W. L. Hults

The effect of alloying on the properties of (Nb, Ti)Cr2 C15 laves phases

Abstract The effect of composition on the ternary (NbCr 2 –TiCr 2 ) C 15 phase properties has been investigated, focusing upon the defect structure, elastic properties, and mechanical behavior. The C 15 phase field is continuous between NbCr 2 –TiCr 2 , with a maximum phase field width of at least 7 at.% solubility. The defect mechanism is governed by anti-site constitutional defects for all alloys. Mechanically, the alloys display a maximum in hardness in the center of the ternary phase field (and a minimum of toughness). The ternary phase field has features that are characteristic of solid-solution strengthening mechanisms. Finally, the elastic properties indicate that the alloys become stiffer in the middle of the ternary phase field. The best compromise of properties occurs furthest from stoichiometry in the ternary phase field at the nominal composition of Nb 19 Ti 19 Cr 62 . The relationships between the defect structure, elastic properties, and mechanical response for the C15 phases are discussed using a combination of atomic size arguments and electronic structure analyses. From these relationships, alloy design strategies for NbCr 2 -based alloys are evaluated.

Development of Tl-1223 superconducting tapes

Abstract A superconducting tape based on the TlBa 2 Ca 2 Cu 3 O 9 (Tl-1223) phase sheathed in silver was fabricated using the powder-in-tube method. Optimal conditions to synthesize powder ( T c =110 K ) and anneal composite tapes were examined. Microstructural examination indicates that cracks are easily healed during a 3 h anneal at the relatively low temperature of 775°C. A transport critical current density J c of 6200 A/cm 2 at 75 K was measured on short lengths of the annealed rolled and pressed 0.20 mm thick tape. The J c value drops to about 250 A/cm 2 at 75 K and 1 T due to the presence of weak links; however, a J c of 1000 A/cm 2 at 45 K is maintained constant to above 6 T. Measurements of magnetic hysteresis loops indicate that the Tl-1223 tape exhibits a high degree of flux pinning. Very little texturing of the superconducting core was evident.

DISLOCATIONS IN MO5SIB2 T2 PHASE

Abstract Dislocation structures in a nearly single phase annealed Mo5SiB2 T2 alloy have been investigated by transmission electron microscopy (TEM). The dislocations have been subjected to Burgers vector and trace analyses to determine the slip directions and planes with the aid of image simulations generated using single crystal elastic constants derived from first principles calculations. The experimental results are compared to predicted slip directions and planes from anisotropic elasticity calculations. Thermal expansion coefficients have been measured by dilatometry and are compared to both calculated and previous experimental values measured using diffraction techniques. Lastly, preliminary compression testing has been performed on the single phase material at 1200°C.

Low-frequency ac measurement of the Seebeck coefficient

We have analyzed the sources of error in the measurement of the Seebeck coefficient and designed a low frequency ac method to reduce them. This method has high precision in a short time period compared to commonly used dc methods while it minimizes some major sources of error that other ac methods do not. Furthermore, the setup can be fit into a 3 mm diam × 7 mm Teflon pressure cell and has minimal side effects due to the heat conductance of the pressure medium. We have also proposed and tested several methods to calibrate the Seebeck coefficient of thermocouples under pressure.

Temperature dependence of the structure of YBa2Cu3O7: The role of the axial oxygens

Abstract It is now generally accepted that the CuO 2 planes in YBa 2 Cu 3 O 7 are responsible for carrying the supercurrents while the CuO chains provide a charge reservoir. The role of the axial oxygens in coupling the two is less clear. Within the last year there have been suggestions from both experiment and theory for the existence of temperature dependent structural anomalies at and below T c , but their nature has not been clearly elucidated. Using Rietveld refinement of structural models from pulsed-neutron powder-diffraction data, we have performed a careful study of the temperature dependence of the structure of YBa 2 Cu 3 O 7 in order to search for possible anomalies involving the axial oxygens in the vicinity of the transition temperature. We find that the axial oxygen gradually moves closer to the CuO 2 planes as the temperature is lowered from 305 to 13 K, indicating a small redistribution in electronic charge from the planes to the chains; however, we find no evidence for anomalous changes in bond lengths involving the axial oxygens near T c , the existence of a double-well potential for the axial oxygens, or evidence that these bonds become either more harmonic or softer as the sample temperature is lowered past T c .

Bi2223 powder-in-tube tapes with different size Ag particles

Abstract Silver powders with different particle sizes were added to Bi2223 powder-in-tube tapes as a follow-up to our similar work on Bi2212. It was found that Ag powders with a 38 μm mean particle size were stretched into long, thin ribbons during deformation. Tapes were rolled as thin as 50 μm with a 20 μm core with no necking or cracking. Transport critical current density of 3.5 × 10 4 A/cm 2 at 75 K and zero magnetic field has been obtained. The Ag powders with a 3 μm mean particle size were deformed into thin plates during tape fabrication. Due to the large surface area, the Ag plates were more effective in promoting Bi2223 phase formation at low heat-treatment temperatures. However, these Ag plates spheridized during heat treatment and distorted Bi2223 grain alignment.

Synthesis of Ag-clad (Bi,Pb)-2223 tapes by an alternative precursor route

Abstract An alternative two-powder process for the synthesis of superconducting (Bi,Pb)-2223/Ag clad wires by the powder-in-the-tube (OPIT) process is described. The first powder, of nominal stoichiometry Bi 1.8 Pb 0.4 Sr 2 CaCu 2 O 8 , is a mixture of (Bi,Pb)-2212 and Ca 2 PbO 4 . The second powder, of nominal stoichiometry CaCuO 2 , is a solution-synthesized mixture of Ca 0.45 Cu 0.55 O 2 and CaO. Using these oxide precursor mixtures, we obtain tapes with well-aligned grains, and reproducible critical current densities J c in the range of 20 000–26 000 A/cm 2 at 75 K in self-field after anneals at 815°C in 10% O 2 in less than 200 h. The decrease in annealing time, relative to other two-powder processes, may be due to the increase in reactivity and possible reaction paths of the Ca 0.45 Cu 0.55 O 2 precursor. Results on tapes prepared using other processing conditions are also reported.

Ag ribbons in Bi2212 powder-in-tube tape

Abstract For an optimum particle size, silver particles added to Bi2212 powder for powder-in-tube tape fabrication stretch into long, thin ribbons during rolling, but not during drawing. This ceramic/metal composite has excellent properties during deformation. Tapes are rolled as thin as 51 μm with a 23 μm core with necking, sausaging, or cracking in the superconductor core. The Ag ribbons also provide additional Ag/superconductor interfaces to help Bi2212 phase deformation and grain alignment. Improved critical current densities are reported both at 4 K and 26.2 K.

Angle-resolved photoemission and first-principles electronic structure of single-crystalline -uranium (001)

Continuing the photoemission study begun with the work of Opeil et al. [Phys. Rev. B textbf{73}, 165109 (2006)], in this paper we report results of an angle-resolved photoemission spectroscopy (ARPES) study performed on a high-quality single-crystal

NMR spin echo experiments as probes of vortex dynamics in YBa2Cu3O7

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