Citrate‐Gel Syntheses in the Y(O)–Ba(O)–Cu(O) System

A synthetic procedure has been developed for the yttrium‐barium‐copper oxides, in terms of a liquid‐mixed citrate precursor and the use of yttrium oxide, barium carbonate, and basic copper carbonate as starting materials. The stability of the citrate gels against phase separation is investigated as a function of the content of metal oxides in the gels, and critical concentrations for the individual metals are derived in terms of the maximum metal‐to‐citric acid ratio across the Y(O)–Ba(O)–Cu(O) phase diagram. Sintering properties and compositions of the products are investigated as functions of the thermal treatment of the citrate precursor. The present citrate‐gel procedure does not make use of internal oxidants like nitrates and polymerization agents like glycol. This approach decreases the incineration temperature and leads to a nanodispersed precursor (except submicrometer Cu), which, e.g., yields virtually phase‐pure (< 1% of impurities) YBa 2 Cu 3 O 7 already after the first firing at 900°C in pure O 2 .