Method for coating small particles

Abstract

The coating method includes the steps of dissolving coating precursor(s) in a solvent to form a precursor solution: adding with mixing a miscible diluent to the precursor solution to form a coating solution; admixing solid particles to the coating solution to form a coating slurry, with the particles surrounded with the coating solution; spraying the coating slurry to form droplets containing at least one particle; passing the droplets through a drying zone where the droplets are dried and form dry particles coated with a coating material formed from the coating precursor(s); heat-treating the coating material on the particles emanating from the drying zone to remove volatile matter on the coating material, to improve integrity of the coating material and/or to effect another objective; and collecting dry coated particles.

1. A method for coating solid particles consisting essentially of the steps of
 (a) adding solid particles wherein solid particles are less than 100 microns in diameter to a liquid precursor coating solution to form a liquid slurry containing a precursor coating material, solvent for the precursor and the solid particles dispersed therein whereby the precursor coating material is not precipitated as a coating on the particle until after spraying, and mixing a precursor solution with a dilutent which dilutent is miscible with the precursor solution to form the coating solution, the precursor solution containing coating precursor during droplet formation, wherein the coating material is selected from the group consisting of indium tin oxide, silicon dioxide, magnesium oxide, sodium phosphate, yttrium-europium oxide, and mixtures thereof; and the precursor is selected from the group consisting of indium methyl (trimethyl) acetyl acetate, tin isopropoxide, tetraethyl orthosilicate, magnesium nitrate, yttrium chloride, europium chloride, sodium phosphate and mixtures thereof and wherein the particles are ZnS:Ag,Cl phosphor particles that can last for about 10,000 hours of continuous operation under accelerating voltages of 50-10,000 volts without losing 50% or more, of the original brightness,  
 (b) then spraying the liquid slurry to form droplets containing at least one particle,  
 (c) then passing the droplets through a zone where the droplets are dried and form dry coated particles wherein the coating material is then formed from the precursor wherein the residence time of the droplets through the zone is 1-5 seconds, the particles are phosphor particles, wherein temperature in the zone is elevated and the heat treatment of the coated particles is conducted at a temperature above the elevated temperature in the zone, and wherein the particles are less than about 50 microns in diameter, temperature in the zone is 100-500° C., dilution ratio in the coating slurry of milliliters of coating solution per gram of phosphor particles is 200-3000, thickness of the coating material on the particles is 2-200 nm, velocity of the droplets in the zone is 1-50 cm/sec. and residence time of the droplets in the zone is 0.1-10 seconds,  
 (d) then heat treating the coating material on the particles to remove volatile matter from the coating material wherein said heat treating step is carried out at 50-2000° C. over a period of 0.01-48 hours, and  
 (e) providing at least one more coating on the coated particles.  
 
     
     
       2. A method consisting essentially of the steps of
 (a) preparing a liquid precursor solution by dissolving a coating precursor in a liquid precursor solvent;  
 (b) mixing the precursor solution with a diluent, that is miscible with the precursor solvent, to form a liquid coating precursor solution;  
 (c) adding with mixing solid particles to the coating precursor solution to form a liquid slurry containing the coating precursor dissolved in the coating precursor solution and the solid particles dispersed therein whereby the precursor is not precipitated until after spraying;  
 (d) spraying the liquid slurry to form droplets containing at least one particle;  
 (e) passing the droplets through a zone where the droplets are dried and form dry particles coating with a coating material formed from the precursor solution;  
 (f) heat-treating the coating material on the particles to remove volatile matter on the coating material and to convert the coating material from electrically non-conducting amorphous to electrically conducting crystalline and/or to improve integrity of the coating material.  
 
     
     
       3. The method of  claim 2  wherein temperature in the zone is elevated and the heat treatment of the coating particles is conducted at a temperature above the elevated temperature in the zone, and the precursor(s) is selected from the group consisting of alkoxides, nitrates, sulfates, acetates, hydroxides, hydrates, chlorides, and mixtures thereof. 
     
     
       4. The method of  claim 3  wherein the particles are less than about 100 microns in diameter, dilution ratio in the coating slurry of milliliters of coating solution per gram of phosphor particles is 100-5000, thickness of the coating material on the particles is 1-1000 nm, velocity of the droplets in the zone is 0.1-100 cm/sec and residence of the droplets in the zone is from instantaneous to a fraction of a minute. 
     
     
       5. The method of  claim 4  wherein the particles are less than about 50 microns in diameter, temperature in the zone is 100-500° C., dilution ratio in the coating slurry of milliliters of coating solution per gram of phosphor particles is 200-3000, thickness of the coating material on the particles is 2-200 nm, velocity of the droplets in the zone is 1-50 cm/sec, and residence time of the droplets in the zone is 0.1-10 seconds. 
     
     
       6. The method of  claim 5  wherein the residence time of the droplets through the zone is 0.1-10 seconds, the particles are phosphor particles, and said heat treating step is carried out at 200-2000° C. over a period of 0.01-48 hours. 
     
     
       7. The method of  claim 6  wherein said heat-treating step is carried out at 300-1500° C. over a period of 0.1-24 hours and wherein the coating material is selected from the group consisting of inorganic, organic and inorganic/organic hybrids. 
     
     
       8. The method of  claim 7  wherein said heat-treating step is carried out at 300-1500° C. over a period of 0.1-24 hours and wherein the coating material is selected from the group consisting of indium tin oxide, silicon dioxide, magnesium oxide, sodium phosphate, yttrium-europium oxide, and mixtures thereof; and the precursor solution includes a precursor selected from the group consisting of indium methyl (trimethyl) acetyl acetate, tin isopropoxide, tetraethyl orthosilicate, magnesium nitrate, yttrium chloride, europium chloride, sodium phosphate and mixtures thereof. 
     
     
       9. A method for coating solid particles comprising the steps of:
 (1) adding solid particles to a liquid precursor coating solution to form a liquid slurry containing a precursor coating material, solvent for the precursor and the solid particles dispersed therein whereby the precursor coating material is not precipitated as a coating on the particle until after spraying;  
 (2) spraying the liquid slurry of step (1) to form droplets containing at least one particle;  
 (3) passing the droplets of step (2) through a zone where the droplets are dried and form dry coated particles wherein the coating material is formed from the precursor; and  
 (4) heat treating the coating material of step (3) on the particles to remove volatile matter from the coating material.  
 
     
     
       10. The method of  claim 9  wherein temperature in the zone is elevated and the heat treatment of the coated particles is conducted at a temperature above the elevated temperature in the zone, and the precursor is selected from the group consisting of alkoxides, nitrates, sulfates, acetates, hydroxides, hydrates, chlorides, and mixtures thereof. 
     
     
       11. The method of  claim 10  wherein the particles are less than about 100 microns in diameter, dilution ratio in the coating slurry of milliliters of coating solution per gram of phosphor particles is 100-5000, thickness of the coating material on the particles is 1-1000 nm, velocity of the droplets in the zone is 0.1-1000 cm/sec and residence of the droplets in the zone is from instantaneous to a fraction of a minute. 
     
     
       12. The method of  claim 11  wherein the particles are less than about 50 microns in diameter, temperature in the zone is 100-500° C., dilution ratio in the coating slurry of milliliters of coating solution per gram of phosphor particles is 200-3000, thickness of the coating material on the particles is 2-200 nm, velocity of the droplets in the zone is 1-50 cm/sec, and residence time of the droplets in the zone is 0.1-10 seconds. 
     
     
       13. The method of  claim 12  including the step of mixing a precursor solution with a diluent which diluent is miscible with the precursor solution to form the coating solution, the precursor solution containing coating precursor during droplet formation. 
     
     
       14. The method of  claim 13  wherein the residence time of the droplets through the zone is 1-5 seconds, the particles are phosphor particles, said heat treating step is carried out at 50-2000° C. over a period of 0.01-48 hours, wherein said heat-treating step is carried out at 300-1500° C. over a period of 0.1-24 hours, wherein the coating material is selected from the group consisting of indium tin oxide, silicon dioxide, magnesium oxide, sodium phosphate, yttrium-europium oxide, and mixtures thereof; and the precursor is selected from the group consisting of indium methyl (trimethyl) acetyl acetate, tin isopropoxide, tetraethyl orthosilicate, magnesium nitrate, yttrium chloride, europium chloride, sodium phosphate and mixtures thereof, and wherein the particles are ZnS:Ag,Cl phosphor particles that can last for about 10,000 hours of continuous operation under accelerating voltages of 50-10,000 volts without losing 50%, or more, of the original brightness. 
     
     
       15. The method of  claim 14  including the step of providing at least one more coating on the coated particles. 
     
     
       16. A method comprising the steps of:
 (1) preparing a liquid precursor solution by dissolving a coating precursor in a liquid precursor solvent;  
 (2) mixing the precursor solution of step (1) with a diluent, that is miscible with the precursor solvent, to form a liquid coating precursor solution;  
 (3) adding with mixing solid particles to the coating precursor solution of step (2) to form a liquid slurry containing the coating precursor dissolved in the coating precursor solution and the solid particles dispersed therein whereby the precursor is not precipitated until after spraying;  
 (4) spraying the liquid slurry of step (3) to form droplets containing at least one particle;  
 (5) passing the droplets of step (4) through a zone where the droplets are dried and form dry particles coated with a coating material formed from the precursor solution;  
 (6) heat-treating the coating material of step (5) on the particles to remove volatile matter on the coating material and to convert the coating material from electrically non-conducting amorphous to electrically conducting crystalline and/or to improve integrity of the coating material.  
 
     
     
       17. The method of  claim 1  wherein temperature in the zone is elevated and the heat treatment of the coated particles is conducted at a temperature above the elevated temperature in the zone, the precursor is selected from the group consisting of alkoxides, nitrates, sulfates, acetates, hydroxides, hydrates, chlorides, and mixtures thereof, wherein the particles are less than about 50 microns in diameter, temperature in the zone is 100-500° C., dilution ratio in the coating slurry of milliliters of coating solution per gram of phosphor particles is 200-3000, thickness of the coating material on the particles is 2-200 nm, velocity of the droplets in the zone is 1-50 cm/sec, and residence time of the droplets in the zone is 0.1-10 seconds, including the step of mixing a precursor solution with a diluent which diluent is miscible with the precursor solution to form the coating solution, the precursor solution containing coating precursor during droplet formation, the particles are phosphor particles, and said heat treating step is carried out at 300-1500° C. over a period of 0.1-24 hours. 
     
     
       18. The method of  claim 17  wherein the coating material is selected from the group consisting of indium tin oxide, silicon dioxide, magnesium oxide, sodium phosphate, yttrium-europium oxide, and mixtures thereof; and the precursor is selected from the group consisting of indium methyl (trimethyl) acetyl acetate, tin isopropoxide, tetraethyl orthosilicate, magnesium nitrate, yttrium chloride, europium chloride, sodium phosphate and mixtures thereof, wherein the particles are ZnS:Ag,Cl phosphor particles that can last for about 10,000 hours of continuous operation under accelerating voltages of 50-10,000 volts without losing 50%, or more, of the original brightness, and including the step of providing at least one more coating on the coated particles. 
     
     
       19. A method for coating solid particles comprising the steps of:
 (a) adding a solid particle to a liquid precursor coating solution to form a liquid slurry;  
 (b) preventing precipitation or gellation of said precursor coating material as a coating on said particle;  
 (b) forming a droplet of said liquid slurry whereby said droplet consists of said particle and said liquid precursor coating solution;  
 (c) forming a coating on said particle;  
 (d) heat treating said coating to remove volatile matter and to convert said coating from an electrically non-conducting amorphous coating to an electrically conducting crystalline and/or to improve integrity of said coating.