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We present a time-resolved ultrafast optical spectroscopy study on BiTeI, a noncentrosymmetric semiconductor with large spin-orbit splitting. By tuning the pump photon energy, hot carriers can be excited into different energy bands, and the hot carriers decay dynamics are measured. The hot carriers excited by an 1.544 eV photon induce a positive differential reflectivity following a single exponential decay, while the hot carriers excited by an 1.651 eV photon show a negative reflectivity following two exponential decays

Multiple ballistic impacts are carried out on a 2024-T4 aluminum alloy by spherical steel projectiles (5-mm diameter) at ∼ 400 m s − 1 , to investigate its dynamic deformation and damage. The ballistic impact process is captured with high-speed photography. Postmortem samples are characterized with optical imaging, three-dimensional laser scanning, microhardness testing and electron backscatter diffraction. With increasing number of impacts, crater diameter increases slightly, but crater depth and crater volume increase significantly, and strain accumulation leads to microhardness increase overall. Crater parameters all follow power-law relations with the number of impacts. Twin-like deformation bands and macroscopic deformation twins are produced by impact as a result of spontaneous dislocation self-pinning under high strain rate, large shear deformation. Under multiple impacts, shear strain accumulation in the arc-shaped region of the crater induces deformation twinning when it exceeds a critical value ( ∼ 1.1 − 1.6 ). It is highly possible that the deformation twins are related to deformation bands, since they both share one set of the { 111 } pole with the initial matrix grain. A finite element method model is optimized to reproduce experimental observations and interpret deformation mechanisms.

Ignition delay times of nitromethane-doped ethanol dual fuels are measured under the micro-combustion engine conditions (8 ​× ​105 ​Pa and 904–1477 ​K). The promotion effect of nitromethane on ethanol ignition is more pronounced at a fuel-lean condition; it first increases and then decreases with increasing blending ratio and temperature. The optimal promotion effect is achieved at 950–1150 ​K with an addition of ~50% nitromethane (mole fraction). A chemical kinetic model is proposed to describe the ignition process of nitromethane-doped ethanol, and can reproduce the measured ignition delay times. Reaction pathway, sensitivity, and rate of production analyses are performed to reveal the promotion mechanisms of nitromethane. The consumption of ethanol via the H-abstraction reactions is enhanced due to the pyrolysis of nitromethane, and the produced ethanol radicals are beneficial to the re-establishment of the radical pool after the first-stage ignition of nitromethane. With decreasing temperature, nitromethane decomposes slowly, while the low-temperature oxidation of ethanol becomes more important. At a moderate temperature, both these two reasons result in a shorter ignition delay time.

The H-abstraction reaction kinetics of methyl acrylate (MA) + H/OH/CH3/HO2 radicals have been investigated theoretically in the present work. For these reactions, the reaction energies and barrier heights are first computed using several density functionals and compared to the coupled cluster CCSD(T)-F12/jun-cc-pVTZ benchmark calculations. The M062X/maug-cc-pVTZ method shows the best performance with the smallest mean unsigned deviation (MUD) of 0.42 kcal mol-1. Combined with the electronic structure calculations using the M062X/maug-cc-pVTZ method, the multistructural canonical variational transition-state theory (MS-CVT) with small-curvature tunneling (SCT) is employed to calculate the reaction rate constants at 500-2000 K. The variational effect is between 0.56 and 1.0, the multistructural torsional anharmonicity factor ranges from 0.004 to 4.57, and the tunneling coefficient is in the range of 0.5-4.70. Notably, given the existence of reactant complexes (RCs) between reactants and transition states for the reaction systems MA + OH/HO2, we further compare the rate constants under the low-pressure limit (LPL) kinetic model, which treats the reaction as a single-step process and neglects RCs, and the pre-equilibrium model, which takes RCs into account in the reaction and treats the reaction as a two-step process. The rate constants calculated by these two models are similar within the combustion temperature range, and apparent differences occur at lower temperatures. In addition, we determine the branching ratios as a function of temperature and find that the methyl site (S3) abstractions by OH and H radicals are dominant in the low- and high-temperature ranges, respectively. Moreover, we update the kinetic model with the calculated H-abstraction rate constants to simulate the ignition delay times of MA. The simulations of the updated model are in good agreement with experimental results. The accurate reaction kinetics determined in this work are useful for the understanding and prediction of consumption branching fractions and ignition properties of the unsaturated methyl esters.

DATAD, a Python-based X-ray diffraction simulation code, has been developed for simulating one- and two-dimensional diffraction patterns of a polycrystalline specimen with an arbitrary texture under an arbitrary deformation state and an arbitrary detection geometry. Pixelated planar and cylindrical detectors can be used. The basic principles and key components of the code are presented along with the usage of DATAD. As validation and application cases, X-ray diffraction patterns of single-crystal and polycrystalline specimens with or without texture, or applied strain, on a planar or cylindrical detector are simulated.

High-speed penetration into carbon fiber composites is of fundamental importance to materials science and impact engineering, but research along this line suffers considerably from the lack of direct experimental observations. Here we investigate such penetration dynamics of a unidirectional carbon fiber reinforced epoxy (UCFRE) composite, with a combination of in situ, ultrafast, synchrotron phase contrast imaging and finite element (FE) analysis. The experiments yield the first direct observation on projectile trajectories and fiber-scale deformation and damage in the UCFRE composites during supersonic microprojectile penetration, for different fiber orientations ( 0 ∘ − 90 ∘ from the impact direction) and projectile velocities 600 − 850 m.s - 1 , at unprecedented temporal ( ∼ 100 ps) and spatial (5 μm) resolutions. The maximum penetration depth decreases with increasing fiber orientation angles, as a result of anisotropic damage evolution in the composite sample. Strain localizations are prone to develop along a direction perpendicular to the fiber orientation, while the damage or cavity region, along the fiber direction. FE modeling with a three-dimensional Hashin criterion yields consistent projectile trajectory and cavity morphology with the experimental results. With increasing fiber orientation angles, damage analyses show a transition in the damage mode from fiber compression to matrix compression damage, in line with the increasing maximum penetration depth.

Methyl pentanoate(MP) was identified as a potential candidate. To facilitate the application of MP with high efficiency in engines, a comprehensive understanding of combustion chemical kinetics of MP is necessary. In this work, the H-abstraction reactions from MP by H and CH3 radicals, critical in controlling the initial fuel consumption, are theoretically investigated at the DLPNO-CCSD(T)/CBS(T-Q)//M06-2X/cc-pVTZ level of theory. The multistructural torsional(MS-T) anharmonicity is characterized using the dual-level MS-T method; the HF/3–21G and M06-2X/cc-pVTZ methods are chosen as the low- and high-level methods, respectively. The conventional transition state theory(TST) is employed to calculate the high-pressure limit rate constants at 298–2000 K with the Eckart tunneling correction. Our calculations indicate that the hydrogen atoms of the methylene functional group are easier to be abstracted by H and CH3 radicals. The multistructural torsional anharmonicities of H-abstraction reactions MP+H/CH3 are significant within the temperature range investigated. The tunneling effects are more pronounced at low temperatures, and contribute considerably to the rate constants below 500 K. The model from the work of Dievart et al. is updated with our calculations, and the simulations of the updated model are in excellent agreement with the reported ignition delay time of MP/O2/Ar and MP/Air mixtures. The sensitivity analysis indicates that the H-abstraction reactions, MP+H-CH3CH2CHCH2C(-O)OCH3/CH3CHCH2CH2C(-O)OCH3+H2, are critical in controlling the initial fuel consumption and ignition delay time of MP.

The pyrolysis of nitromethane, CH3NO2, was studied at ∼3.5 atm and 1013 K–1418 K in a heated shock tube by measuring the key product nitric oxide (NO) using mid-infrared laser absorption spectroscopy. We used a quantum cascade laser (QCL) at 5.26 µm to exploit the strong NO absorption at 1900.08 cm−1. With the NO absorption cross-section data characterized at 1006 K–1789 K and 2.7 atm–3.5 atm behind reflected shock waves, we measured the NO concentration time-histories during the pyrolysis of nitromethane at two different concentrations (1.05% and 0.6%). The absorption interference from other major products such as CO and H2O was analyzed to be negligible, leading to an interference-free NO diagnostic in nitromethane pyrolysis. A recent kinetic model of Shang et al. (2019) was adopted to interpret the shock tube data. All the NO time-histories measured over the entire temperature range 1013 K–1418 K were well-predicted by this model in terms of the initial NO formation rate and the final plateau level. The rate-of-production, sensitivity, and reaction flux analyses were performed to identify four important reactions (CH3NO2 = CH3 + NO2, CH3NO2 ↔ CH3ONO = CH3O + NO, CH3 + NO2 = CH3O + NO, and NO2 + H = NO + OH) that determine the NO formation during CH3NO2 pyrolysis. The satisfactory agreement between the simulation and shock tube/laser absorption measurement further validated the kinetic mechanism of nitromethane decomposition. The developed mid-infrared NO absorption sensor provides a promising diagnostic tool for studying fuel-nitrogen chemical kinetics in the shock tube experiments.

Four azoles with similar element percentages but different heterocyclic rings and functional groups are investigated with fs laser-induced breakdown spectroscopy (LIBS) as regards the influence of the interaction between the functional groups and heterocyclic rings on laser-induced plasma (LIP) emission. Temporally and spatially resolved measurements of the CN and C2 emissions are obtained and analyzed. The functional group affects the π-electron delocalization of the heterocyclic ring and consequently changes the dissociation pathways of the heterocyclic ring. At the early stage, CN is formed from the atomic recombination or native fragments, while at the late stage, through the remaining native fragments. In the whole plasma lifetime, C2 is mainly transformed from CN. Furthermore, dynamic CN and C2 distributions mediated by the laser-induced shock waves are discussed.

The promotion effect of nitromethane (NM) on ethane ignition is studied in a heated shock tube. An unusual promotion effect of NM is observed: the ignition delay time of blends first decreases and then increases with increasing NM blending ratio, and the promotion effect peaks at blending ratio of 0.5. A model from our recent study [Fuel, 2019, 256: 115956] can reproduce the measurements and the abnormal promotion effect of NM. The kinetic analyses indicate that the radical pool establishment is slow for ethane, but much faster for NM due to the weak C–N bond. After the first-stage ignition of NM, most radicals are consumed and the radical pool needs to be re-established. In the ethane/NM blends, ethyl radicals, produced from the H-atom abstraction from ethane, can accelerate the re-establishment of the radical pool and then lead to a faster ignition delay time in comparison to neat ethane and NM, thus resulting in an abnormal promotion effect.

To thoroughly understand low-temperature oxidation of dimethylamine (DMA), the reaction kinetics of triplet ground state O2 addition to DMA radicals ( CH 3 NH C ˙ H 2 and CH 3 N ˙ CH 3 ) are theoretically investigated. In CH 3 NH C ˙ H 2 + O2, the direct H-abstraction reaction is significant due to its extremely low energy barrier, and the dominant channels involving intermediates, CH 3 NHCH 2 O O ˙ = CH 3 NCH 2 + H O ˙ 2 , CH 3 NHCH 2 O O ˙ = C ˙ H 2 NHCH 2 OOH and C ˙ H 2 NHCH 2 OOH = CH 2 NH + CH2O + O ˙ H , also have much lower barriers than those in reaction system CH 3 CH 2 C ˙ H 2 + O2. In CH 3 N ˙ CH 3 + O2, the O2 addition to N atom has a tight transition state, hindering radical consumption at low temperatures. The second O2 addition to C ˙ H 2 NHCH 2 OOH is further investigated because of its comparable production at low temperatures and elevated pressures. The temperature- and pressure-dependent rate constants are calculated; CH3NCH2 + H O ˙ 2 and CH2NCH2OOH + H O ˙ 2 are the dominant products for the first and second O2 addition reactions, respectively, but the formation of formamides/O-heterocycles + O ˙ H is less important. Our results reveal different reaction pathways of DMA radicals from those of alkyls, and shed light on low-temperature oxidation mechanisms of N-containing fuels.

We investigate the onset of detwinning in magnesium alloy Mg-3Al-1Zn under continuous loading with real-time in situ synchrotron X-ray diffraction. Detwinning of the { 10 1 ¯ 2 } extension twins activated both by tension parallel to and compression perpendicular to the c-axis fibers is explored. The experimental results reveal that detwinning of the { 10 1 ¯ 2 } extension twins occurs immediately upon unloading, regardless of whether the twins are activated by tension parallel to or compression perpendicular to the c-axis fibers.

Plate impact experiments are conducted on the carbon nanotube (CNT) reinforced 2024Al composite fabricated by flake powder metallurgy and hot extrusion, to investigate the effects of microstructural anisotropy on its dynamic deformation and damage, as well as the role of CNTs. Three loading directions are explored with the loading axis being parallel to the extrusion, transverse or normal direction. Free-surface velocity histories are measured to evaluate the mechanical properties and damage processes, including the Hugoniot elastic limit (HEL; ∼ 0.8 GPa) and dynamic spall strengths ( 1.4 − 1.9 GPa). Postmortem samples are characterized with synchrotron X-ray computed tomography and scanning electron microscopy. The microstructural anisotropy of the composite (in terms of the orientation of lamellar microstructures) has a negligible effect on HEL but induces an anisotropy in spall strengths; spall strength is the highest for loading along the extrusion direction, the long axis of the lamellar microstructures. CNTs appear to increase the spall strengths of the 2024Al matrix, in contrast to other reinforcing fibers/particles. The crack propagation direction and damage features can be correlated with collinear propagation of microcracks following the lamellar microstructures.

The link between spin-phonon coupling (SPC) and coherent phonon excitation as well as hot carrier decay dynamics is investigated with femtosecond transient optical spectroscopy. Coherent phonon excitation via SPC is directly observed in a van der Waals ferromagnet Cr2Ge2Te6 (CGT). Such coherent phonon excitation is strongly dependent on spin ordering of CGT and facilitates considerably hot carrier decay. While hot carriers decay normally via direct electron-phonon coupling, hot carrier decay in ferromagnetic CGT is also achieved via indirect electron-phonon coupling, with spin ordering acting as the intermediate between hot carriers and coherent phonons.

Thermomechanical, physical, and chemical processes in energetic materials (EMs) during manufacturing and processing or under external stimuli such as shock compression, involve multiple temporal and spatial scales. Discovering novel phenomena, acquiring new data, and understanding underlying mechanisms all require temporally and spatially resolved diagnostics. Here, we present a brief review of novel diagnostics that are either emerging or have existed but rarely been applied to EMs, including two-dimensional (2D) and three-dimensional (3D) X-ray imaging, X-ray diffraction, coherent X-ray diffraction imaging, small angle X-ray scattering, terahertz and optical absorption/emission spectroscopy, and one-dimensional (1D) and 2D laser-based velocity/displacement interferometry. Typical spatial scales involved are lattice (nanometer and micrometer) and typical temporal scales (femtosecond, picosecond, nanosecond, microsecond, and millisecond). The targeted scientific questions and engineering problems include defects, strengths, deformations, hot spots, phase changes, reactions, and shock sensitivities. Basic principles of measurement and data analysis, and illustrative examples of these are presented. Advanced measurements and experimental complexities also necessitate further development in corresponding data analysis and interpretation methodologies, and multiscale modeling.

Annealing twins are synthesised in high purity aluminium processed by dynamic equal channel angular pressing during annealing. Annealing twins and recrystallised grains encircling the twins have sp...

In situ, three-dimensional (3D) characterizations of particle breakage in porous carbonate sands are presented, for the first time, with synchrotron-based micro computed tomography. Evolution of grain-scale characteristics are identified and quantified via elaborate image processing and topology analyses. The sequential 3D images reveal distinctly different fracture mechanisms for carbonate sands from silica sands. The angular shape of carbonate sand particles facilitates bending fracture, and particles with a lower sphericity and a higher porosity are more prone to break. 3D crack networks extracted from fractured particles imply considerable cleavage along initial pores. The fractal dimension of crack networks increases with external loading due to crack branching via cleavage. The resultant fragment size distribution also appears fractal and the fractal feature is valid down to the breakage limit of calcium carbonate. Crack propagation along the initial pores reduces the energy barrier for particle breakage and thus fracture strength of particles.

Recently, methyl pentanoate (MP) was proposed as a viable biodiesel surrogate to petroleum-based fuels. To better understand the pyrolysis chemistry of MP, the unimolecular decomposition kinetics of MP is theoretically investigated on the basis of ab initio calculations; ten primary channels, including four intramolecular H-shifts and six C-C and C-O bond fissions, are identified. The geometries are optimized at the M06-2X/cc-pVTZ level of theory, and accurate barrier heights are determined using the DLPNO-CCSD(T)/CBS(T-Q) method, which shows a good performance against the CCSD(T)/CBS(T-Q) method with an uncertainty of 0.5 kcal mol-1 for small methyl esters. The atomization enthalpy method is adopted to obtain the thermodynamics of involved species. The Rice-Ramsperger-Kassel-Marcus/master equation theory coupled with one-dimensional hindered rotor approximation is employed to calculate the phenomenological rate constants at 500-2000 K and 0.01-100 atm. The branching ratio analysis indicates that two reactions, MP ↔ CH3OC([double bond, length as m-dash]O)CH3 + CH2CHCH3 and MP ↔ CH3OC([double bond, length as m-dash]O)CH2 + CH2CH2CH3, are the dominant channels at low and high temperatures, respectively. The model from Diévart et al. [Proc. Combust. Inst., 2013, 34(1), 821-829] is updated with our calculations, and the modified model can yield a better prediction in reproducing the ignition delay times of MP at high temperatures. This work provides a comprehensive investigation of MP unimolecular decomposition, and can serve as a prototype for understanding the pyrolysis of larger alkyl esters.

Single-mode micro/nanolasers have been of considerable interest for their potential applications in optical communication and high-resolution spectroscopy. However, constrained by large radiation loss and insufficient gain in microcavities, realizing high-performance single-mode micro/nanolasers is still challenging. In this work, single-mode whispering-gallery-mode lasing is demonstrated from single-crystal CdS microflakes with a sharp linewidth (∼0.12 nm) and a high quality factor (Q ∼ 4200) at room temperature. Such lasers are superior to previous CdS lasers in these lasing parameters. Through time-resolved photoluminescence measurements, electron–hole plasma recombination is established to be the lasing mechanism. The radiative recombination rate of CdS microflakes is enhanced by a factor of ∼4.7 due to the Purcell effect. These CdS microflake single-mode lasers can be potentially useful in optoelectronic devices.

The reaction between CO and HO2 plays a significant role in syngas combustion. In this work, the catalytic effect of single-molecule water on this reaction is theoretically investigated at the CCSD(T)/aug-cc-pV(D,T,Q)Z and CCSD(T)-F12a/jun-cc-pVTZ levels in combination with the M062X/aug-cc-pVTZ level. Firstly, the potential energy surface (PES) of CO + HO2 (water-free) is revisited. The major products CO2 + OH are formed via a cis- or a trans-transition state (TS) channel and the formation of HCO + O2 is minor. In the presence of water, the title reaction has three different pre-reactive complexes (i.e., RC2: CO⋯HO2 + H2O, RC3: CO⋯H2O + HO2, and RC4: HO2⋯H2O + CO), depending on the initial hydrogen bond formation. Compared to the water-free process, the reaction barriers of the water-assisted process are reduced considerably, due to more stable cyclic TSs and complexes. The rate constants for the bimolecular reaction pathways CO + HO2, RC2, RC3, and RC4 are further calculated using conventional transition state theory (TST) with Eckart asymmetric tunneling correction. For reaction CO + HO2, our calculations are in good agreement with the literature. In addition, the effective rate constants for the water-assisted process decrease by 1–2 orders of magnitude compared to the water-free one at a temperature below 600 K. In particular, the effective rate constants for the water-assisted and water-free processes are 1.55 × 10−28 and 3.86 × 10−26 cm3 molecule−1 s−1 at 300 K, respectively. This implies that the contribution of a single molecule water-assisted process is small and cannot accelerate the title reaction.

The effects of temperature and grain size on mechanical properties of polycrystalline copper-graphene nanolayered (PCuGNL) composites are investigated by analytical mechanical models and molecular dynamics simulations. The yield of PCuGNL composites under tension depends on temperature, copper grain size, and repeat layer spacing. Graphene-copper interfaces play the dominant role in the ultimate tensile strength of PCuGNL composites. The optimal range for strengthening of repeat layer spacing is 2-10 nm, and the failure stress of PCuGNL composites is weakly dependent on temperature. An analytical model is proposed to accurately characterize the mechanical behaviors of PCuGNL composites.

Significance Intense lasers can be used to drive materials into transient states far from equilibrium. Investigations of such states and processes at the atomic scale are of fundamental significance in understanding a material’s behavior under extreme conditions. Herein, an ultrafast electron diffraction technique is used to track the atomic pathway of the entire melting process of aluminum and reveal a coherent breathing motion of polyhedral clusters in transient liquid aluminum at high temperature and high pressure. The negative expansion behavior of interatomic distances in a superheated liquid state upon heating is observed. These findings provide insight into ultrafast structural transformations and transient atomic dynamics under extreme conditions., Under the irradiation of an ultrafast intense laser, solid materials can be driven into nonequilibrium states undergoing an ultrafast solid–liquid phase transition. Understanding such nonequilibrium states is essential for scientific research and industrial applications because they exist in various processes including laser fusion and laser machining yet challenging in the sense that high resolution and single-shot capability are required for the measurements. Herein, an ultrafast diffraction technique with megaelectron-volt (MeV) electrons is used to resolve the atomic pathway over the entire laser-induced ultrafast melting process, from the initial loss of long-range order and the formation of high-density liquid to the progressive evolution of short-range order and relaxation into the metastable low-density liquid state. High-resolution measurements using electron pulse compression and a time-stamping technique reveal a coherent breathing motion of polyhedral clusters in transient liquid aluminum during the ultrafast melting process, as indicated by the oscillation of the interatomic distance between the center atom and atoms in the nearest-neighbor shell. Furthermore, contraction of interatomic distance was observed in a superheated liquid state with temperatures up to 6,000 K. The results provide an atomic view of melting accompanied with internal pressure relaxation and are critical for understanding the structures and properties of matter under extreme conditions.

Perovskite nanocrystals (NCs) have emerged as attractive gain materials for solution-processed microlasers. Despite the recent surge of reports in this field, it is still challenging to develop low-cost perovskite NC-based microlasers with high performance. Herein, we demonstrate low-threshold, spectrally tunable lasing from ensembles of CsPbBr

The classical $B1(\mathrm{NaCl})\ensuremath{\leftrightarrow}B2(\mathrm{CsCl})$ transitions have been considered as a model for general structural phase transformations, and resolving corresponding phase transition mechanisms under high strain rate shock compression is critical to a fundamental understanding of phase transition dynamics. Here, we use subnanosecond synchrotron x-ray diffraction to visualize the lattice response of single-crystal KCl to planar shock compression. Complete $B1\text{\ensuremath{-}}B2$ orientation relations are revealed for KCl under shock compression along ${⟨100⟩}_{B1}$ and ${⟨110⟩}_{B1}$; the orientation relations and transition mechanisms are anisotropic and can be described with the standard and modified Watanabe-Tokonami-Morimoto model, respectively, both involving interlayer sliding and intralayer ion rearrangement. The current study also establishes a paradigm for investigating solid-solid phase transitions under dynamic extremes with ultrafast synchrotron x-ray diffraction.

The H-abstraction reaction kinetics of methyl acrylate (MA) + H/OH/CH

A special spallation morphology in bulk metallic glass, named as the “cup-cone” structure, is of particular interest since it manifests a unique “ductile–brittle” transition. To gain insights into the underlying mechanism for the formation of a cup-cone structure, we conduct planar impact experiments at various impact velocities, as well as finite element method analysis. Spall strength increases with increasing impact velocity. Scanning electron microscopy and X-ray computed tomography are performed on postmortem samples to characterize cup-cone structures; their average size and spacing decrease as impact velocity increases, and they dominate fracture morphology at high impact velocities. Cups and cones are generally distributed on the side away from and on the side closer to the target free surface, respectively. The initial nucleation sites of voids become the conical vertices of cup-cones, and the subsequent nucleation sites form along the conical surface and coalesce into the cracks and fracture surfaces.

We investigate deformation dynamics in a widely used closed-cell foam of polymethacrylimide under uniaxial compression with in situ X-ray computed tomography and digital volume correlation. Axial strain mapping demonstrates discrete deformation bands nucleated in sequence across the sample. A buckling strength index is proposed to quantify the buckling resistance of cell walls, based on their morphology extracted via an edge-segmentation procedure developed in this work. The spatial distribution of the weakest cell walls is correlated well with the location and nucleation sequence of deformation bands, and can be used for predicting deformation banding.

The influence of segregation bands of alloying elements Cr and Ni on impact response of a 304 stainless steel is investigated under high strain rate impact loading with a single-stage gas gun. Free-surface velocity histories are measured to evaluate the mechanical properties. The Hugoniot elastic limits are 1.0–1.1 GPa, and the spall strengths are 2.1–2.4 GPa for the impact velocities explored. Oriented segregation bands induce negligible anisotropy in the Hugoniot elastic limit (dynamic yield stress) and spall strength. Scanning electron microscopy analyses show that microvoids tend to coalesce along the segregation bands, regardless of the loading direction, indicating segregation bands or the interfaces between segregation bands and matrix are the weakest nucleation sites. The fracture strength is dominated by the segregation bands, and thus independent of strain rate and peak stress. However, the damage degree is related to both peak stress and segregation band orientation. When the peak stress is lower than 5 GPa, damage degree is independent of the loading direction. At higher peak stresses, the damage degree is larger for impact direction aligned along segregation bands.

Dynamic and quasi-static compression experiments are conducted on a high-density porous titanium with in situ X-ray imaging to characterize its deformation dynamics. The porous titanium exhibits higher strain-rate sensitivity than full-density titanium. Strain field mapping indicates that compaction bands are narrower and denser under dynamic loading, leading to a more homogeneous strain distribution and thus a higher flow stress. The mean Poisson’s ratio for dynamic loading is 15% lower than that for quasi-static loading at the same bulk strain, as a result of local inertial confinement which also contributes to the rate hardening of porous titanium.

Ignition delay times for methane/nitromethane binary fuels with oxygen diluted in Ar ( CH 4 / CH 3 NO 2 / O 2 /Ar mixtures) are measured with a heated shock tube behind the reflected shock wave at 995–1962 K and 4–18 atm for different equivalence ratios (0.5, 1.0, and 2.0). Blending ratios for CH 3 NO 2 are varied (0, 0.1, 0.2, 0.5, and 1) to investigate the promotion of CH 3 NO 2 on CH 4 during the ignition process. OH∗ emission histories at a sidewall are used to determine the measured ignition delay times of the binary fuels mixture. Correlations for the measured ignition delay times are obtained using multiple linear regression as a function of pressure, temperature, blending ratio, and equivalence ratio. Experimental results show that a small addition of CH 3 NO 2 dramatically reduces the ignition delay times of CH 4 , and induces a weak two-stage ignition. Three literature mechanisms are examined and compared with the measured ignition delay times for CH 4 / CH 3 NO 2 binary fuels mixtures, and the mechanism from Mathieu et al. is updated and validated against the experimental data. In this mechanism, the rate constants for some important reactions are theoretically determined by quantum chemical calculation. In addition, sensitivity analysis and reaction pathway analysis as well as rate of production are carried out to further elucidate the promotion of CH 3 NO 2 on ignition of the CH 4 .

The effects of loading on spall strength of a mild steel are investigated systematically under the flat-top impact loading, including the effect of tensile stress history and the effect of shock-induced microstructure. Peak stress, strain rate, pulse duration, spall strength and tensile stress history are obtained directly or indirectly from free-surface velocity measurements, and the recovered samples are characterized with electron backscatter diffraction analysis. For different tensile stress histories, spall strength depends on peak stress or strain rate, depending whether spall occurs on the plateau or rising edge of a tensile pulse. Pulse duration has an effect on whether spall occurs (incipient spallation) but not spall strength. Low shock stresses induce small microstructure change (dislocations) which has weak effects on nucleation and propagation of brittle cleavage cracks, while high shock stresses lead to deformation twins (in addition to dislocations) which act as the source of crack nucleation. A model is proposed to predict the spall strength of the mild steel under arbitrary loading conditions (peak stress below the phase transition).

The stability of dynamic fracture is a fundamental and challenging problem in the field of materials science. The grain size effect on dynamic fracture instability in polycrystalline graphene under tear loading is explored via theoretical analysis and molecular dynamics simulations. The fracture stability phase diagram in terms of grain size and crack propagation velocity is obtained, and three regions of crack propagation are identified: stable, metastable, and unstable. For grain size above 2 nm, there exists a critical velocity beyond which fracture instability occurs, and this critical velocity depends linearly on grain size. Decreasing grain size leads to reduced characteristic time for correction of crack path deflection, which plays a dominant role in dynamic fracture instabilities. However, when grain size is below 2 nm, there does not exist a critical velocity for steady propagation of cracks due to discontinuous effects. Our results also provide a valuable insight into dynamic fracture of polycrystalline graphene as well as other 2D and quasi-2D materials.

Deformation twinning in a mild steel is investigated under quasi-isentropic compression (IC) and shock compression (SC) to about 12 GPa, as regards the effects of pulse duration (plateau width) and strain rate (rising edge slope), and associated twin strengthening. The pulse duration ranges from 80 ns to 790 ns; and two rise times are explored, approximately 30 ns for SC and 300 ns for IC. Free-surface velocity histories are measured to obtain strain and strain rate. The postmortem samples are characterized with electron backscatter diffraction, and the yield strengths of the samples pre-deformed by impact are examined with a materials testing system. For SC, twin density and size increase with pulse duration up to 580 ns. At longer pulse durations, the increase in twin density is stagnated as a result of deviatoric stress relaxation, while twin size continues growing. For IC (410 ns), twin density and size are much larger than the SC counterpart, as a result of shallower rising edge. Increased deformation time can compensate the effects of reduced strain rate or applied stress for deformation twinning. Twin strengthening effect is strong in postmortem samples, depends on twin density instead of area fraction, and follows the empirical Hall–Petch relationship.

Vanadium dioxide (VO2), due to its characteristic semiconductor–metal transition (SMT) at 341 K, is a promising material for optical modulators in the terahertz (THz) regime. In this study, doping Co ions into epitaxial VO2 films have been demonstrated to acquire SMT with a reduced critical temperature, a large modulation depth, and a narrow transition region in the THz regime, which would benefit the applications of VO2 film in THz switches and memory devices. Utilizing Co ions, a 77% THz modulation ratio within a 3 °C transition region appeared when the doping content was 4.0 at. %. Moreover, more doping content induced two different phases in VO2 films, identified by X-ray diffraction (XRD), which would decrease the THz modulation ratio. Theoretical analysis based on the Drude-Smith model and the Bruggeman effective medium theory predict that the phase containing more Co ions might be transparent to THz waves and void of switching ability in the temperature range from 30 °C to 80 °C. This phase separation may be responsible for the complex correlation between the Co doping concentration and the SMT properties.

Dimethyl amine (DMA) is identified as a promising nitrogen-containing fuel candidate. To better understand the atmospheric and combustion chemistry of aliphatic amines, we systematically investigate the reaction kinetics of H-abstractions from DMA by H, CH3, OH, and HO2 radicals in a broad temperature range (100–2000 K). The BHandHLYP/cc-pVTZ method is adopted to determine the optimized geometries and frequencies, and the multi-structural torsional anharmonicity method (MS-T) is employed to characterize the effects of multi-conformer and torsional coupling for the involved species. High-level methods CCSD(T) and CCSD(T)-F12 combined with cc-pVXZ (X = D, T, Q), cc-pVXZ-F12 (X = D, T), and jun-cc-pV(T+d)Z basis sets are used to refine the electronic energies. The results of the gold standard method CCSD(T)/CBS(D–T–Q) with the zero point energy correction are adopted for the kinetic calculations. For the DMA + H/CH3 reactions, the conventional transition state theory (cTST) as well as one-dimensional Eckart tunneling correction is adopted. But for the DMA + OH/HO2 reactions, the reactant-complex (RC) is formed with a deep well (−6.4 and −11.7 kcal mol−1 for RC3 and RC4, respectively), due to the strong hydrogen bonding between the reactants. Hence, the variational transition state theory (VTST) combined with cTST is used to calculate the rate constants. The Rice–Ramsperger–Kassel–Marcus/master equation method is employed to determine the pressure-dependent rate constants in the pressure range of 0.001–100 atm. Our calculations are in agreement with previous experimental measurements and show well the trend in a broad temperature range. In addition, a pronounced pressure-dependence is observed under 400 K, indicating that pressure impacts the reaction mechanisms especially at atmospheric or interstellar temperatures.

Plate impact experiments are conducted on a rolled titanium alloy Ti-6Al-4V to investigate the effects of structural anisotropy on dynamic deformation and damage, with the loading axis being parallel to the normal direction or transverse direction, referred to as LA ∥ ND and LA ∥ TD, respectively. Free-surface velocity histories are measured to evaluate the mechanical properties; the Hugoniot elastic limits for the LA ∥ ND and LA ∥ TD loading are 3.38 GPa and 3.75 GPa, respectively; the spall strengths are 4.54–4.71 GPa for the LA ∥ ND loading, and 4.69–4.91 GPa for the LA ∥ TD loading, for the impact velocities explored. The postmortem samples are characterized with scanning electron microscopy, electron backscatter diffraction and synchrotron X-ray computed tomography. The { 10 1 ¯ 2 } extension twins are activated near spall cracks and their number increases with increasing impact velocity. Microvoids nucleate at both grain (in the matrix) and phase boundaries, and grain boundary nucleation is more likely to occur given the small volume fraction of the β -phase. Cracks for the LA ∥ ND loading are generally perpendicular to the shock direction, while they are serrated and distributed more discretely for the LA ∥ TD loading. The damage degree for the LA ∥ ND loading is larger than for the LA ∥ TD loading, consistent with the corresponding spall strengths. At incipient spall, the average size of voids/cracks is larger for the LA ∥ ND loading than for the LA ∥ TD loading. Compared with the LA ∥ ND loading, small voids/cracks are more isotropic and large voids are more anisotropic for the LA ∥ TD loading.