CORRELATION BETWEEN GRAIN BOUNDARY AND PTC EFFECT IN N-TYPE BaTiO_3

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The n-type BaTiO_3 doped with impurities shows a positive temperature coefficient (PTC) effect at the transition point from the tetragonal to the cubic structure (about 400K). This material is used wide as practical sensors such as thermistor and so on. However, the kinetics of the PTC effect is still unknown. In order to establish this kinetics, we have to identify the carrier and also to investigate the contribution of the grain boundary to the PTC effect. These factors correlate one another. For this end, we have prepared the specimens of BaTiO_3 doped with Gd or La and investigated the dielectric properties and dc resistivities at low temperatures. These experiments have a possibility to identify the carrier. Next, we have changed the density of the high resistivity layers at the grain boundaries in the specimens and measured the PTC effects. By this experiment, we can expect to investigate the contribution of the high resistivity-layers at the boundaries to the PTC effect and also … More to have some knowledge on the factors which enhance this effect.The experiments in this project give us the following results and conclusions. As for the carriers responsible for the PTC effect, there have been many discussions since 1960. There are two candidates for the carriers in n-type BaTiO_3, i.e., one is the electron in the conduction band and the other is the polaron. These discussions have been based upon the experiments at the high temperatures around the phase transition point from the tetragonal to the cubic structure, the results of which cannot, however, present the direct evidences as to the characteristics of carriers. Then, the speculation over natures of the carriers which play an important role on the PTC effect requires the experiments at low temperatures. Our results at low temperature indicate directly that non-adiabatic small polarons contribute predominantly to the conduction in this material. The behaviors of the small polarons obtained in our experiments are found to be in very good agreement with the theoretical predictions. Next, we have prepared two sorts of specimens, one is quenched from 1673 K to the room temperature after the sintering and the other is cooled very slowly in the furnace. The quenched sample is expected to involve high concentration of Ba vacancy but the high resistivity layers do not grow so much at boundaries in this material. However, the sample cooled slowly contains high density of the high resistivity layers. The comparison of the dc resistivity between these sample shows that the PTC effect in the sample cooled slowly is greater than the quenched one by approximately 10^4, although the density of small polarons in the sample cooled slowly is a little less than the quenched one.These facts obtained in our projects indicate that the high resistivity layers at boundaries have influences, very sensitively, upon the PTC effect more than the concentration of carriers. Nevertheless, the investigation of the conduction mechanism is very important in the study of the kinetics of the PTC effect. Less