I wish to report a significant improvement in the synthesis of YBa₂Cu₃O_7-d (YBCO), a superconductor commonly used in undergraduate laboratories. Currently, the most common "shake and bake" method for YBCO synthesis consists of grinding together Y₂O₃, BaCO₃, and CuO powders, annealing the mixture at ca. 940 °C for 12+ hours, and slowly cooling to room temperature. However, unless numerous time-consuming "re-grind/anneal" steps are performed, non-superconducting phases, namely Y₂Ba₄O₇, BaCuO₂, Y₂Ba₂O₅, and Y₂BaCuO₅, will be present and the products' physical and electronic properties will be drastically altered (1).

An experiment involving the optimization of the synthesis of YBa₂Cu₃O_7-d superconductor pellets was recently introduced to students at UC Irvine. Attempts to obtain phase-pure YBCO using the well-known barium carbonate procedure were unsuccessful, as confirmed qualitatively by magnet-levitation experiments and quantitatively by powder X-ray diffraction and resistivity measurements. Hence, we began to search the literature for a superior synthetic alternative.

The method that was successfully used in our module was similar to that reported by Costa et al. (2). After Y₂O₃, BaO₂, and CuO were ground together using a mortar and pestle, the mixture was heated to 940 °C at 30 °C min^-1. This temperature was maintained for four hours under a modest flow (< 2 mL min^-1) of oxygen. Neither extending the reaction time nor changing the flow rate of oxygen resulted in any observable differences in superconductivity or phase purity of the product. In accord with the literature, the crucial step governing superconductor quality was the final cooling step. For our samples, this was accomplished using a cooling rate of 5 °C min^-1. The final powders were homogeneously black and were superconducting at 77 K (liquid nitrogen medium) as determined from simple magnet levitation and resistivity measurements. Powder X-ray diffraction also verified the phase purity of the resultant powders.

Although procedures describing the utilization of barium peroxide already exist in the literature (3), the above description should simplify the search for an extremely facile and reliable recipe for YBCO. This procedural information is especially suitable for JCE, as it represents an improvement over alternate procedures that have appeared in this Journal (4). Although two of these methods report an improvement in phase purity relative to that obtained by the carbonate procedure, both use a significantly long reaction time, on the order of 24 h per superconductor pellet. By comparison, our single-step synthetic procedure was completed in a single laboratory period (ca. 7-8 h, including slow cooling). If shorter laboratory periods (4-5 h) are required, the students would easily have enough time to both prepare and anneal their mixture; the samples could then be set to cool overnight for the next day of characterization studies.

Literature Cited

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2. Costa, C. A.; Ferretti, M.; Olcese, C. L.; Cimberle, M. R.; Ferdeghini, C.; Nicchiotti, G. L.; Siri, A. S.; Rizzuto, C. J. Cryst. Growth 1987, 85, 623.
3. Costa, G. A.; Ferretti, M.; Olcese, G. L. J. Cryst. Growth 1988, 91, 392. Parent, L.; Champagne, B.; Cole, K.; Moreau, C. Supercond. Sci. Technol. 1989, 2 (2), 103. Peacor, S. D.; Uher, C.; Francis, A. H.; Shewchun, J. J. Appl. Phys. 1990, 67 (12), 7488. Kao, M.; McKinney, B. L. Mater. Lett. 1991, 11 (3-4), 91.
4. Cogdell, C. D.; Wayment, D. G.; Casadonte, D. J.; Kubat-Martin, K. A. J. Chem. Educ. 1995, 72, 840. Djurovich, P. I.; Watts, R. J. J. Chem. Educ. 1993, 70, 497. Jacob, A. T.; Pechman, C. I.; Ellis, A. B. J. Chem. Educ. 1988, 65, 1094. Ellis, A. B. J. Chem. Educ. 1987, 64, 836.