Low temperature synthesis of 2H-SiC powders via molten-salt-mediated magnesiothermic reduction
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 SiO 2 -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). SiO 2 could be completely converted into SiC after 3 h at 1073 K, this temperature was about 600 K lower than required by the conventional methods, and the relatively content of 2H-SiC in the final SiC sample reached about 45 wt%. Furthermore, the content of 2H-SiC could be increased to 72 wt% upon firing at 1373 K for 3 h and using about 60 wt% excessive Mg. In contrast to this, only 20 wt% 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 3 h firing at 1073 and 1373 K were about 24 and 39 nm, respectively. A formation mechanism of SiC powder in the case of using molten-salt-mediated magnesiothermic reduction was proposed based on the experimental results.
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