Measuring the in-plane thermal diffusivity of moving samples using laser spot lock-in thermography

Abstract

In this work we deal with samples that move at constant speed and are illuminated by a modulated and focused laser beam. We have obtained a general expression for the surface temperature of these moving samples: it is valid not only for opaque and thermally thick materials, but also for thermally thin and semitransparent samples. Moreover, heat losses by convection and radiation are taken into account in the model. Numerical calculations indicate that the temperature (amplitude and phase) profiles in the directions parallel and perpendicular to the sample motion are straight lines with respect to the distance to the laser spot. The slopes of these straight lines depend on sample speed, modulation frequency and in-plane thermal diffusivity of the sample. Provided the two first experimental parameters are known, the in-plane thermal diffusivity can be retrieved in a simple manner. Measurements performed on materials covering a wide range of thermal diffusivity values, from insulators to good thermal conductors, confirm the validity of these linear methods.