Preparation And Investigation Of BST-Based Temperature Stable Dielectric Ceramics For High Voltage Capacitor Applications

The investigations and applications of environment-friendly high voltage capacitors with fine properties are imperative due to the miniaturization of electronic equipments as well as the demand of environment protection. This dissertation focuses on the development of dielectric materials with high permittivity, low dielectric loss, high breakdown voltage and low temperature coefficient of capacitor based on barium strontium titanate(BST) ceramics.Twice burden technology was innovatively introduced into the traditional solid state method and for the first time rare earth oxide RE2O3(RE=Y,La,Nd,Dy) and Sb₂O₃ doped barium strontium titanate (BST) dielectric ceramics were obtained successfully utilizing this preparation procedure. The effects of preparing conditions and sample compositions on the characters of dielectric ceramics were investigated respectively. It is revealed that temperature stable dielectric ceramics for high voltage capacitor applications can be synthesized using suitable calcination temperature, sintering temperature as well as duration time after decreasing Sr content and increasing the addition level of RE2O3 and Sb₂O₃.The X-ray diffraction (XRD) meter and environmental scanning electron microscope (ESEM) were employed to confirm the phases and surface morphologies of specimens. It is found that with increasing Sb₂O₃ doping content the BST based dielectric ceramics are turned into fine-grained multiphase compounds from single-phase perovskites. Nevertheless, Sr concentration as well as RE2O3 addition content has few influences on the phases of current dielectric ceramcis. The substitution characteristics of RE2O3 and Sb₂O₃ in BST perovskite structure were illustrated according to the average lattice constant. It is indicated that rare earth ions RE3+ tend to occupy the A sites while Sb3+ ions initially enter A sites as donor dopant and then inhabit B sites as acceptor dopant with increasing doping content. Furthermore, the defect models of present systems were established based on the above replacement traits.The modification mechanisms of Sb₂O₃ in BST based dielectric ceramics were discussed using breakdown and broadening theories. It is put forward that the enhancement of breakdown voltage in BST based dielectric ceramics after doping Sb₂O₃ is mainly attributed to the decrease of porosity and stress induced by electrostrictive strain. Furthermore, the occurrence of phase transformation diffusion and dilution arising from substitution as well as grain boundaries is responsible for the increase of thermal stability in Sb2O3 doped systems.1.6wt%Sb2O3-doped(Ba0.952Sr0.04La0.008)TiO3.004 ceramics with high dielectric constant(εr=2326), low dielectric loss( tanδ=0.0074), high breakdown voltage ( E b=4.5 kV / mm) and high thermal stability(ΔC /C<±17%,-25 oC⁸5 oC) are promising for applications in high voltage capacitor industry as environment-friendly and temperature stable dielectric materials.