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  • 1
    Electronic Resource
    Electronic Resource
    Properties of niobium–titanium superconducting wires with Nb artificial pinning centers (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 1640-1646 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Artificial pinning center (APC) Nb 47 wt % Ti composite wires containing 24 vol % Nb pins were characterized as the wire diameter was reduced. As the nominal pin diameter dp was reduced from 165 to 15 nm, the nominally round pins transformed into ribbons with a relatively wide distribution in thickness, as compared to the α-Ti ribbons in optimized, conventionally processed Nb 47 wt % Ti. The maximum in the bulk flux pinning force Fp of 25 GN/m3 (4.2 K, 2.5 T) occurred at dp=40 nm, for which the measured Nb ribbon thickness ranged from 1 to 16 nm. This Fp value was about one third higher than that found in the best conventional Nb 47 wt % Ti. The upper critical magnetic field Hc2, measured by magnetization, decreased from 10.3 to 9 T as the Nb pins became proximity-effect coupled to the matrix. To better compare APC and conventional wires, we measured the properties of the best APC wire at a reduced temperature so that its Hc2 was the same as Hc2(4.2 K) for Nb 47 wt % Ti. The peak value of Fp was then 36 GN/m3 at 3 T, almost twice the maximum value yet reported for conventional Nb–Ti (∼19 GN/m3 at 5 T). In spite of the pinning force curve remaining sharply peaked at lower fields, the equalized 5 T critical current density was 4600 A/mm2, some 25% higher than the best values of conventional Nb–Ti. These properties demonstrate the strong potential of APC composites. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362979
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  • 2
    Electronic Resource
    Electronic Resource
    Strongly enhanced critical current density in Nb 47 wt. % Ti having a highly aligned microstructure (1991)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 58 (1991), S. 2984-2986 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The transport critical current density increased from 2610 to 5200 A/mm2 at 5 T, 4.2 K, when a round monofilament Nb 47 wt. % Ti composite was rolled to an aspect ratio of 9.7 and was tested with the broad face of the tape parallel to the field. This value exceeds the previous maximum of 3700 A/mm2 by about 50%. Transmission electron micrographs show that more than 90% of the α-Ti precipitates in the optimum-rolled filament are aligned within 10° of the broad face of the tape, whereas the precipitates had a random azimuthal orientation prior to rolling. The strong alignment caused the elementary pinning force to be greatly enhanced in the parallel-field orientation; however, in the orthogonal orientation the Jc fell to low values characteristic of conventional wires having no α-Ti flux pinning centers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.104689
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  • 3
    Electronic Resource
    Electronic Resource
    Proximity effect depression of the critical temperature in two-phase Nb-Ti superconductors (1987)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 51 (1987), S. 688-689 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The superconducting critical temperature of high critical current density Nb-Ti composites has been measured at various stages in the critical current optimization process. This process involves heat treatment steps which cause precipitation of normal α-Ti and make the matrix more Nb rich. Tc rises from 9.1 to 9.5 K during this stage. The final optimization stage involves extensive wire drawing during which the α-Ti precipitates are reduced to less than a coherence length in thickness. This does not result in a reduction in Jc but Tc is found to fall from 9.44 to 8.7 K during this step. The depression of Tc is found to be in fair agreement with the predicted proximity effect suppression of Tc. Wires of optimum transport critical current density are seen to have Tc of around 9 K.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.98335
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  • 4
    Electronic Resource
    Electronic Resource
    Artificial two-phase Nb-Ti nanostructures using powder metallurgy techniques (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 767-769 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Several techniques to form artifical high critical current density superconducting Nb-Ti nanostructures have been reported. A drawback to virtually all of these techniques is that they require fabrication strains of 30 or more to reduce the second phase pinning center to the optimum 1–10 nm size at which critical current densities of 103–104 A/mm2 are obtained. Here we describe a powder metallurgy process that yields 6020 A/mm2 at 2 T and 1470 A/mm2 at 5 T for an alloy with an upper critical field of ∼8 T within the same strain space (∼13) employed in the conventional Nb-Ti fabrication process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.112224
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  • 5
    Electronic Resource
    Electronic Resource
    Periodic pin array at the fluxon lattice scale in a high-field superconducting wire (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 1298-1300 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A periodic pin array has been made at dimensions approaching the diameter and spacing of the fluxon cores in an "island''-type of artificial pinning center superconducting composite wire. Gun-barrel drilling was used to precisely incorporate a hexagonal array of Nb 1.3 wt. % Ti pinning centers into an ingot of Nb 46.5 wt. % Ti, before extrusion and wire drawing. Transmission electron microscopy images showed little evidence of pin geometry alteration down to 100 nm pin diameter. At ∼30 nm effective pin diameter, the aspect ratio of the pins was 3–5, and the pin array retained its hexagonal geometry. Bulk pinning force curves had temperature-independent structure down to 15 nm effective pin diameter, suggestive of matching between fluxons and pins. Because of the uniform pin geometry, the maximum bulk pinning force exceeded that of the best conventional composites, even though the upper critical field and the volume fraction of pins were significantly lower.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.111940
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  • 6
    Electronic Resource
    Electronic Resource
    Increased critical current density in Nb–Ti wires having Nb artificial pinning centers (1997)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 70 (1997), S. 901-903 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Artificial pinning center (APC) wires containing Nb 47 wt.% Ti with 24 vol.% of round Nb pins have produced very high critical current densities (Jc) which are attributed to a sharply defined, nanometer-scale Nb-pin array. By reducing both the number of warm extrusion steps from four to three and the temperature of the third extrusion from 650 °C to 250 °C, the degree of pin-matrix interdiffusion has been reduced and Jc values at all applied magnetic fields increased by 25–45% over those for a previous composite of almost identical design. The best wire achieved the very high Jc(5 T, 4.2 K) value of 4600 A/mm2. These results underscore the importance of the thermomechanical treatment in determining the maximum flux pinning properties of APC Nb–Ti wires. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.118238
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  • 7
    Electronic Resource
    Electronic Resource
    Quantitative description of a high Jc Nb-Ti superconductor during its final optimization strain. I. Microstructure, Tc, Hc2, and resistivity (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 66 (1989), S. 5962-5970 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A most important step in the critical current density (Jc) optimization of Nb-Ti is the large final drawing strain, in which α-Ti precipitates, initially approximately equiaxed and 100–200 nm in diameter, are drawn into ribbons, whose thickness (1–2 nm) is less than the superconducting coherence length [ξ (4.2 K)∼5 nm]. Using transmission electron microscopy, the precipitate thickness, spacing, cross-sectional area, and circumference were measured over the whole final drawing strain range. Each of these parameters was found to have a simple power dependence on the wire diameter. Tc, Hc2, and the resistivity (ρn) were also change considerably during the refinement of the precipitates. Directly after precipitation, Tc increased, and (dHc2/dT)Tc and ρn were reduced from the single-phase values. Drawing the wire returned these parameters to their single-phase values, as the precipitate thickness was reduced to less than ξ. This observation explains a long-standing peculiarity in this system, namely that the optimum Hc2 of high Jc conductors occurs for a composition close to Nb 46 wt.% Ti, even when the precipitation of 18 vol % of α-Ti shifts the matrix composition to a Nb-rich composition of theoretically lower Hc2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.343624
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  • 8
    Electronic Resource
    Electronic Resource
    Plants lacking the main light-harvesting complex retain photosystem II macro-organization (2003)
    [s.l.] : Macmillian Magazines Ltd.
    Nature 421 (2003), S. 648-652 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Photosystem II (PSII) is a key component of photosynthesis, the process of converting sunlight into the chemical energy of life. In plant cells, it forms a unique oligomeric macrostructure in membranes of the chloroplasts. Several light-harvesting antenna complexes are organized precisely in ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/nature01344
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  • 9
    Electronic Resource
    Electronic Resource
    Nutritional and related experiments on laboratory maintenance of three species of symbiont-bearing, large foraminifera (1991)
    Springer
    Marine biology 109 (1991), S. 417-425 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Factors were examined that affect survival and growth of two common species of large foraminifera from the Red Sea,Amphisorus hemprichii Ehrenberg andAmphistegina lobifera Larsen, 1976. The former is host for dinoflagellate and the latter for diatom zooxanthellae. Experimental conditions were modeled on conditions at 25 m during spring at Wadi Taba, Gulf of Elat, Israel, the season and site where the experimental organisms were collected between 1983 and 1988. The two species responded quite differently in nutritional experiments.A. hemprichii grew, on average, 0.270 mm in diameter in 3 mo on a diet ofNitzschia subcommunis Hustedt,Chlorella sp. (clone AT) orCylindrotheca closterium Rabenhorst isolated from their native habitat. Unfed controls did not grow. In contrast, unfed populations ofA. lobifera grew as well or better than those that were fed unialgal diets. Growth of both species was enhanced on particular mixed algal diets. Both species required photosynthetically active symbionts. Even when fed weekly and supplied with nutrients, neither species survived in the dark. All individuals ofA. hemprichii died after 8 wk incubation in the dark;A. lobifera survived longer, but all were dead by 13 wk. The highest growth rate ofA. hemprichii (0.037 mm wk−1) was obtained when they were fed, the medium was enriched, and the medium was changed weekly. All other conditions being the same, growth rate dropped to 0.009 mm wk−1 when the medium was changed every 3 wk. In contrast,A. lobifera grew fastest when the medium was changed every 3 wk. Food or enrichment with nitrate or phosphate did not stimulate growth (0.03 mm wk−1) over that of the controls. Specimens ofMarginopora kudakajimensis Gudmundsson from Japan, another dinoflagellate-bearing species, were also tested. They grew best (0.02 mm wk−1) when cultured in light, in media enriched with nitrate and phosphate changed weekly, and fed. All three species withdrew nitrate and phosphate from the medium in chemostat experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01313507
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  • 10
    Electronic Resource
    Electronic Resource
    Determination of the flux pinning force of α-Ti ribbons in Nb46.5wt% Ti produced by heat treatments of varying temperature, duration and frequency (1988)
    Springer
    Journal of materials science 23 (1988), S. 3951-3957 
    Details
    ISSN: 1573-4803
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Transmission electron microscopy has been used to investigate the effect of varying the heat treatment temperature, the duration and the frequency on the microstructure of high current density Nb-Ti superconducting composites. It was found that multiple heat treatment and drawing cycles produced between 16 and 25 vol % ofα-Ti precipitate. The longer and more frequent were the heat treatments, the greater was the volume fraction of precipitate. An increase in the number of heat treatment and drawing cycles significantly reduced the size of the precipitates formed at heat treatment size. If the composites were drawn to final wire size following final heat treatment, theα-Ti precipitates were distorted into folded sheets which appeared as ribbons in transverse cross-section, with average thickness varying from 1.6 nm to 2.6 nm. The specific flux pinning force of the ribbons varied from 360 to 260 N m−2 and was found to decrease with ribbon thickness. Some evidence of an extrinsic limit on theJ c was provided by a study of the filament cross-section variability and the resistive transition index.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01106819
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