Study on the Polycrystalline Mechanism of Polycrystalline Diamond Synthesized from Graphite by Direct Detonation Method.

In this paper, a polycrystalline diamond was synthesized by the direct detonation method using graphite as the carbon source. By comparing the numbers of the obtained diamond particles and the original graphite particles, it was found that when the graphite phase transformed into the polycrystalline diamond during the detonation process, a single graphite particle would form multiple diamond nuclei, and the nuclei would grow simultaneously to form polycrystals. Accordingly, a validation experiment was designed, which added different ratios of inert additives while keeping the ratio of graphite to hexogen (RDX) unchanged. It was found that increasing the ratio of inert additives within a certain range could increase the grain size of a polycrystalline diamond, which is consistent with the obtained polycrystalline mechanism.

Wide range temperature-dependent deformation and fracture mechanisms for 8701 under dynamic and static loading.

Although the RDX-based composite explosive 8701 explosive 8701 has been widely used to achieve military goals, its mechanical properties have not been carefully investigated. In the present study, we focused on the mechanical response of 8701 at a wide range temperature from -125 °C to 100 °C under both quasi-static (about 0.001 s-1) and high-rate compression loading (about 600 s-1). The stress-strain curves exhibit different tendencies at different temperatures for both quasi-static and high strain-rate loading. The failure stress and elastic/storage modulus present important temperature-dependence. Differential scanning calorimetry (DSC) tests showed that the glass transition temperature and softening temperature of 8701 are 11.61 °C and 15.14 °C respectively, which is lower than that of the binder (with glass transition temperature of 25 °C and softening temperature 38 °C). For the quasi-static loading, scanning electron microscopy (SEM) observations revealed that 8701 shows an interface debonding failure mode along the binder phase below 15 °C, while the mechanical behavior of 8701 is dominated by softening behavior of the binder above 38 °C. For high-rate loading, 8701 shows a mixture of interface debonding and trans-granular cleavage when below 15.14 °C.

Anisotropic shock sensitivity in a single crystal δ-cyclotetramethylene tetranitramine: a reactive molecular dynamics study.

The anisotropic shock sensitivity in a single crystal δ-cyclotetramethylene tetranitramine (δ-HMX) was investigated using the compress-shear reactive dynamics (CS-RD) computational protocol. Significant anisotropies in the thermo-mechanical and chemical responses were found by measuring the shear stress, energy, temperature, and chemical reactions during the dynamical process for the shock directions perpendicular to the (100), (010), (001), (110), (101), (011), and (111) planes. We predict that δ-HMX is sensitive for the shocks perpendicular to the (111), (011), (110), and (101) planes, which is intermediate to the (100) and (010) plane and is insensitive to the (001) plane. The internal energy accumulated within the duration of the surmounting shear stress barrier is a useful criterion to distinguish the sensitive directions from the less sensitive ones. The molecular origin of the anisotropic sensitivity is suggested to be the intermolecular steric arrangements across a slip plane induced by shock compression. The shear deformation induced by the shock along the sensitive direction encounters strong intermolecular contacts and has small intermolecular free space for geometry relaxation when the molecules collide, leading to high shear stress barriers and energy accumulation, which benefits the temperature increase and initial chemical bond breaking that trigger further reactions.

Effects of defects on thermal decomposition of HMX via ReaxFF molecular dynamics simulations.

Effects of molecular vacancies on the decomposition mechanisms and reaction dynamics of condensed-phase ß-HMX at various temperatures were studied using ReaxFF molecular dynamics simulations. Results show that three primary initial decomposition mechanisms, namely, N-NO(2) bond dissociation, HONO elimination, and concerted ring fission, exist at both high and lower temperatures. The contribution of the three mechanisms to the initial decomposition of HMX is influenced by molecular vacancies, and the effects vary with temperature. At high temperature (2500 K), molecular vacancies remarkably promote N-N bond cleavage and concerted ring breaking but hinder HONO formation. N-N bond dissociation and HONO elimination are two primary competing reaction mechanisms, and the former is dominant in the initial decomposition. Concerted ring breaking of condensed-phase HMX is not favored at high temperature. At lower temperature (1500 K), the most preferential initial decomposition pathway is N-N bond dissociation followed by the formation of NO(3) (O migration), although all three mechanisms are promoted by molecular vacancies. The promotion effect on concerted ring breaking is considerable at lower temperature. Products resulting from concerted ring breaking appear in the defective system but not in the perfect crystal. The mechanism of HONO elimination is less important at lower temperature. We also estimated the reaction rate constant and activation barriers of initial decomposition with different vacancy concentrations. Molecular vacancies accelerate the decomposition of condensed-phase HMX by increasing the reaction rate constant and reducing activation barriers.