CAS OpenIR
液氩的热力学性质计算与组成结构分析
孙晨阳
Thesis Advisor侯超峰, 何艺
2020-07-01
Degree Grantor中国科学院大学
Degree Name硕士
Degree Discipline化学工程
Keyword分子动力学模拟,截断半径,液氩,结构,介科学
Abstract

在固体基础理论和气体理论发展相对成熟的今天,物质的另一种状态-液态的理论却仍然是凝聚态物理中尚未解决的难题。液体具有类似气体流动性的同时,也具有类似固体的原子间强相互作用,这种复杂的特征成为构建精确的液体理论的巨大障碍。基于能量最小多尺度模型(EMMS)的“介科学”理论的提出和发展为解决该问题提出了一种可能的途径。氩体系一级相变的过程可以从介尺度科学的角度进行分析,即气态和固态各有一种机制控制,液态为类气和类固两种状态的混合物,由该两种机制共同控制。本课题基于这样的目标,首先使用分子动力学模拟方法对氩体系进行了系统的模拟,计算了不同状态下的热力学性质,探究了不同截断半径的影响,分析了背后的物理成因和机制。然后,论文从介科学的视角出发,统计分析了液氩体系局域的不均匀结构特征,为简单液体热力学性质的精确描述在方法论上进行了探索。近年来在分子动力学方法研究LJ(Lennard-Jones)势氩系统中,越来越多的计算建议采用4.5σ甚至更大的截断半径(σ为氩原子的直径)。在NPT系综条件下,更大的截断半径可能更准确的描述LJ势氩体系的热力学性质和行为。为此我们研究了不同截断半径对常压下NPT系综氩系统熔沸点相图的影响,计算了不同截断半径下在氩系统的熔点及液相区域不同热力学状态点的径向分布函数和速度自相关函数,发现以熔点为基准,在距离其相同液态温区分率的热力学温度点能获得相同的热力学性质。并且在体积不变的NVT系综和NVE系综情况下,截断半径对径向分布函数和速度自相关函数计算结果的影响较弱。该工作为液氩模拟中截断半径的选择指明了一个新的思考方向,最终发现2.5σ的截断半径在模拟的准确性和计算性能上对于计算径向分布函数和速度自关联函数均能满足要求。然后在此基础上,从势能和动能竞争协调的角度出发分析了不同截断半径对热力学性质影响背后的物理成因和机制。更深入一步,本课题从模拟体系局域数密度和扩散系数出发,探究了液态体系静态结构和动力学上的不均匀性。采用零外压下硬球的扩散系数为参照,定量描述液态氩内部类气组分和类固组分比率的计算,统计了常压下NPT系综不同温度热力学状态下的类气类固组成。同时,以介尺度科学的观点研究了液体的热力学状态的微观过程,可将氩体系固-液-气相变视为体系原子间扩散和振动状态竞争协调的结果。固态可表达为由一种机制A控制,原子振动主导,气态可表达为由一种机制B控制,原子扩散主导,而液态可以看作由机制A(solid-like)与机制B(gas-like)竞争协调导致。该工作是不同于传统热力学的探索和创新,将会拓宽介尺度科学的应用范围,对液态中介尺度结构形成机理获得更深入认识,是介尺度科学理论普遍性的有力补充和完善。;With the continuous improvement of solid theory and gas theory, the theory of liquid, another state of matter, is still an unsolved problem in condensed matter physics. Liquid flows, and in this sense is close to gas. At the same time, interactions between atoms in liquids are as strong as that in solid. The combination of these two features brings abouta huge obstacle to the construction of accurate liquid theory. However, the development of the theory of "Mesoscience" based on the energy-minimization multiscale model (EMMS) provides a possible way to solve this problem. The first-order phase transformation of argon system can be analyzed from the perspective of "Mesoscience", that is, there is a regime domination for gas state and solid state, respectively, and the state of liquid can be simply regarded as a mixture of solid-like and gas-like structure, governed by the compromise in competition between these two regimes. Based on this goal, firstly, the molecular dynamics simulation method is used to simulate the argon system, calculate the thermodynamic properties in different states, explore the influence of different cutoff distance, and analyze the underlying physical mechanisms. Then, from the perspective of "Mesoscience", the characteristics of the local inhomogeneous structure of liquid argon system are analyzed, and a novel methodology for the accurate description of simple liquid thermodynamic properties is explored.In Lennard-Jones (LJ) potential argon system investigated by molecular dynamics simulation, more and more calculations suggest to use 4.5σ or even larger truncation distances (σ is the diameter of argon atom) to obtain the more accurate thermodynamic properties of the systems. Under the condition of NPT ensemble, a larger cutoff distance may more accurately describe the thermodynamic properties and behaviors of the LJ potential argon system. From this perspective, we have studied the influence of different truncation radius on the phase diagram of melting point and boiling point of NPT ensemble of argon system at atmospheric pressure. At the same time, the radial distribution functions (RDF) and velocity autocorrelation function (VACF) at the melting points and different thermodynamic states of the liquid argon with different cutoff distances are analyzed. It is found that, the same thermodynamic properties can be obtained at the corresponding thermodynamic state points with the same proportion of liquid temperature zone under different truncation distances. In the case of constant volume, the cutoff distance has little effect on the results of radial distribution function and velocity autocorrelation function in NVT and NVE ensembles. This work proposes an exploratory way for the selection of the cutoff distance in the simulation of liquid argon, where the truncation distance of 2.5σ can meet the requirements of computational accuracy and performance in the simulations. On this basis, the physical mechanisms behind the influence of different truncation radius on the thermodynamic properties are analyzed from the view point of compromise in competition between potential energy and kinetic energy.Furthermore, based on the local number density and diffusion coefficient of the simulation system, the static structure and dynamic heterogeneity of the liquid system are studied. In terms of this inhomogeneous structure, a computational method is proposed to quantitatively describe the proportion of gas-like component and solid-like component in liquid argon with reference to the diffusion coefficient of hard sphere systems under zero external pressure, and the proportion of gas-like component in different-temperature thermodynamic state is calculated according to the phase diagram of NPT argon system under atmospheric pressure. In addition, the solid-liquid-gas phase transition of argon system can be regarded as the result of the compromise in competition between atomic diffusion and vibration. The solid state is dominated by a mechanism A governing atomic vibration, the gas state is dominated by a mechanism B governing atomic diffusion. The liquid can be regarded as a result of the compromise of mechanism A (solid-like) and mechanism B (gas-like) in the competition. This exploration and innovation is different from the traditional thermodynamics. Our research will broaden the application scope of mesoscale science, gain a deeper understanding of the formation mechanism of mesoscale structure, and is also a powerful supplement of the universality of the mesoscience theory. 

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/49712
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
孙晨阳. 液氩的热力学性质计算与组成结构分析[D]. 中国科学院大学,2020.
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