WB5-4

Effect of boron precursor ultra-sonicated in hexane medium on the critical current density of sintered bulk MgB2
*Sai Srikanth Arvapalli1, Milos Jirsa2, Masato Murakami1, Muralidhar Miryala1

Improving critical current density (Jc) 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 Jc. 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. Tc was close to 39 K, with a sharp transition. The highest Jc 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

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