Investigation of phonon suppression by nanostructuring and doping in thermoelectric half-Heusler materials

Abstract

We live in the age when humanity finds itself on the edge of energy crisis, fossil fuels are consumed and our energy consumption rises every year. One solution would be to obtain energy from renewable sources and to minimize the losses of energy produced, e.g. reuse the waste heat. Thermoelectric materials can convert heat directly and reversibly into electricity and allow therefore to use waste thermal energy more efficiently. Their benefits include the absence of moving parts, quiet operation, reliability, durability, and the fact that they do not produce any polluting emissions, so we can use them in a wide range of applications and they are also attractive from an environmental point of view. Half-Heusler alloys belong to one of the most promising thermoelectric materials composed of relatively non-toxic and abundant elements, with highest ZT = 1.5 at 700 K for Zr0.25Hf0.25Ti0.5Ni1Sn0.998Sb0.002 at% alloy. In our study we try to improve the thermoelectric performance of this alloy by doping it with semiconducting dispersion phase - β-FeSi2, which should reduce the thermal conductivity of the origin alloy. Since thermal conductivity depends to large extent on the propagation of phonons we have investigated how the nanostructuring of the samples by means of ball milling and doping impact the phonon behavior. For this purpose we have conducted inelastic neutron scattering experiments using the time-of flight spectrometer NEAT at Helmholtz Zentrum Berlin. In this paper are presented results of our study demonstrating the effect of phonon suppression by nanostructuring and doping in thermoelectric half-Heusler alloys.