Optimising the performance of SiC-based varistors through composition and microstructure control

SiC varistors are employed as surge arrestors in high power/high energy niche applications. Using a model formulation based on 50 % SiC and 50 % clay plus graphite, the effects of SiC grain size, composition and graphite content were investigated. Disc and toroidal samples were sintered at 1130 °C under a reducing atmosphere; products were ∼70–75 % dense. The main phases detected by SEM and XRD were SiC, SiO2 (quartz), mullite, graphite and porosity. With increasing graphite content (zero to 11 wt%) the non-linear coefficient (α) decreased from 5.9 to 2.7, the breakdown field Eb decreased from 3346 to 36 V cm−1, and bulk electrical resistivity fell by three orders of magnitude. As SiC grain size reduced from 120 μm to 10 μm, non-linear coefficients (α) almost doubled (3.7–6.3) and breakdown field (Eb) increased by an order of magnitude (226–2656 V cm−1) as a result of increasing numbers of resistive grain boundaries between the device electrodes. The impurity content of SiC grains had a modest impact on electrical properties. The resistance of grain boundary regions was typically one to two orders of magnitude larger than that of grain cores.