Transparent, colored cooking surface and method for indicating an operating state of such a surface

Abstract

Method for indicating an operating state of a transparent colored cooking surface (1) with improved colored display capability, comprising a glass ceramic with high quartz mixed crystals as the predominant crystal phase, wherein the glass ceramic contains none of the chemical refining agents arsenic oxide and antimony oxide, except for unavoidable traces, wherein the glass ceramic has transmission values ​​of greater than 0.1% in the visible light range over the entire wavelength range greater than 450 nm, a visible light transmission of 0.8 - 5%, preferably 0.8 - 2.5%, and an infrared transmission at 1600 nm of 45 - 85%, characterized by an equilibrium oxygen partial pressure pO2 of the glass ceramic of 1 bar at a temperature greater than 1580°C, wherein a display device with display elements is provided which emit their light from the rear of the cooking surface (1), wherein the display device has a display unit (8,9), and wherein the display device (8, 9) indicates differently predetermined operating states with different colors.

Description

[0001] The invention relates to a transparent, colored cooking surface consisting of a glass ceramic with high quartz mixed crystals as the predominant crystal phase and to a method for displaying an operating state of such a cooking surface.

[0002] Hobs with a glass-ceramic cooking surface are state-of-the-art. Such glass-ceramic hobs are usually available as flat panels or are three-dimensionally shaped.

[0003] Glass ceramics with high quartz solid solutions as the predominant crystal phase are made from crystallizable lithium aluminum silicate glasses.

[0004] The production of these glass ceramics takes place in several stages.

[0005] In the large-scale production of glass-ceramics, the crystallizable starting glass is first melted from a mixture of cullet and powdered raw materials at temperatures typically between 1500 and 1650°C. Arsenic and / or antimony oxide are typically used as refining agents during the melting process. These refining agents are compatible with the required glass-ceramic properties and result in good bubble quality in the melt. Even though these substances are firmly integrated into the glass structure, they are detrimental from a safety and environmental perspective. Special precautions must be taken during raw material extraction and processing, and due to evaporation during the melting process.

[0006] Recently, the use of SnO2 has been described as a safe refining agent. To achieve good bubble qualities, halide compounds are preferred as additional refining agents alongside SnO2 at conventional melting temperatures (maximum approx. 1680°C). For example, Japanese applications JP 11 100 229 A and JP 11 100 230 A describe the use of 0.1-2 wt.% SnO2 and 0-1 wt.% Cl. According to these documents, the coloring is achieved by adding V2O5 as the sole colorant.

[0007] The addition of 0.05–1 wt.% fluorine (US 2007 0004578 A1) and 0.01–1 wt.% bromine (US 2008 0026927 A1) to assist refining with SnO2 is also disclosed. These documents also describe refining temperatures below 1700°C. The main colorant is V2O5. The addition of halides is disadvantageous, as they evaporate rapidly at the melting temperature, forming toxic compounds such as HF.

[0008] The use of SnO2 in conjunction with high-temperature refining above 1700°C to achieve good bubble quality is described in DE 199 39 787 C2. However, this document provides no indication of achieving good detection capability in the wavelength range above 450 nm.

[0009] After melting and refining, the glass is usually hot-formed by rolling or, more recently, floating to produce sheets. For economical production, a low melting temperature and a low processing temperature are required. A desired, and secondly, the glass must not show any devitrification during the forming process. This means that no disruptive crystals may form that could impair the strength of the starting glass and the glass-ceramics produced from it. Since the forming process takes place near the processing temperature V A (Viscosity 10 4dPas) of the glass, it must be ensured that the upper devitrification temperature of the melt is close to and preferably below the processing temperature in order to avoid the formation of disturbing crystals.

[0010] The starting glass is then converted into the glass-ceramic article through controlled crystallization. This ceramization takes place in a two-stage temperature process, in which nuclei, usually consisting of ZrO2 / TiO2 solid solutions, are first generated by nucleation at a temperature between 680 and 800°C. SnO2 can also be involved in the nucleation process. Upon subsequent temperature increase, the high-quartz solid solutions grow on these nuclei. High crystal growth rates, as required for economical, rapid ceramization, are achieved at temperatures of 850 to 950°C. At this maximum production temperature, the structure of the glass-ceramic is homogenized, and the optical, physical, and chemical properties of the glass-ceramic are adjusted. If desired, the high-quartz solid solutions can subsequently be converted into keatite solid solutions.The transformation into keatite solid solutions occurs with an increase in temperature in a range of approximately 950 to 1200°C. With the transition from high-quartz to keatite solid solutions, the thermal expansion coefficient of the glass-ceramic increases, and transparency decreases due to the light scattering associated with the enlargement of the crystals. Therefore, glass-ceramics with keatite solid solutions as the main phase are generally translucent or opaque, and the associated light scattering negatively impacts the display capability.

[0011] A key property of these glass ceramics with high quartz solid solutions as the main crystal phase is the manufacturability of materials that have an extremely low thermal expansion coefficient in the range from room temperature to 700°C and above of < 0.5 × 10 -6 / K. Due to their low thermal expansion, these glass-ceramics have excellent resistance to temperature differences and thermal shock.

[0012] When used as a cooking surface, technical development has led to very specific, sometimes contradictory, requirements for transmission due to the requirements of practical use.

[0013] To prevent the technical components beneath the glass ceramic cooktop from being seen through and to avoid the glare from radiant heating elements, particularly bright halogen heaters, glass ceramic cooktops have limited light transmission. On the other hand, the radiant heating elements must be clearly visible during operation, even at low power, to eliminate the risk of burns. However, when such cooktops are switched off, the cooling of the heating elements after glowing is no longer visible, even though the heat capacity of the materials used and present still produces considerable heat, sufficient to cause serious injury. A certain degree of light transmission is also required for display capability, as the standard red LEDs are installed beneath the cooktop.To meet these requirements, glass-ceramic cooktops are typically adjusted to light transmission values ​​of 0.5 to 2.5%. This is achieved by adding coloring elements. Regardless of the coloring element used, glass-ceramic cooktops appear black when viewed from above due to their low light transmission, and when viewed through, they usually appear red, reddish-violet, or brownish-orange, depending on the coloring elements used.

[0014] Color displays consist of light-emitting electronic components, usually LEDs, which are installed beneath the cooking surface. They are desirable for ease of use and safe operation. For example, the current heating output or residual heat of the various cooking zones is visually displayed. The residual heat indicator is important for safe handling when the heating elements are not radiating or when, as with inductively heated cooking surfaces, it is generally not apparent that the cooking surface is hot. The usual red LEDs emit at wavelengths around 630 nm. To improve ease of use and technical functions, but also to give household appliance manufacturers an opportunity to differentiate themselves through design, displays in other colors are desired alongside the usual red.

[0015] There are known hobs of Japanese origin that have an LCD display that can be backlit in green, orange and red.

[0016] The various colors used today, with the exception of red, serve purely aesthetic purposes. However, the color red usually always indicates danger.

[0017] Safety-relevant information is only encoded and known via identically colored display elements or symbols on seven-segment displays. In safety-critical situations, the user is forced to think about what the display is trying to tell them. In addition, the high level of technology in kitchens and the wide variety of appliances available in kitchens, such as stoves, ovens, microwaves, grills, extractor hoods, refrigerators and freezers, bread slicers, etc., means that the user is bombarded with a flood of information, with the information varying from appliance to appliance. For example, a flashing red light may indicate danger on one appliance, while it may indicate operation on another.

[0018] Furthermore, a user cannot use the colors to determine the operating and error status, i.e. to what extent the device is ready for operation and whether there is an indication of a possible error condition.

[0019] In the infrared range, a transmission of the cooking surface of 45 - 85% is desired.

[0020] High infrared transmission is advantageous because the radiation hits the bottom of the pot directly, where it is absorbed, and thus allows for faster cooking. If transmission is too high, the area surrounding the cooking surface can overheat if handled incorrectly, for example, if a pan is moved on a free-radiating cooking zone.

[0021] An earlier type of glass ceramic cooking surface, known under the name Ceran Color ® , manufactured by SCHOTT AG, had good color display capability. Ceran Color ®is colored by the addition of NiO, CoO, Fe2O3, and MnO, and refined with Sb2O3. This combination of color oxides achieves a light transmission of typically 1.2% for cooktops with a typical thickness of 4 mm. The transmission in the range from 380 nm to 500 nm is 0.1–2.8%, depending on the wavelength. At a wavelength of 630 nm, which is typical for red LEDs, the transmission is approximately 6%. A disadvantage of this earlier type of glass-ceramic cooktop is that the color oxides used also absorb very strongly in the infrared range. The IR transmission at 1600 nm is less than 20%. This reduces the heating speed. The transmission curve of Ceran Color ®is shown in the book "Low Thermal Expansion Glass Ceramics," edited by Hans Bach, Springer-Verlag Berlin Heidelberg 1995, on page 66 (ISBN 3-540-58598-2). The composition is listed in Tables 2 - 7 in the book "Glass-Ceamic Technology," Wolfram Höland and George Beall, The American Ceramic Society 2002.

[0022] In newer, more advanced glass-ceramic cooking surfaces, V2O5 is usually used for coloring because it has the special property of absorbing visible light and allowing high transmission in the infrared radiation range.

[0023] Coloring by V2O5 is a rather complex process. As shown in previous studies (DE 19939787 C2), a redox process is required for the conversion of vanadium oxide into the coloring state. In the crystallizable starting glass, the V2O5 still colors relatively weakly and leads to a slightly greenish hue. During ceramization, the redox process takes place: the vanadium is reduced and the redox partner is oxidized. The refining agent acts as the primary redox partner. This was demonstrated by Mössbauer studies on Sb and Sn-refined compositions. During ceramization, part of the Sb 3+ or Sn 2+ in the starting glass into the higher oxidation state Sb 5+ or Sn 4+ It is assumed that the vanadium is present in a reduced oxidation state as V 4+ or V 3+is incorporated into the seed crystal, where it intensely colors through electron charge-transfer reactions. TiO2, as a further redox partner, can also enhance the coloration of vanadium oxide. In addition to the type and quantity of redox partners in the starting glass, the redox state established in the glass during melting also has an influence. A low oxygen partial pressure pO2 (reducing melt), e.g., due to high melting temperatures, enhances the color effect of vanadium oxide.

[0024] The ceramization conditions also influence the color effect of vanadium oxide. In particular, high ceramization temperatures and longer ceramization times lead to stronger coloration.

[0025] The described relationships in coloring with V2O5 will be used by the expert to adjust the desired transmission curve through a specific glass composition, specific redox settings of the pO2 in the melt, and the ceramization conditions. However, until now, it has not been possible to meet all requirements, such as specification-compliant light transmission, high IR transmission, and display capability for standard red LEDs, along with the desired improved display capability for other colored LEDs.

[0026] The shape of the absorption band of vanadium oxide and thus the transmission in the visible light range in the entire wavelength range greater than 450 nm up to the upper limit of 750 nm could not be adapted to higher transmissions.

[0027] Examples of such V2O5-tinted glass ceramic cooktop types are the Sb2O3-fined Ceran Hightrans ®and the SnO2-refined Ceran Suprema ® , manufactured by SCHOTT AG. The transmission curves of these two glass-ceramics are published in the book "Low Thermal Expansion Glass Ceramics," Second Edition, Editors Hans Bach, Dieter Krause, Springer-Verlag Berlin Heidelberg 2005, on page 63 (ISBN 3-540-24111-6).

[0028] The transmission value of 0.1% is exceeded by the aforementioned glass-ceramic cooktops and other glass-ceramic cooktops on the market at wavelengths of approximately 450–550 nm, which are important for the visibility of colored displays, especially blue and green. These glass-ceramic cooktops meet other essential transmission requirements: high infrared transmission for high cooking speeds, transmission in accordance with specifications for standard red LEDs at approximately 630 nm, and a light transmission of around 1.5%.

[0029] To remedy this disadvantage, European patent application EP 1465460 A2 discloses a glass-ceramic cooktop that, measured in the CIE color system using standard illuminant C, has a Y value (brightness) of 2.5–15, at a thickness of 3 mm. The terms "brightness" and "light transmittance" correspond to the same measurement. The Y value is identical to the light transmittance value measured according to DIN 5033. This light transmittance achieves improved displays for blue and green LEDs. The disclosed compositions are refined with As2O3 and / or Sb2O3, sometimes in combination with SnO2. The coloring is achieved with V2O5.

[0030] The comparative example demonstrates that, at a light transmission of 1.9%, the display capability for blue and green LEDs with the listed material compositions is inadequate. However, the claimed high light transmission values ​​of at least 2.5% and preferably higher are disadvantageous in terms of covering the electronic components beneath the cooktop. Furthermore, the aesthetic black appearance of the cooktop when viewed from above is compromised.

[0031] The subsequently published document DE 10 2008 050 263 A1 deals with a glass ceramic with improved color display capability. US 2008 / 0099449 A1 generally addresses the issue of information reproduction on cooking surfaces using multi-colored light, particularly using LED light sources.

[0032] It is an object of the invention to provide a cooking surface of the type mentioned at the outset which has an improved display capability so that a user can clearly and intuitively recognize dangerous situations.

[0033] The cooking surfaces according to the invention should meet all other requirements that are placed on cooking surfaces, such as chemical resistance, temperature resistance and high temperature / time resistance with regard to changes in their properties (such as thermal expansion, transmission, build-up of stresses).

[0034] It is a further object of the invention to provide a method for displaying an operating state of a cooking surface which enables hazard detection.

[0035] These objects are achieved by a cooking surface according to claim 1 and by a method according to claim 24. Advantageous further developments are specified in the subclaims.

[0036] The colored cooking surfaces have transmission values ​​of greater than 0.1% in the visible light range in the entire wavelength range greater than 450 nm, a light transmission in the visible range of 0.8 - 5% (preferably 0.8 - 2.5%) and a transmission in the infrared at 1600 nm of 45 - 85%.

[0037] The inventive light transmission of 0.8–2.5% ensures that the technical components beneath the glass-ceramic cooktop are not obstructed and that the aesthetic black appearance is maintained when viewed from above. Radiant heaters are visible during operation, and standard red LED displays are clearly legible. A transmission of more than 0.1% in the visible light range across the entire wavelength range above 450 nm also allows displays of other colors to be clearly legible. Considering the luminosity of commercially available blue, green, yellow, or orange LEDs, this transmission value is sufficient and represents a significant improvement over the state of the art. In particular, blue and green displays are significantly improved. Displays with white light are less subject to color distortion due to the transmission profile across the entire wavelength range above 450 nm.

[0038] According to the invention, colored and especially multicolored displays can be realized on the cooking surface, with the display elements radiating their light from the rear of the cooking surface, which is then perceived by the viewer on the front. The transmission behavior of the cooking surface thus enables a clear representation of various heat states and / or error states.

[0039] To indicate different heat states, the invention can, in particular, utilize a display that conveys a red, orange, or blue color impression depending on the heat. Red represents heat, orange represents warmth, and blue represents cold. For example, the display has a red-illuminated field, a possibly orange-illuminated field, and a possibly blue-illuminated field, which are formed, in particular, from LEDs. Furthermore, it is possible to provide a light field in which a color change occurs continuously within a single field or along different color fields or segments.

[0040] What both designs have in common is that the user can intuitively identify the appliance's heat status via the color coding, without having to familiarize themselves with the control panel or consider the meaning of a display. These colored displays, which indicate the temperature status in steps or continuously and correlate with the heat level, can be located next to each individual cooking or heating zone, adjacent to a corresponding control switch, or in a separate display panel on the cooktop.

[0041] According to the invention, such color fields, which either have several individual color fields or a single color field, can indicate error states. The color gradient can be from red to yellow to green, and in particular, discrete, i.e., stepped, from red to yellow to green. Red indicates a safety-critical error state or operating state, yellow indicates a non-safety-critical error state, and green indicates unrestricted operational readiness.

[0042] This means that the user is informed using color symbols that are familiar to them from everyday life (red-blue = fire-water; red-yellow-green = traffic light colors). This allows them to quickly and reliably identify the appliance's status. The advantage of this is that it significantly improves user safety, ergonomics, and confidence in the appliance, especially in a cooktop.

[0043] Below 350 nm, in the ultraviolet range, the low transmission values ​​of significantly less than 0.01% of known glass-ceramic cooktops are also ensured with the cooktop according to the invention. Blocking UV light is advantageous for protecting organic components, such as adhesives in the technical components beneath the cooktop, as well as for protection during cooking when blue LEDs with a component of ultraviolet radiation are used for display purposes.

[0044] The set infrared transmission of 45–85%, measured at 1600 nm, meets the requirements for high cooking speed and protection against undue heating of the area surrounding the cooking surface. Since the transmission and light transmission values ​​according to the invention are crucial for the function of the cooking surface, they apply regardless of the thickness of the cooking surface, which is usually between 2.5 and 6 mm. Smaller thicknesses are disadvantageous in terms of strength, and greater thicknesses are uneconomical because they require more material and reduce the ceramization speed. The cooking surface thickness is usually around 4 mm. The transmission values ​​specified for commercial cooking surfaces and in the exemplary embodiments therefore refer to this thickness, unless otherwise stated.If the cooktop is manufactured using roller molding, the underside is usually provided with dimples to protect it from damage during production that could reduce strength. The underside of the cooktop is often smoothed with a transparent organic polymer in the area of ​​the colored displays to prevent optical distortion caused by the dimples. On cooktops with a smooth underside without dimples, colored displays are undistorted and brighter to see.

[0045] In order to ensure the requirements for temperature resistance, the cooking surfaces according to the invention have low thermal expansion coefficients of less than 1 10 -6 / K, preferably from (0 ± 0.3) · 10 -6 / K.

[0046] The transparent, colored cooktops according to the invention are characterized by a composition without the use of arsenic and / or antimony oxide as refining agents and are thus technically free of these components, which are detrimental from a safety and environmental perspective. These components are typically present as contaminants in concentrations of less than 500 ppm.

[0047] The method according to the invention for producing a transparent, colored cooking surface with improved color display capability is characterized in that it forms a glass ceramic with high quartz mixed crystal as the predominant crystal phase and that, apart from unavoidable traces, the chemical refining agents arsenic and / or antimony oxide are dispensed with and the cooking surface is adjusted to transmission values ​​of greater than 0.1% in the visible light range in the entire wavelength range greater than 450 nm, a light transmission in the visible range of 0.8 to 2.5% and a transmission in the infrared at 1600 nm of 45 to 85%.

[0048] To improve bubble quality, additional refining additives such as CeO2, sulfate compounds, and halide compounds can be used in addition to the SnO2 used. Their contents are typically limited to amounts of up to 1 wt.%. In the production of cooktops, good bubble qualities are those with bubble counts of less than 10, preferably less than 5, bubbles / kg of glass (based on bubble sizes larger than 0.1 mm).

[0049] Preferably, the transmission of the cooking surface is set to values ​​of > 0.15% at 450 nm > 0.15% at 500 nm > 0.25% at 550 nm 3-9% at 630 nm 50 - 80% at 1600 nm and a light transmission in the visible range of 1.0 - 2.0%.

[0050] At these values, the color display capability is further improved and the various requirements for the transmission curve are further optimized. Further improved coverage of the technical components beneath the cooktop glass ceramic and a particularly aesthetic black appearance in incident light are achieved when the light transmission is less than 1.7%. Transmission values ​​of the cooktop at 630 nm of 3 to 9% correspond to the tolerance values ​​of commercially available cooktops. It is advantageous to set these values ​​so that the appearance of the conventional red LED displays remains unchanged even with the cooktop according to the invention.

[0051] The display capability is further improved if, in a preferred design, the transmission of the cooking surface is set to values ​​of > 0.10% at 400 nm > 0.15% at 450 nm > 0.25% at 500 nm > 0.30% at 550 nm 3 - 9% at 630 nm 50 - 80% at 1600 nm and a light transmission in the visible range of 1.0 - 1.7% is set.

[0052] The cooking surface according to the invention preferably has a composition of the glass ceramic which, in % by weight on an oxide basis, essentially consists of: Li2O 3,0 - 4,2 Σ Na2O+K2O 0,2 - 1,5 MgO 0 - 1,5 Σ CaO+SrO+BaO 0 - 4 ZnO 0 - 2 B2O3 0 - 2 Al2O3 19 - 23 SiO2 60 - 69 TiO2 2,5 - 4 ZrO2 0,5 - 2 P2O5 0-3 SnO2 0,1 - < 0,6 ΣTiO2+ZrO2+SnO2 3,8 - 6 V2O5 0,01 - 0,06 Fe2O3 0,03 - 0,2, with the condition: 1 <Fe2O3 / V2O5<8.

[0053] The term "consists essentially of" means that the listed components should constitute at least 96%, usually 98%, of the total composition. Numerous elements such as F, Cl, the alkalis Rb, Cs, or elements such as Hf are common impurities in the industrially used raw materials for batches. Other compounds such as those of the elements Ge, rare earths, Bi, W, Nb, Ta, and Y may be added in small amounts.

[0054] In addition to the color oxide V2O5 in concentrations of 0.01 to 0.06 wt.%, other coloring components such as chromium, manganese, cobalt, nickel, copper, selenium, rare earth elements, and molybdenum compounds can also be used to enhance the coloration. Their content is limited to a maximum of approximately 1 wt.% because these compounds generally reduce infrared transmission. Furthermore, these largely polyvalent compounds can interfere with the coloration of the V2O5 via redox reactions and complicate the adjustment of the transmission according to the invention.

[0055] By adding 50–400 ppm of Nd2O3, it is possible to mark the glass-ceramic cooktop. The near-infrared absorption band of Nd at 806 nm lies in a region of high transmittance values ​​in the glass-ceramic, making it prominent in the transmission spectrum. This allows the cooktop material to be reliably identified by its manufacturer, and optical shard detection systems enable efficient recycling.

[0056] The water content of the starting glasses used to produce the cooking surfaces according to the invention is typically between 0.015 and 0.06 mol / l, depending on the choice of batch raw materials and the melting process conditions. This corresponds to β-OH values ​​of 0.16 to 0.64 mm. -1 for the output glasses.

[0057] The oxides Li2O, Al2O3, and SiO2, within the preferred, specified limits, are necessary components of high-quartz solid solutions. A minimum Li2O content of 3 wt.% is required, but Li2O contents above 4.2 wt.% often lead to unwanted devitrification during a manufacturing process. A content of 3.2 to 4.0 wt.% produces particularly good results.

[0058] To avoid high viscosities of the starting glass and to suppress the tendency for undesired devitrification of mullite during forming, the Al2O3 content is preferably limited to a maximum of 23 wt.%, in particular 22 wt.%. The SiO2 content should be a maximum of 69 wt.%, because this component greatly increases the viscosity of the glass. This component is preferably further limited to values ​​of a maximum of 68 and a further maximum of 67 wt.%. For good melting of the glasses and for low forming temperatures, higher SiO2 contents are uneconomical. The minimum SiO2 content should be 60 wt.%, in particular 62 wt.%, because this is advantageous for the required cooking surface properties, such as chemical resistance and temperature resistance.

[0059] MgO, ZnO, and P2O5 can be incorporated into the high-quartz solid solutions as additional components. Due to the potential for undesirable crystal phases such as zinc spinel (gahnite) to form during ceramization, the ZnO content is limited to a maximum of 2.0 wt.%, preferably a maximum of 1.8 wt.%. The MgO content is limited to a maximum of 1.5 wt.%, preferably up to 1.2 wt.%, because otherwise it would unduly increase the expansion coefficient of the glass-ceramic. A minimum MgO content of 0.1 wt.% is generally required to prevent the thermal expansion of the glass-ceramic from falling to negative values.

[0060] The addition of alkalis Na2O, K2O, and the alkaline earths CaO, SrO, BaO, as well as B2O3, improves meltability and devitrification stability during glass forming. However, the contents must be limited because these components are not incorporated into the crystal phases but essentially remain in the residual glass phase of the glass-ceramic.

[0061] Excessively high contents increase the thermal expansion of the glass-ceramic to an unacceptable extent and impair the crystallization behavior during the conversion of the crystallizable starting glass into the glass-ceramic, particularly at the expense of faster ceramization rates. Furthermore, higher contents have an adverse effect on the time / temperature resistance of the glass-ceramic. The sum of the alkalis Na2O + K2O should be at least 0.2, preferably at least 0.4 wt.%, and a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%.

[0062] The sum of the alkaline earth metals CaO + SrO + BaO should be a maximum of 4 wt.%, preferably at least 0.2 wt.%.

[0063] The alkalis and alkaline earths mentioned above accumulate not only in the residual glass phase between the crystals but also on the surface of the glass-ceramic. During ceramization, a glassy surface layer approximately 200 to 1000 nm thick forms, which is almost free of crystals and enriched in these elements and depleted in lithium. This glassy surface layer has a positive effect on the chemical resistance of the glass-ceramic.

[0064] The addition of P2O5 can be up to 3 wt.% and is preferably limited to 1.5%. The addition of P2O5 is beneficial for devitrification resistance. Higher contents have a negative impact on acid resistance.

[0065] TiO2, ZrO2, and SnO2 are required as nucleating agents. During ceramization, they form high-density nuclei that serve as a substrate for the growth of the high-quartz solid solutions during crystallization. Higher concentrations than a total of 6 wt.% impair devitrification stability. This is particularly true for the SnO2 component, which is limited to values ​​of less than 0.6 wt.%. Higher concentrations lead to the crystallization of Sn-containing crystal phases on the contact materials (e.g., Pt / Rh) during forming and must be avoided at all costs.

[0066] The ZrO2 content is limited to 2 wt.%, preferably 1.8 wt.%, and more preferably to a maximum of 1.6 wt.%, since higher contents impair the melting behavior of the batch during glass production and devitrification stability during molding can be impaired by the formation of ZrO2-containing crystals. The minimum ZrO2 content should be 0.5 wt.% and preferably 0.8 wt.% to promote a high ceramization rate. The TiO2 content is between 2.5 and 4.0 wt.%, preferably at least 2.8 wt.%. The minimum amount should not be undercut to ensure rapid nucleation for achieving high ceramization rates. The content should not exceed 4 wt.%, because otherwise devitrification stability will be impaired.

[0067] Surprisingly, it has now been found that Fe2O3 contents from 600 ppm, preferably from 700 ppm, in combination with narrowly specified contents of TiO2, V2O5 and SnO2 can influence the transmission curve. Since Fe2O3, especially the proportion of divalent Fe 2+ Since this negatively affects IR transmission, the Fe2O3 content should be a maximum of 0.2 and preferably a maximum of 0.18 wt.%. A Fe2O3 content of 0.08 - 0.15 wt.% is particularly preferred.

[0068] The combination of the color oxides Fe2O3 with V2O5 and its redox partner SnO2 allows for the use of smaller quantities of the expensive and hazardous colorant V2O5. This meets the requirements for transmission at low wavelengths from 450 nm and other requirements such as specified light transmission, infrared transmission, and transmission at 630 nm. The transmission curve is flatter in the visible light range compared to conventional glass ceramics colored with vanadium oxide. To reduce the content of the expensive and hazardous colorant V2O5, it is essential that the Fe2O3 content be at least as high as the V2O5 content and thus meet the requirements. 1 <Fe2O3 / V2O5<8.

[0069] The pigment oxide Fe2O3 is therefore the main colorant in terms of quantity, and the content is preferably twice as high as that of V2O5. This also makes it possible to use more cost-effective batch raw materials.

[0070] Other components that contribute to reducing the required V2O5 content are SnO2 and TiO2. To adjust the transmittance according to the invention, it is therefore necessary to adjust V2O5, SnO2, TiO2, and Fe2O3 within certain narrow limits. In particular, the TiO2 content must exceed a certain minimum amount.

[0071] With the Fe2O3, TiO2, V2O5 and SnO2 contents according to the invention, it is possible to achieve all requirements for the transmission curve, such as specification-compliant light transmission, high infrared transmission, as well as display capability for standard red LEDs together with the desired improved display capability for other colored LEDs.

[0072] Another important result of the addition of Fe2O3 was found to be that it significantly supports the refining process. In combination with SnO2 as the main refining agent, the Fe2O3 also releases oxygen and is converted to Fe 2+ reduced. The conversion rate, which is crucial for the refining effect, increases with the melt temperature. Therefore, temperature treatment of the melt at temperatures above 1700°C and even higher than 1750°C produces further improved results in terms of bubble quality. For the addition of Fe2O3 as an additional refining agent in combination with SnO2 to be particularly beneficial, the content should be at least 20% of the SnO2 content.

[0073] For economical production, rapid ceramization of the starting glass is desired. To achieve this, it is necessary to increase the nucleation and ceramization rates through an appropriately selected composition. Increasing the contents of the nucleating agents TiO2 + ZrO2 + SnO2 has proven advantageous in this regard, while lowering the P2O5 content.

[0074] Furthermore, distortions in the cooktops that occur during ceramization are a limiting factor. Large-scale ceramization furnaces have a certain temperature inhomogeneity, making it difficult to maintain a completely uniform temperature on the top and bottom of the crystallizable glass during crystallization. Small temperature differences of just a few °C lead to earlier crystallization on the side of the plate with the higher temperature. Combined with the linear crystallization shrinkage of approximately 1%, this leads to distortions. Commercially available cooktops are typically specified for a distortion of less than 0.1% of their diagonal dimension.

[0075] To minimize this distortion during a rapid ceramization program, it has proven beneficial to minimize the proportion of crystal phase-forming components such as Li2O and SiO2 and to increase the proportion of components that form the residual glass phase of the glass-ceramic, such as the alkalis Na2O and K2O as well as the alkaline earths CaO, SrO, and BaO. The proportion of the high-quartz solid solution phase is advantageously less than 70 wt.% and ranges from 60 to 70 wt.%. During ceramization, the glass-ceramic plate is placed on a flat base plate. By increasing the proportion of the residual glass phase and reducing the proportion of the crystal phase, distortions caused by viscous flow at high temperatures are reduced as the glass-ceramic plate adheres to the flat base.

[0076] Preferably, the cooking surface according to the invention has, with regard to rapid ceramizability of the starting glass, a composition of the glass ceramic which, in % by weight on an oxide basis, essentially consists of: Li2O 3,2 - 4,2 Na2O 0,1 - 1 K2O 0,1 - 1 Σ Na2O+K2O 0,2 - 1,5 MgO 0 - 1,5 Σ CaO+SrO+BaO 0,1 - 4 ZnO 0 - 2 B2O3 0 - 1 Al2O3 19 - 23 SiO2 60 - 68 TiO2 2,8 - 4 ZrO2 0,8 - 1,8 P2O5 0 - 1,5 SnO2 0,1 - < 0,6 ΣTiO2+ZrO2+SnO2 4,4 - 6 V2O5 0,01 - 0,05 Fe2O3 0,07 - 0,18, with the condition: 2 <Fe2O3 / V2O5<6.

[0077] Rapid ceramizability is understood to mean a thermal treatment for crystallizing the glass ceramic with a duration of less than 2 hours, preferably less than 80 minutes.

[0078] In the ceramization process according to the invention, the thermally relaxed, crystallizable starting glass is heated to a temperature range of approximately 680°C within 3 to 30 minutes. The required high heating rates can be achieved on an industrial scale in roller kilns. This temperature range of approximately 680°C roughly corresponds to the glass's transformation temperature. Above this temperature, up to approximately 800°C, is the range with high nucleation rates. The nucleation temperature range is traversed over a period of 10 to 30 minutes.

[0079] The temperature of the glass containing crystallization nuclei is then increased within 5 to 30 minutes to a temperature of 850 to 950°C, which is characterized by high crystal growth rates of the high-quartz solid solution phase. This maximum temperature is maintained for up to 20 minutes. This homogenizes the structure of the glass-ceramic and adjusts its optical, physical, and chemical properties. The resulting glass-ceramic is cooled to 800°C at cooling rates of approximately 10°C / min and then rapidly cooled to room temperature.

[0080] Batch raw materials such as barium nitrate or barium chloride, which are used as sources of BaO, are environmentally harmful and require special precautions during use. Furthermore, BaO increases the density of the glass ceramic and thus the weight of the cooking surface. To reduce the BaO content or completely remove it, except for impurities, it has proven advantageous to substitute BaO with the chemically related alkaline earth metals CaO and SrO. The CaO content should be 0.2 to 1 wt.% and the SrO content 0.1 to 1 wt.%.

[0081] For economical production, the crystallizable starting glass should be readily meltable and have a high devitrification resistance. The processing temperature should be below 1320°C and preferably below 1310°C. The upper devitrification limit should be at least 30°C, preferably at least 50°C, below the processing temperature. Crystal phases critical to devitrification are mullite (aluminum silicate), baddeleyte (ZrO2), Li2O-Al2O3-SiO2 solid solutions, and SnO2-containing crystal phases. Therefore, higher contents of Li2O, Al2O3, SiO2, ZrO2, and SnO2 are detrimental to devitrification resistance. In order to reduce the viscosity of the glass melt, it has been found necessary to reduce the content of SiO2, Al2O3, ZrO2, while the contents of alkalis Na2O + K2O, alkaline earths CaO + SrO + BaO are chosen at higher values.

[0082] In this regard, a cooking surface according to the invention preferably has the following composition in % by weight on an oxide basis: Li2O 3,2 - 4,0 Na2O 0,2 - 1 K2O 0,1 - 1 Σ Na2O+K2O 0,4 - 1,2 MgO 0,1 - 1,2 CaO 0,2 - 1 SrO 0 - 1 BaO 0 - 3 Σ CaO+SrO+BaO 0,2 - 4 ZnO 0 - 1,8 B2O3 0 - 1 Al2O3 19 - 22 SiO2 62 - 67 TiO2 2,8 - 4 ZrO2 0,5 - 1,6 P2O5 0 - 1,5 SnO2 0,1 - 0,5 ΣTiO2+ZrO2+SnO2 4,2 - 6 V2O5 0,01 - 0,05 Fe2O3 0,08 - 0,15, with the condition: 2 <Fe2O3 / V2O5<6.

[0083] It has been shown that targeted reduced conditions of the crystallizable starting glass, which are applied during melting, have a beneficial effect on the desired transmission curve of the glass-ceramic.

[0084] The various transmission requirements mentioned above can then be even better reconciled. The equilibrium oxygen partial pressure pO2 of 1 bar should be achieved at a temperature of >1580°C, preferably >1640°C. The higher this temperature, the more reducing the resulting glass is and the proportions of low valences in the polyvalent components, such as Sn, are reduced. 2+ , Fe 2+ , Ti 3+are increased. This enhances the color effect of the vanadium oxide.

[0085] At the temperatures for the equilibrium oxygen partial pressure pO2 according to the invention, it is possible to achieve the transmission curve according to the invention with low V2O5 contents. Less than 0.04 and preferably less than 0.03 wt.% is required. Since vanadium is also a costly raw material, it is economically advantageous to minimize the V2O5 content.

[0086] This equilibrium oxygen partial pressure (pO2) can be adjusted in the melt by adding reducing agents in powder and / or liquid form to the initial mixture. Suitable reducing agents include metals, carbon, and / or oxidizable carbon or metal compounds such as Al or Si powder, sugar, charcoal, SiC, TiC, MgS, and ZnS. Gaseous reducing agents, such as forming gas, are also suitable. These reducing agents are suitable for lowering the pO2 of the melt and setting the desired value for the equilibrium oxygen partial pressure.

[0087] A preferred method for adjusting the equilibrium oxygen partial pressure pO2 is to heat treat the glass melt at temperatures greater than 1700°C, preferably greater than 1750°C. This heat treatment can advantageously be carried out as high-temperature refining, which allows the desired low bubble counts of < 10, preferably < 5 bubbles / kg, to be achieved on an industrial scale. The refining effect is so pronounced because SnO2 is a refining agent that releases the oxygen required for refining at elevated temperatures above 1700°C.

[0088] This also applies to the additional refining agent Fe2O3. The combination of the two thus delivers further improved bubble qualities and allows for higher glass throughputs in large-scale melting tanks. The contribution of Fe2O3 to refining becomes significant at contents of at least 20 wt.% of the refining agent SnO2 used. This combines the advantage of good refining with the inventive advantage of adjusting the equilibrium oxygen partial pressure pO2 to the preferred value. The mechanism is that at high temperatures, O2 refining bubbles are formed, which rise in the glass melt and leave it, while other dissolved foreign gases are also removed. With sufficient time, when all the oxygen released during refining is removed from the melt, the temperature at which the equilibrium oxygen partial pressure pO2 reaches 1 bar corresponds to the maximum temperature during treatment.Since this equilibrium cannot be fully achieved in large-scale glass melting and the throughputs used due to time constraints, a certain amount of oxygen refining bubbles always remains in the melt and is reabsorbed during cooling. Furthermore, a small amount of oxygen is reabsorbed from the environment when the melt cools from the maximum temperature to the forming temperature VA. As a result, the measured temperature of the equilibrium oxygen partial pressure pO2 of 1 bar does not correspond to the maximum temperature of the melt, but lies below it. Unless reducing additives are added.

[0089] Preferably, one or more differently colored displays, such as blue, green, yellow, orange, or white, are arranged beneath the inventive cooking surface with improved color display capability, instead of or in addition to the usual red displays. The colored displays consist of light-emitting electronic components, usually LEDs. The underside of the cooking surface can have the usual nubs or be smooth.

[0090] In an advantageous development of the invention, the user is supported in evaluating the error condition by the use of symbols or text.

[0091] For example, there can be a temperature status display field and an error status display field on the hotplate or in the hob, with these two fields each being marked with a unique symbol (heat = flame symbol, error = caution sign (triangle with exclamation mark)). Furthermore, it is also possible - particularly for users with color vision deficiencies - to mark the individual levels with symbols. For example, a temperature display field that lights up red can also have a large flame in the display field. The corresponding orange display field can have a small flame, and the blue field can have no symbol or a symbol symbolizing cold. When using traffic light colors to indicate error states, the little men familiar from pedestrian traffic lights (red = stand still, green = walk) can be used, for example.Of course, the displays can flash in addition to a color or a symbol and, in particular, depending on the error and / or temperature intensity, flash faster or slower or at different rhythms.

[0092] The cooking surface can be heated by radiant heaters, halogen heaters, induction heating, or gas. All types of displays, both point and surface, are possible.

[0093] The cooking surface can be designed not only as a flat plate, but also as three-dimensionally shaped plates, such as beveled, angled, or curved plates. The plates can be rectangular or in other shapes, and can contain flat areas as well as three-dimensionally shaped areas, such as woks.

[0094] The present invention is further illustrated below by examples.

[0095] They show: Table 1: Composition and properties of crystallizable starting and reference glasses and Table 2: Ceramicization conditions and properties of glass ceramics according to the invention and comparison glass ceramics. Fig. 1 is a diagram showing the partial pressure of oxygen plotted against temperature for a glass according to the invention; Fig. 2 two diagrams showing the transmission curve of glass ceramics according to the invention; Fig. 3 a plan view of a hob with temperature and error status information fields according to the invention; Fig. 4 shows another embodiment of the arrangement of the fields; Fig. 5 another embodiment of the fields and Fig. 6 shows an example of the arrangement of displays in a display field with additional symbols.

[0096] For some embodiments, compositions and properties of the crystallizable starting glasses are listed in Table 1. Glasses 1 to 12 are glasses according to the invention, and glass 13 is a comparison glass outside the present invention, the composition of which corresponds to the commercial cooktop glass ceramic Ceran Suprema. ® of SCHOTT AG.

[0097] Due to typical impurities in the industrial batch raw materials used, the compositions do not add up to exactly 100 wt.%. Typical impurities, even if not intentionally introduced into the composition, are F, Cl, B, P, Rb, Cs, and Hf, which usually amount to less than 0.05 wt.%. They are often introduced via the raw materials for chemically related components, for example, Rb and Cs via the Na and K raw materials, or Sr via the Ba raw material, and vice versa.

[0098] The water content of the glasses is 0.03 - 0.05 mol / l, corresponding to βOH values ​​of 0.32 to 0.53 mm -1 .

[0099] Table 1 also shows the properties in the glassy state, such as transformation temperature Tg, processing temperature V A , devitrification temperature, and density are listed. These properties are the same for glasses of the same composition where the refining temperature has been varied.

[0100] To measure the devitrification temperature, the glasses are melted in Pt / Rh10 crucibles. The crucibles are then heated for 5 hours at various temperatures in the range of the processing temperature V A The highest temperature at which the first crystals appear at the contact surface of the glass melt and the crucible wall determines the devitrification temperature.

[0101] Table 1 also lists the maximum melt temperature and the corresponding time, as well as the measured temperature at which the pO2 reaches 1 bar. The pO2 measurement was performed in a conventional manner on the remelted glasses as a function of temperature, and the temperature for the equilibrium oxygen partial pressure pO2 = 1 bar was determined. In pO2 measurements, the pO2 is a function of temperature and depends reversibly on it as long as no oxygen is exchanged with the environment.

[0102] Fig.Figure 1 shows the measurement of pO2(T) for glass No. 9 and the determination of the characteristic temperature T (pO2 = 1 bar). Since the glass melt releases oxygen to the environment at high temperatures close to the equilibrium oxygen partial pressure T (pO2 = 1 bar), this leads to a change in the value of this characteristic temperature. Therefore, during the measurement, the temperature of the glass was increased to approximately 40°C below the characteristic temperature T (pO2 = 1 bar), and the value was determined by extrapolating the measured straight line log pO2 proportional to 1 / T (see Fig. 1).

[0103] The starting glasses in Table 1 were melted from raw materials commonly used in the glass industry at temperatures of approximately 1620°C for 4 hours. After melting the mixture in sintered silica crucibles, the melts were poured into Pt / Rh crucibles with silica inner crucibles and homogenized by stirring at temperatures of 1550°C for 30 minutes. Following this homogenization, various temperature treatments were carried out for refining. The maximum melting and refining temperatures and times are shown in Table 1.

[0104] Glasses Nos. 2, 3, 5, 6, 8, 10, 11, 13 were refined for 2 hours at 1640°C.

[0105] Subsequently, pieces of approximately 140×100×30 mm 3 Size cast.

[0106] Glasses 1, 4, 7, 9, 12, and the reference glass were subjected to high-temperature refining at the temperatures and times specified in Table 1 to achieve good bubble qualities and to establish more reducing conditions corresponding to the specified temperature T (pO2 = 1 bar). Before pouring the refined glass melt, the temperature was lowered to approximately 1700°C. To avoid stresses, the castings were cooled to room temperature in a lehr, starting at a temperature approximately 20°C below the transformation temperature of the glasses. Test specimens for measurements were prepared from the castings.

[0107] The positive influence of high-temperature refining on bubble quality is also evident in laboratory melts. Due to technical reasons (different surface-to-volume ratios in laboratory melts), the absolute values ​​of the bubble counts in laboratory melts are orders of magnitude higher than those in industrial-scale melts. The relative differences are significant. Melts refined conventionally at 1640°C have approximately 1000 to 5000 bubbles / kg of glass, while those refined with high-temperature refining have approximately 200 to 2000 bubbles / kg. With the same composition, high-temperature refining delivers better values. At these values, the desired quality of less than 10 bubbles / kg of glass is achieved on an industrial scale.

[0108] Glasses 1, 2, 3, as well as 7, 8, and 9, 10, have the same composition but were melted differently. Glass 1 was subjected to high-temperature refining. Glass 3 was made more reducing by adding 1 wt.% sugar to the mixture than Glass 2, which was refined at the same temperatures. Consequently, the temperatures for the equilibrium oxygen partial pressure also differ. Due to the associated different reducing conditions of the crystallizable starting glasses, the transmission values ​​of the resulting glass-ceramics differ under the same ceramization conditions. At low maximum melt temperatures, higher proportions of V2O5 or higher ceramization temperatures are required to equalize the transmission values.

[0109] The glasses 1, 4 and the comparison glass 12, as well as shards of the commercially produced glass ceramic Ceran Color ®After refining and standing at 1600°C for 50 hours in a 14-capacity Pt / Rh10 crucible, the samples were formed into plates typical of cooking surfaces using a laboratory roller mill. The laboratory roller consists of shortened original production rollers.

[0110] The bottom roller is structured for the production of a conventional, dimpled cooktop underside. After cooling to relieve stress, the resulting glass ribbon, approximately 200 mm wide, 4 mm thick, and 2 m long, was used to prepare test specimens for measurements and plates measuring 18 cm × 18 cm × 4 mm for flatness tests during rapid ceramization. The plates were ceramized on a flat ceramic base plate in a laboratory furnace with homogeneous, controllable top and bottom heat. They were heated to 750°C at 10°C / min, with a hold time of 15 minutes. They then heated further to 900°C at 4°C / min, with a hold time of 15 minutes, and quickly cooled to room temperature. Throughout the entire program, the upper temperature in the furnace was 6°C higher than the lower temperature. This deliberately deformed the glass-ceramic plates into a dome-like shape. The deviation from flatness for the reference glass No. 13 was 2.1 ± 0.2 mm (6 tests), for Ceran Color ®0.6 ± 0.1 mm (4 tests) and for glass No. 1 and No. 4 for both 0.5 ± 0.1 mm (3 tests). Since for the industrially produced glass-ceramic material Ceran Color ® It has been shown that it can be ceramized with the required flatness in less than 80 minutes, this is also shown for the glasses according to the invention by the relative experimental comparison.

[0111] Table 2 shows the ceramization conditions and properties of the resulting glass-ceramics and the comparative ceramics 2, 4, 17, outside the invention. The ceramization of the starting glasses was carried out using the following temperature / time programs, the values ​​for T max and t max are shown in Table 2: Ceramicization program 1: a) Heating from room temperature to 680°C within 15 minutes; b) Temperature increase from 680 to 800°C within 34.5 min, heating at 10°C / min to 750°C, holding time of 15 min at 750°C, heating at 4°C / min to 800°C; c) Temperature increase from 800°C to Tmax and holding time tmax with heating rate 4°C / min; d) Cooling to 800°C at 10°C / min, then rapid cooling to room temperature. Ceramicization program 2: a) Heating from room temperature to 680°C within 5 minutes; b) Temperature increase from 680 to 800°C within 19 min, heating at 10°C / min to 730°C, further heating at 5°C / min to 800°C; c) Temperature increase from 800°C to T max and holding time t max with heating rate 5°C / min; d) Cooling to 800°C at 10°C / min, then rapid cooling to room temperature.

[0112] In the comparative glass-ceramics, Examples 2 and 4, the adjusted pO2 values ​​in the starting glass lead to light transmission that does not comply with the invention. This demonstrates the influence of the adjusted redox state compared to Example 1, since the examples were produced under the same ceramization conditions.

[0113] This can be corrected by adjusting the ceramization conditions (Examples 3 and 5). Starting glasses Nos. 1, 2, and 3 of Examples 1 to 5 have the same composition and differ only in the redox state established during melting.

[0114] The values ​​for the mean crystallite size and the proportion of the high quartz solid solution phase were determined by X-ray diffraction.

[0115] Due to their content of high-quartz solid solution as the predominant crystal phase, the examples exhibit the desired very low thermal expansion values, measured in the temperature range between room temperature and 700°C. The characteristic values ​​for the invention for the transmission at the various wavelengths, as well as for the light transmission, synonymous with "brightness" Y, are listed in the table. The values ​​were determined on polished plates with a thickness of 4 mm, typical for cooking surfaces. The optical measurements were performed using standard illuminant C, 2 degrees.

[0116] In an additional example No. 18, a composition according to Glass No. 1 was melted on an industrial scale and a temperature for the equilibrium oxygen partial pressure pO2 = 1 bar of 1610°C was achieved by high-temperature refining at approximately 1770°C for 15 minutes. The bubble quality of this glass was excellent, with <3 bubbles / kg of glass. A dimpled glass ribbon 4 mm thick was produced during the forming process and cooled in a lehr to avoid stress. Cooking surfaces measuring 500 × 500 × 4 mm were cut from this glass ribbon and ceramized in a large-scale roller kiln. The ceramization program corresponded to Program 2 (Table 2), and the crystallizable glass plates were placed on a flat ceramic base plate. The resulting glass-ceramics had very good flatness of <0.1% in their diagonal dimensions. The transmission curve of this glass ceramic according to the invention is shown in Fig.2, the comparative glass ceramic of Example 17 and the inventive Example 19. In the wavelength range in visible light of 400 to 600 nm, which is essential for the invention, the transmission curve of the inventive glass ceramic, which is advantageous for the higher display capability, is shown at values ​​above 0.1% and the good UV blocking below 350 nm.

[0117] In the Fig. Figures 3 to 6 illustrate a cooktop according to the invention, which is manufactured on the basis of one of the glass ceramics described above. As these illustrations show, the cooktop has, for example, a flat glass ceramic plate 2 with round cooking zones 3, in the present example, four cooking zones 3.

[0118] An operating panel 4 is also provided at the edge of the plate 2. The operating panel 4 includes a rotary switch 5 for each hotplate 3. In the present example, four rotary switches 5 are provided. The rotary switches 5 are designed in a known manner and can operate the cooking surfaces either continuously or in stages with regard to the heating output. In addition, characteristic fields 6 are provided, which indicate to the user, using simple symbols, which cooking zone 3 the respective switch 5 belongs to.

[0119] In addition, a safety and warning field 7 is provided in one side area of ​​the plate 2, preferably in another side area of ​​the plate 2.

[0120] The safety and warning field 7, for example, comprises two rows of indicators. There are indicators 8, for example, for a device error, and indicators 9, which indicate a temperature condition. Fig.In the example shown in Figure 3, there are three displays 8 arranged one below the other for faults in the entire appliance, and adjacent to them are three displays 9 arranged one below the other for the temperature status of one of the cooking zones 3. In this embodiment, the user is thus informed that one of the surfaces may be at an elevated temperature and / or that the appliance or parts of the appliance have faults.

[0121] In a further advantageous embodiment ( Fig.4) Four cooking zones 3 are also arranged in the glass ceramic plate 2, with the control panel 4 also having four rotary switches 5, and for each rotary switch 5 there is a symbol relating to the assignment of the rotary switch 5. Next to each of the rotary switches 5 are display elements 9 for the temperature status, so that in this embodiment the temperature status of each cooking zone 3 is arranged adjacent to its rotary switch 5 and the symbolism associated with the plate. In the safety and warning panel 7 there are three error displays 8 arranged one above the other, which can indicate an error or the normal status for the entire appliance.

[0122] In a further advantageous embodiment ( Fig.5) dispenses with a separate safety and warning panel 7, with the displays 9 for displaying a temperature status and the error displays 8 for each cooking zone 3 being arranged next to the respective rotary switch 5. Thus, the temperature status of each cooking zone 3 and the error status of each cooking zone 3 are displayed individually to the user.

[0123] If symbols are added to the color markings to indicate a critical temperature condition and / or a device error ( Fig. 6), in a safety and warning field 7, for example, three temperature displays 9 and three error displays 8 are arranged in two columns each. Within the temperature display, for example, a corresponding flame symbol 10a may be present in a display 9a for a high critical temperature, and a reduced flame symbol 10b may be present in a display 9b for a high but non-critical temperature.

[0124] In the display fields 8a, 8b, 8c for an error condition, a symbol 11a indicating a critical error may be present in a field 8a for indicating a critical error condition. A corresponding smaller or less distinct symbol 11b may be present in a display field 8b for indicating an error that, for example, does not affect operational safety.

[0125] In the displays 8c, 9c, which respectively indicate a proper condition and the absence of a critical temperature, corresponding symbols 10c, 11c may be present, which indicate to the user without thinking that the device is in a proper non-critical condition.

[0126] The character arrangement according to Fig. 4 can of course be transferred to the three embodiments, so that the corresponding signals are also arranged next to the rotary switches or other locations of the glass ceramic plate 2.

[0127] In a further embodiment (not shown), only one indicator 8 is provided for a critical error condition and only one indicator 9 is provided for a temperature condition. These indicators 8, 9 have colored light elements that can illuminate the display either in different colors in a stepped manner or continuously in different colors.

[0128] In this case, a single display 8 and a single display 9 can be arranged in each safety and warning field 7. However, it is also possible to provide only one display element 8 and one display element 9 for each rotary switch 5.

[0129] Likewise, each cooking surface, ie each rotary switch 5, can of course be assigned a display 8, 9.

[0130] In a further advantageous embodiment (not shown), the temperature indicator 9 or the error indicator 8 can also be provided in the form of lights or segments of different colors arranged along a circle. For example, the colors are green (for error-free) and blue (for a low temperature) in a non-critical area. A rising temperature and / or a rising error rate are indicated along the circle with changing colors. These temperature profiles can also be displayed along a bar.

[0131] The invention offers the advantage of providing simple and intuitive displays for the user, allowing them to easily identify the dangers posed by heat or thermal conditions or device malfunctions. This significantly increases safety during use. Furthermore, ergonomics and confidence in the device are enhanced.

[0132] The invention was disclosed with reference to a hob, in particular a hob with a glass ceramic plate. However, the invention can be applied with equal success to any type of electrical appliance, particularly in the home. Table 1: Compositions and properties of starting glasses according to the invention and comparison glass 13 Glass No. 1 2 3 4 5 6 7 8 9 10 11 12 13 Compositions in wt% on an oxide basis Like Glass No. 1 Like Glass No. 1 Like glass No. 7 Like glass No. 9 Al2O3 20,8 20,9 21,0 20,90 21,7 19,1 21,0 20,9 21,2 BaO 2,19 2,26 0,97 1,48 1,01 0,77 - 2,30 - CaO 0,44 0,46 0,40 0,40 0,13 0,05 0,5 0,42 - K2O 0,24 0,20 0,25 0,30 0,22 0,13 0,4 0,22 0,15 Li2O 3,61 3,64 3,52 3,48 3,73 3,59 3,7 3,71 3,8 MgO 0,34 0,31 0,31 0,40 0,72 1,16 0,4 0,37 1,0 Na2O 0,58 0,62 0,45 0,50 0,33 0,14 0,6 0,59 0,45 P2O5 - - - - - - 0,6 - - SiO2 65,2 64,9 66,70 66,10 65,3 68,6 65,8 65,14 67,2 SnO2 0,24 0,24 0,24 0,24 0,27 0,27 0,26 0,24 0,2 SrO 0,10 - - - - 0,5 - - TiO2 3,11 3,27 3,23 3,26 3,20 2,58 3,1 3,1 2,5 ZnO 1,52 1,50 1,38 1,36 1,60 1,51 1,6 1,5 1,7 ZrO2 1,38 1,37 1,26 1,31 1,48 1,71 1,35 1,34 1,7 CoO - 0,039 0,069 0,065 0,039 0,04 - Fe2O3 0,12 0,12 0,15 0,15 0,12 0,12 0,09 0,085 0,03 MnO2 0,020 0,028 - - 0,028 0,03 0,02 0,025 - Nd2O3 0,015 - - NiO - 0,013 - - 0,012 0,012 - V2O5 0,024 0,024 0,037 0,042 0,025 0,023 0,027 0,026 0,06 Table 1 (continued): Compositions and properties of starting glasses according to the invention and comparison glass 13 Glass No. 1 2 3 4 5 6 7 8 9 10 11 12 13 Compositions in wt% on an oxide basis Like Glass No. 1 Like Glass No. 1 Like glass No. 7 Like glass No. 9 Additional mixture % wt 1 (sugar) CHARACTERISTICS TG °C 673 666 679 678 671 680 672 670 674 V A °C 1306 1303 1327 1318 1296 1332 1318 1305 1310 density g / cm 3 2,478 2,480 2,448 2,460 2,468 2,443 2,444 2,479 2,438 Devitrification te mp °C 1255 1260 1265 1265 1290 1310 1235 1235 1280 T (pO2 = 1 bar) °C 1643 1582 1798 1654 1608 1617 1664 1548 1696 1595 1564 1600 1569 Maximum melt temperature, time °C h 1850 1 1640 2 1640 2 1850 1 1640 2 1640 2 1850 1 1640 2 1850 1 1640 2 1640 2 1780 0,25 1640 2 Table 2: Ceramicization conditions and properties of glass ceramics according to the invention and comparative glass ceramics, Examples 2, 4, 17 Example No. 1 2 3 4 5 6 7 8 9 Glass No. 1 2 2 3 3 4 4 5 6 Keram.Prog. 1 1 1 1 1 2 2 2 2 T max °C 900 900 940 900 860 890 875 875 875 t max min 15 15 15 15 15 10 10 10 10 Properties ceramized: density g / cm 3 2,547 2,547 2,541 2,546 2,526 2,532 Thermal expansion α 20 / 700 10 -6 / K 0,23 0,58 0,35 0,32 0,01 0,20 Transmission standard light C, 2° 400 nm % 0,04 0,09 0,02 0,01 0,02 0,05 0,06 0,13 0,11 450 nm % 0,16 0,57 0,27 0,03 0,10 0,20 0,25 0,35 0,31 500 nm % 0,31 1,15 0,61 0,05 0,17 0,17 0,22 0,13 0,13 550 nm % 0,78 2,60 1,40 0,15 0,48 0,33 0,43 0,22 0,24 630 nm % 4,81 10,8 6,65 1,57 3,85 4,42 5,32 5,19 5,34 700 nm % 16,7 27,2 19,8 8,78 15,9 18,8 21,2 24,4 24,5 1600 nm % 69,4 70,0 69,6 67,8 67,6 54,8 55,8 48,0 48,8 Light transmission % 1,4 3,9 2,2 0,4 1,0 1,0 1,2 1,0 1,1 X-ray diffraction HQz phase content % 62 50 60 61 65 63 average crystallite size nm 48 - 38 38 32 35 Table 2 (continued): Ceramicization conditions and properties of glass ceramics according to the invention and comparative glass ceramics, Examples 2, 4, 17 Example No. 10 11 12 13 14 15 16 17 18 19 Glass No. 7 8 8 9 10 11 11 13 12 Ceramic. Prog. 2 2 2 2 2 1 2 2 2 2 T max °C 890 915 930 915 935 930 930 930 915 915 t max min 10 10 10 5 10 15 15 15 3 3 Properties ceramized: density g / cm 3 2,548 2,554 2,540 2,544 2,522 2,525 2,527 2,546 2,543 Thermal expansion α 20 / 700 10 -6 / K 0,11 0,07 0,02 -0,04 0,02 0,01 -0,14 0,24 0,23 Transmission standard light C, 2° 400 nm % 0,07 0,16 0,14 0,02 0,06 0,03 0,03 0,01 0,03 0,30 450 nm % 0,20 0,57 0,47 0,16 0,29 0,18 0,19 0,04 0,15 0,38 500 nm % 0,16 0,46 0,37 0,19 0,29 0,35 0,37 0,15 0,28 0,40 550 nm % 0,36 0,86 0,68 0,51 0,67 0,85 0,90 0,92 0,72 0,85 630 nm % 5,37 8,91 7,48 7,90 8,51 5,03 5,21 9,96 4,72 5,19 700 nm % 22,9 29,4 26,4 29,3 29,2 17,4 17,7 28,6 17,0 18,3 1600 nm % 55,2 54,6 54,8 54,6 54,4 72,4 72,5 82,2 71,5 76,7 Light transmission % 1,2 2,2 1,8 1,7 2,0 1,5 1,6 2,5 1,4 1,6 X-ray diffraction HQz phase content % 68 70 75 72 64 64 73 62 63 average crystallite size nm 38 46 43 44 46 44 43 44 42

Claims

[1] Method for indicating an operating state of a transparent colored cooking surface (1) with improved color display capability, consisting of a glass ceramic with high quartz mixed crystals as the predominant crystal phase, wherein the glass ceramic contains, apart from unavoidable traces, none of the chemical refining agents arsenic oxide and antimony oxide, wherein the glass-ceramic has transmission values ​​of greater than 0.1% in the visible light range in the entire wavelength range greater than 450 nm, a light transmission in the visible range of 0.8 - 5%, preferably 0.8 - 2.5%, and a transmission in the infrared at 1600 nm of 45 - 85%, characterized by an equilibrium oxygen partial pressure pO2 of the glass-ceramic of 1 bar at a temperature greater than 1580°C, wherein a display device is provided with display elements which radiate their light from the rear of the cooking surface (1), wherein the display device comprises a display device (8,9), and wherein the display device (8, 9) displays different predetermined operating states with different colors. [2] Method according to claim 1, characterized by that the operating state displayed is a temperature state and / or an error state of the device or part of the device. [3] Method according to claim 1 or 2, characterized by that the temperature of a part of the cooking surface (1) or of a cooking zone (3) of the cooking surface or of a part of the cooking surface is displayed in different colors. [4] Method according to one of the preceding claims, characterized by that an error condition of a hob (1) and / or a part of a hob (1) and / or a cooking area (3) is displayed with different colors. [5] Method according to one of the preceding claims, characterized bythat the different operating states are displayed with differently colored light elements with at least two different colors. [6] Method according to one of the preceding claims, characterized by that different temperature conditions are indicated at least with a red and a blue light element or a single light element which can display red and blue. [7] Method according to one of the preceding claims, characterized by that an error condition is indicated with a red and a green light element or a single light element that can display red and green. [8] Method according to one of the preceding claims, characterized by that the colors used to represent an operating status are displayed continuously. [9] Method according to one of the preceding claims, characterized bythat the colors are displayed with light segments of a circular display or light elements of a bar display. [10] Method according to one of the preceding claims, characterized by that the temperature operating states are represented by the colors red for hot, orange for warm and blue for cold. [11] Method according to one of the preceding claims, characterized by that the error states are displayed in the colors red for a critical error, yellow / orange for an error that is not critical for operational safety and green for error-free. [12] Method according to one of the preceding claims, characterized by that the operating states are also displayed with symbols. [13] Method according to claim 12, characterized by that different flame or fire sizes are used as symbols for temperature operating conditions and different sized warning signs are used for fault operating conditions. [14] Method according to one of the preceding claims, characterized by that in addition to the display with colours and / or symbols, the display devices flash. [15] Method according to claim 14, characterized by that the displays flash faster or slower or with different rhythms depending on the error and / or temperature intensity.

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