EP1514099A1 - Method for calibrating a sensor element for a limiting current probe - Google Patents

Method for calibrating a sensor element for a limiting current probe

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Publication number
EP1514099A1
EP1514099A1 EP03735294A EP03735294A EP1514099A1 EP 1514099 A1 EP1514099 A1 EP 1514099A1 EP 03735294 A EP03735294 A EP 03735294A EP 03735294 A EP03735294 A EP 03735294A EP 1514099 A1 EP1514099 A1 EP 1514099A1
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EP
European Patent Office
Prior art keywords
sensor element
diffusion
gas
pump
camera
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.)
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Application number
EP03735294A
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German (de)
French (fr)
Inventor
Lothar Diehl
Karl-Heinz Friese
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
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Filing date
Publication date
Application filed by Robert Bosch GmbH filed Critical Robert Bosch GmbH
Publication of EP1514099A1 publication Critical patent/EP1514099A1/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Withdrawn legal-status Critical Current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
    • G01N27/4175—Calibrating or checking the analyser

Definitions

  • the invention is based on a method for calibrating a sensor element for a limit current probe, in particular for a planar broadband lambda probe, for determining the concentration of a gas component, in particular oxygen, in a gas mixture, in particular in the exhaust gas of internal combustion engines, according to the preamble of claim 1 ,
  • the diffusion barrier which separates the inner pump electrode of a pump cell from the exhaust gas together with a measuring or Nernst electrode of a Nernst cell, is produced using a screen printing process with a defined layer thickness, the layer thickness being checked in the manufacturing process.
  • the diffusion barrier is thus given a certain diffusion resistance, which is determined, for example, by the limit current in air with oxygen withdrawal from which the inner pump electrode and the Including measuring electrode can be determined by the Diffusio sbamere separated from the exhaust gas or measuring gas space. This diffusion resistance determines the sensitivity of the probe.
  • the layer thickness of the diffusion bamers is subject to production fluctuations, which are particularly amplified by the sintering process to which the sensor element is subjected at the end of its manufacture, so that the limit current supplied by the pump cell fluctuates not inconsiderably with different batches of the sensor elements.
  • the sintered sensor elements must therefore be subjected to calibration.
  • a gas access hole which is passed through oiesen essentially perpendicular to the surface of the solid electrolyte and which is enclosed in the end region by the diffusion camera, is specifically increased in diameter, as a result of which the diffusion resistance of the Diffusion camera is linearly adjustable.
  • a batch of identical, non-sintered sensor elements without a gas access hole is first produced, from which a sensor element is selected.
  • a gas access hole with a defined diameter is made in the selected sensor element and the sensor element is then sintered.
  • the limit current is measured at a preselected pump voltage and the target value of the measured limit current is compared by increasing the diameter of the gas access locomotive. With the optimized diameter of the gas access hole found in this way, the Gas access holes drilled in the remaining sensor elements of the batch and the sensor elements then sintered.
  • the method according to the invention for calibrating a sensor element for a limit current probe with the features of claim 1 has the advantage that the diffusion resistance of the diffusion barrier can be set with high precision while the limit current of the pump cell is being measured. At the same time, the material input achieves a homogenization effect with regard to the diffusion resistance within the diffusion barrier, since the sublimation of the precursor gas causes the material portions from the precursor gas to attach to the surfaces of the porous structure
  • Diffusion barrier comes and the larger pores in the structure are more strongly added than the smaller ones, so that the diffusion channels forming in the diffusion barrier over the layer thickness of the diffusion barrier have approximately the same diffusion resistance.
  • Another advantage of the method according to the invention is that it can also be used in sensor elements in which the diffusion barrier does not directly enclose the end region of the gas access hole to the inner pump electrode, but must be arranged withdrawn from the gas access hole for other reasons, so that the diffusion barrier enclosed annulus has a larger diameter than the bore diameter of the gas access hole in the solid electrolyte.
  • the known method described above with adjustment of the bore diameter of the gas access hole would not be applicable in this case.
  • Advantageous further developments and improvements of the method specified in claim 1 are possible through the measures listed in the further claims.
  • the sensor element is heated with a pump voltage applied to the pump cell to a temperature significantly above the later operating temperature and exposed to a highly concentrated precursor gas with a defined concentration of the gas component, preferably oxygen, until a limit current is passed through the pump cell for the set concentration of the gas component, preferably the oxygen, flows.
  • a limit current is passed through the pump cell for the set concentration of the gas component, preferably the oxygen, flows.
  • the pump voltage When the pump voltage is applied, a pump current flows in the pump cell that reaches saturation, the so-called limit current.
  • the process is terminated, with which the sensor element is calibrated, i.e. its diffusion barrier has the required diffusion resistance, and the probe thus has the required sensitivity to different concentrations of Sensing gas component, preferably oxygen, in the gas mixture with high precision.
  • the heating of the sensor element to the high temperature is achieved by applying an overvoltage above its operating voltage to an electrical resistance heater usually present in the sensor element. This allows the temperature required for the oxidation process to be obtained in a simple manner.
  • a sensor element produced by the method according to the invention is the subject of claim 10.
  • Advantageous embodiments of the sensor element according to the invention can be found in claims 11 to 13.
  • the drawing shows a schematic section of a longitudinal section of a sensor element exposed to a precursor gas for a broadband lambda probe.
  • the sensor element for a broadband lambda probe for measuring the oxygen concentration in the exhaust gas from internal combustion engines which is shown schematically in longitudinal section in the figure, is known per se and is described, for example, in DE 199 41 051 A1 in terms of structure and mode of operation. It has a plurality of oxygen ion conductive
  • Solid electrolyte layers 11 which are designed as ceramic films and laminated together. Two gas spaces are formed in the sensor element, namely a measuring gas space 12 and one Reference gas channel 13, which in the same
  • Solid electrolyte layer 11b arranged and separated from each other by a gas-tight partition 14.
  • a reference gas atmosphere e.g. Air
  • the measuring gas chamber 12 which has a circular shape, is connected to the exhaust gas via a gas access hole 16, which perpendicularly penetrates the solid electrolyte layer 11a.
  • an annular measuring electrode 17 is printed on the solid electrolyte layer 11c, which together with the reference electrode 15 forms a Nernst or concentration cell.
  • the measuring electrode 17 is arranged opposite an inner, also circular pump electrode 18 on the solid electrolyte layer 11a, which together with an outer, on the solid electrolyte layer 11a applied, circular, outer Pumpele trode 19 forms a pump cell.
  • the outer pump electrode 19 is covered by a porous protective layer 20.
  • a porous diffusion chamber 21 is arranged between the end region of the gas access hole 16 and the measurement gas space 12.
  • the porous diffusion camera 21 forms a diffusion resistance with respect to the exhaust gas diffusing into the measurement gas space 12 to the electrodes 17, 18.
  • the diffusion camera 21 consists, for example, of zirconium oxide (Zr0 2 ) or aluminum oxide (A1 2 0 3 ), which is applied, for example, to the solid electrolyte layer 11b using the screen printing method.
  • so-called pore formers for example thermal soot powder, which burns out during the sintering process, and / or evaporating components, such as, for example, theobromine or ammonium carbonate, are present in the Zr0 2 or A1 2 0 3 . and / or thermally decomposable components are added.
  • All electrodes 15, 17, 18, 19 consist of a catalytically active material, for example platinum, the electrode material being used as cermit in order to sinter with the ceramic films of the solid electrolyte layers 11.
  • All electrodes 15, 17, 18, 19 are contacted with a conductor track, of which only the conductor track 22 which is applied to the surface of the solid electrolyte layer 11a and leads to the outer pump electrode 19 can be seen. Between the solid electrolyte layers 11c and lld is one
  • Resistance heater 23 arranged, which is embedded in an electrical insulation 24, which consists for example of aluminum oxide (A1 2 0 3 ).
  • the sensor element is heated to the corresponding operating temperature of, for example, 750 ° C. by means of the resistance heater 23.
  • the layer thickness of the porous is used in the manufacture of the sensor element
  • Diffusion barrier 21 which is printed on the solid electrolyte layer 11c, dimensioned such that the diffusion barrier 21 has a diffusion resistance value that is smaller than a specified value, provided that all manufacturing tolerances are observed.
  • a material input is introduced into the diffusion barrier 21 on the finished, sintered sensor element by means of a precursor gas by sublimation, as a result of which the diffusion resistance of the diffusion barrier 21 increases.
  • the material input is dimensioned so that the diffusion resistance exactly reaches the specified value.
  • the sensor element is exposed in a closed chamber 25 to a precursor gas with a defined concentration.
  • a pump voltage is applied to the pump cell, that is to say between its pump electrodes 16, 19, and the one flowing across the pump cell
  • the sensor element is heated to a very high temperature of, for example, 1000-1200 ° C., which is significantly higher than the later operating temperature of the sensor element, which is, for example, 750 ° C.
  • This heating of the sensor element is brought about in that an overvoltage is applied to the resistance heater 23, which is substantially above the normal operating voltage of the resistance heater 23, for example 13 V.
  • Zirconium chloride (ZrCl 4 ), for example, is used as the precursor gas.
  • the precursor gas m penetrates the diffusion camera 21 (symbolized in the drawing by the arrows 26), the metal zirconium precipitating through the sublimation process reaching the hot structural surfaces inside the diffusion camera 21 and here oxidized.
  • the zirconium oxide (Zr0 2 ) that forms is deposited in layers on the structure, as a result of which the pores in the porous structure are increasingly reduced in size.
  • the limit current flowing in the pump cell is determined by pumping the oxygen out of the measuring gas chamber 12. The process must be carried out in a quasi-stationary manner, since the limit current decreases due to the oxygen requirement of the zirconium oxidation.
  • the calibration process is terminated by removing the sensor element from the chamber 25.
  • the sensor element thus calibrated is subjected to a post-sintering process at temperatures greater than 1200 ° C. in order to achieve the desired one Stabilize the crystal structure of the introduced Zr0 2 .
  • the sensor element thus calibrated has a diffusion resistance due to the material input introduced by sublimation of a precursor gas, which is within very narrow tolerance limits of, for example, ⁇ 2% around the default value, so that the broadband lambda probe has the required measuring accuracy.
  • the sensor element described is not based on a planar broadband lambda probe
  • Oxygen concentration in the exhaust gas from internal combustion engines limited. It can also be used with other gas probes with which the concentration of any gas component in a gas mixture is to be determined.

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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 method for calibrating a sensor element for a limiting current probe, in particular for a planar broadband lambda probe, which comprises a pump cell with two pump electrodes (18,19), an inner pump electrode (18) being screened by a porous diffusion barrier (21). To adjust the diffusion resistance of the diffusion barrier (21) to a default value in a highly accurate manner, the thickness of the diffusion barrier (21) is calculated during the production of the sensor element in such a way that said diffusion barrier (21) has a lower diffusion resistance value than the default value, whilst maintaining the production tolerances. In the finished sensor element, a material charge is introduced into the diffusion barrier (21) by sublimation by means of a precursor gas, thus increasing the diffusion resistance to obtain the default value.

Description

Verfahren zum Kalibrieren eines Sensorelements für eine GrenzstromsondeMethod for calibrating a sensor element for a limit current probe
Stand der TechnikState of the art
Die Erfindung geht aus von einem Verfahren zum Kalibrieren eines Sensorelements für eine Grenzstromsonde, insbesondere für eine planare Breitband-Lambdasonde, zur Bestimmung der Konzentration einer Gaskomponente, insbesondere von Sauerstoff, in einem Gasgemisch, insbesondere im Abgas von Brennkraftmaschinen, nach dem Oberbegriff des Anspruchs 1.The invention is based on a method for calibrating a sensor element for a limit current probe, in particular for a planar broadband lambda probe, for determining the concentration of a gas component, in particular oxygen, in a gas mixture, in particular in the exhaust gas of internal combustion engines, according to the preamble of claim 1 ,
Bei Sensorelementen für Grenzstromsonden oder Zweizellen- Grenzstromsonden, auch Breitband-Lambdasonden genannt, zur Messung der Sauerstoffkonzentration im Abgas vonIn sensor elements for limit current probes or two-cell limit current probes, also called broadband lambda probes, for measuring the oxygen concentration in the exhaust gas from
Brennkraftmaschinen wird die Diffusionsbarriere, die die innere Pumpelektrode einer Pumpzelle zusammen mit einer Meßoder Nernstelektrode einer Nernstzelle vom Abgas trennt, im Siebdruckverfahren mit definierter Schichtdicke hergestellt, wobei die Schichtdicke im Fertigungsprozeß kontrolliert wird. Damit erhält die Diffusionsbarriere einen bestimmten Diffusionswiderstand, der z.B. über den Grenzstrom an Luft bei Ξauerstoffabzug aus dem die innere Pumpelektrode und die Meßelektrode einschließenden, durch die Diffusio sbamere vom Abgas getrennten Hohl- oder Meßgasraum bestimmt werden kann. Dieser Diffusionswiderstand bestimmt die Empfindlichkeit der Sonde.In internal combustion engines, the diffusion barrier, which separates the inner pump electrode of a pump cell from the exhaust gas together with a measuring or Nernst electrode of a Nernst cell, is produced using a screen printing process with a defined layer thickness, the layer thickness being checked in the manufacturing process. The diffusion barrier is thus given a certain diffusion resistance, which is determined, for example, by the limit current in air with oxygen withdrawal from which the inner pump electrode and the Including measuring electrode can be determined by the Diffusio sbamere separated from the exhaust gas or measuring gas space. This diffusion resistance determines the sensitivity of the probe.
Die Schichtdicke der Diffusionsbamere unterliegt jedoch Fertigungsschwankungen, die insbesondere noch durch den Sinterprozeß, dem das Sensorelement am Ende seiner Herstellung unterzogen wird, verstärkt werden, so daß der von der Pumpzelle gelieferte Grenzstrom bei unterschiedlichen Chargen der Sensorelemente nicht unerheblich schwankt. Zur Herstellung der geforderten Meßgenauigkeit müssen daher die fertig gesinterten Sensorelemente einer Kalibirierung unterzogen werden.However, the layer thickness of the diffusion bamers is subject to production fluctuations, which are particularly amplified by the sintering process to which the sensor element is subjected at the end of its manufacture, so that the limit current supplied by the pump cell fluctuates not inconsiderably with different batches of the sensor elements. To produce the required measurement accuracy, the sintered sensor elements must therefore be subjected to calibration.
Bei einem bekannten Verfahren zur Kalibirierung des Sensorelements (DE 198 17 012 AI) wird ein Gaszutrittsloch, das im wesentlichen senkrecht zur Oberflache des Festelektrolyten durch oiesen hindurchgefuhrt ist und das im Endbereich von der Diffusionsbamere umschlossen ist, gezielt im Durchmesser vergrößert, wodurch der Diffusionswiderstand der Diffusionsbamere linear einstellbar ist. Hierzu wird zunächst eine Charge identischer, nicht gesinterter Sensorelemente ohne Gaszutrittsloch hergestellt, aus der ein Sensorelement ausgewählt wird. In das ausgewählte Sensorelement wird ein Gaszutrittsloch mit einem definierten Durchmesser eingebracht und das Sensorelement anschließend gesintert. An dem ausgewählten, gesinterten Sensorelement wird der Grenzstrom bei einer vorgewählten Pumpspannung gemessen und der Zielwert des gemessenen Grenzstroms durch Vergrößern des Durchmessers des Gaszutrittslocns abgeglichen. Mit dem so gefundenen optimierten Durchmesser des Gaszutrittslochs werden die Gaszutrittslöcher in den übrigen Sensorelementen der Charge gebohrt und die Sensorelemente anschließend gesintert.In a known method for calibrating the sensor element (DE 198 17 012 AI), a gas access hole, which is passed through oiesen essentially perpendicular to the surface of the solid electrolyte and which is enclosed in the end region by the diffusion camera, is specifically increased in diameter, as a result of which the diffusion resistance of the Diffusion camera is linearly adjustable. For this purpose, a batch of identical, non-sintered sensor elements without a gas access hole is first produced, from which a sensor element is selected. A gas access hole with a defined diameter is made in the selected sensor element and the sensor element is then sintered. At the selected, sintered sensor element, the limit current is measured at a preselected pump voltage and the target value of the measured limit current is compared by increasing the diameter of the gas access locomotive. With the optimized diameter of the gas access hole found in this way, the Gas access holes drilled in the remaining sensor elements of the batch and the sensor elements then sintered.
Vorteile der ErfindungAdvantages of the invention
Das erfindungsgemäße Verfahren zum Kalibrieren eines Sensorelements für eine Grenzstromsonde mit den Merkmalen des Anspruchs 1 hat den Vorteil, daß der Diffusionswiderstand der Diffusionsbarriere bei laufender Messung des Grenzstroms der Pumpzelle hochgenau eingestellt werden kann. Gleichzeitig wird durch den Materialeintrag ein Vergleichsmäßigungseffekt bezüglich des Diffusionswiderstands innerhalb der Diffusionsbarriere erzielt, da es durch die Sublimation des Precursorgases zu Anlagerung der Materialanteile aus dem Precursorgas an den Oberflächen der porösen Struktur derThe method according to the invention for calibrating a sensor element for a limit current probe with the features of claim 1 has the advantage that the diffusion resistance of the diffusion barrier can be set with high precision while the limit current of the pump cell is being measured. At the same time, the material input achieves a homogenization effect with regard to the diffusion resistance within the diffusion barrier, since the sublimation of the precursor gas causes the material portions from the precursor gas to attach to the surfaces of the porous structure
Diffusionsbarriere kommt und dabei die größeren Poren in der Struktur stärker zugesetzt werden als die kleineren, so daß die in der Diffusionsbarriere über die Schichtdicke der Diffusionsbarriere hinweg sich ausbildenden Diffusionskanäle einen annähernd gleichen Diffusionswiderstand besitzen.Diffusion barrier comes and the larger pores in the structure are more strongly added than the smaller ones, so that the diffusion channels forming in the diffusion barrier over the layer thickness of the diffusion barrier have approximately the same diffusion resistance.
Ein weiterer Vorteil des erfindungsgemäßen Verfahrens besteht darin, auch bei Sensorelementen eingesetzt werden zu können, bei denen die Diffusionsbarriere nicht den Endbereich des Gaszutrittslochs zur inneren Pumpelektrode unmittelbar umschließt, sondern aus anderen Gründen gegenüber dem Gaszutrittsloch zurückgezogen angeordnet sein muß, so daß der von der Diffusionsbarriere umschlossene Ringraum einen größeren Durchmesser aufweist als der Bohrungsdurchmesser des Gaszutrittsloch im Festelektrolyten. Das vorstehend beschriebene bekannte Verfahren mit Abgleich des Bohrungsdurchmessers des Gaszutrittslochs würde in diesem Fall nicht angewandt werden können. Durch die in den weiteren Ansprüche aufgeführten Maßnahmen sind vorteilhafte Weiterbildungen und Verbesserungen des im Anspruch 1 angegebenen Verfahrens möglich.Another advantage of the method according to the invention is that it can also be used in sensor elements in which the diffusion barrier does not directly enclose the end region of the gas access hole to the inner pump electrode, but must be arranged withdrawn from the gas access hole for other reasons, so that the diffusion barrier enclosed annulus has a larger diameter than the bore diameter of the gas access hole in the solid electrolyte. The known method described above with adjustment of the bore diameter of the gas access hole would not be applicable in this case. Advantageous further developments and improvements of the method specified in claim 1 are possible through the measures listed in the further claims.
Gemäß einer vorteilhaften Ausführungsform der Erfindung wird das Sensorelement mit einer an die Pumpzelle angelegten Pumpspannung auf eine erheblich über der späteren Betriebstemperatur liegende Temperatur erwärmt und einem hochkonzentrierten Precursorgas mit einer definierten Konzentration der Gaskomponente, vorzugsweise von Sauerstoff, solange ausgesetzt, bis über die Pumpzelle ein Grenzstrom für die eingestellte Konzentration der Gaskomponente, vorzugsweise des Sauerstoffs, fließt. Durch diese Ausgestaltung des erfindungsgemäßen Verfahrens läßt sich der Materialeinbringungsprozeß in die Diffusionsbarriere kostengünstig und leicht beherrschbar durchführen. Durch die hohe Temperatur kommt es an den heißen Oberflächen innerhalb der porösen Struktur der Diffusionsbarriere zur Oxidation des durch Sublimation aus dem Precursorgas sich niederschlagenden Metalls, und das Metalloxid lagert sich schichtweise auf den Oberflächen innerhalb der porösen Struktur ab und verkleinert damit die in der Struktur enthaltenen Poren. Durch Anlegen der Pumpspannung fließt in der Pumpzelle ein Pumpstrom, der eine Sättigung, den sog. Grenzstrom, erreicht. Sobald der für die eingestellte Konzentration der Gaskomponente, vorzugsweise des Sauerstoffs, gewünschte Grenzstrom erreicht ist, wird der Prozeß abgebrochen, womit das Sensorelement kalibiriert ist, d. h. seine Diffusionsbarriere den geforderten Diffusionswiderstand aufweist, und die Sonde somit die geforderte Empfindlichkeit besitzt, um unterschiedliche Konzentrationen der Gaskomponente, vorzugsweise des Sauerstoffs, in dem Gasgemisch hochgenau zu sensieren. Gemäß einer vorteilhaften Ausfuhrungsform der Erfindung wird die Erwärmung des Sensorelements auf die hohe Temperatur dadurch erreicht, daß an einem üblicherweise im Sensorelement vorhandenen, elektrischen Widerstandsheizer eine über dessen Betriebsspannung liegende Überspannung angelegt wird. Dadurch laßt sicn die für den O idationsprozeß erforderliche Temperatur in einfacher Weise gewinnen.According to an advantageous embodiment of the invention, the sensor element is heated with a pump voltage applied to the pump cell to a temperature significantly above the later operating temperature and exposed to a highly concentrated precursor gas with a defined concentration of the gas component, preferably oxygen, until a limit current is passed through the pump cell for the set concentration of the gas component, preferably the oxygen, flows. This embodiment of the method according to the invention enables the material introduction process into the diffusion barrier to be carried out inexpensively and easily. The high temperature causes the hot surfaces within the porous structure of the diffusion barrier to oxidize the metal which is deposited by sublimation from the precursor gas, and the metal oxide is deposited in layers on the surfaces within the porous structure and thus reduces the size of the structure pores. When the pump voltage is applied, a pump current flows in the pump cell that reaches saturation, the so-called limit current. As soon as the desired limit current for the set concentration of the gas component, preferably the oxygen, is reached, the process is terminated, with which the sensor element is calibrated, i.e. its diffusion barrier has the required diffusion resistance, and the probe thus has the required sensitivity to different concentrations of Sensing gas component, preferably oxygen, in the gas mixture with high precision. According to an advantageous embodiment of the invention, the heating of the sensor element to the high temperature is achieved by applying an overvoltage above its operating voltage to an electrical resistance heater usually present in the sensor element. This allows the temperature required for the oxidation process to be obtained in a simple manner.
Ein nach dem erfmdungsgemaßen Verfahren hergestelltes Sensorelement ist Gegenstand des Anspruchs 10. Vorteilhafte Ausfuhrungsformen des erfmdungsgemaßen Sensorelements finden sich in Anspruch 11 bis 13.A sensor element produced by the method according to the invention is the subject of claim 10. Advantageous embodiments of the sensor element according to the invention can be found in claims 11 to 13.
Zeichnungdrawing
Die Erfindung ist anhand eines m der Zeichnung dargestellten A_usfuhrungsbeιspιels in der nachfolgenden Beschreibung naher erläutert. Dabei zeigt die Zeichnung ausscnnittwe se in schematischer Darstellung einen Längsschnitt eines einem Precursorgas ausgesetzten Sensorelements für eine Breitband- Lambdasonde.The invention is explained in more detail in the following description with the aid of an exemplary embodiment shown in the drawing. The drawing shows a schematic section of a longitudinal section of a sensor element exposed to a precursor gas for a broadband lambda probe.
Beschreibung des AusfuhrungsbeispielsDescription of the exemplary embodiment
Das m der Figur schematisch im Längsschnitt ausschnittweise dargestellte Sensorelement für eine Breitband-Lambdasonde zum Messen der Sauerstoffkonzentration im Abgas von Brennkraftmaschinen ist an sich bekannt und beispielsweise m der DE 199 41 051 AI in Aufbau und Wirkungsweise beschrieben. Es weist eine Mehrzahl von Sauerstofflonen leitendenThe sensor element for a broadband lambda probe for measuring the oxygen concentration in the exhaust gas from internal combustion engines, which is shown schematically in longitudinal section in the figure, is known per se and is described, for example, in DE 199 41 051 A1 in terms of structure and mode of operation. It has a plurality of oxygen ion conductive
Festelektrolytschichten 11 auf, die als keramische Folien ausgeführt und zusammenlaminiert sind. Im Sensorelement sind zwei Gasraume ausgebildet, und zwar ein Meßgasraum 12 und ein Referenzgaskanal 13, die in der gleichenSolid electrolyte layers 11, which are designed as ceramic films and laminated together. Two gas spaces are formed in the sensor element, namely a measuring gas space 12 and one Reference gas channel 13, which in the same
Festelektrolytschicht 11b angeordnet und durch eine gasdichte Trennwand 14 voneinander getrennt sind. Im Referenzgaskanal 15, der an einem Ende aus dem Sensorelement herausgeführt ist und mit einer Referenzgasatmosphare, z.B. Luft, in Verbindung steht, ist eine Referenzelektrode 15 angeordnet. Der kreisnngformig ausgebildete Meßgasraum 12 steht über ein Gaszutrittsloch 16, das senkrecht die Festelektrolytschicht 11a durchdringt, mit dem Abgas m Verbindung. Im Meßgasraum 12 ist auf der Festelektrolytschicht 11c eine ringförmige Meßelektrode 17 aufgedruckt, die zusammen mit der Referenzelektrode 15 eine Nernst- oder Konzentrationszelle bildet. Im Meßgasraum 12 ist der Meßelektrode 17 gegenüberliegend eine innere, ebenfalls kreisrmgformige Pumpelektrode 18 auf der Festelektrolytschicht 11a angeordnet, die zusammen mit einer außen auf der Festelektrolytschicht 11a aufgebrachten, kreisnngformigen, äußeren Pumpele trode 19 eine Pumpzelle bildet. Die äußere Pumpelektrode 19 ist von einer porösen Schutzschicht 20 bedeckt.Solid electrolyte layer 11b arranged and separated from each other by a gas-tight partition 14. In the reference gas channel 15, which is led out of the sensor element at one end and with a reference gas atmosphere, e.g. Air, in communication, a reference electrode 15 is arranged. The measuring gas chamber 12, which has a circular shape, is connected to the exhaust gas via a gas access hole 16, which perpendicularly penetrates the solid electrolyte layer 11a. In the measuring gas chamber 12, an annular measuring electrode 17 is printed on the solid electrolyte layer 11c, which together with the reference electrode 15 forms a Nernst or concentration cell. In the measuring gas chamber 12, the measuring electrode 17 is arranged opposite an inner, also circular pump electrode 18 on the solid electrolyte layer 11a, which together with an outer, on the solid electrolyte layer 11a applied, circular, outer Pumpele trode 19 forms a pump cell. The outer pump electrode 19 is covered by a porous protective layer 20.
Zwischen dem Endbereich des Gaszutrittsloch 16 und dem Meßgasraum 12 ist eine poröse Diffusionsbamere 21 angeordnet. Die poröse Diffusionsbamere 21 bildet einen Diffusionswiderstand bezuglich des n den Meßgasraum 12 zu den Elektroden 17, 18 diffundierenden Abgases. Die Diffusionsbamere 21 besteht z.B. aus Zirkoniumoxid (Zr02) oder Aluminiumoxid (A1203) , das z.B. im Siebdruckverfahren auf die Festelektrolytschicht 11b aufgebracht ist. Zur Erzeugung der porösen Struktur der Diffusionsbamere 21 sind dem Zr02 oder A1203 sog. Porenbildner, z.B. Thermalrußpulver, das beim Sinterprozeß ausbrennt, und/oder ausdampfende Komponenten, wie z.B. Theobromm oder Ammoniniumcarbonat, und/oder thermisch zersetzbare Komponenten beigemischt. Alle Elektroden 15, 17, 18, 19 bestehen aus einem katalytisch aktivem Material, beispielsweise Platin, wobei das Elektrodenmaterial als Cermit eingesetzt wird, um mit den keramischen Folien der Festelektrolytschichten 11 zu versintern. Alle Elektroden 15, 17, 18, 19 sind mit einer Leiterbahn kontaktiert von denen nur die auf der Oberfläche der Festelektrolytschicht 11a aufgebrachte Leiterbahn 22, die zu der äußeren Pumpelektrode 19 führt, zu sehen ist. Zwischen den Festelektrolytschichten 11c und lld ist einA porous diffusion chamber 21 is arranged between the end region of the gas access hole 16 and the measurement gas space 12. The porous diffusion camera 21 forms a diffusion resistance with respect to the exhaust gas diffusing into the measurement gas space 12 to the electrodes 17, 18. The diffusion camera 21 consists, for example, of zirconium oxide (Zr0 2 ) or aluminum oxide (A1 2 0 3 ), which is applied, for example, to the solid electrolyte layer 11b using the screen printing method. To create the porous structure of the diffusion cameras 21, so-called pore formers, for example thermal soot powder, which burns out during the sintering process, and / or evaporating components, such as, for example, theobromine or ammonium carbonate, are present in the Zr0 2 or A1 2 0 3 . and / or thermally decomposable components are added. All electrodes 15, 17, 18, 19 consist of a catalytically active material, for example platinum, the electrode material being used as cermit in order to sinter with the ceramic films of the solid electrolyte layers 11. All electrodes 15, 17, 18, 19 are contacted with a conductor track, of which only the conductor track 22 which is applied to the surface of the solid electrolyte layer 11a and leads to the outer pump electrode 19 can be seen. Between the solid electrolyte layers 11c and lld is one
Widerstandsheizer 23 angeordnet, der in einer elektrischen Isolation 24, die beispielsweise aus Aluminiumoxid (A1203) besteht, eingebettet ist. Mittels des Widerstandsheizers 23 wird das Sensorelement auf die entsprechende Betriebstemperatur von beispielsweise 750°C erwärmt.Resistance heater 23 arranged, which is embedded in an electrical insulation 24, which consists for example of aluminum oxide (A1 2 0 3 ). The sensor element is heated to the corresponding operating temperature of, for example, 750 ° C. by means of the resistance heater 23.
Um den Diffusionswiderstand der porösen Diffusionsbarriere 21, der für die Empfindlichkeit des Sensorelements im späteren Betrieb von wesentlicher Bedeutung ist, abgleichen oder kalibrieren zu können, wird bei der Fertigung des Sensorelements die Schichtdicke der porösenIn order to be able to calibrate or calibrate the diffusion resistance of the porous diffusion barrier 21, which is of essential importance for the sensitivity of the sensor element in later operation, the layer thickness of the porous is used in the manufacture of the sensor element
Diffusionsbarriere 21, die auf die Festelektrolytschicht 11c aufgedruckt wird, so bemessen, daß die Diffusionsbarriere 21 bei Einhaltung aller Fertigungstoleranzen einen gegenüber einem Vorgabewert kleineren Diffusionswiderstandswert besitzt. Zum Abgleichen des Diffusionswiderstands auf den erforderlichen Vorgabewert wird am fertigen, gesinterten Sensorelement mittels eines Precursorgases durch Sublimation ein Materialeintrag in die Diffusionsbarriere 21 eingebracht, wodurch sich der Diffusionswiderstand der Diffusionsbarriere 21 erhöht. Der Materialeintrag wird dabei so bemessen, daß der Diffusionswiderstand exakt den Vorgabewert erreicht. Im einzelnen wird hierzu das Sensorelement m einer geschlossenen Kammer 25 einem Precursorgas mit einer definierten Konzentration ausgesetzt. Dabei wird an die Pumpzelle, also zwischen deren Pumpelektroden 16, 19, eine Pumpspannung gelegt und der über die Pumpzelle fließendeDiffusion barrier 21, which is printed on the solid electrolyte layer 11c, dimensioned such that the diffusion barrier 21 has a diffusion resistance value that is smaller than a specified value, provided that all manufacturing tolerances are observed. To adjust the diffusion resistance to the required default value, a material input is introduced into the diffusion barrier 21 on the finished, sintered sensor element by means of a precursor gas by sublimation, as a result of which the diffusion resistance of the diffusion barrier 21 increases. The material input is dimensioned so that the diffusion resistance exactly reaches the specified value. In detail, the sensor element is exposed in a closed chamber 25 to a precursor gas with a defined concentration. In this case, a pump voltage is applied to the pump cell, that is to say between its pump electrodes 16, 19, and the one flowing across the pump cell
Grenzstrom gemessen. Gleichzeitig wird das Sensorelement auf eine sehr hohe Temperatur von beispielsweise 1000 - 1200°C erwärmt, die wesentlich über der spateren Betriebstemperatur des Sensorelements, die beispielsweise 750°C betragt, liegt. Diese Aufheizung des Sensorelements wird dadurch bewirkt, daß an den Widerstandsheizer 23 eine Überspannung gelegt wird, die wesentlich über der normalen Betriebsspannung des Widerstandsheizers 23 von beispielsweise 13 V liegt. Als Precursorgas wird beispielsweise Zirkoniumchlond (ZrCl4) verwendet. Wahrend der Verweildauer des Sensorelements in der Precursorgas-Atmophare dringt das Precursorgas m die Diffusionsbamere 21 ein (in der Zeichnung durch die Pfeile 26 symbolisiert), wobei das durch den Sublimationsprozeß sich niederschlagende Metall Zirkonium an die heißen Strukturoberflachen im Innern der Diffusionsbamere 21 gelangt und hier oxidiert. Das sich bildende Zirkoniumoxid (Zr02) lagert sich schichtweise an der Struktur ab, wodurch die Poren in der porösen Struktur zunehmend verkleinert werden. Der in der Pumpzelle fließende Grenzstrom wird durch das Abpumpen des Sauerstoffs aus dem Meßgasraum 12 bestimmt. Der Prozeß muß dabei quasistationar gefuhrt werden, da wegen des Sauerstoffbedarfs der Zirkoniumoxidation sich der Grenzstrom verkleinert. Sobald der für die im Precursorgas eingestellte Sauerstoffkonzentration gewünschte Grenzstrom gemessen wird, wird durch Herausnehmen des Sensorelements aus der Kammer 25 der Kalibrierprozeß abgebrochen. Das so kalibrierte Sensorelement wird noch einem Nachsinterprozeß bei Temperaturen großer 1200°C unterzogen, um die gewünschte Kristallstruktur des eingebrachten Zr02 zu stabilisieren. Das so kalibrierte Sensorelement hat durch den mittels Sublimation eines Precursorgases eingebrachten Materialeintrag einen Diffusionswiderstand, der in sehr engen Toleranzgrenzen von beispielsweise ± 2% um den Vorgabewert liegt, so daß die Breitband-Lambdasonde die erforderliche Meßgenauigkeit besitzt.Limit current measured. At the same time, the sensor element is heated to a very high temperature of, for example, 1000-1200 ° C., which is significantly higher than the later operating temperature of the sensor element, which is, for example, 750 ° C. This heating of the sensor element is brought about in that an overvoltage is applied to the resistance heater 23, which is substantially above the normal operating voltage of the resistance heater 23, for example 13 V. Zirconium chloride (ZrCl 4 ), for example, is used as the precursor gas. During the dwell time of the sensor element in the precursor gas atmosphere, the precursor gas m penetrates the diffusion camera 21 (symbolized in the drawing by the arrows 26), the metal zirconium precipitating through the sublimation process reaching the hot structural surfaces inside the diffusion camera 21 and here oxidized. The zirconium oxide (Zr0 2 ) that forms is deposited in layers on the structure, as a result of which the pores in the porous structure are increasingly reduced in size. The limit current flowing in the pump cell is determined by pumping the oxygen out of the measuring gas chamber 12. The process must be carried out in a quasi-stationary manner, since the limit current decreases due to the oxygen requirement of the zirconium oxidation. As soon as the limit current desired for the oxygen concentration set in the precursor gas is measured, the calibration process is terminated by removing the sensor element from the chamber 25. The sensor element thus calibrated is subjected to a post-sintering process at temperatures greater than 1200 ° C. in order to achieve the desired one Stabilize the crystal structure of the introduced Zr0 2 . The sensor element thus calibrated has a diffusion resistance due to the material input introduced by sublimation of a precursor gas, which is within very narrow tolerance limits of, for example, ± 2% around the default value, so that the broadband lambda probe has the required measuring accuracy.
Das beschriebene Sensorelement ist nicht auf eine planare Breitband-Lambdasonde zur Bestimmung derThe sensor element described is not based on a planar broadband lambda probe
Sauerstoffkonzentration im Abgas von Brennkraftmaschinen beschränkt. Es kann auch bei anderen Gassonden eingesetzt werden, mit denen die Konzentration einer beliebigen Gaskomponente in einem Gasgemisch bestimmt werden soll. Oxygen concentration in the exhaust gas from internal combustion engines limited. It can also be used with other gas probes with which the concentration of any gas component in a gas mixture is to be determined.

Claims

Ansprüche Expectations
1. Verfahren zum Kalibrieren eines Sensorelements für eine Grenzstromsonde, insbesondere für eine planare Breitband-Lambdasonde, zur Bestimmung der Konzentration einer Gaskomponente, insbesondere von Sauerstoff, in einem Gasgemisch, insbesondere im Abgas von Brennkraftmaschinen, d e eine Pumpzelle mit zwei auf einem lonenleitenden Festelektrolyten (11) angeordneten Pumpelektroden (18, 19) aufweist, von denen eine äußere Pumpelektrode (19) dem Gasgemisch aussetzbar und eine innere Pumpelektrode (18) durch eine poröse Diffusionsbamere (21) von dem Gasgemisch getrennt ist, dadurch gekennzeichnet, daß bei der Fertigung des Sensorelements die Dicke der Diffusionsbamere (21) so bemessen wird, daß sie bei Einhaltung der Fertigungstoleranzen einen gegenüber einem Vorgabewert kleineren Diffusionswiderstandswert besitzt, und daß am fertigen Sensorelement durch Sublimation eines Precursorgases ein den Diffusionswiderstand auf den Vorgabewerte vergrößernder Mateπalemtrag in die Diffusionsbamere (21) eingebracht wird.1.Method for calibrating a sensor element for a limit current probe, in particular for a planar broadband lambda probe, for determining the concentration of a gas component, in particular oxygen, in a gas mixture, in particular in the exhaust gas of internal combustion engines, de a pump cell with two on an ion-conducting solid electrolyte ( 11) arranged pump electrodes (18, 19), of which an outer pump electrode (19) can be exposed to the gas mixture and an inner pump electrode (18) is separated from the gas mixture by a porous diffusion camera (21), characterized in that during the manufacture of the Sensor element, the thickness of the diffusion camera (21) is dimensioned such that it has a smaller diffusion resistance value than the specified value, and that the finished sensor element by sublimation of a precursor gas increases the diffusion resistance to the specified values onsbamere (21) is introduced.
Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß das Sensorelement mit einer an die Pumpzelle angelegten Pumpspannung bei hoher, erheblich über der spateren Betriebstemperatur des Sensorelements liegenden Temperatur einem hochkonzentrierten Precursorgas mit einer definierten Konzentration der Gaskomponente, vorzugsweise von Sauerstoff, solange ausgesetzt wird, bis uoer die Pumpzelle ein Grenzstrom für die im Precursorgas eingestellte Konzentration der Gaskomponente, vorzugsweise des Sauerstoffs, fließt.A method according to claim 1, characterized in that the sensor element with a pump voltage applied to the pump cell at high, significantly higher than the later Operating temperature of the sensor element is a highly concentrated precursor gas with a defined concentration of the gas component, preferably of oxygen, until it is exposed until the pump cell flows a limit current for the concentration of the gas component, preferably the oxygen, set in the precursor gas.
3. Verfanren nach Anspruch 2, dadurch gekennzeichnet, daß die Temperatur des Sensorelements in der Precursorgas-3. Verkanren according to claim 2, characterized in that the temperature of the sensor element in the precursor gas
Atmosphare mit ca. 1000 bis 1200°C gewählt wird.Atmosphere with approximately 1000 to 1200 ° C is selected.
4. Verfahren nach Anspruch 2 oder 3, dadurch gekennzeichnet, daß zum Einstellen der hohen Temperatur an einem im Sensorelement vorhandenen, elektrischen4. The method according to claim 2 or 3, characterized in that for adjusting the high temperature at an existing in the sensor element, electrical
Widerstandsheizer (23) eine über dessen Betriebsspannung liegende Überspannung angelegt wird.Resistance heater (23) is applied an overvoltage above its operating voltage.
5. Verfahren nach einem der Ansprüche 1 - 4, dadurch gekennzeichnet, daß als Precursorgas Zirkoniumchlond (ZrCl ) verwendet wird.5. The method according to any one of claims 1-4, characterized in that zirconium chloride (ZrCl) is used as precursor gas.
6. Verfahren nach einem der Ansprüche 1 - 5, dadurch gekennzeichnet, daß als Material für die Diffusionsbamere (21) Zirkoniumoxid (Zr02) verwendet wird.6. The method according to any one of claims 1-5, characterized in that zirconium oxide (Zr0 2 ) is used as the material for the diffusion bamers (21).
7. Verfahren nach einem der Ansprüche 1 - 5, dadurch gekennzeichnet, daß als Material für die Diffusionsbamere (21) Aluminiumoxid (A1203) verwendet wird. 7. The method according to any one of claims 1-5, characterized in that aluminum oxide (A1 2 0 3 ) is used as the material for the diffusion bamers (21).
8. Verfahren nach einem der Ansprüche 1 - 7, dadurch gekennzeichnet, daß nach Einbringen des Materialeintrags m die poröse Diffusionsbamere (21) das Sensorelement einem Nachsinterprozeß unterzogen wird.8. The method according to any one of claims 1-7, characterized in that after introducing the material input m, the porous diffusion camera (21), the sensor element is subjected to a post-sintering process.
9. Verfahren nach Anspruch 8, dadurch gekennzeichnet, daß das Sensorelement beim Nachsinterprozeß einer Sintertemperatur oberhalb von 1200°C ausgesetzt wird.9. The method according to claim 8, characterized in that the sensor element is subjected to a sintering temperature above 1200 ° C in the post-sintering process.
10. Sensorelement für eine Grenzstromsonde, insbesondere10. Sensor element for a limit current probe, in particular
Breitband-Lambdasonde, zur Bestimmung der Konzentration einer Gaskomponente, insbesondere von Sauerstoff, in einem Gasgemisch, insbesondere im Abgas von Brennkraftmaschinen, mit einer Pumpzelle, die eine dem Gasgemisch, insbesondere dem Abgas, aussetzbare, äußere Pumpelektrode (19) und eine vom Gasgemisch, insbesondere vom Abgas, durch eine Diffusionsbamere (21) mit poröser Struktur getrennte, innere Pumpelektrode (18) aufweist, dadurch gekennzeichnet, daß der Diffusionswiderstand der Diffusionsbamere (21) mittels eines in die poröse Struktur der Diffusionsbamere (21) durch Sublimation eines Precursorgases eingebrachten Materialeintrags eingestellt ist.Broadband lambda probe for determining the concentration of a gas component, in particular oxygen, in a gas mixture, in particular in the exhaust gas of internal combustion engines, with a pump cell which has an outer pump electrode (19) which can be exposed to the gas mixture, in particular the exhaust gas, and one which is a gas mixture. in particular from the exhaust gas, which has an inner pump electrode (18) separated by a diffusion camera (21) with a porous structure, characterized in that the diffusion resistance of the diffusion camera (21) by means of a material introduction introduced into the porous structure of the diffusion camera (21) by sublimation of a precursor gas is set.
11. Sensorelement nach Anspruch 10, dadurch gekennzeichnet, daß der Materialeintrag Zirkoniumoxid (Zr02) ist, das an den Oberflachen innerhalb der porösen Struktur der Diffusionsbamere (21) angelagert ist.11. Sensor element according to claim 10, characterized in that the material input is zirconium oxide (Zr0 2 ), which is attached to the surfaces within the porous structure of the diffusion camera (21).
12. Sensorelement nach Anspruch 10 oder 11, dadurch gekennzeichnet, daß die Diffusionsbamere (21) aus Zirkoniumoxid (Zr02) besteht. 12. Sensor element according to claim 10 or 11, characterized in that the diffusion camera (21) consists of zirconium oxide (Zr0 2 ).
3. Sensorelement nach Anspruch 10 oder 11, dadurch gekennzeichnet, daß die Diffusionsbarriere (21) aus Aluminiumoxid (A1203) besteht. 3. Sensor element according to claim 10 or 11, characterized in that the diffusion barrier (21) made of aluminum oxide (A1 2 0 3 ).
EP03735294A 2002-05-31 2003-05-12 Method for calibrating a sensor element for a limiting current probe Withdrawn EP1514099A1 (en)

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