Green Mixture Density for Gasless Systems

As shown earlier in Fig. 2b, the dependence of combustion velocity on green mixture density for gasless systems is frequently characterized by a maximum at some intermediate value (Merzhanov, 1983 Rice, 1991). However, combustion theory [Eq. (20)] suggests that the velocity under adiabatic conditions should increase with increasing density, since higher densities correspond to higher thermal conductivities (Aleksandrov et al, 1985 Kottke et al 1990). Different explanations have been suggested to explain this discrepancy. 

Another hypothesis was suggested by Kirdiyashkin et al. (1981) for the combustion synthesis systems characterized by melting of a reactant metal (e.g., Ti-C, H-B), where capillary spreading may control the combustion process (Shkiro and Borovinskaya, 1976). In these cases, it was suggested that an optimal density oc-ciffs where the volume of pores equals the volume of the molten metal. However, an analysis of the experimental data for the Ti-B system showed that this hypothesis may not be valid over the entire range of particle sizes investigated (Munir and Anselmi-Tamburini, 1989). 

It is well known that powders with high surface areas usually have a large amount of adsorbed gas impurities. In Fig. 40, the dependence of combustion velocity on green density in the Ni-Al system is shown for combustion of mixtures with (curve 2) and without (curve 1) heat treatment of powders to remove adsorbed contaminants. While the untreated samples exhibit a maximum, for the pretreated samples, the velocity increases monotonically with increasing density (Kasat skiy et al, 1991). The monotonic increase of combustion velocity and 

Based on these results, an explanation for the combustion velocity dependence on green mixture density has recently been suggested (Vadchenko et ai, 1996). For relatively low sample densities (po 0.6), escape of desorbed gases in the combustion wave is not limited by permeation and occurs without destruction of the reactant medium structure. In this case, increasing the green density 

This type of propagation may follow three different combustion modes, including surface combustion and layer-by-layer combustion with either incomplete or complete conversion. The specific mode followed depends on the initial amount of reactant gas present in the sample pores. 

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