Superconducting properties of MgB 2 tapes and wires

Abstract

An overview of the current state of development of MgB 2 wires and tapes prepared by several techniques is presented, based on the known literature data and our own results. We focus on the powder-in-tube processing method, using pre-reacted MgB 2 powders surrounded by an Fe sheath (ex situ processing route). The study of the effect of the initial MgB 2 particle size shows that after reducing the initial particles by ball milling to sizes of the order of 3 μm, the degree of texturing and the upper critical field, B c2, remain unchanged, while both the critical current density, J c, and the irreversibility field, B irr, show a considerable increase, followed by a decrease after longer milling times. After various recrystallization times and temperatures we show that a critical amount of impurities introduced during the particle reduction process is responsible for the observed maximum of J c and B irr. Our analysis indicates that this is a grain boundary effect, thus pointing the way for further improvement of the transport properties. A preferential orientation along the c-axis has been observed in Fe/MgB 2 tapes by X-ray diffraction. An anisotropy ratio of 1.3 for both B c2 and B irr was found. The Fe/MgB 2 tapes exhibit a very high n factor, which opens up the possibility for a persistent mode operation at 4.2 K and moderate fields. Transport J c values above 10 4 A/cm 2 are obtained in monofilamentary Fe/MgB 2 tapes at 4.2 K and 6.5 T and at 25 K and 2.25 T. Multifilamentary tapes were found to exhibit lower J c values due to the presently used deformation process yielding a lower density with respect to monofilaments. In all measured tapes, quenching was observed at the lowest applied fields. Improvement of the thermal stability of MgB 2 tapes will be one of the major challenges in future developments.