Mechanism of structure formation in self-propagating thermite reactions: comparison between alumina and silica as additives in the starting mixture

The mechanism of the selfpropagating thermite reaction befween aluminum and hemalite or the microstruclure formation of the reaction products with the same alumina and silica content in the starting mixture are investigated by means of the combustion front quenching technique. lt is found that the reactants in powder form are converted into products by passing through different transformation zones with changing microstructure and composition. A relatively detailed structural transformation mechanism for the case of alumina as additive, whose final step is the crystallization of alumina and hercynite from a melt, is postulated. On the other hand, ot least at this stage, a result of the same type seems díffícult to obtain when considering silica as additive.