The mechanism of oxidation in semiconducting a(=Sr, Ca,…)TiO3 ceramic materials

Abstract

Ceramic samples of Nb doped (Sr, Ca)TiO3, with a 1% TiO2 excess with respect to the stoichiometry, have been prepared by using liquid phase sintering at high temperature in a reducing atmosphere. These semiconducting materials are n-type, the electrons being provided by the presence of niobium dopants and oxygen vacancies. The latter are eliminated through an oxidation treatment at moderate temperature (1000–1200°C). The oxidation kinetics of ceramic materials and single crystals are very different. The presence of a TiO2-rich liquid phase at grain boundaries during the sintering process modifies, on cooling, the stoichiometry of thin adjacent layers, which show an excess of strontium vacancies. These defects are so slow that transport through grain boundaries is the limiting step in such a ceramic material. The cation vacancy diffusion coefficient has been determined. The particular microstructure leads also to a complicated band structure of the grain boundaries which are resistive while the grains are semiconducting. Its time evolution is followed by using resistivity and capacitance measurements.