EP0653898B1 - Process for manufacturing ceramic heating elements - Google Patents

Description

The invention relates to a method for producing an electrical heating element, in the case of the metallic heating conductor between ceramic insulating layers are embedded, whereby as current leads and as current leads Contacting recesses in the ceramic insulating layers with electrical conductive mass are filled.

Otsuka et al. describe in CERAMIC BULLETIN 60, pp. 540 ff (1981) that Components made of ceramic materials, mainly aluminum oxide or Contain aluminum nitride, with refractory metals such as tungsten or Molybdenum can be metallized, that this metallization then by a covered another layer of green ceramic and the composite afterwards Material formation can be sintered. The is particularly suitable for this Foil technique.

Components manufactured in this way can be used primarily in the field of electronics and electrical engineering Find application. Thick-walled and large heating elements are available in different Embodiments known. With increasing miniaturization in electronics and electrical engineering, however, manufacturing technology and application problems.

At high heating outputs, the materials used must be resistant to high temperatures be without the heating elements being destroyed by overload (Run away).

So an application as a heating element is possible, in which by the action of a electrical current large amounts of heat preferred at such points in the Metallization patterns are generated that have a high resistance.

From DE-OS 39 02 484 a ceramic heater is known, the one contains ceramic body and a heating element that is on the ceramic Body is formed. This heating element consists of several areas. The first area is a heat generating area. Electrical line areas are used for power supply and power dissipation and connection areas connect the heat generating area with the electrical Line areas. The heat generating area is made from a cermet formed which is a ceramic material and mainly from at least contains a metallic material consisting of a precious metal. The electrical Pipe areas are formed from a metallic material that consists mainly of at least one base metal or one Cermet, which contains a ceramic material and the metallic material.

In the subject of DE 39 07 312 A1, the materials of the Heat generation part and the connecting conductor also. Contains here too the material of the heat generating part as a main component is a precious metal. The interconnector must be specially designed to be more specific Contact resistance is not higher than half the specific one Volume resistance of the heat development conductors.

The production of these known ceramic radiators is complex, especially if each of the named electrical areas consists of one other material and therefore in a separate operation must be applied. The respective use of at least one Precious metal is a cost factor.

The process for producing the invention Heating element is defined in claim 1, and that The heating element according to the invention is in claim 7 Are defined.

In a preferred embodiment of the method according to the invention high temperature resistant metallization paste in thick film technology applied. Layers are made using the screen printing process Layer thicknesses of up to 100 µm achieved. The ceramic insulation layers with the applied metallization pastes are then preferably first dried. The drying conditions depend on the used Screen printing oil, generally over a period of 5 to 30 minutes Temperatures in the range of 40 to 150 ° C is dried.

According to the invention, a metallization paste is used that at least 70 wt .-% metal powder consisting of tungsten or molybdenum or mixtures thereof, and not more than 35% by weight of one containing glass phase-forming ceramic powder or powder mixture Aluminum oxide, aluminum nitride, titanium nitride, titanium carbide or tungsten carbide and additionally contains 5 to 35% by weight of an organic pasting medium. As Organic pasting media are in particular oils such as mineral oil, vegetable Suitable oils or synthetic oils such as screen printing oil or recycling oil, it can but also fats, waxes, agents such as thixotropic agents, rosin or lecithin to achieve better filling levels, bentonites to improve the Strength of the unfired paste and / or organic solvents used become.

To produce the heating element, according to a preferred embodiment of the invention, unburned ceramic foils to enable electricity to be transported perpendicular to the Foil surface contacting recesses, in the context of the present Invention also called "vias", punched or drilled. The metallization paste is using a printing process such as screen printing, roller screen printing, Offset printing or pad printing on still unfired ceramic foils transferred, creating the desired pattern on the film surface becomes. The fully filled vias have a diameter of 0.1 to 0.5 mm, preferably 0.3 mm. The layer thickness of the metallization for the conductor tracks can be between 5 and 100 μm, preferably between 10 and 15 µm. The width of the conductor track should be at least 0.25 mm in order to safely avoid a burnout, preferably about 0.5 mm.

For processing the paste, it is recommended to use the paste to fill the vias to be used with a viscosity of 150 to 500 Pa • s for the flat Metallization printing, however, is advantageous, the paste by appropriate Add further small amounts of screen printing oil to a viscosity in the range from 50 to 90 Pa • s.

It is advantageous, but not necessary, to use the same metallization paste Filling the vias and also to use for the conductor tracks and contact areas, since then the occurrence of errors at the interfaces between vias and Metallization levels are largely avoided.

The metallization paste is matched to the shrinkage of the film so that during sintering, neither due to insufficient shrinkage of the Metallization compared to the shrinkage of the ceramic star cracks in the Ceramic, still due to excessive shrinkage of the metallization paste Voids or star cracks appear in the via. The vote on the Shrinkage occurs through the composition and grain sizes of the powders. The non-glass phase forming contained in the metallization paste Ceramic powder preferably has an average grain size of ≤ 10 µm, particularly preferably ≤ 2 µm. The grain sizes are determined with a Lasergranulometer® CILAS 850 measured by ALCATEL.

In addition, the overall resistance should be as constant as possible, which results from the surface resistance of the burned-in conductor track and the The area of the conductor track in the film layer level is obtained by multiplication. In In practice, total resistances of approximately 1 to 1000 ohms are used for such Miniature heating elements required. The distance between neighboring ones Conductors should be ≥ 0.4 mm if possible to prevent them from burning out avoid. The overall arrangement of the conductor tracks should be chosen so that the loop has a heating temperature as uniform as possible over its extent having. Then the outer metallization parts of the Contact surfaces are nickel-plated without current. For this, a commercially available Metallization bath, for example, based on hypophosphite Reducing agents are used. If necessary, an additional copper and / or silver-containing solder layer can be applied.

Several foils covered with metallizing paste are then stacked on top of one another and pressed together using pressure (usually ≥ 5 • 10 ⁴ hPa), possibly in combination with heat (RT to approx. 150 ° C). To facilitate this process, an adhesive aid made of an organic mixture with a binder can be applied over the entire surface of the ceramic films. Such adhesive aids are known from US Pat. No. 5,021,287 and contain organic resins such as polyvinyl butyral or acrylic resins in an organic solvent and possibly also plasticizers such as phthalic acid esters or polyethylene glycols.

After a multilayer laminate was made, usually several Contains heating elements in the plane laterally offset from each other at the same time Now the separation takes place, whereby at the same time the later form of the Heating element is generated. The separation can, for example, by Cutting or punching is done.

The final size of the heating element is due to the sintering process Temperatures ≥ 1600 ° C generated in a reducing, humid atmosphere. The Furnace atmosphere preferably has a composition of about 75% Hydrogen and 25% nitrogen, the mixture being at a temperature of 55 ° C is saturated with water vapor.

With special miniaturization of the heating element is due to the temperature distribution and heat dissipation to take special care. Must be in the heating area attention should be paid to the most uniform layer thickness of the conductor tracks in order to local overheating at narrow points and places with a smaller layer thickness avoid. In addition, there is already a poor coordination between the geometry and the thermal conductivity of the aluminum oxide material Composition of the metallization and the conductor track design for Burning out due to local overheating.

With heating elements which are produced by the method according to the invention, is a continuous operating temperature between 50 and, depending on Material composition, 1100 h at temperatures up to 1800 ° C possible. The upper operating temperature limit is mainly from the chemical Composition of the ceramic insulating layers and their content of softening phases. Preferably for the ceramic Insulating layers materials such as aluminum oxide, aluminum nitride, zirconium oxide, Silicon dioxide or titanium nitride used.

The heating elements can be used as heating elements for Oxygen sensors or other measuring probes, especially for the Automotive technology, in laboratory measuring devices and infrared measuring transmitters or in the Heating technology, for example as an ignition element for igniting escaping flammable gases or as immersion heaters.

The invention is illustrated below by exemplary embodiments for the person skilled in the art presented in more detail, but without restricting it to the want to derive concrete embodiments.

example 1

A 0.8 mm thick green film contained binder, plasticizer and Dispersant primarily aluminum oxide and 4% of a quartz-containing glass-forming offset. The foil was cut with blades into cards that Recesses for the vias were made mechanically punched. The vias were screen printed with a metallizing paste, which in addition to 84% by weight of tungsten with an average grain size of 2.5 µm 16% by weight of a fine-grained alumina with an average grain size of 1 µm and additionally 15% by weight as an organic pasting medium Screen printing oil, based on the weight of the solids content, contained. For the Processing of the paste was a viscosity of 75 Pa • s for surface pressure and set to 175 Pa • s for the pressure of the vias.

After drying the filled vias in air at 70 ° C was on the unfilled and via-filled cards using a looped structure the paste described using a screen printing machine in thick film technology printed. The printed cards were in air at 70 ° C dried. On other unfired cards was also described Metallization paste, a flat pattern using a screen printing machine printed. These metallization areas should be on the finished heating element on the outside lie and allow the electrical connection as contact surfaces. All printed cards were dried in air at 70 ° C.

Several printed cards were then stacked on top of each other two cards with a bow pattern with their non-metallized back lie on top of each other and another card with the connection pattern on it lies, with the connection patterns facing outwards. A graphical representation of this arrangement is illustrated in Figure 1. The ceramic films 1 with the vias 2 can be recognized by reference numerals. The Vias 2 are filled with via fillings, not shown. The metallizations 4 are arranged in such a way that interconnect leads 5 and heating loops 6 result, the latter forming the heating area 7. Finally, there are also recognizable external contact surfaces 8.

This stack of cards was pressed under a pressure of 90,000 hPa at a temperature of 90 ° C. Several individual parts were cut from the laminate using a cutting tool. The distance of the loop-shaped structure inside the heating element from the lateral outer edge of the heating element was 0.5 mm. The rod-shaped heating elements were sintered under protective gas (moist mixture of nitrogen and hydrogen) at a temperature of 1630 ° C. in a hood furnace. On the one hand, the ceramic material aluminum oxide with a content of 96% by weight Al ₂ O _{3 was} produced, on the other hand, the conductor tracks were also sintered in a co-firing process. The fully filled vias were 0.3 mm in diameter. The layer thickness of the conductor track metallization was 12 μm and its width was 0.5 mm. The surface resistance achieved with the conductor tracks according to Example 1 was 5 mΩ / cm ² . The finished heating element each had a width and height of approximately 2.5 mm and a length of its heating area of approximately 18 mm. The measurements made on the finished heating element are described after the examples and compared in a table.

Comparative Example 1

Analogously to Example 1, the same ceramic material was used made of 96% by weight aluminum oxide and 4% by weight quartz-containing, glass-forming Offset a heating element made with the same dimensions. One and only The difference was that the metallization paste consisted of 100% by weight Tungsten with an average particle size of 2.5 µm plus that for the Processing as a paste required the amount of screen printing oil. It's going on the measurement results referenced to the examples.

Example 2

The manufacturing process for a rod-shaped heating element with one Contact area at each of the rod ends and consisting of only two layers Ceramic film is analogous to the manufacturing process of Example 1. As ceramic material was an aluminum nitride with 3 wt .-% aluminum oxide and 4 wt% yttria. For this, a card was made from a unfired ceramic film with the help of those described below Metallizing paste printed with a wavy or meandering structure. In a second unfired ceramic card, vias were mechanically created with a Stamped metal needle.

The metallization paste consisted of 84% by weight of molybdenum and 8% by weight Aluminum oxide and a further 8% by weight aluminum nitride. The powders were fine Grain sizes as described in Example 1. The metallizing paste was made with Screen printing oil adjusted to the viscosity described in Example 1.

The vias were filled with the paste described and dried. Then a flat or meandering print was printed on one side of this card Apply the paste described and dried again. Both Cards were sealed in a waterproof pouch and using a isostatic press at a temperature of 70 ° C under high Laminated pressure of over 100,000 hPa so that the wave-shaped or meandering Structure comes to lie between the two cards while the Contact surface faces outwards. Such an arrangement is shown in FIG. 2 illustrated. The further production process was as in Example 1 specified carried out.

Example 3

The manufacturing process for a substantially annular heating element was in all points identical to the production processes of Examples 1 and 2. As Ceramic material was aluminum nitride with 10 wt .-% aluminum oxide and 3 wt .-% yttrium oxide. For this purpose, cards from a unfired ceramic film using the metallizing paste of the example 2 with a substantially ring-shaped and if necessary wave or meandering structure printed.

If the superimposed conductor tracks have different shapes or Have lengths, their electrical resistances and Adjust heating temperatures across the cross-section of the conductor tracks. The heatable Zone can be characterized by a slight shift in contact areas and vias the outer edge of the component become almost circular.

To test the load capacity of the heating elements manufactured, two various series of measurements carried out. In measurement series 1, the Contacts of the heating elements applied an electrical voltage of 17 V. while the heating element in a furnace to a temperature of constant 1000 ° C was heated. The current intensity of the current, which is thereby through the Heating element flows, regulates itself and is on an ammeter displayed. During the measurement, however, only the time is measured that passes by the ammeter shows the current 0 A, because then the element is broken.

In the second measurement, a so-called overload test is carried out. An electrical voltage of 30 V is applied to the heating element, in which case the current intensity and temperature are freely set. In this case, too, the time is measured that elapses until the heating element has burnt out and 0 A is displayed as a result. The results are summarized in the table below: Example No. 1 2 3 V 1 17 V / 1000 ° C 199 h 212 h 148 h 25 h 30 V 34 m 69 m 28 m 12 m

Claims (9)

1. Method of producing an electrical heating element in which, prior to sintering, there are embedded, between mutually stacked ceramic insulating layers (1), metallic heating conductors (6) that each have current supply conductors and current drain conductors and wherein there are provided in the ceramic insulating layers, as the current supply conductors and current drain conductors (5) between the layered heating conductors, contacting cutouts (2) that are filled with electrically conductive material, characterized in that there is used both for the heating conductors and for the current supply conductors and current drain conductors, a high-temperature-resistant metallization paste that contains at least 70 wt.% of metal powder composed of tungsten or molybdenum or mixtures thereof and at most 30 wt.% of a ceramic powder or powder mixture that does not form a glass phase, containing aluminium oxide, aluminium nitride, titanium nitride, titanium carbide or tungsten carbide and additionally 5 to 35 wt.% of an organic pasting medium, based on the total solids content.
2. Method according to Claim 1, characterized in that the ceramic powder contained in the metallization paste and not forming a glass phase has a mean particle size of ≤ 10 µm, preferably ≤ 3 µm.
3. Method according to Claim 1 or 2, characterized in that the metallization paste is transferred with the aid of a printing method, such as screen printing, rotary screen printing, offset printing or pad printing to an unfired ceramic web, wherein there is generated on the web surface the desired pattern in which the completely filled contacting cutouts have a diameter of 0.1 to 0.5 mm, preferably of 0.3 mm, in which the layer thickness of the metallization for the conductor tracks is in the range from 5 to 100 µm, preferably between 10 and 25 µm, and in which the width of the conductor track is at least 0.25 mm, preferably at least 0.5 mm.
4. Method according to any of Claims 1 to 3, characterized in that the viscosity of the metallization paste that is used to fill the contacting cutouts is adjusted to a value in the range from 150 to 500 Pa·s and in that the viscosity of the metallization paste that is used for the laminar metallization printing is adjusted to a value in the range from 50 to 90 Pa·s.
5. Method according to any of Claims 1 to 4, characterized in that the metallization paste is applied using thick-film technology and in that the ceramic insulating layers with the applied metallization pastes are stacked above one another and are dried at temperatures of 40 to 150°C.
6. Method according to any of Claims 1 to 5, characterized in that the sintering operation is performed at temperatures of ≥ 1600°C in a reducing, moist atmosphere.
7. Electrical heating element, obtainable by a method according to any of Claims 1 to 6, in which there are embedded, between ceramic insulating layers (1), metallic heating conductors (6) that each have current supply conductors and current drain conductors (5) and wherein there are provided in the ceramic insulating layers, as current supply conductors and current drain conductors between the layered heating conductors, contacting cutouts (2) that are filled with electrically conductive material, characterized in that it has a constant total resistance that results from the sheet resistance of the built-in conductor track and the area of the conductor track in the web layer plane by multiplication and that is in the range from 1 to 1000 Ω, wherein both the heating conductors and the current supply conductors and current drain conductors are made from high-temperature-resistant metallization paste that contains at least 70 wt.% metal powder composed of tungsten or molybdenum or mixtures thereof and not more than 30 wt.% of a ceramic powder or powder mixture that does not form a glass phase, containing aluminium oxide, aluminium nitride, titanium nitride, titanium carbide or tungsten carbide, and additionally 5 to 35 wt.% of an organic pasting medium, based on the total solids content.
8. Heating element according to Claim 7, characterized in that its outer metallization sections of the contact areas are electrolessly nickel-plated and in that, optionally, a copper-containing and/or silver-containing solder layer is additionally applied.
9. Heating element according to Claim 7 or 8, characterized in that it is an oxygen sensor or other measuring sensor that is provided, in particular for use in automobile technology, in engine construction or in laboratory measuring instruments and infrared measuring transmitters, or in that it is an ignition element in heating technology for igniting escaping combustible gases or it is an immersion heater.

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