Grain-boundary migration and dielectric properties of semiconducting SrTiO3 in the SrTiO3-BaTiO3-CaTiO3 system

Abstract

Chemically induced grain-boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO3 have been investigated, Strontium titanate specimens that had been doped with 0.2 mol% of Nb2O5 were sintered in 5H(2)/95N(2). The sintered specimens were diffusion annealed at 1400 degrees C in 5H(2)/95N(2) with BaTiO3 or 0.5BaTiO(3)-0.5CaTiO(3) (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air, In the case of BaTiO3 packing, grain-boundary migration occurred with the diffusion of BaTiO3 along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO3 layer. In the case of 0.5BaTiO(3)-0.5CaTiO(3) packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO3 layer at the grain boundary.