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.
