The mechanism of oxidation in semiconducting A(=Sr, Ca, ...)TiO3 ceramic materials

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 a t high temperature in a reducing atmosphere. These semiconducting materials are n-type, the electrons being provided by the presence of niobium dopant s and oxygen vacancies. The latter are eliminated through an oxidation trea tment ar moderate temperature (1000-1200 degrees 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 proces s modifies, on cooling, the stoichiometry of thin adjacent layers, which sh ow an excess of strontium vacancies. These defects are so slow that transpo rt through grain boundaries is the limiting step in such a ceramic material . The cation vacancy diffusion coefficient has been determined. The particu lar microstructure leads also to a complicated band structure of the grain boundaries which are resistive while the grains are semiconducting. Its tim e evolution is followed by using resistivity and capacitance measurements.