Inert Gas Pressure Effects on Phase Stability of Oxides

Abstract

Phase stability of Y-Ba-Cu-O superconducting phases was evaluated as a function of temperature, oxygen partial pressure and total gas pressure by the hot isostatic pressing method using argon and oxygen mixtures as a surrounding atmosphere. The phase transformation temperature from Y₂Ba₄Cu₇O_y to YBa₂Cu₄O₈ increased with increasing the total gas pressure, even if the oxygen partial pressure was kept constant. It indicates that the equilibrium partial pressure of a gas involving the chemical reaction changes if a solid-gas reaction is progressed under high total gas pressure. Dependence of chemical potential of the reactant gas on the inert gas pressure was thermodynamically discussed on the basis of Lewis-Randall's empirical scaling law and Ishizaki's approximation. These two approximations resembled each other at pressures less than the usual HIPing pressure.