Phosphor material and its method of manufacture

Details Phosphor material and its method of manufacture Phosphor material and its method of manufacture

2002-01-31

2004-01-06

Koslow, C. Melissa (Department: 1755)

Compositions

Inorganic luminescent compositions

Compositions containing halogen; e.g., halides and oxyhalides

Reexamination Certificate

active

06673269

ABSTRACT:

This application is based on Application No. 027552 filed in Japan on Feb 2, 2001, Application No. 030245 filed in Japan on Feb 6, 2001, and Application No. 030246 filed in Japan on Feb. 6, 2001, the contents of which are incorporated hereunto by reference.
BACKGROUND OF THE INVENTION
This invention relates to phosphor materials primarily excited by electron beams accelerated by 1000 V or less, described by the chemical composition formula SrTiO
3
:Pr, Al, and to the method of manufacture of those phosphor materials.
Formerly, ZnS:Ag phosphor materials, with zinc sulfide as the primary phosphor component, were used in cathode ray tubes (CRTs) as blue phosphors which emitted light when excited by low accelerating voltage electron beams. However, these sulfide phosphors emit sulfide gasses when excited by an electron beam and the phosphor material dissociates and scatters. As a result, problems with oxide filament contamination and reduction in phosphor light emitting efficiency easily develop. Further, these phosphors also have the drawback that red light cannot be emitted.
(ZnCd)S:AgCl phosphor materials have been developed as phosphors which emit light in the red to yellow range with low accelerating voltage electron beams. However, these phosphors not only include cadmium, which can cause environmental contamination, but they also have undesirable light emission characteristics due to poor conductivity. Poor conductivity yields non-uniform light emission and reduced luminance. This is because electrons supplied for excitation cannot flow smoothly and the phosphor becomes negatively charged by the electrons. Specifically, the phosphor becomes charged-up and the negative charge becomes an obstacle to the flow of electrons in the electron beam. In particular, a low accelerating voltage electron beam cannot be smoothly supplied to a negatively charged phosphor, and this causes significant light emission characteristic degradation. To eliminate this drawback, a powder such as In
2
O
3
is mixed into (ZnCd)S:AgCl phosphor to improve conductivity. This phosphor has improved conductivity due to the added powder, but since the conductivity of the phosphor itself is not improved, ineffective current through the conducting powder becomes large. This situation is the cause of reduced light emitting efficiency particularly for low accelerating voltage electron beams.
Phosphor materials with SrTiO
3
as their primary component have been developed which do not include cadmium, are applicable with low accelerating voltage electron beams, and emit red light (Japanese Patent Publication HEI 8-85788, 1996). These phosphors have the feature that they do not include the environmental contaminant cadmium, but they do not have desirable light emission characteristics. In particular, they do not have sufficiently long life time. With the objective of eliminating this drawback, phosphors which replace part of the Ti with group IVB elements such as Sn, Si, and Ge have been developed (Japanese Patent Publication HEI
10-273658, 1998
). Lifetime characteristics of the SrTiO
3
:Pr, Al phosphor materials cited in this disclosure can be improved by increasing the amount of Sn added. However, as the amount of added Sn increases, reduction in luminance is a drawback.
The present invention was developed to further resolve these drawbacks. Thus it is the first object of the present invention to provide electron beam excited, light emitting SrTiO
3
:Pr, Al type phosphor material which can be improved in both luminance and life time characteristics, and to provide its method of manufacture.
The present invention was developed to improve non-uniform light emission caused by lower conductivity associated with phosphors having SrTiO
3
as a primary component. Thus it is the second object of the present invention to provide phosphor material which can effectively eliminate light emission non-uniformity with low accelerating voltage electron beams.
SUMMARY OF THE INVENTION
The first electron beam excited, light emitting phosphor material of the present invention has the chemical composition formula (Sr
1−x−y
, Mg
x
, Ca
y
)TiO
3
:Pr,Al, where x+y is specified in the range 0.001 to 0.05. If x+y is less than 0.001, luminance improvement effects become insufficient and effects allowing lifetime improvement become small. In contrast, if x+y is made greater than 0.05, luminance drops abruptly. Consequently, considering both luminance and lifetime, x+y is limited to the extremely small range specified above for the phosphor of the present invention.
The phosphor of the present invention has the value of x+y specified within the range described above and with the possibility of x=0 or y=0. A phosphor with x=0 has part of the Sr replaced by Ca only, and a phosphor with y=0 has part of the Sr replaced by Mg only.
The value of x+y in the formula (Sr
1−x−y
, Mg
x
, Ca
y
)TiO
3
:Pr,Al is specified within the range described above for the phosphor of the present invention, and as shown in
FIG. 1
, lifetime characteristics can be greatly improved while improving luminance characteristics by replacing part of the Sr with trace amounts of Mg and Ca. Further, lifetime characteristics can be greatly improved while improving luminance characteristics by replacing part of the Sr with only Ca for x=0, or by replacing part of the Sr with only Mg for y=0.
The value of x+y in the chemical formula is preferably in the range from 0.003 to 0.02, and this allows marked improvement in lifetime characteristics while further improving luminance characteristics. The phosphor material of the present invention is suitable for phosphor displays with electron beam accelerating voltages of 1000 V or less, or as phosphors used in displays having field emission cathodes as sources of electrons.
The amount of the activating element Pr included is preferably from 0.0001 to 0.1 mole per mole of Sr, and the amount of Al included is preferably from 0.001 to 1.0 mole per mole of Ti. Further, part of the activating element Al may also be replaced by at least one of the elements, Ga and In.
The first phosphor of the present invention has the chemical formula (Sr
1−x−y
, Mg
x
, Ca
y
)TiO
3
:Pr,Al, where x+y is specified in the range 0.001 to 0.05, and by replacing part of the Sr with Mg and Ca, has the feature that lifetime characteristics can be greatly improved while improving luminance characteristics.
The second electron beam excited, light emitting phosphor material of the present invention has the chemical composition formula SrTiO
3
:Pr,Al, and the surface of phosphor particles are diffused with a diffusing agent containing at least one type of the following elements; Be, Mg, Ca, Sr, and Ba. SrTiO
3
:Pr,Al phosphors emit red light when excited by low energy electron beam, but the impact energy of a low energy electron beam is small and light emission is from the surface of phosphor particles. By diffusing a diffusing agent containing at least one type of the elements, Be, Mg, Ca, Sr, and Ba, into the surface region of phosphor particles of the present invention, light emission characteristics of the surface of phosphor particles are improved. Consequently, low accelerating voltage, low energy electron beam light emission characteristics of the SrTiO
3
:Pr,Al phosphor material of the present invention can be improved.
The diffusion depth of the diffusing agent into the phosphor particle interior is preferably in the range of 50 Å to 400 Å from the surface. Diffusion depth of the diffusing agent into the phosphor particle can be adjusted by firing time and temperature during the re-firing process. If firing time and temperature during the re-firing process are increased, the diffusing agent will diffuse deeper into the phosphor particle. Diffusion depth is set from 50 Å to 400 Å because light emission characteristics under low accelerating voltage, low energy electron beam excitation decrease when diffusing agent dif

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