Superconducting properties, microstructure and chemical composition of MgB2 sheathed materials

The superconducting properties, the microstructure and the chemical composition of sheathed MgB2 tapes and of one wire, all synthesized by the powder-in-tube method, were investigated. At 4.2 K critical current densities up to 105 A cm-2 (0 T) and 1.5 x 104 A cm-2 (2.5 T) were obtained by transport measurements in the wire and the tapes, respectively. In the MgB2 matrix of all samples, oxygen was identified and mole fractions of 0-10 at% were determined by electron probe microanalysis. It was found by scanning electron microscopy that only the tapes showed boron-rich secondary phases about 10 μm in size. Comparing different tapes, the critical currents increase with the aspect ratio and decrease with the oxygen mole fraction in the MgB2 material. In the tapes, aspect ratios were inhomogeneous and critical current densities at low fields were limited by insufficient thermal stabilization. For understanding the internal oxidation in the MgB2 tapes, one tape was investigated by analytical transmission electron microscopy (TEM). The combination of energy-dispersive x-ray spectroscopy and electron spectroscopic imaging in the TEM yielded phase maps of this sample. It showed a heterogeneous microstructure, MgB2 grain sizes ranged between 20 nm and 1 μm. Oxygen was primarily bound in 20 nm-1 μm MgO precipitates and secondary phases, and no boron oxides could be evidenced. Randomly distributed 50 nm-1 μm boron-rich secondary phases (MgB4+δ, MgB7+γ) embedded in the MgB2 matrix were identified. The possible reasons for the oxidation of the superconducting matrix are discussed