(144g) Thermodynamics & Kinetics of the Erbium-Hydrogen System

Metal hydrides are materials with a rich history and significant potential for emerging applications, including use as an energy carrier for hydrogen-based technologies[1,2]. Pressure-composition-temperature (PCT) properties and kinetics are critical attributes that help determine the suitability of a particular metal hydride system for a given application. In this report, the erbium-hydrogen system is examined experimentally, primarily using a PCT apparatus and thermal desorption spectroscopy, to characterize its isothermal P-C diagrams and hydriding/dehydriding kinetics. Rare earth metal hydrides, including erbium dihydride, are of recent interest due to unusual electronic and magnetic properties depending on hydrogen concentration[3]. A fundamental understanding of erbium-hydrogen isotherms and kinetics is important for accurately controlling composition following a range of processing steps, from loading films in a batch reactor to thermal vacuum treatment.

A well-calibrated PCT apparatus, using a volumetric technique known as Sieverts' method, can quantify the amount of hydrogen absorbed by the storage material. Simultaneously, a range of gauges are used to measure the equilibrium pressure. The resulting isotherms are useful for predicting the equilibrium composition for a given set of loading parameters. In this study, erbium-hydrogen isotherms have been generated for a range of operating temperatures. The equilibrium plateau pressures for the two-phase region are analyzed to determine differential enthalpy and entropy of reaction. For the α-β transition for a bulk erbium sample, ΔH and ΔS are determined to be -53.7 kcal/mol and -36.7 cal/molK. At present, the PCT has been used to verify and refine the accuracy of Lundin's isotherms for bulk erbium[4,5]. It is anticipated that experimental results for film samples may differ substantially from bulk samples, depending on film purity and processing parameters, particularly with respect to the lower β-phase boundary[3]. In addition to thermodynamic study, pressure versus time profiles generated using the PCT apparatus can be used to study isothermal hydriding kinetics. This analysis is ongoing at present.

Reaction kinetics control the composition of metal hydrides during non-equilibrium processes, such as the ?cool down? stage of the loading process. Thermal desorption spectroscopy (TDS) is an experimental tool ideally suited for studying desorption kinetics. A TDS study[6] of erbium dihydride decomposition for film samples has determined that the activation energy for desorption is roughly 60.0 kcal/mol. In addition, this value has been found to vary considerably with a range of different processing parameters.

1. Latroche, M., Structural and thermodynamic properties of metallic hydrides used for energy storage. Journal of Physics and Chemistry of Solids 65, 517-522 (2004).

2. Sandrock, G., A panoramic overview of hydrogen storage alloys from a gas reaction point of view. Journal of Alloys and Compounds 293-295, 877-888 (1999).

3. Vajda, P., Hydrogen In Rare-Earth Metals, Including RH2+x Phases. Handbook on the Physics and Chemistry of Rare Earths 20, 207-291 (1995).

4. Lundin, C. E., The erbium-hydrogen system. Transactions of the Metallurgical Society of AIME 242, 903-907 (1968).

5. Lundin, C. E., Thermodynamics of the erbium-deuterium system. Transactions of the Metallurgical Society of AIME 242, 1161-1165 (1968).

6. Ferrizz, R. M., Erbium Hydride Thermal Desorption: Controlling Kinetics. Sandia National Laboratories report SAND2007-2659 (2007).