Electrical discharge assisted ball milling: a promising materials synthesis and processing method

摘要

Novel devices for reactive ball milling, incorporating high voltage, low current electrical discharges were constructed and their application for materials synthesis and processing investigated. The effects of low frequency, high voltage electrical impulses during milling on fracturing, nitriding and mechanical alloying were studied for a range of metal and ceramic powders. Samples prepared by both conventional milling techniques and electrical discharge assisted milling were examined and compared using standard techniques of metallography, x-ray diffraction and electron microscopy. Reactive milling experiments were performed under different combinations of glow (cold) and spark (hot) discharge conditions and were found to result in completely different reaction paths for the same reacting species. Glow discharge milling generally promoted formation of metastable and nanostructural products and the enhancement of reactions, such as nitration of solids in nitrogen gas. Spark discharge milling was found to promote different reactions, including the direct formation of new phases, such as NiSi, from elemental ingredients, reduction reactions, including formation of magnetite from hematite and reduction of TiN and Si{sub}3N{sub}4, and rapid solid-liquid reactions, including the direct formation silicon carbide from elemental Si and toluene. Spark discharge milling also promoted formation of product in more stable microcrystalline form. For brittle, low electrical conductivity materials, including hematite, ilmenite and alumina, it was found that the electrical discharges associated with this milling method significantly speeded up fracturing, the fracture mechanism involving the bulk breakdown of individual powder particles. For conductive metals, fracturing was found to occur via chipping and shaving from the surface of particles. Discharge assisted milling was particularly useful for the enhancement of reactions of elemental powders with molecular nitrogen, boron and with hydrocarbon liquids. This resulted in increases in reaction rate, changes in the reaction route and significant extension of the range of materials that can be treated in this way. For metals such as Ti and Zr, nitration in nitrogen gas could be greatly speeded up. Other metals/metalloids, including Si, Ge, Fe, and Al, which are either more difficult or impossible to process by conventional milling in molecular nitrogen, were found to react with N{sub}2 using electrical discharge milling. Within the mill, the electrical discharges cause molecular breakdown of the controlled atmosphere, the formation of monatomic gasses and, depending on the species present and discharge conditions, formation of specific types of plasma in the proximity of powder particles. For discharge milling of in a gas atmosphere, processes have similarities with both conventional ion/plasma surface engineering techniques and direct plasma processing of powder particles. However, it is the combined process of electrical discharge and milling which separates it from other techniques and allow new reaction routes to be explored.