Photothermal characterization of the thermal properties of materials using four characteristic modulation frequencies in two-layer systems.

A simple photothermal methodology for determining simultaneously and in four different ways the thermal diffusivity and thermal effusivity of the illuminated layer in a two-layer system is presented. The method is based on the analysis of the photothermal signal of the whole system normalized by the corresponding signal of the illuminated one-layer system. Our approach uses the modulation frequencies at which the normalized amplitude (phase) passes through unity (zero) and reaches its maximum (minimum). It is shown that these four characteristic modulation frequencies are independent of the thermal effusivities of the layers, and their values can be used to obtain the thermal diffusivity of the illuminated layer. Then, by using the exact complex expression of the normalized photothermal signal with the experimental data for its amplitude and phase, the thermal effusivity of the same layer can be found. In order to show the usefulness of this approach, the method is applied to a glassy carbon sample in thermal contact with three different types of very thick back layers of air, water, and glycerine. It is shown that the highest (lowest) accuracy on the measurement of the thermal properties corresponds to the frequency at which the zero (minimum) of the normalized phase occurs. [ABSTRACT FROM AUTHOR]