Chemistry of inorganic compounds – Rare earth compound
Reexamination Certificate
2000-11-13
2002-05-14
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Rare earth compound
Reexamination Certificate
active
06387339
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to particles of a rare earth oxide having unique characteristics to be suitable as a base material of rare earth-based phosphors as well as to a method for the preparation of the unique rare earth oxide particles. More particularly, the invention relates to particles of a rare earth oxide characterized by unique granulometric and intragranular parameters as well as to an improvement in the method for the preparation of such rare earth oxide particles.
It is well established that various powders of rare earth oxides are widely employed in the applications as a base material for the preparation of luminescent phosphors used in fluorescent lamps, cathode-ray tubes and the like and as an ingredient in sintered ceramic articles. It is usually accepted that the performance of rare earth oxide particles in these applications can be improved by controlling or optimizing the granulometric characteristics of the particles such as the particle configuration, average particle diameter and the like.
While the most conventional method for the preparation of a rare earth oxide powder is calcination of a rare earth oxalate obtained by mixing aqueous solutions of a water-soluble rare earth salt and oxalic acid to effect a precipitation reaction of the rare earth oxalate, improved methods based on this background technology are disclosed in Japanese Patent Kokai 3-271117 and 3-271118, according to which rare earth oxide particles having a spherical particle configuration can be prepared by conducting the precipitation reaction of the rare earth oxalate at a low temperature. Even though these methods are suitable for giving rare earth oxide particles having a generally spherical particle configuration, electron microscopic examinations of high magnification reveal that the surface of the generally spherical particles are rather rugged and not smooth. Presumably due to this surface condition, the phosphors prepared from these spherical rare earth oxide particles cannot always exhibit high performance as a phosphor such as the intensity of luminescence or fluorescence.
SUMMARY OF THE INVENTION
The present invention accordingly has an object, in view of the above described problems and disadvantages in the conventional rare earth oxide particles as a base material of luminescent phosphors, to provide improved rare earth oxide particles capable of giving luminescent phosphors exhibiting upgraded performance as a phosphor.
Thus, the rare earth oxide particles provided by the present invention, which preferably have a spherical particle configuration, are characterized by a pore volume not exceeding 0.02 ml/g and an average crystallite diameter not exceeding 50 nm.
The invention further provides an improvement, in the method for the preparation of particles of a rare earth oxide comprising the steps of: (1) mixing an aqueous solution of a water-soluble salt of a rare earth element and an aqueous solution of oxalic acid to effect precipitation of a rare earth oxalate in an aqueous medium; (2) separating the precipitates of the rare earth oxalate from the aqueous medium to give dried rare earth oxalate particles; and (3) calcining the dried precipitates of the rare earth oxalate in an oxidizing atmosphere, which improvement comprises:
(a) conducting, in step (1), mixing of the aqueous solutions of a rare earth salt and oxalic acid at a temperature not higher than 15° C.;
(b) conducting, in step (2), filtration of the precipitates and washing of the precipitates with water at a temperature not higher than 15° C.;
(c) freezing the precipitates of the rare earth oxalate after washing with water at a temperature not higher than −25° C.; and
(d) drying, prior to the calcination in step (3), the thus frozen precipitates of the rare earth oxalate by vacuum drying until the content of water therein including the wetting water and water of crystallization does not exceed 20% by weight based on the anhydrous rare earth oxalate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As is described above, the method for the preparation of rare earth oxide particles according to the improvement provided by the present invention is characterized in conducting the precipitation reaction of the rare earth oxalate and washing of the oxalate precipitates with water at a specified low temperature and freezing the rare earth oxalate precipitates after washing with water at a specified low freezing temperature prior to the calcination treatment.
The rare earth element of the rare earth oxide particles which should be prepared by the improved method of the present invention can be selected from the group consisting of yttrium and the elements having an atomic number in the range from 57 to 71 on the Periodic Table. The rare earth oxide optionally contains two kinds or more of these rare earth elements in combination so that the rare earth oxide can be, for example, a europium-doped yttrium oxide or terbium-doped yttrium oxide from which a red-emitting or green-emitting, respectively, phosphor can be prepared. When the intended application of the rare earth oxide particles is as a base material of a phosphor, it is a due selection that at least 80% by moles or, in particular, from 85 to 90% by moles of the rare earth element constituting the rare earth oxide is selected, in consideration of the high optical transparency in the visible region, from the group consisting of yttrium, lanthanum, gadolinium and lutetium or, preferably in consideration of the low hygroscopicity and abundance as a natural resource, yttrium and gadolinium or, in particular, yttrium. The dopant or activator rare earth element to be combined with the rare earth oxide of the above named elements as the host phase is selected from the other rare earth elements depending on the luminescence characteristics of the phosphor includes cerium, samarium, europium, terbium, dysprosium and thulium of which europium and terbium are known as a red-emitting and a green-emitting activator, respectively.
When the intended application of the rare earth oxide particles is as a base material of a sintered ceramic article or as a sintering aid of other ceramic materials, on the other hand, it is preferable that the rare earth component in the rare earth oxide is one or a combination of the elements selected from yttrium and the elements having an atomic number of 64 to 71.
The inventors have conducted extensive investigations on the relationship between the microstructure or, namely, the intragranular structure of rare earth oxide particles and the performance thereof as a base material of luminescent phosphors to arrive at a discovery that rare earth oxide particles having a pore volume of as small as 0.02 ml/g or smaller or, preferably, in the range from 0.003 to 0.01 ml/g well give a high-performance phosphor. The reason therefor is presumably that infiltration of the flux material used in the process of phosphor preparation into the rare earth oxide particles can be suppressed when the pore volume of the particles is small enough. This advantageous effect can further be promoted when the rare earth oxide particles have a spherical particle configuration. The pore volume here implied is a parameter determinable by means of the adsorption characteristics of nitrogen gas on the particles at the temperature of liquid nitrogen, i.e. −196° C.
The small pore volume of the rare earth oxide particles is not the only characterizing parameter which the particles as a base material of phosphors should have. For example, it is known that rare earth oxide particles obtained by a calcination treatment of a rare earth oxalate at an unduly high temperature or those obtained by crushing and pulverizing a solidified melt of the oxide have an extremely small pore volume but are not capable of giving a high-performance phosphor presumably due to poor reactivity of the particles even by the combined use of a flux or even by a combination with other oxides such as aluminum oxide, silicon dioxide and vanadium oxide to give a c
Kaneyoshi Masami
Sakai Shigeru
Bos Steven
Shin-Etsu Chemical Co. , Ltd.
Wenderoth , Lind & Ponack, L.L.P.
LandOfFree
Rare earth oxide particles and method for preparation thereof does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Rare earth oxide particles and method for preparation thereof, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Rare earth oxide particles and method for preparation thereof will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2870766