Enhancing and tuning phonon transport at vibrationally mismatched solid-solid interfaces

Abstract

The thermal conductance of interfaces plays a major role in defining the thermal properties of nanostructured materials in which heat transfer is predominantly phonon mediated. Ongoing research has improved the understanding of factors which govern interfacial phonon transport, as well as the ability to predict thermal interface conductance. However, despite this progress, the ability to control interface conductance remains a major challenge. In this manuscript, we present a method to enhance and tune thermal interface conductance at vibrationally mismatched solid-solid interfaces. Enhancement is achieved through the insertion of an interfacial film with mediating vibrational properties, such that the vibrational mismatch at the interface is bridged, and consequently, the total interface conductance is enhanced. This phenomena is explored using non-equilibrium molecular dynamics simulations, where the effects of altering the interfacial film thickness, vibrational spectrum, and the temperature of the system are investigated. A systematic study of these pertinent design parameters explores the ability to enhance and tune phonon transport at both ideal (sharp) and non-ideal (compositionally disordered) interfaces. Results show that interface conductance can be broadly enhanced by up to 53% in comparison to the vibrationally mismatched baseline interface. Additionally, we find that compositional disorder at an interface does not imply a deterministic change in interface conductance, but instead, that the influence of compositional disorder depends on the characteristics of the disordered region itself. These results, in contrast to macroscopic thermal transport theory, imply that it is possible to increase thermal conductance associated with interface scattering by adding more material along the direction of heat flux.

Relationship of thermal boundary conductance to structure from an analytical model plus molecular dynamics simulations

“…13 that the interfacial disorder causes a reduction of the temperature drop at the interface, and as a result, the Kapitza conductance increases from 0.084 GW/m 2 · K for the sharp interface to 0.135 GW/m 2 · K for the diffused interface. Similar enhancement of Kapitza conductance has been observed in previous MD simulations, which is attributed to a phonon bridging effect 67,68 . In this case, an intermediate layer forms at the interface where phonon properties (modes) of both Al and GaN are mixed, resulting in some average phonon dispersion that promotes thermal transport.…”

“…14), the temperature profiles are less uniform on either side of the interface and the temperature drop at the vertical and horizontal interfaces becomes less apparent. This suggests that for strongly-bonded interfaces with small interfacial features, the observed enhancement can be attributed to a "phonon bridge" effect rather than an increase in total interface area [67][68][69] . That is, by nanostructuring the interface, a spatially-graded region exists where the vibrational properties overlap those of both materials A and B.…”

Design rules for interfacial thermal conductance: Building better bridges

“…We expect the same minimization outcome, m j ≈ √ m l m r , when interatomic mixing is added at the boundaries. Mixing can ether suppress [5] or enhance [10,11,13] each boundary conductance. Either way, we still expect a similar increasing trend of MT ω with mass ratio dictated by the frequency minimum of the modes M min instead of the conserving modes M c [13].…”

“…Nevertheless, the full potential of this revolution is still to be seen because there is a gap between our fundamental understanding of heat flow across single and multiple interfaces and the outcome of experimental measurements [6]. For instance, while many simulations predict an enhancement of thermal conductance when a thin layer is inserted at a well bonded interface [7][8][9][10][11][12][13], only one experiment backs up that prediction so far [14]. Other experiments reporting conductance enhancement attribute the increase to a strengthening of the bonds at the boundaries [15][16][17].…”

“…Such a complete quenching of reflection occurs at a single frequency where the layer thickness can function as a quarter wave plate. In the anharmonic regime on the other hand, the layer can act as a bridge that facilitates frequency up and down conversion and increases the chances of phonons crossing the interface [10] (Fig. 1b).…”