High Temperature Operation and Stability of Langasite Resonators
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High Temperature Operation and Stability of Langasite Resonators H. Fritze1, M. Schulz1, H. Seh2, H. L. Tuller2 1 Technische Universität Clausthal, Department of Physics, Metallurgy and Materials Science, Robert-Koch-Straße 42, D-38678 Clausthal-Zellerfeld, Germany, [email protected] 2 Massachusetts Institute of Technology, Department of Materials Science & Engineering, 77 Massachusetts Avenue, Cambridge, MA 02139, USA ABSTRACT Factors limiting potential use of langasite as a gas sensor platform at elevated temperatures include excessive conductive and viscous damping, deviations from stoichiometry and chemical instability. This paper focuses on viscous damping which can be described by an effective viscosity of the resonator material. Based on a one-dimensional description of the resonator device, the material constants of langasite are determined as a function of temperature. The effective viscosity of the resonator material and the bulk conductivity are found to exhibit nearly the same activation energy at temperatures from 350 to 600 °C. Based on that fact, it is most likely that the predominant conduction mechanism, i.e. oxygen ion movement, controls the viscous damping in that temperature range. Therefore, intentional doping must suppress the oxygen conductivity to minimize losses. The effect of such dopants is presently under investigation. Further, pre-annealing specimens for several hours above 1050 °C is necessary, for the given sample dimensions, to establish reproducible materials properties. Based on oxygen diffusion data, it can be concluded that the oxygen stoichiometry of langasite specimens becomes equilibrated during the pre-annealing runs. INTRODUCTION High temperature stable piezoelectric materials such as langasite (La3Ga5SiO14) and gallium orthophosphate (GaPO4) are attractive as the basis of elevated temperature gas phase chemical sensors. Specific surface affinity layers selectively adsorb specified gases which are detected by monitoring shifts in resonance frequency due to mass loading of the piezoelectric resonators. High sensitivity is expected since the mass sensitivity of e.g. langasite resonators at 800 °C has been shown to be on the same order of magnitude as that of quartz resonators at room temperature [1]. To take advantage of the high mass sensitivity, low losses are required. According to our previous investigations, the effective viscosity η controls the overall loss at high temperatures [1]. Consequently, the corresponding mechanisms must be understood to define appropriate improvements of the resonator material, e.g. by doping. Further, the movement of oxygen ions was identified as the predominant electrical transport process at high oxygen partial pressure [2]. Consequently, it is of interest to determine if a correlation between viscous damping and electrical conductivity exists. MODEL A thickness shear mode resonator operating at room temperature can be described by a onedimensional physical model taking into consideration the material properties:
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