DE102005025206A1 - Method for preventing stress damage to glass ceramic cooker hob plates due to excessive thermal expansion has a composite material including a negative thermal expansion coefficient compound - Google Patents

Abstract

The glass ceramic hob plate (11) has a stainless steel frame (15) and radiant heating element (17) and the stresses induced by high temperatures are relieved by incorporating a negative thermal coefficient of expansion material which results in a zero coefficient at cooking temperatures. Electrical insulation coatings (24) may be applied to the top or underneath surface.

Description

field of use and state of the art

The The invention relates to a cooktop panel as mounted over a heater is, for example, when assembling a hob, and a such hob.

It It is known that hobs in many cases a hob plate Have glass ceramic, possibly also tempered glass. It exists the problem that a maximum temperature, which is such a hob plate may reach with a radiant heater arranged underneath, at about 600 ° C lies. It has been tried, this maximum temperature through Material additives to raise. But this does not work in every case. The meaning of Maximum temperature lies in the fact that they are used to maximize power coupling in a standing on the hob plate cooking vessel should be as high as possible.

Of Another problem is that at such high temperatures from above 600 ° C problems due to mechanical stresses or local stress fields in the hob plate, which may even can lead to a breakage of a hob plate of glass ceramic.

The Use of glass ceramic with a negative thermal expansion coefficient in telecommunications as an optical transmission medium is in the WO 2005/009916 A1.

alternative Materials for glass-ceramic have been proposed in the past as cooktop surfaces in particular, the heating has an induction heating see, for example, DE-A-102004041007. Problematic with such alternatives Materials, however, that even at temperatures significantly below 600 ° C the problems with different thermal expansion coefficients lead to tension. Such tensions limit the usability of such alternative materials very much strong.

Task and solution

Of the Invention is therefore based on the object, an aforementioned Cooking field plate as well as an aforementioned hob to create, with which problems of the prior art can be solved and in particular a Hob plate and a hob for higher maximum temperatures than previously created.

Is solved This object is achieved by a hob plate with the features of the claim 1 and a hob with the features of claim 12. Advantageous As well as preferred embodiments of the invention are the subject of further claims and will be closer in the following explained. The wording of the claims is by express Reference made to the content of the description.

A Hob plate according to the invention has a material with a negative thermal expansion coefficient on or contains it in shares. In particular, the expansion coefficient is isotropic.

The material with the negative coefficient of thermal expansion may be of the structure AB ₂ O ₈ or CD ₂ O ₇ . In particular, A = Zr or Hf; B = W or Mo; C = 4-valent metal or transition metal and D = P or V. Such a material is ZrW ₂ O ₈ , for example. In this way, the problem of mechanical expansion of the hob plate due to heating with possible damage due to mechanical stresses or the like. be countered effectively. In particular, the material with the negative coefficient of thermal expansion characterizes the resulting expansion characteristics of the cooktop panel so that their overall expansion coefficient is very low or almost or completely to zero. It can also be provided that the cooktop panel has a total negative thermal expansion coefficient.

A Such hob plate can consist of several materials. It is enough if some of these materials have a coefficient of expansion as mentioned above, may also have an isotropic coefficient of expansion.

The Hobbed plate may be a ceramic material or ceramic properties exhibit. These have especially advantages in use, in particular in terms of strength and scratch resistance. It becomes explicit noted that also hobs based on glass or glass-ceramic can be advantageous.

In order to be able to control the coefficient of expansion as well as possible or to be able to produce a hob plate with ultimately desired properties, it may also be intended to produce it from materials, some of which have a positive coefficient of thermal expansion. Due to the mixing ratio are different properties and especially, ultimately, the resulting thermal expansion coefficient determinable.

There Properties such as the coefficient of expansion in certain temperature ranges can be different should the aforementioned thermal expansion coefficient of the material for the hob plate advantageous up to 1000 ° C available. This is roughly the maximum temperature range on which one for Hobs more usual radiant heaters one about it could heat up running hob plate.

A Hob plate of the aforementioned type may advantageously a crystal lattice having tetrahedral and octahedral crystal structures. These can only be connected to each other at the tips, each tetrahedron has a free and untied tip. Such a structure leaves many Vibration modes and thus the desired and inventively advantageous result Properties.

At a cooktop panel may be an additional layer or coating be provided, which has certain properties, in particular electrical or especially insulating properties. Advantageously covered they mostly do the plate or full surface. This serves primarily to one at high temperatures at a Glass ceramic plate incipient electrical conductivity or conductivity to avoid the hob plate. The coating is in terms of their thermal expansion coefficient to the hob plate customized. If these are advantageously the same, the coating will last permanently the hob plate, even with larger layer thicknesses. So can also realized otherwise functional coatings or layer fields be, for example, as described in DE-A-10064621, whose Content hereby by express Reference is made to the content of this description. Otherwise there is a risk that the coating because of a larger coefficient of expansion tearing off or flaking off. Here is just an advantage that the coating can be much thicker than known coatings, as they for example from EP-A-1505354 are known. The greater thickness leads because of the adapted to the cooktop panel expansion coefficient not to flake off or peel off. A larger layer thickness in turn allows a firmer or more resistant Layer.

By suitable choice of additives the coating can be optimized in its properties. So For example, can achieve a particularly high insulation resistance become. On the other hand, the layer can also be provided with additives be that cause positive conductor properties, if desired. Likewise especially the expansion properties are influenced.

One such coating material may be, for example, on the underside the hob plate in the manner of a coating with certain areas, In particular, fields or points or the like., Are applied. Advantageously, a coating on the top of the hob plate be applied. The coating can be characterized by its thickness be protected from mechanical stress as it is on the top a hob plate by placing cooking vessels or the like. can arise.

When Heating a hob plate or a hob can on the one hand radiant heaters arranged underneath as known to those skilled in the art. on the other hand can Heating elements be applied directly to the glass ceramic, for example through thick film techniques. Alternatively you can Layered metallic heating elements are vapor-deposited. So can the glass ceramic are heated directly and serves as a carrier for a cookware. This is heated by the heating element through the glass ceramic, however by contact heat or corresponding heat transfer. Also Here, by a suitable choice of the expansion coefficient of Glass ceramic and heating element are prevented that the heating element separates or the connection breaks.

These and further features of preferred embodiments of the invention go out from the claims also apparent from the description and the drawing, wherein the individual Features for each alone or in the form of subcombinations an embodiment of the invention and in other fields be realized and advantageous also for protectable versions can represent for the protection is claimed here. The subdivision of the application in individual sections and subheadings limit the not in its generality among these statements.

drawing and sample description

The 1 schematically includes an embodiment with several aspects of the invention, wherein therein a hob is shown with a cooking plate described above.

In 1 is a hob 11 shown with a hob plate 13 by a surrounding frame 15 made of metal. The hob plate consists of an aforementioned material with ceramic properties, in particular glass ceramic. Overall, it has a negligible coefficient of thermal expansion. The material contains, inter alia, ZrW ₂ O ₈ . As a result of that the hob plate 13 As a result of this choice of material at high temperatures does not expand, there are no or only negligible internal stresses or local internal stresses within the hob plate 13 , The danger of a breach of the hob plate 13 in operation alone by local mechanical stresses due to heating or by a rudely placed cooking vessel is very low or not larger than conventional hob plates. Above all, however, such a hob plate can be operated at higher temperatures than normal, since a thermal stress break later or at higher temperatures or even not done.

Below the hob plate 13 are radiant heaters 17 arranged, which may have, for example, conventional construction. They form in a known manner cooking zones, which means that the place above them on the hob plate a cooking vessel such as a saucepan or the like. is placed on for heating therein befindlichem food.

By the operation of a radiant heater 17 In this area, a temperature of a few 100 ° C. can arise relatively quickly, in particular even more than 700 ° C. This creates a localized, very large warming. So that this does not lead to locally limited expansions with corresponding mechanical stresses, usually a temperature limitation is made. These mechanical stresses could either lead to cracks or other damage to the hob plate. Furthermore, there is the danger that in such a mechanical weakening of the hob plate at a cooking place or over a radiant heater 17 putting on or moving a heavy saucepan causes damage.

alternative To such radiant heating devices and induction heaters or Induktionsheizspulen be provided. Here is the problem of one To high temperature on the hob plate, although not quite so big. It does exist still, especially with materials other than glass-ceramic.

About the radiant heaters 17 are upper coatings 24a and 24b available. The upper left coating 24a is large and covers almost the entire left half. It can also cover the entire surface in one embodiment, in particular as an electrical insulation layer. The right upper coating 24b however, it consists of two smaller areas or points. They may have a function according to DE-A-10064621.

The coatings 24 consist of a prescribed material. They are set or designed so that they are either electrically insulating or have certain material properties. Depending on the intended use, they may also be provided on the underside, for example as an insulating coating for protection against the voltage applied to the radiant heaters in the event of breakage of the glass ceramic or in the case of electrical conductivity due to high temperatures.

Claims (13)

Hob plate for mounting over a heater, preferably a radiant heater ( 17 ), to form a cooking appliance ( 11 ), characterized in that the hob plate ( 13 ) has a material with a negative coefficient of thermal expansion. Hob plate according to claim 1, characterized in that that the material with the negative thermal expansion coefficient has an isotropic coefficient of expansion. Hob plate according to claim 1 or 2, characterized that it consists of several materials, part of which having negative thermal expansion coefficient. Hobbed plate according to one of the preceding claims, characterized characterized in that it is ceramic. Hobbed plate according to one of the preceding claims, characterized characterized in that some of the materials have a positive thermal Has expansion coefficients. Hob plate according to one of the preceding claims, characterized by an overall negative thermal expansion coefficient. Cooking field plate according to one of the preceding claims, characterized in that a resulting crystal lattice of the hob plate ( 13 ) Has tetrahedral and octahedral crystal structures. Hobbed plate according to one of the preceding claims, characterized characterized in that the negative thermal expansion coefficient at least up to 1000 ° C given is. Cooking field plate according to one of the preceding claims, characterized in that the material with the negative coefficient of thermal expansion of the structure AB ₂ O ₈ or CD ₂ O ₇ , wherein A = Zr or Hf; B = W or Mo; C = 4-valent metal or transition metal and D = P or V is. Cooking field plate according to one of the preceding claims, characterized in that the material with the negative coefficient of thermal expansion ZrW ₂ O ₈ . Hob plate according to one of the preceding claims, characterized in that it has on one of its sides a coating, preferably with the same coefficient of thermal expansion as the hob plate ( 13 ). Hob with a cooktop panel ( 13 ) according to one of the preceding claims, wherein in particular under the cooktop panel ( 13 ) several heating devices ( 17 ) are arranged side by side. Hob according to claim 12, characterized in that that at least one heater directly on the hob plate is applied, preferably as a flat or branched coating, especially in a thick film process.