Absorption of hydrogen by titanium–cobalt and titanium–nickel intermetallic alloys. Part 2.—Thermodynamic parameters and theoretical models

Abstract

Thermodynamic relationships are derived which allow experimental enthalpy, entropy and free energy data for metal–hydrogen systems to be converted to conditions of constant volume. Discrepancies between these relationships and the equations developed by other workers are discussed. The relationships are used to convert to conditions of constant volume the experimental data for the TiCo–H and TiNi–H systems described in Part 1. The enthalpy at constant volume increases continuously with hydrogen content, indicating that under these conditions no hydride phases would be nucleated. However, for the purposes of comparing different metal–hydrogen systems there is little advantage in using the corrected data rather than the original experimental data. The correction to the entropy term is small and is subsequently omitted.

The enthalpy data for TiCo–H and TiNi–H are interpreted in terms of lattice and electronic factors. Discontinuities in the enthalpy against hydrogen content data for TiCo are probably a consequence of changes in the electronic term. The Miedema model for predicting the stability of metal hydrides is examined and found lacking.

The entropy data for TiCo–H and TiNi–H are analysed and compared with entropy data for the TiFe–H system. In TiFe there are 6 tetrahedral interstitial sites per metal atom accessible to hydrogen atoms. In contrast, only 1 site per metal atom is available to hydrogen in TiCo–H and TiNi–H. This is in spite of the fact that all three intermetallic alloys have the same body-centred-cubic structure. In the TiCo or TiNi lattice hydrogen may occupy special sites which are coordinated by a greater than average number of titanium atoms. The entropy at high hydrogen contents in TiNi is considered, and we propose that the unusual change in slope at a H/M ratio of 0.45 occurs because of an increase in the number of available sites from 1 to ≈ 6.