Pore Growth in Sintered Thoria

Sintered ThO 2 will sometimes undergo a decrease in density, i.e. become more porous, if held at high temperature for a sufficient time. The bulk density of sol‐gel ThO 2 sintered to 99% of theoretical density at 1200°C will decrease steadily at 1800°C in air to <90% of theoretical after ∼ 15 h. Gas‐release measurements from such porous ThO 2 specimens annealed in air or in vacuum indicate that some gases move through the material freely at 1800°C. The only gases that could contribute to the decrease in density are the oxides of sulfur and carbon, which, if they exist, could be produced or eliminated rapidly by controlling the annealing environment. However, the density decrease appears to depend on factors that affect grain growth, rather than on the generation or elimination of postulated gases.