Microstructure Optimization of Thermoelectric τ₁-Al₂Fe₃Si₃ via Graded Temperature Heat Treatments

To investigate the relationship between microstructure, chemical composition, and thermoelectric properties, we have applied graded temperature heat treatments to recently developed τ₁-Al₂Fe₃Si₃-based thermoelectric (FAST) materials formed by a peritectic reaction. We investigated microstructures, chemical compositions, and Seebeck coefficients as continuous functions of heat treatment temperature. The τ1 phase can become p- and n-type semiconductors without doping by changing the Al/Si ratio. The Seebeck coefficient was maximized, exceeding |S| > 140 μVK^-1 for both p- and n-type materials, by heat treatment at 1173 K for 24 h through microstructural optimization. These results show that combining the graded temperature heat treatments and spatial mapping measurements of thermoelectric properties gives effective routes to determine the suitable heat treatment temperature for materials with multiphase microstructure.