Low expansion glass-ceramics

Details Low expansion glass-ceramics Low expansion glass-ceramics

1999-10-06

2001-06-19

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Devitrified glass-ceramics

C501S069000

Reexamination Certificate

active

06248678

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to transparent low expansion spinel-based glass-ceramic materials.
BACKGROUND OF THE INVENTION
U.S. patent application Ser. No. 09/030,863, filed on Mar. 31, 1997 and entitled “Glass-Ceramics,” the disclosure of which is incorporated herein by reference in its entirety, disclosed materials that can be used as substrates for high temperature polysilicon thin films. The transparent glass-ceramic materials disclosed in Ser. No. 09/030,863 have high strain points and coefficients of thermal expansion which are compatible with polycrystalline silicon, especially high temperature poly-Si. The glass-ceramics disclosed in Ser. No. 09/030,863 have expansions that were designed to closely match that of silicon (C.T.E. of 32-42×10
−7
/° C.).
It would be desirable to provide glass ceramics having high strain points, and in addition, having thermal expansion characteristics that are intermediate between those of silica and of silicon. Glass-ceramics having a thermal expansion below about 32×10
−7
/° C. over the temperature range of 25-1000° C. and having strain points in excess of 850° C. can be preferable in various applications. It would also be advantageous to provide a glass-ceramic having a low density in combination with the low thermal expansion and high strain point.
SUMMARY OF THE INVENTION
One aspect of the present invention relates to transparent glass-ceramics containing spinel as a predominant crystal phase, optionally accompanied by minor amounts of zirconia, Mg-petalite, &bgr;-quartz solid solution or titanate, said glass-ceramic comprising the following composition as calculated in weight percent on an oxide basis: 60-75 SiO
2
, 8-20 Al
2
O
3,
2-10 ZnO, 1-6 MgO, 1-10 TiO
2
, 0-6 ZrO
2
, 0-3 BaO, ZnO+MgO in combination being greater than or equal to about 5.5, and TiO
2
+ZrO
2
in combination being greater than about 4.
By transparent, it is meant that 1.1 mm thick sheets of the glass-ceramics of the present invention exhibit greater than 85 percent transmittance over the visible region (400 nm to 700 nm) of the spectrum. Preferably, for certain applications, it is also desirable that these sheets exhibit some transmittance in the near ultraviolet as well, for example, greater than 50 percent over the region 350-400 nm.
The transparent spinel-based glass-ceramics of the present invention exhibit a number of advantages compared to prior art glass-ceramics materials. For example, these glass ceramics exhibit thermal expansion coefficients of about 22 to 32×10
−7
/° C., more preferably 25-30×10
−7
/° C. over the temperature range 25° C. to 300° C. and strain points over 850° C., more preferably over 875° C., and most preferably over 900° C. Spinel solid solution comprises the predominant crystal phase in these materials.
These glass-ceramics consequently possess high thermal stability and can withstand temperatures of 900° C. for 24 hours and 1000° C. for 8 hours without distorting, warping, or losing transparency. The predominant (and usually sole) crystal phase in these materials consists of spinel solid solution (Zn,Mg)Al
2
O
4
, although the spinel may be accompanied by minor amounts of other crystal phases, e.g., zirconia, Mg-petalite, &bgr;-quartz solid solution, or MgTi
2
O
5
with no deleterious effects on transparency or properties. By predominant crystal phase, it is meant that this crystal phase is at least about 75 volume percent, more preferably at least 85 percent, and most preferably at least 95 percent of the total crystalline phases present. The glass-ceramics of the present invention are also preferably at least about 20 weight percent crystalline phases dispersed within a glassy matrix.
The glass-ceramics of the present invention exhibit excellent chemical durability. They also exhibit densities less than 2.70 grams/cc.
Glasses described herein can be melted at temperatures of 1575° to 1650° C. and are compatible with conventional forming processes including casting, pressing, and rolling. The compatibility of the precursor glasses with conventional melting and forming techniques, particularly rolling, renders these materials suitable candidates for the manufacture of large sheets. Their higher rigidity and strength compared to that of glass (Young's modulus greater than 12×10
6
psi, modulus of rupture>10×10
3
psi) would permit the use of thinner sheets when compared to glass materials.
The lower expansion, refractory glass-ceramics of the present invention can be used in a wide variety of applications ranging from substrates employing high temperature thin films, such as active matrix liquid crystal displays (AMLCD's) to components for optical devices such as optical waveguides. They are particularly useful for substrates in AMLCD applications because the peripheral drive circuitry can be placed directly on the substrate, a cost and space savings. The high strain points of these glasses enable processing at high temperatures in excess of 850° C. In addition, the materials of the present invention can find use in a wide variety of applications including but not limited to a substrate material in various electric, electronic, and optoelectronic devices such as, for example, flat panel displays, solar batteries, photomasks, and optomagnetic disks.
Although these glasses require melting temperatures higher than those higher-expansion, lower-silica glasses, they retain relatively smooth viscosity curves and have similar or higher liquidus viscosities of 1000 to over 5000 poise. The glass-ceramics have strain points well in excess of 850° C. and are preferable in applications that require a lower thermal expansion. These higher silica materials also have lower densities, which is advantageous for many applications.
DETAILED DESCRIPTION OF THE INVENTION
The present invention involves transparent spinel-based glass-ceramics with thermal expansion coefficients of about 22-32×10
−7
/° C. and strain points over about 850° C. The predominant and usually sole crystal phase in these materials consists of spinel solid solution (Zn,Mg)(Al,Ti)
2
O
4
.
Glass-ceramics consist of randomly oriented crystals dispersed within a matrix of residual glass and can be produced through controlled internal nucleation and crystallization of a precursor glass body. Hence, a glass forming batch of the desired composition is melted and that melt is cooled and simultaneously formed into a glass shape of a predetermined configuration utilizing conventional glass forming techniques. Glass forming batches for the inventive spinel crystal-containing glass-ceramic precursor glasses described herein are easily melted and the ensuing melts can be shaped into articles of widely varying geometries. The composition of the precursor glass, and the subsequently formed glass-ceramic, comprises or consists essentially of the following ingredients, expressed in weight percent:
SiO
2
60-75
BaO
0-3
Al
2
O
3
 8-20
ZnO
 2-10
MgO
1-6
TiO
2
 1-10
(ZnO + MgO)
≧5.5%
ZrO
2
0-6
(TiO
2
+ ZrO
2
)
≧4%
Fining agents, such as As
2
O
5
or Sb
2
O
3
, may be added to the glass compositions described herein, if needed or desired. In addition, up to 5% of other oxides or fluorides, such as Rb
2
O, WO
3
, Nb
2
O
5
, AlF
3
, B
2
O
3
, CeO
2
, Y
2
O
3
, or P
2
Oand/or up to 8% of Cs
2
O, Bi
2
O
3
, Ta
2
O
3
, Ga
2
O
3
, PbO, or La
2
O
3
, can be added if desired. The levels of K
2
O, Na
2
O, Li
2
O, CaO and SrO should preferably be limited to no more than 3%, and most preferably are zero. It is desirable that the glass-ceramics of the present invention be free of mobile alkali ions.
If the amount of SiO
2
is less than 60%, the proportion of spinel to residual glass increases, causing the thermal expansion coefficient to increase. Al
2
O
3
is a necessary component of the spinel, (Zn, Mg) Al
2
O
4
(1 mole (ZnO, MgO) combines with one mole Al
2
O
3
). Therefore, if Al
2
O
3
is less than 8 percent, too little spinel may be formed, and the thermal

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