WO1990003570A1

WO1990003570A1 - A magerast-type probe for determining the oxygen concentration in exhaust gases

Info

Publication number: WO1990003570A1
Application number: PCT/DE1989/000613
Authority: WO
Inventors: Karl-Hermann Friese; Werner Grünwald; Claudio De La Prieta; Hans-Martin Wiedenmann
Original assignee: Robert Bosch Gmbh
Priority date: 1988-10-01
Filing date: 1989-09-29
Publication date: 1990-04-05
Also published as: DE3833541C1; KR900702363A

Abstract

A Magerast-type probe is proposed for determining the oxygen concentration in exhaust gases in which several μ probes are connected in series so that the output signals from the individual probes are added together. If there are six individual μ probes in series, the output voltage is virtually six times that obtained with an individual Nernst cell, i.e. a much stronger signal than from a single Magerast probe. The Magerast probe of the invention is manufactured by the thin-film technique in which the individual Nernst cells are galvanically interconnected via gastight through-contacts. The Magerast probe may be fitted in known casings and used especially to determine the oxygen concentration in motor vehicle exhaust gases.

Description

Lean-rest probe for the determination of the oxygen concentration in exhaust gases

State of the art

The invention relates to a lean-back probe for the determination of the oxygen concentration in exhaust gases, in particular the exhaust gases of internal combustion engines according to the preamble of the main claim.

It is generally known for determining the oxygen content in exhaust gases, in particular of internal combustion engines, measuring sensors or so-called λ probes with at least one measuring cell with an electrode exposed to the exhaust gas and an electrode normally exposed to air, the so-called reference electrode, and a solid electrolyte between the two electrodes.

Known sensors of this type are based on the principle of the oxygen concentration chain with ion-conducting solid electrolytes. They exist e.g. B. from a tube closed on one side made of stabilized zirconium dioxide, on the outer surface facing the exhaust gas there is a porous platinum layer which forms the electrode and at the same time catalyzes the greatest possible adjustment of the thermodynamic equilibrium. The one The electrode exposed to reference gas is inside the tube. Such sensors or λ probes the so-called.

Finger versions are e.g. B. from DE-PS 28 52 647 and

29 13 633 and DE-OS 27 11 880 known.

With the so-called. Unicellular, d. H. a Nernst cell λ probe of the type described, the maximum output signal at λ = 1.02 is approximately 60 mV, as can be seen, for example, from the SAE Technical Paper Series 840141, reprint from the journal "Sensors and Actuators" SP 567, 11 results.

In the sensor known from DE-PS 27 18 907, both electrodes of the oxygen concentration cell are arranged one behind the other on a massive, electrically insulating carrier body, with an upper surface area of the solid electrolyte between the electrodes. From DE-PS 27 18 907 it is also known to produce sensors with a plurality of measuring cells connected in series, the individual measuring cells alternately having successive electrode and solid electrolyte regions and being combined to form a rod-shaped measuring cell arrangement. With this configuration, the sensor not only reaches its working temperature very quickly, but also provides an increased output voltage through the series connection of several measuring cells, which is easier to evaluate than the output voltage of a single measuring cell, and with which a drop in the output voltage due to aging processes can be better controlled.

It is also known to use planar exhaust gas probes to determine the λ value of gas mixtures, which can be produced in a particularly cost-effective manner using ceramic film and screen printing technology. In this regard, reference is made, for example, to EP-A 0 188 900 and 0 142 993 and DE-OS 30 17 947 and 35 43 759. It is useful to use ceramic films based on ZrO ₂ , CeO ₂ , HfO ₂ and ThO ₂ . The reason why ZrO ₂ , CeO ₂ , HfO ₂ and ThO ₂ moldings can be used as solid electrolytes is due to the presence of oxygen ion vacancies as

Result of the addition of stabilizers, such as. B. Ca ²⁺ and Y ³⁺ . A certain problem in the manufacture of electrochemical sensors and probes for determining the

Oxygen content in gases, especially in the exhaust gases of internal combustion engines, consists in the solid electrolyte substrates being subject to electrolytic decomposition as a result of an excessive current load or short circuits or

must be protected from coupling different galvanic circuits by arranging insulating intermediate layers. So it is z. B. is known to use ceramic aluminum oxide layers as insulating intermediate layers.

From GB-PS 1 048 069 and EP-A 0 115 148 it is also known to improve the electrical resistance of ceramic materials based on stabilized ZrO ₂ , HfO ₂ , CeO ₂ or ThO ₂ by additional incorporation of pentavalent metal ions, such as e.g. B. Nb ⁵⁺ and Ta ⁵⁺ ions in the host lattice, since they can be used to reduce the O ² vacancy concentration.

From DE-OS 37 26 479 a method for producing electrically insulating areas or layers in or on O ^2- ions conductive solid electrolytes doped with trivalent or lower-valent cations is known, in which one insulates electrically Areas of the

Solid electrolyte applies a suspension or paste with a compound with one or more 5- or higher-valent cations and allows the 5- or higher-valent cations to diffuse into the solid electrolyte by heating. The

Method can be used advantageously if such Compounds with 5- or higher-value cations suspensions or pastes for e.g. B. conductor tracks or plated-through holes around which an insulation layer is to be formed during the sintering process.

A disadvantage of the known lean-back or λ probes of the prior art is that the output signal is comparatively low. Another disadvantage of the known lean-back probes of the finger version is that the heating power is relatively large.

Advantages of the invention

A lean-back probe according to the invention with the features of the main claim has significant advantages over a single lean-back probe. For example, it initially has a much higher output signal than a single lean-to-notch probe. It has been found that, if several λ probes are connected in series according to the invention, the output signals of the individual probes add up. For example, with six individual probes connected in series, you can achieve practically 6 times the output voltage of a simple Nernst cell. Since each λ probe delivers a defined output signal with a close tolerance, the reproducibility of the output signal becomes significantly better than that of previously known limit current probes. The signal can be evaluated in a customary and known manner. Since the

If a probe delivers a voltage signal, direct evaluation with microprocessors is possible. The response time corresponds to that of the known λ probes and is therefore significantly faster than that of previously known limit current probes, in which diffusion processes determine the speed. Compared to a lean-foot probe in the finger version, the heating power is considerably reduced by the direct contact of the heating layer with the ceramic. Finally With a lean-break probe according to the invention, precise temperature control is possible via a heating resistor or via the internal resistance of the nerast cells.

According to an advantageous embodiment of the invention, the lean-toe probe consists of at least two, preferably four and in particular four to six Nernst cells or individual probes connected in series, which are gas-tight

Contacts are connected in series. The gas tightness is achieved in that the plated-through holes are covered in a known lamination technique by a foil sealing frame from above. In order to keep measurement signal falsifications caused by leakage currents low, the individual Nernst cells are arranged sufficiently far apart (distance, for example,> 5 x solid electrolyte thickness) and / or there are electrically insulating areas between the

Nernst cells by applying electrically insulating

Layers, e.g. B. Al ₂ O ₃ layers, or by diffusing z. B. Nb ⁵⁺ or Ta ^{5+ created} in the solid electrolyte to suppress an O ^2- ion line, as is known for example from DE-OS 37 26 479.

According to a second advantageous embodiment of the invention, the individual Nernst cells are arranged in a so-called "Janus shape". Lean-rest probes, in which the individual Nernst cells are arranged in a "Janus-like" manner, are those that are the same or practically the same on both sides

Show appearance and z. B. are shown in Figures 4-6, for example. Special advantages of the Janus arrangement of the Nernst cells result from the double electrode area with the same distance between the Nernst cells.

Opposing Nernst cells can be connected through an insulation layer system of the heater corresponding layer system can be largely galvanically decoupled if the heater is not used. The position of the probe in the exhaust gas flow is of little influence. The heating power can easily be concentrated on the electrode area. A double air reference arrangement is also advantageously possible with a Janus arrangement of the Nernst cells. In this case, plated-through holes in the area of the sensor cells can be dispensed with.

Comparatively thin ceramic foils based on are suitable for the production of lean-back probes according to the invention

Oxides of tetravalent metals, such as in particular ZrO ₂ , CeO ₂ , HfO ₂ and ThO ₂ , with a certain content of divalent alkaline earth metal oxides and / or preferably trivalent oxides of rare earths or yttrium oxide with a strength of preferably 0.1 to 0.6 mm, in particular 0.25 to

0.3 mm, which can be printed with the electrodes, associated conductor tracks etc. using known thick-film techniques, subjected to punching operations, laminated together and sintered.

The reference channel z. B. can be produced in a known manner by punching or by printing a layer which can be evaporated or burned out during the sintering process. To produce such layers, customary known cavity formers can be used, for. B. organic compounds, for example based on polyurethane, or theobromine, salts, such as. B. ammonium carbonate or thermal carbon black powder.

Via holes can be created by simply punching them out. The isolation of the via holes can, for. B. by means of an insulating Al ₂ O ₃ layer or by means of a suspension or paste, as is used to form the conductor tracks, but one or more connections with a 5- upper werwer term cation is added, which is found in the

Laminating process subsequent sintering process at temperatures up to 1600 ° C, preferably 1350 to 1500 ° C can diffuse into the solid electrolyte substrate.

To generate the electrodes of the individual Nernst cells and the associated conductor tracks, pastes based on noble metal, in particular platinum-based or noble metal-cermet-based, in particular platinum-cermet-based, are preferably used in thick-film technology. Such pastes can be produced in a known manner using organic binders and / or adhesion promoters, plasticizers and organic solvents.

For example, the electrodes and associated ones

Conductor tracks made from a mixture of 60 vol.% Pt and 40

% By volume of YSZ stabilized with 4 mol%. The pastes used to form the conductor tracks can

again compounds with a 5- or higher-valent cation can be added to create insulating regions.

The insulation of the conductor tracks or electrode feeds, the electrical connection between the individual electrodes including the plated-through holes and, if present, the heating layer is expediently carried out using a Pt-Nb ₂ O ₅ mixture, such as that obtained from the

DE-OS 37 26 479 is known.

The films required for the production of a lean-back probe according to the invention can be advantageously used with interlaminar binders, e.g. B. laminate together on a YSZ basis.

The laminate dressing is then sintered, e.g. B. by heating for 1 to 10 hours at temperatures of 1350 to 1500 ° C. After, but possibly also before The sintering process can be printed with electrical contact areas in the area of the via holes.

drawing

The figures serve to explain the invention in more detail. The following are shown schematically in:

Fig. 1 a lean notch probe according to the invention with simpler

Series connection of the Nernst cells in section;

FIG. 2 shows the lean-to-notch probe according to FIG. 1 in plan view;

3 shows the layout of a lean-to-notch probe according to the invention of the type shown in FIGS. 1 and 2;

Fig. 4 shows a lean-break probe according to the invention with "Janus arrangement" of the Nernst cells in section;

5 shows the layout of a lean-to-notch probe according to the invention of the type shown in FIG. 4;

Fig. 6 shows a further advantageous embodiment of a

Lean-break probe according to the invention with a "Janus arrangement" of the Nernst cells without through-contacts on average;

Fig. 7 is a perspective view of part of a

Lean-break probe according to the invention shown in FIG. 6;

Fig. 8 shows a further advantageous embodiment of a

Lean-break probe according to the invention without through-contacts with lateral channels for supplying part of the electrodes with air and exhaust gas. 1 and 2, according to the invention, is a probe made of foils 1, 2, 3 and 4 with six series-connected Nernst cells with the external electrodes exposed to the exhaust gas E ₁ to E ₆ and the Reference electrodes E ₇ to E ₁₂ arranged in reference channel 5.

The individual electrodes are printed on the solid electrolyte film 2 and compared to this by first applied insulating layers, for. B. based on Al ₂ O ₃ , electrically isolated. The film 2 has plated-through holes 6 with electrical insulation, which create a galvanic connection of the electrodes E ₁ -E ₈ , E ₂ -E ₉ , E ₃ -E ₁₀ , E ₄ -E ₁₁ and E ₅ -E ₁₂ .

The outer electrode E ₆ is on the conductor track 7 and

Reference electrode E ₇ is connected to the conductor track 8. The heater 9 with the Heier connection 10 is printed on the film 4. The outer electrodes E ₁ to E ₆ are covered with a porous cover 11, e.g. B. based on Al ₂ O ₃ or Mg spinel. The conductor tracks 7 and 8 are surrounded by insulating regions, as is indicated in FIG. 2 by the dashed lines.

The air reference channel 5 can have a porous filling. Such porous fillings, e.g. B. based on Al ₂ O ₃ or ZrO ₂ are known.

In the embodiment shown in FIG. 3, which is very similar to the embodiment shown in FIGS. 1 and 2, an opening 12 is provided in the film 1 for producing the cover 11. The cover is designed so that it protects the electrodes E ₁₃ to E ₁₈ , but the

Allows access of the exhaust gas to the electrodes. Advantageously, the film 1 in the embodiment shown in FIG. 3 also has an opening 13 to the reference channel 5 in order to reduce the diffusion resistance. A corresponding opening 13 ′ is also located in the film 2, from which a through hole 14 for the electrical connection of the conductor track 15 to the connection 16 is also punched out. The electrode E ₁₃ is connected via the conductor track 17 to the connection 16 'and the

Electrode E ₂₄ to the conductor track 15. In the case of the embodiment shown in FIG. 3, the film 3 has a punched-out reference channel 5. According to another advantageous embodiment of the invention, however, the channel can also be produced by printing on a cavity former of the type already mentioned and decomposing or burning the same during the sintering process to be carried out later. The probe also has a heater element consisting of the film 4 with a printed heater 9, through holes 19 and connections 20 and 20 '. The individual foils are connected to one another by laroining by means of layers 21, 21 'and 21 "of interlaminar binder, as is usually used for the production of planar probes based on solid electrolytes.

According to an advantageous embodiment of the invention, films based on zirconium dioxide stabilized with Y ₂ O ₃ , so-called YSZ films with z. B. 4 mol%

Y ₂ O ₃ .

The cover of the outer electrodes can e.g. B. by inserting a porous sintering ZrO ₂ film or by impressing an Al ₂ O ₃ -containing porous engobe layer in the intended

Opening 12 or by means of a plasma-sprayed Mg spinel layer. Outer electrodes and reference electrodes can have the usual composition for known planar probes and thus consist for example of noble metal cermet electrodes, e.g. B. Platinum cermet electrodes, at for example with 60 vol% Pt and 40 vol% YSZ (with 4 mol% Y ₂ O ₃ ).

The insulation of the electrodes from the film substrates and the creation of insulating areas between the individual Nernst cells can advantageously be carried out according to the method known from DE-OS 37 26 479, after which a suspension or paste with at least one is applied to the areas of the solid electrolyte substrate to be electrically isolated Connection with one or different 5- or

higher-value cations are applied and the 5- or higher-value cations are allowed to diffuse into the solid electrolyte substrate by heating.

For example, 22 22 'and 22 "Nb ₂ O ₅ suspensions with 5 mol% ZrO ₂ can be used to form the insulations.

The electrical insulation of the conductor tracks from the solid electrolyte films can also be carried out according to the method known from DE-OS 37 26 479 by adding one or more compounds with one or different 5- or higher-valued compounds to the materials used to print the conductor tracks or connections or the via-compound materials Add cations of the type mentioned.

The layers of interlaminar binder that are used to laminate the individual films together have the usual known composition, ie they can consist, for example, of a YSZ composition with 4 mol% of Y ₂ O ₃ .

Laminating and sintering the joined together

Foils are also made in a manner known for the production of planar probes, e.g. B. by heating for 0.5 to 3 hours to a temperature of 1200 to 1400 ° C. The lean-rest probe shown in section in FIG. 4 according to the invention in Janus form has four Nernst cells.

Invention lean-throated probes in "Janus shape" can, however, also be constructed from just as much individual Nernst cells as the embodiment shown in FIGS. 1-3 of a lean-throated probe according to the invention with a simple series connection of the Nernst cells. This means that a lean-back probe according to the invention in Janus form can also be constructed, for example, from six or eight or even more individual Nernst cells.

The embodiment shown in FIG. 4, for example, consists of the solid electrolyte foils 1 ', 2', 3 'and 4' with the E ₂₅ -E ₂₆ electrodes; E ₂₇ -E ₂₈ , E ₂₉ -E ₃₀ and

E ₃₁ -E ₃₂ formed Nernst cells. Between the Nernst cells E ₂₅ -E ₂₆ and E ₂₇ -E ₂₈ on the one hand and the Nernst cells E ₂₉ -E ₃₀ and E ₃₁ -Ε ₃₂ on the other hand there is the air reference channel 5 via which the reference electrodes are supplied with air and via the sealing frame 40 is sealed. In the air reference channel 5 is in a porous insulating material, for. B. porous Al ₂ O ₃ embedded heater 9. The electrode E ₂₅ is with the electrode E ₂₈ on the

Vias 24 and electrode E _{29 are} electrically connected to electrode E ₃₂ via vias 25.

Furthermore, the electrode E _{26 is} galvanically connected to the electrode E ₃₀ via the plated-through hole 26. The conductor tracks 27 and 28, which are electrically insulated from the solid electrolyte foils, connect the electrodes E ₂₇ and B ₃₁ to connections (not shown), the conductor track 28 being guided through the via hole 29. The exposed to the exhaust gas outer electrodes E ₂₅ and E ₂₇ and E ₃₀ and E ₃₂ have like the outer electrodes of the embodiment shown in FIGS. 1 to 3 form porous covers 11. As shown schematically in FIG. 5, the solid electrolyte films 1 'and 4' have openings 12 for covers 11, as in the case of the embodiment shown in FIGS. 1-3, which have the outer electrodes E ₂₅ and E ₂₇ and E, respectively _{Cover 30} and E ₃₂ , but allow the exhaust gas to enter the electrodes.

The film 2 'has plated-through holes 6 and is on one side with the electrodes E ₂₅ and E ₂₇ , the conductor track 27 and the connections 16 and 16' and on the other side with the electrodes E ₂₆ and E ₂₈ and the connection 16 " The electrodes are insulated from the film substrate, as in the case of the embodiment shown in FIGS. 1-3, by means of insulations 22 and 22 'produced by the screen printing process. In the case of the embodiment shown in FIGS the the

Electrodes E ₂₆ and E ₂₈ facing side of the solid electrolyte film 2- a layer 30 of an interlaminar binder with cutouts 31 and 32 are printed. However, this layer can optionally also be omitted. The film 3 'is printed in the same way as the film 1' on one side with the electrodes E ₂₉ and E ₃₁ and the conductor track 28 and on the other side with the electrodes E ₃₀ and E ₃₂ and the connections 34 and 35.

The plated-through holes 6 and 6 'serve to produce plated-through holes 25 and 26, respectively. The plated-through holes produced with the aid of the plated-through holes 6 "and 6"' establish the electrical contact with the connections 34 and 35. A layer 36 of interlaminar binder is introduced between the film 3 'and the film 4'. Two porous sintering Al ₂ O ₃ insulation layers 38 and 39 with printed, hermetically sealing frames 40 and 41 based on ZrO ₂ stabilized with Y ₂ O ₃ are used to produce the heater and the air reference channel. The heater 9 can are printed on the insulation layer 39 using thick-film technology. Pastes suitable for printing are e.g. B. pastes based on Pt / Al ₂ O ₃ ; Pt / Nb ₂ O ₅ or Pt / Al ₂ O ₃ / - Nb ₂ O ₅ cermet. The air reference channel 5 can be protruded from the layer 39 or can be generated by means of a cavity former of the type already mentioned by printing on an appropriate paste and decomposing or burning the cavity former during the sintering process. The lamination and sintering together is again carried out in the manner known and customary for the production of planar probes.

It is characteristic of the embodiment shown schematically in FIG. 6 with a "Janus" arrangement of the Nernst cells that it has no through-contacts.

It consists essentially of the four solid electrolyte foils 1, 2, 3 and 4, the electrodes E ₃₃ and E ₃₆ forming the Nernst cell or λ probe 1, the electrodes E ₃₄ and E ₃₅ forming the Nernst cell or λ probe 2 and the Nernst cell or λ probe 3 forming electrodes E ₃₅ and E ₃₈ and the heater 23, which is electrically insulated from the solid electrolyte foils 2 and 3, as indicated by 43. The electrodes E ₃₃ and E ₃₅ are connected to an upper and the electrode E ₃₇ to a lower air reference channel and covered with a gas-tight cover 51. The electrodes E ₃₄ , E ₃₆ and E ₃₈ are on the exhaust gas.

The latter preferably have a porous cover 11. The electrodes E ₃₃ and E ₃₈ are connected to the conductor tracks 44 and 45, respectively. The electrodes E ₃₄ and E ₃₅ as well as the

Electrodes E ₃₆ and E ₃₇ are galvanically connected. FIG. 7 illustrates the course of the air reference channels via which the reference electrodes E ₃₃ , E ₃₅ and E ₃₇ are supplied with air. In Fig. 7 the heater has been omitted for the sake of clarity. In the case of the embodiment schematically shown in FIG. 8 with the Janus arrangement of the Nernst cells, the insulated heater, in contrast to the embodiment shown in FIGS. 6 and 7, is not arranged centrally between the electrodes, but below the electrodes. At this

It is necessary to supply a part of the electrodes via side channels to the heater arrangement. The lean-break probe essentially consists of the solid electrolyte foils 1, 2 and 3 and the foils 4 and 4 'forming the heater unit, which form the first Nernst cell or λ probe

Electrodes E ₃₉ and E ₄₂ , the electrodes E ₄₀ and E ₄₃ forming the second Nernst cell or λ probe, and the

third Nernst cell or λ probe-forming electrodes E ₄₁ and E ₄₄ . The electrodes E ₄₀ and E ₄₁ on the one hand and E ₄₂ and

E _43, on the other hand, are galvanically connected to one another. The

Electrodes E ₃₉ and E ₄₄ have electrical connections 49 and 50, respectively.

The electrodes E ₄₂ and E _{44 are} supplied with exhaust gas via the lateral channels 46 and 47, which have a porous filling, and the reference electrode E _{43 is} supplied with air via the lateral channel 48. These lateral channels can, for example, be analogous to the reference channel 5 the embodiment shown in Fig. 3 are made.

The lean-back detectors according to the invention can be used, for example, in housings of the type known from DE-OS 32 06 903 and 35 37 051 and can be used to determine the oxygen concentration of the exhaust gases from motor vehicles.

Claims

Patent claims

1. Lean-rest probe for determining the oxygen concentration in exhaust gases with a spacing from each other

Arranged oxygen ion-conducting solid electrolytes, at least two Nernst cells connected in series

forming electrodes, characterized in that it is constructed from at least two Nernst cells electrically connected in series using film technology.

2. Lean-break probe according to claim 1, characterized in that the distance between the individual Nernst cells corresponds to at least 3 times the solid electrolyte thickness and / or that there are electrically insulating regions between the individual Nernst cells.

3. lean notch probe according to claim 2, characterized in that the thickness of the used to build the probe

Solid electrolyte films is 0.1 to 0.6 mm.

4. lean-break probe according to one of claims 1 to 3, characterized in that the individual Nernst cells are connected in series by means of gas-tight plated-through holes.

5. Lean notch probe according to claim 4, characterized in that the via holes are covered by a gas-tight ceramic layer.

6. lean notch probe according to one of claims 1 to 5,

characterized in that it is made up of four to six Nernst cells connected in series.

7. Lean-break probe according to one of claims 1 to 6, characterized in that it is constructed from at least four solid electrolyte foils laminated together with an integrated heater, the Nernst cells being formed by one of the four solid electrolyte foils.

8. lean rest probe according to claim 1, characterized by

a double air reference arrangement without vias in the area of the sensor cells.