•Sonja Schlachter, Bing Liu, Bernhard Obst, Silke Zimmer, Johann Reiner und Wilfried Goldacker
Forschungszentrum Karlsruhe, Institut für Technische Physik, P.O. Box 3640, 76021 Karlsruhe, Germany

Two years after the discovery of superconductivity in MgB₂, wires and tapes with high current carrying capability have been developed by various groups. One method for preparing powder-in-tube (PIT) wires and tapes is the ex-situ method which uses prereacted MgB₂ as precursor. In contrast, the in-situ method starts with a mixture of Mg and B which is reacted within the sheath in a heat treatment after the deformation process. Despite the high current carrying capability the microstructure of the wires and tapes is often very inhomogeneous and in transport current measurements they tend to quench at low magnetic fields much below the critical current values determined from magnetization measurements.

In this paper we present measurements of T_c and the critical current density J_c of ex-situ PIT wires and of in-situ PIT wires with different Mg/B precursor ratio as a function of the magnetic field. Astonishingly, in-situ wires with a Mg/B precursor ratio below the stoichiometric value 1/2 show higher critical current densities than wires with stoichiometric Mg/B precursor ratio being a result of the incomplete penetration of boron grains by Mg during the heat treatment. Although the microstructure of these in-situ wires is still very inhomogeneous, J_c(B) values are even higher than J_c(B) values of ex-situ PIT MgB₂ wires with a much more homogeneous microstructure.