Compositions – Inorganic luminescent compositions – Zinc or cadmium containing
Reexamination Certificate
1999-08-23
2001-06-19
Koslow, C. Melissa (Department: 1755)
Compositions
Inorganic luminescent compositions
Zinc or cadmium containing
C313S503000, C313S506000, C428S403000, C428S404000
Reexamination Certificate
active
06248261
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electroluminescent phosphor and an electroluminescent element using the same.
2. Description of the Related Art
An electroluminescent element is prepared in the following way. A phosphor layer has an electroluminescent phosphor dispersed in a dielectric material. On both surfaces of the phosphor layer, electrodes are disposed. At least one of them is a transparent electrode. An alternating voltage is applied between the electrodes to make the electroluminescent element luminesce. Electroluminescent elements are mainly used in display devices or backlighting of display devices.
An electroluminescent phosphor for use with electroluminescent elements generally comprises zinc sulfide as a phosphor base material, at least one kind of copper and manganese as an activator, and at least one kind of chlorine, bromine, iodine and aluminum as a coactivator.
An electroluminescent phosphor based on zinc sulfide is prepared in the following way, for instance. First, zinc sulfide as a base material is mixed with an activator source and a coactivator source. Further, magnesium chloride, barium chloride, sodium chloride or the like is added thereto as a crystal growth agent, followed by thorough mixing. This mixture is fired at a temperature in the range of from 1000 to 1300° C. to form an intermediate. The intermediate, after being crushed, is heated at a temperature in the range of from 600 to 1000° C. Thereby, a zinc sulfide system electroluminescent phosphor including an activator and a coactivator is obtained (cf. U.S. Pat. No. 2,957,830). The added crystal growth agent is removed by washing the fired body with water (cf. U.S. Pat. No. 4,859,361).
The existing zinc sulfide system electroluminescent phosphor can not necessarily satisfy characteristics demanded for display devices such as brightness and life. That is, there are problems to be improved. Accordingly, so far a variety of kinds of improvements have been applied to zinc sulfide system electroluminescent phosphors.
For instance, zinc sulfide is known to take a crystallographic structure of hexagonal system at a temperature of 1020° C. or more and that of cubic system at a temperature below that. Luminescent efficiency due to electroluminescence is higher for the cubic system. However, rather than getting the cubic system from the beginning, by transforming to the cubic system through the hexagonal system, the luminescent efficiency can be made higher.
Japanese Patent Laid-Open Publication (KOKAI) No. Sho 61-296085 discloses a method of transforming the crystal system of zinc sulfide system electroluminescent phosphor. According to this method, to zinc sulfide mixed with an activator a compound of alkali metal element and a compound of alkaline earth metal element are added to form an intermediate of hexagonal system. Thereafter, by applying high pressure, a zinc sulfide system electroluminescent phosphor of cubic system is obtained.
Japanese Patent Laid-Open Publication (KOKAI) No. Sho 57-145174 discloses a method of improving efficiency and life of zinc sulfide system electroluminescent phosphors. In this method, the efficiency and life of zinc sulfide system electroluminescent phosphors are improved by optimizing the addition amount of the aforementioned activator and coactivator.
As described above, various kinds of methods of improving the luminescent efficiency and life of the existing zinc sulfide system electroluminescent phosphors have been proposed. However, the existing methods are gradually becoming incapable of satisfying the present requirement level for brightness and life. In particular, recent display devices are demanded to improve further the brightness and life. Accordingly, electroluminescent phosphors being used with them are demanded to improve further the brightness and life.
U.S. Pat. No. 2,952,642 discloses a zinc sulfide system electroluminescent phosphor that includes lead and copper as activator, and 0.001% of magnesium. However, the zinc sulfide phosphor including lead is still poor in emission efficiency and life characteristic, accordingly it can not satisfy performance that the market demands.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an electroluminescent phosphor that has high brightness and enables to maintain such high brightness over a long time. Another object of the present invention is to provide an electroluminescent element of higher brightness and longer life.
An electroluminescent phosphor of the present invention comprises a phosphor base material consisting essentially of zinc sulfide, at least one kind of activator selected from copper and manganese that is included in the phosphor base material, and at least one kind of coactivator selected from chlorine, bromine, iodine and aluminum that is included in the phosphor base material. In the electroluminescent phosphor, the content of alkaline earth metal elements is 0.05% by weight or less, and cesium is included in the range of from 0.0001% by weight or more to 0.01% by weight or less.
Another electroluminescent phosphor of the present invention comprises a phosphor base material consisting essentially of zinc sulfide, at least one kind of activator selected from copper and manganese that is included in the phosphor base material, and at least one kind of coactivator selected from chlorine, bromine, iodine and aluminum that is included in the phosphor base material. In the electroluminescent phosphor, the content of alkaline earth metal elements is 0.05% by weight or less.
An electroluminescent element of the present invention comprises a phosphor layer containing the aforementioned electroluminescent phosphor of the present invention. In a concrete constitution of an electroluminescent element of the present invention, an electroluminescent element comprises a phosphor layer containing an electroluminescent phosphor of the present invention, a rear electrode disposed and integrated along one main surface of the phosphor layer through a reflection insulation layer, and a transparent electrode disposed opposite to the rear electrode and integrated along the other main surface of the phosphor layer.
The present inventors have carried out various kinds of experiments, discussions and speculations about how to get a higher brightness and longer life of electroluminescent phosphors. As a result of this, we have reached a conclusion that attention must be paid to the amount of crystal growth agent (flux) which is added to ingredients during synthesis of a zinc sulfide system electroluminescent phosphor and remaining in the phosphor thereafter. So far, in order to prepare zinc sulfide system electroluminescent phosphors of excellent property, compounds of alkali metal elements and compounds of alkaline earth metal elements (halogenides in particular) have been employed as a crystal growth agent. These are extremely suitable for growing zinc sulfide ingredient of a particle diameter of approximately from 1 to 3 &mgr;m to a phosphor of a particle diameter of approximately from 10 to 30 &mgr;m.
However, it was found due to the inventor's experiments that the crystal growth agent remaining in the phosphor deteriorates luminescent efficiency and life thereof. Through more detailed investigation, it was found that to the brightness and life the alkaline earth metal elements are particularly influential.
There, in the course of phosphor manufacture, a process is implemented to remove the alkaline earth metal elements remaining in the phosphor. As a result of this, it was found that electroluminescent phosphors of higher efficiency and longer life, when compared with the existing zinc sulfide system electroluminescent phosphors could be obtained. Further, it was also found that even by reducing the amount of alkaline earth metal compounds added to the phosphor ingredient, the luminescent efficiency and life of the zinc sulfide system electroluminescent phosphors can be improved.
Based on the aforementioned know
Oikawa Mitsuhiro
Shimizu Yohe
Takahara Takeshi
Takemura Hirofumi
Foley & Lardner
Kabushiki Kaisha Toshiba
Koslow C. Melissa
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