Behavior of Bi-2212 Wires Above Liquid Helium Temperature—Critical Current, Irreversibility Field and Filaments Coupling

Since the discovery of High Temperature Superconductors (HTS) a possible cryogen-free scenario has been always wished. Nowadays liquid helium is running out, and it is likely that the cooling by will be a large part of the costs of any superconducting system. Bi-2212 wires at temperature higher than 4.2 K still show a very high irreversibility field and thus a deep investigation of their properties in such a range of temperature is very useful in order to assess the applicability in high field cryogen-free magnets. Here electrical transport and magnetic properties characterization at variable temperature and magnetic field of Bi-2212 wires are reported together with a well described original approach to calculate irreversibility field Hirr [1]. From the magnetic and transport measurements comparison it has also been studied the properties of the network of bridges between the filaments formed by grains grown through the Ag matrix during the partial-melt heat treatment process. This is a feature unique in Bi-2212/Ag wires and, 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 case of charge and discharge of DC magnets, principal applications of this kind of superconductor. Here we report on the behavior of these bridges as a function of applied magnetic field and temperature and the implications they have on the electrical coupling. The 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.<br/>All the experiments have been performed on two multifilamentary wires prepared by GDG-PIT process [2] starting from two commercial Bi-2212 precursor powders: Nexans and Engi-Mat. These studies are devoted to provide reference data on the behaviour of the only isotropic wire for high field application with an eye to the performances at temperatures above 4.2 K. We believe that such findings are very useful in magnet design.<br/>[1] A. Leveratto <i>et al</i>, <i>Scientific Reports</i> <b>11</b> (2021), 11660<br/>[2] A. Leveratto <i>et al</i>, <i>Supercond. Sci. Technol.</i> <b>29</b> (2016), 045005