A low-velocity electron excited phosphor having a composition represented by a general formula ZnO.multidot.(Al.sub.x, Ga.sub.l-x).sub.2 O.sub.3 :Mn, wherein X=0.001 to 0.3 mol. A content of Al in the phosphor is set be to within a range between 0.001 mol and 0.3 mol. The range permits a variation in luminance of the phosphor to be within .+-.30% and initial luminance of the phosphor to be significantly increased, resulting in the phosphor being suitable for use for a fluorescent display device.

BACKGROUND OF THE INVENTION 
This invention relates to a low-velocity electron excited phosphor, and 
more particularly to a phosphor which is excited by low-velocity electrons 
to emit a green luminous color and suitable for use for a fluorescent 
display device. 
Japanese Patent Application Laid-Open Publication No. 149772/1976 discloses 
a low-velocity electron excited phosphor having a composition represented 
by the following general formula: 
EQU A(Zn.sub.1-x,Mg.sub.x)O Ga.sub.2 O.sub.3 :BMn 
wherein 6.ltoreq.A.ltoreq.1.2, 0.ltoreq.B.ltoreq.5.times.10.sup.-2 and 
O.ltoreq.x.ltoreq.1.0. 
The phosphor disclosed is prepared by adding Mn and Mg to a phosphor body 
or matrix of a blue luminous color represented by a formula 
ZnO.multidot.Ga.sub.2 O.sub.3. 
FIG. 3 shows luminance life characteristics of phosphors including a 
phosphor of the present invention depending on a content of Al therein. In 
FIG. 3, a curve of X=0 indicates a relationship between relative luminance 
and operating time in a fluorescent display device including a 
ZnO.multidot.Ga.sub.2 O.sub.3 :Mn phosphor. The phosphor has a composition 
represented by the above-described formula wherein x=0. 
As indicated by the curve of X=0 in FIG. 3, the phosphor is increased in 
luminance until about 3000 hours elapse after an initial stage of 
excitation of the phosphor and kept substantially unchanged thereafter, In 
general, a plurality of display segments arranged in a fluorescent display 
device are varied in frequency of excitations in use. Therefore, use of a 
phosphor exhibiting such luminance characteristics as described above for 
display segments of a fluorescent display device causes a difference in 
luminance to occur with time between the display segments subject to 
long-period excitation and those subject to short-period excitation in the 
same fluorescent display device, resulting in quality of a luminous 
display of the device being substantially deteriorated with time. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the foregoing disadvantage 
of the prior art. 
Accordingly, it is an object of the present invention to provide a 
low-velocity electron excited phosphor of a green luminous color which is 
capable of exhibiting increased initial luminance and reducing a variation 
in luminance depending on an operating period. 
In accordance with the present invention, a low-velocity electron excited 
phosphor is provided. The low-velocity electron excited phosphor has a 
composition represented by the general formula: 
EQU ZnO.multidot.(Al.sub.x, Ga.sub.1-x).sub.2 O.sub.3 :Mn 
wherein X=0.001 to 0.3 mol. 
The phosphor of the present invention constructed as described above 
exhibits a green luminous color when it is excited by low-velocity 
electrons. The phosphor exhibits increased initial luminance and 
stabilized luminance over a long period of operating time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now, a phosphor according to the present invention will be described 
hereinafter with reference to the accompanying drawings. 
First, a process for preparing a phosphor of the present invention will be 
described with reference to FIG. 1. 
Mixing of Starting Material: 
First, ZnO, Al.sub.2 O.sub.3 and Ga.sub.2 O.sub.3 which constitute a 
starting material for a matrix of a phosphor of the present invention are 
mixed with each other at each of ratios shown in TABLE 1. 
TABLE 1 
______________________________________ 
Al Content 
ZnO (g) Al.sub.2 O.sub.3 (g) 
Ga.sub.2 O.sub.3 (g) 
______________________________________ 
0.0005 7.3-8.1 0.005 18.7 
0.001 7.3-8.1 0.01 18.7 
0.01 7.3-8.1 0.10 18.6 
0.05 7.3-8.1 0.51 17.8 
0.1 7.3-8.1 1.02 16.9 
0.2 7.3-8.1 2.04 15.0 
0.3 7.3-8.1 3.06 13.1 
0.5 7.3-8.1 5.10 9.4 
______________________________________ 
Also, any one of fluxes shown in TABLE 2 is mixed with the starting 
material. 
TABLE 2 
______________________________________ 
Flux (mol %/mol) 
NaCl (g) NaF (g) KCl (g) 
KF (g) 
______________________________________ 
10 0.584 0.420 0,746 0.581 
20 1.17 0.840 1.49 1.16 
30 1.75 1.26 2.24 1.74 
40 2.34 1.68 2.98 2.32 
______________________________________ 
For this purpose, a halide of each of elements belonging to Group Ia in the 
periodic table such as NaCl, NaF, KCl, KF, RbCl or RbF, a carbonate of the 
element, a nitrate thereof, a hydroxide thereof, a sulfate thereof, or the 
like may be used for the flux. The flux may be mixed with the matrix at a 
ratio of 10 to 40 mol % per mol of the matrix. 
The mixing may be fully carried out in either a dry way or a wet way. It is 
further desirable that the components for the starting material are mixed 
at a molecular level by coprecipitation or the like. 
Primary Burning: 
Then, the resultant mixture is placed in a heat-resistant vessel such as an 
alumina vessel or the like and then subject to primary burning at 
1300.degree. C. for 3 hours in an air atmosphere, resulting in the matrix 
of the phosphor being synthesized. When the preceding mixing is carried 
out by coprecipitation, the mixture is changed to oxides at a temperature 
as low as hundreds degrees C., followed by the primary burning. 
Dispersion and Washing: 
Agglomerated particles of the matrix burned are subject to crushing or 
breaking by means of a ball mill or the like. Then, the crushed matrix is 
washed with pure water to remove an excess of the flux. 
Mn Coating: 
Then, the matrix thus washed is coated with Mn. When Mn is in the form of a 
water soluble Mn compound such as MnSO.sub.4, MnCl.sub.2, 
Mn(NO.sub.3).sub.2 or the like, it is used in the form of an aqueous 
solution thereof. When Mn is in the form of a compound less soluble to 
water, it is dissolved in 0.1 N HCl. 
The matrix is mixed with pure water, resulting in being formed into a 
slurry, which is then mixed with a solution containing a predetermined 
amount of Mn compound, leading to formation of a mixture. Then, the 
mixture is subject to mixing and drying by means of a rotary evaporator or 
the like, so that the matrix may be coated with Mn. The amount of Mn mixed 
is set to be 0,005 to 0.1 mol as shown in TABLE 3. 
TABLE 3 
______________________________________ 
Mn Content MnSO.sub.4 MnCl.sub.2 
Mn(NO.sub.3).sub.2 
(atm/mol) (g) (g) (g) 
______________________________________ 
0.005 0.076 0.063 0.09 
0.01 0.15 0.13 0.18 
0.02 0.30 0.25 0.36 
0.05 0.76 0.63 0.90 
0.1 1.51 1.26 1.79 
______________________________________ 
As shown in TABLE 1, a content of ZnO in the starting material for the 
matrix is set to be in a range between 1 mol (8.1 g) and 0.9 mol (7.3 g). 
Mixing of the starting material carried out when the ZnO content is within 
the above-described range permits Mn to readily enter the matrix. The ZnO 
content below the above-described range causes the phosphor prepared to be 
decreased in luminance. 
It is not necessarily required that the Mn coating uses Mn in the form of 
an aqueous solution. Mixing of Mn with the matrix in a dry way permits Mn 
to be effectively diffused into the matrix. 
Secondary Burning: 
Subsequently, the matrix coated or mixed with Mn is placed in an alumina 
boat and then subject to secondary burning at 1100.degree. C. for 1 hour 
in a reducing atmosphere of H.sub.2 /N.sub.2 =2/198 (ml/min). This permits 
Mn to be diffused into the matrix, resulting in the matrix being 
activated. Then, the matrix is classified by means of a sieve, so that the 
phosphor of the present invention may be prepared. 
The phosphor prepared according to the above-described procedure has a 
composition of ZnO (Al.sub.x, Ga.sub.1-x).sub.2 O.sub.3 :Mn wherein 
x=0.001 to 0.3 mol. 
Various experiments on luminance of the so-prepared phosphor were made 
while using it for an anode display section of a fluorescent display 
device. The results were as shown in FIGS. 2 to 4. 
FIG. 2 shows a relationship between a concentration (mol) of Al contained 
in the phosphor and a relative value of initial luminance of the phosphor. 
FIG. 2 indicates that a content of Al in the phosphor between about 0.001 
mol and about 0.4 mol leads to an increase in luminance of the phosphor. 
In FIG. 3, X indicates an Al content (mol) in the phosphor. FIG. 3 shows a 
relationship between relative luminance and an operating period in each of 
the phosphor of the present invention containing 0.001 to 0.3 mol of Al 
(X=0.05, 0.1 and 0.2), a phosphor containing Al in an amount exceeding the 
above-described Al content range defined in the present invention (X=0.4) 
and a conventional phosphor free of Al (X=0). 
As is apparent from FIG. 3, the phosphor (X=0.05, 0.1) of the present 
invention tends to be somewhat increased in luminance until the operating 
period reaches 100 hours. Thereafter, the luminance of the phosphor 
remains on substantially the same level for a while. After lapse of 
thousands hours, it is somewhat decreased. Thus, it will be noted that the 
phosphor of the present invention exhibits highly stabilized luminance. 
Also, the phosphor (X=0.2) of the present invention permits luminance to be 
kept substantially at a level of initial luminance until the operating 
period reaches tens hours. Then, the luminance decreases somewhat, 
however, this decrease in luminance does not adversely affect performance 
of a fluorescent display device at all. 
The phosphor (X=0.4) of which an Al content is beyond the above-described 
range defined in the present invention is superior in initial luminance to 
the conventional phosphor free of Al. Nevertheless, the former is 
decreased in luminance as compared with the latter after lapse of 10 
hours, followed by a rapid decrease in luminance. Thus, it will be noted 
that the former is not suitable for use for a fluorescent display device 
because of failing to exhibit stabilized luminance. 
FIG. 4 shows a relationship between a content of Al in each of phosphors 
including a phosphor of the present invention and a variation in luminance 
of the phosphor between an initial stage of excitation of the phosphor and 
the excitation extending over 1000 hours. As is apparent from FIG. 4, a 
content of Al in the phosphor of the present invention is within a range 
between 0.001 mol and 0.3 mol. The range permits a variation in luminance 
of the phosphor to be within .+-.30%. Such variation does not adversely 
affect use of the phosphor for a fluorescent display device. 
When a tolerance limit for the luminance variation is set to be .+-.20%, 
the phosphor of the present invention exhibits further stabilized 
luminance in a fluorescent display device, to thereby permit the device to 
carry out a display of high quality. For this purpose, the Al content may 
be set to be 0.005 mol and 0.2 mol. 
Also, FIG. 4 indicates that the conventional phosphor free of Al is varied 
in luminance by +40%. 
As can be seen from the foregoing, the low-velocity electron excited 
phosphor of the present invention is formed of the ZnO.multidot.Ga.sub.2 
O.sub.3 :Mn or ZnGa.sub.2 O.sub.4 :Mn matrix to which a predetermined 
amount of Al is added. Thus, the phosphor of the present invention is 
increased in initial luminance and significantly reduces a variation in 
luminance depending on the operating period. 
While a preferred embodiment of the invention has been described with a 
certain degree of particularity with reference to the drawings, obvious 
modifications and variations are possible in light of the above teachings. 
It is therefore to be understood that within the scope of the appended 
claims, the invention may be practiced otherwise than as specifically 
described.