Performance Evaluation of an Oxygen Sensor as a Function of the Samaria Doped Ceria Film Thickness
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Performance Evaluation of an Oxygen Sensor as a Function of the Samaria Doped Ceria Film Thickness Rahul P. Sanghavi1, M. Nandasiri2, 3, S. Kuchibhatla2, P. Nachimuthu2, M. H. Engelhard2, V. Shutthanandan2, W. Jiang2, S. Thevuthasan2, A. Kayani3 and S. Prasad1 1 Department of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, Arizona. 2 EMSL, Pacific Northwest National Laboratory, Richland, Washington. 3 Physics Department, Western Michigan University, Kalamazoo, Michigan. ABSTRACT The current demand in the automobile industry is in the control of air-fuel mixture in the combustion engine of automobiles. Oxygen partial pressure can be used as an input parameter for regulating or controlling systems in order to optimize the combustion process. Our goal is to identify and optimize the material system that would potentially function as the active sensing material for such a device that monitors oxygen partial pressure in these systems. We have used thin film samaria doped ceria (SDC) as the sensing material for the sensor operation, exploiting the fact that at high temperatures, oxygen vacancies generated due to samarium doping act as conducting medium for oxygen ions which hop through the vacancies from one side to the other contributing to an electrical signal. We have recently established that 6 atom % Sm doping in ceria films has optimum conductivity. Based on this observation, we have studied the variation in the overall conductivity of 6 atom % samaria doped ceria thin films as a function of thickness in the range of 50 nm to 300 nm at a fixed bias voltage of 2 volts. A direct proportionality in the increase in the overall conductivity is observed with the increase in sensing film thickness. For a range of oxygen pressure values from 0.001 Torr to 100 Torr, a tolerable hysteresis error, good dynamic response and a response time of less than 10 seconds was observed. INTRODUCTION Serious attempts to control pollution and optimize automobile performance demand that emissions of green house and other relevant trace gases into the atmosphere be continuously monitored. For real time air quality monitoring, rugged and inexpensive equipment is required 1. One of the key gases whose composition plays a critical role in optimizing automobile performance is oxygen. Oxygen sensors are widely used in many industrial, automotive and biomedical applications 2-3. Oxygen gas sensor identifies oxygen concentration in the exhaust gas 4-6 . Oxygen sensors are used to detect the stoichiometric point for the operation of a three-waycatalyst in internal combustion engines and controlling fossil-fuelled boilers for maximizing thermal efficiency while minimizing the generation of carbon monoxide 7-8-10. Some other applications of oxygen sensors include industrial processes where gas purity and atmospheres in metallurgical heat treatment furnaces need to be monitored and controlled 11-12. These sensors are also used for monitoring the oxygen content during the reduction of ores, carbon activity duri
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