Synthesis of High T _c Superconductors

Abstract

Before going into details of preparation, it would be worthwhile to note the undermentioned properties of YBCO system, as these are of relevance in determining the preparation procedure: On heating the Y:123 compound, it decomposes peritectically at about 1,020–1,040°C into Y₂BaCuO₅, CuO and BaCuO₂. For this reason, all preparation steps should better be carried out at temperatures below 1,000°C. Y₁Ba₂Cu₃O₇ (i.e. orthorhombic phase) is unstable at temperatures higher than 650°C. At 900–950◦C, the oxygen content per formula unit is less then 6.5 and the structure become tetragonal. If quenched in this form, the material is non-superconducting. To regain the oxygen to the desired value (i.e. close to 7), the material should be given a long anneal in O₂ in the temperature range of 400–500◦C, to enable the oxygen pick up. Otherwise oxygen can be incorporated by annealing at ∼900◦C in flowing oxygen for short durations and then, slow cooling in oxygen atmosphere.