Development of High Accuracy NOx Sensor

This paper presents an improvement in the accuracy of NOx sensors at high NOx concentration regions by optimizing the manufacturing process, sensor electrode materials and structure, in order to suppress the deterioration mechanism of sensor electrodes. Though NOx sensors generally consist of Pt/Au alloy based oxygen pump electrodes and Pt/Rh alloy based sensor electrodes, detailed experimental analysis of aged NOx sensors showed changes in the surface composition and morphology of the sensor electrode. The surface of the sensor electrode was covered with Au, which is not originally contained in the electrode, resulting in a diminished active site for NOx detection on the sensor electrode and a decrease in sensor output. Theoretical analysis using CAE with molecular dynamics supported that Au tends to be concentrated on the surface of the sensor electrode. Microstructural analysis revealed the agglomeration of noble metal particles in the sensor electrode, which was another contributor to the decrease in sensor output. The optimal manufacturing process, electrode material, and structure were proposed by estimating the mechanism of sensor output deterioration. Controlling the oxygen concentration during the calcination process and raising the Rh ratio in the sensor electrode material suppressed Au adhesion to the sensor electrode. A porous electrode structure prevented the agglomeration of noble metal particles. The newly developed NOx sensor showed stable performance even after aging the part at high NOx concentration regions.