Search

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]

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]

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]

A racetrack coil using MgB2 /Cu strand has been fabricated and tested for transport current density at 4.2 K in self field. The monofilamentary strand was 1.0 mm OD and insulated with S-glass braid. Eighty turns of strand (42 m) were wound onto a stainless steel former with outer dimensions 25 cm × 10 cm × 0.86 cm. The resulting racetrack coil was heat treated at 675 ° C for 30 minutes in flowing Ar. The strand, with a superconducting fraction of 26%, occupied 49% of the total coil pack cross sectional area. The coil Ic at 4.2 K and self field was 120 A (using a 1 μV/cm criterion). This led to a Jc,sc (across the whole coil) of 6.12 × 104 A/cm², a Je in the wire of 1.59 × 104A/cm², and an overall winding Je of 7.9 × 10² A/cm& at 4.2 K in self field. [ABSTRACT FROM AUTHOR]

We explored the incorporation of field sweep (constant current and rapidly increasing magnetic field) into the four-probe method as a new technique to detect defects in barrier layers in superconducting MgB 2 wires. This method allows us to observe jumps in the electric field in low magnetic fields. The scanning electron microscopy results indicate that such a jump originates from cracks in Nb barriers and ex-situ MgB 2 barriers. Our research indicates that the field sweep allows us to detect damage to barriers that are made of superconducting materials. This method can be the basis for an industrial method for detecting damages in MgB 2 wires. These defects reduce the critical current of MgB 2 wire. Detection and removal of these defects will allow us to produce MgB 2 wires with ex-situ MgB 2 and Nb barriers that will have improved critical current density. Manufacturing of MgB 2 wires with new ex-situ MgB 2 barriers is a new technological concept. This type of barrier is cheaper and easier to manufacture, leading to cheaper MgB 2 wires. Moreover, we show that critical current can be measured by two methods: current sweep (constant magnetic field and quickly increasing current) and field sweep. [ABSTRACT FROM AUTHOR]

Abstract: Resistive joints are found in many systems using superconductors. Joints are used to connect the superconductor to a normal conducting current terminal or to connect two superconductors, for example in pancake type windings. Knowing the resistance between the contacts is important in studying the heat balance of a superconducting system. We performed several experiments with MgB2–MgB2 lap joints to determine the relation between the contact resistance and solder joint length. Also, the effect of the outer sheath material on the contact resistance was studied. To support the experiments, a computational model using Finite Element Method was created. The measured and computed results showed adequate correlation. In the experiments, the soldered joint length was varied from 3 to 20mm resulting in contact areas between 2.5 and 16.6mm2. The results indicated that the outer sheath material has significant effect on the contact resistance. For a Monel sheathed conductor the measured contact resistances varied between 4 and 16μΩ and if a copper sheath was used, the resistances were an order of magnitude smaller. [Copyright &y& Elsevier]

We have studied the influence of hydrogen-containing argon gas on the structural and superconducting properties of MgB2 wires. Lattice parameters were independent of the concentration level of hydrogen in argon, while the fraction of the secondary phase, MgO, was sensitive. The MgO particles observed by scanning transmission electron microscopy were 30–50nm in size, which is considerably larger than the appropriate size range of flux pinning centers. The sintering atmosphere conditions are a crucial issue for MgB2 wires. [ABSTRACT FROM AUTHOR]

Abstract: Gas phase plasma synthesis methods have been used to prepare nano-scale particles of C doped boron powder that are well suited for powder-in-tube (PIT) fabrication of superconducting wire. Mixtures of H2, BCl3, and CH4 gas are injected into an induction plasma where doped boron powder is formed and collected on a stainless steel screen. The particles typically are ∼20nm in size and collect into lacy agglomerates in the 1–50μm size range. Pressed pellets of these powders heated in a Mg atmosphere are transformed to the MgB2 phase at temperatures ranging from 600°C to 1200°C. Critical current densities, J c, at 5K run from about 106 A/cm2 at self-field to 4×104 A/cm2 at 7T. At 20K, J c ranges from 5×105 A/cm2 at self-field to about 104 A/cm2 at 3T. Samples of pure B powder and B with 7.4%C powder were prepared as PIT wires having a combination Fe inner jacket and a cupro-nickel outer jacket. These PIT J c values are lower than, but comparable to the pressed pellet values. The PIT samples can be reacted in times on the order of 10min and still exhibit greater than 105 A/cm2 at 5K and 2T. [Copyright &y& Elsevier]