Film Conductivity Controlled Variation of the Amplitude Distribution of High-temperature Resonators
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Film Conductivity Controlled Variation of the Amplitude Distribution of High-temperature Resonators Silja Schmidtchen, Denny Richter, Han Xia and Holger Fritze Institute of Energy Research and Physical Technologies and Energy Research Center Niedersachsen, TU Clausthal, 38640 Goslar, Germany ABSTRACT High-temperature measurements of the spatial distribution of the displacement characteristics of a thickness shear mode langasite (La3Ga5SiO14) resonator are obtained using a laser Doppler interferometer. Thereby, the resonator is excited in the fundamental mode and the third overtone. Further, the resonator is coated with a gas sensitive CeO2-x film which exceeds the metal electrode. In reducing atmospheres the conductivity of the film increases and induces an increase of the effective electrode area. This effect leads to a broadening of the mechanical displacement distribution. The latter depends strongly on the size of the excited part of the resonator which is determined by the effective size of the electrodes. The direct determination of the mechanical displacement at different oxygen partial pressures confirms a model as derived from the electrical impedance of resonator devices [1]. Further, information about the mass sensitivity distribution of resonators is obtained since the property is directly proportional to the amplitude. INTRODUCTION High-temperature stable langasite resonators can be used to determine atmosphereinduced changes in the mechanical and electrical properties of metal oxide films at elevated temperatures [1]. Potential devices include gas sensors for high-temperature applications showing an increased selectivity to different reducing gases like H2 and CO in comparison to conventional conductivity based metal oxide sensors [2]. Alternatively, piezoelectric transducers can be used to study the electrical and visco-elastic properties of metal oxides under different conditions [3]. In the presented work, the principles of the conductivity-induced variation of the effective electrode diameter and its influence on the distribution of the mechanical displacement of piezoelectric resonators are determined and discussed. For this purpose a CeO2-x film is applied on top of the metal electrode of the resonator. Since the area of the CeO2-x film is larger than the metal area, a change in the conductivity of the film causes a change of the effective electrode area and therefore of the excited area of the resonator. The spatial distribution of the displacement amplitude and, thereby, of the mass sensitivity depends on the size of the excited part of the resonator as shown schematically in FIGURE 1.
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FIGURE 1: Schematic drawing of a resonator with platinum electrodes and an additional metal oxide film. With increasing conductivity, the effective electrode area increases. The arrows indicate the cases of insulating CeO2-x film (low ) and high conductivity (high ) (left). Distribution of displacement depending on the film conductivity (right).
In this configuration, the frequency shift is not
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