Dylan K. Smith

Reaction Kinetics and Combustion Dynamics of I4O9 and Aluminum Mixtures

Tetraiodine nonoxide (I4O9) has been synthesized using a dry approach that combines elemental oxygen and iodine without the introduction of hydrated species. The synthesis approach inhibits the topochemical effect promoting rapid hydration when exposed to the relative humidity of ambient air. This stable, amorphous, nano-particle material was analyzed using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) and showed an exothermic energy release at low temperature (i.e., 180 °C) for the transformation of I4O9 into I2O5. This additional exothermic energy release contributes to an increase in overall reactivity of I4O9 when dry mixed with nano-aluminum (Al) powder, resulting in a minimum of 150% increase in flame speed compared to Al + I2O5. This study shows that as an oxidizer, I4O9 has more reactive potential than other forms of iodine(V) oxide when combined with Al, especially if I4O9 can be passivated to inhibit absorption of water from its surrounding environment.

The water–iodine oxide system: a revised mechanism for hydration and dehydration

Iodic acids are widely studied in atmospheric and biological applications but their inherent hydrophilic properties introduce complexities that affect their functionality and reactivity. We have shown that iodic acid (HIO3) dehydrates directly into iodine pentoxide (I2O5) in contradiction to the generally accepted multi-step dehydration mechanism where HIO3 dehydrates into HI3O8 first, then dehydrates into I2O5. The generally accepted mechanism is used to determine the concentration of iodic acid by TGA and is only valid for special conditions. The revised mechanism allows for the determination of concentrations of iodic acids under all conditions, and the more specific conditions where the accepted mechanism is valid are shown. The determination of concentration of iodic acid with the revised dehydration mechanism is dependent on assumptions of residual water and initial concentration of HI3O8. The validity of these assumptions is established by studying the absorption and hydration behavior of I2O5 from atmospheric water. These results will have an impact on the handling and use of iodine.

Replacing the Al2O3 Shell on Al Particles with an Oxidizing Salt, Aluminum Iodate Hexahydrate. Part I: Reactivity

Improvements in the reactivity, measured in terms of flame speed, for aluminum-based energetic mixtures are increased by a factor of 2−3 by replacing the Al2O3 passivation layer of aluminum (Al) nanoparticles with aluminum iodate hexahydrate (AIH), an oxidizing salt. The Al−AIH nanoparticles are examined under transmission electron microscopy. An AIH passivation shell surrounding the Al core particle is a more reactive composite structure than Al2O3 passivation around Al which facilitates increased reaction rates with flame speeds as high as 3200 m/s. Flame speed measurements are used to show that reaction rates in AIH mixtures are determined by the AIH/Al2O3 ratio, oxygen balance, and β-HIO3. Further optimization of these properties will ultimately boost significant increases in the reaction rates of the energetic materials presented in this article.

Percolation of a metallic binder in energy generating composites

Indium is introduced here as a metallic binder in energetic composites as an approach for consolidating the media and providing a highly conductive percolating scaffold for enhancing energy transport. Indium can replace traditional polymer binders and provide an approach for not only binding composites but also enhancing energy transport. For energy generation applications, a commonly used formulation in thermal batteries is investigated and comprised of magnesium (Mg) and manganese dioxide (MnO2) powders mixed with varying indium (In) concentrations and pressed into thin sheets. Scanning electron microscopy (SEM) images indicated that during compression In flows through voids in the composite, resulting in a connected network of highly conductive filler. This network produces self-adhered composites that do not require additional binders. The thin sheets are ignited, flame speeds are measured, and specific energy ranged from 2000–5000 kJ kg−1 depending on the In concentration. Energy liberation is maximized for Mg + MnO2 mixed with 19 vol% In. Laser flash analyzer (LFA) thermal conductivity measurements demonstrate a good correlation between high energy propagation rates and optimal thermal conductivity. However, increased thermal conductivity is balanced by decreased heat production, and energy propagation decreased beyond 19 vol% In. Comparison of thermal conductivity measurements with predictions from classical and more recent percolation theories show that percolation in composites most closely aligns with the assumptions of the lattice percolation theory.

Improving the Explosive Performance of Aluminum Nanoparticles with Aluminum Iodate Hexahydrate (AIH)

A new synthesis approach for aluminum particles enables an aluminum core to be passivated by an oxidizing salt: aluminum iodate hexahydrate (AIH). Transmission electron microscopy (TEM) images show that AIH replaces the Al2O3 passivation layer on Al particles that limits Al oxidation. The new core-shell particle reactivity was characterized using laser-induced air shock from energetic materials (LASEM) and results for two different Al-AIH core-shell samples that vary in the AIH concentration demonstrate their potential use for explosive enhancement on both fast (detonation velocity) and slow (blast effects) timescales. Estimates of the detonation velocity for TNT-AIH composites suggest an enhancement of up to 30% may be achievable over pure TNT detonation velocities. Replacement of Al2O3 with AIH allows Al to react on similar timescales as detonation waves. The AIH mixtures tested here have relatively low concentrations of AIH (15 wt. % and 6 wt. %) compared to previously reported samples (57.8 wt. %) and still increase TNT performance by up to 30%. Further optimization of AIH synthesis could result in additional increases in explosive performance.

Discovery of β -HIO 3 : A Metastable Polymorph of HIO 3

The β-HIO3 polymorph, previously difficult to detect and whose existence was questioned, has been structurally characterized. The crystal structure of β-HIO3 was solved in the same space group as α-HIO3 (P212121); however, it was found that the unit cell axes were all different by about 1 A. Similar to that of α and γ phases, the unit cell contains only a single HIO3 molecule in the asymmetric unit with I-O bond lengths ranging from 1.786(5) to 1.903(7) A. The I(V) atom is further coordinated by three oxygen atoms of neighboring acid molecules forming a distorted octahedral with a range of I-O distances (2.498(6) - 2.795(7) A). The one structural difference that separates the β phase from the α and γ phases is that the hydroxyl group is bridging between two I(V) atoms, resulting in a smaller hydrogen bonding distance (O-O distance: 2.559 A (β), 2.665 A (γ) and 2.696 A (α)) and presumably a different crystalline energy. Similar to γ-HIO3, β-HIO3 is metastable and slowly converts to α-HIO3. It is hypothesized that β-HIO3 is a transition step in the formation of α-HIO3 and β-HIO3 is a result of trapped water inside particles during crystallization.