Showing papers on "Conductivity published in 1981"

A self-similar model for sedimentary rocks with application to the dielectric constant of fused glass beads

Abstract: We develop a theory for dielectric response of water‐saturated rocks based on a realistic model of the pore space. The absence of a percolation threshold manifest in Archie’s law, porecasts, electron‐micrographs, and general theories of formation of detrital sedimentary rocks indicates that the pore spaces within such rocks remain interconnected to very low values of the porosity ϕ. In the simplest geometric model for which the conducting paths remain interconnected, each grain is envisioned to be coated with water. The dielectric constant of the assembly of water‐coated grains is obtained by a self‐consistent effective medium theory. In the dc limit, this gives Maxwell’s relation for conductivity σ of the rock σ=2σwϕ/(3-ϕ), where σw is the conductivity of water. In order to include the local environmental effects around a grain, a self‐similar model is generated by envisioning that each rock grain itself is coated with a skin made of other coated spheres; the coating at each level consists of other coate...

Grain boundary effects on ionic conductivity in ceramic GdxZr1–xO2–(x/2) solid solutions

Abstract: Complex admittance measurements are performed on high-purity ceramics prepared by means of the alkoxide synthesis and on less pure ceramics obtained from the citrate synthesis. The results on ceramic materials with grain sizes ranging from 0.4 to 20 µm are compared with those from a single crystal. The activation enthalpy for grain boundary conductivity Hgb = (118 ± 2) kJ/mol for the samples studied, is independent of composition, grain size, and preparation method. Grain boundary conductivity values and consequently the relevant pre-exponential factors are an order of magnitude smaller for the alkoxide materials than for the citrate materials. The ratio of grain bulk and grain boundary conductivity (Qb/Qgb) for alkoxide materials with grain-sizes 0.4 to 0.8 µm varies from 8.5 to 1.0 in the temperature range 500 to 700 °C.

Dielectric response and conductivity of dilute suspensions of colloidal particles

Abstract: A numerical scheme is developed for solving the equations which govern the behaviour of dilute suspensions of colloidal particles in an oscillating electric field. Appropriate expressions for the conductivity and the complex dielectric response of a dilute suspension are also derived. The behaviour of these properties as functions of frequency, zeta potential, particle radius, electrolyte concentration and valency is displayed. The effect of slipping-surface boundary conditions for the electric potential is briefly discussed.

Conductivity of the disordered linear chain

Abstract: The authors develop a fast algorithm for evaluating the Kubo formula for the conductivity of a linear chain, and use it to study the dependence of the conductivity as a function of imaginary frequency. The results for the Anderson model with different degrees of disorder and different energies can all be scaled onto the same curve, which is of the form expected from the theory of localised states. The universal curve obtained provides a simple connection between tight-binding model results and the conductivity which can be calculated for an electron in a white noise potential. Similar, but not identical, results are obtained for tight-binding chains with a Cauchy distribution of site energies.

Conducting complexes of polyphenylene sulfides

Abstract: Poly(p‐phenylene) sulfide, poly(m‐phenylene) sulfide, and the newly synthesized polymer poly(thio‐2,8‐dibenzothiophenediyl) have been treated with strong electron acceptors (AsF5, SbF5) to form conducting complexes with p‐type electronic conductivities up to 3 S/cm. Near IR to UV absorption spectra and temperature‐dependent conductivity measurements suggest a localization of charge carriers even at high doping levels. Elemental analysis and IR spectroscopy demonstrate that heavy exposure to AsF5 causes substantial changes in the backbone structure of these polymers. The dopant appears to predominantly induce the formation of carbon–carbon bonds bridging the sulfur linkages to form thiophene rings. This chemical modification enhances the conductivity of the complex and, in the case of poly(m‐phenylene), is shown to be an actual prerequisite for achieving high conductivity.

On the theory of hopping conductivity in disordered systems

Abstract: Starting with the linearised master equation, the authors present a first-principles theory of conductivity for disordered systems. The theory is valid for all situations to which the master equation description applies. The path summation and configurational average are evaluated by generalising relations obtained from exactly soluble models. Both symmetric and asymmetric energy-dependent transition frequencies are considered. In the latter case they are able to define an energy-dependent conductivity from which it is possible to evaluate the thermopower. All electronic transport properties, including the frequency-dependent conductivity can be evaluated self-consistently, the only input parameter being the density of states. Numerical results for the DC conductivity and thermopower are presented using several model density-of-states functions. For random statistics, the results are in complete agreement with percolation theory for low densities (temperature). The theory is exact in the high-density (temperature) limit.

Ionic conductivity and thermoelectric power of pure and Al2O3-dispersed AgI

Abstract: Ionic and electronic conductivities, and thermoelectric power have been measured for AgI and AgI containing a dispersion of submicron size Al2O3 particles. While the dispersion of Al2O3 enhances the ionic conductivity significantly, it does not affect the electronic properties of the matrix. The enhancement is a strong function of the size and concentration of the dispersoid. Various models have been tested to account for the enhanced conduction. However, the complex behavior of the present results points out the need for more sophisticated theoretical models. Ionic conduction and thermoelectric power data suggest that the dispersed Al2O3 generates an excess of cation vacancies and thereby enhances the conductivity and suppresses the thermoelectric power of the matrix. The individual heats of transport of cation interstitials and vacancies have been estimated and compared to their respective migration energies.

Metal-Insulator Transition in Disordered Germanium-Gold Alloys

Abstract: It has been discovered that the disordered material ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Au}}_{x}$ displays an unusual type of metal-insulator transition as the composition $x$ is varied. The onset of conductivity, as extrapolated to $T=0$ K, is continuous, and thus the system has no "minimum metallic conductivity."

Analysis of flow through heterogeneous random aquifers by the method of embedding matrix: 1. Steady flow

Abstract: The statistical structures of the hydraulic conductivity, head gradient, specific discharge, and head fields are analyzed with the aid of a simplified model consisting of a collection of spherical (in three dimensions) and cylindrical (in two dimensions) blocks of different conductivity and diameter set at random and independently in space. Results of a previous work for the effective conductivity and unbounded formations (Dagan, 1979) are recovered and given a rational derivation. The influence of the aquifer boundary and nonuniform average flows are examined for the first time. The growth of the head variance with distance from boundary in two-dimensional flows is compared with that of the Monte Carlo simulations of Smith and Freeze (1979).

Conductive polymer: reticulate doping with charge-transfer complex

Abstract: Additives which can form charge-transfer (CT) complexes enhance the photoconductivity of polymers. In contrast, the dark conductivity of polymers containing this type of additive increases only slightly. Several crystalline CT complexes exhibit high dark conductivity. When introduced into polymer matrix these smuld therefore significantly increase its conductivity provided that the CT complexes are ordered, because their high conductivity is related to the strong interactions between donor and acceptor stacks in the crystal. We describe here a new method of introducing microcrystalline CT complexes into polymers. It consists of growing microcrystals during film casting. The system, poly (bisphenol A carbonate)/(PC) with microcrystals of the CT complex of 7,78,8-tetra-cyanoquinodimethane (TCNQ) with tetrathiotetracene (TTT), reaches the dark conductivity of 3 × 10−2Ω−1 cm−1 at room temperature at 1 wt% of the complex. This conductivity is only slightly temperature dependent.

Weak‐Electrolyte Models for the Mixed‐Alkali Effect in Glass

Abstract: A weak electrolyte model for the mixed-alkali effect on electrical conductivity and ionic mobility in glass was developed for the dilute foreign alkali region. The basic assumptions are (a) that alkali transport in single-alkali glasses is due to a small concentration of mobile species and (A) adding small amounts of foreign alkali suppresses, by mass action, the mobile species concentration. The model was developed explicitly for the case where the mobile species are interstitial cations or cation pairs and was applied to analysis of conductivity data for 0.242(K2O +Na2O)–0.758SiO2 glasses in the dilute Na+ region. The analysis allows the fraction of mobile cations in the single-alkali glass to be calculated. This varies from 0.5% at 25° to 6% at 500°C for 0.242K2O–0.758SiO2 glass and is in good agreement with estimates of the extent of dissociation from internal friction data and an ionic jump model of conductivity.

Radiation-Induced Trivalent Silicon Defect Buildup at the Si-SiO2 Interface in MOS Structures

Abstract: Electron spin resonance and capacitance versus voltage measurements demonstrate approximately a one-to-one correspondence between the density of radiation-induced trivalent silicon defects at the (111) Si-SiO2 interface and the density of radiation induced electronic interface states.

The interaction of tin oxide films with O2, H2, NO, and H2S

Abstract: A series of experiments was performed to investigate the observed changes in conductivity when sputtered tin oxide films maintained at 525 K are exposed to various ambient gases. X‐ray photoemission spectroscopy (XPS) measurements on quenched samples monitored changes in both the chemical state and stoichiometry which occurs during exposure to O2, H2, NO, and H2S. The XPS results were correlated with in situ measurements of sample resistance. This comparison demonstrated no direct connection between the concentration of an adsorbed impurity, i. e., N or S, and conductivity. However, the bulk concentration of oxygen atoms is strongly correlated with the sample conductivity. We conclude that the predominant response mechanisms involve the oxidation/reduction of the sample with a concommitant decrease/increase in the bulk concentration of oxygen vacancies which mediates the conductivity.

Optical properties and transport in microcrystalline silicon prepared at temperatures below 400 °C

Abstract: Microcrystalline silicon (grain size ∼100 A) films have been prepared at temperatures below 400 °C through an H2‐plasma assisted transport process. The films contain an average of ∼2 at. % of hydrogen which is predominantly situated in the grain boundary regions in the form of Si‐H2. The optical absorption coefficient of these films exceeds that of crystalline Si by up to two orders of magnitude below 3.0 eV. The films can be doped by adding PH3 to the hydrogen plasma. The highest room‐temperature conductivity so achieved was ∼1 (Ω−1 cm−1). From an analysis of the activation energies vs dopant concentration we deduce that the high‐temperature transport (T≳330 °K) is barrier limited. The upper limit for the density of trapping states in the grain boundaries is 1×1013 cm−2.

On the determination of electrical conductivity in polyethylene

Abstract: There are significant differences in the field dependence of conductivity in low-density polyethylene as reported by various investigators. These differences are attributable partly to the presence of different kinds of impurities in variable concentrations in polyethylene samples of various origin, but are mainly due to the different measurement procedures adopted. The significance of the time which elapses after the application of voltage to the electrodes, as well as the effect of conditioning, e.g. thermal and voltage stressing of the specimen before and during the measurement, is discussed. Whether it is always necessary to obtain a steady-state current in order to determine the true conductivity is somewhat debatable.

Comparison of the specific heat and conductivity of Si: P

Abstract: The low-temperature specific heat and electrical conductivity have been measured on samples of Si: P with well-characterized electron densities. The metallic conductivity in the limit of zero temperature is found to vary considerably more rapidly than the electronic density of states for donor densities near the metal-insulator transition. Mott's analysis of the Kubo-Greenwood formulation provides an adequate description of the results only at higher densities than expected.

Effect of Water Content on the Electrical Conductivity of Na2O≅3SiO2 Glass

Abstract: Na2O–3SiO2 glasses with up to ≅12 wt% water were prepared under high-pressure, hydrothermal conditions and their electrical conductivities were measured. The conductivity (σ) was found to depend on H2O content in a manner similar to the “mixed-alkali” effect. At constant temperature, σ decreased initially with increasing H2O content to a minimum at 3≅4 wt% H2O and increased with further increase in water content. Infrared spectroanalysis of these glasses was made to determine the type of water present in the glass and the results were interpreted in the context of the measured activation energy and preexponential factor of dc conductivity. The sodium ion diffusion coefficient for a glass containing 0.76 wt% H2O was less than that of water-free glass and in agreement with the electrical conductivity results.

Aluminium-27 nuclear magnetic resonance studies of the hydrolysis of aluminium(III). Part 2. Gel-permeation chromatography

Abstract: The gel-permeation chromatography of two different type of hydrolysed aluminium(III) solution are described, the separations being monitored by conductivity, pH, pCl, and 27Al n.m.r. measurements. Solutions hydrolysed by dissolution of aluminium metal in AlCl3 solution contain at least four components including [Al(OH2)6]3+ and [AlO4Al12(OH)24(OH2)12]7+ whereas hydrolysis with Na2[CO3] produces only a high-molecular-weight polymeric ionic material and the Al137+ cation. A fraction was obtained which contained only the latter ion and was more stable than the original solution.

Thermal decomposition of ferrous oxalate dihydrate studied by direct current electrical conductivity measurements

Abstract: The feasibility of the use of direct current electrical conductivity,σ, measurements in the study of solid-state reactions involved in the synthesis ofγ-Fe2O3 from ferrous oxalate dihydrate has been reported. The study has been carried out in static air, dynamic air, dry nitrogen and dynamic air + water vapour environments. The conductivity data determined in static air are quite complex; nevertheless, the formation of Fe3O4 andα-Fe2O3 with the probable intermediate formation ofγ-Fe2O3 has been indicated. In dry nitrogen the step corresponding to dehydration is well resolved in the temperature region 145–220° C; the formation of FeO and Fe3O4 is also well characterized. In dry dynamic air the reaction further proceeds to the formation ofα-Fe2O3. In dynamic air containing water vapour there are definite indications of the formation ofγ-Fe2O3 prior to the formation ofα-Fe2O3. Definite experimental conditions have been determined for the formation ofγ-Fe2O3 in dynamic air containing water vapour. The conductivity measurements have been supplemented with infra-red spectroscopy and X-ray diffraction pattern measurements. The electrical conductivity measurements were found to give additional information on the solid-state reaction to that obtained from conventional thermal analytical techniques (such as differential thermal, thermogravimetric and differential thermogravimetric analyses).γFe2O3, obtained from the decomposition of FeC2O4 · 2H2O in dynamic air + water was found to have a coercive force of 3.142 A m−1, a saturation magnetization value of 7.1 T kg−1 and a ratio of remanence to saturation magnetization of 0.64.