Thermodynamics of formation of Th-Cu alloys

Electromotive force cells have been used to determine the Gibbs free energies, enthalpies and entropies of formation for ThCu6, ThCusub(3.6), ThCu2 and Th2Cu over the temperature range 729-1219 K. Solid CaF2 was used as the electrolyte. Comparison of the present measurements with earlier measurements on the most copper-rich phase shows that the free energies are reproducible by this technique to approximately 4%. The magnitudes of the entropies of formation were all measured as less than 1 cal/g-atom.K and are therefore physically reasonable. The values for the entropies of formation of ThCu6 and ThCusub(3.6) were found to be positive, that of ThCu2 to be essentially zero, and that of Th2Cu to be negative. Comparison with data from other thorium systems shows that positive entropies of formation are a typical for binary intermetallic phases of thorium. The free energies of formation of the Th-Cu phases were found to be roughly one-half as negative as values reported for Zr-Cu phases at comparable temperatures and stoichiometries. (author)