Thermodynamic investigation of the LiAl and LiPb systems by the hydrogen titration method

Abstract

The hydrogen titration method (HTM) constitutes a new technique for investigating the thermodynamic properties and phase relations in binary A-B alloys. It is based on the reaction A( in alloy) + ( x 2 ) H 2( g) ⇌ AH x( s) in which constituent A in the alloy is the material being titrated, hydrogen is the titrant and AH x is the titration product. Equivalence points of the titration are indicated by the steeply rising segments in the isotherms of hydrogen pressure versus alloy composition and correspond to the stoichiometries of the intermediate phases in the alloy. For a given temperature and composition the chemical potential of constituent A can be determined from the corresponding hydrogen pressure and the standard free energy of formation of AH x . The method was tested on two lithium-based systems, LiAl and LiPb. The results verified the validity of the HTM by showing that (1) the hydriding reaction (with A  Li, x = 1) is reversible, (2) the anticipated intermediate phases are correctly identified and (3) there is a good agreement with the available e.m.f. data on the chemical potentials of lithium in these alloys. For the α + β field of the LiAl system the lithium activity, calculated from the combined HTM and e.m.f. data, is represented by In a Li = 2.662−5302 T −1. For the LiPb system, HTM results indicated the existence of a new phase having a stoichiometry between Li 22Pb 5 and Li 7Pb 2. Criteria for applying the HTM to other alloy systems are discussed.