In situ measurement of damage evolution in shocked magnesium as a function of microstructure.

Accurate modeling and prediction of damage induced by dynamic loading in materials have long proved to be a difficult task. Examination of postmortem recovered samples cannot capture the time-dependent evolution of void nucleation and growth, and attempts at analytical models are hindered by the necessity to make simplifying assumptions, because of the lack of high-resolution, in situ, time-resolved experimental data. We use absorption contrast imaging to directly image the time evolution of spall damage in metals at ∼1.6-µm spatial resolution. We observe a dependence of void distribution and size on time and microstructure. The insights gained from these data can be used to validate and improve dynamic damage prediction models, which have the potential to lead to the design of superior damage-resistant materials.

A MHz X-ray diffraction set-up for dynamic compression experiments in the diamond anvil cell.

An experimental platform for dynamic diamond anvil cell (dDAC) research has been developed at the High Energy Density (HED) Instrument at the European X-ray Free Electron Laser (European XFEL). Advantage was taken of the high repetition rate of the European XFEL (up to 4.5 MHz) to collect pulse-resolved MHz X-ray diffraction data from samples as they are dynamically compressed at intermediate strain rates (≤103 s-1), where up to 352 diffraction images can be collected from a single pulse train. The set-up employs piezo-driven dDACs capable of compressing samples in ≥340 µs, compatible with the maximum length of the pulse train (550 µs). Results from rapid compression experiments on a wide range of sample systems with different X-ray scattering powers are presented. A maximum compression rate of 87 TPa s-1 was observed during the fast compression of Au, while a strain rate of ∼1100 s-1 was achieved during the rapid compression of N2 at 23 TPa s-1.

Supporting data for impact of filler composition on mechanical and dynamic response of 3-D printed silicone-based nanocomposite elastomers.

This research reports on the physical and mechanical effects of various filler materials used in direct ink write (DIW) 3-D printing resins. The data reported herein supports interpretation and discussion provided in the research article "Impact of Filler Composition on Mechanical and Dynamic Response of 3-D Printed Silicone-based Nanocomposite Elastomers" [1]. The datasheet describes the model structures and the interaction energies between the fillers and the other components by using Molecular Dynamics (MD) simulations. This report includes mechanical responses of single-cubic (SC) and face-centered tetragonal (FCT) structures printed using new DIW resin formulations (polydimethylsiloxane-based silicones filled with aluminum oxide, graphite, or titanium dioxide). Using MD simulations and mechanical data, the overall flexibility and interactions between resin components are fully characterized.

Shock-Driven Decomposition of Polymers and Polymeric Foams.

Polymers and foams are pervasive in everyday life, as well as in specialized contexts such as space exploration, industry, and defense. They are frequently subject to shock loading in the latter cases, and will chemically decompose to small molecule gases and carbon (soot) under loads of sufficient strength. We review a body of work-most of it performed at Los Alamos National Laboratory-on polymers and foams under extreme conditions. To provide some context, we begin with a brief review of basic concepts in shockwave physics, including features particular to transitions (chemical reaction or phase transition) entailing an abrupt reduction in volume. We then discuss chemical formulations and synthesis, as well as experimental platforms used to interrogate polymers under shock loading. A high-level summary of equations of state for polymers and their decomposition products is provided, and their application illustrated. We then present results including temperatures and product compositions, thresholds for reaction, wave profiles, and some peculiarities of traditional modeling approaches. We close with some thoughts regarding future work.

Shockwave compression and dissociation of ammonia gas.

We performed a series of plate impact experiments on NH3 gas initially at room temperature and at a pressure of ∼100 psi. Shocked states were determined by optical velocimetry and the temperatures by optical pyrometry, yielding compression ratios of ∼5-10 and second shock temperatures in excess of 7500 K. A first-principles statistical mechanical (thermochemical) approach that included chemical dissociation yielded reasonable agreement with experimental results on the principal Hugoniot, even with interparticle interactions neglected. Theoretical analysis of reshocked states, which predicts a significant degree of chemical dissociation, showed reasonable agreement with experimental data for higher temperature shots; however, reshock calculations required the use of interaction potentials. We rationalize the very different shock temperatures obtained, relative to previous results for argon, in terms of atomic versus molecular heat capacities.

Extended Lagrangian Born-Oppenheimer molecular dynamics simulations of the shock-induced chemistry of phenylacetylene.

The initial chemical events that occur during the shock compression of liquid phenylacetylene have been investigated using self-consistent tight binding molecular dynamics simulations. The extended Lagrangian Born-Oppenheimer molecular dynamics formalism enabled us to compute microcanonical trajectories with precise conservation of the total energy. Our simulations revealed that the first density-increasing step under shock compression arises from the polymerization of phenylacetylene molecules at the acetylene moiety. The application of electronic structure-based molecular dynamics with long-term conservation of the total energy enabled us to identify electronic signatures of reactivity via monitoring changes in the HOMO-LUMO gap, and to capture directly adiabatic shock heating, transient non-equilibrium states, and changes in temperature arising from exothermic chemistry in classical molecular dynamics trajectories.

Quantitative tradeoffs between spatial, temporal, and thermometric resolution of nonresonant Raman thermometry for dynamic experiments.

The ratio of Stokes to anti-Stokes nonresonant spontaneous Raman can provide an in situ thermometer that is noncontact, independent of any material specific parameters or calibrations, can be multiplexed spatially with line imaging, and can be time resolved for dynamic measurements. However, spontaneous Raman cross sections are very small, and thermometric measurements are often limited by the amount of laser energy that can be applied without damaging the sample or changing its temperature appreciably. In this paper, we quantitatively detail the tradeoff space between spatial, temporal, and thermometric accuracy measurable with spontaneous Raman. Theoretical estimates are pinned to experimental measurements to form realistic expectations of the resolution tradeoffs appropriate to various experiments. We consider the effects of signal to noise, collection efficiency, laser heating, pulsed laser ablation, and blackbody emission as limiting factors, provide formulae to help choose optimal conditions and provide estimates relevant to planning experiments along with concrete examples for single-shot measurements.

Intermolecular stabilization of 3,3'-diamino-4,4'-azoxyfurazan (DAAF) compressed to 20 GPa.

The room temperature stability of 3,3'-diamino-4,4'-azoxyfurazan (DAAF) has been investigated using synchrotron far-infrared, mid-infrared, Raman spectroscopy, and synchrotron X-ray diffraction (XRD) up to 20 GPa. The as-loaded DAAF samples exhibited subtle pressure-induced ordering phenomena (associated with positional disorder of the azoxy "O" atom) resulting in doubling of the a-axis, to form a superlattice similar to the low-temperature polymorph. Neither high pressure synchrotron XRD, nor high pressure infrared or Raman spectroscopies indicated the presence of structural phase transitions up to 20 GPa. Compression was accommodated in the unit cell by a reduction of the c-axis between the planar DAAF layers, distortion of the ß-angle of the monoclinic lattice, and an increase in intermolecular hydrogen bonding. Changes in the ring and -NH2 deformation modes and increased intermolecular hydrogen bonding interactions with compression suggest molecular reorganizations and electronic transitions at ∼ 5 GPa and ∼ 10 GPa that are accompanied by a shifting of the absorption band edge into the visible. A fourth-order Birch-Murnaghan fit to the room temperature isotherm afforded an estimate of the zero-pressure isothermal bulk modulus, K0 = 12.4 ± 0.6 GPa and its pressure derivative K0' = 7.7 ± 0.3.

High-pressure far-infrared spectroscopic studies of hydrogen bonding in formic acid.

Simple molecules such as HCOOH, or formic acid, are suggested to have played important roles in planetary physics due to their possibility for high pressure and temperature chemistry under impact conditions. In this study, we have investigated the effect of pressure (up to 50 GPa) on H-bonding and reactivity of formic acid using synchrotron far infrared spectroscopy. Based on the pressure-induced changes to H-bond ν(O-H···O) stretching and γ(O-H···O) deformations, we observe significant reorganization of H-bonding network beginning at ~20 GPa. This is in good agreement with reports of symmetrization of H-bonds reported at 16-21 GPa from X-ray diffraction and Raman spectroscopy studies as well as molecular dynamics simulations. With further increase in pressure, beyond 35 GPa, formic acid undergoes a polymerization process that is complete beyond 45 GPa. Remarkably, upon decompression, the polymeric phase reverts to the crystalline high-pressure phase at 8 GPa.

Non-invasive timing of gas gun-launched projectiles using external surface-mounted optical fiber-Bragg grating strain gauges.

Non-invasive detection methods for tracking gun-launched projectiles are important not only for assessment of gun performance but are also essential for timing a variety of diagnostics, for example, to investigate plate-impact events for shock compression experiments. Measurement of the time of passage of a projectile moving inside of the gun barrel can be achieved by detection of the transient hoop strain induced in the barrel of a light-gas gun by the passage of the projectile using external, barrel surface-mounted optical fiber-Bragg grating strain gauges. Optical fiber-Bragg gratings have been implemented and their response characterized on single-stage and two-stage light gas guns routinely used for dynamic experimentation at Los Alamos National Laboratory. Two approaches, using either broadband or narrowband illumination, were used to monitor changes in the Bragg wavelength of the fiber-Bragg gratings. The second approach, using narrowband laser illumination, offered the highest sensitivity. The feasibility of using these techniques to generate early, pre-event signals useful for triggering high-latency diagnostics was demonstrated.

The phase diagram of ammonium nitrate.

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH(4)NO(3)] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3-11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy.

Acoustic properties of Kel F-800 copolymer up to 85 GPa.

Acoustic properties of the fluorinated copolymer Kel F-800 were determined with Brillouin spectroscopy up to pressures of 85 GPa at 300 K. This research addresses outstanding issues in high-pressure polymer behavior, as to date the acoustic properties and equation of state of any polymer have not been determined above 20 GPa. We observed both longitudinal and transverse modes in all pressure domains, allowing us to calculate the C(11) and C(12) moduli, bulk, shear, and Young's moduli, and the density of Kel F-800 as a function of pressure. We found the behavior of the polymer with respect to all parameters to change drastically with pressure. As a result, we find that the data are best understood when split into two pressure regimes. At low pressures (less than ∼5 GPa), analysis of the room temperature isotherm with a semi-empirical equation of state yielded a zero-pressure bulk modulus K(o) and its derivative K(0) (') of 12.8 ± 0.8 GPa and 9.6 ± 0.7, respectively. The same analysis for the higher pressure data yielded values for K(o) and K(0) (') of 34.9 ± 1.7 GPa and 5.1 ± 0.1, respectively. We discuss this significant difference in behavior with reference to the concept of effective free volume collapse.

Electronic structure of the water oxidation catalyst cis,cis-[(bpy)2(H2O)Ru(III)ORu(III)(OH2)(bpy)2]4+, the blue dimer.

The first designed molecular catalyst for water oxidation is the "blue dimer", cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+). Although there is experimental evidence for extensive electronic coupling across the µ-oxo bridge, results of earlier DFT and CASSCF calculations provide a model with magnetic interactions of weak to moderately coupled Ru(III) ions across the µ-oxo bridge. We present the results of a comprehensive experimental investigation, combined with DFT calculations. The experiments demonstrate both that there is strong electronic coupling in the blue dimer and that its effects are profound. Experimental evidence has been obtained from molecular structures and key bond distances by XRD, electrochemically measured comproportionation constants for mixed-valence equilibria, temperature-dependent magnetism, chemical properties (solvent exchange, redox potentials, and pK(a) values), XPS binding energies, analysis of excitation-dependent resonance Raman profiles, and DFT analysis of electronic absorption spectra. The spectrum can be assigned based on a singlet ground state with specific hydrogen-bonding interactions with solvent molecules included. The results are in good agreement with available experimental data. The DFT analysis provides assignments for characteristic absorption bands in the near-IR and visible regions. Bridge-based dπ → dπ* and interconfiguration transitions at Ru(III) appear in the near-IR and MLCT and LMCT transitions in the visible. Reasonable values are also provided by DFT analysis for experimentally observed bond distances and redox potentials. The observed temperature-dependent magnetism of the blue dimer is consistent with a delocalized, diamagnetic singlet state (dπ(1)*)(2) with a low-lying, paramagnetic triplet state (dπ(1)*)(1)(dπ(2)*)(1). Systematic structural-magnetic-IR correlations are observed between ν(sym)(RuORu) and ν(asym)(RuORu) vibrational energies and magnetic properties in a series of ruthenium-based, µ-oxo-bridged complexes. Consistent with the DFT electronic structure model, bending along the Ru-O-Ru axis arises from a Jahn-Teller distortion with ∠Ru-O-Ru dictated by the distortion and electron-electron repulsion.

"Stubborn" triaminotrinitrobenzene: unusually high chemical stability of a molecular solid to 150 GPa.

We report an unexpectedly high chemical stability of molecular solid 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) under static high pressures. In contrast to the high-pressure behavior of the majority of molecular solids, TATB remains both chemically stable and an insulator to 150 GPa--well above the predicted metallization pressure of 120 GPa. Single crystal studies have shown that TATB exhibits pressure-induced Raman changes associated with two subtle structural phase transitions at 28 and 56 GPa. These phase transitions are accompanied by remarkable color changes, from yellow to orange and to dark red with increasing pressure. We suggest that the high-stability of TATB arises as a result of its hydrogen-bonded aromatic two-dimensional (2D) layered structure and highly repulsive interlayer interaction, hindering the formation of 3D networks or metallic states.

Pressure induced isostructural metastable phase transition of ammonium nitrate.

The energetic material ammonium nitrate (AN, NH(4)NO(3)) has been studied under both hydrostatic and nonhydrostatic conditions using diamond anvil cells combined with micro-Raman spectroscopy and synchrotron X-ray powder diffraction. The refined powder X-ray data indicates that under hydrostatic conditions AN-IV (orthorhombic, Pmmn) is stable to above 40 GPa. In one nonhydrostatic compression experiment a volume collapse was observed, suggesting an isostructural phase transition to a "metastable" phase IV' between 17 and 28 GPa. The structures of phase IV and IV' are similar with the subtle difference in the hydrogen-bonding network; that is, a noticeably shorter N1···O1 distance seen in phase IV'. This hydrogen bond has a significant component along the b-axis, which proves to be the most compressible until cell axis over the entire pressure range. It is likely that the shear stress of the nonhydrostatic experiment drives the phase IV-to-IV' transition to occur. We compare the present isotherms of phase IV and IV' in both static and nonhydrostatic conditions with the previously obtained Hugoniot and find that the nonhydrostatic isotherm approximately matches the Hugoniot. On the basis of this comparison, we conjecture that a chemical reaction or phase transition may occur in AN under dynamic pressure conditions at 22 GPa.

Effects of interstitial impurities on the high pressure martensitic α to ω structural transformation and grain growth in zirconium.

Static high pressure diamond anvil cell experiments were performed on three polycrystalline Zr samples having varying interstitial impurity concentrations. Systematic increase in transition pressure with the increase in the amount of interstitial impurities is observed for the martensitic α →ω structural phase transition in Zr. Significant room temperature crystal grain growth is also observed for the two highest purity samples at the α →ω transition. In the case of the lowest purity sample interstitial impurities obstruct the α →ω transition, while possibly helping impede grain growth-even as the sample is heated to 1279 K.

Phase transition and chemical decomposition of hydrogen peroxide and its water mixtures under high pressures.

We have studied the pressure-induced phase transition and chemical decomposition of hydrogen peroxide and its mixtures with water to 50 GPa, using confocal micro-Raman and synchrotron x-ray diffractions. The x-ray results indicate that pure hydrogen peroxide crystallizes into a tetragonal structure (P4(1)2(1)2), the same structure previously found in 82.7% H(2)O(2) at high pressures and in pure H(2)O(2) at low temperatures. The tetragonal phase (H(2)O(2)-I) is stable to 15 GPa, above which transforms into an orthorhombic structure (H(2)O(2)-II) over a relatively large pressure range between 13 and 18 GPa. Inferring from the splitting of the nu(s)(O-O) stretching mode, the phase I-to-II transition pressure decreases in diluted H(2)O(2) to around 7 GPa for the 41.7% H(2)O(2) and 3 GPa for the 9.5%. Above 18 GPa H(2)O(2)-II gradually decomposes to a mixture of H(2)O and O(2), which completes at around 40 GPa for pure and 45 GPa for the 9.5% H(2)O(2). Upon pressure unloading, H(2)O(2) also decomposes to H(2)O and O(2) mixtures across the melts, occurring at 2.5 GPa for pure and 1.5 GPa for the 9.5% mixture. At H(2)O(2) concentrations below 20%, decomposed mixtures form oxygen hydrate clathrates at around 0.8 GPa--just after H(2)O melts. The compression data of pure H(2)O(2) and the stability data of the mixtures seem to indicate that the high-pressure decomposition is likely due to the pressure-induced densification, whereas the low-pressure decomposition is related to the heterogeneous nucleation process associated with H(2)O(2) melting.

A molecular dynamics simulation study of crystalline 1,3,5-triamino-2,4,6-trinitrobenzene as a function of pressure and temperature.

Quantum chemistry-based dipole polarizable and nonpolarizable force fields have been developed for 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Molecular dynamics simulations of TATB crystals were performed for hydrostatic pressures up to 10 GPa at 300 K and for temperatures between 200 and 400 K at atmospheric pressure. The predicted heat of sublimation and room-temperature volumetric hydrostatic compression curve were found to be in good agreement with available experimental data. The hydrostatic compression curves for individual unit cell parameters were found to be in reasonable agreement with those data. The pressure- and temperature-dependent second-order isothermal elastic tensor was determined for temperatures between 200 and 400 K at normal pressure and for pressures up to 10 GPa on the 300 K isotherm. Simulations indicate considerable anisotropy in the mechanical response, with modest softening and significant stiffening of the crystal with increased temperature and pressure, respectively. For most properties the polarizable potential was found to yield better agreement with available experimental properties.

Electronic structure and spectroscopy of [Ru(tpy)(2)](2+), [Ru(tpy)(bpy)(H(2)O)](2+), and [Ru(tpy)(bpy)(Cl)](+).

We use a combined, theoretical and experimental, approach to investigate the spectroscopic properties and electronic structure of three ruthenium polypyridyl complexes, [Ru(tpy)(2)](2+), [Ru(tpy)(bpy)(H(2)O)](2+), and [Ru(tpy)(bpy)(Cl)](+) (tpy = 2,2':6',2''-terpyridine and bpy = 2,2'-bipyridine) in acetone, dichloromethane, and water. All three complexes display strong absorption bands in the visible region corresponding to a metal-to-ligand-charge-transfer (MLCT) transition, as well as the emission bands arising from the lowest lying (3)MLCT state. [Ru(tpy)(bpy)(Cl)](+) undergoes substitution of the Cl(-) ligand by H(2)O in the presence of water. Density functional theory (DFT) calculations demonstrate that the triplet potential energy surfaces of these molecules are complicated, with several metal-centered ((3)MC) and (3)MLCT states very close in energy. Solvent effects are included in the calculations via the polarizable continuum model as well as explicitly, and it is shown that they are critical for proper characterization of the triplet excited states of these complexes.

A molecular dynamics simulation study of the pressure-volume-temperature behavior of polymers under high pressure.

Isothermal compression of poly (dimethylsiloxane), 1,4-poly(butadiene), and a model Estane (in both pure form and a nitroplasticized composition similar to PBX-9501 binder) at pressures up to 100 kbars has been studied using atomistic molecular dynamics (MD) simulations. Comparison of predicted compression, bulk modulus, and U(s)-u(p) behavior with experimental static and dynamic compression data available in the literature reveals good agreement between experiment and simulation, indicating that MD simulations utilizing simple quantum-chemistry-based potentials can be used to accurately predict the behavior of polymers at relatively high pressure. Despite their very different zero-pressure bulk moduli, the compression, modulus, and U(s)-u(p) behavior (including low-pressure curvature) for the three polymers could be reasonably described by the Tait equation of state (EOS) utilizing the universal C parameter. The Tait EOS was found to provide an excellent description of simulation PVT data when the C parameter was optimized for each polymer. The Tait EOS parameters, namely, the zero-pressure bulk modulus and the C parameter, were found to correlate well with free volume for these polymers as measured in simulations by a simple probe insertion algorithm. Of the polymers studied, PDMS was found to have the most free volume at low pressure, consistent with its lower ambient pressure bulk modulus and greater increase in modulus with increasing pressure (i.e., crush-up behavior).