The Effect of the Thickness of the Sensitive Layer on the Performance of the Accumulating NO(x) Sensor

A novel and promising method to measure low levels of NO(x) utilizes the accumulating sensor principle. During an integration cycle, incoming NO(x) molecules are stored in a sensitive layer based on an automotive lean NO(x) trap (LNT) material that changes its electrical resistivity proportional to the amount of stored NO(x), making the sensor suitable for long-term detection of low levels of NO(x). In this study, the influence of the thickness of the sensitive layer, prepared by multiple screen-printing, is investigated. All samples show good accumulating sensing properties for both NO and NO(2). In accordance to a simplified model, the base resistance of the sensitive layer and the sensitivity to NO(x) decrease with increasing thickness. Contrarily, the sensor response time increases. The linear measurement range of all samples ends at a sensor response of about 30% resulting in an increase of the linearly detectable amount with the thickness. Hence, the variation of the thickness of the sensitive layer is a powerful tool to adapt the linear measurement range (proportional to the thickness) as well as the sensitivity (proportional to the inverse thickness) to the application requirements. Calculations combining the sensor model with the measurement results indicate that for operation in the linear range, about 3% of the LNT material is converted to nitrate.