Showing papers on "Conductivity published in 1989"

Lattice Parameters, Ionic Conductivities, and Solubility Limits in Fluorite-Structure MO2 Oxide [M = Hf4+, Zr4+, Ce4+, Th4+, U4+] Solid Solutions

Abstract: Changes in the lattice parameters of fluorite type MO2 oxides (M = Hf4+, Zr4+, Ce4+, Th4+, U4+) due to the formation of solid solutions can be predicted by proposed empirical equations. The equations show the generalized relationship between dopant size and ionic conductivity in the binary systems of these oxides, illustrating that the smaller the difference between the dopant ionic radius and the critical dopant radius, the higher the conductivity. The solubility limit of the same periodic group elements in fluorite-structure MO2 oxides decreaes linearly with the square of Vegard's slope for each solute as determined from the proposed equations.

Magnetoresistance measurements on the magnetic semiconductor Nd0.5Pb0.5MnO3

Abstract: We present magnetoresistance data up to 20 T and magnetisation measurements on the mixed-valence cubic perovskite semiconductor Nd 05 Pb 05 MnO 3 , which demonstrate that the material orders ferromagnetically below 184 K, and that around and above this temperature the conductivity is dominated by hopping of localized magnetic polarons, with an activation energy ∼95 meV

Grain-boundary graphite in rocks and implications for high electrical conductivity in the lower crust

Abstract: THE origin of zones of high electrical conductivity in the lower continental crust is a long-standing mystery; possible explanations include the presence of brines1,2, partial melt3, serpentine4 and graphite5. When discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6-10. Here we show that a fine film of graphite is present on the grain boundaries in three rocks from the Laramie Anorthosite Complex. In two of these rocks graphite was stable during igneous crystallization but in the other it was not. We maintain that in all of the rocks the grain-boundary graphite precipitated from a CO2-rich fluid during cooling. The chemical processes that produced the grain-boundary graphite in these rocks are likely to operate in many lower-crustal rocks. We therefore contend that, because the films we observe are capable of producing the high conductivity that is seen in the lower crust, grain-boundary graphite should be considered as a possible cause for some conductivity anomalies.

Electrical Conductivity and Seebeck Coefficient of Nonstoichiometric La1 − x Sr x CoO3 − δ

Abstract: The conductivity and Seebeck coefficient of the perovskite-type oxides La 1−x Sr x CoO 3− δ (x=0-0.7) were measured in 10 −5 -1 atm O 2 gas at temperatures 25 o -1000 o C. The results are discussed in relation to the lattice-parameter and oxygen nonstoichiometry

Electrical conductivity of the earth's lower mantle

Abstract: ELECTRICAL conductivity is an important physical property of the Earth's mantle because it controls the transmission of geomagnetic signals from the core to the surface. The lower mantle, from a depth of 670 km down to the core–mantle boundary (2,990 km), is probably composed of (Mg,Fe)SiO3 perovskite and magnesiowustite, (Mg,Fe)O. Analysis of the transient and secular variations of the geomagnetic field yields values of the lower-mantle conductivity of the order of 1 S m−1 at a depth of 1,000 km, increasing to ∼100 S m−1 at the core–mantle boundary1,2. Information about the physical mechanism of the conductivity and its dependence on temperature and pressure would help to constrain the temperature profile in the Earth. In the only study published so far, Li and Jeanloz3,4 reported values of the conductivity of the silicate perovskite and of a mixture of perovskite and magnesio-wiistite lower than 10−3 S m−1 and concluded that the lower mantle is an insulator, thus casting doubt on the geomagnetic results. We report here the results of measurements of the d.c. electrical conductivity of a mixture of perovskite and magnesiowiistite resulting from disproportionation of olivine, and of pure perovskite. The measurements were made in an externally heated diamond-anvil cell at pressures of ∼40 GPa and temperatures from 25 °C to ∼400 °C. Conductivity increases with increasing iron content, increasing temperature and increasing pressure. The activation energy (0.35 eV for 11% Fe) decreases with increasing iron content. The results are compatible with an electron-hopping conduction mechanism. Extrapolation to the temperature appropriate for a depth of 1,100 km, which corresponds to the pressure of our experiments, yields a conductivity of the lower mantle of the order of 1 S m−1; extrapolation to the temperature and pressure of the core-mantle boundary yields values between 50 and 100 S m−1, in agreement with geomagnetic determinations.

Electrical conduction in olivine

Abstract: This paper reports detailed measurements of electrical conductivity σ and thermoelectric effect S in the mineral olivine and in synthetic forsterite as functions of temperature in the range from 1000° to 1500°C and oxygen partial pressure in the range from 10−10 to 104 Pa. The two most striking observations are strong conductivity anisotropy in forsterite and a sign change in S in olivine at 1390°C. These results are interpreted to show that both materials have mixed ionic and extrinsic electronic conduction under these conditions. On the basis of these interpretations, we infer that forsterite conductivity is dominated by electronic conduction in the a and b directions and probably by movement involving magnesium vacancies in the c direction, where far higher, PO2-independent conductivity is observed. Olivine appears to show mixed conduction under all the circumstances observed; at low temperatures, electron holes dominate but are superseded by magnesium vacancies at high temperatures.

Thermoelectric power and conductivity of heterogeneous conducting polymers

Abstract: We discuss the interpretation of thermoelectric power in heterogeneous media, and using a compilation of many sets of data, we analyze the thermopower of conducting polymers as a function of conductivity and of temperature. For samples of very high conductivity, the thermopower (but not the conductivity) shows typical metallic temperature dependence, which is consistent with a heterogeneous model of metallic fibrils separated by thin electrical barriers. Metallic thermopower is expected rather generally to show significant nonlinearities as a function of temperature, and we demonstrate that the observed thermopower in some highly conducting polymers is very similar to the diffusion thermopower of metals in which a knee is produced at low temperatures by the electron-phonon interaction. The thermopower of moderately doped conducting polymers, like their conductivity, is generally consistent with a significant contribution from variable-range hopping.

Conductivity and conjugation length in poly(3-methylthiophene) thin films

Abstract: Evolution de la morphologie et de la conductivite a differentes etapes de la polymerisation electrolytique

Quantification of frost damage in plant tissues by rates of electrolyte leakage.

Abstract: A method for assessing frost hardiness of plant tissues [using shoots of Picea rubens Sarg. syn P. rubra (Du Roi) Link] has been developed based upon the rate of electrolyte leakage from shoots immersed in distilled water after exposure to a range of freezing treatments. The relationship between conductivity (the electrolyte concentration in solution) and time has been shown to follow an asymptotic curve, which may be represented by a first-order equation: Ct - Co = (Cauto - Co) (1 - e-kt) where Ct is the conductivity at time t, Co is the initial conductivity, Cauto is the conductivity after autoclaving and k is the first-order rate constant (units time-1). The rate of electrolyte leakage (k) varies directly with the extent of tissue damage. In P. rubens a rate of 0.4% h-1 distinguished between shoots which eventually died, and shoots which remained alive. A minimum of 3 conductivity measurements (after 1 day, 5 days and after autoclaving) is required for a reliable estimate of k. This objective, quantitative method of assessing frost hardiness may therefore be used directly to estimate LT50 values within a population.

Thermal analysis of chemically synthesized polyaniline and effects of thermal aging on conductivity

Abstract: Thermal characteristics of chemically synthesized polyaniline with various dopants have been studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared spectroscopy, gel-permeation chromatography (GPC), and chemical titration. The HCl-doped polyaniline shows three major weight losses at around 100, 200, and 500°C which are assigned to removal of H2O and HCl, and decomposition of the polymer, respectively. Thermal aging of the HCl-doped polyaniline performed at 100, 150, and 200°C for various periods of time results in a decrease in conductivity. After the thermal treatments, the polymer can be re-doped with HCl to partially recover the conductivity. However, both the conductivity and the doping level cannot be restored to the level of the original materials owing probably to changes in morphology, crosslinking, or other chemical reactions.

Relation between structure and electronic properties of ion irradiated polymers

Abstract: Ion beam irradiation of saturated polymers leads to the formation of new carbonaceous materials exhibiting enhanced electrical conductivity and increase of the optical absorption, which shifts gradually from the near UV to the visible. Different techniques, such as the analysis of the gas evolved from the polymer during irradiation, RBS and ERDA show the loss of the more volatile elements which results from the dissociation of the molecular structure. The consideration of different types of polymers indicates that some specific structural elements are preserved and that unsaturated bonds appear at high fluences, which are responsible for the loss of the insulating properties of polymers. The chemical bond modifications are discussed in view of infrared spectroscopy and ESCA, which shows the formation of graphite-like phases in polyimide and explains the high level of conductivity reached in this type of polymers. The appearance of conducting properties cannot be described by a simple model of carbonization and are governed both by the molecular structure of the parent polymer and the beam conditions. The behaviour of polyimide will be considered more in details because of the remarkable level of conductivity ( σ = 10 3 S/cm) and heat stability reached upon irradiation with heavy ions, which render this type of surface processing very efficient in order to produce conductive polymers.

Applications of graphite intercalation compounds

Abstract: The properties of graphite intercalation compounds (GIC’s) are discussed with respect to possible applications. Five families of intercalates give high electrical conductivity to GIC’s: pentafluorides leading to high conductivity, 108 S/m (higher than metallic copper); metal chlorides; fluorine and alkali metals with bismuth giving relatively high conductivity of the order of 107 S/m plus stability in air; and residual halogens showing extremely high stability under severe conditions, though the conductivity is only of the order of 106 S/m. Electrodes of different GIC’s have been tried in primary and secondary batteries, where their characteristics are high electrical conductivity and easy diffusion of electrochemically active species between the graphite layers. Primary lithium batteries of a covalent graphite fluoride are now widely used commercially. Secondary batteries using different host graphites and intercalates give interesting results. Large amounts of hydrogen can be stored in the functional space in alkali metal-GIC’s. The same GIC’s show high coefficients of isotope separation of hydrogen at liquid nitrogen temperature. The structure and texture of the host graphite play a decisive role in the absorption and separation behaviors of GIC’s. Exfoliated graphite prepared by rapid heating of GIC’s or their residue compounds leads to flexible graphite sheets which have great industrial applications. Some problems connected with the production and use of these sheets are discussed.

On the conductivity of phosphoric acid electrolyte

Abstract: The specific electric conductivity and viscosity of 7.5–100 wt% aqueous phosphoric acid electrolyte have been measured in the temperature range 25–200°C. The product of conductivity and viscosity was found to decrease exponentially with increasing temperature. This suggests that the electric conduction in phosphoric acid follows the Grotthus proton switching mechanism. The measured conductivity data were correlated by the following empirical equation: $$\begin{array}{*{20}c} {k\mu = {\rm A} exp [BT]}\\ {A = 702.7X^{1.5} - 1734.2X^2 + 1446.5X^{2.5} - 350.7X^3 }\\ {B = - 0.010163 + 0.011634X - 0.08313X^2 }\\ \end{array}$$ wherek is conductivity in mho cm−1; μ is viscosity in centipoise;T is the absolute temperature in K; andX is mole fraction of phosphoric acid in the electrolyte. The standard deviation for the percentage difference between the calculated and experimental values of 188 data points was less than 7.5%.

Spin dynamics in the conducting polymer, polyaniline.

Abstract: The frequency dependences of both proton NMR ${T}_{1}$ and ESR linewidth in polyaniline give evidence for quasi-1D spin diffusion. The on-chain diffusion rate, ${\mathit{D}}_{\mathrm{\ensuremath{\parallel}}}$, is independent on the protonation level, while the transverse diffusion, ${D}_{\ensuremath{\perp}}$, exhibits a sudden drop at the percolation threshold. This behavior confirms the conducting-island picture, but it is concluded that a given conducting island just consists of a single conducting chain. The room-temperature data show a strong correlation between spin dynamics and transport properties, and suggest that conductivity is governed by interchain hoppings.

Fluctuation conductivity of high- T c superconductors

Abstract: The leading correction to the normal-state conductivity due to superconducting fluctuations is calculated as a function of temperature, externally applied magnetic field, and internal pair breaking for anisotropic high-${T}_{c}$ superconductors.

Binding constants of .beta.-cyclodextrin/surfactant inclusion by conductivity measurements

Abstract: Formation d'un complexe d'inclusion entre la β. cyclodextrine et differents agents de surface anioniques

Preparation and superconducting properties of ultrathin YBa2Cu3O7−x films

Abstract: Ultrathin films of YBa2Cu3O7−x with thicknesses down to 2 nm were grown on(100)SrTiO3 and (100)MgO by inverted cylindrical magnetron sputtering. Metallic behavior and zero resistance temperatures above 4.2 K were obtained in 3‐nm‐thick films on SrTiO3. Thinner films revealed temperature‐activated conductivity and only partial transitions to superconductivity due to inhomogeneities in the film morphology. On MgO, the critical film thickness leading to deteriorations of the transport properties was 6 nm. An analysis of the fluctuation‐enhanced conductivity near Tc in terms of the Aslamazov–Larkin theory [Phys. Lett. A 26, 238 (1968)] revealed three‐dimensional behavior even in the thinnest fully superconducting films.

Strength and conductivity ofin situ Cu-Fe alloys

Abstract: Alloys of Cu-Fe with iron contents from 10 to 30 wt % have been prepared by casting plus mechanical reduction. A series of heat treatments was done at various stages of the mechanical reduction to promote precipitation of the iron from the copper matrix with the hope of optimizing electrical conductivity at a given strength level. A curve of optimum tensile strength against electrical conductivity was determined. It was found to lie significantly below the available data for Cu-Nb alloys and it is suggested that further improvements may be possible in Cu-Fe alloys by improved thermal mechanism processing.

Lateral conductivity modulated MOSFET

Abstract: A conductivity modulated MOSFET, having a semiconductor substrate of a first conductivity type, a semiconductor layer of a second conductivity type formed on the semiconductor substrate and having a high resistance, a base layer of the first conductivity type formed in the semiconductor layer, a source layer of the second conductivity type formed in the base layer, a gate electrode formed on a gate insulating film which is formed on a channel region, the channel region being formed in a surface of the base layer between the semiconductor layer and the source layer, a source electrode ohmic-contacting the source layer and the base layer, and a drain electrode formed on the surface of the semiconductor substrate opposite to the semiconductor layer, characterized in that the conductivity modulated MOSFET has a saturation current smaller than a latch-up current when a predetermined gate voltage is applied to the gate electrode.

The Electrical Resistivity of the Vadose Zone — Field Survey

Abstract: An expansion of Archie's law relates the electrical resistivity of the unsaturated zone to pore-water resistivity, porosity, saturation, electrical matrix conductivity, and lithological constants. With decreasing saturation the effect of the matrix conductivity increases relative to the electrical conduction of the pore fluid. The bulk electrical resistivity depends on water quality and quantity due to its dependence on pore-water resistivity and porosity. It is also influenced by the hydraulic conductivity due to the surface conductivity of the grain matrix, which is proportional to grain size. Pore-water conductivity and grain-surface conductivity can be distinguished from each other if resistivity measurements at various degrees of saturation are available. Observations in test wells through glacial stream deposits with a neutron moisture probe show, as expected, a decrease in the saturation of the vadose zone during the transition from late spring to the dry summer season. This decrease was correlated with the transverse resistance of the unsaturated zone derived from an interpretation of geo-electrical depth soundings near the test wells. In the fine to medium sand of the glacial outwash material, the surface conduction of the grains was found to be negligibly small.