Green-emitting phosphor composition and cathode ray tube...

Electric lamp and discharge devices – Cathode ray tube – Screen

Reexamination Certificate

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C252S30160S

Reexamination Certificate

active

06465946

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a green-emitting phosphor composition and a cathode ray tube (CRT) manufactured using the same, and more particularly, to a green-emitting phosphor composition having improved luminance characteristics, degradation characteristics and afterglow characteristics, and a CRT adopting the same.
2. Description of the Related Art
Recently, with the advent of HDTV (high definition television) broadcasting, demands for projection type CRTs which can attain 40-inch or larger screens have been increasing for both home and business purposes. In the case of a projection type CRT, the fluorescent screen of a projection type CRT requires a brightness of several to several tens of times that of a direct viewing type CRT because an image is projected onto a 40 inch, 50 inch or larger screen. Thus, the fluorescent screen of a projection type CRT should be excited under the condition in which a high-density current of several to several tens of times that of a direct viewing type CRT is applied. However, this results in deterioration of luminance, which is particularly serious in green-emitting phosphors which contribute to about 70% of the luminance.
As components of the conventional green-emitting phosphor, Y
3
Al
5
O
12
:Tb, Y
3
(Al, Ga)
5
O
12
:Tb, LaOCl:Tb, Zn
2
SiO
4
:Mn, InBO
3
:Tb or Y
2
SiO
5
:Tb, which have improved characteristics compared to Y
2
O
2
S:Tb, Gd
2
O
2
S:Tb or ZnS:Cu, Al, are used alone. However, these single-substance green-emitting phosphors do not fulfil all characteristics required for a projection type CRT. Thus, mixed green-emitting phosphors obtained by mixing Zn
2
SiO
4
:Mn or InBO
3
:Tb with Y
3
(Al, Ga)
5
O
12
:Tb, Y
2
SiO
5
:Tb or pigment-attached Y
3
(Al, Ga)
5
O
12
:Tb in an appropriate mixture ratio are put into practical use.
For example, Japanese Patent Laid-Open Publication No. hei 4-161483 describes a mixed green-emitting phosphor obtained by mixing Y
3
(Al, Ga)
5
O
12
:Tb with Zn
2
SiO
4
:Mn in order to improve color purity. U.S. Pat. No. 4,559,469 describes a mixed green-emitting phosphor obtained by mixing Zn
2
SiO
5
:Mn with Y
2
SiO
5
:Tb in order to improve color purity.
However, the Zn
2
SiO
4
:Mn phosphor used for improving color purity has poor degradation, luminance and afterglow characteristics under the condition in which a high-density current is applied, while exhibiting excellent color purity. Thus, the Zn
2
SiO
4
:Mn phosphor mixed for improving color purity considerably deteriorates luminance and degradation characteristics of the mixed green-emitting phosphor.
SUMMARY OF THE INVENTION
To solve the above problems, it is an object of the present invention to provide a new green-emitting phosphor composition which can replace a conventional green-emitting phosphor composition containing a Zn
2
SiO
4
:Mn phosphor.
It is another object of the present invention to provide a cathode ray tube (CRT) manufacturing method using the new green-emitting phosphor composition.
Accordingly, to achieve the first object, there is provided a green-emitting phosphor composition including 5 to 50 wt % of a SrGa
2
S
4
:Eu phosphor and 50 to 95 wt % of a Y
2
(Al, Ga)
5
O
12
:Tb phosphor.
According to another aspect of the present invention, there is provided a green-emitting phosphor composition including 5 to 20 wt % of a SrGa
2
S
4
:Eu phosphor, 60 to 90 wt % of a Y
2
(Al, Ga)
5
O
12
:Tb phosphor and 5 to 20 wt % of a LaOCl:Tb phosphor.
To achieve the second object, there is provided a cathode ray tube manufactured using the green-emitting phosphor composition.
DETAILED DESCRIPTION OF THE INVENTION
A mixed green-emitting phosphor composition comprising a mixture of a SrGa
2
S
4
:Eu phosphor and a Y
3
(Al, Ga)
5
O
12
:Tb phosphor or a mixture of a SrGa
2
S
4
:Eu phosphor, a Y
3
(Al, Ga)
5
O
12
:Tb phosphor and a LaOCl:Tb phosphor according to the present invention has excellent luminance, afterglow and degradation characteristics under the condition in which a high-density current is applied, so that it is capable of replacing a conventional green-emitting phosphor composition comprising a mixture of a Zn
2
SiO
4
:Mn phosphor, Y
3
(Al, Ga)
5
O
12
:Tb phosphor and/or a LaOCl:Tb phosphor.
That is to say, since the Y
3
(Al, Ga)
5
O
12
:Tb phosphor has poor color purity while exhibiting excellent luminance saturation characteristic and excellent degradation characteristic under high-voltage and high-current conditions, it is used in a mixture with a Zn
2
SiO
4
:Mn phosphor having good color purity. However, the Zn
2
SiO
5
:Mn phosphor has a very long afterglow time, that is, about 30 ms, and is poorest in view of luminance and degradation characteristics among single-substance phosphors for a projection type CRT. Therefore, the green-emitting phosphor compositions according to the present invention have improved luminance, degradation and afterglow characteristics by replacing a Zn
2
SiO
4
:Mn phosphor contained in the conventional green-emitting phosphor composition comprising the Zn
2
SiO
4
:Mn phosphor, a Y
3
(Al, Ga)
5
O
12
:Tb phosphor and/or a LaOCl:Tb phosphor, with a SrGa
2
S
4
:Eu phosphor having excellent luminance and degradation characteristics and a short afterglow time, that is, about 300 &mgr;s.
First, a method of manufacturing a SrGa
2
S
4
:Eu phosphor of the present invention will be described in detail.
0.5 to 1.5 mol of a strontium salt such as strontium carbonate or strontium sulfate, 1.5 to 2.5 mol of a gallium salt such as gallium oxide or gallium sulfate and 0.001 to 0.05 mol of europium oxide are mixed and then an appropriate amount of a solvent such as sodium bromide, ammonium chloride or sodium chloride is mixed therewith. Subsequently, the mixture is put into an alumina vessel and then fired at 700 to 900° C. under a hydrogen sulfide gas atmosphere for 1 to 4 hours. Then, the resultant material is cooled, washed, dried and sieved to obtain a SrGa
2
S
4
:Eu phosphor.
After screen layers are manufactured using a SrGa
2
S
4
:Tb phosphor prepared in the above-described manner and prior art Y
3
(Al, Ga)
5
O
12
:Tb phosphor, a LaOCl:Tb phosphor and a Zn
2
SiO
4
:Mn phosphor by sedimentation, the emission characteristics are evaluated using a demountable system BM-7 and the evaluation result is described in the following Table 1. The evaluation conditions are an acceleration voltage of 20 kV, a driving current I
k
of 60 &mgr;A, a raster size of 2.0 cm×2.0 cm and a degradation evaluation time of 600 seconds.
TABLE 1
Luminance
Luminance
1)
Color coordinate
persistence
Kind of phosphor
(%)
(x/y)
ratio (%)
Y
3
(Al, Ga)
5
O
12
: Tb
100
3550/5550
97.0
LaOCl: Tb
 90
3250/5800
88.5
SrGa
2
S
4
: Tb
120
2677/6900
64.0
Zn
2
SiO
4
: Mn
 80
2100/7000
35.0
In Table 1, Luminance
1)
represents a relative luminance when the luminance of the Y
3
(Al, Ga)
5
O
12
:Tb phosphor is set to 100%.
Referring to Table 1, under high voltage and high current conditions, the Y
3
(Al, Ga)
5
O
12
:Tb phosphor exhibits an excellent luminance characteristic, specifically an excellent degradation characteristic, while exhibiting considerably poor color purity. Thus, the Y
3
(Al, Ga)
5
O
12
:Tb phosphor has been conventionally used in a mixture with a Zn
2
SiO
4
:Mn phosphor having excellent color purity. However, the Zn
2
SiO
4
:Mn phosphor is very poor in view of luminance and degradation characteristics. On the other hand, a SrGa
2
S
4
:Eu phosphor is excellent in view of color purity and luminance characteristics and is relatively superior in view of degradation characteristics than the Zn
2
SiO
4
:Mn phosphor, and thus a mixed green-emitting phosphor composition thereof exhibits excellent luminosity characteristics. However, the degradation characteristics of the SrGa
2
S
4
:Eu phosphor are lower than those of the Y
3
(Al, Ga)
4
O
12
:Tb phosphor. Thus, the present invention provides a mixed phosphor composition of 5 to 50% by weight of a SrGa
2
S
4
:Eu phosphor and 50 to 95% by weight of a Y
3
(Al, Ga)
4
O
12
:Tb phosphor, for suppressing degradation characteristic of th

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