Silicon-29 magic-angle-spinning NMR characterization of silicon carbide polytypes

Abstract

The 29Si MAS-NMR spectra of the 3C, 6H, and 15R silicon carbide polytypes are presented and interpreted. It is shown that there are a finite number of inequivalent silicon sites which are determined by the second-and third-order carbon and first-and second-order silicon coordinations. The spectra of any other S i c polytype may now be predicted. IntroductionHigh-resolution solid-state 29Si magic angle sample spinning nuclear magnetic resonance (MAS-NMR) spectroscopy has become a powerful and popular tool for investigating structures particularly in silicates and zeolites. This technique is equally useful for the study of non-oxide materials. To date, only 3C and 6H S i c polytypes have been studied by MAS-NMR. In this paper

Preparation of Si-Al-N-C Ceramic Composites by Pyrolysis of Blended Precursors

“…2A exhibits two signals at ࢬ16 and ࢬ20 ppm and two shoulders centered atࣽࢬ14 ppm and ࣽࢬ23 ppm, which are péÉÅá~ä fëëìÉ Äó dìÉëí bÇáíçêëW lêÖ~åáÅ-íç-fåçêÖ~åáÅ`çåîÉêëáçå mêçÅÉëë Ñçê mçäóãÉê-aÉêáîÉÇ`Éê~ãáÅë attributable to the SiC 4 environments. [32][33][34][35] The SiC 4 resonances at ࢬ16 ppm seem to be close to the reported chemical shifts for b-SiC. 32 In terms of the resonances of the SiC 4 environments in a-SiC, which has various polytypes e.g., 2H, 4H, 6H, 15R, etc., the signal positions depend on the crystal structures.…”

“…[33][34][35] The intensity ratios high to low frequency were 1শ1শ1 6H and 1শ2শ2 15R . In the spectrum of 4H polytypes, two SiC 4 resonances were observed at ࢬ19.7 ppm and ࢬ22.5 ppm and the intensity ratio was 1শ1.…”

Atomic relaxation and dynamical generation of ordered and disordered chemical vapour infiltration (CVI) SiC polytypes

“…More precisely the first return map suggests to divide them into four groups, on the criterion of both U n and U n+1 . These groups may be referred to as A, B, C and D in exact agreement with the groups of chemical shifts in silicon carbide NMR [19,20]. Going further these groups are split again on the criterion of U n +2 , as shown in fig.…”

“…Careful X-ray investigations of 2H-SiC [17] and 6H-SiC [18] have led to estimates of non-ideal bond lengths and angles in the bulk structure. 29 Si and 13 C NMR studies [19] reported the existence of different kinds of sites in common polytypes (3C, 4H, 6H, 15R), and have been related to the orientations of their layer and the neighbouring ones [19,20]. Ab initio pseudopotential calculations including relaxation effects have been made for 3C, 2H, 4H, 6H and 15R structures [21]: they led to an estimation of the bond lengths in reasonable agreement with the X-ray data, mainly for the 6H polytype.…”

“…Correspondence between the deformations  n , the orientation sequences U n U n+1 U n+2 … and the NMR shift groups of ref. [19]. …”