Temperature–pressure induced nano-structural inhomogenities for vortex pinning in bulk MgB2 of different connectivity

Abstract Higher critical current densities, j c , (up to 1.6–0.15 MA/cm 2 at 10–35 K) at low magnetic fields can be attained in MgB 2 -based materials, if a high manufacturing temperature (1050 °C) is used, while low temperatures (600–800 °C) usually lead to higher critical currents in high magnetic fields (10–4 kA/cm 2 in 6–10 T at 10 K). This tendency was observed for MgB 2 -based materials having 55–99% density and 17–98% connectivity, which were prepared by different methods from different precursors in a wide range of pressure (0.1 MPa–2 GPa). The variation of the manufacturing temperature led to a redistribution of the magnesium, boron, and impurity oxygen. At 2 GPa, its increase results in the segregation of the oxygen in MgB 2 and the transformation of 15–20 nm thick layers of MgB 0.6–0.8 O 0.8–0.9 into separate MgB 0.9–3.5 O 1.6–2 grains and to a reduction of the size of MgB 11–13 O 0.2–0.3 inclusions located in the MgB 2 (MgB 2.2–1.7 O 0.4–0.6 ) matrix. The size reduction of B-enriched inclusions and the localization of O in MgB 2 seem to be the reason for the increase of j c in low fields and for the shift from grain boundary to point pinning of vortices witnessed by an increase of the k -ratio.