Conductometric Soot Sensors: Internally Caused Thermophoresis as an Important Undesired Side Effect

Abstract

Particulate matter sensors are of interest for application in the exhaust of any combustion processes, especially for automotive aftertreatment systems. Conductometric soot sensors have been serialized recently. They comprise planar interdigital electrodes (IDE) on an insulating substrate. Between the IDEs, a voltage is applied. Soot deposition is accelerated by the resulting electric field due to electrophoresis. With increasing soot deposition, the conductance between the IDE increases. The timely derivative of the conductance can serve as a sensor signal, being a function of the deposition rate. An increasing voltage between the IDE would be useful for detecting low particle exhausts. In the present study, the influence of the applied voltage and the sensor temperature on the soot deposition is investigated. It turned out that the maximum voltage is limited, since the soot film is heated by the resulting current. An internally caused thermophoresis that reduces the rate of soot deposition on the substrate follows. It reduces both the linearity of the response and the sensitivity. These findings may be helpful for the further development of conductometric soot sensors for automotive exhausts, probably also to determine real driving emissions of particulate matter.

Modeling of the signal of a resistive soot sensor, influence of the soot nature and of the polarization voltage

“…Moreover, it must be noted that bridge temperature is not the average temperature of the substrate of the sensor, which is measured thanks to the platinum heater (figure 2) and remains very close to the gas temperature (180°C). Recently, Hagen et al [12] showed that thermophoresis can explain loss of sensitivity for polarization voltage high enough to induce overheating by Joule effect. We were able to verify that this effect is indeed present for overheating above 10°C.…”

Maintaining Transparency of a Heated MEMS Membrane for Enabling Long-Term Optical Measurements on Soot-Containing Exhaust Gas

“…Thermophoresis has been explored in conductometric PM sensors as a means of sensitivity control. A reduced sensitivity with increased temperature was confirmed [12]. Thermophoresis can also be exploited for keeping a selected surface area clean of deposits by intentionally locally heating of the sensor surface up to temperatures slightly above the exhaust gas temperature for repelling airborne particles.…”

“…A direct current (DC) bias voltage of about 45 V is applied across two interdigitated electrodes (IDEs). Soot particles in the exhaust gas flowing along the sensor surface are pulled towards the sensor surface, because of the combined effect of the thermophoretic pulling force (due to the colder wall surface on which the sensor is mounted) [12] and the electrophoretic force (due to the electrostatic field) acting on charged particles [13]. As a result, soot particles are deposited and form dendrites (connected strings of particles) on the sensor surface, eventually bridging the two IDEs.…”

“…Integrated resistors are used in the conductometric PM sensor for regularly heating of the sensor up to a temperature of about 600 • C for periodically oxidizing (burning) the soot, which is referred to as 'regeneration of the surface' [12]. This known mechanism for surface regeneration can in principle be directly implemented for periodically restoring window transparency.…”

Impedance Spectroscopy for Enhanced Data Collection of Conductometric Soot Sensors

“…This sensor consists of two Inter-Digitated Electrodes (IDEs, the combination is sometimes referred to as an Inter-Digitated Transducer IDT, or Finger Electrode Structure -FES), which are biased at approximately 45 V DC [6]. Upon exposure to a soot containing gas flowing over the sensor surface, soot particles are deposited on the sensor surface by a combination of thermophoretic and electrophoretic forces [7].…”