Improving critical current density (J_c) of bulk MgB₂ has been a popular agenda in the past 2 decades. Despite many efforts, we have not yet reached the limits of the bulk MgB₂ capability. The large coherence length of MgB₂ material allows large defects such as grain boundaries to act as strong pinning centers, while in most high-temperature superconductors grain boundaries are weak links obstructing the passage of critical currents. Considering this, we synthesized MgB₂ bulk with commercial nano amorphous boron precursor, which then resulted in a nano-sized grain structure in the final bulk that showed a tremendous J_c. However, due to the high cost of commercial nano precursors, we devised a novel ultra-sonication technique to refine a cheap rough boron precursor and avoid the use of the expensive commercial nano amorphous boron powder. Recently, we showed that this novel low-cost technique (with ethanol medium) has helped to improve the critical current density of MgB₂. However, the ultra-sonication is in an embryonic stage and must be optimized further in terms of power, frequency, ultrasonication medium, etc. Here, we report on the influence of hexane medium ultra-sonicated boron on the superconducting properties of MgB₂ bulks prepared using this new precursor. Cheap commercial boron was ultrasonicated in hexane for 15, 30, 45, and 60 minutes and used in MgB₂ fabrication. XRD showed major peaks corresponding to MgB₂, with small fractions of MgO and unreacted Mg. T_c was close to 39 K, with a sharp transition. The highest J_c was found in the sample with boron ultra-sonicated for 30 minutes as high as 375, 328, and 270 kA/cm² at 10, 15, and 20 K, respectively. scanning electron microscopy (SEM) analysis revealed tiny MgB₂ particles (see Fig. 1) and layers randomly dispersed in the microstructure, while energy dispersive X-ray (EDX) analysis indicated that these particles were secondary phases of Mg, B, and O. Flux pinning diagrams showed the peak of the normalized pinning force located close to b=0.2, depicting dominance of grain boundary pinning mechanism. Based on the comparison of superconducting properties with previous results, we speculate that the viscosity of the ultrasonication medium might have a crucial role in the refinement of boron precursor powder.

Keywords: Boron Ultrasonication, Bulk MgB2, Hexane medium, Critical current density