Effect of Al₂O₃ addition on phase reaction of the AIN-Y₂O₃ system

Abstract

It is well known that the existence of oxygen in the sintering of aluminium nitride (AIN) causes the degradation of thermal conductivity. To clarify the role of the oxygen, the effect of Al₂O₃ addition on the phase reaction of the AIN-Y₂O₃ system at high temperatures of 1900 and 1950°C was investigated. In the AIN-Al₂O₃ system, γ-AION and 27R polytypoid were formed at 1900 and 1950°C, respectively, and the ratio of each crystal increased with increasing Al₂O₃ content. The resultant phase was found to be in agreement with the phase diagram given by McCauley and Corbin. In the AIN-Y₂O₃ system, YAM phase or YAM plus unreacted Y₂O₃ were identified. Phase reaction in the AIN-Al₂O₃-2Y₂O₃ system was quite different from that of the AIN-Al₂O₃ system: namely, γ-AION and 27R coexisted even at 1950°C, which is inconsistent with McCauley and Corbin's diagram. It was confirmed by the dilatometric method and phase reaction data using some couples, that Al₂O₃ strongly affected the phase reaction in the AIN-Y₂O₃ system, indicating that the shrinkage of the Al₂O₃-doped specimen was initiated at somewhat lower temperatures. The grain morphology depended on the phase changes shown above, in which AIN and γ-AION were granular, and 27R polytypoid was plate-like.