ENHANCEMENT OF GASLESS COMBUSTION SYNTHESIS BY COUNTERFLOW GAS FILTRATION

We study the propagation of combustion waves through porous samples in which two reactions occur. The first is a gasless solid-solid reactio n between two solid species in the porous solid matrix which react to synthesize a solid product. The second is a solid-gas reaction in whic h gas, delivered to the reaction site through the pores of the sample, reacts with one of the solid species to form gaseous products. We con sider the case of counter-flow filtration, in which the direction of g as flow is opposite to the direction of propagation of the reaction wa ve. Our purpose in considering such systems is that the solid-solid re action is sometimes only weakly exothermic or even endothermic, so tha t propagation, and therefore synthesis, in the absence of enhancement of the reaction rate would not be possible. Enhancement is achieved by arranging for counterflow gas filtration, which leads to the solid-ga s reaction. Heat released in the solid-gas reaction raises the tempera ture in the combustion wave, thus increasing the solid-solid reaction rate. Using the large activation energy approximation, which describes narrow reaction zones, we determine the structure and characteristics of the combustion wave.