Resonance properties of surface acoustic wave resonator in supercritical CO2

A supercritical fluid is a fluid at a temperature and pressure above its critical point. Supercritical CO₂ functions as a moderate solvent, has zero surface tension, and has high permeability in fine structures. These properties are regarded as ideal for the processing of nano- and microscale substances. A problem of applying supercritical fluids is fluctuations in their density. By utilizing a surface acoustic wave (SAW), it may be possible to realize a sensor for measuring the density fluctuation. In this study, the resonance property of a shear-horizontal-type SAW resonator fabricated on 36° Y-X LiTaO₃ substrate and the impedance of interdigitated electrode (IDE) fabricated on a nonpiezoelectric substrate were measured in high-pressure CO₂. Abrupt changes in the resonance property due to discontinuous changes in the density and viscosity were observed at a certain CO₂ pressure between gas and liquid phases. The impedance change of the IDE due to discontinuous changes in the permittivity was also observed.