The quartz crystal resonator: Analysis of the mechanical and electrical loading sensitivities on modified-electrode surface
Abstract
QCR devices have been used as sensors to detect the mechanical and electrical properties of a loading medium. Recent studies report that the electrical loading sensitivity of the QCR device can be significantly improved by modifying the electrode structure of the QCR in terms of the dimension and geometry. This device is referred to as a modified-electrode QCR. The device produces relatively high strength electric fringing fields near the edge of the modified-electrode. These electric fringing fields are utilized to enhance the electrical loading sensitivity of the QCR device. Results clearly indicate that the modified-electrode QCR device can be used as an effective detector in monitoring the electrical properties of a medium adjacent to the QCR surface. However, it appears that further characterizations of the modified-electrode QCR devices are necessary to explain the electrical loading mechanism and to improve the performance of the device. Moreover, investigation on the crystal vibration mechanism of the device is also necessary to optimize the design of the modified-electrode QCR device. For example, the modified-electrode QCR does not sustain oscillation as easily as a typical QCR. This is because most of the acoustic energy is trapped under the narrow or modified-electroded region. As a result, the resonance condition of the QCR device becomes easily unstable when any change in the boundary conditions on the narrow or modified-electroded region occurs. This is even more serious when the device is used in liquid environments where damping occurs due to viscous coupling. Although the modified-electrode QCR device shows relatively high sensitivity to electrical loading, the difficulty in maintaining oscillation can cause severe limitations in its applications. Therefore, a complete understanding of the crystal vibration and detection mechanisms of the modified-electrode QCR device is needed for an optimized design. This dissertation formulated a theory representing the crystal vibration amplitude profiles and the differential mass sensitivity profiles of the various modified-electrode QCR devices. To analyze the electrical loading mechanism of the QCR devices, a study of the behavior of the electric fringing field due to the change in the electrical properties of the loading medium was carried out using the Finite Element Analysis Method. Changes in the electrostatic capacitance of the various QCR devices loaded with various conductive dielectric loading media were also calculated. It is shown that the effective surface area of the electrode increases as the conductivity of the loading medium increases which, in turn, results in a change in the electrical load of the QCR. Using the network analysis method, various critical frequencies of the loaded modified-electrode QCR were theoretically and experimentally characterized. It is shown that these critical frequencies respond differently to the changes in the material properties of the loading medium. It has been found that the antiresonance frequency, $f\sb{a},$ is the most sensitive frequency to the variations in the electrical properties of the loading medium, and therefore can be utilized in sensor applications. On the other hand, the results also indicate that the series resonance frequency, $f\sb{s},$ can be utilized to monitor the mechanical properties of the crystal/liquid interface. Note that the change in the mechanical properties at that interface is directly related to changes in the electrical properties.
This paper has been withdrawn.