DE102005018838A1 - Sensor element for particle sensors and method for operating the same - Google Patents

Abstract

The invention relates to a sensor element for gas sensors, in particular for the determination of particles in gas mixtures, with at least one measuring element exposed to the gas to be determined and at least one temperature measuring element (30) integrated into the sensor element, wherein the temperature measuring element (30) is designed as a thermocouple.

Description

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

Around the functionality current exhaust aftertreatment systems used in motor vehicles to check or to be monitored Sensors needed, with which even in long-term operation an accurate determination of in a concentration of particulates present in the combustion exhaust gas is allowed can. About that In addition, by means of such sensors, a loading forecast, for example a diesel particulate filter provided in an exhaust system, to achieve a high system security and thus more cost-effective To be able to use filter materials.

So is out of the DE 10 2005 003118 a sensor for detecting particles in a fluid flow, which is carried out on the basis of a ceramic multilayer substrate. It comprises two spaced-apart measuring electrodes, which are exposed to the combustion exhaust gas to be examined. If soot is deposited between the two measuring electrodes, a current flow between the measuring electrodes occurs when a voltage is applied to the measuring electrodes. A layered heating element makes it possible to free the electrodes or their surroundings by thermal means from deposited soot particles. The sensor further comprises a temperature measuring element with which the temperature of the sensor can be detected. The heating element is located within the laminate of the sensor between the temperature measuring element and the measuring electrodes. The disadvantage here is the relatively complex construction of the sensor, since separate and mutually insulated layer planes must be provided in the sensor for the measuring electrodes, the temperature measuring and the heating element.

task The present invention is a sensor element for sensors and a method for determining the concentration of particles in gas mixtures that provide accurate temperature control allowed and yet on a simple overall design of the sensor element recourse.

Advantages of invention

The Sensor element or the method with the characterizing features which has independent claims the advantage that the problem underlying the invention in solved in an advantageous manner. This is based in particular on the simple structure of the sensor element and on a smaller number of required electrical contacts for controlling the sensor element. The sensor element comprises at least a measuring element exposed to the gas to be determined, at least a heating element integrated in the sensor element and at least a temperature measuring element integrated in the sensor element. there is in particular a combination of an electrical contact of the temperature measuring element with an electrical contact of a the other two elements provided.

In an advantageous embodiment the temperature measuring element is designed as a thermocouple. there are taking advantage of the so-called Seebeck effect two metallic Conductor tracks of different thermoelectric voltage in the area of measuring temperature brought into electrical contact with each other and it becomes the ends exposed to a reference temperature the conductor tracks adjusting potential difference determined. The potential difference provides a measure of that The advantage is that as the first Trace of the thermocouple a trace can be used, for electrical contacting of the measuring or heating element is provided.

Further advantageous embodiments the present sensor element or method for operating the same emerge from the dependent claims.

So is advantageous if the thermocouple is a conductor of a Platinum-rhodium alloy in particular having the composition Pt13Rh, since this alloy has a Temperature determination up to 1600 ° C allowed.

Farther is advantageous if the sensor element has two measuring elements, the ones on opposite sides Exterior surfaces of the Sensor element are arranged, since the resulting measurement results then on two independent measurements based and thus gain in accuracy.

In a further advantageous embodiment of the present invention, an evaluation device is provided which determines a change in the voltage applied between the measuring electrodes of the measuring element current flow and this as a measure for the particle concentration.

The Sensor element or the method for operating the same is more advantageous Way suitable for monitoring the operation of a diesel engine or to check the functionality or the loading state of a particulate filter.

drawing

Three embodiments of the sensor element according to the invention are shown schematically in the drawing and simplified explained in more detail in the following description. It shows

1 a sensor element according to a first embodiment in an exploded view,

2 a section of the in 1 shown sensor element in a plan view,

3 a sensor element according to a second embodiment in an exploded view,

4 a sensor element according to a third embodiment in an exploded view and

5 a correlation of the thermal voltage of a Pt13Rh thermocouple with the temperature to be determined in ° C.

description the embodiments

In 1 a basic structure of an embodiment of the present invention is shown. With 10 is a ceramic sensor element referred to, which serves the determination of particles, such as soot particles in a gas mixture surrounding the sensor element. The sensor element 10 For example, it includes a plurality of oxygen ion conductive solid electrolyte layers 11a . 11b and 11c , The solid electrolyte layers 11a and 11c are carried out as ceramic films and form a planar ceramic body. They preferably consist of an oxygen-ion-conducting solid electrolyte material, such as Y ₂ O ₃ stabilized or partially stabilized ZrO ₂ .

The solid electrolyte layer 11b in contrast, by means of screen printing of a pasty ceramic material, for example on the solid electrolyte layer 11a generated. As a ceramic component of the pasty material preferably the same solid electrolyte material is used, from which also the solid electrolyte layers 11a . 11c consist.

Furthermore, the sensor element 10 For example, a plurality of electrically insulating ceramic layers 12a . 12b . 12c . 12d . 12e and 12f on. The layers 12a - 12f are also by screen printing of a pasty ceramic material, for example, on the solid electrolyte layers 11a . 11b . 11c generated. As a ceramic component of the pasty material, for example, barium-containing aluminum oxide is used, since this has a largely constant high electrical resistance even with thermal cycling over a long period of time. Alternatively, the use of ceria or the addition of other alkaline earth oxides is possible.

The integrated shape of the planar ceramic body of the sensor element 10 is laminated by laminating with the solid electrolyte layer 11b and with functional layers as well as the ceramic layers 12a - 12f printed ceramic films and subsequent sintering of the laminated structure in a conventional manner.

The sensor element 10 also has a ceramic heating element 40 on, which is designed in the form of an electrical resistance track and the heating of the sensor element 10 in particular to the temperature of the gas mixture to be determined or the burnup of the large areas of the sensor element 10 Deposited soot particles serves. The resistor track is preferably made of a cermet material; preferably as a mixture of platinum or a platinum metal with ceramic portions, such as alumina. The resistance conductor track is furthermore preferably designed in the form of a meander and has plated-through holes at both ends 42 . 44 as well as electrical contacts 46 . 48 on. By applying an appropriate heating voltage to the contacts 46 . 48 The resistance track can control the heating power of the heating element 40 be regulated accordingly.

On a large surface of the sensor element 10 are for example two measuring electrodes 14 . 16 applied, which are preferably designed as interdigitated interdigital electrodes and form a measuring element. The use of interdigital electrodes as measuring electrodes 14 . 16 advantageously allows a particularly accurate determination of the electrical resistance or the electrical conductivity of the between the measuring electrodes 14 . 16 surface material. For contacting the measuring electrodes 14 . 16 are in the region of a gas mixture facing away from the end of the sensor element contacts 18 . 20 in front seen. Here are the supply areas of the electrodes 14 . 16 preferably by the electrically insulating layer 12a against the influences of the sensor element 10 shielded surrounding gas mixture.

On the with the measuring electrodes 14 . 16 provided large area of the sensor element 10 may additionally be provided for reasons of clarity not shown porous cover or protective layer, the measuring electrodes 14 . 16 shielded in their interlocked region against direct contact with the gas mixture to be determined. The layer thickness of the porous protective layer is preferably greater than the layer thickness of the measuring electrodes 14 . 16 , The porous protective layer is preferably made open-porous, wherein the pore size is selected so that the particles to be determined in the gas mixture can diffuse into the pores of the porous protective layer. The pore size of the porous protective layer is preferably in a range of 2 to 10 microns. The porous protective layer is made of a ceramic material, preferably the material of the layer 12a is similar or corresponds to this and can be prepared by screen printing. The porosity of the porous protective layer can be adjusted accordingly by adding pore formers to the screen printing paste.

During operation of the sensor element 10 gets to the measuring electrodes 14 . 16 a voltage applied. Because the measuring electrodes 14 . 16 on the surface of the electrically insulating layer 12b are arranged, there is initially substantially no current flow between the measuring electrodes 14 . 16 ,

Contains one the sensor element 10 flowing gas mixture particles, in particular soot, so this superimposed on the surface of the sensor element 10 from. Since soot has a noticeable electrical conductivity, it comes with sufficient loading of the surface of the sensor element 10 or the porous protective layer with soot to an increasing current flow between the measuring electrodes 14 . 16 which correlates with the extent of loading.

Will now contact the measuring electrodes 14 . 16 a preferably constant DC or AC voltage applied and that between the measuring electrodes 14 . 16 ascertained current flow or the increase of the current flow over time, so can from the quotient of current flow increase and time or from the differential quotient of the current flow to the deposited particle mass or the current particle mass flow, in particular soot mass flow, and the particle concentration be closed in the gas mixture. A calculation of the particle concentration is possible on the basis of the measured values, as long as the flow velocity of the gas mixture is known. This or the volume flow of the gas mixture can be determined, for example, by means of a suitable further sensor.

In addition, the sensor element comprises 10 a temperature measuring element 30 , which is designed in the form of a thermocouple. Its function is based on the so-called Seebeck effect, in which two tracks made of two metals or metallic materials, which have different thermal stresses are connected to each other in a suitable manner. If this connection or soldering point is exposed to a first temperature and the free ends of the two strip conductors are exposed to a second temperature which is different from the first temperature, then a voltage in the form of a thermo-voltage or thermo-power can be measured between the two free ends of the strip conductors ,

A representation of the thermocouple 30 is 2 refer to. Here, the same reference numerals designate the same component components as in FIG 1 ,

To the number of necessary contacts of the sensor element 10 is to keep low, preferably one of the measuring electrodes 14 . 16 as the first trace of the thermocouple 30 used. This is made of a first metal or metallic material, in particular of platinum. Furthermore, the measuring electrode 16 for example, a branch 32 on, leading to a solder joint 34 leads. At the soldering point 34 stands the branch 32 preferably in area contact with a second conductor track 36 of the thermocouple 30 , This is made of a second metal or metallic material, which is unlike the first metal or metallic material. In this case, a noble metal alloy, in particular a platinum alloy such as PtxRh is preferably used as the second metallic material, wherein x represents a number from 6 to 30, preferably from 6 to 20 and in particular from 10 to 18. A significant advantage of the use of these metallic materials is the fact that they can withstand high temperatures of over 1000 ° C permanently. Alternative thermocouples may be based on nickel-chromium alloys in contact with a nickel track, or as copper-constantan thermocouples.

The second track 36 of the thermocouple 30 is preferably in a region remote from the gas to be determined region of the sensor element via a further electrical contact 38 electrically contacted. The further electrical contact 38 is for example made of the material of the second conductor track 36 executed, this also applies to the not shown further electrical connection between the further electrical contact 38 and an evaluation device, not shown, by means of which one between the first and the second conductor track 32 . 36 determined thermoelectric voltage is determined and assigned by means of a map of a temperature of the gas mixture. A cheaper solution is to contact 18 and / or the electrical connection of the contact 18 to perform with the evaluation of a metallic material having a comparable Seebeck coefficient as the material of the branch 32 , but lower material costs. The same applies to the further electrical contact 38 or the electrical connection of the further electrical contact 38 with the evaluation unit

The determination of the solder joint 34 prevailing temperature takes place by means of the evaluation unit between the two free ends of the first and second conductor track 32 . 26 measured potential difference is correlated with a corresponding measurement temperature. For this purpose, for example, a correlation of possible measurement temperatures with expected voltage values based on a thermocouple with a defined design is stored in the evaluation unit. An example of such a correlation is in 5 displayed. There are for thermocouples, which are designed as Pt13Rh / Pt thermocouples, which at certain measurement temperatures in the range of 0-ca. 1700 ° C expected potential differences in millivolts listed. The potential difference attributable to a specific temperature is obtained by adding the temperature in ° C referred to as the column heading of the considered potential difference to the temperature in ° C mentioned as the row heading of the considered potential difference. It can be seen that Pt13Rh / Pt thermocouples show characteristic voltage values for the respective measuring temperatures over the entire temperature measuring range.

there the thermoelectric voltage measured at the thermocouple becomes arbitrary for a measuring temperature from 0 ° C set equal to 0 mV. To avoid the measured thermoelectric voltage depends on the ambient temperature is, which prevails in the area of the evaluation is by means of a Compensation in the form of a so-called cold-junction compensation of the influence of Ambient temperature calculated from the measured thermoelectric voltage eliminated. The cold junction compensation is preferred integrated into the evaluation unit.

In 3 a sensor element according to a second embodiment is shown. In this case, the same reference numerals designate the same component components as in the 1 and 2 ,

The sensor element shown in an exploded view according to the second embodiment is another possibility, such as a thermocouple as a temperature measuring element 30 can be integrated into the sensor element. Here is the temperature measuring element 30 at the same time as a heating element 40 designed. This is indicated by the thermocouple 30 a first and a second conductor track 36 . 37 on, with the tracks 36 . 37 preferably from those already at 1 for the second trace 36 or the branch 32 described materials are executed. The contacting of the first and second conductor track via the vias 42 . 44 or the contacts 46 . 48 ,

If a heating of the sensor element is required, then the temperature measuring element 30 temporarily switched as a heating element. This is to the contacts 46 . 48 in this period a corresponding heating voltage applied.

Because the second trace 36 is preferably made of a platinum-rhodium alloy, this has due to the so-called alloy effect on a higher resistivity than a uniformly sized conductor made of platinum. In order to prevent unilateral heating of the sensor element during heating, therefore, the second conductor 36 , which is made of a platinum-rhodium alloy, provided with a comparatively larger cross-section than the first conductor track 37 made of platinum, so that both tracks 36 . 37 have a comparable electrical resistance.

Alternatively, one of the tracks 36 . 37 by means of a suitable via instead of the contacts 46 . 48 also with one of the contacts 18 . 20 the measuring electrodes 14 . 16 be connected. In this way, the number of required electrical contacts of the sensor element is reduced to three contacts. The contacting of the thermocouple is preferably carried out in such a way that when using the thermocouple as a heating element 40 in addition to the Joule heating due to the electrical resistance of the tracks 36 . 37 Warming by the Peltier effect results.

Another embodiment of a sensor element is shown in FIG 4 shown. Here, the same reference numerals designate the same component components as in the 1 to 3 ,

At the in 4 illustrated embodiment is as in the 3 mapped sensor element, the temperature measuring element 30 additionally designed as a heating element. To the latter To be able to cope with the function particularly well are preferably both conductor tracks 36 . 37 formed in the region of a desirably good heating of the sensor element meandering.

Furthermore, the in 4 shown sensor element on a second measuring element, which the further measuring electrodes 14 ' . 16 ' and preferably on a large surface of the sensor element opposite the first measuring element, for example on the ceramic layer 12e is provided. The second measuring element preferably has a further electrical contact 50 on. The use of two independently designed measuring elements increases the measuring accuracy of the sensor element and its measurement results are largely independent of flow conditions in a gas mixture surrounding the sensor element.

The present invention is not applicable to those in 1 to 5 illustrated embodiments of a sensor element limited, but it can be made numerous modifications of this sensor element. Thus, it is possible, for example, to provide additional ceramic layers in the sensor element or to simplify the multi-layer structure of the sensor element in relation to the application, as well as to provide further measuring electrodes. The use of several heating and temperature measuring elements is possible.

The Application of the described sensor element is not on the determination of soot particles in exhaust gases of internal combustion engines, but it can be general for determining the concentration of particles containing the electric conductivity a ceramic substrate when stored change, for example, in chemical Manufacturing processes or exhaust aftertreatment systems used become.

Claims (13)

Sensor element for gas sensors, in particular for the determination of particles in gas mixtures, with at least one measuring element exposed to the gas to be determined and at least one temperature measuring element integrated in the sensor element (US Pat. 30 ), characterized in that the temperature measuring element ( 30 ) is designed as a thermocouple. Sensor element according to claim 1, characterized in that the thermocouple of a first and a second interconnected interconnect ( 32 . 36 . 37 ), wherein the first conductor as a metallic component contains a platinum-rhodium alloy or a nickel-chromium alloy. Sensor element according to claim 2, characterized that the platinum-rhodium alloy has a rhodium content of 6 to 30 wt.%. Sensor element according to claim 2 or 3, characterized that the platinum-rhodium alloy the composition has Pt10Rh or Pt13Rh. Sensor element according to one of the preceding claims, characterized characterized in that the second conductor track as a metallic component Contains platinum. Sensor element according to one of the preceding claims, characterized in that the measuring element has a first and a second measuring electrode ( 14 . 16 ), wherein one of the measuring electrodes ( 14 . 16 ) is in electrical contact with the thermocouple. Sensor element according to one of claims 1 to 5, characterized in that an electrical heating element ( 40 ) is provided, wherein the thermocouple with one of the electrical connections of the heating element ( 40 ) is in electrical contact. Sensor element according to one of the preceding claims, characterized in that two measuring elements ( 14 . 16 ) are provided on opposite outer surfaces of the sensor element. Sensor element according to one of the preceding claims, characterized in that an evaluation device is provided, which is a change of between the measuring electrodes ( 14 . 16 ) determines adjacent current flow and / or resistance and outputs this as a measure of the particle concentration. Method for determining particles in gas mixtures, in particular soot in exhaust gases of internal combustion engines, by means of a sensor element according to one of the preceding claims, wherein at least two measuring electrodes ( 14 . 16 ) a voltage is applied and between the measuring electrodes ( 14 . 16 ) adjusting current flow or electrical resistance is determined and output as a measure of the particle concentration or the particle mass flow. Method according to claim 10, characterized in that between the measuring electrodes ( 14 . 16 ) adjusting current flow or electrical resistance with a via a thermocouple ( 30 ) is correlated to a specific temperature and by evaluating a characteristic diagram in which the temperature dependence of the temperature between the measuring electrodes ( 14 . 16 ) adjusting current flow or elektri rule resistance is deposited as a function of the particle loading, a measure of the particle concentration or the particle mass flow is output. Gas sensor for the determination of particles in gas mixtures, especially soot sensor, with a sensor element according to one of claims 1 to 9, characterized that an evaluation unit is included, in which a correlation between a determined by the sensor element thermoelectric voltage and a measuring temperature to be determined prevailing on the sensor element is integrated. Use of a sensor element according to one of claims 1 to 9, a method according to any one of claims 10 or 11 or a sensor according to Claim 12 for monitoring the operation of a diesel engine or to monitor the functionality and / or the loading state of a particulate filter.