METHOD OF COATING MICROMETER SIZED INORGANIC PARTICLES

Abstract: A method for coating moisture sensitive inorganic particles, such as phosphors, comprising mixing the inorganic particles which have a particle size of about 1 to 100 micrometers with nanometer sized coating particles, as of silica, and firing to soften or melt the nanometer sized particles, forming a moisture-impervious inorganic coating on the inorganic particles. The coated particles are washed to remove any uncoated particles and dried.

Description: - 1 - METHOD OF COATING MICROMETER SIZED INORGANIC PARTICLES This application claims priority from Provisional application Serial No. 60/134,918 filed May 19, 1999. BACKGROUND OF THE INVENTION Inorganic particles that are sensitive to atmospheric materials such as moisture can be coated using liquid based or vapor phase methods. For example, binders and mineral or metallic colloids can be dispersed in liquid media and distributed over the surface of the inorganic particles to be coated. However, the coating materials must be insoluble in, and non-reactive with, the solvent used. Vapor phase coating methods, such as using fluidized bed reactors, require high temperatures and lengthy reaction times which generally result in some surface reaction or modification of the particles to be coated. Further, the equipment is complex and expensive. As an example, long persistence phosphors are known which, after having been struck by light, continue to emit light after the initial light source is extinguished. For example, europium activated alkaline earth metal sulfides, such as SrS.Eu and CaS.Eu, are known that emit in the orange- red portions of the spectrum. Red-emitting phosphors having long persistence are very rare, but they are very useful, particularly for safety equipment such as "EXIT" signs and the like. However, these phosphors are water sensitive, and thus cannot be exposed to moisture without deleterious effects on their light emission. Thus they must be protected to prevent exposure to moisture. Coating or encapsulating water sensitive phosphors with moisture insoluble materials has been tried. The coating particles can be mixed with the phosphor particles to be coated in a liquid medium, but both the coating material and the phosphor must be insoluble in, and non-reactive with, the liquid. In the case of phosphors, moisture sensitive phosphors cannot be immersed in solutions including any moisture . Vapor phase coating has also been tried; for example tetraethoxysilane or tetramethoxysilane can be passed through a fluidized bed including phosphor particles to deposit a silica layer, but this is an expensive process. Thus it would be desirable to be able to apply a coating of an inorganic material onto inorganic particles to form a protective coating, or a coating that can modify particular surface properties, or impart a particular surface functionality to the inorganic particles. In particular, a method of applying a water impervious coating onto phosphor particles that are sensitive to moisture without the aid of an aqueous liquid medium, a method that does not change the color or the strength of the phosphor emission, would be highly desirable. SUMMARY OF THE INVENTION The present invention is directed to a method of coating inorganic particles with a thin film of an inorganic coating material that does not require any liquid media for its application. Inorganic particles having a particle size of from 1 up to about 100 micrometers can be coated using a sufficient amount of nanometer sized inorganic powders to completely cover the larger inorganic particles. The coated inorganic particles are then fired at a temperature high enough to soften or melt the nanometer-sized coating particles surrounding the coated inorganic particles, - 3 - but a temperature insufficient to change the surface properties of the larger inorganic particles to be coated. The present method does not require that any solvent be used. BRIEF DESCRIPTION OF THE DRAWING The Figure is an illustration of how to produce the coated particles of the invention. DETAILED DESCRIPTION OF THE INVENTION In order for inorganic coatings to uniformly coat inorganic particles such as phosphors, the inorganic coating particles must be much smaller than the particles to be coated. Preferably the particles to be coated are in the micrometer range, and the coating particles are in the nanometer range. Thus the coating particles are very much smaller than the particles to be coated. As used herein, the phosphor or other inorganic particles to be coated has a particle size of from about 1 to about 100 micrometers. They can be screened or ground to obtain the desired particle size in known manner . The inorganic coating particles, on the other hand, suitably have a particle size of from 1-2 to about 100 nanometers. Preferably, they have a particle size of about 3- 50 nanometers. The softening point or melting point of such small particles is generally lower than that of the bulk material . Suitable coating particles, depending on the need for the coated product, can be of metal oxides such as alumina, zinc oxide, titania and the like, or dielectrics such as silica or organic polymers. In accordance with the coating method of the invention, the micrometer sized particles to be coated and the nanometer sized particles of the coating material are mixed together dry in a ratio that permits a uniform distribution of the coating particles over the core inorganic particles to be coated. The resultant particles are then fired so as to form a thin layer of the coating material surrounding the inorganic core particles. The fired powders are rinsed or sprayed to remove any uncoated portion of the core particles, and the washed coated particles are dried. The mixing step is important. It must be conducted so as to ensure that the grains of the particles to be coated are fully surrounded with the nanometer sized coating particles, and mixing must be done so as to avoid the formation of agglomerates of the nanometer- sized coating particles and/or of the micrometer sized particles. For example, ceramic milling balls can be used during the mixing step to disperse the nanosized coating particles and to ensure that they are evenly distributed in the inorganic particle volume. Ultrasonication can also be used to prevent allomeration of the coating particles. When phosphor particles are to be coated with silica, the weight ratio of silica to phosphor can range from about 0.05 to 0.3, depending on the sizes of each type of particle. A motor driven blender or mechanical roller is suitable for the mixing step. Suitably, blending is carried out for at least about 20 minutes to ensure uniform coating of the inorganic micrometer-size particles. After blending the particles and milling to uniformly coat the inorganic micrometer-sized inorganic particles, the particles are fired at a temperature high enough so that the coating particles will soften or melt to form a thin layer on the inorganic particles, but low enough to leave the inorganic material substantially unchanged. In this regard, because of the very small size of the coating particles compared to the size of the particles to be coated, the melting point or softening point of the coating particles is generally significantly lowered relative to the bulk materials . For forming a silica layer on phosphor powders, firing can be carried out at a temperature of from about 500 to about 1100°C for from about 20 to 120 minutes. After the firing step, the coated particles are washed in a suitable solvent that will dissolve or degrade any uncoated particles remaining, leaving only water- impervious coated particles. The uncoated particles will be only a small portion of the fired particles. Lastly, the washed coated particles are dried, as by heating in air below or near the vaporization temperature of the washing solvent used. This invention will be further described in the following Examples, but the invention is not meant to be limited to the details described therein. Example 1 Fifty grams of dry SrS:Eu:Er:Dy phosphor powder that emits orange light for a few tens of minutes when excited with UV or visible light, and that has a particle size of from about 1 to 100 micrometers, was blended with five grams of fumed silica particles having an average particle size of about 7 nanometers. Fumed silica particles having a particle size of about 2-3 to 50 nanometers are commercially available as Aerosil 300 ® from the DeGussa Corporation. They have a softening point of about 800°C. The phosphor and silica particles were placed in a 200 ml cap sealed container and blended together for 30 minutes using a motor driven blender (TURBULA, a tradename of Glen Mills, Inc. of New Jersey) with the assistance of cylindrical ceramic milling balls about inch by % inch, also placed in the container. The ceramic milling balls prevent agglomeration of the powders and particles and are readily removed from the coated inorganic particles using an appropriately sized sieve. The blended powders were fired in a quartz tube-type furnace at 800°C under nitrogen. After firing, the volume of the powder mixture had shrunk in size, but no sintering was apparent. The phosphor body color remained unchanged. The coated phosphor was washed with water for ten minutes and dried at 59°C in air for 3 hours. The dried sample was immersed in water for four days. The phosphor body color remained orange and still glowed in the dark, confirming that the phosphor particles were now coated with a moisture-impermeable layer. In comparison, a sample of the same but uncoated phosphor particles was also immersed in water for four days. The body color became a yellowish white, and the glow was found to fade completely within one-half hour of immersion in water. Luminescent test results for uncoated and coated phosphor particles, before and after wetting, are given below in Table I. TABLE I Sample Description Relative Emission Intensity Initial After 15 minutes uncoated 0.88 0.77 B silica coated, unwetted 1.00 0.88 silica coated, wetted 0.82 0.72 Example 2 A fifty gram sample of a moisture sensitive long persistence red phosphor, CaS : Eu +2 Tm +3 , commercially available as "LUMILUX ® " from AlliedSignal, Inc, and 3 grams of Aerosil 300 ® were blended as in Example 1 in a 200 ml cap sealed container. This phosphor has a pink body color and emits red light for a few tens of minutes after being excited with UV/Visible light. The blended particles were fired for 40 minutes in a tube-type furnace at 800°C under a nitrogen purge gas. After firing, the phosphor was found to have shrunk in its volume, but no sintering was visible. The body color remained unchanged. The coated phosphor was washed with water, followed by drying at room temperature in air overnight. The dried sample was tested by immersing in water for two months. After this test, the body color remained pink and the nature of the emission was unchanged. An uncoated control sample of the same phosphor was also immersed in water. The body color turned pinkish-red and the light emission faded away within two days of immersion in water. Although the invention has been described in terms of particular coating materials and particular phosphors, the invention is not to be so limited. Providing the relative sizes of the particles to be coated and the coating particles disclosed are maintained, other materials can be coated or other inorganic nanometer sized particles can be used to impart other surface properties. Thus the invention is only meant to be limited by the appended claims.

Claims: I Claim : 1. A method of coating inorganic particles comprising mixing inorganic particles having a particle size of about 1 to 100 micrometers with inorganic coating particles having a particle size of about 1 to 100 nanometers, firing the mixture at a temperature high enough to soften or melt the nanometer-sized coating particles, but low enough so the surface properties of the micrometer-sized particles remain unchanged, removing any uncoated micrometer-sized particles by reacting or dissolving them in a solvent, and drying the coated particles. 2. A method according to claim 1 wherein the nanometer-sized particles have a particle size of from about 3-50 nanometers. 3. A method according to claim 1 wherein inert milling balls are added to the powders during the mixing step to prevent agglomeration of the nanometer-sized particles. 4. A method according to claim 1 wherein ultrasonication is used to prevent agglomeration of the nanometer-sized particles. 5. A method according to claim 1 wherein the micrometer-sized particles are phosphor particles. 6. A method according to claim 5 wherein the nanometer-sized particles are selected from the group consisting of silica, alumina, titania, zinc oxide and organic polymers. 7. A method according to claim 1 wherein the firing step is carried out at a temperature range of from about 500 to about 1100°C.

Inventors: TIAN YONGCHI, TIAN, YONGCHI
Applicants: SARNOFF CORP, SARNOFF CORPORATION
Application year: 2000
Main classification: C09K11
IPC codes: B01J2, B05D3, B05D7, C01B17, C09C3, C09K11