EP1342078A2 - Sensor element of a gas sensor - Google Patents

Sensor element of a gas sensor

Info

Publication number
EP1342078A2
EP1342078A2 EP01997695A EP01997695A EP1342078A2 EP 1342078 A2 EP1342078 A2 EP 1342078A2 EP 01997695 A EP01997695 A EP 01997695A EP 01997695 A EP01997695 A EP 01997695A EP 1342078 A2 EP1342078 A2 EP 1342078A2
Authority
EP
European Patent Office
Prior art keywords
sensor element
element according
electrodes
layer
electrode
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP01997695A
Other languages
German (de)
French (fr)
Inventor
Berndt Cramer
Carsten Springhorn
Detlef Heimann
Gudrun Oehler
Margret Schuele
Bernd Schumann
Thorsten Ochs
Sabine Thiemann-Handler
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Robert Bosch GmbH
Original Assignee
Robert Bosch GmbH
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Robert Bosch GmbH filed Critical Robert Bosch GmbH
Publication of EP1342078A2 publication Critical patent/EP1342078A2/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/416—Systems
    • G01N27/417—Systems using cells, i.e. more than one cell and probes with solid electrolytes
    • G01N27/419—Measuring voltages or currents with a combination of oxygen pumping cells and oxygen concentration cells

Definitions

  • the invention relates to a sensor element of a gas sensor for determining the concentration of a component of a gas mixture according to the preamble of claim 1 and its use.
  • electrodes of very different compositions are provided in gas sensors based on solid electrolytes.
  • catalytically active electrodes are made from platinum or platinum / rhodium alloys
  • catalytically inactive electrodes are made from gold or gold alloys.
  • These electrodes are located, among other things, in the inner gas spaces of the sensor, and electrodes of different compositions can be located in one and the same gas space, depending on the design of the sensor. Since the manufacture of such sensors involves at least one sintering process at high temperatures, the electrodes may become contaminated with metal components of the other electrode.
  • a sensor for determining the NO x content of gas mixtures is known from EP 678 740 A1, in which electrodes which serve to control the oxygen content within the sensor consist of a gold / platinum alloy and electrodes for the decomposition of nitrogen oxides from rhodium. Both types of electrodes are optionally spatially separated from each other by a diffusion barrier.
  • the object of the present invention is to provide a gas sensor for determining the concentration of a constituent of a gas mixture, in which mutual contamination of electrodes is prevented without the access of measurement gas to the electrodes being made considerably more difficult.
  • the sensor element according to the invention with the characterizing features of claim 1 has the advantage that the sensor element has a means for avoiding the metal diffusion between the electrodes of the sensor element and thus prevents the mutual contamination of electrodes of different composition at higher temperatures. This enables the provision of a sensor with a high sensitivity to the gas to be determined. The access of the measuring gas to the electrodes is not made significantly more difficult.
  • a further advantageous embodiment has an extended diffusion path between electrodes of different types, the electrodes in question preferably being arranged in different layer planes of the sensor.
  • a layer of a metal vapor-absorbing material as a means of preventing the spreading of metal vapors, since in this way the metal vapors are not only prevented from accessing the electrodes, but rather are removed from the gas space.
  • FIG. 1 shows a cross section through a section on the measuring gas side of a sensor element according to the invention in accordance with a first exemplary embodiment
  • FIGS. 2 to 4 show cross sections through sections of sensor elements on the measuring gas side in accordance with three further exemplary embodiments.
  • FIG. 1 shows a basic structure of a first embodiment of the present invention.
  • 10 designates a planar sensor element of an electrochemical gas sensor, which is used, for example, to determine oxygen-containing gases, in particular the nitrogen oxide content of exhaust gases. It has a plurality of oxygen-ion-conducting solid electrolyte layers 11a, 11b, 11c, lld and lle, which are designed, for example, as ceramic foils and form a planar ceramic body. They consist of a solid electrolyte material which conducts oxygen ions, such as Zr0 2 stabilized or partially stabilized with Y 2 0 3 .
  • the integrated shape of the planar ceramic body of the sensor element 10 is produced by laminating together the ceramic foils printed with functional layers and then sintering the laminated structure in a manner known per se.
  • the sensor element 10 has a first inner gas space 13 which is in contact with the gas mixture to be determined via an opening 12.
  • the opening 12 is made in the solid electrolyte layer 11a perpendicular to the surface of the sensor element 10.
  • a second inner gas space 15 is provided, which is preferably connected to the first inner gas space 13 in a gas-permeable manner via a diffusion barrier 32, and a reference gas channel 19.
  • the reference gas channel 19 is through a gas inlet 17 which protrudes from the planar body at one end of the sensor element 10 leads out, in contact with a reference gas atmosphere.
  • a first and a second inner electrode 20, 21 are arranged in the first inner gas space 13.
  • An outer electrode 25 is located on the outer side of the solid electrolyte layer 1 directly facing the measurement gas, which can be covered with a porous protective layer (not shown).
  • the inner electrodes 20, 21, 22 together with the outer electrode 25 form electrochemical pump cells.
  • a respective constant oxygen partial pressure in the inner gas spaces 13, 15 of the sensor element 10 is set by means of the pump cells.
  • At least one of the inner electrodes 20, 21, 22 is additionally provided with a reference electrode 26, which is arranged in the reference gas channel 19. net is interconnected to so-called Nernst or concentration cells. These enable a direct comparison of the oxygen potential of the inner electrodes 20, 21, 22, which is dependent on the oxygen concentration in the inner gas spaces 13, 15, with the constant oxygen potential of the reference electrode 26 in the form of a measurable electrical voltage.
  • the level of the pump voltages to be applied to the pump cells is selected so that a constant voltage is established at the corresponding concentration cells.
  • a further inner electrode 23 which, together with the outer electrode 25 or the reference gas electrode 26, forms a further pump cell.
  • This pump cell serves to detect the gas to be determined, the gas to be determined decomposing on the surface of the inner electrode 23 and the oxygen released being pumped out.
  • the pump current between the electrodes 23, 25 and 23, 26 is used as a measure of the concentration of the gas to be determined.
  • the electrodes 20, 21, 22 are made from a catalytically inactive material. This can be gold or a gold / platinum alloy, for example.
  • the electrode 23 is catalytically active and consists, for example, of rhodium or a platinum / rhodium alloy.
  • the outer electrode 25 and the reference electrode 26 likewise consist of a catalytically active material such as platinum.
  • the electrode material for all electrodes is in a manner known per se as
  • a resistance heater 40 is also embedded in the ceramic base body of the sensor element 10 between two electrical insulation layers (not shown here). The resistance heater 40 is used to heat the sensor element 10 to the necessary operating temperature of, for example, 750 ° C.
  • a porous diffusion barrier 30 is arranged in front of the inner electrodes 20, 21 within the inner gas space 13 in the diffusion direction of the gas mixture.
  • the porous diffusion barrier 30 forms a diffusion resistance with respect to the gas diffusing to the inner electrodes 20, 21.
  • the inner gas spaces 13, 15 are separated from one another, for example, by the further porous diffusion barrier 32; this enables the setting of different oxygen concentrations in the inner gas spaces 13, 15.
  • a diffusion barrier 34 is arranged over one or both electrodes 22, 23.
  • This is made of a porous ceramic material and can also contain elemental platinum as a substance that traps metal vapors from the gas phase.
  • the diffusion barrier 34 is designed in its layer thickness and porosity so that the access of the gas to be determined to the surface of the electrode 23 is not significantly restricted.
  • FIG. 2 shows a second exemplary embodiment of the present invention, the reference symbols used in FIG. 2 denoting the same components as in FIG. 1.
  • a layer 36 of a metal vapor-absorbing material is provided in the inner gas space 15, which layer is applied to one of those surfaces that delimit the inner gas space 15.
  • the layer 36 can be made porous and / or as a mesh and contains, for example, platinum as a metal vapor-absorbing component.
  • the layer 36 can also be arranged over one of the electrodes 22, 23, a porous intermediate layer being provided between the layer 36 and the electrode 22, 23 to avoid direct contact of the layer 36 with the surface of the electrode 22, 23.
  • An arrangement of the layer 36 on the diffusion barrier 32 or on a diffusion barrier 34 according to the first exemplary embodiment is also possible.
  • the effect of platinum as a metal vapor-absorbing material is primarily based on the fact that it is able to form stable alloys or at least intercalation compounds with both rhodium and gold, depending on the concentration range.
  • FIG. 3 shows a third exemplary embodiment of the present invention, the reference symbols used in FIG. 3 denoting the same components as in FIG. 1.
  • the diffusion path 39 between the electrodes 22, 23 is extended, so that diffusion of metal vapors between the electrodes 22, 23 is made more difficult.
  • the arrangement of the electrode 23 in a separate layer plane 11d of the sensor element is particularly advantageous since this leads to a significant lengthening of the diffusion path 39 between the two Electrodes 22, 23 lead without increasing the length of the sensor element.
  • This arrangement further enables the arrangement of two electrodes 22 and 23 within the inner gas space 15 and the use of an additional diffusion barrier 38 between the electrodes 22, 23.
  • the diffusion path 39 is particularly effective when it has at least one up to several times the chamber height of the inner gas space 15.
  • FIG. 4 shows a fourth exemplary embodiment of the present invention, the reference symbols used in FIG. 4 denoting the same components as in FIGS. 1 to 3.
  • the additional diffusion barrier 38 be equipped with a metal vapor-absorbing, preferably metallic component such as platinum.
  • the porosity of the diffusion barrier 38 is preferably selected so that it does not oppose the gas mixture diffusing to the electrode 23 with any significant diffusion resistance.
  • the porosity and the concentration of the metal vapor-absorbing component can be varied in the flow direction of the diffusing gas mixture within the diffusion barrier 38.
  • a combination of the measures on which the fourth or third exemplary embodiment is based with the measures of the first and second exemplary embodiments are entirely the subject of the invention and lead to particularly effective avoidance of mutual contamination of the electrodes during the production process.
  • bar which contain electrodes of different material compositions and require protection against mutual contamination of the electrodes. This applies, for example, to sensors with mixed potential electrodes for determining gaseous hydrocarbons or hydrogen.

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  • Chemical & Material Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Health & Medical Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Molecular Biology (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Measuring Oxygen Concentration In Cells (AREA)

Abstract

The invention relates to a sensor element of a gas sensor, said sensor element being used to determine the concentration of at least one component of a gas mixture, in particular in the exhaust gases of internal combustion engines. The element contains at least two electrodes (22, 23), which are arranged in an internal gas chamber (15) that is in direct contact with the gas mixture, one electrode (22) consisting of a first material and the second electrode (23) consisting of a second material. The inner gas chamber (15) contains a physically and/or chemically active agent (34, 36, 38, 39) for preventing a metallic diffusion between the two electrodes (22, 23).

Description

Sensorele ent eines GassensorsSensor element of a gas sensor
Die Erfindung betrifft ein Sensorelement eines Gassensors zur Bestimmung der Konzentration eines Bestandteils eines Gasge- mischs nach der Gattung des Anspruchs 1 und dessen Verwendung.The invention relates to a sensor element of a gas sensor for determining the concentration of a component of a gas mixture according to the preamble of claim 1 and its use.
Stand der TechnikState of the art
In Gassensoren auf Festelektrolytbasis sind je nach Anwendungs- fall Elektroden ganz unterschiedlicher Zusammensetzung vorgesehen. So werden katalytisch aktive Elektroden beispielsweise aus Platin oder Platin/Rhodiumlegierungen hergestellt, katalytisch inaktive Elektroden dagegen aus Gold oder Goldlegierungen. Diese Elektroden befinden sich unter anderem in inneren Gasräumen des Sensors, wobei sich je nach Bauart des Sensors in ein und demselben Gasraum Elektroden unterschiedlicher Zusammensetzung befinden können. Da die Herstellung derartiger Sensoren mindestens einen Sinterprozeß bei hohen Temperaturen beinhaltet, kann es dabei zu einer Kontaminierung der Elektroden mit Metallbestand- teilen der jeweils anderen Elektrode kommen. Dies führt beispielsweise zu einer Inaktivierung von katalytisch aktiven Elektroden durch Goldeinlagerungen oder umgekehrt zu einer erhöhten katalytischen Aktivität von Goldelektroden durch Kontaminierung mit Rhodium oder gegbenenfalls mit Platin. Beide Effekte beein- trächtigen die sensitiven Eigenschaften des Sensors. Aus der EP 678 740 AI ist ein Sensor zur Bestimmung des NOx- Gehaltes von Gasgemischen bekannt, bei dem Elektroden, die der Kontrolle des Sauerstoffgehalts innerhalb des Sensors dienen, aus einer Gold/Platinlegierung bestehen und Elektroden zur Zersetzung von Stickoxiden aus Rhodium. Beide Elektrodentypen sind wahlweise durch eine Diffusionsbarriere räumlich voneinander getrennt .Depending on the application, electrodes of very different compositions are provided in gas sensors based on solid electrolytes. For example, catalytically active electrodes are made from platinum or platinum / rhodium alloys, while catalytically inactive electrodes are made from gold or gold alloys. These electrodes are located, among other things, in the inner gas spaces of the sensor, and electrodes of different compositions can be located in one and the same gas space, depending on the design of the sensor. Since the manufacture of such sensors involves at least one sintering process at high temperatures, the electrodes may become contaminated with metal components of the other electrode. This leads, for example, to inactivation of catalytically active electrodes by means of gold deposits or, conversely, to an increased catalytic activity of gold electrodes by contamination with rhodium or possibly with platinum. Both effects affect the sensitive properties of the sensor. A sensor for determining the NO x content of gas mixtures is known from EP 678 740 A1, in which electrodes which serve to control the oxygen content within the sensor consist of a gold / platinum alloy and electrodes for the decomposition of nitrogen oxides from rhodium. Both types of electrodes are optionally spatially separated from each other by a diffusion barrier.
Aufgabe der vorliegenden Erfindung ist es, einen Gassensor zur Bestimmung der Konzentration eines Bestandteils eines Gasgemischs bereitzustellen, bei dem eine gegenseitige Kontaminierung von Elektroden verhindert wird, ohne daß der Zutritt von Meßgas zu den Elektroden wesentlich erschwert ist.The object of the present invention is to provide a gas sensor for determining the concentration of a constituent of a gas mixture, in which mutual contamination of electrodes is prevented without the access of measurement gas to the electrodes being made considerably more difficult.
Vorteile der ErfindungAdvantages of the invention
Das erfindungsgemäße Sensorelement mit den kennzeichnenden Merk- malen des Anspruchs 1 hat den Vorteil, daß das Sensorelement ein Mittel zur Vermeidung der Metalldiffusion zwischen den Elektroden des Sensorelements aufweist und somit die gegenseitige Kontaminierung von Elektroden unterschiedlicher Zusammensetzung bei höheren Temperaturen verhindert. Dies ermöglicht die Bereitstel- lung eines Sensors mit hoher Sensitivität gegenüber dem zu bestimmenden Gas . Der Zutritt des Meßgases zu den Elektroden wird dabei nicht wesentlich erschwert.The sensor element according to the invention with the characterizing features of claim 1 has the advantage that the sensor element has a means for avoiding the metal diffusion between the electrodes of the sensor element and thus prevents the mutual contamination of electrodes of different composition at higher temperatures. This enables the provision of a sensor with a high sensitivity to the gas to be determined. The access of the measuring gas to the electrodes is not made significantly more difficult.
Durch die in den Unteransprüchen aufgeführten Maßnahmen sind vorteilhafte Weiterbildungen und Verbesserungen des im Hauptanspruch angegebenen Sensorelements möglich. So ermöglicht beispielsweise die Verwendung einer auf der Oberfläche mindestens einer der Elektroden aufgebrachten Diffusionsbarriere eine effektive Vermeidung einer Kontaminierung der Elektroden. Eine weitere vorteilhafte Ausführungsform weist zwischen Elektroden unterschiedlicher Bauart eine verlängerte Diffusionsstrecke auf, wobei die betreffenden Elektroden bevorzugterweise in verschiedenen Schichtebenen des Sensors angeordnet sind.Advantageous further developments and improvements of the sensor element specified in the main claim are possible through the measures listed in the subclaims. For example, the use of a diffusion barrier applied to the surface of at least one of the electrodes effectively prevents contamination of the electrodes. A further advantageous embodiment has an extended diffusion path between electrodes of different types, the electrodes in question preferably being arranged in different layer planes of the sensor.
Besonders vorteilhaft ist die Verwendung einer Schicht eines me- talldampfabsorbierenden Materials als Mittel zur Vermeidung der Ausbreitung von Metalldämpfen, da auf diesem Wege die Metalldämpfe nicht nur am Zutritt zu den Elektroden gehindert, sondern vielmehr aus dem Gasraum entfernt werden.It is particularly advantageous to use a layer of a metal vapor-absorbing material as a means of preventing the spreading of metal vapors, since in this way the metal vapors are not only prevented from accessing the electrodes, but rather are removed from the gas space.
Zeichnungdrawing
Vier Ausführungsbeispiele der Erfindung sind in der Zeichnung dargestellt und in der nachfolgenden Beschreibung näher erläutert. Es zeigen Figur 1 einen Querschnitt durch einen meß- gasseitigen Abschnitt eines erfindungsgemäßen Sensorelements gemäß einem ersten Ausführungsbeispiel und die Figuren 2 bis 4 Querschnitte durch jeweils meßgasseitige Abschnitte von Sensorelementen gemäß dreier weiterer Ausführungsbeispiele.Four embodiments of the invention are shown in the drawing and explained in more detail in the following description. FIG. 1 shows a cross section through a section on the measuring gas side of a sensor element according to the invention in accordance with a first exemplary embodiment, and FIGS. 2 to 4 show cross sections through sections of sensor elements on the measuring gas side in accordance with three further exemplary embodiments.
Ausführungsbeispieleembodiments
In Figur 1 ist ein prinzipieller Aufbau einer ersten Ausführungsform der vorliegenden Erfindung dargestellt. Mit 10 ist ein planares Sensorelement eines elektrochemischen Gassensors bezeichnet, das beispielsweise der Bestimmung sauerstoffhaltiger Gase, insbesondere des Stickoxidgehaltes von Abgasen dient. Es weist eine Mehrzahl von sauerstoffionenleitenden Festelektrolytschichten 11a, 11b, 11c, lld und lle auf, die beispielsweise als keramische Folien ausgeführt sind und einen planaren keramischen Körper bilden. Sie bestehen aus einem sauerstoffionenleitenden Festelektrolytmaterial, wie beispielsweise mit Y203 stabilisiertem oder teilstabilisiertem Zr02. Die integrierte Form des planaren keramischen Körpers des Sensorelements 10 wird durch Zusammenlaminieren der mit Funktionsschichten bedruckten keramischen Folien und anschließendem Sin- tern der laminierten Struktur in an sich bekannter Weise hergestellt.1 shows a basic structure of a first embodiment of the present invention. 10 designates a planar sensor element of an electrochemical gas sensor, which is used, for example, to determine oxygen-containing gases, in particular the nitrogen oxide content of exhaust gases. It has a plurality of oxygen-ion-conducting solid electrolyte layers 11a, 11b, 11c, lld and lle, which are designed, for example, as ceramic foils and form a planar ceramic body. They consist of a solid electrolyte material which conducts oxygen ions, such as Zr0 2 stabilized or partially stabilized with Y 2 0 3 . The integrated shape of the planar ceramic body of the sensor element 10 is produced by laminating together the ceramic foils printed with functional layers and then sintering the laminated structure in a manner known per se.
Das Sensorelement 10 weist einen ersten inneren Gasraum 13 auf, der über eine Öffnung 12 im Kontakt mit dem zu bestimmenden Gas- gemisch steht. Die Öffnung 12 ist in der Festelektrolytschicht 11a senkrecht zur Oberfläche des Sensorelements 10 angebracht. Darüber hinaus ist ein zweiter innerer Gasraum 15 vorgesehen, der vorzugsweise über eine Diffusionsbarriere 32 mit dem ersten inneren Gasraum 13 gasdurchlässig verbunden ist, und ein Refe- renzgaskanal 19. Der Referenzgaskanal 19 steht durch einen Gaseinlaß 17, der an einem Ende aus dem planaren Körper des Sensorelements 10 herausführt, in Kontakt mit einer Referenzgasatmosphäre.The sensor element 10 has a first inner gas space 13 which is in contact with the gas mixture to be determined via an opening 12. The opening 12 is made in the solid electrolyte layer 11a perpendicular to the surface of the sensor element 10. In addition, a second inner gas space 15 is provided, which is preferably connected to the first inner gas space 13 in a gas-permeable manner via a diffusion barrier 32, and a reference gas channel 19. The reference gas channel 19 is through a gas inlet 17 which protrudes from the planar body at one end of the sensor element 10 leads out, in contact with a reference gas atmosphere.
Im ersten inneren Gasraum 13 ist eine erste und eine zweite innere Elektrode 20, 21 angeordnet. Im zweiten inneren Gasraum 15 befindet sich eine weitere innere Elektrode 22. An der äußeren, dem Meßgas unmittelbar zugewandten Seite der Festelektrolytschicht lle befindet sich eine äußere Elektrode 25, die mit ei- ner nicht dargestellten porösen Schutzschicht bedeckt sein kann.A first and a second inner electrode 20, 21 are arranged in the first inner gas space 13. There is a further inner electrode 22 in the second inner gas space 15. An outer electrode 25 is located on the outer side of the solid electrolyte layer 1 directly facing the measurement gas, which can be covered with a porous protective layer (not shown).
Die inneren Elektroden 20, 21, 22 bilden mit der äußeren Elektrode 25 zusammen elektrochemische Pumpzellen. Mittels der Pumpzellen wird ein jeweils konstanter Sauerstoffpartialdruck in den inneren Gasräumen 13, 15 des Sensorelements 10 eingestellt.The inner electrodes 20, 21, 22 together with the outer electrode 25 form electrochemical pump cells. A respective constant oxygen partial pressure in the inner gas spaces 13, 15 of the sensor element 10 is set by means of the pump cells.
Zur Kontrolle des eingestellten Sauerstoffpartialdrucks sind mindestens eine der inneren Elektroden 20, 21, 22 zusätzlich mit einer Referenzelektrode 26, die im Referenzgaskanal 19 angeord- net ist, zu sogenannten Nernst- bzw. Konzentrationszellen zusammengeschaltet. Diese ermöglichen einen direkten Vergleich des von der Sauerstoffkonzentration in den inneren Gasräumen 13, 15 abhängigen Sauerstoffpotentials der inneren Elektroden 20, 21, 22 mit dem konstanten Sauerstoffpotential der Referenzelektrode 26 in Form einer meßbaren elektrischen Spannung. Die Höhe der an den Pumpzellen anzulegenden Pumpspannungen wird so gewählt, daß sich an den entsprechenden Konzentrationszellen eine konstante Spannung einstellt.To check the set oxygen partial pressure, at least one of the inner electrodes 20, 21, 22 is additionally provided with a reference electrode 26, which is arranged in the reference gas channel 19. net is interconnected to so-called Nernst or concentration cells. These enable a direct comparison of the oxygen potential of the inner electrodes 20, 21, 22, which is dependent on the oxygen concentration in the inner gas spaces 13, 15, with the constant oxygen potential of the reference electrode 26 in the form of a measurable electrical voltage. The level of the pump voltages to be applied to the pump cells is selected so that a constant voltage is established at the corresponding concentration cells.
Im inneren Gasraum 15 befindet sich eine weitere innere Elektrode 23, die zusammen mit der äußeren Elektrode 25 oder der Referenzgaselektrode 26 eine weitere Pumpzelle bildet. Diese Pumpzelle dient dem Nachweis des zu bestimmenden Gases, wobei das zu bestimmende Gas an der Oberfläche der inneren Elektrode 23 zersetzt und der freiwerdende Sauerstoff abgepumpt wird. Als Maß für die Konzentration des zu bestimmenden Gases wird der Pumpstrom zwischen den Elektroden 23, 25 bzw. 23, 26 herangezogen.In the inner gas space 15 there is a further inner electrode 23 which, together with the outer electrode 25 or the reference gas electrode 26, forms a further pump cell. This pump cell serves to detect the gas to be determined, the gas to be determined decomposing on the surface of the inner electrode 23 and the oxygen released being pumped out. The pump current between the electrodes 23, 25 and 23, 26 is used as a measure of the concentration of the gas to be determined.
Um zu gewährleisten, daß an den Elektroden 20, 21, 22 keine Zersetzung des zu bestimmenden Gases auftritt, werden die Elektroden 20, 21, 22 aus einem katalytisch inaktiven Material gefertigt. Dies kann beispielsweise Gold oder eine Gold/Platin- Legierung sein. Die Elektrode 23 ist dagegen katalytisch aktiv ausgeführt und besteht beispielsweise aus Rhodium oder einer Platin/Rhodium-Legierung. Die äußere Elektrode 25 sowie die Referenzelektrode 26 bestehen ebenfalls aus einem katalytisch aktiven Material wie beispielsweise Platin. Das Elektrodenmaterial für alle Elektroden wird dabei in an sich bekannter Weise alsIn order to ensure that no decomposition of the gas to be determined occurs at the electrodes 20, 21, 22, the electrodes 20, 21, 22 are made from a catalytically inactive material. This can be gold or a gold / platinum alloy, for example. In contrast, the electrode 23 is catalytically active and consists, for example, of rhodium or a platinum / rhodium alloy. The outer electrode 25 and the reference electrode 26 likewise consist of a catalytically active material such as platinum. The electrode material for all electrodes is in a manner known per se as
Cermet eingesetzt, um mit den keramischen Folien zu versintern.Cermet used to sinter with the ceramic foils.
In den keramischen Grundkörper des Sensorelements 10 ist ferner zwischen zwei hier nicht dargestellten elektrischen Isolationsschichten ein Widerstandsheizer 40 eingebettet. Der Widerstandsheizer 40 dient dem Aufheizen des Sensorelements 10 auf die notwendige Betriebstemperatur von beispielsweise 750°C.A resistance heater 40 is also embedded in the ceramic base body of the sensor element 10 between two electrical insulation layers (not shown here). The resistance heater 40 is used to heat the sensor element 10 to the necessary operating temperature of, for example, 750 ° C.
Innerhalb des inneren Gasraums 13 ist in Diffusionsrichtung des Gasgemischs den inneren Elektroden 20, 21 eine poröse Diffusionsbarriere 30 vorgelagert. Die poröse Diffusionsbarriere 30 bildet einen Diffusionswiderstand bezüglich des zu den inneren Elektroden 20, 21 diffundierenden Gases aus. Die inneren Gasräu- me 13, 15 sind beispielsweise durch die weitere poröse Diffusionbarriere 32 voneinander getrennt; dies ermöglicht die Einstellung von unterschiedlichen Sauerstoffkonzentrationen in den inneren Gasräumen 13 , 15.A porous diffusion barrier 30 is arranged in front of the inner electrodes 20, 21 within the inner gas space 13 in the diffusion direction of the gas mixture. The porous diffusion barrier 30 forms a diffusion resistance with respect to the gas diffusing to the inner electrodes 20, 21. The inner gas spaces 13, 15 are separated from one another, for example, by the further porous diffusion barrier 32; this enables the setting of different oxygen concentrations in the inner gas spaces 13, 15.
Während der Herstellung des Sensorelements ist mindestens einThere is at least one during the manufacture of the sensor element
Sinterprozeß bei hohen Temperaturen vorgesehen. Da sich im inneren Gasraum 15 die inneren Elektroden 22, 23 mit unterschiedlichen Materialzusammensetzungen befinden, kommt es während des Sinterprozesses zu einer Kontaminierung der Elektroden 22, 23 mit Metallbestandteilen der jeweils anderen Elektrode. Dies führt beispielsweise zu einer partiellen Inaktivierung der katalytisch aktiven Elektroden 23 durch Goldeinlagerungen oder umgekehrt zu einer erhöhten katalytischen Aktivität der inneren Elektrode 22 durch Kontaminierung mit Rhodium. Beide Effekte be- einträchtigen deutlich die sensitiven Eigenschaften des Sensors.Sintering process provided at high temperatures. Since the inner electrodes 22, 23 with different material compositions are located in the inner gas space 15, the electrodes 22, 23 become contaminated with metal components of the other electrode during the sintering process. This leads, for example, to a partial inactivation of the catalytically active electrodes 23 by gold deposits or, conversely, to an increased catalytic activity of the inner electrode 22 by contamination with rhodium. Both effects significantly impair the sensitive properties of the sensor.
Um dies zu verhindern, ist gemäß einem ersten Ausführungsbei- spiel der vorliegenden Erfindung eine Diffusionsbarriere 34 über einer oder beiden Elektroden 22, 23 angeordnet. Diese ist aus einem porösen keramischen Material gefertigt und kann zusätzlich noch elementares Platin als eine Substanz enthalten, das Metalldämpfe aus der Gasphase abfängt. Die Diffusionsbarriere 34 ist in ihrer Schichtdicke und Porosität so ausgeführt, daß der Zutritt des zu bestimmenden Gases zur Oberfläche der Elektrode 23 nicht in nennenswertem Umfang eingeschränkt wird. Figur 2 zeigt ein zweites Ausführungsbeispiel der vorliegenden Erfindung, wobei die in Figur 2 verwendeten Bezugszeichen die gleichen Komponenten bezeichnen wie in Figur 1 .To prevent this, according to a first embodiment of the present invention, a diffusion barrier 34 is arranged over one or both electrodes 22, 23. This is made of a porous ceramic material and can also contain elemental platinum as a substance that traps metal vapors from the gas phase. The diffusion barrier 34 is designed in its layer thickness and porosity so that the access of the gas to be determined to the surface of the electrode 23 is not significantly restricted. FIG. 2 shows a second exemplary embodiment of the present invention, the reference symbols used in FIG. 2 denoting the same components as in FIG. 1.
Um eine gegenseitige Kontaminierung der Elektroden während des Herstellungsprozesses zu verhindern, ist im inneren Gasraum 15 eine Schicht 36 aus einem metalldampfabsorbierenden Material vorgesehen, die auf eine derjenigen Flächen aufgebracht ist, die den inneren Gasraum 15 begrenzen. Die Schicht 36 kann porös und/oder als Netz ausgeführt sein und beinhaltet als metalldampfabsorbierende Komponente beispielsweise Platin. Die Schicht 36 kann auch über einer der Elektroden 22, 23 angeordnet sein, wobei zur Vermeidung eines direkten Kontaktes der Schicht 36 mit der Oberfläche der Elektrode 22, 23 eine poröse Zwischenschicht zwischen der Schicht 36 und der Elektrode 22, 23 vorgesehen ist. Eine Anordnung der Schicht 36 auf der Diffusionsbarriere 32 oder auf einer Diffusionsbarriere 34 gemäß dem ersten Ausführungsbei- spiel ist ebenfalls möglich. Die Wirkung von Platin als metall- dampfabsorbierendem Material beruht vor allem darauf, daß es in der Lage ist, sowohl mit Rhodium als auch mit Gold je nach Konzentrationsbereich stabile Legierungen oder zumindest Einlagerungsverbindungen zu bilden.In order to prevent mutual contamination of the electrodes during the manufacturing process, a layer 36 of a metal vapor-absorbing material is provided in the inner gas space 15, which layer is applied to one of those surfaces that delimit the inner gas space 15. The layer 36 can be made porous and / or as a mesh and contains, for example, platinum as a metal vapor-absorbing component. The layer 36 can also be arranged over one of the electrodes 22, 23, a porous intermediate layer being provided between the layer 36 and the electrode 22, 23 to avoid direct contact of the layer 36 with the surface of the electrode 22, 23. An arrangement of the layer 36 on the diffusion barrier 32 or on a diffusion barrier 34 according to the first exemplary embodiment is also possible. The effect of platinum as a metal vapor-absorbing material is primarily based on the fact that it is able to form stable alloys or at least intercalation compounds with both rhodium and gold, depending on the concentration range.
In Figur 3 ist ein drittes Ausführungsbeispiel der vorliegenden Erfindung dargestellt, wobei die in Figur 3 verwendeten Bezugszeichen die gleichen Komponenten bezeichnen wie in Figur 1.FIG. 3 shows a third exemplary embodiment of the present invention, the reference symbols used in FIG. 3 denoting the same components as in FIG. 1.
Als Mittel zur Vermeidung einer gegenseitigen Kontamination der Elektroden wird hier die Diffusionsstrecke 39 zwischen den Elektroden 22, 23 verlängert, so daß eine Diffusion von Metalldämpfen zwischen den Elektroden 22, 23 erschwert wird. Besonders vorteilhaft ist die Anordnung der Elektrode 23 in einer separaten Schichtebene lld des Sensorelements, da dies zu einer deut- liehen Verlängerung der Diffusionstrecke 39 zwischen beiden Elektroden 22, 23 führt, ohne daß die Längenausdehnung des Sensorelements zunimmt. Diese Anordnung ermöglicht weiterhin die Anordnung von jeweils zwei Elektroden 22 bzw. 23 innerhalb des inneren Gasraums 15 sowie die Verwendung einer zusätzlichen Dif- fusionsbarriere 38 zwischen den Elektroden 22, 23. Erfahrungsgemäß ist die Diffusionsstrecke 39 dann besonders wirkungsvoll, wenn sie mindestens ein ein- bis mehrfaches der Kammerhöhe des inneren Gasraums 15 beträgt.As a means of avoiding mutual contamination of the electrodes, the diffusion path 39 between the electrodes 22, 23 is extended, so that diffusion of metal vapors between the electrodes 22, 23 is made more difficult. The arrangement of the electrode 23 in a separate layer plane 11d of the sensor element is particularly advantageous since this leads to a significant lengthening of the diffusion path 39 between the two Electrodes 22, 23 lead without increasing the length of the sensor element. This arrangement further enables the arrangement of two electrodes 22 and 23 within the inner gas space 15 and the use of an additional diffusion barrier 38 between the electrodes 22, 23. Experience has shown that the diffusion path 39 is particularly effective when it has at least one up to several times the chamber height of the inner gas space 15.
In Figur 4 ist ein viertes Ausführungsbeispiel der vorliegenden Erfindung dargestellt, wobei die in Figur 4 verwendeten Bezugszeichen die gleichen Komponenten bezeichnen wie in den Figuren 1 bis 3.FIG. 4 shows a fourth exemplary embodiment of the present invention, the reference symbols used in FIG. 4 denoting the same components as in FIGS. 1 to 3.
Zur Vermeidung einer gegenseitigen Kontamination der Elektroden 22, 23 ist hier vorgesehen, die zusätzliche Diffusionsbarriere 38 mit einer metalldampfabsorbierenden, vorzugsweise metallischen Komponente wie beispielsweise Platin auszustatten. Die Porosität der Diffusionsbarriere 38 wird bevorzugt so gewählt, daß sie dem zur Elektrode 23 diffundierenden Gasgemisch keinen • wesentlichen Diffusionwiderstand entgegensetzt. Dabei kann jedoch in Strömungsrichtung des eindiffundierenden Gasgemischs innerhalb der Diffusionsbarriere 38 deren Porosität und die Konzentration der metalldampfabsorbierenden Komponente variiert werden.To avoid mutual contamination of the electrodes 22, 23, it is provided here that the additional diffusion barrier 38 be equipped with a metal vapor-absorbing, preferably metallic component such as platinum. The porosity of the diffusion barrier 38 is preferably selected so that it does not oppose the gas mixture diffusing to the electrode 23 with any significant diffusion resistance. However, the porosity and the concentration of the metal vapor-absorbing component can be varied in the flow direction of the diffusing gas mixture within the diffusion barrier 38.
Eine Kombination der dem vierten bzw. dritten Ausführungsbei- spiel zugrundeliegenden Maßnahmen mit den Maßnahmen des ersten und zweiten Ausführungsbeispiels sind durchaus Gegenstand der Erfindung und führen zu einer besonders effektiven Vermeidung der gegenseitigen Kontaminierung der Elektroden während des Herstellungsverfahrens .A combination of the measures on which the fourth or third exemplary embodiment is based with the measures of the first and second exemplary embodiments are entirely the subject of the invention and lead to particularly effective avoidance of mutual contamination of the electrodes during the production process.
Neben den beschriebenen Ausführungsbeispielen der vorliegenden Erfindung sind weitere Ausgestaltungen von Sensorelementen denk- bar, die Elektroden unterschiedlicher Materialzusammensetzungen beinhalten und eines Schutzes vor gegenseitiger Kontaminierung der Elektroden bedürfen. Dies trifft beispielsweise bei Sensoren mit Mischpotentialelektroden zur Bestimmung von gasförmigen Kohlenwasserstoffen oder von Wasserstoff zu. In addition to the described exemplary embodiments of the present invention, further configurations of sensor elements are also possible. bar, which contain electrodes of different material compositions and require protection against mutual contamination of the electrodes. This applies, for example, to sensors with mixed potential electrodes for determining gaseous hydrocarbons or hydrogen.

Claims

Patentansprüche claims
1. Sensorelement eines Gassensors zur Bestimmung der Konzentration mindestens eines Bestandteils eines Gasgemischs, insbesondere in Abgasen von Verbrennungsmotoren, mit mindestens zwei Elektroden, die in einem in direktem Kontakt mit dem Gasgemisch stehenden inneren Gasraum angeordnet sind, wobei die eine Elektrode ein erstes Material und die zweite Elektrode ein zweites Material enthält, dadurch gekennzeichnet, daß der innere Gasraum (15) ein physikalisch und/oder chemisch wirkendes Mittel (34, 36, 38, 39) zur Vermeidung einer Metalldiffusion zwischen den Elektroden (22, 23) auf- weist.1. Sensor element of a gas sensor for determining the concentration of at least one component of a gas mixture, in particular in exhaust gases from internal combustion engines, with at least two electrodes which are arranged in an inner gas space in direct contact with the gas mixture, the one electrode being a first material and the second electrode contains a second material, characterized in that the inner gas space (15) has a physically and / or chemically acting means (34, 36, 38, 39) to avoid metal diffusion between the electrodes (22, 23).
2. Sensorelement nach Anspruch 1, dadurch gekennzeichnet, daß das Mittel eine Diffusionsbarriere (34, 38) aus einem keramischen porösen Material ist, das eine metalldampfabsor- bierende Komponente aufweist.2. Sensor element according to claim 1, characterized in that the means is a diffusion barrier (34, 38) made of a ceramic porous material which has a metal vapor-absorbing component.
3. Sensorelement nach Anspruch 2, dadurch gekennzeichnet, daß die Diffusionsbarriere (34) schichtförmig auf der Oberfläche mindestens einer der Elektroden (22, 23) angeordnet ist. 3. Sensor element according to claim 2, characterized in that the diffusion barrier (34) is arranged in layers on the surface of at least one of the electrodes (22, 23).
4. Sensorelement nach Anspruch 1, dadurch gekennzeichnet, daß das Mittel eine Schicht (36) ist, die ein metalldampfab- sorbierendes Material enthält und die auf einer den inneren Gasraum (15) begrenzenden Fläche (11a, 11b, 11c) angeordnet ist .4. Sensor element according to claim 1, characterized in that the means is a layer (36) which contains a metal vapor-absorbing material and which is arranged on a surface (11a, 11b, 11c) delimiting the inner gas space (15).
5. Sensorelement nach Anspruch 2 oder 4 , dadurch gekennzeichnet, daß das metalldampfabsorbierende Material eine Le- gierung und/oder Einlagerungsverbindung mit einem jener Materialien bildet, die von der jeweils anderen Elektrode (22, 23) freisetzbar sind.5. Sensor element according to claim 2 or 4, characterized in that the metal vapor absorbing material forms an alloy and / or intercalation compound with one of those materials which can be released from the other electrode (22, 23).
6. Sensorelement nach Anspruch 5, dadurch gekennzeichnet, daß das metalldampfabsorbierende Material Platin enthält.6. Sensor element according to claim 5, characterized in that the metal vapor absorbing material contains platinum.
7. Sensorelement nach einem der Ansprüche 4 bis 6, dadurch gekennzeichnet, daß die Schicht (36) porös und/oder als Netz ausgeführt ist .7. Sensor element according to one of claims 4 to 6, characterized in that the layer (36) is porous and / or is designed as a network.
8. Sensorelement nach einem der Ansprüche 4 bis 7, dadurch gekennzeichnet, daß die Schicht (36) auf der Oberfläche mindestens einer der Elektroden (22, 23) angeordnet ist.8. Sensor element according to one of claims 4 to 7, characterized in that the layer (36) on the surface of at least one of the electrodes (22, 23) is arranged.
9. Sensorelement nach Anspruch 8, dadurch gekennzeichnet, daß zwischen der Schicht (36) und der Oberfläche der Elektrode (22, 23) eine poröse, isolierende Zwischenschicht ausgebildet ist.9. Sensor element according to claim 8, characterized in that a porous, insulating intermediate layer is formed between the layer (36) and the surface of the electrode (22, 23).
10. Sensorelement nach einem der Ansprüche 4 bis 7, dadurch gekennzeichnet, daß die Schicht (36) in eine Diffusionsbarriere (32, 38) integriert ist.10. Sensor element according to one of claims 4 to 7, characterized in that the layer (36) is integrated in a diffusion barrier (32, 38).
11. Sensorelement nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß die Schicht (36) in einer Diffu- sionsstrecke (39) angeordnet ist, die sich zwischen den Elektroden (22, 23) erstreckt.11. Sensor element according to one of the preceding claims, characterized in that the layer (36) in a diffusion Sions stretch (39) is arranged, which extends between the electrodes (22, 23).
12. Sensorelement nach Anspruch 1, dadurch gekennzeichnet, daß das Mittel eine Diffusionsstrecke (39) zwischen der ersten und der zweiten Elektrode (22, 23) ist, wobei die Länge der Diffusionsstrecke (39) mindestens der Kammerhöhe des inneren Gasraums (15) entspricht.12. Sensor element according to claim 1, characterized in that the means is a diffusion path (39) between the first and the second electrode (22, 23), the length of the diffusion path (39) corresponding at least to the chamber height of the inner gas space (15) ,
13. Sensorelement nach Anspruch 12, dadurch gekennzeichnet, daß sich die Diffusionsstrecke (39) über mindestens zwei Schichtebenen (11b, lld) des Sensorelements erstreckt.13. Sensor element according to claim 12, characterized in that the diffusion path (39) extends over at least two layer planes (11b, lld) of the sensor element.
14. Verwendung eines Sensorelements nach einem der Ansprü- ehe 1 bis 13 für einen Sensor zur Bestimmung von Stickoxiden, Kohlenwasserstoffen oder von Wasserstoff. 14. Use of a sensor element according to one of claims 1 to 13 for a sensor for determining nitrogen oxides, hydrocarbons or hydrogen.
EP01997695A 2000-11-23 2001-11-20 Sensor element of a gas sensor Withdrawn EP1342078A2 (en)

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US7037415B2 (en) 2006-05-02

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