石墨烯纳米通道内二维受限水的相态与相变

发布时间：2018-01-29 19:44

  本文关键词： 二维受限水 石墨烯纳米通道 压缩极限 侧向压强 方形冰 过热熔化 相变 相图 分子动力学模拟　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：水是地球上最常见也是最普通的物质之一,被誉为是生命之源。地球上的水绝大部分是以体相水的形式存在,主要为气态、液态和固态(冰)。然而水却更多地以界面或受限水的形式参与各种物理、化学和生物过程,因此研究微纳米结构中受限水的成键构型、物理化学性质以及流动行为等,进而面向材料科学、水资源、环境和能源利用等领域的实际需求系统地开展受限水的应用研究,成为当前水科学研究的重要领域和国际前沿热点。近年来,石墨烯以其独特的力学、电磁学与物理等特性而备受关注。氧化石墨烯薄膜不同寻常的渗透行为和离子海绵效应不仅使得其在海水淡化和物质输运等领域具有广阔的应用前景,而且相应的机理研究也是学术界关注的前沿科学问题。实验和模拟发现,氧化石墨烯薄膜的限域效应使得水分子能够进入间隙只有0.35纳米左右的石墨烯纳米通道中,而且自发形成二维的晶体状类固体结构。Algara-Siller等在实验中观察到纳米水滴在两片石墨烯之间形成单层方形冰,这种二维方形结构是常温下水的一种全新存在形式,突破了长久以来人们对冰的已有认识。模拟表明这种特殊的晶体相主要是由于石墨烯受限空间内高达GPa量级的vanderWaals压强(类毛细压)作用,即纳尺度限域效应。纳尺度限域效应会明显影响受限水的结构形态、动力学行为以及热力学特性等,相关的内在机理还未得到全面的认识。从力学的角度来看,水在两片石墨烯之间由液态到固态的相变具有二维液体的亚稳态极限(压缩极限)的特征。基于此,本文通过经典分子动力学模拟研究二维受限水在石墨烯纳米通道内的相态和相变。第一次系统地研究了二维受限水的压缩稳定极限,发现多种新的二维冰相和相变过程,首次得到了侧向压强作用下二维材料的压缩极限相图,同时发现了侧向压强主导的两阶段过热熔化现象。本文首先研究了两片石墨烯包裹形成的单层方形冰,从结构特征与动力学特性的角度来揭示两片石墨烯包裹的单层方形冰的形成,用统计平均的方法证明单层方形冰固有的方形特征。根据单元特点对模拟中得到的单层方形冰的结构给出定义,具体分析了有限温度作用下单层方形冰中水分子自发翻转行为和翻转引发的结构演化,结合几何平均和时间平均概念,使用统计平均的方法研究单层方形冰的方形特征。随后,本文对石墨烯纳米通道内二维受限水的压缩相变和过热熔化相变进行了详细的研究。分子动力学模拟研究发现,在不同的纳米通道宽度或温度条件下,侧向压强作为一个可控因素,能够实现二维受限水的一阶相变和连续型相变,得到新的不同结构的冰相。在模拟中共观察到12种不同的二维冰相,分别是:平的单层方形冰(fMSI)、之字形褶皱的单层方形冰(pMSI)、单层三角形非晶态冰(ML-T)、AB堆垛结构双层方形冰(BL-ABI)、AB堆垛结构双层方形非晶态冰(BL-AB2)、AB堆垛结构双层菱形冰(BL-AB1)、AA堆垛结构双层正六边形冰(BL-iceI)、AA堆垛结构双层方形管状冰(BL-VHDI)、AA堆垛结构双层三角形冰(BL-AAI)、ABA堆垛结构三层方形冰(TL-ABAI)、ABA堆垛结构三层菱形冰(TL-ABA)和AAA堆垛结构三层三角形冰(TL-AAAI)。根据一系列的热力学平衡状态和Clapeyron方程,在通道宽度-侧向压强(h-P)平面内得到了二维受限水的压缩极限相图,压缩极限曲线具有多个局部极大值。从双层液态水到BL-VHDI和BL-AA1的相变分别是一阶相变和连续型相变,这两种不同相变的温度阈值是～275 K。从BL-VHDI到BL-AAI的结构转变可以看作是方形冰纳米管的屈曲失稳。ABA堆垛结构的三层冰结构类似于双层笼形水合物,其中间层水分子被当作客体分子。对于AAA堆垛结构的三层冰,与石墨烯壁面相邻的两层水分子比中间层的扩散更快。在单层方形冰的过热熔化过程中,共有四种不同的熔化相变情形,其中在较高的侧向压强条件下,发现两种不同的两阶段熔化现象以及单层冰和液相的共存状态,并给出了单层方形冰的与侧向压强相关的过热熔化相图。"What is the structure of water"是Scienc杂志公布的125个最具挑战性的科学前沿问题之一。研究受限水的相态行为是水科学研究领域的重要分支,对受限水的物质形态、固液界面多种物理机制耦合以及纳尺度限域传质等基础科学研究和海水淡化、物质输运、生物科学、材料科学以及传统产业转型升级等实际应用需求具有重要的学术价值。
[Abstract]:Water is the most common one of the material is the most common, is known as the source of life. Most of the water on the earth is in the form of bulk water, mainly for gaseous, liquid and solid (ice). However, the water is more limited to interface or water in the form of a variety of physical and chemical and biological processes, therefore limited water research on micro nano structure in the bonding configuration, physical and chemical properties and flow behavior, and for materials science, water resources, research and application of the system to carry out the actual needs of the field of confined water environment and energy utilization, as an important field of water science research and international frontier in recent years, graphene with its unique mechanics, electromagnetics and physical characteristics have attracted much attention. The ion permeation behavior and sponge effect of graphene oxide film is unusual not only makes the transport in desalination of sea water and material etc. Has broad application prospects, and the corresponding mechanism is also concern the academic frontier scientific issues. Experimental and simulation show that the confinement effect of graphene oxide film which water molecules can enter the gap is only 0.35 nm graphene nano channel, and the spontaneous formation of two-dimensional crystalline solid structure.Algara-Siller to observe the nano water forming a layer of ice in the square between two sheets of graphene in the experiment, the two dimensional structure is a new form of water at normal temperature, break the people long ice has been understanding. Simulation shows that this special crystal phase is mainly due to the pressure of vanderWaals graphene in confined space of up to GPa the order (type of capillary pressure), namely the nanoscale confinement effect. Nano scale limit structure domain effect can significantly affect the dynamic behavior of confined water, and heat The mechanical properties, mechanism of correlation has not been fully understood. From the mechanical point of view, the metastable limit of water between two sheets of graphene from liquid to solid phase transition with two-dimensional liquid (compression limit) characteristics. Based on this, the phase and phase transition through classical molecular dynamics simulation of 2D the limited water in graphene nano channel. The first systematic study of the two-dimensional water confined compression stability limit, found a variety of new two-dimensional ice phase and phase transformation, obtained two-dimensional material under the action of lateral pressure limit of compression phase diagram, and found two stage overheating melting phenomenon. Firstly, lateral pressure leading research two pieces of graphene wrapped to form single-layer square ice, forming a layer of ice to reveal two square pieces of graphene package from the characteristics and dynamic characteristics of the structure of the angle, with the statistical average Method that features a square single square ice. According to the definition of the inherent characteristics of the single unit structure are square ice obtained in simulation, detailed analysis of the structural evolution of the finite temperature under the action of single square ice water molecules in spontaneous behavior and triggered flip flip, combined with the geometric mean and the average time of conception, characteristics of single square square ice use the statistical average. Subsequently, the graphene nano channel in two-dimensional confined water compression phase and overheating melting phase transition is studied in detail. The molecular dynamics simulation study found that in different nano channel width or temperature conditions, the lateral pressure as a controllable factor, a first-order phase transition and continuous phase can the realization of two-dimensional confined water, different structures of the new ice phase. In the simulation were observed in 12 kinds of two-dimensional ice phase, are: single layer flat. The shape of ice (fMSI), single square ice zigzag fold (pMSI), triangular amorphous ice (ML-T), AB stacking structure double square ice (BL-ABI), AB square double stacking structure of amorphous ice (BL-AB2), AB stacking structure of double diamond ice (BL-AB1), AA double hexagonal stacking structure ice (BL-iceI), AA stacking structure double square tubular ice (BL-VHDI), AA stacking structure (BL-AAI), double triangle ice ABA stacking structure of three layers of ice, ABA square (TL-ABAI) stacking structure of three layers of diamond ice (TL-ABA) and AAA stacking structure of three layers of triangular ice (TL-AAAI). According to the state equation and Clapeyron equation a series of thermodynamic equilibrium, the channel width of lateral pressure (h-P) in the plane of the two-dimensional water confined compression limit diagram, compression limit curve has multiple local maxima. The double liquid water phase transition to the BL-VHDI and BL-AA1 are the first order phase transition and continuous phase, the two Different kinds of transformation temperature threshold is 275 ~ K. from BL-VHDI to BL-AAI transition can be seen as buckling of square ice nanotubes lost three layer of ice structure stability.ABA stacking structure similar to the double clathrate hydrate, the interlayer water molecules are as guest molecules. The three layer of ice AAA stacking structure, diffusion and two water layer graphene wall adjacent to the intermediate layer faster. In hot melting process of ice in the single square, a total of four different melting transition, in which the high lateral pressure conditions, found two different two stages of melting phenomenon and monolayer ice and liquid phase coexistence. And gives a single square ice associated with the lateral pressure of melt. "What is the structure phase of water 125" is one of the most challenging scientific frontier issues of Scienc magazine published. The phase study of confined water for Water is an important branch of scientific research in the field of confined water, physical form, physical mechanism and variety of solid-liquid interface coupling nanoscale confinement and mass transfer of basic scientific research and desalination, material transport, biological science, material science and the practical needs of the transformation and upgrading of traditional industries has an important academic value.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O613.71;TB383.1
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本文编号：1474202