Simultaneous optimization of thermoelectric and mechanical properties in p-type NbFeSb via entropy engineering

Abstract

NbFeSb-based half-Heusler (HH) alloys have drawn significant interest for their potential as thermoelectric (TE) materials due to their high-temperature stability and substantial potential for power generation. Nevertheless, their practical application remains limited by relatively low electrical conductivity (σ) and high intrinsic lattice thermal conductivity (κ_L). Herein, we employ entropy engineering as a core strategy to synergistically optimize the TE and mechanical properties of NbFeSb-based HH alloys. By introducing Ti/Zr/Hf co-doping, the σ is enhanced by several orders of magnitude, yielding an excellent peak power factor (PF) of ∼58.7 µW cm⁻¹ K⁻² for Nb_0.91M_0.09FeSb_0.98Sn_0.02 (M = Ti, Zr, and Hf) at 373 K. Entropy-driven multi-scale defects act as efficient phonon scatterers, leading to a low κ_L of ∼3.1 W m⁻¹ K⁻¹, while maintaining an outstanding PF of ∼31.9 µW cm⁻¹ K⁻². Consequently, a peak figure of merit (zT) of ∼0.52 is achieved for Nb_0.82M_0.18FeSb_0.98Sn_0.02 medium-entropy HH alloy at 923 K. Meanwhile, the entropy-driven mechanisms induce solid-solution strengthening, dislocation strengthening, precipitation strengthening, and grain refinement, endowing an exceptional Vickers hardness of ∼1118 HV for Nb_0.82M_0.18FeSb_0.98Sn_0.02. This work demonstrates entropy engineering as an effective strategy for the synergistic optimization of TE and mechanical properties of NbFeSb-based HH alloys, advancing their prospects for practical power generation from waste heat.