Showing papers on "Conductivity published in 1972"

ac Conductivity of Scandium Oxide and a New Hopping Model for Conductivity

Abstract: The ac conductivity of scandium-oxide thin films in the audio-frequency range at temperatures between 4 and 295 K has been measured. The frequency-dependent component of the conductivity was found to obey an equation of the form ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})=A{\ensuremath{\omega}}^{s}$, where $\ensuremath{\omega}$ is the circular frequency and $s$ is a temperature-dependent quantity whose value is close to, but less than, unity. Interpretation of the results in terms of a single-phonon hopping theory does not yield satisfactory agreement. To account for the data a new hopping model is proposed. The conductivity is calculated for classical hopping of carriers between localization sites over potential barriers with a height distribution caused by the random spatial distribution of these sites. This model yields the ${\ensuremath{\omega}}^{s}$ behavior at high frequencies with the quantity ($1\ensuremath{-}s$) increasing almost linearly with temperature. In addition, it is predicted that a thermally activated dielectric-loss peak should occur for very low frequencies. It is suggested that this model may find broad application in the interpretation of ac conductivity results in amorphous materials.

Thermal conductivity of Earth materials at high temperatures

Abstract: The total thermal conductivity (lattice plus radiative) of several important earth materials is measured in the temperature range from 500 to 1900 K. A new technique is used in which a CO2 laser generates a low-frequency temperature wave at one face of a small disk-shaped sample, and an infrared detector views the opposite face to detect the phase of the emerging radiation. Phase data at several frequencies yield the simultaneous determination of the thermal diffusivity and the mean extinction coefficient of the material. The lattice, radiative, and total thermal conductivities are then calculated. Results for single-crystal and polycrystalline forsterite-rich olivines and an enstatite indicate that, even in relatively pure large-grained material, the radiative conductivity does not increase rapidly with temperature. The predicted maximum total thermal conductivity at a depth of 400 km in an olivine mantle is 0.020 cal/cm/sec/deg C, which is less than twice the surface value.

Homogeneous Dynamical Conductivity of Simple Metals

Abstract: Within the jellium model the zero wave vector, frequency-dependent conductivity is expressed in terms of a regular memory function. This quantity is calculated in lowest order in the impurity concentration and the electron-phonon coupling, thus yielding a reasonable approximation for the conductivity valid in the complete frequency regime. The standard results for the static conductivity including vertex corrections are reproduced. Deviations from Drude's formula a because of spin-flip scattering in a magnetic field, because of resonance scattering, because of phonon creation at low temperatures, and because of breaking of the screening cloud attached to charged impurities are discussed.

ac Conductivity of Glasses

Abstract: We investigate the ac conductivity of glasses due to atomic and electronic hopping between sites with random space and energy separations. Several examples are considered including the structural model of glasses recently introduced by Anderson, Halperin, and Varma, and by Philips. We find that glassy materials should quite generally exhibit a nearly linear conductivity-frequency relationship. The details of any particular conductivity mechanism should be contained in the deviation from linearity.

Electrical Relaxation in Na2O·3SiO2 Glass

Abstract: The electrical relaxation associated with alkali diffusion in Na2O·3SiO2 glass was studied from 0.2 Hz to 700 kHz at –1° to 163°C. A formalism for analysis of electrical relaxation in conducting dielectrics which associates the nonexponential decay of the electric field to zero and the dispersions in the dielectric constant and the conductivity with a distribution of relaxation times for the electric field was developed and is shown to be in qualitative accord with current molecular theories of electrical relaxation in alkali silicate glasses. A relation between the dc conductivity, the limiting high-frequency dielectric constant, and the average electric field or conductivity relaxation time was derived and is verified experimentally for the Na2O·3SiO2 glass. The distribution of electric-field relaxation times for the glass is broad, asymmetric on a logarithmic scale, and weighted in favor of the shorter relaxation times; the distribution narrows with increasing temperature. A reduced electrical relaxation curve which can be used to compare electrical and mechanical relaxations in Na2O·3SiO2 glass was generated.

Deformation of Nematic Liquid Crystals in an Electric Field

Abstract: The problem of a nematic liquid crystal in an electric field is solved. Effects due to conductivity of the liquid have been neglected. Numerical solutions are given for various values of parameters. The solution is compared with experimental data taken from the literature. A computer program has been developed to do a non-linear least-squares fit of experimental data with the solution given in the present paper.

High oxide ion conduction in sintered Bi2O3 containing SrO, CaO or La2O3

Abstract: Ionic conduction in sintered oxides of the system Bi2O3-SrO was investigated by measuring the conductivity and ion transference number under various conditions. The ion transference numbers were measured by an oxygen concentration cell employing the specimen as the electrolyte.

Electrical resistivity of conducting particles in an insulating matrix

Abstract: When conducting particles are added to a nonconducting matrix, conductivity increases abruptly at a critical concentration. The theory of infinite chains is used to predict the variation of conductivity with particle concentration. Factors which determine the critical concentration are discussed.

The effect of diameter on the electrical constants of frog skeletal muscle fibres.

Abstract: 1. Electrical constants were determined on isolated single fibres or on fibres from bundles from frog's twitch muscles by analysing the low frequency cable properties.2. The sarcoplasmic conductivity (G(i)) was 5.9 mmho/cm at 20 degrees C, and its temperature coefficient (Q(10)) was 1.37.3. The Q(10) of the membrane conductance (G(M)) was 1.49, and that of the membrane capacity (C(M)) was 1.02.4. C(M) increases with diameter (D) in an approximately linear manner: the values were 4.6 muF/cm(2) at D = 50 mu, and 8.5 muF/cm(2) at D = 130 mu.5. G(M) also increases with diameter, being 0.21 mmho/cm(2) at D = 50 mu and 0.37 mmho/cm(2) at D = 130 mu.6. These results suggest that the transverse tubular system contributes substantially to the values of low frequency capacity and conductance measured at the surface membrane.

The Viscosity and Thermal Conductivity Coefficients of Gaseous and Liquid Argon

Abstract: Data for the viscosity and thermal conductivity of gaseous and liquid argon have been evaluated and represented by empirical functions. Tables for the viscosity from 86 to 500 K for pressures to 400 MPa, and for the thermal conductivity from 90 to 500 K for pressures to 200 MPa are presented. For the viscosity, uncertainties of 2% or better for pressures below 100 MPa, and 3% for higher pressures are assigned. For the thermal conductivity the uncertainties are 4% for temperatures below 150 K and 3% or better for temperatures above. The enhancement in the conductivity close to the critical point has been accounted for. The status of the argon transport data and the philosophy of fitting them are reviewed.

Thermally fixed holograms in linbo3

Abstract: Normally recorded phase holograms in undoped LiNbO3 can be made optically stable, i.e. fixed, by heating the crystal to roughly 100°C. A detailed study of the temperature dependence of this process, coupled with conductivity measurements, suggests a model in which the fixed holograms are associated with ionic charge patterns formed by drift of thermally activated ions in the electric field pattern of the original hologram. These ions would have an activation energy of ∼ 1.1 eV and a concentration in excess of 2 × 1013 cm-3 in fair agreement with previous measurements of ionic conductivity. The room temperature conductivity of the LiNbO3 used in our experiments is concluded to be less than 10-17 Ω cm)-1.

Variable range hopping in a non-uniform density of states

Abstract: An expression is derived for the conductivity due to variable range hopping in a density of states which varies as a power of the energy from the Fermi level. The derivation is also given for the case of two-dimensional hopping.

Water vapour adsorption and surface conductivity in solids

Abstract: The water adsorption and voltage dependence of the surface conduction current at room temperature has been investigated at various relative humidities for quartz, Teflon and cellulose acetate. The results were analysed using the BET equation, so that the monolayer adsorption could be determined and the surface conductivity related to the number of water layers adsorbed. This showed that for Teflon, the surface of which has a weak attraction for water molecules, the conductivity increased exponentially with the number of sorbed layers of water. For quartz, for which the attraction is strong, the conductivity only commenced to increase exponentially after the completion of the first layer.

Influence of water content on electrical conductivity of the soil

Abstract: MEASUREMENT OF DYNAMIC SALINITY AND WATER CONTENT PROFILES UNDER LABORATORY CONDITIONS WERE NEEDED TO TEST A MODEL DEVELOPED TO CHARACTERIZE THE SIMULTANEOUS FLOW OF WATER AND SALTS IN SOILS. METHODS WERE NEEDED FOR MEASURING SALINITY IN PLACE IN A SYSTEM WHERE RAPID WATER MOVEMENT OCCURRED. BECAUSE OF THE TIME LAG IN SALINITY SENSORS, OTHER METHODS WERE CONSIDERED FOR EVALUATING SALINITY. THE FOUR-PROBE SYSTEM OF MEASURING ELECTRICAL CONDUCTIVITY OF SOIL ELIMINATES THE TIME LAG ERROR, BUT INTRODUCES THE PROBLEM OF CORRECTING THE CHANGES IN CONDUCTIVITY CAUSED BY WATER CONTENT. UNITS WERE CONSTRUCTED AND SAMPLES PREPARED TO EVALUATE THE INFLUENCE OF WATER CONTENT, AND SALT CONCENTRATIONS OF THE FOUR-PROBE CONDUCTIVITY. TWO SOILS WERE STUDIED. REGRESSION EQUATIONS DEVELOPED TO ESTIMATE SATURATION OR 1:5 EXTRACT ELECTRICAL CONDUCTIVITY FROM FOUR- PROBE CONDUCTIVITY AND WATER CONTENT GAVE CORRELATION COEFFICIENTS OF 0.75 AND 0.90, RESPECTIVELY. CORRELATIONS FOR INDIVIDUAL SOILS WERE GENERALLY HIGHER THAN THE COMBINED SOILS.

Thermal conductivity of polyethylene: The effects of crystal size, density and orientation on the thermal conductivity

Abstract: Thermal conductivity measurements were made on polyethylene samples which had been crystallized at different temperatures in order to vary density and lamellar crystal thickness. Tests were also made on polyethylenes which were oriented by mechanical shearing and by crystallization in a temperature gradient. The thermal conductivity increased linearly by 50% as the density went from 0.96 to 0.99 gm/cc. By orientation the conductivity was increased ten fold in the direction of molecular orientation and decreased two fold in the perpendicular direction. For samples oriented by deformation at different temperatures, the conductivity apparently correlates with birefringence.

Electrical conduction in evaporated amorphous silicon films

Abstract: We report the results of measurement and analysis of the electrical conductivity as a function of temperature and anneal for amorphous silicon films. The resistivity of the films between 77 and 300°K increasesc with annealing. Refractory electrodes were used. The extrapolated portions of T − 1 4 fits to the log of the conductivity give physically unreasonable parameters for the T − 1 4 formula.

On the Correlation of Electrical Conductivity Properties of Porous Systems with Viscous Flow Transport Coefficients

Abstract: This study emphasizes the role that surface conductivity plays in electrical transport processes in porous media. It shows that this term has to be included in the calculations where phase distributions or porosities are to be deducted from formation factor type measurements even in clean formations when electrolyte concentrations are low. in applying a parallel resistance model of conduction processes through porous media, it can be demonstrated that the pertinent parameters are a conductivity ratio function k s /k f = f(C) and the extent of the internal specific pore surface area S p . The conductivity ratio function will also depend on the nature of the internal surface area which determines the mechanism of charge fixation at the solid-solution interface. Based on the assumption that electrical and viscous flow paths in the porous medium are identical, the electrical transport coefficients can be -connected to viscous flow parameters by way of introducing the Kozeny-Carman hydraulic radius concept of porous interstices and linking the internal surface area term to the permeability of the medium.

Direct‐Current Conductivity and Iron Tracer Diffusion in Hematite at High Temperatures

Abstract: The dc conductivity of natural single-crystal α-Fe2O3 was measured as a function of O partial pressure from 10−4 to 1 atm at 950° to 1422°C. The conductivity was independent of O2 partial pressure, indicating that hematite is an intrinsic semiconductor with lattice defect concentrations much lower than the concentration of intrinsic electrons (holes). The activation energy of the dc conductivity was 1.18 eV. The iron tracer (55Fe) diffusion coefficients, measured as a function of O2 partial pressure at 1200° and 1300°C, increased as the O2 partial pressure decreased, with a pressure dependence of -0.75; the iron therefore migrates interstitially.

Dielectric behaviour in a vanadium phosphate glass

Abstract: The dielectric constant and conductivity of 80% V 2 O 5 : 20% P 2 O 5 glass has been measured in the frequency range 10 2 to 10 9 Hz and in the temperature range 80 to 350°K. It is shown that the dielectric behaviour over these ranges is described by a Debye type relaxation process with distribution of relaxation times. A method is proposed to determine the width of distribution from the data at fixed frequencies and different temperatures. The width of distribution increases at frequencies ω > 10/ τ , which leads to an a.c. conductivity at these frequencies almost linearly proportional to frequency and independent of temperature. The estimated value of the static dielectric constant of about 30 was found to decrease with temperature while the infinite frequency dielectric constant of 10 was independent of temperature. The carrier concentration calculated from the dielectric relaxation time and the d.c. conductivity through a thermal diffusion model shows reasonable agreement with direct measurement using electron paramagnetic resonance.

Electrical Properties of Silver Doped As-Se Glasses

Abstract: The electrical properties of silver doped As-Se glasses up to 1.0 atomic % have been studied. The d.c. conductivity of the doped glasses is represented by a single activation energy which is smaller than that of the parent glass. The d.c. conductivity at room temperature increases with increase of silver content. However the glass containing smaller amount of silver is less conductive than that of the parent glass above a certain temperature. Pressure dependence of the conductivity shows electronic and ionic characteristics. A dielectric relaxation which is closely related to the d.c. conduction is found. Above results can be related to the formation of defects which contain silver.

X-Ray-Induced Photoconductivity in Dielectric Films

Abstract: We have measured the conductivity induced in films of polyethylene, epoxy, polytetrafluoroethylene, polyethylene terephthalate, polyimide, and glass by x rays at dose rates between 109 and 1010 rad/sec (dose in air). The films were 0.05 to 1.25 mm thick. The x-ray spectrum peaked in the vicinity of 10 keV, and the x-ray pulse width was about 40 nsec FWHM. X-ray induced photocurrents were found to obey Ohm's law at low bias voltages (less than 1 kV). Above 1 kV, however, we observed that the peak photoconductivity signals from some of the 0.05-mm-thick materials began to increase at a slightly faster than linear rate with bias voltage. The glass samples exhibited no apparent delayed conductivity, while the other sample materials showed various amounts. The magnitude of the delayed conductivity in polytetrafluoroethylene, polyethylene terephthalate, and polyimide depended on the electric field, an effect that is consistent with Poole-Frenkel field assisted carrier generation. We have qualitatively described the magnitude and time dependence of the conductivity signals by a simple trapping model, using reasonable values for mobility, trap density, capture cross sections and trap depths.