Formation of thallium based high temperature superconductors using aerosol flow reacted precursor powders

摘要

Highly anisotropic particles of various thallium containing superconductors were grown. The Tl free precursor powders with the nominal compositions Ba(sub 1)Ca(sub 2)Cu(sub 3)Ag(sub 0.37)O(sub 6) and Ba(sub 2)Ca(sub 2)Cu(sub 3)Ag(sub 0.37)O(sub 7) were prepared using an aerosol flow reactor. These precursor powders were post-annealed in 0.1 atm oxygen at 700(degree)C for 4h to reduce the carbon present, and subsequently mixed with Tl(sub 2)O(sub 3) (typical composition of Tl(sub x); x = 0.6 (minus) 1.0). The Tl-containing powders were heat-treated in semi-sealed gold tubes between 650--890(degree)C with various short and long heat-treatments. X-ray diffraction showed that the Tl(sub 1)Ba(sub 2)Ca(sub 1)Cu(sub 2)O(sub 7) (referred to as Tl-1212) and Tl(sub 1)Ba(sub 2)Ca(sub 2)Cu(sub 3)O(sub 9) (Tl-1223) phases formed below 700(degree)C while Tl(sub 2)Ba(sub 2)Ca(sub 1)Cu(sub 2)O(sub 8) (Tl-2212) formed between 700--830(degree)C, and Tl(sub 2)Ba(sub 2)Ca(sub 2)Cu(sub 3)O(sub 10) (Tl-2223) formed above 830(degree)C. Scanning electron microscopy showed evidence for the presence of high aspect-ratio particles. These highly anisotropic particles may be of interest for the preparation of powder-in-tube and other powder-deposited conductors, for current leads, and for grain alignment studies.