SYNTHESIS OF ALUMINUM NITRIDE WHISKERS BY REDUCTION OF ALUMINA-CALCIA COMPOUNDS AND THEIR PROPERTIES

Synthesis of aluminum nitride (AlN) whiskers by reduction-nitridation of alumina-calcia compounds and their chemical properties were investigated. Five kinds of the oxide compounds (CaA₁₂O₁₉, CaAl₄O₇, CaAl₂O₄, Ca₁₂Al₁₄O₃₃, Ca₃Al₂O₆) which were prepared by firing at 1300°C for 240 min in air, were fired at 1800 to 1900°C for 60 to 240 min in a nitrogen (N₂) atmosphere. Transparent AlN whiskers were deposited on the inner surface of carbon crucible in the specimens of CaA₁₂O₁₉, CaAl₄O₇ and CaAl₂O₄. However, no whisker was produced in the specimens of Ca₁₂Al₁₄O₃₃ and Ca₃Al₂O₆. Both diameter and length of the whiskers produced in this study increased with increasing CaO content in the oxide compounds. Since the system in this study includes no iron and/or other metals at the tip of the whiskers, the growth mechanism is thought to be vapor-solid (VS). Transparency of AlN whiskers was maintained after exposing in air for 6 months, and no weight gain was observed through oxidation in air. The whiskers had such a stability in air that extreme low oxidation rates were observed, obeying the parabolic law. The activation energies were 109 kJ/mol at 900-1000°C and 237 kJ/mol at 1000-1200°C, respectively.