title: Temperature-Dependent Mean Free Path Spectra of Thermal Phonons Along the c-Axis of Graphite.
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.
authors: Division of Engineering and Applied Science, California Institute of Technology , Pasadena, California 91125, United States.|Institute of Engineering Thermophysics, Chinese Academy of Sciences , 11 Beisihuanxi Road, Beijing 100190, China.
authors: Hang
authors: H
authors: Zhang
authors: Division of Engineering and Applied Science, California Institute of Technology , Pasadena, California 91125, United States.
authors: Xiangwen
authors: X
authors: Chen
authors: School of Information and Communications, Gwangju Institute of Science and Technology , Gwangju 500-712, Korea.
authors: Young-Dahl
authors: YD
authors: Jho
authors: Division of Engineering and Applied Science, California Institute of Technology , Pasadena, California 91125, United States.
authors: Austin J
authors: AJ
authors: Minnich
keywords: Mean free path; cross-plane; graphite; phonon; thermal conductivity; thickness dependent