Direct Visualization of Thermal Conductivity Suppression Due to Enhanced Phonon Scattering Near Individual Grain Boundaries

TL;DR

This study uses spatially-resolved measurements to directly observe how individual grain boundaries in polycrystalline diamond significantly suppress local thermal conductivity due to enhanced phonon scattering, extending up to 10 micrometers from the boundary.

Contribution

It provides the first localized measurements of thermal conductivity near individual grain boundaries and introduces a theoretical model for phonon scattering at these defects.

Findings

Thermal conductivity drops from ~1000 to ~400 W/m-K near grain boundaries.

Suppression of thermal transport extends up to ~10 micrometers from GBs.

A model explains the local reduction in phonon mean-free-paths due to diffuse scattering.

Abstract

Understanding the impact of lattice imperfections on nanoscale thermal transport is crucial for diverse applications ranging from thermal management to energy conversion. Grain boundaries (GBs) are ubiquitous defects in polycrystalline materials, which scatter phonons and reduce thermal conductivity. Historically, their impact on heat conduction has been studied indirectly through spatially-averaged measurements, that provide little information about phonon transport near a single GB. Here, using spatially-resolved time-domain thermoreflectance (TDTR) measurements in combination with electron backscatter diffraction (EBSD), we make localized measurements of thermal conductivity within few \mu m of individual GBs in boron-doped polycrystalline diamond. We observe strongly suppressed thermal transport near GBs, a reduction in conductivity from ~1000 W/m-K at the center of large grains to ~400 W/m-K in the immediate vicinity of GBs. Furthermore, we show that this reduction in conductivity is measured up to ~10 \mu m away from a GB. A theoretical model is proposed that captures the local reduction in phonon mean-free-paths due to strongly diffuse phonon scattering at the disordered grain boundaries. Our results provide a new framework for understanding phonon-defect interactions in nanomaterials, with implications for the use of high thermal conductivity polycrystalline materials as heat sinks in electronics thermal management.