Kinetics of hydrogen absorption and thermodynamics of dissolved hydrogen in Tb 1x Zr x Fe 3 system R. Sivakumar a , S. Ramaprabhu a, *, K.V.S. Rama Rao a , H. Anton b , P.C. Schmidt b a Magnetism and Magnetic Materials Laboratory, Department of Physics, Indian Institute of Technology, Madras 600 036, India b Technische universitat Darmstadt, Institut fu¨r Physikalische Chemie, Petersenstraße 20, D64287, Darmstadt, Germany Abstract The kinetics of hydrogen absorption in the PuNi 3 type rhombohedral Tb 1x Zr x Fe 3 (x = 0.1, 0.2, 0.3) system are studied in the temperature range 75 R T(8C) R 150 at an initial pressure of 20 bar. By fitting the experimental kinetic data to a second order rate equation, the dierent phases [a,(a+b ), b ] and phase boundaries [a 4 (a+b ), (a+b ) 4 b ] of the alloy-hydrogen system are identified. The thermodynamical parameters viz. the relative molar enthalpy (DH H ) and the relative molar entropy (DS H ) of dissolved hydrogen are evaluated using the vant Ho equation and are found to be in the ranges (8 to 28) kJ (mol H) 1 and (32 to 66) J K 1 (mol H) 1 respectively. From the dependence of DH H with hydrogen concentration of the alloy-hydrides, the dierent phases [a,(a+b ), b ] and phase boundaries [a 4 (a+b ), (a+b ) 4 b ] of the alloy-hydrogen system are identified. The dierent phases identified by both kinetic and thermodynamic studies confirm those seen in the hydrogen absorption isotherms of the respective alloy. # 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. 1. Introduction The study of hydrogen absorption properties of intermetallic compounds is significant in regard to the general eort to develop ‘metal hydrides’, with the following characteristics—should have plateau pressure around 1 bar at temperatures slightly above room temperature and large reversible hydro- gen storage capacity. In addition to satisfying these criteria, the metal hydride should also have mini- mum plateau slope, minimum hysteresis, cost eec- tiveness, minimum activation, fast hydrogen absorption/desorption kinetics and good cyclic stab- ility. The Zr based alloys have good hydrogen sto- rage characteristics in addition to the fact that their hydrogen solubility properties can be modified by partially replacing Zr by other suitably chosen el- ements and, hence, the studies of hydrogen solubi- lity properties of Zr based alloys have been the focus of attention today. In our laboratory, we have initiated a program to study the hydrogen solubility studies on Zr based C15 Laves phase AB 2 alloys [1–6]. In our earlier work, we have presented, in detail, the hydrogen solubility, plateau pressure and the structural properties of the Zr based PuNi 3 type rhombohedral Tb 1x Zr x Fe 3 (x = 0.1, 0.2, 0.3) system International Journal of Hydrogen Energy 25 (2000) 463–472 0360-3199/00/$20.00 # 2000 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S0360-3199(99)00050-6 * Corresponding author. Tel.: +91-044-4458680; fax: +91- 044-2350509. E-mail address: ramp@mmm.iitm.ernet.in (S. Rama- prabhu).