CARBOTHERMAL FORMATION OF BETA'-SIALON FROM KAOLINITE AND HALLOYSITE STUDIED BY SI-29 AND AL-27 SOLID-STATE MAS NMR

Al-27 and Si-29 magic-angle spinning(MAS) nuclear magnetic resonance(NMR)and complementary X-ray diffraction (XRD) studies of carbothermal formation of sialons from kaolinite and halloysite confirm that the reaction involves the initial formation of mullite (3Al2O3.2SiO2) and amorphous silica. In the presence of carbon, Si-N bonds are formed at almost-equal-to 1200-degrees-C, giving a continuum of silicon oxynitride compositions which become progressively more N-rich. These do not become sufficiently ordered to be detected by XRD until later in the reaction, when crystalline silicon oxynitride, possibly containing a little Al(O'-sialon) and x-phase sialon are formed, followed by beta'-sialon. The O'- and x-phase sialons are transitory, but the beta'-sialon persists throughout the reaction. Si-O bonds survive the destruction of the mullite and persist throughout the reaction, especially with kaolinite starting material. The Si-29 MAS NMR results indicate that Si-C bonds are formed later in the reaction than previously suggested, the SiC phase behaving more like a secondary product than a transitory intermediate. Al-N bonds are not detectable by Al-27 MAS NMR until very late in the reaction (after 8 h firing at 1400-degrees-C), and coincide with the appearance of the secondary product AlN. The implications for the carbothermal reaction sequence in kaolinite and halloysite are discussed.