温稠密甲烷流体物性的第一性原理分子动力学研究

发布时间：2018-03-03 12:33

  本文选题：第一性原理分子动力学　切入点：非金属-金属转变　出处：《西南大学》2017年硕士论文　论文类型：学位论文

【摘要】：含氢量高的富氢化合物是较好的储能材料,而甲烷是富氢体系中最简单的饱和碳氢化合物。甲烷也是地幔、天王星和海王星等巨行星的主要成分,在这些行星内部温度处于2000 K-8000 K,压强处于20 GPa-600 GPa。研究甲烷在高温高压下电子结构、相态(包括固相、液相、气相和等离子相等)的变化、甲烷分子的解离和原子重组,及其对物理性质(包括热、电、光性质等)的影响,对于人们认识极端条件下凝聚态物质具有重要的科学意义;也利于近代自然科学和工程技术中的几个重大问题,例如:能源问题、天体演化问题、金属氢等问题的解决。本文将采用基于密度泛函理论的第一性原理模拟,研究高温高压下甲烷的物态方程、离解相变和导电性,具体内容包括以下几个方面:1.我们采用基于密度泛函理论的分子动力学模拟计算了密度为0.60g/cm3-2.5g/cm3、温度为1000K-8000K,对应的压强为3.6GPa-332GPa范围内流体甲烷的物态方程,并作出不同密度下的等容线,我们观察到除0.60 g/cm3以外的各个密度下都存在压强滞涨或降低的区域,即((?)P/(?)T)≤0。通过对计算得到的数据进行拟合,得到十分吻合的拟合公式。2.通过构型抽取和径向分布函数分析甲烷在高温高压下化学组分变化情况和原子键合情况,发现在极端条件下甲烷发生化学分解,形成饱和烷烃如乙烷、丁烷等,还有不饱和烷烃如烯烃、炔烃等,最终会聚合成长烃链和一些氢气或者自由的氢原子,但是在甲烷的分解过程中我们并没有发现金刚石结构碳形成。除此之外发现等容线上压强滞涨或降低的区域与甲烷的化学分解过程紧密相关。3.通过计算电子能带和直流电导率随温度和压强的变化规律,发现甲烷在高温高压下发生非金属-金属转变,并给出了体系发生非金属-金属转变的临界温度为5000 K,此时的压强为8 GPa。通过对电子分波态密度分析,发现体系具有很高的直流电导率主要是自由的氢原子和碳链上碳原子的贡献。最后我们给出了甲烷流体非金属-金属的相变曲线。本文采用基于密度泛函理论的分子动力学模拟计算温稠密甲烷热力性质的影响,研究在高温高压下甲烷流体的物态方程、化学分解和非金属-金属转变。
[Abstract]:Hydrogen rich compounds with high hydrogen content are better materials for energy storage, while methane is the simplest saturated hydrocarbon in hydrogen rich systems. Methane is also a major component of giant planets such as the mantle, Uranus and Neptune. The internal temperature of these planets is 2 000 K and the pressure is 20 GPa-600 GPA. The changes of electronic structure, phase states (including solid, liquid, gas and plasma), dissociation and atomic recombination of methane at high temperature and high pressure are studied. And its effects on physical properties (including thermal, electrical, optical properties, etc.) are of great scientific significance to people's understanding of condensed matter under extreme conditions, and are conducive to several important problems in modern natural science and engineering technology, For example, the energy problem, the evolution of celestial bodies, the solution of metal hydrogen, etc. In this paper, the first principle simulation based on density functional theory is used to study the equation of state, dissociation phase transition and electrical conductivity of methane at high temperature and high pressure. The specific contents include the following aspects: 1.We use the molecular dynamics simulation based on density functional theory to calculate the equation of state of the fluid methane in the range of 3.6GPa-332GPa with density of 0.60g / cm3-2.5gcm3, temperature of 1000K-8000K, and the isovolumetric lines at different densities. We have observed that there are areas where the pressure stagflation or decrease exists at all densities except 0.60 g / cm3. P / P? T) 鈮，

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