Solid-flame: Experimental validation

The tantalum-carbon reactive system possesses a high energy of reaction with an adiabatic combustion temperature of 2743 K, which is significantly below the melting points of the reactants, as well as any intermediate phases and final products. It was suggested that a combustion wave could propagate in Ta+C mixtures solely owing to a solid-solid reaction. However, this combustion process has never been shown to occur without gas-assisted transport. Here, we report preparation of highly pure and pore-free Ta/C composite particles, which were used for experimental validation of the solid-flame. Preparation of these composite particles involves short term high-energy ball milling (HEBM) of tantalum and carbon powders. High-resolution microscopy coupled with three-dimensional reconstruction techniques were used to characterize the volume nanostructure of mechanically fabricated composite particles. It was quantitatively shown that the particles have nano-scale mixing of the reagents and possess high contact surface area between tantalum and carbon. Experiments revealed that the ignition temperature of as fabricated composite particles is 1243 ± 15 K and maximum combustion temperature was shown to be 2487 ± 50 K, which is well below any possible solid-liquid transitions. Utilizing results obtained with composite particles prepared under different HEBM conditions, it is shown that carbon diffusion through the tantalum grain boundaries and subsequent formation of a Ta(C) solid solution defines low temperature ignition of mechanically fabricated particles. The high surface area contact between the Ta and C nano-scale reagents allows the reaction to propagate in a self-sustained manner, owing solely to a solid-state diffusion mechanism.