Structure and Properties of MgB 2 - and MT-YBaCuO-based Materials Manufactured Under Pressure

The influences of technological parameters (temperature, pressure, type of cladding), composition of precursor powders, and type of additions (Dy2O3, Zr, Ti, Ti-O, SiC, Nb) on the superconducting characteristics (transition temperature, critical current density, critical magnetic fields) of MgB2 wires produced by Hyper Tech Research Inc. and of bulk materials synthesized under ambient, elevated, and high pressures in correlation with their microstructures, were studied. Microstructural observations are in good agreement with ab-initio modeling [1, 2]. The relevant pinning centers of Abrikosov vortices in MgB2-based materials are oxygen-enriched Mg-B-O inclusions (or nanolayers) and inclusions of high borides MgBx. Their number and size depends on the synthesis parameters and dopants. Ti, Ta, and Zr bind hydrogen, make the formation of magnesium hydride impossible, and thus improve the mechanical characteristics bulk MgB2. Ti, Ta, Zr also affects the formation of higher borides, and Ti, Ta, Zr, SiC, TiC - on the redistribution of oxygen. The critical current density, jc of melt textured YBa2Cu3O7-d (or Y123)-based superconductors (MT-YBaCuO) depends on the perfection 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 micro-cracks increases with the reduction of the distance between Y2BaCuO5 (Y211) inclusions in Y123. A high level of critical current density demonstrated MT-YBCO oxygenated under 16 MPa oxygen pressure and MgB2 high pressure (2 GPa) - high temperature synthesized: jc (MT-YBaCuO) at 77 K was nearly 105 A/cm2 in self-field and 103 A/cm2 at 10 Т, and jc (MgB2) at 20 K was 106 at 1 T and 103 A/cm2 at 8.5 T. The MgB2 wires with added Dy2O3 (4 wt.%, 100 nm) demonstrated Jc=11 kA/cm2 at 20 K in 5 T and at 4.2 K in 12.5 T fields.