Standard Gibbs Free Energy Changes for the Formation of YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>6.5+<I>x</I></SUB>, Y<SUB>2</SUB>BaCuO<SUB>5</SUB>, and Y<SUB>2</SUB>Cu<SUB>2</SUB>O<SUB>5</SUB> from Y<SUB>2</SUB>O<SUB>3</SUB>, BaCO<SUB>3</SUB>, and CuO

Abstract

Standard Gibbs free energy changes for the syntheses of YBa2Cu3O6.5+x, (a high temperature superconductor), Y2BaCuO5, and Y2Cu2O5 were investigated by using chemical thermodynamic methods.For the reaction where YBa2Cu3O6.5+x was synthesized from a Y2Cu2O5-BaCO3-CuO mixture with CO2 evolution, the relationship between the equilibrium temperature and the partial pressure of CO2 was examined by the thermogravimetric analysis. The equilibrium temperature (1080-1190 K) was determined by observing the change of sample weight at a given temperature in a stream of a CO2-O2-N2 gas mixture. The relationship between the equilibrium temperature and the partial pressure of CO2 for a Y2BaCuO5-BaCO3-CuO-YBa2Cu3O6.5+x mixture was also examined by the same method and it was confirmed that the mixture was more stable than a Y2Cu2O5-BaCO3-CuO-YBa2Cu3O6.5+x mixture in the equilibrium state.A similar experiment was performed for the synthesis of Y2BaCuO5 from a Y2Cu2O5-Y2O3-BaCO3 mixture in a temperature range 1090-1210 K.Measurements of electromotive force of the galvanic cell with a zirconia solid electrolyte were conducted for the synthesis of Y2Cu2O5 from Y2O3 and CuO. The construction of the galvanic cell was Pt|Y2Cu2O5, Y2O3, Cu2O |ZrO2, (Y2O3)|CuO, Cu2O|Pt, and the standard Gibbs free energy change for the synthesis of Y2Cu2O5 was derived from the partial pressure of O2 in the equilibrium among Y2Cu2O5, Y2O3, and Cu2O in a temperature range 1025-1220 K.By combining these results, the standard Gibbs free energy changes for the synthesis of YBa2Cu3O6.5+x and Y2BaCuO5 from Y2O3, BaCO3 and CuO were obtained.