Synthesis of intermetallic compounds by reduction of multicomponent solutions of liquid ammonia

For many years metallothermic reduction has been used in the primary extraction of metals. Recent work in this laboratory has found the commonly held view that metallothermic reduction is strictly a chemical reaction and that the process is rate limited by mass transfer to be incomplete. In a study of the production of tantalum powder by the reaction of K ₂TaF ₇ with sodium in a molten salt medium at 850 °C, it has been shown that there are two dominant kinetic pathways, both involving electron transfer. Furthermore, the overall rate of reaction is limited by electron transport between the reactants. Hence, we have dubbed metallothermic reduction an `electronically mediated reaction' (EMR). The location of the tantalum deposit and its morphology are both governed by the reaction pathway. With knowledge of the EMR concept, a parallel set of experiments was conducted in which tantalum powder was produced by the reaction of TaCl ₅ with sodium, both dissolved in liquid ammonia at -35 °C. Sodium dissolves in liquid ammonia to give Na ⁺ and a solvated electron. The result is a medium that is very potent for metallothermic reduction. The production of tantalum powder by reaction of a solution of TaCl ₅ dissolved in liquid ammonia with a solution of Na dissolved in liquid ammonia has been demonstrated. Beyond this, co-precipitation/co-reduction of metals by first dissolving multiple compounds of the metals in liquid ammonia and then reducing the solution to promote the precipitation of mixed metal product has been demonstrated. Various intermetallic compounds have been produced in this manner, e.g., Nb ₃Al.