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) YBa2Cu3O7 already after the first firing at 900 degrees C in pure O-2.