Dopant-induced structural modifications and thermoelectric properties in Al1.2Fe1.9M0.1B₂ intermetallic borides (M = Ag, Ni, Sb, Ga, Ge)

Abstract

AlFe₂B₂, a ternary transition metal boride doped with Ag, Ni, Sb, Ga, and Ge, was investigated for the phase constituents, microstructure, and thermoelectric (TE) properties. The parent compound with 20% excess Al (Al_1.2Fe₂B₂), prepared by vacuum arc melting, contains orthorhombic AlFe₂B₂ and FeB. Additional phases, such as Ag₅Al, AlNi₃, Al₃Ni₂, AlSb, and AlB₂, were formed upon 5% doping (Ag, Ni, Sb, etc.) at the Fe site. The change in lattice parameters (a, b, and c) of the Al_1.2Fe₂B₂ phase with Ag, Ni, and Sb-doping is small; however, it increases noticeably in Ga and Ge-doped compounds. The Ga-doped sample shows only FeB as a secondary phase; however, the AlB₂ phase is observed in Ge-doped AlFe₂B₂. The microstructure investigated in Field Emission Scanning Electron Microscope (FE-SEM) with Energy Dispersive Spectroscopy (EDS) shows the AlFe₂B₂ matrix is chemically homogenous in all samples, except the Ga-doped one, with uniformly distributed single or multiple secondary phases. The Differential Scanning Calorimetry (DSC) and thermogravimetry (TG) results show that all the dopants lower the peritectic reaction temperature of the AlFe₂B₂ formation, indicating the structure destabilizes. The measured TE properties show that AlFe₂B₂ is an n-type compound with electrical conductivity in the range of 0.35–0.46 × 10⁶ S/m. Adding Ag, Ni, Ga, and Ge only marginally alters the Seebeck coefficient and electrical conductivity. The noticeable improvement in the Seebeck coefficient, with negligible change in electrical conductivity, resulted in the highest power factor of 0.4mW/mK² for the Sb-doped sample. The thermal conductivity of AlFe₂B₂, which ranges from 6.4 to 9.1 W/m.K between 300 and 773 K, decreases with Sb doping to 5.2–8.5 W/m·K, resulting in a maximum zT of 0.04 at 773 K.