【作者】 叶宏飞；

【导师】 张洪武； 郑勇刚；

【作者基本信息】 大连理工大学 ， 计算力学， 2012， 博士

【摘要】 近年来,材料在纳米尺度下的性质和行为已经引起了越来越多的关注。作为自然界中最为常见的流体之一,水在纳米尺度下的性质和行为与宏观尺度下相比展现出了很多非常规的特点。例如,有序的分子构型、较高的传输速率以及特殊的扩散形式等等。这些特点促使纳米尺度下的水在分子输运、纳米器械冷却等方面都有着良好的应用前景。因此,深入研究水在纳米尺度通道内所呈现出的变化规律具有重要的科学意义。本文工作基于分子动力学方法展开,研究了水在碳基材料构成的纳米尺度通道内的传输性质和流动行为,包括水在碳纳米管内的粘性性质、水在碳纳米管内的扩散行为以及水在石墨层片驱动下的剪切流动行为,具体内容如下：首先,本文开展了水在碳纳米管内的粘性性质研究。现有分子动力学中的计算方法难以准确计算限制于特征尺寸极小的纳米尺度通道内的流体粘性,为了研究水在碳纳米管内的粘性性质的尺度效应和温度效应,本文将经典的Eyring粘性理论和分子动力学方法相结合,提出了一种半经验性的"Eyring-MD"粘性计算方法。数值算例证实了该方法的正确性和高效性。利用"Eyring-MD"方法,研究了室温下水在直径为8-54A的碳纳米管内的粘性性质。计算结果表明处于限制状态下的水的粘性要低于常规尺度下水的粘性,其会随着碳纳米管直径的增加而升高并逐渐趋于常规尺度下水的粘性。随后,通过考虑新的数值实验算例,对"Eyring-MD"方法中的公式系数进行了补充和修正。利用改进的"Eyring-MD"方法研究了水的粘性性质的温度依赖性。计算结果表明水在碳纳米管内的粘性在不同温度下随碳纳米管直径的变化趋势基本相同。在直径相同的碳纳米管中,水的相对粘性随着温度的升高而增加。此外,氢键数目的计算结果显示除了在(8,8)和(9,9)碳纳米管内,处于限制状态下的水的氢键数目的变化趋势与其粘性的变化趋势基本吻合。水在(8,8)和(9,9)碳纳米管内的氢键数目呈现了一定的增加。最后,根据获得的计算结果,本文给出了一个同时考虑了尺度效应和温度效应的水粘性性质的计算公式。其次,本文开展了水在碳纳米管内的扩散行为研究。构建了一套真实客观、高效准确的碳纳米管内水分子扩散行为研究的分子动力学计算模型。计算结果显示在室温时水在小直径碳纳米管(d<12.2A)内的扩散机理呈现出了两阶段的形式。在初始阶段中水分子遵循着一系列非常规的扩散机理,随后水分子在(6,6)碳纳米管内转变为单列扩散机理,而在其它碳纳米管内则转变为流体常规的扩散机理即Fickian扩散机理。随着温度的升高,两阶段之间的转折逐渐变得模糊。在大直径碳纳米管中,水的扩散机理与常规尺度下的流体扩散机理基本一致。对于扩散系数而言,除了在(8,8)和(9,9)碳纳米管中,水的扩散系数随着碳纳米管直径的增加而增加直至直径达到约40A,随后逐渐趋近于自由状态下水的扩散系数。在(8,8)和(9,9)碳纳米管中,水分子的扩散系数有明显的降低。当温度变化时,处于限制状态下的水的扩散系数的变化趋势几乎不变,但是其数值随着温度的升高而升高。此外,研究表明碳纳米管的尺寸限制和表面属性之间的协同作用导致了小直径碳纳米管内的非常规扩散机理,而它们之间的竞争作用导致了扩散系数的非单调性的变化趋势。基于这两种特性的定性考量,本文给出了一个同时体现了尺度效应和温度效应的水的扩散系数计算公式。最后,本文基于分子动力学方法开展了水在石墨层片驱动下的剪切流动行为研究。为了考察电在纳米尺度下对极性水分子流动的影响,本文在纯石墨层片的模拟基础上进一步考虑了一种在碳原子上粘附电荷的带电石墨层片。计算结果显示纯石墨层片的相对运动可以使水的内部形成线性的速度分布,但是水与管壁之间存在着巨大的速度损耗,且水在靠近边界的部分出现了内部分子层之间的相对滑移现象。粘附电荷后的石墨层片可以更为有效地驱动水分子的运动并明显地削减了水与通道管壁之间的速度损耗。随着粘附电荷的增加,管壁与水分子之间的作用有所增强,水分子的速度梯度也随之逐渐增加。对于电量相同而电性相反的石墨层片,水分子整体的剪切流动行为几乎一致,但是其内部的牵拉机理有所不同。此外,本文还计算了水分子在两种石墨层片表面的滑移长度。计算结果显示滑移长度在低剪切速率时变化缓慢,但是当剪切速率达到阈值后会急剧增加。这样的变化趋势可以归因于石墨层片有限的牵拉能力。对于纯石墨层片,滑移长度的变化范围约在500-2500A之间；对于碳原子粘附电荷为0.10e的石墨层片,滑移长度的变化范围约在50-350A。可以看出,电荷的加入可以有效地降低边界的滑移效应。根据计算结果,本文给出了两种情况下滑移长度和剪切速率的关系公式。更多还原

【Abstract】 The properties and behaviors of materials at the nanoscale have attracted much attention in recent years. As one of the most common fluids in nature, the water possesses many extraordinary characteristics at the nanoscale compared with that at the micro-or macroscale, such as the ordered molecular configuration, the high transport rate, the special diffusion behavior and so on. These nanoscale characteristics make the water have widely promising applications in molecule delivery, nanoequipment cooling and so on. Hence, it is very significant to investigate the unexpected changes and rules of the water at the nanoscale. By using the molecular dynamics (MD) method, this paper focuses on the transport properties and flow behaviors of the water inside the carbon-based nanochannels, including the viscosity of the water confined in carbon nanotubes (CNTs), the diffusion behavior of the water confined in CNTs and the shear flow of the water driven by the graphene sheets. The detailed contents are presented as follows:Firstly, the viscosity of the water inside the CNTs is investigated. The existing methods in the MD simulations are difficult to calculate the viscosity of fluids inside the channels of extremely small size. In this paper, to study the size and temperature effects on the viscosity of the water inside the CNTs, a semi-empirical "Eyring-MD" method is proposed on the basis of the classical Eyring theory and the MD simulations. The numerical examples demonstrate the correctness and the efficiency of the proposed method. Based on the "Eyring-MD" method, the viscosity of the water inside the CNTs of diameter within8～54A is calculated. The computational results indicate that the viscosity of the water inside SWCNTs is lower than that of the bulk water, which increases nonlinearly with enlarging diameter of SWCNTs and gradually approaches the viscosity of the bulk water. Then, through considering some new numerical experiments, the coefficients in the "Eyring-MD" method are supplemented and corrected. The temperature dependence of the viscosity is studied by the modified "Eyring-MD" method. The results reveal that size-dependent trend of the viscosity of the water inside the CNTs is almost independent on the temperature. For the CNTs of the same diameter, the relative viscosity increases with increasing the temperature. Furthermore, except for the (8,8) and the (9,9) CNTs, the amounts of the hydrogen bonds of the water confined in the CNTs exhibit similar profiles with the curves of the viscosity. There is an increment in the amounts of the hydrogen bond of the water inside the (8,8) and the (9,9) CNTs. According to the computational results, we provide a formula for the size-and temperature-dependent viscosity.Secondly, the diffusion behavior of the water inside the CNTs is investigated. A natural, real and efficient MD model is constructed to study the diffusion behavior of the water molecules inside the CNTs. The computational results suggest that the motions of water molecules inside the CNTs of diameter smaller than12.2A follow a two-stage diffusion mechanism. Initially, the water diffusion exhibits some unconventional diffusion mechanisms, and thereafter it transits to the single-file type inside the (6,6) CNT and shifts to the normal type (Fickian diffusion mechanism) inside the larger CNTs. As the temperature increases, the crossover between the two stages becomes obscure. As for the CNTs of diameter larger than12.2A, the diffusion of the confined water occurs through the Fickian mechanism, which is identical to that of the bulk water. Except for the (8,8) and the (9,9) CNTs, the diffusion coefficient of the confined water increases with increasing the diameter up to about40A, and then gradually approaches that of bulk water. A sharp reduction can be observed in the diffusion coefficient of the water inside the (8,8) and the (9,9) CNTs. As the temperature increases, the diffusion coefficient of the confined water gradually increases, but their variation trends with the diameter are almost unchanged. Furthermore, it is suggested that the extraordinary diffusion mechanism of the water molecules inside the CNTs is attributed to the cooperation of the size confinement and the surface property, and the nonmonotonic variation of the diffusion coefficient is ascribed to the competition of the size confinement and the surface property. Based on the qualitative examination of these two effects, we provide a formula for the diffusion coefficient with consideration of the size and the temperature effects.Finally, through the MD simulations, the shear flow of water driven by the graphene sheets is investigated. To explore the influence of the electricity on the flow behavior of the polar water molecules, the graphene sheets with the adsorbed charges are further considered based on the simulations on the pure graphene sheets. The computational results reveal that the pure graphene sheets can result in a linear velocity profile in the water. But there is enormous velocity dissipation between the wall and the water. It can also be seen that the internal slip exists between the molecular layers near the solid boundary. The graphene sheets with the adsorbed charges can drive the shear flow of the water more effectively, which can greatly reduce the velocity dissipation between the wall and the water. As the charge increases, the coupling strength between the tube wall and the water molecules is enhanced, and the velocity gradient of the water molecules gradually increases. For the graphene sheets with the same electric quantity but different electric properties, the flow behaviors are almost identical but the driven mechanisms are completely different. The slip lengths of the water molecules on the two types of the graphene sheets are also calculated. The results reveal that the slip length slightly increases at the low shear rate, and then sharply increase after the shear rate reaches the threshold value. For the pure graphene sheets, the slip length ranges from500A to2500A; for the graphene sheets with0.10e per carbon atoms, the slip length ranges from50A to350A. The present results demonstrate that the introduction of the electricity can effectively suppress the slip phenomenon. According to the computational results, the relationships between the slip length and the shear rate in the two cases are given.更多还原