AU637621B2 - Electrochemical sensor - Google Patents

Description

OPI DATE 03/04/91 PiPPLN. I D 594j14 PCT AOJP DATE 02/05/91 PCT NUMBER PCI/DE90/00539

INTERNATIO.

INTERNATIONALE ZUSAMMENARBE1T AUF DEM GEBIET DES PATENTWESENS (PI) (51) Internationale Patentklassifikation 5 (11) Internationale Vcrdffentlichungsnummer: WO 91/02970 GOIN 27/406 Al (43) Internationales Veriiffentlichungsdatum: 7. Mfirz 1991 (07.03.91) (21) Internationales Aktenzcichen: PCT/DE9O/00539 (81) Bestimmungsstaaten: AT (europ~isches Patent), AU, BE (europtisches Patent), CH (europisches Patent), DE (22) Internationales Anmeldedatum: 18. Juli 1990 (18.07.90) (europiiisches Patent)*, DK (europiiisches Patent), ES (europflisches Patent), FR (europilisches Patent), GB (europtiisches Patent), IT (europiisches Patent), JP, KR, Prioritiitsdaten: LU (europiiisches Patent), NL (europiiisches Patent), SE P 39 27 283.4 18. August 1989 (18.08.89) DE (europiiisches Patent), US.

(71) Anmelder (flir alle Bestiminungsstaaten ausser US): RO- Vertiffentlicht BERT BOSCH GMBH [DE/DE]; Postfach 10 60 50, D- Mit internauionalem Recherclienberic/t.

7000 Stuttgart 10 (DE).

(72) Erfinder; und Erfinder/Anmelder (nurfdr US) :FRIESE, Karl-Herrnann2 [DE/DE]; Strohg~ustr, 13, D-7250 Leonberg (DE).

WEYL, Helmut IDE/DEl; Peter v. Koblenzstr. 34, D-3 72 7141 Schwieberdingen WIELAND, Werner [DE/ DEJ; Daimlerstr. 7, D-7014 Kornwestheim (DE).

(54) Title: ELECTROCHEMICAL SENSOR I (54) Bezeichnung: ELEKTROCHEMISCHER MESSFOHLER (57) Abstract An electrochemical sensor for determining the oxygen content of exhaust gases comprises a sensor stone made of an ionconducting solid electrolyte in the form of a tube closed at one end, an inner electrode and an outer electrode. The outer electrode, which is arranged on the outer surface of the sensor stone, has a connection-side conductirng track with a covering layer. The sensor also has a ceramic protective coating and a metal gasket which seals the reference chamber from the exhaust gas chamber and establishes an electrical contact with the housing in which the sensor is inserted.

The covering layer which covers the conducting track is extends on the reference side of the sensor stone below the metal gasket. As a result, the service life of the sensor is appreciably prolonged.

(57) Zusammenfassung Es wird emn elektrochemischer Mel~fhifier fijr die Bestimmung des Sauerstoffgehaltes in Abgasen mit einem Sondenstein aus einem ionenleitenden Festelektrolyten in Form eines einseitig geschlossenen Rohres, einer Innenelektrode, einer auf der 'augeren Oberfliiche des Sondensteins angeordneten AuIgenelektrode mit einer anschlugseitigen Leiterbahn, die eine Deckschicht aufweist, einer keramischen Schutzschicht sowie einem metallischen Dichtring zur Abdichtung des Referenzraumes vom Abgasraum und zur Herstellung eines elektrischen Kontaktes mit dem Gehituse, in das der Megfiiler eingesetzt werden soil, vorgeschlagen, bei dem die die Leiterbahn abdekkende Deckschicht auf der Referenzseite des Sondensteins bis unter den m~etallischen Dichtring gefiihrt ist. Hierdurch li59t sich die Lebensdauer des Mel~fthlers entscheidend verltingern.

Siehe Rflckseite Electrochemical measuring sensor Prior art The invention relates to an electrochemical measuring sensor for determining the oxygen content in exhaust gases.accordingR tC th pr..ch-a terisi-. pa :t of- -the main clail-.--- It is generally known to use electrochemical measuring sensors often also referred to as A-probes for determining the oxygen content in exhaust gases, in particular in exhaust gases of internal combustion engines.

Known measuring sensors of this type are based on the principle of the oxygen concentration cell incorporating an ion-conducting solid electrolyte. They are composed, for example, of a tube, closed at one end, made of an ion-conducting solid electrolyte on whose external surface, facing the exhaust gas, there is an electrode which may be composed, for example, of a porous platinum layer which simultaneously catalyses an as extensive an esta 'ishment as possible of the thermodynamic equilibrium. Such an establishment of the gas equilibrium is necessary since the gas is generally not in thermodynamic equilibrium at the outset, but this is a condition for an as sharp a potential change as possible to occur at A 1 or, in the case of a polarographic diffusion current probe (cf. German Patent Specification 27 11 880), the respective actual A-value is determined.

In these measuring sensors, the electrode layer, for example platinum layer, is very thin and although it generally carries a porous ceramic protective layer, it is nevertheless subject to a corrosive attack after prolonged use by some of the exhaust gas constituents, for example soot, lead and phosphorous compounds and sulphur compounds. This corrosive attack takes place over the entire surface of the measuring sensor, but is i Z 4 particularly severe towards the open end, where, as a 2 consequence of a lower temperature, these harmful constituents deposit more easily and do not volatilise again so readily, and where the electrode layer is under some circumstances no longer completely covered by the porous protective layer, which is consequently only of limited effectiveness in any case.

German Auslegeschrift 26 19 746 discloses an electrochemical measuring sensor which has, on the external surface, facing the exhaust gas, of the solid electrolyte body or probe block, an electron-conducting layer in the form of a conductor track made of a mixture of electron-conducting material, which catalyses the establishment of the gas equilibrium, and, optionally, ceramic material or glass as a supporting structure which extends from the closed end to the open end and in which the part of the conductor track facing the open end of the tube is covered with a glaze, for example, made of potassium aluminosilicate, barium aluminosilicate or barium calcium aluminosilicate.

It has been found, however, that the covering of the conductor track with glaze as described in German Auslegeschrift 26 19 746 has several serious disadvantages. A disadvantage of using such covering layers is, first of all, that said covering layers can be applied only after the sintering process. A disadvantage is, furthermore, that cracks which easily expose the conductor tracks to a local attack easily arise during the operation of the measuring sensors in the covering layers produced on the glaz-ings because of their very different material composition from the probe ceramic and, consequently, their different thermal expansion.

In the measuring sensor disclosed by German Patent Specification 37 35 298, the disadvantages of the measuring sensor disclosed by German Auslegeschrift 26 19 747 are eliminated by producing the covering layer from the raw material mixture of the probe block ceramic with the same or enhanced sintering activity, for example from stabilized ZrO 2 to which a flux is optionally added.

It has, however, been found that, even in the level

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3 case of these measuring sensors, in which the covering layers of the conductor tracks on the reference gas side of the probe block are not brought down to below the metallic sealing ring, the service life is still not completely satisfactory, in particular in hot applications or when operated with Pb petrol.

Advantages of the invention Compared with the known electrochemical measuring sensors, for example, of the type disclosed by German Patent Specification 37 35 298 and German Auslegeschrift 26 19 746, the electrochemical measuring sensor according to the invention, in which the covering layer covering the conductor track on the reference gas side of the probe block is brought down to below the metallic sealing ring, has substantial advantages.

Surprisingly, it has been found that the service life of the measuring sensor of the type described can be very substantially prolonged by protecting the connection side of the outer conductor track against chemical and erosive attack in the contact region by bringing the covering layer down to below the metallic sealing ring.

It is only here that the actual conductor track is exposed and is brought from the protective region to the contact point on the sealing ring.

The covering layer may cover the outer conductor i track completely or only partially on the exhaust gas side since a complete interruption should primarily be prevented.

The probe block, which is as a rule fingershaped, of the measuring sensor according to the invention may be composed of the ion-conducting solid electrolyte which is standard for producing measuring sensors of this type. Typically, the probe block is composed, for example, of stabilised zirconium dioxide, for example of ZrO 2 stabilised with yttrium oxide or calcium oxide. Apart from stabilised ZrO 2 the probe block may, however, also be made of solid electrolytes based on CeO 2 HfO 2 or Th0 2 which may be stabilised, for example, S with CaO, MgO, SrO, YbO 3 and/or Sc 2 0 3 L41 a'Nr U 4 According to an advantageous development of the invention, the covering layer is composed of a dense, 51 Ct e sEnr-iqLovering layer made of the raw material mixture of the probe block ceramic having the same or enhanced sintering activity.

If, for example, the probe block is consequently composed of ZrO 2 stabilised with 5 mol-% yttrium oxide, a ZrO z stabilised in this way can also be used to produce the covering layer.

The percentage composition of the raw material mixture used to form the covering layer does not, however, need to correspond to the composition of the probe block. Advantageously, however, the same or similar raw materials are used, but the percentage composition may also differ from one another. This means that the stabiliser content of the raw material mixture used to form a covering layer may differ from the stabiliser content of the raw material mixture of the probe block, for example, by up to To produce the covering layer, a raw material mixture having enhanced sintering activity can, for example, also be used. An enhanced sintering activity is achieved, for example, by a more intense grinding and/or by adding a silicate flux, for example by adding Al silicate, Ba silicate or Ba Al silicate. Such additives may be added to the raw material mixture, for example, in amounts of about 5% by weight, referred to the raw material mixture.

Advantageously, spraying suspensions or printing pastes such as those described in greater detail in German Patent Specification 37 35 298 can be used to produce the covering layers.

Preferably, the layer thickness of the covering layers produced is 5 to 50 pm. It has as a rule been found particularly advantageous to largely match the layer thickness of the covering layer to the conductor track thickness, but not to allow the covering layer to become thinner than the conductor track in doing so.

\r .The conductor track and the contact areas are 5 composed of an electron-conducting material the [sic] normally used to produce conductor tracks and contact areas, for example platinum, a platinum alloy, for example a platinum-rhodium alloy or a noble metal cermet, for example a platinum cermet.

Particularly advantageously, the conductor track and contact areas may have a supporting structure which has sintering activity and which makes a largely porefree structure possible. Substances suitable for forming supporting structures with sintering activity are, for example, stabilised zirconium dioxide powder. Typically, the conductor track may consequently consist, for example, of up to about 60-80% by volume of a Pt/Rh alloy and about 40-20% by volume of a stabilised ZrO 2 powder.

A complete integration of the conductor track in the probe block just below the probe block surface is possible as a result of using the covering layer according to the invention and if a Pt cermet conductor track with a supporting structure having sintering activity is used. Optionally, before the application of the covering layer to the conductor track, an additional insulating layer, for example of porously sintering A1 2 0 3 which ensures the electrical insulation of the conductor track from the probe housing even at elevated temperatures 500 0 can also be applied to the conductor track beforehand.

The porous ceramic protective layer is of standard, known type.

Drawing Two exemplary embodiments of the invention are shown in the drawing and explained in greater detail in the description below. The measuring sensors shown, for example, in Figures 1 and 2 have the form of tubes which are closed at one end and of which the external surfaces are exposed to the gas to be measured and the internal surfaces to the reference gas. The measuring sensors are intended for installation in the exhaust gas pipe of an internal combustion engine of a motor vehicle for determining the x-value and they are so constructed that -6 they can be screwed into a threaded bore provided for the purpose and connected to the electronic control via a connecting cable.

Description of the exemplary embodiments The electrochemical measuring sensors shown in Figures 1 and 2 are composed in both cases of a probe block 1 made of an ion-conducting solid electrolyte of the type described in the form of a tube closed at one end, an inner electrode which is not shown and an outer electrode 2 disposed on the external surface of the probe block 1 and having a conductor track 3 on the connection side, the covering layer 4, the ceramic protective layer the metallic sealing ring 6 and the contacting areas 7.

The essential difference between the two electrochemical measuring sensors shown is that, in the case of the measuring sensor shown in Figure 1, two contact areas 7 projecting from the region of the covering layer 4 are provided and these make contact with the metallic sealing ring 6, whereas, in the case of the measuring sensor shown in Figure 2, a contact area 7' which extends round in the sealing groove 8 of the probe block 1 is provided.

The embodiment shown in Figure 1 is particularly efficient since the noble metal consumption, for example platinum consumption, for producing the contact areas is limited. The embodiment shown in Figure 2 is, on the other hand, notable for making a particularly reliable contact, which in turn ensures a particularly favourable service life of the measuring sensor.

ILL

Claims (10)

1. Electrochemical measuring sensor for determining the oxygen content in exhaust gases, in particular of internal combustion engines, having a probe block having a sealing groove and made of an ion-conducting solid electrolyte in the form of a tube closed at one end, an inner electrode, an outer electrode disposed on the external surface of the probe block and having a conductor track on the connection side which has a covering layer, a ceramic protective layer and a metallic sealing ring in the sealing groove for sealing the reference gas space from the exhaust gas space and for making an electrical contact to the housing in g which the measuring sensor is inserted, the covering layer covering the conductor track being brought down on the reference gas side of the probe block to below the metallic sealing ring, said sensor further including at least one contact area as a continuation of the conductor track, said contact area projecting out of the region of the covering layer on the reference gas side of the probe block and making 15 contact with the metallic sealing ring.

2. Measuring sensor according to Claim 1, wherein it has two contact areas projecting out of the region of the covering layer which make the contact to the metallic sealing ring.

3. Measuring sensor according to Claim 1, wherein it has a contact area S which extends round in the sealing groove of the probe block.

4. Measuring sensor according to any one of Claims 1 to 3, wherein the c covering layer is composed of a dense sinterable covering layer made of the raw -8- material mixture of the probe block ceramic having the same or enhanced sintering activity.

L Measuring sensor according to any one of Claims 1 to 4, wherein the covering layer is composed of stabilised ZrO 2 optionally with flux addition.

6. Measuring sensor according to Claim 5, wherein said stabilised ZrO 2 includes flux addition.

7. Measuring sensor according to any one of Claims 1 to 6, wherein the layer thickness of the covering layer corresponds at least to the conductor track layer thickness.

8. Measuring sensor according to Claim 7, wherein the covering layer has a layer thickness of 5 to 50 pm.

9. Measuring sensor according to any one of Claims 1 to 8, wherein the conductor track on the connection side of the outer electrode is a cermet conductor S track and it has a supporting structure of stabilised zirconium dioxide which has as least the same sintering activity as the probe block itself.

10. Electrochemical measuring sensor for determining the oxygen content S* in exhaust gases substantially as hereinbefore described with reference to the accompanying drawings. S S DATED this 24th day of March 1993. 0 ROBERT BOSCH GMBH By their Patent Attorneys: CALLINAN LAWRIE -9- Abstract An electrochemical measuring sensor is proposed for determining the oxygen content in exhaust gases which has a probe block made of an ion-conducting solid electrolyte in the form of a tube closed at one end, an inner electrode, an outer electrode disposed on the external surface of the probe block and having a conductor track on the connection side which has a covering layer, a ceramic protective layer and a metallic sealing ring for sealing the reference space from the exhaust gas space and for making an electrical contact to the housing in which the measuring sensor is to be inserted, in which electrochemical measuring sensor the covering layer covering the conductor track is brought down on the reference side of the probe block to below the metallic sealing ring. This makes it possible to prolong the service life of the measuring sensor significantly.