Behavior of Surface Acoustic Wave Resonators in Supercritical CO2

By utilizing a surface acoustic wave (SAW), it may be possible to realize a sensor for measuring the density fluctuation of supercritical fluids. In this study, the behaviors of a Rayleigh-SAW resonator fabricated on 128° Y--X LiNbO 3 and a pure shear-horizontal-type SAW resonator fabricated on ST 90°-X quartz were measured in high-pressure CO 2 . Abrupt changes in resonance properties were observed at a certain pressure. The rate of change of antiresonance frequency at a pressure between 5 and 6 MPa in the case of LiNbO 3 reached 1,320 ppm at approximately 199 MHz. For both the cases of LiNbO 3 and quartz, it was found that the impedance, particularly antiresonance impedance, was effective for sensing the difference between gas and liquid phases without affecting the temperature. The causes of the changes in the frequency and impedance were discussed.