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.

Critical current density, J _c, in superconductors is strongly connected with size of defects in the material. Frequently, the smaller defects, the higher J _c. In this work, we tried to reduce the size of cheap commercial boron precursor powder using high energy ultra-sonication in ethanol media. The resulting powder was then utilized in synthesizing bulk MgB₂ via sintering at 775 °C. Effect of boron powder ultra-sonication on superconducting properties of the bulk MgB₂ was studied and discussed. SEM of ultra-sonicated boron showed fine particles with sharp edges (high-energy surfaces), irregular shapes and clustering of fine particles occurred for longer ultra-sonication durations. XRD proved a high quality of MgB₂ with only small traces of MgO. Around 36% improvement in J_c at 20 K and T_c close to 39 K were observed in MgB₂ bulk prepared with boron ultra-sonicated for 15 min. Microstructure studies showed numerous nanometre sized MgB₂ grains in the bulk. Other bulks (made of boron ultra-sonicated longer, for 30, 45, and 60 min) have larger grains. It resulted in slightly lower J _c, anyway, still by 22% higher than in reference bulk. The present results demonstrate that the high performance bulk MgB₂ can be achieved without reduction in T _c via employing a cheap boron, reduced in size by high-energy ultra-sonication.

The improved performance of bulk MgB₂ material with added nanometer‐sized MgB₄ particles is presented. Bulk polycrystalline MgB₂ samples with varying amount of MgB₄ x (x = 0, 1, 2, 3, 4, 5, and 10 wt%) are fabricated by solid‐state sintering at 775 °C for 3 h in pure argon gas. Microstructural studies indicate formation of nanometer‐sized grains when MgB₄ is added. Density of nano‐grains is inversely proportional to the MgB₄ content. The MgB₂ sample with 1 wt% of MgB₄ shows the best performance, with its self‐field critical current density reaching 385 and 315 kAcm⁻² at 15 and 20 K, respectively. Flux pinning diagrams reveal the domination of grain boundary pinning mechanism.

In bulk high-temperature superconductors, Ag is usually used as an additive thus improving mechanical performance. In MgB₂ doped with Ag, the Ag reacts with Mg, forming Mg–Ag phases acting as a vortex pinning medium. In this work, we analyze the electromagnetic and pinning properties of bulk MgB₂ doped with 1 wt% MgB_4, 4 wt% Ag, and 1 wt% Dy₂O₃, prepared at the Shibaura Institute of Technology (SIT), Tokyo. In three compounds of Mg_xB₂ + 4 wt% Ag with x = 1, 1.075, and 1.1, the effect of Mg excess was studied. The magnetic moment was measured by a vibrating sample magnetometer (VSM). Pinning was studied in terms of a pinning diagram, i.e. the field dependence of the normalized pinning force density, F _n = F/F _max. In all studied samples, the peak of the F _n (b) dependence (b= B/B _irr, B _irr being the irreversibility field) was observed at around b= 0.2, indicating a prevailing flux pinning at grain boundaries. A slight shift of the peak with decreasing temperature indicated a defect size distribution in the pinning landscape. Transmission electron microscopy (TEM) showed a granular structure of all samples composed of MgB₂ grains of about 230 nm (average size), with ensembles of small grains (22 nm in average) of Ag₃Mg, AgMg, Ag, Dy₂O₃, and MgB₄. While the large MgB₂ grains control the main pinning mechanism, the small precipitates seem to determine details of the current flow through the grain boundaries.