ABSTRACT
1,3,5-tri-amino-2,4,6-tri-nitrobenzene (TATB) is a typical and wide- studied explosive molecular and its single crystal is a typical triclinic lattice with a, b, c respectively 9.010, 9.028, 6.812 angstrom and α, β, γ 108.59, 91.82, 119.97. This explosive crystal is a stiff and anisotropic material. Packing of polymer on its crystal surface can obviously improve its mechanics properties. As a powerful tool, molecular dynamics (MD) simulation can be used to calculate the mechanics properties mainly elastic properties. But when applied the software package of Materials Studio (MS) to carry out MD, only the isotropic elastic properties which were averaged in x, y and z directions of materials can be obtained. To calculate the elastic properties in one direction of anisotropic materials, we developed a method which is similar to the experimental determination of elastic properties in one direction. Firstly, a P1 periodic super cell of TATB with 34.06×36.04 ×28.838 angstroms which ab plane was designated as (0 1 0) planar of the crystal was constructed. After several fixing and relaxing steps, the cell was pre-equilibrated 500 ps and performed 100 ps MD at 298 K within NVT ensemble. The averaged isotropic tensile modulus and Poisson’ ratio were calculated by MS analysis module of elastic properties (static) and they were 1796±90 GPa and 0.231 respectively. Secondly, to obtain the elastic properties in each direction, NPT ensemble was chosen and different stresses in six different directions were added through many tries to keep the cell parameters fluctuating around those values in NVT ensemble while performing constant pressure MD. The MS averaged isotropic tensile modulus and Poisson’ ratio of well equilibrated NPT’s system were then 2361 ±30 GPa and 0.273 respectively. Finally, while carrying out subsequent constant pressure MD, different magnitude of compressive or tensile stress was applied to the cell in one direction. The elastic properties were then obtained via the strain-stress profile. The calculated anisotropic tensile modulus was separately 1423, 2558 and 1955 GPa in x, y and z direction and the averaged value was 1979 GPa while averaged Poisson’s Ratio was 0.217. These two averaged values were well agreed with the MS calculated ones within about 20% deviation. The result revealed that our method can be applied to calculate elastic properties of anisotropic materials.
