Papers by Author: Jan Dellith

Abstract: 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.

Abstract: Superconducting (SC) and mechanical properties of spark plasma (or SPS) produced MgB₂ –based materials allow their efficient applications in fault current limiters, superconducting electromotors, pumps, generators, magnetic bearings, etc. The synthesized from Mg and B at 50 MPa, 1050 °C for 30 min material has a density of 2.52 g/cm³, critical current density, j_c = 7.1•10⁵ A/cm² at 10 K , 5.4 •10⁵ A/cm² at 20 K, and 9•10⁴ A/cm² at 35 K in zero magnetic field; at 20 K its field of irreversibility B_irr(20)=7 T and upper critical field B_c2(20)=11 T; microhardness H_V=10.5 GPa and fracture toughness K_1C =1.7 MPa•m^1/2 at 4.9 N-load. SPS-manufactured in- situ MgB₂-based materials usually have somewhat higher jc than sintered ex-situ. The pressure variations from 16 to 96 MPa during the SPS-process did not affect material SC characteristics significantly; the j_c at 10-20 K was slightly higher and the material density was higher by 11%, when pressures of 50-96 MPa were used. The structure of SPS-produced MgB₂ material contains Mg-B-O inclusions and inclusions of higher borides (of compositions near MgB₄, MgB₇, MgB₁₂, MgB₁₇, MgB₂₀), which can be pinning centers. The presence of higher borides in the MgB₂ structure can be revealed by the SEM and Raman spectroscopy.