Carbothermal formation of β′-sialon from kaolinite and halloysite studied by 29Si and 27Al solid state MAS NMR

Abstract

27Al and 29Si 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 ≈1200 °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 β′-sialon. The O′- and x-phase sialons are transitory, but the β′-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 29Si 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 27Al MAS NMR until very late in the reaction (after 8 h firing at 1400 °C), and coincide with the appearance of the secondary product AlN. The implications for the carbothermal reaction sequence in kaolinite and halloysite are discussed.