Structure and properties of metal hydrides prepared by mechanical alloying
Our research examines the structure and reversible hydrogen storage capacity of alloys based on the LaNi{sub 5} intermetallic. The alloys are prepared by mechanical alloying (MA), a technique particularly useful when alloying LaNi{sub 5} with low melting point elements such as tin and calcium. In LaNi{sub 5-y}Sn{sub y}, x-ray diffraction and Rietveld analysis show that tin preferentially occupies the Ni(3g) sites in the LaNi{sub 5} structure, and the unit cell volume increases linearly with tin content to a maximum tin solubility of 7.33 atomic percent (LaNi{sub 4.56}Sn{sub 0.44}). The addition of tin to LaNi{sub 5} causes (a) a logarithmic decrease in the plateau pressures for hydrogen absorption and desorption, which is consistent with the corresponding increase in the volume of the LaNi{sub 5} unit cell; (b) a decrease in the hysteresis between the pressures for hydride formation and decomposition, which is in agreement with a recent theoretical model for the effect; and (c) a linear decrease in the hydrogen storage capacity. Effect (c) is explained by a rigid-band model whereby electrons donated by the tin atoms occupy holes in the 3d band of LaNi{sub 5}, which could otherwise be occupied by electrons donated by the hydrogen atoms. Thermodynamic van`t Hoff analysis for these alloys show an increase in hydride formation enthalpy and no change in entropy with increasing tin concentration. LaNi{sub 5} with calcium additions shows enhanced kinetics of hydrogen absorption/desorption. The powder particles prepared by MA have a larger surface area than particles of the same overall size prepared by arc casting. All LaNi{sub 5}-based alloys prepared by MA in an inert environment require no activation for hydrogen absorption and suffer less comminution upon hydriding/dehydriding.
- Research Organization:
- Los Alamos National Lab., NM (United States)
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- W-7405-ENG-36
- OSTI ID:
- 106459
- Report Number(s):
- LA-UR--95-2653; CONF-950798--1; ON: DE95016965
- Country of Publication:
- United States
- Language:
- English
Similar Records
Electrochemical properties of LaNi{sub 5{minus}x}Ge{sub x} alloys in Ni-MH batteries
Reaction kinetics and x-ray absorption spectroscopy studies of yttrium containing metal hydride electrodes