EP0870190A1 - Gas sensor - Google Patents

Gas sensor

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Publication number
EP0870190A1
EP0870190A1 EP96946098A EP96946098A EP0870190A1 EP 0870190 A1 EP0870190 A1 EP 0870190A1 EP 96946098 A EP96946098 A EP 96946098A EP 96946098 A EP96946098 A EP 96946098A EP 0870190 A1 EP0870190 A1 EP 0870190A1
Authority
EP
European Patent Office
Prior art keywords
layer
sensor
gas
sensor element
oxygen
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Withdrawn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
EP96946098A
Other languages
German (de)
French (fr)
Inventor
Dieter Hahn
Hermann Leiderer
Birgitta Hacker
Hans Meixner
Susanne Kornely
Bertrand Lemire
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.)
Siemens AG
Original Assignee
Siemens AG
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 Siemens AG filed Critical Siemens AG
Publication of EP0870190A1 publication Critical patent/EP0870190A1/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Withdrawn legal-status Critical Current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
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
    • G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/0004—Gaseous mixtures, e.g. polluted air
    • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
    • G01N33/0011—Sample conditioning
    • G01N33/0014—Sample conditioning by eliminating a gas
    • 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
    • G01N27/403—Cells and electrode assemblies
    • G01N27/406—Cells and probes with solid electrolytes
    • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
    • G01N27/4077—Means for protecting the electrolyte or the electrodes

Definitions

  • Exhaust gas sensors are generally exposed to a gas mixture containing several reactive components. If the gas-sensitive element consists of a metal oxide, then the reversible interactions (volume reactions, adsorption and desorption processes) of the sensor material with the target gas, which usually take place at higher temperatures, are used to measure its concentration or partial pressure. Often, however, the metal oxide also interacts with other components of the gas mixture. In particular, these can be chemical reactions which ultimately lead to the destruction of the sensor layer, which is only a few ⁇ m thick, or which can irreversibly change its properties. In order to ensure the required long service life and reliability of the gas sensors, such reactions must be avoided at all costs. This problem can be solved, for example, by covering the gas-sensitive sensor areas with a porous protective layer, the material of which chemically binds the substances which damage the metal oxide.
  • the oxygen sensor of a fast ⁇ probe known from [1], shown in cross section in FIG. 1, essentially consists of the two comb electrodes 2/2 'arranged on an A1 2 0 3 substrate 1, the oxygen-sensitive SrTi0 3 layer 3 and a porous SrTi0 3 protective layer 4.
  • the protective layer 4 completely covering the oxygen-sensitive sensor areas is exposed to the exhaust gas of an internal combustion engine.
  • the exhaust gas contains due to the abrasion and the additives added to the fuel or engine oil include Si0 2 , Mn0 2 , Fe 2 0 3 , P 2 0 5 , Cl 2 and S0 2 .
  • the gaseous compounds react with the strontium (Sr) and the titanium (Ti) of the protective layer 4, for example to form Ti0 2 / Sr 3 (P0 4 ) 2 , TiCl 4 and SrS0 4 and therefore do not reach the sensitive layer 3.
  • the protective layer 4 intercepts the Si0 2 , Mn0 2 and Fe 2 0 3 particles.
  • the SrTi0 3 protective layer 4 extends the life of the known oxygen sensor considerably. However, the observed drift of the sensor signal in long-term operation is disadvantageous.
  • the invention relates to a sensor which can be exposed to a gas mixture containing aggressive components for a long time without being damaged and whose output signal shows only a negligible drift even in long-term operation.
  • a gas sensor with the features specified in claim 1 has these properties.
  • the dependent claims relate to further developments and refinements of the sensor.
  • the invention enables, for example, the construction of a rapid ⁇ probe for cylinder-selective control of the air number.
  • the output signal of the ⁇ probe only depends on the resistance or conductance of the oxygen-sensitive sensor layer contacted by electrodes. It is no longer influenced by the contamination and the gradual degradation of the protective layer of the sensor exposed to the exhaust gas.
  • FIG. 1 shows the known oxygen sensor in cross section
  • Figure 2 shows the oxygen sensor according to the invention in cross section
  • FIG. 3 shows the structure and sequence of the layers in the oxygen sensor according to the invention
  • FIG. 4 shows the characteristic curves of an oxygen sensor without a protective layer and of the oxygen sensor according to the invention
  • Figure 5 shows the characteristic of a Ta-doped SrTi0 3 sensor layer.
  • the sensor shown in cross section in FIGS. 2 and 3 also has two comb electrodes 2, 2 "arranged on an A1 2 0 3 or BeO substrate 1, for example made of platinum.
  • the material-sensitive element is a layer 3 of strontium titanate (SrTi0 3 ) that connects the comb electrodes 2/2 'in a conductive manner.
  • the layer 3, which is about 1 ⁇ m to 50 ⁇ m thick, can be produced by sputtering, screen printing or using a CVD process it is deposited with an electrically insulating, porous layer 34.
  • the layer 34 which preferably consists of aluminum oxide (Al 2 O 3 ), magnesium oxide (MgO) or porous silicon oxide (SiO 2 ), is sieved using a sieve pressure or by using another method of thick-layer technology, which carries the approximately 5 ⁇ m-100 ⁇ m thick screen-printed protective layer 4.
  • the porous shot is produced in the simplest case tz ⁇ layer 4 and the oxygen-sensitive layer 3 made of the same material.
  • the protective layer 4 thus consists in particular of strontium titanate (SrTi0 3 ), the SrTi0 3 optionally also containing additives such as Ca or Mg.
  • temperature-resistant materials adhering to the insulator layer 34 are also suitable, which behave chemically similar to the oxygen-sensitive titanate with regard to the reaction with the pollutants present in the exhaust gas.
  • Examples include barium titanate (BaTi0 3 ) and calcium titanate (CaTi0 3 ). O 97/24609
  • the contaminated area of the protective layer 4 is designated by 40 in FIG.
  • the products TiO2, TiCl4, Sr3 (P04> 2 and SrSO4) formed by the reaction of the titanium and the strontium with the exhaust gas components P2O5, CI2 and SO2 are deposited, the composition and thickness of the contaminated layer 40 also changing as a result of the contamination constantly changes Fe2 ⁇ 3 ⁇ SiO 2 and MnO 2 ⁇ particles.
  • the measured sensor resistance R total D2W * conductance 1 / Rtotal 9 em AESS equation (1) also depends on the resistance or conductance of the contaminated layer 40.
  • the Al 2 O 3 layer 34 provides electrical insulation of the layer 3 contacted by the comb electrodes 2/2 ', so that the measured conductance 1 / Rtotal is only a function f (Po2' of the oxygen partial pressure pQ2 i st (s Equation (2)).
  • FIG. 4 shows the characteristic curves of a SrTi0 3 sensor without a protective layer and the oxygen sensor according to the invention.
  • Squares symbolize the measured values of the SrTi0 3 layer, triangles the measured values of the sensor according to FIG. 2. It can be seen that the structure consisting of the insulator layer 34 and the protective layer 4 does not change the sensor characteristic.
  • the SrTi0 3 layer 3 is covered with a donor (Ta,
  • the Ta concentration is about 0.1% - 1%.
  • a n-type sensor material for example Ce0 2 ) for all occurring 0 2 partial pressures does not require any doping.
  • the oxygen sensor described above should be as large as possible.
  • the catalyst can be applied, for example, by wet chemical means (impregnation with H 2 PtCl 6 / tempering), sputtered on or evaporated on. It causes the gas mixture to be measured to react before it reaches the sensor layer 3. Since the sensor then has to detect very high oxygen partial pressures, Ta-doped SrTi0 3 as sensor material. Other metal oxides such as Ce0 2 in the relevant 0 2 partial pressure range are also suitable.
  • the invention is not restricted to the exemplary embodiments described. It is used in all areas of gas sensors where an electrical decoupling of a sensor element from a cover, protective or sacrificial layer is necessary or desirable.
  • the layer structure described above can also be used in other oxygen sensors (metal oxide: BaTi0 3 / Ga 2 0 3 , Ce0 2 , Ti0 2 , W0 3 ), probes for monitoring the Catalyst function (metal oxide: SrTi0 3 , BaTi0 3 , Ti0 2 , Ga 2 0 3 ), probes for nitrogen oxide detection (metal oxide: A1V0 4 , FeV0 4 ), ammonia sensors (metal oxide: W0 3 , AlV0 4 , FeV0 4 ) and others, Realize sensors exposed to exhaust gases.
  • the protective layer 4 itself can also consist of several layers of chemically binding substances that are different from each other.

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  • Chemical & Material Sciences (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Analytical Chemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • Pathology (AREA)
  • Immunology (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Biochemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Combustion & Propulsion (AREA)
  • Medicinal Chemistry (AREA)
  • Food Science & Technology (AREA)
  • Molecular Biology (AREA)
  • Investigating Or Analyzing Materials By The Use Of Fluid Adsorption Or Reactions (AREA)

Abstract

To protect the sensor element, consisting of a titanate, from the chlorine, phosphorus and sulphur compounds included in the waste gas, it is proposed to cover the oxygen-sensitive sensor zones with a porous SrTiO3 layer. Appropriate μ probes have to date not been used in spite of their long working life, since the output signal thereof demonstrates an excessively large drift due to contamination of the protective layer. A thick, porous Al2O3 film (34) covers the proposed oxygen sensor SrTiO3 layer (3) bonded by two Pt electrodes (2, 2') and precipitated on a Al2O3 substrate (1). The electrically insulating Al2O3 layer carries the protective layer (4) preferably also consisting of SrTiO3 and exposed to the waste gas. Said structure ensures that the sensor output signal representing the partial oxygen pressure still only depends on the resistance or conductance of the non-contaminated SrTiO3 sensor layer (3). The invention relates, in particular, to a rapid oxygen sensor, a μ probe, cylinder-selective regulation of the air ratio, and gas sensors which are exposed to a gas mixture containing aggressive constituents.

Description

BesehreibungDescription
GassensorGas sensor
1. Einleitung1 Introduction
Abgassensoren sind in der Regel einem mehrere reaktive Kompo¬ nenten enthaltenden Gasgemisch ausgesetzt. Besteht das gas- empfindliche Element aus einem Metalloxid, so nutzt man die üblicherweise bei höheren Temperaturen ablaufenden reversib¬ len Wechselwirkungen (Volumenreaktionen, Adsorbtions- und Desorptionsprozesse) des Sensormaterials mit dem Zielgas, um dessen Konzentration bzw. Partialdruck zu messen. Häufig wechselwirkt das Metalloxid aber auch noch mit anderen Kom¬ ponenten des Gasgemisches. Hierbei kann es sich insbesondere um chemische Reaktionen handeln, welche letztendlich zur Zer¬ störung der nur wenige μm dicken Sensorschicht führen bzw. deren Eigenschaften in irreversibler Weise ändern können. Um die geforderte hohe Lebensdauer und Zuver-lässigkeit der Gas¬ sensoren zu gewährleisten, müssen solche Reaktionen unbedingt vermieden werden. Lösen läßt sich dieses Problem beispiels¬ weise durch Abdecken der gasempfindlichen Sensorbereiche mit einer porösen Schutzschicht, deren Material die das Metall- oxid schädigenden Stoffe chemisch bindet.Exhaust gas sensors are generally exposed to a gas mixture containing several reactive components. If the gas-sensitive element consists of a metal oxide, then the reversible interactions (volume reactions, adsorption and desorption processes) of the sensor material with the target gas, which usually take place at higher temperatures, are used to measure its concentration or partial pressure. Often, however, the metal oxide also interacts with other components of the gas mixture. In particular, these can be chemical reactions which ultimately lead to the destruction of the sensor layer, which is only a few μm thick, or which can irreversibly change its properties. In order to ensure the required long service life and reliability of the gas sensors, such reactions must be avoided at all costs. This problem can be solved, for example, by covering the gas-sensitive sensor areas with a porous protective layer, the material of which chemically binds the substances which damage the metal oxide.
2. Stand der Technik2. State of the art
Der aus [1] bekannte, in Figur 1 im Querschnitt dargestellte Sauerstoffsensor einer schnellen λ-Sonde besteht im wesentli¬ chen aus den beiden auf einem A1203-Substrat 1 angeordneten Kammelektroden 2/2', der sauerstoffempfindlichen SrTi03- Schicht 3 und einer porösen SrTi03-Schutzschicht 4. Die die sauerstoffempfindlichen Sensorbereiche vollständig abdeckende Schutzschicht 4 ist dem Abgas eines Verbrennungsmotors ausge¬ setzt. Neben Stickoxiden (NOx) , Kohlenmonoxid (CO) und Koh¬ lenwasserstoffen (CHX) enthält das Abgas aufgrund des Abriebs und der dem Kraftstoff bzw. Motoröl zugesetzten Additive un¬ ter anderem auch Si02, Mn02, Fe203, P205, Cl2 und S02. Die gas¬ förmigen Verbindungen reagieren mit dem Strontium (Sr) und dem Titan (Ti) der Schutzschicht 4 beispielsweise zu Ti02/ Sr3(P04)2, TiCl4 und SrS04 und gelangen daher nicht zur sensi¬ tiven Schicht 3. Außerdem fängt die Schutzschicht 4 die Si02-, Mn02- und Fe203-Partikel ab. Die SrTi03-Schutzschicht 4 verlängert die Lebensdauer des bekannten Sauerstoffsensors erheblich. Von Nachteil ist allerdings die beobachtete Drift des Sensorsignals im Langzeitbetrieb.The oxygen sensor of a fast λ probe known from [1], shown in cross section in FIG. 1, essentially consists of the two comb electrodes 2/2 'arranged on an A1 2 0 3 substrate 1, the oxygen-sensitive SrTi0 3 layer 3 and a porous SrTi0 3 protective layer 4. The protective layer 4 completely covering the oxygen-sensitive sensor areas is exposed to the exhaust gas of an internal combustion engine. In addition to nitrogen oxides (NO x ), carbon monoxide (CO) and hydrocarbons (CH X ), the exhaust gas contains due to the abrasion and the additives added to the fuel or engine oil include Si0 2 , Mn0 2 , Fe 2 0 3 , P 2 0 5 , Cl 2 and S0 2 . The gaseous compounds react with the strontium (Sr) and the titanium (Ti) of the protective layer 4, for example to form Ti0 2 / Sr 3 (P0 4 ) 2 , TiCl 4 and SrS0 4 and therefore do not reach the sensitive layer 3. In addition, the protective layer 4 intercepts the Si0 2 , Mn0 2 and Fe 2 0 3 particles. The SrTi0 3 protective layer 4 extends the life of the known oxygen sensor considerably. However, the observed drift of the sensor signal in long-term operation is disadvantageous.
3. Gegenstand / Vorteile der Erfindung3. Object / advantages of the invention
Gegenstand der Erfindung ist ein Sensor, den man einem ag- gressive Komponenten enthaltenden Gasgemisch längere Zeit ohne Schaden zu nehmen aussetzen kann und dessen Ausgangs¬ signal auch im Langzeitbetrieb nur eine vernachlässigbar kleine Drift zeigt. Ein Gassensor mit den in Patentanspruch 1 angegebenen Merkmalen besitzt diese Eigenschaften. Die ab- hängigen Ansprüche betreffen Weiterbildungen und Ausgestal¬ tungen des Sensors.The invention relates to a sensor which can be exposed to a gas mixture containing aggressive components for a long time without being damaged and whose output signal shows only a negligible drift even in long-term operation. A gas sensor with the features specified in claim 1 has these properties. The dependent claims relate to further developments and refinements of the sensor.
Die Erfindung ermöglicht beispielsweise den Bau einer schnel¬ len λ-Sonde zur zylinderselektiven Regelung der Luft-zahl. Das Ausgangssignal der λ-Sonde hängt nur noch vom Widerstand bzw. Leitwert der durch Elektroden kontaktierten sauerstoff¬ empfindlichen Sensorschicht ab. Es wird nicht mehr von der Kontamination und dem allmählichen Abbau der dem Ab-gas aus¬ gesetzten Schutzschicht deε Sensors beeinflußt.The invention enables, for example, the construction of a rapid λ probe for cylinder-selective control of the air number. The output signal of the λ probe only depends on the resistance or conductance of the oxygen-sensitive sensor layer contacted by electrodes. It is no longer influenced by the contamination and the gradual degradation of the protective layer of the sensor exposed to the exhaust gas.
4. Zeichnungen4. Drawings
Die Erfindung wird im folgenden anhand der Zeichnungen erläu¬ tert. Es zeigen: Figur 1 den bekannten Sauerstoffsensor im Querschnitt; Figur 2 den erfindungsgemäßen Sauerstoffsensor im Quer¬ schnitt; Figur 3 die Struktur und Abfolge der Schichten im erfindungs¬ gemäßen Sauerstoffsensor;The invention is explained below with reference to the drawings. FIG. 1 shows the known oxygen sensor in cross section; Figure 2 shows the oxygen sensor according to the invention in cross section; FIG. 3 shows the structure and sequence of the layers in the oxygen sensor according to the invention;
Figur 4 die Kennlinien eines Sauerstoffsensors ohne Schutz¬ schicht und des erfindungsgemäßen Sauerstoffsensors; Figur 5 die Kennlinie einer Ta-dotierten SrTi03-Sensor- schicht.FIG. 4 shows the characteristic curves of an oxygen sensor without a protective layer and of the oxygen sensor according to the invention; Figure 5 shows the characteristic of a Ta-doped SrTi0 3 sensor layer.
5. Ausführungsbeispiele der Erfindung5. Embodiments of the invention
5.1 Aufbau eines Sauerstoffsensors5.1 Structure of an oxygen sensor
Wie der aus [1] bekannte Sauerstoffsensor besitzt auch der in den Figuren 2 und 3 im Querschnitt dargestellte Sensor zwei auf einem A1203- oder BeO-Substrat 1 angeordnete, beispiels weise aus Platin bestehende Kammelektroden 2, 2". Als sauer- stoffempfindliches Element dient eine die Kammelektroden 2/2' leitend verbindende Schicht 3 aus Strontiumtitanat (SrTi03) . Die etwa 1 μm - 50 μm dicke Schicht 3 kann man durch Aufsput¬ tern, Siebdrucken oder Anwendung eines CVD-Verfahrens erzeu- gen. Auf ihr ist eine elektrisch isolierende, poröse Schicht 34 abgeschieden. Die vorzugsweise aus Aluminiumoxid (Al203) , Magnesiumoxid (MgO) oder porösem Siliziumoxid (Si02) beste¬ hende, etwa 3 μm - 100 μm dicke Schicht 34 wird mittels Sieb¬ druck oder durch Anwendung eines anderen Verfahrens der Dick- Schichttechnologie hergestellt. Sie trägt die dem Abgas aus¬ gesetzte, etwa 5 μm - 100 μm dicke siebgedruckte Schutz¬ schicht 4. Im einfachsten Fall fertigt man die poröse Schutz¬ schicht 4 und die sauerstoffsensitive Schicht 3 aus demselben Material. Die Schutzschicht 4 besteht also insbesondere aus Strontiumtitanat (SrTi03), wobei das SrTi03 gegebenenfalls noch Zusätze wie beispielsweise Ca oder Mg enthalten kann. In Frage kommen aber auch temperaturbeständige, auf der Isola¬ torschicht 34 haftende Materialien, welche sich hinsichtlich der Reaktion mit den im Abgas vorhandenen Schadstoffen che- misch ähnlich verhalten wie das sauerstoffempfindliche Tita- nat. Zu nennen sind hier beispielsweise Bariumtitanat (BaTi03) und Calziumtitanat (CaTi03) . O 97/24609Like the oxygen sensor known from [1], the sensor shown in cross section in FIGS. 2 and 3 also has two comb electrodes 2, 2 "arranged on an A1 2 0 3 or BeO substrate 1, for example made of platinum. The material-sensitive element is a layer 3 of strontium titanate (SrTi0 3 ) that connects the comb electrodes 2/2 'in a conductive manner. The layer 3, which is about 1 μm to 50 μm thick, can be produced by sputtering, screen printing or using a CVD process it is deposited with an electrically insulating, porous layer 34. The layer 34, which preferably consists of aluminum oxide (Al 2 O 3 ), magnesium oxide (MgO) or porous silicon oxide (SiO 2 ), is sieved using a sieve pressure or by using another method of thick-layer technology, which carries the approximately 5 μm-100 μm thick screen-printed protective layer 4. The porous shot is produced in the simplest case tz¬ layer 4 and the oxygen-sensitive layer 3 made of the same material. The protective layer 4 thus consists in particular of strontium titanate (SrTi0 3 ), the SrTi0 3 optionally also containing additives such as Ca or Mg. However, temperature-resistant materials adhering to the insulator layer 34 are also suitable, which behave chemically similar to the oxygen-sensitive titanate with regard to the reaction with the pollutants present in the exhaust gas. Examples include barium titanate (BaTi0 3 ) and calcium titanate (CaTi0 3 ). O 97/24609
4 Aus Gründen der Übersichtlichkeit sind die den Kammelektroden 2/2' zugeordneten Anschlußleitungen, deren Passivierung, der Temperaturfühler, die als Heizelemente dienenden Pt-Wider- standsschichten und das den Sensor aufnehmende Gehäuse nicht dargestellt. Eine Beschreibung dieser Komponenten findet sich in [1] (siehe hier insbesondere die Figuren 2 bis 4 und den zugehörigen Beschreibungsteil in Spalte 2, Zeile 30 ff.).4 For reasons of clarity, the connecting lines assigned to the comb electrodes 2/2 ', their passivation, the temperature sensor, the Pt resistance layers serving as heating elements and the housing accommodating the sensor are not shown. A description of these components can be found in [1] (see here in particular FIGS. 2 to 4 and the associated description part in column 2, lines 30 ff.).
Der kontaminierte Bereich der Schutzschicht 4 ist in Figur 2 mit 40 bezeichnet. Hier lagern sich die durch Reaktion des Titan und des Strontiums mit den Abgaskomponenten P2O5, CI2 und SO2 entstehenden Produkte Tiθ2, TiCl4, Sr3(P04>2 und SrSθ4 ab, wobei sich die Zusammensetzung und Dicke der kontaminierten Schicht 40 auch als Folge der Verunreinigung durch Fe2θ3~ Siθ2~ und Mnθ2~Partikel ständig ändert. Im be¬ kannten Sensor führt dieser Effekt zu einer Drift seines Aus¬ gangssignals, da der gemessene Sensorwiderstand Rtotal D2W* Leitwert 1/Rtotal 9emäß Gleichung (1) (Reihenwiderstände sind nicht berücksichtigt) auch vom Widerstand bzw. Leitwert der kontaminierten Schicht 40 abhängt.The contaminated area of the protective layer 4 is designated by 40 in FIG. The products TiO2, TiCl4, Sr3 (P04> 2 and SrSO4) formed by the reaction of the titanium and the strontium with the exhaust gas components P2O5, CI2 and SO2 are deposited, the composition and thickness of the contaminated layer 40 also changing as a result of the contamination constantly changes Fe2θ3 ~ SiO 2 and MnO 2 ~ particles. In be¬ known sensor of this effect results in a drift of his Aus¬ transition signal, since the measured sensor resistance R total D2W * conductance 1 / Rtotal 9 em AESS equation (1) (series resistors are not taken into account ) also depends on the resistance or conductance of the contaminated layer 40.
---/Rtotal = 1/RSchutzschicht + 1/Rkontam.Schicht + --- / Rtotal = 1 / R protective layer + 1 / R contam. Layer +
!/RSensorschicht (-*-) ! / R sensor layer ( - * - )
Die Kontamination und der Abbau der Schutzschicht 4 beein¬ flussen das Ausgangssignal des erfindungsgemäßen Sensors hin¬ gegen nicht. Hier sorgt die Al2θ3~Schicht 34 für eine elek¬ trische Isolation der von den Kammelektroden 2/2' kontaktier¬ ten Schicht 3, so daß der gemessene Leitwert 1/Rtotal nur noch eine Funktion f(Po2' des Sauerstoffpartialdrucks pQ2 ist (s. Gleichung (2) ) .The contamination and the degradation of the protective layer 4, however, do not influence the output signal of the sensor according to the invention. Here, the Al 2 O 3 layer 34 provides electrical insulation of the layer 3 contacted by the comb electrodes 2/2 ', so that the measured conductance 1 / Rtotal is only a function f (Po2' of the oxygen partial pressure pQ2 i st (s Equation (2)).
---/Rtotal = i/Rsensorschicht = f(P02> (2) Die Figur 4 zeigt die Kennlinien eines SrTi03-Sensors ohne Schutzschicht und des erfindungsgemäßen Sauerstoffsensors. Dargestellt ist die Leitfähigkeit der Sensoren in Abhängig¬ keit vom Sauerstoffpartialdruck, wobei die Sensortemperatur jeweils T = 900°C betrug. Quadrate symbolisieren die Meßwerte der SrTi03-Schicht, Dreiecke die Meßwerte des Sensors gemäß Figur 2. Man erkennt, daß die aus der Isolatorschicht 34 und der Schutzschicht 4 bestehende Struktur die Sensorkennlinie nicht verändert.--- / Rtotal = i / Rsensorschicht = f ( P02> ( 2 ) FIG. 4 shows the characteristic curves of a SrTi0 3 sensor without a protective layer and the oxygen sensor according to the invention. The conductivity of the sensors is shown as a function of the oxygen partial pressure, the sensor temperature in each case being T = 900 ° C. Squares symbolize the measured values of the SrTi0 3 layer, triangles the measured values of the sensor according to FIG. 2. It can be seen that the structure consisting of the insulator layer 34 and the protective layer 4 does not change the sensor characteristic.
5.2 Sauerstoffsensor mit eindeutiger Kennlinie5.2 Oxygen sensor with a clear characteristic
Um auch bei hohen Sauerstoffkonzentrationen noch eine ein¬ deutige Abhängigkeit der Leitfähigkeit vom Partialdruck zu erhalten, wird die SrTi03-Schicht 3 mit einem Donator (Ta,In order to obtain a clear dependence of the conductivity on the partial pressure even at high oxygen concentrations, the SrTi0 3 layer 3 is covered with a donor (Ta,
La, W, Nb) dotiert. Als Folge dieser Dotierung bleibt das 02- sensitive Material innerhalb des interessierenden Meßbereichs immer n-leitend und die Leitfähigkeit nimmt mit steigendem Sauerstoffpartialdruck stetig ab. In Figur 5 ist die ent- sprechende Kennlinie einer Ta-dotierten SrTi03-Schicht darge¬ stellt (Sensortemperatur: T = 900°C) . Die Ta-Konzentration beträgt etwa 0,1% - 1%. Ein für alle auftretenden 02-Par- tialdrücke n-leitendes Sensormaterial (z.B. Ce02) bedarf dem¬ gegenüber keiner Dotierung.La, W, Nb) doped. As a result of this doping, the 0 2 - sensitive material always remains n-conducting within the measuring range of interest and the conductivity decreases steadily with increasing oxygen partial pressure. FIG. 5 shows the corresponding characteristic curve of a Ta-doped SrTi0 3 layer (sensor temperature: T = 900 ° C.). The Ta concentration is about 0.1% - 1%. In contrast, a n-type sensor material (for example Ce0 2 ) for all occurring 0 2 partial pressures does not require any doping.
Falls man den oben beschriebenen Sauerstoffsensor als schnelle λ-Sonde einsetzen will, sollte sein Signalhub (Ausganssignal in mageren Abgasgemischen/Ausgangssignal in fetten Abgasgemischen) möglichst groß sein. Dies läßt sich durch Versehen der isolierenden Zwischenschicht 34 und/oder der Schutzschicht 4 mit einem Katalysator (Pt, Rh oder Mi¬ schungen dieser Stoffe) erreichen. Den Katalysator kann man beispielsweise naßchemisch aufbringen (Imprägnieren mit H2PtCl6 /Tempern), aufsputtern oder aufdampfen. Er bewirkt eine Ausreaktion des zu messenden Gasgemisches, bevor es die Sensorschicht 3 erreicht. Da der Sensor dann sehr hohe Sauer¬ stoffpartialdrücke detektieren muß, verwendet man vor-teil- hafterweise Ta-dotiertes SrTi03 als Sensormaterial. In Frage kommen aber auch andere, im relevanten 02-Partial-druckbe- reich n-leitende Metalloxide wie Ce02.If you want to use the oxygen sensor described above as a fast λ probe, its signal swing (output signal in lean exhaust gas mixtures / output signal in rich exhaust gas mixtures) should be as large as possible. This can be achieved by providing the insulating intermediate layer 34 and / or the protective layer 4 with a catalyst (Pt, Rh or mixtures of these substances). The catalyst can be applied, for example, by wet chemical means (impregnation with H 2 PtCl 6 / tempering), sputtered on or evaporated on. It causes the gas mixture to be measured to react before it reaches the sensor layer 3. Since the sensor then has to detect very high oxygen partial pressures, Ta-doped SrTi0 3 as sensor material. Other metal oxides such as Ce0 2 in the relevant 0 2 partial pressure range are also suitable.
5.3 Ausgestaltungen und Weiterbildungen5.3 Developments and further training
Die Erfindung ist nicht auf die beschriebenen Ausführungsbei- spiele beschränkt. Sie findet in allen Bereichen der Gassen- sorik Anwendung, wo eine elektrische Entkopplung eines Sen- sorelements von einer Deck-, Schutz- oder Opferschicht er¬ forderlich oder wünschenswert ist. So läßt sich der oben be¬ schriebene Schichtenaufbau (Sensorelement-Isolator-Deck- schicht) beispielsweise auch in anderen SauerstoffSensoren (Metalloxid: BaTi03/ Ga203, Ce02, Ti02, W03), Sonden zur Ober- wachung der Katalysatorfunktion (Metalloxid: SrTi03, BaTi03, Ti02, Ga203) , Sonden zur Stickoxiddetektion (Metall-oxid: A1V04, FeV04), Ammoniaksensoren (Metalloxid: W03, AlV04, FeV04) und anderen, Abgasen ausgesetzten Sensoren verwirkli¬ chen. Die Schutzschicht 4 selbst kann hierbei auch aus mehre- ren, jeweils andere Schadstoffe chemisch bindenden Schichten bestehen.The invention is not restricted to the exemplary embodiments described. It is used in all areas of gas sensors where an electrical decoupling of a sensor element from a cover, protective or sacrificial layer is necessary or desirable. For example, the layer structure described above (sensor element-insulator cover layer) can also be used in other oxygen sensors (metal oxide: BaTi0 3 / Ga 2 0 3 , Ce0 2 , Ti0 2 , W0 3 ), probes for monitoring the Catalyst function (metal oxide: SrTi0 3 , BaTi0 3 , Ti0 2 , Ga 2 0 3 ), probes for nitrogen oxide detection (metal oxide: A1V0 4 , FeV0 4 ), ammonia sensors (metal oxide: W0 3 , AlV0 4 , FeV0 4 ) and others, Realize sensors exposed to exhaust gases. The protective layer 4 itself can also consist of several layers of chemically binding substances that are different from each other.
6. Literatur6. Literature
[1] DE 43 39 737 Cl [1] DE 43 39 737 Cl

Claims

Patentansprüche claims
1. Gassensor mit einem Sensorelement (3), dessen elektrischer Widerstand bzw. Leitwert vom Partialdruck eines nachzuweisen- den Gases abhängt, einem das Sensorelement (3) kontaktieren¬ den Elektrodenεystem (2, 2') und einer einem Gasgemisch aus¬ gesetzten porösen ersten Schicht (4), wobei die erste Schicht (4) aus einem Material besteht, das eine das Sensorelement (3) schädigende Komponente des Gasgemisches chemisch bindet, dadurch gekennzeichnet , daß die erste Schicht (4) auf einer zumindest die gasempfind¬ lichen Bereiche des Sensorelements (3) abdeckenden, porösen, elektrisch isolierenden zweiten Schicht (34) ange-ordnet iεt.1. Gas sensor with a sensor element (3), the electrical resistance or conductance of which depends on the partial pressure of a gas to be detected, an electrode system (2, 2 ') contacting the sensor element (3) and a porous first one which is exposed to a gas mixture Layer (4), wherein the first layer (4) consists of a material that chemically binds a component of the gas mixture that damages the sensor element (3), characterized in that the first layer (4) on at least the gas-sensitive areas of the Porous, electrically insulating second layer (34) covering sensor element (3) is arranged.
2. Gassensor nach Anspruch 1, dadurch gekennzeichnet , daß die erste und /oder die zweite Schicht (4, 34) mit einem2. Gas sensor according to claim 1, characterized in that the first and / or the second layer (4, 34) with a
Katalysator versehen sind.Catalyst are provided.
3. Gassensor nach Anspruch 1 oder 2, dadurch gekennzeichnet , daß die zweite Schicht (34) aus A1203, MgO oder Si02 besteht.3. Gas sensor according to claim 1 or 2, characterized in that the second layer (34) consists of A1 2 0 3 , MgO or Si0 2 .
4. Gassensor nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, daß das Sensorelement (3) und die erste Schicht (4) aus dem¬ selben Material bestehen.4. Gas sensor according to one of claims 1 to 3, characterized in that the sensor element (3) and the first layer (4) consist of the same material.
5. Gassensor nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet , daß das Sensorelement (3) aus einem halbleitenden Metalloxid besteht.5. Gas sensor according to one of claims 1 to 4, characterized in that the sensor element (3) consists of a semiconducting metal oxide.
6. Gassensor nach Anspruch 5, dadurch gekennzeichnet, daß das Sensorelement (3) aus einem mit einem Donator dotier¬ ten Metalloxid besteht. 6. Gas sensor according to claim 5, characterized in that the sensor element (3) consists of a metal oxide doped with a donor.
7. Gassensor nach Anspruch 5 oder 6, dadurch gekennzeichnet, daß das Sensorelement (3) aus SrTi03, BaTi03, CaTi03, Ce02, Ti02, Ga203/ W03, A1V04 oder FeV04 besteht.7. Gas sensor according to claim 5 or 6, characterized in that the sensor element (3) consists of SrTi0 3 , BaTi0 3 , CaTi0 3 , Ce0 2 , Ti0 2 , Ga 2 0 3 / W0 3 , A1V0 4 or FeV0 4 .
8. Gassensor nach einem der Ansprüche 1 bis 1 , dadurch gekennzeichnet , daß die erste Schicht (4) unmittelbar auf der das Sensorele- ment (3) vollständig abdeckenden zweiten Schicht (34) ange¬ ordnet ist.8. Gas sensor according to one of claims 1 to 1, characterized in that the first layer (4) is arranged directly on the second layer (34) completely covering the sensor element (3).
9. Verwendung eines Gassensors nach einem der vorhergehenden Ansprüche zur Messung des Sauerstoffpartialdrucks im Abgas einer Brennkraftmaschine oder zur zylinderselektiven Regelung der Luftzahl λ. 9. Use of a gas sensor according to one of the preceding claims for measuring the oxygen partial pressure in the exhaust gas of an internal combustion engine or for cylinder-selective control of the air ratio λ.
EP96946098A 1995-12-29 1996-12-18 Gas sensor Withdrawn EP0870190A1 (en)

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