Thermodynamics and Kinetics Analysis of Hydrogen Absorption by Zr_0.8Ti_0.2Co Alloy

The ZrCo-based alloy is considered one of the most promising materials for hydrogen isotope storage in the conceptual design of a fusion reactor. However, there are few systematic studies on the thermodynamic and kinetic models of hydrogen absorption in the new Zr_0.8Ti_0.2Co alloy. The aim of this study is to computationally derive the general mathematical equations for the thermodynamics and kinetics of hydrogen absorption by Zr_0.8Ti_0.2Co. In order to obtain the thermodynamic and kinetic data quickly, a constant-flow hydrogen absorption test was used in this study. The thermodynamic performance test revealed that the Zr_0.8Ti_0.2Co hydrogen absorption transition process was switched from ZrCo to ZrCoH_x (metastable phase) and then to ZrCoH₃ with an enthalpy of hydrogenation (ΔH) of 66.59 kJ·mol⁻¹ H₂, which was obviously lower than that of the ZrCo-based alloy due to the metastable phase.

A mathematical model of the hydrogen absorption coupled with the kinetic equations was established by kinetic process analysis. The hydrogen absorption process was divided into two stages, and the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model could fit the two stages of the Zr_0.8Ti_0.2Co hydrogen absorption well. In the first stage, the JMAK index was n₁ = 1.04, activation energy Ea₁ = 7594.6 J/mol, and rate coefficient of reaction k₀₁ = 1.958E-4 s⁻¹. While in the second stage, it was n₂ = 1.39, Ea₂ = 5221 J/mol, and k₀₂ = 9.938E-5 s⁻¹. Based on the range of n values, it can be inferred that both the nucleation and growth mechanisms or the diffusion mechanism were expressed as the rate-limiting steps. Combined with the simulation software, metal hydride bed performance could be better investigated and the structural design could be guided by the obtained mathematical equation of Zr_0.8Ti_0.2Co hydriding.