Understanding the manner in which vibrational energy flows between molecular and lattice vibrations is of great interest in physical chemistry due to its central role in reactivity and energy dissipation in molecular materials. In this feature article, we highlight our recent efforts employing ultrafast broadband infrared spectroscopy toward understanding the interplay between molecular and lattice vibrations in energetic materials, motivated by the open questions surrounding the role of vibrational energy transfer (VET) in reaction initiation in these materials. Our work addresses the ongoing debate on the participation of doorway modes in VET. We further present new results from high-pressure ultrafast experiments on RDX, a hydrogen-bonded material, and BNFF, a hydrogen-free material, to explore how intermolecular interaction strength governs VET pathways and time scales. Collectively, our findings reveal that vibrational dynamics in these systems occurs across three distinct time regimes, with VET being incomplete out to hundreds of picoseconds, suggesting the importance of considering nonstatistical reactions in the modeling of these materials. These time scales vary as intermolecular interaction strength is indirectly modified by application of static pressure, indicating dramatic changes to the vibrational structure of these materials under shock-relevant conditions. Our results thus shed light on how intermolecular interactions shape vibrational energy redistribution in molecular materials, and highlight the need for further theoretical and experimental investigation.

Energy conversion in energetic materials from shock-wave-induced lattice compression to bond breaking critically depends on vibrational coupling and energy transfer between intra- and intermolecular vibrations, though the details of the mechanisms remain unknown. Herein, we indirectly tune the strength of intermolecular interactions in 3,4-bis(3-nitrofurazan-4-yl)furoxan (BNFF), a hydrogen-free energetic material characterized by van der Waals interactions, by applying high static pressure using a diamond anvil cell and monitoring vibrational energy transfer (VET) with ultrafast broadband infrared pump-probe spectroscopy. As BNFF is compressed from ambient pressure to 9 GPa, we find that VET accelerates by ∼ 0.9 ps/GPa. Density functional theory is applied in tandem with experiments to assign mode character and elucidate VET pathways. We find that furazan ring O-N-O vibrations, which are high-frequency detonation-relevant vibrational modes, experience increased sensitivity to lattice compression under shockwave pressures. These findings provide new mechanistic insight into how intermolecular interactions govern the rate and selectivity of VET.

Understanding the interfacial properties of high energy explosives blended with polymer binders is important for optimizing material performance and predicting aging. In the case of PBX 9502, direct measurements of the TATB - Kel-F 800 interface have been limited by the lack of high-fidelity model systems. This paper reports the fabrication of a new thin film, model TATB sample to enable neutron reflectometry measurements of this interface. The fabricated TATB films were characterized using X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The film deposits in the same polymorph as the TATB crystallites in PBX 9502, but in the form of highly textured nanopillars that are similar to 200 nm wide. The buried interface of the TATB film (i.e., bottoms of the pillars) has an rms roughness of similar to 3 nm and is the (001) crystal face. The TATB films retain their features when deposited on Kel-F 800 thin films, enabling the formation of layered PBX 9502 model samples. Neutron reflectometry measurements showed increasing roughness of the TATB - Kel-F 800 interface with thermal aging, consistent with void formation at this interface.

Aging of energetic materials may change performance and affect their safety and reliability, but the relationship between microstructure changes induced by aging and consequent performance changes has not been fully established. This work presents results of phase-field method simulations used to model microstructure evolution of vapor-deposited pentaerythritol tetranitrate (PETN) thin films. Simulated aging is shown to induce grain coarsening and substantial changes of the configuration of porosity in the film: Specifically, we show that porosity tends to concentrate in large pores to a greater degree in aged films, a state that is arrived at by closure or consolidation of small pores. To evaluate the performance of the as-deposited and aged films, we perform two-dimensional hydrocode flyer-film impact simulations that incorporate the phase-field output microstructures directly, permitting us to connect features therein to changes in reactivity, a key metric of energy output for shock initiation. The results demonstrate that declining sensitivity obtained for the simulated aged films can be correlated with the loss of fine-structured pores relatively early in the aging process, while long-term microstructure evolution that gradually alters the shape of large, branching pores is less impactful. Finally, we discuss commonalities and discrepancies between our simulation results and high-throughput initiation experiments on shock initiation of aged PETN thin films.

Understanding the thermophysical properties and fracture mechanics of energetic materials is critical for ensuring safety and reliability during manufacturing, transport, and operation. In this work, we utilized digital image correlation (DIC) at high magnification to study the thermal expansion of energetic materials and simulants. The experimental setup and DIC algorithm was validated by measuring thermal expansion of common materials over a range of temperatures. Coefficients of thermal expansion (CTEs) obtained from DIC strain calculations for each material agreed well with available literature values. The validated high magnification experimental setup was then used to measure the CTE of TKP pellets, pentaerythritol crystals (PE), and pentaerythritol tetranitrate crystals (PETN). Additionally, crack formation in single crystals of sucrose and PETN were also imaged with the experimental setup and analyzed with DIC. Overall, this work demonstrates how this robust diagnostic can be used to study thermophysical properties and fracture mechanics in energetic materials. Not only will this contribute to a better understanding of the operation of energetic materials, but also to a better understanding of the effects of aging on the structure and performance.

Explosive detonation products equation-of-state (EoS) models are typically derived from experimental data on explosive acceleration of metal. The cylinder expansion test has served as the standard method for decades to obtain this information. Recently, the Disk Acceleration eXperiment (DAX) has been developed by researchers at Lawrence Livermore National Laboratory (LLNL) to compliment the cylinder expansion test and provide data on metal acceleration with less explosive. We present experimental work on an explosive formulation of CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) with PDMS (poly(dimethyl siloxane), Sylgard (R) 182) using DAX to measure the detonation velocity and metal disk acceleration performance of this explosive.

Vapor-deposited explosive films with variable substrate interfacial roughness were studied through combined detonation thickness experiments and 3D simulations using Sandia National Laboratories' hydrocode, CTH. Prior computational work assumed smooth interfaces in 2D and investigated the detonation velocity as a function of film thickness. However, lack of interfacial roughness and the 2D geometry limited the applicability for model comparisons with experiments, which show that non-ideal interfaces have a significant impact on the detonation behavior. The current computational effort leverages a novel capability in CTH for voxel-map import of an image stack, e.g., containing a 3D rendering of a non-ideal interface from analytical equations or a profilometer scan. Here, the effects of the roughness intensity between the vapor-deposited film and inert substrate are explored, and the results show how non-ideal 3D interfaces increase the failure thickness of an explosive film. Additionally, the shock front curvature is analyzed for each type of interface, and parallels are drawn to Detonation Shock Dynamics theory. Finally, the concept of transverse kinetic energy is proposed as a potential loss mechanism contributing to the detonation velocity deficit. Overall, this work analyzes the effects of individual contributions to the detonation velocity of unconfined vapor-deposited explosives, and it proposes a mechanism to explain the observed behaviors that have so far remained elusive to model.

Understanding early chemical reaction steps in detonation environments is critical to the development of robust detonation theories and non-phenomenological models. Towards this end, we present initiation studies of 3,4-bis(3-nitrofurazan4-yl)furoxan (BNFF), a hydrogen-free explosive with the chemical formula C(6)N(8)0(8). BNFF is of interest for its unique chemistry, high detonation temperature, and formation of nanoscale carbon structures upon detonation. These studies are performed on Sandia's High Throughput Initiation (HTI) experimental platform, using laser-driven flyer plates to rapidly investigate the reaction threshold of vapor-deposited BNFF samples as a function of thickness using photonic Doppler velocimetry diagnostics. Initial cutback style experiments, where the thickness of the sample is reduced in order to see how far along a reaction is at a given distance into the explosive, indicate that BNFF is sub-detonative at a depth of 25 mu m up to our upper limit of impact velocity at around 4200 m/s using a 25 mu m thick Parylene C flyer. When thicker films, 100 150 mu m thick, are impacted, growth to detonation begins to occur more promptly at impact velocities below 3000 m/s. Future work will involve incorporation of streak spectroscopy into the HTI platform for emission spectroscopy to further elucidate initial chemistry.