Thermal parameter measurements of bulk YBCO superconductor using PVDF transducer

A theory combining both the pyroelectric and the piezoelectric photoacoustic methods is used to determine thermal diffussivity of a bulk superconducting YBCO sample at room temperature. We use polyvinylidene difluoride (PVDF) as the detector for the thermal waves. As PVDF has both piezoelectric and pyroelectric properties we find that the signal from PVDF detected using the lock-in amplifier is a combination of the piezoelectric and pyroelectric signals. 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 more than the pyroelectric contribution, and vice versa when the sample is thermally thin. This behaviour of the combined signal is made use of in determining the thermal diffussivity of a thermally thick superconductor at low temperatures. The thermal diffussivity curve shows a dip at the resistivity transition onset temperature and a cusp at the offset temperature where the electrical resistance disappears. Thus, we find that the PVDF transducer is very effective in determining the normal-to-superconductor transition and also for the measurement of the thermal parameters of the superconducting samples at low temperatures such as thermal diffussivity, thermal conductivity and the specific heat capacity.