Nanostructured $Co1−xNix(Sb1−yTey)3$ skutterudites: Theoretical modeling, synthesis and thermoelectric properties

The properties of Te-doped $Co(Sb1−yTey)3$ and Te-Ni double-doped $Co1−xNix(Sb1−yTey)3$ nanostructured skutterudites were evaluated by means of x-ray powder diffraction, and transport properties measured on the synthesized samples have been compared with ab initio theoretical modeling. Theoretical optimal dopant contents have been evaluated according to the maximum value of the power factor, calculating the electronic transport properties from the ab initio material band structure using semiclassical Boltzmann transport theory. The samples have been synthesized by chemical alloying with Te substitution for Sb up to $2.5at.%$ and Ni substitution for Co up to $2.0at.%$⁠. X-ray powder diffraction has been performed on all samples to reveal information about phase purity and Rietveld refinement was performed for the phase composition and cell parameter. The thermoelectric properties of the resulting consolidates were investigated in a temperature range from $300to723K$ using various measurement facilities. A standardization and round robin program was started among the participating evaluation laboratories in order to ensure reliability of the data obtained. The significant reduction in thermal conductivity, when compared to highly annealed $CoSb3$⁠, could be proved which is caused by the nanostructuring, resulting in a high concentration of grain boundaries. A combination of substitution levels for Ni and Te has been found resulting in the largest $ZT$ value of 0.65 at $680K$ among unfilled skutterudite materials.