Mirko Schoenitz

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.

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.

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 .

COMBUSTION OF AEROSOLIZED SPHERICAL ALUMINUM POWDERS AND FLAKES IN AIR

Combustion rates and completeness of aerosolized spherical aluminum powders and flakes are compared using constant volume explosion experiments. The comparison of particles and flake sizes was made based on their specific surface areas determined using the Brunauer–Emmett–Teller (BET) method and respective “BET diameters.” It is observed that the rates of pressure rise and respective rates of flame propagation were higher for spherical aluminum powders with BET diameters of about 2 to 5 µm compared to aluminum flakes for which the respective BET diameters were under 1 µm. In agreement with the flame propagation rates, the overall completeness of combustion was also higher for spherical powders compared to flakes. It is suggested that aerosolized flakes could be agglomerated in gas flows more than spherical particles causing their inferior combustion performance.

REFLECTED SHOCK IGNITION AND COMBUSTION OF ALUMINUM AND NANOCOMPOSITE THERMITE POWDERS

A comparison of the ignition and combustion characteristics of Al-Fe2O3 and Al-MoO3 nanocomposite powders and two sizes of aluminum powder in inert and oxidizing environments was performed in the region behind a reflected shock in a shock tube. Radiation intensity was monitored by photometry, and temporal information on the particle temperatures was obtained using high-speed pyrometry. In addition, emission spectra were collected to identify intermediate species produced during combustion. In inert environments, both thermite materials showed evidence of ignition within 1–2 ms at 1400 and 1800 K. Particle temperatures during reaction ranging from 2700–3350 K were observed, with Al-MoO3 having generally higher temperatures than Al-Fe2O3. Addition of oxygen in the ambient environment reduced ignition times and increased combustion temperatures to 3350–3800 K as well, suggesting that heterogeneous reactions can enhance the combustion performance of the thermite materials. In air at 3 atm, the nanocomposite thermites and nanoscale aluminum all showed extremely rapid ignition: on the microsecond time scale and under 2000 K. The bulk of the material, however, ignited and burned on much longer time scales of the order of 1 millisecond. Bulk nanocomposites were found to ignite as quick or more quickly than bulk, agglomerated nanoscale aluminum and significantly faster than a 5–10 micron aluminum powder.

Morphology and composition of the fly ash particles produced in incineration of municipal solid waste

This paper describes the results of experiments using a pilot-scale, 140,000 Btu/h, solid fuel continuous feed laboratory incinerator. A synthetic fuel representative of the municipal solid waste in the United States was formulated and used in this research. The fuel contained Fe and SiO2, and was doped with trace amounts of Al, Ni, Cr, Hg, and PbO. Experiments were performed with varying fuel–air ratio, and both gaseous and condensed products were collected and analyzed. This work focuses on the characterization of composition and morphology of fly ash particles captured in a fabric filter. Particle size distributions were obtained using optical microscopy and sieving. Atomic absorption (AA) was used to determine bulk compositions of the size-classified ash fractions. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to study the morphology and surface compositions of the ash particles. It was observed that the fly ash particles have bimodal size distribution and, most interestingly, that the ash particles of different sizes have different elemental and phase compositions. Concentrations of Cr, Ni, and Fe were greater in the coarse particles (up to 1-mm diameter), whereas concentrations of Al and Si were higher in the finer particles (less than 75 μm). Maximum concentrations of Pb and Hg occurred in the 150–300-μm particles. It is suggested that if a correlation between the composition and size of the ash particles similar to that observed in this research exists in the products of industrial combustors, a technique of ash processing based on the particle size classification could be developed. Applying such a technique could result in the efficient and inexpensive removal of the lead- and mercury-rich particulates from the produced ash. The processed, environmentally benign ash portions will therefore be useful for a variety of the recycling-based manufacturing, and metal recovery processes.

Oxidation Processes and Phase Changes in Metastable Al-Mg Alloys

Oxidation behavior of metastable mechanical alloys in the Al-Mg binary system has been examined in the context of high-energy density materials and combustion applications. Mechanical alloy powders with compositions ranging from Al 0 . 9 5 Mg 0 . 0 5 to Al 0 . 5 Mg 0 . 5 , as well as the component metals, were heated at 20 K/min in oxygen. Differential thermal analysis and thermogravimetric analysis showed that oxidation proceeds in two separate steps. During the first step occurring over the range of 550-600°C, Mg is oxidized and thereby quantitatively removed from the metallic phase. The selective removal of Mg from the alloy was identified by correlation of weight gain with the Mg concentration of the alloy and by x-ray diffraction and scanning electron microscopy, of intermediate products. The second step, during which the remainder of the metallic phase is oxidized, occurs over a wider range of temperatures (900-1200°C). The temperatures of both effects decrease slightly with increasing Mg content in the alloy. Oxidation is increasingly incomplete as the Mg concentration of the alloy decreases below 30 at.%. It was concluded that the low-temperature selective oxidation of Mg is controlled by the volatilization of Mg from the alloy. No correlation could be established between the oxidation reactions and subsolidus phase transitions, which occur over the temperature range of 100-400°C and are associated with the relaxation of the metastable state of the mechanical alloys.

Combustion of Boron-Titanium Nanocomposite Powders in Different Environments

Combustion of nanocomposite powders with the bulk composition 2B+Ti was compared with combustion of blended boron and titanium powders with the same bulk composition and with combustion of aluminum in wet and dry gas environments. Nanocomposite powders were prepared by Arrested Reactive Milling. The gas environments were N2/O 2/CH 4 mixtures with oxygen concentration fixed at 22.5 % and methane concentration varied from 0 to 12 %. The experiments were conducted in a constant volume explosion vessel. The mass loads of metallic fuel were determined from thermodynamic calculations to ensure the maximum flame temperature for each metal fuel – gas mixture combination. The calculations showed that despite the higher adiabatic flame temperatures for Al than for 2B+Ti, a greater energy per unit mass of metal fuel was released to produce heated gaseous combustion products in combustion of 2B+Ti as compared to Al. Experiments with Al powders showed that the flame temperature did not change noticeably as a function of gas composition and remained close to 2560 K. The combustion temperature for the nanocomposite 2B+Ti increased from about 2180 to 2370 K as the methane concentration increased from 0 to 12 %. The bulk burn rates inferred from the rates of pressure rise were consistently higher for the nanocomposite 2B+Ti powder, followed by Al and then by the blended 2B+Ti powder. The efficiency of combustion for all the fuels was assessed by comparing the predicted and experimental portions of the combustion energy used to produce the heated gaseous products. Based on this assessment, nanocomposite boron-based fuels outperformed Al for all environments, with the difference increasing at the increased methane concentrations. Nearly complete combustion was observed for both 2B+Ti fuels (nanocomposite and blended powders) at high methane concentration, when the highest rates of combustion were also observed. Thus, the effect of kinetic trap associated with formation of HOBO could not be detected. It was concluded that nanocomposite 2B+Ti powders enable one to achieve rapid and highly efficient combustion in both dry and wet gaseous environments.