bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimization of the Properties of Undoped a-Ge:H
Authors: Turner W. A. , Pang D. , Wetsel A. E. , Jones S. J. , Paul W. , Chen J.H.
Published: March 2011

High density, non-porous, highly photoconductive films of undoped a-Ge:H showing minimal microstructure have been prepared out of an r.f. glow discharge of GeH4 on substrates mounted on the powered electrode of a diode reactor. These films, prepared under the systematic variation of substrate temperature, discharge power and dilution of the plasma by H2 are markedly different from those produced on the unpowered electrode. An optimum level of applied power is found which does not differ significantly from that used to prepare state-of-the-art a-Si:H. For the reactor geometry used, dilution of the plasma with H2 is found to be essential to the preparation of high quality a-Ge:H. These films contain similar amounts of bound hydrogen to that found in good a-Si:H, and also relatively large quantities of unbound hydrogen. Transport is activated with a Fermi level near mid-gap, the dangling bond spin density is 5 × 1016 spins/cm3, and the samples exhibit photoluminescence. The electron μÄ has been measured using the TOF technique. The conditions for preparation of optimized material are quite different for those found to optimize the preparation of high quality a-Si:H in this reactor. This is asserted to be a principal cause for our finding, along with other laboratories, of inferior properties for a-SiGe:H alloys when compared with those of a-Si:H.
Fabrication of Device-Quality Wide-Gap a-Si:H Films at Very Low Substrate Temperatures
Authors: Hishikawa Yoshihiro , Tsuge Sadaji , Nakamura Noboru , Tsuda Shinya , Nakano Shoichi , Ohnishi Michitoshi , Kuwano Yukinori
Published: March 2011

Wide-gap a-Si:H films with device quality (Tauc’s optical gap > 1.9eV, σph under AMI.5, 100mW/cm2 illumination ≥ 10−5, Ω−1cm−1, a σph/σ a≥106) have been fabricated. These films are deposited at low substrtate temperatures (TS≤80°C ) either by diluting SiH4 with H2 or optimizing the plasma parameters in a capacitively–coupled RF plasma–CVD reactor. Reduction in the SiH2 bond density and the ESR spin density are also observed. In this study, good film quality is always accompanied by a small deposition rate. Furthermore, σph is nearly the same if the deposition rate and Ts is the same, regardless of other deposition parameters. This suggests that the surface reactions or structural relaxations at the film-growing surface can produce high–quality a–Si:H films even at low TsS, if the deposition rate is low. Results in thermal annealing, light exposure, and solar cell performance confirm that these films have device quality and wide bandgap.
Reduction of Si-H2 Bond Content in Hydrogenated Amorphous Silicon Prepared by Mercury-Sensitized Photochemical Vapor Deposition
Authors: Sakuma N. , Nozaki H. , Niiyama T. , Ito H.
Published: March 2011

The ratio of Si-H2 bonds to hydrogen content in hydrogenated amorphous silicon films, prepared by mercury-sensitized photochemical vapor deposition, depends on the deposition conditions, in particular on the distance between the substrate and the light-transparent window.

The ratio is reduced from 20 % to 8 % by decreasing the distance from 30 mm to 8 mm. On the other hand, the hydrogen content remains constant at 15 at.%. Decreasing the distance has been found to be almost equivalent to increasing the light intensity, especially 254 nm-light intensity.
Etching Selectivity of SiF4 and H2 Plasmas for c-Si, a-Si:H and SiO2
Authors: Okada Y. , Wagner S.
Published: March 2011

The etching of a-Si:H, c-Si and SiO2 in glow discharges of SiF4 and H2 was studied. The order of the etch rates in a SiF4 plasma is a-Si:H > c-Si > SiO2. The etch rates of these materials are nearly independent of temperature. Only a-Si:H is etched in a H2 plasma.
Comparison of the Properties of Sputtered and Glow Discharge a-Ge:H Films
Authors: Wetsel A. E. , Jones S. J. , Turner W. A. , Pang D. , Paul W. , El Zawawi I. , Bouizem Y. , Chahed L. , Theye M. L. , Marques F. C. , Chambouleyron I.
Published: March 2011

Undoped a-Ge:H films have been made both by sputtering and by glow discharge under various combinations of deposition parameters, and characterized structurally, electrically, and optically. A detailed comparison of subband gap spectra of sputtered and glow discharge samples has been carried out using photothermal deflection spectroscopy and the constant photocurrent method. The results show subband gap absorption in a-Ge:H to be an order of magnitude larger than in device-quality a-Si:H. For some of the films the absorption spectra deduced from transmission and PDS fit very well onto the absorption spectra inferred from CPM measurements, while for others it is clear that the match is very poor. We infer that the photoconductivity-derived spectrum does not necessarily reflect the dominant absorption process, and that this is related to the existence of a two-phase structure clearly observed by transmission and scanning electron microscopy.
The Effect of Sample Substrate on the Structural Properties of Co-Deposited Films of A-Ge:H
Authors: Jones S.J. , Turner W.A. , Pang D. , Paul W.
Published: March 2011

Results from structural measurements on r.f. glow discharge produced a-Ge:H films have been found to be substrate dependent. The variations in the results were found to depend on both the substrate temperature, Ts, and the substrate yield strength. Differential scanning calorimetry results were particularly affected by these parameters. For films prepared at Ts = 150°C, the DSC spectra contain two exothermic peaks when the films are deposited on low yield strength substrates while only one exothermic peak is present for films deposited on high yield strength substrates. One exothermic DSC peak is seen in spectra for all films prepared at Ts = 300°C no matter what substrates were used. This DSC spectral dependence is attributed to differences in the microstructure of films deposited at the two substrate temperatures, as seen in TEM micrographs. X-ray diffraction measurements performed on films annealed to various temperatures show that all of the exothermic DSC peaks described above are associated with the crystallization process. Thus, for the films prepared at low Ts, crystallization is either a one or two step process depending on the yield strength of the substrate.
Optimization of a-Ge:H Films
Authors: Eberhardt K. , Schubert M. B. , Eicke A. , Bauer G.H.
Published: March 2011

Amorphous hydrogenated germanium films have been deposited in a conventional glow discharge reactor (diode as well as triode configuration) from GeH4 with and without additional H2 dilution. In this study special emphasis was put on samples deposited at different substrate temperatures, different neutral gas densities and dilution ratios. The deposition rate has been varied from 0.2 to 2.5 Å/s. Optical, structural and electronic properties have been studied by VIS/NIR thin film spectroscopy, PDS, Raman backscattering, FTIR, as well as temperature dependent dark and photoconductivity. Hydrogen content was monitored by quantitative SIMS analysis and FTIR spectroscopy.

By ‘soft’ deposition, i.e. high pressure and H2 dilution ratio, Fermi level positions near midgap (Eact =0.48eV at Eg=1.0ev) are achieved. Nevertheless the optimum ratio σph/σd is restricted to 0.6 at room temperature (λ=632.8nm). To point out the influence of Fermi level position for transport properties, we investigated two samples with different Fermi level positions in detail. σd(T) is strongly dependent on Fermi level positions, as well as the absolute values of σph(T), whereas the temperature dependence of σph(T) does not differ greatly. This is an indication, that Fermi level position and recombination centers of a-Ge:H can be effectively influenced by changing deposition parameters.
The Influence of Deposition Conditions on the Properties of a-Sic:H Thin Films
Authors: Magafas L. , Girginoudi D. , Georgoulas N. , Thanailakis A.
Published: March 2011

The dependence of chemical composition, structure and optoelectronic properties of sputtered a-SiC:H thin films on substrate temperature, Ts, and hydrogen flow rate has been studied. The films are amorphous for the growth conditions used in this work. The chemical composition of the alloys is very little influenced by the Ts, whereas the hydrogen content and the optical absorption coefficient depends strongly on Ts and hydrogen flow rate.
1.00 Mev Proton Induced Defects in a-Si:H thin Films and Solar Cells
Authors: Payson J. Scott , Abdulaziz Salmam , Li Yang , Woodyard James R.
Published: March 2011

Irradiation of thin films and solar cells with 1.00 MeV protons has been investigated for a fluence of 5.0E14 cm−2. We have used photothermal deflection spectroscopy and light conductivity to characterize the effect of irradiation on thin films; current-voltage and quantum efficiency measurements have been used to determine the effects of irradiation on solar cells.

Irradiation introduces increases in the sub-band-gap optical absorption and decreases in the photoconductivity of thin films. The major effect on solar cells is to reduce the short-circuit current. The spectral dependence of the quantum efficiency is reduced in a somewhat uniform manner with irradiation and also recovers in a uniform manner with annealing. Investigations of the annealing behavior of both thin films and solar cells show similar behaviors in the measured properties. The properties of both the films and cells are essentially restored with a one-hour anneal at 200 °C. Attempts have been made to use simple models to calculate defect densities and carrier transport.
Trace Impurities and Metastable States in a-Si:H
Authors: Unold Thomas , David Cohen J.
Published: March 2011

The effect of trace amounts of impurities such as carbon and nitrogen on the density of states in undoped a-Si:H has been investigated using drive-level capacitance profiling and transient photocurrent methods. The results indicate a significant correlation between impurity levels and the susceptibility to metastable state creation after light-soaking. We found this correlation to be independent of bandtail broadening and hydrogen content in the material.
The Generation and Bleaching of Positive Charge in Gate-Quality Nitrogen-Rich Amorphous Silicon Nitride By Sub-Bandgap Illumination
Authors: Kanicki J. , Sankaran M.
Published: March 2011

We report, for the first time, on the stretched-exponential time dependence of the generation and bleaching of positive charge in gate-quality nitrogen-rich amorphous silicon nitride films subjected to the sub-bandgap illumination at room temperature in vacuum. We also propose a mechanism which we believe is responsible for the generation and bleaching of the positive charge in the nitride films.
Fluorinated Silicon Nitride (a-Si:N,F,H) Films Using NF3 for Amorphous Silicon Based Solar Cells
Authors: Goh Freddy. H. C. , Tan S. M. , Ng K. , Naseem H. A. , Brown W. D. , Hermann A. M.
Published: March 2011

The large amounts of methane commonly used to prepare wide-gap a-SiC:H films present a lot of problems. This study was aimed at investigating the properties of fluorinated silicon nitride as an alternative to silicon carbide and to study the thermal stability of the films.

The RF glow discharge of NF3 (an etchant gas) and pure SiH4 produced a-Si:H,N,F films. Films were deposited with a gas ratio of NF3 over (NF3 + SiH4) from 0% to 15%. The substrate temperature, chamber pressure, and power were kept constant; only flow rate was varied. Characterizations of the films included AES, FTIR, visible/UV spectroscopy, dark conductivity, and photoconductivity.

Deposition rate increased about eight times with an addition of 6% NF3. The deposition rate increased from 1.22 A/s to 10.5 A/s. The Tauc optical gap increased from 1.72 eV to about 2.06 eV, suggesting excellent N incorporation. The dark conductivity decreased one order of magnitude while the photoconductivity decreased by as much as five orders of magnitude with ELH light. FTIR spectra showed the nitrogen incorporation. The Si-N bonds replaced the Si-Si bonds with increasing NF3 concentration. Also with increasing NF3 content in the gas phase, the hydrogen concentration in the material decreased and the films became more thermally stable.
Recombination at Dangling-Bonds and Fermi Level Effects on Steady-State Photoconductivity and Light-Induced ESR in a-Si:H
Authors: Morgado E.
Published: March 2011

A simple recombination model is proposed in order to explain Fermi level effects on steady-state photoconductivity in a-Si:H. The model assumes positively correlated dangling-bonds as the only localized states in the gap and takes into account the occupation statistics of correlated defect centers in the dark and under illumination. The steady-state photoconductivity and its dependence on the light intensity are investigated as a function of the Fermi level position in the gap. The results show a good agreement with experimental data from undoped, phosphorus and boron doped a-Si:H material. The quenching of the g≈2.0055 Electron Spin Resonance signal upon illumination in undoped samples and its enhancement in doped ones are also reproduced by the model.
Bonding and Electronic Structure of Nb/NbC and Nb/NbSi2 Interface
Authors: Donovan M. M. , Maclaren J. M. , Eberhart M. E. , Barron A. R.
Published: March 2011

The atomic origins of the intrinsic mechanical and thermal properties of the interfaces between niobium and its carbides and silicides are explored using both the layer Korringa-Kohn-Rostoker (LKKR) technique and model cluster calculations. The relative stability of an idealised [100] interface is predicted from charge transfer arguments.
Theoretical Study of High Pressure Metallic Hydrogen
Authors: Barbee Troy W. , García Alberto , Cohen Marvin L.
Published: March 2011

A study of the zero temperature phase transitions in hydrogen under megabar pressures using a first-principles total-energy method is presented. An anisotropic primitive hexagonal phase is found to be particularly stable relative to other monatomic phases for pressures between 4 and 8 megabars. Calculations of the vibrational frequencies show that this phase is unstable with respect to a distortion tripling the unit cell along the c-axis. Results for this distorted hexagonal phase will be presented, including a calculation of its superconducting transition temperature Tc.
A Comparison of the Direct Configurational Averaging and Connolly-Williams Methods of Obtaining Effective Pair Interactions in Substitutionally Disordered Alloys
Authors: Wolverton C. , Dreysse H. , Fontaine D. De
Published: March 2011

In order to calculate thermodynamic properties of disordered alloys, it is necessary to extract certain parameters (namely effective cluster interactions) from electronic band structure models. Several of these methods exist, thus necessitating a comparison of their accuracy and convergence. Direct configurational averaging is performed in both a canonical (concentration-dependent) and grand canonical (concentration- independent) scheme. These results are compared with those derived from the Connolly-Williams Method and “exact” results obtained on a simple, tight-binding, d-band model.
Atomic Structure Calculations of the Interaction between Lattice Dislocations and Grain Boundaries
Authors: Pestman B. J. , De Hosson J. Th. M. , Vitek V. , Schapink F. W.
Published: March 2011

The interaction between screw dislocations and [1 1 0] symmetric tilt boundaries is investigated by atomistic calculations. In order to study the differences between fcc and ordered alloys and to study the effect of increasing ordering tendency, many-body potentials representing Cu, Cu3Au and Ni3Al were used. For the ordered alloys, the different possible ordering configurations of the boundaries that were studied are discussed.
Calculation of Free Energy from the Internal-Energy Distribution Function
Authors: Phillpot S. R. , Rickman J. M.
Published: March 2011

We outline a new method for the calculation of free energies from a cumulant analysis of the internal-energy distribution function obtained from a Monte-Carlo simulation. The new methodology is validated by comparing the free energy calculated for a perfect crystal from this approach with the free energy calculated by a temperature integration scheme. By comparing the two methods for an inhomogeneous system containing grain boundaries, we investigate the effects of the local-strain distribution on the free energy; for the superlattice of twist grain boundaries studied here, the effects are found to be small.
GaAs Total Energy Tight Binding Hamiltonians for use in Molecular Dynamics
Authors: Broughton Jeremy , Pederson Mark , Papaconstantopoulos Dimitrios , Singh David
Published: March 2011

A self-consistent non-orthogonal semi-empirical tight binding Hamiltonian is proposed for GaAs, or any sp system, which is simple, reliable, transferable, accurate and fast to evaluate. Matrix elements are functions of charges, distances between atoms and simple cosines of angles between s and p-electron densities and interatomic vectors which maintain the simplicity of Slater-Koster parameterizations. The tight binding scheme is fit against a large data base of local density functional derived total energies for systems of differing coordination and geometry. The Hamiltonian fulfills the correct Virial constraint, invokes the physically correct relationship between overlap and kinetic energy matrix elements and defines charges via Mulliken or Löwdin schemes. Such Hamiltonians will allow the reliable simulation of statistical mechanically interesting systems of order hundred or more atoms over physically useful periods of time of order tens to hundreds of thousands of time steps within not unreasonable supercomputer budgets.
Calculation of the Atomic Structure of Grain Boundaries in Metals and Alloys
Authors: Foiles Stephen M.
Published: March 2011

This paper reports two applications of computer simulations based on the embedded atom method to the structure of grain boundaries. First, the equilibrium segregation at symmetric twist grain boundaries in Ni-Cu alloys is computed. These simulations include both compositional rearrangements and atomic displacements. It is found that Cu segregates to the boundary and that the change in composition is confined to a few atomic layers near the boundary. The degree of segregation was found to be greater for Ni-rich alloys and for large angle boundaries. The variation of the concentration within the planes parallel to the boundary for a low angle twist boundary is studied and the variations are correlated with the positions of the boundary dislocations. The second application addresses the possibility of structural disorder in the Σ5(001) twist boundary in Au. These results show that defects corresponding to local motion of the boundary normal to the interface are low in energy. The energetics of these defects are then mapped onto the body-centered-solid-on-solid model of surface roughening. This model predicts that a roughening transition will occur in this grain boundary at 100 K or 500 K depending on the choice of interatomic interactions. The effect of this roughening transition on the x-ray scattering from the boundary is also computed.
Interaction of Phosphorus With Dislocation Cores in Silicon
Authors: Heggie Malcolm I. , Jones Bob , Umerski Andrey
Published: March 2011

Silicon is an ideal model for strong covalent solids because it is an elemental semiconductor amenable to many well-defined structural, mechanical and electronic experiments. It was shown some 25 years ago that electronically active dopants enhance dislocation mobility in silicon, but recently Sumino and co-workers have shown that phosphorus also strongly pins dislocations at low stresses. Almost paradoxically the same dislocations move with enhanced mobility once they are in motion at higher stresses.

We have performed local density functional pseudopotential calculations on a molecular fragment taken from the core of a 90° partial dislocation of silicon. The surface dangling bonds were hydrogenated and phosphorus was substituted for silicon in various positions. Our results suggest that there is a strong valence effect in operation that causes phosphorus to bind very strongly to three fold coordinated sites in the core, simultaneously losing electrical activity. It is this binding that inhibits dislocation motion, because motion would require the coordination of phosphorus to change from three-fold to the more energetic fourfold.
Order/Disorder and Phase Diagram of H on Pd(100)
Authors: Tibbits P. , Karimi M. , Ila D. , Dalins I. , Vidali G.
Published: March 2011

An atomistic simulation of H-Pd(100) provided a phase diagram for the c2×2 H overlayer phase. The Embedded Atom Method (EAM) calculated energy of each configuration of atoms and the Metropolis Monte Carlo algorithm equilibrated the structure and generated configurations from which to sample the structure factor for the H overlayer. The procedure provided the expectation of the square of the structure factor modulus, < |S2| >, as a function of temperature at three coverages. The inflection point of the < |S2| > versus T curve estimated the critical temperature for disordering, Tc,, for one value of coverage, θ. The plot of Tc versus θ, the phase boundary for the c2×2 phase, lay about 125 K below the experimentally determined boundary. A comparison of the energies of ordered and disordered phases showed ΔE = 0.016 eV per hydrogen atom. Equating this unrealistically small energy difference to thermal kinetic energy (3/2)kBTc at the critical temperature implies Tc ≈ 100 K. Obtaining – |S2| > values relatively free of noise at such low temperatures required large numbers of Monte Carlo steps. The c2×2 phase is the experimentally determined stable low temperature phase, and was assumed to be the lowest-energy phase possible in this simulation. The very small ΔE indicates that some other ordered phase may be more stable than c2×2 in the EAM model.
Surface and Near-Surface Atom Dynamics During Low Energy Xe Ion Bombardment of Si and Fcc Surfaces
Authors: Murty M. V. R. , Lee H. S. , Atwater Harry A.
Published: March 2011

Surface and near-surface processes have been studied during low energy Xe ion bombardment of Si (001) and fcc surfaces using molecular dynamics simulations. Defect production is enhanced near the surface of smooth Si (001) surfaces with respect to the bulk in the energy range 20–150 eV, but is not confined exclusively to the surface layer. The extent and qualitative nature of bombardment-induced dissociation of small fcc islands on an otherwise smooth fcc (001) surface is found to depend strongly on island cohesive energy.
Ab-Initio Simulations to Study the Configurational Entropy of Column IV Microclusters
Authors: Klemm Stefan , Drabold David A. , Sankey Otto F.
Published: March 2011

We directly simulate the dynamics of carbon and silicon clusters to study the relevance of the probability distribution of coordinates to experimentally measured phenomena. We believe that the high temperature conformations are not necessarily related to the ground state minimum, and that care should be exercised when comparing ground state T = 0 calculations to experiments performed at high temperature.
Monte-Carlo Investigation of Avalanche in Adhesion
Authors: Good Brian S. , Banerjea Amitava , Smith John R. , Bozzolo Guillermo H. , Ferrante John
Published: March 2011

Recently it has been proposed that as two metal surfaces are gradually brought toward contact, atomic layers can collapse or avalanche together when the interfacial spacing falls below a critical distance. This avalanche can occur regardless of the stiffness of external supports and is accompanied by a discontinuous drop in the adhesive binding energy. Here we re-examine this phenomenon using a simple zero-temperature Monte-Carlo simulation using the Equivalent Crystal Method. In particular, we investigate the effect of loss of registry between the two contacting surfaces in order to delimit the circumstances under which avalanche can occur. We find that the avalanche is suppressed when the two surfaces are sufficiently far out of registry, at least when only a few layers near the surface are allowed to relax.
Transport Properties of Granular Nix(SiO2)100−x Thin Films
Authors: Beamish John R. , Patterson B.M. , Unruh K.M.
Published: March 2011

We have studied the electrical transport behavior of sputter deposited Nix(SiO2)100−x thin films between room temperature and 100 mK and, at selected temperatures, in applied magnetic fields up to 6 T. As the Ni concentration x is reduced, the resistivity increases systematically. At a Ni concentration (nominal) of about x–70 atomic percent (38 volume percent) the room temperature coefficient of resistivity changes sign. For Ni concentrations greater than 70 percent the resistance first decreases with temperature then increases logarithmically at, low temperatures. This increase becomes smaller and the resistivity minimum moves to progressively lower temperatures as the Ni concentration increases. In films with less than x–70 percent Ni, the resistivity has a temperature dependence of the form ρ(T)–ρo exp \(To/T)α] between room temperature and about 5 K. The exponent a is about 1/2 and To increases with decreasing Ni content. Below 1 K, however, the resistivity increases much less rapidly, with a temperature dependence independent of Ni concentration. In all films the magnetoresistance is small and negative.
Far Infrared Magneto-Optical Absorption in Small Bismuth Particles
Authors: Devaty Robert P. , Sherriff Ralph E.
Published: March 2011

A semiclassical model based on the measured properties of the free carriers in bulk bismuth successfully accounts for far infrared (ῶ ≤ 60 cm−1) magnetic field dependent resonances observed in free-standing powders of ∼0.5 μm diameter Bi particles prepared by inert gas evaporation. In earlier work, we found that particles with each of the three principal axes aligned parallel to the applied field were required to explain the data. The conclusion that these particles are bulk-like in nature remains valid when an ensemble of randomly oriented particles is treated using a Monte Carlo method.
Anderson Localization in Anisotropically Random Media
Authors: Sheng Ping , Xue Weige , Zhang Zhao-Qing , Chu Q. J.
Published: March 2011

Dimensional cross-over behavior of Anderson localization is presented. By delineating the physical basis of localization, it is shown that the localization phenomenon is sensitive to the spatial dimension of the randomness. Analytic and numerical results demonstrate that in an anisotropically random medium there is a critical amount of anisotropy which separates the system behavior into a ID-like regime and a 3D-like regime. Dimensional crossover is proposed as a viable experimental approach to observe the mobility edge.
A Quantitative Analysis of Cermet Resistivity Data Using the General Effective Media (GEM) Equation
Authors: Mclachlan David S.
Published: March 2011

A General Effective Media (GEM) equation is used to quantitatively describe the resistivity of the W - Al203 and Ni - Si02 cermet systems as a function of the relative volume fractions. The two percolation morphology parameters (øc and t) characterise the microstructure. A constant resistivity pl is assumed for the metal (W or Ni) while, in some cases, it is necessary that the resistivity of the insulator be modeled as the tunneling of electrons, through an oxide barrier, from a thermally excited charged grain to a neutral grain.
Bulk Effective Moduli: Their Calculation and Usage for Describing Physical Properties of Composite Media
Authors: Bergman David J.
Published: March 2011

While not telling the whole story, the representation of a composite medium by a homogeneous effective medium is often an excellent approximation for describing its macroscopic physical properties. Modern methods for calculating the effective medium properties are reviewed with special emphasis on understanding both successes and limitations. Outstanding problems that can and should be tackled are identified. The successes include calculations of the electrical conductivity, dielectric coefficient, and elastic stiffness moduli of composites with a periodic microstructure, and the simulation of those properties for disordered composites near a percolation threshold by means of discrete models such as a random-resistor-network. For composites where the microstructure is either unknown or very complicated, a whole class of exact bounds have been found for these properties based on various types of limited information. Recently, advances have been made in calculating the weak field magneto-transport and the thermoelectric behavior of two-component composites, and also some types of nonlinear properties. An important challenge remains the calculation of magneto-transport at high magnetic fields. Another is the theoretical treatment of multicomponent composites. A third is to find relations between different effective properties of a composite that can enable us to learn about property A by measuring a different property B. This is especially important when the measurement of A would destroy the sample, as when A is the yield stress, whereas the measurement of B is nondestructive, as when B is a small, nonlinear correction to the usual elastic stiffness moduli.
An Extended Thermodynamic Approach to Transport Phenomena in Porous Media
Authors: Rio J. A. del , Haro M. López de
Published: March 2011

A formalism of extended irreversible thermodynamics (EIT) is used to study the physical aspects of heat, momentum and mass transport through porous media.

The thermodynamic space is enlarged with respect to that of classical linear irreversible thermodynamics (LIT) to include the mass, heat and momentum fluxes as independent variables. The time evolution equations for such variables are derived self-consistently and reduce to the usual constitutive equations of LIT when the appropriate limits are taken. Equations that involve effects characterized by terms of second order in the gradients of conserved variables (such as the Darcy-Brinkman law) may also be derived within the same formalism. Finally, EIT provides the natural framework beyond LIT to introduce non-isothermal effects in the study of transport phenomena in porous media.
Granular Aspects of High Tc Superconductivity
Authors: Deutscher Guy
Published: March 2011

As Bednorz and Muller noted in their original publication reporting on the discovery of high temperature superconductivity, their oxides present many of the features of granular superconductors. This behavior was first primarily ascribed to poor connectivity of the grains in the bulk ceramic samples. but later studies have pointed out to more fundamental reasons for these similarities. We will discuss them after first reviewing the well established properties of low Tc granular superconductors.
Studying Superconducting Granular Aluminum with Microwaves: An Apprenticeship
Authors: MÜller K. Alex
Published: March 2011

A substantial part of my scientific career has been devoted to using the methods of electron paramagnetic (EPR) and electron spin resonance (ESR) in solids. The former describe investigations of microwave transitions between Zeeman levels of paramagnetic ions, whereas the latter indicate transitions between nearly free, but stationary spins in radicals and semiconductors as well as itinerant carriers in semimetals and metals. As early as 1962 an ESR study in intercalated graphites was undertaken in the latter. ESR was observed in C8K, C24K and C28Rb, but not in C8Rb, C8Cs and C24Cs [1]. From the observed linewidths, which are larger for heavier alkali intercalates with larger spin-orbit coupling, it was concluded that the carrier wave function was composed not only of carbon π but also of alkali-metal s orbitals, as theories later corroborated.
Granular Magnetism
Authors: Chien C.L.
Published: March 2011

Recent studies of granular magnetic solids involving Fe and its alloys embedded in either an insulating medium (e.g. SiO2 A12O3) or a metallic medium are described. Below the percolation threshold, isolated magnetic granular exhibit single-domain properties and superparamagnetic relaxation. Giant coercivity has been uncovered in granular Fe-SiO2, with Hc- as high as 3 kOe at 5 K and 1.1 kOe at 300 K. Some recent results on granular Fe in a metallic medium, and other granular magnetic systems will also be discussed.
Digitized Direct Simulation Model of the Microstructural Development of Cement Paste
Authors: Bentz Dale P. , Garboczi Edward J.
Published: March 2011

The complex microstructure of hardened cement paste is produced by hydration reactions between cement particles and the water in which they are suspended. In recent years, algorithms like the diffusion-limited aggregation (DLA) and Eden models have demonstrated that simple growth rules can result in complex aggregated structures. The model described in this paper simulates, via simplified growth rules, the microstructural development ofhydrating cement paste. This model has similarities to DLA, but with the additional novel features of dissolution of solid particles, and a free-space nucleation probability. The percolation aspects and transport properties of the model's pore space are computed and discussed.
Optical Properties of Granular Metallic Media Near the Percolation Transition
Authors: Lafait J. , Berthier S. , Gadenne M. , Gadenne P.
Published: March 2011

This review covers most of new physical aspects opened by the recent interest in the optical properties of heterogeneous media close to the percolation transition: relevant parameters and lengths, caracteristic optical behaviour (transmittance, reflectance and absorptance), theoretical models, dimensionali ty effects, discrimination between optical, electrical and morphological percolation transitions. Morphological aspects are emphasized as the key of optical properties at percolation.
Transport Properties of Granular Porous Media
Authors: Schwartz Lawrence M.
Published: March 2011

This paper is concerned with two related problems: (1) the construction of geometrical models of porous media relevant to granular composites and (2) the description of transport processes in these model systems. We will show that a variety of interesting porous media can be generated by the packing and subsequent modification of spherical grains. This modification may involve a change in either the grain's size, shape, or both. Steady state transport processes such as the flow of electrical current or viscous fluids are controlled by the distribution of pore throat sizes and, within the presentframework, can be studiedefficiently by random walk simulations of diffusion. The techniques developed here are also of interest in connection with dynamic transport processes such as the filtration of fine grained particles into consolidated granular networks. Wediscuss briefly the modeling of such processes as well as the interaction of steady state and dynamic transport.
Effect of Granularity on CuInSe2 Solar Cell Response
Authors: Sites James R.
Published: March 2011

Polycrystalline CuInSe2 solar cells, fabricated by evaporation or by selenization of metal films, are granular and relatively porous. Grains are a few hundred nanometers in dimension, and hence about 1% of the atoms are at a surface. Despite the granularity, quantum efficiency is quite high and implies a diffusion length exceeding the grain dimension. The primary photovoltaic loss is excessive forward recombination current. The proposed model consists of single crystal CuInSe2 granules with an indium rich surface layer. When properly passivated, the otherwise uncoordinated indium bonds are terminated by oxygen. However, residual non-passivated crystalline surface states distributed throughout the depletion region provide the paths for enhanced recombination.
Microstructural Changes and Straining Behavior in Transformation Superplasticity in Iron and Steels
Authors: Saotome Yasunori , Iguchi Nobuhiro
Published: March 2011

The microstructural changes associated with phase transformation and the straining behavior in polycrystalline structures during transformational superplastic deformation have betransformatonal superplasticen investigated. In-situ observations have been carried out with specially designed hot-stage microscopes. The strain distribution has been examined by microscopic strain analyses using a micro-grid pattern with 12.7 and 6 μm intervals in pure iron, 0.1%C steel and eutectoid steel. The results are as follows: (1) In the initial stage of Ac3 transformation in pure iron, superplastic strain is induced by the sliding at o/y interface along the prior ferrite grain boundaries and by the grain rotation. In the intermediate stage, the sliding deformation is generated at the migrating tranformation interface associated with the growth of austenite grains. (2) Accumulated strain by sliding has been observed within the previously transformed region during Ac3 transformation in 0.1%C steel. (3) The characteristic straining behavior in the eutectoid steel is due to the behavior at the transformation interface in the microstructure including secondary phase Fe3C particles. These observations suggest that a sliding mechanism at the migrating interface is a principal mechanism of transformation superplasticity.
The Mechanism of Internal Stress Superplasticity
Authors: Derby B.
Published: March 2011

The phenomenon of internal stress superplasticity is reviewed and the theoretical mechanical foundations of current mechanism models of the process examined. These models are shown to be based on two different principles: i.e. a biased internal plastic flow, or an enhanced creep generated by a superposition of internal and external stresses. Mechanical test data is shown to be more consistent with enhanced plasticity models in zinc and metal matrix composites for deformation during thermal cycling.
Superplasticity in Use: A Critical Review of Its Status, Trends and Limits
Authors: Winkler Peter-J.
Published: March 2011

The transformation of the phenomenon Superplasticity into a manufacturing technique has been pushed ahead over the last two decades mainly by the aerospace industries. The possibility of producing complicated structures in one step offers well known economic advantages. The combination of superplastic forming and diffusion bonding further enhances design flexibility thanks to the ability of fabricating multisheet structures.

This paper provides an overview of the practical application of Superplasticity in forming components. It focusses on the major aerospace materials i.e. Ti alloys and Al alloys and discusses specific characteristics, the material specifications, and supply situation.

Superplastic forming processing is presented including the techniques used for diffusion bonding; design concepts and exemplary SPF and SPF/DB parts are presented.

Finally a critical discussion concerning current development trends, and the factors limiting broad application of this manufacturing technique are given. Future prospects are outlined and further research needs emphasized.
Diffusion-Accommodated Grain Boundary Sliding in a Polycrystal
Authors: Hazzledine P. M. , Schneibel J. H.
Published: March 2011

A two-dimensional model for diffusional creep (Coble creep) and diffusionaccommodated grain boundary sliding in polycrystals has been developed. The results obtained for small symmetrical clusters of grains reproduce Spingam and Nix's work [Acta Metall. 2M, 1389 (1978)]. For clusters of irregularly shaped grains the material deposition and removal rates, the fluxes along the grain boundaries, the grain boundary tractions as well as the grain boundary sliding rates are irregular and can all be calculated.
Superplastic Forming Characteristics and Microstructural Development of an Aluminium Lithium Alloy (8090)
Authors: Mahon G. J. , Keyte R. , Parson N. C. , Ricks R. A.
Published: March 2011

Superplastic testing of the aluminium-lithium alloy 8090 has been performed in the temperature range 485°C to 550°C. Up to 530°C, raising the test temperature has the effect of increasing the m-value and decreasing the flow stress. Changes in superplastic forming characteristics and microstructure have been followed during straining using a variety of techniques. Jump-strain rate testing reveals that in the optimum temperature range (515°C to 525°C).there is a steady decrease in m-value. Cavitation also occurs, but can be suppressed effectively by applying a hydrostatic pressure during testing. Raising the test temperature above this optimum range to 550°C produces a significant decrease in the m-value and a more rapid fall-off with strain. These observations are explained in terms of recrystallisation and grain growth, and the effectiveness of precipitates in pinning grain boundaries at the various temperatures.
Mechanical Properties of 5083 SPF After Superplastic Deformation
Authors: Dunwoody Barry J , Stracey R J , Barnes A J
Published: March 2011

Superplastic aluminium alloys may be divided into two categories: those which recrystallise dynamically during superplastic deformation, such as the Supral alloys (2004 SPF) and 8090 SPF and, secondly those in which the fine grain necessary for superplasticity is produced by static recrystallisation during thermo-mechanical processing.

Both 7475 SPF and 5083 SPF belong to the second category.

This paper describes the superplastic forming characteristics of 5083 SPF and its post-formed mechanical properties.

Some typical applications will be shown to demonstrate the degree of complexity which is possible with this non-heat treatable alloy.
The Influence of Grain Boundary Structure on Strain-Induced Grain Growth During Superplastic Deformation
Authors: Frost H. J. , Raj R.
Published: March 2011

A model is presented to explain the grain growth that is often observed during superplastic deformation. The atomic structure of grain boundaries leads to a coupling between boundary sliding and boundary migration. There is a similar coupling between the absorption or emission of vacancies from a boundary and boundary migration. Because of these couplings, the grain boundary sliding and diffusional flow of superplastic deformation produce extensive boundary migration. We propose that this forced migration leads to random changes in the sizes of grains, and that this evolution of the grain size distribution leads to grain growth.
Superplastic Deformation of an Al-Li-Cu-Zr Alloy under Uniaxial and Biaxial Tension
Authors: Lee Ho-Sung , Mukherjee Amiya K.
Published: March 2011

The superplasticity of an Al-Li-Cu-Zr alloy has been studied under uniaxial tension and biaxial bulging conditions using hydrostatic pressure. An optimum tensile elongation of 850% at a strain rate of 3×10−4 /sec was obtained at 758 K. The maximum true thickness strain was 1.22 for a biaxially deformed specimen under the same conditions. It was shown that strain rate dependent flow hardening was related to necking profile. It was observed that cavity growth during superplastic deformation depended on the type of loading. Under biaxial bulging conditions, the cavities grew by forming cavity fissure networks along the grain boundaries. It was found that, by the superimposition of hydrostatic pressure, cavitation was retarded and the superplastic ductility was increased in both tensile and biaxial deformation.
Instability of Superplastic Aluminium Alloys
Authors: Iwasaki H. , Hayami S. , Higashi K. , Tanimura S.
Published: March 2011

The influence of the strain hardening coefficient,γ,and the strain rate sensitivity exponent, m, on the onset and growth of necking has been pointed out in many models about plastic instability controlled by the mechanical properties of materials. In this paper, the instability parameter I (=(l-γ-m)/m) proposed by Hart is used to relate the values of m (=∂lnσ/∂lnέ) and γ (=∂σ/(σ·∂ε)) for some superplastic aluminium alloys. These values of m and γ were calculated from the load-elongation curve at various strain-rates in constant strain-rate tests with periodic m determinations by strain-rate departures of small magnitude. It will be shown that the strain hardening due to grain growth contributes to an increase in the stability of plastic deformation during superplastic flow. However the decrease of m due to dynamic grain growth with strain causes superplastic deformation to be unstable near the onset of fracture. The transition of I values from negative to positive coincides with a decrease in m value and flow stress.

It is concluded that the development of necks can be characterized by an instability parameter I which include the effect of strain hardening caused by strain induced grain growth.
Effect of Hydrogen on the Superplastic Behavior of Ti-6Al-4V Alloy
Authors: Gong B. , Zeang C. B. , Lai Z. H. , Xu Z. S.
Published: March 2011

The superplastic behaviour of Ti-6Al-4V alloy containing various amounts of hydrogen (0.07 ∼ 0.33wt%) has been investigated by tensile tests at temperatures of 800 ∼ 950°C. Results show that the solution of hydrogen in the alloy helps to reduce the flow stress with strain rate in the range of 1.67 ×10−4 ∼ 1.67×10−2 s−1 and makes the temperature of superplastlc deformation become Lower (810∼870°C). Nevertheless, it also causes a decrease of the strain rate sensitivity exponent and tensileelongation, However, by a suitable alloying with hydrogen (∼ 0.1wt%), the alloy can be made to undergo superplastlc deformation at a rather tow temperature (840°C) with an acceptable loss of superptastic ductility.
Fatigue Properties of SPF/DB Joints in 7475 Aluminum Alloy and Ti-6A1-4V Alloy
Authors: Mutoh Yoshiharu , Kobayashi Masaru , Tatemura Makoto , Yoshida Hideo
Published: March 2011

Bonding strength and fatigue strength tests of SPF/DB joints in 7475 aluminum alloy and Ti-6Al-4V alloy were carried out. Their strengths of the joints using cryogenic rolled sheet were higher than those using conventional cold rolled sheet. Surface finish for removing oxide film was essential to obtain high-quality bond in aluminum alloy.
Superplastic Deformation Mechanisms in a Fine-Grained, Yttria-stabilized Tetragonal Zirconia Polycrystal (Y-TZP)
Authors: Nieh T. G. , Wadsworth J.
Published: March 2011

Concurrent grain growth, and in particular, dynamic grain growth, was observed to take place during superplastic deformation of Y-TZP. As a result of this concurrent grain growth, the measured strain rate sensitivity was found to be lower than that measured under constantstructure conditions. In the present paper, data obtained from the superplastic deformation of YTZP under constant-structure conditions are presented. It is demonstrated that the strain rate sensitivity values are generally higher than 0.5, when measured from the grain size-compensated data; this result suggests a grain boundary sliding mechanism. Microstructures from samples prior to and after superplastic deformation reveal grains which are essentially equiaxed; this observation is also consistent with a grain boundary sliding mechanism. Both high-resolution images of grain boundary triple points using transmission electron microscopy, and fracture surface studies using Auger electron spectroscopy and X-ray photoelectron spectroscopy indicate that there is no evidence for the presence of glassy phases at grain boundaries in Y-TZP.
Fractal Characterization of Cavity Shape Changes During Superplastic Deformation of High Strength Aluminium Alloy 7475
Authors: Xinggang Jiang , Jianzhong Cui , Longxiang Ma
Published: March 2011

The fractal dimensions of cavities during superplastic deformation of high strength aluminium alloy 7475 were measured with different strain, temperature, strain rate, and grain size. The results show that the fractal dimension of cavities increases with the increase of strain. Deformation under lower temperature, higher strain rates, or for larger grain sizes corresponds to higher fractal dimensions of cavities. The higher the fractal dimension of cavities, the more complex the cavities' shapes and the more possible it is for the specimen to rupture.