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A MEMS-Based Gas Sensor for Detecting the NOx of Automobile Exhaustion Gas
Keywords: Gas Sensor, MEMS, Indium oxide
Microelectromechanical systems (MEMS)-based gas sensors with semiconducting materials have many advantages of small dimension, low cost, high sensitivity, short response time, and low power consumption. In this study, the micro gas sensors for NOx gas were fabricated by using a MEMS technology and sol-gel process. The sensing electrode and micro heater were designed to be a co-planar typed structure in the Pt thin film layer. The thermal characteristics for a micro heater array were analyzed by a finite element method (FEM). The chip size of gas sensor was about 2 mm 2 mm. The fabricated micro platform had a low power consumption of 67 mW at 2.0 V of heater voltage and 300 of operating temperature. Indium oxide as a sensing material for NOx gas was synthesized by a sol-gel process with three different precursors of indium isopropoxide, indium nitrate and indium hydroxide. The particle size of synthesized In2O3 was identified as about 50 nm by FE-SEM and XRD measurements. The maximum gas sensitivity as relative resistance (Rs = Rgas/Rair) occurred at 300 with the value of 8.0 at 1ppm NO2 gas. Response time (70 % saturation) was within 60 seconds. The recovery time was within 4 minutes after NO2 gas was removed. The sensing property of NO2 gas showed a linear behavior with the increase of gas concentration. The fabricated gas sensor did not react on CO gas which is one of the major automobile exhaust gases. The sensing mechanism of the gas sensor was explained by the variation of the electron depletion layers and adsorption of gas molecules on the In2O3 particle surface. The present study shows that a MEMS-based gas sensor is a potential candidate for the automobile AQS (air quality system) gas sensor with many advantages of small dimension, high sensitivity, short response time and low power consumption.
Jung-Sik Kim, Professor
University of Seoul
Seoul 130-743 ,

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