A shell passivation strategy for micro-scale aluminum particles and power generation applications

Aluminum (Al) particles are widely used as solid fuels in energy generating applications such as rocket propellants, batteries, and powering MEMS devices. The particles are naturally a core-shell composite of aluminum-alumina (Al 2 O 3 ). The passivation shell is a strong barrier to oxygen diffusion and thus limits power generation potential. Recent studies on nano-aluminum (nAl) particles replaced the Al 2 O 3 shell with an oxygen-rich, halogen-based shell composed of [Al(H 2 O) 6 ](IO 3 ) 3 (HIO 3 ) 2 called aluminum iodate hexahydrate (AIH). The abundance of oxidizing species in molecular scale proximity to the Al core, and the relatively low decomposition energy of AIH provided the conditions necessary to increase power from the nAl reactions. However, AIH passivated nAl was highly ignition sensitive and unsafe to handle. Micron-sized aluminum (µAl) particles are less ignition sensitive than nAl particles and are a suitable alternative for extending nAl-shell modification approaches. This study used µAl particles to synthesize a controlled concentration of AIH shell (µAl@AIH). The precipitation reactions required for shell synthesis examined three key variables to control AIH concentration: H 2 O to I 2 O 5 solution ratio, solution temperature, and time in solution. Thermal analysis of the µAl@AIH particles confirmed AIH dehydration and decomposition followed by core oxidation. The µAl@AIH powder showed up to 92% increase in apparent oxidation efficiency compared to standard Al indicating the AIH shell provides increased oxygen accessibility to the core. Accelerated aging studies revealed physical and chemical transformation of AIH, but the new shell structure resulted in a further 1992% increased apparent oxidation efficiency than standard Al. The apparent activation energy for the unaged and aged µAl@AIH powder is 16-34% lower in equilibrium and non-equilibrium conditions compared to standard Al. The results shown here pave the way for the development of new solid fuels with tremendous power generation capabilities compared to the yesteryear particles plagued by a nearly impenetrable native oxidation passivation layer crippling the speed of energy release.