The use of experimental thermodynamic data in the phase equilibria verification

This presentation summarises shortly the following experimental techniques: emf with liquid and solid electrolytes, vapour pressure measurements and c alorimetric studies in application to metallic alloys. It is shown that usu ally one experimental technique is not sufficient for a reliable separation of the Gibbs energy into appropriate: entropy and enthalpy contributions, which is necessary if one seeks the correlation between thermodynamic prope rties and phase equilibria. Critical assessment of the data available from different sources must be performed to get the proper thermodynamic informa tion. Several specific examples will be analysed for systems with the misci bility gap and with intermetallic compounds to demonstrate this point of vi ew. In some cases the correlation will be extended on the physical properti es of alloys as well as the structure. There are, however, some cases for which new experimental data are still ne eded to clarify the phase equilibria. Two examples from the field of calori metry and emf measurements will be discussed to show how thermodynamic info rmation must be coupled with the phase diagram investigations to obtain pro per results: Calorimetric measurements. Results obtained in Cu-As and Cre-As systems wil l be shown and compared with existing binary phase diagrams. New data on th e enthalpy of formation of solid and liquid Ge-As alloys let the descriptio n of the thermodynamic representation of the binary system be improved and express some doubts about the shape of the Ge-As phase diagram. emf studies. Results of electrochemical measurements with solid CaF2 electr olytes on Yb-Ba-Cu-O systems are analysed. This example indicates how emf r esults can be misinterpreted if simultaneous investigations of phase equili bria are not conducted. (C) 1999 Elsevier Science B.V. All rights reserved.