Search

We have succeeded in the fabrication of high– J c MgB 2 wires applying hot isostatic pressure process through a reduction of the wire diameter. The results show that annealing at 570 °C in pressure of 1.1 GPa allows for high critical current density in undoped MgB 2 wires e.g. at 4.2 K-1000 A/mm 2 in 8.1 T, 100 A/mm 2 in 13 T and at 20 K–100 A/mm 2 in 5.2 T. The reduction of the MgB 2 wire diameter induces the number of connections between the grains retaining the circular shape of the wires and increasing the density of pinning centers active at 20 K. [ABSTRACT FROM AUTHOR]

Abstract: Uniaxial hot-pressing was applied as an alternative way to further improve the grain connectivity of both un-doped and carbon doped MgB2/Nb/Monel wires. In this study, hot-pressing of 100MPa resulted in the improvement of the mass density, the critical current density, and the grain connectivity of the wires. However, this also caused additional critical current density anisotropy, which is associated with the texture of a- and b-axes oriented grains. In particular, the anisotropy factors of critical current density for the un-doped and the carbon doped conductors were estimated to be 2.6 and 1.7, respectively, under an external magnetic field of 14T. [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]

Abstract: In a continuation of a series of reports on the fabrication and properties of several demonstration coils and solenoids wound with MgB2 strand this paper describes the winding, testing, and load-line comparisons for a racetrack coil (R-4) and two solenoids (S-1 and S-2). Racetrack R-4 was wound with 19-filament strand, S-1 and S-2 with 7-filament strand. The filaments were clad in Nb-lined Cu and the restack bundle was enclosed in a monel sheath; some Cu stabilization was included. Transport J c measurements were performed initially in liquid He, and then as a function of temperature up to 30K. Racetrack R-4 achieved 240A in self-field at 7.7K corresponding to a J c of 240kA/cm2, leading to a strand J e of 43kA/cm2, and a winding J e,w of 22kA/cm2. At 4K, S-1 (310m of strand) achieved an I c of 138A and a bore field of 3.4T; S-2 (742m of strand) an I c of 85A and a bore field of 3.9T. The magnetic field profiles for all coils were calculated using a finite element approach in conjunction with the Biot–Savart equation. The windings were treated as a series of small segments, each tangential to the winding, numerical errors were estimated. These calculations were used to generate load-line comparisons for the coils. [Copyright &y& Elsevier]

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]

The tube filling/forming powder-in-tube (PIT) technique was employed at Hyper Tech Research for the continuous processing of MgB2/Fe composite strand. Demonstration lengths of up to 70 m of 1.2 mm diameter precursor strand––an Fe-clad Mg + B elemental powder mixture––have so far been produced. Short samples of this strand, heat treated for 3–30 min/800–950 °C, have yielded transport critical current densities (Jc) of 2–3×104 A/cm2 at 4 K/4 T, 4×104 A/cm2 at 4 K/2 T, and at 4 K in self field an extrapolated Jc of at least 2×105 A/cm2. Batch-processed PIT MgB2/Fe strands and tapes were made at the University of Wollongong’s Institute for Superconducting and Electronic Materials. These were the focus of magnetization- and magnetic-shielding studies. Depending on the thickness of the Fe sheath, the superconducting core could be completely shielded at low fields (e.g. below 2 kOe). Taking this into account, the magnetic Jc is extracted, and found to be comparable to the extrapolated transport value. [Copyright &y& Elsevier]

Vibrating sample magnetization (VSM) measurements have been made on round Fe-clad monocore MgB2 wire fabricated by a powder-in-Fe-tube technique. Magnetization, AC loss, and transport measurements are reported. The M–H and loss measurements were made with a field sweep amplitude of 17 kOe over a temperature range of 4–40 K. In a round monocore wire the superconducting core was completely shielded against an external field of ±2 kOe; at higher fields a fixed ΔH was observed (partial shielding). It was found that the signal picked up by the VSM was itself also partially shielded, i.e. reduced in magnitude during its ‘return passage’ through the sheath, leading to a low field dip in the magnetization. FEM calculations of this latter effect are presented. The fact that the field which reaches the inside of the conductor is less than the external field leads to a real loss suppression, even at moderate fields. The low field dip caused by a suppression of the signal emanating from the superconducting core leads to an apparent loss suppression, i.e., the magnetically measured loss is less than the real (e.g., calorimetric) loss. The difference between real and apparent loss is estimated by FEM calculation, and the field sweep amplitude dependence of these two components was computed neglecting the effects of partial flux penetration. [Copyright &y& Elsevier]