Study of Phase Composition of a Siilcon Nitride (Si3N4)-Rare-Earth Oxide Ceramic Sintered under High Pressures

The phase compositions of silicon nitride-rare-earth oxide ceramic (3-Si3N4+0.5La2O3+0.5Pr6O11 (mol.%)) sintered in a pressure range from 5 to 7 GPa, temperature from 600 ℃ to 1 800 ℃ has been studied by X-ray diffraction and SEM observation. The observed results show that the phase forming rule at high p-T is different from that of Si3N4-Al2O3-AlN system. Under high pressure and below 1 600 ℃, the sintering products are mixture of Si3N4 and the polymorphs of the rare-earth oxides. Si3N4 remains -structure even when certain amount of rare earth oxides has dissolved in. when the temperature exceeds 1 600 ℃ and pressure is above 6 GPa, a new single phase belonging to orthorhombic system is formed with lattice parameters a=1.298 3 nm, b=0.814 0 nm, c=0.428 5 nm.