Processing of the Tl–Ca–Ba–Cu–O high-Tc phase via the sol–gel route

Abstract

Tl₂CaBa₂Cu₂O_{8 +x}, and Tl₂Ca₂Ba₂Cu₃O_{10 +x} superconductors have been produced via a sol–gel route. Thallium butoxide, calcium methoxyethoxide, barium methoxyethoxide, and copper(II) ethoxide were the alkoxide precursors for the synthesis. 2-Amino-2-methylpropanol was added to the copper(II) ethoxide in order to enhance its solubility in butan-1-ol, 2-methoxyethanol, methyl ethyl ketone, and toluene. Characterization of sols was done by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Samples were fabricated by heating the sols under vacuum, prefiring the resulting gels to 798 K, pelletizing, and firing at 1158 K in closed containers. DTA was performed to observe decomposition (432 K) and crystallization (691 K) reactions of the gels. The superconductors were characterized by X-ray diffraction, magnetic susceptibility, four-point probe resistivity, chemical analysis and scanning electron microscopy. The 2122 and 2223 superconducting phases were produced using 3 : 2 : 2 : 3 and 1 : 3 : 1 : 3 starting ratios, respectively. T_c(zero) values were 105 (2122) and 115 (2223) K with increases in these transitions occurring with firing times. Microstructural analysis revealed that a large degree of directional plate-like crystal growth occurred on the surface of the pellets.