Unexpected enhanced reactivity of aluminized nanothermites by accelerated aging

We examine the thermal aging of four Al based thermites chosen among the most commonly used in microenergetic systems: Al/CuO, Al/Fe2O3, Al/Fe3O4 and Al/Co3O4. For each nanothermite system, we applied the modified Friedmann isoconversional method from DSC signals to calculate the kinetic parameters of reactions and the function of the reaction progress in air and argon. As result, it is found that all thermites should be thermally stable for one century when stored at temperatures below 200 degrees C, except for Al/Fe3O4 which readily converts into Al/Fe2O3 before the thermite reaction onset. Then, we designed annealing experiments to a fixed reaction progress : (i) as simulated after 100 years storage at the ambient temperature, (ii) up to 5%. The pressure development and burning rate of aged thermites are compared with as-prepared ones. After annealing at 200 degrees C in air and 400 degrees C in argon, an unforeseen faster reaction and pressurization rates are observed for Al/CuO and Al/Fe2O3 thermites. STEM and EDX show that the annealing provokes a modification of the aluminum particles structure accompanied with a softening of the alumina which explains the enhanced pressure performances and burn rates. Whereas the same nanothermites, Al/CuO and Al/Fe2O3, annealed at 400 degrees C in air, feature a reduced reactivity due to the over-oxidations of the aluminum core and the formation of crystalline alumina shell. These results reveal the high complexity of aging processes in powdered nanothermites as it provokes, not only a consumption of the heat reservoir, but also a modification of the Al/oxidizer interface morphology and chemistry which has a greater impact on the material reactivity than cumulative heat release and reaction progress evolution.

Automated summary

The study of aging processes in powdered nanothermites reveals the high complexity, as it not only leads to a consumption of the heat reservoir, but also results in a modification of the Al/oxidizer interface morphology and chemistry. The annealing experiments show that Al/CuO and Al/Fe2O3 thermites exhibit increased reaction and pressurization rates after annealing at 200 degrees C in air and 400 degrees C in argon, while the same nanothermites annealed at 400 degrees C in air feature reduced reactivity due to over-oxidations of the aluminum core and the formation of crystalline alumina shell.