Structure and stability of superconducting core of single-core Mg[B.sub.2]/Cu,Nb tube composite with a high critical current

• Published in: Physics of metals and metallography Vol. 115; no. 6; p. 538
• Main Authors: Krinitsina, T.P, Kuznetsova, E.I, Blinova, Yu.V, Rakov, D.N, Belotelova, Yu.N, Sudareva, S.V, Degtyarev, M.V, Romanov, E.P
• Format: Journal Article
• Language: English
• Published: Springer 01.06.2014
• Subjects: Analysis; Annealing; Superconductors
• ISSN: 0031-918X; 1555-6190
• DOI: 10.1134/S0031918X1406009X

The core of a single-core Mg[B.sub.2]/Cu,Nb composite, which has been prepared by the ex-situ technique and exhibits a high critical current equal to 427 A (at 0 T and 4.2 K, [j.sub.c] ≥ [10.sup.5] A/[cm.sup.2]), has been studied using various structural methods. Two kinds of Mg[B.sub.2] crystals were observed; those of the first kind is large, highly dense crystals characterized by a low oxygen content (2-8 at %) and the others are fine, weakly coupled crystallites characterized by high oxygen content (4-21 at %). To perform a comparative analysis of the structures, we have also studied an Mg[B.sub.2] bulk sample synthesized at 1000°C. It was found that two phases with the same lattice are formed; they differ in the magnesium and boron contents (within the homogeneity range), impurity oxygen content and microstructure as well but differ slightly in the lattice parameters. The two-phase state of Mg[B.sub.2] bulk sample is due to the mechanism of its formation, which includes the melting of magnesium, the dissolution of solid boron in it, and the crystallization of Mg[B.sub.2] from the melt with the formation of dendrite-like structure characterized by corresponding redistribution of components and impurities. To a certain degree, the two-phase structure of Mg[B.sub.2] bulk sample is inherited by the Mg[B.sub.2]/Cu, Nd composite prepared by ex-situ technique (annealing of composite at 700°C). It was shown that oxygen in the Mg[B.sub.2] compound is the destabilizing factor and leads to the transformation of the superconductor into MgO. Keywords: composites, microstructure, superconductors DOI: 10.1134/S0031918X1406009X