Thermal diffusivity measurement of silicon samples by a combined piezoelectric and pyroelectric method

Abstract

In the past, when thermal diffusivity measurement of materials were carried out by photoacoustic signal detection using transducers, only the piezoelectric or the pyroelectric property of the transducers was considered. In case the transducer exhibits both piezoelectric and pyroelectric properties, one of these properties had been suppressed during the experimentation, obviously more errors are introduced this way. We use polyvinylidene difluoride ~PVDF! as the detector for thermal waves. Since PVDF has both piezoelectric and pyroelectric properties, we show in this article that the signal detected by the transducer is a sum of both the piezoelectric and pyroelectric effects. Silicon semiconductor samples are considered in this article to compare the theory with experimental results. Although both the piezoelectric and pyroelectric properties are found in the resultant signal at all the frequency ranges considered, we find that when the samples are thermally thick, the piezoelectric contribution to the detected signal is slightly more than the pyroelectric contribution and vice versa when the sample is thermally thin. This behavior of the combined signal can be explained by the fact that in an optically opaque solid heat is generated very close to the surface, following absorption. This heat is communicated to the PVDF as long as the thermal diffusion length is larger than the thickness ~i.e., the sample is thermally thin!. At high frequencies the solid becomes thermally thick and the pyroelectric nature decreases as both the optical and thermal contact of the sample with the detector diminishes. Since both the properties are considered in our theory, we can measure the thermal diffusivity of a general sample without ‘‘artificial suppression.’’ Moreover, from our analysis we can deduce the physical thickness of the sample from the critical frequency, which is the frequency at which the sample changes from thermally thin to thermally thick. This transition is clearly evident in the amplitude curve as a change in slope is detected at the critical frequency. © 1999 American Institute of Physics.