Enhancement of critical current density and flux pinning force of MgB2 by Bi2212 addition

Abstract

As far as current interest is concerned, the pinning centers of non-ideal type II superconductors are mainly structural defects that apply attractive potential wells to the motion of magnetic flux lines, including non-superconducting second phase particles, grain boundaries, etc. In this paper, high-temperature superconductor Bi2212 particles are introduced into MgB₂ to explores the behavior of superconducting pinning centers played by superconducting particles that apply repulsive potential barriers to the motion of magnetic flux lines. By mixing ground Bi2212 single crystal with Mg and B powders, (MgB₂)_1-x(Bi2212)_x (x=0, 3, 5, 7, and 10 wt%) composites were prepared and characterized with XRD, SEM, TEM, and physical property measurements. The irreversible magnetic field H_irr, critical current density J_c, and flux pinning force F_p were enhanced at x = 3 wt %. The best in-field-J_c@( 4.2 K, 6 T) reaches 8.1×10³ A/cm², which improved the performance by 55.8% compared to 5.2×10³ A/cm² of pure MgB₂ sample; and the J_c@(20 K,2 T) reaches 8.0×10⁴ A/cm², which is 1.60 times of the 5.0×10⁴ A/cm² for the pure MgB₂ sample under the same conditions. Although the addition of Bi2212 enhances the pinning strength of MgB₂, it has no qualitative effect on the flux pinning characteristics, i.e., the flux pinning behavior is still dominated by grain boundary pinning mechanism. It is argued that in this study, Bi2212 particles are likely to exist in the form of nanoparticles in the MgB₂ matrix, which contribute to the collective pinning as weak pinning centers.