Patent Publication Number: US-2009224651-A1

Title: Phosphor mixture and plasma display panel comprising the phosphor mixture

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2008-0020568, filed on Mar. 5, 2008 in the Korean Intellectual Property Office, the entire content of which is incorporated herein in by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to phosphor mixtures and plasma display panels comprising the phosphor mixtures. 
     2. Description of the Related Art 
     Display apparatuses can be classified into emissive type displays that emit light themselves and non-emissive type displays that use an additional lamp. The non-emissive type displays include liquid crystal displays (LCDs), and the emissive type displays include plasma display panels (PDPs), cathode ray tubes (CRTs), and organic light emitting diodes (OLEDs). 
     The emissive type displays include phosphors which can be classified into light emission type (PL) phosphors, cathode ray emission type (CL) phosphors, and field emission type (EL) phosphors. Plasma display panels have recently received much attention as emissive type displays which use light emission type phosphors. 
     In order to realize and emissive type full color display, red, green, and blue colors of light are used. Of the three colors of light, the green color of light plays the most important role for realizing an image. This is because the human eye photonic response has peak sensitivity at approximately 535 nm (a green component of the visible spectrum). 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, a phosphor mixture increases color reproducibility and increases luminous efficiency, thereby increasing brightness characteristics by reducing the decay time. 
     In another embodiment of the present invention, a plasma display panel includes the phosphor mixture. 
     According to an embodiment of the present invention, a phosphor mixture includes a first compound represented by Chemical Formula 1 or 2 below, and a second compound represented by Chemical Formula 3 below. The first compound and the second compound are mixed in a weight ratio ranging from about 3:7 to about 6:4. 
       Gd 1-y Tb y Al 3 (BO 3 ) 4  (0.05≦Y≦0.8)   Chemical Formula 1 
       YBO 3 :Tb 3+   Chemical Formula 2 
       Zn(Ga 1-x Al x ) 2 O 4 :Mn 2+  (0.2≦X≦0.8)   Chemical Formula 3 
     According to another embodiment of the present invention, a plasma display panel includes a first substrate and a second substrate facing each other, a phosphor layer disposed in a discharge space between the first substrate and the second substrate, a discharge electrode to which a voltage is applied to generate a discharge in the discharge space, and a discharge gas injected in the discharge space, wherein the phosphor layer includes a phosphor having a first compound represented by Chemical Formula 1 or 2 above and a second compound represented by Chemical Formula 3 above mixed in a weight ratio ranging from about 3:7 to about 6:4. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent by reference to the following detailed description when considered in conjunction with the attached drawing in which: 
         FIG. 1  is a partial cutaway exploded perspective view of a plasma display panel according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described with reference to the accompanying drawings in which exemplary embodiments of the invention are shown. 
     Phosphor Mixture 
     A phosphor mixture according to an embodiment of the present invention generates green light, and is made by mixing a first compound represented by Chemical Formula 1 or 2 and a second compound represented by Chemical Formula 3 in a weight ratio ranging from about 3:7 to about 6:4. 
       Gd 1-y Tb y Al 3 (BO 3 ) 4  (0.05≦Y≦0.8)   Chemical Formula 1 
       YBO 3 :Tb 3+   Chemical Formula 2 
       Zn(Ga 1-x Al x ) 2 O 4 : Mn 2+  (0.2≦X≦0. 8)   Chemical Formula 3 
     If the weight ratio is less than about 3:7, for example, the weight ratio is 2:8 etc., the decay time is 9 ms or more, and thus luminous efficiency is reduced. The decay time directly affects the luminous efficiency. That is, the shorter the decay time, the higher the luminous efficiency, and accordingly, the brightness of a corresponding display apparatus can be increased. Also, the decay time directly affects a latent image and/or a flicker phenomenon. The longer the decay time, the more severe the latent image and/or the flicker phenomenon, thereby reducing the reliability of the display apparatus. Thus, the weight ratio must not be less than about 3:7. 
     If the weight ratio exceeds about 6:4, for example, 7:3, 8:2 etc., color reproducibility is reduced, and thus, desired image quality cannot be realized. 
     A phosphor mixture according to one embodiment of the present invention can be formed by mixing the first compound of Chemical Formula 1 or 2 with the second compound of Chemical Formula 3 in a weight ratio ranging from about 3:7 to about 6:4. 
     The compound of Chemical Formula 3 can be manufactured using the following method. A phosphor mother-body is formed by mixing a zinc compound, a gallium compound, and an aluminum compound in a mixing ratio that satisfies the conditions of Chemical Formula 3. The compounds that constitute the phosphor mother-body can be nitrates, acetates, chlorides, oxides, carbonates, or sulfides. Powders of the compounds that constitute the phosphor mother-body are mixed, and then, the powders are doped with Mn, which is an activator. Afterwards, the first compound of Chemical Formula 3 can be manufactured according to a well known solid-state reaction by annealing the powders. In another embodiment, the compound can be manufactured using spray pyrolysis instead of the solid-state reaction. These methods of manufacturing the compound of Chemical Formula 3 are well known. 
     Also, a compound of Chemical Formula 1 can be manufactured by the following method. That is, a phosphor mother-body is formed by mixing a gadolinium compound, a terbium compound, an aluminum compound, and a boric acid compound in a mixing ratio that satisfies the conditions of Chemical Formula 1, and is annealed in the same manner as the compound of Chemical Formula 3. 
     Thus, the phosphor mixture according to one embodiment can be manufactured by mixing a first compound represented by Chemical Formula 1 (manufactured as described above) and the second compound represented by Chemical Formula 3 in a weight ratio ranging from about 3:7 to about 6:4. 
     Also, the phosphor mixture according to another embodiment of the present invention can be manufactured by mixing a first compound represented by Chemical Formula 2 and a second compound of Chemical Formula 3 in a weight ratio ranging from about 3:7 to about 6:4. 
     The compound of Chemical Formula 2 can be manufactured such that, after a phosphor mother-body is formed by mixing a yttrium compound and a boric acid compound in a mixing ratio that satisfies the conditions of Chemical Formula 2, the phosphor mother-body is doped with terbium, and then annealed in the same manner as the compound of Chemical Formula 3. 
     Thus, the phosphor mixture according to another embodiment of the present invention can be manufactured by mixing a first compound of Chemical Formula 2 and a second compound of Chemical Formula 3 in a weight ratio ranging from about 3:7 to about 6:4. 
     A green phosphor mixture can be manufactured by mixing the above phosphor mixtures with a phosphor selected from Zn 2 SiO 4 :Mn, BaMgAl 10 O 17 :Mn, BaMgAl 12 O 19 :Mn, and Li 2 ZnGe 3 O 8 :Mn. 
     Plasma Display Panel 
       FIG. 1  is a cutaway exploded perspective view of a plasma display panel (PDP) according to an embodiment of the present invention. A three-electrode surface discharge type plasma display panel is shown, however, the present invention is not limited thereto. That is, the plasma display panel may be any plasma display panel on which a phosphor layer can be disposed. 
     Referring to  FIG. 1 , the PDP according to one embodiment includes a front panel  210  and a rear panel  220 . The front panel  210  includes a first substrate  211 , a plurality of sustain electrode pairs  214  disposed parallel to each other on the first substrate  211 , a first dielectric layer  215  covering the sustain electrode pairs  214 , and a protective film  216 . 
     The rear panel  220  includes a second substrate  221  disposed facing the first substrate  211 , a plurality of address electrodes  222  disposed on the second substrate  221  generally perpendicular to the sustain electrode pairs  214 , a second dielectric layer  223  covering the address electrodes  222 , and a barrier rib structure  224  disposed in a matrix shape on the second dielectric layer  223 . 
     When the front panel  210  and the rear panel  220  are combined, a lower part of the barrier rib structure  224  contacts the second dielectric layer  223  and an upper part of the barrier rib structure  224  contacts the first dielectric layer  215 . Thus, the barrier rib structure  224  defines a discharge space between the first substrate  211  and the second substrate  221  forming a plurality of discharge cells  226 . In one embodiment, the discharge cells  226  have a rectangular shape since the barrier rib structure  224  is disposed in a matrix shape. 
     Phosphor layers  225  are disposed in the discharge cells  226 . More specifically, the PDP according to one embodiment includes red phosphor layers  225   a,  green phosphor layers  225   b,  and blue phosphor layers  225   c.    
     Each red phosphor layer  225   a  includes a red phosphor, nonlimiting examples of which include (Y,Gd)BO 3 :Eu, or Y(V,P)O 4 :Eu. Each blue phosphor layer  225   c  includes a blue phosphor, nonlimiting examples of which include BaMgAl 10 O 17 :Eu or CaMgSi 2 O 6 :Eu. 
     Each green phosphor layer  225   b  includes a phosphor mixture in which the first compound of Chemical Formula 1 or 2 and the second compound of Chemical Formula 3 are mixed in a weight ratio ranging from about 3:7 to about 6:4. If the first and second compounds are mixed in a weight ratio less than about 3:7, for example, 2:8, 1:9 etc., the decay time can be 9 ms or more. The decay time directly affects the luminous efficiency of the phosphor, and if the decay time is greater than 9 ms, the luminous efficiency cannot reach a level useful in a display product. Also, the decay time of the phosphor affects the flicker phenomenon, which is noticeable to the viewer. In particular, if the decay time is greater than 9 ms, the display product cannot be used since the flicker phenomenon will increase the viewer&#39;s eye fatigue. 
     Also, if the first and second compounds are mixed in a weight ratio exceeding about 6:4, that is, 7:3 or 8:2 etc., the color reproducibility of the display apparatus is greatly reduced. In a PDP having high color reproducibility, the green phosphor has an x color coordinate value of 0.2589 and a y color coordinate value of 0.6884. In the case of the green phosphor, the color reproducibility increases as the x coordinate value is lowered and the y coordinate value is increased. Therefore, in order to produce a PDP having high color reproducibility, the x coordinate value should be below 0.2598 and the y coordinate value should be greater than 0.6884. This is standard for the green phosphor to widen the range of color reproducibility. The color reproducibility range is an area value calculated from the area of a color coordinate system triangle formed from the color coordinates of the red, green and blue phosphors. The area of the triangle can be increased by minimizing the x coordinate value of the green phosphor and maximizing the y coordinate value of the green phosphor. Thus, the green phosphor mixture according to embodiments of the present invention includes the first compound of Chemical Formula 1 or 2 and the second compound of Chemical Formula 3 mixed in a weight ratio not exceeding about 6:4. 
     The phosphor layer  225  can be formed by first forming a phosphor paste, and coating the phosphor paste in the discharge cells  226 . Then, the phosphor paste is dried and sintered. The phosphor paste can be formed by mixing the phosphor with a binder resin and a solvent. In particular, a green phosphor paste can be manufactured by mixing the phosphor mixture including the first compound of Chemical Formula 1 or 2 and the second compound of Chemical Formula 3 in a weight ratio ranging from about 3:7 to 6:4 with a binder resin and a solvent. 
     The binder resin can be a cellulose group resin, an acryl group resin, or a mixture of these resins. Nonlimiting examples of suitable cellulose group resins include methyl cellulose, ethyl cellulose, propyl cellulose, hydroxy methyl cellulose, hydroxy ethyl cellulose, hydroxy propyl cellulose, hydroxy ethyl propyl cellulose, and combinations thereof. Nonlimiting examples of suitable acryl group resins include methacrylate group materials (such as poly methyl methacrylate, poly isopropyl methacrylate, and poly isobutyl methacrylate), polymers of acryl group monomers (such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, hexyl methacrylate, 2-ethyl hexyl methacrylate, benzyl methacrylate, dimethyl amino ethyl methacrylate, hydroxy ethyl methacrylate, hydroxy propyl methacrylate, hydroxy butyl methacrylate, phenoxy 2-hydroxy propyl methacrylate, glycidyl acrylate, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, hexyl acrylate, 2-ethyl hexyl acrylate, benzyl acrylate, dimethyl amino ethyl acrylate, hydroxyl ethyl acrylate, hydroxy propyl acrylate, hydroxy butyl acrylate, phenoxy 2-hydroxy propyl acrylate, glycidyl acrylate), and combinations thereof. In some cases, the mixture can include a small amount of inorganic binder. The content of the binder resin can range from about 2 to about 8 parts by weight based on 100 parts by weight of the phosphor paste. 
     The solvent can be an alcohol group, an ether group, an ester group, or a mixture of these materials. In one embodiment, the solvent is selected from butyl cellusolve (BC), butyl carbitol acetate (BCA), terpineol, and combinations thereof. If the content of the solvent is too high or too low, the flowability characteristic of the mixture is not appropriate, and thus, the process for forming the green phosphor layer may not be straightforward. Accordingly, the content of the solvent in the phosphor paste may range from about 25 to about 75 parts by weight based on 100 parts by weight of the phosphor paste. 
     The phosphor paste can further include other additives for improving flowability characteristics and processing characteristics. The additives can be, for example, photosensitizer (e.g., benzophenone), dispersing agents, defoamers, leveling agents, plasticizers, and anti-oxidants. These additives can be used alone or in combination, and are commercially available. 
     After the phosphor is mixed and stirred with a binder resin and a solvent, the phosphor paste is coated on the discharge cells  226 . The coated phosphor paste is dried at a temperature ranging from about 120 to about 140° C., and then sintered at a temperature ranging from about 200 to about 600° C. for from about 1 to about 4 hours, thereby forming the phosphor layer  225 . 
     A discharge gas is filled in the discharge cells  226 . The discharge gas can be, for example, a Ne—Xe gas mixture containing from about 5 to about 10% Xe. As necessary, at least a portion of the Ne can be replaced with He. 
     Hereinafter, evaluation results of decay time, light emission characteristics, and color reproducibility of phosphor mixtures according to embodiments of the present invention will be provided. 
     Evaluation 1 Decay Time Evaluation 
     Gd 0.6 Tb 0.4 Al 3 (BO 3 ) 4  and Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+  were mixed in various weight ratios as summarized in Table 1, and decay times were measured. A green phosphor including Zn 2SiO   4 :Mn 2+  and YBO 3 :Tb mixed in a weight ratio of 80:20 was used for comparison. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Gd 0.6 Tb 0.4 Al 3 (BO 3 ) 4   
                 Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+   
                 Decay time (ms) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 100 
                 0 
                 4 
               
               
                 80 
                 20 
                 6.1 
               
               
                 70 
                 30 
                 6.5 
               
               
                 60 
                 40 
                 7.0 
               
               
                 40 
                 60 
                 8.1 
               
               
                 30 
                 70 
                 8.7 
               
               
                 20 
                 80 
                 9.4 
               
               
                 0 
                 100 
                 10.2 
               
            
           
           
               
               
            
               
                 Comparative phosphor 
                 7.6 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, it can be seen that as the content of Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+  increases, the decay time increases. In particular, it is seen that when Gd 0.6 Tb 0.4 Al 3 (BO 3 ) 4  and Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+  are mixed in a weight ratio of 20:80, the decay time exceeds 9 ms. Accordingly, in order to reduce the decay time below 9 ms, the weight ratio of Gd 0.6 Tb 0.4 Al 3 (BO 3 ) 4  and Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+  should not exceed about 30:70. 
     Evaluation 2 Light Emission Characteristic Evaluation 
     Light emission characteristics of the phosphors listed in Table 1 were measured. More specifically, relative brightness and brightness saturation were measured. The relative brightness was measured (calculated) by comparing the values of phosphors in a conventional PDP, and brightness saturation was calculated by measuring brightness while increasing sustain frequencies. The results are shown in Table 2 below. 
     As a result, it is seen that the phosphor mixtures according to embodiments of the present invention have increased brightness saturation compared to the comparative phosphor while the inventive phosphor mixtures have brightness levels generally equivalent to the comparative phosphor. Thus, it can be seen that the phosphor mixtures according to embodiments of the present invention can increase brightness saturation characteristics without reducing brightness, thus increasing panel brightness and sensitivity when the phosphor mixtures are applied to PDPs. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Relative 
                 Brightness 
               
               
                   
                   
                 Brightness 
                 saturation 
               
               
                 Gd 0.6 Tb 0.4 Al 3 (BO 3 ) 4   
                 Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+   
                 (%) 
                 (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 100 
                 0 
                 101 
                 92 
               
               
                 80 
                 20 
                 100 
                 92 
               
               
                 70 
                 30 
                 99 
                 92 
               
               
                 60 
                 40 
                 99 
                 91 
               
               
                 40 
                 60 
                 97 
                 90 
               
               
                 30 
                 70 
                 96 
                 90 
               
               
                 20 
                 80 
                 96 
                 89 
               
               
                 0 
                 100 
                 94 
                 88 
               
            
           
           
               
               
               
            
               
                 Comparative group 
                 100 
                 81 
               
               
                   
               
            
           
         
       
     
     Evaluation 3 Color Reproducibility Evaluation 
     Color reproducibilities of the phosphors listed in Table 1 were measured, and the results are summarized in Table 3. The lower the x coordinate value and the higher the y coordinate value in a color coordinate, the higher the color reproducibility of the green phosphor. Thus, a phosphor mixture can have color reproducibility equal to or greater than that of the comparative group by including Gd 0.6 Tb 0.4 Al 3 (BO 3 ) 4  and Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+  in a weight ratio not exceeding about 60:40. 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Composition 
                 CIE coordination 
               
            
           
           
               
               
               
               
            
               
                 Gd 0.6 Tb 0.4 Al 3 (BO 3 ) 4   
                 Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+   
                 x 
                 y 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 100 
                 0 
                 0.3365 
                 0.5871 
               
               
                 80 
                 20 
                 0.2966 
                 0.6226 
               
               
                 70 
                 30 
                 0.2767 
                 0.6404 
               
               
                 60 
                 40 
                 0.2567 
                 0.6581 
               
               
                 40 
                 60 
                 0.2169 
                 0.6935 
               
               
                 30 
                 70 
                 0.1970 
                 0.7112 
               
               
                 20 
                 80 
                 0.1770 
                 0.7290 
               
               
                 0 
                 100 
                 0.1371 
                 0.7645 
               
            
           
           
               
               
               
            
               
                 Comparative group 
                 0.2597 
                 0.6884 
               
               
                   
               
            
           
         
       
     
     Overall, from the evaluations, the phosphor mixtures according to embodiments of the present invention, in which Gd 0.6 Tb 0.4 Al 3 (BO 3 ) 4  and Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+  are mixed in a weight ratio ranging from about 3:7 to about 6:4, have greatly improved relative brightness and brightness saturation, and thus can be used as the green phosphor. The phosphor mixtures also have high color reproducibility and can increase luminous efficiency by having a reduced decay time. The reduction in decay time can prevent the flicker phenomenon and/or latent image problem. 
     As described above, the phosphor mixtures according to embodiments of the present invention are used as green phosphors. In one embodiment, the phosphor mixture includes Gd 0.6 Tb 0.4 Al 3 (BO 3 ) 4  and Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+  mixed in a weight ratio ranging from about 3:7 to about 6:4, and exhibits a decay time of 9 ms or less, thereby increasing luminous efficiency and preventing latent image and/or flicker phenomena when used in a display apparatus. Also, with regard to color reproducibility, since the phosphor mixture can effectively realize a green color, the phosphor mixture can be used in emissive display apparatuses, in particular, in plasma display apparatuses. 
     In the above evaluations, green phosphors in which x is 0.5 and y is 0.4 are described, however, the present invention is not limited to these green phosphors. Indeed, other green phosphors, including Zn(Ga 1-x Al x ) 2 O 4 :Mn 2+  (0.2≦x≦0.8) and Gd 1-y Tb y Al 3 (BO 3 ) 4  (0.05≦y≦0.8) exhibit similar and/or better effects as the green phosphors of the above evaluations. 
     Also, green phosphors including YBO 3  and Zn(Ga 1-x Al x ) 2 O 4 :Mn 2+  in weight ratios ranging from 3:7 to 6:4 have decay times of 9 ms or less, and can as effectively represent the green color with color reproducibility as the phosphors including Zn(Ga 1-x Al x ) 2 O 4 :Mn 2+  and Zn(Ga 0.5 Al 0.5 ) 2 O 4 :Mn 2+ . 
     While the present invention has been illustrated and described with reference to certain exemplary embodiments, it is understood by those of ordinary skill in the art that various changes and modifications to the described embodiments may be made without departing from the spirit and scope of the present invention as defined by the following claims.