Abstract
Using hydrogenography, we investigate the thermodynamic parameters and hysteresis behavior in Mg thin films capped by Ta/Pd, in a temperature range from 333 K to 545 K. The enthalpy and entropy of hydride decomposition, ∆Hdes = −78.3 kJ/molH2, ∆Sdes = −136.1 J/K molH2, estimated from the Van't Hoff analysis, are in good agreement with bulk results, while the absorption thermodynamics, ∆Habs = −61.6 kJ/molH2, ∆Sabs = −110.9 J/K molH2, appear to be substantially affected by the clamping of the film to the substrate. The clamping is negligible at high temperatures, T > 523 K, while at lower temperatures, T < 393 K, it is considerable. The hysteresis at room temperature in Mg/Ta/Pd films increases by a factor of 16 as compared to MgH2 bulk. The hysteresis increases even further in Mg/Pd films, most likely due to the formation of a Mg-Pd alloy at the Mg/Pd interface. The stress–strain analysis of the Mg/Ta/Pd films at 300–333 K proves that the increase of the hysteresis occurs due to additional mechanical work during the (de-)hydrogenation cycle. With a proper temperature correction, our stress–strain analysis quantitatively and qualitatively explains the hysteresis behavior in thin films, as compared to bulk, over the whole temperature range.
Highlights
In the search for light-weight hydrogen storage materials, Mg and Mg-based alloys play a prominent role [1,2,3,4,5,6,7]
We investigate the thermodynamic properties and hysteresis behavior of MgH2 thin films
We show that desorption thermodynamics of Mg/Ta/Pd films is similar to that of MgH2 bulk, whereas absorption parameters differ substantially from it
Summary
In the search for light-weight hydrogen storage materials, Mg and Mg-based alloys play a prominent role [1,2,3,4,5,6,7]. Large mechanical compressive stresses in thin films during (de-)hydrogenation—arising as a result of the rigid substrate—could influence the thermodynamic properties of thin films as compared to bulk materials. This so-called clamping effect could lead to false results and, needs to be analyzed in more detail. M = Ta, Ti, Fe) multilayers to determine the enthalpy and entropy of the hydride formation and decomposition of Mg. We show that desorption thermodynamics of Mg/Ta/Pd films is similar to that of MgH2 bulk, whereas absorption parameters differ substantially from it. A thorough analysis of the hysteresis behavior reveals that the large difference between hydride formation and decomposition at low temperatures is caused by the clamping effect.
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