Piezoelectric quartz crystal resonators (QCRs) have been investigated as detectors in liquid environments. In all the applications, mass loading and viscous coupling are the main interaction mechanisms which result in changes in the QCR resonant frequency. However, other interaction mechanisms such as the acoustoelectric interaction due to fringing fields at electrode ends arise which contribute to the total change in frequency, in particular, the parallel resonant frequency. In the present work, it is shown that by modifying the geometry of the electrode at the QCR surface in contact with the solution, a transition region can be created in which the lateral decaying acoustic field is enhanced. The electric field can then interact with an adjacent conductive/dielectric solution which will result in relatively large changes in the parallel resonance conditions of the QCR. An equivalent circuit is proposed to analyze the loaded QCR with a modified electrode geometry. It is shown that this circuit is a general circuit which can be used to analyze all cases of a loaded QCR with one side in contact with a given viscous, conductive, or dielectric liquid. Especially, expressions are obtained for the parallel resonant frequency of the loaded QCR in terms of the solution dielectric constant and conductivity. It is shown, using 11-MHz devices on AT-cut quartz, that the modified QCRs can be used as effective and reliable detectors in conductive liquid environments to detect ionic solutes and their dielectric properties. Other applications are suggested.Acoustic wave devices are increasingly being studied both for the physical measurement of liquid properties and in biosensor applications. This is because a need has been recognized for rapid, sensitive, inexpensive high-performance microsensors capable of performing in liquid environments. Initial work relating to the use of acoustic wave devices in liquid-phase sensing applications utilized conventional bulk acoustic wave (BAW) piezoelectric crystal resonator.1•2 Commonly used in these applications is the AT-cut quartz crystal resonator (QCR) in which thickness-shear horizontal vibrations are excited to set up standing waves. The AT-cut QCR typically consists of a thin quartz disk sandwiched between two circular, concentric, metallic electrodes of equal diameters. The electrodes are used to excite the resonator whose frequency is determined by the resonance of the standing wave across the thickness of the plate. The device is then used as the frequency-determining element of an oscillator circuit. The acoustic wave is at an antinode at the surface of the quartz plate and thus can interact with an adjacent medium.

An analytical solution for the resonance condition of a piezoelectric quartz resonator with one surface in contact with a viscous conductive liquid is presented. The characteristic equation that describes the resonance condition and accounts for all interactions including acoustoelectric interactions with ions and dipoles in the solution is obtained in terms of the crystal and liquid parameters. A simple expression for the change in the resonance frequency is obtained. For viscous nonconductive solutions, the frequency change is reduced to a relationship in terms of the liquid density and viscosity. For dilute conductive liquid, the change in frequency is derived in terms of the solution conductivity and dielectric constant. The boundary conditions for the problem are defined with and without the electrical effects of electrodes. Experiments were conducted with various viscous and conductive chemical liquids using a fabricated miniature liquid flow cell containing an AT-cut quartz crystal resonator. The results, which show good agreement with the theory, on the use of quartz crystal resonators as conductivity and/or viscosity sensors are reported.

Effects of liquid relaxation time on the propagation characteristics of shear horizontal (SH)surface waves at the interface between a piezoelectric crystal and a fluid medium are theoretically analyzed for liquid sensor applications. General closed-form expressions are derived for both velocity change and attenuation coefficient for a Maxwellian viscoelastic fluid that can be treated as a Newtonian fluid or as an amorphous solid, depending on the liquid shear relaxation time. It is shown that relaxation time becomes important as the liquid viscosity and/or the wave frequency increases. The result is a saturation level in the propagation loss and the device sensor frequency shift as the liquid relaxation time approaches the wave period. The effect of liquid relaxation is thus to set detection limits on the range of liquid vicosity that can be measured. However, it is also shown that in actual sensor applications, a trade-off can be performed between the maximum measurable viscosity and the SH surface wave device frequency of operation, and thus the device sensitivity.

No abstract

Surface shear horizontal wave propagation at the boundary of a piezoelectric substrate with a viscous fluid is theoretically analyzed. The analytical solution clearly shows the dependence of the velocity change, the wave propagation loss, and the wave amplitude profile (thus the energy confinement near the surface), in terms of the liquid viscosity and density, the layer t. hickness, and the wave frequency. It is shown that the viscous fluid loading produces some guidance near the surface but also some damping of the wave. The propagation loss is due only to viscous coupling and not to a mode conversion and viscous coupling as is the case with Rayleigh surface acoustic waves (SAWs). Closed-form expressions are derived for the attenuation coefficient and the fractional velocity change (thus the frequency change) in terms of the piezoelectric crystal and viscous liquid parameters. The theory, applicable to both a Bleustein-Gulyaev (BG) wave and a surface skimming bulk wave (SSBW), indicates that surface shear waves could be used in the implementation of sensitive acoustic wave liquidphase-based detectors, viscosity sensors, and/or biosensors. These sensors will not experience the high propagation loss associated with Rayleigh SAWs.