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

The nitridation of elemental silicon powder at 900-1475 degrees C was studied by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), XRD, thermal analysis and (29)Si MAS NMR. An initial mass gain of about 12% at 1250-1300 degrees C corresponds to the formation of a product layer about 0.2 mu m 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 Si(2)N(2)O. XRD shows that above 1300 degrees C the Si is rapidly converted to a mixture of alpha-and beta-Si(3)N(4), the latter predominating > 1400 degrees 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 degrees C, the (29)Si MAS NMR chemical shift of the elemental Si changes progressively from about -80ppm to -70ppm, in tandem with the growth of the Si(3)N(4) resonance at about -48ppm. Possible reasons for this previously unreported change in the Si chemical shift are discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.