Influence of thorium dioxide powder synthesis methods on conventional and spark plasma sintering†

We investigated the influence of the synthesis method on the properties of ThO₂ powders and on their sinterability in conventional and spark plasma sintering (SPS). The powders were obtained by precipitation of thorium oxalate or thorium hydroxide, and further processed by low-temperature calcination at 600 °C or hydrothermal conversion at 250 °C. All powders had nanocrystallites <10 nm and a high surface area, which allow the onset of sintering at reduced temperatures. However, the presence of larger voids and non-ideal particle packing hinder full densification in the final sintering step. The powder obtained by hydrothermal decomposition of thorium hydroxide shows the highest specific surface area among all (SSA > 100 m² g⁻¹) and reaches a relative density of ∼90% in conventional pressure-less sintering at 1100 °C.