Investigation of transport mechanisms induced by filament-coupling bridges-network in Bi-2212 wires

One of the features unique in Bi-2212/Ag wires is the network of bridges between the filaments formed by grains grown through the Ag matrix during the partial-melt heat treatment process. Although these interconnections favor a redistribution of the current among the filaments allowing high critical current density, they represent a strong electrical coupling between the filaments themselves. Such a coupling increases the AC losses, present also in the case of the charge and discharge of DC magnets, and principal applications of this kind of superconductor. In this work, through transport and magnetic measurements and their comparison, we study the behavior of these bridges as a function of applied magnetic field and temperature and the implications they have on the electrical coupling. The experiment has been performed on two multifilamentary wires prepared by Groove-Die-Groove Powder In Tube process starting from two commercial Bi-2212 precursor powders: Nexans and Engi-Mat. The reported results show that the effective length scale on which the filaments are coupled is dependent on the field and temperature, passing from the filaments-bundle diameter at low field and temperature to single filament diameter at high field and temperature. Although the two samples have different Jc values, such filaments coupling behavior is common to both. We believe that these findings are very useful in magnet design.