La-Doped BaTiO<sub>3</sub> Ceramics with Bi<sub>2</sub>O<sub>3</sub> as a Grain Boundary Element

Abstract

It is known that sintered bodies of semiconducting barium titanates or its solid solutions have a large positive temperature coefficient of resistivity above and near the Curie temperature. They have a negative temperature coefficient of resistivity normal as usual semiconductors below the Curie temperature. At the Curie temperature the tetragonal to cubic transition takes place and the resistivity anomaly around the temperature has been related to the transition. At the transition temperature, orthorhombic to tetragonal, however, no anomaly in the resistivity-temperature characteristic has been found on the sintered bodies reported so far.The present authors modified the texture of sintered bodies of semiconducting barium titanate with bismuth oxide penetrated along grain boundaries. When non-ohmic contact electrodes were applied to the texture modified specimens another anomaly in the resistivity-temperature characteristic appeared above the orthorhombic to tetragonal transition temperature in addition to the anomaly above the Curie temperature. The nonlinear current-voltage and capacitance-voltage characteristics measured on present specimens with asymmetric electrodes (non-ohmic contact electrode to the one side and ohmic contact to the other side), showed the existence of a surface barrier-layer in the interface between the non-ohmic electrode and the body with texture modified. The nonlinear curve in the capacitance-voltage characteristic was not expressed by the relation 1/C2=2(Vd-V)/eεsNd (Vd is a built-in voltage at a barrier, εs is a dielectric constant in a barrier-layer and Nd is the density of ionized donor centers) above a certain reverse bias which was dependent on a measuring temperature. It seems reasonable to assume that the surface barrier-layer is made up of a inversion layer and a bismuth oxide layer. The barrier gave rise to the anomaly around a room temperature. Under a higher electric field the effect of the surface barrier becomes negligible small compared with the effects by barriers given across the grain boundaries in the sintered body which is considered to give rise to the anomaly above the Curie temperature. It may be considered that by the addition of bismuth oxide the concentration of trap centers changed; the concentration of trap centers affects the height and width of the barrier between semiconducting grains and of the barrier between the non-ohmic electrode and the pellet surface.