New synthesis route of highly porous InxCo4Sb12 with strongly reduced thermal conductivity

Abstract

Highly porous, In-filled CoSb 3 skutterudite materials with an attractive thermoelectric figure of merit (ZT * 1) and corresponding dense samples were fabricated through the cost-effective method of reduction in oxides in dry hydrogen and the pulsed electric current sintering (PECS) method, respectively. The reduction process was described in detail using in situ thermogravimetric analysis of Co 2 O 3 , Sb 2 O 3 and In(NO 3) 3 Á5H 2 O separately and in a mixture. Two methods to synthesise the same material were examined: (a) free sintering of an initially reduced powder and (b) PECS. The free-sintered materials with higher porosities (up to * 40%) exhibited lower values of electrical conductivity than the dense PECS samples (porosity up to * 5%), but the benefit of an even sixfold reduction in thermal conductivity resulted in higher ZT values. The theoretical values of thermal conductivity for various effective media models considering randomly oriented spheroid pores are in good agreement with the experimental thermal conductivity data. The assumed distribution and shape of the pores correlated well with the scanning electron microscope analysis of the microstructure. The lowest value of thermal conductivity, equal to 0.5 W/m K, was measured at 523 K for In 0.1 Co 4 Sb 12 with 41% porosity. The highest value of ZT max = 1.0 at 673 K was found for the In 0.2 Co 4 Sb 12 sample in which the porosity was 36%.

Structure Optimization and Multi-frequency Phonon Scattering Boosting Thermoelectrics in Self-Doped CoSb3-Based Skutterudites

“…From the EDS spectra of precipitate (see Figure S2 and Table S2 in Supporting Information), the contents of In and Sb are higher than those of the scanned grains (Figure S1 and Table S1). Furthermore, according to XRD results of the impurity phase and similar descriptions about InSb in related reports 41,42 allow us to reasonably determine that the precipitated phase is InSb.…”

Thermoelectric properties of In1Co4Sb12+δ: role of in situ formed InSb precipitates, Sb overstoichiometry, and processing conditions

“…It is worth noting that within the XRD detection limit, all the obtained samples do not contain the CoSb 2 phase, which usually deteriorates the electrical transport of the main phase and is formed due to the Sb deficiency in the skutterudite matrix attributed to the formation of InSb inclusions. 10,14,16,20,21 However, in our samples, the formation of the CoSb 2 phase was suppressed by the Sb overstoichiometry. Another point that we want to note is that there is a slightly higher content of indium in the skutterudite matrix in the MS and MSA samples.…”

“…4–8 Among various kinds of filled skutterudites, huge attention has been paid to In-filled skutterudites, where another additional mechanism of lattice thermal conductivity reduction can be realized to achieve zT > 1. 9–14 Initially, In atoms go into the filler site and the lattice thermal conductivity is greatly decreased along with the noticeable increase in the electrical conductivity. 9–14 However, when the solubility limit (maximum filling level in the cage of the framework) of In in In x Co 4 Sb 12 is exceeded (0.16 ≤ x max ≤ 0.27), 9,11,12,15,16 InSb secondary phase precipitates are formed predominantly at the grain boundaries of the main skutterudite phase.…”

“…9–14 Initially, In atoms go into the filler site and the lattice thermal conductivity is greatly decreased along with the noticeable increase in the electrical conductivity. 9–14 However, when the solubility limit (maximum filling level in the cage of the framework) of In in In x Co 4 Sb 12 is exceeded (0.16 ≤ x max ≤ 0.27), 9,11,12,15,16 InSb secondary phase precipitates are formed predominantly at the grain boundaries of the main skutterudite phase. 10,13,14,16–18 This leads to an even more pronounced decrease in the lattice thermal conductivity and the electrical resistivity (so-called compositing effect).…”

“…9-14 However, when the solubility limit (maximum lling level in the cage of the framework) of In in In x Co 4 Sb 12 is exceeded (0.16 # x max # 0.27), 9,11,12,15,16 InSb secondary phase precipitates are formed predominantly at the grain boundaries of the main skutterudite phase. 10,13,14,[16][17][18] This leads to an even more pronounced decrease in the lattice thermal conductivity and the electrical resistivity (so-called compositing effect). 19 Both the mechanisms account for excellent TE performance of In-lled skutterudites and the zT of 1.4-1.5 at 700 K. 13,20 However, the formation of InSb precipitates obviously leads to an Sb deciency in the skutterudite phase and thus the formation of a CoSb 2 impurity phase, which usually results in the deterioration of the transport properties of the material.…”