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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]

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]

Raw MgB2 wires have been manufactured by Hyper Tech Inc. Each of their 18 cores contained undoped in situ powder surrounded by an Nb barrier and placed in a Cu sheath, and cores were coated in a common Monel sheath. Wires of diameters 0.63 and 0.83 mm were annealed under high argon pressure (up to 1 GPa). Parameters of such HIP process (temperature, time, and pressure) were varied in order to determine their optimal values. Superconducting properties of such samples were investigated by means of four-probe critical current j c measurement. Bitter magnet producing magnetic field up to 14 T was used. It provided place for samples 70 mm long in parallel field and 20 mm long in perpendicular field. Results include critical current j c and pinning force density F p dependencies on magnetic field as well as Kramer plots. Critical current of 104 A/cm 2 was achieved at 12 T magnetic field. SEM pictures of wire cross sections were also taken to determine quality of Nb barrier and microstructure of superconducting material. [ABSTRACT FROM PUBLISHER]

we report the synthesis and characterization of \MgB2 made from nano-boron and doped with graphene in the following mole percentages, x=0, 3.0 and 12.0. The effect of graphene doping on the normal state resistivity (\rho), superconducting transition temperature (Tc), irreversibility and upper critical fields (Hirr and Hc2), and critical current density (Jc), as well as the pinning force (Fp) were evaluated. [ABSTRACT FROM AUTHOR]

A model helical undulator 250 mm in length, with a period of 14 mm, was designed, fabricated and tested. The helical coil was wound with multifilamentary, internal-Sn type Nb3Sn strand. The 0.7 mm OD strand was insulated with S-glass, wound onto the former, and reacted, after which the coil underwent vacuum epoxy impregnation. The beam aperture was 7 mm, the winding bore diameter was 8 mm, and the OD of the complete winding was 18 mm. The helical poles were made from 1016 low carbon steel, and projected slightly above the coil pack, with an OD of 19.06 mm. The coil Ic and the magnetic field in the bore were measured at 4.2 K in a liquid helium bath. The bore field was also measured as a function of position along the length of the undulator. [ABSTRACT FROM AUTHOR]

A short model helical undulator, 250 mm long, with a period of 14 mm was designed, fabricated and then subsequently tested in a liquid helium bath. The helical coil was wound with stabilized multi-filamentary MgB2 strand, 0.5 mm in diameter, in a wind and react mode with S-glass insulation followed by vacuum epoxy impregnation. Beam aperture of the coil was 7 mm, winding bore diameter 8mm, coil winding OD = 18 mm and the 1010 Fe pole (flange) has an OD = 19.05 mm. The coil was measured for Ic and magnetic field in the bore at 4.2 K in a liquid helium bath. The bore field as a function of depth was also determined near the critical current at 4.2 K. The load line of the coil was compared to short sample Ic values. [ABSTRACT FROM AUTHOR]

Niobium tin was one of the first superconductors to be made into relatively small high field magnets forty-six years ago. Conductors made from it are widely used for a range of different applications and development efforts are still being carried out to improve properties and reduce costs. The paper describes some of the work being carried out by the authors to make such improved conductors for fusion, high-energy physics and other applications. Three different types of niobium-tin conductors are described: 1. Low loss material with modest current density for fusion magnets. 2. High current density, higher loss material for high field high energy physics magnets. 3. Tubular type, low cost material with intermediate current density and small subelement size that could have application in a wide range of different magnets. Some of the advantages and drawbacks of each type of conductor are described together with illustrative properties. [ABSTRACT FROM AUTHOR]

Tube type composites containing up to 167 subelements were studied in terms of their reaction systematics in an attempt to enhance their properties. These conductors had transport Jc. values in excess of 2200 A/mm2 at 12 T and 4.2 K. Samples were heat treated at temperatures between 650-800° C for varying amounts of time. The phase formations as well as the growth kinetics of the AlS layer during reactive diffusion between the Nb and a liquid Sn-rich bronze alloy were experimentally investigated at 650°C and 725°C. The A15 layer growth exponent `n', proportionality constant 'k', and the rate controlling process were determined at these temperatures. Compositional analysis was performed using EDS on samples with various initial Cu-Sn ratios. The variation of grain size and morphology with temperature and composition were studied in detail. The activation energy for grain growth in the fine and coarse grain A15 regions were determined. [ABSTRACT FROM AUTHOR]

MgB2 samples were prepared using as-supplied commercial 96% boron with strong crystalline phase and the same 96% boron (B) after ball milling. The effects of the properties of the starting B powder on the superconductivity were evaluated. We observed that samples using ball-milled 96% B, in comparison with the one made from the as-supplied 96% B, were characterized by small grain size, broadened full width at half maximum (FWHM), and enhanced magnetic critical current density (Jc). reached 2 × 103 Acm-2 at 5 K and 8 T. The improved pinning of these samples seems to be caused by enhanced grain boundary pinning at high field. [ABSTRACT FROM AUTHOR]

Two standard internal-Sn type rod-in-tube (RIT) and two newly developed Tube Type conductors were investigated. Both types of strand were doped with Ta. A series of heat-treatments with varying temperature and duration time were performed to study their influence on the microstructure and properties. In this work, the fully reacted strands were analysed. Non-Cu Jc was measured and Bc2 values were estimated via Kramer plot extrapolation. Bc2 values were also measured by resistive transitions at high magnetic fields, and compared with the values from Kramer plot extrapolations. High resolution Scanning Electron Microscopy (SEM) was used to observe the cross-section and the composition across the strands' cross-section was measured by using Energy Dispersive Spectroscopy (EDS). Scanning Transmission Electron Microscopy (STEM) was used to measure the composition variation in the individual filaments in the RIT strands. Superconducting properties were correlated to strand composition. [ABSTRACT FROM AUTHOR]

A process designed to produce Nb3 Sn conductors by simply restacking Nb7.Swt.%Ta tubes with Cu-clad Sn cores is being explored. Previous work has shown that, in this tubular process, the A-15 compound forms in a somewhat different manner from that in the conventional internal-tin process. It was necessary therefore to determine if this had a significant effect on the proper- ties of the A-15 formed. This was done with subelements containing different percentages of Nb7.Swt.%Ta, Sn and Cu. The initial work on single subelements has been extended to include restacks with relatively large numbers of subelements. The critical current density (Jc) of the various designs is reported and some of the factors involved in optimizing the properties are discussed. [ABSTRACT FROM AUTHOR]

Multifilamentary MgB2 wire was used to construct three coils that were tested for transport Jc and magnetic field in the bore between 4.2 K and 30 K. The solenoid coil achieved 85 A at 4.2 K with 3.9 T in a 3.8 cm bore. A racetrack coil was also fabricated with an attached compression plate to test the effects of compressive stress on an MgB2 racetrack coil. At compressive stresses up to 52 MPa, no degradation was observed in transport Jc at 20 K. A react and wind coil with a 56 cm bore and 828 m of MgB2 wire was also tested for transport Jc and magnetic field in the bore to demonstrate the feasibility of the react and wind process. At 5 K, the react and wind coil achieved 168 A and 0.17 T. [ABSTRACT FROM AUTHOR]

We have investigated magnesium diboride wires as an element in a resistive superconducting fault current limiter using a pulsed system, which could deliver a fixed number of AC cycles to avoid burnout due to thermal instabilities. Experiments have been carried out in both liquid helium and at 27 K on magnesium diboride mono-core wires in stainless steel tubes, in a CuNi sheath with an Fe barrier and also on multifilamentary wires with Nb barriers. Experiments showed good current limiting in the first cycle and no damage after six cycles. Also wires with a number of voltage contacts spaced at about 1 cm intervals to assess the effect of inhomogeneity were tested. Relatively small differences in local Jc led to large differences in the local temperature rise although in no case did this lead to failure of the conductor. The results were compared with those on Bi-2223/Ag tapes and YBCO coated conductors measured in liquid nitrogen. [ABSTRACT FROM AUTHOR]

The advantages of titanium as a third element in Nb3Sn have been known for many years. It increases the upper critical field and lowers the critical current density at lower fields, thus reducing the losses, which are usually measured in this field region. Several methods for the introduction of titanium are listed and discussed briefly. The two methods discussed in more detail are both introduction by way of Nb47wt. %Ti. One is by wrapping foils around the filaments and the other by introducing rods into the filament array. A relatively uniform distribution of Ti in the filaments, results from both methods. Scanning Electron Microscopy (SEM) was used to illustrate the various phases, produced under different heat treatment conditions. Energy Dispersive Spectroscopy, (EDS) and Wavelength Dispersive Spectroscopy, (WDS) were both used to obtain compositional data. [ABSTRACT FROM AUTHOR]

Studies the influence of primary and secondary heat treatment on the superconductivity of Nb[sub 3]Al superconductors. Magnetic field dependence of the critical current density; Critical current density as a function of the departure of maximum temperature from the reaction temperature during rapid heating.

The critical current density (Jc) of the monofilament Monel/Fe/MgB2 wire was measured between 4.2 K and 30 K with a Variable Temperature Insert (VTI) in a 20 T superconducting magnet using the analog DC pulse four-wire method with 1 μv/cm criterion. The critical current density, Jc was measured to be 0.16 × 105 A/cm² at 4 T at 21 K for a MgB2 sample which was annealed at 700° C for 5 min and at 900°C for 5 min under Ar gas flow. Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) analysis were used to characterize the microstructure of the samples. [ABSTRACT FROM AUTHOR]

The critical current densities of MgB2 monofilamentary strands with and without SiC additions were measured at 4.2 K. Additionally, magnetic Jc at B=1 T was measured from 4.2 to 40 K. Various heat treatment times and temperatures were investigated for both short samples and small helical coils. SiC additions were seen to improve high field transport Jc at 4.2 K, but improvements were not evident at 1 T at any temperature. Transport results were relatively insensitive to heat treatment times and temperatures for both short samples and coils in the 700–900 °C range. [ABSTRACT FROM AUTHOR]

Abstract: We report detailed field, temperature and strain dependence of the critical current for a commercial multi-filamentary in-situ MgB2 wire. The field dependence of the critical current presented in the form of a Kramer plot was not linear near the irreversibility field for all temperatures measured. Instead, we found that the field dependence of the critical current can be well described by a recent model incorporating the Kramer model with a percolation theory proposed by Eisterer et al. [Phys. Rev. Lett. 90 (2003) 247002]. The temperature dependence of fitting parameters, the upper critical field along the ab-plane and c-axis is compared with that of the irreversibility field. It is further shown that the strain dependence of the critical current can also be understood by the percolation model as related to a variation in the anisotropy factor of the upper critical field. [Copyright &y& Elsevier]

MgB2 samples were prepared using as-supplied commercial 96% boron with strong crystalline phase and the same 96% boron (B) after ball milling. The effects of the properties of the starting B powder on the superconductivity were evaluated. We observed that samples using ball-milled 96% B, in comparison with the one made from the as-supplied 96% B, were characterized by small grain size, broadened full width at half maximum (FWHM), and enhanced magnetic critical current density (Jc). reached 2 × 103 Acm-2 at 5 K and 8 T. The improved pinning of these samples seems to be caused by enhanced grain boundary pinning at high field. [ABSTRACT FROM AUTHOR]

Abstract: We studied the effects of MgB2 with embedded multiwall carbon nanotubes (MWCNTs) on the crystallinity, lattice parameters, critical current density (J c), upper critical field (H c2), irreversibility field (H irr), and microstructure of MgB2. Fe sheathed un-doped and MWCNT doped MgB2 wires were fabricated by the powder-in-tube (PIT) method and sintered at the high sintering temperatures of 900°C. We observed that for the MWCNT doped sample high temperature sintering resulted in depressed crystallinity, shrinkage of the a-lattice parameter, higher J c up to 12T, and lower critical temperature (T c) values. Specifically, MWCNT doped samples sintered at 900°C exhibited excellent J c, ∼104 Acm−2 up to 9T at 4.2K. This can be explained by lattice distortion and poor crystallinity due to carbon (C) substitution from the MWCNT. [Copyright &y& Elsevier]