Improvement of the sensitivity of a conductometric soot sensor by adding a conductive cover layer
Diesel particulate filters are emission-relevant devices of the exhaust gas aftertreatment system. They need to be monitored as a requirement of the on-board diagnosis. In order to detect a malfunction, planar sensors with interdigital electrodes on an insulating substrate can be installed downstream of the filter. During the loading phase, soot deposits onto the electrodes, but the sensor remains blind until the percolation threshold has been reached (initiation time) and the sensor current starts to flow. In order to detect small soot concentrations downstream of the filter from small defects, this initiation time needs to be as low as possible. One may reduce the initiation time by covering the interdigital electrodes with an electrically conductive layer. Using finite element method (FEM) simulations, the influence of conductivity and thickness of such a coating on the initiation time are determined. It is found that a thin, screen printable coating with a thickness of 20 μm and a conductivity in the range of 10 −3 to 10 −1 S m −1 may reduce the initiation time by about 40%. The FEM results were verified by a commercially available thick film resistor paste with a conductivity of 0.45 mS m −1, showing an improvement of about 40% compared to an uncoated sensor.