Showing papers on "Conductivity published in 1997"

P-type electrical conduction in transparent thin films of CuAlO2

Abstract: Optically transparent oxides tend to be electrical insulators, by virtue of their large electronic bandgap (⩾3.1 eV). The most notable exceptions are doped versions of the oxides In2O3, SnO2 and ZnO—all n-type (electron) conductors—which are widely used as the transparent electrodes in flat-panel displays1,2. On the other hand, no transparent oxide exhibiting high p-type (hole) conductivity is known to exist, whereas such materials could open the way to a range of novel applications. For example, a combination of the two types of transparent conductor in the form of a pn junction could lead to a ‘functional’ window that transmits visible light yet generates electricity in response to the absorption of ultraviolet photons. Here we describe a strategy for identifying oxide materials that should combine p-type conductivity with good optical transparency. We illustrate the potential of this approach by reporting the properties of thin films of CuAlO2, a transparent oxide having room-temperature p-type conductivity up to 1 S cm−1. Although the conductivity of our candidate material is significantly lower than that observed for the best n-type conducting oxides, it is sufficient for some applications, and demonstrates that the development of transparent p-type conductors is not an insurmountable goal.

Heat transport in thin dielectric films

Abstract: Heat transport in 20–300 nm thick dielectric films is characterized in the temperature range of 78–400 K using the 3ω method. SiO2 and SiNx films are deposited on Si substrates at 300 °C using plasma enhanced chemical vapor deposition (PECVD). For films >100 nm thick, the thermal conductivity shows little dependence on film thickness: the thermal conductivity of PECVD SiO2 films is only ∼10% smaller than the conductivity of SiO2 grown by thermal oxidation. The thermal conductivity of PECVD SiNx films is approximately a factor of 2 smaller than SiNx deposited by atmospheric pressure CVD at 900 °C. For films <50 nm thick, the apparent thermal conductivity of both SiO2 and SiNx films decreases with film thickness. The thickness dependent thermal conductivity is interpreted in terms of a small interface thermal resistance RI. At room temperature, RI∼2×10−8 K m2 W−1 and is equivalent to the thermal resistance of a ∼20 nm thick layer of SiO2 .

The electrical conductivity of human cerebrospinal fluid at body temperature

Abstract: The electrical conductivity of human cerebrospinal fluid (CSF) from seven patients was measured at both room temperature (25/spl deg/C) and body temperature (37/spl deg/C). Across the frequency range of 10 Hz-10 kHz, room temperature conductivity was 1.45 S/m, but body temperature conductivity was 1.79 S/m, approximately 23% higher. Modelers of electrical sources in the human brain have underestimated human CSF conductivity by as much as 44% for nearly two decades, and this should be corrected to increase the accuracy of source localization models.

Highly Conductive PEO-like Polymer Electrolytes

Abstract: Polymer electrolyte membranes comprising poly(vinylidene fluoride)−hexafluoropropene (PVdF−HFP) copolymer plasticized with a solution of LiSO3CF3, LiN(SO2CF3)2, or LiPF6 in oligomeric poly(ethylene glycol) dimethyl ethers (PEGDME, Mw = 250, 400, and 500) were prepared by hot-melt-rolling or solvent-casting techniques. Since the electrolytes containing PEGDME400 and PEGDME500 are “dry” with essentially no volatile components up to 150 °C, we have dubbed them PEO-like. Their thermal stability, mechanical strength, conductivity, electrochemical stability window, and Li/electrolyte interface stability were characterized. Plasticizing PVdF−HFP with the PEGDME/LiX solutions disordered the polymer structure leading to polymer electrolytes having lower crystallinity than the polymer host itself. The mechanical strength of the electrolyte membranes varied depending on the PVdF content. Tensile strength (stress) as high as 420 psi at an elongation-at-break value (strain) of 75% was observed. The conductivities of t...

Carrier concentrations and relaxation spectroscopy : new information from scaling properties of conductivity spectra in ionically conducting glasses

Abstract: A new kind of scaling analysis for the conductivity spectra of glasses without any arbitrary parameters is presented. By applying this method to sodium borate glasses of different compositions, we find strong indications for the existence of a universal ionic relaxation process as well as for a strong electrolyte behavior. Our results enable us to show that the often used electric modulus formalism is misleading when relaxation mechanisms on a microscopic level are concerned. A more meaningful discussion can be based on the log-log dependence of the conductivity on frequency.

Nitrogen modification of hydrogenated amorphous carbon films

Abstract: The effect of nitrogen addition on the structural and electronic properties of hydrogenated amorphous carbon (a-C:H) films has been characterized in terms of its composition, sp3 bonding fraction, infrared and Raman spectra, optical band gap, conductivity, and paramagnetic defect. The variation of conductivity with nitrogen content suggests that N acts as a weak donor, with the conductivity first decreasing and then increasing as the Fermi level moves up in the band gap. Compensated behavior is found at about 7 at. % N, for the deposition conditions used here, where a number of properties show extreme behavior. The paramagnetic defect density and the Urbach tailwidth are each found to decrease with increasing N content. It is unusual to find alloy additions decreasing disorder in this manner.

Electron Transport in the Nanostructured TiO2-Electrolyte System Studied with Time-Resolved Photocurrents

Abstract: Laser flash induced photocurrent transient measurements have been used to investigate the electron transport in nanostructured TiO2 (anatase) thin film electrodes in contact with an electrolyte. The shape and the time domain for the current transients were found to be dependent on film thickness and electrolyte conductivity. The experimental results are discussed using a diffusion model. If the experimental results are interpreted as diffusion, a chemical diffusion coefficient for the electrons in the nanostructured system is determined to 1.5 × 10-5 cm2/s using 700 mM LiClO4 in ethanol as electrolyte.

Charge transport of the mesoscopic metallic state in partially crystalline polyanilines

Abstract: Charge transport properties, including temperature-dependent dc conductivity, thermoelectric power, electron paramagnetic resonance, microwave frequency dielectric constant and conductivity, and electric-field-dependent conductance of partially crystalline (``physically'' cross-linked) HCl-doped polyaniline correlated with x-ray structure studies, demonstrate that charge delocalization in physically cross-linked polyaniline systems is structurally controlled. Further, we observe a positive dielectric constant at room temperature which increases (to values $g~{10}^{4})$ with increasing percent crystallinity, the size of crystalline regions, and polymer chain alignment in the disordered regions, supporting the establishment of mesoscopic metallic regions. We propose an inhomogeneous disorder model for this system in which ordered (crystalline) regions, described by three-dimensional metallic states, are connected through amorphous regions of polymer chains where one-dimensional disorder-induced localization is dominant. We utilize the metallic box, interrupted metallic strands, and Nakhmedov's phonon-induced delocalization models to account for the temperature dependence of charge transport properties of the various partially crystalline polyanilines. Analyses for the sample and temperature-dependent electron paramagnetic resonance linewidth and thermoelectric power are presented.

Rechargeable lithium batteries

Abstract: The market for lithium batteries is undergoing a rapid expansion as new applications demand higher densities of energy and power storage. Simple theoretical estimates show that lithium and lithium ion cells can reach specific energies of 880 and 500 W h kg–1 respectively. With an electrolyte conductivity above 3 × 10–4 S cm–1 and thickness below 0.01 cm, a power density of 300 W dm–3 can be obtained without excessive energy losses. Diffusion in porous or polymer composite electrodes is enhanced by an interpenetrating electrolyte provided the electrode particles are small. Batteries using transition metal oxide positive electrodes and carbon negative electrodes are expected to give practical specific energies up to 180 W. h kg–1 including packaging and other essential additional materials in the near future.

Nature of Conduction in Doped Silicon

Abstract: Via ultrafast optoelectronic THz techniques, we are able to test alternative theories of conduction by precisely measuring the complex conductivity of doped silicon from low frequencies to frequencies higher than the plasma frequency and the carrier damping rate. These results, obtained for both $n$ and $p$-type samples, spanning a range of more than 2 orders of magnitude in the carrier density, do not fit any standard theory. We only find agreement over the full frequency range with the complex conductivity given by a Cole-Davidson type distribution applied here for the first time to a crystalline semiconductor, and thereby demonstrate that fractal conductivity is not just found in disordered material.

Electrophysical properties of polymer electrolyte membranes : a random network model

Abstract: A random network model of charge transport in porous polymer membranes is proposed and investigated by means of an effective medium theory. Specific conductivity and geometrical capacity (dielectric constant) of membranes are calculated in dependence of water content, taking swelling effects into account. The conductivity shows a quasi-percolation type dependence, growing above a certain critical water content, while taking below it a value typical of residual conductivity in dry membranes. The geometrical capacity can show an increase with water content or it can undergo a maximum at the percolation threshold, depending on the model parameters. The theoretical findings are in line with experimental observations of the membrane complex impedance as a function of water content. The results suggest a frame for classification of different types of polymer membranes with regard to their performance, depending on swelling properties.

Nonstoichiometry and Electrical Conductivity of Nanocrystalline CeO $${2 - x} $$

Abstract: We synthesized dense CeO $${2 - x} $$ polycrystals of ∼10 nm grain size and characterized their electrical conductivity, in order to determine whether the defect properties of nanocrystalline solids fundamentally differ from those of conventional materials The nanocrystals exhibit enhanced electronic conductivity, greatly reduced grain boundary impedance, and a heat of reduction more than 24 eV lower per oxygen vacancy compared to their coarse-grained counterparts We propose that defect formation at low energy grain boundary sites is responsible for these properties, and that nanocrystalline oxides represent bulk materials possessing the defect thermodynamics of interfaces

Nonaqueous electrolytes for electrochemical capacitors: Imidazolium cations and inorganic fluorides with organic carbonates

Abstract: Electrolytes based on 1-ethyl-3-methylimidazolium cation (EMI{sup +}) and either the hexafluorophosphate (EMIPF{sub 6}) or tetrafluorborate (EMIBF{sub 4}) anion in organic alkyl carbonate solvents have been evaluated for use in electrochemical capacitors. The conductivity, capacitance, limiting oxidation and reduction potentials, and thermal stability were assessed. High conductivity and capacitance values were found regardless of whether cyclic (high viscosity/high dielectric constant) or acyclic (low viscosity/low dielectric constant) alkyl carbonates were used. The best correlation with conductivity for the EMIPF{sub 6} salt was found to be the molecular weight and to a lesser degree the viscosity of the solvent. The high specific capacitance (130 F/g) and excellent stability (>3.5 V, >130 C) make these electrolytes well suited for use in electrochemical double-layer capacitors.

Correlated ion hopping in single-crystal yttria-stabilized zirconia

Abstract: A study of the effect of correlated ion motion on the electrical conductivity relaxation in single-crystalline yttria-stabilized zirconia is presented. Complex admittance in the radio frequency range show power-law dependencies in the real part of the conductivity at high frequencies of the form ω^(n) and asymmetric electric modulus plots as a result of correlations. An analysis of the frequency dependence of the electric modulus is conducted to obtain time decay functions of the form exp[-(t/τ)^(β)] from an analytical distribution of relaxation times. Correlation times, and parameters n and β characterizing the relaxation in time and frequency domains are compared to show the equivalence of time and frequency representations. The common origin of ac and dc processes is discussed in view of the frequency dependence of the complex conductivity. From a macroscopic activation energy for ion motion E = 1.16 eV and a β value of 0.43, a single-ion microscopic activation energy E_(a) = 0.5 eV is obtained as βE according to Ngai’s coupling model. The microscopic activation energy is related to the association energy of oxygen vacancies.

Conductivity of Quasiperiodic Systems: A Numerical Study

Abstract: We develop a new real-space method which allows one to evaluate the Kubo-Greenwood formula for dc conductivity of independent electrons in a static potential. We apply it to a numerical study of propagation modes in three dimensional quasiperiodic systems. These modes are strikingly different from those of periodic ones with regard to the effect of disorder. In particular, for Fermi energies in pseudogaps the conductivity can be stable or can even increase when disorder increases.

A new transparent conducting oxide in the ga2o3-in2o3-sno2 system

Abstract: A new transparent conducting oxide (TCO), which can be expressed as Ga3−xIn5+xSn2O16; 0.2⩽x⩽1.6, has been identified. The equilibrium phase relationships of this new material with respect to three other TCOs in Ga2O3–In2O3–SnO2 are reported. The optical properties of this phase are slightly superior to Sn-doped indium oxide (ITO) and depend on composition. A room-temperature conductivity of 375 Ω cm−1 was obtained for H2-reduced Ga2.4In5.6Sn2O16. This value is an order of magnitude lower than commercial ITO films, but comparable to values reported for bulk, polycrystalline Sn-doped In2O3.

Synthesis and electrical characterisation of doped perovskite titanates as potential anode materials for solid oxide fuel cells

Abstract: This work reports the synthesis and electrical characterisation over a range of oxygen partial pressures (10–20–1 atm) of the A-site deficient perovskites Sr1–3x/2LaxTiO3–δ, with a view to establishing their potential as anode materials for solid oxide fuel cells. Single phase samples were observed for synthesis in air for 0≤x≤0.6, and the materials remained phase pure for both high and low oxygen partial pressures at the measurement temperature of 930 °C. Good electrical conductivity, which increased with increasing La content, was observed on reduction in low oxygen partial pressures, with values as high as 7 S cm–1 [ P(O2)= 10–20 atm], similar to values observed for the related system, Sr1–x/2Ti1–xNbxO3–δ, examined previously. The conductivity of the fully reduced samples showed metallic character from 100 to 930 °C. As the oxygen partial pressure was raised, the conductivity dropped, showing an approximate [P(O2)]–1/6 dependence for porous samples. New samples, Sr1–y/2–3x/2LaxTi1–yNbyO3–δ, with both La and Nb substitutions, were also studied, and these phases showed similar electrical behaviour. Further results for the Sr1–x/2Ti1–xNbxO3–δsystem are presented and compared with the La doped systems.

Thermal conductivity of thin metallic films measured by photothermal profile analysis

Abstract: Thermal conductivity of nickel and gold films on quartz (thickness 0.4–8 μm) was measured by a modulated thermoreflectance technique recording the surface temperature profile. Model calculations predict an optimum frequency for measuring thermal transport within the film. Measurements on films with various thicknesses reveal a thermal conductivity close to the bulk value for nickel while gold films exhibit a reduced conductivity with decreasing film thickness.

Processing and Electrical Properties of Alkaline Earth‐Doped Lanthanum Gallate

Abstract: Oxides exhibiting substantial oxygen ion conductivity are utilized in a number of high-temperature applications, including solid oxide fuel cells, oxygen separation membranes, membrane reactors, and oxygen sensors Alkaline earth-doped lanthanum gallate powders were prepared by glycine/nitrate combustion synthesis Compacts of powders synthesized under fuel-rich conditions were sintered to densities greater than 97% of theoretical Appropriate doping with Sr or Ba on the A-site of the perovskite structure, and Mg on the B-site, resulted in oxygen ion conductivity higher than that of yttria-stabilized zirconia (YSZ), and high ionic transference numbers Doping with Ca and Mg resulted in lower conductivity than YSZ Thermal expansion coefficients of the doped gallates were higher than that of YSZ