Molecular dynamics simulation of ice nanocluster in supercooled water

The properties of the interface between an ice nanocluster and the surrounding water shell were investigated by the molecular simulation method for the SPC/E model in the temperature interval from 200 to 230?K. The melting point of the ice core was determined on the basis of the caloric curve and the behaviour of the diffusion coefficient. The change of the local structure was described by the orientational distribution functions and by the radial profiles of the local density, energy, and normal pressure.     

Pressure variation by a magnetohydrodynamic method at the surface of a body placed in a supersonic flow

This study is aimed at investigating the possibility of pressure variation near the surface of a body placed in a supersonic flow as a model of an aerofoil or the nose of an aircraft by organizing a surface gas discharge in a magnetic field transverse to the flow. The flow parameters and pressure are mainly affected by the ponderomotive Lorentz force acting on the gas in the direction orthogonal to the direction of the organized discharge current and leading to the removal or compression of the gas at the surface of the body and, hence, a variation of pressure. Experimental data on the visualization of the flow and on the pressure at the surface of the body are considered for various configurations of the current and intensities of the gas discharge and magnetic field; it is demonstrated that such configurations of the current and magnetic field near the surface of the body under investigation can be organized in such a way that the pressure at the front part as well as the upper and lower surfaces of the body under investigation can be increased or decreased, thus changing the aerodynamic drag and the aerofoil lift. Such a magnetohydrodynamic control over aerodynamic parameters of the aircraft can be used during takeoff and landing as well as during steady-state flight and also during the entrance into dense atmospheric layers. This will considerably reduce the thermal load on the surface of the body in the flow.     

Suppression of ice nucleation in supercooled water under temperature gradients

Understanding the behaviours of ice nucleation in non-isothermal conditions is of great importance for the preparation and retention of supercooled water. Here ice nucleation in supercooled water under temperature gradients is analyzed thermodynamically based on classical nucleation theory (CNT). Given that the free energy barrier for nucleation is dependent on temperature, different from a uniform temperature usually used in CNT, an assumption of linear temperature distribution in the ice nucleus was made and taken into consideration in analysis. The critical radius of the ice nucleus for nucleation and the corresponding nucleation model in the presence of a temperature gradient were obtained. It is observed that the critical radius is determined not only by the degree of supercooling, the only dependence in CNT, but also by the temperature gradient and even the Young's contact angle. Effects of temperature gradient on the change in free energy, critical radius, nucleation barrier and nucleation rate with different contact angles and degrees of supercooling are illustrated successively. The results show that a temperature gradient will increase the nucleation barrier and decrease the nucleation rate, particularly in the cases of large contact angle and low degree of supercooling. In addition, there is a critical temperature gradient for a given degree of supercooling and contact angle, at the higher of which the nucleation can be suppressed completely.     

过冷水法冰浆制取的实验设计与分析

动态冰蓄冷技术是目前蓄能技术重要的研究方向之一,其中冰浆制取方式是研究的关键课题。冰浆具有良好的流动性,大的相变潜热和快速的释冷特性,是常规载冷剂的潜在替代物。该文给出了过冷水法冰浆制取的实验装置,采用纯净水和自来水为试样,在不同的条件下进行了对比实验,并分析了管内冻结情况。     

An analysis of surface acoustic wave propagation in a plate of functionally graded materials with a layered model

In a homogeneous plate, Rayleigh waves will have a symmetric and anti-symmetric mode regarding to the mid-plane with different phase velocities. If plate properties vary along the thickness, or the plate is of functionally graded material (FGM), the symmetry of modes and frequency behavior will be modified, thus producing different features for engineering applications such as amplifying or reducing the velocity and deformation. This kind of effect can also be easily realized by utilizing a layered structure with desired material properties that can produce these effects in terms of velocity and displacements, since Rayleigh waves in a solid with general material property grading schemes are difficult to analyze with known methods. Solutions from layered structures with exponential and polynomial property grading schemes are obtained from the layered model and comparisons with known analytical results are made to validate the method and examine possible applications of such structures in engineering. Supported by the National Natural Science Foundation of China (Grant Nos. 10432030, 10125209, and 10572065) and the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions, Ministry of Education of China, and also supported by Qianjiang River Fellow Fund established by Zhejiang Provincial Government and Ningbo University and administered by Ningbo University, Zhejiang, China     

Tailoring the structure of water at a metal surface: a structural analysis of the water bilayer formed on an alloy template

Recent studies show that structures based on the traditional "icelike" water bilayer are not stable on flat transition metal surfaces and, instead, more complex wetting layers are formed. Here we show that an ordered bilayer can be formed on a SnPt(111) alloy template and determine the structure of the water layer by low energy electron diffraction. Close agreement is found between experiment and the structure calculated by density functional theory. Corrugation of the alloy surface allows only alternate water molecules to chemisorb, stabilizing the H-down water bilayer by reducing the metal-hydrogen repulsion compared to a flat surface.     

Theoretical analysis of the effect of crystallization temperature on structure formation in flexible-chain polymers

Theoretical analysis of flexible-chain polymer crystallization was carried out over a wide temperature range from glass transition temperature T_g to melting temperature T_m. Temperature dependence of dynamic behavior manifesting in the decrease of crystallizing length with decreasing temperature was taken into account. The dependence of crystallizing length on temperature was obtained using chosen values of it at T_g and T_m. The crystallization with the formation of folded-chain crystals (type I) and uncoiled-chain crystals (type II) was considered. The analysis of thermodynamical favorability of both types of crystals with respect to temperature made it possible to obtain a flexible-chain crystallizing polymer phase diagram. This diagram shows the existence of two ranges where type II crystal formation is more favorable: a narrow range near T_m and the wider one near T_g, separated by the temperature range of crystallization with chain folding. Temperature dependences of type II crystals fraction in the system and their size were calculated. It is shown that the crystallization at considerable supercooling leads to the appearance of a great number of type II small crystals connected by tie chains. The system formed is characterized by a high degree of crystallite interconnection.     

Nonstationary heat conduction over a blunt body with surface mass exchange placed in a supersonic flow

Interaction of a high-temperature gas with a heat-reflecting coating is accompanied by many interrelated processes. The need for thermal protection arises when an unprotected structure is bound to inevitably fail under the action of heat fluxes. It seems that heat fluxes on the order of 2.5 × 10 ⁵ W/m², which corresponds to equilibrium surface temperatures exceeding 1500 K, set the upper operating temperature limit for unprotected refractory metals. However, this limit is to some extent conditional, since mechanical and corrosion effects may often aggravate the thermal action, causing the structure to fail at much lower temperatures.     

Recent experimental aspects of the structure and dynamics of liquid and supercooled water

Liquid water, the most familiar liquid, is still not completely understood, even less so all the processes in which it participates. The directionality of the bonds and quantum aspects make the establishment of a complete theory difficult, particularly in the case of effective potentials built with spherical electrostatic forces. Recent work has focused on the hydrogen bonds formed between water molecules or with hydrophilic substrates. We describe the present situation of research concerning the so-called anomalies of liquid water at low temperature. Although without direct applications, this problem is consistently an object of discussion, enhanced by results from molecular dynamics simulations. Conversely, because in many situations where water plays a major role, such as, for example, in biology, only a few molecules are involved, the study of confined water is extremely important, sometimes decoupled from the more fundamental studies of bulk water. A short, but far from exclusive, summary of some of the more active domains of research concerning liquid water is given, mainly concerning interactions with other media.     

Theoretical analysis of ARUPS of a Si(100) surface at room temperature

ARUPS spectra of a Si(100) surface with disorder perpendicular to the dimer row are studied by the tight-binding method for the ensemble of surface buckled dimers. We assume order along the row. Results of ARUPS experiment on the surface at room temperature agree qualitatively with calculated results for disordered structure in which the distance to the next buckled dimer along the row and the height of the buckled dimer change alternately along the row. Peaks of two branches observed in a recent experiment at room temperature appear distinctly in calculated results as long as the short range order among the rows is not weak. The intensity of the two branches is very small compared to the other for conventional structure of the surface.     

Kinetics of the formation and structure of oligoperoxide nanolayers and grafted polymer brushes on glass plate surface

Functional oligoperoxide surfactants and coordinating oligoperoxide metal complexes were studied as modifiers of glass flat plates to provide the localization of radical forming sites and other functional fragments in adsorbed polymeric layers of a nanoscale thickness. Both the kinetics of the coating formation and properties of the nanolayers witness the dependence of the packing density of oligoperoxide molecules in the coatings on the oligoperoxide natures, concentrations and conditions of the sorption modification. The availability of definite amount of peroxide groups in formed nanolayer provides the possibility of controlled radical graft polymerization initiated from modified surface leading to reliable surface protection, functionality and targeted surface hydrophilic-hydrophobic properties.      

SH surface acoustic wave propagation in a cylindrically layered piezomagnetic/piezoelectric structure

SH surface acoustic wave (SH-SAW) propagation in a cylindrically layered magneto-electro-elastic structure is investigated analytically, where a piezomagnetic (or piezoelectric) material layer is bonded to a piezoelectric (or piezomagnetic) substrate. By means of transformation, the governing equations of the coupled waves are reduced to Bessel equation and Laplace equation. The boundary conditions imply that the displacements, shear stresses, electric potential, and electric displacements are continuous across the interface between the layer and the substrate together with the traction free at the surface of the layer. The magneto-electrically open and shorted conditions at cylindrical surface are taken to solve the problem. The phase velocity is numerically calculated for different thickness of the layer and wavenumber for piezomagnetic ceramics CoFe₂O₄ and piezoelectric ceramics BaTiO₃. The effects of magnetic permeability on propagation properties of SH-SAW are discussed in detail. The distributions of displacement, magnetic potential and magneto-electromechanical coupling factor are also figured and discussed.     

Impact of water and ice 1h formation in a photonic crystal fiber grating

The impact of ice 1h formation inside the holes of a photonic crystal fiber Bragg grating was analyzed and discussed. As a result of the ice's expansion, a broadening of the grating spectrum was observed that corresponds to internal microbending of the fiber and after some temperature cycling leads to failure of the fiber. An analytical model with which to estimate the internal compression forces is proposed, and the calculated values are found to be in agreement with reported data.     

Theoretical investigation of the resonance properties of an active ring made of a ferrite-ferroelectric layered structure

A theoretical model of a microwave active ring resonator based on a ferrite-ferroelectric layered structure serving as a waveguide for hybrid electromagnetic-spin waves is developed. The resonator is made in the form of a closed ring consisting of a ferrite-ferroelectric delay line, a microwave amplifier, and a variable attenuator. The resonance properties of this system are studied theoretically. The feasibility is demonstrated of effectively controlling the resonance frequencies of the ring by varying the permittivity of the ferroelectric layer.     

Preliminary analysis of surface radiation measurements recorded at the Nansen ice sheet (Antarctica)

Summary An experiment on radiation and surface energy balance was conducted during the 9th Italian expedition in Antarctica at the Nansen ice sheet, a glacier situated close to the Italian base at Terra Nova Bay, to correlate surface balances to the formation and development of katabatic winds. Measurements were taken by radiometers covering the whole spectra of solar and terrestrial emissions and by fast sensors of atmospheric wind velocity and humidity for the application of the eddy correlation technique. A preliminary analysis of the radiotive energy balance during the Antarctic summer in clear-sky conditions is reported and discussed. The findings show the very low available energy (mean about 1 W/m²), in terms of net radiation, for the physical processes such as sensible- and latent-heat fluxes. Long-wave radiation balance was applied to estimate the reliability of the Swinbank's parametrization, relative to general conditions of the atmosphere.     

Theoretical investigation of superconductivity in ternary silicide NaAlSi with layered diamond-like structure

We have investigated the electronic structure, phonon modes and electron–phonon coupling to understand superconductivity in the ternary silicide NaAlSi with a layered diamond-like structure. Our electronic results, using the density functional theory within a generalized gradient approximation, indicate that the density of states at the Fermi level is mainly governed by Si p states. The largest contributions to the electron–phonon coupling parameter involve Si-related vibrations both in the x–y plane as well as along the z-axis in the x–z plane. Our results indicate that this material is an s-p electron superconductor with a medium level electron–phonon coupling parameter of 0.68. Using the Allen–Dynes modification of the McMillan formula we obtain the superconducting critical temperature of 6.98 K, in excellent agreement with experimentally determined value of 7 K.     

离子浓度及表面结构对岩石孔隙内水流动特性的影响

酸性环境引发的岩石孔隙表面溶解增加了孔隙内水溶液的盐离子浓度,破坏了孔隙的表面结构.本文采用分子动力学模拟的方法研究了纳米级岩石孔隙内水溶液的流动特性,分析了盐离子浓度和孔隙表面结构对水流速度分布的影响及原因.研究结果表明:纳米级岩石孔隙内的水溶液流动符合泊肃叶流动特性,流速呈"抛物线"分布;随盐离子浓度增加,水溶液内部氢键网络变得更为致密,水黏度随其呈线性增长;水溶液中离子浓度越大,孔隙表面对水流动的阻力越大,最大流速越小,速度分布的"抛物线"曲率半径越大;岩石孔隙表面结构的破坏改变了流动表面的粗糙程度,增加了孔隙表面对H2O分子的吸引力.随表面结构破坏程度的增大,水溶液在近壁区域的密度增大,流速降低;当表面破坏程度达到50%时,水溶液在近壁区域出现了明显的负边界滑移现象.