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
  • C03C3/089—Glass compositions containing silica with 40% to 90% silica, by weight containing boron
  • C03C3/091—Glass compositions containing silica with 40% to 90% silica, by weight containing boron containing aluminium

Definitions

• the invention relates to an alkali-free aluminoborosilicate glass.
• the invention also relates to uses of this glass.
• High requirements are placed on glasses for applications as substrates in flat-panel liquid-crystal display technology, for example in TN (twisted nematic)/STN (supertwisted nematic) displays, active matrix liquid-crystal displays (AMLCDs), thin-film transistors (TFTs) or plasma-addressed liquid crystals (PALCs).
• TN twisted nematic
• STN supertwisted nematic
• AMLCDs active matrix liquid-crystal displays
• TFTs thin-film transistors
• PLCs plasma-addressed liquid crystals
• the glasses should have high transparency over a broad spectral range (VIS, UV) and, in order to save weight, a low density.
• a-Si amorphous silicon
• the resulting, partially crystalline poly-Si layer is characterized by a thermal expansion coefficient of ⁇ 20/300 ⁇ 3.7 ⁇ 10 ⁇ 6 /K.
• the thermal expansion coefficient ⁇ 20/300 may vary between 2.9 ⁇ 10 ⁇ 6 /K and 4.2 ⁇ 10 6 /K.
• substantially crystalline Si layers are generated by high temperature treatments above 700° C. or direct deposition by CVD processes, which is likewise desired in thin-film photovoltaics, a substrate is required which has a significantly reduced thermal expansion of 3.2 ⁇ 10 ⁇ 6 /K or less.
• applications in display and photovoltaics technology require the absence of alkali metal ions. Sodium oxide levels of less than 1500 ppm as a result of production can be tolerated in view of the generally “poisoning” action due to diffusion of Na + into the semiconductor layer.
• the glass needs to have a suitable temperature-dependent viscosity characteristic line: with respect to thermal process and shape stability, it should have a sufficiently high glass transition temperature, i.e. T g >700° C., while on the other hand not having excessively high melting and processing (V A ) temperatures, i.e. a V A of ⁇ 1350° C.
• Glass substrates for LCD display technology or thin-film photovoltaics technology are also described in “Glass substrates for AMLCD applications: properties and implications” by J. C. Lapp, SPIE Proceedings, Vol. 3014, invited paper (1997), and in “Photovoltaik—Strom aus der Sonne” by J. Schmid, Verlag C. F. Muller, Heidelberg 1994, respectively.
• the transition to larger display formats places new requirements on the mechanical stability and the specific gravity of the glass substrates.
• the transition from present-day 600 mm ⁇ 720 mm sheets to sheets having dimensions of e.g. 1 m ⁇ 1 m and more would lead to a corresponding increase in weight which would have an effect on, inter alia, the robot handling for transporting the glass sheet from one production process step to another.
• a glass is desirable which has a high modulus of elasticity of >80 GPa, preferably ⁇ 85 GPa, combined with a low density of ⁇ 2.55 g/cm 3 . This also minimizes the risk of sagging of the sheet during coating of the glass substrate with an active silicon layer.
• Some of these glasses and also the glasses of DE 197 36 912 C1 and, according to the examples, the glasses of JP 9-48 632 A contain relatively high amounts of the heavy alkaline earth metal oxides BaO and/or SrO which leads to poor meltability. Moreover, such glasses have an undesirably high density, which is disadvantageous in particular for large-format displays.
• the glass contains >58-70% by weight of SiO 2 . At lower contents, the chemical resistance is impaired, while at higher levels, the thermal expansion becomes too low and the crystallization tendency of the glass increases. Preference is given to a maximum content of 68% by weight.
• the glass contains 10-25% by weight of Al 2 O 3 . This has a positive effect on the devitrification stability of the glass and the heat resistance increases without excessively increasing the processing temperature. Preference is given to a content of 14-24% by weight of Al 2 O 3 .
• the B 2 O 3 content is 0.5- ⁇ 9% by weight.
• the B 2 O 3 content is restricted to the maximum content specified in order to achieve a high mechanical stability. Higher contents would also impair the chemical resistance to hydrochloric acid solutions.
• the minimum B 2 O 3 content specified serves to ensure that the glass has good meltability and good devitrification stability. Preference is given to a content of 1-8.5% by weight. Particular preference is given to a maximum content of 5% by weight.
• An essential glass component are the network-modifying alkaline earth metal oxides. With a sum of alkaline earth metal oxides of between >8 and 18% by weight, a coefficient of thermal expansion ⁇ 20/300 of between 2.8 ⁇ 10 ⁇ 6 /K and 3.9 ⁇ 10 ⁇ 6 /K is achieved. MgO is always present, while CaO, SrO and BaO are optional components. Preferably at least two alkaline earth metal oxides are present. This second alkaline earth oxide is particularly preferably CaO. Particularly preferably at least three alkaline earth metal oxides are present.
• the glass contains >8-15% by weight of MgO. These relatively high levels make it possible to obtain a glass having a modulus of elasticity which is sufficient for the increased requirements, and a low density.
• the B 2 O 3 content preferably depends on the MgO content because B 2 O 3 and MgO have opposite effects on the modulus of elasticity.
• the MgO/B 2 O 3 ratio by weight is thus preferably >1, particularly preferably >1.35.
• the glass may furthermore contain up to ⁇ 10%, preferably ⁇ 9%, by weight of CaO. Higher levels would lead to an excessive increase in density and to an increase in crystallization tendency. It is preferred that the glass contains CaO, specifically preferably in an amount of at least 0.5% by weight, particularly preferably at least 1% by weight.
• the glass may furthermore contain BaO, which has a positive effect on its devitrification stability.
• the maximum content is restricted to ⁇ 2% by weight to keep the density of the glass low.
• the BaO content of the glass is particularly preferably between 0 and 0.5% by weight.
• the glass is most preferably free of BaO.
• the glass may furthermore contain SrO. Its presence likewise has a positive effect on the devitrification stability.
• the maximum SrO content is restricted to ⁇ 3% by weight to keep the density of the glass low.
• the glass contains particularly preferably between 0 and 1% by weight and most preferably between 0 and 0.5% by weight.
• the sum of the two heavy alkaline earth metal oxides SrO and BaO is preferably limited to a maximum of 4% by weight.
• the glass may furthermore contain up to ⁇ 2% by weight of ZnO.
• ZnO has an effect on the viscosity characteristic line which is similar to that of boric acid, has a network-loosening function and has less effect on the thermal expansion than the alkaline earth metal oxides.
• the maximum ZnO level is preferably limited to 1.5% by weight, in particular when the glass is processed by the float method. Higher levels would increase the risk of unwanted ZnO coatings on the glass surface which may form by evaporation and subsequent condensation in the hot-shaping range.
• the glass is alkali-free.
• alkali-free as used herein means that it is essentially free from alkali metal oxides, although it can contain impurities of less than 1500 ppm.
• the glass may contain up to 2% by weight of ZrO 2 +TiO 2 , where both the TiO 2 content and the ZrO 2 content can each be up to 2% by weight.
• ZrO 2 advantageously increases the heat resistance of the glass. Owing to its low solubility, ZrO 2 does, however, increase the risk of ZrO 2 -containing melt relicts, so-called zirconium nests, in the glass. ZrO 2 is therefore preferably omitted.
• Low ZrO 2 contents originating from the corrosion of zirconium-containing trough material are unproblematic.
• TiO 2 advantageously reduces the solarization tendency, i.e. the reduction in transmission in the visible wavelength region because of UV-VIS radiation. At contents of greater than 2% by weight, colour casts can occur due to complex formation with Fe 3+ ions which are present in the glass at low levels as a result of impurities of the raw materials employed.
• the glass may contain conventional refining agents in the usual amounts: it may thus contain up to 1.5% by weight of As 2 O 3 , Sb 2 O 3 , SnO 2 , CeO 2 , Cl ⁇ , F ⁇ and/or SO 4 2 ⁇ .
• the sum of the refining agents should, however, not exceed 1.5% by weight. If the refining agents As 2 O 3 and Sb 2 O 3 are omitted, the glass can be processed not only using a variety of drawing methods, but also by the float method.
• Glasses were produced in Pt/Ir crucibles at 1620° C. from conventional raw materials which were essentially alkali-free apart from unavoidable impurities.
• the melt was refined at this temperature for one and a half hours, then transferred into inductively heated platinum crucibles and stirred at 1550° C. for 30 minutes for homogenization.
• the melts were poured into preheated graphite moulds and cooled down to room temperature.
• the table shows eight examples of glasses according to the invention (A 1 -A 8 ) and an example of a comparative glass (C) with their compositions (in % by weight, based on oxide) and their most important properties.
• the following properties are given:
• the glasses according to the invention have the following advantageous properties:
• the glasses are thus highly suitable for use as substrate glass in display technology, in particular for TFT displays, and in thin-film photovoltaics, in particular on the basis of polycrystalline Si, and as substrate glass for hard disks.

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• Chemical & Material Sciences (AREA)
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• General Chemical & Material Sciences (AREA)
• Geochemistry & Mineralogy (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Glass Compositions (AREA)

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• 2001
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