Acoustic modeling of perforated concrete using the dual porosity theory

Perforated concrete shows nowadays a high potential for many construction and building engineering applications. This work is devoted to the analysis of the acoustic properties of perforated concrete made from arlite lightweight aggregates. Concrete produced from these materials is an environmentally friendly alternative to traditional materials and offers a higher durability, excellent strength-to-weight ratio and low cost. In particular, it is shown that the acoustic behavior of perforated concrete can be modeled using a dual porosity approach based on the knowledge of the non-acoustic properties of the matrix granular material and geometrical data. To this end, various non-perforated and perforated samples were prepared and characterized in an experimental test facility, their acoustic properties being determined through the transfer function impedance tube method. Experimental and estimated results related to the acoustic properties of a number of prepared specimens are presented, showing a good agreement. Results suggest that this approach is suitable for practical design of such materials as part of noise control systems.     

离散颗粒层被横向推移过程中的力学行为研究

研究发现,颗粒物质层被匀速推移挤压过程中,所需推移力先以线性规律增加,在某一确定点后,则会以指数规律增加.而颗粒物质是由众多离散颗粒组成的软凝聚态物质,其宏观上反映的是离散颗粒的个体性质和凝聚态物质的集体效应.颗粒与颗粒之间以及颗粒与边界之间的细观尺度接触力链的构成以及演变规律将会直接影响各种宏观受力情况,其摩擦力与挤压力便是力链的主要构成形式.围绕着定量描述细观力链特征,从而揭示力的变化规律这一目标,采用计算机模拟的方法,依照球形颗粒Hertz法向接触理论和Mindlin-Deresiewicz切向接触理论,对重力作用下不同数目的三维等径球体颗粒层的推移情况进行了离散元仿真模拟,量化分析了推移力变化规律、各摩擦力变化规律以及力链分布规律,发现摩擦力与挤压力在颗粒堆积的不同阶段对力链的构成起到了不同的主次作用,使得力链发生强弱演变,从而发现了推移颗粒物质层时推移力的变化规律及原因.这些结果有利于从力链角度揭示颗粒内部和颗粒与各边界之间的受力情况.     

Breakdown of energy equipartition in a 2D binary vibrated granular gas

We report experiments on the equipartition of kinetic energy in a mixture of pairs of different types of grains vibrated in two dimensions. In general, the two types of grains do not attain the same granular temperature, T(g) = 1/2m. However, the temperature ratio is constant in the bulk of the system and independent of the vibration velocity. The ratio depends strongly on the ratio of mass densities of the grains, but not on their inelasticity. Also, the temperature ratio is insensitive to compositional variables such as the number fraction of each component and the total number density. We conclude that a single granular temperature, as traditionally defined, does not characterize a multicomponent mixture.     

Correlations and aggregate statistics in granular packs

We study how the aggregate statistical properties for density fluctuations in granular aggregates scale with the sample size and how such a scaling is associated with the correlations between grains. Correlations are studied both between grain positions and between Vorono? cell volumes, showing distinct behaviors and properties. A non-linear scaling in the aggregate volume fluctuations as function of the sample size is discovered and the connection between such anomalous scaling and correlations is explained. It emerges that volume fluctuations might be described by means of a single universal equation for all samples at all cluster sizes.     

振动颗粒物质“巴西果”分离效应实验和理论研究

本文通过实验和理论研究了影响"巴西果"分离的因素及其物理机理. 分析了振动加速度、大小颗粒尺寸和密度对分离时间的影响, 并利用流体模型对分离时间作估算, 对实验结果进行定性解释. 结果表明在振动频率固定时, 调节振动加速度是控制"巴西果"分离的一个主要手段. 振动加速度存在一个临界值, 当高于此临界值时, "巴西果"分离的主要物理机理由对流机理转变为几何填空机理, 且振动加速度对分离影响变小, 大颗粒尺寸对分离的影响增大. 可通过调节大颗粒的尺寸来改变分离效果. 当大、 小颗粒密度比为1时, 仍会出现"巴西果"分离现象. 增大小颗粒尺寸或密度可以促进"巴西果"分离.     

Variations in luminescence properties of single quartz grains and their consequences for equivalent dose estimation

In this paper examples of variability of luminescence properties between single grains of quartz extracted from natural samples or from crushed single crystals are shown. It is demonstrated how measurements of glow curves, PSL (photon stimulated luminescence) decay curves, thermal activation characteristics using multi-grain aliquots can lead to a misinterpretation of the obtained data. The possible consequences for different D_E estimation techniques applied to multi-grain aliquots or to single grains and normalisation techniques applied to multi-grain aliquots are discussed. In particular pre-dose, multi-aliquot additive, and single grain PSL SAR D_E estimation methods are considered.     

Effect of starting powder on screen-printed YSZ films used as electrolyte in SOFCs

Screen-printing technology was developed to fabricate dense YSZ electrolyte films onto NiO–YSZ porous anode substrates. A single fuel cell of Ni-YSZ/YSZ (31 μm)/LSM-YSZ was successfully prepared by screen-printing technology. Using humidified hydrogen as fuel and ambient air as oxidant, the fuel cell provided the maximum power densities of 0.18, 0.33, 0.58, 0.97 and 1.3 W/cm² at 650, 700, 750, 800 and 850 °C, respectively. The properties of the starting YSZ powder exerted a significant effect on the characteristics of the screen-printed YSZ electrolyte films. The aggregates of the powder could be partially broken by ball milling. The YSZ powder with a small particle size and a narrow particle size distribution helped to obtain dense YSZ films. The films prepared from the YSZ powder with high aggregates were very porous, which resulted in a low open circuit voltage, a high ohmic resistance, a high polarization resistance and thus a poor cell performance.     

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.     

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.     

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.     

Dynamic photoelastic studies of wave propagation in granular media

The optical method of dynamic photoelasticity is used to visualize the load transfer profiles due to explosive loading in granular aggregates. The granular media are simulated by using arrays of disks fabricated from a brittle polyester material—Homalite 100. Attention is focused on the effect of microstructure or the geometrical packing of the grains on the wave propagation phenomena. The experimental data are analyzed to obtain the stress wave attenuation, wave velocities and contact stresses as a function of time along various directions in the assembly of grains. Dynamic load transfer functions are developed to predict dynamic contact loads in any systematic or random assembly of grains for any given loading. The predicted results are compared with the experimental data. The effect of local inhomogeneities on the wave propagation phenomena is also shown.     

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：     

The effective absorption cross-section of thermal neutrons in a medium containing strongly or weakly absorbing centres

The structure of a heterogeneous system influences diffusion of thermal neutrons. The thermal-neutron absorption in grained media is considered in the paper. A simple theory is presented for a two-component medium treated as grains embedded in the matrix or as a system built of two types of grains (of strongly differing absorption cross-sections). A grain parameter is defined as the ratio of the effective macroscopic absorption cross-section of the heterogeneous medium to the absorption cross-section of the corresponding homogeneous medium (consisting of the same components in the same proportions). The grain parameter depends on the ratio of the absorption cross-sections and contributions of the components and on the size of grains. The theoretical approach has been verified in experiments on prepared dedicated models which have kept required geometrical and physical conditions (silver grains distributed regularly in Plexiglas). The effective absorption cross-sections have been measured and compared with the results of calculations. A very good agreement has been observed. In certain cases the differences between the absorption in the heterogeneous and homogeneous media are very significant. A validity of an extension of the theoretical model on natural, two-component, heterogeneous mixtures has been tested experimentally. Aqueous solutions of boric acid have been used as the strongly absorbing component. Fine- and coarse-grained pure silicon has been used as the second component with well-defined thermal-neutron parameters. Small and large grains of diabase have been used as the second natural component. The theoretical predictions have been confirmed in these experiments.     

Shear and compressional wave speeds in Hertzian granular media

It is shown that the shear wave speed in a granular medium is less than that in an elastic solid of the same shear modulus-to-density ratio. Shear and compressional wave speeds are derived for granular media using a conservation of energy approach. The grains are assumed to be spherical with elastic Hertzian contacts of constant stiffness. The affine approximation is used to determine the relative displacements of grain centers, and it is also assumed that the grains are small compared to a wavelength, consistent with the effective medium approximation. Potential and kinetic energies associated with linear motion are the same as those in an elastic solid, but it is found that shear wave propagation in a granular medium involves additional energies associated with grain rotation. The partition of energies results in a reduction in the shear wave speed, relative to an elastic solid of the same shear modulus-to-density ratio. It is shown that the reduction is an inherent property of granular media, independent of any departure from the affine approximation or fluctuations in coordination number or contact stiffness. The predicted wave speed ratios are consistent with published measurements.     

Optical properties of plasmonic silver nanoparticles exposed to organic solvents

The influence of ethanol and acetic acid on the structure and optical properties of silver granular films formed by physical vapor deposition in a high vacuum on sapphire substrates was studied via optical spectroscopy and scanning electron microscopy. It was found that irregularly shaped oblate silver grains transforms into almost spherical nanoparticles. Simultaneously, optical extinction spectra shift in the short wavelength range and become narrower. The same effect was observed when liquid crystal was poured on such film. It was noticed, that nanoparticle shapes change under the action of these fluids in the same way as in the process of thermal annealing. This analogy suggests that the observed effects could be explained by acceleration of atomic diffusion over the islet surfaces. It was noticed also that the resistance of thick granular films changes abruptly several minutes after pouring such film with ethanol. This jump of resistance is also very similar to the resistance jump observed previously in the case of annealing.     

Acoustic absorption modeling of porous concrete considering the gradation and shape of aggregates and void ratio

The results of acoustic absorption modeling of porous concrete considering the gradation and shape of aggregates and void ratio are presented. To model the void texture of porous concrete, the multi-layered micro-perforated rigid panel model considering air gaps [1] and [2] is adopted. The parameters used in this acoustic absorption modeling are determined by a geometrical and experimental approach considering the gradation and shape of aggregates and void ratio. The predicted acoustic absorption spectra are compared with experimental results to verify the proposed acoustic absorption modeling approach. Finally, a parametric study is conducted to investigate the influence of design factors on the acoustic absorption properties of porous concrete.     

Structure–conductivity relations of simulated highly porous nanoparticle aggregate films

Electrical conductivity of porous films composed of nanoparticle aggregates is theoretically evaluated with respect to aggregate structure and film packing density. The aggregates are fractals composed of 5–30 primary particles with diameter of 10 nm. The film properties are derived from simulated boxes in the range of 0.5–1 μm. The electrical conductivity across the films of packing densities ranging from 0.01 to 0.15 was studied. All films prepared by an aerosol deposition technique, which uses nanoparticle aggregates, exhibited percolation behavior between planes parallel to the moving direction of the aggregates. They also followed the classical percolation relation for electrical conductivity while the critical percolation packing density depends on the aggregate size and structure used to build the films. Films using larger aggregates as building blocks have higher electrical conductance than smaller aggregates close to the percolation limit. For validation and supplementary information, two independent models are developed: one model follows the percolation theory to get detailed physical insights and another one computes the exact conductivities but at the cost of some details. This analysis gives new insights into the conduction backbone structures of these films with regard to neck contacts within an aggregate and grain boundary contacts between aggregates. The results shown are important for solar application of these films and especially for gas sensors where high sensitivity is often counteracted by low conductivity.     

Mössbauer study of ilmenite-limonite concrete for nuclear reactor shield

A heavy type concrete for nuclear reactor shield prepared by adding to the portland cement Egyptian iron ores has been studied. This concrete contains heavy elements and a large amount of water of crystallization that make it more effective than ordinary concrete for neutron and gamma attenuation. The measurement of the Mössbauer parameters allows a deeper understanding of the relations between attenuation and physical properties.     

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.     

氧氩比对钴掺杂氧化锌薄膜光电性能的影响

以氧化锌陶瓷靶和金属钴靶为靶材,利用磁控共溅射方法制备钴掺杂氧化锌(Co-ZnO)薄膜。研究了氧氩比对薄膜的结构、光学和电学性能的影响。结果表明:薄膜具有类似于ZnO的六方纤锌矿结构,并沿c轴择优生长;当氧氩比为2:8时,薄膜具有较好的纳米晶粒和表面结构,其霍尔迁移率为2.188×104cm2/V·s,最小电阻率为1.326×104Ω·cm,其薄膜透光率最高,且在紫外区有一个相对较强的发射峰。