Low Gravity Combustion Synthesis: Theoretical Analysis of Experimental Evidences

The results obtained during a parabolic flight campaign sponsored by ESA on March 2002, and related to the combustion synthesis of TiB2 - xTiAl and TiB2 - xTiAl3 composites are reported. Besides classical SHS experiments performed using cylindrical pellets, combustion front quenching experiments using conical samples placed inside a copper block are also conducted. Similar experiments are carried out under terrestrial conditions, for the sake of comparison. As expected, under both low-gravity and terrestrial conditions, it is found that combustion temperature and front propagation velocity decrease as the system exothermicity is reduced, that is, when the aluminide/diboride molar ratio is augmented. However, it is observed that reaction front propagates relatively slower under low-gravity conditions. Consistently, the extinction of the combustion front occurs earlier when the reaction is performed under reduced gravity conditions. A theoretical analysis performed by means of appropriate dimensionless numbers is proposed to provide possible explanations of the main experimental evidences reported in the literature on this subject, including those shown in this work. Specifically, this analysis reveals that the relatively higher-propagation velocity observed under terrestrial conditions may be due to the correspondingly lower-sample porosity change, which in turn limits the thermal conductivity decreasing in the reaction zone. In addition, free convection phenomena taking place in the molten phase formed at the reaction zone are able to justify the difference in combustion wave velocity experimentally observed under terrestrial conditions between top and bottom ignition configurations. Moreover, it is found that finer microstructure typically observed under low-gravity is likely a consequence of the reduced coalescence phenomena taking place in such conditions.