Surface Diffusion-Controlled Jonhson–Mehl–Avrami–Kolmogorov Model for Hydrogenation of Mg-based Alloys

The use of the Jonhson–Mehl–Avrami–Kolomogorov (JMAK) model has become prevalent in the kinetic analysis for Mg-based hydrogen storage alloys. However, the classical JMAK model has shown inadequate results in certain cases, resulting in poor fitness and/or inexplicably low Avrami exponent n. To address this issue, this paper proposes a new model based on the classic JMAK model. The new model considers the preferential growth of the hydride phase from the surface into the bulk, resulting in a diffusion barrier effect during the hydrogenation of Mg-based hydrogen storage alloys. A new parameter, surface diffusion barrier index b, is introduced to quantify the surface diffusion barrier effect. The governing equation is expressed as ln( – ln [1 – Y(t)]) = n ln k + n ln t – bt. The modified JMAK model demonstrates improved fitting performance for the hydrogenation analysis in multiple Mg-based systems. Additionally, the relationship between diffusion barrier index b and the reaction rate is discussed. The results suggest that the faster initial reaction rate leads to a stronger diffusion barrier, which inhibits the subsequent hydrogenation for the alloy.