Abstract:
An apparatus for electrostatically coating powder on a flat panel and a method for using such apparatus are provided. In the apparatus, a heating device is used to heat a panel to be coated and positioned on a powder spray chamber to a temperature of at least 100° C. and simultaneously charging the panel with a negative voltage of at least 20,000 volts. The high temperature and the high voltage of the otherwise non-conductive panel result in a panel that is electrostatically conductive. Powder particles that are charged with a high positive voltage are then injected toward the panel surface by an air pressure of at least 0.8 kg/cm 2  until particles are adhered to the panel surface. The present invention novel method can be used advantageously at low costs due to its simple processing steps and as an environmentally friendly process since no solvent needs to be evaporated and released into the atmosphere. The present invention novel method further provides a low cost method for fabricating flat panel display devices due to its simplified processing steps.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to an apparatus and a method for coating a powder material on a flat panel and more particularly, relates to an apparatus and a method for coating a fluorescent powder on a substantially flat, non-electrically conducting panel by an electrostatic coating method in which the panel is heated and negatively charged by a gas flame. 
     BACKGROUND OF THE INVENTION 
     In recent years, flat panel display devices have been developed and widely used in electronic applications such as personal computers. One of such devices is a field emission display (FED) device that overcomes some of the limitations of liquid crystal displays and provides significant advantages over the traditional LCD devices. For instance, the field emission display devices have higher contrast ratio, larger viewing angle, higher maximum brightness, lower power consumption and a wider operating temperature range when compared to a conventional thin film transistor liquid crystal display panel. 
     One of the most drastic difference between a FED and a LCD is that, unlike the LCD, FED produces its own light source utilizing colored phosphor. The FEDs do not require complicated, power-consuming backlights and filters and as a result, almost all the light generated by a FED is visible to the user. Furthermore, the FEDs do not require large arrays of thin film transistors, and thus, a major source of high cost and yield problems for active matrix LCDs is eliminated. 
     In a FED, electrons are emitted from a cathode and impinge on a fluorescent coating layer, such as a phosphor layer on the back of a transparent cover plate to produce an image. Such a cathodoluminescent process is known as one of the most efficient methods for generating light. Contrary to a conventional CRT device, each pixel or emission unit in a FED has its own electron source, i.e., typically an array of emitting microtips. A voltage difference existed between a cathode and a gate attracts electrons from the cathode and accelerates them toward the phosphor coating. The image produced by the phosphor coating is therefore largely dependent on the quality and the uniformity of the phosphor coating. The process for coating a phosphor layer on a transparent glass plate is therefore an important step in the total fabrication process of a FED. 
     Conventionally, the coating of a fluorescent powder layer on a flat transparent plate (or on a flat bulb) is accomplished by either a screen printing or a dip coating process. In either process, the screen printing paste or the dip coating emulsion contains a large amount of solvent which is used in forming the paste or the emulsion. After a paste is printed on a glass plate or after a glass plate is dip coated in an emulsion, the glass plate must be oven baked to evaporate all the solvent. The screen printing and the dip coating process are therefore not only complicated fabrication processes, but also processes that generate pollution for the environment for release of evaporated solvent into the atmosphere. The complicated fabrication process further result in a low throughput of the device fabricated. 
     It is therefore an object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel that does not have the drawbacks or shortcomings of the conventional apparatus. 
     It is another object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel that operates on an electrostatic coating principle. 
     It is a further object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel electrostatically that utilizs gas flames for heating the panel and negatively charging the panel. 
     It is another further object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel that is capable of rotating a flat panel mounted on a powder spray chamber during the electrostatic coating process. 
     It is still another object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel that is capable of heating the panel to a temperature of at least 100° C. and simultaneously charging the panel with a negative voltage of at least 20,000 volts. 
     It is yet another object of the present invention to provide an apparatus for coating a fluorescent powder on a flat panel that is capable of electrostatically coating the powder on a panel that is electrically non-conductive. 
     It is still another further object of the present invention to provide a method for coating a fluorescent powder on a flat panel by simultaneously heating a glass panel to a high temperature and negatively charging the panel to a high negative voltage and directing positively charged powder particles at the panel surface. 
     It is yet another further object of the present invention to provide a method for coating a fluorescent powder on a flat panel by simultaneously heating, rotating and negatively charging the panel and directing positively charged powder particles under a high air pressure toward the panel surface. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an apparatus and a method for coating a fluorescent powder on a substantially flat panel are provided. 
     In a preferred embodiment, an apparatus for electrostatically coating powder on a panel is provided which includes a chamber that has a top opening, a bottom opening and a cavity contained therein, a panel positioned on top of the chamber substantially sealing the top opening, the panel has a bottom surface to be coated which is exposed in the top opening to the cavity, a heating means for heating the panel on a top surface opposite to the bottom surface and for electrically grounding the flat panel, a powder spray nozzle adapted for engaging the bottom opening of the chamber and for receiving a positive voltage forming a closed-loop circuit with the heating means, means for rotating the chamber and the panel positioned on the chamber, and high pressure means for flowing a powder through the spray nozzle and for charging the powder with a positive electrostatic charge such that the powder is attracted toward the negatively charged bottom surface of the panel. 
     The apparatus for electrostatically coating powder on a flat panel may further include a chamber that has a columnar shape with a diameter at the top larger than a diameter at the bottom. The apparatus may further include a chamber fabricated of glass that is capable of withstanding a temperature of at least 100° C. The panel may be a glass panel for coating a fluorescent powder thereon. The panel may be a glass panel for fabricating a flat panel display device. The apparatus may further include a heating means that includes a plurality of gas nozzles for forming a layer of gas flame when ignited. The apparatus may further include a heating means that includes a plurality of gas nozzles for heating the panel to a temperature between about 100° C. and about 250° C., and preferably between about 100° C. and about 200° C. 
     The apparatus for electrostatically coating powder on a panel may furter include a heating means that includes a plurality of gas nozzles for heating the panel and for providing electrical conductance to the panel such that the panel may be grounded. The powder spray nozzle may further include a multiplicity of apertures for passing powder therethrough, the multiplicity of apertures may each have a passageway formed at an angle between about 0° and about 45° when measured from a vertical axis. The positive voltage received by the powder spray nozzle is between about 20,000 volts and about 50,000 volts. 
     The apparatus for electrostatically coating powder on a panel may further include a rotor member engaging the chamber for providing a rotational motion of the chamber at between about 1 RPM and about 20 RPM. The apparatus may further include a gas supply tank for delivering a flammable gas to the heating means. The apparatus may further include a powder recovery unit for recovering powder that does not stick to the bottom surface of the panel from the chamber cavity, the powder recovery member is situated juxtaposed to the powder spray nozzle. The high pressure means flows a powder through the powder spray nozzle at a pressure between about 0.8 kg/cm 2  and about 8 kg/cm 2 . The apparatus may further include a powder supply means for delivering a powder to the spray nozzle under a compressed air pressure, the powder supply means may further include an agitation means for insuring a free flow of the powder. 
     The present invention is further directed to a method for electrostatically coating a powder on a panel that includes the steps of first providing a powder spray chamber that has a top opening for engaging a panel to be coated thereon and for exposing a first surface of the panel to be coated to a cavity in the chamber, heating the panel on a second surface opposite to the first surface with a gas flame to at least 100° C. and simultaneously charging the second surface with a negative voltage of at least 20,000 volts, rotating the chamber with the panel on top by a rotating means, and injecting a positively charged powder into the cavity by air pressure through a bottom opening of the chamber such that powder is attracted towards and adheres to the negatively charged first surface of the panel. 
     The method for electrostatically coating a powder on a panel may further include the step of heating the panel with a gas flame from a plurality of gas nozzles positioned juxtaposed to the second surface of the panel until a temperature between about 100° C. and about 250° C. is reached. The method may further include the step of rotating the chamber with a rotating means which frictionally engaging the powder spray chamber at a rotational speed between about 1 RPM and about 20 RPM. The method may further include the step of charging the second surface of the panel with a negative voltage between about 20,000 volts and about 50,000 volts. The method may further include the step of injecting the positively charged powder into the chamber by an air pressure of at least 0.8 kg/cm 2 . 
     The method for electrostatically coating a powder on a flat panel may further include the step of injecting the positively charged powder that is charged with a voltage of at least 20,000 volts into the cavity. The method may further include the step of injecting the powder through a powder spray nozzle equipped with a plurality of apertures. The method may further include the step of supplying the powder from a powder supply hopper equipped with a stirrer. The method may further include the step of recovering unused powder from the chamber cavity for recycling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph illustrating the present invention apparatus for electrostatically coating a flat panel mounted on a powder spray chamber. 
     FIG. 1A is an enlarged, cross-sectional view of the powder spray chamber of FIG. 1 illustrating the rotating mechanism for the chamber. 
     FIG. 2 is a perspective view of a flat panel that is provided with a side frame. 
     FIG. 3 is a perspective view of a flat panel that is provided with spacers mounted on the surface of the panel. 
     FIG. 4 is a graph illustrating the dependence of percent transmittance on the air pressure used for injecting powder into the chamber. 
     FIG. 5 is a graph illustrating the dependence of light transmittance on the air pressure for blowing powder from a powder supply hopper into the powder supply line to the chamber when the apertures in the powder spray nozzle are formed at 60°. 
     FIG. 6 is a graph illustrating the dependence of light transmittance on the air pressure used for feeding powder from a powder supply hopper into the supply line to the chamber when the apertures in the powder spray nozzle are formed at 70°. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention discloses an apparatus and a method for coating a fluorescent powder on a substantially flat panel that is electrically non-conductive. In the apparatus, a novel heating apparatus is utilized for heating the panel and for negatively charging the panel simultaneously such that positively charged powder particles can be attracted and adhered to the panel. 
     The present invention is provided to eliminate the drawbacks of the conventional coating methods which involve time-consuming processing steps and environmentally polluting solvents. The method utilizes an electrostatic coating principle wherein a high voltage current and a gas flame are utilized to change the electric characteristics of a non-conductive substrate such that it becomes conductive for the electrostatic coating process. 
     The present invention novel method effectively reduces or eliminates the use of organic solvents in a paste or in an emulsion and furthermore, increases the throughput of a fabrication process. The present invention novel method is therefore especially suited for a mass production process that can be carried out at low cost. 
     The present invention novel method can be carried out by first heating a glass substrate, and then charging the substrate with a high voltage through the ions contained in the gas flame such that a negative electrostatic charge is produced on the glass substrate. A positively charged fluorescent powder, such as phosphor is then injected toward the glass plate by high pressure compressed air through a powder spray nozzle for coating the glass substrate. The uniformity and the thickness of the coating layer obtained can be determined by adjusting the high pressure air flow. 
     In a flat panel display device, a small amount of an inert gas is charged into a sealed cavity between two flat plates, after a high voltage is inputted at the two extreme ends of the cavity, a UV radiation is emitted from the inert gas. The UV radiation activates the fluorescent powder coated surfaces of the two flat plates such that the UV frequency is transformed into visible light frequency by the fluorescent powder. 
     Referring initially to FIG. 1, wherein a present invention apparatus  10  for coating a fluorescent powder on a flat panel is shown. The apparatus  10  consists mainly of a powder spray chamber  20 , a heating device  30 , a powder spray nozzle  40  and a powder recovery unit  50 . In the powder spray chamber  20 , a top opening  22 , a bottom opening  24  and a cavity  26  contained therein are provided. A substantially flat panel  28  with a surface  32  to be coated is positioned on the top opening  22  with the surface  32  facing the cavity  26 . A powder spray nozzle  40  is engaged at the bottom opening  24  and substantially seals the bottom opening with a channel  36  for collecting unused powder and delivering to the powder recovery unit  50 . Magnetic shut-off valves  38 ,  42  and  44  are further utilized to control the flow of the recovered powder and the flow of the powder being fed into the cavity  26 , respectively. 
     The flat panel  28  which has a surface  32  to be coated, when positioned on the top opening  22 , substantially seals the opening to form an air tight enclosure for cavity  26 . On the uncoated side  34  of the flat panel  28 , a heating device  30  which consists of a brass plate  46  and a multiplicity of gas nozzles  48  are utilized to supply heat thereto. A multiplicity of gas flames are generated from the multiplicity of gas nozzles  48  such that a uniformly distributed heat can be applied on the panel surface  34 . 
     Simultaneously, a negative voltage of between about 20,000 volts and about 50,000 volts is applied to the brass plate  46  by a power source  52 . A flammable gas is supplied from the gas supply  54  into the multiplicity of gas nozzles  48  for heating the flat panel  28 . A suitable flammable gas for use in the present invention novel apparatus is propane. 
     To practice the present invention novel method, the flat panel  28  should be heated to a temperature of between about 100° C. and about 250° C., and preferably between about 100° C. and about 200° C. to carry out the electrostatic coating process. The voltage utilized from the power source  52  should be between about 20,000 volts and about 50,000 volts, and preferably between about 30,000 volts and about 40,000 volts. 
     The powder spray chamber  20  can be constructed in a columnar shape with a larger diameter at the top and a smaller diameter at the bottom. A suitable size for the chamber is approximately 6 inches in height and 3 inches in diameter measured at the center of the chamber. 
     A powder supply hopper  60  is utilized to supply a powder coating material to the input line  62  and then into the powder spray nozzle  40 . The powder supply hopper  60  is equipped with a stirrer  64  for insuring an uninterrupted flow of powder through the input conduit  62 . The feeding of powder is made possible by a high pressure compressed air source  70  which is capable of supplying an air pressure of between about 0.8 kg/cm 2  and about 8 kg/cm 2 . The input conduit  60  utilized may have a diameter of about 10 mm. The flow of the compressed air from source  70  can be controlled by a magnetic shut-off valve  44  by reading the pressure gauge  56 . The pressure indicated on the pressure gauge  56  is R 1 , while the pressure indicated on the pressure gauge  58  is R 2 . As shown in FIG. 1, the R 1  pressure is controlled to feed powder from the powder supply hopper  60  into the input conduit  62 , while the R 2  pressure is used to feed powder from input conduit  62  into the powder spray nozzle  40 . The powder spray nozzle  40  is equipped with a multiplicity of apertures (not shown) such that fine particles  66  of the powder can be injected into the cavity  26 . The plurality of apertures provided in the powder spray nozzle  40  are normally formed at an angle of between about 0° and about 45° when measured from A vertical axis such that powder particles  66  may be sprayed out in a fan pattern toward the flat panel  28 . 
     It should be noted that, during the electrostatic coating process, the flat panel  28  is first heated to a temperature of at least 100° C. and simultaneously, charged with a negative voltage of at least 20,000 volts such that an electrostatic charge is produced in the flat panel  28 . The flat panel  28 , as shown in FIG. 1A, is further rotated at a rotational speed of between about 1 RPM and about 10 RPM by a mechanism shown in FIG.  1 A. 
     As shown in FIG. 1A, in order to advantageously rotate the flat panel  28  during the coating process, the powder spray chamber  20  is first mounted on a rotor member  80  which consists of a silicon rubber coated collar  82  for frictionally engaging the bottom opening  24  of the chamber  20 , a sleeve  84  into which the silicon rubber coated collar  82  is mounted, and a pulley  86  which is driven by a belt  88  by an electric motor  90 . The powder spray chamber  20  frictionally engages the silicon rubber coated collar  82  and rotates with the collar  82  at a suitable rotational speed between about 1 RPM and about 10 RPM during the electrostatic coating process. 
     It should be noted that the powder spray nozzle  40  is mounted on a fixed shaft  76  and thus does not rotate with chamber  20  during the electrostatic spraying process. As shown in FIG. 1, the unused powder, i.e., the powder which does not adhere to the surface  32  of the flat panel  28 , is collected by the recovery channel  36 , controlled by the magnetic shut-off valve  38  into a powder recovery unit  50  by a vacuum force provided (not shown). After each spray coating process, magnetic shut-off valves  42  and  44  are closed, while the magnetic shut-off valve  38  is opened to recover the unused powder from the chamber cavity  26 . 
     Each of the electrostatic coating process takes approximately between about 1 second and about 5 seconds, and preferably between about 1 second and about 2 seconds. A suitable coating thickness obtained on the flat panel  28  is between about 10 μm and about 50 μm, and preferably between about 20 μm and about 40 μm. In the powder material stored in the powder supply hopper  60 , less than 5 wt % of a flow enhancer is also added to insure the free flow of powder through the input conduit  62  to the powder spray nozzle  40  under air pressure. A suitable particle size of the powder used is between about 1 μm and about 20 μm and preferably between about 3 μm and about 7μm. A frequently used fluorescent powder material is of the phosphor type. The size of the flat panel coated in the illustration shown in FIG. 1 is normally between about 2 inches and about 5 inches in length on adjacent sides. 
     FIGS. 2 and 3 illustrate the types of flat panels that can be suitably processed by the present invention novel apparatus shown in FIGS. 1 and 1A. For instance, FIG. 2 shows a flat panel  92  that is equipped with a frame member  94 , while FIG. 3 illustrates a flat panel  96  which is equipped with spacers  98 . Neither of these panels  92 ,  96  can be processed in a conventional apparatus for screen printing. While a dip coating method is not only time consuming, but also environmentally hazardous due to the evaporation of solvents contained in the coating material. 
     In operating the present invention novel apparatus, the dependency of percent transmittance through the coating layer on the air pressure (R 2 ) for feeding into the chamber cavity is shown in FIG.  4 . When the air pressure is formed at 0.5 kg/cm 2  with the apertures in the powder spray nozzle maintained at 60° angle, the percent transmission increases with an increase in R 2  pressure. This is indicative of the fact that a thinner coating is formed on the flat panel surface which causes an increase in the percent transmission. At higher R 2  pressures, lesser coating material is being coated on the panel surface since the powder is being blown into a more scattered pattern. 
     FIGS. 5 and 6 shows an effect that is contrary to that shown in FIG. 4 when the R 1  pressure (the pressure for carrying powder from the powder supply hopper) is increased. FIG. 5 illustrates data obtained when the R 2  pressure is kept constant at 0.8 kg/cm 2  with the apertures at 60° angle. FIG. 6 is obtained from data with the R 2  pressure kept constant at 0.8 kg/cm 2  while the aperture angle at 70°. It is assumed that more powder material is carried by higher R 1  pressure into the cavity, resulting in a heavier coating being formed on the flat panel surface, and thus the percent transmission decreases accordingly. The dependency is different for two powder spray nozzles which have different aperture angles, i.e., 60° and 70°. 
     The present invention novel apparatus and a method for using the apparatus have therefore been amply described in the above descriptions and in the appended drawings of FIGS.  1 ˜ 6 . 
     While the present invention has been described in an illustrative manner, it should be understood that the terminology used is intended to be in a nature of words of description rather than of limitation. 
     Furthermore, while the present invention has been described in terms of a preferred embodiment, it is to be appreciated at those skilled in the art will readily apply these teachings to other possible variations of the inventions.