Nanocomposite Thermite Research Articles

Thermo-chemical and energetic properties of layered nano-thermite composites

Thermochemical properties and microstructures of layered aluminum (Al) and iron oxide (Fe2O3) nano-thermite were investigated via thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) for developing a new type of metastable intermolecular composites (MIC). The nanoparticles of Al and Fe2O3 were dispersed in solution and deposited separately on stainless steel electrodes by means of the electrophoretic deposition (EPD) process. These nanoparticles were dispersed and deposited without any surfactant and additive, which eliminates potential contaminates in products. In order to produce layered structures, nanoparticles were deposited in a sequence which promotes effective bonding between two different types of nanoparticles. SEM images demonstrate a layered structure with the intimate contact between Al and Fe2O3. DSC data was collected to characterize the onset temperature and energy release per unit mass from the layered composites with different molar ratios of Al/Fe2O3. It is revealed, based on the consistent onset temperatures from different composites, that the ignition of Al and Fe2O3 layers is closely associated with oxygen which is produced from thermal decomposition of Fe2O3. In addition, the thickness of the reaction zone is restricted by the diffusion length of oxygen across the layer-to-layer interface, which results in a varying degree of reaction completion within the composite and subsequently the distinct energy release values per unit mass. The decomposition of Fe2O3 nanoparticles, diffusion path of oxygen and interfacial contact between Al and Fe2O3 layers are found to determine the energetic properties of layered thermite composites.

Combustion Characteristics of Stoichiometric Al-CuO Nanocomposite Thermites Prepared by Different Methods

ABSTRACTThree nanocomposite materials with the same nominal stoichiometric thermite composition of 2Al-3CuO were prepared by three different methods: ultrasonic mixing (USM) of constituent nanopowders, electrospraying (ES), and arrested reactive milling (ARM). Prepared powders were placed in a 6.7-mm diameter, 0.5-mm-deep cavity in a brass substrate and ignited by electro-static discharge. The experiments were performed in air, argon, and helium. The mass of powder removed from the sample holder after ignition was measured in each test. Using a multi-anode photo-multiplier tube coupled with a spectrometer, time-resolved light emission traces produced by the ignited samples were recorded in the range of wavelengths of 373–641 nm. Time-resolved temperatures were then determined by fitting the recorded spectra assuming Planck’s black body emission. Temporal pressure generated by ignition events in the enclosed chamber showed that the powder’s combustion properties were tied to both their preparation technique as well as the environment they were ignited in. We found that agglomeration of nanoparticles hindered combustion of USM powders; while it was not observed for the ES powders. In oxygen-free gas environments, lower temperatures and pressures were observed for USM and ES powders prepared using starting nano-particles. For the ES powders, the effect of gas environment was less significant, which was interpreted considering that ES materials included gasifying nitrocellulose binder, enhancing heat, and mass transfer between individual Al and CuO particles. Higher pressures and temperatures were observed in inert environments for the ARM-prepared powder.

Modes of Ignition of Powder Layers of Nanocomposite Thermites by Electrostatic Discharge

ABSTRACTNanocomposite powders with aluminum as a fuel and oxides of molybdenum, copper, bismuth, and iron as oxidizers were prepared by arrested reactive milling. The powders were placed in 0.5-mm-thick layers and ignited by electrostatic discharge (ESD) in air. In different tests, time-resolved light emission was recorded at 568 nm or in the range of 373–641 nm. The amount of material consumed was recorded as well. Time-resolved temperatures were determined. Two distinct ignition regimes were observed. Prompt ignition occurred within 10 µs of the electric discharge, comparable to what had previously been observed for corresponding powder monolayers. This ignition mode was observed for composites with Bi2O3 and Fe2O3 ignited with a 12-kV discharge, whereas it only occurred at higher spark voltage (20 kV) and energy for CuO and MoO3 composites. Delayed ignition, occurring after 0.1–1 ms following the discharge, was observed for all composites with consistently stronger light emission. Analysis of quenched, partially burned particles showed that the original nanostructure was preserved after prompt ignition but not after delayed ignition. It is proposed that prompt ignition represents direct ESD initiation of composite particles rapidly and adiabatically preheated to high temperatures while keeping the nanostructure intact, resulting in a heterogeneous reaction consuming most of the aluminum. Delayed ignition occurs when particles preheated to lower temperatures start oxidizing at much lower rates, leading to cloud combustion, in which thermal interaction between individual aerosolized burning particles is substantial. During this process, the nanostructure may be lost. Temperature measurements show that nanocomposites with CuO and MoO3 burned superadiabatically with flame temperatures exceeding thermodynamic predictions.

Energy storage materials with oxide-encapsulated inclusions of low melting metal

A composite energy storage material (ESM) with inclusions of a low-melting point metal encapsulated in protective metal oxide shells is prepared and characterized. ESM precursor is a metal matrix composite with metal oxide inclusions. Controlled redox reaction between the matrix and inclusions yields a composite with inclusions of an oxide encapsulated, easy to melt metal. Al/Bi2O3 nanocomposite thermites served as precursors for ESM with Al matrix and Bi inclusions encapsulated with Al2O3. Thermal and mechanical properties of the ESM were compared to those of reference binary Al/Bi and ternary Al/Bi/Al2O3 materials. The material containing encapsulated Bi inclusions stored energy by melting and solidifying Bi inclusions repeatedly. The inclusions did not drift upon heating; composite structure remained largely intact upon thermal cycling. Conversely, the structure of reference materials with similar compositions but without encapsulated Bi inclusions was not preserved upon thermal cycling. Molten Bi tended to separate from Al and form coarse network of inclusions. Upon cycling, Bi accumulated on surface of the material. Difference in mobility between encapsulated and non-encapsulated Bi inclusions caused differences in the mechanical properties of the composites. Materials with encapsulated Bi inclusions were much stronger and capable of retaining their structure and strength despite thermal cycling.

A micro-chip initiator with controlled combustion reactivity realized by integrating Al/CuO nanothermite composites on a microhotplate platform

The interfacial contact area between the fuel and oxidizer components plays an important role in determining the combustion reactivity of nanothermite composites. In addition, the development of compact and reliable ignition methods can extend the applicability of nanothermite composites to various thermal engineering fields. In this study we report the development of a micro-chip initiator with controlled combustion reactivity using concepts usually applied to microelectromechanical systems (MEMS) and simple nanofabrication processes. The nanothermite composites fabricated in this study consisted of aluminum nanoparticles (Al NPs) as the fuel and copper oxide nanoparticles (CuO NPs) as the oxidizer accumulated on a silicon oxide substrate with a serpentine-shaped gold (Au) electrode. The micro-chip initiator rapidly ignited and exploded when minimal current was supplied. The effects of stacking structures of Al and CuO-based multilayers on the combustion properties were systematically investigated in terms of the pressurization rate, peak explosion time, and heat flow. Pressurization rates of 0.004–0.025 MPa μs−1 and heat flows of 2.0–3.8 kJ g−1 with a commonly fast response time of less than 20 ms could be achieved by simply changing the interfacial structures of the Al and CuO multilayers. The controllability of combustion reactivity of micro-chip initiator can be made for general nanothermite composites composed of Al and various metal oxides (e.g. Fe2O3, CuO, KMnO4, etc). The micro-chip initiator fabricated in this study was reliable, compact, and proved to be a versatile platform, exhibiting controlled combustion reactivity and fast response time, which could be used for various civilian and military thermal engineering applications, such as in initiators and propulsion, welding, and ordinance systems.

Preparation and Properties of Boron‐Based Nano‐B/NiO Thermite

AbstractBoron particles have several major burning problems, such as incomplete combustion, poor ignitability, and a complex burning process in solid propellants. It is documented that the low ignitability and combustion efficiency of boron are caused by the oxidation of its surface. In order to improve the combustion efficiency of boron particles, a precipitation method was employed to prepare nanometer‐sized NiO and coat it on boron particles. The morphology and coating results of the B/NiO nanocomposite thermite were characterized using different approaches such as SEM, X‐ray Diffraction (XRD), and EDS. The results indicated that the boron particles were well distributed and coated completely by nanocomposite NiO. The B/NiO nanocomposite thermite reaction process was tested by TG‐DTA. The results showed that the reaction temperature of B/NiO particles is about 30 °C lower than that of boron particles. The B/NiO thermite and boron powder were added to Mg/PTFE propellant to be measured for their respective combustion performance. The results showed that the burning rate of the B/NiO‐Mg/PTFE propellant increased by 22.8–25.2 %, mass burning rate by 26.7–30.8 %, and combustion temperature increased by 8–56 °C compared to the B‐Mg/PTFE propellant. The above results indicate that NiO coating of boron particles has a significant effect on the combustion behavior and increases the combustion performance of the propellant compared with uncoated particles.

Electro-Static Discharge Ignition of Monolayers of Nanocomposite Thermite Powders Prepared by Arrested Reactive Milling

Reactive nanocomposite powders with aluminum as a fuel and copper, bismuth, iron, and molybdenum oxide as oxidizers: 2Al·3CuO, 2.35Al·Bi2O3, 2Al·Fe2O3, and 2Al·MoO3, were prepared by arrested reactive milling, placed in monolayers on a conductive substrate, and ignited by an electro-static discharge (ESD) or spark in air, argon, and vacuum. The ESD was produced by discharging a 2000 pF capacitor charged to a voltage, which varied from 5 to 20 kV. Emission from ignited particles was monitored using a photomultiplier equipped with an interference filter. Experimental variables included particle sizes, milling time used to prepare composite particles, surrounding environment, and starting ESD voltage. All materials ignited in all environments, producing individual burning particles that were ejected from the substrate. The spark duration varied from 1 to 5 μs; the duration of the produced emission pulse was in the range of 80–250 μs for all materials studied. The longest emission duration was observed for the nanocomposite thermite using MoO3 as an oxidizer. The reaction rates of the ESD-initiated powders were defined primarily by the scale of mixing of and reactive interface area between the fuel and oxidizer in composite materials rather than by the external particle surface or particle dimensions. In vacuum, particles were heated by ESD while remaining on the substrate until they began generating gas combustion products. In air and argon, particles initially pre-heated by ESD were lifted and accelerated to ca. 100 m/s by the generated shock wave; the airborne particles continued self-heating due to heterogeneous redox reactions.

Electro-static discharge ignition of monolayers of nanocomposite thermite powders prepared by Arrested Reactive Milling

AbstractReactive nanocomposite powders with compositions 2Al∙3CuO, 2.35Al∙Bi2O3, 2Al∙Fe2O3, and 2Al∙MoO3 were prepared by arrested reactive milling, placed in monolayers on a conductive substrate and ignited by an electro-static discharge (ESD) or spark in air, argon, and vacuum. The ESD was produced by discharging a 2000 pF capacitor charged to a voltage varied from 5 to 20 kV. Emission from ignited particles was monitored using a photomultiplier equipped with an interference filter. Experimental variables included particle sizes, milling time used to prepare composite particles, surrounding environment, and starting ESD voltage. All materials ignited in all environments, producing individual burning particles that were ejected from the substrate. The spark duration varied from 1 to 5 µs; the duration of the produced emission pulse was in the range of 80 – 250 µs for all materials studied. The longest emission duration was observed for the nanocomposite thermite using MoO3 as an oxidizer. The reaction rates of the ESD-initiated powders were defined primarily by the scale of mixing of and reactive interface area between the fuel and oxidizer in composite materials rather than by the external particle surface or particle dimensions. In vacuum, particles were heated by ESD while remaining on the substrate until they began generating gas combustion products. In air and argon, particles initially pre-heated by ESD were lifted and accelerated to ca. 100 m/s by the generated shock wave; the airborne particles continued self-heating due to heterogeneous redox reactions.

Ignition of Nanocomposite Thermites by Electric Spark and Shock Wave

AbstractNanocomposite 8Al ⋅ MoO3 thermite particles were prepared using arrested reactive milling and ignited using two experimental techniques. In spark ignition, a monolayer of powder was placed on a conductive substrate and heated in air by a pulsed electrostatic discharge. In shock ignition, an individual particle was targeted by a miniature, laser‐driven flyer plate accelerated to a speed in the range of 0.5–2 km s−1. In both experiments, time‐dependent optical emission produced by the ignited material was monitored and recorded. The heating rates achieved in the present experiments are on the order of 109−–1011 K s−1. These ignition methods result in a very fast combustion with characteristic burn times reduced by 1–3 orders of magnitude compared to the burn times measured previously for the same material ignited in the CO2 laser beam, where it was heated at a much lower rate of about 106−–107 K s−1. Ignition delays observed in both shock and spark ignition experiments are close to each other and vary in the range of 120–200 ns. The times of characteristic rapid increase in the optical emission of the ignited particles are also close to each other for the two experiments; however, these times are somewhat shorter (less than one μs) for the spark ignition tests compared to few μs observed for the shock initiated particles. Preliminary ideas enabling one to interpret the present results are discussed. This work establishes an approach for systematic studies of high rate ignition and respective combustion of nanocomposite reactive materials.

Ignition of Fully Dense Nanocomposite Thermite Powders by an Electric Spark

Electrostatic discharge ignition of nanocomposite thermites prepared by Arrested Reactive Milling was investigated. The powders 2Al–3CuO and were placed in a sample holder and subjected to an electrostatic discharge from a discharging capacitor. Their optical emission was monitored. Two ignition modes were detected: immediate ignition of individual particles and delayed ignition of a cloud of aerosolized powder. Experiments addressed the effects of material composition and morphology, aging, energy input, layer thickness, and environment on the ignition mode and ignition delays. Prepared thermites were blended with pure metal powders to study ignition of such blends. Individual particle ignition was observed for samples prepared as monolayers in air and argon and for any samples in vacuum. For thicker samples in air and in argon, a delayed powder ignition mode was observed. The delays varied from to several milliseconds. Powders of 2Al–3CuO had shorter ignition delays compared with . For both materials, delays decreased for the powders prepared using longer milling times. For 2Al–3CuO, aged powders ignited after longer delays compared with the freshly prepared powders; however, shorter delays were observed for the aged . Electrostatic discharge energy and layer thickness (except for a monolayer) did not affect the ignition delay. Blending nanocomposite thermites with Al and Ti powders reduced their electrostatic discharge ignition sensitivity and caused longer ignition delays.

Aluminum Hydride as a Fuel Supplement to Nanothermites

An experimental study was conducted in which aluminum hydride (alane, ) replaced nanoaluminum incrementally as a fuel in a nanocomposite thermite based on CuO, , and . Pressure cell and burn tube experiments demonstrated enhancements in absolute pressure, pressurization rate, and burning velocity when micron-scale aluminum hydride was used as a minor fuel component in a nanoaluminum–copper-oxide thermite mixture. Peak pressurization rates were found when the aluminum hydride made up about 25% of the fuel by mole. Pressurization rates increase by a factor of about two with the addition of , whereas burn tube velocities increase by about 25%. The enhancement in pressurization rate appears to primarily be a result of the increased pressure associated with the decomposition in the nanocomposite thermite system and an enhancement in convective heat transfer. Similar experiments were conducted with micron-scale aluminum in place of the aluminum hydride, which resulted in a reduction of all the previously mentioned parameters with respect to the baseline nanoaluminum–copper-oxide thermite. The addition of any amount of alane to iron oxide based thermite resulted in a reduction in performance in pressure cell testing. The performance of based thermite was largely unaffected by alane until alane became the majority fuel component. These results have been found to correlate with changes in the combustion mechanism through equilibrium calculations.

High-Performance Nanothermite Composites Based on Aloe-Vera-Directed CuO Nanorods

In this work, we demonstrate the development of high-performance nanothermite composites derived from super-reactive CuO nanorods oxidizers fabricated by simple biogenic routes using Aloe vera plant extracts. Nanorods of various length scales have been realized via simple sonoemulsion and solid-state biosynthesis routes using Aloe vera gel as a green surfactant promoting the directional growth of CuO nanorods in both solid and emulsion phase. The biosynthesized CuO nanorods (oxidizers)/fuel (nanoaluminum) composites ignited vigorously with abundant gas generation, developing high heat of reaction of 1.66 kJ g(-1) and very high pressurization rate of around 1.09 MPa μs(-1) and peak pressure of 65.4 MPa when blasted inside a constant volume pressure cell with a charge density of 0.2 g cm(-3). The pressurization rates so obtained are four times higher with twice the peak pressure in comparison to such nanothermites formulated via other available state of the art wet-chemical techniques, which reflects the catalytic role of Aloe vera surface functional groups (A. vera-sfg) enhancing the reactivity of CuO oxidizers with excess gas release rate during exothermic reaction with nanoaluminum. Through this work, Aloe vera gel has for the first time been identified as a novel biotemplate for green synthesis of nanorod structures of metal oxides, and we have also studied the utility of A. vera-sfg in the creation of super-reactive CuO oxidizers producing excellent heat of reaction and dynamic pressure characteristics as demanded in propellants, explosives, and pyrotechnics.

A pressure-driven flow analysis of gas trapping behavior in nanocomposite thermite films

This article is in direct response to a recently published article entitled Electrophoretic deposition and mechanistic studies of nano-Al/CuO thermites (K. T. Sullivan et al., J. Appl. Phys., 112(2), 2012), in which we introduced a non-dimensional parameter as the ratio of gas production to gas escape within a thin porous thermite film. In our original analysis, we had treated the problem as Fickian diffusion of gases through the porous network. However, we believe a more physical representation of the problem is to treat this as pressure-driven flow of gases in a porous medium. We offer a new derivation of the non-dimensional parameter which calculates gas velocity using the well-known Poiseuille's Law for pressure-driven flow in a pipe. This updated analysis incorporates the porosity, gas viscosity, and pressure gradient into the equation.

Correlation of optical emission and pressure generated upon ignition of fully-dense nanocomposite thermite powders

Ignition was studied experimentally for four nanocomposite materials prepared by Arrested Reactive Milling (2Al·3CuO, 2.35Al·Bi2O3, 4Al·Fe2O3, and 8Al·MoO3). Thin layers of the prepared powders were coated on an electrically heated Ni–Cr filament and ignited at heating rates between 200 and 17,000K/s in a miniature vacuum chamber. The ignition was monitored based on both photodiode and pressure transducer signals recorded simultaneously. Ignition temperatures were found to be weakly dependent of the heating rate and similar for all thermites (in the range of ∼800–950K) except for that using Bi2O3 as an oxidizer, which ignited at lower temperatures. Both maximum pressure and the rate of pressure rise (dP/dt) decreased with increasing heating rate. The onsets of pressure pulses produced by ignited thermites occurred prior to those of the respective emission pulses, with the effect increasing at higher heating rates. It was observed that 2.35Al·Bi2O3 did not ignite at heating rates above 5000K/s in vacuum, while it readily ignited when heated in air. A qualitatively similar observation was made for 8Al·MoO3, which could not be ignited in vacuum when heating rates reached 13,000K/s. The results are interpreted to suggest that the low-temperature redox reactions occurring prior to ignition modify the oxidizers, making them less stable and prone to rapid decomposition upon further heating. Ignition, triggered by a change in transport properties of the growing interfacial Al2O3 layer is not directly affected by the oxygen release from the decomposing oxides. Both oxygen release and growth of the interfacial Al2O3 layer are directly affected by the rates of low-temperature redox reactions preceding ignition.

Electrophoretic deposition and mechanistic studies of nano-Al/CuO thermites

Electrophoretic deposition was used to deposit thin films (∼10–200 μm) of nano-aluminum/copper oxide thermites, with a density of 29% the theoretical maximum. The reaction propagation velocity was examined using fine-patterned electrodes (0.25 × 20 mm), and the optimum velocity was found to correspond to a fuel-rich equivalence ratio of 1.7. This value did not correlate with the calculated maximum in gas production or temperature, and it is suggested that it is a result of enhanced condensed-phase transport, which is speculated to increase for fuel-rich conditions. A ∼25% drop in propagation velocity occurred above an equivalence ratio of 2.0, where Al2O3 is predicted to undergo a phase change from liquid to solid. This is expected to hinder the kinetics by decreasing the mobility of condensed-phase reacting species. The effect of film thickness on propagation velocity was investigated, using the optimum equivalence ratio. The velocity was seen to exhibit a two-plateau behavior, with one plateau between 13 and 50 μm film thickness, and the other above ∼120 μm. The latter had nearly an order of magnitude faster velocity than the former, 36 m/s vs. 4 m/s, respectively. For film thicknesses in the 50-120 μm range, a linear transitional regime was observed. Images from the combustion studies showed an increase in forward-transported particles as the film thickness increased, along with more turbulent behavior of the flame. It was suggested that the two-plateau behavior indicated a shift in the energy transport mechanism. While nanocomposite thermites have been traditionally thought to exhibit convective energy transport, we find in this work that particle advection may also be important. The velocity of particles ejected through a thin slit mounted above a thermite strip was measured, and was found to be even faster (∼2-3×) than the flame propagation velocity. The morphology of captured particles was examined with an electron microscope, and indicated that reactive sintering had occurred. A non-dimensional number (A) was used to relate the rate of gas pressurization (1/τp) to the rate of gas escape by Fickian diffusion (D/L2), A = L2/(D*τp). For small values of A, gases rapidly escape and do not accumulate within the thermite films. Thus, the resultant energy transport is relatively slow. For large values of A, gases are entirely trapped, thus activating enhanced energy transport via oscillating pressure buildup and unloading of the material. This analysis is suitable for thermites which can produce sufficient gas to raise the local pressure above some critical value to overcome material adhesion strength, inducing pressure-driven unloading and resulting in enhanced energy transport. We suggest that further improvements in nanocomposite thermites can be made by examining the coupling of multiple length scales. Not only is nano-scale mixing important, in this work we found that a second length scale (∼120 μm) was necessary to fully activate a pressure buildup/unloading mechanism, which significantly enhances the reactivity.

ON GAS RELEASE BY THERMALLY-INITIATED FULLY-DENSE 2Al·3CuO NANOCOMPOSITE POWDER

A recently developed model for low-temperature exothermic reactions in nanocomposite Al-CuO thermites described the evolution of an alumina layer growing between Al and CuO and changes in its diffusion resistance as critically affecting ignition of the composite reactive material. The model was successful in describing ignition of individual composite particles in a CO2 laser beam. However, it was unable to conclusively predict ignition of the same powder particles coated onto an electrically heated filament. In this work, ignition of fully-dense 2Al·3CuO nanocomposite powder prepared by arrested reactive milling was studied using a modified electrically heated filament experiment, located in a miniature vacuum chamber. Thin layers of the powders coated on a nickel-chromium filament were ignited at heating rates between 200 and 16,000 K/s. The ignition was accompanied by both optical emission and pressure signals. The pressure signals occurred before emission, with increasing delay at higher heating rates. Ignition temperatures were only slightly affected by the heating rate. The results are interpreted proposing that the low-temperature redox reaction produces a metastable CuOl−x phase with 0 < x ≤ 1 which releases oxygen upon heating. It is shown that despite a relatively small heat release, the low-temperature reactions in nanocomposite thermites are important as producing destabilized, partially reduced oxides that decompose with gas release upon heating. In the present experiments, the gas release changed thermal properties of the powder coating, reducing the efficiency of heat exchange with the supporting filament and thus enabling its thermal runaway and ignition.

Low-temperature exothermic reactions in fully dense Al–CuO nanocomposite powders

A hypothesis stating that the Cabrera–Mott kinetics describes low temperature redox reactions in fully dense nanocomposite thermite powders is explored. Earlier experiments involving differential scanning calorimetry (DSC) and new isothermal microcalorimetry measurements were interpreted using a reaction model employing the Cabrera–Mott mechanism. Following earlier work, the effect of Mott potential on the mass transport rate is considered to be temperature dependent. Simultaneous processing of both experimental data sets enabled us to determine parameters for the Cabrera–Mott mechanism describing the redox reaction in a nanocomposite Al–CuO thermite prepared by arrested reactive milling. The reaction model developed enables one to describe the initial portions of the DSC curves (up to 600 K) measured at different heating rates as well as microcalorimetry traces recorded in the temperature range of 303–413 K. The model is expected to be useful for describing both aging and ignition of fully dense, nanocomposite reactive materials.

Reactive sintering: An important component in the combustion of nanocomposite thermites

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/Bi 2O 3 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.

Thermal initiation of consolidated nanocomposite thermites

Reactive nanocomposite powder with bulk composition of 8Al·MoO 3 was prepared by arrested reactive milling and cold-pressed into 0.635 cm diameter cylindrical pellets. The material was consolidated to densities up to 99% of theoretical maximum density, or 3.217 g/cm 3, while maintaining high reactivity. Pellets were ignited by a CO 2 laser beam and the ignition delays increased at lower laser powers and greater pellet densities. A simple numerical model was developed to describe the heating and thermal initiation of the reactive pellets. The model described adequately the observed effects of both laser power and pellet density on the measured ignition delays. The exothermic reaction governing thermal initiation of the prepared consolidated samples is described by the same Arrhenius term as that used to describe ignition of thin layers of respective unconsolidated nanocomposite powders coated on an electrically heated filament. An exothermic reaction with the same activation energy is also detected by earlier thermo-analytical studies with Al–MoO 3 nanocomposite thermites and is suggested to be rate-limited by the reduction of MoO 3.

Reactions leading to ignition in fully dense nanocomposite Al-oxide systems

Aluminum-metal oxide energetic compositions with components mixed on the nano-scale are substantially more reactive than conventional thermites and are of interest as potential additives to propellants, explosives, and pyrotechnics. For such nanocomposite materials prepared by Arrested Reactive Milling (ARM), the exothermic reactions leading to ignition were detected to begin at relatively low temperatures. These materials are prepared by mechanical processing at room temperature, and the nature of the interface present between aluminum and the oxidizer (metal oxide, e.g., CuO, MoO3, Bi2O3, etc.) is unknown. Experiments using a Thermal Activity Monitor (TAM III) quantify the reaction rates between aluminum and CuO at temperatures between 303 and 373 K. Results of the present TAM III measurements and results of earlier measurements using differential scanning calorimetry for the same 2Al·3CuO nanocomposite are interpreted considering two different reaction models. The rate-limiting step is described either as a conventional thermally activated diffusion, or using the Cabrera–Mott model developed originally for oxidation of fresh metal surfaces. It is shown that the thermally activated diffusion model is inadequate for description of the low-temperature reactions observed in nanocomposite thermites prepared by ARM. The Cabrera–Mott model provides a description qualitatively matching the experimental results; achieving the quantitative match is expected to be possible by adjusting the model parameters.