Microstructure-hydrogen sorption property relationships in LaNi 5 -based alloys
- PDF / 1,379,689 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 62 Downloads / 217 Views
BB5.57.1
Microstructure-hydrogen sorption property relationships in LaNi5-based alloys Haruyuki Inui, Hiroki Sakamoto, Tokujiro Yamamoto and Masaharu Yamaguchi Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT The relationship between microstructures and hydrogen absorption-desorption properties described in terms of pressure-composition (P-C) isotherms has been investigated for binary and Co- and Al-added LaNi5-based alloys as a function of the number of hydrogen sorption cycles. From microstructure observations, factors determining absorption pressures are deduced and variations of P-C isotherms with cycle number for these three LaNi5-based alloys are discussed. INTRODUCTION Alloys based on LaNi5 with the hexagonal CaCu5-type structure have been used as negative electrode materials of rechargeable nickel-metal hydride (Ni-MH) batteries because of their fast activation, high storage capacity, long cycle-life and excellent electrochemical charge-discharge kinetics [1]. In order to further increase the storage capacity and cycle life of LaNi5-based alloys, Ni is partially replaced with Al [2,3] and transition metals such as Co and Mn [4], both of which are believed to be effective in improving the cycle life, and La is usually replaced with misch metals (Mm) to reduce the production cost [5,6]. Thus, alloys of Mm-Ni-Co-Al-Mn are commonly used for Ni-MH battery applications [6,7]. Although alloys based on LaNi5 are recognized to be rather easily activated, activation of these alloys usually involves a few to a several cycles of hydrogen absorption and desorption, after which reproducible absorption-desorption characteristics are obtained. This has been attributed mostly to the formation of fresh and clean surfaces that are free from adsorption gas molecules as a result of cracking and pulverization of LaNi5 particles due to hydride formation and dissolution, which involves lattice expansion of more than 20 % [1]. However, neutron and X-ray diffraction studies have indicated that extensively anisotropic line broadening occurs for {hki0}-type reflections even after the first cycle of hydrogen sorption [8-10]. This clearly indicates that lattice defects such as dislocations are introduced in the bulk of LaNi5 during activation. This is indeed confirmed by transmission electron microscopy (TEM) [11-13]. In our previous studies [13], numerous a-type edge dislocations of the order of 1012 cm-2 are observed to be introduced during the first absorption cycle of virgin LaNi5. The absorption pressure for the subsequent cycles is substantially smaller than that for the first cycle. This clearly indicates that hydrogen sorption properties are affected by microstructures to a large extent. In the present study, we investigate microstructure evolution in Co- and Al-added LaNi5 as a function of the number of hydrogen sorption cycles in order to establish the relationship between microstructures and hydrogen absorption-desorption properties. EXPERIMENTAL PROCEDURES Ingots wit
Data Loading...
