Formation of nanostructure in magnesium diboride based materials with high superconducting characteristics

Abstract

The paper presents an analysis of the properties of bulk superconducting magnesium diboride-based materials obtained by heating at high quasi-hydrostatic pressures (1–2 GPa), hot pressing (30 MPa), spark plasma sintering (16–96 MPa) and loose powder sintering. It is shown that the optimization of impurity distribution in MgB2 can be achieved by varying the synthesis conditions and introducing dopants. In particular, polycrystalline MgB2 materials synthesized at 2 GPa and containing a high amount of impurity oxygen demonstrates high critical current densities (106 and 103 A/cm2 at 20 K in magnetic fields of 1 and 8.5 T, respectively). It is found that the oxygen impurities are mainly localized in nanolayers or nanoinclusions, homogeneously distributed in the matrix. They act as pinning centers, while the MgB2 matrix also contains small amounts of dissolved oxygen. Impurity or intentionally added carbon entering the magnesium diboride structure leads to an increase in the critical magnetic fields up to Bc2 (22 K) = 15 T and Birr (18.5 K) = 15 T. The results of ab initio calculations of the electronic structure and stability of the magnesium diboride compounds with partial oxygen or carbon substitution for boron show that it is energetically favorable for carbon to distribute homogeneously in MgB2 structure, while oxygen atoms replace boron pairwise in neighboring positions or form zigzag chains.