Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B3/00—Ohmic-resistance heating
  • H05B3/68—Heating arrangements specially adapted for cooking plates or analogous hot-plates
  • H05B3/74—Non-metallic plates, e.g. vitroceramic, ceramic or glassceramic hobs, also including power or control circuits
  • H05B3/748—Resistive heating elements, i.e. heating elements exposed to the air, e.g. coil wire heater

Definitions

• the invention relates to a ceramic hob with a hotplate made of glass ceramic or glass, with an electrical heating conductor layer, and with a thermally sprayed insulating layer between the hotplate and the heating conductor layer.
• Such a ceramic hob is known for example from DE 31 05 065 C2 or from US 6 037 572.
• the known ceramic hob has a hotplate made of glass ceramic, the underside of which is provided with a thermally sprayed, grounded metal layer onto which a ceramic insulating layer is sprayed, on the underside of which a heating conductor layer with a heating conductor is applied, for example by a screen printing process.
• Such a ceramic hob has compared to conventional ceramic hobs, which were previously heated essentially above the glass ceramic plate at a distance from the heating conductor via radiant heating, a significantly improved parboiling behavior, since the heat is now transferred by heat conduction and generated directly on the underside of the glass ceramic becomes.
• a glass ceramic suitable for a hob such as CERAN® from Schott, has an NTC characteristic, ie that the electrical conductivity increases noticeably as the temperature rises, there is a ceramic insulating layer between the heating conductor layer and the glass ceramic hotplate.
• a particular problem with such a ceramic hob is the different coefficients of thermal expansion of the individual layers.
• glass ceramics such as CERAN® have an expansion coefficient ⁇ which is close to zero ( ⁇ 0.15 • 10 "6 K “ 1 ).
• metals have significantly higher coefficients of expansion, which are well above 10 " 5 K 1.
• Ceramics have a lower coefficient of expansion (eg about 8 • 10 " 6 K “1 for Al 2 0 3 ), but this also leads to greater layer thicknesses considerable problems due to the thermal stresses occurring during operation.
• the dielectric strength of the insulating layer must be 3,750 V during cooking. This requires a relatively large layer thickness for the ceramic insulating layer, which must be about 300 ⁇ m or more for aluminum oxide.
• Such a thick ceramic insulating layer in turn, cannot be easily applied to a glass ceramic surface by thermal spraying, since cracks are usually observed or delamination occurs.
• the invention is therefore based on the object of providing an improved ceramic hob which avoids the disadvantages indicated above and is designed as a stable layer system which on the one hand has the necessary electrical safety and on the other hand ensures high stability in long-term operation.
• a thermally sprayed adhesive layer made of a ceramic material is provided on the hotplate.
• the object of the invention is completely achieved in this way.
• the insulating layer can now consist of cordierite, mullite or mixtures thereof or other thermally sprayable ceramics with a similarly low coefficient of thermal expansion.
• Cordierite and mullite have a coefficient of thermal expansion that is significantly lower than the thermal expansion coefficient of aluminum oxide. While the thermal expansion coefficient for cordierite is about 2.2 to 2.4 10 "6 K 1 , the thermal expansion coefficient for mullite is about 4.3 to 5.0 • 10 " 6 K "1. Thus, using this Materials significantly reduce the problem of thermal stresses during operation due to the lower thermal expansion coefficient.
• a layer made of aluminum oxide, titanium oxide or mixtures thereof is particularly suitable as an adhesion promoter layer.
• the layer thickness of the adhesion promoter layer, which is applied by thermal spraying, is preferably between approximately 10 ⁇ m and 150 ⁇ m, preferably approximately 30 to 100 ⁇ m, in particular in a range between approximately 40 and 70 ⁇ m.
• Such a thin adhesion promoter layer has practically no adverse influence on the overall system due to the thermal stresses caused thereby, but has an extremely good adhesion to the glass ceramic surface without damaging it in the area of the interface.
• a ceramic layer which preferably consists of cordierite, of mullite, possibly also of magnesium oxide or mixtures thereof, can now be applied directly to such an adhesion promoter layer by thermal spraying in the required layer thickness.
• a thermally sprayed, electrically conductive intermediate layer which is preferably grounded, is applied between the adhesion promoter layer and the insulating layer.
• This intermediate layer preferably consists of an electrically conductive ceramic or a cermet.
• An electrically conductive ceramic can be produced, for example, by thermal spraying of TiO 2 , since such a high oxygen loss occurs during thermal spraying that the material is electrically conductive. becomes capable.
• the volume conductivity for Ti0 2 at room temperature is between about 10 3 ⁇ cm to about 5 • 10 2 ⁇ cm.
• a suitable cermet has, for example, a metal matrix made of a nickel / chromium / cobalt alloy in which carbide particles, e.g. Tungsten carbide or chrome carbide are dispersed.
• Such a cermet layer does indeed have a coefficient of thermal expansion which is in the range from approximately 4 * 10 " 6 K “ 1 to 11 • 10 "6 K “ 1 , and thus somewhat above aluminum oxide, but still below the coefficient of expansion of conventional ones metals.
• the heating conductor layer is produced by thermal spraying, preferably by laser spraying.
• Heating conductor layers produced in the screen printing process have a glass portion of mostly more than 5% in the metallic conductor, so that the flow temperatures can be reduced during layer penetration.
• the low-melting glass solders in mixed paste ensure that a sealed, closed conductor layer is formed at baking temperatures between 500 and 850 ° C.
• the proportion of the glass frit reduces the metallically conductive proportion.
• Sub-segments of the conductor track, which locally have an increased proportion of glass, are areas with a higher resistance, so that it can possibly lead to overheating and material failure when the current flows through.
• the laser spraying method is particularly suitable, since this is particularly advantageous for producing a web-shaped application.
• the hotplate has on its side facing the heating conductor layer an annularly closed depression which runs in the vicinity of the edge region of the layer sprayed onto the hotplate.
• the individual layers have a decreasing area towards the heating conductor layer. This measure also counteracts the risk of delamination in the edge region of the layers.
• Fig. 1 shows a cross section of a first embodiment of a ceramic hob according to the invention
• FIG. 2 shows a cross section of a modified embodiment of the ceramic hob according to FIG. 1.
• a ceramic hob according to the invention is generally designated by the number 10. It has a flat hotplate 12, which preferably consists of a glass ceramic, such as CERAN® from Schott. It goes without saying that the illustration is only exemplary and that, in particular, the proportions are not to scale.
• This hotplate 12 is used to hold cooking vessels. On the underside of the hotplate 12, a hotplate is generated at different locations. For household purposes, typically four or possibly five hotplates are provided on a ceramic hob. 1 and 2, only one hotplate is shown in each case.
• an adhesion promoter layer 14 is applied by thermal spraying, preferably by atmospheric plasma spraying (APS), at least at the points where an insulating layer and a heat conductor layer are to be applied later.
• thermal spraying preferably by atmospheric plasma spraying (APS)
• the application is preferably limited to the areas of the hotplates in order to keep the total voltages as low as possible.
• This adhesion promoter layer 14 preferably consists of aluminum oxide, titanium oxide or mixtures thereof.
• aluminum oxide and mixtures of aluminum oxide and titanium oxide with a small proportion of titanium oxide, for example 97% by weight A1 2 0 3 with 3% by weight Ti0 2 have particularly good adhesion to the surface of the glass ceramic and have a very good chemical Compatibility with this.
• the adhesion promoter layer 14 is applied with a layer thickness between approximately 10 and 150 ⁇ m, preferably between approximately 40 and 70 ⁇ m, for example with approximately 50 ⁇ m.
• Insulation layer 16 which preferably consists of cordierite (2MgO • 2A1 2 0 3 ) or mullite (3A1 2 0 3 • 2Si0 2 ), is applied by thermal spraying with the necessary layer thickness in order to achieve the desired dielectric strength of 3,750 V at an operating temperature of 450 ° C to ensure.
• the layer thickness is preferably up to approximately 500 ⁇ m, preferably approximately 200-400 ⁇ m.
• the surface of the glass ceramic plate 12 is not pretreated by roughening, as is otherwise customary, since this would lead to damage to the surface of the hotplate 12. Instead, the surface of the hotplate 12 is only cleaned, e.g. degreased with acetone.
• An electrical heating conductor layer 18 is then applied to the underside of the insulating layer 16 by thermal spraying, the necessary structuring of the heating conductor layer 18 being achieved in a manner known per se by a masking process. In this way, a heating conductor 20, for example wound in a meandering shape, can be produced.
• FIG. 2 A variant of the ceramic hob according to the invention is shown in FIG. 2 and is designated overall by number 10 '.
• the insulating layer 16 is not applied directly to the adhesion promoter layer 14, but that an electrically conductive intermediate layer 22 is first sprayed onto it, onto which the insulating layer 16 'is then applied.
• This electrically conductive intermediate layer 22 is grounded, as indicated in FIG. 2 by the connection to ground 24.
• a fuse of the hotplate 12 which is known per se and is not shown, is triggered when the electric conductor breaks from the heating conductor 20 to the hotplate 12 as a result of its grounding.
• the insulating layer 16 'can be designed for a lower dielectric strength, whereby according to VDE about 1,500 V at operating temperature is sufficient. Therefore, the thickness of the insulating layer 16 'can be reduced accordingly.
• the heating conductor layer 18 is in turn sprayed on the underside of the insulating layer 16 'as already described above.
• the electrically conductive intermediate layer 22 preferably consists of a cermet, for example of an alloy based on nickel / chromium / cobalt, in which carbide particles, for example tungsten carbide and chromium carbide, are embedded.
• a cermet has a lower coefficient of thermal expansion than conventional metals due to the carbide inclusions, which leads to reduced problems due to thermal stresses.
• an electrically conductive ceramic can also be used for this intermediate layer instead of a cermet, provided sufficient electrical conductivity can be achieved with this.
• a layer thermally sprayed from Ti0 2 could be used, since during the thermal spraying process the Ti0 2 loses oxygen in such a way that it becomes electrically conductive.
• the electrical conductivity (volume conductivity) of the Ti0 2 thus formed. x between 10 3 ⁇ cm to 5 • 10 2 ⁇ cm at RT) still significantly lower than the electrical conductivity of metals.
• the individual layers 14, 16 according to FIG. 1 and 14, 22, 16 'according to FIG. 2 have a surface that decreases towards the heating conductor layer 20.
• the individual layers run out gently in their edge area, so they continuously pass to the layer underneath.
• annular recess 26 on the underside of the hotplate 12, which annularly surrounds the edge region of the adhesion promoter layer 14. Tensions which are transmitted in the edge area between the hotplate 12 and the adhesion promoter layer 14 can be better absorbed or reduced through this recess 26.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Ceramic Engineering (AREA)
• Coating By Spraying Or Casting (AREA)
• Cookers (AREA)
• Electric Stoves And Ranges (AREA)
• Laminated Bodies (AREA)
• Inorganic Insulating Materials (AREA)
• Devices For Post-Treatments, Processing, Supply, Discharge, And Other Processes (AREA)
• Control Of Motors That Do Not Use Commutators (AREA)
• Baking, Grill, Roasting (AREA)

Applications Claiming Priority (3)

Publications (2)

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• 2001
  • 2001-03-06 DE DE10112236A patent/DE10112236C1/de not_active Expired - Fee Related
• 2002
  • 2002-02-19 CN CNA028060008A patent/CN1494817A/zh active Pending
  • 2002-02-19 CA CA002439141A patent/CA2439141A1/fr not_active Abandoned
  • 2002-02-19 AT AT02724173T patent/ATE333204T1/de not_active IP Right Cessation
  • 2002-02-19 EP EP02724173A patent/EP1366643B1/fr not_active Expired - Lifetime
  • 2002-02-19 WO PCT/EP2002/001742 patent/WO2002071801A1/fr active IP Right Grant
  • 2002-02-19 DE DE50207493T patent/DE50207493D1/de not_active Expired - Lifetime
• 2003
  • 2003-08-27 US US10/649,177 patent/US20040108307A1/en not_active Abandoned

Non-Patent Citations (1)

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