Low temperature liquid-phase-assisted sintering of Si3N4 ceramics as an inert matrix for confinement of minor actinides

Abstract

Silicon nitride ceramics is a candidate for inert matrix, since it is very tolerant for neutron irradiation and it has relatively high thermal conductivity. For these reasons, Si3N4 ceramics containing large amounts of CeO2, as a simulant of transuranium elements, were manufactured by sintering. The low-temperature sintering behavior and chemical and thermal properties of the obtained ceramics are reported. CeO2 (16 or 30 mass%), SiO2 (3 mass%) and MgO (5 mass%) were mixed with fine Si3N4 powder. The mixed powder was uniaxially pressed into a pellet under 60 MPa. These pellets were sintered at 1400~1750°C for 2~4 h in flowing N2 (2 L/min). The pellet density rapidly increased after the sintering between 1400 and 1430°C in the case of 4 h keeping and bulk density of 3.34 g/cm3 was attained at 1450°C (Relative density ~95%), in the case of 16mass% CeO2. Room temperature thermal conductivity increased with sintering temperature from ~10 to ~40W/mK after sintering at 1450°C and 1650°C, respectively. High temperature thermal conductivity of sintered ceramics at 1650°C was approximately four times as high as that of UO2 up to 800°C. The dissolution rate of the sintered pellets for 3mol/L HNO3 solution at 80°C was measured. After 200 h, almost 25% of grain boundary phase was eluted from the pellet containing 16mass% CeO2.