Molecular dynamics study on the correlation between structure and sensitivity for defective RDX crystals and their PBXs.

Molecular dynamics simulation was applied to investigate the sensitivities of perfect and defective RDX (cyclotrimethylene trinitramine) crystals, as well as their PBXs (polymer-bonded explosives) with the polymeric binder F(2311), in the NPT (constant number of particles, constant pressure, constant temperature) ensemble using the COMPASS force field. Five kinds of defects-two dislocations, one vacancy, and two types of doping-were considered separately. The bond length distribution and the maximum (L (max)) and average (L (ave)) bond lengths of the N-NO(2) trigger bonds in RDX were obtained and their relationships to the sensitivities of RDX and PBXs are discussed. L (max) was found to be an important structural parameter for judging the relative sensitivity, and defects were observed to have little effect on the sensitivities of PBXs, due to the strong desensitizing effect of the polymer F(2311).

Theoretical studies of solid bicyclo-HMX: effects of hydrostatic pressure and temperature.

On the basis of density functional theory (DFT) and molecular dynamics (MD), the structural, electronic, and mechanical properties of the energetic material bicyclo-HMX have been studied. The crystal structure optimized by the LDA/CA-PZ method compares well with the experimental data. Band structure and density of states calculations indicate that bicyclo-HMX is an insulator with the band gap of ca. 3.4 eV and the N-NO(2) bond is the reaction center. The pressure effect on the bulk structure and properties has been investigated in the range of 0-400 GPa. The crystal structure and electronic character change slightly as the pressure increases from 0 to 10 GPa; when the pressure is over 10 GPa, further increment of the pressure determines significant changes of the structures and large broadening of the electronic bands together with the band gap decreasing sharply. There is a larger compression along the c-axis than along the a- and b-axes. To investigate the influence of temperature on the bulk structure and properties, isothermal-isobaric MD simulations are performed on bicyclo-HMX in the temperature range of 5-400 K. It is found that the increase of temperature does not significantly change the crystal structure. The thermal expansion coefficients calculated for the model indicate anisotropic behavior with slightly larger expansion along the a- and c-axes than along the b-axis.

Molecular dynamics simulations of trans-1,4,5,8-tetranitro-1,4,5,8-tetraazadecalin-based polymer-bonded explosives.

Molecular dynamics has been applied to investigate the low-sensitivity explosive TNAD (trans-1,4,5,8-tetranitro-1,4,5,8-tetraazadecalin)-based polymer-bonded explosives (PBXs) with four typical fluorine polymers, PVDF (polyvinylidenedifluoride), PCTFE (polychlorotrifluoroethylene), F(2311) (fluorine rubber), and F(2314) (fluorine resin). The elastic constants, mechanical properties (tensile modulus, bulk modulus, shear modulus, and Poission ratio), binding energies, and detonation performances are first reported for the TNAD-based PBXs. The results show that the mechanical properties of TNAD can be effectively improved by the addition of small amounts of fluorine polymers, and the overall effect of fluorine polymers on the mechanical properties of the PBXs along three crystalline surfaces is (001) > (010) > (100). On each crystal surface, improvement in the ductibility made by the fluorine polymers changes approximately in the sequence of PVDF > F(2311) > F(2314) > PCTFE. The binding energies between different TNAD crystalline surfaces and different polymer binders with the same chain segment or mass fraction both decrease in the order of (010) > (100) > (001). The binding properties of the polymers with the same chain segment on each crystal surface of TNAD increase as PVDF < F(2311) < F(2314) < PCTFE, while those of different polymers in the same content decrease in the sequence of PVDF > F(2311) > F(2314) > PCTFE. The detonation performances of the PBXs decrease in comparison with the pure crystal, but they are superior to those of TNT.

Ab initio and molecular dynamics studies of crystalline TNAD (trans-1,4,5,8-tetranitro-1,4,5,8-tetraazadecalin).

The structural and electronic properties of the energetic crystal TNAD (trans-1,4,5,8-tetranitro-1,4,5,8- tetraazadecalin) have been studied using plane-wave ab initio calculations based on the density function theory method with the ultrasoft pseudopotentials. It is found that the predicted crystal structure is in good agreement with experimental data and there are strong inter- and intramolecular interactions in bulk TNAD. Band structure calculations indicate that TNAD is an insulator with the band gap of ca. 3.3 eV. The hydrostatic compression effect on TNAD has been studied in the pressure range of 0-600 GPa. The results show that a pressure less than 10 GPa does not significantly change the geometric parameters, charge distributions, and electronic bands. When the pressure is over 10 GPa, increasing the pressure determines significant changes of the geometrical and electronic structures and large broadening of the electronic bands together with a sharp decrease of the band gap. Isothermal-isobaric molecular dynamics simulations at atmospheric pressure were further performed on the TNAD crystal in the temperature range 5-500 K. Average equilibrium lattice parameters and elastic properties as functions of temperature were determined. The thermal expansion coefficients calculated for the crystal indicate anisotropic behavior with the largest expansion along the b axis.

Molecular dynamics simulations for pure epsilon-CL-20 and epsilon-CL-20-based PBXs.

Molecular dynamics has been employed to simulate the well-known high energy density compound epsilon-CL-20 (hexanitrohexaazaisowurtzitane) crystal and 12 epsilon-CL-20-based PBXs (polymer bonded explosives) with four kinds of typical fluorine polymers, i.e., polyvinylidenedifluoride, polychlorotrifluoroethylene, fluorine rubber (F(2311)), and fluorine resin (F(2314)) individually. The elastic coefficients, isotropic mechanical properties (tensile moduli, bulk moduli, shear moduli, and poission's ratios), and bonding energy are first reported for epsilon-CL-20 crystal and epsilon-CL-20-based polymer bonded explosives (PBXs). The mechanical properties of epsilon-CL-20 can be effectively improved by blending with a small amount of fluorine polymers, and the whole effect of the adding fluorine polymers to improve mechanical properties of PBXs along the three crystalline surfaces of epsilon-CL-20 is found to be (100) approximately (001) > (010). The interaction between each of the crystalline surfaces and each of the fluorine polymers is different, and the ordering of binding energy for the three surfaces is (001) > (100) > (010); F(2314) always has the strongest binding ability with the three different surfaces. F(2314) can best improve the ductibility and tenacity of PBX when it is positioned on epsilon-CL-20 (001) crystal surface. The calculations on detonation performances for pure epsilon-CL-20 crystal and the four epsilon-CL-20-based PBXs show that adding a small amount of fluorine polymer into pure epsilon-CL-20 will lower detonation performance, but each detonation parameter of the obtained PBXs is still excellent.