Miniaturized resonant sensors for harsh environments

Abstract

Miniaturized active structures for operation temperatures between 500 and 1000 °C are presented. They base on langasite single crystals (La 3 Ga 5 SiO 14 ) which exhibit piezoelectrically excited bulk acoustic waves up to at least 1000 °C. Those devices enable new high-temperature sensing approaches. Resonant microbalances are of particular interest since they correlate very small gas composition-dependent mass changes of thin films already deposited onto the resonators with the resonance frequency shift of such devices. Thereby, high-temperature processes as occurring in combustion systems can be monitored in-situ. Miniaturization of those sensor devices improves the sensitivity due to higher operation frequencies. Arrays consisting preferably of miniaturized devices increase the selectivity. Miniaturization of high-temperature devices requires even more stable materials due to the increased effect of e. g. diffusion processes. Further, the resonator design, the arrangement of electrodes and sensor films, the vibration profiles etc. must be reviewed critically in order to take account for their miniaturization. Beside the characterization of the electromechanical properties such as temperature dependent resonance frequency and loss, the specific vibration profiles of devices like membranes of different shape, cantilevers and tuning forks are determined. For this purpose a novel measurement system based on a laser Doppler vibrometer is used to characterize different types of resonant sensor devices in-situ at high temperatures and in different atmospheres. Mapping of the sample surfaces provides the spatial distribution of the mechanical displacement and, thereby, the vibration modes.