INVESTIGATION OF KINETICS AND STRUCTURAL-CHANGES IN TIFE0.8NI0.2 AFTER PROLONGED CYCLING

Changes in the crystal structure and isothermal sorption kinetics of the hydride-forming alloy TiFe0.8Ni0.2 were investigated after prolonged cyclic sorption. The changes were followed by X-ray diffraction, reaction rate and pressure-composition-temperature (P-C-T) measurements, of samples after various numbers of cycles, up to 65000. After 65000 sorption cycles, a decrease of 16% in the hydrogen absorption capacity was detected. Residual, trapped hydride was indicated, the amount of which increased as the cycling advanced. Overall isothermal reaction rate measurements were interpreted by assuming a first-order rate law. The decomposition rates of TiFe0.8Ni0.2H after cycling were found to be lower than those obtained for non-cycled (activated only) alloy. The changes found in the kinetics and in the X-ray diffraction pattern of the alloy are discussed. A relationship between the reacted fraction-alpha and f(P), the pressure function, was established for closed measuring systems.