Patent Abstract:
MgO pellets are provided for use as a protective layer for a plasma display panel providing improved physical properties. The plasma display panel includes first and second substrates facing each other. A plurality of first and second electrodes are internally formed on the first and the second substrates. Dielectric layers cover the first and the second electrodes and a MgO protective layer covers one of the dielectric layer. The MgO protective layer has 400 columnar crystals per μm 2 .

Full Description:
CROSS-REFERENCE 
   The present application is based on and claims priority to Korean Patent Application No. 10-2003-0073531 filed in the Korean Intellectual Property Office on Oct. 21, 2003, the entire contents of which are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to MgO pellets used for providing a protective layer for a plasma display panel, and also to a plasma display panel using such pellets whereby the discharge delay time is minimized. 
   BACKGROUND OF THE INVENTION 
   Generally, a plasma display panel (referred to hereinafter simply as a “PDP”) is a display device which displays images with phosphors excited by the plasma discharge. When voltages are applied to the electrodes arranged within the discharge space of the PDP, a plasma discharge is generated between the electrodes and generates ultraviolet rays. The ultraviolet rays excite the phosphors with a predetermined pattern, thereby displaying the desired images. 
   A PDP is generally classified as an AC-type, a DC-type or a hybrid-type.  FIG. 4  is an exploded perspective view of a discharge cell for a common AC-type PDP. As shown in  FIG. 4 , the PDP  100  includes a bottom substrate  111 , a plurality of address electrodes  115  formed on the bottom substrate  111 , a dielectric layer  119  formed on the bottom substrate  111  over the address electrodes  115 , a plurality of barrier ribs  123  formed on the dielectric layer  119  and phosphor layers  125  formed between the barrier ribs  123 . The barrier ribs maintain the discharge distance and prevent cross talk between the cells. 
   A plurality of discharge sustain electrodes  117  are formed on the lower surface of a top substrate  113  facing the bottom substrate  111  and spaced apart from the address electrodes  115  formed on the bottom substrate  111 . The address electrodes are oriented perpendicular to the sustain electrodes. A dielectric layer  121  and a protective layer  127  sequentially cover the discharge sustain electrodes  117  on the side opposite the top substrate. While other materials may be used, the protective layer  127  is often formed of MgO. 
   The MgO protective layer is a transparent thin film, which reduces the effect of the ion collision caused by the discharge gas during operation, thereby protecting the dielectric layer. The MgO layer also emits secondary electrons so that the discharge voltage is lowered. The MgO protective layer is generally formed on the dielectric layer to a thickness of 3000-7000 Å. The MgO protective layer is generally formed using a sputtering method, electron beam deposition, ion beam assisted deposition (IBAD), chemical vapor deposition (CVD), or a sol-gel method. Recently, an ion plating method has been developed and has been used to form a MgO protective layer. 
   With regard to the electron beam deposition method, electron beams accelerated by electromagnetic fields collide against the MgO deposition material in order to heat and vaporize it, thereby forming a MgO protective layer. Although the sputtering method is preferred over the electron beam deposition method because the resulting protective layer is more densely formed with favorable crystalline alignment, the production costs are unfavorably high For the sol-gel method, the MgO protective layer is formed from a liquid phase. 
   As an alternative to these various methods for forming a MgO protective layer, an ion plating method has been recently developed. In the ion plating method, vaporized particles are ionized and form a target layer. Although the ion plating method is similar to the sputtering method with respect to the adhesion and crystallinity of the MgO protective layer, there is an advantage in that it is capable of rather high speed deposition at 8 nm/s. 
   According to such a processes, single crystal of MgO or sintered MgO is used. However, it is difficult to control the suitable amount of a specific dopant due to the difference of the solid solution limit in cooling process to manufacture a single crystal of MgO. Namely, a specific dopant for controlling the quality of MgO layer is precipitated without being solved in a single crystal of MgO during cooling process. For this reason, the MgO protective layer is generally formed by the ion plating method using a sintered MgO combined with a suitable amount of an appropriate dopant. Pellet-shaped materials may be used to deposit the MgO protective layer. The dissolution speed of the MgO generally depends upon the size and the shape of the pellets. Therefore, various attempts have been made to optimize the size and the shape of the MgO pellets. 
   SUMMARY OF THE INVENTION 
   In one embodiment of the present invention, MgO pellets with improved physical properties are provided and used for forming a MgO protective layer for a PDP. The use of such MgO pellets in forming a PDP protective layer enhances the discharge quality of the PDP. 
   In one embodiment of the present invention, the PDP includes first and second substrates facing each other. A plurality of first and second electrodes are internally formed on the first and the second substrates, respectively with the first and the second electrodes running in directions perpendicular to one another. Dielectric layers cover the first and the second electrodes. A MgO protective layer covers at least one of the dielectric layers. In one embodiment of the invention, the density of columnar crystals in the MgO protective layer is 400 columnar crystals or less per μm 2 . 
   In one embodiment, the MgO protective layer preferably has a refractive index of 1.45-1.74. 
   In another embodiment, the protective layer has (111) planes and (110) planes in a mixed manner. 
   According to the invention, the MgO pellets may be used to form a protective layer with a bulk density of 2.80-2.95 g/cm 3 . 
   In yet another embodiment, the MgO pellets preferably have a mean crystal grain size of 30-70 μm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other advantages of the present invention will become more apparent by describing preferred embodiments thereof in detail with reference to the accompanying drawings in which: 
       FIG. 1  is a perspective view of an upper panel of a PDP according to an embodiment of the present invention; 
       FIG. 2  schematically illustrates the process of depositing a MgO layer according to an embodiment of the present invention; 
       FIG. 3  is a SEM photograph illustrating the crystal planes of a MgO protective layer according to an embodiment of the present invention; and 
       FIG. 4  is an exploded perspective view of a discharge cell of a PDP according to the prior art. 
   

   DETAILED DESCRIPTION 
   The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which various embodiments of the invention are shown. 
     FIG. 1  is a perspective view of an upper panel of a PDP according to an embodiment of the present invention. 
   As shown in  FIG. 1 , the upper panel of a PDP according to an embodiment of the present invention is shown. A plurality of first electrodes  17 , a dielectric layer  21  and a protective layer  27  are sequentially formed on a top substrate  13 . The lower panel of the PDP is manufactured according to the prior art as set forth in  FIG. 4 . For clarity of illustrating the invention, the upper panel of  FIG. 1  has been flipped 180 degrees compared to the PDP of  FIG. 4 . A plurality of second electrodes  115  are formed on a bottom substrate  111  facing the top substrate  13  and are positioned to run in a direction perpendicular to the first electrodes  17 . A dielectric layer  119  covers the second electrodes. Barrier ribs  123  are formed on the dielectric layer, and phosphor layers  125  are formed between the barrier ribs. 
   Frits are coated on the peripheries of the upper and the lower panel, which are then sealed to each other. A discharge gas such as Ne or Xe is injected between the panels, thereby completing the PDP. 
   With regard to the PDP according to one embodiment of the present invention, upon application of driving voltages to the electrodes, an address discharge is made between the electrodes, thereby forming a wall charge at the dielectric layer. With the discharge cells selected by the address discharge, a sustain discharge is made between a pair of electrodes formed on the upper panel by the current signals alternately fed thereto. Consequently, the discharge gas filled within the discharge space forming the discharge cells is excited and shifted, thereby generating ultraviolet rays. Phosphors are excited by the ultraviolet rays to thereby generate visible rays, and display the desired images. 
   As shown in  FIG. 1 , in the PDP according to this embodiment of the present invention, a plurality of electrodes cross each other within the protective layer to thereby form pixels, which form a display area together surrounded by a non-display area. The plurality of electrodes  17  formed on the substrate  13  are illustrated to the left and the right of the dielectric layer  21  at their terminal portions where they are connected to a flexible printed circuit board (FPC, not shown). 
   With the PDP according to an embodiment of the present invention, the MgO protective layer  27  is formed by depositing MgO pellets in a MgO deposition chamber. The MgO pellets for the protective layer of the PDP according to the embodiment of the present invention are made by the following method. 
   First, a MgO powder with a purity of 90.0-92.0% is prepared, and a doping material is added thereto to form Mg(OH) 2 . Sufficient doping material is added to improve the purity thereof to 99.0%. 
   The Mg(OH) 2  has a moisture content of 50.0%, and is dried in an oven with hot air to remove the water. After drying, the Mg(OH) 2  is electrically fused in a bell type low temperature sintering furnace at 2800° C. for 60 hours, thereby calcinating it. In this way, the water of crystallization is removed from the Mg(OH) 2  to thereby obtain a MgO powder. The electrically fused MgO powder is then cooled and solidified again. 
   The solidified MgO powders are broken using a breaker, and are mixed with an adjunct of a solvent and an additive to form a slurry. The mixing is made using a wet mill technique, and 99.5% or more of an anhydrous solvent and Aldrich reagent are used as the additives. Zirconia balls and urethane ports are used in the wet milling. 
   The MgO slurry is dried by the spray drying method using an explosion proof spray dryer to form MgO granules. In the agglomeration process, MgO powder with a mean particle size of 3-5 μm is spherically agglomerated by 80 μm. 
   Then, the MgO granules are press-formed using a rotary press. The press-formed MgO granules are sintered and crystallized in a high temperature sintering furnace at 1700° C. When the sintering is made at that temperature, the surfaces of the MgO granules are molten and are adhered to those of other MgO granules so that the density of the MgO granules is increased and the pores thereof are reduced, thereby forming MgO pellets with a dense structure. 
   The bulk density of the MgO pellets is preferably from 2.80 to 2.95g/cm 3 . The bulk density of the MgO pellets is obtained through the mathematical formula 1. A sample of the MgO pellets is dried at 100° C. for 24 hours or more, and is calculated by kerosene immersion.
 
Bulk density (g/cm 3 )= k ×mass of dried sample (g)/(mass of moisture-contained sample (g)−mass of moisture content (g))  Formula 1
 
   where k is 0.796 g/cm 3 , the specific gravity of kerosene. 
   The bulk density of MgO pellets for the protective layer of the PDP according to the embodiment of the present invention can be controlled through the steps of drying a MgO slurry mixed by the spray drying method to form MgO granules, press-forming the MgO granules, and sintering the MgO granules in a high temperature sintering furnace. 
     FIG. 2  schematically illustrates the process of forming a MgO protective layer using MgO pellets. The electron beam deposition method is introduced here to form the MgO protective layer on a substrate sequentially overlaid with electrodes and a dielectric layer. 
   In the electron beam deposition method, electron beams are accelerated by electromagnetic fields and collide against the deposition material to thereby heat and vaporize it, and form a protective layer. In this case, the energies of the electron beams are concentrated on the material surface, thereby enabling the high speed deposition and the high purity deposition.  FIG. 2  illustrates an exemplary process of forming the protective layer, and the process of forming the protective layer is not limited to the electron beam deposition method. 
   In the process of forming the MgO protective layer  27  shown in  FIG. 2 , the substrate  13  is transferred from the left to the right by rollers  51 , and loaded into an inlet port  23  of the deposition chamber  20 . After the MgO protective layer  27  is deposited on the substrate  13 , it is discharged through the outlet port  25  of the deposition chamber  20 . If there is something wrong with the substrate  13 , it is possible to unload the substrate  13  from the inlet port of the deposition chamber  23 . Since the deposition chamber  20  should be in a vacuum state, a vacuum pump (not shown) is attached thereto to exhaust the interior gas continuously. The deposition chamber  20  is isolated from the outside using shutters  33 . An electron gun  31  is operated to form the electromagnetic fields. The ions emitted from the electron gun  31  collide against the MgO pellets  57  placed at the bottom of the deposition chamber  20  to thereby deposit a MgO layer on the substrate  13  placed at the top of the deposition chamber  20 . The MgO pellets  57  have a tendency to overheat due to the ion collisions, and therefore, the MgO protective layer  27  is formed while cooling the MgO pellets  57  with a cooler  29 . 
   In the process of depositing a MgO protective layer  27 , if the bulk density of the MgO pellets is less than 2.80 g/cm 3 , a numbers of pores are present in the MgO pellets making it impossible to manufacture a MgO protective layer having a dense crystal structure. In contrast, if the bulk density of the MgO pellets exceeds 2.95 g/cm 3 , the MgO pellets are so densely formed that the decomposition speed of MgO is lowered, thereby deceasing the decomposition speed when forming the MgO protective layer. Although the MgO protective layer is commonly deposited at 60-110 Å/s, if the bulk density of the MgO pellets is controlled to be in the range of 2.80-2.95 g/cm 3 , its deposition speed can be increased to 130 Å/s. The relatively low bulk density can be controlled by reducing the splash phenomenon due to the thermal shock such that the substrate is not damaged during the deposition. In this case, the mean crystal grain size of the MgO pellets is preferably from 30 to 70 μm. Therefore, the MgO protective layer can be deposited onto the PDP substrate while reducing the splash phenomena. 
     FIG. 3  is a scanning electron microscope (SEM) photograph of a MgO protective layer according to an embodiment of the present invention. The MgO protective layer shown in  FIG. 3  is formed while maintaining the partial pressure ratio of oxygen to hydrogen at about 6:1. As known from the SEM photograph of  FIG. 3 , the triangle-shaped crystal planes and the rectangle-shaped crystal planes are uniformly mixed in the MgO protective layer according to the embodiment of the present invention. The triangle-shaped crystal plane is a plane (111), and the rectangle-shaped crystal plane is a plane (110). By controlling the partial pressure of oxygen and hydrogen when depositing the MgO protective layer on the substrate of the PDP, the number of columnar crystals is varied. In order to evaluate the influence of the number of columnar crystals in the MgO protective layer on the discharge quality of the PDP, several experiments were made as set forth below. 
   EXPERIMENTAL EXAMPLES 
   In order to evaluate the features of the MgO protective layer as a function of the columnar crystal density (measured as the number of columnar crystals per μm 2 ), the discharge delay times as a function of the respective numbers of columnar crystals in a 1 μm 2  area of a MgO protective layer were measured. The time required for applying the driving voltage to the PDP through scanning electrodes is referred to as the scanning time. Although the discharge occurs during the scanning time, the discharge does not instantly occur as soon as the driving voltage is applied so that the discharge is delayed. This is referred to as a discharge delay time. The discharge delay time is divided into a formation delay time and a statistical delay time. The MgO protective layer is intimately related to the discharge of secondary electrons. Therefore, in the Experimental Examples of the present invention, the discharge delay time according to the number of columnar crystals per μm 2  was measured so that the proper range for the density of columnar crystals could be derived therefrom. It is to be noted that the following Experimental Examples merely illustrate specific embodiments of the present invention, and the scope of the present invention is not limited thereto. 
   Experimental Example 1 
   MgO pellets were loaded into a MgO deposition chamber, and a MgO layer was deposited on a dielectric layer formed on a substrate. The deposited MgO protective layer had a thickness of approximately 7000 Å. The pressure inside the deposition chamber was set at 1×10  −4  Pa except during deposition when it was increased to 5.3×10 −2  Pa. The substrate was maintained at 200±5° C. while supplying oxygen at a rate of 100 sccm. Electron beams were emitted from an electron gun set at a current of 390 mA and a voltage of −15 kV DC to deposit the MgO protection layer. As a result of depositing the MgO protective layer, 200 columnar crystals per μm 2  were obtained, and the discharge delay time of the PDP with the MgO protective layer was 265 ns. 
   Experimental Example 2 
   A partial pressure ratio of oxygen to hydrogen was set at approximately 6:1 and the other conditions were maintained as set forth in Experimental Example 1. As a result of depositing the MgO protective layer, 400 columnar crystals per μm 2  were obtained, and the discharge delay time of the PDP with the MgO protective layer was 284 ns. 
   Experimental Example 3 
   A partial pressure ratio of oxygen to hydrogen was set at approximately 30:1 and the other conditions were maintained as set forth in Experimental Example 1. As a result of depositing the MgO protective layer, 1200 columnar crystals per μm 2  were obtained, and the discharge delay time of the PDP with the MgO protective layer was 322 ns. 
   Experimental Example 4 
   A partial pressure ratio of oxygen to hydrogen was set at approximately 50:1 and the other conditions were maintained as set forth in Experimental Example 1. As a result of depositing the MgO protective layer, 2100 columnar crystals per μm 2  were obtained, and the discharge delay time of the PDP with the MgO protective layer was 339 ns. 
   Experimental Example 5 
   A partial pressure ratio of oxygen to hydrogen was set at approximately 100:1 and the other conditions were maintained as set forth in Experimental Example 1. As a result of depositing the MgO protective layer, 3400 columnar crystals per μm 2  were obtained, and the discharge delay time of the PDP with the MgO protective layer was 345 ns. 
   Experimental Example 6 
   A partial pressure ratio of oxygen to hydrogen was set at approximately 150:1 and the other conditions were maintained as set forth in Experimental Example 1. As a result of the MgO protective layer, 5000 columnar crystals per μm 2  were obtained, and the discharge delay time of the PDP with the MgO protective layer was 368 ns. 
   The results of the Experimental Examples 1 to 6 are summarized in Table 1. 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
               Partial pressure 
               Number of 
                 
             
             
               Experimental 
               ratio of oxygen to 
               columnar crystals 
               Discharge delay 
             
             
               Example 
               hydrogen 
               per μm 2   
               time 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Experimental 
                3:1 
               200 
               265 ns 
             
             
               Example 1 
             
             
               Experimental 
                6:1 
               400 
               284 ns 
             
             
               Example 2 
             
             
               Experimental 
                30:1 
               1200 
               322 ns 
             
             
               Example 3 
             
             
               Experimental 
                50:1 
               2100 
               339 ns 
             
             
               Example 4 
             
             
               Experimental 
               100:1 
               3400 
               345 ns 
             
             
               Example 5 
             
             
               Experimental 
               150:1 
               5000 
               368 ns 
             
             
               Example 6 
             
             
                 
             
           
        
       
     
   
   As shown in Table 1, for Experimental Example 2, the discharge delay time was reduced to less than 300 ns, and the discharge quality was improved. In this case, the density of columnar crystals in the MgO protective layer was about 400 columnar crystals per μm 2  or less. If the density of columnar crystals is in this range, the address discharge delay during the plasma discharge can be minimized, thereby improving the display quality. 
   Meanwhile, the thickness of the MgO protective layer obtained in the Experimental Examples 1 and 2 was about 6400 Å, and the refractive index thereof was 1.45-1.74. The (111) planes and the (110) planes were mixed in the MgO protective layer, and improved discharge quality was obtained. 
   As described above, when the density of columnar crystals of the MgO protective layer is about 400 columnar crystals per μm 2  or less, the discharge delay time is minimized, thereby improving the discharge quality of the PDP. 
   Furthermore, if the refractive index of the MgO protective layer is 1.45-1.74, the discharge delay time can be reduced. Also, if the (111) planes and the (110) planes are mixed in the MgO protective layer, the above effects are obtained. 
   Meanwhile, when the bulk density of the MgO pellets for the protective layer of the PDP is 2.80-2.95 g/cm 3 , the deposition speed of the MgO layer is increased, thereby enhancing the productivity of the PDP while reducing the splash phenomena. 
   If the mean crystal grain size of the MgO pellets is 30-70 μm, the productivity of the PDP is further enhanced, and the splash phenomenon is significantly reduced. 
   Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concept herein taught which may appear to those skilled in the art will still fall within the spirit and scope of the present invention, as defined in the appended claims.

Technology Classification (CPC): 7