Mechanisms describing multi-phase diffusion reactions in energetic mixtures focused on interface sciences

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2023-05

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Abstract

Solid energetic materials are composed of a solid fuel and solid oxidizer. Solid fuels include metals such as aluminum (Al) which possess a high energy density (31 kJ/g, 84 kg/cm3). Al is used in combination with solid oxidizers such as copper oxide (CuO), molybdenum trioxide (MoO3), ammonium perchlorate (AP), and ferric oxide (Fe2O3). Diffusion forms an important step in determining reaction kinetics during fuel-oxidizer reactions. Diffusion occurs at key locations such as the Al2O3 passivating surface covering the Al particle and at the surface of AP during its decomposition. In this dissertation, methods of enhancing/controlling diffusion-based processes and subsequent energetic heat release are described. Diffusion at the Al2O3 passivation shell was enhanced using an alloy of Al with silicon (Si) i.e., Al-Si. Al-Si particles had a ~0.1 ms earlier achievement of steady flame speed in fast-heating rate flame speed experiments when mixed with MoO3 oxidizer (Al-Si+MoO3) and 5.8% greater early temperature rise in closed Parr bomb experiments on Al/Al-Si. Early heat release for Al-Si is attributed to larger early exotherms seen in Al-Si+MoO3 in thermal equilibrium Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) experiments. Diffusion at the AP surface was enhanced by the development of a new energetic additive with catalytic properties- an amorphous form of a Metal Inorganic Framework (amorphous MIF or a-MIF). This new a-MIF – called amorphous Aluminum Iodate Hexahydrate (a-AIH) is a modified form of a recently developed MIF energetic additive aluminum iodate hexahydrate iodic acid (AIH). a-AIH consists of a rough, porous surface containing Al+ ions acting as catalytic sites, combined with iodine and oxygen that can off-gas to interact energetically with the products of AP decomposition. Open crucible DSC/TGA experiments show that addition of a-AIH to AP in a mixture of 70:30 AP:a-AIH ratio by mass (AP+a-AIH) resulted in an increase in average net heat released during mixture decomposition from +240 J/g net endothermic for pure AP to -1040 J/g net exothermic in AP+a-AIH. Additionally, the mass loss rate during decomposition increases with the addition of a-AIH - i.e., 36°C decrease in peak mass loss rate from 17%/min at 401°C to 18%/min at 365°C. Microscopy and spectroscopy of mixtures and their components were analyzed to identify mechanisms of AP+a-AIH interaction. Enhanced exothermic decomposition in AP+a-AIH was found to be driven by the Al+ ion-rich surface of a-AIH. Al in combination with solid oxidizers in thermite mixtures react to form gaseous aluminum suboxides. In nano-sized Al, gaseous sub oxide Al2O is formed at the surface of the Al particle. This formation of Al2O allows an energetic pathway for oxidation of Al to Al2O to Al2O3 that allows Al to escape the passivation shell otherwise limited by diffusion of condensed Al fuel outwards through the passivating Al2O3 shell. A theoretical analysis of the thermal equilibrium compositions of Al thermite mixtures – Al+CuO, Al+MoO3 and Al+Fe2O3 was conducted using NASA CEA thermal equilibrium analysis software to examine the formation of Al2O. Al2O formation was found to be proportional to the difference in electronegativity between Al and the reduced metal from the metal oxide since it is easier for Al to reduce oxides of more electronegative oxides to form Al2O. Electronegativity of copper (Cu) > molybdenum (Mo) > iron (Fe) > Al corresponding with a peak mass ratio of Al2O to initial Al of 0.76, 0.7 and 0.49 for Al+CuO, Al+MoO3 and Al+Fe2O3 respectively. Al2O energetic pathway was also found suitable to provide heat for other energetic transport processes like reduced oxide (Cu,Fe) metal vapor generation. Addition of alkali metal additives (K, Cs, Na), less electronegative than Al, were found to also increase Al2O formation since such metals create a more reducing environment for Al2O formation. The results of this dissertation, summarized above, provide theoretical and experimental reasoning and proof for the enhancement/control of diffusion-based reactions in energetic mixtures. Such energetic enhancement was achieved using alloyed fuel particles, catalytic/energetic additives for AP and the identification of conditions for gaseous Al suboxide formation to accelerate the diffusion-limited interaction of Al with oxidizer regulated by the passivating Al2O3 shell. Results have application in energetic systems that require enhancement and/or control of energetic heat release such as primers and propellants.


Embargo status: Restricted until 06/2028. To request the author grant access, click on the PDF link to the left.

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Keywords

energetics, solid combustion, diffusion, aluminum combustion, powder, nanomaterials, catalyst, alloy, thermite, solid propellant, ammonium perchlorate, burn rate modifier

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