<p dir="ltr">Developing high-nitrogen materials is highly important to industries, such as pharmaceuticals and energetic materials development. The production of such materials is often wrought with hazardous conditions that raise manufacturing costs and produce toxic waste streams. This is often observed using heavy-metal redox reagents such as potassium permanganate, lead acetate, zinc, and silver metal. Elucidating novel synthesis techniques to alleviate these issues is highly important for reducing environmental toxicity and lowering reaction costs. One green technique that has gained popularity in the past few decades is synthetic organic electrochemistry. Electrochemistry is a technique that utilizes the direct flow of electrons to drive chemical reactions. This is advantageous as the direct use of electrons supplied from an electrode is an inherently cheap and environmentally friendly redox reagent. Additionally, electrochemistry allows for unique reaction pathways that would be difficult, if not impossible, to obtain via traditional chemical methodologies.</p><p dir="ltr">Herein, I discuss our work on advancing electrochemical synthesis for synthesizing high-nitrogen and energetic materials, which includes: An overview of potential reaction pathways toward developing promising high-nitrogen heterocyclic small molecules and polymers. Studies of the reaction efficiency of the decagram scale electrochemical production of useful energetic feedstocks, such as potassium dinitroethane. Novel azo bridged energetic materials produced via electrochemical amine oxidation reactions that were further utilized to prepare a series of energetic <i>N</i>-nitramines. Finally, a novel sequential electrochemical-photochemical methodology has been developed that has produced annulated heterocycles with promising pharmaceutical and energetic applications.</p>

<p dir="ltr">Polyvinylidene fluoride (PVDF) and its copolymer poly(vinylidene fluoride-co-trifluoroethylene) P(VDF-TrFE) has attracted great interest due to its ability to be utilized as a matrix binder capable of producing modified ignition sensitivity in energetic systems. While there have been studies on the combustion of fluoropolymer/aluminum systems, there is still a shortage of knowledge on what roles the electromechanical properties of the piezoelectric P(VDF-TrFE) and nano-aluminum (nAl) composite films play in the processes leading to their ignition upon a mechanical impact. To help bridge this gap, we conducted experiments and computational simulations to elucidate the underlying electromechanical properties that the films exhibit and to quantify the time duration it takes to commence ignition (i.e., ignition time). Based on our systematic assessment, we conclude that both piezoelectricity and flexoelectricity of the P(VDF-TrFE) can influence the ignition sensitivity as measured by the ignition time by locally enhancing the electric field near the nAl particles (by a factor of ~6.0) beyond the binder’s breakdown strength, resulting in concentrated channels of heat dissipation and ultimately ignition reactions. This suggests that the piezoelectric effect can catalyze the ignition process. The effect of poling films was also investigated by comparing how the sensitivity of the poled films differs from that of the unpoled films, thereby offering a mechanism to tune the ignition sensitivity by varying the level of piezoelectricity in the films. Results indicate that poling the films can enhance sensitivity by decreasing the minimum ignition energy (MIE) by 8%.</p><p dir="ltr">Additionally, we employed a mini drop-weight and shaker setup to investigate the response of the films to pressure and vibrations. Our findings demonstrate that these piezo-energetic films can replicate the behavior of a commercial PVDF gauge at relatively low-pressure impacts, indicating their potential use as shock or pressure sensors in various fields, as well as an accelerometer gauge. Furthermore, aging studies of up to one year indicated minimal loss in the energetic content of the created films, enabling the use of energetic gauges for an extended period. Our findings support the effectiveness of piezo-energetic composite films as pressure sensors or accelerometers and highlight their potential for energetic applications.</p><p dir="ltr">Solid propellants are used in propulsion systems for their high performance, feasibility to manufacture, long shelf life, and ease of storage and handling; however, they are limited by the inability to actively control the burning rate. The overall burning rate of solid propellant is predetermined based on geometry (surface area) and initial conditions such as temperature and pressure. This work proposes two methods to tailor the effective burning rate of solid composite ammonium perchlorate/hydroxyl-terminated polybutadiene (AP/HTPB) propellants using embedded shape memory alloy (SMA) and Nichrome^TM wires through Ohmic heating. First, Nitinol^TM wires, coiled into a spring, are demonstrated to increase the burning surface area upon expanding inside of the solid propellant. The result of this geometry modification was shown to increase the rate of pressurization (dP/dt) by over 180%, thereby throttling the propellant sample. Second, Nichrome^TM wire was embedded in the samples to demonstrate varying the heat flux (between 24.4 kW/m2 and 153.6 kW/m2) also affects the rate of pressurization under heated conditions. Preheating the solid propellant in this manner increased the rate of pressurization from 69 MPa/s to 110 MPa/s. This concept is useful for many applications such as in electro-explosive devices (EEDs) or actuators where it is desired to have the resulting gas expansion rate tailored.</p>

<p dir="ltr">Energetic materials are substances with considerable amounts of energy that can detonate under shock, pressure, or high temperature conditions, making them acceptable candidates for applications such as explosives, propellants, and fuels. One example of an energetic material is the explosive known as cyclotetramethylene tetranitramine (HMX). When subjected to impact, HMX can undergo thermo-mechanical responses that may lead to deflagration or, in the most severe cases, detonation. Due to the multiscale nature of these phenomena and the varying impact velocity magnitudes, replicating such responses can be challenging or even unattainable in an experimental setting. Consequently, computational models capable of predicting real-world conditions beyond experimental reach are highly valuable to the explosives research community.</p><p dir="ltr">This study continues the work from previous analyses (Duarte 2021) by developing a finite element model of HMX combined with an aluminum rod, predicting damage evolution and dynamic response under shock compression. The impact velocities applied in the model ranged from 0.1 km/s to 0.6 km/s using three different crystal orientations to investigate their corresponding effects. The results indicate that impacting in the direction normal to the HMX plane [110], which exhibited high levels of plastic energy, had the most resistant to cracking near the HMX-aluminum interface. Furthermore, these findings show that elastic energy accumulation is the primary driver in this analysis of crack propagation and bulk damage in HMX crystals.</p><p dir="ltr">Additionally, the HMX and aluminum results were compared against two additional models: a homogeneous HMX sample without discontinuities and an HMX sample with a void in place of the aluminum rod. Comparisons of the models show that the most severe damage field occurs in the void model, while the shock wave accelerated through the aluminum rod but also decelerated significantly in the presence of a void due to wave refraction at traction free boundaries. These results provide another level of understanding into the role of material interfaces and voids in the dynamic response of HMX under shock loading. Experimental validation of these findings is recommended for future studies, assuming the conditions are feasible for testing.</p>

<p dir="ltr">Energetic materials are a special class of materials which are immensely useful across applications due to their high energy density. However, energetics present unique challenges in manufacturing, processing, and handling. Sensitivity depends on microstructural features (i.e., porosity) established during manufacturing processes, external stimulation (i.e., electrostatic discharge, shock), environmental conditions (i.e. humidity), etc. Improving control of microstructure with new, safe manufacturing techniques such as powder deposition could expand the capabilities of energetic materials and improve sensitivity. Industries such as pharmaceuticals and additive manufacturing routinely use vibration to control dispensing of fine powders, but this has not been applied often to energetics. This research uses a DC vibration mini motor and Luer-lock nozzle tips to investigate controlled dispensing of sugar, soda lime, and nylon powders as energetic surrogates or potential binders. Changes in powder flow due to powder characteristics (size, shape, density), orifice sizes (0.2-1.6 mm), nozzle geometry (tapered and blunt-end), and motor voltages (1-3.4V) were quantified with high-speed image data and novel image processing scripts. Free flowing powders (> 150 µm) formed natural bridges in nozzles 2-4x larger. Finer, more cohesive powders bridged across larger orifice diameters. Vibration was applied to toggle flow by disrupting bridging. Higher vibration voltages created erratic dispensing patterns, while lower motor voltages (< 2V) yielded smaller cone angles and cyclic behavior tied to the motor frequency. A mixture of sugar and nylon was dispensed, and partial segregation was observed over time. This research demonstrated the range of vibration-controlled deposition conditions applicable to energetic materials which are currently lacking in literature.</p>

<p>Raman spectroscopy is a molecular spectroscopy technique that uses monochromatic light to provide a fingerprint to identify structural components and chemical composition. Depending on the changes in the unit-cell parameters and volume under the application of stress and temperature, the Raman spectrum undergoes changes in the wavenumber of Raman-active modes that allow identification of sample characteristics. Due to the various advantage of mechanical Raman spectroscopy (MRS), the use of this technique in the characterization and modeling of chemical changes under stress and temperature have gained popularity. </p> <p> Quantitative information regarding the local behavior of interfaces in an inhomogeneous material during shock loading is limited due to challenges associated with time and spatial resolution. Recently, we have extended the use of MRS to high-strain rate experiments to capture the local thermomechanical response of mock energetic material and obtain material properties during shock wave propagation. This was achieved by developing a novel method for <i>in‑situ</i> measurement of the thermo‑mechanical response from mock energetic materials in a time‑resolved manner with 5 ns resolution providing an estimation on local pressure, temperature, strain rate, and local shock viscosity. The results show the solid to liquid phase transition of sucrose under shock compression. The viscous behavior of the binder was also characterized through measurement of shock viscosity at strain rates higher than 10⁶/s using microsphere impact experiments.</p> <p> This technique was further extended to perform Raman spectral imaging over a microscale domain of the sample with a nano-second resolution. This was achieved by developing a laser-array Raman spectral imaging technique where simultaneous deconvolution of Raman spectra over the sample domain was achieved and Raman spectral image was reconstructed on post-processing. We developed a Raman spectral imaging system using a laser array and analysis was performed over the interface of sucrose crystals bonded using an epoxy binder. This study provides the Raman spectra over the microstructure domain which enabled the detection of localized melting under shock compression. The distribution of shock pressure and temperature over the microstructure was obtained using mechanical Raman analysis. The study shows the effects of an actual interface on the propagation of shock waves where a higher dissipation of shock energy was observed compared to an ideal interface. This increase in shock dissipation is accompanied by a decrease in both the maximum temperature, as well as the maximum pressure within the microstructure during shock wave propagation.</p>

<p dir="ltr">The modern world faces many challenges related to sustainability, including the ability to make high-level decisions using a sustainability-oriented framework, a matter of increasing importance to the United States military with respect to energetic materials (EMs). Although a few pieces - process flowsheet optimization, life cycle assessment (LCA) studies, and the use of optimization tools to identify an option - have been studied and utilized, there exists no systematic approach that combines all these pieces to create a framework that allows for holistic decision making. This is especially true with EMs, other key critical materials, and new methods of manufacture. An interconnected framework for LCA-based decision making is developed and a tool based on this framework created for use with novel materials. The interconnected framework and tool are utilized in two case studies related to the manufacture of RDX- a lab-scale, batch-mode configuration and a simplified continuous-mode configuration, to determine the optimal reaction temperature.</p>

Structural energetic materials (SEM) are a class of multicomponent materials which may react under various conditions to release energy. Fragmentation and impact induced reaction are not well characterized phenomena in SEMs. The structural energetic systems under consideration here combine aluminum with one or more of the following: nickel, tantalum, tungsten, and/or zirconium. These metal+Al systems were formulated with powders and consolidated using explosive compaction or the gas dynamic cold spray process. Fragment size distributions of the indicated metal+Al systems were explored; mean fragment sizes were found to be smaller than those from homogeneous ductile metals at comparable strain rates, posing a reduced risk to innocent bystanders if used in munitions. Extensive interface failure was observed which suggested that the interface density of these systems was an important parameter in their fragmentation. Existing fragmentation models for ductile materials did not adequately capture the fragmentation behavior of the structural energetic materials in question. A correction was suggested to modify an existing fragmentation model to expand its applicability to structural energetic materials. Fragment data demonstrated that the structural energetic materials in question provided a significant mass of combustible fragments. The potential combustion enthalpy of these fragments was shown to be significant. Impact experiments were utilized to study impact induced reaction in the indicated metal+Al SEM systems. Mesoscale parametric simulations of these experiments indicated that the topology of the microstructure constituents, particularly the stronger phase(s), played a significant role in regulating impact induced reactions. Materials in which the hard phase was topologically connected were more likely to react at a lower impact velocity due to plastic deformation induced temperature increases. When a compliant matrix surrounded stronger, simply connected particles, the compliant matrix accommodated nearly all of the deformation, which limited plastic deformation induced temperature increases in the stronger particles and reduced reactivity. Decreased difference between the strength of the constituents in the material also increased reactivity. The results presented here demonstrate that the fragmentation and reaction of metal+Al structural energetic materials are influenced by composition, microstructure topology, interface density, and constituent mechanical properties.

Sous l'effet d'une forte impulsion mécanique, d'une chaleur très forte ou d'une décharge électrostatique, un explosif comme le TNT ou le RDX peut accidentellement être initié. L'énergie apportée à l'explosif est convertie en chaleur, appelée point-chaud, dans des endroits spécifiques, contenant des impuretés, bulles de gaz, pores ouverts ou autres hétérogénéités. La taille d'un point-chaud de quelques micromètres peut être déjà suffisante pour initier une déflagration ou même une détonation. En réduisant la taille des particules de l'explosif, la formation des points-chauds est empêchée conduisant à un matériau moins sensible. Au sein de ce travail, un procédé continu est développé, fondé sur le principe de la cristallisation-flash, et permettant la préparation de particules énergétiques submicroniques en quantité de plusieurs grammes. Le procédé repose sur une opération de séchage par atomisation, au cours de laquelle une solution surchauffée est atomisée d'une manière continue. Afin de diminuer la taille moyenne des particules et d'obtenir une distribution de taille des particules très étroite, une étude paramétrique est réalisée. Au moyen de la cristallisation-flash, la préparation de composites énergétiques de haute qualité en grandes quantités est un succès. La qualité et quantité de ce composite énergétique sont uniques. Grâce au potentiel de ce procédé, la cristallisation-flash peut permettre la préparation de nombreuses substances et compositions énergétiques ou inertes / High explosives, such as TNT or RDX, may be accidentally initiated under the influence of a strong mechanical impulse, great heat or an electrostatic discharge. Smallest impurities, open pores, entrapped gases or other inhomogeneities within the explosive matrix may convert the delivered energy into heat, causing the formation of a so called hot-spot. A hot-spot size of a few micrometers can already be sufficient to initiate a deflagration or even a detonation of the explosive. By decreasing the particle size of the explosive, the formation of hot-spots is inhibited, resulting in a less sensitive material. In this work, a continuous operating flash-crystallization process was developed, being able to produce energetic submicron particles in a multigram scale. The process bases on a spray drying process where superheated solutions are continuously atomized. A parametric study was performed on this process in order to decrease the particle size and obtaining a narrower particle size distribution. By means of this flash-crystallization process, highly homogeneous energetic composites were prepared in a large scale. The quality and amount of the energetic composite are unique. The versatility of the flash-crystallization process allows the preparation of a large number of energetic and inert substances and compositions

In this study, dynamic crack propagation in brittle materials has been studied using a regularized phase field approach.The phase field model used has been validated using specific experimental results of a dynamic in-plane fracture. The crack branching phenomena and existence of a limiting crack tip velocity has been validated using a mode I simulation set-up. A parametric study has also been performed so as to normalize the various numerical parameters that affect the velocity at the crack tip. Following the validation of the phase field model a stochastic analysis of a PBX microstructure has been performed. The microstructure has a high HMX volume fraction of 79\%. The energetic material is HMX and the binder used is Sylgard. Artificial defects are introduced in the system using phase field cracks. The analysis uses a finite element framework that accounts for various thermal-mechanical processes like deformation, heat generation, conduction, fracture and frictional heating at the crack surfaces. The effect on the temperature and damage field due to varying parameters like loading velocities and critical energy release rates is studied. Critical hotspot formation due to localized frictional heating is also studied. A concept of dirty binder is introduced to increase the grain volume fraction of the energetic in the composite. This amounts to a homogenized binder that accounts for the influence of the subsume particles that do not contribute to fracture but affect material properties of the binder.

<div>A combined experimental and computational study has been performed in order to understand the effect of interface chemical composition on the shock induced mechanical behavior of an energetic material (EM) system consisting of Hydroxyl-Terminated Polybutadiene (HTPB) binder and an oxidizer, Ammonium Perchlorate (AP), particle embedded in the binder. The current study focuses on the effect of interface chemical composition between the HTPB binder material and the AP particles on the high strain rate mechanical behavior. The HTPB-AP interface chemical composition was changed by adding cyanoethylated polyamine (HX-878 or Tepanol) as a binding agent. A power law viscoplastic constitutive model was fitted to nanoscale impact based experimental stress-strain-strain rate data in order to obtain the constitutive behavior of the HTPBAP interfaces, AP particle, and HTPB binder matrix. An in-situ mechanical Raman spectroscopy framework was used to analyze the effect of binding agent on cohesive separation properties of the HTPB-AP interfaces, AP particle, and HTPB binder matrix. In addition, a combined mechanical Raman spectroscopy and laser impact set up was used to study the effect of strain rate, as well as the interface chemical composition on the interface shock viscosity. Finally, high velocity strain rate impact simulations were performed using an explicit cohesive finite element method framework to predict the effect of strain rate, interface strength, interface friction, and interface shock viscosity on possible strain rate dependent temperature rises at high strain rates approaching shock velocities. </div><div><br></div><div>A modified stress equation was used in the cohesive finite element framework in order to include the effect of shock viscosity on the shock wave rise time and shock pressure during impact loading with strain rates corresponding to shock impact velocities. It is shown that increasing the interface shock viscosity, which can be altered by changing the interface chemical composition, increases the shock wave rise time at the analyzed interfaces. It is shown that the interface shock viscosity also plays an important role in determining the temperature increase within the microstructure. Interface shock viscosity leads to a decrease in the overall density of the possible hot-spots which is caused by the increase in dissipation at the shock front. This increase in shock dissipation is accompanied by a decrease in the both the maximum temperature, as well as the plastic dissipation energy, within the microstructure during shock loading.</div>