OPTIMIZATION OF SUPERCONDUCTIVITY IN THE HIGH-PRESSURE SR-CA-CU-O SYSTEM

Samples of nominal composition (Sr0.7Ca0.3)1-xCuOy with 0 less-than-or-equal-to x less-than-or-equal-to 0.10 and (Sr0.7Ca0.3)3Cu2Oy were treated under pressure 4-6 GPa in various conditions of temperature, post-annealing and oxygen pressure. For x = 0 and in the absence of an oxidizer, the result is single-phase, non-superconducting layered (Sr, Ca)CuO2 with infinite CuO2 planes (Cu(infinity) phase). In the presence of an oxidizer, as well as excess CuO (x > 0), the multiphase product consists mostly of Cu(infinity) and (Sr, Ca)14Cu24O41, according to X-ray diffraction. A systematic study of the experimental parameters showed that large superconducting volumes are only obtained under specific conditions of high-pressure treatment, temperature, pressure and duration, followed by quenching and post-annealing. Samples with T(c) = 100 K and volume fractions up to 31% of the theoretical (-1/4pi) value were obtained.