Defects Generated by Hydrogen Absorption/Desorption in Lani 5 and Derivatives
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DEFECTS GENERATED BY HYDROGEN ABSORPTION/DESORPTION IN LANI5 AND DERIVATIVES B. Décamps1, J.-M. Joubert1, R. Cerny2 and A. Percheron-Guégan1 1 Laboratoire de Chimie Métallurgique des Terres Rares, UPR 209 - CNRS, 2-8 Rue Henri Dunant, 94320 Thiais cedex, France 2 Laboratoire de Cristallographie, Université de Genève, 24 Quai E. Ansermet, 1211 Genève 4, Switzerland ABSTRACT The combination of bright-field and weak-beam transmission electron microscopy (TEM) techniques has been used to analyse the dislocation systems activated in LaNi5 and derivatives after absorption/desorption hydrogen cycling. The TEM results are discussed and compared with those obtained from the modelling of the anisotropic diffraction peak broadening using only two dislocation slip systems of the hexagonal structure (1). INTRODUCTION Due to their ability to form reversible hydrides at room temperature and ambient pressure, LaNi5 and its substitutional derivatives are commonly used for hydrogen storage. They are commercially developed as negative electrode in rechargeable nickel-metal hydride (Ni-MH) batteries due to their excellent properties (fast activation, high storage capacity, long cycle life and good charge/discharge kinetics). During hydrogen absorption/desorption cycling, defects such as dislocations are produced as a consequence of the interaction of hydrogen with the material. In particular, due to the difference of lattice parameters between the intermetallic compound and its hydride, misfit dislocations may be formed at their interface. In order to understand the material behaviour, it is essential to analyse the activated dislocation systems that are directly related with the hydride growth mechanisms and aging properties. It is well known that, after cycling, LaNi5 exhibits an intense anisotropic diffraction peak broadening that is reduced in several substitutional compounds (2, 3) and almost inexistent in a commercially corrosion resistant compound (4). Recently, this broadening has been studied in LaNi5 by means of neutron powder diffraction (5), and in LaNi5 and derivatives by means of synchrotron powder diffraction (1). By modelling this broadening effect, using the theory developed by Krivoglaz (6) and the procedure described by Klimanek et al. (7) for hexagonal systems (Table I), with only two slip systems [E1 (edge a-type 1/3 , basal {0001}) yielding isotropic broadening and E2 (edge a-type 1/3 , prismatic {0-110}) yielding anisotropic broadening (hkl broadened and 00l not broadened)], a good agreement has been obtained with the experimental data (1). Such systems together with c-type dislocations have been identified by transmission electron microscopy (TEM) in LaNi5 either deformed (8) or
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Table I. Dislocations and slip systems considered in hexagonal materials (7). Dislocation
Type
Burgers vector
Slip type
Slip plane
S1 S2 S3 E1 E2 E3 E4 E5 E6 E7 E8
Screw Screw Screw Edge Edge Edge Edge Edge Edge Edge Edge
1/3
1/3 1/3 1/3
1/3 1/3 1/3 1/3 1/3
Basal Prismatic Prismatic Prismatic Pyramidal Py
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