Optimized Thermoelectric Properties in Ta-Doped NbFeSb Alloys via the Lanthanide Contraction Effect for Wearable Applications

NbFeSb thermoelectric materials require ultrahigh carrier concentrations (∼10²¹ cm^-3) to optimize their electrical transport properties due to their high density-of-state effective mass, yet the heavy doping-induced atomic radius mismatch disrupts lattice potentials, degrading carrier mobility while simultaneously enhancing point defect and phonon scattering, creating a critical trade-off between electronic and phononic performance optimization. This work optimizes the thermoelectric performance of Ta-doped NbFeSb-based half-Heusler alloys via the lanthanide contraction effect. The Nb_0.82-xTa_xTi_0.06Zr_0.06Hf_0.06FeSb (x = 0-0.25) alloys, synthesized through levitation melting and spark plasma sintering, exhibit exceptional room-temperature electrical conductivity (5000 S cm^-1) and carrier concentrations (2 × 10²¹ cm^-3). Ta doping enhances mass fluctuation scattering, reducing the lattice thermal conductivity by 24% while maintaining high power factors of 40 μW cm^-1 K^-2 across temperatures. The x = 0.1 composition achieves a peak zT of 0.8 at 973 K while maintaining excellent room-temperature electrical transport properties that are crucial for low-ΔT applications. Leveraging this material, a wearable thermoelectric wristband integrating 40 × 8 p-n modules (NbFeSb/ZrNiSn) was designed. Finite element simulations under ΔT = 16 °C demonstrate a maximum output power of 15.6 μW. Furthermore, the output power shows a positive correlation with the applied temperature gradient, highlighting its adaptability. This work highlights the synergy between lanthanide contraction-driven material optimization and device engineering, offering a robust solution for high-performance wearable thermoelectric applications.