Robust, selective NOx species sensor for tailpipe emissions

Diagnostic characterization of tailpipe emission of nitrogen oxides (NOx) is becoming increasingly important as environmental concern and government regulations are requiring the use of complex NOx monitoring and destruction schemes, especially in diesel fueled engines. As these systems become more efficient, tailpipe emissions can retreat to the parts per million (ppm) level, which requires an equally precise NOx sensor to provide feedback for the active destruction system in the presence of the many components of post catalyzer exhaust gas. Equally important to maintaining the NOx destruction efficiency is the ability to provide diagnostic information on an ever shorter timescale to tighten the engine control loop about its most efficient operating point. Finally, reliability, cost and manufacturability are crucial to helping these systems achieve widespread implementation and acceptance. New robust transducer structures that have high sensitivity, quick responses selective molecular determination and high concentration precision determination are needed to go along with cost effective manufacturability and robustness in the face of the extreme environmental conditions present in the exhaust of internal combustion engines. During the past eighteen months, Nanohmics has been developing a gas multiplexed detection system that involves bonding of a patterned of an array of electrodes to a novel nanoporous semiconducting fiber thin paper comprising metal oxide semiconductor material prepared in collaboration with manufacturing partner, Unifrax. The paper provides a means for lateral entrainment and molecular separation with real-time detection based on gas affinity for surface adsorption to the novel “semiconductor stationary phase” material that serves as both a real-time molecular separation and sensing film. The fiber paper is continuously resettable when molecular desorption is induced providing a means for continuous monitoring of complex exhaust gas plumes. Nanohmics recently demonstrated the basic principles of the compact, semiconducting stationary phase gas detection and has been developing methods to integrate the paper with electrode arrays on ceramic circuit boards. For this Phase I program, Nanohmics proposes to develop a high-temperature and environmentally stable sensor array device based on this core detection technology. The NOx Sensor array device will include a high-temperature ceramic base (e.g. Al2O3, BN) with nanodimension semiconductor transducer structures comprising an film of nanoporous tin oxide fibers that are bonded to electrodes forming independently, electrically- addressable regions of the matrix that serve as real time sensing zones for complex separation and concentration profile when gas molecules adsorb to the local active region and undergo carrier exchange with the semiconductor. The method will further include the development of a method that provides a means to batch impart unique specificity to different gas classes within the high temperature environment during a single processing step. In addition to high temperature combustion diagnostics, the ability to perform rapid, highly sensitive and selective detection of a complex chemical profiles is important for applications ranging from acute situational health and safety assessment, industrial process control to suspect/target identification.