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The preparation of nickel substrates rolled to greater than 95% reduction and annealed in 5% H2/95% He at 300-1000 deg C, for high current coated YBa2Cu3Ox superconductors is described.

A modified form of the rod-in-tube method (MRT) and the conventional jelly-roll (JR) method were used in fabricating niobium-aluminum precursor strands. Warm hydrostatic extrusion was explored for use in the next phase of strand production although most of the material produced used the fully cold drawn method. Pulsed resistive heating to temperatures of 1800 to 2100 degrees C followed by either liquid-Ga quenching or radiative cooling were used to test the JR and MRT strands.

The conventional jelly roll method with two Niobium-aluminum (Nb-Al) volume ratios, 3:1, and 5:1, were used to fabricate Nb-Al precursor strands. The mono-elements were restacked in 7-, 19- and 36-core arrangements and drawn down to wires with 0.4 mm and 0.8 mm diameter. The wires were then resistively heated and allowed to cool by radiation plus lead conduction or by quenching in liquid gallium. The best results obtained were for a 5:1 Nb:Al volume ratio sample Ga quenched from 2000 degrees C.

High field pinning centers in MgB2 doped with 2 at. % carbon under a low and a high hot isostatic pressures have been investigated by transport measurements. The field dependence of the transport critical current density was analyzed within the different pinning mechanisms: surface pinning, point pinning, and pinning due to spatial variation in the Ginzburg-Landau parameter (Δκ pinning). Research indicates that a pressure of 1 GPa allows similar pinning centers to Δκ pinning centers to be obtained. This pinning is very important, because it makes it possible to increase the critical current density in high magnetic fields at 20K and 25 K. Our results indicate that the δTc and dl pinning mechanisms, which are due to a spatial variation in the critical temperature (Tc) and the mean free path, l, respectively, create dislocations. The high density of dislocations with inhomogeneous distribution in the structure of the superconducting material creates the δl pinning mechanism. The low density of dislocations with inhomogeneous distribution creates the δTc pinning mechanism. Research indicates that the hot isostatic pressure process makes it possible to obtain a high dislocation density with a homogeneous distribution. This allows us to obtain the δTc pinning mechanism in MgB2 wires. In addition, a high pressure increases the crossover field from the single vortex to the small vortex bundle regime (Bsb) and improves the δTc pinning mechanism. Our research has proved that a high pressure significantly increases the crossover field from the small bundle to the thermal regime (Bth), with only a modest decrease in Tc of 1.5K, decreases the thermal fluctuations, increases the irreversibility magnetic field (Birr) and the upper critical field (Bc2) in the temperature range from 4.2K to 25K, and reduces Birr and Bc2 above 25K. [ABSTRACT FROM AUTHOR]

Field, temperature, and strain dependences of the critical current for a SiC doped multifilamentary in situ MgB2 wire have been studied. Measurement results were compared with that of the undoped wire, and the origin of the difference in the critical current is discussed. The critical current can be calculated with the percolation model considering the effect of anisotropy. The temperature dependence of the fitting parameters, the upper critical field along the ab-plane, and c-axis is compared with the dirty-limit two-gap theory. To assess the validity of the fitting parameters, resistive transition has been measured especially to extract the upper critical field directly. It is shown that even the resistive broadening can be well explained by a simple parallel path model using the fitting parameters obtained from the critical current analysis. [ABSTRACT FROM AUTHOR]

Studies of the magnetic field dependence of the transport critical current density (Jct) and the grain connectivity of MgB2/Fe wires fabricated from ball-milled boron have been conducted in detail, and strong correlations have been found, as evidenced by differences in grain size, critical transition temperature, and resistivity. It was observed that the samples fabricated by ball milling had relatively small grain sizes, resulting in a strong field dependence of the Jct in the high field region. On the other hand, the ball-milled boron was associated with poor connectivity between adjacent grains. It is clearly shown that the observed reduction in low field Jct is related to the reduction in the superconducting area fraction (AF) that is reflected by the connectivity factor. Even if high temperature sintering in all the samples can compensate for the degradation of the Jct in the low field region, the subsequent grain growth is mainly responsible for the degradation of Jct in the high field region. [ABSTRACT FROM AUTHOR]

MgB2/Fe wires with 10 at. % excess Mg produced by in situ powder-in-tube processing were compared to normal stoichiometric MgB2/Fe wires prepared by the same method. It was found that the critical current density (Jc) and the irreversibility field (Hirr) were significantly enhanced for MgB2/Fe wires with excess Mg. The transport Jc for 10 at. % Mg excess samples sintered at 800 °C, measured at a field of up to 14 T, increased by a factor of 2 compared to that for the normal MgB2 wires. The best Jc results for the 10 at. % Mg excess sample were obtained by heating the sample at 600 °C for 1 h; the Jc at a field of 8 T and at temperature of 10 K reached 3×104 A/cm2. The detailed analysis of the effect of excess Mg on the microstructures, the Jc, and the Hirr, of MgB2/Fe wires are presented in this paper. [ABSTRACT FROM AUTHOR]

MgB2 samples were prepared by using 96% boron (B) powder with strong crystalline phase that had been ball milled for various times. We observed samples that contained ball-milled 96% B in comparison with one made from as-supplied commercial 96% B, with the results showing a significant enhancement in the high field critical current density (Jc) due to small grain size and better reactivity. Specifically, many grain boundaries for MgB2 could be acting as strong flux pinning centers. Based on Rowell connectivity analysis, when the ball-milling time increased, the connectivity factor, described as the active cross-sectional area fraction (AF), was decreased. This implies that the intergrain connectivity became worse. These properties could lead to poor Jc in low field. However, the pinning force strength, Jc1/2×B1/4, of samples using ball-milled 96% B is larger than that of the reference sample using commercial 96% B powder. These results accompany enhanced irreversibility (Hirr) and upper critical fields (Hc2). [ABSTRACT FROM AUTHOR]

Field, temperature and strain dependence of the critical current for a SiC doped multifilamentary in situ Mg[B.sub.2] wire are analyzed by using percolation model. The validity of the fitting parameters is assessed by measuring the resistive transition to extract the upper critical field directly.

A detailed study on the magnetic field dependence of the transport critical current density ([J.sub.ct]) and the grain connectivity of Mg[B.sub.2]/Fe wires fabricated from ball-milled boron is presented. The observed reduction in low field [J.sub.ct] is found to be related to the reduction in the superconducting area fraction (AF) that is reflected by the connectivity factor.