Suppression of superconductivity in granular metals

We investigate the suppression of the superconducting transition temperature due to Coulomb repulsion in granular metallic systems at large tunneling conductance between the grains, g(T)>1. We find the correction to the superconducting transition temperature for 3D granular samples and films. We demonstrate that, depending on the parameters of superconducting grains, the corresponding granular samples can be divided into two groups: (i). the granular samples that belong to the first group may have only insulating or superconducting states at zero temperature depending on the bare intergranular tunneling conductance g(T), while (ii). the granular samples that belong to the second group in addition have an intermediate metallic phase where superconductivity is suppressed while the effects of the Coulomb blockade are not yet strong.     

The effect of blending granular aggregates of different origin on the strength of concrete

This work correlates the resistance of concrete cores with the physical properties of the granular material forming it. A basic physical characterization is conducted, taking into account the origin of the grains involved in the concrete mixture; that is, if they come from natural degradation (natural granular aggregates) or from a grinding process (crushed granular aggregates). Apparent and real densities, shape factors, packing densities, and specific surface areas of the grains are measured. The results are discussed as a function of size and origin of the grains. Several mixtures are prepared following a standard protocol and using different ratios of natural and crushed aggregates. For each ratio, six cores are prepared and uniaxial compression tests are performed. A non-monotonic relation between the resistance of concrete and the percentage of crushed aggregate present in it is obtained, with an optimum ratio depending on the physical properties of the grains.     

Discrete particle simulation of radial segregation in horizontally rotating drum: Effects of drum-length and non-rotating end-plates

The radial segregation phenomena of a mixture of two different size grains in a horizontal rotating drum are studied by DEM simulations. The grano-dynamics of radial segregation phenomena is examined as a function of the axial length and the friction between grains and not-rotating end-plates of the drum. The results indicate that, in the longer drums, the radial segregation ratio is higher and the friction on the end-plates shows little effect. Whereas in the shorter drums, the radial segregation is very slow or negligible; however, decreasing the friction on non-rotating end-plates increases the segregation ratio. If we increase the friction further (greater than the frictions between the grain-grain and the grains and the inner wall), the segregation ratio drops in the longer drums while in the shorter drums mixing is seen instead. The cause of these phenomena lies in the mechanism of diffusion in granular flows due to shearing strain by the end-plates. For more roughened end-plates, this shearing activity increases the granular temperature of the system and only the mixing can be observed instead of the segregation.     

垂直振动颗粒混合气体的振荡现象研究

运用事件驱动算法研究颗粒混合物在垂直振动容器中的振荡现象.容器被具有一定高度的隔板分成相等大小的两个小室,并采用半径相差一倍的两种颗粒.研究结果表明,颗粒振荡周期随两种颗粒密度比的减少而急剧增加.通过计算在垂直方向上两种颗粒高度之比随颗粒密度的变化关系,说明决定颗粒振荡与否的主要因素并不是"巴西坚果效应"或"反巴西坚果效应".通过计算颗粒温度,发现颗粒振荡取决于颗粒混合气体的小颗粒温度.当小颗粒温度大于一定值时,颗粒混合气体发生颗粒振荡现象.根据Viridi等提出的流体动力学模型,文中对该模型做出相应的修改,加入密度因子,从而可以解释颗粒振荡周期与颗粒密度比的关系.     

Upward penetration of grains through a granular medium

We study experimentally the creeping penetration of guest (percolating) grains through densely packed granular media in two dimensions. The evolution of the system of the guest grains during the penetration is studied by image analysis. To quantify the changes in the internal structure of the packing, we use Voronoï tessellation and a certain shape factor which is a clear indicator of the presence of different underlying substructures (domains). We first consider the impact of the effective gravitational acceleration on upward penetration of grains. It is found that the higher effective gravity increases the resistance to upward penetration and enhances structural organization in the system of the percolating grains. We also focus our attention on the dependence of the structural rearrangements of percolating grains on some parameters like polydispersity and the initial packing fraction of the host granular system. It is found that the anisotropy of penetration is larger in the monodisperse case than in the bidisperse one, for the same value of the packing fraction of the host medium. Compaction of initial host granular packing also increases anisotropy of penetration of guest grains. When a binary mixture of large and small guest grains is penetrated into the host granular medium, we observe size segregation patterns.     

Size segregation in granular media induced by phase transition

In order to study analytically the nature of the size segregation in granular mixtures, we introduce a mean field theory in the framework of a statistical mechanics approach, based on Edwards' original ideas. For simplicity we apply the theory to a lattice model for a hard sphere binary mixture under gravity, and we find a new purely thermodynamic mechanism that gives rise to the size segregation phenomenon. By varying the number of small grains and the mass ratio, we find a crossover from the Brazil nut to the reverse Brazil nut effect, which becomes a true phase transition when the number of small grains is larger then a critical value. We suggest that this transition is induced by the effective attraction between large grains due to the presence of small ones (depletion force). Finally the theoretical results are confirmed by numerical simulations of the 3d system under taps.     

Dynamic Properties of Two-Dimensional Polydisperse Granular Gases

We propose a two-dimensional model of polydisperse granular mixtures with a power-law size distribution in the presence of stochastic driving. A fractal dimension D is introduced as a measurement of the inhomogeneity of the size distribution of particles. We define the global and partial granular temperatures of the multi-component mixture. By direct simulation Monte Carlo, we investigate how the inhomogeneity of the size distribution influences the dynamic properties of the mixture, focusing on the granular temperature, dissipated energy, velocity distribution, spatial clusterization, and collision time. We get the following results： a single granular temperature does not characterize a multi-component mixture and each species attains its own ＂granular temperature＂; The velocity deviation from Gaussian distribution becomes more and more pronounced and the partial density of the assembly is more inhomogeneous with the increasing value of the fractal dimension D; The global granular temperature decreases and average dissipated energy per particle increases as the value olD augments.     

Oscillations, cessations, and circulation reversals of granular convection in a densely filled rotating container

We study spontaneously forming convection in a container that is almost completely filled with a bidisperse granular mixture. The container with an aspect ratio close to 1 rotates slowly about a horizontal axis. In this geometry, single vortex rolls are observed in the cell plane, after a spontaneous symmetry breaking. The circulation of grains produces nonuniform segregation patterns of the mixture that in turn interact with the convective flow. We describe oscillatory modulations of the convection velocity, cessations and spontaneous reversals of the circulation. All these features are absent in multiroll granular convection.     

Dynamic Properties of Two-Dimensional Polydisperse Granular Gases

We propose a two-dimensional model of polydisperse granular mixtures with a power-law size distribution in the presence of stochastic driving. A fractal dimension D is introduced as a measurement of the inhomogeneity of the size distribution of particles. We define the global and partial granular temperatures of the multi-component mixture. By direct simulation Monte Carlo, we investigate how the inhomogeneity of the size distribution influences the dynamic properties of the mixture, focusing on the granular temperature, dissipated energy, velocity distribution, spatial clusterization, and collision time. We get the following results: a single granular temperature does not characterize a multi-component mixture and each species attains its own "granular temperature"; The velocity deviation from Gaussian distribution becomes more and more pronounced and the partial density of the assembly is more inhomogeneous with the increasing value of the fractal dimension D; The global granular temperature decreases and average dissipated energy per particle increases as the value of D augments.     

A critical path approach for elucidating the temperature dependence of granular hopping conduction

We revisit the classical problem of granular hopping conduction’s σ∝exp[–(T₀/T)] temperature dependence, where σ denotes conductivity, T is temperature, and T₀ is a sample-dependent constant. By using the hopping conduction formulation in conjunction with the incorporation of the random potential that has been shown to exist in insulator-conductor composites, it is demonstrated that the widely observed temperature dependence of granular hopping conduction emerges very naturally through the immediate-neighbor critical-path argument. Here, immediate-neighbor pairs are defined to be those where a line connecting two grains does not cross or by-pass other grains, and the critical-path argument denotes the derivation of sample conductance based on the geometric percolation condition that is marked by the critical conduction path in a random granular composite. Simulations based on the exact electrical network evaluation of finite-sample conductance show that the configurationaveraged results agree well with those obtained using the immediate-neighbor critical-path method. Furthermore, the results obtained using both these methods show good agreement with experimental data on hopping conduction in a sputtered metal-insulator composite Ag_x(SnO₂)_1–x, where x denotes the metal volume fraction. The present approach offers a relatively straightforward and simple explanation for the temperature behavior that has been widely observed over diverse material systems, but which has remained a puzzle in spite of the various efforts made to explain this phenomenon.     

Temperature Properties in Polydisperse Granular Mixtures

Numerical simulations are employed to consider the problem of determining the granular temperatures of the species of a homogeneous heated granular mixture with a power-law size distribution. The partial granular temperature ratios are studied as functions of the fractal dimension D, the restitution coefficient e, the rescaled viscosity time, the average occupied area fraction φ, the total particle number N and the number fraction. Different species of particles in a power-law system typically do not have the same mean kinetic energy, namely the granular temperature. It is found that the extent of nonequipartition of kinetic energy is determined by the fractal dimension D, the restitution coefficient e and the rescaled viscosity time, while is insensitive to the total particle number N , the area fraction φ and the number fraction.     

Temperature oscillations in a compartmentalized bidisperse granular gas

A granular clock is observed in a vertically vibrated compartmentalized granular gas composed of two types of grains with the same size. The dynamics of the clock is studied in terms of an unstable evaporation or condensation model for the granular gas. In this model, the temperatures of the two types of grains are considered to be different, and they are functions of the composition of the gas. Oscillations in the system are driven by the asymmetric collisions properties between the two types of grains. Both our experiments and model show that the transition of the system from a homogeneous state to an oscillatory state is via a Hopf bifurcation.     

Magneto-transport properties of dilute granular ferromagnets

We present magnetoresistance (MR) measurements performed on quench-condensed granular Ni thin films which are on the verge of electric continuity. In these systems, the electric conductivity is believed to be governed by the resistance between a very small number of grains. The films exhibit sharp resistance jumps as a function of magnetic field. We interpret these findings as being the result of magneto-mechanical distortions that occur in single grains which act as bottlenecks in the dilute percolation network. The observed features provide a unique measure of magnetostriction effects in nano-grain structures as well as being able to shed light on some of the properties of regular granular magnetic films.     

二维颗粒流通道宽度效应的分子动力学模拟

通过用分子动力学方法对颗粒物质流的计算机模拟，研究发现增大通道宽度可以使二维颗粒流从稀疏流转变为密集流状态.通过对不同通道宽度下，固定开口为9.5d的颗粒流和 漏斗口以上9.5d×8d区域记录的模拟结果分析，发现随通道宽度增大，密度变大、温 度降低.当“颗粒温度”T较低时(T/m<0.05 J/kg),颗粒流内部接触数开始超过1.2 ，同时出现较为牢固的横向链状颗粒团簇，是造成流量突变以及密集流的原因. 关键词： 颗粒物质 颗粒流 计算机模拟     

Flame Temperature Effect on the Structure of SiC Nanoparticles Grown by Laser Pyrolysis

Small SiC nanoparticles (10 nm diameter) have been grown in a flow reactor by CO₂ laser pyrolysis from a C₂H₂ and SiH₄ mixture. The laser radiation is strongly absorbed by SiH₄ vibration. The energy is transferred to the reactive medium and leads to the dissociation of molecules and the subsequent growth of the nanoparticles. The reaction happens with a flame. The purpose of the experiments reported in this paper is to limit the size of the growing particles to the nanometric scale for which specific properties are expected to appear. Therefore the effects of experimental parameters on the structure and chemical composition of nanoparticles have been investigated. For a given reactive mixture and gas velocity, the flame temperature is governed by the laser power. In this study, the temperature was varied from 875°C to 1100°C. The chemical analysis of the products indicate that their composition is a function of the temperature. For the same C/Si atomic ratio in the gaseous phase, the C/Si ratio in the powder increases from 0.7 at 875°C up to 1.02 at 1100°C, indicating a growth mechanism limited by C₂H₂ dissociation. As expected, X-ray diffraction has shown an improved crystallisation with increasing temperature. Transmission electron microscopy observations have revealed the formation of 10 nm grains for all values of laser power (or flame temperature). These grains appear amorphous at low temperature, whereas they contain an increasing number of nanocrystals (2 nm diameter) when the temperature increases. These results pave the way to a better control of the structure and chemical composition of laser synthesised SiC nanoparticles in the 10 nm range.     

剪切振动下湿颗粒的力学谱

在恒温25 ℃剪切振动条件下,测量不同水分含量的NaCl湿颗粒体系的力学谱（能量耗散tanφ和剪切模量G）.研究发现,随着剪切振幅增大,NaCl湿颗粒体系的剪切模量G和能量耗散tanφ都表现出类似于干颗粒体系的阻塞（Jamming）转变行为.随着体系中水含量的增大,湿颗粒体系的剪切模量G和能量耗散tanφ在质量分数约等于11%的临界水浓度下均出现一个峰值,且峰位与应变振幅无关,表明此时颗粒之间主要的作用力发生了变化.     

Arches and contact forces in a granular pile

Assemblies of granular particles mechanically stable under their own weight contain arches. These are structural units identified as sets of mutually stable grains. It is generally assumed that these arches shield the weight above them and should bear most of the stress in the system. We test such hypothesis by studying the stress born by in-arch and out-of-arch grains. We show that, indeed, particles in arches withstand larger stresses. In particular, the isotropic stress tends to be larger for in-arch grains whereas the anisotropic component is marginally distinguishable between the two types of particles. The contact force distributions demonstrate that an exponential tail (compatible with the maximization of entropy under no extra constraints) is followed only by the out-of-arch contacts. In-arch contacts seem to be compatible with a Gaussian distribution consistent with a recently introduced approach that takes into account constraints imposed by the local force balance on grains.     

Hall effect in granular materials

We show that in granular materials the Hall coefficient cannot, in general, be used to estimate directly the density of carriers, since it will depend on the ratio of the mobility within the grains to the conductivity mobility. The Hall voltage is predominantly determined within the individual grains, where the mobility may be quite large. On the other hand, the conductivity mobility may be considerably smaller due to a strong potential barrier between grains. The Hall coefficient depends on the ratio of these mobilities as well as on the carrier density.     

二维颗粒膜超导电性

我们对二维排列的Sn颗粒膜进行了低温R-T特性研究。样品是在充有氧气的真空室内用热蒸发方法制备的。因为颗粒尺寸、颗粒分布和颗粒间距的随机性,使得颗粒之间的Josephson耦合和热激发电子的hopping过程的强弱,在不同的样品中不同,观测到的结果可以理解为以下三类:1)向零电阻的K-T转变;2)电阻极小转变;3)金属-绝缘转变。 关键词：     

Multi-scale analysis of the stress state in a granular slope in transition to failure

By means of contact dynamics simulations, we analyze the stress state in a granular bed slowly tilted toward its angle of repose. An increasingly large number of grains are overloaded in the sense that they are found to carry a stress ratio above the Coulomb yield threshold of the whole packing. Using this property, we introduce a coarse-graining length scale at which all stress ratios are below the packing yield threshold. We show that this length increases with the slope angle and jumps to a length comparable to the depth of the granular bed at an angle below the angle of repose. This transition coincides with the onset of dilation in the packing. We map this transition into a percolation transition of the overloaded grains, and discuss it in terms of long-range correlations and granular slope metastability.