METHOD FOR MANUFACTURING A SENSOR ELEMENT FOR AN EXHAUST AIR SENSOR

Description

State of the art

[0001] From the prior art, for example from DE102013211796 A1, a planar ceramic sensor element for an exhaust gas sensor for determining a physical property of an exhaust gas is already known, comprising a first and a second ceramic foil which are arranged on top of one another and connected to one another by lamination, wherein the first of the ceramic foils is provided with a functional structure on its large surface facing the second ceramic foil.

[0002] Ceramic sensor elements and their production are also known from DE 41 43 539 C2, DE 29 20 268 A1, DE 199 37 163 A1, DE 42 17 605 A1, DE 101 22 27 A1 and US 2017 / 0122 898 A1. Disclosure of the invention

[0003] The present invention with the features of independent claim 1 is based on the finding that, in connection with the technology explained above, leaks can occur in the area between the ceramic foils. Such leaks can result, for example, from the functional structure having a non-constant height or from the functional structure extending only along part of the large surface. A gap then preferably forms next to the functional surface or in areas where the height of the functional structure is reduced.

[0004] The inventors' investigations have shown that such leaks are problematic because they are correlated with malfunctions or early failures of the exhaust gas sensor.

[0005] By partially or completely enclosing the functional structure with an embedding layer, such gaps are filled and leaks eliminated. The functionality and longevity of the exhaust gas sensors are ensured.

[0006] In the present context, a functional structure of a ceramic sensor element of an exhaust gas sensor is in particular a structure that can be distinguished from the ceramic base material of the sensor element and that directly serves the function of determining the physical property of the exhaust gas or is at least indirectly related to this determination.In particular, the functional structure can be one of the following objects and / or comprise several of the following objects, wherein the objects can in turn be arranged next to one another and / or one above the other without further restriction: an electrical conductor track, for example made of a material containing a noble metal, a flat electrode, for example made of a material containing a noble metal and a metal oxide, a cavity, a space filled with a porous material through which flow can pass, for example containing a metal oxide, an insulation layer, for example made of a material containing a metal oxide, separating webs, barrier webs, sealing frames, for example made of a material containing a metal oxide.

[0007] According to the invention, the functional structure is partially or completely surrounded by an embedding layer. The embedding layer is thus arranged at least below and / or above and next to the functional structure. The functional structure is embedded in the embedding layer, i.e., the embedding layer not only partially or completely covers a top and / or bottom side of the functional structure, but at the same time, the embedding layer also borders the functional structure laterally, in particular along the entire lateral outer circumference of the functional structure.

[0008] In particular, the embedding layer fills gaps that would otherwise exist between the ceramic foils, between a ceramic foil and the functional structure, within the functional structure, or between different functional structures.

[0009] It is preferred that the embedding layer has an average layer thickness of not less than 5 µm, preferably not less than 10 µm. The average layer thickness is preferably not greater than 25 µm.

[0010] An average layer thickness is understood here as a value that results from averaging the local height of the embedding layer over the areal extent of the embedding layer, in particular over the entire areal extent of the embedding layer. The average layer thickness can also be determined, in particular, by averaging over the entire large area of ​​the ceramic foil on which the embedding layer is arranged.

[0011] The embedding layer preferably consists of a material that differs only slightly from the base material of the ceramic foils.

[0012] According to the invention, the embedding layer consists of >50 wt% YSZ.

[0013] The printing of at least one of the large areas of the first ceramic green film with at least one functional structure can also provide that one or both of the large areas of the first and / or the second and / or further ceramic films are printed with one functional structure or with several functional structures.

[0014] For this purpose, the printing of at least one large surface of the first ceramic green sheets with an embedding paste can be performed either later or earlier. It can also be provided that both large surfaces of the first and / or the second and / or further ceramic green sheets are each printed with an embedding paste, in particular over the entire surface, in particular all those large surfaces of the green sheets that are directly opposite one another during subsequent lamination of the ceramic green sheets.

[0015] It is preferred that printing of the at least one large area of ​​the first ceramic green foils with an embedding paste takes place in at least two, preferably even at least three printing steps in which printing layers arranged one on top of the other are produced.

[0016] The printing of at least one large area of ​​the first ceramic green film can be carried out over the entire surface. This can be done, for example, by screen printing.

[0017] According to the invention, an embedding paste is used whose solids content is 55-75% by weight. It can, for example, be 60-70%.

[0018] In particular, a screen printing paste is used which comprises 55 - 75 wt.% (in particular 60 - 70%) of an oxide ceramic solid, and whose rheological behavior is determined in particular by a dynamic viscosity of 20 - 60 Pa s (in particular 30 - 50 Pa s) at a shear rate of γ = 30 1 / s and / or by a tan delta value: 10 - 20 at w = 1 rad / s.

[0019] Such screen printing pastes, although still printable, are nevertheless suitable for reliably closing potential gaps in the adjacent structure. In contrast to previously known screen printing pastes, this is - figuratively speaking - a ceramic filler that flows during printing, thus compensating for unevenness and height differences, and evenly transmitting the contact pressure required for the lamination process.

[0020] In addition to the behavior of this screen printing paste during printing (i.e. in particular the flowability vs. edge stiffness under shear stress during squeegee pull / screen passage), the behavior during a lamination process (i.e. flowability of the partially dried paste in the layer composite due to pressure load in the lamination press) is of great importance in the application described here.

[0021] For this reason, the binder / solvent system of the paste was also adapted so that the dried paste retains residual flowability. By increasing the binder and plasticizer content, reducing the solvent content, and simultaneously selecting a solvent with a low vapor pressure, favorable residual flowability is ensured in this process step. Short description of the drawing

[0022] The Figures 1aand 2a each show schematically a cross section through a sensor element not manufactured according to the invention.

[0023] The Figure 1b and 2b each show schematically a cross section through a sensor element manufactured according to the invention.

[0024] Figure 3 shows schematically the method according to the invention. Embodiments

[0025] In Figure 1aA cross-sectional view of the basic structure of a sensor element 10 of a lambda probe is shown. The ceramic sensor element 10 comprises a first and a second ceramic foil 21, 22 made of YSZ, which are arranged one on top of the other and bonded together by lamination. Arranged on these foils are: a first electrode 23a and a second electrode 23b, which are part of an electrochemical cell, a separating web 23c made of YSZ, a porous region 23d, and an insulating region 23e. As can be seen, in this sensor element there is a horizontal gap 30 between the separating web 23c and the first solid electrolyte film 21 as well as a vertical joint 31 between the separating web 23c and the insulating region 23e or between the separating web 23c and the first electrode 21. Such gaps 30 and joints 31 have so far been unavoidable due to inaccuracies in the manufacture of the functional structures 23.

[0026] In contrast, the Figure 1ba sensor element 10 manufactured according to the invention. In contrast to the sensor element 10 described above, this has an embedding layer 24 which completely fills the potential gaps 30, 31 between the separating web 23c and the insulating region 23e or between the separating web 23c and the first electrode 21 in the example.

[0027] Figure 2a shows a cross-sectional view of the basic structure of another sensor element 10 of a lambda sensor. The ceramic sensor element 10 comprises a first and a second ceramic foil 21, 22 made of YSZ, which are arranged one on top of the other and bonded together by lamination. Arranged on these foils 21, 22 are: a first electrode 23a and a second electrode 23b, which are part of an electrochemical cell, an oxygen-ion-conducting layer 23f, a separating web 23c made of YSZ, a porous region 23d, and an insulating region 23e. As can be seen, in this sensor element 10 there is a horizontal gap 30 between the separating web 23c and the first solid electrolyte foil 21, which was previously unavoidable due to inaccuracies in the manufacture of the functional structures 23.

[0028] In contrast, Figure 2b shows a sensor element 10 according to the invention. In contrast to the sensor element 10 described above, this one has an embedding layer 24 arranged over its entire surface in a layer plane below the first electrode 23a and above the second ceramic foil 22. The potential gap 30 between the separating web 23c and the first electrode 23a is completely filled in the example.

[0029] The method according to the invention, with which, for example, the sensor elements 10 shown in Figures 2a and 2b can be produced, is shown schematically in Figure 3. In a first method step 101, a first and a second ceramic green film are provided and, in a second method step 102, a functional structure is printed on each large surface, which may include, for example, electrodes, insulation, porous layers, a separating web, and so on. In a third method step, before or after the second method step 102, an embedding paste is printed over the entire surface or only along a portion of the green films provided with functional structures or onto the large surfaces of the green films to be provided with functional structures.In a fourth process step 104, the ceramic green sheets are laminated together such that the large surfaces provided with functional structures and embedding paste lie on top of each other. In a fifth process step 105, the ceramic sensor element 10 is completed by sintering. It is then ready for further installation in an exhaust gas sensor.

Claims

1. Method of producing a planar ceramic sensor element (10) for an exhaust gas sensor for determination of a physical property of an exhaust gas, comprising at least a first ceramic film (21) and a second ceramic film (22) that have been laminated onto and connected to one another, wherein the first ceramic film (21) has been provided with a functional structure (23) over its large area facing the second ceramic film (22), wherein the functional structure (23) is partly or fully surrounded by an embedding paste (24), wherein the functional structure (23) is disposed on the films (22, 21), wherein the functional structure (23) consists of a first electrode (23a) and a second electrode (23b) that are part of an electrochemical cell, a separation pad (23c) composed of YSZ, a porous region (23d) and an insulating region (23e), with complete filling of a horizontal gap (30) between the separation pad (23c) and the first solid electrolyte film (21) and of a vertical join (31) between the separation pad (23c) and the insulating region (23e) and / or between the separation pad (23c) and the first electrode (23a) by the embedding paste (24), wherein the method involves the following steps: - providing (101) at least a first and a second ceramic green film (21, 22); - printing (102) at least one of the large areas of the first ceramic green film with at least one functional structure (23); - printing (103) the at least one large area of the first ceramic green films with an embedding paste, where the solids content of the embedding paste is 55-75% by weight of YSZ, - co-laminating (104) the first ceramic green film (21) and the second ceramic green film (22) such that the at least one large area of the first ceramic green film faces the second ceramic green film and - sintering (104) the co-laminated green films.

2. Method of producing a sensor element (10) according to Claim 1, characterized in that the embedding layer (24) has at least an average layer thickness of at least 5 µm, preferably at least 15 µm.

3. Method of producing a sensor element (10) according to Claim 1 or 2, characterized in that the printing (103) of the at least one large area of the first ceramic green films with the embedding paste creates a layer having an average layer thickness before sintering of at least 10 µm, in particular at least 20 µm, and at most 50 µm, for example at most 40 µm.

4. Method of producing a sensor element (10) according to any of Claims 1 to 3, characterized in that the printing is effected in at least 2, preferably in at least 3, superposed layers.

5. Method of producing a sensor element (10) according to any of Claims 1 to 4, characterized in that the embedding paste has the following rheological characteristics: - dynamic viscosity: 20-60 Pa s at a shear rate of γ = 30 1 / s - edge stiffness: tan delta: 10-20 at ω = 1 rad / s)