Zhenguo Yang

Synthesis of nanocrystalline SiC at ambient temperature through high energy reaction milling

Abstract This study investigated the in-situ synthesis of nanosized crystalline SiC powders at room temperature through high energy ball milling of elemental silicon and carbon mixtures. Milling conditions including the mill design, the milling speed, the milling time and the ball-to-powder weight ratio (i.e. the charge ratio) necessary for the in-situ synthesis were studied. It was found that uniform formation of nanosized crystalline SiC powders within the powder charge could be achieved with a correctly designed attritor and the contamination could be minimized with proper selections of milling conditions. The crystalline β-SiC powders synthesized were themselves in nanosize scale, quite different from many previous studies which have shown that it is the internal grain structure of milled powders that is the “nanocrystalline” component of the powders (typically 5–20 nm), while the powders are themselves typically 0.1 μm to > 1 μm in size. Furthermore, it was found that the product structures generated by high energy reaction milling depended strongly on the milling speed, the charge ratio and the milling time.

Synthesis of nanostructured chromium nitrides through mechanical activation process

Abstract High energy milling prior to nitriding chromium metals has led to the formation of nanostructured CrN. Furthermore, high energy milling has substantially enhanced the chromium nitridation process. Under the same nitridation condition, nearly 100% CrN is formed for Cr powder milled in NH 3 for 24 hours prior to nitridation, whereas little CrN is produced for Cr powder without milling. More nitrides are formed as milling time increases. The enhanced nitridation process is believed to be primarily related to the enhanced reaction kinetic of the milled powder due to the presence of the sorbed nitrogen (when milled in NH 3 ), the reduced crystallite size, large grain boundary areas associated with ultrafine grains, the reduced correlation length of ordered stacking, and possibly the increased defect concentration induced by high energy milling.

Synthesis of nanostructured Si3N4/SiC composite powders through high energy reaction milling

In this study, synthesis of Si 3 N 4 /SiC nanocomposite powders through high energy reaction milling was investigated. Graphite and silicon powders were used as the source of carbon and silicon, respectively, while the source of nitrogen was from either nitrogen or ammonia gases. Effects of milling conditions including the milling speed, milling time, the ball-to-powder weight ratio, the powder mixture composition and cooling conditions on the formation of Si 3 N 4 and SiC were studied. It was found that milling silicon and graphite powders in an ammonia atmosphere followed by annealing in a nitrogen atmosphere is a viable approach for preparing nanostructured Si 3 N 4 /SiC composite powders. Further, the formation of Si 3 N 4 and SiC was found to be affected by the milling speed, milling time, the ball-to-powder weight ratio, the powder mixture composition, cooling conditions and the milling atmosphere. Explanations of these effects were provided based on the fundamental processes that occurred during reaction milling.

Nanostructured TiN powder prepared via an integrated mechanical and thermal activation

Abstract A new process for large-scale production of nanostructured TiN powder is described in this paper. In this process TiO 2 and graphite are used as the starting materials and the carbothermic reduction and the simultaneous nitriding reaction are enhanced by high-energy milling prior to the reactions. The product from this process is nanostructured TiN powder. It is found that under the same nitridation condition, 100% TiN is formed for TiO 2 and graphite powder mixtures with high-energy milling, whereas no TiN is obtained for powder mixtures without milling. Furthermore, more TiN is formed as the time of high-energy milling increases. The enhanced carbothermic reduction and nitridation process have been related to the reduced crystallite size, large grain boundary area, large specific surface area, presence of amorphous phases and the increased defect concentration in the powder mixture introduced by high-energy milling.