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Modeling of the phase evolution in Mg1-xAlxB2 (0<x<0.5) and its experimental signatures

Description: Despite the chemical and structural simplicity of MgB{sub 2}, at 39 K this compound has the highest known {Tc} of any binary compound. Electron doping by substituting Al for Mg leads to decreasing Tc and the observed concentration dependent rate of decrease has been proposed to arise from the non-ideal character of MgB{sub 2}-AIB{sub 2} solid solutions, which derives from the existence of an ordered Mg{sub 0.5}Al{sub 0.5}B{sub 2} compound. Heterogeneous nano-scale structure patterns in solid solutions have emerged as an important concept for complex materials, ranging from actinide alloys and oxides to high-temperature cuprate superconductors and mallganite-based materials exhibiting colossal magnetoresistivity. In this work we investigate the formation of structural heterogeneities in Mg{sub 1-x}AI{sub x}B{sub 2}, which take the form of nano-scale AI-AI and AI-Mg domains of different geometry and size, using molecular statics/dynamics simulations and in particular we study the corresponding signatures in diffraction experiments. In order to undertake this task we first derive appropriate Mg-AI-B semi-empirical potentials within the Modified Embedded Atom Method formalism. These potentials are also applied to explore the equilibrium Mg{sub 1-x}AI{sub x}B{sub 2} phase diagram for 0 &lt; x &lt; 0.5. Additionally, density functional theory calculations were utilized to study the influence of heterogeneities on the electronic structure and charge distribution in Mg{sub 1-x}AI{sub x}B{sub 2}.
Date: January 1, 2009
Creator: Andersson, Anders David; Casillas, Luis; Lezama Pacheco, Juan & Conradson, Steven D
Partner: UNT Libraries Government Documents Department

Characterization of Tank 48H Samples for Alpha Activity and Actinide Isotopics

Description: This document reports the total alpha activity and actinide isotopic results for samples taken from Tank 48H prior to the addition of sodium tetraphenylborate and MST in Batch {number_sign}1 of the ITP process. This information used to determine the quantity of MST for Batch {number_sign}1 of the ITP process and the total actinide content in the tank for dose calculations.
Date: December 4, 1995
Creator: Hobbs, D.T.; Coleman, C.J. & Hay, M.S.
Partner: UNT Libraries Government Documents Department

Structural and magnetic characterization of actinide materials

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The authors have successfully used neutron scattering techniques to investigate physicochemical properties of elements, compounds, and alloys of the light actinides. The focus of this work is to extend the fundamental research capability and to address questions of practical importance to stockpile integrity and long-term storage of nuclear material. Specific subject areas are developing neutron diffraction techniques for smaller actinide samples; modeling of inelastic scattering data for actinide metal hydrides; characterizing actinide oxide structures; and investigating aging effects in actinides. These studies utilize neutron scattering supported by equilibrium studies, kinetics, and x-ray diffraction. Major accomplishments include (1) development of encapsulation techniques for small actinide samples and neutron diffraction studies of AmD{sub 2.4} and PuO{sub 2.3}; (2) refinement of lattice dynamics model to elucidate hydrogen-hydrogen and hydrogen-metal interactions in rare-earth and actinide hydrides; (3) kinetic studies with PuO{sub 2} indicating that the recombination reaction is faster than radiolytic decomposition of adsorbed water but a chemical reaction produces H{sub 2}; (4) PVT studies of the reaction between PuO{sub 2} and water demonstrate that PuO{sub 2+x} and H{sub 2} form and that PuO{sub 2} is not the thermodynamically stable form of the oxide in air; and (5) model calculations of helium in growth in aged plutonium predicting bubble formation only at grain boundaries at room temperature. The work performed in this project has application to fundamental properties of actinides, aging, and long-term storage of plutonium.
Date: December 31, 1998
Creator: Cort, B.; Allen, T.H. & Lawson, A.C.
Partner: UNT Libraries Government Documents Department

Band description of materials with localizing orbitals

Description: Density functional theory is a form of many-body theory which maps the problem onto an equivalent single particle-like system by limiting to the ground state (or some limited ensemble). So it should be surprising that this ground state theory could have any relevance whatsoever to the excitation properties of a material - and yet it does when used carefully. However, the most interesting materials involve active orbitals which are at least partially localized in space and this has profound effects both on the ground state and the excitation spectrum. My long term interest is in Ce and actinide compounds such that the popular concerns are mixed valence, heavy fermions, and the various forms of magnetic transitions. Band structure calculations can give a great deal of information concerning the mechanisms and degree of the localization as shown by examples using the Ce and U Ll/sub 2/ structured materials and the Ce cubic Laves phase materials. There are some difficulties due to an incomplete knowledge of the functionals involved which causes an underestimate of the local character. This is illustrated and discussed.
Date: March 1, 1986
Creator: Koelling, D.D.
Partner: UNT Libraries Government Documents Department

Thermal simulation of quenching uranium-0. 75% titanium alloy in water

Description: A computer model, The Quench Simulator, has been developed to simulate and predict in detail the behavior of U-0.75 Ti alloy when quenched at high temperature (about 850/sup 0/C) in cold water. The code allows one to determine the time- and space-dependent distributions of temperature, residual stress, distortion, and microstructure that evolve during the quenching process. The nonlinear temperature- and microstructure-dependent properties, as well as the cooling rate-dependent heats of transformation, are incorporated into the model. The complex boiling heat transfer with its various regimes and other thermal boundary conditions are simulated. Experiments have been performed and incorporated into the model. Both sudden submersion and gradual controlled immersion can be applied. A parametric and sensitivity study has been performed demonstrating the importance of the thermal boundary conditions applied for achieving certain product characteristics. The thermal aspects of the model and its applications are discussed and demonstrated.
Date: January 1, 1985
Creator: Siman-Tov, M.; Llewellyn, G.H.; Childs, K.W.; Ludtka, G.M. & Aramayo, G.A.
Partner: UNT Libraries Government Documents Department

Potentiometric determination of percent porosity in nickel electroplates on uranium metal

Description: A potentiometric procedure has been developed to determine the percent porosity of nickel electroplates on uranium metal. In this case, percent porosity is defined as the ratio of the exposed uranium surface to the nickel electroplate surface times 100. Basically, this procedure exploits the high chemical reactivity between uranium metal and chloride ions in aqueous solution. It involves the potentiometric measurement of the corrosion potentials of coupled uranium/nickel metal electrodes in 0.01 molar potassium chloride solution versus a double-junctioned calomel electrode. A mathematical treatment of observed potentiometric measurements from a two-point calibration technique permits a best-fit calibration for a particular porosity determination on the same nickel-plated sample.
Date: January 1, 1982
Creator: Dietrich, W.C. & Johnson, E.E.
Partner: UNT Libraries Government Documents Department

Evaluation of two styles of slotted, flat-head screws. [U--6% Nb alloys]

Description: A series of torque tests were performed to evaluate the relative merits of two different flat-head screws fabricated from a uranium--6% niobium alloy. The screws tested were machined with both normal, straight-through slots in the head and with slots having radiused bottoms. Test results indicate that both designs easily surpass the required 20-inch-pound-proof torque.
Date: April 30, 1979
Creator: Reeves, C.A. Jr. & Johnson, W.B.
Partner: UNT Libraries Government Documents Department

X-ray diffraction study of reversible deformation mechanisms in the aged uranium-6. 5 niobium alloy

Description: The x-ray diffraction (XRD) data from 200/sup 0/C/2h-aged uranium-6.5 wt % niobium (U-6.5Nb) alloys, taken under stress as a function of strain, revealed a gamma-zero (..gamma../sup 0/)..-->.. alpha prime-prime (..cap alpha..'') thermoelastic martensitic phase transformation. It was concluded that the primary reversible deformation modes consisted of the movement of ..gamma../sup 0//..cap alpha..'' interphase interfaces and ..cap alpha..'' intervariant interfaces. Specimen elasticity at low strains was associated with the retreat of interphase interfaces. At higher strains, interphase interfaces did not recover significantly on unloading, and elasticity was due primarily to the retreat of ..cap alpha..'' intervariant interfaces.
Date: June 21, 1985
Creator: Carpenter, D.A.
Partner: UNT Libraries Government Documents Department

Anomalous vomume expansion of plutonium alloys

Description: A simple heuristic model for the valence of Pu in its various phases is used to explain the large positive deviations from Vegard's Law in ..cap alpha..-Pu(Al), ..cap alpha..-Pu(Ti), and delta-Pu(Sc).
Date: January 1, 1981
Creator: Fisk, Z.; Elliott, R.O.; Tate, R.E. & Roof, R.B.
Partner: UNT Libraries Government Documents Department

Magnetic properties and electronic structure of the actinide systems UAl/sub 2/, NpAl/sub 2/, and PuAl/sub 2/

Description: The electronic ground states of UAl/sub 2/, NpAl/sub 2/ and PuAl/sub 2/ have been determined by band theory techniques. These calculations indicate that both actinide d-f hybridization and spin-orbit coupling are important in these systems. Using a new scheme for obtaining the paramagnon parameters for UAl/sub 2/ it is shown that calculated and experimentally derived 5f band width are consistent. The fact that orbital angular momentum may not be quenched in these systems is suggested as a possible mechanism for damping spin-fluctuations in NpAl/sub 2/ and PuAl/sub 2/.
Date: January 1, 1985
Creator: Albers, R.C.; Boring, A.M. & Koelling, D.D.
Partner: UNT Libraries Government Documents Department

Literature study of metallic ternary and quanternary hydrides

Description: Metallic hydrides are being considered as an attractive solution to storage problems involved in using hydrogen as an alternative fuel. In this application, the unclassified literature was surveyed to find metallic ternary hydride systems of the type MM'H/sub x/; M is a transition element, a member of the rare earth element series, or a member of the actinide element series, and M' is a transition element. Reported quaternary hydrides of the same metallic element series were also reviewed. The hydride-forming systems found and some of their most interesting properties are summarized. Recent advances in the understanding of the absorption characteristics and the stability of these hydride materials are reviewed. (auth)
Date: September 19, 1975
Creator: Newkirk, H.W.
Partner: UNT Libraries Government Documents Department

THERMODYNAMIC AND KINETIC MODELING OF ADVANCED NUCLEAR FUELS - FINAL LDRD-ER REPORT

Description: This project enhanced our theoretical capabilities geared towards establishing the basic science of a high-throughput protocol for the development of advanced nuclear fuel that should couple modern computational materials modeling and simulation tools, fabrication and characterization capabilities, and targeted high throughput performance testing experiments. The successful conclusion of this ER project allowed us to upgrade state-of-the-art modeling codes, and apply these modeling tools to ab initio energetics and thermodynamic assessments of phase diagrams of various mixtures of actinide alloys, propose a tool for optimizing composition of complex alloys for specific properties, predict diffusion behavior in diffusion couples made of actinide and transition metals, include one new equation in the LLNL phase-field AMPE code, and predict microstructure evolution during alloy coring. In FY11, despite limited funding, the team also initiated an experimental activity, with collaboration from Texas A&amp;M University by preparing samples of nuclear fuels in bulk forms and for diffusion couple studies and metallic matrices, and performing preliminary characterization.
Date: November 28, 2011
Creator: Turchi, P
Partner: UNT Libraries Government Documents Department

THERMODYNAMICS AND KINETICS OF PHASE TRANSFORMATIONS IN PLUTONIUM ALLOYS - PART I

Description: In this report we investigate order, stability, and phase transformations for a series of actinide-based alloys. The statics and kinetics of precipitation and ordering in this class of alloys are modeled with a scheme that couples fundamental information on the alloy energetics obtained from experimental and assessed thermo-chemical data to the CALPHAD approach commonly used in industry for designing alloys with engineering specificity with the help of the Thermo-Calc software application. The CALPHAD approach is applied to the study of the equilibrium thermodynamic properties of Pu-based alloys, Pu-X, where X=Al, Fe, Ga. The assessment of the equilibrium phase diagrams in the whole range of alloy composition has been performed with the PARROT module of the Thermo-Calc application software. Predictions are made on the low temperature and Pu-rich side of the phase diagrams of Pu-Ga and Pu-Al for which controversy has been noted in the past. The validity of the assessed thermo-chemical database will be discussed by comparing predicted heats of transformation for pure Pu with measured values from differential scanning calorimetry analysis. An overall picture for the stability properties of Pu-Ga and Pu-Al that reconciles the results of past studies carried out on these alloys is proposed. Results on phase stability in the ternary Fe-Ga-Pu and Al-Fe-Pu alloys are discussed. The information collected in this study is then used to model metastability, long-term stability and aging for this class of alloys by coupling Thermo-Calc with DICTRA, a series of modules that allow the analysis of DIffusion Controlled TRAnsformations. Kinetics information is then summarized in so-called TTT (temperature-time-transformations) diagrams for the most relevant phases of actinide alloys. Specifically, results are presented on kinetics of phase transformations associated with the eutectoid-phase decomposition reaction occurring at low temperature, and with the martensitic transformation that takes place at low Ga content in Pu-Ga alloys. ...
Date: August 18, 2004
Creator: Turchi, P A; Kaufman, L; Liu, Z & Zhou, S
Partner: UNT Libraries Government Documents Department

Homogenization of arc-melted uranium-6 weight percent niobium alloy ingots

Description: Microsegregation and macrosegregation in uranium-6 niobium arc-melted ingots were characterized by electron microprobe analysis. Homogenization studies determined that microsegregation can be eliminated by heat treating the alloy for six hours at 1100/sup 0/C. Extensive homogenization did not eliminate macrosegregation (banding) from the as-cast alloy, but a mathematical model describing banding and coring indicated that banding could possibly be eliminated by working prior to homogenization.
Date: January 31, 1978
Creator: Snyder, W. B., Jr.
Partner: UNT Libraries Government Documents Department

Gallium in the alpha and delta plutonium lattice

Description: Some general ideas are offered to further an understanding of the nature of the delta-phase plutonium--gallium alloys and their stability. Attention is given to a hypothesis relating to the role of gallium atom positions in the plutonium lattice. (GHT)
Date: January 1, 1976
Creator: Fischer, R. B.
Partner: UNT Libraries Government Documents Department

Thermal cnductivity and Lorenz number of plutonium and plutonium-gallium alloys

Description: The thermal diffusivities of Pu, Pu-2.7 at. % Ga, Pu-3.5 at. % Ga, and Pu-6.6 at. % Ga were measured from 25/sup 0/C to around 500/sup 0/C using a laser flash technique and electronic data acquisition. Although the Lorenz number, L, of pure Pu is well below the Sommerfeld value, L/sub 0/, except for the ..cap alpha..-phase, L exceeds L/sub 0/ for the alloys at all temperatures and has a pronounced minimum around 200/sup 0/C. At the lower temperatures we attribute the excess to lattice conduction and at temperature above 200/sup 0/C the excess is attributed to an electronic component. The negative deviation of L from L/sub 0/ for pure Pu is ascribed to a reduction of the electron mobilities in those energy ranges where the 5f bands overlap the conduction bands.
Date: January 1, 1981
Creator: Andrew, J.F. & Klemens, P.G.
Partner: UNT Libraries Government Documents Department

Stress corrosion cracking of uranium--niobium alloys

Description: The stress corrosion cracking behavior of U-2/sup 1///sub 4/, 4/sup 1///sub 2/, 6 and 8 wt % Nb alloys was evaluated in laboratory air and in aqueous Cl/sup -/ solutions. Thresholds for crack propagation were obtained in these environments. The data showed that Cl/sup -/ solutions are more deleterious than air environments. Tests were also conducted in pure gases to identify the species in the air responsible for cracking. These data showed the primary stress corrodent is water vapor for the most reactive alloy, U-2/sup 1///sub 4/% Nb, while O/sub 2/ is primarily responsible for cracking in the more corrosion resistant alloys, U-6 and 8% Nb. The 4/sup 1///sub 2/% alloy was found to be susceptible in both H/sub 2/O and O/sub 2/ environments.
Date: March 1, 1978
Creator: Magnani, N.J.
Partner: UNT Libraries Government Documents Department

Magnetic anisotropy in the U/sub x/Th/sub 1-x/Zn/sub 8. 5/ system

Description: We have investigated the source of the anisotropic susceptibility of UZn/sub 8.5/ by preparing single crystals of U/sub x/Th/sub 1-x/Zn/sub 8.5/ for small x. Preliminary indications are that anisotropic exchange, rather than crystal field effects, are responsible for the anisotropy, but possible impurity contributions to the (anisotropic) susceptibility of the ThZn/sub 8.5/ do not permit a stronger statement. Further measurements for small x show that both the electronic specific heat and the susceptibility at low temperature are strongly enhanced, and that Kondo-like resistivity minima are observed. The enhancements are somewhat reduced in the concentrated system UZn/sub 8.5/.
Date: January 1, 1986
Creator: Willis, J.O.; Fisk, Z.; Aikin, R.M.; McElfresh, M.W.; Thompson, J.D.; Zirngiebl, E. et al.
Partner: UNT Libraries Government Documents Department

Spectrochemical analysis of plutonium using direct current plasma emission spectrometry

Description: One year ago, LLNL was just completing the installation of a Direct Current Plasma (DCP) spectrometer for the analysis of Pu and Pu alloys. The installation was completed in December 1982 and has been utilized regularly for Pu analysis since then. This paper discusses the experience with the instrument and some data demonstrating its performance.
Date: December 3, 1983
Creator: Morris, W.F.; Fadeff, S.K. & Torres, S.
Partner: UNT Libraries Government Documents Department

Formation of Pu amorphous alloys or metastable structures in Pu-Fe, Pu-Ta, and Pu-Si alloys

Description: Sputter deposition technique was used to study the possible formation of amorphous structures in Pu-Fe, Pu-Ta, and Pu-Si systems. A triode sputtering system was used to prepare sputtered coatings: 13 to 59 at. % (a/o) Fe, 10 to 50 a/o Si, and 15 to 65 a/o Ta. Structure of the coatings was determined by x-ray diffraction techniques. The temperature stability of the obtained structures was determined by Differential Scanning Calorimetry (DSC) measurements. The Pu-Fe and Pu-Si binary systems showed strong evidence for the formation of amorphous phases in the sputtered coatings. X-ray analyses indicated the presence of Pu6Fe in the 13 to 20 a/o Fe range of Pu-Fe alloys and no apparent crystalline phases over the entire 10 to 50 a/o Si range of Pu-Si alloys. In the Pu-Ta system, the DSC data obtained for compositions below 50 a/o Ta did not show typical crystallization exotherms. At compositions above 50 a/o Ta, a metastable bcc alpha Ta structure was observed with an expanded lattice parameter. The calculated volume expansion (2.9%) corresponds to 29 a/o of Pu in solid solution if the lattice parameter is assumed to follow Vegards Law. After storage in a nitrogen glovebox atmosphere for over two years, the Pu-Si and Pu-Ta coatings have maintained a metallic luster and have shown no visible evidence of surface oxidation.
Date: August 20, 1985
Creator: Rizzo, H.F. & Echeverria, A.W.
Partner: UNT Libraries Government Documents Department

Muon spin relaxation and Knight shift in the heavy-fermion superconductor UPt/sub 3/

Description: Positive muon spin relaxation experiments have been conducted on the heavy-fermion superconductor UPt/sub 3/ in both the normal and superconducting states for zero, transverse, and longitudinally applied magnetic fields. Below 6 K in zero applied field, the ..mu../sup +/ relaxation rate is approximately twice that expected from /sup 195/Pt nuclear dipolar relaxation alone. Transverse- and longitudinal-field measurements show that the observed relaxation rate depends on magnetic field and is quasistatic in origin. It is suggested that the onset of very weak (approx.10/sup -3/ ..mu../sub B//U atom) magnetic ordering below approximately 6 K is responsible for the observed increase in the relaxation rate. ..mu../sup +/ Knight shift measurements in the normal state of UPt/sub 3/ show a temperature dependent shift K/sub ..mu../ which tracks the bulk susceptibility chi. From the K/sub ..mu../ vs chi plot, a ..mu../sup +/ hyperfine field of approximately 100 Oe/..mu../sub B/ is extracted.
Date: January 1, 1986
Creator: Cooke, D.W.; Heffner, R.H.; Hutson, R.L.; Schillaci, M.E.; Smith, J.L.; Willis, J.O. et al.
Partner: UNT Libraries Government Documents Department

Magnetization density in paramagnetic UGe/sub 3/

Description: The magnetization density was measured in the actinide intermetallic compound UGe/sub 3/ (cubic AuCu/sub 3/ structure). The induced moment at 4.2/sup 0/K with H (applied) = 48 k0e is 0.012 ..mu../sub B//mole. The most significant result is that a small positive moment of approx. 0.0018 ..mu../sub B/ is induced at and near the Ge site, which is unusual because elemental Ge is diamagnetic. The spin density at the Ge site is highly aspherical, showing the nature of the bonding with the uranium wave functions.
Date: January 1, 1979
Creator: Lander, G.H.; Reddy, J.F.; Delapalme, A. & Brown, P.J.
Partner: UNT Libraries Government Documents Department

Shape memory effect in uranium-niobium alloys below room temperature. Final report. [6. 2 to 7. 0 wt % Nb]

Description: The temperature dependence of the martensitic ..gamma../sup 0/ to ..cap alpha../sup 11/ phase transformation between 25/sup 0/C and -198/sup 0/C in water-quenched uranium-niobium alloys between 6.2 and 7.0 wt % niobium was determined using cold-stage x-ray diffraction experiments. Lowering the temperature promoted the transformation to ..cap alpha../sup 11/ but complete transformation was never observed. The transformation was reversible with little or no hysteresis. Aging the alloys for two hours at 200/sup 0/C stabilized the ..gamma../sup 0/ phase and retarded the martensitic transformation. The compositions where M/sub s/ and M/sub f/ equaled room temperature were 5.80 and 7.04 wt % Nb respectively. At -196/sup 0/C these values were approximately 0.25 wt % higher. Two alloys were selected as potential candidates for a uranium alloy clamp.
Date: March 1, 1985
Creator: Vandermeer, R.A.
Partner: UNT Libraries Government Documents Department