碳化硼力学特性的大尺度分子动力学研究

The boron carbide B4C has received intense experimental and theoretical attention which has wide engineering and military applications due to its combinations of unique properties, i.e. high hardness (~25-40 GPa), low density (2.5 g/cm3), high melting temperature (2447 ℃). The mechanical properties response of B4C to compression, shear deformation, tensile loading, and shock waves are examined with combination of density functional theory and large scale molecular dynamics simulation in this project. Firstly, we employ the density functional theory to calculate the equation of states of B（α-B12，γ-B28，T-B50）and B4C candidate structures ((B11Cp)CBC, (B11Ce)CBC, (B12)CBC), heat of formation and shear deformations which serve as training sets to fit a reliable ReaxFF potential. This is extremely important since we can apply this ReaxFF potential to other B-C systems. Secondly, with millions of atoms adopted, we explore the radial distribution function, stress-strain curve and examine the atomic structures subjected to compression, shear deformation, tensile loading, and shock waves with large scale molecular dynamics in order to resolve the current controversy issue about the abrupt drop of shear strength under hypervelocity impact (> 907 m/s). For nanocrystalline B4C, different grain size (2nm, 5nm, 10nm, and 20nm) samples will be prepared using voroni tessellation methods with the aim to study the grain size dependence. Introducing the defects, such as vacancies, or replacing atoms are reasonable ways to enhance the ductility of the B4C. We will also focus on possible ways to design a B4C composite with enhanced mechanical properties which are useful for future experiments.

B4C具有很多优异性能，硬度大（约25-40 GPa），密度小（2.5 g/cm3），熔点高(2447 ℃)，抗化学侵蚀能力优越,因此在工业和军事上有重要的用途。本项目拟结合第一性原理和大尺度分子动力学模拟晶体和纳米B4C在高压，剪切形变，拉伸形变和冲击波作用下力学性质的变化。首先，我们利用第一性原理计算晶体B各相和B4C各种可能结构的物态方程，结合能，剪切形变，这些数据用来拟合得到可靠的ReaxFF经验势。后续采用大尺度分子动力学（几十万个原子）研究晶体B4C和纳米B4C在上述四种作用下力学性质的响应。通过分析径向分布函数，应力应变曲线关系，形变后的原子结构。致力于解决以下问题（1）帮助理解现有文献关于压制非晶化机理的争论（2）在理论上探索结晶颗粒尺寸对纳米B4C在上述四种作用下力学性质的影响；（3）进一步提出改善B4C力学特性的可行性方法，比如引入空位，杂质原子，改变结晶颗粒尺寸等。

陶瓷材料碳化硼具有很多独特性质: 硬度大（约 25-40 GPa），热传导系数大(40 W/m∙K)，密度小（ 2.5 g/cm3） ，熔点高(2447 ℃ )， 抗化学侵蚀能力优越,因此在工业和军事上有重要的用途。我们对碳化硼的三种可能的结构（B11Ce)CBC，(B11Cp)CBC 和(B12)CCC应用密度泛函理论计算总能量，确定基态结构为(B11Cp)CBC。.外尔半金属里含有外尔费米子和费米弧，是在拓扑绝缘体之外的又一种非平凡拓扑材料。首次报道的TaAs家族的外尔半金属材料NbAs ,NbP, TaP，实验上观察到巨磁阻，不寻常的运输特性，因而引起了全球范围内广泛的科研关注。.压力常用来改变材料的化学环境，以产生新的物理性质和新的结构。我们基于CALYPSO结构预测平台，采用粒子群优化算法，结合第一性原理计算能量，确定外尔半金属材料NbAs ,NbP, TaP的晶体结构。加静水压至200 GPa，这三种材料呈现了三种不同的相变序列，.NbAs: I41md → P-6m2(相变压强23 GPa) → P21/c (相变压强38 GPa) → Pm-3m (相变压强73 GPa); NbP: I41md → Cmcm (相变压强63.5 GPa)；TaP: I41md → Pmmn(相变压强67 GPa) → P21/c (相变压强90 GPa)→ Pm-3m(相变压强125 GPa); 这些高压相均满足热力学稳定性和动力学稳定性条件，我们还发现NbAs ,NbP, TaP，在压力下实现了由半金属到金属的转变。通过对电子局域函数的计算分析，我们知道化学成键的离子性随压力的增大而增强。.这一工作填补了外尔半金属材料NbAs, NbP, TaP在高压下的结构空白，对于TaAs家族的外尔半金属的在高压或极限条件下的研究和应用提供了理论依据.