Electric lamp and discharge devices – Cathode ray tube – Envelope
Reexamination Certificate
2002-05-31
2004-11-09
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Envelope
Reexamination Certificate
active
06815882
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color cathode ray tube to be used for e.g. a display for a television broadcast receiver (hereinafter referred to as a television) or a computer, and a glass bulb to be used for such a cathode ray tube.
2. Discussion of the Background
Firstly, the construction of a color cathode ray tube will be described referring to the attached drawings.
FIG. 1
is a partially cross-sectional view of the entirety of the color cathode ray tube.
FIG. 2
is an enlarged view of
FIG. 1
at a portion S including the sealing portion and its vicinity. Here, in the present invention, a cathode ray tube is meant for a color cathode ray tube unless otherwise specified.
The envelope of the cathode ray tube
1
is constituted by a glass bulb
2
which basically comprises a panel
3
for displaying picture images, a funnel-shaped funnel
4
sealingly bonded to the panel
3
and a neck
5
accommodating an electron gun
17
. The panel
3
is constituted by an approximately rectangular face portion
7
constituting a picture image-displaying screen and a skirt portion
6
extending in a direction substantially perpendicular to the face portion
7
from its periphery via a blend R portion
9
.
An explosion proof reinforcing band
8
is wound around the circumference of the skirt portion
6
to maintain the panel strength and to prevent scattering upon breakage. On the inner surface side of the face portion
7
, a phosphor screen
12
which emits fluorescence by electron beam bombardment from an electron gun
17
and an aluminum film
13
to reflect the fluorescence emitted from the phosphor screen
12
towards the rear side of the cathode ray tube (towards the funnel
4
side), to the front side (to the face
7
side), are laminated, and a shadow mask
14
which regulates the position for electron beam bombardment, is further provided. The shadow mask
14
is fixed to the inner surface of the skirt portion
6
by stud pins
15
. Further, A in
FIG. 1
indicates a tube axis connecting the center axis of the neck
5
and the center axis of the panel
3
.
Such a panel
3
is sealingly bonded to a seal edge portion
16
′ of the funnel
4
by a sealing material such as a solder glass provided at the seal edge portion
16
corresponding to the end portion of the skirt portion
6
, whereby a sealing portion
10
is formed.
The glass bulb
2
for a cathode ray tube having the above construction, is used as a vacuum vessel, whereby atmospheric pressure is exerted to the outer surface. The glass bulb is in unstable deformed state due to an asymmetrical shape as is different from a spherical shell, and a stress is exerted over a relatively wide range (a stress formed when the glass bulb is vacuumized, will hereinafter be referred to as a vacuum stress). In such a state that a high tensile vacuum stress is applied to the outer surface, a delayed fracture may take place due to the effect by moisture in the atmosphere, which may cause decrease in safety and reliability.
The face portion
7
as a portion which displays picture images has the highest flatness in the cathode ray tube and thereby has a low rigidity, and it is most significantly deformed when the inside of the cathode ray tube is depressurized and an atmospheric pressure is applied thereto. Further, the face portion
7
is supported by the blend R portion
9
having a high rigidity, whereby a high tensile vacuum stress is likely to generate in the vicinity of the blend R portion
9
along with the deformation of the face portion
7
. Further, the deformation of the face portion
7
functions as a force to deform the skirt portion
6
towards the outside via the blend R portion
9
, and accordingly a high tensile vacuum stress is generated also at the sealing portion
10
.
However, the sealing portion sealingly bonded by means of a sealing material has the lowest allowance against the tensile vacuum stress in the glass bulb, and the allowable stress at the sealing portion becomes lower when the accuracy of the flatness at the sealing surface between the panel and the funnel is low.
A television employing a cathode ray tube has a demerit of being heavy as compared with a plasma display and a liquid crystal display, whereby weight reduction of a glass bulb has been desired. Further, in recent years, a cathode ray tube having a face portion having a higher flatness has been desired to decrease distortion of picture images as far as possible to improve visibility. However, by making the face portion flat, asymmetry of the glass bulb shape increases, and the glass bulb is in a further unstable deformed state, whereby tensile vacuum stress generated to the respective portions tends to increase. In addition, the amount of glass used tends to decrease as compared with conventional ones due to weight reduction, whereby a higher deformation energy tends to be accumulated on the glass bulb, thus increasing possibility of destruction.
Accordingly, if the panel thickness is made thin and the face portion is made flat at the same time to accomplish such weight reduction, a tensile vacuum stress generated at the face portion will significantly increase as described above. To overcome the above problem, tempering methods to produce a compressive stress to the panel surface have been developed.
Heretofore, as a means to reduce the weight of the glass bulb for a cathode ray tube, it has been practically proposed to form a compressive stress layer on the surface of a panel in a thickness of about ⅙ of the glass by means of e.g. a physical tempering method, as disclosed in Japanese Patent No. 2904067. However, it is impossible to uniformly quench a panel or funnel having a three dimensional structure and a non-uniform thickness distribution. Consequently, due to the non-uniform temperature distribution, a large tensile residual stress will be formed together with the compressive stress, whereby the compressive stress is rather limited to a level of 30 MPa at best, and it has been impossible to produce a large compressive stress. Namely, when a physical tempering method is employed, the weight reduction of the glass bulb is limited, since the compressive stress which can be produced, is relatively small.
On the other hand, it is known to reduce the weight by tempering the surface of a glass bulb by an ion-exchange method. This method is a method wherein certain alkali ions in glass are substituted by ions larger than the alkali ions at a temperature of not higher than the distortion point, and a compressive stress layer is formed on the surface by the volume increase. For example, it can be accomplished by immersing a strontium/barium/alkali/alumina/silicate glass containing from about 5 to about 8% of Na
2
O and from about 5 to about 9% of K
2
O, in a molten liquid of KNO
3
at about 450° C. In the case of such ion-exchange method, a large compressive stress at a level of from 50 to 300 MPa can be obtained, and it is advantageous for the weight reduction over the physical tempering in that no necessary tensile stress will be formed.
The ion-exchange method is usually carried out in the process of panel production, i.e. it is carried out after press molding and polishing, whereby a high compressive stress can be produced to the face portion and the skirt portion. However, the sealing portion is provided in such a manner that after e.g. shadow mask is attached to the inside of the panel, the seal edge portion of the panel and the seal edge portion of the funnel are put together and welded by means of a sealing material such as a solder glass, whereby no compressive stress can be produced by means of an ion-exchange method, and accordingly the difference in strength between the face portion and the seal portion tends to further widen.
On the other hand, the face portion to which a high compressive stress is produced by an ion-exchange method, can tolerate a high tensile vacuum stress as compared with a conventional one, and consequently, the face portion can significantly be mad
Murakami Toshihide
Sugawara Tsunehiko
Asahi Glass Company Limited
Macchiarolo Peter
Patel Nimeshkumar D.
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