Nitridation of silicon powder studied by XRD, ²⁹Si MAS NMR and surface analysis techniques

The nitridation of elemental silicon powder at 900-1475 °C was studied by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), XRD, thermal analysis and ²⁹Si MAS NMR. An initial mass gain of about 12% at 1250-1300 °C corresponds to the formation of a product layer about 0·2 μm thick (assuming spherical particles). XPS and XAES show that in this temperature range, the surface atomic ratio of N/Si increases and the ratio O/Si decreases as the surface layer is converted to Si₂N₂O. XRD shows that above 1300 °C the Si is rapidly converted to a mixture of α- and β-Si₃N₄, the latter predominating >1400 °C. In this temperature range there are only slight changes in the composition of the surface material, which at the higher temperatures regains a small amount of an oxidised surface layer. By contrast, in the interval 1400-1475 °C, the ²⁹Si MAS NMR chemical shift of the elemental Si changes progressively from about -80 ppm to -70 ppm, in tandem with the growth of the Si₃N₄ resonance at about -48 ppm. Possible reasons for this previously unreported change in the Si chemical shift are discussed.