Coating processes – Heat decomposition of applied coating or base material
Patent
1995-11-22
1998-02-17
Bell, Jancye
Coating processes
Heat decomposition of applied coating or base material
427229, 427230, 4273766, 427404, 4274192, B05D 302, B05D 722
Patent
active
057189463
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an induction heating substrate for a receptacle made of ceramic or of vitreous ceramic (porcelain, earthenware, terra cotta, brownware, etc.).
BACKGROUND OF THE INVENTION
Induction heating is becoming more and more widespread both in domestic cookers and in catering cookers, and it is tending to replace, in part, heating by transmission.
The coil of the induction top establishes an alternating magnetic field whose frequency generally lies in the range 25 kHz to 45 kHz. When heat production takes place, it relies on two different mechanisms depending on the properties of the materials concerned: current generation, with the flow thereof giving rise to considerable Joule effect heating which is used for cooking or warming food; ferromagnetic properties), then it is dissipated magnetic energy that gives rise to heating.
The magnitude of the eddy currents is a function of the strength and the frequency of the external magnetic field. Also, the direction of the eddy currents is such that the magnetic field that they generate tends to oppose the external magnetic field, thereby preventing it from penetrating to the core of the conductor. This is known as the "skin effect" and the thickness of the material affected by eddy currents increases with deceasing frequency in the external field, so commercially available induction tops are of relatively low frequency.
Unfortunately, the ceramic materials that constitute a large fraction of receptacles for holding food are neither magnetic nor conductors of electricity. It is therefore not possible, a priori, to use them for induction heating.
Proposals have already been made in FR-A-2 671 709 to place a heat-conducting multi-layer film on a receptacle made of non-magnetic material in order to absorb electromagnetic waves and microwave energy. To this end, a layer 1 picks up heat from a layer 2 that absorbs energy from electromagnetic waves, with said layers being covered by a protective layer 3. The layer 2 is made of gold in the form of a paste, silver powder, and oil. The forming and depositing of those layers require a long series of operations.
GB-A-2 010 054 proposes a complete integrated cooking system in which the induction coil and the receptacle are included in a thermally insulated enclosure in order to limit heat losses from the receptacle.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to mitigate that drawback and to enable a magnetic and/or electrically conductive layer to be put into place in a single operation, as is the case for an enamel slip using a paintbrush or silkscreen printing, hot or cold pad printing. Deposition is preferably performed in the form of a transfer including the conductive substrate.
For the above reasons, it is not necessary for the layer deposited in this way to be ferromagnetic or ferrimagnetic in order for the receptacle to heat up; it suffices for it to be mainly a good conductor of electricity.
According to the invention, the method of manufacturing a receptacle of ceramic material capable of being heated by induction consists in depositing on the bottom of the receptacle by painting, silkscreen printing, or transfer, a thin layer of material possessing properties of conducting electrically and/or magnetic properties leading to the production of heat when exposed to an electromagnetic field.
Such deposition performed by a technique that is conventional in the field of ceramics may be performed either after optional decoration has been applied or simultaneously therewith. Only one additional firing operation may be required, and that does not significantly alter the traditional process whereby ceramic objects are manufactured and decorated.
According to another characteristic of the invention, the film capable of being heated by electromagnetic induction and applied to the bottom of the ceramic receptacle is electrically conductive or possesses magnetic properties, and in particular ferro-or ferrimagnetic properties.
When the layer is merely a conductor of ele
REFERENCES:
patent: 4385082 (1983-05-01), Eichelberger et al.
patent: 5002826 (1991-03-01), Pollart et al.
patent: 5155316 (1992-10-01), Chiu
Bell Jancye
F. Deshoulieres (S.A.)
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