Toward design of the pre-stressed nano- and microscale aluminum particles covered by oxide shell

Prediction based on the recently developed melt-dispersion mechanism of reaction for nanometric (nano) and micrometer (micron) scale aluminum (Al) particles suggests a possible increase in particle reactivity if the alumina shell is pre-compressed and the Al core is pre-expanded. This prediction was checked experimentally by measuring the flame speed for Al and molybdenum trioxide (MoO3) thermites in a semi-confined tube. Pre-stressing was produced by heating particles to several elevated temperatures, holding them at a temperature for 10 min to relax thermal stresses, and cooling them at several rates to room temperature. For the optimal thermal treatment conditions (heating to 105 °C and cooling at 0.13 °C/s), flame propagation speed increased by 31% for nanoparticles and for 41% for micron particles. Cooling at 0.06 °C/s after heating to 105 °C and cooling at 0.06 °C/s and 0.13 °C/s after heating to 170 °C either did not change the flame speed or increased it significantly less. Results are quantitatively consistent with the theoretical predictions based on the melt-dispersion mechanism.