Solid state 29Si magic angle spinning NMR: investigation of bond formation and crystallinity of silicon and graphite powder mixtures during high energy milling

In this study, solid state 29Si nuclear magnetic resonance (NMR), transmission electron microscopy (TEM), and X-ray diffraction (XRD) have been used to study the bond formation and structural changes of Si in Si–graphite powder mixtures during high energy milling. Both TEM and NMR analyses have shown that amorphization of Si occurred under the milling conditions used in this study. Furthermore, the NMR spectra have provided evidence that even under milling conditions that do not result in the formation of crystalline SiC in-situ during milling, most of the Si and C atoms are intimately mixed at atomic levels in the amorphous phase and the nominal composition of the amorphous phase is about 1:1 of the Si:C molar ratio, corresponding to the Si:C molar ratio of the starting powder mixture. Comparison among XRD, TEM, and NMR analyses indicates that the reduction of the integrated area of Si reflections in the XRD patterns is primarily due to amorphization of Si. Based on these results, it is proposed that amorphization of Si is primarily induced by crystallite refinement and mixing of Si and C at atomic levels is assisted by micro-diffusion during milling.