Imaging Nanoscale Kirkendall Effect on Oxidation Kinetics of Mg and MgH2 Particles by in Situ TEM

Abstract Understanding how Mg and MgH2 nanoparticles evolve under reactive and nonreactive environments is critical for hydrogen storage, energetic materials, and other functional applications where the native oxide shell governs mass transport and stability. Prior in situ studies were limited to subatmospheric O2 or high vacuum, where evaporation obscures oxidation-driven hollowing. Here, we use in situ transmission electron microscopy to compare the thermal evolution of Mg and Mg/MgH2 nanoparticles under atmospheric O2, atmospheric Ar, and a high vacuum. Under O2, both systems showed an inward oxide growth, a passivation window, and subsequent Kirkendall void formation via outward Mg transport. The vacuum promoted pressure-assisted rupture and sublimation, while Ar favored liquid-phase Mg extrusion. Relative to Mg, hydrogenated Mg exhibited a higher outward Mg flux and faster oxidation at lower temperature, indicating that hydrogenation enhances transport and accelerates oxidation. These results establish an atmospheric-pressure stability map, clarifying how oxygen activity, pressure, and hydrogenation govern oxidation.