Thermodynamic properties and phase equilibria for Pt-Rh alloys

Abstract

The activity of rhodium in solid Pt-Rh alloys is measured in the temperature range from 900 to 1300 K using the solid-state cell Pt-Rh, Rh + Rh₂O₃/(Y₂O₃)ZrO₂/Pt_1-xRh_x + Rh₂O₃, Pt-Rh The activity of platinum and the free energy, enthalpy, and entropy of mixing are derived. Activities exhibit moderate negative deviation from Raoult's law. The mixing properties can be represented by a pseudosubregular solution model in which excess entropy has the same type of functional dependence on composition as the enthalpy of mixing, ΔH = X_Rh (1 - X_Rh)[-10,970 + 45X_Rh] J/mol ΔS^E = X_Rh (1- X_Rh)[-3.80 + 1.55 × 10^-2 X_Rh] J/mol·K The negative enthalpy of mixing obtained in this study is in qualitative agreement with predictions of semiempirical models of Miedema and co-workers and Colinet et al. The results of this study do not support the solid-state miscibility gap suggested in the literature, but are consistent with liquidus data within experimental uncertainty limits.