基于分子动力学模拟的单晶硅冲击压缩相变研究

晶体硅具有复杂的相变机制，在相图研究中受到广泛关注，其在动载荷下的变形机制是当前研究热点。为揭示晶体硅在强动加载下的变形和相变行为特征，基于分子动力学方法，采用平板冲击加载方式，模拟研究了单晶硅在初始环境温度为300 K时分别沿001]、110]和111]晶向的不同强度下的冲击压缩行为，冲击粒子速度为0.3～3.2 km/s。研究发现，随着冲击粒子速度的增加，单晶硅剪切应力在逐渐增加后由于结构相变发生急剧下降，相变阈值和相变机制均呈现各向异性。其中，沿001]晶向冲击压缩下观察到多种固-固相变以及固-液相变，并观察到与最新文献的实验高度一致的固-液共存现象。研究结果可为动加载下晶体硅的相变研究提供纳米尺度的结果支撑。

Study on shock compression phase transition of single crystal siliconbased on molecular dynamics simulation

Crystalline silicon has a complicated phase transition mechanism, which has received extensive attention in the research field of phase diagram, and the deformation mechanism of silicon crystals under dynamic loading is the current research hotspot. In order to reveal its deformation and phase transition behaviors under intensive dynamic loading, molecular dynamics method was used to simulate the shock compression behavior of single crystal silicon along the crystal directions 001], 110] and 111] at an initial ambient temperature of 300 K, respectively. All simulations were carried out basing on the classical open-source codes LAMMPS and a Tersoff interatomic potential was adopted to describe the material responses of silicon under dynamic compression. Before shock loading, periodic boundary conditions were applied along the three independent directions, and an NPT ensemble was used to equilibrate the systems; then shock compression was applied by using the piston method, where a virtual piston wall impinges the sample such that the particle velocity in the sample is the same as the piston speed after the shock reaches a steady state. The shock particle velocities varied from 0.3 km/s to 3.2 km/s, and a timestep of 0.001 ps was adopted. During the stress wave formation and propagation, the simulation system was in the NVE ensemble with the absence of temperature control. The loading method and effect are similar to typical plane impact experiments. The results show that with the increase of shock particle velocity, the shear stress of single crystal silicon increases gradually and then decreases sharply due to the structural phase change. Both the phase transition threshold and the phase transition mechanism are anisotropic. Among them, a variety of solid-solid phase transitions and solid-liquid phase transitions are observed under shock compression along the 001] crystal direction. The phenomenon of solid-liquid coexistence is highly consistent with the recent international experiments. The research results provides new nano-scale results to support the study of phase transition of crystalline silicon under dynamic loading.