Bulk Nanostructured Silicide Thermoelectric Materials by Reversible Hydrogen Absorption-Desorption

The production of bulk nanostructured silicide thermoelectric materials by a reversible hydrogen absorption-desorption process is demonstrated. Here, high-pressure reactive milling under 100 bar hydrogen is used to decompose the Ca₂Si phase into CaH₂ and Si. Subsequent vacuum heat treatment results in hydrogen desorption and recombination of the constituents into the original phase. By changing the heat treatment temperature, recombination into Ca₂Si or Ca₅Si₃ can be achieved. Most importantly, the advanced synthesis process enables drastic and simple microstructure refinement by more than two orders of magnitude, from a grain size of around 50 µm in the initial ingot to 100-200 nm after the hydrogen absorption-desorption process. Fine precipitates with sizes ranging from 10-50 nm are forming coherently inside the grains. Thus, the route is promising and can be used for reducing thermal conductivity due to phonon scattering from grain boundaries as well as through nanostructuring with second-phase precipitates. Moreover, the process is environmentally friendly since hydrogen is reversibly absorbed, desorbed, and can be fully recovered.