Ceramic sensor element for exhaust gas sensor and method for manufacturing the same

Abstract

The invention relates to a ceramic sensor element (10) for an exhaust gas sensor, wherein the ceramic sensor element (10) is constructed in a layered and elongated manner, wherein 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 its interior, wherein the ceramic sensor element (10) has an electrochemical pump unit (38) having a first electrode (16) subjected to the exhaust gas (100), a second electrode (18) arranged in the cavity (30), and a solid electrolyte (14) which connects the first electrode (16) to the exhaust gas (100). 16) is connected to the second electrode (18), wherein the ceramic sensor element (10) has a gas inlet hole (64) in the end area facing the exhaust gas, the gas inlet hole extends perpendicularly to the layer direction into the ceramic sensor element (10) and connects the cavity (30) to the exhaust gas (100), wherein the ceramic sensor element (10) has a porous thermal shock protection layer (62) in the end area facing away from the exhaust gas, the thermal shock protection layer covering the gas inlet hole (64), characterized in that the cross-sectional area of ​​the gas inlet hole (64) toward the thermal shock protection layer (62) is larger than the cross-sectional area toward the cavity (30).

Description

Background Art

[0001] From the prior art DE 10 2014 204 124 A1, a ceramic sensor element for an exhaust gas sensor is known. The ceramic sensor element is constructed in a layered and elongated manner, has an end region facing the exhaust gas in the longitudinal direction, has a cavity extending in the layer direction, and includes an electrochemical pump cell having a first electrode exposed to the exhaust gas and a second electrode arranged in the cavity, and a solid electrolyte connecting the first and second electrodes. The ceramic sensor element has a gas inlet opening in the end region facing the exhaust gas, which extends perpendicularly to the layer direction into the ceramic sensor element, connects the cavity to the exhaust gas, and is covered by a porous thermal shock protection layer.

[0002] In this way, the end region of the ceramic sensor element including the gas inlet opening is protected from thermal shock, ie from damage to the ceramic, which could in principle occur, for example, when water droplets impinge on the heated ceramic.

[0003] The oxygen concentration in the exhaust gas can be measured by pumping the amount of oxygen through the gas inlet opening to the second electrode using an electrochemical pump cell and sensing the resulting pump current. Summary of the Invention

[0004] The present invention is based firstly on the inventors' recognition that the amount of oxygen reaching the second electrode through the gas inlet holes is related not only to the geometry and filling of the gas inlet holes and the cavity but also to the properties of the thermal shock protection layer covering the gas inlet holes.

[0005] The inventors have also recognized that the geometric dimensions of the gas inlet holes and cavities, as well as any porous elements contained therein, such as the properties of the diffusion barrier, can be achieved relatively simply and with high precision using methods known in principle, such as machining or screen printing using a specially adapted printing paste formulation. In contrast, the thermal shock protection layer covering the gas inlet holes, when applied using methods that are economically viable in large-scale production (e.g., dipping, spraying, etc.), is subject to significant fluctuations in its thickness and porosity, and thus its permeability to oxygen. Furthermore, the thermal shock protection layer is particularly exposed during operation of the exhaust gas sensor, which can further affect its properties over its service life.

[0006] These fluctuations in the thermal shock protection layer reduce the accuracy of measuring the oxygen concentration in the exhaust gas using the sensor element.

[0007] According to the present invention, this problem is solved by having the gas inlet openings have a larger cross-sectional area facing the thermal shock protection layer than facing the cavity. The area of ​​the thermal shock protection layer relevant for oxygen transport (diffusion, flow) to the second electrode is essentially determined by the cross-sectional area of ​​the gas inlet openings adjacent thereto. Because this area is large according to the present invention, the associated resistance (diffusion resistance / flow resistance) of the thermal shock protection layer is low.

[0008] The total resistance between the exhaust gas and the second electrode (total diffusion resistance / total flow resistance) is composed of the sum of the resistances of the thermal shock protection layer, the gas inlet holes and the cavity in the upstream part of the second electrode.

[0009] As mentioned above, the resistance of a thermal shock protection layer is typically subject to relative fluctuations caused by manufacturing and / or operation. A reduction in the resistance of the thermal shock protection layer (which is accompanied by a larger cross-section of the gas inlet opening on the side facing the thermal shock protection layer) reduces not only the absolute fluctuations in the total resistance but also the relative fluctuations in the total resistance. This improves the accuracy with which the oxygen concentration in the exhaust gas can be measured.

[0010] An advantageous embodiment of the invention provides that the cross-sectional area of ​​the gas inlet opening facing the thermal shock protection layer is at least 10 times (or at least 50 times, or even at least 100 times) greater than its cross-sectional area facing the cavity. The diameter of the gas inlet opening facing the thermal shock protection layer can be greater than the diameter of the gas inlet opening facing the cavity by a minimum factor of 3, 7, or even 10.

[0011] One advantageous embodiment of the present invention provides that the gas inlet opening is designed as a stepped hole having a section facing the cavity and a section facing the thermal shock protection layer, wherein the cross-section of the stepped hole in the section facing the cavity is smaller than the cross-section of the stepped hole in the section facing the thermal shock protection layer. For example, the cross-section of the stepped hole in the section facing the cavity may be at most 1 / 10, at most 1 / 50, or even at most 1 / 100 of the cross-section of the stepped hole in the section facing the thermal shock protection layer. Similarly, the diameter of the stepped hole in the section facing the cavity may be at most 1 / 3, at most 1 / 7, or even at most 1 / 10 of the diameter of the stepped hole in the section facing the thermal shock protection layer.

[0012] Advantageously, the height of the section of the stepped hole facing the thermal shock protection layer can be small, for example no greater than the diameter of the stepped hole in the section facing the cavity, for example no greater than 0.2 mm. Therefore, the mechanical strength of the sensor element is relatively slightly reduced by introducing the stepped hole.

[0013] If the thermal shock protection layer bulges outward in the region of the gas inlet opening Then its stability in the region covering the gas inlet hole is just improved.In particular, this convex arch prevents the thermal shock protection layer from collapsing into the interior of the gas inlet hole.

[0014] The advantageous effects of the present invention are particularly achieved when the intrusion of material of the thermal shock protection layer into the gas inlet opening is excluded. In order to prevent the intrusion of material of the thermal shock protection layer into the gas inlet opening and to stabilize the thermal shock protection layer during production and on the finished sensor element, it is advantageous to arrange a bridging layer between the gas inlet opening and the thermal shock protection layer, the bridging layer being structurally different from or distinguishable from the thermal shock protection layer.

[0015] A gas inlet opening is understood in principle to mean a geometry that can be produced by a drill (eg a mechanical step drill), but independently of whether the gas inlet opening is actually introduced into the specific sensor element by drilling, milling, countersinking or similar methods.

[0016] Accordingly, the sensor element according to the present invention can be manufactured by the following steps:

[0017] - Printing the green ceramic film with a printing paste;

[0018] - producing an unsintered stack by laminating a plurality of printed green ceramic films in layers;

[0019] - introducing a gas inlet opening into the end region of the green stack facing the exhaust gas;

[0020] - Printing the bridging layer separately onto the transfer film;

[0021] - Drying the bridging layer;

[0022] - transferring the bridging layer from the transfer film to the unsintered stack such that the bridging layer covers the gas inlet holes;

[0023] - sintering the green stack into a sintered stack;

[0024] - Applying a thermal shock protection layer to the end region of the stack facing the exhaust gas.

[0025] Thus, a sintered ceramic sensor element is produced as a sintered stack, onto which the thermal shock protection layer is applied. This transfer technology enables bridging of the gas inlet opening during the production process, so that material of the thermal shock protection layer cannot penetrate into the gas inlet opening.

[0026] On the one hand, the green stack can be sintered first, and then the thermal shock protection layer can be applied to the end region of the sintered stack facing the exhaust gas. Alternatively, the material forming the thermal shock protection layer can also be applied to the green stack. During the subsequent sintering, the green stack transitions into the sintered stack, and the material transitions into the thermal shock protection layer.

[0027] If the gas inlet holes are filled with a cavity-forming component before the bridging layer is transferred, the bridging layer is initially supported on the cavity-forming component and cannot sink into the gas inlet holes. A cavity-forming component is understood to be a material that evaporates without residue during sintering (e.g., above 1100° C.), such as graphite, glassy carbon, theobromine, etc.

[0028] The inventors have also discovered that it is possible to selectively produce camber of the bridging layer and, consequently, of the thermal shock protection layer. If the solvent or water content in the cavities and gas inlet holes, or in the green stack, is selected to be relatively high when the bridging layer is applied to the green stack, then during subsequent heating, for example, during transfer of the bridging layer or at the start of sintering, an increase in the vapor pressure in the cavities and gas inlet holes occurs, leading to camber of the bridging layer or thermal shock protection layer. Drying or degreasing the green stack before transferring the bridging layer to the green stack results in a non-cambered, i.e., flat, bridging layer or thermal shock protection layer. During subsequent heating, for example, at the start of sintering, no or minimal increase in the vapor pressure in the cavities and gas inlet holes, leading to camber of the bridging layer or thermal shock protection layer, occurs.

[0029] As an alternative to the above-mentioned transfer techniques, the sensor element according to the invention can also be produced by printing techniques.

[0030] Especially in the following steps:

[0031] - Printing the green ceramic film with a printing paste;

[0032] - producing an unsintered stack by laminating a plurality of printed green ceramic films in layers;

[0033] - introducing a gas inlet opening into the end region of the green stack facing the exhaust gas;

[0034] - introducing a cavity-forming component into the gas inlet pore, in particular filling the gas inlet pore with the cavity-forming component;

[0035] - Covering the gas inlet holes with a bridging layer;

[0036] - sintering the green stack into a sintered stack;

[0037] - Applying a thermal shock protection layer to the end region of the stack facing the exhaust gas.

[0038] Thus, a sintered ceramic sensor element is also produced as a sintered stack, onto which the thermal shock protection layer is applied. This transfer technology also enables bridging of the gas inlet opening during the production process, so that material of the thermal shock protection layer cannot penetrate into the gas inlet opening.

[0039] Here, on the one hand, the unsintered stack can again be sintered first and the thermal shock protection layer can then be applied to the end region of the sintered stack facing the exhaust gas.

[0040] Alternatively, the material forming the thermal shock protection layer can also be applied to the unsintered stack. During the subsequent sintering, the unsintered stack transitions into the sintered stack and the material transitions into the thermal shock protection layer.

[0041] Introducing the cavity-forming component into the gas inlet hole, in particular filling the gas inlet hole with the cavity-forming component, can be achieved by pressing the cavity-forming component into the gas inlet hole from the outside. Here, the cavity-forming component preferably ends flatly with the outer surface of the unsintered stack. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 shows a sensor element according to the present invention;

[0043] Figure 2 and 3 A first method according to the invention for producing a sensor element according to the invention is shown by way of example;

[0044] Figure 4 and 5 A second method according to the invention for producing the sensor element according to the invention is shown by way of example. DETAILED DESCRIPTION

[0045] Figure 1 A schematic cross section through an end region of a ceramic sensor element 10 according to the invention for an exhaust gas sensor, the end region facing the exhaust gas, is shown.

[0046] The sensor element 10 is of layered and elongated construction, wherein the layer direction is Figure 1 The sensor element 10 extends in one plane from left to right and perpendicular to the drawing plane. The sensor element 10 has a cavity 30 in its interior.

[0047] Sensor element 10 further comprises an electrochemical pump cell 38. This pump cell comprises a first electrode 16, for example, annular, exposed to exhaust gas 100, a second electrode 18, for example annular, arranged in cavity 30, and a solid electrolyte body 14, which connects first electrode 16 and second electrode 18 in an oxygen ion-conducting manner.

[0048] Concentrically with the first electrode 16 and the second electrode 18, a gas inlet opening 64, in this example implemented as a blind stepped hole, passes through the solid electrolyte body 38. From the bottom of the gas inlet opening 64, a porous diffusion barrier 36 branches outward, to which the annular cavity 30 in which the second electrode 18 is arranged in turn adjoins outward.

[0049] The gas inlet holes 64 are covered by a porous bridging layer 60, such as Figure 1 As shown, the bridge layer extends from left to right, for example, over the entire width of the sensor element 10. The porous thermal shock protection layer 62 is arranged on the bridge layer. Figure 1 In the cross section shown, the thermal shock protection layer 62 is arranged on all sides of the sensor element 10. The thickness of the thermal shock protection layer 62 can be greater than the thickness of the bridging layer 60, for example.

[0050] The gas inlet hole 64 is configured as a stepped hole having a section 64a directed toward the cavity 30 (at Figure 1 The section 64b pointing to the thermal shock protection layer 62 (in the middle and lower part) Figure 1 upper center).

[0051] In this example, the diameter d of the section 64 a pointing toward the cavity 30 before (after) sintering is 210 μm (168 μm). In this example, the diameter D of the section 64 b pointing toward the thermal shock protection layer 62 is 2100 μm (1680 μm). In this example, the height h of the section 64 a pointing toward the cavity 30 is 145 μm (116 μm).

[0052] The function of the sensor element 10 is to connect the exhaust gas 100 and the molecular oxygen O2 contained therein via the thermal shock protection layer 62 and via the bridging layer 60 to the gas inlet opening 64 and further to the cavity 30 via the diffusion barrier 36, as shown in FIG. Figure 1 If a sufficient pump voltage is now applied to the electrochemical pump unit 38, the oxygen in the cavity 30 is always released by the oxygen ions O - Depending on the existing partial pressure gradient, oxygen enters (flows / diffuses) from the exhaust gas 100 through the thermal shock protection layer 62, the bridging layer 60, the gas inlet holes 64 and the diffusion barrier 36 into the cavity 30, where it is electrochemically pumped out again.

[0053] Therefore, the generated pump current is directly proportional to the oxygen partial pressure in the exhaust gas 100 and inversely proportional to the resistance (flow / diffusion resistance) along the path of oxygen entering the cavity 30 .

[0054] To accurately measure the oxygen partial pressure in exhaust gas 100, the resistance along the path of oxygen into cavity 30 must have minimal fluctuations due to manufacturing and aging. In this example, this is achieved by manufacturing the diffusion barrier 36 and gas inlet opening 64 with minimal fluctuations and operating with minimal aging effects. In contrast, the bridging layer 60 and thermal shock protection layer 62 are implemented with relatively large effective cross-sectional areas. Therefore, their contribution to the resistance along the path of oxygen into cavity 30 is relatively small relative to the resistance of the diffusion barrier 36 and the resistance of the gas inlet opening 64. Therefore, even if the thermal shock protection layer 62 is subject to fluctuations due to manufacturing and aging, it only makes a small absolute and relative contribution to the fluctuations in the resistance along the path of oxygen into cavity 30.

[0055] Therefore, the sensor element 10 according to the present invention is able to accurately measure the oxygen concentration in the exhaust gas 100 during its service life.

[0056] In one embodiment of the present invention, sensor element 10 according to the present invention is manufactured in such a way that the penetration of material of thermal shock protection layer 62 into gas inlet opening 64 is reliably excluded.

[0057] Reference Figure 1 ,exist Figure 2 and Figure 3 , the individual method steps according to the first example are shown.

[0058] In this example, in a first method step V1, a plurality of green YSZ (yttrium-stabilized zirconia) films are printed using a printing paste. Subsequently (method step V2), the printed films are laminated in layers to form an unsintered stack. Gas inlet holes 64 are introduced into the end region of the unsintered stack facing the exhaust gas 100, for example, using a mechanically rotating step drill (method step V3).

[0059] The gas inlet holes 64 are filled (in method step V4) with a paste containing a cavity-forming component, such as glassy carbon, or with a UV lacquer. The unsintered stack can optionally be dried, depending on whether a convex, domed, or flat covering of the gas inlet holes 64 is desired on the finished sensor element after sintering.

[0060] In parallel (method step V5), the bridging layer 60 is printed separately on the transfer film 110. An ink with ceramic and organic components is used for printing. This ink contains a short-chain binder and the solvent diethylene glycol, which in this way gives the bridging layer 60 increased strength and makes it transferable. Optionally, the strength of the bridging layer 60 can be further increased by drying. In this example, a commercially available product called PLUS membrane.

[0061] The bridging layer 60 is then transferred from the transfer film 110 to the unsintered stack so that it covers the gas inlet openings 64 (method step V6 ). The transfer is accomplished, for example, in a heated press 120 at 80° C. and 30 kN / (200 mm*220 mm).

[0062] After and / or before the ceramic is sintered (method step V7), a thermal shock protection layer 62 can be applied to the entire end region of sensor element 10 facing the exhaust gas and to bridging layer 60 (method step V8) by methods known per se (e.g., dipping, spraying, lamination, etc.). The presence of bridging layer 60 ensures that the material of thermal shock protection layer 62 cannot penetrate into gas inlet openings 64.

[0063] refer to Figure 1 ,exist Figure 4 and Figure 5 , the individual method steps according to the second example are shown.

[0064] In this example, in a first method step V1, a plurality of green YSZ films are printed with a printing paste. Subsequently (method step V2), the printed films are laminated in layers to form an unsintered stack. Gas inlet holes 64 are introduced into the end region of the unsintered stack facing the exhaust gas 100, for example, using a mechanically rotating step drill (method step V3).

[0065] The gas inlet holes 64 are filled (in method step V4) with a paste containing a cavity-forming component, such as glassy carbon, or with a UV lacquer. Alternatively, the gas inlet holes 64 can be completely filled so that the cavity-forming component 64h ends flat with the outer surface of the unsintered stack, for example, by means of a press 120.

[0066] Subsequently (in method step V14 ), the gas inlet openings 64 are bridged by a bridging layer 60 , for example using a printing paste, as is known per se for producing porous protective layers on sensor elements.

[0067] After and / or before the ceramic is sintered (method step V7), a thermal shock protection layer 62 can be applied to the entire end region of sensor element 10 facing the exhaust gas and to bridging layer 60 (method step V8) by methods known per se (e.g., dipping, spraying, lamination, etc.). The presence of bridging layer 60 ensures that the material of thermal shock protection layer 62 cannot penetrate into gas inlet openings 64.

Claims

1. A ceramic sensor element for an exhaust gas sensor, wherein: The ceramic sensor element (10) is constructed in a layered and elongated manner, wherein 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 its interior, wherein the ceramic sensor element (10) has an electrochemical pump unit (38) having a first electrode (16) subjected to the exhaust gas (100), a second electrode (18) arranged in the cavity (30), and a solid electrolyte (14) connecting the first electrode (16) to the second electrode (18), wherein the ceramic sensor element (10) has a gas inlet opening (64) in the end region facing the exhaust gas, which gas inlet opening An inlet hole extends perpendicularly to the layer direction into the ceramic sensor element (10) and connects the cavity (30) to the exhaust gas (100), wherein the ceramic sensor element (10) has a porous thermal shock protection layer (62) in an end region facing away from the exhaust gas, which thermal shock protection layer covers the gas inlet hole (64), characterized in that the cross-sectional area of ​​the gas inlet hole (64) toward the thermal shock protection layer (62) is larger than the cross-sectional area toward the cavity (30), wherein the cross-sectional area of ​​the gas inlet hole (64) toward the thermal shock protection layer (62) is at least 10 times its cross-sectional area toward the cavity (30), and wherein the thermal shock protection layer (62) bulges outward in the region of the gas inlet hole (64).

2. The ceramic sensor element according to claim 1, characterized in that The cross-sectional area of ​​the gas inlet hole (64) toward the thermal shock protection layer (62) is at least 50 times the cross-sectional area of ​​the gas inlet hole (64) toward the cavity (30).

3. The ceramic sensor element according to claim 1 or 2, characterized in that The gas inlet hole (64) is configured as a stepped hole, the stepped hole having a section (64a) directed toward the cavity (30) and a section (64b) directed toward the thermal shock protection layer (62), wherein a cross section of the stepped hole in the section (64a) directed toward the cavity (30) is smaller than a cross section of the stepped hole in the section (30b) directed toward the thermal shock protection layer (62).

4. The ceramic sensor element according to claim 3, characterized in that A quotient of a cross section of the stepped hole in a section (64b) directed toward the thermal shock protection layer (62) and a cross section of the stepped hole in a section (64a) directed toward the cavity (30) is 50 or greater.

5. The ceramic sensor element according to claim 3, characterized in that The diameter (d) of the stepped hole in a section (64a) facing the cavity (30) is greater than the height (h) of the section (64b) of the stepped hole facing the thermal shock protection layer (62).

6. The ceramic sensor element according to any one of claims 1, 2, 4 and 5, characterized in that A bridging layer (60) is arranged between the gas inlet hole (64) and the thermal shock protection layer (62).

7. A method for producing a ceramic sensor element (10) according to any one of claims 1 to 6, comprising: - printing a green ceramic film with a printing paste; - producing an unsintered stack by laminating a plurality of printed green ceramic films in layers; - introducing a gas inlet opening (64) into the end region of the green stack facing the exhaust gas; - printing the bridging layer (60) separately onto the transfer film (110); - transferring the bridging layer (60) from the transfer film (110) to the green stack so that the bridging layer (60) covers the gas inlet holes (64); - sintering the green stack to form a sintered stack; - applying a thermal shock protection layer (62) to the end region of the sintered stack or the unsintered stack facing the exhaust gas.

8. The method according to claim 7, characterized in that Before transferring the bridge layer (60), a cavity-forming component (64h) is filled into the gas inlet hole (64), so that the cavity-forming component (64h) supports the bridge layer (60) during the transfer.

9. The method according to claim 7 or 8, characterized in that Prior to transferring the bridging layer (60), the green stack is dried.

10. The method according to claim 7 or 8, characterized in that The unsintered stack contains moisture before sintering, so that the bridging layer (60) bulges outward during sintering.

11. A method for producing a ceramic sensor element (10) according to any one of claims 1 to 6, comprising: - printing a green ceramic film with a printing paste; - producing an unsintered stack by laminating a plurality of printed green ceramic films in layers; - introducing a gas inlet opening (64) into the end region of the unsintered stack facing the exhaust gas; - introducing a cavity-forming component (64h) into the gas inlet opening (64); - bridging the gas inlet holes (64) with a bridging layer (60); - sintering the unsintered stack into a sintered stack; - applying a thermal shock protection layer (62) to the end region of the sintered stack or the unsintered stack facing the exhaust gas.

12. The method according to claim 11, characterized in that The cavity-forming component (64h) is introduced into the gas entry hole (64) by filling the gas entry hole (64) with the cavity-forming component (64h).

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