PTCR Characteristics of Semiconducting (Ba, Ca)TiO<sub>3</sub> Ceramics Fired in Reducing-Reoxidizing Atmospheres

Abstract

Three electrode materials, nickel, palladium and platinum were tested about the ohmic contact to semiconducting BaTiO3 ceramics for the application in multilayer BaTiO3 semiconductors with a positive temperature coefficient (PTC) of resistivity. Semiconducting BaTiO3 ceramics with nickel electrodes showed a good ohmic contact, while palladium and platinum electrodes showed rectifying contacts at the junctions. Semiconducting BaTiO3 ceramics with a PTC characteristic of resistivity were fired at 1350°C in a reducing atmosphere and then annealed at various temperatures in the cooling process and in various oxidizing atmospheres, with a view to fabricate multilayer PTC ceramics having Ni metal or Pd-Ni alloy as internal electrodes. The resistivity anomaly of PTC characteristics increased with increasing oxygen concentration in the annealing atmosphere and with increasing annealing temperature below 1200°C. The resistivities at room temperature were almost independent of the annealing conditions. The reoxidation in annealing occurs probably only at grain boundaries.