Superconducting properties, microstructure and chemical composition of MgB₂sheathed materials

The superconducting properties, the microstructure and the chemical composition of sheathed MgB 2 tapes and of one wire, all synthesized by the powder-in-tube method, were investigated. At 4.2 K critical current densities up to 10 5 A cm −2 (0 T) and 1.5 × 10 4 A cm −2 (2.5 T) were obtained by transport measurements in the wire and the tapes, respectively. In the MgB 2 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 MgB 2 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 MgB 2 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, MgB 2 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 (MgB 4+δ , MgB 7+γ ) embedded in the MgB 2 matrix were identified. The possible reasons for the oxidation of the superconducting matrix are discussed.