High-temperature carboreduction of kaolins of different crystallinity

Abstract

The high-temperature (1600–1700°C) carboreduction of `kaolin/carbon black' raw mixture has been processed under argon atmosphere with two kaolins of quite different crystallinity. Although no transition metal catalyst was added, the growth of SiC whiskers (SiCw) proceeded via the vapor–liquid–solid (VLS)-like mechanism, due to the well-known particular role played by iron and titanium impurities contained in the raw kaolin samples. This work shows that the whisker yield essentially depends on the crystallinity of high-temperature phases (mullite and silica), which is obtained with kaolin having the highest degree of crystallinity. This later observation and complementary experiments made by varying the carbon/kaolin ratio indicate that the carboreduction reaction must be processed with well-crystallized kaolin sample and carbon excess to produce hot-pressed high-strength `alumina/SiC whisker' ceramic composites.