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

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 (κ). 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 κ within few μm of individual GBs in boron-doped polycrystalline diamond. We observe strongly suppressed thermal transport near GBs, a reduction in κ 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 κ is measured up to ∼10 μm 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-κ polycrystalline materials as heat sinks in electronics thermal management.

Quasi-Ballistic Thermal Transport Across MoS2 Thin Films

“…An integrated dark-field microscope helped locate the samples under the laser spots. 24 Since some of the samples have lateral dimensions as small as 15 µm (especially for the thinnest films), it is important to position the laser spot well between the edges of the flake. To do this, a precision two-axis translation stage was used to map out the TDTR signal and probe beam reflectivity over the area of the sample at a fixed delay time (see Figure 1b,c).…”

A steady-state thermoreflectance method to measure thermal conductivity

“…This variation could be due to local thermal conductivity reduction from grain boundaries. Sood et al 12 showed that local thermal conductivities in CVD-grown boron-doped polycrystalline diamond (an average grain size of 23 µm) can decrease κ by nearly 60% near grain boundaries. Because grain sizes in the diamond measured in this study range from 10 to 100 µm, we do not expect to see the same extent of variation observed by Sood et al…”

“…Finally, these techniques fundamentally measure the thermal conductance across a bulk specimen rather than within a locally probed area as has been shown in FDTR 9,10 and Review of Scientific Instruments ARTICLE scitation.org/journal/rsi TDTR. 11,12 Given the benefits inherent in transient pumpprobe techniques and the simplicity of steady-state techniques, we develop a steady-state thermoreflectance (SSTR) technique based on a continuous wave (CW) laser pump and probe. We show that SSTR is robust, capable of measuring materials having thermal conductivities ranging from 1 to >2000 W m −1 K −1 , showing excellent agreement with literature values.…”

The Heat Conduction Renaissance

“…This was achieved using electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). Using this experimental platform, Sood et al [44] observed a nearly twofold reduction in the local thermal conductivity immediately adjacent to single GBs in boron-doped CVD-diamond. While the influence of near-boundary disorder (including dopants, dislocations, and extended defects) on local thermal transport would certainly depend on the specific details of the material, this work shows that in general, it may be important to consider heterogeneities in the local thermal environment due to variations in the microstructure of the heat-spreading layer.…”