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

掺杂的n型BaTiO_3在四方相到立方相的转变点（约400K）表现出正温度系数（PTC）效应。这种材料在热敏电阻等传感器中有着广泛的应用，但其PTC效应的动力学机制尚不清楚。为了建立这种动力学，我们必须确定载体，也要调查的PTC效应的晶界的贡献。这些因素相互关联。为此，我们制备了掺Gd或La的BaTiO_3样品，并研究了它们的低温介电性能和直流电阻率。这些实验有可能识别出携带者。其次，我们改变了样品中晶界处高电阻率层的密度，并测量了PTC效应。通过本实验，我们可以期望研究边界处的高磁导率层对PTC效应的贡献， ...更多信息 通过本项目的实验，我们得到了以下结果和结论。关于PTC效应的载流子，自1960年以来一直有很多讨论。在n型BaTiO_3中有两种候选载流子，一个是导带中的电子，另一个是极化子。这些讨论都是基于从四方相到立方相的高温相变点附近的实验结果，但其结果并不能提供有关载流子特性的直接证据。其次，对PTC效应起重要作用的载流子性质的推测需要低温实验。我们在低温下的结果直接表明，非绝热小极化子在这种材料的导电中占主导地位。实验中得到的小极化子的行为与理论预言非常一致。其次，我们制备了两种试样，一种是烧结后从1673 K淬火到室温，另一种是在炉中缓慢冷却。淬火的样品预计会涉及高浓度的Ba空位，但高电阻率层不会在该材料的边界处生长得太多。然而，缓慢冷却的样品包含高密度的高电阻率层。直流电阻率的比较表明，尽管慢冷样品中的小极化子密度略小于淬灭样品，但慢冷样品的PTC效应比淬灭样品的PTC效应大约10^4倍，这些事实表明，边界处的高阻层对PTC效应的影响非常敏感，对PTC效应的影响大于载流子浓度。然而，在PTC效应动力学的研究中，导电机制的研究是非常重要的。少

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