Distribution by Scientific Domains
Distribution within Physics and Astronomy

Kinds of Resistivity

  • contact resistivity
  • electrical resistivity
  • high resistivity
  • low resistivity
  • surface resistivity
  • volume resistivity

  • Terms modified by Resistivity

  • resistivity imaging
  • resistivity index
  • resistivity measurement
  • resistivity survey
  • resistivity tomography
  • resistivity value

  • Selected Abstracts

    Growth of bismuth tri-iodide platelets by the physical vapor deposition method

    A. Cuña
    Abstract The work reports the growth of single BI3 crystals with platelets habit. Platelets were grown by physical vapor deposition (PVD) in a high vacuum atmosphere and with argon, polymer or iodine as additives. Crystals grew in the zone of maximum temperature gradient, perpendicular to the ampoule wall. Crystals grown with argon as additive show a very shining surface, have hexagonal (0 0 l) faces, sizes up to 20 x 10 mm2 and thicknesses up to 100 ,m. They were characterized by optical microscopy and scanning electron microscopy (SEM). Dendritic-like structures were found to be their main surface defect. SEM indicates that they grow from the staking of hexagonal unities. Electrical properties of the crystals grown under different growth conditions were determined. Resistivities up to 2 x 1012 ,cm (the best reported value for monocrystals of this material) were obtained. X-ray response was measured by irradiation of the platelets with a 241Am source of 3.5 mR/h. A comparison of results according to the growth conditions was made. Properties of the crystals grown by this method are compared with the ones measured for others previously grown from the melt. Also, results for bismuth tri-iodide platelets are compared with the ones obtained for mercuric and lead iodide platelets. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Growth and electrical properties of flash evaporated AgGaTe2 thin films

    B. H. Patel
    Abstract Thin films have been prepared by flash evaporation technique of a stoichiometric bulk of AgGaTe2 compound in vacuum and analysed using X-ray diffraction, transmission electron microscopy, selected area diffraction and energy dispersive analysis of X-rays. The effect of substrate temperature on the structural properties , grain size, film orientation, composition, and stoichiometry of the films have been studied. It was found that the polycrystalline, stoichiometric films of AgGaTe2 can be grown in the substrate temperature range of 473K < Ts < 573K. The influence of substrate temperature (Ts) on the electrical characteristics- Resistivity, Hall Mobility, Carrier concentration of AgGaTe2 thin films were studied. The electrical resistivity was found to decrease with increase in substrate temperature up to 573K and then increases. The variation of activation energy of AgGaTe2 thin films were also investigated. The implications are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Measurement of the Electric Resistivity of Metals up to and Above the Melting Temperature,

    R. Brandt
    A device for high-accuracy measurements of electric resistivity is introduced by the authors. Also, a layout for its use in high-temperature experiments presented to record resistivity changes around the melting temperature. A demonstration of the apparatus' precision (performed on steel samples at low temperature, with a deviation of merely 0.5 % from reference date) is given and first high-temperature results are discussed. [source]

    Positive Temperature Coefficient of Resistivity in Donor-Doped BaTiO3 Ceramics derived from Nanocrystals synthesized at Low Temperature,

    ADVANCED MATERIALS, Issue 5 2008
    L. Brutchey
    Small, donor-doped Ba1- xLaxTiO3nanocrystals are successfully prepared for the first time. These nanocrystals are suitable precursors for small-grain Ba1- xLaxTiO3 ceramics with a sizeable positive temperature coefficient of resistivity (PTCR) that is comparable to that of traditional large-grain ceramics derived from solid-state routes. [source]

    Electrolytic Synthesis of Al-Doped ZnO Nanopowders With Low Electrical Resistivity

    Takeshi Takaki
    Al-doped ZnO (AZO) nanopowders with different Al content were fabricated by a galvanostatic electrolytic method. The electrical resistivities were measured by a cell method, which reached its minimum (28 ,·cm) at 0.93 at.% of Al with its grain size of ,30 nm. Microstructures of powders were characterized by X-ray diffraction and transmission electron microscopy (TEM), which showed a decrease in grain size with the increase of Al content. In addition, the distributions of Al and the chemical bonding nature of Al atoms were examined by STEM-EDS and by X-ray photoelectron spectra, respectively, which suggested the substituational incorporation of Al atom into the ZnO lattice. Absorption properties were investigated for wavelength ranging from 250 to 2500 nm, which showed that the film coated with AZO nanopowders exhibited a rapid decrease in transmittance below 370 nm to ,0% and beyond 1250 nm to ,40% (at 2000 nm). [source]

    Negative Temperature Coefficient of Resistivity in Lightweight Conductive Carbon Nanotube/Polymer Composites

    Zhi-Dong Xiang
    Abstract It is particularly difficult to prepare a foam CPC material because its porous structure makes it hard to form a conductive network. We utilized acetone-assisted dispersion to disperse CNTs into PU foam and successfully prepared a lightweight conductive CNT/assembled PU foam composite. The NTC effect, which exclusively exists in the melt state CPC materials, has unexpectedly been observed in the solid-state lightweight conductive CNT/sPU composite. Higher gas fraction and lower matrix modulus could result in stronger NTC effect. The mechanism that thermal expansion of gas wrapped in the cellular structure induces more perfect conductive paths has been proposed to satisfactorily elucidate the NTC effect and its gas fraction and matrix modulus dependence. [source]

    Electrical Resistivity and Phase Transformation in Steels

    O. N. Mohanty
    Elektrischer Widerstand; Phasenumwandlung; Magnetische Eigenschaften Abstract Changes in electrical resistance accompanying transformations in steels with magnetic change (e.g. , , martensite/bainite) and without magnetic change (e.g. , , ,, above Curie temperature) have been examined; the former class affects the resistivity the latter does not. Next, while the efficacy of electrical resistivity measurement in capturing the well-known features of austenite stabilization (e.g. over , ageing, reversibility, and influence of prior martensite amount and so on) in high carbon steels has been reported in an earlier publication, new features (e.g. increase in resistance -increase at very low temperatures, change in temperature co-efficient of resistivity in the stabilized material etc.) are highlighted here. Finally, the work shows that a quantitative estimate of precipitation in the copper bearing, age-hardenable HSLA-100 steel during tempering can be done by continuous electrical resistivity measurement. These data also allow an in-depth kinetic analysis using the Johnson-Mehl-Avrami equation. Zusammenhang zwischen elektrischem Widerstand und Phasenumwandlungen in Stählen Änderungen des elektrischen Widerstands wurden im Zusammenhang mit Phasenumwandlungen, die zu Änderungen der magnetischen Eigenschaften führen (z.B. , , Martensit/Bainit), und solchen, die keine Änderungen der magnetischen Eigenschaften bewirken (z.B. , , , oberhalb der Curie-Temperatur), untersucht. Während sich die erste Umwandlungsart auswirkt, gilt dies für die andere nicht. Während über die Empfindlichkeit von Widerstandsmessungen zur Erfassung der Austenitstabilisierung (z.B. Überalterung, Umkehrbarkeit, Einfluß des vorher vorhandenen Martensitanteils, usw.) in übereutektoiden Stählen in einer früheren Veröffentlichung berichtet wurde, wird im zweiten Teil der hier vorgelegten Veröffentlichung auf neuerdings erfassbare Merkmale (z.B. der Anstieg des Widerstands bei sehr niedrigen Temperaturen, die Änderung der Temperaturabhängigkeit des Widerstands im stabilisierten Material, usw.) eingegangen. Schließlich zeigt die Arbeit, daß durch kontinuierliche Widerstandsmessungen während des Anlassens eine quantitative Abschätzung der Ausscheidungsgehalte im kupferhaltigen, ausscheidungsverfestigenden HSLA-100-Stahl erfolgen kann. Die Daten hierzu erlauben zudem eine vertiefte kinetische Analyse auf Basis der Johnson-Mehl-Avrami-Gleichung. [source]

    Study of prelocalized graphite/styrene acrylonitrile conducting composites for device applications

    V. K. Sachdev
    Abstract Conductive polymer composites were prepared by compression molding of prelocalized graphite on to styrene acrylonitrile (SAN) particles. The electrical conductivity is found to be strongly dependent on the graphite content. Three different series were prepared for different processing and material parameters. A low percolation threshold has been noticed when only 1 wt% of graphite is incorporated. Resistivity as low as ,14 , cm has been achieved in a composite with SAN resin particles of 180,212 µm size and graphite 10,20 µm at 90 °C, 105 MPa and 15 min. An electrically conducting network of graphite channels has been observed using scanning electron microscopy. V,I characteristic reveals that at a lower percentage of graphite the increase in current with increase in electric field is due to the hopping/tunneling of electrons, while for higher percentages of graphite ohmic behavior similar to metals has been observed. The data has been analyzed using percolation model. The value of the exponent t that determines the increase in electrical conductivity above the percolation threshold is found to be close to the values given in the literature. The theoretically calculated values of conductivity are found to be in satisfactory agreement with the experimental ones. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Variation of electrical resistance in metallic glasses subjected to 130 MeV 28Si ion irradiation

    H. Narayan
    Abstract The change in the electrical resistivity of 2605SC (Fe81B13.5Si3.5C2) and 2705M (Co69B12Si12Fe4Mo2Ni1) metallic glasses (MGs) resulting from 130 MeV 28Si ion irradiation has been investigated. Resistivity as a function of temperature has been measured off-line and in situ before and after irradiation, and on-line as a function of ion fluence. The results show that for the 2605SC MG electrical resistivity increases by 5.2% (fluence = 3.7 × 1014 ions/cm2) and 4.5% (fluence = 1.6 × 1014 ions/cm2). This has been explained on the basis of the ,two-hit model'. For the 2705M MG, however, an unexpected decrease of electrical resistivity of about ,9.6% (fluence = 1.1 × 1016 ions/cm2) and ,8.7% (1.3 × 1014 ions/cm2) is observed, which has been attributed to irradiation-induced structural modification. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Carbon black filled PET/PMMA blends: Electrical and morphological studies

    J. G. Mallette
    In this work, the electrical and morphological properties of blends of poly(ethylene terephthalate) (PET), poly(methyl methacrylate) (PMMA), and carbon black (CB) were analyzed. Resistivity decreases similarly in both PET and PMMA with CB concentration. Similarly in the PET/PMMA blend, extensive modification to this behavior occurs, since resistivity becomes a function of morphology and specific location of CB in the polymers. A minimum in the resistivity of the blend with 5% CB (PET basis) is observed at 100% PET, whereas with an increase in the CB content to 20%, the minimum in the resistivity shifts to 60% PET. High conductivity is observed when PET is the continuous phase (having the larger viscosity). Large stresses lead to a large dispersion of CB and a high deformation and rupture of the dispersed PMMA phase. This situation itself promotes an increase of surface area of droplets and high CB concentrations at the interface. Consideration is given to models that predict a selective location of conductive particles in the PET matrix based on its lower interfacial tension with CB. [source]

    Resistivity and lifetime variation along commercially grown Ga- and B-Doped Czochralski Si ingots and its effect on light-induced degradation and performance of solar cells

    V. Meemongkolkiat
    Abstract A systematic study of the variation in resistivity and lifetime on cell performance, before and after light-induced degradation (LID), was performed along ,900-mm-long commercially grown B- and Ga-doped Czochralski (Cz) ingots. Manufacturable screen-printed solar cells were fabricated and analyzed from different locations on the ingots. Despite the large variation in resistivity (0·57,2·5,,,cm) and lifetime (100,1000,µ,s) in the Ga-doped Cz ingot, the efficiency variation was found to be ,,0·5% with an average efficiency of ,17·1%. No LID was observed in these cells. In contrast to the Ga-doped ingot, the B-doped ingot showed a relatively tight resistivity range (0·87,1·22,,,cm), resulting in smaller spread in lifetime (60,400,µ,s) and efficiency (16·5,16·7%) along the ingot. However, the LID reduced the efficiency of these B-doped cells by about 1·1% absolute. Additionally, the use of thinner substrate and higher resistivity (4·3,,,cm) B-doped Cz was found to reduce the LID significantly, resulting in an efficiency reduction of 0·5,0·6%, as opposed to >1·0% in ,1,,,cm ,17% efficient screen-printed cells. As a result, Ga-doped Cz cells gave 1·5 and 0·7% higher stabilized efficiency relative to 1 and 4·3,,,cm B-doped Cz Si cells, respectively. Copyright © 2005 John Wiley & Sons, Ltd. [source]

    Geophysical investigation of the site of the former monastic settlement, Clonard, County Meath, Ireland

    Paul J. Gibson
    Abstract Clonard, in County Meath, Ireland was a major ecclesiastical centre from the early sixth century to the twelfth century and buildings associated with the monastery were in existence until the late eighteenth century. However, today no extant buildings or features associated with the monastery are known. The geophysical investigation undertaken in this project has uncovered many anomalies which may be related to the monastic settlement at Clonard. A significant number of linear anomalies have been detected east of St Finian's church using magnetic gradiometry and twin electrode resistance surveying. Many of these anomalies are probably field boundaries; however, a much greater diversity of geophysical responses is located south and west of St Finian's church. A subsquare enclosure of 60,m sides is adjacent to a 300,m long palaeochannel. A fine network of intersecting low resistance anomalies probably represent former artificial drainage channels. Three large areas associated with anomalous magnetic readings were located which might indicate sites of human activity. In addition, there are four distinct zones which have large concentrations of high resistance values suggesting the presence of walls or buildings. Resistivity and ground-penetrating radar depth slices show that one of these zones has characteristics which indicate the presence of an east,west aligned building approximately 7,m wide in a north,south direction and about 15,m long in an east,west direction. Copyright © 2005 John Wiley & Sons, Ltd. [source]

    MOCVD of Platinum Films from (CH3)3CH3CpPt and Pt(acac)2: Nanostructure, Conformality, and Electrical Resistivity,

    J. Goswami
    Abstract A potentially manufacturable liquid-source MOCVD process was applied to deposit platinum (Pt) films (12,140,nm) on thermally oxidized Si substrates. The deposition of Pt films was carried out at a substrate temperature of 350,°C by oxygen-assisted pyrolysis of complex precursors in a low-pressure, hot-wall reactor. The effects of two different metal,organic precursors, a) trimethyl methyl cyclopentadienyl platinum [(CH3)3CH3CpPt], and b) platinum acetylacetonate [Pt(acac)2], on the properties of Pt films were studied. Although the polycrystalline Pt films deposited from Pt(acac)2 exhibited a preferred (111) orientation with a X-ray intensity ratio of I(111)/I(200),=,40, the films deposited from (CH3)3CH3CpPt were highly (111) oriented with I(111)/I(200),=,270. The following properties were typical of Pt films from Pt(acac)2 as compared to Pt films from (CH3)3CH3CpPt: finer grain size (25,nm vs. 50,nm), smaller root mean square (rms) surface roughness (5,nm vs. 15,nm), and better step coverage (95,% vs. 35,%). These experimental findings indicated that growth of Pt films from Pt(acac)2 occurred under the kinetically-limited regime, whereas the deposition of Pt from (CH3)3CH3CpPt was limited by the mass transport rate. Additionally, the temperature (4.2,293,K) dependence of the electrical resistivities (,) of Pt films was measured and the electron mean free paths were estimated. It was observed that ,(T) deviated from Matthiessen's rule. [source]

    ChemInform Abstract: Origin of the Metallization of c-Axis Resistivity upon Iodine Intercalation into Bi2Sr2CaCu2O8+,

    CHEMINFORM, Issue 32 2001
    Jin-Ho Choy
    Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

    Biodegradable Porous Silicon Barcode Nanowires with Defined Geometry,

    Ciro Chiappini
    Abstract Silicon nanowires are of proven importance in such diverse fields as energy production and storage, flexible electronics, and biomedicine due to the unique characteristics that emerge from their 1D semiconducting nature and their mechanical properties. Here, the synthesis of biodegradable porous silicon barcode nanowires by metal-assisted electroless etching of single-crystal silicon with resistivities ranging from 0.0008 to 10,, cm is reported. The geometry of the barcode nanowires is defined by nanolithography and their multicolor reflectance and photoluminescence is characterized. Phase diagrams are developed for the different nanostructures obtained as a function of metal catalyst, H2O2 concentration, ethanol concentration, and silicon resistivity, and a mechanism that explains these observations is proposed. These nanowires are biodegradable, and their degradation time can be modulated by surface treatments. [source]

    A radiomagnetotelluric survey on an oil-contaminated area near the Brazi Refinery, Romania

    B. Tezkan
    ABSTRACT Scalar radiomagnetotelluric measurements were carried out on a contaminated test area close to the Brazi Refinery in Romania in order to detect and to monitor a 1 m thick oil layer expected at 5 m depth. Radio transmitters broadcasting in a frequency range from 10 kHz to 300 kHz were selected to observe the apparent resistivity and the phase data associated with the E- and B-polarizations. They were located parallel and perpendicular to the assumed strike direction of the contamination plume. The data were interpreted by a 2D inversion technique from which the conductivity structure of the area was derived. The 2D inversion models of all profiles on the contaminated area show a poor-conductivity zone above the groundwater table which could be associated with the oil contamination. A first attempt was also made to monitor the contaminated layer: the radiomagnetotelluric measurements were repeated on the same profiles a year later, but this time in a dry period, not in a rainy one. The 2D inversion results of the measurements in the dry period indicate that the high-resistivity layer moved closer to the surface. Additional reference measurements were then carried out on a non-contaminated area situated at a distance from the refinery, in the opposite direction to the flow of the groundwater. These reference measurements were used for the derivation of the unperturbed geology and they were also compared with the measurements of the contaminated test area. There is a significant difference in the frequency dependences of the apparent resistivities of the reference and contaminated areas, which could indicate a contamination at shallow depth. The 2D inversion results show the increase of resistivity at a depth of about 5 m beneath the contaminated area where the oil contamination is expected according to the information from the boreholes. [source]

    A field test of imaging properties of rotational invariants of the magnetotelluric impedance tensor

    László Szarka
    ABSTRACT A part of the Békés Basin (an extensional sub-basin of the Pannonian Basin, where the basement under thick Pannonian sediments is well known from deep boreholes and from seismic measurements, and where many magnetotelluric (MT) soundings have been carried out for frequencies ranging from 1 to 10,3 Hz) was selected as a test area to assess the imaging performances of various apparent-resistivity definitions computed with rotational invariants of either the real part of the complex impedance tensor, or its imaginary part, or both. A comparison (based on earlier 3D numerical studies) has been made between the magnetotelluric images obtained in this way and the depths to the high-resistivity basement, as known from boreholes and seismic investigations. The correlation coefficient between the series of basement depth values at 39 MT sites and the apparent-resistivity values was found to be stronger and high correlation appeared at a shorter period when it was computed with apparent resistivities based on the real tensor rather than with apparent resistivities based on the imaginary tensor. In the light of our studies, ,ReZ and the impedance phase seem to be more informative than any other combination of magnetotelluric interpretation parameters. [source]

    The effect of the electrical anisotropy on the response of helicopter-borne frequency-domain electromagnetic systems

    Changchun Yin
    ABSTRACT Helicopter electromagnetic (HEM) systems are commonly used for conductivity mapping and the data are often interpreted using an isotropic horizontally layered earth model. However, in regions with distinct dipping stratification, it is useful to extend the model to a layered earth with general anisotropy by assigning each layer a symmetrical 3 × 3 resistivity tensor. The electromagnetic (EM) field is represented by two scalar potentials, which describe the poloidal and toroidal parts of the magnetic field. Via a 2D Fourier transform, we obtain two coupled ordinary differential equations in the vertical coordinate. To stabilize the numerical calculation, the wavenumber domain is divided into two parts associated with small and large wavenumbers. The EM field for small wavenumbers is continued from layer to layer with the continuity conditions. For large wavenumbers, the EM field behaves like a DC field and therefore cannot be sensed by airborne EM systems. Thus, the contribution from the large wavenumbers is simply ignored. The magnetic fields are calculated for the vertical coaxial (VCX), horizontal coplanar (HCP) and vertical coplanar (VCP) coil configurations for a helicopter EM system. The apparent resistivities defined from the VCX, VCP and HCP coil responses, when plotted in polar coordinates, clearly identify the principal anisotropic axes of an anisotropic earth. The field example from the Edwards Aquifer recharge area in Texas confirms that the polar plots of the apparent resistivities identify the principal anisotropic axes that coincide well with the direction of the underground structures. [source]

    Using DC resistivity tomography to detect and characterize mountain permafrost

    Christian Hauck
    ABSTRACT Direct-current (DC) resistivity tomography has been applied to different mountain permafrost regions. Despite problems with the very high resistivities of the frozen material, plausible results were obtained. Inversions with synthetic data revealed that an appropriate choice of regularization constraints was important, and that a joint analysis of several tomograms computed with different constraints was required to judge the reliability of individual features. The theoretical results were verified with three field experiments conducted in the Swiss and the Italian Alps. At the first site, near Zermatt, Switzerland, the location and the approximate lateral and vertical extent of an ice core within a moraine could be delineated. On the Murtel rock glacier, eastern Swiss Alps, a steeply dipping boundary at its frontal part was observed, and extremely high resistivities of several M, indicated a high ice content. The base of the rock glacier remained unresolved by the DC resistivity measurements, but it could be constrained with transient EM soundings. On another rock glacier near the Stelvio Pass, eastern Italian Alps, DC resistivity tomography allowed delineation of the rock glacier base, and the only moderately high resistivities within the rock glacier body indicated that the ice content must be lower compared with the Murtel rock glacier. [source]

    3D resistivity inversion using 2D measurements of the electric field

    P.D. Jackson
    Field and ,noisy' synthetic measurements of electric-field components have been inverted into 3D resistivities by smoothness-constrained inversion. Values of electrical field can incorporate changes in polarity of the measured potential differences seen when 2D electrode arrays are used with heterogeneous ,geology', without utilizing negative apparent resistivities or singular geometrical factors. Using both the X - and Y -components of the electric field as measurements resulted in faster convergence of the smoothness-constrained inversion compared with using one component alone. Geological structure and resistivity were reconstructed as well as, or better than, comparable published examples based on traditional measurement types. A 2D electrode grid (20 × 10), incorporating 12 current-source electrodes, was used for both the practical and numerical experiments; this resulted in 366 measurements being made for each current-electrode configuration. Consequently, when using this array for practical field surveys, 366 measurements could be acquired simultaneously, making the upper limit on the speed of acquisition an order of magnitude faster than a comparable conventional pole,dipole survey. Other practical advantages accrue from the closely spaced potential dipoles being insensitive to common-mode noise (e.g. telluric) and only 7% of the electrodes (i.e. those used as current sources) being susceptible to recently reported electrode charge-up effects. [source]

    Depth of detection of highly conducting and volume polarizable targets using induced polarization

    A. Apparao
    We define the apparent frequency effect in induced polarization (IP) as the relative difference between apparent resistivities measured using DC excitation on the one hand and high-frequency excitation (when the IP effect vanishes) on the other. Assuming a given threshold for the minimum detectable anomaly in the apparent frequency effect, the depth of detection of a target by IP can be defined as that depth below which the target response is lower than the threshold for a given electrode array. Physical modelling shows that for the various arrays, the depth of detection of a highly conducting and volume polarizable target agrees closely with the depth of detection of an infinitely conducting and non-polarized body of the same shape and size. The greatest depth of detection is obtained with a two-electrode array, followed by a three-electrode array, while the smallest depth of detection is obtained with a Wenner array when the array spread is in-line (i.e. perpendicular to the strike direction). The depth of detection with a Wenner array improves considerably and is almost equal to that of a two-electrode array when the array spread is broadside (i.e. along the strike direction). [source]

    Artificial neural networks for parameter estimation in geophysics

    Carlos Calderón-Macías
    Artificial neural systems have been used in a variety of problems in the fields of science and engineering. Here we describe a study of the applicability of neural networks to solving some geophysical inverse problems. In particular, we study the problem of obtaining formation resistivities and layer thicknesses from vertical electrical sounding (VES) data and that of obtaining 1D velocity models from seismic waveform data. We use a two-layer feedforward neural network (FNN) that is trained to predict earth models from measured data. Part of the interest in using FNNs for geophysical inversion is that they are adaptive systems that perform a non-linear mapping between two sets of data from a given domain. In both of our applications, we train FNNs using synthetic data as input to the networks and a layer parametrization of the models as the network output. The earth models used for network training are drawn from an ensemble of random models within some prespecified parameter limits. For network training we use the back-propagation algorithm and a hybrid back-propagation,simulated-annealing method for the VES and seismic inverse problems, respectively. Other fundamental issues for obtaining accurate model parameter estimates using trained FNNs are the size of the training data, the network configuration, the description of the data and the model parametrization. Our simulations indicate that FNNs, if adequately trained, produce reasonably accurate earth models when observed data are input to the FNNs. [source]

    Experimental Approaches for Controlling Current Flowing through Metal,Molecule,Metal Junctions,

    ADVANCED MATERIALS, Issue 10 2006
    E. Tran
    Abstract Two experimental approaches that enable control of current flow through metal,molecules,metal junctions are described. A number of studies using two-electrode metal,molecules,metal junctions have shown that the current between the electrodes depends on the structures of the incorporated molecules. When a tunneling mechanism dominates electron transport through organic molecules, the molecules behave similar to resistors with resistivities that can be controlled by changing the structure. Incorporation of molecules with increasing conjugation into Hg-based junctions increases the current flow dramatically. Alternatively, by using four-electrode electrochemical junctions that allow the potential of the electrodes to be controlled with respect to the energy levels of the incorporated molecules, it is possible to change the mechanism of electron transfer and produce abrupt increases in the current flow. These signals, analogous to solid-state diodes, are particularly significant for molecular electronics. Electrochemical junctions also permit prediction of the value of the applied potential at which the current will start taking off and to identify the mechanism of charge transport. New and recently published results obtained using junctions based on Hg electrodes in an "electrochemical" mode show that two junctions incorporating redox centers by different interactions behave as current switches, with the current flow dominated by different charge-transport mechanisms. [source]

    Dipole moments of polyenic oligomeric systems.

    Part II, allenes, molecular organic wire resistivities: polyacetylenes, polyynes
    Abstract Polyacetylenic, allenic and polyynic molecular wire series, containing electron-donor (D) and electron-acceptor (A) groups as two terminal units of the oligomeric bridge (D,wire,A), can be well described by means of a one-dimensional conduction model, which considers a scattering process of electrons through the charge-transfer conduction bridge. The conduction constants (,i) of the oligomeric structures of the three molecular series under study were determined from the functional dependence between the dipole moment of the oligomers (,n) and the ,-molecular orbital bridge length (L). According to our one-dimensional molecular organic wire model: where ,o is the dipolar moment of the first compound of the oligomeric series without a bridge unit (n,=,0) and ,, is a limit value for L,,,,. By means of the Landauer theoretical expression for the conductance of a metallic one-dimensional conductor and our molecular wire conduction constants (,i), we determined the intrinsic resistivities associated with the molecular resistances of these oligomeric wires. Using this approach we determined, for the first time, the linear and non-linear contributions to the net molecular resistivity. The order of magnitudes of the linear resistivities determined in these oligomeric systems agrees very well with the expected results of experimental measurements for macroscopic wires. Copyright © 2005 John Wiley & Sons, Ltd. [source]

    Analysis of different factors affecting cathodic protection for deep well casings

    I. A. Metwally
    Abstract This paper introduces a comparative theoretical investigation of the conventional cathodic protection (CP) and the pulse cathodic protection (PCP) systems to show how both of them behave under different operating conditions. The effectiveness of the PCP system is also highlighted for a typical large-scale configuration as well as some field measurements have been carried out. The performance of PCP system has been analyzed in the light of getting better protection-current distribution along the protected well casing at reduced anode current together with reducing the stray current (corrosion) at any nearby unprotected structure(s). Many factors have been investigated to show their effects on the performance of the CP system, namely, soil resistivity, voltage pulse waveform and frequency, and multi-layer soil. In addition, the performance of both the conventional CP and the PCP systems has been compared to that when utilizing unused/abandoned well casing as anode energized by the conventional CP system. The PCP system shows better performance than that of the conventional CP, and a similar performance at high soil resistivity to that when using unused/abandoned well casing. On the other hand, the utilization of the unused/abandoned well casing gives superior performance, especially at low soil resistivities, where the protection-current profiles of both the conventional CP and the PCP systems decay sharply. [source]

    Permafrost distribution in talus slopes located within the alpine periglacial belt, Swiss Alps

    Christophe Lambiel
    Abstract The extent of permafrost in nine talus slopes located within the alpine periglacial belt in the western Swiss Alps was investigated using ground surface temperature measurements and one- dimensional geoelectrical profiles. Based on the thermal data, permafrost appears likely in the lower parts of the slopes. At the same locations, an electrically resistive layer, interpreted as frozen sediments, was identified beneath 3,5,m of surface materials. Further upslope, resistivities were lower and ground surface temperatures were warmer, suggesting that permafrost was absent. The cooling of the ground in the lower parts of some of the talus slopes investigated was apparently due to the chimney effect. It is inferred that this mechanism plays an important role in permafrost development in the lower half of talus slopes located within the discontinuous mountain permafrost belt. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Permafrost and Little Ice Age glacier relationships, Posets Massif, Central Pyrenees, Spain

    Ralph Lugon
    Abstract This paper contributes to the study of permafrost in the Pyrenees by reporting geoelectrical investigations and thermal measurement on the Little Ice Age (LIA) forefields of two glaciers. The aim was to assess the internal composition of sedimentary bodies (debris rock glaciers and moraine deposits) located in this proglacial environment. Ground ice was prospected using two DC resistivity techniques: vertical electrical soundings and resistivity mapping at a fixed pseudo-depth. Extreme specific resistivities ranging between 1 and 25,M,,m were detected under a thin (1,2,m) unfrozen layer, indicating the presence of a massive ice layer, certainly buried glacier ice. This ice of glacial origin probably covers former permafrost bodies, i.e. a much thicker layer of perennially frozen sediments. Low subsurface temperatures measured on the deposits indicate that buried glacier ice could have been preserved on top of permafrost since the end of the LIA or earlier Holocene glacier advances. This stratigraphy demonstrates that glaciers and pre-existing perennially frozen sediments (permafrost) were in contact during the LIA. Copyright © 2004 John Wiley & Sons, Ltd. [source]

    Inversion and interpretation of two-dimensional geoelectrical measurements for detecting permafrost in mountainous regions

    Christian Hauck
    Abstract The use of tomographic geoelectrical measurements for the detection of ground ice occurrences in mountainous regions is evaluated. Because isolated ground ice occurrences in a warming climate may be subject to rapid thawing, they present a future hazard in the context of thawing-permafrost-induced landslides and rockfalls. Isolated permafrost occurrences are difficult to detect with commonly used one-dimensional sounding techniques, as plane-layer approximations for the inversion are often invalid. Therefore, recently developed two-dimensional tomographic inversion schemes are used to yield realistic near-surface geological models. The method is applied to various types of permafrost occurrences in the European Alps. The influence of topography, measurement geometry and different inversion parameters are discussed. In addition, complimentary seismic survey results are used to exclude the possibility of air-filled cavities, which exhibit similarly high resistivities as ground ice occurrences. The results show a large variety of influencing parameters which have to be considered in resistivity inversion and interpretation. Copyright © 2003 John Wiley & Sons, Ltd. [source]

    Highly conductive and optically transparent GZO films grown under metal-rich conditions by plasma assisted MBE

    H. Y. Liu
    Abstract We demonstrate a critical effect of a metal-to-oxygen ratio on the electrical, optical, and structural properties of ZnO films heavily doped with Ga (carrier concentration in the range of 1020,1021 cm,3) grown by plasma-assisted molecular beam epitaxy. The as-grown layers prepared under the metal-rich conditions exhibited resistivities below 3 × 10,4 , cm and an optical transparency exceeding 90% in the visible spectral range as well as a large blue shift of the transmission/absorption edge attributed to the Burstein,Moss shift of the Fermi level deep into the conduction band, indicating high donor concentration. In contrast, the films grown under the oxygen-rich conditions required thermal activation and showed inferior properties. Furthermore, electrical measurements point to the nonuniform depth distribution of free carriers. An oxygen-pressure-dependent surface disordering is suggested to be responsible for the drastic effect of the metal-to-oxygen ratio on the film properties. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Improved electrical, optical, and structural properties of undoped ZnO thin films grown by water-mist-assisted spray pyrolysis

    L. Martínez Pérez
    Abstract Undoped ZnO thin films were prepared using the ultrasonic spray pyrolysis deposition technique with zinc acetylacetonate dissolved in N,N-dimethylformamide as the source materials solution. The addition of water mist in a parallel flux to the spray solution stream was also used during deposition of the films. The addition of water mist improved the electrical characteristics of the ZnO films. Fresh ZnO samples were then thermally annealed in a H2 reducing atmosphere. X-ray diffraction patterns show mainly the wurzite crystalline ZnO phase in the films. An electrical resistivity (, ) of around 2.7 × 10,2 , cm was measured at room temperature for the best undoped ZnO film. , is approximately one order of magnitude lower than the resistivities found in undoped ZnO films obtained by means of similar non-vacuum deposition techniques. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]