Showing papers on "Conductivity published in 1984"

Determining conductivity by boundary measurements

Abstract: A. P. Calderon pose la question: est-il possible de determiner la conductivite thermique d'un objet a partir de mesures statiques de la temperature et du flux de chaleur a la limite? On demontre que ces mesures determinent de facon unique la conductivite thermique et toutes ses derivees a la limite

An electron conductivity model for dense plasmas

Abstract: An electron conductivity model for dense plasmas is described which gives a consistent and complete set of transport coefficients including not only electrical conductivity and thermal conductivity, but also thermoelectric power, and Hall, Nernst, Ettinghausen, and Leduc–Righi coefficients. The model is useful for simulating plasma experiments with strong magnetic fields. The coefficients apply over a wide range of plasma temperature and density and are expressed in a computationally simple form. Different formulas are used for the electron relaxation time in plasma, liquid, and solid phases. Comparisons with recent calculations and available experimental measurement show the model gives results which are sufficiently accurate for many practical applications.

Conductivity and permeability of rocks

Abstract: The electrical conductivity of salt-water-saturated rocks is modeled by a random resistance network which has a zero percolation threshold. The porosity is varied by a random bond-shrinkage mechanism. Numerical and analytical calculations of the model in different dimensions show an Archie's-law behavior: ${\ensuremath{\sigma}}_{r}=a{\ensuremath{\sigma}}_{w}{\ensuremath{\varphi}}^{m}$, where $\ensuremath{\varphi}$ is the porosity of the rock, and ${\ensuremath{\sigma}}_{r}$ and ${\ensuremath{\sigma}}_{w}$ are the conductivities of the rock and water, respectively. We find that the Archie's exponent $m$ is always greater than unity and is related to the skewness of the "pore-size distribution" of the rock. Applying the same model to fluid-flow permeability (${k}_{r}$) gives ${k}_{r} \ensuremath{\propto}{\ensuremath{\varphi}}^{{m}^{\ensuremath{'}}}$, where ${m}^{\ensuremath{'}}=m(m+1)$ in one dimension, and ${m}^{\ensuremath{'}}=2m$ in higher dimensions. This power-law form is consistent with the well-known Kozeny equation and has been frequently suggested by empirical studies. Experimental tests of the model are performed on artificial rocks, made by fusing small glass beads, as well as real rocks. From resistivity measurements, we demonstrate that $m$ is larger in samples with a wider fluctuation of pore sizes, which is qualitatively consistent with the model. From fluid-flow experiments on fused glass beads, we find quantitative support for the ${m}^{\ensuremath{'}}=2m$ prediction.

The extraction of ionic conductivities and hopping rates from a.c. conductivity data

Abstract: Over a wide range of frequencies, the a.c. conductivity of ionic materials shows two regions of frequency-dependent conductivity. These are each characterized by a term Kω p 1− ω n where K, n are constants, ω p is a fundamental frequency identified with the hopping rate and ω is the measuring frequency. This behaviour is an example of Jonscher's Law of Dielectric Response for ionic conductors. In many cases, the region of low-frequency dispersion approximates to a frequency-independent plateau which may be taken as the d.c. conductivity. In others, a significant low-frequency dispersion is present and cannot be ignored in determining the effective d.c. conductivity. A method for the extraction of d.c. conductivities, hopping rates and for estimating carrier concentration effects is described. Data for three different types of material, single-crystal LiGaO2, β″-alumina and Na/Ag β-alumina are used to illustrate the method.

Conductivity and permeability from microgeometry

Abstract: We show how the electrical conductivity and the fluid flow permeability of a disordered random medium may be calculated from the microscopic geometry of the pore space. Serial sections of the pore space are used to determine an equivalent random network of cylinders, the transport coefficients of this network are related to the conductivity of an analog random resistor network, and the latter is calculated using an effective medium approximation. The procedure is illustrated by detailed analysis of a Massilon sandstone.

Electrical conductivity of single crystal and polycrystalline yttria-stabilized zirconia

Abstract: The conductivity of several single crystal and polycrystalline Y2O3-ZrO2 samples has been studied by complex impedance and four-probe direct current techniques. For single crystals only one arc, due to lattice conductivity, was observed in the complex impedance representation. Polycrystalline materials showed a second arc, due to grain boundary resistance, the extent of which decreased as the impurity concentration was reduced and as the electrolyte microstructure improved. The activation energies for the volume and total conductivity of the purest polycrystalline samples were similar and agreed with those for the single crystals. These values, however, decreased by 20 to 25 kJ mol−1 on going from low ( 850° C) temperatures. The change in the activation energy with temperature is thought to be due to a gradual transition between an association region, where vacancies are bound to dopant cations, and a dissociation region where vacancies are free and mobile.

Conductivity anisotropy in oriented poly(p-phenylene vinylene)

Abstract: The study of charge transport mechanisms in highly conjugated conducting polymers has historically been hampered by the complex and invariant morphologies of the best conductors We have prepared amorphous and uniaxially oriented films of poly(p-phenylene vinylene) (PPV) which exhibit a large conductivity anisotropy proportional to the degree of molecular orientation The conductivity of the AsF5 doped PPV, together with wide angle x-ray and IR characterization of these samples is reported

Texture effects on megahertz dielectric properties of calcite rock samples

Abstract: Dielectric measurements have been made from 0.5 to 1300 MHz on Whitestone, a quarried calcite rock, saturated with salty water. Whitestone shows a large increase in dielectric permittivity (dispersion) at the low end of this frequency range. When the conductivity of the water is varied, the dielectric permittivity of Whitestone is found to scale as water conductivity/frequency, i.e., as the complex dielectric constant of water. This is believed to be unique in measurements on insulator‐conductor mixtures, and establishes that the dispersion is primarily caused by the geometry of the sample. Two other calcite samples show much lower dielectric dispersion. Micrographs indicate that the variation in dispersion among the three samples is in rough proportion to grain platiness. This is consistent with the platey grain mechanism, one of three mechanisms proposed by Sen to explain dielectric dispersion in water‐saturated rocks. A model consisting of water containing insulating spheroids of identical aspect ratio...

Position and pressure effects in rf magnetron reactive sputter deposition of piezoelectric zinc oxide

Abstract: c‐axis oriented thin films of piezoelectric ZnO have been deposited on room‐temperature substrates by rf magnetron sputtering of zinc in a reactive ambient of 100% oxygen over a pressure range from 5–70 mTorr. Below 10 mTorr the film structure, orientation, and physical properties depend on the position of the substrates with respect to the target. This is attributed primarily to resputtering of the film in the region of the target erosion area by high‐energy neutral oxygen. At higher pressures where the mean free path inhibits such effects, the film structure and properties are uniform. At 700‐W sputtering power, the optimum pressure lies between 20–30 mTorr leading to highly oriented ZnO over a large area of plasma with a dielectric constant of 11, conductivity at 300 °C=2×10−10 Ω−1 cm−1, and Rayleigh wave velocity=2600 m/s. The films are characterized using optical transmittance, electrical conductivity, dielectric constant and SAW, and acoustic microscope measurements.

Some properties of polypyrrole-paper composites

Abstract: It is shown that porous materials such as paper can be impregnated with a conducting polymer by first soaking the paper in a solution of metal salt which acts as an initiator for subsequent polymerization. By using this procedure the conductivity of filter paper can be raised to as high as 2 (ohm cm)−1 for an FeCl3/H20/pyrrole combination. The role of the metal ion is discussea and it is pointed out that the impregnated papers exhibit both ionic and electronic conductivity. The polypyrrole paper composites exhibit some properties similar to carbon black-polymer composites.

Electrical Conductivity and Defect Structure of Y2O3 as a Function of Water Vapor Pressure

Abstract: The electrical conductivity of several sintered yttria ceramics with different levels of lower valent cation impurities was studied as a function of the partial pressure of water vapor (3 to 1600 Pa) at 500° to 1300°C in oxygen or air. At the higher temperatures yttria is a P-type conductor, but at the lowest temperatures and highest water vapor pressures, an ionic contribution becomes significant. The p-type conductivity decreases with increasing water vapor pressure. This is interpreted in terms of a model involving dissolution of hydrogen as interstitial protons and which counterbalance the effective charges of the lower valent impurities. At the higher water vapor pressures and lower impurity levels excess protons are probably compensated by interstitial hydroxide or oxygen ions. Indications of grain boundary segregation of impurities are reported.

The mixed alkali effect in lithium-sodium borate glasses

Abstract: The electrical conductivity of a series of 0.35 (Li, Na)2O·B2O3 glasses shows a minimum at the composition Na/(Na+Li)∼0.6, which becomes stronger as the temperature is decreased; the activation enthalpy for electrical conductivity shows a maximum at this composition. In general, replacing 1% of the total oxygen concentration by chlorine or bromine (keeping the total alkali content fixed) in these glasses increases the conductivity; fluorine doping has an opposite effect. The mixed alkali effect, expressed in terms of the compositional dependence of the activation enthalpy for conductivity, is enhanced when borate glass is doped with fluorine, but is slightly diminished when doped with chlorine or bromine. The results are explained in terms of the structure of halogenated alkali-borate glasses, and discussed in relation to the origin of the mixed alkali effect.

Incipient localisation and electron-electron correlation effects in metallic glass alloys

Abstract: The general form of the temperature dependence of resistivity in metallic glassy alloys is discussed in detail using the results of the scaling model of localisation and models of electron-electron correlation effects. It is shown that for a metallic glass of relatively low resistivity there would normally exist a resistance minimum at low temperatures, while for glasses of higher resistivity the conductivity below theta D should be directly proportional to T, changing to a square root T dependence for higher temperatures. These predictions are then shown to be in accordance with new measurements on Cu50Ti50 and Ti50Be40Z10 alloys.