Anisotropic thermal diffusivity measurement of thin films: From a few to hundreds of microns

The thermal transport properties of thin films have garnered significant research interest due to their diverse applications in new energy sources and thermal management. As the thickness of anisotropic thin films decreases to a few microns, conventional laser flash analyzer and infrared lock-in thermography encounter difficulties in characterizing their thermophysical properties. In response to this challenge, we propose a laser spot periodic heating technique based on sub-region phase fitting for measuring anisotropic thin films. In this method, a thermocouple monitors the temperature response on the sample back surface, and the amplified voltage is subsequently measured using a lock-in amplifier to record the spatial distribution of the phase signal. Finite element simulations indicate that the impact of contact thermal resistance between the thermocouple and the sample can be negligible when measuring in variable locations. The anisotropic thermal diffusivity is then fitted using one-dimensional and three-dimensional unsteady-state heat conduction models, with the spatial distribution of phase lag being partitioned into linear and non-linear regions, respectively. We measure the in-plane and out-of-plane thermal diffusivity of a stainless-steel film, demonstrating the reliability and accuracy of the method. As a showcase study demonstrating the capabilities of our methodology, we test four anisotropic thin films, covering a large spectrum of thermal diffusivity, ranging from 5.5 × 10⁻⁸ m²/s to 9.6 × 10⁻⁴ m²/s. It is verified that the present method can accomplish anisotropic thermal diffusivity measurements for thin films with thicknesses ranging from a few to hundreds of microns, a capability crucial for advancing the characterization of the thermophysical properties of materials.