We investigated the influence of fullerene (C 60 ) addition on the superconducting parameters of MgB 2 bulk samples obtained by spark plasma sintering (SPS). It was found that 1.5 wt% C 60 addition leads to the reduction of the critical temperature by a few K, whereas the magnetically determined critical current density increases by approximately one order of magnitude in an applied magnetic field H = 70 kOe at temperature T = 10 K. This indicates substitution of B by C in C 60 added MgB 2 processed by SPS, contrary to some data reported in the literature obtained using conventional methods. For our relatively large specimens, the occurrence of macroscale flux jumps was observed over a wide (H, T ) domain. The relaxation of the irreversible magnetization suggests that the macroscale flux jumps transform into microscale flux jumps with increasing H and T .
High density bulks (97%–99%) of MgB2 were prepared by spark plasma sintering (SPS) in nitrogen (N2) atmosphere for different heating rates (10, 20 and 100 °C min−1) and compared with reference samples processed in vacuum and Ar. N2 reacts with MgB2 and forms MgB9N along the MgB2 grain boundaries. The high-field critical current density is enhanced for the sample processed in N2 with a heating rate of 100 °C min−1. At 2–35 K, this sample shows the strongest contribution of the grain boundary pinning (GBP). All samples are in the point pinning (PP) limit and by increasing temperature the GBP contribution decreases.
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