Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2001-02-15
2003-08-19
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S070000, C501S073000, C065S030140, C313S480000
Reexamination Certificate
active
06607999
ABSTRACT:
TECHNICAL BACKGROUND
1. Field of the Invention
The present invention relates to (1) a glass for a cathode-ray tube, a glass panel for a cathode-ray tube for which the above glass is applied, a cathode-ray tube and a process for the production thereof, (2) a glass for a cathode-ray tube, a process for the production thereof and a glass panel for a cathode-ray tube and a cathode-ray tube to which the glass for a cathode-ray tube is applied, and (3) a strengthened glass, a process for the production thereof, a glass made of the above strengthened glass for a display and a glass panel for a cathode-ray tube and a cathode-ray tube for which the above glass is applied.
2. Related Art Statement
It is conventional practice to use a glass having large contents of PbO, SrO and BaO as a glass for a cathode-ray tube (CRT). For example, JP-B-59-27729 discloses a CRT face plate (glass panel) containing BaO, SrO and BaO. However, a glass of this type has a bending strength of approximately 50 to 100 MPa, and it is required to increase the thickness thereof for ensuring strength, so that the face plate is to have an extraordinarily large weight for satisfying a larger display screen (for example, a glass panel for a 36-inch CRT has a thickness of 20 mm or more and has a weight of approximately 40 kg). The weight of a cathode-ray tube is greatly dependent upon a glass panel, so that there is caused a problem that a cathode-ray tube using such a glass panel comes to have a considerably large weight.
In a flat-surface cathode-ray tube which has been available in recent years, a glass panel having a flat face is required to support vacuum inside the CRT and atmospheric pressure outside unlike a conventional curved glass panel, so that the panel is increased in thickness or there is employed a method using a physically strengthened glass.
With regard to the physically strengthened glass, Japanese Patent No. 2,671,766 discloses a glass bulb for a cathode-ray tube, which glass bulb is produced by physical strengthening.
In a physical strengthening method, however, a stress-strain layer (including a compression stress layer) having a moderate profile is formed, so that the physical strengthening method, if anything, is suitable for strengthening a glass having a large thickness. However, no sufficient stress-strain layer can be secured in a glass having a small thickness, so that it is difficult to attain high strength. As a consequence, it is difficult to decrease the thickness of a glass panel by physical strengthening, and a decrease in the weight of a glass panel cannot be expected. Further, since this physically strengthened glass has a strain point of approximately 470° C. as described in Japanese Patent No. 2,671,766, if it is partly exposed to a higher temperature for some time when heated to approximately 450° C. in a frit-sealing step, the stress-strain may be alleviated and the desired bending strength may not be obtained. Further, the panel glass disclosed in Japanese Patent 2,671,766 has a Young's modulus of approximately 76 GPa, and the glass is liable to be deformed under atmospheric pressure when decreased in thickness.
Japanese Patent No. 2,904,067 discloses a CRT panel constituted of a physically strengthened panel.
It is said that the physically strengthened glass such as an air-cooled strengthened glass generally has a bending strength of 200 to 300 MPa. This physically strengthened glass is obtained by a method in which a glass is rapidly cooled from a temperature around its softening point to a temperature around its strain point to provide a temperature difference between the inside of the glass and the glass surface, whereby a compression stress layer is formed in the glass surface. The merit of this method is that a stress-strain layer having a thickness approximately ⅙ as large as the thickness of the glass panel can be formed. However, this method involves a problem that it is not suitable for a glass having such a small thickness that it is difficult to provide a temperature difference between the inside and the surface of the glass or a glass having such a complicated form that no uniform temperature distribution can be obtained. A glass panel for a CRT generally has a complicated form consisting of a flat face portion (image display screen) and a junction portion that is located outside the flat face portion and is to be bonded to a skirt (frame)-shaped funnel. Particularly, while the outer surface of the face portion is flat, the inner surface of the face portion has a curvature due to the scanning of an electron gun. The glass panel is non-uniformly cooled so that the panel surface is liable to be distorted or undulated. Further, an air-cooled strengthened glass internally has a tensile-stress layer having a size approximately ½ as large as a compression stress. A general air-cooled strengthened glass has a compression stress of approximately 50 to 150 MPa and a tensile stress of approximately 25 to 75 MPa. When cracking develops inside, therefore, the above large tensile stress is instantly released and a phenomenon called self-fracture is caused to take place in some cases. It therefore involves a problem to apply a glass that is only physically strengthened as above to a CRT panel.
For the above reasons, in Japanese Patent No. 2,904,067, the above rapid cooling is not carried out, but the cooling is moderately carried out by applying cold air to a glass while the glass is cooled from an annealing point to a strain point. The tensile stress in the center is small. However, the compression stress in the surface is small as well, and the bending strength of the glass also decreases. Such a glass has a compression stress of approximately 5 to 30 MPa, a tensile stress of 2 to 15 MPa and a bending strength of 100 to 150 MPa. For supporting a difference between pressures inside and outside a CRT with the glass having a bending strength of 100 to 150 MPa, it is inevitable to increase the glass thickness, so that the glass comes to have a large thickness and a large weight.
Japanese Patent 2,904,067 also discloses a method for chemically strengthening a glass panel for a CRT. As is concluded in the above Japanese Patent, the chemical strengthening fails to produce a sufficient stress-strain layer, and it is said that the thus-chemically-strengthened glass is unsuitable as a glass for a CRT. Further, JP-A-1-319232 discloses a chemically strengthened glass for a CRT. However, when the stress-strain layer has a small thickness, if the glass is damaged due to an external shock during the production of a CRT or use of a CRT as a product, the crack may penetrate the stress-strain layer and the glass may break.
Further, Japanese Patent No. 2,837,134 discloses a chemically strengthened glass having a high ion exchange ratio. The glass disclosed in this Patent has a stress-strain layer having a thickness of 200 &mgr;m or more and a bending strength of at least 800 MPa. However, it has been found that the above glass has a low X-ray absorption coefficient and fails to satisfy the X-ray absorption coefficient of 28/cm that is the standard for glass panels for a CRT. More specifically, the above glass is not required to contain any other component than ZrO
2
for increasing the X-ray absorption coefficient. For example, when the content of ZrO
2
is increased to its maximum amount of 15% by weight, the above glass shows an X-ray absorption coefficient of only approximately 25/cm. Moreover, ZrO
2
is a component that is not easily soluble in a glass, and if it is incorporated in an amount of more than 10% by weight, most of such an additional portion remains undissolved.
In actuality, there has been found no glass that satisfies a stress-strain layer having a thickness of at least 100 &mgr;m, a high bending strength of at least 300 MPa and an X-ray absorption coefficient of at least 28/cm. Therefore, a CRT glass panel inevitably has a large thickness and a large weight, and for example, a TV receiver has a very large weight. Specifically, a 36-inch CRT panel has a cen
Bolden Elizabeth A
Hoya Corporation
Nixon & Vanderhye P.C.
Sample David
LandOfFree
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