Compositions: ceramic – Ceramic compositions – Yttrium – lanthanide – actinide – or transactinide containing
Reexamination Certificate
2001-07-03
2004-01-13
Koslow, C. Melissa (Department: 1755)
Compositions: ceramic
Ceramic compositions
Yttrium, lanthanide, actinide, or transactinide containing
C423S263000, C252S30140R
Reexamination Certificate
active
06677262
ABSTRACT:
This invention relates to a rare earth oxide, a basic rare earth carbonate, methods for preparing them, and a phosphor and ceramic obtained therefrom.
BACKGROUND OF THE INVENTION
Among rare earth oxides, a spherical rare earth oxide having an average particle diameter of 0.2 to 1 &mgr;m as measured by a Fisher sub-sieve sizer (this average particle diameter is sometimes referred to as Fisher diameter, hereinafter) is obtainable by a known method as disclosed in Wataya et al., U.S. Pat. No. 5,879,647 (JP-A 10-139427). Also, a method for preparing finer spherical particles having an average particle diameter of 0.1 to 0.3 &mgr;m is known from JP-A 10-139427. A method for preparing spherical particles having an average particle diameter of 2 to 6 &mgr;m is disclosed, for example, in JP-A 8-59233.
These methods, however, are difficult to produce spherical rare earth oxide particles having a Fisher diameter from more than 0.5 &mgr;m to less than 2 &mgr;m.
Yttrium-europium oxide phosphors and yttrium-gadolinium-europium oxide phosphors are used in plasma display and medical diagnostic x-ray systems as the red phosphor. The plasma display is promising as a large-size flat display panel. Yttrium-europium oxide phosphors and yttrium-gadolinium-europium oxide phosphors are attractive as plasma display red phosphors having a high luminous efficiency to excitation light of 147 nm emitted by xenon plasma.
While yttrium-gadolinium-europium borate phosphors are also known as the red phosphor for plasma displays, the yttrium-europium oxide phosphors and yttrium-gadolinium-europium oxide phosphors are potential candidates since they are superior in color purity and lifetime despite a lower luminous efficiency.
For medical diagnostic x-ray systems, yttrium-gadolinium-europium oxide phosphors are regarded promising because of high luminous efficiency to x-rays.
For such display panels as plasma display panels (PDP), to increase their brightness is an important task in improving display performance.
The brightness of panels can be increased, for example, by increasing the brightness of phosphor itself. It is believed that the brightness of panels is largely affected by the coating property of phosphor to cells. With respect to the coating property of phosphor, those phosphors which can be applied to plasma display cells in a uniform, least irregular state are regarded preferable. With respect to the shape of phosphor, particles of small size, equal diameter and identical shape are regarded preferable because uniform coating property is improved.
The particle size and shape of phosphor, especially the particle size of phosphor, depend on the particle size of a raw material. In general, using a raw material having a less variation of particle diameter or a sharper particle size distribution, a phosphor having a sharper particle size distribution is obtained. The raw material powder is thus required to have a sharper particle size distribution.
However, a microscopic observation of conventional raw material oxide revealed that even a raw material powder having a sharper particle size distribution contained particles of differing size. Such a raw material powder was regarded to have a sharper particle size distribution for the mere reason that the difference in particle size was relatively small or particles had somewhat similar shapes. The phosphor prepared from such a raw material powder contains particles of differing size.
SUMMARY OF THE INVENTION
An object of the invention is to provide a spherical rare earth oxide and spherical basic rare earth carbonate having an average particle diameter from more than 0.5 &mgr;m to less than 2 &mgr;m which is difficult to obtain in the prior art, methods for preparing them, a phosphor and ceramic obtained therefrom.
Another object of the invention is to provide a phosphor having a uniform particle diameter and a sharp particle size distribution, suitable for typical use as a red phosphor in displays and in medical diagnostic x-ray systems, and a method for preparing the same.
From a study on precipitating conditions including controlled concentrations of rare earth ions, carbonic acid or carbonate ions, and ammonia or ammonium ions in aqueous solution, we have reached the present invention. More specifically, when rare earth oxides are used as a raw material to form ceramics and phosphors, the characteristics of products are largely affected by the particle shape, particle size and particle size distribution of rare earth oxides. The rare earth oxide particles have a variety of shapes including irregular, tabular, angular, and spherical shapes. Of these, spherical particles are one of the particle shapes regarded most preferable as the raw material. Among such spherical particles of rare earth oxide, a fraction of particles having a diameter from more than 0.5 &mgr;m to less than 2 &mgr;m has never been available in the art, and we tried to obtain this fraction of particles. We reached the basic concept of synthesis that better results are obtainable by generating a suitable amount of particle nuclei serving as crystal seeds in a liquid phase and thereafter, controlling the concentration of a precipitant in the liquid phase such that the initially generated particle nuclei may be grown without generating new particle nuclei. Since amorphous particles were believed preferable to obtain spherical particles, a choice was made of basic rare earth carbonates from which amorphous rare earth salts were readily obtainable. It was found that an effective means for obtaining a basic carbonate was to homogeneously add ammonia or ammonium ions and carbonic acid or carbonate ions to a rare earth ion-containing liquid phase.
The addition of a precipitant is readily accomplished by adding urea to the solution and heating the solution at a temperature of 80° C. or higher.
However, the method of obtaining a precipitate of basic rare earth carbonate by adding urea to a solution of rare earth salt and heating the solution is difficult to produce particles having a Fisher diameter in excess of 1 &mgr;m because an excessive amount of precipitate forms in the solution if the concentration of urea in the solution is too high.
Paying attention to the change of the urea concentration in the solution, we have found that the number of particles generated in the solution at an initial stage of reaction can be reduced by adjusting the urea concentration so as not to become high, and that the initially generated particles can be grown larger by suppressing further particle generation after the initial stage of reaction.
Moreover, using a spherical rare earth oxide of uniform particle diameter and uniform particle shape having an average particle diameter of 0.5 to 2 &mgr;m which is obtained by the above method, we tried to produce a consistent yttrium-europium oxide phosphor or yttrium-gadolinium-europium oxide phosphor having a uniform particle diameter of 0.5 to 2 &mgr;m.
Nowadays, it becomes customary in the medical field to store digital radiographic data for diagnosis. To increase the resolution of diagnostic images, the phosphor is required to have a high luminous efficiency to x-rays, a fine particle size, luminous characteristics having a high sensitivity to the detector, and good coating property. We thus tried to produce a consistent yttrium-gadolinium-europium oxide phosphor having a uniform particle diameter of 0.5 to 2 &mgr;m as the phosphor capable of meeting these requirements.
We have found that a yttrium-europium or yttrium-gadolinium-europium oxide phosphor having a uniform particle diameter can be produced by heating a consistent, spherical, coprecipitated yttrium-europium or yttrium-gadolinium-europium oxide having a particle diameter of 0.5 to 2 &mgr;m, obtained as above, at a temperature of 1,100 to 1,800° C. Advantageously, the oxide used as the raw material has a so small particle diameter of 0.5 to 2 &mgr;m that crystal growth is facilitated. The oxide particles used as the raw material have a uniform size so that equal crystal growth occurs among individual par
Ohyama Miyuki
Wataya Kazuhiro
Koslow C. Melissa
Millen White Zelano & Branigan P.C.
Shin-Etsu Chemical Co. , Ltd.
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