The Effect of the Thickness of the Sensitive Layer on the Performance of the Accumulating NOx Sensor

Abstract

A novel and promising method to measure low levels of NOx utilizes the accumulating sensor principle. During an integration cycle, incoming NOx molecules are stored in a sensitive layer based on an automotive lean NOx trap (LNT) material that changes its electrical resistivity proportional to the amount of stored NOx, making the sensor suitable for long-term detection of low levels of NOx. 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 NO2. In accordance to a simplified model, the base resistance of the sensitive layer and the sensitivity to NOx 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.

Overview on conductometric solid-state gas dosimeters

“…Groß et al demonstrated the dependency of the dosimeter-type NO x sensing properties on the thickness of the carbonate-based sorbent. Since nitrates form mainly at the surface of the sorbent, the sensitivity of the NO x accumulating sensor decreases with a growing layer thickness as the linear measurement range increases (Groß et al, 2012b). Additionally, the sensor response time concerning the concentration detecting properties was found to increase with the layer thickness.…”

“…7b, whereas it is even 18.5 h for the annual value of 40 µg m −3 . Besides the sensitivity, the linear measurement range of the presented sensor was found to be affected by temperature as well as by the thickness (Groß et al, 2012b) of the sensitive layer. The linear range always ends at a sensor response change of about 30 to 40 %.…”

“…The irreversible, selective, and progressive analyte sorption resulting in analyte accumulation can be caused by strong analyte sorption or a chemical reaction between the analyte and the receptor. Hence, gas dosimeters working in the accumulation mode (Yamazoe and Shimanoe, 2007) are often also denoted as "integrating" or "accumulating" sensors (Tanaka et al, 1999;Maruo 2007;Geupel et al, 2010;Groß et al, 2012b). For the explanation of the accumulating sensing principle, a planar setup consisting of an insulating support with electrodes, e.g., interdigitated electrodes, covered with a sensitive layer, was chosen.…”

Dosimeter-Type NOx Sensing Properties of KMnO4 and Its Electrical Conductivity during Temperature Programmed Desorption

“…During the NO x loading stage, Δ R rel is expected to increase in the presence of NO x at T sorption due to progressive NO x accumulation, without recovery (Figure 4(a)). In the case of a constant flow rate, the cumulated NO x exposure (or dose) A NOx,in is given by the time integral of c NOx,in as sketched in Figure 4(b), resulting in the unit ppm·s [6,16,37]. …”

“…Small deviations of the NO 2 related data points from linearity at the initial stage of exposure may originate from NO 2 adsorption on the inner surface of the feed lines. The sensing characteristics and in particularly the sensitivity of NO x dosimeter with a comparable sensitive material were found to be dependent on the temperature as well as on the thickness of the sensitive layer [6,16]. …”