The structure and phase composition of nano-silicon as a function of calcination conditions of diatomaceous earth

Abstract

The powder characteristics of nanostructured silicon produced via Magnesiothermic Reduction Reaction (MRR) of raw (amorphous) and calcined (crystalline) diatomite powders were studied. Magnesiothermic reduction reaction of diatomaceous earth samples was carried out in an argon-filled electric furnace at 700 °C for 2.5 h. Both X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) analysis of calcined diatomite powders confirmed the crystallization of opal to cristobalite and the preservation of the nano-scale morphology of natural diatomite after calcination at 1200 °C for 4 h. Raw and calcined diatomite after MRR contained product nanostructured silicon, unreacted silica, and by-product MgO. Silicon with nano-scale morphology was obtained after etching the reduced powder with 1.0 M HCl and 5% HF severally. Among the different grades of nanostructured Si produced via different routes, the one obtained from the diatomite powder calcined under argon flow showed nano-scale morphology identical to that of the precursor powder. X-ray diffraction analysis and Raman spectroscopy confirmed the prevalence of nanostructured silicon. All samples showed intense Raman signals with variations in peak positions confirming a difference in crystallinity between the silicon types. This work provides insights into the effects of different calcination conditions on the final structure of product silicon.