Glass for a substrate

Details Glass for a substrate Glass for a substrate

1999-02-24

2001-11-06

Sample, David R (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S070000, C501S072000

Reexamination Certificate

active

06313052

ABSTRACT:

The present invention relates to a glass used as a substrate for a flat display such as a field emission display (FED), a plasma display panel (PDP) or the like, and also as a substrate for a solar cell.
Various display systems have been developed with regard to a flat display, and among them, a notable one is a self-luminescent system display wherein electrodes are formed on a front face glass substrate and a back face glass substrate and a phosphor is excited to emit light by using an electron beam or discharge generated between the two substrates. Typical examples include FED and PDP.
Various systems have been proposed with regard to FED, and all of them employ a system of irradiating an electron beam generated at an electrode on a back face substrate towards a transparent electrode on a front face substrate and exciting a phosphor patterned on the transparent electrode to emit light. Since it is necessary to maintain the irradiation space in a vacuum state in the same manner as in a cathode ray tube, the front face glass substrate and the back face glass substrate are fixed so as to face to each other and its surrounding is frit-sealed and evacuated to vacuum, and a mixed gas of Xe—Ne is charged therein.
It is common in both systems of FED and PDP (1) to form a transparent electrode directly on a front face glass substrate and (2) to frit-seal the surrounding of a front face glass substrate and a back face glass substrate fixed so as to face to each other. ITO (In
2
O
3
:Sn) or SnO
2
is usually used as the transparent electrode.
Also, a solar cell which is a clean energy source is greatly expected as an energy source of next generation. Particularly, a thin film type solar cell using amorphous silicon has been recently noticeable since a solar cell can be produced at a relatively low cost.
Since a solar cell is used often outdoors, its substrate is required to be transparent to light and abrasion-resistant, and soda lime silica glass excellent in these properties is now used. In this case also, a transparent electrode such as SnO
2
is formed on a glass substrate.
With regard to glass used as a substrate for a flat display such as FED, PDP or the like, there is known a glass having a high strain point and an improved heat resistance in addition to a soda lime silica glass having an average thermal expansion coefficient (hereinafter simply referred to as “thermal expansion coefficient”) of 85×10
−7
/° C. at 50 to 350° C. For example, JP-A-7-257937 discloses a glass composition consisting essentially of from 56 to 60 wt % of SiO
2
, from 10 to 16 wt % of Al
2
O
3
, from 1 to 4 wt % of ZrO
2
, from 0 to 3 wt % of TiO
2
, from 0 to 4 wt % of MgO, from 3 to 9 wt % of CaO, from 0 to 4 wt % of SrO, from 2 to 8 wt % of BaO, from 0 to 3 wt % of ZnO, from 3 to 7 wt % of Na
2
O, from 5 to 10 wt % of K
2
O and at most 16 wt % of MgO+CaO+SrO+BaO+ZnO, and the glass illustrated in the Example has a thermal expansion coefficient of from 77.3×10
−7
to 87.3×10
−7
/° C.
However, ITO or SnO
2
used as a transparent electrode has a thermal expansion coefficient of about 40×10
−7
/° C., while glass has a thermal expansion coefficient of at least 77×10
−7
/° C. Thus, a thermal expansion coefficient difference between the two materials is large. For this reason, there are problems that the transparent electrode formed on the glass substrate is separated (peeled) or causes cracks due to expansion or shrinkage caused by temperature change.
On the other hand, there is known a low expansion glass having a thermal expansion coefficient of at most 50×10
−7
/° C., which is used as a substrate for a liquid crystal display. For example, U.S. Pat. No. 5,348,916 discloses an alkali-free glass comprising SiO
2
, Al
2
O
3
, B
2
O
3
, MgO, CaO, SrO and BaO as the essential components, and the glass illustrated in the Example disclosed therein has a thermal expansion coefficient of from 34×10
−7
to 49×10
−7
/° C. In this case, an expansion coefficient difference between the glass and a transparent electrode is small, and there is less fear of raising problems of causing separation of the transparent electrode or cracks in the transparent electrode. However, a frit material suitable for frit-sealing two sheets of substrates of such a low expansion glass is a frit material for sealing an alumina ceramic substrate, for example, but such a frit material requires a very high baking temperature, at which there is a fear that a material of forming picture elements is subjected to chemical change or softened to flow.
In addition, there is known a glass having a thermal expansion coefficient of from 60×10
−7
to 75×10
−7
/° C., which is used as a substrate for PDP or the like.
JP-A-3-170343 discloses a glass composition having a strain point of at least 680° C., a glass softening point of at least 900° C. and a thermal expansion coefficient of from 60×10
−7
to 75×10
−7
/° C., which consists essentially of from 52 to 65 wt % of SnO
2
, from 1 to 10 wt % of Al
2
O
3
, from 30 to 45 wt % of MgO+CaO+SrO+BaO, from 0.5 to 6 wt % of ZrO
2
, from 0 to 3 wt % of SnO
2
+Tio
2
, from 0 to 0.5 wt % of SO
3
+As
2
O
3
+Sb
2
O
3
, and from 0 to 5 wt % of La
2
O
3
.
JP-A-9-249430 discloses a glass composition for a substrate, which consists essentially of from 56 to 65 wt % of SiO
2
, from more than 15 to 23 wt % of Al
2
O
3
, from 0 to 7 wt % of MgO, from 0 to 8 wt % of CaO, from 4 to 15 wt % of MgO+Cao, from 0 to 9 wt % of Na
2
O, from 0 to 11 wt % of K
2
O, from 8 to less than 12 wt % of Na
2
O+K
2
O and from 0 to 2 wt % of ZrO
2
.
Further, D263 of Schott and #0211 of Corning Incorporated are known to be a glass having a thermal expansion coefficient of about 72×10
−7
/° C. for a substrate.
If these glasses are used as a substrate, there is less fear of raising problems of causing separation of a transparent electrode or cracks in the transparent electrode since a thermal expansion coefficient difference between the glasses and the transparent electrode is relatively small. Also, in the case of using these glasses, it is possible to conduct frit-sealing without using a frit material requiring a high baking temperature. However, the glass disclosed in JP-A-3-170343 has a high specific gravity, and it is difficult to lighten elements. Actually, all of the glasses illustrated in the Examples disclosed therein have specific gravities of at least 2.9 calculated by mathematical model of glass composition-physical property, which exceeds the upper limit value 2.6 of a preferable specific gravity. Also, the glass disclosed in JP-A-9-249430 has a high Al
2
O
3
content of exceeding 15 wt %, and raises a problem of corroding an Al
2
O
3
—ZrO
2
—SiO
2
type electrocast brick or an Al
2
O
3
type electrocast brick at a part in direct contact with molten glass in a glass melting furnace. Further, each of D263 of Schott and #0211 of Corning Incorporated has a high ZnO content of at least 6 wt %, and consequently, there is a fear of causing devitrification in a case of carrying out formation by float process.
On the other hand, as a substrate for a solar cell, soda lime silica glass is conventionally used. However, soda lime silica glass contains alkali oxides such as Na
2
O, K
2
O and the like in a total amount of 14.5 wt %, and these alkali components are diffused into a transparent electrode or into amorphous silicon through the transparent electrode, thereby deteriorating performances of a solar cell. Further, when soda lime silica glass is used outdoors for a long term, the alkali components in the glass and moisture in air cause “stain” phenomenon, thereby lowering transparency and raising problems of causing separation of the transparent electrode. Still further, due to a large expansion coefficient difference between the glass and the transparent electrode, the transparent electrode is separated (peeled) or cracks

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