含能材料冲击起爆机理的量子分子动力学研究

Despite decades of research, the microscopic details and extreme states of matter found within a detonating high explosive have yet to be elucidated. To simulate the process of shock initiation of energetic materials, the development of an efficient and relatively high accurate quantum molecular dynamics (QMD) model has been suggested in this proposal. QMD simulations provide a potentially powerful means of exploring the details of processes that are difficult to study in the lab, because they require no knowledge or simplifying assumptions about the chemistry of the system. QMD works by calculating the forces on each of the atoms in a system from first principles using the laws of quantum mechanics, which are then used to determine how the atoms should move according to Newton's laws of motion. QMD not only provides information about the dynamical evolution of a system's atomic structure but also its electronic structure. This is very important for investigating the occurrence of a transition from an insulating to a conducting state during the detonation..The model is a direct dynamics simulation method, which means the energies of system and the force arising between atoms are calculated on the fly. The chemical and electronic dynamics in a shock wave are calculated for up to several nanoseconds behind the shock front represented by the density-functional-based tight-binding method (DFTB). This represents a reasonable compromise, being orders of magnitude faster than a complete density-functional description, but one that has been proved to give reliable structural and energetic data that are comparable to a full ab initio description, and consistent with experimental data of molecular systems and condensed matter. Such a long-duration simulation is enabled by multi-scale shock-wave molecular-dynamics technique, which opens the door to longer-duration detonation simulations than previously possible by several orders of magnitude. Modeling will provide detailed information about the fundamental chemical and physical processes that control the conversion of an energetic material after shock to products, including reaction and failure waves, phase transitions, and defect formation and propagation. This study will provide the designer with information with which he can tailor a material for specific performance. Such predictive capabilities also allow for screening of notional materials upon conception, allowing for elimination of poor candidates before investing in synthesis and testing.

对于含能材料在极端条件下爆轰这一快速剧烈反应，几十年来尽管许多实验与理论方法都致力于揭示其微观细节，但人们对其反应机理的了解却依然甚少。本项目建议发展一个高效且相对高精度量子分子动力学模型模拟含能材料冲击起爆的过程。该模型是一种直接动力学模拟方法，即不预先构建势能面，体系的能量和作用于原子核的力采取随用随算（on the fly）模式。体系的电子结构计算选用基于紧束缚近似的密度泛函方法（DFTB），对冲击波的描述选择多尺度近似方法，该方法在长时间尺度的模拟中比传统方法快几个数量级，模拟时间可长达几个纳秒。模拟结果将呈现反应中化学和物理变化过程，给出从反应物到产物的化学反应方式、热力学性质随时间变化等详细信息，以阐明分解爆炸机理。本项目的研究将为现有含能材料的改进，新型含能材料的筛选、设计、优化等提供先期的理论预测和科学依据。

由于含能材料爆轰的瞬间性，运用实验手段、实验技术来跟踪爆轰过程，对其爆轰性能进行跟踪和描述非常困难。理论研究大多局限于结构、物理性质等方面的静态研究，而在动态研究中，经典分子动力学模拟精度不够，从头算分子动力学模拟计算量巨大，模拟的过程仅能到达几个皮秒，与爆炸过程化学反应的时间相差甚远，因而不能揭示引爆时含能材料性质变化的全貌。高效率且相对高精度的模拟含能材料化学和物理变化的量子分子动力学模型，并实现其程序化是本项目的核心和目标。选择冲击波为起爆条件，运用多尺度冲击波技术（Multi-Scale Shock Technique, MSST）模拟稳定的平面冲击波作用，该方法通过调节从连续介质计算得到压力和能量来模拟冲击波通过计算体系时产生的影响，计算体系的大小不再受冲击波的影响，计算的工作量与要模拟的化学反应的时间成正比。电子结构信息由电荷自洽的基于紧束缚近似的密度泛函 (self-consistent charge density functional based tight bonding，SCC-DFTB) 方法计算得到。在SCC-DFTB计算中，原子间排斥势等以参量的形式提前确定并贮存在程序中，计算量较纯粹DFT方法大大降低，能够有效地用于大体系的量子分子动力学模拟。MSST程序代码取自LAMMPS程序包，我们改写部分代码，将其作为一个模块嵌入到DFTB+程序中，优化程序结构，完成了一个针对材料完美晶体进行动力学模拟的程序，可以模拟冲击起爆条件下含能材料的化学变化过程，计算效率较CP2K中类似程序提高了数倍。应用新程序对三氨基三硝基苯（TATB）冲击起爆的最初过程作用了研究，提出了一个TATB冲击起爆的新机理：TATB分解的第一步是N-O键的断裂，之后C环先被N原子连接成簇，然后C环再打开，形成了含N的杂环并一直稳定存在数百皮秒。该起爆机理与冲击波速度无关。此外应用新程序模拟了三亚甲基三硝铵（RDX）冲击起爆下完整的爆炸过程，并对新型含能材料二硝酰胺铵（ADN）的冲击起爆条件做了初步研究。