Carrier Engineering via Al and Ga Doping Enhances Thermoelectric Properties in Si-Rich Higher Manganese Silicide

Higher manganese silicide (HMS) is regarded as a promising material for medium-temperature thermoelectric (TE) applications due to its superior electronic properties, high abundance, and eco-friendliness. In this work, we report carrier-engineering-driven TE properties of MnSi1.8−xGax and MnSi1.8−xAlx (x = 0.00, 0.025, 0.05, and 0.10) prepared by vacuum arc melting followed by resistive hot-pressing. Powder X-ray diffraction and FE-SEM studies confirmed the phase formation and successful substitution of Ga and Al at the Si site in the crystal lattice of HMS. Furthermore, the aliovalent acceptor dopants Ga and Al notably enhance the electrical conductivity by increasing the hole concentration, as evidenced by Hall measurements, and subsequently scatter low-wavelength phonons effectively, which leads to a substantial reduction in lattice thermal conductivity (κlat). The maximum TE figure of merits, zTs of 0.34 and 0.36, were obtained at 773 K for compositions of MnSi1.75Ga0.05 and MnSi1.775Al0.025, respectively. Moreover, the optimized compositions of MnSi1.75Ga0.05 and MnSi1.775Al0.025 have shown excellent mechanical hardness of ≈19.5 and ≈18.2 GPa, respectively, supporting their stability for durable mid-temperature TE applications.