Optimizing hydrogenation and optical switching performances of FC/Pd/Mg films via vacuum annealing: A Hall-Petch relationship perspective on Mg layer intermediate-temperature recovery

Mg-based films are promising candidates for hydrogen storage and switchable mirror applications, but their practical use is hindered by sluggish hydrogenation/dehydrogenation kinetics. The fluorocarbon (FC)/Pd/Mg film with an equiaxed Mg layer exhibits superior room-temperature hydrogen-chromic properties, and annealing is a critical strategy to tailor its microstructural and functional performances. In this work, vacuum annealing experiments were systematically conducted on FC/Pd/Mg films at temperatures ranging from 50 °C to 250 °C for different durations. The correlation between annealing-induced microstructural evolution of the Mg layer, hydrogenation/optical properties, and the Hall-Petch relationship was established. The Hall-Petch-derived logic, emphasizing grain boundary regulation of mass transport, was employed to interpret the grain size dependent hydrogen diffusion behavior. Among all annealing conditions, the film annealed at 100 °C for 0.5 h achieves the optimal comprehensive performance, with a reflectance conversion range of 67%, a transmittance conversion range of 40%, a complete hydrogenation time of 60 s, and a dehydrogenation sensitivity factor of 7.1. This enhancement is attributed to the intermediate-temperature recovery of the Mg layer, which induces dislocation recombination and grain coarsening (average grain size of 6.7 nm). Guided by the Hall-Petch principle, this grain size balances the grain boundary density and hydrogen diffusion path length, forming a microstructure with moderate defect density and favorable grain boundary density for hydrogen diffusion. This work provides a fundamental understanding of the annealing-microstructure property relationship in FC/Pd/Mg films from the perspective of the Hall-Petch relationship and offers a feasible approach to optimize the functional performances of Mg-based films for hydrogen energy-related applications.