Photoacoustic detection of phase transitions with a resonant piezoelectric scheme with extreme sensitivity to small volume changes

Abstract

Resonant piezoelectric photoacoustic detection is demonstrated to be a sensitive tool for the determination of phase transitions. A model is presented that describes the changes in the signal expected during phase transitions when resonant detection is used. The technique is applied to the study of first-order martensitic diffusionless transformations in copper-based shape-memory alloys. The model takes into account the signal changes arising from two sources. One behaves like an effective change in the heat capacity, and arises due to the enthalpy of the reaction, and the other can be described as an effective change in the thermal expansion coefficient, and arises from the volume change during the transformation. Due to the relative high frequency used (around 20 kHz), the transformation lags behind the temperature oscillations, yielding a phase shift in the acoustic signal as the transformation temperature is passed. The relative sign of the phase angle and amplitude as the transformation proceeds is an indication as to whether the signal arises from volume changes or heat exchange (enthalpy). Huge signals from very small volume changes (smaller than 0.5%) were observed.