Particle Size Effects on the Burning Velocity and the Range of Flammability in the SHS Process for Mo-Si System

Abstract

Relevant to Self-propagating High-temperatuzre Synthesis (SHS) process for Mo-Si system, burning velocity and range of flammability are examined not only experimentally but also theoretically, by varying Mo particle size in a relatively wide range, as well as mixture ratio of Mo-to-Si, degree of dilution by MoSi2, and initial temperature. Although effects of the mixture ratio, degree of dilution, and initial temperature on the burning velocity are the same as those for other representative systems, the particle size effect has turned out to be quite different from those for these systems. It is found that with decreasing Mo particle size, the burning velocity first increases gradually, reaches a plateau, and then decreases steeply. The promoting effect of this burning behavior can be attributed to an increase in the heat generation rate, caused by an increase in the total particle surface that participates in the reaction; while the suppressing effect can be attributed to dilution by oxides, the portion of which in the particle increases with decreasing particle size. By the same token, it is found that the range of flammability for steady combustion is restricted, due to the heat loss from the flame. In order to further elucidate effects of dominant parameters on the burning velocities, range of flammability, and/or limit of steady combustion, theoretical study has also been conducted, based on the heterogeneous theory for the SHS flame propagation, with oxide-layer thickness over Mo particles taken into account in the formulation. It has turned out that the theoretical result for the burning velocity fairly represents the experimental result. Furthermore, range of steady combustion is found to be bounded by limits, upper one of which is due to suppression of the heat generation rate by increasing particle size, and lower one of which is due to dilution by the oxide-layer over Mo particles. It is also demonstrated that fair degree of agreement exists between the experimental and theoretical results, as far as the trend and approximate magnitude are concemed.