Influence of the oxidation on the surface properties of silicon carbide

The effect of oxidation temperatures on the surface chemical properties of silicon carbide powders has been investigated. The various SiC powders have been firstly characterized using granulometry, X-ray diffraction, thermogravimetric analysis, diffuse reflectance infrared Fourier transform spectroscopy, 29Si and 13C MAS NMR spectroscopy. These materials seem to consist of a mixture of the 6H polytype, other polytypes β-SiC and some α-SiC. The experimental results suggest that the high-temperature treatment eliminates the graphitic region and leads to an enrichment in SiOxCy and especially SiO2 units, without any important structural changes. Nitrogen and NH3 gases adsorption data indicate that this conversion (from graphitic region to SiOxCy and SiO2) gives rise to an important decrease of the specific surface area while the hydrophilic character of silicon carbide powders increases. The surface density of acidic sites on SiC treated at 350, 800 and 1000 °C represents respectively about 73, 79 and 91% of the total surface sites occupied by the adsorbing NH3 molecules. These information concerning the dependence of both the hydrophilic–hydrophobic character and the extent of surface acidic sites with the oxidation conditions are confirmed by immersion calorimetry experiments. Finally the calorimetric data, especially the effect of the pH on the enthalpy of immersion of SiC into water, allow additional conclusions to be drawn concerning the dependence of the electrical double layer formation with pH.