Thermoelectric properties of chemically substituted Heusler-type Ru_2-xNb_1+xGa and Ru₂NbGa_1-xM_x (M = In, Ge, and Sn) alloys

Pallab Bag^1*, W. Z. Liu¹, Yung-Kang Kuo¹, C. N. Kuo², C. S. Lue²

¹Department of Physics, National Dong Hwa University, Hualien-97401, Taiwan
²Department of Physics, National Cheng Kung University, Tainan-70101, Taiwan

* Presenter:Pallab Bag, email:pallabbag@gms.ndhu.edu.tw

Investigation of thermoelectric (TE) properties in full Heusler alloys (FHAs) with cubic L2₁-type crystal structure is an active research area for the last few years due to its multifunctional applications [1]. The TE properties in FHAs strongly depend on the structural order and atomic arrangements in the crystals and the number of the valence electrons per formula unit of the constituents [1-5]. Lately, the nonmagnetic Ru-based FHAs such as Ru₂NbGa, Ru₂NbAl, Ru₂VAl, and Ru₂TaAl have considerable interest in thermoelectricity, as they exhibited narrow bandgap semiconducting or semimetallic behavior [2-5]. A substantial increase of ZT values by chemical substitutions in Ru₂TaAl and Ru₂VAl alloys were studied [3,5]. Therefore, a similar study on Ru₂NbGa alloy is highly anticipated for the improvement of ZT value. Here, we present the TE properties of the two series of Ru₂NbGa alloys by attempting chemical substitutions, (i) making the off-stoichiometry between Ru and Nb in Ru_2-xNb_1+xGa (x = 0.00, ±0.05, and 0.10) and (ii) doping the sp-elements M = Sn, In, and Ge onto the Ga sites in Ru₂NbGa_1-xM_x (x = 0.05, 0.10, 0.20) [6]. All these alloys show a semiconducting-like feature. Interestingly, the x = -0.05 sample in Ru_2-xNb_1+xGa series exhibits a substantial decrease in electrical resistivity compared to the pristine Ru₂NbGa sample and demonstrates metallic behavior at low temperatures. Besides, the Seebeck coefficient for the x = 0.10 alloy displays a positive sign, indicating that the hole-type carriers dominate the TE transport. Thermal conductivity (κ) reveals the behavior of the excitation of phonon modes with temperature and indicates a dominant lattice phonons heat conduction in all alloys. Moreover, the value of κ was found to be little affected by the chemical substitutions due to the mass-difference phonon scattering. As a result, the TE performances, i.e., the power factor and figure of merit values in Ru₂NbGa_0.90Sn_0.10, Ru₂NbGa_0.90Ge_0.10, and Ru₂NbGa_0.95In_0.05 alloys, show a substantial enhancement as compared to the parent compound.

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Keywords: Full Heusler alloys, Semiconductors, Transport properties, Thermoelectric properties, Point defects