Embedded nanocrystallites prepared by ion implantation

Nanocrystallites embedded in substrates (granular materials) prepared by ion implantation have many unusual properties in comparison with bulk materials. Features are found such as high magnetic coercivity, size effect of the effective magnetic moment per atom, dependence of the interface magnetic hyperfine field on the Co concentration, nonlinear photoluminescence as well as charge transfer. We show that CEMS and emission Mössbauer spectroscopy are powerful tools for studying these granular systems.     

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.     

Microstructure characterization of granular materials

This paper presents techniques and algorithms to compute microstructure properties of irregular-shaped granulate assemblies utilizing 3D images. The techniques are capable of extracting microstructure properties of particles such as centeroid, particle size distribution, shape indices (i.e., sphericiy and angularity), number of contacts (i.e., distribution of coordination numbers), contact network, packing efficiency, distribution of local void ratio and radial distribution function. Such properties are critical parameters for micromechanical-based numerical models to capture micro- and macromechanical behavior of geomaterials. X-ray microtomography was used to reconstruct high-resolution 3D image of a natural sand system to represent granular materials. Microstructure properties of the sand system were computed and compared with properties of a computer-simulated image of periodic random spheres. Findings indicate that the use of simplified systems of idealized spheres to model micro- and macromechanical behavior of granular systems can lead to inaccurate results due to the differences in the microstructure between both systems. Methods presented in this paper enabled capturing a more realistic microstructure that can be incorporated in micromechanical models to better simulate, understand, or explain macroscale behavior of granular materials based on their actual microstructure.     

Rheological properties of dense granular flows

Recent progresses in understanding the behavior of dense granular flows are presented. After presenting a bulk rheology of granular materials, I focus on the new developments to account for non-local effects, and on ongoing research concerning the surface rheology and the evolution of mechanical properties for heterogeneous systems.     

Scales and kinetics of granular flows

When a granular material experiences strong forcing, as may be the case, e.g., for coal or gravel flowing down a chute or snow (or rocks) avalanching down a mountain slope, the individual grains interact by nearly instantaneous collisions, much like in the classical model of a gas. The dissipative nature of the particle collisions renders this analogy incomplete and is the source of a number of phenomena which are peculiar to "granular gases," such as clustering and collapse. In addition, the inelasticity of the collisions is the reason that granular gases, unlike atomic ones, lack temporal and spatial scale separation, a fact manifested by macroscopic mean free paths, scale dependent stresses, "macroscopic measurability" of "microscopic fluctuations" and observability of the effects of the Burnett and super-Burnett "corrections." The latter features may also exist in atomic fluids but they are observable there only under extreme conditions. Clustering, collapse and a kinetic theory for rapid flows of dilute granular systems, including a derivation of boundary conditions, are described alongside the mesoscopic properties of these systems with emphasis on the effects, theoretical conclusions and restrictions imposed by the lack of scale separation. (c) 1999 American Institute of Physics.     

二维颗粒体系单轴压缩形成的力链结构

从接触力、能量分布和接触网络结构特点出发,提出了强力链的力大小(Fc)判据(Fc大于平均接触力〈F〉)和角度θc判据(θc=180/〈Z〉,其中〈Z〉是平均配位数),指出强力链和弱力链是本质不同的两类结构存在于颗粒体系中,其中强力链网络与体系的宏观性质直接相关.以二维颗粒体系的单轴压缩为例,计算发现了强力链长度的幂率分布规律,分析了侧向压力系数与相应强力链网络结构的关系:当内部强力链网络充分发育而不再变化时,侧向压力系数趋于稳定数值.     

The collision of particles in granular systems

Collisions between granular particles are irreversible processes which cause dissipation of mechanical energy by fragmentation or heating of the colliders. The knowledge of these phenomena is essential for the understanding of the behaviour of complex systems of granular particles. We have developed a model for inelastic collisions of granular particles and calculated the velocity restitution coefficients, which describe all possible collisions in the system. The knowledge of these coefficients allows for event-driven many-particle simulations which cannot be performed in the frame of molecular dynamics. This approach has the advantage that very large particle numbers can be treated which are necessary for the understanding of intrinsic large-scale phenomena in granular systems.     

沙子里的图案

文章从物理学家的角度探讨颗粒体系中有序图案的产生和机理,探寻对广泛存在于自然界的非平衡态体系有序结构生成的理解.非平衡态体系的随机扩散与能量耗散的特性是有序结构生成的必备条件,所生成的图形则由这两个因素的权重比值和体系内部的对称性决定.具备以上因素,还需加上最后的一个决定性的关键因素,这就是熵的驱动.熵最大的态决定了体系是有序还是无序的排列.     

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.     

Long-time relaxation of the magnetization and tunneling magnetoresistance of granular ferromagnets

Magnetic anisotropy and orientational variance as well as shape diversity of granules largely determine the magnetic properties of granular ferromagnetic metals. The model of magnetically anisotropic ellipsoidal granules explains the glassy nature of the magnetic state of such systems. The relaxation of the magnetization and the magnetoresistance of granular ferromagnetic metals is examined on the basis of this model.     

Acoustical properties of double porosity granular materials

Granular materials have been conventionally used for acoustic treatment due to their sound absorptive and sound insulating properties. An emerging field is the study of the acoustical properties of multiscale porous materials. An example of these is a granular material in which the particles are porous. In this paper, analytical and hybrid analytical-numerical models describing the acoustical properties of these materials are introduced. Image processing techniques have been employed to estimate characteristic dimensions of the materials. The model predictions are compared with measurements on expanded perlite and activated carbon showing satisfactory agreement. It is concluded that a double porosity granular material exhibits greater low-frequency sound absorption at reduced weight compared to a solid-grain granular material with similar mesoscopic characteristics.     

Dynamics of wet granular matter

Recent advances in understanding model systems of wet granular materials are presented, with particular emphasis on statistical concepts, dynamics, and phase transitions. It is demonstrated that although wet granular systems are quite complex, their main features may be understood on the basis of rather simple concepts. The significance of these systems for investigating fundamental problems of non-equilibrium dynamics are shortly discussed.     

Dressed Josephson junction network models and low-field magnetic response of high-Tc granular superconductors

A bare three-dimensional model, in which grains are reduced to points, cannot fully account for the magnetic properties of granular superconductors. A dressed version of these network models is proposed to discuss the quantitative link between the low-field magnetic response of high-T_c superconducting granular samples and the characteristic properties of Josephson junction network models. By means of dressed models, the temperature dependence of the d.c. field-cooled susceptibility of a simple three-dimensional granular system, consisting of eight grains in a cubic arrangement, is studied.     

Nonlinear dynamics of density waves in granular flows through narrow vertical channels

We study granular flows through narrow channels driven by gravity in the framework of the kinetic theory for dissipative dense gases. We derive equations of motion for quasi-one-dimensional systems. In a certain range of flow density, the steady homogeneous regime is found to be unstable against the formation of density waves. We show moreover that near the onset of the instability, the governing equation for the flow density is a mixture of the Korteweg-de-Vries equation, which leads to soliton, and the Bürger equation which exhibits spatio-temporal chaos. The competition between chaos and solitons may lead either to regular spatially ordered density waves or to chaotic dynamics. We argue that these two types of dynamics can be encountered experimentally according to the channel width and the dissipative properties of the granular media. Received: 11 March 1998 / Revised and Accepted: 3 July 1998     

Bose systems in spatially random or time-varying potentials

Bose systems, subject to the action of external random potentials, are considered. For describing the system properties, under the action of spatially random potentials of arbitrary strength, the stochastic mean-field approximation is employed. When the strength of disorder increases, the extended Bose-Einstein condensate fragments into spatially disconnected regions, forming a granular condensate. Increasing the strength of disorder even more transforms the granular condensate into the normal glass. The influence of time-dependent external potentials is also discussed. Fastly varying temporal potentials, to some extent, imitate the action of spatially random potentials. In particular, strong time-alternating potential can induce the appearance of a nonequilibrium granular condensate.     

Anomalous electric-field effect and glassy behaviour in granular aluminium thin films: electron glass?

We present a study of non-equilibrium phenomena observed in the electrical conductance of insulating granular aluminium thin films. An anomalous field effect and its slow relaxation are studied in some detail. The phenomenology is very similar to the one already observed in indium oxide. The origin of the phenomena is discussed. In granular systems, the present experiments can naturally be interpreted along two different lines. One relies on a slow polarisation in the dielectric surrounding the metallic islands. The other one relies on a purely electronic mechanism: the formation of an electron Coulomb glass in the granular metal. More selective experiments and/or quantitative predictions about the Coulomb glass properties are still needed to definitely distinguish between the two scenarios.     

Two scenarios for avalanche dynamics in inclined granular layers

We report experimental measurements of avalanche behavior of thin granular layers on an inclined plane for low volume flow rate. The dynamical properties of avalanches were quantitatively and qualitatively different for smooth glass beads compared to irregular granular materials such as sand. Two scenarios for granular avalanches on an incline are identified, and a theoretical explanation for these different scenarios is developed based on a depth-averaged approach that takes into account the differing rheologies of the granular materials.     

Diffusion of two-dimensional particles on an air table

Rapid granular flow is diffusive in character. This diffusive nature is important in the generation of constitutive properties such as effective shear and bulk viscosities, effective conductivity of particle fluctuation energy, and self-diffusion coefficients. Experiments were performed in a large air-table apparatus that can sustain up to a few thousand small, light disks just above a horizontal porous plane. Experiments on 2D diffusion processes were performed using this apparatus. Measurements of self-diffusion coefficients and granular temperature in systems having different areal concentrations are in good agreement with predictions based on the kinetic theory and on numerical simulations.