Characterisation of ceramic reactions by solid state NMR

The identification of local structural coordinations in aluminosilicates is possible by solid state 29Si and 27Al NMR spectroscopy. The sensitivity of NMR to local structure (nearest neighbour, next nearest neighbour) only, allows both crystalline and amorphous phases to be equally well studied, providing complementary information to traditional characterisation techniques, including X-ray diffraction (XRD) and electron microscopy. A combination of these techniques provides a powerful methodology for materials characterisation. This is illustrated by examining the thermal decomposition of the clay mineral halloysite (Al2Si2O5(OH)4.2H2O). In the amorphous phase formed by dehydroxylation at 600 deg C aluminium is present in AlO4, AlO5 and AlO6 coordinations. The silica and alumina layers become increasingly separated in the amorphous phase with heat treatment between 600 deg C and 1000 deg C. It is shown that the exotherm at 1000 deg C corresponds to the loss of AlO5 units and the formation of very finely divided (<5 nm) γ-Al2O3 which is not readily observable by XRD. Further heat treatment causes increasing recombination of alumina and silica unit at 1400 deg C 3:2 mullite and cristobalite crystallize out. 10 refs., 1 tab., 4 figs