Elastic stress in composite FeTi hydrogen storage materials
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Elastic stress in composite FeTi hydrogen storage materials P. Tessiera) Centre for the Physics of Materials, Department of Physics, McGill University, 3600 University Street, Montr´eal, Qu´ebec, H3A 2T8, Canada
R. Schulz Technologie des mat´eriaux, IREQ – Institut de recherche d’Hydro-Qu´ebec, 1800 boul. Lionel-Boulet, Varennes, Qu´ebec, J3X 1S1, Canada
J. O. Str¨om-Olsen Centre for the Physics of Materials, Department of Physics, McGill University, 3600 University Street, Montr´eal, Qu´ebec, H3A 2T8, Canada (Received 26 February 1997; accepted 8 August 1997)
A simple model of the elastic stress in a composite hydrogen absorbing material is developed to account for the hydrogen storage properties of nanocrystalline FeTi with a network of intergranular phase having a wide storage site energy distribution. The model accounts for the equilibrium properties of nanocrystalline FeTi hydrogen absorbers made by ball-milling such as the narrowing of the miscibility gap and changes in plateau pressure. A second model is proposed for disconnected inclusions of the second phase. The effect of chemical disorder is also briefly examined. I. INTRODUCTION
The study of nanocrystalline metals has revealed properties which can differ considerably from those of coarse-grained polycrystalline materials of the same composition. The potential for applications of some of these properties has recently led to an intensification of the research effort on nanocrystalline metals and other nanophase materials. Hydrogen storage is one of the promising applications of nanocrystalline materials. Experiments on FeTi, LaNi5 , and Mg2 Ni1–3 have resulted in dramatic progress which was long overdue in the field. Series of x-ray diffraction, transmission electron microscopy, and hydrogen absorption experiments have also been performed on nanocrystalline palladium prepared by the inert gas condensation method. Those studies were mostly aimed at a fundamental understanding of the structure of grain boundaries in nanocrystalline palladium.4 –10 The process of ball-milling, applied to FeTi, causes the formation of an amorphous layer between crystallites.1,11 This amorphous layer results in a wide energy distribution of hydrogen sites concentrated at grain boundaries. We develop a model to describe such a composite storage material. This model might be applied to other nanocrystalline materials. II. OVERVIEW OF HYDROGEN STORAGE RESULTS
We present in this section the equilibrium behavior of four samples prepared by high energy ball-milling: a)
Present address: National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki, 305, Japan.
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http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 6, Jun 1998
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coarse-grained FeTi made by milling the intermetallic compound for only 2 min, two nanocrystalline samples made from the intermetallic compound and from elemental powders and called nanocrystalline Fe50 Ti50 IC and nanocrystalline Fe50 Ti50 -EP, respec
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