Effect of microstructure on the electrical conductivity of p-type Fe–Al–Si thermoelectric materials

Developing cost-effective thermoelectric (TE) materials that do not rely on toxic and expensive elements is crucial to widespread use in practical applications. Earth-abundant Fe3Al2Si3-based thermoelectric materials exhibit moderate thermoelectric properties near room temperature. Hence, improving the thermoelectric properties of these materials may lead to practical applications. In this study, we investigated the microstructural factors controlling the electrical conductivity (σ) in p-type Fe3Al2Si3-based thermoelectric (FAST) material to find ways to improve the TE performance. The σ of a large-grained stoichiometric τ1-Fe3Al2Si3 compound is low, ~1 × 104 Ω–1 m–1 at room temperature, whereas a high σ of ~6.7 × 104 Ω–1 m–1 is obtained in Al-lean alloys. Microstructure observation revealed that the formation of a composite structure consisting of FeSi (ԑ-phase) precipitates dispersed in the τ1-matrix results in an increase in σ while maintaining thermal conductivity and sufficiently high Seebeck coefficient to improve the power factor. We found that the Ohmic-contacted ԑ-phase can modify the surrounding τ1-matrix to an Al-lean τ1-phase, which contributes to the carrier concentration increment and brings six times the enhancement of σ. This led to a power factor of ~880 µW m–1 K–2, more than three times larger than that of the single-phase compound (~250 µW m–1 K–2), the highest for the earth-abundant Fe–Al–Si system. This work demonstrates that the control of τ1-Fe3Al2Si3/ԑ-FeSi composite microstructure is critical to developing a higher power factor in p-type FAST materials.