A Novel Optical-Based AC Calorimetry Method to Measure In-Plane Thermal Conductivity of Small-Scale Samples

Abstract

Conventional ac-calorimetry method could accurately measure the in-plane thermal diffusivity of suspended thin films but cannot be applied to small-scale samples due to the large sample size required by this method. This work proposes a novel optical-based ac-calorimetry method that circumvents the limitations of the conventional ac-calorimetry method. This new method uses a modulated continuous-wave laser to heat the sample periodically, and another continuous-wave laser to detect the sample’s temperature response on the surface at different distances from the heating spot via the principle of thermoreflectance. Since this method uses highly focused laser beams with micrometer-scale spot diameters for optical heating and temperature sensing, and this method uses the state-of-the-art thermal model of 3D anisotropic multilayers for signal processing, this method can accurately measure the in-plane thermal conductivity of suspended and supported thin films and bulk materials with sub-millimeter-scale lateral sizes, with a typical measurement uncertainty of less than 5%. This paper describes the details of this technique, including the basic principle, test device, signal acquisition, data processing, and uncertainty analysis. This technique is demonstrated through measurements of a standard sapphire sample, with the phase and amplitude approaches of ac-calorimetry compared. This technique is also compared with some other related optical-based thermal methods for their pros and cons in the end.