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Investigation of epitaxial silicon layers as a material for radiation hardened silicon detectors

Description: Epitaxial grown thick layers (>100 {mu}m) of high resistivity silicon (Epi-Si) have been investigated as a possible candidate of radiation hardened material for detectors for high-energy physics. As grown Epi-Si layers contain high concentration (up to 2{center_dot}10{sup 12} cm{sup {minus}3}) of deep levels compared with that in standard high resistivity bulk Si. After irradiation of test diodes by protons (E{sub p} = 24 GeV) with a fluence of 1.5{center_dot}10{sup 11} cm{sup {minus}2}, no additional radiation induced deep traps have been detected. A reasonable explanation is that there is a sink of primary radiation induced defects, in epitaxial layers. The {open_quotes}sinking{close_quotes} process, however, becomes non-effective at high radiation fluences (10{sup 14} cm{sup {minus}2}) due to saturation of epitaxial defects by high concentration of radiation induced ones. As a result, at neutron fluence of 1{center_dot}10{sup 14}cm{sup {minus}2} the deep level spectrum corresponds to well-known spectrum of radiation induced defects in high resistivity bulk Si. The net effective concentration in the space charge region equals to 3{center_dot}10{sup 12} cm{sup {minus}3} after 3 months of room temperature storage and reveals similar annealing behavior for epitaxial as compared to bulk silicon.
Date: November 1, 1997
Creator: Li, Z.; Eremin, V.; Ilyashenko, I. & Ivanov, A.
Partner: UNT Libraries Government Documents Department

Theory of point-defects, non-stoichiometry, and solute additions in SmCo{sub 5+x}-Sm{sub 2}Co{sub 17{minus}y} and related compounds

Description: There is considerable interest in the possibility of producing Sm-Co-based nanocomposite magnets by rapid solidification and other far-from-equilibrium processing methods. Thermodynamic and kinetic models are quite valuable in understanding and optimizing such methods. This paper describes a method of estimation, utilizing tight-binding-based bond-order interatomic interaction potentials, of the thermodynamic properties of point defects such as vacancies, interstitials, antisite defects, and solute additions in the SmCo{sub 5+x} and Sm{sub 2}Co{sub 17{minus}y} phases and related rare-earth-transition metal compounds. Illustrative calculations for point defects in SmCo{sub 5} will be presented. The results suggest a unified model of the thermodynamic properties of the SmCo{sub 5+x}-Sm{sub 2}Co{sub 17{minus}y} region of the phase diagram, based on the 1-5 structure and the replacement of Sm by interacting dumb-bell interstitials to form the 2-17 structure; the model is similar in nature to theories of the thermodynamics of metal hydrides.
Date: August 1, 1998
Creator: Welch, D.O.
Partner: UNT Libraries Government Documents Department

Development of a Fundamental Understanding of Chemical Bonding and Electronic Structure in Spinel Compounds

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos national Laboratory (LANL). Hundreds of ceramic compounds possess the spinel crystal structure and exhibit a remarkable variety of properties, ranging from compounds that are electrical insulators to compounds that are superconducting, or from compounds with ferri- and antiferromagnetic behavior to materials with colossal magnetoresistive characteristics. The unique crystal structure of spinel compounds is in many ways responsible for the widely varying physical properties of spinels. The objective of this project is to investigate the nature of chemical bonding, point defects, and electronic structure in compounds with the spinel crystal structure. Our goal is to understand and predict the stability of the spinel structure as a function of chemical composition, stoichiometry, and cation disorder. The consequences of cation disorder in spinel materials can be profound . The ferromagnetic characteristics of magnesioferrite, for instance, are entirely attributable to disorder on the cation sublattices. Our studies provide insight into the mechanisms of point defect formation and cation disorder and their effects on the electronic band structure and crystal structure of spinel-structure materials. our ultimate objective is to develop a more substantive knowledge of the spinel crystal structure and to promote new and novel uses for spinel compounds. The technical approach to achieve our goals is to combine first-principles calculations with experimental measurements. The structural and electronic properties of spinel samples were experimentally determined primarily with X-ray and neutron scattering, optical and X-ray absorption, and electron energy-loss spectroscopy. Total energy electronic structure calculations were performed to determine structural stability, band structure, density of states, and electron distribution. We also used shell-model total -energy calculations to assess point-defect formation and migration energies in magnesio-aluminate spinel.
Date: May 14, 1999
Creator: Sickafus, K.E.; Wills, J.M.; Chen, S.-P.; Terry, J.H., Jr.; Hartmann, T. & Sheldon, R.I.
Partner: UNT Libraries Government Documents Department

Development of a Fundamental Understanding of Chemical Bonding and Electronic Structure in Spinel Compounds

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Hundreds of ceramic compounds possess the spinel crystal structure and exhibit a remarkable variety of properties, ranging from compounds that are electrical insulators to compounds that are superconducting, or from compounds with ferri- and antiferromagnetic behavior to materials with colossal magnetoresistive characteristics. The unique crystal structure of spinel compounds is in many ways responsible for the widely varying physical properties of spinels. The objective of this project is to investigate the nature of chemical bonding, point defects, and electronic structure in compounds with the spinel crystal structure. Our goal is to understand and predict the stability of the spinel structure as a function of chemical composition, stoichiometry, and cation disorder. The consequences of cation disorder in spinel materials can be profound . The ferromagnetic characteristics of magnesioferrite, for instance, are entirely attributable to disorder on the cation sublattices. Our studies provide insight into the mechanisms of point defect formation and cation disorder and their effects on the electronic band structure and crystal structure of spinel-structure materials. Our ultimate objective is to develop a more substantive knowledge of the spinel crystal structure and to promote new and novel uses for spinel compounds. The technical approach to achieve our goals is to combine first-principles calculations with experimental measurements. The structural and electronic properties of spinel samples were experimentally determined primarily with X-ray and neutron scattering, optical and X-ray absorption, and electron energy-loss spectroscopy. Total energy electronic structure calculations were performed to determine structural stability, band structure, density of states, and electron distribution. We also used shell-model total -energy calculations to assess point-defect formation and migration energies in magnesio-aluminate spinel.
Date: June 3, 1999
Creator: Sickafus, K.E.; Wills, J.M.; Chen, S.-P.; Terry, J.H., Jr.; Hartmann, T. & Sheldon, R.I.
Partner: UNT Libraries Government Documents Department

Polyester-based thin films with high photosensitivity

Description: A great deal of research has been done to understand the photosensitive optical response of inorganic glasses, which exhibit a permanent, photo-induced refractive index change due to the presence of optically active point defects in the glass structure. In the present work, the authors have performed a preliminary study of the intrinsic photosensitivity of a polyester containing a cinnamylindene malonate group (CPE, a photo- and thermal-crosslinkable group) for use in photonic waveguide devices. Thin films of CPE (approximately 0.5 microns thick) were spun onto fused silica substrates. Optical absorption in the thin films was evaluated both before and after exposure to UV radiation sources. It was found that the polyester exhibits two dominant UV absorption bands centered about 240 nm and 330 nm. Under exposure to 337 nm radiation (nitrogen laser) a marked bleaching of the 330 nm band was observed. This band bleaching is a direct result of the photo-induced crosslinking in the cinnamylindene malonate group. Exposure to 248 nm radiation (excimer laser), conversely, resulted in similar bleaching of the 330 nm band but was accompanied by nearly complete bleaching of the higher energy 240 nm band. Based on a Kramers-Kronig analysis of the absorption changes, refractive index changes on the order of {minus}10{sup {minus}2} are estimated. Confirmation of this calculation has been provided via ellipsometry which estimates a refractive index change at 632 nm of {minus}0.061 {+-} 0.002. Thus, the results of this investigation confirm the photosensitive potential of this type of material.
Date: February 29, 2000
Creator: POTTER,KELLY SIMMONS; POTTER JR.,BARRETT G.; WHEELER,DAVID R. & JAMISON,GREGORY M.
Partner: UNT Libraries Government Documents Department

Theory of point-defects, non-stoichiometry, and solute additions in SmCo{sub 5+x}-Sm{sub 2}Co{sub 17{minus}y} and related compounds

Description: There is considerable interest in the possibility of producing Sm-Co-based nanocomposite magnets by rapid solidification and other far-from-equilibrium processing methods. Thermodynamic and kinetic models are quite valuable in understanding and optimizing such methods. This paper describes a method of estimation, utilizing tight-binding-based bond-order interatomic interaction potentials, of the thermodynamic properties of point defects such as vacancies, interstitials, antisite defects, and solute additions in the SmCo{sub 5+x} and Sm{sub 2}Co {sub 17}-y phases and related rare-earth-transition metal compounds. Illustrative calculations for point defects in SmCo{sub 5} will be presented. The results suggest a unified model of the thermodynamic properties of the SmCo{sub 5+x} -- Sm{sub 2} Co{sub 17{minus}y} region of the phase diagram, based on the 1-5 structure and the replacement of Sm by interacting dumb-bell interstitials to form the 2-17 structure; the model is similar in nature to theories of the thermodynamics of metal hydrides.
Date: September 3, 1998
Creator: WELCH,D.O.
Partner: UNT Libraries Government Documents Department

A practical approach for modeling EUVL mask defects

Description: An approximate method is proposed to calculate the EUV scattering from a defect within a multilayer coating. In this single surface approximation (SSA) the defective multilayer structure is replaced by a single reflecting surface with the shape of the top surface of the multilayer. The range of validity of this approximation has been investigated for Gaussian line defects using 2D finite-difference-time-domain simulations. The SSA is found to be valid for sufficiently low aspect ratio defects such as those expected for the critical defects nucleated by particles on the mask substrate. The critical EUVL defect size is calculated by combining the SSA with a multilayer growth model and aerial image simulations. Another approximate method for calculating the aerial image of an unresolved defect is also discussed. Although the critical substrate defects may be larger than the resolution of higher NA cameras, the point defect approximation provides a useful framework for understanding the printability of a wide range of defects.
Date: June 1, 2001
Creator: Gullikson, E. M.; Cerjan, C.; Stearns, D. J.; Mirkarimi, P. B. & Sweeney, D. W.
Partner: UNT Libraries Government Documents Department

Understanding the role of defect production in radiation embrittlement of reactor pressure vessels.

Description: Comparative experiments using high energy (10 MeV) electrons and test reactor neutrons have been undertaken to understand the role that primary damage state has on hardening (embrittlement) induced by irradiation at 300 C. Electrons produce displacement damage primarily by low energy atomic recoils, while fast neutrons produce displacements from considerably higher energy recoils. Comparison of changes resulting from neutron irradiation, in which nascent point defect clusters can form in dense cascades, with electron irradiation, where cascade formation is minimized, can provide insight into the role that the in-cascade point defect clusters have on the mechanisms of embrittlement. Tensile property changes induced by 10 MeV electrons or test reactor neutron irradiations of unalloyed iron and an Fe-0.9 wt.% Cu-1.0 wt.% Mn alloy were examined in the damage range of 9.0 x 10{sup {minus}5} dpa to 1.5 x 10{sup {minus}2} dpa. The results to date showed the ternary alloy experienced substantially greater embrittlement in both the electron and neutron irradiated samples relative to unalloyed iron. Surprisingly, despite their disparate nature of defect production, similar embrittlement trends with increasing radiation damage were observed for electrons and neutrons in both the ternary and unalloyed iron.
Date: August 4, 1999
Creator: Alexander, D. E.
Partner: UNT Libraries Government Documents Department

Surface morphology evolution in silicon during ion beam processing

Description: The Semiconductor Industry Association (SIA) projects that the semiconductor chips used in personal computers and scientific workstations will reach five times the speed and ten times the memory capacity of the current pentium-class processor by the year 2007. However, 1 GHz on-chip clock speeds and 64 Gbits/Chip DRAM technology will not come easy and without a price. Such technologies will require scaling the minimum feature size of CMOS devices (the transistors in the silicon chip) down to below 100nm from the current 180 to 250 nm. This requirement has profound implications for device manufacturing. Existing processing techniques must increasingly be understood quantitatively and modeled with unprecedented precision. Indeed, revolutionary advances in the development of physics-based process simulation tools will be required to achieve the goals for cost efficient manufacturing, and to satisfy the needs of the defense industrial base. These advances will necessitate a fundamental improvement in our basic understanding of microstructure evolution during processing. In order to cut development time and costs, the semiconductor industry makes extensive use of simple models of dopant implantation, and of phenomenological models of defect annealing and diffusion. However, the production of a single device often requires more than 200 processing steps, and the cumulative effects of the various steps are far too complex to be treated with these models. The lack of accurate process modeling simulators is proving to be a serious impediment to the development of next generation devices. New atomic-level models are required to describe the point defect distributions produced by the implantation process, and the defect and dopant diffusion resulting from rapid thermal annealing steps. In this LDRD project, we investigated the migration kinetics of defects and dopants in silicon both experimentally and theoretically to provide a fundamental database for use in the development of predictive process simulators. The results ...
Date: August 1, 1999
Creator: P, Bedrossian; Caturla, M; Diaz de la Rubia, T & Johnson, M
Partner: UNT Libraries Government Documents Department

Microstructure of laterally overgrown GaN layers

Description: Transmission electron microscopy study of plan-view and cross-section samples of epitaxial laterally overgrown (ELOG) GaN samples is described. Two types of dislocation with the same type of Burgers vector but different line direction have been observed. It is shown that threading edge dislocations bend to form dislocation segments in the c-plane as a result of shear stresses developed in the wing material along the stripe direction. It is shown that migration of these dislocations involves both glide and climb. Propagation of threading parts over the wing area is an indication of high density of point defects present in the wing areas on the ELOG samples. This finding might shed new light on the optical properties of such samples.
Date: April 3, 2001
Creator: Liliental-Weber, Zuzanna & Cherns, David
Partner: UNT Libraries Government Documents Department

Defects in Ga, Cr, and In-doped CoO

Description: From simulation, trivalent cations, Ga(3+), Cr(3+), Co(3+) and In(3+), bind with Co vacancy to form singly pairs with binding energies of about 0.7 to 0.8 eV. These binding energies are in reasonable agreement with experimental measurement of about 0.5 eV. In ion prefers the second nearest neighbor position from a Co vacancy, while other cations prefer the third nearest neighbor sites. Two cations can also forma triplet with a Co vacancy with binding energies of about 1.2 to 1.5 eV. These valves are in fair agreement with the 0.8 to 1.1 eV measured from the tracer diffusion experiments.
Date: July 1, 1995
Creator: Chen, S.P.; Yan, M.; Grimes, R.W. & Vyas, S.
Partner: UNT Libraries Government Documents Department

Evolution of deep centers in GaN grown by hydride vapor phaseepitaxy

Description: Deep centers and dislocation densities in undoped n GaN, grown by hydride vapor phase epitaxy (HVPE), were characterized as a function of the layer thickness by deep level transient spectroscopy and transmission electron microscopy, respectively. As the layer thickness decreases, the variety and concentration of deep centers increase, in conjunction with the increase of dislocation density. Based on comparison with electron irradiation induced centers, some dominant centers in HVPE GaN are identified as possible point defects.
Date: April 18, 2001
Creator: Fang, Z.-Q.; Look, D.C.; Jasinski, J.; Benamara, M.; Liliental-Weber, Z. & Molnar, R.J.
Partner: UNT Libraries Government Documents Department

On the Existence of Our Metals-Based Civilization: I. Phase Space Analysis

Description: The stability of the barrier layers of bilayer passive films that form on metal and alloy surfaces, when in contact with oxidizing aqueous environments, is explored within the framework of the Point Defect Model (PDM) using phase-space analysis (PSA), in which the rate of growth of the barrier layer into the metal, (dL{sup +}/dt), and the barrier layer dissolution rate, (dL{sup -}/dt), are plotted simultaneously against the barrier layer thickness. A point of intersection of dL{sup -}/dt with dL{sup +}/dt indicates the existence of a metastable barrier layer with a steady state thickness greater than zero. If dL{sup -}/dt > (dL{sup +}/dt){sub L=0}, where the latter quantity is the barrier layer growth rate at zero barrier layer thickness, the barrier layer cannot exist, even as a metastable phase, as the resulting thickness would be negative. Under these conditions, the surface is depassivated and the metal may corrode at a rapid rate. Depassivation may result from a change in the oxidation state of the cation upon dissolution of the barrier layer, such that the dissolution rate becomes highly potential dependent (as in the case of transpassive dissolution of chromium-containing alloys, for example, in which the reaction Cr{sub 2}O{sub 3} + 5H{sub 2}O {yields} 2CrO{sub 4}{sup 2-} + 10H {sup +} + 6e{sup -} results in the destruction of the film), or by the action of some solution-phase species (e.g., H{sup +}, Cl{sup -}) that enhances the dissolution rate to the extent that dL{sup -}/dt > (dL{sup +}/dt){sub L=0}. The boundaries for depassivation may be plotted in potential-pH space to develop Kinetic Stability Diagrams (KSDs) as alternatives to the classical Pourbaix diagrams for describing the conditions under which metals or alloys exist in contact with an aqueous environment. The advantage of KSDs is that they provide kinetic descriptions of the state of ...
Date: June 22, 2005
Creator: Macdonald, D.D.
Partner: UNT Libraries Government Documents Department

Corrosion Behavior of Plasma-Passivated Cu

Description: A new approach is being pursued to study corrosion in Cu alloy systems by using combinatorial analysis combined with microscopic experimentation (the Combinatorial Microlab) to determine mechanisms for copper corrosion in air. Corrosion studies are inherently difficult because of complex interactions between materials and environment, forming a multidimensional phase space of corrosion variables. The Combinatorial Microlab was specifically developed to address the mechanism of Cu sulfidation, which is an important reliability issue for electronic components. This approach differs from convention by focusing on microscopic length scales, the relevant scale for corrosion. During accelerated aging, copper is exposed to a variety of corrosive environments containing sulfidizing species that cause corrosion. A matrix experiment was done to determine independent and synergistic effects of initial Cu oxide thickness and point defect density. The CuO{sub x} was controlled by oxidizing Cu in an electron cyclotron resonance (ECR) O{sub 2} plasma, and the point defect density was modified by Cu ion irradiation. The matrix was exposed to 600 ppb H{sub 2}S in 65% relative humidity air atmosphere. This combination revealed the importance of oxide quality in passivating Cu and prevention of the sulfidizing reaction. A native oxide and a defect-laden ECR oxide both react at 20 C to form a thick Cu{sub 2}S layer after exposure to H{sub 2}S, while different thicknesses of as-grown ECR oxide stop the formation of Cu{sub 2}S. The species present in the ECR oxide will be compared to that of an air oxide, and the sulfide layer growth rate will be presented.
Date: July 9, 1999
Creator: Barbour, J.C.; Braithwaite, J.W.; Son, K.A.; Sullivan, J.P.; Missert, N, & Sorensen, N.R.
Partner: UNT Libraries Government Documents Department

Donor and acceptor concentrations in degenerate InN

Description: A formalism is presented to determine donor (N{sub D}) and acceptor (N{sub A}) concentrations in wurtzitic InN characterized by degenerate carrier concentration (n) and mobility ({mu}). The theory includes scattering not only by charged point defects and impurities, but also by charged threading dislocations, of concentration N{sub dis}. For an 0.45-{micro}m-thick InN layer grown on Al{sub 2}O{sub 3} by molecular beam epitaxy, having N{sub dis} = 5 x 10{sup 10} cm{sup -2}, determined by transmission electron microscopy, n(20 K) = 3.5 x 10{sup 18} cm{sup -3}, and {mu}(20 K) = 1055 cm{sup 2}/V-s, determined by Hall-effect measurements, the fitted values are N{sub D} = 4.7 x 10{sup 18} cm{sup -3} and N{sub A} = 1.2 x 10{sup 18} cm{sup -3}. The identities of the donors and acceptors are not known, although a comparison of N{sub D} with analytical data, and also with calculations of defect formation energies, suggests that a potential candidate for the dominant donor is H.
Date: January 28, 2002
Creator: Look, D.C.; Lu, H.; Schaff, W.J.; Jasinski, J. & Liliental-Weber, Z.
Partner: UNT Libraries Government Documents Department

Ultrathin Alumina Film Al-Sublattice Structure, Metal Island Nucleation at Terrace Point Defects, and How Hydroxylation Affects Wetting

Description: In this paper, we include for discussion three topics of current interest in metal oxide surface science. Using first principles density functional theory (DFT) [1] calculations, we have investigated: (1) the atomic-scale structure of experimentally-relevant ultrathin alumina films, (2) the role of common point defects in metal island nucleation on oxide terraces, and (3) the growth and morphology of metals on oxide surfaces which have high concentrations of a common impurity.
Date: August 9, 1999
Creator: Bogicevic, A. & Jennison, D.R.
Partner: UNT Libraries Government Documents Department

Silicon Three-Dimensional Photonic Crystal and its Applications

Description: Photonic crystals are periodically engineered ''materials'' which are the photonic analogues of electronic crystals. Much like electronic crystal, photonic crystal materials can have a variety of crystal symmetries, such as simple-cubic, closed-packed, Wurtzite and diamond-like crystals. These structures were first proposed in late 1980's. However, due mainly to fabrication difficulties, working photonic crystals in the near-infrared and visible wavelengths are only just emerging. In this article, we review the construction of two- and three-dimensional photonic crystals of different symmetries at infrared and optical wavelengths using advanced semiconductor processing. We further demonstrate that this process lends itself to the creation of line defects (linear waveguides) and point defects (micro-cavities), which are the most basic building blocks for optical signal processing, filtering and routing.
Date: November 1, 2001
Creator: LIN, SHAWN-YU; FLEMING, JAMES G. & LYO, SUNGKWUN K.
Partner: UNT Libraries Government Documents Department

Hardening in AlN induced by point defects

Description: Pressureless-sintered AIN was neutron irradiated and the hardness change was examined by Vickers indentation. The hardness was increased by irradiation. When the samples were annealed at high temperature, the hardness gradually decreased. Length was also found to increase and to change in the same way as the hardness. A considerable density of dislocation loops still remained, even after the hardness completely recovered to the value of the unirradiated sample. Thus, it is concluded that the hardening in AIN is caused by isolated point defects and small clusters of point defects, rather than by dislocation loops. Hardness was found to increase in proportion to the length change. If the length change is assumed to be proportional to the point defect density, then the curve could be fitted qualitatively to that predicted by models of solution hardening in metals. Furthermore, the curves for three samples irradiated at different temperatures and fluences are identical. There should be different kinds of defect clusters in samples irradiated at different conditions, e.g., the fraction of single point defects is the highest in the sample irradiated at the lowest temperature. Thus, hardening is insensitive to the kind of defects remaining in the sample and is influenced only by those which contribute to length change.
Date: December 31, 1991
Creator: Suematsu, H.; Mitchell, T. E.; Iseki, T. & Yano, T.
Partner: UNT Libraries Government Documents Department

Structure, stability, and mechanical properties of intermetallic phases

Description: The importance of the structural stability of intermetallics with regard to their mechanical properties is illustrated with two case studies. First, the importance of structural and thermal defects for the strength of (weakly ordered) FeAl and (strongly ordered) NiAl is shown. Several inconsistencies and unresolved issues in the present understanding of point defects in FeAl are addressed. Since point defects alone may not explain the mechanical differences between these two materials, the role of dislocations is considered as well. It is shown that the differences in the atomic bonding of FeAl and NiAl, which deter-mine the active slip systems, are likely to influence the compositional dependence of the strength of these two intermetallics. Second, the class of the trialuminides is reviewed with emphasis on Al{sub 3}Ti. In addition to stabilizing a cubic crystal structure, the ratio of K/G, where K is the bulk modulus and G the shear modulus, needs to be increased in order to achieve extensive plastic deformation at room temperature. It is not clear, at the present time, to what extent macroalloying of trialuminides can achieve this goal, although promising results have been reported for Al{sub 3}Ti containing relatively high concentrations (14 at. %) of chromium.
Date: December 31, 1993
Creator: Schneibel, J. H. & Liu, C. T.
Partner: UNT Libraries Government Documents Department

Bulk and defect properties of ordered intermetallics

Description: First-principles calculations based on the local-density-functional theory have been used to investigate the factors governing strength of ordered intermetallic alloys. Unlike Ni{sub 3}Al, calculated elastic constants and shear fault energies indicate anomalous yield strength behavior is not likely to occur in Ni{sub 3}Si. This suggests the inadequacy of cross-slip-pinning model to explain the strength anomaly in the L1{sub 2} structure. For strongly ordered NiAl, the defect structure is dominated by two types of defects -- monovacancies on the Ni sites and substitutional antisite defects on the Al sites. By contrast, for Ni{sub 3}Al, absence of structural vacancies and deviations from stoichiometry are accommodated by substitutional antisite defects on both sublattices. Intrinsic strengthening mechanisms in TiAl are discussed in terms of calculated elastic constants and shear fault energies. Because of reduced fault energies at the {gamma}/{alpha}{sub 2} interface, slip and twinning contribute significantly to high shear deformation of the lamellar structure.
Date: December 31, 1993
Creator: Fu, C. L. & Yoo, M. H.
Partner: UNT Libraries Government Documents Department

Fundamental studies on passivity and passivity breakdown

Description: Using photoelectrochemical impedance and admittance spectroscopies, a fundamental and quantitative understanding of the mechanisms for the growth and breakdown of passive films on metal and alloy surfaces in contact with aqueous environments is being developed. A point defect model has been extended to explain the breakdown of passive films, leading to pitting and crack growth and thus development of damage due to localized corrosion.
Date: June 1, 1993
Creator: Macdonald, D. D. & Urquidi-Macdonald, M.
Partner: UNT Libraries Government Documents Department

ATOMISTIC SIMULATION OF VACANCY AND SELF-INTERSTITIAL DIFFUSION IN Fe-Cu ALLOYS

Description: Neutron hardening and embrittlement of pressure vessel steels is due to a high density of nanometer scale features, including Cu-rich precipitates which form as a result of radiation enhanced diffusion. High-energy displacement cascades generate large numbers of both isolated point defects and clusters of vacancies and interstitials. The subsequent clustering, diffusion and ultimate annihilation of primary damage is inherently coupled with solute transport and hence, the overall chemical and microstructural evolutions under irradiation. In this work, we present atomistic simulation results, based on many-body interatomic potentials, of the migration of vacancies, solute and self-interstitial atoms (SIA) in pure Fe and binary Fe-0.9 and 1.0 at.% Cu alloys. Cu diffusion occurs by a vacancy mechanism and the calculated Cu diffusivity is in good agreement with experimental data. Strain field interactions between the oversized substitutional Cu solute atoms and SIA and SIA clusters are predominantly repulsive and result in both a decreased activation energy and diffusion pre-factor for SIA and small (N < 5) SIA cluster migration, which occurs by three-dimensional motion. The Cu appears to enhance the reorientation of the SIA clusters to different <111> directions, as well as the transition from <110> to mobile <111> configurations. The migration behavior of larger SIA clusters, which undergo only one-dimensional diffusion during molecular dynamics timescales, is largely unaffected by the Fe-Cu alloy, although SIA clusters are effectively repelled by coherent Cu precipitates.
Date: January 25, 2001
Creator: Marian, J; Wirth, B D; Perlado, J M; Odette, G R & Diaz de la Rubia, T
Partner: UNT Libraries Government Documents Department

EFFECT OF SURFACE PREPARATION TECHNIQUE ON THE RADIATION DETECTOR PERFORMANCEOF CDZNTE

Description: Synthetic CdZnTe (CZT) semiconducting crystals are highly suitable for the room temperature-based detection of gamma radiation. The surface preparation of Au contacts on surfaces of CZT detectors is typically conducted after (1) polishing to remove artifacts from crystal sectioning and (2) chemical etching, which removes residual mechanical surface damage however etching results in a Te rich surface layer that is prone to oxidize. Our studies show that CZT surfaces that are only polished (as opposed to polished and etched) can be contacted with Au and will yield lower surface currents. Due to their decreased dark currents, these as-polished surfaces can be used in the fabrication of gamma detectors exhibiting a higher performance than polished and etched surfaces with relatively less peak tailing and greater energy resolution. CdZnTe or ''CZT'' crystals are attractive to use in homeland security applications because they detect radiation at room temperature and do not require low temperature cooling as with silicon- and germanium-based detectors. Relative to germanium and silicon detectors, CZT is composed of higher Z elements and has a higher density, which gives it greater ''stopping power'' for gamma rays making a more efficient detector. Single crystal CZT materials with high bulk resistivity ({rho}>10{sup 10} {Omega} x cm) and good mobility-lifetime products are also required for gamma-ray spectrometric applications. However, several factors affect the detector performance of CZT are inherent to the as grown crystal material such as the presence of secondary phases, point defects and the presence of impurities (as described in a literature review by R. James and researchers). These and other factors can limit radiation detector performance such as low resistivity, which causes a large electronic noise and the presence of traps and other heterogeneities, which result in peak tailing and poor energy resolution.
Date: May 23, 2007
Creator: Duff, M
Partner: UNT Libraries Government Documents Department