STANDARD GIBBS FREE-ENERGY OF FORMATION OF SUPERCONDUCTING YBA2CU3OX AND RELATED-COMPOUNDS IN THE Y2O3-BAO-CUO SYSTEM

Four galvanic cells were constructed with single-crystal CaF2 solid electrolytes. The cell e.m.f. was measured and expressed as E202 = - 0.074 + 8.8 × 10 -5 TE211 = - 0.061 + 7.8 × 10 -5 TE123 = - 0.011 + 1.6 × 10 -4 TE011 = - 0.063 + 1.43 × 10 -4 T all in volts at 950-1200 K. From the cell e.m.f., the standard Gibbs free energy of formation of Y2Cu2O5(202), Y2BaCuO5(211), YBa2Cu3Ox(123) and BaCuO2(011) from the constituent oxides was calculated and expressed as ΔG°202 = 14.3-0.017 TΔG°211 = -24.4-0.038 TΔG°123+δ = 109.0 + 4.6 × 10-3T + 6.7 × 10-3δT AG°011 = -43.9 + 7.3 × 10-3 T all in kilojoules per mole. The phase relationship in the region of CuOY2Cu2O5Y2BaCuO5YBa2Cu3Ox was determined and the directed reaction process in preparing single-phase YBa2Cu3Ox was discussed. © 1990.