Temperature Dependent Mean Free Path Spectra of Thermal Phonons Along the c-axis of Graphite

TL;DR

This study quantitatively measures the phonon mean free path spectra along the c-axis of graphite at various temperatures, revealing a narrow distribution and temperature-dependent scattering mechanisms, advancing understanding of heat conduction in anisotropic materials.

Contribution

First quantitative measurement of c-axis phonon MFP spectra in graphite across temperatures, revealing a narrow distribution and temperature-dependent scattering mechanisms.

Findings

c-axis phonon MFPs are a few hundred nanometers at room temperature

Phonon scattering is dominated by grain boundaries at low temperatures

Narrow MFP distribution compared to isotropic crystals

Abstract

Heat conduction in graphite has been studied for decades because of its exceptionally large thermal anisotropy. While the bulk thermal conductivities along the in-plane and cross-plane directions are well known, less understood are the microscopic properties of the thermal phonons responsible for heat conduction. In particular, recent experimental and computational works indicate that the average phonon mean free path (MFP) along the c-axis is considerably larger than that estimated by kinetic theory, but the distribution of MFPs remains unknown. Here, we report the first quantitative measurements of c-axis phonon MFP spectra in graphite at a variety of temperatures using time-domain thermoreflectance measurements of graphite flakes with variable thickness. Our results indicate that c-axis phonon MFPs have values of a few hundred nanometers at room temperature and a much narrower distribution than in isotropic crystals. At low temperatures, phonon scattering is dominated by grain boundaries separating crystalline regions of different rotational orientation. Our study provides important new insights into heat transport and phonon scattering mechanisms in graphite and other anisotropic van der Waals solids.