Effects of Gravity on Combustion Synthesis in Heterogeneous Gasless Systems

Abstract

The first systematic experimental investigation of combustion synthesis from elements conducted under low (10 -4 m/s 2 ) gravity conditions is reported. Several classes of gasless heterogeneous reaction systems were studied, e.g.,Ni-Al,Ti-C, and Ni-Al-Ti-B. A variety of gravity-related effects were observed. It was shown that the convection of inert gas, occurring in the reaction chamber during the combustion process in terrestrial conditions, leads to instability of combustion front propagation along the sample. Also, composite materials (Ni 3 Al-TiB 2 ) produced in microgravity have finer size of refractory phase (TiB 2 ) that is more uniformly distributed in the Ni 3 Al intermetallic matrix. The grains formed in terrestrial conditions were approximately 50% larger. The results obtained with quenched samples showed that this difference is related to the buoyancy enhanced coalescence process. For the Ti-C system, dynamics of sample expansion have been determined, and materials with final porosity up to 90% have been synthesized in microgravity conditions without the use of any gasifying additives. Finally, it was observed that the combustion velocity for loose Ni-Al mixtures in microgravity is about three times larger than in terrestrial conditions. This effect may be explained by a change in the primary mechanism of heat transfer through the reaction medium.