Structure and stability of superconducting core of single-core MgB2/Cu,Nb tube composite with a high critical current

The core of a single-core MgB 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 c ≥ 10 5 A/cm 2 ), has been studied using various structural methods. Two kinds of MgB 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 MgB 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 MgB 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 MgB 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 MgB 2 bulk sample is inherited by the MgB 2 /Cu,Nd composite prepared by ex-situ technique (annealing of composite at 700°C). It was shown that oxygen in the MgB 2 compound is the destabilizing factor and leads to the transformation of the superconductor into MgO.