Classifications

• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C3/00—Glass compositions
  • C03C3/04—Glass compositions containing silica
  • C03C3/076—Glass compositions containing silica with 40% to 90% silica, by weight
  • C03C3/083—Glass compositions containing silica with 40% to 90% silica, by weight containing aluminium oxide or an iron compound
  • C03C3/085—Glass compositions containing silica with 40% to 90% silica, by weight containing aluminium oxide or an iron compound containing an oxide of a divalent metal
  • C03C3/087—Glass compositions containing silica with 40% to 90% silica, by weight containing aluminium oxide or an iron compound containing an oxide of a divalent metal containing calcium oxide, e.g. common sheet or container glass

Definitions

• the invention relates to soda-lime-silica glass compositions capable of being converted into glass ribbon in which plates which are heat-resistant can be cut. These plates are used in particular as a substrate for the manufacture of display screens, in particular plasma screens.
• a plasma screen generally consists of two glass sheets, more commonly called “substrates”, 2.8 to 3 mm thick, welded together by a glass frit and separated by a space in which a gas mixture is trapped.
• Plasmagen Ne, Xe, Ar
• the front substrate comprises, on its inner face, a deposit of indium oxide and tin (ITO) serving as a cathode.
• ITO indium oxide and tin
• the rear substrate is provided on its inner face with an anode (Ni, Ag) and phosphors which, when excited by the ultraviolet radiation emitted by the plasma gas mixture during the plasma discharge between the two substrates , produce visible light radiation (red, green, blue). The image produced from this radiation is projected through the front substrate.
• the glass used for producing substrates is a silico-soda-lime glass, of the type used for the manufacture of automotive glazing and for building.
• This glass has interesting properties in terms of flatness, glass defects, optical properties and chemical resistance. However, it has been found that the temperature resistance is not very high.
• the substrate is subjected to several heat treatments which are intended to stabilize the dimensions of said substrate and to fix a series of layers of different compounds deposited on its surface as already mentioned. Fixing these layers of greater or lesser thicknesses requires that the substrate be heated to temperatures above 550 ° C.
• the silico-soda-lime glass has a lower annealing temperature generally close to 510 ° C, so that at the indicated treatment temperatures, the glass does not have sufficient strength and it is necessary to put it on a ground slab to avoid any deformation during treatments. Since the trend is to increase the size of the screens and therefore the substrates, this disadvantage becomes unacceptable.
• the soda-lime-silica glass contains a high proportion of Na 2 O which is likely to react with the silver constituting the electrodes.
• a permeation exchange phenomenon of the Na + and Ag + ions can occur, and these ions can react by forming an Ag ° colloid which absorbs a part of the UV radiation, thus causing an effect yellowing of the glass.
• the yellowing effect can be avoided by interposing a layer of SiO 2 between the glass and the silver electrodes.
• these glasses may present risks of breakage during heat treatments of the layers deposited on the glass, especially at the edges of the substrate.
• the substrate cut from the glass ribbon obtained under the conditions of the method of float glass on molten metal bath undergoes a step of shaping the edges consisting, by a mechanical treatment, to remove the sharp edges because they have numerous defects from which cracks can propagate under the effect of thermal stresses. These cracks are all the more important as the substrate is subjected to repeated thermal gradients.
• the mechanical quality of the shaping is directly related to the nature of the glass, including its ability to resist damage by indentation. Added to this is the problem of weight reduction which, because of the constant increase in the dimensions of the substrates, becomes a major concern on the part of screen manufacturers.
• An object of the present invention is to provide a glass composition for obtaining substrates, in particular intended for producing screens for plasma-type visualization, which makes it possible to obtain a good compromise between the density and the mechanical properties, this composition advantageously having a temperature of annealing at least equal to 570 ° C. and a coefficient of thermal expansion of between 70 and 90 10 " 7 / ° C.
• another object of the present invention is to provide a glass composition allowing to obtain a substrate which has improved mechanical properties including improved resistance to thermal gradients, which reduces the risk of breakage
• Another object of the invention is to provide a glass composition which allows the production of a glass substrate having a better ability to resist scratching.
• composition according to the invention comprises the constituents below in the following weight proportions: SiO 2 55 - 75%
• composition having a weight ratio Al 2 O 3 / ZrO 2 ranging from 0.7 to 1.8, and preferably 0 to , 7 to 1, 2.
• composition according to the invention has a density which varies from 2.5 to 2.7, preferably is less than or equal to 2.65 and advantageously is greater than or equal to 2.55.
• the composition of the invention preferably has a strain point of at least 570 ° C and a thermal expansion coefficient ⁇ 2O - oo 3 between 70 and 90 10 "7 / ° C.
• the glasses corresponding to this definition have a lower annealing temperature which is at least 50 ° C higher than that of a silico-sodo glass conventional calcium, preferably at least 60 ° C.
• the lower annealing temperature is above 580 ° C.
• the combination of constituents also makes it possible to obtain a glass whose thermal coefficient of expansion (CTE) remains of the same order of magnitude as that of a traditional silico-soda-lime glass.
• the coefficient of thermal expansion of the glass composition is between 75 and 85 10 "7 / ° C. Such values are particularly interesting for their compatibility with the glass frits usually used to weld the substrates in Plasma screens.
• the constituents used in the glass composition according to the invention are described below.
• SiO 2 plays a vital role as a trainer of the basic glass network. Below 55% by weight of SiO 2 , the stability of the glass is insufficient, which results in particular in a low chemical and hydrolytic resistance. The SiO 2 content does not exceed 75% by weight; beyond that, the melting of the glass batch and the refining of the glass require high temperatures which accelerate the wear of the furnace refractories. On the other hand, it has been observed that the increase in the SiO 2 content does not affect the rise in the lower annealing temperature of the glass.
• the most easily melting glasses according to the invention which have a viscosity suitable for float glass production on a bath of molten metal and have the lowest annealing temperature contain at least 60% SiO 2, preferably between 65% and 70%.
• AI 2 O 3 acts as a stabilizer; it helps to improve the chemical resistance of the glass and promotes the increase of the lower annealing temperature.
• the percentage of AI 2 O 3 does not exceed 5% in order to avoid an excessive reduction in the coefficient of thermal expansion.
• the content of Al 2 O 3 is greater than or equal to 1%, and advantageously between 2 and 4%.
• ZrO 2 also plays a stabilizing role. Like AI 2 O 3 , it improves the chemical resistance of glass and promotes the increase of the lower annealing temperature. Above 5%, the risk of devitrification increases and the coefficient of thermal expansion decreases.
• the ZrO 2 content is preferably greater than 1%, advantageously varies from 2 to 4% and more preferably ranges from 2.5 to 3.5%.
• the sum of the contents of SiO 2 , Al 2 O 3 and ZrO 2 is less than or equal to 78% to allow the melting of the glass to remain within acceptable temperature limits for production by the float process. glass on a bath of molten metal. It is considered that these limits are acceptable since the temperature of the glass corresponding to a viscosity ⁇ of 100 poises does not exceed 1550 ° C. and preferably 1510 ° C.
• the sum of the contents of SiO 2 , Al 2 O 3 and ZrO 2 is preferably greater than or equal to 65%. Na 2 O and K 2 O make it possible to maintain the melting temperature and the viscosity at high temperatures within the limits given above. They also control the coefficient of thermal expansion.
• the total content of Na 2 O and K 2 O is generally at least 8%, preferably at least 10%. Beyond 12%, the lower annealing temperature decreases significantly.
• the simultaneous presence of these two oxides in the glass composition according to the invention makes it possible to considerably increase its chemical resistance, more specifically its hydrolytic resistance, as well as its electrical resistivity.
• Increasing the electrical resistivity of the glasses makes it possible to reduce the diffusion of ions, for example silver, into the glass originating from the layers deposited on the surface of the substrates, as has already been mentioned.
• the increase in electrical resistivity is also of interest in applications where glasses are involved as substrates for cold cathode screens.
• the increase in the total content of Na 2 O and K 2 O is generally done by increasing the proportion of K 2 O which has the advantage of fluidifying the glass without decreasing the lower annealing temperature.
• the K 2 O / Na 2 O weight ratio is at least 1, preferably at least 1, 2.
• the alkaline earth oxides CaO and SrO have the effect of reducing the melting point and the viscosity of the glass at high temperatures. They also allow raising overall lower annealing temperature. The total content of these oxides is generally at least 12%. Beyond 17%, the risk of devitrification increases and may become incompatible with the conditions of the glass floating process on molten metal bath.
• SrO helps to raise the lower annealing temperature and increases the chemical resistance of the glass. Its content varies from 3 to 9%, preferably from 3.5 to 9%, advantageously from 4.5 to 9%, and more preferably ranges from 4.5 to 6.6%.
• the total content of BaO and MgO in the composition according to the invention does not exceed 1% and preferably is zero.
• the glass composition according to the invention leads to a low corrosion of refractories of the AZS (alumina-zirconia-silica) type conventionally used in this type of furnace.
• the glass thus allows an optimization of the duration of use of the oven.
• the glass composition according to the invention also has the advantage of being able to be melted and transformed into a glass ribbon by floating the glass on a bath of molten metal at temperatures close to those adopted for the manufacture of a silico-sodo glass. -calcic classic.
• This difference is advantageously at least 10 ° C to 30 ° C remains "accessible" without significant modification or taking risk in the oven.
• the glass ribbon is then cut to the appropriate dimensions, in particular to form substrates for a display screen.
• the substrates then undergo a shaping step by mechanical treatment of the edges to limit the risk of breakage.
• the glass obtained from the composition according to the invention has improved mechanical properties, in particular a better resistance to scratching and thermal gradients.
• the scratch resistance is evaluated by measuring the value of the ratio c / a defined by the brittleness test described in detail in the examples.
• the c / a ratio measures the sensitivity of glass to Vickers indentation surface damage: a glass is more resistant to scratching than its c / a value is low.
• the glass obtained from the composition according to the invention has a value of c / a of less than 3.85, preferably less than 3.70 and advantageously less than 3.60 and better still less than 3.50.
• E is the modulus of elasticity
• v is the Poisson's ratio
• the product ⁇ 2 .c / a reflects the sensitivity of the glass plate to thermal stresses, especially during the manufacture of the screen, in particular at the shaped edges.
• the glass is even more resistant to thermal gradients than the value ⁇ 2 .c / a is low.
• the glass obtained from the composition according to the present invention has a value of ⁇ 2 .c / a of less than 2.75 MPa 2 / ° C 2 , preferably less than 2.30 MPa 2 / ° C 2 , and advantageously less than 2.20 MPa 2 / ° C 2 .
• Glasses are produced by melting glass compositions comprising the constituents given in Table 1, expressed in weight percent. On the obtained glass, we measure:
• the modulus of elasticity E and the Poisson's ratio determined by the following test a test piece of glass measuring 100 ⁇ 10 mm 2 and having a thickness of less than 6 mm is bent 4 points on the outside supports are separated by 90 mm and internal supports by 30 mm. A strain gauge is glued to the center of the glass plate. We deduce the main displacements (in the length of the plate and in its width). From the applied force, the applied stress is calculated. The relationships between stress and principal displacements make it possible to determine the modulus of elasticity E and the Poisson's ratio, and to calculate the value of ⁇ .
• the glass is first annealed in order to eliminate the residual stresses.
• the glass is heated to its Annealing Point for one hour and then cooled to room temperature at a rate of 2 ° C / minute.
• the test piece of glass to be tested is indented under a load of 1000 g for 30 seconds at room temperature.
• the measurement of the diagonals of the Vickers footprint, as well as the size of the radial fissures (Lawn and Marshall, J. Am., Cer Soc., 62, pp. 347-350 (1979), Sehgal et al, J. Matt Sci 14, p.197-169 (1995)) is carried out 1 hour after indentation.
• the c / a ratio of the length of the radial cracks to the half-diagonal is measured on indentation to obtain a sufficient statistic.
• compositions of Examples 1 to 8 and 10 according to the invention make it possible to obtain glasses whose strain point and coefficient of thermal expansion are compatible with use as a display screen substrate.
• Example 5 has an acceptable density and values of c / a and ⁇ 2 .c / a lower than those of Comparative Example 9, thus demonstrating a better resistance of the glass to scratching and thermal gradients. .
• Example 10 has a lower density than that of Comparative Example 9 and comparable scratch resistance and thermal gradients.
• the glass compositions of Examples 5 and 10 also have viscosity and liquidus characteristics compatible with conventional soda-lime glass production conditions. TABLE 1

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