High Temperature Wireless Surface Acoustic Wave Gas Sensors Using Zinc Oxide and Tin Oxide Thin Films

Sensors that are able to withstand high temperatures are needed to monitor industrial processes for efficient operation. Langasite surface acoustic wave (SAW) sensors have been shown to operate at temperatures up to 1000 °C. This thesis reports progress on the development of wireless langasite SAW sensors for oxygen gas sensing applications.;For a wireless application we examined compact antennas that include the loop antenna, the half wavelength dipole antenna, the inductively loaded dipole antenna, the folded dipole antenna and the meander dipole antenna. We compared the input impedance of the different antennas to find the best match to the impedances of the SAW device that will produce the maximum power transfer. The meander antenna was fabricated using thermocouple wires and connected to the SAW device for temperature sensing up to 650 °C. We have also demonstrated the wireless SAW device being interrogated by a dipole antenna up to a distance of 225 cm.;Implementation of the SAW devices as a gas sensor is realized by applying a metal oxide sensing thin film on the surface of SAW devices. ZnO and SnO 2 metal oxides are discussed and characterized. Resistivity measurements of oxygen gas sensing thin films were performed up to 650 °C. We compared the resistivity measurements of the thin films on fused silica substrate, on langasite substrate and on a buffer layer on langasite substrate. Diffusion between the thin film and langasite substrate is discussed and the importance of a buffer layer between the sensing film and the langasite substrate is shown. Silicon nitride and silicon oxynitride buffer layers were investigated and their applicability at high temperature environments was discussed.;Oxygen gas sensing measurements were performed on the SAW gas sensors, up to 650 °C. The phase measurements were compared to the resistivity values through theoretical equations. We found a good match between the phase measurements for a langasite SAW sensor with SnO2 thin film and resistivity measurements for a SnO2 thin film on langasite substrate.