Removal of supermicron particles from precursor powders for PIT fabrication of Nb3Sn and Bi-2212 multifilament conductors

A technique is being developed for separating all particles larger than ∼1 micron from the precursor powders that are used in powder-in-tube (PIT) fabrication of superconductors. For both Nb₃Sn and Bi-2212, elimination of large particles enables drawing a multi-filament restack to smaller final filament size, with benefits for overall current density and suppression of persistent current effects. The technique uses virtual impact (VI) sizing, in which the powder is dispersed in an aerosol stream and passed through a nozzle array which separates particles larger and smaller than a critical size into separate flow channels. Unlike other separation techniques, VI exhibits a sharp threshold and yields extremely clean removal of large particles. Successful operation of a pilot system, studies of the separation dynamics, and issues relating to scale-up are presented.