Steven F. Son

Combustion velocities and propagation mechanisms of metastable interstitial composites

Combustion velocities were experimentally determined for nanocomposite thermite powders composed of aluminum (Al) fuel and molybdenum trioxide (MoO3) oxidizer under well-confined conditions. Pressures were also measured to provide detailed information about the reaction mechanism. Samples of three different fuel particle sizes (44, 80, and 121nm) were analyzed to determine the influence of particle size on combustion velocity. Bulk powder density was varied from approximately 5% to 10% of the theoretical maximum density (TMD). The combustion velocities ranged from approximately 600 to 1000m∕s. Results indicate that combustion velocities increase with decreasing particle size. Pressure measurements indicate that strong convective mechanisms are integral in flame propagation.

Reaction Propagation of Four Nanoscale Energetic Composites (Al/MoO3, Al/WO3, Al/CuO, and B12O3)

Nanoscale composite energetics (also known as metastable intermolecular composites) represent an exciting new class of energetic materials. Nanoscale thermites are examples of these materials. The nanoscale thermites studied consist of a metal and metal oxide with particle sizes in the 30-200 nm range. They have potential for use in a wide range of applications. The modes of combustion and reaction behavior of these materials are not yet well understood. This investigation considers four different nanoaluminum/metal-oxide composites. The same nanoscale aluminum was used for each composite. The metal oxides used were molybdenum oxide (MoO 3 ), tungsten oxide (WO 3 ), copper oxide (CuO), and bismuth oxide (Bi 2 O 3 ). The reaction performance was quantified by the pressure output and propagation velocity using unconfined (or open burn) and confined (burn tube) experiments. We examine the optimization of each composite in terms of pressure output and propagaton speed (or burn rate) for the open burn experiment. We find that there is a correlation between the maximum pressure output and optimum propagation speed (or burn rate). Equilibrium calculations are used to interpret these results. We find that the propagation speed depends on the gas production and also on the thermodynamic state of the products. This suggests that condensing gases or solidifying liquids could greatly enhance heat transfer. We also vary the density of these composites and examine the change in performance. Although the propagation wave is likely supersonic with respect to the mixture sound speed, the propagation speed decreases with density. This behavior is opposite of classical detonation in which propagation (detonation) speed increases with density. This result indicates that the propagation mechanism may differ fundamentally from classical detonations.

Melt dispersion mechanism for fast reaction of nanothermites

An unexpected mechanism for fast oxidation of Al nanoparticles covered by a thin oxide shell (OS) is proposed. The volume change due to melting of Al induces pressures of 0.1–4GPa and causes spallation of the OS. A subsequent unloading wave creates high tensile pressures resulting in dispersion of liquid Al clusters, oxidation of which is not limited by diffusion (in contrast to traditional mechanisms). Physical parameters controlling this process are determined. Methods to promote this melt dispersion mechanism, and consequently, improve efficiency of energetic nanothermites are discussed.

Combustion of Nanoscale Al/MoO3 Thermite in Microchannels

Abstract : Microscale combustion is of interest in small-volume energy-demanding systems, such as power supplies, actuation, ignition, and propulsion. Energetic materials can have high burning rates that make these materials advantageous, especially for microscale applications in which the rate of energy release is important or in which air is not available as an oxidizer. In this study we examine the combustion of mixtures of nanoscale aluminum with molybdenum trioxide in microscale channels. Nanoscale composites can have very high burning rates that are much higher than typical materials. Quartz and acrylic tubes are used. Rectangular steel microchannels are also considered. We find that the optimum mixture ratio for the maximum propagation rate is aluminum rich. We use equilibrium calculations to argue that the propagation rate is dominated by a convective process where hot liquids and gases are propelled forward heating the reactants. This is the first study to report the dependence of the propagation rate with a tube diameter for this class of materials.Wefind that the propagation rate decreases linearly with 1=d. The propagation rate remains high in tubes or channels with dimensions down to the scale of 100 m, which makes these materials applicable to microcombustion applications.

Mechanochemical mechanism for fast reaction of metastable intermolecular composites based on dispersion of liquid metal

An unexpected mechanism for fast reaction of Alnanoparticles covered by a thin oxide shell during fast heating is proposed and justified theoretically and experimentally. For nanoparticles, the melting of Al occurs before the oxide fracture. The volume change due to melting induces pressures of 1–2 GPa and causes dynamic spallation of the shell. The unbalanced pressure between the Al core and the exposed surface creates an unloading wave with high tensile pressures resulting in dispersion of atomic scale liquid Al clusters. These clusters fly at high velocity and their reaction is not limited by diffusion (this is the opposite of traditional mechanisms for micron particles and for nanoparticles at slow heating). Physical parameters controlling the melt dispersion mechanism are found by our analysis. In addition to an explanation of the extremely short reaction time, the following correspondence between our theory and experiments are obtained: (a) For the particle radius below some critical value, the flame propagation rate and the ignition time delay are independent of the radius; (b) damage of the oxide shell suppresses the melt dispersion mechanism and promotes the traditional diffusive oxidation mechanism; (c) nanoflakes react more like micron size (rather than nanosize) spherical particles. The reasons why the melt dispersion mechanism cannot operate for the micron particles or slow heating of nanoparticles are determined. Methods to promote the melt dispersion mechanism, to expand it to micron particles, and to improve efficiency of energetic metastable intermolecular composites are formulated. In particular, the following could promote the melt dispersion mechanism in micron particles: (a) Increasing the temperature at which the initial oxide shell is formed; (b) creating initial porosity in the Al; (c) mixing of the Al with a material with a low (even negative) thermal expansion coefficient or with a phase transformation accompanied by a volume reduction; (d) alloying the Al to decrease the cavitationpressure; (e) mixing nano- and micron particles; and (f) introducing gasifying or explosive inclusions in any fuel and oxidizer. A similar mechanism is expected for nitridation and fluorination of Al and may also be tailored for Ti and Mg fuel.

The role of gas permeation in convective burning

Abstract Convective burning is commonly identified in the literature as the key step in deflagration-to-detonation transition (DDT) of granular explosives. The prevalent physical picture of convective burning is of rapid and deep penetration of hot gases which controls the propagation rate via convective heat transfer. This investigation includes a review of relevant literature, new transient measurements of permeability at high pressures, and analysis of the experimental results. Results presented here show that deep penetration (many particle diameters) of gas at high velocities is not physically plausible for the low porosity granular beds of interest. The measured permeabilities are consistent with measurements made at lower pressures in similar materials, but are significantly lower than predictions based on beds of spherical particles. The important time and space scales of this experiment are identified. The interface region between the reservoir and porous bed is analysed. The wave hierarchy of the permeation experiment is studied, and short- and long-time limits are investigated using simplified asymptotic analysis. The low-speed flow approximation is also considered for flow within the bed. It is shown that drag dissipation terms in the energy equation cannot be neglected under adiabatic conditions as is commonly done. These results indicate that compaction processes play a larger role than commonly thought, and motivate the consideration of an asymptotic large drag limit of two-phase, two-velocity models.

Hypergolic Ionic Liquids to Mill, Suspend and Ignite Boron Nanoparticles

Boron nanoparticles prepared by milling in the presence of a hypergolic energetic ionic liquid (EIL) are suspendable in the EIL and the EIL retains hypergolicity leading to the ignition of the boron. This approach allows for incorporation of a variety of nanoscale additives to improve EIL properties, such as energetic density and heat of combustion, while providing stability and safe handling of the nanomaterials.

Kinetics of high temperature reaction in Ni-Al system: influence of mechanical activation.

High temperature (>1000 K) reaction kinetics in the stoichiometric (1:1 by molar ratio) Al-Ni system was investigated by using the, so-called, electrothermal analysis (ETA) method. ETA is the only technique that allows studying kinetics of a heterogeneous gasless reaction at temperatures above the melting points of the precursors. Special attention was focused on methodological aspects of the ETA method. Two different reaction systems were studied: (i) initial Al/Ni clad particles; (ii) the same powders but after 15 min of high energy ball milling. Analysis of the obtained results leads to the conclusion that such mechanical treatment decreases the apparent activation energies of the reaction in the Ni-Al system, from 47 +/- 7 kcal/mol for the initial powder to 25 +/- 3 kcal/mol after ball milling. Comparison of these data with those reported previously was also made.

Quasi-steady combustion modeling of homogeneous solid propellants

Abstract Classical, linearized quasi-steady (QS) theory of unsteady combustion of homogeneous solid propellants (both pressure- and radiation-driven) is reexamined. Zeroth order, high activation energy (E/RT ⪢ 1), decomposition is assumed. Many prevailing ideas about condensed-phase pyrolsis are challenged and several misconceptions are corrected. The results show the following: (1) the inadequacy of simple Arrhenius surface pyrolysis; (2) that the common assumption of zero Jacobian (δ or ns) parameter is physically unrealistic for pressure-driven combustion except perhaps in plateau regions; (3) that classical quasi-steady theory is not necessarily incompatible with observed pressure instability [Re{Rp} > 0] in mesa propellants; (4) that measured steady-state combustion parameters (e.g., Ec = 2Es = 40 kcal/mol for double base propellant) and quasi-steady theory can model T-burner data reasonably well; (5) that the preexponential parameters in the pyrolysis expression play a critical role in the dynamic response (particularly T0 and Qc); (6) that thermal radiation also plays an important role through its effect on the steady state sensitivity parameters, particularly the k (or σp) parameter. An approach is outlined for modeling dynamic combustion response based on zeroth order pyrolysis which allows difficult parameters, such as r and δ (or A and ns) to be obtained from relatively easily measured ones, k and ν (or B and n), Ec, Qc, and Ts. An approach for determining these fundamental combustion parameters using radiation-driven unsteady burning tests is described.

Combustion and Conversion Efficiency of Nanoaluminum-Water Mixtures

An experimental investigation on the combustion behavior and conversion efficiency of nanoaluminum and liquid water mixtures was conducted. Burning rates and chemical efficiency of aluminum-water and aluminum-water-poly(acrylamide-co-acrylic acid) mixtures were quantified as a function of pressure (from 0.12 to 15 MPa), nominal aluminum particle size (for diameters of 38, 50, 80, and 130 nm), and overall equivalence ratios (0.67 < φ < 1.0) under well-controlled conditions. Chemical efficiencies were found to range from 27 to 99% depending upon particle size and sample preparation. Burning rates increased significantly with decreased particle size attaining rates as high as 8 cm/s for the 38 nm diameter particles above approximately 4 MPa. Burning rate pressure exponents of 0.47, 0.27, and 0.31 were determined for the 38, 80, and 130 nm diameter particle mixtures, respectively. Also, mixture packing density varied with particle size due to interstitial spacing, and was determined to affect the burning rates at high pressure due to inert gas dilution. The presence of approximately 3% (by mass) poly(acrylamide-co-acrylic acid) gelling agent to the nAl/H2O mixtures had a small, and for many conditions, negligible effect on the combustion behavior.