THERMAL-CONDUCTIVITY IMPROVEMENT OF YAG ADDED ALN CERAMICS IN THE GRAIN-BOUNDARY ELIMINATION PROCESS

The primary cause of the thermal conductivity improvement in Y2O3-cont aining AlN ceramics caused by elimination of the grain boundary phase was investigated. Y3Al5O12 (YAG)-containing AlN powder compacts were s ufficiently densified by firing at 1800 degrees C for 30 min, but had low thermal conductivity (90-110 Wm(-1) K-1). Improvement to 245 Wm(-1 ) K-1 and elimination of the grain boundary phases was brought about b y annealing in a carbo-reductive atmosphere at 1850 degrees C for up t o 100 h. During annealing, YAlO3, Y4Al2O9 and Y2O3 appeared as a resul t of the reduction-nitridation of the Al2O3 constituent in the YAG. Su bsequently, these yttrium rich phases acted as oxygen getters for the dissolved Al2O3 which was originally included in the AlN grains. The A l2O3 continuously changed into AlN by reduction-nitridation. The cycli c oxygen removal from the AlN through the grain boundary phase composi tion change is seen as the primary cause for the thermal conductivity improvement.