WO2021239401A1

WO2021239401A1 - Ceramic sensor element for an exhaust gas sensor, and method for producing same

Info

Publication number: WO2021239401A1
Application number: PCT/EP2021/061650
Authority: WO
Inventors: Hans-Joerg Renz; Uwe Glanz; Thorsten Sahm; Katharina Zorn; Julian Keller; Thomas Moser; Michael Piwonski; Oliver Dotterweich; Hartwig Lehle; Christoph GLAWE
Original assignee: Robert Bosch Gmbh
Priority date: 2020-05-28
Filing date: 2021-05-04
Publication date: 2021-12-02
Also published as: JP2023528008A; KR20230016660A; CN115698695A; EP4158326A1; DE102020206662A1

Abstract

The invention relates to a ceramic sensor element (10) for an exhaust gas sensor. The ceramic sensor element (10) has a layered design and is elongated, and the ceramic sensor element (10) has an end region facing the exhaust gas in the longitudinal direction, wherein the ceramic sensor element (10) has a cavity (30) extending in the layer direction in the interior, and the ceramic sensor element (10) has an electrochemical pump cell (38), which has a first electrode (16) that is exposed to the exhaust gas (100), a second electrode (18) that is arranged in the cavity (30), and a solid electrolyte (14), said solid electrolyte connecting the first electrode (16) to the second electrode (18). The ceramic sensor element (10) has a gas entry bore (64) in the end region facing the exhaust gas, said gas entry bore extending into the ceramic sensor element (10) perpendicularly to the layer direction and connecting the cavity (30) to the exhaust gas (100), and the ceramic sensor element (10) has a porous thermal shock protection layer (62) in the end region facing away from the exhaust gas, said protection layer covering the gas entry bore (64). The invention is characterized in that the gas entry bore (64) has a greater cross-sectional area towards the thermal shock protection layer (62) than towards the cavity (30).

Description

description

title

Ceramic sensor element for an exhaust gas sensor and method for its manufacture

State of the art

A ceramic sensor element for an exhaust gas sensor is already known from the prior art, DE 102014204 124 A1. It is constructed in layers and is elongated, has an end region facing the exhaust gas in the longitudinal direction, has a cavity extending in the layer direction in its interior, has an electrochemical pump cell with a first electrode that is exposed to the exhaust gas and with a second electrode , which is arranged in the cavity, and with a solid electrolyte that connects the first electrode to the second electrode. In the end region facing the exhaust gas, the ceramic sensor element has a gas inlet bore which extends perpendicular to the direction of the layer into the ceramic sensor element, connects the cavity with the exhaust gas and which is covered by a porous thermal shock protective layer.

In this way, the end area of the ceramic sensor element including the area of the gas access hole is protected from thermal shock, that is, from damage to the ceramic which, for example, basically threatens to occur when drops of water hit the heated ceramic.

The oxygen concentration in the exhaust gas can be measured by using the electrochemical pump cell to pump out the amount of oxygen reaching the second electrode through the gas inlet hole and to record the electrical pump current that occurs in the process.

Advantages of the invention The present invention is initially based on the inventors' knowledge that the amount of oxygen reaching the second electrode through the gas access hole depends not only on the geometry and the filling of the gas inlet hole and the cavity but also on the nature of the thermal shock protective layer covering the gas inlet opening.

The inventors have also recognized that the geometrical dimensions of the gas inlet bore and the cavity and the properties of any porous elements contained in them, such as diffusion barriers, can be implemented relatively easily with high accuracy using methods that are known in principle, for example by means of mechanical processing or screen printing using individually tailored formulations for printing pastes. The thermal shock protective layer covering the gas access hole, on the other hand, is subject to greater fluctuations in terms of its thickness and its porosity and thus its permeability for oxygen if it is applied using methods that can be economically viable in mass production (e.g. dipping, spraying, etc.). In addition, the thermal shock protective layer is particularly exposed when the exhaust gas sensor is in operation, which can further influence its properties over the service life.

These fluctuations in the thermal shock protective layer reduce the accuracy with which the sensor element can measure the oxygen concentration in the exhaust gas.

According to the invention, this problem is solved in that the gas access bore towards the thermal shock protective layer has a larger cross-sectional area than towards the cavity. The area of the thermal shock protective layer that is relevant for the oxygen transport (diffusion, flow) to the second electrode is essentially given by the cross-sectional area of the gas inlet bore adjoining it. Since this area is large according to the invention, the associated resistance (diffusion resistance / flow resistance) of the thermal shock protective layer is small.

The total resistance (total diffusion resistance / total flow resistance) between the exhaust gas and the second electrode is made up of the respective resistances of the thermal shock protective layer, the gas access hole and the part of the cavity upstream of the second electrode. As already explained above, the resistances of the thermal shock protective layer are mostly provided with production-related and / or operational-related fluctuations.

The reduction in the resistance of the thermal shock protective layer, which is associated with the large cross section of the gas access hole on the side facing the thermal shock protective layer, thus reduces not only the absolute fluctuation of the total resistance but also the relative fluctuation of the total resistance. The latter improves the accuracy with which the oxygen concentration in the exhaust gas can be measured.

An advantageous further development of the invention provides that the gas access hole towards the thermal shock protective layer has a cross-sectional area that is at least 10 times (or at least 50 times or even at least 100 times) the value of its cross-sectional area towards the cavity. A diameter of the gas access hole towards the thermal shock protective layer can have a diameter which is a minimum factor of 3 or 7 or even 10 larger than a diameter of the gas access hole towards the cavity.

An advantageous development of the invention provides that the gas inlet bore is designed as a stepped bore with a section facing the cavity and a section facing the thermal shock protective layer, the cross section of the stepped bore in the section facing the cavity being smaller than the cross section of the stepped bore in the section facing the thermal shock protective layer. For example, the cross section of the stepped bore in the section facing the cavity can be at most 1/10 of the cross section of the stepped bore in the section facing the thermal shock protective layer, or at most 1/50, or even at most 1/100. Analogously, the diameter of the stepped bore in the section facing the cavity can be at most 1/3 of the diameter of the stepped bore in the section facing the thermal shock protective layer, or at most 1/7 or even at most 1/10.

The height of the section of the stepped bore facing the thermal shock protective layer can advantageously be small, for example not greater than the diameter of the stepped bore in the section facing the cavity, for example not greater than 0.2 mm. The mechanical strength of the sensor element is then comparatively little reduced by the introduction of the stepped bore. If the thermal shock protective layer bulges outward in the area of the gas access hole, its stability is increased in the area in which it covers the gas access hole. In particular, the protrusion prevents the thermal shock protective layer from breaking into the interior of the gas access hole.

The advantageous effects of the invention occur to a particular extent when it is impossible for a material of the thermal shock protective layer to penetrate into the gas inlet bore. In order to prevent the material of the thermal shock protective layer from penetrating into the gas access hole and to stabilize the thermal shock protective layer during production and on the finished sensor element, it can therefore be advantageous for a bridge layer to be arranged between the gas access hole and the thermal shock protective layer, which differs structurally from the thermal shock protective layer or is distinguishable from it.

A gas access hole is basically understood to be a geometry that can be produced by a drill (e.g. mechanical step drill), regardless of whether it is actually made in a specific sensor element by drilling, milling, countersinking or a similar process.

Accordingly, a sensor element according to the invention can be produced by the following steps:

- Printing of green ceramic foils with printing pastes;

Producing an unsintered stack by laminating several of the printed green ceramic foils in layers;

- Introducing a gas inlet hole in an end region of the unsintered stack facing the exhaust gas;

- Separate printing of a bridge layer on a transfer film;

- drying the bridge layer;

- Transferring the bridge layer from the transfer film to the unsintered stack, so that the bridge layer covers the gas access hole;

Sintering the green stack into a sintered stack;

Application of a thermal shock protective layer to the end region of the stack facing the exhaust gas.

The sintered ceramic sensor element thus results as the sintered stack on which the thermal shock protective layer is applied. This transfer technique enables it is to bridge the gas access hole in the manufacturing process in such a way that the material of the thermal shock protective layer cannot penetrate into it.

It is possible on the one hand to first sinter the unsintered stack and then to apply the thermal shock protective layer to the end region of the sintered stack facing the exhaust gas. Alternatively, it is also possible to apply a material from which the thermal shock protective layer is formed to the unsintered stack. In the subsequent sintering process, the unsintered stack then merges into the sintered stack and the material merges into the thermal shock protective layer.

If a cavity former is filled into the gas access hole before the bridge layer is transferred, the bridge layer is initially supported on the cavity former and cannot sink into the gas access hole. A cavity former is understood to mean a material that during sintering (that is to say for example above 1100 ° C.) evaporates without leaving any residue, that is for example graphite, glass carbon, theobromine or the like).

The inventors have also found that a bulging of the bridge layer and thus the thermal shock protective layer can be produced in a targeted manner. If the content of solvent or water in the cavity and in the gas inlet bore or in the unsintered stack at the point in time at which the bridging layer is applied to the unsintered stack is selected to be rather high, subsequent heating occurs, for example during the transfer of the bridge layer or at the beginning of sintering, in the cavity and in the gas access hole, to an increase in the vapor pressure in the cavity and in the gas access hole and thereby to the bulging of the bridge layer or the thermal shock protective layer. A non-arched, i.e. flat bridge layer or thermal shock protective layer is obtained by subjecting the unsintered stack to drying or debinding before the bridge layer is transferred to it. In the case of subsequent heating, e.g. at the beginning of sintering, there is no or hardly any increase in the vapor pressure in the cavity and in the gas inlet bore and thus no bulging of the bridge layer or the thermal shock protective layer.

As an alternative to the transfer technique described above, a sensor element according to the invention can also be produced by a printing technique, especially in the following steps:

- Printing of green ceramic foils with printing pastes;

Introducing a cavity former into the gas inlet bore, in particular filling the gas inlet bore with a cavity former;

- Overpressure of the gas access hole with a bridge layer;

Sintering the green stack into a sintered stack;

This also results in the sintered ceramic sensor element as the sintered stack on which the thermal shock protective layer is applied. This technology also makes it possible to bridge the gas access hole in the manufacturing process in such a way that the material of the thermal shock protective layer cannot penetrate it.

It is again possible on the one hand to first sinter the unsintered stack and then to apply the thermal shock protective layer to the end region of the sintered stack facing the exhaust gas.

Alternatively, it is again possible to apply a material to the unsintered stack from which the thermal shock protective layer is formed. In the subsequent sintering process, the unsintered stack then merges into the sintered stack and the material merges into the thermal shock protective layer.

The introduction of the cavity former into the gas inlet bore, in particular the filling of the gas inlet bore with a cavity former, can take place in that the cavity former is pressed into the gas inlet bore from the outside. The cavity former preferably ends flush with the outer surface of the unsintered stack.

Figures Figure 1 shows schematically a sensor element according to the invention

FIGS. 2 and 3 show, by way of example, a first method according to the invention for producing the sensor element according to the invention. FIGS. 4 and 5 show, by way of example, a second method according to the invention for producing the sensor element according to the invention.

Description of the exemplary embodiments

FIG. 1 schematically shows a cross section through an end region facing the exhaust gas of a ceramic sensor element 10 according to the invention for an exhaust gas sensor.

The sensor element 10 is layered and elongated, the layer direction in FIG. 1 extending in a plane from left to right and perpendicular to the image plane. In its interior, the sensor element 10 has a cavity 30.

The sensor element 10 also has an electrochemical pump cell 38. It consists of a first electrode 16 exposed to the exhaust gas 100, in the example ring-shaped, a second electrode 18 arranged in the cavity 30, in the example ring-shaped, and a solid electrolyte body 14 which connects the first electrode 16 to the second electrode 18 in an oxygen-ion-conducting manner.

Concentrically with the first electrode 16 and the second electrode 18, a gas inlet bore 64, designed as a stepped blind hole in this example, extends through the solid electrolyte body 38 outside - the annular cavity 30 adjoins, in which the second electrode 18 is arranged.

The gas access bore 64 is covered with a porous bridge layer 60 which, as shown in FIG. 1, extends from left to right, for example over the entire width of the sensor element 10. A porous thermal shock protective layer 62 is arranged on the bridge layer and in the cross section shown in FIG. 1 on all side surfaces of the sensor element 10. The thickness of the thermal shock protective layer 62 can be greater than the thickness of the bridge layer 60, for example.

The gas inlet bore 64 is designed as a stepped bore with a section 64a facing the cavity 30 (bottom in FIG. 1) and with a section 64b facing the thermal shock protective layer 62 (top in FIG. 1). The section 64a facing the cavity 30 in this example has a diameter d of 210 pm (168 pm) before sintering (after sintering). The section 64b facing the thermal shock protective layer 62 has a diameter D of 2100 pm (1680 pm) in this example. The height h of the section 64a facing the cavity 30 is 145 pm (116 pm) in this example.

The function of the sensor element 10 is that exhaust gas 100 and the molecular oxygen O2 contained therein communicates through the thermal shock protective layer 62 and through the bridge layer 60 with the gas inlet hole 64 and furthermore through the diffusion barrier 36 with the cavity 30, as in FIG. 1 marked with an arrow. If a sufficient pump voltage is now applied to the electrochemical pump cell 38, the oxygen located in the cavity 30 is always removed electrochemically by the transport of oxygen ions O. According to the existing partial pressure gradient, oxygen from the exhaust gas 100 flows (flows / diffuses) via the thermal shock protective layer 62, the bridge layer 60, the gas access hole 64 and the diffusion barrier 36 into the cavity 30, where it is again pumped out electrochemically.

The resulting pumping current is therefore proportional to the oxygen partial pressure in the exhaust gas 100 and inversely proportional to the resistance (flow / diffusion resistance) along the path on which oxygen reaches the cavity 30.

In order to be able to measure the oxygen partial pressure in the exhaust gas 100 precisely, it is necessary that the resistance along the path along which oxygen reaches the cavity 30 fluctuates little due to manufacturing and aging reasons. In the example, this is implemented in that the diffusion barrier 36 and the gas access bore 64v can be produced with small fluctuations and operated with low aging effects. The bridge layer 60 and the thermal shock protection layer 62, on the other hand, are designed with a comparatively very large effective cross-sectional area. Their contribution to the resistance along the path on which oxygen reaches the cavity 30 is therefore small relative to the resistance of the diffusion barrier 36 and the resistance of the gas access bore 64. Even if the thermal shock protective layer 62 is subject to manufacturing-related fluctuations and aging effects, this therefore makes only a small absolute and relative contribution to a fluctuation in the resistance along the path on which oxygen enters the cavity 30. The sensor element 10 according to the invention is therefore able to precisely measure an oxygen concentration in the exhaust gas 100 over its lifetime.

In a development of the invention, the sensor element 10 according to the invention is produced in such a way that penetration of the material of the thermal shock protective layer 62 into the gas inlet bore 64 is reliably excluded.

The individual method steps according to a first example are shown in FIGS. 2 and 3 with regard to FIG.

In the example, several green YSZ foils are printed with printing pastes in a first process step VI. Subsequently (method step V2) the printed foils are laminated in layers to form an unsintered stack. The gas inlet bore 64 is introduced into the end region of the unsintered stack facing the exhaust gas 100, for example with a mechanical rotating step drill (method step V3).

The gas inlet bore 64 is filled (in method step V4) with a paste containing a cavity former, e.g. glass carbon, or with UV varnish. Optionally, the unsintered stack can be dried, depending on whether a protruding or planar covering of the gas access bore 64 is desired on the finished sensor element after sintering.

In parallel to this (method step V5), a bridge layer 60 is printed separately on a transfer film 110. An ink with a ceramic and an organic component is used for printing. It contains a short-chain binder and the solvent diethylene glycol. In this way, the bridging layer 60 has increased strength and is transferable. Optionally, the strength of the bridge layer 60 can be increased further by drying. As the transfer film 110 in this example, a film is used, which is commercially available under the trade name PACOTHANE ^® PLUS.

The bridge layer 60 is then transferred from the transfer film 110 to the unsintered stack in such a way that the bridge layer 60 covers the gas access hole 64 (method step V6). The transfer succeeds, for example, at 80 ° C and 30 kN / (200 mm * 220 mm) in a heated press 120. After and / or before sintering the ceramic (method step V7), a thermal shock protective layer 62 can be applied to the entire end region of the sensor element 10 facing the exhaust gas and to the bridge layer 60 using methods known per se (e.g. dipping, spraying, lamination, etc.) (method step V8). The presence of the bridge layer 60 has the effect that the material of the thermal shock protective layer 62 cannot penetrate into the gas access bore 64.

The individual method steps according to a second example are shown in FIGS. 4 and 5 with regard to FIG.

In the example, several green YSZ foils are printed with printing pastes in a first process step VI. Subsequently (method step V2) the printed foils are laminated in layers to form an unsintered stack. The gas inlet bore 64 is introduced into the end region of the unsintered stack facing the exhaust gas, for example with a mechanical step drill (method step V3).

The gas access bore 64 is filled (in method step V4) with a paste containing a cavity former 64h, e.g. glass carbon, or with UV varnish. Optionally, the gas inlet bore 64 can be completely filled in such a way that the cavity former 64h is flush with the outer surface of the unsintered stack, for example by means of a press 120

Subsequently (in method step V14) the gas access bore 64 is overprinted with a bridge layer 60, for example with a printing paste as is known per se for producing porous protective layers on sensor elements.

After and / or before sintering the ceramic (method step V7), a thermal shock protective layer 62 can be applied to the entire end region of the sensor element 10 facing the exhaust gas and to the bridge layer 60 using methods known per se (e.g. dipping, spraying, lamination, etc.) (method step V8). The presence of the bridge layer 60 has the effect that the material of the thermal shock protective layer 62 cannot penetrate into the gas access bore 64.

Claims

Expectations

1. Ceramic sensor element for an exhaust gas sensor, the ceramic sensor element (10) being constructed in layers and elongated, the ceramic sensor element (10) having an end region facing the exhaust gas in the longitudinal direction, the ceramic sensor element (10) in its interior having an in Has cavity (30) extending in the direction of the layer, the ceramic sensor element (10) having an electrochemical pump cell (38) which has a first electrode (16) which is exposed to the exhaust gas (100) and which has a second electrode (18) which is arranged in the cavity (30) and which has a solid electrolyte (14) which connects the first electrode (16) to the second electrode (18), the ceramic sensor element (10) in the end region facing the exhaust gas Has gas access bore (64) which extends perpendicular to the layer direction into the ceramic sensor element (10) and connects the cavity (30) with the exhaust gas (100), wherein the ceramic sensor element (10) in the end region facing away from the exhaust gas has a porous thermal shock protective layer (62) which covers the gas inlet hole (64), characterized in that the gas inlet hole (64) has a larger cross-sectional area towards the thermal shock protective layer (62) has than towards the cavity (30).

2. Ceramic sensor element according to claim 1, characterized in that the gas inlet bore (64) towards the thermal shock protective layer (62) has a cross-sectional area which is at least 50 times the value of its cross-sectional area towards the cavity (30).

3. Ceramic sensor element according to claim 1 or 2, characterized in that the gas inlet bore (64) is designed as a stepped bore with a section (64a) facing the cavity (30) and a section (64b) facing the thermal shock protective layer (62) ), the cross section of the stepped bore in the section (64a) facing the cavity (30) being smaller than the cross section of the stepped bore in the section (30b) facing the thermal shock protective layer (62).

4. Ceramic sensor element according to claim 3, characterized in that the quotient of the cross section of the stepped bore in the section (64b) facing the thermal shock protective layer (62) and the cross section of the Stepped bore in the section (64a) facing the cavity (30) is 50 or more.

5. Ceramic sensor element according to claim 3 or 4, characterized in that the stepped bore in the section (64a) facing the cavity (30) has a diameter (d) which is greater than the height (h) of the thermal shock protective layer (62) facing section (64b) of the stepped bore.

6. Ceramic sensor element according to one of the preceding claims, characterized in that the thermal shock protective layer (62) is bulged outward in the region of the gas inlet bore (64).

7. Ceramic sensor element according to one of the preceding claims, characterized in that a bridge layer (60) is arranged between the gas inlet bore (64) and the thermal shock protective layer (62).

8. A method for producing a ceramic sensor element (10) according to one of the preceding claims by means of the following steps:

- Printing of green ceramic foils with printing pastes;

- Introducing a gas inlet bore (64) into an end region of the unsintered stack facing the exhaust gas;

- Separately printing a bridge layer (60) on a transfer film (110);

- Transferring the bridge layer (60) from the transfer film (110) to the unsintered stack, so that the bridge layer (60) covers the gas access hole (64);

Sintering the green stack into a sintered stack;

- Application of a thermal shock protective layer (62) to the end region of the sintered or unsintered stack facing the exhaust gas.

9. The method according to claim 8, characterized in that a cavity former (64h) is filled into the gas access hole (64) before the transfer of the bridge layer (60), so that the cavity former (64h) supports the bridge layer (60) during transfer.

10. The method according to claim 8 or 9, characterized in that the unsintered stack is dried prior to the transfer of the bridge layer (60).

11. The method according to any one of claims 8 to 10, characterized in that the unsintered stack contains a moisture before sintering, so that the

Bridge layer (60) bulges outward during sintering.

12. A method for producing a ceramic sensor element (10) according to one of claims 1 to 7 by means of the following steps: printing green ceramic foils with printing pastes;

- Introducing a gas inlet bore (64) into an end region of the unsintered stack facing the exhaust gas; - Introducing a cavity former (64h) into the gas inlet bore (64), in particular filling the gas inlet bore (64) with a cavity former (64h);

- Overpressure of the gas access bore (64) with a bridge layer (60);

Sintering the green stack into a sintered stack;