EP0319079B1 - Vitroceramic heating element - Google Patents

Description

The invention relates to a glass ceramic heating element comprising at least one flat electric heating body applied to one face of a glass ceramic plate, this heating body including from this face a first insulating layer, a second conductive layer for forming power supply lines, and a third resistive layer to form a heating resistor.

The invention finds its application in the production of household appliances for which it is sought to associate a glass-ceramic plate appreciated for its ease of maintenance, with a heating stove at high temperature greater than or equal to 650 ° C.

It is already known from the state of the art by United States patent no. 3,978,316 to produce a heating element on a glass-ceramic substrate. This heating element is produced by means of conductive films applied directly to a surface of the glass-ceramic substrate to form a heating resistor.

This cited document teaches having discovered two problems associated with the direct application of conductive layers on a glass-ceramic surface to form a heating element.

First, the modulus of rupture of the ceramic glass decreases, particularly when these films include metallic constituents in combination with ceramic phases.

Then, it proves difficult to obtain a good mechanical bond between the metallic films and the surface. smooth of the ceramic glass material, when the film is free of glassy phase. Therefore, this document teaches that attempts were first made to resolve this problem by applying a layer of sintered ceramic to the surface of the glass ceramic before applying the metallic film to provide a sufficiently rough bonding surface. . But then another problem appeared, because this sintered ceramic layer had an interaction with the glass ceramic and therefore tended to weaken the mechanical properties of the plate.

So the cited document teaches to make a buffer layer which allows the attachment of metal layers on a glass ceramic surface without however weakening the mechanical properties of the plate.

But with or without this bonding layer, the cited document teaches that the glass ceramic exhibits a high resistivity even at high temperature, which means that no layer of electrical insulation should be necessary. The present invention simultaneously poses and solves a problem which was hitherto completely unknown in the state of the art and which is as follows:
When an electrical resistance is produced by screen printing on a glass ceramic material, then supplied with electricity to produce a heating element by thermal transfer, it appears that, at these high temperatures used in hotplate hearths, the glass ceramic support material becomes conductive electricity while the ceramic material alone retains a high resistivity. It therefore seems that the combination of a screen printed high temperature electrical resistance and a ceramic glass material is impossible to use for the production of a consumer hob, since it does not meet safety standards. But it also seems that one should not be able to use a screen-printed insulating layer placed between the glass-ceramic plate and the screen-printed resistance for it seems that if we are looking for an insulating material with a zero coefficient of expansion, we will obviously arrive at the formulation of the glass-ceramic material itself which has previously been found to be electrically non-insulating at high temperatures.

The present invention however solves this problem by providing a formulation for an electrically insulating layer at high temperatures and which also has a coefficient of expansion quite suitable for the glass-ceramic support at these high temperatures.

It is known to the general knowledge of a person skilled in the art that in order to produce a screen-printing ink, a compound of a glassy phase and a ceramic phase is generally used. In the search for a compound capable of constituting the insulating layer, an additional problem appeared. To produce a resistive layer, the material chosen must have a positive or zero resistance temperature coefficient (CTR) for the temperature range considered. And this resistance coefficient should not change over time, especially when the device ages. However, if one chooses to make the insulating layer a material comprising an excessively large vitreous phase, or a material whose ceramic phase decomposes at high temperature to supply glass, this glass tends to rise in the resistive layer and, coating conductive particles, to cause the temperature coefficient to decrease, possibly even causing this temperature coefficient to become less than O. This would then lead to rapid deterioration of the heating element, leading to breakdown of the resistance. The present invention solves this problem by providing an insulating layer which does not react at high temperatures with the resistive layer.

According to the invention these problems are solved by a heating element as described in the preamble and characterized in that, to give the first layer insulating material having characteristics suitable for preventing the production of current leaks from the heating resistor to the ceramic plate at high temperatures, and in order to additionally give it properties of compatibility with both the glass-ceramic plate and the other layers, this insulating layer is produced by screen printing from a starting mixture for screen printing ink comprising on the one hand:
a glassy phase consisting in molar proportions of: ZnO + MeO 50 to 65% WHERE 10 to 20% Al₂O₃ 0 to 10% If 40 to 5%
in which MeO is an oxide chosen from refractory oxides such as:
MgO, CaO, and in which MeO is associated with ZnO in the molar proportions 0 to 10% of the whole of the glassy phase such that the proportions ZnO + MeO constitute 50 to 65% in moles of said glassy phase, and comprising d on the other hand an amorphous phase formed of amorphous silica, and in that the vitreous phase is associated with the amorphous phase in the volume proportions of 3 to 13% for the vitreous phase and from 97 to 87% for the amorphous phase.

Consequently, the heating resistor is perfectly insulated at high temperatures, its temperature coefficient is positive and the entire device supports aging well.

It is also known from patent FR-2 410 790, a ceramic glass hob comprising an electric heating body arranged in the form of a spiral below the plate as well as a thermostatic probe thermally coupled to the baking plate. inside the cooking surface area. On the marginal zone of the cooking surface is provided an unheated zone for the thermal coupling of the probe, the rest of the surface being covered of the two-wire heating element, the connections of which are located on the periphery of the cooking surface.

However, a cooking plate thus equipped offers several disadvantages. First of all the heating device is always of a high price because it is of a complex assembly. Then it is located at a certain distance from the ceramic hob, which leads to heat losses. Thus, it is subjected to a time constant for cooling and heating mainly due to poor thermal conduction of the air, which makes this kind of hob less flexible to use than hobs with adjustable flames. for example.

The present invention provides a heating element which is free from this type of drawback, since the heating resistor is directly in contact with the ceramic hob.

To this end, the invention presents a new formulation for a high temperature resistive ink. Indeed, it was also necessary that the coefficient of expansion of this ink, at the firing temperature, or at the temperature of use, be as close as possible to that of the glass-ceramic support, which is practically zero. This is difficult to achieve for a resistive material which contains conductive particles. The invention however solves this problem.

• la figure 1 qui représente un élément chauffant schématiquement et en coupe ;
• les figures 2a et 2b qui représentent les schémas de deux exemples de circuit de résistance électrique selon l'invention vus du dessus ;
• la figure 3 qui représente schématiquement une coupe des figures 2 selon l'axe I-I ;
• la figure 4 qui représente schématiquement une coupe des figures 2 selon l'axe II-II ;
• la figure 5a qui représente en fonction de la température T les variations linéaires relatives $\frac{\text{Δℓ}}{\text{ℓ }}$
  
  du matériau isolant, et la figure 5b qui représente en fonction de la température T les variations linéaires relatives $\frac{\text{Δℓ}}{\text{ℓ }}$
  
  du matériau vitrocéramique.

The invention will be better understood by means of the following description illustrated by the appended figures, in which:

• Figure 1 which shows a heating element schematically and in section;
• Figures 2a and 2b which represent the diagrams of two examples of electrical resistance circuit according to the invention seen from above;
• Figure 3 which schematically shows a section of Figures 2 along axis II;
• Figure 4 which schematically shows a section of Figures 2 along the axis II-II;
• FIG. 5a which represents as a function of the temperature T the relative linear variations $\frac{\text{Δℓ}}{\text{ℓ}}$
  
  of the insulating material, and FIG. 5b which represents as a function of the temperature T the relative linear variations $\frac{\text{Δℓ}}{\text{ℓ}}$
  
  glass ceramic material.

As shown in section in FIG. 1, the glass ceramic heating element according to the invention comprises a glass ceramic support plate 10 serving to work surface on its upper face 11, and substrate for the heating element 20 on its lower face 12.

Such a heating element has the advantage of forming a very smooth work surface and therefore easy to clean, as not showing any crevices, into which solid or liquid food particles can be introduced, for example, from the overflow of culinary containers. This very flatness is an advantage for receiving the culinary containers which always rest in a very stable manner on the work surface, which allows good heat exchange.

The underside 12 of the ceramic hob is coated with at least one heating hearth constituted by a heating element as shown seen from above in FIGS. 2.

The ceramic glass material has been chosen to date to produce cooktops because of its aesthetic appearance, the practical qualities mentioned above, and above all because of the fact that it has a zero coefficient of expansion which makes it very resistant to thermal shock. On the other hand, it has the disadvantage of being a poor conductor of heat, which means that, if the heating element is at all slightly away from the surface to be heated, there is a considerable temperature gradient in the air. Hence the advantage provided by the present invention which makes it possible to produce a heat source for the heating hearth in direct contact with the ceramic hob, which reduces the thermal resistances.

The poor thermal conductivity of the glass-ceramic material is used as an advantage to preserve between the hotplates, outside each heating hearth, non-hot zones, where electrical contacts can be made at leisure with traditional welding materials, therefore cheap.

According to the invention, to avoid the phenomenon of electrical conduction which appears not insignificant at temperatures above 300 ° C in the glass ceramic material of the plate, an insulating layer 21 is first deposited directly on the surface 12. This material is developed to first present a coefficient of expansion practically identical to that of the plate 10 and that of the upper layers 23, and that at the highest temperatures. This material is also developed to present excellent electrical insulation at these same high temperatures. This material is finally produced so that it does not diffuse into the resistant layers 23 either at cooking temperatures or at high temperatures, thus avoiding changing the temperature coefficient (CTR) of the resistant layers during aging.

du matériau isolant 21 en fonction de la température T, et la courbe C_V de la figure 5b montre les variations correspondantes du matériau vitrocéramique 10 aux mêmes températures. Ces courbes sont toutes deux très voisines de 0.Curve C _I in Figure 5a shows the relative linear variations $\frac{\text{Δℓ}}{\text{ℓ}}$

of the insulating material 21 as a function of the temperature T, and the curve C _V of FIG. 5b shows the corresponding variations of the glass-ceramic material 10 at the same temperatures. These curves are both very close to 0.

As shown in FIGS. 2, the insulating material 21 is deposited over the entire surface of the zone constituting the heating hearth of the cooking plate.

Figures 3 and 4 which are respectively schematic layers of Figures 2 along axes I-I and II-II show that the insulating layer 21 is a uniform layer of thickness 100 microns or more.

Two supply lines C₁ and C₂ for the electrical supply of the heating element are produced in the form of a screen-printed ribbon in a layer of thickness approximately 50 μm, depending on the applied voltage and the desired temperature, on the surface. of the insulating layer 21. These lines are formed of a conductive compound 22.

On the surface of the layer 21 and between the supply lines 22, extend ribbons R of a resistive compound 23 deposited in a screen-printed layer of thickness approximately 10 to 50 µm. The resistive material constituting the layer 23 is designed to have a coefficient of expansion as close as possible to that of the glass-ceramic material at high temperatures.

Figures 2 show two advantageous diagrams of the arrangement of these resistive ribbons between the supply lines. These diagrams are given purely by way of example, since the method for producing the heating element according to the invention is very flexible to use and makes it possible to carry out absolutely all types of configuration for this kind of circuit.

However, it will be advantageous for reasons of longevity of the circuit, to avoid as far as possible to provide a path having acute angles for producing the resistive tapes.

The circuit can thus cover a hearth forming a square area as illustrated in FIG. 2b, rectangular, oval or circular as illustrated in FIG. 2a. It may moreover be free according to the request of the consumer, or customer, to produce a cooking plate provided with several hearths showing a different shape. In addition, all the areas of hearth surface are achievable, and not only the surfaces with the two standard diameters currently marketed.

The circuit according to the invention being produced on the lower face 12 of the ceramic hob, the upper face 11 serving as a work surface remains blank.

In another application of the heating element according to the invention, the circuit can also be produced in small dimensions on a ceramic glass support to serve as a plunger heating element; for example to quickly bring a liquid to a given temperature. To avoid current losses in the liquid, the circuit can then be coated with an upper insulating layer 24 similar to layer 21.

For this application to a heating element plunger, the supply terminals are also fitted with waterproof and insulating sleeves like any conventional heated plunger.

In another application, the heating element according to the invention can also be used to make a high or low heating plate (floor or ceiling) in a convection or fan-assisted oven, or else in a multi-microwave oven.

In order to distance the welds of electrical conductors from the heating zone of the hearth, the lines C₁ and C₂ are extended sufficiently so that their end is placed in a relatively cold zone. In view of the poor thermal conductivity of the glass ceramic material, a few centimeters are sufficient to bring the lines C₁ and C₂ to an area where the temperature will always be low enough for the glass ceramic support material to be absolutely non-conductive of electricity. When the lines C₁ and C₂ have reached this so-called cold zone, the insulating layer 21 is interrupted under the layer 22 which constitutes these terminals so that this layer 22 is in direct contact with the glass-ceramic material.

This arrangement is not obligatory but it is advantageous for producing soft solder between the end of lines C₁ and C₂ and electrical conductors intended to bring the supply current to the heating resistor R. Indeed layer 22 is better attached, more resistant mechanically, when it is carried out directly on the ceramic glass material. This then makes it possible to produce such solderings.

According to the invention, the layers 21, 22, 23 and possibly 24 are produced by a screen printing technology by means of compounds whose formulation is given below.

A starting mixture for an insulating composition capable of forming a high temperature insulating screen printing ink for cooking under nitrogen is known from the prior art. Patent EP-0 016 498. According to this document, the mixture comprises a glassy phase consisting of the molar proportions of the following oxides: If 30 to 55% ZnO 20 to 40% WHERE 0 to 20% Al₂O₃ 0 to 10% SrO, BaO, CaO 5 to 40% CoO 0 to 10%
and ceramic phase consisting of ZnO + CoO, the glassy phase accounting for 85 to 60%, and the ceramic phase accounting for 15 to 40% by volume of the mixture.

However, this mixture has a coefficient of expansion at high temperature which is close to that of alumina, that is to say very far from that of the glass-ceramic material itself.

This is why, according to the invention, a starting mixture for a screen-printing ink capable of producing layer 21, that is to say both insulating at high temperature, with a coefficient of expansion close to that of the glass-ceramic material, and not diffusing into the upper resistive layer, will firstly comprise a glassy phase constituted in molar proportions by: ZnO + MeO 50 to 65% WHERE 10 to 20% Al₂O₃ 0 to 10% If 40 to 5%
in which MeO is an oxide chosen from refractory oxides such as MgO, CaO, MeO being associated with ZnO in molar proportions 0 to 10% of the entire glass phase and such that the proportions ZnO + MeO constitute 50 to 65 % in moles of said vitreous phase.

In an exemplary embodiment, a glassy phase can be found, made up in molar proportions of: ZnO + MeO 62% WHERE 17% If 21%

In another embodiment, we can find a glassy phase consisting in molar proportions of: ZnO + MeO 62% WHERE 12% Al₂O₃ 5% If 21%

The starting mixture for such an insulating composition will also comprise an amorphous phase. The glassy phase and the amorphous phase are associated in volume proportions such as: Glassy phase 3 to 13% and preferably 5% Amorphous phase 97 to 87% and preferably 95%.

According to the invention, the amorphous phase will consist of amorphous silica chosen for its low coefficient of expansion.

To carry out the production of an insulating screen-printing ink according to the invention, a glass is first of all produced, the molar proportions of which correspond to the ranges indicated above or to one of the examples cited. The glass thus obtained is ground. During this operation is incorporated to obtain a homogeneous mixture the powder forming the amorphous phase in the chosen volume proportions.

This grinding can be carried out in a liquid medium such as water. The result of the grinding is then dried and then dispersed in an organic vehicle.

As an organic vehicle capable of making this starting mixture screen-printing, it is possible to use a solution containing a polymer, for example a solution of ethylcellulose in a terpineol or a mixture based on terpineol. This organic vehicle can represent before cooking 10 to 40% of the weight of the screen-printing ink. The proportions of the organic vehicle relative to the ink are chosen according to the desired rheological behavior.
As in the present case, none of the materials chosen to make the heating device on ceramic glass presents the risk of oxidizing in air, the ink is baked in the open air. The organic vehicle is thus consumed using oxygen from the air. Cooking at around 900 ° C. is carried out in a so-called pass-through oven for about 10 minutes.

On the other hand, it is known from patent EP-0 048 063 a starting mixture for a resistive composition of CTR of the order of: ± 100 10⁻6 ° C⁻¹. This composition comprises an active phase consisting of a mixture of bivalent and / or trivalent metallic hexaborides, and a glass frit formed of calcium borate and optionally of silica.

However, in this resistive ink, the composition of the glass is not intended to constitute a heating resistance, and more particularly a heating resistance capable of being brought to 650 ° C. by the Joule effect, and of presenting a positive CTR and which remains so in aging.

This is why, according to the invention, a starting mixture for a screen-printing ink capable of producing the layer 23, endowed with this property and with a coefficient of expansion close to that of the glass-ceramic material, will firstly comprise an active phase consisting of volume proportion of the total mixture of:
RuO₂ ≃ 15 to 40% and in particular preferably ≃ 30%
CuO ≃ 0 to 5%
and a glassy phase made up of a composition similar to that of melted vitroceram and quenched in volume proportions complementary to the above mixture. Thus, thanks to a judicious thermal cycle, the glass melts ensuring the function of binder then during this same cycle recrystallizes from vitroceram. The glass-ceramic thus formed makes it possible to obtain the appropriate coefficient of expansion.

On the other hand, the CTR of this resistance, when it is carried out with the preferred proportions, is:
+ 520 ppm ° C⁻¹ between 20 and 300 ° C
and + 150 ppm ° C⁻¹ between 300 and 650 ° C.

To make the resistive screen-printing ink, the glassy phase is ground and the oxides constituting the phase active are incorporated as it was said previously for the production of insulating ink. Following this, the mixture is incorporated into a rheological vehicle already described.

According to the invention, a screen-printing ink capable of producing lines C₁ and C₂ in layer 22, will be formed in one example of a silver powder (Ag) + palladium (Pd) or platinum (Pt), or even in another example of a silver powder (Ag) alone, to which a small proportion of copper oxide (CuO) is added, this powder then being incorporated into a rheological vehicle as described above.

Tableau II mélange de départ pour la couche 23 résistive Composition du mélange en proportions volumiques Exemple préférentiel Composition générale Phase vitreuse = composition semblable au vitrocéram ≃ 65 % Complément à 100 % RuO₂ ≃ 30 % ≃ 15 à 40 % Phase active CuO ≃ 5 % ≃ 0 à 5 % Tableau III mélange de départ pour la couche 22 conductrice Composition du mélange en proportions volumiques Exemple I Exemple 2 Ag 80 à 100 % Ag 80 à 100 % CuO 20 à 0 % Pd/Pt 20 à 0 % CuO en proportions complémentaires The tables below summarize the compositions of the starting mixtures for layers 21, 22 and 23.

Table II starting mixture for resistive layer 23 Composition of the mixture in volume proportions Preferred example General composition Glassy phase = composition similar to vitroceram ≃ 65% 100% supplement RuO₂ ≃ 30% ≃ 15 to 40% CuO active phase ≃ 5% ≃ 0 to 5% starting mixture for conductive layer 22 Composition of the mixture in volume proportions Example I Example 2 Ag 80 to 100% Ag 80 to 100% CuO 20 at 0% Pd / Pt 20 to 0% CuO in complementary proportions

Claims (6)

1. A glass-ceramic heating element comprising at least a flat electric heating member provided on one surface of a glass-ceramic plate, said member comprising, starting from said surface, an insulating first layer (21), a conductive second layer (22) for the formation of two electrical supply lines (C₁, C₂), and a resistive third layer (23) for the formation of a heating resistor (R), characterized in that, for giving the insulating first layer (21) appropriate characteristics to preclude leakage of current from the heating resistor to the ceramic plate at high temperatures and, in addition, for giving said first layer properties compatible with both the glass-ceramic plate and the other layers, said insulating layer is formed by a screen-printing process using a starting mixture for screen-printing ink, which mixture comprises, on the one hand: a vitreous phase formed by molar proportions of: ZnO + MeO 50 to 65 % B₂O₃ 10 to 20 % Al₂O₃ 0 to 10% SiO₂ 40 to 5% in which MeO is an oxide chosen from refractory oxides such as:
   MgO, CaO, and in which MeO is associated with ZnO in the molar proportions from 0 to 10% of the total of the vitreous phase, such that the quantities of ZnO + MeO constitute 50 to 65% in moles of said vitreous phase, and which comprises, on the other hand, an amorphous phase formed by amorphous silicon dioxide, and the vitreous phase is associated with the amorphous phase in proportions of 3 to 13 % by volume for the vitreous phase and 97 to 87 % for the amorphous phase.
2. A heating element as claimed in Claim 1, characterized in that in the starting mixture for the formation of the insulating layer (21) the vitreous phase comprises, in molar proportions: ZnO + MeO 62 % SiO₂ 21 % B₂O₃ 17 %
3. A heating element as claimed in Claim 1, characterized in that in the starting mixture for the formation of the insulating layer (21) the vitreous phase comprises, in molar proportions: ZnO + MeO 62 % SiO₂ 21 % B₂O₃ 12 % Al₂O₃ 5 %
4. A heating element as claimed in any one of the Claims 1 to 3, characterized in that in the starting mixture for the formation of the insulating layer (21) the vitreous phase is in a proportion of 5% and the amorphous phase in a proportion of 95% by volume of the total mixture.
5. A heating element as claimed in any one of the Claims 1 to 4, characterized in that, for giving the resistive third layer (23) properties compatible with the glass-ceramic plate and the other layers, said third layer is formed by a screen-printing process using a starting mixture for screen-printing ink, which mixture has an active phase
   which in proportions by volume of the total mixture comprises: RuO₂ 15 to 40 % CuO 0 to 5 % and a vitreous phase comprising a composition similar to that of the glass-ceramic in complementary proportions by volume.
6. A heating element as claimed in any one of the Claims 1 to 5, characterized in that, for giving the conductive second layer (22) properties compatible with the glass-ceramic plate and the other layers, said second layer is formed by a screen-printing process using a starting mixture for screen-printing ink, which starting mixture comprises silver powder (Ag) associated with powder of either copper oxide (CuO) or palladium (Pd) in respective proportions by volume of from 80 to 100 % for the silver (Ag) and from 20 to 0 % for the copper oxide (CuO) or the palladium (Pd).

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