Dramatic enhancement of interfacial thermal transport by mass-graded and coupling-graded materials

With the dimension of materials shrinking into nanoscale, there has been growing interest in the Kapitza resistance which inhibits overall thermal transport. In this work, using the non-equilibrium Green's function method, we systematically investigate the optimized interfacial couplers with various gradient materials for phonon transport across one-dimensional atomic hetero-junction models. Relative to the optimized homogenous couplers, the mass-graded or coupling-graded structures are found to be applicable to improve the interfacial thermal conductance of two lead materials with both mismatched impedance and mismatched cutoff frequencies. For the couplers with both geometric graded mass and geometric graded coupling, the interfacial thermal conductance can be maximum enhanced (nearly up to sixfold enhancement on interfacial thermal conductance compared to the optimized homogenous case). The underlying mechanism of phonon transport enhancement by the optimized coupler is investigated by the phonon transmission coefficient: on the one hand, this kind of coupler is able to maximally suppress the destructive interference for transmitted phonon waves; on the other hand, the constructive interference for the transmitted phonon is also largely improved. Our findings may offer guidance for advanced thermal interface materials design.