Barium aluminosilicate glasses

This invention is directed to glass compositions particularly designed for use as substrates in flat panel display devices and, more expressly, for use as substrates in LCDs which employ polycrystalline silicon thin film transistors. The glass compositions are essentially free from alkali metal oxides and consist essentially, in mole percent, of ______________________________________ SiO.sub.2 65-76 MgO 0-5 ZrO.sub.2 0-2.5 Al.sub.2 O.sub.3 7-11 CaO 0-10 TiO.sub.2 0-3 BaO 12-19 SrO 0-10 Ta.sub.2 O.sub.5 0-3 B.sub.2 O.sub.3 0-5 MgO + 0-15 ZrO.sub.2 + 0.5-5. CaO + TiO.sub.2 + SrO Ta.sub.2 O.sub.5 ______________________________________

RELATED APPLICATION 
U.S. application Ser. No. 08/008,560, filed concurrently by us with the 
predecessor of this application, viz., Ser. No. 08,008,561, under the 
title HIGH LIQUIDUS VISCOSITY GLASSES FOR FLAT PANEL DISPLAYS, is directed 
to glasses particularly designed for use as substrates in flat panel 
display devices. Those glasses exhibit strain points higher than 
650.degree. C., liquidus temperatures no higher than 1,125.degree. C., 
viscosities at the liquidus temperature greater than 600,000 poises 
(60,000 Pa.multidot.s), weight losses of less than 2 mg/cm.sup.2 after 
immersion for 24 hours in an aqueous 5% by weight HCl solution at 
95.degree. C., and melting viscosities of about 200 poises (20 
Pa.multidot.s) at a temperature below 1,675.degree. C., the glasses being 
essentially free from alkali metal oxides and consisting essentially, 
expressed in terms of mole percent on the oxide basis, of 
______________________________________ 
SiO.sub.2 
64-70 MgO 0-5 
Al.sub.2 O.sub.3 
9.5-12 CaO 3-13 
B.sub.2 O.sub.3 
5-10 SrO 0-5.5 
TiO.sub.2 
0-5 BaO 2-5.5 
Ta.sub.2 O.sub.5 
0-5 MgO + CaO + SrO + BaO 
10-20. 
______________________________________ 
FIELD OF THE INVENTION 
This invention is directed to the production of glass compositions 
exhibiting such properties as high strain points, high viscosities at 
their liquidus temperature, and long term stability against 
devitrification at processing temperatures, coupled with a temperature 
capability and chemical durability necessary to withstand liquid crystal 
display manufacture, thereby rendering them eminently suitable for use as 
substrates for liquid crystal display (LCD) devices which employ 
polysilicon (poly-Si) thin film transistors (TFTs) as switches. 
BACKGROUND OF THE INVENTION 
Liquid crystal displays (LCDs) are passive displays which depend upon 
external sources of light for illumination. They are manufactured as 
segmented displays or in one of two basic configurations. The substrate 
needs (other than being transparent and capable of withstanding the 
chemical conditions to which it is exposed during display processing) of 
the two matrix types vary. The first type is intrinsic matrix addressed, 
relying upon the threshold properties of the liquid crystal material. The 
second is extrinsic matrix or active matrix (AM) addressed, in which an 
array of diodes, metal-insulator-metal (MIM) devices, or thin film 
transistors (TFTs) supplies an electronic switch to each pixel. In both 
cases, two sheets of glass form the structure of the display. The 
separation between the two sheets is the critical gap dimension, of the 
order of 5-10 .mu.m. 
Intrinsically addressed LCDs are fabricated using thin film deposition 
techniques at temperatures .ltoreq.350.degree. C., followed by 
photolithographic patterning. As a result, the substrate requirements 
therefor are often the same as those for segmented displays. 
Soda-lime-silica glass with a barrier layer has proven to be adequate for 
most needs. A high performance version of intrinsically addressed LCDs, 
the super twisted nematic (STN) type, has an added requirement of 
extremely precise flatness for the purpose of holding the gap dimensions 
uniform. Because of that requirement, soda-lime-silica glass used for 
those displays must be polished or, alternatively, a precision formed, 
barium aluminoborosilicate glass marketed by Corning Incorporated, 
Corning, N.Y., as Code 7059 may be used without polishing. 
Extrinsically addressed LCDs can be further subdivided into two categories; 
viz., one based on MIM or amorphous silicon (a-Si) devices, and the other 
based on polycrystalline silicon (poly-Si) devices. The substrate 
requirements of the MIM or a-Si type are similar to the STN application. 
Corning Code 7059 sheet glass is the preferred substrate because of its 
very low sodium content, i.e., less than 0.1% Na.sub.2 O by weight, its 
dimensional precision, and its commercial availability. Devices formed 
from poly-Si, however, are processed at higher temperatures than those 
that are employed with a-Si TFTs. Substrates capable of use temperatures 
(taken to be 25.degree. C. below the strain point of the glass) of 
600.degree.-800.degree. C. are demanded. The actual temperature required 
is mandated by the particular process utilized in fabricating the TFTs. 
Those TFTs with deposited gate dielectrics require 600.degree.-650.degree. 
C., while those with thermal oxides call for about 800.degree. C. Both 
a-Si and poly-Si processes demand precise alignment of successive 
photolithographic patterns, thereby necessitating that the thermal 
shrinkage of the substrate be kept low. Those temperatures have mandated 
the use of glasses exhibiting higher strain points than soda-lime-silica 
glass and Corning Code 7059 glass in order to avoid thermal deformation of 
the sheet during processing. As can be appreciated, the lower the strain 
point, the greater this dimensional change. Thus, there has been 
considerable research to develop glasses demonstrating high strain points 
so that thermal deformation is minimized during device processing at 
temperatures greater than 600.degree. C., and preferably, higher than 
650.degree. C. 
U.S. Pat. No. 4,824,808 (Dumbaugh, Jr.) lists four properties which have 
been deemed mandatory for a glass to exhibit in order to fully satisfy the 
needs of a substrate for LCDs: 
First, the glass must be essentially free of intentionally added alkali 
metal oxide to avoid the possibility that alkali metal from the substrate 
can migrate into the transistor matrix; 
Second, the glass substrate must be sufficiently chemically durable to 
withstand the reagents used in the TFT matrix deposition process; 
Third, the expansion mismatch between the glass and the silicon present in 
the TFT array must be maintained at a relatively low level even as 
processing temperatures for the substrates increase; and 
Fourth, the glass must be capable of being produced in high quality thin 
sheet form at low cost; that is, it must not require extensive grinding 
and polishing to secure the necessary surface finish. 
That last requirement is most difficult to achieve inasmuch as it demands a 
sheet glass production process capable of producing essentially finished 
glass sheet, such as the overflow downdraw sheet manufacturing process 
described in U.S. Pat. No. 3,338,696 (Dockerty) and U.S. Pat. No. 
3,682,609 (Dockerty). That process requires a glass exhibiting a very high 
viscosity at the liquidus temperature plus long term stability, e.g., 
periods of 30 days, against devitrification at melting and forming 
temperatures. 
Corning Code 7059 glass, supra, is currently employed in the fabrication of 
LCDs. That glass, consisting essentially, in weight percent, of about 50% 
SiO.sub.2, 15% B.sub.2 O.sub.3, 10% Al.sub.2 O.sub.3, and 24% BaO, is 
nominally free of alkali metal oxides, and exhibits a linear coefficient 
of thermal expansion (25.degree.-300.degree. C.) of about 
46.times.10.sup.-7 /.degree. C. and a viscosity at the liquidus 
temperature in excess of 600,000 poises (6.times.10.sup.-4 Pa.multidot.s). 
The high liquidus viscosity of the glass enables it to be drawn into sheet 
via the overflow downdraw sheet processing technique, but its relatively 
low strain point (.about.593.degree. C.) is adequate only for processing 
a-Si devices and not for poly-Si devices. 
The glasses of U.S. Pat. No. 4,824,808, supra, were designed to meet the 
requirements for use in fabricating poly-Si devices, including the 
capability of being formed into sheet by the overflow downdraw sheet 
processing technique, and linear coefficients of thermal expansion as low 
as about 36.5.times.10.sup.-7 /.degree. C. (25.degree.-300.degree. C.), 
such as to closely match that of silicon, thereby enabling a silicon chip 
to De sealed directly thereon, but their strain points were less than 
650.degree. C. 
The glasses of U.S. Pat. No. 4,409,337 (Dumbaugh, Jr.) were also considered 
for LCD substrates, but their long term stability against devitrification 
was feared to be insufficient for their use in the overflow downdraw sheet 
processing technique. 
The glasses of U.S. Pat. No. 5,116,787 (Dumbaugh, Jr.) are essentially free 
from alkali metal oxides and MgO and demonstrate strain points of 
655.degree. C. and higher, with viscosities at the liquidus greater than 
1.5.times.10.sup.5 poises (1.5.times.10.sup.4 Pa.multidot.s). Although 
designed for use in the overflow downdraw sheet processing technique, 
their long term stability against devitrification was found to be marginal 
when employed in the process, some crystallization being formed in the 
glass during manufacture. 
U.S. Pat. No. 5,116,788 (Dumbaugh, Jr.) discloses other glasses exhibiting 
high strain points, i.e., greater than 675.degree. C., but having such 
relatively low viscosities at the liquidus temperature, viz., 
20,000-200,000 poises (2,000-20,000 Pa.multidot.s), as to be subject to 
devitrification when formed utilizing the overflow downdraw sheet 
processing technique. 
SUMMARY OF THE INVENTION 
The present invention arose from the discovery of glasses in the barium 
aluminosilicate composition system which exhibit strain points higher than 
660.degree. C., liquidus temperatures no higher than 1,175.degree. C., 
exceptionally good chemical durability, exhibiting weight losses of less 
than 0.5 mg/cm.sup.2 after immersion for 24 hours in an aqueous 5% by 
weight HCl solution at 95.degree. C., and long term stability against 
devitrification at melting and forming temperatures. The glass 
compositions are essentially free from alkali metal oxides and consist 
essentially, expressed in terms of mole percent on the oxide basis, of 
______________________________________ 
SiO.sub.2 
65-76 SrO 0-10 
Al.sub.2 O.sub.3 
7-11 MgO + CaO + SrO 
0-15 
BaO 12-19 ZrO.sub.2 0-2.5 
B.sub.2 O.sub.5 
0-5 TiO.sub.2 0-3 
MgO 0-5 Ta.sub.2 O.sub.5 
0-3 
CaO 0-10 ZrO.sub.2 + TiO.sub.2 + Ta.sub.2 O.sub.5 
0.5-5. 
______________________________________ 
Compliance with those specified composition intervals has been found 
necessary in order to obtain glasses illustrating the desired matrix of 
chemical, forming, and physical properties, as is demonstrated below. 
Thus, where the SiO.sub.2 concentration is below 65%, the strain point will 
fall below 660.degree. C. and the resistance of the glass to attack by 
acid suffers. Conversely, when the content of SiO.sub.2 is greater than 
76%, melting of the glass becomes difficult at customary glass melting 
temperatures. 
The presence of Al.sub.2 O.sub.3 plays a vital role in controlling the 
temperature of the liquidus. Hence, Al.sub.2 O.sub.3 contents outside of 
the designated 7-11% interval cause the liquidus temperature to rise to 
high levels. 
The high concentrations of BaO are required to assure the demanded low 
liquidus temperature. Nevertheless, Levels in excess of 19% can lead to 
liquidus temperatures higher than desired. 
B.sub.2 O.sub.3 is advantageous in lowering the high temperature melting 
viscosity of the glass, thereby facilitating melting. It also reduces the 
strain point of the glass so that additions thereof will be limited to a 
maximum of about 5%. 
The other alkaline earth metal oxides can be useful in modifying the 
melting and physical properties of the glasses. A substitution of CaO 
and/or SrO for a portion of the BaO serves to reduce the linear 
coefficient of thermal expansion of the glass and, in some instances, 
raise the strain point thereof; but, however, it also raises the liquidus 
temperature. MgO appears to reduce the liquidus temperature of the glasses 
when included in amounts less than about 5% but at greater levels the 
liquidus temperature appears to rise. In general, the total MgO+CaO+SrO 
will not exceed about 15%, with CaO and SrO being useful in concentrations 
up to 10% each. 
We have found several unexpected benefits through additions of as little as 
0.5% of Ta.sub.2 O.sub.5, TiO.sub.2, and/or ZrO.sub.2. That is, the 
inclusion of Ta.sub.2 O.sub.5 at levels up to about B% raises the strain 
point of the glasses significantly, while lowering their linear 
coefficient of thermal expansion, without substantially affecting the 
liquidus temperature thereof. ZrO.sub.2 behaves in a similar manner at 
concentrations up to about 2.5%. At levels above that amount, however, the 
liquidus temperature rises steeply. TiO.sub.2 contents up to about 3% 
appear to raise the strain point of the glasses slightly and are quite 
effective in reducing the thermal expansion thereof. The sum of ZrO.sub.2 
+TiO.sub.2 +Ta.sub.2 O.sub.5 will total about 5%, with a minimum level of 
about 0.5% assuring a substantial effect upon the strain point and linear 
coefficient of thermal expansion of the glass. 
The preferred glasses have compositions consisting essentially, expressed 
in terms of mole percent on the oxide basis, of about 
______________________________________ 
SiO.sub.2 
68-76 ZrO.sub.2 0-2 
Al.sub.2 O.sub.3 
7-10 TiO.sub.2 0-2.5 
BaO 14-19 Ta.sub.2 O.sub.5 
0-2.5 
B.sub.2 O.sub.3 
0-5 ZrO.sub.2 + TiO.sub.2 + Ta.sub.2 O.sub.5 
0.5-5. 
______________________________________ 
PRIOR ART 
U.S. Pat. No. 5,116,789 (Dumbaugh, Jr. et al.) is drawn to strontium 
aluminosilicate glasses especially designed for use as substrates for LCD 
devices which utilize poly-Si TFTs. The compositions of the glasses are 
essentially free from alkali metal oxides and MgO and consist essentially, 
in mole percent on the oxide basis, of 
______________________________________ 
SiO.sub.2 
65-75 CaO and/or BaO 0-10 
Al.sub.2 O.sub.3 
6-10 B.sub.2 O.sub.3 0-5 
SrO 15-26 (CaO and/or BaO) + B.sub.2 O.sub.3 
0-12 
______________________________________ 
Not only are those compositions high in SrO and low in BaO compared to the 
glasses of the present invention, but also there is no recognition therein 
of the very beneficial effects resulting from additions of Ta.sub.2 
O.sub.5, TiO.sub.2, and/or ZrO.sub.2. 
U.S. Pat. No. 5,116,789 (Shell) discloses glass compositions essentially 
free from alkali metal oxides especially designed for the encapsulation 
and sealing of electronic equipment and for the formation of a variety of 
containers where superior chemical durability and high electrical 
necessity are desired. The glasses consisted essentially, expressed in 
terms of mole percent on the oxide basis, of 
______________________________________ 
SiO.sub.2 
64-75.9 BaO 7-16.4 
Al.sub.2 O.sub.3 
6-9.1 MgO 0-4 
CaO 0-26 CaO + BaO + MgO .gtoreq.16.4. 
______________________________________ 
Ta.sub.2 O.sub.5, TiO.sub.2 and ZrO.sub.2 are nowhere mentioned in the 
patent so there is no appreciation of the capability of those materials in 
raising the strain point of the base BaO.Al.sub.2 O.sub.3.SiO.sub.2 glass 
while reducing the linear coefficient of thermal expansion.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Table I reports a number of glass compositions, expressed in terms of parts 
by weight on the oxide basis, illustrating the compositional parameters of 
the present inventive glasses. Inasmuch as the sum of the individual 
components totals or very closely approximates 100, for all practical 
purposes the listed values may be considered to reflect weight percent. 
The actual batch materials may comprise any materials, either an oxide or 
other compound, which, when melted together with the other batch 
constituents, will be converted into the desired oxide in the proper 
proportions. For example, CaCO.sub.3 and BaCO.sub.3 can supply the source 
of CaO and BaO, respectively. 
The batch ingredients were compounded, tumble mixed together thoroughly to 
assist in obtaining a homogeneous melt, and charged into platinum 
crucibles. After placing lids thereon, the crucibles were introduced into 
furnaces operating at temperatures of 1,600.degree. C. To assure the 
formation of inclusion- and cord-free glasses, a two-step melting practice 
was undertaken. In the first step the batch was melted for about 16 hours, 
stirred, and thereafter poured as a fine stream into a bath of tap water 
to form finely-divided particles of glass, a process termed "drigaging" in 
the glass art. In the second step the finely-divided glass particles 
(after drying) were remelted at 1,600.degree. C. for about four hours, the 
melts stirred in both directions, i.e., both clockwise and 
counterclockwise, and the melts then poured onto steel plates to make 
glass slabs having the approximate dimensions 18".times.6".times.0.5" 
(.about.45.7.times.15.2.times.1.3 cm), and those slabs transferred 
immediately to an annealer operating at about 750.degree. C. 
It must be recognized that the above description reflects a laboratory 
melting procedure only. Thus, the inventive glasses are quite capable of 
being melted and formed utilizing large scale, commercial glass melting 
and forming equipment. Where desired, fining agents such as the oxides of 
arsenic and antimony may be added in customary amounts. The small residual 
remaining in the glass has no substantial effect upon the properties of 
the glass. 
The compositions of four glasses commercially available from Corning 
Incorporated are also recorded in Table I, as analyzed in weight percent, 
for comparison purposes. Code 7059 has been discussed above. Code 1724, 
Code 1733, and Code 1735 glasses are included within U.S. Pat. No. 
4,409,337, U.S. Pat. No. 4,804,808, and U.S. Pat. No. 5,116,787, 
respectively, which patents were reviewed briefly above. 
Table I also recites measurements of several chemical and physical 
properties determined on the glasses in accordance with measuring 
techniques conventional in the glass art. Hence, the linear coefficient of 
thermal expansion (Exp) over the temperature range 0.degree.-300.degree. 
C. expressed in terms of .times.10.sup.-7 /.degree. C., and the softening 
point (S.P.), the annealing point (A.P.), and the strain point (St.P.) 
expressed in terms of .degree.C., were determined via fiber elongation. 
The durability (Dur) in HCl was evaluated by determining the weight loss 
(mg/cm.sup.2) after immersion in a bath of aqueous 5% by weight HCl 
operating at 95.degree. C. for 24 hours. 
The liquidus temperatures of the glasses were measured via two different 
methods. The standard liquidus method (Liq.) involves placing crushed 
glass particles in a platinum boat, placing the boat in a furnace having a 
region of gradient temperatures, heating the boat in an appropriate 
temperature region for 24 hours to melt the glass in at least a section of 
the boat length, withdrawing the boat from the furnace, allowing the melt 
to cool in the boat to a length of glass, removing said length of glass 
from the boat, and determining by means of microscopic examination the 
highest temperature at which crystals appear in the interior of the glass. 
A second method termed the "meltback liquidus" (M.Liq.) contemplates 
placing a glass sample which has been precrystallized by holding at a 
temperature of 1,000.degree. C. for 24 hours in a platinum boat, heating 
the boat in an appropriate temperature region in a gradient furnace for 24 
hours, withdrawing the boat from the furnace, removing the glass sample 
from the boat, and then determining by microscopic examination the lowest 
temperature at which crystals are not observed in the interior of the 
glass. Generally, the Liquidus temperatures measured by these two 
techniques do not differ by more than 50.degree. C., with the "meltback 
liquidus" typically being higher than the standard liquidus temperature. 
Table IA records the same glass compositions but reported in terms of mole 
percent on the oxide basis. 
TABLE I 
______________________________________ 
7059 1724 1733 1735 1 
______________________________________ 
SiO.sub.2 
50 56.8 57.0 57.1 53.0 
Al.sub.2 O.sub.3 
10 16.4 15.2 14.5 9.1 
B.sub.2 O.sub.3 
15 4.7 12.4 4.7 1.7 
MgO -- 5.8 1.4 -- -- 
CaO -- 7.8 3.9 11.1 -- 
SrO -- -- 3.6 -- 0.6* 
BaO 25 8.0 52 12.5 33.8 
Exp. 46 43.5 36.5 48.8 55.2 
S.P. 844 920 918 924 914 
A.P. 639 720 695 717 707 
St.P. 593 675 640 671 660 
Dur. 12 0.3 4 0.1 0.04 
Liq. 960 1100 980 1055 -- 
M.Liq. 
955 -- 1035 1090 1090 
______________________________________ 
2 3 4 5 6 
______________________________________ 
SiO.sub.2 
60.5 53.2 56.7 52.6 52.2 
Al.sub.2 O.sub.3 
11.0 9.2 10.3 9.1 9.0 
B.sub.2 O.sub.3 
-- -- 3.4 3.3 3.3 
SrO 0.5* 0.6* 0.4* 0.5* 0.5* 
BaO 27.8 33.9 26.0 33.6 33.6 
ZrO.sub.2 
-- 3.0 3.0 0.8 1.5 
Exp. 47.0 53.7 45.5 54.4 53.9 
S.P. 1054 1000 1008 921 928 
A.P. 793 781 760 712 717 
St.P. 730 724 704 664 669 
Dur. -- 0.04 0.03 0.02 0.02 
M.Liq. 
12O0 1100 1100 1090 1075 
______________________________________ 
7 8 9 10 
______________________________________ 
SiO.sub.2 
51.5 50.6 52.5 48.1 
Al.sub.2 O.sub.3 
8.9 8.7 9.0 8.3 
B.sub.2 O.sub.3 
3.2 3.2 3.3 3.0 
SrO 0.5 0.5 0.4 0.4 
BaO 32.8 32.4 29.6 27.1 
ZrO.sub.2 
3.0 4.4 3.0 2.8 
TiO.sub.2 
-- -- 2.0 -- 
Ta.sub.2 O.sub.5 
-- -- -- 2.0 
Exp. 52.9 52.4 49.9 47.0 
S.P. 941 952 949 991 
A.P. 730 740 729 772 
St.P. 679 690 675 718 
Dur. 0.03 0.03 0.03 0.03 
M.Liq. 
1175 1240 -- -- 
Liq. -- -- -- -- 
*Present as an impurity in the BaCO.sub.3 batch material. Not added 
intentionally. 
TABLE IA 
______________________________________ 
7059 1724 1733 1735 1 
______________________________________ 
SiO.sub.2 
63.4 62.8 65.0 66.0 71.1 
Al.sub.2 O.sub.3 
8.0 10.7 10.4 9.9 7.2 
B.sub.2 O.sub.3 
16.1 4.6 12.3 4.7 3.9 
MgO -- 9.2 2.5 -- -- 
CaO -- 9.2 4.9 13.8 -- 
SrO -- -- 2.5 -- -- 
BaO 12.5 3.5 2.5 5.7 17.8 
______________________________________ 
2 3 4 5 6 
______________________________________ 
SiO.sub.2 
77.7 72.5 73.3 70.7 70.4 
Al.sub.2 O.sub.3 
8.3 7.4 7.8 7.2 7.1 
B.sub.2 O.sub.3 
-- -- 3.8 3.8 3.8 
SrO -- -- -- -- -- 
BaO 14.0 18.1 13.2 17.7 17 6 
ZrO.sub.2 
-- 2.0 1.9 0.5 1.0 
______________________________________ 
7 8 9 10 
______________________________________ 
SiO.sub.2 
69.7 69.0 69.7 69.7 
Al.sub.2 O.sub.3 
7.1 7.0 7.1 7.0 
B.sub.2 O.sub.3 
3.8 3.7 3.8 3.8 
BaO 17.4 17.3 15.4 15.4 
ZrO.sub.2 
2.0 2.9 2.0 2.0 
TiO.sub.2 
-- -- 2.0 -- 
Ta.sub.2 O.sub.5 
-- -- -- 2.0 
______________________________________ 
An examination of the above glasses illustrates the care in composition 
control that must be exercised in preparing glasses satisfying the 
objectives of the present invention. For example, Code 7059 and Code 1733 
demonstrate strain points which are too low and the chemical durabilities 
thereof are less than desired. Whereas the strain points and the chemical 
durabilities of Code 1724 and Code 1735 are satisfactory, their long term 
stability against devitrification has not proven satisfactory for use in 
the overflow downdraw sheet process. 
Examples 1 and 5 demonstrate the very dramatic effect which the inclusion 
of B.sub.2 O.sub.3 alone exerts on the strain point of the inventive 
glasses and yielding a low liquidus temperature, and the action of 
ZrO.sub.2 to raise the strain point while leaving the liquidus temperature 
undisturbed. Example 2 illustrates that a high SiO.sub.2 content raises 
the liquidus temperature to too high a value. Examples 3-10 are 
particularly interesting in demonstrating the substantial effects upon the 
properties of the inventive glasses exerted by ZrO.sub.2, TiO.sub.2, and 
Ta.sub.2 O.sub.5. Thus, each addition significantly reduces the thermal 
expansion of the glass while not deleteriously affecting the strain point. 
Examples 7 and 8 illustrate the need to restrict the level of ZrO.sub.2, 
however, in order to avoid a great increase in the liquidus temperature. 
Example 10 comprises the most preferred composition based upon its overall 
combination of chemical, physical, and melting properties, and being 
ideally suited to be drawn into thin sheet employing the downdraw sheet 
processing technique.