WO2006032558A1

WO2006032558A1 - Method for embedding a metallic wire in a ceramic element

Info

Publication number: WO2006032558A1
Application number: PCT/EP2005/053569
Authority: WO
Inventors: Alexander Klonczynski; Laurent Jeannel; Martin KÖHNE
Original assignee: Robert Bosch Gmbh
Priority date: 2004-09-22
Filing date: 2005-07-22
Publication date: 2006-03-30
Also published as: DE102004045815A1; EP1794502A1

Abstract

The invention relates to a method for embedding a metallic wire in a ceramic element during the formation of a ceramic precursor from a green body, obtained by the pyrolysis of elemental organic polymer precursor moulded parts that contain oxygen. A polymer is at least partially injection moulded around the metallic wire during or after the production of the green body, prior to pyrolysis.

Description

Process for embedding a metallic wire in a ceramic element

The invention relates generally to a method of embedding a metallic wire in a ceramic heating element according to the preamble of the independent claim. Specifically, the invention relates to the contacting of a metallic wire with a ceramic heating element in a ceramic glow plug.

State of the art

In the production of ceramic glow plugs from ceramic composite materials, amorphous SiOC ceramics are obtained by pyrolysis of elemental organic precursors. Advantages of the precursor thermolysis process compared to the conventional production methods for ceramics (sintering) are the significantly lower process temperatures and the simple processability and moldability of polysiloxane resins.

Starting material for the production of the ceramic composite material is a polysiloxane, ie, a polymer of Si, C, O and H, which is mixed with various fillers. Due to the choice of fillers, the electrical and physical property profile of the resulting after the pyrolysis ceramic composite material of the glow plug is tailored exactly to the requirements profile. The use of an oxygen-containing polysiloxane precursor as starting material allows easy processability in air and thus the production of low-cost products. The pyrolysis product of the filled polysiloxane has a good strength, high chemical stability, for example. Against oxidation and corrosion and is harmless to health. In general, one of the major advantages of the precursor thermolysis process over conventional ceramic composite fabrication techniques is the possibility of having a wider range of fillers available. This is partly due to the fact that pyrolysis is generally essential Lower temperatures than the sintering process, whereby liquid or volatile fillers can still be used and occurring at higher temperatures phase reactions are avoided. On the other hand, the polysiloxane resins, as meltable thermosetting polymers which are soluble in organic solvents, allow a simple and extremely homogeneous incorporation of fillers into the precursor, for example by kneading or dissolving. This is so interesting because with a large selection of fillers, the properties of the precursor composite material can be adjusted over a wide range. In addition to polysiloxanes, polysilsesquioxanes, polysilanes, polycarbosilanes, polyborosilazanes, mixtures of aminoplasts with nanopowders of metals, intermetallics and metal alloys, and aluminoxanes may also be used.

For the electrical contacting of the ceramic heater in a glow plug different methods are currently used.

One possibility is contacting via a, e.g. conductive layer applied by electrodeposition after the pyrolysis process and the subsequent connection by interference fit with the contact pin, i.e., with the metallic connector.

Another possibility is to solder the joint between wire and ceramic pin.

All these methods require some additional process steps after pyrolysis. When contacting the ceramic pin by means of a metallic wire, the ceramic pin must be drilled first in the green body before pyrolysis or in the ceramic after pyrolysis, and then contacted in a second step by a soldering process.

The aim of the present invention is therefore to avoid the disadvantages of the prior art and to provide a solderless connection between ceramic heating element and metallic wire. It should be noted that the present invention is not can only be applied to ceramic glow plugs, but can be made using the inventive method any heating elements.

Advantages of the invention

The inventive method for embedding a metallic wire in a ceramic component of precursor ceramic has the advantage over the prior art that the ceramic can be mechanically reinforced in a simple manner.

Another advantage is that it can be realized in an easy way an electrical connection between the ceramic pin and metallic wire without an additional drilling step.

Furthermore, it is advantageous that the integration of the contacting wire can take place during the shaping process.

Another advantage is that in addition to the drilling step, the previously required soldering step can be omitted.

Furthermore, it is advantageous that the effective fading of the precursor ceramic can be kept very low on the contact wire, but at the same time a subsequent penetration of air is prevented.

Advantageous developments of the invention will become apparent from the measures mentioned in the dependent claims.

Ausiührungsbeispiele

The core of the invention is the realization of a mechanical and an electrical connection between a metallic wire and a ceramic heating element. The compound is realized by the pyrolysis of a green body and a wire integrated into this green belt. First, the metallic wire is coated prior to the production of the green body with a plastic layer of defined thickness to compensate for the shrinkage of the precursor ceramic during the subsequent heat treatment. The coating decomposes during pyrolysis before the shrinkage of the precursor ceramic begins and thereby creates a cavity between the contact wire and the precursor ceramic. Through this cavity, the effective fading of the precursor ceramic is limited to a value as low as possible, but at the same time high enough to prevent a later penetration of air and to avoid the associated oxidation. In this way, a thermo-oxidatively very stable electrical contact is created. The production of the above-mentioned cavity immediately after decomposition of the coating allows the precursor ceramic to disappear until the cavity is closed, without thereby aufzuschwinden on the contact wire. Only when the cavity is closed does the precursor ceramic shrink onto the contact wire. Only by this disappearance arise stresses in the precursor ceramic.

The coating of the contact wires is preferably carried out by means of suitable solvent-containing lacquer-like preparations, which are applied to the wire in particular by means of the technique of wire coating.

The layer thickness of the plastic coating must be chosen so that it largely compensates for the shrinkage of the precursor ceramic and only so much

Swelling of the precursor ceramic onto the wire allows a defined low tensile stress to be established in the surrounding precursor ceramic.

The wire should be chosen for its properties such that its coefficient of thermal expansion (X _{D) is} less than or equal to the thermal expansion coefficient α p of the precursor ceramic.

(XD = (Xp • x with 0.8 <x <1,0.

This ensures that the contact surface between wire and precursor ceramic does not open at low temperatures. Would the coefficient of thermal expansion of the wire be greater as the thermal expansion coefficient of Precursorkeramik, could be at low

Temperatures form a gap between the wire and precursor ceramic. In this gap, oxygen can penetrate, which oxidizes the contact surface and thereby significantly increase the electrical contact resistance between the contact wire and precursor ceramic. For the calculation of the layer thickness S _{L of} the coating of a contact wire which is to be integrated in a cylindrical pin made of precursor ceramic, the following details are important:

Layer thickness S _{R of} the residue of the polymer after decomposition in an inert atmosphere

Diameter d of the wire to be integrated

linear shrinkage 1 of the precursor mass of a cylindrical pin in the radial direction after the heat treatment in percent

As an approximation, for S _L «d: S _L = ((d / 2) ^« 1/100 + S _R ) • x with preferably 0.95 <x <1.00

The shrinkage must be so far advanced after the heat treatment in an inert atmosphere that the contact wires sit airtight in the precursor ceramic and the Nachschwindung the precursor ceramic during further heat treatment in air is so low that can build up no harmful tensile stresses.

Example:

It is a wire made of molybdenum (coefficient of thermal expansion CTE of 5.1 x 10 ^"6) having a diameter of 0.5 mm (Stock No. 10038, Alfa Aesar, Germany). This is the wire enamel Terebec KF 287-45 (Altana Electrical Insulation, Germany) The coated wire is then cut to 1.5 cm pieces. A piece of wire is placed in the corresponding recess at the edge of the cavity of the molding mold that protrude 0.5 cm into the cavity. The injection mold is closed and the injection process is carried out.

Subsequently, the molded article of the precursor precursor ceramic is subjected to a heat treatment process as follows:

pyrolysis:

Furnace: Type HTK8 (Gero GmbH, Germany) Atmosphere: Argon 4.9 (purity 99.9999% argon)

Flow rate: 0.6 l / h

Heating at 100 K / min until 1300 ° C are reached; 2 h hold at 1300 ° C;

Cooling at 300 K / min until room temperature is reached

This is followed by a heat treatment in air for densification and the formation of an oxide layer on the precursor ceramic surface. Before this heat treatment in air, the wire is provided with a protective layer against oxidation.

Example of a preparation of the protective layer:

Weighing of:

68.7% by mass GR 650 F (manufacturer: Techneglas Co., United States of America, composition: polymethylsilsesquioxane)

30.0% by weight of glycol ether Dowanol ™ PMM (manufacturer: Dow Chemical Co.,

Germany, composition: polypropylene glycol methyl ether)

1, 0 mass% boron (manufacturer: HC-Starck)

0.3 mass% AEROSIL® R 812 S (manufacturer: Degussa, composition: fumed silica) - Weigh 100 g of the mixture into a mixing vessel;

10 min mixing with Ultraturax (manufacturer: IKA-Werke, mixing principle: RotorStator);

Immersing the free ends of the contact wires, that is, the portion of the contact wires protruding from the molded part made of the precursor of the ceramic glow pencil by plastic molding into the weighed mixture

Dry the coating of the contact wires in the air.

An alternative to the oxidation protection layer described above is e.g. the coating with an aqueous suspension containing alumina and silica as fillers and inorganic binders. This coating produces an aluminosilicate layer during the heat treatment which prevents the oxygen from reaching the contact wire.

Example:

The wire is immersed in the aqueous suspension Ceramic-Coating HE (manufacturer: Office of Applied Mineralogy Germany, composition: aluminosilicate) and the coating is allowed to air-dry. Subsequently, the heat treatment is carried out in air.

Heat treatment in air:

Oven: Type HTRH (Gero GmbH, Germany) - Heating with lOOK / min, until 1300 ° C are reached;

2 h hold at 1300 ° C; Cooling at 300 K / min until room temperature is reached

If the metallic wire is then at least partially encapsulated with a polymer after the coating, it is during the pyrolysis of the bonding wire with the ceramic pin by means of the resulting pressure caused by the shrinkage of the material during pyrolysis, ie, the conversion of the polymer into the ceramic, is applied, pressed. Thus, no hole must be introduced into the ceramic, and a soldering process is thus obsolete.

So that the occurring stresses due to the shrinkage of the material do not lead to stress cracks in the ceramic, the thermomechanical

Material properties, eg. The coefficient of thermal expansion of the wire, as well as its geometry can be selectively varied.

An alternative to coating the wire with a polymer is the coating with active solder. With the aid of a suitable device, for example a lead frame, the tubular wire coated with active solder is positioned and held in the cavity of the injection molding tool. The active solder is intended to provide a suitable and good connection (contact) to the ceramic material. For coating, all materials can be used that meet the requirements mentioned below, eg Ni, W or MoSi ₂ . Finally, in the production of the green body, the wire is overmolded with an organometallic polymer. The elemental organic polymer may in this case be any transfermolding or injection-moldable polymer with suitable fillers. Examples can be found in the European patent EP 0 412 428 B1. In connection to the transfer molding process, the shaped body is ground and then pyrolyzed. Due to the shrinkage occurring during the pyrolysis of the wire is pressed into the ceramic. The wire is compressed by the resulting pressure, at the same time can be reduced by the tubular shape of the wire voltages. Therefore, cracking does not occur in the ceramic material.

The material of the wire must meet the following basic requirements:

• Resistant in inert gas atmosphere and reducing atmosphere (carburization) up to approx. 1300 ° C

• Sufficient electrical or metallic conductivity Other wires are destroyed in the thermal processes used by carburization and / or decomposition.

For the shape of the wire to be used, the following possible variants result:

• Cylindrical

• Spiral

• tubular

• Massive

• Trapezoidal • Prismatic

• Knurled

• Coated

• Special surfaces, for example, modified surfaces or surfaces with undercuts to better reduce the stresses and to increase the pull-out strength after pyrolysis

Claims

claims

1. A method for embedding a metallic wire in a ceramic element in the formation of a precursor ceramic from a green body by pyrolysis of element organic precursor polymer moldings, characterized in that the metallic wire during or after the production of the green body prior to pyrolysis at least partially encapsulated with a polymer.

2. The method according to claim 1, characterized in that the metallic wire is additionally coated with a plastic layer before encapsulation.

3. The method according to claim 1 or 2, characterized in that the metallic wire is electrically contacted during embedding.

4. The method according to any one of claims 1 to 3, characterized in that the element organic precursor polymer is selected from the group consisting of

Polysiloxane, polysilsesquioxane, polysilane, polycarbosilane, polysilazane, polyborosilazane, mixtures of aminoplasts with nanopowders of metals, intermetallics and metal alloys, and aluminoxane.

5. The method according to claim 5, characterized in that the element organic precursor polymer is a mixture of several constituents of the group.

6. The method according to any one of the preceding claims, characterized in that the ceramic element is a heating element.

7. The method according to claim 6, characterized in that the ceramic heating element is part of a ceramic glow plug.

8. The method according to claim 2, characterized in that the coating is carried out by solvent-containing paint formulations by wire coating or dipping.

A method according to claim 2, characterized in that the coating is applied by a plastic molding method, e.g. Injection molding, or by an electrostatic coating process takes place.

10. The method according to any one of claims 2 to 9, characterized in that the

Thermal expansion coefficient (X _{D of} the wire and the thermal expansion coefficient αp of the precursor ceramic of the equation (X _D = αp • x is sufficient, where 0.8 <x <1.0.

11. The method according to any one of claims 2 to 10, characterized in that the layer thickness S _{L of} the plastic layer and the diameter d of the wire to be used satisfies the equation ((d / 2) ^« 1/100 + S _R ) • x, where S _{R is} the layer thickness of the polymer after decomposition and 0.95 <x <1.00.

12. The method according to any one of the preceding claims, characterized in that after the pyrolysis, a further heat treatment takes place in air, in which the wire with a

Oxidation protective layer is provided.

13. The method according to claim 12, characterized in that instead of the oxidation protection layer, an aqueous suspension is applied, which contains aluminum oxide and silicon dioxide and inorganic binder.

14. The method according to any one of the preceding claims, characterized in that the contact wire is coated with an active solder.

15. The method according to any one of the preceding claims, characterized in that the material of the contact wire is selected from the group consisting of nickel, Mo, Ni alloys and W alloys.

16. The method according to any one of the preceding claims, characterized in that the polymer, which is converted by pyrolysis in the Precursorkeramik and used to overmold the contact wire, is selected from the group consisting of polysiloxane, polysilsesquioxane, polysilane, polycarbosilane, polysilazane, polyborosilazane or Aluminoxane and mixtures of these compounds.

17. The method according to any one of claims 2 to 16, characterized in that the

Polmer for coating the wire, a thermoplastic selected from the group consisting of polyphenylsulfide (PPS), polyetheretherketone (PEEK), polyurethane (TPU) and polymethyl methacrylate (PMMA) or a thermoset selected from the group consisting of epoxy resin (EP), phenolic Formaldehyde resin (PF), polyurethane resin (PUR) and polyester resin (UP) is.

18. Veriahren according to claim 17, characterized in that one or more fillers are added to the polymer.

19. The method according to claim 18, characterized in that the fillers are chosen so that their thermal decomposition is completed during the pyrolysis before the ceramic fades onto the metallic wire.

20. Ceramic heating element, consisting of a ceramic pin and a metallic wire embedded therein, characterized in that the interface between the metallic wire and the ceramic pin is free of solder.