The high pressure (50 MPa - 2 GPa) – high temperature synthesized MgB2 bulk materials are characterized by nearly theoretical density (1-2% porosity), 80-98% connectivity, extremely high critical current densities (e.g. at 20 K, in 0–1 T jc=1.3–1.0 MA·cm-2 (with 10% SiC) and jc= 0.92 – 0.73 MA·cm-2 (without doping)), large irreversibility fields (Birr(18.4 K) =15 T and Birr (0 K) = 32.5 T) and high upper critical fields (Bc2 (22 K) =15 T and Bc2(0 K) ~ 42.1 T). The transformation of grain boundary pinning to point pinning in MgB2-based materials with increasing manufacturing temperature (from 800 to 1050 oC) under pressures from 0.1 to 2 GPa correlates well with an increase in critical current density in low external magnetic fields caused by the redistribution of boron and the oxygen impurities in the material structure. As the manufacturing temperature increases, the discontinuous oxygen enriched layers transform into distinct Mg-B-O inclusions and the size and amount of inclusions of higher magnesium borides MgBX (X>4) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC additions.
The superconducting characteristics, such as the critical current density and the critical magnetic fields, of MgB2-based materials, which in fact belong to the Mg-B-O system because of the high concentration of admixed oxygen (up to 17 wt. %), depend on the inhomogeneity of the oxygen and boron distribution, which can be controlled by the synthesis temperature (600-1200 oC) and pressure (up to 2 GPa) as well as by SiC and Ti additions (10 wt%). With increasing manufacturing temperature grain boundary pinning transforms into point pinning, which is well correlated with the transformation of discontinuous oxygen enriched layers into separately located Mg-B-O inclusions in the MgB2 nanostructure and with a reduction of the size and amount of inclusions of higher magnesium borides MgBX (X>2). Ti or SiC additions can influence the oxygen and boron distribution as SEM and Auger structural studies showed.
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