Phase evolution and sinterability of lanthanum phosphate – Towards a below 600 °C Spark Plasma Sintering

Lanthanum phosphate, due to its interesting thermal and mechanical properties is a widely studied material for refractory applications. Sintering processes have already been proposed to densify this material and drive its microstructure. Inspired by recent progress on low temperature sintering, we investigate a low temperature Spark Plasma Sintering (LowT-SPS) using hydrated precursor. First, lanthanum phosphate thermal behaviour was studied using TGA/DTA and XRD analysis on various heat-treated powders. Their SPS behaviour were explored by in situ dilatometry measurements. As hydrated precursor showed a low temperature densification, samples were sintered at temperatures from 160 °C to 350 °C under 400 MPa. Even if dense and nano-scaled microstructures were obtained, a residual hydration was observed. Finally, a well densified and fine-grained monazite type lanthanum phosphate was obtained at 550 °C and under 200 MPa. Its mechanical properties are then compared to conventional and Spark Plasma Sintered materials.