DE102022119488A1 - Method for producing a heating element made of metal foam, in particular for an exhaust gas heater, heating element and exhaust gas heater - Google Patents

Abstract

The invention relates to a method for producing a heating element (14) made of metal foam, in particular for an exhaust gas heating device (10), by means of the following steps:
a base element (1) made of a foam is provided, in particular made of a PU foam,
the base element (1) is locally compressed, resulting in a higher density of cell connections,
the base element (1) with a metal coating applied to it is sintered.
The invention also relates to a heating element (14) made of metal foam, in particular for an exhaust gas heating device (10), wherein it has sections produced by plastic deformation in which the density of the electrical conductor tracks (S) is higher than in undeformed sections. The invention finally relates to an exhaust gas heating device (10) with such a heating element (14).

Description

The invention relates to a method for producing a heating element made of metal foam, in particular for an exhaust gas heating device. The invention further relates to such a heating element and an exhaust gas heating device with such a heating element.

Heating elements for exhaust gas heaters are known from the prior art. An example can be found in the DE 10 2020 130 657 .

In general terms, the heating element is geometrically designed in such a way that a current conduction path is defined between two electrical connections. When a voltage is applied to the two electrical terminals, a current flows through the metal foam. Due to the electrical resistance that opposes the current flow, the metal foam heats up. The heat generated in the metal foam can be transferred to an exhaust stream that comes from an internal combustion engine. In this way, the exhaust gas stream is heated in order, for example, to allow a cleaning device arranged downstream of the exhaust gas heating device to respond more quickly. The cleaning device can be a catalyst. In addition, the metal foam itself can be used as an active catalytic element because it can have a catalytically active coating.

For some applications it is desirable to mechanically support the heating element at various points, for example to prevent damage from vibrations or the heating element from resonance. Small insulators are used for support, which are passed through recesses in the heating element and then attached to a support bridge. The recesses in the metal foam can be created, for example, by laser or water jet cutting.

It has been found that the temperature distribution in the heating element is uneven in that temperature peaks occur in the area of the mechanical supports.

The object of the invention is to improve the heating element in such a way that a more uniform temperature distribution results and heat concentrations are avoided, since these significantly limit the function of the component

To solve this problem, according to the invention a method for producing a heating element made of metal foam is provided, in particular for an exhaust gas heating device, wherein in a first step a base element made of a foam is provided, in particular made of a PU foam. The base element is locally compressed, resulting in a higher density of cell connections. Furthermore, the base element is sintered with a metal coating applied to it. The invention is based on the knowledge that the temperature peaks of the heating elements examined are due to the fact that there is less conductive material available in the area of the mechanical supports than in the areas in which there are no recesses for the mechanical supports. In other words: By cutting out the recesses for the mechanical supports, conductive material is removed, so that more current per cross-sectional unit of material has to flow through the remaining material. Due to the higher current density, more heat is generated. The invention counteracts this effect in that the recesses are no longer cut out, but are obtained through plastic deformation. Accordingly, no conductive material is removed, but only displaced “laterally” so that, for example, the recess is created. As a result, just as much material is available on both sides of the recess as in the adjacent sections of the heating element, so that the overall electrical resistance in the area of the recesses is no higher than “in front of” and “behind” the corresponding recess. Accordingly, the heating foam does not offer any higher resistance to the current flow in the area of the recess, so that there is no temperature peak there.

The idea according to the invention can also be extended to designs in which a higher density or current conductivity of the heating element is desired for other reasons, for example in order to electrically contact the heating element. For this purpose, it can be provided that the base element has an oversize in the area that is locally compressed and is compressed to the target size using a tool. In this way, a higher current conductivity can be generated with very little effort, as would otherwise have to be produced in a complex manner using methods known from the prior art in that the metal foam is impregnated with different materials or has to be coated several times.

In the embodiment in which local recesses are to be provided, the base element is preferably pre-cut in the area that is locally compressed, with the pre-opening created by the pre-cutting then being expanded using a tool. When pre-cutting, the pre-opening becomes just that far enough that a spreading tool can be inserted into the pre-opening. As a result, the current conduction paths specified by the carrier material and the coating remain essentially undamaged.

Preferably, the base element is impregnated with a liquid metal before it is locally compressed. This is advantageous in that the first process steps can then be carried out in the same way as in known processes for producing heating elements made of metal foam. In principle, however, it is also conceivable that the base element is first locally deformed and then impregnated with the liquid metal.

According to a preferred embodiment of the invention, it is provided that excess liquid material is removed from the base element after the base element has been locally compressed and while the tool continues to interact with the base element. During this step, the tool remains on or in the base element so that the locally deformed areas cannot spring back.

In order to remove the excess liquid material from the base element, it has proven to be particularly expedient for the base element to be centrifuged. This step can be carried out comparatively easily even if the tool remains on or in the base element.

Preferably, the tool is removed after drying and before sintering so that it can be used again in the production of further heating elements. Alternatively, it would also be conceivable to use tools that are destroyed during sintering, similar to a lost mold in casting processes.

To solve the above-mentioned problem, a heating element made of metal foam is also provided, in particular for an exhaust gas heating device, wherein the heating element can be produced using a method of the type explained above. The heating element has sections created by plastic deformation in which the density of the electrical conductor tracks is higher than in undeformed sections. With regard to the resulting advantages, reference is made to the explanations above.

As an alternative to plastic deformation of the base elements before sintering, the sections produced by plastic deformation can also be produced by sintering the base element in a conventional manner and then locally plastically deforming them in order to obtain areas with a higher cell density.

The sections produced by plastic deformation can be arranged on an outside of the heating element, for example in order to be used for electrical contacting of the heating element.

The sections used for contacting and created by plastic deformation can be designed to run circumferentially or only in sections, particularly in the area of the electrical contacts.

The sections created by plastic deformation may also surround an opening intended to receive a support element. The higher density of the electrical conductor tracks in the area of the opening ensures that no temperature peaks occur there.

• - 1 in einer schematischen perspektivischen Ansicht einer Abgasheizvorrichtung mit einem Heizelement;
• - 2 einen Querschnitt durch einen Teil eines erfindungsgemäßen Heizelements, in dem ein Abstützelement angebracht ist;
• - 3 in einer schematischen, vergrößerten Ansicht einen Abschnitt eines erfindungsgemäßen Heizelements im Bereich einer Öffnung zur Aufnahme eines Abstützelements;
• - 4 eine Fotografie des Basiselements in einem Ausschnitt ähnlich 3;
• - 5 einen Ausschnitt von 4 in vergrößertem Maßstab;
• - 6 einen Abschnitt eines Heizelements aus dem Stand der Technik entsprechend der Darstellung von 4;
• - 7 eine Fotografie des Basiselements des Heizelements von 6;
• - die 8a bis 8c zwei Schritte bei der Herstellung eines erfindungsgemäßen Heizelements;
• - die 9a bis 9d in jeweils einer Seitenansicht und einer Draufsicht die Schritte des Erzeugens von lokal komprimierten Bereichen bei einem Basiselement zur Herstellung eines Heizelements gemäß einer ersten Ausführungsform;
• - die 10a bis 10c in einer schematischen Draufsicht weitere Schritte bei der Herstellung eines Heizelements;
• - die 11a bis 11c in einer schematischen Draufsicht verschiedene Schritte bei der Herstellung eines Heizelements gemäß einer weiteren Ausführungsform; und
• - die 12a und 12b in jeweils einer schematischen Draufsicht und einer Schnittansicht verschiedene Schritte der Bearbeitung eines Basiselements zur Herstellung eines Heizelement gemäß einer dritten Ausführungsform.

The invention is described below using various embodiments, which are shown in the accompanying drawings. Show in these:

• - 1 in a schematic perspective view of an exhaust gas heating device with a heating element;
• - 2 a cross section through a part of a heating element according to the invention, in which a support element is attached;
• - 3 in a schematic, enlarged view a section of a heating element according to the invention in the area of an opening for receiving a support element;
• - 4 a photograph of the base element in a similar detail 3 ;
• - 5 a section of 4 on an enlarged scale;
• - 6 a section of a heating element from the prior art as shown in 4 ;
• - 7 a photograph of the base element of the heating element of 6 ;
• - the 8a until 8c two steps in the production of a heating element according to the invention;
• - the 9a until 9d in each case a side view and a top view, the steps of producing locally compressed areas in a base element for producing a heating element according to a first embodiment;
• - the 10a until 10c in a schematic top view further steps in the production of a heating element;
• - the 11a until 11c in a schematic top view various steps in the production of a heating element according to a further embodiment; and
• - the 12a and 12b in each case a schematic top view and a sectional view different steps of processing a base element for producing a heating element according to a third embodiment.

In 1 1, an exhaust gas heater 10 is shown, which is intended to be arranged in an exhaust system for an internal combustion engine, in particular upstream of an exhaust gas treatment device, for example upstream of a catalytic converter.

The exhaust gas heating device 10 has a holding frame 12 in which a heating element 14 is arranged. This consists of a metal foam that is porous so that exhaust gas can flow through it. The metal foam is particularly open-pored.

The heating element 14 is divided into webs 16 that are electrically insulated from one another and are electrically connected in series in such a way that a current conduction path results from an electrical connection 18 to a second electrical connection 20. When a voltage is applied to the connections 18, 20, a current flows through the entire heating element 14.

The geometric shape of the webs 16 is no longer relevant here; Instead of straight bars, curved bars can also be used.

The exhaust gas heating device 10 has a support bridge 22 on which a plurality of support elements 24 are arranged, which serve to mechanically support the heating element 14.

Each support element 24 extends through an opening 26 (see also 2 , which shows a section through a part of the heating element 14 at the level of a support element 24).

In 3 a section of the heating element 14 is shown in the area of the opening 26 for a support element. The two webs 16 can be seen, which extend parallel to one another at a small distance. The opening 26 is located between the two webs. A special feature of the heating element 14 is that the opening 26 was created by moving the heating element in the direction of the arrows P (see 4 ) was locally plastically deformed so that the opening 26 was created.

The open-pore metal foam from which the heating element 14 consists can be imagined as a multitude of current conduction paths that are formed by the cell walls of the metal foam. Accordingly, there is a specific density of current conduction paths in the heating element in the areas that have not been deformed. In the area of the opening 26, the specific density is higher because the same amount of metal foam is present as in the undeformed sections; the metal foam just has a higher density. This is in 3 symbolized by the lines S, which indicate the current conduction paths in the respective web 16. The density of the current conduction paths S is greater in the area of the opening 26 than in the undeformed areas of the webs 16, which lie “in front of” or “behind” the opening 26 in the current flow direction.

Due to the higher density of the current conduction paths S in the area of the opening 26, it is ensured that the webs 16 do not provide a higher electrical resistance to the current flow there than in front of or behind the opening 26. This prevents local temperature peaks from occurring.

By locally compressing the material of the heating element 14, the density of cell connections is higher in the compressed areas than in the non-compressed areas. This is in 5 to recognize. Area I, which represents the edge of the opening 26 and has been compressed, has a higher density of cell connections than area II, which is remote from the compressed sections.

Another advantage of the locally compressed areas is a higher mechanical strength of the heating element 14 where the support elements 24 attack the heating element 14.

It should be emphasized that the change in the density of cell connections does not occur suddenly, but rather there is a seamless transition from the areas in which the density is highest (i.e. where the material of the heating element 14 was acted upon) to the undeformed ones Areas in which the density of cell connections corresponds to the initial state.

In the 6 and 7 is according to the representation of the 3 and 4 a heating element from the prior art is shown. The opening 26 is created by laser or water jet cutting. Since the material has been removed there, the density of cell connections is immediately adjacent at the edge of the opening 26 just as high as in the rest of the heating element 14.

Based on 8th until 10 The following describes the production of a first embodiment of a heating element 14 for the exhaust gas heating device 10 1 explained.

First, a base element 1, in particular a PU foam, is soaked with a liquid metal material 2 and arranged on a base 3 (see 8a) .

A tool 4 is then used, which creates the openings 26 in the base element 1 soaked in the liquid metal. This will be in the direction of the arrows in the 8a , 8b pressed into the base element 1.

The tool 4 has several expanding or expanding mandrels 5, each of which is provided with a cutting edge 6 (see in 8a expanding mandrel 5 shown enlarged. The expanding mandrels 5 are pressed into the base material.

First, an incision 7 is created by the cutting edges 6 (see 9b below). Then the expanding mandrels 5 create a pre-opening 8, which is expanded into a complete opening, which is surrounded by a ring 9 made of compressed material (see 9d below).

After the tool 4 is pressed into the base element 1, excess metal material is removed from the base element. For this purpose, the base element 1 can be centrifuged together with the tool 4. It is recommended to leave the tool in the base element in this process step, otherwise the openings created would close again (at least largely) due to the restoring forces acting.

In a next step, the coated base element 1 is dried. When dry, the tool 4 can be removed without the risk of the openings in the base element 1 closing again. The base element 1 coated with the metal material 2 is in 10a to see.

The dried base element (the “green body”) is then sintered, whereby the workpiece shrinks overall (compare 10a and 10b) . The sintering shrinkage, which is in the order of 15% to 20%, is taken into account when dimensioning the expanding mandrels and their arrangement on the tool 4 in order to obtain a finished heating element with the desired dimensions.

After sintering, slots 17 are cut to define sections 16.

In the 11a until 11c a coated base element 1, 2 can be seen, with which a heating element according to a second embodiment can be produced. The same reference numbers are used for the components and features known from the first embodiment, and reference is made to the above explanations.

The essential difference between the first and second embodiments is that in the second embodiment, a locally compressed region 30 is created at the points where the current path is later redirected from one section 16 to the next. For this purpose, asymmetrical expanding mandrels (not shown here) are used, which displace the material to one side when inserted into the base element 1. The locally compressed areas are symbolized here by a darker area with a higher cell density.

As an alternative to asymmetrical expanding mandrels, it is also conceivable that the expanding tool is displaced transversely to the insertion direction when inserted.

After the locally compressed areas 30 are created, the base element is sintered. The slots 17 are then cut (see 11b) in order to produce the adjacent sections 16 and thus the meandering current conduction paths through the heating element 14.

The locally compressed areas 30 serve to ensure a higher cell density in the area of the deflection of the current conduction path at the ends of each section 16 than in the area of the sections 16. In this way, local temperature peaks are avoided.

In the second embodiment, openings 26 can also be used, which are surrounded by locally compressed areas of the material of the heating element 14 and serve to accommodate support elements 24. The openings 24 are in 11c shown.

In the 12a and 12b The steps for producing a heating element according to a third embodiment are shown. The same reference numbers are used for the components and features known from the previous embodiments, and reference is made to the above explanations.

In 12a a base element 1 can be seen, which is designed with an oversize beyond the addition to compensate for the sintering shrinkage.

The base element 1 is soaked with liquid metal material and clamped between two fixing plates 40. It is then compressed by means of several pressing tools 42, so that a circumferential zone 44 is created in which the material of the base element is compressed.

It was found that the base element 1 is compressed from the outside in according to the force exerted by the pressing tools 42 and the center remains uncompressed. This results in a ring area in which the cell density is higher than in the initial state. At the same time, the higher cell density does not change suddenly to the original cell density, but rather gradually. This is advantageous in terms of mechanical strength.

In addition to the circumferential compressed zone 44, further locally compressed areas can be provided, as are known from the other two embodiments, i.e. for receiving support elements and in the area of current deflection.

The base element 1 is then sintered.

The zone 44 serves to mechanically connect the heating element 14 in the holding frame 12. It also makes it easier to attach the electrical connections 18, 20 because they provide more cable cross-section.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102020130657 [0002]

Claims (15)

Method for producing a heating element (14) made of metal foam, in particular for an exhaust gas heating device (10), by means of the following steps: a base element (1) made of a foam is provided, in particular made of a PU foam, the base element (1) is locally compressed, resulting in a higher density of cell connections, the base element (1) with a metal coating applied to it is sintered. Procedure according to Claim 1 , characterized in that the base element (1) has an oversize in the area that is locally compressed and is compressed to the target size by means of a tool (8). Procedure according to Claim 1 , characterized in that the base element (1) is pre-cut in the area that is locally compressed and then the pre-opening created by the pre-cutting is expanded using a tool (4). Method according to one of the preceding claims, characterized in that the base element (1) is impregnated with a liquid metal before it is locally compressed. Procedure according to Claim 4 , characterized in that excess liquid material is removed from the base element (1) after the base element (1) has been locally compressed and while the tool (4; 8) continues to interact with the base element (1). Procedure according to Claim 5 , characterized in that the base element (1) is centrifuged. Method according to one of the preceding claims, characterized in that the base element (1) coated with the liquid metal is dried after it has been locally compressed. Procedure according to Claim 7 , characterized in that the tool (4; 5; 6; 8) is removed after drying and before sintering. Heating element (14) made of metal foam, in particular for an exhaust gas heating device (10), in particular produced using a method according to one of the preceding claims, wherein it has sections produced by plastic deformation in which the density of the electrical conductor tracks (S) is higher than in undeformed ones sections. heating element Claim 9 , characterized in that the sections produced by plastic deformation are arranged on an outside of the heating element (14). heating element Claim 9 , characterized in that the sections created by plastic deformation surround an opening (26) intended to receive a support element (24). heating element Claim 9 , characterized in that the sections generated by plastic deformation are arranged in the area of a current deflection between two adjacent sections (16) of the heating element (14). Exhaust gas heating device (10) with a heating element (14) according to one of Claims 9 until 12 . Exhaust gas heater Claim 13 , characterized in that the plastically deformed section is connected to an electrical connection (18, 20). Exhaust gas heater Claim 13 , characterized in that a support bridge (22) is provided which extends over the flow cross section of the heating element, with a plurality of support elements (24) being provided on the support bridge (22), each of which extends through an opening (26) which located within plastically deformed sections of the heating element (14).

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