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[en] A 10 H superconducting MgB2 coil, suitable for an induction heating application, has been wound, cooled and initially tested. Eight kilometres of MgB2 tape were first insulated and then wound into double pancake coils before being assembled into the complete coil. The process is described in detail, focusing on hands-on experience gained, especially regarding the different steps in the wet-winding technique used. The insulation, soldering, winding and cooling techniques applied proved practical for manufacturing of the large MgB2 coil. The complete coil was cooled by conduction cooling to 8 K and tested with a current of 185 A.
[en] Current transfer length (CTL) and current sharing with the normal paths for a metal sheathed MgB2 superconductor are studied by analytical and numerical methods. The study was aimed at wire geometries with short sample lengths. The computations show low CTL values for Fe sheathed MgB2 and significantly higher CTL for Cu sheathed MgB2. For short length samples a considerable part of the applied current is not transferred into the superconducting layer and flows through the sheath in Cu sheathed conductors. Close to the current contacts the heat generated due to current sharing with normal paths is significantly higher for the Fe sheath than for the Cu sheath.
[en] We report results of a dc magnetic and ac linear low-frequency study of a polycrystalline MgB2 sample. AC susceptibility measurements at low frequencies, performed under dc fields parallel to the sample surface, provide clear evidence for surface superconducting states in a polycrystalline MgB2
[en] We report accurate measurements of the resistive transition in two different polycrystalline samples of superconducting MgB2. The results are described in terms of the temperature derivative of the resistivity, d ρ/d T. A method is developed to analyse these data. A structure consisting of two closely separated maxima is observed in d ρ/d T near the transition. This splitting is suppressed by applying magnetic fields above 10 kOe parallel to the current orientation. We discuss the possible mechanisms underlying the split resistive transition observed in our samples
[en] In this paper we present some experimental results concerning the current noise produced during the resistive transition in MgB2 thin films. Preliminary investigations evidenced the presence of electrical noise whose power spectrum has a region of the 1/fn type with n ∼ 3. We suggest that the noise may originate from abrupt rearrangement of the current distribution inside the specimen during the percolative process of a diphasic system. Experimental measurements of the spectral components of the current noise taken during the resistive transition will be given and discussed
[en] The novel superconductor MgB2 (TC = 39 K) is available as nanowires. Their synthesis has been achieved by a two-step protocol: Slightly Si-doped boron nanowires were generated by transport reaction, and treated with Mg vapor to form polycrystalline nanowires of MgB2 with diameters of 50-400 nm and lengths of several tens of micrometers. Superconductivity was proven by a strong Meissner effect at 33 K. (orig.)
[en] By controlling growing process, we have succeeded in making MgB2 single crystals with different electronic properties. In the crystals with smaller c/a, larger residual resistivity ratio, higher Tc, and higher anisotropy in Hc2 are observed. In superconducting state, a difference of anisotropy is attributable to Hc2(parallel c), suggesting a considerable change beyond the disorder effect due to lattice strain. We discuss possible origins for the changes
[en] The newly discovered BCS-like superconductor MgB2 has rapidly attracted the interest of the Raman community. Many contributions explored the E2g phonon mode of MgB2 with a predicted strong coupling to the electronic conduction σ-bands. The published Raman spectra of MgB2 samples both polycrystalline and small single crystals contain mainly the same structures: broad bands extending from 300 to 1600 cm-1. The main peak at ∼600 cm-1 is generally associated with the sought E2g mode, but its unusually broad width (∼270 cm-1), as well as the presence of other broad peaks in the spectra make the assignment difficult. On the other hand, TEM, X-ray photoemission and diffraction studies indicated the presence of impurity phases such as elemental Mg, MgO, B2O3, Mg(OH)2 . MgCO3 and MgB4. We investigated these impurity phases and present an analysis of the second-order features in the spectra of MgB2. (orig.)
[en] Full text: The pinning behaviour of bulk MgB2 superconductors is peculiar in many respects. Pinning seems to be stronger than in classical high TC materials and there seems to be no weak link problem in these compounds, giving hope to produce bulk samples and wires with current densities appropriate for technical applications. But, although many studies concerning the pinning behaviour in this compound appeared in recent years, the results are still contradictory. In the present work we present results of an investigation of the pinning behaviour by magnetic relaxation measurements of three MgB2 samples: a pure one, a sample with 8 at% Al substitution and a sample with 10 wt% of SiC admixture. A comparison of different analyses methods is given. (author)
[en] MgB2 superconducting sheets have been fabricated by powder rolling method using mixture of Mg and B powders. Sheet-type MgB2 bulk samples of ∼10 mm width and 50-100 mm long were squeezed out after compacted by two rotating rolls of 130 mm diameter with gap distance of 0.5 mm and speed of ∼40 cm/min (∼1 rpm). The nominal composition of Mg, which is ductile metal, was added up to 30% to facilitate forming the MgB2 sheets. The annealed samples at 900 degree C and 3 hrs showed superconducting transition temperature of ∼32 K and critical current densities at zero fields were ∼105 A/cm2 at 5 K and ∼5 x 104 A/cm2 at 20 K.