JP7623629B2 - Method for manufacturing alkali-free glass substrate - Google Patents

Description

The present invention relates to a method for manufacturing an alkali-free glass substrate, and more particularly to a method for manufacturing an alkali-free glass substrate suitable for displays equipped with thin film transistors (TFTs) having an oxide film such as indium gallium zinc oxide (IGZO).

In flat panel displays, a glass substrate is generally used as the support substrate. Electrical circuit patterns such as TFTs are formed on the surface of this glass substrate. For this reason, non-alkali glass substrates that contain substantially no alkali metal components are used for this type of glass substrate so as not to adversely affect the TFTs and other components.

Known methods for forming alkali-free glass substrates include the down-draw method, typified by the overflow down-draw method.

The down-draw method is a method of stretching molten glass downward to form it into a sheet shape. The overflow down-draw method, which is one type of down-draw method, is a method of forming a glass ribbon by stretching molten glass overflowing from both sides of a forming body with a roughly wedge-shaped cross section downward. The molten glass overflowing from both sides of the forming body flows down along both sides of the forming body and joins below the forming body. Therefore, in the overflow down-draw method, the surface of the glass ribbon does not come into contact with anything other than air and is formed by surface tension, so that a glass substrate with a flat surface can be obtained without having to polish the surface after forming and without foreign matter adhering to the surface. The overflow down-draw method also has the advantage of being easy to form thin glass substrates.

Methods for melting alkali-free glass substrates include electric melting, which is described in Patent Document 1.

JP 2014-88306 A

However, the glass described in Patent Document 1 had the problem of being prone to blemishes.

The present invention was made in consideration of these circumstances, and its purpose is to provide a method for manufacturing an alkali-free glass substrate with a small number of bumps.

The method for producing alkali-free glass of the present invention is a method for continuously producing an alkali-free glass substrate of SiO ₂ -Al ₂ O ₃ -RO (RO is one or more of MgO, CaO, BaO, SrO and ZnO), which comprises the steps of preparing a raw material batch containing a tin compound and substantially free of arsenic compounds and antimony compounds, electrically melting the prepared raw material batch in a melting furnace capable of electrically heating with electrodes, and forming the molten glass into a plate by a down-draw method, and is characterized in that the β-OH value of the obtained glass is 0.05/mm or more and the number of metal particles is 20/ton or less. Here, the "metal particles" are calculated by observing with a stereomicroscope, and if metal particles of 1 μm or more are observed within the observation field, they are counted as metal particles, and the number of metal particles per ton is calculated from the size of the glass used for the measurement.

As a result of various investigations, the inventors have found that when glass is electrically melted, SnO2 in the glass is reduced by Fe, Cr, Ni, etc. contained in the raw material batch, and metal particles containing Sn precipitate in the glass, as shown in formula ₁ (x and y are coefficients, and M represents Fe, Cr, and/or Ni). Furthermore, they have also found that by increasing the β-OH value of glass, Fe, Cr, Ni, etc. bond to OH groups in the glass, as shown in formula 2 (z is a coefficient, and M represents Fe, Cr, and/or Ni), making it difficult to reduce _SnO2 and difficult to precipitate metal particles in the glass.

(Formula 1) SnO ₂ + (2x/y)M⇒ Sn+(2/y)MxOy
(Formula 2) (z)Si-OH+(z)M⇒ (z)Si-OM+(z/2)H ₂

"Alkali-free glass" refers to glass to which no alkali metal oxide components have been intentionally added, specifically glass in which the content of alkali metal oxides (Li ₂ O, Na ₂ O, and K ₂ O) in the glass composition is 2000 ppm (by mass) or less. "Continuously produced" refers to glass being continuously produced for a certain period of time in a continuous melting furnace such as a tank furnace. "SiO ₂ -Al ₂ O ₃ -RO system" refers to a glass composition system in which SiO ₂ , Al ₂ O _3, and RO are essential components. "Electric melting" refers to a melting method in which electricity is passed through glass and the glass is melted by the Joule heat generated by the passage of electricity. Note that this does not exclude melting methods that supplementarily utilize radiant heating by a heater or burner. "Substantially free of arsenic and antimony" refers to glass raw materials and glass cullets containing these components not being intentionally added to the glass batch. More specifically, it means that the resulting glass contains, on a molar basis, 50 ppm or less of arsenic as As ₂ O ₃ and 50 ppm or less of antimony as Sb ₂ O _3. The "down-draw method" is a general term for a forming method in which molten glass is formed while being continuously drawn downward.

The present invention is also characterized in that the glass is melted using electrical heating. When electrical heating is used, the amount of energy per mass required to obtain molten glass is reduced, thereby reducing the environmental impact.

In the present invention, it is preferable to adjust the β-OH value and the number of metal particles of the resulting glass by adjusting the glass raw materials and/or the melting conditions.

In the present invention, the effect is more pronounced when radiant heating by burner combustion is not used in combination. "Radiant heating by burner combustion is not used in combination" means that radiant heating by burner combustion is not used at all during normal production, and does not exclude the use of a burner during start-up of production (when the temperature is rising). It also does not exclude the use of radiant heating by a heater in combination during start-up of production or normal production. Note that "start-up of production" refers to the period from when the raw material batch melts to become molten glass and becomes electrically heated.

In the present invention, when producing an alkali-free glass substrate further containing B ₂ O ₃ as a glass composition, it is preferable to use orthoboric acid in at least a part of the glass raw material serving as a boron source.

The adoption of the above-mentioned configuration makes it easier to adjust the water content of the obtained glass. Moreover, since the boron component (B ₂ O ₃ ) is a component that improves the meltability of glass, the adoption of the above-mentioned configuration makes it easier to obtain glass with excellent productivity.

In the present invention, it is preferable that the raw material batch contains a hydroxide raw material.

In the present invention, when manufacturing an alkali-free glass substrate by adding glass cullet to a raw material batch, it is preferable to use glass cullet consisting of glass having a β-OH value of 0.05/mm or more as at least a part of the glass cullet. Here, "glass cullet" means defective glass generated during glass manufacturing, or recycled glass collected from the market. "β-OH value" refers to the value obtained by measuring the transmittance of glass using FT-IR and using the following formula.

β-OH value = (1/X)log(T ₁ /T ₂ )
X: Glass thickness (mm)
T ₁ : Transmittance (%) at a reference wavelength of 3846 cm ⁻¹
T ₂ : Minimum transmittance (%) at a hydroxyl group absorption wavelength of about 3600 cm ⁻¹

In the present invention, it is preferable to carry out the process of removing metals by passing the glass cullet through a magnetic separator two or more times.

Recycled glass cullet contains metals including Fe, Cr, Ni, etc., which are used in blending equipment. By passing the glass cullet through a magnetic separator two or more times, metals including Fe, Cr, Ni, etc. can be sufficiently removed, making it difficult for metals including Fe, Cr, Ni, etc. to be mixed into the raw material batch. As a result, metal particles including Sn are less likely to precipitate in the glass.

In the present invention, it is preferable that the strain point of the resulting glass is 690°C or higher. Here, "strain point" is a value measured based on the method of ASTM C336-71.

In the present invention, it is preferable that the thermal shrinkage of the obtained glass is 25 ppm or less. Here, "thermal shrinkage" refers to the value measured under conditions in which the glass is heated from room temperature to 500°C at a rate of 5°C/min, held at 500°C for 1 hour, and then cooled at a rate of 5°C/min.

The thermal shrinkage is measured by the following method. First, as shown in FIG. 1(a), a rectangular sample G of 160 mm×30 mm is prepared as a sample of the glass plate 1. Markings M are formed on both ends of the long side of the rectangular sample G at positions 20 to 40 mm away from the edge using #1000 waterproof abrasive paper. Then, as shown in FIG. 1(b), the rectangular sample G on which the markings M are formed is folded in two along the direction perpendicular to the markings M to prepare sample pieces Ga and Gb. Then, only one of the sample pieces Gb is subjected to a heat treatment in which the temperature is raised from room temperature (25° C.) to 500° C. at a rate of 5° C./min, and the sample is held at 500° C. for 1 hour, and then the temperature is lowered to room temperature at a rate of 5° C./min. After the above heat treatment, as shown in Fig. 1(c), a sample piece Ga that has not been subjected to heat treatment and a sample piece Gb that has been subjected to heat treatment are arranged in parallel, and the positional deviations ( _ΔL1 , _ΔL2 ) of the markings M of the two sample pieces Ga and Gb are read by a laser microscope, and the thermal shrinkage is calculated by the following formula, where _l0 is the initial distance between the markings M.

Heat shrinkage rate = [{ΔL ₁ (μm) + ΔL ₂ (μm)}×10 ³ ]/l ₀ (mm) (ppm)

By adopting the above configuration, a glass substrate suitable for forming oxide TFTs can be obtained.

In the present invention, it is preferable to use it in the manufacture of a glass substrate on which an oxide TFT is formed.

FIG. 4 is a plan view for explaining a procedure for measuring the thermal shrinkage rate of a glass substrate. 1 is a graph showing the relationship between β-OH value and metal particles.

The method for producing alkali-free glass of the present invention is described in detail below.

The method of the present invention includes the steps of preparing a raw material batch, electrically melting the prepared batch, and forming the molten glass into a sheet shape.

(1) Step of preparing raw material batch First, glass raw materials are prepared so as to have a composition of SiO ₂ —Al ₂ O ₃ —RO (RO is one or more of MgO, CaO, BaO, SrO and ZnO). A suitable glass composition will be described later.

Silica sand, stone powder (SiO ₂ ), etc. can be used as the silicon source.

As the aluminum source, alumina (Al ₂ O ₃ ), aluminum hydroxide (Al(OH) ₃ ), etc. can be used.

As the boron source, orthoboric acid (H ₃ BO ₃ ) or boric anhydride (B ₂ O ₃ ) can be used. Since orthoboric acid contains water of crystallization, when the ratio of orthoboric acid used is large, the water content of the glass can be adjusted to be relatively high. For this reason, it is preferable to use both orthoboric acid and boric anhydride and adjust the ratio of their use according to the target β-OH content.

Examples of the alkaline earth metal source that can be used include calcium carbonate ( _CaCO3 ), magnesium oxide (MgO), magnesium hydroxide (Mg(OH) ₂ ), barium carbonate ( _BaCO3 ), barium nitrate (Ba( _NO3 ) ₂ ), strontium carbonate ( _SrCO3 ), and strontium nitrate (Sr( _NO3 ) ₂ ).

Zinc oxide (ZnO) etc. can be used as a zinc source.

Zircon (ZrSiO ₄ ) and the like can be used as the zirconia source. When a Zr-containing refractory such as a zirconia electrocast refractory or dense zircon is used as the refractory material constituting the melting furnace, zirconia components may be eluted from the refractory material. These eluted components may also be used as the zirconia source.

Titanium oxide (TiO ₂ ) or the like can be used as the titanium source.

As the phosphorus source, aluminum metaphosphate (Al(PO ₃ ) ₃ ), magnesium pyrophosphate (Mg ₂ P ₂ O ₇ ), or the like can be used.

As the tin compound, tin oxide (SnO ₂ ) or the like can be used. When tin oxide is used, it is preferable to use tin oxide having an average particle size D ₅₀ in the range of 0.3 to 50 μm, 2 to 50 μm, and particularly 5 to 50 μm. If the average particle size D ₅₀ of the tin oxide powder is small, aggregation between particles occurs, and clogging is likely to occur in the blending plant. On the other hand, if the average particle size D ₅₀ of the tin oxide powder is large, the dissolution reaction of the tin oxide powder in the molten glass is delayed, and the clarification of the molten liquid does not progress. As a result, oxygen gas cannot be sufficiently released at the appropriate time of glass melting, bubbles tend to remain in the glass product, and it becomes difficult to obtain a product with excellent bubble quality. In addition, it is easy to cause a situation in which undissolved SnO ₂ crystals appear in the glass product.

In the present invention, the raw material batch may contain a carbonate raw material. The carbonate raw material can make the fining agent _SnO2 function efficiently. As the carbonate raw material, for example, calcium carbonate ( _CaCO3 ), barium carbonate ( _BaCO3 ), strontium carbonate ( _SrCO3 ), etc. can be used.

In the present invention, the raw material batch may contain a nitrate raw material. The nitrate raw material can make the fining agent _SnO2 function efficiently. As the nitrate raw material, for example, barium nitrate (Ba( _NO3 ) ₂ ), strontium nitrate (Sr( _NO3 ) ₂ ), etc. can be used.

In the present invention, the raw material batch may contain a hydroxide raw material. 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 ), etc. can be used.

In the present invention, it is preferable that the batch is substantially free of arsenic compounds and antimony compounds. If these components are present, they will corrode the electrodes, making it difficult to perform stable electrical melting over a long period of time. Furthermore, these components are undesirable from an environmental standpoint.

In the present invention, in addition to the glass raw materials described above, it is preferable to use glass cullet. When glass cullet is used, the proportion of glass cullet used relative to the total amount of the raw material batch is preferably 1 mass% or more, 5 mass% or more, and particularly 10 mass% or more. There is no upper limit to the proportion of glass cullet used, but it is preferably 50 mass% or less, 40 mass% or less, and particularly 30 mass% or less. It is also preferable that at least a part of the glass cullet used is glass cullet made 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 cullet is too high, there is a risk that the strain point of the glass will be too low, so the upper limit of the β-OH value of the glass cullet is preferably 0.4/mm or less.

In addition, it is preferable to carry out the process of removing metals by passing the glass cullet through a magnetic separator two or more times, preferably three or more times, and particularly preferably five or more times. In this way, metals including Fe, Cr, Ni, etc. that reduce _SnO2 in the glass are less likely to be mixed into the raw material batch. As a result, metal particles including Sn are less likely to precipitate. Note that, if the number of times of the above process is increased, the amount of metals including Fe, Cr, Ni, etc. that are mixed in is reduced, but from the viewpoint of cost, it is preferable that the number of times of the above process is 10 or less.

(2) Step of Electrically Melting the Prepared Raw Material Batch Next, the prepared raw material batch is placed in a melting furnace and electrically melted.

The melting furnace has multiple electrodes. By applying electricity between the electrodes, electricity flows through the molten glass, and the resulting Joule heat melts the glass continuously. Radiation heating from a heater or burner may also be used as an auxiliary method.

Molybdenum electrodes are preferably used as the electrodes because they offer a high degree of freedom in terms of the location and shape of the electrodes, and because the optimal electrode location and shape can be adopted even for non-alkali glass, which does not easily conduct electricity, and because this makes it easy to apply electrical heating. The electrode shape is preferably rod-shaped. If the electrodes are rod-shaped, it is possible to arrange the desired number of electrodes at any position on the side wall or bottom wall of the melting furnace while maintaining the desired distance between the electrodes. The electrodes are preferably arranged in pairs on the walls (side wall, bottom wall, etc.) of the melting furnace, especially on the bottom wall, with the distance between the electrodes being short. If the glass contains arsenic or antimony components, molybdenum electrodes cannot be used because they will corrode, and instead it is necessary to use tin electrodes, which are not corroded by these components. However, tin electrodes have very little freedom in terms of the location and shape of the electrodes, making it difficult to electrically melt non-alkali glass.

The raw material batch fed into the melting furnace is melted by electrical heating to become molten glass (molten glass). At this time, the tin compounds contained in the raw material batch dissolve in the molten glass and act as a fining agent. More specifically, the tin component releases oxygen bubbles during the temperature rise process. The released oxygen bubbles expand the bubbles contained in the molten glass, causing them to rise to the surface and be removed from the glass. Furthermore, the tin component absorbs the oxygen bubbles during the temperature fall process, eliminating any bubbles remaining in the glass.

The glass melted in the melting furnace is supplied to the forming device, but a fining tank, stirring tank, conditioning tank, etc. may be placed between the melting furnace and the forming device, and the glass may be passed through these tanks before being supplied to the forming device. In addition, it is preferable that at least the surface of the communication flow path connecting the melting furnace and the forming device (or each tank placed between them) that comes into contact with the glass is made of platinum or a platinum alloy to prevent contamination of the glass.

(3) Step of forming molten glass into a sheet Next, the glass molten in the melting furnace is supplied to a forming device and formed into a sheet by a down-draw method.

As the down-draw method, it is preferable to adopt the overflow down-draw method. The overflow down-draw method is a method in which molten glass is made to overflow from both sides of a trough-shaped refractory having a wedge-shaped cross section, and the overflowed molten glass is stretched downward while joining at the lower end of the trough-shaped refractory to form the glass into a plate shape. In the overflow down-draw method, the surface to be the surface of the glass substrate is not in contact with the trough-shaped refractory and is formed in a free surface state. For this reason, it is possible to inexpensively manufacture a glass substrate that is unpolished and has good surface quality, and it is also easy to make the glass larger and thinner. The structure and material of the trough-shaped refractory used in the overflow down-draw method are not particularly limited as long as they can achieve the desired dimensions and surface accuracy. In addition, the method of applying force when performing downward stretching is not particularly limited. For example, a method in which a heat-resistant roll having a sufficiently large width is rotated in contact with the glass to stretch it may be adopted, or a method in which a plurality of pairs of heat-resistant rolls are brought into contact with only the vicinity of the end face of the glass to stretch it may be adopted. In addition to the overflow downdraw method, other methods such as the slot down method can also be used.

The glass formed into a plate in this manner is then cut to a specified size and, if necessary, subjected to various chemical or mechanical processes to produce a glass substrate.

(4) Composition of alkali-free glass The composition of alkali-free glass to which the manufacturing method of the present invention can be suitably applied is, in mass%, SiO ₂ 50-70%, Al ₂ O ₃ 15-25%, B ₂ O ₃ 2-7.5%, MgO 0-10%, CaO 0-10%, SrO 0-10%, BaO 0-15%, ZnO 0-5%, ZrO ₂ 0-1%, TiO ₂ 0-5%, P ₂ O ₅ 0-10%, SnO ₂ 0.1-0.5%, and glass that does not substantially contain As ₂ O ₃ and Sb ₂ O _3. The reasons for limiting the content of each component as described above are as follows. In the description of the content of each component, the % indication represents mass % unless otherwise specified.

SiO ₂ is a component that forms the skeleton of glass. The lower limit of the content of SiO ₂ is preferably 50%, 51%, 51.5%, 52%, 55%, 56%, 57%, and particularly preferably 58%. The upper limit of the content of SiO ₂ is preferably 70%, 69%, 68%, 67%, 66%, 65%, 64%, 63%, and particularly preferably 62%. If the content of SiO ₂ is too small, the density becomes too high and the acid resistance is easily reduced. On the other hand, if the content of SiO ₂ is too large, the high-temperature viscosity becomes high and the melting property is easily reduced. In addition, devitrified crystals such as cristobalite are easily precipitated, and the liquidus temperature is easily increased.

Al ₂ O ₃ is a component that forms the skeleton of glass, increases the strain point and Young's modulus, and suppresses phase separation. The lower limit of the content of Al ₂ O ₃ is preferably 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, and particularly preferably 18%. The upper limit of the content of Al ₂ O ₃ is preferably 25%, 24%, 23%, 22%, 21.5%, and particularly preferably 21%. If the content of _{Al 2} O ₃ is too small, the strain point and Young's modulus tend to decrease, and the glass tends to separate. On the other hand, if the content of Al ₂ O ₃ is too large, devitrified crystals such as mullite and anorthite tend to precipitate, and the liquidus temperature tends to increase.

B ₂ O ₃ is a component that enhances melting property and devitrification resistance. The lower limit of the content of B ₂ O ₃ is preferably 2%, 2.2%, particularly 2.5%. The upper limit of the content of B 2 O ₃ is preferably 7.5%, particularly 7%. If the content of _{B 2 O 3} is too small, the melting property and devitrification resistance are likely to decrease, and the resistance to hydrofluoric acid-based chemicals such as buffered hydrofluoric acid is likely to decrease. There is also a risk that the amount of moisture carried over from the batch will be too small. On the other hand, if the content of B ₂ O ₃ is too large, the strain point and Young's modulus are likely to decrease.

MgO is a component that reduces high-temperature viscosity and increases melting properties, and among alkaline earth metal oxides, it is a component that significantly increases Young's modulus. The lower limit of the MgO content is preferably 0%, 0.1%, 0.5%, 1%, or 1.5%, and particularly preferably 2%. The upper limit of the MgO content is preferably 10%, 9%, 8%, 7.5%, 7%, or 6%, and particularly preferably 5%. If the MgO content is too low, the melting properties and Young's modulus are likely to decrease. On the other hand, if the MgO content is too high, the devitrification resistance is likely to decrease and the strain point is likely to decrease.

CaO is a component that reduces high-temperature viscosity and significantly improves melting properties without lowering the strain point. In addition, among alkaline earth metal oxides, CaO is a component that reduces raw material costs because the raw materials used are relatively inexpensive. The lower limit of the CaO content is preferably 0%, 0.1%, 1%, 2%, or 3%, and particularly 3.5%. The upper limit of the CaO content is preferably 10%, 9%, 8%, and particularly 7%. If the CaO content is too low, it becomes difficult to enjoy the above effects. On the other hand, if the CaO content is too high, the glass is prone to devitrification and the thermal expansion coefficient is likely to increase.

SrO is a component that suppresses phase separation and enhances devitrification resistance. Furthermore, it is a component that reduces high-temperature viscosity and enhances melting property without lowering the strain point. It is also a component that suppresses the rise in liquidus temperature. The lower limit of the SrO content is preferably 0%, 0.1%, and particularly 0.3%. The upper limit of the SrO content is preferably 10%, 9%, 8%, 7%, 6%, and particularly 5%. If the SrO content is too low, it becomes difficult to enjoy the above effects. On the other hand, if the SrO content is too high, the density becomes too high and devitrification crystals containing SrO are easily precipitated, which makes it easy for devitrification resistance to decrease.

BaO is a component that significantly improves devitrification resistance. The lower limit of the BaO content is preferably 0%, 0.1%, or 0.5%, and particularly preferably 1%. The upper limit of the BaO content is preferably 15%, 14%, 13%, 12%, or 11%, and particularly preferably 10.5%. If the BaO content is too low, it becomes difficult to obtain the above effects. On the other hand, if the BaO content is too high, the density becomes too high and the melting property tends to decrease. In addition, devitrification crystals containing BaO are more likely to precipitate, and the liquidus temperature tends to increase.

ZnO is a component that enhances meltability. The ZnO content is preferably 0-5%, 0-4%, 0-3%, and particularly 0-2%. If the ZnO content is too high, the glass becomes more susceptible to devitrification and the strain point becomes more likely to decrease.

_ZrO2 is a component that enhances chemical durability. The lower limit of the content of _ZrO2 is preferably 0%, particularly preferably 0.01%. The upper limit of the content of _ZrO2 is preferably 1%, 0.5%, 0.2%, 0.1%, particularly preferably 0.05%. If the content of _ZrO2 is too high, devitrification of _ZrSiO4 is likely to occur.

_TiO2 is a component that reduces high-temperature viscosity and enhances melting property. It is also a component that suppresses solarization. The content of _{TiO2 is preferably 0-5%, 0-4%, 0-3%, 0-2%, 0-1%, and particularly preferably 0-0.1%. If the content of TiO2 is} too high, the glass becomes colored and the transmittance is easily reduced.

P ₂ O ₅ is a component that increases the strain point and can suppress the precipitation of devitrified crystals of alkaline earth aluminosilicates such as anorthite. The content of P ₂ O ₅ 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 _{P 2} O ₅ is too high, the glass is prone to phase separation.

_SnO2 is a component that has a good clarification effect in the high temperature range, and also increases the strain point and reduces the high temperature viscosity. It also has the advantage of not corroding the molybdenum electrode. The lower limit of the _SnO2 content is preferably 0.1%, particularly 0.15%. The upper limit of the SnO2 content is preferably 0.5%, 0.45%, 0.4%, 0.35%, particularly 0.3%. If the _SnO2 content is too low, it is difficult to enjoy the above effects. On the other hand, if _{the SnO2} content is too high, the devitrified crystals of _SnO2 are easily precipitated, and the precipitation of the devitrified crystals of _ZrO2 is easily promoted.

It is preferable that As ₂ O ₃ and Sb ₂ O ₃ are not substantially contained from the viewpoint of the environment and from the viewpoint of preventing the erosion of electrodes. Here, "substantially not contained" means that glass raw materials or glass cullets containing these components are not intentionally added to the glass batch. More specifically, it means that arsenic is 50 ppm or less as As ₂ O ₃ and antimony is 50 ppm or less as Sb ₂ O ₃ in the obtained glass. These components are useful as fining agents, but should not be used because they corrode molybdenum electrodes and make electric melting on an industrial scale difficult. It is also preferable not to use them from the viewpoint of the environment.

In addition to the above ingredients, other ingredients may be contained in a total amount of up to 5%.

The glass may contain Cl and F, but the Cl content is preferably less than 0.1%, particularly less than 0.05%, and the F content is preferably less than 0.1%, particularly less than 0.05%. Also, it is preferable that Cl+F (the total amount of Cl and F) is less than 0.1%.

(5) Characteristics of the Alkali-Free Glass Substrate Next, the alkali-free glass substrate obtained by the method of the present invention will be described.

The alkali-free glass substrate obtained by the method of the present invention is made of 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, it is possible to sufficiently reduce the number of metal particles. If the β-OH value is too large, the strain point of the glass does not become sufficiently high, making it difficult to reduce the thermal shrinkage rate, so the upper limit of the β-OH value is preferably 0.4/mm or less, and particularly 0.35/mm or less.

The alkali-free glass substrate obtained by the method of the present invention is made of glass having a number of metal particles of 20 or less per ton, 10 or less per ton, 5 or less per ton, and particularly 3 or less per ton. There is no particular lower limit for the number of metal particles, but in practice it is 0.1 or more per ton.

The alkali-free glass plate obtained by the method of the present invention 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, and particularly 13 ppm or less when the glass is heated from room temperature to 500°C at a rate of 5°C/min, held at 500°C for 1 hour, and then cooled at a rate of 5°C/min. If the thermal shrinkage is large, it becomes difficult to use the glass as a substrate for forming an oxide TFT. There is no lower limit to the thermal shrinkage, but it is preferably 2 ppm or more, and particularly preferably 3 ppm or more.

The alkali-free glass plate obtained by the method of the present invention is preferably made of glass having a strain point of 690°C or higher, 700°C or higher, 705°C or higher, and particularly 710°C or higher. In this way, it becomes easier to suppress thermal shrinkage of the glass plate in the manufacturing process of the oxide TFT. If the strain point is too high, the temperature during molding and melting becomes too high, which tends to increase the manufacturing cost of the glass plate, so the upper limit of the strain point is preferably 750°C or lower, 740°C or lower, and particularly 730°C or lower.

The alkali-free glass plate obtained by the method of the present invention is preferably made of glass having a temperature at 10 ^2.5 dPa·s of 1630°C or less, 1620°C or less, 1610°C or less, 1600°C or less, 1590°C or less, particularly 1580°C or less. If the temperature at ^{10 2.5} dPa·s is too high, the glass becomes difficult to melt, the manufacturing cost of the glass plate rises, and defects such as bubbles are easily generated. If the temperature at ^{10 2.5} dPa·s is too low, it becomes difficult to design the viscosity at the liquidus temperature to be high, so the lower limit of the temperature at 10 ^2.5 dPa·s is preferably 1500°C or more, 1510°C or more, particularly 1520°C or more. The "temperature corresponding to 10 ^2.5 dPa·s" is a value measured by a platinum ball pulling method.

The alkali-free glass plate obtained by the method of the present invention is preferably made of 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, and particularly less than 1200°C. In this way, devitrification crystals are less likely to occur during glass production, making it easier to prevent a decrease in productivity. Furthermore, since it is easier to form the glass by the overflow downdraw method, it is easier to improve the surface quality of the glass plate and reduce the manufacturing cost of the glass plate. And, from the viewpoint of the recent increase in the size of glass plates and the high definition of displays, it is very important to improve devitrification resistance in order to minimize devitrification products that can become surface defects. The liquidus temperature is an index of devitrification resistance, and the lower the liquidus temperature, the better the devitrification resistance. "Liquid phase temperature" refers to the temperature at which devitrification (crystal inclusions) is observed in the glass after placing glass powder that passes through a standard sieve of 30 mesh (500 μm) and remains on the 50 mesh (300 μm) in a platinum boat and holding it in a temperature gradient furnace set at 1100°C to 1350°C for 24 hours, and then removing the platinum boat.

The alkali-free glass plate obtained by the method of the present invention is preferably made of 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 during forming, making it easier to form the glass plate by the overflow downdraw method, and as a result, it is possible to improve the surface quality of the glass plate and reduce the manufacturing cost of the glass plate. The liquidus viscosity is an index of formability, and the higher the liquidus viscosity, the better the formability. The "liquidus viscosity" refers to the viscosity of glass at the liquidus temperature, and can be measured, for example, by the platinum sphere pull-up method.

The alkali-free glass plate obtained by the method of the present invention preferably has a substrate area of 4 ^m2 or more. If the substrate area is too small, it becomes difficult to efficiently manufacture large LCD and OLED displays equipped with TFTs having an oxide film such as IGZO.

Next, we will explain the glass produced using the method of the present invention. Table 1 shows examples of the present invention (No. 1 to 6).

First, silica sand, aluminum oxide, orthoboric acid, boric anhydride, calcium carbonate, strontium carbonate, strontium nitrate, barium carbonate, and stannic oxide were mixed and prepared to obtain the composition shown in Table 1. Glass cullet with the same composition as the target composition (β-OH value 0.2/mm, used at 35% by mass relative to the total amount of the raw material batch) was also used. The glass cullet was passed through a magnetic separator twice to remove metals.

Next, the glass raw materials were fed into an electric melting furnace that did not use burners and melted, and then in a fining tank and adjustment tank, the molten glass was clarified and homogenized, and the viscosity was adjusted to a level suitable for molding.

The molten glass was then fed to an overflow downdraw forming device, formed into a plate, and cut to obtain a glass sample with a thickness of 0.5 mm. The molten glass leaving the melting furnace was fed to the forming device while in contact with only platinum or platinum alloy.

The obtained glass samples were observed under a stereo microscope at 50x magnification. If metal particles of 1 μm or more were observed within the observation field, they were counted as metal particles, and the number of metal particles per ton was calculated based on the size of the glass used for the measurement. The results are shown in Table 1.

As is clear from Table 1, samples No. 1 to 6 had a large β-OH value of 0.12/mm or more, and therefore had a small number of metal particles of 13.8 particles/ton or less. Figure 2 shows a plot of β-OH value on the horizontal axis and metal particles on the vertical axis. As is clear from Figure 2, the larger the β-OH value, the fewer the number of metal particles.

The β-OH value of the glass was calculated by measuring the transmittance of the glass using FT-IR and using the following formula.

β-OH value = (1/X)log10(T ₁ /T ₂ )
X: Glass thickness (mm)
T ₁ : Transmittance (%) at a reference wavelength of 3846 cm ⁻¹
_T2 : Minimum transmittance (%) at a hydroxyl group absorption wavelength of about 3600 cm ^-1

Claims (10)

A method for continuously producing an alkali-free glass substrate of SiO ₂ —Al ₂ O ₃ —RO (RO is one or more of MgO, CaO, BaO, SrO and ZnO), comprising the steps of:
preparing a feed batch containing tin compounds and substantially free of arsenic and antimony compounds;
A step of electrically melting the prepared raw material batch in a melting furnace capable of electrically heating using electrodes;
and forming the molten glass into a sheet shape by a down-draw method.
A method for producing an alkali-free glass substrate, characterized in that the β-OH value of the obtained glass is 0.18/mm or more and the number of metal particles containing reduced Sn is 5/ton or less . The method for producing an alkali-free glass substrate according to claim 1, characterized in that the β-OH value and the number of metal particles of the resulting glass are adjusted by the glass raw materials and/or the melting conditions. The method for producing an alkali-free glass substrate according to claim 1 or 2, characterized in that no radiant heating by burner combustion is used in combination. The method for producing an alkali-free glass substrate according to any one of claims 1 to 3, further comprising the step of: as a glass composition, producing an alkali-free glass substrate further containing B ₂ O ₃ ; and using orthoboric acid in at least a part of a glass raw material serving as a boron source. The method for manufacturing an alkali-free glass substrate according to any one of claims 1 to 4, characterized in that the raw material batch contains a hydroxide raw material. A method for producing an alkali-free glass substrate by adding glass cullet to a raw material batch, characterized in that at least a portion of the glass cullet is made of glass having a β-OH value of 0.05/mm or more, according to any one of claims 1 to 5. The method for producing an alkali-free glass substrate according to claim 6, characterized in that the step of removing metals by passing the glass cullet through a magnetic separator is carried out two or more times. The method for producing an alkali-free glass substrate according to any one of claims 1 to 7, characterized in that the strain point of the resulting glass is 690°C or higher. The method for producing an alkali-free glass substrate according to any one of claims 1 to 8, characterized in that the thermal shrinkage of the resulting glass is 25 ppm or less. The method for manufacturing an alkali-free glass substrate according to any one of claims 1 to 9, characterized in that it is used for manufacturing a glass substrate on which an oxide TFT is formed.