The prediction of the hydriding thermodynamics of Pd-Rh-Co ternary alloys
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I. INTRODUCTION
PALLADIUM-BASED alloys provide an excellent system to explore the effects of alloying on the hydriding thermodynamics of hydrogen storage materials, because a wealth of literature exists documenting the behavior of Pd alloys. Models developed for predicting the hydriding thermodynamics of palladium-based alloys may then be applied to other more-economically feasible hydrogen storage materials. Palladium and its alloys would be attractive hydrogen storage materials due to their fast absorption/desorption kinetics, high storage capacities (H/M , 0.6), and structural stability with hydrogen cycling; however, the high cost and high density of palladium prevents the widespread use of palladium as a hydrogen storage material. Alloying palladium with rhodium increases the plateau pressures and also increases the hydrogen storage capacity.[1] In particular, the capacity is maximized at about Pd–7 at. pct Rh. However, recent studies[2–5] have shown that hydrogen causes metastable Pd-Rh solid solutions to phase separate. The established Pd-Rh phase diagram shows a miscibility gap with a lower boundary of about 12 pct Rh at 600 8C.[6,7,8] No data exist which delineate the boundary at lower temperatures because of the sluggish kinetics of the phase separation, but extrapolations to room temperature suggest that the boundary would be approximately Pd 5 to 7 at. pct Rh. Therefore, Pd alloys containing greater than 5 pct Rh are considered metastable and have the potential to undergo hydrogen-induced phase separation. Consequently, this may produce an altered thermodynamic response over time. To ensure a reliable alloy D.F. TETER and D.J. THOMA, Technical Staff Members, are with the Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545 This article is based on a presentation made at “The Milton Blander Symposium on Thermodynamic Predictions and Applications” at the TMS Annual Meeting in San Diego, California, on March 1–2, 1999, under the auspices of the TMS Extraction and Processing Division and the ASM Thermodynamics and Phase Equilibrium Committee. METALLURGICAL AND MATERIALS TRANSACTIONS B
with constant hydrogen absorption/desorption and hydriding/dehydriding thermodynamic properties over time, the alloy is required to be a stable solid solution (less than ,5 pct Rh addition). Unfortunately, decreasing the Rh content also decreases the plateau pressures. Cobalt forms an fcc solid solution with Pd to at least 50 at. pct Co.[9] A metastable ordered L12 compound, CoPd3, has been observed in thin films,[10] but has not been observed in bulk samples and can be considered to be a thin-film artifact. The ternary Pd-RhCo phase diagram is unknown; however, since cobalt forms a solid solution with palladium, cobalt would be expected to decrease the miscibility phase field in ternary space. Cobalt also increases the plateau pressures.[11,12] Therefore, with small additions of cobalt to a “lean” Rh alloy, it may be possible to approach the thermodynamic properties of a Pd–7 at. p
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