Fabrication of C Doped Wire Using a Mixture of In-situ and Ex-situ Powders
ABSTRACT MgB2 wires were fabricated using a mixture of doped in-situ and ex-situ powders. The effects of the ratio of ex-situ to in-situ powders on the microstructure and critical properties were evaluated. The wires were processed using a powder-in-tube technique with a series of (MgB2)x + (Mg + 1.99B + 0.01C)1-x, where x = 0, 0.3, 0.5, 0.7, and 1.0. The critical current density (Jc) was measured using both magnetization (5 and 20 K, 1-6.5 T) and transport methods (4.2 K, 4-10 T). Microstructural observations revealed a decrease in the number of Kirkendall voids and an increase in core density, while cracks grew along the longitudinal direction of the wire with increasing x. In addition, the Mg from the in-situ powder acted as a sintering aid, healing cracks and enhancing the connection between the in-situ and ex-situ regions. At x = 0.5 and 1.0, the sample had the highest and lowest Tc and Jc, respectively. On the other hand, the other samples produced using the mixed powders had moderate critical properties, probably due to the mixed morphology of the in-situ and ex-situ microstructure.
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ABSTRACT: Polycrystalline MgB2 samples with GaN additions were prepared by reaction of Mg, B, and GaN powders. The presence of Ga leads to a low melting eutectic phase which allowed liquid phase sintering and produces plate-like grains. For low-level GaN additions (5% at. % or less), the critical transition temperature, Tc, remained unchanged and in 1T magnetic field, the critical current density, Jc was enhanced by a factor of 2 and 10, for temperatures of \~5K and 20K, respectively. The values obtained are approaching those of hot isostatically pressed samples. Comment: 12 pages, 1 table, 4 figures, accepted in Applied Physics LettersApplied Physics Letters 04/2005; · 3.79 Impact Factor
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ABSTRACT: This review paper illustrates the main normal and superconducting state properties of magnesium diboride, a material known since the early 1950s but only recently discovered to be superconductive at a remarkably high critical temperature Tc = 40 K for a binary compound. What makes MgB2 so special? Its high Tc, simple crystal structure, large coherence lengths, high critical current densities and fields, and transparency of grain boundaries to current promise that MgB2 will be a good material for both large-scale applications and electronic devices. During the last seven months, MgB2 has been fabricated in various forms: bulk, single crystals, thin films, tapes and wires. The largest critical current densities, greater than 10 MA cm-2, and critical fields, 40 T, are achieved for thin films. The anisotropy ratio inferred from upper critical field measurements is yet to be resolved as a wide range of values have been reported, gamma = 1.2-9. Also, there is no consensus on the existence of a single anisotropic or double energy gap. One central issue is whether or not MgB2 represents a new class of superconductors, which is the tip of an iceberg awaiting to be discovered. To date MgB2 holds the record for the highest Tc among simple binary compounds. However, the discovery of superconductivity in MgB2 revived the interest in non-oxides and initiated a search for superconductivity in related materialss; several compounds have since been announced to be superconductive: TaB2, BeB2.75, C-S composites, and the elemental B under pressure.Superconductor Science and Technology 01/2001; 14(11). · 2.76 Impact Factor
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ABSTRACT: A series bulk samples of (MgB2)x(Mg+2B)1−x (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared by the single step reaction (x=0) and the two-step reaction (x≠0) in Ar atmosphere. The compositions, porosity and microstructure in MgB2 samples were investigated by using X-ray diffraction (XRD) and scanning electronic microscope (SEM). XRD analysis indicates that the MgB2 is easy oxidized during heating. SEM observations show that if comparing with the sample by single step reaction the porosity of sample by two-step reaction decreases clearly due to the effective decrease of Mg content in precursors. The grain connection of sample by two-step reaction is worse because the recrystallization behavior of pre-reacted MgB2 powders in precursors is not sufficient under our sintering processing. The voluminal expansion rates of bulks decrease when x varies from 0 to 1. We propose that for the sample of x=0 the increase in volume is resulted from the formation of MgB2 phase and a possible Kirkendall effect during Mg and B converting into MgB2, and the decrease in volume of sample of x=1.0 is attributed to the sintering shrinkage effect of the pre-reacted MgB2 powders. These evidences indicate that the two-step reaction is a good choice for improving the density and decreasing the voluminal expansion rate of MgB2 sample.Physica C Superconductivity 01/2007; 466(1):96-100. · 0.72 Impact Factor