Study On Performance Degradation Under DC Field And Grain Boundary Layer Structure Of SrTiO₃ Ceramic Capacitors

Strontium titanate grain boundary ceramic capacitors have high dielectric constant,good high-frequency characteristics,high voltage resistance,capacitance and microwave characteristics,and are widely used in microwave communication circuit,oscillation circuit,timing or delay circuit,coupling circuit,balanced filter circuit,high frequency noise suppression circuit,radio frequency bypass circuit and microwave integrated circuit.They are suitable for bonding process which make them unreplaceable in many applications.However,there are some service reliability problems in this product at present,which the deterioration of electrical performance under long-term operation is disadvantageous to reliability.So it is significant to research the characteristics of performance changes under aging treatment to improve the reliability.In this thesis,SrTiO₃ grain boundary layer ceramic capacitors were fabricated by two-step sintering method,and the characteristics difference between SrTiO₃ semi-conducting ceramic substrate and SrTiO₃ grain boundary layer ceramic substrate were compared.After oxidized by Bi₂O₃ in air,the density of SrTiO₃ semi-conducting ceramic substrate got improved,the resistivity increased more than 8 orders of magnitude,and the frequency characteristics become stable.In this thesis,the effect of DC bias field on the degradation of electrical properties of SrTiO₃grain boundary layer ceramic capacitors was studied.With the increase of aging time under DC bias,the resistivity decreases continuously,and the degradation behavior of resistivity is directional.When the direction of electric field is the same as that of aging electric field?positive measurement?,the decrease of resistivity is smaller than that of resistivity measured by reverse measurement?reverse measurement?when the direction of electric field is opposite to that of aging electric field.That is to say,the degree of resistivity degradation in the direction of electric field is lower than that in the direction of inverse electric field.Scanning microwave impedance microscopy measurements directly observed the carrier concentration differences on both sides of grain boundaries after aging.The carrier concentration at the grain boundary on the positive side is higher than that on the negative side when DC aging occurs.The difference of carrier concentration on both sides of grain boundary causes the potential barrier asymmetry on both sides of grain boundary,which leads to the difference of positive and reverse resistivity after aging.In this thesis,the grain boundary microstructures of SrTiO₃ semiconducting ceramic substrates and grain boundary layer ceramic substrates after Bi oxidation were observed and analyzed by transmission electron microscopy and electron energy loss spectroscopy.SrTiO₃ceramic substrates have a variety of grain boundaries,ranging in width from several nanometers to hundreds of nanometers.The distribution of Bi at different grain boundaries is different.In some grain boundaries,Bi exists only in the center of grain boundaries,and in some grain boundaries,besides the high concentration of Bi in the center of grain boundaries,Bi diffusion zones with a certain width exist in the grains on both sides of grain boundaries.There are Sr vacancies in Bi diffusion region,and the range of Sr vacancies is smaller than that of Bi diffusion region.The geometrical form of grain boundary region,the distribution of impurity elements and vacancy defects are directly related to the formation and distribution of grain boundary barriers,and the properties and aging properties of grain boundary layer capacitors are related to them.