Enhancement of thermoelectric properties of CoSb₃ skutterudite by addition of Ga and In

Abstract

Filled skutterudites are known as excellent thermoelectric (TE) materials. The voids in the structure of skutterudites such as cobalt antimonide (CoSb3) can be filled or partially filled with a variety of different atoms. In the present study, we tried to fill both gallium (Ga) and indium (In) into the voids of CoSb3. The polycrystalline samples with nominal compositions of Ga0.2InxCo4Sb12 (x = 0.15, 0.20, 0.25, and 0.30) were prepared and their TE properties were examined from room temperature to 773 K. Ga and In added to CoSb3 partly filled the voids and most of the residual precipitated as nanoparticles at the grain boundaries. A tiny amount of Ga substituted for Sb at the Sb site of CoSb3, while it was more likely for In to fill the voids than Ga. All the samples exhibited negative values for the Seebeck coefficient. The lattice thermal conductivity was reduced effectively by the co-addition of Ga and In without degrading the power factor. The maximum dimensionless figure of merit (ZT) value of 0.95 at 725 K was obtained for the sample with the nominal composition Ga0.2In0.3Co4Sb12.

Enhanced thermoelectric properties of Ga and In Co-added CoSb3-based skutterudites with optimized chemical composition and microstructure

“…Generally, the presence of Ga and In atoms inside the voids expands the skutterudite lattice, 37,39 while substituting the Sb sites with Ga atoms tends to contract it. 27 Thus, the increase in the lattice parameter observed in the present study can be caused by the increase in the In filling fraction (a detailed analysis of the lattice parameter will be presented later). As a result, high-density bulk samples with densities exceeding 98% of the theoretical value were obtained.…”

“…Owing to the combination of a high S 2 ρ −1 magnitude and significantly reduced κ lat , the sample with x = 0.45 exhibited the maximum of zT > 1.1 at 675 K, corresponding to the enhancement of the best value obtained in the previous study (equal to approximately 14%). 27 To confirm the origin of high zT achieved at x = 0.45, Ga and In site occupancies in the skutterudite phase, the existing states of Ga and In atoms added in the amounts over the filling limits, and sample's microstructure were investigated in detail. Figure 4 shows the observed and calculated powder XRD patterns and the difference profile obtained for the samples with x = 0.35 and 0.45.…”

“…In particular, CoSb 3 -based filled skutterudites are considered promising TE materials. Alkali metals, 11,12 alkaline earth elements, [13][14][15] lanthanides, [16][17][18][19][20][21][22][23][24] electronegative elements, 25,26 and group 13 elements [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] are known as good candidates for filler elements. Among group 13 elements, indium (In), as a co-dopant in multiple filled skutterudites, show excellent TE properties of maximum zT values of 1.2-1.4, in which co-dopants such as Barium, Cerium and Ytterbium easily react with air and thus the special process is required to apply for mass production.…”

“…Furthermore, since Ga indicates the unique behavior in CoSb 3 such as dual-site occupancy and the desirable electronic band engineering, it is worth studying as a co-dopant with In in CoSb 3 -based skutterudites. 29,33,34,45 Our group has previously investigated the TE properties of Ga and In co-added CoSb 3 -based skutterudites 27 and found that (1) Ga and In could be simultaneously introduced into the voids of the skutterudite structure; (2) Ga could also occupy Sb sites; and (3) Ga and In added in the amounts exceeding the filling limit precipitated as (Ga,In)Sb nanoinclusions. These three phenomena contributed to the κ lat reduction significantly, and thus the maximum value of zT close to unity was obtained for the nominal composition Ga 0.20 In 0.30 Co 4 Sb 12 .…”

“…These three phenomena contributed to the κ lat reduction significantly, and thus the maximum value of zT close to unity was obtained for the nominal composition Ga 0.20 In 0.30 Co 4 Sb 12 . 27 In the present study, we have attempted to improve the zT of Ga and In co-added CoSb 3 by optimizing both its chemical composition and microstructure. After characterizing the TE properties of (Ga 0.8 In 0.2 ) x Co 4 Sb 12 (x = 0.050.45), we found that the sample corresponding to the stoichiometric composition of x = 0.45 has an optimized carrier concentration together with a preferable microstructure, leading to the maximum enhancement of zT equal to 14% (> 1.1).…”