Characterizing the thermal reactivity of novel inorganic energetic formulations

Date

2021-05

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Abstract

Characterizing the thermal reactivity of newly synthesized energetic materials mainly inorganic materials such as energetic salts, binders, and metal fuel particles is the means to enhance the combustion of formulations with chemistry tailored for specific purposes under non-equilibrium conditions. The reactivity of new fuel passivation chemistry was examined that is being considered to replace the alumina passivation shell for the core-shell aluminum fuel particle. Before resolving the complexities of reaction in the core-shell composite particle formulated with new shell chemistry, the combustion of the shell chemistry alone was studied. The new shell chemistry is aluminum iodate hexahydrate ((Al(H2O)6(IO3)3(HIO3)2), AIH). To examine the compatibility of AIH with common energetic ingredients, AIH is combined with metal oxides often used in energetic formulations. Three metal oxides: bismuth (III) oxide (Bi2O3), copper (II) oxide (CuO), and iron (III) oxide (Fe2O3) were investigated with AIH. Thermal stability and reactivity were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) and powder X-ray diffraction (XRD) was used to identify the product species at various stages of heating corresponding to exothermic activity. The reactions were exothermic and produced metal iodates, which in turn are also strong oxidizers when combined with metal fuel particles like aluminum (Al). This study showed that metal iodates are the more stable form of metal oxides when reacting in-situ with AIH and could offer an additional chemical pathway for contributing energy to the overall reaction. The DSC-TGA and XRD analysis were performed to resolve the thermal reactivity of a newly synthesized iodinated binder: hexamethylene diamine (HMD) trimer (C20H30N4O2I4) when combined with aluminum and iron oxide (Al + Fe2O3) thermite. Laser ignition studies were also performed to evaluate ignition time and energy, as well as in-situ monitoring of the reaction using a high-speed camera. Thermal analysis of HMD showed four stages of exothermic decomposition plus generation of gas phase iodine species released upon decomposition. Thermal analysis of HMD + Al + Fe2O3 is consistent with laser ignition studies that showed HMD aids ignition and reaction of the ternary mixture. Overall, HMD contributed to heating and ignition as well as provided a source for iodine gas in multiple stages of the thermal reaction. The iodine gas has benefits for neutralizing spore forming bacteria thereby sterilizing environments that may be biologically contaminated. While Al is a common fuel in energetic formulations, magnesium (Mg) particles have similar core-shell composite structures and show potential for many combustion applications. This study showed that the Mg particle surface can be used to regulate fluorination or oxidation reactions depending on the applied heating rate and Mg particle size. The reactivity of 800 nm Mg particles (nMg) was compared to 44 µm Mg particles (µMg) when combined with Perfluoropolyethyer (PFPE), providing both fluorine and oxygen for Mg reactions through slow heating rates (10°C/min) using a DSC-TGA to examine reaction kinetics and fast heating rates (6.0 x 105 °C/min) used a hot wire to ignite a thermal run-away reaction. For nMg powders, the outer Mg(OH)2 surface layer dehydrates and creates highly reactive sites for surface oxidation reactions and greater consumption of Mg through oxidation reactions. For µMg, higher Mg(OH)2 dehydration temperatures stabilize µMg particles and the bulk of reactions occur at elevated temperatures producing higher MgF2 concentrations. Under high heating rate conditions, MgF2 formation is favored over MgO formation for both particle sizes owing to the high reaction temperatures that promote gas phase reactions favoring MgF2 formation. Theoretical analysis using density functional theory (DFT) through cluster models show that the Mg(OH)2 surface is more reactive with fluorine species than MgO, especially at elevated temperatures and also explain the high heating rate reaction pathway that favors fluorination reactions independent of Mg particle size. All these studies provide unique contributions to our understanding of reactions that are important in understanding the overall energy generation processes of energetic formulations. The challenge is extrapolating the kinetic pathways resolved through thermal analysis to the reaction mechanisms associated with the more dynamic, higher heating rate ignition events. These challenges can be better addressed with the foundational kinetics framework provided here.


Embargo status: Restricted until 06/2026. To request the author grant access, click on the PDF link to the left.

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Keywords

Thermal Reactivity, Inorganic, Novel, Characterization, Energetic Materials

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