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.
[Show abstract][Hide abstract] ABSTRACT: A review of current developments in the study of chemical doping effect on the superconducting properties of MgB<sub>2</sub> wires and tapes is presented, based on the known literature data and our own results. The critical current density of MgB<sub>2</sub> can be improved through various kinds of dopants. Among these dopants, doping with carbon-containing materials seems to be the most effective way to improve the J<sub>c</sub> performance. The doping effect of carbon in different forms and carbon-based compounds such as SiC, nano-C, metal carbides, as well as aromatic hydrocarbon and carbohydrate on the J<sub>c</sub>-B characteristics of MgB<sub>2</sub> was discussed in detail. The C can be incorporated into the MgB<sub>2</sub> crystal lattice by replacing boron, and thus Bc<sub>2</sub> is significantly enhanced due to selective tuning of impurity scattering of the ?? and ?? bands in the two-band MgB<sub>2</sub>. Besides the efforts of increasing Bc<sub>2</sub> by carbon doping, the fine grain size and nano-size inclusions caused by doping would create many flux pinning centers improving the J<sub>c</sub>-B property of MgB<sub>2</sub>. Based on these considerations, we suggested some principles for the selection of dopants.
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