Processing of bulk MgB₂ superconductors for application in fault current limiters

Fault current limiters (FCL) require superconducting (SC) materials which can provide a definite rate of response to a fault event resulting in the SC – normal state reversible transition. The main characteristics determined the material suitability are the critical current density, jc, thermal conductivity and capacity which are strongly determined by manufacturing technology, in particular, of MgB₂. In the paper we estimate the jc of bulk MgB₂ samples by the vibrating magnetometer and inductive, contactless transformer, method using ring samples. The bulk MgB₂ samples were produced under 30 MPa (hot pressing) and 2 GPa (quasihydrostatic pressing) at 800- 1050 ºC from different initial ingredients (Mg and B or MgB₂ with and without additions). It is shown that the technology process and initial ingredients strongly influence the distribution of boron- and oxygen-enriched nanosized inhomogenities in MgB₂ matrix, connectivity between SC grains, material porosity and, as result, the SC properties. The transformer method gives the jc in the range from 1.6·10⁴ up to 6.3·10⁴ A/cm² at about 4 K while using magnetometer measurements the jc is estimated from 2.24·105 up to 5.1·105 A/cm² at 10 K in self magnetic fields. The contradictions in the jc estimated by different methods can be explained by instability of the SC state of MgB₂, caused by variation of the applied magnetic field. Using the transformer method AC losses per a cycle before quenching for the best materials were estimated around 0.75-1 J/cm³, while the power of losses was about 200 W. The FCL model with rings cut out from SC MgB₂ materials prepared using various technologies demonstrated that MgB₂ is a promising material for application in inductive FCLs.