EP1366641B1 - Ceramic cooktop - Google Patents

Abstract

The invention relates to a ceramic cooktop comprising a cooking plate (12) made of glass ceramic or glass. The ceramic cooktop also comprises an electrical heat conductor layer (18), an insulating layer (16) that is located between the cooking plate (12) and the heat conductor layer (18), and an electrically conductive intermediate layer (14) between the cooking plate (12) and the insulating layer (16). The intermediate layer (14) is a thermally sprayed layer consisting of an electrically conductive ceramic material, preferably of an oxide layer that is rendered electrically conductive by oxygen loss occurring during thermal spraying, or of a cermet.

Description

The invention relates to a ceramic hob with a hotplate made of glass ceramic or glass, with an electrical heating conductor layer, with an insulating layer between the hotplate and the heating conductor layer, and with an electrically conductive Intermediate layer between the hotplate and the insulating layer.

Such a ceramic hob is known from DE 31 05 065 C2 and known from US 6 037 572.

The hotplate according to DE 31 05 065 C2 consists of glass ceramic, a metallic layer on the underside, for example is applied by a spray process on which again a ceramic insulating layer by a Spraying process is applied, finally on a heating element evaporated or applied in a spraying process is.

As is known, have glass ceramics used for hobs an NTC characteristic, i.e. with increasing Temperatures noticeably increase in electrical conductivity. To a current flow between a metallic pot or the To prevent the surface of the hotplate and the heating conductor, is therefore an electrical insulation layer for operation of such a cooking system requirement. To the necessary The system must meet security requirements Operating temperatures a dielectric strength of 3 750 volts exhibit.

Since such ceramic hobs must be designed for operating temperatures of up to about 600 ° C, considerable problems can arise due to the differences in the thermal expansion coefficients of the materials used. While the thermal expansion coefficient for a glass ceramic, for example for a Ceran® glass ceramic from Schott, is in the order of magnitude of ± 0.15 x 10 ^-6 K ^-1 , the thermal expansion coefficient of ceramic materials is significantly higher. For example, the thermal expansion coefficient "for Al ₂ O ₃ is approximately 8 x 10 ^-6 K ^-1 . In contrast, the thermal expansion coefficients of metals are significantly higher.

The application procedure for the individual shifts includes the thermal spraying has become known because of this on relative applied a variety of materials in a cost-effective manner can be. Because of the high speed and the high temperature is usually a sufficiently good one Liability reached.

However, layers should be thicker than about 100 microns are applied because of the differences the coefficient of thermal expansion between the Glass ceramic and the other layers mostly substantial Adhesion problems. For example, aluminum oxide layers, which have the necessary dielectric strength and thus a thickness of the order of a few hundred have µm, although easily produced by thermal spraying, however, there are usually cracks or the layers tend to flake off during use, because of the rapid changes in temperature during the Operating significant thermal stresses arise.

The dielectric strength requirements can be reduced be when according to DE 31 05 065 C2 or according to US 6 037 572 between the insulating layer and the hotplate an electrically conductive layer is applied that is grounded becomes. In such a case, enough for the ceramic Insulation layer has a dielectric strength of approximately 1500 volts to ensure the necessary operational safety in accordance with VDE guarantee.

In this way, the layer thickness of the ceramic insulating layer be significantly reduced, causing the problems due to of the different thermal expansions is reduced become.

On the other hand, the use of a metallic intermediate layer according to DE 31 05 065 C2 or according to US 6 037 572 the disadvantage that another layer is introduced into the composite which is again a significantly higher thermal Expansion coefficient than the hotplate has, whereby the stability of the overall system is adversely affected.

The invention is therefore based on the object of a ceramic hob to improve in accordance with the type mentioned at the beginning, that the operational safety of the ceramic hob improves and a good long-term resistance in rough everyday use is guaranteed.

This task is performed on a ceramic hob according to the above mentioned type according to the invention solved in that the Intermediate layer a thermally sprayed layer from a is electrically conductive ceramic or from a cermet.

The object of the invention is completely achieved in this way.

By forming the intermediate layer in the form of a electrically conductive ceramics will be a much better match of the coefficient of expansion of the intermediate layer to the Coefficient of expansion of the hotplate almost reached is zero because the coefficient of expansion of suitable ceramic materials is significantly lower than the coefficient of expansion of metals. Even when using a The cermet layer results from a metallic Matrix embedded ceramic particles a reduced thermal expansion, reducing the thermal stresses be reduced.

While using a cermet layer a special one good electrical conductivity can be achieved if necessary when using an electrically conductive Ceramics accepted a somewhat reduced conductivity become. However, the use of an electrically conductive Ceramic as an intermediate layer has the further advantage that the ceramic from the choice of materials better to the glass ceramic of the Hotplate can be adjusted, with a targeted Material selection a particularly good adhesion and low thermal Tensions can be achieved in use.

In an advantageous development of the invention, the intermediate layer is an oxide layer, which is lost due to oxygen loss thermal spraying is electrically conductive.

Here, the intermediate layer can be made, in particular, of TiO ₂ , of a mixture of Al ₂ O ₃ with a TiO ₂ content of at least 50% by weight, preferably of at least 90% by weight, of ZrO ₂ , of a mixture of Al ₂ O ₃ with ZrO ₂ with a proportion of ZrO ₂ of at least 50 wt .-%, preferably of at least 90 wt .-%, of a mixture of TiO ₂ and ZrO _2, or of a mixture of Al ₂ O ₃ with TiO ₂ and ZrO ₂ with a proportion of at least 50% by weight, preferably at least 90% by weight, of TiO ₂ and ZrO ₂ .

These intermediate layers made of TiO _2-x , ZrO _2-x or from mixtures of Al ₂ O ₃ with TiO _2-x and / or ZrO _2-x have particularly good adhesion to a glass ceramic surface. Thermal spraying reduces the oxygen content to such an extent that this material becomes electrically conductive.

0,1 eine Volumenleitfähigkeit von etwa 10³ Ohm x cm bis etwa 5 x 10² Ohm x cm (bei Raumtemperatur). Infolge der relativ geringen thermischen Ausdehnung von TiO_2-x und der besonders guten Affinität von TiO_2-x zur Glaskeramik erscheint besonders TiO_2-x zur Verwendung als leitfähige Zwischenschicht geeignet.For example, for TiO _2-x with x

0.1 a volume conductivity of about 10 ³ ohm x cm to about 5 x 10 ² ohm x cm (at room temperature). As a result of the relatively low thermal expansion of TiO _2-x and the particularly good affinity of TiO _2-x for the glass ceramic, TiO _2-x appears to be particularly suitable for use as a conductive intermediate layer.

In addition, however, are the other materials mentioned readily usable, with other chemically similar ones Oxides that appear suitable during thermal Have a sufficiently high oxygen loss, to maintain sufficient electrical conductivity.

As already mentioned, the intermediate layer can also consist of one Cermet can be made with a metal matrix. It points the metal matrix preferably at least one of the components Nickel, cobalt and chrome.

In an advantageous development of this embodiment, the Intermediate layer made of a cermet with a metal matrix, which is an alloy of the main components nickel, Is cobalt and chrome.

Particles of carbide, such as from tungsten carbide, chromium carbide or the like, be stored.

With such a cermet, good electrical results Conductivity of the intermediate layer, at the same time by the ceramic inclusions the coefficient of thermal expansion compared to a pure metal matrix. The metal matrix in question also has good adhesion on a glass ceramic surface and is increased due to the Suitable for ductility, certain thermal stresses that occur in the Operation occur to catch or dismantle.

In an additional development of the invention is between the electrically conductive intermediate layer and the hotplate ceramic adhesive layer is provided.

This adhesion promoter layer preferably consists of Aluminum oxide, titanium oxide or mixtures thereof and is preferably applied by thermal spraying.

Especially when using a cermet material as Intermediate layer leads to an adhesion promoter layer again improved adhesion to the glass ceramic surface, which overall, an extremely stable layer composite results, which has a very good temperature resistance and Has resistance to temperature changes.

The insulating layer applied to the intermediate layer is preferably made of cordierite or mullite and is preferably applied by thermal spraying.

The use of these ceramics to produce the insulating layer has the advantage of a relatively low coefficient of thermal expansion which is between approximately 4.3 and 5.0 x 10 ^-6 K ^-1 for mullite and between approximately 2.2 and 2.4 x 10 ^-6 K ^-1 for cordierite. As a result of the low coefficient of thermal expansion, there are lower stresses in connection with the hotplate made of glass ceramic.

In principle, of course, other ceramic materials can also be used to produce the ceramic insulating layer, for example Al ₂ O ₃ , but there are particular advantages in the aforementioned materials because of the low thermal expansion coefficient and the sufficiently high breakdown field strength.

It is understood that the above and those below Features to be explained not only in each case specified combination, but also in other combinations or can be used alone without the scope of the Leaving invention.

Fig. 1

einen Querschnitt durch ein erfindungsgemäßes Keramik-Kochfeld in einer ersten Ausführung und

Fig. 2

einen Querschnitt durch ein erfindungsgemäßes Keramik-Kochfeld in einer gegenüber Fig. 1 abgewandelten Ausführung.

Further features and advantages of the invention result from the following description of preferred exemplary embodiments with reference to the drawing. Show it:

Fig. 1

a cross section through an inventive ceramic hob in a first embodiment and

Fig. 2

a cross section through a ceramic hob according to the invention in a modified version compared to FIG. 1.

In Fig. 1 is an inventive ceramic hob in cross section shown and designated overall by the number 10.

It is understood that the illustration is only exemplary Is nature and that especially the proportions are not are to scale.

The ceramic hob has a hotplate 12 made of glass ceramic, such as Ceran®. This hotplate 12 is used to hold cooking vessels. On the underside of the hotplate 12, a hotplate is generated at different locations. For household purposes, typically four or possibly five hotplates are provided on a ceramic hob. Only one hotplate is shown in FIGS. 1 and 2. An intermediate layer of TiO _{2 was} applied to the underside of the hotplate 12 by thermal spraying. This can be done, for example, by means of atmospheric plasma spraying (APS) with a layer thickness of approximately 50-250 µm. The respective layers are preferably applied only in the area of the respective hotplates in order to keep the total voltages as low as possible.

Before thermal spraying, the glass ceramic is cleaned, e.g. degreased with acetone. On thermal spraying otherwise usual pretreatment by sandblasting is omitted, as this would damage the glass ceramic.

After the generation of the intermediate layer 14 an insulating layer 16 is on in turn by atmospheric plasma spraying sprayed, which preferably consists of cordierite (2MgO · 2Al ₂ O ₃ · 5SiO ₂₎ or consists of mullite (3Al ₂ O ₃ · 2SiO _2).

The layer thickness of the insulating layer 16 depends on the desired one Dielectric strength and the material used from time to time is between about 100 and 500 microns, preferably between about 150 and 300 µm.

A heating conductor layer is then placed on the insulating layer 18, approximately in the form of a meander-shaped heating conductor 20 generated. The heating conductor 20 can be in a known manner a screen printing process are applied, whereby by a glassy content of mostly more than 5% the flow temperatures can be reduced in such a way in the layer penetration that Baking temperatures between about 500 and 850 ° C, where a dense, closed conductor layer is created.

As an alternative to this, the heating conductor layer 18 can also be passed through thermal spraying are generated. To do this, start with a part that is not to be coated in a conventional masking process masked and then the exposed parts by thermal Syringes coated with the heating conductor material.

The previously covered part can then be removed, see above that a winding heating conductor 20 is formed, the individual Heating conductor tracks are insulated from each other.

The intermediate layer 14, which is applied from TiO ₂ by thermal spraying, becomes electrically conductive due to the high oxygen loss of the titanium oxide during the spraying process. This results in a volume conductivity of about 10 ³ ohm x cm to about 5 x 10 ² ohm x cm (at RT). This is sufficient to be able to effectively ground the intermediate layer 14, as is indicated by the connection to ground 22 in FIG. 1. This reduces the dielectric strength required for the insulating layer 16 to approximately 1500 volts. In the event of a fault, a safety switch which is known per se and is not shown here is triggered when the heating conductor 20 strikes the hotplate 12.

A modification of the ceramic hob is shown in Fig. 2 and designated overall by the number 10 '.

Again, there is glass ceramic, such as Ceran® Cooking plate 12 on the underside of an electrically conductive Intermediate layer 14 'applied. This intermediate layer 14 ', at which is a cermet layer, is however through an adhesion promoter layer sprayed onto the hotplate 12 24 separated.

The adhesion promoter layer 24 preferably consists of Al ₂ O ₃ or a mixture of Al ₂ O ₃ and TiO ₂ , for example 97% by weight Al ₂ O ₃ and 3% by weight TiO ₂ . The adhesion promoter layer 24 is thermally sprayed with a layer thickness of approximately 10 to 150 μm, preferably by APS. The preferred layer thickness is of the order of about 30 to 100 μm. A cermet layer consisting of a nickel / cobalt / chromium alloy with embedded carbide particles (tungsten carbide, chromium carbide etc.) is then sprayed onto the adhesion promoter layer 24. The intermediate layer 14 'is produced with a layer thickness of approximately 50 to 250 μm, preferably approximately 50 to 100 μm. Then the insulating layer 16 and the heat conductor layer 18 are then applied in the manner already described with reference to FIG. 1.

As can be seen from the figures according to FIGS. 1 and 2, run the individual layers lying on top of each other on Gradually out of the border area and go continuously to each underlying layer above. In addition, the Total area of the individual layers towards the heating conductor layer from each. In this way there are favorable Tension relationships in the marginal areas of the respective Layers so as to delaminate the layers counteract.

In Fig. 1 there is also an annular recess 26 shown, the intermediate layer 14 at its edge region encloses in a ring.

Due to this slight depression, tensions between the hotplate 12 and the intermediate layer 14 are formed, be taken up and partially dismantled.

Claims (9)

1. A ceramic cooktop comprising a cooking plate (12) of glass ceramic or glass, an electric heat conductor layer (18), an insulating layer (16) between the cooking plate (12) and the heat conductor layer (18), and an electrically conducting grounded intermediate layer (14) between the cooking plate (12) and the insulating layer (16), characterized in that the intermediate layer (14) is a thermally sprayed layer consisting of an electrically conductive ceramic or of a cermet.
2. The ceramic cooktop of claim 1, characterized in that the intermediate layer (14) is an oxide layer that is electrically conductive by oxygen loss during thermal spraying.
3. The ceramic cooktop of claim 2, characterized in that the intermediate layer (14) is made of TiO₂, of a mixture of Al₂O₃ with a portion of at least 50 wt.-% of TiO₂, preferably at least 90 wt.-%, of ZrO₂, of a mixture of Al₂O₃ with ZrO₂ with a portion of at least 50 wt.-% of ZrO₂, preferably at least 90 wt.-%, of a mixture of TiO₂ and ZrO₂, or of a mixture of Al₂O₃ with TiO₂ and ZrO₂ with a portion of at least 50 wt.-%, preferably at least 90 wt.-% of TiO₂ and ZrO₂.
4. The ceramic cooktop of claim 1, characterized in that the intermediate layer (14) is made of a cermet having a metal matrix, comprising at least one of the components nickel, cobalt and chromium.
5. The ceramic cooktop of claim 4, characterized in that the intermediate layer (14) is made of a cermet having a metal matrix, which is an alloy comprising the major components nickel, cobalt and chromium.
6. The ceramic cooktop of claim 4 or 5, characterized in that particles of carbide, such as tungsten carbide, chromium carbide etc., are dispersed within the metal matrix.
7. The ceramic cooktop of any of the preceding claims, characterized in that a ceramic bonding layer (24) is provided between the electrically conductive intermediate layer (14) and the cooking plate.
8. The ceramic cooktop of claim 7, characterized in that the bonding layer (24) is thermally sprayed of aluminum oxide, of titanium oxide or of mixtures thereof.
9. The ceramic cooktop of any of the preceding claims, characterized in that the insulating layer (16) consists of cordierite or of mullite and is, preferably, applied by thermal spraying.

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