Dynamics of supercooled water in confined geometry

As with most liquids, it is possible to supercool water; this generally involves cooling the liquid below its melting temperature (avoiding crystallization) until it eventually forms a glass. The viscosity and related relaxation times (tau) of glass-forming liquids typically show non-Arrhenius temperature (T) dependencies. Liquids with highly non-Arrhenius behaviour in the supercooled region are termed 'fragile'. In contrast, liquids whose behaviour is close to the Arrhenius law (In tau infinity 1/T) are termed 'strong'. A unique 'fragile-strong' transition around 228 K has been proposed for supercooled water; however, experimental studies of bulk supercooled water in this temperature range are generally hampered because crystallization occurs. Here we use broad-band dielectric spectroscopy to study the relaxation dynamics of supercooled water in a wide temperature range, including the usually inaccessible temperature region. This is possible because the supercooled water is held within a layered vermiculite clay-the geometrical confinement and presence of intercalated sodium ions prevent most of the water from crystallizing. We find a relaxational process with an Arrhenius temperature dependence, consistent with the proposed strong nature of deeply supercooled bulk water. Because water that is less supercooled has been established as highly fragile, our results support the existence of a fragile-strong transition.     

First-order transition in confined water between high-density liquid and low-density amorphous phases

Supercooled water and amorphous ice have a rich metastable phase behaviour. In addition to transitions between high- and low-density amorphous solids, and between high- and low-density liquids, a fragile-to-strong liquid transition has recently been proposed, and supported by evidence from the behaviour of deeply supercooled bilayer water confined in hydrophilic slit pores. Here we report evidence from molecular dynamics simulations for another type of first-order phase transition--a liquid-to-bilayer amorphous transition--above the freezing temperature of bulk water at atmospheric pressure. This transition occurs only when water is confined in a hydrophobic slit pore with a width of less than one nanometre. On cooling, the confined water, which has an imperfect random hydrogen-bonded network, transforms into a bilayer amorphous phase with a perfect network (owing to the formation of various hydrogen-bonded polygons) but no long-range order. The transition shares some characteristics with those observed in tetrahedrally coordinated substances such as liquid silicon, liquid carbon and liquid phosphorus.     

Thermal conductivity of nanoscale thin nickel films

The inhomogeneous non-equilibrium molecular dynamics (NEMD) scheme is applied to model phonon heat conduction in thin nickel films. The electronic contribution to the thermal conductivity of the film is deduced from the electrical conductivity through the use of the Wiedemann-Franz law. At the average temperature of T=300 K, which is lower than the Debye temperature ΘD=450 K, the results show that in a film thickness range of about 1?11 nm, the calculated cross-plane thermal conductivity decreases almost linearly with the decreasing film thickness, exhibiting a remarkable reduction compared with the bulk value. The electrical and thermal conductivities are anisotropic in thin nickel films for the thickness under about 10 nm. The phonon mean free path is estimated and the size effect on the thermal conductivity is attributed to the reduction of the phonon mean free path according to the kinetic theory.     

Cu/Ni多层膜的电阻与位错

采用电桥及X射线衍射线形分析法研究磁控溅射Cu／Ni多层膜（CLI及Ni单层厚度均为5nm）的室温电阻率及平均位错密度随对层（bilayer）层数的变化规律．结果表明,随层数增加,电阻率及平均位错密度减少;多层膜电阻率大于单层膜电阻率;单层膜电阻率大于同质块状体的电阻率．并对其微观机制进行分析．     

A thermodynamic connection to the fragility of glass-forming liquids

Although liquids normally crystallize on cooling, there are members of all liquid types (including molecular, ionic and metallic) that supercool and then solidify at their glass transition temperature, Tg. This continuous solidification process exhibits great diversity within each class of liquid-both in the steepness of the viscosity-temperature profile, and in the rate at which the excess entropy of the liquid over the crystalline phase changes as Tg is approached. However, the source of the diversity is unknown. The viscosity and associated relaxation time behaviour have been classified between 'strong' and 'fragile' extremes, using Tg as a scaling parameter, but attempts to correlate such kinetic properties with the thermodynamic behaviour have been controversial. Here we show that the kinetic fragility can be correlated with a scaled quantity representing excess entropy, using data over the entire fragility range and embracing liquids of all classes. The excess entropy used in our correlation contains both configurational and vibration-related contributions. In order to reconcile our correlation with existing theory and simulations, we propose that variations in the fragility of liquids originate in differences between their vibrational heat capacities, harmonic and anharmonic, which we interpret in terms of an energy landscape. The differences evidently relate to behaviour of low-energy modes near and below the boson peak.     

有机蒙脱土的制备及其结构表征

为增加蒙脱土与有机相的相容性,制备有机蒙脱土,并观察蒙脱土的层状结构在有机化前后的变化.以钠基蒙脱土为原料,用季铵盐、有机胺和复合试剂作为有机插层剂与蒙脱土层间的Na+交换,制备出一系列有机蒙脱土,并对其结构进行表征.FTIR证明有机插层剂已进入蒙脱土的层间.XRD结果表明蒙脱土的层间距由1.2 nm增加到1.7～4.2 nm.TEM观察也表明蒙脱土的层间距增大,粒层厚度约为50 nm左右.研究结果表明选择高交换容量的钠基蒙脱土和合适的插层剂,制备的有机蒙脱土层间距增加明显.     

多层结构有机物薄膜制备及光致发光特性

利用有机物真空沉积技术制备了8-羟基喹啉铝（Alq）和叔丁基联苯基苯基二唑（PBD）交替生长的多层结构薄膜．低角X射线衍射测量表明，样品具有良好的层状结构；光吸收及光致发光测量结果表明，随着PBD层厚度的减薄，激发态能量发生了从PBD向Alq层中转移的现象．     

Spreading of nanofluids on solids

Suspensions of nanometre-sized particles (nanofluids) are used in a variety of technological contexts. For example, their spreading and adhesion behaviour on solid surfaces can yield materials with desirable structural and optical properties. Similarly, the spreading behaviour of nanofluids containing surfactant micelles has implications for soil remediation, oily soil removal, lubrication and enhanced oil recovery. But the well-established concepts of spreading and adhesion of simple liquids do not apply to nanofluids. Theoretical investigations have suggested that a solid-like ordering of suspended spheres will occur in the confined three-phase contact region at the edge of the spreading fluid, becoming more disordered and fluid-like towards the bulk phase. Calculations have also suggested that the pressure arising from such colloidal ordering in the confined region will enhance the spreading behaviour of nanofluids. Here we use video microscopy to demonstrate both the two-dimensional crystal-like ordering of charged nanometre-sized polystyrene spheres in water, and the enhanced spreading dynamics of a micellar fluid, at the three-phase contact region. Our findings suggest a new mechanism for oily soil removal--detergency.     

匀胶法制备TiO_2薄膜的亲水性研究

利用匀胶机制备不同厚度TiO2薄膜,分别用X射线衍射仪（X-ray Diffraction,XRD）、拉曼（Raman）光谱、原子力显微镜（Atomic Force Microscope,AFM）和X射线光电子能谱（X-ray photoelectron Spectroscopy,XPS）表征薄膜微观结构;利用紫外-可见分光光度计（UV-Vis）研究薄膜厚度对光透过率的影响;通过表面接触角测试仪研究TiO2薄膜厚度对其亲水性的影响.实验结果表明：匀胶二层的TiO2薄膜（厚度约为170nm）可达到最佳亲水性;TiO2表面形貌和表面羟基官能团（-OH）的含量会对薄膜亲水性造成影响;继续增加TiO2薄膜厚度不利于改善薄膜亲水性.     

溶胶-凝胶方法制备铁酸铋薄膜及膜厚的影响

以溶胶-凝胶方法制备不同厚度的铁酸铋(BiFeO₃, BFO)薄膜,XRD分析表明,BFO薄膜呈钙钛矿相结构,且随着厚度的增加,薄膜的结晶性变好.而薄膜厚度为4层(200 nm)至8层(400nm)之间,其微观结构可能发生由赝立方相向三方相的转变.同时,随着膜厚的增加,BFO薄膜的介电性能得到改善,绝缘性能得到提高,铁电性能也相应地好转.     

Molecular dynamics study on thermal conductivity of nanoscale thin films

A simple and effective model of heat conduction across thin films is set up and molecular dynamics simulations are implemented to explore the thermal conductivity of nanoscale thin dielectric films in the direction perpendicular to the film plane. Solid argon is selected as the model system due to its reliable experimental data and potential function. Size effects of the thermal conductivity across thin films are found by computer simulations: in a film thickness range of 2–10 nm, the conductivity values are remarkably lower than the corresponding bulk experimental data and increase as the thickness increases. The consistency between the approximate solution of the phonon Boltzmann transport equation and the simulation results ascribes the thermal conductivity size effect to the phonon scattering at film boundaries.     

超薄润滑膜界面滑移现象的分子动力学研究

为研究超薄润滑膜的流变和滑移特性 ,采用了分子动力学模拟方法。模拟系统由 2个固体壁面和介于壁面间的润滑剂分子构成 ,分子模型为正癸烷。结果表明 :薄膜中粒子密度沿膜厚方向呈周期性变化 ,存在某种有序结构 ;薄膜中润滑剂分子的平均速度仍大体呈线性分布 ,但在固液界面和液体层间可以观察到滑移现象 ;壁面滑移率随着剪切率的增加而上升 ,并在高剪切率区迅速增大 ;分子级薄膜中一个重要现象是滑移可能在较低的剪切率下发生 ;薄膜中润滑剂经历着向固态转化的相变过程 ,低剪切率下的滑移率可作为衡量薄膜固化程度的定量判据     

Molecular dynamics simulation of a single polymer in hydrophilic nano-slits

The behavior of a single polyethylene polymer in aqueous solution confined between two hydrophilic walls is studied with molecular dynamics （MD） simulations. The thickness of the nano-slit ranges from 1.26 to 3.15 nm, which is comparative to the polymer dimension. A monotonic transition from 3D- to 2D-like configurations is observed as the distance between the two walls narrows. Monomers are compressed into several layers and the preferred bond orientations alternate between parallel and normal to the walls accordingly. The diffusivity in the direction parallel to the wall is always larger than the one perpendicular to it. Calculation of the entropy and enthalpy changes during the folding of the polymer chain alone cannot explain the spontaneous process. The corresponding increase in water entropy due to volume expansion may be large enough to result in the overall free energy decrease.     

Stability of organic carbon in deep soil layers controlled by fresh carbon supply

The world's soils store more carbon than is present in biomass and in the atmosphere. Little is known, however, about the factors controlling the stability of soil organic carbon stocks and the response of the soil carbon pool to climate change remains uncertain. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6-0.8 m depth) stimulated the microbial mineralization of 2,567 +/- 226-year-old carbon. Our results support the previously suggested idea that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile could however stimulate the loss of ancient buried carbon.     

表面力仪及油性添加剂薄膜流变特性

为研究超薄润滑膜的摩擦特性和添加剂的影响 ,采用自制的表面力仪进行了润滑剂基础油和油性添加剂的薄膜流变实验。结果表明 :当膜厚减薄到纳米量级时 ,润滑油呈现非牛顿剪切响应 ,即剪切稀释现象 ,其等效粘度随着膜厚变薄而增加 ,并在某个临界膜厚处急剧上升。加入添加剂后 ,润滑油等效粘度降低 ,临界膜厚变小 ,说明薄膜流变特性与界面的摩擦状况有关。指出油性添加剂的功能在于形成摩擦系数小但厚度较薄的吸附层 ,与界于壁面间的润滑流体构成夹层结构 ,从而较好地解释了实验规律     

钢筋混凝土桥墩的延性分析

采用解析的方法探讨了箍筋对钢筋混凝土桥墩延性抗震性能的影响内容包括箍筋对混凝土的有效约束,本构关系以及曲率延性同配箍率的关系通过回归分析提出了一个配箍率同曲率延性的近似关系     

Formation of ordered ice nanotubes inside carbon nanotubes

Following their discovery, carbon nanotubes have attracted interest not only for their unusual electrical and mechanical properties, but also because their hollow interior can serve as a nanometre-sized capillary, mould or template in material fabrication. The ability to encapsulate a material in a nanotube also offers new possibilities for investigating dimensionally confined phase transitions. Particularly intriguing is the conjecture that matter within the narrow confines of a carbon nanotube might exhibit a solid-liquid critical point beyond which the distinction between solid and liquid phases disappears. This unusual feature, which cannot occur in bulk material, would allow for the direct and continuous transformation of liquid matter into a solid. Here we report simulations of the behaviour of water encapsulated in carbon nanotubes that suggest the existence of a variety of new ice phases not seen in bulk ice, and of a solid-liquid critical point. Using carbon nanotubes with diameters ranging from 1.1 nm to 1.4 nm and applied axial pressures of 50 MPa to 500 MPa, we find that water can exhibit a first-order freezing transition to hexagonal and heptagonal ice nanotubes, and a continuous phase transformation into solid-like square or pentagonal ice nanotubes.     

Cu/TiO_2超晶格的拉曼光谱

用拉曼光谱对Cu／TiO2超晶格微结构进行研究．结果表明,TiO2的声子振动被限制在自身层内,形成准二维声子振动体系．对类体模和界面模进行分析表明,TiO2和Cu在界面发生键合,形成短程有序化合物,其振动频率为91cm－1．     

Cu块材及Cu膜的光学常数研究

用反射式动态椭圆偏振光谱技术对Cu块材、Cu薄膜及Cu厚膜的光学常数进行了测试分析。研究结果表明:与Roberts块材、Johnson厚膜数据相比,不同方法得到Cu的光学常数在谱线形状上基本相似,但在数值上存在一定差别;在波长为250～830nm范围内,Cu块材和膜的折射率n与消光系数k分别在0.1～1.5和1.5～5.0之间;随膜厚增加,n值增大,k值减小;厚膜的n、k值与块材的更为接近。同时讨论了光学常数与微结构的关系。     

Thin Films for Coating Nanomaterials

Introduction The extremely high surface to volume ratio of nano- materials (particles, fullerenes, and fibers) results in increased chemical reactivity, environmental degrada- tion, tendency to agglomerate, and resistance touniform dispersion in any matrix. Therefore, under- standing ways to modify the surface to enhance its properties is essential if these materials need to be incorporated in modern technology. Surface modification in general is not new. The need for combining the desirable b…