Herein, we prepare phase-pure ZrB 2 -SiC composite powders by molten-salt-mediated reduction of ZrSiO 4 /B 2 O 3 /activated carbon mixtures with Mg, showing that the phase composition and morphology of the above composites is influenced by firing temperature, B:Zr and C:Si molar ratios, and the amount of excess Mg. Notably, phase-pure ZrB 2 -SiC powder with a ZrB 2 :SiC weight ratio of ~75:25 could be obtained by 3-h firing at 1200 °C, i.e., at a temperature lower than that used for conventional carbothermal reduction by at least 200 °C. As-prepared ZrB 2 -SiC composites exhibited grain sizes of several microns and comprised SiC nanoparticles well distributed in the ZrB 2 matrix. Finally, the oxidation activation energies of the prepared ZrB 2 and ZrB 2 -SiC powders were determined as 326 and 381 kJ/mol, respectively, which demonstrated that the introduction of SiC improved the oxidation resistance of monolithic ZrB 2 .
Highly dispersible SiC powder was prepared using sodium humate as a modifier. The results showed that the ζ-potential absolute value of the modified SiC powder increased from 13.1 to 38.7 mV when the adsorption capacity of sodium humate on the SiC surface was 2.368 mg/g. The slurry with a maximum solid content of 65 vol % was obtained using the modified SiC powder, and the minimum viscosity value was 0.322 Pa s at a solid content of 50 vol %. The adsorption kinetics equation of sodium humate on the SiC surface is t/qt = 0.6253t + 11.0472, which is in accordance with the pseudo-second-order kinetics model. In addition, the isotherm equation, ln qe = 0.461 ln Ce – 0.334, agrees with the Freundlich model. The adsorption energy of sodium humate on the SiC surface was 0.8280 kJ/mol, indicating that the adsorption type was physical adsorption.
Cao Tan,Jianghao Liu,Haijun Zhang,Junkai Wang,Saisai Li,Jianbo Song,Yuxin Zhang,Shaowei Zhang
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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