Feng-Lei Huang

Experiments and modeling of HMX granular explosives subjected to drop-weight impact

The drop-weight impacts on a thin layer of cyclotetramethylene tetranitramine (HMX) explosive particles have been performed. The drop-weight impact machine is equipped with high-speed photographic techniques. Effects of drop heights and the amount of particles on compaction, deformation, and thermal responses were investigated. Considering the contact plasticity, friction, melting, fracture, as well as chemical reaction, an analytical model based on heating and kinematics equations for a single layer of explosive particles has been developed. The calculated average pressure, loading time and time-to-ignition agree well with experimental measured ones. Threshold conditions for ignition can be derived from the predicted temperature history curves. Individual particles’ pressure–strain exhibited stress–strain-like relationships, from which close relations can be found between experimental phenomena and the materials softening/hardening characteristics.

Hydrogen storage and selective carbon dioxide capture in a new chromium(III)-based infinite coordination polymer

A new chromium(III)-based infinite coordination polymer (ICP) was synthesized under solvothermal conditions. The morphology, particle size and the textural porosity of the resulting solids could be monitored and controlled by adjusting the volume ratio of the solvent mixture (1,4-dioxane/N,N′-dimethylformamide). Two differently textured samples were further compared using powder X-ray diffraction, scanning electron microscopy, FT-IR spectroscopy, solid-state UV-Vis spectroscopy and thermogravimetric analysis. Their sorption capacities for H2, CO2 and CH4 were measured by using volumetric gas adsorption, while the isosteric heats of adsorption for the same gasses were calculated based on gas sorption data at different temperatures. Both samples showed a remarkable selectivity for CO2 over CH4 at 273 K. H2 and CO2 sorption capacities, (respectively 16.9 mg g−1 at 77 K and 817 mm Hg and 168 mg g−1 at 273 K and 760 mm Hg) of the sample composed of the nanosized particles, are comparable or even superior to values reported for most MOFs and ZIFs under similar conditions.

All-atom, non-empirical, and tailor-made force field for α-RDX from first principles

A general strategy is presented to develop an all-atom, non-empirical, and tailor-made force field (NETMFF) for high explosives (HEs). The central part of the strategy is a self-consistent force field (SCFF) optimization technique. The consistence of the force field is ensured by iterating the parameterization procedure. The generation of reliable ab initio reference data for optimizing the NETMFF parameters and the SCFF technique are discussed in detail. Starting with the crystal structure obtained from either experiment or crystal structure prediction (CSP), NETMFF can be developed only by first principles, including conventional DFT, the periodic DFT-D model, and the SAPT(DFT) plus Williams–Stone–Misquitta (WSM) methods. The development strategy of NEMTFF has been applied to α phase hexahydro-1,3,5-trinitro-1,3,5-triazine (α-RDX), an important high explosive. The force field obtained for α-RDX can yield the correct geometries for all RDX conformers found by the DFT calculations in a good way, and also can identify the transition states obtained from the DFT calculations by force field-based vibration analysis. More importantly, it can accurately predict densities of high explosives under the environmental conditions to which they are often subjected, a long-standing issue in the field of energetic materials. The parameterization strategy described in this paper can be easily generalized toward other known HEs or the new HEs whose crystal structures can be obtained by CSP.

A reactive molecular dynamics study on the anisotropic sensitivity in single crystal α-cyclotetramethylene tetranitramine

The anisotropic shock sensitivity in single crystal α-cyclotetramethylene tetranitramine (α-HMX) was investigated using the compress-shear reactive dynamics (CS-RD) computational protocol. Anisotropy in thermo-mechanical and chemical responses is found by measuring shear stress, energy, temperature, and chemical reactions during the dynamical process for shock directions perpendicular to the (010), (001), (100), (110), (011), (111), and (101) planes. We suggest that the internal energy accumulated within the duration of the surmounting shear stress barrier can be used as a useful criterion to distinguish the anisotropic sensitivity among various shock orientations. Accordingly, the α-HMX single crystal is predicted to be sensitive for the shock normal to the (010) plane, is intermediate to the (001) plane, and is insensitive to the (100), (110), (011), (111), and (101) planes. The molecular origin of the anisotropic sensitivity is considered to be the different intermolecular steric arrangements on the two sides of the slip plane induced by shock compression along various orientations. The shear deformation induced by shock compression along sensitive directions encounters strong intermolecular contact and has little intermolecular free space for geometry relaxation when molecules collide, leading to a high shear stress barrier and energy accumulation, which benefit a temperature increase and initial chemical bond dissociation that trigger further reactions. This validation of CS-RD indicates that this approach would be valuable in examining the anisotropic sensitivity of new energetic crystals and in evaluating which one would be least sensitive.

Anisotropic constitutive model and numerical simulations for crystalline energetic material under shock loading

A dynamic crystal plasticity model for a low-symmetric β-cyclotetramethylene-tetranitramine single crystal with only limited operative slip systems has been developed, accounting for nonlinear elasticity, volumetric coupling with deviatoric behavior and thermo-dynamical consistence. Based on the decomposition of the stress tensor, a modified equation of state for anisotropic materials is employed. Simulations of the planar impact on the β-cyclotetramethylene-tetranitramine single crystal show good agreement with existing particle velocity data in the case of (110) and (011). Pressure snapshots, the dislocation density, the shear stress and the strain localization for β-cyclotetramethylene-tetranitramine single crystal under shocked loading are discussed. The present model provides more insights into a range of complex, orientation-dependent elastic and inelastic behaviors in shocked explosive crystals than isotropic elastic–plastic constitutive descriptions. The proposed formulation and algorithms can also be applied to study other low-symmetric crystals under high-pressure shocked loading that deform mainly by crystallographic slip.