bluuebunny/crossref_metadata_2025 huggingface dataset:
Achievement of More than 10% Efficiency for a Single-Junction 100cm2 a-Si Solar Cell and Development of a New-Type Module Structure
Authors: Hishikawa Yoshihiro , Ohnishi Michitoshe , Kuwano Yukinori
Published: March 2011

A total area conversion efficiency of 10.2% has been achieved for a 1Ocm×1Ocm integrated-type single-junction amorphous silicon (a-Si) solar cell submodule. It is the highest conversion efficiency ever reported for an a-Si solar cell with an area of 100cm2, including multi-junction cells. The effective area conversion efficiency is as high as 11.3%. The high efficiency is obtained by improving the quality of the i-layer and the p/i buffer layer, as well as by utilizing a highly textured, high-quality transparent electrode. The quality of the i-layer plays a dominant role in the performance of a-Si solar cells, especially in high efficiency cells. Techniques that control the properties of the high-quality a-Si films for the i-layer are described. Electric conductivity, ESR spin density and the Raman spectra of high-quality a-Si:H films are investigated as well as their thickness-dependence and substrate-dependence.

A Through-Hole Contact (THC) integrated-type submodule has been developed as a new-type a-Si solar cell module structure. Numerical simulations on the output power of the structure show that the output power can be significantly improved by the THC structure.
Effect of Bias on Recovery Rates of Stressed Amorphous Silicon Mis Structures
Authors: Jackson W. B. , Hack M.
Published: March 2011

The effect of Fermi level position on the annealing of defects created by electron accumulation is investigated. Results indicate that holes accelerate the annealing of defects created by electrons confirming the theoretical understanding of the proximity compensated layer. Capacitance-voltage curves indicate that hole accumulation tends to create defect states located closer to the conduction band than electron accumulation–a result in agreement with energetics of defect formation.
Field Profile Tailoring in a-Si:H Radiation Detectors
Authors: Fujieda I. , Cho G. , Drewery J. , Jing T. , Kaplan S. N. , Perez-Mendez V. , Qureshi S. , Wildermuth D. , Street R. A.
Published: March 2011

The capability of tailoring the field profile in reverse-biased a-Si:H diodes by doping and/or manipulating electrode shapes opens a way to many interesting device structures. Charge collection in a-Si:H radiation detectors is improved for high LET particle detection by inserting thin doped layers into the i-layer of the usual p-i-n diode. This buried p-i-n structure enables us to apply higher reverse-bias and the electric field is enhanced in the mid i-layer. Field profiles of the new structures are calculated and the improved charge collection process is discussed. Also discussed is the possibility of field profile tailoring by utilizing the fixed space charges in i-layers and/or manipulating electrode shapes of the reverse-biased p-i-n diodes.
The Role of Carbon in Amorphous Silicon Nip Photodiode Sensors
Authors: Weisfield R. L. , Tsai C. C.
Published: March 2011

We have been investigating the dark current and quantum efficiency of nip and pin photodiodes for document scanning applications. Alloying the top doped layer with carbon or making it microcrystalline was found to enhance the spectral response without causing significant deleterious effects to dark current. We also observed lower dark currents in both pin and nip(μc-Si:C:H) sensors compared to nip(a-Si:C:H) sensors, which we attribute to a more graded p-i interface in the former devices.
Effects of Buffer Layers on Performance of Amorphous Silicon Image Sensors
Authors: Miyagaki S. , Ri S. , Takasaki K.
Published: March 2011

This paper reports the effects of a-SiC:H buffer layers on a-Si:H photodiode properties, especially the dark current and decay image lag. The photodiode has an ITO/p-type a-SiC:H/a-Si:H/i-type a-SiC:H/metal structure.

Using two heterojunctions–p-SiC:H/a-Si:H at the surface contact and a-Si:H/i-SiC:H at the back contact–significantly decreases the dark current. The p-SiC:H layer blocks electron injection, and the i-SiC:H layer suppresses the reaction between the a-Si:H layer and metal electrode. The use of both a-SiC:H layers may increase decay image lag. Excess boron doping to the p-SiC:H layer induces boron diffusion into the a-Si:H photo-electric conversion layer, which increases trap levels in the a-Si:H layer. The i-SiC:H layer, because of its high resistivity, decreases the electric field applied to the a-Si:H photo-electric conversion layer.
Optimizing the amount of boron in the p-SiC:H layer and the resistance of the i-SiC:H layer enabled us to produce the characteristics required for high-definition television (HDTV) image sensors–a dark current on the order of 10−10 A/cm2 and a 1% level decay image lag.
A-Si:H Deposited by Direct Photo-Cvd Using a Microwave-Excited Xe Lamp
Authors: Matsunami Hiroyuki , Shirafuji Tatsuru , Fuyuki Takashi , Yoshimoto Masahiro
Published: March 2011

A-Si:H films were deposited by direct decomposition of Si2H6 using vacuum ultra-violet light (147nm) emitted from a microwave-excited Xe resonance lamp. The substrate temperature was varied between 200 and 300 °C . The hydrogen content in the films is estimated to be comparable or lower than that in high-quality glow discharge (GD) films based on results from infrared absorption measurements and hydrogen effusion measurements. Hydrogen atoms are incorporated in the form of Si-H bonds (2000 cm−1 mode), and Si-Hn bonds of the 2090 mode In a unit volume was one or two order less than that of Si-H bonds (2000 cm−1 mode). In spite of the lower hydrogen content, the defect density was lower than that of GD films. The mobility gap of the photo-CVD film was closer to the optical gap. For both n- and p-type doping, the dark conductivity increased with the dopant gas composition monotonously, and reached the order of 10−3 S/cm.
Post Initial Light Induced Degradation Stability and Performance of Amorphous Silicon Modules
Authors: Berry W. B. , Hahn M.J. , Mrig L.
Published: March 2011

Data is presented on the field test of amorphous silicon photovoltaic modules. The modules have been obtained by SERI under the field test and performance evaluation program to track the changes in performance following the continuing development of improvements in their manufacture. The data have been gathered for up to five years. Procurement time intervals are used to distinguish between the sets of modules under test. The results show that more recent technogies have a more rapid initial loss of performance but that stability sets in at an earlier time than that observed for the earlier modules. Data are presented to show efficiency and fill factor degradation from both the date of deployment and a date selected for post initial degradation(pi). Performance models are presented and compared, including models which account for annual variations in insolation and temperature and, therefore, generation of light induced defects and annealing.
¼c Silicon Thin Films Deposited by Remote Plasma Enhanced Chemical Vapor Deposition Process
Authors: Wang C. , Parsons G. N. , Kim S. S. , Buehler E. C. , Nemanich R. J. , Lucovsky G
Published: March 2011

In an earlier study, we deposited ¼c-Si thin films by reactive magnetron sputtering (RMS). Here we extend our studies to the deposition of both undoped and high conductivity N-type and P-type ¼c-Si thin films by a remote PECVD. We show that ¼c-Si films can be deposited by bringing hydrogen, H2, into the source gas mixtures. The H2 could introduced by either upstream in a He/H2 mixture and directly plasma excited, or downstream, and be remotely excited along with the silane, SiH4, feed gas. The degree of crystallinity is shown to depend on the hydrogen dilution, the substrate temperature and the substrate material.
Improved Blue Response of Amorphous Silicon Alloy Solar Cells
Authors: Banerjee A. , Guha S.
Published: March 2011

A two-layer MgF2/ITO antireflection (AR) coating has been used to reduce the reflection losses from the surface of a hydrogenated amorphous silicon alloy solar cell. This has resulted in a higher efficiency device primarily due to an improved blue response. The relative thicknesses of the MgF2and ITO layers have been tailored to give the highest overall quantum efficiency (Q) values, which are higher than that obtained with a single-layer antireflection coating. Typically, the 0 value at 400 nm (Q400) has been increased from 0.58 to 0.68 for a single a:SiH cell. Incorporation of the double-layer AR coating in conjunction with μc-SiC p-layer has yielded Q400value of 0.77. The total current density obtained by adding the individual contribution of the component cells of a dual bandgap triple amorphous silicon alloy solar cell has been increased from 21.90 to 23.27 mA/cm2using the double-layer AR coating.
Reduction of Internal Stress of a-Si:H Films by in Situ Measurements of Optical Emission Intensity from SiH4 Plasma
Authors: Tamahashi K. , Wakagi M. , Ishikawa F. , Kaneko T. , Tamura K. , Satoh A. , Hanazono M.
Published: March 2011

A parameter which determine internal stress of hydrogenated amorphous silicon (a-Si:H) prepared by plasma CVD method has been investigated to prevent a peeling off or crack forming problem. It is clarified that the internal stress changes from tension to compression as a function of supplied rf power density during deposition process. Supplied rf power density is closely connected to H o and SiH* optical emission intensity ratio (H α /SiH* ) of CVD plasma. Ve find that the internal stress can be reduced remarkably by controlling this ratio to 0.3. This H α/SiH* parameter has a superiority that the internal stress can be in situ controlled.
Hydrogen Dilution of Silane: Correlation Between the Structure and Optical Band Gap in Gd a-Si:H Films
Authors: Meiling H. , Van Den Boogaard M. J. , Schropp R.E.I. , Bezemer J. , Van der Weg W.F.
Published: March 2011

The influence of diluting SiH4 with H2 on the optical and structural properties of a-Si:H has been investigated. The major effects of the dilution are a pronounced decrease of the amount of (Si-H2)n-chains, an increase of the macroscopic density, and a decrease of the total amount of incorporated hydrogen in the films. Different models for the structure as a function of the hydrogen concentration are deduced. We also present new data on the dependence of the optical band gap Eg on the hydrogen concentration and density of the films. An important result from this work is that Eg appears to correlate with the distance between the silicon atoms, rather than with the overall hydrogen concentration.
Systematic Errors in the Analysis of the Infrared Transmission Data Of Hydrogenated Amorphous Silicon
Authors: Maley N. , Szafranek I.
Published: March 2011

The validity of the Brodsky, Cardona and Cuomo (BCC) [1] and the Connell and Lewis (CL) [2] methods to analyze infrared transmission data of hydrogenated amorphous silicon (a-Si:H) was examined using computer simulations. Transmission spectra for a-Si:H films 0-5¼m thick and containing up to 30 atomic% hydrogen were simulated assuming coherent reflections in the film and incoherent reflections in the c-Si substrate. Analysis of the simulated data for the 640cm−1 Si-H wagging mode shows that the BCC and CL techniques systematically overestimate the absorption coefficeint, α, and hence hydrogen content, CH, when the film thickness, d, is less than ∼l¼m. The error is nearly independent of CH and is as large as 80% in the limit d→0. On this basis, previously reported experimental evidence for the dependence of CH on d is shown to be an analysis artifact. A simple method to correct the hydrogen content determined by the BCC or CL analysis using only the film thickness is presented.
Transmission Electron Microscopy of Excimer Laser Crystallized Amorphous Si Thin Films
Authors: Anderson G. B. , Bachrach R. Z. , Winer K. , Boyce J. B. , Ponce F. A. , Johnson R. I. , Ready S. E.
Published: March 2011

The results of a transmission electron microscopy (TEM) study of excimer laser crystallized a-Si:H thin films are described in this paper. High resolution TEM has shown the crystallization threshold of 10−2 P a-Si:H, grown by PECVD, to be 85 mj/cm2. TEM reveals that PECVD a-Si:H films crystallize in a stratified structure at laser energy densities greater than 120 mj/cm2. This stratified structure consists of a polycrystalline Si region near the surface, a microcrystalline region below the polycrystalline region and uncrystallized a-Si near the substrate. The depth of the crystallized region increases as the laser energy density increases. Spherical voids are also present in the film in the microcrystalline region. These voids are formed by H2 evolving from the film during crystallization. TEM also reveals that thermal annealing a 50 nm film of a-Si:H followed by laser crystallization produces 100 nm Si grains with low defect density.
A New Technique for Determining Midgap States and Hole Localization in a-Si:H Devices
Authors: Dalal Vikarm L. , Knox Ralph D.
Published: March 2011

We describe a technique for measuring localization of holes in mid-gap states in high quality a-Si:H devices. The localization of holes is determined by measuring quantum efficiency of a-Si:H devices as a function of reverse bias voltage and wavelength of light. It is shown that the QE of localized holes increases significantly upon application of high electric fields, whereas the QE of de-localized holes does not show such a behavior. The voltage-induced increase in QE is explained using a Frenkel-Poole tunneling model. It is also shown that the density of mid-gap states (states in which holes are localized) increases significantly upon light soaking, and that a major consequence of this increase in mid-gap density is a decrease in the electric field in the device. The decrease in electric field is experimentally estimated by fitting the increased current due to tunneling to the expression for Frenkel-Poole tunneling.
Does ion Bombardment Induce a Degradation of the Electronic Properties of a-Si:H Films?
Authors: Roca i Cabarrocas P. , Morin P. , Conde J. , Chu V. , Liu J.Z. , Park H.R. , Wagner S.
Published: March 2011

We present a detailed study of the effects of a negative bias applied to the substrate on the electronic properties of a-Si:H films deposited by r.f. glow discharge. Two series of samples deposited at 30 and 100 mTorr respectively have been studied. For each series the negative d.c. bias applied to the substrate was decreased from 0 to −100 V in steps of 25 V. We observe for both series of samples an improvement of the electronic properties of the films as we decrease the substrate bias (increase the ion energy) down to − 50 V. We have found a clear correlation between the negative bias applied to the substrate and the subgap absorption, the valence band tail slope and the electron and hole μτ products.
Saturation behavior of the Light-Induced Defect Density in Hydrogenated Amorphous Silicon
Authors: Park H. R. , Liu J. Z. , i Cabarrocas P. Roca , Maruyama A. , Isomura M. , Wagner S. , Abelson J. R. , Finger F.
Published: March 2011

Using a Kr ion laser (λ = 647.1 nm) to produce a carrier generation rate G of 3 × 1020 cm−3s−1, we have saturated the light-induced defect generation in hydrogenated (and fluorinated) amorphous silicon (a-Si:H(F)), within a few hours near room temperature. While the defect generation rate scales roughly with 1/G2, the saturation defect densities Ns,sat are essentially independent of G. The saturation is not due to thermal annealing. We have further measured Ns,sat m 37 a-Si:H(F) films grown in six different reactors under different conditions. The results show that Ns,sat lies between 5 × 1016 and 2 × 1017 cm−3, that Ns,sat drops with decreasing optical gap and hydrogen content, and that Ns,sat is not correlated with the initial defect density or with the Urbach energy.
Study of Temperature and Light Bias Effects on the Occupied Near Surface Defect Density in Hydrogenated Amorphous Silicon
Authors: Aljishi Samer , David Cohen J. , Ley Lothar
Published: March 2011

The occupied density of states distributions in doped and undoped a-Si:H are investigated. Results reveal the presence of a distinct defect sub-band at 0.6 eV above the valence band edge Ey in n-type and in undoped films. In p-type films, this band is absent. Instead, a new band centered at 0.8 eV above Ev develops. This band is normally unoccupied and is therefore only seen under non-equilibrium conditions. A sub-band at this energy is also discovered in undoped a-Si:H films. The effects of temperature and light bias on these defect sub-bands are discussed.
Light-Induced Metastable Effects in a-Ge:H
Authors: Santos P. V. , de O. Graeff C. F. , Marcano G. , Chambouleyron I.
Published: March 2011

We report on light induced metastable conductivity changes in a-Ge:H thin films deposited by rf-sputtering. The dark- and the photoconductivity of the samples decrease after light soaking. The changes are metastable and both the defect formation and the defect annealing processes are temperature activated. As in the case of a-Si:H films, the time evolution of the conductivity changes is well represented by a stretched exponential type of decay, with activated time constants.
Static Dipole Polarizabilities of C, N, O and F: The Importance of Spin Projection
Authors: Sosa C. , Ferris K. F.
Published: March 2011

The static dipole polarizability for the ground state of C(3Π ), N(4∑+), O(3Π ) and F(2∑+ ) have been calculated using ab initio molecular orbital techniques. The polarizabilities are obtained from unrestricted Hartree-Fock and Many-Body Perturbation Theory wave functions with and without spin projection, which are computed by finite-field differentiation. The effect of spin contamination is shown to be important on the calculation of static dipole polarizabilities for the ground state of C, N, O and F.
Electronic Structure Calculations of Σ5 Grain Boundaries in Cu
Authors: Sowa Erik C. , Gonis A. , Zhang X.-G.
Published: March 2011

Extended Abstract

In a recent publication [1], we presented results of electronic-structure calculations on Σ5 grain boundaries in Cu. These calculations were performed using the real-space multiplescattering theory (RSMST), a first-principles method which does not rely on translational invariance and Bloch's theorem [2, 3], and hence avoids the problems associated with treating interfaces in a periodic repeating slab geometry. A brief summary of this method follows.

The RSMST is based on the concept of semi-infinite periodicity (SIP), defined as the regular repetition along a given direction of a scattering unit (atom, planes of atoms, etc.), or a set of such units. Systems with SIP possess the property of removal invariance, which states that the scattering properties (scattering matrices) of any such system remain invariant when an integral number of scattering units is removed from, or added to, the free end of the system. Using this property in conjunction with multiple-scattering theory, one can determine the electronic Green function, and hence all one-particle quantities such as the density of states (DOS), directly in real space.

The essence of the method consists in a prescription for the proper renormalization of the scattering properties of the boundary sites of a cluster of atoms. Unrenormalized or “bare” sites in the interior of the cluster describe the region of interest, such as a grain boundary, while the renormalized boundary sites represent the infinite medium surrounding the cluster. This “dressing up” of the cluster is done independently for each part of a system that is characterized by its own SIP, so that grain boundaries between essentially arbitrary crystal structures can be treated.

The calculations described previously [1] used given atomistic configurations and electronic one-particle potentials and computed the local DOS at atoms of interest within the grain boundaries. We are currently extending our method to incorporate charge self-consistency and total-energy capabilities. In connection with this, we have implemented four different methods of expressing mathematically the invariance properties of semi-infinite systems, and are evaluating them for optimal rates of convergence and computational efficiency. Calculations on Cu (100) monolayers will be used to illustrate these methods.
Interatomic Forces and Bonding Mechanisms in MgO Clusters
Authors: Wright Nancy F. , Painter Gayle S.
Published: March 2011

We report results from a first-principles local spin density quantum mechanical study of the energetics and elastic properties of a series of magnesium-oxygen clusters of various morphologies. The role of quantum effects, e.g. covalency, in the bonding character of diatomic MgO is determined by comparison of classical and quantum restoring force curves. The dependence of binding properties on geometry and metal to oxygen ratio is determined by comparison of binding energy curves for a series of clusters. Results show that while gross features of the binding curves may be represented by simple interatomic potentials, details require the many body corrections of a full quantum treatment.
Molecular Dynamics Studies of Surface Indentation in Two Dimensions
Authors: Belak J. , Stowers I. F.
Published: March 2011

Non-equilibrium molecular dynamics is used to study the indentation of an ideal metallic surface by a hard spherical ball. The problem is symmetric about the axis of the ball and we choose to study the deformation in two dimensions. We use the simple embedding function F(ρ) = 2eρln ρ along with the Lennard-Jones two body potential to model a material resembling copper. We present a calculation of the st'ress field (from the internal atomistic description) for both elastic and plastic deformations. The material deforms plastically through the creation of edge dislocations and the flow of material around the sides of the indenter. The creation of dislocations is reversible—they return to the surface (anneal out) when the indenter is removed. The plastic flow leads to a permanent indentation in the surface.
Multiple Scattering Theory for Space Filling Potentials
Authors: Butler W. H. , Brown R. G. , Nesbet R. K.
Published: March 2011

Multiple scattering theory (MST) provides an efficient technique for solving the wave equation for the special case of muffin-tin potentials. Here MST is extended to treat space filling non- muffin tin potentials and its validity, accuracy and efficiency are tested by application of the two dimensional empty lattice test. For this test it is found that the traditional formulation of MST does not converge as the number of partial waves is increased. A simple modification of MST, however, allows this problem to be solved exactly and efficiently.
Molecular Water in Silica
Authors: Fairbrother Peter J. , Heggie Malcolm I. , Tole Philip , Jones Bob , Oberg Sven
Published: March 2011

Both silica (SiO2 ) and water are ubiquitous in our environment and our technologies. They come together in two important processes: the oxidation of silicon by steam, which is a process step in integrated circuit fabrication, and the hydrolytic weakening of quartz, which helps us understand plate tectonics. In the former process water molecules must diffuse through an amorphous silica layer and in the latter they must diffuse through quartz to reach dislocations and bubbles to promote plasticity.

We have taken a molecular fragment of SiO2 based on an a quartz c channel, with a hydrogenated external surface and with an internal water molecule. Ab initio, local density, total energy calculations, relaxing the water molecule and the atoms lining the channel, have been applied to discover the insertion energy of water in quartz. In addition, this quantity has been examined in a c axis dislocation core, in order to evaluate the ease of pipe diffusion in the case of hydrolytic weakening and to better simulate amorphous SiO2 in the case of silicon oxidation.
A Molecular Dynamics Study of Zirconium Based on An N-Body Potential: HCP/BCC Phase Transformation and Diffusion Mechanisms in the BCC-Phase
Authors: Willaime Francois , Massobrio Carlo
Published: March 2011

We propose a simple interatomic potential model for zirconium which successfully reproduces the phonon dispersion curves in both the α- and the β-phases as well as the temperature-induced phase transformation. Two interesting phenomena that help to understand the anomalous diffusion in the high-temperature β-phase are observed: the spontaneous formation of Frenkel-pairs and the highly correlated walk of the vacancy.
Theoretical Model for Adsorbate-Induced Surface Reconstructions: A Comparison of Potassium and Oxygen on Copper
Authors: Jacobsen K. W.
Published: March 2011

A Cu(110) surface can exhibit missing-row type reconstructions if potassium or oxygen atoms are chemisorbed on the surface. In the case of potassium every second of the closepacked rows in the top layer of the copper surface is removed. The reconstruction induced by oxygen on the same surface is of a very different kind where the missing rows are formed by picking out every second atom in the close-packed rows of the copper surface. The energetics and the driving force behind the two different reconstructions are discussed and compared on the basis of effective medium calculations.
Energetics and Structures of Pt, Pd, and Ni Adatom Clusters on the Pt (001) Surface
Authors: Wright Alan F. , Daw Murray S. , Fong C. Y.
Published: March 2011

Using the embedded atom method (EAM), we have calculated the stable configurations of Pt, Pd, and Ni clusters, containing up to nine adatoms, on the Pt (001) surface. For Pt, we predict that the stable configurations are linear chains oriented along the <110> directions for three and five adatoms, and close-packed islands otherwise. These results, for clusters containing up to seven adatoms, were reported previously in reference 4. For Pd, the results are the same except that the stable configuration predicted for five adatoms is a close-packed island. For Ni, we predict that linear chains are the stable configurations for all numbers of adatoms. In determining these stable configurations, we allowed substrate relaxations. To assess the importance of substrate relaxations, we also performed calculations on an unrelaxed substrate. We found that substrate relaxations are important with regard to the stable configurations, and that allowing relaxation lowers the energies of chains with respect to islands.
Electronic Structure Studies of Diamond/Metal Interfaces
Authors: Erwin Steven C. , Pickett Warren C.
Published: March 2011

We have begun to investigate theoretically the electronic properties of several ideal epitaxial interfaces of diamond with Ni and Cu, both of which enjoy a close lattice match. Of particular interest is the mechanism responsible for formation of the Schottky barrier, which is not yet fully understood at the microscopic level. We find that both the barrier height and the chemical bonding at the interface are strongly dependent on interface orientation (i.e., the relative positioning of the two surfaces). For orientations near the minimum total energy geometry, the calculated Fermi level is apparently pinned around 1.7 and 2.1 eV for the (111) and (001) interfaces, respectively, relative to the valence band maximum. For orientations not near the total energy minimum, the calculated barrier height is zero. A tentative explanation for this difference is proposed.
Magnetoresistance in Granular Metals
Authors: Mclean W.L.
Published: March 2011

We review the origins of the magnetoresistance in granular metals with emphasis on the quantum coherence effects arising from the interference of electrons traveling by different paths. We discuss the maximum in the magnetoresistance expected from scaling arguments and observed in granular aluminum. Two theories of the magnetoresistance in the variable range hopping regime, one based on the magnetic-field induced shift of the mobility edge and the other dealing with ‘directed’ hopping, are compared with experiments.
Thermoelectric Properties of Two Component Composites
Authors: Levy Ohad , Bergman David J.
Published: March 2011

An explicit expression for the bulk effective thermoelectric coefficient αe of a two component composite is derived. This coefficient is found to depend only on the bulk effective electric conductivity σe and thermal conductivity γe, as well as on the moduli of the pure components. Using this expression and making standard scaling assumptions about the forms of σe and γe, the scaling behavior of αe, and of the effective figure of merit Ze are investigated. This behavior depends strongly on the thermal and electric conductivities ratios in the pure components. Upper and lower bounds for αe are calculated from its analytical properties.
Tunneling and Percolation Behavior in Granular Metals
Authors: Balberg I. , Wagner N. , Goldstein Y. , Weisz S.Z.
Published: March 2011

The nature of the percolation process in granular metals is examined for the first time by a computer simulation of a system of metallic grains embedded in an insulating matrix. Assuming that the intergrain conduction is due to quantum mechanical tunneling it is found that a percolation-like critical behavior of the conductivity is obtained, but that a percolation universal behavior will be found only in a very special case. In contrast, the behavior of the electrical noise does not deviate substantially from the universal one. Comparison of these results with recent experimental observations suggests that in the metallic range, both transport properties are controlled by the continuous metallic network rather than by intergrain tunnelin.. We propose that the metallic network resembles the previously studied system of ‘inverted random voids’.
Inverse Transport Problems for Composite Media
Authors: Mcphedran Ross C. , Milton Graeme W.
Published: March 2011

For two-component ceramic metal composites we address the inverse problem of estimating the volume fraction occupied by each component from measurements of the transmission and absorption coefficients at various frequencies of the applied radiation. We devise the algorithms, each based on representation formulas and bounds for the complex dielectric constant that were developed by Bergman and Milton. One inversion algorithm works remarkably well when the measurements are exact but fails completely when small errors are present. The second random throw algorithm works well when the measurement errors are large but is computationally intensive and is limited to small data sets. The third method based on single and two-point inversion, while not so accurate, is simple and efficient. The last two algorithms are implemented with experimental data and good agreement is obtained with the experimentally measured true volume fraction.
Effective-Medium Approach for Composite Materials Containing Conductive Sticks
Authors: Sarychev A.K. , Smychkovich Y.R.
Published: March 2011

There is a growing interest in composites in which conductive component has the elongated shape (sticks) because these systems can be used to describe various physical objects [1]. The main tool to investigate the systems is MC simulation. For example in papers [2-4] the percolation threshold in such systems has been explored and it was obtained that pcα r/L, where r is the radius of stick and L is it's length. In this work, we are suggest the calculation of dielectric constant and conductivity method based on effective medium approximation.
Infrared Absorption by Granular Metals
Authors: Tanner D.B. , Kim Y.H. , Carr C.L.
Published: March 2011

The infrared properties of granular metals and superconductors are qualitatively in accord with effective medium ideas, with insulating behavior below a percolation transition and metallic response above. An exception is the far-infrared absorption at low metallic concentrations, which is much stronger than theoretical predictions. Measurements of superconductors and of normal metals in different hosts suggest that this absorption is predominately electric dipole rather than the magnetic dipole (eddy current) absorption which is expected to be the dominant low-frequency loss in highly conducting particles. Measurements of clustered and non-clustered samples suggest that the strong far-infrared absorption does not arise from the clustering together of the individual metallic particles, although clustering does lead to about a tenfold increase in absorption.
Superconducting and Mechanical Properties of YBa2Cu3O7−x / Silver Granular Solids
Authors: Niculescu H. K. , Gielisse P. J. , Hascicek Y. S. , Testardi L. R.
Published: March 2011

In this paper an experimental study of YBa2Cu3O7−x/silver composites is presented and the relationship between microstructure, superconducting and mechanical properties is discussed. Silver contents ranging from 0 to 17 vol% fell all below the percolation treshold. X-ray and Wavelength Dispersive Spectrometry analyses indicate that Ag remains in a free state, and is located mainly at the grain boundaries. A tenfold increase in Jc. and a more than twofold increase in strength are the benefits of the improved microstructure of silver composites. The Tc remains unaffected.
Indium-Impregnated Porous Glass: Magnetotransport and Superconducting Transition
Authors: Graf M. J. , Huber C. A. , Huber T. E. , Salzberg A. P.
Published: March 2011

Measurements of the resistive superconducting transition and magnetoresistance of indium metal in the restricted geometry of 50 Å size porous Vycor glass are reported. We have mapped out the upper critical magnetic field versus temperature phase diagram for these transitions for fields up to 20 T and temperatures down to 100 mK. The results are consistent with magnetization measurements by previous authors and can be generally interpreted in terms of the description of inhomogeneous type-II superconductors. The field-induced resistive transitions exhibit unusual behavior not probed by previous bulk-type measurements which may be related to the microstructure of the composite.
Spin Glass Magnetic Behavior of Iron/Silica Gel Nanocomposites
Authors: Shull Robert D. , Ritter Joseph J.
Published: March 2011

Homogeneous gelled composites of iron and silica containing 5-40 wt.%Fe prepared by low temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst) were heated to 380°C in the presence of hydrogen gas. X-ray diffraction and M6ssbauer effect measurements, and transmission electron microscope (TEM) observations show these materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix. Magnetic susceptibility data indicate the materials became either superparamagnetic or ferromagnetic at room temperature. On cooling, the magnetization data furthermore show that the hydrogenated materials containing ll-30% Fe become magnetic spin glasses at temperatures less than 30 K. Magnetic history effects are observed in addition to displaced hysteresis loops below their spin freezing temperatures (Tf). For field-cooled materials at 10 K, the displacement of the hysteresis loops along the field axis indicates the presence of a unidirectional anisotropy which decreases with the cooling field. Both superparamagnetic-to-spin glass and ferromagnetic-to-spin glass transitions are observed in these nanocomposites. Tf varies with the Fe content from ∼30 K for the 11%Fe nanocomposite to ∼10 K for a content near 33%Fe.
Magnetic Properties of Nanostructured Fe/Ag Composite Metal Films
Authors: Liou S.H. , Zhang Y.X. , Deangelis R.J.
Published: March 2011

We have synthesized a series of nanostructured Fe/Ag composite metal films. The much enhanced magnetic coercivity was found to scale with the particle size which is process controlled. At a low substrate temperature(< 200°C), the film consists of small clusters of Fe(< 100Å) which behave superparamagnetically with a blocking temperature above 100K.
Wave Velocities in Sediments
Authors: Marion Dominique , Nur Amos , Yin Hezhu
Published: March 2011

Systematic relations between porosity and compressional velocity Vp in the three component (sand, grains, clay and brine) systems (1) porous sandstone, (2) sands, and (3) suspensions, were obtained using experimental data and models. In Cemented Shaley Sandstones Vp was found to correlate linearly with porosity and clay content. The velocities in clean sandstones are about 7% higher than those predicted by the linear fit, indicating that a small amount of clay significantly reduces the elastic moduli of sandstones.

For uncemented shaley sand, a model for the dependence of sonic velocity and porosity on clay content and compaction was developed for sand with clay dispersed in the pore space and for shale with suspened sand grains. The model closely mimics the experimentally observed minimum for porosity and the peak in velocity versus clay content. The results explain much of the scatter in velocity data in-situ. Velocity in suspensions at ϕ = 39% of grains in brine is close to values predicted by the Reuss (Isostress) average. Velocity dispersion, as suggested by Biot (1956 a,b) is calculated and observed in coarser sediments such as sand, whereas velocities in the finer clay and silt follow Biot's low frequency value.

In total, our results provide the complete dependence of velocity on porosity in brine saturated sediment with clays, ranging from pure quartz to pure clay and water. Our results also highlight the crucial role of the critical porosity ϕ at about 39%, and the transition from cemented to uncemented sands.
Effect of Sodium Ions on the Dielectric Conductivity of Porous Silica in Humid Environments
Authors: Cao Wanqing , Gerhardt Rosario , Wachtman John B.
Published: March 2011

Partially sintered silica gels were immersed in different NaCI solutions to deliberately introduce a small amount of sodium ions ranging from 387 to 9900 ppm. Dielectric measurements were carried out in the frequency range 12 to 107Hz under various relative humidity conditions. Two relaxation processes were observed in the impedance and modulus planes and were assigned to two separate regions in the hydrated silica surface. The top region is expected to be rich in hydrated sodium since its modulus relaxation time depends strongly on the relative humidity as well as the sodium content. The interfacial region is believed to be composed of structured water since its dielectric conductivity is affected mainly by the amount of water adsorbed on the pore surfaces.
Observation of Large-Scale Structures in Unsaturated Materials
Authors: Maneval J. E. , Mccarthy M.J. , Whitaker S.
Published: March 2011

We report here the use of nuclear magnetic resonance imaging in the observation of liquid-phase fraction distributions in a partially-wetted sample of glass beads. By combiningboth large- and small-scale imaging techniques, we can study the transition from local-averaged saturations to large-scale averaged saturations. The image data allows us to assess the utility of the large-scale measurements We comment on the reliability and generality of the measurements for our specific system.
The Granular Nature of High Rank Coals
Authors: Cody George D. , Larsen John W. , Siskin Michael , Cody George D.
Published: March 2011

We present optical evidence in the visible and mid-infrared for the coal to graphite transformation proceeding via a nucleation mechanism. In the visible region, we show that changes in reflectivity that occur with increasing coal rank can be understood by an increase in the volume fraction of sub-microscopic “graphite-like” nuclei and a subsequent increase of the complex part of the dielectric constant with increasing coal rank. Optical phenomena associated with the granular nature of coal is also observed in the mid-infrared; specifically, measurements of absorption across the mid-infrared show a significant increase of absorption with increasing coal rank. This increase can be ascribed to absorption arising from a plasma resonance “graphite-like” material of the granular composite. A broad range of the optical repsroopnearntciees foofr hitghhe rank coals in the visible and mid-infrared can thus be readily understood and quantitatively modeled in terms of a granular medium consisting of nuclei of “graphite-like” material imbedded in an insulating organic matrix. If validated by further experimentation, this model should greatly expand current capabilities in the opticalcharacterization of high rank carbonaceous materials.
Melting Behavior in Granular Metal Thin Films
Authors: Unruh Karl M. , Patterson B.M. , Shah S.I.
Published: March 2011

Granular metal films consisting of small Sn, Bi, and Pb particles, typically from less than 100 Å to several 1000 Å in size and embedded in a SiO2 matrix, have been fabricated over a range of metal compositions by RF sputter deposition. Two different film geometries have been prepared, homogeneous films and multilayer films consisting of alternating layers of granular metal and SiO2. These films have been characterized by x-ray diffraction and transmission electron microscopy and their melting behavior studied by differential scanning calorimetry. As the concentration of the metal component is decreased, the average particle size decreases and the particle size distribution becomes more narrow. When the solid-liquid transition is studied, the melting temperature has been found to be increasingly depressed as the particle size is reduced. In the smallest particles the relative reduction in the melting temperature is greater than 10 percent. No strong evidence for melting point enhancements, due to pressureeffects arising from the different thermal expansions of the metal particles and the SiO2 matrix, has been observed in either the homogeneous or multilayer films.
Particle Shape Effects on Optical Absorption in Semiconductor Colloids
Authors: Persans P. D. , Lu E. , Haus J. , Wagoner G. , Ruppert A. F.
Published: March 2011

The absorption spectrum of small MoS2 particles in colloidal dispersions differs significantly from that of bulk material. In particular the amplitude and position of exciton absorption peaks shifts when small particles are suspended in a dielectric fluid. Our results are compared with predictions of a Maxwell-Garnett effective medium theory for ellipsoidal particles. We find that particle aspect ratio is an important parameter for interpreting these changes.
Recent Advances in Semiconductor Nanocluster Preparation
Authors: Alivisatos A. P. , Colvin V. L. , Goldstein A. N. , Olshavsky M. A. , Shiang J. J.
Published: March 2011

Semiconductor nanocrystals have received an extraordinary degree of attention during thepast few years. The electronic, optical, photochemical, and thermodynamic properties of these materials are strongly size dependent, providing a rich area of research, with many potential applications1. It remains true that the range of phenomena that can be studied in nanocrystals is limited by the quality of available samples. In this paper we describe two advances in the preparation of nanocrystalline semiconductor samples. First is the preparation of CdS nanocrystals covalently bound to a metal surface; in this configuration it is possible to study the nanocrystals by photoemission. Second, we report the organometallic synthesis of GaAs nanocrystals.
Pt-SiO2 Granular Metals for Cryogenic Thermometry in High Magnetic Fields
Authors: Obaldia Elida De , Moustakas T.D. , Hettengier Jeff , Brooks J.S.
Published: March 2011

Pt-SiO2 granular metal films were produced by RF diode sputtering and characterized as thermistors for cryogenic temperature measurements at high magnetic fields. The devices were found to have high sensitivity and small magnetoresistance. This, together with ease of fabrication and stability, makes such thermistor useful candidates for cryogenic thermometry.
Influence of the Morphology on the Optical Properties of Nanocermet Films: A Renormalization Approach
Authors: Berthier S. , Driss-Khodja K.
Published: March 2011

In order to take into account the actual morphology of the inhomogeneous media, we have developed, effective medium models based on a 2D and 3D position space renormalization /1,2/. These models predict the dipolar resonance and the percolation transition with critical exponents in good agreement with theoretical values and fairly reproduce most of the experimental results, whatever the concentration is. Further more, this allows a valuable comparison of the predictions of our models when applied on different lattices like real digitized TEM of cermet films or randomly occupied lattices.
Simulation of Composite Optical Properties Close to Percolation Threshold
Authors: Brouers F. , Clerc J.P. , Giraud G.
Published: March 2011

We analyze the optical properties of a metal-insulator composite thin film close to the percolation transition pc. Using a mean field approximation, it is possible to predict the existence of an optical threshold at a concentration p* slightly larger than pc. At that concentration defined by the vanishing of the real part of the dielectric constant and which depends of the relaxation time and therefore of the microgeometry, the composite optical absorption is frequency independent up to the near infrared frequencies. This property has been observed in a number of granular and cermet films.

We show that this property is quite general and can be obtained by generalizing the percolation scaling laws to a mixture of resistors, inductors and capacitors. The quality factor is shown to be a relevant physical quantity.

We report and discuss the results of simulations on real analogic LCR circuits and on computer which confirm and explicit these conclusions.
Classical and Nonclassical Optical Diffusion
Authors: Drake J. M. , Genack A. Z.
Published: March 2011

We present a brief summary of our recent work on classical and nonclassical optical diffusion in disorder metal oxides. Results on the optical transmission through a slab of titania spheres is presented and discussed in some detail illustrating the experimental method for obtaining the optical diffusion coefficient and photon absorption time from the transmission data. We conclude that the optical field-field correlation function with frequency is the Fourier transform of the time of flight distribution.
Phonon Dispersion in Suspensions of Hard Sphere Colloids*
Authors: Weitz D.A. , Liu J. , Ye L. , Sheng Ping
Published: March 2011

We use Brillouin scattering to measure the dispersion of the propagating acoustic modes in a suspension of hard sphere colloids. We find two distinct longitudinal modes when the sound wavelength becomes comparable to the sphere diameter. The higher frequency mode has a velocity intermediate between those of the pure solid and the pure liquid phases, and its velocity increases with increasing volume fraction, ø. The lower frequency mode has a velocity less than the velocities in either the pure fluid or pure solid phases, and its velocity decreases with increasing ø. We interpret the higher frequency mode as a compressional wave which propagates through both the solid and the fluid, as expected for a composite medium. The lower frequency mode has not been observed before, and is interpreted as a surface acoustic mode, which propagates between adjacent spheres through a decaying portion of the excitation in the fluid.
An Investigation of Diffusion Bonding for Ti–6AI–4V Titanium Alloys
Authors: Huang Wei-Dong , Zhang Zong-Yuan , Lin Zhao-Rong
Published: March 2011

Based on current theories of diffusion and creep cavity growth (or closure) at high temperatures, a physical analysis of diffusion bonding for the Ti–6Al–4V titanium alloy is given. The diffusion bonding is mainly pictured as a two—stage process:

1. The shrinkage process of cavities with shape change from elliptical to cylindrical;

2. In the second stage, the radii of the cylindrical cavities arc reduced and eliminated.

In addition, according to experimental work and previous theoretical models of diffusion bonding, a new model has been established in this paper, which can be used to predict the effect of process variables, such as temperature, pressure, surface roughness and bonding time on the bonding degree for Ti–6Al–4V by using mechanisms of creep and diffusion.