In this study, we report on a further improvement of the critical current density of the sintered bulk MgB ₂ material utilizing the optimized sintering temperature combined with a varying content of carbon in carbon-encapsulated boron. The MgB ₂ bulk was prepared from high-purity commercial powder of Mg metal and a carbon-encapsulated boron with 0 wt.%, 2.8 wt.%, 4.5 wt.%, and 7.3 wt.% of carbon, using a single-step solid-state reaction at 805°C for 3 h in pure argon atmosphere. The magnetization measurements confirmed a sharp superconducting transition with onset T _c at around 38.5 K, decreasing with increasing carbon content. For 7.3 wt.% of carbon, the bulk MgB ₂ reached the superconducting transition at around 33 K. Scanning electron microscopy of the fractured bulk MgB ₂ cross section showed a dispersion of 100-200 nm large grains. Due to the carbon doping and optimized processing, the critical current density (J _c ) in bulk MgB ₂ samples with the carbon-coated boron was improved both in low and high magnetic fields. The highest J _c values at 20 K, of 375 and 220 kA/cm ² , in the self-field and 1 T, respectively, were achieved in the MgB ₂ sample with 2.8 wt.% of carbon in the carbon-encapsulated boron. The present results clearly demonstrate that the optimized sintering temperature combined with the appropriate amount of carbon in carbon-coated boron is able to improve the entire J _c performance of the bulk MgB ₂ material.

Although the fascinating effect of superconductivity was discovered more than 100 years ago, only a few low-temperature superconductors, especially Nb₃Sn and NbTi, have reached everyday-life use. After the discovery of superconductivity in MgB₂ in 2001, a boom appeared in interest in employing MgB₂ in commercial applications, facilitated by simple crystallographic structure, high critical temperature, low-cost raw ingredients, simple and fast production, and other advantages for moderate temperature applications. It only lacks high flux pinning and good mechanical performance. In this review, the implications of manufacturing procedures on the superconducting properties, crystal and electronic structure of MgB₂ are discussed. The advantages and drawbacks of various manufacturing processes will be presented, as well as the ways how to improve MgB₂ superconducting performance. The impact of oxygen reduction, elements doping, the introduction of defects and additives, and the effect of precursor refinement on superconducting properties are here compiled to give an overview of the progress of flux pinning enhancement in bulk MgB₂.

The authors design and develop a prototype DC superconducting cable for railways. For the preliminary evaluation 2 m Bi-2223 DC cable is used. The maximum current flowing in the cable cooled by liquid nitrogen was 1720 A in the inner sheet wire and 2430 A in the outer sheet. The experimental results and simulations led to the conclusion important for practical applications that the leakage of magnetic field was negligible. This result is of crucial importance, in particular for use in railway systems.