Study On Ni/ceramics PTCR Composites

In this paper, the mechanism, mathematic model and influencing factors of Ni/(Ba,Sr)TiO3 PTC composites are present. According GEM theory, the influence of processing on the properties of composites and the useful ways to improve the properties are discussed. And some glass additives which can lower the room-temperature resistivity and improve the resistivity-jump are investigated. At last, a novel process in which NiO is introduced instead of Ni is explored. This process has potential to excellent performance PTCR. By research on the relation between resistivity and metal volume fraction of composites, it is concluded that Ni particles and (Ba,Sr)TiO3 particles are even distributed in composites which resistance pattern is series & parallel one and agrees with GEM equation. The resistivity of composites can be approximately calculated according to. Deducing from above formula, we get the resistivity-jump formula. The resistivity-jump of composites is dominated by the resistivity and resistivity-jump of ceramic matrix, the resistivity and volume fraction of Ni and structural factor. By analyzing related formula, it is concluded that the resistivity of composites can be controlled by the volume fraction of Ni, and the PTC effect of composites originates the PTC effect of ceramic matrix. And it will improve the properties of composites such as lowering the resistivity of ceramic matrix and Ni, increasing the resistivity-jump of ceramic matrix, decreasing the structural factor.The influence of processing on the properties of composites is complicated. In reducing phase, the room-temperature resistivity of ceramic matrix is lowered and the PTC effect is diminished, and the contact-resistance of composites decreases with the increasing sintering temperature. In oxidizing phase, oxygen diffuse in ceramic and metal particles which causes the ceramic matrix resistivity increased and a little amount of metal oxidized and the PTC effect of ceramic matrix restored.A suitable amount of glass-additives can lower the room-temperature resistivity and enhance the PTC effect which forms liquid phase that aids the solution and diffusion of solid atoms.This novel process has the following advantages: the uniform distribution and the good control over the particle size of metal material, and the high reactivity. This process supplies a new way to metal/ceramic PTC composites with good performance.