Study on Simultaneous Photoacoustic Measurement of Thermal Diffusivity and Thermal Effusivity of Solids

Abstract

The goal of this study is the simultaneous measurement of the thermal diffusivity and thermal effusivity of thick samples using the photoacoustic method that is nondestructive, quick, and simple. In their original paper, Rosencwaig and Gersho showed that it is possible to measure thermal properties of a thermally thin sample, but it is impossible to measure those of a thermally thick sample. To improve this situation, a new method is proposed, in which the position of the sample and reference (thin plate) are exchanged, and it is analyzed theoretically whether simultaneous measurement is possible or not when samples are transparent, opaque, and semitransparent. When a sample is transparent, it absorbs little light, thereby the sample produces little photoacoustic waves. When a sample is opaque, light is totally absorbed by the sample. Therefore, thermal properties of the plate almost do not affect the phase-lag output. In these cases, it is shown that the simultaneous measurement is impossible. Hence, only the case when both the plate and sample are semitransparent is simulated. From theoretical analysis, in the case of a PVC plate, it is shown that the simultaneous measurement is likely to be possible when the ratio of the absorption coefficient of the plate to sample lies between $10^{-3}\hbox { and }10^{3}$, the thermal diffusivity between $10^{-4}\hbox { and }10^{4}$, and the thermal effusivity between $10^{-2}\hbox { and }10^{2}$.