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(N MR)and complementary X-ray diffraction (XRD) studies of carbothermal f ormation of sialons from kaolinite and halloysite confirm that the rea ction involves the initial formation of mullite (3Al2O3.2SiO2) and amo rphous silica. In the presence of carbon, Si-N bonds are formed at alm ost-equal-to 1200-degrees-C, giving a continuum of silicon oxynitride compositions which become progressively more N-rich. These do not beco me sufficiently ordered to be detected by XRD until later in the react ion, 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 per sists throughout the reaction. Si-O bonds survive the destruction of t he mullite and persist throughout the reaction, especially with kaolin ite starting material. The Si-29 MAS NMR results indicate that Si-C bo nds are formed later in the reaction than previously suggested, the Si C phase behaving more like a secondary product than a transitory inter mediate. Al-N bonds are not detectable by Al-27 MAS NMR until very lat e in the reaction (after 8 h firing at 1400-degrees-C), and coincide w ith the appearance of the secondary product AlN. The implications for the carbothermal reaction sequence in kaolinite and halloysite are dis cussed.