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The field of multifunctional energetics encompasses a range of materials including propellants, explosives, and pyrotechnics that possess the ability to be manipulated through various characteristics that can be switched between go and no-go, or those that have controllable energy release levels or have additional functions beyond energetic output. The development of multifunctional energetics harnessing electromechanical or piezoelectric properties of polymeric materials or binder systems has garnered increasing interest in recent years. Among polymers, fluoropolymers such as poly(vinylidene fluoride) (PVDF) and copolymers such as poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], which are used as the binder and oxidizer in the energetic formulations, have demonstrated the highest piezoelectric coefficients. In this study, we fabricated piezo-energetic composite films using aluminum nanopowders (10 wt. % active content) as fuel and P(VDF-TrFE) (70/30) as an oxidizer and investigated the piezoelectric response using a small-scale drop weight setup. Additionally, we employed a shaker setup to investigate the response of the films to 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. Additionally, 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. [ABSTRACT FROM AUTHOR]

We investigated the relationship between porosity and flexoelectricity for aluminum (Al)/polyvinylidene fluoride (PVDF) composites. Neat PVDF, composites of micron aluminum (μAl)/PVDF, and composites of nano aluminum (nAl)/PVDF were 3D printed, and the flexoelectric response was measured using a cantilever beam test setup. Voids (up to 72.4 mm3) were incorporated into the samples by decreasing the infill percent of the 3D printed material. We found that increasing the porosity via millimeter scale voids incorporated into the infill pattern decreased the average effective flexoelectric coefficient relative to the near full-density (100% infill) control samples. This contrasts with other studies that have shown increasing micron scale porosity increases the flexoelectric coefficient. In addition, we measured higher flexoelectric responses for nAl/PVDF than μAl/PVDF as well as for samples printed by the Hyrel 3D SR printer as opposed to the Ender 3 V2 printer. These results indicate that charge generation due to flexoelectricity can be altered by changing parameters such as porosity, particle size of inclusions, or manufacturing method. Smaller voids and fine particles can induce larger strain gradients than larger inhomogeneities, leading to increased flexoelectric coefficients. A competing effect is that more porosity leads to less materials, which can decrease the flexoelectric coefficient. [ABSTRACT FROM AUTHOR]

Burning rate as a function of pressure is one of the primary evaluation metrics of solid propellants. Most solid propellant burning rate measurements are made at a nearly constant pressure using a variety of measurement approaches. This type of burning rate data is highly discretized and requires many tests to accurately determine the burning rate response to pressure. It would be more efficient to measure burning rate dynamically as pressures are varied. Techniques used to make transient burning rate measurements are reviewed briefly and initial results using a microwave interferometry (MI) technique are presented. The MI method used in tandem with a closed bomb enables nearly continuous measurement of burning rates for self‐pressurizing burns, capturing burning rate data over a wide range of pressures. This approach is especially useful for characterization of propellants with complex burning behaviors (e. g., slope breaks or mesa burning). The burning rates of three research propellants were characterized over a pressure range of 0.101–24.14 MPa (14–3500 psi). One research propellant exhibited a slope break at a pressure of 6.63 MPa (960 psi). Using MI in a closed pressure vessel, 14 propellant strand burns resulted in a nearly continuous burning rate curve over a pressure range of 0.41–24.13 MPa (60–3500 psi) that reasonably matched conventional burning rate measurements. The development of this technique provides an opportunity to quickly characterize the burning rate curve of solid propellants with greater fidelity and efficiency than traditional quasi‐static pressure testing techniques. [ABSTRACT FROM AUTHOR]

Inhomogeneities within explosives affect the sensitivity and detonation waveshape of energetic materials. The influence of voids on explosive initiation has been well documented; however, the effects that voids between 0.1 mm and 10 mm have on a propagating detonation wave remains largely unexplored. The effect of single cylindrical voids on detonation waveshape and re‐initiation was examined here using manufactured voids in a rubberized 1,3,5‐trinitro‐1,3,5‐triazinane (RDX) explosive. Two streak imaging techniques were fielded to investigate void influence. For the first, back‐surface streak imaging, the location of the void on the samples was changed and the resulting change in detonation waveshape at the downstream breakout was captured using a streak camera in cut‐back experiments. The results from this experiment showed the effects of an initial jet form for short cut‐back distances and as shock propagation progressed, the jet formation was absorbed by the unaffected portions of the wave front. The second method, top‐surface streak imaging, was used to investigate the re‐initiation/downstream propagation of the detonation front and the detonation velocity of the rubberized explosive. These experiments were compared to similar experimental results from machined voids in PBX 9501, an 1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane (HMX)‐based explosive, to investigate the interaction of a detonation wave with a 0.5 mm void for different explosives. The experiments were also compared to simulations using a multi‐dimensional and multi‐material hydrodynamic code. These results showed the influence that small features can have on detonation waveshaping and how explosive properties play a key role in that interaction. [ABSTRACT FROM AUTHOR]

Using multiphysics simulations and experiments, we demonstrate that dielectric breakdown due to electric charge accumulation can lead to sufficient hotspot development leading to the initiation of chemical reactions in P(VDF-TrFE)/nAl films comprising a poly(vinylidene fluoride-co-trifluoroethylene) binder and nano-aluminum particles. The electric field (E-field) development in the material is driven by the flexoelectric and piezoelectric responses of the polymer binder to mechanical loading. A two-step sequential multi-timescale and multi-physics framework for explicit microscale computational simulations of experiments is developed and used. First, the mechanically driven E-field development is analyzed using a fully coupled mechanical–electrostatic model over the microsecond timescale. Subsequently, the transient dielectric breakdown process is analyzed using a thermal–electrodynamic model over the nanosecond timescale. The temperature field resulting from the breakdown is analyzed to establish the hotspot conditions for the onset of self-sustained chemical reactions. The results demonstrate that temperatures well above the ignition temperatures can be generated. Both experiments and analyses show that flexoelectricity plays a primary role and piezoelectricity plays a secondary role. In particular, the time to ignition and the time to pre-ignition reactions of poled films (possessing both piezoelectricity and flexoelectricity) are ∼10% shorter than those of unpoled films (possessing only flexoelectricity). [ABSTRACT FROM AUTHOR]

Nanothermite NiO–Al is a promising material system for low gas emission heat sources; yet, its reactive properties are highly dependent on material processing conditions. In the current study, sputter deposition is used to fabricate highly controlled nanolaminates comprised of alternating NiO and Al layers. Films having an overall stoichiometry of 2Al to 3NiO were produced with different bilayer thicknesses to investigate how ignition and self-sustained, high temperature reactions vary with changes to nanometer-scale periodicity and preheat conditions. Ignition studies were carried out with both hot plate and laser irradiation and compared to slow heating studies in hot-stage x-ray diffraction. Ignition behavior has bilayer thickness and heating rate dependencies. The 2Al/3NiO with λ ≤ 300 nm ignited via solid/solid diffusion mixing (activation energy, Ea = 49 ± 3 kJ/mole). Multilayers having λ≥ 500 nm required a more favorable mixing kinetics of solid/liquid dissolution into molten Al (Ea = 30 ± 4 kJ/mole). This solid/liquid dissolution Ea is a factor of 5 lower than that of the previously reported powder compacts due to the elimination of a passivating Al oxide layer present on the powder. The reactant mixing mechanism between 300 and 500 nm bilayer thicknesses was dependent on the ignition source's heating rate. The self-propagating reaction velocities of 2Al/3NiO multilayers varied from 0.4 to 2.5 m/s. Pre-heating nanolaminates to temperatures below the onset reaction temperatures associated with forming intermediate nickel aluminides at multilayer interfaces led to increased propagation velocities, whereas pre-heating samples above the onset temperatures inhibited subsequent attempts at laser ignition. [ABSTRACT FROM AUTHOR]

We prepared poly(vinylidene fluoride) (PVDF) and aluminum composite dense films using a tape caster and investigated the microstructural, thermal, and electrical properties and the combustion behavior of the films as a function of nano-aluminum (nAl) solids loading (5–30 wt. %). We found that the addition of nAl facilitates the formation of piezoelectric β and γ phases of PVDF as determined by x-ray diffraction and Fourier transform infrared spectroscopy. At higher nAl solid loadings, the lower onset temperature of the pre-ignition and decomposition reactions have been observed. Moreover, the intentionally incorporated porosity into the films slightly affected the thermal decomposition behavior. While the dielectric constant of the films increases with higher nAl content, the dielectric breakdown strength of the films decreases significantly. The critical active nAl content for the films to exhibit self-propagated reaction was determined to be between 10 and 15 wt. %. Thermochemical calculations using the NASA CEA code showed the maximum flame temperature of 1750 °C near the stoichiometric ratio (∼20 wt. %). The burning rate of the films is enhanced drastically at ambient conditions with further addition of nAl. However, the films with active nAl content over 20 wt. % showed lower flame temperatures, which is due to the reduction of hydrofluoric acid gas generation and the incomplete combustion of Al to form aluminum monofluoride (AlF), instead of aluminum fluoride (AlF3) gas. The fabrication of energetic thin films with tunable properties could enable their use in multifunctional energetic material systems. [ABSTRACT FROM AUTHOR]

Electroactivity of polymers used in energetic materials may result in charge separation that could result in safety concerns (unintentional ignition) or be exploited for multifunctional applications. We measured the flexoelectric properties of several polymers and energetic composites including poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)], nanosized aluminum (nAl)/P(VDF-TrFE), poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)], micron aluminum (μAl)/P(VDF-HFP), hydroxyl-terminated polybutadiene (HTPB), ammonium perchlorate (AP)/HTPB, μAl/AP/HTPB, polytetrafluoroethylene (PTFE), and polydimethylsiloxane (PDMS). The presence of flexoelectricity in PTFE (Teflon®) and the relatively high flexoelectric coefficient of P(VDF-HFP) (Viton®) measured in this work may help explain accidents involving the production and use of Magnesium-Teflon-Viton (MTV) that in many instances have been attributed to electro-static discharge. The addition of aluminum nanopowders to the P(VDF-TrFE) increased the flexoelectric coefficient by ∼30%. However, the addition of aluminum micrometer particles (10 wt. %) to P(VDF-HFP) decreased the effective flexoelectric coefficient, while an increase was observed when the aluminum loading was increased from 10 to 20 wt. %. The effective flexoelectric coefficient of HTPB and two propellant compositions (AP/HTPB and μAl/AP/HTPB) were measured to be in the same range as each other. The effect of particle addition (nAl, μAl, and AP) on flexoelectricity was different depending on the binder, further illustrating the complexity of flexoelectric properties in composite energetics. This may be somewhat explained by competing effects where particle additions (nAl, μAl, and AP) create additional strain gradients that contribute to flexoelectricity, but the particle additions also replace the mass of flexoelectric polymer binders (P(VDF-TrFE, P(VDF-HFP), and HTPB) with particles (nAl, μAl, and AP) that are less flexoelectric. [ABSTRACT FROM AUTHOR]

The favorable properties of aluminum nanoparticles have led to their application in areas ranging from propellants and explosives to plasmonics. In the area of energetics research, where the heating of aluminum (Al) nanoparticles (nAl) is involved, producing materials that maximize heat flow with a low onset temperature is a sought-after goal. This study examined the ability of gallium (Ga) to increase the reactivity of nAl. Three Al/Ga composite nanoparticles were formed by the addition of Ga salt (GaCl3) at different time points during the nAl synthesis. The timing of Ga addition was found to correlate with the quantity of Ga present in the resulting composite with earlier addition yielding a higher Ga concentration. The thermal behavior of the Al/Ga composites was assessed by using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The Al/Ga composite material produced with early Ga addition produced particles with a TGA trace showing an early onset exothermic peak near 480 °C. This coincided with a peak seen at this temperature in the DSC trace for Ga nanoparticles (nGa). Complete oxidization of this material was achieved below 600 °C, while achieving a higher maximum heat flow, a significant improvement over that of pure nAl. Experiments suggest the reduction of gallium (Ga3+) to neutral metal Ga0, whose presence was detected spectroscopically in the resulting product concentrated between the active Al core and the exterior aluminum oxide shell. [ABSTRACT FROM AUTHOR]

Optical sources such as lasers provide a means for precise temporal and spatial control of the ignition of energetic materials through customized deposition of excitation energy. Using coupled microscale electromagnetic (EM) and thermal simulations, we analyze the interactions between the EM waves and the microstructures of PVDF/nAl composites with weight fractions of nAl particles (or solids loadings), wf, ranging from 10 to 40 wt. %. Statistically equivalent microstructure sample sets with multiple random microstructure instantiations are generated and used for each solid loading, thereby allowing the statistical variations in the material heating behavior due to microstructure randomness to be analyzed. Maxwell’s equations are solved to characterize the interactions between the materials and EM waves at wavelengths of 266, 532, and 1064 nm. The resulting energy deposition rate is calculated, accounting for Joule heating, dielectric heating, and magnetic induction heating. The coupled thermal analysis accounts for the energy deposition and thermal diffusion, yielding the temperature fields in the materials. The energy deposition and heating are characterized using three measures: the skin depth of the EM wave, the depth of the significant temperature increase in the material, and the average temperature. An empirical relation is developed for the average temperature increase in the heated layer of the material as a function of the intensity of the input laser, solids loading, and time. It is found that trends in the average temperature and depth of significant heating correlate well with the ignition trends observed in experiments. [ABSTRACT FROM AUTHOR]

The influence on 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX) shock sensitivity resulting from damage due to phase transition versus that of the polymorphic conformer and crystal lattice change in heated HMX are explored in this work. Samples of class III HMX at 1.24 g/cm3 are shocked in a modified gap test, which yields quantitative, rather than 'go/no-go,' results. Microstructural characterization of similarly prepared samples is used to draw conclusions about the primary driver of the difference in sensitivity between pristine β -HMX, thermally induced δ -HMX, and HMX that has spontaneously reverted from δ → β -HMX. It is found, surprisingly, that large crystals of HMX incur a slight decrease in shock sensitivity after undergoing δ -phase transition and/or spontaneous reversion from δ → β -HMX. The difference between shock sensitivity results of a significantly smaller HMX particle size distribution are inconclusive using this test configuration. Further testing of the small particle size distribution using a different experimental design that allows lower input pressures may be more useful for comparison. The results of this study indicate that while the increased crack cross-sectional area in HMX crystals drives an increase in impact sensitivity, it drives a decrease in sensitivity in the shock regime for the conditions studied. [ABSTRACT FROM AUTHOR]

Internal features of varying scale and geometry are always present in explosives systems. Below a critical length, dependent on the explosive, these features can operate as a driving force for energy concentration and reaction, known as hotspots. At larger length scales, internal features result in jetting and detonation wave shaping, allowing for bulk work to be done by the explosive as is seen in shape charges. To date, a large volume of work has been performed to simulate hotspot ignition and large‐scale wave shaping. However, little experimental data exists on the effects of features in intermediate length scales, 0.1 to 1 millimeter. It has been observed in many tests that these small‐scale features can influence the high explosive (HE) performance and in some cases cause substantial damage to adjacent systems. This work provides quantitative data on the effects of machined voids moderately above more typical hotspot lengths, 0.3–0.8 mm in diameter, in PBX 9501 pressed to 1.785 g cm−3±2.5 mg cm−3. Streak imaging was used to visualize void collapse, jet velocity, re‐initiation time, and wave shape evolution. Delay in detonation front propagation time was found to be linearly dependent on the void diameter and jet velocity was found to be independent within the tested range and resolution. Cut‐back experiments were used to investigate wave shape distortion and evolution downstream of the void, showing consistent growth and decay shapes across all void sizes. Simulations using CTH, a hydrocode by Sandia National Laboratory, were used to investigate void collapse showing agreement with the trends of experimental results but yielded inaccurate wave shape development delay values. Both simulation and experimental results identified several re‐initiation mechanisms with jetting and subsequent double shocking of localized HE being the dominant mechanisms for the void sizes that were studied. [ABSTRACT FROM AUTHOR]

When formulating a new solid propellant, one of the most important aspects of its performance is the burning rate's response to a change in pressure. For this reason, it is useful to be able to predict the burning rate response of a given propellant before the propellant formulation is created such that experimental trade studies are minimized or reduced in scale. While many theoretical and phenomenological models have been proposed to explain various aspects of energetic material combustion, little work has been made publicly available in the application of machine learning models to predicting solid propellant burning rates. To facilitate model creation, the material formulation and burning rate parameters for over 600 publicly available propellant formulations have been collected into a coherent data set. This work utilizes the large amount of publicly available data to inform a random forest machine learning (ML) model in the prediction of solid propellant burning rate parameters. This ML model operates over a large parameter space including ammonium perchlorate composite, plastic bonded high explosive, and double‐base propellants. The model's accuracy, adaptability, and prediction capabilities are presented and discussed. The effects of different materials on a propellant's expected burning rate are examined. [ABSTRACT FROM AUTHOR]