Anton Shaternik

Pinning in

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, <i>J</i>_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, <i>J</i>_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.

\hbox{MgB}_{2}

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.

- and YBaCuO-Based Superconductors: Effect of Manufacturing Pressure and Temperature

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.

Structure and Functional Properties of Bulk MgB2 Superconductors Synthesized and Sintered under Pressure

This paper describes the established correlations between the characteristics of MgB₂-based superconducting materials and their structural features, in particular the oxygen distribution and content. As indicated by SEM and Auger analysis, a sizeable amount of oxygen is usually present in superconducting MgB₂ -based materials (bulks, thin films, and wires). The matrix phase of bulk MgB₂ contains rather small amounts of oxygen, but contains a high amount of dispersed inclusions or areas with compositions close to MgBO. X-ray phase analysis with Rietveld refinement (using the FullProf Suite program package) of several highly dense MgB₂ -based bulk samples (with high critical current densities) showed that the superconducting phase had a composition within the range MgB_1.68O_0.26 and MgB_1.74O_0.32 instead of pure MgB₂. Besides, a small amount of a phase with MgO structure was observed in the materials by X-rays as well. The calculation of the enthalpy of formation confirms the possibility of oxygen solubility in MgB₂ and shows that the formation of MgB_1.75B_0.25 is most favorable. The results of <italic>ab initio</italic> calculations of the electronic structure and stability of the MgB₂ compounds with partial oxygen substitution for boron show that it is energetically preferable for oxygen atoms to replace boron pairwise.

Influence of oxygen and boron distribution on superconducting characteristics of nanostructural Mg-B-O ceramics

MgB2-based bulk materials synthesized under 30 MPa-2 GPa pressure demonstrated an enhanced connectivity AF ~50%-98%,as compared tomostother reports (below50%), and a shielding fraction of 75%-100%. The materials inhibit high critical current densities jc, but no strict correlation was found between AF and jc. On the other side, we found correlations between jc and the distribution of nanostructural inhomogeneities such as impurity oxygen in the form of oxygen-enriched Mg-B-O nanolayers or nanoinclusions and its solution in the MgB2 matrix and as well with the amount and size of higher magnesium borides MgBx, x ≥ 4 inclusions. The distribution of boron- and oxygen-enriched (as compared to stoichiometric MgB2) inhomogeneities affects the type of pinning and, thus, jc, and to some extent, it can be regulated by synthesis temperatures and pressures and by Ti addition. Auger spectroscopy, scanning electron microscopy, and X-ray with Rietveld refinement were used for structural study.

Structure and Properties of MgB 2 Bulks, Thin Films, and Wires

Charge transport in superconductor-insulator-superconductor junctions has been studied. To relate technological parameters of the deposition of junction layers and barrier’s structure to model parameters of charge transport in these junctions for the first time has been made possible by the use of superhard material, leucosapphire, as the substrates to produce Josephson junction. The results have been considered of experimental studies of nanosized structures fabricated on the basis of thin films of MoRe superconductors. The charge transport in these heterostructures has been analyzed in the framework of the modified theoretical model of multiple Andreev reflections taking into account the function of the barrier transparencies distribution. Such approach allows one to describe the evolution of the shape of the real current-voltage characteristics of heterostructures depending on technological conditions of manufacturing the heterostructures.