Steady State and Modulated Heat Conduction in Layered Systems Predicted by the Phonon Boltzmann Transport Equation

TL;DR

This paper derives analytical expressions for heat conduction in layered systems using the phonon Boltzmann transport equation, revealing regimes where Fourier law applies and providing tools for thermal analysis of thin films.

Contribution

It extends the classical Fourier law approach by incorporating phonon transport physics, offering new formulas and invariants for analyzing heat conduction in layered thin films.

Findings

Heat conduction depends on the ratio of film thickness to phonon mean free path.

Diffusive behavior emerges when the ratio is large, ballistic when small.

Derived a simple formula for cross-plane thermal conductivity of thin films.

Abstract

Based on the phonon Boltzmann transport equation under the relaxation time approximation, analytical expressions for the temperature profiles of both steady state and modulated heat conduction inside a thin film deposited on a substrate are derived and analyzed. It is shown that both steady state and modulated components of the temperature depend strongly on the ratio between the film thickness and the average phonon mean free path, and they exhibit the diffusive behavior as predicted by the Fourier law of heat conduction when this ratio is much larger than the unity. In contrast, in the ballistic regime when this ratio is comparable to or smaller than the unity, the steady-state temperature tends to be independent of position, while the amplitude and the phase of the modulated temperature appear to be lower than those determined by the Fourier law. Furthermore, we derived an invariant of heat conduction and a simple formula for the cross-plane thermal conductivity of dielectric thin films, which could be a useful guide for understanding and optimizing the thermal performance of the layered systems. This work represents the Boltzmann transport equation-based extension of the Rosencwaig and Gerko work [J. Appl. Phys. 47, 64 (1976)], which is based on the Fourier law and has widely been used as the theoretical framework for the development of photoacoustic and photothermal techniques. This work might shed some light on developing a theoretical basis for the determination of the phonon MFP and relaxation time using ultrafast laser-based transient heating techniques.