阅读说明：本技术 无碱玻璃板 (Alkali-free glass plate ) 是由 虫明笃 齐藤敦己 林昌宏 村田隆 于 2019-10-07 设计创作，主要内容包括：本发明的无碱玻璃板的特征在于,作为玻璃组成,以摩尔％计含有SiO-2 60～74％、Al-2O-3 6～20％、B-2O-3 0～9％、MgO 1～13％、CaO 1～13％、SrO 0～7％、BaO 0～8％、Y-2O-3+La-2O-3 0％以上且小于1.0％,实质上不含碱金属氧化物,且应变点为650℃以上。(The alkali-free glass plate of the present invention is characterized by containing SiO in mol% as a glass composition 2 60～74％、Al 2 O 3 6～20％、B 2 O 3 0～9％、MgO 1～13％、CaO 1～13％、SrO 0～7％、BaO 0～8％、Y 2 O 3 +La 2 O 3 0% or more and less than 1.0%, substantially free of alkali metal oxide, and having a strain point of 650 ℃ or more.)

1. An alkali-free glass plate characterized by containing SiO in mol% as a glass composition₂60％～74％、Al₂O₃ 6％～20％、B₂O₃ 0％～9％、MgO 1％～13％、CaO 1％～13％、SrO 0％～7％、BaO 0％～8％、Y₂O₃+La₂O₃0% or more and less than 1.0%, substantially free of alkali metal oxide, and having a strain point of 650 ℃ or more.

2. The alkali-free glass sheet of claim 1, wherein the SrO + BaO is present in an amount of 0 to 3 mol%.

3. The alkali-free glass sheet according to claim 1 or 2, wherein the strain point is 700 ℃ or higher.

4. The alkali-free glass sheet according to any one of claims 1 to 3, wherein the Young's modulus is 79GPa or more.

5. The alkali-free glass sheet according to any one of claims 1 to 4, wherein the coefficient of thermal expansion is 30 x 10^-7/℃～45×10^-7/℃。

6. The alkali-free glass sheet according to any one of claims 1 to 5, wherein the high temperature viscosity is 10^2.5The temperature at dPa · s is 1600 ℃ or lower.

Technical Field

The present invention relates to an alkali-free glass plate, and more particularly to an alkali-free glass plate suitable for a substrate for forming a TFT circuit in a flat panel display such as a liquid crystal display or an organic EL display, or a carrier glass for holding a resin substrate for forming a TFT circuit.

Background

As is well known, a liquid crystal panel or an organic EL panel includes a Thin Film Transistor (TFT) for driving control.

As a thin film transistor for driving a display, amorphous silicon, low-temperature polysilicon, high-temperature polysilicon, and the like are known. In recent years, with the spread of large-sized liquid crystal displays, smart phones, tablet PCs, and the like, there has been an increasing demand for higher resolution of displays. The low-temperature polysilicon TFT can meet the requirement, but is subjected to a high-temperature film forming process at 500-600 ℃. However, the conventional glass plate has large thermal shrinkage before and after the high-temperature film forming process, and thus causes a pattern shift of the thin film transistor. Therefore, a glass plate with low thermal shrinkage is required for high resolution of a display. In recent years, further improvement in definition of a display has been studied, and in this case, it is required to further reduce thermal shrinkage of a glass plate.

Documents of the prior art

Patent document

Patent document 1: japanese patent No. 5769617

Disclosure of Invention

Problems to be solved by the invention

As methods for reducing the thermal shrinkage of the glass sheet, two methods are mainly used. The first method is a method of holding a glass plate in advance at a temperature near the heat treatment temperature of the film forming process and annealing the same. In this method, the glass structure relaxes and shrinks during annealing, and therefore the amount of thermal shrinkage in the subsequent high-temperature film formation process can be suppressed. However, this method increases the number of manufacturing steps and the manufacturing time, and thus the manufacturing cost of the glass plate increases.

The second method is a method of increasing the strain point of a glass sheet. The overflow downdraw process typically cools from the melting temperature to the forming temperature in a relatively short period of time. Under the influence of this, the virtual temperature of the glass sheet becomes high, and the thermal shrinkage of the glass sheet becomes large. Therefore, if the strain point of the glass plate is increased, the viscosity of the glass plate at the heat treatment temperature of the film forming process increases, and the structure is less likely to be relaxed. As a result, thermal shrinkage of the glass sheet can be suppressed. Further, the higher the heat treatment temperature in the film formation process, the greater the effect of increasing the strain point on reducing the thermal shrinkage. Therefore, in the case of a low-temperature polysilicon TFT, it is desirable to make the strain point of the glass plate as high as possible.

For example, patent document 1 discloses a method for reducing B in the glass composition₂O₃The content (c) of (b) is such that the strain point of the glass sheet is increased. Further, patent document 1 discloses Y introduced into the glass composition₂O₃And/or La₂O₃To avoid low B₂O₃The amount is reduced with the decrease in devitrification resistance. However, since Y is₂O₃And La₂O₃Since they are rare earth elements, the cost of raw materials is high, and the manufacturing cost of the glass sheet is high.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an alkali-free glass plate having a high strain point and capable of reducing the production cost.

Means for solving the problems

The present inventors have made extensive studies and as a result, have found that the above-mentioned technical problems can be solved by strictly limiting the content of each component and limiting the strain point to a predetermined value or more, and have come to be proposed as the present invention. That is, the alkali-free glass plate of the present invention is characterized by containing SiO in mol% as a glass composition₂ 60～74％、Al₂O₃ 6～20％、B₂O₃0～9％、MgO 1～13％、CaO 1～13％、SrO 0～7％、BaO 0～8％、Y₂O₃+La₂O₃0% or more and less than 1.0%, substantially free of alkali metal oxide, and having a strain point of 650 ℃ or more. Here, "Y₂O₃+La₂O₃"means Y₂O₃And La₂O₃The total amount of (a) and (b). The term "substantially free of alkali metal oxide" means an alkali metal oxide (Li) in the glass composition₂O、Na₂O、K₂O) is less than 0.5 mol% (preferably less than 0.1 mol%). "Strain Point" means measured according to ASTM C336The resulting value.

In addition, the alkali-free glass sheet of the present invention preferably contains SrO + BaO in an amount of 0 to 3 mol%. Here, "SrO + BaO" means the total amount of SrO and BaO.

The alkali-free glass sheet of the present invention preferably has a strain point of 700 ℃ or higher.

Further, the alkali-free glass plate of the present invention preferably has a Young's modulus of 79GPa or more. Here, the "young's modulus" can be measured by a bending resonance method.

Further, the alkali-free glass plate of the present invention preferably has a thermal expansion coefficient of 30X 10^-7～45×10^-7V. C. In this way, it is possible to suppress occurrence of local dimensional changes in the glass plate due to temperature unevenness in the high-temperature film formation process.

Further, the alkali-free glass sheet of the present invention preferably has a high-temperature viscosity of 10^2.5The temperature at dPa · s is 1600 ℃ or lower. This can reduce the melting cost.

Detailed Description

The alkali-free glass plate of the present invention is characterized by containing SiO in mol% as a glass composition₂ 60～74％、Al₂O₃ 6～20％、B₂O₃ 0～9％、MgO 1～13％、CaO 1～13％、SrO 0～7％、BaO 0～8％、Y₂O₃+La₂O₃0% or more and less than 1.0%, and substantially no alkali metal oxide. The reason why the contents of the respective components are limited as described above will be described below. In the description of the content of each component, the expression% means mol% unless otherwise specified.

SiO₂The component forming the glass skeleton is a component for increasing the strain point. Thus, SiO₂The content of (b) is preferably 60% or more, 62% or more, 64% or more, particularly 66% or more. On the other hand, if SiO₂When the content (b) is too large, the high-temperature viscosity increases and the meltability tends to decrease. Thus, SiO₂The content of (b) is preferably 74% or less, 72% or less, 70% or less, particularly 68% or less.

Al₂O₃The component forming the glass skeleton is a component for increasing the strain point, and is a component for suppressing phase separation. Thus, Al₂O₃The content of (b) is preferably 6% or more, 8% or more, 10% or more, particularly 12% or more. On the other hand, if Al₂O₃When the content (b) is too large, the high-temperature viscosity increases and the meltability tends to decrease. Thus, Al₂O₃The content of (b) is preferably 20% or less, 18% or less, 16% or less, particularly 14% or less.

B₂O₃Is an optional component, and is a component that significantly improves the meltability. Thus, B₂O₃The content of (b) is preferably 0% or more, 0.01% or more, 0.1% or more, 0.2% or more, 0.3% or more, 0.4% or more, particularly 0.5% or more. On the other hand, if B₂O₃If the content of (A) is too large, the strain point is greatly reduced, and the beta-OH value is greatly increased. More specifically, as described later, when the β -OH value increases, the thermal shrinkage in a temperature range equal to or lower than the strain point increases. Thus, B₂O₃The content of (b) is preferably 9% or less, 7% or less, 5% or less, particularly 4% or less.

Molar ratio of SiO₂/B₂O₃Preferably 50 or less, 40 or less, 30 or less, 25 or less, and particularly preferably 20 or less. SiO in terms of molar ratio₂/B₂O₃If it is too large, it is difficult to achieve both high strain point and high melting property. Note that "SiO" is used₂/B₂O₃"means SiO₂Is divided by B₂O₃The content of (b).

MgO is a component that reduces high-temperature viscosity and improves meltability, and improves devitrification resistance by balancing with other components. In addition, the component is a component for remarkably improving the young's modulus in view of mechanical properties. Therefore, the content of MgO is preferably 1% or more, 3% or more, 5% or more, 6% or more, and particularly 7% or more. On the other hand, if the content of MgO is too large, the strain point tends to decrease, or the composition is unbalanced with other components and the devitrification tendency tends to increase. Therefore, the content of MgO is preferably 15% or less, 13% or less, 10% or less, and particularly 8% or less.

CaO is a component that reduces high-temperature viscosity to improve meltability, and improves devitrification resistance in balance with other components. Therefore, the content of CaO is preferably 1% or more, 3% or more, particularly 5% or more. On the other hand, if the content of CaO is too large, the strain point is easily lowered. Therefore, the content of CaO is preferably 15% or less, 13% or less, 10% or less, 8% or less, and particularly 7% or less.

SrO is a component that reduces high-temperature viscosity and improves meltability, and is a component that improves devitrification resistance in balance with other components. Therefore, the SrO content is preferably 0% or more, 0.5% or more, 1% or more, particularly 1, 5% or more. On the other hand, if the SrO content is too large, the strain point is likely to decrease. Therefore, the SrO content is preferably 7% or less, 5% or less, 3% or less, and particularly 2% or less.

BaO is a component that reduces high-temperature viscosity and improves meltability, and is a component that improves devitrification resistance by balancing with other components. Therefore, the content of BaO is preferably 0% or more, 0.5% or more, 1% or more, and particularly 1.5% or more. On the other hand, if the content of BaO is too large, the strain point is liable to decrease. Therefore, the content of BaO is preferably 8% or less, 6% or less, 4% or less, 3% or less, and particularly 2% or less.

The total amount of SrO and BaO is preferably 0% or more, 0.5% or more, 1% or more, 1.2% or more, 1.4% or more, and particularly 1.6% or more. If the total amount of SrO and BaO is too small, the meltability tends to decrease. On the other hand, if the total amount of SrO and BaO is too large, the compositional balance of the glass composition is impaired, and the devitrification resistance is liable to decrease. Therefore, the total amount of SrO and BaO is preferably 6% or less, 4% or less, 3% or less, 2.5% or less, 2.2% or less, and particularly 2% or less.

Molar ratio B₂O₃the/BaO is preferably 1 or more, 2 or more, 5 or more, 8 or more, particularly 10 or more. In a molar ratio B₂O₃if/BaO is too small, the glass composition becomes unbalanced in the glass system of the present application, and the devitrification resistance is liable to decrease. In addition, "B" is₂O₃The term "BaO" meansB₂O₃Is divided by the content of BaO.

The molar ratio BaO/(SrO + BaO) is preferably 1 or less, 0.8 or less, 0.6 or less, 0.4 or less, and particularly 0.2 or less. If the molar ratio BaO/(SrO + BaO) is too large, the glass composition becomes unbalanced in the glass system of the present application, and the devitrification resistance is liable to decrease. "BaO/(SrO + BaO)" means a value obtained by dividing the content of BaO by the total amount of SrO and BaO.

Molar ratio (SiO)₂+Al₂O₃+B₂O₃) preferably,/(SrO + BaO) is 10 or more, 15 or more, 20 or more, 25 or more, particularly 30 or more. Molar ratio (SiO)₂+Al₂O₃+B₂O₃) If SrO + BaO is too small, it is difficult to achieve both a high strain point and a high young's modulus. "(SiO)₂+Al₂O₃+B₂O₃) /(SrO + BaO) "means SiO₂、Al₂O₃And B₂O₃Is divided by the total amount of SrO and BaO.

If (CaO + SrO + BaO) - (Al)₂O₃+B₂O₃) When the amount is too large, the amount of non-bridging oxygen in the glass increases, and the heat shrinkage ratio increases. Thus, (CaO + SrO + BaO) - (Al)₂O₃+B₂O₃) Preferably 5% or less, 3% or less, 1% or less, 0% or less, -1% or less, -3% or less, particularly-4% or less. On the other hand, if (CaO + SrO + BaO) - (Al)₂O₃+B₂O₃) If the amount is too small, the strain point becomes low, and the heat shrinkage ratio becomes large. Thus, (CaO + SrO + BaO) - (Al)₂O₃+B₂O₃) Preferably-20% or more, -15% or more, -10% or more, -7% or more, particularly-6% or more. Note that "(CaO + SrO + BaO) - (Al)₂O₃+B₂O₃) "means that Al is subtracted from the total amount of CaO, SrO and BaO₂O₃And B₂O₃The sum of (a) and (b).

Y₂O₃The amount of the component for increasing the strain point and Young's modulus is too large, and the density is highThe degree and the raw material cost are liable to increase. Thus, Y₂O₃The content of (B) is preferably 0 to 0.8%, 0 to 0.7%, 0 to 0.5%, 0 to 0.2%, particularly 0% or more and less than 0.1%.

La₂O₃The amount of the component is too large, which tends to increase the density and the raw material cost. Thus, La₂O₃The content of (B) is preferably 0 to 0.8%, 0 to 0.7%, 0 to 0.5%, 0 to 0.2%, particularly 0% or more and less than 0.1%.

Y₂O₃And La₂O₃The total amount of (A) is preferably 0% or more and less than 1.0%, 0 to 0.8%, 0 to 0.7%, 0 to 0.5%, 0 to 0.2%, particularly 0% or more and less than 0.1%. However, if Y₂O₃And La₂O₃When the total amount of (A) is too large, the density and the raw material cost tend to increase.

The alkali-free glass sheet of the present invention may contain the following components in addition to the above components in the glass composition.

Although ZnO is a component for improving the meltability, when ZnO is contained in a large amount, the glass is easily devitrified and the strain point is easily lowered. The content of ZnO is preferably 0 to 5%, 0 to 3%, 0 to 0.5%, 0 to 0.3%, particularly 0 to 0.2%.

P₂O₅A component whose liquid phase temperature is significantly lowered for devitrifying and crystallizing Al while maintaining the strain point, but if P is contained in a large amount₂O₅The Young's modulus decreases, or the glass phase separates. In addition, P may diffuse from the glass to affect the performance of the TFT. Thus, P₂O₅The content of (B) is preferably 0 to 5%, 0 to 3%, 0 to 1%, particularly 0 to 0.5%.

TiO₂A component for reducing high-temperature viscosity and improving meltability and a component for suppressing solarization, but if TiO is contained in a large amount₂The glass is colored and the transmittance is liable to decrease. Thus, TiO₂The content of (B) is preferably 0 to 3%, 0 to 1%, 0 to 0.1%, particularly 0 to 0.02%.

SnO₂Is a component having a good clarifying action in a high-temperature region, andis a component that increases the strain point and reduces the high-temperature viscosity. SnO₂The content of (B) is preferably 0 to 1%, 0.001 to 1%, 0.05 to 0.5%, particularly 0.08 to 0.2%. If SnO₂When the content of (A) is too large, SnO₂Devitrified crystals of (2) are liable to precipitate. If SnO is required₂The content of (b) is less than 0.001%, the above-mentioned effects are hardly enjoyed.

SnO₂Suitable as fining agents, SnO may also be used as long as the glass properties are not substantially impaired₂Other clarifying agents. Specifically, As may be added in a total amount₂O₃、Sb₂O₃、CeO₂、F₂、Cl₂、SO₃For example, the total amount of C and C may be 0.5%, or metal powder such as Al and Si may be added to the total amount of C and C, for example, 0.5%.

As₂O₃And Sb₂O₃Is excellent, but from the viewpoint of environment, it is preferable not to introduce the compound as much as possible. Further, if the glass contains As in a large amount₂O₃It tends to have a reduced resistance to solarization, and therefore, As₂O₃The content of (b) is preferably 0.5% or less, particularly 0.1% or less, and ideally substantially none. Here, "substantially not containing As₂O₃", means As in the glass composition₂O₃The content of (B) is less than 0.05%. In addition, Sb₂O₃The content of (b) is preferably 1% or less, particularly 0.5% or less, and ideally substantially none. Here, "substantially no Sb contained₂O₃", means Sb in the glass composition₂O₃The content of (B) is less than 0.05%.

Cl has an effect of promoting melting of the alkali-free glass, and when Cl is added, the melting temperature can be lowered and the action of the fining agent can be promoted, and as a result, the melting cost can be reduced and the life of the glass manufacturing furnace can be prolonged. However, if the Cl content is too large, the strain point is likely to decrease. Therefore, the Cl content is preferably 0.5% or less, particularly 0.1% or less. As a raw material for introducing Cl, a chloride of an alkaline earth metal oxide such as strontium chloride, aluminum chloride, or the like can be used.

The alkali-free glass sheet of the present invention preferably has the following characteristics.

The thermal expansion coefficient is preferably 30X 10^-7～45×10^-7/℃、30×10^-7～42×10^-7/℃、30×10^-7～40×10^-7/℃、30×10^-7～38×10^-7/° C, especially 30X 10^-7～36×10^-7V. C. If the thermal expansion coefficient is too high, local dimensional changes tend to occur in the glass plate due to temperature variations in the high-temperature film formation process.

The density is preferably 2.80g/cm³2.75g/cm below³2.70g/cm or less³2.65g/cm below³2.60g/cm below³2.55g/cm below³Below, especially 2.45 to 2.50g/cm³. If the density is too high, the amount of deflection of the glass plate increases, and thus pattern shift due to stress is likely to be promoted in a manufacturing process of a display or the like.

The strain point is preferably 650 ℃ or higher, 680 ℃ or higher, 700 ℃ or higher, 710 ℃ or higher, 720 ℃ or higher, 730 ℃ or higher, and particularly 740 ℃ or higher. If the strain point is too low, the glass sheet is likely to thermally shrink in the high-temperature film forming process.

The annealing point is preferably 720 ℃ or higher, 750 ℃ or higher, 780 ℃ or higher, and particularly 800 ℃ or higher. If the annealing point is too low, the glass sheet is likely to thermally shrink in the high-temperature film forming process.

The softening point is preferably 940 ℃ or higher, 960 ℃ or higher, 980 ℃ or higher, particularly 1000 ℃ or higher. If the softening point is too low, the glass sheet is likely to thermally shrink in the high-temperature film forming process.

High temperature viscosity 10^2.5The temperature at dPa · s is preferably 1656 ℃ or lower, 1620 ℃ or lower, 1600 ℃ or lower, 1590 ℃ or lower, 1580 ℃ or lower, particularly 1570 ℃ or lower. If 10^2.5When the temperature at dpas is high, the meltability and the fining property tend to be low, and the production cost of the glass sheet increases.

The Young's modulus is preferably 76GPa or more, 78GPa or more, 79GPa or more, 80GPa or more, 81GPa or more, 82GPa or more, particularly 83GPa or more. If the young's modulus is too low, the deflection amount of the glass plate becomes large, and thus pattern shift due to stress is likely to be promoted in a manufacturing process of a display or the like.

The specific Young's modulus is preferably 29GPa/g cm^-3Above 30GPa/g cm^-3Above 31GPa/g cm^-3Above 32GPa/g cm^-3Above, especially 33GPa/g cm^-3The above. If the young's modulus is too low, the amount of deflection of the glass plate tends to increase, and thus pattern shift due to stress tends to be promoted in a manufacturing process of a display or the like.

The liquid phase temperature is preferably 1450 ℃ or lower, 1300 ℃ or lower, 1200 ℃ or lower, particularly 1150 ℃ or lower. When such a method is used, devitrification crystals are less likely to occur during molding. Further, since the glass sheet can be easily formed by the overflow down-draw method, the surface quality of the glass sheet can be easily improved, and the manufacturing cost of the glass sheet can be reduced. The "liquidus temperature" refers to a temperature at which the glass powder passing through a standard sieve of 30 mesh (500 μm) and remaining in a 50 mesh (300 μm) is put into a platinum boat and kept in a temperature gradient furnace for 24 hours, followed by crystallization.

The liquid phase viscosity is preferably 10^4.610 dPas or more^5.010 dPas or more^5.2dPas or more, especially 10^5.5dPas or more. In this way, devitrification is less likely to occur during molding, and therefore, molding by the overflow down-draw method is easier, and as a result, the surface quality of the glass sheet can be improved, and the manufacturing cost of the glass sheet can be reduced. The "liquidus viscosity" refers to the viscosity of glass at a liquidus temperature, and can be measured by the platinum ball pulling method.

The β -OH value is an index indicating the amount of water in the glass, and when the β -OH value is decreased, the strain point can be increased. In addition, even when the glass composition is the same, the thermal shrinkage in a temperature range of not more than the strain point becomes smaller in the case of a smaller β -OH value. The beta-OH value is preferably 0.30/mm or less, 0.25/mm or less, 0.20/mm or less, 0.15/mm or less, particularly 0.10/mm or less. If the β -OH value is too small, the meltability tends to be low. Therefore, the beta-OH value is preferably 0.01/mm or more, particularly 0.03/mm or more. Here, the "β — OH value" refers to a value obtained by measuring the transmittance of glass using FT-IR and using the following equation 1.

[ mathematical formula 1]

beta-OH value ═ (1/X) log (T)₁/T₂)

X: plate thickness (mm)

T₁: reference wavelength 3846cm^-1Transmittance (%) of

T₂: hydroxyl absorption wavelength of 3600cm^-1Near minimum transmittance (%)

The alkali-free glass plate of the present invention preferably has an overflow converging surface at the center in the plate thickness direction. That is, it is preferably formed by the overflow downdraw method. The overflow down-draw method is a method of forming a flat plate shape by overflowing molten glass from both sides of a wedge-shaped refractory, joining the overflowing molten glass at the lower end of the wedge, and extending and forming the molten glass downward. In the overflow down-draw method, the surface of the glass sheet to be formed is formed in a free surface state without contacting the refractory. Therefore, a glass plate having good surface quality and no polishing can be produced at low cost. Further, it is easy to increase the area and reduce the thickness.

In addition to the overflow downdraw method, the sheet can be formed by, for example, a slot down-draw method, a redraw method, a float method, or a roll-out method.

In the alkali-free glass sheet of the present invention, the sheet thickness is not particularly limited, but is preferably 1.0mm or less, 0.7mm or less, 0.5mm or less, and particularly 0.05 to 0.4 mm. As the thickness is smaller, the liquid crystal panel or the organic EL panel is more easily reduced in weight. The sheet thickness can be adjusted by the flow rate, forming speed (drawing speed), and the like in glass production.

The method for industrially producing the alkali-free glass sheet of the present invention preferably comprises: a melting step of putting a glass batch prepared from the glass composition into a melting furnace, and heating the glass batch by energization of a heating electrode to obtain molten glass; and a molding step of molding the obtained molten glass into an alkali-free glass sheet by an overflow down-draw method.

The glass sheet production process generally includes a melting process, a fining process, a supplying process, a stirring process, and a forming process. The melting step is a step of melting a glass batch containing glass raw materials to obtain molten glass. The fining step is a step of fining the molten glass obtained in the melting step by the action of a fining agent or the like. The supply step is a step of conveying molten glass between the steps. The stirring step is a step of stirring and homogenizing the molten glass. The forming step is a step of forming the molten glass into a glass plate. If necessary, a step other than the above-described steps, for example, a state adjustment step of adjusting the molten glass to a state suitable for forming, may be incorporated into the stirring step.

In the case of industrially producing an alkali-free glass sheet, the glass sheet is generally melted by heating with a combustion flame of a burner. The burner is generally disposed above the melting furnace, and fossil fuel, specifically, liquid fuel such as heavy oil, gas fuel such as LPG, or the like is used as fuel. The combustion flame may be obtained by mixing fossil fuel with oxygen. However, in this method, a large amount of water is mixed into the molten glass during melting, and therefore the β — OH value is likely to increase. Therefore, in the production of the alkali-free glass sheet, it is preferable to perform electric heating by the heating electrode, and it is more preferable to melt, that is, to completely electrically melt, by electric heating by the heating electrode without heating by the combustion flame of the burner. Therefore, the beta-OH value is easily limited to 0.30/mm or less, 0.25/mm or less, 0.20/mm or less, 0.15/mm or less, and particularly 0.10/mm or less because water is not easily mixed into the molten glass during melting. Further, when the electric heating by the heating electrode is performed, energy per unit mass for obtaining the molten glass is reduced, and the amount of the volatile matter to be melted is reduced, so that the environmental load can be reduced.

In addition, in the electric heating, the smaller the moisture content in the glass batch, the easier the β -OH value in the glass sheet is to be reduced. And, B₂O₃The introduced raw material (2) is likely to be the largest source of mixing of moisture. Therefore, from the viewpoint of producing an alkali-free glass plate having a low β -OH value, it is preferable to use B₂O₃The content of (A) is as small as possible. Further, as the amount of moisture in the glass batch material is smaller, the glass batch material is more likely to spread uniformly in the melting furnace, and thus a homogeneous and high-quality glass sheet is easily produced.

The energization heating by the heating electrode is preferably performed by: an alternating voltage is applied to a heating electrode provided at the bottom or side of the melting furnace so as to be in contact with the molten glass in the melting furnace. The material used for the heating electrode is preferably a material having heat resistance and corrosion resistance to molten glass, and for example, tin oxide, molybdenum, platinum, rhodium, or the like can be used.

Examples

The present invention will be described below based on examples. However, the following examples are only illustrative. The present invention is not limited to the following examples.

Tables 1 to 28 show examples of the present invention (sample Nos. 1 to 391).

[ Table 1]

[ Table 2]

[ Table 3]

[ Table 4]

[ Table 5]

[ Table 6]

[ Table 7]

[ Table 8]

[ Table 9]

[ Table 10]

[ Table 11]

[ Table 12]

[ Table 13]

[ Table 14]

[ Table 15]

[ Table 16]

[ Table 17]

[ Table 18]

[ Table 19]

[ Table 20]

[ Table 21]

[ Table 22]

[ Table 23]

[ Table 24]

[ Table 25]

[ Table 26]

[ Table 27]

[ Table 28]

First, a glass batch prepared by blending glass raw materials so as to have a glass composition shown in the table was put into a platinum crucible, and then melted at 1600 to 1650 ℃ for 24 hours. When the glass batch was melted, the mixture was homogenized by stirring with a platinum stirring rod. Next, the molten glass was caused to flow out onto a carbon plate to be formed into a plate shape, and then annealed at a temperature near the annealing point for 30 minutes. For each of the obtained samples, the thermal expansion coefficient, density, strain point, annealing point, softening point, high temperature viscosity were 10^4.0Temperature, high temperature viscosity at dPa · s 10^3.0Temperature, high temperature viscosity at dPa · s 10^2.5Temperature, Young's modulus, and specific Young's modulus at dPa · s were evaluated. To be explainedSome of the glass characteristics are estimated values calculated from past data, not actual measured values.

The thermal expansion coefficient is a value obtained by measuring an average thermal expansion coefficient in a temperature range of 30 to 380 ℃ by using an dilatometer.

The density is a value measured by a known archimedes method.

The strain point, annealing point, and softening point are values measured by the methods of ASTM C336 and C338.

High temperature viscosity 10^4.0dPa s, high temperature viscosity 10^3.0dPa s, high temperature viscosity 10^2.5The temperature at dPa · s is a value measured by a platinum ball pulling method.

The young's modulus is a value measured by a flexural resonance method.

The specific young's modulus is a value obtained by dividing the young's modulus by the density.

As is clear from the table, sample Nos. 1 to 391 contained no Y in the glass composition₂O₃And La₂O₃And a strain point of 650 ℃ or higher. Therefore, sample Nos. 1 to 391 are considered to be suitable for a substrate for forming a TFT circuit in a flat panel display such as a liquid crystal display and an organic EL display, or a carrier glass for holding a resin substrate for forming a TFT circuit.