Alkali-free glass substrate

Abstract

To provide an alkali-free glass substrate that enables easy removal of bubbles included in molten glass under a reduced-pressure atmosphere.SOLUTION: An alkali-free glass substrate contains, as represented by mass% based on oxides: SiO2: 54-68%, Al2O3: 10-25%, B2O3: 1.8-5.5%, and MgO+CaO+SrO+BaO: 8-26%. The alkali-free glass substrate has β-OH of 0.15 to 0.32 mm-1, and a Cl content of 0.15 to 0.25 mass%. A bubble growth index I of the alkali-free glass substrate given by the following formula is 320 or more: I=590.5×[β-OH]+874.1×[Cl]-5.7×[B2O3]-33.3. In the formula, [β-OH] is β-OH (mm-1) of the alkali-free glass substrate, [Cl] is the Cl content (mass%) of the alkali-free glass substrate, and [B2O3] is a 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 Art

[0002] For glass substrates for various displays, particularly those having a metal or oxide thin film or the like formed on the surface, if they contain alkali metal oxides, alkali metal ions diffuse into the thin film and deteriorate the film characteristics. Therefore, it is preferable to use an alkali-free glass substrate substantially free of alkali metal ions.

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

[0004] In the clarification of molten glass, a vacuum defoaming method is known in which the molten glass is introduced into a vacuum atmosphere, and bubbles in the continuously flowing molten glass stream are grown large under this vacuum atmosphere to cause the bubbles contained in the molten glass to float and burst and be removed, and then discharged from the vacuum atmosphere. For example, Patent Document 1 discloses a glass manufacturing method including a step of vacuum defoaming molten glass in a vacuum defoaming tank.

[0005] For the purpose of glass recycling and improved solubility, cullets of used glass are used as part of the glass raw material.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Non-Patent Documents

[0007]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] By the way, in recent years, the demand for larger liquid crystal displays has increased, and larger glass substrates used in liquid crystal displays are desired. In order to efficiently manufacture large glass substrates, it may be necessary to increase the size of the vacuum defoaming device in order to increase the flow rate of the molten glass. In particular, when a molten glass conduit (vacuum defoaming tank, riser or downcomer) made of platinum or platinum alloy is used in the vacuum defoaming device, there is a problem that the investment cost of the equipment increases.

[0009] Also, even if the average density of bubble defects in the 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 increase the size of the glass substrate to be manufactured, it is necessary to reduce the bubble density in the substrate more than ever.

[0010] In order to solve the above problems, an object of the present invention is to provide an alkali-free glass substrate capable of easily removing bubbles contained in molten glass under a reduced pressure atmosphere.

Means for Solving the Problems

[0011] In order to achieve the above object, the present invention provides an alkali-free glass substrate containing, in terms of mass% based on oxides, SiO2: 54 to 68%, Al2O3: 10 to 25%, B2O3: 0.1 to 5.5%, 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%, and provides an alkali-free glass substrate characterized in that the bubble growth index I represented by the following formula (1) is 320 or more. 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.

Effect of the Invention

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

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0014] [Alkali-Free Glass Substrate] Hereinafter, the alkali-free glass substrate in one embodiment of the present invention will be described. The alkali-free glass refers to a glass that substantially does not contain alkali metal oxides such as Na2O and K2O. Here, substantially not containing alkali metal oxides means that the total content of alkali metal oxides is 0.1% by mass or less.

[0015] The alkali-free glass substrate according to the present invention contains, in terms of mass% based on oxides, SiO2: 54 to 68%, Al2O3: 10 to 25%, B2O3: 0.1 to 5.5%, and MgO + CaO + SrO + BaO: 8 to 26%. Hereinafter, in this specification, the mass% based on oxides will be simply described as “%”.

[0016] Next, the composition ranges of each component will be described. When the SiO2 content is 54% or more, the strain point of the alkali-free glass substrate is improved, and the chemical resistance is good. A content of 57% or more is preferred, 58.5% or more is more preferred, and 59.5% or more is even more preferred. When the SiO2 content is 68% or less, the solubility during glass melting is good. A content of 67.5% or less is preferred, 66% or less is more preferred, and 65% or less is even more preferred.

[0017] When the Al2O3 content is 10% or more, phase separation is suppressed and the strain point of the alkali-free glass substrate is improved. A content of 16% or more is preferred, 17% or more is more preferred, and 18% or more is even more preferred. When the Al2O3 content is 25% or less, the solubility during glass melting is good. A content of 24% or less is preferred, 23% or less is more preferred, and 22% or less is even more preferred.

[0018] When the B2O3 content is 0.1% or more, the solubility during glass melting is good. A content of 0.5% or more is preferred, 1% or more is more preferred, and 1.8% or more is even more preferred. When the B2O3 content is 5.5% or less, the strain point of the alkali-free glass substrate is improved. A content of 4.5% or less is preferred, 3.0% or less is more preferred, and 2.5% or less is even more preferred.

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

[0020] MgO can be contained to improve the solubility during glass melting. The content is preferably 2% or more, more preferably 3% or more, even more preferably 4% or more, and particularly preferably 4.5% or more. When the MgO content is 12% or less, phase separation is suppressed, which is preferable. More preferably, it is 8.5% or less, still more preferably 7.5% or less, and particularly preferably 7% or less.

[0021] CaO can be contained to improve the solubility during glass melting. The content is preferably 1.5% or more, more preferably 3% or more, and still more preferably 4% or more. When the CaO content is 15% or less, it is preferable because the incorporation of phosphorus, which is an impurity in limestone (CaCO3) as a CaO raw material, is small. More preferably, it is 8.5% or less, still more preferably 8% or less, and particularly preferably 7% or less.

[0022] SrO can be contained to improve the solubility during glass melting. The content is preferably 0.5% or more, more preferably 2% or more, and still more preferably 2.5% or more. When the SrO content is 16% or less, it is preferable because the acid resistance is good. More preferably, it is 10% or less, still more preferably 8.5% or less, and particularly preferably 8% or less.

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

[0024] The first aspect of the preferred composition of the non-alkali glass substrate according to the present invention is SiO2: 58.5 - 67.5%, Al2O3: 18 - 24%, B2O3: 0.1 - 1.7%, MgO: 4 - 8.5%, CaO: 3 - 8.5%, SrO: 2 - 10%, BaO: 0 - 2.5% is contained.

[0025] The second aspect of the preferred composition of the non-alkali glass substrate according to the present invention is SiO2: 57 - 67.5%, Al2O3: 17 - 25%, B2O3: 1.8 - 5.5%, MgO: 2 - 8.5%, CaO: 1.5 - 8%, SrO: 0.5 - 8.5%, Contains BaO: 0 - 1%.

[0026] The alkali - free glass substrate of the present invention has β - OH of 0.15 - 0.35 mm -1 It is. β - OH is used as an index of the moisture content in the glass. If β - OH is 0.15 mm -1 or more, the moisture in the glass flows into the bubbles under a reduced - pressure atmosphere, and the bubbles tend to grow. 0.18 mm -1 or more is preferable, 0.2 mm -1 or more is more preferable, 0.22 mm -1 or more is even more preferable. If β - OH is 0.35 mm -1 or less, the excessive growth of the bubbles can be suppressed, so that during the reduced - pressure defoaming process, the penetration of bubbles due to the hypertrophy of the bubble layer can be suppressed. 0.32 mm -1 or less is preferable, 0.3 mm -1 or less is more preferable, 0.28 mm -1 or less is even more preferable.

[0027] β - OH can be obtained by measuring the transmittance of a plate - shaped alkali - free glass test piece formed from the molten glass after reduced - pressure defoaming using a Fourier transform infrared spectrophotometer (FT - IR) and using the following formula.

[0028] β - OH = (1 / X)log 10 (T1 / T2) X: Glass plate thickness (mm) T1: Transmittance (%) at the reference wave number 4000 cm -1 T2: Minimum transmittance (%) near the hydroxyl absorption wave number 3570 cm -1 ​​β-OH is governed by the moisture content in the glass raw materials, the water vapor concentration in the melting tank, the burner combustion method (oxygen combustion, air combustion) in the melting tank, etc. In particular, β-OH can be easily adjusted by adjusting the burner combustion method. Specifically, to increase β-OH, increase the oxygen combustion ratio of the burner combustion, and to decrease β-OH, increase the air combustion ratio of the burner combustion.

[0029] The alkali-free glass substrate of the present invention has a Cl content of 0.15 to 0.3% by mass with respect to the base composition of the glass. Cl in the glass is a component that facilitates the growth of bubbles contained in the molten glass under a reduced-pressure atmosphere. If the Cl content is less than 0.15% by mass, the growth of bubbles tends to be insufficient. Preferably it is 0.18% by mass or more, more preferably 0.2% by mass or more. If the Cl content exceeds 0.3% by mass, during the reduced-pressure defoaming treatment, due to the enlargement of the bubble layer, the bubble layer is likely to be formed. Preferably it is 0.28% by mass or less, more preferably 0.25% by 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×[B2O3] - 33.3 (1) In formula (1), [β-OH] represents β-OH (mm -1 ) of the alkali-free glass substrate, [Cl] represents the Cl content (% by mass) of the alkali-free glass substrate, and [B2O3] represents the B2O3 content (% by mass) of the alkali-free glass substrate.

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

[0032] In this specification, the bubble growth index I is correlated with the value of the pressure P defined as follows. Under the condition of constant temperature, when the vacuum degassing tank is depressurized, the volume (bubble diameter) of the bubbles present in the molten glass in the vacuum degassing tank increases according to Boyle's law. However, when the vacuum degassing tank is depressurized to a certain pressure, the volume (bubble diameter) of the bubbles in the molten glass deviates from Boyle's law and increases rapidly. This pressure is defined as the pressure P.

[0033] The pressure P can be obtained by the following procedure. In order to reproduce the situation in the vacuum degassing tank, a quartz glass crucible containing a cullet of non-alkali glass is placed in a vacuum pressure reduction vessel. The crucible is heated to 1450 °C to melt the non-alkali glass. After the non-alkali glass is completely melted, while reducing the pressure in the vacuum pressure reduction vessel, the diameter of the bubbles in the molten glass is observed. To observe the diameter of the bubbles in the molten glass, for example, the bubbles in the molten glass may be photographed using a CCD camera through a viewing window provided in the vacuum pressure reduction vessel. The number of bubble samples for measuring the bubble diameter is 20 or more. When the pressure in the vacuum pressure reduction vessel is decreased, the diameter of the bubbles in the molten glass increases according to Boyle's law. However, when the vacuum pressure reduction vessel is depressurized to a certain pressure, the diameter of the bubbles in the molten glass deviates from Boyle's law and increases rapidly. The pressure in the vacuum pressure reduction vessel at this time is defined as the pressure P. Based on the measurement results of the pressure P in a plurality of types of non-alkali glass substrates, the present inventors obtained the bubble growth index I. As a result, for non-alkali glass substrates that can easily remove the bubbles contained in the molten glass under a reduced-pressure atmosphere, it was found that the bubble growth index I represented by the formula (1) is 320 or more.

[0034] The alkali-free glass substrate of the present invention may contain Fe2O3 as a trace component. Fe2O3 is preferably 0.1% by mass or less, and more preferably 0.05% by mass or less.

[0035] When the strain point of the alkali-free glass substrate of the present invention is 690 ° C or higher, thermal shrinkage during panel manufacturing can be suppressed, which is preferable, and 700 ° C or higher is more preferable. When the strain point is 750 ° C or lower, it is not necessary to raise the temperature in the float bath and at the float bath outlet so much, and it is less likely to affect the life of the metal members located in the float bath and on the downstream side of the float bath, which is preferable. 740 ° C or lower is more preferable, and 730 ° C or lower is even more preferable. The strain point was measured by the fiber elongation method according to the method specified in JIS R3103-2 (2001).

[0036] When the Young's modulus of the alkali-free glass substrate of the present invention is 78 GPa or more, the deflection of the glass substrate during conveyance is suppressed as the glass substrate is made larger and thinner, which is preferable. 79 GPa or more is more preferable, 80 GPa or more is even more preferable, and 82 GPa or more is particularly preferable. On the other hand, if the Young's modulus is too high, the cuttability of the glass deteriorates, so 95 GPa or less is preferable, 92 GPa or less is more preferable, and 90 GPa or less is even more preferable. The Young's modulus was measured by the ultrasonic method.

[0037] The alkali-free glass substrate of the present invention preferably has a substrate size of 2100 mm or more on the short side and 2400 mm or more on the long side, more preferably 2800 mm or more on the short side and 3000 mm or more on the long side, and even more preferably 2900 mm or more on the short side and 3200 mm or more on the long side. As the substrate size increases, it is necessary to reduce the bubble density in the substrate. However, since the alkali-free glass substrate of the present invention has a low bubble density, even when the substrate size is large, the product yield is less likely to decrease, and it is suitable when the substrate size is large. Further, the substrate size is preferably 6000 mm or less on the short side and 6500 mm or less on the long side. This is because if the substrate size is too large, the investment cost of the equipment will increase due to the enlargement of the equipment, and it will be difficult to transport the glass substrate.

[0038] The alkali-free glass substrate of the present invention preferably has a bubble density of more than 100 μm in bubble diameter of 0.06 pieces / kg or less, more preferably 0.03 pieces / kg or less, and even more preferably 0.01 pieces / kg or less. Note that the bubble density of more than 100 μm in bubble diameter was calculated by examining the number of bubble defects with a size of more than 100 μm by edge light inspection in which light is irradiated from the side surface of the glass substrate in a dark room and the main surface of the glass substrate is inspected for the glass substrate.

[0039] The alkali-free glass substrate of the present invention is preferably float glass. The float method is excellent in picking up a glass substrate with a large substrate size as compared with the fusion method.

[0040] The alkali-free glass substrate of the present invention melts the alkali-free glass substrate, and while holding at 1450 °C, reduces the pressure at a constant rate from atmospheric pressure to 44 kPa in 20 minutes, and holds at 44 kPa for 5 minutes. The bubbles having a diameter of 0.1 mm to 0.3 mm contained in the molten glass at 1450 °C before starting the pressure reduction are used as initial bubbles, and the bubbles corresponding to the initial bubbles after holding at 44 kPa for 5 minutes are used as growth bubbles. It is preferable that the diameter of the growth bubbles is 3 times or more the diameter of the initial bubbles. When the diameter of the growth bubbles is 3 times or more the diameter of the initial bubbles, the bubbles contained in the molten glass are likely to grow and float in the molten glass under a reduced pressure atmosphere. Therefore, the bubbles contained in the molten glass can be easily removed.

[0041] The diameters of the initial bubbles and the growing bubbles can be determined by the following procedure.

[0042] A quartz cell containing a cullet of a non-alkali glass substrate is placed in a vacuum decompression vessel. The quartz cell is heated to 1450 °C to melt the cullet, and then the inside of the vacuum decompression vessel is decompressed. The 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 the bubble diameter) is measured by image analysis. Note that it is not necessary to measure the bubble diameter for all of the plurality of bubbles observed in the image. Specifically, bubbles adhering to the wall surface of the quartz cell, bubbles formed by the coalescence of a plurality of bubbles, etc. are excluded from the measurement targets. The ratio of the diameter of the growing bubbles to the diameter of the initial bubbles (hereinafter referred to as the bubble growth rate) in one embodiment of the present invention is obtained by calculating the bubble growth rate for each of the plurality of bubbles observed in the image and finding their average value.

[0043] For the evaluation, the pressure is reduced from atmospheric pressure to 44 kPa at a constant decompression rate in 20 minutes. When the molten glass is decompressed to 44 kPa, the bubbles contained in the molten glass can be sufficiently grown, so it is suitable for evaluating the growth of the bubble diameter. Also, when the decompression time is 20 minutes, the growth of the bubble diameter can be appropriately evaluated while shortening the evaluation time.

[0044] As the initial bubbles, those with a bubble diameter of 0.1 mm to 0.3 mm are selected. When the diameter of the initial bubbles is 0.1 mm or more, it becomes easy to measure the bubble diameter with a CCD camera. When the diameter of the initial bubbles is 0.3 mm or less, it is possible to prevent the growing bubbles from growing too large and bursting, so 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 the growing bubbles. When the holding time is 5 minutes, the grown bubbles become sufficiently large compared to the initial bubbles, and it is possible to prevent the bubbles from growing too large and bursting, so the growth of the bubble diameter can be appropriately evaluated.

[0046] For the alkali-free glass substrate of the present invention, it is more preferable that the diameter of the growing bubbles is 3.5 times or more, and more preferably 4 times or more, the diameter of the initial bubbles.

[0047] For the alkali-free glass substrate of the present invention, it is preferable that the diameter of the growing bubbles is 15 times or less the diameter of the initial bubbles. When it is 15 times or less, during the vacuum defoaming process, the bubble layer that usually exists on the surface of the molten glass at about 10 mm or less can be suppressed from growing to 10 mm to several hundred mm, so that the escape of bubbles can be prevented. Here, the escape of bubbles refers to the phenomenon that the bubbles reaching the glass surface do not burst but form a bubble layer and exist stably for a long time, and the bubbles rising later do not burst and flow out directly to the subsequent process. When the escape of bubbles occurs, there will be a problem that bubbles remain in the molten glass after vacuum defoaming. For the alkali-free glass substrate of the present invention, it is more preferable that the diameter of the growing bubbles is 10 times or less the diameter of the initial bubbles, more preferably 8 times or less, and particularly preferably 6 times or less.

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

[0049] [Method for manufacturing an alkali-free glass substrate] Next, a method for manufacturing 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 a configuration example of a glass manufacturing apparatus used for manufacturing the alkali-free glass substrate of the present invention.

[0050] The glass manufacturing apparatus 1 includes a melting tank 10 and a vacuum defoaming apparatus 20. A forming apparatus such as a float bath is arranged subsequent to the vacuum defoaming apparatus 20. The forming apparatus may be a forming apparatus used in the down-draw method.

[0051] The manufacturing method of an alkali-free glass substrate involves producing molten glass G by melting glass raw materials in a melting tank 10, subjecting the molten glass G to a vacuum degassing treatment in a vacuum degassing apparatus 20, gradually cooling and cutting a strip-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 raw materials. The burner forms a flame by mixing and burning a fuel such as natural gas or heavy oil with gas. 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 raw materials from above by radiating the flame toward the glass raw materials. Further, the melting tank 10 may be equipped with electrodes for heating the glass raw materials.

[0053] For example, silica sand, boric acid, limestone, aluminum oxide, strontium carbonate, magnesium oxide, etc. can be used as the glass raw materials, and they can be formulated to have the composition of the target alkali-free glass substrate. As described above, since the alkali-free glass substrate of the present invention can easily remove the bubbles contained in the molten glass under a reduced pressure atmosphere, it is preferable to use cullets generated during the production of the alkali-free glass substrate or cullets of used alkali-free glass substrates as the glass raw materials.

[0054] It is preferable to add a chloride-based fining agent to the glass raw materials. From the viewpoint of no concern about deliquescence, BaCl2·2H2O, SrCl2·6H2O, CaCl2, MgCl2·6H2O or NH4Cl is preferable as the chloride-based fining agent.

[0055] For adjusting the bubble growth rate appropriately, other than chloride-based fining agents, other fining agents may be used. In this case, examples of other fining agents include SO3, F, SnO2, etc. The content of these other fining agents in the glass raw materials is preferably 2% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less.

[0056] The vacuum degassing device 20 includes a vacuum housing 21, a vacuum degassing tank 22, a riser pipe 23, a downcomer pipe 24, and a heat insulating material 25.

[0057] The cylindrical vacuum degassing tank 22 is housed and arranged in the vacuum housing 21 such that its major axis is oriented in the horizontal direction. A riser pipe 23 oriented vertically is attached to the lower surface of one end of the vacuum degassing tank 22, and a downcomer pipe 24 is attached to the lower surface of the other end. A part of the riser pipe 23 and the downcomer pipe 24 is located inside the vacuum housing 21.

[0058] The riser pipe 23 communicates with the vacuum degassing tank 22 and introduces the molten glass G from the melting tank 10 into the vacuum degassing tank 22. The downcomer pipe 24 communicates with the vacuum degassing tank 22 and discharges the molten glass G after vacuum degassing to the next processing tank. Inside the vacuum housing 21, a heat insulating material 25 such as heat insulating bricks for heat insulatingly covering the vacuum degassing tank 22, the riser pipe 23, and the downcomer pipe 24 is disposed around them.

[0059] Since the vacuum degassing tank 22, the riser pipe 23, and the downcomer pipe 24 are conduits for molten glass, they are made of a material excellent in heat resistance and corrosion resistance to molten glass. For example, they are made of platinum, platinum alloy, or reinforced platinum obtained by dispersing a metal oxide in platinum or platinum alloy. Also, they may be made of a ceramic-based non-metallic inorganic material, that is, a dense refractory. Further, they may be a structure in which a dense refractory is lined with platinum or platinum alloy.

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

[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 internal temperature is in the 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 by vacuum degassing means such as a vacuum pump from the outside through a suction opening provided at a predetermined position of the vacuum housing 21. Thereby, the air in the vacuum degassing tank 22 accommodated in the vacuum housing 21 is indirectly exhausted, and the inside of the vacuum degassing tank 22 is decompressed to a predetermined pressure.

[0063] The pressure inside the vacuum degassing tank 22 is preferably 15 to 55 kPa.

Example

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

[0065] [Experimental Example 1] Using the glass manufacturing apparatus 1 shown in FIG. 1, a molten glass G was produced by melting a glass raw material having a non-alkali glass composition in a melting tank 10, the molten glass G was subjected to a vacuum degassing treatment in a vacuum degassing apparatus 20, and the molten glass was formed into a strip-shaped glass ribbon by the float method. The glass ribbon was gradually cooled and cut to prepare a non-alkali glass substrate (Example 1, Example 2) having a plate thickness of 0.50 mm.

[0066] The glass composition of Example 1 was, in terms of mass% based on oxides, SiO2: 61.2%, Al2O3: 20.0%, B2O3: 2.0%, MgO: 5.3%, CaO: 4.5%, SrO: 7.0%, BaO: 0.1% (MgO + CaO + SrO + BaO = 16.9%), and the Cl content relative to the mother composition of the glass was 0.234 mass%. Also, β-OH measured by the following method was 0.329 mm -1 It was. (β-OH) β-OH measures the absorbance of the glass sample with respect to light having a wavelength of 2.75 μm to 2.95 μm, and the maximum value β of the absorbance maxIt was determined by dividing by the thickness (mm) of the sample. Therefore, the bubble growth index I = 590.5×0.329 + 874.1×0.234 - 5.7×2.0 - 33.3 = 354.

[0067] The glass composition of Example 2 is expressed in mass% based on oxides: SiO2: 62%, Al2O3: 18%, B2O3: 9.5%, MgO: 2%, CaO: 7%, SrO: 1.2%, BaO: 0.1% (MgO + CaO + SrO + BaO = 10.3%), SnO2: 0.2%, and the Cl content with respect to the base composition of the glass was 0 mass%. Also, β-OH measured by the above method was 0.47 mm -1 It was. Therefore, the bubble growth index I = 590.5×0.47 + 874.1×0 - 5.7×9.5 - 33.3 = 190.

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

[0069] Figure 2 shows the change in bubble diameter over time under the reduced pressure conditions of Experimental Example 1. Comparing Example 1 and Example 2, it can be seen that while being held at 44 kPa, the bubble diameter in Example 1 grew larger. In Example 1, the initial bubble diameter was 0.24 mm, the diameter of the growing bubble was 0.97 mm, and the diameter of the growing bubble was 4.0 times that of the initial bubble. On the other hand, in Example 2, the initial bubble diameter was 0.25 mm, the diameter of the growing bubble was 0.47 mm, and the diameter of the growing bubble was 1.9 times that of the initial bubble. According to the alkali-free glass substrate of Example 1, it was found that bubbles contained in the molten glass can be easily removed under a reduced pressure atmosphere. Note that in Experimental Example 1 and Experimental Example 2 described below, the temperature of the molten glass differed between 1450 °C and 1400 °C, and the pressure in the vacuum reduced pressure vessel (reduced pressure defoaming tank) differed between 44 kPa and 33.33 kPa. This is because if Experimental Example 1 is carried out at a pressure of 33.33 kPa, the bubbles in the molten glass will swell too much and it will be difficult to take pictures using a CCD camera.

[0070] [Experimental Example 2] The clarification effect of the molten glass in the reduced pressure defoaming tank was evaluated by simulation. The rising speed of the bubbles growing in the molten glass flow and floating in the molten glass is related to the bubble diameter by Stokes' formula. Therefore, in the simulation, the bubble diameter was calculated based on the bubble growth rate, and the behavior of bubble floating was analyzed based on Stokes' formula. Note that the bubbles were assumed to be generated at the center of the position where the riser pipe was attached to the reduced pressure defoaming tank, and the calculation was performed with the initial bubble diameter set to 0.2 mm.

[0071] The dimensions of the reduced pressure defoaming tank and the height of the molten glass liquid surface are as follows. Horizontal length from the center of the riser pipe to the center of the downcomer pipe in the reduced pressure defoaming tank: 10 m Inner diameter of the reduced pressure defoaming tank: 500 mm Height of the molten glass liquid surface: 250 mm The molten glass passing through the reduced pressure defoaming tank was assumed as follows. The glass composition is as follows. Note that for the Cl content, refer to Table 1 described later. (The glass base composition is expressed in mass% based on oxides, and the Cl content is in mass% relative to the glass base composition) Examples 3 to 14 SiO2: 61.2%, Al2O3: 20.0%, B2O3: 2.0%, MgO: 5.3%, CaO: 4.5%, SrO: 7.0%, BaO: 0.1% (MgO + CaO + SrO + BaO = 16.9%) Example 15 SiO2: 61.6%, Al2O3: 20.9%, B2O3: 0.1%, MgO: 6.1%, CaO: 4.6%, SrO: 6.8%, BaO: 0.1% (MgO + CaO + SrO + BaO = 17.6%) Example 16 SiO2: 61.2%, Al2O3: 20.1%, B2O3: 1.8%, MgO: 5.4%, CaO: 4.5%, SrO: 7.0%, BaO: 0.1% (MgO + CaO + SrO + BaO = 17.0%) Example 17 SiO2: 60.9%, Al2O3: 19.5%, B2O3: 3.5%, MgO: 4.9%, CaO: 4.4%, SrO: 7.1%, BaO: 0.1% (MgO + CaO + SrO + BaO = 16.6%) Example 18 SiO2: 60.7%, Al2O3: 19.1%, B2O3: 5.5%, MgO: 4.6%, CaO: 4.4%, SrO: 7.3%, BaO: 0.1% (MgO + CaO + SrO + BaO = 16.3%) Flow rate of molten glass: 0.6 m 3 / h or 1.5 m 3 / h Pressure in the vacuum degassing tank: 33.33 kPa Temperature (average) when passing through the vacuum degassing tank: 1400 °C Viscosity when passing through the vacuum degassing tank: 150 - 200 Pa·s Density when passing through the vacuum degassing tank: 2380 kg / m 3 Regarding the degassing performance, the distance (floating distance) from the central part of the riser pipe in the vacuum degassing tank to the position where the bubbles float to the liquid surface of the molten glass was evaluated. The smaller the floating distance, the better the degassing performance.

[0072] The results are shown in Table 1. Table 1 shows the flow rate of the molten glass, the B2O3, β-OH, Cl contents, the bubble growth index I, the bubble growth rate, and the floating distance. The bubble growth rate was measured under the same conditions as in Experimental Example 1.

[0073] [Table 1]

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

[0075] On the other hand, for the glasses with a bubble growth index I of 320 or more (Examples 4 to 7, 9, 10, 12, 13, 15, 16, 18), the floating distance was 10 m or less. From this result, it is assumed that the bubble density with a bubble diameter of more than 100 μm on the glass substrate is 0.06 pieces / kg or less.

[0076] From the above results, the glass of the example can solve the problem that the investment cost of the equipment increases without the need to deliberately increase the size of the vacuum defoaming device for efficiently manufacturing a large glass substrate. In addition, since the bubble density in the substrate can be reduced, it can be assumed that the problem that the product yield decreases as the substrate size increases is solved.

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

[0078] The uses of the non-alkali glass substrate include those for liquid crystal displays, organic EL displays, flat panel displays, or other various uses. [Explanation of Signs]

[0079] 1 Glass manufacturing apparatus 10 Melting tank 20 Vacuum degassing apparatus 21 Vacuum housing 22 Vacuum degassing tank 23 Uptake pipe 24 Downcomer 25 Heat insulating material

Claims

1. In terms of mass percentage based on oxides, SiO 2 : 58.5 to 67.5%, Al 2 O 3 : 18 to 24%, B 2 O 3 : 1.8 to 5.5%, MgO + CaO + SrO + BaO: 8 to 26%, and in terms of mass percentage based on oxides, it contains MgO: 4 to 8.5%, CaO: 3 to 8.5%, SrO: 2 to 10%, BaO: 0 to 2.5%. It is an alkali-free glass substrate, β-OH is 0.15 to 0.32 mm -1 and the Cl content is 0.15 to 0.25 mass%, An alkali-free glass substrate, characterized in that the bubble growth index I represented by the following formula (1) is 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 β-OH (mm -1 ) of the alkali-free glass substrate, [Cl] represents the Cl content (mass %) of the alkali-free glass substrate, and [B 2 O 3 represents the B 2 O 3 content (mass %) of the alkali-free glass substrate.

2. An alkali-free glass substrate containing, in terms of mass% based on oxides, SiO₂: 57 to 67.5%, Al₂O₃: 17 to 25%, B₂O₃: 1.8 to 5.5%, MgO + CaO + SrO + BaO: 8 to 26%, and containing, in terms of mass% based on oxides, MgO: 2 to 8.5%, CaO: 1.5 to 8%, SrO: 0.5 to 8.5%, BaO: 0 to 1%, where β-OH is 0.15 to 0.32 mm⁻¹ and the Cl content is 0.15 to 0.25 mass%, An alkali-free glass substrate, characterized in that the bubble growth index I represented by the following formula (1) is 320 or more. I = 590.5×[β-OH] + 874.1×[Cl] - 5.7×[B₂O₃] - 33.3 (1) In 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.

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

4. The alkali-free glass substrate according to any one of claims 1 to 3, wherein the strain point is 690 to 750 °C.

5. The alkali-free glass substrate according to any one of claims 1 to 4, wherein the Young's modulus is 78 GPa or more.

6. The alkali-free glass substrate according to any one of claims 1 to 5, wherein the plate thickness is 0.1 mm to 0.5 mm.

7. The alkali-free glass substrate according to any one of claims 1 to 6, wherein the substrate size is 2100 mm or more on the short side and 2400 mm or more on the long side.

8. The alkali-free glass substrate according to any one of claims 1 to 7, wherein the substrate size is 2900 mm or more on the short side and 3200 mm or more on the long side.

9. The alkali-free glass substrate according to any one of claims 1 to 8, wherein the bubble density of bubbles with a diameter exceeding 100 μm is 0.06 pieces / kg or less.

10. When the alkali-free glass substrate is melted, held at 1450 °C, and depressurized from atmospheric pressure to 44 kPa at a constant depressurization rate in 20 minutes and then held at 44 kPa for 5 minutes, Using bubbles with diameters ranging from 0.1 mm to 0.3 mm contained in molten glass at 1450 °C before starting decompression as initial bubbles, After holding at 44 kPa for 5 minutes, using the bubbles corresponding to the initial bubbles as grown bubbles, and the diameter of the grown bubbles is 3 times or more the diameter of the initial bubbles. The alkali-free glass substrate according to any one of claims 1 to 9.

11. The diameter of the grown bubbles is 15 times or less the diameter of the initial bubbles. The alkali-free glass substrate according to claim 10.

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