Correlations of the relationships between hydrogen content, hydrogen chemical potential and electrical resistivity for palladium alloy-hydrogen systems: Possible catastrophe theory representation of relationships

Abstract

At temperatures at or near 25 °C, isothermal p- n relationships between the equilibrium hydrogen pressure p and the hydrogen content n (the atomic ratio of hydrogen to palladium) for the Pd-H system exhibit an initial region of markedly increasing p with increasing n (the mainly α phase region) followed by a region of substantial increases in n over which p is relatively invariant (the mainly α + β phase region) and finally a region where further increases in n require increasingly larger increases in p (the mainly β phase region). The forms of the gradual overall changes of p- n isotherms when palladium has been alloyed with increasing amounts of an alloying addition are now known for a substantial number of alloying elements. With the apparent exception of the Pd-Rh series of alloys, a general consequence of increasing the concentration of any of these various alloying elements has seemed to be a reduction in the range of n corresponding to the main region of coexistence of the α and β phases. A complementary consequence has been a reduction in the critical temperature T c (α, β) above which hysteretic differences can no longer be detected to exist between the detailed forms of p- n relationships derived during hydrogen absorption and desorption cycles. However, more complex behaviour appears to apply in the case of the Pd-Ni series of alloys. Some measurements of electrical resistivity changes during the electrolytic charging of Pd-Ni alloys with hydrogen are reported in this paper. These are discussed with respect to earlier allied information and the general problem of the phase relationships of the Pd-Ni-H system. The hydriding characteristics of nickel, palladium and platinum are also compared in this paper, and recent attempts to correlate the form of the p- n relationships with more precise details of the processes of phase transitions in such metal—hydrogen systems are discussed. Tentative suggestions that the progress of these phase transitions can be justifiably described in terms of “catastrophe theory” types of representation are advanced.