Design rules for interfacial thermal conductance - building better bridges

TL;DR

This paper investigates how adding an intermediate layer affects thermal conductance at solid-solid interfaces, revealing conditions for maximizing conductance and proposing a graded layer design for improved heat transfer.

Contribution

It introduces a comprehensive analysis of interfacial thermal conductance with intermediate layers, including the effects of phonon interactions and a novel graded layer approach.

Findings

Maximum conductance occurs near the geometric mean of contact masses.

Adding a layer can increase or decrease conductance depending on phonon interactions.

Graded layers can serve as broadband impedance matching networks.

Abstract

We study the thermal conductance across solid-solid interfaces as the composition of an intermediate matching layer is varied. In absence of phonon-phonon interactions, an added layer can make the interfacial conductance increase or decrease depending on the interplay between (1) an increase in phonon transmission due to better bridging between the contacts, and (2) a decrease in the number of available conduction channels that must conserve their momenta transverse to the interface. When phonon-phonon interactions are included, the added layer is seen to aid conductance when the decrease in resistances at the contact-layer boundaries compensate for the additional layer resistance. For the particular systems explored in this work, the maximum conductance happens when the layer mass is close to the geometric mean of the contact masses. The surprising result, usually associated with coherent antireflection coatings, follows from a monotonic increase in the boundary resistance with the interface mass ratio. This geometric mean condition readily extends to a compositionally graded interfacial layer with an exponentially varying mass that generates the thermal equivalent of a broadband impedance matching network.