Enhancing the Performance of SiC-based Varistors through the Use of SPS Processing and Fluxes

SiC varistors, which are used as surge arrestors in high power/high energy applications, often contain high levels of porosity (~25%) which can degrade varistor performance. Starting with SiC-clay varistors (~43 µm grain size SiC) we employed spark plasma sintering (SPS) and flux additions (to enable liquid phase sintering) to reduce the level of porosity and understand its impact on the microstructure and the electrical properties. Sintering by SPS at 1200 °C for 5 minutes increased the density to 94% theoretical, increasing energy density capability by almost a factor of 10 (to ~1100 J cm^-3), but the non-linear coefficient (α) fell by 40% (to 3.5) and breakdown field (Eb) fell by 75% (to ~200 V cm-1). Varistors prepared with nepheline syenite as a flux (replacing clay) which were pressureless sintered at 1200 oC had maximum densities of 80%. The samples in which all the clay was replaced by the flux exhibited exceptional energy density performance (over 10,000 J cm^-3) but suffered reductions in other parameters. Using small grain size SiC (~3 µm) and 40% syenite flux yielded consistently superior properties: low power range α value 12.2, high power range α value 32.5, breakdown field ~5880 (V cm^-1) and very low leakage currents ~10^-3 (mA/cm²).