Effect of rare earth additives on properties of silicon nitride ceramics

Silicon nitride ceramics with rare earth (Re) compound (5 wt-%) and MgO (3 wt-%) additives were fabricated by spark plasma sintering and following heat treatment. The Re compounds included two groups: ReF3 ((Re=La, Nd, Gd) and Re2O3 (Re=La, Nd, Gd). Specimens show the same tendency in the sintering shrinkage rate, relative density, grain size and bending strength with the increasing Re cation (Re3+) radius both in ReF3 and Re2O3 added samples. However, as to aspect ratios and thermal conductivity, the change rules are completely opposite between the two groups of specimens.