Magnetic monitoring of SmCo5 thin film hydriding kinetics

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Abstract
Sufficiently thin films of hydrogen storage alloys are found to not experience decrepitation upon hydriding and thus provide a well defined metal‐gas interface for the study of hydriding surface kinetics. An in situ magnetic torque method was developed to monitor hydriding‐dehydriding rates. The films produced by r.f. sputtering in argon were hydrided using 490 psia hydrogen gas and dehydrided against vacuum. The 1080Å film exhibited rate behavior described by the Equation, F(t) =1 −exp [−(t/τ)1/2]. The rate constant, τ, was 120 sec for hydriding and 574 sec for dehydriding. The rate behavior of a 1420Å film is best described by F(t) =1 −exp [−(t/τ)2], with τ=685 sec for hydriding and 214 sec for dehydriding. Partial pressures of 1, 2, and 4% nitrogen gas were added and the reaction rates measured again. The rate constants were about 1200 sec, 2450 sec, and 5900 sec, respectively, for 1, 2, and 4% partial pressures of N2. The rates were almost identical for both hydriding and dehydriding. The reaction rates did not fully recover after the nitrogen was removed. This indicates a blockage, which is not completely reversible, of the hydrogen entry points.

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