Improvement of the Thermoelectric Properties in the FeSi2 Semiconductor Through Cu and Al Doping

The nontoxic β-FeSi2 semiconductor is gaining renewed interest as a thermoelectric material for waste heat recovery. Its earth-abundant elemental composition, consisting of iron (Fe) and silicon (Si), aligns well with the United Nations Sustainable Development Goals. However, the use of the β-FeSi2 semiconductor is limited by its high electrical resistivity. To improve the thermoelectric properties of the β-FeSi2 phase, specimens of FeSi2 were doped with Cu and Al and analyzed. X-ray diffraction and thermal analysis showed that small amounts (up to at least 2%) of Cu and Al dissolved into the FeSi2 phase. Cu doping reduced the electrical resistivity of FeSi2 but also lowered the Seebeck coefficient. In contrast, Al doping did not lower the Seebeck coefficient of FeSi2, while still reducing the electrical resistivity of FeSi2. Al doping thus improved the power factor of FeSi2 with 156 μW/mK2 and 314 μW/mK2 at room temperature for the 1% and 2% Al-doped specimens, respectively. Further thermal conductivity revealed that the Al-doped FeSi2 specimens showed lower thermal conductivity than the undoped FeSi2 specimen. Unlike in the case of the electrical resistivity, the 1% Al-doped specimen showed lower thermal conductivity than the 2% Al-doped specimen. The ZT of the 1% Al-doped specimen increased from 0.021 at room temperature to 0.052 at 600 K. This value was slightly higher than that of the Mn-doped β-FeSi2 but smaller than that of the Co-doped β-FeSi2.