Sintering behaviour of nanocrystalline ThO2 powder using spark plasma sintering

Thorium dioxide is a refractory ceramic material and is difficult to sinter to high density. Commercial powder starts to sinter in SPS under 70 MPa above 1000 °C and compaction is not finished at 1600 °C. In contrast, a 13 nm nanopowder synthesised via low temperature decomposition of thorium oxalate starts to sinter at ∼500 °C; the onset of sintering shifts to higher temperatures for powders with larger crystallites. The initial crystallite size solely affects the onset of sintering, whereas the final stage of sintering is independent of the initial crystallite size for the powders used in this study. Pellets with density well above 95% of the theoretical density were prepared using SPS at sintering temperature of 1600 °C, 70 MPa pressure and 10 min dwell time.