Engineered Porosity-Induced Burn Rate Enhancement in Dense Al/ CuO Nanothermites

This work investigates the combustion of porous Al/CuO thermites, that is, nanolaminates fabricated with various densities of micron-sized air-filled pores in the range of 0-20 vol %. High-speed videography and pyrometry of the high-temperature propagating flame were used to analyze the effect of porosity on propagation velocity. Incorporating micron-sized pores in Al/CuO nanolaminates results in a faster burn rate (burn rate enhancement of 18% for a pore loading of 20 vol %) while the flame temperature remains the same. Microscopic observations of the flame front in porous nanolaminates show hotspots around each pore upstream of the flame but no advection. Conduction remains the dominant heat transfer mechanism in dense thermite configuration (80% theoretical maximum density) and the causes of burn rate enhancement when pores are embedded into the nanolaminate are found to be the convection of the trapped air inside the pores upon heating together with a possible modification of the reaction chemistry leading to a lowering of the ignition threshold of the thermite around each micron-sized pores. Indeed, this hot gaseous O2 species trapped into the pores diffuse and react with solid Al on the inner wall of the pores to form Al2O3. This gas-phase mediated reaction mechanism in the pores occurs at a lower temperature than the diffusionbased mechanism of the aluminum cations and oxide anions across the alumina shell as in fully dense Al/CuO nanolaminates. The critical size of the pores beyond which their beneficial effect disappears is difficult to estimate, but this study showed that 100 x 100 mu m2 pores have almost no effect on the combustion with an average burn rate increase of only similar to 4% compared to the fully dense nanolaminate part.