EP1366643B1 - Ceramic hob - Google Patents

Abstract

The invention discloses a ceramic cooktop comprising a cooking plate made of a glass ceramic or glass. The ceramic cooktop also comprises an electrical heat conductor layer and an insulating layer that is located between the cooking plate and the heat conductor layer. Onto the cooking plate a thermally sprayed bonding layer is applied, before further layers are applied.

Description

The invention relates to a ceramic hob with a cooking plate made of glass ceramic or glass, with an electrical heating conductor layer, and with a thermally sprayed insulating layer between the hotplate and the Heizleiterschicht.

Such a ceramic hob is known for example from DE 31 05 065 C2 or from US 6,037,572. The known ceramic cooktop has a cooking plate of glass ceramic, the underside of which is provided with a thermally sprayed, grounded metal layer, on which a ceramic insulating layer is sprayed, on the underside of a heat conductor layer is applied with a heat conductor as by a screen printing process.

Such a ceramic hob has over conventional ceramic hobs, which were previously heated substantially above below the glass ceramic plate from this spaced heating conductor via radiant heating, a significantly improved cooking behavior, since the heat is now transmitted by heat conduction and generated directly on the bottom of the glass ceramic becomes. Since a glass ceramic suitable for a cooktop, such as Schott CERAN®, has an NTC characteristic, i. that at increasing temperatures, the electrical conductivity increases significantly, is located between the Heizleiterschicht and the hotplate of glass ceramic, a ceramic insulating layer.

A particular problem with such a ceramic hob is the different thermal expansion coefficients of the individual layers. As is known, a glass-ceramic such as CERAN® has a coefficient of expansion α which is close to zero (± 0.15 · 10 ^-6 K ^-1 ). In contrast, metals have significantly higher expansion coefficients, which are well above 10 ^-5 K ¹ . Although ceramics have a lower coefficient of expansion (for example about 8 · 10 ^-6 K ^-1 for Al ₂ O ₃ ), this also leads to considerable problems with larger layer thicknesses because of the thermal stresses occurring during operation.

To ensure the required operational safety according to VDE, the dielectric strength of the insulating layer must be 3,750 V during cooking operation.

This requires a relatively large layer thickness for the ceramic insulating layer, which must be about 300 microns or more for alumina.

In turn, such a thick ceramic insulating layer can not be easily applied to a glass-ceramic surface by thermal spraying, since in this case cracks are usually observed or delamination occurs.

On the other hand, if one uses, as known from DE 31 05 065 C2, an electrically conductive, grounded intermediate layer between the insulating layer and the cooking plate made of glass ceramic, as a result of the grounding only a dielectric strength of the insulating layer of about 1500 V is required, whereby the thickness of Insulating layer can be reduced accordingly. However, the application of a metal layer between the insulating layer and the glass-ceramic plate leads to further problems due to the high thermal expansion coefficient of the metal layer.

The invention is therefore an object of the invention to provide an improved ceramic hob, which avoids the above-mentioned disadvantages and is designed as a stable layer system, on the one hand has the necessary electrical safety and on the other hand ensures high stability in long-term operation.

This object is achieved in that a thermally sprayed adhesive layer of a ceramic material is disposed on a surface of the cooking plate, which is covered by the insulating layer on which the Heizleiterschicht is added.

The object of the invention is completely solved in this way. Namely, according to the invention, it is possible to use other, more suitable materials for the production of the insulating layer by thermal spraying on the glass-ceramic cooking plate instead of aluminum oxide. Namely, according to the present invention, the insulating layer may now be made of cordierite, mullite or mixtures thereof or other thermally moldable ceramics having a similar low thermal expansion coefficient.

In the thermal spraying of these materials directly on the surface of a glass ceramic, this is damaged. In the case of thermal spraying of cordierite or mullite on the glass-ceramic surface, for example, micro-cracks are produced which impair the stability of the overall system.

Cordierite and mullite have a thermal expansion coefficient that is significantly lower than the thermal expansion coefficient of aluminum oxide. While the thermal expansion coefficient of cordierite is about 2.2 to 2.4 10 ^-6 K ^1, the thermal expansion coefficient of mullite is about 4.3 to 5.0 x 10 ^-6 K ^-1. Thus, using these materials, the problem of thermally induced stresses in operation can be significantly reduced due to the lower coefficients of thermal expansion.

As a primer layer is particularly suitable a layer of alumina, titanium oxide or mixtures thereof. In this case, the layer thickness of the adhesion promoter layer, which is applied by thermal spraying, preferably between 10 .mu.m and 150 .mu.m, preferably at 30 to 100 microns, in particular in a range between 40 and 70 microns.

Such a thin adhesion promoter layer has practically no adverse effect by the resulting thermal stresses on the overall system, but has an exceptionally good adhesion to the glass-ceramic surface, without damaging them in the interface.

A ceramic layer, which preferably consists of cordierite, of mullite, if appropriate also of magnesium oxide or mixtures thereof, can now be applied by thermal spraying in the necessary layer thickness to such an adhesion promoter layer.

In an alternative embodiment of the invention, a thermally sprayed electrically conductive intermediate layer is applied between the bonding agent layer and the insulating layer, which is preferably grounded.

As a result, as previously mentioned, the dielectric strength requirement of the insulating layer is reduced, which is reduced to about 1,500 V in the event that the interlayer is grounded and coupled to a rollover shutdown circuit breaker. This intermediate layer is preferably made of an electrically conductive ceramic or of a cermet. An electrically conductive ceramic can be produced for example by the thermal spraying of TiO ₂ , since during the thermal spraying such a high loss of oxygen occurs that the material is electrically conductive becomes. Thus, the volume conductivity for TiO ₂ at room temperature is between about 10 ³ Ωcm to about 5 · 10 ² Ωcm.

When using a cermet to produce the electrically conductive intermediate layer naturally results in a significantly higher electrical conductivity, whereby a secure grounding can be achieved. Applying the cermet layer to the adhesion promoter layer avoids adhesion problems on the glass ceramic layer. A suitable cermet comprises, for example, a metal matrix of a nickel / chromium / cobalt alloy in which carbide particles, e.g. Tungsten carbide or chromium carbide are dispersed.

Although such a cermet layer has a thermal expansion coefficient in the range of about 4 · 10 ^-6 K ^-1 to 11 · 10 ^-6 K ^-1 , and thus slightly above alumina, but still below the expansion coefficient of conventional metals.

This also results in advantages over the use of a conventional metal layer as an electrically conductive intermediate layer.

According to a further embodiment of the invention, the heat conductor layer is produced by thermal spraying, preferably by laser spraying.

By this measure, problems are avoided, which occur in the conventional production of a heat conductor layer by screen printing. Namely, heat conductor layers produced by screen printing generally have a glass content of more than 5% in the metallic conductor, so that the flow temperatures during layer burn-in can be lowered. The low-melting glass solders in mixed paste ensure that at a baking temperature between 500 and 850 ° C, a dense closed conductor layer is formed. However, the proportion of glass frit reduces the metallic conductive portion. Sub-segments of the track, which have an increased amount of glass locally, are areas with higher resistance, so that it may possibly lead to overheating and material failure in the current flow.

These disadvantages are avoided in a thermally sprayed heating element. The necessary structuring of the heating conductor is produced here by a masking process.

The laser spraying method is particularly suitable since this is particularly advantageous for producing a web-shaped application.

According to a further variant of the invention, the hotplate has, on its side facing the heat conductor layer, an annularly closed depression which runs in the vicinity of the edge region of the layer sprayed onto the hotplate.

In this way, the stresses that occur in particular in the edge region of the sprayed onto the hot plate insulating layer can be significantly reduced. Thus, the risk of delamination in this area is counteracted.

In accordance with a further advantageous embodiment of the invention, the individual layers have a decreasing surface towards the heating conductor layer. This measure also counteracts the risk of delaminations in the edge region of the layers.

Fig. 1

einen Querschnitt einer ersten Ausführung eines erfindungsgemäßen Keramik-Kochfeldes und

Fig. 2

einen Querschnitt einer abgewandelten Ausführung des Keramik-Kochfeldes gemäß Fig. 1.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the drawings. Show it:

Fig. 1

a cross section of a first embodiment of a ceramic hob according to the invention and

Fig. 2

a cross section of a modified embodiment of the ceramic hob according to FIG. 1.

In Fig. 1, a ceramic hob according to the invention is designated overall by the numeral 10. It has a flat cooking plate 12, which preferably consists of a glass ceramic, such as Schott CERAN®.

It is understood that the illustration is merely exemplary and in particular that the size ratios are not to scale.

This hotplate 12 serves to accommodate cooking vessels. On the underside of the hotplate 12 a hotplate is generated at different locations. For domestic purposes typically four or possibly five hotplates are provided on a ceramic hob. In Figs. 1 and 2, only one hotplate is shown.

On the underside of the hotplate 12, at least at the points where an insulating layer and a heat conductor layer are to be applied later, an adhesion promoter layer 14 is applied by thermal spraying, preferably by atmospheric plasma spraying (APS).

The application is preferably limited to the areas of the cooking zones in order to keep the total stresses as low as possible.

This adhesion promoter layer 14 is preferably made of aluminum oxide, titanium oxide or mixtures thereof. In particular, aluminum oxide and mixtures of aluminum oxide and titanium oxide with a low proportion of titanium oxide, for example 97 wt .-% Al ₂ O ₃ with 3 wt .-% TiO ₂ , have a particularly good adhesion to the surface of the glass ceramic and have a very good chemical Compatibility with this. The adhesion promoter layer 14 is applied with a layer thickness of between about 10 and 150 μm, preferably between about 40 and 70 μm, for example with about 50 μm. On this primer layer 14 is now a Insulation layer 16, which is preferably made of cordierite (2MgO · 2Al ₂ O ₃ ) or mullite (3Al ₂ O ₃ · 2SiO ₂ ), applied by thermal spraying with the necessary layer thickness to the desired dielectric strength of 3750 V at an operating temperature of 450 ° C. to ensure. For cordierite and mullite, the layer thickness is preferably up to about 500 μm, preferably about 200-400 μm.

Immediate application of the cordierite or mullite layer to the surface of the glass-ceramic would not be possible since this would result in damage in the form of microcracks or the like on the glass-ceramic surface.

Prior to the thermal spraying, the surface of the glass-ceramic plate 12 is not pretreated by roughening, as would otherwise be customary, since this would lead to damage to the surface of the cooking plate 12. Instead, the surface of the cooking plate 12 is merely cleaned, e.g. degreased by acetone.

On the underside of the insulating layer 16, an electrical heating conductor layer 18 is then applied by thermal spraying, wherein the necessary structuring of the heating conductor layer 18 is achieved by a masking method in a conventional manner. In this way, for example, a meandering wound heating element 20 can be generated.

Here, as a method for thermal spraying, a laser spraying method is preferred because it can be particularly advantageous to achieve a web-shaped order.

A variant of the ceramic hob according to the invention is shown in Fig. 2 and generally designated by numeral 10 '.

The difference from the embodiment shown in FIG. 1 is that the adhesive layer 14 is not directly applied to the insulating layer 16, but that this is first an electrically conductive intermediate layer 22 is sprayed, then in turn the insulating layer 16 'is applied.

This electrically conductive intermediate layer 22 is grounded, as indicated in Fig. 2 by the connection to ground 24. In the event of a fault, a known per se, not shown fuse the hot plate 12 is triggered in the electrical breakdown of the heating element 20 on the hot plate 12 as a result of their grounding.

For this reason, the insulating layer 16 'may be designed for a lower dielectric strength, with about 1500 V being sufficient at operating temperature according to VDE. Therefore, the thickness of the insulating layer 16 'can be reduced accordingly.

On the underside of the insulating layer 16 ', in turn, the heating conductor layer 18 is sprayed on as already described above.

The electrically conductive intermediate layer 22 preferably consists of a cermet, for example of a nickel / chromium / cobalt-based alloy, in which carbide particles, for example tungsten carbide and chromium carbide, are embedded. Such a cermet has a lower coefficient of thermal expansion compared to conventional metals due to the carbide inclusions, resulting in reduced problems due to thermal stress.

Alternatively, instead of a cermet, an electrically conductive ceramic can be used for this intermediate layer, provided that sufficient electrical conductivity can be achieved hereby. For example, a TiO ₂ thermally sprayed layer could be used since, during the thermal spray process, the TiO ₂ loses oxygen such that it becomes electrically conductive. However, the electrical conductivity (volume conductivity) of resulting TiO ₂ -x between 10 ³ Ωcm to 5 · 10 ² Ωcm at RT) is still significantly lower than the electrical conductivity of metals.

The single layers 14, 16 according to FIG. 1 or 14, 22, 16 'according to FIG. 2 have a surface which decreases towards the heating conductor layer 20. Furthermore, the individual layers run out gently in their marginal area, that is, they are continuously transferred to the underlying layer.

These measures counteract delamination of the layers in the edge region.

FIG. 2 also shows a possibility with which the partially considerable stresses in the edge region of the layers can be partially reduced.

For this purpose, located on the underside of the cooking plate 12 is an annular recess 26 which surrounds the edge region of the bonding agent layer 14 in an annular manner. Tensions that are transmitted in the edge region between the cooking plate 12 and the adhesion promoter layer 14 can be better absorbed or reduced by this recess 26.

Claims (9)

1. A ceramic cooktop comprising a cooking plate (12) of glass ceramic or glass, an electric heat conductor layer (20), and a thermally sprayed insulating layer (16; 16') between the cooking plate (12) and the heat conductor layer (20), characterized in that on a surface of the cooking plate (12) a thermally sprayed bonding layer (14) of a ceramic material is provided, which is covered by the insulating layer (16, 16'), whereon the heat conductor layer (20) is received.
2. The ceramic cooktop of claim 1, characterized in that the bonding layer (14) consists of aluminum oxide, of titanium oxide or of mixtures thereof.
3. The ceramic cooktop of claim 1, characterized in that the insulating layer (16; 16') consists of cordierite, of mullite or of mixtures thereof.
4. The ceramic cooktop of any of the preceding claims, characterized in that the bonding layer (14) has a layer thickness of 10 to 150 µm, preferably of 30 to 100 µm, in particular of 40 to 70 µm.
5. The ceramic cooktop of any of the preceding claims, characterized in that between the bonding layer (14) and the insulating layer (16') a thermally sprayed electrically conductive intermediate layer (22) is applied that is preferably grounded.
6. The ceramic cooktop of claim 5, characterized in that the intermediate layer (22) consists of an electrically conductive ceramic or of a cermet.
7. The ceramic cooktop of any of the preceding claims, characterized in that the heat conductor layer (20) is produced by thermal spraying, preferably by laser spraying.
8. The ceramic cooktop of any of the preceding claims, characterized in that the layers (14, 16; 14, 22, 16') occupy an area diminishing toward the heat conductor layer (20).
9. A ceramic cooktop comprising a cooking plate (12) of glass ceramic or glass, an electric heat conductor layer (20) and a thermally sprayed insulating layer (16, 16') between the cooking plate (12) and the heat conductor layer (20), according to any of the preceding claims, characterized in that the cooking plate (12) comprises an annular closed recess (26) at its side facing the heat conductor layer (20), and extending adjacent to the rim region of the layer (14) sprayed onto the cooking plate (12).

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