Showing papers on "Conductivity published in 1998"

Quantum Transport in Two-Dimensional Graphite System

Abstract: In a self-consistent Born approximation, the density of states and the conductivity are calculated in a two-dimensional graphite sheet in magnetic fields. Two different cases of scatterers are considered, the short-range case where the range is smaller than the lattice constant and the long-range case where it is comparable or slightly larger. The quantum theory provides results quite different from the results of Boltzmann transport theory even in the absence of a magnetic field. In high magnetic fields, the conductivity exhibits a series of peaks, whose values depend only on the natural constants and the Landau level index. The conductivity of undoped systems is always given by a universal conductivity \(e^{2}/\pi^{2}\hbar\) independent of a magnetic field.

Room-temperature molten salts based on the quaternary ammonium ion

Abstract: The properties of a family of novel quaternary ammonium salts based on the bis(trifluoromethylsulfonyl)imide and triflate anions are reported. Binary phase diagrams for some of their mixtures and their electrochemical windows of stability are also reported. The highest conductivity observed in the pure salt systems at 25 °C was 7 × 10-4 S cm-1. An electrochemical window of stability of up to 5 V was measured on graphite electrodes. The effect of salt structure and solvent on conductivity of the salts is also discussed.

Electrical conductivity in shaly sands with geophysical applications

Abstract: We develop a new electrical conductivity equation based on Bussian's model and accounting for the different behavior of ions in the pore space. The tortuosity of the transport of anions is independent of the salinity and corresponds to the bulk tortuosity of the pore space which is given by the product of the electrical formation factor F and the porosity ϕ. For the cations, the situation is different. At high salinities, the dominant paths for the electromigration of the cations are located in the interconnected pore space, and the tortuosity for the transport of cations is therefore the bulk tortuosity. As the salinity decreases, the dominant paths for transport of the cations shift from the pore space to the mineral water interface and consequently are subject to different tortuosities. This shift occurs at salinities corresponding to ξ/t(+)f ∼ 1, where ξ is the ratio between the surface conductivity of the grains and the electrolyte conductivity, and t(+)f is the Hittorf transport number for cations in the electrolyte. The electrical conductivity of granular porous media is determined as a function of pore fluid salinity, temperature, water and gas saturations, shale content, and porosity. The model provides a very good explanation for the variation of electrical conductivity with these parameters. Surface conduction at the mineral water interface is described with the Stern theory of the electrical double layer and is shown to be independent of the salinity in shaly sands above 10−3 mol L−1. The model is applied to in situ salinity determination in the Gulf Coast, and it provides realistic salinity profiles in agreement with sampled pore water. The results clearly demonstrate the applicability of the equations to well log interpretation of shaly sands.

Controlled doping of phthalocyanine layers by cosublimation with acceptor molecules: A systematic Seebeck and conductivity study

Abstract: We investigate the doping of vanadyl–phthalocyanine by a fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic dye films by cosublimation of matrix and dopant. The electrical parameters of the films derived from conductivity and Seebeck measurements show that the results largely follow standard models used to describe the doping of crystalline semiconductors; e.g., a smooth shift of the Fermi level towards the valence states with increasing doping is observed. Other effects, like the superlinear increase of conductivity with the molar doping ratio, need the inclusion of additional effects like percolation.

Measurement of resonant frequency and quality factor of microwave resonators : comparison of methods

Abstract: Precise microwave measurements of sample conductivity, dielectric, and magnetic properties are routinely performed with cavity perturbation measurements. These methods require the accurate determination of quality factor and resonant frequency of microwave resonators. Seven different methods to determine the resonant frequency and quality factor from complex transmission coefficient data are discussed and compared to find which is most accurate and precise when tested using identical data. We find that the nonlinear least-squares fit to the phase versus frequency is the most accurate and precise when the signal-to-noise ratio is greater than 65. For noisier data, the nonlinear least-squares fit to a Lorentzian curve is more accurate and precise. The results are general and can be applied to the analysis of many kinds of resonant phenomena.

Extended Electrokinetic Characterization of Flat Solid Surfaces

Abstract: An experimental setup has been developed and applied for the combined determination of the electrokinetic potential and the surface conductivity of flat surfaces. The key feature of the new device (designated as microslit electrokinetic setup) is the variability of the distance between two parallel flat sample surfaces (10 mm x 20 mm) forming a slit channel. The setup allows us to decrease this distance down to about 1 µm keeping the surfaces parallel. In consequence, streaming potential measurements can be performed at a given solid/liquid interface both at conditions where surface conductivity is negligible and at conditions where surface conductivity significantly contributes to the total channel conductivity. The zeta potential is calculated at different channel geometries based on streaming potential and channel conductivity data and, alternatively, based on streaming current measurements and the dimensions of the cross section of the slit channel. The results obtained were found to agree well if correct conductivity values for the calculation of the zeta potential based on the streaming potential data are used. The surface conductivity is determined from the extrapolation of the channel conductance values gained at a number of sufficiently small distances between the parallel sample surfaces to the distance zero. An additional feature of the developed microslit electrokinetic setup is the assessability of the hydrodynamic thickness of adsorbed layers of macromolecules or particles at the investigated flat surface. In a series of measurements a plasma-deposited fluoropolymer (PDFP) layer on top of a glass carrier and an adsorption layer of the blood protein fibrinogen on top of the PDFP layer were characterized by zeta potential and surface conductivity measurements in different aqueous electrolyte solutions (KCl, KOH, HCl). For the PDFP/solution interfaces zeta potential up to -100 mV were obtained in solutions of neutral pH exclusively due to preferential ion adsorption. After adsorption of fibrinogen the zeta potential is considerably reduced. For the PDFP/solution interfaces surface conductivities were determined in the range of (1-2) x 10(-9) S. The contribution of the diffuse layer to the surface conductivity has been calculated from the zeta potential according to the approach of Bikerman (Kolloid Z. 72, 100 (1935)) and compared with the experimentally determined surface conductivity. Based on this comparison ions in hydrodynamically immobile interfacial layers were concluded to contribute considerably to the surface conductivity in all investigated cases. This so-called additional surface conductivity is attributed to the accumulation of hydroxide and hydronium ions in the Stern layer. Both the high specific mobility of these ions (as compared to the potassium and the chloride ions) and the conductivity of the charge determining species may contribute to the experimental observations. After adsorption of fibrinogen onto the PDFP surface the additional surface conductivity is increased by about an order of magnitude. The latter fact is assumed to be caused by the presence of mobile ions in the interfacial volume of the adsorbed protein layer. In addition to the electrochemical characterization of the adsorbed protein layer its hydrodynamic thickness has been determined by means of liquid flow measurements with the microslit electrokinetic setup. The obtained value of 48 +/- 5 nm correlates well with the protein dimensions given in the literature and is in the order of magnitude of the optical layer extension determined by ellipsometry. Copyright 1998 Academic Press.

Identification of conductivity imperfections of small diameter by boundary measurements. Continuous dependence and computational reconstruction

Abstract: We derive an asymptotic formula for the steady-state voltage potential in the presence of a finite number of diametrically small inhomogeneities with conductivity different from the background conductivity. We use this formula to establish continuous dependence estimates and to design an effective computational identification procedure.

Proton Conductivity in Nafion® 117 and in a Novel Bis[(perfluoroalkyl)sulfonyl]imide Ionomer Membrane

Abstract: A study of proton conductivity in a commercial sample of Nation® 117 and a structurally similar bis[(perfluoroalkyl)sulfonyl]imide ionomer membrane under variable temperature and humidity conditions is reported. The sulfonyl imide ionomer was synthesized using a novel redox‐initiated emulsion copolymerization method, and conductivities were measured using a galvanostatic four‐point‐probe electrochemical impedance spectroscopy technique. Both materials exhibited a strong dependence of conductivity on temperature and humidity, with conductivity in both cases being strongly diminished with decreasing humidity (at constant temperature) and increasing temperature (at constant water partial pressure). The observed behavior is consistent with a "liquid‐like" mechanism of proton conductivity whereby protons are transported as hydrated hydronium ions through water‐filled pores and channels in the ionomer.

Broadband ac conductivity of conductor-polymer composites

Abstract: The electrical conductivity of a composite model system formed by highly structured carbon black ~CB! filled, within an amorphous polymer, poly~ethylene terephtalate! composite is studied. The dc conductivity as a function of CB content follows a scaling law of the type s}(p2pc) t yielding for the percolation concentration, pc50.011 and for the exponent, t52.17. The analysis of the temperature dependence of the conductivity suggests that for temperatures larger than 45 K, conduction can be ascribed to thermal fluctuation induced tunneling of the charge carriers through the insulating layer of polymer separating two CB aggregates. At lower temperatures, conductivity becomes temperature independent, which is typical of conventional tunneling. The frequency dependence of the conductivity is also studied between dc and 10 Hz. By the introduction of a shift factor ap , a procedure for the construction of a master curve based on a ‘‘time-length equivalence principle’’ is proposed. Finally, a model is introduced to describe the frequency dependence of the conductivity of CB-filled composites based on the behavior of charge carriers placed in a fractal object. @S0163-1829~98!06304-8#

The role of microstructure and processing on the proton conducting properties of gadolinium-doped barium cerate

Abstract: The influence of grain boundary conductivity and microstructure on the electrical properties of BaCe0.85Gd0.15O3–δ have been examined. Grain sizes were varied by sintering at various temperatures. Impedance data were analyzed using the brick layer model, and some new consequences of this model are presented. The specific grain boundary conductivity exhibits an activation energy of ~0.7 eV, and for similar processing routes, is independent of grain size. An isotope effect was observed, indicating that protons (or deuterons) are the mobile species. TEM investigations showed the intergranular regions to be free of any glassy phase that could account for the differences in bulk and grain boundary properties. Single-crystal fibers, grown by a modified float zone process, were notably barium deficient, and exhibited a low conductivity, comparable to that of polycrystalline Ba0.96Ce0.85Gd0.15O3–δ.

A Comparative Study of the Determination of the Critical Micelle Concentration by Conductivity and Dielectric Constant Measurements

Abstract: Conductivities and dielectric constant measurements in water at 25 °C have been made on the amphiphilics sodium n-dodecyl sulfate, n-dodecyltrimethylammonium bromide, and chlorpromazine hydrochloride. By using the conductivity/concentration data, critical micelle concentrations (cmc) have been determined by applying the Williams definition and two forms of the Phillips method. This first Phillips form consists of an approximation to Gaussians of the second derivative of the conductivity/concentration data followed by two consecutive integrations. The second form, which is proposed here, consists of the application of a combination of the Runge−Kutta numerical integrations method and the Levenberg−Marquardt least-squares fitting algorithm. The proposed method permits the determination of the cmc in systems with low aggregation numbers and with slow variations of physical property/concentration curves allowing the determination of the so-called second cmc. A comparative study with results obtained by dielec...

Electrical Conductivity of Olivine, Wadsleyite, and Ringwoodite Under Upper-Mantle Conditions

Abstract: Geophysical models show that electrical conductivity in Earth9s mantle rises about two orders of magnitude through the transition zone in the depth range 410 to 660 kilometers. Impedance measurements obtained on Mg1.8Fe0.2SiO4 olivine, wadsleyite, and ringwoodite at up to 20 gigapascals and 1400°C show that the electrical conductivities of wadsleyite and ringwoodite are similar and are almost two orders of magnitude higher than that of olivine. A conductivity-depth profile to 660 kilometers, based on these laboratory data, shows a conductivity increase of almost two orders of magnitude across the 410-kilometer discontinuity; such a profile favors a two-layer model for the upper mantle. Activation enthalpies of 1.2 to 1.7 electron volts permit appreciable lateral variations of conductivity with lateral temperature variations.

Effect of Salt Concentration on the Conductivity of PEO-Based Composite Polymeric Electrolytes

Abstract: The effect of salt concentration on the molal conductivity of various composite electrolytes was studied. Conductivity enhancement is achieved for composite electrolytes over the basic poly(ethylen...

Thermal conductivity of thermal barrier coatings

Abstract: In thermal barrier coatings and other ceramic oxides, heat is conducted by lattice waves, and also by a radiative component which becomes significant at high temperatures. The theory of heat conduction by lattice waves is reviewed in the equipartition limit (above room temperature). The conductivity is composed of contributions from a spectrum of waves, determined by the frequency dependent attenuation length. Interaction between lattice waves (intrinsic processes), scattering by atomic scale point defects and scattering by extended imperfections such as grain boundaries, each limit the attenuation length in different parts of the spectrum. Intrinsic processes yield a spectral conductivity which is independent of frequency. Point defects reduce the contribution of the high frequency spectrum, grain boundaries and other extended defects that of the low frequencies. These reductions are usually independent of each other. Estimates will be given for zirconia containing 7wt% Y 2 O 3 , and for yttrium aluminum garnet. They will be compared to measurements. The effects of grain size, cracks and porosity will be discussed both for the lattice and the radiative components. While the lattice component of the thermal conductivity is reduced substantially by decreasing the grain size to nanometers, the radiative component requires pores or other inclusions of micrometer scale.

Electrical conductivity of dense copper and aluminum plasmas

Abstract: Measurements are reported of the electrical conductivity of dense copper and aluminum plasmas in the temperature range 10--30 kK, in a density range from about one-fifth solid density down to $0.02{\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$. Plasmas were created by rapid vaporization of metal wires in a water bath. At temperatures below about 15 kK, as density decreases from the highest values measured, the conductivity falls roughly as the cube of density, reaches a minimum, and subsequently rises to approach the Spitzer prediction at low density. This minimum is not seen for temperatures above about 20 kK. These results are compared with several theoretical predictions.

Raman Spectra and Transport Properties of Lithium Perchlorate in Ethylene Carbonate Based Binary Solvent Systems for Lithium Batteries

Abstract: Electrolyte solutions formed by the addition of lithium perchlorate to a binary solvent mixture obtained by mixing ethylene carbonate (EC) with propylene carbonate (PC), diethyl carbonate (DEC), or dimethyl carbonate (DMC) were studied. The Raman spectra, conductivity, and viscosity of the EC based binary solvent electrolytes were measured. The salt concentration was optimized for maximum conductivity. Conductivity increases with increasing lithium perchlorate concentration in the EC/PC/LiClO4 electrolyte until a maximum of 6.1 kΩ-1 cm-1 is reached at 0.82 M LiClO4 (EC/PC/LiClO4 = 8/8/1). For the optimized salt concentration, the conductivity and viscosity dependence on the percent EC content of each electrolyte was measured at 25 °C. The maximum conductivity is observed for the EC/DMC/LiClO4 electrolyte at about 60% EC. The temperature dependence of conductivity and viscosity in the −30 ° to 60 °C range was also examined. The spectroscopic evidence for specific Li+ coordination is supported by quantum ch...

Ionic Interactions in Polymeric Electrolytes Based on Low Molecular Weight Poly(ethylene glycol)s

Abstract: It is shown that ionic conductivity of polymeric electrolytes based on low molecular weight amorphous polyglycols can be modified by the addition of α-Al2O3 fillers containing surface groups of the Lewis acid type. An enhancement of conductivity over pure PEG−LiClO4 electrolyte is observed for PEG−α-Al2O3−LiClO4 composite electrolytes containing from 0.5 to 3 mol/kg of the lithium salt. This increase in conductivity is coupled with the lowering of the viscosity of composite electrolytes and increasing chain flexibility when compared to the PEG−LiClO4 system as shown by rheological and DSC experiments. A decrease in the fraction of ionic aggregates is also seen from the FT-IR experiments for composite electrolyte in this salt concentration range. FT-IR studies of the C−O−C stretching mode has shown reduction in the transient cross-link density obtained after the addition of α-Al2O3 in the salt concentration range corresponding to the conductivity enhancement. The phenomena observed are explained in view of...

Theoretical interpretation of conductivity measurements of a thiotolane sandwich. a molecular scale electronic controller

Abstract: Quantum density functional theory and classical molecular dynamics studies of tolane molecules are carried out to interpret results of conductivity measurements on a monolayer of thiotolane molecules self-assembled on a gold surface and sandwiched by a titanium layer. Density functional theory techniques have been used to determine the ground state conformations and electronic structure, while classical molecular dynamics accounts for the effects of pressure and temperature for a cluster of five thiotolane molecules arranged between titanium and gold surfaces used to simulate the experimental system. On the basis of the theoretical results, it can be concluded, in agreement with the experimental findings, that the relative angle between two benzene rings in each tolane molecule determines its conductivity, with a maximum at 0° and a minimum at 90°. Therefore, this system would work as an unbiased controller, where the current through the molecule is controlled by the angle of one phenyl ring with respect ...