Alkali-free glass substrate and method for manufacturing alkali-free float glass substrate

Abstract

To provide a non-alkali glass substrate having a thin plate thickness and excellent strength; and to provide a production method of a non-alkali float glass substrate.SOLUTION: In a non-alkali glass substrate, a plate thickness is 0.75 mm or less, and a depth direction profile of internal standardization hydrogen count determined from secondary ion mass spectrometry in which Cs+ is used as primary ion from one surface side of the glass substrate, satisfies a prescribed condition.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an alkali-free glass substrate and a method for producing an alkali-free float glass substrate. [Background technology]

[0002] There is a demand for flat panel displays such as liquid crystal displays and organic EL displays to be thinner and larger. Accordingly, there is a demand for glass substrates for flat panel displays (FPDs) to be thinner and larger. Alkali-free glass is used for FPD glass substrates. Glass substrates are more susceptible to breakage as they become thinner, and therefore there is a demand for further improvements in strength. Summary of the Invention [Problem to be solved by the invention]

[0003] One known method for increasing the strength of glass substrates is chemical strengthening, which involves ion-exchanging alkali ions in the glass to form a compressive stress layer on the surface of the glass substrate. However, alkali-free glass is not suitable for chemical strengthening because it does not contain alkali metal oxides. Furthermore, one known method for increasing the strength of glass substrates is physical strengthening, in which low-temperature air is blown onto a high-temperature glass substrate to form a compressive stress layer on the surface of the glass substrate. However, when the glass substrate is thin, such as in the case of FPD glass substrates, it is difficult to form a compressive stress layer, and physical strengthening is therefore not suitable.

[0004] The present invention has been made in view of the above problems, and has an object to provide an alkali-free glass substrate that is thin and has excellent strength, and a method for manufacturing an alkali-free float glass substrate. [Means for solving the problem]

[0005] The alkali-free glass substrate according to the present invention has a thickness of 0.75 mm or less, and is formed by depositing Cs on one surface of the glass substrate.+ Secondary ion mass spectrometry (D-SIMS) was performed using primary ions, and the horizontal axis was the depth X (nm) from one of the surfaces, and the vertical axis was H - / Si - A depth profile (plotting interval: 10 nm or less) is created as a secondary ion intensity ratio, and when the following (a) to (d) are identified from the depth profile, the c value obtained from the following (c) satisfies -0.0110 or less, and the s value obtained from the following (d) satisfies 1.5 to 3.5. (a) In the depth profile, H in the internal region where the depth X is 450 nm or more and 500 nm or less - / Si - The average value of the secondary ion intensity ratios is calculated, and this value is taken as the internal hydrogen count. (b) H normalized to the internal hydrogen count of 1 - / Si - A new depth profile is created with the secondary ion intensity ratio as the internal normalized hydrogen count Y, the horizontal axis as the depth X (nm), and the vertical axis as the internal normalized hydrogen count Y. (c) In the depth direction profile of (b) above, the exponential approximation curve equation (1) is determined from the plot of the outermost surface region where the depth X is 50 nm or more and 150 nm or less, and c in the equation (1) is defined as the c value. logY=cX+logb (1) (d) In the depth profile of (b), the average value of the internal normalized hydrogen count Y in the surface region at a depth X of 50 nm or more and 450 nm or less is defined as the s value. The method for producing an alkali-free float glass substrate according to the present invention is a method for producing an alkali-free float glass substrate, which includes a forming step of continuously supplying molten glass onto a bath surface of molten metal contained in a bath to form a glass ribbon, In the forming step, a roof brick is provided above the molten metal, In an upstream region where the viscosity η (dPa s) at the center in the width direction of the glass ribbon is less than logη = 7.65, the hydrogen concentration in the space between the molten metal and the roof bricks is more than 6.0 volume % and not more than 12.0 volume %, In a downstream region where the viscosity η (dPa·s) at the center of the width of the glass ribbon is logη=7.65 or more, the hydrogen concentration in the space between the molten metal and the roof bricks is set to 6.0% by volume or less. [Effects of the Invention]

[0006] According to the present invention, it is possible to provide an alkali-free glass substrate that is thin and has excellent strength, and a method for producing an alkali-free float glass substrate. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing depth profiles (2) of Examples 1 to 6, with the horizontal axis representing depth and the vertical axis representing internal normalized hydrogen count. [Figure 2] FIG. 2 is a diagram showing a depth profile (2) of Example 1, with the horizontal axis representing depth and the vertical axis representing internal normalized hydrogen count. [Figure 3] 1 is a diagram showing an exponential approximation curve obtained from plots of the outermost surface region at a depth of 50 nm or more and 150 nm or less in the depth direction profile (2) of Example 1. FIG. [Figure 4] 1 is a graph showing the relationship between the c value and the BoR average breaking load in Examples 1 to 6. FIG. [Figure 5] FIG. 1 is a cross-sectional view showing a float glass manufacturing apparatus used in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Alkali-free glass substrate] An alkali-free glass substrate according to one embodiment of the present invention will now be described. Alkali-free glass refers to glass that is substantially free of alkali metal oxides such as NaO and KO. Here, "substantially free of alkali metal oxides" means that the total content of alkali metal oxides is 0.1% by mass or less.

[0009] The alkali-free glass substrate according to the present invention has a thickness of 0.75 mm or less and is suitable as a glass substrate for FPD. The thickness may be 0.55 mm or less. In order to ensure strength, the alkali-free glass substrate according to the present invention preferably has a thickness of 0.05 mm or more.

[0010] The present inventors have found that the depth profile of hydrogen ions near the surface of a non-alkali glass substrate, determined by secondary ion mass spectrometry (D-SIMS), affects the strength of the non-alkali glass substrate.

[0011] In the present specification, the hydrogen ion depth profile (2) by D-SIMS is obtained by the following steps 1) to 3). Note that, as a pre-treatment before D-SIMS, cleaning with acetone, UV ozone cleaning, low-temperature ashing with oxygen plasma, or the like may be performed as necessary to remove organic contamination from the surface of the alkali-free glass substrate.

[0012] 1) Cs is applied to one surface of the non-alkali glass substrate. + Secondary ion mass spectrometry (D-SIMS) was performed using primary ions, and the horizontal axis was the depth X (nm) from one surface, and the vertical axis was H - / Si - A depth profile (1) is created as a secondary ion intensity ratio with plot intervals of 10 nm or less. To convert the horizontal axis from sputtering time to depth X, it is necessary to determine the sputtering rate of the primary ions. This sputtering rate can be determined by measuring the depth of the analysis crater formed after D-SIMS.

[0013] 2) In the depth profile (1), H in the internal region at a depth X of 450 nm or more and 500 nm or less - / Si - The average value of the secondary ion intensity ratios is calculated and this value is taken as the internal hydrogen count. In addition, H near the surface of the non-alkali glass substrate - / Si -The secondary ion intensity ratio decreases with the depth X, but becomes almost constant in the inner region where the depth X is 450 nm or more and 500 nm or less.

[0014] 3) H normalized to internal hydrogen count of 1 - / Si - A new depth profile (2) is created with the secondary ion intensity ratio as the internal normalized hydrogen count Y, the horizontal axis as the depth X (nm), and the vertical axis as the internal normalized hydrogen count Y.

[0015] 4) In the depth profile (2), the exponential approximation curve equation (1) is found from the plot of the outermost surface region where the depth X is 50 nm or more and 150 nm or less, and c in equation (1) is defined as the c value. logY=cX+logb (1) In equation (1), logY and logb are natural logarithms. The reason why the start point of the outermost layer region is set at a depth X of 50 nm is because detection by D-SIMS varies in regions with a depth X of less than 50 nm due to factors such as contamination on the substrate surface.

[0016] 5) In the depth profile (2), the average value of the internal normalized hydrogen count Y in the surface region at a depth X of 50 nm or more and 450 nm or less is defined as the s value.

[0017] As a result of extensive investigations, the inventors of the present application have confirmed that when the s value determined in 5) is in the range of 1.5 or more and 3.5 or less, there is a correlation between the c value determined in 4) and the strength of the alkali-free glass substrate. The alkali-free glass substrate according to the present invention has a c value determined in 4) of −0.0110 or less and an s value determined in 5) of 1.5 or more and 3.5 or less. When the c value and s value satisfy the above conditions, the alkali-free glass substrate has excellent strength.

[0018] The reason why the alkali-free glass substrate has excellent strength is presumed to be as follows.

[0019] The hydrogen ions detected by D-SIMS originate from Si-OH, which is formed when the Si-O-Si network of the glass is cut. It is speculated that cracks propagate in the depth direction along the cut sites of the Si-O-Si network of the glass.

[0020] If the c value is -0.0110 or less, H - / Si - The secondary ion intensity ratio drops sharply. This indicates that in the outermost surface region of the alkali-free glass substrate, the number of sites where the Si-O-Si network of the glass is broken rapidly decreases in the depth direction. This is thought to make it more difficult for cracks to propagate in the depth direction, improving the strength of the alkali-free glass substrate.

[0021] In the alkali-free glass substrate according to the present invention, the value c determined in 4) preferably satisfies −0.0150 or more and −0.0110 or less. Within the s-value range of 1.5 to 3.5, the smaller the c-value determined in 4), the stronger the alkali-free glass substrate tends to be. On the other hand, if the c-value is too small, the Si-OH formed by breaking the Si-O-Si network of the glass becomes concentrated in the outermost layer region of the alkali-free glass substrate. In this state, there is a risk that regions with different refractive indices will be formed in the outermost layer region of the alkali-free glass substrate, or that the substrate will be more susceptible to scratches during handling. If the c-value determined in 4) is −0.0150 or greater, the risk of these problems occurring is reduced. In the alkali-free glass substrate according to the present invention, the value c determined in 4) more preferably satisfies the range of −0.0150 to −0.0115, and even more preferably the range of −0.0150 to −0.0120.

[0022] In the alkali-free glass substrate according to the present invention, the s value determined in 5) preferably satisfies the range of 1.5 to 3.0, more preferably 1.5 to 2.6.

[0023] In the alkali-free glass substrate of the present invention, b in formula (1) is the b value, and the b value preferably satisfies 10.0 or more and 25.0 or less. When the s value is in the range of 1.5 or more and 3.5 or less, if the b value is small, the c value tends to be large, which may ultimately result in a decrease in the strength of the alkali-free glass substrate. On the other hand, if the b value is large, the H in the outermost surface layer region of the alkali-free glass substrate may be reduced. - / Si - The increase in the secondary ion intensity ratio may result in regions with different refractive indices between the outermost surface region and the surface regions other than the outermost surface region of the alkali-free glass substrate, and may make the substrate more susceptible to scratches during handling. If the b value is 10.0 or more and 25.0 or less, the risk of these problems occurring is reduced.

[0024] The alkali-free glass substrate of the present invention is preferably an alkali-free float glass substrate manufactured by the procedure described below. The alkali-free float glass substrate has a bottom surface that comes into contact with molten metal during forming and a top surface opposite to the bottom surface. When the alkali-free glass substrate of the present invention is an alkali-free float glass substrate, it is preferable that the top surface satisfies the c value determined in 4) above is −0.0110 or less and the s value determined in 5) above is 1.5 or more and 3.5 or less.

[0025] In the present specification, the surface strength measured by a ball-on-ring test is used as an index of the strength of an alkali-free glass substrate. In the alkali-free glass substrate according to the present invention, the surface strength F (N) measured by a ball-on-ring test and the thickness t (mm) of the glass substrate preferably satisfy the formula (2). F≧1100×(t / 0.7) 2 (2)

[0026] The alkali-free glass substrate according to the present invention can be appropriately selected from a wide range of compositions as long as it does not substantially contain alkali metal oxides. Note that alkali metal oxides (RO) may break the Si-O-Si network in the glass and exist as Si-OR. Therefore, it is predicted that in glass substrates containing alkali metal oxides, not only Si-OH but also Si-OR in the outermost layer region contribute to strength changes. On the other hand, glass that does not substantially contain alkali metal oxides can be appropriately selected from a wide range of compositions because it is not necessary to consider the contribution of Si-OR. Specific examples of the composition of the alkali-free glass substrate according to the present invention are shown below.

[0027] A specific example of an alkali-free glass substrate according to the present invention contains, in mass % on an oxide basis, SiO2: 54 to 66%, Al2O3: 10 to 23%, B2O3: 6 to 12%, and MgO+CaO+SrO+BaO: 8 to 26%. Hereinafter, in the present specification, mass % based on oxides will be simply referred to as "%".

[0028] Next, the composition range of each component will be described. When SiO2 is 54% or more, the strain point of the alkali-free glass substrate is improved and the chemical resistance is improved, preferably 55% or more, more preferably 57% or more, and even more preferably 58% or more. When SiO2 is 66% or less, the melting property during glass melting is good, preferably 64% or less, more preferably 62% or less, and even more preferably 61% or less.

[0029] When Al2O3 is 10% or more, phase separation is suppressed and the strain point of the alkali-free glass substrate is improved, preferably 12% or more, more preferably 14% or more, and even more preferably 16% or more. When Al2O3 is 23% or less, the melting property during glass melting is good, preferably 22% or less, more preferably 21% or less, and even more preferably 20% or less.

[0030] If B2O3 is 6% or more, the solubility during glass melting is good and the chemical resistance of the alkali-free glass substrate is improved. B2O3 is preferably 6.5% or more, more preferably 7% or more, and even more preferably 7.4% or more. If B2O3 is 12% or less, the strain point of the alkali-free glass substrate is improved, preferably 11% or less, more preferably 10% or less, and even more preferably 9% or less.

[0031] When the total amount of MgO, CaO, SrO, and BaO (i.e., MgO+CaO+SrO+BaO) is 8% or more, the melting property of the glass is good. The total amount is preferably 9% or more, more preferably 10% or more, and even more preferably 12% or more. When MgO+CaO+SrO+BaO is 26% or less, the strain point of the alkali-free glass substrate is improved, preferably 24% or less, more preferably 22% or less, and even more preferably 20% or less.

[0032] MgO can be contained to improve the melting property of the glass during melting. The content is preferably 0.1% or more, more preferably 1% or more, even more preferably 2% or more, and particularly preferably 3% or more. A content of 12% or less is preferable because phase separation can be suppressed, 10% or less is more preferable, 8% or less is even more preferable, and 6% or less is particularly preferable.

[0033] CaO can be contained to improve the solubility of the glass during melting. The content is preferably 0.1% or more, more preferably 1.5% or more, even more preferably 3% or more, and particularly preferably 3.5% or more. A CaO content of 12% or less is preferable because it reduces the amount of phosphorus, an impurity in limestone (CaCO3), the CaO raw material. A CaO content of 10% or less is more preferable, 8% or less is even more preferable, and 6% or less is particularly preferable.

[0034] SrO can be contained to improve the solubility of the glass during melting. The content is preferably 0.1% or more, more preferably 1% or more, even more preferably 3% or more, and particularly preferably 5% or more. A content of 16% or less is preferable because acid resistance is good, 14% or less is more preferable, 12% or less is even more preferable, and 10% or less is particularly preferable.

[0035] BaO can be contained to improve solubility, and its content is preferably 0.1% or more. A content of 16% or less is preferable because segregation is less likely to occur when the raw material is melted, 13% or less is more preferable, 10% or less is even more preferable, and 7% or less is particularly preferable.

[0036] Another specific example of an alkali-free glass substrate according to the present invention contains SiO2: 54 to 73%, Al2O3: 10.5 to 24%, B2O3: 0.1 to 12%, MgO: 0 to 8%, CaO: 0 to 14.5%, SrO: 0 to 24%, BaO: 0 to 13.5%, ZrO2: 0 to 5%, and MgO+CaO+SrO+BaO: 8 to 29.5%.

[0037] In another specific example of the alkali-free glass substrate according to the present invention, when both a high strain point and high solubility are to be achieved, the glass preferably contains, in mass % on an oxide basis, 58 to 66% SiO2, 15 to 22% Al2O3, 5 to 12% B2O3, 0 to 8% MgO, 0 to 9% CaO, 0 to 12.5% ​​SrO, and 0 to 2% BaO, with a total of MgO+CaO+SrO+BaO of 9 to 18%.

[0038] In another specific example of the alkali-free glass substrate according to the present invention, when a particularly high strain point is desired, the composition is preferably, in mass % on an oxide basis, SiO: 54 to 73%, Al2O3: 10.5 to 22.5%, B2O3: 0.1 to 5.5%, MgO: 0 to 8%, CaO: 0 to 9%, SrO: 0 to 16%, BaO: 0 to 9%, or MgO+CaO+SrO+BaO: 8 to 26%.

[0039] [Method for manufacturing alkali-free float glass substrates] A method for producing an alkali-free float glass substrate according to one embodiment of the present invention will now be described. The method for producing an alkali-free float glass substrate according to the present invention includes a forming step of continuously supplying molten glass onto a bath surface of molten metal contained in a bath to form a glass ribbon. Molten glass obtained by melting glass raw materials in a melting furnace is supplied onto the bath surface of the molten metal. The formed glass ribbon is drawn out from the bath surface of the molten metal and slowly cooled in an annealing furnace.

[0040] Roof bricks are provided above the molten metal contained in the bath. Float baths having a bath and roof bricks are described in, for example, JP 2006-16291 A, JP 2016-98160 A, and JP 2019-94222 A.

[0041] When the melting furnace side is defined as the upstream side and the annealing furnace side is defined as the downstream side, the glass ribbon moves from the upstream side to the downstream side on the bath surface of the molten metal. During this process, the temperature of the glass ribbon decreases and the viscosity increases.

[0042] The space between the molten metal and the roof bricks is maintained in a reducing atmosphere by introducing hydrogen to suppress oxidation of the molten metal, whereas the atmosphere in the downstream annealing furnace is not maintained in a reducing atmosphere and oxygen is present in the atmosphere.

[0043] Due to the structure of the float bath, it is difficult to completely prevent oxygen from the outside air or the annealing furnace from entering the space between the molten metal and the roof bricks. Oxygen that enters the space between the molten metal and the roof bricks reacts with hydrogen in the reducing atmosphere to produce water. When the resulting water comes into contact with the top surface of the glass ribbon, the Si-O-Si network of the glass is broken, forming Si-OH. The longer the contact time between water and the top surface of the glass ribbon, the deeper the water penetrates from the top surface, and the deeper the cut sites in the Si—O—Si network are formed.

[0044] Therefore, shortening the contact time between water and the top surface of the glass ribbon makes it difficult for water to penetrate from the top surface, which makes it difficult for the Si-O-Si network to be broken to form deeper. The contact time between the water and the top surface of the glass ribbon can be reduced by reducing the hydrogen concentration in the space between the molten metal and the roof bricks.

[0045] However, in the upstream region where the viscosity of the glass ribbon is low, no correlation is observed between the hydrogen concentration in the space between the molten metal and the roof bricks and the severance of the Si-O-Si network. The inventors of the present application speculate that the reason for this is as follows.

[0046] In the upstream region where the viscosity of the glass ribbon is low, the glass ribbon is softened, and the glass ribbon spreads on the surface of the molten metal bath from the upstream side to the downstream side. In this softened state of the glass ribbon, compositional flow occurs, so the surface of the glass ribbon is always in a new state even if the Si-O-Si network is broken.

[0047] On the other hand, in the downstream region where the viscosity of the glass ribbon is high, the glass ribbon has hardened and compositional flow does not occur, so the surface of the glass ribbon does not return to a new state even if the Si-O-Si network is cut. Therefore, a correlation is recognized between the hydrogen concentration in the space between the molten metal and the roof bricks and the cutting of the Si-O-Si network. Therefore, by lowering the hydrogen concentration in the space between the molten metal and the roof bricks, the cut portion of the Si-O-Si network is less likely to form deeper, and an alkali-free float glass substrate with excellent strength can be obtained.

[0048] In the method for producing an alkali-free float glass substrate according to the present invention, the hydrogen concentration in the space between the molten metal and the roof bricks is more than 6.0 volume % and not more than 12.0 volume % in the upstream region where the viscosity η (dPa s) at the center in the width direction of the glass ribbon is less than log η = 7.65, and the hydrogen concentration in the space between the molten metal and the roof bricks is not more than 6.0 volume % in the downstream region where the viscosity η (dPa s) at the center in the width direction of the glass ribbon is log η = 7.65 or more. Here, log η = 7.65 is used as an index of the viscosity of the glass ribbon because it is the viscosity that corresponds to the softening point of alkali-free glass.

[0049] By setting the hydrogen concentration in the space between the molten metal and the roof bricks in the upstream region to more than 6.0% by volume and not more than 12.0% by volume, oxidation of the molten metal can be suppressed, thereby reducing dross defects caused by oxides in the molten metal. By setting the hydrogen concentration in the space between the molten metal and the roof bricks in the downstream region to 6.0% by volume or less, the Si-O-Si network is less likely to be broken to a greater depth, resulting in an alkali-free float glass substrate with excellent strength. The lower limit of the hydrogen concentration in the space between the molten metal and the roof bricks in the downstream region is not particularly limited, but is preferably 0.1% by volume or more. If the hydrogen concentration in the space between the molten metal and the roof bricks in the upstream and downstream regions is set to 6.0% by volume or less, an alkali-free float glass substrate with excellent strength can be obtained, but oxidation of the molten metal cannot be suppressed and dross defects cannot be reduced.

[0050] In the float bath disclosed in JP 2016-98160 A, a mixed gas of hydrogen and nitrogen is introduced as a reducing gas from a gas inlet into the space above the roof bricks, and the proportion of hydrogen in the mixed gas introduced is varied at different locations in the space. In the method for producing an alkali-free float glass substrate according to the present invention, the hydrogen concentration in the space between the molten metal M and the roof bricks 30 can be controlled within the above range by varying the proportion of hydrogen in the mixed gas introduced through the gas inlet 43 located in the upstream region and the gas inlet 43 located in the downstream region, as shown in Fig. 5 . For example, in the method for producing an alkali-free float glass substrate according to the present invention, only nitrogen may be introduced through the gas inlet located in the upstream region and the gas inlet located in the downstream region.

[0051] The method for producing an alkali-free float glass substrate of the present invention may be carried out in a conventional manner, except that in the forming step, the hydrogen concentration in the space between the molten metal and the roof bricks is set to more than 6.0 volume % and not more than 12.0 volume % in an upstream region where the viscosity (dPa s) at the center in the width direction of the glass ribbon is less than log η = 7.65, and the hydrogen concentration in the space between the molten metal and the roof bricks is set to not more than 6.0 volume % in a downstream region where the viscosity (dPa s) at the center in the width direction of the glass ribbon is log η = 7.65 or more. That is, glass raw materials prepared so as to have the composition of the desired alkali-free float glass substrate are melted in a melting furnace to form molten glass, and then a forming step is carried out to form a glass ribbon. The glass ribbon is then drawn out from the surface of the molten metal bath and annealed in an annealing furnace and cut to the desired dimensions.

[0052] When the alkali-free float glass substrate is used as a glass substrate for FPDs, at least one of the two surfaces of the alkali-free float glass substrate is further polished to remove minute irregularities and waviness present on the surface of the alkali-free float glass substrate. In this case, of the two surfaces of the alkali-free float glass substrate, only the bottom surface is often polished. [Example]

[0053] The present invention will be described in detail below with reference to examples. Examples 1 to 3 are working examples, and Examples 4 to 6 are comparative examples. However, the present invention is not limited to these examples.

[0054] In this example, a float glass manufacturing apparatus having a melting furnace, a float bath, and an annealing furnace was used. Figure 5 is a cross-sectional view showing the float glass manufacturing apparatus used in the example. However, the melting furnace and the annealing furnace are omitted. The float glass manufacturing apparatus 10 shown in Figure 5 has a bath 20, a spout lip 22, a twill 23, a roof brick 30, a roof casing 40, a partition 42, a top roll 50, and a heater 60.

[0055] The bath 20 contains molten metal M on which a glass ribbon G floats. The spout lip 22 continuously supplies molten glass onto the surface of the molten metal M contained in the bath 20 at a flow rate corresponding to the distance from the spout lip 22 to the tweel 23, forming a glass ribbon G. The roof bricks 30 are disposed above the bath 20 and cover the space 21 above the bath 20. A mixed gas of hydrogen and nitrogen is supplied to the space 21 above the bath 20 through the through holes 31 in the roof bricks 30 to prevent oxidation of the molten metal M. The roof casing 40 forms an internal space 41 between itself and the roof bricks 30. The mixed gas of hydrogen and nitrogen is supplied from a gas inlet 43 provided at the top of the roof casing 40 into the internal space 41 of the roof casing 40, and then supplied to the space 21 above the bath 20 through the through holes 31 in the roof bricks 30. The partitions 42 divide the internal space 41 of the roof casing 40 into 10 sections, sequentially from upstream in the flow direction of the glass ribbon G, and the supply amount and blending ratio of the hydrogen and nitrogen mixed gas can be changed for each section. The top rolls 50 apply tension to the glass ribbon G in the width direction by supporting the side edges of the glass ribbon G. This limits the shrinkage of the glass ribbon G in the width direction, allowing a thin glass ribbon G to be formed. The heaters 60 are inserted into the through holes 31 in the roof bricks 30 and protrude downward from the roof bricks 30 to heat the glass ribbon G and the like. In this embodiment, the temperatures of the molten glass and glass ribbon G passing through the bath 20 were set so that sections 1 to 6 were on the upstream side and sections 7 to 10 were on the downstream side. A shielding plate 32 was installed at the boundary between sections 6 and 7 so as to protrude downward from the roof brick 30, and could partially block the atmospheres upstream and downstream. In Examples 1 to 6, the hydrogen concentrations in the mixed gas introduced from Sections 1 to 6 on the upstream side and Sections 7 to 10 on the downstream side were different. Because the introduced mixed gas was partially mixed upstream and downstream due to the airflow within the bath, measurement tubes were installed in the flow direction of the glass ribbon at positions 100°C above and below the softening point (approximately 930°C) where the viscosity (dPa·s) at the center of the width of the glass ribbon reached logη = 7.65 (1030°C upstream, 830°C downstream). The tip of the measurement tube was located 1 m from the side edge of the float bath, and the hydrogen concentration of the aspirated atmosphere was measured using a thermal conductivity gas analyzer (CALOMAT, manufactured by Japan Air Gassys Co., Ltd.). The results are shown in Table 1 below.

[0056] The molten glass obtained by melting glass raw materials with an alkali-free glass composition in a melting furnace was formed into a glass ribbon in a float bath. The glass ribbon was then slowly cooled in an annealing furnace and cut to obtain alkali-free glass substrates with a thickness of 0.7 mm. The alkali-free glass substrate contains, in mass % oxide content, 60% SiO2, 17% Al2O3, 8% B2O3, 3% MgO, 4% CaO, and 8% SrO.

[0057] D-SIMS was performed on the top surface of the obtained alkali-free glass substrate, and depth profiles (2) were obtained according to the above procedures 1) to 3). Figure 1 shows the depth profiles (2) of Examples 1 to 6, with the horizontal axis representing depth and the vertical axis representing internal normalized hydrogen count. Furthermore, following the procedure in 4) above, the exponential approximation curve formula (1) was determined from the plot of the outermost surface region at a depth of 50 nm to 150 nm in the depth profile (2). Figure 2 shows the depth profile (2) of Example 1, and Figure 3 shows the exponential approximation curve determined from the plot of the outermost surface region at a depth of 50 nm to 150 nm in the depth profile (2) of Example 1. The c value determined from formula (1) was -0.0117, and the b value was 16.0. Furthermore, in accordance with the procedure of 5) above, the s value, which is the average value of the internal normalized hydrogen count in the surface region from a depth of 50 nm to 450 nm in the depth profile (2), was determined to be 2.3. The D-SIMS measurement conditions were as follows: Equipment: ULVAC-PHI ADEPT1010 Primary ion species: Cs + Primary ion accelerating voltage: 5 kV Primary ion current value: 100nA Incident angle of primary ions: 60° to the normal of the sample surface Primary ion raster size: 300 x 300 μm 2 Secondary ions monitored: 1 H - , 30 Si - Secondary ion detection area: 90 x 90 μm 2 (9% of the primary ion raster size) Use of neutralization gun: Yes Method for converting the horizontal axis from sputtering time to depth X: The depth of the analyzed crater was measured using a stylus surface profiler (Dektak150 manufactured by Veeco) to determine the primary ion sputtering rate. Using this sputtering rate, the horizontal axis was converted from sputtering time to depth X. Plot interval: 5nm or less 1 H - Field Axis Potential at the time of detection: The value was set so that the background was sufficiently cut. Vacuum level of measurement chamber: 5.0 x 10 -9 Less than Torr The c value, b value, and s value were determined in the same manner for Examples 2 to 6. The results are shown in Table 1 below. The device may be an IMS 7f manufactured by AMETEK CAMECA.

[0058] Using the alkali-free glass substrates obtained in Examples 1 to 6, the fracture load was measured five times by the ball-on-ring (BoR) method using a SUS ring with a diameter of 30 mm and a radius of 2.5 mm and a SUS ball with a diameter of 10 mm, and the BoR average fracture load was calculated from the measurement results. The results are shown in Table 1 below. The relationship between the c value and the BoR average fracture load for Examples 1 to 6 is also shown in Figure 4.

[0059] [Table 1]

[0060] The alkali-free glass substrates of Examples 1 to 3, which had an upstream hydrogen concentration of more than 6.0 vol% and 12.0 vol% or less and a downstream hydrogen concentration of 6.0 vol% or less, all had a c value of -0.0110 or less and an s value of 1.5 to 3.5. The alkali-free glass substrates of Examples 1 to 3 had an BoR average fracture load of 1100 N or more and were excellent in surface strength. The alkali-free glass substrates of Examples 4 to 6, in which the downstream region hydrogen concentration exceeded 6.0% by volume, all had c values ​​exceeding −0.0110 and BoR average fracture loads less than 1100 N, indicating poor surface strength. [Explanation of symbols]

[0061] 10. Float glass manufacturing equipment 20 Bathtub 21 Space above the bathtub 22 Spout Trip 23 Tweel 30 Roof Bricks 31 Through hole 32 Shielding plate 40 Roof casing 41 Roof casing internal space 42 partitions 43 Gas inlet 50 Top Roll 60 Heater G Glass ribbon M Molten metal

Claims

1. The thickness of the glass substrate is 0.75 mm or less, and Cs is + Secondary ion mass spectrometry (D-SIMS) was performed using primary ions, and the horizontal axis was the depth X (nm) from the one surface, and the vertical axis was H - / Si - A depth profile (plotting interval: 10 nm or less) is created as a secondary ion intensity ratio, and when the following (a) to (d) are identified from the depth profile, the c value calculated by the following (c) is −0.0110 or less, and the s value calculated by the following (d) is 1.5 or more and 3.5 or less, an alkali-free glass substrate, wherein a surface strength F (N) of the one surface side measured by a ball-on-ring test satisfies the following formula (2) relative to a thickness t (mm) of the glass substrate: (a) In the depth profile, H in the internal region where the depth X is 450 nm or more and 500 nm or less - / Si - The average value of the secondary ion intensity ratios is calculated, and this value is taken as the internal hydrogen count. (b) H normalized to the internal hydrogen count of 1 - / Si - A new depth profile is created with the secondary ion intensity ratio as the internal normalized hydrogen count Y, the horizontal axis as the depth X (nm), and the vertical axis as the internal normalized hydrogen count Y. (c) In the depth direction profile of (b), the exponential approximation curve of the equation (1) is determined from the plot of the outermost surface region where the depth X is 50 nm or more and 150 nm or less, and c in the equation (1) is defined as the c value. logY=cX+logb (1) (d) In the depth profile of (b), the average value of the internal normalized hydrogen count Y in the surface region at a depth X of 50 nm or more and 450 nm or less is defined as the s value. F≧1100×(t / 0.7) 2 (2)

2. 2. The alkali-free glass substrate according to claim 1, wherein the value c obtained by (c) satisfies −0.0150 or more and −0.0110 or less.

3. 3. The alkali-free glass substrate according to claim 1, wherein b in formula (1) is a b value, and the b value calculated by (c) is 10.0 or more and 25.0 or less.

4. A float glass substrate having a bottom surface that contacts molten metal during forming and a top surface opposite to the bottom surface, 2. The alkali-free glass substrate according to claim 1, wherein the top surface satisfies the following conditions: a value c determined in (c) is −0.0110 or less; and a value s determined in (d) is 1.5 or more and 3.5 or less.

5. A method for producing an alkali-free glass substrate according to any one of claims 1 to 4, comprising: the method for producing an alkali-free glass substrate includes a forming step of continuously supplying molten glass onto a bath surface of molten metal contained in a bath to form a glass ribbon, In the forming step, roof bricks are provided above the molten metal, In an upstream region where the viscosity η (dPa s) at the center in the width direction of the glass ribbon is less than log η = 7.65, the hydrogen concentration in the space between the molten metal and the roof bricks is more than 6.0 vol% and not more than 12.0 vol%, a hydrogen concentration in a space between the molten metal and the roof bricks is set to 6.0 volume % or less in a downstream region where the viscosity η (dPa s) at the center in the width direction of the glass ribbon is log η = 7.65 or more.

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