Rheological study of a gel-forming precursor for superconducting YBa2Cu3O7−x

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Abstract
Rheological measurements are used to monitor the sol‐gel behavior of a superconducting precursor. A precursor made with an aged yttrium component [Y(OH)3] has a much higher elastic modulus (G’) and complex viscosity (η*) but the same loss modulus (G‘) compared to that made with a fresher yttrium solution. The G‘/G’ ratio is less than unity for the older solution but greater than 1 for the fresher sample, indicating that the former solution has a more gel‐like character. The high viscosity sample gives better fired thick‐film superconducting properties but has a shorter shelf life. Measurements on yttrium components of different age show differences in viscosity and G‘/G’ which correlate with the behavior of the oxide precursors. Reshearing the precursor solution at any stage during its gelation process can cause irreversible changes in the gel structure.