Summary:

For many years, surface acoustic waves (SAW) have been used in electronic components for industrial applications (filtering, signal processing) and in sensors. Thanks to on-board electronics and the possibility of wireless communication, this technology can offer an excellent solution for detection in harsh environments such as high pressure, temperature and humidity. In the case of refractory concretes, optimising drying cycles has a significant economic impact (reduction in production line downtime, energy costs) while making them more efficient and limiting the risk of structural damage. The sensors currently on the market are too restrictive in terms of conditions of use (temperature range, pressure, environment) and therefore do not fully meet the needs of manufacturers in the refractory concrete sector. The aim of this thesis is therefore to fill these gaps by proposing SAW-based pressure sensors capable of operating at much higher temperatures (around 500°C), in wider pressure measurement ranges and in an environment compatible with that of refractory concretes. Developing these types of SAW sensors involves choosing compatible piezoelectric materials, the optimum IDT (Inter Digital Transducer) configuration and the right geometry to withstand these harsh conditions. Lithium niobate was chosen as the basic piezoelectric substrate to carry out modelling studies and optimise IDT configurations and manufacture the first low-temperature pressure sensor prototypes. Electroacoustic modelling tools such as the impulse response model and mode coupling theory (COM) were used to study and optimise the electrical response of LARs (delay lines) and SAW resonators with a view to integrating them into a measurement chain. Several lithium niobate-based pressure sensor prototypes were tested. These enabled us to validate the sensitivity of surface acoustic waves to pressure. The sensitivities obtained for these sensors ranged from 5 to 8 kHz/bar. In collaboration with partners in the CUBISM project, other prototypes of the pressure sensor based on fresnoite glass-ceramics were tested and showed great potential for high-temperature applications.

Abstract:

For many years, surface acoustic waves (SAW) have been used in electronic components for industrial applications (filtering, signal processing) and for the realization of sensors. Due to embedded electronics and the possibility of wireless communication, this technology can offer an excellent solution for sensing in harsh environments such as high pressure, temperature and humidity. In the case of refractory concretes, the optimization of drying cycles has a significant economic impact (reduction of production line downtime, energy cost) while making them more efficient and limiting the risk of structural damage. The sensors currently on the market are too restrictive on the conditions of use (temperature range, pressure, environment) and therefore do not fully meet the needs of the refractory concrete industry. The objective of this thesis is therefore to fill these gaps by proposing SAW-based pressure sensors capable of operating at much higher temperatures (around 500°C), in wider pressure measurement ranges and in an environment compatible with that of refractory concrete. The development of these types of SAW sensors involves selecting compatible piezoelectric materials, the optimal IDT (Inter Digital Transducer) configuration, and the proper geometry that can withstand these harsh conditions. Lithium niobate was chosen as the basic piezoelectric substrate to perform modeling studies and optimization of IDT configurations and fabricate the first low temperature pressure sensor prototypes. Electroacoustic modeling tools such as impulse response model and mode coupling theory (COM) were used to study and optimize the electrical response of LARs (delay line) and SAW resonators in order to integrate them into a measurement chain. Several prototypes of pressure sensors based on lithium niobate have been tested. These allowed us to validate the sensitivity of surface acoustic waves to pressure. The sensitivities obtained for these sensors were between 5 and 8 kHz/bar. In collaboration with partners of the CUBISM project, other prototypes of the pressure sensor based on fresnoite glass-ceramics have been tested and have shown a very interesting potential for high temperature applications.