Effects of rare-earth oxide additions and heat-treatment temperature on the transformation and microstructure of silicon nitride

In this paper, mixtures of Si3N4 powder with different added amounts of single rare-earth oxide Re2O3 (Re = Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb) were heat treated at 1600-1800 degreesC and the effects of Re2O3 on the transformation and the microstructure of Si3N4 were investigated. Compared with the original Si,N, powder, the transformation ratio of the Si3N4 powders obviously increased and the phase transformation finishing temperatures decreased by about 100 degreesC when different rare-earth oxides were added. When the heat-treatment temperature was 1700 degreesC, the transformation ratio of the Si3N4 powder was about 100%. When the heat-treatment temperature was 1600 degreesC and the amount of Re-2,O-3 addition was 3 mol%, an anomalous transformation ratio occurred. In addition, it was found that the transformation ratio of Si3N4 changed periodically with the increase of the atomic number of the lanthanide when the heat-treatment temperature was 1650 degreesC or less. The SEM image of the Si3N4 powders obtained at 1700 degreesC indicated that the heavy lanthanide oxides such as Er2O3 and Yb2O3 were very helpful to develop rod-like beta-Si3N4 particles.