WO1999009791A1

WO1999009791A1 - Ceramic hob

Info

Publication number: WO1999009791A1
Application number: PCT/SE1997/001356
Authority: WO
Inventors: Rudolf Buchta
Original assignee: Aktiebolaget Electrolux (Publ)
Priority date: 1996-07-25
Filing date: 1997-08-15
Publication date: 1999-02-25
Also published as: AU4140197A; SE506968C2; SE9602866D0; SE9602866L

Abstract

A ceramic cooking hob comprises a ceramic substrate (3) with one or several heating zones. Each heating zone is formed by a film layer covering in the shape of a thin film (2). The thin film consists of a metal silicide which is joined to the substrate via a silicon layer (4) and is preferably covered with a passivating layer (6). By means of the silicon layer (4) a good adhesion to the substrate (3) is secured, partly due to the fact that the silicon layer does not actively generate heat and partly in that the silicon layer has not reacted with the active heat generating layer of metal silicide (2). Moreover, a good self-healing property is achieved in the metal silicide layer (2) in the case of damage to the passivating layer (6).

Description

Ceramic hob

Technical field

The present invention refers to a ceramic cooking hob comprising a ceramic substrate having one heating zone or a plurality of such zones each having the shape of an active heat-generating film layer covering on the surface of the substrate.

State of the prior art

There are known ceramic heating elements, e.g. disclosed in US-A-4 690 872, in which a thin layer of a metal silicide (selected from groups IVb, Vb and Vib of the periodic system) is directly applied to the surface of the ceramic substrate to be used as the film layer covering. Examples of film layers given in the cited document include suicides of molybdenum and tungsten wherein the thickness of the layer is indicated to be between 2 and 5μm.

Obviously, the thought behind the use of a silicide as the active heat generator, according to the publication referred to, is based on the good heat-resisting property of the silicide. However, there are annoying problems associated with the known design if used in a cooking hob due to the fact that the ceramic material of the hob has a coefficient of thermal expansion which essentially equals zero resulting in considerable differences to arise between the coefficients of thermal expansion for the metal silicide used and the supporting ceramic substrate. Accordingly, during heating serious stress would arise in the transition area betweeen the metal silicide and the ceramic substrate which easily leads to the metal silicide cracking. It is also known to give the active heating element the shape of a thin film of pure silicon, see for example WO91/10336, which is applied directly to the supporting ceramic substrate. However, silicon is more inclined to oxidize which causes a certain change in resistance after some time in use. Accordingly, a silicon thin film is less suitable as an active heating element.

Object of the invention and description of same

Accordingly, the object of the invention is to improve the heat generating part for use in ceramic cooking hobs of the kind indicated above. Among other things the desire of a reliable function and a long life of the film layers applied to the ceramic substrate of the cooker hob is to be met without any increase of the manufacturing costs for the cooker hob. The object is achieved, according to the invention, in that the film layer covering is connected to the surface of the ceramic substrate via an intermediate silicon layer.

By means of such intermediate silicon layer a good mechanical joint is achieved between the ceramic substrate and the active film layer which, accordingly, is not in direct contact with the ceramic substrate. As will be discussed more in detail below, this is achieved in that during manufacture a metal is allowed to react with the upper part of the silicon layer, which has previously been applied to the ceramic substrate, in order to form the active film layer covering. In this way an optimal bond is obtained between the film layer and the remaining silicon layer which has not reacted with the metal and where also the highest thermo-mechanical gradient appears. Normally, the adhesion between the silicon layer and the ceramic substrate is good even if less good than between the silicon layer and the active film layer. However, because of the fact that the stress appears in the active film layer due to current flow and associated temperature variations the more sensitive interface between the ceramic substrate and the silicon layer is not influenced by these stresses and strains. The silicon layer, which is mainly electrically passive, can be considered to provide a buffer between the surface of the ceramic substrate and the active film layer, said buffer, per se, acting to even the stress variations that may occur. Advantageously and in a manner known per se, the film layer covering according to the invention comprises a silicide based on one metal or a plurality of metals selected from the groups IVb, Vb and Vib of the periodic system. Preferably, titanium silicide is used as the film layer which is suitably covered by an outer passivating layer of a suitable material, such as silicon dioxide.

Another important feature of the cocking hob according to the invention is the self-healing process obtained by the application of the intermediate silicon layer. Damage in the passivating layer will then lead to the growth of new oxide by the atmospheric oxygen joining with diffused silicon in the active film layer of titanium silicide. Permanent damage in the titanium silicide will not occur until all silicon of the silicon layer has been exhausted. However, theoretically, it will take approximately 1000 years to oxidize 1 μm of silicon at 400°C. As a result extremely thin layers can be used which give low stress between the heating layer and the ceramic substrate.

Another object of the invention is to provide an economically favourable method for producing a film layer on the surface of a ceramic cooking hob with the features indicated above. The object has been achieved according to the invention by applying a thin silicon layer to the surface of the ceramic substrate of the cooking hob in order to form a heating zone, said silicon layer being subsequently covered with a thin layer of a metal selected from any of the groups IVb, Vb and Vib of the periodic system and said silicon layer and the metal being then heated to a suitable temperature in order to form a metal silicide in the interface between the silicon layer and the metal used. Advantageously, the film layer thus obtained can be covered with a passivating layer.

A preferred end product obtained by use of the method according to the invention comprises, as seen from the surface of the ceramic substrate of the cooking hob, an intermediate layer of silicon (Si) fixedly secured to the surface of the substrate and having a thickness of about 1 μm, a thin layer of titanium silicide (TiSi0₂) formed by reaction with the silicon in the surface of the intermediate layer and having a thickness of about 0.2 μm and in use of the cooking hob being connected to an electric power supply, and, possibly, a passivating layer of silicon dioxide (Si0₂) of a thickness of about 1 μm.

Description of a preferred embodiment

The ceramic cooking hob according to the invention will be described more in detail below by means of illustrative preferred embodiments the purpose of which is to exemplify, only, the realization of the invention. The structure of the ceramic cooking hob according to the invention is shown in the appending drawing, in which:

Fig. 1 is a schematic sketch showing in perspective a complete cooking hob having four heating zones; Fig. 2 is a section A-A in fig. 1 trough part of a heating coil of a heating zone without any passivating layer;

Fig. 3 is a corresponding section A-A in fig. 1 through part of a heating coil of a heating zone with a passivating layer; Fig. 4 shows the heating coil of Fig. 3 in a preceding manufacturing stage, and

Fig. 5 is a section A-A through an alternative embodiment of the heating coil according to the invention.

In fig. 1 a complete coking hob 1 is shown in a schematic perspective view, said hob having four heating zones H1-H4 each comprising a plurality of heating elements L. The heating zones

H1-H4 can have any shape, accordingly not limited to the circular shape shown in Fig. 1.

The present invention relates to a ceramic cooking hob of the kind shown in fig. 1 wherein each of the heating elements L comprises a thin film 2 as shown in the section A-A of various exemplifying embodiments according to Figs. 2, 3 and 5. The thin film 2 is an electrically powered heating unit, in the preferred embodiment of the invention being a metal silicide. The basic material of the metal silicide is deposited on the lower side of the ceramic substrate 3 of the cooking hob. The deposition takes place in a vacuum process of the same kind as in manufacture of micro-electronic circuits.

According to the invention and as indicated in fig. 4 the deposition can take place in that a thin layer, about 1 μm, of silicon 4 is applied on the ceramic substrate 3 with the desired shape (rings, spirals, finite straight or wave-shaped stripes etc.). The silicon layer is covered with a thin, about 0.1 μm, titanium layer 5. The application or deposition of the different layers 4 and 5 can be performed by means of various vacuum processes such as sputtering, vacuum deposition or CVD (Chemical Vapor Deposition) .

When the deposition of the above layers 4 and 5 has been carried out the ceramic substrate 3, with the applied layers 4 and 5, is heated to a suitable temperature (840°C) for an adequate time period (30 seconds). The values indicated have been taken from tests performed in connection with the manufacture of a cooking hob in which a halogen lamp was used as heating source for said heating process. Of course, this heating can take place in different ways and in addition to the use of a halogen lamp another type of radiation heater can be used or an oven. Thereby the surface of the silicon layer 4 reacts with the titanium layer 5 forming a thin layer of titanium silicide which constitutes the thin film 2 referred to above, as shown for the various alternative embodiments according to figs. 2, 3 and 5. The intermediate silicon layer 4 has got a number of tasks for the purpose of securing a reliable function of the cooking hob 1. Accordingly, the silicon layer 4 has a relatively good adhesive capacity against the surface of the ceramic substrate 3. Moreover, the silicon layer 4 forms a mechanically strong connection with the thin film 2 due to its reaction with the titanium metal. In other words, a good adherence between the heating metal silicide/thin film 2 and the surface of the substrate 3 is achieved in spite of the great difference in thermic expansion coefficient due to the fact that the intermediate silicon layer 4 functions as a stress absorbing layer because the thermic expansion coefficient of this layer has a value between those of the thin film 2 and the substrate 3. In addition, the silicon layer 4 has the advantage of offering a self-healing process.

Even if the embodiment of the heating coil L shown in Fig. 2, the outer surface of which being constituted by the metal silicide 2, is an advantageous embodiment where a connection for power supply of the active thin film can be made directly on said film, there are occasions in which, suitably, as the outer surface a passivating surface 6 is provided. Such an embodiment is shown in Fig. 3. A defect in the passivating layer 6 will only lead to the growth of new oxide in the silicide layer 2 by atmospheric oxygen uniting to diffused silicon.

During manufacture of a cooking hob 1 having heating coils L formed as shown in Fig. 3 deposition of the layers 4 and 5 takes place as indicated above in connection with Fig. 2, the titanium layer subsequently being also covered with a passivating layer 6 prior to the above-described heating process taking place. Suitably, the passivating layer 6 consists of silicon dioxide thermo-mechanically matching the ceramic substrate and having good insulating and oxidation hampering properties. Pure Si0₂ as well as SJgO doped with phosphorus, called PSG, or boron, called BSG, can be used. Alternatively, silicon nitride, oxide nitride or aluminum nitride can possibly be used.

Even if a preferred embodiment of the ceramic cooking hob and of the method of making same have been described in connection with Figs. 1-4, the invention must not be considered to be limited to said embodiment. The deposition of the layers on the ceramic substrate in the desired shape can take place directly by the use of e.g. a shadow mask or by depositing a wholly covering layer which is patterned in a photolithographic process and etched. It is also possible to deposite the metal 5 in several thin layers with intermediate silicon 4 for the purpose of obtaining the desired silicon layers 2 as shown in Fig. 5. The outer layer can either be a silicide layer 2, as shown, or a silicon layer 4 or, alternatively, a passivating layer 6.

Also other metals than titanium can be used for the metal silicide structure.

Accordingly, the invention as defined in the claims to follow is intended to include also those modifications and embodiments easily identified by the man skilled in the art after having assimilated the content of the description given above .

Claims

C l a i m s

1. Ceramic cooking hob comprising a ceramic substrate (3) having one heating zone or a plurality of heating zones (H1-H4), each of which having the shape of an active heat- generating film layer covering (2) provided on the surface of the ceramic substrate (3) and connectable to an electric power supply, characterized in that the active film layer covering (2) is joined to the surface of the ceramic substrate (3) of the cooking hob (1) via an intermediate silicon layer (4).

2. Ceramic cooking hob according to claim 1, characterized in that the film layer covering (2) has the shape of a thin film comprising a silicide of a metal selected from any of the groups IVb, Vb or Vib of the periodic system.

3. Ceramic cooking hob according to claim 1 or claim 2, characterized in that the film layer covering (2) is covered with a passivating layer (6).

4. Ceramic cooking hob according to any of the claims 1-3, characterized in that the film layer covering (2) consists of titanium silicide.

5. Ceramic cooking hob according to any of the claims 1-4, characterized in that the intermediate silicon layer (4) comprises polycrystalline silicon.

6. Method for producing a film layer covering (2) on the surface of a ceramic cooking hob (1), characterized in that a thin layer of silicon (4) is deposited on the surface of the ceramic substrate (3) of the cooking hob to form a heating zone

(H1-H4), said silicon layer being subsequently covered with a thin layer (5) of a metal selected from any of the groups IVb,

Vb or Vib of the periodic system, and the silicon layer (4) and the metal (5) being then heated to a suitable temperature to form a metal silicide (2) in the interface between the silicon layer and the metal used.

7. Method according to claim 6, characterized in that the thickness of the deposited silicon layer (4) is between 0.5 ╬╝m and 2 ╬╝m, preferably 1 ╬╝m.

8. Method according to claim 6 or claim 7, characterized in that the thickness of the deposited metal layer (5) is less than 0.5 ╬╝m and preferably amounts to about 0.1 ╬╝m.

9. Method according to any of claims 6-8, characterized in that the heating of the silicon layer (4) and the metal (5) proceeds so that the metal silicide (2) formed in the interface between the silicon layer and metal reaches a thickness of about 0.2 ╬╝m.

10. Method according to any of claims 6-9, characterized in that the deposition of the different layers is performed by use of a vacuum process, e.g. a sputtering, a vaporization or a CVD process .

11. Method according to any of claims 6-10, characterized in that the film layer (2) obtained is covered with a passivating layer (6) .

12. Method according to any of claims 6-10, characterized in that the metal (5) deposited comprises several thin layers of silicon (4 ) .