Phonons, Localization, and Thermal Conductivity of Diamond Nanothreads and Amorphous Graphene

Abstract

Recently, the domains of low-dimensional (low-D) materials and disordered materials have been brought together by the demonstration of several new low-D, disordered systems. The thermal transport properties of these systems are not well-understood. Using amorphous graphene and glassy diamond nanothreads as prototype systems, we establish how structural disorder affects vibrational energy transport in low-dimensional, but disordered, materials. Modal localization analysis, molecular dynamics simulations, and a generalized model together demonstrate that the thermal transport properties of these materials exhibit both similarities and differences from disordered 3D materials. In analogy with 3D, the low-D disordered systems exhibit both propagating and diffusive vibrational modes. In contrast to 3D, however, the diffuson contribution to thermal transport in these low-D systems is shown to be negligible, which may be a result of inherent differences in the nature of random walks in lower dimensions. Despite the lack of diffusons, the suppression of thermal conductivity due to disorder in low-D systems is shown to be mild or comparable to 3D. The mild suppression originates from the presence of low-frequency vibrational modes that maintain a well-defined polarization and help preserve the thermal conductivity in the presence of disorder.

Thermal Conductivity of Amorphous Materials

“…Zhu et al calculated the thermal conductivity of amorphous graphene using equilibrium MD (EMD) and a generalized model. [110,111] As shown in Figure 13a, the thermal conductivity is suppressed by the introduction of disorder, as expected. However, the degree of reduction in the thermal conductivity is much smaller than that in 3D materials.…”

“…The thermal transport properties of amorphous graphene sheets are quite different from those of 3D amorphous materials. Zhu et al calculated the thermal conductivity of amorphous graphene using equilibrium MD (EMD) and a generalized model . As shown in Figure a, the thermal conductivity is suppressed by the introduction of disorder, as expected.…”

“…a) EMD results for the thermal conductivity of pristine and amorphous graphene as a function of temperature. Adapted with permission . Copyright 2016, American Chemical Society.…”

“…Adapted with permission. [110] Copyright 2016, American Chemical Society. b) Temperature dependence of the thermal conductivity of amorphous graphene according to the generalized model and EMD.…”

The contribution of propagons and diffusons in heat transport through calcium-silicate-hydrates

“…To investigate the thermal activation degree of phonons, we compute the participation ratio of each vibrational mode. 28 This ratio characterizes the fraction of atoms that vibrate at a given vibrational mode. It can be expressed as…”