Thermal Diffusivity Measurements for High Thermal Conductive Materials by Applying Undersampling to Lock-in Thermography

Abstract

In this study, a method employing lock-in thermography is proposed for measuring the thermal diffusivity distribution of materials with high thermal conductivity. In this method, the thermal response distribution induced by periodic laser heating is analyzed, and the thermal diffusivity in the out-of-plane direction over the material surface is mapped. An undersampling method is applied during lock-in thermography to measure the thermal diffusivity distribution of materials with low thermal resistance at high frequencies. Additionally, a principle is developed to eliminate the inherent phase lag generated by the measurement system. The accuracy of the proposed method is validated by quantitatively measuring the thermal diffusivity of a pure copper sheet that exhibits an isotropic thermal diffusivity distribution. Results reveal that its average thermal diffusivity agreed with the reference value within + 2.4 %. The proposed method is also used to measure the thermal diffusivity of an isotropic graphite sheet with an inhomogeneous thermal diffusivity distribution. Results reveal that its average thermal diffusivity agreed with the reference value within + 7.0 %, and local areas with high thermal diffusivity are successfully visualized.