Thermodynamics of Hydrogen Solution and Hydride Formation in Binary Pd Alloys
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Basic and Applied Research: Section I
Thermodynamics of Hydrogen Solution and Hydride Formation in Binary Pd Alloys Ted B. Flanagan and S. Luo
(Submitted October 24, 2006) Some recent thermodynamic results for the solution of H2 and hydride formation in fcc disordered Pd-rich alloys based on a combined equilibrium-calorimetric technique are reviewed in this paper. This dual approach is more powerful than employing only one of these techniques. Some interesting thermodynamic results connected with these systems are presented, such as minima in DHH and DSH as a function of the H content of the alloys.
Keywords
metal-hydrogen, thermodynamics, hydrides
1. Introduction Combined equilibrium-calorimetric measurements will be described for the determination of thermodynamic properties of H2 absorption by random binary substitutional fcc Pd alloys. An earlier review covering this type of research was published by Kleppa[1] in 1983 for a large series of Pd-rich alloys. Most data were obtained at T ‡ 555 K, where the H2 solubilities are small and hydride phases do not form. They did, however, carry out measurements for Pd-H at relatively high pressures over the temperature range, 518-623 K, where thermodynamic data for hydride phase formation was determined calorimetrically.[2] The research to be reviewed here uses the same dual approach employed by Kleppa and co-workers,[1] where thermodynamic parameters are determined using calorimetry and equilibrium pH2 measurements. This dual approach has advantages as compared to the usual technique of employing only equilibrium pH2 measurements at a series of temperatures because the calorimetric data are unaffected by hysteresis in the dilute M hydride two-phase region[3] and it is more accurate in the phase transition regions while the equilibrium pH2 measurement method is more accurate in the dilute phase.
Based on their lattice constants relative to Pd, fcc Pd alloys fall into expanded or contracted categories and the former have a more exothermic and the latter, a less exothermic solution of H2 than Pd. This classification does not work well for borderline cases where the lattice changes are small but, otherwise, it is reliable.[4] Based on entropy changes it is generally agreed that some octahedral interstitial sites must be preferred to others.[1,5,6] Wagner and coworkers[7,8] have shown directly from Mo¨ssbauer measurements that H avoids interstices with Fe, Sn, Au, Pt, Co, etc., as nearest neighbors, however, it does not avoid interstices with Rh nearest neighbors. Except for the Pd-Rh and Pd-Ni fcc substitutional alloys, all of the other Pd alloys have decreased miscibility gap widths as compared to Pd-H and all fcc, disordered Pd alloys have greater dilute/(dilute + hydride) phase boundaries (THS, terminal H solubilities) than that for Pd-H. Another interesting feature are the minima found in plots of DHH and DSH against H/(Pd(1-x)Mx), mol ratio, = r for Pd alloys above their critical compositions, XM,c. A minima is also found for Pd-H above its Tc.[9] After the minima, t
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