Timothy J. Foley

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.

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.

Electrostatically self-assembled nanocomposite reactive microspheres.

Nanocomposite reactive microspheres with diameters of approximately 1-5 mum were created via electrostatic self-assembly of aluminum and cupric oxide nanoparticles. The ability to utilize this novel approach of bottom-up assembly to create these reactive materials allows for the potential for a more intimate mixture between the two nanoreactants and, thus, an overall more energetic combustion process. Experiments with the self-assembled material demonstrate the ability to achieve ignition and sustain a combustion wave in rectangular microchannels, which does not occur with material having similar amounts of organics mixed via the traditional sonication method.

The Effect of Added Al2O3 on the Propagation Behavior of an Al/CuO Nanoscale Thermite

Three types of experiments were performed on an Al/CuO nanoscale thermite to understand the effect of adding a diluent (40 nm Al2O3 particles) to the mixture: the constant volume pressure cell, the unconfined burn tray, and the instrumented burn tube. The addition of Al2O3 decreased the pressure output and reaction velocity in all three experiments. Burn tube measurements showed three reaction velocity regimes: constant velocity observed when 0% (633 m/s) and 5% (570 m/s) of the total weight is Al2O3, constant acceleration observed at 10% (146 m/s to 544 m/s over a distance of 6 cm) and 15% (69 m/s to 112 m/s over a distance of 6 cm) Al2O3, and an unstable, spiraling combustion wave at 20% Al2O3. The pressure measurements correlated to these three regimes showing a dropoff in peak pressure as Al2O3 was added to the system, with relatively no pressure increase observed when 20% of the total weight was Al2O3. Equilibrium calculations showed that the addition of Al2O3 to an Al/CuO mixture lowered the flame temperature, reducing the amount of combustion products in the gas phase, thus, hindering the presumed primary mode of forward heat transfer, convection.

Combustion of Silicon/Teflon/Viton and Aluminum/Teflon/Viton Energetic Composites

DOI: 10.2514/1.46182 The combustion of Si- and Al-based systems using polytetrafluoroethylene (PTFE) as the oxidizer and Fluorel FC 2175 (a copolymer of hexafluoropropylene and vinylidene fluoride) as a binder has been studied. Experimental data were obtained using two methods: 1) instrumented tube burns and 2) pressed pellets inside a windowed pressure vessel. Loose-powder burning rates were seen to optimize at slightly-fuel-rich mixture ratios for Si/PTFE/FC-2175 (SiTV). Al/PTFE/FC-2175 (AlTV) burning rates optimized near a stoichiometric ratio. Pressures calculated by assuming constant-volume combustion equilibrium were seen to match experimental values from burn-tube experiments when burning rates were at or near peak values. The pressure dependence of SiTV and AlTV pellet burning rates was also characterized and compared with reported Mg/PTFE/Viton (MTV) results. SiTV showed power-law dependence with a constant-pressure exponent over the experimental range of pressures. AlTV was showntoexhibitnonconstant-pressureexponentbehavior.SiTVburningratesoptimizedatmixtureratiossimilarto that of the tube burns. AlTV burning rates increased well past a stoichiometric ratio and decreased at a fuel-rich ratio, which is a similar trend to MTV burning rates.

Overview of Nanoscale Energetic Materials Research at Los Alamos

Metastable Intermolecular Composite (MIC) materials are comprised of a mixture of oxidizer and fuel with particle sizes in the nanometer range. Characterizing their ignition and combustion is an ongoing effort at Los Alamos. In this paper we will present some recent studies at Los Alamos aimed at developing a better understanding of ignition and combustion of MIC materials. Ignition by impact has been studied using a laboratory gas gun using nano-aluminum (Al) and nano-tantalum (Ta) as the reducing agent and bismuth (III) oxide (Bi 2 O 3 ) as the oxidant. As expected from the chemical potential, the Al containing composites gave higher peak pressures. It was found, for the Al/Bi 2 O 3 system, that impact velocity under observed conditions plays no role in the pressure output until approximately 100 m/s, below which speed, impact energy is insufficient to ignite the reaction. This makes the experiment more useful in evaluating the reactive performance. Replacing the atmosphere on impact with an inert gas reduced both the amount of light produced and the realized peak pressure. The combustion of low-density MIC powders has also been studied. To better understand the reaction mechanisms of burning MIC materials, dynamic electrical conductivity measurements have been performed on a MIC material for the first time. Simultaneous optical measurements of the wave front position have shown that the reaction and conduction fronts are coincident within 160 μm.

Semi-Empirical Model for Reaction Progress in Nanothermite

Calculations of thermite reaction progress were made using the Cheetah thermochemistry package. Progress was modeled by fractionally substituting reactants with an inert species with identical thermodynamic properties. The results of this model have been used to create a semi-empirical model of regression rate. The acceptability of the D 2 particle regression law for nanothermite systems of a scale on the order of 10 nm has been demonstrated. Additionally a strong sensitivity to composition is demonstrated.

Instrumented Burn Tube: Experimental Observations and Analysis of Data

experimental techniques to measure the various properties of nanoenergetic systems. Among these techniques, the instrumented burn tube is a simple experiment that is capable of obtaining data related to the combustion of these materials. The purpose of this paper is to review the current state of the conned burn tube experiment, including the drawbacks of the technique and possible remedies. As this report is intended to focus on the specic experimental technique, data from many dierent energetic materials and experimental congurations will be presented. The qualitative and quantitative data that can be gathered using conned burn tube experiments include burning rate, total impulse, and pressurization rate. All of these measurements lend insight into the combustion properties and mechanisms of specic nanoenergetic powders.

Isolator fragmentation and explosive initiation tests

Three tests were conducted to evaluate the effects of firing an isolator in proximity to a barrier or explosive charge. The tests with explosive were conducted without barrier, on the basis that since any barrier will reduce the shock transmitted to the explosive, bare explosive represents the worst-case from an inadvertent initiation perspective. No reaction was observed. The shock caused by the impact of a representative plastic material on both bare and cased PBX9501 is calculated in the worst-case, 1-D limit, and the known shock response of the HE is used to estimate minimum run-to-detonation lengths. The estimates demonstrate that even 1-D impacts would not be of concern and that, accordingly, the divergent shocks due to isolator fragment impact are of no concern as initiating stimuli.

THERMAL IGNITION OF DETONABLE HYDROGEN PEROXIDE COMPOSITIONS

Hydrogen peroxide can be mixed with a variety of fuels to produce detonable compositions. These compositions can be thermally unstable and their behavior can be difficult to predict. Furthermore, the addition of some acids to the mixture could increase its sensitivity. Presented here are the outcomes of cookoff experiments performed on hydrogen peroxide and fuels compositions, as well as an acid‐sensitized mixture. Soak temperatures of 88° C, 84° C and 82° C were used, with reaction times of 3010 seconds, 3560 seconds and 3230 seconds, accordingly. The acid‐sensitized experiment, when soaked at 82° C, reacted after just 2450 seconds.