Glass substrate for a display

Details Glass substrate for a display Glass substrate for a display

2000-08-23

2002-12-31

Fiorilla, Christopher A. (Department: 1731)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S053000

Reexamination Certificate

active

06500778

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a glass substrate for a display which is particularly useful as a substrate for a flat panel display such as a plasma display panel (PDP) or a field emission display (FED).
2. Discussion of Background
In recent years, a flat panel display, particularly PDP which is one type of thin flat plate type gas discharge display panels, has attracted an attention and has been actively developed. PDP has a cell constituted and defined by a front glass substrate, a rear glass substrate and partition walls, so that plasma discharge is generated in the cell, whereby a phosphor layer on an inner wall of the cell will emit light to form an image.
For the front glass substrate and the rear glass substrate for PDP, a glass having a strain point higher than the strain point of soda lime silica glass, has been employed in order to minimize deformation of the glass substrate during the heat treatment step in the process for the production of PDP.
However, the conventional glass substrate having a high strain point has had a problem that it is susceptible to fracture in the process for the production, as compared with the soda lime silica glass substrate.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a glass substrate for a display, whereby this problem can be solved.
The present invention provides a glass substrate for display, which has a strain point of at least 550° C., an average linear expansion coefficient of from 65×10
−7
to 100×10
−7
/°C. within a range of from 50 to 350° C. and a ratio K
IC
/d of at least 280 Pa·kg·m
7/2
where K
IC
is the fracture toughness and d is the density.
The present inventors have analyzed in detail the substrates fractured during the process for the production of PDP. As a result, with most of the fractured substrates, impact impressions have been observed which are considered to have formed when the edge portions have received a shock during the handling in an upstream step in the process for the production of PDP. It has been found that with such impact impressions serving as fracture origins, the substrates have been fractured by e.g. a thermal stress in the process for the production of PDP. Accordingly, it is considered possible to solve the problem of fracture of the substrate in the process for the production of PDP by employing a substrate whereby such impact impressions will scarcely form.
The present inventors have investigated the relation between the probability for formation of the above-mentioned impact impressions and the physical properties of the glass substrate, and as a result, have found that the probability for formation of the impact impressions depends on the ratio K
IC
/d where the K
IC
is the fracture toughness of the glass and d is the density. Namely, such a probability decreases as K
IC
/d increases.
The reason is considered to be explained as follows.
K
IC
is a quantity representing the resistance against fracture of the glass and shows the degree of resistance against the same impact force.
On the other hand, the impact force exerted to a substrate during the handling of the substrate, is represented by the impulse given to the substrate i.e. the change of momentum. The momentum is given by mass ×velocity. Accordingly, the impulse exerted to a substrate having a certain velocity change created, is determined by its mass. For example, the impact force given to a substrate of the same size dropped from the same height, is greater as the specific gravity of the substrate is larger.
On the basis of the foregoing discovery, the present inventors have arrived at the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The strain point of the glass substrate for a display of the present invention (hereinafter referred to imply as the glass substrate of the present invention) is at least 550° C. If it is less than 550° C., deformation during the heat treatment step tends to be large. It is preferably at least 560° C.
The average linear expansion coefficient of the glass substrate of the present invention within a range of from 50 to 350° C. is from 65×10
−7
to 100×10
−7
/°C. If it is less than 65×10
−7
/°C. or more than 100×10
−7
/°C., matching in the expansion coefficient with the material which is commonly used for the production of a display (such as glass frit) tends to be difficult. It is preferably from 75×10
−7
to 95×10
−7
/°C., more preferably from 80×10
−7
to 90×10
−7
/°C. Hereinafter, the average linear expansion coefficient within a range of from 50 to 350° C. will be referred to simply as an expansion coefficient.
In the glass substrate of the present invention, the ratio K
IC
/d where K
IC
is the fracture toughness and d is the density, is at least 280Pa·kg·m
7/2
. If it is less than 280 Pa·kg·m
7/2
, the glass substrate tends to be susceptible to fracture. It is preferably at least 290 Pa·kg·m
7/2
.
The glass substrate of the present invention is preferably one prepared by a float method which is capable of producing plate glass of high quality in a large amount.
The glass substrate of the present invention preferably consists essentially of:
SiO
2
45 to 70
wt %,
Al
2
O
3
2 to 20
wt %,
B
2
O
3
0 to 6
wt %,
MgO
1 to 10
wt %,
CaO
1 to 10
wt %,
SrO
0 to 9
wt %,
BaO
0 to 9
wt %,
MgO + CaO + SrO + BaO
10 to 25
wt %,
ZnO
0 to 5
wt %,
Na
2
O + K
2
O
5 to 15
wt %,
Li
2
O
0 to 2
wt %, and
ZrO
2
0 to 10
wt %.
More preferably, the glass substrate of the present invention consists 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 %,
ZnO
0 to 5
wt %,
Na
2
O + K
2
O
5 to 15
wt %,
Li
2
O
0 to 2
wt %, and
ZrO
2
0 to 7
wt %.
The reasons for defining the above compositions will be described below, wherein wt % will be represented simply as %.
SiO
2
is essential as a network former. If it is less than 45%, the heat resistance or the chemical durability tends to deteriorate. It is preferably at least 51%. If it exceeds 70%, K
IC
/d tends to be too small. It is preferably at most 65%, more preferably at most 60%, particularly preferably at most 55%.
Al
2
O
3
is essential as a component to increase the strain point. If it is less than 2%, the strain point tends to be too low. It is preferably at least 6%, more preferably at least 10%. If it exceeds 20%, the viscosity of the molten glass tends to be too high, whereby forming, particularly float forming, tends to be difficult. It is preferably at most 16%.
B
2
O
3
is not essential, but may be incorporated up to 6% to increase K
IC
/d or to lower the viscosity of the molten glass at the time of melting the glass. If it exceeds 6%, the strain point tends to be too low. It is more preferably at most 5%. When B
2
O
3
is incorporated, the content is preferably at least 0.5%, more preferably at least 1%, particularly preferably at least 2%.
MgO is essential as a component to increase K
IC
/d and to lower the viscosity of the molten glass at the time of melting the glass. If it is less than 1%, K
IC
/d tends to be too small, or the viscosity of the molten glass at the time of melting tends to be too large. It is preferably at least 2%, more preferably at least 3%. If it exceeds 10%, devitrification is likely to result. It is preferably at most 5%, more preferably at most 4%.
CaO is essential as a component to lower the viscosity of the molten glass at the time of melting the glass. If it is less than 1%, the viscosity of the molten glass at the time of melting tends to be too high. It is preferably at least 5%. If it exceeds 10%, devitrification is likely to result. It is preferably at most 9%.
SrO is not essential, but may be incorporated up to 9% to lower the viscosity of the molten glass at the time of melting the glass. If it exceeds 9%, K
IC
/d tends to be sma

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