Flux pinning and microstructure of a bulk MgB₂ doped with diverse additives

In bulk high-temperature superconductors, Ag is usually used as an additive thus improving mechanical performance. In MgB₂ doped with Ag, the Ag reacts with Mg, forming Mg–Ag phases acting as a vortex pinning medium. In this work, we analyze the electromagnetic and pinning properties of bulk MgB₂ doped with 1 wt% MgB_4, 4 wt% Ag, and 1 wt% Dy₂O₃, prepared at the Shibaura Institute of Technology (SIT), Tokyo. In three compounds of Mg_xB₂ + 4 wt% Ag with x = 1, 1.075, and 1.1, the effect of Mg excess was studied. The magnetic moment was measured by a vibrating sample magnetometer (VSM). Pinning was studied in terms of a pinning diagram, i.e. the field dependence of the normalized pinning force density, F _n = F/F _max. In all studied samples, the peak of the F _n (b) dependence (b= B/B _irr, B _irr being the irreversibility field) was observed at around b= 0.2, indicating a prevailing flux pinning at grain boundaries. A slight shift of the peak with decreasing temperature indicated a defect size distribution in the pinning landscape. Transmission electron microscopy (TEM) showed a granular structure of all samples composed of MgB₂ grains of about 230 nm (average size), with ensembles of small grains (22 nm in average) of Ag₃Mg, AgMg, Ag, Dy₂O₃, and MgB₄. While the large MgB₂ grains control the main pinning mechanism, the small precipitates seem to determine details of the current flow through the grain boundaries. (paper)