Speaker
Description
Complex metallic alloys (CMAs) are long range ordered materials, characterized by large cells, comprising several hundreds of atoms and cluster building blocks. A key property of CMAs is the low lattice thermal conductivity (1.3 W/m.K), which suggests a potential application for CMAs for thermoelectricity.
Engineering lattice thermal conductivity commonly implies controlling the heat flow carried by waves of atomic vibrations called phonons. It is thus fundamental to quantify the heat transmitted and the distance travelled by a single phonon. However, this requires the knowledge of its lifetime, the determination of which is still at the limits of instrumental and numerical capabilities.
Here, we present quantitative measurement and calculation of phonon lifetimes in a single crystal of the clathrate-I phase Ba7.81Ge40.67Au5.33 and o-Al13Co4 which is an approximant of the quasicrystal, decagonal phase AlNiCo CMAs renowned for its puzzling glass-like thermal conductivity. Surprisingly, we find acoustic phonons with long lifetimes travelling over distances from a hundred to tens of nanometers and which are found to dominate the thermal transport. Considering only the three low-energy acoustic phonons, and the observed energy dependence of their lifetime leads to a calculated thermal conductivity in very good agreement with the experimental one. Atomistic simulations show that the finite phonon lifetime is an anharmonic effect, due to structural disorder, explaining the weak temperature of the phonon lifetime. This findings prove that structural complexity is at the origin of the low thermal conductivity in these systems, leading to a drastic reduction of the phase space for long-living heat carrier. Our results provide a novel picture of thermal transport in the systems and underline the state of the art for the simulations to reproduce the observed phonons lifetime and thermal conductivity.
