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Effects of electronically neutral impurities on muonium in germanium

Description: Low-temperature measurements of muonium parameters in various germanium crystals have been performed. We have measured crystals with different levels of neutral impurities, with and without dislocations, and with different annealing histories. The most striking result is the apparent trapping of Mu by silicon impurities in germanium.
Date: April 1, 1983
Creator: Clawson, C.W.; Crowe, K.M.; Haller, E.E.; Rosenblum, S.S. & Brewer, J.H.
Partner: UNT Libraries Government Documents Department

Measurement of viscosity, density and gas solubility of refrigerant blends

Description: Liquid/liquid miscibilities of four different 32 ISO VG polyolesters and one alkylbenzene at three concentrations were determined. Also a full vapor lubricant equilibrium (VLE) viscosity reduction of a 32 ISO VG mineral oil with HCFC-22 is complete. Partial viscosity reduction information by the fractionate components from R-502 in 32 ISO VG mineral oil is presented from 40C (104C) and 70C (158F) isotherms.
Date: April 1, 1993
Creator: Cavestri, R.C. & Munk, J. (Air-Conditioning and Refrigeration Technology Inst., Inc., Arlington, VA (United States))
Partner: UNT Libraries Government Documents Department

Structure of borate glasses by Raman spectroscopy

Description: Raman spectra of the alkali borate glasses are presented with emphasis on the implications of detailed frequency shifts and line shapes. The results are interpreted on a rather complicated three-level hierarchy of glass structures which include boron-oxygen polyhedra, topologically closed rings of polyhedra, and larger scale interconnected clusters of alkali-rich and alkali-poor structures. Both the model and the argument are deliberately vague. The borate glasses appear to be highly disordered materials with a size scale such that no presently available tool gives a really clear view of the structural detail.
Date: January 1, 1977
Creator: White, W.B.; Brawer, S.A.; Furukawa, T. & McCarthy, G.J.
Partner: UNT Libraries Government Documents Department

Pore structure and growth kinetics in carbon materials

Description: Pore structure of glassy carbon (GC) and pyrolytic graphite (PG) have been investigated. GC is one of the most impervious of solids finding applications in prosthetic devices and fuel cells while PG is used extensively in the aerospace industry. One third of the microstructure of GC consists of closed pores inaccessible to fluids. The microstructure of this material has been characterized using x-ray diffraction (XRD) and high resolution electron microscopy. Small angle x-ray scattering (SAXS) has been used to measure the angstrom sized pores and to follow the evolution of pore surface area as a function of heat treatment temperature (HTT) and heat treatment time (HTt) at constant temperature. From these measurements an analysis of the surface area kinetics was made to find out if rate processes are involved and to locate graphitization occurring at pore surfaces. PG on the other hand has been found to have larger sized pores that comprise five percent of its volume. In addition to being closed these pores are oriented. Some pore models are proposed for PG and the existing scattering theory from oriented ellipsoids is modified to include the proposed shapes.
Date: April 1, 1978
Creator: Bose, S.
Partner: UNT Libraries Government Documents Department

Crystalline to amorphous transformation in silicon

Description: In the present investigation, an attempt was made to understand the fundamental mechanism of crystalline-to-amorphous transformation in arsenic implanted silicon using high resolution electron microscopy. A comparison of the gradual disappearance of simulated lattice fringes with increasing Frenkel pair concentration with the experimental observation of sharp interfaces between crystalline and amorphous regions was carried out leading to the conclusion that when the defect concentration reaches a critical value, the crystal does relax to an amorphous state. Optical diffraction experiments using atomic models also supported this hypothesis. Both crystalline and amorphous zones were found to co-exist with sharp interfaces at the atomic level. Growth of the amorphous fraction depends on the temperature, dose rate and the mass of the implanted ion. Preliminary results of high energy electron irradiation experiments at 1.2 MeV also suggested that clustering of point defects occurs near room temperature. An observation in a high resolution image of a small amorphous zone centered at the core of a dislocation is presented as evidence that the nucleation of an amorphous phase is heterogeneous in nature involving clustering or segregation of point defects near existing defects.
Date: September 1, 1982
Creator: Cheruvu, S.M.
Partner: UNT Libraries Government Documents Department

Hydrogen in germanium

Description: Hydrogen is shown to form molecular acceptors and donors in pure germanium. Piezospectroscopy reveals that the hydrogen-related shallow donor D has non-tetrahedral symmetry. One hydrogen atom is incorporated in D as shown with an isotope shift in the donor ground state. Oxygen is also involved in the formation of D. The hole emission rates of two copper--hydrogen acceptor complexes were determined with Deep Level Transient Spectroscopy. They are e/sub 1//sup Cu-H/ = 1.5 x 10/sup 8/ T/sup 2/ exp (-.195 eV/kT) K/sup -2/ s/sup -1/ and e/sub 3//sup Cu-H/ = 5.0 x 10/sup 8/ T/sup 2/ exp (-.068/kT) K/sup -2/ s/sup -1/.
Date: September 1, 1978
Creator: Haller, E.E.
Partner: UNT Libraries Government Documents Department

Study of short-range motion of atomic hydrogen in amorphous silicon by neutron reflectometry

Description: Preliminary results of neutron reflectometry (NR) measurements on rf sputter-deposited a-Si:H/a-Si:D bilayers indicate that this technique may be used to monitor H and D motions over distances of {approx} 10 to 200 {Angstrom} with a nominal resolution of 5--10 {Angstrom}. In studying rf sputter-deposited thin films containing a high density of microvoids annealed at 270 C, we found that the hydrogen diffused a distance of only {approx} 100 {Angstrom}. Further annealing at 270 and 280 C produced no additional motion. This result is consistent with a model of this system in which the hydrogen is trapped in microvoids after moving a relatively short distance.
Date: May 1, 1992
Creator: Dozier, W.D. (Argonne National Lab., IL (United States)); Herwig, K.W. (Missouri Univ., Columbia, MO (United States). Research Reactor Facility); Shinar, R. (Iowa State Univ. of Science and Technology, Ames, IA (United States). Microelectronics Research Center); Jia, H. & Shinar, J. (Ames Lab., IA (United States))
Partner: UNT Libraries Government Documents Department

New method to determine the chemical composition and structure of non-elemental acceptor and donor centers in ultra-pure germanium

Description: A new approach towards the understanding of hitherto unknown, non-elemental acceptors and donors which can limit the purity of ultra-pure germanium has been explored for a specific pair of shallow centers, designated A/sub 2/ and D. Using photoelectric spectroscopy, we have demonstrated that an isotope shift in the ground-state binding energy occurs when the germanium crystals are grown in pure deuterium instead of in the usual pure hydrogen atmosphere. This isotope shift is the most direct proof of the presence of hydrogen atoms in the centers A/sub 2/ and D. Applying uniaxial stress to Ge samples containing A/sub 2/ and D, we show that the symmetry and structure of the centers can be explored. The knowledge of the chemical composition and the structure of the non-elemental centers will allow development of methods to reduce and keep their concentrations to acceptable levels.
Date: October 1, 1977
Creator: Haller, E.E.
Partner: UNT Libraries Government Documents Department

3-D structures and intrinsic defects in trans-polyacetylene

Description: We report first-principles local-density functional pseudopotential calculations of static and dynamic electronic structure properties of crystalline 3-D trans--(CH)/sub x/. We find a broken symmetry ground state of P2/sub 1//a symmetry with in-phase dimerizations. Using a Green's function technique, we show that the 3-D character of the electronic band edge states strongly suppresses self-trapping, destabilizing polarons and possibly bipolarons as well in perfectly ordered 3-D trans--(CH)/sub x/. 26 refs.
Date: January 1, 1989
Creator: Vogl, P. & Campbell, D.K.
Partner: UNT Libraries Government Documents Department

Structure and stability of microvoids in A-Si:H

Description: Microvoids appear to be universally present in a-Si:H as demonstrated by small angle X-ray scattering including the presence of microvoids in device quality glow discharge a-Si:H. We have studied the structural properties of these microvoids with molecular dynamics simulations. Using molecular dynamics simulations with classical potentials, we have created microvoids by removing Si and H atoms from a computer generated a-Si:H network. The internal surfaces of the microvoids were passivated with additional H atoms and the microvoids were fully relaxed. Microvoids over a limited range of sizes (5--90 missing atoms) were examined. We obtained a relaxed microvoid structure with no dangling bonds for a microvoid with 17 missing atoms, whereas other sizes examined produced less relaxed models with short H-H distances at the microvoid surface. The strains near the microvoid surface are described. The microvoid model was stable to local excitations on weak bonds in the vicinity of the microvoid.
Date: January 1, 1993
Creator: Biswas, R. (Iowa State Univ. of Science and Technology, Ames, IA (United States). Microelectronics Research Center) & Kwon, I. (Los Alamos National Lab., NM (United States))
Partner: UNT Libraries Government Documents Department

Growth kinetics of grain-boundary induced melting: A molecular-dynamics study

Description: The growth kinetics of melting nucleated at a high-angle twist boundary in silicon are investigated using molecular dynamics. Melting is found to be a two-stage process. In the first stage order is lost within a single plane at the interface and the density of the solid increases to that of the liquid. In the second stage the atomic coordination changes and an isotropic liquid is formed. 11 refs., 4 figs., 1 tab.
Date: December 1, 1988
Creator: Phillpot, S.R.; Lutsko, J.F.; Wolf, D. & Yip, S.
Partner: UNT Libraries Government Documents Department

New theories for smectic and nematic liquid-crystal polymers: Backbone LCPs (liquid crystalline polymers) and their mixtures and side-chain LCPs

Description: A summary of predictions and explanations from statistical-physics theories for both backbone and side-chain liquid crystalline polymers (LCPs) and for mixtures with backbone LCPs are presented. Trends in the thermodynamic and molecular ordering properties have been calculated as a function of pressure, density, temperature, and molecule chemical structures (including degree of polymerization and the following properties of the chemical structures of the repeat units: lengths and shapes, intra-chain rotation energies, dipole moments, site-site polarizabilities and Lennard-Jones potentials, etc.) in nematic and multiple smectic-A LC phases and in the isotropic liquid phase. The theoretical results are found to be in good agreement with existing experimental data. These theories can also be applied to combined LCPs. Since these theories have no ad hoc or arbitrarily adjustable parameters, these theories can be used to design new LCPs and new solvents as well as to predict and explain properties. 27 refs., 4 tabs.
Date: January 1, 1987
Creator: Dowell, F.
Partner: UNT Libraries Government Documents Department

Use of electrical conductivity experiments to study the phase diagram of carbon

Description: Recent theoretical and experimental work on the properties of carbon at very high pressures indicates that the diamond phase may dominate the carbon phase diagram. The work of Yin and Cohen indicates that diamond may be the stable carbon phase to pressures as high as 20 Mbar. Recent work by Shaner, et al. is compatible with solid diamond on the Hugoniot at pressures up to 1.4 Mbar. Using previously developed techniques, we have carried out conductivity measurements on shock-compressed graphite to pressures in excess of 600 kbar and temperatures up to 5000/sup 0/K. No diamond - liquid transition was observed under the experimental conditions achieved.
Date: January 1, 1985
Creator: Mitchell, A.C.; Shaner, J.W. & Keeler, R.N.
Partner: UNT Libraries Government Documents Department

Remarks on solid state amorphizing transformations

Description: Amorphous solids can now be produced through a variety of laboratory synthesis techniques as well as through many naturally occurring processes. In general, we can classify the methods of synthesis of amorphous solids as follows: (1) rapid solidification of melts or vapors; (2) atomic disordering of crystalline lattices; (3) solid state reactions between pure elements; (4) solid-state transformations from metastable crystalline states; and (5) deposition from electrolytes. We give a short summary of the historical development of methods (1)-(4) (method (5) is clearly outside the focus of this conference) and we discuss the basic physical principles behind the methods. 21 refs., 1 fig.
Date: January 1, 1987
Creator: Schwarz, R.B. & Johnson, W.L.
Partner: UNT Libraries Government Documents Department

Influence of substrate topography on the nucleation of diamond thin films

Description: Polycrystalline diamond films are of interest because of their unique materials properties. However, depositing films with the desired morphology is hindered by the difficulty in controlling the nucleation process. A popular method of enhancing the nucleation rate and density is to abrade the substrate with diamond powder before deposition. Although effective, this procedure is difficult to analyze since it may leave residual powder, may introduce damage, and may modify the surface topography. In order to separate these effects, we have studied the nucleation on chemically etched silicon substrates. Our results show that the majority of nucleation events occur on protruding surface features.
Date: January 1, 1991
Creator: Dennig, P.A. & Stevenson, D.A.
Partner: UNT Libraries Government Documents Department

Transport properties of disordered porous media from the microstructure

Description: Progress has been made in five general areas: (1) the development of cross-property relations, i.e., relationships between one effective property and a different effective property; (2) the development of a systematic means to represent and calculate statistical correlation functions that arise in rigorous property relations for heterogeneous media, using statistical-mechanical methods; (3) the derivation and calculation of property relations which depend upon this microstructural information; (4) sterically hindered fragmentation in reactive solids using percolation theory and simulations; and (5) the use of Brownian-motion simulation techniques to exactly'' predict the effective properties of disordered media. We embarked on a new area of research, namely, the study of passive advection of tracer particles in random velocity fields.
Date: November 1, 1991
Creator: Torquato, S.
Partner: UNT Libraries Government Documents Department

Dopant profile changes induced by pulsed laser annealing

Description: Secondary ion mass spectrometry (SIMS) and Rutherford ion backscattering was used to investigate the effects of pulsed laser annealing (Q switched ruby laser, 50 x 10/sup -9/ sec. pulse duration time) on silicon crystals implanted by B, P, As, Sb, Cu, and Fe. The results show that B, P, As, and Sb undergo substantial redistribution in the absence of significant surface segregation during the laser annealing process. Calculations strongly suggest that the crystal can be melted to a depth of approx. 1..mu.. and the implanted region remains in the melted state for several hundred nanoseconds. Profiles calculated for liquid phase diffusion of the dopant are shown to be in excellent agreement with the experimental results. Arsenic profiles after laser annealing are shown to be very sensitive to the laser photon energy density. Profiles for implanted Cu and Fe show significant segregation to the surface after pulsed laser annealing and this may be related to their very low segregation coefficient from the liquid.
Date: August 1, 1978
Creator: White, C.W.; Christie, W.H.; Pronko, P.P.; Appleton, B.R.; Wilson, S.R.; Young, R.T. et al.
Partner: UNT Libraries Government Documents Department

Orientational epitaxy in adsorbed monolayers

Description: The ground state for adsorbed monolayers on crystalline substrates is shown to involve a definite relative orientation of the substrate and adsorbate crystal axes, even when the relative lattice parameters are incommensurate. The rotation angle which defines the structure of the monolayer-substrate system is determined by the competition between adsorbate-substrate and adsorbate-adsorbate energy terms, and is generally not a symmetry angle. Numerical predictions are presented for the rare gas-graphite systems, whose interaction potentials are rather well known. Recent LEED data for some of these systems appear to corroborate these predictions.
Date: January 1, 1977
Creator: Novaco, A. D. & McTague, J. P.
Partner: UNT Libraries Government Documents Department

New theories for smectic and nematic liquid crystalline polymers

Description: A summary of results from new statistical-physics theories for both backbone and side-chain liquid crystalline polymers (LCPs) and for mixtures with LCPs is presented. Thermodynamic and molecular ordering properties (including odd-even effects) have been calculated as a function of pressure, density, temperature, and molecule chemical structures (including degree of polymerization and the following properties of the chemical structures of the repeat units: lengths and shapes, intra-chain rotation energies, dipole moments, site-site polarizabilities and Lennard-Jones potentials, etc.) in nematic and multiple smectic-A LC phases and in the isotropic liquid phase. These theories can also be applied to combined LCPs. Since these theories have no ad hoc or arbitrarily adjustable parameters, these theories have been used to design new LCPs and new solvents and to predict and explain properties.
Date: January 1, 1987
Creator: Dowell, F.
Partner: UNT Libraries Government Documents Department

Shock compression of pyrolytic, ceylon natural, and a hot-pressed synthetic graphite to 120 GPa

Description: Recent improvements have made the two-stage, light-gas gun at Lawrence Livermore Laboratory into a dependable source of very energetic planar shocks. Also, the inclined-prism technique has proved useful in observing anomalous behavior of materials, especially at low pressure. The availability of these improved techniques presented an opportunity to re-examine the low pressure, shock-compression characteristics for graphite and, in a search for an additional transformation, to extend some of the data to higher pressures.
Date: June 6, 1977
Creator: Gust, W. H. & Young, D. A.
Partner: UNT Libraries Government Documents Department

X-ray diffraction on radioactive materials

Description: X-ray diffraction studies on radioactive materials are discussed with the aim of providing a guide to new researchers in the field. Considerable emphasis is placed on the safe handling and loading of not-too-exotic samples. Special considerations such as the problems of film blackening by the gamma rays and changes induced by the self-irradiation of the sample are covered. Some modifications of common diffraction techniques are presented. Finally, diffraction studies on radioactive samples under extreme conditions are discussed, with primary emphasis on high-pressure studies involving diamond-anvil cells.
Date: January 1, 1978
Creator: Schiferl, D. & Roof, R.B.
Partner: UNT Libraries Government Documents Department

Stress-displacement relation during fiber pullout

Description: During fiber pullout tests of fiber-reinforced composites, initial debonding, partial debonding, complete debonding at the interface, and fiber pullout occur sequentially. Adopting the shear lag model for stress analyses and the strength criterion for interfacial debonding, a bond length dependence of the initial debond stress is derived. During partial debonding, the stress initially increases with the increasing fiber displacement. The partial debond stress reaches a maximum value and begins to decrease with an accompanying decreasing fiber displacement until the interface is completely debonded. Theoretically, the stress-displacement curve shows a nose'' at the maximum debond stress. However, the pullout test is generally conducted under the condition of an increasing fiber displacement. Hence, at the maximum debond stress, the observed stress drops abruptly as the increasing fiber displacement type test obscures the nose-type characteristic.
Date: January 1, 1990
Creator: Hsueh, C.H.
Partner: UNT Libraries Government Documents Department

Study of pore growth in glassy carbon using small angle x-ray scattering

Description: Small-angle x-ray scattering was used to study the average pore size in glass-like carbon as a function of both heat-treatment time and heat-treatment temperature. A pore-growth model based on graphitization processes is presented. The simple mechanism shows that the change in the average radius of gyration with time is related to the total number of pores as a function of time, which in turn depends on the irreversible thermal-expansion phenomenon. The results of this study are inconsistent with a vacancy-migration pore-growth mechanism proposed earlier.
Date: July 1, 1982
Creator: Hoyt, J.
Partner: UNT Libraries Government Documents Department