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 degrees Y-X LiNbO3 and a pure shear-horizontal-type SAW resonator fabricated on ST 90 degrees-X quartz were measured in high-pressure CO2. 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 LiNbO3 reached 1,320 ppm at approximately 199 MHz. For both the cases of LiNbO3 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. (c) 2011 The Japan Society of Applied Physics