Advancing energetic material science through high-energy density formulation development and characterization

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2022-05

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Abstract

Metallic fuels, specifically aluminum powder (Al), are prominent in propulsion and ordinance systems. In fact, Al is commonly used to enhance the deflagration zone of explosive formulations. However, due to incomplete combustion, the alumina passivation barrier, and the consumption of available oxygen in energetic formulations, Al still poses as a diffusion limitation to ideal reaction rates. Enter aluminum iodate hexahydrate (AIH) – a high-energy dense oxidizing salt that is shown to increase the response of the same late time explosive effects that Al is known to slow down. AIH has also demonstrated the added effect of participating in reactions on the microsecond timescale versus only the deflagration effects of Al on the millisecond timescale. AIH can be formed on Al (Al-AIH) by chemically stripping the naturally occurring passivation shell with a low pH iodic acid followed by crystallization of precursor material at the core shell interface of Al particles. AIH can also be synthesized as a high-energy dense oxidizer through a similar acid-base precipitation reaction with aluminum hydroxide (Al(OH)3) instead of Al powder. The objective of this dissertation is to enhance Al reactivity through iodinated chemistries and to understand the resulting reaction pathways that open door to more oxidative reactions in energetic formulations. This goal is accomplished by analyzing the thermal behavior of different Al-AIH formulations in equilibrium conditions and theoretically evaluating the molecular interactions occurring at elevation temperatures. A second objective is to characterize AIH for safety considerations and evaluate its sensitivities in comparison to other common oxidizers in propulsion applications.

Al-AIH particles are synthesized from 80 nm Al submerged in iodic acid to form the reactive crystalline layer AIH. The low temperature onset of AIH decomposition was found to contribute to the fast energy release in Al-AIH due to its low apparent activation energy (Ea) of 105.1 kJ/mol, as measured by differential scanning calorimetry (DSC) – similar to other conventional energetic materials.

While the alumina shell is considered a barrier to aluminum oxidation, it can also exothermically react with halogenated species and therefore contribute to the overall energy generated during aluminum particle combustion. Fluorination reactions with alumina have been studied because fluorine is abundant in binder formulations that commonly surround aluminum particles in an energetic mixture. However, iodine has emerged as an alternative halogenated-based binder or oxidizer because iodine gas provides ancillary benefits such as chemical neutralization of biological agents or sterilization of contaminated environments. Density functional theory (DFT) calculations were used to evaluate potential reaction pathways for aluminum-iodine combustion. Relative to fluorinated fragments such as HF and F-, the adsorption energies associated with HI and I- are nearly triple the exchange reaction energy available from fluorination reactions with alumina (-189 and -278 kJ/mol for HI and I-, respectively). However, exchange reactions between iodinated species and the alumina surface are energetically unfavorable. These results explain that through adsorption, alumina surface exothermic reactions with iodine are more energetic than with fluorine fragments. Experiments performed with differential scanning calorimetry (DSC) confirm the higher magnitude of energy generated for iodination compared with fluorination reactions with alumina. Additionally, strong adsorption energies can promote synthesis of new shell chemistries. Adsorption in solution will promote alumina dissolution and iodine precipitation reactions to produce hydroxyl complexes and iodinated species synthesized on the surface of the particle, thereby replacing alumina with alternative passivation shell chemistry.

Neat AIH is also characterized for the first time as an energetic material. The compound was characterized physically and chemically by microscopy, pycnometry, and X-ray crystallography. Further energetic characterization was performed by bomb calorimetry, and impact, friction, and electrostatic discharge ignition sensitivity. AIH demonstrates potential as an alternative propellant owing to its high gas generation properties and therefore its characterized parameters are compared to ammonium perchlorate (AP) and ammonium nitrate (AN).


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Keywords

Aluminum Iodate Hexahydrate, Aluminum Combustion, Reactive Materials, Propellants, Iodates, Reaction Kinetics, Pre-Ignition Reaction, Energetic Materials

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