Showing papers on "Conductivity published in 1995"

Acid-doped polybenzimidazoles : a new polymer electrolyte

Abstract: Polybenzimidazole films doped with phosphoric acid are being investigated as potential polymer electrolytes for use in hydrogen/air and direct methanol fuel cells. In this paper, we present experimental findings on the proton conductivity, water content, and methanol vapor permeability of this material, as well as preliminary fuel cell results. The low methanol vapor permeability of these electrolytes significantly reduces the adverse effects of methanol crossover typically observed in direct methanol polymer electrolyte membrane fuel cells.

Organic transistors: two-dimensional transport and improved electrical characteristics.

Abstract: The thiophene oligomer α-hexathienylene (α-6T) has been successfully used as the active semiconducting material in thin-film transistors. Field-induced conductivity in thin-film transistors with α-6T active layers occurs only near the interfacial plane, whereas the residual conductivity caused by unintentional doping scales with the thickness of the layer. The two-dimensional nature of the field-induced conductivity is due not to any anisotropy in transport with respect to any molecular axis but to interface effects. Optimized methods of device fabrication have resulted in high field-effect mobilities and on/off current ratios of > 106. The current densities and switching speeds are good enough to allow consideration of these devices in practical large-area electronic circuits.

Metal oxide semiconductor device forming a pn junction with a thin film transistor of metal oxide semiconductor of copper suboxide and manufacture thereof

Abstract: PURPOSE: To obtain an excellent PN junction by doping controlled metal oxide semiconductor with impurities, by controlling defects by introducing hydrogen or the like in the defects due to the excessive oxygen in a part of metal oxide semiconductor of copper suboxide or the like, and controlling the carrier density and the conductivity type. CONSTITUTION: A metal oxide semiconductor 25 is metal semiconductor obtained by oxidizing metal films 24, 24'. An insulating protective film is formed on the surfaces of an insulating film 26 and the metal oxide semiconductor 25. By leading out electrodes connected with source drain electrodes 24, 24', a transistor having a gate electrode 22 is formed. The carrier density and the conductivity type are controlled by eliminating oxygen defects. The P-type conductivity or the N-type conductivity, and the resistivity can be controlled by impurity doping. In these cases, ion implantation method or the like can be applied. Thereby a thin film transistor of high mobility can be formed in a large area by low temperature treatment.

Thermal conductivity of sputtered oxide films

Abstract: The thermal conductivity of oxide thin films deposited using dc, rf, and ion-beam sputtering is measured in the temperature range 80--400 K using the 3{omega} method. Thermal conductivity data for amorphous thin films of SiO{sub 2} are nearly identical to bulk {ital a}-SiO{sub 2}. Data for amorphous Al{sub 2}O{sub 3}, while having a magnitude and temperature dependence similar to bulk amorphous oxides, show a dependence on deposition method; rf sputtering of an Al{sub 2}O{sub 3} target produces films with a thermal conductivity 35% smaller than films prepared by ion-beam sputtering. Microcrystalline thin films show a rich variety of behavior: the conductivity of TiO{sub 2} films depends on the substrate tempreature {ital T}{sub {ital s}} and approaches the thermal conductivity of bulk TiO{sub 2} ceramics when {ital T}{sub {ital s}}{congruent}400 {degree}C; HfO{sub 2} films show glasslike thermal conductivity independent of annealing temperature up to 900 {degree}C; and MgO films display a crystalline thermal conductivity that is greatly reduced relative bulk values.

Correlation between structure and thermal conductivity of organic aerogels

Abstract: Organic aerogels are derived from the sol-gel polymerization of resorcinol with formaldehyde. While these materials are usually produced as monoliths, this paper describes a new method for the production of organic aerogel microsphere powders. Supercritical drying provides highly porous aerogels which have an open-cell structure consisting of interconnected solid particles with typical diameters of 10 nm. The structure is controlled by the sol-gel polymerization conditions. This paper addresses the correlation between structure and thermal conductivity of these novel materials. Thermal conductivity measurements have been performed on both monoliths and powders using a hot-wire device. The measurements under variation of gas pressure as well as spectral infrared transmission measurements allow the determination of the solid, gaseous and radiative thermal conductivity as a function of density and catalyst concentration. The results show that the thermal conductivity components are clearly correlated with the aerogel structure: porosity and connectivity between the particles determine the solid conductivity, while the pore size influences the gaseous conductivity and radiative transport depends on the mass specific infrared absorption of the building units.

The Measurement of a Complete Set of Transport Properties for a Concentrated Solid Polymer Electrolyte Solution

Abstract: Polymer electrolytes based on alkali metal salts in poly(ethylene oxides) are important for possible use in rechargeable batteries for both electric vehicle and consumer electronics applications. The authors measure a complete set of transport properties for one particular binary salt solution: sodium trifluoromethanesulfonate in poly(ethylene oxide), over a wide range of salt concentrations (0.1 to 2.6M) at 85 C. The properties measured include the conductivity, the salt diffusion coefficient, and the Na ion transference number. The mean molar activity coefficient of the salt is also determined. The conductivity and diffusion coefficients of NaCF{sub 3}SO{sub 3} are similar in magnitude to those of LiCF{sub 3}SO{sub 3} in (polyethylene oxide). The transference number and thermodynamic factor are found by combining concentration cell data with the results of galvanostatic polarization experiments. A theoretical analysis of the experimental method based on concentrated-solution theory is given. The study verifies that the transference numbers derived from the experiments retain fundamental significance in applications involving both steady and transient processes and in systems coupling the polymer electrolyte with electrodes of all types (stoichiometries). The relevant transference numbers can be determined independently of any knowledge of speciation of the polymer electrolyte. The transference numbers found here for themore » sodium ion are much lower than those reported for the lithium ion, especially in the concentrated solutions. The transference number of the sodium ion is negative in the more concentrated solutions and levels off at its maximum value of 0.31 in the dilute concentration range. The transference number results are interpreted in terms of complexation of the sodium ion with the anionic species.« less

Partial conductivities in SrTiO3 : bulk polarization experiments, oxygen concentration cell measurements, and defect-chemical modeling

Abstract: Knowledge of the exchange kinetics of O2 in SrTiO3 allows us to design appropriate strategies to separate the ionic and the electronic conductivity. In the low-temperature range, where the overall surface reaction is very slow compared to bulk diffusion and measuring time, electrochemical cells of the type Pt|SrTiO3|Pt are self-blocking and self-sealing and a Wagner–Hebb-type polarization succeeds without the necessity of using selectively blocking electrodes. In the present study the ionic conductivity data obtained for Feand Ni-doped SrTiO3 in this way are compared to data obtained from the analysis of the oxygen partial pressure dependence of the total conductivity as well as to defect chemical calculations. In complete contrast to the low temperature situation, at high temperatures, where the surface reaction is fast, the emf technique is conveniently applicable. Results are presented for Pt, O2|SrTiO3|O2, Pt cells. The conductivity behavior of SrTi(Fe)O3 as a function of temperature (20°–1000°C) is complex, due to partially frozen-in equilibria, but even details can be quantitatively understood in terms of a simple defect chemistry. The turnover of the diffusion-controlled regime to the surface reaction-controlled regime can be shifted to significantly lower temperatures by using YBa2Cu3O7–8 electrodes.

Theoretical and experimental determination of cell constants of planar-interdigitated electrolyte conductivity sensors

Abstract: In this paper, an analytical expression is presented for the cell constant of planar-interdigitated electrodes used as electrolyte conductivity sensors. The result of this expression is compared with results of measurement carried out with several differently shaped planar probes provided with a thin Ta2O5 insulating film, showing good agreement. More than 10 different devices have been fabricated with predicted cell constants ranging from 0.14 to 4.44 cm?1. The measured cell constants are typically 10?20% smaller, possibly due to fringing effects.

Ionic Conductivity of Ln10(SiO4)6O3 (Ln = La, Nd, Sm, Gd and Dy)

Abstract: Electrical properties were investigated for lanthanoid-silicates of Ln10(SiO4)6O3(Ln = La, Nd, Sm, Gd and Nd). The conductivity at 773 K was 2.3 × 104 S cm−1 for Nd10(SiO4)6O3. The sole carrier is the O2− ion, which was determined using an O2 gas concentration cell.

Hopping transport on a fractal: ac conductivity of porous silicon.

Abstract: We have measured the frequency dependence of the conductivity and the dielectric constant of various samples of porous Si in the regime 1 Hz-100 kHz at different temperatures. The conductivity data exhibit a strong frequency dependence. When normalized to the dc conductivity, our data obey a universal scaling law, with a well-defined crossover, in which the real part of the conductivity sigma' changes from an sqrt(omega) dependence to being proportional to omega. We explain this in terms of activated hopping in a fractal network. The low-frequency regime is governed by the fractal properties of porous Si, whereas the high-frequency dispersion comes from a broad distribution of activation energies. Calculations using the effective-medium approximation for activated hopping on a percolating lattice give fair agreement with the data.

Scale Dependency of Hydraulic Conductivity Measurements

Abstract: The hydraulic conductivity of five stratigraphic units in a carbonate aquifer has been measured with slug, pressure, and pumping tests, and with two calibrated digital models. The effective test radii range from less than one to greater than 10,000 meters. On log-log plots hydraulic conductivity increases approximately linearly with test radius to a range between 20 and 220 meters, but thereafter, it is constant with scale. The increase in magnitude of hydraulic conductivity is similar to scaling effects reported at seven additional sites in a variety of geologic media. Moreover, the increase in magnitude correlates with an increase in variance of log-hydraulic conductivity measured at successively greater separation distances. The rate of increase in both parameters, and particularly the range, have characteristic values for different pore systems. The larger ranges are consistently present in units with greater secondary porosity. Therefore, scaling effects provide a qualitative measure of the relative importance of secondary and primary permeability, and they can potentially be used to distinguish the dominant type of pore system.

GdVO4 as a new medium for solid-state lasers: some optical and thermal properties of crystals doped with Cd3+, Tm3+, and Er3+ ions

Abstract: The refractive indices of new Nd3+ : GdVO4, Tm3+ : GdVO4, and Er3+ : GdVO4 laser crystals were determined with an error of ±5×10-5 in the range 400–1100 nm. The birefringence of these crystals was on the average 0.24, and the dispersion was described by the formula 1/Δn2 = a+ b/λ2. The thermal conductivity of the Nd : GdVO4 crystal (atomic neodymium concentration 1.3%) was determined in the temperature range 77-300 K. At 300 K the conductivity along the direction was 11.7 W m-1 K-1, which was higher than the conductivity of Nd : YAG crystals.

Ion-beam-induced transformation of diamond

Abstract: The ion-beam-induced transformation of insulating diamond to a conducting form of carbon is explored by performing measurements of the electrical conductivity of diamond subject to ion damage. A wide range of implantation temperatures (150--690 K) with both Xe (320 keV) and C (100 keV) ions are employed. The dose dependence of the conductivity, R(D), is found to scale with the nuclear energy deposited in the irradiated volume, thus demonstrating that it is the density of collisionally induced defects that governs the electrical conductivity. The data are analyzed in terms of a model that proposes that the passage of an ion through the solid leaves in its wake conducting spheres of varying radii. The average radius of these spheres is found to decrease from about 2.05 nm for irradiation at 150 K with 320-keV Xe ions to zero for implantation at about 815 K. At critical doses ${\mathit{D}}_{\mathit{c}}$, which depend on the implantation temperature and the ion species, these spheres overlap to form a continuous conductive pathway through the irradiated diamond. In some cases this transition is sharp enough to be well accounted for by a simple percolation theory. Below ${\mathit{D}}_{\mathit{c}}$, R(D) displays complicated nonmonotonic behavior, which is explained as being due to the competition of the contribution of different types of defects to the observed electrical conductivity. Remarkable similarities between the conductivity induced in diamond and fused quartz implanted with C ions under identical conditions are reported.

Thermal conductivity and diffusivity of free‐standing silicon nitride thin films

Abstract: The thermal conductivity and diffusivity of free‐standing silicon nitride (Si‐N) films of 0.6 and 1.4 μm in thickness are measured. A new experimental technique, the amplitude method, is proposed and applied to measurement of the thin‐film thermal diffusivity. The thermal diffusivity is determined by three independent experimental approaches: the phase‐shift method, the amplitude method, and the heat‐pulse method. Good agreement among the measured thermal diffusivities obtained by the three methods indicates the validity of the amplitude method. High‐resolution electron microscopy studies show a large quantity of voids in the 1.4 μm Si‐N films. In contrast, very few voids are found in the 0.6 μm films. This difference may be responsible for the measured lower conductivity of the 1.4 μm Si‐N films as compared to the 0.6 μm thin films.

Gas sensing properties of porous silicon

Abstract: Conductivity of porous silicon layers (p-type) has been investigated for organic vapor sensing. A many orders of magnitude increase in conductivity in response to a vapor pressure change from 0 to 100% has been measured for some compounds. The conductivity (at a constant pressure) varies exponentially with the compound's dipole moment The temporal response of the porous silicon layers is in the seconds range, and the recovery is much slower (minutes). However, due to the tremendous conductivity changes and the low background noise, a complete recovery is not needed for sensing purposes. The mechanism of conductivity enhancement has been studied using several methods. It is attributed to an increase in the density of charge carriers. An additional mechanism based on increased diffusivity may take place in microporous silicon. The observed characteristics suggest the application of porous silicon to future chemical sensors. The sensors have the potential to be integrated monolithically with other silicon devices using current technologies.

Polyaniline Doped by the New Class of Dopant, Ionic Salt: Structure and Properties

Abstract: Polyaniline (PAn) can be doped through a protonation by protonic acids, in addition to an oxidation doping by Lewis acid as for the other conjugated conducting polymers. This work reports the new class of dopant, ionic salt, such as LiClO 4 , LiBF 4 , LiPF 6 , and Zn(ClO 4 ) 2 , for PAn. The ionic salt can be used to dope PAn by mixing an ionic salt with PAn in the common solvent 1-methyl-2-pyrrolidone and then casting the solution into a film. The structure and properties of ionic salt doped-PAn are investigated by UV-visible, IR and XPS spectroscopies, dynamic mechanical analysis, scanning electron microscopy, and conductivity measurement. It is found that the PAn therein is doped via pseudoprotonation of the imine nitrogen by the metal cation. As in the case of HCl-doped PAn, polarons/bipolarons are generated as reflected in the presence of UV-visible absorption peaks at 420 and 865 nm. The LiBF 4 -doped PAn film retains a conductivity at the level of 10 -2 S/cm in the temperature range 25-140 °C. The conductivity then decays to 10 -3 S/cm as temperature increases to 175 °C, resulting from the increased ring distorsion in the PAn subchains due to the glass transition.

Ionic conductivities and structure of lithium phosphorus oxynitride glasses

Abstract: Lithium phosphorus oxynitride glasses with different lithium contents have been prepared by melting base glasses at high temperature in a flowing ammonia atmosphere for 16–72 h. The melt was then furnace-cooled to room temperature to avoid bubble formation in the sample. The structure of the lithium phosphorus oxynitride glasses was probed by X-ray photoelectron spectroscopy and high performance liquid chromatography. The results of ac impedance measurements show that nitrogen incorporation into the glass structure increases the ionic conductivity. The highest conductivity was found in Li0.99PO2.55N0.30 glass with σ ∼ 3.0 × 10−7 S cm−1 at 25°C and Ea = 0.60 eV.

Static conductivity and superconductivity of carbon nanotubes: Relations between tubes and sheets

Abstract: We relate the static conductivity of carbon nanotubes to the static in-plane conductivity of a graphite sheet and conclude that isolated single-wall nanotubes are excellent conductors. In contrast, multiwall tubes at low doping may possess conductivities substantially below that of the sum of the constituent tubes. The curvature of small tubes opens new electron-phonon scattering channels that are not available to sheets. This increases the electron-phonon coupling and yields superconducting transition temperatures for small doped tubes intermediate between those of intercalated graphite and alkali-metal-doped ${\mathrm{C}}_{60}$.

Optical properties along the c-axis of YBa2Cu3O6+x, for x = 0.50 → 0.95 evolution of the pseudogap

Abstract: The reflectance of high-quality, unpolished single crystals of YBa2Cu3O6+x, for the doping range x = 0.50 → 0.95, has been measured with radiation polarized along the c-axis from ≈ 50 cm−1 to 5000 cm−1 between 10 K and 300 K. In highly doped (x = 0.95) material, the normal-state conductivity shows a metallic response. For intermediate dopings (x = 0.85 → 0.90) the conductivity is no longer metallic, increasing with temperature, and for low dopings (x = 0.50 → 0.80) this behavior is clearly seen to be caused by a broad pseudogap at ≈ 290 cm−1, which develops well above Tc. In the superconducting state, in the optimally doped material, a gap-like depression develops below Tc but there is residual conductivity to very low frequency indicating either an anisotropic gap, or a gap with nodes. In the underdoped materials the superconductivity-induced changes to the conductivity are harder to see, but it appears that the weight of the condensate originates at frequencies much higher than the conventional weak-coupling BCS value of 3.5kBTc.