Porous to dense LaPO4 sintered ceramics for advanced refractories

Lanthanum phosphate (LaPO4), a low thermal conducting and machinable ceramic, exhibits attractive non-reactivity with molten metals, which makes it a promising functional refractory for metallurgical field. However, the high thermal expansion and abnormal grain growth during sintering limit its potential applications in industrial refractories. In the present work, the effect of calcination temperature on sintering of LaPO4 ceramics was investigated. The LaPO4 powder was synthesized via a precipitation method using lanthanum nitrate and orthophosphoric acid. The LaPO4 precipitate was calcined at different temperatures 400, 600, 800, 1000, 1200 and 1400 °C and the effects of calcination temperature on phase stability, particle growth and morphology were studied. The calcined powders were sintered as compacted discs at different temperatures, 1350, 1450 and 1550 °C. The micro-structural changes and densification behavior of the sintered samples were systematically investigated. Chemical resistance of sintered LaPO4 ceramics under acidic and basic conditions was also carried out. The thermal shock resistance of the material was evaluated by a water quenching method. Chemically inert, fine-grained and dense LaPO4 refractory ceramic was successfully fabricated with thermal expansion coefficient controlled to a moderate value of 5×10−6 K−1 from the original value of 10×10−6 K−1.