Evaluation of persistent-mode operation in a superconducting MgB2 coil in solid nitrogen

We report the fabrication of a magnesium diboride (MgB2) coil and evaluate its persistent-mode operation in a system cooled by a cryocooler with solid nitrogen (SN2) as a cooling medium. The main purpose of SN2 was to increase enthalpy of the cold mass. For this work, an in situ processed carbon-doped MgB2 wire was used. The coil was wound on a stainless steel former in a single layer (22 turns), with an inner diameter of 109 mm and height of 20 mm without any insulation. The two ends of the coil were then joined to make a persistent-current switch to obtain the persistent-current mode. After a heat treatment, the whole coil was installed in the SN2 chamber. During operation, the resultant total circuit resistance was estimated to be \u3c7.4x10−14 Ω at 19.5 K±1.5 K, which meets the technical requirement for magnetic resonance imaging application

Superconducting properties of MgB2 wire using ball milled low purity boron

MgB2/Fe wire samples were prepared by using ball-milled 96% boron (B) powder with strong semi-crystalline phase. 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 transport critical current density (Jct) due to small grain size and better reactivity. However, the inter-grain connectivity became worse, which could lead to poor Jct in low field and an increased level of disorder

Increased superconductivity for CNT doped MgB2 sintered in 5T pulsed magnetic field

The superconductivity of carbon nanotube (CNT) doped MgB2 sintered in pulsed magnetic field (PMF) was investigated with Raman scattering measurements and Raman spectral fit analysis. Although the carbon (C) substitution for the boron (B) is increased for the sample sintered in PMF, its superconductivity is advanced comparing with the sample sintered without PMF. The improved critical transition temperature, Tc, is attributed to the strengthening of the electron-phonon coupling (EPC) in MgB2, as reflected by the broadened E 2g mode in the Raman spectra, because the carbon atoms are homogeneously distributed in the boron planes

Influence of annealing temperature and isostatic pressure on microstructure and superconducting properties of isotopic Mg\u3csup\u3e11\u3c/sup\u3eB2 wires fabricated by internal Mg diffusion method

Here, we report superconducting Mg11B2 wires made by using the internal Mg diffusion technique with isotopic amorphous nano boron (11B) as the precursor material. We show the influence of annealing temperature and isostatic pressure of 0.1 MPa and 1.1 GPa on Mg diffusion into 11B layer, microstructure of superconducting filament, critical current density (Jc) at 20 K and 25 K, critical temperature (Tc) and irreversible magnetic induction (Birr) in mono (single-core) - and multi-filament Mg11B2 wires. Our research shows that thermal treatment at 700 °C and 0.1 MPa for 60 min yields a superconducting phase with low Tc, Birr and Jc in single-core Mg11B2 wire. A higher annealing temperature (740 °C and 0.1 MPa for 60 min) significantly accelerates the diffusion of Mg into the 11B layer and increases the Tc, Birr and Jc. However, the distribution of Mg in 11B layer is very heterogeneous (places with high and low Mg concentration). This leads to heterogeneity in the superconducting material and inhomogeneous Tc, Birr and Jc. Further studies showed that higher annealing temperatures at 770 °C and 800 °C significantly accelerates the diffusion of Mg into the 11B layer and leads to the increase of Tc, Birr and Jc in single-core Mg11B2 wire. In single-core wire annealing treatment under isostatic pressure of 1.1 GPa for 60 min at temperature of 800 °C it causes Mg penetrates into the 11B layer very heterogeneously and large amount of 11B remains unreacted. However, the same heat treatment at 800 °C and 1.1 GPa for 60 min in multi-filament wires leads to a complete Mg synthesis reaction with the 11B layer. This leads to increase Birr and Jc. This work shows that the formation and accumulation of pinning centers is essential to further increase Jc in Mg11B2 wires

The new resistance jump: the detection of damage in Nb barrier in MgB2 wires

We present a new method for detecting damage to Nb barriers in MgB2 wires by using a four-contact probe. The transport measurements and scanning electron microscope images indicate that a newly identified jump in resistance means that there is damage to the Nb barrier. Damage detection is important for application because it allows us to avoid reactions between the filament and the sheath material, and to develop implementation methods for MgB2 with high critical current density. Our methods for damage detection proposed in this paper are simple, fast, and easy to use

Increased superconductivity for CNT doped MgB2 sintered in 5T pulsed magnetic field

The superconductivity of carbon nanotube (CNT) doped MgB2 sintered in pulsed magnetic field (PMF) was investigated with Raman scattering measurements and Raman spectral fit analysis. Although the carbon (C) substitution for the boron (B) is increased for the sample sintered in PMF, its superconductivity is advanced comparing with the sample sintered without PMF. The improved critical transition temperature, Tc, is attributed to the strengthening of the electron-phonon coupling (EPC) in MgB2, as reflected by the broadened E2g mode in the Raman spectra, because the carbon atoms are homogeneously distributed in the boron planes