Development of a reliable materials base for superconducting electronics

Abstract

Careful studies of the corrosion, redox, galvanic, and oxygen evolution/uptake reactions associated with YBa 2 Cu 3 O 72d and related compounds have been completed. These studies have led to an understanding of the many factors that contribute to the poor material characteristics exhibited by these popular high-T c phases. With knowledge of the structure-reactivity relationships, a powerful crystal engineering approach has been developed that is capable of producing cation substituted versions of YBa 2 Cu 3 O 72d ; the resulting compounds therefrom produced exhibit markedly improved processability, oxygen stability, and durability characteristics. These materials have been combined in thin film structures so as to make prototype SNS junctions and SQUID sensors which exhibit promising device performance characteristics.

Evidence for high stability against water corrosion of NdBa2Cu3O7±δ relative to YBa2Cu3O7−δ and EuBa2Cu3O7−δ

“…Cu 3 O 7Ϫ␦ and YBa 2 Cu 3 O 7Ϫ␦ decompose over similar time frames, a trend also found in previous work with other rare earth ions like Gd 3ϩ (Rϭ1.05 Å) 2. Here, there is little or no effect of increasing the RE 3ϩ ion size.…”

“…3͑a͔͒. 2,8 Thus, it is conjectured here that water diffusion into the interior region of NdBa 2 Cu 3 O 7Ϯ␦ may be hampered by the structural disorder with respect to the oxygen components of the lattice, as previously documented 2 for Y 0.6 Ca 0.4 Ba 1.6 La 0.4 Cu 3 O 7Ϫ␦ ͓Fig. 3͑b͔͒.…”

“…2 These problems have stimulated the development of superconducting phases, such as Y 0.6 Ca 0.4 Ba 1.6 La 0.4 Cu 3 O 7Ϫ␦ , with improved corrosion resistance, higher thermodynamic stability, and lower oxygen diffusion rates. 2,7,8 However, the substitution of La for Ba and Ca for Y leads to a suppression of T C ͑ϳ80 K͒ relative to the parent compound YBa 2 Cu 3 O 7Ϫ␦ (T C ϳ92 K). Thus, it would be desirable to find simple REBa 2 Cu 3 O 7Ϯ␦ ͑REϭrare earth͒ formulations that exhibit higher intrinsic corrosion resistance while retaining a high T C value.…”

“…In addition, the magnetic measurements also yield a second independent a͒ Author to whom correspondence should be addressed; also with: Texas Materials Institute and Center for Nano and Molecular Science and Engineering; electronic mail: mcdevitt@mail.utexas.edu The corrosion sequence for YBa 2 Cu 3 O 7Ϫ␦ is believed to occur through a series of four steps. 2,6 In the first step, water enters into the structure along the b axis in the vacancies left at the O͑5͒ site ͓Fig. 3͑a͔͒.…”

“…The strong importance of the redox step is exemplified by the surprising observation that corrosion is halted almost completely for YBa 2 Cu 3 O 7Ϫ␦ samples submerged in water by biasing them to 0.8 V versus sodium-saturated calomel reference electrode. 2 Following the redox step, Ba 2ϩ ions leach out into the water phase due to the high hydration energy of Ba 2ϩ and the high solubility of Ba͑OH) 2 in water. The corrosion process is further stimulated by the internal stress that exists in these cuprate ceramic samples.…”