THE FORMATION OF BETA'-SIALON FROM A MONTMORILLONITE POLYACRYLONITRILE COMPOSITE BY CARBOTHERMAL REDUCTION - AN NMR, TGA, XRD AND EM STUDY

The products of carbothermal reduction in N2 of a nanocomposite between dodecylammonium-exchanged montmorillonite and polyacrylonitrile (PAN) have been studied by solid-state Al-27 and Si-29 NMR spectroscopy, X-ray diffraction, transmission electron microscopy and thermogravimetry. Comparison with analogous reactions involving sodium-exchanged montmorillonite and dodecylammonium-exchanged montmorillonite (without PAN) shows that in the presence of PAN, the formation of silica, cordierite or mullite is almost completely suppressed. The only crystalline phase detected between 1000 and 1300-degrees-C was a beta'-sialon, having a much higher Si:Al ratio (7.05:1) than that of the precursor clay (2.44:1). Reduction of the octahedral AlO6 begins near 1200-degrees-C, forming increasing amounts of Al(N,O)4 tetrahedra with temperature, so that by 1600-degrees-C, complete reduction to AIN4 (i.e. bulk AIN) has occurred. In contrast, reduction of the tetrahedral SiO4 is appreciable at 1100-degrees-C, and is almost complete (SiN4 tetrahedra only) by 1200-degrees-C. No intermediate Si(N,O)4 environments are found. By 1600-degrees-C, only the SiC4 environment (i.e. bulk SiC) remains. A mechanism is suggested, involving the formation of alternating slabs of an amorphous aluminosilicate and carbon at 1000-degrees-C, followed by diffusion of silicon from the outer regions of the aluminosilicate band towards the centre, and sequential reduction of Si(OSi)4 and Si(OSi)3 (OAl) groups.