SYNTHESIS OF CARBIDE CERAMICS VIA REDUCTION AND CARBURIZATION OF OXYANIONS ADSORBED ONTO AN ACTIVATED CARBON MATRIX

Carbide ceramics rank high among the hardest and most chemically resistant materials. Their ability to resist physical and chemical attack under conditions where more traditional materials fail make them very desirable for a number of industrial applications. Their use is limited, however, due to the expensive and energy-intensive methods required to produce them commercially. A more versatile and energy-efficient process for commercial carbide production has been developed by synthesizing micron/sub-micron carbide ceramic particles through the adsorption and subsequent carburization of anions on an activated carbon matrix. Oxyanion solutions containing sodium tungstate, sodium molybdate, or sodium metasilicate are adsorbed onto activated carbon to produce anion-loaded precursors. These precursors are carburized in the presence of a reducing atmosphere consisting of hydrogen, carbon monoxide, and methane to produce carbide crystals on the activated carbon surface. In this study, tungstate (WO42-), molybdate (MoO42-), and silicate (SiO32-) anions were evaluated. Silicon carbide (SiC) whiskers and mixed crystals of tungsten carbide (WC), tungsten semicarbide (W2C), and tungsten (W) were formed via carbothermal reduction using inert and reducing gas atmospheres at temperatures much lower than current commercial practice. Mixed crystals of WC, W2C, and W were synthesized at 950 °C under an atmosphere of 80% CH4, 10% H2, and 10% CO. Molybdenum carbide (Mo2C) was synthesized at temperatures as low as 850oC under an atmosphere consisting of 80% CH4, 10% CO, and 10% H2. Under optimal conditions, conversion to Mo2C and WC exceeded 90%. SiC was synthesized at temperatures as low as 1200 °C under H2. Separation of the WC/W2C/W crystals from the activated carbon matrix has been demonstrated using surfactant-aided density separation methods. Response surface modeling was used to determine optimal conditions for tungstate, molybdate, and silicate adsorption as well as the optimal carburization conditions for the W-loaded and Mo-loaded precursors. Results show that the carburization process is feasible and that it is possible to mathematically model and statistically optimize the production and carburization of the activated carbon precursors