(753a) Reduced Aging, Mechanically Activated Aluminum Reactives Incorporating Alternative Fluorocarbons

Though metal-based energetics are widely used for their ease of ignition, high flame temperatures, and large condensed fraction of hot particulate combustion products, many formulations suffer from aging complications due to the porosity/permeability of oxide coatings to oxygen. In particular, magnesium oxide coatings are particularly susceptible to further oxidation. Significant effort has been expended investigating techniques to improve the aging properties of magnesium and other metal fuels. Previous efforts have involved application of organic/fluorocarbon coatings [1] and application of inorganic aluminum oxide coatings have been considered as a technique to stabilize magnesium [2]. However, these techniques generally result in the formation of an ignition-inhibiting barrier between metal particles. 

Arrested reactive milling or mechanical activation has been widely used to enhance the reactivity of heterogeneous energetic materials [3]. Specifically, milling has allowed formulation of magnesium/aluminum alloys [4] and aluminum/poly(tetrafluoroethylene) (PTFE) mechanically activated particles [5] with enhanced ignition and aging properties. However, considering mechanically activated aluminum/PTFE, though the resulting particles have relative combustion enthalpies 60% higher than mixtures of nAl/nPTFE, they posses noticeably worse aging characteristics than micrometer-scale mixtures. Electron microscopy images and surface area measurements from previous work suggest that while particle surface area is lower than comparable mixtures of nanoparticles, oxidation accessible cracks may still be present within particles. This is expected to be due to poor particle consolidation as a result of the low adhesion of PTFE.

In this effort we investigate the effect of alternative fluorocarbon oxidizers having improved adhesion properties as well as a new milling technique incorporating partially solvated fluorocarbon in order to improve the consolidation and aging characteristics of aluminum/fluorocarbon composite energetics. We explore as-milled particle structure using electron microscopy and surface area analysis and measure the aging stability of fuels through microcalorimetry accelerated aging experiments. The ignitability of composite particles pre- and post-aging are explored using an electrically heated filament t-jump ignition experiment and laser ignition.

REFERENCES

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[2]      Wilharm CK. Combustion Performance of Coated Magnesium. The International Pyrotechnics Seminar, Fort Collins, CO: 2008, pp. 1–8.

[3]      Dreizin EL. Metal-based reactive nanomaterials. Prog. Energ Combust 2009;35:141–67. doi:10.1016/j.pecs.2008.09.001.

[4]      Aly Y, Dreizin EL. Ignition and combustion of Al·Mg alloy powders prepared by different techniques. Combust Flame 2014. doi:10.1016/j.combustflame.2014.11.010.

[5]      Sippel TR, Son SF, Groven LJ. Altering Reactivity of Aluminum with Selective Inclusion of Polytetrafluoroethylene through Mechanical Activation. Propell Explos Pyrot 2012;38:286–95. doi:10.1002/prep.201200102.