Assembly Technologies for Piezoelectric Sensors Operating up to 1000 °C

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Piezoelectric single crystals can be used as resonators to measure temperature, pressure or gas compositions at temperatures up to at lest 1000°C. Suitable sensor systems have to show an adequate functionality and reliability. This in turn requires stable assembly and interconnection technologies and compatible ceramic housings for the sensor elements. Up to now there are no interconnection technologies available for temperatures up to 1000 °C. For this reason the suggested proposal encompasses the generation of appropriate fundamental knowledge in terms of material science and system concepts for such sensors. In the frame of the project, a resonant sensor element with basic functionality will be integrated into a fully functional housing to operate as a model system. This allows investigations on fundamental effects of materials, design and manufacturing processes on the function and the long-term stability up to 1000°C. Initially it is necessary to describe the sensor functions in terms of electrical and thermo-mechanical models to gain understanding of phenomena such as signal transmission and the generation of thermo-mechanical stresses. As basic technologies, stacked ceramics with appropriate metallization layers will be evaluated as housing components. Furthermore glass-ceramic brazes and suitable joining technologies will be developed in order to assemble the sensor and the housing. The housed sensor enables a thorough investigation of the joined components. The analysis of inter-diffusion processes between the different materials allows the evaluation of their stability. Additionally it is necessary to evaluate hermeticity and degradation mechanisms. Resonators made of CTGS (Ca3TaGa3Si2O14), which operate as bulk oscillators, are chosen as model sensors. Their proven operating stability up 1000°C allows an investigation of packaging effects on the sensor signal by utilizing the temperature dependency of the frequency as sensor signal. One of the scientific challenges is the development of an appropriate equivalent circuit accompanied by its characterization and validation.The project includes common work packages for the conceptual design of the whole sensor system as well as the preparation and characterization of the resonator elements. The development of ceramic housings including a functional metallization will be done at the University of Freiburg. Technologies for temperature stable glass-ceramics brazing, metallization and interconnection will be provided. The preparation and characterization of the sensors in the temperature range up to 1000 °C will be performed at the TU Clausthal. Based on these tests, the University of Freiburg will contribute to the modelling of functional behaviour and degradation models based on physics-of-failure concepts. In addition, the sensor behaviour and interfaces of the materials are investigated at TU Clausthal to obtain information on thermomechanical stability.