Combustion characteristics of molybdenum–silicon mixtures

Abstract

Molybdenum disilicide was produced from elemental powder compacts of molybdenum and silicon by self-propagating high-temperature synthesis. The effects of the reactant ratio, particle size of silicon, and density of the compact were investigated in a high-pressure microreactor. In order to get the relationships among them, the atomic ratio of silicon to molybdenum (Si/Mo) was changed from the values of 1.0 to 2.6 with steps of 0.2. Stable combustion was observed for values of atomic ratio of silicon to molybdenum of 1.8, 2.0, and 2.2. The SHS-produced material consisted of uniform and single-phased molybdenum disilicide (MoSi2) for the atomic ratio of silicon to molybdenum of 2.0. In the meantime unstable combustion such as oscillatory and spinning combustion was detected for values of atomic ratio of silicon to molybdenum beyond the limits of stable combustion. The SHS-produced material under unstable combustion includes impurities of Mo5Si3, unreacted Mo and Si resulting from the layered structure which gives a lower degree of reaction and poor electrical properties of MoSi2 heating element. A significant amount of unreacted molybdenum (≈17%) was found for the atomic ratio of silicon to molybdenum of 1.0. This may result from the spinning combustion as well as from the low combustion temperature below the melting point of silicon (≈1414°C). The value of criterion α suggested by Shkadinskii et al. to differentiate stable combustion from unstable one, is found to be 0.74 for the production of molybdenum disilicide by self-propagating high-temperature synthesis. Stable combustion was observed for α≥0.74 to give uniform products while unstable combustion was detected for α<0.74 to result in non-uniform and multi-phased products. This critical value should be useful for the industrial production of uniform molybdenum disilicide by self-propagating high-temperature synthesis.