Influence analysis of the packaging technology for piezoelectric sensors operating up to 1000 °C

Abstract

Sensors working at high temperatures of 1000 °C are required in various fields like automobile, aerospace and materials industry. However, reliable sensors working at such high temperature are still not sufficiently available. Developing a high temperature stable package imposes new challenges due to thermal cross-sensitivity and temperature induced stresses. Other maj or issues are to identify stable materials for high temperatures and stress-tolerant sensor mounting techniques. This research work focuses on the demonstration of a packaging technology for piezoelectric sensors operating up to 1000 °C. In addition to the resonator as the active sensor element the assembly and interconnection technology are decisive for the sensor's functional performance. Electrical temperature-dependent and material-specific parameters of the package and the interconnections such as resistance, capacitance and loss factor (tan δ) can influence the sensor behaviour. The parameters determined are included in an extended Butterworth-Van-Dyke-model [1]–[3] which includes the sensor element together with the assembly and interconnection technology. The potential factors influencing the sensor behaviour are the conductance and the capacitance of sapphire substrate, sensor element, ceramic enclosure and spacer. The investigation of these parameters could be realised by the appropriate measurement set-ups in an oven. The parameters determined are included in an extended Butterworth-Van Dyke (BvD) model. The realised sensor is shown in Fig. 1 a. The detailed assembly and interconnection technology for the resonators discussed here is described in [4]. The sensor elements used in this work were manufactured and pre-characterised by Dr. M. Schulz. The equivalent circuit diagram shown can be derived with its influencing variables. The corresponding equivalent circuit model is an extended Butterworth- Van-Dyke model in Fig. 1 b. The validation of the model and the measured parameters take place through the successive assembly of a prototype. Between the steps of fixing the sensor element and housing it with a cap, measurements are carried out to determine the resonance behaviour of the sensor.