Abstract:
A plasma display panel with low firing voltage is disclosed. The plasma display panel includes an upper panel and a lower panel facing each other through barrier ribs wherein the upper panel includes a first protective film composed of magnesium oxide and a second protective film formed on the first protective film and composed of a secondary electron-emitting material.

Description:
[0001]     This application claims the benefit of Korean Patent Application No. 10-2006-0000849, filed on Jan. 4, 2006 and Korean Patent Application No. 10-2006-0001884, filed on Jan. 6, 2006, which are hereby incorporated by reference in their entirety.  
       BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     This document relates to a plasma display panel, and more particularly, to protective films of a plasma display panel.  
         [0004]     2. Discussion of the Related Art  
         [0005]     Plasma display panels include an upper panel, a lower panel, and barrier ribs formed between the upper and lower panels to define respective discharge cells. A major discharge gas, such as neon, helium or a mixed gas thereof, and an inert gas containing a small amount of xenon (Xe) are filled within the discharge cells. When a high-frequency voltage is applied to produce discharge in the discharge cells, vacuum ultraviolet rays are generated from the inert gas to cause phosphors present between the barrier ribs to emit light, and, as a result, images are created. Such plasma display panels have attracted more and more attention as next-generation display devices due to their small thickness and light weight.  
         [0006]      FIG. 1  is a perspective view showing the structure of a plasma display panel. As shown in  FIG. 1 , the plasma display panel includes an upper panel  100  and a lower panel  110  integrally joined in parallel to and at a certain distance apart from the upper panel. The upper panel  100  includes an upper glass plate  101  as a display plane on which images are displayed and a plurality of sustain electrode pairs, each of which consists of a scan electrode  102  and a sustain electrode  103 , arranged on the upper glass plate  101 . The lower panel  110  includes a lower glass plate  111  and a plurality of address electrodes  113  arranged on the lower glass plate  111  so as to cross the plurality of sustain electrode pairs.  
         [0007]     Barrier ribs  112 , which may be, for example, stripe type or well type, for forming a plurality of discharge spaces or discharge cells are arranged parallel to each other on the lower panel  110 . A plurality of address electrodes  113 , which act to perform address discharge, are arranged in parallel with respect to the barrier ribs to generate vacuum ultraviolet rays. Red (R), green (G) and blue (B) phosphors  114  are applied to upper sides of the lower panel  110  to emit visible rays upon address discharge, and, as a result, images are displayed. A lower dielectric layer  115  is formed between the address electrodes  113  and the phosphors  114  to protect the address electrodes  113 .  
         [0008]     An upper dielectric layer  104  is formed on the sustain electrode pairs  103 , and a protective layer  105  is formed on the upper dielectric layer  104 . The upper dielectric layer  104 , which is included in the upper panel  100 , is worn out due to the bombardment of positive (+) ions upon discharge of the plasma display panel. Thus, short circuiting of the electrodes may be caused by metal elements, such as sodium (Na). To address this problem, a magnesium oxide (MgO) thin film as the protective layer  105  is formed on the upper dielectric layer  104  by coating to protect the upper dielectric layer  104 . Magnesium oxide sufficiently withstands the bombardment of positive (+) ions and has a high secondary electron emission coefficient, thus achieving a low firing voltage. Accordingly, the protective layer is formed to operate the plasma display panel at a low voltage. This low-voltage operation leads to a reduction in the power consumption of the panel, thus contributing to a reduction in the production costs of the panel as well as an improvement in the discharge efficiency and brightness of the panel.  
         [0009]     However, such a conventional protective layer of magnesium oxide may fail to sufficiently lower the discharge voltage of plasma display panels, on account of its material characteristics. Specifically, magnesium oxide has a low secondary electron emission coefficient with respect to ions escaping from plasma.  
       SUMMARY OF THE INVENTION  
       [0010]     In one general aspect, a plasma display panel that has improved secondary electron emission characteristics and a method for producing such a plasma display panel are provided.  
         [0011]     Implementations of the plasma display panel may have low firing voltage, high brightness, improved discharge efficiency and reduced power consumption, which result from improved secondary electron emission characteristics.  
         [0012]     Implementations of the plasma display panel may emit an increased number of secondary electrons due to the bombardment of electrons.  
         [0013]     Additional features will be apparent from the description which follows, including the drawings, and the claims.  
         [0014]     In another general aspect, a plasma display panel includes an upper panel and a lower panel facing each other through barrier ribs, wherein the upper panel includes a first protective film composed of magnesium oxide and a second protective film formed on the first protective film and composed of a secondary electron-emitting material.  
         [0015]     In another general aspect, a method for producing a plasma display panel includes forming a first protective film composed of magnesium oxide on a dielectric layer of an upper panel and forming a second protective film composed of a secondary electron-emitting material on the first protective film.  
         [0016]     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the subject matter claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a perspective view of a plasma display panel;  
         [0018]      FIG. 2  is a graph showing changes in the firing voltages of different plasma display panels, each of which includes a protective layer composed of magnesium oxide and another oxide; and  
         [0019]      FIG. 3  is a section view of an upper panel of a plasma display panel. 
     
    
     DETAILED DESCRIPTION  
       [0020]     A plasma display panel includes a protective layer having a bilayer structure. Hereinafter, a layer formed on one surface of an upper dielectric layer is referred to as a ‘first protective film’, and a layer formed on the first protective film is referred to as a ‘second protective film’.  
         [0021]      FIG. 2  is a graph showing changes in the firing voltages of different plasma display panels, each of which includes a protective layer composed of magnesium oxide and another oxide. As is apparent from the graph of  FIG. 2 , the firing voltages of the plasma display panels can be lowered by the addition of various kinds of oxides other than magnesium oxide to the respective protective layers.  FIG. 2  also shows changes in the firing voltages of the plasma display panels with increasing amounts of Y 2 O 3 , SrO, ZrO 2 , ZnO, CaO, Al 2 O 3  and TiO 2  added as additives. Although there is a difference depending on the kind of the additives, the firing voltages of the plasma display panels generally decrease to the lowest values when the number of moles of the additive constituting each of the protective layers reaches about 10% of the total number of moles of the additive and magnesium oxide.  
         [0022]     Based on these results, a plasma display panel may include a protective film which is composed of a mixture of magnesium oxide and another oxide. Other implementations provide a plasma display panel including an additional protective film composed of a crystalline oxide and formed over a protective film that comprises magnesium oxide or a protective film which comprises a mixture of magnesium oxide and another oxide. Implementations are not limited to using a protective film formed from the materials shown in  FIG. 2 .  
         [0023]      FIG. 3  is a view of an upper panel of a plasma display panel. The plasma display panel includes sustain electrode pairs  390  included in an upper panel and a dielectric layer  375  formed thereon. Each of the sustain electrode pairs  390  includes a transparent electrode  390   a  and a bus electrode  390   b  formed on the transparent electrode. A black electrode  390   c  may be interposed between the transparent electrode  390   a  and the bus electrode  390   b . A first protective film  380   a  and a second protective film  380   b  are sequentially formed on the dielectric layer  375 . The first protective film  380   a  is composed of magnesium oxide, and the second protective film  380   b  is composed of a secondary electron-emitting material.  
         [0024]     In certain implementations, crystalline oxide is used as the secondary electron-emitting material. The crystalline oxide is a material that serves to increase the number of secondary electrons emitted to lower the firing voltage of a plasma display panel. The crystalline oxide may be at least one material selected from alkaline earth metal oxides, alkali metal oxides and transition metal oxides. Examples of alkaline earth metal oxides include MgO, BeO, CaO, SrO and BaO, examples of alkali metal oxides include LiO 2 , Na 2 O, K 2 O, Rb 2 O and CsO, and examples of transition metal oxides include TiO 2 , Y 2 O 3 , ZrO 2 , Ta 2 O 5 , ZnO, CoO and MnO. In addition to these materials, materials such as Al 2 O 3 , SiO 2 , GeO 2 , SnO 2 , La 2 O 3 , CeO 2 , Eu 2 O 3 , and Gd 2 O 3  may be used as the crystalline oxide. More generally, any material that is able to be used to increase the number of secondary electrons emitted by the bombardment of ions upon plasma discharge may be used.  
         [0025]     In certain implementations, the first protective film  380   a  has a thickness of 400 to 1,000 nm, and the crystalline oxide constituting the second protective film  380   b  has a size of 50 to 1,000 nm. The crystalline oxide may have a shape of a cube or a sphere. If the shape of the crystalline oxide is a cube, the size of the crystalline oxide refers to the length of one side of the cube. Meanwhile, if the shape of the crystalline oxide is a sphere, the size of the crystalline oxide refers to the diameter of the sphere. The surface area of the second protective film composed of the crystalline oxide may be established to be as large as possible to increase the number of secondary electrons emitted. To this end, the first protective film  380   a  may not be completely covered by the second protective film  380   b . Specifically, the second protective film  380   b  may cover about 80% or between 30 to 80% of the surface area of the first protective film  380   a . The second protective film  380   b  may be formed in such a manner that it has a regular or irregular pattern.  
         [0026]     Particles of the crystalline oxide, e.g., particles of an alkaline earth metal, are formed on the first protective film, and as a result, the surface of the second protective film is rugged rather than flat. Accordingly, the surface area of the second protective film where ions collide upon discharge increases, resulting in an increase in the number of secondary electrons emitted. This increase in the number of secondary electrons emitted leads to an improvement in the discharge efficiency of the plasma display panel and a reduction in the firing voltage of the plasma display panel. Further, when the second protective film is composed of Gd 2 O 3 , UV light having a wavelength of about 250 nm is emitted from vacuum ultraviolet (VUV) light of a wavelength of about 147 nm, which is generated from a discharge gas, e.g., Xe, during discharge, resulting in an improvement in the brightness of the plasma display panel.  
         [0027]     Next, an explanation of how the second protective film formed on the first protective film serves to increase the number of secondary electrons emitted by the bombardment of electrons is provided below.  
         [0028]     The second protective film  380   b  is composed of a material having a secondary electron emission coefficient, which results from the bombardment of electrons, higher than that of magnesium oxide. The material constituting the second protective film  380   b  may be single crystalline or polycrystalline. Examples of such single-crystal materials include KBr, KCl, KI, NaBr, NaCl, NaF, NaI and LiF, and examples of such polycrystalline materials include CsCl, KCl, KI, NaBr, NaCl, NaF, NaI, LiF, RbCl, Al 2 CO 3 , BaO, BeO, BaF 2 , CaF 2 , BiCs 3 , GeCs, Rb 3 Sb, and SbCs 3 .  
         [0029]     The secondary electron emission coefficient of magnesium oxide varies depending on the measurement conditions. Magnesium oxide is measured to have a secondary electron emission coefficient lower than 1 under routine conditions. The secondary electron emission coefficients of the single-crystal materials are as follows: KBr=14, KCl=12, KI=10, NaBr=24, NaCl= 14 , NaF=14, NaI=19, and LiF=8.5. The secondary electron emission coefficients of the polycrystalline materials are as follows: CsCl=6.5, KCl=7.5, KI=5.6, NaBr=6.3, NaCl=6.8, NaF=5.7, NaI=5.5, LiF=5.6, RbCl=5.8, Al 2 CO 3 =2-9, BaO=2.3-4.8, BeO=3.4, BaF 2 =4.5, CaF 2 =3.2, BiCs 3 =6, GeCs=7, Rb 3 Sb=7.1, and SbCs 3 =6. The secondary electron emission coefficient of a material is defined as the number of electrons ejected from the material when one electron collides with the material.  
         [0030]     In certain implementations, the first protective film  380   a  may have a thickness of 400 to 1,000 nm, and the single-crystal or polycrystalline oxide constituting the second protective film  380   b  may have a size of 50 to 1,000 nm. If the single-crystal or polycrystalline particles are spherical, the size of the particles refers to the diameters of the spheres. Meanwhile, if the single-crystal or polycrystalline particles are cubic, the size of the particles refers to the length of one side of the cube. Increasing the surface area of the second protective film composed of the single-crystal or polycrystalline oxide serves to increase the number of secondary electrons emitted. In general, the first protective film  380   a  is not completely covered by the second protective film  380   b . Specifically, the surface area of the second protective film  380   b  may be less than 80% or between 30 to 80% of that of the first protective film  380   a . That is, the second protective film  380   b  is formed on the first protective film  380   a  such that it has an island shape. Since the material constituting the second protective film  380   b  is not satisfactorily resistant to the bombardment of ions, the second protective film  380   b  is formed only on portions of the surface of the first protective film  380   a . Accordingly, the magnesium oxide constituting the first protective film  380   a  functions to protect the second protective film  380   b , and the second protective film  380   b  functions to effectively increase the number of secondary electrons emitted by the bombardment of ions and electrons.  
         [0031]     A method for producing a plasma display panel such as is described above is different from conventional methods in that a protective layer having a bilayer structure is formed. Specifically, a plasma display panel is produced by the following procedure. First, sustain electrode pairs are formed on a glass substrate. Thereafter, a dielectric layer is formed on the glass substrate and the sustain electrode pairs. A first protective film and a second protective film are sequentially formed on the dielectric layer. At this time, the first protective film is composed of magnesium oxide, and the second protective film is composed of a secondary electron-emitting material. The kind and size of the secondary electron-emitting material and the shape of the second protective film are as described above. That is, the secondary electron-emitting material is a crystalline oxide and is present in the form of particles within the second protective film. Also, the second protective film is composed of a material having a secondary electron emission coefficient, which results from the bombardment of electrons, higher than that of magnesium oxide.  
         [0032]     The second protective film composed of crystalline oxide particles may be formed by preparing a liquid paste, applying the liquid paste on the first protective film, and drying and calcining the applied liquid paste. The liquid paste used to form the second protective film may be applied to portions of the surface of the first protective film. This application of the liquid paste is performed by a process selected from spray coating, bar coating, spin coating, blade coating, and inkjet printing. The liquid paste is prepared by milling a crystalline oxide powder, such as BeO powder, and mixing the milled powder with a solvent and a dispersant. As the amount of the powder increases (i.e. the content of the powder in the final liquid paste increases), the area of the second protective film formed on the first protective film increases.  
         [0033]     The formation of a second protective film composed of a material having a secondary electron emission coefficient, which results from the bombardment of electrons, higher than that of magnesium oxide is achieved by the following example procedure. First, a first protective film essentially composed of magnesium oxide is formed by a conventional process selected from e-beam deposition, ion plating, sputtering and screen printing. Subsequently, a second protective film is formed on the first protective film such that it has an island shape. The second protective film may be formed by liquid-phase deposition, green sheet lamination or spray coating. When it is intended to form the second protective film having an island shape by green sheet lamination, patterning may be performed in subsequent processing. According to liquid-phase deposition, the concentration of a powder in a liquid paste can be controlled. According to spray coating, the material for the second protective film can be sprayed through a mask disposed on the first protective film.  
         [0034]     An example method for forming the second protective film by liquid-phase deposition includes preparing a liquid paste, applying the liquid paste on the first protective film, and drying and calcining the applied first protective film. First, a crystalline powder, such as a single-crystal KBr or polycrystalline CsCl powder, is milled. The milled powder is mixed with a solvent and a dispersant to prepare a liquid paste. At this time, the powder may be present in an amount of 1 to 30% by weight with respect to the total weight of the liquid paste, and the dispersant may be present in an amount of 5 to 60% by weight with respect to the weight of the powder. As the amount of the powder increases (i.e. the content of the powder in the final liquid paste increases), the area of the second protective film formed on the first protective film increases.  
         [0035]     Subsequently, the liquid paste is applied to the first protective film. The application of the liquid paste may be performed by screen printing, dipping, dye coating or spin coating. Thereafter, the applied liquid paste is dried and calcined to complete the formation of the second protective film. The second protective film thus formed emits an increased number of secondary electrons due to the bombardment of electrons, and as a result, the firing voltage and power consumption of a plasma display panel including the second protective film can be reduced.  
         [0036]     Other implementations are within the scope of the following claims.