In the field of color display devices used for displaying images such as computers and televisions, display units provided with plasma display panels (hereinafter referred to as PDP) enjoy popularity in recent years as color display devices of large screen with capability of realizing thin profile and light weight.
A plasma display device provided with a PDP gives full color display by mixing so-called three primary colors (i.e., red, green and blue). To display images in full color, the plasma display device is provided with phosphor layers for emitting individually the three primary colors, or red (R), green (G) and blue (B), in which phosphor particles composing the phosphor layers are excited by ultraviolet rays generated inside discharge cells of the PDP, to produce visible light of the individual colors.
Examples of chemical compounds known as phosphors of the individual colors noted above include (YGd)BO3; Eu3+ for emitting red light, Zn2SiO4:Mn2+ for emitting green light, and BaMgAl10O17:Eu2+ for emitting blue light. Each of these phosphors is produced by means of solid phase reaction in which prescribed raw materials are mixed and fired at a temperature as high as 1000° C. (refer to, for example, “Phosphor Handbook”, published by Ohmsha, Ltd., PP.219–220). The phosphor particles obtained by this firing are ground and sifted before use (so that the red and green phosphor particles have a mean particle diameter of 2 μm to 5 μm, while the blue phosphor particles have a mean particle diameter of 3 μm to 10 μm). The phosphor particles are ground and sifted (classified) for the following reason. In general, paste including the phosphor particles having each color is applied by screen printing to form the phosphor layer in the PDP. The surface of the paste applied easily becomes smoother if the particle diameters of the phosphor are smaller and more uniform (in particle size distribution). In other words, the smaller and the more uniform the particle diameters of the phosphor and the more spherical the phosphor particles, the smoother the applied surface. Accordingly, packing density and a light-emitting surface area of the phosphor particles in the phosphor layer conceivably increase, thus increasing luminance of the plasma display device.
However, reducing the particle diameters of the phosphor particles increases the surface area of the phosphor and thus increases the number of defects in the phosphor. For this reason, a large amount of water, carbon dioxide or hydrocarbon-containing organic substances easily adheres to the surface of the phosphor. For a blue phosphor, including a divalent Eu ion as a luminescence center, such as Ba1-xMgAl10O17:Eux or Ba1-x-ySryMgAl10O17:Eux, in particular, its crystal structure has a layer structure (refer to, for example, “Display and Imaging”, 1999, Vol. 7, pp. 225–234). This layer structure includes an oxygen (O) vacancy in the vicinity of a layer (Ba—O layer) including a Ba atom, and the smaller the particle diameters, the more these vacancies problematically increase in number (refer to, for example, “OYO BUTSURI (Applied Physics)”, Vol. 70, No. 3, 2001, p. 310). FIG. 6 schematically illustrates the structure of the Ba—O layer of the blue phosphor, Ba1-xMgAl10O17:Eux.
For the above reason, water existing in the air or hydrocarbon-containing gas selectively adsorbs on the surface of such a Ba—O layer. Such water and gas are released into the panel in large amounts in a panel manufacturing process and react with the phosphor and MgO during discharge, thus problematically degrading the luminance, causing a change in chromaticity (which leads to a color shift or a burn on a screen), reducing a drive margin and raising discharge voltage.
Moreover, since water and hydrocarbon group gases are selectively adsorbed by the blue phosphor, they make ethyl cellulose in binder difficult to be adsorbed by the blue phosphor in the process of making paste and ink. This causes the phosphor and the ethyl cellulose easily separable. If the ethyl cellulose and the phosphor separate, the phosphor accumulates in the vicinity of an opening in a nozzle of thin diameter where the velocity has no gradient when the phosphor ink is being sprayed from the nozzle, thereby giving rise to a problem of choking up the nozzle opening.
After a phosphor layer is formed by such a method as screen printing or inkjet printing, it is necessary to fire the phosphor layer at 500 to 600° C. to remove the organic binder and solvent elements in the ink or the paste. It is also necessary to fire the phosphor layer again at 400 to 450° C. in a sealing process and the like in which a front glass substrate and a rear glass substrate are bonded together with frit glass. These firing processes cause degradation in luminance of the ordinary phosphor such as Ba1-xMgAl10O17:Eux and Ba1-x-ySryMgAl10O17:Eux used previously. There was therefore disclosed a technique to reduce the number x of Eu atoms to 0.1 or less, in order to alleviate the luminance degradation (e.g., Japanese Patent Unexamined Publication, No. H11-323325).
Furthermore, it is also known that deficiencies are produced in the phosphor by ultraviolet rays having a wavelength of 147 nm generated by electrical discharge while the panel is operated (refer to Technical Report, EID99-94, published on Jan. 27, 2000 by The Institute of Electronics, Information and Communication Engineers).
To solve these problems, there was proposed a method of compensating oxygen vacancies in the blue phosphor by firing the phosphor in an oxidative environment. Although this method corrects the oxygen vacancies, it gives another problem that luminance of the phosphor decreases.
There was also proposed another method in which crystals of Al2O3 are coated over the entire surface of the phosphor, to repair the deficiencies in the Ba—O layer. However, the coating layer absorbs ultraviolet rays and prevents them from reaching the phosphor in this case, thereby giving rise to still another problem of reducing the luminous luminance.
In addition, there is another method of reducing the oxygen vacancies, in which a part of bivalent Eu (15% or less) is replaced with trivalent Eu by firing blue phosphor of Ba1-xMgAl10O17:Eux having 0.2 or less in amount x of Eu atoms in an environment containing oxygen gas. However, this method changes only about 15% of the bivalent Eu. It is thus insufficient to avoid the problem of choke-up in the nozzle, and it still gives the problem of reducing luminance.
The present invention addresses the problems discussed above, and it aims at preventing luminance degradation and chromaticity change of the phosphor, and improving a discharge characteristic by eliminating the oxygen vacancies in the vicinity of the layer containing amount of Ba (i.e., Ba—O layer), to restrain adsorption of water and hydrocarbon to the surface of the blue phosphor.