A NEW SYNTHESIS OF BETA'-SIALON USING THE VAPOR-PHASE TECHNIQUE REDUCTION OF KAOLIN

Silicon-aluminum oxynitrides and/or aluminum nitride were prepared by the reduction-nitridation of kaolin in graphite crucibles under hydrog en/nitrogen flow at temperatures in the range 1100-degrees-C-1450-degr ees-C. Almost pure beta'-SiAlON was obtained in less than 24 h at 1200 -degrees-C. At high temperatures (e.g., 1450-degrees-C) and for long r eaction times (e.g., 10 h), beta'-SiAlON is fully reduced to AlN. In m ost preparations, beta'-SiAlON is formed together with small amounts o f AlN. However, the formation of AlN can be limited by using short rea ction times and/or by adjusting the reducing power of the atmosphere, i.e., the N-2/H-2 ratio. Compared to the carboreduction of aluminosili cates which always yields mixtures, the present method leads to almost pure products under appropriate conditions. The temperature for the f ormation of beta'-SiAlON is at least 200 to 400-degrees-C lower than t he temperatures used for the carboreduction of aluminosilicates or sin tering of powders, respectively. A gaseous species formed by the react ion of hydrogen with graphite is suspected to be responsible for the n itriding reduction of kaolinite.