Yasmine Aly

Combustion of Mechanically Alloyed Al∙Mg Powders in Products of a Hydrocarbon Flame

Burn times and temperatures were measured optically for a set of mechanically alloyed Al·Mg powders injected into a laminar and a turbulent air-acetylene flame. Magnesium concentrations varied from 10 to 53 mole%; particle sizes were in the range of 1–50 μm. Emission from the burning particles at 700 nm, 800 nm, and 900 nm was captured using three filtered photomultiplier tubes. The burn times were correlated with particle sizes using measured statistical distributions for both times and sizes. The measured trends for burn times, t, as a function of particle size, d, for all alloys were approximated by a t = a·dn law, where the exponent n varied from 0.6 to 1. Shorter burn times were measured in more turbulent flows; respectively, the values of pre-exponent, a, decreased and exponent, n, increased slightly with an increased level of turbulence. An increase in Mg concentration led to longer burn times for the alloy particles for all flame conditions. For all compositions, alloy particles burned longer than similarly sized Al particles except for the alloy with the smallest concentration of Mg, Al0.9Mg0.1, for which particles less than ˜4 μm burned faster than similarly sized Al. This effect was observed for laminar and turbulent flames. The optically measured temperatures were lower for Al0.47Mg0.53 alloy (˜2400 K) compared to ˜2700–2800 K obtained for other alloys. Turbulent mixing resulted in a slight increase in the measured temperature.

Aluminum-Metal Reactive Composites

Three binary Al-based reactive composite powders are prepared by mechanical milling. The particles have an aluminum matrix and inclusions of Fe, Ni, or Zn comprising 10 at % of the bulk composition. For additives of Ni and Zn, only short milling times can be used to prepare composites; intermetallic phases form at longer milling. Short milling times yield relatively coarse particles with flake-like shapes. Prepared powders are characterized using electron microscopy and X-ray diffraction. Oxidation and ignition of the materials are studied using thermal analysis and heated-filament ignition, respectively. Thermogravimetric analysis shows selective oxidation of Zn and Ni at low temperatures, prior to a characteristic first step of Al oxidation. At higher temperatures, the powders oxidize following, qualitatively, the stepwise process reported earlier for the pure Al powders. The magnitude and kinetics of the low-temperature aluminum oxidation steps are substantially affected by the presence of the metal inclusions. Heated-filament ignition experiments showed that all three prepared composite powders ignite at lower temperatures than pure Al powder. Comparison of the Al-metal composites with Al · Al2O3 reference composites prepared with similar milling conditions suggests that the altered Al morphology, such as developed grain boundary network produced in the milled powders, is primarily responsible for their accelerated low-temperature oxidation. It is further observed that simply an increase in the low-temperature oxidation rate detected in thermo-analytical experiments is insufficient to achieve ignition of the material under rapid heating conditions. It is concluded that the improved ignition dynamics for the prepared Al-metal composites is due to a combination of the accelerated low-temperature oxidation with reaction mechanisms altered by the presence of metal inclusions.

Reactive, Mechanically Alloyed Al·Mg Powders with Customized Particle Sizes and Compositions

Previous work showed that particles of mechanically alloyed Al·Mg powders burn faster than aluminum. However, such powders were coarser than fine aluminum commonly used in energetic formulations. This work addresses preparation of mechanically alloyed Al·Mg powders in which both internal structures and particle size distributions are adjusted. Powders with 50–90 at.% Al were prepared and characterized. Milling protocol was optimized to prepare equiaxial, micron-scale particles. Ignition temperatures measured using an electrically heated filament were much lower than those of pure Al powders and are close to those of Mg. Powders were aerosolized and ignited in air; the maximum pressure was higher, rates of pressure rise were greater, and ignition delays were shorter for the mechanically alloyed powders than for pure Al with directly comparable particle size distributions. Individual particle combustion experiments used laser ignition and showed that the alloyed particles burn in two stages, with the first ...

Characterization of Fine Aluminum Powder Coated with Nickel as a Potential Fuel Additive

Oxidation, ignition, and combustion processes are studied and compared for a fine nickelcoated aluminum powder and reference uncoated aluminum powder with a similar particle size distribution. Oxidation is studied by thermogravimetry in argon-oxygen mixtures. Ignition processes are studied for powders coated on an electrically heated metal filament. Combustion is characterized in constant volume explosion tests. Both ignition and combustion experiments were performed in air. Thermogravimetric measurements showed selective oxidation of Ni at low temperatures, where oxidation of Al remains undetected. At higher temperatures, oxidation for both, nickel-coated and uncoated powders occurs in a characteristic stepwise process with individual oxidation steps associated with polymorphic phase changes in the growing alumina layer and with growth of individual alumina polymorphs. The activation energies for individual oxidation steps appear to be unaffected by the Ni coating; however the oxidation occurs somewhat faster for the coated powder, indicating an increase in the pre-exponential coefficients in Arrhenius formulations describing respective oxidation processes. Ignition kinetics for both coated and uncoated powders are similar, however, ignition is more readily detected and appears to be more violent for the coated powders. Finally, powder combustion experiments showed substantially reduced ignition delays and somewhat increased overall burn rates for the coated powders.