阅读说明：本技术 无碱玻璃基板 (Alkali-free glass substrate ) 是由 广濑元之 榎本高志 于 2019-06-21 设计创作，主要内容包括：本发明涉及无碱玻璃基板。本发明提供一种能够在减压气氛下容易地除去熔融玻璃中所包含的气泡的无碱玻璃基板。以氧化物基准的质量％表示,所述无碱玻璃基板含有54％～66％的SiO<Sub>2</Sub>、10％～23％的Al<Sub>2</Sub>O<Sub>3</Sub>、6％～12％的B<Sub>2</Sub>O<Sub>3</Sub>、8％～26％的MgO+CaO+SrO+BaO,其特征在于,所述无碱玻璃基板的β-OH为0.15mm<Sup>-1</Sup>～0.5mm<Sup>-1</Sup>、Cl含量为0.1质量％～0.35质量％,并且所述无碱玻璃基板的由下述式(1)表示的气泡生长指数I为280以上,I＝590.5×[β-OH]+874.1×[Cl]-5.7×[B<Sub>2</Sub>O<Sub>3</Sub>]-33.3 (1)其中,在式(1)中,[β-OH]表示所述无碱玻璃基板的β-OH(mm<Sup>-1</Sup>)、[Cl]表示所述无碱玻璃基板的Cl含量(质量％)、[B<Sub>2</Sub>O<Sub>3</Sub>]表示所述无碱玻璃基板的B<Sub>2</Sub>O<Sub>3</Sub>含量(质量％)。(The present invention relates to an alkali-free glass substrate. The invention provides an alkali-free glass substrate which can easily remove bubbles contained in molten glass under a reduced pressure atmosphere. The alkali-free glass substrate contains 54 to 66% of SiO in terms of mass% based on oxides 2 10 to 23 percent of Al 2 O 3 6 to 12 percent of B 2 O 3 8 to 26% of MgO + CaO + SrO + BaO, wherein the alkali-free glass substrate has β -OH of 0.15mm ‑1 ～0.5mm ‑1 A Cl content of 0.1 to 0.35% by mass, and a bubble growth index I represented by the following formula (1) of the alkali-free glass substrate is 280 or more, wherein I is 590.5 × [ β -OH [ ]]+874.1×[Cl]‑5.7×[B 2 O 3 ]-33.3 (1) wherein, in formula (1), [ β -OH]β -OH (mm) representing the alkali-free glass substrate ‑1 )、[Cl]Represents the Cl content (mass%) and [ B ] of the alkali-free glass substrate 2 O 3 ]B represents the alkali-free glass substrate 2 O 3 Content (mass%).)

1. An alkali-free glass substrate comprising 54 to 66% of SiO in terms of mass% based on oxides₂10 to 23 percent of Al₂O₃6 to 12 percent of B₂O₃8 to 26 percent of MgO + CaO + SrO + BaO, and is characterized in that,

said is free ofβ -OH of the alkali glass substrate is 0.15mm^-1～0.5mm^-1And a Cl content of 0.1 to 0.35 mass%, and

the alkali-free glass substrate has a bubble growth index I represented by the following formula (1) of 280 or more,

I＝590.5×[β-OH]+874.1×[Cl]-5.7×[B₂O₃]-33.3 (1)

wherein, in the formula (1), [ β -OH]β -OH (mm) representing the alkali-free glass substrate^-1)、[Cl]Represents the Cl content (mass%) and [ B ] of the alkali-free glass substrate₂O₃]B represents the alkali-free glass substrate₂O₃Content (mass%).

2. The alkali-free glass substrate according to claim 1, wherein the alkali-free glass substrate has a bubble growth index I represented by the formula (1) of 360 or less.

3. The alkali-free glass substrate of claim 1 or claim 2, wherein the alkali-free glass substrate has an β -OH of 0.15mm^-1～0.40mm^-1。

4. The alkali-free glass substrate according to claim 1 or 2, wherein the alkali-free glass substrate contains, in mass% on an oxide basis, 0% to 12% of MgO, 0% to 12% of CaO, 0% to 16% of SrO, and 0% to 16% of BaO.

5. The alkali-free glass substrate of claim 1 or claim 2, wherein the alkali-free glass substrate has a strain point of 650 ℃ to 700 ℃.

6. The alkali-free glass substrate according to claim 1 or 2, wherein the substrate size of the alkali-free glass substrate is 2100mm or more in the short side and 2400mm or more in the long side.

7. The alkali-free glass substrate according to claim 1 or 2, wherein the substrate size of the alkali-free glass substrate is 2900mm or more in the short side and 3200mm or more in the long side.

8. The alkali-free glass substrate according to claim 1 or 2, wherein the alkali-free glass substrate has a bubble density of 0.06 bubbles/kg or less with a bubble diameter of more than 100 μm.

9. The alkali-free glass substrate according to claim 1 or 2, wherein, when the alkali-free glass substrate is melted, held at 1400 ℃, decompressed from atmospheric pressure to 33.33kPa at a constant decompression rate for 20 minutes, and held at 33.33kPa for 5 minutes,

bubbles having a diameter of 0.1mm to 0.3mm contained in the 1400 ℃ molten glass before the start of decompression are set as initial bubbles,

the bubble corresponding to the initial bubble after being maintained at 33.33kPa for 5 minutes was set as a growth bubble,

the diameter of the growing bubble is more than 3 times the diameter of the initial bubble.

10. The alkali-free glass substrate of claim 9, wherein the grown bubbles have a diameter that is 20 times or less the diameter of the initial bubbles.

11. The alkali-free glass substrate according to claim 1 or 2, wherein the thickness of the alkali-free glass substrate is 0.45mm or less.

Technical Field

The present invention relates to an alkali-free glass substrate suitable as a glass substrate for various displays.

Background

In the case where the glass substrate for various displays, particularly a glass substrate on which a metal or oxide thin film or the like is formed, contains an alkali metal oxide, alkali metal ions diffuse in the thin film to deteriorate film characteristics, and therefore, an alkali-free glass substrate substantially not containing alkali metal ions is preferably used.

The alkali-free glass substrate used for the above purpose is obtained by: glass raw materials prepared in a predetermined ratio are heated and melted in a melting tank to be vitrified, the molten glass is clarified, and then a glass ribbon having a predetermined thickness is formed by a float method or a fusion method, and the glass ribbon is cut into a predetermined shape.

For fining of molten glass, a vacuum degassing method is known: molten glass is introduced into a reduced-pressure atmosphere, bubbles in a continuously flowing molten glass flow are greatly grown in the reduced-pressure atmosphere, bubbles contained in the molten glass are floated and broken to remove the bubbles, and then the molten glass is discharged from the reduced-pressure atmosphere. For example, patent document 1 discloses a glass production method including a step of vacuum-degassing molten glass in a vacuum degassing vessel.

Scrap of used glass is used as a part of a glass raw material for the purpose of recovering glass and improving meltability.

Disclosure of Invention

Problems to be solved by the invention

Incidentally, in recent years, the demand for large-sized liquid crystal displays has increased, and large-sized glass substrates used in liquid crystal displays have been desired. In order to efficiently produce a large-sized glass substrate and increase the flow rate of molten glass, there is a case where the size of the vacuum degassing apparatus needs to be increased. In particular, when a molten glass conduit (a vacuum degassing vessel, a riser or a downcomer) made of platinum or a platinum alloy is used in the vacuum degassing apparatus, there is a problem that the investment cost of the apparatus increases.

In addition, with respect to the glass substrate, even if the average density of bubble defects in the substrate is the same, the yield of products decreases as the substrate size increases (see the drawing of non-patent document 1). Therefore, in order to increase the size of the glass substrate to be manufactured, it is necessary to further reduce the bubble density in the substrate than in the conventional case.

In order to solve the above-described problems, an object of the present invention is to provide an alkali-free glass substrate in which bubbles contained in molten glass can be easily removed in a reduced-pressure atmosphere.

Means for solving the problems

In order to achieve the above object, the present invention provides an alkali-free glass substrate comprising 54 to 66% of SiO, expressed in mass% based on oxides₂10 to 23 percent of Al₂O₃6 to 12 percent of B₂O₃8 to 26 percent of MgO + CaO + SrO + BaO, and is characterized in that,

the β -OH of the alkali-free glass substrate is 0.15mm^-1～0.5mm^-1And a Cl content of 0.1 to 0.35 mass%, and

the alkali-free glass substrate has a bubble growth index I represented by the following formula (1) of 280 or more,

I＝590.5×[β-OH]+874.1×[Cl]-5.7×[B₂O₃]-33.3 (1)

wherein, in the formula (1), [ β -OH]β -OH (mm) representing the alkali-free glass substrate^-1)、[Cl]Represents the Cl content (mass%) and [ B ] of the alkali-free glass substrate₂O₃]B represents the alkali-free glass substrate₂O₃Content (mass%).

Effects of the invention

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

Drawings

Fig. 1 is a cross-sectional view showing one configuration example of a glass manufacturing apparatus for manufacturing an alkali-free glass substrate of the present invention.

Fig. 2 is a graph showing a temporal change in bubble diameter under the reduced pressure condition of experimental example 1.

Reference numerals

1: glass manufacturing device

10: melting tank

20: vacuum defoaming device

21: pressure reducing machine shell

22: decompression defoaming groove

23: ascending pipe

24: down pipe

25: heat insulating material

Detailed Description

[ alkali-free glass substrate ]

An alkali-free glass substrate according to an embodiment of the present invention will be described below. The alkali-free glass means that Na is not substantially contained₂O、K₂And alkali metal oxide glasses such as O. Here, the substantial absence of the alkali metal oxide means that the total content of the alkali metal oxide is 0.1 mass% or less.

The alkali-free glass substrate of the present invention contains 54 to 66% of SiO in terms of mass% based on oxides₂10 to 23 percent of Al₂O₃6 to 12 percent of B₂O₃8 to 26 percent of MgO + CaO + SrO + BaO.

Hereinafter, in the present specification, mass% based on oxide is simply referred to as "%".

Next, the composition ranges of the respective components will be explained.

SiO₂When the content is 54% or more, the strain point of the alkali-free glass substrate increases, and the chemical resistance becomes good. Preferably 55% or more, more preferably 57% or more, and further preferably 58% or more.

SiO₂When the content is 64% or less, the meltability of the glass during melting becomes good. Preferably 63% or less, more preferably 62% or less, and further preferably 61% or less.

Al₂O₃When the content is 10% or more, phase separation is suppressed, and the strain point of the alkali-free glass substrate is increased. Preferably 12% or more, more preferably 14% or more, and further preferably 16% or more.

Al₂O₃When the content is 23% or less, the meltability of the glass during melting becomes good. Preferably 22% or less, more preferably 21% or less, and further preferably 20% or less.

B₂O₃When the content is 6% or more, the meltability at the time of glass melting becomes good, and the chemical resistance of the alkali-free glass substrate is improved. Preferably 6.5% or more, more preferably 7% or more, and further preferably 7.4% or more.

B₂O₃When the content is 12% or less, the strain point of the alkali-free glass substrate is increased. Preferably 11% or less, more preferably 10% or less, and still more preferably 9% or less.

When the total amount of MgO, CaO, SrO, and BaO (i.e., MgO + CaO + SrO + BaO) is 8% or more, the meltability of the glass during melting becomes good. Preferably 9% or more, more preferably 10% or more, and further preferably 12% or more.

When MgO + CaO + SrO + BaO is 26% or less, the strain point of the alkali-free glass substrate increases. Preferably 24% or less, more preferably 22% or less, and further preferably 20% or less.

MgO may be contained to improve the meltability of the glass during melting. The content is preferably 0.1% or more, more preferably 1% or more, further preferably 2% or more, and particularly preferably 3% or more.

When MgO is 12% or less, phase separation is suppressed, and therefore, it is preferable. More preferably 10% or less, still more preferably 8% or less, and particularly preferably 6% or less.

CaO may be contained to improve the meltability of the glass during melting. The content is preferably 0.1% or more, more preferably 1.5% or more, further preferably 3% or more, and particularly preferably 3.5% or more.

Limestone (CaCO) as a CaO raw material when CaO is 12% or less₃) The impurity phosphorus in (b) is preferably less mixed. More preferably 10% or less, further preferablyThe concentration is preferably 8% or less, more preferably 6% or less.

SrO may be contained to improve the meltability of the glass during melting. The content is preferably 0.1% or more, more preferably 1% or more, further preferably 3% or more, and particularly preferably 5% or more.

When SrO is 16% or less, acid resistance is good, and therefore, SrO is preferable. More preferably 14% or less, still more preferably 12% or less, and particularly preferably 10% or less.

BaO may be contained for improving the meltability. The content is preferably 0.1% or more.

When BaO is 16% or less, segregation is not likely to occur when the raw material is melted, and therefore, BaO is preferable. More preferably 13% or less, still more preferably 10% or less, and particularly preferably 7% or less.

The alkali-free glass substrate of the invention has β -OH of 0.15mm^-1～0.5mm^-1β -OH was used as an indicator of the moisture content in the glass β -OH was 0.15mm^-1In the above case, moisture in the glass flows into the bubbles in the reduced pressure atmosphere, and the bubbles grow easily. Preferably 0.20mm^-1Above, more preferably 0.25mm^-1The above.

β -OH is 0.5mm^-1Since the growth of bubbles can be suppressed from becoming excessively large, escape of bubbles generated by expansion of the bubble layer can be suppressed in the vacuum degassing treatment (すリ removal け). Preferably 0.45mm^-1Below, more preferably 0.40mm^-1The following.

β -OH can be determined as follows: the transmittance of an alkali-free glass test piece obtained by forming a molten glass after vacuum degassing into a plate shape was measured using a fourier transform infrared spectrophotometer (FT-IR), and β — OH was determined using the following formula.

β-OH＝(1/X)log₁₀(T₁/T₂)

X: glass plate thickness (mm)

T₁: reference wave number 4000cm^-1Transmittance (%) of

T₂: hydroxyl absorption wave number of 3570cm^-1Near minimum transmissionPercentage (%)

The beta-OH is controlled by the moisture content in the glass raw material, the water vapor concentration in the melting tank, the burner combustion method (oxygen combustion supporting, air combustion supporting) in the melting tank and the like. In particular, the beta-OH can be easily adjusted by adjusting the burner combustion method. Specifically, the oxygen combustion supporting ratio of the burner combustion is increased to increase β -OH, and the air combustion supporting ratio of the burner combustion is increased to decrease β -OH.

The alkali-free glass substrate of the present invention has a Cl content of 0.1 to 0.35 mass% based on the basic composition of the glass. Cl in the glass is a component that facilitates growth of bubbles contained in the molten glass in a reduced pressure atmosphere. When the Cl content is less than 0.1 mass%, the growth of bubbles tends to be insufficient. When the Cl content is more than 0.35 mass%, the cell layer is easily formed due to expansion of the cell layer in the vacuum degassing treatment. More preferably 0.15 to 0.35% by mass, and still more preferably 0.2 to 0.35% by mass.

The alkali-free glass substrate of the present invention has a bubble growth index I represented by the following formula (1) of 280 or more.

I＝590.5×[β-OH]+874.1×[Cl]-5.7×[B₂O₃]-33.3 (1)

In formula (1), [ β -OH]β -OH (mm) representing an alkali-free glass substrate^-1)、[Cl]Represents the Cl content (mass%) and [ B ] of the alkali-free glass substrate₂O₃]B represents an alkali-free glass substrate₂O₃Content (mass%).

The bubble growth index I is an index of the ease of growth of bubbles contained in the molten glass in a reduced pressure atmosphere. When the bubble growth index I is 280 or more, bubbles contained in the molten glass easily grow in a reduced-pressure atmosphere. Preferably 290 or more, more preferably 300 or more.

When the bubble growth index I is 360 or less, the occurrence of reboiling in the molten glass flowing in the vacuum degassing vessel can be prevented, and therefore, it is preferable. Here, reboiling refers to a phenomenon in which bubbles are generated at the interface of glass that is in contact with a platinum, platinum alloy, or dense refractory molten degassing vessel. The bubble growth index I is more preferably 340 or less, and still more preferably 320 or less.

In the present specification, the bubble growth index I is correlated with a value of the pressure P defined in the following manner.

When the vacuum degassing vessel is depressurized under a constant temperature, the volume of bubbles (bubble diameter) present in the molten glass in the vacuum degassing vessel increases according to the boyle's law. However, when the pressure in the reduced-pressure degassing vessel is reduced to a certain pressure, the volume of bubbles (diameter of bubbles) in the molten glass rapidly increases in a manner deviating from the boyle's law. This pressure is set as pressure P.

The pressure P can be determined by the following procedure.

In order to reproduce the state in the vacuum degassing vessel, a crucible made of quartz glass containing cullet of alkali-free glass was placed in a vacuum degassing vessel. The crucible was heated to 1400 ℃ to melt the alkali-free glass. After the alkali-free glass was completely melted, the inside of the vacuum pressure reducing vessel was reduced in pressure and the diameter of the bubbles in the molten glass was observed. In order to observe the diameter of the bubbles in the molten glass, the bubbles in the molten glass may be photographed from an observation window provided in the vacuum decompression vessel using, for example, a CCD camera. The number of samples of the bubbles to be measured for the bubble diameter was 20 or more.

When the pressure in the vacuum decompression vessel is reduced, the diameter of bubbles in the molten glass increases according to boyle's law. However, when the inside of the vacuum decompression vessel is decompressed to a certain pressure, the diameter of the bubbles in the molten glass is sharply increased deviating from the boyle's law. The pressure in the vacuum reduced-pressure vessel at this time was set to a pressure P.

The present inventors have found that the bubble growth index I represented by formula (1) is 280 or more for an alkali-free glass substrate from which bubbles contained in molten glass can be easily removed in a reduced pressure atmosphere, based on the measurement results of the pressure P in a plurality of alkali-free glass substrates.

The alkali-free glass substrate of the present invention may contain Fe₂O₃As a minor component. Fe₂O₃Preferably 0.1 mass% or less, more preferably 0.05 mass% or less.

The alkali-free glass substrate of the present invention is preferably used because it can suppress thermal shrinkage during panel production when the strain point is 650 ℃. More preferably 670 ℃ or higher, and still more preferably 680 ℃ or higher.

When the strain point is 700 ℃ or lower, it is not necessary to excessively increase the temperature in the float kiln and at the outlet of the float kiln, and the life of the metal member located in the float kiln and downstream of the float kiln is less affected, and therefore, it is preferable.

The strain point was measured by a fiber drawing method in accordance with the method specified in JIS R3103-2 (2001).

The substrate size of the alkali-free glass substrate of the present invention is preferably 2100mm or more in the short side and 2400mm or more in the long side, more preferably 2800mm or more in the short side and 3000mm or more in the long side, and still more preferably 2900mm or more in the short side and 3200mm or more in the long side. The alkali-free glass substrate of the present invention has a low bubble density, and therefore, even when the substrate size is large, the yield of the product is not easily lowered, and the alkali-free glass substrate is suitable for a case where the substrate size is large. The substrate size is preferably 6000mm or less in the short side and 6500mm or less in the long side. This is because, when the substrate size is too large, the equipment investment cost increases due to the enlargement of the equipment, and it is difficult to transport the glass substrate.

The density of bubbles having a bubble diameter of more than 100 μm in the alkali-free glass substrate of the present invention is preferably 0.06 bubbles/kg or less, more preferably 0.03 bubbles/kg or less, and still more preferably 0.01 bubbles/kg or less.

It should be noted that the bubble density of the bubble diameter greater than 100 μm is calculated as follows: the glass substrate was irradiated with light from the glass substrate side in a dark room, and the number of bubble defects having a size of more than 100 μm was investigated by edge light inspection for inspecting the main surface of the glass substrate, thereby calculating the bubble density having a bubble diameter of more than 100 μm.

The alkali-free glass substrate of the present invention is preferably float glass. The float process is superior to the fusion process in that it is superior to a glass substrate having a large substrate size in terms of collecting the glass substrate (folding the glass substrate).

In the alkali-free glass substrate of the present invention, when the alkali-free glass substrate is melted, held at 1400 ℃, and reduced in pressure from atmospheric pressure to 33.33kPa for 20 minutes at a constant reduced pressure rate, and held at 33.33kPa for 5 minutes, bubbles having a diameter of 0.1mm to 0.3mm contained in the 1400 ℃ molten glass before the start of the reduction in pressure are set as initial bubbles, bubbles corresponding to the initial bubbles after held at 33.33kPa for 5 minutes are set as growth bubbles, and the diameter of the growth bubbles is preferably 3 times or more the diameter of the initial bubbles.

When the diameter of the grown bubbles is 3 times or more the diameter of the initial bubbles, the bubbles contained in the molten glass easily grow and easily float up in the molten glass in the reduced-pressure atmosphere. Therefore, bubbles contained in the molten glass can be easily removed.

The diameter of the initial bubble and the diameter of the grown bubble can be determined by the following procedure.

A quartz cell containing cullet of an alkali-free glass substrate was placed in a vacuum pressure reduction vessel. The quartz cell was heated to 1400 ℃ to melt the cullet, and the pressure in the vacuum vessel was reduced. Bubbles contained in the molten glass were photographed from an observation window provided in the vacuum decompression vessel using a CCD camera, and the diameter of the bubbles (hereinafter referred to as bubble diameter) was measured by image analysis. 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 merging a plurality of bubbles, and the like are excluded from the measurement target. The ratio of the diameter of the grown bubble to the diameter of the starting bubble (hereinafter referred to as a bubble growth rate) in one embodiment of the present invention is obtained as follows: the bubble growth rate was calculated for each of a plurality of bubbles observed in the image, and the average value of the bubble growth rates was obtained.

The reduced pressure conditions used for the evaluation were: the pressure was reduced from atmospheric pressure to 33.33kPa at a constant rate of pressure reduction over 20 minutes. When the pressure of the molten glass was reduced to 33.33kPa, bubbles contained in the molten glass could be sufficiently grown, and therefore, it was suitable for evaluating the growth of the bubble diameter. When the decompression time is 20 minutes, the growth of the bubble diameter can be appropriately evaluated while shortening the evaluation time.

The initial bubble is 0.1 mm-0.3 mm in bubble diameter. When the diameter of the initial bubble is 0.1mm or more, the measurement of the bubble diameter by the CCD camera becomes easy. When the diameter of the initial bubble is 0.3mm or less, the growth of the bubble diameter can be appropriately evaluated because the growth bubble can be prevented from expanding and breaking.

The bubble corresponding to the initial bubble after being held at 33.33kPa for 5 minutes was set as a growth bubble. When the holding time is 5 minutes, the grown bubbles become sufficiently large relative to the initial bubbles, and the bubbles can be prevented from expanding and collapsing, so that the growth of the bubble diameter can be appropriately evaluated.

The diameter of the grown bubbles of the alkali-free glass substrate of the present invention is more preferably 5 times or more, still more preferably 7 times or more, and particularly preferably 9 times or more the diameter of the initial bubbles.

The diameter of the grown bubbles of the alkali-free glass substrate of the present invention is preferably 20 times or less the diameter of the initial bubbles. At 20 times or less, the bubble layer, which is usually present on the surface of the molten glass at about 10mm or less, can be suppressed from expanding to 10mm to several hundred mm in the vacuum degassing treatment, and therefore, the escape of bubbles can be prevented. Here, the slippage of bubbles refers to a phenomenon in which bubbles reaching the glass surface stably exist for a long time by forming a bubble layer without breaking, and then the floating bubbles flow out to the subsequent step without breaking. When the generated bubbles escape, there is a problem that the bubbles remain in the molten glass after the vacuum degassing.

The diameter of the grown bubbles of the alkali-free glass substrate of the present invention is more preferably 17 times or less, still more preferably 15 times or less, and particularly preferably 13 times or less the diameter of the initial bubbles.

The alkali-free glass substrate of the present invention is preferably 0.75mm or less in thickness, more preferably 0.55mm or less, and further preferably 0.45mm or less in thickness for use as a glass substrate for a liquid crystal display. The thickness is preferably 0.01mm or more. This is because, when the thickness is too thin, the rigidity of the glass substrate is lowered, and it is difficult to roll-convey the glass substrate.

[ method for producing 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 one configuration example of a glass manufacturing apparatus for manufacturing an alkali-free glass substrate of the present invention.

The glass manufacturing apparatus 1 includes a melting tank 10 and a vacuum degassing apparatus 20. A forming apparatus such as a float furnace is disposed after the vacuum degassing apparatus 20. The forming apparatus may be a forming apparatus used in a down-draw method.

In the method for producing an alkali-free glass substrate, a glass raw material is melted in a melting tank 10 to produce a molten glass G, the molten glass G is subjected to a vacuum degassing treatment by a vacuum degassing apparatus 20, and a glass ribbon shaped like a ribbon by a forming apparatus is gradually cooled and cut to obtain an alkali-free glass substrate.

The melting tank 10 has a burner for melting the supplied glass raw material. The burner forms a flame by mixing and burning fuel such as natural gas or heavy oil with gas. A burner mainly using air as gas is called an air-assisted burner, and a burner mainly using oxygen as gas is called an oxygen-assisted burner. The burner heats the glass raw material from above by emitting a flame to the glass raw material. Further, the melting tank 10 may have an electrode for heating the glass raw material.

The glass raw material may be prepared, for example, using silica sand, boric acid, limestone, alumina, strontium carbonate, magnesium oxide, or the like so as to obtain a composition of the target alkali-free glass substrate.

As described above, the alkali-free glass substrate of the present invention can easily remove bubbles contained in molten glass in a reduced-pressure atmosphere, and therefore, cullet generated during production of the alkali-free glass substrate or cullet of a used alkali-free glass substrate is preferably used as a glass raw material.

It is preferable to add chlorine to the glass raw materialA compound clarifying agent. The chloride-based clarifying agent is preferably BaCl from the viewpoint of not worrying about deliquescence₂·2H₂O、SrCl₂·6H₂O、CaCl₂、MgCl₂·6H₂O or NH₄Cl。

As the fining agent, in order to appropriately adjust the bubble growth rate, a fining agent other than the chloride-based fining agent may also be used. In this case, SO may be mentioned, for example₃、F、SnO₂Etc. as other clarifying agents. The content of these other fining agents in the glass raw material is preferably 2% by mass or less, more preferably 1% by mass or less, and still more preferably 0.5% by mass or less.

The vacuum degassing apparatus 20 includes a vacuum casing 21, a vacuum degassing vessel 22, an ascending pipe 23, a descending pipe 24, and a heat insulator 25.

The vacuum degassing vessel 22, which is provided in a cylindrical shape, is accommodated and arranged in the vacuum casing 21 so that the long axis thereof is oriented in the horizontal direction. A vertically oriented ascending pipe 23 is attached to the lower surface of one end of the vacuum degassing vessel 22, and a descending pipe 24 is attached to the lower surface of the other end. The riser 23 and a portion of the downcomer 24 are located within the pressure letdown enclosure 21.

The riser pipe 23 communicates with the vacuum degassing vessel 22, and guides the molten glass G from the melting vessel 10 to the vacuum degassing vessel 22. The downcomer 24 communicates with the vacuum degassing vessel 22, and guides the vacuum-degassed molten glass G to the next processing vessel. In the vacuum casing 21, a heat insulator 25 such as a heat insulating brick is disposed around the vacuum degassing vessel 22, the rising pipe 23, and the falling pipe 24 to insulate and cover them.

The vacuum degassing vessel 22, the rising pipe 23, and the falling pipe 24 are conduits for molten glass, and are made of materials having excellent heat resistance and corrosion resistance to molten glass. For example, the metal oxide is made of platinum, a platinum alloy, or reinforced platinum obtained by dispersing a metal oxide in platinum or a platinum alloy. Further, the refractory may be made of a ceramic-based non-metallic inorganic material, that is, a dense refractory. Further, the refractory may be a material obtained by lining a dense refractory with platinum or a platinum alloy.

The vacuum degassing is performed by passing the molten glass G supplied from the melting tank 10 through a vacuum degassing tank 22 that is depressurized to a predetermined pressure. The molten glass G is preferably continuously supplied and discharged to the vacuum degassing vessel 22. From the viewpoint of productivity, the flow rate of the molten glass is preferably 1 ton/day to 200 ton/day.

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 the inside thereof reaches a temperature range of 1200 to 1600 ℃, particularly 1350 to 1550 ℃.

In performing the vacuum degassing, air in the vacuum chamber 21 passes through a suction opening provided at a predetermined position of the vacuum chamber 21, and is sucked from the outside by a vacuum pressure reducing means such as a vacuum pump. As a result, the air in the vacuum degassing vessel 22 housed in the vacuum casing 21 is indirectly drawn out, and the inside of the vacuum degassing vessel 22 is depressurized to a predetermined pressure.

The pressure inside the reduced-pressure defoaming tank 22 is preferably 13.33kPa to 53.33 kPa.