Trapped-energy high-frequency piezoelectric resonators

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Trapped-energy high-frequency piezoelectric resonators

 

Author: Wong, Hon-tung
Title: Trapped-energy high-frequency piezoelectric resonators
Degree: M.Phil.
Year: 2007
Subject: Hong Kong Polytechnic University -- Dissertations.
Piezoelectric devices.
Electric resonators.
Department: Dept. of Applied Physics
Pages: 1 v. (various pagings) : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2321627
URI: http://theses.lib.polyu.edu.hk/handle/200/4101
Abstract: The resonance responses of two types of AT-cut quartz resonators, 4-MHz and 27-MHz (provided by Hong Kong X'tals Ltd.), have been studied using the commercial finite element (FEM) code ANSYS. For the 4-MHz resonator with full electrode, there exist spurious modes (thickness-twist mode, flexural mode, and inharmonic thickness-shear mode) in the vicinity (+-0.2 MHz) of the fundamental thickness-shear mode resonance (f111). After the use of an electrode of size smaller than the quartz plate, the spurious modes are not suppressed or shift to higher frequencies, and still affect significantly the vibration at f111. The simulations also show that the vibration at f111 is not effectively trapped in the electroded region and spreads over the whole quartz plate, thus, the silver epoxy introduced at the ends of the quartz plate absorbs the vibration energy and the impedance at f111 is increased. The poor energy trapping efficiency should be due to the small difference between the cutoff frequencies in the electroded and unelectroded regions as well as the low frequency of the vibration. Due to the mass loading effect, a thicker electrode (e.g. 2 um) can decrease the cutoff frequency in the electroded region (i.e. fm) and hence improve the energy trapping efficiency. Similar to the 4-MHz resonators, there exist spurious modes (the flexural mode and the inharmonic thickness-shear mode) in the vicinity of fjn for the 27-MHz resonator with full electrodes. However, all the spurious modes are suppressed or shift to higher frequencies after the use of an electrode of size smaller than the quartz plate. The simulations show that the vibration at f111 is pure thickness-shear mode and it is effectively trapped in the electroded region. As a result, f111 is dependent relatively stronger on the electrode length, and the impedance at f111 does not increase significantly after the introduction of the silver epoxy. The better energy trapping efficiency should be due to the high frequency of the vibration. A new experimental method has been developed to investigate the vibration distribution of the resonators. It is based on the fact that an external load can disturb the vibration and hence change the resonance frequency as well as the impedance at resonance of a quartz plate. Good agreements between the observed and simulated vibration distributions at f111 are obtained for both the 4-MHz and 27-MHz resonators. The dependence of f111 on the electrode length has also been confirmed by experiments on the "commercial" resonators with different electrode lengths. Again, good agreements between the experimental and simulation results are obtained for both the 4-MHz and 27-MHz resonators. A third-overtone thickness-shear resonator with resonance frequency around 58 MHz has been designed and successfully fabricated. In the new design, two sub-electrodes are added right next to each end of the main electrode of the resonator to amplify the "leaked" vibration at fm from the main electrode and subsequently damped by the silver epoxy at the ends of the resonator. On the other hand, the vibration at the third-overtone thickness-shear resonance f111 is mainly trapped in the main-electroded region and hence is not affected significantly by the sub-electrode and the silver epoxy. As a result, the impedance at f111 becomes much larger than that at f111 and the resonator will resonate at its third-overtone thickness-shear mode.

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