Synthesis and multinuclear magnetic resonance studies of some nitrogen-containing ceramic phases

Abstract

This Thesis describes the synthesis of a range of sialon ceramic phases, and their characterisation using multinuclear magnetic resonance and powder X-ray diffraction.Sihcon-29 and aluminium-27 MAS NMR spectroscopies have been used in the past on a range of inorganic systems, with considerable success. In this study, they were applied to phases in M-Si-Al-O-N systems, with M=La, Y, Li, Mg and Ca, leading to an improved understanding of the factors affecting chemical shifts and other NMR parameters. It was found that some structural information on, for example, 0/N ordering was not available from (^29)Si and (^27)Al NMR, but could be obtained from studies of other nuclei. Nitrogen-15 and oxygen-17 were selected for further study. The low natural abundances of these nuclei meant that isotopically enriched materials had to be prepared. The synthesis of a-Si(^15)(_3)N(_4), Mg(^17)O and Si(^17)O(_2), and of enriched sialon materials is described, and also (^15)N and (^17)0 MAS NMR studies of many sialon phases. In particular, the (^15)N and (^29)Si spectra of lanthanum new phase, a phase of previously unknown structure, were combined with a Patterson map from powder XRD data to allow the crystal structure of this phase to be determined. Finally, the feasibility of using other nuclei to study ceramic structures has been investigated, and (^9)Be, (^7)Li and (^139)La NMR spectra of several phases are reported.