Diffusion of oxygen out of AlN polytypoid grains

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.