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Showing papers on "Conductivity published in 1996"


Journal ArticleDOI
TL;DR: In this article, the proton conductivity of Nafion 117 was measured under various conditions of humidity and temperature using a four-electrode ac impedance method, and it varied strongly with the humidity and heat-treatment of the membrane.
Abstract: Nafion 117{reg_sign} is a proton conducting ion-exchange membrane which is now receiving much attention due to its use as an electrolyte in the polymer-electrolyte-membrane fuel cell (PEMFC), which is of great interest for electric vehicle propulsion. The proton conductivity of Nafion 117 was measured under various conditions of humidity and temperature using a four-electrode ac impedance method. The conductivity of this membrane without heat-treatment was ca. 7.8 {times} 10{sup {minus}2} S/cm at ambient temperature and 100% relative humidity; it varied strongly with the humidity and heat-treatment of the membrane. After heat-treatment, the membrane showed a slight dependence of conductivity on temperature. From 21 to 45 C, its conductivity at a given relative humidity decreased with increasing temperature, while from 45 to 80 C it increased with temperature.

851 citations


Journal ArticleDOI
TL;DR: In this paper, the ac impedance spectra of polymer electrolyte fuel cell (PEFC) cathodes measured under various experimental conditions are analyzed and compared by simultaneous least squares fitting of a set of spectra measured for several cathode potentials.
Abstract: The ac impedance spectra of polymer electrolyte fuel cell (PEFC) cathodes measured under various experimental conditions are analyzed. The measurements were carried out in the presence of large dc currents. The impedance spectrum of the air cathode is shown to contain two features : a higher frequency loop or are determined by interfacial charge-transfer resistance and catalyst layer properties and a lower frequency loop determined by gas-phase transport limitations in the backing. The lower frequency loop is absent from the spectrum of cathodes operating on pure oxygen. Properties of measured impedance spectra are analyzed by a PEFC model to probe the effect of ac perturbation. Comparison of model predictions to observed data is made by simultaneous least squares fitting of a set of spectra measured for several cathode potentials. The spectra reveal various charge and mass-transfer effects in the cathode catalyst layer and in the hydrophobic cathode backing. Three different types of losses caused by insufficient cell hydration, having to do with interfacial kinetics, catalyst layer proton conductivity, and membrane conductivity, are clearly resolved in these impedance spectra. The data reveal that the effective tortuous path length for gas diffusion in the cathode backing is about 2.6 times the backing thickness.

685 citations


Journal ArticleDOI
TL;DR: In this paper, a mixed oxide (Zn1−x Al x )O exhibits promising thermoelectric properties attaining a dimensionless figure of merit ZT of 0.30 at 1000°C, which value is much superior to other oxides and quite comparable to conventional state-of-the-art thermolectricmaterials.
Abstract: A mixed oxide (Zn1−x Al x )O exhibits promising thermoelectricproperties attaining a dimensionless figure of merit ZT of 0.30 at 1000 °C, which value is much superior to other oxides and quite comparable to conventional state‐of‐the‐art thermoelectricmaterials. The addition of a small amount of Al2O3 to ZnO results in a large power factor of 10–15×10−4 W/mK2, showing a marked increase in the electrical conductivity while retaining moderate thermoelectric power. A large product of the carrier mobility and density of states would be responsible for the favorable electrical properties of the present oxide. A figure of merit Z=0.24×10−3 K−1 is attained by (Zn0.98Al0.02)O at 1000 °C, even with a high thermal conductivity. A predominant proportion of the phononthermal conductivity promises a further improvement in the thermoelectric performance by selective enhancement of phonon scattering.

563 citations


Journal ArticleDOI
TL;DR: In this paper, the structural properties, thermal expansion, electrical conductivity, thermodynamic stability, and surface properties of bismuth oxide and solid solutions with face centred cubic, rhombohedral, tetragonal or layer structures are reviewed.

554 citations


Journal ArticleDOI
R. D. Gould1
TL;DR: In this article, the structure and electrical conduction properties of evaporated phthalocyanine thin films are critically reviewed, and a summary of the various dc conduction processes observed is given, together with the relevant theoretical conductivity equations.

299 citations


Journal ArticleDOI
TL;DR: In this paper, self-assembled monolayers of long chain alkanes deposited on silicon wafers using an optimally designed procedure exhibit very large energy barriers to carrier tunneling and demonstrate that the tunneling contribution to the overall conductivity can be made negligible when the organic structure is densely packed and highly ordered.
Abstract: Self-assembled monolayers of long chain alkanes deposited on silicon wafers using an optimally designed procedure exhibit very large energy barriers ( $\ensuremath{\sim}45$ eV) to carrier tunneling The dc conductivity is found to be $\ensuremath{\sim}46\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ S/cm, close to that of bulk polyethylene, and independent of monolayer thickness This demonstrates that the tunneling contribution to the overall conductivity can be made negligible when the organic structure is densely packed and highly ordered Distinctly higher conductivity values are observed with disordered monolayers

282 citations


Journal ArticleDOI
TL;DR: In this article, the effects of microstructure, atmosphere, and several dopants on the electrical conductivity of bismuth titanate (Bi4Ti3O12, BIT) were investigated.
Abstract: A study was conducted on the effects of microstructure, atmosphere, and several dopants on the electrical conductivity of bismuth titanate (Bi4Ti3O12, BIT), Increased grain size increased the conductivity in undoped BIT as did acceptor dopants that substituted for either Bi (Ca and Sr) or Ti (Fe), A donor dopant (Nb) decreased the conductivity in BIT by as much as 3 orders of magnitude, The increased resistivity of Nb-doped BIT improved the polarization in an electric field, A piezoelectric coefficient, d(33), of 20.0 pC/N was achieved with a Nb-doped BIT composition corresponding to Bi4Ti2.86Nb0.14O12.

281 citations


Journal ArticleDOI
TL;DR: In this paper, Nafion samples were loaded with fuel-cell grade platinum black and heated under atmospheres of nitrogen, 5% hydrogen, or air in a thermal gravimetric analyzer and the products of decomposition were taken directly into a mass spectrometer for identification.
Abstract: Nafion{reg_sign} is an important polymer electrolyte for polymer electrolyte fuel cell applications due to its inertness and high proton conductivity. Operation of these fuel cells for extended periods of time at temperatures approaching 100 C introduces concerns of the thermal stability of the Nafion electrolyte. To simulate the conditions in a fuel cell, Nafion samples were loaded with fuel-cell grade platinum black and heated under atmospheres of nitrogen, 5% hydrogen, or air in a thermal gravimetric analyzer. The products of decomposition were taken directly into a mass spectrometer for identification. In all cases, Nafion was found to be thermally stable up to 280 C, at which temperature the sulfonic acid groups began to decompose. A mechanism for the decomposition is proposed which explains many of the evolved compounds observed during heating.

274 citations


Journal ArticleDOI
TL;DR: In this article, the ionic conductivity and electrochemical intercalation properties of La 2 3−x Li 3 x TiO 3 solid solutions (for 0.07 ≤ x ≤ 0.13) have been studied.

271 citations


Journal ArticleDOI
TL;DR: In this article, a working hypothesis to find wide gap oxide semiconductors was proposed on the basis of simple considerations, which predicts that amorphous double oxides composed of heavy metal cations (HMCs) with an electronic configuration (n − 1)d10s0 are promising candidates for a novel class of ammorphous semiconductor.
Abstract: A working hypothesis to find wide gap oxide semiconductors was proposed on the basis of simple considerations. The hypothesis predicts that amorphous double oxides composed of heavy metal cations (HMCs) with an electronic configuration (n − 1)d10s0 are promising candidates for a novel class of amorphous semiconductors. Electrical and optical properties of three amorphous double oxides composed of the HMCs, a-AgSbO3, Cd2GeO4 and Cd2PbO4, were examined, following this hypothesis. It was found that when carrier electrons are generated via the formation of oxygen vacancies or doping of excess cations by ion implantation, these three wide band gap amorphous oxides show high electrical conductivities of 10−1 to 102 S cm−1 at ∼ 300 K and the conductivity remains almost constant down to 77 or 4 K for high carrier concentrations (> 1018 cm−3). This high conductivity originates primarily from a large Hall mobility of ∼ 10 cm2 V−1 s−1, which is higher by several orders of magnitude than that in amorphous transition metal oxides, Si:H and chalcogenides. A variety of chemical compositions for a novel oxide amorphous semiconductor are suggested.

249 citations


Journal ArticleDOI
TL;DR: In this article, an overview of the main factors controlling the performance of the solid oxide fuel cell (SOFC) electrodes, emphasizing the most widely chosen anodes and cathodes, Ni-YSZ and LSM-YSZ.

Journal ArticleDOI
TL;DR: In this paper, electrical conductivity measurements on cement composites containing carbon fibres or steel fibres were conducted, and percolation phenomena associated with electrical conductivities were observed.
Abstract: Electrical conductivity measurements on cement composites containing carbon fibres or steel fibres were conducted. Percolation phenomena associated with electrical conductivity were observed. The conductivity of the systems studied increased by several orders of magnitude, at a specific concentration of conductive fibre, i.e. the percolation concentration. The percolation concentration is shown to be dependent on conductive fibre geometry instead of system composition. The results provide an important guide for the manufacture of conductive cement composites containing conductive fibres.

Journal ArticleDOI
TL;DR: A new technique for separating bioparticles from suspended mixtures is demonstrated for pure suspensions and mixtures of the bacteria Bacillus subtilis, Escherichia coli and Micrococcus luteus.

Journal ArticleDOI
TL;DR: In this article, the electrical conductivity of Sr1−xLaxTiO3 ceramics (x ≥ 0.4) and single crystals (x≤ 0.1) was investigated in the temperature range between 19 and 1673 K.
Abstract: The electrical conductivity of Sr1−xLaxTiO3 ceramics (x≤0.4) and single crystals (x≤0.1) was investigated in the temperature range between 19 and 1673 K. The mobility was calculated from the carrier concentration, n, which was determined by Hall measurements as well as by a chemical Ti3+ titration. The mobility of the single crystals agrees well with a model originally developed for undoped strontium titanate. At low T and high n scattering by ionized donor centers predominates. Above room‐temperature phonon scattering becomes predominant. Sr1−xLaxTiO3 ceramics follow the same model, but only if they had been previously reduced in water‐free and oxygen‐free hydrogen atmospheres. The behavior during the reducing and cooling process can be explained by a defect chemical model, using a set of constants developed in a former work. However, reduced in water saturated hydrogen the conductivity at low T decreases by decades compared to single‐crystal data and cannot be explained anymore by the above‐mentioned models. In conjunction with impedance spectroscopy experiments it was found, that in this case the electrical behavior of the ceramics is completely governed by grain‐boundary phenomena. These high‐ohmic grain boundaries may be depletion layers, which are formed or annihilated depending on the conditions of the preceding high‐temperature process.

Journal ArticleDOI
01 Jul 1996-Polymer
TL;DR: In this article, the optimum polymerization temperature to give a polymer with the highest yield and molecular weight has been determined, and solution state 13C n.m.r. spectroscopy shows that the improved polyaniline has fewer defect sites compared to the standard polymer.

Journal ArticleDOI
TL;DR: Using conductivity and density data for different host glasses mixed with various metal-halide salts, a remarkable common cubic scaling relation is found between the conductivity enhancement and the expansion of the network forming units induced by salt doping, suggesting that the glass network expansion is a key parameter determining the increase of the high ionic conductivity in this type of fast ion conducting glasses.
Abstract: It is well known that fast ionic conductivity can be obtained by doping modified oxide glasses with a metal-halide salt. The role of the dopant salt, apart from providing additional charge carriers, for the ionic conductivity has been a much debated issue. Using conductivity and density data for different host glasses mixed with various metal-halide salts, we find a remarkable common cubic scaling relation between the conductivity enhancement and the expansion of the network forming units induced by salt doping. This suggests that the glass network expansion, which is related to the available free volume, is a key parameter determining the increase of the high ionic conductivity in this type of fast ion conducting glasses. [S0031-9007(96)01369-5] High room temperature ionic conductivity in solid materials is technologically interesting for various solid state electrochemical devices, e.g., batteries, “smart windows”. Apart from some exotic crystalline materials such as Rb4AgI5, the highest ionic conductivity at room temperature has been observed in some salt doped oxide, sulfur, and halide glasses. The salt doped oxide glasses are particularly interesting for applications because of their ease of preparation, their stability, and the large available composition ranges, and have also become model materials for investigations of diffusion in disordered solids.

Journal ArticleDOI
TL;DR: In this article, the conductivity behavior of a quaternary water-in-oil microemulsion of a cationic surfactant (cetyltrimethylammonium bromide, CTAB), n-hexane, water, and n-pentanol has been investigated using conductivity, quasi-elastic light scattering, near-infrared absorption spectroscopy, and pulsed field gradient spin-echo NMR measurements.
Abstract: Quaternary water-in-oil microemulsion of a cationic surfactant (cetyltrimethylammonium bromide, CTAB), n-hexane, water, and n-pentanol has been investigated using conductivity, quasi-elastic light scattering, near-infrared absorption spectroscopy, and pulsed field gradient spin-echo NMR measurements The conductivity behavior shows features characteristic of the migration of charged droplets Consequently, using the charge fluctuation model, the conductivity data were correlated with the droplet radius obtained from self-diffusion coefficients for different obstruction factors Conductivity and self-diffusion measurements were found to be self-consistent for spherical droplets with hard-sphere interactions Comparison between collective diffusion and self-diffusion coefficients fully supports this conclusion The average head-group area of CTAB, the amount of water free in the organic bulk, and the fraction of alcohol present into the aggregates were evaluated together with the thickness of both the inter

Journal ArticleDOI
TL;DR: In this paper, a defect model was proposed to elucidate the oxygen partial pressure dependence of the measured conductivity and the reported oxygen nonstoichiometry, which was successfully described by a diffusion model with consideration of partial control by surface reaction.

Journal ArticleDOI
TL;DR: In this paper, the effective conductivity of composites containing aligned spheroids of finite conductivity is determined with the pair interaction rigorously taken into account, and the results are in the form of virial expansion in the inclusion volume fraction f, truncated at the O(f2) term.
Abstract: The effective conductivity of composites containing aligned spheroids of finite conductivity is determined with the pair interaction rigorously taken into account. The pair interaction is evaluated by solving a boundary‐value problem involving two aligned spheroids with a boundary collocation scheme. Our results are in the form of virial expansion in the inclusion volume fraction f, truncated at the O(f2) term. Effective conductivities obtained from this study agree well with Willis’s [J. Mech. Phys. Solids 25, 185 (1977)] bounds and Kim and Torquato’s [J. Appl. Phys. 74, 1844 (1993)] first‐passage‐time simulation results. An expression for the effective conductivity tensor based on an equivalent inclusion method originated by Hasselman and Johnson [J. Compos. Mater. 21, 508 (1987)] is also derived, from which a criterion for testing the virial expansion validity is constructed. The relevant series ratio is found to be a good measure of intensity of the system thermal interaction. The smaller the value of the series ratio, the better agreement of the virial expansion with the simulation results. Explicit expressions for the second‐order virial expansion for the parallel and perpendicular components of the effective conductivity tensor are tabulated for inclusion aspect ratios 1, 10/9, 2, 5, 10 and phase conductivity ratios σ=0, 0.1, 2, 10, 100, 10 000, and ∞.

Journal ArticleDOI
TL;DR: In this article, a review of the recent progress in the study of liquid foams is presented, which further improves upon the approximation recently proposed by us, in accounting for the nonlinearity in the dependence of conductivity on liquid fraction.
Abstract: Some recent progress in the study of liquid foams is reviewed in outline. Calculations of foam conductivity are presented, which further improve upon the approximation recently proposed by us, in accounting for the nonlinearity in the dependence of conductivity on liquid fraction.

Journal ArticleDOI
TL;DR: In this paper, the percolation behavior of the conductivity of composite was investigated and the degree of polymer-filler interaction was found to be a determinant of conductivity.
Abstract: Conductive composites based on PE, PP, PS, PMMA and PA filled with carbon black have been studied. It was found that percolation behavior of the conductivity depended on the degree of polymer-filler interaction. Equations taking into account the polymer-filler interactions were proposed to describe the conductivity of composite above and below the percolation threshold.

Journal ArticleDOI
TL;DR: In this paper, the concept of reactive codoping was introduced to achieve p-type room temperature conductivities as high as 50/Ω cm in cubic GaN layers by using Be as the acceptor species and O as the reactive donor.
Abstract: P‐type room‐temperature conductivities as high as 50/Ω cm are achieved in cubic GaN layers by the concept of reactive codoping. We use Be as the acceptor species and O as the reactive donor to render isolated Coulomb scatterers into dipole scatterers. This concept allows us to achieve high hole mobilities and thus p‐type conductivities.

Journal ArticleDOI
TL;DR: In this paper, the effect of pH, temperature, monomer/oxidant molar ratio, catalysts and ionic strength on the reaction yield and electrical conductivity of chemically synthesized poly (o-methoxyaniline) was studied.

Journal ArticleDOI
TL;DR: In this paper, LiMn2O4 films were prepared by RF magnetron sputtering on interdigitated microarray electrodes and in situ conductivity-potential profiles and cyclic voltammograms during extraction/insertion processes of Li ions were obtained simultaneously in nonaqueous and aqueous electrolyte solutions (1M LiClO4/propylene carbonate and 1 M LiCl/water).
Abstract: Thin films of LiMn2O4 have been prepared by RF magnetron sputtering on interdigitated microarray electrodes. In situ conductivity–potential profiles and cyclic voltammograms during extraction/insertion processes of Li ions were obtained simultaneously in nonaqueous and aqueous electrolyte solutions (1 M LiClO4/propylene carbonate and 1 M LiCl/water). The electronic conductivity of Li1–xMn2O4 was found not to show metallic transition and maintain its semiconducting state during the extraction/insertion of Li ion. A slight decrease in conductivity was observed with increasing the anodic potential, i.e., with increasing x (lithium extraction) and recovered reversibly when the potential returned to the cathodic side (re-insertion of Li ions). Similar results were obtained in both aqueous and nonaqueous electrolyte solutions.

Journal ArticleDOI
TL;DR: In this article, the effect of temperature on the plane-wave SE of a lightly doped (0.006 M p-TS) polypyrrole (PPy) film was also investigated.

Journal ArticleDOI
TL;DR: In this article, the conductivity of graphite platelets with diameter-to-thickness ratios of the order of 100 was characterized by X-ray diffraction and electron microscopy: orientation of filler and porosity of the polymer matrix were observed.

Journal ArticleDOI
TL;DR: In this paper, the dielectric properties of a series of smectite clays, namely, Na+, Li + and Cu ++ -fluorohectorites, Na + montmorillonite and Na + laponite have been measured as a function of water content over the frequency range 40−10 5 Hz.

Journal ArticleDOI
TL;DR: In this paper, the electrical conductivity of polyaniline films prepared via the camphorsulphonic acid (CSA)m-cresol solution processing route has been synthesized with doping levels in the range of 10 - 90%.
Abstract: Polyaniline films prepared via the camphorsulphonic acid (CSA)m-cresol solution processing route have been synthesized with doping levels in the range of 10 - 90%. The electrical conductivity of these films has been measured as a function of temperature between 10 and 300 K. At a doping level of 30% the onset of metallic transport is observed, and at 60% the films are found to exhibit metallic transport down to 135 K, and have a maximum room-temperature conductivity of . The results are modelled in terms of a heterogeneous model of fluctuation induced tunnelling (FIT) and metallic transport.

Journal ArticleDOI
TL;DR: It has been shown, that in the range of relative humidity values (0-100%), changes in the hydration state of melanin have predominant influence on electrical conductivity in comparison to temperature.
Abstract: The dependence of the d.c. conductivity on the hydration and temperature (293-343 K) for synthetic DOPA-melanin polymer is presented. The hydration state of the melanin has been changed by varying the humidity conditions around the sample. It has been shown, that in the range of relative humidity values (0-100%), changes in the hydration state of melanin have predominant influence on electrical conductivity (10-13-10 -5S cm-1) in comparison to temperature. The influence of the two forms of water on the conductivity-the first form adsorbed mainly on the melanin surface and easily removed by drying, and the second one incorporated into the inner structure of the polymer, has been investigated. The temperature dependence of the conductivity in vacuum (0.8 and 0.04 mb) and thermal activation energy values (0.49-0.76 eV) for cooling and heating curves have been determined. The relationship between thermal activation energy and preexponential factor σ0 (compensation effect) and possible charge transport mechani...

Journal ArticleDOI
TL;DR: In this paper, the transport, thermal and structural properties of composite solid electrolytes (1 − x )CsHSO 4 xSiO 2 (where x = 0-0.8) were investigated.