Symposium FF06 : Advances in the Fundamental Understanding and Functionalization of Reactive Materials

Subcritical reactions comprise phase transformations in reactive nanolaminates leading to the point of ignition and typically occur under high heating rates above 500 K/s. Harnessing the reaction attributes and the specific metallurgy of subcritical reactions in metallic nanolaminates represents a promising pathway for creating novel types of thin film nanocomposites. Based on this notion we develop so called “Pulsed Metallurgy”. The nanolaminates with their microstructure serve as templates which are transformed into nanocomposites via millisecond short temperature pulses. Thermal pulsing enables us to precisely control the precipitation of hard intermetallic compounds in the soft metallic matrix at the nanoscale. The pulse shape defines the amount as well as morphology of the intermetallic phase. Here, we present first design guidelines for thin film composites based on Al-rich Al/Ni nanolaminates. For the chosen system, we require fundamental knowledge on the thermodynamics and kinetics of the underlying phase transformations during subcritical reactions. Especially, the effect of high heating rates on the phase transformation is explored. We performed chip-based nanocalorimetry and time-resolved X-ray diffraction with synchrotron radiation to in situ explore the phase transformations. In addition to the thermodynamic/kinetic information, this allows us to trace structural changes with a temporal resolution of up to 50 µs. Heating rates from 10 K/s up to 10,000 K/s are chosen to determine the onset and the growth rate of intermetallic phases. Effects of chemical composition are studied. Complementary kinetic studies are performed under isothermal conditions. The analysis of two dimensional X-ray diffraction data gives further insight in the structural evolution during the reaction and their influence on the reaction mechanism. Selected Al/Ni multilayer samples are finally characterized via electron microscopy. Eventually, the calorimetry in combination with the structural data enables us to derive guidelines for microstructure development of Al-based thin film nanocomposites.

Despite the thermodynamic advantages of boron, its long ignition delay and burn time hinder its widespread application as a practical fuel. The ignition is delayed by the nascent shell of hydrolyzed oxide acting as a diffusion barrier protecting the boron core. The combustion at higher temperatures occurs at the oxide-free boron surface; the rate of this heterogeneous reaction is relatively low. To address these shortcomings, modifying the chemistry of the oxidative process encompassing ignition and combustion, by introducing fluorination was found to be effective. The addition of fluorides like bismuth fluoride (BiF₃) and cobalt fluoride (CoF₂) depressed the ignition temperatures and improved burn times of micron-sized boron-fluoride composites. The focus of this work is to introduce small quantities of bismuth fluoride as an oxidizer to maintain the thermodynamic potential of boron while improving its ignition and combustion rates. Towards this end, different boron-bismuth fluoride composites, (100-x) B-xBiF_3, with 5, 10, 30 and 40 wt. % of bismuth fluoride were prepared. Composites with 10, 30 and 40 wt. % of BiF₃ were prepared by arrested reactive milling. Additionally, composites with 10 and 5 wt. % of BiF₃ were prepared by the solvent based chemical deposition technique. The milled samples were found to have a small quantity of reduced Bi and were contaminated by iron from the milling media and vial walls. Based on recent work, the iron introduced by milling is expected to accelerate combustion of boron. Here, a similar effect of reduced bismuth left behind after fluorination, on boron’s combustion behavior was explored experimentally. Milled boron-bismuth composites were prepared including 92B-8Bi and 69B-31Bi, containing bismuth and iron content comparable to the boron-fluoride composites, 90B-10BiF_3, and 60B-40BiF₃ respectively.
A systematic reduction in ignition temperatures with the content of BiF₃ was observed with all milled composites igniting at much lower temperatures than boron. At a heating rate of 2500 K/s, milled composites 90B-10BiF₃, 70B-30BiF₃, and 60B-40BiF₃ ignited at 650, 600 and 580 °C respectively. The coated sample 90B-10BiF₃ ignited in a range of temperatures, with an average of 650 °C, while 95B-5BiF₃ did not ignite at temperatures under 1200 °C. The thermogravimetric analysis in aerobic conditions shows an oxidation step around 500 °C for all composites. The reference boron sample had an oxidative weight gain at a higher temperature, 710 °C. All the composite particles were heated and ignited by a CO₂ laser beam in air. They exhibited shorter burn times compared to similar-sized boron particles. A reduction in burn-times with an increase in bismuth fluoride content was observed. For a particle of 1-micron size, boron had a burn-time of 2.1 ms. The milled composites 90B-10BiF₃, 70B-30BiF₃, and 60B-40BiF₃ exhibited burn-times of 0.38, 0.33, and 0.28 ms respectively, almost an order of magnitude faster than boron. The boron-bismuth composites were found to burn slightly faster than boron. Analysis of partially reacted products shows bismuth retained in particle till boron melting point. The combustion mechanism and reaction pathways involving the species will be discussed.

Phenomenon of self-sustained chemical reactions that sub-sonically propagate along the high-energy density material is widely used for fabrication of different ceramics [1,2]. The specifics of synthesis condition in such combustion front are as follows: short (0.1-10 ms) reaction time, high temperatures (2000-4000 K) and extremely high rate for temperature change (10³-10⁶ K/s). These features allow fabrication of materials with unique microstructures and properties. In this work, we report recent results on combustion synthesis (CS) of different materials including ultra-high temperature ceramics, high entropy ceramics, metastable phases, as well as cubic boron nitride. Special attention is paid to such novel synthetic routes as reactive spark plasma sintering (SPS), CS with mechanical stimulation, as well as solution combustion synthesis.
Specifically it is shown that SPS of reactive materials [3] allows one-step rapid fabrication of dense bulk nanostructured ceramics (e.g. SiC. B₄C, HfCN) with high mechanical properties. Application of similar approach in combination with high-energy ball milling permits production of high entropy carbides and nitrides. It is also demonstrated that CS in B-TiN reactive system under shock-wave conditions leads to formation of c-BN phase [4]. Finally, using self-sustained waves in reactive solutions of metal nitrates and different fuels (e.g. hexamethylenetetramine and glycine) allows synthesis of metastable metal nitrides phases [5].

1. Rogachev AS and Mukasyan AS, Combustion for Materials Synthesis, CRC Press, Taylor & Francis, Boca Raton, London, New York, 2015.
2. Concise Encyclopedia of Self-Propagating High-Temperature Synthesis, 1^st- edition, Edited by: I. Borovinskaya, A. Gromov, E. Levashov, A. Mukasyan, A. Rogachev, Elsevier, Amsterdam, Netherlands, 2017.
3. D.O. Moskovskikh, K.A. Paramonov, A.A. Nepapushev, N.F. Shkodich, A.S. Mukasyan, Bulk Boron Carbide Nanostructured Ceramics by Reactive Spark Plasma Sintering, Ceramic International, 43 (11) 8190-8194, 2017.
4. MT. Beason, JM. Pauls, IE. Gunduz, S. Rouvimov, KV. Manukyan, K. Matous, SF. Son, AS Mukasyan, Shock-induced reaction synthesis of cubic boron nitride, APL, 112(17) 171903 , 2018.
5. A.S. Mukasyan, S. Roslyakov, J. Pauls, L. Gallington, T. Orlova, X. Liu, M. Dobrowolska, J. Furdyna,, K Manukyan, Khachatur, Nanoscale Metastable ε-Fe₃N Ferromagnetic Materials by Self-Sustained Reactions, Inorganic Chemistry, 58(9), 5583-5592, 2019.

Glassy amorphous organic energetics is a new field offering several novel areas for study. While practical application might be limited due to low density and propensity to crystallization of amorphous energetics, the scientific opportunities are fertile. For example, they allow an in depth view of crystallization in real time, even of novel systems such as cocrystals, and the in situ characterization of pores, cracks and other gross defects as they form. The ability to quantify and characterize the crystallization of an organic material would not only be an important step in the science of crystallization, but could have broader application to the pharmaceutical, biological science, and energetic materials communities. Amorphous energetic formulations with relatively low excipient (i.e. polymer used to inhibit crystallization) loadings were studied as they crystallized. This allowed several key determinations to be made that linked porosity and crystallinity of the material during this dynamic process. This is especially critical, as porosity is a critical parameter in the sensitivity and mechanical properties of an explosive, two key drivers for future munitions applications. Furthermore, the results will have an impact on the study of crystallization from both a practical and theoretical standpoint.

Aluminum powders are commonly used in the formulation of reactive materials [1]. In those materials, there is a definitive interest in improving the reactivity of aluminum powders and the actual trend is to produce powders with a high specific surface areas (higher than 10m²/g) i.e. aluminium nanopowders. Without any consideration of the scale, aluminium particles are covered by a thin alumina layer whose thickness does not vary much with the particle size [2]. The enhanced reactivity due to the use of nanosized powders is proposed to be linked to size reduction effects of the particles and to the structural and microstructural characteristics of both the metallic core and this alumina layer.

The standard synthesis routes for nanopowders are atomization and wire electro-explosion techniques. Such techniques produce powders with spherical morphology. High-energy ball milling has been recently proposed as an alternative procedure to produce micro- or nano- aluminum powders. They possess a flake like morphology and have different reactivity compared to the spherical ones [3]. This technique has also been shown to be able to produce reactive composite materials [4].

In this contribution, we propose a comparative study of the oxidation mechanisms of aluminum powders as a function of their shape (spheres versus flakes) and size (micro versus nano). Thermogravimetric and differential thermal analysis performed under dry Air up to 1500°C are used to evaluate the oxidation behaviour of the powders. The morphology of the various powders is followed for different oxidation temperatures using gas adsorption, transmission electron microscopy (TEM) and energy filtered transmission electron microscopy (EFTEM) [5]. Finally, the oxidation mechanisms are studied using kinetically controlled thermal analysis [6]. In this mode, keeping the oxidation rate constant, the kinetics equations are simplified which facilitates the modeling of the experimental data. The observed differences between the oxidation mechanisms are discussed as a function of the shape and the size of the particles.


The Direction Générale des Armées (DGA) and the University of Aix-Marseille (AMU) are acknowledged for funding the PhD of P.H. ESPOSITO.

[1] Dreizin E. L., Prog. Energy Combust. Sci. 3 (2009) 141-147
[2] Rufino B. et al., Acta Materialia 55 (2007) 2815-2827.
[3] André B. et al., Materials Letters 110 (2013) 108-110.
[4] Dreizin E. L. et al., Journal of Materials Science 52 (2017) 11789-11809.
[5] Coulet M.-V. et al. J. Phys Chem. C. 119 (2015) 25063−25070
[6] Rouquerol, J.; Toft Sorensen, O. Sample Controlled Thermal Analysis, Kluwer Academic, (2003).

The addition of catalyst is the most common method for enhancing the combustion performance of solid and liquid fuels for a wide range of civil, military and space applications. Various types of catalyst have been used among which the transition metal oxides are the most preferred because of their ease of production, low cost and insensitivity. The use of graphene foam-supported catalyst can provide a more efficient contact between the propellant and the catalyst in terms of the uniformity and surface area of the catalyst exposed and thus enhance the catalytic effect. Moreover, the graphene foam-based structures provide an additional benefit of increased thermal transport. It is well-known that heat transport also plays an important role in the combustion of solid fuels; higher thermal conductivity or diffusivity usually results in higher burn rates. Most solid fuels have a thermal conductivity in the range of 0.1-1 W/m-K, which is much lower than that of these graphene-based materials (up to a few thousand W/m-K). Indeed, carbon nanomaterials such as carbon nanotubes and graphene, because of their high thermal conductivity and large surface-to-volume ratio, have been used as nano-fillers to enhance the thermal conductivity of various composites and as heat exchangers in nano-electronic devices.

Motivated by the above, we explored the potential of GF structures, which were functionalized with a transition metal oxide manganese dioxide such as MnO₂ or CuO using a hydrothermal approach. The propellants we chose were Nitrocellulose (NC) and hybrid Ammonium Perchlorate-Nitrocellulose (AP-NC). For nitrocellulose with GF supported MnO₂, burn rate enhancements up to 9 times were obtained with the activation energies being lowered by 17%. However, without the use of GF as the supporting structure, using only MnO₂ oxide nanoparticles, burn rate enhancements only up to 2 times were obtained with the activation energies being lowered by 3.6%. Although, the presence of GF in itself does not have any catalytic effect on the propellant decomposition, the GF supported metal-oxide particles provided a more efficient contact between the propellant and the metal-oxide. This was in contrast to the traditional solid-fuel/metal-oxide mixture prepared in which a continuous contact between the propellant and the catalyst was not achieved because of the random mixing of metal oxide particles into the fuel matrix. For AP-NC, the CuO-functionalized GF micro-structures significantly enhanced the burn rates, up to 7 times the bulk value, which was attributed to the combined physical (increased thermal transport) and chemical (enhanced thermal decomposition) effects. TG (thermogravimetric) and DSC (differential scanning calorimetry) analysis showed the activation energy of AP-NC was lowered by 22% with the use of the CuO-functionalized GF structures.
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Surface functionalization provides efficient strategy to significantly improve material surfaces to overcome the infectious problems from microbial fouling in many medical devices and food processing equipment. Anti-biofouling has been improved by passive or active ways. Passive antifouling strategies aim to prevent the initial adsorption of foulants, while active strategies aim to eliminate proliferative fouling by destruction of the chemical structure and inactivation of the cells. However, neither passive antifouling strategies nor active antifouling strategies can solely resist biofouling due to their inherent limitations. To achieve highly effective anti-biofouling surfaces, integration of bactericidal and bacteria-repellency is highly in demand.
Herein, we successfully developed multimodal antibacterial surfaces for waterborne and airborne bacteria with the benefit of combination of anti-adhesion (passive) and bactericidal (active) properties of the surfaces. We elaborated multi-functionalizable porous amine-reactive (PAR) polymer films from poly(pentafluorophenyl acrylate) (PPFPA). Pentafluorophenyl ester groups in the PAR films facilitate to create multiple functionalities through a simple post-modification under mild condition, based on their high reactivity towards various primary amines. To retain lubricant in films, porous structures of PPFPA films were developed by vapor-induced phase separation (VIPS) of polystyrene (PS)/PPFPA mixture and selective removal of PS. Attributing to excellent amine-reactive property of PPFPA, porous amine-reactive (PAR) films enable imparting versatile physiochemical functionalities on surfaces via sequential post-polymerization modification. Amine-containing molecules (i.e., amine-polydimethylsiloxane (amine-PDMS) and dopamine) and silver nanoparticles (AgNPs) were introduced on PAR films in serial order, followed by silicone oil infusion into PAR films.
Antibacterial efficiency of the developed surfaces was assessed for both Gram-negative bacteria (Escherichia coli, E. coli) and Gram-positive (B. cereus and S. aureus). We confirmed that lubricant-infused PAR films inhibit adhesion of not only waterborne bacteria but also airborne bacteria. To the best of our knowledge, this is the first study to utilize slippery surfaces as antibacterial surfaces for airborne bacteria. According to previous studies, contact-killing strategy is necessary to minimize airborne bacterial adhesion, the lubricant infused surfaces were insufficient to prevent airborne bacteria adhesion thoroughly. On the contrary, the AgNPs incorporated lubricant infused surface showed nearly perfect antibacterial efficiency for airborne bacteria owing to bacterial resistance of AgNPs. Furthermore, the L-infused Ag@DP-PAR possessing contact-killing activity has potential to be sustained longer than biocide-leaching system and to have lower toxicity. The aim of this work is to develop efficient and multimodal antibacterial surfaces toward both waterborne and airborne bacteria as the first proof of concept for application of PAR films with multi-functionalities by post-modification. Thus, the platform presented in this study suggests a new door to develop an effective multimodal anti-biofouling surface.

Reactive materials aim to release large amounts of energy in a rapid yet controlled fashion. These materials include but are not limited to; thermites, intermetallics, or metal polymers which can be prepared from a variety of different metals and metal precursors. Titanium borides have drawn a great deal of interest as reactive materials; however, the high temperatures often required for their preparation results in materials with high hardness but low energy. Our group has previously reported a low temperature method for preparing hydrogen containing Ti-B materials from the decomposition titanium borohydride (Ti(BH₄)₃). This presentation details our work to optimize and expand upon this concept. Isolating and stabilizing the borohydride precursor affords greater control of the final materials. This allows for improved purity, more facile processing, and ultimately higher energy content. We can also control the inclusion of other metal additives such as aluminum. The resulting in Ti/Al/B materials have distinct properties compared to reactives prepared via other methods. The synthesis, processing, and characterization of these materials will be discussed.

Development of nanoporosity on metals and alloys is a well-known pathway for making three dimensional high surface area structures especially useful for catalysis¹. Majority of the techniques are based on selective removal of a sacrificial metal from an alloy, a method known as dealloying. The driving force for the traditional electrochemical/chemical dealloying is difference of redox potentials and dissolution tendencies amongst the constituent metals. This limits the process to few noble metals² e.g. Pt, Pd, Au etc. that can make homogeneous alloys with less noble metals. More recently liquid metal dealloying³ and vapor phase dealloying⁴ have been also identified to mimic the process of traditional dealloying based on difference of affinities of thermal dissolution in a liquid metal, and vapor pressures of the constituent metals, respectively. However, these are still restricted to few metals and the primary challenges lie with obtaining homogeneity in precursor alloy and limiting the pore size within 100 nm upon spinodal decomposition. Here, we present gas phase thermal decomposition of transition metal dichalcogenides (TMDs) as an alternative to dealloying that generates nanopores for a broader class of metals including refractory metals like W, Mo, Re etc. The chalcogen is removed from the surface by both reductive reaction with hydrogen and evaporation at elevated temperatures, which leads to the rearrangement and surface diffusion of the remaining metal atoms that evolve into an interconnected bicontinuous nanoporous network. Based on varying dynamics of pore formation and residual chalcogen contents for different TMDs, we have proposed a mechanism that emulates the decomposition process. The availability of vast library of TMDs having inherent atomistic homogeneity makes it a universal technique that can be utilized to make nanoporous metals.

References.
1. Luc, W. & Jiao, F. Nanoporous Metals as Electrocatalysts: State-of-the-Art, Opportunities, and Challenges. ACS Catal. 7, 5856–5861 (2017).
2. Zhang, Z. et al. Generalized fabrication of nanoporous metals (Au, Pd, Pt, Ag, and Cu) through chemical dealloying. J. Phys. Chem. C 113, 12629–12636 (2009).
3. Wada, T., Yubuta, K., Inoue, A. & Kato, H. Dealloying by metallic melt. Mater. Lett. 65, 1076–1078 (2011).
4. Lu, Z. et al. Three-dimensional bicontinuous nanoporous materials by vapor phase dealloying. Nat. Commun. 9, (2018).

Previously we demonstrated that the sonochemically-mediated decomposition of in-situ generated complex metal hydrides is a versatile methodology for the synthesis of unique metastable reactive mixed-metal nanopowders (RMNPs) that can be tuned for high energy content while also possessing other highly desirable solid fuel characteristics. In this latest iteration of our efforts to further improve the Ti-Al-B RMNP fuels, we made the effort to eliminate any parasitic impurities and focused on the sonochemically-mediated reaction process to evaluate the effects of the various synthetic and processing parameters on the final fuel properties. To that end we took the time to better purify the LiAlH₄, and then the syntheses were carried out using both the original approach of adding etheral solutions of LiAlH₄ and LiBH₄ to TiCl₄, as well as the reverse. We also employed a different approach of synthesizing the ether adduct of Ti(BH₄)₃, which was then reacted with LiAlH₄. The product powders were characterized for particle sizing and slow oxidation via thermogravimetric analysis, and subjected to careful elemental analysis via both ICP-OES of digests for metals content and combustion analysis for C, H, and N. New approaches were also explored in the compatibilization of the powders with HTPB cross-linking chemistry to produce RMNP-HTPB composites, the combustion behavior of which was then characterized in oxygen bomb calorimetry, as well as heterogeneous strand-burn experiments. While the powders produced by different methods had similar elemental compositions, each of the newly produced materials reproducibly exhibited surprisingly unique combustion behavior.

In energetic, pharmaceutical and other industries, dealing with spherical powders is attractive because of the ease of their handling and high flowability. Spherical powders are also of interest as feedstock materials for additive manufacturing. For metals, it is common to prepare spherical powders by atomization of melts, which is not suitable for most reactive materials. For non-metals, extrusion, spheronization, and emulsion polymerization are used, which are not compatible with many reactive materials sensitive to heating or having limited solubility in polymers. This work describes preparation of spherical composite powders of reactive materials by high-energy mechanical milling. Powders are ball milled in presence of two immiscible fluids (hexane and acetonitrile). It is observed that spherical composite powders are formed at a specific solid/liquid ratio, when the volume of solid is matched to that of acetonitrile. It is also found that the total volume of liquid must be at least twice as big as the volume of the solid powder. Reactive spherical powders were prepared with compositions of aluminum-rich thermites with copper and iron oxides as oxidizers, with an aluminum-boron composite as well as pure with aluminum, titanium, and boron. Additionally, spherical powders were prepared using fumed silica and iron oxide. Some of the formed spheres, such as those of fumed silica were not mechanically stable and easily fell apart, while others, such as those of aluminum or thermites were stable and could be easily handled. The powders were observed using optical and scanning electron microscopy. For selected powders, particle size distributions were measured. For both aluminum and boron the particle sizes became smaller at longer milling times. It is hypothesized that the spherical powders are formed from Pickering emulsions generated in the milling vial when the loaded liquid/solid mixture is mechanically agitated. Droplets of acetonitrile are expected to form in the continuous hexane phase and be stabilized by solid particles adsorbed to the hexane/acetonitrile interfaces. It is further hypothesized that because of the large amount of the loaded solid powder, the Pickering emulsion in this case coexists with a dense suspension of powder in hexane, the continuous phase. The presence of suspended powder makes it difficult to destabilize the emulsion droplets mechanically: when the surface is stretched and becomes unstable it rapidly adorbs a suspended particle to become stabilized. The interaction of a dense suspension with Pickering emulsion droplets is proposed to lead to accumulation of the solid phase inside droplets, which causes formation of the spherical powders as observed here. Preparation, characterization, and reactivity of the prepared powders will be discussed.

The growth of polycyclic aromatic hydrocarbons (PAH) is the chemistry dominating the formation of interstellar organic substances, various biogeochemical cycles and modern industry processes, but their mechanisms remain unclear. In this work, we study various cycloaddition mechanisms at intermediate temperatures (300-800 °C), including hydrogen abstract acetylene addition and polynuclear oligomerization, using first-principles approaches. Based on gas chromatography mass spectrometry measurements, we identify proxy molecules for heavy hydrocarbons such as coal tar pitch that is often used as a precursor for carbon-fiber processing. For a given molecular component and byproduct, rather than using the conventional bottom-up approach that hypothesizes growth from reactants, we develop a top-down framework to automatically enumerate all possible pathways through splitting products to possible reactants, with molecular symmetry considered. These individual pathways are evaluated in a high-throughput manner, based on which a Bayesian description of cycloaddition mechanism is constructed. Moreover, the construction of such Bayesian mechanism at varying temperature reveals different mechanisms at different temperatures, suggesting different dominant pathways in the formation of large aromatic clusters in coal tar pitch processing leading to further insights towards the optimization of the synthesis of pitch-based carbon fibers. Moreover, the growth mechanisms could also give insights to other PAH chemical reactions, such as soot formation.

Reactive inorganic nanolaminates containing metastable interfaces, such as those between metals and metal oxides, can undergo exothermic reactions when disturbed by heat or shock. While the chemical thermodynamics associated with reactivity in these systems is relatively straight forward, the mechanisms by which this chemistry is initiated are unknown. This is due in large part to the potential complexity of the dynamics and the challenges associated with experimental probes of dynamics at buried interfaces.

Using molecular dynamics simulations, we investigated the mechanisms by which the energy of a shock pulse delivered by a simulated flyer plate can initiate chemical mixing at a buried metal-metal interface. The simulations show plastic damage in the form of dislocations that are initiated at the edges of the flyer plate due to shear forces. This damage at the plate edges is consistent with recent experiments by Dlott and co-workers in which the initiation of buried exothermic reactivity in a reactive nanolaminate is observed in a ring pattern that matches the flyer plate shape. Depending on the crystal orientation and the energy delivered by the flyer plate, the simulated dislocations can reach a buried interface. In the case of a Cu-Zr nanolaminate containing two buried layers with thicknesses of about 35Å, the dislocations were observed in the simulations to be terminated at the first interface, but with an elastic energy pulse that is transferred into subsequent layers. Hence the mechanisms by which energy is delivered across a series of buried interfaces may be different depending on the interface location with respect to the shock pulse. In these initial simulations little interfacial mixing was observed on a picosecond timescale. We will also discuss related simulations in which a simulated shock pulse is delivered across a buried metal metal-oxide interface, where charge transfer and significantly exothermic chemistry play a large role that is missing from the dynamics of the buried metal interfaces.

This work was supported by the U.S. Department of Defense, Multidisciplinary University Research Initiative through the Army Research Office, Grant No. W911NF-16-1-0406.

Reactive materials based on metallic elements are produced by various methods. The basic method for the production of multilayer reactive nano-foils is the layer-by-layer magnetron deposition. The thickness of each layer may vary from several nanometers to microns, while the number of the layers "stacked" in one foil may vary from ten to thousands. An alternative and less expensive method for the production of reactive nano-foils is a mechanical method combining high-energy ball mill and cold rolling. The ball milled and cold rolled foils possess relatively high non-uniformity, intermittence, and tortuosity, as compared to the magnetron deposited films. However, nanometer-scale Ni and Al layers are present in the material. A very peculiar property of reactive multilayer nano-foils is that a reaction between pure metals initiated at one edge of the sample will propagate throughout the system in a self-sustained way. The heat released locally allows the reaction to proceed without any further supply of energy. During this talk, we will review the mean features of ignition and reaction in nano-layered metallic systems Me/Al, with Me=Ni or Ti. We will also give some insight into the effect of production by a mechanical method.

The understanding of the reaction mechanisms in reactive nano-composites relies on the study of reaction kinetics, heat and mass transfer, as well as the dynamics of structural transformations. Besides the usual multi-physics description, the atomistic approach provided by Molecular Dynamics (MD) simulations becomes a useful tool in describing the behavior of reactive materials. MD simulations indeed give the basic atomistic steps leading to observed microstructure beyond any thermodynamic or kinetic modeling. MD simulations can be considered as a model that offers a counterpart to in-situ experiments to explore elemental mechanisms. In the case of nanometric metallic multilayers, MD proves to be a very appropriate method for numerical studies, as the accessible time and length scales are in the same range of magnitude as in “real” experiments. During this talk, we will review the main features of combustion synthesis in nanofoils considering Ni/Al as a model system [1]. The reactivity of Ti/Al nanofoils will also be analyzed. We will show how modeling and experiments are complementary approaches in order to detect intrinsic behaviors and reactive mechanisms.

In the second part of the talk, we will focus on the mechanical method of production. The idea here is to develop molecular dynamics simulations in order to study the possible elemental mechanisms (compaction, shear, plastic deformation, welding, fracturing,…) that can be observed during high energy ball milling. We will present our calculations to mimic the first impact due to grinding balls. A set of spherical Al and Ni (Ti) particles were submitted to a rapid compression. The compression produces compaction of the powder by removing empty spaces between the particles. Once the particles are compacted, they start to undergo plastic deformation. Indeed, the ductile Al mostly deforms whereas Ni remains more spherical. An increase in the number of atoms in mixing zones was observed. The detailed atomistic analysis allows us to follow the progressive amorphization, the formation of defects, the induced chemical mixing and the possibility of recrystallization due to this first hit. The ignition of such a system will be characterized.

Finally, we will show how large-scale MD simulations can handle more and more complex systems. We were able to consider complex nanostructures in order to understand the role of defects or intricate microstructure in nanostructured samples. As an example, we will show self-sustained propagation in a system made of disordered nanostructured grains.


[1] F. Baraset al., Adv. Eng. Mater. 20 (2018) 1–20
[2] F. Baras, O. Politano, Acta Mater. 148 (2018) 133–146

Among energetic materials, nanothermites are known for their high volumetric energy densities (up to 16 kJ/cm3), adiabatic flame temperature (> 2600 °C) and high reaction (burn) rates. One of the promising materials for integration into micro devices is Al/CuO nanocomposite because of its high enthalpy of oxidation-reduction reaction [1]. Increasing the contact surface between both components of the thermite composite should lead to improved properties. The control of the particles size, the morphology and the distribution of the nanoparticles inside the composite is therefore of paramount importance.

Recently, an original synthesis approach has been proposed to produce ultra-small CuO nanoparticles (ca. 5 nm) functionalized with octylamine ligands. This approach consists in the formation of the CuO nanoparticles by the controlled hydrolysis and/or oxidation of an organometallic precursor (i.e. copper amidinate), in the presence of octylamine. [2] These usual ligands stabilize the CuO nanoparticles which can be beneficial to enhance the coating of Al particles with smaller CuO ones during the physical mixing. Thermally characterized by Differential Scanning Calorimetry (DSC), Al/CuO nanothermites thus produced react differently compared to those produced following the same mixing process but from commercial CuO nanoparticles.
Using a variety of characterization techniques, including microscopy, spectroscopy, mass spectrometry and calorimetry (ATG/DSC), the structural and chemical evolution of CuO nanoparticles stabilized with octylamine ligands are characterized upon heating. This enables us to depict the main decomposition processes taking place at the CuO surface at low temperature (< 500 °C) [3]: the ligands fragment into organic species accompanied with CO2 release, which promotes the CuO reduction into Cu2O and further Cu. Then an optimization process to overcome the ligands-induced CuO degradation at low temperature is proposed. Al/CuO nanothermite reaction is analysed, in terms of onset temperature and energy released.
References:
[1] Glavier et al.,Combust. Flame., 162, 1813, (2015)
[2] Jonca et al.,ChemPhysChem, 18, 2658, (2017)
[3] Palussiere et al.,PCCP, submitted, (2019)

Hydrogen is considered as a promising alternative energy carrier due to its high energy density, great variety of potential sources, light weight and low environmental impact. Though the storage of hydrogen via Mg-based nanomaterials is very promising, the high reactivity of Mg towards oxygen leads to the easy formation of MgO, which blocks the penetration of hydrogen in the material, limiting the performance of the device.
In this work, in order to overcome this limitation, we propose to synthesize bilayer nanostructured Mg/polymer thin films using cold plasma-based processes. The process consists of two steps: (i) the synthesis of Mg nanostructured films by magnetron sputtering at grazing incidence and (ii) the coverage of the metallic nanostructured film by plasma polymerization from ethylene precursor. This method should allow (i) to control the porosity of Mg films by simply modifying depositions parameters such as temperature, incidence angle of the depositing particles (α) or pressure, and (ii) to avoid oxidation of the Mg-based nano-objects by protecting them with a highly cross-linked plasma polymer.
SEM characterization of the samples has revealed the presence of isolated Mg-based nanocolumns. Both the intercolumnar space (i.e. from 20 to 100 nm) and the diameter of the nano-objects (i.e. from 100 to 300 nm) depend on the incidence angle of the depositing particles. This evolution is directly related to the competition between surface diffusion and shadowing effect as confirmed by kinetic Monte Carlo methods. Mg-based nanostructures were homogeneously covered by a carbon/hydrogen-based plasma polymer which protects the films against oxidation. The obtained material reveals a high potential for efficient hydrogen storage.

Beyond their most common use for structural and catalytic purposes, metals are also great fuels due to their higher heats of combustion compared to traditional hydrocarbons. In particular, boron has been of interest as a high-energy density fuel additive in explosives and propellants. Some of the challenges are that boron particles tend to agglomerate, have lengthy ignition delays and very low combustion rates. Researchers have attributed the ignition delays to boron’s naturally occurring inhibiting oxide layer impeding further oxidation at low temperatures. For full-fledged combustion occurring at high temperatures when boron oxide is gasified, current studies report that boron particles burn in two consecutive stages; however, the actual reaction mechanism is poorly understood.
Despite many years of relevant research, quantitative combustion data on practically interesting, micron-sized boron particles are limited. Most proposed modifications to boron were designed to reduce its ignition delays by adding new chemical components and thus substantially diminished the energy density of the resulting composite material. Research aimed to accelerate boron combustion at high temperatures is generally lacking.
The motivation of this work was to achieve higher burn rates for boron powders without jeopardizing their thermochemical performance, safety and stability and to develop an experimentally validated model adequately describing oxidation kinetics for boron that can be used in practical simulations for a broad range of temperatures. This study also was aimed to close the gap in data for combustion of fine boron particles in varying oxidizing environments. Commercial boron powders were modified, and both as received and modified powders were used in oxidation, ignition, and combustion experiments. Materials were prepared by mechanical milling or wet synthesis techniques and burned in air, in hydrocarbon combustion products, and in steam/nitrogen environments. Combustion and ignition characteristics were determined from optical measurements and images and described as trends of burn times or ignition delays versus particle size. Strategies to modify boron’s heterogeneous reactions by functionalizing its surface by organic solvents and using transition metals as “shuttle catalysts” have been explored. It was found washing powders with acetonitrile removes hydrated surface oxide and reduces their ignition delays while not leading to rapid aging and re-oxidation at ambient conditions. Doping boron with as low as 5 wt% of transition metals (Fe or Hf) accelerates surface reaction rates leading to shorter particle burn times compared to the starting commercial powder. A kinetic model was derived from low-temperature thermo-analytical measurements to describe the oxidation of complex aggregated boron particles accounting for their surface morphology. Comparison with particle combustion experiments showed that the same model can describe reactions at high temperatures typical of full-fledged boron combustion, suggesting that the same heterogeneous reactions govern both ignition and combustion of boron. Further, results suggest that the morphology of as received boron powders comprising micron-sized agglomerates of finer primary particles does not always change to spherical droplets even at temperatures exceeding the boron melting point. This leads to variation in burn rates and temperatures for various particles.

Reactions in nanocomposite thermites prepared by Arrested Reactive Milling (ARM) occur at the interfaces between fuel and oxidizer and are affected by structure and properties of the interfacial layers formed during milling. For many materials prepared by ARM, an organic process control agent (PCA) is used to limit cold welding which serve as heat sink for the mechanically initiated exothermic reactions. While PCA directly controls shapes and sizes of the powders being milled, the presence of an organic substance can also modify and functionalize the forming interfaces, e.g., by forming carbide, oxycarbide, or nitride inclusions or even sublayers. Such possible effects are explored here for Al-CuO thermite. Previously, such thermites were prepared by ARM using hexane as PCA. Here, samples of Al-CuO thermites were prepared using staged milling. In the first stage, Al and CuO powders were individually pre-milled in acetonitrile, a polar PCA. The products were recovered, dried, and used in the second stage milling, for which hexane served as PCA. In the second stage, pre-milled Al and CuO powders were combined together or with respective as-received powders. The products were characterized by scanning electron microscopy, thermal analysis and heated filament ignition experiments. Pre-milling CuO in acetonitrile did not affect properties of the obtained thermites. The samples including aluminum powder subjected to the first stage milling in acetonitrile were found to ignite at a lower temperature and have stronger low-temperature exothermic reactions. When as received aluminum was processed using hexane only, the ignition temperature did not depend on heating rate, suggesting that the ignition was governed by a phase change occurring at the Al-CuO interface. Conversely, for thermites prepared using aluminum pre-milled in acetonitrile, the ignition temperature increased with heating rate, suggesting that a reaction with thermally activated kinetics lead to ignition.

Physical vapor deposition (PVD) of energetic materials, including high explosives such as pentaerythritol tetranitrate (PETN), has enabled an unprecedented level of control over energetic material morphology and microstructure compared to traditional powder processing techniques. Previous research demonstrated the ability to control the crystal orientation, grain size, and subsequent density and porosity of PETN films, through manipulation of substrate surface energy alone. This has profound implications for initiation sensitivity and detonation wave velocity in crystalline high explosives, which are strongly dependent on grain size, porosity, and unreacted explosive material density, respectively.

In this study, we investigate novel methods to create functionally graded density in thin films of PETN for the purpose of detonation wave shaping. Specifically, we alter local substrate surface energy to attain desired morphology of PETN, with low surface energy areas used to create porous regions in the PETN film, and high surface energy areas to create denser regions. The technique is demonstrated with both simple linear geometries as well as more complex shapes using different masking and deposition techniques. Energetic film thicknesses range from approximately 10 µm to 100 µm, with relative, controlled density variations of 10-20% achieved in a single deposition. Characterization results for the energetic films, including scanning electron microscopy, surface profilometry, and X-ray diffraction, are presented. Scanning electron microscopy of surfaces and of fracture cross sections portrays the variation in microstructure and density achieved across PETN films. Pole figures taken at different locations on the energetic films demonstrate differences in crystalline orientation that contribute to observed density changes.


Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. SAND2019-6744 A.

Reactive composite metal fuels synthesized from varying amounts of Al, Mg, and Zr have previously shown promise in bio-agent defeat applications due to their high combustion efficiency, low ignition thresholds, and tunable combustion processes. Here, we report preliminary results on the efficacy of these powders for the related application of chemical agent defeat. Composite metal powders synthesized using high energy ball milling were ignited using PETN explosive charges in a combustion chamber containing chemical agent simulants Diisopropyl methylphosphonate (DIMP) and Triethyl phosphate (TEP), separately. Once the powders are ignited, the heat from combustion vaporizes the liquid simulant and then continues to burn. Burn times and reactive metal particle temperatures were evaluated using two-color pyrometry in conjunction with high speed videography. The concentration of DIMP and TEP were monitored over time, utilizing quantum cascade lasers (QCLs) for infrared spectroscopy, and we observe a reduction in the concentration of both species over the time period of microseconds. The composition of the powders was varied to monitor the effect of Al/Mg/Zr ratios on the rate of decomposition and the overall surrogate neutralization capability. Comparisons were also made between the synthesized composite powders and similarly sized Al powder. Lastly, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to observe the microstructure of post-reaction oxide materials. Taken together, the collective data set demonstrates the ability of composite metal powders to neutralize these chemical agent surrogates.

Both commercial and military explosives frequently rely on time delays, devices which are triggered from some external event and, after a precisely controlled time, initiate detonation of the explosive charge. The design of these devices typically includes a delay column which controls the time between the external trigger and the propagated signal by means of a controlled exothermic reaction. Typically, these reactions involve hazardous materials such as lead oxides, chromates, and azides. While there has been a push to remove or mitigate the hazardous materials used in these delays, no comparable low-cost environmentally friendly alternatives have yet been widely adopted. Nanolaminates such as Ni/Al are known to react in a controllable and consistent self-propagating manner and can be fabricated with environmentally friendly materials. However, their reaction velocities are typically too high for chemical time delays manufactured into small and simple geometries. Here we present amethod to dramatically reduce the reaction propagation velocity of nanolaminate films through architectural control of the sample substrates. By depositing reactive nanolaminates on woven polymeric meshes, we can reduce the velocity by orders of magnitude while maintaining the desirable levels of sensitivity, shelf-stability, and scalability of production of typical multilayer films. In this work, we explore the reaction behavior of these on-mesh delays as a function of the reactive thin film properties, as well as the properties of the underlying substrate. We will present a model for the underlying mechanisms of the interrupted propagation which slows the net velocity, and we propose designs for integrated delays based on these materials.

The future of energetic systems may require on-command tuning of energy output: In-mission control of solid rocket motor thrust to meet revised trajectories, dynamic control of countermeasure light emission to overwhelm advanced ordnance guidance systems, and in-flight sensitization/tuning of ordnance energy release. This talk presents recent materials fabrication strategies through which electromagnetic control can be designed into energetic materials. Materials strategies enabling localization of microwave energy deposition to the gas phase flame, the condensed phase, or the burning surface are discussed with application to solid propellant burning rate control, ignition, and extinguishment; dynamic control of pyrotechnic luminosity and color chroma; and thermites and propellants with thermally switchable microwave ignitability. Functional grading of the condensed phase as well as models of condensed phase absorption are also discussed. Finally, future directions and applications of dynamically controllable energetics are proposed, including synergies with other recent advances.

Ni/Al nanolaminates are reactive composite materials with highly customizable combustion characteristics. These characteristics can be modified by altering system nanostructure through high precision synthesis techniques like physical vapor deposition (PVD). PVD has also been shown to form grains with diameters on the order of a single layer thickness and grain boundaries (GB) that may extend through the Ni/Al interface across multiple layers. The current work uses molecular dynamics simulations to investigate the effects of grain size and specific GB structures on Ni/Al nanolaminate combustion. Reaction rates are shown to increase with decreasing grain size on the same order of magnitude as increases caused by reductions in bilayer thickness, the fundamental attribute typically used to characterize nanolaminate combustion. Reaction peak temperatures are also shown to increase with decreasing grain size. To assess the role of specific GB structures, minimum energy GB structures are constructed and simulated in Ni/Al nanolaminate combustion. Effective diffusion coefficients are computed for each GB. Arrhenius descriptions of diffusion are constructed and shown to be a function of both GB structure and concentration, revealing stronger GB effects at temperatures below the Al melting point. Finally, GB effects on diffusion are shown to affect published continuum-scale calculations of both Ni/Al nanolaminate ignition temperature and combustion wave velocity.

Reactive nanolaminate (RNL) multilayer thin films, as a family of energetic materials, attract large attention recently due to the large storing capacity over a long term and its capability of tuning energy and power output. Despite the termite reaction has been known for decades, the fundamental transport mechanisms during oxygen exchange between reactants needs to be examined more carefully to understand how the released energy can be tuned and controlled. This work studies the phase transformations and properties of ZnO/Zr RNL films grown by physical vapor deposition (PVD) as the main synthesis method in producing well-defined interfacial areas and excellent control of reactant layer spacing and stoichiometry. Thermal annealing was performed to ignite the exothermic reaction between individual layers, and the resulting phase transformations and reaction products were analyzed by means of X-ray diffraction (XRD) analysis. We found that the RNLs reacted to completion, as seen by the presence of the terminal metal Zn and the terminal oxide ZrO₂, measured by XRD. Furthermore, XRD reveals that in the RNL films, adjacent layers react and form an intermetallic intermediate, which provides additional energy to the system and assists the thermite reaction to completion. In addition, differential scanning calorimetry (DSC) and ultrafast shock spectroscopy were used to study how numbers of layers and interface density influence the reactive and thermal behavior of RNLs. The DSC results showed the activation energy decreases with increasing numbers of bilayers. In other words, larger interfacial area, increases the RNL reactivity.

Wrought aluminum alloys, particularly those exhibiting high strength, are already seeing extensive use in aerospace and automotive applications given their high strength-to-weight ratio. Additive manufacturing (AM) of these alloys has grown more commonplace in an effort to meet the demands of these industries while overcoming the limitations of conventional manufacturing processes. However, many alloys of this class possess a propensity for solidification cracking, resulting in reduced ductility and tensile strength, which tends to make them unsuitable for welding and AM processes. Also, especially for aerospace applications, higher stiffness and more robust high-temperature properties are desired of these alloys. Addition of ceramic particulates to an aluminum alloy matrix to form a metal matrix composite (MMC) has been shown to both mitigate solidification cracking and achieve these desired properties. For ideal minimization of solidification cracking and to ensure uniform mechanical properties, the reinforcement phase must be uniformly distributed and possess a small mean diameter. Reaction synthesis refers to the reaction between precursor materials to form nanoscale particles that act as grain refiners or material reinforcement. Particles are formed in-situ during welding and AM processing through an exothermic reaction. Reaction synthesis can be used to form ceramic or intermetallic products that serve as the reinforcing phase in a MMC. Reaction synthesis is also able to address the poor wettability of the ceramic phase with the aluminum matrix through the presence of much higher temperatures during the reaction.

In this work, aluminum-based reactive additive manufacturing (RAM) powder blend feedstocks are employed in welding, laser powder-bed fusion (PBF), and electron beam freeform fabrication (EBF³) processes to examine the effect of feedstock composition and morphology on microstructure and mechanical properties. Blended powder was the feedstock for the laser PBF process and powder core filled wire was the feedstock for welding and the EBF³ process. The RAM feedstocks were characterized using scanning electron microscopy and particle size analysis. X-ray diffraction confirmed the presence of ceramic and intermetallic secondary phases in material fabricated using the RAM feedstocks. Light optical microscopy and electron back-scatter diffraction were used to examine texturing and the extent of grain refinement among the various feedstock compositions and the different fabrication processes. ASTM subsize tensile and Charpy specimens were used to measure the tensile strength and impact energy of materials produced by each process. Finally, differential thermal analysis was used to compare the heat evolution during solidification of the RAM powder feedstocks compared to inert aluminum powders.

Ni/Al multilayers are reactive nanostructures consisting of alternating Ni and Al layers that can release heat through a self-sustained propagating exothermic reaction. Sparked by advances in physical vapor deposition, the renewed interest towards these materials propelled over the past two decades the development of applications where the multilayers’ heat release is targeted towards a specific functionality. Prominent examples include the development of miniaturized heat sources for micro-propulsion and on-demand healing of metal thin films. In many cases, however, Ni/Al multilayers are designed to work in intimate thermal contact with other components, or on a specific substrate, which act as heat sinks by extracting heat from the propagating reaction front. These conductive losses may reduce both the maximum temperature and the propagation velocity of the reaction until a point where the propagation is ultimately quenched, a process that so far has only been qualitatively observed. In this work, we study the influence of thin film heat sinks such as gold, copper and silicon, on the propagation velocity and temperature of the self-sustained heat wave produced by the NiAl intermetallic-forming reaction. Both the reaction temperature and propagation speed are shown to be linearly decreasing functions of the heat sink thickness up to a critical thickness for which the reaction propagation is completely quenched (~350nm for gold and copper, ~1.3µm for silicon). Further, we demonstrate that a multilayer Al₂O₃/Zr/Al₂O₃ thermal barrier between the heat source and the heat sink prevents reaction quenching and enables stable propagation in otherwise quenched conditions. The results of this study provide a detailed overview of the propagation behavior of Ni/Al multilayers on thin heat sinks and demonstrate that the reaction propagation can be critically controlled and eventually stopped by varying the thickness of a thin heat sink. Together with the introduction of a nanoscale thermal barrier, these results will facilitate the integration of Ni/Al multilayers as intrinsic heat sources on different substrates for applications in micro/nano-devices.

High-strain mechanical loading of polymer-bonded explosives can produce significant localized heating due to microstructural heterogeneities. Detailed in-situ observations of the processes governing the generation of heat in these materials are challanging, yet they are important for improving predictive modeling tools for safety and performance. Here we report on a novel high-speed, high-resolution in-situ synchrotron x-ray diagnostic, combining phase contrast imaging with diffraction during heating of an explosive particle in a binder. We demonstrate that the morphology changes can help identify hetaing mechanisms, whereas the measured diffraction spot shifts from the single crystal can be used to track in-situ temperatures, verified by thermal modeling based on infrared camera measurements on the outside surface of the samples.

Hard magnetic nanoparticles have been synthesized by a solution-based combustion method. Strontium hexaferrite (SrFe₁₂O₁₉) was prepared by a sol-gel approach that allowed stoichiometric control of the nitrate-citrate gel by complexing specific ratios of metal nitrates with citric acid, and the subsequent auto-combustion reaction resulted in nanometer range particles. Material structure was analyzed by x-ray diffraction (XRD), and the nanometer-range particle size and shape were characterized by scanning transmission electron microscopy (STEM) and dynamic light scattering (DLS). The effects of calcining on the crystal structure and particle coarsening were investigated and characterized by XRD, STEM, and DLS. The electromagnetic permeability of toroids, pressed from pre- and post-calcined nanopowders, were measured with a 7mm coaxial airline.

The need to counter weapons of mass destruction with reduced collateral damage in urban environments has necessitated the development of countermeasures containing pre-formed, dense reactive materials. A crucial first step in understanding the impact and combustion performance of high-density reactive materials compacts is to characterize the effect that processing has on the microstructure and mechanical properties of the compacts. Here, we discuss the fabrication and characterization of high-density, reactive metallic composites formed through radial forging (swaging). The composites are formed by mixing dense, elemental powders of tantalum or tungsten with ball-milled reactive composite powders comprised of aluminum, magnesium, and zirconium; these powder mixtures are then compacted to near theoretical density through room temperature mechanical swaging. We manipulate process parameters such as the size of the elemental and composite powders and the degree of swaging, and we characterize the impact on the resulting microstructures and mechanical properties of the dense composites. These properties will, in turn, be used to understand the fragmentation of the composites on impact, as well as their reaction products and energy release.

It has been shown recently that cubic boron nitride (c-BN) can be formed through the ultra-fast (0.1-5 µs) shock-initiated reaction of TiN and B, leading to the synthesis of TiB₂ and BN. In this study, we have further investigated the synthesis of boron nitride using different precursor nitrogen sources, ZrN, GaN, Cr₂N. The shock-induced reaction of a ZrN+3B system will be reported herein. The composite ZrN+3B nano-powders were produced through high-energy ball milling to provide intimate, homogenous, mixing and were subsequently shocked to ~20 GPa using a copper recovery capsule, copper flyer, and a PETN based explosive. X-Ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) has been used to identify the phases and chemical bonding characteristics of the shocked powder. XRD results confirm that the precursor powder system has successfully reacted and formed hexagonal BN (h-BN). FTIR spectra corroborated the formation of h-BN and revealed a band near 1100 cm^-1, which suggests the existence of c-BN or wurtzitic BN, or both. These results could pave the way for in-situ creation of impact resistant ceramics.

The reactive-fragment concept involves explosive or gun launch of a reactive material, which then travels some distance before impact on a target. Upon impact, the fragment should shatter into a fine debris cloud and begin combusting, creating either thermal or overpressure damage. These competing requirements create a challenge for material design, as the fragment must be sufficiently tough to survive energetic launch without failure but still brittle enough to break up catastrophically when impacting a target. The majority of materials that work well in this role are pure metal compositions that rely on aerobic combustion for energy release. Traditional powder metallurgy techniques like hot isostatic pressing have been the most widely employed for fabrication of reactive fragments.

In this talk I review several years of research (especially within the Navy) on the material properties, energy release, and fragmentation of this subclass of reactive materials. The degree to which the material shatters on impact is critical for aerobic combustion, but efforts to rationally tune the energy release of reactive frags has been hindered by the lack of experimental fragmentation data. I will discuss our efforts to directly recover and analyze brittle debris from high-velocity reactive frag impact or explosive launch using soft catch media. This data has allowed the development of analytic fragmentation models which provide a first step towards more advanced continuum and mesoscale modeling efforts.

The mixing process of multimaterial systems can impart microstructure that can affect the overall material properties such as mechanical response and reactivity. In-line mixing has been implemented with a variety of feedstocks including siloxanes, sol-gel formulations, and reactive materials where the chemistry has been optimized for extrusion-based 3D printing. The behavior of the feedstocks requires additional characterization due to the complex fluid behavior present when introducing additional shear energy through mixing. This is especially true when mixing materials with vastly different densities, rheological behavior, and mechanical properties. The use of an active mixing printhead in 3D printing processes requires an understanding of how the processing dictates the microstructure and correlates to the resulting material properties. Characterization techniques such as mechanical testing and reactivity measurements are used to determine the connection between the microstructure due to the printing parameters, and the macro-scale behavior of printed structures. While in-line mixing has been utilized for a diverse set of materials, reactive materials are of special interest. Reactive materials benefit immensely from in-line mixing since the reaction kinetics can be controlled by printing parameters such as flow rate, mixing volume, and mixing speed. Additionally, the reaction is initiated in the mixing volume, reducing safety concerns when working with hazardous materials. An overview of multimaterial printing processes with an emphasis on reactive materials is presented.

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-778259

Controlling the surface chemistry of inorganic-oxide nanoparticles and their dispersion in water is an essential requirement for many technologies including ink and paint formulations.¹ Competing requirements are typically encountered, when considering the necessity of water wettability of the nanoparticle surface in order to render low-viscosity aqueous dispersion on the one hand, and the need for water-barrier properties in the dried film on the other hand. Building on nearly a decade of advances in the surface modification and tailoring of surface chemistry of inorganic-oxide nanoparticles,^2-4 which include poly(1→4)-β-glucan, alkylsilane-hexamethyldisilazane, and polysiloxane surface modification strategies, here we describe the synthesis of nanoparticles that exhibit a combination of hydrophilic and hydrophobic character. These nanoparticles are hydrophilic in the wet state, and hydrophobic in the dry state, as characterized by capillary rise, contact angle, and water-uptake measurements. We describe the synthesis of such nanoparticles, and the importance of both electrostatic effects as well as steric effects of stabilization in aqueous dispersion, the latter characterized by ²⁹Si CP/MAS NMR spectroscopy and thermogravimetric analysis. We also study the flow behavior of the concentrated aqueous dispersion of modified-nanoparticles by rheometry, which reveals a low viscosity of the dispersions, thus making them ideal for applications.




References:
[1] S. M. Sarrica, Paints: types, components and applications, Nova Science Publishers, Inc.
[2] J. Jankolovits, O. M. Gazit, M. M. Nigra, J. Bohling, J. A. Roper III, A. Katz . Adv. Mater. Interfaces_, 2015, 2, 1400465.
[3] J. Jankolovits, A. Kusoglu, A. Z. Weber, A. V. Dyk, J. Bohling, J. A. Roper, III, C. J. Radke, and A. Katz, Langmuir 2016, 32, 1929−1938.
[4] Y. Guo, M.K. Mishra, F. Wang, J. Jankolovits, A. Kusoglu, A.Z. Weber, A. Van Dyk, K. Beshah, J.C. Bohling, J.A. Roper III, C.J. Radke, Langmuir 2018, 34, 11738−11748.

Vibration Assisted 3D Printing (VAP) is a new direct-write manufacturing approach in which a nozzle resonates to enable dynamic flow control with constant driving pressure. Recent work suggests that VAP is capable to print highly viscous composite materials and propellants with applications to variable geometry solid rocket propellant, gun propellant, biological materials, ceramics and more. Developing a dual-nozzle VAP printer integrates two materials in a variety of configurations to explore topics such as combined energetic materials. An open-source Fused Deposition Modeling (FDM) 3D printer is modified to incorporate two VAP printing heads, each including a transducer and a 5cc syringe in contact with an ultrasonic transducer tip. The transducers are connected to a function generator and linear amplifier to generate the vibration frequency and amplitude needed for extrusion. A dual output compressor generates back pressure to keep a constant flow of material. A UV light is also incorporated to cure UV-curable propellants. Either of the VAP heads can be swapped with a FDM head to incorporate hybrid VAP/FDM prints. Such application includes printing various geometries of a support structure for a reactive material. Developing a dual nozzle VAP is a desirable step to investigate a wider range of energetics applications as well as implementations outside of the field. Here we present initial results of a dual nozzle VAP printer with examples of potential applications.

Breakthroughs in additive manufacturing of energetic materials, particularly direct ink write 3D printing, have allowed for greater control of performance by manipulation of architecture and/or composition. Here, we demonstrate control over the self-propagating reaction properties of 3D printed Al/CuO thermite by generating lattice structures with various void contents and filament sizes. The thermite is produced via on-the-fly static mixing of constituent Al and CuO inks, decreasing the risk associated with handling such materials. By reducing the fundamental print width (i.e. filament size) and introducing even small amounts of engineered porosity, we show that energy release rates can be increased by more than 100x over full-density burn strips. We also report on unique channel structures, which display propagation velocities of over 100 m/s due to confinement of the hot gas and molten particles released by the thermite reaction. We present Ashby plots which demonstrate the design space for 3D printed thermites and discuss the wide level of control we achieve over the energy release rate using architecture with a single thermite formulation.

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-776741.

Monomers containing acrylate/methacrylate functional groups have attracted intensive interest due to the ability to polymerize readily under ultraviolet exposure. However, it would have great benefits for many applications if a network could be pre-established to support the monomers. A promising approach is to use low-molecular-weight gelators, which are capable of forming a physical 3D network to entrap and immobilize liquids. This might potentially form a dual network, modifying physical properties (rheology, morphology and others) of composites. The aim of this study is to obtain comprehensive results via investigating the interaction between gelators and monomers.
Methodology: Nine thermo-responsive gelators (A-I) with different structures were investigated. Each gelator was mixed with 29 monomers at 10 mg/ml. Gel tests were performed using a Crystallisation Systems Crystal16. Samples were heated gradually from 20°C to 120°C and cooling back to 25°C. Light transmission was recorded for each sample to determine gelation temperatures. Gel stiffness was characterized using an oscillatory rheometer and xerogels morphology were characterised via SEM.
Statistical analysis has been performed to build the relationship between gelation and 10 monomer parameters including: molecular weight, atom account, partition coefficient, hydrophilic-lipophilic balance (HLB), polar surface area, refractivity, Reichardt’s polarity (E_T(30)), Kamlet-Taft polarizability, hydrogen bonding donor, and hydrogen bonding acceptor. Hansen’s HSPiP software was also used to understand the gelation.
Results and Discussion: There were three bis-urea gelators that formed gels with a significant amount of monomers: A (20 monomers), B (18 monomers), C (12 monomers), and a cyclohexanone derivative (D) formed gels with 19 monomers. The bis-urea molecules provided strong hydrogen bonding as the driving force for molecules to self-assemble into fibrous network. However, the acrylate/methacrylate monomers may mediate the process of hydrogen bonding molecular recognition pathways. The cyclohexanone derivative was of particular interests because its supramolecular mechanism was primarily relying on π-π stacking and London dispersion force contributed by the alkyl chains. Linear symmetrical bis-ureas demonstrated their interaction with monomers was most influential by E_T(30) (p value = 0.004, F value = 5.72). As for the remaining gelators, partition coefficient and HLB were the solvent properties that had the most significant impact on correlating gelator-monomer interaction.
The asymmetrical bis-urea exhibited morphologies of self-assembly into thick fiber bundles whereas the cyclohexanone derivative formed plate-shaped morphologies. However, the linear symmetrical bis-ureas formed intertwining flexible networks with primarily single-strand fibrils. The results indicated that hydrogen bonding tended to favor self-assemble into single-strand fibers whereas self-assembly via aromatic interaction lead to non-fibrous arrangement. The gels formed from bis-ureas and cyclohexanone derivative were in the same range of storage modulus with the highest achieved above 4000 Pa at 1 Hz and the lowest below 50 Pa. However, none of the gels formed from gelator C was above 800 Pa. Linear symmetrical bis-urea gelator with longer alkyl chain resulting in higher gelation temperature and lower stiffness due to partially forming crystalline precipitation. Extensive precipitation has been observed on the surfaces of the gels formed from L-cysteine-derived double hydrocarbon chain.

Recent developments in additive manufacturing have drastically improved the ability to rapidly prototype and assess product design and viability. However, for products with integrated electronics, the ability to 3D print devices incorporating electronic components is limited by the small number of conductive inks, such as nanosilver. As the most common conductive ink, nanosilver's high surface diffusivity allows it to sinter at low temperatures, resulting in electric interconnects with low resistivity. However, that same surface diffusivity causes poor durability as the printed lines can fail due to electromigration even at moderate current densities. Here, we demonstrate a new pathway to creating stable, electrically conductive printer interconnects, wherein the powders used to formulate the ink have stored chemical energy which assists in the sintering process and produces a low-resistivity product phase. The particles contain pairs of elements, such as Zr-C, Ti-C, and V-C, that have high heats of reaction, embedded in a soft, low melting temperature and low resistivity matrix, such as Al or a Cu-Ag alloy. This technology allows for the use of stable conductive materials, such as carbides, which would otherwise necessitate very high processing temperatures. To investigate the selection of chemistries suitable for these reaction-assisted inks, samples with highly controlled composition and microstructure were generated via physical vapor deposition (PVD) and the effects of chemistry and ignition method on the product phases, resistivity, and microstructure were assessed. These PVD materials serve as model materials and provide a toolbox for tuning stoichiometry when developing commercially viable powders for reaction assisted additive manufacturing applications.

As typical energy storage materials, nanothermites which contain Al and oxide, have attracted much more attention for their broad potential applications in both civilian and military fields. They exhibit not only better combustion efficiencies and better ignitability compared to traditional monomolecular explosives but also the reaction outputs can be tuned thanks to the selections of fuels, oxidizers, architectures and reactant size allowing multiple actions. The performance and sensitivity of these materials are critically dependent on the properties of both active materials, Al and oxide, and their interfaces which control the reactants diffusion and thus reaction kinetics. Among nanothermites, sputter-deposited Al/CuO multilayers represent the state-of-the-art of energetic nanomaterials for tunable ignition to replace old hot-wire ignitor. To apply these 2D layered materials into products, it is now important to modulate the layers features in a controllable manner to fit the application requirements which are often contradictory in terms of ignition threshold, expected high for safety reasons, and reactivity. For example, decreasing the bilayer thickness below 150 nm permits to reach self-propagation velocities ~100 m/s but also led to instabilities and potential non-desired ignition, so that the materials can not be handled in a reliable manner. In this paper, we propose a new strategy based on utilizing uniformly distributed gold nanoparticles at the first interface to - upon thermal stimulation - generate reactive hot spots serving as ignition precursors. We first report a fast, low-cost and scalable approach to produce uniformly distributed gold nanoparticles onto the first sputtered CuO nanolayer using a photo-deposition method before sputtering the Al/CuO nanothermite on top. Then, we discuss the thermal-mechanical energy-transfer mechanisms that govern the initiation of redox reaction. Under thermal stimulation, Gold nanoparticles serve as localized-heat absorbers triggering locally the redox reaction and thus producing reactive micro-sized hot-spots. These hot-spots generate stress-induced mechanical rupture of the structure. The following disintegration of the layered material project Al droplets that burns into the environment.
The ignition and flame characterization including emission spectra analyses as well as electronic microscopy observation completed in this work have allowed the construction of the following fundamental description of the ignition and reaction scenario in Al/CuO nanolaminates being ignited upon ultra-fast ramping. Upon heating the redox reaction begins between the Al and CuO reactant layers in the bulk material. In the reaction zone, the temperature quickly increases and produces localized highly stressed zones (hot-spots) leading to the disintegration of the nanolaminate before the entire Al reservoir has reacted. This explains why the observed flame temperature is above the Al vaporization point and, even, in some particular conditions, above the Al₂O₃ vaporization point. When the multilayer structure breaks apart, it disperses in the ambient unreacted, melted Al and CuO. Both continue to react, now with the additional possibility for unreacted Al to burn with environmental oxidizers such as those found in air.
Finally, we provide a perspective on the advantages of using gold nanopartciles as ignition triggers and suggest design strategies for highly efficient, stable while tunable Al/CuO nanothermites for commercial applications.

Micron-scale and nanometer-scale aluminum (nAl) particles are often considered attractive choices for fuels in energetic materials. In general, reaction rates increase as particle size decreases, owing to the increased surface area and reduced diffusion lengths between reactants. The oxidation behavior for aluminum nanoparticles >10 nm has been widely studied, and so has the oxidation behavior of clusters <1 nm (primarily for catalysis applications). These two regimes exhibit vastly different reaction mechanisms, but there is no experimental work observing oxidation behavior for intermediate size particles from 1-10 nm. The present study investigates this transition regime by producing unpassivated aluminum particles of this size range using Superfluid Helium Droplet Assembly (SHeDA), and then oxidizing the particles by rapidly transferring them from ultra-high vacuum (UHV) to ambient air. Scanning Transmission Electron Microscopy with EDS (STEM/EDS) and X-ray Photoelectron Spectroscopy (XPS) showed that particles < 4 nm vaporize upon oxidation while particles > 4 nm do not. We have hypothesized that this is a critical diameter, based on a rapid decrease in cohesive energy at this size, and is the threshold between the oxygen-etching mechanism of clusters and the heterogeneous oxidation of nanoparticles.

The introduction of novel reactive metal fuels into various explosive, pyrotechnic, and agent defeat applications requires an understanding of how ignition, combustion, and large-scale performance properties are coupled and how they depend on powder properties. Here, we will discuss the last five years of research on Al and Zr-based composite metal fuel powders that are synthesized via the scalable method of high energy ball milling. We will describe the entire lifecycle of these materials, including their synthesis, initiation, and combustion at both the individual particle and macroscopic fireball scales. These composite fuels are unique in that they have independently tunable ignition and combustion properties that includes a two-stage burn process (vapor then condensed phase). The composite metal powders demonstrate a significant decrease in ignition thresholds and substantial increases in combustion efficiency and burn time relative to pure Al powders of similar or smaller sizes. Through collaborations with a variety of research groups at ARL, NSWC-IHEODTD, Argonne National Lab, and several universities, we have explored the fundamental ignition and combustion mechanisms of these composites using multiple methods such as laser-induced plasmas, heated filaments, and explosive launch tests. In this presentation, we will summarize the results of these collaborations, as well as our own work, characterizing the dependence of ignition and burn mechanisms on powder parameters that include microstructure, size, chemistry, and environment.

Confocal Raman microscopy is an important tool to characterize two dimensional (2D) materials, but the low flux of generated Raman scattered photons significantly hinders its expanding applications. For metastable materials such as graphene oxide (GO), the low signal flux is aggravated by the requirement of an extremely low laser dose to avoid or minimize laser-induced sample damage. In this work, we introduce algorithm-improved Confocal Raman Microscopy (ai-CRM), a fast and non-invasive confocal Raman microscopy (CRM) method based on a combination of an electron multiplying charge-coupled device (EMCCD), and principal component analysis (PCA) guided data de-noising. Using graphene and GO as a proof-of-concept, we demonstrate that ai-CRM can increase the Raman scanning rate by at least one order of magnitude with respect to state-of-the-art works. Meanwhile, GO can be imaged at a laser dose that is 2 to 3 orders of magnitude lower than previously reported, to the extent that the influence of laser illumination on the properties of the material can be avoided. We show that ai-CRM enables fast and spatially resolved quantitative analysis, such as mapping the defect density of graphene sheets. It is also demonstrated that the method can be extended to fast imaging of other 2D materials including Molybdenum Sulfide (MoS₂), Tungsten Sulfide (WS₂) and Boron Nitride (BN), and the three-dimensional distributions of nano-composite materials. Since ai-CRM is based on general mathematical principles, it is cost-effective, facile-to-implement and universally applicable to many other materials as well as other hyperspectral imaging methods such as surface-enhanced Raman scattering, hyperspectral infrared microscopy, and photo-luminescent microscopy.

Vapor-deposited thin-film explosives provided a pathway to fundamentally understand the physics of explosives initiation and detonation. However, the physical properties of vapor-deposited thin-films deviate from their bulk counterparts and must be carefully characterized. Specifically, density, which is a primary physical parameter in detonation performance, varies depending on the substrate and across the thickness of a vapor-deposited thin-films. A gravimetric measurement of density can be obtained by dividing the measured mass of the thin-film by its measured volume. Unfortunately, variations in film thickness and limitations in balance resolution result in large uncertainty in these calculations and limit our ability to reliably measure density.

In this work, we present a novel method for non-contact technique for density and thermal conductivity measurements of encased organic thin-films and thin-film explosives. Four films of indomethacin and an array of varying thickness pentaerythritol tetranitrate (PETN) thin-films were vapor-deposited on silicon and fused silica substrates. The samples with encased with aluminum and gold thin-films and the density of the samples was measured with Frequency Domain Thermoreflectance (FDTR). Our results are compared to those obtained through gravimetric measurements and are in good agreement and prove to be a more reliable method for local density measurements of thin-film explosives and demonstrate the applicability of our technique for future microenergetics research.

We have developed methods for laser-driven shock loading of energetic materials and direct observation of shock-induced energetic material decomposition [1-3]. Small RDX crystallites embedded in polymer hosts are subjected to focusing quasi-2D shocks, and shock propagation and material responses are observed with a multi-frame camera that can record up to 16 frames in a single event with as little as 3 ns between frames. The results from many shocked crystals consistently show decomposition occurring initially along a single well-defined crystalline plane, with additional decomposition following depending on the shock pressure that is reached. Chemical and physical decomposition are apparent from the images and from photoemission that is also measured in real time. Post-mortem SEM and ultrasmall-angle x-ray scattering reveal shock-induced voids whose sizes and shapes depend on the extent of chemical as well as physical decomposition.
Increases in RDX sensitivity also are observed under some conditions. When multiple crystallites are in contact, decomposition occurs preferentially at the regions of contact where stress localization is likely. Multiple shocks also have profound effects on sensitivity, with the extent of decomposition induced by a first weak shock far less than the decomposition induced by a second weak shock delivered to the same crystal as the first. The method is well suited for study of aging effects, either through the use of multiple shocks, application of heat, or combinations of stress and heat.
[1] "Direct visualization of laser-driven focusing shock waves," T. Pezeril, G. Saini, D. Veysset, S. Kooi, R. Radovitzky, and K.A. Nelson, Phys. Rev. Lett. 106, 214503 (2011). https://doi.org/10.1103/PhysRevLett.106.214503
[1] “Machine learning to analyze images of shocked materials for precise and accurate measurements,” L. Dresselhaus-Cooper, M. Howard, M. C. Hock, T. B. Meehan, K. Ramos, C. Bolme, R. L. Sandberg, and K. A. Nelson, J. Appl. Phys. 122, 104902 (2017). http://doi.org/10.1063/1.4998959
[2] “Single-shot multi-frame imaging of cylindrical shock waves in a multi-layered assembly,” L. Dresselhaus-Cooper, J. E. Gorfain, C. T. Key, B. K. Ofori-Okai, S. J. Ali, D. J. Martynowych, A. Gleason, S. Kooi, and K. A. Nelson, Sci. Rep. 9, 3689 (2019). doi.org/10.1038/s41598-019-40037-3

Synthesis of functional materials in flame reactors is a reliable method to produce pure nanoparticles or coatings with unique physical and chemical properties, which are of great interest to a wide field of applications as additives, e.g. in edibles and medicine. The properties of flame-synthesized nanoparticles strongly depend on the particle size. The size can be influenced, for example, by the temperature of the synthesis and precursor concentration. The network of chemical reactions of the precursor decomposition interacts with the flame chemistry and forms key intermediates that are involved in particle growth. Consequently, the fundamental understanding of the decomposition of the iron based precursor to key intermediate species and stable products and the inception of particles during flame synthesis is of major importance for planned synthesis processes. Time-of-flight mass spectrometric in-situ measurements of iron pentacarbonyl doped model flames can identify which species occur in the synthesis flames. This information is the first step in generating a reaction model of the chemical processes in flame synthesis.
In this study, the decomposition of iron pentacarbonyl (Fe(CO)₅) and the reaction pathways in laminar H₂/C₂H₄/O₂/Ar synthesis flames at low pressure are investigated. For a chemically controllable synthesis and functionalization of tailored iron oxide particles in flames, gaseous intermediates that act as building units during particle formation have to be identified. The analysis of the neutral intermediates is performed with aid of electron ionization molecular beam mass spectrometry (EI-MBMS). The extraction of a representative sample from the particle-laden flow of a synthesis flame by using an intrusive sampling technique for the mass spectrometric analysis is an ongoing challenge, because easily condensing iron containing species can obstruct the sampling probe. As a result, species with the structure Fe-O-H are still difficult to detect. Naturally occurring cationic iron containing species can be extracted from the flame with a novel sampling technique, which reveals cationic reaction pathways in the flame. The sampling of naturally occurring cations from flames allows measurements with high sampling efficiency for iron containing intermediates, thus overcoming one of the major difficulties in the EI-MBMS work. Here, cations with the structure Fe(OH)₂⁺/ Fe(OH)₂H⁺ revealing the presence of neutral Fe(OH)₂ and the cations Fe(OH)₃⁺/ Fe(OH)₃H⁺ allude to the presence of Fe(OH)₃ and hydroxide clusters with the structure Fe_xO_yH_z. This study sheds some light on the gap between precursor decomposition and formation of nascent particles in iron oxide synthesis flames. Data sets with concentration profiles of neutrals and cations are provided for the validation of kinetic reaction mechanisms in combustion synthesis processes.

Polyaromatic carbonaceous materials (such as coal, tar, and pitch) are a family of materials with extremely rich chemistry and complex structures, representing a massive opportunity for their use in a range of potential applications. However, a deeper understanding of their electronic and structural properties at the microscopic scale is essential before they can be utilized in applications that require targeted properties and performance as well as large-scale manufacturing. In this talk, we present our recent work using laser ablation as a processing tool to induce extremely high local temperatures in thin films of natural carbon materials. Our results reveal the laser thermal effect can control the H:C ratio, sp2 concentration, and graphitic stacking which are key structural fingerprint determining the electrical properties of processed carbon materials. The laser ablation of carbonaceous materials involves two key steps dehydrogenation and restacking behavior of polyaromatic hydrocarbons (PAHs). The broad tunability of the two factors result in a wide distribution of carbon materials crystallinity from amorphous to highly graphitic, with a sequential broad distribution of conductivity up to 10³ S/m. Semiconducting carbonaceous materials require relative high H:C, while highly graphitized carbon is desired for conductivity electrodes. Both experimental and simulation results indicate a graphitic cluster formation mechanism of NPAHs involving aromatic corea, alkane “glue” molecules, and additive aromatic agents. These results shed light on the potential of natural carbon to be utilized in active components, and can be used to guide the tuning of their electrical, structural and chemical properties, paving the way towards their use far beyond simply disposal or burning.

Frontal Polymerization is a reaction that, following initiation, releases sufficient energy to further polymerize, creating a spatially propagating wave front. The present work utilizes this process in epoxy formulations where frontal polymerization is driven by the exotherm released from cationic ring opening of the epoxide monomers. Accordingly, this technology is advantageous in creating high-T_g, high-performing thermosets in that curing takes place without the use of costly energy sources or VOCs. Frontally polymerizable epoxies are envisioned to have relevance in a variety of applications where mechanical strength and chemical resistance is required. More specifically, the use of frontally polymerizable epoxies as adhesives is investigated herein.

In this paper, the adhesion performance of a frontally polymerized formulation to a wide range of substrates is investigated. Results discussing the influence of substrate properties, adhesive morphology, and boundary conditions on adhesion will be presented in this work. Adhesion testing utilized a lap shear configuration and the shear stress distribution was calculated using a shear lag model. This model was formulated to enable the calculation of the stress distribution when adhering two substrates with different material properties. Consequently, an adhesion investigation was facilitated where adhesion to one substrate was exceptional thereby promoting failure at another material interface. Further, in all cases at least one interface was selected to be transparent, allowing for in-situ measurements of the polymerization propagation rates and resulting morphology. Herein, the mechanisms of adhesion to the various substrates will also be discussed.

The reaction mechanisms in thermite powders is a highly complex process, and involves various non-equilibrium and equilibrium processes as fuel and oxide particles react to form mixed-phase products on a rapid time scale. Understanding this process involves understanding of the various length and time scales and how they contribute to the reaction processes. Fundamental studies can help better our understanding of the reaction mechanisms, specifically as they pertain to safety and performance. In this work, we present a small scale test designed to evaluate the reaction violence of thermites at the formulation scale. A small charge is placed at the sealed end of a clear, acrylic tube, and ignited. This results in rapid entrainment and flow of the powder. The velocity of this flow can give a relative ranking of the pressurization, as intermediate gases are released and drive this flow. The burn time of the dispersed powder can yield information about the thermal profile, which is important to know when evaluating heating effects such as damage to a target or safety shielding requirements. The test can easily be modified as-needed, but is an important first step in evaluating the safety characteristics of new materials, and towards developing a better fundamental understanding of how the material attributes can control this behavior.
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-760652.

Understanding structure-function relationships is exceedingly difficult because the characteristic length scale of the microphysics (composition and thermal gradients) is usually much smaller than the characteristic length scale of the measurement. To address this mismatch we employ a new high-speed microscopy/thermometry capability that enables ~ µs time and ~ µm spatial resolution as applied to highly exothermic reaction propagation. This enabled us to directly observe reactive sintering and the reaction front at high spatial and temporal resolution.
We show the usefulness of this approach for three specific examples. An important proposed mechanism in nanothermites reactions – reactive sintering – plays a significant role on the combustion performance of nanothermites. Experiments on the Al + CuO nanocomposite system reveals a reaction front thickness to be ~30 μm and temperatures in excess of 3000 K, resulting in a thermal gradient in excess of 10⁷ K m^-1. The local microscopic reactive sintering velocity is found to be an order of magnitude higher than macroscale flame velocity. In this observed mechanism, propagation is very similar to the general concept of laminar gas reaction theory in which reaction front velocity ~ (thermal diffusivity x reaction rate)^1/2.
In a second example, recent studies have demonstrated a counterintuitive result in that additives with thermally insulating properties – notably SiO₂ particles – can enhance the propagation rate in solid propellants. Using high-speed microscopy and thermometry on 3D printed solid propellant films containing both thermally conducting (graphite) and insulating (SiO₂) to investigate the role of these additives on film propagation rate. It was found that addition of SiO₂ particles increased the effective surface area of the reaction front through inhomogeneous heat transfer in the films, and that such corrugation of the reaction front area on the micron scale manifests itself as a global increase in the propagation rate on the macro scale. Graphite additive was observed to have a substantially lower burning surface area and propagation rate, suggesting that the effect of reaction front surface area is larger than the effect of thermal diffusivity for low-weight percent additives in solid propellants.
In the last example we will demonstrate how the use of microwaves can be used to initiate and moderate burn rate using metal coated nanoparticles that optimize absorption.

Metallic nanoparticles, such as nano-aluminum (nAl), are promising candidates as the next generation energetic additives for improved combustion and detonation effects in explosive/propellant formulations. Their high theoretical energy content and potentially rapid burning of nAl, enabled by the high surface area to volume ratio, are very desirable characteristics for energetics. This work aims to enhance fundamental understanding of the chemical and structural properties for reactive aluminum-aluminum iodate hexahydrate (AIH) composite nanoparticles. The composites were produced by treating commercial nAl particles (60-100 nm average particle diameter) with helium and argon inert plasmas using custom-designed atmospheric dielectric barrier discharge reactors followed by subsequent mixing with acidic iodic solution (HIO₃) to form the energetic oxidizing salt, aluminum iodate hexahydrate (AIH) on the nAl surface. High resolution transmission electron microscopy (HRTEM) micrographs revealed unique miniature crystalline structures within the amorphous alumina (Al₂O₃) shell surrounding the crystalline Al core and at the outermost particle surface with only one to two monolayer thickness. Elemental maps obtained from scanning TEM (STEM) experiments demonstrated distinctly different distributions of AIH crystallites on the nAl surface for the nAl-AIH composites produced by different gases during the plasma treatment. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were used to study surface functional groups of the nAl-AIH nanoparticles. This paper emphasizes the significance of exploiting advanced materials characterizations to enhance in-depth understanding of reactive nAl-AIH composites by revealing the plasma-dependent altered surface chemical and structural properties.

The propagation of sputter deposited Co/Al nanolaminates is known to have a bilayer thickness dependent instability. The specific 2D instability observed involves the transverse propagation of a band in front of a stalled front commonly referred to as a “spin band.” Previous laser ignition studies have revealed that preheating to the point of pre-reaction removes enough stored chemical energy to slow propagation. Here, a single step diffusion limited reaction model is implemented into a time dependent heat spreading model to calculate the magnitude of pre-reaction ahead of a propagating front. To determine the amount of stored chemical energy lost as a function of pre-reaction thickness, films with product phase diffusion barriers were grown and characterized. This pre-reaction model is combined with an analytical calculation of propagation velocity, which allows for a prediction of quench length. The energy-depletion based model for spin band width is then tested on normally stable bilayer designs where instabilities have been induced through the addition of diluent product phase layers within the reactant layers.

This work was supported by the Sandia National Laboratory Directed Research and Development
(LDRD) program. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract No. DE-NA0003525. This work describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S.
Department of Energy or the U.S. Government.

Reactive multilayers are uniformly structured materials that may be ignited to produce rapid and localized heating. While this class of material is well-known, and has long been studied for a variety of applications (e.g. joining, brazing, or near-net shape forming of intermetallic parts), only recently have the related thermochemical reactions been implemented in large 3D computer simulations. Original modeling efforts through the early 2000s considered flame propagation in one dimension; this approach is useful to compare with the measured reaction front velocities and flame front thicknesses. Such 1D models provide physical insights into the material behavior, as velocity and front thickness are varied experimentally by controlling the multilayer periodicity and total film thickness. However, high speed videography has shown that the reaction fronts can be non-planar [1], for certain periodic spacings (i.e. bilayer), ambient temperatures, gaseous environments, as well as other design conditions. In particular, various instability modes have been observed, including auto-oscillation of the reaction front (1D instability), spinning reaction fronts (2D or 3D instability), and chaotic reaction propagation (3D instability). Such behavior may only be explored in large finite element analysis (FEA) codes (e.g. [2]), where the current state-of-the-art is thought to be on the order of 10 to 100 million elements for a reactive thermal analysis on modern high-performance computing resources.

In this work, an empirically-driven reduced order model for 1:1 Co/Al multilayers is implemented in Aria, a Galerkin FEA code developed by Sandia National Laboratories, and part of the SIERRA computational framework. The reduced model is a diffusion-limited reaction model based on the one by Hardt and Phung [3]; this model has been previously used to predict the ignition delay, propagation velocity, and quench limits for different Co/Al and Al/Pt multilayer designs [4]. Now, the reaction model is coupled to empirical thermophysical property data obtained from years of testing at Sandia. This includes: an anisotropic thermal conductivity model, mixture heat capacity, melting and phase change with formation enthalpies, piecewise continuous diffusivity with an Arrhenius dependence on temperature, and bilayer-dependent species concentration profiles. To summarize this implementation, we present the most up-to-date model fits for 1:1 Co/Al and the associated equations which are assumed to govern the reaction front dynamics. Next, large 3D simulation results show that the fully detailed model is able to predict planar as well as non-planar fronts, which are then examined for their heating rates and thermal gradients. From these simulations, it is hypothesized that real instabilities originate from nucleation sites. In order to artificially induce this nucleation, we show numerical initiation schemes that break the symmetry of the calculation, with different boundary conditions and multilayer edge geometries. Finally, one case of a spinlike reaction front is discussed in greater detail to compare and contrast with the mechanism originally proposed in [5].

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

¹J. P. McDonald, V. C. Hodges, E. D. Jones, and D. P. Adams, Appl. Phys. Lett. 94, 034102 (2009).
²L. Alawieh, O. M. Knio, and T. P. Weihs, J. Appl. Phys. 110, 013509 (2011).
³A. P. Hardt and P. V. Phung, Combust. Flame 21, 77 (1973).
⁴D. E. Kittell, C. D. Yarrington, M. L. Hobbs, M. J. Abere, and D. P. Adams, J. Appl. Phys. 123, 145302 (2018).
⁵R. V. Reeves and D. P. Adams, J. Appl. Phys. 115, 044911 (2014).

Nanoenergetic materials deposited in the form of nanolaminates have been developed for welding purposes or as micro-initiators. One major advantage lies in their ability to be integrated on a chip via conventional MEMs fabrication techniques such as sputter deposition. Their integration and miniaturization in circuits open a number of new functional perspectives in miniaturized systems (security, actuation, local heat source…). One of the main obstacles to use these materials and deploy them at a large-scale is to guarantee and preserve their performance and safety under long term storage. To this end, predicting and quantifying the impact of thermal aging on nanolaminate structures and their energetic performance is of crucial importance.
Recently, an experimental investigation dedicated to the identification of a reaction scenario of Al/CuO nanolaminate at very low heating rate, typically used in Differential Scanning Calorimetry (a few tens of degree per sec.) has been proposed, establishing a number of basic mechanisms [1]. From low to medium range temperatures (< 500°C), we evidenced that degradation of the nanolaminate is governed by the transport of oxygen across the growing layer of Al2O3 separating Al and CuO, exhibiting multiple steps with corresponding structural variants in the interface. On this basis, we developed a heterogeneous reaction model [2] integrating an ensemble of basic reaction mechanisms (oxygen diffusion, structural transformations, polymorphic phase changes) as the simulation core of a modelling platform, NICAM (for Nanolaminates Ignition Combustion and Aging Model) to: (i) simulate accelerated thermal aging, (ii) predict the effect of aging on nanolaminate performance in terms of combustion, flame propagation and initiation delay. We discuss “in silico” experiments with varying parameters such as layer thicknesses, stoichiometric ratio, initiation power, substrate thermal conductivity, or the size of the heating zone. For each case studied, we provide the nominal conditions of aging and how performance is altered, in terms of initiation delay, flame propagation and heat release.


[1] Structure and chemical characterization at the atomic level of reactions in Al/CuO multilayers, Iman Abdallah, James Zapata, Guillaume Lahiner, Benedicte Warot-Fonrose, Jeremy Cure, Yves Chabal, Alain Esteve, Carole Rossi, ACS Appl. Energy Mater., 1 (4), pp 1762–1770, 2018
[2] A redox reaction model for self-heating and aging prediction of Al/CuO multilayers G. Lahiner, J. Zapata, J. Cure, N. Richard, M. Djafari Rouhani, A. Estève, C. Rossi, Combustion Theory and Modelling 1 2019

Interest in reactive nanocomposites has experienced massive growth due to the proclivity of these energetic materials to provide highly adaptable, attractive solutions to pyrotechnical problems requiring large amounts of energy in the form of heat or pressure. These nanothermites, constructed of a fuel-oxidizer pairing, produce extremely exothermic reduction-oxidation reactions with highly tunable properties including the gas production, flame temperature, burn rate, or visual effect. By adapting the thermite couple materials to produce the desired reactions, these nanoenergetic materials have applications in MEMS as heat sources or micro-initiators, the aerospace industry as solid propellant, propulsion systems, or space vehicle separation, mining time delay elements, among others. The study and characterization of different nanothermites and their reactions by experiment can be costly, difficult to standardize or quantify, and even dangerous to manufacture. Up to this point, many experimental studies and most numerical simulation of nanothermites have been based on the Al/CuO nanothermite due to the abundance of the materials and elevated base of understanding in the community, but it is not applicable for all possible domains. Computational modelization provides an economical, relatively quick, and safe method to characterize the different options available for these applications.
This work presents a novel phenomenological model based on the Deal-Grove diffusion/reaction scheme for mixed powder-based nanothermites. It is based purely on the condensed phase reaction, including diffusion of oxygen atoms across interfaces in addition to reactive sintering. These cause the initial agglomeration and melting of the mixed nanoparticles due to an external heating source that begins the self-sustaining reaction due to the exothermic reduction and oxidation of the metal oxide and metal fuel, respectively. With inputs such as the initial particle materials, size and nature, and associated properties, such as diffusion coefficient, the wetting contact angle of coalescence, the stoichiometric ratio, the compaction rate (as a percentage of the Theoretical Max Density), and the initiation method, results like the initiation delay, total burn time, and speed of steady-state propagation are produced. We first discuss the role of condensed phase mechanisms and sintering on the initiation of an Al/CuO mixture as a function of the heating ramp or initiation mode (resistive versus laser heating). This leads to a comparative study of the driving force in initiation between condensed or gas phase mechanisms. [1] A benchmark study is then completed on commonly used thermite couples, their initiation and propagation, with an emphasis on low-gas or gasless thermites for specific applications in mining or space domains. These simulated results are validated with available experiment where possible, with the hope that this information can provide a framework reference for new or existing applications looking for a certain range of pyrotechnical effects.

[1] A condensed phase model of the initial Al/CuO reaction stage to interpret experimental findings. Sarah Brotman, M. Djafari Rouhani, C. Rossi, A. Estève. J. Appl. Phys. 125, 035102 (2019); https://doi.org/10.1063/1.5063285