Acoustic and thermal transport properties of hard carbon formed from C60 fullerene

TL;DR

This study investigates the complex thermal and acoustic properties of hard carbon derived from C60 fullerene, revealing inhomogeneous structures, phonon localization, and a temperature-dependent thermal conductivity consistent with fractal and amorphous models.

Contribution

It provides detailed insights into the inhomogeneous structure and vibrational dynamics of hard carbon from C60, introducing a phonon-fracton hopping model for disordered materials.

Findings

Inhomogeneous, fractal-like structure with amorphous clusters and microcrystallites.

Evidence of phonon localization and crossover to fracton excitations.

Thermal conductivity follows a power law T^(1.4) from 0.1 to 10 K and is linear in T from 20 to 300 K.

Abstract

We report on extended investigation of the thermal transport and acoustical properties on hard carbon samples obtained by pressurization of C60 fullerene. Structural investigations performed by different techniques on the same samples indicate a very inhomogeneous structure at different scales, based on fractal-like amorphous clusters on the micrometer to submillimetre scale, which act as strong acoustic scatterers, and scarce microcrystallites on the nanometer scale. Ultrasonic experiments show a rapid increase in the attenuation with frequency, corresponding to a decrease in the localization length for vibrations. The data give evidence for a crossover from extended phonon excitations to localized fracton excitations. The thermal conductivity is characterized by a monotonous increase versus temperature, power law T^(1.4), for T ranging from 0.1 to 10 K, without any well-defined plateau, and a strictly linear-in-T variation between 20 and 300K. The latter has to be related to the linear-in-T decrease of the sound velocity between 4 and 100 K, both linear regimes being characteristic of disordered or generally aperiodic structures, which can be analysed by the "phonon-fracton hopping" model developed for fractal and amorphous structures.