The Impact of Nanostructuring on the Thermal Conductivity of Thermoelectric CoSb₃

Abstract

The high concentration of grain boundaries provided by nanostructuring is expected to lower the thermal conductivity of thermoelectric materials, which favors an increase in their thermoelectric figure‐of‐merit, ZT. A novel chemical alloying method has been used for the synthesis of nanoengineered‐skutterudite CoSb3. The CoSb3 powders were annealed for different durations to obtain a set of samples with different particle sizes. The samples were then compacted into pellets by uniaxial pressing under various conditions and used for the thermoelectric characterization. The transport properties were investigated by measuring the Seebeck coefficient and the electrical and thermal conductivities in the temperature range 300 K to 650 K. A substantial reduction in the thermal conductivity of CoSb3 was observed with decreasing grain size in the nanometer region. For an average grain size of 140 nm, the thermal conductivity was reduced by almost an order of magnitude compared to that of a single crystalline or highly annealed polycrystalline material. The highest ZT value obtained was 0.17 at 611 K for a sample with an average grain size of 220 nm. The observed decrease in the thermal conductivity with decreasing grain size is quantified using a model that combines the macroscopic effective medium approaches with the concept of the Kapitza resistance. The compacted samples exhibit Kapitza resistances typical of semiconductors and comparable to those of Si–Ge alloys.

Fabrication and characterization of nanostructured thermoelectric FexCo1-xSb3

“…Having a synthesis route with high yield, reproducibility and low energy consumption is one of the key issues that have to be tackled and overcome. Chemical synthesis routes usually take less energy, time and have less impact on environment [5,6]. CoSb 3 , known as skutterudites, exhibit high TE figure of merit.…”

“…Iron substituted CoSb 3 , FexCo 1-x Sb 3 , have been reported to display good performance as a p-type TE material, in terms of its TE figure of merit (ZT), and abundance of the constituent elements [8][9][10][11][12][13][14]. The ZT can be improved via introduction of dopants, nanoengineering and in case of skutterudites, introducing rare earth elements in the crystal cages to reduce the thermal conductivity [1,[4][5][6] due to rattling of the fillers.…”

“…Following this a thermochemical process of no more than 3 to 4 hours of heating will result in high purity skutterudites [5,[19][20][21]. In this work, iron has been selected as the substituent element and the materials have been synthesized via a novel chemical synthesis technique.…”

Nanoscale granular boundaries in polycrystalline Pb0.75Sn0.25Te: an innovative approach to enhance the thermoelectric figure of merit

“…We are observing very similar reductions in thermal conductivity to those of the Si nanowires with a roughened surface as reported in Refs. [12] and [13]. Previously, Chen also addressed the effect of interface roughness on the crossplane thermal transport in a superlattice structure [30].…”

“…They asserted that the embedded nanostructures allow a filtering effect that allows the 'alloy lower limit' to be overcome in the reduction of the thermal conductivity. Toprak et al also observed a substantial reduction in the thermal conductivity of CoSb 3 with decreasing grain size in the nanometer region [12]. A ZT value of 1.4 in BiSbTe alloy was reported lately, in which the authors believed that the ZT improvement is the result of low thermal conductivity caused by the increased phonon scattering by grain boundaries and defects [13].…”