WO2012156991A2

WO2012156991A2 - Glass composition and glass substrate for display devices

Info

Publication number: WO2012156991A2
Application number: PCT/IN2012/000343
Authority: WO
Inventors: Jeetendra SHEGAL
Original assignee: Sterlite Technologies Ltd
Priority date: 2011-05-11
Filing date: 2012-05-11
Publication date: 2012-11-22
Also published as: WO2012156991A3; WO2012156991A4

Abstract

The present invention discloses a substrate glass and a glass composition with reduced β-ΟΗ, reduced bubbles or inclusion, wherein said glass composition comprises Si02 from about 63.3 to about 71.3 mole %; AI₂O₃ from about 8.1 to about 12.1 mole %; B₂O₃ from about 6.7 to about 12.7 mole %; CaO from about 5 to 12 mole %; BaO from about 0.2 to about 3.0 mole %; SrO from about 0.2 to about 4.5 mole %; and Na₂O from about 0.001 to about 0.1 mole %; wherein inclusion of the alkali metal Na₂ decreases the melting point of the substrate glass composition; fluorine from about 0.001 to about 0.1 mole % and chlorine from about 0.001 to about 0.5 mole %, help to improve refining and reduce

Description

GLASS COMPOSITION AND GLASS SUBSTRATE FOR DISPLAY DEVICES

FIELD OF THE INVENTION

Embodiments in general relate to a glass composition and a glass substrate for use in display and in particular to a glass composition and a glass substrate for use in liquid crystal display [LCD], an electroluminescence display [ELD], a field emission display [FED], plasma display [PD] organic light emitting diode [OLED] etc.

BACKGROUND OF THE INVENTION

Over the years active matrix liquid crystal displays (AMLCDs) have become ubiquitous features of our life, wherein they are used in cellular phones, mobiles, desktops, laptops computer screens and in large screen televisions.

One aspect of all the electronic devices that has turned out to be noteworthy over the years is energy utilization by the device which is obviously desired to be as low as possible. It is quite clear that CRT has been replaced by flat panel display and in particular by AMLCD and organic light-emitting diode (OLED)- based devices.

A typical AMLCD comprises, inter alia, two sheets of glass, the first one is vapor-deposited with silicon that forms the basis for the thin-film transistor (TFT) array and whereas the second for the color filter (CF) materials that enables RGB displays. A gap between the two sheets of glass holds a very thin layer of liquid crystal, wherein the gap between the two sheets has to be maintained within tight specifications. The glass (also referred to as "glass substrate") used for depositing the silicon for the thin-film transistor array has a unique material composition, which is specifically designed for the application.

One method of producing these substrate glasses is the down draw-fusion process [also known as the down-draw process] and is capable of producing a precision fire-polished surface that requires no additional modification such as grinding or polishing prior to use. The US patent numbers 3,338,696 and 3,682,609 disclose fusion downdraw processes which include allowing flow of molten glass over the edges or weirs of a forming wedge, referred to as isopipe. The molten glass flows over converging forming surfaces of the isopipe and the separate flows reunite at the apex or root where the two converging forming surfaces meet to form a glass sheet. Thus, in the fusion process the glass which has been in contact with the forming surfaces is located in the inner portion of the glass sheet and the exterior surfaces of the glass sheet are contact free. Pulling rolls positioned downstream of isopipe root capture edge portions of the glass sheet so formed to control the rate at which the glass sheet leaves the isopipe and thus aids in controlling the thickness of the finished sheet. As the glass sheet descends from the root of the isopipe past the pulling rolls, it cools to form a solid elastic glass sheet, which may be processed further. In order to achieve a good quality substrate glass using the fusion technique, various material properties of the glass in the molten stage must be controlled within specified limits during the downdraw process.

The molten glass that is supplied to the isopipe has to be free of any inclusion and/or gas bubbles. If the bubbles remain in the final glass sheet that is drawn, the bubbles may cause undesired distortion in the final LCD panel made from the glass sheet. It is thereby desired that the glass sheets drawn are bubble free.

In order to remove gas bubbles that are entrapped in the molten glass, refining of the molten glass is carried out, the bubbles being formed during the reaction by evolution of one or more reaction products in gas phase.

Attempts have been made to refine molten glass, that is, remove the bubbles or gas entrapped therein. One way to remove the bubbles from the molten glass is to let it flow through a distance, wherein the flow is maintained laminar. Many apparatus have been designed to maintain the molten glass flow laminar from the melting furnace to the isopipe. Further, the longer the length of the refining channel the better the . chances that the molten glass is bubble free by mechanism of rising of bubbles explained by Stoke 's Law.

However, to hold the molten glass the apparatus are to be made up of noble metals such as platinum and titanium which are exorbitantly expensive and hence increase the cost of making apparatus. Any increase in the dimensions will obviously lead to increase in the apparatus cost.

In addition to physical refining, it is known in the prior art that use of some chemical compounds enhances or improves the refining of molten glass. These compounds are known as the refining agents. Notable among them are arsenic oxide and antimony oxides. These refining agents are added with the raw materials and then melted along with the other glass component. These refining agents aids in removal of bubbles or gas entrapped in the molten glass by releasing oxygen that combines with the gas bubbles and increase their buoyancy thereby enhancing the chances of the bubbles to rise in short time span.

Another significant aspect in the glass is the β-ΟΗ content in the glass. , β-ΟΗ which is a part of glass structure, causes generation of oxygen bubbles at the platinum-glass interface due to the catalytic conversion of β-ΟΗ into 0₂ and H₂. H₂ diffuses out of the platinum wall and 0₂ remains in the glass as bubbles. It is thereby desired to have minimum β-ΟΗ content in the glass.

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Thus, there is a need to have a glass composition with the drying agents that reduce the concentration of β-ΟΗ resulting in improved refining and decreased foaming of the glass. OBJECTS OF THE PRESENT INVENTION

Some of the objects of the present disclosure are described herein below:

An object is to provide a glass composition and a substrate glass.

Another object is to provide a glass composition and a substrate glass for display devices.

Still another object is to provide a glass composition and a substrate glass compatible with the down draw process.

Another object is to provide a glass composition with a refining agent that are efficient in removal of bubbles or glass in molten stage.

Another object is to have a glass composition with minimum β-ΟΗ content, and in particular below 300 ppm.

Another object is to have a glass composition for use in liquid crystal display [LCD], an electroluminescence display [ELD], a field emission display [FED], plasma display [PD], organic light emitting diode [OLED] etc.

Other objects and advantages of the present invention will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present invention.

SUMMARY OF THE INVENTION

These and other objects of the present disclosure are to a great extent dealt within the disclosure.

In accordance with the present invention a substrate glass and a glass composition is disclosed with reduced β-ΟΗ, reduced bubbles or inclusion, wherein said glass composition comprises Si0₂ from about 63.3 to about 71.3 mole %; Al₂ ⁽¾ from about 8.1 to about 12.1 mole %; B2O3 from about 6.7 to about 12.7 mole %; CaO from about 5 to 12 mole %; BaO from about 0.2 to about 3.0 mole %; SrO from about 0.2 to about 4.5 mole %; wherein inclusion of the alkali metal Na₂0 from about 0.001 to 0.1 mole % decreases the melting point of the substrate glass composition; fluorine from about 0.001 to about 0.1 mole % and chlorine from about 0.001 to about 0.5 mole %.

In accordance with the present invention the inclusion of chlorine and fluorine helps to dry the glass melt by reducing β-ΟΗ. Additionally, the fluorine also acts as a refining agent.

In accordance with the present invention the glass composition further comprises of at least one of the refining agents chosen from the group consisting of As₂0₃ in the range from 0 to 0.7 mole % and Sb₂0₃ in the range from 0 to 0.8 mole % along with N₂0₅ in the range from about 0.1 to 0.4 mole%. It is observed that use of N2O5 in conjunction with AS2O3 and Sb₂0₃ increases the refining efficiency of the refining agents.

In accordance with the present invention the glass has a density of less than about 2.5 g/cm³, a coefficient of thermal expansion less than 45 x 10^"7/°C, a logarithm of liquidus viscosity of greater than 5, a strain point of greater than 620 °C, an annealing temperature of about 650°C or greater, and a liquidus temperature of less than 1230 °C.

In accordance with one embodiment of the present invention the glass is down- drawable or slot drawn or press-formable in a mold or drawable through roller press or drawable through float process.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein

The present invention is concerned with enhanced glasses for use as substrates in displays. In particular, the glasses meet the various property requirements of such substrates.

In accordance with the present invention, the glass substrate composition comprises of the major components of the glass, for use in glass substrates, namely, Silica (Si0₂), Alumina (AI2O3), Boron Oxide (B2O3), trace amount of alkali oxides (AO) such as Na20 and alkaline earth oxides (AEO) such as CaO, SrO and BaO.

In accordance with the present invention silica (S1O2) behaves as the basic glass former, wherein the former (silica in this case) forms the basic skeleton or network. Formers alone may form glass however the melting point in some cases (particularly when the former is silica) will be so high so as to make it impractical to commercially melt the glasses. In other cases wherein B2O3 or P2O5 is utilized as former the chemical durability will be so poor that it is impractical to use it in any application in its hundred percent forms.

In accordance with the present invention modifiers such as alkali oxides (AO) and Alkaline earth oxides (AEO) are utilized to modify the network structure formed by the glass formers so as to reduce the melting temperature and thereby improve other glass forming processes e.g., refining. In accordance with the present invention trace amounts of alkali oxides such as Na₂0 is utilized as the modifier for reasons discussed herein below.

In accordance with one embodiment of the present invention alkaline earth oxides such as CaO, BaO and SrO are added as modifiers.

Further, in accordance with the present invention intermediates such as alumina is added to the glass composition.

In accordance with one embodiment the present invention, the substrate glass composition, comprises Si0₂ from about 63.3 to about 71.3 mole %; AI2O3 from about 8.1 to about 12.1 mole %; B₂0₃ from about 6.7 to about 12.7 mole %; CaO from about 5 to 12 mole %; BaO from about 0.2 to about 3.0 mole %; SrO from about 0.2 to about 4.5 mole %; and Na₂0 from about 0.001 to about 0.1 mole %; wherein inclusion of the trace amount of the alkali metal Na₂0 decreases the melting point of the substrate glass composition; fluorine from about 0.001 to about 0.1 mole % and chlorine from about 0.001 to about 0.5 mole % resulting in drying the glass, hence reducing β-ΟΗ, which thereby reduces foaming behavior of the glass.

In accordance with this embodiment of the present invention, the trace mole % of Na₂0 is limited to a cap value of 0.1 mole % as a greater amount than this may lead to diffusion of sodium into the TFT which may interfere with the electrical working of the TFT. Presence of trace amount of Na₂0 reduces the viscosity of glass melt, resulting in easier refining and reduced foaming.

In accordance with one embodiment the substrate glass composition may contain AS2O3 in the range from about 0 to about 0.7 mole %, wherein AS2O3 is acts as a fining agent and aids in removal of bubbles or gaseous inclusions from the molten glass.

In accordance with one embodiment the substrate glass composition may contain Sb₂0₃ in the range from about 0 to about 0.8 mole %, wherein Sb₂0₃ also acts as a fining agent and aids in removal of bubbles or gaseous inclusions from the molten glass.

In accordance with the present invention the substrate glass composition may contain N2O5 in the range from about 0.1 to 0.4 mole % in conjunction with AS2O3 and Sb₂03, wherein addition of N₂0₅ enhances the refining efficiency of As₂0₃ and Sb₂0₃.

In accordance with one embodiment of the present invention the amount of As₂0₃ and Sb₂03 is maintained to a minimum level in the glass composition.

The alkaline earth oxides SrO and BaO both contribute to low liquidus temperatures (high liquidus viscosities) and hence the glasses of the invention include at least one of these oxides. However, both oxides are known to raise the coefficient of thermal expansion (CTE) and density of the glass. It is desired to have both the CTE and density of the glasses to be low. Further, both SrO and BaO lowers the modulus and strain point of the glass in comparison with CaO.

In accordance with one embodiment the mole percentage of non-green components such as BaO and SrO is kept to a minimum so as to produce an environmentally friendly "green" product, and in particular lessening or getting rid of barium is preferred since barium is one of the listed metals in the Resources Conservation and Recovery Act (RCRA) and is therefore classified by the US EPA as hazardous.

In accordance with the present invention, it is observed that inclusion of chlorine, helps in refining and also helps to dry the glass melt by reducing β- OH. The term "β-ΟΗ value" is used to indicate the hydroxyl concentration of a glass as measured in the conventional manner by infrared spectroscopy according to U.S. Pat. No. 4,072,489 (i.e., at an appropriate wave length of a few microns). β-ΟΗ value of 0.04 indicates a hydroxyl concentration of about 40 ppm. In the glass structure, hydroxyl ions are a part of the network structure. Chlorides react with hydroxyl ions to form Hydrogen Chloride gas, which escapes from the glass melt. This reaction of Chlorides helps to reduce the β- OH in glass, thereby resulting in an improved refining and reduced foaming of the glass melt.

In accordance with the present invention, it is observed that inclusion of Fluorine helps in refining and also helps to dry the glass melt by reducing β- OH. As described before a β-ΟΗ value of 0.04 indicates a hydroxyl concentration of about 40 ppm. In the glass structure, hydroxyl ions are a part of the network structure. Fluorides react with hydroxyl ions to form Hydrogen Fluoride gas, which escapes from the glass melt. This reaction of Fluorides helps to reduce the β-ΟΗ in glass, thereby resulting in an improved refining and reduced foaming of the glass melt.

In accordance with the present invention, it is observed that the three components, i.e., Na₂0, F and CI individually assist in directly or indirectly assist in refining. But, their combination is more effective because of the combined multiplying effects of the individual components towards a common final process improvement, i.e., refining. If the same amount of effect (as it is obtained by the combined effect of the 3 components) is desired from any one component then the quantity of one single component will have to be very high and this will lead to other side effects, e.g., if Na₂0 alone is added then the Na₂0 contamination of TFT will happen. If F and CI alone are added then environmental hazard will be a problem. In accordance with the present invention an advantage of the glass from the glass composition is that it exhibit higher viscosity, typically of the order of 10⁶ poise.

In accordance with the present invention the glass from the glass composition exhibit a high-strain point greater than about 620 °C.

In accordance with the present invention the glass from the glass composition has lower liquidus temperature of less than about 1230 °C.

In accordance with the present invention the glass from the glass composition exhibits a high liquidus viscosity of about 10⁵ or greater.

In accordance with the present invention the glass from the glass composition exhibits an annealing temperature of greater than about 650 °C

In accordance with the present invention the trace amount of Na₂0 does not chemically interact with the TFTs due to very low percentages and but still reduce the melting point of the glass.

In accordance with the present invention, the substrate glass has light-weight owing to the low density of less than about 2.5 g/cm³. In accordance with the present invention, the substrate glass has lower coefficient of thermal expansion of less than about 40 x 10^"7/°C

Thus, the present invention provides a glass composition for display devices compatible with the down draw fusion process.

In accordance with the present invention the substrate glass composition may be used in liquid crystal display [LCD], an electroluminescence display [ELD], a field emission display [FED], plasma display [PD], organic light emitting diode display [OLED] etc.

Exemplary compositions of the glass of the present disclosure are listed in Table I . Table 1

Table 1 continued ...

Table 1 continued ...

In the Table I provided herein above, examples 1 to XXVIII are glass compositions in accordance with the embodiments of the present invention, whereas C-I is a comparative example of glass composition in accordance with known prior art. It is observed that owing to presence of β-ΟΗ in . C-I, there is foam formation at about 1420 °C, whereas due to drying effect of chlorine and fluorine, the value of β-ΟΗ is expected to be reduced, whereas no foam formation was observed in examples I to XXVIII due to the drying and refining effect of chlorine and fluorine.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

Dated this 09^m Day of May 2012

Claims

We claim:

1. A glass composition with reduced foaming, wherein said glass composition comprises:

Si0₂ from about 63.3 to about 71.3 mole %;

AI2O3 from about 8.1 to about 12.1 mole %;

B₂0₃ from about 6.7 to about 12.7 mole %;

CaO from about 5 to 12 mole %;

BaO from about 0.2 to about 3.0 mole %;

SrO from about 0.2 to about 4.5 mole %;

Na₂0 from about 0.001 to about 0.1 mole %;

fluorine from about 0.001 to about 0.1 mole % and

chlorine from about 0.001 to about 0.5 mole %;

wherein inclusion of the alkali metal Na₂0 decreases the melting point of the substrate glass composition.

2. The glass as claimed in claim 1, wherein the glass further comprises of at least one of the refining agents selected from a group consisting of As₂0₃ in the range from 0 to 0.7 mole % and Sb₂0₃ in the range from 0 to 0.8 mole % along with N₂Os in the range from 0.1 to 0.4 mole %.

3. The glass as claimed in claim 1, wherein said glass has a density of less than about 2.5 g/cm³.

4. The glass as claimed in claim 1, wherein said glass has a coefficient of thermal expansion less than 45 x 10^"7/°C.

5. The glass as claimed in claim 1, wherein said glass has a logarithm of liquidus viscosity of greater than 5.

6. The glass as claimed in claim 1, wherein said glass has a strain point greater than 620 °C.

7. The glass as claimed in claim 1, wherein said glass has a liquidus temperature of less than 1230 °C.

8. The glass as claimed in claim 1, wherein said glass has an annealing temperature of greater than 650 °C.

9. The glass as claimed in claim 1, wherein said glass is down-drawable or slot drawn or press-formable in a mold or drawable through roller press or drawable through float process.

10. A glass sheet prepared from the glass as claimed in claim 1.

1 1. An electronic device, the electronic device comprising a glass substrate, the glass substrate comprising the glass composition as claimed in claim 1.