| Ultrafine AlN powders were synthesized from two different precursors; namely, aluminum isopropoxide and aluminum nitrate. The aluminum isopropoxide was dissolved in water + citric acid, and polymerized in ethylene glycol. The aluminum nitrate was dissolved in water + glucose. The starting solutions were rapidly spray dried to form homogeneous precursor powders, which were thermochemically converted to AlN powders by calcining in flowing nitrogen or ammonia. Uncombined carbon was eliminated from the product powder by heating in flowing ammonia. This treatment avoided undesirable oxidation of the AlN particles.;The synthesized powders were characterized by XRD, SEM, TEM, BET and elemental analysis. It was found that nitridation began at much lower calcination temperatures (1300;AlN powder in which a sintering aid (yttrium oxide or nitride) is uniformly dispersed was obtained by mixing yttrium nitrate in the starting solution, prior to spray drying and calcination. The powder was pressureless sintered at 1800;The measured thermal conductivity of the sintered samples was 183 W/mK. This result compares favorably with the best values reported in the literature. However, on theoretical grounds, the thermal conductivity of pure AlN at room temperature is predicted to be 320 W/mK. Future work will attempt to achieve this theoretical goal, using the recently developed 'gas condensation processing' method for synthesizing ultrapure ceramic powders. |
