High Temperature Inductively Coupled Wireless Oxygen Sensor

High temperature sensors are used in many places such as oxy-fuel combustors, and automobiles. In this thesis we developed a wireless oxygen sensor for measurement of oxygen concentration in oxy-fuel combustors. Wireless sensors reduce electrical connections and penetrations, and therefore are desirable in high temperature environments. In this thesis, a high temperature inductively coupled oxygen sensor is designed and characterized.;The sensor uses a screen-printed ZnO thick film as a resistive oxygen sensor. We developed a multi-layer screen printing process for simultaneously fabricating ZnO thick films, spiral inductors and parallel plate capacitors. Measurements show that the ZnO thick film is n-type at 500 °C -- 900 °C, i.e., the film resistance rises when the oxygen concentration increases. The response time of the ZnO thick film falls when the operation temperature increases.;We explored Fe-doped SrTiO3 thick films as an alternative oxygen-sensing layer. The activation energy of the Fe-doped SrTiO3 thick films was smaller than ZnO thick films. ZnO thick films were easier to fabricate than Fe-doped SrTiO3 thick films so ZnO films were used in inductively coupled oxygen sensors.;We report the evaluation of several different oxygen sensor designs screen-printed on an alumina substrate. The resistance of the screen-printed conducting film contributed significantly to the loss of the resonant circuit, reducing the magnitude of the oxygen response. Another problem encountered was the oxygen response of the capacitor dielectic itself;In order to develop a more practical sensor, we propose a figure of merit for comparing the performance of different sensor designs. According to the figure of merit, it is hard to further improve the performance of a screen printed inductively coupled sensor. Accordingly, an improved helical coil design which has better performance according to the figure of merit was proposed. COMSOL simulations were used to predict the series resistance of helical coil inductors, the self inductance and mutual inductance of the helical coil inductors. The coupling coefficient of helical coil inductors was calculated for given gap between the two coils, the wire diameters and the coil diameters. The calculation shows that the improved designs have better figure of merit.;Finally, a prototype sensor was designed and tested at high temperatures from 650 °C to 850 °C. The helical coils were wound on ceramic tubes using thermocouple wires. It was found that the prototype sensor has good performance for measuring oxygen concentrations at high temperatures. The maximum impedance magnitude rises when the oxygen concentration increases. The minimum impedance phase falls when the oxygen concentration increases. The frequency of the sensor decreases when the temperature increases. This is a potential method for temperature compensation of the sensor. Further improvements in performance are possible, potentially leading to a practical inductively coupled sensor.