Anomalously Strong Size Effect on Thermal Conductivity of Diamond Microparticles

TL;DR

This study reveals an unexpectedly strong size-dependent increase in thermal conductivity of diamond microparticles, challenging the assumption that larger particles have bulk-like properties, with implications for material design.

Contribution

It uncovers a significant size effect on diamond microparticles' thermal conductivity linked to growth-related defects, which was previously underestimated.

Findings

Thermal conductivity increases from 400 to 2000 W/m·K as particle size grows from 20 to 300 micrometers.

Long-range defects during growth significantly influence thermal conductivity.

Size effect is attributed to defect structures and thermal penetration depth dependence.

Abstract

Diamond has the known highest thermal conductivity of around \SI{2000}{\watt\per\meter\per\kelvin} and is therefore widely used for heat dissipation. In practical applications, synthetic diamond microparticles are usually assumed to have similar thermal conductivity to that of bulk diamond because the particle size is larger than theoretical phonon mean free path so that boundary scattering of heat-carrying phonons is absent. In this report, we find the thermal conductivity of diamond microparticles anomalously depends on their sizes. Thermal conductivity of diamond microparticles increases from \SI{400}{\watt\per\meter\per\kelvin} to \SI{2000}{\watt\per\meter\per\kelvin} with the size growing from \SI{20}{\micro\meter} to \SI{300}{\micro\meter}. We attribute the abnormally strong size effect to the long-range defects during the growth process based on analysis of point defects, dislocations, and thermal penetration depth dependence of thermal conductivity. Our results play a vital role in the design of diamond composites and in the improvement of thermal conductivity of synthetic diamonds.