Pinning in $\hbox {MgB}_{2}$ - and YBaCuO-Based Superconductors: Effect of Manufacturing Pressure and Temperature

Bulk MgB₂- and YBaCuO-based materials are competitive candidates for applications. The properties of both compounds can be significantly improved by high temperature-high pressure preparation methods. The transformation of grain boundary pinning to point pinning in MgB₂-based materials with increasing manufacturing temperature from 800 to 1050^°C under pressures from 0.1 MPa to 2 GPa correlates well with an increase in critical current density in low and intermediate magnetic fields and with the redistribution of boron and oxygen in the material structure. As the manufacturing temperature increases (to 2 GPa), the discontinuous oxygen-enriched layers transform into distinct Mg-B-O inclusions, and the size and amount of inclusions of higher borides MgB_X (X>;2) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC addition. The oxygenation of melt-textured YBa₂Cu₃O_{7 - δ} (MT-YBaCuO) under oxygen pressure (16 MPa) allows one to increase the oxygenation temperature from 440°C to 700-800°C, which leads to an increase of the twin density in the Y123 matrix and to a decrease of dislocations, stacking faults, and the density of microcracks, and as a result, to an increase of the critical current density, J_c, and the trapped magnetic field. In MT-YBaCuO, practically free form dislocations and stacking faults and with a twin density of 22-35 μm^-1, J_c of 100 kA/cm² (at 77 K, 0 T) has been achieved, and the importance of twins in Y123 for pinning was demonstrated experimentally.