bluuebunny/crossref_metadata_2025 huggingface dataset:
The Role of the Species Formed in Pecvd Systems on the Density of States of a-Si:H Films
Authors: Martins R. , Rodrigues L. , Vieira M. , Fortunato E. , Santos M. , Dirani E. , Carvalho N. , Baía I. , Guimarães L.
Published: March 2011

Here the species detected by a quadropole mass analyser and resulting from the decomposition of SiH4 induced by a plasma [using a diode-like PECVD (Plasma Enhanced Chemical Vapour Deposition) system, in which has been varied the following experimental conditions: temperature of the substrate (Ts); deposition pressure (pd); power density (dp) and electrode geometries], have been correlated with the electro-optical properties and Density of States (DOS), inferred from Constant Photocurrent Measurements (CPM). The mass spectra analysis reveal the existence of atomic and molecular hydrogen as well as on the SiH3 and SiH2 species whose ratios are strongly dependent on electrode geometries used, that also roles the adequated deposition conditions for producing good quality materials. Thus, for the recessed electrode we observe an enhancement on plasma density at low self bias voltage (Vdc), leading to the production of high quality films at Ts = 180 °C, using moderate dp and pd in the range of 75–80 mtorr, with Vdc ≃ 8 V. On the other hand, for the grid electrode we observe an enhancement in molecular hydrogen at growth surface for high Ts, leading to the production of the best films for Ts in the range of 280–300 °C, using dp ≃ 25 mWcm−3, pd ≃ 100 mtorr and Vdc ≃ 30 V. For both electrodes we observe that high bombardment is important for getting microstructures, defect free and dense. The CPM data show that DOS increases with pd indicating that the enhancement on the number of collisions in the gas phase lead to the increase on chain polymerization formation. We also observe that the hydrogen coverage onto the growth surface is important for activating the growth mechanism, producing homogeneous material with a low DOS.
Thermal Equilibration Between Band Tail and Near Surface Defect States in Hydrogenated Amorphous Silicon and Silicon-Germanium Alloys
Authors: Aljishi Samer , Jin Shu , Ley Lothar , Wagner Sigurd
Published: March 2011

We employ total yield photoelectron spectroscopy to measure the density of occupied states at the clean a-SixGe1_x:H alloy surface. The near surface defect states are observed to lie at 0.57 eV above the valence band edge with a density of 4×l017 cm−3, independent of Ge content. The valence band tail characteristic energy is also measured to be independent of alloy composition with an average value of 54 meV. We demonstrate that thermodynamic equilibrium at the surface between weak bonds (forming the valence band tail) and the dangling bonds provides an excellent description of the experimental data and explains why the surface state density in a-Si:H cannot be lowered below the 1011 to 1012 cm−2 range.
Structure of a-Si:H/SnO2 Interface Characterized By Xps
Authors: Koinuma H. , Nakano M. , Gonda S.
Published: March 2011

The interface structure of a-Si:H films deposited by glow discharge decomposition of Si2H6 on SnO2 was characterized on an atomic scale with the use of in situ XPS. The method analyzes the variation of the intensity of photoelectron emission originating from the bottom SnO2 layer and transmitting through the upper a-Si:H layers of various thicknesses. It was evaluated that a-Si:H was partially oxidized as it grew to a thickness of about 15 A, while the SnO2 bottom layer was reduced to metallic Sn and SnOx (0 < x < 2) states as deep as 2 A and 12 A from the interface, respectively,
Experimental Determination for the Transition Temperature Between Ballistic and Hopping Controlled Recombination in a-Si:H
Authors: Zvanut M. E. , Wang K. , Han D. , Silver M.
Published: March 2011

The transient forward bias current of a-SI:H p-i-n diodes has been measured as a function of temperature and voltage. The transient shows an initial decay followed by a substantial rise which suggests a large value for the ratio of the carrier lifetime to the transit time as demonstrated by Shapiro. We found that both the time at which the rise begins and the ratio of the final saturated current to the minimum current increase with decreasing temperature for temperature greater than a characteristic temperature, Tc. For T < Tc, both the time and the ratio abruptly decrease with temperature. We attribute this change in temperature dependence to a change in the transport kinetics from ballistic to hopping controlled recombination.
The Influence of Tunneling Transitions on Recombination and Photoconductivity of a-Si:H
Authors: Cleve B. , Thomas P.
Published: March 2011

Spin dependent photoconductivity measurements provide evidence for the existence of tunneling transitions from localized tail states into the dangling bond recombination centers. These processes can be included approximately into the conventional model based on rate equations. Satisfactory values for the activation energy of the photoconductivity at low temperatures are obtained. Our model produces thermal quenching and an enhancement of the photoconductivity with rising dark Fermi level.
Effect of the Depletion Region of a-Si:H Schottky Diodes on the Time-of-Flight Mobility-Lifetime Product
Authors: Wyrsch N. , Roca i Cabarrocas P. , Wagner S. , Viret V.
Published: March 2011

The effect of the depletion region in amorphous silicon Schottky diodes on the mobility-deep trapping lifetime product μτ of electrons has been investigated using transient photoconductivity techniques. We varied the laser wavelength of the exciting laser in time-of-flight (TOF) measurements. The results are correlated with measurements of the internal field by means of the charge collection efficiency, and with measurement of the deep-trapping time by the Delayed-Field TOF technique (DFTOF). We find that the electron μτ decreases from the bulk toward the top surface.
Amorphous Hydrogenated Silicon P-I-N Solar Cells Grown from Hydrogen-Diluted Silane
Authors: Bennett Murray S , Tu Jacob C
Published: March 2011

We investigated p-i-n solar cells in which the i-layer was grown from hydrogen-diluted silane. The deposition parameters which were varied include flow rates, deposition pressure and power, and the degree of hydrogen dilution. We found that high quality devices could be made in which the i-layer was grown under a wide range of deposition conditions but that over this range neither the initial performance nor the stability of the devices differed significantly from those of cells having the same structure, but in which the i-layer was deposited from SiH4 with no H2-dilution.
Properties of Catalytic-Cvd Amorphous Silicon-Germanium (a-SiGe:H)
Authors: Matsumura Hideki , Yamaguchi Masaaki , Morigaki Kazuo
Published: March 2011

Hydrogenated amorphous silicon-germanium (a-SiGe:H) films are prepared by the catalytic chemical vapor deposition (Cat-CVD) method using a SiH4, GeH4 and H4 gas mixture. Properties of the films are investigated by the photo-thermal deflection spectroscopy (PDS) and electron spin resonance (ESR) measurements, in addition to the photo-conductive and structural studies. It is found that the characteristic energy of Urbach tail, ESR spin density and other photo-conductive properties of Cat-CVD a-SiGe:H films with optical band gaps around 1.45 eV are almost equivalent to those of the device quality glow discharge hydrogenated amorphous silicon (a-Si:H).
B(Ch3)3 as P Layer Doping Gas
Authors: Shen D. S. , Chatham H. , Schropp R. E. I.
Published: March 2011

The boron doped p-layer is a critical part of a-Si:H solar cells. Trimethylboron (B(CH3)3) has been suggested to be a better doping gas and has a better thermal stability than B2H6. Single junction a-Si:H solar cells and a-Si:H/a-Si:H tandem cells with the p-layers deposited using B(CH3)3 have resulted in conversion efficiencies of 11.4% and 10.4%, respectively. Using these new p+-layers, we also reached 10% efficiencies in single junction a-Si:H solar cells with the i-layer deposited at a high deposition rate of ∼ 2 nm/s from either SiH4 or Si2H6 as a source gas.
The Influence of Doping on the Electronic Properties of Hydrogenated Amorphous Silicon.
Authors: Herm D. , Wetzel H. , Kunst M.
Published: March 2011

The results of transient photoconductivity and photoinduced absorption measurements in lightly phosphorus and boron doped (0-10 ppm ) a-Si:H films at T = 300 K are presented. From the observed increase of minority carrier trapping with doping it is concluded that even light doping leads to a strong increase of the density of deep traps. Furthermore an abrupt change of the behaviour of the transient photoconductivity between 2 ppm and 3 ppm B2H6 is observed. This change may be interpreted as the transition to a p-type semiconductor.
Hydrogen Bonding, and Microvoids in a-Si:H: a Proton NMR Study
Authors: Zheng M. , VanderHeiden E.J. , Taylor P.C. , Shinar R. , Mitra S. , Shinar J.
Published: March 2011

The cw proton NMR spectra, spin lattice relaxation times Tx and free induction decay times T*2 were measured in various a-Si:H films deposited by rf-sputtering. All spectra exhibited broad and narrow components, of T*2 ∼ 20 and 45 μsec, respectively. In samples deposited at high rf power, the ratio of the integrated intensities of the broad and narrow lines was ∼ 1.5. These samples, which apparently contain mainly bulk monohydride bonds and a low volume fraction of microvoids, exhibit long-range H motion. In a sample deposited at low rf power, this ratio was ∼ 3. These samples, which contain significant dihydride and microvoid content, show a dramatic suppression of the long-range H motion. The T1 of both types of samples exhibits a clear minimum at ∼ 30 K, indicative of relaxation by H2 trapped in microvoids. The absolute values of T1, however, are smaller in the sample deposited at low rf power (0.1 sec at the minimum as opposed to 0.3 to 1.0 sec at the minimum), apparently due to a larger ratio of molecular hydrogen to bonded hydrogen. After annealing for 24 hours at 294°C, the ratio of broad-to-narrow components increases to > 10 in all samples, and the magnitudes of the T1 minima increase to about 1 sec in all samples.
Long Range Hydrogen Motion, Evolution, and Bonding in a-Si:H and a-Ge:H
Authors: Shinar R. , Wu X.-L. , Mitra S. , Shinar J.
Published: March 2011

The results of secondary ion mass spectrometry and IR studies of hydrogen diffusion in a-Si:H and a-Ge:H are reviewed and discussed. In a-Si:H, the diffusion is significantly slower at low total H content. The exponent α of the power-law time dependent diffusion constant D(t) = Doo(ωt)−α does not decrease with temperature as 1-T/To. D(tL), for constant diffusion length L, thus deviates from an Arrhenius behavior. The “apparent” activation energy Ea and prefactor Do derived from lnD(tL) vs 1/T yield a Meyer-Neldel relation Do = Aooexp(Ea/kToo) irrespective of L, H content, or microstructure. Aoo ≅ 3.3 × 10−14cm2/s and Too ≅ 730K for 2.5 × 10−5 ≤ Do ≤ 102cm2/s and 1.3 ≤ Ea ≤ 2.2eV. The preliminary results on a-Ge:H are similar, with Aoo ≅ 1.3 × 10−15cm2/s and Too = 530K. It is suspected that deviations from an exponential distribution of H site energies, structural relaxation, and deep H trapping sites related to microvoids may all contribute to the deviation of α from a 1-T/To behavior. Measurements following prolonged annealing suggest structural relaxation that affects H-site energies.
Effect of Thermal Equilibration on Dark- and Photo-Conductivities in Undoped Amorphous Silicon-Germanium Alloys
Authors: Liu J. Z. , Shen D. S. , i Cabarrocas P. Roca , Park H. , Wagner S.
Published: March 2011

We report the effect of thermal equilibration on the dark- and photo- conductivities of an un-doped a-Si,Ge:H,F with optical gap of 1.47 eV. Annealing at high temperature and subsequent quenching can freeze in the equilibrium configuration at the annealing temperature. The characteristic glass-like transition behavior of the conductivities was observed and used to estimate a freeze-in temperature of about 140°C. As the annealing temperature increases above the freeze-in temperature, the frozen-in dark- and photo- conductivities decrease, the photo- to dark- conductivity ratio increases, and the photoconductivity-generation rate exponent increases. These changes in conductivities are explained by a model calculation, which assumes that the quenching introduces new defect states to die lower energy flanks of the Gaussian defect distributions.
Capacitance Studies of Metastable States in Light-Soaked, Quench-Cooled, and Bias-Annealed N-Type Hydrogenated Amorphous Silicon
Authors: Leen Thomas M. , David Cohen J. , Gelatos Avgerinos V.
Published: March 2011

We have investigated the metastable states produced in n-type a-Si:H samples by 3 kinds of treatments: light soaking followed by partial annealing, fast (quench) cooling, and bias-annealing. We employed voltage pulse photocapacitance to examine the deep defect state distribution, ND, and drive level capacitance profiling to determine the occupied conduction bandtail states NBT. Our results indicate that: (1) Although the most heavily doped films exhibit a larger increase in ND with light soaking than the corresponding decrease in NBT (consistent with some proposed defect creation reactions), in more lightly doped films we observe significant changes in NBT for much smaller corresponding differences in ND; (2) for quench cooling we observe increases by factors of up to over 2 in NBT without any measurable changes in ND for all samples; and (3) we observe increased NBT in bias annealed samples with no changes in ND. All of these results are inconsistent with a purely silicon bond breaking mechanism for metastable defect creation. We consider a significant contribution from a dopant activation mechanism which does not involve changes in ND).
Electron Trapping in Amorphous Silicon - A Quantum Molecular Dynamics Study
Authors: Yang Lin H. , Kalia Rajiv K. , Vashishta Priya
Published: March 2011

Quantum molecular dynamics (QMD) simulations provide the real-time dynamics of electrons and ions through numerical solutions of the time-dependent Schrodinger and Newton equations, respectively. Using the QMD approach we have investigated the localization behavior of an excess electron in amorphous silicon at finite temperatures. For time scales on the order of a few picoseconds, we find the excess electron is localized inside a void of radius ∼ 3 Å at finite temperatures.
An Extension of the Embedded Atom Method
Authors: Pasianot R. , Savino E. J. , Rao S. , Farkas D.
Published: March 2011

For the class of materials in which covalent effects are important, there is still no simple and reliable scheme, adapted to computer simulations, that can handle angle de- pendent forces. Either they are based on the introduction of three body (or higher) [1] interactions, or demand unphysical behavior from the many body functions used [2,3]. In the first case, computer efficiency is considerably low due to the large amounts of calculations required; in the second case a negative curvature of the embedding function must be assumed for materials in which the Cauchy pressure is negative, and this is contrary to the current interpretations of that function.

In the present work we derive a method to introduce many body shear forces, suited to computer simulations, which is free from the shortcomings mentioned above.
Structural and Electronic Properties of Narrow Gap Zincblende and Chalcopyrite Compounds
Authors: Pascale T.M. de , Meloni F. , Serra M. , Massidda S. , Continenza A. , Freeman A. J.
Published: March 2011

Indium compounds and corresponding epitaxially grown superlattices have provided good single crystals suitable for accurate experimental measurements and have added new interest to the study of the constituent bulk semiconductors and the II-IV-V2 chalcopyrite crystal phases. This paper reports results of structural and electronic properties of narrow gap binary and ternary semiconductors determined selfconsistently from first principles using both the full potential linearized augmented plane wave (FLAPW) and norm-conserving pseudopotentials (PP) total-energy methods.
Dislocation Glide in Ice Hi
Authors: Fairbrother Peter J. , Heggie Malcolm I. , Jones Bob
Published: March 2011

The field of geological materials is rapidly opening up to computer modelling and ice poses stimulating new problems. The normal pressure and 0°C form of ice is ice Ih in which the oxygen sublattice is wurtzite. Imagining first that the protons occupy O–O “bond-centre” sites, it is generally believed that in ice Ih these protons are displaced along the bond direction in a correlated fashion such that each oxygen has two close protons (at about 1Å ) and two more distant ones (at about 1.7Å ). The correlated “disorder” of protons causes frustration of certain atomic movements, such as water molecule reorientation (as measured by dielectric relaxation times) and dislocation glide. Frustration can be relieved (and dislocation glide facilitated) by proton order defects, such as ionic defects, like an oxygen surrounded by only one close proton (OH−) or three (H3O+), or Bjerrum defects, like an O–O link with no intervening proton (L defect) or two protons (D defect).

We present results of ab initio total energy calculations on the formation of these defects.
Ab Initio Molecular Dynamics of Column IV Microclusters
Authors: Drabold David A. , Klemm Stefan , Sankey Otto F.
Published: March 2011

We report the results of ab-initio molecular dynamics simulations for small clusters of Si and C atoms. Ground-state geometries and vibrational spectra are presented. We also describe a Bayesian spectral estimation technique which we have found to be useful in analyzing molecular dynamics trajectories.
Molecular-Dynamics Study of Grain-Boundary Migration in Metals
Authors: Rickman J. M. , Phillpot S. R. , Wolf D.
Published: March 2011

The migration of a (100) Θ = 43.6° (Σ29) twist grain boundary is observed during the course of a molecular-dynamics (MD) simulation. The migration is investigated by calculating changes in the planar structure factor Sp(k), the orientation of interparticle bonds in the migrating plane, and the motion of the center of mass of planes near the grain boundary. It is found that (a) migration is accompanied by sliding, and (b) each migration step consists of a series of local bond rearrangements. It appears that, as a precursor to migration, Frenkel-like defects are produced at the grain boundary.
Energetics and Configurations of Lattice Defects in CuTi
Authors: Shoemaker James R. , Wesley David , Wharton William R. , Oehrli Michael L. , Sabochick Michael J. , Lam Nghi Q.
Published: March 2011

The energies and configurations of interstitials and vacancies in the ordered compound CuTi were calculated using atomistic simulation. Vacancies created by the removal of either a Cu or Ti atom resulted in a vacant Cu site, with an antisite defect in the latter case. The vacancy at the Cu site was found to be very mobile within two adjacent (001) Cu planes, resulting in two dimensional migration. Interstitials created by inserting either a Cu or Ti atom had complicated configurations containing one or more antisite defects.
First Principles Calculation of Oxygen Ordering in YBa2Cu3O7-y
Authors: Sterne P. A. , Wille L. T.
Published: March 2011

We have performed a number of first principles electronic structure calculations for YBa 2Cu 3O7_y with different oxygen orderings and concentrations. The resulting total energies have been used to assess the applicability of some of the proposed models for oxygen ordering in this system. We find that the results are consistent with an Ising-like model with asymmetric next-neighbor interactions between oxygen sites. We determine effective interaction parameters from the first principles calculations and use them to compute the phase diagram for the system, which is in excellent agreement with experiment for the tetragonal-orthorhombic I transition.
Theoretical Study of a new Transition Sequence in III-V Compounds: High-Pressure Phases of InSb
Authors: García Alberto , Cohen Marvin L. , Zhang S. B.
Published: March 2011

A detailed study of the pressure-induced phase transitions at zero temperature in InSb up to 40 GPa using a first-principles pseudopotential total-energy method is presented. In addition to InSb(I) (cubic) and (II) (polar β-Sn), we identify InSb(III) as a hexagonal phase (found earlier for GaSb) and (VI) as a polar bcc phase in agreement with recent experiments. New structural models, orthorhombic polar β-Sn and body-centered orthorhombic, are proposed as candidates for the InSb(IV) and (V) phases based on total-energy minimizations. These findings are compared with recent results for GaAs to illustrate the trends in transition paths among III-V compounds.
First-Principles Interatomic Potentials for Transition Metals and Their Surfaces
Authors: Moriarty John A. , Phillips Robert B.
Published: March 2011

For bulk transition metals, a first-principles generalized pseudopotential theory (GPT) of interatomic potentials has been developed in which the cohesive-energy unctional takes the form of a volume term plus sums over widely transferable two-, three-, and four-ion potentials. The GPT has been further extended to surfaces by making an internal transformation of this functional to an embedded-atom-like format in which the embedding function is identified as the bulk volume term and the atomic volume is replaced by an average electron density. Applications of the bulk and surface GPT to the calculation of structural, vacancy-formation, and surface energies in Cu and Mo, and to the investigation of surface relaxation and reconstruction in Mo are discussed.
Survey of Eutectic Systems as Potential Intermetallic Matrix Composites for High Temperature Application
Authors: Mazdiyasni S. , Miracle D. B.
Published: March 2011

The lack of low-temperature damage tolerance along with the desire to achieve lower densities and higher creep strengths over monolithic intermetallic materials, has sparked significant interest in the research community for the development of an intermetallic matrix composite suitable for high temperature structural applications. Eutectic systems are desirable as composites due to the inherent thermodynamic stability between matrix and reinforcement. It also allows the opportunity of controlling the microstructure by control of solidification conditions. This paper presents a qualitative examination of the toughness behavior of the Cr-Cr2Zr, Cr-Cr2Hf, Cr-Cr2Ta, Cr-Cr3Si, Ta-Cr2Ta, and Ta-Ta5Si eutectics. The evaluation is based on microcracking from microhardness indentations and fracture appearance. Relative oxidation resistance at 800°C and 1200°C is also presented.
Dislocation Morphologies in TiB2/NiAl
Authors: Wang L. , Arsenault R. J.
Published: March 2011

The addition of 20 volume percent titanium diboride in particulate form (1-3 μm) to nickel aluminide (TiB2/NiAl) results in a twofold increase in the high temperature strength of NiAl. Theories that have been proposed to account for the high temperature strength of discontinuous reinforced metal matrix composites can not be adequately used as a basis to explain the observed strengthening.

An investigation was undertaken of NiAl, 10 V% TiB2/NiAl and 20 V% TiB2/NiAl in the annealed condition and after deformation, allowed to cool slowly. There is a low dislocation density in the annealed samples and the dislocation density did increase slightly as a result of deformation. However, deformation did produce some intriguing dislocation arrangements; for example, it was found that there was a high dislocation density within the TiB2 in the deformed higher volume fraction composites and the dislocation density within NiAl matrix was not uniform.
The High Temperature Behaviour of TiAl Containing Carbide Reinforcements
Authors: Rösler J. , Valencia J. J. , Levi C. G. , Evans A. G. , Mehrabian R.
Published: March 2011

The mechanical behaviour of γ-TiAl reinforced with Ti2AIC platelets is investigated in the temperature range between 293K and 1255K. Comparison with the particle-free material of same matrix composition reveals a pronounced strengthening effect of the platelike reinforcements up to about 1000K. However, under creep loading the composite is remarkably weak. It is shown that this weakness is caused by diffusional creep along the particle/matrix interface. Modelling of this process indicates that the dimensions of the Ti2AlC particles (thickness d=lμm, length l=20μm) are too small to avoid this unwanted deformation mechanism. It is concluded that thicknesses well above 10±m are needed to maintain reinforcement strengthening at high temperature in this system.
Strength and Toughness of Single-Phase and Dual-Phase High-Temperature Intermetallics
Authors: Bruemmer Stephen M. , Brimhall John L. , Henager Charles H.
Published: March 2011

The strength and toughness of several high-temperature beryllium-niobium (Be12Nb and Be17Nb2) and iridium-niobium (lr3Nb) intermetallic compounds have been evaluated. Materials were prepared by high-rate sputter deposition and characterized by x-ray diffraction and analytical electron microscopy before and after heat treatment. As-sputtered microstructures were varied from amorphous to microcrystalline by control of substrate temperature. Grain size, degree of order, and second-phase distribution were modified by subsequent heat treatment. Strength as a function of temperature was assessed by hot hardness tests and toughness estimated from indentation-induced crack lengths. Intermetallics exhibited very high strength and low toughness up to temperatures of ˜1000 C. Mechanical behavior was a direct function of the microstructural parameters including matrix phase composition, second phase distribution, and degree of order.
Effect of XD™ TiB2 Volume Fraction on the Microstructure of a Cast Near-gamma titanium aluminide alloy
Authors: Larsen Donald E. , kampe Stephen , Christodoulou Leo
Published: March 2011

TiB2 particulate levels ranging from 0.1 to 4.6 volume percent were incorporhted into the near-gamma titanium aluminide alloy Ti-47.5Al-2Nb-2Mn (at%) via the XD™ process. Material was prepared by Vacuum ArcRemelting (VAR) followed by investment casting into cylindrical bars. Results show that grain refinement occurs in alloys with XD™ TiB additions equal to or greater than 1.0 volume percent. Alloys with ad itions of less than 1.0 volume percent TiB2 contained boride phases having lacey and blocky morphologies. Alloys with additions of 1.0 volume percent TiB2 or greater contained needle shaped borides, blocky equiaxed borides and lacey borides. Determination of boride phase morphology, boride type and microstructural response were determined by backscattep electron imaging (BEI), electron microprobe analysis (EMPA), X-ray diffraction (XRD) and optical metallography.
Innovative Processing Techniques for Intermetallic Matrix Composites
Authors: Stoloff N. S. , Alman D. E.
Published: March 2011

Although intermetallics based upon aluminum or silicon tend to have a very attractive combination of low density and excellent oxidation resistance, they suffer from lack of adequate creep strength and, in most cases, from inadequate ductility and toughness. It has been recognized for several years that an approach which could simultaneously solve both problems, without degrading other properties, is to utilize the intermetallics as matrices for composite materials. The consequence has been an explosion of interest in two-phase intermetallicbased alloys, as is manifested in the current symposium.
A Study of the Volume Fraction, temperature and Pressure Dependence of the Resistivity in a Ceramic–Polymer Composite using a General Effective media Equation
Authors: McLachlan David S. , Blaszkiewics Michael , Yoshikawa Shoko , Newnham Robert E.
Published: March 2011

A General Effective Media (GEM) equation is used to quantitatively describe the resistivity of an Fe304-eccogel composite system as a function of the relative volume fractions. The two percolation morphology parameters (øc and t) characterise the microstructure. Preliminary models, also based on the GEM equation, are used to describe the positive temperature coefficient of resistivity (PTC) and the piezoresistivity (uniaxial pressure) of the composite when the composition is near the percolation threshold.
Microwave Properties of Non-Percolating Metal-Insulator Composites
Authors: Jacobs I.S. , Patchen H.J. , Miller S.A. , Rachford F.J. , Hanson J.O.
Published: March 2011

Permittivity and permeability have been studied at microwave frequencies (1 to 20 GHz) on artificial dielectrics of metal particles randomly loaded in a polymer up to 40 vol%. Ni-Cr alloy powder (∼1 to 37 μm) in various size fractions was prepared by gas-water atomization, providing an oxide coating for interparticle isolation. The alloy is ferromagnetic below 165 K, enabling a magnetic measure of volume loading. The complex permeability results are in very good agreement with calculations of induced magnetic dipole effects as functions of frequency and particle diameter. Permittivity results are independent of particle size and essentially constant with frequency. When compared with a highly successful effective spherical cluster model and a well-founded empirical curve, the present alloy powder exhibits a residual upward deviation attributed to nonspherical morphology. In sum, the microwave electromagnetic behavior of such non-percolating composites is rather well understood.
The Formation of New Critical System by Electric Breakdown in Percolation System
Authors: Garanov V.A. , Kalachev A.A. , Karimov A.M. , Lagar´kov A.N. , Matitsin S.M. , Pakhomov A.B. , Peregood B.P. , Sarychev A.K. , Vinogradov A.P. , Virnik A.M.
Published: March 2011

A new way of obtaining binary composite materials having some properties of the critical percolation system is presented..These materials can be obtained by high-density current action on an initially well conducting mixture.This action destroys part of contacts between adjacent conducting granules bringing the system to the quasi-critical state.It is shown that quasi-critical s stems may have high values of dielectric constant ε′ ∼ 102-106 101 in the frequency range 0<f<1010Hz.Formation and dielectric properties of the systems are studied both by computer simulation and experi mentally.
Effective Medium Theory of Dielectric Constant of Granular Materials in the Presence of Skin Effect
Authors: Panina L.V. , Lagar´kov A.N. , Sarychev A. Ќ , Smychkovich Y.R. , Vinogradov A.P.
Published: March 2011

We consider the behavior of the dielectric constant of percolation composite systems. An example of such systems is a composite material consisting of a disordered mixture of metallic and insulating particles. A reduction in concentration P of the metallic (conducting) component reduces the static conductivity of the composite, so that it vanishes at some critical concentration PC known as the percolation threshold.
The Response of Two-Dimensional Granular Superconducting Films to Light
Authors: Culbertson J. C. , Strom U. , Wolf S. A.
Published: March 2011

We have examined the photoresponse of two-dimensional granular superconducting films of high Tc superconductors (Y-Ba-Cu-O). We see evidence for the existence of a twodimensional Kosterlitz-Thouless transition. The photoresponse and resistance have been measured as a functions of temperature, excitation power, bias current, and photon energy. The response to photons having energies above and below the superconducting energy gap(s) has been measured. Photoresponse features have been observed that are not explainable in terms of a conventional phonon bolometric mechanism. Comparison will be made with similar observations made on low temperature superconducting films (NbN-BN).
Phase Transitions in Dissipative Josephson Chains
Authors: Bobbert P.A. , Fazio R. , Schon Gerd , Zimanyi GT.
Published: March 2011

We study the zero temperature phase transitions of a chain of Josephson junctions, taking into account the quantum fluctuations due to the charging energy and the effects of an Ohmic dissipation. We map the problem onto a generalized Coulomb gas model, which then is transformed into a sine-Gordon field theory. Apart from the expected dipole unbinding transition, which describes a transition between globally superconducting and resistive behavior, we find a quadrupole unbinding transition at a critical strength of the dissipation. This transition separates two superconducting states characterized by different local properties.
Non-Ohmic Normal State Behavior and the Onset of Global Superconductivity in Partially Annealed Quench–Condensed Films of Tin
Authors: Shiffman Carl A. , Markiewicz Robert S. , Ho Men
Published: March 2011

Films of tin condensed onto 4K pyrex substrates can be driven from “insulating” (i.e. dR/dT < 0) to superconducting behavior by low temperature annealing. Global superconductivity occurs when the normul state sheet resistance, R□, shrinks to a value very near R□≡ h/4ea, as in experiments where R□is changed by increasing the film thickness. When R□> R□, conduction is decidedly non-ohmic, with R□(T,V) = R□(T) + R1(T)exp(−m(T)|V|).
Optical Properties of Granular Materials: How to Model Coatings for Energy-Efficient Windows
Authors: Granqvist C.G.
Published: March 2011

I discuss coatings for energy efficient windows of several different kinds with a view to the possibility of modelling their optical properties by effective medium theory. Special attention is given to spectrally selective noble-metal-based coatings with induced transmission due to voids, angular selective metal coatings with oblique columnar microstructure, and tentative theoretical descriptions of chromogenic materials having dynamic radiative properties.
Surface Impedance and Granularity in Superconducting Cuprates
Authors: Halbritter J.
Published: March 2011

The rf residual surface resistance Rres is an important figure of merit for superconducting microwave devices and for establishing the homogeneity of superconducting surfaces. In granular superconductors not only are high residual rf surface resistance values Rres observed, but also strong field dependencies of Rres(T,ω, Hrf) and the penetraton depth λres(T, ω, Hrf) are encountered even at very low field levels (of order 1 Oe). These observations can be explained in terms of granularity consisting of weak links and of insulating interfaces, which provide additional degrees of freedom due to fluxoid motion and electromagnetic strip line modes. These degrees of freedom cause additional interaction mechanisms with applied rf fields.
A Chord Distribution Description of Porous Glass
Authors: Lin M. Y. , Sinha S. K.
Published: March 2011

Small angle neutron scattering data of Vycor is analyzed using a chord distribution model describing the microporous structure. In addition, the same model is applied in interpreting the data taken when capillary condensation takes place in the porous medium. In both cases, the results are in good agreement with other measurements, and shows a promising potential in describing such a bicontinuous system.
Monte Carlo Simulations of Effective Diffusivities in Three—Dimensional Pore Structures
Authors: Reyes Sebastian C. , Iglesia Enrique , Chiew Yee C.
Published: March 2011

A hybrid discrete/continuum Monte Carlo technique combining random walk simulations with first passage time (FPT) concepts is developed here in order to estimate diffusion properties of randomly-assembled sintered porous structures. This work combines the creation of realistic porous solid structures with controlled pore size, shape, and tortuosity features with the application of an efficient algorithm for calculating effective diffusivities in all diffusion regimes (Knudsen, transition, and molecular). The hybrid simulation technique consists of creating a “protective” boundary layer surrounding solid surfaces within which discrete random motion simulations are performed while continuum FPT results are used in the remaining pore space. The boundary layer thickness reflects a characteristic length scale, of the order of a few mean free paths, over which the FPT approximation breaks down. This procedure significantly reduces the computations required to cover statistically representative regions of the porous structure,a serious shortcoming in previous studies of gas diffusion through porous solids; it leads to effective diffusivity estimates that are in excellent agreement with experimental measurements.
Melting and Thermal Characteristics of Small Granular Particles
Authors: Sheng Ping , Zhou Min-Yao
Published: March 2011

We present results of theoretical calculations on the thermal characteristics of small granular particles. By using the selfconsistent Einstein model with a Morse interaction potential, it is shown that Debye temperature drops significantly for particle sizes below 50Å, a phenomenon observed experimentally. Melting temperature of small particles is calculated using a generalized Lennard-Jones-Devonshire theory. Depression of the melting point by as much as 60% is obtained from particles <10Å in size. Consideration of the constraining effect of the elastic matrix show that under the condition of tight coupling with the matrix the melting point of small particles may be significantly enhanced.
Preparation and Properties of Co/SiO2 and Co3O4/SiO2 Composites
Authors: Win W. , Logothetis E. M. , Soltis R. E. , Plummer H. K. , Wenger L. E.
Published: March 2011

Films of Co/SiO2 composites were prepared by sequential sputtering from Co and SiO2 targets in Ar. As-grown films as well as films annealed in H2/N2 atmospheres consist of hcp Co particles of uniform size embedded in the SiO2 matrix. The measured optical absorption spectra of these composites are consistent with the Maxwell-Garnett theory except for the absence of the predicted structure that arises from the interband transitions of cobalt. Composites consisting of semiconducting Co3 O4 particles embedded in SiO2 were prepared by post-growth annealing of Co/SiO2 films in air. The optical properties of these composites were found to be comparable to those of CO3O4.
Preparation and Mechanical Properties of Ultrafine Grained Metals
Authors: Gunther B. , Baalmann A. , Weiss H.
Published: March 2011

Ultrafine—grained polycrystalline metallic components (Cu, Au, Fe) have been prepared by means of the inert gas evaporation technique combined with an integrated uniaxial cold compaction device. The average grain sizes ranaed typically from 20 nm to about 100 nm. The microstructure and Imourity content of the as-pressed samples have been investigated by means of TEM and AES, respectively. The yield strength of ultrafine (30 nm) grained Cu specimens obtained in tensile tests compares well with respective values for heavily cold—worked coarse grained copper. Al slight heat treatment (150ºC/30min) improves the strain—to—fracture at slightly reduced yield strength values. The results are discussed within the picture of two concurrent processes determining the strength of ultrafine grained metals: Coble creep vs. grain boundary strengthening effect.
The Optical Properties of Semi-Continuous Metal Films: A Scaling Based Model
Authors: Yagil Y. , Yosefin M. , Bergman D. J. , Deutscher G.
Published: March 2011

We present a calculation of the optical properties of thin semi-continuous metal films near the percolative metal-insulator transition. The model is based on scaling assumptions, reflecting the fractal nature of these films. The film is divided into small squares of linear size L and the local complex conductivity of each square is calculated, using finite size scaling arguments and taking into account both ohmic resistance within the metallic clusters and intercluster capacitance. The size L, over which the finite size scaling is done, is related to the optical frequency by the anomalous diffusion relation, i.e. L(ω) α ωl/(2+θ). In this calculation two types of conductivities are found : good ones for the ‘metallic’ squares, showing that large clusters are present within these squares, and poor conductivities for ‘dielectric’ squares, where only small clusters are present. Moreover, the ‘metallic’ and ‘dielectric’ squares are not identical, thus a certain distribution of each type has to be considered. The width of the distribution is quite large close to the percolation threshold and decreases to zero when the film becomes homogeneous. The optical properties of the whole sample are obtained by summing the contribution from all squares, using a wide bimodal distribution function. Comparison with recent experimental results shows good agreement between this model and the experimental data.
A Quantitative Study of Cavity Evolution in An Al-Cu-Zr Alloy
Authors: Zhao Xiaoyu , Langdon Terence G.
Published: March 2011

Experiments were conducted on a superplastic aluminum-based alloy containing 6% Cu and 0.5% Zr. Specimens were pulled in tension under selected experimental conditions and then polished and examined with an optical microscope connected through a video camera to an image monitor and a personal computer. A software package was used to provide quantitative information on the cavity morphologies including the number of cavities in a selected area, their size, their deviation from a circle, and the orientation of their longest length with respect to the tensile axis. Results are presented showing the change in cavity morphology with elongation and the effect of testing at different strain rates.
The Superplastic m-δ Relationships of the T10 Steel
Authors: Liu Chin , Wei Wong , Fu Liu Wan
Published: March 2011

At 700°C, with strain rates (έ) of 0.02, 0.05 and 0.08 min−1, the superplastic m-δ curves of T10 steel determined were all of the type (the general type). Their Values of rr (0.241 → 0.252 → 0.267), mL (0288 → 0.241 → 0.252 ),mF (0236 → 0.251 → 0.268), δL(38→. 535 →.58%), and δF (84.7 → 92 → 99%) all increased with increases in value. At the same temperature with έ =0.1 min−1, an ascending type of curve, mL=mwn = mo(=0.245) δx (=94%) was obtained from the transformation of δL and mL into the δF and mL of the general type of curve. Similar results have been obtained at 730°C.

At 600°,650°, 700° and 730°C, with έ =0.05 min−1 the same general type of curves were obtained. Their values of mo. (0.213 → 0.230 → 0.252 → 0296), mL (0.19 5 → 0.208 → 0.241 → 0.256 ).mF (0.204 → 0.234 → 0.251 → 0.280). δL (44 → 49 → 53.5 → 63.5%)and δF (77 → 81 → 117 percnt) increased with increases in temperature. For the same strain rate, while at 780°C, an ascending type of curve, mL.= mmis = mo (= 0.198) < mF (0.236).δo (=o) = δL < δx (=73%)was obtained from the transformation of the general type of curve in the same way as mentioned above.

The shape constants, C values, which vary with strain have been calculated.

Symbols: Refer to the List of Symbols.
Influence of Specimen Notices on the Superplastic m-δ Relationship of Zn-Ni-Mg Alloy
Authors: Liu Chin , Jin Tao
Published: March 2011

With AM/Ao = 1, the superplastic m- δ curves of Zn-Ni-Mg alloy have been studied. When T=220°C and έ (initial)=1.33×10−2 min−1, the curve was of the type. It was transformed into the mL =mmin =mo<mF,δL=δL<εF type of curve, when T=200°–230°C and έ (initial)=3.33×10−2 min−1 and further transformed the type at 240°–250°C with the same strain rate.

With AN/Ao<1 and >0.595, T=230°C and έ (initial)=3.33×10−2 min−1 the curves were all of the mL=mmax =mo <mF, δ.(=O)= δL < δF type. The δF values increased with increase in AN/Ao value. The effect mainly reflected on the CF value and the CL m-δ equation can be expressed to account for the notch effect as follows:

where and AN/Ao < 1 and > 0.595.

Symbols: Refer to the List of Symbols.
Superplastic Deformation of YBa2Cu3O7−x in a Superplastic Metal Matrix
Authors: Moreno J. Emilio , Torres-Villasefor G.
Published: March 2011

The new class of high-transition-temperature ceramic superconductors (e.g. Y-Ba-Cu-O) show a fine grain size polycrystalline structure, similar to that shown by the superplastic metals. The material behaves in a brittle manner with a strain to fracture below 0.5 % at room temperature. One of the reasons for this mechanical behavior is that the grain boundaries are easily separated when a stress is applied. It was found in this work that a deformation of the superconductor ceramic (scc) in superplastic metal (spin) matrix reduces the separation of the grain boundaries in such materials so that they can be deformed at room temperature. The spin matrixes used in this work were Zn-Cd, Bi-Sn and Cd-Sn. It was found that the highest the yield point of the matrix the highest the deformation induced in the scc. The Cd-Zn alloy was the most effective in avoiding the separation of the grain boundaries during the deformation of a composite formed by a cylinder of scc embedded in a spin matrix. Meissner effect was observed in the scc, after more than 160 % of plastic deformation. SEM observations show that deformation takes place by grain boundary sliding and some grain refinement was observed.
Superplasticity of Non-Oxide Ceramics
Authors: Wakai Fumihiro , Kodama Yasuharu , Sakaguchi Shuji , Murayama Norimitu , Izaki Kansei , Niihara Koichi
Published: March 2011

Superplasticity is a mode of deformation for fine-grained polycrystalline solids at elevated temperatures: included are metals, ionic polycrystals (e.g. oxides) and covalent polycrystals (e.g. non-oxide ceramics). Plastic deformation of covalent crystals by dislocation glide is not easy because of their high Peierls force. This paper describes high ductilities in Si3N4 and SiC, and also reports superplastic elongations in Si3N4/SiC composite.

Fine-grained Si3N4/SiC composites (20 wt% SiC) were fabricated by hot-pressing amorphous Si-C-N powder with 6 wt% Y2O3 and 2 wt% A12O3 as sintering aids. The composites were composed of equiaxed grains (<200 nm) and elongated grains. A composite exhibited a superplastic elongation larger than 150 % at 1600 °C and at an initial strain rate of 4 × 10−5 s−1. The superplasticity of the composite is probably related to the presence of an intergranular liquid phase.
A Comparative Study of Superplastic Deformation and Cavitation Failure in a Yttria Stabilized Zirconia and a Zirconia Alumina Composite
Authors: Chokshi A. H. , Schissler D. J. , Nieh T.-G. , Wadsworth J.
Published: March 2011

A fine grained yttria stabilized zirconia and a zirconia composite containing 20% alumina exhibit superplastic characteristics, with optimum elongations to failure of ≥500%. There is microstructural evidence for deformation enhanced grain growth and concurrent cavitation in both of these superplastic ceramics. Grain sizes after superplastic deformation may be several times larger than the initial grain size, and the level of cavitation can attain values as high as ≥30% under some experimental conditions. This report presents in a comparative format some recent experimental results on the microstructural aspects of superplastic deformation and concurrent cavitation in these superplastic ceramics.
Superplastic Deformation and Mechanisms of the Commercial Aluminium Alloy Al-4Cu-I.6Mg-0. 8Mn
Authors: Liu Yuwen , Yang Guang , Lian Jianshe , Zeng Xianwen
Published: March 2011

The superplastic deformation behavior and mechanisms of commercial Al-4cu-1.6 Mg-0.0Mn have been studied, using tensile tests with microstructuraL observations being carried out by optical and transmission etectronmicroscopy. The results of tensile tests show that considerable superplasticflow can be obtained without any pretreatment to this atloy.The maximum elongaton achieved is 42096 and the strain rate sensitivity index, m, is 0.49. Microstructural analysis reveals that the dominant mechanism in the early stage of deformation is diffusion. As strain increases, grain-borundary sliding and grain rearrangment take the dominant part in thedeformation process, and diffusion becomes an accommodation mechanism. Second phase particles effectively restrict grain growth during deformation. Cavity initiation and growth during deformation are the direct cause of specimen failure.