Study of the MgB2 grain size role in ex situ multifilamentary wires with thin filaments

The MgB2 superconductor has already demonstrated its potential, in particular for DC applications such as magnetic resonance imaging (MRI) magnets, thanks to the low cost of the raw materials and to its simple production process. However, further efforts have still to be made in order to broaden its employment in AC applications such as superconducting fault current limiters (SFCLs), motors, transformers etc. The main issues are related to the reduction of AC losses. Some of these can be faced by obtaining multifilamentary conductors with a large number of very fine filaments and, in this context, the powder's granulometry can play a crucial role. We have prepared MgB2 starting powders with different granulometries and by the ex situ powder-in-tube (PIT) method we have realized multifilamentary wires with a number of filaments up to 361 and an average size of each filament lowered down to 30 mu m. In particular we have studied the relationship between grain and filament size in terms of transport properties and have shown that the optimization of this ratio is possible in order to obtain suitable conductors for AC industrial applications.