Stock material or miscellaneous articles – Composite – Of quartz or glass
Reexamination Certificate
2002-03-27
2004-01-13
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of quartz or glass
C428S001100, C428S001620, C428S689000, C428S702000, C501S004000, C501S055000, C501S065000, C501S066000, C501S068000, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06677046
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a glass ceramic, a glass ceramic substrate, an opposite substrate for a liquid crystal panel and a dustproof substrate for a liquid crystal panel. More specifically, the present invention relates to a glass ceramic having properties such as a low thermal expansion property, a high transmittance in a visible light region and a low specific gravity, a glass ceramic substrate that is made of the above glass ceramic and is suitable for use as a dustproof substrate for a liquid crystal projector or a substrate (TFT-opposite substrate) facing a substrate with a thin film transistor in a liquid crystal device, and an opposite substrate and a dustproof substrate for a liquid crystal panel, each of which comprises the above glass ceramic.
PRIOR ART
A glass ceramic formed by depositing a fine crystal phase in a glass is widely known as a glass having the property of low expansion. For example, JP-B-47-5558 discloses a glass ceramic having a thermal expansion coefficient of ±0.2×10
−7
/° C. at and around room temperature. The glass ceramic is used in the fields of a reflecting mirror of a large-sized astronomical telescope, a laser gyroscope, a standard or a surface plate and heat-resistant cooking utensils.
Conventionally, glasses of this type are colored in yellow or brown even if no colorant is contained, and they have poor transmittance as compared with general glass, so that their transmittance is often a barrier against the use thereof as a substitute for quartz glass.
In recent years, a liquid crystal projector is commercially available as one of large-screen television sets. The key portion of the liquid crystal projector is a liquid crystal panel made of a quartz substrate, and dustproof glass substrates for defocusing are attached to both the surfaces of the liquid crystal panel for preventing the projection of a foreign matter if such a foreign matter adheres to the liquid crystal panel surfaces. The liquid crystal substrate is formed of quartz, and essentially, it is desirable to use quartz as the above dustproof glass. Since, however, quartz glass is expensive, low-expansion transparent glass ceramic is substituted in some liquid crystal projectors. Similarly, of the two glass substrates constituting a liquid crystal panel, one on the side where no TFT is formed (“opposite substrate” hereinafter) is replaced with a low-expansion transparent glass ceramic substrate in some liquid crystal projectors. However, conventional low-expansion transparent glass ceramics have poor transmittance to light in a short wavelength region as compared with quartz glass, so that such ceramics degrade the performance of the liquid crystal projector with regard to screen image qualities. Particularly, conventional low-expansion transparent glass ceramics have low transmittance at and around 400 nm, and such glass ceramics look yellow or brown when visually observed. For this reason, projected images are inevitably affected by the coloring of the glass ceramics.
Further, no ultraviolet-curable resin can be used for bonding a dustproof glass due to a low transmittance at a wavelength of 400 nm or shorter. It is therefore general practice to use a heat-curable resin for the bonding. However, the heat-curing procedure takes a time and causes a productivity problem.
Further, a molten glass of a low-expansion transparent glass ceramic of the above type has a high viscosity, and convection of the molten glass does not easily occur, so that it is difficult to produce a homogeneous glass. Further, another defect is that the melting temperature thereof is high, so that a melting apparatus is greatly limited or that an ultra-high temperature melting furnace is required, which increases the production cost thereof.
Further, the transparent glass ceramic of the above type has another problem that its crystallization takes a time, so that its productivity is low. In the above JP-B-477558, Examples describe glass ceramics for which the holding time period is 4 to 100 hours at a temperature elevation rate of 8° C./hour. For example, in Example in which the holding time period is 24 hours at 800° C., it takes 32 hours before cooling is started.
Further, most of transparent glass ceramics of the above type have a specific gravity of 2.5 or more. The specific gravity is an important factor in use of a transparent glass ceramic including the use in a liquid crystal display. Since quartz glass has a specific gravity of 2.2, such a specific gravity of the glass ceramic sometimes comes to be a barrier against a use thereof as a substitute for the quartz glass.
Under the circumstances, attempts have been made in various ways to improve low-expansion transparent glass ceramics in transmittance and melting properties.
For example, JP-A-323237 and JP-A-2293345 describe glass ceramics containing no TiO
2
. It is said that Ti ion in the co-presence of impurities such as Fe ion, etc., greatly colors a glass ceramic, and it has been attempted to prevent the coloring by incorporating such Ti ion. In the above glass ceramic, TiO
2
is a nucleating component of the glass ceramic and is therefore replaced with ZrO
2
. Since, however, ZrO
2
is a component that is not easily dissolved in glass, it is liable to remain, and it is difficult to produce a homogeneous glass, so that its melting requires a high temperature of approximately 1,600° C.
Japanese Patent 2,516,537 discloses a low-expansion transparent glass ceramic containing 3 to 6% by weight of Li
2
O and substantially containing none of Na
2
O and K
2
O. However, since the above glass contains none of Na
2
O and K
2
O, and further since the content of Li
2
O is 6% by weight or less, it has a high melt viscosity, so that homogenization takes a time. Further, since the above glass ceramic contains none of Na
2
O and K
2
O that are components for suppressing the crystallization rate, the crystallization rate of the glass increases, and the glass during the crystallization is liable to undergo cracking. It is therefore required to decrease the temperature elevation rate, or hold the glass at a relatively low temperature for a long period of time, so that the crystallization does not sharply take place and that the cracking is accordingly prevented. For example, Example of the above Japanese Patent describes holding time periods of 10 hours at a nucleating temperature and 10 hours at a crystal growth temperature. In any case, the crystallization takes a long time and is liable to decrease the productivity of the glass ceramic. The glass ceramic is improved in transparency as compared with conventional products, while the transmittance thereof is low at and around 400 nm, and its color is yellowish. The glass ceramic of the above Japanese Patent is therefore hardly suitable for use in a display.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a glass ceramic which is easily meltable and has properties such as a low thermal expansion property, a high transmittance in a visible light region and a low specific gravity imparted by carrying out crystallization treatment for a short period of time, a glass ceramic substrate formed of the above glass ceramic, and an opposite substrate and a dustproof substrate for a liquid crystal panel, each of which comprises the above glass ceramic.
For achieving the above object, the present inventors have made diligent studies and as a result has found that a glass ceramic having a crystal phase containing &bgr;-quartz solid solution precipitated by heat treatment of a glass ceramic matrix glass having a specific composition or a glass ceramic having a crystal phase containing a &bgr;-quartz solid solution and having specific physical properties suits the above objects. The present invention has been completed on the basis of the above findings.
That is, the present invention provides:
(1) A glass ceramic having a crystal phase containing a &bgr;-quartz solid solution precipitated by heat treatment of a matrix glass for a glass ceramic, the ma
Hachitani Yoichi
Hosoda Keiji
Matsumoto Kenji
Blackwell-Rudasill G. A.
Hoya Corporation
Jones Deborah
Nixon & Vanderhye P.C.
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