THE MICROSTRUCTURE DEVELOPMENTS AND ELECTRICAL-PROPERTIES OF CALCIUM-MODIFIED BARIUM-TITANATE CERAMICS

The microstructure and electrical behaviour of calcium-modified BaTiO3 ceramics of compositions (a) (Ba1-xCa(x)TiO3, (b) Ba(Ti1-xCa(x)O3-x and (c) (Ba1-x/2Ca(x)/2) (Ti1-x/2Ca(x)/2)O3-x/2 have been investigated. These characteristics are observed to vary systematically with the cationic ratio, alpha = (Ba + Ca)/Ti, rather than the amount (x-value) and sites that Ca2+ ions are supposed to occupy. A large uniform grain microstructure and normal dielectric behaviour are obtained for alpha less-than-or-equal-to 1.01 samples (groups I and II), whereas an ultra-fine grain microstructure and diffuse phase transformation (DPT) characteristics are observed for alpha greater-than-or-equal-to 1.08 samples (groups III). The latter is proposed to be the suppression of tetragonal to cubic transformation due to the formation of second phase. When the sintering atmosphere is changed from air to H2/N2 mixture, the electrical resistivity decreases and dielectric loss increases tremendously for alpha = 0.99 samples (group I), whereas the high electrical resistivity and low dielectric loss characteristics are preserved for alpha greater-than-or-equal-to 1.01 samples (groups II and III). The mechanism by which the cationic ratio improves the resistance of the materials to a reducing atmosphere is proposed to be the formation of hexagonal BaTiO3-delta phase which consumes the oxygen vacancies generated.