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Impact of the formation of NpO{sub 2} and Np{sub 2}O{sub 5} on the Np solubility in YM waters

Description: The objective of this activity is to evaluate the impact of different Np solids on the solubility of neptunium in YM waters. Special interest is on the impact of Np{sub 2}O{sub 5} found in solubility experiments as the Np solid phase. The formation and stability of Np{sub 2}O{sub 5} is discussed and its stability calculated in J-13 based upon new solubility data. Clearly, the results show that Np{sub 2}O{sub 5} is the most stable Np(V) solid phase that determined the solubility of neptunium under aerobic conditions. Both solubility experiments and geochemical modeling show a decrease of the neptunium solubility in J-13 of about two orders of magnitude (compared to previously reported results) based on the new solubility data. Under anaerobic conditions NpO{sub 2} is the most stable Np bearing solid phase. Its formation would decrease the Np solubility by several orders of magnitude.
Date: January 1, 1998
Creator: Runde, W.; Janecky, D.; Kaszuba, J. & Tait, C.D.
Partner: UNT Libraries Government Documents Department

Pressure-induced crossover from long-to-short-range order in [Pb(Zn{sub 1/3})Nb{sub 2/3}O{sub 3}]{sub 0.905}(PbTiO{sub 3}){sub 0.095} single crystal

Description: A pressure-induced crossover from normal Ferroelectric-to-Relaxer behavior has been observed in single crystal [Pb(Zn{sub 1/3}Nb{sub 2/3})O{sub 3}]{sub 0.905}(PbTiO{sub 3}){sub 0.0095}, or PZN - 9.5% PT. Analogy with similar observations for other perovskites indicates that this crossover is a general feature of compositionally-disordered soft mode ferroelectrics. The Pressure-Temperature phase diagram has been also determined.
Date: January 25, 2000
Partner: UNT Libraries Government Documents Department

Ancient Blacksmiths, The Iron Age, Damascus Steels, and Modern Metallurgy

Description: The history of iron and Damascus steels is described through the eyes of ancient blacksmiths. For example, evidence is presented that questions why the Iron Age could not have begun at about the same time as the early Bronze Age (i.e. approximately 7000 B.C.). It is also clear that ancient blacksmiths had enough information from their forging work, together with their observation of color changes during heating and their estimate of hardness by scratch tests, to have determined some key parts of the present-day iron-carbon phase diagram. The blacksmiths' greatest artistic accomplishments were the Damascus and Japanese steel swords. The Damascus sword was famous not only for its exceptional cutting edge and toughness, but also for its beautiful surface markings. Damascus steels are ultrahigh carbon steels (UHCSs) that contain from 1.0 to 2.1%. carbon. The modern metallurgical understanding of UHCSs has revealed that remarkable properties can be obtained in these hypereutectoid steels. The results achieved in UHCSs are attributed to the ability to place the carbon, in excess of the eutectoid composition, to do useful work that enhances the high temperature processing of carbon steels and that improves the low and intermediate temperature mechanical properties.
Date: September 11, 2000
Creator: Sherby, O.D. & Wadsworth, J.
Partner: UNT Libraries Government Documents Department

Magnetic phase diagram of layered manganites in the highly doped regime.

Description: The naturally layered colossal magnetoresistive (CMR) manganites La{sub 2{minus}2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} exhibit an extremely varied range of magnetic and electronic behavior over a very narrow composition range between x = 0.3 and x = 0.5. The successful synthesis in our laboratories of compounds with nominally greater than 50 percent Mn{sup 4+} concentration has now allowed the study of this heretofore unexplored region of the phase diagram. Here we present detailed neutron diffraction measurements of these compounds with doping levels 0.5 < x <1.0. As predicted by simple theories, the type-A layered antiferromagnetic (AF) structure is found at x{approximately}0.5 and the type-G ''rocksalt'' AF structure at x = 1.0. Between these two extremes is found a C-type structure (ferromagnetic rods parallel to b coupled antiferromagnetically to all neighboring rods) stabilized by orbital ordering of y{sup 2} states. Also in this Mn{sup 4+}-rich regime is found a region in which no long-range magnetic order is observed. We discuss how semi-empirical models can explain the variety of magnetic structures and how structural trends as a function of doping corroborate the unifying notion of a shift from in-plane to axial orbital occupation as the Mn{sup 4+} concentration is decreased.
Date: November 2, 2000
Creator: Mitchell, J. F.; Ling, C. D.; Millburn, J. E.; Argyriou, D. N.; Berger, A. & Medarde, M.
Partner: UNT Libraries Government Documents Department


Description: The energy and entropy curves for unalloyed Pu mark the extent of our progress toward an understanding of the phase diagrams presented at the beginning of the paper; it is clear that there is still a long way to go. It would be desirable to evaluate the vibrational thermodynamic functions from measured phonon spectra. However, this may require the growth of single crystals, if the PDF method using powder diffraction proves impractical for Pu. The measurement of inelastic phonon densities of states from polycrystalline samples might be a practical method for all phases of Pu. Estimates of the electronic contributions to the thermodynamics have been derived from the work of Wallace (1998). It would be of interest to compare these with theoretical estimates to evaluate the role of electron correlation in the phase stability of plutonium metal. As emphasized by Wallace, this will be an ''enormous challenge'' to electronic structure theory. In particular, we need a more predictive understanding of how narrow bands in Pu stabilize idiosyncratic crystal structures (Soederlind et al., 1995 and Soederlind, 1998) Materials Properties.
Date: May 1, 2001
Creator: LAWSON, A.; ROBERTS, J. & AL, ET
Partner: UNT Libraries Government Documents Department

The Dirac Experiments - Results and Challenges

Description: The 1997 international Dirac II Series held at Los Alamos National Laboratory involved low temperature electrical transport and optical experiments in magnetic fields exceeding 800%, produced by explosive flux compression using Russian MC-1 generators. An overview of the scientific and technical advances achieved in this Series is given, together with a strategy for future work in this challenging experimental environment. A significant outcome was achieved in transport studies of microfabricated thin-film YBCO structures with the magnetic field in the CuO plane. Using a GHz transmission line technique at an ambient temperature of 1.6 K, an onset of dissipation was observed at 150 T (a new upper bound for superconductivity in any material), with a saturation of resistivity at 240 T. Comparison with the Pauli limit expected at B=155 T in this material suggests that the critical field in this geometry is limited by spin paramagnetism. In preparation for a Diract III series, a systematic temperature-dependent transport study of YBCO using in-plane magnetic fields of 150 T generated by single-turn coils, at temperatures over the range 10-100 K, has been undertaken in collaboration with the Japanese Megagauss Laboratory. The objective is to map out the phase diagram for this geometry, which is expected to be significantly different than the Werthamer-Helfand-Hohenberg model, due to the presence of paramagnetic limiting. Nanofabricated magnetometers have also been developed in a UNSW-LANL collaboration for use in Dirac III for Fermi surface measurements of YBCO in megagauss fields, which are described.
Date: October 24, 1998
Creator: Clark, R.G.; O'Brien, J.L.; Dzurak, A.S.; Kane, B.E.; Lumpkin, N.E.; Reilley, D.J. et al.
Partner: UNT Libraries Government Documents Department

The effect of disorder on the critical points in the vortex phase diagram of YBCO

Description: The effect of line disorder induced by heavy ion irradiation and of point disorder induced by proton and electron irradiation on the upper and lower critical points in the vortex phase diagram of YBCO is presented. The authors find that dilute line disorder induces a Bose glass transition at low fields which is replaced at the lower critical point by first order melting at higher fields. Strong pinning point defects raise the lower critical point, while weak pinning point defects have little or no effect on the lower critical point. The upper critical point is lowered by point disorder, but raised by line disorder. First order melting is suppressed by point disorder in two ways, by lowering of the upper critical point only for weak point pins, or by merging of the upper and lower critical points for strong point pins. The differing responses of the upper and lower critical points to line and point disorder can be understood in a picture of transverse and longitudinal spatial fluctuations.
Date: January 19, 2000
Creator: Crabtree, G. W.; Kwok, W. K.; Paulius, L. M.; Petrean, A. M.; Olsson, R. J.; Karapetrov, G. et al.
Partner: UNT Libraries Government Documents Department

First-principles approaches to materials stability

Description: Parameter-free electronic structure approaches are now being used to predict chemical order, and to a lesser extent, structural transformations in multi-component alloys, as a function of temperature, concentration and pressure. The underlying state-of-the-art framework will be briefly reviewed, and applications to specific aspects of the statics and kinetics of alloy transformations will be discussed. Finally special emphasis will be put on the relations between stability and mechanical properties in substitutional alloys with examples pertaining to the energetics of antiphase boundaries and interfaces.
Date: December 1, 1994
Creator: Turchi, P.E.A.
Partner: UNT Libraries Government Documents Department

Phase diagram of the random-field Ising system Fe{sub 0.60}Zn{sub 0.40}F{sub 2} at intense fields

Description: The critical and irreversibility phase boundaries of the d = 3 diluted uniaxial antiferromagnet Fe{sub 0.60}Zn{sub 0.40}F{sub 2} have been determined under strong external magnetic fields by means of magnetization measurements. The data reveal that the random-field-induced glassy phase, previously observed in the upper part of the (H,T) phase diagram for highly diluted samples (x {approx_equal} 0.3), is extended to higher values of x.
Date: October 1, 1997
Creator: Montenegro, F.C.; Lima, K.A.; Torikachvili, M.S. & Lacerda, A.H.
Partner: UNT Libraries Government Documents Department

Magnetic phase diagrams of UNiGe

Description: UNiGe undergoes two magnetic transitions in zero field. Here, the magnetic diagrams of UNiGe for B {parallel} b and B {parallel} c are reported. We performed temperatures scans of the magnetization in static magnetic fields up to 19.5T applied along the b and c axes. For both orientations 3 magnetic phases have been identified in the B-T diagrams. We confirmed the previously reported phase boundaries for B {parallel} c, and in addition we determined the location of the phase boundaries for B {parallel} b. We discuss a possible relationship of the two zero-field antiferromagnetic phases (commensurate: T<42K; incommensurate: 42K<T<50K) and the field-induced phase, which, at low temperatures, occurs between 18 and 25T or 4 and 10T for B {parallel} b or B {parallel} c, respectively. Finally, we discuss the field dependence of the electronic contribution {gamma} to the specific heat for B {parallel}c up to 17.5T, and we find that its field dependence is similar to the one found in more itinerant uranium compounds.
Date: November 1, 1997
Creator: Nakotte, H.; Hagmusa, I.H.; Klaasse, J.C.P.; Nakotte, H.; Lacerda, A.H.; Hagmusa, I.H. et al.
Partner: UNT Libraries Government Documents Department

Study of the potential use of carburized niobium in plutonium processing

Description: Carburized refractory metals, especially tantalum, have been shown to possess properties useful for application as hardware in the plutonium-processing environment. These applications are driven in part by a desire to minimize the production of radioactively contaminated waste. The current use of ceramics as containment materials for Pu processing are not ideal due to the short service life of the hardware, placing an additional burden on the contaminated waste stream. Carburized niobium has been examined for use as an improved hardware material. The Nb-C system is analogous to the previously studied Ta-C system. The low density of niobium relative to tantalum will improve the ergonomics of the glovebox environment. The choice of the Nb-C system will be supported by a thermodynamic and kinetic analysis. Preliminary results of the processing investigation also will be presented.
Date: December 1, 1998
Creator: Johnson, M.J.; Soderquist, S.D. & Axler, K.M.
Partner: UNT Libraries Government Documents Department

Disordered vortex phases in YBa{sub 2}Cu{sub 3}O{sub x}

Description: The disordered vortex phases induced by line and point pinning in YBa{sub 2}Cu{sub 3}O{sub x} are explored. At high defect densities there is a single disordered solid separated from the liquid phase by a melting line. At low defect densities the topology of the phase diagram changes dramatically, with a vortex lattice phase adjoining disordered phases at high or low field. Critical points at the termination of first order melting separate the lattice and disordered phases. The line defect disordered phases follow the expected Bose glass behavior, while the point defect disordered phases do not exhibit the expected vortex glass behavior.
Date: May 22, 2000
Creator: Crabtree, G. W.; Kwok, W. K.; Olsson, R. J.; Karapetrov, G.; Paulius, L. M.; Petrean, A. et al.
Partner: UNT Libraries Government Documents Department

Transition between the 1 x 1 and ({radical}3 x 2{radical}3)R30{degree} surface structures of GaN in the vapor-phase environment

Description: Out-of-plane structures of the GaN(0001) surface in the metal-organic chemical vapor deposition (MOCVD) environment have been determined using in situ grazing-incidence X-ray scattering. The authors measured 11{bar 2}{ell} crystal truncation rod intensities at a variety of temperatures and ammonia partial pressures on both sides of the 1 x 1 to ({radical}3 x 2{radical}3)R30{degree} surface phase transition. The out-of-plane structure of the ({radical}3 x 2{radical}3)R30{degree} phase appears to be nearly independent of temperature below the transition, while the structure of the 1 x 1 phase changes increase rapidly as the phase transition is approached from above. A model for the structure of the 1 x 1 phase with a partially-occupied top Ga layer agrees well with the data. The observed temperature dependence is consistent with a simple model of the equilibrium between the vapor phase and the surface coverage of Ga and N. In addition, the authors present results on the kinetics of reconstruction domain coarsening following a quench into the ({radical}3 x 2{radical}3)R30{degree} phase field.
Date: January 12, 2000
Creator: Munkholm, A.; Thompson, C.; Stephenson, G. B.; Eastman, J. A.; Auciello, O.; Fini, P. et al.
Partner: UNT Libraries Government Documents Department

Solidification analysis of a centrifugal atomizer using the Al-32.7wt.% Cu alloy

Description: A centrifugal atomizer (spinning disk variety) was designed and constructed for the production of spherical metal powders, 100--1,000 microns in diameter in an inert atmosphere. Initial atomization experiments revealed the need for a better understanding of how the liquid metal was atomized and how the liquid droplets solidified. To investigate particle atomization, Ag was atomized in air and the process recorded on high-speed film. To investigate particle solidification, Al-32.7 wt.% Cu was atomized under inert atmosphere and the subsequent particles were examined microscopically to determine solidification structure and rate. This dissertation details the experimental procedures used in producing the Al-Cu eutectic alloy particles, examination of the particle microstructures, and determination of the solidification characteristics (e.g., solidification rate) of various phases. Finally, correlations are proposed between the operation of the centrifugal atomizer and the observed solidification spacings.
Date: February 23, 1998
Creator: Osborne, M.G.
Partner: UNT Libraries Government Documents Department

Structural chemistry of some phases in the YC-Ni-B system

Description: NiB, monoclinic Ni{sub 4}B{sub 3}, Ni{sub 2}B and Ni{sub 3}B were prepared by arc-melting and their room-temperature crystal structures were refined by Rietveld analysis of neutron powder diffraction data. The NiB refinement is altogether new data. Although the B atoms in NiB form characteristic zigzag chains, the primary coordination of each atom by atoms of the other kind is similar and distinctively sevenfold, with one short (2.117 {angstrom}), two intermediate (2.152 {angstrom}), and four long (2.163 {angstrom}) bonds. Other samples with stoichiometries (YC)nNi{sub 2}B{sub 2}, n = 3, 4, did not yield single-phase material, but both x-ray and neutron powder diffraction suggest that the n = 4 structure is present in both of these samples. Phase-pure samples of these homologues may require non-stoichiometry and a more controlled thermal history than is attainable by arc melting. To screen samples for superconductivity, ac susceptibility measurements were made using the mutual inductance method with ac signal of 200 Hz.
Date: December 31, 1994
Creator: Chakoumakos, B.C.
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

Microstructural development of rapid solidification in Al-Si powder

Description: The microstructure and the gradient of microstructure that forms in rapidly solidificated powder were investigated for different sized particles. High pressure gas atomization solidification process has been used to produce a series of Al-Si alloys powders between 0.2 {mu}m to 150 {mu}m diameter at the eutectic composition (12.6 wt pct Si). This processing technique provides powders of different sizes which solidify under different conditions (i.e. interface velocity and interface undercooling), and thus give different microstructures inside the powders. The large size powder shows dendritic and eutectic microstructures. As the powder size becomes smaller, the predominant morphology changes from eutectic to dendritic to cellular. Microstructures were quantitatively characterized by using optical microscope and SEM techniques. The variation in eutectic spacing within the powders were measured and compared with the theoretical model to obtain interface undercooling, and growth rate during the solidification of a given droplet. Also, nucleation temperature, which controls microstructures in rapidly solidified fine powders, was estimated. A microstructural map which correlates the microstructure with particle size and processing parameters is developed.
Date: November 1, 1995
Creator: Jin, F.
Partner: UNT Libraries Government Documents Department

Layered manganese oxide intergrowth electrodes for rechargeable lithium batteries: Part 2. Substitution with Al

Description: The structural and electrochemical characterization of layered Li{sub x}Al{sub y}Mn{sub 1-y}O{sub 2} compounds prepared from sodium-containing precursors is described. A quaternary phase diagram showing composition ranges for pure P2 and P3 structures and P2/P3 intergrowths obtained in the Na-Al-Mn-O system is presented. Upon ion exchange, these compounds change to O2, O3 or O2/O3 stacking arrangements, respectively. The oxygen array in O3 and spinel structures is similar, and most of the O3 structures convert to spinel rapidly upon electrochemical cycling in lithium cells. This process is delayed somewhat by increased Al substitution, but not completely inhibited. More effective suppression of the phase transformation is observed in O2/O3 intergrowth electrodes. Additionally, the capacity retention upon cycling and the rate behavior of cells containing intergrowth electrodes is superior to those with pure O2 structures.
Date: September 8, 2004
Creator: Patoux, Sebastien; Dolle, Mickael & Doeff, Marca M.
Partner: UNT Libraries Government Documents Department

Multicomponent three-phase equilibria

Description: This paper presents the relations that describe thermodynamic equilibrium in a three-phase system. Multiple components, including air, water, and oil components, are considered in three phases: (1) aqueous, (2) oil, and (3) gas. Primary variables are specified for each of seven possible phase combinations. These primary variables are then used to determine the necessary secondary variables to completely describe the system. Criteria are also developed to check the stability of each phase configuration and determine possible transitions from one phase configuration to another phase configuration via phase appearances and disappearances.
Date: June 1995
Creator: Ho, C. K.
Partner: UNT Libraries Government Documents Department

Paramagnetism and reentrant behavior in quasi-1D superconductors at high magnetic fields

Description: The thermodynamics of quasi-one-dimensional superconductors in the presence of large magnetic fields is studied. When the quantum effects of the magnetic field are taken into account, several reentrant phases persist at very high fields. In the last reentrant phase, the free energy, the specific heat jump and the excess magnetization are estimated near the critical temperature. In particular, the excess magnetization is found to be paramagnetic as opposed to diamagnetic (in weak fields) and its sign is found to be controlled by the slope of H{sub c2}. This result is further generalized to the entire phase diagram (including all quantum phases) and to different physical systems using general thermodynamic relations which show that the sign of the excess magnetization {Delta}M near H{sub c2}(T) follows dH{sub c2}(T)/dT. These relations provide a scenario for the evolution of the sign of {Delta}M from weak fields to strong fields.
Date: April 1, 1995
Creator: Se de Melo, C.A.R.
Partner: UNT Libraries Government Documents Department

Layered manganese oxide intergrowth electrodes for rechargeable lithium batteries: Part 1-substitution with Co or Ni

Description: Lithium manganese oxides substituted with nickel or cobalt were characterized electrochemically in lithium cell configurations. The compounds studied were either single-phase layered structures with either primarily O2 or O3 stacking arrangements, or O2/O3 intergrowths, prepared from P2, P3 and P2/P3 sodium-containing precursors, respectively. The stacking arrangements are extremely sensitive to the Na/T. M. (T. M. = transition metal) ratios and the level of substitution. Phase diagrams showing the stability regions of the various arrangements for the Na-Ni-Mn-O system are presented. A possible correlation between vacancies and electrochemical performance is suggested. For high levels of substitution with Ni, fewer defects are possible for materials containing more O3 component and higher discharge capacities can be achieved, but spinel conversion upon cycling also occurs more rapidly as the O3 content increases. Intergrowths show intermediate behavior and represent a potential route towards designing stable, high capacity electrodes.
Date: September 8, 2004
Creator: Dolle, Mickael; Patoux, Sebastien & Doeff, Marca M.
Partner: UNT Libraries Government Documents Department

Phase equilibria in strong polar fluids using a perturbed hard-sphere-chain equation of state combined with three different association models

Description: Goal of this work is the extension of a Perturbed-Hard-Sphere-Chain equation of state (PHSC EOS) to systems containing strong polar components. Three different types of association models (ten Brinke/Karasz, SAFI, modified Veytsman) were used to calculate the contribution of specific interactions like hydrogen bonding to thermodynamic quantities. Pure component parameters obtained from regression of temperature dependent density and vapor pressure data allow the prediction of VLE and LLE data. The results of simple fluids and polymer solutions were compared with experimental data. The SAFT and the modified Veytsman extension give similar results for pure fluids and mixtures with components of similar segment size. Differences increase with increasing difference of segment size.
Date: October 1, 1994
Creator: Schaefer, B.; Lambert, S.M.; Song, Y. & Prausnitz, J.M.
Partner: UNT Libraries Government Documents Department

Transition from single-domain to vortex state in soft magnetic cylindrical nanodots.

Description: The authors have investigated the magnetic properties of submicron soft magnetic cylindrical nanodots using an analytical model as well as three dimensional numerical finite element simulations. A detailed comparison of the magnetic vortex state shows the differences between these two models. It appears that the magnetic surface charges play a crucial role in the equilibrium magnetization distribution especially for shifted vortices. In addition magnetic volume charges, which arise from a radial component of the magnetization, have been found. Finally, the magnetic phase diagram for soft magnetic particles with varying aspect ratio is presented.
Date: October 9, 2002
Creator: Scholz, W.; Guslienko, K. Yu.; Novosad, V.; Suess, D.; Schrefl, T.; Chantrell, R. W. et al.
Partner: UNT Libraries Government Documents Department

Probing the phase diagram of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+delta} with tunneling spectroscopy.

Description: Tunneling measurements are performed on Ca-rich single crystals of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} (Bi2212), with various oxygen doping levels, using a novel point contact method. At 4.2 K, SIN and SIS tunnel junctions are obtained with well-defined quasi-particle peaks, robust dip and hump features and in some cases Josephson currents. The doping dependence of tunneling conductances of Ca-rich Bi2212 are analyzed and compared to stoichiometric Bi2212. A similar profile of energy gap vs. doping concentration is found although the Ca-rich samples have a slightly smaller optimum T{sub c} and therefore smaller gap values for any doping level. The evolution of tunneling conductance peak height to background ratios with hole concentration are compared. For a given doping level, the Ca-rich spectra showed more broadened features compared to the stoichiometric counterparts, most likely due to increased disorder from the excess Ca. Comparison of the dip and hump features has provided some potential insights into their origins.
Date: August 29, 2002
Creator: Ozyuzer, L.; Zasadzinski, J. F.; Gray, K. E.; Hinks, D. G. & Miyakawa, N.
Partner: UNT Libraries Government Documents Department