3+/Ce4-ions affect the types and states of defect dipoles([Ti4+·e-CeSr¨-e·Ti4+]and[CeSr·-e·Ti4+])in SCT ceramics,which localize the charge carriers,facilitating the acquisition of both the CP and low tanδ in the ceramics.The reduction and re-oxidation process can significantly affect the structure of grain boundary barrier,and the defect chemistry state at the grain boundary in the SCT10-N-O ceramics,which promotes the improvement of p.In order to explore the differences caused by different doped ions in many aspects of ceramics,different rare earth elements(Ln=Ce,Er,Y,Ho,Sm)were doped into ST ceramics according to the Sr1-1.5xLnxTiO3(valence equilibrium)ratio.Sr0.985Ln0.01TiO3 ceramics were selected for comparative study.It is found that Ce3+/Ce4+,Er3+,and Y3+ions may have entered both A and B-sites,showing an amphoteric site-occupation behavior,while Ho3+,Sm3+ only entered A-sites.It makes the Sr0.985Ce0.01TiO3,Sr0.985Er0.01TiO3,and Sr0.985Y0.01TiO3 ceramics have more abundant species of defect dipoles,and can become the favorable stable with colossal permittivity.MgO was selected to be added into SYT14 ceramics with relatively excellent performance.Taking advantage of the high insulation and the physical characteristics of easy formation of the second phase at the grain boundary of MgO,it is intended to introduce an interface into SYT14 ceramics to change the structure of grain boundary barrier.Using the variable temperature in-situ resistance monitoring equipment,during the re-oxidation process of ceramics at high temperature with applied voltage,the resistance degradation process of ceramics is monitored,and the dynamic laws of oxygen vacancy diffusion and electron migration are analyzed.While MgO forms the second phase at the grain boundary,some Mg2+ ions enter the Ti4+ion site of the ST lattice to form point defects MgTi",which can associate with the positively charged point defects in ceramics to form defect dipoles[MgTi"-Vo¨]and[MgTi″-2YSr·].It makes the SYT14-3M,SYT145M and SYT14-7M ceramics having more abundant defect dipoles and promotesεr further improvement.SYT14-7M ceramic has an excellent comprehensive performance,its CP is 28156,tanδ is 0.0136,ρ is 7.02 × 1011 Ω·cm.The second phase with high insulation at the grain boundary of SYT14-xM ceramics can significantly inhibit oxygen vacancy diffusion and electron migration.The diffusion rate and activity of oxygen vacancies in SYT14-xM ceramics are the highest in the temperature range of 400～500℃.