Hydriding kinetics analysis by the frequency response method.: Application to the ZrNi-H2(g) system

Hydrogen absorption-desorption reactions in the ZrNi-H-2(g) system were carried out with high-precision calorimeter-volumetric devices specifically devoted to simultaneous thermodynamic and kinetic analysis. Strict control of the experimental parameters ensures quasi-isothermal conditions during the phase transformation. The effect of the operating temperature in the range 400-500 K and the activation process on the kinetics of phase transformation was studied. Time-domain analysis of the kinetics was performed using an empirical rate law based on a nucleation and growth mechanism. Up to 500 K, the fit of the experimental kinetics is satisfactory. However, differences remain between the measured and calculated curves, especially in the early stages of the trans formation. To demonstrate evidence for a possible surface rate limitation, the kinetics were then analyzed in the frequency domain using the frequency response method (FRM). It is shown that the high-frequency shape of the transfer function plotted in Nyquist coordinates consists of a semi-circle located along the real axis, the diameter of which is directly related to the operating temperature and correlated with the activation process. The present possibilities and limitations of the FRM are discussed. (C) 2002 Elsevier Science BY. All rights reserved.