Fabrication and basic characterization of silicon nitride ceramics as an inert matrix

Silicon nitride ceramics containing cerium as a simulating element of americium were fabricated to clarify proper sintering conditions. Basic properties of sintered specimens were evaluated for utilization to an inert matrix. Commercial powders of silicon nitride and cerium dioxide (16 or 24.6 wt%), and a powder of aluminum oxide or zirconium oxide as a sintering additive (5 wt%) for some specimens were mixed by ball milling in ethanol. Small amounts of stearic acid as a lubricant were also added. The mixed powder was uniaxially pressed into cylindrical pellets. Then, the pellets were embedded in a packing powder composed of 50 wt%-Si₃N₄ and 50 wt%-BN, and sintered at 2023 or 2073 K for 2 h in a 0.1 MPa N₂ atmosphere. Most of the sintered specimens had high densities (>95% TD). Sintered bodies consisted of columnar silicon nitride grains and grain-boundary phase. XRD analysis clarified that the grain-boundary phase contained crystalline compounds of cerium. The thermal conductivities of sintered specimens except for specimens containing aluminum oxide were about 40 W/m K at room temperature.