Glass for a substrate

Details Glass for a substrate Glass for a substrate

1999-07-22

2001-10-02

Sample, David R (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S067000, C501S070000, C501S072000

Reexamination Certificate

active

06297182

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a glass for a substrate having high resistance against progress of fracture i.e. a glass for a substrate having high fracture toughness.
2. Discussion of Background
In recent years, large sized flat display panels represented by color plasma display panels (hereinafter referred to as color PDP) have been increasingly used, and glasses to be used as substrates thereof have been diversified. Heretofore, usual soda lime silica glass has been widely used for substrates for large sized flat display panels. One of the reasons is such that the thermal expansion coefficients of various glass frit materials to be used as constituting part materials for panels, including inorganic sealing materials, can easily be adjusted to the thermal expansion coefficient of soda lime silica glass.
On the other hand, in order to reduce deformation or thermal shrinkage of glass substrates in the heat treatment process during the production of large sized flat display panels, it is strongly desired to improve the thermal resistance of the glass for substrates. For this purpose, a so-called high strain point glass has been widely used as a substrate, which has a thermal expansion coefficient of the same level as soda lime silica glass and has a higher strain point (at least about 550° C.) and which has an alkali content controlled to be low in order to improve the electrical insulating property.
Further, a glass substrate is being practically employed as a substrate for information recording media, particularly as a substrate for magnetic discs (hard discs), since it is excellent in the surface smoothness or the mechanical strength such as impact resistance.
However, such a high strain point glass is brittle as compared with soda lime silica glass and thus has had a problem that it is likely to break in the production process. Further, such a high strain point glass has had another problem that its density is large, whereby it has been difficult to reduce the weight of the large sized flat display panel.
To solve such problems, a glass for a substrate has been proposed which has a low density and is hardly scratched, and which is suitable as a substrate for a flat panel display (JP-A-9-301733). The characteristic of being hardly scratched is effective in a case where a flaw as a fracture origin is likely to be imparted in the process for producing a panel, but it is not necessarily effective in a case where a flaw is imparted during processing treatment such as cutting prior to such a production process. It is usual that many flaws which are likely to be fracture origins, are already present at an edge portion of the glass in the processing treatment such as cutting.
Further, also in such a case where glass is employed as a substrate for magnetic discs, many processing treatments are required such as circular processing, center cut, inner and outer peripheral edge chamfering, etc. During such processing treatments, many flaws which are likely to be fracture origins, will be formed along the edge, etc., of the glass, and there has been a problem that not only during the production process, but also during the use of magnetic discs, the glass is likely to break from such flaws as fracture origins. Recently, it has been desired to increase the speed of rotation of discs in order to improve the read/write speed, and especially in such a case, the problem of fracture of glass becomes more serious.
In such a case, in order to prevent fracture of the glass, it has been necessary to present a glass for a substrate which essentially has high resistance against the progress of fracture due to a tensile stress, i.e. a glass for a substrate having high fracture toughness.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the above problems and to provide a glass for a substrate which has a high glass transition point and a thermal expansion coefficient being substantially equal to soda lime silica glass and which has high resistance against the progress of fracture i.e. high fracture toughness and scarcely fractures during the production process or during the use.
The present invention provides a glass for a substrate consisting essentially of:
SiO
2
45 to 65 wt %,
Al
2
O
3
6 to 20 wt %,
B
2
O
3
0.5 to 6 wt %,
MgO
2 to 5 wt %,
CaO
1 to 10 wt %,
SrO
0 to 6.5 wt %,
BaO
0 to 2 wt %,
MgO + CaO + SrO + BaO
10 to 17 wt %,
ZrO
2
0 to 7 wt %, and
Na
2
O + K
2
O
7 to 15 wt %.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, the present invention will be described in detail with reference to the preferred embodiments.
In the present invention, the glass for a substrate is a glass to be used for a substrate for an information recording medium such as a magnetic disc, or for a substrate for a flat panel display such as color PDP, plasma address liquid crystal (PALC) or field emission display (FED).
The glass for a substrate of the present invention is suitable for float forming.
The glass for a substrate of the present invention preferably consists essentially of:
SiO
2
45 to 65 wt %,
Al
2
O
3
10 to 20 wt %,
B
2
O
3
1 to 6 wt %,
MgO
2 to 5 wt %,
CaO
1 to 10 wt %,
SrO
0 to 4.5 wt %,
BaO
0 to 2 wt %,
MgO + CaO + SrO + BaO
10 to 17 wt %,
ZrO
2
0 to 5 wt %, and
Na
2
O + K
2
O
7 to 15 wt %.
In the present invention, the reasons for defining the composition are as follows. (In the following, “%” means “wt %” unless otherwise specified.) SiO
2
is essential as a network former. If it exceeds 65%, the average thermal expansion coefficient from 50 to 350° C. (hereinafter referred to simply as the thermal expansion coefficient) tends to be too small. It is preferably at most 59%, more preferably at most 55%, particularly preferably at most 54%. If it is less than 45%, the heat resistance and chemical durability tend to deteriorate. It is preferably at least 50%.
Al
2
O
3
is an essential component and is effective to increase the glass transition point and to increase the heat resistance. If it exceeds 20%, the thermal expansion coefficient tends to be too small, and the viscosity of the molten glass tends to be too high to carry out float forming. It is preferably at most 16%. If it is less than 6%, the above-mentioned effects tend to be small. It is preferably at least 10%, more preferably at least 12%.
Further, in a case where the glass for a substrate of the present invention is melted in a glass melting furnace wherein an AZS (Al hd
2
O
3
—ZrO
2
—SiO
2
) type electrocast brick is used at a portion which will be in contact directly with molten glass, Al
2
O
3
is preferably from 6 to less than 10%. If it is 10% or more, the corrosiveness of molten glass against the AZS type electrocast brick tends to be substantial.
B
2
O
3
is an essential component and is effective to increase the fracture toughness and to lower the viscosity of the molten glass at the time of melting the glass. If it exceeds 6%, the thermal expansion coefficient tends to be too small, or vaporization of B
2
O
3
at the time of melting the glass tends to be too much. It is preferably at most 5%. If it is less than 0.5%, the above effects tend to be small. It is preferably at least 1%, more preferably at least 2%. In order to increase the above effects, it is preferably from 2.6 to 5%. Further, in order to prevent a damage to the material of the glass melting furnace by evaporated B
2
O
3
, it is preferably from 0.5 to 2.5%, more preferably from 0.5 and less than 1%.
MgO is an essential component and is effective to increase the fracture toughness and to reduce the viscosity of the molten glass at the time of melting the glass. If it exceeds 5%, the glass tends to be unstable. It is preferably at most 4%. If it is less than 2%, the above effects tend to be small. It is preferably at least 3%.
CaO is an essential component and is effective to increase the thermal expansion coefficient and to lower the viscosity of the molten glass at the time of melting the glass. If it exceeds 10%, the glass tends to be unst

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