Blaine W. Asay

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.

A constitutive model for the non-shock ignition and mechanical response of high explosives

Abstract An understanding of the non-shock ignition properties of energetic particulate composite materials, high explosives such as PBX-9501 is an important part of the safety assessments for conventional handling (transportation, storage, etc.) of weapons systems including assembly operations. This paper develops and demonstrates the use of a numerical constitutive model for PBX-9501 that includes viscoelastic response, statistical fracture mechanics, and an ignition hot-spot mechanism. The intent is that this model can be used in safety analyses involving accidents to prevent undesirable dispersion of Pu. The parameters have been determined that will predict the mechanical response and ignition:non-ignition of a set of experiments that have explored the non-shock properties of this material.

The β-δ phase transition in the energetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine: Thermodynamics

In this paper we present second harmonic generation (SHG) experiments designed to confirm the mechanism and quantify the transformation kinetics of the β–δ solid state phase transition in the organic nitramine molecule octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The β phase adopts a centrosymmetric crystallographic configuration (P21/c) while the δ phase adopts a noncentrosymmetric one (P61(P65)). As expected, this results in a very poor generation of SHG intensity from the β phase, while the δ phase is very efficient, rivaling KH2PO4 in absolute efficiency. SHG thus provides a very high sensitivity zero background probe of the δ phase. We discuss the use of this signal as a quantitative measure of the δ phase mole fraction in ensembles of free HMX crystals and crystals embedded in a visco–elastic polymer matrix. We report imaging experiments where the spatial characteristics of the transformation are shown to be consistent with nucleation from a low density of initial sites, followed by rapid...

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.

High-irradiance laser ignition of explosives

A current issue important to high explosive safety is deflagration-to-detonation transitions (DDTs) in accident scenarios. In order to better understand the reactive mechanisms involved in DDT and to begin to approach the fast ignition and heating rates seen in DDT, high-irradiance ( h 800W/cm 2 ) CO 2 laser ignition experiments were performed on the common high explosives octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Reported data include time to ignition as a function of laser irradiance, energy, and ignition temperature. A simple dual ignition criteria model (DICM) was used to interpret the HMX results. The DICM requires two basic criteria for ignition: (1) a minimum surface temperature must be reached and (2) a minimum energy concentration must exist within the solid. The DICM sucessfully predicted the slope transition trend and the critical ignition energy for HMX to within 10% of the measured values. TATB had a single dependence on irradiance over the entire range of heating rates.

On the nucleation mechanism of the β-δ phase transition in the energetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

In our previous work on the mechanism for the β-δ solid-solid phase transition in octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), we used an empirical mechanism for the nucleation step and hypothesized a defect mechanism that was greatly affected by the presence of a nitroplasticizer/estane binder in the HMX formulation. Since then, we have acquired further evidence for this and have separated out the components of the binder to confirm that it is the nitroplasticizer that controls the nucleation energy in HMX formulations containing a nitroplasticizer/estane binder. While the exact distribution of nucleation energies as a function of synthesis route/defect type has not been worked out, it is likely that the solubility of the HMX in the nitroplasticizer is responsible for lowering the nucleation energy at the crystal surface, and therefore determines the nucleation rate for the formulation.