Beating the amorphous limit in thermal conductivity by superlattices design.

Hideyuki Mizuno,Stefano Mossa,Jean-Louis Barrat

doi:10.1038/srep14116

Abstract

The value measured in the amorphous structure with the same chemical composition is often considered as a lower bound for the thermal conductivity of any material: the heat carriers are strongly scattered by disorder, and their lifetimes reach the minimum time scale of thermal vibrations. An appropriate design at the nano-scale, however, may allow one to reduce the thermal conductivity even below the amorphous limit. In the present contribution, using molecular-dynamics simulation and the Green-Kubo formulation, we study systematically the thermal conductivity of layered phononic materials (superlattices), by tuning different parameters that can characterize such structures. We have discovered that the key to reach a lower-than-amorphous thermal conductivity is to block almost completely the propagation of the heat carriers, the superlattice phonons. We demonstrate that a large mass difference in the two intercalated layers, or weakened interactions across the interface between layers result in materials with very low thermal conductivity, below the values of the corresponding amorphous counterparts.

Highlights

A crucial issue[4] is whether thermal conductivity can be lowered below the glass limit through nanoscale phononic design[3,11]
Thermal conductivities have been estimated by the Green-Kubo formulation[47,48]. We have calculated both components of the superlattices thermal conductivities, κCP and κIP, by varying the pattern repetition period W
For the first time to our knowledge, a clear demonstration of very low thermal conductivities in superlattices, below the glassy limit of the corresponding amorphous structures

Summary

Introduction

A crucial issue[4] is whether thermal conductivity can be lowered below the glass limit through nanoscale phononic design[3,11]. For instance, that very recent numerical work[21], has shown that a superlattice composed by layers with randomized thicknesses can show a κ below the value pertaining to the disordered-alloy with the same composition. This limit, is generally higher than that in the corresponding glass[5,6,7,8], which should be considered the true amorphous limit to be beaten. As the lifetime of heat carriers is already minimum in glasses[8], we demonstrate that the key to even lower thermal conductivities is to suppress their propagation across the interfaces between the constituent layers

Methods

Results

Conclusion