A new synthesis of β’-SiAION using the vapor phase technique reduction of kaolin

Abstract

Silicon-aluminum oxynitrides and/or aluminum nitride were prepared by the reduction-nitridation of kaolin in graphite crucibles under hydrogen/nitrogen flow at temperatures in the range 1100 °C–1450 °C. Almost pure β’-SiAION was obtained in less than 24 h at 1200 °C. At high temperatures (e.g., 1450 °C) and for long reaction times (e.g., 10 h), β’-SiAION is fully reduced to AlN. In most preparations, β’-SiAION is formed together with small amounts of AlN. However, the formation of AlN can be limited by using short reaction times and/or by adjusting the reducing power of the atmosphere, i.e., the N₂/H₂ ratio. Compared to the carboreduction of aluminosilicates which always yields mixtures, the present method leads to almost pure products under appropriate conditions. The temperature for the formation of β’-SiAION is at least 200 to 400 °C lower than the temperatures used for the carboreduction of aluminosilicates or sintering of powders, respectively. A gaseous species formed by the reaction of hydrogen with graphite is suspected to be responsible for the nitriding reduction of kaolinite.