Study On Sintering Performance And Dielectric Properties Of SrTiO₃ Boundary Layer Ceramic Capacitors

With the miniaturization of electronic components,higher requirements have been placed on the electrical performance of SrTiO₃boundary layer ceramic capacitors（BLCC）.It is hoped that SrTiO₃ BLCC exibit a weak temperature-and frequency-dependent graint permittivity and as well as a very low dielectric loss over a broad range of temperature and frequency.The electrical properties of materials are closely related to their microstructure and defects.BLCC with large grains and thin grain boundaries tend to have the characteristics of a high dieletric constant,and BLCC with high grain boundary barriers and appropriate concentration of defects usually have low dielectric loss and stable dielectric properties.Firstly,SrTiO₃ powders with controllable size and morphology were prepared by molten salt systhesis.The reaction process and the effects of flux type were analysized.The whole reaction mainly included solid state reaction and molten salt reaction.A small amount of molten salt ions replaced the Sr site of SrTiO₃,and the substitution amount of Na⁺was more than that of K⁺.The morphology of SrTiO₃synthesized with Na Cl flux tended to be cubic.As the content of KCl in the molten salt increased,the content of high-index faces of SrTiO₃grains gradually increased.Secondly,the as-prepared SrTiO₃ powder was sintered to obtain SrTiO₃ceramics.The sintering thermodynamic process of SrTiO₃ceramics was studied,and the effects of composition and sintering atmosphere on the sintering performance of strontium titanate ceramics were studied as well.The results showed that temperature and sintering atmosphere were two important parameters affecting the growth rate of sintering process.The higher the temperature or the higher the oxygen partical pressure,the larger the grain growth constant of SrTiO₃ceramics.For the temperature below 1360℃,the growth of SrTiO₃ceramics followed the classical Arrhenius-type temperature dependence in an air atmosphere,and the growth rate of 0.005 mol Nb/0.005 mol La co-doped SrTiO₃was higher than others.Undoped and Nb-doped samples showed typical liquid sintering and had abnormal grain growth phenomenon,while La doping inhibited the formation of liquid phase,making grain growth relatively difficult and the grain size distribution relatively inhomogenerous.Finally,the sintered ceramics with large grain size and high density were placed in reducing atmosphere for semiconducting treatment,and then placed in an oxide atmosphere to prepare SrTiO₃ BLCC.The grains of SrTiO₃ BLCC were semiconducting and the grain boundaries were insulation.Not only the phase composition,microstructure,element distribution,element valence and defects of the samples,but the barrier height and electrical properties such as dielectric constant,dielectric loss and resistivity were analyzed.The results showed that the prepared SrTiO₃ BLCC had a relatively dense microstructure.Bi penetrated along the grain boundary and partially replaced Sr to form titanium-strontium-bismuth compound.Compared with undoped SrTiO₃ BLCC,the grain boundary barrier and dielectric permittivity of donor-doped samples were higher.Donor-doped samples also had lower dielectric loss and better temperature and frequency stability.This was caused by the existence of an appropriate content of defect dipoles and defect clusters with heterovalent charges in materials.The influence of donor doping on grain conductivity was small,but it would decrease the conduction activation energy of grain boundary,thereby reducing the grain boundary resistance.