Low Lattice Thermal Conductivity in a Wider Temperature Range for Biphasic-Quaternary (Ti,V)CoSb Half-Heusler Alloys

Abstract

Intrinsically high lattice thermal conductivity has remained a major bottleneck for achieving a high thermoelectric figure of merit (zT) in state-of-the-art ternary half-Heusler (HH) alloys. In this work, we report a stable n-type biphasic-quaternary (Ti,V)CoSb HH alloy with a low lattice thermal conductivity κL ≈ 2 W m–1 K–1 within a wide temperature range (300–873 K), which is comparable to the reported nanostructured HH alloys. A solid-state transformation driven by spinodal decomposition upon annealing is observed in Ti0.5V0.5CoSb HH alloy, which remarkably enhances phonon scattering, while electrical properties correlate well with the altering electronic band structure and valence electron count (VEC). A maximum zT ≈ 0.4 (±0.05) at 873 K was attained by substantial lowering of κL and synergistic enhancement of the power factor. We perform first-principles density functional theory calculations to investigate the structure, stability, electronic structure, and transport properties of the synthesized alloy, which rationalize the reduction in the lattice thermal conductivity to the increase in anharmonicity due to the alloying. This study upholds the new possibilities of finding biphasic-quaternary HH compositions with intrinsically reduced κL for prospective thermoelectric applications.

Low-Lattice Thermal Conductivity in Zr-Doped Ti2NiCoSnSb Thermoelectric Double Half-Heusler Alloys

“…Ti 0.7 NiSb with inherent Ti vacancies exhibited ultralow-lattice thermal conductivity (κ l ) of 1.05 W/(m·K), close to the Debye–Cahill limit in HH alloys . While κ l as low as 1.5 W/(m·K) was obtained in isoelectronic Ta-doped NbFeSb, in ZrNiSn and ZrCoSb alloy systems, minimum κ l of 2 W/(m·K) with Hf doping has been observed. , Phase separation in V-doped TiCoSb yielded a low κ l value of 2 W/(m·K) due to spinodal decomposition during heat treatment . The combination of lighter element Ti and heavier element Hf in ZrNiSn further lowers κ l owing to an increase in the density of defects .…”

Strong electron–phonon coupling and high lattice thermal conductivity in half-Heusler thermoelectric materials

“…Traditionally, crystal defects were considered as a major factor behind the lower k l values. 4 However, as already mentioned, vacancy scattering may fail to fully explain such differences in 17,34 TiNiSb, 15 NbCoSb, 18 NbFeSb, 37 TiCoSb, 27 ZrCoSb 38 and HfCoSb 38 ).…”

“…In a recent study by Chauhan et al, the calculated phonons of VCoSb exhibited softened acoustic branches, and the large mode Gru ¨neisen parameters suggested strong anharmonicity, though the theoretical k l value was not calculated. 34 Interestingly, Hori et al calculated the lattice thermal conductivity of MnCoSb, a nominal 22-electron metallic half-Heusler compound. 35 The calculated k l value of MnCoSb (8.2 W m À1 K À1 ) was also significantly lower than the 18-e half-Heusler compounds TiCoSb (24.6 W m À1 K À1 ) and ZrCoSb (22.7 W m À1 K À1 ) due to stronger anharmonicity.…”

“…(a) Crystal structure of half-Heusler compounds of the general formula XYZ. (b) Experimental room temperature lattice thermal conductivity of various half-Heusler compounds (VCoSb,17,34 TiNiSb,15 NbCoSb,18 NbFeSb,37 TiCoSb,27 ZrCoSb 38 and HfCoSb38 ).…”