Exotic Inverse Kinetic Isotopic Effect in the Thermal Decomposition of Levitated Aluminum Iodate Hexahydrate Particles.

Aluminum iodate hexahydrate ([Al(H2O)6](IO3)3(HIO3)2; AIH) represents a novel, oxidizing material for energetic applications. Recently, AIH was synthesized to replace the aluminum oxide passivation layer of aluminum nanoenergetic materials (ALNEM). The design of reactive coatings for ALNEM-doped hydrocarbon fuels in propulsion systems requires fundamental insights of the elementary steps of the decomposition of AIH. Here, through the levitation of single AIH particles in an ultrasonic field, we reveal a three-stage decomposition mechanism initiated by loss of water (H2O) accompanied by an unconventional inverse isotopic effect and ultimate breakdown of AIH into gaseous elements (iodine and oxygen). Hence, AIH coating on aluminum nanoparticles replacing the oxide layer would provide a critical supply of oxygen in direct contact with the metal surface thus enhancing reactivity and reducing ignition delays, further eliminating decades-old obstacles of passivation layers on nanoenergetic materials. These findings demonstrate the potential of AIH to aid in the development of next-generation propulsion systems.

Adsorption and exchange reactions of iodine molecules at the alumina surface: modelling alumina-iodine reaction mechanisms.

Harnessing aluminum oxidation energy requires navigating the particle's passivation shell composed of alumina. The shell is a barrier to aluminum oxidation but 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. This study used density functional theory (DFT) calculations 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-1 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.

Synthesis of metal iodates from an energetic salt.

Iodine containing oxidizers are especially effective for neutralizing spore forming bacteria by generating iodine gas as a long-lived bactericide. Metal iodates have been shown to be strong oxidizers when combined with aluminum fuel particles for energy generating applications. One method to produce metal iodates in situ is by using metal oxides and an energetic salt: aluminum iodate hexahydrate (Al(H2O)6(IO3)3(HIO3)2), which is called AIH. In this study, the thermal stability and reactivity of AIH with metal oxides commonly used in energetic formulations was investigated. Three metal oxides: bismuth(iii) oxide (Bi2O3), copper(ii) oxide (CuO), and iron(iii) oxide (Fe2O3) were investigated because of their different oxygen release properties. Each metal oxide powder was combined with AIH powder. Thermal stability and reactivity were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) and reactive properties calculated to supplement experimental observations. Powder X-ray diffraction (XRD) was also used to identify the product species at various stages of heating corresponding to exothermic activity. Results show that AIH decomposition is entirely endothermic but, with the addition of metal oxide powder to AIH, exothermic reactions transform metal oxides into more stable metal iodates. This analysis provides an understanding of the compatibility of AIH with metal oxides and contributes to the development of novel energetic composites that have the advantages of both thermal and biocidal mechanisms for spore neutralization.