Minimum thermal conductance of twisted-layer graphite nanofibers

TL;DR

This study investigates how twisting layers in graphite nanofibers affects phonon thermal conductance, revealing significant reductions at specific angles and highlighting the role of optical modes and size effects.

Contribution

It provides new insights into the dependence of thermal conductance on twisted angles and layer overlap, identifying critical angles and the influence of finite-size effects.

Findings

Thermal conductance can be reduced by about 50% at specific twisted angles.

Optical phonon modes are mainly affected by twisting, while acoustic modes remain largely unchanged.

Critical angles for minimum conductance depend on nanofiber size and edge proportions.

Abstract

We study the thermal transport properties of twisted-layer graphite nanofibers. We show that in the presence of a twisted layer, the phonon thermal conductance of a graphite nanofiber varies remarkably with the twisted angle and can reach minimum values either at two critical angles $\theta_1$ and $\theta_2$ that conform to the rule $\theta_1$ + $\theta_1$ = $180^0$ or exactly at the angle $\theta$ = $90^0$. A reduction of roughly 50% of the phonon thermal conductance can be achieved in some structures. We unveil that the twisting effect mainly influences the optical modes, leaving almost unaltered the acoustic ones. The effect is also visible in the higher and more numerous van Hove singularities of the phonon density of states. We also point out that the behavior of the thermal conductance with the twisted angle is associated with and dominated by the alteration in the overlap area between the twisted and non-twisted layers. The finite-size effect is demonstrated to play an essential role in defining the critical angles at the local minimums, where these angles are dependent on the size of the investigated nanofibers, in particular on the proportion between the widths of zigzag and armchair edges.