Role of the Grain Oxidation in Improving the In-Field Behavior of ${\rm MgB}_{2}$ Ex-Situ Tapes

The grain size and the grain boundaries play an important role in determining the pinning properties and the current carrying capability of MgB₂. Here we present an experimental study on the effect of grain oxidation on the superconducting properties of MgB₂ ex-situ Powder-in-Tube (PIT) tapes. We fabricated Fe-sheathed monofilamentary tapes using a variety of powders synthesized in inert (IA) or oxidizing (OA) atmosphere. In order to understand the role of magnesium oxide (MgO) we studied the effect of the presence of the oxidation layer on the field dependence of the critical current density. To this purpose, we developed a process for the powder synthesis and tube filling and sealing in controlled atmosphere to reduce the oxygen contamination; with this process the connectivity of the grains in the final tapes proved to be increased. Then, we introduced a controlled oxidation step. We will show how the presence of the MgO layer enhances the critical current field dependence. High energy X-ray diffraction measurements were performed on the samples, and the grain size of the unsintered and sintered powders in the tapes was measured by means of Scanning Electron Microscopy (SEM). The pinning force was studied, and different mechanisms were found for the samples prepared in IA and OA. The different behavior shown by the two different typologies of tapes was correlated with the presence or absence of the MgO layer.