Dispersion modeling by kinematic simulation: Cloud dispersion model

A new technique has been developed to compute mean and fluctuating concentrations in complex turbulent flows (tidal current near a coast and deep ocean). An initial distribution of material is discretized into any small clouds which are advected by a combination of the mean flow and large scale turbulence. The turbulence can be simulated either by kinematic simulation (KS) or direct numerical simulation. The clouds also diffuse relative to their centroids; the statistics for this are obtained from a separate calculation of the growth of individual clouds in small scale turbulence, generated by KS. The ensemble of discrete clouds is periodically re-discretized, to limit the size of the small clouds and prevent overlapping. The model is illustrated with simulations of dispersion in uniform flow, and the results are compared with analytic, steady state solutions. The aim of this study is to understand how pollutants disperses in a turbulent flow through a numerical simulation of fluid particle motion in a random flow field generated by Fourier modes. Although this homogeneous turbulent is rather a “simple” flow, it represents a building block toward understanding pollutant dispersion in more complex flow. The results presented here are preliminary in nature, but we expect that similar qualitative results should be observed in a genuine turbulent flow.     

Numerical analysis of reversible A B C reaction-diffusion systems

We develop an effective numerical method of studying large-time properties of reversible reaction-diffusion systems of type A + B ↔ C with initially separated reactants. Using it we find that there are three types of asymptotic reaction zones. In particular we show that the reaction rate can be locally negative and concentrations of species A and B can be nonmonotonic functions of the space coordinate x, locally significantly exceeding their initial values. Received 6 June 2002 / Received in final form 20 January 2003 Published online 7 May 2003     

Surface morphology and reaction at Cu/Si interface—Effect of native silicon suboxide

Copper thin films are deposited by thermal evaporation on unetched and etched monocrystalline silicon. The study by alpha particles backscattering (RBS) raises a strong diffusion of copper in silicon substrates with and without native suboxide layer. On the other hand, the X-rays diffraction shows the formation and the growth of Cu₃Si and Cu₄Si silicides. Whereas the scanning microscopy underlines large crystallites growth surrounded by black zones of silicon coming from the uncovered substrate, independently to the surface state of the substrate, after annealing at high temperature. The presence of native silicon suboxide at Cu/Si interface, influences in a drastic way the minimal temperature to which the interfacial reaction occurs. The oxygen impurities detected by microanalysis, after heat treatment under vacuum, are closely related to the growth of silicides crystallites.     

追念钱临照先生--《钱临照文集》读后感

文章叙述了钱临照教授的科学生涯，展示了他对物理学所作的主要贡献和对现代中国科学和教育事业的发展所起的重要作用。文中也包含了作者的一些个人回忆。     

Effects of annealing time on infrared emissivity of the Pt film grown on Ni alloy

Platinum films were sputter-deposited on polished nickel alloy substrates. The platinum thin films were applied to serve as low-emissivity layers to reflect thermal radiation. The platinum-coated samples were then heated in the air at 600 °C to explore the effects of annealing time on the emissivity of platinum films. The results show that the grain size of the Pt films increased with the increasing annealing time while their dc electrical resistivity decreased. Besides, the IR emissivitiy of the films gradually decreased with the increasing annealing time. Especially, when the annealing time reached 150 h, the average IR emissivity at the wavelength of 3-14 μm was only about 0.1. Moreover, the chemical analysis indicated that the Pt films on Ni-based alloy exhibit a good resistance against oxidation at 600 °C.     

Diffusion-induced growth of compositional heterogeneity in polymer blends containing random copolymers

The compositional relaxation in random copolymer systems on a macroscopic scale is considered in theory. A set of diffusion equations is derived that describes the motion of chains of different composition and then converted into coupled equations for statistical moments of the compositional distribution. Several ways to solve the closure problem for these equations are discussed. The simplest is the situation when the shape of the transient compositional distribution can be predicted a priori, for example, a bimodal distribution is kept during interdiffusion of two copolymers that are not very close in composition. For a general case, it is shown that the cumulant-neglect closure based on the truncation of high-order cumulants is an effective method to get an approximate solution in terms of the time-dependent local mean composition and its dispersion. This method is applied to non-homogeneous compatible polymer systems, such as a random copolymer AB of a composition varying in space, a bilayer of Bernoullian copolymers AB of different composition, and a bilayer of homopolymers A and B, in which an autocatalytic polymer-analogous reaction A → B takes place, with possibility of the neighbor group effect. It is found that the interdiffusion can lead to a substantial broadening of the local compositional distribution, which, in turn, accelerates the system dynamics and promotes chemical reactions.     

Characterization of thin Ta-Si-Nx layers of different nitrogen content using XPS, UPS and STM

Reactively sputtered Ta-Si-N_x barrier systems of different nitrogen content on copper were investigated by photoelectron spectroscopy (XPS, UPS) and scanning tunnelling microscopy (STM). The measured photoelectron spectra (excitation He-I) showed a clear dependence of the electron state density near the Fermi edge on the content of nitrogen. These results correlate with the I(U) characteristics of the STM measurements and the electrical conductivity of these layers.     

Measurements of isomeric cross-section ratios for neutron capture in the resonances region

The creation of isomeric nuclei by neutron capture in the resonances energy range was studied with the lead slowing-down assembly CIRENE. The isomeric-ratio measurement of 8 nuclei allows to determine the spin of the compound nucleus at the binding energy. An experiment on ¹⁷⁷Lu underlines the role of the spin of a resonance on the isomeric-state feeding probability.     

Nuclear parity violation beyond the two-state approximation: Criteria for quantitative studies above neutron threshold

Issues involved in quantifying polarised neutron transmission studies of nuclear parity violation (PV) in the isolated resonance regime are considered. The adaptation of the appropriate theoretical tool (the on-resonance ensemble method) to PV studies with spinnon-zero targets is discussed. An approximate treatment utilising all the spectroscopic information available in practice is presented. The consequences of the restricted size of data samples for the reliable extraction of a PV spreading width are explored. A criterion for the minimum number of independent measurements required with a given target is established, and a simple figure of merit for the selection of targets deduced. The implications for other reaction studies of PV are addressed.H.L. Harney is thanked for a rewarding discussion. This work has been supported by NSF grant PHY-87-23182.     

Nucleon form factors in dispersion theory

In the initial stage of the bottom-up picture of thermalization in heavy-ion collisions, the gluon distribution is highly anisotropic which can give rise to plasma instability. This has not been taken into account in our original paper (Phys. Lett. B 632, 257 (2006) hep-ph/0505164). It is shown that in the presence of instability there are scaling solutions, which depend on one parameter, that match smoothly onto the late stage of bottom-up when thermalization takes place.-1