RHEOLOGY OF PRECURSORS FOR SOL-GEL DERIVED YBA2CU3O7-X SUPERCONDUCTORS

Aqueous-based precursors for high Tc superconductors were prepared by mixing solutions of copper acetate, barium acetate and yttrium hydroxide. The time-dependent dynamic moduli show that the kinetics and extent of gelation of the precursor solution are directly related to the age of yttrium hydroxide component; age being the length of time between the preparation of the yttrium component and its use in the precursor solution. The viscosity of the colloidal suspension of the yttrium hydroxide at low shear rates increases with age by an order of magnitude in two months. Dynamic oscillatory and controlled-stress experiments on the yttrium hydroxide suspension also reveal increases in moduli and formation of network structure as this material ages. The particle size of the yttrium hydroxide almost doubles in two months of aging. Superconducting films made from different batches of precursors containing fresh and old yttrium components show differences in their morphology and electrical behavior. Fired films obtained from precursors containing aged yttrium hydroxide had a single phase perovskite and showed superconducting behavior whereas films made from fresh yttrium had an additional structural phase and never achieved zero resistance. A possible mechanism relating the changes in the yttrium hydroxide properties as it ages to the sol-gel kinetics of the precursor solution and the characteristics of the fired film is presented.