Some Factors Affecting the Magnitude of the PTCR Effect in Semiconducting Barium Titanates

Abstract

Large PTCR (positive temperature coefficient of resistivity) effects of more than seven orders of magnitude have been observed in porous semiconducting barium titanate ceramics with small average grain sizes in the range of 2 to 5μm and porosities between 10 and 20%. It has been shown that the magnitude of the PTCR effect in the porous materials was very sensitive to both the average grain size and the degree of sintering, where densification to a high relative sintered density above 90% which was normally associated with a significant grain growth led to a drastic degradation of the PTCR effect. This paper is concerned with factors affecting the magnitude of the PTCR effect in barium titanate ceramics, in which the influences of both the inclusions and the ununiformity of grain size existing in the starting material on the effect were specifically investigated. They often cause exaggerated graingrowth leading to a small PTCR effect. Mixtures of Al₂O₃ and SiO₂, SiO₂ and TiO₂ were intentionally added in the starting material as inclusions in this study.