Vern K. Hoffmann

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.

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.

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.

Inactivation of Aerosolized Bacillus Atrophaeus (BG) Endospores and MS2 Viruses by Combustion of Reactive Materials

Accidental release of biological agents from a bioweapon facility may contaminate large areas, possibly causing disastrous environmental consequences. To address this issue, novel halogen-containing reactive materials are being designed with the added capability to inactivate viable airborne microorganisms. This study determined the efficiency of combustion products of such materials to inactivate aerosolized bacteria and viruses. Spores of Bacillus atrophaeus and MS2 viruses dispersed in dry air were exposed for subsecond time intervals to hydrocarbon flames seeded with different reactive powders so that bioaerosol particles interacted with the combustion products in a controlled high-temperature environment. The experiments were designed to quantify differences in the biocidal effects of different reactive material powders including Al and Mg, a B•Ti nanocomposite, an 8Al•MoO(3) nanothermite, and a novel Al•I(2) nanocomposite. Compared to pure hydrocarbon flame, powder-seeded flame (with no iodine) produced about an order of magnitude greater inactivation of bacterial spores. The iodine-containing material increased the spore inactivation by additional 2 orders of magnitude. The aerosolized MS2 viruses (generally not as stress-resistant as spores) were fully inactivated when exposed to combustion of either the iodinated or noniodinated powders. Overall, the study suggests a great biocidal potential of combustion products generated by novel iodine-containing nanocomposite materials.

Constant pressure combustion of aerosol of coarse magnesium particles in microgravity

The combustion mechanisms of clouds of metal particles are addressed in this research. A microgravity environment was used to create a stationary model aerosol consisting of relatively.large (100-300 μm diameter), initially motionless particles. The development of individual particle flames, motion of individual particles, and overall aerosol combustion process could be observed simultaneously. The experiments used the 2.2-s Drop Tower at the NASA Lewis Research Center. Various image analysis procedures were employed to extract information on the flame structure from the high-speed movie and video records. Mg particle aerosol combustion at constant pressure was addressed. The observed flame structure contained preheat and combustion zones typical of the volatile type aerosol flames. The preheat and combustion zones were identified by differences in intensity and spectral content of the emitted radiation. The velocity of propagation of the preheat zone into the unburnt mixture was in the range of 0.15-0.30 m/s, consistent with the microgravity flame speed measurements reported in the literature. The combustion zone propagated at a slower rate of less than 0.1 m/s. The width of the preheat zone increased and the width of the combustion zone decreased during the flame propagation. Particle inertia caused significant velocity lag relative to the cold gas that was pushed ahead of the flame. The particles were, however, efficiently entrained by the hot gas in the preheat and combustion zones. Thus, the particle number density in the preheat and combustion zones increased as the flame propagated, eventually resulting in flame quenching due to oxygen deficiency. Also, collective particle motion was observed in the direction opposite to that of the flame propagation; the nature of this motion needs further investigation. Unburnt metal particles were observed to reignite when fresh air from the constant pressure ballast reservoir returned to the combustion chamber as it cooled.

Constant pressure flames of aluminum and aluminum-magnesium mechanical alloy aerosols in microgravity

In an effort to develop new aluminum-basedenergetic materials for advanced metallizedpropellants, explosives, pyrotechnics, and incen-diaries, metastable Al-Mg mechanical alloyshave been recently prepared and characterized[1]. In this work, the experimental setup de-signed for the constant pressure experiments onaerosol flame propagation in microgravity [2–4]was exploited to compare the flame propagationin the aerosols of pure Al versus Al-Mg me-chanical alloys. In addition, combustion prod-ucts produced with different metallic fuels werecollected and compared with one another.

Characteristics of Aluminum Combustion Obtained from Constant-Volume Explosion Experiments

Combustion of aluminum powders was studied using a constant volume explosion (CVE) experiment with varied powder mass loads, particle sizes, and environment compositions. A simplified model of aerosol combustion in CVE experiment was used to extract the information about the burning velocity from the measured pressure traces; additional assumptions were used to evaluate the flame thickness. It was observed that an increase in oxygen concentration always results in higher rates of pressure rise, shorter induction periods, and shorter aerosol combustion times. Similarly, adding methane to the gas mixture always results in shorter induction times, greater rates of pressure rise, and higher maximum combustion pressures. It was further observed that at increased oxygen concentrations, the kinetics of Al combustion may be faster than that of gas-phase combustion of methane. In most experiments, there was a period when the flame propagation occurred in a quasi-steady mode, with nearly constant burning velocity and flame thickness. The burning velocities measured for Al aerosols vary approximately from 0.25 to 1.3 m/s and compare well with those reported earlier for similar size Al powders. The aluminum aerosol flame thickness was evaluated to vary approximately from about 2.5 to 12 mm. Experimental results suggest that radiation is the dominant mechanism of heat transfer for the aerosol flames.

Experiments on magnesium aerosol combustion in microgravity

Abstract An experimental study of the combustion of an aerosol of coarse magnesium particles in microgravity is reported. Particles with sizes between 180–250 μm were aerosolized in a 0.5-L combustion chamber and ignited in a constant-pressure, microgravity environment. Two flame images were produced simultaneously using interference filters separating adjacent MgO and black body radiation bands at 500 and 510 nm, respectively. The characteristic MgO radiation was used as an indicator of the gas-phase combustion. Comparison of the two filtered flame images showed that preheat and combustion zones can be distinguished in the flame. Experiments have also shown that in microgravity the flame speed depends on the initial particle speeds varied in the range of 0.02–0.4 m/s. This dependence is, most likely, due to the role the moving particles play in the heat transfer processes. Product analyses showed an oxide coating on the surfaces of particles collected after experiments in which the flame speeds were higher than 0.1 m/s. No oxide coating was detected in the products collected after experiments in which a slower flame propagation was observed. However, the particles collected after such experiments contained significant amounts of dissolved oxygen. Strong MgO radiation and production of dense MgO smoke clouds were observed in all the experiments, including those with the slowly propagating flames. Therefore, it has been suggested that the MgO produced in the vapor-phase flame is not the primary source of the MgO coating found on the burnt particle surfaces. An alternative mechanism of forming the oxide coating is, consistent with the earlier single metal particle combustion studies, via the formation of a metal–oxygen solution followed by a phase separation occurring within the burning particles.

Experimental technique for studying high-temperature phases in reactive molten metal based systems

Containerless, microgravity experiments for studying equilibria in molten metal–gas systems have been designed and conducted onboard of a NASA KC-135 aircraft flying parabolic trajectories. An experimental apparatus enabling one to acoustically levitate, laser heat, and splat quench 1–3 mm metal and ceramic samples has been developed and equipped with computer-based controller and optical diagnostics. Normal-gravity testing determined the levitator operation parameters providing stable and adjustable sample positioning. A methodology for optimizing the levitator performance using direct observation of levitated samples was developed and found to be more useful than traditional pressure mapping of the acoustic field. In microgravity experiments, spherical specimens prepared of pressed, premixed powders of ZrO2, ZrN, and Zr, were acoustically levitated inside an argon-filled chamber at one atmosphere and heated by a CO2 laser up to 2800 K. Using a uniaxial acoustic levitator in microgravity, the location of...

High-temperature phases in ternary Zr–O–N systems

Zirconium aerosol was ignited and burned in atmospheric pressure air in microgravity using a 2.2-s drop tower. Combustion products were collected and analyzed using electron microscopy. The elemental composition analyses indicated that combustion product compositions fell along two linear traces on a ternary Zr–O–N diagram. Currently, the equilibrium Zr–O–N phases are not characterized at temperatures above 2000 °C, typical of zirconium combustion in air, and it is suggested that the phases detected in zirconium combustion products can serve as a guide to further studies of the Zr–O–N system at high temperatures. It is also suggested that experimental metal combustion techniques can be adopted for studying high-temperature metal–gas phase equilibria.