Abstract
Randomizing the layer thickness of superlattices (SL) can lead to localization of coherent phonons and thereby reduces the lattice thermal conductivity κl. In this work, we propose strategies that can suppress incoherent phonon transport in the above random multilayer (RML) structure to further reduce κl. Molecular dynamics simulations are conducted to investigate phonon heat conduction in SLs and RMLs with lattice imperfections. We found that interfacial species mixing enhances thermal transport across single interfaces and few-period SLs through the phonon “bridge” mechanism, while it substantially reduces the κl of many-period SLs by breaking the phonon coherence. This is a clear manifestation of the transition from incoherent-phonon-dominated to coherent-phonon-dominated heat conduction in SLs when the number of interface increases. In contrast, interfacial species mixing always increases the κl of RMLs owing to the dominance of incoherent phonons. Moreover, we found that doping a binary RML with impurities can reduce κl significantly, especially when the impurity atom has an atomic mass lower or higher than both of the two base elements. This work reveals the critical effect of lattice imperfections on thermal transport in SLs and RMLs, and provides a unique strategy to hierachically suppress coherent and incoherent phonon transport concurrently.
Highlights
We found that interface species mixing can increase the κl of random multilayer (RML), because the mixed interfacial region has an intermediate phonon vibrational density of states (vDOS) between the adjacent layers and facilitates phonon transport across the interface through the phonon “bridge” effect
This is because interface disorder does not kill all the phonon coherence and the remaining coherent phonons lead to a higher κl of rough SLs than that of rough RMLs
We revealed that interface roughness reduces the thermal resistance of a single interface or SLs with few periods, while it increases the thermal resistance of SLs containing many periods
Summary
This inspires us to investigate whether we can further reduce the κl of RMLs by nanostructuring to minimize incoherent phonon transport, for example, by introducing interfacial species mixing or lattice impurities, which have been demonstrated to reduce the κl of materials effectively. We strive to understand the effect of structural disorders, namely, random layer thickness, interfacial species mixing, and impurities on phonon transport in SLs and RMLs. We conduct molecular dynamics simulations to systematically investigate the above issues and provide strategies on further reducing the κl of RMLs. We conduct non-equilibrium molecular dynamics (NEMD) simulations to investigate phonon heat conduction in the structures shown, namely, SL with rough interfaces (rough SL), rough RML, RML doped with impurities (doped RML), and binary random alloy.
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