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

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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.