Synthesis of carbon-free Si3N4/SiC nanopowders using silica fume

We have investigated a possible method of synthesizing carbon-free, nano-silicon nitride–silicon carbide (Si3N4/SiC) powders from the waste silica fume for the first time, using the integrated mechanical and thermal activation (IMTA) process. This novel process results in the formation of nano-Si3N4/SiC powders at 1465 °C with crystallite sizes as small as 45 nm. In order to synthesize carbon-free nano-Si3N4/SiC powders, two different approaches, one using the H2 gas and the other using air, have been studied for their effectiveness in removing the free carbon present. It is found that the H2 treatment is not very effective although both Si3N4 and SiC are stable during the H2 treatment. In contrast, removing the free carbon using air is effective, and the limited oxidation of nano-Si3N4 and SiC can be achieved if the air treatment is terminated soon after the free carbon is eliminated. This study has provided a clear pathway and understanding for effectively synthesizing carbon-free, nano-Si3N4/SiC powders from the silica fume.