Effective thermal conductivity of granulated two-phase systems at interstitial air pressures

Considering the htermal conduction through molecular collisions an expression for the effective thermal conductivityλ _e of loose and granular two-phase materials at different interstitial air pressure has been derived. The dependence ofλ _e on pore and particle sizes, characteristic pressure and radiative heat transfer is also discussed. Calculated values ofλ _e of glass beads and loose building materials are compared with reported results.     

Thermal conduction in a homogeneous two-phase system

Assuming a regular geometry of dispersed phase (λ ₂) an integrated theory for the effective thermal conductivityλ _e of all kind of two-phase materials (including loose materials) is developed. The flux modification is carried out by considering the effective neighbouring interactions in the solution of Poisson’s equation. A comparison of calculatedλ _e values with the reported experimental results over a wide range of two-phase materials shows a good agreement.     

Thermal conduction through porous and dispersed three-phase systems

A loose three-phase system made of metal, non-metal and air is considered resulting from small successive dispersions in effective continuous medium (ecm). The effective thermal conductivity of loose three-phase systems is estimated by extending theecm approach to multi-phase systems. The unsteady state line source (needle) method is employed to determine the effective thermal conductivity of some selected three-phase materials. The calculated and observed values show good agreement suggesting that the continuous medium approach can be applied to estimate effective thermal conductivity of multi-phase systems.     

Prediction of heat storage coefficient of some loose granular materials on the basis of structure and packing of the grains

A theoretical expression is presented to predict the heat storage coefficient (HSC) of two-phase granular systems from the values of HSC of the constituent phases and their volume fractions. Particles are assumed to be a simple cube, arranged in a regular manner in a cubic geometry. Theoretical calculations of the HSC of loose granular materials using the proposed model have been carried out. The HSCs of the materials have been determined experimentally by using a plane heat source. The values obtained are predicted by the proposed model. The model is also tested on other reported values of the HSC. A close agreement has been found between predicted and experimental values.     

The current status and future of granular media

The current status of CoPtCr-SiO₂ granular media is briefly reviewed, and challenges relative to media technology exceeding 1 Tb/in² are discussed. It is effective to enhance grain isolation using oxide materials that easily precipitate at the grain boundary, and this technique is adopted in commercially available perpendicular recording media. Although some difficulties such as reduction of the grain size retaining the switching field remain, 1 Tb/in² could be achieved by using granular exchange-coupled composite-type media or related technologies. Discrete track media could be used for more than 1 Tb/in² recording. It could also take several years to further develop nano-processing technology and establish a cost-effective infrastructure. BPM offers great potential in achieving high recording density, although some necessary technologies are still too primitive to consider commercial production.     

冲击荷载下颗粒物质缓冲性能的试验研究

颗粒物质是一种复杂的能量耗散体系. 颗粒间的摩擦和黏滞作用可使冲击荷载引起的能量有效衰减, 颗粒间的力链结构又可将瞬时局部冲击荷载进行空间扩展和时间延长, 达到良好的缓冲效果. 为研究颗粒物质对冲击荷载的缓冲性能, 本文采用重力作用下球体冲击筒内颗粒物质的试验系统, 研究了筒体底部作用力在颗粒材料、颗粒厚度等因素影响下的变化规律. 试验结果表明: 非规则颗粒具有更加良好的缓冲性能, 粗颗粒的缓冲性能略高于细颗粒. 颗粒厚度H是影响缓冲性能的重要因素, 并存在一个临界厚度H_c. 当H<H_c时, 缓冲性能随H的增加而增强; 当H>H_c时, H对缓冲效果的影响不再显著. 以上研究是在同一冲击能量下进行的, 而对于不同冲击能量下的H_c还需要深入开展. 通过颗粒物质对冲击荷载缓冲性能的试验研究, 可揭示颗粒材料的基本物理力学行为, 为其在缓冲减振领域中的应用提供依据.     

Acoustic properties of unconsolidated granular mixes

Simple empirical relations have been proposed to relate a limited number of directly measurable non-acoustic properties of an unconsolidated granular mix to its characteristic acoustic impedance and propagation constant. These properties are: characteristic particle dimension, porosity, tortuosity and the density of the grain base. It is believed that the model accounts heuristically for the mechanical friction between the elements of the rigid frame, the absorption in the frame micro-pores, and the degree of compaction. These effects can be important and are linked to the value of material density. This work presents practical applications of the proposed model for the prediction of the acoustic characteristics of hard-backed layers of loose granular mixes which can be used for acoustic absorption and insulation. It is shown that the predictions are in excellent agreement with the measured data for a representative range of loose granular mixes. A comparison of the results of the 4-parameter Attenborough model for the acoustic properties of porous media and the experimental data is made also. This model is used extensively to predict the acoustic properties of porous ground and granular media. The accuracy of this model with respect to loose granular materials is discussed.     

重力驱动下运动物体在颗粒介质中的最大穿透深度

通过高速摄像的跟踪，研究了重力场中球形下落物体对松散颗粒床进行撞击并进入颗粒床内的运动过程.运用已得到的颗粒体系中运动物体的阻力模型，分析了物体质量对穿透深度的影响.研究表明，当物体与颗粒床撞击的初始速度较小时，物体在颗粒床中的最大穿透深度与物体质量呈近似线性关系. 实验得到的结果与模型符合很好. 关键词： 颗粒体系 阻力 动力学过程     

A new empirical model for the acoustic properties of loose granular media

This paper examines physical parameters of loose granular mixes and their empirical relations to the acoustic performance of these mixes. In this work a new classification of granular media has been proposed which is related to the characteristic particle dimension and the specific density of the grain base. It has been shown that this classification is a useful characteristic for rapid evaluation of the acoustic performance of loose granular mixes. The characteristic impedance and propagation constant have been measured for a representative selection of grain mixes and used to develop a new empirical model. This model relates the above acoustic characteristics to the characteristic particle dimension, porosity, tortuosity and specific density of the grain base, which are routinely measurable parameters. A very good agreement with the experimental data is illustrated in the frequency range of 250-4000 Hz for materials with the grain base of 0.4-3.5 mm and specific densities between 200 and 1200 kg/m³. Unlike many theoretical models for the prediction of the acoustic properties of porous media, the proposed expressions do not involve any special functions of complex argument, empirical shape factors or sophisticated characteristics of porous structure. These are practical enough to be of interest to acoustic and noise control engineers and material manufacturers.     

基于分形多孔介质三维网络模型的非混溶两相流驱替数值模拟

提出了一个描述多孔介质孔隙尺寸分布的三维分形网络模型,利用该模型对多孔介质中的非混溶两相流驱替进行了数值模拟,研究了孔隙尺寸分布分维D_f和两相流黏滞比M对驱替前沿指进型的影响,结果表明指进型容量维数D_h随着孔隙尺寸分布分维D_f以及黏滞比M的增大而减少,并通过曲线拟合得到了它们之间的定量关系. 关键词： 多孔介质 三维网络 黏滞指进 非混溶两相流     

Preparation and characterization of Yb-doped Ba(Ce,Zr)O<Subscript>3</Subscript> nanopowders with high sinterability

The Ba(Ce_0.8Zr_0.2)_0.95Yb_0.05O_2.975 ceramics electrolyte was prepared via a Pechini method using metal nitrate salts as starting materials. An optimum annealing temperature of 1,400 °C was needed to obtain a pure perovskite-like phase with orthorhombic structure. Particle size distribution showed a bimodal distribution that corresponds to the loose powders and agglomerates size. Scanning electron micrograph revealed that the loose powders were in the nanosize range (70–200 nm). These ultrafine loose powders enhanced the densification of a pellet with relative density ∼95% obtained at 1,400 °C. The sample formed clear and compact grains with submicron sizes. Impedance results showed that the impedance semicircle of the grain was observed only at T ≤ 250 °C. The introduction of 20 mol% Zr improved the chemical stability of BaCe_0.95Yb_0.05O_2.975 sample in atmosphere containing carbon dioxide at 600 °C. The sample also exhibited high proton conductivity in wet hydrogen.     

Precursors in two-phase detonation: Occurrence of a contact discontinuity

This paper is concerned with detonation-propelled shocks (precursors) occurring in gas-permeable charges. It is shown that the basic aspects of the event may successfully be reproduced within a one-dimensional two-phase picture widely employed in the modelling of porous energetic materials. Precursor shocks are sustained by intense gasification of the solid phase, provided the ignition pressure (P_ign) exceeds a certain critical level. At high enough P_ign, the post-shock flow is found to acquire a contact discontinuity. Although this pattern is compatible with the associated self-similar solution, the question of the mechanism governing its formation remains open.     

Testing heat storage capability of some loose granular materials

The role of the heat storage coefficient of some loose granular materials in determining the amount of heat retained during the transient state has been investigated. An experimental study has been done in the laboratory for testing the heat storage capability of some loose granular materials such as marble powder, dune sand, ash, high refractory cement, wheat husk and sawdust. For this purpose a wooden box of sides 30.5 × 51 × 53 cm was fabricated. In the middle of this a rectangular plastic box (heat store) of sides 28 × 23 × 6.5 cm, surrounded by loose granular materials, was placed. The upper portion of the wooden box was covered by thermal insulation. Two containers, similar to the one placed in the materials, were lying above the wooden box at a height. Using a battery of electric light bulbs the temperature of water in these containers was raised to about 70°C. The water from one of the containers was transferred to the container lying in the material. A study of heat loss from this container (heat store) and from the other one which was lying in the open, was conducted at time intervals. In the heat store a thermocouple was also placed along the z-axis. It was found that the fall in temperature of the materials surrounding the heat store is determined by the heat storage coefficients of the surrounding materials, which were measured by the plane heat source method. It was observed that for the low-values of heat storage coefficient surrounding materials, the heat dissipation from the heat store was low, whereas for high valued heat storage coefficient material surrounding the store, heat loss increased.     

Superconductivity of thin films of lead,indium, and tin prepared in the presence of oxygen

We have studied the influence of oxygen on the superconducting properties of thin films of lead, indium and tin deposited on glass or sapphire substrates. In addition, the morphological microstructure was investigated by scanning electron microscopy. The film thickness was 1.0 μm, and the partial pressure of O₂ during the film deposition was raised up to 1×10⁻⁴ Torr. In all three materials the development of a granular structure and a strong increase in the residual electric resistivity was observed due to the O₂-treatment. Whereas in the Pb films no change of the critical temperature was found, the In films deposited on glass substrates showed a slight increase ofT _c due to the oxygen. The strongest increase ofT _c (up to 8%) was observed in the O₂-treated Sn films. These results are discussed in terms of the McMillan theory. From our measurements of the critical current densityj _c we conclude that edge pinning is dominant in the undoped films. All three materials showed a strong increase ofj _c due to the O₂-treatment which must be interpreted in terms of bulk pinning.     

Study of optical and magneto-optical properties of CoFe–HfO2 granular magnetic films

Granular films of composition (CoFe)_x(HfO₂)_1−x with continuous variation of the Co_0.5Fe_0.5 volume fraction in the range 0.1<x<0.9 have been prepared by e-beam co-evaporation. The optical properties of the thin granular composite films have been determined from transmission and reflection spectra in the 300–1100 nm region. The optical characteristics of the granular films as a function of x were also determined by the ellipsometrical method at λ=632.8 nm. We find that the transmission and reflection spectra and the dielectrical function have anomalies near the percolation threshold at x_p≈0.45. The Faraday rotation in the films was determined at λ=632.8 nm. The spectral dispersion of Faraday effects have peak in the region of the plasmon frequency. The Faraday effect is greatly enhanced near x_p. The calculated optical and magneto-optical parameters, described in the framework of the effective medium approximation, are in qualitative agreement with experimental data.     

Factors influencing the density profiles of granular flux in a two-dimensional inclined channel

The two-dimensional dilute granular flow on a smooth incline bounded by rough sidewalls is investigated experimentally, and the transverse density profiles of granular flux (ζ = ρ v) across the channel are measured. The results show that the transverse density profiles of granular flux are symmetric about the channel center and that the density of granular flux near the boundary is clearly lower than that of the center. There is a critical width of channel W _c for the transition of the density of granular flux. The density of granular flux near the boundary decays with the increasing of inclination (sinθ) of the channel. Supported by the National Natural Science Foundation of China (Grant No. 10274071) and the Department of Education of Zhejiang Province (Grant Nos. 20050916 and y200804093)     

Utilization of Fly Ash Nanofluids in Two-phase Closed Thermosyphon for Enhancing Heat Transfer

This study investigates how fly ash nanofluids affect the thermal performance of a two-phase closed thermosyphon at various states of operation. The utilization of nanofluids obtained from X₂O₃-type oxides, such as Al₂O₃, Fe₂O₃, or CuO, on the improvement of two-phase closed thermosyphon performance was reported in a number of studies in the literature. The present study experimentally demonstrated the effect of using a nanofluid obtained from fly ash comprised of various types of metal oxides in varying ratios on improving the performance of a two-phase closed thermosyphon. The fly ash was obtained from the flue gas that was captured in the cyclones of the Yatagan thermal power plant (Turkey). Triton X-100 (Dow Chemical Company) dispersant was used in the study to produce the 0.2% (wt) fly ash/water nanofluid via direct synthesis. A straight copper tube with an inner diameter of 13 mm, outer diameter of 15 mm, and length of 1 m was used as the two-phase closed thermosyphon. The nanofluid filled 33.3% (44.2 ml) of the volume ofthe two-phase closed thermosyphon. Three heating power levels (200, 300, and 400 W) were used in the experiments with three different flow rates of cooling water (5, 7.5, and 10 g/s) used in the condenser for cooling the system. A increase of 26.39% was achieved in the efficiency of the two-phase closed thermosyphon when 4% (wt) fly ash containing nanofluid was used to replace deionized water at a heat load of 200 W and with a cooling water flow rate of 5 g/s.     

Organic-ligand-assisted supercritical hydrothermal synthesis of titanium oxide nanocrystals leading to perfectly dispersed titanium oxide nanoparticle in organic phase

Titanium oxide (TiO₂) nanocyrstals which are perfectly dispersed in organic solvents are synthesized by organic-ligand-assisted supercritical hydrothermal synthesis. The addition of hexaldehyde to the supercritical hydrothermal synthesis of TiO₂ leads to the in-situ surface modification, which enables the synthsized TiO₂ nanocrystals to be perfectly dispersed in iso-octane because of its hydrophobic nature. Further, the one-pot synthesis of hybrid materials results in the significant reduction of the particles size, probably due to the capping effect of hexaldehyde to suppress the particles growth.     

静电力显微镜研究二相材料及其界面介电特性

利用静电力显微镜(EFM)研究了二相材料不同区域的介电特性. 制备了高定向石墨/聚乙烯、云母/聚乙烯等层叠状二相材料复合物, 在EFM相位检测模式下观测二相材料过渡界面处, 可以发现二相材料中介电常数较大的材料会引起较大的相位滞后角Δθ该相位滞后角正切值tan(Δθ) 与探针电压V_EFM存在二次函数关系, 且函数二次项系数与样品的介电常数存在增函数关系, 进而可在微纳米尺度下区分不同微区域内材料的介电常数差异. 研究表明EFM 可用于对材料介电特性的微纳米尺度测量, 这对分析复合材料二相界面区域特性有积极意义.     

A comparative study of gamma-ray interaction and absorption in some building materials using Zeff-toolkit

The gamma-ray shielding behaviour of a material can be investigated by determining its various interaction and energy-absorption parameters (such as mass attenuation coefficients, mass energy absorption coefficients, and corresponding effective atomic numbers and electron densities). Literature review indicates that the effective atomic number (Z_eff) has been used as extensive parameters for evaluating the effects and defect in the chosen materials caused by ionising radiations (X-rays and gamma-rays). A computer program (Z_eff-toolkit) has been designed for obtaining the mean value of effective atomic number calculated by three different methods. A good agreement between the results obtained with Z_eff-toolkit, Auto_Z_eff software and experimentally measured values of Z_eff has been observed. Although the Z_eff-toolkit is capable of computing effective atomic numbers for both photon interaction (Z_eff,PI) and energy absorption (Z_eff,En) using three methods in each. No similar computer program is available in the literature which simultaneously computes these parameters simultaneously. The computed parameters have been compared and correlated in the wide energy range (0.001–20?MeV) for 10 commonly used building materials. The prominent variations in these parameters with gamma-ray photon energy have been observed due to the dominance of various absorption and scattering phenomena. The mean values of two effective atomic numbers (Z_eff,PI and Z_eff,En) are equivalent at energies below 0.002?MeV and above 0.3?MeV, indicating the dominance of gamma-ray absorption (photoelectric and pair production) over scattering (Compton) at these energies. Conversely in the energy range 0.002–0.3?MeV, the Compton scattering of gamma-rays dominates the absorption. From the 10 chosen samples of building materials, 2 soils showed better shielding behaviour than did other 8 materials.