Diffusion of oxygen out of AlN polytypoid grains

The phase change from 27R AlN polytypoid to AlN in the presence of oxide has been investigated in order to determine the qualitative behavior of oxygen 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 started at about 1350°C in the absence of a liquid phase, since the lowest liquidus temperature in the system AlN–Al2O3–Y2O3 is reported to be about 1690°C. The result revealed that the diffusion of oxygen out of 27R grains can progress in part through solid-state reaction at low temperatures. However, the reaction was complete at temperatures exceeding 1700°C in the presence of liquid phase. Thermal conductivity of the hot-pressed samples was also measured and discussed in terms of the phase change as well as microstructural evolution.