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We work on ways for improving the heat-to-electricity conversion efficiency of thermoelectrics. The materials of our current interests include 'defective' Heusler alloys, high-entropy half-Heusler alloys and superionic thermoelectrics. The experimental techniques used in this research include sample preparation by variety of methods, thermopower and electrical resistivity measurements from room temperature to 1250 K, and thermal diffusivity from 300 K to 1250 K using a Laser flash apparatus. The Hall effect measurement is done using a home-built, highly precise lock-in based set-up in vacuum and inert atmosphere up to 750 K.
'Defective' and entropy stabilized half-Heuslers
These are defect stabilized alloys that adopts the cubic hH structure in the presence of a large concentration of vacancies. The best example to illustrate this is that of NbCoSb which is stable in the stoichiometry Nb0.8CoSb. Presence of defects lowers the thermal conductivity of these materials by more than 100%. Below, some pictures from our work are shown. Left: The left panel highlights the presence of short-range ordering in the SAED pattern, manifested in the form of diffused bands (the wavy bands running between the diffraction spots due to the hH structure. Right: The right panel shows the thermoelectric figure-of-merit (zT) in Sn doped Nb0.8CoSb (slight Nb excess is taken for carrier concentration optimization.
Liquid-like Superionic Thermoelectrics
Recently, the superionic thermoelectrics, which typify the novel `phonon-liquid electron-crystal' concept, have attracted enormous attention due to their ultralow thermal conductivity and high figure-of-merit (zT). However, their high zT is generally obtained deep inside the superionic phase, e.g., near 1000 K in Cu2X (X: chalcogen atom) family where the superioinic transition is close to 400 K. At such high temperatures, the liquid-like flow of the metal ions under an electric eld or a temperature gradient, both of which are integral to the working of a thermoelectric device, results in device degradation. To harness the full potential of the superionic thermoelectrics, it is therefore necessary to reach high zT at low temperatures where the metal-ion diffusion is not an issue.
We have developed a novel all-room-temperature route to fabricate 100% dense, nanostructured Ag2Te with highly reproducible thermoelectric properties and a high zT of 1.2 at 570 K, i.e., merely 150 K above its superionic transition. The samples show a broad particle-size distribution ranging from a few nanometer to a few micrometer. This hierarchical nanostructuring is shown to suppress the thermal conductivity of Ag2Te beyond the phonon-liquid electron-crystal limit to ultra low values, leading to a remarkable enhancement of 87% in the zT over that of the ingot sample. These values supersede the zT of any Ag2Te previously reported.
Below we have shown some representative images from this work. Top: Ball-milled nanoparticles of Ag2Te showing a spectrum of shapes and degree of crystallinity which is one of the most important ingredients for an all-scale hierarchical nanostructuring. Bottom: Ultralow thermal conductivity obtained by combining 'phonon-liquid, electron-crystal' concept with hierarchical nanostructuring. Panel (d) shows the calculated phonon dispersion in the normal phase and (e) shows the effect of grain-size on the calculated thermal conductivity. These calculations are performed by Dr. Ankita Katre at the Center for Modeling and Simulation, Pune University.
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