The phase change from 27R AIN polytypoid to AlN in the presence of oxide ha
s been investigated in order to determine the qualitative behavior of oxyge
n release from AlN grains. The 27R polytypoid (Al9O3N7) powders were mixed
with a stoichiometric amount of Y2O3 and, then hot-pressed at 20 MPa. X-ray
diffraction analysis revealed that the phase change from 27R to AlN starte
d at about 1350 degrees C in the absence of a liquid phase, since the lowes
t liquidus temperature in the system AlN-Al2O3-Y2O3 is reported to be about
1690 degrees C. The result revealed that the diffusion of oxygen out of 27
R grains can progress in part through solid-state reaction at low temperatu
res. However, the reaction was complete at temperatures exceeding 1700 degr
ees C in the presence of liquid phase. Thermal conductivity of the hot-pres
sed samples was also measured and discussed in terms of the phase change as
well as microstructural evolution. (C) 2000 Elsevier Science Ltd and Techn
a S.r.l. All rights reserved.