WO2008025602A1

WO2008025602A1 - Sensor for resistively determining concentrations of conductive particles in gas mixtures

Info

Publication number: WO2008025602A1
Application number: PCT/EP2007/057053
Authority: WO
Inventors: Ulrich Hasenkox; Sabine Roesch; Peter Bartscherer
Original assignee: Robert Bosch Gmbh
Priority date: 2006-08-29
Filing date: 2007-07-10
Publication date: 2008-03-06
Also published as: DE102006040351A1

Abstract

The invention relates to a sensor (20) for resistively determining concentrations of conductive particles (26) in gas mixtures. Said sensor has a surface (23) that is exposed to the gas mixture and is provided with at least two electrodes (21, 22) that are arranged at a distance from each other on said surface. The distance between the electrodes can be bridged by the conductive particles from the gas mixture that are disposed on the surface of the sensor. As a result, a conductive connection can be established between the electrodes of the sensor and a measured variable for the concentration of the conductive particles in the gas mixture can be derived from said connection. Said sensor is characterised in that a material (24) is arranged at least in the region between the electrodes, said material being selected such that it has conductive sections that promote the formation of a conductive connection between the electrodes when conductive particles from the gas mixture are deposited.

Description

Description title

Sensor for the resistive determination of concentrations of conductive particles in gas mixtures.

The present invention relates to a sensor for the resistive determination of concentrations of conductive particles in gas mixtures according to the preamble of claim 1.

State of the art

To reduce the particulate emissions of a diesel engine, soot particulate filters have been used recently. In order to monitor the effectiveness of these filters, often sensors are used, the particle content of the filter passing

Measure exhaust gases (on-board diagnostics, OBD). For this purpose, a resistive particle sensor is often used as the sensor type, which uses the change in resistance of an interdigital electrode structure as a measured variable by the addition of conductive soot particles. Due to its mode of operation, the resistive particle sensor settles with the collecting principles (cf., for example, DE 10149333 A1, WO 2003006976 A2). At present, resistive particle sensors for conductive particles are known, in which two or more metallic electrodes are formed, wherein the adhering particles, in particular soot particles, short-circuit the comb-like interdigitated electrodes and thus with increasing particle concentration on the sensor surface a decreasing resistance (or an increasing resistance) Current at a constant applied voltage) between the electrodes becomes measurable. The measured current or resistance can be correlated with the accumulated soot quantity and thus also with the soot particle concentration prevailing in the exhaust gas. In this case, a threshold is usually defined and the time taken until reaching the threshold as a measure of the accumulated soot amount. The faster this threshold is reached, the higher the soot particle concentration in the exhaust gas.

Alternatively, the current resistance value of the sensor or the decrease in the resistance value over time can also be utilized as a graduated measure of the soot particle concentration. The more conductive connections are formed between the electrodes, the lower the measured resistance.

For regeneration of the sensor element after particle accumulation, the sensor element must be burned free. During burnout, the sensor can not detect the amount of soot.

For the production of the interdigital electrodes of the particle sensor, the screen printing technique is suitable. This has been retained for economic reasons and provides reliable sensors. The problem with sensors produced in this way is that the minimum electrode spacing that can be produced with this technique amounts to 50 μm. Even finer structures can only be produced with photolithographic methods. However, photolithography requires considerably more process steps than screen printing technology and is therefore out of the question for economic reasons.

Due to the relatively high electrode spacing are therefore relatively large amounts of soot in sensors from the prior art required to reach the above threshold. The respective sensors therefore have a relatively low sensitivity. The period until a conductive connection is established between two electrodes and thus a measurement signal is generated ("blind phase") is therefore comparatively long.

Disclosure of the invention

The object of the present invention is therefore to provide a sensor for the resistive determination of concentrations of conductive particles in gas mixtures, which has a higher measuring sensitivity and a shorter reactive phase and is nevertheless inexpensive to manufacture and reliable in operation.

This object is achieved by the features of present claim 1. The dependent claims indicate preferred embodiments.

Accordingly, a sensor is provided for the resistive determination of concentrations of conductive particles in gas mixtures, comprising a surface exposed to the gas mixture with at least two electrodes which are spaced from each other on the surface, such that the distance between the electrodes through itself on the surface of the sensor depositing conductive particles from the gas mixture can be bridged and in this way a conductive connection between the electrodes of the sensor can be produced, from which a measured variable for the concentration of the conductive particles in the gas mixture can be derived. The sensor is characterized in that at least in the region between the electrodes, a material is arranged, which is selected so that it has conductive portions, the addition of conductive particles from - A -

promote the formation of a conductive connection between the electrodes of the gas mixture, and in this way shorten the Auslosezeit or reduce the triggering threshold.

It should be noted that, as already mentioned, measuring time is the time that elapses until a conductive contact between the two electrodes is produced by the deposited conductive particles, ie the resistance at an applied voltage from infinity to a finite value reduced or a measurable current flows. The arranged at least in the region between the electrodes material that is selected so that it has conductive sections, which favor the formation of a conductive connection between the electrodes when attaching conductive particles from the gas mixture, thus reducing the distances that are for the formation of a conductive Connection between the electrodes must be bridged conductive, thus reducing the number of conductive particles required, thus increasing the sensitivity of the sensor.

In a preferred embodiment of the sensor according to the invention, it is provided that the material arranged in the region between the electrodes is an electrically conductive material whose composition is selected such that it fades and / or cracks after being applied to the sensor in such a way that a conductive connection between the electrodes possibly caused by the freshly applied material is interrupted by shrinkage or crack formation.

This material can be, for example, a conductive lacquer or a conductive paste that is applied, and the after application shrinkage cracks forms, for example by drying or evaporation of a solvent contained in the paint or the paste. For the person skilled in the art, it is an easy matter to select a material with suitable conductivity, adhesion and shrinkage properties on the basis of the technical teaching according to the invention. Since the shrinkage cracks occurring during drying are randomly arranged, an average distance to be bridged is established in the region between the electrodes that is far below the distance that the electrodes have from one another. In this way, the amount of soot particles required to produce a conductive connection is reduced and the measuring sensitivity of the sensor increases.

Likewise, the material arranged in the region between the electrodes may be a material comprising electrically conductive particles whose concentration in the material is selected to be below the percolation threshold.

This material may be, for example, conductive ceramic or metal particles in an insulating ceramic matrix, such as platinum particles in an alumina matrix. Since the conductive particles in the matrix are randomly arranged, an average distance to be bridged is established in the region between the electrodes that is far below the distance that the electrodes have from one another. Also in this way, the amount of soot particles required to make a conductive connection is reduced, and the measuring sensitivity of the sensor increases. The term percolation is understood in electrical engineering to mean that conductive fillers in a nonconductive matrix can impart conductivity to the composite of filler and matrix by forming a three-dimensional network. From a certain filler concentration, which as

Percolation threshold is called, the formation of such a network leads directly to a reduction in electrical resistance.

For the skilled person it is an easy to select a matrix material with suitable properties, and also the particle density and size to choose so that the percolation threshold is more or less well below, so as to adjust the triggering sensitivity of the sensor due to the technical teaching of the invention.

An example of such a composite material is e.g. a matrix of insulating ceramic material, e.g. Alumina, with platinum particles embedded therein.

In a first embodiment, the relevant material can be applied to the surface of the sensor already provided with electrodes, thus depositing on the electrodes and in the region between the electrodes.

In a further embodiment, the material can also be applied to the surface of the sensor not yet provided with the electrodes. The electrodes are then printed on the material. This embodiment has the same advantages as the previously discussed embodiment. Moreover, the material may be chosen to serve as an adhesive layer between the surface and the electrodes. In a further embodiment of this idea according to the invention, it is provided that the material arranged in the region between the electrodes is electrically conductive particles which form dendritic structures between the electrodes.

These dendritic structures are structures which protrude from the electrodes into the region between the electrodes without establishing a connection (short circuit) between the electrodes. You can e.g. finger-shaped, triangular or branched.

By means of these structures, the distances between the electrodes are reduced and thus favors the formation of conductive connections between the electrodes. The said electrically conductive particles should have a high thermal stability so that they are not removed during the thermal regeneration of the sensor and / or the soot particle filter.

Such a sensor may e.g. be prepared in which the

Surface of the sensor is exposed to a gas stream containing said electrically conductive particles. These store themselves - similar to the later, to be detected soot particles - on the surface and form the dendritic structures mentioned (see FIG. 5).

In order to support the deposition of the particles, it can be provided that a DC voltage (so-called suction voltage) is applied between the two electrodes, so that the particles accumulate on the surface in the region of the electrodes by electrostatic attraction. Under the microscope, a proper "growth" of the dendritic structures of the Observe electrodes in the direction of the area between the electrodes.

Subsequently, the dendritic structures are fixed on the surface, e.g. by annealing or forming interactions between the surface and the particles.

The particles may be e.g. to act platinum particles or electrically conductive ceramic particles.

Particularly preferably, it is provided that the electrodes are arranged to one another in the form of interdigital electrodes.

This type of embodiment as well as the resulting

Advantages are known per se. Often, and this is preferred, the electrodes are in maanderformiger arrangement.

In a likewise preferred embodiment of the sensor according to the invention, it is provided that the electrodes are applied to the surface of the sensor by means of screen printing technology. The resulting advantages are already explained above. Other suitable methods for applying the electrodes are stencil printing, pad printing, ink jet printing, transfer film. Common to all these methods is that the minimum spacing of the electrodes which can be achieved with one another is not sufficient for a satisfactory improvement in the sensitivity of the sensor, so that the technical teaching of the present invention can be advantageously applied to all sensors produced by one of these methods.

Preferably, the conductive particles to be determined are soot particles. Accordingly, the sensor is preferred a soot particle sensor. The gas mixture is preferably a combustion gas mixture. Particularly preferably, the sensor is arranged in the exhaust gas flow of a diesel engine.

In a further preferred embodiment of the sensor according to the invention, it is provided that the sensor has a heating device for thermal regeneration of the sensor. With their help, the soot particles deposited on the sensor surface can be burned off and the conductive connection between the electrodes is interrupted, so that the sensor can be used again.

In order to improve the correlation of the measured signal with the particle concentration present in the gas and / or to control the regeneration (temperature-controlled / limited), a temperature sensor (in the form of a resistance mander) can additionally be integrated in the sensor, since many particles (in particular soot) have a dependent on the temperature electrical conductivity.

In a particularly preferred embodiment of the present invention, it is provided that the sensor has at least two sensor sections each having at least two electrodes, wherein the electrodes of the at least two sensor sections each have different distances and / or configurations, the at least two sensor sections are operated with different voltages or the electrodes of the at least two sensor sections have different materials. In this way, different measuring ranges or measuring sensitivities can be achieved at the different sensor sections. A method is also provided for the resistive determination of concentrations of conductive particles in gas mixtures, which is characterized in that a sensor according to the invention as described above is used in this method.

drawings

The present invention will be explained in more detail by the figures shown and discussed below. It should be noted that the figures have only descriptive character and are not intended to limit the invention in any way.

Show it:

Figure 1 is a plan view of a sensor according to the invention.

FIG. 2 a cross-sectional view of a first embodiment of a sensor according to the invention; FIG.

3 shows a first variant of a second embodiment of a sensor according to the invention in cross-sectional view;

4 shows a second variant of a second embodiment of a sensor according to the invention in cross-sectional view; and

Fig. 5 shows a third embodiment of an inventive sensor in an enlarged view.

Fig. 1 shows a sensor 10 according to the invention with the electrodes 11, 12 on a surface 13. In the electrodes 11, 12 acts they are so-called interdigital electrodes, which are arranged comb-like interlocking. The sensor is arranged, for example, in the exhaust gas flow of a diesel engine, not shown, so that soot particles are deposited from the exhaust gas flow on the surface of the sensor. When sufficient soot particles have deposited on the surface, the two electrodes are short-circuited and a decreasing resistance (or increasing current at a constant applied voltage) between the electrodes can be measured.

Figs. 2a, 2b show a sensor 20 according to the invention in two cross-sectional views of different scales. Shown are the electrodes 21, 22 on a surface 23. In the region between the electrodes 21, 22, an electrically conductive material 24 is arranged, the composition of which is selected so that it forms cracks 25 after application to the sensor, such that a through the freshly applied material is interrupted if necessary lead conductive connection between the electrodes.

In the present example, this material 24 is a conductive lacquer which forms shrinkage cracks 25 after application. Since the shrinkage cracks occurring during drying are randomly arranged, an average distance to be bridged is established in the region between the electrodes that is far below the distance that the electrodes have from one another. In this way, the amount of soot particles 26 required to produce a conductive connection is reduced and the measuring sensitivity of the sensor increases. Figs. 3a, 3b show a first variant of a second embodiment of the inventive sensor 30 in two cross-sectional views of different scales. Shown are the electrodes 31, 32 on a surface 33. In the region between the electrodes 31, 32, a material 34 is arranged, which has electrically conductive particles 35. Their concentration in the material 34 is chosen to be below the percolation threshold. In this way, the amount of soot particles 36 required to produce a conductive connection is reduced and the measuring sensitivity of the sensor increases.

The material 34 in the present example is an aluminum oxide insulating matrix with platinum particles 35 embedded therein. Since the conductive particles in the matrix are randomly randomized, an average gap to be bridged is established in the region between the electrodes which is wide is below the distance that the electrodes have to each other.

Figs. 4a, 4b show a second variant of a second embodiment of the inventive sensor 40 in two cross-sectional views of different scales. Shown are the electrodes 41, 42 on a surface 43. In the region between the electrodes 41, 42, a material 44 is arranged, the electrically conductive particles 45 has. In contrast to the variant shown in FIG. 3, in this variant the material 44 was applied to the surface of the sensor not yet provided with the electrodes, and the electrodes were then printed on the material. This embodiment has the same advantages as the previously discussed variant. Moreover, can the material 44 may be chosen so that it serves as an adhesive layer between the surface 43 and the electrodes 41, 42.

5 shows a third embodiment of an inventive sensor 50 in enlarged plan view. The sensor has the two interdigital electrodes 51, 52, between which dendritic structures 53 are formed, which consist of conductive particles 54 and with the aid of which the distance between the two interdigital electrodes 51, 52 is shortened. Particles, electrodes and electrode spacing are not shown to scale.

Such a sensor may e.g. in which the surface of the sensor 50 is exposed to a gas stream containing said electrically conductive particles. These store themselves - similar to the later, to be detected soot particles - on the surface and form the dendritic structures mentioned (see FIG. 5).

In order to support the deposition of the particles, it can be provided that a DC voltage (so-called suction voltage) is applied between the two electrodes 51, 52, so that the particles 54 accumulate on the surface in the region of the electrodes by electrostatic attraction. Under the microscope, a proper "growth" of the dendritic structures 53 from the electrodes in the direction of the area between the electrodes can be observed, after which the dendritic structures 53 are fixed on the surface of the sensor 50.

Claims

1. Sensor for the resistive determination of concentrations of conductive particles in gas mixtures, comprising a surface exposed to the gas mixture with at least two electrodes which are arranged at a distance from each other on the surface,

in such a way that the distance between the electrodes can be bridged by conductive particles depositing on the surface of the sensor from the gas mixture and in this way a conductive connection can be produced between the electrodes of the sensor, from which a measured variable for the concentration of the conductive particles in the gas mixture is derivable

characterized in that a material is arranged at least in the region between the electrodes, which is selected such that it has conductive sections which favor the formation of a conductive connection between the electrodes when conductive particles of the gas mixture accumulate.

2. Sensor according to claim 1, characterized in that it is arranged in the region between the electrodes

Material is an electrically conductive material whose composition is chosen so that it fades after application to the sensor and / or cracks.

3. Sensor according to claim 1, characterized in that it is in the region between the electrodes arranged material comprising a material electrically conductive Particles whose concentration in the material is chosen to be below the percolation threshold.

4. Sensor according to claim 1, characterized in that the material arranged in the region between the electrodes is electrically conductive particles which form dendritic structures between the electrodes.

5. Sensor according to one of claims 1 - 4, characterized in that the electrodes are arranged to each other in the form of interdigital electrodes.

6. Sensor according to one of the preceding claims, characterized in that the electrodes are applied by means of screen printing technology on the surface of the sensor.

7. Sensor according to one of the preceding claims, characterized in that it is to be determined conductive particles are soot particles.

8. Sensor according to one of the preceding claims, characterized in that it is the sensor is a soot particle sensor.

9. Sensor according to one of the preceding claims, characterized in that it is the gas mixture is a combustion gas mixture.

10. Sensor according to one of the preceding claims, characterized in that the sensor is arranged in the exhaust stream of a diesel engine.

11. Sensor according to one of the preceding claims, characterized in that the sensor has a heating device for thermal regeneration of the sensor.

12. Sensor according to one of the preceding claims, characterized in that the sensor has a temperature sensor.

13. Sensor according to one of the preceding claims, characterized in that the sensor has at least two sensor sections, each having at least two electrodes, wherein

(A) the electrodes of the two sensor sections each have different spacings and / or configurations; (B) the two sensor sections with different

Voltages are operated; or (c) the electrodes of the two sensor sections comprise different materials.

14. A method for the resistive determination of concentrations of conductive particles in gas mixtures, characterized in that in this method, a sensor according to any one of claims 1-12 is used.