Improved microstructure in Ag/Bi-2223 composite tapes by systematic variation of heat treatment parameters

Multifilament-type, silver-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox (Ag/Bi-2223) composite tapes produced by the powder-in-tube (PIT) method were given a first heat treatment that employed either a standard (STD) single oxygen pressure/temperature (pO2/T) set point or a novel variable pO2/T treatment referred to as thermal sliding heat treatment (TSHT). X-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy and transmission electron microscopy were employed to provide a comparative analysis of the Bi-2223 grain colony microstructure and connectivity, nonsuperconducting second phase (NSP) composition and distribution, and grain boundary character in the STD- and TSHT-type post-first-heat-treatment Ag/Bi-2223 tape specimens. The dominant NSPs in STD and TSHT specimens were (Ca, Sr)2CuO3, (Ca, Sr)14Cu24O41, and amorphous phases that were randomly distributed in the filaments. The number and size of the NSPs in the STD specimens were sufficient to cause substantial misalignment of Bi-2223 grain colonies throughout the filament cores. However, the TSHT specimens (when compared to the STD specimens) expressed an improved microstructure with fewer/smaller NSPs that were localized mainly in the interior regions of the filaments. Also, the Bi-2223 grain colonies in TSHT specimens were more robust and better aligned from the silver-sheath/Bi-2223 interface to the mid-core region of each filament. (author)