Particle dispersion and connectivity in energetic thin films



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Thermite energetic reactions have been the subject of concentrated study for some time. Although recent advances have suggested that conduction, convection, and hot particle advection play a role in moving the reaction front forward, the factors determining which of these thermodynamic phenomena is dominant is still a subject of much inquiry.

This work deals with the synthesis and characterization of thin film energetic materials consisting of magnesium (Mg) and manganese dioxide (MnO2). Composites were blade cast onto a stainless steel foil substrate with polyvinylidene fluoride (PVDF) binder and n-methyl pyrrolidone (NMP) solvent. The effects of mixing conditions, such as equivalence ratio, film thickness, and solids content on the heat generation and flame speeds of such depositions were examined in order to better determine how mixing parameters affect the reaction properties of thin film energetics. Additives of indium were also added to powdered Mg and MnO2, and the resultant mixture was pressed to a high theoretical maximum density (TMD) to form free standing thin-sheets which maintained their shapes without the need for supplemental filler material. These thin films were ignited, and the impact of varying filler content on the reaction rate was investigated.

The subject of particle connectivity in random media -- or percolation theory -- has a rich history of study from an analytical, computational and experimental perspective. Early theoretical work demonstrated that percolating systems of a single type generally have constant area fractions, although what factor governs systems of polydisperse objects is still a subject of study. Computational Monte Carlo studies found the constant area fractions for many systems, including some with high degrees of polydispersity. Empirical models for percolation thresholds generated by such studies are often heavily caveated, and limited in application. Experimental studies are costly and time consuming, and the assumptions of classical percolation theory often do not reflect the reality of experimental conditions.

Here, we present a computational work that attempts to clarify the effect of polydispersity on percolation thresholds, and an experimental compliment that vets the validity of the assumptions of percolation theory for a thin film energetic system. The computational study demonstrated that the percolation threshold for systems containing disks of two sizes was maximized when the ratio of the disk radii was balanced with the number density of the larger disks and suggested that percolation thresholds for polydisperse systems of disks would be maximized when polydispersity was maximized. Experimentally, indium was added to powdered Mg and MnO2, and it was demonstrated that the highest flame speeds were achieved for the indium contents near the onset of percolation. It was further shown that the percolation behavior of indium in the composite was most consistent with the assumptions of lattice percolation theory.

The main goal of this work is to explore how assembly, mixing and deposition enable thermite composites to burn more efficiently, and from this, extrapolate knowledge about how the heat wave is propagated through the composite during the reaction. To that end, multiple experimental studies were conducted, and predictive percolation models were expanded and applied to the experimental systems to attempt to link observed behaviors to developed theory.



Thermite, Thermal conductivity, Polydispersity, Percolation threshold, Indium