Temperature gradient and thermal conductivity in superdiffusive materials

TL;DR

This paper investigates the validity of using the temperature gradient method for calculating thermal conductivity in superdiffusive materials, demonstrating its limitations and inconsistency with system size dependence in such systems.

Contribution

The study shows that the temperature gradient method is unreliable for superdiffusive materials like nanocomposites, highlighting the need for alternative approaches.

Findings

Temperature gradient method is inconsistent in superdiffusive systems.

Conductivity in superdiffusive materials depends on system size.

Central linear temperature profiles do not guarantee accurate conductivity measurement.

Abstract

Thermal conductivities are routinely calculated in molecular dynamics simulations by keeping the boundaries at different temperatures and measuring the slope of the temperature profile in the bulk of the material, explicitly using Fourier's law of heat conduction. Substantiated by the observation of a distinct linear profile at the center of the material, this approach has also been frequently used in superdiffusive materials, such as nanotubes or polymer chains, which do not satisfy Fourier's law at the system sizes considered. It has been recently argued that this temperature gradient procedure yields worse results when compared with a method based on the temperature difference at the boundaries -- thus taking into account the regions near the boundaries where the temperature profile is not linear. We study a realistic example, nanocomposites formed by adding boron nitride nanotubes to a polymer matrix of amorphous polyethylene, to show that in superdiffusive materials, despite the appearance of a central region with a linear profile, the temperature gradient method is actually inconsistent with a conductivity that depends on the system size, and, thus, it should be only used in normal diffusive systems.