Ultrahard BC5 -- an efficient nanoscale heat conductor through dominant contribution of optical phonons

TL;DR

This study reveals that BC5, a superhard semiconductor, exhibits exceptionally high thermal conductivity primarily due to optical phonons, making it promising for nanoscale heat management applications.

Contribution

The paper demonstrates that optical phonons dominate heat conduction in BC5, with a detailed analysis of their high-frequency behavior and weak temperature dependence, which is a novel insight.

Findings

Thermal conductivity of BC5 is 169 W/mK at 300K.

Optical phonons contribute about 54% to heat conduction at 500K.

High nanoscale thermal conductivity of 77 W/mK at 100 nm length scale.

Abstract

In this work, we study thermal conductivity (k) of BC5, an ultra-hard diamondlike semiconductor material, using first-principles computations and analyze the effect of both isotopic disorder as well as length scale dependence. k of isotopically pure BC5 is computed to be 169 Wm- 1K-1 (along a-axis) at 300K; this high k is found to be due to the high frequencies and phonon group velocities of both acoustic and optical phonons owing to the light atomic mass of Carbon (C) and Boron (B) atoms and strong C-C and B-C bonds. We also observe a dominance of optical phonons (~ 54%) over acoustic phonons in heat conduction at higher temperatures (~500 K). This unusually high contribution of optical phonons is found to be due to a unique effect in BC5 related to a weaker temperature dependence of optical phonon scattering rates relative to acoustic phonons. The effect is explained in terms of high frequencies of optical phonons causing decay into other high frequency phonons, where low phonon populations cause the decay term to become insensitive to temperature. The effect further leads to high nanoscale thermal conductivity of 77 Wm-1K-1 at 100 nm length scale due to optical phonon meanfreepaths being in nanometer regime. These results provide avenues for application of BC5 in nanoscale thermal management.