Preparation and Formation Processes of HfC Fine Powders by Mg-Thermite Method

Abstract

In the ZrO2-Mg-CH4 system, ZrC fine powders have been prepared by two different methods; (1) direct preparation from ZrO2 and Mg in a CH4 gas flow and (2) reduction of ZrO2 with Mg in an Ar flow and subsequent carbonization of the reduced materials in a CH4 flow. The formation process of the ZrC powders of single phase has also been studied. In the method (1), the appropriate synthesis conditions were Mg/ZrO2 molar ratio≥4, heating temperature≥700°C and heating time=1h. In the method (2), on the other hand, the optimum conditions were as follows; Mg/ZrO2 molar ratio=4, and reduction time=1h in an Ar flow and carbonization time=1h in a CH4 flow at 800°C. The formation process of ZrC powders in the ZrO2-Mg-CH4 system was as follows: in the case of Mg/ZrO2≥4 molar ratio, (1) 600°-; ZrO2+2Mg→α-Zr+2MgO, (2) 650°-800°C; 3CH4+2Mg→Mg2C3+6H2, (3) 650°-750°C; α-Zr+H2→δ-ZrH2, (4) 700°C-; Mg2C3→2Mg+3C, (5) 750°C-; δ-ZrH2→α-Zr+H2 and (6) 700°C-; α-Zr+C→ZrC. Comparison of the two methods (1) and (2) indicated that the former was highly efficient because of an exothermic reaction by the thermite reaction.