Low temperature synthesis of 2H-SiC powders via molten-salt-mediated magnesiothermic reduction

Abstract

A novel molten-salt-mediated magnesiothermic reduction method was used to prepare 2H-SiC ultrafine powders by using silicon dioxide, active carbon and magnesium powders as raw materials. Phase compositions in the SiO2-2Mg-C system were thermodynamically evaluated by using the FactSage 6.2 package, and the effects of firing temperature, molar ratio of n(C)/n(Si) and amount of Mg on the synthesis of 2H-SiC powders were discussed. Phase compositions and microstructures of as-prepared ultrafine SiC powder were characterized by using X-ray diffraction (XRD), field emission gun scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). SiO2 could be completely converted into SiC after 3h at 1073K, this temperature was about 600K lower than required by the conventional methods, and the relatively content of 2H-SiC in the final SiC sample reached about 45wt%. Furthermore, the content of 2H-SiC could be increased to 72wt% upon firing at 1373K for 3h and using about 60wt% excessive Mg. In contrast to this, only 20wt% SiC was formed in the sample fired under the identical conditions but without using a molten salt medium. The average crystalline sizes of 2H-SiC in the product powders resultant from 3h firing at 1073 and 1373K were about 24 and 39nm, respectively. A formation mechanism of SiC powder in the case of using molten-salt-mediated magnesiothermic reduction was proposed based on the experimental results.