Electrical resistivity of NaPb compound-forming liquid alloy using abinitio pseudopotentials

The study of electrical resistivity of compound-forming liquid alloy, NaPb, is presented as a function of concentration. Hard sphere diameters of Na and Pb are obtained through the interionic pair potentials evaluated using Troullier and Martinsab initio pseudopotential, which have been used to calculate the partial structure factors S(q). Considering the liquid alloy to be a ternary mixture, Ziman formula, modified for complex formation has been used for calculating resistivity of binary liquid alloys. Form factors are calculated usingab initio pseudopotentials. The results suggest that Ziman formalism, when used withab initio pseudopotentials, are quite successful in explaining the electrical resistivity data of compound-forming binary liquid alloys.     

Ca掺杂氧化物超导体正常态电阻反常及其磁输运行为

本文研究了对Y1-xCaxBa2Cu3O7-δ进行Ca掺杂后样品的输运特性．实验中发现：随Ca含量增加，样品正常态电阻随之下降．出现反常电阻现象原因可能是Ca替代Y位，导致体系载流子浓度发生变化，从而使得晶界附近的电子能带结构发生改变．另外；低温下的样品伏安特性在外磁场下出现反常，动态电阻dV／dI随磁场增加而下降．有关该现象的物理机制有待于进一步研究．     

Exploring the third dimension: Volume electron microscopy comes of age

Groundbreaking advances in volume electron microscopy and specimen preparation are enabling the 3-dimensional visualisation of specimens with unprecedented detail, and driving a gratifying resurgence of interest in the ultrastructural examination of cellular systems. Serial section techniques, previously the domain of specialists, are becoming increasingly automated with the development of systems such as the automatic tape-collecting ultramicrotome, and serial blockface and focused ion beam scanning electron microscopes. These changes are rapidly broadening the scope of biomedical studies to which volume electron microscopy techniques can be applied beyond the brain. Further innovations in microscope design are also in the pipeline, which have the potential to enhance the speed and quality of data collection. The recent introduction of integrated light and electron microscopy systems will revolutionise correlative light and volume electron microscopy studies, by enabling the sequential collection of data from light and electron imaging modalities without intermediate specimen manipulation. In doing so, the acquisition of comprehensive functional information and direct correlation with ultrastructural details within a 3-dimensional reference space will become routine. The prospects for volume electron microscopy are therefore bright, and the stage is set for a challenging and exciting future.     

Liquid Co–Sn alloys at high temperatures: structure and physical properties

The Co–Sn system is an important subsystem for Sn-based anode materials of lithium-ion batteries. Experimental results on the physical–chemical properties of this system in the liquid state, however, are rather sparse. In this work, the atomic structure and structure-sensitive thermophysical properties (viscosity, electrical resistivity, and thermoelectric power) of liquid Co–Sn alloys were investigated in a wide temperature range with special attention to the melting-solidification region. The obtained experimental results were combined with differential thermal analysis (DTA) data in order to verify the liquidus curve in the Sn-rich part of the Co–Sn phase diagram.     

Investigation of Donnan equilibrium in charged porous materials—a scale transition analysis

We propose a new theory describing how the macroscopic Donnan equilibrium potential can be derived from the microscale by a scale transition analysis. Knowledge of the location and magnitude of the charge density, together with the morphology of the pore space allows one to calculate the Donnan potential, characterizing ion exclusion in charged porous materials. Use of the electrochemical potential together with Gauss’ electrostatic theorem allows the computation of the ion and voltage distribution at the microscale. On the other hand, commonly used macroscopic counterparts of these equations allow the estimation of the Donnan potential and ion concentration on the macroscale. However, the classical macroscopic equations describing phase equilibrium do not account for the non-homogeneous distribution of ions and voltage at the microscale, leading to inconsistencies in determining the Donnan potential (at the macroscale). A new generalized macroscopic equilibrium equation is derived by means of volume averaging of the microscale electrochemical potential. These equations show that the macroscopic voltage is linked to so-called “effective ion concentrations”, which for ideal solutions are related to logarithmic volume averages of the ion concentration at the microscale. The effective ion concentrations must be linked to an effective fixed charge concentration by means of a generalized Poisson equation in order to deliver the correct Donnan potential. The theory is verified analytically and numerically for the case of two monovalent electrolytic solutions separated by a charged porous material. For the numerical analysis a hierarchical modeling approach is employed using a one-dimensional (1D)macroscale model and a two-dimensional (2D)microscale model. The influence of various parameters such as surface charge density and ion concentration on the Donnan potential are investigated.     

Some refinements concerning the boundary conditions at the macroscopic level

The study of boundary effects initiated in a previous paper is continued. New assumptions regarding the geometrical structure of the boundary surface are introduced. Under these assumptions, it is shown that macroscopic Neumann conditions do not generally affect the determination of the macroscopic field in the case of the transport process considered — heat conduction. For this type of boundary condition, the boundary effect is generally confined within a thin layer near the boundary. When heat sources are taken into account within the porous domain, the result is different. In this case, making use of a Neumann boundary condition, expressed in terms of macroscopic variables, amounts to introducing an extra flux. Under normal circumstances, however, this additional flux is negligible.Roman Letters A cross-sectional area of a unit cell - A _e cross-sectional area of a unit cell at the boundary surface - A _sf interfacial area of the s-f interface contained within the averaging volume - surface area per unit volume (A _sf/ ) - A _sf interfacial area of the s-f interface contained within the macroscopic system - g closure vector - h closure vector - k heat transfer coefficient at the s-f interface - K_eff effective thermal conductivity tensor - _x unit cell length - n unit vector - n_e outwardly directed unit normal vector at the boundary - n_sf outwardly directed unit normal vector for thes-phase at f-s interface - q heat flux density - T ^* macroscopic temperature defined by the macroscopic problem - s closure variable - V volume of the macroscopic system - V boundary surface of the macroscopic domain - V ₁ macroscopic sub-surface of the boundary surface - x local coordinate Greek Letters _s,_f volume fraction - _s, gl_f microscopic thermal conductivities - true microscopic temperature - ^* microscopic temperature corresponding toT ^* - microscopic error temperature - vector defined by Equation (34) - < > spatial average     

Facile fabrication of high-performance polyimide nanocomposites with <Emphasis Type="Italic">in situ</Emphasis> formed “impurity-free” dispersants

Polyimide/carbon black (PI/CB) nanocomposite films were fabricated via the direct ball-milling method with poly(amic acid) (PAA), the precursor of PI, as an in situ formed impurity-free dispersant. FTIR and Raman spectral results reveal that, besides physical adsorption, chemical grafting of PAA chains onto the CB surface occurs during the ball-milling process. Comparative studies show that introduction of various commercial dispersants improves the dispersion of CB. However, the mixtures exhibit poor reproducibility, unstable electrical properties, and decreased tensile strength; these issues may be attributed to interfacial pollution brought about by differences in the chemical structures of the dispersant and the matrix. The impurity-free dispersant is effective not only in ensuring the uniform dispersion of CB particles but also in enhancing filler-matrix interfacial adhesion. High-molecular weight PAA chains are effective reagents for impurity-free modification and can therefore be used to improve the electrical and mechanical properties of the resultant composite.     

An application of Kozeny–Carman flow resistivity model to predict the acoustical properties of polyester fibre

Modelling of the acoustical properties of polyester fibre materials is usually based on variations of the Bies and Hansen empirical model [1], which allows the calculation of the air flow resistivity of a porous material. The flow resistivity is the key non-acoustical parameter which determines the ability of this kind of materials to absorb sound. The main scope of this work is to illustrate that an alternative theoretical model based on the Kozeny–Carman equation can be used to predict more accurately the flow resistivity from the fibre diameter and bulk material density data. In this paper the flow resistivity is retrieved from the acoustic absorption coefficient data for polyester fibre samples of different densities and fibre diameters. These data agree closely with the flow resistivity predicted with the proposed Kozeny–Carman model.     

Magnetic properties and resistivity of a 2:17-type SmCo magnet doped with ZrO_2

In order to counteract the demagnetization caused by eddy current loss, widespread attention has been devoted to increasing the resistivity of permanent magnets. We prepared 2:17-type Sm Co magnets doped with different ZrO_2 contents and investigated the influence of the ZrO_2 content on the magnetic properties and resistive anisotropism. The results showed that not only was the resistivity of the magnet improved, but, in addition, the coercivity of the magnet was significantly increased. The microstructure was studied with TEM, which showed that ZrO_2 doping was able to cause a decrease in the lamellar phase density and the growth of cellular structures. The increased grain boundaries and Sm_2O_3 phases were favorable to the improvement of resistivity. The decrease of the lamellar phases caused a narrowing of the resistive anisotropism. The additional Cu in the center of the cellular boundaries was the main reason for the enhancement of Hcj.However, an excessive amount caused an increase of the Zr_6(Fe Co)_(23) phase and a deterioration of the cellular structure,thereby leading to a decrease in coercivity.     

Reduced and Surface‐Modified Graphene Oxide with Nonlinear Resistivity

Field‐grading materials (FGMs) are used to reduce the probability for electrical breakdowns in critical regions of electrical components and are therefore of great importance. Usually, FGMs are heavily filled (40 vol.%) with semi‐conducting or conducting particles. Here, polymer‐grafted reduced graphene oxide (rGO) is used as a filler to accomplish percolated networks at very low filling ratios (<2 vol.%) in a semi‐crystalline polymer matrix: poly(ethylene‐co‐butyl acrylate) (EBA). Various simulation models are used to predict the percolation threshold and the flake‐to‐flake distances, to complement the experimental results. A substantial increase in thermal stability of rGO is observed after surface modification, either by silanization or subsequent polymerizations. The non‐linear DC resistivity of neat and silanized rGO and its trapping of charge‐carriers in semi‐crystalline EBA are demonstrated for the first time. It is shown that the polymer‐grafted rGO improve the dispersibility in the EBA‐matrix and that the graft length controls the inter‐flake distances (i.e. charge‐carrier hopping distances). By the appropriate selection of graft lengths, both highly resistive materials at 10 kV mm^‐1 and FGMs with a large and distinct drop in resistivity (six decades) are obtained, followed by saturation. The nonlinear drop in resistivity is attributed to narrow inter‐flake distance distributions of grafted rGO.