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.

Bulk MgB2 Superconducting Materials: Technology, Properties, and Applications

“…Figure 6 presents one of the important characteristics of superconductors, which determines the field of their application, the upper critical magnetic field, B c2 . Figure 6 shows the temperature dependences of the highest upper critical magnetic fields for the HP, SPS, and HotP materials [ 120 , 132 ].…”

“…The dependences of the superconducting properties on the manufacturing temperature are associated with variations in the MgB 2 structures [ 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 85 , 109 , 110 , 117 ]. The typical structures of MgB 2 materials synthesized at low (800 °C) and high (1050 °C) temperatures under 2 GPa are shown in Figure 9 a,b [ 132 ].…”

“…These facts stimulated the authors of [ 10 , 11 , 117 , 130 , 132 ] to perform detailed structural studies of MgB 2 and modeling of electron density in MgB 2-x Ox structures, binding energy, structure variations, and enthalpy of solid solutions formation.…”

“…Calculations of the density of electronic states N ( E ) (DOS) for different concentrations of oxygen substituting for boron were performed in [ 132 ], which assumed that the oxygen atoms were in the same positions as the substituted boron ones. The authors of [ 132 ] found changes in the positions of the N ( E ) peaks, marked I, II, and III in Figure 20 . The calculated DOS N ( E ) for the Mg-B-O supercells revealed significant hybridization of the s and p states of Mg, B, and O.…”

“…An increase in N ( E F ) with the oxygen concentration (x→1) leads to an increase in the total energy, and the minimum of free energy cannot be realized. This may explain the appearance of separate oxygen-enriched inclusions with increasing oxygen concentration, such as MgO and Mg-B-O [ 132 ].…”