Fractal Nanoparticles of Phase-Separating Solid Solutions: Nanoscale Effects on Phase Equilibria, Thermal Conductivity, Thermoelectric Performance

Abstract

In recent years, thermoelectric materials and energy converters have attracted considerable attention, especially as a part of advanced “green” energy and space technologies. One of the most promising ways of obtaining high values of the thermoelectric figure of merit is the formation of nanostructured 3D materials with nanoparticles of phase-separating alloys. In this chapter, using the example of low-temperature thermoelectric Bi1−x-Sbx alloys for the application in space engineering, we have shown how nanoscale effects on phase equilibria in nanoparticles influence on their thermoelectric properties. Such effects consist in nonlinear changes in mutual solubilities of components at a given temperature, phase transition temperatures and even the total suppression of the phase separation depending on the morphology of a nanoparticle as well as on some other factors. The combination of thermodynamic and ab initio approaches has been used while the nanoparticle shape has been determined using the methods of fractal geometry. A method has been suggested in order to calculate the optimal morphology of nanoparticles, at which their equilibrium phase composition leads to a dramatic reduction of the phonon thermal conductivity, favoring the growth of the thermoelectric figure of merit. A decrease in the phonon thermal conductivity in nanoparticles of a pure substance depending on their morphology as well as an approach of calculating the equilibrium size and shape distribution within a nanoparticle ensemble have also been discussed.