PROCESS FOR PRODUCING RARE EARTH OXYSULFIDE PHOSPHOR

Abstract: A process for producing rare earth oxysulfide phosphor by firing a mixture of a rare earth oxide and/or a rare earth compound which is readily converted into the oxide at high temperature, a sulfiding agent and a flux, characterized by adding a seed of the rare earth oxysulfide to the mixture for firing.

Description: TECHNICAL FIELD The present invention relates to a method for preparing a rare earth oxysulfide phosphor. BACKGROUND ART Since the rare earth oxysulfide phosphor (in the form of Ln₂O₂S:Ln') is good in emission efficiency, chemically stable, easy in handling, etc., Y₂O₂S:Eu is practically used as a red-emitting phosphor for color televisions and Gd₂O₂S:Tb is practically used as a green-emitting phosphor for projection cathode-ray tubes. As a method for preparing this kind of phosphor, it has been known to employ a method wherein Ln₂O₃ and Ln'₂O₃ as rare earth oxide raw materials are dissolved in a mineral acid and then reacted with oxalic acid to coprecipitate as their oxalates, which are pyrolysed to form a Ln₂O₃·Ln'₂O₃ mixed crystal, and then a sulfiding agent and a flux are added to the mixed crystal, followed by baking, and a method wherein a sulfiding agent and a flux are added directly to Ln₂O₃ and Ln'₂O₃, followed by taking (US Patent No. 3,502,590). In these methods, the flux serves to let the above synthesis proceed easily and accelerate the growth of particles. Under these circumstances, in order to improve the emission efficiency of phosphors and accelerate the growth of particles, attempts have been made to add various kinds of phosphates as the flux, for example, YPO₄ (Japanese Patent Publication No. 35555/1976), Na₄P₂O₇ (Japanese Patent Publication No. 46182/1979), AlPO₄ (Japanese Patent Publication No. 192484/1982), etc. On the other hand, with the trend to high quality of color televisions and display tubes in recent years, it has been highly demanded to improve the quality of fluorescent screens of cathode-ray tubes. It is important that the dispersibility is good and giant particles are not contained to obtain a fluorescent film (dot, stripe) excellent in packing property. This is because that the presence of the giant particles is apt to cause defects such as pinholes. For fluorescent film properties, it is important to form three pure colors (dots or stripes) by coating. If a red-emitting phosphor particle is deposited on a blue or green picture element formed in the previous step and remains thereon, so-called color cross contamination which reduces color purity is caused. The color cross contamination is mainly caused by fine particles and thus it is desired to reduce the content of the fine particles. Namely, as the powder properties of phosphors, it is desired that the dispersibility is good, the contents of the large particles and fine particles are low and the particle size distribution is narrow. It is also desired to obtain large particles (i.e. 7 µm or more) which can hardly be obtained by conventional synthetic methods, and thus it is required to provide a phosphor having a particle size suitable for particular use of the phosphor. However, conventional techniques for synthesizing phosphors are not adequate because there is a limit in the particle size control by adjusting the baking temperature or the kind or the amount of the flux added. Particularly, the synthesis of large particle phosphors is difficult and, if the baking temperature is made high, the synthetic conditions in a baking container become uneven which lead to the production of phosphors containing considerable amounts of the giant particles and fine particles and having a wide particle size distribution. DISCLOSURE OF INVENTION The present invention is to overcome the above drawbacks and to provide a method for preparing a rare earth oxysulfide phosphor which has good dispersibility, a narrow particle size distribution and a desired particle size. The present invention provides a method for preparing a rare earth oxysulfide phosphor represented by, for example, the formula Ln₂O₂S:Ln', by adding a sulfiding agent and a flux to rare earth oxides such as Ln₂O₃ and Ln'₂O₃, etc. wherein Ln is an element such as Gd, Y, Sc, Lu or La and Ln' is an element such as Eu, Tb, Sm, Pr, Dy or Tm, followed by baking, characterized in that seeds (seed crystals) of a rare earth oxysulfide (Ln₂O₂S:Ln' and/or Ln₂O₂S) are added to the above mixture before baking, as defined in claim 1. It is preferred to add 1 to 60% by weight, particularly 5 to 30% by weight, of the rare earth oxysulfide seeds to the total amount of the rare earth oxide and the rare earth oxysulfide seeds from the viewpoint of the particle size distribution, the particle size or the like of the rare earth oxysulfide phosphor. Also, it is recommended to add 1 to 40% by weight, more preferably 3 to 30% by weight, of the above seeds taking into account the points of the production cost and the production efficiency. In the conventional synthetic methods, the formation of nuclei for the growth of particles, coupled with the unevenness of the synthetic conditions in the baking container, causes a deviation in the formation and also a great difference in the growth of particles. It is considered that a phosphor having a wide particle size distribution is thereby produced. According to the present invention wherein, in the course of mixing a sulfurizing agent and a flux with a rare earth oxide and/or a rare earth compound which easily becomes its oxide at a high temperature (e.g. carbonates, oxalates, sulfites or the like of rare earth elements) followed by heating and synthesizing, an appropriate amount of rare earth oxysulfide seeds (seed crystals) having an appropriate particle size are added, it is possible to make the conditions of the growth of particles uniform, obtain particles having a good dispersibility with less fusing of particles, achieve a narrow particle size distribution with less contents of fine particles and giant particles, and produce a rare earth oxysulfide phosphor having a desired particle size. The particle size of the rare earth oxysulfide seeds added in the present invention is less than the particle size of the objective rare earth oxysulfide phosphor. With respect to the particle size of the above seeds, usually used is a particle size which is 80% or less of the median particle size (d₅₀) of the phosphor. Particularly, it is preferred to use the seeds, the majority of which has a particle size within the range of 10 to 70% of the median particle size. Particularly, for the efficiency of the growth of particles, it is optimum that the majority of the particle sizes of the phosphor is within the range of 25% to 65%. For the improvement of the effect to the amount of the above seeds, it is recommended to use the seeds, the majority of which has a particle size within the range of 10% to 40%. In the present invention, it is recommended to use a phosphor represented by Ln₂O₂S:Ln' among the rare earth oxysulfide seeds because of its merit in use. The method of the present invention provides a meaningful effect only to the rare earth oxysulfide phosphor. No such effect is observed on the conventional sulfide series phosphors (e.g. phosphors such as ZnS:Ag, ZnS:CuAl or ZnCdS:Cu), oxide phosphors (e.g. phosphors such as Ln₂O₃:Eu), etc. In the present invention, the median particle size (d₅₀) is represented by the weight distribution. According to the present invention, wherein the above construction is employed, it is possible to obtain a rare earth oxysulfide phosphor which is good in dispersibility, easy in controlling the particle size within a narrow particle size distribution and is excellent in coating properties. BRIEF DESCRIPTION OF DRAWINGS Figures 1 and 2 are graphs showing the particle size d₅₀ and the particle size distribution of the phosphor obtained by changing the mixing ratio of the seeds in Example 1. BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1 226 g of Y₂O₃ and 13.7 g of Eu₂O₃ were dissolved in nitric acid and diluted with pure water to about 1.8 ℓ, followed by heating at 70°C. Then, a 70°C solution of 565 g of oxalic acid dissolved in 1 ℓ of pure water was added while stirring to the previously prepared solution to conduct coprecipitation and form oxalates of Y and Eu. After washing with water several times, it was heated at 900°C for 1 hour to form an oxide. To 160 g of (Y,Eu)₂O₃, 70 g of sulfur, 70 g of sodium carbonate and 20 g of potassium phosphate, a (Y,Eu)₂O₂S seed having the same molar ratio of Y and Eu as the molar ratio of the rare earth oxide and having the particle size d₅₀ of 4.5 µm was added which changing the added amount successively, for example, 10 g when the mixing ratio was 5% and 20 g when the mixing ratio was 10%. The mixture was thoroughly mixed, then put into an alumina crucible, and baked at 1,150°C for 2 hours. The baked product was repeatedly washed with water, and the residual alkali was neutralized with nitric acid. The resulting product was subjected to dispersing treatment by a ball mill, dehydrated, filtered, and then dried and at 120°C for 13 hours, followed by sieving with a nylon mesh of 250 mesh to obtain Y₂O₂S:Eu red-emitting phosphors corresponding to the respective mixing ratios. With respect to the phosphors obtained under the above synthetic conditions provided that the mixing ratio of the seeds was changed from 5% to 80%, their effects to the particle size distribution were examined. The particle size distribution in Figure 2 is a particle size distribution when the mixing ratio of the seeds was changed while taking σlog(L) = log(d 84.1 /d₅₀) and σlog(S) = -log(d 15.8 /d₅₀) as ordinates. σlog(L) shows the extent of incorporation of giant particles. The larger the value, the higher the content. σlog(S) shows the extent of incorporation of fine particles. The larger the particle, the higher the content. It is evident from Figure 1, when seeds having the particle size of 4.5 µm is added, the particle size is changed with the mixing ratio of the seeds. Particularly, the median particle size d₅₀ of the phosphor obtained at the mixing ratio of 5% by weight indicates the highest value of 11.4 µm, and, when the mixing ratio is further increased, the value is gradually decreased and indicates the lowest value of 8 µm at the mixing ratio of 60% by weight. It is also evident from Figure 2 that the value of σlog(L) becomes small by the addition of the seeds and the content of giant particles is reduced. Particularly, the tendency is remarkable within the range of 5 to 30% by weight. σlog(S) also has the essentially same tendency as σlog(L). In this Example, when a part of Y was replaced with 10 mol% of Gd, the effect was essentially same, and, when 1,000 ppm of Sm was added, no change was observed. EXAMPLE 2 226 g of Y₂O₃ and 13.7 g of Eu₂O₃ were dissolved in nitric acid, diluted with pure water to about 1.8 ℓ, and then heated at 70°C. When a 70°C solution of 565 g of oxalic acid dissolved in 1 ℓ of pure water was added to the previously prepared solution to coprecipitate oxalates of Y and Eu. After washing with water several times, heating was conducted at 900°C for 1 hour to form an oxide. To 160 g of (Y,Eu)₂O₃ thus obtained, 70 g of sulfur, 70 g of sodium carbonate and 20 g of potassium phosphate, 40 g of a (Y,Eu)₂O₂S seeds having the particle size d₅₀ of 4.5 µm, which has the same molar ratio of Y and Eu as the molar ratio of the rare earth oxide, was added. The mixture was thoroughly mixed, then put into an alumina crucible, and baked at 1,150°C for 2 hours. The baked product was repeatedly washed with water, and its residual alkali was neutralized with nitric acid. The resulting product was subjected to dispersing treatment by a ball mill, dehydrated and filtered, and then dried at 120°C for 13 hours, followed by sieving with a nylon mesh of 250 mesh to obtain a Y₂O₂S:Eu red-emitting phosphor. For the purpose of comparison, in Comparative Example 1, a phosphor was prepared under the above-mentioned conditions provided that the seed was not used, and, in Comparative Example 2, large particles were prepared by using no seed and changing the baking temperature to 1,250°C to conduct the baking at a higher temperature. The powder properties and coating properties of the phosphors thus obtained are shown in Table 1. In this Table, the sediment volume in water is obtained in such a manner that 5 g of a phosphor and 30 ml of pure water were placed in a 30 ml settling tube, well shaken, left to stand and measured its sediment volume after 1 hour. The packing property of the coating properties indicates the extent of packing of the phosphor particles which constitute the fluorescent film (dot, stripe), wherein ○ indicates to have excellent packing property and X indicates not to have necessary packing property. The color mixing indicates the extent of the red-emitting phosphor particles attached on blue picture elements (dot, stripe), thereby indicating the effect to the color purity of blue color. ○ indicates that no essential influence to the color purity is observed and X indicates that the influence is large. It is found that, although the phosphor in Example 2 is grown as considerably large as 8 µm in the particle size d₅₀, σlog(S) and σlog(L) are small, whereby the particle size distribution is narrow. It is also found that the sediment volume in water is small, the dispersibility is good, and the packing property and color cross contamination are excellent. The phosphor in Comparative Example 2 was obtained by the method wherein no seed was used under the preparation conditions in Example 2, and its particle size d₅₀ is as small as 4.9 µm. On the other hand, in Comparative Example 1 wherein the baking temperature is rised, although the phosphor thus obtained has a particle size d₅₀ of 8.0 µm which is close to that of Example 2, other powder properties and coating properties are considerably inferior to those of Example 2, similarly as in the case of Comparative Example 2. When a part of Y was replaced with 20 mol% of Sc in this Example, the effect was substantially same. EXAMPLE 3 160 g of Gd₂O₃ and 2.8 g of Tb₄O₇ were added with 60 g of sulfur, 60 g of sodium carbonate and 10 g of potassium phosphate, and further added with 40 g of (Gd,Tb)₂O₂S seeds having the particle size d₅₀ of 4.5 µm which has the same molar ratio of Gd and Tb as of the molar ratio of the rare earth oxide. The mixture was thoroughly mixed, and put into an alumina crucible, followed by baking at 1,150°C for 2 hours. The baked product was repeatedly washed with water, and its residual alkali was neutralized with nitric acid. The resulting product was subjected to dispersing treatment by a ball mill, dehydrated and filtered, and then dried at 120°C for 13 hours, followed by sieving with a nylon mesh of 250 mesh to obtain a Gd₂O₂S:Tb green-emitting phosphor. For the purpose of comparison, in Comparative Example 4, a phosphor was prepared by the method wherein no seed was used under the above-mentioned preparation conditions, and, in Comparative Example 3, the preparation was conducted by using no seed and changing the baking temperature to 1,200°C. The powder properties and coating properties of the phosphors thus obtained are shown in Table 2. It is found, from the comparison between Comparative Examples 3 and 4, that the phosphor of Comparative Example 3 is superior to that of Comparative Example 4 in the powder properties and coating properties as shown in the Table. In the above, as typical examples, the case wherein Y, Gd or Sc is used as a host element (Ln) and the case wherein Eu, Tb or Sm is used as an activator element (Ln') are exemplified. However, according to the present invention, the effect of the present invention can be obtained also in the case wherein La or Lu as other host element is used, the case where a solid solution including these elements is used, the case wherein Pr, Dy or Tm is used as other activator element (Ln'), further the case of coactivation by two or more elements including these elements. In addition, by using the technique of the present invention, a phosphor of large particle size which have not been obtained in the conventional manner can be obtained, i.e. a phosphor of 20 µm can be obtained by using seeds of 10 µm, whereby the present invention is extremely useful.

Claims: A method for preparing a rare earth oxysulfide phosphor which comprises baking a mixture containing a rare earth oxide and/or a rare earth compound which becomes a rare earth oxide easily at a high temperature, a sulfurizing agent and a flux, wherein rare earth oxysulfide seeds are added in an amount of 1 to 60% by weight to the total amount of the rare earth oxide and the rare earth oxysulfide seeds to the mixture before baking, wherein the particle size of the rare earth oxysulfide seeds is less than the particle size of the objective rare earth oxysulfide phosphor. The method according to Claim 1, wherein Ln₂O₃ and Ln'₂O₃ or their mixed crystal, or a rare earth compound which becomes these rare earth oxides easily at a high temperature is used as the rare earth oxide, and Ln₂O₂S:Ln' and/or Ln₂O₂S is used as the rare earth oxysulfide seeds to prepare a rare earth oxysulfide phosphor represented by the formula Ln₂O₂S:Ln', wherein Ln is at least one element of Gd, Y Sc, Lu and La and Ln' is at least one element of Eu, Tb, Sm, Pr, Dy and Tm. The method according to Claim 1, wherein the rare earth oxysulfide seeds are added in an amount of 3 to 30% by weight. The method according to Claim 1, wherein the particle size of the rare earth oxysulfide seeds are 80% or less of the median particle size of the resulting are earth oxysulfide phosphor. The method according to Claim 1, wherein the majority of the particle size of the rare earth oxysulfide seeds is within the range of 10 to 70% of the median particle size of the rare earth oxysulfide phosphor. The method according to Claim 1, wherein the rare earth oxysulfide seeds are Ln₂O₂S:Ln', where Ln is yttrium and/or gadolinium and Ln' is an element containing at least one of europium or terbium.

Inventors: IWASAKI KAZUHITO, TONO HIDEO, TUKIHASHI YOUJI, IWASAKI, KAZUHITO, TONO, HIDEO, TUKIHASHI, YOUJI
Applicants: KASEI OPTONIX, KASEI OPTONIX, LTD.
Application year: 1991
Main classification: C09K11
IPC codes: C09K11, C01F17, H01J29