On hydrogen solubilities in Pd/alumina composites prepared by internal oxidization of Pd/Al alloys

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Internal oxidation of a Pd–Al alloy results in a composite consisting of Al 2O 3 precipitates within a Pd matrix. In accord with earlier studies (Huang et al., 1988. Scripta Met. 22, 1114), H dissolved in the Pd matrix becomes strongly and weakly trapped by the alumina precipitates. It is shown that the extent of strong H trapping is only weakly dependent on the Al content from X Al=0.005 to 0.06. H can be removed from the strong traps by evacuation at 573 K (2 h) but not at 473 K. After evacuation at 973 K, the strong traps themselves disappear but when the alloys are hydrided/dehydrided (cycled) at 323 K, traps reappear along with weak-dislocation traps due to dislocation formation resulting from the cycling. Although the former traps, which appear after cycling, are deep enough so that the corresponding P H 2 ( equil) cannot be readily measured (273 K), they are not as deep as the initial ones produced after internal oxidation since H can be removed from them by evacuation at 323 K. The number of weak-dislocation traps due to cycling the composite formed from internal oxidation (1073 K) is greater than for cycled pure Pd. A higher temperature is needed to anneal out the weak-dislocation traps arising from cycling the composites as compared to Pd; thus the alumina precipitates hinder the movement and annihilation of the dislocations. The enthalpies for H 2 solution in the dilute phase of the internal oxidation Pd–Al alloys have been determined directly by reaction calorimetry and they have a spectrum of trapping enthalpies.