Temperature changes in the reaction bed were detected during the hydriding and dehydriding reactions of FeTi. There were large heat effects even when small amount of speciemen reacted in a stainless steel reactor. It is observed how the experimental data approach to the intrinsic reaction rates as the heat effect is reduced sequentially by mixing the sample with inert metal powder, Mn, and by using a specially designed Cu tube reactor which is highly heat conductive. Near intrinsic rate data with minimum heat effect were obtained using the Cu tube reactor and the sample with mixture of FeTi and Mn powder. Empirical rate equations are derived, which are helpful in the qualitative expectation and analysis of heat effect for a given set of experimental data.
The kinetic behaviors of the hydriding reaction of FeTi vary with temperature, hydrogen pressure and reacted fraction. That is explained by mixed rate controlling theory of both the surface reaction (dissociative chemisorption of hydrogen molecules) and the diffusion of hydrogen atoms through β phase. It is suggested that the characteristic behavior of nucleation and growth process is observed because the surface reactivity increases which the extent of reaction. The apparent activation energy of the surface reaction is measured to be very small, i.e. less than 1,500 cal/mol. The apparent activation energy of diffusion is about 8,550 cal/mol.
In the dehydriding reaction, initial reaction rate is very fast and the overall kinetic behavior varies with ($Peq-P_{H_2}$). The characteristic behavior of nucleation and growth is observed when ($Peq-P_{H_2}$) is low, while, at high ($Peq-P_{H_2}$), the reaction rate decreases monotonically with the extent of reaction. It is suggested that above phenomenon is due to the variation of the growth morphology of α phase with ($Peq-P_{H_2}$).
When a small amount of Mn or Ni is substituted for Fe in FeTi, hydriding reaction rate is increased significantly. For $Fe_0.9Ni_0.1Ti$, diffusion of hydrogen atoms is the rate controlling step, and the apparent activation energy is 7,500-7,800 cal/mol. For $Fe_0.85Mn_0.15Ti$, kinetic mechanism is found to be the same as that of FeTi.
