Phonon propagation inhibition in nanocomposites

In the search of novel materials with optimized thermal properties, and more specifically with low thermal conductivity, nanocomposites have proved to be very promising. However, a large panoply of thermal properties have been reported in literature on nanocomposites, with thermal conductivities extremely small as well as much larger than in the parent materials. Today, theory still struggles for giving a microscopic understanding of such variety.

The ILM members, together with other colleagues, have used molecular dynamics for studying the propagation of the elementary waves of heat, phonons, in nanocomposites made of an amorphous matrix with nanocrystalline inclusions.

They have shown that nanoinclusions strongly scatter phonons with a wavelength comparable with the nanostructure, reducing their life time, so that they cannot propagate anymore. As such, there are less propagative phonons in the nanocomposite than in the pure material. This reduction dramatically affects thermal transport when propagative phonons are the main heat carriers. If this is not true, as in amorphous silicon, the effect may be negligible and thermal conductivity may even be enhanced thanks to the contribution from the more conductive nanoparticles. The thermal behavior of the nanocomposite is thus the result of a delicate balance between the role played in thermal transport by propagative phonons and the impact of the nanostructure on their number.

At low frequency, long wavelength, phonons propagate similarly in the amorphous (left) and in the nanocomposite (right – dashed lines mark the nanoinclusions position). However at high frequency and wavelength comparable with the nanostructure, this is not true anymore and the phonon does not propagate in the nanocomposite.