Molecular Dynamics Studies On The Structure And Properties Of Energetic Systems

The present dissertation is devoted to systematic research on the structures and properties of the energetic composites, which are mainly composed of the typical cyclic nitramines such as hexhydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), ammonium perchlorate (AP: NH₄ClO₄) and polymers. Molecular dynamics (MD) simulations have been performed to investigate such energetic composites and provide plentiful information for the practical formulation design of energetic materials. The whole work can be divided into two parts:The first part focuses on theoretical studies of RDX and RDX-based polymer bonded explosives (PBXs). Periodic NPT-MD simulations in the temperature range of 195-445K are performed on RDX using COMPASS force field to study their crystal structures and properties. The effects of temperature on the crystal structures, thermal expansion, and mechanical properties of RDX are observed. Their lattice parameters and cell volume all increase with the increasing temperature, while the linear and volume expansion coefficients increase slightly as the temperature increases. Elastic constants and mechanical moduli all decrease with the increasing temperature, indicating that the system's rigidity weakens and ductibility decreases.The "cutting surface" method as implemented in the MS program package is chosen to cleave the RDX supercells along three different crystalline surfaces (001), (010), and (100). Four types of typical fluorine polymer binders PVDF, PCTFE, F₂₃₁₁, and F₂₃₁₄ are then put on three crystalline surfaces of RDX, and a dozen of RDX-based PBX models are obtained. MD simulations are carried out on them with the COMPASS force field to get their structures and properties. Mechanical properties, binding energies, detonation performances, and their variation trend with the types and contents of polymer binders and different crystalline surfaces are first reported for the RDX-based PBXs. The results show that the mechanical properties of the PBXs can be effectively improved by adding small amounts of fluorine polymers and the overall effect of fluorine polymers on the mechanical properties of the PBXs along three crystalline surfaces is different and PVDF is regarded to best improve the mechanical properties of the PBXs on three surfaces. The interactions between each fluorine polymer and different crystalline surfaces of RDX all decrease in the order of PVDF > F₂₃₁₁ > F₂₃₁₄ > PCTFE. The additions of small amounts of fluorine polymers slightly decrease the detonation properties compared with the pure crystal, but they are still superior to the commonly used explosive TNT and hence can be used as good energetic materials with high exploitation and application values.The better PBX's model (RDX/F₂₃₁₁) is chosen as an example to investigate the influences of temperature on the performances of the PBX. The "cutting" model was used and NPT-MD simulations at various temperatures have been performed. The results show that the temperature can affect mechanical properties but influence little on binding energy. By the analysis of pair correlation function g(r), the essence of the interaction between the blinders and RDX is revealed.The second part concentrates on the structures and properties of various energetic composites. NVT-MD simulations have been performed on a lot of mixed and complex systems of propellants in MPCFF force field. A theoretical criterion of sensitivity has been suggested, which can help judge or predict the relative safety of energetic composites.We parameterized an all-atom force field for AP from ab initio quantum-chemical calculations and experimental data with the framework of PCFF force field. The modified PCFF force field parameters for AP were validated by calculating the structural parameters of AP molecule and crystal. The results show good agreement between the force-field-based results and experimental values. MD simulations have been performed to investigate AP/HMX composite at different temperatures. The binding energies and thermal expansion coefficient of AP/HMX composite have been obtained. By the analysis of pair correlation function g(r), the essence of the interactions between the AP and HMX is revealed.In order to find out the theoretical criterion for the sensitivity of energetic composites, MD simulations have been performed to investigate AP/HMX composites at various concentrations and temperatures in NVT ensemble using the modified PCFF force field. The binding energies and the average and maximum bond lengths of pyrogenation trigger bond N-NO2 in the AP/HMX composites were obtained and it is suggested that the maximum trigger bond length of the most sensitive component in energetic composites can be used as a theoretical criterion of thermal or impact sensitivity to judge or predict the relative safety of energetic composites.NVT-MD simulations have been performed on a lot of mixed and complex systems of propellants, such as AP/NG, AP/BTTN, HMX/BTTN, HMX/NG, PEG/AP, PEG/HMX, PEG/NG, PEG/BTTN, (NG/BTTN)/AP and (PEG/NG/BTTN)/AP/HMX.The binding energies and the average and maximum bond lengths of pyrogenation trigger bond in such composites were obtained. These can provide abundant information and reference evidence for judging or predicting the relative safety of energetic composites. All in all, MD simulations are employed to investigate the relationship between structures and properties of PBXs and energetic composites. These studies are precursory and originally innovative in front of multi-subject crossing research fields of energetic materials, and successfully complete the research tasks assigned by the national "973" projects and National Natural Science Foundations.