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.

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Filled skutterudites constitute an important class of efficient and stable thermoelectric materials for power generation; however, their commercialization has been hampered due to the usage of expensive rare-earth elements as “fillers” and the nonavailability of the efficient and compatible p-type counterpart. In view of this, we report a state-of-the-art thermoelectric figure of merit (ZT) in rare-earth-free p-type unfilled CoSb3 skutterudite co-doped with Fe and Se, synthesized using a facile process of arc-melting and spark plasma sintering, which is both fast and scalable. The doping of Fe and Se have been chosen in accordance with the first-principles-based density functional theory (DFT) calculations which suggested that Fe leads to p-type conduction in CoSb3, while Se strengthens the thermoelectric properties. The experimental results also suggest that the optimized partial substitutional doping of Fe at the Co-site and Se at the Sb-site in CoSb3 leads to a favorable tuning of the electrical and thermal transport properties, which resulted in a high ZT ∼ 0.7 at 870 K in an optimized skutterudite composition of Fe0.25Co0.75­Sb2.965Se0.035, which is the highest value reported thus far for unfilled CoSb3-based p-type skutterudites. The resulting ZT of Fe0.25Co0.75­Sb2.965Se0.035 is higher by 2 orders of magnitude than that for its pristine counterpart. In addition, the theoretically estimated transport properties of pristine and doped CoSb3, calculated employing the density functional theory (DFT) and Boltzmann transport equations, were found to be in good qualitative agreement with those measured experimentally.

Thermally stimulated depolarization current characteristics of Kapton‐H electrets are studied in the temperature range 303 to 573 K as functions of the polarizing field (133.3 to 500.0 kV cm−1), polarizing temperature (333 to 453 K), polarizing time (2.7 × 103 to 16.2 × 103 s), specimen thickness (2.5 × 106 to 12.5 × 106 nm), heating rate (0.017 to 0.10 K s−1), and storage time (0 to 720 h). The TSDC spectra comprise three maxima in all, namely, β, α, and ρ with their respective locations around 373, 453, and 553 K. These are attributed to dipole‐orientation, motion of space charges, and surface charge injection, respectively. A shoulder (β′) emerging around 343 K is suggested to be akin to the main β‐peak. The space‐charge peak is found to be more sensitive to the forming and storage parameters than to that involving dipoles. The observed dependence of the peak temperature as well as the activation energy for various peaks on polarizing temperature and time is indicative of a continuous distribution of relaxations.