Showing papers on "Conductivity published in 2000"
TL;DR: A first-principles investigation, based on density functional theory, produces strong evidence that hydrogen acts as a source of conductivity: it can incorporate in high concentrations and behaves as a shallow donor.
Abstract: Zinc oxide, a wide-band-gap semiconductor with many technological applications, typically exhibits n-type conductivity. The cause of this conductivity has been widely debated. A first-principles investigation, based on density functional theory, produces strong evidence that hydrogen acts as a source of conductivity: it can incorporate in high concentrations and behaves as a shallow donor. This behavior is unexpected and very different from hydrogen's role in other semiconductors, in which it acts only as a compensating center and always counteracts the prevailing conductivity. These insights have important consequences for control and utilization of hydrogen in oxides in general.
2,970 citations
TL;DR: In this article, the authors evaluated thermodynamic and electrical conductivity data to select the most appropriate electrolyte composition for IT-SOFC operation at 500°C and found that the Gd 3+ ion is the preferred dopant, compared to Sm 3+ and Y 3+, at this temperature.
1,888 citations
TL;DR: In this article, the authors show that tunnel contacts can dramatically increase spin injection and solve the problem of the mismatch in the conductivities of a ferromagnetic (FM) metal and a semiconductor microstructure.
Abstract: Theory of electrical spin injection from a ferromagnetic (FM) metal into a normal (N) conductor is presented. We show that tunnel contacts (T) can dramatically increase spin injection and solve the problem of the mismatch in the conductivities of a FM metal and a semiconductor microstructure. We also present explicit expressions for the spin-valve resistance of FM-T-N- and FM-T-N-T-FM-junctions with tunnel contacts at the interfaces and show that the resistance includes both positive and negative contributions (Kapitza resistance and injection conductivity, respectively).
1,133 citations
TL;DR: The preparation of defined heterolayered films composed of CaF2 and BaF2 that exhibit ionic conductivity increasing proportionally with interface density are reported, in excellent agreement with semi-infinite space-charge calculations, assuming a redistribution of fluoride ions at the interfaces.
Abstract: Ion conduction is of prime importance for solid-state reactions in ionic systems, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors Ionic conductivity in solid electrolytes can be improved by dissolving appropriate impurities into the structure or by introducing interfaces that cause the redistribution of ions in the space-charge regions Heterojunctions in two-phase systems should be particularly efficient at improving ionic conduction, and a qualitatively different conductivity behaviour is expected when interface spacing is comparable to or smaller than the width of the space-charge regions in comparatively large crystals Here we report the preparation, by molecular-beam epitaxy, of defined heterolayered films composed of CaF2 and BaF2 that exhibit ionic conductivity (parallel to the interfaces) increasing proportionally with interface density--for interfacial spacing greater than 50 nanometres The results are in excellent agreement with semi-infinite space-charge calculations, assuming a redistribution of fluoride ions at the interfaces If the spacing is reduced further, the boundary zones overlap and the predicted mesoscopic size effect is observed At this point, the single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated Our results should lead to fundamental insight into ionic contact processes and to tailored ionic conductors of potential relevance for medium-temperature applications
735 citations
TL;DR: A family of solid oxides based on the parent compound La2Mo 2O9 (with a different crystal structure from all known oxide electrolytes) which exhibits fast oxide-ion conducting properties and is comparable to that of stabilized zirconia, the most widely used oxide electrolyte.
Abstract: The ability of solid oxides to conduct oxide ions has been known for more than a century, and fast oxide-ion conductors (or oxide electrolytes) are now being used for applications ranging from oxide fuel cells to oxygen pumping devices1,2. To be technologically viable, these oxide electrolytes must exhibit high oxide-ion mobility at low operating temperatures. Because of the size and interaction of oxygen ions with the cationic network, high mobility can only be achieved with classes of materials with suitable structural features. So far, high mobility has been observed in only a small number of structural families, such as fluorite3,4,5, perovskites6,7, intergrowth perovskite/Bi2O2 layers8,9 and pyrochlores10,11. Here we report a family of solid oxides based on the parent compound12 La2Mo2O9 (with a different crystal structure from all known oxide electrolytes) which exhibits fast oxide-ion conducting properties. Like other ionic conductors2,13, this material undergoes a structural transition around 580 °C resulting in an increase of conduction by almost two orders of magnitude. Its conductivity is about 6 × 10-2 S cm-1 at 800 °C, which is comparable to that of stabilized zirconia, the most widely used oxide electrolyte. The structural similarity of La2Mo2O9 with β-SnWO4 (ref. 14) suggests a structural model for the origin of the oxide-ion conduction. More generally, substitution of a cation that has a lone pair of electrons by a different cation that does not have a lone pair—and which has a higher oxidation state—could be used as an original way to design other oxide-ion conductors.
635 citations
TL;DR: The hemicylindrical wires prepared by this technique were self-uniform, and the wires prepared in a particular electrodeposition (in batches of 10(5) to 10(7)) were narrowly distributed in diameter.
Abstract: Metallic molybdenum (Moo) wires with diameters ranging from 15 nanometers to 1.0 micrometers and lengths of up to 500 micrometers (0.5 millimeters) were prepared in a two-step procedure. Molybdenum oxide wires were electrodeposited selectively at step edges and then reduced in hydrogen gas at 500°C to yield Moo. The hemicylindrical wires prepared by this technique were self-uniform, and the wires prepared in a particular electrodeposition (in batches of 105 to 107) were narrowly distributed in diameter. Wires were obtained size selectively because the mean wire diameter was directly proportional to the square root of the electrolysis time. The metal nanowires could be embedded in a polystyrene film and lifted off the graphite electrode surface. The conductivity and mechanical resiliency of individual embedded wires were similar to those of bulk molybdenum.
570 citations
TL;DR: First principles electronic structure calculations indicate that lambda-DNA chains should present large resistance values, and it is shown that low energy electron bombardment induces a rapid contamination and dramatically affects the measured conductivity, thus providing an explanation to recent reports of high DNA conductivity.
Abstract: The electrical conductivity of biomaterials on a molecular scale is of fundamental interest in the life sciences. We perform first principles electronic structure calculations, which clearly indicate that lambda-DNA chains should present large resistance values. We also present two direct procedures to measure electrical currents through DNA molecules adsorbed on mica. The lower limit for the resistivity is 10(6) Omega . cm, in agreement with our calculations. We also show that low energy electron bombardment induces a rapid contamination and dramatically affects the measured conductivity, thus providing an explanation to recent reports of high DNA conductivity.
541 citations
TL;DR: The detailed experimental measurements of the velocity of fluid flow on microelectrodes at frequencies below the charge relaxation frequency of the electrolyte are presented and discussed in terms of a linear model of ac electroosmosis.
Abstract: Under the influence of an ac electric field, electrolytes on planar microelectrodes exhibit fluid flow. The nonuniform electric field generated by the electrodes interacts with the suspending fluid through a number of mechanisms, giving rise to body forces and fluid flow. This paper presents the detailed experimental measurements of the velocity of fluid flow on microelectrodes at frequencies below the charge relaxation frequency of the electrolyte. The velocity of latex tracer particles was measured as a function of applied signal frequency and potential, electrolyte conductivity, and position on the electrode surface. The data are discussed in terms of a linear model of ac electroosmosis: the interaction of the nonuniform ac field and the induced electrical double layer.
496 citations
TL;DR: In this paper, a series of exposure tests was carried out with anode substrates used in SOFC development at the Research Centre Julich, where the changes in electrical conductivity as well as in the microstructure of the material were investigated.
457 citations
TL;DR: The phonon thermal conductivity of a multilayer is calculated for transport perpendicular to the layers and it is shown that the conductivity has a minimum value for a layer thickness somewhat smaller then the mean free path of the phonons.
Abstract: The phonon thermal conductivity of a multilayer is calculated for transport perpendicular to the layers. There is a crossover between particle transport for thick layers to wave transport for thin layers. The calculations show that the conductivity has a minimum value for a layer thickness somewhat smaller then the mean free path of the phonons.
456 citations
TL;DR: In this paper, the performance of perovskites based on alkaline-earth containing lanthanum cobaltites was investigated in the system La1−xSrxCo1−yFeyO3−δ.
TL;DR: Preliminary results on microcrystalline silicon clearly indicate the critical role of amorphouslike tissue in transport in microcrystals, and Contrary to time of flight, this method can be used even for relatively conductive materials.
Abstract: The transport properties of microcrystalline silicon, namely, mobility and conductivity, are investigated by a new method, for which the simple theory as well as numerical modeling is presented. The basic idea of the new method is verified on amorphous hydrogenated silicon by comparison with the widely used time-of-flight method. Contrary to time of flight, the new method can be used even for relatively conductive materials. Preliminary results on microcrystalline silicon clearly indicate the critical role of amorphouslike tissue in transport in microcrystalline silicon.
TL;DR: In this paper, the authors investigated the effect of electrochemical oxidation of poly(3,4-ethylenedioxythiophene) (PEDOT) by cyclic voltammetry combined with conductivity measurements, UV-vis-NIR and Raman spectroelectrochemical studies.
TL;DR: In this paper, evidence is provided that an organic thin-film transistor (OTFT) can be used as a novel gas sensor when exposed to chemical species at room temperature, four parameters can be measured: the bulk conductivity of the organic thin film, the field-induced conductivity, the transistor threshold voltage and the field effect mobility.
Abstract: In this communication, evidence is provided that an organic thin-film-transistor (OTFT) can be used as a novel gas sensor. When exposed to chemical species at room temperature, four parameters can be measured: the bulk conductivity of the organic thin film, the field-induced conductivity, the transistor threshold voltage and the field effect mobility. Measurements of these parameters may allow for recognition of molecular species.
TL;DR: It is observed that the hopping frequency can be used as the scaling frequency in the absence of well-defined dielectric loss peaks, and a universality of the scaling of the conductivity can be obtained for compositions with a similar structure.
Abstract: A scaling approach in the conductivity formalism is applied to lithium tellurite glasses of different compositions. We observe that the hopping frequency can be used as the scaling frequency in the absence of well-defined dielectric loss peaks, and a universality of the scaling of the conductivity can be obtained for compositions with a similar structure. Further, the reasons behind the inapplicability of the scaling approach reported recently are elucidated in terms of structure of the glasses.
TL;DR: In this article, the role of quantum corrections to the classical thermal conduction was examined, and it was shown that these effects are small for fairly harmonic systems such as diamond, and that vacancies dramatically decrease the thermal conductivity, and can be described by a reciprocal relation with a scaling as n va, with a50.6960.
Abstract: Based on the Green‐Kubo relation from linear response theory, we calculated the thermal current autocorrelation functions from classical molecular dynamics ~MD! simulations. We examined the role of quantum corrections to the classical thermal conduction and concluded that these effects are small for fairly harmonic systems such as diamond. We then used the classical MD to extract thermal conductivities for bulk crystalline systems. We find that ~at 300 K! 12 C isotopically pure perfect diamond has a thermal conductivity 45% higher than natural ~1.1% 13 C! diamond. This agrees well with experiment, which shows a 40%‐50% increase. We find that vacancies dramatically decrease the thermal conductivity, and that it can be described by a reciprocal relation with a scaling as n va , with a50.6960.11 in agreement with phenomenological theory (a51/2 to 3/4!. Such calculations of thermal conductivity may become important for describing nanoscale devices. As a first step in studying such systems, we examined the mass effects on the thermal conductivity of compound systems, finding that the layered system has a lower conductivity than the uniform system. © 2000 American Institute of Physics.@S0021-9606~00!70140-1#
TL;DR: In this paper, the authors describe the stepwise, ligand-directed assembly, characterization, and prospective applications of three-dimensional Au and Ag nanoparticle, multilayered films.
Abstract: This manuscript describes the stepwise, ligand-directed assembly, characterization, and prospective applications of three-dimensional Au and Ag nanoparticle, multlilayered films. Films were prepared by successive treatments of a Au nanoparticle monolayer with a bifunctional cross-linker and colloidal Au or Ag solutions. Changes in film electrical and optical properties are reported for a series of bifunctional cross-linkers of varying molecular lengths. Interestingly, these films exhibit Beer's law behavior despite the presence of strong interparticle optical coupling. Multilayer films with greater than six exposures to 2-mercaptoethylamine and Au colloid were highly conductive and resembled bulk Au in appearance. In contrast, films of similar particle coverage generated using a longer cross-linker (1,6-hexanedithiol) exhibited higher transmission in the near-infrared region and exhibited a reduced conductivity. Measurement of the multilayer morphology with atomic force microscopy , electrostatic force mi...
TL;DR: In this paper, the authors compared simulation results with experimental data for Bellcore PLION® cells using the graphite/1 M LiPF6 in EC:DMC (2:1)/LiMn2O4 system.
TL;DR: In this article, the authors investigated the performance of several compositions in the scandia-zirconia system (Sc 2 O 3 content between 7.0 and 11.0 mol%) as a function of temperature and time at the nominal solid oxide fuel cell operating temperature of 850°C.
TL;DR: In this article, slow photoconductivity transients were comprehensively studied in ZnO films prepared by spray pyrolysis of the zinc-nitrate solution, and it was possible to reversibly change the conductivity by short-term annealing in hydrogen and oxygen.
Abstract: Slow photoconductivity transients were comprehensively studied in ZnO films prepared by spray pyrolysis of the zinc-nitrate solution. Surface charge controlled the film conductivity, and it was possible to reversibly change the conductivity by many orders of magnitude using short-term annealing in hydrogen and oxygen. Under illumination, the conductivity of as-grown films may increase by several orders of magnitude, depending on the dark conductivity. Photoconductivity was due to the capture of nonequilibrium holes at surface oxygen states to produce an equivalent number of excess electrons in the conduction band. Reverse process of the photoconductivity relaxation is determined by an electron tunneling mechanism to the surface oxygen states.
TL;DR: The p-type behavior was confirmed by the Seebeck effect as discussed by the authors, which indicated that the p-layer conductivity of the transparent CuScO2+x films is a function of the temperature dependence of the conductivity.
Abstract: Transparent films of CuScO2+x have been prepared which show p-type electrical conductivity. The temperature dependence of the conductivity indicates semiconducting behavior with an apparent room temperature activation energy of 0.11 eV. The highest room temperature conductivity observed was 30 S cm−1. Films 110 nm thick show 40% transparency in most of the visible spectrum and become much more transparent in the infrared spectrum. The p-type behavior was confirmed by the Seebeck effect.
TL;DR: Water uptake and proton conductivity as a function of temperature were determined for three aromatic-based, sulfonic acid-bearing polymers, plus the perfluoroalkyl sulfonic acids Nafion{reg_sign} 117.8 x 10{sup {minus}3} S/cm as mentioned in this paper.
Abstract: Water uptake and proton conductivity as a function of temperature were determined for three aromatic-based, sulfonic acid-bearing polymers, plus the perfluoroalkyl sulfonic acid Nafion{reg_sign} 117. Water uptake of submerged, equilibrated samples ranged from less than five water molecules per acid group for a high equivalent weight, sulfonated polyethersulfone to almost fifty waters per acid for a low equivalent weight, sulfonated polyetheretherketone. The most conductive aromatic-based polymer, sulfonated polyphenylquinoxaline (S-PPQ), had a room temperature conductivity of 9.8 x 10{sup {minus}3} S/cm, about an order of magnitude less than that of a perfluoroalkyl sulfonic acid under identical conditions. The slope of the S-PPQ Arrhenius conductivity plot was sufficiently steep that at 180 C, the proton conductivity, 1.3 x 10{sup {minus}1} S/cm, was only a factor of two lower than that of Nafion under similar conditions. The lower conductivity of the aromatic-based sulfonic acid polymers can be attributed to chain rigidity, lack of ion channels, and lower acidity.
TL;DR: In this article, the authors used a variety of spatial averaging schemes for mixtures of the mantle minerals and incorporated effects of oxygen fugacity to estimate the electrical conductivity of the Earth's mantle.
Abstract: Recent laboratory measurements of electrical conductivity of mantle minerals are used in forward calculations for mantle conditions of temperature and pressure. The electrical conductivity of the Earth's mantle is influenced by many factors, which include temperature, pressure, the coexistence of multiple mineral phases, and oxygen fugacity. In order to treat these factors and to estimate the resulting uncertainties, we have used a variety of spatial averaging schemes for mixtures of the mantle minerals and have incorporated effects of oxygen fugacity. In addition, to better calculate lower mantle conductivities, we report new measurements for electrical conductivity of magnesiowustite (Mg0.89Fe0.11)O. Because the effective medium theory averages lie between the Hashin-Shtrikman bounds for the whole mantle, a laboratory-based conductivity-depth profile was constructed using this averaging scheme. Comparison of apparent resistivities calculated from the laboratory-based conductivity profile with those from field geophysical models shows that the two approaches agree well.
TL;DR: The conductivity of the materials LaAlO3, LaGaO3 and LaScO3 with 10% strontium on the A-site and 10% magnesium at the B-site has been measured at different temperatures and oxygen partial pressures as mentioned in this paper.
TL;DR: In this paper, the electrical conductivities of polysiloxanes [RSiO1.5]n with R=CH3 (PMS) and C6H5 (PPS), respectively, were measured at room temperature by using a shielded two point configuration.
Abstract: Polysiloxanes [RSiO1.5]n with R=CH3 (PMS) and C6H5 (PPS), respectively, were transformed to Si–O–C ceramics of variable composition and structure upon pyrolysis in inert atmosphere at 800–1500°C. The electrical conductivities of the Si–O–C ceramics in air were measured at room temperature by using a shielded two point configuration. In situ measurements of the dc-conductivity during the pyrolytic conversion from the polymer to the ceramic phase were carried out up to 1500°C with four point contacted carbon electrodes in inert atmosphere. During polymer-ceramic conversion excess carbon precipitates above 400°C (PPS)–700°C (PMS). At temperatures above 800°C (PPS) and 1400°C (PMS) coagulation and growth of the carbon clusters results in a percolation network formation. While below the percolation threshold electrical conductivity can be described according to Motts mechanism by variable-range-hopping of localized charge carriers, regular electron band conduction due to the instrinsic conductivity of turbostratic carbon (8×10−4 (Ωcm)−1) predominates above. Thus, the in situ measurement of non-linear electrical property changes can be used as a microprobe of high sensivity to detect microstructural transformations during the pyrolysis of preceramic polymers.
TL;DR: The thermal conductivity of a chain of coupled rotators is discussed, showing that it is the first example of a 1D nonlinear lattice exhibiting normal transport properties in the absence of an on-site potential.
Abstract: We discuss the thermal conductivity of a chain of coupled rotators, showing that it is the first example of a 1D nonlinear lattice exhibiting normal transport properties in the absence of an on-site potential. Numerical estimates obtained by simulating a chain in contact with two thermal baths at different temperatures are found to be consistent with those based on linear response theory. The dynamics of the Fourier modes provides direct evidence of energy diffusion. The finiteness of the conductivity is traced back to the occurrence of phase jumps. Our conclusions are confirmed by the analysis of two variants of this model.
TL;DR: Composite membranes based on PWA adsorbed on silica (SiO2) and polybenzimidazole (PBI) have been prepared and their physico-chemical properties have been studied as discussed by the authors.
TL;DR: In this article, a systematic study on the properties of the superior oxide-ion conductor Sr and Mg-doped LaGaO3 (LSGM) and its performance in a single fuel cell is presented.
TL;DR: In this article, overpotential and AC impedance spectra were measured to study the reaction model of dense Sm 0.5Sr0.5CoO3 (SSC) as SOFC cathode.
TL;DR: In this article, the electrical conductivity of chemically prepared polypyrrole in aqueous solution was found to be strongly dependent on the preparation technique and polymer additive, and significant enhancement of the conductivity up to 90 S cm −1 by using poly(ethylene glycol) as an additive during the polymerization could be achieved.