Spray combustion from a thermite reaction is a new area of research relevant to localized energy generation applications, such as welding or cutting. In this study, we characterized the heat flux of combustion spray impinging on a target from a nozzle for three thermite mixtures. The reactions studied include aluminum (Al) with iron oxide (Fe2O3), Al with copper oxide (CuO), and Al with molybdenum oxide (MoO3). Several standoff distances (i.e., distance from the nozzle exit to the target) were analyzed. A fast response heat flux sensor was engineered for this purpose and is discussed in detail. Results correlated substrate damage to a threshold heat flux of 4550 W/cm(2) for a fixed-nozzle configuration. Also, higher gas-generating thermites were shown to produce a widely dispersed spray and be less effective at imparting kinetic energy damage to a target. These results provide an understanding of the role of thermal and physical properties (i.e., such as heat of combustion, gas generation, and particle size) on thermite spray combustion performance measured by damaging a target substrate.

Composite energetic material response to electrical stimuli was investigated and a correlation between electrical conductivity and ignition sensitivity was examined. The composites consisted of micrometer particle aluminum combined with another metal, metal oxide, or fluoropolymer. Of the nine tested mixtures, aluminum (Al) with copper oxide (CuO) was the only mixture to ignite by electrostatic discharge. Under the loose powder conditions of these experiments, the Al–CuO minimum ignition energy (MIE) is 25mJ and exhibited an electrical conductivity two orders of magnitude higher than the next composite. This study showed a similar trend in MIE for ignition triggered by a discharged spark compared with a thermal hot wire source.

anometer aluminum fuel particles demonstrate orders of magnitude higher ignition sensitivity and reactivity than micrometer aluminum particles. This enhanced ignition sensitivity could enable a submerged reaction to propagate and react to completion without quenching. Energetic composites consisting of micrometer Al fuel particles cannot react submerged because too much energy from the reaction is lost to the surrounding water, causing the reactants to quench. However, Al nanoparticles can be synthesized with significantly higher surface-area-to-volume ratios that enable them to exhibit new and unique combustion properties that are much improved over their micron scale counterparts. One such property is enhanced ignition sensitivity, which has the potential to open a new realm of underwater reaction applications. These could include fuses, propulsion, underwater ordnance, torch or metal cutting technologies, offshore oil drilling, blasting, and welding, as well as alternative power sources in oceanic environments. Polytetrafluoroethylene (PTFE) (CnF2n+2) is an ideal oxidizer for underwater reactions because it is a fluorine compound and it is hydrophobic. An oxidizer is a reactant that either releases oxygen atoms or gains electrons in an oxidation-reduction (redox) reaction. Fluorine compounds are powerful oxidizers because fluorine is the most electronegative element and gains electrons more readily than oxygen. Also, because PTFE is hydrophobic, a mixture containing PTFE should reduce or eliminate permeation of water through the reactants, thus preventing loss of reaction energy to the water, permitting the reaction to go to completion. Combustion reactions can be categorized by their speed. Reactions that take place at supersonic speeds via shock compression are called detonations; whereas reactions that take place at subsonic speeds are called deflagrations. Most flames observed in daily life are deflagrations, and they propagate via modes of thermal energy transport (conduction, convection, radiation, for example). The Al-PTFE reactions discussed here fall into the deflagration category of combustion reactions. The submerged mixture is ignited by a heated Nichrome wire. As the PTFE is heated, the CnF2n+2 chain decomposes and releases fluorine gas, which is highly reactive with aluminum. No current studies are available in the literature that demonstrate a naturally hydrophobic solid energetic composite’s ability to burn or deflagrate while submerged underwater. However, this study proves that with nanometric particles, this reaction underwater is possible. Furthermore, we have characterized the submerged reaction dynamics. Understanding the reaction dynamics will help engineers design equipment for various underwater applications.

Characterizing the combustion behaviors of energetic materials requires diagnostic tools that are often not readily or commercially available. For example, a jet of thermite spray provides a high temperature and pressure reaction that can also be highly corrosive and promote undesirable conditions for the survivability of any sensor. Developing a diagnostic to quantify heat flux from a thermite spray is the objective of this study. Quick response sensors such as thin film heat flux sensors can not survive the harsh conditions of the spray, but more rugged sensors lack the response time for the resolution desired. A sensor that will allow for adequate response time while surviving the entire test duration was constructed. The sensor outputs interior temperatures of the probes at known locations and utilizes an inverse heat conduction code to calculate heat flux values. The details of this device are discussed and illustrated. Temperature and heat flux measurements of various thermite spray conditions are reported. Results indicate that this newly developed energetic material heat flux sensor provides quantitative data with good repeatability.

Nanometer aluminum fuel particles demonstrate orders of magnitude higher ignition sensitivity and reactivity than micrometer aluminum particles. This enhanced ignition sensitivity could enable a submerged reaction to propagate and react to completion without quenching. Energetic composites consisting of micrometer Al fuel particles cannot react submerged because too much energy from the reaction is lost to the surrounding water, causing the reactants to quench. However, Al nanoparticles can be synthesized with significantly higher surface-area-to-volume ratios that enable them to exhibit new and unique combustion properties that are much improved over their micron scale counterparts.

Nanoparticles of tungsten trioxide (WO3) were synthesized using sol−gel chemistry and combined with nanoscale aluminum (Al) particles to form a thermite. The as-made sol−gel-derived WO3 aerogel contains impurities such as hydroxyl groups that are inherent to the synthesis process. These impurities can be removed via heat-treating the powder, and both as-made and heat-treated mixtures were examined for ignition and flame propagation as a function of the mixture bulk density. Results showed that the hydroxyl impurity impedes flame propagation in high density (compressed pellets) by acting as a heat sink and absorbing energy during flame propagation. In loose powders (low density mixtures), convection plays a more dominant role in flame propagation, such that the reduced particle size of the as-made WO3 produces higher flame propagation speeds than the enhanced thermal transport properties associated with the heat-treated WO3.

In the field of composite energetic materials, properties such as ingredient distribution, particle size, and morphology affect both sensitivity and performance. Since the reaction kinetics of composite energetic materials are typically controlled by the mass transport rates between reactants, one would anticipate new and potentially exceptional performance from energetic nanocomposites. We have developed a new method of making nanostructured energetic materials, specifically explosives, propellants, and pyrotechnics, using sol-gel chemistry. A novel sol-gel approach has proven successful in preparing nanostructured metal oxide materials. By introducing a fuel metal, such as aluminum, into the nanostructured metal oxide matrix, energetic materials based on thermite reactions can be fabricated. Two of the metal oxides are tungsten trioxide and iron(III) oxide, both of which are of interest in the field of energetic materials. Due to the versatility of the preparation method, binary oxidizing phases can also be prepared, thus enabling a potential means of controlling the energetic properties of the subsequent nanocomposites. Furthermore, organic additives can also be easily introduced into the nanocomposites for the production of nanostructured gas generators. The resulting nanoscale distribution of all the ingredients displays energetic properties not seen in its micro-scale counterparts due to the expected increase of mass transportmore » rates between the reactants. The unique synthesis methodology, formulations, and performance of these materials will be presented. The degree of control over the burning rate of these nanocomposites afforded by the compositional variation of a binary oxidizing phase will also be discussed. These energetic nanocomposites have the potential for releasing controlled amounts of energy at a controlled rate. Due to the versatility of the synthesis method, a large number of compositions and physical properties can be achieved, resulting in energetic nanocomposites that can be fabricated to meet specific safety and environmental considerations.« less

Making solid energetic materials requires the physical mixing of solid fuels and oxidizers or the incorporation of fuel and oxidizing moieties into a single molecule. The former are referred to as composite energetic materials (i.e., thermites, propellants, pyrotechnics) and the latter are deemed monomolecular energetic materials (i.e., explosives). Mass diffusion between the fuel and oxidizer is the rate controlling step for composite reactions while bond breaking and chemical kinetics control monomolecular reactions. Although composites have higher energy densities than monomolecular species, they release that energy over a longer period of time because diffusion controlled reactions are considerably slower than chemistry controlled reactions. Conversely, monomolecular species exhibit greater power due to more rapid kinetics than physically mixed energetics. Reducing the diffusion distance between fuel and oxidizer species within an energetic composite would enhance the reaction rate. Recent advances in nanotechnology have spurred the development of nano-scale fuel and oxidizer particles that can be combined into a composite and effectively reduce diffusion distances to nano-scale dimensions or less. These nanocomposites have the potential to deliver the best of both worlds: high energy density of the physically mixed composite with the high power of the monomolecular species. Toward this end, researchers at Lawrence Livermore National Laboratory (LLNL) developed nano-particle synthesis techniques, based on sol-gel chemistry, for the production of thermite nanocomposites.

Nanocomposite thermites are attractive materials for their diverse applications from metallurgy to ordnance technologies. While there are a plethora of combinations of fuel and oxidizers, this work shows that the composite's overall performance is intimately tied to how the fuel and oxidizer are prepared and combined. Comparison of the combustion velocities of two separate ternary mixtures of Al-Fe(2)O(3)-SiO(2), one prepared in situ using sol-gel processing and the other prepared by physically mixing discrete nanoscale particles, demonstrated different burning behaviors as a result of preparation technique. The stoichiometry of the two sets of thermite was varied to examine the influence of SiO(2) on combustion velocity as a means to control the reaction behavior. For pure Fe(2)O(3) + Al reactions, results show that the sol-gel synthesized materials (40 m/s) exhibit increased velocities over the physically mixed materials (9 m/s) by approximately 4 times. This trend is not observed, however, upon addition of SiO(2) to the thermite mixture; ternary thermites with 40 wt % SiO(2) showed decreased burn velocities of 0.02 m/s for sol-gel prepared thermites compared to 0.2 m/s for their physically mixed counterparts. The observed trends are believed to be caused by the unique mixing between the Fe(2)O(3) and SiO(2) phases resulting from the two synthesis techniques.