ELECTRICAL FEEDTHROUGH WITH POROUS CERAMIC LAYER AND A PORE FILLER

Description

Technical field

[0001] The invention relates to a feedthrough for an electrical conductor for electrically connecting an electrically heatable heating disk, particularly in an exhaust system of an internal combustion engine, through a housing. The feedthrough comprises an inner conductor, an outer sleeve, and at least one insulating means. The insulating means is arranged between the inner conductor and the outer sleeve such that the inner conductor is electrically insulated from the outer sleeve. Furthermore, the invention relates to a method for producing a feedthrough. State of the art

[0002] Electric heating elements are now routinely used to heat exhaust gases in an exhaust system downstream of a combustion engine. The goal is to more quickly reach a temperature threshold at which effective conversion of the pollutants carried in the exhaust gas can occur. This is necessary because the catalytically active surfaces of the catalysts used for exhaust gas aftertreatment only allow sufficient conversion of the respective pollutants above a minimum temperature, the so-called light-off temperature.

[0003] Known solutions in the prior art include so-called heating catalysts, which have a metallic structure connected to a voltage source and which can be heated by utilizing the ohmic resistance (see e.g. US2015 / 011115A1).

[0004] For the purpose of electrically connecting the heatable structure, an electrical conductor must be routed through the exhaust system housing at least once. It must be ensured that the feedthrough is gas-tight, that electrical insulation is provided between the housing and the electrical conductor, and that sufficient durability is guaranteed.

[0005] Electrical feedthroughs are known in the prior art that have a conical inner conductor, which is partially coated, for example, with a porous ceramic layer. The porous coating is followed by a metallic sleeve with a conical inner cross-section. The inner conductor serves to contact the structure to be heated, while the porous layer provides electrical insulation. The metallic sleeve ultimately serves to fix the feedthrough in the housing, but the sleeve is electrically insulated from the inner conductor.

[0006] A particular disadvantage of the prior art devices is that the electrical insulation effect is insufficient as soon as the inner conductor is subjected to a higher voltage and the feedthrough through the housing is exposed to a humid, salty atmosphere. The porous ceramic used has a tendency to absorb saline solutions from the area surrounding the feedthrough. This creates an electrolytic bridge through the ceramic layer, and electrical insulation is no longer guaranteed.

[0007] Organic sealants are known to be used to reduce the porosity of the ceramic layer. However, these are not durable enough under the stresses encountered. Alternative inorganic sealants regularly lead to cracking under thermal stress due to different thermal expansion coefficients. Description of the invention, task, solution, advantages

[0008] Therefore, it is the object of the present invention to provide a feedthrough for at least one electrical conductor through the housing of an exhaust gas line, which has a durable ceramic layer for insulating the inner conductor from an outer sleeve or the housing.

[0009] A preferred embodiment of the invention can be used in the exhaust aftertreatment of an internal combustion engine in an automotive application. The invention can also be used in stationary systems, such as power generators.

[0010] The problem with regard to the implementation is solved by an implementation having the features of claim 1.

[0011] An embodiment of the invention relates to a feedthrough for an electrical conductor for the electrical connection of an electrically heatable heating disc, in particular in an exhaust system of an internal combustion engine, through a housing, wherein the feedthrough has an inner conductor, an outer sleeve and at least one insulating means, wherein the insulating means is arranged between the inner conductor and the outer sleeve such that the inner conductor is electrically insulated from the outer sleeve, wherein the insulating means is formed by a porous ceramic layer, wherein pores of the ceramic layer are at least partially filled with a pore filler.

[0012] The ceramic layer used as an insulation material has a certain degree of porosity due to the material used. This can be larger or smaller depending on the ceramic used, so that the pore sizes or the average pore size are larger or smaller. The pores in the ceramic, if they are unfilled or filled with air, make the ceramic layer less stable when exposed to alternating thermal stresses. Furthermore, the pores cause the ceramic layer to become saturated or saturated with a liquid medium, such as a saline solution. This can create conductive bridges that destroy the electrical insulating effect of the ceramic layer. Furthermore, structural damage to the ceramic layer can occur, for example due to erosion.

[0013] Therefore, the insulation material is preferably coated with a pore filler, whereby the pore filler settles into the pores of the ceramic layer and thus completely or at least partially fills the pores. The filled pores can no longer absorb the saline solution, preventing the formation of conductive bridges. Furthermore, the pore filler makes the ceramic layer significantly more resistant to alternating thermal stresses, thus improving its durability.

[0014] It is particularly advantageous if the pore filler has a thermal expansion coefficient which corresponds to the joint thermal expansion coefficient of the inner conductor and the unfilled porous ceramic layer.

[0015] This is advantageous for achieving a structure that is as homogeneous as possible in terms of its respective thermal expansion coefficients. Significantly different thermal expansion coefficients in neighboring structures can lead to significantly faster structural damage, especially under alternating thermal loading, because individual structures expand to different degrees under the influence of heat. These different expansions create stresses at the boundary layers, which can lead to cracks or fractures in the material.

[0016] The term "common coefficient of thermal expansion" refers to a coefficient of thermal expansion that is as similar as possible to the two individual coefficients of thermal expansion of the inner conductor material and the ceramic layer material. This is intended to avoid excessive differences in the thermal expansion coefficients of the three elements. In particular, the difference between the thermal expansion coefficients of the pore filler and the ceramic layer should be as similar as possible to avoid generating large stresses in the boundary layer area between the pore filler in the pores of the ceramic layer and the ceramic layer, which could lead to the ceramic layer cracking from within.

[0017] It is also advantageous if the pore filler is formed by nanoparticles that are filled into the pores of the porous ceramic layer. Nanoparticles are characterized by their small size. This is particularly advantageous because the average pore size of the ceramic layers commonly used is particularly small, so a very fine material must be used to partially or completely fill the pores. Nanoparticles preferably have an average size of 1 to 100 nanometers.

[0018] A preferred embodiment is characterized in that the size of the nanoparticles depends on the average pore size distribution of the ceramic layer, wherein the nanoparticles are 10% to 80% smaller than the average pore size of the ceramic layer.

[0019] The size difference between the average pore size and the average size of the nanoparticles ensures that the nanoparticles can easily and simply penetrate the pores of the ceramic layer and accumulate there. A smaller size difference could also lead to the nanoparticles jamming against the pore walls, preventing them from being sufficiently filled. Significantly smaller nanoparticles are therefore advantageous for optimally filling the pores.

[0020] Furthermore, it is advantageous if the porous ceramic layer is formed from at least two layers. Two layers can, in particular, have different material properties, whereby, for example, a boundary layer on the inner conductor has different material properties than the boundary layer on the outer sleeve. This can prevent the occurrence of stresses or damage. In particular, the two layers can have different thermal expansion coefficients or different pore sizes.

[0021] A difference in pore size can, for example, be used to ensure that the pores fill with the pore filler to varying degrees. The filler level in the pores can further influence the properties of the individual layers. The overall thermal expansion coefficient of one layer, resulting from the combination of the base material and the pore filler, can thus differ from that of the other layer or be close to it.

[0022] Furthermore, it is advantageous if the layers have the same thermal expansion coefficients. Identical or similar thermal expansion coefficients are advantageous in order to produce a material that is as homogeneous as possible and, in particular, to avoid the formation of stress-induced cracks or fractures.

[0023] It is also preferable if the ceramic layer is formed by a plasma sprayed ceramic, which is thermostable up to 1200 degrees Celsius.

[0024] The problem with regard to the method is solved by a method having the features of claim 7.

[0025] An embodiment of the invention relates to a method for producing a feedthrough according to one of the preceding claims, wherein the porous ceramic layer is exposed to the pore filler in several successive passes, the degree of filling of the pores being increased with each pass.

[0026] Filling or saturating the ceramic layer with the pore filler can preferably take place in several consecutive passes, with each pass allowing the pore filler to be embedded in the pores of the ceramic layer. Repeating the application can, in particular, increase the degree of pore filling, since after the initial deposition of a nanoparticle, additional nanoparticles can be deposited in the next pass. This allows a more homogeneous layer to be created overall, and the material properties created by the pore filler can be specifically enhanced to a desired degree.

[0027] Furthermore, it is advantageous if the average size of the nanoparticles used is reduced from pass to pass. Reducing the average size of the nanoparticles used from pass to pass can also improve the filling level of the individual pores. The pores occupied by a first nanoparticle have a smaller internal volume than the unoccupied pores. By using significantly smaller nanoparticles, even these reduced volumes can be filled in subsequent passes.

[0028] It is also advisable to subject the feedthrough with the porous ceramic layer filled with pore filler to a sintering process. The sintering process serves to solidify the ceramic layer created by filling it with nanoparticles.

[0029] Advantageous further developments of the present invention are described in the subclaims.

Claims

1. Feedthrough for an electrical conductor for electrical connection of an electrically heatable heating disk, especially in an exhaust gas system of an internal combustion engine, through a housing, where the feedthrough has an internal conductor, an outer sleeve and at least one insulation means, where the insulation means is disposed between the internal conductor and the outer sleeve such that the internal conductor is electrically insulated from the outer sleeve, characterized in that the insulation means is formed by a porous ceramic layer, where pores in the ceramic layer are at least partly filled by a pore filler.

2. Feedthrough according to Claim 1, characterized in that the pore filler has a coefficient of thermal expansion corresponding to the common coefficient of thermal expansion of the internal conductor and the unfilled porous ceramic layer.

3. Feedthrough according to either of the preceding claims, characterized in that the pore filler is formed by nanoparticles introduced into the pores of the porous ceramic layer.

4. Feedthrough according to Claim 3, characterized in that the size of the nanoparticles is dependent on the average pore size distribution of the ceramic layer, where the nanoparticles are 10% to 80% smaller than the average pore size of the ceramic layer.

5. Feedthrough according to any of the preceding claims, characterized in that the porous ceramic layer is formed from at least two layers.

6. Feedthrough according to Claim 5, characterized in that the two layers have different porosities.

7. Feedthrough according to either of the preceding Claims 5 and 6, characterized in that the layers have equal coefficients of thermal expansion.

8. Feedthrough according to any of the preceding claims, characterized in that the ceramic layer is formed by a plasma-sprayed ceramic which is thermally stable up to 1200 degrees Celsius.

9. Process for producing a feedthrough according to any of the preceding claims, characterized in that the porous ceramic layer is contacted with the pore filler in two or more successive passes, increasing the filling level of the pores with each pass.

10. Process according to Claim 8, characterized in that the average size of the nanoparticles used is reduced from pass to pass.

11. Process according to either of the preceding Claims 8 and 9, characterized in that the feedthrough with the porous ceramic layer filled with pore filler is subjected to a sintering process.