Study of vanadium doped strontium bismuth niobate tantalate ferroelectric ceramics and thin films.

摘要

First part of the dissertation is the research on the material system strontium bismuth niobate vanadates, SrBi₂(Nb,V)₂O ₉, (SBNV) ferroelectrics. Present research includes two parts: (1) enhancement of ferroelectric and dielectric properties through partial vanadium substitution and (2) thin films of SBNV ferroelectrics by sol-gel processing. The experimental results demonstrated that the partial incorporation of vanadium into the crystal structure resulted in a significantly enhanced ferroelectric and dielectric properties, which include approximately 300% increase in remanent polarization and 100% reduction in coercive field. Such a significant property enhancement was attributed to the fact that the incorporation of V 5+ with smaller radius (58 pm), in comparison with that of Nb 5+ (69 pm), resulted in an increased “rattling space” for spontaneous polarization. It was also found that the incorporation of vanadium improved other properties of the ferroelectrics including reduced DC conductivity and tangent loss. In addition, some preliminary work has been done on the sol-gel processing and film deposition of SBNV ferroelectrics. A sol-gel process has been successfully developed and single phase SBNV ferroelectrics have been obtained after heat treatment at 600–800°C. Smooth dense thin films of SBNV ferroelectrics with an average grain size of ∼100 nm were obtained through sol-gel coating. Second part of the dissertation is the study on the influence of the vanadium doping on the strontium bismuth tantalate, SrBi₂Ta₂O₉, (SBT) system. Partial substitution (10 at%) of pentavalent tantalum ions by pentavalent vanadium ions with a relatively smaller ionic radius in the SBT layered perovskite ferroelectrics leads to an enhanced dielectric constants, a broadened peak, and a reduced stability of layered tetragonal perovskite structure, as evidenced by an increased para-ferroelectric transition temperature. It was found that the frequency dependence of para-ferroelectric transition temperature and broadening of dielectric constant peak in the strontium bismuth tantalate vanadate (SBTV) system resulted from oxygen-vacancy-induced dielectric relaxation, not from diffused phase transition. Post-sintering annealing at 850°C in air appreciably reduced the concentration of oxygen vacancies and, thus, led to a reduced dielectric constants and tangent loss, particularly at high temperatures. In addition, DC conductivity of the SBTV sample was reduced with post-sintering annealing.