Glass composition and substrate for plasma display

A glass composition comprising from 45 to 66 wt % of SiO.sub.2, from 0 to 15 wt % of Al.sub.2 O.sub.3, from 10 to 24 wt % of Li.sub.2 O+Na.sub.2 O+K.sub.2 O, from 14 to 26 wt % of CaO+MgO+SrO+BaO+ZnO, and from 0 to 1 wt % of SO.sub.3 +Sb.sub.2 O.sub.3, said glass composition containing substantially no zirconia and having a strain point of at least 560.degree. C. and a linear thermal expansion coefficient of at least 80.times.10.sup.-7 /.degree.C. within a temperature range of from 50.degree. to 350.degree. C.

BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
The present invention relates to a glass composition suitable for 
substrates for display devices such as fluorescent character display 
tubes, plasma display panels, flat cathode ray tubes and liquid crystal 
display tubes. 
As a glass substrate for plasma display, a soda lime glass having a strain 
point of about 510.degree. C., formed by float process, is commonly used. 
A typical plasma display panel is produced by the following process. 
Firstly, on a glass substrate for the display surface side, display 
electrodes will be printed, and dielectric layer will be printed thereon, 
followed by baking. Further, a protective film will be vapor-deposited on 
this dielectric layer. On the other hand, on the opposing rear-side glass 
substrate, Al, Ag or Ni electrodes, and stripe-shaped partition walls (low 
melting point glass) to prevent electric discharge between electrodes and 
to prevent color mixing of red-, green- and blue-phosphors, will be formed 
by baking at a temperature of from 500.degree. to 600.degree. C. Further, 
printed circuits will be formed, and red-, green- and blue-phosphors will 
be printed. 
The display side and rear side glass substrates will be bonded by means of 
a low melting temperature glass frit within the same temperature range as 
the above-mentioned temperature, and a gas mixture of xenon and neon as 
the main discharge gas, will be sealed in to obtain a plasma display 
panel. 
The glass substrate for plasma display is subjected to heat treatment at a 
temperature equal to or higher than the strain point of soda lime glass at 
a level of from 500.degree. to 600.degree. C., and thermal deformation is 
likely to take place. Therefore, when soda lime glass substrates are used, 
a 40 inch panel is almost at the limit, and it is substantially difficult 
to use soda lime glass substrates for high definition TV which requires a 
panel of a larger size with a high level of resolution. 
A ZrO.sub.2 -containing glass is also known which undergoes a less degree 
of deformation by such heat treatment (Japanese Unexamined Patent 
Publication No. 40933/1991). However, this glass is susceptible to 
scratching, and it is necessary not to polish the glass or to polish with 
a due care not to form a substantial scratch mark. Further, even with a 
glass having no scratch mark, a scratch mark may form during the 
production process. Therefore, a due care will be required for handling in 
the process. 
Each of these methods reduces the amount of glass substrate which can be 
produced per unit period of time. In other words, each method has a 
problem that the production cost of glass increases. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a glass composition 
which scarcely undergoes thermal deformation and which is hardly 
susceptible to scratching. 
That is, the present invention provides a glass composition comprising from 
45 to 66 wt % of SiO.sub.2, from 0 to 15 wt % of Al.sub.2 O.sub.3, from 10 
to 24 wt % of Li.sub.2 O+Na.sub.2 O+K.sub.2 O, from 14 to 26 wt % of 
CaO+MgO+SrO+BaO+ZnO, and from 0 to 1 wt % of SO.sub.3 +Sb.sub.2 O.sub.3, 
said glass composition containing substantially no zirconia and having a 
strain point of at least 560.degree. C. and a linear thermal expansion 
coefficient of at least 80.times.10.sup.-7 /.degree.C. within a 
temperature range of from 50.degree. to 350.degree. C. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, the reason for the definition of the composition of the glass 
composition of the present invention will be described. 
In the present invention, if the glass composition contains ZrO.sub.2 
substantially, the glass tends to be susceptible to scratching, and the 
breakage frequency during the production process tends to be high, whereby 
the production yield will be poor. Therefore, the glass composition of the 
present invention contains substantially no zirconia. Here, "contains 
substantially no zirconia" means that the zirconia content is less than 
0.2 wt %. 
Further, if the glass composition contains As.sub.2 O.sub.3 substantially, 
arsenic will dissolve in the polishing waste liquid resulting from 
polishing in the commercial production, whereby a substantial cost will be 
required for treatment of the waste liquid. Therefore, the glass 
composition preferably contains substantially no arsenic. Here, "contains 
substantially no arsenic" means that the As.sub.2 O.sub.3 content is less 
than 0.1 wt %. 
Further, if the glass composition contains B.sub.2 O.sub.3 substantially, 
the linear expansion coefficient tends to be small, and boron tends to 
evaporate during the production process, whereby it tends to be difficult 
to produce a uniform glass. Further, evaporated boron is likely to erode 
the bricks of the furnace. Accordingly, it is preferred that the 
composition does not substantially contain B.sub.2 O.sub.3. Here, "does 
not substantially contain B.sub.2 O.sub.3 " means that the content is less 
than 0.1 wt %. 
If the linear thermal expansion coefficient is less than 80.times.10.sup.-7 
/.degree.C., within a temperature range of from 50.degree. to 350.degree. 
C., cracks are likely to form in the electrodes and the partition walls. 
Therefore, the linear thermal expansion coefficient is at least 
80.times.10.sup.-7 /.degree.C., preferably at least 85.times.10.sup.-7 
/.degree.C. Further, the linear thermal expansion coefficient is 
preferably less than 120.times.10.sup.-7 /.degree.C., more preferably less 
than 95.times.10.sup.-7 /.degree.C. 
If the strain point is less than 560.degree. C., thermal deformation is 
likely to form during the heat treatment of e.g. electrodes and partition 
walls. Therefore, the strain point is usually at least 560.degree. C., 
preferably at least 570.degree. C. 
SiO.sub.2 is a main component for the glass. If the content of SiO.sub.2 is 
less than 45 wt %, the chemical durability tends to be poor, particularly 
corrosion by hydrofluoric acid tends to be substantial. On the other hand, 
if it exceeds 66 wt %, the linear thermal expansion coefficient will be 
less than 80.times..sup.-7 /.degree.C. Therefore, the SiO.sub.2 content is 
from 45 to 66 wt %. It is preferably within a range of from 50 to 63 wt % 
in order to obtain a higher strain point (at least 570.degree. C.). 
Al.sub.2 O.sub.3 which may optionally be used, is a component which is 
effective to increase the strain point without substantially reducing the 
linear thermal expansion coefficient. If the content of Al.sub.2 O.sub.3 
exceeds 15 wt %, the liquidus temperature tends to be too high. Therefore, 
the content of Al.sub.2 O.sub.3 is usually within a range of from 0 to 15 
wt %, preferably from 0.5 to 12 wt %, more preferably from 5.5 to 10 wt %. 
Li.sub.2 O, Na.sub.2 O and K.sub.2 O are components which are effective for 
adjusting the linear thermal expansion coefficient of glass or the 
viscosity at a high temperature. These components may not all be 
incorporated simultaneously. However, if their total content is less than 
10 wt %, the linear thermal expansion coefficient tends to be less than 
80.times.10.sup.-7 /.degree.C., and if it exceeds 24 wt %, it becomes 
difficult to bring the strain point to a level of at least 560.degree. C. 
Therefore, their total content is within a range of from 10 to 24 wt %. 
The content of Na.sub.2 O is preferably from 0 to 6 wt %, and the content 
of K.sub.2 O is preferably from 0 to 0.5 wt %. 
Among them, K.sub.2 O has an effect of increasing the strain point, and its 
content is preferably from 4 to 20 wt %, particularly from 9 to 16 wt %. 
SrO, BaO, ZnO, CaO and MgO may not all be incorporated simultaneously. 
However, if their total content is less than 14 wt %, it tends to be 
difficult to bring the strain point to a level of at least 560.degree. C. 
On the other hand, if it exceeds 26 wt %, the strain point tends to be too 
low. Therefore, their total content is usually within a range of from 14 
to 26 wt %, preferably from 17 to 23 wt %, more preferably from 18 to 20 
wt %. 
The content of CaO is preferably from 0 to 14 wt %, the content of MgO is 
preferably from 0 to 6 wt %, the content of BaO is preferably from 0 to 14 
wt %, and the content of ZnO is preferably from 0 to 6 wt %. 
Among them, SrO has an effect of increasing the strain point, and its 
content is preferably from 1 to 14 wt %, particularly from 4 to 10 wt %. 
Further, CaO is preferably contained in an amount of from 1 to 14 wt %. 
Sb.sub.2 O.sub.3 and SO.sub.3 which may optionally be incorporated, are 
refining agents. If their total content exceeds 1 wt %, their effects of 
refining will be saturated. Therefore, their total content is within a 
range of from 0 to 1 wt %. 
In addition to the above components, the following components may be 
incorporated. 
CeO.sub.2 may be incorporated for the purpose of suppressing browning which 
is likely to result when the glass is irradiated with X-rays. TiO.sub.2 
and Bi.sub.2 O.sub.3 may be incorporated for the purpose of suppressing 
solarization which is likely to result when the glass is irradiated with 
ultraviolet rays. PbO has an effect of suppressing solarization, but even 
if it is added excessively, the effect will be saturated. Therefore, the 
content is preferably less than 0.3 wt %, more preferably less than 0.1 wt 
%. Further, the color of glass can be controlled by incorporating a small 
amount of Fe.sub.2 O.sub.3, CoO, Cr.sub.2 O.sub.3 or NiO. 
On the other hand, it is preferred that fluorine, zirconium, boron, 
phosphorus and arsenic are not substantially contained, although they may 
be incorporated as impurities. 
The glass composition of the present invention can be produced by supplying 
glass batch formulated to have the desired composition, into a melting 
furnace, followed by vitrification and forming into a transparent and not 
substantially crystallized sheet glass having a predetermined thickness by 
float process.

Now, the present invention will be described in further detail with 
reference to Examples. However, it should be understood that the present 
invention is by no means restricted by such specific Examples. 
EXAMPLES 1 to 6 and COMATIVE EXAMPLES 7 to 9 
550 g of glass batch formulated to have the desired composition, were put 
into a platinum crucible and heated for 4.5 hours for vitrification in a 
furnace of 1510.degree. C. while stirring from time to time. Then, the 
molten glass was cast in a graphite mold and then annealed to reduce 
strain. Table 1 shows the composition by weight % of oxides. The amounts 
of CoO and NiO were so small that they were represented by ppm. 
With respect to such a sheet glass, the linear thermal expansion 
coefficient, the strain point, the brittleness, the temperature at 
10.sup.2 poise, which is an index for the solubility, the temperature at 
10.sup.4 poise, which is an index for formability, the liquidus 
temperature, the chemical durability and the electrical resistance were 
measured, and the results are shown in the respective columns in Table 1. 
Table 1 also shows Comparative Examples. 
Various properties were measured as follows. 
The brittleness was determined in such a manner that a Vickers indentator 
was pressed against a mirror-polished glass surface under a load of 500 g, 
whereupon the diagonal length (a) of the indentation and the crack length 
(C) were measured, and their ratio C/a was obtained. The larger the ratio 
C/a, the higher the brittleness [Sehgal et al. J. Mat. Sci. letters [14], 
p.167-169, 1995]. 
The chemical durability was represented by the weight reduction per unit 
area (mg/cm.sup.2) when a test sample of 50 mm.times.50 mm.times.3 mmt was 
immersed in a 15 wt % HF aqueous solution at 40.degree. C. for 90 seconds. 
The linear thermal expansion coefficient is represented by a value within a 
temperature range of from 50.degree. to 350.degree. C., and the electrical 
resistance is represented by a logarithmic value of the resistance (unit: 
.OMEGA..multidot.cm) at 150.degree. C. Other physical properties were 
measured in accordance with methods which are commonly employed in the 
glass industry. 
The liquidus temperatures of all glasses in Examples and Comparative 
Examples in Table 1, are all lower than the temperatures of the respective 
glasses at 10.sup.4 poise. This indicates that they can be formed by float 
process. The ratio C/a of the soda lime glass in Comparative Example 9 is 
lower than C/a of the glasses in Examples 1 to 8, which indicates that the 
strain point is low as compared with that of the glasses in the Examples, 
and such soda lime glass can not be used for a plasma display substrate of 
a large size with high resolution, although it is scarcely brittle. The 
glass in Comparative Example 10 is equivalent to the glasses in Examples 1 
to 8 with respect to both the strain point and the linear expansion 
coefficient, and the ratio C/a is also small, which indicates that the 
glass is scarcely brittle. However, the temperature at 10.sup.2 poise is 
higher by as much as 200.degree. C. than those of the glasses of Examples 
1 to 8, which indicates that melting is very difficult. The ratio C/a of 
the glass in Comparative Example 11 which contains zirconia, is higher 
than the ratios C/a of the glasses in Examples 1 to 8, which indicates 
that it is readily brittle. 
As shown by the foregoing Examples, the glass composition of the present 
invention has a high linear thermal expansion coefficient and a high 
strain point, and it can relatively easily be melted and scarcely brittle. 
TABLE 1 
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Comparative 
Examples Examples 
Composition 1 2 3 4 5 6 7 8 9 10 11 
__________________________________________________________________________ 
SiO.sub.2 64.4 
57.2 
61.2 
62.4 
53.8 
62.6 
46 64.1 
72.3 
62.4 
57.5 
Al.sub.2 O.sub.3 
1 5.6 5.7 6.1 5.6 6.1 15 4.2 2 17.1 
7 
Li.sub.2 O 0 0 0 0 0 0.2 0 0.2 0 0 0 
Na.sub.2 O 0 0 0 0 1.0 0 3.2 1.4 12.5 
12.2 
0 
K.sub.2 O 11 10.6 
13.4 
11.6 
9.4 11.6 
19.1 
10 1 3.8 4.3 
SrO 13 5 5.8 9.2 4.3 9.2 6.7 0 0 0 6 
BaO 0.4 12.8 
0 0 11.4 
0 0 7 0 0 9 
ZnO 5 0 2.7 0.3 10.2 
0.3 0 0 0 0 4 
CaO 0 3.4 8.1 10 1.4 10.0 
3.6 7.7 8 0.5 0 
MgO 5 5 2.8 0 2.9 0 5 5 4 3.5 7 
ZrO.sub.2 0 0 0 0 0 0 0 0 0 0 2 
CeO.sub.2 0.5 0 0 0 0 0 0 0 0 0 3 
TiO.sub.2 0.4 0 0.1 0 0 0 0 0 0 0 0 
PbO 0 0.2 0 0 0 0 0 0 0 0 0 
Bi.sub.2 O.sub.3 
0 0 0 0 0 0 0.3 0 0 0 0 
Fe.sub.2 O.sub.3 
0 0 0 0 0 0 0.2 0.1 0 0 0 
CoO (ppm) 18 0 0 0 0 0 0 0 0 0 0 
NiO (ppm) 140 0 0 0 0 0 0 0 0 0 0 
SO.sub.3 0.1 0.2 0.2 0 0 0 0.4 0 0.2 0 0 
Sb.sub.2 O.sub.3 
0.2 0 0 0.4 0 0 0.5 0.3 0 0.5 0.2 
Linear expansion 
82 85 85 82 83 83 116 82 85 88 82 
coefficient (.times. 10.sup.-7 /.degree.C.) 
Strain point (.degree.C.) 
567 587 587 594 571 586 570 572 511 576 589 
Brittleness 2.61 
2.97 
2.62 
2.64 
2.91 
2.65 
3.07 
2.63 
2.43 
2.43 
3.16 
Temp. at 10.sup.2 poise 
1565 
1560 
1560 
1565 
1549 
1550 
1518 
1564 
1470 
1770 
1504 
Temp. at 10.sup.4 poise 
1145 
1161 
1161 
1155 
1169 
1143 
1118 
1165 
1044 
1270 
1155 
Liquidus temp. (.degree.C.) 
1125 
1110 
1110 
1107 
992 1107 
1042 
1112 
1040 
1230 
1150 
Chemical durability 
2.2 4.3 3.4 3.3 6.9 3.3 10 0.8 4.2 3.8 5.2 
Electrical resistance 
11.4 
11.7 
11.3 
11.5 
12.4 
11.5 
11.6 
11 8.8 8.4 10.9 
__________________________________________________________________________ 
The glass composition of the present invention is scarcely brittle and has 
a strain point of at least 560.degree. C. and a linear thermal expansion 
coefficient close to that of soda lime glass, whereby it can be used for 
an application where a high strain point is required among glass 
substrates for which soda lime glass has heretofore been used. It is 
particularly useful for a plasma display substrate, but it is also useful 
for other substrates such as substrates for e.g. liquid crystal devices. 
Further, the glass composition of the present invention has a high 
electrical resistance of at least 10.sup.11 .OMEGA..multidot.cm at 
150.degree. C., and it is accordingly useful for an application where 
electrical insulation is particularly required.