Adjusting SiO2 : C mole ratios in rice hull ash (RHA) to control carbothermal reduction to nanostructured SiC, Si3N4 or Si2N2O composites

Abstract

We report here extracting SiO₂ as spirosiloxane [(CH₃)₂C(O)CH₂CH(O)CH₃]₂Si from rice hull ash (RHA) to carefully control the SiO₂: C mole ratios, allowing direct carbothermal reduction to SiC, Si₃N₄, or Si₂N₂O without the need to add extra carbon and as a mechanism to preserve the original nanocomposite structure. We can adjust SiO₂: C ratios from 2 : 15 to 13 : 35 simply by reacting RHA with hexylene glycol (HG) with catalytic base to distillatively extract SiO₂ to produce silica depleted RHA (SDRHA) with SiO₂ contents of 40–65 wt% and corresponding carbon contents of 60–35 wt% with specific surface areas (SSAs) of >400 m² g⁻¹. On heating SDRHA_40–65 at 1400–1500 °C in an Ar, N₂, or N₂–H₂ atmosphere, XRD patterns reveal formation of SiC, Si₃N₄, or Si₂N₂O as the major phase with some residual hard carbon. SEM studies reveal mixtures of particles and whiskers in the products, which show BET specific surface areas >40 m² g⁻¹ after oxidative removal of excess carbon. Dilute acid and boiling water prewashing of RHA with milling eliminates typical product impurities compared to those found using conventional carbothermal reduction of agricultural wastes, which qualifies the resulting composites as components for electrochemical energy storage devices among other applications, to be reported elsewhere.