In this comprehensive study, we enhanced the superconducting properties of bulk MgB2 using in-situ techniques through three key approaches: boron processing, silver addition, and advanced spark plasma sintering (SPS) techniques. Both boron processing and silver addition were executed using solid-state sintering, while advanced sintering methods were later employed to further optimize the material. Initially, our efforts were concentrated on sieving crystalline boron following ball milling, which reduced powder size and improved uniformity. This process led to enhanced inter-grain connectivity and microstructural homogeneity, as confirmed by scanning electron microscope (SEM) analysis and contributed to the high purity of the MgB2 phase, as verified by x-ray diffraction (XRD). As a result, the critical current density (Jc) increased by 116% at 20 K, reaching 156 kA/cm2. In the subsequent phase, silver (Ag) was added into the boron precursor. By optimizing the silver content, we enhance Jc by creating additional pinning centers within the Ag-Mg phase [1], as confirmed by XRD results. Incorporating 4.5 wt.% Ag led to the highest Jc values reported for MgB2, achieving 227 kA/cm2 at 20 K and up to 379 kA/cm2 at 7 K under self-field conditions. Finally, we applied spark plasma sintering (SPS) to densify MgB2 samples. This technique allowed us to achieve 95% of the theoretical density [2], resulting in Jc values of 407 kA/cm2 at 10 K and 261 kA/cm2 at 20 K (see Fig.1). These improvements represent a 67% improvement over traditional solid-state sintering of undoped MgB2, demonstrating the significant impact of combining boron processing with SPS in enhancing the superconducting properties of bulk MgB2.
[1] M. Muralidhar, K. Inoue, M. R. Koblischka, A. Murakami, M. Murakami, Effects of silver addition on critical current densities and mechanical properties in bulk MgB2, Adv. Eng. Mater. 17 (2015) 831-838.
[2] Y. Xing, P. Bernstein, M. Miryala, J. G. Noudem, High critical current density of nanostructured MgB2 bulk superconductor densified by spark plasma sintering, Nanomaterials. 12(15) (2022) 2583.
Malik Shadab, one of the authors, acknowledges SIT for its financial support during his doctoral program.
Figure 1. Comparison of Jc at 20 K for bulk MgB2 using normal boron precursor versus processed boron, silver addition, and advanced sintering methods.
Keywords: Bulk MgB2, Ball milling, Sieving, Ag addition, Microstructural analysis, Critical current density (Jc)