Implications of Interfacial Bond Strength on the Spectral Contributions to Thermal Boundary Conductance across Solid, Liquid, and Gas Interfaces: A Molecular Dynamics Study
A Giri and JL Braun and PE Hopkins, JOURNAL OF PHYSICAL CHEMISTRY C, 120, 24847-24856 (2016).
The modal contributions to interfacial heat flow across Lennard-Jones based solid/solid, solid/liquid, and solid/gas interfaces are predicted via molecular dynamics simulations. It is found that the spectral contributions to the total heat flux from the solid that comprises the interface are highly dependent on the phase of the adjoining matter and the interfacial bond driving the interaction between the solid and the adjacent matter. For solid/solid interfaces, along with low temperatures, weak cross-species interaction strength can severely limit the conductance owing to the inhibition of inelastic channels that otherwise facilitate heat flow across the interface via anharmonic interactions. The increase in the cross-species interaction strength is shown to shift the modal contributions to higher frequencies, and most of the inelastic energy exchange is due to the longitudinal vibrational coupling across the interface. For solid/liquid interfaces, the increase in the cross-species interaction enhances the coupling of transverse vibrational frequencies in the interfacial solid region, which leads to an increase in the total heat current across the interface. Our modal analysis suggests that very high frequency vibrations (with frequencies greater than 80% of the maximum frequency in the bulk of the solid) have negligible contribution to heat flow across solid/liquid interfaces, even for a strongly bonded interface. In the limit of weakly interacting solid/gas interfaces, the modes coupling in the solid to the gas have signatures of reduced dimensionality, as evident by the surface-like density-of-states in the solid. Increasing the interfacial interaction shows similar trends to the solid/liquid case up to the limit in which gas atoms adsorb to the surface, enhancing the contribution of transverse phonons coupling at the solid interface. Our work elucidates general similarities in the influence of interfacial bond strength to thermal boundary conductance across solid/solid, solid/liquid, and solid/gas interfaces. In general, we find that the mode softening with a decrease in interfacial bond strength is more pronounced in the longitudinal modes as compared to transverse modes, and we consistently observe a decrease in the transverse mode contribution from the solid across the interface as the interfacial bond strength is decreased, regardless of the phase of matter on the other side.
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