Materials Design for the Improvement of SiC Field Effect Gas Sensor performance in High Temperature Process Control applications

Sammanfattning: Following growing concerns about the effect these last decades of accelerating energy consumption, escalating use of natural (not the least fossil) resources, and waste generation has had on earth’s climate, human health, and the environment, industry is facing mounting pressure to increase the energy-, resource-, and cost-efficiency of both processes and products while drastically reducing pollutant emissions. In response, industry is, besides increasing their utilization of renewable energy and introducing new bio-based products, taking advantage of the rapid development in 5G technology and associated wireless interconnectivity to move towards full automation of production processes and facilities. With the ability to connect and collect information from a large number of instruments and process steps as well as to analyse huge amounts of multi-dimensional data, entire production lines can be automatically controlled and/or adapted, e.g., through the application of machine learning, for optimized efficiency and minimized greenhouse gas and pollutant emissions. In order to fully benefit from this so-called 4th industrial revolution and the Industrial Internet of Things (IIoT), however, access to cost-efficient and long-term reliable means for measuring various process parameters in multiple locations is required.The basis for this thesis is therefore the characterization of silicon carbide-based Metal Oxide Semiconductor Field Effect Transistor (MOSFET) gas sensors for their applicability in real-time monitoring of process gases/ gas mixtures and pollutant emissions in high-temperature applications, e.g., control of flue gas/ exhaust after-treatment systems, as well as investigations performed to gain a better understanding of the corresponding sensing mechanisms and to improve sensor performance in terms of e.g., long-term reliability.Besides demonstrating the general applicability of SiC MOSFET gas sensors in ammonia (NH3) slip detection and control of Selective Catalytic Reduction and similar flue gas/ exhaust emissions abatement systems to minimize the release of nitrogen oxides into the atmosphere from the energy production and transport sectors, sensors for the selective monitoring of nitrogen oxides with negligible interference from (sensitivity/ cross-sensitivity to) ammonia have been developed. Through variation of the gold/ iridium composition of the MOSFET gate contact, the interaction of which with gaseous substances in the ambient determines the sensor signal, sensors selective to ammonia and nitrogen oxides (NOx; NO and NO2), respectively, could be developed. Furthermore, as both sensor-types exhibit decent sensitivity to the respective substance over a common temperature interval, the realization of simultaneous NH3 and NOx monitoring with one and the same sensor probe would be facilitated. In relating the sensor response under different conditions to the composition and micro- (/nano-) structure of the gate contact material another piece in the puzzle towards an understanding of the ammonia sensing mechanism of field effect sensor devices could also be obtained.Through the investigation of co-deposited platinum group metals/ metal oxides in different ratios it was also shown possible to improve the long-term stability of carbon monoxide (CO) sensors intended for emissions monitoring and combustion control in industrial processes. Both the materials composition and structure were thereby evaluated and their effect on other important sensor parameters, e.g., sensitivity and temperature dependence, analysed. To further improve sensitivity and selectivity of CO as well as NH3 sensors for combustion control and ammonia slip monitoring applications, respectively, the sensor characteristics were also studied under different ambient conditions, e.g., variations in oxygen concentration, and related to the microstructure of the deposited gate materials. From the simultaneous tuning of film structure and operating temperatures, the selectivity could be enhanced for both the CO and NH3 sensors.In summary, the obtained research results from this thesis work have contributed to a further understanding of the sensing mechanisms of silicon carbide-based MOSFET sensors for CO, NOx, and NH3 monitoring and through the tuning of materials properties and sensor operation parameters generated improvements in sensor reliability, sensitivity, and selectivity of importance to the further realization of sensors for process control and emissions reduction applications.