Pinning and trapped field in MgB2- and MT-YBaCuO bulk superconductors manufactured under pressure

The relevant pinning centers of Abrikosov vortices in MgB$_{2}$–based materials are oxygen-enriched Mg-B-O inclusions or nanolayers and inclusions of MgB$_{x}$ (x>4) phases. The high critical current densities, j$_{c}$, of 10$^{6}$ and 10$^{3}$A/cm$^{2}$ at 1 and 8.5 T, respectively, at 20 K can be achieved in polycrystalline materials (prepared at 2 GPa) containing a large amount of admixed oxygen. Besides, oxygen can be incorporated into the MgB$_{2}$ structure in small amounts (MgB$_{1.5}$O$_{0.5}$), which is supported by Auger studies and calculations of the DOS and the binding energy. The j$_{c}$ of melt textured YBa$_{2}$Cu$_{3}$O$_{7-δ }$ (or Y123)-based superconductors (MT-YBaCuO) depends not only on the perfectness of texture and the amount of oxygen in the Y123 structure, but also on the density of twins and micro-cracks formed during the oxygenation (due to shrinking of the c-lattice parameter). The density of twins and microcracks increases with the reduction of the distance between Y$_{2}$BaCuO$_{5}$ (Y211) inclusions in Y123. At 77 K j$_{c}$=8⋅10$^{4}$ A/cm$^{2}$ in self-field and j$_{c}$=10$^{3}$ A/cm$^{2}$ at 10 Т were found in materials oxygenated at 16 MPa for 3 days with a density of twins of 22–35 per μm (thickness of the lamellae: 45-30 nm) and a density of micro-cracks of 200–280 per mm. ... mehrPinning can occur at the points of intersection between the Y123 twin planes and the Y211 inclusions. MTYBaCuO at 77 K can trap 1.4 T (38×38×17 mm, oxygenated at 0.1 MPa for 20 days) and 0.8 T (16 mm in diameter and 10 mm thick with 0.45 mm holes oxygenated at 10 MPa for 53 h). The sensitivity of MgB$_{2}$ to magnetic field variations (flux jumps) complicates estimates of the trapped field. At 20 K 1.8 T was found for a block of 30 mm in diameter and a thickness of 7.5 mm and 1.5 T (if the magnetic field was increased at a rate of 0.1 T) for a ring with dimensions 24×18 mm and a thickness of 8 mm.