Isotropic Resistivity Research Articles

New Bounds on Strong Field Magneto-Transport in Multiphase Columnar Composites

This paper provides new bounds on the effective properties in dimension two of multiphase composites with isotropic phases having nonsymmetric conductivities. This is relevant to classical strong field magneto-transport in heterogeneous conductors with columnar microstructure, with a magnetic field in the direction of the columns. On the one hand, a polygon-like G-closure for multiphase composites without prescribed volume fractions is derived for isotropic effective resistivities. On the other hand, the G-closure of the possible average currents associated with a given applied electric field is found for anisotropic multiphase composites with prescribed volume fractions. This extends the G-closure result obtained by Rai˘tum and Tartar for symmetric conductivities.

Residual resistivity and its anisotropy in random CoNi and CuNi ferromagnetic alloys

Residual resistivities of random CoNi and CuNi ferromagnetic alloys are calculated by a recently developed ab initio technique based on the linear muffin-tin orbital (LMTO) method with perturbative inclusion of spin-orbit interaction. The obtained results compare well with those of existing fully relativistic techniques. Particular attention has been paid to the isotropic resistivity and the spontaneous magnetoresistance anisotropy; a high value of the resistance anisotropy is predicted for Ni-rich CoNi alloys.

Mass Transports in an Air-Breathing Cathode of a Proton Exchange Membrane Fuel Cell

Mass transport in an air-breathing cathode of a proton exchange membrane (PEM) fuel cell was investigated numerically. The porous cathode in contact with a perforated current collector breathes fresh air through an array of orifices. The diffusions of reactant species in the porous cathodes are described using the Stefan–Maxwell equation. The electrochemical reaction on the surfaces of the porous cathode is modeled using the Butler–Volmer equation. Gas flow in the air-breathing porous cathodes is described using isotropic linear resistance model with constant porosity and permeability. The electron/ion transports in the catalyst/electrolyte are handled using charge conservation based on Ohm’s law. A finite-element scheme is adopted to solve these coupled equations. The effects of electrode porosity (0.4<ε<0.6) on the fluid flow, mass transport, and electrochemistry are examined. Detailed electrochemical/mass characteristics, such as flow velocities, species mass fraction, species flux, and current density distributions are presented. These details provide a solid basis for optimizing the geometry of a PEM fuel cell stack that is run in passive mode.

Magnetohydrodynamic dissipation range spectra for isotropic viscosity and resistivity

Dissipation range spectra for incompressible magnetohydrodynamic turbulence are derived for isotropic viscosity μ and resistivity η. The spectra are obtained from heuristic closures of spectral transfer correlations for cases with Pm=μ/η≤1, where Pm is the magnetic Prandtl number. Familiar inertial range power laws are modified by exponential factors that dominate spectral falloff in the dissipation range. Spectral forms are sensitive to alignment between flow and magnetic field. There are as many as four Kolmogorov wavenumbers that parametrize the transition between inertial and dissipative behavior and enter corresponding spectral forms. They depend on the values of the viscosity and resistivity and on the nature of alignment in inertial and dissipation ranges.

Possible Verwey-Type Transition in Pb3Rh7O15

Crystals of the mixed-valence compound Pb3Rh43+Rh34+O15 have been grown in a PbO flux. These hexagonal crystals show an isotropic electrical resistivity of about 1 × 10−3 ohm cm at room temperature. This resistivity slowly increases with decreasing temperature until 185 K, where the resistivity begins increasing more rapidly as temperature is further decreased. Specific heat measurements on polycrystalline Pb3Rh7O15 show a sharp, lambda-shaped peak at 185 K. Magnetic susceptibility and thermopower measurements on Pb3Rh7O15 also show a discontinuity at about 185 K. Structural analyses of X-ray diffraction data obtained above and below 185 K indicate that a change in space group has occurred at 185 K. It is likely that the transition at 185 K is related to a change in charge modulation. The phase transition at 185 K fades away as Bi is substituted for Pb in Pb3Rh7O15.

Mass/electron co-transports in an air-breathing cathode of a PEM fuel cell

Mass/electron transports in a passive cathode of a proton exchange membrane (PEM) fuel cell have been studied numerically. The porous cathode in contact with a perforated current collector breathes fresh air through an array of orifices. Diffusions of reactant species in the porous cathodes are described by the Stefan–Maxwell equation. Electrochemical reaction on the surfaces of the porous cathode is depicted via the Butler–Volmer equation. Gas flow in the air-breathing porous cathodes is governed by isotropic linear resistance model with constant porosity and permeability. The electron/ion transports in the catalyst/electrolyte are dealt with the charge conservations based on the Ohm's law. A finite-element method is employed to solve the above-coupled equations. The effect of overpotential on the fluid flow, mass transport and electrochemistry is examined. Detailed electrochemical/mass characteristics such as flow velocities, species mass fraction, species flux and current density distributions are presented. They can provide a solid basis for optimizing the geometry of the PEM fuel cell stack running with a passive mode.

Mass/Charge Transfer in Mono-Block-Layer-Built-Type Solid-Oxide Fuel Cells

The mass/charge transfer characteristics in a simulated MOLB (mono-block-layer built)-type solid-oxide fuel cells have been studied numerically. The transport phenomena within a linear MOLB module, including flow channels, active porous electrodes, electrolyte, and interconnections, are simulated using the finite volume method. The gas flow in the porous electrodes is governed by the isotropic linear resistance model with constant porosity and permeability. The diffusions of reactant species in the porous electrodes are described by the Stefan-Maxwell relation. Effective diffusivities for porous layers follow the Bruggman model. Porous electrochemistry is depicted via surface reactions with a constant surface-to-volume ratio, tortuosity, and average pore size. Results of the cathode-supported cell and the anode-supported cell are obtained, discussed, and compared thereafter for the first time.

Electrical and mechanical properties of melamine-formaldehyde-based laminates with shungite filler

Processing issues and the electrical and mechanical properties of a novel combination of a natural carbonaceous filler, Karelian shungite, and a water soluble thermosetting polymer, melamine-formaldehyde, were investigated. Two types of shungite with different carbon contents were investigated and compared to a commercial carbon black. The larger particle size and chemically more active surface of the shungites compared to carbon black leads to easy handling with little dusting and allows water to be used as dispersant. Laminates of melamine-formaldehyde reinforced with random cellulose fibers and filler were prepared by film stacking. The layered structure results in anisotropic resistivities for the shungites with fairly low in-plane percolation thresholds and a dissipative resistivity above the percolation transition. In comparison, carbon black had a lower percolation threshold and a low isotropic resistivity above the percolation transition. The mechanical properties of the composites were slightly deteriorated by all three fillers, indicating poor interfacial adhesion. The results of Fourier transform infrared (FTIR) measurements are interpreted as indicating hydrogen bonding and thus relatively weak adhesion between filler and polymer. Aqueous mixtures of melamine-formaldehyde and shungite filler had lower viscosities than corresponding melamine-formaldehyde and carbon black filler mixtures. POLYM. COMPOS., 26:552–562, 2005. © 2005 Society of Plastics Engineers

PALEOEVIRONMENT OF NIGERIA'S AJALI SANDSTONES: A PEBBLE MORPHOMETIC APPROACH.

The Ajali Sandstone is a major clastic formation of Campanian-Maastrichtian age occuring within the Anambra Basin, Southeastern Nigeria. The Sandstones have a high incidence of quartz and feldspar pebbles. Clasts of vein quartz pebbles were selected for morphometric study of decipher the depositional environment of the sandstone. The three mutually perpendicular axes of each pebble were measured and the roundness estimate with the aid of a roundness image set. Morphometric parameters such as size, flatness ratio, elongation ratio, elongation ratio, maximum projection sphericity, form geometry and oblate index were computed. Bivarite scattergrams of roundness versus oblate-prolate index were plotted. Except for the flatness ratio which is non-diagnostic, all the other results of the morphometric studies indicated a fluvial – dominated depositional environment. The result is in line with earlier conclusions of fluvial or fluviodeltaic depositional environment. The result is in line with earlier conclusions of fluvial or fluviodeltaic depositional environment based on analysis of faceis, sedimentary rock which are suitable for morphometrical research: those with isotropic constitution and high resistivity to wear such as cherts, quartz sandstones and some related types. KEY WORDS: Clastic formation, Fluviatile, Fluviodeltaic, Elongation ratio, Oblate index. Global Jnl Geological Sciences . Vol.2(1) 2004: 37-43

Two-Dimensional Basin Flow with Irregular Bottom Configuration

Two-dimensional flow from a point or line source is simulated in an irrigated basin with a nonlevel soil surface; the goal is to predict the distribution uniformity of infiltrated depths. The zero-inertia approximation to the equations of motion allows computation in both wet and dry areas. A fully implicit, nonlinear finite-difference scheme is developed for the solution, but practical numerical considerations suggest local linearization instead. Both isotropic and anisotropic resistance to flow are considered. Results in basins with irregular bottom configurations and small inflows show stream advance confined to the lowest elevations in the basin.

Brillouin light scattering from collective spin waves in Fe–Al–O granular films

Spin waves (SWs) propagating along the inplane easy and hard directions of highly resistive and soft magnetic Fe 67Al 13O 20 granular films of ∼1 μm in thickness have been studied at room temperature by Brillouin light scattering in magnetic fields of up to 4.0 kOe. The granular films possess an isotropic inplane resistivity of ρ∼150 μΩ cm and exhibit an inplane magnetic anisotropy of H K ∼150 Oe. We also examined SWs of a polycrystalline Fe film of ∼1 μm in thickness. SW spectra observed from the granular films consist of a bulk SW peak on both frequency sides and a surface SW peak only on the positive frequency side in this study, and are very similar to the SW spectra of the Fe film. Also, the SW peak widths of the granular films are found to be comparable to the SW peak widths of the Fe film. Magnetostatic analyses for a uniform medium with an anisotropic exchange coupling and an inplane uniaxial magnetic anisotropy successfully reproduce the SW frequencies as a function of the magnetic field. We found that the SW stiffness constants in the Fe–Al–O granular films are highly anisotropic; the easy-axis constant can be more than 10 times larger than the hard-axis constant.

Uniaxial ratcheting of SS304 stainless steel at high temperatures: visco-plastic constitutive model

The uniaxial ratcheting of SS304 stainless steel at high temperatures (300, 600 and 700 °C) were analyzed experimentally, and described by a cyclic constitutive visco-plasticity model. The rate dependence of the material was accounted for by introducing a viscous term. The cyclic hardening and cyclic flow behavior of the material under asymmetrical stress-controlled cycling were described by the evolution rules of kinematic hardening back stress and isotropic deforming resistance. Under the isothermal condition, temperature effect was included by terms involving temperature in the evolution equations of isotropic deforming resistance. The effect of load history on ratcheting was also considered by introducing a fading memory function of the maximum inelastic strain amplitude and isotropic deformation resistance. After the material constants were determined from the experimental data, the uniaxial ratcheting of SS304 stainless steel was numerically simulated and compared with the corresponding experimental results at high temperatures. The predicted results agree well with the experimental ones.

On axisymmetric resistive magnetohydrodynamic equilibria with flow free of Pfirsch–Schlüter diffusion

The equilibrium of an axisymmetric magnetically confined plasma with anisotropic resistivity and incompressible flows parallel to the magnetic field is investigated within the framework of the magnetohydrodynamic (MHD) theory by keeping the convective flow term in the momentum equation. It turns out that the stationary states are determined by a second-order elliptic partial differential equation for the poloidal magnetic flux function ψ along with a decoupled Bernoulli equation for the pressure identical in form with the respective ideal MHD equations; equilibrium consistent expressions for the resistivities η∥ and η⊥ parallel and perpendicular to the magnetic field are also derived from Ohm’s and Faraday’s laws. Unlike in the case of stationary states with isotropic resistivity and parallel flows [G. N. Throumoulopoulos and H. Tasso, J. Plasma Phys. 64, 601 (2000)] the equilibrium is compatible with nonvanishing poloidal current densities. Also, although exactly Spitzer resistivities either η∥(ψ) or η⊥(ψ) are not allowed, exact solutions with vanishing poloidal electric fields can be constructed with η∥ and η⊥ profiles compatible with roughly collisional resistivity profiles, i.e., profiles having a minimum close to the magnetic axis, taking very large values on the boundary and such that η⊥>η∥. For equilibria with vanishing flows satisfying the relation (dP/dψ)(dI2/dψ)>0, where P and I are the pressure and the poloidal current functions, the difference η⊥−η∥ for the reversed-field pinch scaling, Bp≈Bt, is nearly two times larger than that for the tokamak scaling, Bp≈0.1Bt (Bp and Bt are the poloidal and toroidal magnetic-field components). The particular resistive equilibrium solutions obtained in the present work, inherently free of—but not inconsistent with—Pfirsch–Schlüter diffusion, indicate that parallel flows might result in a reduction of the diffusion observed in magnetically confined plasmas.

Uniaxial and non-proportionally multiaxial ratcheting of SS304 stainless steel at room temperature: experiments and simulations

The uniaxial and non-proportionally multiaxial ratcheting behaviors of SS304 stainless steel at room temperature were initially researched by experiment and then were theoretically described by a cyclic constitutive model in the framework of unified visco-plasticity. The effects of cyclic stress amplitude, mean stress, and their histories on the ratcheting were experimentally investigated under uniaxial and different multiaxial loading paths. The shapes of non-proportional loading paths were linear, circular, elliptical and rhombic, respectively. In the constitutive model, the rate-dependent behavior of the material was reflected by a viscous term; the cyclic flow and cyclic hardening behaviors of the material under asymmetrical stress-controlled cycling were reflected by the evolution rules of kinematic hardening back stress and isotropic deforming resistance, respectively. The effect of loading history on the ratcheting was also considered by introducing two fading memorization functions for maximum inelastic strain amplitude and isotropic deformation resistance, respectively, into the constitutive model. The effect of multiaxial loading path on the ratcheting was reflected by a non-proportional factor defined in this work. The predicting ability of the developed model was proved to be good by comparing the simulations with corresponding experiments.

On the effects of viscosity and anisotropic resistivity on the damping of Alfvén waves

The equations of magnetohydrodynamics (MHD) are written for non-uniform viscosity and resistivity – the latter in the cases of Ohmic and anisotropic resistivity. In the case of Ohmic (anisotropic) diffusivity, there is (are) one (two) transverse components of the velocity and magnetic field perturbation(s), leading to a second-order (fourth-order) dissipative Alfvén- wave equation. In the more general case of dissipative Alfvén waves with isotropic viscosity and anisotropic resistivity, the fourth-order wave equation may be replaced by two decoupled second-order equations for right- and left-polarized waves, whose dispersion relations show that the first resistive diffusivity causes dissipation like the viscosity, whereas the second resistive diffusivity causes a change in propagation speed. The second resistive diffusivity invalidates the equipartition of kinetic and magnetic energy, modifies the energy flux through the propagation speed, and also changes the ratio of viscous to resistive dissipation. If the directions of propagation and polarization are equal (i.e. for right-polarized upward-propagating or left-polarized downward-propagating waves), the magnetic energy increases relative to the kinetic energy, the resistive dissipation increases relative to the viscous dissipation, and the total energy density and flux increase relative to the case of isotropic resistivity; the reverse is the case for opposite directions of propagation, i.e. upward-propagating left-polarized waves and downward-propagating right-polarized waves, which can lead to the existence of a critical layer. The role of the viscosity and first and second resistive diffusiveness on the dissipation of Alfvén waves is discussed with reference to the solar atmosphere.

Rate dependent constitutive relations based on Anand model for 92.5Pb5Sn2.5Ag solder

A rate dependent constitutive model, the Anand model, was applied to represent the inelastic deformation behavior for a Pb-rich solder 92.5Pb5Sn2.5Ag used in electronic packaging and surface mount technology. This rate dependent model is a unified viscoplastic constitutive model using an internal state variable, the deformation resistance, to describe the averaged isotropic resistance to macroscopic plastic flow. In order to obtain the acquired data for the fitting of the material parameters of this unified model for 92.5Pb5Sn2.5Ag solder, a series of experiments of constant strain rate test and constant load creep test were conducted under isothermal conditions at different temperatures ranged from -65/spl deg/C to 250/spl deg/C. A procedure for the determination of material parameters was proposed in this paper. Model simulations and verifications revealed that there are good agreements between model predictions and experimental data. Moreover, some discussions on using this rate dependent model in the finite element simulation of stress/strain responses of solder joints under thermal fatigue loading were presented.

The influence of spin–orbit coupling and a current dependent potential on the residual resistivity of disordered magnetic alloys

It has been shown recently, for a number of various magnetic disordered alloy systems, that the spin–orbit coupling (SOC) may have an important influence on the isotropic residual resistivity and that it is the primary source of the galvano-magnetic properties spontaneous magnetoresistance anisotropy (SMA) and anomalous Hall resistivity (AHR). Here it is demonstrated that—in contrast to many other spin–orbit induced phenomena—all these findings stem from the part of the spin–orbit coupling that gives rise to a mixing of the two spin sub-systems. In line with this result it is shown that inclusion of a current dependent potential within a calculation of the underlying electronic structure hardly affects the transport properties if the corresponding magnetic vector potential does not lead to a mixing of the spin sub-systems.

Anisotropic States of Two-Dimensional Electron Systems in High Landau Levels: Effect of an In-Plane Magnetic Field

We report the observation of an acute sensitivity of the anisotropic longitudinal resistivity of two-dimensional electron systems in half-filled high Landau levels to the magnitude and orientation of an in-plane magnetic field. In the third and higher Landau levels, at filling fractions $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9/2$, 11/2, etc., the in-plane field can lead to a striking interchange of the ``hard'' and ``easy'' transport directions. In the second Landau level the normally isotropic resistivity and the weak $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5/2$ quantized Hall state are destroyed by a large in-plane field and the transport becomes highly anisotropic.

Effective epicardial resistance of rabbit ventricles.

This study evaluated effective resistances on the ventricular surfaces of arterially-perfused rabbit hearts. Effective resistances were determined with a four-electrode array that was parallel or perpendicular to epicardial fibers. Resistance along or across epicardial fibers was determined by applying current to the epicardium with two parallel line electrodes and measuring potentials in the region between the electrodes. Computer simulations were performed to gain insight into the distribution of current in the ventricular wall. The effective resistances were not different along versus across fibers. Simulations showed that transmural rotation of fibers causes current to be distributed differently when the electrode is oriented perpendicular versus parallel to epicardial fibers. When the array is oriented so that epicardial current is across fibers, the fraction of current that flows transmurally and along the deeper fibers increases while the fraction of current that flows epicardially decreases. This introduces isotropy of the effective resistance. Thus, in contrast to isolated cardiac fibers, the ventricular epicardium exhibits isotropic effective resistance due to transmural rotation of fibers. The rotation and isotropic resistance may be important for cardiac electrical behavior and effects of electrical current in the ventricles.

Interplay of magnetic order and electronic transport in CeCu 6− xAu x

The electrical resistivity ρ( T) of magnetically ordered CeCu 6− x Au x shows strong x-dependent anisotropies below the Néel temperature T N. ρ( T) can be qualitatively explained by an overall (isotropic) resistivity decrease due to the partial suppression of the Kondo effect by the local magnetization generated by ordered moments, and by a change in the electronic band structure in the magnetically ordered state.