FR2939427A1 - TRANSPARENT AND LOW-DENSE GLASS-CERAMIC AND USE THEREOF - Google Patents

Abstract

The invention relates to a transparent and low-density glass-ceramic of the Li O-Al O -SiO system, which comprises β-quartz mixed crystals as the main crystalline phase, which has a density p of less than 2.5 g / cm. , which contains, in weight percent relative to the oxides, 3 to 4.5% of Li O, 18 to 24% of Al O, 55 to 70% of SiO, 0.1 to less than 2.3% of TiO, 0.1 to less than 1.8% ZrO, 0.2 to 4% total TiO and ZrO, 0 to 1.5% BaO, 0 to 0.5% CaO, 1 to 6% total of MgO and ZnO, more than 5 to 10% of BO, 0 to less than 1% in total of NaO and KO and 0 to 1.5% of usual refining agents (SnO, As O, Sb O, CeO). Such a glass-ceramic has a clarity index Y (A / 2 °), measured according to DIN 5031, of more than 80, and in particular, more than 87.

Description

B09-4519FR

Society called: SCHOTT AG Transparent and sparse glass ceramic, and its use Invention of: SIEBERS Friedrich SCHAUPERT Kurt ZACHAU Thilo DAHLMANN Ulf

Priority of a patent application filed in Germany on December 8, 2008, under number 10 2008 054 386.1

This invention relates to a transparent glass ceramic and low density. It is known that the density of vitroceramics of the Li2O-Al2O3-SiO2 system comprising mixed crystals of quartz 13 (high temperature quartz) can be varied to a small extent for the same composition and the same proportion of crystalline phase. as the main crystalline phase, since one can, through the ceramization program, exert a small influence on the contraction (compaction) undergone by the starting glass during its ceramization. However, despite all the measures proposed, the density of the glass-ceramics of the Li 2 O -Al 2 O 3 -SiO 2 system comprising mixed crystals of quartz 13 as the main crystalline phase always remains greater than 2.5 g / cm 3. Thus, for example, the glass ceramic marketed by Schott AG under the name Zerodur, designed in particular for optical applications, has a density of 2.53 g / cm3.

If it is desired to prepare a glass ceramic of the Li 2 OAl 2 O 3 -SiO 2 system comprising mixed crystals of quartz 13 as the main crystalline phase and having a density of less than 2.5 g / cm 3, a new composition must be found for this glass-ceramic. Patent EP 1 840 093 A1 discloses a glass ceramic which comprises spodumene 13 as the main crystalline phase, but to which a density of less than 2.5 g / cm 3 can not be conferred. The object of this invention is to find a vitroceramic system Li2O-Al2O3-SiO2 comprising mixed crystal quartz 13 as the main crystalline phase, which has a density less than 2.5 g / cm3. This glass ceramic must also have a high degree of transparency and a low clean color.

This object is achieved by means of a glass ceramic of the Li 2 O -Al 2 O 3 -SiO 2 system, which comprises mixed crystals of quartz 13 as the main crystalline phase, which has a density of less than 2.5 g / cm 3, and which contains, in particular % by weight relative to the oxides: Li2O Al2O3 SiO2 TiO2 ZrO2 Total TiO2 + ZrO2 BaO CaO Total MgO + ZnO B2O3 Total Na2O + 1 (20 Usual refining agents (SnO2, As2O3, Sb2O3, CeO2) 3 ù 4.5 18ù24 55 0.1 to less than 2.3 0.1 to less than 1.8 to 0.4 to 1.5 to 0.5 more than 5 to 10 to less than 1

0 to 1.5 The oxides of lithium Li 2 O, aluminum Al 2 O 3 and silicon SiO 2 are indispensable constituents of a glass ceramic having mixed crystals of quartz and / or keatite. When the Li 2 O content is less than 3% by weight, the proportion of crystalline phase is too low for the intended application (it is desirable that the mixed quartz crystal fraction 13 is less than 60% by volume), but when the Li 2 O content is greater than 4.5% by weight, the crystal growth rate increases, which creates a risk for stability with respect to devitrification. It is advantageous that the Li 2 O content is from 3 to 4% by weight, since when it is in this range, a particularly favorable compromise is reached between the proportion of accessible crystalline phase and a satisfactory stability with respect to of devitrification.

The Al2O3 content should be 18 to 24% by weight, because when it is in this range, the melt has a viscosity low enough to ensure that it can be properly worked. If the Al.sub.2 O.sub.3 content is less than 18% by weight, a drop in the accessible ceramization rate is observed, and the degree of transparency of the glass-ceramic obtained is too low. Preferably, the Al2O3 content is from 19.5 to 21.5% by weight. The SiO2 content should be 55 to 70% by weight. A higher SiO2 content would certainly lead to a lower value of the density, but the upper limit to be respected is 70% by weight, and more preferably 66% by weight, because otherwise one must, in order to be able to process the raw glass, wear it at a temperature above 1550 ° C, at which it is technically difficult to work. On the other hand, when the SiO 2 content is less than 55% by weight, the ceramization rate decreases. It is preferred that the SiO 2 content be at least 56% by weight. It is known that the use of alkali metal oxides, sodium oxide Na2O and potassium oxide K2O, improves during the manufacture of the glass, the fusibility thereof and its devitrification behavior. However, the total content of Na2O and K2O must remain below 1% by weight, because if these oxides are used in a larger proportion, this leads to an increase in the coefficient of thermal expansion of the glass-ceramic. Preferably, the total content of Na2O and K2O is from 0 to 0.9% by weight, and in particular from 0.3 to 0.5% by weight. The alkaline earth metal oxides CaO and BaO also, in general, have similar positive effects on the melting behavior of the glass. It is preferred, however, that there be no calcium oxide CaO in the compositions of the present invention, although a content of at most 0.5% by weight can be tolerated. The presence of CaO in the compositions of this invention leads to the formation of parasitic phase precipitates during ceramization and should therefore be avoided. Barium oxide BaO also has similar effects on the melting behavior of glass, but a relatively high BaO content leads to an increase in density, so that the BaO content must be at most 1.5 % in weight. It is preferable to provide from 0.1 to 0.8 wt.% BaO in the compositions of the invention in order to improve the fusibility and degree of transparency thereof. It is however particularly preferable that a composition of the invention does not contain BaO, but this is not always feasible for reasons of fusibility. As is known, titanium oxides TiO 2 and zirconium ZrO 2 act as seed formers, and it is then necessary that the TiO 2 content is less than 2.3% by weight, that the ZrO 2 content is less than 1%. , 8% by weight and that the sum of TiO2 and ZrO2 contents is from 0.2 to 4% by weight. If these limit values are exceeded in plus or minus, the formation of germ-forming precipitates takes place too slowly, and the result is a decrease in the quartz mixed crystal content 13 and the degree of transparency, which is not satisfactory. Preferably, the minimum contents of TiO2 and ZrO2 are 1% by weight in each case, and the sum of the contents of TiO2 and ZrO2 is from 2 to 4% by weight. The sum of the magnesium oxide MgO and zinc ZnO contents must be from 1 to 6% by weight, because these oxides reinforce the coloration taken by the glass-ceramic in the presence of Fe 2 O 3 iron oxide traces. It is preferable that in this overall value the MgO content does not exceed 0.5% by weight and the ZnO content does not exceed 2.5% by weight. It is particularly preferred that the sum of the MgO and ZnO contents be from 1 to 2.5% by weight. Boron oxide B2O3 is an important component for obtaining a low density glass ceramic, since it has the effect of making the structure looser and reducing the densification of the glass ceramic during ceramization. It must be present in a proportion by weight of more than 5% to 10%. With higher B2O3 contents, there is a risk that the transmission rate of the glass ceramic will decrease or that the tendency to devitrification during shaping (Va) will increase. It is preferred that the B2O3 content be more than 5% to 9% by weight.

In the glass composition, there may be 0 to 1% by weight tin oxide SnO 2, which may act as a refining agent as well as as a seed former. However, it is preferable that there is no SnO2.

Due to the color problems mentioned about the MgO and ZnO components, the Fe 2 O 3 iron oxide content must be less than 200 ppm by weight, and preferably less than 130 ppm by weight. It must therefore be ensured that only iron-deficient raw materials of this type are used.

Furthermore, it is preferable that the glass contains no calcium oxide, no fluorine, no lead oxides or coloring oxides. The glass-ceramic is manufactured in a well-known manner by melting the raw materials together, and then refining and shaping the glass. The refining can be carried out chemically or physically. If known refining agents, such as arsenic oxides As2O3, antimony Sb2O3, cerium CeO2 and tin SnO2, sulphates or chlorides are used in usual proportions, from 0.1 to 1.5% by weight. For physical refining, there are for example processes where one operates under reduced pressure or at high temperature. After shaping the glass, for example in the form of a plate, this glass is subjected to the well-known ceramizing process to make it a glass-ceramic. To do this, the glass is treated in a known manner, for example at a temperature of 630 to 770 ° C. for a period of time of more than 15 minutes, in order to precipitate with a high density of seeds the phase of preferred nuclei, TiZrO4 titanium zirconate, and crystallization thereof is crystallized into a quartz crystal mixed glass ceramic 13, treating it at a temperature of 700 to 950 ° C for a period of at least 5 minutes. In this way, the size of crystallites of quartz mixed crystals 13 within the glass-ceramic is less than 100 nm, and in particular less than 80 nm, which gives the glass-ceramic a remarkable transparency. In the range of 20 to 300 ° C, the thermal expansion coefficient of the glass ceramic is less than 0.3 × 10 -6 to 0.6 × 10 -6 K -1, and preferably less than 0.5 × 10 -6 K -1. . The clarity index Y (A / 2 °) of the glass ceramic, measured according to the German standard DIN 5031, is, for a plate of 4 mm thick, more than 80 and preferably more than 87. The clarity index Y the CIE xyY color measurement system is given each time for a normalized light A transmitted at an observation angle of 2 °, and this index is calculated from the wavelength resolved transmission spectra, using the ocular sensitivity curves defined by the CIE (tristimulus curves). Sometimes, this index is also calculated as the spectral transmission rate of light. An advantageous method of ceramizing, which makes it possible to obtain a glass-ceramic from a green glass, consists in first heating the green glass to a temperature of 650 ° C. at a heating rate of 10 to 15 K. / min, then continue heating this glass to a temperature of 710 ° C to 50 ° C at a heating rate of 1 to 5 K / min, to maintain the glass at this temperature for 60 minutes for the formation of germs then, to bring the glass to a crystallization temperature of 850 ° C, at a heating rate of 0.5 to 5 K / min, and to hold the glass at that temperature for 15 minutes for crystallization. The glass-ceramic is then cooled at an arbitrary cooling rate, but which is generally from 1 to 20 K / min. The present invention will now be explained in more detail by way of examples. The starting glasses are prepared by melting the usual raw materials in the glass industry at 1620 ° C with the refining agents. After the melting, carried out in sintered quartz glass crucibles (Quarzal from Schott AG), the melts are poured into platinum crucibles and stirred at 1600 ° C. for 30 minutes to homogenize them. After two hours at 1640 ° C., 140 mm × 100 mm × 30 mm pieces are poured and then cooled in a cooling oven, starting from 650 ° C. the ambient temperature, so that the voltages of thermal origin are relaxed. It is from these castings that samples are made for testing, such as bars for the measurement of thermal expansion coefficient and platelets for measuring transmission. The vitroceramics, under the conditions of formation of seeds and crystallization indicated below, are then transformed into glass samples of the dimensions necessary for the investigations carried out on vitroceramics. For this purpose, the samples are heated to 650 ° C., at the rate of 10 K / min, and once this temperature is reached, they are heated to a temperature of 20 K higher than their glass transition temperature, and they are kept for 30 minutes at this temperature so that crystallization seeds are formed. The samples are then heated at a rate of 10 K / min at a temperature in the vicinity of the maximum formation of quartz mixed crystal precipitates 13, determined by differential scanning calorimetry (EDA), and maintained for 10 minutes at room temperature. this temperature, and then they are cooled in air to room temperature, at an average cooling rate of 5 K / min. The density p, expressed in g / cm 3, is measured for the starting glasses and the glass-ceramics obtained, as well as, for the latter, the crystalline phase fraction FPC and the clarity index Y, in normalized light A under a angle of 2 °. The data of the examples are summarized below in Table 1.

Table 1 Example No. 1 2 3 Al2O3 21.26 21.17 19.70 As2O3 0.34 0.35 0.27 B2O3 5.14 5.35 5.14 BaO 0.00 0.24 Fe2O3 0.01 0 , 01 Li2O 3.66 3.68 3.72 MgO 0.58 0.56 0.43 Na2O 0.53 <0.1 SiO2 62.96 63.48 65.90 TiO2 2.20 2.21 2.23 ZnO 1.65 1.25 0.86 ZrO 2 1.68 1.71 1.74 p (glassy phase) 2.4069 2.4005 2.3798 p (ceramic phase) 2.4876 2.4968 2.4731 FPC 62 65 68 Y (A / 2 °) 87.2 86.8 88.7 Due to their low inherent color and their high Y (A / 2 °) clarity corresponding to a high value of the spectral transmission, the glass-ceramics of the present invention are well suited for use in a variety of devices such as hotplates, oven or chimney panes, semiconductor substrates, or glazing components, e.g. , observation windows for high-temperature equipment, etc. The glass-ceramics of the present invention are particularly suitable for use as components in mass-resistant glazing systems and as protection against vandalism and the effects of shots or pressure waves. Of course, it is also possible, if necessary, to color these glass-ceramics and the corresponding starting glasses by means of the usual dye oxides, to achieve particular aesthetic effects.

Claims (4)

REVENDICATIONS1. Vitroceramic system Li2O-Al203-SiO2, which has mixed crystals of quartz 13 as the main crystalline phase, which has a density p less than 2.5 g / cm3, and which contains, in weight percent relative to the oxides 15 Li 2 O Al 2 O 3 SiO 2 TiO 2 ZrO 2 Total TiO 2 + ZrO 2 BaO CaO Total MgO + ZnO B 2 O 3 Total Na 2 O + 1 (20 customary refining agents (SnO 2, As 2 O 3, Sb 2 O 3, CeO 2) 3 to 4.5 18 to 24 55 to 70 0.1 or less from 2.3 0.1 to less than 1.8 0.2 to 4.0 to 1.5 to 0.5 more than 5 to 10 to less than 1.0 to 1.5 2. Glass ceramic according to claim 1, which contains, in weight percent relative to the oxides: Li 2 O Al 2 O 3 SiO 2 Total TiO 2 + ZrO 2 BaO ZnO MgO Total MgO + ZnO B 2 O 3 Total Na 2 O + 1 (20 Usual refining agents (SnO 2 As 203, Sb 2 O 3, CeO 2) 3, 19.5, 21.5, 56, 66, 0.10, 0.80, 2.5, 0.5, 2.5, more than 5, 9, 0, 0, 9 0 to 1.5 3. Glass ceramic according to claim 1 or 2, whose clarity index Y (A / 2 °), measured according to DIN 5031, is more than 80, and more particularly more than 87. 4. Use of a glass-ceramic according to one or more of claims 1 to 3 for cooking plates, oven or chimney panes, semiconductor substrates, glazing components, anti-glass glazings, fire, observation windows for apparatus operating at high temperatures, or glazing systems resistant to mass effects and used as protection against vandalism and the effects of shots or pressure waves.