多孔石墨烯时间维度反渗透滤盐机理研究

时间维度选择性反渗透原理虽然克服了反渗透膜微孔尺寸的限制,一定程度上突破了渗透性和选择性之间的平衡,但多层反渗透膜时间维度的滤盐机理尚未明晰.本文采用分子动力学方法,揭示了多孔石墨烯反渗透膜的厚度和剪切速度对盐水反渗透特性的影响规律.结果表明,随着多孔石墨烯反渗透膜旋转速度的增加,离子截留率增加而水通量先增加后降低;反渗透膜厚度的增加会提高离子截留率,但阻碍了水通量的上升.本文创新性地对三层石墨烯反渗透膜上的纳米孔结构进行了设计研究,发现梯度孔结构在保证高选择性的同时提高了渗透性;供给端最内层纳米孔径的变化对水通量的影响最为显著,水通量随该孔径的增加而快速上升.研究结果进一步阐明并验证了时间维度反渗透滤盐机理,利用梯度孔的设计提升了相同膜厚度情况下的水通量,为大尺度滤盐设备的设计研发提供了理论基础.     

柱状石墨烯膜反渗透滤盐特性及机理

柱状石墨烯在能源气体的存储运输和气体净化分离等方面备受关注,但其在海水淡化方面受到了大面积制备的限制,其反渗透特性和机理尚未明晰.本文运用分子动力学方法研究了不同压强、温度和膜的剪切运动对柱状石墨烯膜反渗透滤盐特性的影响规律.结果表明:压强较大时,水通量随着压强的增加而线性增加;温度的升高能提升水分子渗透率,但对离子截留率的影响不大;反渗透膜的剪切运动虽然会阻碍水分子的渗透,但相应地可以提高离子截留率.对氢键和离子水合结构的分析表明,反渗透膜的剪切运动可以提高氢键和离子水合壳的稳定性,但温度的升高会产生相反的效果.本文结果有助于深入理解柱状石墨烯膜在不同条件下的脱盐性能,进一步验证了柱状石墨烯膜在海水淡化领域的巨大应用潜力.     

石墨烯碳纳米管复合结构渗透特性的分子动力学研究

采用经典分子动力学方法研究了压力驱动作用下水在石墨烯碳纳米管复合结构中的渗透特性.研究结果表明,水分子渗透通过石墨烯碳纳米管复合结构的渗透率明显高于石墨烯碳纳米管组合结构.水在石墨烯碳纳米管复合结构中的渗透率随着压强的升高而增大,随着电场强度的增大而减小.考虑了温度和复合结构中双碳管轴心距对水渗透性的影响规律.系统温度越高,水的渗透率越高;随着双碳管轴心距的增加,水的渗透率逐渐降低.通过计算分析水流沿渗透方向的能障分布,解释了各参数变化对水在石墨烯碳管复合结构中渗透特性的影响机理.研究结果将为基于石墨烯碳管复合结构的新型纳米水泵设计提供一定的理论依据.     

石墨烯纳米孔道内受限水介电常数影响因素的分子动力学模拟

受限水的介电特性对于石墨烯电容器的储能效率有着重大的影响.因此本文使用分子动力模方法,研究了不同石墨烯纳米孔道宽度(0.812～10 nm)下内受限水介电常数的分布及其影响因素.结果表明,在石墨烯纳米孔道内部受限水双电层结构可分为空隙层、界面层和体相层三个部分.其整体的介电常数随孔道大小的降低而线性减少.这是由内部体相层的宽度的降低而导致的.此外电极电压大小和石墨烯-水相互作用参数ε_CO的强弱也会显著改变电极表面的双电层(Electric Double Layers, EDLs)结构.其中电压的增大使得介电常数分布的震荡的程度也随之增加,最终导致了整体介电常数的减小.与之类似,亲水态的石墨烯表面(高ε_CO)下受限水分布的震荡程度也显著增加,这导致了整体介电常数的降低.     

石墨烯在强激光作用下改性的拉曼研究

采用化学气相沉积法制备了不同层数的石墨烯样品.根据石墨烯透过率曲线分析石墨烯样品层数与550 nm处透过率关系的同时,利用拉曼光谱法分析了不同层数石墨烯样品在强激光辐照下的损伤特性.结果表明:单层石墨烯样品经强激光辐照后,G带和2D带均向高频移动;多层石墨烯样品经强激光辐照后只有G带发生了略微的频移;石墨烯样品拉曼光谱G带与2D带强度比值表征了石墨烯的层数,此比值随激光辐照时间的增加而减小,这表明强激光对石墨烯样品具有明显的剥离现象.     

单层石墨烯杨氏模量的理论模型

石墨烯力学性能的研究对其在半导体技术中的应用是十分重要的,本文基于半连续体模型并结合石墨烯纳米结构特性,通过对原子的描述构建了石墨烯形变分量和位移分量的新关系,从而给出了单层石墨烯结构形变能,并计算了不同尺寸单层石墨烯的杨氏模量值.通过对不同方向杨氏模量的分析,讨论了单层石墨烯的手性行为.结果表明:随着尺寸的增加,单层石墨烯两个方向的杨氏模量分别趋于0.746 TPa和0.743 TPa,当尺寸相同时,两方向杨氏模量的最大差值不超过0.003 TPa,此结果与文献报道结果相符.在小应变情况下,单层石墨烯薄膜呈各向同性,且薄膜尺寸变化对该特性影响不大.该计算结果对研究石墨烯的其它力学特性提供一定的参考价值.     

高质量大面积石墨烯的化学气相沉积制备方法研究

石墨烯因其奇特的能带结构和优异的物理性能而成为近年来大家研究的热点, 但是目前单层石墨烯的质量与尺寸制约了其实际应用的发展. 本文采用常压化学气相沉积(CVD)方法, 基于铜箔衬底, 利用甲烷作为碳源制备了高质量大面积的单层与多层石墨烯. 研究发现: 高温度、稀薄的甲烷浓度、较短的生长时间以及合适的气体流速是制备高质量、大面积石墨烯的关键. Raman光谱, 扫描电子显微镜、透射电子显微镜等表征结果表明: 制备的石墨烯主要为单层, 仅铜箔晶界处有少量多层石墨烯. 电学测试表明CVD制备的石墨烯在低温时呈现出较明显的类半导体特性; 薄膜电阻随外界磁场的增大而减小.     

基于温度的亚稳态氧化石墨烯性能

单片层氧化石墨烯由于其优异的物理化学性能,在离子和分子筛选、水脱盐和净化、气体分离、生物传感、质子导体、锂电池和超级电容等领域有巨大的潜在应用.然而普遍采用的Hummers法等化学、物理方法制备的氧化石墨烯是一种亚稳态材料.其最终形态的物理化学性能的转变与调控至关重要,亟需系统研究.本文采用恒温处理方法对氧化石墨烯亚稳态的转变进行调控,利用X射线光电子吸收谱、傅里叶红外吸收谱、扫描电子显微镜等方法检测氧化石墨烯含氧基团的含量、类型和形貌与温度的变化关系,并利用Zeta电位、紫外吸收谱、拉力测试分析温度对氧化石墨烯在转变过程中的溶液悬浮稳定性、光子能带、拉伸强度等性能的影响.所得定量测试结果发现,氧化石墨烯亚稳态的转变过程中存在环氧减少、羟基增加,以及整体含氧量下降的现象,而在此过程中氧化石墨烯的单片层形貌并无明显变化.但是这种结构的转变使得悬浮液黏稠度和亲水性大幅度增强,能带减小和拉伸强度增强效应明显.而当转变过程足够长时,氧化石墨烯的亲水性转而下降,并出现沉淀现象,表明羟基之间进一步发生了脱水转变.另外,本文还分析了恒温处理的时间、悬浮液的浓度对这种转变过程的影响.相关研究结果有利于理解亚稳态氧化石墨烯悬浮液随温度变化的性能转变,对氧化石墨烯具体应用有一定参考价值.     

硅功能化石墨烯热导率的分子动力学模拟

采用Tersoff势函数与Lennard-Jones势函数,结合速度形式的Verlet算法和Fourier定律,对单层和两层硅功能化石墨烯沿长度方向的导热性能进行了正向非平衡态分子动力学模拟.通过模拟发现,硅原子的加入改变了石墨烯声子的模式、平均自由程和移动速度,使得单层硅功能化石墨烯模型的热导率随着硅原子数目的增加而急剧地减小.在300 K至1000 K温度变化范围内,单层硅功能化石墨烯的热导率呈下降趋势,具有明显的温度效应.对双层硅功能化石墨烯而言,少量的硅原子嵌入,起到了提高热导率的作用,但当硅原子数目达到一定数量后,材料的导热性能下降.     

黑磷纳米通道内压力驱动流体流动特性

运用分子动力学方法探索了水-黑磷流-固界面各向异性、水流驱动力、黑磷通道宽度和黑磷层数等对黑磷通道内Poiseuille水流流动特性的影响规律.研究结果表明:随着驱动力的增加,边界滑移速度随之增加各向异性也会对压力驱动作用下纳米通道内的水分子的流动特性产生影响,具体表现为边界滑移速度会随着手性角度的增加而减小,而水分子黏度系数却不受各向异性的影响.发现黑磷表面天然的褶皱结构所产生的粗糙势能表面,是导致流固界面各向异性特性的本质原因.在加速度值保持不变的情况下,研究纳米通道宽度和黑磷层数对水分子流动特性的影响,发现随着纳米通道宽度的增加,水分子滑移速度随之减小;双层模型中水分子的速度分布与单层模型差异微小,而随着层数的增加,黑磷-水流固交互界面能随之增加,各向异性规律依然保持不变.研究结果将为水-黑磷流体器件设计与制备提供理论基础.     

Ultrasound-assisted forward osmosis desalination using inorganic draw solutes

Internal concentration polarization (ICP) represents a serious challenge in forward osmosis (FO) desalination since it causes a significant decline in the water flux across the membrane. Mitigation of ICP is cumbersome since the phenomenon occurs within the membrane porous support layer and mitigation procedures such as inducing turbulence or changing the hydrodynamic conditions tend to be ineffective. In this study, the effect of 40 kHz ultrasound on FO desalination of synthesized brackish and seawater was investigated. The studied process utilizes two different inorganic draw solutes (magnesium and copper sulfate) that are available commercially, can generate high osmotic pressures, and can be easily separated from the product water. Different concentrations of the draw solutions were considered. Results show that the applied ultrasound was effective in partially mitigating the ICP effects and enhancing the water flux. Depending on the feed and the draw solution concentration, flux enhancements of up to 34.6% and 43.9% were observed with magnesium sulfate and copper sulfate draw solution, respectively. In addition, it was observed that the effect of ultrasound on flux enhancement was more evident at lower draw solution concentrations. Although water flux was enhanced, ultrasound resulted in an increased reverse draw solute flux across the membrane.     

The use of ultrasound to reduce internal concentration polarization in forward osmosis

Unlike reverse osmosis (RO) that is dominated by the hydraulic pressure differential, forward osmosis (FO) uses the osmotic pressure gradient as the driving force between a dilute feed solution and a concentrated draw solution across a membrane. High pressure is not required in FO, which means that FO can be used as an alternative to RO as an energy-saving separation process in desalination technology. However, a major limiting factor of the FO process is the internal concentration polarization (ICP). Because of the stagnant environment inside the porous supporting layer of a FO membrane, it is difficult to mitigate the ICP by simply increasing the shear stress or promoting turbulence. In this study, the ICP is reduced by ultrasound. The effect of the ultrasound frequency and output power on the ICP coefficient is investigated in a flat-sheet FO membrane module with counter-current flow. The ultrasound frequency and output power are varied between 25, 45, and 72 kHz and over the range of 10–70 W, respectively. NaCl solution is used as both the feed and draw solution. The results illustrate that moderate ultrasonic irradiation is effective for reducing the ICP in a FO process. A modified solution–diffusion model based on film theory is used to assess the effect of ultrasound on the ICP in a FO process. The ICP coefficient is estimated using this model.     

Molecular understanding of osmosis in semipermeable membranes

We investigate single-file osmosis of water through a semipermeable membrane with an uncharged, a positively and a negatively charged nanopore. Molecular dynamics simulations indicate that the osmotic flux through a negatively charged pore (J_) is higher compared to the osmotic flux in a positively charged pore (J+) followed by the osmotic flux in the uncharged pore (J(0)), i.e., J_ > J+ > J(0). The molecular mechanisms governing osmosis, steady state osmosis, and the observed osmotic flux dependence on the nanopore charge are explained by computing all the molecular interactions involved and identifying the molecular interactions that play an important role during and after osmosis. This study helps in a fundamental understanding of osmosis and in the design of advanced nanoporous membranes for various applications of osmosis.     

Heterogeneous oxidization of graphene nanosheets damages membrane

Graphene-based materials exhibit unique properties that have been sought to utilize for various potential applications. Many studies suggest that graphene-based materials can be cytotoxic, which may be attributed to destructive effects on cell membranes.However, there still are conflicting results regarding interactions between graphene-based materials and lipid membranes. Here,through cryo-electron microscopy(Cryo-EM) and dye-leakage experiments along with in silico methods, we found that graphene oxide nanosheets induce significant membrane damage, while the effect of pristine graphene is negligible. We revealed the importance of heterogeneous oxidization of graphene-based nanosheets in damaging vesicle membranes. Moreover, that not only the oxidization degree but also the oxidization loci and membrane tension play important roles in the cytotoxicity of the graphene-based nanosheets.     

Impacts of zeolite nanoparticles on substrate properties of thin film nanocomposite membranes for engineered osmosis

In this work, microporous substrates modified by zeolite nanoparticles were prepared and used for composite membrane making with the aim of reducing internal concentration polarization (ICP) effect of membranes during engineered osmosis applications. Nanocomposite substrates were fabricated via phase inversion technique by embedding nanostructured zeolite (clinoptilolite) in the range of 0–0.6 wt% into matrix of polyethersulfone (PES) substrate. Of all the substrates prepared, the PES0.4 substrate (with 0.4 wt% zeolite) exhibited unique characteristics, i.e., increased surface porosity, lower structural parameter (S) (from 0.78 to 0.48 mm), and enhanced water flux. The thin film nanocomposite (TFN) membrane made of this optimized substrate was also reported to exhibit higher water flux compared to the control composite membrane during forward osmosis (FO) and pressure-retarded osmosis (PRO) test, without compromising reverse solute flux. The water flux of such TFN membrane was 43% higher than the control TFC membrane (1.93 L/m² h bar) with salt rejection recorded at 94.7%. An increment in water flux is ascribed to the reduction in structural parameter, leading to reduced ICP effect.     

微观结构对膜换湿能力影响研究

对多孔膜换湿过程进行了理论分析,得到了透湿量和湿阻的计算式.同时对三种膜包括PVDF,PES和纤维素膜进行了湿阻的测量实验,通过对实验数据的回归处理,得到了多孔膜的结构参数.结果表明,膜厚、孔径分布、孔隙率及曲折因子对膜的换湿特性都有较大影响,在平均孔径相同的情况下,其它因素仍然导致了不同膜换湿能力的较大差距。在所获得的结构参数基础上,通过数值模拟的方法研究了平均孔径对膜换湿能力的影响。结果表明湿阻随平均孔径的增大而减小,减小趋势逐渐平缓最后趋向一定值。     

Membrane-water structural interactions in reverse osmosis transport

Reverse-osmotic water permeabilities, equilibrium water sorption levels, and rates of approach to sorption equilibrium were measured for a series of polymers, including hydroxyethyl methacrylate (HEMA), copolymers of HEMA and ethyl methacrylate (EMA), cellulose acetate, cellulose nitrate, and poly(urethans). Pronounced equilibrium solvent clustering behavior was observed for these systems as vapor saturation was approached in sorption experiments. However, clustering tendency was not found to be a function of total membrane water content at saturation but rather appears to be a function of the chemical nature of the polymer in question. Moreover, clustering of water molecules in (relatively) hydrophobic membranes resulted in low effective diffusivities (reverse osmotic permeability divided by equilibrium water content) whereas clustering in hydrophilic membranes led to higher effective water diffusivities. Clustering tendency was not as strong in the case of the weakly interacting membranes (i.e., the cellulose acetates). These conclusions were supported by theoretical diffusivity calculations. Predictions were based on analyses of transient sorption data, employing a dual-mode sorption model, and considering ordinary Fickian diffusion with simultaneous first-order reversible penetrant localization at water-binding sites in the polymer matrices. Means were found for correcting these diffusivity predictions to those values obtained experimentally under reverse osmosis conditions by accounting for the nonideality of the water flux under the latter conditions.     

Improving protein resistance of α-Al2O3 membranes by modification with POEGMA brushes

A kind of protein-resistant ceramic membrane is prepared by grafting poly(oligo (ethylene glycol) methyl ether methacrylate) (POEGMA) brushes onto the surfaces and pore walls of α-Al₂O₃ membrane (AM) by surface-initiated atom-transfer radical polymerization (SI-ATRP). Contact-angle, Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and field-emission scanning electron microscopy (FESEM) were measured to confirm that the surfaces and pore walls of the ceramic porous membranes have been modified by the brushes with this method successfully. The protein interaction behavior with the POEGMA modified membranes (AM-POEGMA) was studied by the model protein of bovine serum albumin (BSA). A protein-resistant mechanism of AM-POEGMA was proposed to describe an interesting phenomenon discovered in the filtration experiment, in which the initial flux filtrating BSA solution is higher than the pure water flux. The fouling of AM-POEGMA was easier to remove than AM for the action of POEGMA brushes, indicated that the ceramic porous membranes modified with POEGMA brushes exhibit excellent protein resistance.     

Poly(vinylidene fluoride) membranes by an ultrasound assisted phase inversion method

Poly(vinylidene fluoride) (PVDF) membranes were prepared by an ultrasound assisted phase inversion process. The effect of ultrasonic intensity on the evolution of membrane morphology with and without the addition of pore former LiCl during precipitation process was comprehensively investigated. Besides the inter-diffusion between the solvent and nonsolvent, the ultrasonic cavitation was thought to have significant influences on phase inversion and the resultant membrane morphology. The mutual diffusion between water and solvent during the ultrasound assisted phase inversion process was measured. The crystalline structure was detected by wide angle X-ray diffractometer (WAXD). The thermal behavior was studied by differential scanning calorimeter (DSC). The mechanical strength, forward and reverse water flux, rejection to bovine serum albumin (BSA) and pepsin were also investigated. By the ultrasound assisted phase inversion method, ultra-filtration membrane was successfully prepared, which exhibited more preferable morphology, better mechanical property and more favorable permeability without sacrificing the rejection and thermal stability.