BACKGROUND OF THE INVENTION The present invention relates to dry-powdered, double-coated blue-color emitting(B) phosphor particles with silica and titanic-coupling for use in manufacturing a CRT screen assembly, a method of manufacturing them and a CRT comprising a screen assembly manufactured by using them, and more particularly to dry-powdered, double-coated B phosphor particles with silica and titanic-coupling having improved chargeability thereof in the triboelectrical charging or the corona charging and improved flow characteristics thereof, a method of manufacturing the silica-and-titanic-coupling coated B phosphor particles and a CRT comprising a screen assembly manufactured by using the silica-and-titanic-coupling coated B phosphor particles.

Referring to FIG. 1, a color CRT 10 generally comprises an evacuated glass envelope consisting of a panel 12, a funnel 13 sealed to the panel 12 and a tubular neck 14 connected by the funnel 13, an electron gun 11 centrally mounted within the neck 14 and a shadow mask 16 removably mounted to a sidewall of the panel 12. A three color phosphor screen is formed on the inner surface of a

display window or faceplate 18 of the panel 12.

The electron gun 11 generates three electron beams 19a or 19b, said beams being directed along convergent paths through the shadow mask 16 to the screen 20 by means of several lenses of the gun and a high potential applied through an anode button 15 and being deflected by a deflection yoke 17 so as to scan over the screen 20 through apertures or slits 16a formed in the shadow mask 16.

In the color CRT 10, the phosphor screen 20, as shown in FIG. 2, comprises an array of three phosphor elements R, G and B of three different emission colors arranged in a cyclic order of a predetermined structure of multiple-stripe or multiple-dot shape and a matrix of light-absorptive material surrounding the phosphor elements R, G and B.

A thin film of aluminum 22 overlies the screen 20 in order to provide a means for applying the uniform potential to the screen 20, increase the brightness of the phosphor screen and prevent the burning of the phosphor screen due to ions that are produced as the result of a decomposition by collision of gases residing on the tube with the electron beams emitted from the electron gun and the decreasing of the potential of the phosphor screen.

And also, a film of resin such as lacquer(not shown) may be applied between the aluminum thin film 22 and the phosphor screen to enhance the flatness and reflectivity of the aluminum thin film 22.

In a photolithographic wet process, which is well known as a prior art process for forming the phosphor screen, a slurry of a photosensitive binder and phosphor particles is coated on the inner surface of the faceplate. It does not meet the higher resolution demands and requires a lot of complicated processing steps and a lot of manufacturing equipments, thereby necessitating a high cost in manufacturing the phosphor screen. And also, it discharges a large quantity of effluent such as waste water, phosphor elements, 6th chrome sensitizer, etc., with the use of a large quantity of clean water.

To solve or alleviate the above problems, the improved process of eletrophotographically manufacturing the phosphor screen by utilizing dry-powdered phosphor particles is developed.

Our copending Korean patent application Serial No.

95-10420 filed on April 29, 1995 and assigned to the assignee of the present invention describes one method of electrophotographically manufacturing the phosphor screen assembly using dry-powdered phosphor particles sprayed through a nozzle from a hopper to the interior surface of the faceplate and charged by a corona charger, as is briefly explained in the following.

FIGs. 3A through 3E schematically show various steps in the above-described manufacturing method. FIG. 3A represents a coating step that forms an electrically conductive layer 132 is formed on the inner surface of the

faceplate 18 and overlies an photoconductive layer 134 on the conductive layer 132.

The conductive layer 132, for example, can be formed by conventionally applying a volatilizable organic conductive material consisting of about 1 to 50 weight % of a polyelectrolyte commercially known as Catfloc-c, available from Calgon Co., Pittsburgh, Pa., to the inner surface of the faceplate 18 in an aqueous solution containing about 1 to 50 weight % of 10% poly vinyl alcohol and drying the solution. Said conductive layer 132 serving as an electrode for the overlying photoconductive layer 134. The photoconductive layer 134 is formed by conventionally applying to the conductive layer 132, a novel photoconductive solution containing ultraviolet-sensitive material and by drying it.

The ultraviolet-sensitive material can consist of bis dimethyl phenyl diphenyl butatriene, and one of trinitro fluorenone(TNF), ethylanthraquinone(EAQ) and their mixture. The photoconductive solution is prepared by dissolving 0.01 to 10 % by weight of the ultraviolet-sensitive material and 1 to 30 % by weight of polystyrene as a polymeric binder in a suitable solvent such as toluene or xylene.

FIG. 3B schematically illustrates a charging step, in which the photoconductive layer 134 is charged to a positive potential of less than 1 Kvolt, preferably above 700 volts by a corona discharger 3b. The charging step does not require a dark environment since the

photoconductive layer 134 is sensitive to ultraviolet rays below about 450nm of wave length.

FIG. 3C schematically shows an exposing step. The shadow mask 16 is inserted in the panel 12 and the positively charged photoconductive layer 134 is selectively exposed through an ultraviolet-transmissive lens system 140 and apertures or slits 16a of the shadow mask 16 to the ultraviolet rays from a ultraviolet lamp 138 with each predetermined incident angle with respect to each aperture or slit 16a. The charges of the exposed areas are discharged through the grounded conductive layer 132 and the charges of the unexposed areas remain in the photoconductive layer 134, thus establishing a latent charge image in a predetermined array structure. This exposing step also does not require a dark environment since the ultraviolet rays are used. Three exposures with three different incident angles of the three electron beams, respectively are required for forming a light-absorptive matrix.

FIG. 3D diagrammatically illustrates the outline of a developing step, in which, after removing the shadow mask 16, suitably charged, dry-powdered particles such as particular color-emitting phosphor particles or light-absorptive material particles are sprayed by compressed air toward a photoconductive layer 134 through a venturi tube 146 and a nozzle 144b from a hopper 148 and attracted to one of the charged or unexposed areas. The discharged or exposed areas depend upon the polarity of

the charged particles due to electrical attraction or repulsion, thus one of the two areas is developed in a predetermined array pattern. Below the nozzle 144b, there is provided a discharge electrode 144a such as a corona discharger for charging dry-powdered particles to be sprayed in the nozzle 144b. The light-absorptive material particles for directly developing the unexposed or positively charged areas are negatively charged and the phosphor particles are positively charged for reversely developing the exposed or discharged areas. The charging of the dry-powdered particles may be executed by a triboelectrical charging method disclosed in U.S. Pat. No.

4,921,767 issued to Datta at al. on May 1, 1990 using surface-treated carrier beads.

FIG. 3E schematically illustrates a fixing step using a vapor swelling method. In the fixing step, the surface of the polymers-contained photoconductive layer 134, in which the particles are attracted in a predetermined array, is applied to solvent vapor such as acetone, methyl isobutyl ketone, etc. Then, polymers contained in the photoconductive layer 134 are dissolved and the dry-powdered particles deposited on the developed areas of the photoconductive layer 134 are fixed by an adhesive property of said dissolved polymers.

Turning to FIG. 3D, since the dry-powdered particles should flow from the hopper 148 through the venturi tube to the nozzle 144b without adherence and be sprayed at the nozzle 144b toward and over the whole surface of the

photoconductive layer 134 by the compressed air, and should be sufficiently charged with a corona discharge electrode 144a in the developing step, excellent flow and charge characteristics are required on the surfaces of the dry-powdered particles. However, although the dry-powdered phosphor particles are coated with a polymethyl methacrylate layer and an overlying polyacrylamide layer in order to provide charge characteristics, said coatings still do not meet a uniform and sufficient chargeability demand and a fluidity demand in the phosphor particles.

Therefore, there are problems in the above-described process that the phosphor particles are not charged uniformly and sufficiently with a corona discharge electrode 144a in the aforementioned developing step or by a triboelectrical charging method as in the developing step described in U.S. Pat. 4,921,767, and adhere to each other and the wall of the hopper 148 or the tube between the hopper 148 and the nozzle 144b. Also, the fluidity problem is caused between the phosphor particles and the carrier beads when they are mixed and generate the triboelectric charge in the developing step described in U.S. Pat. 4,921,767, cited above.

It is an object of the present invention to provide silica-and-titanic-coupling coated B phosphor particles having excellent flow and charge characteristics, for use in manufacturing a screen assembly for a CRT.

It is another object of the present invention to provide a method of manufacturing the

silica-and-titanic-coupling coated B phosphor particles having the excellent flow and charge characteristics.

It is yet another object of the present invention to provide a CRT comprising a screen assembly comprising silicon and titanium in the surface thereof by being manufactured using the above silica-and-titanic-coupling coated B phosphor particles.

SUMMARY OF THE INVENTION In accordance with one object of the present invention, it is provided with dry-powdered B phosphor particles for use in manufacturing a CRT screen, said B phosphor particles having silica below 0.5 weight % and titanic-coupling below 2 weight % which are dispersed and coated on the surface thereof for improved flow and charge characteristics.

In accordance with another object of the present invention, a method of manufacturing dry-powdered, silica and titanic-coupling coated B phosphor particles having improved flow and charge characteristics for use in manufacturing a CRT screen comprises (A) first polymer- coating steps of: (a) dispersing polymers of 5 weight % in organic solvent; (b) slowly adding dry-powdered B phosphor particles to said polymer-dispersed organic solvent; (c) filtering said resultant organic solvent of the adding step; (d) drying the filtered resultant of the filtering step; and (e) sieving, through a screen, said dried resultant of the drying step, thereby obtaining

dry-powdered, polymer-coated B phosphor particles; (B) silica-coating steps of: (f) first-dispersing silica particles in methanol; (g) slowly adding said dry-powdered, polymer-coated B phosphor particles to said silica-dispersed methanol; (h) second-dispersing methanol in the silica-dispersed, B phosphor particles-adding methanol; (i) filtering said resultant methanol of the second-dispersing step; (j) drying the filtered resultant of the filtering step; and (k) sieving, through a screen, said dried resultant of the drying step, thereby obtaining dry-powdered, silica-coated B phosphor particles; and (C) third titanic-coupling-coating steps of: (1) first-dispersing titanic-coupling particles in organic solvent like N-nucleic acid; (m) slowly adding the dry-powdered, silica-coated B phosphor particles to said titanic-coupling-dispersed solvent; (n) second-dispersing organic solvent like N-nucleic acid in the dry-powdered, silica-coated B phosphor particles-adding solvent; (o) filtering said resultant methanol of the second-dispersing step(n); (p) drying the filtered resultant of the filtering step(o); and (q) sieving, through a screen, said dried resultant of the drying step(p), thereby obtaining dry-powdered, silica and titanic-coupling coated B phosphor particles.

It is desirable that said first-dispersing steps and said second-dispersing steps be performed by ultrasonic waves; said filtering steps be performed by a glass frit filter; and said drying steps be performed below 100

degrees centigrade for about 3 to 5 hours.

In accordance with still another object of the present invention, a method of electrophotographically manufacturing a luminescent screen on an interior surface of a faceplate panel for a CRT comprises the steps of: (a) first-coating said surface of the panel with a volatilizable conductive layer; (b) second-coating said conductive layer with a volatilizable photoconductive layer; (c) establishing a substantially uniform electrostatic charge over the whole area of the inner surface of said photoconductive layer; (d) exposing selected areas of said photoconductive layer to visual lights or ultraviolet rays through a shadow mask to discharge the charge from the selected areas of the inner of the photoconductive layer; (e) developing the discharged, exposed areas with charged first color-emitting phosphor particles after removing the shadow mask; (f) repeating said steps (c) to (e) for charged second and third color-emitting phosphor particles; and (g) fixing said developed three color-emitting phosphor particles to said photo conductive layer to form a luminescent screen comprising picture elements of triads of color-emitting phosphors; said first to third color-emitting phosphor particles comprising dry- powdered B phosphor particles on the surface of which silica below 0.5 weight % and titanic-coupling below 2 weight % are dispersed and coated for improved flow and charge characteristics.

In accordance with still another object of the present invention, a CRT comprises a luminescent viewing screen and means for selectively exciting areas of said screen to luminescence, said screen comprising picture elements of red or green color emitting phosphors on the surface of which silicon and titanium are dispersed and coated. It is desirably that said silicon be below 0.5 weight % and titanium be below 2 weight %.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view partially in axial section of a color cathode-ray tube.

FIG. 2 is a section of a screen assembly of the tube shown in FIG.1.

FIGs. 3A through 3E show a series of steps in electrophotographically manufacturing the screen assembly with dry-powdered, silica-and-titanic-coupling coated B phosphor particles in accordance with the present invention.

FIG. 4A is an enlarged section of one B phosphor particle according to an example of the present invention for electrophotographically manufacturing the screen, and FIG. 4B is an enlarged section of one B phosphor particle constituting a completed phosphor screen assembly made using B phosphor particles of FIG. 4A.

FIG. 5 is a graph of the resistivity of R phosphor particles according to various examples of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A double-coated B phosphor particle P with silica C1 and titanic-coupling C2 according to one embodiment of the present invention is schematically shown in FIG. 4A, and a B phosphor particle P on which silicon Si and titanium Ti are remained after baking is schematically shown in FIG. 4B.

First, a method of manufacturing dry-powdered, polymer-coated B phosphor particles according to one embodiment of the present invention is as follows.

5 weight % polymers of B phosphor particles are dispersed and dissolved in organic solvent and then dry-powdered B phosphor particles are slowly added to said polymer-dispersed organic solvent. After said resultant organic solvent of the adding step is filtered and dried, the dried resultant is sieved through a screen, thereby obtaining dry-powdered, polymer-coated B phosphor particles.

Said polymers are selected from polymethyl- methacrylate, polystyrene oxazoline copolymer, poly a- methylstyrene, PIBMA, etc., and said organic solvent includes xylene, toluene, MIBK, etc. The reason of coating B phosphor particles with polymer is that it has been found out that, during dispersing of B phosphor particles in a suitable solvent, organic polymer compound contained in the B phosphor particles is dissolved in the solvent, thereby decreasing electric resistivity of the B phosphor particles.

Second, a method of manufacturing dry-powdered, silica-coated B phosphor particles is as follows.

Initially, 1 gram of silica particles is dispersed in one liter of methanol by conventional mixing method and preferably ultrasonic waves. Then, 1 kilogram of said dry-powdered, polymer-coated B phosphor particles is slowly added into the silica-dispersed methanol.

After said B phosphor particles are added, 0.5 liter of methanol is secondly dispersed into the resultant B phosphor particles-added, silica-dispersed methanol by the conventional mixing method and preferably by ultrasonic waves. And then, the desired B phosphor particles are filtered from the resultant mixture through the conventional filter and preferably through the glass frit filter, dried at the temperature of 60 to 80 degrees centigrade for 2 to 3 hours and then sieved through a 400 mesh screen. The resultant dry-powdered B phosphor particles have silica particles C1 dispersively coated on the B phosphor particles.

The desirably silica-coated, dry-powdered B phosphor particles obtained by the above-described manufacturing method may be used in the developing step in the electrophotographic process of manufacturing a phosphor screen assembly as described in relation to FIGs. 3A to 3E, in case that, prior to said silica coating, B phosphor particles are coated with a polymethyl methacrylate layer and an overlying polyacrylamide layer by a conventional method to provide the electrical charge characteristics on

the B phosphor particles.

Since the resultant dry-powdered, silica-coated B phosphor particles have the excellent flow characteristics due to the slippery nature of the silica particles, the cohesion phenomena of the B phosphor particles as described in the above as one problem of the electrophotographic process is almost removed.

Accordingly, the B phosphor particles do not easily adhere to each other and the wall of the hopper or the tube during the develop step.

Also, in the baking step after the screen is formed using said resultant B phosphor particles, the volatilizable constituents of the screen including the conductive layer 132, the photoconductive layer 134, the polymethyl methacrylate layer, the polyacrylamide layer and the remaining solvents can be easily driven off even in the shortened heating time or on the lower temperature and the desirable array of the B phosphor particles can be obtained since the silica is dispersed and coated on the surfaces of the B phosphor particles.

Third, a titanic-coupling C2, as shown in FIG. 4A, is dispersed and coated on the resultant dry-powdered B phosphor particles with silica C1 coated, instead of prior coating of the polymethyl methacrylate layer and the overlying polyacrylamide layer for providing the electrical charge characteristics on the B phosphor particles. The titanic-coupling comprises hydrophobic property group and hydrophilic property group. The

structural formula of the titanic-coupling is as follows for an example: Since stearyl(C17H35) is hydrophobic property group and isopropoxy(C3H70) is hydrophilic property group, said titanic coupling is dispersed and coated on B phosphor particles and joined with metal of metallic sulfur compound or acid sulfur compound of B phosphor particles.

As it were, in said structural formula, SiO2 of B phosphor particle surface is combined instead of hydrophilic property group ( C3H70).

Thus the phosphor particles with titanic-coupling coated are easily charged with positive electricity by C17H35 of hydrophobic property group as shown in FIG. 4A.

But, the electrical chargeability is come down when titanic-coupling is above 1 % of weight.

One embodiment of the titanic-coupling coating method on the resultant silica-coated B phosphor particles is as follows.

Initially, 10 grams of titanic-coupling are dispersed in 1 liter of organic solvent like N-nucleic acid by conventional mixing method and preferably ultrasonic

waves. The titanic-coupling is commercially known as KR TTS, KR 46B, KR 55, KR 41B, KR 38S, KR 138S, KR 238S, 338X, KR 12, KR44, KR 9SA, KR34S, ETC.

Then, 1 kilogram of the aforementioned dry-powdered, silica-coated B phosphor particles are slowly added into the titanic-coupling dispersed N-nucleic acid, and then 0.5 liter of organic solvent like N-nucleic acid is dispersed by conventional mixing method and preferably by ultrasonic waves.

After the resultant is filtered by glass frit filter, the filtered resultant is dried at the temperature of 60 to 80 degrees centigrade for 2 to 3 hours, and then sieved through a 400 mesh screen.

The resultant dry-powdered, silica-and-titanic-coupling coated B phosphor particles have high electrical charge characteristics with excellent fluidity. In FIG. 5, said double-coated B phosphor particles with 1 wt. % of titanic-coupling KRTTS has higher electrical resistivity than that of said double- coated B phosphor particles with 0.5 wt. % of titanic-coupling KRTTS, and thereby has higher electrical charge characteristics of excellent electrical resistivity by more than 1014 ohms-cm. But, the electrical chargeability decreases when titanic-coupling is above 2 % of weight.

After the screen is formed using said resultant double coated B phosphor particles with silica C1 and titanic-coupling C2 shown in FIG. 4A by the method

described in relation to FIGs. 3A to 3E, a spray film of lacquer is applied to the screen and then an aluminum thin film is vapor-deposited onto the lacquer film, as is known in the art. The screen is baked at a high temperature, as is known in the art and then the volatilizable constituents of the screen including the conductive layer 132, the photoconductive layer 134, the hydrophobic group, the hydrophilic group, etc., are driven off, thus the colorless transparent silicon particles Si and titanium particles Ti dispersively remaining on the surfaces of the B phosphor particle P as shown in FIG. 4B.

Since the resultant B phosphor particles are easily charged with positive electric charge on the surface of the phosphor particles by rubbing or discharge electrode, therefore the time of developing is shortened in the developing step, volatile constituents are easily removed in the baking step because silica and titanic-coupling are dispersed and coated on the surface of B phosphor particles, and the manufactured screen structure has an even thickness.

It should be clear to one skilled in the art that the present B phosphor particles can be used for electrophotographically manufacturing the screen by the method as described in U.S. Pat. 4,921,767 and can also be used in the conventional wet process, and that the present process for obtaining the phosphor particles with the flow and charge characteristics can be modified within the scope of the present invention.