Effect of the absorption coefficient of aluminium plates on their thermoelastic bending in photoacoustic experiments

Abstract

The open-cell photoacoustic signal measured in the transmission configuration for aluminum thin plates with thicknesses of 280 mu m, 197 mu m, and 112 mu m is experimentally and theoretically analyzed, in the 20 Hz-7 kHz modulation frequency range. It is shown that the observed differences between the predictions of the standard thermoelastic model and the experiment data of both the amplitude and phase of the photoacoustic signal can be overcome by considering the aluminum samples coated with a thin layer of black paint as volume-absorber materials. This new approach provides a quite good agreement with the obtained experimental data, in the whole frequency range, and yields an effective absorption coefficient of (16 +/- 2) mm(-1), for a 280 mu m-thick sample. The introduction of the finite absorption coefficient led to the correct ratio between the thermal diffusion and thermoelastic components of the photoacoustic signal. Furthermore, it is found that the volume-absorber approach accurately describes the behavior of the amplitude, but not that of the phase recorded for a 112 mu m-thick sample, due to its relatively strong thermoelastic bending, which is not considered by this theory. Within the approximation of the small bending, the proposed volume-absorber model provides a reliable description of the photoacoustic signal for Al samples thicker than 112 mu m, and extends the applicability of the classical opaque approach. (C) 2015 AIP Publishing LLC.