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

Higher critical current densities, j_c, (up to 1.6-0.15 MA/cm² 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/cm² in 6-10 T at 10 K). This tendency was observed for MgB₂-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 MgB₂ and the transformation of 15-20 nm thick layers of MgB_0.6-0.8O_0.8-0.9 into separate MgB _0.9-3.5O_1.6-2 grains and to a reduction of the size of MgB_11-13O_0.2-0.3 inclusions located in the MgB₂ (MgB_2.2-1.7O_0.4-0.6) matrix. The size reduction of B-enriched inclusions and the localization of O in MgB₂ 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.