Role of Mg-B-O Nanostructural Inhomogenities on the Performance of Superconducting MgB2

Abstract

Auger and SEM studies show that with increasing of MgB2 manufacturing temperature from 600÷800 °C to 1050÷1100 °C the Mg-B-O nanolayers which are present in the MgB2 matrix transform into distinct dispersed Mg-B-O inclusions. On the other hand the sizes of inclusions of higher magnesium borides (MgBx, x=7 ÷ 25) which are also present in the MgB2 matrix. The tendency is observed in a wide range of synthesis pressures (0.1 MPa-2 GPa). The described structural transformations are accompanied by an increase in critical current density, jc, in low and medium magnetic fields and by transition from the grainboundary to the point pinning. The Ti addition results in a further increase in jc due to: Ti promotes the formation of higher magnesium boride inclusions and localization (or segregation) of oxygen in MgB2 matrix, and, hence, facilitates the formation of a homogeneous MgB2 matrix with lower oxygen content, but with an increased number of Mg-B-O and MgBx pinning centers. At low synthesis temperature Ti absorbs hydrogen forming titanium hydrides, thus preventing the formation of MgH2 and provides the material densification. The positive effect of Ti addition is connected with the high ability of Ti to absorb hydrogen, oxygen, and magnesium. The results of the critical current and AC loss study by transformer method using rings from MgB2 are discussed.