Conversion of 124 into 123 + CuO

This paper reports on conversion of 124 into 123 + CuO because it can produce non-superconducting CuO islands and highly strained local regions, both of which may act as flux pinning centers. Microstructural studies (TEM) show localized regions with high strains fields resulting from the lattice mismatch between 124 and 123 along the c axis. Enhanced Tc (95 K) was found in partly converted samples. The partial pressure of oxygen necessary for conversion from YBa2Cu4O8 (124) to YBa2Cu3O7 (123) decreases with decreasing temperatures, and below 850 degrees C the boundary between 123 and 124 phase regions is found to fall approximately as log P(O2) = 19.3 - 22000/T. The phase diagram of the Y- Ba-Cu-O system is given as a function of temperature and partial pressure of oxygen over the range between 500 degrees C and 1000 degrees C and 10-6 < P[O2] < 102 bar