Collective Effect of Fe and Se To Improve the Thermoelectric Performance of Unfilled p-Type CoSb₃ Skutterudites

Abstract

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.

Band Structure Modification and Mass Fluctuation Effects of Isoelectronic Germanium-Doping on Thermoelectric Properties of ZrNiSn

“…The calculated S and power factors are qualitatively comparable with experimentally measured values, although the quantitative alteration in the estimated and measured electronic properties is attributed to the defects generation during the synthesis process in the lattice. This may also be understood and attributed to the several approximations used in the theoretical calculations, such as constant relaxation time assumption and occurrence of synthesis defects induced atomic disorder, in a manner similar to observation made previously. ,, …”

“…For the pristine compound 4735 k points in IBZ were used, and for the alloys 595–624 k points were used. The other details of the calculations can be found in our previous work …”

“…This may also be understood and attributed to the several approximations used in the theoretical calculations, such as constant relaxation time assumption and occurrence of synthesis defects induced atomic disorder, in a manner similar to observation made previously. 49,60,61 Figure 7a exhibits temperature dependent κ of pristine ZrNiSn as well as Ge-doped samples. The total thermal conductivity was calculated by multiplying the measured thermal diffusivity, specific heat (C p ) and density.…”

Computationally Modelling the Use of Nanotechnology to Enhance the Performance of Thermoelectric Materials

“…Neither SiGe-20% SiMo nor Yb 13.5 La 0.5 ZnSb 11 manage to achieve ZT values above 0.9, with maximum values of 0.88 at 1273 K and 0.77 at 1213 K, respectively. Although Li et al (2019) found SiGe-20% SiMo performs almost 50% better than all other additions of SiMo to SiGe when it comes to electrical conductivity, the downfall of SiGe-20% SiMo is its high thermal conductivity over the entire temperature range [45]. It starts at 4.9 Wm −1 K −1 at 273 K, only dropping below 4 Wm −1 K −1 for the first time at 1100 K, and finishes at 3.9 Wm −1 K −1 at 1273 K. Although Yb 13.5 La 0.5 ZnSb 11 does not perform the best here, the maximum ZT of 0.77 recorded for this material is almost six times greater than the maximum value of 0.13 recorded when testing Yb 14 ZnSb 11 .…”

“…A promising thermoelectric material can be formed by doping with Bi and then producing a Mg 3 Sb 1.8 Bi 0.2 /graphene nanosheet (GNS) nanocomposite with a mass ratio of 80:1. Mg 3 Sb 1.8 Bi 0.2 /GNS (80:1) was modelled here after an investigation by Bhardwaj et al (2015) found this material to have a ZT of 1.30 at 773 K [45]. The modelling carried out using COMSOL produced a similar ZT level of 1.34 at 773 K. However, Bhardwaj et al (2015) did not test at temperatures above 773 K, and when Mg 3 Sb 1.8 Bi 0.2 /GNS (80:1) was modelled using COMSOL between 773 K and 873 K the maximum ZT value recorded was 1.65 and the maximum efficiency was 22.23%, both at 873 K. Mg 2.69 Li 0.01 Cd 0.5 Sb 2 and Mg 2.9875 Na 0.0125 Sb 2 recorded maximum ZT values of 0.84 and 0.62 respectively, meaning they both struggle to compete with their magnesium counterpart Mg 3 Sb 1.8 Bi 0.2 /GNS (80:1).…”