Anisotropic and inhomogeneous thermal conductivity of sub-micrometer polycrystalline diamond thin films: A Monte Carlo ray tracing simulation study

• Published in: International communications in heat and mass transfer Vol. 156; p. 107683
• Main Authors: Baek, Jongwon, Bae, Junyoung, Hori, Takuma, Cho, Jungwan
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.08.2024
• Subjects: Diamond thin films; Phonon heat conduction; Polycrystalline material; Ray tracing simulation; Thermal conductivity; Thermal conductivity; Ray tracing simulation; Diamond thin films; Polycrystalline material; Phonon heat conduction
• ISSN: 0735-1933; 1879-0178
• DOI: 10.1016/j.icheatmasstransfer.2024.107683

Diamond has attracted significant attention for power electronics cooling. However, understanding thermal transport in polycrystalline diamond (PCD), particularly in thin-film form, is still challenging, predominantly owing to its complicated microstructure. In this study, we predict the anisotropic and inhomogeneous thermal conductivity of sub-micrometer PCD thin films using Monte Carlo ray tracing simulations. Based on the columnar grain structure of PCD thin films, we construct three film structure models (nanowire, cubic, and Voronoi models) and compare the mean free paths (MFPs) of phonons due to grain boundary scattering, quantified via the ray tracing method, for both cross-plane and in-plane directions. This comparison suggests that a simple cubic structure with periodic and mirror boundary conditions can be a reasonable structural approximation of PCD thin films. Building upon this geometric simplification, we combine our ray tracing calculations with Born von Karman-based acoustic phonon dispersion and bulk scattering models to predict the cross-plane and in-plane thermal conductivities of PCD thin films as a function of film thickness, yielding values that are consistent with previous experimental data exhibiting both anisotropy and inhomogeneity. Our methodology can be applied broadly to polycrystalline materials with structural complexity such as diamond microchannels for enhanced convective cooling. •The ray tracing method computes phonon boundary scattering mean free paths in sub-1 µm polycrystalline diamond (PCD) films.•A simple cube with periodic and mirror boundary conditions can be a reasonable structural approximation of PCD thin films.•Our method based on ray tracing considers effects involving initial grain evolution and grain boundary thermal resistance.•Our PCD thermal conductivity model successfully captures essential features associated with anisotropy and inhomogeneity.•Our approach can be applied broadly to polycrystalline materials with structural complexity, such as PCD microchannels.