Resistance Sensor Based on Thermophoresis for Soot in Diesel Exhaust
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Resistance Sensor Based on Thermophoresis for Soot in Diesel Exhaust
Robert Bjorklund1, Mats L. Johansson2, Ann Grant3, Peter Jozsa3, Per-Erik Fägerman4, Jaska Paaso5, Andreas Larsson6, Doina Lutic7 and Anita Lloyd Spetz1 1
Dept Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden Volvo Car Corp., Dept 97621 PV3 C1, SE-405 31 Göteborg, Sweden 3 Volvo Technology Corp, Dept 06100, SE-412 88 Göteborg, Sweden 4 Mandalon Technologies AB, Westmansgatan 49, SE 582 16 Linköping, Sweden 5 Selmic Oy, Box 350, FI-90550 Oulu, Finland 6 SINTEF ICT, Box 124, NO-0314 Blindern, Norway 7 Dept Chemistry, Alexandru Ioan Cuza University of Iasi, 700506 Iasi, Romania 2
ABSTRACT A resistance sensor for use in diesel exhaust is reported. Several soot deposition mechanisms contribute to collection on the sensing electrodes. The sensor is designed to enhance the temperature difference between the electrode surface and the ambient. The resulting thermophoretic force on nanoparticles enhances soot deposition. Exhaust soot concentrations were shown to correlate with resistance decreases and the effect of thermophoresis was studied.
INTRODUCTION Employment of particle filters in diesel driven vehicles, first introduced as standard equipment in 2000, has greatly reduced soot emissions. Current reporting of the filter operating status (OBD=on board diagnostics) is by monitoring the pressure drop over the filter in order to detect clogging or cracks. The proposed California Code of Regulations Title 13, Section 1971.1.e.8 requirement for OBD after 2013 where all types of deterioration or failures must be detected before tailpipe emissions exceed 0.03 g/BHP-hr (BHP=brake horse power, engine horse power without loss caused by auxiliary components) will require new types of monitoring techniques probably involving particle sensors. This anticipated technology shift has led to development of a number of innovative sensors for monitoring soot based on charge [1], the escaping current principle [2] and resistance when collected on a surface [3-5]. Electrical resistance in soot is controlled by the conductivity of the primary particles, the conductivity across interparticle contacts and the presence of heteroatoms such as charge trapping oxygen [6]. One aerosol deposition mechanism where the collection surface is held at a lower temperature than the ambient is thermophoresis. It is a weak force asserted on aerosol particles when located in a temperature gradient. Collisions with more energetic gas molecules from the warmer zone result in a net movement toward the colder area. The force is used in thermophoretic collectors to obtain aerosol samples from air [7]. In this work it is used to enhance the deposition of soot particles on the sensor.
EXPERIMENTAL DETAILS Sensors with interdigitated finger electrodes (width/gap, 150/100 µm) were fabricated by screen printing PtPdAu conductor on one end of alumina substrates (90x5x1 mm), shown in figure 1. A heater for burning off collected soot and Pt100 thermal ele
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