Preferential phonon scattering and low energy carrier filtering by interfaces of in situ formed InSb nanoprecipitates and GaSb nanoinclusions for enhanced thermoelectric performance of In0.2Co4Sb12

Abstract

Filling the voids of cage forming compounds with loosely bound electropositive elements and by incorporating nano-sized secondary phases are promising approaches to enhance the thermoelectric figure of merit of these materials. Hence, in this work, by combining these two approaches—inserting In into the voids of skutterudite Co₄Sb₁₂ as well as dispersing nanoparticles (GaSb)—we have synthesized various samples via ball-milling and spark plasma sintering. InSb as the secondary phase of the matrix, mixed with GaSb, forms the solid solution Ga_1−xIn_xSb. Nanocrystalline grains together with a few larger grains (10–30 μm) are found to be spread in In_0.2Co₄Sb₁₂. The former is comprised of either InSb, GaSb or Ga_1−xIn_xSb. Because of their identical space group and similar lattice parameters, InSb, GaSb and Ga_1−xIn_xSb could not be detected separately in EBSD. High resolution transmission electron microscopy (HRTEM) was used to resolve different phases, which showed GaSb grains of size ∼10–30 nm and InSb grains of size ∼30–100 nm. Scattering of charge carriers at the interfaces of InSb, GaSb and Ga_1−xIn_xSb as well as the matrix phases increased both the electrical resistivity and Seebeck coefficient. The multi-scale size distribution of grains, of both the matrix phase and the secondary phases, scattered phonons within a broad wavelength range, resulting in very low lattice thermal conductivities. As a result, an enhanced figure of merit of 1.4 was achieved for the (GaSb)_0.1 + In_0.2Co₄Sb₁₂ nanocomposite at 773 K.