A multiscale approach to enhance the thermoelectric properties of α-SrSi2 through micro-/nano-structuring and Ba substitution

This study investigates the impact of nanostructuring and Ba substitution on the thermoelectric performance and lattice dynamics of cubic SrSi2. The solubility limit of Ba in alpha-SrSi2 is investigated under varied synthesis conditions, revealing a notable increase beyond previous reports. Mechanical alloying enables a remarkable enhancement, surpassing the conventional solubility limit of 13-14 %at. Ba. After spark plasma sintering (SPS), secondary phases appear above 22 % at. Ba, extending the solubility limit to 22 % at. Ba. High-pressure sintering at 1 GPa further extends this limit to 40 % at. Ba with excellent relative density. Raman and infrared spectroscopy were explored and unveil shifts in lattice dynamics, impacting phonon scattering and indicating a reduction in lattice thermal conductivity. From the electrical resistivity and Seebeck coefficient we were able to determine the evolution of the bandgap with the Ba content which led to an increase of the power factor by about 10 %. Increasing of Ba content also led to a strong decrease of the thermal conductivity leading to an increase of the figure of merit by 68 % compared to pure cubic SrSi2.