Oxidation of aluminum nanopowders obtained by electro-exploded wires is studied. Particle size distributions are obtained from transmission electron microscopy (TEM) images. Thermo-gravimetric (TG) experiments are complemented by TEM and XRD studies of partially oxidized particles. Qualitatively, oxidation follows the mechanism developed for coarser aluminum powder and resulting in formation of hollow oxide shells. Sintering of particles is also observed. The TG results are processed to account explicitly for the particle size distribution and spherical shapes, so that oxidation of particles of different sizes is characterized. The apparent activation energy is obtained as a function of the reaction progress using model-free isoconversion processing of experimental data. A complete phenomenological oxidation model is then proposed assuming a spherically symmetric geometry. The oxidation kinetics of aluminum powder is shown to be unaffected by particle sizes reduced down to tens of nm. The apparent activation energy describing growth of amorphous alumina is increasing at the very early stages of oxidation. The higher activation energy is likely associated with an increasing homogeneity in the growing amorphous oxide layer, initially containing multiple defects and imperfections. The trends describing changes in both activation energy and pre-exponent of the growing amorphous oxide are useful for predicting ignition delays of aluminum particles. The kinetic trends describing activation energies and pre-exponents in a broader range of the oxide thicknesses are useful for prediction of aging behavior of aluminum powders.
Stable ternary powders of Al•B•I 2 and Mg•B•I 2 composites of interest for agent defeat applications were prepared by mechanical milling. All powders contained 20 wt % of iodine. Powder ignition was characterized using a heated filament experiment. Ignition kinetics was compared to the kinetics of events occurring upon slow heating of these materials in thermoanalytical experiments. Individual particle combustion was studied by seeding the powder into a premixed hydrocarbon-air flame. Both particle burn times and temperatures were measured optically. Aerosol combustion of the powders was tested in a constant volume explosion chamber. Ignition temperatures for the Mg•B•I 2 composites were lower than those for the Al•B•I 2 composites. Iodine release occurring due to the formation of AlB 2 and MgB 2 was a likely ignition trigger for Al•B•I 2 and Mg•B•I 2 composites, respectively. The burn times of these composites were longer than those for pure Al and Mg powders. Burn times for Mg•B•I 2 particles were shorter than for the same size particles of Al•B•I 2. Combustion temperatures of the composite powders were lower than those of pure Al and Mg. In aerosol combustion, the rate of pressurization and maximum pressure were inversely proportional to the concentration of boron. The combustion efficiency was expressed through a ratio of the experimental maximum pressure to that predicted by a thermodynamic equilibrium calculation. This efficiency was the same for Al and Al•B•I 2 composites. The efficiency for Mg•B•I 2 composites exceeded that of pure Mg.
Energetic materials generating biocidal combustion products to disable airborne pathogenic microorganisms (including bio-threat agents) were designed as compounds of halogens and metals with high heats of oxidation. Thermally stable Al-based powders containing iodine and chlorine were prepared using ball-milling at room and cryogenic temperatures. Such powders can replace pure aluminum in metallized energetic formulations. Their stability and halogen release were quantified using thermo-gravimetric analysis. Ignition temperatures were determined by coating prepared powders onto an electrically heated filament. All prepared composites had lower ignition temperatures and longer combustion times compared to pure Al. In separate experiments, combustion products generated by injecting the prepared powders into an air-acetylene flame were mixed with a well-characterized bioaerosol. Inactivation of viable bioaerosol particles exposed to the heated combustion products for a short period of time (estimated to be 0.33 s) was quantified. The combustion products of materials investigated in this study effectively inactivated the aerosolized spores of two tested surrogates of Bacillus anthracis (B. atrophaeus and B. thuringiensis var kurstaki). A ternary composite with 20 wt% of iodine, 40 wt% of aluminum and 40 wt% of boron was found to be most attractive based on both its stability and efficiency in inactivating the aerosolized spores. The inactivation achieved was primarily attributed to chemical stresses as the thermal effect could not solely produce the high measured levels of inactivation. The findings point to a possible synergy of the thermal and chemical spore inactivation mechanisms.
Mechanically alloyed aluminum–iodoform composites were prepared with iodine concentration of 20 wt%. Ball milling at both room temperature and liquid nitrogen temperature was explored. Material characterization by electron microscopy and X‐ray diffraction showed no difference between samples milled at different temperatures. However, samples prepared at room temperature aged rapidly. Thermo‐gravimetric measurements quantifying release of iodine upon heating confirmed that cryogenic milling was necessary to stabilize iodoform in the Al‐matrix. Iodine was released upon heating in four distinct stages. The oxidation of the prepared materials was also studied using thermo‐gravimetric analysis and two main oxidation steps were detected. The ignition temperatures were determined for powders coated onto a metal filament heated electrically at 103–104 K min−1. The ignition temperatures of the prepared materials were noticeably lower compared to the Al · I2 composite prepared using a similar cryo‐milling approach. The combustion characteristics determined using constant volume explosions of aerosolized powders were found to be similar to those of Al · I2 composite. The maximum pressure and rate of pressure rise observed in the latter experiments were greater than for pure aluminum powders with comparable particle sizes.
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