Conversion of 124 into 123 + CuO: microstructure and phase diagram
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Conversion of 124 into 123 + Cut: microstructure and phase diagram Donald E. Morris, Janice H. Nickel, Andrea G. Markelz, Ronald Gronsky, Mark Fendorf and Christopher P. Burmester 2-300 Lawrence Berkeley Laboratory, Berkeley, CA 94720 Abstract Conversion of 124 into 123 + CuO is interesting 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 strain 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 YBa 2Cu 4 O 8 (124) to YBa2Cu3O7 (123) decreases with decreasing temperatures, and below 850TC the boundary between 123 and 124 phase regions is found to fall approximately as log P(0 2 ) = 19.3 22000/F. The phase diagram of the Y-Ba-Cu-0 system is given as a function of temperature and partial pressure of oxygen over the range between 500WC and 1000'C and 10-6< P[02] < 102 bar. 1. Introduction: The motivation for this study of 124 -- 123 +CuO conversion is the possibility of enhanced intragrain critical currents due to flux pinning by small inclusions of non-superconducting CuO within the superconducting 123 grains converted from 124 [1]. In fact, the most important use for the 124 compound may be as a precursor for preparation of 123. The 124 phase is stable at elevated oxygen pressures (because of its higher oxygen content at higher temperatures), the 123 at lower pressures, and the 247 is stable in a narrow intermediate range [2,3]. The conversion process is interesting because the starting compound as well as the principal end product are superconductors, and the conversion is not dependent on the addition of elements that are not compatable with superconductivity. Thermal treatment of 124 in air or vacuum removes oxygen and causes conversion into 123 plus excess CuO, which may migrate to the grain boundaries. However, if the low P(O2) treatment is done at low temperature, or for a short time, the Cu will not reach the grain boundaries, but will be left within the 123 grains as CuG 'islands'. These intragranular non-superconducting islands may be effective as flux pinning centers [1] to increase Jc. Locally strained regions in the partially converted material may also act as flux pinning centers. The stability boundary between the 124 and 123 phases has been determined down to lower temperatures, by converting 124 into 123 at reduced P(0 2). (Conversion of 123 + CuO into 124 had been demonstrated earlier by heating 123 + CuG at high oxygen pressure [41, and recently at I bar [51.)
2. Sample Preparation and Experiments YBa2Cu408 was prepared by solid state reaction of a stochiometric mixture of the constituent oxides, following the procedure described earlier [6]. The samples were heated to 200'C and held for 5 minutes (to expel residual H2 0), then, the temperature was rapidly increased to the specified soak temperature and
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