RESUMEN
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 Ga1-xInxSb. Nanocrystalline grains together with a few larger grains (10-30 µm) are found to be spread in In0.2Co4Sb12. The former is comprised of either InSb, GaSb or Ga1-xInxSb. Because of their identical space group and similar lattice parameters, InSb, GaSb and Ga1-xInxSb 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 â¼10-30 nm and InSb grains of size â¼30-100 nm. Scattering of charge carriers at the interfaces of InSb, GaSb and Ga1-xInxSb 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 + In0.2Co4Sb12 nanocomposite at 773 K.
RESUMEN
The thermoelectric efficiency of skutterudite materials can be improved by lowering the lattice thermal conductivity via the uniform dispersion of a nanosized second phase in the matrix of filled Co4Sb12. In this work, nanocomposites of Ba0.3Co4Sb12 and InSb were synthesized using ball-milling and spark plasma sintering. The thermoelectric transport properties were studied from 4.2 to 773 K. The InSb nanoparticles of â¼20 nm were found to be dispersed in the Ba0.3Co4Sb12 grains with a few larger grains of about 10 µm due to the agglomeration of the InSb nanoparticles. The +2 oxidation state of Ba in Co4Sb12 resulted in a low electrical resistivity, ρ, value of the matrix. The enhancement of the Seebeck coefficient, S, and the electrical resistivity values of Ba0.3Co4Sb12 with the addition of InSb can be credited to the energy-filtering effect of electrons with low energy at the interfaces. The power factor of the composites could not be enhanced compared to the matrix because of the very high ρ value. A minimum possible lattice thermal conductivity (0.45 W/m·K at 773 K) was achieved due to the combined effect of rattling of Ba atoms in the voids and enhanced phonon scattering at the interfaces induced by nanosized InSb particles. As a result, the (InSb)0.15 + Ba0.3Co4Sb12 composite exhibited improved thermoelectric properties with the highest zT of 1.4 at 773 K and improved mechanical properties with a higher hardness, higher Young's modulus, and lower brittleness.