Chapter 134 Thermodynamic properties of metallic systems

Abstract

This chapter presents a compilation of the phase diagrams, the structure of possible intermetallic compounds and the thermodynamic data, enthalpies and entropies of formation or of mixing, of alloys based on rare earths and actinides for the purpose of comparing the behavior of rare earths and actinides on alloying. The experimental method that has most often been used is measurement of the electromotive force (emf) with liquid or solid electrolyte. This method leads to the values of the partial Gibbs energy of one of the constituents. Under certain conditions, the Gibbs energies of the formation of compounds present in the system under study are obtained. However, the enthalpies and entropies of formation of these compounds are obtained indirectly from the variation of the Gibbs energy of formation with temperature. This requires measurements in a large temperature range, but unfortunately the experiments do not always fulfill this condition and unlikely values of enthalpies and entropies of formation are sometimes obtained. While calorimetric methods, direct reaction calorimetry, or solution calorimetry using tin or aluminum as solvent have been performed in rare earth based alloys, this has not been the case in actinide-based alloys, where acid solution calorimetry has sometimes been used. This method, even when performed at room temperature, requires several precautions to ensure good dissolution of the compounds. To a lesser extent, vapor pressure measurements have been performed on rare-earth-and actinide-based alloys. These measurements present the same disadvantage as the emf method because the enthalpies and entropies of formation are obtained indirectly.