Glasses for display panels

This application claims the benefit of U.S. Provisional Application No. 
60/035,171, filed Jan. 9, 1997. 
FIELD OF THE INVENTION 
The invention relates to a family of aluminosilicate glass compositions 
exhibiting physical and chemical properties suitable for use in flat panel 
displays, in particular plasma display panels (PDPs). 
BACKGROUND OF THE INVENTION 
There is a rapidly growing interest in flat panel display devices. Thus 
far, commercial activity has centered on small units such as those used in 
laptop computers. For this purpose, the liquid crystal display (LCD) 
device has been the dominant product. 
Increasing attention is being given to larger units that may be used in 
information and entertainment applications. LCDs tend to require critical 
accuracy in construction and, therefore, do not readily lend themselves to 
large size screens. Accordingly, as interest shifts to larger size units, 
attention is being directed to alternative types of display devices. One 
such alternative is a plasma display device. In its simplest form, a 
plasma display device embodies two insulating glass substrates maintained 
in opposed, spaced relationship. One substrate has anode electrodes formed 
on its interface. The other substrate has cathode electrodes formed on its 
interface. The electrodes may be applied in stripes that are perpendicular 
to one another. They may be printed, or may be formed by a 
photolithographic process. Barriers are formed between the electrodes in 
each set to prevent cross talk. 
The substrates are maintained in a fixed relationship facing each other. A 
rare gas, such as neon, argon, or helium is enclosed around and between 
the electrodes. When a voltage, which may be up to 100 V, is applied 
between the electrode sets, the gas undergoes a glow discharge. This is 
based on a principle commonly known as the neon glow discharge principle. 
The light generated by this discharge may be used to form the image on the 
display. The electrodes may contain materials that generate the primary 
red, green and blue (RGB) colors under influence of the discharge. In 
another form, fluorescent phosphors are coated on repeating structures and 
are affected by the discharge to produce the desired colors. Typically, 
the gas discharge generates UV light which excites the phosphors, which in 
turn give off RGB light in the visible region of the spectrum. 
Heretofore, the insulating substrates employed in emissive display devices 
have been sheets of soda lime glass. Soda lime glasses have been used 
because they provide a relatively high coefficient of thermal expansion 
(CTE), thereby more closely matching the expansion of glass frits 
typically used in producing electrodes and barriers in a display device. 
For example, the electrodes and barriers may be applied as a paste and 
dried or fired. The paste will contain a conductive component, such as a 
metal powder, a low melting point glass frit, and organics as a solvent 
and as a binder. The dried paste is fired to burn out any residual organic 
and to soften the glass frit to adhere to the substrate. 
While traditional soda lime glasses have the necessary high CTEs, they also 
have low strain points and low resistivities. Consequently, a soda lime 
substrate may shrink and/or undergo distortion during thermal processing. 
This processing includes firing the electrodes and/or sealing the 
substrates together. The high soda content also leads to sodium ion 
migration which degrades the display electronics (e.g. electrodes). 
It would, therefore, be desirable to provide a glass substrate having a CTE 
of 79.degree.-88.degree..times.10.sup.-7 /.degree. C. over the range of 
25.degree.-300.degree. C. and a strain point greater than 600.degree. C. 
At the same time, it would be desirable for the glass to be capable of 
being manufactured using the float process in order to minimize 
manufacturing costs. 
SUMMARY OF THE INVENTION 
One aspect of the present invention relates to a glass having a coefficient 
of thermal expansion over the temperature range of 25.degree.-300.degree. 
C. between 60.degree. and 90.degree..times.10.sup.-7 /.degree. C., and a 
strain point higher than 600.degree. C., whose composition consists 
essentially of, as calculated in weight percent on an oxide basis, 42-62 
SiO.sub.2, 15-28 Al.sub.2 O.sub.3, 0-4 B.sub.2 O.sub.3, 3-10 Na.sub.2 O, 
1-11 K.sub.2 O, 0-6 MgO, 9.5-24 CaO, 0-8 SrO, 0-16 BaO, 0-4 ZrO.sub.2 and 
4-16 Li.sub.2 O+Na.sub.2 O+K.sub.2 O. 
The present glasses employ 42-62% by weight SiO.sub.2 as the primary glass 
former. Increasing SiO.sub.2 content generally improves durability, but 
raises the melting and forming temperatures. The glasses also comprise 
15-28% Al.sub.2 O.sub.3. As the Al.sub.2 O.sub.3 content increases, glass 
durability increases, but CTE decreases and the melting and forming 
temperatures increase. Consequently Al.sub.2 O.sub.3 is maintained between 
about 15 and 28 weight percent, more preferably between about 16.5 to 27 
weight percent, and most preferably between about 20 and 26 weight 
percent. Boric oxide (B.sub.2 O.sub.3) decreases melting temperature, but 
is generally detrimental to durability, strain point, and CTE. Therefore, 
B.sub.2 O.sub.3 is limited to 4%, and is most preferably essentially 
omitted. It is desirable to maintain the molar ratio of Na.sub.2 O:K.sub.2 
O at approximately 1:1 in order to limit alkali mobility (due to the mixed 
alkali effect). However, a molar ratio of approximately 2.5:1 is desirable 
to minimize the liquidus temperature. Consequently, in the present 
invention, it is desirable to maintain the Na.sub.2 O:K.sub.2 O molar 
ratio at 0.8-2.7:1, more preferably 1.0-2.5:1. Alkali are used in order to 
maintain a high coefficient of thermal expansion (CTE). The role of MgO 
and CaO is to limit alkali mobility and flux the melt at relatively high 
temperatures, while helping to provide a high strain point, a low density 
and enhanced chemical durability and Young's modulus. Other ingredients 
may include, for example, the transition metal oxides, particularly those 
in period 4 (such as ZnO and TiO.sub.2), as well as Y.sub.2 O.sub.3, 
La.sub.2 O.sub.3, and P.sub.2 O.sub.5, and those ingredients employed for 
fining (e.g. CaSO.sub.4, NaSO.sub.4, halides, and so forth). These, and 
other ingredients should preferably not exceed 5 weight percent in total, 
and most preferably should be less than or equal to 3 weight percent in 
total. 
The preferred glasses are those which consist essentially of (expressed in 
weight percent): 42-59 SiO.sub.2, 16.5-27 Al.sub.2 O.sub.3, 0-3 B.sub.2 
O.sub.3, 3-10 Na.sub.2 O, 2-9 K.sub.2 O, 0-5 MgO, 9.5-24 CaO, 0-8 SrO, 
0-13 BaO, 0-3 ZrO.sub.2 and 5-15 Li.sub.2 O+Na.sub.2 O+K.sub.2 O and no 
more than 5 wt % of optional ingredients such as those listed above. 
The most preferred glasses are those which consist essentially of 
(expressed in weight percent): 42-59 SiO.sub.2, greater than 20 and less 
than 26 Al.sub.2 O.sub.3, 0-2 B.sub.2 O.sub.3, 3-9 Na.sub.2 O, 2-7 K.sub.2 
O, 0-4 MgO, 10-24 CaO, 0-7 SrO, 0-8 BaO, 0-2 ZrO.sub.2, 6-15 Li.sub.2 
O+Na.sub.2 O+K.sub.2 O, and no more than 4 wt % of the optional 
ingredients listed above. 
The preferred glasses in accordance with the present invention have a CTE 
in the range of 60.degree.-90.degree..times.10.sup.- /.degree. C., more 
preferably 70.degree.-90.degree..times.10.sup.-7 /.degree. C., and most 
preferably 79.degree.-88.degree..times.10.sup.-7 /.degree. C. over the 
temperature range of 25.degree.-300.degree. C. The desire for such a CTE 
is primarily driven by the desire to match the CTEs of glass frits used in 
electrodes, barrier ribs, overcoats, sealing operations, and other 
materials. The glasses of the present invention preferably have a strain 
point greater than 615.degree. C., more preferably greater than about 
625.degree. C., and most preferably greater than about 650.degree. C. A 
high strain point is desired to help prevent panel distortion due to 
compaction/shrinkage or viscous flow during subsequent thermal processing. 
Such processing includes firing of electrodes, sealing of panels and 
application of coatings. In the most preferred embodiments, the glasses 
exhibit a combination of desirable CTE's and strain point. For example, 
the most preferred glasses exhibit a CTE in between 
79.degree.-88.degree..times.10.sup.-7 /.degree. C., in combination with a 
strain point greater than 625.degree. C., and most preferably greater than 
about 650.degree. C. The glasses of the present invention also exhibit low 
density (less than 3.0 g/cm.sup.3, more preferably less than 2.8 
g/cm.sup.3) in order to minimize display weight, relatively high 
resistivity (log resistivity greater than or equal to 6.5 ohm cm. at 
250.degree. C., more preferably greater than or equal to 7.0 ohm cm at 
250.degree. C.) in order to ensure extended display lifetimes. These 
glasses also exhibit good chemical durability and optical clarity, as well 
as low batch cost. As a means to optimize the market opportunity for PDP's 
it is desirable that the batch cost of a PDP glass composition should 
preferably be low, and it is believed that the most economical means of 
panel/substrate manufacture is the float glass process. The present 
invention disclosure outlines a glass compositional area which is believed 
to be ideally suited for substrates for plasma display panels. It is 
believed that many of the glasses of the present invention are capable of 
being formed using the float glass manufacturing process. 
Additionally, the value of Al.sub.2 O.sub.3 /ZrO.sub.2 should preferably 
not fall below 3 (calculated in wt %). Moreover, the total of Al.sub.2 
O.sub.3 +ZrO.sub.2 should preferably exceed 10 wt %, but not more than 28 
wt %. SiO.sub.2 +Al.sub.2 O.sub.3 should preferably range from 60-82 wt %, 
more preferably 64-76 wt %.