Significant enhancement of the critical current density for cubic BN addition into<i>ex situ</i>spark plasma sintered MgB₂

Abstract

High density (above 93%) superconducting bulks of MgB2 with addition of hexagonal BN (h-BN) and cubic BN (c-BN) with compositions ((MgB2) + (BN) x , x = 0.01, 0.03, 0.05) were obtained by ex situ spark plasma sintering. All the investigated samples have the critical temperature Tc = 38.8 K. The variation of the critical current density Jc with the external magnetic field H for h-BN added sample is almost overlapping the Jc(H) dependence for the pristine MgB2 sample. On the other hand, Jc for the samples added with c-BN is larger at high magnetic fields, while the decrease of Jc at low H is very small. At T = 20 K, a Jc of 102 A cm−2 is determined for the sample with xc-BN = 0.005 at H = 58 kOe, and for the sample with xc-BN = 0.01 at 54 kOe. Magnetic relaxation measurements indicate a significant flux pinning enhancement in MgB2 samples added with c-BN. It is proposed that the disorder at the interface caused by the convenient lattice matching relationship between the lateral plane of the MgB2 crystal prism and the face of the c-BN crystal cube is responsible for the observed vortex pinning increase.