Length-dependent thermal transport and ballistic thermal conduction

Bor-Woei Huang,Kung-Hsuan Lin,Tzu-Kan Hsiao,Dah-Wei Chiou,Chih-Wei Chang

doi:10.1063/1.4914584

Abstract

Probing length-dependent thermal conductivity of a given material has been considered as an important experimental method to determine the length of ballistic thermal conduction, or equivalently, the averaged phonon mean free path (l). However, many previous thermal transport measurements have focused on varying the lateral dimensions of samples, rendering the experimental interpretation indirect. Moreover, deducing l is model-dependent in many optical measurement techniques. In addition, finite contact thermal resistances and variations of sample qualities are very likely to obscure the effect in practice, leading to an overestimation of l. We point out that directly investigating one-dimensional length-dependent (normalized) thermal resistance is a better experimental method to determine l. In this regard, we find that no clear experimental data strongly support ballistic thermal conduction of Si or Ge at room temperature. On the other hand, data of both homogeneously-alloyed SiGe nanowires and heterogeneously-interfaced Si-Ge core-shell nanowires provide undisputed evidence for ballistic thermal conduction over several micrometers at room temperature.

Highlights

Probing length-dependent thermal conductivity of a given material has been considered as an important experimental method to determine the length of ballistic thermal conduction, or equivalently, the averaged phonon mean free path (l)
Many previous thermal transport measurements have focused on varying the lateral dimensions of samples, rendering the experimental interpretation indirect
We find that no clear experimental data strongly support ballistic thermal conduction of Si or Ge at room temperature

Summary

Introduction

Probing length-dependent thermal conductivity of a given material has been considered as an important experimental method to determine the length of ballistic thermal conduction, or equivalently, the averaged phonon mean free path (l). Wilson and Cahill recently pointed out the discrepancy of previous results originated from anisotropic thermal transport in-plane and cross-plane.[11] Yet deducing the l’s is not at all straightforward, as experimental complexities associated with laser spot sizes, laser modulation frequencies, temperature profiles, and interface phonon scatterings are quite involved.[11] theoretical interpretations of the optical measurement results remain controversial.

Results

Conclusion