Previously we have presented a joining method whereby materials of widely differing CTE were successfully bonded with no deformation at room temperature using reactive multilayer foils as the local heat source. This is unique in as far as creating a metallic bond with any other technique results in significant deformation upon cooling back to room temperature. For many large area applications this deformation results in having to use mechanical methods to flatten these surfaces to their original shape, which is highly undesirable as such techniques are unpredictable and do cause yield loss when used. When reactive multilayer foils are used in the bonding process, no such deformation of materials occurs resulting in assemblies that can be effectively used as soon as they are bonded. Critical to the use of parts at higher temperature is the need to minimize/manage joint stress in assemblies at a particular application temperature. This paper will demonstrate how this can be done effectively, with the use of a reactive multilayer foil in creating the bond.

Exothermic formation reactions can occur in multilayer systems consisting of alternating layers of two or more reactants that have a high heat of mixing. When the reactant spacing is sufficiently small, these reactions can self-propagate at speeds ranging from 0.1 - 100 m/s. Freestanding foils consisting of 10s to 1000s of individual layers are often fabricated using physical vapor deposition to ensure a consistent, repeatable microstructure. Recently, high-speed imaging techniques have been used to monitor self-propagating reactions in these materials in situ. Experiments have shown two distinct modes of reaction propagation: a steady mode in which the combustion front moves at a constant rate and an unsteady mode where the front proceeds in a piece-wise fashion via a series of narrow bands in which the reaction front moves perpendicular to the net propagation direction. While a number of different chemistries and a broad range of foil geometries have been investigated, previous studies have not reported any transitions between steady and unsteady reaction modes with variations in foil geometry (bilayer spacing, total thickness, etc.) Here, we report on the first observed transitions from steady to unsteady reaction resulting from variations in bilayer spacing in Al/Ni and Al/Co multilayer foils with 1:1 stoichiometries. In 7.5 - 9.0 micron thick Al/Ni foils, reactions propagate in a steady fashion at bilayer spacings le; 100 nm before transitioning to an unsteady propagation mode at a 150 nm bilayer spacing. In 7.5 micron thick Al/Co foils, steady reaction propagation was only observed over a small window in bilayer spacing located near the peak in the velocity vs. bilayer spacing curve. Potential mechanisms for these transitions will be discussed in the context of rates of heat generated and heat lost in the vicinity of the combustion front.

Reactive nanolaminates are comprised of alternating layers of materials that react exothermically. Self-propagating fronts, traveling at speeds that can exceed 10m/s, can be initiated in these materials with an external heat source. The wide range of length and time scales involved in such reactions presents a typical modeling challenge due to the inherent interplay of the different scales in the underlying dynamics and the eventual end-product. In this presentation, we will discuss the development of a reduced reaction model for Ni/Al nanolaminates. The model incorporates a generalized, anisotropic description of thermal transport that also accounts for the dependence of thermal conductivity on composition and temperature. A generalized description of intermixing is also developed, that incorporates information derived from homogeneous ignition experiments, nanocalorimetry experiments, measurements of front propagation velocity, and molecular dynamics (MD) computations. Using insights gained from MD computations, intermixing is described using a simplified, temperature-dependent diffusivity relation that enables us to reproduce measurements of low-temperature ignition, homogeneous reactions at intermediate temperatures, as well as the dependence of reaction fronts on microstructural parameters.

Abstract not available.

Molecular dynamics (MD) simulation of Al/Ni multilayered foils reveals a range of different reaction pathways depending on the temperature of the reaction. At the highest temperatures Fickian interdiffusion dominates. At intermediate temperatures Ni dissolution into the Al liquid is accompanied by Al interdiffusion into the Ni solid prior to formation of the B2 intermetallic phase. At low temperatures the B2 intermetallic forms early in the reaction process. Interdiffusion rates and activation barriers are extracted from MD data and compared with experimental observations.

An Al/Pt reactive nanolaminate is composed of many alternating layers of aluminum and platinum having a single out-of-plane periodicity. These nanolaminates are made by direct current magnetron sputter deposition of high-purity polycrystalline metals; and the layer periodicity is uniform in thickness. A premixed ~100 Å thick Al_xPt_y alloy layer forms between each Al and Pt layer during deposition, and these interfacial layers are a diffusion barrier for the reaction between the Al and the Pt. At high temperature, the diffusion rates increase, especially when the metals melt, and a reaction front propagates into the unreacted nanolaminate. The reaction front velocity can be controlled using the bilayer thickness, the overall thickness of the nanolaminate, and the substrate. In the current work, the nanolaminate is either free-standing (not attached to a substrate), or resides on a substrate composed of Al₂O₃, SiO₂, or W. The front speed depends on the passivating layer, the amount of available Al and Pt for reaction, and the rate of diffusion. Fronts fail to propagate when the bilayer thickness is less than a few hundred angstroms regardless of the total film thickness due to the passivating layer. Front speeds increase when the bilayer thickness is between a few hundred angstroms and about 500 Å as a result of more Al and Pt available for reaction. Front speeds decrease as bilayer thicknesses increase above 500 Å as diffusive resistance increases. Eventually, the front ceases to propagate when the diffusive resistance is too great. We refer to the bilayer thickness where the front fails to propagate due to the large diffusive resistance as the extinction point. In the current work, we focus on the extinction of the reactive front as the bilayer thickness increases. We use experiments and models to show three trends of the extinction point with increasing bilayer thickness: the extinction point 1) increases with overall thickness of the nanolaminate, 2) increases as the substrate thermal conductivity is decreased, and 3) increases when there is no substrate (free-standing) versus reaction on a SiO₂ substrate. The reactive film is modeled as a continuum using a reactive constitutive model similar to Hardt and Phung** in conjunction with a finite element code. Melting of the Al and Pt layers are modeled with an effective capacitance model. Radiative heat loss to the surroundings and conductive heat loss to the substrate are also considered. This work is relevant to understanding the parameters that effect reaction extinction and is useful to those wishing to control reaction front velocity with bilayer thickness. *Work performed at Sandia National Laboratories (SNL). Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. **Hardt AP and Phung PV, Comb. and Flame, 21, 77-89 (1973).

Nanoheaters are reactive composite materials that can generate heat in calculated amounts through exothermic reactions. Due to the metastable nature of these materials there are a limited number of fabrication techniques that do not employ high temperatures. Ultrasonic powder consolidation (UPC) is an ultrasonic welding technique in which particles, instead of sheets or wires, are joined by the action of ultrasonic vibrations at low to moderate temperatures. In this study, we investigated the fabrication of various nanoheaters such as bimetallic Al-Ni, thermite Al-Fe2O3, Al-CuO, and hybrid Al-Ni-Fe2O3, Al-Ni-CuO, Al-Ni-NiO. These composites were produced by UPC at a maximum temperature of 573 K and a maximum pressure of 100 MPa in just 1 s. Microstructural analyses showed that the particles were consolidated to full density without causing any reactions between the constituents which allows these materials to be utilized as ignitable nanoheaters. Nanoheaters produced by UPC can be used as heat sources for joining of metallic sheets. Joining between Al and Sn-5wt.% Pb was achieved by using an Al-CuO thermite composite. Also, two Al sheets, each with a thickness of 500 µm, were joined together by roll bonding Ni nanoflakes on one of the Al sheets at room temperature and subsequent heating of the Al-Ni-Al sandwich structure up to the ignition temperature (820 K).

Microstructures of titanium-carbon mixtures subjected to short-term (minutes) high energy ball milling are investigated as a function of milling time for dry and wet grinding conditions. Two different reaction mechanisms, i.e. sudden ignition (dry milling) and gradual solid state diffusion (wet grinding) for mill-induced reactions are observed. SEM/XRD/BET studies show that at dry milling graphite flakes crushed to fine amorphous carbon particles at the initial stages of milling. The fine carbon uniformly distributed on the surface of flattened titanium particles. Carbon-coated titanium particles cold-welded with each other to form sandwich-like structures. TEM studies reveal that scale of titanium-carbon mixing in such composite particles is on the order of several nanometers. Some tiny (~5 nm) nucleus of titanium carbide phase (TiC) has been also found. The critical time for mixture self-ignition under the dry ball milling conditions is ~ 9 minutes. The wet grinding in hexane hinders mass transfer processes that allow much longer mechanical treatment without self-ignition. As a result significant amount of TiC forms by solid state diffusion mechanism during the milling, which was detected by both TEM and XRD techniques. The investigation of evolution of microstructures and reaction kinetics suggests that the reactivity enhancement in this system is primary related to formation of intimate contact between the reactants and solid-state dissolution of carbon into the titanium lattice. Those microstructural modifications make ball-milled Ti+C mixtures extremely sensitive to thermal ignition (Tig), as the Tig decreases from 1940K for the initial mixture to 600K for ball milled composite particles.

During self-propagating reactions in most multilayer material systems, freestanding foils rapidly heat and cool within 0.5 s. For some applications, including bio-agent defeat, longer reaction durations are desired. We have previously demonstrated that multilayer foils of certain chemistries are capable of having a slow and highly exothermic oxidation reaction following a rapid intermetallic formation reaction. Foils with Al:Zr chemistry are one salient example, reacting in air with temperatures in excess of 1400 K for a duration of 2 s. Based on an analysis of temperature-time profiles for these reactions in air, we have determined that the energy released is approximately four times greater than that of the formation reaction alone. However, application of these materials requires additional knowledge of the reactions involved. Here we report on the properties of these reactions and elucidate some of the mechanisms and phases involved. Using hot-plate and uniform ignition tests, we have modeled the activation energy and the pre-exponential term in the Arrhenius equation for low temperature diffusivity that leads to the initial formation of the intermetallic compounds. We have also measured the velocity of the formation reactions as they self-propagate along the foils. Lastly, we present in-situ X-ray diffraction of the phase evolution that occurs during oxidation and cross-sectional TEM of the final oxidized structure with the goal of identifying the mechanisms that control and terminate the oxidation process.

It is axiomatic that the burning time dependence on particle size follows an integer power law dependence. However, a considerable body of experimental data show a power dependence less than unity. In this paper, we focus on what might be responsible for the fractional power dependence observed for the burning time for nanoaparticles ( e.g. Al and B). Specifically we employ reactive molecular dynamics simulations of oxide-coated aluminum nanoparticles (Al-NPs). Since most nanomaterials experimentally investigated are aggregates, we study the behavior of the simplest aggregate - a doublet of two spheres. The thermo-mechanical response of an oxide coated Al-NP is found to be very different than its solid alumina counterpart, and in particular we find that the penetration of the core aluminum cations into the shell significantly softens it, resulting in sintering well below the melting point of pure alumina. For such coated nanoparticles, we find a strong induced electric field exists at the core-shell interface. With heating, as the core melts, this electric field drives the core Al cations into the shell. The shell, now a sub-oxide of aluminum, melts at a temperature that is lower than the melting point of aluminum oxide. Following melting, the forces of surface tension drive two adjacent particles to fuse. The characteristic sintering time (heating time + fusion time) is seen to be comparable to the characteristic reaction time, and thus it is quite possible for nanoparticle aggregates to sinter into structures with larger length scales, before the bulk of the combustion can take place. This calls into question what the appropriate ‘effective size&’ of nanoparticle aggregates is.

Over the past two decades the focus of energetic materials research has been on the possible benefits that nanomaterials can offer for reactive composite materials. It has been well documented that the use of nano reactants instead of micron-size reactants has had a significant effect on the improvement of combustion characteristics. Traditional nanothermites, which have been already well characterized, consist of a nanoscale fuel (80 nm aluminum) and a submicron or nanoscale oxidizer(s), such as Bi₂ O₃ , MoO₃ , CuO, WO₃ or Fe₂ O₃ . Applications of different kinds of nanoenergetic composites have been limited due to their ignition sensitivity (electrostatic discharge (ESD), friction and impact) and low resistivity of nano aluminum to oxidation, especially water. Another elemental fuel, that has been of interest in metalized energetic, is nano silicon. While silicon is traditionally used in electronics and MEMS devices it has similar energy density as aluminum and it is less susceptible to aging. Another significant advantage of silicon is its higher ignition temperature which decreases the ignition sensitivity of resulting nanothermite materials. Silicon and aluminum-silicon nano powders were recently produced by Innovative Materials and Processes, LLC (IMP) and the South Dakota School of Mines and Technology (SDSM&T). The special attrition milling technique allows the controlled formation of these nanoscale fuels. Specific surface area of these nanopowders can be adjusted by controlling key operating conditions in order to meet desired combustion characteristics of reactive nanocomposites. The nanothermite materials used in this work consisted of Si and Si/Al with several different specific surface areas mixed with both micron and nano Bi₂ O₃ . In this contribution the ignition (ESD, friction, impact and laser ignition delay) as well as dynamic combustion characterization in a closed bomb and open or semi-open channels will be presented for Si-Bi₂ O₃ and Si/Al-Bi₂ O₃ nanothermites. In addition, the use of Resodyn mixing technology for the processing of nanothermites will be also addressed.

We have performed spatial in-situ temperature measurements of aluminum-copper oxide thermite reactions using high-speed color pyrometry. Electrophoretic deposition was used to create thermite microstructures. Tests were performed with micron- and nano-sized particles at different stoichiometries. The color pyrometry was performed using a high-speed color camera. The color filter array on the image sensor collects light within three spectral bands. Assuming a gray-body emission spectrum a multi-wavelength ratio analysis allows a temperature to be calculated. An advantage of using a two-dimensional image sensor is that it allows heterogeneous flames to be measured with high spatial resolution. Light from the initial combustion of the Al-CuO can be differentiated from the light created by the late time oxidization with atmosphere. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

In several recent studies, it was reported that ignition in nanocomposite thermites is accompanied by oxygen release attributed to the decomposition of metal oxides serving as oxidizers. This oxygen release occurs at temperatures substantially lower than expected for decomposition of respective oxides. It may affect ignition of nanocomposite thermites, especially when they are used as consolidated reactive structures and components. The objective of this study is to characterize such oxygen release experimentally comparing its timing with that of the optical signature produced by the igniting thermites. It is further desired to establish the role this oxygen release plays in triggering ignition and in ensuing combustion processed. The experiments are performed with several nanocomposite thermites prepared by Arrested Reactive Milling (ARM). Aluminum serves as a fuel in all compositions, oxidizers are CuO, MoO3, Fe2O3, and Bi2O3. The experimental apparatus includes a miniature vacuum chamber containing an electrically heated metal filament. The filament is coated with a thermite of interest. The thermite ignites upon heating and both pressure and optical emission are monitored simultaneously. It is observed that the pressure pulses produced by the ignited materials begin before the optical emission pulses with the time difference increasing at higher heating rates. Further experiments address effect of structural refinement in the ARM-prepared materials with the same bulk compositions on their ignition and respective gas release. Results are interpreted considering a recently developed multi-step model for ignition in nanocomposite thermites. Modifications of this model accounting for the gas release will also be discussed.

Recent advances in nanoscience are providing capabilities to synthesize materials with complex molecular patterns using various novel assembly approaches. This talk will review our current progress towards developing a framework of principles for design and fabrication of nano-tailored energetic materials and study their ignition sensitivity, energy/gas release and reaction rate. The investigation of the exothermic reactions always complicated due to high temperature nature and fast reaction rate. We used different techniques, such as High-Speed Temperature Scanner (HSTS) build at the Institute of Chemical Physics NAS RA, Differential Scanning Calorimetry (DSC) and Dynamic Gas Measurement (DGM) to characterize the exothermic reactions at different heating rate. The direct resistive heating of a metal foil heater in the HSTS system allows to control sample heating rate from 10 to 10.000 °C/min. The test sample in the form of powder mixture was placed into a heater made from Pt thin foil as a holder. Due to the inherent thermal characteristics of sample geometries and the direct heating system of the HSTS instrument, thermal and physical material testing under a variety of extreme experimental conditions is possible. The comparison of investigation of thermite and Al-Teflon reactions with different methods shows that the HSTS instrument, in some instances, can give more information as to the mechanism, kinetics and thermodynamics of an exothermic reaction than the conventional calorimetry techniques can provide. It has also been demonstrated to be more sensitive than conventional DTA analysis, enabling less valuable time to be spent on stability or reactivity testing.

Gas-generating compositions usually include a solid compound that decomposes at high temperatures and an energetic additive (e.g., a metal fuel) that provides heat for self-sustained combustion. The present paper investigates the feasibility of using nanocomposite reactive materials produced by Arrested Reactive Milling as energetic additives in oxygen and hydrogen generators. In such reactive materials, components are mixed at the scale of about 100 nm, but are not chemically bound. Thermodynamic calculations of the adiabatic flame temperature and combustion products have been conducted for the mixtures of sodium chlorate and ammonia borane with various reactive compositions. Analysis of the obtained results indicates the most promising additives. Experimental studies are ongoing.

Adding aluminum to propellants, pyrotechnics, and explosives is a common way to boost their energy density. A number of approaches have been investigated that shorten aluminum ignition delay, increase combustion rate, and decrease the tendency of aluminum droplets to agglomerate. Previous work showed that particles of mechanically alloyed Al-Mg powders burn faster than similarly sized particles of pure aluminum. However, preparation of mechanically alloyed powders with particle sizes matching those of fine aluminum used in energetic formulations was not achieved. This work is focused on preparation of mechanically alloyed, composite Al-Mg powders in which both internal structures and particle size distributions are adjusted simultaneously. Powders with compositions in the range of 50 - 90 at. % were prepared and characterized. Milling protocol includes separate stages for mechanically alloying Al and Mg and for adjusting the particle sizes of the produced powder. The protocol is optimized for each composition to prepare equiaxial, micron-scale particles suitable for laboratory evaluations of their oxidation, ignition, and combustion. Oxidation of the prepared powders is studied using thermo-analytical measurements. Ignition is characterized experimentally using an electrically heated filament setup. Combustion is studied using a constant volume explosion setup, for the powder cloud combustion, and a laser ignition setup for characterization of combustion rates and temperatures for individual particles. For all materials, ignition and combustion characteristics are compared to each other and to those of pure Al. Compositions with improved performance are identified. Results will be presented and discussed in this talk.

Previous investigations of the oxidation, ignition and combustion of aluminum particles in steam, CO₂, and mixed environments have shown qualitatively different behavior compared to experiments in air: aluminum oxidizes at lower temperatures in the presence of water, and significant oxidation occurs at the aluminum melting point. The addition of CO₂ makes this oxidation step more prominent, but the effect changes in a nonlinear way with the CO₂ concentration. At the same time, vapor flames are only sustained for larger particle sizes in the presence of H₂O and CO₂, and ignition thresholds and combustion times change accordingly. Motivated by the ongoing development of aluminum-based alloys and composites, oxidation in H₂O and CO₂ is studied for magnesium and mechanically alloyed Al-Mg powders. Initial results show similar effects: oxidation of Mg completes at lower temperatures, and the melting point changes the oxidation rate. Results for varied alloy compositions and mixed gases will be presented. Implications for combustion as well as for hydrogen generation will be discussed.

The system integration and packaging of micro electro mechanical systems (MEMS) is increasingly affected by three dimensional chip and/or wafer stacking. Particularly smart systems consist of multiple modules, components and materials that must be combined and manufactured within several production locations. If different functionalities such as electronics, mechanics or optics have to be combined the bonding process has to fulfill several conditions. The most important requirements on the joint interface in MEMS packaging are the mechanical strength and in most cases also hermetical tightness. Typical bonding processes are silicon direct bonding, anodic bonding or adhesive bonding. In most applications, however, the bonding temperature must not exceed 400 °C. Therefore new low temperature processes have been under investigation recently. Using a localized heat source in form of an exothermic reaction in nano scale multilayer to fabricate soldered connections minimizes heat and stress put into the components being bonded. The heat is applied directly to the joint interface so that external heating with a furnace is not necessary. This paper introduces a new method of local heating for wafer bonding processes which is based on nano scale energetic material systems. Such systems typically consist of several alternating layers of two different nano scale metallic films. With the application of an initial energy pulse, the system starts to rapidly form intermetallic phases by interdiffusion of adjacent material layers. By choosing material combinations with negative enthalpy of formation this reaction can be running exothermic and self-propagating. This paper focuses on the direct wafer deposition and patterning of reactive and nano scale multilayer films. These systems are called integrated reactive material systems (RMS). In contrast to the typically used Ni/Al systems, our total multilayer film thicknesses had to be smaller than 2.5 µm to reduce stress within the multilayer and, most important, deposition costs. Thus, new high energetic RMS have to be introduced. These systems were deposited by using alternating magnetron sputtering from high purity Al- and Pd-targets to obtain films with a defined Al:Pd atomic ratio. We will show reaction characteristics and reaction velocities ranging up to 75 m/s in bond frames with lateral dimensions to a minimum of 25 µm. In addition to that, the micro structure and the possibility for patterning by using wet etching and lift-off techniques will be demonstrated. Furthermore, the feasibility for 150 mm Si-Si hermetic and metallic wafer bonding at room-temperature is presented. We will show that by using this bonding technology strong (shear strengths up to 150 MPa) and hermetically sealed bond interfaces can be achieved.

Combustion reactions in nanomaterials are challenging to study because they occur with short characteristic time scales (microseconds to milliseconds) and sometimes are highly localized (such as self-propagating reaction fronts in nanolaminates). In this talk we review recent applications of synchrotron x-ray radiation to the study of rapid combustion reactions in reactive nanolaminates and nanopowders. Examples will include x-ray microdiffraction with microsecond temporal resolution to study self-propagating reaction fronts in Al/Ni and Al/Zr nanolaminates, phase-contrast imaging studies of thermite reactions in reactive nanopowders, millisecond time-resolved studies of reaction ignition in Al/Ni nanolaminates, and a newly-developed x-ray reflectivity technique to study the earliest stages of interdiffusion during rapid heating in nanolaminates. We will close with a discussion of the prospects for future work in this area, taking advantage of new and proposed x-ray sources and detectors.

We report the latest results from a nanocalorimetry study of solid-state mixing in Ni/Al laminates with simultaneous electron imaging using the dynamic transmission electron microscope (DTEM). This system allows time-resolved, simultaneous thermodynamic and structural measurements of materials during rapid exothermic reactions such as those which occur in reactive materials. Nanocalorimetric studies of reactive materials provide access to controlled heating rates in the range of 10,000 to 100,000 K/s, approaching those observed in self-propagating reactions. In this heating rate regime, we have identified transitions in the sequence of phase transformations in Al/Ni reactive foils evidenced by changes in the number of exothermic peaks during reaction. Post-reaction electron backscattered diffraction (EBSD) results indicate that all samples react to completion regardless of heating rate, so the differences in reaction kinetics can only be attributed to changes in the phase formation sequence. This phenomenon was the motivation for the development of the coupled nanocalorimeter - DTEM system, which we have used to determine the changes in phase transformations which lead to the observed differences in reaction exotherms. The results obtained are being used to develop a theory describing the response of reactive systems to different heating rates, with a focus on phase suppression due to large concentration gradients. More generally, the nanocalorimetry/DTEM system provides opportunities to investigate energetic systems in which simultaneous observations of structural transformations and energy release during reaction has been previously impossible, giving new insight into the events governing reaction initiation and propagation in these systems.

Reactive nanolaminate or multilayer films (RMFL) can rapidly release stored energy into a small volume, making them useful in applications such as soldering temperature sensitive devices and igniting other reactions. The localized heat is released via self-sustained reactions that propagate driven by the release their stored chemical energy. These self-propagating reactions are initiated by an external heating source that causes the interdiffusion of the species, which in turn releases heat due to this intermixing and results in subsequent mixing of the layers in surrounding regions and a reaction front propagating parallel to the layers. The zone of dynamic mixing, or reaction front, can have extremely high heating rates of ~106-109 K/s. Reaction front velocity and temperature can be manipulated by tailoring layer thickness, foil chemistry, and the extent of pre-mixing. However, little is known as to how the reactants mix and transform into the final reaction products, limiting predictive modeling and the tailoring of reactive laminate performance for specific applications. Using the high-time resolution capabilities of the dynamic transmission electron microscope (DTEM) at LLNL, we are able to capture the transient reaction with direct imaging and electron diffraction, providing data to improve existing models. We have conducted experiments in two different systems, Ti:2B and Ni:Al based RMLF laminates. Images of the reaction front in the off-equiatomic compositions in Ni-Al based RMLF reveal dark striations trailing behind the front that are thought to form as the reacted material passes through a two-phase field of liquid + NiAl during cooling. Though Ti-2B has much higher adiabatic reaction temperature (~3500 K), no liquid phase was observed trailing the reaction front as opposed to Ni-Al system. Post-mortem chemical analysis reveals a preferential loss of Ti, most likely due to evaporation, which is conceivable having a vapor pressure 20 Torr at 3500K. The significant loss in Ti and rapid quenching via evaporated cooling can explain the lack of an observable liquid phase. In paper, we will discuss in detail these dynamics in the context of the direct nanosecond scale imaging and electron diffraction data obtained by the DTEM. Work preformed at LLNL under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported in part by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and sponsored in part by the Department of Defense, Defense Threat Reduction Agency.

Nanometric metallic multilayers constitute a class of nano-energetic systems made essentially of 2d layers of metals separeted by interfaces. They support the self-sustained propagation of exothermic reactions associated with phase transformations. The ignition temperature is much lower and the propagation velocity much higher than for usual microstructured systems. Il is crucial to understand how their specific propreties are related to the nano-heterogeneity. Using the thermodyamics background of nucleation of alloys under strong concentrations gradients, we propose to revisit the main basic atomistic mechanisms leading to phase transformations at nanoscale. We will illustrate these aspects in thé case of Ni/Al system.

Intermolecular reactive composites find diverse applications in defence, microelectronics and medicine where strong, localized sources of heat are required. However, there is much to be understood regarding the mechanism of their propagation, which is a roadblock to their widespread application. We describe molecular dynamics simulations of both crystalline and amorphous Ni/Al nano-laminates across different laminate periods, ignition temperatures and geometries. We find that the reaction is diffusion controlled in the bulk phase but the presence of defects like pores or free surfaces speed up the reaction by as much as five times. Not only are the defective geometries faster, they scale differently from the bulk samples and for the first time, we see evidence of ballistic transport in these type of reactions. Also studied is the effect of using amorphous Ni/Al and we find that the reactions are faster and also more energetic than the crystalline samples. Recent experimental work has shown that high energy ball milling can significantly improve the reactivity as well as the ease of ignition of Ni/Al inter-metallic composites. Ball milling a mixture of ductile materials like Ni and Al leads to the formation of an intimately mixed nanostructure of Ni/Al that shows increased sensitivity to initiation. Significant reduction in both ignition temperature, which can be as low as 600 K and the threshold impact energy have been observed. However, neither the rationale behind the surprisingly low impact energies for these nano-engineered composites, nor the mechanism of the reaction&’s propagation is fully understood. We present large scale (~4 million atom) molecular dynamics simulations of shock induced chemistry in porous Ni/Al nanocomposites that mimic the observed structure of the ball milled particles. We describe the importance of pores as sites of initiation, where local temperatures approach the melting temperature of Aluminium. We find that this rapid, localized heating is a consequence of both plastic deformation and accelerated chemistry along the surfaces of each pore.

Beside their relevance in commercial applications metallic reactive multilayers also serve as model materials for exploring irreversible phase transformations under rapid heating conditions. Heating rates up to 107 K/s can be achieved by uniformly or locally igniting self-sustaining or self-propagating exothermic reactions in multilayer foils and films. Recent in situ experiments suggest that three major factors influence the sequence of transformations that occur during these rapid reactions: the nature of the interfaces (solid/solid or solid/liquid), the heating rates and the maximum reaction temperature. The effect of heating rate is particularly important for reactions that remain in the solid state, such as though performed on nanocalorimeters. Here we use analytical models and thermodynamic data to predict the first phase to nucleate in Al:Ni multilayer films as a function of heating rate. The effort draws on the concept of that steep concentration gradients impede nucleation, and it uses both a closed analytical solution and numerical calculations of the energy barriers to predict the first intermetallic phase to nucleate, as a function of heating rate. We present a parametric study of various physical quantities such as interfacial energy to assess their importance and we compare predictions with recent experimental results and MD simulations.

To investigate the process by which runaway self-sustaining reactions are initiated in Al/Ni nanolaminates, we have performed a series of in situ x-ray diffraction experiments with millisecond temporal resolution, on specimens subjected to uniform Joule heating. By varying the power input, we can observe a reproducible series of behaviors. At low power inputs, Joule heating causes intermixing of Al and Ni resulting in further heating which continues for up to 300 ms after the input current is turned off. Above a threshold current (which increases with bilayer period of the specimen) the specimen heating is sufficient to initiate a runaway self-sustaining reaction that results in a rapid increase to temperatures >850 °C. Increasing the power input beyond this critical level causes an increase in the ultimate maximum temperature of the reaction. The x-ray diffraction observations allow us to track the sequence of phase formation during the self-sustaining reaction in detail. We observe three different behaviors, depending on the maximum temperature attained during the reaction sequence. More interestingly, we are also able to track the structural changes occurring prior to initiation of the runaway reaction. In particular, we can identify the formation of metastable phases (such as Al9Ni2) and, by tracking the evolution of the Al and Ni lattice parameters, we can monitor the overall degree of intermixing of Al and Ni during preheating.This allows us to compare the behavior of specimens in which the heating is insufficient to initiate a runaway reaction with those in which a runaway reaction is initiated, and identify phase transformation mechanisms that promote (or prevent) the runaway reaction.

We have been conducting experiments to understand the effects of electrically heated Ni/Al laminate foils at heating rates as high as 10^12 K/s, including the measurement of emission spectra resolved temporally over 350 ns as reported in the 2011 Fall MRS meeting. These results provided qualitative evidence of rapid exothermic, vapor phase mixing of Ni and Al, given their much brighter emission compared to control experiments containing only Ni or Al. These results were significant, because thermal diffusion processes normally limit Ni/Al reactions to much slower energy release rates, potentially limiting their applications. Here we present further evidence of exothermic Ni/Al mixing, quantified by experimental velocity measurements of encapsulation material ejected from the reaction zone, and interpreted by numerical calculations of energy partitioning into different processes. Heating Ni/Al laminates at these rates caused a rapid vaporization of the material, and a physical acceleration of the encapsulation layer away from the substrate. By measuring the velocity of this layer using photonic Doppler velocimetry, we quantified the conversion of electrical to kinetic energy. We facilitated the interpretation of these results by modeling the split of electrical energy between atomization and ionization processes, the internal energy of the vapor phase metal particles, the kinetic energy of these particles, and the kinetic energy of the ejected encapsulation layer. Although the model required some empirical fitting to account for processes such as additional heating and melting of material outside the intended reaction zone, it successfully reproduced a variety of experimental conditions involving different materials and heating rates. Most significantly, it allowed the quantification of energy released from the vapor phase mixing of Ni and Al, which correlated with reference values for heats of mixing. Alternative reactive materials with higher intrinsic heats of mixing promise correspondingly higher efficiency gains. These results will likely enable new, high efficiency bridge wire applications where the initiation of subsequent energetic reactions may be accomplished with less electrical energy than is currently required.

Novel combustible materials are of great interest for the US ARMY, in particular the Propulsion Research and Engineering Branch of ARDEC (Armament Research, Development and Engineering Center). ARDEC, in-conjunction with Polymer Processing Institute (PPI), has developed a novel combustible material, Foamed Celluloid. Foamed Celluloid Technology is based on celluloid, a plastic-like material made primarily of Nitrocellulose and Camphor that is readily processed into different shapes and is classified as a 4.1 flammable solid by the U.S.D.O.T. Traditional commercial Celluloid can be improved and made far more superior by foaming the material, which reduces the product density to much less than 1 g/cc, and create a porous structure. Foaming this material enhances the transformation of chemical energy into combustion energy at a higher rate while still being considered a completely insensitive material. It helps the Department of Defense (DoD) by leaving no debris on the ground which mitigates unnecessary waste and hazard issues encountered on both the battlefield and training grounds. Foamed Celluloid material can be obtained in the form of sheets and cylindrical pellets which also enables the production of parts with complex geometries while maintaining similar combustion characteristics. In addition, Foamed Celluloid materials mechanical properties can be tailored depending on the product requirement. This material offers war fighters a customizable weight reducing and cost saving alternative for a wide range of product applications.

The effects of high-pressure dynamic loading on Ni-Al mixtures in various configurations and any subsequent phase transformations were investigated. Ni-Al mixtures represent an example of a Structural Energetic Material (SEM) that combines strength and stiffness with the energy-release capabilities of a molecular explosive. High-pressure shock-loading can cause intimate constituent mixing, leading to chemical reactions, forming an intermetallic, and releasing energy. Experiments were performed on different forms of nickel-aluminum mixtures (i.e. powder compacts, vapor-deposited foils, single layers) over a range of shock pressures using a laser-accelerated flyer system in attempt to characterize the processes leading to complete reaction. VISAR and PDV interferometry were used to infer the occurrence of any shock-induced chemical reactions. Post-mortem microstructural characterization was performed to identify the reaction products and isolate the processes that lead to reaction initiation. In this presentation, the influence of flyer momentum and Ni-Al reactant configuration on the reaction response will be discussed.

Boron and boron-carbon nanomaterials with BET surface areas as high as 535 m²/g were prepared by the sonochemical reduction of boron halides with alkali metals. The as-prepared solids were heat treated under vacuum to anneal the material and to remove salts by sublimation. Higher surface areas correlate to lower ignition temperatures, and the highest surface area boron materials spontaneously combust in air. Ignition and combustion characteristics were studied by TGA/DSC for both freshly made and air exposed nanomaterials. Additional characterization involved oxygen bomb calorimetry, SEM, bulk elemental analysis, solid state NMR, IR, and gas absorption measurements.

Previous our studies have shown the possibility of the laser control of the self-propagated high-temperature synthesis (SHS) of AlN and TiN phases during selective laser sintering/melting (SLS/M) of Al and Ti powders, accordingly, in the high pressure nitrogen gas environment [1, 2]. Optimal regimes of SLS/M were determined. Optical metallography, SEM, XRD and EDX analysis allowed justify the structure and phase composition of the synthesized nitrides. At the first time, 3D articles containing TiN were fabricated from titanium powder. But synthesis of TiN during SLM with article bed temperature up to 300 0C leads to non-controlled ignition of titanium powder in N2. Synthesis of AlN layers during SLM with article bed temperature 150 0C or less results in curling of the 3D article layers. So at present report we developed the self- consistent two-dimensional mathematical model describing the overlap of the SHS of nitrides phases with SLS process, controlled by moving the laser irradiation spot, but without a gas component transport in reaction zone. Practically, model is described thermal processes into powder, similar developed us numerical model in [3]. The model allows the estimation to correspond to the geometrical characteristics of the system and values of the laser irradiation velocity at which the single layer of the green mixture completely reacts in a neighbor area of the moving laser irradiation spot. The results produced are in good agreement with experimental data obtained earlier [1, 2]. [1] Shishkovsky I.V., I. Yadroitsev, Y. G. Morozov, I. Smurov. Titanium and Aluminum Nitride Synthesis via layer by layer LA-CVD // Applied Surface Science, Vol. 255. Issue 24. 2009. p. 9847-9850. [2] I. Shishkovsky, L. Kholpanov, S. Zakiev. Layer by layer synthesis of 3D parts from TiN via SLS method. // Physics and Chemistry of Material Treatment, 2005. Issue 3. p. 71-78 (in Russian). [3] Zakiev S.E., Kholpanov L.P., Shishkovsky I.V. et al. Modeling of the thermal processes that occur during laser sintering of reacting powder compositions. // Applied Physics A: Materials Science & Processing. 2006. V. 84 (1-2). p. 123-129. Keywords: numerical modeling, laser control self-propagated high-temperature synthesis, TiN, AlN.

Aluminum powder is widely used as a fuel additive in propellants, explosives, and pyrotechnics due to its high volumetric combustion enthalpy and relatively low cost. However, the thermodynamically predicted benefits of aluminum combustion are rarely achieved because of extended ignition delays associated with heterogeneous reactions involving protective alumina layer, which is always present on the aluminum surface. In order to fully exploit aluminum&’s high reaction energy, this effort focuses on adjusting its combustion dynamics by modifying its surface and structure. The modification is achieved by cryo-milling aluminum with cyclooctane, which is liquid at room temperature, but solid when cooled by liquid nitrogen. A previous study showed that fine, equal-axial Al-cyclooctane composite powder with small amount of cyclooctane retained by aluminum can be prepared by mechanical milling. Aluminum surface of the prepared sample is coated with a cyclooctane-modified layer with properties significantly different from those of regular alumina. The powder ignites at substantially reduced temperatures, produces shorter ignition delays, and higher aerosol burn rates compared to an unmilled spherical Al powder with similar particle sizes. Its oxidation kinetics, as observed from thermo-analytical measurements, is also different from that of pure aluminum. Further modification of the surface layer of the cryomilled aluminum is achieved by its annealing in an inert atmosphere. The oxidation, ignition and combustion processes will be compared for the annealed and as-prepared samples. Results will be presented and discussed in this paper. The oxidation mechanism of the prepared materials is studied in the present work. Experiments are carried out at varied atmospheres to study reactions of surface-modified aluminum with different oxidizers. Micro-calorimetry is used to quantify the reaction rate at constant low temperatures; these rates are compared to the reaction rates obtained from differential scanning calorimetry and thermogravimetry, covering oxidation at higher temperatures. Reaction products are characterized using energy-dispersive X-ray spectroscopy and X-ray diffraction.

The goal of this general effort is to use the rapid, exothermic formation reactions commonly found in multilayer foils to drive oxidation at high temperatures and produce controlled bursts of energy. In this particular study we focus on extending and controlling the oxidation of nano-layered metallic foils by optimizing foil geometry and chemistry. The most successful foils to date are composed of alternating layers of Al and Zr or Al-Mg and Zr, all of which are deposited via magnetron sputtering. Total heat output is primarily a function of oxidation time and temperature, both of which are measured for a given reaction using two-color pyrometry. Bomb calorimetry is also employed to measure total heat output, and XRD, SEM-EDS, and electron microprobe microscopy analyses are used to characterize phase and compositional information. The duration and magnitude of oxidation is found to increase with wider and thicker foils, and the presence of Mg speeds the oxidation process and increases the heat released as Mg sublimes upon reaction and oxidizes in air.

Efforts to ensure reliability of current munitions and to develop new munitions require multiscale modeling of energetic materials from formulation to application. Integration of computational methods across the size scales of scientific and engineering interest, from atomistic through micro- and meso-scale to the continuum level, has already begun. The ultimate objective is to be able to approach energetic design problems, to the greatest extent possible, from a firm scientific and engineering standpoint. For these efforts to be successful modeling must not only reproduce bulk material responses but important microstructural features that arise at the mesoscale during processing that have commensurate affects on composite materials&’ responses. Given the micron-scale particle size and high concentration of the explosive constituents in the polymer matrix, a detailed understanding of crystal-polymer interfaces holding the PBX together is critical for accurate model development. Here, we study Composition A-3 as a representative PBX, chosen both for relevance to the field and as an ostensibly uncomplicated formulation to model. We find that key aspects of the PBX formulation process affect the microstructure and thermal properties of the finished product. Specifically, residual solvents and oils from the binder emulsification create a diffuse interface between the crystal and binder. The diffuse interfaces, along with some of the chemicals that remain in the composite after manufacture, create a heterogeneous multicomponent system that may be difficult to model. We also describe initial attempts to experimentally quantify structure-processing-property relationships through multiscale characterization that is commensurate with our multiscale modeling efforts.

Mechanical stimuli in energetic materials initiate chemical reactions at shock fronts prior to detonation. Shock sensitivity measurements provide widely varying results, and quantum-mechanical calculations are unable to handle systems large enough to describe shock structure. We report multi-million atom reactive force field molecular dynamics (REAXFF-MD) simulation of shock-induced chemical reaction of l,3,5-trinitro-l,3,5-triazine (RDX) crystals. Our REAXFF-MD simulations reveal that the shock-induced chemical reaction is preceded by a transition from well-ordered molecular dipoles with a diffuse shock front to a disordered dipole distribution with a sharp front. We find the RDX crystal has a bimodal response to shock loading. A group of molecules with NO2 group facing away from the shock undergoes rotation upon the arrival of the shock front, whereas another group of molecules with NO2 group facing toward the shock front shows deformation. In the presence of a nanometric void, we observe the formation of a nanojet in the void. Combined with the excitation of vibrational modes through enhanced intermolecular collisions in the void, the nanojet catalyzes chemical reactions that do not occur otherwise. When the particle velocity is increased, the shock wave front undergoes a pinning-depinning transition. We will discuss two characteristic regimes during the void collapse: 1) NO2 formation until the void closes completely, and 2) the subsequent production of N2 and H2O. This work is supported by the Office of Naval research, Air warfare and Weapons Department, grant No. N00014-12-1-0555.

Reactive materials offer the rapid release of heat through exothermic reactions and are gaining commercial acceptance. While these materials are commonly fabricated using physical vapor deposition, ball milling, cold spray and cold rolling, alternative processing methods are often desired. Here we present an inexpensive, mechanical process in which elemental Al and Ni powders are packed into steel tubes and then radially forged or swaged to reduce the average reactant spacing. Through a series of radial reductions of the tube diameter we observed the formation of a fully-dense compact and a decrease in the average reactant spacing. This microstructural refinement was observed directly through cross-sectional imaging. It is also inferred from reaction properties. Differential scanning calorimeter (DSC) experiments show a shift of exothermic peaks to lower temperatures, reactions are ignited at lower temperatures, and the reactions self-propagate faster as the average reactant spacing is reduced with additional swaging. In addition to microstructural refinement, the presentation also considers the chemistry and shape of the initial reactant powders. By substituting Al-Mg powders for the elemental Al powders, ignition temperatures drop and reaction velocities rise. However, the substitution of Ni flake for Ni powder does not improve reaction properties.

Efforts are underway to replace inert fragment materials with reactive materials that deliver kinetic energy and chemical energy on impact. In addition to delivering chemical energy, the reactive material is intended to fragment into particles of controlled size with an explosive launch or on initial impact. Here we describe a two-step mechanical process for fabricating reactive materials that fragment into particles of preferred size. In the first step powders of Ni, Al-Mg, and W are packed into a steel tube and then they are radially forged (swaged) at room temperature into a fully dense rod. The rod is then ground and sieved into particles that measure 1 mm to 2.36 mm in their smallest dimension. In the second step these particles are re-compacted into a steel tube and are re-swaged to form a second rod. This rod is then machined into a cylinder for open field explosive fragmentation tests. An Al cylinder was also formed and tested in the same manner for comparison. Fragments were collected using a soft catch method and the reactive material samples showed a higher concentration of larger particles than the Al samples, as expected. To complement these particle distributions, the reactive materials were also characterized both before and after testing using a combination of differential scanning calorimetry and x-ray diffraction. An assessment of the fabrication method and plans for future directions will be presented.

Reactive multilayers grown by sputter deposition have recently attracted interest for emerging applications including joining (soldering, brazing) and energy sources. For these applications, a metal-metal multilayer is typically designed to have a composition that corresponds to the peak enthalpy for a given material system as this maximizes heat output. With the focus on a single composition, it is not surprising that little work has explored the full range of composition that gives rise to self-sustained, high temperature synthesis (SHS) reactions for a given reactive metal pair. With this poster, we describe a detailed study of the reactive Al/Pt system in which the net composition of multilayers is varied over a large range from Al0.2Pt0.8 to Al0.8Pt0.2. For multilayers having a total thickness of 1.6 microns, self-sustained, high temperature reactions occur when the net multilayer composition is in the range of Al0.33Pt0.67 to Al0.75Pt0.25. Equiatomic Al/Pt multilayers have the maximum velocity of all films investigated in this study, consistent with the maximum heat of formation (revealed by calorimetry). Multilayers having a net composition of Al0.2Pt0.8, Al0.25Pt0.75 and Al0.8 Pt 0.2 did not react when stimulated at a point. In addition, we describe the phases formed for each of the 12 compositions that exhibited self-sustained reactions. Multilayers reacted in air formed intermetallic compounds, and no oxides were detected by x-ray diffraction. Phase formation was largely consistent with published equilibrium phase diagrams. However, multilayers having near-equiatomic compositions reacted to form a metastable, large unit-cell, rhombohedral phase. An approximate solubility range has been determined for this metastable phase. * Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Nanoparticles (NPs) have larger specific surface area and higher activity faces than bulk material. We succeeded in synthesizing NPs with high dispersiblity, small size distribution and high crystallinity by hydrothermal treatment using oleate-metal complexes[1-3] and named this method as “Oleic acid modified hydrothermal growth method”. Normally, NPs synthesized by this method have oleic group on their surfaces facing non-polar group outward and can disperse only in non-polar organic solvent. However, we found YVO4 NPs synthesized by this method have amphipathic property dispersing in both of polar and non-polar solvents. This novel property might be obtained by the formation of “Oleic acid double layer” on their surfaces. The primary layer is formed by chemical bonding of the carboxyl group onto the surface active sites of nanoparticles, and the secondary layers aligned antiparallel between the primary layers by hydrophobic interactions among alkyl chains of the oleate.[4] In this study, we investigated to find some of hydrothermal conditions to obtain amphipathic property for nanoparticles synthesized using oleate-metal complexes. We found CeO2 NPs could also exhibit amphipathic property by changing hydrothermal condition of oleic / metal ion ratio to 1/4, while the NPs synthesized that of 1/1 did not exhibit amphipathic property. On the other hand, YVO4 NPs could exhibit amphipathic property only when they were synthesized from the solution with oleic / metal ion ratio of 1/1. Based on the obtained results, the optimization of oleic/metal ion ratio for each crystallite phase would be one of key parameters to synthesize amphipathic NPs by oleic acid modified hydrothermal growth method. [1] Taniguchi, T., et al European Journal of Inorganic Chemistry 2009, (14) 2054-2057 [2] Ohno, Y., et al., Crystal Growth & Design 2008, 11(11), 4831-4836 [3] Taniguchi, T., et al.,Crystal Growth & Design 2008, 8(10), 3725-3730 [4] Taniguchi, T., et al.,The Journal of Physical chemistry C 2010, 114(9), 3763-3769

Reactive energetic plasticizers (REPs) containing an activated terminal alkyne group are synthesized by using the aldol condensation and esterification. These REPs are found to work properly as a general plasticizer in GAP-based polyurethane formulation process capable of exhibiting prominent plasticization effect on GAP prepolymer and then complete the click reaction under Cu-free condition with azide groups of GAP prepolymer in PU curing process. The miscibility between REPs and GAP prepolymer is quantitatively evaluated by modified Fox equation. Effect of REP structure on the plasticization to GAP prepolymer in respect to the reduction on viscosity and glass transition temperature of GAP prepolymer and kinetics study of in-situ click reaction with PTG prepolymer are investigated in detail. We find that REPs with ester linkage behave stronger ability on decreasing viscosity of GAP prepolymer and the distance between electron withdrawing groups and terminal alkyne determines the reactivity of REPs reaction with azido prepolymer. Thermal and mechanical properties of REP-integrated GAP-based PUs are also discussed.

Passivation is critical for safe handling and protection of molten Mg surfaces in casting operations. Recent studies have explored the ability of certain fluorine-containing cover gases to protect molten Mg from excessive vaporization and burning during casting by modifying the native oxide (MgO) through interaction with these gas atmospheres. These challenges are due to the high vapor pressure and pyrophoricity at high temperatures of molten Mg, especially for fresh melt surfaces that react with air and humidity. Such hazards are compounded for gas atomization of Mg that creates expanded powder surface. The present study sought to develop a version of this melt protection strategy for use in the much more dynamic situation of in-situ passivation during gas atomization of spherical Mg powders, i.e., to form a protective reaction film on the powders that resists ignition at elevated temperatures and during powder handling. Both conventional SF6 and novel NF3 gas treatments were tried in these experiments. The objective was to convert fragile MgO to a robust oxy-fluoride protective surface film. Small-scale trials on Mg “puddle” samples evaluated their passivation behavior in short timescales for use in subsequent gas atomization trials. Gas atomization parameters were designed to produce Mg powders larger than 200 µm to minimize surface area production. Spherical powders were captured in passivation process gas and cooled to ambient temperature to further enhance the safety of the experiments. Tests of the ignition temperature of the resulting spherical powders, compared to commercially available Mg powders, demonstrated that the novel passivation technique resisted ignition in air to 635 C, 110 C above commercial powders. Auger depth profiling revealed the new protective film to be only 20-40 nm thick, at least 10 times thinner than the typical MgO on commercial Mg powders. Some preliminary Mg powder compaction experiments were also conducted to test the green strength and (vacuum) sintered strength of test parts made from these powders. Supported by the U.S. Army Armament Research, Development and Engineering Center (ARDEC), Picatinny Arsenal, on a Work for Others contract through Ames Lab Contract No.DE-AC02-07CH11358.

Downscaling nanoenergetic materials in such a way to allow for “nanoenergetics on a chip” offers new perspectives in micro and nanosystems. In this context, controlling the barrier layer formation during technological processes and subsequently, predicting final performances appear as one of the main challenges in multilayered nanoenergetic materials. Indeed designing barrier layers should lead to tailored nanoenergetic performances for targeted applications. In this context, modeling is expected to play a key role for the fundamental understanding of the elementary mechanisms that are impacting the growth and structuring of the interfaces as well as for the proper calibration of process parameters. In this modeling study, we adopt a bottom-up multiscale modeling approach to develop a macroscopic 1D model based on environment dependent chemical kinetics. This model has been achieved with the goal of establishing the link between the intermixed interfacial layer and its impact on ignition capabilities. It has been fitted on the Al/Ni couple. We propose a fully physico-chemical based model of the ignition process. Specifically, we detail the basic atomic scale reactions identified through DFT calculations and subsequent mesoscale equation set. From these equations, the impact of intermixing layers and defects on ignition properties is evaluated. We study the intermixing and interface atomic composition under different temperatures. Notably we highlight the major role played by pre-existing vacancies coming from the deposition procedure and intrinsic lattice mismatch between Al and Ni materials, that decrease the ignition temperature. We also observe a thermal stability (temperature and delay) induced by premixed barrier layers.

Reaction between metal and metal oxide nanoparticles can be extremely fast. Previously, it has been shown that metal oxides will release O2 at high temperature and that condensed phase reactions can occur as well, but the relative role of each process during ignition and reaction propagation is not clear. This work probes the issue by altering the distances between the two components through changes to the degree of mixing. Changes in the distance between components affect gas-solid reaction and condensed phase reaction in significantly different ways. Therefore, by varying the degree of mixing, changes in combustion characteristics can provide insight into the nature of these reactions. In this work several different mixing procedures are tested for reactivity and segregation of components. Reactivity is tested using a constant volume pressure cell that simultaneously records both pressure and optical signal. Segregation in the samples is studied using scanning electron microscopy (SEM) with backscattering and energy dispersive X-ray spectroscopy (EDS) capabilities.

Finding the optimal fuel with the highest selectivity for catalytic platinum nanoparticles would enhance the performance and better sustain the chemical reaction within micro-scale reactors. Platinum nanoparticles ignite small liquid alcohols and gaseous alkane fuel-air mixtures at room temperature. This combustion reaction yields a high conversion of fuel, due to the high catalytic activity and large surface-to-volume ratio of nano-sized platinum catalysts. For this study, a variety of fuels were investigated for catalytic combustion system. The Pt nanoparticle were synthesized via a colloidal synthesis technique (dp ~ 7 nm), suspended in solution, and deposited on cordierite monolith substrates. Catalytic performance in the oxidation of methanol, ethanol, methane, propane, and butane was investigated. Catalytic activity was measured by bulk substrate temperature within a small-scale continuous flow reactor. Each fuel was evaluated for repeated catalytic cycling from room temperature ignition to stable operational temperatures and for the range of flow rates at which ignition occurred. The overall outcomes indicate micro-scale reactor device design can be optimized for thermal management and device integration using the fuel-based catalysis results of this work. Alternatively, the study contributes to the library of hydrocarbon nanocatalysis studies using platinum.

Aluminum coated tungsten-oxide nanowires, forming a thermite nanocomposite, are fabricated using a novel combined flame and solution synthesis method. Such geometry not only presents an avenue to tailor heat release characteristics due to anisotropic arrangement of fuel and oxidizer, but also possibly eliminates or minimizes the presence of an Al2O3 passivation layer between the aluminum and metal oxide. Such a nanocomposite would be very useful for fundamental study of aluminothermic reactions. This work encompasses detailed characterization of the nanocomposites, where the Al layer thickness is ~16 nm, covering WO2.9 nanowires of diameters from 20-50 nm and of lengths>10 µm. Ignition and combustion of the thermite arrays are also investigated.

We use molecular dynamics simulations to investigate the mechanisms of energy absorption, localization and transfer in the high energy density nitramine HMX and how chemistry initiation is affected by way energy if inputed into the system. Our ultimate goal is to understand how energy can be localized spatially and in the frequency domain to trigger chemical reactions with minimum insults. We use a combination of reactive and inert force fields to characterize how energy is distributed among vibrational modes after electromagnetic excitation targeted at specific vibrations. Reactive MD simulations are used to characterize the initial chemical events in HMX after thermal and electromagnetic insults including the role of defects such as grain boundaries and interfaces with a polymer binder.

We report the effect of Ni in polycrystalline ceramic solid solution of Lead Zirconium Titanate (PZT) with chemical composition of Pb1-xNix(Zr0.52Ti0.48 )O3 with x = 0,0.05,0.10,0.15,0.20 prepared by solid state reaction technique. The X-ray diffraction study shows the formation of homogeneous polycrystalline tetragonal structure. The lattice strain (c/a) ratio increases linearly with an increase in the doping concentration of Ni however crystallite size is decrease due to significant change in lattice parameters may be attributed to lateral shrink & vertical expansion in oxygen octahedral. In FTIR four distinct absorption bands at 580 cm -1 (v1), 616 cm -1 (v2), 669 cm -1 (v3) and 794 cm -1 (v4) were observed for all compositions. The ferroelectric and magnetic properties were discussed. The dielectric permittivity & loss tangent of ceramics were measured as a function of frequency and temperature. The Ni doped solid solution undergo into ferroelectric-paraelectric diffuse phase transition. Maximum dielectric constant decrease with increase in Ni concentration. The activation energy (Ea) of different compositions were estimated and in ferroelectric phase. The saturation polarization (Ps) for pure PZT was found to be ~16µC/cm2.

In this work we propose a macroscopic (continuum) simulations scheme for shock response of porous inter-metallic materials. The proposed simulation model includes (1) a simplified equation of state for porous solids that includes the evolution of porosity in the material as a function of shock pressure and, (2) a rate dependent plasticity of the matrix material that accounts for the deviatoric strength of the material at weak to moderate shock strengths. The numerical scheme employs cold-mixture theory to predict the shock response of porous inter-metallic mixtures. It is envisioned that the simulation model will lend itself as a predictive tool to delineate the threshold and sub-threshold velocities required to initiate chemical reactions in nano-structured energetic materials.

In the future missions to the Moon and Mars, it would be attractive to produce construction materials in situ from regolith. High-temperature sintering needs a lot of energy. An alternative approach involves mixing the regolith with materials that can react exothermally either between each other or with the regolith, leading to self-sustained combustion. The generated high temperatures may result in the formation of dense ceramic products, like in self-propagating high-temperature synthesis. An important advantage of this method is that only small amount of energy is required for ignition. The use of magnesium as an additive to lunar regolith ensures self-sustained propagation of the combustion wave over the mixture pellet, i.e., the regolith behaves as a reactive material in respect to magnesium, with various combustion regimes, including spin combustion, being observed. At the same time, using aluminum requires additional additives such as iron oxides. The experiments with regolith/magnesium mixtures have shown that the properties of the products are affected by the environment. For example, submerging the pellet in silica significantly increases the product strength.

Aluminum is commonly used in energetic materials. The protective oxide film on its surface, however, often complicates the use of aluminum as a reactive material. Current methods for aluminum activation involve harmful and expensive materials. The nanoscale aluminum powders also remain very expensive and have problems such as a large amount of oxide on the surface. The present paper investigates the preparation of an activated aluminum powder from aluminum foil that is widely available as scrap and waste. The obtained results demonstrate that a highly reactive, fine aluminum powder can be obtained from aluminum foil by high-energy ball milling. The process involves environment friendly additives that can easily be removed after milling and recycled. Characterization of the obtained powder was conducted using laser diffraction particle size analysis, Brunauer-Emmett-Teller surface area analysis, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The obtained powder readily reacts with hot water, releasing hydrogen. The kinetics of this reaction was studied using an inverted graduated cylinder. The effective activation energy of the reaction rate is in a good agreement with the literature data for Al-H2O reaction. The combustion characteristics of the obtained Al powder are currently being studied.

In this study, hollow CuO spheres with thin shell thickness have been prepared by a simple “droplet-to-particle” aerosol spray pyrolysis method. Hollow structure is produced by adding sucrose and H2O2 in the precursor solution as gas blowing agents. The resultant hollow CuO spheres are comprised by small nanosized building blocks with crystallite size of ~10 nm. The nanoaluminum thermite with hollow CuO spheres as oxidizer ignites in a very violent manner and demonstrates a high pressurization rate of 108 psi/mu;s and transient peak pressure of 129.9 psi, which significantly outperforms commercial CuO nanoparticles. Temperature jump mass spectrometry results show that hollow CuO spheres display a faster oxygen release behavior than commercial CuO nanoparticles.

Silicon-based reactive materials are of interest because of silicon's good thermochemical properties, relatively high active content, higher reaction temperatures with select oxidizers, a thin passivation layer, and surface modification potential. Nanoscale silicon (nSi) based reactive materials and mechanically activated silicon-based reactive materials have the capability to improve fuzes, initiation systems, and high radiant intensity applications. The primary objective of this research was to determine the dominant parameter (oxygen content, SSA, morphology, or other) in nanoscale silicon systems influencing silicon reactivity with fluoropolymers. Likewise, we explore the parameters affecting the performance of mechanically activated silicon reactives. While various oxidizers have been observed to show promising combustion behavior with silicon, the high heat of fluorination for Si/polytetrafluoroethylene (PTFE) combustion suggests that this system is of particular interest. In this work, various nSi powders with varied specific surface area (SSA), oxygen content, and morphology were mixed with PTFE and FC-2175 (SiTV) and compared on the basis of temperature, burning rate, and spectral emission (visual to infrared). In addition, mechanically activated silicon, and also aluminum silicon alloy reactive materials with PTFE as the oxidizer were also characterized.

Porous silicon (PS) has become an attractive energetic material due to its high energy density and recent demonstration of flame propagation speeds of over 3000 m/s. However, much work remains to determine the controlling factors in PS combustion. This study probes the combustion of a porous silicon/sodium perchlorate composite and the effects of various PS material properties, including pore size, porosity, film depth, and surface area. The porous silicon samples are etched into crystalline silicon wafers through a tunable process that results in variation of silicon material characteristics. Analysis of the porous silicon material properties is performed using Brunauer, Emmett, Teller (BET) porosimetry, SEM, and the spectroscopic liquid infiltration method (SLIM). Combustion characterization is primarily performed with a Photron Fastcam SA5 high-speed camera at rates of 930,000 frames per second. Using the above characterization techniques the combustion flame speeds can be correlated to the changes in porosity, film depth, surface area, etc. A relationship has been observed for the dependence of reaction rate on PS pore size as the reaction rate varies from several kilometers per second to 300 m/s as pore size increases from 2 nm to 5 nm.

Aluminum (Al) is an important solid fuel for energy conversion and propulsion systems due to its large volumetric energy density, earth abundance, and low cost. These applications involving Al will benefit greatly from a simple, nonintrusive and multi-point ignition method to improve ignition reliability and to increase heat release rate. Previously, we have applied a xenon flash lamp to successfully ignite Al nanoparticles (NPs) due to the photothermal effect, but not Al micron particles (MPs) because Al MPs have smaller light absorption due to increased light scattering and simultaneous higher ignition temperatures. Flash ignition of Al MPs was achieved previously but with very high-power xenon flash lamps (around or above 25 J/cm2). Herein, we report a successful low-power (1 J/cm2) flash ignition of Al MPs by addition of WO3 nanoparticles (NPs). We believe that the main roles of WO3 NPs in facilitating Al MPs flash ignition is to increase light absorption and effectively supply oxygen due to their large specific surface area as well as their close contact with Al MPs for the following reasons. First, the light absorption measurement shows that WO3 NPs increase the total amount of light absorption mainly in the ultraviolet region (< 450 nm). Second, the minimum ignition energies (MIEs) of the mixture of Al MPs and WO3 NPs are around 0.7 to 0.9 J/cm2 and insensitive to the ambient gaseous oxygen concentration for the Al-lean (Phi;=0.5) to stoichiometry range, indicating that the major oxygen source comes from WO3, not ambient O2. Third, on the other side, when the mixture of Al MPs and WO3 is Al rich, the MIEs are lower in igniting in Air than those in inert N2, suggesting that atmospheric O2, in addition to WO3, is also an important oxygen source for Al MPs. In summary, this flash ignition study shows that WO3 NPs not only enhance the light absorption of Al MPs, but also are much more effective oxygen source than gaseous O2 in oxidizing Al MPs upon exposed to a xenon flash lamp.

This work reports on the synthesis and reactivity of new metastable intermolecular composites (MICs) which employ periodate salts nanoparticles as oxidizers and nanoaluminum as fuel. The periodate salts (KIO4 and NaIO4) nanoparticles are synthesized by an aerosol spray-drying method which enables the formation of periodate salts nanoparticles by a “droplet to particle” process. The as prepared Al/KIO4 and Al/NaIO4 MICs are characterized by combustion cell and rapid heating wire experiments. The results show that these new MICs formulations have pressurization rates of >100.0 psi/us and peak pressure of >600.0 psi, which significantly outperform reported aluminum based MICs in both pressurization rate and peak pressure. The results indicate peroidate salts based MICs are very good candidates for nanoenergetic gas generators.

Aluminized fluorinated acrylate (AlFA) nanocomposites were recently introduced by the Air Force Research Laboratory, Materials and Manufacturing Directorate (AFRL/RX) as a new class of reactive fluoropolymer composites. Prepared through a two step functionalization-polymerization process the AlFA composites demonstrate comparable reactivity to Al/PTFE composites yet are highly tunable, less sensitive and display thermoplastic behavior thus allowing them to be readily processed through conventional polymer processing techniques such as melt extrusion and compression molding. Articles including infiltrated metallic foams and reactive liners have been successfully prepared and will be presented. Observation of the reactive performance of these composites has been investigated through multiple tests including open air pellet burns, instrumented tube burns, high strain rate impact studies, as well as constant volume explosion (blast enhancement) tests. Experimental results suggest the composite material is capable of reacting on the microsecond timescale.

The objectives of this work are to prepare and characterize iodine-rich thermites and reactive materials for potential application in bio-agent defeat. Iodine-rich compositions were prepared using metal iodate oxidizers in combination with aluminum fuel. Higher iodine contents were achieved using iodocarbon additives, e.g., tetraiodoethylene. Reactivity during rapid combustion was evaluated for both nanoscale and micron-scale materials. The nanoscale materials were evaluated directly using a spark-initiated pan dent test. The micron-scale materials were mixed with 50% of nano Al/MoO3 and also evaluated with the pan dent test. The results for the mixed material were shown to fit well to a linear combination of the expected dent for each component, based on a rapid reaction. Results of the pan dent test were used to down-select micron thermites for further testing. Bismuth iodate was synthesized by precipitation from nitric acid solutions. The average particle size was controlled by the addition rate, and sizes included 95 nm (amorphous structure), and 180 nm and 3 micron (both crystalline). Additional sizes were produced by ball milling the 3 micron material, giving 1 micron and 350 nm sizes. Fluoropolymers, including poly(tetrafluoroethylene), were included in compositions to provide additional biocidal products, namely HF, that could be produced from reaction of AlF3 product with water.

Architectural features in the micron and nanometer regime influence many properties observed for the combustion behavior of binary and monomolecular energetic materials (EM). The flame propagation speed, one measure of reaction rate, is a key indicator of the combustion dynamics of such systems and has been traditionally conducted on loose powders, pressed pellets, and laminates. Morphology of reactants as well their stoichiometric ratios have also shown to influence the combustion behavior of such binary composites. In this study, highly reactive powders composed of amorphous and/or crystalline iodine (V) oxide and nano-aluminum were intermixed by sonication in denatured ethanol or a blend of denatured ethanol and ethyl acetate with/without the addition of cyclohexanone. The sensitivity of these powders was measured with respect to electrostatic discharge (ESD) values under ambient conditions. Thermal gravimetric analysis, differential scanning calorimetry, and powder X-ray diffraction show that samples containing cyclohexanone proceed by a different ignition mechanism than neat nano-aluminum/iodine (V) oxide composites. Furthermore, composites containing cyclohexanone with crystalline iodine (V) oxide vary to some degree with respect to their sensitivity (via measured ESD values) to that of composites with an amorphous oxide. This effort reports on the tradeoff between reductions in sensitivity due to variations in composite morphology, addition of cyclohexanone, and their effects to the combustion chemistry.

Reactive, metal-metal nanolaminates are a class of energetic materials that, if designed and selected appropriately, have potential utility for reserve batteries. In this presentation we discuss nanolaminate materials properties necessary for reserve battery application, including high adiabatic reaction temperatures (in excess of 1000 oC), high reaction propagation rates, ignition sensitivity, high electronic conductivity, and gasless reaction mechanisms. We will also describe nanolaminate material properties that impact battery manufacture, and demonstrate battery fabrication incorporating these materials. We will show how incorporation of these materials into the battery improves overall battery performance, leading to increases in both specific energy and specific power of the reserve battery.

Heterogeneous reactive materials with nano-sized solid reactants possess very high rates of reactions and low temperature of the reaction initiation. Comparison of four classes of nano-heterogeneous reactive materials (NHRM), including mechanically structured powder mixtures and compacts, deposited multilayer films, sol-gel systems and powders mixtures is made. A variety of experimental methods, e.g., high speed video, quenching of reaction wave, time-resolved X-rays and Synchrotron-rays diffraction, was applied to study exothermic reaction wave propagation in nano-heterogeneous media. An overview of experimental results creates ground for consideration hypothetic mechanisms of these processes. It is shown that direct interaction of solid and solid-liquid reactants, without formation of continuous layer of solid product can explain extraordinary rates of reactions and velocities of reaction wave propagation in the NHRM. As a rule, self-sustained reaction initiates at temperature below melting points of all reactants, therefore, solid state reactions trigger self-ignition (thermal explosion) in the NHRMs. It was assumed that nano-clusters and metastable phases formed in the vicinity of reactive boundary can play role of initiators of the reaction at low temperatures. Probable mechanisms of formation of such nano-structures are discussed. Peculiarities of reaction wave propagation in the NHRMs leads to the question; can this process be recognized as gasless combustion wave? It is shown that some features of process go out of the frames of current combustion theory. Recent experimental results on thermal structure of reaction waves and product structure formation process are also presented. Finally, some examples of existing and prospective applications of the NHRMs are critically overviewed.

An efficient approach that combines short-term (minutes) high-energy dry ball milling and wet grinding to tailor the nanostructure of Ni+Al, Ti+C, Si+C and Ta+C composite reactive particles is reported. Depending on the system and milling conditions two different reaction mechanisms, i.e. sudden ignition and gradual solid-state diffusion for milling-induced reactions are observed. Varying the ball milling conditions different microstructures can be obtained. The reactivity and ignition sensitivity of ball milled materials are investigated as function of milling time. A direct correlation between the milling-induced nanostructures and ignition characteristics (ignition temperature, ignition delay time and minimum kinetic energy thresholds for impact ignition) is found. Analysis of the microstructures and reaction kinetics suggests that different factors may be responsible for reactivity enhancement. In Ni+Al and Si+C systems such enhancement is primary related to the formation of intimate contact between the reactants. The fine mixing of reactants creates fresh oxygen-free boundaries which reduces the energy barrier for diffusion processes. In carbide forming systems (e.g. Ti+C, Ta+C) the solid-state dissolution of carbon in the metal crystal lattice is another important factor for reactivity enhancement. These and other specific features of high energy ball milling influence on the microstructure and properties of different powder systems are discussed.

This presentation describes in detail the mechanical fabrication, nonuniform microstructure and reaction properties of Ni:Al laminate composites. The reactive composites were fabricated by repeatedly cold-rolling Ni and Al foils that were stacked together with initial thicknesses of 25 µm and 18 µm, respectively. The rolling process consists of multiple 50% thickness reductions wherein the first reduction was followed by cutting, restacking and rerolling to achieve a total of three, six or nine 50% thickness reductions. However, some of the laminates also received a more mild series of six 20% thickness reductions without restacking. A numerical code was written and used to quantify the distribution of layer thicknesses, bilayer thicknesses and local chemistries for the complex laminate microstructures, while also preserving positional information for the constituent layers. The resulting distributions show that laminates with only 50% thickness reductions have a higher percentage of large bilayers, relative to the volume mean bilayer, compared to laminates with the additional 20% thickness reductions. Differential Scanning Calorimetry (DSC) was performed on the laminates to determine the temperatures of their exothermic peaks and total energy release during controlled heating. Peak temperatures correlate with the volume average bilayer thickness, while the energy release also correlates with the bilayer thickness distribution. Reaction velocity and maximum temperature were also measured for the laminates and were found to vary according to processing conditions and not according to the volume average bilayer thickness. Foils with 20% thickness reductions have both hotter and faster reactions compared to samples with only 50% thickness reductions. We use the distributions of layer thicknesses, bilayer thicknesses and local chemistries within the laminates to predict their reaction velocities and maximum temperatures.

This talk will include a brief review of the development of continuum models for modeling homogeneous and self-propagating reactions in nanostructured materials. Starting from simplified analytical models of steady reaction fronts, we shall discuss the evolution of physical representations of intermixing and thermal transport, as well as the construction of efficient computational solvers that effectively defeat the stiffness of the equations of motion. Attention is then focused on recent reduced order representations, and insight gained from these representations into the transient behavior of self-propagating fronts in three dimensions. Open questions and potential extensions are finally outlined.

Reaction characteristics of nanoscale multilayer foils of Nickel (Ni) and Aluminum (Al) were investigated using a new numerical model based on sequential diffusion limited growth of Ni₂Al₃ and NiAl with diffusivity values obtained from previous diffusion experiments. The model makes use of percentages of each phase that can exist at each grid point, which simplifies the enthalpy calculations and incorporation of phase changes. Furthermore, a dynamic grid consisting of a fine and coarse zone was used to improve numerical efficiency. The results show excellent agreement with measured velocity values, observed in-situ morphology of the thermal fronts and temperatures. The incorporation of an intermixing layer with a thickness of 2.35 nm resulted in a reduction in front velocity and temperatures at bilayer thicknesses lower than 20 nm. Thermal front oscillations were observed for bilayer thicknesses lower than 12 nm, when the adiabatic formation temperatures fell below the melting point of Ni (1728 K). The effect of convective and radiative losses was negligible on the front temperatures and velocities.

Reaction front instabilities are known to occur in gasless reactive systems when certain conditions reducing the reaction temperature are met. This generally occurs when the heat losses overwhelm the heat generated from the reaction. However, the relative importance of total heat release to lower heat generation rates by inhibited chemical kinetics is not well understood. In this study, the reaction behaviors of Co/Al gasless reactive systems with lowered heats of reaction and with decreased reaction rates through limitation of mass diffusion rates are compared. Reactive films were grown through sputter depositions. The foils consist of reactant layers of Co and Al, as well as diluent layers of reaction product CoAl, also deposited by sputtering using a stoichiometric alloy target. By varying the location and thickness of the CoAl layers, the effects of mass diffusion and heat losses on reaction front propagation dynamics can be individually monitored. Foils containing the diluent layer within a reactant layer (e.g. Co/CoAl/Co/Al/CoAl/Alhellip;) allow controlled reduction of the bulk exothermicity of the foil without affecting the reactant interfaces or diffusion distances, while placing thin diluent layers at the Co/Al interface provides a mass diffusion barrier with little reduction in heat of reaction. Using differential scanning calorimetry to partially react and quench samples and x-ray diffraction to identify the formed phases, the reaction pathway was determined as a function of diluent layer thickness and location and compared to undiluted foils. High speed imaging, both visible light and infrared was used to determine the individual effects of mass diffusion control and exothermicity reduction on reaction propagation rate and on instability formation in the reaction front. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Spark ignition and self-propagating reactions in green compacts of continuously low-energy ball-milled aluminum (Al) and nickel (Ni) powders at the NiAl composition for durations up to 13 hours (h) were investigated. The microstructure of the as-milled powders show uniform mechanical mixing and refinement of alternating Al and Ni layers with increasing milling time. XRD analysis of the as-milled powders shows nanoscale grain formation and solid-state diffusion of Al into Ni-rich solid solution for milling times beyond 6 h. Interrupted Differential Scanning Calorimetry (DSC) in combination with X-Ray Diffraction (XRD) analysis revealed that the milled powders have an identical phase formation sequence to those of nanoscale magnetron sputtered multilayer foils (MF). Green compacts of powders milled for 11 and 12 h with a packing factor of 0.6 could be ignited using a low-energy spark from a battery, similar to nanoscale magnetron MF's and they form metallurgically bonded compacts upon completion of the self-propagating reactions. The maximum thermal front velocity measured using a high-speed optical camera was ~ 0.25 m/s. Infrared camera measurements show that the temperatures reach 1911 K, indicating near adiabatic reactions. Post-reaction X-Ray Diffraction (XRD) analysis of green compacts show nearly complete conversion to the NiAl phase.

Ni/Al multilayers were prepared for the following SHS(Self-propagating High Temperature Synthesis). Sputtering followed by pressurizing method combination of sputtering and pressurizing, is tried in this study, whereas others are applying a single process between sputtering or cold rolling. It is expected that suggested sputtering followed by pressurizing method may reduce the production time compared with the process with sputtering only. Further in practice, the production cost of suggested sputtering followed by pressurizing method is considered to be cheaper than the other ones using a single preparation process, sputtering or cold rolling. In preparation of a single layer, sputtering of Ni on the 6 um Al foil, as a substrate, was proceeded before pressurizing the foils. Both of the Ni sputtered Al layers, which means one bilayer, were stacked tens of times, respectively. The Ni/Al layers were pressurized and bonded mechanically using Instron 5583. Fabricated pressurized Ni/Al layer (hereafter "Ni/Al layer") was compared and characterized using SEM (Scanning Electron Microscope) and XRD (X-ray Diffractometer). 50kW continuous fiber laser (YLS-10,000-S4) was applied as a local heat source in igniting the foils in order to produce the SHS. We found appropriate thickness for the homogeneous composition IMC after ignition by mechanical bonding. Make homogeneous composition IMC is very important for most intensive exothermic heat, uniform and continuity reaction. IMC's were analyzed by XRD in accordance with the thickness of the foil. The microstructures of the cross-section in processed multilayer foils and in the foils after ignition were investigated by SEM. Differences in the phase-evolution sequence of Ni/Al layer foil and Co/Al layer foil were studied by DSC (Differential Scanning Calorimetry).

NanoEnergetic materials offer unique perspectives for controlling exothermic reaction rates and energy outputs as well as for integrating them into Microsystems, thus opening the era of “Nanoenergetics on a Chip”. To be successful, this experimental effort must rely on a proper multi-model strategy in which interface formation or barrier layers and the related atomistic mechanisms responsible for the temporal evolution of these interface structures can be evaluated precisely at the atomic scale. This is mandatory for guiding the experimental processing conditions and to allow for subsequent predictive modeling of overall thermal characteristics. In our work we describe how the combination of DFT calculations, to shed light into the elementary chemical mechanisms, with conventional Kinetic Monte Carlo (KMC) procedures, can achieve the atomic scale simulation of experimental processes and give new insights into the understanding of interface formation. However, state-of-the-art KMC techniques are dramatically limited to account for exothermic chemical reactions that lead to specific “hyperthermal” atomistic motions. We will give new directions to overcome this difficulty and to implement these hyperthermal motions into conventional KMC procedures. We will illustrate and draw the main issues and goals of such a multi-model strategy through given examples on Ni/Al and CuO/Al multi-layer systems.

Metals such as nanoaluminum (nAl) contain and release a large amount of stored energy due to their chemical composition and size. Unfortunately, the energetic properties of these materials are often limited by the mass transport, diffusion distance, and poor assembly of reactive components. Biology, on the other hand, uses an assortment of biomolecules to precisely control the interface and assembly of inorganic materials into exquisitely structured nanomaterials. As a result, this strategy has been highly effective for the assembly of complex nanostructures, protein-nanoparticle interfaces, and hybrid nanocomposites. In this study, we have used the binding affinity and loading capacity of protein cages (ferritins) to enhance the energetic properties of aluminum nanomaterials through a layer-by-layer assembly process. Using this approach, we can control the reaction stoichiometry of nAl by dialing in the number of protein layers and ultimately tune the energetic properties. Consequently, using ferritin in a single or multi-layer structure, we achieved enhanced reaction rates and increased energy output of nAl materials as demonstrated by the assembly of two types of materials (ammonium perchlorate/nAl and iron oxide/nAl). Notably, the combination of multifunctional protein cages with nAl represents a new approach in creating thermite materials.

Understanding of the combustion of metastable intermolecular composites, including the burning of aluminum nanoparticles, is critical for applications such as propulsions, explosives and other pyrotechnics. Aluminum nanorods (Al-NR) with oxidized shells are good candidates for stable fuel-oxidizer combinations. We investigate the oxidation dynamics of Al-NRs of different diameters (26, 36 and 46 nm) but same aspect ratio using molecular dynamics simulations. We heat one end of the Al-NR to 1100 K and then study the oxidation reaction at the interface of the alumina shell and the Al core. We find that the heat produced along the oxidation cause the melting of nanorods, the acceleration of heat release is due to absorption of environmental oxygen atoms and the larger surface-to-volume ratio causes faster burning of thinner nanorods. We will present results for 1) the oxidation speed and 2) the melting speed of nanorods of different diameters. This work is supported by the Office of Naval research, Air warfare and Weapons Department, grant No. N00014-12-1-0555.

Interface layers between reactive materials in nanolaminates or nanostructured systems are believed to play a crucial role in the characteristics of nanoenergetic systems. Typically, in the case of Metastable Interstitial Composite nanolaminates, the interface layer between the metal and oxide controls the onset reaction temperature, reaction kinetics and stability at low temperature. So far, the formation of these interfacial layers is not well-understood for lack of in-situ characterization, leading to a poor control of important properties. We have combined in-situ infrared spectroscopy and ex-situ X-ray photoelectron spectroscopy, differential scanning calorimetry, and high resolution transmission electron microscopy, in conjunction with first-principles calculations to identify the stable configurations that can occur at the interface and determine the kinetic barriers for their formation. We thus can determine the composition of the interfacial layer and its role as a barrier. For instance, we find that an interface layer is formed during sputter deposition of alumimum, composed of a mixture of Cu, O and Al through Al penetration into CuO, and constitutes a poor diffusion barrier (spurious exothermic reactions at lower temperature); and in contrast (ii) atomic layer deposition (ALD) of Al2O3 using trimethylaluminum (TMA) produces a conformal coating that effectively prevents Al diffusion even for ultra-thin layer thicknesses (~0.5 nm), resulting in controlled ignition and better stability at low temperature. Importantly, the initial reaction of TMA with CuO leads to the extraction of oxygen from CuO to form an amorphous interfacial layer that is an important component for superior protection properties of the interface leading to a high system stability. Thus, while both Al sputter deposition and Al2O3 growth on CuO lead to CuO reduction, the reduction mechanism is different, directly affecting the ability to prevent Al diffusion. This work illustrates that the nature of the monolayer interface between CuO and Al2O3/Al controls the kinetics of Al diffusion rather than the thickness of the Al2O3 layer, underscoring the importance of the chemical bonding at the interface in these energetic materials. In-situ characterization and first principles modeling clearly show that ALD, by producing conformal diffusion barriers, provides a means for tailoring materials interface for safe and controlled use of Metastable Interstitial Composite nanolaminates and all reactive unstable materials systems, nanostructured or nanolaminate systems.

Nanothermites is a class of energetic material composed of a metal and a metal oxide (the oxidizer) that can be processed as a stacking of successive nanolayers. In this frame, the Al/CuO couple is particularly interesting since its compatibility with standard microelectronics processes makes it as a good candidate for the integration of energetic materials into Microsystems. The great energy density associated with the combustion reaction between Al and CuO (21 kJ/cm3) ensure that, in principle, a small amount of material will lead to a substantial delivery of energy. But this theoretical value is valid under the condition that the fabrication process is optimized and gives a perfect nanolayer stacking, without any defects and exhibiting a maximized intimacy between the reactants. In reality, microscopy pictures reveal that during the experimental deposition processes, such as PVD, an interdiffusion exists and has a non-negligible role in the final structure and, as a consequence, in the device performances. Numerical simulation can help us to attack the problem of simulating the experimental process at the atomic scale via Kinetic Monte-Carlo technique. This kind of simulation has proven its efficiency on a number of applications and materials (e.g. Atomic Layer Deposition of high-k materials on silicon surface [1] and silicon oxidation [2]). We will present a preliminary version of a Kinetic Monte-Carlo package being able to simulate the initial stages of the deposition of CuO onto Al(111). We will detail the list of basic mechanisms which have been derived from First Principles calculations on the interaction of Cu, O, and their interactions in contact with Al(111). We will then discuss their implementation in the KMC and give simulation results as a function of experimental parameters (temperature and partial pressures). [1] A. Dkhissi et al., “Multiscale Modeling of the Atomic Layer Deposition of HfO2 Thin Film Grown on Silicon: How to Deal with a Kinetic Monte Carlo Procedure,” Journal of Chemical Theory and Computation 4 (2008): 1915-1927. [2] Anne Hémeryck et al., “A Kinetic Monte Carlo Study of the Initial Stage of Silicon Oxidation: Basic Mechanisms-induced Partial Ordering of the Oxide Interfacial Layer,” Surface Science 603 (2009): 2132-2137.

Multi-layer nano metal-oxide/metal thin films have the potential to augment and improve existing energetic materials used in the military and industry. Here we report on a series of reactive oxygen exchange nanolaminates between an oxygen source, CuO, and a reactive metal oxygen sink where the propensity for energy release is tailored by material selection and by multilayer geometry. These results suggest it is possible to create a class of energetic materials whose yield can be tailored for specific applications. In one set of experiments we demonstrate that by considering anion transport in the terminal oxide, we can produce multilayers that are unstable at room temperature, or those which require substantial thermal energy to ignite. We first explored this terminal phase hypothesis by comparing CuO-metal laminates with the reactive metals: Mg, Zr, and Al. The nanolaminates were exposed to rapid furnace anneals and subsequent x-ray diffraction to identify the onsets of oxygen exchange. Zr-CuO laminates were the least stable, owing to the fast oxygen transport through the ZrO2 terminal oxide, while CuO-Al laminates were the most stable, owing to the excellent diffusion barrier properties associated with Al2O3. CuO-Mg exhibited intermediate stability. A second demonstration is made for laminates of CuO and Al1-xTix where x is varied systematically between the pure end members. We identify a composition of ~ 35% Ti, above which the laminates react at RT, and below which temperatures of 300 C and above are require to initiate oxygen exchange. We will present sets of x-ray diffraction, SEM, and TEM data that illustrate the evolution of phase and microstructure in these “rapid” and “slow” oxygen exchange multilayer systems. In a second set of experiments we explore how geometry can be used to regulate the exothermic exchange reactions. For the reactive laminate systems of CuO-Al, CuO-Zr, and CuO-Mg, we prepared sets of multilayers of constant total thickness, but where the individual layer thicknesses are varied such that the number of metal oxide interfaces range from1 to 7. Using ex-situ x-ray diffraction and rapid furnace anneals, we identify that the minimum temperature needed to initiate the reaction drops in all cases on the order of 200°C as the number of interfaces increases. Calorimetry data for these samples show multiple exotherms that exhibit thickness-dependent temperatures. The exotherms can be attributed to the oxygen exchange from CuO and Cu2O to the reactive metal. The magnitude of the Cu2O exotherm and its temperature are strongly dependent upon the number of interfaces. Finally, we demonstrate that an applied voltage can be used to joule heat the electrically conductive laminate constituent and initiate the exothermic oxygen exchange. In all cases, small applied voltages could initiate the reaction. X-ray diffraction and optical images reveal that such reactions occur homogeneously over sample areas greater than 20 cm2.

Reactive composite nanolaminates are nanostructured materials composed of tens to hundreds ultra-thin bilayers of one metal (typically Al) and oxide (as CuO, Fe2O3). Once ignited by an external stimulus (commonly thermal impulse), the nanolaminate exhibits a rapid, high temperature and self-sustained exothermic reaction. Integrated on an electronic chip, reactive composite nanolaminate can therefore provide high energy density micro sources interesting for a large numbers of applications such as miniature safe initiators, thermal batteries, in-situ welding and microsoldering. Our research team at LAAS-CNRS works on the integration of Al/CuO nanolaminate on MEMS chip to produce tunable heat and gas at variable rate. Thin Al/CuO bilayers are magnetron sputter deposited on silicon or glass chip at ambient from alternatively Al and Cu targets. Each individual layer is controlled in thickness between 25nm-1µm with an accuracy of 5nm. In this work, we examine the relationship between nanolaminate structures and the resulting ignition and reaction properties by DTA, DSC and high speed camera. On chip ignition measurements have been also performed on glass, silicon and polymeric substrate. To evaluate the effect of reactant ratio, the DSC, DTA analysis and the burn rate of stoichiometric Al/CuO samples are compared to fuel rich and fuel poor ones. To evaluate the effect of layers thickness, different Al/CuO stoichiometric bilayer thickness ranging from 75nm to 3µm are also compared from thermal analysis and burn rate measurements. Results demonstrate experimentally that it is possible to tailor accurately the generated heat, burn rate and on-chip ignition of Al/CuO nanolaminates as desired by controlling the Al to CuO stochiomerty and the bilayer thickness. Compared to microstructured bilayers counterparts, the Al/CuO nanolaminates reaction is characterized by a highest reactivity and lowest ignition temperature Finally, to quantitatively evaluate the impact of interface thickness, we consider stoichiometric Al/CuO bilayer and characterized its thermal decomposition and burn rate after the sputter deposition, after different thermal treatment conditions (200°C during 24h, 48h and 1 week). Results show that the nanolaminate reactivity quickly decreases with the thickening of bilayer interface. It drops from 130m/s down to 27m/s. After a one day at ambient, the nanolaminate reactivity becomes stable since any further thermal heating at 200°C does not affect the reactivity. In consequence, for 300nm stoichiometric bilayer thickness, the natural interface composed of a mixture of Cu, O and Al constitutes a good diffusion barrier leading to a high material stability. This work opens the perspective to integrate with electronics and sensors, insensitive and long-shell life energetic layer providing tunable heat (hundreds to thousands °C, variable rate) under low-energy ignition with microsecond initiation response.

This work examines the synthesis and reaction mechanism of particle composite thermites. Direct ink writing (DIW) is used to prepare fine-featured conductive substrates, and electrophoretic deposition (EPD) is then used to deposit thin films of well-mixed thermites onto the substrates. A variety of electrode designs are investigated; both to explore the versatility of this combination of techniques, as well as to probe the reaction mechanism. Specifically, we find that the gas-trapping and degree of confinement can play a large role on the reactivity by impacting the forward energy transport of intermediate gases and particles. A nano-Al / nano-CuO thermite has been shown to exhibit over two orders of magnitude in propagation velocity by changing the confinement. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. # LLNL-ABS-561184

Impact initiation of reactions in various aluminum-based intermetallic-forming materials systems are investigated using instrumented gas-gun impact experiments under conditions of uniaxial-strain and uniaxial-stress loading. Time-resolved stress and particle velocity measurements as well as high-speed imaging are used for monitoring the deformation and reaction states to obtain evidence of reaction, based on changes in compressibility and shock velocity in the case of uniaxial-strain, and via direct light emission in the case of uniaxial-stress experiments. Meso-scale numerical simulations with CTH multimaterial hydrocode are also performed on actual (imported) and synthetically-generated microstructure. The simulations allow qualitative and quantitative probing of the local configurational changes and their effects on impact-initiated reaction mechanisms, following validation of macroscopic properties by correlations with experiments. The heterogeneous nature of wave-propagation through reactants of dissimilar elastic and plastic properties and morphological characteristics, produce effects that give rise to localized and turbulent flow, vortex formation, and dispersion of reactants across large distances. This presentation will provide an overview of our experimental and modeling activities in understanding the mechanistic aspects of impact-initiation of reactions in various aluminum-based intermetallic-forming materials for design of reactive materials systems. *Funding provided by ONR MURI Grant No. N00014-07-1-0740 and DTRA Grant No. HDTRA1-10-1-0038

A comparison of cold sprayed and explosively compacted Ni+Al reactive materials is conducted to elucidate the effects of microstructure morphology on performance. Rod-on-anvil impact experiments provided a measure of reactivity under uni-axial stress loading. Parallel plate impact tests, representing a uni-axial strain state, were also utilized to measure the shock propagation characteristics (Hugoniot) of the two types of Ni+Al samples. CTH, a multi-material Eulerian hydrocode, was utilized to study mesoscale deformation during simulated rod-on-anvil experiments and parallel plate impact experiments in real microstructures. Parametric studies of artificial microstructures were also conducted to study the influence of different microstructure parameters in a controlled fashion. The results demonstrate the different roles of metric and topological microstructure properties which are shown to influence diverse behaviors such as reactivity, deformation behavior, and shock wave propagation in explosively compacted and cold sprayed Ni+Al. In this presentation, the various experimental and computational approaches and results to-date will be described.

Heterogeneities at the meso-scale strongly influence the shock compression response of composite materials. Laminated geometries with full density and intimate particle contacts provide a unique system to investigate the effects of microstructure on a propagating shock wave. Computational analysis is used to understand the effects of configurational changes on the shock compression response of cold-rolled Ni and Al multilayers. Changes in the orientation, bilayer spacing, and material properties are varied in order to understand the resulting changes in the shock compression response. Real heterogeneous microstructures, obtained from optical micrographs, are incorporated into the Eulerian, finite volume code CTH. The results show a marked difference in the dissipation and dispersion of the shock wave as the underlying microstructure varies. Research funded by ONR/MURI grant No. N00014-07-1-0740.

This work reports on an experimental and simulation effort to understand and predict the shock response of reactive materials. More specifically, binder-less powdered compacts of the reactive material combination Ni/Al with varying particle size and particle morphologies were fabricated and their initial microstructures extensively characterized using electron microscopy and X-ray computed tomography. Results from this characterization were input to the Eulerian code GEODYN and simulations of strong shocks were performed. In parallel, shock experiments were performed using both impact via gas gun and laser shock and the samples recovered and their microstructure characterized. A variety of shocks pressures were utilized in an attempt to characterize the evolution of the microstructure at successively increasing sub-initiation shocks. Results from the experimental efforts will be compared with those from the simulations. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-561782

We investigate the dynamic behavior under unaxial stress and strain loading conditions of heterogeneous mixtures of Ti/Al/B via impact simulations on real and simulated microstructures. We simulate a range of Al concentrations and mixture porosities to determine their effects on the shock-compressed state of the material. These simulations are validated using shock-compression data from parallel-plate impact experiments. The configuration effects of each constituent are investigated using stereological tools to track the microstructural evolution during shock and high strain-rate loading. We also study particle-level effects such as mixing, extreme deformation, and hot spot formation due to void collapse, as a function of microstructure. Our goal is to determine how accurately our microstructure simulation scheme is able to predict shock-wave compression wave-dispersion effects to design Ti/Al/B-based reactive material systems with the desired reaction response.

The addition of metal particles can be used to enhance effectiveness of traditional explosives. A class of these reactive materials combine the enhanced energy release with an increased density and strength. For example, Aluminum-Tungsten (AL-W) granular composites can be used as to enhance the effectiveness of traditional explosives while at the same time having the same density as steel and the strength of solid Al alloys. The energy enhancement is derived from oxidation of in situ generated small size, fast reacting Al particles. The composites highly heterogeneous nature gives it the ability to generate small sized fragments of Al that are able to rapidly oxidize with the surrounding air and/or detonation products after fragmentation. Small scale dynamic experiments have been performed with Al-W composite rings comprised of Al powders and W powders/rods with a range of powder sizes from 4 to 30 microns that have been processed using cold isostatic pressing. Recovered fragments from the experiment showed a significant reduction in the fragment size in comparison to a homogeneous material from a fragment size on the order of 10 mm to 100 microns. In the fragments, clusters consisting of 5-10 W particles/rods are observed. These W particle clusters may be responsible for blocking of fragmentation to the particle level. This work compares numerical simulations with the dynamic experiments to examine the processes of fragment generation during the dynamic loading, the development of W particle clusters and their possible repression of fragmentation to the particle level. Behavior of Al-W composite rings comprised of Al powders and W powders will be compared to the fracture of granular materials comprised of A powders and W rods processed using cold isostatic pressing. The support for this project provided by the Office of Naval Research Multidisciplinary University Research Initiative Award N00014-07-1-0740, Program Manager Dr. Clifford D. Bedford.

We investigate dynamics of oxidation in a linear chain of three alumina coated aluminum nanoparticles (ANPs) using multimillion-atom molecular dynamics simulations. The two outer ANPs are heated above the melting temperature of pure aluminum, and we study the mode and mechanism behind the heat and mass transfer from the hot ANPs to the central ANP. The hot ANPs oxidize first and subsequently ejected Al atoms from the outer ANPs penetrate the middle ANP. Consequently, the central ANP has a higher oxidation rate than the other two ANPs. The calculated penetration speed, 54 m/s, is within the range of experimentally measured flame propagation rates. The penetration front consists of three layers: 1) atoms from the shell of the central ANP; 2) atoms from the shell of the outer ANPs; and 3) atoms from the core of the outer ANPs. In addition to convectional heating of the central ANP, the ejected hot Al atoms from the outer ANPs initiate exothermic oxidation reactions inside the central ANP, which cause further heating within the central ANP. All three ANPs fuse together in about 1 ns, forming a single ellipsoidal aggregate with an average temperature of ~5,500K. This work is supported by the Office of Naval research, Air warfare and Weapons Department, grant No. N00014-12-1-0555.