Extended phase homogeneity and electrical properties of barium calcium titanate prepared by the wet chemical methods

Ca-substituted BaTiO3 with extended homogeneity range upto similar to50 mol% CaTiO3 have been prepared by three different chemical routes namely carbonate-oxalate (COBCT), gel-carbonate (GCBCT), and gel-to-crystallite conversion (GHBCT) followed by heat treatment above 1150 degreesC. X-ray powder diffraction (XRD) data show continuous decrease in the tetragonal unit cell parameters as well as co/ao ratio with CaTiO3 content, which are in accordance with the substitution of smaller sized Ca2+ ions at the barium sites. The microstructure as well as the dielectric properties are greatly influenced by the cationic ratio, alpha = (Ba + Ca)/Ti. The grain size decreases with CaTiO3 content for the stoichiometric samples (alpha = 1), whereas ultrafine microstructure is observed in the case of off-stoichiometric samples (a > 1) for the whole compositional range of CaTiO3 concentrations. Sharper epsilon(r)-T characteristics at lower calcium content and broader epsilon(r)-T with decreased epsilon(max), in the higher calcium range are observed in the case of alpha = 1. Whereas nanometer grained ceramics exhibiting diffuse epsilon(r)-T characteristics are obtained in the case of a > 1. The positive temperature coefficient of resistivity (PTCR) is realized for barium calcium titanate ceramics having 0.3 at .% Sb as the donor dopant for higher CaTiO3 (typically 30 mol%) containing samples (alpha = 1), indicating that Ca (2+) ions do not behave as acceptors if they were to substitute at the Ti4+ sites. Whereas the off-stoichiomettic (alpha > 1) ceramics retained high resistivity, indicative of the Ti-site occupancy for Ca2+ in fine grain ceramics. (C) 2004 Elsevier B.V. All rights reserved.