Cathode-ray tube including a white phosphor screen

A cathode-ray tube according to the present invention is provided with a phosphor screen comprising phosphors A, B, and C, wherein each of the phosphors A and B constitute 5 to 15% by weight and, the phosphor C constitutes 70 to 90% by weight. The phosphor A is a bluish white-emitting phosphor (Y.sub.1-x-y Gd.sub.x Tb.sub.y).sub.2 O.sub.2 S (0.ltoreq.X.ltoreq.0.999999 and 0.000001.ltoreq.Y.ltoreq.0.001), the phosphor B is at least one of a blue-emitting silver-activated hexagonal zinc sulfate (ZnS:Ag) phosphor, etc., and the phosphors C is a yellow-emitting phosphor (In.sub.1-p-r M.sub.p TB.sub.q Eu.sub.r BO.sub.3) (wherein M is at least one element selected from the group consisting of Sc, Lu, Y, Gd, and Ga, and p, q, and r satisfy 0.ltoreq.p.ltoreq.0.2, 0.0005.ltoreq.q.ltoreq.0.05, and 0.001.ltoreq.r.ltoreq.0.1). The cathode-ray tube of the present invention is a phosphor screen which is free from toxicity and has good flickering and brightness characteristics and high current dependence of brightness.

BACKGROUND OF THE INVENTION 
The present invention relates to a cathode-ray tube and, more particularly, 
to a cathode-ray tube used in a display for a computer terminal or the 
like. 
A phosphor mixture prepared by mixing a plurality of different 
color-emitting phosphors is used for a white phosphor screen in a display 
such as a computer terminal. Typical examples of the conventional phosphor 
mixture are listed below: 
(1) a phosphor mixture of a blue-emitting silver-activating cubic zinc 
sulfate (ZnS:Ag) phosphor and a yellow-emitting (as a complementary color 
of blue) copper-activated zinc-cadmium sulfate {(Zn,Cd)S:Cu}; 
(2) a phosphor mixture of the above-mentioned blue-emitting phosphor 
(ZnS:Ag), a green-emitting manganese-arsenic-activated zinc silicate 
phosphor (Zn.sub.2 SiO.sub.4 :Mn,As), and a red-emitting 
manganese-activated zinc phosphate phosphor {Zn.sub.3 (PO.sub.4).sub.2 
:Mn}or a manganese-activated zinc magnesium phosphate phosphor 
{(Zn,Mg).sub.3 (PO.sub.4).sub.2 :Mn}; 
(3) a phosphor mixture (Japanese Patent Disclosure (Kokai) No. 84-122578) 
of the above-mentioned blue-emitting phosphor (ZnS:Ag), the 
above-mentioned green-emitting phosphor (Zn.sub.2 SiO.sub.4 :Mn,As), and 
an orange-emitting cadmium halophosphate phosphor {Cd.sub.5 
(PO.sub.4).sub.3 Cl:Mn}; and 
(4) a phosphor mixture (Japanese Patent Disclosure (Kokai) No. 85-38490) of 
the above-mentioned blue-emitting phosphor (ZnS:Ag) and a yellow-emitting 
phosphor In.sub.l-u-v-w M.sub.u Tb.sub.v Eu.sub.w BO.sub.3 (wherein M is 
at least one element of Sc, Lu, Y, Gd, and Ga and u, v, and w satisfy 
u.gtoreq.0, v.gtoreq.0, w&gt;0, and u+v+w&lt;1). 
Since each of the phosphors of phosphor mixture (1) has a 10% afterglow 
time of 10 msec or less, flickering of the cathode-ray tube typically 
occurs. 
Since, in phosphor mixture (2), the 10% afterglow time of each of the 
green-emitting (Zn.sub.2 SiO.sub.4 :Mn,As) and red-emitting {Zn.sub.3 
(PO.sub.4).sub.2 :Mn}and {(Zn,Mg).sub.3 (PO.sub.4)2:Mn}is relatively long, 
flickering of the screen is reduced. However, phosphor mixture (2) 
contains the red-emitting phosphor having a low luminous efficacy, and 
thus, the brightness of the screen is degraded. In addition, phosphor 
mixture (2) contains arsenic, which is highly toxic, and is thus 
undesirable from the viewpoint of safety. 
In phosphor mixture (3), brightness and flickering characteristics are 
improved as compared with phosphor mixture (2). However, mixture (3) 
contains arsenic and cadmium which are highly toxic, thus posing a safety 
problem. 
Phosphor mixture (4) is free from toxicity and has a high luminance. 
However, the afterglow time of the In.sub.l-u-v-w M.sub.u Tb.sub.v 
Eu.sub.w BO.sub.3 phosphor is shorter than that of each of the 
aforementioned Zn.sub.2 SiO.sub.4 :Mn,As, Zn.sub.3 (PO.sub.4).sub.2 :Mn, 
(Zn,Mg).sub.3 (PO.sub.4).sub.2 :Mn, and Cd.sub.5 (PO.sub.4).sub.3 Cl:Mn 
phosphors. Therefore, flickering characteristics of phosphor mixture (4) 
are degraded. 
In order to improve the flickering characteristics of phosphor mixture (4), 
the present inventors replaced the cubic zinc sulfate phosphor as the 
blue-emitting phosphor with a hexagonal zinc sulfate phosphor (ZnS:Ag) 
disclosed in Japanese Patent Disclosure Nos. 83-115024 and 83-129083 and 
the like. As a result, the flickering characteristics could be improved. 
However, the hexagonal zinc sulfate phosphor has poor current dependence of 
brightness. If the screen is made bright by increasing an exciting current 
in a cathode-ray tube using the above phosphor mixture, white color cannot 
be satisfactorily saturated and becomes yellowish. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a cathode-ray tube with 
a phosphor screen free from toxicity and having good flickering and 
brightness characteristics and high current dependence of brightness. 
According to the present invention, there is provided a cathode-ray tube 
with a phosphor screen containing phosphors A, B, and C or D, wherein, in 
relation to 100% of the phosphor mixture, phosphors A and B each 
constitute 5 to 15% by weight, and phosphor C or D constitutes 70 to 90% 
by weight of the content thereof, respectively; 
A being a bluish white-emitting phosphor (Y.sub.l-x-y Gd.sub.x 
Tb.sub.y).sub.2 O.sub.2 S (0.ltoreq.x.ltoreq.0.999999 and 
0.000001.ltoreq.y.ltoreq.0.001), 
B being at least one of a blue-emitting silver-activated hexagonal zinc 
sulfate (ZnS:Ag) phosphor, a blue-emitting silver-activated cubic zinc 
sulfate phosphor (ZnS:Ag,X) (wherein X is at least one element selected 
from Ga, In, and Sc), and a blue-emitting silver-activated cubic zinc 
sulfate-scandium oxide phosphor (ZnS.multidot.zSc.sub.2 O.sub.3 :Ag 
(wherein 1.times.10.sup.-5 .ltoreq.z .ltoreq.8.times.10.sup.-2)), 
C being a yellow-emitting phosphor (In.sub.l-p-q-r M.sub.p Tb.sub.q 
Eu.sub.r BO.sub.3) (wherein M is at least one element selected from the 
group consisting of Sc, Lu, Y, Gd, and Ga, and p, q, and r satisfy 0 
p.ltoreq.0.2, 0.0005.ltoreq.q.ltoreq.0.05, and 0.001.ltoreq.r.ltoreq.0.1), 
and 
D being a phosphor mixture of 
(d-1) which is a green-emitting phosphor (In.sub.l-p-q M.sub.p Tb.sub.q 
BO.sub.3) (wherein M is at least one element selected from the group 
consisting of Sc, Lu, Y, Gd, and Ga, and p, and q satisfy 
0.ltoreq.p.ltoreq.0.2 and 0.0005.ltoreq.q.ltoreq.0.05), and 
(d-2) which is a red-emitting phosphor (In.sub.l-p-r M.sub.p Eu.sub.r 
BO.sub.3) (wherein M is at least one element selected from the group 
consisting of Sc, Lu, Y, Gd, and Ga, and p, and r satisfy 
0.ltoreq.p.ltoreq.0.2 and 0.001.ltoreq.r.ltoreq.r.ltoreq.0.1), a weight 
ratio of (d-1) to (d-2) being 3 : 7 to 7 : 3.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows an emission spectrum of phosphor A (Y.sub.l-x-y Gd.sub.x 
Tb.sub.y).sub.2 O.sub.2 S used in the present invention. If concentration 
y of Tb is less than 0.000001, brightness is degraded. However, if 
concentration y of Tb exceeds 0.01, a peak of the green component at a 
wavelength of 540 nm is undesirably too high. Concentration y preferably 
falls within the range of 0.0001.ltoreq.y.ltoreq.0.005. When the 
concentration x of Gd is increased, the emission color becomes greenish. 
However, since preferable concentration y remains unchanged, concentration 
x should be adjusted subject to concentration y. Ten % afterglow time of 
phosphor A is about 1 msec and is longer than that (10 msec) of the cubic 
ZnS:Ag phosphor. Therefore, flickering can be reduced. However, when only 
phosphor A as the blue-emission component is used together with 
yellow-emission phosphor C, emission of white color can be obtained only 
if the content of phosphor A is large. As a result, flickering 
characteristics of the resultant phosphor screen are degraded, and 
brightness is also impaired. 
According to the present invention, blue-emitting phosphor B is employed, 
which comprises at least one of ZnS:Ag, ZnS:Ag,X (wherein X is at least 
one element selected from the group consisting of Ga, In, and Sc), and 
ZnS.multidot.zSc.sub.2 O.sub.3 :Ag (wherein 1.times.10.sup.-5 
.ltoreq.z.ltoreq.8.times.10.sup.-2), so as to obtain white color even if 
the content of phosphor A is very small. 
Phosphor C, used together with phosphors A and B in the present invention, 
is represented by general formula In.sub.l-p-q-r M.sub.p Tb.sub.q Eu.sub.r 
BO.sub.3 (wherein M is at least one element selected from the group 
consisting of Sc, Lu, Y, Gd, and Ga, and p, q, and r satisfy 
0.ltoreq.p.ltoreq.0.2, 0.0005.ltoreq.q.ltoreq.0.05, and 
0.001.ltoreq.r.ltoreq.0.1). Phopshor C is a yellow-emitting phosphor 
having a crystal structure such as a calcite. Values p, q, and r are 
determined by considering brightness and emission colors. However, the 
following conditions are generally preferred: 0.ltoreq.p.ltoreq.0.1, 
0.001.ltoreq.q.ltoreq.0.03, and 0.002.ltoreq.r.ltoreq.0.08. 
Phosphor D, used together with phosphors A and B in the present invention, 
is a phosphor mixture of a green-emitting phosphor represented by general 
formula (d-1) In.sub.l-p-q M.sub.p Tb.sub.q BO.sub.3 (wherein M is at 
least one element selected from the group consisting of Sc, Lu, Y, Gd, and 
Ga, and p and q satisfy 0.ltoreq.p.ltoreq.0.2 and 
0.0005.ltoreq.q.ltoreq.0.05) and a red-emitting phosphor represented by 
general formula (d-2) In.sub.l-p-r M.sub.p Eu.sub.r BO.sub.3 (wherein M is 
at least one element selected from the group consisting of Sc, Lu, Y, Gd, 
and Ga, and p and r satisfy 0.ltoreq.p.ltoreq.0.2 and 
0.001.ltoreq.r.ltoreq.0.1). When the mixing ratio of (d-1) to (d-2) is 3 : 
7 to 7 : 3, phosphor D emits yellow light. Phosphors (d-1) and (d-2) have 
the same crystal structure as that of calcite. Preferable values p, q, and 
r are the same as those in phosphor C. The mixing ratio of (d-1) to (d-2) 
is preferably 4 : 6 to 6 : 4. 
Phosphors A and B each constitute 5 to 15% by weight, and phosphor C or D 
constitutes 70 to 90% by weight of the total content of the resultant 
phosphor product. If the phosphors fall outside the above content ranges, 
brightness and emission colors are degraded. Phosphors A and B, as 
contents of the phosphor product, should preferably fall within the ranges 
of 8 to 12% by weight, and phosphor C or D, 76 to 84% by weight, 
respectively. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Example 1 
FIG. 2 is a sectional view of cathode-ray tube 13 according to the present 
invention. Envelope 11 comprises panel 3 having phosphor screen 1 on its 
inner surface, neck 7 incorporating electron gun 5, and funnel 9 for 
connecting panel 3 to neck 7. Phosphor screen 1 contains 8% by weight of 
phosphor A represented by (Y.sub.0.899 Gd.sub.0.1 Tb.sub.0.001).sub.2 
O.sub.2 S, 12% by weight of phosphor B represented by ZnS:Ag,Ga, 35% by 
weight of phosphor (d-1) represented by In.sub.0.995 Tb.sub.0.005 BO.sub.3 
and 45% by weight of phosphor (d-2) represented by In.sub.0.97 Eu.sub.0.03 
BO.sub.3. 
Cathode-ray tube 13 is fabricated as follows. The above phosphor mixture 
was suspended in an aqueous solution of water glass and was sufficiently 
stirred to prepare a dispersion. An aqueous solution of heavy metal ions 
such as barium ions was filled in envelope 11 before electron gun 5 was 
mounted therein. This aqueous solution is called a cushion solution. The 
dispersion was mixed with the cushion solution and was left to stand for 
an hour to precipitate the phosphor mixture. Envelope 11 was inclined to 
pour the supernatant liquid therefrom. After the deposited film was dried, 
it was heated to 400.degree. to 500.degree. C. to obtain phosphor screen 1 
in panel 3. Electron gun 5 was then mounted in neck 7, and envelope 11 was 
evacuated and sealed to prepare cathode-ray tube 13. 
Example 2-7 
Following the same procedures as in Example 1, cathode-ray tubes having 
different phosphor screens with different mixing ratios and kinds of 
phosphors A, B and C or D were prepared. 
Relative brightness, critical fusion frequencies, x and y chromatic values 
at an excitation current of 50.mu.A, changes .DELTA.x and .DELTA.y in x 
and y chromatic values upon a change in excitation current from 50 to 
300.mu.A, and the presence/absence of toxicity of the cathode-ray tubes 
using the phosphors in Examples 1 to 7 were measured and are summarized in 
Table 1. Test results of conventional cathode-ray tubes are also 
summarized as Control 1 to 7 in Table 1. Relative luminous intensities are 
measured with reference to the luminous intensity of Control 3. The 
critical fusion frequency is defined as a frequency causing screen 
flickering when a pulsed excitation current is supplied with a certain 
frequency to the phosphor screen. If a commercial frequency having an 
upper limit is taken into consideration, a lower frequency provides good 
flickering characteristics. Test samples that were turned out to be toxic 
were not subjected to measurements of changes in chromatic values. 
All phosphors of the present invention were not toxic. The critical fusion 
frequencies of Examples 1 to 7 can be decreased to 55 Hz or less while the 
.DELTA.x values, the .DELTA.x values, the .DELTA.y values, and the 
relative luminance intensities were maintained as 0.008 or less, 0.003 or 
less, and 74% or more, respectively. It is apparent that the Examples 1 to 
7 are superior to the Controls according to total evaluation results. 
TABLE 1 
__________________________________________________________________________ 
CIE x 
Mixing 
Rela- 
Critical 
and y 
.DELTA.x 
Ratio 
tive 
Fusion 
Chro- 
and 
(% by 
Bright- 
Frequen- 
matic 
.DELTA.y 
Phosphor wt.) 
ness(%) 
cy (Hz) 
Value 
Value 
Toxicity 
__________________________________________________________________________ 
Example 
(Y.sub.0.899 Gd.sub.0.1 Tb.sub.0.001).sub.2 O.sub.2 S 
8 75 54 x = .DELTA.x = 
None 
1 ZnS:Ag,Ga 12 0.355 
0.007 
In.sub.0.995 Tb.sub.0.005 BO.sub.3 
35 y = .DELTA.y = 
In.sub.0.97 Eu.sub.0.03 BO.sub.3 
45 0.395 
0.003 
Example 
(Y.sub.0.999 Tb.sub.0.001).sub.2 O.sub.2 S 
12 76 55 x = .DELTA.x = 
None 
2 hexagonal ZnS:Ag 
8 0.352 
0.005 
In.sub.0.995 Tb.sub.0.005 BO.sub.3 
38 y = .DELTA.y = 
In.sub.0.97 Eu.sub.0.03 BO.sub.3 
42 0.399 
0.001 
Example 
(Y.sub.0.998 Tb.sub.0.002).sub.2 O.sub.2 S 
9 74 54 x = .DELTA.x = 
None 
3 ZnS:Ag,Ga 11 0.355 
0.006 
In.sub.0.995 Tb.sub.0.005 BO.sub.3 
35 y = .DELTA.y = 
In.sub.0.97 Eu.sub.0.03 BO.sub.3 
45 0.395 
0.002 
Example 
(Y.sub.0.5 Gd.sub.0.499 Tb.sub.0.001).sub.2 O.sub.2 S 
11 76 55 x = .DELTA.x = 
None 
4 ZnS:Ag,Ga 9 0.355 
0.006 
In.sub.0.996 Eu.sub.0.003 Tb.sub.0.001 BO.sub.3 
80 y = .DELTA.y = 
0.395 
0.001 
Example 
(Y.sub.0.999 Tb.sub.0.001).sub.2 O.sub.2 S 
12 75 55 x = .DELTA.x = 
None 
5 hexagonal ZnS:Ag 
9 0.353 
0.008 
In.sub.0.995 Tb.sub.0.005 BO.sub.3 
37 y = .DELTA.y = 
In.sub.0.87 Y.sub.0.1 Eu.sub.0.03 BO.sub.3 
42 0.395 
0.003 
Control 
(Y.sub.0.999 Tb.sub.0.001).sub.2 O.sub.2 S 
13 78 57 x = .DELTA.x = 
None 
1 cubic ZnS:Ag 
7 0.356 
0.005 
In.sub.0.995 Tb.sub.0.005 BO.sub.3 
35 y = .DELTA.y = 
In.sub.0.97 Eu.sub.0.03 BO.sub.3 
45 0.394 
0.002 
Control 
(Gd.sub.0.999 Tb.sub.0.001).sub.2 O.sub.2 S 
13 74 57 x = .DELTA.x = 
None 
2 cubic ZnS:Ag 
8 0.357 
0.005 
In.sub.0.995 Tb.sub.0.005 BO.sub.3 
37 y = .DELTA.y = 
In.sub.0.86 La.sub.0.1 Eu.sub.0.04 BO.sub.3 
42 0.390 
0.002 
Control 
cubic ZnS:Ag 
40 100 59 x = Not Toxic 
3 (ZnCd)S:Cu 60 0.355 
meas- 
y = ured 
0.394 
Control 
cubic ZnS:Ag 
15 50 53 x = Not Toxic 
4 Zn.sub.2 SiO.sub.4 :Mn,As 
31 0.355 
meas- 
Zn.sub.3 (PO.sub.4).sub.2 :Mn 
54 y = ured 
0.394 
Control 
cubic Zn:Ag 15 78 53 x = Not Toxic 
5 Zn.sub.2 SiO.sub.4 :Mn,As 
22 0.352 
meas- 
Cd.sub.5 (PO.sub.5).sub.2 Cl:Mn 
63 y = ured 
0.397 
Control 
cubic ZnS:Ag 
20 75 57 x = .DELTA.x = 
None 
6 In.sub.0.995 Tb.sub.0.005 BO.sub.3 
35 0.354 
0.010 
In.sub.0.97 Eu.sub.0.03 BO.sub.3 
45 y = .DELTA.y = 
0.398 
0.008 
Control 
ZnS:Ag,Ga 21 74 55 x = .DELTA.x = 
None 
7 In.sub.0.995 Tb.sub.0.005 BO.sub.3 
34 0.351 
0.027 
In.sub.0.97 Eu.sub.0.03 BO.sub.3 
45 y = .DELTA.y = 
0.398 
0.026 
Example 
(Y.sub.0.999 Tb.sub.0.001).sub.2 O.sub.2 S 
12 77 55 x = .DELTA.x = 
None 
6 ZnS.0.002Sc.sub.2 O.sub.3 :Ag 
8 0.354 
0.004 
In.sub.0.995 Eu.sub.0.003 Tb.sub.0.002 BO.sub.3 
80 y = .DELTA.y = 
0.389 
0.002 
Example 
(Y.sub.0.998 Tb.sub.0.002).sub.2 O.sub.2 S 
11 75 55 x = .DELTA.x = 
None 
7 ZnS:Ag,Ga 9 0.352 
0.005 
In.sub.0.995 Eu.sub.0.003 Tb.sub.0.002 BO.sub.3 
80 y = .DELTA.y = 
0.388 
0.003 
__________________________________________________________________________ 
Example 8 
FIG. 3 is a graph showing chromatic changes of emission colors according to 
changes in currents supplied to the cathode-ray tube of the present 
invention and the conventional cathode-ray tube. The x chromatic values 
are plotted along the abscissa and the y chromatic values are plotted 
along the ordinate. Arrow S indicates chromatic changes of a cathode-ray 
tube of the present invention with a phosphor screen containing 10% by 
weight of phosphor A represented by formula (Y.sub.0.999 
Tb.sub.0.001).sub.2 O.sub.2 S, 10% by weight of phosphor B represented by 
formula ZnS:Ag,Ga, 35% by weight of phosphor (d-1) represented by formula 
In.sub.0.995 Tb.sub.0.005 BO.sub.3, and 45% by weight of phosphor (d-2) 
represented by formula In.sub.0.97 Eu.sub.0.03 BO.sub.3 when the 
excitation current is increased from 50 to 300 .mu.A. Arrow T indicates 
chromatic changes in a cathode-ray tube with a phosphor screen (Control 8) 
containing 20% by weight of a cubic phosphor represented by formula 
ZnS:Ag,Ga, 35% by weight of a phosphor represented by formula In.sub. 
0.995 Tb.sub.0.005 BO.sub.3, and 45% by weight of a phosphor represented 
by formula In.sub.0.97 Eu.sub.0.03 BO.sub.3 when the excitation current is 
increased in the same manner as in Example 8. Chromatic changes depending 
on changes in current supplied to the cathode-ray tube of the present 
invention are smaller than those to the conventional cathode-ray tube.