Showing papers on "Conductivity published in 1993"

Models proposed to explain the electrical conductivity of mixtures made of conductive and insulating materials

Abstract: The electrical conductivity of mixtures of conductive and insulating materials is reviewed In general, the conductivity of such mixtures increases drastically at a certain concentration of the conductive component, the so-called percolation concentration Among the parameters influencing the percolation concentration, the filler distribution, filler shape, filler/matrix interactions and the processing technique are the most important ones On the basis of these parameters, different models have been proposed aimed at the prediction of the conductivity or the percolation concentration It will be shown here that statistical, geometric or thermodynamic models explain the conductivity behaviour of specific mixtures on the basis of insufficient assumptions However, the conductivity seems to be predictable with the help of structure-oriented models

Rubbery solid electrolytes with dominant cationic transport and high ambient conductivity

Abstract: EFFORTS to develop a high-voltage, lightweight rechargeable battery for electrically powered vehicles have focused on batteries based on solid electrolytes, which offer good mechanical strength, easy packaging and high energy densities. An important objective is to identify an electrolyte with the desired combination of mechanical properties, electrical conductivity and stability against powerfully oxidizing and reducing electrodes (lithium is preferred for the anode). Among the most promising materials are rubbery 'salt-in-polymer' electrolytes and highly conducting but brittle superionic glass electrolytes. In the latter category are salts with good lithium-ion conductivity, which are compatible with lithium-anode systems. Here we describe new ionic conductors — 'polymer-in-salt' materials — in which lithium salts are mixed with small quantities of the polymers polypropylene oxide and polyethylene oxide. These materials have glass transitions low enough to remain rubbery at room temperature while preserving good lithium-ion conductivities and high electrochemical stability.

Unconventional charge gap formation in FeSi

Abstract: We report the first infrared and optical measurements of the absolute conductivity of FeSi Above 200 K the ac conductivity of FeSi resembles that of a dirty metal, while at low temperature most of the conductivity below 60 meV is depleted, consistent with an energy gap of that magnitude Thus in contrast to what is expected from a picture based on simple thermal activation, the gap disappears at a temperature which is low relative to its size Moreover, the spectral weight lost below the gap does not reappear just above the gap, but is instead distributed over an energy range of order eV

Mechanism of quasi‐one‐dimensional electronic conductivity in discotic liquid crystals

Abstract: Recently, it has been shown that a new class of quasi‐one‐dimensional conductors can be created by doping discotic liquid crystals with appropriate oxidants. This paper reports the elucidation of the mechanism of conduction in these new materials. In particular, the ac conductivity of 2,3,6,7,10,11‐hexahexyloxytriphenylene (HAT6) doped with the Lewis acid AlCl3, has been measured as a function of frequency (10−3–107 Hz), and temperature in its crystalline solid (K), hexagonal discotic liquid crystal (Dho), and isotropic liquid (I) phases. In all three phases the conductivity is independent of frequency at low frequencies, but shows a power law dependence on frequency [σ(ω)∼ωs, s∼0.7–0.8] at higher frequencies. This behavior is characteristic of charge carrier transport by a hopping mechanism. The conductivity data have been analyzed in terms of the Scher and Lax theory to obtain the parameters describing this process. In macroscopically aligned K and Dho phases, the conductivity measured along the column ...

The Electrical Properties of Ceramic Electrolytes for LiM x Ti2 − x ( PO 4 ) 3 + yLi2 O , M = Ge , Sn , Hf , and Zr Systems

Abstract: The electrical properties of systems of , were examined in detail. The conductivity and the sinterability increased with the amount of excess lithium oxide in the phosphate. The secondary phase acts as a flux to accelerate the sintering process and to obtain high conductivity grain boundaries. The conductivity decreased and the activation energy of the bulk component for Li+ migration increased by the partial substitution of Tr4+for M4+ in systems of . A minimum activation energy of 0.28–0.30 eV, was obtained for the sample with ca. 1310 A3 in the cell volume. has the most suitable tunnel size for a Li+ migration through the NASICON‐type network structure.

Effects of oxygen and illumination on the in situ conductivity of C60 thin films.

Abstract: We report in situ measurements of dc conductivity versus temperature in polycrystalline films of C 60 . Films sublimed using C 60 powders from different batches synthesized according to standard techniques were used in conductivity versus temperature measurements in the range 20-180 o C. The films showed clear semiconductor behavior. Activation energies E a and room-temperature dark conductivities a were in the range 0.54-0.58 eV and 10 -6 -10 -7 (Ωcm) -1 , respectively

Li Ion Conductive Electrolytes Based on Poly(vinyl chloride)

Abstract: New Li ion conductive solid electrolytes with a conductivily of about 10 -3 Ω -1 .cm -1 at room temperature have been prepared. Li ion conductivity in the electrolytes has been demonstrated with the cycling of Li/LiMn 2 O 4 and C/LiMn 2 O 4 solid-state cells

Contamination indications derived from electrical properties in the low frequency range1

Abstract: Electrical measurements are an important and integrated component of geophysical investigations connected with environmental problems. As a result of an analysis of the electrical conductivity, basic experiments on sandstones at frequencies below 10 kHz show that the complex behavior of conductivity is caused exclusively by a complex interface conductivity. Its value is determined mainly by the internal rock interface to porosity ratio, the composition of the pore fluid and connected matrix-water interactions resulting in a specific microstructure of the interface. Therefore, it can be expected that the interface region of a soil or rock material is very sensitive to changes in composition caused by contamination. Contaminated sandstone and clay samples were investigated using a low-frequency measurement system. The investigations are directed at the influence of different contaminants and their concentration. Results show that the complex electrical conductivity (real and imaginary parts) is influenced by properties of the pore-filling contaminant. This influence results in a change of the level of both parts and the shape of their frequency dependence. The imaginary part in particular seems to provide important secondary information; in some cases this part alone allows a differentiation of the various contaminants. The different behavior of various rock types showsmore » that the effects observed are the result of interactions between pore fluid properties and the internal pore surface structure.« less

Sulfonated polymers for solid polymer electrolytes

Abstract: Sulfonated polyphenylene solid polymer electrolytes having high proton conductivity are provided, which are stable and which maintain high conductivity at temperatures of 100° C. and above. These membranes are useful as solid polymer electrolytes for fuel cells and can be operated above the boiling point of water.

Universal low-temperature ac conductivity of macroscopically disordered nonmetals

Abstract: This paper discusses a macroscopic model for ac conduction in electronically or ionically conducting disordered solids. The model considers ac conduction in an inhomogeneous solid that is characterized by a spatially randomly varying thermally activated (frequency-independent) conductivity. Discretizing Maxwell's equations leads to an equivalent electrical circuit that is a simple-cubic lattice where each pair of nodes are linked by a resistor and a capacitor in parallel. The values of the resistors are determined by the local resistivity while the capacitors are all equal, given by the infinite-frequency dielectric constant. It is shown that the capacitor currents are Maxwell's displacement currents

Electron mobility in single- and polycrystalline Ga2O3

Abstract: Ga2O3 is a purely n‐conducting metal oxide at high temperatures. Increasing interest is being shown in this material as the basic material for gas sensors since its electrical conductivity is gas dependent. To investigate its electrical conductivity mechanism in the temperature range between 800 and 1000 °C, conductivity and Hall measurements were performed on single crystals and on polycrystalline ceramics. In the conductivity measurements, identical results were obtained with dc and low‐frequency ac, thus confirming the notion of purely electron current transport. A thermally activated specific conductivity is observed in both cases, that of the single crystals lying around three and a half orders of magnitude above that of the ceramic. The carrier densities are determined from the Hall measurements, a thermally activated behavior always being observed. The carrier density of the ceramic is only 2×1013 cm−3 at 1000 °C, that of the single crystals lying somewhat more than three orders of magnitude above ...

Electrical conduction and polaron mobility in Fe-bearing olivine

Abstract: SUMMARY Electrical conductivity u of Mg,(, _Y)Fe2YSi04 olivines was measured as a function of Fe content (Y = 0.09, 0.1 1, 0.18, 0.34) at temperatures between 1423-1573 K and at oxygen fugacities slightly below the wustite-magnetite buffer. Conductivity was proportional to Xk," for the conditions examined. In accord with current and previous experiments and defect models at these conditions, conductivity is dominated by small polaron hopping of holes from Fe'+ to Fe2' on the Mg sublattice. From quantitative equilibrium defect chemistry calculations, we find that the Fe" concentration is proportional to XFz3. From measured conductivity and calculated defect concentrations, the hopping mobility is proportional to XbL and is in the range (1-6) X cm2 V-' s-'. This 1.1 power dependence on Fe content for mobility is close to the value of -1.0 independently deduced from polaron hopping theory for mixed valence solid solution systems. Therefore, small polaron conduction self-consistently explains the observed 1.8 power dependence of u on Fe content. Using the measured dependence of conductivity on Fe content, a change in composition from Fa,(, to Fal2 produces a conductivity increase by a factor of 1.39; near 1473K such a conductivity change could be misinterpreted as an equivalent temperature increase of 52 K in the upper mantle.

Electrical conductivity measurements using microfabricated interdigitated electrodes

Abstract: Microfabricated interdigitated electrode arrays were constructed and evaluated for use as miniature conductivity cells. The arrays were 2×3 mm in size and had digits ranging in size from 10 to 80 μm. The complex electrical impedance of the devices was measured at frequencies spanning from 100 to 100 kHz in solutions with resistivities ranging from 10 to 67 000 Ω/cm. An electromagnetic field model of the interdigitated electrode geometry was used to calculate cell constants. The resulting values ranged from 0.07 to 0.55 cm -1 , and they agreed well with the experimental data. The frequency dependence of the complex electrical impedance could be represented by an equivalent circuit incorporating an interfacial impedance consisting of a double-layer capacitance in series with a Warburg-type diffusion impedance

The effective thermal conductivity of composites with coated reinforcement and the application to imperfect interfaces

Abstract: An analytical model is developed to predict the effective thermal conductivity of multiphase composites reinforced with coated fillers. The coated fillers are modeled as confocal spheroids, thus enabling the simulation of a wide range of reinforcement geometry ranging from thin flake to continuous fiber. The orientation of the coated fillers is described by a density distribution function which can simulate completely random, in‐plane random, and aligned distributions as special cases. The analytical approach appears to be the only one in the literature that renders closed‐form predictions of the effective thermal conductivity of composites with misoriented coated reinforcement and thus recourse to a numerical scheme is not required. The analytical approach is specialized to consider the effects of a thermal resistance at the filler‐matrix interface of a composite. Results of the proposed model are compared with those obtained by self‐consistent and differential estimates [Y. Benveniste and T. Miloh, J. Appl. Phys. 69, 1337 (1991)] for the effective thermal conductivity of a composite reinforced by completely random coated short fibers. The proposed model is in close agreement with the other two models for low conductivity coatings, but differs appreciably for high conductivity coatings. Finally, good agreement is shown to exist between analytical predictions and experimental results obtained from the literature for a diamond particle/zinc‐sulfide matrix composite with an interfacial thermal resistance.

Electrical conductivity of annealed polyaniline

Abstract: The electrical conductivity of chemically synthesized and annealed polyaniline has been investigated. The room‐temperature conductivity varies with the time and the temperature of annealing, and exhibits a maximum peak on its time‐dependence curves for the polymer annealed above 120 °C. The time showing the maximum peak of the conductivity shifts to the shorter side with higher annealing temperature. The linear dependence of the conductivity on temperature with T−1/2 is observed in all samples as explained for this polymer with the quasi‐one‐dimensional variable‐range‐hopping processes. In the initial stage of annealing, the conductivity increases in the whole temperature range without varying the exponential factor of the variable range hopping. This indicates that the density of states remains unchanged in this stage. The results concerning the pre‐exponential factor of the quasi‐one‐dimensional variable‐range‐hopping mechanism demonstrate the increase of the mean free time, the transverse localization length, and the interchain transfer integral with annealing. X‐ray‐diffraction patterns exhibit the generation of regularized amorphous structure in annealed samples, which leads to the enlargement of the interchain interaction. The results show the increase of regularity of the polymer chain at first, and the phase transition to the loose‐packed and ordered structure in next stage of annealing.

Electrical Conductivity and Defect Structure of Calcium‐Doped Lanthanum Chromites

Abstract: Electrical conductivity of calcium-doped lanthanum chromites, La 1-x Ca x CrO 3-δ , was determined as a function of composition, temperature, and oxygen partial pressure, P o2 , to determine its defect structure and understand its redox behavior. The conductivity was independent of P o2 and was proportional to the dopant concentration at high P o2 . The activation energy of conductivity was 0.12 to 0.14 eV and the mobility was 0.066 to 0.075 cm 2 /V/s in the temperature range of 900 to 1050 o C, which was ascribable to small-polaron hopping. Under reducing conditions, the conductivity decreased exponentially with decreasing P o2 and asymptotically approached a P 02 1/4 relationship

Plasma-polymerized C60/C70 mixture films : electric conductivity and structure

Abstract: Polymerization of a mixture of C60/C70 in rf plasma is reported. The electric dark‐current conductivity of the plasma‐polymerized mixture of C60/C70, which is approximately 10−7 S/cm in the atmosphere, does not depend on the applied voltage at least in the range of −25 to 25 V. A semiconductor‐type temperature dependence of the conductivity in the higher‐temperature domain was observed, and the band‐gap energy was estimated to be 2.1 eV. The conductivity increased with increasing temperature from 25 to 230 °C in vacuum, whereas in the atmosphere the conductivity increased upon decreasing the temperature below 80 °C. It is supposed that in this temperature domain the electric conductivity is enhanced by the existence of water molecules on the film, the surface of which is characterized by a high hydrophilicity. The surface morphology of the polymerized film was characterized by the presence of aggregates with diameters of about 300 A and the surface was highly hydrophilic, polar, and heterogeneous. The surface has a completely amorphous molecular structure. When the C60 molecules polymerize each other, it has been shown that the polymerization of C60 molecules proceeds, at least primarily, by the formation of 1,2‐cyclobutane structures between the cyclohexatrienyl parts of neighboring C60 molecules. The addition of hydrogens, hydroxyl groups, carbonyl groups, and aryl peroxide groups to the radical sites occurs but the free radicals of the order of magnitude 1017 spins/g were found to remain without quenching.