# Thermodynamic Mixing Properties and Solid-State Immiscibility in the Systems Pd-Rh and Pd-Rh-O

Jacob, KT and Priya, S and Waseda, Y (1998) Thermodynamic Mixing Properties and Solid-State Immiscibility in the Systems Pd-Rh and Pd-Rh-O. In: Journal of Phase Equilibria, 19 (4). pp. 340-350.

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## Abstract

The thermodynamic activity of rhodium in solid Pd-Rh alloys is measured in the temperature range 950 to 1350 K using the solid-state cell: $Pt-Rh, Rh + Rh_2O_3/(Y_2O_3)ZrO_2/Pd_{1-x} Rh _x + Rh_2O_3, Pt-Rh.$ The activity of palladium and the free energy, enthalpy, and entropy of mixing are derived. The activities exhibit strong positive deviation from Raoult's law. The activities obtained by the electro- chemical technique, when extrapolated to 1575 K, are found to be significantly lower than those obtained from vapor pressure measurements. The mixing properties can be represented by a pseudosubregular solution model in which excess entropy has the same type of function dependence on composition as the enthalpy of mixing:$\Delta H = X_{Rh}(1 - X_{Rh})(31 130 + 4 585X_{Rh} J/mol and \DeltaS^{ex} = X_{Rh}(1 - X_{Rh})(10.44+ 1.51X_{Rh}) J/mol·K.$ The positive enthalpy of mixing obtained in this study in qualitative agreement with predictions of semiempirical models. The results predict a solidstate miscibility gap with $T_c = 1210(\pm5)K at X_{Rh} = 0.55(\pm0.02)$. The computed critical temperature is approximately 100 K higher than that reported in the literature. The oxygen chemical potential for the oxidation of Pd-Rh alloys under equilibrium conditions is evaluated as a function of composition and temperature. The Gibbs energy of formation of PdO is measured as a function of temperature. At low temperatures, the alloys are in equilibrium with $Rh_2O_3$, and PdO coexists with Pd and $Rh_2O_3$. At high temperatures, PdO is unstable and Pd-rich alloys are in equilibrium with diatomic oxygen gas.

Item Type: Journal Article Division of Mechanical Sciences > Materials Engineering (formerly Metallurgy) 13 Dec 2006 27 Aug 2008 12:28 http://eprints.iisc.ernet.in/id/eprint/9061
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