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Here we present analysis of a novel reactive material system that employs dinitrogen tetroxide (N2O4) as a liquid oxidizer with metal powder fuels. The oxidizer was added to micron scale aluminum and zirconium powders by a remote injection system. When ignited with a high voltage spark, the mixtures were observed to possess reactivity comparable to nanocomposite reactive materials, with open-tube flame expansion velocities from 500 to 1400 m/s depending on fuel/oxidizer ratio and tube diameter. Temperatures were observed to range from 3000 to 3500 K as measured with gray body fits to 16-channel time-resolved pyrometer and time-integrated spectrometer data. These values were significantly below calculated adiabatic flame temperatures, which we attribute to local deviations from stoichiometry and kinetic/energetic limitations similar to those observed in studies of particles burning in high pressures gaseous oxidizers. Al/N2O4 reactivity was found to be most likely limited by the vaporization of the metal from the particle surface and Zr/N2O4 was limited by slower burning and complex interactions involving the solubility of nitrogen and oxygen in the molten Zr. We also discuss the potential for these materials to be used to create an “on/off” reactive material, since N2O4 can be added remotely and driven to evaporate via vacuum or purge of inert gases to return it to a safe condition.

Time-resolved x-ray imaging is used to capture high-resolution images of the Al/Fe2O3 thermite reaction as it proceeds in an extended burn tube. The observed thermite reaction products are generally large, multi-phase composite particles. Image processing algorithms are developed to extract a continuous record of composite particle size as a function of time and distance from the ignition point. Small particles are observed immediately after ignition, followed by a rapid increase in particle size and a gradual decline toward late times. Correlating the dynamic particle size with visible light emission from the reaction, we propose a two-stage mechanism where the combination of early ignition and delayed expansion causes the material closest to the ignition point to undergo substantial coalescence before it begins moving down the tube. We also identify two mechanisms that may contribute to particle size refinement at late times, both based on the collapse of large gas bubbles that are observed preferentially in particles at earlier times. A new approach to extracting burn times from luminous intensity data is explored, and we find that a more nuanced analysis of burn time and luminous intensity may help to improve reactivity assessments when data from in situ studies is unavailable.

A common method for measuring the reactivity of rapidly deflagrating materials has been to loosely pack a desired mixture into an approx. 10 cm long×3 mm diameter acrylic tube, ignite the material on one end, and report the observed self-propagating flame velocity through the material. While this method can yield qualitative information, linking this to quantitative intrinsic properties, such as particle burn time, has remained challenging. In this work, we significantly redesign the traditional burn tube experiment. Between 25 and 250 mg of nanocomposite aluminum/copper oxide (Al/CuO) thermite is loosely packed into the capped end of a 1.8 m long tube, and the remainder of the tube is left unfilled. The material is ignited using a hot wire, resulting in a steadily-propagating luminous front, which extends part, or all, of the way down the length of the tube. We suggest the behavior is a result of “reactive entrainment”, which occurs when the reaction time scale becomes longer than the characteristic momentum relaxation time scale. When this criterion is met, and when there are significant pressure gradients present or produced during the reaction, material will be entrained by the gases before and/or during the reaction; a behavior very different from conventional thermites, which use larger particle sizes. The effect of sample mass and tube diameter on propagation velocity is investigated, and we find a linear scaling with mass and a power law scaling with tube radius (r−1.3) between 1.56 and 4.76 mm radii. For several conditions, we observe the reaction complete; defined by the distance where the propagation velocity decreases to a fixed fraction of its steady value. The ratio of quench distance to propagation velocity was found to approach a constant value of 3.29±0.70 ms, which we suggest is the burn time of the Material. For the range of masses studied in this work, the burn time was found to be independent of mass, implying that it is an intrinsic particle burn time. We expect other quantitative information can be deduced from this experiment, so long as the chosen sample mass and tube dimensions allow one to observe the full extent of reaction.

This study explores the reaction between Al and Bi2O3 nanoparticles under high heating rate conditions with temperature-jump/time of flight mass spectrometry (T-Jump/TOFMS), high speed imaging, and rapid sample heating within a scanning electron microscope (SEM). Comparison of rapid heating of Al/Bi2O3 thermite and neat Bi2O3 shows that initiation of the Al/Bi2O3 reaction occurs at a much lower temperature than the point where oxygen is released from the neat Bi2O3 powder. Thus, without the presence of a gas phase oxidizer, it can be concluded that a condensed phase initiation mechanism must be at play in the Al/Bi2O3 nanothermite. C/Bi2O3 heating experiments were used for a mechanistic comparison between two different fuel types since the carbon represents a nonvolatile fuel in contrast to the aluminum. This formulation showed a similar condensed phase initiation as was seen with the aluminum nanothermite. Qualitative comparison of high speed imaging for Al/Bi2O3 and Al/CuO indicates that the initiation ...

The effect of aluminum size on confined flame propagation velocities in thermite composites was investi- gated between 108 mm and 80 nm, and in all cases using nanometric copper oxide as the oxidizer. It was found that the velocity exhibited two distinct regimes; between 108 and 3.5 mm the velocity scaled as the particle diameter to the � 0.56 power, and becomes invariant of size below this. One explanation for the invariance is that the pressure- driven flow reaches some peak velocity, controlled by the pressure gradient, pore size, and fluid viscosity. Another ex- planation is that the system becomes limited by the inter- nal gas heating rate, defined by the intrinsic kinetic time scale, and which can significantly impact the effective parti- cle heating time. The particle heating time was calculated as a function of particle size, and as a function of gas heat- ing rates ranging from 10 5 Ks � 1 to infinity. It was found that at any finite gas heating rate, there exists a critical par- ticle diameter below which all sizes take the same amount of time to heat. This is a direct artifact of the characteristic thermal relaxation time scale; if the heating rate is not suffi- ciently fast, then the particle will rapidly equilibrate with the gas at each time step. The inverse of thermal relaxation time was used to calculate a critical heating rate defining a transition point, and which exhibits a dp 2 scaling. This scaling sets a constraint on the kinetics, which must at least scale with dp 2 to remain in the size-dependent regime.

In this study we investigate the role of gas phase oxygen on ignition of nanothermite reactions. By separately evaluating the temperature at which ten oxidizers release gas phase species, and the temperature of ignition in an aluminum based thermite, we found that ignition occurred prior to, after or simultaneous to the release of gas phase oxygen depending on the oxidizer. For some nanothermites formulations, we indeed saw a correlation of oxygen release and ignition temperatures. However, when combined with in situ high heating stage microscopy indicating reaction in the absence of O2, we conclude that the presence of free molecular oxygen cannot be a prerequisite to initiation for many other nanothermites. This implies that for some systems initiation likely results from direct interfacial contact between fuel and oxidizer, leading to condensed state mobility of reactive species. Initiation of these nanothermite reactions is postulated to occur via reactive sintering, where sintering of the particles can commence at the Tammann temperature which is half the melting temperature of the oxidizers. These results do not imply that gas phase oxygen is unimportant when full combustion commences.

This work investigated Ag2O as a potential oxidizer in antimicrobial energetic systems. Ultrafine Ag2O was synthesized, and its performance in nanoaluminum-based thermite systems was evaluated using a constant volume combustion cell. The Ag2O alone was found to be a relatively poor oxidizer, but it performed well when blended with more reactive oxidizers, CuO and AgIO3. Time-resolved mass spectrometry was used to investigate the reaction mechanism in more detail. Post-reaction analysis confirmed the production of Ag, but it was seen to exist in a matrix with Cu in the Al/CuO/Ag2O ternary system. The product in surface contact with Al2O3 suggested a reactive sintering mechanism occurred. The results indicate that Ag2O, while a poor oxidizer itself, can be integrated into more reactive systems to produce high yields of biocidal silver. The morphology of the final product, however, indicates that a large amount of the silver may not be surface-exposed, a result which would negatively impact the biocidal activity. 2012 Published by Elsevier Inc. on behalf of The Combustion Institute.

One of the open questions in understanding the reactivity of nanometric metal/metal oxide particulate composites is the relative role of gas–solid vs. condensed state reactions. This work is an investigation of several nano-Al based thermites subjected to very rapid heating rates. The ignition temperature of thermites, as measured by the onset of optical emission, was measured using a rapidly heated Pt wire. Generally, ignition was seen to occur above the melting temperature of aluminum. The exception was Al/Bi2O3 which ignited slightly below this point. Samples were also rapidly heated in situ within electron microscopes to provide direct imaging before and after heating. The sintering of aggregated and/or agglomerated particles into characteristically larger structures was experimentally observed in all cases, and the fuel and oxidizer were found to be in surface contact suggesting that a condensed-phase reactive sintering mechanism had occurred. High resolution image sequences of thermites ignited on the Pt wire were collected using a real time phase contrast imaging technique at the Advanced Photon Source of Argonne National Lab. The results suggest that reactive sintering occurs on a fast timescale, and relatively early in the reaction, leading to rapid melting and coalescence of aggregated particles. This dramatically changes the initial size and morphology of the constituents before the remainder of the material burns. The results question the idea that a decrease in particle size will necessarily lead to an enhancement in reactivity, since large amounts of sintering occurs early in the reaction, and alters the morphology. It is suggested that improvements in reactivity can be achieved by designing architectures to improve the interfacial contact upon sintering, as well as by selecting oxidizers based on their ability to liberate and transport oxygen in the condensed phase, while producing volatile species to assist in convective energy transport.

The authors investigated the ignition and reaction of Al/AgIO3 thermites for potential use in biocidal applications. Rapid-heating wire experiments were performed to measure the ignition temperature and investigate the thermal decomposition of the oxidizer using a T-Jump/TOF Mass Spectrometer and an optical emission setup. Combustion experiments inside a constant-volume pressure cell were also carried out, and the relative performance was compared with other thermite systems. The ignition temperature in air at atmospheric pressure was found to be 1215 ± 40 K. The AgIO3 was found to significantly outperform CuO and Fe2O3 oxidizers in pressurization tests, and this is attributed to the enhanced gas release as the AgIO3 thermally decomposes to release iodine in addition to oxygen. The reacted product was collected to investigate the final state of the products. Transmission electron microscopy and X-ray diffraction were performed to show that the major Ag product species was AgI, and not elemental Ag and I2....

Aluminum is traditionally used as the primary fuel in nanocomposite energetic systems due to its abundance and high energy release. However, thermodynamically boron releases more energy on both a mass and volumetric basis. Kinetic limitations can explain why boron rarely achieves its full potential in practical combustion applications, and thus has not replaced aluminum as the primary fuel in energetic systems. In particular, the existence of the naturally formed boron oxide (B 2 O 3 ) shell is believed to play a major role in retarding the reactivity by acting as a liquid barrier if it cannot be efficiently removed. In this paper we demonstrate from constant-volume combustion experiments that nanoboron can be used to enhance the reactivity of nanoaluminum-based Metastable Intermolecular Composites (MICs) when the boron is 50 mol % of the fuel. It was also observed that an enhancement was not achieved when micronboron (700 nm) was used. Thermodynamic calculations showed that the aluminum reaction with CuO was sufficient to raise the temperature above ∼ 2350 K in those mixtures which showed an enhancement. This is above both the boiling point of B 2 O 3 (2338 K) and the melting point of boron (2350 K). A heat transfer model investigates the heating time of boron for temperatures > 2350 K (the region where the enhancement is achieved), and includes three heating times; sensible heating, evaporation of the B 2 O 3 oxide shell, and the melting of pure boron. The model predicts the removal of the B 2 O 3 oxide shell is fast for both the nano- and micronboron, and thus its removal alone cannot explain why nanoboron leads to enhancement while micronboron does not. The major difference in heating times between the nano- and micronboron is the melting time of the boron, with the micronboron taking a significantly longer time to melt than nanoboron. Since the oxide shell removal time is fast for both the nano- and micronboron, and since the enhancement is only achieved when the primary reaction (Al/CuO) can raise the temperature above 2350 K, we conclude that the melting of boron is also necessary for fast reaction in such formulations. Nanoboron can very quickly be heated relative to micronboron, and on a timescale consistent with the timescale of the Al/CuO reaction, thus allowing it to participate more efficiently in the combustion. The results indicate that sufficiently small boron can enhance the reactivity of a nanoaluminum-based MIC when added as the minor component (

An experimental investigation of the combustion characteristics of boron nanoparticles in the post flame region of a flat flame burner has been conducted. Boron is attractive as a fuel or a fuel supplement in propellants and explosives due to its high heats of combustion on both a gravimetric and volumetric basis. A relatively large database exists for combustion characteristics of large (greater than 1 μm) boron particles, but very little exists for nano-sized boron. Ignition and combustion characteristics have been studied in the post flame region of a fuel lean CH4/Air/O2 flame, with burner temperatures ranging from about 1600 K to 1900 K, and oxygen mole fractions ranging between 0.1 and 0.3. As in earlier investigations on boron combustion, a two-stage combustion phenomenon was observed. Ensemble-averaged burning times of boron nanoparticles were obtained, while the ignition time measurements for boron nanoparticles were extended into a lower temperature range previously unavailable in the literature. The measured burning times were between 1.5 ms and 3.0 ms depending on both the temperature and oxygen mole fraction. The ignition times were relatively insensitive to oxygen concentration in the range studied, and were affected only by temperature. The measured ignition times were inversely related to the temperature, ranging from 1.5 ms at 1810 K to 6.0 ms at 1580 K. The burning time results were compared to both diffusion and kinetic limited theories of particle combustion. It was found that the size dependence on particle burning times did not follow either theory.

Nanocomposite thermite powders can be used in reactive parts and components. Manufacturing these components requires tailoring powder particle size distributions, particle shapes, and powder flowability. Specifically, an improved flowability is desired to use such powders as feedstock in additive manufacturing. Arrested reactive milling (ARM) offers a versatile and practical approach for preparing nanocomposite thermites with fully dense particles, which will retain their structures and mixedness between reactive components while being stored, handled, and processed. However, ARM products usually have broad particle size distributions, rock-like particle shapes, and poor flowability. Here, ARM is modified to include a low-energy milling step to tune the shapes and flowability of the prepared powders. Experiments are performed with aluminum-rich Al·Fe2O3 thermites. After the initial nanocomposite thermite is prepared in a planetary mill, it is additionally milled at a reduced rotation rate, replacing milling balls with smaller glass beads, and adding different liquid process control agents. Powders with modified particle shapes and size distributions are obtained, which have substantially improved flowability compared to the initial material. The reactivity of the initial and modified powders is evaluated using their ignition on a heated wire, by electro-static discharge, by thermal analysis (DSC) and in constant volume explosion tests. The reactivity of the modified powders is not diminished; instead, an improved reactivity is observed for selected samples.