Alkali-free glass substrate

Abstract

To provide an alkali-free glass substrate that allows bubbles included in molten glass to be removed easily in a reduced-pressure atmosphere.SOLUTION: Provided is an alkali-free glass substrate containing BaO, and containing, in mass% based on oxides, SiO2: 54-68%, Al2O3: 10-25%, B2O3: 1-1.7%, and MgO+CaO+SrO+BaO: 8-26%, the alkali-free glass substrate having β-OH of 0.15-0.35 mm-1, a Cl content of 0.15-0.3 mass%, and a bubble growth index I of 320 or more as represented by the following formula (1): I=590.5×[β-OH]+874.1×[Cl]-5.7×[B2O3]-33.3 (1). In the formula (1), [β-OH] represents β-OH (mm-1) of the alkali-free glass substrate, [Cl] represents the Cl content (mass%) of the alkali-free glass substrate, and [B2O3] represents the B2O3 content (mass%) of the alkali-free glass substrate.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an alkali-free glass substrate suitable as a glass substrate for various displays. [Background technology]

[0002] For glass substrates for various displays, particularly those on which a thin metal or oxide film is formed on the surface, if the substrate contains an alkali metal oxide, alkali metal ions will diffuse into the thin film and deteriorate the film properties. Therefore, it is preferable to use an alkali-free glass substrate that is substantially free of alkali metal ions.

[0003] The alkali-free glass substrate used for the above purpose is obtained by heating and melting glass raw materials prepared in a predetermined compounding ratio in a melting tank to vitrify the glass, clarifying the molten glass, and forming the glass ribbon having a predetermined thickness by a float method or a fusion method, and cutting the glass ribbon into a predetermined shape.

[0004] For fining molten glass, a vacuum degassing method is known in which molten glass is introduced into a reduced pressure atmosphere, bubbles in a continuously flowing molten glass flow are allowed to grow large under this reduced pressure atmosphere, and the bubbles contained in the molten glass are caused to float up, break, and removed, and then the molten glass is discharged from the reduced pressure atmosphere. For example, Patent Document 1 discloses a glass manufacturing method including a step of vacuum degassing molten glass in a vacuum degassing vessel.

[0005] In order to recycle glass and improve its melting property, cullet from used glass is used as part of glass raw materials. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2008 / 093580 [Non-patent literature]

[0007] [Non-Patent Document 1] Chikao Tanaka, "Environmentally Friendly Glass Melting Technology: Especially on Degassing Technology," NEW GLASS 83 Vol. 21 No. 4 (2006), p. 31-36 Summary of the Invention [Problem to be solved by the invention]

[0008] In recent years, the demand for larger liquid crystal displays has been increasing, and there is a demand for larger glass substrates for use in liquid crystal displays. In order to efficiently manufacture large glass substrates, it may be necessary to increase the dimensions of a vacuum degassing apparatus in order to increase the flow rate of molten glass. In particular, when a molten glass conduit (vacuum degassing tank, riser or downcomer) made of platinum or a platinum alloy is used in the vacuum degassing apparatus, there is a problem in that the investment cost of the equipment is large.

[0009] In addition, even if the average density of bubble defects in a glass substrate is the same, the product yield decreases as the substrate size increases (see the figure in Non-Patent Document 1). Therefore, in order to manufacture larger glass substrates, it is necessary to reduce the bubble density in the substrate more than ever before.

[0010] In order to solve the above problems, an object of the present invention is to provide an alkali-free glass substrate from which bubbles contained in molten glass can be easily removed under a reduced pressure atmosphere. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention provides a SiO 2 : 54-68%, Al 2 O 3 : 10~25%, B 2 O 3 : 0.1 to 5.5%, MgO + CaO + SrO + BaO: 8 to 26%, β-OH is 0.15~0.35mm -1 and the Cl content is 0.15 to 0.3 mass %, The present invention provides an alkali-free glass substrate having a bubble growth index I, represented by the following formula (1), of 320 or more. I=590.5×[β-OH]+874.1×[Cl]-5.7×[B 2 O 3 ]-33.3 (1) In the formula (1), [β-OH] represents the β-OH (mm -1 ), [Cl] represents the Cl content (mass%) of the alkali-free glass substrate, and [B 2 O 3 ] is the B of the non-alkali glass substrate 2 O 3 The content (mass%) is shown. Effect of the Invention

[0012] According to the alkali-free glass substrate of the present invention, bubbles contained in molten glass can be easily removed under a reduced pressure atmosphere. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a cross-sectional view showing one configuration example of a glass manufacturing apparatus used for manufacturing an alkali-free glass substrate of the present invention. [Diagram 2] FIG. 2 is a graph showing the change in bubble diameter over time under reduced pressure conditions in Experimental Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] [Non-alkali glass substrate] Hereinafter, an alkali-free glass substrate according to one embodiment of the present invention will be described. 2 OK 2 The term "glass that is substantially free of alkali metal oxides such as O" refers to glass that is substantially free of alkali metal oxides, such as O. Here, "substantially free of alkali metal oxides" means that the total content of alkali metal oxides is 0.1 mass % or less.

[0015] The alkali-free glass substrate according to the present invention is made of, in terms of mass % based on oxide, SiO 2: 54-68%, Al 2 O 3 : 10~25%, B 2 O 3 : 0.1 to 5.5%, MgO+CaO+SrO+BaO: 8 to 26%. Hereinafter, in this specification, mass % based on oxide will simply be referred to as "%".

[0016] Next, the composition range of each component will be described. SiO 2 When the content is 54% or more, the strain point of the alkali-free glass substrate is improved and the chemical resistance is improved. It is preferably 57% or more, more preferably 58.5% or more, and even more preferably 59.5% or more. SiO 2 When the content is 68% or less, the solubility during glass melting is good. The content is preferably 67.5% or less, more preferably 66% or less, and further preferably 65% ​​or less.

[0017] Al 2 O 3 When the content is 10% or more, phase separation is suppressed and the strain point of the alkali-free glass substrate is improved. It is preferably 16% or more, more preferably 17% or more, and even more preferably 18% or more. Al 2 O 3 When the content is 25% or less, the solubility during glass melting is good. The content is preferably 24% or less, more preferably 23% or less, and further preferably 22% or less.

[0018] B 2 O 3 When the content is 0.1% or more, the solubility during glass melting is good. The content is preferably 0.5% or more, more preferably 1% or more, and even more preferably 1.8% or more. B 2 O 3 When the content is 5.5% or less, the strain point of the alkali-free glass substrate is improved. It is preferably 4.5% or less, more preferably 3.0% or less, and further preferably 2.5% or less.

[0019] When the total amount of MgO, CaO, SrO, and BaO (i.e., MgO+CaO+SrO+BaO) is 8% or more, the solubility during glass melting is good. The total amount of 10% or more is preferable, 12% or more is more preferable, and 14% or more is even more preferable. When MgO+CaO+SrO+BaO is 26% or less, the strain point of the alkali-free glass substrate is improved, preferably 24% or less, more preferably 22% or less, and further preferably 20% or less.

[0020] MgO can be contained in order to improve the solubility of the glass during melting. The content is preferably 2% or more, more preferably 3% or more, further preferably 4% or more, and particularly preferably 4.5% or more. It is preferable that MgO is 12% or less since phase separation is suppressed, more preferably 8.5% or less, further preferably 7.5% or less, and particularly preferably 7% or less.

[0021] CaO can be contained in order to improve the solubility of glass during melting. The content of CaO is preferably 1.5% or more, more preferably 3% or more, and further preferably 4% or more. If CaO is less than 15%, the raw material for CaO, limestone (CaCO 3 It is preferable that the content of phosphorus, which is an impurity in the aluminum alloy, is small. It is more preferable that the content of phosphorus is 8.5% or less, further preferably 8% or less, and particularly preferably 7% or less.

[0022] SrO can be contained in order to improve the solubility of the glass during melting. The content is preferably 0.5% or more, more preferably 2% or more, and further preferably 2.5% or more. If SrO is 16% or less, the acid resistance is good, so this is preferable, 10% or less is more preferable, 8.5% or less is even more preferable, and 8% or less is particularly preferable.

[0023] BaO can be contained to improve solubility. The content is preferably 0.1% or more. BaO of 15% or less is preferable because segregation is less likely to occur when the raw material is melted, 10% or less is more preferable, 8% or less is even more preferable, 4% or less is even more preferable, and 2.5% or less is particularly preferable.

[0024] A first aspect of a suitable composition of the alkali-free glass substrate according to the present invention is SiO 2 :58.5~67.5%, Al 2 O 3 : 18~24%, B 2 O 3 :0.1~1.7%, MgO: 4-8.5%, CaO: 3-8.5%, SrO: 2-10%, BaO: Contains 0-2.5%.

[0025] A second embodiment of a suitable composition of the alkali-free glass substrate according to the present invention is SiO 2 :57~67.5%, Al 2 O 3 : 17~25%, B 2 O 3 : 1.8~5.5%, MgO: 2-8.5%, CaO: 1.5-8%, SrO: 0.5-8.5%, BaO: Contains 0-1%.

[0026] The alkali-free glass substrate of the present invention has a β-OH value of 0.15 to 0.35 mm. -1 β-OH is used as an index of the amount of water in glass. -1 If the thickness is more than 0.18 mm, the moisture in the glass will flow into the bubbles under reduced pressure, making the bubbles easier to grow. -1 More than 0.2mm is preferable. -1 More preferably, 0.22 mm or more -1 The above is even more preferable. β-OH is 0.35mm-1 If the thickness is less than 0.32 mm, the bubbles can be prevented from growing too large, and therefore the bubbles can be prevented from slipping through due to the thickening of the bubble layer during the reduced pressure degassing process. -1 Less than 0.3mm is preferable -1 Less than 0.28mm is more preferable. -1 The following is even more preferred:

[0027] The β-OH can be calculated by measuring the transmittance of an alkali-free glass test piece obtained by forming the molten glass after degassing under reduced pressure into a plate shape using a Fourier transform infrared spectrophotometer (FT-IR) and using the following formula.

[0028] β-OH = (1 / X)log 10 (T 1 / T 2 ) X: Glass thickness (mm) T 1 :Reference wave number 4000cm -1 Transmittance (%) T 2 : Hydroxyl group absorption wave number 3570cm -1 Minimum transmittance (%) near β-OH is governed by the amount of water in the glass raw materials, the steam concentration in the melting tank, the burner combustion method in the melting tank (oxygen combustion, air combustion), etc. In particular, β-OH can be easily adjusted by adjusting the burner combustion method. Specifically, to increase β-OH, the oxygen combustion ratio in the burner combustion is increased, and to decrease β-OH, the air combustion ratio in the burner combustion is increased.

[0029] The alkali-free glass substrate of the present invention has a Cl content of 0.15 to 0.3 mass% relative to the matrix composition of the glass. Cl in glass is a component that facilitates the growth of bubbles contained in molten glass under reduced pressure atmosphere. If the Cl content is less than 0.15 mass%, the growth of bubbles tends to be insufficient. It is preferably 0.18 mass% or more, more preferably 0.2 mass% or more. If the Cl content exceeds 0.3 mass%, a bubble layer tends to be formed due to the enlargement of the bubble layer during reduced pressure degassing treatment. It is preferably 0.28 mass% or less, more preferably 0.25 mass% or less.

[0030] The alkali-free glass substrate of the present invention has a bubble growth index I, represented by the following formula (1), of 320 or more. I=590.5×[β-OH]+874.1×[Cl]-5.7×[B 2 O 3 ]-33.3 (1) In formula (1), [β-OH] is the β-OH (mm -1 ), [Cl] is the Cl content (mass%) of the alkali-free glass substrate, [B 2 O 3 ] is the B of non-alkali glass substrate 2 O 3 The content (mass%) is shown.

[0031] The bubble growth index I is an index of the ease with which bubbles contained in molten glass grow in a reduced pressure atmosphere. If the bubble growth index I is 320 or more, bubbles contained in molten glass tend to grow in a reduced pressure atmosphere. The bubble growth index I is preferably 330 or more, and more preferably 340 or more. A bubble growth index I of 400 or less is preferable because it can prevent reboiling in the molten glass flowing through the vacuum degassing vessel. Here, reboiling refers to a phenomenon in which bubbles are generated at the glass interface in contact with a melting degassing vessel made of platinum or platinum alloy, or dense refractory. The bubble growth index I is more preferably 390 or less, and even more preferably 380 or less.

[0032] As used herein, the bubble growth index I is correlated to a value of pressure P, which is defined as follows: When the vacuum degassing vessel is depressurized under a constant temperature condition, the volume (bubble diameter) of bubbles present in the molten glass in the vessel increases according to Boyle's law. However, when the pressure inside the vessel is reduced to a certain level, the volume (bubble diameter) of bubbles in the molten glass increases rapidly, deviating from Boyle's law. This pressure is called pressure P.

[0033] The pressure P can be calculated by the following procedure. In order to reproduce the conditions in the vacuum degassing tank, a quartz glass crucible containing alkali-free glass cullet is placed in the vacuum degassing vessel. The crucible is heated to 1450°C to melt the alkali-free glass. After the alkali-free glass is completely melted, the pressure in the vacuum degassing vessel is reduced while observing the diameter of bubbles in the molten glass. To observe the diameter of bubbles in the molten glass, for example, the bubbles in the molten glass can be photographed through a viewing window in the vacuum degassing vessel using a CCD camera. The number of bubble samples used to measure the diameter of bubbles is 20 or more. When the pressure inside the vacuum vessel is reduced, the diameter of the bubbles in the molten glass increases according to Boyle's law. However, when the pressure inside the vacuum vessel is reduced to a certain level, the diameter of the bubbles in the molten glass deviates from Boyle's law and increases rapidly. The pressure inside the vacuum vessel at this time is called pressure P. The inventors calculated the bubble growth index I based on the measurement results of the pressure P for several types of alkali-free glass substrates, and found that the bubble growth index I, represented by formula (1), is 320 or more for alkali-free glass substrates from which bubbles contained in molten glass can be easily removed in a reduced pressure atmosphere.

[0034] The alkali-free glass substrate of the present invention contains Fe as a trace component. 2 O 3 It may contain Fe. 2 O 3 is preferably 0.1 mass % or less, and more preferably 0.05 mass % or less.

[0035] The alkali-free glass substrate of the present invention preferably has a strain point of 690° C. or higher, since this can suppress thermal shrinkage during panel production, and more preferably has a strain point of 700° C. or higher. If the strain point is 750°C or lower, it is not necessary to increase the temperature inside the float bath and at the outlet of the float bath very high, and this is preferable because it has little effect on the life of metal members located inside the float bath and downstream of the float bath. A strain point of 740°C or lower is more preferable, and a strain point of 730°C or lower is even more preferable. The strain point was measured by the fiber stretching method according to the method specified in JIS R3103-2 (2001).

[0036] In the alkali-free glass substrate of the present invention, a Young's modulus of 78 GPa or more is preferable since bending of the glass substrate during transportation is suppressed as the glass substrate becomes larger and thinner, more preferably 79 GPa or more, even more preferably 80 GPa or more, and particularly preferably 82 GPa or more. On the other hand, if the Young's modulus is too high, the cuttability of the glass deteriorates, so that the Young's modulus is preferably 95 GPa or less, more preferably 92 GPa or less, and even more preferably 90 GPa or less. The Young's modulus was measured by an ultrasonic method.

[0037] The alkali-free glass substrate of the present invention has a substrate size of preferably 2100mm or more in short side and 2400mm or more in long side, more preferably 2800mm or more in short side and 3000mm or more in long side, and even more preferably 2900mm or more in short side and 3200mm or more in long side. As the substrate size increases, it is necessary to reduce the bubble density in the substrate. However, the alkali-free glass substrate of the present invention has a low bubble density, so that even if the substrate size is large, the product yield is not likely to decrease, and is suitable for a large substrate size. In addition, the substrate size is preferably 6000mm or less in short side and 6500mm or less in long side. If the substrate size is too large, the investment cost of the equipment increases due to the enlargement of the equipment, and it becomes difficult to transport the glass substrate.

[0038] In the alkali-free glass substrate of the present invention, the density of bubbles having a diameter of more than 100 μm is preferably 0.06 bubbles / kg or less, more preferably 0.03 bubbles / kg or less, and even more preferably 0.01 bubbles / kg or less. The density of bubbles with diameters exceeding 100 μm was calculated by counting the number of bubble defects with a size exceeding 100 μm using edge light inspection, in which light is irradiated from the side of a glass substrate in a darkroom to inspect the main surface of the glass substrate.

[0039] The alkali-free glass substrate of the present invention is preferably float glass, since the float method is superior to the fusion method in producing glass substrates having a large substrate size.

[0040] In the alkali-free glass substrate of the present invention, when the alkali-free glass substrate is melted and held at 1450°C, the pressure is reduced from atmospheric pressure to 44 kPa at a constant pressure reduction rate over 20 minutes, and the substrate is held at 44 kPa for 5 minutes, bubbles having a diameter of 0.1 mm to 0.3 mm contained in the molten glass at 1450°C before the start of pressure reduction are defined as initial bubbles, and bubbles corresponding to the initial bubbles after holding at 44 kPa for 5 minutes are defined as grown bubbles, and the diameter of the grown bubbles is preferably three times or more the diameter of the initial bubbles. When the diameter of the grown bubbles is three times or more the diameter of the initial bubbles, the bubbles contained in the molten glass tend to grow and rise in the molten glass in a reduced pressure atmosphere, and therefore the bubbles contained in the molten glass can be easily removed.

[0041] The diameter of the initial bubble and the diameter of the grown bubble can be obtained by the following procedure.

[0042] A quartz cell containing cullets of an alkali-free glass substrate is placed in a vacuum decompression vessel. The quartz cell is heated to 1450°C to melt the cullets, and then the vacuum decompression vessel is decompressed. Bubbles contained in the molten glass are photographed using a CCD camera through a viewing window provided in the vacuum decompression vessel, and the diameter of the bubbles (hereinafter referred to as bubble diameter) is measured by image analysis. It is not necessary to measure the bubble diameter for all bubbles among the multiple bubbles observed in the image. Specifically, bubbles attached to the wall surface of the quartz cell and bubbles formed by the merging of multiple bubbles are excluded from the measurement target. In one embodiment of the present invention, the ratio of the diameter of a grown bubble to the diameter of an initial bubble (hereinafter referred to as bubble growth rate) is calculated by calculating the bubble growth rate for each of the multiple bubbles observed in the image and calculating the average value of the calculated bubble growth rates.

[0043] The decompression conditions for the evaluation were to reduce pressure from atmospheric pressure to 44 kPa at a constant decompression rate over 20 minutes. Reducing the pressure of the molten glass to 44 kPa allows the bubbles contained in the molten glass to grow sufficiently, making it suitable for evaluating the growth of bubble diameter. Furthermore, a decompression time of 20 minutes allows the bubble diameter growth to be appropriately evaluated while shortening the evaluation time.

[0044] Select initial bubbles with a diameter of 0.1mm to 0.3mm. If the diameter of the initial bubbles is 0.1mm or more, it is easy to measure the bubble diameter using a CCD camera. If the diameter of the initial bubbles is 0.3mm or less, it is possible to prevent the growing bubbles from becoming enlarged and breaking, so that the growth of the bubble diameter can be appropriately evaluated.

[0045] The bubbles corresponding to the initial bubbles after holding at 44 kPa for 5 minutes are defined as grown bubbles. If the holding time is 5 minutes, the grown bubbles become sufficiently large compared to the initial bubbles, and the bubbles can be prevented from becoming enlarged and breaking, so the growth of the bubble diameter can be appropriately evaluated.

[0046] In the alkali-free glass substrate of the present invention, the diameter of grown bubbles is more preferably at least 3.5 times, and even more preferably at least 4 times, the diameter of the initial bubbles.

[0047] In the alkali-free glass substrate of the present invention, the diameter of the grown bubbles is preferably 15 times or less than the diameter of the initial bubbles. If it is 15 times or less, the bubble layer that is usually about 10 mm or less and exists on the molten glass surface during the reduced pressure degassing process can be suppressed from enlarging to 10 mm to several hundred mm, and thus the bubbles can be prevented from slipping through. Here, the bubble slipping through is a phenomenon in which bubbles that reach the glass surface do not break and form a bubble layer, thereby remaining stable for a long time, and bubbles that rise later do not break and flow to the subsequent process as they are. If the bubble slipping through occurs, a problem occurs in that bubbles remain in the molten glass after the reduced pressure degassing. In the alkali-free glass substrate of the present invention, the diameter of grown bubbles is more preferably 10 times or less, further preferably 8 times or less, and particularly preferably 6 times or less, the diameter of the initial bubbles.

[0048] The alkali-free glass substrate of the present invention preferably has a thickness of 0.75 mm or less for use as a glass substrate for liquid crystal displays, more preferably 0.55 mm or less, further preferably 0.5 mm or less, particularly preferably 0.45 mm or less. The thickness is preferably 0.1 mm or more, more preferably 0.2 mm or more.

[0049] [Method of manufacturing alkali-free glass substrate] Next, a method for producing an alkali-free glass substrate according to an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a cross-sectional view showing an example of the configuration of a glass production apparatus used for producing the alkali-free glass substrate of the present invention.

[0050] The glass manufacturing apparatus 1 includes a melting tank 10 and a vacuum degassing apparatus 20. A forming apparatus such as a float bath is disposed downstream of the vacuum degassing apparatus 20. The forming apparatus may be a forming apparatus used in a down-draw process.

[0051] The method for manufacturing an alkali-free glass substrate includes melting glass raw materials in a melting tank 10 to produce molten glass G, subjecting the molten glass G to a vacuum degassing treatment in a vacuum degassing apparatus 20, and slowly cooling and cutting a band-shaped glass ribbon formed by a forming apparatus to obtain an alkali-free glass substrate.

[0052] The melting tank 10 is equipped with a burner for melting the supplied glass frit. The burner forms a flame by mixing a fuel such as natural gas or heavy oil with gas and burning it. A burner that mainly uses air as the gas is called an air combustion burner, and a burner that mainly uses oxygen as the gas is called an oxygen combustion burner. The burner heats the glass frit from above by radiating a flame toward the glass frit. The melting tank 10 may also be equipped with electrodes for heating the glass frit.

[0053] The glass raw materials may be, for example, silica sand, boric acid, limestone, aluminum oxide, strontium carbonate, magnesium oxide, etc., and may be mixed to obtain the desired composition of the alkali-free glass substrate. As described above, in the alkali-free glass substrate of the present invention, bubbles contained in molten glass can be easily removed under a reduced pressure atmosphere. Therefore, it is preferable to use, as the glass raw material, cullet generated during the production of an alkali-free glass substrate or cullet of a used alkali-free glass substrate.

[0054] It is preferable that a chloride-based fining agent is added to the glass raw material. The chloride-based fining agent is preferably BaCl 2 2H2 O, SrCl 2 6H 2 O, CaCl 2 , MgCl 2 6H 2 O or NH 4 Cl is preferred.

[0055] The fining agent may be other than the chloride-based fining agent in order to appropriately adjust the bubble growth rate. In this case, the other fining agent may be, for example, SO 3 , F, SnO 2 The content of these other fining agents in the glass raw materials is preferably 2 mass % or less, more preferably 1 mass % or less, and further preferably 0.5 mass % or less.

[0056] The vacuum degassing apparatus 20 includes a vacuum housing 21 , a vacuum degassing vessel 22 , an uprising pipe 23 , a downcomer pipe 24 and a thermal insulator 25 .

[0057] The cylindrical vacuum degassing vessel 22 is housed and arranged in the vacuum housing 21 with its long axis oriented horizontally. A vertically oriented uprising pipe 23 is attached to the underside of one end of the vacuum degassing vessel 22, and a downcomer pipe 24 is attached to the underside of the other end. A portion of the uprising pipe 23 and the downcomer pipe 24 are located within the vacuum housing 21.

[0058] The uprising pipe 23 communicates with the vacuum degassing vessel 22 and introduces the molten glass G from the melting vessel 10 into the vacuum degassing vessel 22. The downcomer pipe 24 communicates with the vacuum degassing vessel 22 and delivers the molten glass G after vacuum degassing to the next treatment vessel. Inside the vacuum housing 21, heat insulating materials 25 such as insulating bricks are arranged around the vacuum degassing vessel 22, the uprising pipe 23 and the downcomer pipe 24 to insulate them.

[0059] The vacuum degassing vessel 22, the uprising pipe 23, and the downcomer pipe 24 are conduits for the molten glass, and are therefore made of materials that are excellent in heat resistance and corrosion resistance to the molten glass. For example, they are made of platinum, a platinum alloy, or reinforced platinum obtained by dispersing a metal oxide in platinum or a platinum alloy. They may also be made of a ceramic-based nonmetallic inorganic material, that is, a dense refractory material. They may also be dense refractory material lined with platinum or a platinum alloy.

[0060] The vacuum degassing is performed by passing the molten glass G supplied from the melting tank 10 through a vacuum degassing tank 22 whose pressure has been reduced to a predetermined level. The molten glass G is preferably continuously supplied to and discharged from the vacuum degassing tank 22. From the viewpoint of productivity, the flow rate of the molten glass is preferably 1 to 200 tons / day.

[0061] In order to prevent a temperature difference from occurring with the molten glass G supplied from the melting tank 10, the vacuum degassing tank 22 is preferably heated so that its inside is in the temperature range of 1200°C to 1600°C, particularly 1350°C to 1550°C.

[0062] When performing vacuum degassing, the air in the vacuum housing 21 is exhausted from the outside by a vacuum degassing means such as a vacuum pump through a suction opening provided at a predetermined position of the vacuum housing 21. This indirectly exhausts the air in the vacuum degassing tank 22 contained in the vacuum housing 21, and the inside of the vacuum degassing tank 22 is depressurized to a predetermined pressure.

[0063] The pressure inside the reduced pressure degassing vessel 22 is preferably 15 to 55 kPa. EXAMPLES

[0064] The present invention will be further described below using examples and comparative examples. Note that the present invention is not limited to these descriptions. Note that Examples 1, 4 to 7, 9, 10, 12, 13, 15, 16, and 18 are examples, and Examples 2, 3, 8, 11, 14, and 17 are comparative examples.

[0065] [Experimental Example 1] Using the glass manufacturing apparatus 1 shown in FIG. 1 , glass raw materials having an alkali-free glass composition were melted in a melting tank 10 to produce molten glass G, the molten glass G was subjected to a vacuum degassing treatment in a vacuum degassing apparatus 20, the molten glass was formed into a band-shaped glass ribbon by a float process, and the glass ribbon was slowly cooled and cut to prepare alkali-free glass substrates (Example 1 and Example 2) having a thickness of 0.50 mm.

[0066] The glass composition of Example 1 is expressed as mass% on an oxide basis: SiO 2 : 61.2%, Al 2 O 3 :20.0%, B 2 O 3 : 2.0%, MgO: 5.3%, CaO: 4.5%, SrO: 7.0%, BaO: 0.1% (MgO + CaO + SrO + BaO = 16.9%), and the Cl content in the glass matrix composition was 0.234 mass%. In addition, β-OH measured by the following method was 0.329 mm -1 It was. (β-OH) β-OH is measured by measuring the absorbance of a glass sample at wavelengths of 2.75 μm to 2.95 μm, and the maximum absorbance value β max was calculated by dividing by the thickness (mm) of the sample. Therefore, the bubble growth index I = 590.5 x 0.329 + 874.1 x 0.234 - 5.7 x 2.0 - 33.3 = 354.

[0067] The glass composition of Example 2 is expressed as mass% on an oxide basis: SiO 2 : 62%, Al 2 O 3 :18%, B 2 O 3 :9.5%, MgO:2%, CaO:7%, SrO:1.2%, BaO:0.1% (MgO+CaO+SrO+BaO=10.3%), SnO 2 The content of Cl in the glass matrix composition was 0 mass%. The β-OH measured by the above method was 0.47 mm -1 It was. Therefore, the bubble growth index I = 590.5 x 0.47 + 874.1 x 0 - 5.7 x 9.5 - 33.3 = 190.

[0068] In order to reproduce the atmosphere in which degassing was performed, a quartz cell containing 50 g of cullet of non-alkali glass substrate was placed in a vacuum degassing vessel. The vacuum degassing vessel was a Glass Service HTO (High Temperature Observation) Furnace. The quartz cell was heated from room temperature to 1450°C to melt the cullet, and then depressurization of the vacuum degassing vessel was started. While maintaining the vacuum degassing vessel at 1450°C, the pressure was reduced from atmospheric pressure to 44 kPa at a constant depressurization rate in 20 minutes, and the vessel was maintained at 44 kPa for 10 minutes. During this period, the start of depressurization was set as 0 minutes, and bubbles in the molten glass were photographed using a CCD camera through a viewing window provided in the vacuum degassing vessel at 0, 3, 6, 9, 12, 15, 18, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, and 30 minutes, and the diameter of the bubbles was measured by image analysis. In Experimental Example 1, the initial bubbles were formed at 0 minutes, and the bubbles were grown at 25 minutes from the start of depressurization.

[0069] FIG. 2 is a diagram showing the change in bubble diameter over time under reduced pressure conditions in Experimental Example 1. Comparing Example 1 and Example 2, it can be seen that the bubble diameter in Example 1 grows larger while the pressure is maintained at 44 kPa. In Example 1, the initial bubble diameter was 0.24 mm, and the grown bubble diameter was 0.97 mm, which was 4.0 times the initial bubble diameter. On the other hand, in Example 2, the initial bubble diameter was 0.25 mm, and the grown bubble diameter was 0.47 mm, which was 1.9 times the initial bubble diameter. It was found that the alkali-free glass substrate of Example 1 can easily remove bubbles contained in molten glass under reduced pressure atmosphere. In Experimental Example 1 and Experimental Example 2 described below, the temperatures of the molten glass are different, 1450°C and 1400°C, and the pressures in the vacuum decompression vessel (decompression degassing vessel) are different, 44 kPa and 33.33 kPa. This is because, when Experimental Example 1 is performed at a pressure of 33.33 kPa, bubbles in the molten glass expand too much, making it difficult to photograph them using a CCD camera.

[0070] [Experimental Example 2] The clarification effect of molten glass in a vacuum degassing vessel was evaluated by simulation. The speed at which bubbles grow in the molten glass flow and rise up in the molten glass is related to the bubble diameter by Stokes' equation. In the simulation, the bubble diameter was calculated based on the bubble growth rate, and the behavior of the bubble floating up was analyzed based on Stokes' equation. Note that the bubbles were assumed to be generated in the center of the position where the riser tube was attached to the vacuum degassing vessel, and the calculation was performed with the initial bubble diameter set to 0.2 mm.

[0071] The dimensions of the vacuum degassing vessel and the height of the molten glass surface are as follows: Horizontal length from the center of the ascending pipe to the center of the descending pipe in the vacuum degassing tank: 10m Inner diameter of vacuum degassing tank: 500mm Molten glass level: 250mm The molten glass passing through the vacuum degassing vessel was assumed as follows. The glass composition is as follows. For the Cl content, see Table 1 below. (Glass matrix composition is expressed as mass% based on oxides, Cl content is mass% based on glass matrix composition) Examples 3 to 14 SiO 2 : 61.2%, Al 2 O 3 :20.0%, B 2 O 3 :2.0%, MgO:5.3%, CaO:4.5%, SrO:7.0%, BaO:0.1% (MgO+CaO+SrO+BaO=16.9%) Example 15 SiO 2 : 61.6%, Al 2 O 3 :20.9%, B 2 O 3 :0.1%, MgO:6.1%, CaO:4.6%, SrO:6.8%, BaO:0.1% (MgO+CaO+SrO+BaO=17.6%) Example 16 SiO 2 : 61.2%, Al 2 O 3 :20.1%, B 2 O 3:1.8%, MgO:5.4%, CaO:4.5%, SrO:7.0%, BaO:0.1% (MgO+CaO+SrO+BaO=17.0%) Example 17 SiO 2 : 60.9%, Al 2 O 3 :19.5%, B 2 O 3 :3.5%, MgO:4.9%, CaO:4.4%, SrO:7.1%, BaO:0.1% (MgO+CaO+SrO+BaO=16.6%) Example 18 SiO 2 : 60.7%, Al 2 O 3 :19.1%, B 2 O 3 :5.5%, MgO:4.6%, CaO:4.4%, SrO:7.3%, BaO:0.1% (MgO+CaO+SrO+BaO=16.3%) Molten glass flow rate: 0.6m 3 / h or 1.5m 3 / h Pressure in the vacuum degassing tank: 33.33kPa Temperature when passing through the vacuum degassing tank (average): 1400℃ Viscosity when passing through vacuum degassing tank: 150-200 Pa s Density after passing through the vacuum degassing tank: 2380kg / m 3 The degassing performance was evaluated based on the distance (floating distance) from the center of the riser pipe in the vacuum degassing vessel to the position where the bubbles floated to the surface of the molten glass. The smaller the floating distance, the better the degassing performance.

[0072] The results are shown in Table 1. In Table 1, the flow rate of the molten glass, B 2 O 3 , β-OH, Cl content, bubble growth index I, bubble growth rate and floating distance are shown. The bubble growth rate was measured under the same conditions as in Experimental Example 1.

[0073] [Table 1]

[0074] As shown in Table 1, the glass (Examples 3, 8, 11, 14, and 17) with a bubble growth index I of less than 320 had a floating distance of more than 10 m. From this result, it is estimated that the bubble density of the glass substrate with a bubble diameter of more than 100 μm will exceed 0.06 bubbles / kg.

[0075] In contrast, the glass with a bubble growth index I of 320 or more (Examples 4 to 7, 9, 10, 12, 13, 15, 16, and 18) had a floating distance of 10 m or less. From this result, it is estimated that the bubble density of the glass substrate with a bubble diameter of more than 100 μm will be 0.06 bubbles / kg or less.

[0076] From the above results, the glass of the embodiment does not need to enlarge the dimensions of the vacuum degassing apparatus in order to efficiently manufacture large glass substrates, and can solve the problem of large investment costs for equipment. In addition, since the bubble density in the substrate can be reduced, it is expected that the problem of the decrease in product yield as the substrate size increases will be solved.

[0077] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. [Industrial Applicability]

[0078] The alkali-free glass substrate can be used for liquid crystal displays, organic EL displays, flat panel displays, and various other applications. [Explanation of symbols]

[0079] 1. Glass manufacturing equipment 10 Dissolution tank 20. Vacuum degassing device 21 Pressure reducing housing 22 Decompression degassing tank 23 Ascending pipe 24 Downcomer 25 Insulation

Claims

1. Contains BaO, and is expressed as mass% on an oxide basis: SiO 2 :54-68%, Al 2 O 3 : 10-25%, B 2 O 3 : 1 to 1.7%, MgO + CaO + SrO + BaO: 8 to 26%, β-OH is 0.15 to 0.35 mm -1 and the Cl content is 0.15 to 0.3 mass %; An alkali-free glass substrate having a bubble growth index I, represented by the following formula (1), of 320 or more: I=590.5×[β-OH]+874.1×[Cl]-5.7×[B 2 O 3 ]-33.3(1) In the formula (1), [β-OH] represents the β-OH (mm -1 ), [Cl] represents the Cl content (mass%) of the alkali-free glass substrate, and [B 2 O 3 ] is B of the non-alkali glass substrate 2 O 3 The content (mass %) is shown.

2. 2. The alkali-free glass substrate according to claim 1, wherein the bubble growth index I represented by the formula (1) is 400 or less.

3. 3. The alkali-free glass substrate according to claim 1, having a strain point of 690 to 750°C.

4. The alkali-free glass substrate according to any one of claims 1 to 3, containing, in mass% on an oxide basis, MgO: 0 to 12%, CaO: 0 to 15%, SrO: 0 to 16%, and BaO: 15% or less.

5. In terms of mass% based on oxide, SiO 2 :58.5~67.5%, Al 2 O 3 : 18-24%, B 2 O 3 5. The alkali-free glass substrate according to claim 1, further comprising: 1 to 1.7% of CaO, 4 to 8.5% of MgO, 3 to 8.5% of CaO, 2 to 10% of SrO, and 2.5% or less of BaO.

6. In terms of mass% based on oxide, SiO 2 :57-67.5%, Al 2 O 3 : 17-25%, B 2 O 3 5. The alkali-free glass substrate according to claim 1, further comprising: 1 to 1.7% of CaO, 2 to 8.5% of MgO, 1.5 to 8% of CaO, 0.5 to 8.5% of SrO, and 1% or less of BaO.

7. The alkali-free glass substrate according to any one of claims 1 to 6, having a Young's modulus of 78 GPa or more.

8. The alkali-free glass substrate according to any one of claims 1 to 7, having a thickness of 0.1 mm to 0.5 mm.

9. The alkali-free glass substrate according to any one of claims 1 to 8, wherein the substrate has a short side of 2100 mm or more and a long side of 2400 mm or more.

10. The alkali-free glass substrate according to any one of claims 1 to 9, wherein the substrate has a short side of 2900 mm or more and a long side of 3200 mm or more.

11. The alkali-free glass substrate according to any one of claims 1 to 10, wherein the density of bubbles having a diameter of more than 100 µm is 0.06 bubbles / kg or less.

12. The alkali-free glass substrate was melted and held at 1450° C. while being depressurized from atmospheric pressure to 44 kPa at a constant depressurization rate over 20 minutes, and held at 44 kPa for 5 minutes. Bubbles having a diameter of 0.1 mm to 0.3 mm contained in the molten glass at 1450° C. before the start of decompression are defined as initial bubbles.

12. The alkali-free glass substrate according to claim 1, wherein a bubble corresponding to the initial bubble after holding at 44 kPa for 5 minutes is defined as a grown bubble, and a diameter of the grown bubble is three times or more the diameter of the initial bubble.

13. The alkali-free glass substrate according to claim 12 , wherein a diameter of the grown bubble is 15 times or less than a diameter of the initial bubble.

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