Method for manufacturing an alkali-free glass substrate

Abstract

Provided is a method for producing an alkali-free glass substrate having a small number of pinholes. The method is a method for continuously producing an alkali-free glass substrate of the SiO2-Al2O3-RO (RO represents one or more of MgO, CaO, BaO, SrO, and ZnO) system, which includes a step of preparing a raw batch material containing a tin compound and substantially not containing an arsenic compound and an antimony compound; a step of electrically melting the prepared raw batch material in a melting furnace capable of performing electric heating using an electrode; and a step of forming the molten glass into a plate shape by a down-draw method, and the obtained glass has a β-OH value of 0.05 / mm or more and a number of metallic pinholes of 20 / t or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a manufacturing method of an alkali-free glass substrate, and particularly to a manufacturing method of an alkali-free glass substrate suitable for a display or the like having a thin film transistor (TFT) having an oxide film such as indium gallium zinc oxide (IGZO). BACKGROUND

[0002] In a flat panel display, a glass substrate is generally used as a support substrate. A circuit pattern such as a TFT is formed on the surface of the glass substrate. Therefore, in such a glass substrate, in order not to cause adverse effects on the TFT or the like, an alkali-free glass substrate which does not substantially contain an alkali metal component is used.

[0003] As a forming method of the alkali-free glass substrate, a down-draw method typified by a fusion down-draw method or the like is known.

[0004] The down-draw method is a method of forming a glass sheet by drawing a molten glass downward. The fusion down-draw method is a method of forming a glass ribbon by drawing a molten glass overflowing from both sides of a forming body having a substantially wedge-shaped cross section downward. The molten glass overflowing from both sides of the forming body flows down along both side surfaces of the forming body and merges at the lower side of the forming body. Therefore, in the fusion down-draw method, the surface of the glass ribbon is not contacted with air but is formed by surface tension, and thus a glass substrate having a surface to which no foreign matter is attached and a flat surface can be obtained even without polishing the surface after forming. In addition, the fusion down-draw method has an advantage that a thin glass substrate can be easily formed.

[0005] In addition, as a melting method of the alkali-free glass substrate, an electric melting or the like described in Patent Literature 1 can be given.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT LITERATURE

[0008] Patent Literature 1: Japanese Patent Application Laid-Open No. 2014-88306 SUMMARY

[0009] PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] However, the glass described in Patent Literature 1 has a problem that a number of pits is easily generated.

[0011] The present application has been achieved in view of the above-described circumstances, and an object thereof is to provide a manufacturing method of an alkali-free glass substrate having a small number of pits.

[0012] MEANS OF SOLVING THE PROBLEM

[0013] The manufacturing method of the alkali-free glass substrate of the present application is a method of continuously manufacturing an SiO2-Al2O3-RO (RO is one or more of MgO, CaO, BaO, SrO, and ZnO) system alkali-free glass substrate, which includes a step of preparing a raw batch material containing a tin compound and substantially not containing an arsenic compound and an antimony compound; a step of electrically melting the prepared raw batch material in a melting furnace capable of performing electric heating by using an electrode; and a step of forming the molten glass into a plate shape by a down-draw method, and the obtained glass has a β-OH value of 0.05 / mm or more and a number of metal pits of 20 per ton or less. Here, the "metal pit" is counted as a metal pit when a metal pit of 1 μm or more is observed in the field of view by using a stereomicroscope, and the number of metal pits per 1 ton of glass is calculated based on the size of the glass used at the time of measurement.

[0014] As a result of various studies by the present inventors and others, it was found that, if glass is electrically melted, SnO2 in the glass is reduced by Fe, Cr, Ni, and the like contained in the raw batch material, as shown in Formula 1 (x, y are coefficients, and M represents Fe, Cr, and / or Ni), and Sn-containing metal pits are precipitated in the glass. It was further found that, by increasing the β-OH value of the glass, Fe, Cr, Ni, and the like combine with OH groups in the glass, as shown in Formula 2 (z is a coefficient, and M represents Fe, Cr, and / or Ni), and thus it becomes difficult to reduce SnO2, and metal pits are not easily precipitated in the glass.

[0015] (Formula 1)

[0016] (Formula 2)

[0017] "Alkali-free glass" means a glass which does not intentionally contain an alkali metal oxide component, and specifically means a glass which contains an alkali metal oxide (Li20, Na20, and K20) in an amount of 2000 ppm or less by mass. "Continuously producing" means continuously producing a glass in a fixed period in a continuous melting furnace such as a tank furnace. "Si02-Al203-RO system" means a glass composition system which contains "Si02, Al203, and RO" as essential components. "Electric melting" means a melting method in which a glass is melted by Joule heat generated by electric current. Here, a melting method which additionally uses radiant heat generated by a heater or a burner is not excluded. "Substantially free of arsenic and antimony" means that a glass raw material or glass cullet containing these components is not intentionally added to a glass batch. More specifically, it means that, in the resulting glass, arsenic is 50 ppm or less as As203, and antimony is 50 ppm or less as Sb203, on a molar basis. "Down-draw method" means a general term for a forming method in which a molten glass is continuously drawn downward while being formed.

[0018] Further, the present application is characterized in that the glass is melted by electric heating. If the glass is melted by electric heating, the energy per unit mass for obtaining the molten glass is reduced, and thus the environmental load can be reduced.

[0019] In the present application, it is preferable to adjust the β-OH value and the number of metallic streaks of the resulting glass by using a glass raw material and / or a melting condition.

[0020] In the present application, the effect becomes more significant without using radiant heat generated by combustion of a burner. "Without using radiant heat generated by combustion of a burner" means that radiant heat generated by combustion of a burner is not used at all during normal production, and does not exclude use of a burner during start-up of production (during warming-up). Further, it does not exclude use of radiant heat generated by a heater during start-up of production and during normal production. Note that start-up of production means a period from the time when a raw material batch is dissolved to become a glass melt until the time when electric heating can be performed.

[0021] In the present application, when an alkali-free glass substrate which further contains B203as a glass component is produced, it is preferable to use orthoboric acid as at least a part of a glass raw material which becomes a boron source.

[0022] If the above-described configuration is employed, the water content of the resulting glass can be easily adjusted. Further, the boron component (B203) is a component which improves the meltability of a glass, and thus if the above-described configuration is employed, a glass having excellent productivity can be easily obtained.

[0023] In the present application, it is preferable to contain a hydroxide raw material in a raw material batch.

[0024] In this invention, when glass shavings are added to the raw material batch to manufacture alkali-free glass substrates, it is preferable to use glass shavings containing glass with a β-OH value of 0.05 / mm or higher as at least a portion of the glass shavings. Here, "glass shavings" refers to defective glass generated during glass manufacturing or recycled glass recovered from the market. "β-OH value" refers to the value obtained by measuring the transmittance of the glass using FT-IR and calculating it using the following formula.

[0025] β-OH value = (1 / X)log(T1 / T2)

[0026] X: Glass wall thickness (mm)

[0027] T1: Reference wavelength 3846cm -1 Transmittance at (%)

[0028] T2: Hydroxyl absorption wavelength 3600 cm⁻¹ -1 Minimum transmittance (%) in the vicinity

[0029] In this invention, it is preferable to perform the process of removing metal by passing glass shavings through a magnetic separator more than twice.

[0030] Recycled glass shavings may contain metals such as Fe, Cr, and Ni, which are used in blending equipment. By passing the glass shavings through a magnetic separator more than twice, these metals can be thoroughly removed, making it difficult for them to contaminate the raw material batch. As a result, metal pitting containing Sn is less likely to precipitate in the glass.

[0031] In this invention, the strain point of the obtained glass is preferably 690°C or higher. Here, "strain point" is a value determined based on the method of ASTM C336-71.

[0032] In this invention, the heat shrinkage rate of the obtained glass is preferably 25 ppm or less. Here, "heat shrinkage rate" refers to the value measured under the following conditions: the glass is heated from room temperature to 500°C at a rate of 5°C / minute, held at 500°C for 1 hour, and then cooled at a rate of 5°C / minute.

[0033] It should be noted that the heat shrinkage rate is determined using the following method. First, as... Figure 1 As shown in (a), a strip specimen G measuring 160 mm × 30 mm is prepared as the glass plate specimen. Marks M are formed at both ends of the strip specimen G along its long side using #1000 water-resistant abrasive paper, at positions 20–40 mm from the edge. Then, as... Figure 1(b) As shown, the strip-shaped test sample G on which the mark M is formed is folded in two along a direction orthogonal to the mark M to produce test pieces Ga and Gb. Then, only one test piece Ga is subjected to heat treatment of increasing the temperature from normal temperature (25°C) to 500°C at a rate of 5°C / min and decreasing the temperature to normal temperature at a rate of 5°C / min after keeping at 500°C for 1 hour. After the heat treatment, as shown in Figure 1 (c) As shown, the position displacement amounts (ΔLl, ΔL2) of the marks M of the two test pieces Ga and Gb are read by a laser microscope in a state where the test piece Gb not subjected to the heat treatment and the test piece Ga subjected to the heat treatment are arranged side by side, and the thermal shrinkage is calculated by the following equation. In the equation, lo is the initial distance between the marks M.

[0034] Thermal shrinkage = [{ΔLl (μm) + ΔL2 (μm)} x 10 3 ] / lo (mm) (ppm)

[0035] If the above structure is employed, a glass substrate suitable for forming an oxide TFT can be obtained.

[0036] In the present application, it is preferable to use in the production of a glass substrate for forming an oxide TFT. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 is a plan view for explaining the steps of measuring the thermal shrinkage of a glass substrate.

[0038] Figure 2 is a graph showing the relationship between the β-OH value and the metal scum. DETAILED DESCRIPTION

[0039] Hereinafter, the method for producing an alkali-free glass substrate of the present application will be described in detail.

[0040] The method of the present application includes a step of preparing a raw batch, a step of electrically melting the prepared batch, and a step of shaping the molten glass into a plate shape.

[0041] (1) Step of preparing a raw batch

[0042] First, a glass raw material is prepared so as to have a composition of SiO2-Al2O3-RO (RO is one or more of MgO, CaO, BaO, SrO, and ZnO). Note that the preferable glass composition is described later.

[0043] As the silicon source, silica sand, stone powder (SiO2), or the like can be used.

[0044] As the aluminum source, alumina (Al2O3), aluminum hydroxide (Al(OH)3), or the like can be used.

[0045] As the boron source, orthoboric acid (H3BO3), boric anhydride (B2O3) can be used. Orthoboric acid contains crystal water, and therefore, in the case where the proportion used is large, the water content of the glass can be adjusted to be high. Therefore, it is preferable to use both orthoboric acid and boric anhydride, and adjust the proportion thereof used in accordance with the β-OH content targeted.

[0046] As the alkaline earth metal source, calcium carbonate (CaCO3), magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), barium carbonate (BaCO3), barium nitrate (Ba(NO3)2), strontium carbonate (SrCO3), strontium nitrate (Sr(NO3)2), and the like can be used.

[0047] As the zinc source, zinc oxide (ZnO), and the like can be used.

[0048] As the zirconia source, zircon (ZrSiO4), and the like can be used. Note that, in the case where a zirconia electrocasted refractory, a dense zircon, or the like containing Zr is used as the refractory constituting the melting furnace, there are cases where the zirconia component from the refractory dissolves out. These dissolved-out components can also be utilized as the zirconia source.

[0049] As the titanium source, titanium oxide (TiO2), and the like can be used.

[0050] As the phosphorus source, aluminum metaphosphate (Al(PO3)3), magnesium pyrophosphate (Mg2P2O7), and the like can be used.

[0051] As the tin compound, tin oxide (SnO2), and the like can be used. Note that, in the case where tin oxide is used, it is preferable to use tin oxide having an average particle diameter D 50 of 0.3 to 50 μm, 2 to 50 μm, particularly 5 to 50 μm. If the average particle diameter D 50 of the tin oxide powder is small, agglomeration between the particles occurs, and clogging in the compounding equipment easily occurs. On the other hand, if the average particle diameter D 50 of the tin oxide powder is large, the dissolution reaction of the tin oxide powder into the glass melt is delayed, and the melt cannot be clarified. As a result, oxygen cannot be sufficiently released at the appropriate period of glass melting, and bubbles easily remain in the glass product, and a product having excellent bubble quality is difficult to obtain. In addition, cases where SnO2crystals appear as undissolved specks in the glass product easily occur.

[0052] In the present application, a carbonate raw material can also be contained in the raw material batch. The carbonate raw material can cause SnO2, which is a fining agent, to effectively function. As the carbonate raw material, for example, calcium carbonate (CaCO3), barium carbonate (BaCO3), strontium carbonate (SrCO3), and the like can be used.

[0053] In the present application, a nitrate raw material can also be contained in the raw material batch. The nitrate raw material can make Sn02as a fining agent effectively function. As the nitrate raw material, for example, barium nitrate (Ba(N03)2), strontium nitrate (Sr(N03)2), or the like can be used.

[0054] In the present application, a hydroxide raw material can also be contained in the raw material batch. The hydroxide raw material can increase the water content in the glass. As the hydroxide raw material, aluminum hydroxide (Al(OH)3), magnesium hydroxide (Mg(OH)2), calcium hydroxide (Ca(OH)2), or the like can be used.

[0055] In the present application, it is preferable that the batch does not substantially contain arsenic compounds and antimony compounds. If these components are contained, the electrode is corroded, and thus it is difficult to perform electric melting stably for a long period of time. In addition, these components are not preferable from the environmental viewpoint.

[0056] In the present application, in addition to the above-described glass raw materials, glass scraps are preferably used. In the case where the glass scraps are used, the use ratio of the glass scraps with respect to the total amount of the raw material batch is preferably 1 mass% or more, 5 mass% or more, and particularly preferably 10 mass% or more. The upper limit of the use ratio of the glass scraps is not limited, but is preferably 50 mass% or less, 40 mass% or less, and particularly preferably 30 mass% or less. In addition, it is preferable that at least a part of the used glass scraps be glass scraps composed of glass having a β-OH value of 0.05 / mm or more, 0.07 / mm or more, and particularly 0.1 / mm or more. If the β-OH value of the glass scraps is too high, the strain point of the glass can excessively decrease, and thus the upper limit of the β-OH value of the glass scraps is preferably 0.4 / mm or less.

[0057] In addition, the process of removing metals by passing the glass scraps through a magnetic separator is performed two or more times, preferably three or more times, and particularly preferably five or more times. In this way, metals including Fe, Cr, Ni, and the like, which reduce Sn02in the glass, are difficult to mix into the raw material batch. As a result, Sn-containing metal seeds become less likely to precipitate. Note that if the number of times of the above-described process is increased, the amount of the metals including Fe, Cr, Ni, and the like, which are mixed, decreases, but from the cost viewpoint, the number of times of the above-described process is preferably 10 times or less.

[0058] (2) Process of electrically melting the prepared raw material batch

[0059] Next, the prepared raw material batch is charged into a melting furnace, and electric melting is performed.

[0060] The melting furnace has a plurality of electrodes, and by passing electric current between the electrodes, electric current is passed in the glass melt, and the glass is continuously melted by the Joule heat thereof. Note that radiation heating by a heater, a burner, or the like can also be used in an auxiliary manner.

[0061] The configuration site and the degree of freedom of the electrode shape are high, and even an alkali-free glass, which is difficult to be electrically fused, can adopt an optimal electrode configuration and electrode shape. In order to make the electrical fusion heating easy, it is preferable to use a molybdenum electrode as the electrode. As the electrode shape, it is preferable to be a rod shape. If it is a rod shape, it is possible to maintain a desired electrode distance at any position of the side wall surface and the bottom wall surface of the melting furnace and to configure a desired number of electrodes. The configuration of the electrodes is preferably to configure a plurality of pairs of electrodes by shortening the electrode distance on the wall surface (the side wall surface, the bottom wall surface, and the like) of the melting furnace, particularly the bottom wall surface. Note that in the case where the glass contains an arsenic component or an antimony component, the molybdenum electrode is corroded, and thus cannot be used. Instead, it is necessary to use a tin electrode that is not corroded by these components. However, the degree of freedom of the configuration site and the electrode shape of the tin electrode is very low, and thus it is difficult to electrically fuse an alkali-free glass.

[0062] The raw material batch that is put into the melting furnace is melted by the electrical fusion heating and becomes a glass melt (molten glass). At this time, the tin compound contained in the raw material batch is dissolved in the glass melt and functions as a fining agent. In detail, the tin component releases oxygen bubbles during the temperature increase. The released oxygen bubbles enlarge and float the bubbles contained in the glass melt and remove them from the glass. In addition, the tin component absorbs the oxygen bubbles during the temperature decrease, and thus the bubbles remaining in the glass disappear.

[0063] Note that the glass that is melted in the melting furnace is supplied to the forming device, but a fining tank, a stirring tank, a state adjusting tank, or the like can be configured between the melting furnace and the forming device, and after passing through them, it is supplied to the forming device. In addition, in order to prevent the glass from being contaminated, it is preferable that the contact surface of the communication flow path that connects the melting furnace and the forming device (or each tank provided therebetween) is made of platinum or a platinum alloy.

[0064] (3) A step of forming the molten glass into a plate shape

[0065] Next, the glass that is melted in the melting furnace is supplied to the forming device and is formed into a plate shape by the down-draw method.

[0066] As the down-draw method, an overflow down-draw method is preferably employed. The overflow down-draw method is a method in which molten glass is caused to overflow from both sides of a trough-shaped refractory having a wedge-shaped cross section, and the overflowed molten glass is caused to flow together at the lower end of the trough-shaped refractory while extending downward to be shaped into a sheet shape. In the overflow down-draw method, the surface that is to become the surface of the glass substrate is not brought into contact with the trough-shaped refractory, but is shaped in a free surface state. Thus, a glass substrate that is not polished and has a good surface quality can be inexpensively produced, and the glass can be easily made large and thin. Note that the structure and material of the trough-shaped refractory used in the overflow down-draw method are not particularly limited as long as the desired size and surface accuracy can be achieved. Further, the method of applying a force when extending downward is not particularly limited. For example, a method in which a heat-resistant roller having a sufficiently large width is rotated in contact with the glass to stretch the glass, or a method in which a plurality of pairs of heat-resistant rollers are brought into contact with the glass only near the end surfaces to stretch the glass can be employed. Note that, in addition to the overflow down-draw method, a slot down-draw method or the like can be employed, for example.

[0067] Further, the glass shaped into a sheet shape is cut into a predetermined size, and various chemical or mechanical processes or the like are performed as needed to become a glass substrate.

[0068] (4) Composition of Alkali-Free Glass

[0069] As the composition of the alkali-free glass to which the production method of the present application is suitably applied, a glass containing, in mass%, Si02 50 to 70%, Al203 15 to 25%, B203 2 to 7.5%, MgO 0 to 10%, CaO 0 to 10%, SrO 0 to 10%, BaO 0 to 15%, ZnO 0 to 5%, Zr02 0 to 1%, Ti02 0 to 5%, P205 0 to 10%, Sn02 0.1 to 0.5%, and substantially no As203 and Sb203 can be exemplified. The reasons for limiting the content of each component as described above are shown below. Note that, in the description of the content of each component, the expression of % indicates mass% unless otherwise specified.

[0070] Si02 is a component that forms the skeleton of the glass. The lower limit of the content of Si02 is preferably 50%, 51%, 51.5%, 52%, 55%, 56%, 57%, and particularly preferably 58%. Further, the upper limit of the content of Si02 is preferably 70%, 69%, 68%, 67%, 66%, 65%, 64%, 63%, and particularly preferably 62%. If the content of Si02 is too small, the density becomes too high, and the acid resistance easily decreases. On the other hand, if the content of Si02 is too large, the high-temperature viscosity becomes high, and the meltability easily decreases. Further, devitrification crystals such as cristobalite easily precipitate, and the liquidus temperature easily increases.

[0071] Al2O3 is a component that forms the skeleton of the glass, and is a component that increases the strain point, Young's modulus, and suppresses phase separation. The lower limit of the content of Al2O3 is preferably 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, and particularly preferably 18%. In addition, the upper limit of the content of Al2O3 is preferably 25%, 24%, 23%, 22%, 21.5%, and particularly preferably 21%. If the content of Al2O3 is too small, the strain point and Young's modulus easily decrease, and in addition, the glass easily separates into phases. On the other hand, if the content of Al2O3 is too large, it becomes easy to precipitate devitrification crystals such as mullite and anorthite, and the liquidus temperature easily increases.

[0072] B2O3 is a component that increases the melting property and increases the resistance to devitrification. The lower limit of the content of B2O3 is preferably 2%, 2.2%, and particularly preferably 2.5%. In addition, the upper limit of the content of B2O3 is preferably 7.5%, and particularly preferably 7%. If the content of B2O3 is too small, the melting property and the resistance to devitrification easily decrease, and in addition, the resistance to hydrofluoric acid-based solutions such as buffered hydrofluoric acid easily decreases. In addition, the amount of moisture from the batch that is taken in can become too small. On the other hand, if the content of B2O3 is too large, the strain point and Young's modulus easily decrease.

[0073] MgO is a component that decreases the high-temperature viscosity, increases the melting property, and among the alkaline earth metal oxides, is a component that significantly increases the Young's modulus. The lower limit of the content of MgO is preferably 0%, 0.1%, 0.5%, 1%, 1.5%, and particularly preferably 2%. In addition, the upper limit of the content of MgO is preferably 10%, 9%, 8%, 7.5%, 7%, 6%, and particularly preferably 5%. If the content of MgO is too small, the melting property and the Young's modulus easily decrease. On the other hand, if the content of MgO is too large, the resistance to devitrification easily decreases, and the strain point easily decreases.

[0074] CaO is a component that does not decrease the strain point, decreases the high-temperature viscosity, and significantly increases the melting property. In addition, among the alkaline earth metal oxides, since the raw material to be introduced is relatively inexpensive, it is a component that makes the raw material cost inexpensive. The lower limit of the content of CaO is preferably 0%, 0.1%, 1%, 2%, 3%, and particularly preferably 3.5%. In addition, the upper limit of the content of CaO is preferably 10%, 9%, 8%, and particularly preferably 7%. If the content of CaO is too small, the above effects are difficult to obtain. On the other hand, if the content of CaO is too large, the glass easily devitrifies, and the coefficient of thermal expansion easily becomes high.

[0075] SrO is a component that suppresses phase separation and improves resistance to devitrification. Further, it is a component that lowers high-temperature viscosity and improves meltability without lowering the strain point. It is also a component that suppresses an increase in liquidus temperature. The lower limit of the content of SrO is preferably 0%, 0.1%, and particularly preferably 0.3%. In addition, the upper limit of the content of SrO is preferably 10%, 9%, 8%, 7%, 6%, and particularly preferably 5%. If the content of SrO is too small, the above effects are difficult to obtain. On the other hand, if the content of SrO is too large, the density becomes too high, and devitrification crystals containing SrO easily precipitate, and the resistance to devitrification easily decreases.

[0076] BaO is a component that significantly improves resistance to devitrification. The lower limit of the content of BaO is preferably 0%, 0.1%, 0.5%, and particularly preferably 1%. In addition, the upper limit of the content of BaO is preferably 15%, 14%, 13%, 12%, 11%, and particularly preferably 10.5%. If the content of BaO is too small, the above effects are difficult to obtain. On the other hand, if the content of BaO is too large, the density becomes too high, and meltability easily decreases. In addition, devitrification crystals containing BaO easily precipitate, and the liquidus temperature easily increases.

[0077] ZnO is a component that improves meltability. The content of ZnO is preferably 0 to 5%, 0 to 4%, 0 to 3%, and particularly preferably 0 to 2%. If the content of ZnO is too large, the glass easily devitrifies, and in addition, the strain point easily decreases.

[0078] Zr02 is a component that improves chemical durability. The lower limit of the content of Zr02 is preferably 0%, and particularly preferably 0.01%. In addition, the upper limit of the content of Zr02 is preferably 1%, 0.5%, 0.2%, 0.1%, and particularly preferably 0.05%. If the content of Zr02 is too large, devitrification nodules of ZrSi04 easily occur.

[0079] Ti02 is a component that lowers high-temperature viscosity and improves meltability. In addition, it is a component that suppresses the effects of exposure to light. The content of Ti02 is preferably 0 to 5%, 0 to 4%, 0 to 3%, 0 to 2%, 0 to 1%, and particularly preferably 0 to 0.1%. If the content of Ti02 is too large, the glass is colored, and the transmittance easily decreases.

[0080] P205 is a component that improves the strain point, and is a component that can suppress the precipitation of devitrification crystals of alkaline earth aluminosilicate such as anorthite. The content of P205 is preferably 0 to 10%, 0 to 9%, 0 to 8%, 0 to 7%, 0 to 6%, 0 to 5%, 0 to 4%, and particularly preferably 0 to 3%. If the content of P205 is too large, the glass easily phase separates.

[0081] Sn02is a component that has a good fining effect in a high temperature region, and is a component that increases the strain point, and is a component that reduces the high temperature viscosity. In addition, it has the advantage of not corroding molybdenum electrodes. The lower limit of the content of Sn02is preferably 0.1%, and particularly preferably 0.15%. In addition, the upper limit of the content of Sn02is preferably 0.5%, 0.45%, 0.4%, 0.35%, and particularly preferably 0.3%. When the content of Sn02is too small, the above effects are difficult to enjoy. On the other hand, when the content of Sn02is too large, devitrification crystals of Sn02easily precipitate, and precipitation of devitrification crystals of Zr02is easily promoted.

[0082] From the viewpoint of the environment, and the viewpoint of preventing corrosion of the electrodes, it is preferable to substantially not contain As203, Sb203. Here, "substantially not contain" means that glass raw materials, glass scraps containing these components are not intentionally added to the glass batch. More specifically, in the obtained glass, arsenic is 50 ppm or less as As203, and antimony is 50 ppm or less as Sb203. These components are useful as fining agents, but should not be used because they corrode molybdenum electrodes, and make electric melting at an industrial scale difficult. In addition, from the viewpoint of the environment, it is also preferable not to use.

[0083] In addition to the above components, other components can be contained in a total amount of 5% or less.

[0084] In addition, Cl and F can also be contained in the glass, but the content of Cl is preferably less than 0.1%, and particularly preferably less than 0.05%, and the content of F is preferably less than 0.1%, and particularly preferably less than 0.05%. In addition, Cl + F (the total amount of Cl and F) is preferably less than 0.1%.

[0085] (5) Properties of Alkali-Free Glass Substrate

[0086] Next, the alkali-free glass substrate obtained by the method of the present application will be described.

[0087] The alkali-free glass substrate obtained by the method of the present application contains a glass having a β-OH value of 0.05 / mm or more, 0.07 / mm or more, 0.1 / mm or more, 0.12 / mm or more, 0.15 / mm or more, 0.18 / mm or more, and particularly 0.2 / mm or more. In this way, the number of metal mottle can be sufficiently reduced. Note that when the β-OH value is too large, the strain point of the glass does not sufficiently increase, and it is difficult to reduce the thermal shrinkage, so the upper limit of the β-OH value is preferably 0.4 / mm or less, and particularly preferably 0.35 / mm or less.

[0088] The alkali-free glass substrate obtained by the method of the present application contains glass having a number of metal pits of 20 pieces per ton or less, 10 pieces per ton or less, 5 pieces per ton or less, particularly 3 pieces per ton or less. Note that the lower limit of the number of metal pits is not particularly limited, and is practically 0.1 pieces per ton or more.

[0089] The alkali-free glass substrate obtained by the method of the present application preferably has a thermal shrinkage of 25 ppm or less, 20 ppm or less, 19 ppm or less, 18 ppm or less, 17 ppm or less, 16 ppm or less, 15 ppm or less, 14 ppm or less, particularly preferably 13 ppm or less, when the glass is heated from room temperature to 500°C at a rate of 5°C / min and held at 500°C for 1 hour, and then cooled at a rate of 5°C / min. If the thermal shrinkage is large, it is difficult to use as a substrate for forming an oxide TFT. Note that the lower limit of the thermal shrinkage is not limited, and is preferably 2 ppm or more, particularly preferably 3 ppm or more.

[0090] The alkali-free glass substrate obtained by the method of the present application preferably contains glass having a strain point of 690°C or higher, 700°C or higher, 705°C or higher, particularly 710°C or higher. In this way, in the manufacturing process of an oxide TFT, it is easy to suppress thermal shrinkage of the glass substrate. If the strain point is too high, the temperature at the time of molding and at the time of dissolving becomes too high, and the manufacturing cost of the glass substrate easily increases, and thus the upper limit of the strain point is preferably 750°C or lower, 740°C or lower, particularly preferably 730°C or lower.

[0091] The alkali-free glass substrate obtained by the method of the present application preferably contains glass having a viscosity at 10 2.5 dPa·s of 1630°C or lower, 1620°C or lower, 1610°C or lower, 1600°C or lower, 1590°C or lower, particularly 1580°C or lower.10 2.5 If the temperature at 10 2.5 dPa·s is too high, the glass becomes difficult to dissolve, the manufacturing cost of the glass substrate increases, and defects such as bubbles are easily generated.10 2.5 dPa·s is too low, it is difficult to design the viscosity at the liquidus temperature to be high, and thus the lower limit of the temperature at 10 2.5 The "temperature at 10

[0092] The alkali-free glass substrate obtained by the method of the present application preferably contains a glass having a liquidus temperature of less than 1250°C, less than 1240°C, less than 1230°C, less than 1220°C, less than 1210°C, particularly less than 1200°C. In this way, devitrification crystallization is less likely to occur in the manufacture of the glass, and a decrease in productivity can be prevented. Furthermore, since the glass is easily formed by the overflow down-draw method, the surface quality of the glass substrate can be improved, and the manufacturing cost of the glass substrate can be reduced. Moreover, from the viewpoint of the recent large-scale of the glass substrate and the high definition of the display, it is very important to improve the devitrification resistance in order to suppress devitrification, which can become a surface defect. Note that the liquidus temperature is an index of the devitrification resistance, and the lower the liquidus temperature, the more excellent the devitrification resistance. The "liquidus temperature" is the temperature at which devitrification (crystalline foreign matter) is confirmed in the glass after the glass powder passing through a standard sieve of 30 mesh (500 μm) and remaining on a 50 mesh (300 μm) is put in a platinum boat, and the platinum boat is kept in a temperature gradient furnace set to 1100°C to 1350°C for 24 hours.

[0093] The alkali-free glass substrate obtained by the method of the present application preferably contains a glass having a liquidus viscosity of 10 4.0 dPa-s or more, 10 4.2 dPa-s or more, 10 4.4 dPa-s or more, 10 4.5 dPa-s or more, 10 4.6 dPa-s or more, 10 4.7 dPa-s or more, 10 4.8 dPa-s or more, 10 4.9 dPa-s or more, particularly 10 5.0 dPa-s or more. In this way, devitrification is less likely to occur at the time of forming, and thus the glass substrate can be formed by the overflow down-draw method, as a result, the surface quality of the glass substrate can be improved, and the manufacturing cost of the glass substrate can be reduced. Note that the liquidus viscosity is an index of the formability, and the higher the liquidus viscosity, the more improved the formability. Note that the "liquidus viscosity" is the viscosity of the glass at the liquidus temperature, and can be measured by, for example, the platinum ball pull method.

[0094] The substrate area of the alkali-free glass substrate obtained by the method of the present application is preferably 4 m 2 or more. If the substrate area is too small, it is difficult to efficiently manufacture a large-sized LCD, OLED display having a TFT with an oxide film such as IGZO.

[0095] Example

[0096] Next, the glass manufactured using the method of the present application is described. Table 1 shows the examples (No. 1-6) of the present application.

[0097] [Table 1]

[0098] Mass % No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 SiO2 61.5 62.0 60.0 60.3 59.2 59.1 Al2O3 19.0 20.0 19.0 20.0 19.0 18.0 B2O3 2.0 2.3 3.0 4.0 6.0 7.0 MgO 5.5 2.6 3.0 3.5 2.5 3.0 CaO 4.5 4.5 5.0 5.5 6.5 6.0 SrO 5.2 1.5 1.0 2.5 1.0 2.0 BaO 2.2 6.9 8.8 4.0 5.5 4.8 SnO2 0.1 0.15 0.2 0.2 0.25 0.15 β-OH value [mm] 0.12 0.15 0.25 0.28 0.30 0.33 Metallic specks (pieces / ton) 13.8 6 1.8 1 0.3 0.3

[0099] First, the silica sand, alumina, orthoboric acid, boric anhydride, calcium carbonate, strontium carbonate, strontium nitrate, barium carbonate, and tin dioxide were mixed in a manner to become the composition of Table 1, and were prepared. In addition, glass chips having the same composition as the target composition (β-OH value of 0.2 / mm, 35 mass% used with respect to the total amount of the raw material batch) were used. Note that, with respect to the glass chips, a process of removing metal by passing the glass chips through a magnetic separator was performed twice.

[0100] Next, the glass raw material was supplied to an electric melting furnace that did not use a burner for combustion, and was melted, and then the molten glass was clarified and homogenized in a clarifying tank and a conditioning tank, and was adjusted to a viscosity suitable for forming.

[0101] Next, the molten glass was supplied to an overflow down-draw forming apparatus, was formed into a plate shape, and then was cut, thereby obtaining a glass sample of 0.5 mm thickness. Note that the molten glass that had come out of the melting furnace was supplied to the forming apparatus while being in contact with platinum or a platinum alloy alone.

[0102] With respect to the obtained glass sample, observation was performed using a 50-fold body microscope, and when metal specks of 1 μm or more were observed in the field of view, the number of metal specks per 1 ton of glass was calculated based on the size of the glass used in the measurement. The results are shown in Table 1.

[0103] As is clear from Table 1, the β-OH value of Samples No. 1-6 was as large as 0.12 / mm or more, and thus the number of metal specks was as small as 13.8 or less per ton. In addition, in Figure 2 a graph in which the β-OH value is plotted on the horizontal axis and the number of metal specks is plotted on the vertical axis is shown in FIG. 1. As is clear from Figure 2 , the larger the β-OH value, the smaller the number of metal specks.

[0104] Note that the β-OH value of the glass was determined using FT-IR by measuring the transmittance of the glass, and was calculated using the following equation.

[0105] β-OH value = (1 / X) log10(T1 / T2)

[0106] X: thickness of the glass (mm)

[0107] T1: transmittance (%) at a reference wavelength of 3846 cm -1

[0108] T2: minimum transmittance (%) near a hydroxyl absorption wavelength of 3600 cm -1 .​

Claims

1. A method for manufacturing an alkali-free glass substrate, characterized in that, The manufacturing method is a method for continuously manufacturing alkali-free glass substrates based on the SiO2-Al2O3-RO system, wherein RO is one or more of MgO, CaO, BaO, SrO, and ZnO. The manufacturing method includes: The process of preparing raw material batches in a manner that contains tin compounds and is substantially free of arsenic and antimony compounds; A process of electromelting a prepared batch of raw materials in a melting furnace that can be heated by electricity using electrodes; and The process of shaping molten glass into a plate shape using a downward pulling method. The obtained glass has a β-OH value of 0.18 / mm or higher, and the number of metallic pits containing reduced Sn is less than 5 per ton.

2. The method for manufacturing an alkali-free glass substrate according to claim 1, characterized in that, The β-OH value and the number of metallic pits in the obtained glass are adjusted by using glass raw materials and / or melting conditions.

3. The method for manufacturing an alkali-free glass substrate according to claim 1 or 2, characterized in that, It does not use radiant heating from combustion in a burner.

4. The method for manufacturing an alkali-free glass substrate according to claim 1 or 2, characterized in that, The manufacturing method is a method for manufacturing an alkali-free glass substrate that also contains B2O3 as a glass component. Orthoboric acid is used as at least a portion of the glass raw material that serves as a boron source.

5. The method for manufacturing an alkali-free glass substrate according to claim 1 or 2, characterized in that, The raw material batch contains hydroxides.

6. The method for manufacturing an alkali-free glass substrate according to claim 1 or 2, characterized in that, The manufacturing method involves adding glass shavings to a batch of raw materials to produce an alkali-free glass substrate. Glass shavings containing glass with a β-OH value of 0.05 / mm or higher are used as at least a portion of the glass shavings.

7. The method for manufacturing an alkali-free glass substrate according to claim 6, characterized in that, The process involves passing glass shards through a magnetic separator more than twice to remove metal.

8. The method for manufacturing an alkali-free glass substrate according to claim 1 or 2, characterized in that, The strain point of the obtained glass is above 690℃.

9. The method for manufacturing an alkali-free glass substrate according to claim 1 or 2, characterized in that, The resulting glass has a thermal shrinkage rate of less than 25 ppm.

10. The method for manufacturing an alkali-free glass substrate according to claim 1 or 2, characterized in that, The manufacturing method is used to manufacture a glass substrate for forming oxide TFTs.

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