Structure and cluster formation in granular media

The two most important phenomena at the basis of granular media are excluded volume and dissipation. The former is captured by the hard sphere model and is responsible for, e.g., crystallization, the latter leads to interesting structures like clusters in non-equilibrium dynamical, freely cooling states. The freely cooling system is examined concerning the energy decay and the cluster evolution in time. Corrections for crystallization and multi-particle contacts are provided, which become more and more important with increasing density.     

Structure formation in electromagnetically driven granular media

We report structure formation in submonolayers of magnetic microparticles subjected to periodic electrostatic and magnetic excitations. Depending on the excitation parameters, we observe the formation of a rich variety of structures: clusters, rings, chains, and networks. The dynamics and shapes of the structures are strongly dependent on the amplitude and frequency of the external magnetic field. We find that for pure ac magnetic driving the low-frequency magnetic excitation favors compact clusters, whereas high frequency driving favors chains and netlike structures. An abrupt phase transition from chains to a network phase was observed for a high density of particles.     

三元Co-Cu-Pb偏晶合金的快速凝固组织形成规律研究

在自由落体条件下实现了三元Co-Cu-Pb合金的液相分离与快速凝固. 实验发现,随液滴直径减小,Co₅₁Cu₄₇Pb₂合金液滴发生由枝晶→两层壳核→枝晶组织的转变,Co₄₇Cu₄₄Pb₉合金液滴的组织形态由壳核组织演化为均匀组织. 两种合金的快速凝固组织均由α(Co),(Cu)和(Pb)固溶体三相组成,α(Co)和(Cu)相主要以枝晶方式生长,(Pb)相分布在(Cu)枝晶间. 关键词： 液相分离 偏晶合金 快速凝固 自由落体     

Structure formation on the surface of alloys irradiated by femtosecond laser pulses

Laser-induced periodic surface structures with different spatial characteristics have been observed after multiple linearly polarized femtosecond laser pulse (120 fs, 800 nm, 1 Hz to 1 kHz pulse repetition frequency) irradiation on alloys. With the increasing number of pulses, nanoripples, classical ripples and modulation ripples with a period close to half of classical ripples have all been induced. The generation of second-harmonic has been supposed to be the main mechanism in the formation of modulation ripples.     

Investigation of the magnetic properties of homogeneous copper-manganese alloys

The magnetic characteristics of homogeneous copper—manganese alloys Cu_{1 ? x} Mn _x are studied by the muon spin relaxation technique for the first time. It is revealed that the specific magnetic phase, which is most likely characterized by a fast spin dynamics and the absence of long-range order, is formed in alloys with concentrations 0.2 < x < 0.7 in the temperature range 10–330 K. The complete magnetic phase diagram is constructed.     

Structure of the 3d-band and thermal emf of nickel and its alloys

The laws governing the change in thermal emf of nickel and some of its alloys after they are doped are considered. It is concluded that important data on the structure of the d-band can be obtained by measuring the absolute thermal emf of transition metals and their alloys.     

CoFe-based granular alloys: the role of the metallic matrix

RF-sputtered CoFe-NM granular alloys (NM=Ag, Cu) with CoFe volume content, x_v, ranging from 0.10 to 0.45 have been studied. These two series of samples show similar features depending on the synthesis conditions and post-deposition annealing treatments, revealing the strong dependence of magnetotransport properties on microstructure. Three different regimes have been observed as x_v is increased: the classical giant magnetoresistance (GMR) regime at low ferromagnetic contents; at intermediate x_v, a domain structure appears, and GMR and anisotropic magnetoresistance (AMR) together with domain wall scattering are observed; and a third regime at x_v close but below the volume percolation threshold, where the two latter contributions still coexist, while the GMR contribution has been suppressed by strong magnetic correlations. The role of the metallic matrix is crucial to determine the crossover ferromagnetic contents between these three regimes, which depend on the relative immiscibility of CoFe either in the Ag or Cu matrices and the diffusivity of Ag and Cu. Moreover, the metallic matrix settles the degree of CoFe segregation, sample crystallisation and texture, which are responsible for the magnetotransport properties.     

Architecture of columnar nacre, and implications for its formation mechanism

We analyze the structure of Haliotis rufescens nacre, or mother-of-pearl, using synchrotron spectromicroscopy and x-ray absorption near-edge structure spectroscopy. We observe imaging contrast between adjacent individual nacre tablets, arising because different tablets have different crystal orientations with respect to the radiation's polarization vector. Comparing previous data and our new data with models for columnar nacre growth, we find the data are most consistent with a model in which nacre tablets are nucleated by randomly distributed sites in the organic matrix layers.     

Magnetic properties and structure of electrodeposited Zn-Co alloys granular thin films

We present several results concerning the preparation by means of electrolysis and characterization of Zn-Co alloys thin films. Films of Zn, Co and Zn-Co with various compositions (8-16 at% Co) were prepared in sulfate baths, using potentiostatic control, envisaging applications in the domain of corrosion resistant magnetic sensors. The effects of applied voltage on the magnetic properties, microstructure and phase content of the electrodeposited Zn-Co films were investigated. The applied voltage significantly influenced the film composition and their magnetic properties. These films, when deposited at an applied voltage of 4.5 V exhibited multiphase behavior due to the inclusion of new phases (cobalt hydroxide), whereas at 3.0 V, only Zn-Co alloys were deposited. The structure and morphology of the samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).     

CoNiZ系列合金的结构和马氏体相变性质

利用X射线衍射研究了CoNiZ(Z=Si,Sb,Sn,Ga等)合金在不同热处理条件下的相组成.当Z元素为Sn,Sb时,材料是完全的B2结构;但Z为Si时,材料变成面心立方的γ相.形成B2还是γ相由电子浓度和原子尺寸效应两种因素共同决定.而CoNiGa的研究结果表明,在合金中除了形成B2结构的同时还容易形成γ相,常表现出两相共存的特性.对材料进行不同方式的热处理可以使合金中两相的含量有所消长,γ相含量的多少对CoNiGa合金的马氏体相变有很大的影响.分析指出,两相共存及其所带来的物性变化是CoNiGa铁磁性形状记忆合金非常有利用价值的物理性质.     

Role of air in granular jet formation

A steel ball impacting on a bed of very loose, fine sand results in a surprisingly vigorous jet which shoots up from the surface of the sand [D. Lohse, Phys. Rev. Lett. 93, 198003 (2004)10.1103/PhysRevLett.93.198003]. When the ambient pressure p is reduced, the jet is found to be less vigorous [R. Royer, Nature Phys. 1, 164 (2005)10.1038/nphys175]. In this Letter we show that p also affects the rate of penetration of the ball: Higher pressure increases the rate of penetration, which makes the cavity created by the ball close deeper into the sand bed, where the hydrostatic pressure is stronger, thereby producing a more energetic collapse and jetting. The origin of the deeper penetration under normal ambient pressure is found to lie in the extra sand fluidization caused by the air flow induced by the falling ball.     

Correlation of spin and velocity in the homogeneous cooling state of a granular gas of rough particles

In a granular gas of rough particles the spin of a grain is correlated with its linear velocity. We develop an analytical theory to account for these correlations and compare its predictions to numerical simulations, using Direct Simulation Monte Carlo as well as Molecular Dynamics. The system is shown to relax from an arbitrary initial state to a steady-state, which is characterized by time-independent, finite correlations of spin and linear velocity. The latter are analyzed systematically for a wide range of system parameters, including the coefficients of tangential and normal restitution as well as the moment of inertia of the particles. For most parameter values the axis of rotation and the direction of linear momentum are perpendicular like in a sliced tennis ball, while parallel orientation, like in a rifled bullet, occurs only for a small range of parameters. The limit of smooth spheres is singular: any arbitrarily small roughness unavoidably causes significant translation-rotation correlations, whereas for perfectly smooth spheres the rotational degrees of freedom are completely decoupled from the dynamic evolution of the gas.     

On the electron mechanism of suppression of the magnetic moment of nickel in its alloys with copper and zinc

For the first time, the electronic structure of Ni, Cu, and Zn in Ni_0.27Cu_0.73, Ni_0.18Cu_0.51Zn_0.31, and Cu_0.66Zn_0.34 and some other chemical compounds has been studied using the method of shifts of the K X-ray lines. Statistically reliable shifts (from the positions for the metal) of all diagram K-lines of Ni have been found for both alloys, whereas the shifts of the K-lines of Cu and Zn (also from the positions for the metals) are close to zero for all the alloys. For the first time, estimates have been obtained for the 3d- and/or 4s-facsimiles, i.e., shifts of all diagram lines caused by the removal of one 3d- and/or 4s-electrons. On the basis of these facsimiles, the found shifts of the K-lines of Ni in Ni_0.27Cu_0.73 and Ni_0.18Cu_0.51Zn_0.31 have been explained by the transference of ≲0.10 and 0.15 of its 4s-electron to 3d-band of the same atom.     

Structure and kinematics in dense free-surface granular flow

We show that the structure of a dense, free-surface boundary layer granular flow is similar to the structure of a laminar liquid flow: There is a strong component of order (stratification parallel to the mean flow) superposed with a mild component of disorder (self-diffusion perpendicular to the mean flow). We also show that the self-diffusion coefficient scales with the mean velocity and propose a model that relates this scaling to the ordered structure of the flow. Last, we show that the structure of the flow imprints an oscillatory signature (similar to that found in confined granular flow) on the mean velocity profile.     

Structure and physicomechanical properties of Al-Si alloys

Al-Si alloy samples with a silicon content of 8–15 wt % are grown by the Stepanov method at a solidification rate of 10² and 10³ μm/s. The microstructure of the samples is examined, and the stress-strain curves obtained during tension and bending at a strain rate of 10^?4 s^?1 are studied. The behavior of Young’s modulus, the modulus defect, and ultrasonic attenuation is investigated. The silicon content in a eutectic structure is found to increase with the solidification rate. The yield strength and the ultimate tensile strength increase with the silicon content up to a eutectic composition. The quality index (which characterizes the strength and plasticity of the material) of Stepanov-grown samples is higher than the quality indices of traditional modified ingots.     

Energy of formation for and liquid binary alloys

We have investigated the free energy of formation for Ag_xIn_1-x and Ag_xSn_1-x liquid binary alloys at temperatures 1173 and 1250 K, respectively. A microscopic theory based on the first order perturbation has been applied. The interionic interaction and a reference liquid are the fundamental components of the theory. The interionic interaction is described by a local pseudopotential. A liquid of hard spheres (HS) of two different effective diametres and charges is used to describe the reference system. The results of the calculations for energy of formation agree very well with the available experimental data. Our calculations also reveal that a simple perturbative approach along with appropriate effective pair potentials can produce nearly quantitative results for the concerned alloys.     

Structure and electronic properties of niobium-ruthenium alloys

The electronic specific heat, the Debye temperature, the paramagnetic susceptibility and the superconducting transition temperature have been measured in the niobium-rich region of the Nb-Ru system. Structural investigation proved that the single phase region extends up to at least 45 at. % Ru at low temperatures, with a gradually increasing tetragonal deformation above 37% Ru. The results reveal a flat minimum in the density of states near a concentration of 5.7 valence electrons per atom. Comparison of specific heat and susceptibility data lends support to the suggestion that a van Vleck type paramagnetic contribution might be important in this and related alloy systems. The superconducting transition temperature of very pure niobium was found to be 9.49°K.