STRUCTURE AND PINNING CENTERS IN MGB2 BULKS, WIRES, THIN FILMS AND MT-YBCO

Abstract

The structure and composition of MgB2-based materials (wires, bulks, and thin films) prepared at different pressure (0.1 MPa - 2 GPa) – temperature (600 - 1050 oC) conditions and MT-YBCO oxidized in the gas flow at 440 oC under ambient pressure and under hydrostatic 16 MPa pressure at 800 oC witch demonstrated high critical current densities, jc, were analyzed by X-ray, JAMP−9500F Auger spectrometer (after removing the oxidized layers at the sample surfaces by Ar ion etching performed directly in the vacuum chamber of microscope) and TEM. Regularly distributed inhomogeneities connected with Mg, B and admixture O content variation on the nanolevel were observed in all types of the MgB2-based materials and are responsible for pinning. In MT-YBCO such defects as twins, dislocations and staking faults to high extent are responsible for pinning. Their densities are strongly correlated with the distances between dispersed Y2BaCuO5 inclusions. The correlations between the character of the materials inhomogeneities which can be pinning centers and the attained superconducting characteristics are discussed. The superconducting matrices of magnesium diboride bulks with AlB2 structures contain some oxygen in their unite cells what has been shown experimentally by X ray and Auger study and supported by ab-inito simulation.