Enhancing aluminum reactivity by exploiting surface chemistry and mechanical properties

Date

2015-08

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Abstract

Metal-fluoropolymer based systems have drawn quite a bit of attention in the combustion community in recent years due to the exothermic surface chemistry between fluorine (F) and the alumina (Al2O3) shell surrounding aluminum (Al) fuel particles promotes aluminum reactivity. Incorporating a liquid fluorinated oligomer, specifically perfluoropolyether (PFPE), exhibits this surface chemistry while increasing the proximity of fuel and oxidizer.
Flame speeds, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and quadruple mass spectrometry (QMS) were performed for a variety of Al particle sizes blended with PFPE. The results show that the combustion performance of these blends is highly dependent on the Al2O3 concentration and exposed surface area. As Al particle diameter increases from 80 to 120 nm, the Al-PFPE blends exhibit an increase in flame speeds by 48%, but from 120 to 5500 nm, flame speeds decrease by 93%. There is a balance to optimizing Al particle reactivity with PFPE coating between activating Al particles with exothermic surface chemistry versus the unreacted alumina that contributes a thermal heat sink during energy generation. Many Al fueled energetic composites use solid oxidizers that induce no alumina surface exothermicity, such as molybdenum trioxide (MoO3) or copper oxide (CuO). To further understand the role of PFPE in energetic systems, varying concentrations were blended with Al/CuO and Al/MoO3 thermites. Flame speeds, differential scanning calorimetry (DSC) and quadruple mass spectrometry (QMS) were performed for varying percent PFPE blended with Al/MoO3 or Al/CuO in order to examine reaction kinetics and combustion performance. X-ray photoelectron spectroscopy (XPS) was performed to identify product species. Results show that the performance of the thermite-PFPE blends is highly dependent on the bond dissociation energy of the metal oxide. Fluorine-aluminum based surface exothermic chemistry with MoO3 produce an increase in reactivity while the blends with CuO show a decline when increasing the PFPE loadings. These results provide new evidence that optimizing aluminum combustion can be achieved through activating exothermic Al surface chemistry that produces aluminum fluoride.
Another avenue for increasing Al reactivity is to alter its mechanical properties. In bulk material processing, annealing and quenching metals such as Al can relieve residual stress and improve mechanical properties. On a single particle level, affecting mechanical properties may also affect Al particle reactivity. Aluminum particles underwent thermal treatment in order to examine the effect of annealing and quenching on the strain of Al particles and the corresponding reactivity of Al and CuO composites. Synchrotron X-ray diffraction (XRD) analysis of the particles reveals the thermal treatment increased the dilatational strain of the aluminum-core, alumina-shell particles. Flame propagation experiments also show thermal treatments effect reactivity when combined with CuO. An effective annealing/quenching treatment for increasing aluminum reactivity was identified. These results show that altering the mechanical properties of Al particles affects their reactivity.

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Keywords

Aluminum powder, Combustion, Aluminum fluoride, Oligomers, Energetic materials, Exothermic surface chemistry, Dilatational strain, Mechanical properties, Reactivity, Equilibrium kinetics, Flame propagation, Annealing, Quenching

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