Ajay Dhar

Half-Heuslers (HH) represent an emerging family of thermoelectric (TE) materials, wherein intrinsic doping enables a wide range of electronic functionalities. In recent years, the solid-state transformation phenomenonon of spinodal decomposition has been actively explored as an effective paradigm to attain bulk nanostructured TE materials via induced phase separation. In the present work, the implication of intrinsic doping and spinodal decomposition on the thermal and electrical transport parameters of nonstoichiometric (Ti, Zr)CoSb HH systems is examined and corroborated with the help of microstructural characteristics. The synthesized HH nanocomposites were found to contain coherent nanoscale heterogeneities along with nanoscale grain, which severely depress the lattice thermal conductivity, while the intrinsic doping due to interstitial Co and defects induced by excess Co off-stoichiometry favorably tunes the electrical transport. A maximum ZT of ∼0.7 at 873 K was obtained for optimized p-type ZrCo_1.03Sb_0.8Sn_0.2 HH nanocomposites, which is among the highest obtained in p-type HH alloys. The present work thus provides a fundamental basis to the understanding of defect engineering and to achieve highly efficient and cost effective HH compositions.

Compositional tailoring enables fine tuning of thermoelectric (TE) transport parameters by synergistic modulation of electronic and vibrational properties. In the present work, aspects of compositionally tailored defects have been explored in ZrNiSn based Half-Heusler (HH) TE material, to achieve high thermoelectric performance and cost effectiveness in n-type Hf-free HH alloys. In off-stoichiometric Ni-rich ZrNi1+xSn alloys in low Ni-doping limit (x<0.1), excess Ni induces defects (Ni-Vacancy antisite + Interstitials), which tends to cause band structure modification. In addition, structural similarity of HH and full-Heusler (FH) compounds and formation energetics lead to intrinsic phase segregation of FH nano-scale 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 873K for the optimized ZrNi1.03Sn composition. Our findings highlight a realistic prospect of enhancing thermoelectric performance via compositional engineering approach for wide applications of thermoelectrics.

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-earths elements as “fillers” and the non-availability 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 to the first principles based density functional theory (DFT) calculations which suggested that Fe leads to p-type conduction in CoSb3, while Se strengthen 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 favourable tuning of the electrical and thermal transport properties, which resulted 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 resulted ZT of Fe0.25Co0.75Sb2.965Se0.035 is higher by two orders of magnitudes than 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.