Cost-effective nano-boron engineering: Tap water ultrasonication, sedimentation, and Ag addition for high-performance bulk MgB2 superconductors

This study presents a cost-effective and environmentally friendly approach for producing nano-sized boron using tap water as the ultrasonication medium, replacing conventional organic solvents such as ethanol and propanol. Furthermore, it is the first report to demonstrate the use of a sedimentation technique for the post-ultrasonication refinement of boron particles. The effects of ultrasonication power and duration on particle size were systematically investigated. Increasing the ultrasonication power progressively reduced the boron particle size, with the optimum condition identified at 100% power for 2 h, decreasing the average particle size from 508 nm to 168 nm. Further ultrasonication beyond 2 h resulted in particle coarsening, indicating an optimum processing window. The refined boron significantly enhanced the superconducting performance of MgB 2 bulk, increasing the critical current density ( J c ) from 268 to 379 kA/cm 2 at 20 K under self-field conditions. However, the use of ultrasonicated boron reduced the bending strength of MgB 2 due to increased porosity. To overcome this limitation, Ag was incorporated as a reinforcing additive, successfully improving the mechanical strength while preserving the superconducting properties. Additional refinement of the boron precursor through sedimentation selectively removed coarse particles, further reducing the average particle size from 168 to 128 nm and improving particle size uniformity. Consequently, the J c increased to 461 and 657 kA/cm 2 at 20 K and 10 K, respectively, under self-field conditions. These findings demonstrate an economical, scalable, and sustainable strategy for producing high-quality nano-boron from commercially available precursors, providing a practical route to simultaneously enhance the superconducting and mechanical performance of MgB 2 for advanced cryomagnetic applications.