非对称双原子分子在输运扩散中的取向效应

本文建立了同时具有化学势梯度和温度梯度的非平衡系统,研究非对称双原子分子的输运扩散行为.研究发现,双原子分子在非平衡输运中具有取向效应.浓度梯度与温度梯度使双原子分子在输运中产生的大小原子取向的方向刚好相反,沿着梯度的正方向,前者使小原子在前,后者使大原子在前.通过最小熵产生原理,解释了取向的物理机制.研究结果对于深刻理解非平衡条件下物质的输运与其形态的关系具有理论意义.     

The preferential growth of pyrite films prepared by thermal sulfuration of Fe2O3 films

Pyrite thin films were prepared by the sol-gel dip coating process and sulfuration treatment. The evolution of crystal orientation for the pyrite films was investigated as a function of sulfuration temperature. And the effect of crystal orientation on the electrical and optical properties was studied. It was found that films show (1 1 1) preferred orientation after sulfurized at low temperature. However, the (2 0 0) and (3 1 1) mixed preferred orientations were observed when pyrite films were sulfurized at higher temperature. Experimental results also indicate that the carrier concentration is high when the films show (1 1 1) preferred orientation. And the optical absorption coefficient is also large when the films grow with (1 1 1) preferred orientation. It is speculated that surface free energy could play a more important role in determination of preferred orientation when films were sulfurized at low temperature. However, the strain energy plays a more important role in determination of preferred orientation when films were sulfurized at higher temperature.     

Long-range order and dynamic structure factor of a nematic under a thermal gradient

We use a fluctuating hydrodynamic approach to calculate the orientation fluctuations correlation functions of a thermotropic nematic liquid crystal in a nonequilibrium state induced by a stationary heat flux. Since in this nonequilibrium stationary state the hydrodynamic fluctuations evolve on three widely separated times scales, we use a time-scale perturbation procedure in order to partially diagonalize the hydrodynamic matrix. The wave number and frequency dependence of these orientation correlation functions is evaluated and their explicit functional form on position is also calculated analytically in and out of equilibrium. We show that for both states these correlations are long-ranged. This result shows that indeed, even in equilibrium there is long-range orientational order in the nematic, consistently with the well known properties of these systems.We also calculate the dynamic structure of the fluid in both states for a geometry consistent with light scattering experiments. We find that as with isotropic simple fluids, the external temperature gradient introduces an asymmetry in the spectrum shifting its maximum by an amount proportional to the magnitude of the gradient. This effect may be of the order of 7 per cent. Also, the width at half height may decrease by a factor of about 10 per cent. Since to our knowledge there are no experimental results available in the literature to compare with, the predictions of our model calculation remains to be assessed.     

Density functional theory for molecular orientation of hard rod fluids in hard slits

A density functional theory (DFT) is used to investigate molecular orientation of hard rod fluids in a hard slit. The DFT approach combines a modified fundamental measure theory (MFMT) for excluded-volume effect with the first order thermodynamics perturbation theory for chain connectivity. In the DFT approach, the intra-molecular bonding orientation function is introduced. We consider the effects of molecular length (i.e. aspect ratio of rod) and packing fraction on the orientations of hard rod fluids and flexible chains. For the flexible chains, the chain length has no significant effect while the packing fraction shows slight effect on the molecular orientation distribution. In contrast, for the hard rod fluids, the chain length determines the molecular orientation distribution, while the packing fraction has no significant effect on the molecular orientation distribution. By making a comparison between molecular orientations of the flexible chain and the hard rod fluid, we find that the molecular stiffness distinctly affects the molecular orientation. In addition, partitioning coefficient indicates that the longer rodlike molecule is more difficult to enter the confined phase, especially at low bulk packing fractions.     

腺嘌呤、嘌呤和四并苯的表面增强拉曼光谱随温度的可逆变化

用硝酸腐蚀法处理得到了具有表面增强拉曼光谱 (SERS)活性的银表面。在表面温度于 - 1 90℃～30℃变化时 (上升和下降 ) ,分别测定了吸附在银表面上的腺嘌呤、嘌呤和四并苯的表面增强拉曼光谱 ,观察到一系列有关拉曼峰频率和强度的可逆变化。结果表明 ,温度降低 ,吸附分子SERS中受电磁增强机制作用的振动的拉曼位移发生蓝移 ,同时峰强度也有变化 ;而受化学增强机制作用的振动的拉曼位移则不受温度的影响。温度的变化导致分子平面可弯曲分子在金属表面的取向发生变化 ,如腺嘌呤和嘌呤在高温下取直立态 ,而平面的对称分子 (如四并苯 )在表面上的取向则不受温度的影响。     

晶体取向对定向凝固枝晶生长的影响

采用类金属透明模型合金丁二腈-1.0 wt%乙醇(SCN-1.0 wt% Eth)合金, 考察了晶体取向对定向凝固过程中晶粒的平界面失稳孕育时间、枝晶形态演化以及枝晶一次间距的影响. 结果表明, 随着枝晶择优生长方向与温度梯度方向夹角的增大, 晶粒的平界面失稳孕育时间增加, 界面的稳定性增强; 对于不同晶体取向的枝晶形态演化, 枝晶择优生长方向与温度梯度方向夹角越大, 枝晶二次臂不对称生长越严重, 同时, 具有生长优势的枝晶二次臂对相邻枝晶的生长的抑制越强烈; 至于不同晶体取向的枝晶一次间距, 随着枝晶择优生长方向与温度梯度方向夹角的增大, 枝晶一次间距增大. 关键词： 定向凝固 平界面失稳 枝晶间距 晶体取向     

CRYSTALLIZATION OF A LOW MOLECULAR WEIGHT POLYETHYLENE AND PARAFFINS UNDER A TEMPERATURE GRADIENT

A low molecular weight polyethylene (PE) and even-number paraffins were crystallized under a temperature gradient. Highly oriented crystalline textures were developed by the temperature slope crystallization. The in situ crystallizing surface was observed by an optical microscope and X-ray diffraction. Polyethylene has a b-axis orientation in which the lamellar normal and crystalline c-axis are perpendicular to the temperature gradient. On the contrary, in the even-number paraffins, both axes are parallel to the temperature gradient. The results of the in situ measurements and the crystalline orientation are compared and discussed for both cases.     

Bandgap engineering of rare earth element doped nanostructured cadmium oxide thin films

Rare earth element (gadolinium) doped cadmium oxide (CdO:Gd) thin films were deposited using the pulsed laser deposition technique. X-ray diffraction analysis reveals that growth temperature has large impact on the preferred orientation of the films. The films grown at low temperature show (1 1 1) preferred orientation, while films grown at high temperature have (2 0 0) orientation. The effect of substrate temperature on optical and electrical properties shows widening in optical bandgap and improvement in electron mobility with increase in growth temperature. These wide bandgap transparent conducting films could be used in optoelectronic applications.     

Effect of Shear Flow on Nanoparticles Migration near Liquid Interfaces

The effect of shear flow on spherical nanoparticles (NPs) migration near a liquid–liquid interface is studied by numerical simulation. We have implemented a compact model through which we use the diffuse interface method for modeling the two fluids and the molecular dynamics method for the simulation of the motion of NPs. Two different cases regarding the state of the two fluids when introducing the NPs are investigated. First, we introduce the NPs randomly into the medium of the two immiscible liquids that are already separated, and the interface is formed between them. For this case, it is shown that before applying any shear flow, 30% of NPs are driven to the interface under the effect of the drag force resulting from the composition gradient between the two fluids at the interface. However, this percentage is increased to reach 66% under the effect of shear defined by a Péclet number Pe = 0.316. In this study, different shear rates are investigated in addition to different shearing times, and we show that both factors have a crucial effect regarding the migration of the NPs toward the interfacial region. In particular, a small shear rate applied for a long time will have approximately the same effect as a greater shear rate applied for a shorter time. In the second studied case, we introduce the NPs into the mixture of two fluids that are already mixed and before phase separation so that the NPs are introduced into the homogenous medium of the two fluids. For this case, we show that in the absence of shear, almost all NPs migrate to the interface during phase separation, whereas shearing has a negative result, mainly because it affects the phase separation.     

Long-range boundary effects in simple fluids

We discuss long-range boundary effects in simple two- or three-dimensional fluids. These boundary effects are due to the existence of long-range correlations in nonequilibrium fluids and can be computed either by means of kinetic theory or phenomenological mode-coupling theories. In particular, we use kinetic theory to compute the stress tensor and heat flux vector for a fluid in a nonequilibrium steady state in a finite geometry and show that both the effective shear viscosity and effective heat conductivity have contributions due to the walls of the container that influence the behavior of the system far into the fluid. We also show that the mechanocaloric effect is present in the bulk of a three-dimensional fluid and that there are normal stresses in a fluid whenever the temperature gradient is nonzero.Work performed under National Science Foundation grant No. CHE 77-16308.     

Molecular aspect ratio and anchoring strength effects in a confined Gay–Berne liquid crystal

Phase diagrams for Gay–Berne (GB) fluids were obtained from molecular dynamics simulations for GB(2, 5, 1, 2) (i.e. short mesogens) and GB(3, 5, 1, 2) (i.e. long mesogens), which yield isotropic, nematic, and smectic-B phases. The long-mesogen fluid also yields the smectic-A phase. Ordered phases of the long-mesogen fluid form at higher temperatures and lower densities when compared to those of the short-mesogen fluid. The effect of confinement under weak and strong substrate couplings in slab geometry was investigated. Compared to the bulk, the isotropic–nematic transition does not shift in temprature significantly for the weakly coupled substrate in either mesogen fluid. However, the strongly coupled substrate shifts the transition to lower temperature. Confinement induces marked stratification in the short-mesogen fluid. This effect diminishes with distance from the substrate, yielding bulk-like behaviour in the slab central region. Fluid stratification is very weak for the long-mesogen fluid, but the strongly coupled substrate induces ‘smectisation’, an ordering effect that decays with distance. Orientation of the fluid on the substrate depends on the mesogen. There is no preferred orientation in a plane parallel to the substrate for the weakly coupled case. In the strongly coupled case, the mesogen orientation mimics that of adjacent fluid layers. Planar anchoring is observed with a broad distribution of orientations in the weakly coupled case. In the strongly coupled case, the distribution leans toward planar orientations for the short-mesogen fluid, while a marginal preference for tilting persists in the long-mesogen fluid.     

High-frequency driven capillary flows speed up the gas-liquid phase transition in zero-gravity conditions

Under weightlessness conditions, the phase transition of fluids is driven only by slow capillary flows. We investigate the effect of high-frequency vibrations to reproduce some features of gravity effects and show that such vibrations can greatly modify the phase transition kinetics. The investigation is performed in H2 near its critical point (critical temperature 33 K) where critical slowing down enables the phase transition process to be carefully studied. Gravity effects are compensated in a strong magnetic field gradient.     

Director reorientation in twisted nematic liquid crystals due to thermomechanical effect

We have studied molecular director reorientation in a twisted nematic liquid crystal induced by a two-dimensional temperature gradient. We studied the effect of rate change between the temperature gradients in two directions. Our obtained director reorientations are in the range that can be observed experimentally very easily.     

Thermal orientation effect in a nematic liquid crystal

The system of equations describing the thermal orientation effect in nonchiral liquid crystals, which was observed earlier, is proposed and substantiated. The effect of the director reorientation under the action of the temperature gradient is in many ways analogous to the Freedericksz transition in electric or magnetic fields. The angle of deviation of the director from the initial position is determined by the square of the temperature gradient. The effect is observed for a nematic liquid crystal, initially uniformly oriented, and is not accompanied by macroscopic fluxes of the medium under stationary conditions.     

A simple model for ordering in adsorbed layers

Order-disorder phase transitions in adsorbed and confined fluids with directional interactions are studied using lattice density functional theory. A new model is developed that is capable of predicting both order-disorder and condensation phase transitions. For systems with weak interactions, the results of this model are compared with both lattice Monte Carlo simulation data and simple isotherm theories that are commonly used to fit experimental data. The results show that these simple isotherms are incapable of duplicating complex behaviour exhibited by anisotropic molecules. When sufficiently strong interactions are present, confined fluids with directional interactions may spontaneously form ordered structures. It is shown that the ordering predicted by this model can result in the assembly of long chains. Such ordering has been observed experimentally in magnetorheological fluids in a magnetic field. It is shown that the orientation of the chains predicted by this model can be controlled by adjusting the molecule-surface interaction. It may be possible to create nanoscale devices that exploit this type of molecular switching.     

Configurational temperature thermostat for fluids undergoing shear flow: application to liquid chlorine

Non-equilibrium molecular dynamics (NEMD) simulations play a major role in characterizing the rheological properties of fluids undergoing shear flow. However, all previous studies of flows in molecular fluids either use an ‘atomic’ thermostat which makes incorrect assumptions concerning the streaming velocity of atoms within their constituent molecules, or they employ a centre of mass kinetic (COM) thermostat which only controls the temperature of relatively few degrees of freedom (3) in complex high molecular weight compounds. In the present paper we show how recently developed configurational expressions for the thermodynamic temperature can be used to develop thermostatting mechanisms which avoid both of these problems. In this work, we propose a thermostat based on a configurational expression for the temperature and apply it to NEMD simulations of chlorine undergoing Couette flow. The results so obtained are compared with those obtained using a COM kinetic thermostat. At equilibrium the properties of systems thermostatted in the two different ways are of course equivalent. We show that the two responses only differ far from equilibrium. In particular, we show that the formation of a string phase for extremely high shear rates is an artefact of the COM thermostat. At the largest shear rates studied with the configurational thermostat, no string phase is observed.     

Metal–organic frameworks: a novel SERS substrate

We report the direct observation of surface‐enhanced Raman scattering (SERS) effect using metal–organic frameworks (MOFs) as substrates. Without the aid of any metal colloids or enhancing agents, the SERS signals of methyl orange (MO) adsorbed in MOFs were observed and even remained active if the organic linkers in MOFs were completely removed by high temperature and O₂ plasma treatments. It implies that the SERS active site is at the metal oxide clusters. The ultraviolet‐visible spectra of MO, MOFs, and MO–MOF complexes show that absorption peaks are far from laser excitation line. Thus, conventional resonance enhancement effect should be ruled out, and charge‐transfer mechanism is the most likely scenario responsible for the observed SERS effect. Density functional theory (DFT) was used to interpret the chemical enhancement mechanism and the adsorption orientation‐dependent SERS spectra in our observation. The preferred adsorption orientations calculated by DFT method are consistent with the observed SERS results. Copyright © 2013 John Wiley & Sons, Ltd.     

Orientation evolution of single-crystal superalloys under different solidification interface

The single-crystal superalloys with different deviation angle are prepared by using the liquid metal cooling high-temperature gradient directional solidification furnace, and the orientation evolution of single-crystal superalloys under different solidification interface is studied. The results indicate that the increase of the deviation angle will make the planar interface unstable. With the increase of the solidification rate, the growth direction of the crystal will deviate from the direction of heat flow to the preferred orientation direction. Under the developed dendrite crystal growth, the branches of the dendrite will follow the preferred orientation, nearly free from the effect of the heat flow.     

Effects of annealing on structural,optical and electrical properties of Al-doped ZnO thin films

Transparent conducting oxide (TCO) thin films such as SnO2, In2O3, and Cd2SnO4, have been used extensively as sensor devices, surface acoustic wave devices, coating to heat glass windows and transparent electrodes for solid state display devices, solar cells[1,2] because of their high optical transparency in the visible range, infrared reflec-tance and low d.c. resistivity. Although SnO2 film was developed early, nowadays Sn-doped In2O3 (ITO) films are the predominant TCO thin film in …