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This study explores the formation of functionalized carbon surfaces through shock compression of graphite in the presence of water, modeled using molecular dynamics and the ReaxFF reactive force field. The shock compression method produces activated carbon with surface functionalities, primarily hydroxyl groups, and varying morphological properties. Two approaches, unidirectional and isotropic compression, yield distinct surface structures: the former preserves a relatively flat surface, while the latter generates corrugated features with valleys and ridges. These features significantly impact the adsorption properties of methylene blue (MB), a commonly used dye. Simulations reveal that MB molecules are highly mobile on flat surfaces, aligning with a mobile adsorption model. However, on corrugated surfaces, MB exhibits localized adsorption, with the deepest valleys effectively immobilizing the dye molecules. Additionally, the study highlights the influence of surface hydroxyl groups, which, through interactions with water molecules, prevent MB from occupying these regions. The findings underscore that traditional adsorption models may not fully capture the dynamics of MB adsorption on activated carbons with complex morphologies. These insights are critical for advancing carbon-based adsorbents in water purification applications. [ABSTRACT FROM AUTHOR]

Experiments were conducted in a stoichiometric hydrogen-oxygen mixture at initial pressure of 1 atm to study the flame acceleration and deflagration-to-detonation transition (DDT) in a 20 mm high channel equipped with continuous triangular obstacles. Effect of blockage ratio (br) was explored by considering different obstacle heights of 0, 1 mm, 3 mm, 5 mm, and 7 mm, corresponding to br = 0, 0.1, 0.3, 0.5, and 0.7, respectively. High-speed schlieren photography and OH* chemiluminescence recording were used to visualize the processes. Results show that blockage ratio has a significant effect on the scales and velocities of the vortices generated in the gaps between neighboring obstacles and consequently on the subsequent flame-vortex interactions. Higher br leads to faster flame acceleration via stronger flame-vortex interactions. The intricate shock-flame interactions cause successive local explosions in the br = 0.3, 0.5, and 0.7 cases, creating conditions for DDT. An eventual choking regime occurs in the br = 0.7 case due to energy losses by continuous diffractions at the obstacle vertices. It was found that the onset of DDT is triggered when the critical height of the unobstructed passage is approximately greater than three detonation cell sizes. The mechanism of DDT changes as the br decreases. One important feature observed is that the br = 0.1 case has the shortest DDT distance. In addition, the final flame speed (~740 m/s, 1.4 times unburned sound speed) before DDT for br = 0.1 is significantly smaller than those (~1300 m/s, 2.4 times unburned sound speed) for br ≥ 0.3. The reason is that the low obstacles retain the characteristics of the obstacle, while mimicking the wall roughness, resulting in both shock compression of unburned gas and viscous heating in the boundary layer ahead of the flame front. The combined effects promote the occurrence of DDT for the case with br = 0.1. [ABSTRACT FROM AUTHOR]

Goethite (α-FeOOH) is an iron-oxyhydroxide mineral that is commonly found in soils and is of importance within the context of industrial mineralogy and aqueous geochemistry. The structure of goethite is such that vacant rows of octahedral sites form “channels” or nanopores. This study aims to investigate the response of goethite to dynamic shock compression in order to advance our understanding of minerals as potential shock-absorbing media. Shock compression of synthetic goethite powdered samples was achieved by using an inverted shock microscope and laser driven “flyer plates”. With this setup, a high-energy laser launches small aluminum discs as projectiles or flyer plates at velocities of the order of a few km/s towards the sample. The resulting impact sends a shock wave through the sample, thereby compressing it. The compression is precisely controlled by the plate-impact speed, which in turn is controlled by laser-power. In this work, 25 µm aluminum flyer plates with 3.5 km/s impact velocities were used. The impact resulted in a planar shock wave with shock velocity (Us) ~ 6.78 km/s and an estimated pressure of ~ 41.6 GPa. The shock wave compressed the target goethite for 5 ns. Subsequent, post-shock investigations via transmission electron microscopy (TEM) documented that crystal morphology persisted, and that goethite’s “bird’s nest” texture was maintained. Lattice fringe images revealed localized zones of distortion and amorphous regions within single goethite particles. Raman spectra appear to indicate structural changes after shock compression with the shocked goethite spectra matching that of synthetic hematite. X-ray diffraction (XRD) interestingly identified two major phases: goethite and magnetite. Irrespective of the mineral phases present, the goethite particles persist post shock. A thixotropic-like model for accompanying shock compression is proposed to account for goethite’s shock resistant behavior. [ABSTRACT FROM AUTHOR]

Polymorphic phase transition in metallic materials under high pressure is a critical aspect of dynamic properties and has been attracting a great interest. Despite the extensive researches have been made on understanding of this phase transition in traditional single-principal element alloys, little is known about the phase transition in recently emergent multi-principal medium and high entropy alloys, especially compressed under high strain rates. In this work, based on molecular dynamic simulations, three impact loading strategies with distinct loading paths, such as single-shock, double-shock and ramp-wave loading are carried out on the single crystalline CoCrNi medium-entropy alloy (MEA) to investigate the phase transition under high strain-rate compression. Careful characterizations show that the phase transition of CoCrNi MEA is loading-path dependent, as evidenced by the significant differences in macroscopic pressure evolution and microscopic structural phase transition among the samples under various thermodynamic paths. An intriguing pressure “overshoot” is found and demonstrated as the characteristic of the critical structural phase transition from face-centered cubic (FCC) structure to hexagonal-close-packed (HCP) structure mediated by body-centered cubic (BCC) like clusters. We show that such loading-path dependence is attributed to the strain rate and temperature rise in the loading process, which control the evolution of microstructure and deformation field. The inherent correlation between the atomistic process of phase transition and loading strategies results in polymorphic phase transition under high strain rates. These findings shed new light on the nature of impact phase transition of multi-principal alloys.

To investigate the dynamic mechanical properties of water-saturated fissure rock at different strain rates, prefabricated sandstone specimens with a 45° dip angle were treated with water saturation and the impact compression test was performed with a Split Hopkinson Pressure Bar (SHPB) test device at different impact pressures. The results show that the clusters of dynamic stress–strain curves of water-saturated and natural sandstone specimens with a 45° dip angle of prefabricated fissures are basically similar under different impact air pressures. A distinct strain rate effect was observed for dynamic strain and dynamic compressive strength, both of which increased with increasing strain rate. From the failure pattern of the specimen, it can be seen that cracks appeared from the tip of the prefabricated fissure under axial stress, spreading to both ends and forming wing cracks and anti-wing cracks associated with shear cracks. As the strain rate increased, the energy dissipation density of the specimen gradually increased, and the macroscopic cracks cross-expanded with each other. The fracture form of the specimen showed a small block distribution, and the average particle size of the specimen gradually decreased. The specimen crushing energy dissipation density was negatively correlated with fracture size, reflecting a certain rate correlation. The sandstone fragments' fractal dimension increases with the increase in crushing energy dissipation density, and the fractal dimension may be applied as a quantitative index to characterize sandstone crushing. [ABSTRACT FROM AUTHOR]

Measurements of the electrical resistance of shock-compressed aluminum are used in the present study to estimate the concentration of point defects generated by the shock wave front. The parameters of the physical state of a thin metal sample are found by means of modeling the shock wave processes in the measurement cell. Experimental values of the specific electrical resistance of aluminum are compared with predictions of the equilibrium electrical resistance model. The proposed model ensures an adequate description of currently available reference data on equilibrium isothermal compression and isobaric heating of aluminum. At the same time, the shock wave experiment yields a higher specific electrical resistance than that predicted by the model of the electrical resistance of an equilibrium defectless crystal. The detected difference in the specific electrical resistances testifies to generation of defects of the crystal structure of the metal subjected to dynamic compression. Under the assumption of predominant formation of vacancies, the concentration of defects in aluminum is estimated as a function of the shock wave pressure. The number of defects in the metal increases with an increase in the shock wave pressure. The data obtained are qualitatively consistent with available results for copper and silver, which allows one to claim that generation of defects under shock compression has common specific features for these metals. The physical state of shock-compressed aluminum is thermodynamically nonequilibrium and includes numerous defects. [ABSTRACT FROM AUTHOR]

YAG transparent ceramic has great potential in the applications to transparent armour protection modules. To study the dynamic behaviour and obtain the parameters for the equation of state of YAG under the load of longitudinal stress ranging from 0 to 20 GPa, ramp wave and shock compression experiments were conducted based on the electromagnetic loading test platform. The Hugoniot data, isentropic data, dynamic strength, and elastic limit of YAG were obtained. The results showed that the relationship between the longitudinal wave speed and the particle velocity of YAG was linear when the longitudinal stress was lower than the elastic limit. The quasi-isentropic compression and shock Hugoniot compression curves were coincident when the stress in YAG was below 10 GPa; however, a separation of the two curves occurred when the stress in YAG ranged from 10 GPa to the elastic limit. Moreover, the effect of strain rate on the fracture stress of YAG under a moderate strain rate of 10 5 –10 6 s - 1 was more evident than in other strain rate ranges. The amplitude of the precursor wave decayed with increasing sample thickness. [ABSTRACT FROM AUTHOR]

The mechanical properties of alloys Cu–0.03 wt % Zr and Cu–0.10 wt % Cr with a submicrocrystalline structure formed after dynamic channel angular pressing and subsequent annealing. The properties of the alloys were studied under conditions of shock compression with a pressure of 4.7–7.0 GPa and a deformation rate of (1.3–3.2) × 105 s–1. It is shown that grain size refinement from 200–400 to 0.3–1.0 μm increases the dynamic elastic limit and the dynamic yield strength of the Cu–0.03% Zr alloy by factors of 1.9 and 1.8, respectively. At the same time, the spall strength is reduced by a factor of 1.4. Subsequent annealing at 400 and 450°C can increase the characteristics of the elastic–plastic transition by factors of 3.0 and 3.7, respectively. This elevates the spall strength to the level of a large crystal analogue. It is determined that the process of dispersing the Cu–0.10% Cr alloy structure to 1.0–5.0 μm leads to an increase in the spall strength by a factor of 1.5 with respect to this value in the coarse grained state, while the dynamic elastic limit and the dynamic yield strength are increased by factors of 3.7 and 2.6, respectively. [ABSTRACT FROM AUTHOR]

The equation of state (EOS) for energetic structural materials (ESMs) has been drawn a great attention due to the absent comprehensive understanding on the effect of the shock-induced chemical reaction. In this paper, the shock compression behavior of Ni/Al ESM is investigated by developing the EOS, which mainly considers the effects of the chemical reaction and the reaction products. The chemical reaction is based on the Avram-Erofeev kinetic law and the Arrhenius equation. The study concerns the shock pressure, the relative volume, the temperature, and the chemical reaction during the shock compression. The effects of the initial porosities, the stoichiometric ratios and inert additives were mainly discussed. The results showed that high porosity would induce high temperature rise. Different stoichiometric ratios would produce different temperature rise. When the stoichiometric ratio Ni: Al ​= ​1:1, the temperature rise is highest. In addition, the inert additive material would obviously reduce the temperature rise. Finally, the developed model improved the temperature calculation, compared with the existing model.

Shock-induced structural transformations in copper exhibit notable directional dependence and anisotropy, but the mechanisms that govern the responses of materials with different orientations are not yet well understood. In this study, we employ large-scale non-equilibrium molecular dynamics simulations to investigate the propagation of a shock wave through monocrystal copper and analyse the structural transformation dynamics in detail. Our results indicate that anisotropic structural evolution is determined by the thermodynamic pathway. A shock along the ⟨100⟩ orientation causes a rapid and instantaneous temperature spike, resulting in a solid–solid phase transition. Conversely, a liquid metastable state is observed along the ⟨111⟩ orientation due to thermodynamic supercooling. Notably, melting still occurs during the ⟨110⟩ -oriented shock, even if it falls below the supercooling line in the thermodynamic pathway. These results highlight the importance of considering anisotropy, the thermodynamic pathway and solid-state disordering when interpreting phase transitions induced by shock. This article is part of the theme issue 'Dynamic and transient processes in warm dense matter'. [ABSTRACT FROM AUTHOR]

Both the nanoscale helium (He) bubble and grain boundaries (GBs) play important roles in the dynamic mechanical behavior of irradiated nanocrystalline materials. Using molecular dynamics simulations, we study the shock-induced deformation and spallation failure of bicrystal copper with a nanoscale He bubble. Two extreme loading directions (perpendicular or parallel to the GB plane) and various impact velocities (0.5–2.5 km/s) are considered. Our results reveal that the He bubble shows hindrance to the propagation of shock waves at lower impact velocities but will accelerate shock wave propagation at higher impact velocities due to the local compression wave generated by the collapse of the He bubble. The parallel loading direction is found to have a greater effect on He bubble deformation during shock compression. The He bubble will slightly reduce the spall strength of the material at lower impact velocities but has a limited effect on the spallation process, which is dominated by the evolution of the GB. At lower impact velocities, the mechanism of spall damage is dominated by the cleavage fracture along the GB plane for the perpendicular loading condition but dominated by the He bubble expansion and void growth for the parallel loading condition. At higher impact velocities, micro-spallation occurs for both loading conditions, and the effects of GBs and He bubbles can be ignored. [ABSTRACT FROM AUTHOR]

Potentials of the embedded atom model (EAM) for solid and liquid magnesium are proposed. Properties of magnesium are studied by means of molecular dynamics (MD) at binodals of up to 1500 K, and under conditions of static and shock compression. The main characteristics of bcc and liquid magnesium (structure, density, energy, compressibility, speed of sound, and coefficients of self-diffusion) are calculated. The static compression isotherm at 298 K up to a pressure of 108 GPa and the Hugoniot adiabat up to a pressure of 80 GPa are calculated with allowance for electron contributions. Values of the excess energy of the surfaces of magnesium nanoclusters with 13 to 2869 particles are found, and the Gibbs–Helmholtz equation for the relationship between surface tension and surface energy is estimated. [ABSTRACT FROM AUTHOR]

YAG (Y3Al5O12) transparent ceramics have attractive application prospects for transparent armor protection modules because of their excellent light transmittance and anti-ballistic capability. Understanding the fracture behavior and damage mechanism of YAG is necessary for armor design. To explore the damage characteristics of YAG under compression and tension, shock compression and shockless spalling experiments with soft recovery technique are conducted. The spall strength of YAG is obtained and the recovered samples are observed by CT and SEM. It is shown that the macroscopic damage characteristic of YAG under compression is vertical split cracks with oblique fine cracks distributed in the entire sample, while that under tension is horizontal transgranular cracks concentrated near the main spall surface. The cracks generated by macroscopic compression, tension and shear stress extend in similar tensile form at the microscale. The proportion of transgranular fractures on spall surfaces is higher than that of cracks induced by macroscopic compression. Meanwhile, higher loading rate and longer loading duration increase the transgranular fracture percentage.

A new approach for performing quantitative structure-factor analysis and density measurements of liquids using X-ray diffraction with a pink-spectrum X-ray source is described. The methodology corrects for the pink beam effect by performing a Taylor series expansion of the diffraction signal. The mean density, background scale factor, peak X-ray energy about which the expansion is performed, and the cutoff radius for density measurement are estimated using the derivative-free optimization scheme. The formalism is demonstrated for a simulated radial distribution function for tin. Finally, the proposed methodology is applied to experimental data on shock compressed tin recorded at the Dynamic Compression Sector at the Advanced Photon Source, with derived densities comparing favorably with other experimental results and the equations of state of tin.

To investigate the dynamic mechanical properties of water-saturated fissure rock at different strain rates, prefabricated sandstone specimens with a 45° dip angle were treated with water saturation and the impact compression test was performed with a Split Hopkinson Pressure Bar (SHPB) test device at different impact pressures. The results show that the clusters of dynamic stress–strain curves of water-saturated and natural sandstone specimens with a 45° dip angle of prefabricated fissures are basically similar under different impact air pressures. A distinct strain rate effect was observed for dynamic strain and dynamic compressive strength, both of which increased with increasing strain rate. From the failure pattern of the specimen, it can be seen that cracks appeared from the tip of the prefabricated fissure under axial stress, spreading to both ends and forming wing cracks and anti-wing cracks associated with shear cracks. As the strain rate increased, the energy dissipation density of the specimen gradually increased, and the macroscopic cracks cross-expanded with each other. The fracture form of the specimen showed a small block distribution, and the average particle size of the specimen gradually decreased. The specimen crushing energy dissipation density was negatively correlated with fracture size, reflecting a certain rate correlation. The sandstone fragments’ fractal dimension increases with the increase in crushing energy dissipation density, and the fractal dimension may be applied as a quantitative index to characterize sandstone crushing.

Experimental data on the electrical resistance of aluminum under shock compression are analyzed. The electrical resistance of two types of aluminum foil located in dielectric materials with different shock impedances is measured by the electrocontact method. The resultant dependences of the electrical resistance of aluminum on the shock wave pressure are monotonically increasing functions of pressure. However, the dependence of the specific electrical resistance of aluminum on the shock wave pressure can be monotonic (foil in Plexiglas) or nonmonotonic (foil in fluoroplastic). In the latter case, the specific electrical resistance first slightly decreases with an increase in pressure and then increases. This behavior can be explained by the competing effects of compression and temperature heating on the specific electrical resistance. Due to shock compression of metal foil in the dielectric with a smaller shock impedance (Plexiglas), the measured electrical resistance is greater than that in the dielectric with a greater shock impedance (fluoroplastic). This result is caused by the greater temperature heating of metal foil in Plexiglas. The reasons for the qualitative difference in the behavior of the specific electrical resistance of metal under static and dynamic compression are discussed. [ABSTRACT FROM AUTHOR]

Materials respond to shock in many ways such as plastic flow in metals and amorphization in ceramics. It is very challenging to characterize the deformation mechanisms of ceramics under shock compression due to extreme loading conditions. Here we report the shock response of nanocrystalline boron carbide (n -B 4 C) using large-scale molecular dynamics simulations with a machine-learning force field. We identify three quasiplastic deformation mechanisms in shocked n -B 4 C: grain boundary (GB) sliding, intergranular amorphization, and intragranular amorphization. As the deformation mechanism changes from GB sliding to intergranular amorphization at ∼40 GPa, a bilinear Hugoniot behavior occurs, consistent with experimental observations. At higher pressure (>∼100 GPa), intragranular amorphization becomes dominant, causing a complete loss of shear strength. These quasiplastic mechanisms may play an important role in the shock behaviors of ceramics. [ABSTRACT FROM AUTHOR]

Results from calculating the properties of liquid tin using the EAM (Embedded Atom Model) interparticle potential are analyzed, and the surface properties of tin are calculated according to molecular dynamics (MD). Calculations based on the EAM generally agree better with experiments for the properties of liquid tin than ones based on the MEAM. The accuracy of the Gibbs–Helmholtz equation for the relationship between surface tension and surface energy is evaluated. [ABSTRACT FROM AUTHOR]

The previously developed wide-range multiphase equation of state for Fe was used to calculate the shock pressure leading to vaporization of iron under isentropic unloading to 10-4 GPa (1 atm). Calculations were made for three initial states of the material: pressure 1 atm and temperature 298 K ("cold" initial state), 1 GPa and 1500 K ("warm" state), and 40 GPa and 4000 K ("hot" state). The shock pressure is 359, 261, and 132 GPa, respectively. These values are generally lower than the estimates by other authors. Arguments in support of the obtained values are provided. [ABSTRACT FROM AUTHOR]

An experimental investigation of the shock wave structure, Hugoniot states, and spall strength of a shock-compressed porous carbon fiber–epoxy composite was conducted. To generate high dynamic pressures in the material, the impact of flat-plate aluminum projectiles accelerated by explosive planar shock wave generators to velocities ranging from 0.65 to 5.05 km/s was used. Particle velocity profiles were recorded on the composite surface–water window interface with a multichannel VISAR laser interferometer. On the velocity profiles for the composite with a transverse fiber orientation, a single shock wave was recorded, while for the parallel orientation, a two-wave structure was observed. It was found that the shock wave compressibility of the porous composite did not depend on the fiber orientation relative to the direction of shock wave propagation. A kink on the Hugoniot curve was observed at the pressure of 19 GPa. The results obtained for the porous composite were compared with data for a non-porous carbon–epoxy composite and epoxy resin used as a matrix in the composites. When analyzing dynamic fracture of the porous composite under shock compression, it was found that the spall strength of the material was significantly lower than that of epoxy resin. [ABSTRACT FROM AUTHOR]

The phase stability of carbonates under mantle conditions is important for understanding the global carbon cycle. In this study, the Hugoniot data of a natural siderite (FeCO 3) were measured up to 90 GPa using the plane-plate impact method. Two successive phase transitions were observed at 38–40 GPa and 65–69 GPa, respectively. In comparison with the static compression results, the first phase transition was identified as a spin transition, and the second is attributed to the self-redox reaction. The volume change during the self-redox transition is consistent with the reaction products of tetrairon orthocarbonate Fe 4 C 3 O 12 and diamond. Using the measured Hugoniot data, we estimated the density of Fe 4 C 3 O 12 along the lower mantle conditions and found it to be higher than the seismic values. Our results suggest siderite plays an important role in the deep carbon cycle. [Display omitted] • Hugoniot of a natural siderite has been measured up to ∼90 GPa and ∼ 1700 K. • Spin transition and self-redox reaction were identified at 38–40 GPa and 65–69 GPa, respectively. • Siderite plays an important role in deep carbon cycle. [ABSTRACT FROM AUTHOR]

• Three improvements are introduced to refine the physical descriptions for the cold component, melting, and vaporization in the Gray EOS, thereby an improved multiphase EOS is developed. • Full material parameters of the improved EOS for aluminum are obtained. • The prediction differences of the improved EOS, Gray EOS, and Tillotson EOS are compared in the thermodynamic behavior of aluminum during shock compression and isentropic release. • The improved EOS can best predict the thermodynamic properties of aluminum in a wide range of states, from the high-pressure dense state (<1 TPa) to the low-density expanded fluid state (>0.1 g/cm3). • Using the improved EOS and the AUTODYN-SPH hydrocode, the phase evolution process of aluminum in HVI is displayed. Equation of state (EOS) contains information about the relationships between the thermodynamic variables of materials, and is widely applied in the field of hypervelocity impact (HVI) study. In this paper, based on the framework of the Gray EOS, three improvements are introduced to refine its physical descriptions, including the correction of the entropy function of metals in the liquid phase, the more reasonable description for the cold term, and the modification of the Young-Alder EOS, thereby an improved multiphase EOS is developed, accounting for solid, liquid, gas and mixed-phase (melting and vaporization). Further to this, supported by numerous existing experimental data and molecular dynamics simulation results, the complete parameters of the improved EOS for aluminum are obtained, and the thermodynamic behaviors of aluminum during shock compression and isentropic release are studied, simultaneously a systematic comparison is made in the prediction differences of three EOSs, i.e., the improved EOS, the Gray EOS, and the Tillotson EOS. The results show that the improved EOS can best predict the thermodynamic properties of aluminum in a wide range of states, from the high-pressure dense state (<1 TPa) to the low-density expanded fluid state (> 0.1 g/cm3). Then, the improved EOS is embedded into the AUTODYN-SPH hydrocode, and the phase evolution process of aluminum in HVI is displayed. [ABSTRACT FROM AUTHOR]

(a)Hugoniot curves of P-u p and t-u p for disordered thin-graphite sample under shock compression obtained by MD using BOP,where the elastic deformation,structural transition,liquefaction regions are marked in white,light gray and dark gray,respectively.(b)Pressure–temperature (P-T) phase diagram including three typical phase structures of the sample,with sp, sp2and sp3 hybridization marked in white, grey and black,respectively.(c)Distributions of shock pressure during the passage of shock wave throughout the sample at u p = 9 km/s, with visualizations of the phase transition, see a two −wave structure at 1.6 ps. [Display omitted] • The graphite-to-diamond transition is activated at shock pressure of about 30 GPa and completely liquefied at about 250 GPa. • Grain boundaries have a significant effect on the dynamic mechanical behavior of graphite, with phase transitions and damage always emerging from grain boundaries. • After exceeding the Hugoniot limit, the appearance of double wave structure can be observed, and further increasing the piston velocity will cause the plastic wave to catch up with the precursor elastic wave and form a single overdriven wave. Dynamic structural response of nano-polycrystalline graphite under shock compression is investigated using molecular dynamics (MD) simulations. Hugoniot data shows that the structural transition is activated at shock pressure P ∼30 GPa (experimental range, 20–50 GPa), resulting in the formation and extension of hexagonal diamond nuclei along grain boundaries, embedded incoherently among thin-graphite grains. As P increases from 130 GPa, the structure starts to liquefy, accompanied by a decrease in shear stress τ from approximately 5.3 GPa, and completely liquefies at P ∼250 GPa (melting pressure of graphite, 180–280 GPa) and τ ∼ 0 GPa. In ultrahigh-pressure region, a two-wave structure is generated consisting of an elastic shock wave and a phase transition wave, and when the piston velocity exceeds 5.2 km/s, the latter wave can catch up with the elastic one, eventually becoming a single over-driven wave. During the relaxation of compressed nano-polycrystalline graphite, void nucleation inside the sample induces the initiation of visible cracks when piston velocity is higher than 1 km/s. At low piston velocities, the cracks propagate gradually along grain boundaries due to shear-slip effects. While at high piston velocities, direct spall of the nano-polycrystalline graphite makes it into multiple fragments by ultrahigh strain rate tensile forces. This study provides a useful guide to the structural transition and dynamic damage evolution of nano-polycrystalline graphite under shock compression. [ABSTRACT FROM AUTHOR]

[Display omitted] • Shock response of lightweight multicomponent alloy is simulated by AIMD with MSST. • Both migration of atoms and electronic transition are involved in shock compression. • Multicomponent alloy can be disordered due to different migration ability of atoms. • Crystal orbitals contributions and density of states are changed after compression. • Electrons response more quickly to the shock compression than crystal structure. The dynamic response of shock wave impact on single crystal aluminium and lightweight multicomponent alloy Al-Cu-Li-Mg is simulated by using the combination of Ab initio Molecular Dynamics (AIMD) and Multi-Scale Shock Technique (MSST), with the analysis carried out at the atomic/electronic levels. The simulation is verified by comparing the particle velocity of single crystal obtained in this work with the data in literature. The shock compression process not only involves the migration of atoms, but also is related to electronic transition. Two stages could be found in the shock compression process: oscillatory compression of the crystal cell and oscillatory migration of the atoms. The crystal structure of the multicomponent alloy could be disordered even at low shock speed, due to the difference in the ability to migrate between different kinds of atoms. As the sample is shock-compressed, the contribution proportion of crystal orbitals shows a sharp decrease for D orbital, while it increases significantly for S orbital and P orbital. The electron structure shows a quicker response to the shock wave compression process than the crystal structure. The orbital contribution from P orbital of the crystal is mainly due to the P orbital of Al atoms, while the orbital contribution from D orbital of the crystal is mainly due to the D orbital of Cu atoms. Total Density of States (TDOS) is mainly contributed by the Projected Density of State (PDOS) of Cu atoms in the occupied state of energy levels, while it is close to the PDOS of Al atoms in the non-occupied state of energy levels. [ABSTRACT FROM AUTHOR]

In inertial confinement fusion using central ignition, the ignition hot spot is generated through self-heating during fuel compression. In contrast, fast ignition creates the hot spot through external heating. This difference allows the fast ignition approach to use a solid sphere as the fusion fuel shape. The implosion of a solid sphere is one form of laser-direct-drive slow implosion. Solid sphere fuel exhibits tolerance to hydrodynamic instability and can be mass-produced relatively easily, offering significant advantages for developing inertial fusion energy. Achieving high fuel peak and areal densities of with a solid sphere requires quasi-isentropic compression, which involves multiple shock waves. Our results show the critical role of uniform laser irradiation in initiating weak shock waves in the early phase, which is essential for forming a uniform and dense fuel core with solid spheres. Furthermore, dynamically adjusting the laser spot diameter could be crucial in optimizing the effectiveness of laser-direct-drive and fast ignition techniques when using solid sphere fuel. [ABSTRACT FROM AUTHOR]

A new approach for performing quantitative structure-factor analysis and density measurements of liquids using X-ray diffraction with a pink-spectrum X-ray source is described. The methodology corrects for the pink beam effect by performing a Taylor series expansion of the diffraction signal. The mean density, background scale factor, peak X-ray energy about which the expansion is performed, and the cutoff radius for density measurement are estimated using the derivative-free optimization scheme. The formalism is demonstrated for a simulated radial distribution function for tin. Finally, the proposed methodology is applied to experimental data on shock compressed tin recorded at the Dynamic Compression Sector at the Advanced Photon Source, with derived densities comparing favorably with other experimental results and the equations of state of tin. [ABSTRACT FROM AUTHOR]

Shock responses of Mg–Al–Zn alloy are investigated by the molecular dynamics (MD) method. The wave propagation, plastic deformation behavior and failure mechanism along the [0001] and [ 10 1 ¯ 0 ] orientations are analyzed. For both orientations, simulation results show that the shock wave has an obvious double-wave structure (plastic-elastic) under a piston velocity of 1200 m/s. A higher Hugoniot elastic limit (HEL) is observed for [0001]-oriented shock. When the shock pressure is along the [ 10 1 ¯ 0 ] direction, the distance between plastic and elastic waves is closer, and higher dislocation density and more twins are observed. Moreover, the spall strength for [ 10 1 ¯ 0 ]-oriented shock is predicted to be higher. In addition, the wave interactions, HEL and spall strength predicted for Mg–Al–Zn alloy are compared with the experimental results and MD simulation results of Mg single crystal in the literature. It is concluded that the shock performance of Mg–Al–Zn is better than that of Mg single crystal. [ABSTRACT FROM AUTHOR]

We have designed an experiment for the National Ignition Facility to measure the Hugoniot of materials such as plastic at extreme pressures. The design employs a strong spherically converging shock launched through a solid ball of material using a hohlraum radiation drive. The shock front conditions can be characterized using X-ray radiography until background from shock coalescence overtakes the backlit signal. Shock coalescence at the center is predicted to reach tens of Gbars and can be further characterized by measuring the X-ray self-emission and 2.45 MeV neutrons emitted from the shock flash region. In this simulation design work the standard plastic sphere is replaced with a deuterated polyethylene sphere, CD2, that reaches sufficiently high densities and temperatures in the central hot spot to produce neutrons from Deuterium-Deuterium (DD) fusion reactions that can be measured by a neutron time of flight spectrometer (nTOF) and act as a temperature diagnostic. This paper focuses on the design of these experiments, based on an extensive suite of radiation-hydrodynamics simulations, and the interpretation of the predicted DD neutron signals. The simulations predict mean temperatures of 1 keV in the central hot spot with mean densities of 33 g/cc and mean pressures of 25 Gbar. A preliminary comparison with early experimental results looks promising with an average ion temperature of 1.06 ± 0.15 keV in the central hot spot estimated from the nTOF spectral width and measured neutron yield of 7.0 (±0.5) × 109 DD neutrons.

The effect of dynamic high-pressure loading on the decomposition reaction and negative thermal expansion of Ag2O was investigated by X-ray diffraction (XRD) analysis and differential scanning calorimetry (DSC). The XRD pattern of the sample shocked at 6.4 GPa indicated that the sample was composed of cubic phase Ag2O and metallic Ag. These XRD patterns indicated that the shock-induced decomposition reaction of Ag2O occurred when the sample was shock-loaded at 6.4 GPa and above. The DSC curves of the shocked Ag2O revealed that an additional exothermic reaction occurred at around 478 K in addition to an endothermic reaction at around 700 K, which corresponds to the decomposition of Ag2O. The exothermic reaction at around 478 K was probably caused by the release of shock-induced residual energy. Synchrotron XRD performed from 300 to 130 K clarified the suppression of negative thermal expansion in the shocked Ag2O. [ABSTRACT FROM AUTHOR]