Development of Bi alloy insulator composites exhibiting metal-insulator-transition-like resistivity change

在本项目中，我们研究了“金属-绝缘子过渡式PTCR材料：双绝缘子复合材料”，开发了一种新型的热敏过流限制器。该复合材料利用铋微粒子组成的金属网络随温度变化的自固定过程。制作了两个原型，不仅用于验证PTCR机制，而且用于制造电气元件。此外，还研究了微波加热对改善普通陶瓷材料PTCR性能的影响。所得结果总结如下。1)双聚酰亚胺复合材料的室温电阻率ρ_<RT>在加热和冷却一个循环后增大。另一方面，在双玻璃复合材料中，即使在反复的热应力作用下，ρ_<RT>仍稳定在~ 1 Ωcm。双玻复合材料的高温电阻率ρ_<HT> > 10^6 Ωcm。2)基于对双玻璃复合材料微观结构的观察，提出了双金属微网络的PTCR机制。用Bi微粒的冻结过程来表示ρ - T关系的滞后特性。3)铋锡合金-玻璃复合材料的电阻率变化临界温度比铋玻璃复合材料低~ 100K，与PTCR BaTiO_3接近。但ρ - T关系表现出半导体特性，ρ_<HT>/ρ_<RT> < 1.4)。一种是填充铋的微管，另一种是由铋溅射在玻璃板上设计的微图案。在这两种元素中，为获得PTCR性能，应将微米或微米以下的铋微粒填充在玻璃基体中。5)微波加热虽然降低了BaTiO_3陶瓷的ρ_<RT>，但PTCR性能不如Bi-glass复合材料。

In this project, we have investigated "metal-insulator transition like PTCR material : Bi-insulator composite" to develop a new type of thermo-sensitive over-current limiter. The composite utilizes self-fixing process in metal-networks composed of Bi micro-particles depending on temperature change. Two prototypes have been made not only for verifying the PTCR mechanism but also for fabricating the electrical element. Besides, effect of microwave heating was studied to improve the PTCR property of the usual ceramics material. Obtained results are summarized as follows.1) The ρ_<RT>, resistivity at room temperature, of Bi-polyimide composite increases after one cycle of heating and cooling process. On the other hand, in Bi-glass composite the ρ_<RT> was kept stable at 〜 1 Ωcm even with repeated thermal stresses. The ρ_<HT>, resistivity at high temperature, of the Bi-glass composite was higher than 10^6 Ωcm.2) We proposed the PTCR mechanism for the Bi-glass composite in terms of "Bi-metal micro-networks" based on the microstructure observations. The hysterisis characteristic in ρ - T relation was expressed by freezing process of Bi micro-particles.3) The critical temperature of the resistivity change in Bi-Sn alloy-glass composite was 〜 100K lower than that of Bi-glass composite, which is close to that of a PTCR BaTiO_3. However, the ρ - T relation shows semiconductor characteristics and the ρ_<HT>/ρ_<RT> is lower than 1.4) We have made two prototypes of Bi-glass with PTCR property. One is micro-tube filled with Bi, and another is micro-pattern designed by sputtered Bi on a glass plate. In both elements Bi micro- particles with micron or lower in size should be packed in glass matrix for obtaining PTCR property.5) Although ρ_<RT> of BaTiO_3 ceramic was reduced by microwave heating, the PTCR property is inferior to that of Bi-glass composite.