bluuebunny/crossref_metadata_2025 huggingface dataset:
Rheed Intensity Observation of AlAs and GaAs by in situ Etching Using Arsenic Tribromide
Authors: Kaneko T. , Säger T. , Eberl K.
Published: April 2011

The first in situ layer-by-layer etching of AlAs(100) surfaces has been observed by using RHEED intensity oscillations technique and is contrasted with the results obtained for the etching of GaAs(100). The experiments were conducted by introducing the etchant, arsenic tribromide, directly into a conventional MBE chamber without the use of any carrier gas. RHEED intensity oscillations during the etching of AlAs are observed between 350 and 760°C indicating a continuous increase in the etching rate with temperatures, with no supply rate limiting conditions being reached. Conversely, oscillations from GaAs reveal a reaction rate limited region at low temperatures (≤500°C) and a supply rate limited region at higher temperature(>500°C). The maximum selectivity in the etching rates between GaAs and AlAs is obtained at 450°C (40:1). The selectivity, and the ability to monitor the layer-by-layer process by RHEED intensity oscillations is foreseen to be of great importance for more controlled fabrications of AlAs and GaAs heterointerfaces.
Growth and Characterization of Pseudomorphic Gel-yCy and Si1-yCy Alloy Layers on Si Substrates
Authors: Brunner K. , Eberl K. , Winter W.
Published: April 2011

Pseudomorphic Gel-yCy and Sil-yCy alloy layers have been synthesized by solidsource molecular beam epitaxy on Si (001) substrates. High quality short-period Gel-y Cy/Si superlattice structures with a carbon content up to about 5 % are grown at low substrate temperature. The partial compensation of strain within Ge964C036 layers of 7 Å thickness improves the thermal stability against lattice relaxation, compared to pure Ge layers. Band-edge related photoluminescence is observed from pseudomorphic Sil-yCy/Si multiple quantum well structures at low temperature. The two predominant luminescence lines are attributed to no-phonon transitions and Si-Si TO phonon replicas of bound excitons confined within the Sil-yCy alloy quantum well layers. The tensile strain within the Sil-yCy layers shifts the twofold degenerate Δ(2) conduction band valley down in energy and is mainly responsible for the linear band gap reduction which is observed for increasing C content.
Damage, Strain and Quantum Confinement Issues in Dry Etched Semiconductor Nanostructures
Authors: Tang Y. S. , Torres C. M. Sotomayor
Published: April 2011

Semiconductor quantum wires and dots have been fabricated in GaAs/AlGaAs, CdTe/CdMnTe and Si/SiGe multiple quantum wells using electron beam patterning and reactive ion etching and studied by photoreflectance, photoluminescence and Raman scattering. It was found that the smaller the lateral size of the nanostructure, the smaller the fabrication induced residual strain. In all cases, the dominant strain component is found to be parallel to the sample growth direction, i.e., along the etched sidewalls of the etched wires and dots. The lateral confining potential is found to be quasi-parabolic for polar semiconductor systems. Possible ways of using and controlling the damage and residual strain in nanostructures are discussed in the context of device applications of nanostructures.
Afm/Tem Investigation of Low Temperature Polycrystalline Silicon Grown by ECR-CVD
Authors: Hsiao H. L. , Wang K. C. , Cheng L. W. , Yang A. B. , Yew T. R. , Hwang H. L.
Published: April 2011

The polycrystalline silicon films were deposited by electron cyclotron resonance chemical vapor deposition (ECR-CVD) with hydrogen dilution at 250°C and without any thermal annealing. The surface morphology and the microstructure of the poly-Si films are investigated by atomic force microscopy (AFM), plan-view transmission electron microscopy (TEM), crosssectional TEM and high resolution TEM (HRTEM). The low temperature poly-Si films deposited by ECR-CVD show a special leaf-like grain shape (plan-view) and an upside-down cone shape (3-dimensional view). The grains in the poly-Si films have preferred orientation of <112> and the longer side of the leaf-like grain is direction and the shorter side is direction. Lattice bending and interruption are found in the films. The arrangement of the atoms on the grains are well ordered, while atoms in the interfacial regions are randomly distributed. A simple grain formation model based on growth rate differences between different planes and etching effect can explain the film growth mechanism and the formation of the special grain geometry.
Room Temperature Contactless Electromodulation Characterization of a Wafer-Sized InGaAs/GaAs/GaAlAs Grinsch Laser Structure
Authors: Krystek Wojciech , Leibovitch M. , Pollak Fred H. , Gumbs Godfrey , Konopelski T.
Published: April 2011

Using contactless electroreflectance at 300K we have characterized the complete potential profile of a pseudomorphic 0.98 μm InGaAs/GaAs/GaAlAs graded index of refraction separate confinement heterostructure laser fabricated by molecular beam epitaxy. Signals were detected from all three relevant portions of the sample. Comparison of the observed transitions from the InGaAs single quantum well (SQW) section with an envelope function calculation (including the effects of strain) made it possible to evaluate the In composition and width of SQW. These values were in good agreement with the intended parameters. The energy of 11H, the fundamental conduction to heavy-hole level (which is closely related to the lasing frequency) can easily be determined to less than ± 1 nm at 300K. From the period of the observed Franz-Keldysh oscillations from the graded GaAlAs region it was possible to directly evaluate the built-in electric field in the structure.
Steplike Lineshape of Low Temperature Photoreflectance Spectra of InAlAs
Authors: Baltagi Y. , Bearzi E. , Bru-Chevallier C. , Benyattou T. , Guillot G. , Harmand J. C.
Published: April 2011

We have performed low temperature photoreflectance spectra on several MBE-InAlAs layers lattice matched to InP. Unusual lineshapes of PR spectra are observed at temperatures below 40K, characterized by a step at the InAlAs band gap energy. This step is shown to be related to a strong modification of the photo luminescence background of the photoreflectance spectra. This modification is attributed to an effect of carrier localization due to clustering effects in the InAlAs layers
Spectroellipsometry Studies of Znl-x.cdxSe: From Optical Functions to Heterostructure Characterization
Authors: Lee Joungchel , Hong Byungyou , Burnham J. S. , Collins R. W. , Flack F. , Samarth N.
Published: April 2011

The dielectric functions of 0.5–1.5 μm Znl-xCdxSe (0≤x≤0.34) epilayers on GaAs have been determined over the photon energy range 1.5≤E≤5.3 eV. These spectra have been parameterized using the Sellmeier approximation for energies less than the fundamental gap at E0 and using a sum of Lorentz oscillators above the gap region. As an example of the usefulness of the approach, expressions are provided that yield the index of refraction of Znl-xCdxSe (0≤x≤ 0.34) for 1.5 eV≤E<E0- As a second example, we have determined the composition and thickness of a Znl-xCdxSe quantum well between ZnSe barrier layers.
Monitoring Of The Intermediate Products In The Thermal Decomposition Of SiH4, Si2H6, SiF4 AND SiH2F2
Authors: Han Jae Hyun , Ryu Hyun-Kyu , Moon Sang Heup
Published: April 2011

Thermal decomposition of SiH4, Si2H6, SiF4 and SiH2F2 used as the reactants in the Si-CVD process was studied by monitoring the gaseous products with a quadrupole mass spectrometcr (QMS). The rates of SiH4 pyrolysis showed a large compensation effect at low pressures, which agreed with the theory of unimolecular reaction. Si2H6 decomposed at relatively low temperatures producing SiH4 as the intermediate products. The decomposition temperatures of the other compounds increased in the order of SiH4, SiH2F2 and SiF4. The pyrolysis rates of SiH2F2 and silanes, i.e., SiH4 or Si2H6, were enhanced synergically when they were mixed together. The results were explained by a reaction mechanism proposed based on analysis of the intermediates produced in the process.
Trace-Element Accelerator Mass Spectrometry: A New Technique for Low-Level Impurity Measurements in Semiconductors
Authors: Datar S. A. , Renfrow S. N. , Anthony J. M. , Mcdaniel F. D.
Published: April 2011

SIMS is a well-established technique for the measurement of impurities in semiconductor materials. It is the technique of choice for the measurement of very low levels of impurities, especially depth profiles. Accelerator Mass Spectrometry (AMS) is also a well-established technique and is primarily used for measurements of abundance ratios of long-lived radionuclides, such as 14C and 36C1 to their respective stable isotopes. A combination of these techniques holds the potential for impurity determination in semiconductors at the ppt level. The new technique, Trace- Element Accelerator Mass Spectrometry (TEAMS), eliminates the problem of molecular interference which is the major constraint limiting the sensitivity of SIMS. Depth profiles for B, P and As implants in Si, obtained by TEAMS are presented below.
A Comparative Study of the Reflectance Difference Spectrum from Si(001) Using Reflectance Difference Spectroscopy /Low-Energy Electron Diffraction/Scanning Tunneling Microscopy
Authors: Lin Jia-Ling , Jaloviar S. G. , Mantese L. , Aspnes D. E. , McCaughan L. , Lagally M. G.
Published: April 2011

From a comparative study using scanning tunneling microscopy, high-resolution lowenergy electron diffraction, and reflectance difference spectroscopy, we investigate the optical anisotropy on clean and H-covered Si(001) surfaces. We demonstrate a high sensitivity of reflectance difference spectroscopy to the surface tructural anisotropy and surface chemistry.
Photoluminescence and Raman Scattering from (CdSe)m,(ZnSe)n-ZnSe Multiple Quantum Wells Under Hydrostatic Pressure
Authors: Zhang J. Q. , Liu Z. X. , Wang Z. P. , Han H. X. , Li G. H. , Peng Z. L. , Yuan S. X.
Published: April 2011

The photoluminescence and Raman scattering of {|(CdSe)1 (ZnSe)3|14-(ZnSe)130} × 5 multiple quantum well structure have been investigated at 77 K and under hydrostatic pressure up to 7 GPa. Resonant excitations have been accomplished by turning the electronic levels under hydrostatic high pressure. Two kind of excitons and ZnSe-like LO phonon modes as well as their pressure behavior are presented.
Thermal Stability in Pd-Based Contacts to p-Type In0.53Ga0.47as Characterized by Rbs
Authors: Leech P. W. , Ressel P. , Reeves G. K. , Zhou W. , Kuphal E.
Published: April 2011

The resistivity and interfacial characteristics of Pd/Zn/Pd/Au and Pd/Zn/Au/LaBd6/Au contacts to p-In0.53Ga0.47As have been investigated. Annealing of the contacts at 375–425°C yielded a minimum in specific contact resistance, p,, of 2 × 10-7 Δ cm2 for the Pd/Zn/Pd/Au contacts and 1 Δ cm2 for the Pd/Zn/Au/LaB6/Au configuration. This is the lowest reported measurement of pc for an ohmic contact to p-In0.53Ga0.47As doped to ≤1 × 1019 cm−3. In the Pd/Zn/Au/LaB6/Au scheme, the minimum in pc was the same irrespective of whether the Zn was incorporated as a structural layer or as Zn ions implanted into the interfacial Pd prior to metallization. The effect of thickness of the Zn layer on pc has been determined for the Pd/Zn/Au/LaB6/Au scheme. RBS measurements have shown that during annealing, the LaB6 layer acted as a barrier to the indiffusion of Au and to the degradation of the In0.53Ga0.47As substrate.
Determining Thin Film Density by Energy-Dispersive X-Ray Reflectivity: Application to a Spin-On-Glass Dielectric
Authors: Wallace W. E. , Wu W. L.
Published: April 2011

A novel method for determining thin film density by energy dispersive x-ray reflectivity is demonstrated for a polymer-derived spin-on-glass dielectric intended for microelectronics applications. The effects of sample misalignment limit the accuracy of x-ray reflectivity as typically practiced. These effects may be properly accounted for by measuring the critical angle for reflection at many different x-ray wavelengths simultaneously. From this measurement, thin film density can be ascertained with much improved accuracy. The results of the x-ray reflectivity measurement are compared to those derived from MeV ion scattering.
Optical Second Harmonic Generation Method For Silicon Material Monitoring And Characterization During Ion Implantation And Annealing Processes
Authors: Gu Y. , Chou Y. C. , Vu T. , Yen K. , Lin Y. S. , Li G. P.
Published: April 2011

Non-linear optical second harmonic generation (SHG) from bulk silicon is very weak because of the inversion symmetry of the silicon lattice structure. However, when silicon material is subjected to the ion implantation and thermal annealing processes, the inversion symmetry will be broken. As a result, the optical second harmonic generation from the material will increase, and the characteristics of the second harmonic signal are related to the material conditions. In this study, we compare SHG results with Transmission Electron Microscope (TEM) observations for silicon material that has been treated with phosphorous ion implantation and rapid thermal annealing, and suggest that the SHG method may be used for the detection and monitoring of impurities and defects during ion implantation and thermal annealing processes.
Structural relaxations in a Simple Model Molten SaltM
Authors: Fuchs Matthias
Published: April 2011

The structural relaxations of a dense, binary mixture of charged hard spheres are studied using the Mode Coupling Theory (MCT). Qualitative differences to non-ionic systems are shown to result from the long-range Coulomb interaction and charge ordering in dense molten salts. The presented non-equilibrium results are determined by the equilibrium structure, which is input using the well studied Mean Spherical Approximation.
A Low Frequency Study of the Vibrational Modes in Alkalisilicate Glasses by Raman Spectroscopy
Authors: Sommer R. , Toulouse J. , Jain H.
Published: April 2011

We have performed a study on low frequency modes in several alkali silicate glasses by Raman spectroscopy. The Boson peak region is analyzed with a single parameter ω0 which is believed to characerize the density of states of the system. Analysis of the dependence of ω0 on the nature and concentration of the alkali suggests that the position of the Boson peak is essentially governed by the ratio “force constant” over “mass” of localized oscillator modes. At lower frequency (below 30 cm−1), the “excess” intensity can be explained by considering secondorder processes of the same vibrational modes, superimposed on other (possibly relaxational) modes.
Fractal and Non-Fractal Surfaces in Ion Sputtering
Authors: Barabási A.-L. , Cuerno R.
Published: April 2011

Recently a number of experimental studies focusing on the scaling properties of surfaces eroded by ion bombardment provided apparently contradictory results. A number of experiments report the observation of self-affine fractal surfaces, while others provide evidence about the development of a non-fractal periodic ripple structure. To explain these discrepancies, here we derive a stochastic nonlinear equation that describes the evolution and scaling properties of surfaces eroded by ion bombardment. The coefficients appearing in the equation can be calculated explicitly in terms of the physical parameters characterizing the sputtering process. We find that transitions may take place between various scaling behaviors when experimental parameters, such as the angle of incidence of the incoming ions or their average penetration depth, are varied.
Topological Disorder and Conductance Fluctuations in Granular Thin Films
Authors: Abkemeier Kristin M. , Grier David G.
Published: April 2011

Topological disorder places constraints on the local flow of currents in granular thin films of metals and semiconductors. These constraints in turn influence measurable transport properties such as conductance and conductance fluctuations for these films. We quantify disorder in the disposition of grains within real and simulated thin films by applying methods originally developed studying foam evolution. For simulated Voronoi resistor networks the overall conductance of a film with a given grain density achieves a minimum value for an intermediate degree of disorder. Films of equal conductances on either side of this minimum can have strikingly different current distributions. Short range inhomogeneities in the size and placement of grains lead to large scale conductivity inhomogeneities in disordered films. This renders the disordered films more susceptible to 1/f noise for systems with nonlinear intergrain coupling due to factors such as hydrogen diffusion. We discuss the results of simulations of such systems in the context of transport and scanning probe microscopy measurements on a-Si:H thin films.
Nonequlibrium Statistical Mechanics of An Ensemble of Vesicles
Authors: Golubović Leonardo
Published: April 2011

We investigate far-from-equilibrium dynamics of a polydisperse ensemble of vesicles ( such as liposomes). For that purpose, we construct of a Smoluchowsky-type transport equation incorporating vesicle diffusion and the processes of vesicle fusions and fissions. This approach is used to study the time evolution of an initially monodisperse vesicle ensemble and its important quantities such as the internal aqueous, encapsulated volume. We find three stages of the evolution: (i) an early stage during which EV remains nearly constant, followed by (ii) a stage with a rapid decay of the EV, and, finally (iii) a late stage in which the ensemble approaches the thermodynamic equilibrium. In the far-fromequilibrium stages (i) and (ii), the vesicle ensemble is a “two-fluid” system composed of a polydisperse fluid of small, nearly equilibrated vesicles, coexisting with a fluid of nearly monodisperse vesicles which size evolves due to evaporation-recondensation of the small vesicles. Our picture agrees with experimental data on the EV of lecithin liposomes.
Pitting Corrosion in Two-Dimensional Aluminium Thin Layers
Authors: Balázs L. , Gouyet J-F.
Published: April 2011

We have investigated the influence of C1− and Fe3+ ions on the morphology of pits grown in aluminium thin films, under open circuit conditions. We observed various morphologies ranging from percolation-cluster-like patterns to dense holes with smooth perimeters. The experimental behavior has been appropriately simulated by a spreading percolation model with feed-back. This model reproduced all the morphologies observed and sustains the mechanisms proposed.
Atomic Kinetics and Dynamical Phason Disorder in a Quasicrystal
Authors: Dzugutov M.
Published: April 2011

Slow relaxation dynamics in strongly coupled systems is known to be universally described by the stretched exponential Kohlrausch law. However, this phenomenon, observed in various condensed systems, still eludes a comprehensive microscopic interpretation. I discuss here the relaxation dynamics in an equilibrium dodecagonal quasicrystal which demonstrates a pronounced stretched exponential behaviour. The quasicrystal, simulated by Molecular Dynamics, reveals the presence of dynamical phason disorder, a generic form of relaxation dynamics associated with incommensurate, or phason degrees of freedom. This dynamics brings about a novel form of atomic diffusion, which is examined here by calculating the self part of intermediate scattering function Fs(Q, t). The latter displays a non-exponential decay which is found to be described by the Kohlrausch law with the stretching exponent β = 0.47. I discuss here a possible similarity between the dynamics of quasicrystals observed and that of supercooled liquids and glasses.
Structure of Silane Films and their Adhesion Properties
Authors: Araujo Y. Carolina , Toledo Pedro G.
Published: April 2011

Silane films, their structure and stability, are of great interest in processes such as flow in porous media, mineral flotation, chromatography and corrosion. Here, the structure of octadecyl thriclorosilane (OTS) films on glass surfaces is studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM), their adhesion properties by contact angle measurements and adhesion tests. Complete glass surface coverage by the silane is attained after an immersion time tc characteristic of the OTS compound. The time evolution of the OTS films regarding surface coverage is monitored by SEM with a BSE detector, by measuring the OTS film thicknesses from XPS data, by AFM and by contact angle measurement. At tc the structure of the films changes from micromolecular to macromolecular. Below tc the glass coverage has a fractal geometry and various degrees of hydrophobicity are possible. At t > tc the surface coverage is complete and the contact angle achieved a well defined constant value.
Morphological changes of Branched Ge Clusters caused by Diffusion Fields and Surface Roughness of Au Underlayer
Authors: Sugawara A. , Kikukawa T. , Haga Y. , Nittono O.
Published: April 2011

The formation of polycrystalline Ge clusters, during annealing of amorphous Ge/polycrystalline Au bilayers, has been studied by in-situ transmission and scanning electron microscopy. The experimentally observed generation of branching patterns, and the evolution mechanism of branches, are discussed on the basis of finite diffusion length aggregation simulations.
A Simple Model for Anomalous Relaxation in Porous Media
Authors: Araujo Mariela , Gonzalez Orlando
Published: April 2011

We present a simple model to explain anomalous relaxation in random porous media. The model, based on the properties of random walks on a disordered structure, is able to describe essential features of the relaxation process in terms of a one body picture, in which the many body effects are approximated by geometrical restrictions on the particles diffusion. Disorder is considered as a random variable (quenched and annealed) taken from a power-law distribution |μ|ξμ−1. Quantities relevant to relaxation phenomena, such as the characteristic function and the particle density are calculated. Different regimes are observed as a function of the disorder parameter μ. For μ > 1 the relaxation is of exponential or Debye type, and turns into a stretched exponential as μ decreases. We compare numerical predictions (based on Monte Carlo simulations) with experimental data from porous rocks obtained by Nuclear Magnetic Resonance, and numerical data from other disordered systems.
Interaction Of Dilute Colloidal Particles In A Mixed Solvent
Authors: Maher James V. , Kurnaz M. Levent
Published: April 2011

Introduction

Colloidal particles in mixed solvents can show reversible aggregation in the one-phase regime of the mixture near the mixture's phase separation temperature [1–5]. This aggregation condition has been shown to be related to the affinity of the colloidal surfaces for one of the solvent components. In particular, for a 2,6 lutidine plus water (LW) mixture with colloidally dispersed polystyrene latex spheres (PLS) in a temperature range near the critical temperature, Tc, in the mixture's two-phase region, the particles will partition into one of the solvent phases, with the meniscus between the liquid phases clear to the eye and showing no sign of population by colloidal particles. Which phase of the solvent attracts the particles depends on the surface charge density of the particles, with high surface charge density particles preferring the water-rich phase and low charge density particles preferring the lutidine rich phase. As temperature is advanced deeper into the two-phase region (all effects discussed here are equilibrium effects), there is a temperature, Tw, at which particles appear on the meniscus (most particles remain in the preferred phase, whose population depletion is too small to measure). Tw changes with the surface charge density of the particles [4], but not with radius or with number density of the particles in the sample. The aggregation observed in the one-phase region [5] is then restricted to the side of the solvent's coexistence curve poor in the component which is rich in the partitioning-favored phase.
Immittance Data-Handling/Analyzing Criteria For Heterogeneous Systems
Authors: Alim Mohammad A.
Published: April 2011

The factors determining electrical characteristics using immittance data of heterogeneous systems are not identical to those established with devices based on single-crystal/single-junction (SCSJ) technology. The ”state of normalization” using ”physical geometrical factors” of the asmeasured immittance quantities does not provide a SCSJ-like straightforward interpretation of simultaneously operative phenomena. The normalization procedure can the vitiate identity of each phenomenon occurring within the series-parallel microstructural network of electrical paths. The advantage of using the as-measured immittance data in delineating simultaneously operative phenomena is emphasized. An approach to data-handling/analyzing is proposed considering the limitations and conditions of utilizing the as-measured immittance data. The ”state of normalization” using ”physical geometrical factors” can only be executed for a specific type of phenomenon when isolated from the total electrical behavior.
Electrical Characterization of the Structure and Other Phenomena in Superionic PbSnF4
Authors: Milova Galina , Denes Georges , Madamba M. Cecilia , Perfiliev M.
Published: April 2011

The structure of PbSnF4 is derived from that of fluorite-type β-PbF2 The replacement of half the lead by tin results in a thousand-fold increase of the fluoride ion mobility, even though tin-fluorine bonds are strongly covalent. This is due to an increase of fluoride ion disorder on some sites. The structure of α-PbSnF4 can be interpreted as resulting from the insertion of two layers of [SnF]+ cations between each pair of layers of PbF8 cubes. This has profound bearings on the texture and properties of this material, since the tin(II) stereoactive lone pairs create very effective cleavage planes. In addition, the fluorine covalently bonded to tin occupy all vacant sites used to create Frenkel defects. However, superionicity is observed by complex impedance measurements. Transport number measurements show that fluoride ions are the charge carriers.
Resistivity as a Tool for Characterizing Point Defects in Nonstoichiometric Metallic Ceramics
Authors: Williams Wendell S.
Published: April 2011

Compounds of IVb and Vb transition metals and carbon are electronically conductive, hard materials that crystallize in the NaC1 structure but are very nonstoichiometric (carbon-poor), as in TiCx, where x is the carbon/metal ratio and is less than unity. Carbon atom vacancies are randomly located in the fcc carbon sublattice and act as scattering centers for conduction electrons. Data on the electrical resistivity as a function of the concentration of these point defects could be used in the opposite sense–i.e., to determine the defect concentration, (l-x), through a measurement of resistivity. The chemical composition (carbon/metal ratio) is then given directly by x. The random vacancies can be ordered by slow cooling from high temperatures, thereby eliminating the residual resistivity. The carbon/metal ratios of these ordered phases are either 3:4, 5:6 or 7:8 and thus can specify the chemical composition precisely.
Electrical and Optical Properties of Defects by Complementary Spectroscopies
Authors: Castaldini A. , Cavallini A. , Fernandez P. , Fraboni B. , Piqueras J. , Polenta L.
Published: April 2011

Deep levels in II-VI compounds were investigated by complementary junction and optical spectroscopy methods to assess the characteristics of the traps as well as the limits and the reliability of the techniques applied. The electrical properties have been investigated by current and capacitance transient spectroscopy, while the optical properties have been studied by cathodoluminescence. A critical and comparative analysis of the results obtained with the various methods allowed the determination of the parameters and the nature (majority or minority carrier trap) of most of the detected levels.
Composition Profiling of Graded Dielectric Function Materials by Spectroscopic Ellipsometry
Authors: Trolier-Mckinstry S. , Koh J.
Published: April 2011

Spectroscopic ellipsometry (SE) is a powerful optical tool for non-destructively depth-profiling the dielectric function of samples with a resolution in the Ångstrom range. In this work, the characterization of graded composition layers in transparent materials by SE is described. In modeling studies on the sensitivity limits of the technique, it has been shown that for known index profile shapes, the depth sensitivity can be quite high. For example, diffusion of Na2O into the surface of a vitreous silica sample can be detected for layers as thin as 100Å when the maximum refractive index contrast is only 0.005. The sensitivity of the technique increases as the diffusion depth and refractive index contrast increases. The accuracy of the thickness determination depends on the total thickness of the graded layer; for a given system, the accuracy of the composition measurements do not depend on the surface concentration.

Leached alkali-aluminosilicate and modified lead silicate glasses were examined by SE to experimentally confirm the modeling predictions on the composition depth profiling. Extremely good correlation between the SE-determined depth profile and SIMS measurements on similar samples was obtained for the case of the modified lead silicate glass. For the aluminosilicate glass, simultaneous roughening of the glass surface during etching makes composition profiling more difficult.
Dielectric Properties Dependence on Residual Sodium and Potassium Content in Silica Xerogels
Authors: Meyers K. S. , Zhang G. , Gerhardt R. A.
Published: April 2011

As sodium and potassium are removed from silica xerogels through leaching, the dielectric properties of gels are significantly altered. Work has been done to relate the magnitude of the change in alkali ion content to the change in dielectric properties. Gels were prepared through a colloidal processing method and leached in ammonium nitrate, nitric acid, and water to reduce, to different degrees, the content of sodium and potassium ions. Atomic absorption (AA) was used to determine the actual sodium and potassium content of each gel after its capacitance and dielectric loss were measured as a function of frequency. All electrical measurements were carried out at room temperature and constant humidity. At least three samples for each leaching treatment were evaluated. Data obtained shows a definite relationship between the sodium and potassium content measured by AA and the dielectric constant.
Three Electrode Measurements on Solid Electrolytes
Authors: Pham A. Q. , Glass R. S.
Published: April 2011

AC impedance spectroscopy and chronopotentiometry have been used to study solid state ionic conductors. Results obtained using three electrodes are compared to those using a two electrode configuration. The uncompensated resistance was shown to depend strongly on the geometric placement of the electrodes. The optimal configuration is similar to the Luggin capillary arrangement in the liquid phase. The effect of non-negligible geometric capacitance on interpretation of results is discussed.
Spatially Resolving Impedance Spectroscopy
Authors: Jamnik J. , Fleig J. , Maier J.
Published: April 2011

Two techniques for measurement of local electrical conductivity of inhomogeneous materials are described. I) A novel variant of the impedance technique for thin/thick film characterization was developed; due to the two-dimensionality of the cell set up, at different frequencies different parts of the material are probed. The technique was experimentally verified by measuring the position coordinate of a Ag strip artificially added to a AgCl film. Its application to the Ag/AgCl boundary is touched upon. II) Micro-electrodes were used to probe surface conductances and local subsurface conductivities. The technique was implemented by a home-made high impedance adapter and combined with AFM to measure the contact area. Several examples of application are shown, viz. measurements of a) the enhanced surface conductivity of mechanically polished AgCl crystals, b) the interdiffusion coefficient of Cd2+ in AgCl, and c) the grain conductivity of polycrystalline AgCl.
Ionic Conductivities of Doped CeO2 Thin Films as Related to Their Microstructure
Authors: Tian Chunyan , Chan Siu-Wai
Published: April 2011

Thin films of 4% Y2O3 doped CeO2/Pd film/(001)LaA103 with a very low pinhole density were successfully prepared using electron-beam deposition technique. The microstructure of the films was characterized by x-ray diffraction and the electrical properties were studied as a function of temperature with AC impedance spectroscopy. A brick layer model was adopted to correlate the electrical properties to the microstructure of the films, which can be simplified as either a series or a parallel equivalent circuit associated with either a fine grain or a columnar grain structure, respectively. The conductivities of the films fell between the conductivities derived from the two circuit models, suggesting that the films are of a mixed fine grain and columnar grain structure. The measured dielectric constants of the films were found smaller than that of the bulk.
Structure and Electrical Properties of Conjugated Polymer Composites
Authors: Gong K. C. , Ma W. S.
Published: April 2011

The microstructure and electrical property relationships in conjugated polymer composites( polyaniline (PAn) or polypyrrole(PPy)/thiokol rubber (TR)) have been characterized by 4-probe method , scanning electron microscope (SEM), x-ray energy dispersive spectrometer (EDS) and attenuated total reflection(ATR). The electrical conductivities of the two surfaces of the film, with film thicknesses from 0.01mm to 0.30mm, prepared by electropolymerization , were measured . The differences were related to the thickness of the film. As for a similar PPy/TR film, when trichloroacetic acid was regarded as the supporting electrolyte the film thickness was 0.05mm and the electrical conductivity of the surface on the solution side was reduced by about 21.3%. The studies of SEM, x-ray EDS , ATR for the chemical composition and microstructure of the two surfaces of the composite films showed that a reaction layer was present in the films. Moreover , the conductivity changes between two sides of a fold mark in the conducting composite film is discussed.
Current Density Effects on the Corrosion of Ceramic and Metallic Electrode Materials in Waste Glasses
Authors: Wang E. , Mohr R. K. , Buechele A. C. , Pegg I. L.
Published: April 2011

Vitrification by joule-heating requires suitable electrode materials. Molybdenum electrodes are used often in high temperature (∼1500°C) joule-heated melters producing commercial glass while Inconel 690 has been the material of choice for lower temperature (∼1150°C) high-level nuclear waste vitrification. Vitrification of diverse waste streams at higher-temperatures places increasingly severe demands upon the electrode materials. Some commercially available ceramics possess sufficient conductivity at high temperatures to be used as electrodes. Many mixed- and low-level waste vitrification applications involve significant amounts of reducible species that accelerate the corrosion process for metallic electrodes. In addition, the corrosion rate of all electrode materials depends on the electrode current density. A special test rig was designed that measures the dependence of corrosion rate on both current density and temperature over extended periods of time. Molybdenum, Inconel 690, and three types of commercial ceramic coupons were tested over a range of conditions in simulated waste glasses containing key reducible species (e.g., PbO, CuO, ZnO, SO3). The corrosion rates show strong dependencies on the current density. Coupons were sectioned and the glass-coupon interface examined using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS). Cu, Pb, Ni and Sn were found to be reduced to their metallic states at the molybdenum coupon surfaces and were active in the corrosion process.
The Release of Technetium from Defense Waste Processing Facility Glasses
Authors: Ebert W. L. , Wolf S. F. , Bates J. K.
Published: April 2011

Laboratory tests are being conducted using two radionuclide-doped Defense Waste Processing Facility (DWPF) glasses (referred to as SRL 131A and SRL 202A) to characterize the effects of the glass surface area/solution volume (SN) ratio on the release and disposition of Tc and several actinide elements. Tests are being conducted at 90°C in a tuff ground water solution at S/V ratios of 10, 2000, and 20,000 m−1 and have been completed through 1822 days. The formation of certain alteration phases in tests at 2000 and 20,000 m−1 results in an increase in the dissolution rates of both glasses. The release of Tc parallels that of B and Na under most test conditions and its release increases when alteration phases form. However, in tests with SRL 202A glass at 20,000 m−1, the Tc concentration in solution decreases coincidentally with an increase in the nitrite/nitrate ratio that indicates a decrease in the solution Eh. This may have occurred due to radiolysis, glass dissolution, the formation of alteration phases, or vessel interactions. Technetium that was reduced from Tc(VII) to Tc(IV) may have precipitated, though the amount of Tc was too low to detect any Tc-bearing phases. These results show the importance of conducting long-term tests with radioactive glasses to characterize the behavior of radionuclides, rather than relying on the observed behavior of nonradioactive surrogates.
Schoepite and Dehydrated Schoepite
Authors: Hawthorne F. C. , Finch R. J. , Ewing R. C.
Published: April 2011

Schoepite, [(UO2)8O2 (OH)12]·12H2O, dehydrates irreversibly to a defect structure-derivative of α-UO2 (OH)2 with composition UO3·0.75H2O (dehydrated schoepite) according to the reaction.[(UO2)8O2 (OH) 12]+12H2O ↠ 8[(UO2 )O0.25 (OH)1 5.] + 12H2O.Natural crystals change from translucent yellow schoepite to opaque yellow, polycrystalline dehydrated schoepite. The complete transformation occurs in three steps: (I) loss of interlayer H20 in schoepite causing collapse of the layers; (2) atomic rearrangement within the sheets from a schoepite-type arrangement to a configuration which may be similar to that of rnetaschoepite, UO3.2H2O; (3) a second rearrangement to the defect α-UO2 (OH)2-type sheet. Although dehydrated schoepite may transform to stoichiometric α-UO 2(OH)2 in water above approximately 290°C, it does not hydrate at any temperature. Schoepite transforms slowly in air at ambient temperature to metaschoepite, although this may take geologically long periods. The transformation of schoepite to dehydrated schoepite can occur rapidly at room temperature if crystals, which have only partially converted to metaschoepite. are subjected to external stress.
Thermodynamic Modelling of the Sorption of Radioelements onto Cementitious Materials
Authors: Heath T. G. , Ilett D. J. , Tweed C. J.
Published: April 2011

A model has been developed for the sorption of radioelements onto cementitious materials based on the diffuse-layer modelling approach. The model assumes that silicon sites (>SiOH) and calcium sites (>CaOH) dominate the surface chemistry and the sorption of radioelements onto the cementitious materials. Both types of site may undergo surface protonation and deprotonation reactions. Cement-based systems vary greatly in their chemistry depending on their calcium-tosilicon molar ratio, and the corresponding variation in the surface chemistry has been incorporated by allowing sorption of calcium ions onto silicon sites. This process results in a change from a silica-type surface, at very low calcium-silicon ratios, to a calcium hydroxide-type surface for high-calcium cement-based materials. The predicted variation in the surface chemistry is consistent with literature data on measured zeta potentials of cements. The model has been applied successfully to describe the sorption of simple caesium and iodide ions at varying calciumsilicon ratios. In a Nirex repository for low and intermediate level wastes, a high-calcium cementitious backfill would be specified. This model has allowed a consistent interpretation of experimental data for sorption of key radioelements, including uranium and plutonium, onto the backfill, under saline and non-saline conditions.
A Computer Program to Facilitate Performance Assessment of Underground Low-Level Waste Concrete Vaults
Authors: Snyder K. A. , Clifton J. R. , Pommersheim J.
Published: April 2011

A computer program (4SIGHT) to facilitate performance assessment of underground concrete vaults for low level waste (LLW) disposal facilities is being developed at the National Institute of Standards and Technology (NIST). Specifically, the program predicts the hydraulic conductivity and the service life of an underground concrete vault. The hydraulic conductivity estimate is based upon empirical relations. The service life is estimated from consideration of three major degradation processes: steel reinforcement corrosion, sulfate attack, and leaching. The performance prediction is based upon ion transport equations for both diffusion and advection. Most importantly, the computer program incorporates the synergistic degradation effects of all three processes, and their effect upon the transport coefficients.
Evaluation of Factors Affecting Diffusion in Compacted Bentonite
Authors: Lehikoinen J. , Carlsson T. , Muurinen A. , Olin M. , Salonen P.
Published: April 2011

The information available from the open literature and our studies on exclusion, sorption and diffusion mechanisms of ionic and neutral species in bentonite has been compiled and re-examined in relation to the microstructure of bentonite. The emphasis is placed on a more thorough understanding of the diffusion processes taking place in compacted bentonite. Despite the scarcity of experiments performed with neutral diffusants, these imply that virtually all the pores in compacted bentonite are accessible to neutral species. Anion exclusion, induced by the overlap of electrical double layers, may render the accessible porosity for anions considerably less than the porosity obtained from the water content of the clay. On the basis of the compiled data, it is highly probable that surface diffusion plays a significant role in the transport of cations in bentonite clays. Moreover, easily soluble compounds in bentonite can affect the ionic strength of porewater and, consequently, exclusion, equilibrium between cations, and surface diffusion.
A Capillary Network Model for Coupled Gas and Water Flow in Engineered Barriers
Authors: Impy M. D. , Grindrod P. , Clark K. J. , Takase H.
Published: April 2011

A two-dimensional capillary network model for gas migration through a water-saturated medium is presented. The model is an extension of previously developed capillary bundle models, and provides a discrete alternative to classical continuum Darcy models. The need for such an alternative has become apparent from recent experimental results that suggest gas migrates through low permeability water-saturated media via a small number of preferential pathways.
Experimental Study on Groundwater Flow and Mass Transport in a Heterogeneous Porous Medium
Authors: Hatanaka Koichiro , Watari Shingo , Uchida Masahiro , Takase Hiroyasu , Impey Michael D.
Published: April 2011

In performance assessment of geological disposal systems the phenomenon of dispersion in geological media is one of the most important processes to be modeled and is dependent on the heterogeneity of the media. In order to understand the dispersion process in a well defined heterogeneous field, laboratory experimental apparatus, named MACRO, was constructed to study fluid flow and mass transport. A synthetic heterogeneous field was constructed in a flow-bed using six kinds of glass beads with diameters of 0.1, 0.15, 0.2, 0.4, 0.6 and 0.8 mm. Both dye (brilliant blue) and NaCI solution were used as tracers in the experiment. A particle tracking method, considering advection, was used in numerical simulations of the experiment, and the dispersion effect was assumed to be expressed by the variability of the local advective velocity in a high resolution numerical grid. The simulated results obtained by the particle tracking method agreed with the measurement, confirming the applicability of modelling approach used in this study.
Discrete-Fracture Modeling of Thermal-Hydrological Processes at Yucca Mountain and the Llnl G-Tunnel Heater Test
Authors: Nitao John J. , Buscheck Thomas A.
Published: April 2011

An in situ heater test was performed at G-Tunnel, Nevada Nuclear Test Site, to investigate the thermal-hydrological response of unsaturated, fractured volcanic tuff under conditions similar to those at Yucca Mountain. The NUFT flow and transport code was used to model the test using discrete-fracture and equivalent-continuum approaches. Nonequilibrium fracture flow and thermal buoyant gas-phase convection were found to be the likely causes for observed lack of condensate imbibition into the matrix. The potential repository at Yucca Mountain was also modeled. Disequilibrium fracture flow is predicted to occur for less than a hundred years after emplacement followed by a period of fracture-matrix equilibrium, during which the equivalent-continuum and discrete-fracture models give almost identical results.
Cleaning of the Moscow Sia “Radon” Surface Water Runoff: Problems and Solutions
Authors: Barinov A. S. , Volkov A. S. , Karlin Y. V. , Kropotov V. N.
Published: April 2011

This paper describes a three-barrier system of engineered safety features for the cleaning of the Moscow SIA “Radon” surface water runoff.
Alteration History Studies in the Exploratory Studies Facility, Yucca Mountain, Nevada, USA
Authors: Levy S. S. , Norman D. I. , Chipera S. J.
Published: April 2011

By mid-1995, the Exploratory Studies Facility (ESF) extended about 1.1 km from Exile Hill westward toward Yucca Mountain, mostly within densely welded, devitrified Tiva Canyon Tuff. Secondary mineral occurrences in this unit include breccia cements of mordenite, a fibrous zeolite, and vapor-phase deposits of silica, alkali feldspar, apatite, hollandite, amphibole, and zircon. Calcite is also a common secondary mineral in faults and fractures. Studies of water and gas contents in fluid inclusions in calcites from a fault in nonwelded tuff and a fracture in densely welded tuff suggest mineral deposition under transient locally saturated conditions. Calcite in the nonwelded tuff incorporated air from the unsaturated tuff adjacent to the fault.

A highly altered interval within pre-Pah Canyon tuffs just above the top of the Topopah Spring Tuff may be a fossil fumarole or other hydrothermal feature associated with cooling pyroclastic deposits, overprinted by later zeolitic alteration. The observed quartz, cristobalite, opal-CT, and fluorite have been widely identified as products of syngenetic devitrification and vapor-phase alteration in and above the Topopah Spring Tuff. Smectite, also an abundant secondary mineral at the ESF site, has been observed elsewhere at this stratigraphic level.

Zeolitic alteration of nonwelded tuffs above the Topopah Spring Tuff, as seen in the ESF, has also been noted in drill core and outcrop at northeastern Yucca Mountain. The hydrologic and geochemical conditions that favored zeolitization only in certain areas of this stratigraphic interval have yet to be determined.