Thermal hydrogen absorption kinetics of metallic yttrium at high temperatures

Metallic yttrium hydride (YHₓ) is a candidate moderator material for new nuclear reactors due to high-temperature stability, high hydrogen density, and small neutron absorption cross-section. However, there is a gap in the hydrogen absorption kinetics data of yttrium in the high-temperature range of 700℃ to 900℃ in existing studies, and the correlation mechanism between microstructure and hydrogen absorption rate has not been clarified. For this purpose, this study used pure metal yttrium as the object and conducted experiments using the constant temperature and constant volume hydrogen absorption method combined with metallographic characterization techniques to systematically explore the kinetic laws of hydrogen absorption reactions at high temperatures. The results show that the hydrogen absorption reaction of metallic yttrium and yttrium hydrides is more in line with the first-order kinetic model. The reaction rate constant decreases significantly with the increase of hydrogenation degree (H/Y ratio), while the increase in temperature increases the rate constant significantly, and pure yttrium can achieve "instantaneous equilibrium" at 900℃. The activation energy of the metal yttrium hydrogen absorption reaction is 138.01 kJ/mol, and the hydrogen absorption phase transition follows the process of "α-Y → α-Y+δ-YHx → δ-YHx", with the grain boundaries being the preferred nucleation sites of the δ-YHx phase. This study fills the gap in high-temperature hydrogen absorption kinetics data, clarifies the rate control mechanism, and provides key data support for the application of yttrium-based materials in high-temperature hydrogen storage systems and nuclear reactors.