NASA NTRS1992
Experimental work has continued on the development and characterization of bulk and hot pressed powders and tapecast materials in the Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems. A process for producing warp-free, sintered, superconducting tapes of Bi composition Bi2Sr2Ca2Cu3O(x) with a mixed oxide process was established. This procedure required a triple calcination at 830 C for 24 hours and sintering at 845 C from 20 to 200 hours. Hot pressing the triple calcined powder at 845 C for 6 hours at 5000 psi yielded a dense material which on further heat treatment at 845 C for 24 hours exhibited a Tc of 108.2 K. A further improvement in the processing of the bismuth materials was achieved via a chemical coprecipitation process wherein the starting nitrate materials were coprecipitated with oxalic acid, thus yielding a more chemically homogeneous, more reactive powder. With the coprecipitated powders, only one calcine at 830 C for 12 hours and a final sinter at 845 C for 30 hours was sufficient to produce a bulk superconducting material with a Tc of 108.4 K. SAFIRE-type grounding links were successfully fabricated from sintered, tapecast, coprecipitated BSCCO 2223 powders. Compositional and processing investigations were continued on the Tl-based superconductors. Manganese and lithium additions and sintering temperature and time were examined to determine their influence on superconducting properties. It was found that lithium substitutions for copper enhance the transition temperatures while manganese additions produced deleterious effects on the superconducting properties. A suitable procedure for producing reproducible bulk and tapecast material of Tl composition Tl2Ba2Ca2Cu3O(x) was developed and used in fabricating uniform superconducting tapes. The highest transition temperature for Tl-based tapes was measured at 110.2 K. Thallium superconducting SAFIRE-type grounding links were fabricated from the tapes.