WO2014167854A1

WO2014167854A1 - Glass sheet and method for producing glass sheet

Info

Publication number: WO2014167854A1
Application number: PCT/JP2014/002044
Authority: WO
Inventors: Satoshi Tanaka; Kiyomi FUKUSHIMA; Kazuishi Mitani; Yasuhiro Saito
Original assignee: Nippon Sheet Glass Company, Limited
Priority date: 2013-04-09
Filing date: 2014-04-09
Publication date: 2014-10-16

Abstract

A glass sheet of the present invention is a glass sheet including at least one surface subjected to dealkalization. The surface has repeating projections and depressions formed therein. In the projections and the depressions, a distance between a projection reference level and a depression reference level in a thickness direction of the glass sheet is 100 to 200 nm. In a region between the projection reference level and the depression reference level, the change rate of the porosity representing the proportion of voids is -3.0 to 2.0%/nm over the region from the projection reference level to the depression reference level.

Description

GLASS SHEET AND METHOD FOR PRODUCING GLASS SHEET

The present invention relates to a glass sheet including a surface subjected to dealkalization, and a method for producing a glass sheet.

Conventionally, a technique of forming a thin film having a high transmittance on a surface of a glass sheet is known as one method for increasing the transmittance of the glass sheet. For example, Patent Literature 1 describes a glass sheet including a thin film whose main component is silicon nitride and that contains at least one of carbon and hydrogen. In addition, Patent Literature 2 describes a glass sheet having an improved transmittance provided by a thin film having a multi-layer structure in which one of the layers is a vanadium oxide layer having tungsten and fluorine added thereto.

As another example of a method for increasing the transmittance of a glass sheet, Patent Literature 3 proposes a method relating to a glass sheet formed by a float process, the method including removing a predetermined thickness of an outermost surface portion of the bottom side of the glass sheet so as to remove a layer containing tin cations. This method makes it possible to obtain a glass sheet that exhibits a high transmittance intrinsic to the glass composition.

{Patent Literature 1} Japanese Laid-Open Patent Publication No. 2003-221257
{Patent Literature 2} Japanese Laid-Open Patent Publication (Translation of PCT Application) No. 2002-516813
{Patent Literature 3} Japanese Laid-Open Patent Publication No. 2006-206400

In the case of film-coated glass sheets as proposed in Patent Literature 1 and Patent Literature 2, a certain degree of improvement in transmittance can be expected. However, a step of applying the film is necessarily added, which leads to high cost. In addition, the delaminability and durability of the applied film may constitute a problem. For the glass sheet proposed in Patent Literature 3, the step of removing a surface portion is required, which results in high cost.

Therefore, the present invention aims to provide a glass sheet having an increased transmittance without a significant increase in cost.

The present invention provides a glass sheet including at least one surface subjected to dealkalization. The surface has repeating projections and depressions formed therein. In the projections and the depressions, a distance between a projection reference level and a depression reference level in a thickness direction of the glass sheet is 100 to 200 nm. In a region between the projection reference level and the depression reference level, a change rate of a porosity representing a proportion of voids is -3.0 to 2.0%/nm over the region from the projection reference level to the depression reference level. The projection reference level is a level in the thickness direction at which the porosity is 80%, and the depression reference level is a level in the thickness direction at which the porosity is 20%.

The present invention further provides a method for producing a glass sheet, the method including the steps of: (I) forming a molten glass raw material into a glass ribbon on a molten metal; and (II) bringing an acid gas containing a fluorine element (F)-containing acid and water vapor into contact with a surface of the glass ribbon on the molten metal so as to subject the surface of the glass ribbon to dealkalization. A volume ratio of the water vapor to the acid in the acid gas is more than 0 and not more than 1.

A surface of the glass sheet of the present invention has repeating projections and depressions formed therein. These projections and depressions meet requirements that the distance between the projection reference level and the depression reference level in the thickness direction of the glass sheet be 100 to 200 nm, and that the change rate of the porosity in the region between the projection reference level and the depression reference level be -3.0 to 2.0%/nm over the region from the projection reference level to the depression reference level. In a region where projections and depressions presenting a pattern that meets such requirements are formed, the refractive index changes gradually in the thickness direction of the glass, and light reflection is therefore less likely to occur. Consequently, the glass sheet of the present invention can achieve a high transmittance. In addition, since the glass sheet of the present invention can be produced using a dealkalization step conventionally carried out for production of glass sheets, the cost does not increase significantly. Thus, according to the present invention, a glass sheet having an increased transmittance can be provided without a significant increase in cost.

With the method of the present invention for producing a glass sheet, the above glass sheet of the present invention can be produced without a significant increase in cost.

Fig. 1 is a schematic diagram showing an example of a system capable of producing a glass sheet of the present invention. Fig. 2 is a scanning electron microscope (SEM) photograph of a dealkalized surface of a glass sheet of Example 1. Fig. 3 is a SEM photograph of a dealkalized surface of a glass sheet of Example 2. Fig. 4 is a SEM photograph of a dealkalized surface of a glass sheet of Example 3. Fig. 5 is a SEM photograph of a dealkalized surface of a glass sheet of Comparative Example 1. Fig. 6 is a SEM photograph of a dealkalized surface of a glass sheet of Comparative Example 2. Fig. 7 is a SEM photograph of a dealkalized surface of a glass sheet of Comparative Example 3. Fig. 8 is a SEM photograph of a dealkalized surface of a glass sheet of Comparative Example 4. Fig. 9 is a SEM photograph of a dealkalized surface of a glass sheet of Comparative Example 5. Fig. 10 is a SEM photograph of a dealkalized surface of a glass sheet of Comparative Example 6. Fig. 11 is a SEM photograph of a dealkalized surface of a glass sheet of Comparative Example 7. Fig. 12 is a SEM photograph of a dealkalized surface of a glass sheet of Comparative Example 8. Fig. 13 is a SEM photograph of a dealkalized surface of a glass sheet of Comparative Example 9. Fig. 14A is a binarized image of a cross-section of the glass sheet of Example 1 taken along the thickness direction. Fig. 14B is a binarized image of a cross-section of the glass sheet of Example 2 taken along the thickness direction. Fig. 14C is a binarized image of a cross-section of the glass sheet of Example 3 taken along the thickness direction. Fig. 15A is a graph showing the porosity calculated based on the result of Fig. 14A. Fig. 15B is a graph showing the porosity calculated based on the result of Fig. 14B. Fig. 15C is a graph showing the porosity calculated based on the result of Fig. 14C. Fig. 16A is a graph showing the porosity change rate calculated based on the result of Fig. 15A. Fig. 16B is a graph showing the porosity change rate calculated based on the result of Fig. 15B. Fig. 16C is a graph showing the porosity change rate calculated based on the result of Fig. 15C. Fig. 17A is a binarized image of a cross-section of the glass sheet of Comparative Example 1 taken along the thickness direction. Fig. 17B is a binarized image of a cross-section of the glass sheet of Comparative Example 2 taken along the thickness direction. Fig. 17C is a binarized image of a cross-section of the glass sheet of Comparative Example 3 taken along the thickness direction. Fig. 17D is a binarized image of a cross-section of the glass sheet of Comparative Example 5 taken along the thickness direction. Fig. 17E is a binarized image of a cross-section of the glass sheet of Comparative Example 6 taken along the thickness direction. Fig. 18A is a graph showing the porosity calculated based on the result of Fig. 17A. Fig. 18B is a graph showing the porosity calculated based on the result of Fig. 17B. Fig. 18C is a graph showing the porosity calculated based on the result of Fig. 17C. Fig. 18D is a graph showing the porosity calculated based on the result of Fig. 17D. Fig. 18E is a graph showing the porosity calculated based on the result of Fig. 17E. Fig. 19A is a graph showing the porosity change rate calculated based on the result of Fig. 18A. Fig. 19B is a graph showing the porosity change rate calculated based on the result of Fig. 18B. Fig. 19C is a graph showing the porosity change rate calculated based on the result of Fig. 18C. Fig. 19D is a graph showing the porosity change rate calculated based on the result of Fig. 18D. Fig. 19E is a graph showing the porosity change rate calculated based on the result of Fig. 18E.

Hereinafter, an embodiment of the present invention will be described in detail.

A glass sheet of the present embodiment includes at least one surface subjected to dealkalization, and the surface has repeating projections and depressions formed therein. The projections and the depressions respectively project outwardly and are depressed inwardly in the thickness direction of the glass sheet. These projections and depressions are formed so as to meet at least the following requirements.
(1) In the projections and the depressions, a distance between a projection reference level and a depression reference level in the thickness direction of the glass sheet is 100 to 200 nm (the projection reference level can be regarded as a reference level from which the depressions are depressed inwardly, while the depression reference level can be regarded as a reference level from which the projections project outwardly).
(2) In a region between the projection reference level and the depression reference level, the change rate of the porosity representing the proportion of voids is -3.0 to 2.0%/nm over the region from the projection reference level to the depression reference level.

First, the porosity and the change rate of the porosity in the requirement (2) will be described with reference to Fig. 14A to Fig. 14C and Fig. 15A to Fig. 15C. Fig. 14A to Fig. 14C are respectively binarized images of cross-sectional views of glass sheets of later-described Examples 1 to 3 taken along the thickness direction. These binarized images can each be obtained as follows: a SEM photograph of a cross-section of the glass sheet taken along the thickness direction is scanned by a scanner; and the scanned image is then subjected to binarization. The upper surface in each image is a glass surface subjected to dealkalization, and the vertical direction is the thickness direction of the glass sheet. These binarized images show results of binarization carried out in such a manner that portions where glass is present are indicated in white, while portions where glass is not present, in other words, void portions, are indicated in black.

Fig. 15A to Fig. 15C are graphs generally called bearing curves, and are those calculated respectively based on the results of Fig. 14A to Fig. 14C. Bearing curves are curves specified in JIS B 0601. In Fig. 15A to Fig. 15C, the horizontal axis represents a distance (depth) from the glass sheet surface (the highest projection point) in the thickness direction of the glass sheet, and the vertical axis represents porosities at different levels in the thickness direction. The porosities at different levels in the thickness direction mean the proportions of voids at different levels in the thickness direction in a cross-section of the glass sheet taken along the thickness direction. Specifically, the porosities can be calculated by counting the number of black pixels, i.e., pixels representing voids, in the binarized image data. The change rate of the porosity means the rates of change of the porosity at different levels in the thickness direction.

Next, the projection reference level and the depression reference level in the requirement (1) will be described. The projection reference level means a level in the thickness direction at which the porosity is 80%, and corresponds to a level A in Fig. 15A to Fig. 15C. The depression reference level means a level in the thickness direction at which the porosity is 20%, and corresponds to a level B in Fig. 15A to Fig. 15C. The reason why the level of the most protruding portion (highest projection point) in the thickness direction of the glass sheet is not used as the projection reference level is that the surface of the glass sheet is consistently uneven, and thus that using the highest projection point as the projection reference level results in erroneous data (data that does not accurately represent the pattern of the projections and depressions). A detailed description will be given. The effect of the present invention is largely contributed to by the features that repeating projections and depressions formed in a surface of the glass sheet extend within predetermined boundaries (the distance between which is 100 to 200 nm) in the thickness direction of the glass sheet, and that the change rate of the porosity is in a specified range (-3.0 to 2.0%/nm) over the distance between the boundaries. That is, large protrusions present only in a part of the glass sheet surface do not contribute to the effect of the present invention. Although such protrusions should act as noise, when the level of such a protrusion is used as the projection reference level for specifying the distance between the boundaries within which the projections and depressions extend and for specifying the change rate of the porosity within the boundaries, erroneous data is obtained. The same goes for the reason why the level of the deepest point of voids in the thickness direction of the glass sheet is not used as the depression reference level. This will be specifically described using Fig. 15B. In the curve of Fig. 15B, a region where the porosity is not zero can be observed at levels for which the distance from the projection reference level in the thickness direction of the glass sheet is around 250 to 300 nm. This region is caused by non-uniformity of dealkalization. Therefore, erroneous data is obtained when such a region is used as the depression reference level for specifying the distance between the boundaries within which the projections and depressions extend and for specifying the change rate of the porosity within the boundaries.

The repeating projections and depressions formed in a dealkalized surface of the glass sheet of the present embodiment present a pattern that meets the requirement that the distance between the projection reference level and the depression reference level in the thickness direction of the glass sheet be 100 to 200 nm, and preferably 120 to 180 nm. By virtue of such a configuration, the glass sheet of the present embodiment can be expected to have an improved transmittance at a wavelength of 400 to 800 nm. The reason for this is as follows: since the distance of 100 to 200 nm corresponds to 1/4 of the wavelength of 400 to 800 nm, the reflection amount of incident light is reduced, which results in an increase in the transmission amount of incident light. On the other hand, when the distance between the projection reference level and the depression reference level in the thickness direction of the glass sheet falls outside the range of 100 to 200 nm, the reflection amount of incident light is increased, which results in a decrease in the transmission amount.

Although it is essential that repeating projections and depressions be formed, the adjacent projections and depressions do not need to be continuous with each other. It is desirable that a distance between one projection or depression and its adjacent projection or depression be equal to or less than 400 nm corresponding to the shortest wavelength among the wavelengths of 400 to 800 nm for which the effect of the present invention on transmittance increase is expected. The shape of a surface between one projection or depression and its adjacent projection or depression is not particularly limited, and may be, for example, approximately flat. Here, the distance between one projection or depression and its adjacent projection or depression means a distance from the terminating edge of the one projection or depression to the starting edge of the adjacent projection or depression. When the distance between one projection or depression and its adjacent projection or depression is 400 nm or more, it may happen that the haze ratio is increased due to scattering of incident light.

A distance from a depression located at the depression reference level to its adjacent depression is not particularly limited. The distance is desirably equal to or less than 400 nm corresponding to the shortest wavelength among the wavelengths of 400 to 800 nm for which the effect of the present invention on transmittance increase is expected. The longer the distance between the depressions is, the higher the possibility of increase in haze ratio due to scattering of incident light is.

The projections and depressions present a pattern that meets not only the requirement (1) but also the requirement (2) that the change rate of the porosity in a region between the projection reference level and the depression reference level be -3.0 to 2.0%/nm over the region from the projection reference level to the depression reference level. When the change rate of the porosity in the region between the projection reference level and the depression reference level is too large, the region where the projections and depressions are formed has a structure similar to that which is composed of a plurality of layers formed of materials having different components from each other. When the change rate of the porosity in the region between the projection reference level and the depression reference level is -3.0 to 2.0%/nm, the refractive index in the region where the projections and depressions are formed changes gradually in the thickness direction of the glass. This reduces light reflection due to varying refractive indices which is similar to that occurring at an interface between layers made of materials having different components. The change rate of the porosity is desirably -2.5 to 1.5%/nm over the region from the projection reference level to the depression reference level. On the other hand, when the change rate of the porosity does not fall within the range of -3.0 to 2.0%/nm, the porosity sharply increases or decreases. This causes light reflection similar to that caused at an interface between layers made of materials having different components, because of which the reflection amount cannot be reduced.

The projections and depressions can be formed by dealkalization. By virtue of the above characteristic structure that can be formed by dealkalization, the glass sheet of the present embodiment can achieve a high visible-light transmittance. In the case of this glass sheet, the average value of transmittance gains for 400 to 800-nm wavelength visible light can be increased to 2.5% or more or even 3.0% or more. Here, a transmittance gain is a value obtained by subtracting a measured value of the transmittance of the glass sheet before dealkalization from a measured value of the transmittance of the glass sheet after dealkalization. In general, the transmittance gain is calculated for every 1-nm wavelength interval. The average value of transmittance gains is a value obtained by determining the values of transmittance gains at the corresponding wavelengths in the wavelength range (the wavelength range of 400 to 800 nm in the present embodiment) for which the average value is to be calculated, and then by performing simple averaging of the determined values.

The term "dealkalization" as used herein means a treatment in which the glass surface is brought into contact with an acid gas to elute and remove an alkali component of the glass surface. The pattern of the projections and depressions formed by dealkalization can be controlled by appropriately selecting the conditions for the dealkalization (the type of the acid gas, the treatment temperature, the treatment time, etc.).

Hereinafter, an example of a method for producing the glass sheet of the present embodiment will be described.

The glass sheet of the present embodiment can be formed by a float process which is a continuous production method of glass sheets. In the float process, a glass raw material melted in a melting furnace (float furnace) is formed into a sheet-shaped glass ribbon on a molten metal (e.g., molten tin) in a float bath, the glass ribbon obtained is annealed in an annealing furnace, and then the glass ribbon is cut into glass sheets having a predetermined size. The production of glass sheets by this method can be carried out, for example, using a system shown in Fig. 1.

A glass raw material melted (molten glass) in a float furnace 11 flows from the float furnace 11 into a float bath 12, forms into a semisolid glass ribbon 10 while traveling on molten tin 15, and is then drawn out of the float bath by a roller 17 to be fed into an annealing furnace 13. The glass ribbon solidified in the annealing furnace 13 is cut into glass sheets having a predetermined size by a cutting device which is not shown.

A predetermined number of coaters 16 (three

coaters

16a, 16b, and 16c in the system shown) are disposed in the float bath 12 at a predetermined distance from a surface of the high-temperature glass ribbon 10 on the molten tin 15. An acid gas for dealkalization is continuously supplied onto the glass ribbon 10 from at least one coater of the coaters 16a to 16c, and an alkali component is sufficiently removed. The acid used is a fluorine element (F)-containing acid (desirably hydrogen fluoride (HF)). Furthermore, water is also supplied at the same time as the fluorine element (F)-containing acid is supplied onto the glass ribbon 10. That is, the dealkalization is carried out, for example, by supplying an acid gas containing hydrogen fluoride (HF) and water vapor to the surface of the glass ribbon 10 from the coater 16.

Since the temperature of the glass ribbon 10 on the molten tin 15 is much higher than the glass-transition point, modification of the glass surface is effectively achieved. When the acid gas containing hydrogen fluoride (HF) and water vapor is brought into contact with the surface of the hot glass ribbon 10, alkali ions in the glass surface are eluted, and components contained in the acid gas enter the glass in various forms, such as in the form of proton (H⁺), water (H₂O), and oxonium ion (H₃O⁺). Thereafter, the water having entered the glass exits from the glass by dehydration condensation, and projections and depressions are formed in the surface of the glass sheet as a result of the dealkalization. At this time, the glass surface is densified. Since hydrogen fluoride breaks Si-O bonds that are basic structures of the glass, it is easy for water and oxonium ions to enter the glass, in addition to which phenomena such as erosion of glass by hydrogen fluoride and reprecipitation of glass occur in a complicated manner. For these and other reasons, projections and depressions presenting a characteristic pattern that allows high transmittance for visible light are formed in the surface of the glass sheet as a result of the dealkalization.

In the acid gas used in the dealkalization, the volume ratio of the water vapor to the acid is, for example, 5 or less, desirably 3 or less, more desirably 2 or less, and even more desirably 1 or less. By adjusting the amount of water within this range, a glass sheet having a surface with a projection-depression pattern that allows high transmittance for visible light can be fabricated.

The acid concentration in the acid gas used is desirably 2.0 vol% or more, and more desirably 3.0 to 10.0 vol%. Adjusting the acid concentration within this range makes it possible to further control the pattern of the projections and depressions to be formed in the surface of the glass sheet, and makes it easy to form a desired projection-depression pattern. The acid gas may contain, for example, nitrogen gas in addition to the fluorine element (F)-containing acid and water vapor.

The time of contact between the acid gas and the glass (the treatment time) is desirably 3.0 to 10.0 seconds, and more desirably 4.0 to 8.0 seconds. The treatment temperature is desirably 580 to 740 degrees Celsius, and more desirably 600 to 680 degrees Celsius. Adjusting the treatment time and the treatment temperature within these ranges makes it possible that the pattern of the projections and depressions to be formed in the surface of the glass sheet is controlled to be a more desired pattern, thus making it easy to form a desired projection-depression pattern.

The acid gas is desirably recovered after the dealkalization. In this case, the acid gas may be recovered using the coater that supplies the acid gas. Alternatively, at least one of the coaters 16a to 16c may be used to recover the acid gas. When the acid gas is supplied to the molten tin 15, there may arise a problem of the molten tin 15 in the float bath 12 being oxidized by reaction with the acid gas. Recovering the acid gas allows avoiding the occurrence of such a problem and thereby producing a glass sheet having a surface with projections and depressions safely by dealkalization.

As described above, the projection and depressions in the surface of the glass sheet of the present embodiment can be formed using a conventional production process of glass sheets, in addition to which a conventional system can be used. Therefore, the glass sheet of the present embodiment can be produced relatively inexpensively.

It can be said that the glass sheet of the present embodiment can be produced, for example, by a production method including the steps of:
(I) forming a molten glass raw material into a glass ribbon on a molten metal (molten tin); and
(II) bringing an acid gas containing a fluorine element (F)-containing acid and water vapor into contact with a surface of the glass ribbon on the molten metal so as to subject the surface of the glass ribbon to dealkalization,
the volume ratio of the water vapor to the acid in the acid gas being more than 0 and not more than 1. This method may further include a step of recovering the acid gas after the step (II).

The glass sheet may be glass that can be produced by a float process or figured glass. For example, common soda-lime glass or the like can be used, and its composition is not particularly limited. For example, common clear glass, low-iron glass, or the like, can be used. The thickness of the glass sheet is not particularly limited, and can be, for example, 0.33 to 10.0 mm.

Hereinafter, the present invention will be described in more detail using examples. However, the present invention is not limited to the examples given below, and other examples are possible as long as they do not depart from the gist of the present invention.

(Examples 1 to 3)
[Method for producing glass sheet]
Soda-lime glass sheets were produced by a float process. For the production of each glass sheet, a system having the same configuration as the system shown in Fig. 1 was used. First, a glass raw material was prepared so as to have the following main composition of glass: 70.8 wt% of SiO₂, 1.0 wt% of Al₂O₃, 5.9 wt% of MgO, 8.5 wt% of CaO, and 13.2 wt% of Na₂O. The glass raw material was melted, the glass raw material melted was formed into a glass ribbon on molten tin in the float bath, and dealkalization was carried out by supplying an acid gas containing hydrogen fluoride (HF) and water vapor to a surface of the glass ribbon using a coater. The acid concentration (concentration of hydrogen fluoride (HF)) in the acid gas and the ratio of the water vapor concentration to the acid concentration are shown in Table 1. The treatment time and the treatment temperature are also shown in Table 1. The thicknesses of the glass sheets obtained are also shown in Table 1.

(Comparative Examples 1 to 3)
Glass sheets were fabricated by the same procedures as in Examples 1 to 3, except that water vapor was not contained in the gas blown onto the surface of the glass ribbon for dealkalization, and that the concentration of the acid (concentration of hydrogen fluoride (HF)) contained in the gas, the treatment time, and the treatment temperature were varied. The acid concentration in the gas, the treatment time, and the treatment temperature which were employed in Comparative Examples 1 to 3 are shown in Table 1.

(Comparative Examples 4 to 9)
Glass sheets were fabricated by the same procedures as in Examples 1 to 3, except that the acid concentration (concentration of hydrogen fluoride (HF)) in the acid gas, the ratio of the water vapor concentration to the acid concentration, and the treatment time and treatment temperature in the dealkalization were varied. The acid concentration in the gas, the treatment time, and the treatment temperature which were employed in Comparative Examples 4 to 9 are shown in Table 1.

[Average value of transmittance gains]
For Examples 1 to 3 and Comparative Examples 1 to 9, average values of transmittance gains for 400 to 800-nm wavelength visible light were determined. The results are shown in Table 1. The method employed is as follows. First, in order to determine transmittance gains, the transmittance of the glass sheet before dealkalization and the transmittance of the glass sheet after dealkalization were measured in the wavelength range of 400 to 800 nm for every 1-nm wavelength interval using a spectrophotometer, U4100 manufactured by Hitachi High-Technologies Corporation. For each measurement wavelength, a transmittance gain was calculated by subtracting the transmittance of the glass sheet before dealkalization from the transmittance of the glass sheet after dealkalization. This was followed by simple averaging of the transmittance gains in the wavelength range of 400 to 800 nm to determine the average value of the transmittance gains.

[SEM Observation]
The surfaces of the glass sheets obtained in Examples 1 to 3 and Comparative Examples 1 to 9 were observed by a SEM. Figs. 2 to 13 are respectively SEM photographs of the glass sheets of Examples 1 to 3 and Comparative Examples 1 to 9.

In Examples 1 to 3, it was confirmed that repeating projections and depressions were formed in the surfaces of the glass sheets. On the other hand, in the surfaces of the glass sheets of Comparative Examples 1 to 9, the presence of projections and depressions was not confirmed, or even when the presence was confirmed, it was presumed that the distance in the thickness direction of the glass sheet between the highest projection point and the deepest depression point in the projections and depressions was obviously shorter than 100 nm. The distance from the projection reference level to the depression reference level in a glass sheet is shorter than the distance between the highest projection point and the deepest depression point in the thickness direction of the glass sheet. Therefore, it can be determined that, in the glass sheets in which the distance between the highest projection point and the deepest depression point in the thickness direction of the glass sheet was obviously shorter than 100 nm, the distance from the projection reference level to the depression reference level was less than 100 nm.

[Binarization of SEM observation result]
SEM photographs of cross-sections of the glass sheets obtained in Examples 1 to 3 and Comparative Examples 1 to 3, 5, and 6, were scanned by a scanner, and the data obtained by the scanning were subjected to binarization. Photoshop (CS 6) manufactured by Adobe Systems Incorporated was used as the scanner. Fig. 14A, Fig. 14B, and Fig. 14C are respectively binarized images of cross-sections of the glass sheets of Examples 1, 2, and 3 taken along the thickness direction. Figs. 17A to 17E are respectively binarized images of cross-sections of the glass sheets of Comparative Examples 1 to 3, 5, and 6 taken along the thickness direction. The binarization was performed in such a manner that portions where glass was present were indicated in white, while portions where glass was not present, in other words, void portions, were indicated in black.

[Distance from projection reference level to depression reference level]
First, porosities at different levels in the thickness direction were determined with respect to the distance (depth) from the glass sheet surface (highest projection point) in the thickness direction of the glass sheet. The porosities at different levels in the thickness direction mean the proportions of voids at different levels in a cross-section of the glass sheet taken along the thickness direction. Specifically, the porosities were calculated by counting the number of black pixels, i.e., pixels representing voids, in the binarized image data. The porosities of the surfaces of the glass sheets obtained in Examples 1, 2, and 3, which were calculated using the data of Fig. 14A, Fig. 14B, and Fig. 14C, are respectively shown in Fig. 15A, Fig. 15B, and Fig. 15C. The porosities of the surfaces of the glass sheets obtained in Comparative Examples 1 to 3, 5, and 6, which were calculated using the data of Fig. 17A to Fig. 17E, are respectively shown in Fig. 18A to Fig. 18E. The projection reference levels and the depression reference levels were determined using Fig. 15A to Fig. 15C and Fig. 18A to Fig. 18E, and the distances from the projection reference levels to the depression reference levels were determined. The results are shown in Table 1. In Fig. 15A to Fig. 15C and Fig. 18A to Fig. 18E, the level A is the projection reference level (at which the porosity is 80%), and the level B is the depression reference level (at which the porosity is 20%). For Comparative Examples 4 and 7 to 9, since the presence of projections and depressions was not confirmed in the surfaces of the glass sheets, the calculation of the distance from the projection reference level to the depression reference level and the below-described calculation of the change rate of the porosity were not performed.

[Change rate of porosity]
The change rates of the porosity in Examples 1 to 3 and Comparative Examples 1 to 3, 5, and 6 were respectively calculated from the data of Fig. 15A to Fig. 15C and Fig. 18A to Fig. 18E. The change rates of the porosity obtained for Examples 1 to 3 and Comparative Examples 1 to 3, 5, and 6 are respectively shown in Fig. 16A to Fig. 16C and Fig. 19A to Fig. 19E. The ranges of the change rate of the porosity are shown in Table 1. In Fig. 16A to Fig. 16C and Fig. 19A to Fig. 19E, the level A is the projection reference level, and the level B is the depression reference level.

The glass sheets of Examples 1 to 3 showed a transmittance gain of more than 2.5%. The glass sheets of Comparative Examples 1 to 9 were not able to achieve a transmittance gain of 2.5% or more.

From the above results, it was confirmed that the glass sheet of the present invention can achieve a high transmittance, specifically, an average value of transmittance gains of 2.5% or more for 400 to 800-nm wavelength visible light.

By virtue of having a high visible-light transmittance, the glass sheet of the present invention is suitable for use as cover glass for solar cells required to utilize sunlight efficiently, for use as Low-E glass having an increased transmittance, and for use as glass for displays. In addition, the glass sheet of the present invention has a reduced reflectivity, and is therefore expected to be suitable also for use as an automobile windshield endowed with image-reflection preventing function, for use as glass for show windows, and for use as glass for displays.

Claims

A glass sheet comprising at least one surface subjected to dealkalization, wherein
the surface has repeating projections and depressions formed therein,
in the projections and the depressions, a distance between a projection reference level and a depression reference level in a thickness direction of the glass sheet is 100 to 200 nm,
in a region between the projection reference level and the depression reference level, a change rate of a porosity representing a proportion of voids is -3.0 to 2.0%/nm over the region from the projection reference level to the depression reference level,
the projection reference level is a level in the thickness direction at which the porosity is 80%, and
the depression reference level is a level in the thickness direction at which the porosity is 20%.
The glass sheet according to claim 1, wherein, in the region between the projection reference level and the depression reference level, the change rate of the porosity is -2.5 to 1.5%/nm over the region from the projection reference level to the depression reference level.
A method for producing a glass sheet, comprising the steps of:
(I) forming a molten glass raw material into a glass ribbon on a molten metal; and
(II) bringing an acid gas containing a fluorine element (F)-containing acid and water vapor into contact with a surface of the glass ribbon on the molten metal so as to subject the surface of the glass ribbon to dealkalization,
wherein a volume ratio of the water vapor to the acid in the acid gas is more than 0 and not more than 1.
The method for producing a glass sheet according to claim 3, further comprising a step of (III) recovering the acid gas after the step (II).