Thermal, mechanical and chemical properties of sintered monazite-(La, Ce, Nd or Sm)

Monazite powder, prepared by heating rhabdophane-(La, Ce, Nd or Sm) at 600°C for 2 hours in air, was dry-pressed to disks and bars. The disks and bars of monazite could be sintered to the relative density (100·bulk density/true density) of larger than 99 % at 1200°C in air without cracking. The linear thermal expansion coefficient of the sintered monazite-(R) ceramics at 1000°C was 10.0×10−6/°C for R=La, 9.9×10−6/°C for R=Ce, 9.8×10−6/°C for R=Nd and 9.7×10−6/°C for R=Sm. The bending strength was 100±29 MPa for R=La, 183±18 MPa for R=Ce, 97±34 MPa for R=Nd and 135±22 MPa for R=Sm. The monazite did not react with SiO2 or Al2O3 even at 1600°C in air, however, the monazite began to react with CaO above 700 °C in air, forming Ca3(PO4)2 and R2O3 (or CeO2). The monazite ceramics were stable in water, however, they were decomposed by acidic or alkaline aqueous solutions at high temperatures.