MULLITE FORMATION IN KAOLINITE-ALPHA-ALUMINA

The primary and secondary mullite formation in high purity kaolinite-a lpha-alumina have been studied by conventional processing, XRD, TEM, A EM, and optical microscopy. After 4 h firing at 1300 degrees C or belo w, only monosized primary mullite was formed in platy kaolinite, leavi ng alpha-alumina largely inert. Bimodal mullite crystals demonstrating secondary mullite nucleation and primary mullite growth were observed after 4 h firing at 1400 degrees C. At 1400 degrees C and above, the rapid kinetics of secondary mullite formation, the characteristics of the co-existing glassy phase and the microstructure, as well as the ev idence of liquid phase sintering strongly support that solution-precip itation via a transitory liquid phase was the major mechanism for seco ndary mullite formation. Secondary mullite formation via inter-diffusi on,though also observable, was considered as a minor route in the kaol inite-alpha-alumina reaction.