Edward L. Dreizin

Experimental study of stages in aluminium particle combustion in air

Abstract An experimental study of Al particle combustion in air is presented. Uniform Al particles were formed and ignited in air using a pulsed micro-arc discharge. Burning particle color temperatures were measured using a three-wavelength pyrometer, partially burned particles were quenched and cross-sectioned. Particle internal compositions were studied using a scanning electron microscope equipped with an x-ray energy dispersive spectroscopy detector and wavelength dispersive spectroscopy scan. Temporal variations of the particle diameter and the shape and size of the smoke cloud surrounding a burning particle were determined. The effect of an external electric field on Al particle combustion was also tested. Three distinct stages were identified in Al particle combustion, which correspond to different temperatures, internal particle compositions, and flame shapes. The transitions between the stages were shown to correlate with the internal phase transformations occurring in the burning Al droplets. Growing on spinning Al particles “oxide caps” were shown to cause rapid changes of trajectories of the burning particles. The temperature histories of burning Al particles were affected by the electric field, and a reduction in the total combustion time due to external electric fields was observed.

Effect of polymorphic phase transformations in alumina layer on ignition of aluminium particles

The mechanism of aluminium oxidation is quantified and a simplified ignition model is developed. The model describes ignition of an aluminium particle inserted in a hot oxygenated gas environment: a scenario similar to the particle ignition in a reflected shock in a shock tube experiment. The model treats heterogeneous oxidation as an exothermic process leading to ignition. The ignition is assumed to occur when the particles temperature exceeds the alumina melting point. The model analyses processes of simultaneous growth and phase transformations in the oxide scale. Kinetic parameters for both direct oxidative growth and phase transformations are determined from thermal analysis. Additional assumptions about oxidation rates are made to account for discontinuities produced in the oxide scale as a result of increase in its density caused by the polymorphic phase changes. The model predicts that particles of different sizes ignite at different environment temperatures. Generally, finer particles ignite at lower temperatures. The model consistently interprets a wide range of the previously published experimental data describing aluminium ignition.

Phase changes in metal combustion

Abstract This paper describes an expanded mechanism of metal combustion that emphasizes reactions and phase changes occurring within the burning metal in addition to those occurring on and above the metal surface. Selected recent experimental work showing the importance of these internal processes in metal combustion is surveyed. Gas dissolution within the burning metal, ensuing phase changes, and their effects on combustion are discussed. Metal–gas phase diagrams are used to interpret the experimentally observed metal combustion behavior. Some concepts aimed at designing improved metal-based high energy density materials and controlling metals flammability are briefly addressed.

On the mechanism of asymmetric aluminum particle combustion

Abstract The results of an experimental study of the combustion of single aluminum particles in N2/O2, Ar/O2, and He/O2 gas mixtures, and in pure O2 are presented and interpreted. This research focuses on identifying conditions under which asymmetric combustion of the aluminum particles develops. It also illustrates the relationship between aluminum particle combustion behavior and particle internal and surface structure and composition. The experimental technique used in this work is based on a micro-arc generator of monodisperse metal droplets and has been previously employed for metal particle combustion experiments. Liquid aluminum particles of 90 and 250 μm diameter were produced and ignited in a transparent chamber containing controlled gas atmospheres. The burning times, radiation histories, and color temperatures of the free falling particles were measured using optical sensors. Partially burned particles were rapidly quenched and their surfaces and interiors were examined by electron microscopy. It was found that brightness oscillations indicative of asymmetric particle burning developed reproducibly in the N2/O2 gas mixtures, consistent with the previous observations. Similar brightness oscillations developed occasionally in both Ar/O2 and He/O2 mixtures. Oxide caps were found on the surface of particles burning in all the environments; however the size of the oxide caps detected on the particles quenched in the Ar/O2 and He/O2 mixtures was markedly smaller than that found in the N2/O2 gas mixtures. Dissolved oxygen was detected in the interiors of all the partially burned particles independent of the gas environment in which the particles burned. A qualitative mechanism of diffusive oxygen transport to the surface of burning aluminum particles is discussed in which a higher rate of oxygen transport is expected in the N2/O2, as compared to the Ar/O2 and He/O2 gas mixtures, due to production of significant amounts of NO.

Preparation and characterization of energetic Al-Mg mechanical alloy powders

Abstract Metals such as Al and Mg have high combustion enthalpies and they are widely used as additives in energetic materials for propellants, explosives, and pyrotechnics. However, long ignition delays and slow combustion kinetics limit their current applications. An approach suggested in this work is to design new metal-based materials in which pre-determined phase changes will occur and trigger ignition at a desired temperature and also accelerate the rate of heat release during combustion. As a first step, metastable solid solutions of Mg in Al (10–50% of Mg) have been produced by mechanical alloying. The ignition temperatures of the produced alloys in air were determined using digital imaging and three-color pyrometry of the electrically heated filaments coated with different alloy powders. Combustion of mechanical alloys in air was studied using a laminar, premixed flame aerosol burner. The ignition temperatures were around 1,000 K, much lower than the pure aluminum ignition temperature of about 2,300 K. The steady flames of mechanical alloy powders were produced at lower equivalence ratios and had higher propagation velocities than similar pure aluminum powder flames. Phase compositions of the combustion products were determined using X-ray diffraction. In addition to Al 2 O 3 and MgO, significant amounts of Al 2 MgO 4 were found in experiments.

Aluminum-Rich Al-MoO3 Nanocomposite Powders Prepared by Arrested Reactive Milling

Fuel-rich Al-MoO 3 nanocomposites were prepared using arrested reactive milling. Powder composition was varied from 4Al + MoO 3 to 16Al + MoO 3 . Powders were evaluated using electron microscopy, thermal analysis, x-ray diffraction, heated-filament-ignition experiments, and constant-volume-explosion experiments. Uniform mixing of MoO 3 nanodomains in the aluminum matrix was achieved for all prepared powders. Multiple and overlapping exothermic processes were observed to start when the nanocomposite powders were heated to only about 350 K. In heated-filament experiments, all nanocomposite powders ignited at temperatures well below the aluminum melting point. Ignition temperatures for these powders were estimated for the higher heating rates that are typical of fuel-air explosions. Constant-volume-explosion experiments indicated that flame propagation in aerosols of nanocomposite thermite powders in air is much faster than that in pure aluminum aerosols. The energy release, normalized per unit mass of aluminum, was higher for the nanocomposite materials with bulk compositions 4Al + MoO 3 and 8Al + MoO 3 and lower for pure aluminum and for the 16Al + MoO 3 nanocomposite sample. The reaction rate was the highest for the 8Al + MoO 3 nanocomposite powder. The combustion efficiency inferred from the measured pressure traces correlated well with the phase compositions of the analyzed condensed combustion products.

Structure and properties of Al–Mg mechanical alloys

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 (Al 1 2 Mg 1 7 ), and additional amorphous material. The strongest supersaturation of Mg in the a 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 (Al 3 Mg 2 ) 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.

Experimental study of aluminum particle flame evolution in normal and micro-gravity

Abstract This research addresses the flame structure of single aluminum particles burning in air with the emphasis on the transition from spherically symmetric to non-symmetric combustion regime. The unique feature of this work is that free motionless aluminum particles were produced and ignited in both normal and microgravity environments. That allowed us to observe whether the particle flame non-symmetry develops when effects of convection and buoyancy are minimized. The particles were produced using a novel micro-arc device ensuring repeatable formation and ignition of uniform metal droplets with controllable initial temperatures and velocities. For the microgravity experiments the device was modified to produce motionless metal particles that made it possible to study the flame structure unaffected by particle motion or buoyancy. Burning droplet temperature was measured in real time with a three wave-length pyrometer. The evolution of flame shape at normal and microgravity was studied using high-speed video imaging and correlated with the heterogeneous combustion processes. It was found that combustion times and temperatures are similar for normal and microgravity environments. A non-symmetric flame structure and brightness oscillations were observed to develop at the same combustion times around nearly motionless aluminum particles burning in air in both normal and microgravity environments. Therefore, flame non-symmetry is an intrinsic feature of aluminum particle burning rather than the result of forced or natural convection flows. It was observed that in addition to the particle spinning, actual periodic changes in the flame emission occur during aluminum particle combustion in air. It was also observed that the onset of the non-symmetric burning is accompanied by the formation of highly radiative condensed products in several locations within the particle flame zone. This was found to be consistent with an aluminum combustion mechanism in which Al-O solution forms inside burning aluminum particles shortly after ignition.

Constant Volume Explosions of Aerosols of Metallic Mechanical Alloys and Powder Blends

High-energy ball milling was used to prepare sets of mechanical alloys in the systems Al-Mg, Al-Mg-H, B-Mg, and Ti-B. X-ray diffraction, electron microscopy, and low-angle laser diffraction were used to characterize structures, morphology, and sizes of the alloys, respectively. The produced materials were metastable and nanocrystalline; the particle sizes were in the range of 1-50 μm. A constant volume explosion technique was used to evaluate performance of the mechanical alloys and to compare it to the performance of blends of elemental powders of the same bulk composition. For reference, samples of mechanical alloys were annealed to produce stable intermetallic phases and tested in the same explosion experiments. Pressure traces recorded in real time served as the main piece of experimental information. After selected experiments, combustion products were collected and analyzed. The results have shown that the combustion rates of mechanical alloys are appreciably higher than those of the respective powder blends, thermodynamically stable intermetallics, and pure metals. The analyses of the combustion products also showed that combustion was more complete for mechanical alloys. It was found that combustion parameters of mechanical alloys compared to other metallic fuels were significantly improved even though their particle sizes were identical or greater than those of the reference metals. The use of mechanical alloy powders with relatively large particle sizes is expected to be advantageous in many practical applications requiring mixing and handling of energetic formulations.

Kinetic Analysis of Thermite Reactions in Al-MoO3 Nanocomposites

Reactions in energetic Al-MoO 3 nanocomposites prepared by arrested reactive milling were investigated by scanning calorimetry and heated filament ignition experiments. The calorimetry data were processed to obtain kinetic parameters describing the reaction between Al and MoO 3 . The reaction was treated as a combination of four subreactions, which were described by a combination of a diffusion-controlled reaction model and first-order reactions. The activation energies determined in this study allowed the comparison to reference values for the decomposition of MOO 3 and the diffusion of oxygen through an Al 2 O 3 product layer. The kinetic model was extrapolated to high heating rates in the 10 3 -10 6 K/s range and compared to ignition data. It was concluded that ignition of Al-MoO 3 nanocomposites prepared by arrested reactive milling is primarily controlled by oxygen diffusion in Al 2 O 3 .