Processing and characterization of calcined and sintered donor doped barium titanate

Abstract

Formation and development of microstructure of Nb doped BaTiO3 was investigated through the mixed oxides route via the process of calcination based on the formula BaTi1-xNbxO3 where x=0.004, 0.008, 0.016. Calcination temperature of 1300°C was found to be appropriate by XRD data regarding the formation of perovskite structures when Nb doped BaTiO3 patterns were compared with standard BaTiO3 pattern. From the XRD patterns, peaks corresponding to (002) and (200) planes suggested that increasing Nb doping level reduced the tetragonality of barium titanate as found by plotting c/a ratio against doping level. Non-uniform distribution of Nb was observed when the calcined product showed different Nb concentrations at different regions, found in Energy-dispersive X-ray spectroscopy study. Particle size analysis data indicated bimodal distributions of calcined product for all three doping levels being calcined from BaCO3, TiO2 and Nb2O5 where starting raw materials had different sizes and distributions. Moreover, submicron-size particles were produced in each case of the calcined product for all three doping levels. However, with the addition of Nb, particle size was reduced gradually probably due to the pinning effect of Nb on particle size. Above 90% of theoretical density was achieved for samples having different doping levels sintered around 1450°C. Consistency in the grain growth behavior was observed when microstructures were studied by scanning electron microscope. Maximum dielectric constant at room temperature was recorded above 10,000 for the samples containing 0.4 mol% Nb being sintered at 1450°C. Shifting of Curie temperature occurred towards lower temperature than that of pure BaTiO3 due to the addition of Nb. Nb addition has resulted low dielectric constant values beyond 0.8 mol% Nb doping due to loosing of tetragonality and stabilizing cubic or pseudocubic structure which are detrimental for polarization of dielectric materials. In order to generate the significance of variables, a statistical modeling by ANOVA (analysis of variance) was performed on different sintering parameters and frequencies for some particular set of data. It was evident that sintering temperature had stronger effect than doping level of Nb in determining the density of niobium doped BaTiO3 whereas Nb concentration had stronger effect in determining the grain size over sintering temperature. Furthermore, profound effect of frequencies in determining dielectric constant of BaTiO3 was observed when it was compared with holding time during sintering, as applied frequency strongly controlled the polarization of material.