Compositional tailoring enables fine-tuning of thermoelectric (TE) transport parameters by synergistic modulation of electronic and vibrational properties. In the present work, the aspects of compositionally tailored defects have been explored in ZrNiSn-based half-Heusler (HH) TE materials to achieve high TE performance and cost effectiveness in n-type Hffree HH alloys. In off-stoichiometric Ni-rich ZrNi 1+x Sn alloys in a low Ni doping limit (x < 0.1), excess Ni induces defects (Ni/vacancy antisite + interstitials), which tend to cause band structure modification. In addition, the structural similarity of HH and full-Heusler (FH) compounds and formation energetics lead to an intrinsic phase segregation of FH nanoscale precipitates that are coherently dispersed within the ZrNiSn HH matrix as nanoclusters. A consonance was achieved experimentally between these two competing mechanisms for optimal HH composition having both FH precipitates and Ni/vacancy antisite defects in the HH matrix by elevating the sintering temperature up to the solubility limit range of the ZrNiSn system. Defect-mediated optimization of electrical and thermal transport via carrier concentration tuning, energy filtering, and possibly all scale-hierarchical architecture resulted in a maximum ZT ≈ 1.1 at 873 K for the optimized ZrNi 1.03 Sn composition. Our findings highlight the realistic prospect of enhancing TE performance via compositional engineering approach for wide applications of TE.

Despite Hf-free half-Heusler (HH) alloys being currently explored as an important class of cost-effective thermoelectric materials for power generation, owing to their thermal stability coupled with high cost of Hf, their figure-of-merit (ZT) still remains far below unity. We report a state-of-the-art figure-of-merit (ZT) ∼ 1 at 873 K in Hf-free n-type V-doped Zr1–x V x NiSn HH alloy, synthesized employing arc-melting followed by spark plasma sintering. The efficacy of V as a dopant on the Zr-site is evidenced by the enhanced thermoelectric properties realized in this alloy, compared to other reported dopants. This enhancement of ZT is due to the synergistic enhancement in electrical conductivity with a simultaneous decrease in the thermal conductivity, which yields ZT ∼ 1 at 873 K at an optimized composition of Zr0.9V0.1NiSn, which is ∼70% higher than its pristine counterpart and ∼25% higher than the best reported thus far in Hf-free n-type HH alloys. The enhancement of the electrical conductivity is due to the modification of the band structure by suitable tuning of the electronic band gap near the Fermi level, through optimized V-doping in ZrNiSn HH alloys. The reduction in the thermal conductivity has been attributed to the mass fluctuation effects and the substitutional defects caused by V-doping, which results in an abundant scattering of the heat-carrying phonons. The optimized V-doped ZrNiSn HH composition, therefore, strikes a favorable balance between cost and thermoelectric performance, which would go a far way in the realization of a cost-effective (Hf-free) HH based thermoelectric generator for power generation through waste heat recovery.

Less-expensive and abundantly available Hffree half-Heusler (HH) alloys are promising candidates for mid-temperature thermoelectric (TE). In the present work, we combine experimental outcomes with theoretical estimates to understand, design, and synthesize, Hf-free ZrNiSn 1−x Ge x based HH alloys with enhanced TE performance. A state-ofthe-art TE figure-of-merit (ZT) ∼ 0.92 at around 873 K was achieved for the optimal ZrNiSn 0.97 Ge 0.03 HH composition, wherein Ge atoms substitute Sn interstitial sites, as confirmed and understood by X-ray analysis and first-principles calculations, respectively. The isoelectronic Ge-doping improves electronic transport due to enhancement in carrier mobility. Concurrently, the reduction in thermal conductivity is attained by enhanced phonon scattering owing to mass fluctuation and strain field effects. The present work exhibits the efficacy of Ge as an effective dopant for HH alloys and strengthens the possibility of developing Hf-free cost-effective HH materials with high TE performance.