Thermal Stability of Hydrides of Disordered and Amorphous Alloys

Abstract

Hydrides of amorphous/glassy alloys have received considerable attention recently because of their potential applications as energy carriers, chemical storage of hydrogen, heat pumps, fuel cells, and heat engines (1,2). In all of these applications the uncharged intermetallic compound and the corresponding ternary hydride are subjected to a large number of charging and decharging cycles. A major disadvantage to using ternary hydrides which results after a relatively large number of cycles is the decomposition or disproportionation of the material so that it no longer absorbs hydrogen gas in a reversible way. It has been shown that this decomposition is a reaction by part of the ternary hydride to form the corresponding binary hydride of the more stable (stronger hydrogen-attracting component) plus free metal of the less hydrogen-attracting component (1). It is not well understood why this disproportionation reaction is significant for some systems and almost insignificant for other systems. In some cases an intermetallic compound is formed along with the more stable binary hydride instead of the free metal. Buschow, Bouten and Miedema (1) list over 100 intermetalic hydrides that have been prepared and partially characterized. There are several times this many that are known, yet few are really satisfactory for the applications listed above. This paper is limited to transition metal-transition metal type alloys where one metal (A) is the stronger hydrogen-attracting and (B) is the weaker hydrogen-attracting. Examples of A-type metals are early (IIIb, IVb, Vb such as Sc, Y, La, Ti, Zr, Hf, V, Nb, etc.) and B-type (late) (VIIIb, Ib such as Fe, Co, Ni, Cu, Rh, Pd, Ir, Pt, etc.). The alloys may be intermetallic compounds but often are all compositions that may be prepared by the melt-spinning technique and are only limited in composition by the range of glass-stability. These alloys are designated by the atom percent composition, such as Ti₄₅Cu₅₅ for an alloy of 45 atom percent Ti and 55 atom percent Cu. Both the crystalline (c−) and amorphous/glass (a−) alloys will be discussed in this paper. The systems that are discussed here are: a-TiCuH_x, c-TiCuH_x, c-Ti₂CuH_x, a-Zr₂PdH_x, c-Zr₂PdH_x, a-Zr₃RhH_x, where x refers to noninteger values for the hydrogen composition. In addition, the intermetallic alloys will be included in the properties discussed wherever appropriate. The experimental methods used to study thermal stability are differential scanning calorimetry (DSC), isothermal annealing, and powder X-ray diffraction (XRD). All XRD data have been taken at room temperature following quenching of the DSC or annealing studies.