Processing Pnictide Superconductors

Since 2008, the discovery of superconductivity in pnictide compounds with transition temperatures (Tc) above 50 K and upper critical fields (Hc2) exceeding 100 T promoted a considerable research effort to synthesize and process these materials in forms that may be suitable for conductor applications (Kamihara et al. 2008; Putti et al. 2010). Of the dozens of pnictide superconductors discovered, the AEFe2As2 (122) and REFeAs(O,F) (1111) compounds (where AE and RE stand for alkaline earth and rare earth, respectively) listed in Table E3.12.1 have attracted the most attention due to their intermediate transition temperatures between 20 K and 58 K and very high upper critical fields (Putti et al. 2010). Early experiments showed grain boundaries intrinsically block current transport in Co-doped 122 bicrystals, but this problem is less pronounced than in the cuprates (Lee et al. 2009; Katase et al. 2011). Wires and tapes have since been produced by the powder-in-tube (PIT) technique that have critical current densities (Jc) around 104–105 Acm−2 at 4.2 K and 10 T (Gao et al. 2015; Weiss et al. 2012; Zhang et al. 2014). The weak field dependence of Jc above 10 K and 10 T suggests these wires may be suitable for applications in operating fields and temperatures inaccessible by low temperature superconductors without the technical challenges inherent in coated conductors.