Phonon-mediated thermal transport at exponentially mass-graded interfaces: A computational study

TL;DR

This study uses computational methods to show that exponentially mass-graded interfaces can significantly enhance thermal conductance, with effects influenced by geometric properties and anharmonicity, revealing complex phonon transport mechanisms.

Contribution

It demonstrates that exponential mass grading at interfaces can greatly improve thermal conductance, highlighting the roles of anharmonicity and geometric factors in phonon transport.

Findings

Exponential mass grading enhances thermal conductance by up to 68%.

Anharmonic effects facilitate phonon thermalization, affecting conductance.

Elastic transmission and anharmonicity have competing influences on thermal transport.

Abstract

We numerically investigate thermal transport at solid-solid interfaces with graded intermediate layers whose masses vary exponentially from one side to the other. Using Non-Equilibrium Green's Function and Non-Equilibrium Molecular Dynamics simulations, we show that an exponentially mass-graded junction with a finite thickness can result in 68\% of enhancement in thermal conductance larger compared to a single bridging layer (29\%) and a linear mass-graded junction (64\%) of similar thickness. We examine how the thermal conductance at such interfaces is influenced by geometric qualities and strength of anharmonicity. For geometric properties, we tested the effects from number of layers and the junction thickness. In the absence of anharmonicity, increasing the number of layers results in better elastic phonon transmission at each individual boundary, countered by the decrease of available conducting channels. Consequently, in the harmonic regime, conductance initially increases with number of layers due to better bridging, but quickly saturates. The presence of slight anharmonic effects (at ultra-low temperature T = 2 K) turns the saturation into a monotonically increasing trend. Anharmonic effects can facilitate interfacial thermal transport through the thermalization of phonons. At high temperature, however, the role of anharmonicity as a facilitator of interfacial thermal transport reverses. Strong anharmonicity introduces significant intrinsic resistance, overruling the enhancement in thermal conduction at the boundaries. Our analysis shows that in our model Lennard-Jones system, the influence of a mass-graded junction on thermal conductance is dominated by the phonon thermalization through anharmonic effects, while elastic phonon transmission plays a secondary role.