WO2008031654A1 - Sensor element for gas sensors and method for the operation thereof - Google Patents

Abstract

The present invention relates to a sensor element for gas sensors for determining particles in gas mixtures, particularly for soot sensors, with at least two electrodes (1), (2) being exposed to the gas mixture, and a substrate (3) carrying said electrodes, wherein an electrically conductive base (4) is provided between the substrate (3) and the electrodes (1), (2) are connected in an electrically conductive manner by the conductive base (4). The present invention further refers to a method for determining particles in gas mixtures with the inventive sensor element and use of the inventive sensor element.

Description

 Sensor element for gas sensors and method for operating the same

The invention is based on a sensor element and a method for the determination of particles in gas mixtures and their use according to the type defined in the preamble of the independent claims.

State of the art

At present, resistive particle sensors for conductive particles are known, in which two or more metallic electrodes are formed, wherein the accumulating particles, in particular soot particles, which preferably short comb-like interdigitated electrodes and thus with decreasing particle concentration on the sensor surfaces, a decreasing resistance between the electrodes measurable becomes.

Currently, the sensor has a blind time ("blind collection time"), namely the time it takes for the particles to deposit to form first conductive paths and thus a measurable resistance between the electrodes.

Also known is the use of conductive protective layers on the particle sensors, for example from DE 103 19 664 A1. This document discloses a sensor for detecting particles in a gas stream, in particular of soot particles in an exhaust gas flow, comprising at least two measuring electrodes, which are arranged on a substrate of an insulating material and wherein the protection of the measuring electrodes, these are covered by a protective layer. The protective layer may have an electrical conductivity lower than that of the particles to be detected.

However, the production of such coated sensors is difficult because such a protective layer must be applied to the electrodes of the finished sensor. The Downstream firing does not survive all electrode materials and other components of the sensor without damage.

The temperature measurement in conventional sensors is currently carried out via an additional Temperaturmessmäander, which causes additional costs. The alternative measurement of the temperature across the heating resistor, which is realized for cost reasons with a 2-point measurement, is subject to a high measurement inaccuracy due to the influence of the leads.

Furthermore, with a temperature meander, the high temperatures that occur during the regeneration of the particulate filter in an on-board application behind the filter can not be determined with sufficiently high accuracy. This is because the temperature meander is designed for a lower temperature range, namely below the Rußabbrandtemperatur.

Consequently, there is still a need in the art for a particle sensor that has a significantly reduced blind time and is thus ready for use sooner after regeneration.

Disclosure of the invention

According to the invention, a sensor element for gas sensors for the determination of particles in gas mixtures, in particular for soot sensors, with the features according to the preamble of patent claim 1, wherein between the substrate and the electrodes, an electrically conductive pad is provided and the electrodes by the conductive pad electrically conductive connected to each other.

The inventive arrangement of the electrodes and the conductive substrate, a parallel connection of the electrical resistances of the conductive substrate and the layer of deposited on the electrodes particles is achieved. This is metrologically favorable, since even small changes in the total electrical resistance of the arrangement on the sensor element, which result from the deposition of particles on the regenerated electrodes, can be detected. Thus, one achieves the advantage that the ratio of measuring time to blank time in comparison to previous sensors significantly larger, that is cheaper, and the regenerated sensor has a shorter response time than previously achievable.

Preferably, the specific conductivity of the layer over the entire, for the

Application required temperature range smaller than that of the particles to be measured. So each attached particle contributes to the conductivity. Furthermore, it is advantageous if the conductive pad has a well-known or very low temperature dependence of the electrical resistance.

In addition, the conductive pad provides over their defined resistance a way to check the completeness of the electrodes after manufacture. By comparing with reference values at defined temperatures, it also offers the possibility to calibrate the electrode. Such a test or calibration can be used both in the factory as end of line testing, as well as in

Vehicle over the lifetime of the sensor to detect or compensate for damage and aging effects of the electrode. In the vehicle, this check can always be carried out when there are no conductive particles on the sensor, ie every time the sensor is regenerated.

In the regenerated state of the sensor, it is also possible to calibrate the inaccurate temperature signal, which can be obtained from the resistance of the heating meander, with the exact temperature signal from the conductive carrier layer. This measure eliminates the need for an additional temperature meander for determining the sensor temperature during measurement operation.

Alternatively, the temperature dependence of the conductivity of the electrically conductive base can be chosen so that it allows for a more accurate temperature measurement, especially for high temperatures, such as occur in the regeneration of the particulate filter than the not optimized for this area

Temperaturmessmäander.

In a preferred embodiment of the present invention, the specific conductivity of the conductive surfaces is at 500 ⁰ C in a range of> 1 x 10 ^-9 (ohm-cm) ^"1 to <1 x ^{10" 2} (Ωcmf ^1, preferably> 1 X 10 ^"8 (Ωcm) ^{" 1} to <1 x 10 ^"3 (Ohm-cm) ^"1, more preferably of> 1 × 10 ^-7 (Ωcmf ¹ to <1 x ^{10" 4} (ohm-cm) ^{". 1} Such specific conductivity of the substrate allow for the typical, required in use and currently produced layer thicknesses For example, resistances of more than 30 MΩ can be achieved with layer thicknesses of 1 to 10 μm and a typical applied voltage of 30 V. This conductivity can be achieved in the Range of operating temperature can be achieved, which is about 500 ⁰ C.

In an advantageous embodiment of the present invention, the specific

Conductivity of the conductive surfaces at 50 ⁰ C in a range of> 1 x 10 ^-9 (ohm-cm) ^"1 to <1 x lO ^ ^{ohm-cm)" 1,} preferably> 1 × 10 ⁸ (ohm-cm) ^"1 Ws <1 x lO ^ ohm-cm) ^1, more preferably of> 1 x 10 ^{"7 (ohm-cm)" 1} to <1 x lO ^ (ohm-cm) ^'1. Such specific conductivities of the underlay make it possible to obtain electrical resistances in the typical layer thicknesses of the underlay which are required in use and can currently be produced, and which stir during operation into easily measurable and readily evaluable flows of current. For example, with layer thicknesses of 1 to 10 μm and a typical applied voltage of 30 V, resistances of more than 30 MΩ can be achieved. This conductivity can be reached in the region of the exhaust gas temperature directly after the start, which is about 50 ⁰ C. Thus, the function of the sensor element can also be ensured directly after the start of an internal combustion engine.

In a further advantageous embodiment of the present invention, the material of the conductive substrate comprises aluminum oxide doped with metal oxides, preferably with metal oxides selected from the group consisting of Fe ₂ O 3, ZrO ₂ , Cr ₂ O 3, MgO and / or MnO. The addition of these metal oxides to the non-conductive alumina makes it possible to produce a desired electrical conductivity without degrading the structural properties of the high temperature resistant ceramic.

In a further advantageous embodiment of the present invention makes the

Concentration of the metal oxides in the alumina a total content of> 0.01 mol% to <20 mol%, preferably from> 0.1 mol% to <10 mol%, more preferably from> 1 mol% to <5 mol -% out. With these doping concentrations, a defined dependence of the electrical conductivity on the temperature is achieved in the temperature range which corresponds to the operating temperature of the particle sensor. For example, with a doping of 4.3 mol% of Fe ₂ O ₃ in the range between 200 ^° C. and 500 ^° C., a conductivity is achieved which has a linear course with semilogarithmic plotting of the electrical conductivity against the inverse temperature. Doping with over 20 mol% of foreign oxides are not appropriate, since a lower and a non-regular temperature dependence of the electrical

Conductivity can occur.

In a further advantageous embodiment of the present invention, the electrodes of the sensor element are designed as interdigital electrodes. Such electrodes have the advantage that with a small base area of the substrate, a large area of the opposing electrodes can be achieved and thus a correspondingly high sensitivity of the sensor is realized.

In a further advantageous embodiment of the present invention, the conductive pad is electrically isolated from the substrate by insulation. These

Arrangement makes it possible to use substrates which have an undesirable intrinsic conductivity during operation, but due to their other, for example mechanical, properties for the construction of a sensor element would be favorable.

It is further possible within the scope of the present invention to provide, in addition to the sensor element according to the invention, an evaluation device which determines a change in the current flow and / or resistance present between the electrodes and outputs this as a measure of the particle concentration or the particle mass flow. This makes it possible, the invention

Integrate sensor element in the control system of the engine electronics and to use for efficient engine operation management.

The present invention further provides a method for the determination of particles in gas mixtures, in particular of soot in exhaust gases of internal combustion engines by means of a sensor element according to the present invention, wherein at least two electrodes, an electrical voltage is applied and the adjusting between the electrodes current flow or electrical Resistance is determined and output as a measure of the particle concentration or the particle mass flow. The method according to the invention benefits from the advantages of the sensor element according to the invention that the ratio of measuring time to blank time is shorter and that the sensor element responds more quickly after the regeneration. Thus, more accurate particle determinations can be carried out.

In an advantageous embodiment of the method according to the invention, the method according to the invention is integrated into a method for monitoring a system comprising a diesel engine and particle filter with regard to parameters which are selected from the group comprising the mode of operation of the diesel engine, the functionality of the particulate filter and / or the loading state of the particulate filter.

For example, a sensor element mounted in front of the particulate filter can provide conclusions as to when the filter will be blocked and thus initiate regeneration. A mounted behind the particulate filter sensor element can serve to check whether the particulate filter still retains enough particles or if it is damaged. Furthermore, it can be determined based on the particle measurement, how well the combustion takes place in the diesel engine and are readjusted accordingly in the engine control. This is especially important with changing fuel qualities.

Another object of the present invention is the use of a

Sensor element according to the present invention for monitoring a system comprising diesel engine and particulate filter with respect to characteristics which are selected from the group comprising the operation of the diesel engine, the functionality of the particulate filter and / or the loading state of the particulate filter. The possibilities and advantages of such use have already been described above.

drawing

Two embodiments of a sensor element according to the invention are shown schematically simplified in the drawing and are in the following

Description explained in more detail. Show it

Figure 1 is a schematic plan view of an inventively ausgestaltetes sensor element; FIG. 2 shows a section through a sensor element configured according to the invention;

3 shows a section through another inventively ausgestaltetes

Sensor element;

FIG. 4 shows a sensor element configured according to the invention after being loaded with

Soot particles as well

FIG. 5 shows an equivalent circuit diagram for a resistive DC current measurement.

Description of the embodiments

FIG. 1 shows a sensor element according to the invention for gas sensors for determining particles in gas mixtures. The sensor element or the gas sensor according to the invention is used for installation in an exhaust system of a motor vehicle and is preferably arranged downstream of a soot filter of a motor vehicle with a diesel internal combustion engine.

The sensor element comprises a plate-like carrier layer which serves as a substrate (3) and which consists of a highly insulating material, for example of a ceramic, such as

Alumina, is made. It is also conceivable to manufacture the substrate (3) from an alternative material, such as, for example, yttrium-stabilized zirconium dioxide. On this substrate (3), a conductive pad (4) is applied. For better representation of the substrate (3), the conductive pad (4) is partially drawn. The conductive substrate may be made of, for example, doped with iron oxide alumina.

On the conductive pad (4) now a pair of comb electrodes or interdigital electrodes (1), (2) are applied. The electrodes can be made of platinum and can be connected via corresponding contacts with a measuring and control unit. The electrodes (1) and (2) are now exposed to the gas mixture. Under

Gas mixture is to be understood here as the exhaust gas of an internal combustion engine. In addition to gaseous combustion products, this also contains particles resulting from incomplete combustion of the fuel. On the electrodes (1), (2), the particles separate and cause an electrically conductive connection between them. The sensor element can be manufactured by screen printing pasty ceramic precursor compounds. As a result, reproducible and thin layer thicknesses can be achieved. Thus, for example, the substrate (3) can be formed by screen printing, dried to a green body and then provided with the conductive substrate (4). After drying, the layer structure with the

Electrodes (1), (2) are provided and burned. The electrodes (1), (2) can be applied as wires. Alternatively, metal nanoparticles, for example concentrated colloidal sols, can also be printed by jet-printing techniques (ink jet) to the electrodes. As a result, a large variety of electrode shapes can be realized.

FIG. 2 shows a sectional view of the sensor element arrangement shown in FIG. The section is perpendicular to the fingers of the interdigital electrodes (1) and (2) shown in FIG. One recognizes the substrate (3), on which the conductive base (4) is applied, as well as on this the electrode sections of the

Interdigital electrodes (1), (2). In this case, it becomes clear that in the unloaded state, as it exists for example directly after production or after regeneration, the electrodes are electrically connected to one another only via the conductive base (4).

FIG. 3 shows a further embodiment of the present invention in a sectional view. In addition to the embodiment shown in FIG. 2, a further layer (5), which is electrically insulating, is provided in FIG. 3 between the substrate (3) and the conductive base (4). This layer (5) can be constructed, for example, of aluminum oxide. By this measure it is achieved that as substrate (3) any material can be selected which for the respective

Purpose is best suited to have to accept without restrictions because of the eventual electrical conductivity.

FIG. 4 shows a sectional view of a sensor element according to the invention analogous to that shown in FIG. 2, wherein the electrodes (1), (2) exposed to the gas mixture are now covered by a layer of soot particles (6). The soot particles create an electrical contact between the two electrodes (1), (2). With a low loading of the electrodes (1), (2), accordingly, there is only a slight electrical connection via the soot particles (6), which, however, are good at metrology capture is. With increasing loading, the electrodes are connected to each other more and more electrically conductive.

The electrical resistance ratios of the arrangement shown in Figure 4 with an electrically conductive layer (4) and the soot particles (6) are shown in Figure 5. It is a parallel connection of two resistors. The first resistance is determined by the resistance of the layer 4 and is represented by the section Rl -4-2. The second resistance is determined by the resistance of the soot particles 6 and is represented by the symbol Rl -6-2. With increasing amount of soot on the electrode surface, the resistance Rl -6-2 decreases, from which it can be concluded that the state of the exhaust gas in question. The resistance Rl -4-2 remains constant.

The application of the sensor element described is not limited to the determination of soot particles in exhaust gases of internal combustion engines, but it can be used in general for the determination of the concentration of condensable and electrically conductive particles.

Claims

claims 1. Sensor element for gas sensors for the determination of particles in Gas mixtures, in particular for soot sensors, with at least two electrodes (1), (2) exposed to the gas mixture and a substrate (3) carrying these electrodes, characterized in that between the substrate (3) and the electrodes (1), (2) an electrically conductive pad (4) is provided and that the electrodes (1), (2) are electrically conductively connected to one another by the conductive pad (4). 2. Sensor element according to claim 1, wherein the specific conductivity of the conductive substrate (4) at 500 ⁰ C in a range of> 1 X 10 ^"9 (Ωcm) ^{" 1} to <1 X 10 ^"2 (Ωcm) ^{" 1} , preferred from> 1 x 10 ^"8 (Ωcm) ^{" 1} to <1 x 10 ^"3 (Ωcm) ^{" 1} , more preferably from> 1 x 10 ^"7 (Ωcm) ^{" 1} to <1 x 10 ^"4 (Ωcm) ^{" 1} lies. 3. Sensor element according to claim 1 or 2, wherein the specific conductivity of the conductive substrate (4) at 50 ⁰ C in a range of> 1 X 10 ^"9 (Ωcm) ^{" 1} to <1 X 10 ^"2 (Ωcm) ^{" 1} , preferred from> 1 x 10 ^"8 (Ωcm) ^{" 1} to <1 x 10 ^"3 (Ωcm) ^{" 1} , more preferably from> 1 x 10 ^"7 (Ωcm) ^{" 1} to <1 x 10 ^"4 (Ωcm) ^{" 1} lies. 4. Sensor element according to one of the preceding claims, wherein the material of the conductive substrate (4) comprises metal oxides doped alumina, preferably with metal oxides selected from the group comprising Fe ₂ θ3, ZrÜ ₂ , Cr ₂ θ3, MgO and / or MnO. 5. Sensor element according to claim 4, wherein the concentration of the metal oxides in the Alumina has a total content of> 0.01 mol% to <20 mol%, preferably from> 0.1 mol% to <10 mol%, more preferably from> 1 mol% to <5 mol%. 6. Sensor element according to one of the preceding claims, wherein the electrodes (1), (2) are designed as interdigital electrodes. 7. Sensor element according to one of the preceding claims, wherein the conductive Base (4) is electrically insulated from the substrate (3) by an insulation (5). 8. A method for the determination of particles in gas mixtures, in particular of Soot in exhaust gases of internal combustion engines, by means of a sensor element according to one of claims 1 to 7, characterized in that on at least two electrodes (1), (2) an electrical voltage is applied and extending between the electrodes (1), (2) adjusting current flow or electrical resistance is determined and output as a measure of the particle concentration or the particle mass flow. 9. The method of claim 8, wherein the method is included in a method for monitoring a system comprising diesel engine and particulate filter with respect to characteristics that are selected from the group comprising the operation of the diesel engine, the functionality of the particulate filter and / or the Loading condition of the particulate filter. 10. Use of a sensor element according to one of claims 1 to 7 for monitoring a system comprising diesel engine and particulate filter with respect to characteristics which are selected from the group comprising the operation of the Diesel engine, the functionality of the particulate filter and / or the loading condition of the particulate filter.