WO2015029870A1

WO2015029870A1 - Method for production of non-alkali glass

Info

Publication number: WO2015029870A1
Application number: PCT/JP2014/071887
Authority: WO
Inventors: 博文 ▲徳▼永; 和孝小野
Original assignee: 旭硝子株式会社
Priority date: 2013-08-26
Filing date: 2014-08-21
Publication date: 2015-03-05
Also published as: JP2016188147A; CN105492395B; TW201512122A; KR20160046809A; CN105492395A; CN109970318A

Abstract

　The present invention pertains to a method for production of non-alkali glass in which a glass starting material is loaded into a melting furnace and heated to a temperature of 1350 to 1750°C to form molten glass, and the molten glass is thereafter molded into a plate shape using a float glass process, the method for production of non-alkali glass comprising selecting the glass starting material and a refractory material so that Rb > Rg, using for heating in the melting furnace both heating by a combustion flame of a burner and energizing heating of the molten glass using a heating electrode disposed so as to be immersed in the molten glass inside the melting furnace, where Rg (Ωcm) is the electrical resistivity at T_3.3, which is the clarifying temperature of the molten glass (the temperature at which glass viscosity is 10^3.3dPa・s; units: °C), and Rb (Ωcm) is the electrical resistivity of the refractory material constituting the melting furnace at T_3.3.

Description

Method for producing alkali-free glass

The present invention relates to a method for producing alkali-free glass suitable as various display substrate glasses and photomask substrate glasses.
Hereinafter, in the present specification, the term “non-alkali” means that the content of alkali metal oxides (Li ₂ O, Na ₂ O, K ₂ O) is 2000 ppm or less.

Conventionally, the following characteristics have been required for various display substrate glasses, particularly those in which a metal or oxide thin film is formed on the surface.
(1) When alkali metal oxide is contained, alkali metal ions diffuse into the thin film and deteriorate the film characteristics, so that the content of alkali metal oxide is extremely low. The oxide content is 2000 ppm or less.
(2) When exposed to a high temperature in the thin film forming process, the strain point is high so that the deformation (thermal shrinkage) associated with glass deformation and glass structural stabilization can be minimized.

(3) Sufficient chemical durability against various chemicals used for semiconductor formation. In particular, buffered hydrofluoric acid (BHF: liquid mixture of hydrofluoric acid and ammonium fluoride) for etching SiO _x and SiN _x , and chemicals containing hydrochloric acid used for etching ITO, various acids used for etching metal electrodes (Nitric acid, sulfuric acid, etc.) Resistant to alkali of resist stripping solution.
(4) There are no defects (bubbles, striae, inclusions, pits, scratches, etc.) inside and on the surface.

In addition to the above requirements, in recent years, there are the following situations.
(5) The weight reduction of the display is required, and the glass itself is desired to have a low density glass.
(6) A reduction in the weight of the display is required, and a reduction in the thickness of the substrate glass is desired.

(7) In addition to the conventional amorphous silicon (a-Si) type liquid crystal display, a polycrystalline silicon (p-Si) type liquid crystal display having a slightly higher heat treatment temperature has been produced (a-Si). : About 350 ° C. → p-Si: 350 to 550 ° C.).
(8) A glass having a small average thermal expansion coefficient is required to increase productivity and thermal shock resistance by increasing the temperature raising / lowering rate of the heat treatment for producing a liquid crystal display.

On the other hand, dry etching has progressed, and the demand for BHF resistance has become weaker. Conventionally, glass containing 6 to 10 mol% of B ₂ O ₃ has been often used in order to improve BHF resistance. However, B ₂ O ₃ tends to lower the strain point. Examples of non-alkali glass that does not contain B ₂ O ₃ or have a low content are as follows.

Patent Document 1 discloses a glass containing 0 to 5 mol% of B ₂ O ₃ , but the average coefficient of thermal expansion at 50 to 300 ° C. exceeds 50 × 10 ⁻⁷ / ° C.

The alkali-free glass described in Patent Document 2 has a high strain point and can be molded by a float process, and is said to be suitable for applications such as a display substrate and a photomask substrate.

Alkali-free glass used for applications such as display substrates and photomask substrates, specifically, plate glass with non-alkali glass composition is prepared so that the raw materials of each component become target components, and this is used in a melting kiln. Continuously charged, heated to a predetermined temperature and dissolved. This molten glass can be obtained by forming it into a predetermined plate thickness and cutting it after slow cooling.

In the case of a glass having a high strain point, it is necessary to heat to a high temperature of 1350 to 1750 ° C. when the raw material is melted. As a heating means at the time of melting the raw material, it is generally heated to a predetermined temperature by heating with a combustion flame of a burner disposed above the melting furnace, but when heated to a high temperature of 1350 to 1750 ° C., There is a risk that the refractory constituting the kiln will be eroded. When refractory erosion occurs, the components of the refractory melt into the molten glass, leading to a deterioration in the quality of the produced glass.
As described above, as a heating means at the time of melting the raw material, it is common to heat to a predetermined temperature by a combustion flame of a burner disposed above the melting furnace, but as an additional heating means, There is a method in which a heating electrode is provided so as to be immersed in the molten glass, and the molten glass in the melting furnace is energized and heated by applying a DC voltage or an AC voltage to the heating electrode (see Patent Documents 3 and 4). Such combined use of heating by a burner combustion flame and energization heating of molten glass is effective in suppressing erosion of the refractory constituting the melting kiln. Erosion of the refractory constituting the melting furnace is likely to occur particularly near the interface between the molten glass and the upper space. For this reason, the combined use of energization heating that heats only the molten glass without raising the atmospheric temperature of the upper space is effective in suppressing erosion of the refractory.

Japanese Patent Laid-Open No. 5-232458 Japanese Patent Laid-Open No. 10-45422 Japanese Unexamined Patent Publication No. 2005-132713 Japan Special Table 2009-523697

However, there is a solid-phase crystallization method as a method for producing a high-quality p-Si TFT. In order to implement this, it is required to further increase the strain point.
On the other hand, due to demands in the glass production process, particularly melting and molding, it is required to lower the temperature T _{4 at} which the viscosity of the glass, particularly the glass viscosity, becomes 10 ⁴ dPa · s.

However, when the non-alkali glass is energized and heated, it is necessary to pay attention to the following points.
Compared to alkali glass like soda lime glass, alkali-free glass like soda lime glass has lower alkali metal oxide content, so less alkali metal ions are present in the molten glass. And current is difficult to flow during energization heating. For this reason, there exists a possibility that an electric current may flow not only from molten glass but to the refractory which comprises a melting kiln from the heating electrode provided in the melting kiln.
If a current flows through the refractory constituting the melting furnace, it is not possible to use all of the charged electricity for current heating of the molten glass, which is not preferable from the viewpoint of utilization efficiency of the charged electricity. Further, when an electric current flows through the refractory constituting the melting kiln, the electric current also flows through a metal member (for example, a metal frame) around the melting kiln and there is a risk of electric shock. In addition, current heating of the refractory material may occur, and the temperature of the refractory material may rise and melt.

The object of the present invention is to produce an alkali-free glass that solves the above-mentioned drawbacks, has a high strain point, and has a low viscosity, particularly a low temperature T _{4 at} which the glass viscosity is 10 ⁴ dPa · s, and is particularly easy to float. It is in providing a suitable method.

In the present invention, a glass raw material is prepared so as to have the following glass composition, put into a melting furnace, heated to a temperature of 1350 to 1750 ° C. to form molten glass, and then the molten glass is formed into a plate shape A method for producing alkali-free glass,
For heating in the melting furnace, heating by a combustion flame of a burner and electric heating of the molten glass by a heating electrode arranged to be immersed in the molten glass in the melting furnace are used in combination.
The electrical resistivity at T _3.3 (the temperature at which the glass viscosity becomes 10 ^3.3 dPa · s, unit: ° C.), which is the clarification temperature of the molten glass, is Rg (Ωcm), and the melting furnace at T _3.3 is Provided is a method for producing an alkali-free glass in which the glass raw material and the refractory are selected so that Rb> Rg when the electrical resistivity of the refractory to be configured is Rb (Ωcm).
SiO ₂ 63 to 74 in terms of mol% based on oxide,
Al ₂ O ₃ 11.5-16,
B ₂ O ₃ greater than 1.5 and less than 5,
MgO 5.5-13,
CaO 1.5-12,
SrO 1.5-9,
BaO 0 ~ 1,
Containing ZrO ₂ 0-2 and containing 200-2000 ppm of alkali metal oxide,
MgO + CaO + SrO + BaO is 15.5-21,
MgO / (MgO + CaO + SrO + BaO) is 0.35 or more, CaO / (MgO + CaO + SrO + BaO) is 0.50 or less, and SrO / (MgO + CaO + SrO + BaO) is 0.50 or less.

According to the method of the present invention, the strain point is 680 to 735 ° C., the average thermal expansion coefficient at 50 to 350 ° C. is 30 × 10 ⁻⁷ to 43 × 10 ⁻⁷ / ° C., and the glass viscosity is A non-alkali glass having a temperature T _{2 of} 10 ² dPa · s of 1710 ° C. or lower and a temperature T _{4 of} 10 ⁴ dPa · s of 13 10 ° C. or lower can be preferably produced.
The alkali-free glass produced by the method of the present invention is particularly suitable for a display substrate, a photomask substrate and the like for high strain point use, and is a glass that is particularly easy to float.

In the present invention, the heating in the melting furnace is combined with the heating by the combustion flame of the burner and the electric heating of the molten glass in the melting furnace to constitute the melting furnace at the time of high temperature heating of 1350 to 1750 ° C. Erosion of refractories can be suppressed. Thereby, it is suppressed that the component of a refractory melts into a molten glass, and the quality of the alkali free glass manufactured improves.
In this invention, at the time of the electric heating of molten glass, it is suppressed that an electric current flows into the refractory material which comprises a melting kiln from a heating electrode. Thereby, the utilization efficiency of the electric quantity supplied at the time of energization heating improves. In addition, if a current flows through the refractory constituting the melting furnace, the current also flows through a metal member (for example, a metal frame) around the melting furnace, and there is a risk of electric shock. However, in the present invention, these fears are eliminated.

FIG. 1 is a graph showing measurement results of electrical resistivity of molten glass (glass 1) and refractories (refractory 1, refractory 2) in Examples. FIG. 2 is a graph showing measurement results of electrical resistivity of molten glass (glass 2) and refractory (refractory 1, refractory 2) in the examples. FIG. 3 is a graph showing measurement results of electrical resistivity of molten glass (glass 3) and refractory (refractory 1, refractory 2) in the examples.

Hereinafter, a method for producing the alkali-free glass of the present invention will be described.

In the method for producing alkali-free glass of the present invention, a glass raw material prepared so as to have the following glass composition is used.
SiO ₂ 63 to 74 in terms of mol% based on oxide,
Al ₂ O ₃ 11.5-16,
B ₂ O ₃ greater than 1.5 and less than 5,
MgO 5.5-13,
CaO 1.5-12,
SrO 1.5-9,
BaO 0 ~ 1,
Containing ZrO ₂ 0-2 and containing 200-2000 ppm of alkali metal oxide,
MgO + CaO + SrO + BaO is 15.5-21,
MgO / (MgO + CaO + SrO + BaO) is 0.35 or more, CaO / (MgO + CaO + SrO + BaO) is 0.50 or less, and SrO / (MgO + CaO + SrO + BaO) is 0.50 or less.

Next, the composition range of each component will be described. If the SiO ₂ content is less than 63% (mol%, the same unless otherwise specified), the strain point is not sufficiently increased, the thermal expansion coefficient is increased, and the density is increased. It is preferably 64% or more, more preferably 65% or more, further preferably 66% or more, and particularly preferably 66.5% or more. In 74 percent, the solubility decreases, the temperature _{T 4} which is a temperature _{T 2} and ¹⁰ 4 dPa · s glass viscosity becomes ¹⁰ 2 dPa · s is increased, the liquidus temperature rises. 70% or less is preferable, 69% or less is more preferable, and 68% or less is more preferable.

Al ₂ O ₃ suppresses the phase separation property of the glass, lowers the thermal expansion coefficient, and increases the strain point. However, this effect does not appear at less than 11.5%, and the glass composition has a component that increases the thermal expansion coefficient ( For example, since the ratio of BaO, SrO) becomes high, the coefficient of thermal expansion of the glass increases as a result. It is preferably 12% or more, 12.5% or more, and more preferably 13% or more. If it exceeds 16%, the solubility of the glass may be deteriorated, or the devitrification temperature may be increased. It is preferably 15% or less, more preferably 14% or less, and further preferably 13.5% or less.

B ₂ O ₃ improves the meltability of the glass, lowers the devitrification temperature, and improves the BHF resistance, but this effect is not sufficiently exhibited at 1.5% or less, and the strain point is excessive. Or become a haze problem after treatment with BHF. 2% or more is preferable, and 3% or more is more preferable. However, if it exceeds 5%, the strain point becomes low and the Young's modulus becomes small. 4.5% or less is preferable and 4% or less is more preferable.

MgO has the feature of increasing the Young's modulus while keeping the density low while keeping the density low in alkaline earths, and improves the solubility. However, if it is less than 5.5%, this effect appears sufficiently. Furthermore, in the glass composition, the density increases because the ratio of other alkaline earths increases. It is preferably 6% or more, more preferably 7% or more, more preferably 7.5% or more and 8% or more, and particularly preferably 8.5% or more. If it exceeds 13%, the devitrification temperature rises. It is preferably 12% or less, more preferably 11% or less, and particularly preferably 10% or less.

CaO has the characteristics that it does not increase the expansion in alkaline earth after MgO, and does not excessively lower the strain point, and also improves the solubility.
If it is less than 1.5%, the above-described effect due to the addition of CaO is not sufficiently exhibited. It is preferably 2% or more, more preferably 3% or more, further preferably 3.5% or more, and particularly preferably 4% or more. However, if it exceeds 12%, the devitrification temperature may increase, or a large amount of phosphorus, which is an impurity in limestone (CaCO ₃ ), which is a CaO raw material, may be mixed. It is preferably 10% or less, more preferably 9% or less, further preferably 8% or less, and particularly preferably 7% or less.

SrO improves the solubility without increasing the devitrification temperature of the glass, but if it is less than 1.5%, this effect does not appear sufficiently. 2% or more is preferable, 2.5% or more is more preferable, and 3% or more is more preferable. However, if it exceeds 9%, the expansion coefficient may increase. It is preferably 7% or less, more preferably 6% or less and 5% or less.

BaO is not essential, but can be contained to improve solubility. However, if the amount is too large, the expansion and density of the glass are excessively increased, so the content is made 1% or less. 0.5% or less is preferable, 0.3% or less is more preferable, 0.1% or less is further preferable, and it is particularly preferable that it is not substantially contained. “Substantially not contained” means not containing any inevitable impurities.

ZrO ₂ may be contained up to 2% in order to lower the glass melting temperature or to promote crystal precipitation during firing. If it exceeds 2%, the glass becomes unstable or the relative dielectric constant ε of the glass increases. Preferably it is 1.5% or less. 1% or less is more preferable, 0.5% or less is more preferable, and it is desirable not to contain substantially.

MgO, CaO, SrO, when BaO is less than 15.5% in total, the higher the temperature _{T 4} which glass viscosity of ¹⁰ 4 dPa · s, Ya housing structure of the float bath during the float forming There is a risk of extremely shortening the life of the heater. 16% or more is preferable, and 17% or more is more preferable. If it exceeds 21%, there is a risk that the thermal expansion coefficient cannot be reduced. It is preferably 20% or less, 19% or less, and more preferably 18% or less.

When the total amount of MgO, CaO, SrO, and BaO satisfies the above and satisfies the following conditions, the Young's modulus and the specific elastic modulus are increased without increasing the devitrification temperature. In particular _{T 4} can be lowered.
MgO / (MgO + CaO + SrO + BaO) is 0.35 or more, preferably 0.37 or more, and more preferably 0.4 or more.
CaO / (MgO + CaO + SrO + BaO) is 0.50 or less, preferably 0.48 or less, and more preferably 0.45 or less.
SrO / (MgO + CaO + SrO + BaO) is 0.50 or less, preferably 0.40 or less, more preferably 0.30 or less, more preferably 0.27 or less, and further preferably 0.25 or less.

In the alkali-free glass of the present invention, Al ₂ O ₃ × (MgO / (MgO + CaO + SrO + BaO)) is preferably 4.3 or more because the Young's modulus can be increased. 4.5 or more is preferable, 4.7 or more is more preferable, and 5.0 or more is further more preferable.

In the method for producing an alkali-free glass of the present invention, an alkali metal oxide is contained in the glass raw material in an amount of 200 to 2000 ppm (mole) in order to heat and heat the molten glass in the melting furnace.
Alkali-free glass has lower alkali metal oxide content than alkali glass such as soda lime glass, and less alkali metal ions are present in molten glass. Not suitable.
In the present invention, by containing 200 ppm or more of the alkali metal oxide in the glass raw material, as a result of the increase of alkali metal ions in the molten glass, the electrical resistivity of the molten glass decreases. As a result, the electrical conductivity of the molten glass is improved, and current heating is possible.
Here, when the content of the alkali metal oxide is increased, alkali metal ions diffuse into the thin film and deteriorate the film characteristics. This causes a problem when used as a substrate glass for various displays. If the content of the metal oxide is 2000 ppm or less, preferably 1500 ppm or less, more preferably 1300 ppm or less, and even more preferably 1000 ppm or less, such a problem does not occur.
The glass raw material used in the present invention preferably contains an alkali metal oxide of 1500 ppm or less, more preferably 1300 ppm or less, further preferably 1000 ppm or less, more preferably 700 ppm or less, and more preferably 200 to 500 ppm. preferable.
In addition, examples of the alkali metal oxide include Na ₂ O, K ₂ O, and Li ₂ O. The balance between the effect that Na ₂ O and K ₂ O lower the electrical resistivity of the molten glass, the raw material cost, and the balance. From the viewpoint of, Na ₂ O is more preferable.

In order to prevent the deterioration of the properties of the metal or oxide thin film provided on the glass surface during panel manufacture, it is preferable that the glass raw material does not substantially contain P ₂ O ₅ . Furthermore, in order to facilitate recycling of the glass, it is preferable that the glass raw material does not substantially contain PbO, As ₂ O ₃ , or Sb ₂ O ₃ .

In order to improve the solubility, clarity and formability of the glass, the glass raw material contains ZnO, Fe ₂ O ₃ , SO ₃ , F, Cl, SnO ₂ in a total amount of 1% or less, preferably 0.5% or less. it can. It is preferable that ZnO is not substantially contained.

In the present invention, the glass raw material prepared to have the above composition is continuously charged into a melting furnace and melted by heating to 1350 to 1750 ° C.
Here, for the heating in the melting furnace, heating by a burner flame and electric heating of the molten glass in the melting furnace are used in combination.

The burner is disposed above the melting kiln, and is heated by a combustion flame of fossil fuel, specifically, a liquid fuel such as heavy oil and kerosene, or a gaseous fuel such as LPG. During the combustion of these combustions, the fuel can be mixed and burned with oxygen gas, or the fuel can be mixed and burned with oxygen gas and air. By using these methods, moisture can be contained in the molten glass, and the β-OH value of the alkali-free glass to be produced can be adjusted.

On the other hand, the electric heating of the molten glass in the melting furnace is performed by applying a DC voltage or an AC voltage to a heating electrode provided on the bottom or side of the melting furnace so as to be immersed in the molten glass in the melting furnace. . However, as will be described later, it is preferable to maintain the potential difference between the electrodes at 100 to 500 V when conducting the heating by heating. Therefore, it is preferable to apply an AC voltage.

At the time of energization heating of the molten glass, applying an AC voltage to the heating electrode so as to satisfy the following can suppress the electrolysis of the molten glass in the melting furnace and the generation of bubbles thereby, and the energization heating It is preferable from the point of efficiency of time.
Local current density: 0.1 to 2.0 A / cm ²
Potential difference between electrodes: 20-500V
AC voltage frequency: 10 to 90 Hz
Local current density is more preferably 0.2 ~ 1.7A / cm ^2, further preferably 0.3 ~ 1.0A / cm ^2.
The potential difference between the electrodes is more preferably 30 to 480V, and further preferably 40 to 450V.
The frequency of the AC voltage is more preferably 30 to 80 Hz, and further preferably 50 to 60 Hz.

In addition to being excellent in conductivity, the material used for the heating electrode is required to be excellent in heat resistance and corrosion resistance to the molten glass because it is immersed in the molten glass in the melting furnace.
Examples of the material satisfying these include rhodium, iridium, osmium, hafnium, molybdenum, tungsten, platinum, and alloys thereof.

In the present invention, when the total amount of heating by the combustion flame of the burner and current heating of the molten glass in the melting furnace is T ₀ (J / h), the heat amount T (J / h) by current heating. Preferably satisfies the following formula.
0.10 × T ₀ ≦ T ≦ 0.40 × T ₀
When T is smaller than 0.10 × T ₀ , there is a possibility that the effect by the combined use of the electrically heated heating of the molten glass, that is, the effect of suppressing the erosion of the refractory constituting the melting kiln may be insufficient.
If T is larger than 0.40 × T ₀ , the temperature at the bottom of the melting furnace rises and erosion of the refractory may proceed.

Since the melting furnace is heated to a high temperature of 1300 to 1700 ° C. or 1350 to 1750 ° C. when the glass raw material is melted, a refractory is used as a constituent material. In addition to heat resistance, the refractory constituting the melting furnace is required to have corrosion resistance, mechanical strength, and oxidation resistance against molten glass.
As the refractory constituting the melting kiln, a zirconia refractory containing 90% by mass or more of ZrO ₂ has been preferably used since it has excellent corrosion resistance against molten glass.
However, the above zirconia refractory contains alkali components (Na ₂ O and K ₂ O) in a total amount of 0.12% by mass or more as components for reducing the viscosity of the matrix glass. When heated to a high temperature of 1350 to 1750 ° C., it exhibits ionic conductivity due to the presence of the alkali component. For this reason, at the time of energization heating, current may flow not only from the molten glass but also to the refractory constituting the melting kiln from the heating electrode provided in the melting kiln.

In the present invention, the electrical resistivity at T _3.3 (the temperature at which the glass viscosity becomes 10 ^3.3 dPa · s, unit: ° C.), which is the glass refining temperature, is Rg (Ωcm), and the melting at T _3.3 When the electrical resistivity of the refractory constituting the kiln is Rb (Ωcm), the glass raw material and the refractory constituting the melting kiln are selected so that Rb> Rg.
As shown in the examples described later, the electrical resistivity of the molten glass and the refractory decreases as the temperature increases, but the decrease in the electrical resistivity with respect to the temperature increase is larger in the molten glass than in the refractory. Therefore, if the electrical resistivity at T _3.3 is Rb> Rg, a higher temperature range (for example, T ₂ which is the melting temperature of glass (temperature at which the glass viscosity becomes 10 ² dPa · s). , Unit: ° C)), the refractory always has a higher electrical resistivity than the molten glass.
Therefore, if the glass raw material and the refractory constituting the melting kiln are selected so that Rb> Rg at T _3.3 , the current flows from the heating electrode to the refractory constituting the melting kiln during energization heating. Flow is suppressed.

In the present invention, the ratio of Rb to Rg (Rb / Rg) preferably satisfies Rb / Rg> 1.00, more preferably satisfies Rb / Rg> 1.05, and Rb / Rg> It is more preferable to satisfy 1.10.

In the case of the alkali-free glass having the above-described composition, Rg can be adjusted by changing the content of the alkali metal oxide within the range of 200 to 2000 ppm. Rg becomes low, so that there is much content of an alkali metal oxide.
Rg can also be adjusted by changing _T3.3 of the alkali-free glass to be produced. The lower T _3.3 is, the lower Rg is.

In the case of a suitable composition of a refractory described later, Rb can be adjusted by changing the content of alkali components (Na ₂ O, K ₂ O). Moreover, Rb can be adjusted by changing the ratio of K ₂ O in the alkali component. Rb becomes higher as the content of alkali components (Na ₂ O, K ₂ O) is lower. Rb increases as the proportion of K ₂ O in the alkali component increases.

With respect to the alkali-free glass having the above-described composition, refractories satisfying Rb> Rg are ZrO ₂ 85 to 91%, SiO ₂ 7.0 to 11.2%, and Al ₂ O ₃ 0% by mass. 0.85-3.0%, P ₂ O ₅ 0.05-1.0%, B ₂ O ₃ 0.05-1.0%, and the total amount of K ₂ O and Na ₂ O is 0.8. Examples thereof include high zirconia molten cast refractories containing 01 to 0.12% and containing K ₂ O in an amount of Na ₂ O or more.

The high zirconia molten cast refractory having the above composition is a refractory consisting mainly of zirconia (ZrO ₂ ) of 85 to 91% of the chemical component, and has a badelite crystal as a main constituent, It exhibits excellent corrosion resistance, has a low alkali component content, and mainly contains K ₂ O having a large ionic radius and a small mobility as an alkali component, and therefore has an electrical resistivity in a temperature range of 1350 to 1750 ° C. large.

Next, the composition range of each component will be described.
As the high zirconia molten cast refractory, the higher the content of ZrO _{2 in} the refractory, the better the corrosion resistance to the molten glass, so 85% or more, preferably 88% or more. However, if the content of ZrO ₂ is more than 91%, the amount of matrix glass is relatively small and the volume change associated with the transition (ie transformation) of the baderite crystal cannot be absorbed, and the heat cycle resistance deteriorates. 91% or less.

SiO ₂ is an essential component for forming a matrix glass that relieves stress generated in the refractory, and in order to obtain a molten cast refractory having a practical size without cracks, it is necessary to contain 7.0% or more. is there. However, if the content of the SiO ₂ component is more than 11.2%, the corrosion resistance to the molten glass becomes small, so it is 11.2% or less, preferably 10.0% or less.

Al ₂ O ₃ plays the role of adjusting the relationship between the temperature and viscosity of the matrix glass, and also shows the effect of reducing the content of ZrO _{2 in} the matrix glass. When the content of ZrO ₂ in the matrix glass is small, the precipitation of zircon (ZrO ₂ · SiO ₂ ) crystals found in conventional refractories in the matrix glass is suppressed, and the cumulative tendency of residual volume expansion is significantly reduced. .

In order to effectively reduce the content of ZrO ₂ in the matrix glass, the content of Al ₂ O ₃ in the refractory is set to 0.85% or more, preferably 1.0% or more. In addition, when casting or using a refractory, Al ₂ O ₃ is used so that crystals such as mullite precipitate in the matrix glass and the matrix glass is not altered and cracks are not generated in the refractory. The content of is set to 3.0% or less.

Therefore, the content of Al ₂ O ₃ in the high zirconia molten cast refractory is 0.85 to 3.0%, preferably 1.0 to 3.0%. In high zirconia molten cast refractories cast with the refractory composition adjusted to such a range, heat cycle resistance, that is, volume increase due to accumulation of residual volume expansion is suppressed within a practically no problem range. At the same time, the chip-off phenomenon is remarkably improved.

In addition, B ₂ O ₃ and P ₂ O ₅ are included in addition to a small amount of alkali component, so that the viscosity of the matrix glass at 800 to 1250 ° C. is adjusted to an appropriate level even if the alkali component content is small. Therefore, even when the thermal cycle that passes through the transition temperature range of the badelite crystal is repeatedly used during use, the residual volume expansion becomes small, and thus there is no tendency to cause cracks due to the accumulation of the residual volume expansion.

B ₂ O ₃ is contained mainly in the matrix glass with _P 2 _{O 5,} as well as soften the matrix glass in cooperation with the _P 2 _{O 5} in place of the alkali components, the refractory at the temperature range of 1350 ~ 1750 ° C. It is a component that does not reduce the electrical resistivity.

If the content of B ₂ O ₃ is 0.05% or more because the amount of the matrix glass in the high zirconia molten cast refractory is small, an effect of adjusting the viscosity of the matrix glass is exhibited. However, if the content of B ₂ O ₃ is too large, a dense melt-cast refractory cannot be cast. Therefore, the content of B ₂ O ₃ is 0.05 to 1.0%, preferably 0.10 to 1. 0%.

P ₂ O ₅ is mostly contained in the matrix glass together with B ₂ O ₃ and the alkali component, and the volume change accompanying the transition of the badelite crystal is adjusted (soft) by adjusting the viscosity of the matrix glass in the transition temperature range of the badelite crystal. Prevents the occurrence of cracks due to the stress caused by. Further, P ₂ O ₅ and B ₂ O ₃ is, when the refractory is used in a glass melting furnace, which is no possibility of components for coloring glass even if the leach into the glass. Furthermore, when P ₂ O ₅ is added to the refractory raw material, the refractory raw material is easily melted, so that there is an advantage that the amount of electric power required for casting the refractory can be reduced.

Here, since the amount of matrix glass in the high zirconia molten cast refractory is small, even if the content of P ₂ O ₅ in the refractory is small, the content of P ₂ O ₅ in the matrix glass is relatively In particular, the effect of adjusting the viscosity of the matrix glass can be obtained if 0.05% or more of P ₂ O ₅ is contained in the refractory. Further, if the content of P ₂ O ₅ is more than 1.0%, the property of the matrix glass changes and tends to promote the residual volume expansion of the refractory and the generation of cracks accompanying the accumulation. The content of P ₂ O ₅ in the refractory suitable for adjusting the viscosity is 0.05 to 1.0%, preferably 0.1 to 1.0%.

Further, the content of the alkali component consisting of K ₂ O and Na ₂ O is 0.12 in terms of the total amount as an oxide so that the electrical resistivity of the refractory in the temperature range of 1350 to 1750 ° C. has a sufficiently large value. %, And 50% or more, preferably 70% or more of the alkali component is K ₂ O having a low ion mobility in the glass. However, if the total amount of K ₂ O and Na ₂ O is less than 0.01%, it becomes difficult to produce a melt-cast refractory without cracks, so the total amount of K ₂ O and Na ₂ O is 0.8. 01% or more. Further, the content of K ₂ O is made larger than the content of Na ₂ O so that a high zirconia molten cast refractory without cracks can be stably cast. It is preferable that the Na ₂ O content is 0.008% or more and the K ₂ O content is 0.02 to 0.10%.

Further, if the total content of Fe ₂ O ₃ and TiO ₂ contained as impurities in the raw material is 0.55% or less, there is no problem of coloring in the melting furnace of the alkali-free glass having the above glass composition. Preferably, the total amount does not exceed 0.30%. Moreover, it is not necessary to contain alkaline earth oxides in the refractory, and the total content of alkaline earth oxides is preferably less than 0.10%.

As the refractory constituting the melting furnace, as chemical components, ZrO ₂ is 88 to 91%, SiO ₂ is 7.0 to 10%, Al ₂ O ₃ is 1.0 to 3.0%, P ₂ O ₅ High zirconia molten cast refractories containing 0.10 to 1.0% and B ₂ O ₃ containing 0.10 to 1.0% are preferred.

In the present invention, the glass composition prepared to have the above composition is continuously charged into a melting furnace, heated to 1350 to 1750 ° C. to form molten glass, and then the molten glass is formed into a plate shape by a float process. Thus, alkali-free glass can be obtained. More specifically, an alkali-free glass can be obtained as a plate glass by forming it to a predetermined plate thickness by a float process, and cutting it after slow cooling.
The forming method for the plate glass is preferably a float method, a fusion method, a roll-out method, or a slot down draw method, and the float method is particularly preferable in consideration of productivity and enlargement of the plate glass.

The alkali-free glass obtained by the method of the present invention (hereinafter referred to as “the alkali-free glass of the present invention”) has a strain point of 680 to 735 ° C. and can suppress thermal shrinkage during panel production. Further, a solid phase crystallization method can be applied as a method for manufacturing a p-Si TFT.
In the alkali-free glass of the present invention, the strain point is more preferably 685 ° C or higher, and further 690 ° C or higher. When the strain point is 690 ° C. or higher, it is used for a high strain point (for example, a display substrate or lighting substrate for organic EL having a plate thickness of 0.7 mm or less, preferably 0.5 mm or less, more preferably 0.3 mm or less. Or a thin display substrate or lighting substrate having a thickness of 0.3 mm or less, preferably 0.1 mm or less. When forming a sheet glass having a plate thickness of 0.7 mm or less, further 0.5 mm or less, further 0.3 mm or less, and further 0.1 mm or less, the drawing speed at the time of forming tends to increase. Increases and the glass compaction (heat shrinkage rate) tends to increase. In this case, compaction can be suppressed when the glass is a high strain point glass. However, if the strain point exceeds 735 ° C., the glass temperature at the time of conveying the glass after molding becomes high, which may affect the equipment life. The strain point is preferably 730 ° C. or lower, and more preferably 725 ° C. or lower.

Further, the alkali-free glass of the present invention has a glass transition point of preferably 750 ° C. or higher, more preferably 760 ° C. or higher, and further preferably 770 ° C. or higher.

The alkali-free glass of the present invention has an average coefficient of thermal expansion at 50 to 350 ° C. of 30 × 10 ⁻⁷ to 43 × 10 ⁻⁷ / ° C., has high thermal shock resistance, and has high productivity during panel production. it can. In the alkali-free glass of the present invention, the average thermal expansion coefficient at 50 to 350 ° C. is preferably 35 × 10 ⁻⁷ to 43 × 10 ⁻⁷ / ° C.

Furthermore, the alkali-free glass of the present invention has a specific gravity of preferably 2.65 or less, more preferably 2.64 or less, and further preferably 2.62 or less.

The alkali-free glass of the present invention has a T ₂ of 1710 ° C. or less, preferably less than 1710 ° C., more preferably 1700 ° C. or less, and even more preferably 1690 ° C. or less. is there.

The alkali-free glass of the present invention has a T _3.3 of 1430 ° C. or less, preferably less than 1420 ° C., more preferably 1410 ° C. or less, and even more preferably 1400 ° C. or less. Easy.

Further, the alkali-free glass of the present invention has a temperature T _{4 at} which the viscosity becomes 10 ⁴ dPa · s is 1310 ° C. or less, preferably 1305 ° C. or less, more preferably 1300 ° C. or less, still more preferably less than 1300 ° C., 1295 ° C. or less, It is 1290 ° C. or lower and is suitable for float forming.
In addition, the alkali-free glass of the present invention preferably has a devitrification temperature of 1315 ° C. or lower because molding by the float method is easy. Preferably they are 1300 degrees C or less, 1300 degrees C or less, 1290 degrees C or less, More preferably, it is 1280 degrees C or less. The difference between the temperature T ₄ (temperature at which the glass viscosity is 10 ⁴ dPa · s, unit: ° C.) and the devitrification temperature (T ₄ −devitrification temperature), which is a standard for float moldability and fusion moldability, is It is preferably −20 ° C. or higher, −10 ° C. or higher, further 0 ° C. or higher, more preferably 10 ° C. or higher, still more preferably 20 ° C. or higher, and particularly preferably 30 ° C. or higher.
In this specification, the devitrification temperature is obtained by putting crushed glass particles in a platinum dish and performing heat treatment for 17 hours in an electric furnace controlled at a constant temperature. It is an average value of the maximum temperature at which crystals are deposited inside and the minimum temperature at which crystals are not deposited.

The alkali-free glass of the present invention has a Young's modulus of preferably 78 GPa or more, 79 GPa or more, 80 GPa or more, more preferably 81 GPa or more, and further preferably 82 GPa or more.

The alkali-free glass of the present invention preferably has a photoelastic constant of 31 nm / MPa / cm or less.
Due to the birefringence of the glass substrate due to stress generated during the manufacturing process of the liquid crystal display panel and the liquid crystal display device, a phenomenon in which the black display becomes gray and the contrast of the liquid crystal display decreases may be observed. By setting the photoelastic constant to 31 nm / MPa / cm or less, this phenomenon can be suppressed small. Preferably it is 30 nm / MPa / cm or less, More preferably, it is 29 nm / MPa / cm or less, More preferably, it is 28.5 nm / MPa / cm or less, Most preferably, it is 28 nm / MPa / cm or less.
The alkali-free glass of the present invention has a photoelastic constant of 23 nm / MPa / cm or more, more preferably 25 nm / MPa / cm or more, considering the ease of securing other physical properties.
The photoelastic constant can be measured by a disk compression method at a measurement wavelength of 546 nm.

The β-OH value of the alkali-free glass can be appropriately selected according to the required characteristics of the alkali-free glass. In order to increase the strain point of the alkali-free glass, it is preferable that the β-OH value is low. For example, when the strain point is 725 ° C. or more, the β-OH value is preferably 0.3 mm ⁻¹ or less, more preferably 0.25 mm ⁻¹ or less, and 0.2 mm ⁻¹ or less. More preferably.
The β-OH value can be adjusted by various conditions at the time of melting the raw material, for example, the amount of water in the glass raw material, the water vapor concentration in the melting kiln, the residence time of the molten glass in the melting kiln, and the like. As a method for adjusting the amount of water in the glass raw material, a method using a hydroxide instead of an oxide as a glass raw material (for example, magnesium hydroxide (Mg (OH) ₂ instead of magnesium oxide (MgO) as a magnesium source) )). In addition, as a method for adjusting the water vapor concentration in the melting furnace, there are a method in which fossil fuel is mixed with oxygen gas and burned, and a method in which it is burned with oxygen gas and air at the time of combustion in a burner.

The electrical resistivity of the molten glass and refractory (zirconia electrocast refractory) in the temperature range of 1300 to 1700 ° C. was measured.
Molten glass (Glass 1, Glass 2, Glass 3) was prepared by mixing the raw materials of each component so as to have the following composition, and was melted at a temperature of 1600 ° C. using a platinum crucible. In melting, the mixture was stirred using a platinum stirrer to homogenize the glass. The electrical resistivity was measured by the method described in the following document while the molten glass thus obtained was maintained in a temperature range of 1300 to 1700 ° C.
"Conductivity measurement method for ionic melt, Norio Ota, Mitsuo Miyanaga, Kenji Morinaga, Tsutomu Yanagase, Journal of the Japan Institute of Metals, Vol. 45 No. 10 (1981) 1036-1043"

[Glass 1]
Composition (expressed as mol% based on oxide)
SiO ₂ 67.5
Al ₂ O ₃ 12.7
B ₂ O ₃ 3.5
MgO 6.2
CaO 6.5
SrO 3.6
BaO 0

ZrO

₂ 0
MgO + CaO + SrO + BaO 16.3
MgO / (MgO + CaO + SrO + BaO) 0.38
CaO / (MgO + CaO + SrO + BaO) 0.40
SrO / (MgO + CaO + SrO + BaO) 0.22

[Glass 2]
Composition (expressed as mol% based on oxide)
SiO ₂ 66.9
Al ₂ O ₃ 13.0
B ₂ O ₃ 1.7
MgO 8.8
CaO 5.1
SrO 4.5
BaO 0

ZrO

₂ 0
MgO + CaO + SrO + BaO 18.4
MgO / (MgO + CaO + SrO + BaO) 0.48
CaO / (MgO + CaO + SrO + BaO) 0.28
SrO / (MgO + CaO + SrO + BaO) 0.24
[Glass 3]
Composition (expressed as mol% based on oxide)
SiO ₂ 66.8
Al ₂ O ₃ 13.8
B ₂ O ₃ 2.8
MgO 8.4
CaO 5.0
SrO 3.2
BaO 0

ZrO

₂ 0
MgO + CaO + SrO + BaO 16.6
MgO / (MgO + CaO + SrO + BaO) 0.51
CaO / (MgO + CaO + SrO + BaO) 0.30
SrO / (MgO + CaO + SrO + BaO) 0.19

In addition to these, the Na ₂ O content was added in two ways of 200 ppm and 1000 ppm based on the oxide.
In addition, zirconia-based electrocast refractories (refractory 1 and refractory 2) having the following chemical composition and mineral composition also have an electrical resistivity of “JIS C2141 for electrical insulation” in a temperature range of 700 to 1600 ° C. The measurement principle of the volume resistivity (Section 14) of “Ceramic material test method” was developed at a high temperature (the sample was placed in an electric furnace and heated) and measured.
[Refractory 1]
Chemical composition (mass%)
ZrO ₂ 88
SiO ₂ 9.3
Al ₂ O ₃ 1.5
P ₂ O ₅ 0.1
B ₂ O ₃ 0.8
Fe ₂ O ₃ 0.05
TiO ₂ 0.15
Na ₂ O 0.02
K ₂ O 0.04
Mineral composition (mass%)
Vatelite 88
Glass phase 12
[Refractory 2]
Chemical composition (mass%)
ZrO ₂ 94.5
SiO ₂ 4.0
Al ₂ O ₃ 0.8
P ₂ O ₅ 0.10
B ₂ O ₃ 0.8
Fe ₂ O ₃ 0.05
TiO ₂ 0.15
Na ₂ O 0.4
K ₂ O 0.01
Mineral composition (mass%)
Vatelite 88
Glass phase 12

The measurement result of the electrical resistivity of the glass 1 is shown in FIG. 1, the measurement result of the electrical resistivity of the glass 2 is shown in FIG. 2, and the measurement result of the electrical resistivity of the glass 3 is shown in FIG. In FIGS. 1, 2, and 3, the results of Na ₂ O = 200 ppm and 1000 ppm are actually measured values, and the other values are calculated values. _{T 3.3} Glass 1 1393 ° C., _{T 3.3} of the glass 2 is 1378 ° C., _{T 3.3} of the glass 3 is 1396 ° C.. As is apparent from FIGS. 1, 2 and 3, the refractory 1 has an electrical resistivity Rb at T _3.3 when the glass 1, glass 2 and glass 3 have a Na ₂ O content of 200 ppm or more. the electric resistivity Rg of the molten glass in the T _3.3, satisfied the relationship of Rb> Rg. Further, even in the temperature range of T _3.3 or higher, the refractory 1 had a higher electrical resistivity than the molten glass. If a melting kiln is comprised with such a refractory 1, it will be thought that it is suppressed that an electric current flows into the refractory which comprises a melting kiln from a heating electrode at the time of energization heating.
When the Na ₂ O content of Glass 1, Glass 2, and Glass 3 was less than 200 ppm, the electrical resistivity Rb, Rg at T _{3.3 was} in a relationship of Rb <Rg.
On the other hand, electrical resistivity Rb of refractory 2 _{T 3.3} is a Glass 1, a glass 2, 200 ppm is the content of Na ₂ O of the glass 3, in each case 1000ppm of the molten glass in the _{T 3.3} The relationship was Rb <Rg with respect to the electrical resistivity Rg. Further, even in the temperature range of T _3.3 or higher, the refractory 2 had a lower electrical resistivity than the molten glass. When a melting kiln is configured with such a refractory 2, it is considered that a current flows from the heating electrode to the refractory constituting the melting kiln during energization heating.

In the following, Examples 1 to 23 and Examples 27 to 28 are Examples, and Examples 24 to 26 are Comparative Examples. A mixture of raw materials of each component so as to have a target composition was put into a melting furnace composed of the refractory 1, and was melted at a temperature of 1500 to 1600 ° C. For heating of the melting furnace, heating by a burner flame and electric heating of the molten glass by a heating electrode arranged so as to be immersed in the molten glass in the melting furnace were used in combination. At the time of energization heating, an alternating voltage was applied to the heating electrode at a local current density of 0.5 A / cm ² , a potential difference between the electrodes of 300 V, and a frequency of 50 Hz.
In addition, when the total amount of heating by the burning flame of the burner and current heating of the molten glass in the melting furnace is T ₀ (J / h), the heating amount T (J / h) by current heating is The relationship of T = 0.30 × T ₀ was satisfied.

Tables 1 to 4 show the glass composition (unit: mol%), the coefficient of thermal expansion at 50 to 350 ° C. (unit: × 10 ⁻⁷ / ° C.), the strain point (unit: ° C.), and the glass transition point (unit: ° C.). ), Specific gravity, Young's modulus (GPa) (measured by ultrasonic method), high temperature viscosity value, temperature T ₂ (temperature at which glass viscosity becomes 10 ² dPa · s, unit: ° C.) _(temperature at which the glass viscosity is 10 ^3.3 dPa · s, the unit: ° C.) temperature T _3.3 which is a measure of the clarity and the float method, a fusion method, a roll out method, moldability such as a slot down draw method Temperature T ₄ (temperature at which the glass viscosity becomes 10 ⁴ dPa · s, unit: ° C.), devitrification temperature (unit: ° C.), photoelastic constant (unit: nm / MPa / cm) (plate Wavelength measured by disk compression method using a slowly cooled sample molded into a shape And a relative dielectric constant (measured by a method described in JIS C-2141 using a slowly cooled sample formed into a plate shape). Evaluation of the heat shrinkage rate was performed according to the following procedure. The sample is held at a temperature of glass transition point + 100 ° C. for 10 minutes and then cooled to room temperature at 40 ° C. per minute. Here, the total length of the sample is measured. Thereafter, the sample is heated at 100 ° C./hour to 600 ° C., held at 600 ° C. for 80 minutes, cooled to room temperature at 100 ° C./hour, and the total length of the sample is measured again. The ratio of the shrinkage of the sample before and after the heat treatment at 600 ° C. to the total length of the sample before the heat treatment at 600 ° C. was defined as the heat shrinkage rate.
In Tables 1 to 4, the values shown in parentheses are calculated values.

As is apparent from the table, all the glasses of the examples have a low thermal expansion coefficient of 30 × 10 ⁻⁷ to 43 × 10 ⁻⁷ / ° C. and a high strain point of 680 to 735 ° C., which is sufficient for heat treatment at high temperatures. I can see that it can withstand.

Temperature T ₂ which is a measure of the solubility 1710 ° C. or less and relatively low solubility is easy. Moreover, _T3.3 is 1430 degrees C or less, and clarification is comparatively easy. In addition, the temperature T ₄ that is a measure of moldability is 1310 ° C. or less, and molding by the float method is particularly easy. Further, the devitrification temperature is 1320 ° C. or lower, and it is considered that there is no trouble such as devitrification generated particularly during float forming.

A photoelastic constant is 31 nm / MPa / cm or less, and when used as a glass substrate of a liquid crystal display, a decrease in contrast can be suppressed.
Further, the relative dielectric constant is 5.6 or more, and the sensing sensitivity of the touch sensor is improved when used as a glass substrate of an in-cell type touch panel.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on Japanese Patent Application No. 2013-174621 filed on August 26, 2013, the contents of which are incorporated herein by reference.

The alkali-free glass of the present invention has a high strain point and is suitable for uses such as a display substrate and a photomask substrate. Moreover, it is suitable also for uses, such as a substrate for solar cells and a glass substrate for magnetic disks.

Claims

A glass raw material is prepared so as to have the following glass composition, put into a melting furnace, heated to a temperature of 1350 to 1750 ° C. to form molten glass, and then the alkali-free glass for forming the molten glass into a plate shape A manufacturing method comprising:
For heating in the melting furnace, heating by a combustion flame of a burner and electric heating of the molten glass by a heating electrode arranged to be immersed in the molten glass in the melting furnace are used in combination.
The electrical resistivity at T 3.3 (the temperature at which the glass viscosity becomes 10 3.3 dPa · s, unit: ° C.), which is the clarification temperature of the molten glass, is Rg (Ωcm), and the melting furnace at T 3.3 is When the electrical resistivity of the refractory to be configured is Rb (Ωcm), the glass raw material and the method for producing an alkali-free glass that selects the refractory so that Rb> Rg are satisfied:
SiO 2 63 to 74 in terms of mol% based on oxide,
Al 2 O 3 11.5-16,
B 2 O 3 greater than 1.5 and less than 5,
MgO 5.5-13,
CaO 1.5-12,
SrO 1.5-9,
BaO 0 ~ 1,
Containing ZrO 2 0-2 and containing 200-2000 ppm of alkali metal oxide,
MgO + CaO + SrO + BaO is 15.5-21,
MgO / (MgO + CaO + SrO + BaO) is 0.35 or more, CaO / (MgO + CaO + SrO + BaO) is 0.50 or less, and SrO / (MgO + CaO + SrO + BaO) is 0.50 or less.
The manufacturing method of the alkali free glass of Claim 1 whose composition is the following:
SiO 2 63 to 74 in terms of mol% based on oxide,
Al 2 O 3 11.5-14,
B 2 O 3 greater than 1.5 and less than 5,
MgO 5.5-13,
CaO 1.5-12,
SrO 1.5-9,
BaO 0 ~ 1,
Containing ZrO 2 0-2 and containing 200-2000 ppm of alkali metal oxide,
MgO + CaO + SrO + BaO is 15.5-21,
MgO / (MgO + CaO + SrO + BaO) is 0.35 or more, CaO / (MgO + CaO + SrO + BaO) is 0.50 or less, and SrO / (MgO + CaO + SrO + BaO) is 0.30 or less.
The method for producing an alkali-free glass according to claim 1 or 2, wherein the glass raw material and the refractory are selected so that a ratio (Rb / Rg) between the Rb and the Rg satisfies the following formula. .
Rb / Rg> 1.00
When the sum of the heating amount by the burner combustion flame and the heating amount by the electric heating of the molten glass in the melting furnace is T 0 (J / h), the heating amount T (J / h) by the electric heating has the following formula: The method for producing an alkali-free glass according to any one of claims 1 to 3, which satisfies the above.
0.10 × T 0 ≦ T ≦ 0.40 × T 0
The refractory that constitutes the melting furnace includes, as chemical components of the refractory, ZrO 2 of 85 to 91%, SiO 2 of 7.0 to 11.2%, and Al 2 O 3 of 0.85 to 0.5% by mass. 3.0%, P 2 O 5 0.05-1.0%, B 2 O 3 0.05-1.0%, and the total amount of K 2 O and Na 2 O is 0.01-0 The method for producing an alkali-free glass according to any one of claims 1 to 4, which is a high zirconia molten cast refractory containing 12% and containing K 2 O in an amount of Na 2 O or more.
6. An AC voltage having a frequency of 10 to 90 Hz is applied to the heating electrode so that a local current density is 0.1 to 2.0 A / cm 2 and a potential difference between the electrodes is 20 to 500 V. The manufacturing method of the alkali free glass as described in any one of these.