The advantage of aluminum powder as a fuel additive in energetic formulation includes its high volumetric combustion enthalpy and relatively low cost. However, the thermodynamically predicted benefits of aluminum combustion are rarely achieved because of extended ignition delays associated with heterogeneous reactions occurring at the alumina surface which surrounds the aluminum particle. In order to fully exploit aluminum’s high reaction energy, this effort focuses on adjusting its combustion dynamics by modifying its surface and structure. The modification is achieved by cryo-milling aluminum with cyclooctane, which is liquid at room temperature, but solid when cooled by liquid nitrogen. The prepared materials consist of micron-sized, equiaxial, mostly Al particles with a small amount of cyclooctane. Aluminum surface in the prepared sample is coated with a cyclooctane-modified layer with properties significantly different from those of regular alumina. Its oxidation kinetics, as observed from thermo-analytical measurements, is different from that of pure aluminum. The powder ignites at substantially reduced temperatures, produces shorter ignition delays, and higher aerosol burn rates compared to a regular spherical Al powder with similar particle sizes.

Adding aluminum to propellants, pyrotechnics, and explosives is a common way to boost their energy density. A number of approaches have been investigated that shorten aluminum ignition delay, increase combustion rate, and decrease the tendency of aluminum droplets to agglomerate. Previous work showed that particles of mechanically alloyed Al-Mg powders burn faster than similarly sized particles of pure aluminum. However, preparation of mechanically alloyed powders with particle sizes matching those of fine aluminum used in energetic formulations was not achieved. This work is focused on preparation of mechanically alloyed, composite Al-Mg powders in which both internal structures and particle size distributions are adjusted. Binary powders with compositions in the range of 50 - 90 at. % Al were prepared and characterized. Milling protocol is optimized to prepare equiaxial, micron-scale particles suitable for laboratory evaluations of their oxidation, ignition, and combustion characteristics. Quantitative particle size analyses are done using low-angle laser light scattering. Electron microscopy and x-ray diffraction are used to examine particle morphology and phase makeup, respectively. Combustion of aerosolized powder clouds is studied using a constant volume explosion setup. For all materials, ignition and combustion characteristics are compared to each other and to those of pure Al. Compositions with improved performance (i.e., shorter ignition delay and faster pressurization rate) compared to pure Al are identified.

Powders of Zr, ZrO2, and ZrN were mixed and pressed to produce samples with different bulk stoichiometries in the ternary Zr–O–N systems. The samples were laser heated above melting, maintained at a high temperature, and quenched. The processed samples were cross-sectioned and studied using scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction. The results pointed to the location of the ternary invariant point Liquid + Gas + ZrO2 + ZrN on the high-temperature portion of the Zr–ZrO2–ZrN phase diagram. The ternary liquidus in the Zr–O–N system was further constrained based on the comparison of the results obtained in this work with composition histories of zirconium particles burning in air reported earlier. Elemental analysis of nitrogen-rich inclusions found in the samples showed the existence of an extended compositional range for ternary solid Zr–O–N solutions. X-ray diffraction analysis of the quenched samples indicated that these solutions are likely to be derived from the ZrN phase. A preliminary outline of the subsolidus ternary Zr–ZrO2–ZrN phase diagram is constructed based on these findings and the interpretations of the well-known binary Zr–O and Zr–N phase diagrams.

Recently, nanometer-sized aluminum powders became available commercially and their use as potential additives to propellants, explosives, and pyrotechnics has attracted significant interest. It has been suggested that very low melting temperatures are expected for nano-sized aluminum powders and that such low melting temperatures could accelerate oxidation and trigger ignition much earlier than for regular, micron-sized aluminum powders. The objective of this work was to investigate experimentally the melting and oxidation behavior of nano-sized aluminum powders. Powder samples with three different nominal sizes of 44, 80, and 121 nm were provided by Nanotechnologies Inc. The particle size distributions were measured using small angle x-ray scattering. Melting was studied by differential scanning calorimetry where the powders were heated from room temperature to 750 °C in argon environment. Thermogravimetric analysis was used to measure the mass increase indicative of oxidation while the powders were heated in an oxygen-argon gas mixture. The measured melting curves were compared to those computed using the experimental particle size distributions and thermodynamic models describing the melting temperature and enthalpy as functions of the particle size. The melting behavior predicted by different models correlated with the experimental observations only qualitatively. Characteristic step-wise oxidation was observed for all studied nanopowders. The observed oxidation behavior was well interpreted considering the recently established kinetics of oxidation of micron-sized aluminum powders. No correlation was found between the melting and oxidation of aluminum nanopowders.

Mechanically alloyed powders with the composition Al0.7Li0.3 are synthesized. Materials milled for different times are studied using electron microscopy, x-ray diffraction, and thermal analysis. A solid solution of Li in Al (α-phase) is formed with as much as 10 at % of dissolved Li. The LiAl intermetallic δ-phase is readily produced by mechanical alloying but disappears after extended milling times. The final product of milling for 102 hours consists of an x-ray amorphous phase. Mechanically alloyed powders heated in inert environment exhibit several weak exothermic reactions between 420 and 700 K, and two endothermic reactions, around 810 and 870 K. All the observed relaxation processes become less pronounced and eventually become undetectable as the milling time increases and an amorphous material is produced. Ignition experiments performed for the powders coated on an electrically heated filament showed that the powders ignited in the vicinity of 1250 K. An experimental setup for studying combustion of reactive mechanically alloyed powders is developed and initial experimental results are described.

Al-MoO3 nanocomposites were synthesized based on arrested reactive milling. Altering the milling parameters leads to different scales of refinement in the structure of the reactive nanocomposites. The objective of this work is to determine the range in which the degree of structural refinement can be changed in a controlled manner. The milling intensity was controlled by using different milling media along with varying amounts of process control agent (PCA). XRD, SEM, DSC and wire-ignition tests were performed to analyze the Al-MoO3 nanocomposites. Results indicate that there is a decrease in the crystallite size with increase in the milling intensity. However increase in milling intensity also stimulates the undesired reaction between Al and MoO3. Milling conditions resulting in the highest structural refinement and lowest ignition temperatures were identified.

Mechanically alloys in the Al–Mg binary system in the range of 5–50 at.% Mg were produced for prospective use as metallic additives for propellants and explosives. Structure and composition of the alloys were characterized by x-ray diffraction microscopy (XRD) and scanning electron microscopy. The mechanical alloys consisted of a supersaturated solid solution of Mg in the α aluminum phase, γ phase (Al12Mg17), and additional amorphous material. The strongest supersaturation of Mg in the α phase (20.8%) was observed for bulk Mg concentrations up to 40%. At 30% Mg, the γ phase formed in quantities detectable by XRD; it became the dominating phase for higher Mg concentrations. No β phase (Al3Mg2) was detected in the mechanical alloys. The observed Al solid solution generally had a lower Mg concentration than the bulk composition. Thermal stability and structural transitions were investigated by differential scanning calorimetry. Several exothermic transitions, attributed to the crystallization of β and γ phases were observed. The present work provides the experimental basis for the development of detailed combustion and ignition models for these novel energetic materials.

Oxidation of Al-Ti mechanical alloys with Ti concentrations from 5 to 25 at% was studied and compared to the oxidation of Al powders using thermal analysis in the temperature range of 300–1500 °C in oxygen. Differential scanning calorimetry (DSC) and differential thermal analysis (DTA) with simultaneous thermo-gravimetric analysis (TGA) were used to monitor phase changes and oxidation reactions. Intermediate reaction products were recovered at different temperatures and analyzed using x-ray diffraction (XRD) and scanning electron microscopy (SEM). Oxidation of all samples occurred stepwise. The temperature of the first oxidation step for Al-Ti mechanical alloys correlates with the exothermic formation of the Al3Ti intermetallic at T>700 °C and has a higher rate and greater degree of oxidation than the first oxidation step observed for pure aluminum. The second and third oxidation steps in mechanical alloys and aluminum occur at higher temperatures and both appear to be controlled by changes in the permeability of the Al2O3 films. The effect of the Al2O3 film becomes less noticeable at increased Ti concentrations.

MgSiO3 - rich perovskite is expected to dominate the Earth's lower mantle (pressures > 25 GPa), with iron and aluminum as significant substituents. The incorporation of trivalent ions, M3+, may occur by two competing mechanisms: MgA+ SiB = MA + MB and SiB = AlB + 0.5 VO. Phase synthesis studies show that both substitutions do occur, and the nonstoichiometric or defect substitution is prevalent along the MgSiO3 - MgAlO2.5 join. Oxide melt solution calorimetry has been used to compare the energetics of both substitutions. The stoichiometric substitution, represented by the reaction 0.95 MgSiO3 (perovskite) + 0.05 Al2O3 (corundum) = Mg0.95Al0.10Si0.95O3 (perovskite), has an enthalpy of -0.8±2.2 kJ/mol. The nonstoichiometric reaction, 0.90 MgSiO3 (perovskite) + 0.10 MgO (rocksalt) + 0.05 Al2O3 (corundum) = MgSi0.9Al0.1O2.95 (perovskite) has a small positive enthalpy of 8.5±4.6 kJ/mol. The defect substitution is not prohibitive in enthalpy, entropy, or volume, is favored in perovskite coexisting with magnesiowüstite, and may significantly affect the elasticity, rheology and water retention of silicate perovskite in the Earth.