WO2008047199A2

WO2008047199A2 - Ceramic or glass cooktop with integrated heater

Info

Publication number: WO2008047199A2
Application number: PCT/IB2007/003016
Authority: WO
Inventors: Gianfranco Cirri; Maria Prudenziati
Original assignee: Inglass S.P.A.
Priority date: 2006-10-18
Filing date: 2007-10-05
Publication date: 2008-04-24
Also published as: WO2008047199A3; ITMO20060336A1

Abstract

Ceramic or glass cooktop with a heater integrated and self-regulated in temperature, characterized by an interface (2) presenting a metallization layer, having the function of providing a layer for supporting and adhesion to the ceramic plate of the subsequent layers that form the integrated heating system, as well as of obtaining a conductive layer which is able to receive an electrical earthing connection that is industrially reliable. The integrated heating system is obtained by adapting APS technology.

Description

"CERAMIC OR GLASS COOKTOP WITH INTEGRATED HEATER"

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Field of the invention The present invention relates to glass and/or ceramic cooktops for domestic cookers. State of the prior art

Glass-ceramic cooking surfaces or cooktops are commercial products that have been available now for a long time. The most appreciated aspect of these cooktops is the planarity of the surface and the total absence of expansion joints (tops with electrical hot plates) and holes (gas stoves). Consequently, these cooktops allow convenient movement of the cooking containers and an appreciable simplicity of cleaning. A further advantage of these cooktops consists in the possibility of varying the colours and decorations of the cooking surface itself to integrate it with the design of its surroundings. In brief, the commercial success of glass-ceramic cooktops derives from the planarity of the surfaces, the absence of any porosities or irregularities, and the aesthetic appearance of the heating area.

From the technological standpoint the use of glass ceramics in this application is intimately linked to the availability of glass ceramics with a low coefficient of thermal expansion (CTE) in the temperature range from room temperature to 700⁰C. CTE values of a few tenths of part per million per degree centigrade are characteristic of different glass ceramics (e.g., aluminosilicate glass ceramics with crystalline phases of the β-spodumene, β-spodurnene-β-eucryptite or h-quartz type). With an adequate choice of the composition of the precursor glass and of the process of ceramization, it is possible to obtain CTE values close to zero. A contained CTE is necessary to prevent failure of a brittle material that is subjected to differences in temperature of 600⁰C in points that are just a few centimetres apart and to thermal shock, e.g., when a cold liquid drips onto the hot plate or a cold pan or other container is set on the plate at 600⁰C.

Two different procedures are currently used for heating the plate and transmitting heat to the cooking containers: a) a system based upon the parasitic currents induced by the electromagnetic field in a cooking container made of ferromagnetic material (or having at least the bottom made of ferromagnetic material); ahd b) an infrared system for heating by irradiation with radiation emitted by electrical resistors brought to high temperature and set under the plate, at some distance from the bottom surface of the plate itself. The heating elements are thus generally of a " discrete element" type, such as the ones described in the U.S, Patent No. 3,889,021 and British Patent No. 1,391,076. This method is consequently of wider application, but its efficiency calls for a glass-ceramic plate with high transparency to IR radiation.

Units with heating elements that form an integral part of the cooktop would offer substantial advantages in terms of efficiency, economy of space and ease of assembly. However, the development of said hot plates with integrated heating elements presents some technological problems that up to now have not been satisfactorily solved. Amongst these problems, particular importance lies in the inadequate characteristics of glass ceramics, when it is considered that the electrical resistivity of such materials decreases rapidly as the temperature increases, and consequently a heating element directly applied thereon would not be sufficiently insulated at the operating temperatures. Consequently, for insulation, it is necessary to set between the two (plate and heating element) a dielectric with high dielectric-breakdown voltage. The latter will have to reach, for 120-220 VAC electrical supply, values of 3000-3500 V in the absence of earthing. or else of 1000 V if an. electrically conductive layer, earthed, is available between the plate and the heating element.

The approaches adopted up to now for developing electrical-heating units of the integral type on glass ceramics, which will satisfy the requirements mentioned above, have been basically the ones described in the patents Nos. US- 3,978,315 and US-6,921,S82.

The patent US-3,978,315 describes a glass-ceramic plate provided with a zinc-aluminosilicate semicrystalline coating that should to protect the plate from any undesirable interaction with the ceramic (e.g., corundum) applied subsequently; the task of the latter ceramic is to provide the necessary electrical insulation between the heating element and the top surface of the plate itself. The heating element consists of a thin conductive film. No method is described for providing the electrical connections between the heating element and the electrical-supply circuit and no practical applications of this invention are known.

Instead, the patent No. US-6,921,882 describes a cooktop consisting of a plate made of glass ceramics or glass, comprising a heating element made up of a conductive path deposited on a plurality of insulating layers each of which has a porosity decreasing towards the path itself. The dielectric layers are applied with a thermal-spraying process. The heating element itself is preferably deposited by thermal spray or alternatively by serigraphy. Also in this case (as in the previous patent), it is not indicated how the electrical connections are made.

As may be readily inferred, the possibility of providing a plurality of layers with decreasing porosity using the APS technique as likewise the possibility of obtaining safe and industrially reproducible levels of insulation of 3000-3500 V, is, as is well known to persons skilled in the sector, particularly difficult to obtain at an industrial level.

Summary of the invention

A first purpose of the present invention is to provide of a cooktop made of ceramic and/or glass with integrated and self-regulated heater having improved characteristics in terms of safety of operation, keeping the plate separate from the corresponding integrated and self-regulated heating system, with a purposely provided earthed metal interface.

A second purpose of the invention is to produce a cooking plate with good long-term stability, by choosing the most appropriate system for the production of said interface, on the basis of the ceramic selected. A third purpose of the invention is to produce a cooktop wherein the heating system is obtained with simple and already consolidated cost-effective processes, such as the APS processes described in the patent applications Nos. IT- A-TO2004A0002S3, dated April 23, 2004, US-A-2005/0257367, dated November 24, 2005 and IT-A-MO2006A000314, dated October 5, 2006 (the latter not published at the date of priority of the present application).

A fourth purpose of the invention is the use of a technique designed to improve the dielectric properties of the intrinsically porous insulating layers deposited by APS₃ e.g., AI₂O₃, according to what is described in the Italian patent application No. IT-A-MO2006A00322 (not published at the date of priority of the present application),

The above purposes are achieved, according to the invention, thanks to a cooktop as defined in the ensuing claims.

The interface with which the cooktop according to the invention is equipped has the dual advantage both of providing a layer for supporting and engagement to the ceramic plate of the subsequent layers deposited by APS that form the integrated heating system and of providing a conductive layer which is able to receive an industrially reliable earthing connection, thus overcoming the problems related to the electrical characteristics of ceramic-glass cooktops. Brief description of the drawings The peculiar characteristics and advantages of the invention can be inferred from the ensuing description with reference to the annexed plate of drawings, which is provided purely by way of non-limiting example and in which:

- Figure 1 is a schematic top plan view illustrating a cooktop; and

- Figure 2 shows the cooktop in cross-sectional view. Detailed description of the invention

With reference to the drawings, a cooktop with integrated and self- regulated heater according to the invention comprises a plate 1 made of glass and/or ceramic, on the bottom face of which designated by 2 is a purposely provided innovative interface, provided with a metallization layer, designated by 3 is a first insulating layer consisting of 100 μm of AI2O3 deposited by APS, designated by 4 is a nickel path of appropriate section for the heating element, deposited by APS, designated by 5 is a second insulating layer consisting of 100 μm of AI₂O₃ deposited by APS, designated by 6 is a sealing layer of serigraphed and remolten glazing, and finally designated by 7 is an anti-emissive gold flash plating.

Innovative methods for obtaining a heater integrated with the ceramic plate and self-regulated are based upon the production of a suitable interface with a metallization layer between the ceramic plate and the heating element and are funded in the technology of industrial and microwave transmitting tubes (valves) and in the technology of ultrasound delay lines.

The choice of the method of production of the interface is made on the basis of the physico-chemical characteristics of the plate of glass and/or ceramic to be heated, whilst the integrated heating system is obtained by adapting the APS technology described, for example, in the documents Nos. IT-A-TO2004A000253, US-A-2005/0257367 and IT-A- MO2006A000314 .

Illustrated hereinafter are four different processes envisaged according to the invention for obtaining a suitable interface with a metallization layer between the ceramic plate and the heating element obtained with APS technology, which are applicable on the basis of the physico-chemical characteristics of the plate to be heated. 1st Process (Pi) - On the bottom surface of the ceramic plate a lapping is performed, with appropriate grain size, followed by a cleaning sputtering process, by deposition of a chrome thin film, and by deposition of a copper film subsequently coated with Galvanic or electroless nickel-plating. 2nd Process (P₂) - This process is an extension of the preceding process: once the copper film has been deposited, an indium film is deposited. At the same time, a copper lamina (< 0.1 mm) of the same width as the plate is gold plated.

Said copper lamina is rested on the indium film and is subjected to pressure, in a vacuum, at a temperature lower than 18O⁰C for a time sufficient to obtain Ih/ Au diffusion and hence brazing between the ceramic and the copper lamina.

3rd Process (P₃) - After carrying out lapping, the bottom surface of the ceramic plate is subjected to a physico-chemical activation and then coated with a chemical nickel layer.

4th Process (P₄) - This process for obtaining the interface is based upon application of the "moly-manganese process", well known to persons skilled in the sector of production of ceramic transmitting tubes; it consists in sintering a suspension of Mo and Mn powders for metallization of the surface of the ceramic. After sintering at a high temperature, the surface of the bottom face of the plate thus metallized is nickel-plated and sintered once again. On said interfaces, obtained with one of the processes Pj, P₂, P3, P45 i* is intrinsically easy to provide industrially reliable electrical connections.

Once the necessary interface has been obtained with the process most suitable for the ceramic plate, there follow:

- deposition with APS technology, according to the processes described in the patent applications cited above, of a first insulating layer of Al₂O₃ of a thickness of approximately 100 μm, sufficient for insulation at 1000 V in so far as the metal interface is earthed;

- deposition on this first insulating layer, once again using APS technology, of the conductive path of the heater made of nickel or equivalents, with a high temperature coefficient of the resistance and of appropriate dimensions, and subsequent coating thereof with a protective layer of AI₂O₃, deposited by APS and with a thickness of approximately 100 μm;

- deposition by serigraphy of a glazing on said second Al₂Og layer, and remelting thereof in order to seal the intrinsic porosity of alumina spray, thus rendering the heating system immune from any pollution; - final gold flash plating, e.g., electroless gold plating, in order to improve the energy efficiency of the system.

It will emerge clearly from the foregoing description that the interface with which the cooktop according to the invention is provided enables both a layer for supporting and engagement to the ceramic plate of the subsequent layers deposited by APS that form the integrated heating system and a conductive layer able to receive an electrical earthing connection, thus overcoming the problems linked to the electrical characteristics of glass-ceramic cooktops.

Claims

I) A cooktop with integrated heater, characterized in that it comprises:

- a ceramic and/or glass plate (1); - an interface (2) adherent to the bottom surface of said plate and designed to separate electrically said heater from said plate and to ensure at the same time an optimal transfer of heat from the heater to the plate, said interface presenting a metallization layer.

2) The cooktop according to Claim 1, characterized in that said interface includes:

- a first thin insulating layer (3) obtained by APS;

- a conductive path (4) made of a material with high positive temperature coefficient, obtained by APS on said first insulating layer (3);

- a second thin insulating layer (5) obtained by APS deposited on the conductive path (4);

- a glazing (6) deposited by serigraphy on said second insulating layer (5) and subsequently reroolten; and

- a gold flash (7) deposited on said remolten glazing (6).

3) The cooktop as per Claim 1 or Claim 2, wherein the metallization layer of said interface is obtained according to a process including a step of lapping of the bottom face of said plate followed by a cleaning sputtering step and by a step of deposition of a chrome thin film and of a copper ϋlm subsequently coated with galvanic or electroless nickel-plating.

4) The cooktop as per Claim 3, wherein the metallization layer of said interface is obtained according to a process moreover including, following upon deposition of the copper film_} deposition of an indium film and provision of a thin copper lamina having the same width of said plate, which is rested on said indium film and subjected to pressure at a temperature below ISO⁰C and then brazed on said plate. 5)The cooktop as per Claim 1 or Claim 2, wherein the metallization layer of said interface is obtained according to a process including a step of lapping of Hie bottom face of said plate followed by a step of physico-chemical activation and by a step of coating with a chemical nickel layer. 6) The cooktop as per Claim 1 or Claim 2, wherein the metallization layer of said interface is obtained according to a process including a step of sintering at high temperature of a suspension of powders of molybdenum and manganese on the bottom face of said plate, followed by a step of nickel-plating and further sintering of said bottom face of said plate,

7) The cooktop as per Claim 1 or Claim 2, wherein the metallization layer of said interface is earthed.

8) The cooktop as per Claim 2, wherein the first insulating layer is obtained by APS and consists of AI₂O₃ or the like with a thickness of approximately 100 μm, deposited on the metal layer of the interface obtained with one of the processes according to Claim 3 or Claim 4 or Claim 5 or Claim 6. respectively chosen according to the highest compatibility with the plate selected-

9) The cooktop as per Claim 2, wherein the conductive path obtained by APS is made of nickel or equivalent metal or alloy, with high temperature coefficient of the resistance, in order to self-regulate the temperature and the dissipated power.

10) The cooktop as per Claim 2, wherein the second insulating layer is obtained by APS of AI₂O₃ or equivalent, with a thickness of the order of 100 μm.

11) The cooktop as per Claim 2, wherein the sealing glazing is obtained by serigraphy of a paste containing glass with a coefficient of thermal expansion compatible with said plate and with the corresponding heating system.

12) The cooktop as per Claim 2, wherein the sealing glazing is remolten at an adequate temperature in order to seal properly the intrinsic porosity of the insulator, preferably AI₂O₃, obtained by APS.

13) The cooktop as per Claim 2, wherein the remolten glazing is coated by an anti- emissive gold flash, preferably electroless gold plating,

14) A cooktop including a glass and/or ceramic plate with heater integrated and self-regulated in temperature according to Claim 2, said cooktop being characterized in that said insulating layer of separation between said plate and the heating system is designed to withstand insulation voltages of 3000-3500 V and is obtained with a technique that improves the dielectric characteristics of insulating layers deposited by APS. 15) The cooktop as per Claim 2, wherein the insulating layer of separation has high dielectric strength and is obtained by successive layers, preferably of AI2O3 or equivalents, deposited by APS and coated with an appropriate glazing, remolten, layer after layer, for sealing the porosities. 16) The cooktop as per Claim 1 or Claim 2. wherein the integrated and self- regulated heating system is obtained by depositing on said interface with high dielectric strength, obtained by sealing with purposely provided glazing the porosities of the insulator used, a first insulating layer OfAl₂O₃ or equivalents with a thickness of approximately 100 μm.

17) The cooktop as per Claim 2, wherein the conductive path is made of nickel or the like and deposited by APS.

18) The cooktop as per Claim 2, wherein the second insulation layer, preferably made OfAl₂Os. deposited by APS, has a thickness of approximately 100 μm.

19) The cooktop as per Claim 2, wherein a thin layer of glass powder is deposited, preferably by serigraphy, on the insulating layer OfAIzO₃ deposited by APS, 20) The cooktop as per Claim 2, wherein the glazing is remolten in order to close the pores of the underlying insulating layer.