Trip of a Lifetime
Following phonons to understand thermoconductivity in complex materials
Joanna Cummings |
For industrial materials especially, low thermoconductivity is a useful property, but the principles behind how heat propagates through materials is not fully understood. A new study of clathrates (complex chemical substances containing cages that trap atoms) has revealed the source of their heat-insulating characteristics at the atomic level. Marc de Boissieu, Senior Scientist at France’s National Center for Scientific Research (CNRS) and coordinator of the research, explains his findings.
How is heat conducted through a material?
Heat is carried by quasiparticles named phonons (related to atomic vibration), which travel in the material at the speed of sound. While traveling, phonons collide with defects or with other phonons, like billiard balls. The more collisions there are, the lower the thermal conductivity. The mean distance a phonon travels without collision is called the phonon mean free path; the mean time between two phonon collisions is the phonon lifetime.
How are phonon lifetimes usually recorded?
Phonon lifetimes are most easily measured using inelastic neutron or x-ray scattering. However, current instrumental resolution only allows the measurement of a relatively short lifetime (5 ps) and mean free path (of the order 5–10 nm at most).
What were the goals of your study?
Understanding how the heat propagates in complex systems is a key issue in many different fields; however, an understanding of the detailed mechanism is still lacking. We measured experimentally, for the first time, the phonon lifetime in a thermoelectric material called clathrate, which is renowned for its very low “glass-like” thermal conductivity.
Were the findings what you expected?
The result came as a real surprise, although previous measurements had already provided some hints. We found unexpectedly long mean free paths ranging from tens to hundreds of nanometers – much longer than the 0.5 nm phonon mean free path that is commonly associated with such thermal conductivity. The study demonstrates a large reduction in the number of phonons effectively carrying heat.
To confirm all measurements were correct, we did a careful comparison with pure Ge single crystal (clathrates contain 75 percent Ge), that are already known to have very long lifetimes. We also carried out a detailed structural analysis to fully understand the atomic structure and the disorder. By combining this with atomic scale simulations, we were able to understand the distribution of phonon velocities.
What instrumentation enabled the study?
We used a high-resolution inelastic neutron scattering instrument and compared the data with pure germanium to detect a clear signal, indicating a mean free path over 20 nm. We then obtained an almost 10-times better accuracy by using a neutron resonance spin echo (NRSE) technique, which allowed us to measure phonon lifetimes up to 50 ps and a mean free path of up to 100 nm.
Why was such a big collaborative effort needed for this project?
The collaboration was set up within a 24-laboratory network dedicated to the study of complex intermetallic compounds (European C-MAC; www.eucmac.eu). Cutting-edge experimental and theoretical facilities were required for single crystal growth, in addition to atomic structure measurements of disorder, physical property measurements, inelastic neutron scattering measurement, and atomic scale simulations – expertise that belongs to different teams. The work highlights the importance of collaboration at the European level, which certainly goes against current research policies promoting competition between teams...
What are your next steps?
We have demonstrated a very long phonon lifetime, but the atomic-scale mechanisms responsible are not yet understood. This requires further experiments involving the same sample, other samples, and new atomic-scale simulations. Ultimately, we hope to derive general principles explaining thermal conductivity in structurally complex materials.
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