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John O | September 2017

New study gives picture of thermal conductivity in complex materials


researchers from the institut laue langevin (ill) in grenoble, france and the laboratoire léon brillouin in saclay, france used inelastic neutron scattering (ins) and neutron resonant spin-echo (nrse) experiments to study the phonon lifetime of a clathrate to describe the thermal conductivity of complex materials.

 


 ba7.81ge40.67au5.33 was the focus of the neutron study. (institut laue langevin)

 

according to a report from the ill, this study highlights the importance of neutron techniques to overcome the challenges in accessing and measuring phonon lifetimes.

 

“whereas the ‘glass-like’ thermal conductivity of the clathrate ba7.81ge40.67au5.33 has frequently been associated with a short phonon lifetime, this study measured for the first time to date a very long phonon lifetime using a large single crystal sample of high quality,” the article explained.

 

“the study also reveals a dramatic reduction of the number of phonons carrying heat, as a result of structural complexity, allowing a simple and general explanation of the low thermal conductivity of complex materials.”

 

the thermal conductivity of semiconductor materials is a critical component of nanotechnology because a low thermal conductivity means a material can be used as an insulator, reducing heat transfer, and preventing components from overheating.

 

clathrates, such as ba7.81ge40.67au5.33, have thermoelectric properties that are used in harvesting waste heat and converting it to electricity. prior to this study, little was known about what caused the low thermal conductivity in these materials.

 

focusing on the neutrons, allowed the researchers to validate computational and theoretical understandings of the thermal properties of clathrates.

 

the research was recently published in nature communications. the abstract stated:

 

“engineering lattice thermal conductivity requires to control the heat carried by atomic vibration waves, the phonons. the key parameter for quantifying it is the phonon lifetime, limiting the travelling distance, whose determination is however at the limits of instrumental capabilities.

 

“here, we show the achievement of a direct quantitative measurement of phonon lifetimes in a single crystal of the clathrate ba7.81ge40.67au5.33, renowned for its puzzling ‘glass-like’ thermal conductivity. surprisingly, thermal transport is dominated by acoustic phonons with long lifetimes, travelling over distances of 10 to 100 nm as their wave-vector goes from 0.3 to 0.1 å−1.

 

“considering only low-energy acoustic phonons, and their observed lifetime, leads to a calculated thermal conductivity very close to the experimental one.

 

“our results challenge the current picture of thermal transport in clathrates, underlining the inability of state-of-the-art simulations to reproduce the experimental data, thus representing a crucial experimental input for theoretical developments.”

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