Structure evolution during thermal processing of high-Tc ceramic superconductors produced using sol–gel techniques

Abstract

Characterization of the decomposition and phase development reactions for Cu^II methoxyethoxide, 2Ba–3Cu, Y–3Cu, and Y–2Ba–3Cu gels has been performed using in situ neutron diffraction and differential thermal analysis (DTA). The structual evolution process of a Y–2Ba–3Cu gel to the superconducting YBa₂Cu₃O_{7 –x} phase was elucidated using the acquired spectra. Precursor gel powder was produced from an alkoxide sol–gel system having a methoxyethanol, methyl ethyl ketone and toluene solvent mixture. Neutron diffraction results indicate initial YBa₂Cu₃O_{7 –x} formation below 700 °C, and that a firing temperature of 850 °C is sufficient to produce essentially single-phase YBa₂Cu₃O_{7 –x}. The conversion of BaCO₃ to BaCO₃ polymorph was observed to occur between 790 and 800 °C, after which rapid formation of the YBa₂Cu₃O_{7 –x} tetragonal phase occurred. The results also indicate an incomplete conversion of the non-superconducting tetragonal phase to the superconducting orthorhombic phase during the cool-down and annealing segments of the heat treatment.