Abstract:
A plasma display panel is provided with a front board ( 2 ) wherein a dielectric layer ( 8 ) is formed to cover a display electrode ( 6 ) formed on a front glass substrate ( 3 ) and a protection layer ( 9 ) is formed on the dielectric layer ( 8 ); and a back board, which faces the front board ( 2 ) so as to form a discharge space, forms an address electrode in a direction intersecting with the display electrode and has a partitioning wall for partitioning the discharge space. The protection layer ( 9 ) forms a base film ( 91 ) on the dielectric layer ( 8 ) and is constituted by adhering agglomerated particles ( 92 ) wherein a plurality of crystal grains composed of a metal oxide are agglomerated, on the base film ( 91 ) so that the agglomerated particles are distributed over the entire surface. The agglomerated particles are adhered so that the ratio of the transmissivity of the front board ( 2 ) with the agglomerated particles ( 92 ) adhered thereto to the transmissivity of said front board without the agglomerated particles adhered thereto is 85% or more but not more than 99%. Thus, the PDP, which has improved electron discharge characteristics, charge retention characteristics, and achieves high image qualities, low cost and low voltage at the same time, is provided.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a plasma display panel used in a display device, and the like. 
       BACKGROUND ART 
       [0002]    Since a plasma display panel (hereinafter, referred to as a “PDP”) can realize a high definition and a large screen, 65-inch class televisions are commercialized. Recently, PDPs have been applied to high-definition television in which the number of scan lines is twice or more than that of a conventional NTSC method. Meanwhile, from the viewpoint of environmental problems, PDPs without containing a lead component have been demanded. 
         [0003]    A PDP basically includes a front panel and a rear panel. The front panel includes a glass substrate of sodium borosilicate glass produced by a float process; display electrodes each composed of striped transparent electrode and bus electrode formed on one principal surface of the glass substrate; a dielectric layer covering the display electrodes and functioning as a capacitor; and a protective layer made of magnesium oxide (MgO) formed on the dielectric layer. On the other hand, the rear panel includes a glass substrate; striped address electrodes formed on one principal surface of the glass substrate; a base dielectric layer covering the address electrodes; barrier ribs formed on the base dielectric layer; and phosphor layers formed between the barrier ribs and emitting red, green and blue light, respectively. 
         [0004]    The front panel and the rear panel are hermetically sealed so that the surfaces having electrodes face each other. Discharge gas of Ne—Xe is filled in discharge space partitioned by the barrier ribs at a pressure of 400 Torr to 600 Torr. The PDP realizes a color image display by selectively applying a video signal voltage to the display electrode so as to generate electric discharge, thus exciting the phosphor layer of each color with ultraviolet rays generated by the electric discharge so as to emit red, green and blue light (see patent document 1). 
         [0005]    In such PDPs, the role of the protective layer formed on the dielectric layer of the front panel includes protecting the dielectric layer from ion bombardment due to electric discharge, emitting initial electrons so as to generate address discharge, and the like. Protecting the dielectric layer from ion bombardment is an important role for preventing a discharge voltage from increasing. Furthermore, emitting initial electrons so as to generate address discharge is an important role for preventing address discharge error that may cause flicker of an image. 
         [0006]    In order to reduce flicker of an image by increasing the number of initial electrons from the protective layer, an attempt to add Si and Al into MgO has been made for instance. 
         [0007]    Recently, televisions have realized higher definition. In the market, low cost, low power consumption and high brightness full HD (high definition) (1920×1080 pixels: progressive display) PDPs have been demanded. Since an electron emission property from a protective layer determines an image quality of a PDP, it is very important to control the electron emission property. 
         [0008]    In PDPs, an attempt to improve the electron emission property has been made by mixing impurities in a protective layer. However, when the electron emission property is improved by mixing impurities in the protective layer, electric charges accumulate on the surface of the protective layer, thus increasing a damping factor, that is, reducing electric charges to be used as a memory function with the passage of time. Therefore, in order to suppress this, it is necessary to take measures, for example, to increase a voltage to be applied. Thus, a protective layer should have two conflicting properties, high electron emission performance and a high electric charge retention property, i.e., a property of reducing the damping factor of electric charges as a memory function.
   [Patent document 1] Japanese Patent Unexamined Publication No. 2007-48733   
 
       SUMMARY OF THE INVENTION 
       [0010]    A PDP of the present invention includes a front panel including a substrate, a display electrode formed on the substrate, a dielectric layer formed so as to cover the display electrode, and a protective layer formed on the dielectric layer; and a rear panel disposed facing the front panel so that discharge space is formed and including an address electrode formed in a direction intersecting the display electrode, and a barrier rib for partitioning the discharge space. The protective layer is formed by forming a base film on the dielectric layer and attaching aggregated particles of a plurality of aggregated crystal particles of metal oxide over the entire surface of the base film. Furthermore, the aggregated particles are attached so that the ratio of a transmittance of the front panel to which the aggregated particles are attached to a transmittance of the front panel to which the aggregated particles are not attached is not less than 85% and not more than 99%. 
         [0011]    With such a configuration, a PDP having an improved electron emission property and an electric charge retention property and being capable of achieving a high image quality, low cost, and low voltage is provided. Thus, a PDP with low electric power consumption, and high-definition and high-brightness display performance can be achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view showing a structure of a PDP in accordance with an exemplary embodiment of the present invention. 
           [0013]      FIG. 2  is a sectional view showing a configuration of a front panel of the PDP. 
           [0014]      FIG. 3  is an enlarged sectional view showing a protective layer part of the PDP. 
           [0015]      FIG. 4  is an enlarged view illustrating aggregated particles in the protective layer of the PDP. 
           [0016]      FIG. 5  is a graph showing a measurement result of cathode luminescence of a crystal particle. 
           [0017]      FIG. 6  is a graph showing an examination result of electron emission performance of a PDP and a Vscn lighting voltage in the result of experiment carried out to illustrate the effect by the present invention. 
           [0018]      FIG. 7  is a graph showing a relation between a transmittance ratio and the electron emission performance. 
           [0019]      FIG. 8  is a graph showing a relation between a transmittance ratio and the Vscn lighting voltage. 
           [0020]      FIG. 9  is a graph showing a relation between a particle diameter of a crystal particle and the electron emission performance. 
           [0021]      FIG. 10  is a graph showing a relation between a particle diameter of a crystal particle and the rate of occurrence of damage in a barrier rib. 
           [0022]      FIG. 11  is a graph showing an example of the particle size distribution of aggregated particles in a PDP in accordance with the exemplary embodiment of the present invention. 
           [0023]      FIG. 12  is a chart showing steps of forming a protective layer in a method of manufacturing a PDP in accordance with the exemplary embodiment of the present invention. 
       
    
    
     REFERENCE MARKS IN THE DRAWINGS 
       [0000]    
       
         
           
               1  PDP 
               2  front panel 
               3  front glass substrate 
               4  scan electrode 
               4   a ,  5   a  transparent electrode 
               4   b ,  5   b  metal bus electrode 
               5  sustain electrode 
               6  display electrode 
               7  black stripe (light blocking layer) 
               8  dielectric layer 
               9  protective layer 
               10  rear panel 
               11  rear glass substrate 
               12  address electrode 
               13  base dielectric layer 
               14  barrier rib 
               15  phosphor layer 
               16  discharge space 
               81  first dielectric layer 
               82  second dielectric layer 
               91  base film 
               92  aggregated particles 
               92   a  crystal particle 
           
         
       
     
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0047]    Hereinafter, a PDP in accordance with an exemplary embodiment of the present invention is described with reference to drawings. 
       Exemplary Embodiment 
       [0048]      FIG. 1  is a perspective view showing a structure of a PDP in accordance with the exemplary embodiment of the present invention. The basic structure of the PDP is the same as that of a general AC surface-discharge type PDP. As shown in  FIG. 1 , PDP  1  includes front panel  2  including front glass substrate  3 , and the like, and rear panel  10  including rear glass substrate  11 , and the like. Front panel  2  and rear panel  10  are disposed facing each other. The outer peripheries of PDP  1  are hermetically sealed together with a sealing material made of a glass frit, and the like. In discharge space  16  inside the sealed PDP  1 , discharge gas such as Ne and Xe is filled at a pressure of 400 Torr to 600 Torr. 
         [0049]    On front glass substrate  3  of front panel  2 , a plurality of display electrodes  6  each composed of a pair of band-like scan electrode  4  and sustain electrode  5  and black stripes (light blocking layers)  7  are disposed in parallel to each other. On glass substrate  3 , dielectric layer  8  functioning as a capacitor is formed so as to cover display electrodes  6  and blocking layers  7 . Furthermore, protective layer  9  made of, for example, magnesium oxide (MgO) is formed on the surface of dielectric layer  8 . 
         [0050]    Furthermore, on rear glass substrate  11  of rear panel  10 , a plurality of band-like address electrodes  12  are disposed in parallel to each other in the direction orthogonal to scan electrodes  4  and sustain electrodes  5  of front panel  2 , and base dielectric layer  13  covers address electrodes  12 . In addition, barrier ribs  14  with a predetermined height for partitioning discharge space  16  are formed between address electrodes  12  on base dielectric layer  13 . In grooves between barrier ribs  14 , every address electrode  12 , phosphor layers  15  emitting red, green and blue light by ultraviolet rays are sequentially formed by coating. Discharge cells are formed in positions in which scan electrodes  4  and sustain electrodes  5  intersect address electrodes  12 . The discharge cells having red, green and blue phosphor layers  15  arranged in the direction of display electrode  6  function as pixels for color display. 
         [0051]      FIG. 2  is a sectional view showing a configuration of front panel  2  of PDP  1  in accordance with the exemplary embodiment of the present invention.  FIG. 2  is shown turned upside down with respect to  FIG. 1 . As shown in  FIG. 2 , display electrodes  6  each composed of scan electrode  4  and sustain electrode  5  and light blocking layers  7  are pattern-formed on front glass substrate  3  produced by, for example, a float method. Scan electrode  4  and sustain electrode  5  include transparent electrodes  4   a  and  5   a  made of indium tin oxide (ITO), tin oxide (SnO 2 ), or the like, and metal bus electrodes  4   b  and  5   b  formed on transparent electrodes  4   a  and  5   a , respectively. Metal bus electrodes  4   b  and  5   b  are used for the purpose of providing the conductivity in the longitudinal direction of transparent electrodes  4   a  and  5   a  and formed of a conductive material containing a silver (Ag) material as a main component. 
         [0052]    Dielectric layer  8  includes at least two layers, that is, first dielectric layer  81  and second dielectric layer  82 . First dielectric layer  81  is provided for covering transparent electrodes  4   a  and  5   a , metal bus electrodes  4   b  and  5   b  and light blocking layers  7  formed on front glass substrate  3 . Second dielectric layer  82  is formed on first dielectric layer  81 . In addition, protective layer  9  is formed on second dielectric layer  82 . Protective layer  9  includes base film  91  formed on dielectric layer  8  and aggregated particles  92  attached to base film  91 . 
         [0053]    Next, a method of manufacturing a PDP is described. Firstly, scan electrodes  4 , sustain electrodes  5  and light blocking layers  7  are formed on front glass substrate  3 . Transparent electrodes  4   a  and  5   a  and metal bus electrodes  4   b  and  5   b  thereof are formed by patterning by, for example, a photolithography method. Transparent electrodes  4   a  and  5   a  are formed by, for example, a thin film process. Metal bus electrodes  4   b  and  5   b  are formed by firing a paste containing a silver (Ag) material at a predetermined temperature to be solidified. Furthermore, light blocking layer  7  is similarly formed by a method of screen printing a paste containing a black pigment, or a method of forming a black pigment over the entire surface of the glass substrate, then carrying out patterning by a photolithography method, and firing thereof. 
         [0054]    Next, a dielectric paste is coated on front glass substrate  3  by, for example, a die coating method so as to cover scan electrodes  4 , sustain electrodes  5  and light blocking layer  7 , thus forming a dielectric paste layer (dielectric material layer). Since a dielectric paste is coated and then stood still for a predetermined time, the surface of the coated dielectric paste is leveled and flattened. Thereafter, the dielectric paste layer is fired and solidified, thereby forming dielectric layer  8  that covers scan electrode  4 , sustain electrode  5  and light blocking layer  7 . The dielectric paste is a coating material including a dielectric material such as glass powder, a binder and a solvent. Next, protective layer  9  made of magnesium oxide (MgO) is formed on dielectric layer  8  by a vacuum deposition method. In the above-mentioned steps, predetermined components, that is, scan electrode  4 , sustain electrode  5 , light blocking layer  7 , dielectric layer  8 , and protective layer  9  are formed on front glass substrate  3 . Thus, front panel  2  is completed. 
         [0055]    On the other hand, rear panel  10  is formed as follows. Firstly, a material layer as a component of address electrode  12  is formed on rear glass substrate  11  by, for example, a method of screen-printing a paste containing a silver (Ag) material, or a method of forming a metal film on the entire surface and then patterning it by a photolithography method. Then, the material layer is fired at a predetermined temperature. Thus, address electrode  12  is formed. Next, on rear glass substrate  11  on which address electrode  12  is formed, a dielectric paste is coated so as to cover address electrodes  12  by, for example, a die coating method. Thus, a dielectric paste layer is formed. Thereafter, by firing the dielectric paste layer, base dielectric layer  13  is formed. Note here that the dielectric paste is a coating material including a dielectric material such as glass powder, a binder, and a solvent. 
         [0056]    Next, by coating a barrier rib formation paste containing a material for the barrier rib on base dielectric layer  13  and patterning it into a predetermined shape, a barrier rib material layer is formed. Then, the barrier rib material layer is fired to form barrier ribs  14 . Herein, a method of patterning the barrier rib formation paste coated on base dielectric layer  13  may include a photolithography method and a sand-blast method. Next, a phosphor paste containing a phosphor material is coated on base dielectric layer  13  between neighboring barrier ribs  14  and on the side surfaces of barrier ribs  14  and fired. Thereby, phosphor layer  15  is formed. With the above-mentioned steps, rear panel  10  including rear glass substrate  11  provided with predetermined component members is completed. 
         [0057]    In this way, front panel  2  and rear panel  10 , which include predetermined component members, are disposed facing each other so that scan electrodes  4  and address electrodes  12  are disposed orthogonal to each other, and sealed together at the peripheries thereof with a glass frit. Discharge gas including, for example, Ne and Xe, is filled in discharge space  16 . Thus, PDP  1  is completed. 
         [0058]    Herein, first dielectric layer  81  and second dielectric layer  82  forming dielectric layer  8  of front panel  2  are described in detail. A dielectric material of first dielectric layer  81  includes the following material compositions: 20 wt. % to 40 wt. % of bismuth oxide (Bi 2 O 3 ); 0.5 wt. % to 12 wt. % of at least one selected from calcium oxide (CaO), strontium oxide (SrO) and barium oxide (BaO); and 0.1 wt. % to 7 wt. % of at least one selected from molybdenum oxide (MoO 3 ), tungsten oxide (WO 3 ), cerium oxide (CeO 2 ), and manganese oxide (MnO 2 ). 
         [0059]    Instead of molybdenum oxide (MoO 3 ), tungsten oxide (WO 3 ), cerium oxide (CeO 2 ) and manganese oxide (MnO 2 ), 0.1 wt. % to 7 wt. % of at least one selected from copper oxide (CuO), chromium oxide (Cr 2 O 3 ), cobalt oxide (CO 2 O 3 ), vanadium oxide (V 2 O 7 ) and antimony oxide (Sb 2 O 3 ) may be included. 
         [0060]    Furthermore, components other than the above-mentioned components may include material compositions, for example, 0 wt. % to 40 wt. % of zinc oxide (ZnO), 0 wt. % to 35 wt. % of boron oxide (B 2 O 3 ), 0 wt. % to 15 wt. % of silicon oxide (SiO 2 ) and 0 wt. % to 10 wt. % of aluminum oxide (Al 2 O 3 ), which do not include a lead component. The contents of such material compositions are not particularly limited and may be around the range of those in conventional technologies. 
         [0061]    The dielectric materials including these composition components are ground to have an average particle diameter of 0.5 μm to 2.5 μm by using a wet jet mill or a ball mill to form dielectric material powder. Then, 55 wt % to 70 wt % of the dielectric material powders and 30 wt % to 45 wt % of binder components are well kneaded by using a three-roller to form a paste for the first dielectric layer to be used in die coating or printing. 
         [0062]    The binder component is ethyl cellulose, or terpineol containing 1 wt % to 20 wt % of acrylic resin, or butyl carbitol acetate. Furthermore, in the paste, if necessary, dioctyl phthalate, dibutyl phthalate, triphenyl phosphate and tributyl phosphate may be added as a plasticizer; and glycerol monooleate, sorbitan sesquioleate, Homogenol (Kao Corporation), an alkylallyl phosphate, and the like, may be added as a dispersing agent, so that the printing property may be improved. 
         [0063]    Next, this first dielectric layer paste is printed on front glass substrate  3  by a die coating method or a screen printing method so as to cover display electrodes  6  and dried, followed by firing at a temperature of 575° C. to 590° C., that is, a slightly higher temperature than the softening point of the dielectric material. 
         [0064]    Next, second dielectric layer  82  is described. A dielectric material of second dielectric layer  82  includes the following material compositions: 11 wt. % to 20 wt. % of bismuth oxide (Bi 2 O 3 ); furthermore, 1.6 wt. % to 21 wt. % of at least one selected from calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO); and 0.1 wt. % to 7 wt. % of at least one selected from molybdenum oxide (MoO 3 ), tungsten oxide (WO 3 ), and cerium oxide (CeO 2 ). 
         [0065]    Instead of molybdenum oxide (MoO 3 ), tungsten oxide (WO 3 ) and cerium oxide (CeO 2 ), 0.1 wt. % to 7 wt. % of at least one selected from copper oxide (CuO), chromium oxide (Cr 2 O 3 ), cobalt oxide (CO 2 O 3 ), vanadium oxide (V 2 O 7 ), antimony oxide (Sb 2 O 3 ) and manganese oxide (MnO 2 ) may be included. 
         [0066]    Furthermore, as components other than the above-mentioned components, material compositions, for example, 0 wt. % to 40 wt. % of zinc oxide (ZnO), 0 wt. % to 35 wt. % of boron oxide (B 2 O 3 ), 0 wt. % to 15 wt. % of silicon oxide (SiO 2 ) and 0 wt. % to 10 wt. % of aluminum oxide (Al 2 O 3 ), which do not contain a lead component, may be included. The contents of such material compositions are not particularly limited and may be around the range of those in conventional technologies. 
         [0067]    The dielectric materials including these composition components are ground to have an average particle diameter of 0.5 μm to 2.5 μm by using a wet jet mill or a ball mill to form dielectric material powder. Then, 55 wt % to 70 wt % of the dielectric material powders and 30 wt % to 45 wt % of binder components are well kneaded by using a three-roller to form a paste for the second dielectric layer to be used in die coating or printing. The binder component is ethyl cellulose, or terpineol containing 1 wt % to 20 wt % of acrylic resin, or butyl carbitol acetate. Furthermore, in the paste, if necessary, dioctyl phthalate, dibutyl phthalate, triphenyl phosphate and tributyl phosphate may be added as a plasticizer; and glycerol monooleate, sorbitan sesquioleate, Homogenol (Kao Corporation), an alkylallyl phosphate, and the like, may be added as a dispersing agent so that the printing property may be improved. 
         [0068]    Next, this second dielectric layer paste is printed on first dielectric layer  81  by a screen printing method or a die coating method and dried, followed by firing at a temperature of 550° C. to 590° C., that is, a slightly higher temperature than the softening point of the dielectric material. 
         [0069]    Note here that it is preferable that the film thickness of dielectric layer  8  in total of first dielectric layer  81  and second dielectric layer  82  is not more than 41 μm in order to secure the visible light transmittance. In first dielectric layer  81 , in order to suppress the reaction between metal bus electrodes  4   b  and  5   b  and silver (Ag), the content of bismuth oxide (Bi 2 O 3 ) is set to be 20 wt % to 40 wt %, which is higher than the content of bismuth oxide in second dielectric layer  82 . Therefore, since the visible light transmittance of first dielectric layer  81  becomes lower than that of second dielectric layer  82 , the film thickness of first dielectric layer  81  is set to be thinner than that of second dielectric layer  82 . 
         [0070]    In second dielectric layer  82 , it is not preferable that the content of bismuth oxide (Bi 2 O 3 ) is not more than 11 wt % because bubbles tend to be generated in second dielectric layer  82  although coloring does not easily occur. Furthermore, it is not preferable that the content is more than 40 wt % for the purpose of increasing the transmittance because coloring tends to occur. 
         [0071]    As the film thickness of dielectric layer  8  is smaller, the effect of improving the panel brightness and reducing the discharge voltage is more remarkable. Therefore, it is desirable that the film thickness is set to be as small as possible within a range in which withstand voltage is not reduced. From such a viewpoint, in the exemplary embodiment of the present invention, the film thickness of dielectric layer  8  is set to be not more than 41 μm, that of first dielectric layer  81  is set to be 5 μM to 15 μm, and that of second dielectric layer  82  is set to be 20 μm to 36 μm. 
         [0072]    In the thus manufactured PDP, even when a silver (Ag) material is used for display electrode  6 , it is confirmed that less coloring phenomenon (yellowing) of front glass substrate  3  occurs, bubbles are not generated in dielectric layer  8  and dielectric layer  8  having excellent withstand voltage performance can be realized. 
         [0073]    Next, in the PDP in accordance with the exemplary embodiment of the present invention, the reason why these dielectric materials suppress the generation of yellowing or bubbles in first dielectric layer  81  is considered. That is to say, it is known that by adding molybdenum oxide (MoO 3 ) or tungsten oxide (WO 3 ) to dielectric glass containing bismuth oxide (Bi 2 O 3 ), compounds such as Ag 2 MoO 4 , Ag 2 Mo 2 O 7 , Ag 2 Mo 4 O 13 , Ag 2 WO 4 , Ag 2 W 2 O 7 , and Ag 2 W 4 O 13  are easily generated at such a low temperature as not higher than 580° C. In this exemplary embodiment of the present invention, since the firing temperature of dielectric layer  8  is 550° C. to 590° C., silver ions (Ag + ) dispersing in dielectric layer  8  during firing react with molybdenum oxide (MoO 3 ), tungsten oxide (WO 3 ), cerium oxide (CeO 2 ), and manganese oxide (MnO 2 ) in dielectric layer  8  so as to generate a stable compound and are stabilized. That is to say, since silver ions (Ag + ) are stabilized without undergoing reduction, they do not aggregate to form a colloid. Consequently, silver ions (Ag + ) are stabilized, thereby reducing the generation of oxygen accompanying the formation of colloid of silver (Ag). Thus, the generation of bubbles in dielectric layer  8  is reduced. 
         [0074]    On the other hand, in order to make these effects be effective, it is preferable that the content of molybdenum oxide (MoO 3 ), tungsten oxide (WO 3 ), cerium oxide (CeO 2 ), and manganese oxide (MnO 2 ) in the dielectric glass containing bismuth oxide (Bi 2 O 3 ) is not less than 0.1 wt. %. It is more preferable that the content is not less than 0.1 wt. % and not more than 7 wt. %. In particular, it is not preferable that the content is less than 0.1 wt. % because the effect of suppressing yellowing is reduced. Furthermore, it is not preferable that the content is more than 7 wt. % because coloring occurs in the glass. 
         [0075]    That is to say, in dielectric layer  8  of the PDP in accordance with the exemplary embodiment of the present invention, the generation of yellowing phenomenon and bubbles is suppressed in first dielectric layer  81  that is brought into contact with metal bus electrodes  4   b  and  5   b  made of a silver (Ag) material, and high light transmittance is realized by second dielectric layer  82  formed on first dielectric layer  81 . As a result, it is possible to realize a PDP in which dielectric layer  8  as a whole has extremely reduced generation of bubbles and yellowing and which has high transmittance. 
         [0076]    Next, as the feature in accordance with the exemplary embodiment of the present invention, a configuration and a manufacturing method of a protective layer are described. 
         [0077]    In the PDP in accordance with the exemplary embodiment of the present invention, as shown in  FIG. 3 , protective layer  9  includes base film  91  and aggregated particles  92 . Base film  91  made of MgO containing Al as an impurity is formed on dielectric layer  8 . Aggregated particles  92  of a plurality of aggregated crystal particles  92   a  of MgO as metal oxide are discretely scattered on base film  91  so that aggregated particles  92  are distributed over the entire surface substantially uniformly. 
         [0078]    Herein, aggregated particle  92  is a state in which crystal particles  92   a  having a predetermined primary particle diameter are aggregated or necked as shown in  FIG. 4 . In aggregated particles  92 , a plurality of primary particles are not bonded as a solid form with a large bonding strength but they are combined as an assembly structure by static electricity, Van der Waals force, or the like. That is to say, a part or all of crystal particles  92   a  are combined by an external stimulation such as ultrasonic wave to such a degree that they are in a state of primary particles. The particle diameter of aggregated particles  92  is about 1 μm. It is desirable that crystal particle  92   a  has a shape of polyhedron having seven faces or more, for example, truncated octahedron and dodecahedron. 
         [0079]    Furthermore, the primary particle diameter of crystal particle  92   a  of MgO can be controlled by the production condition of crystal particle  92   a . For example, when crystal particle  92   a  of MgO is produced by firing an MgO precursor such as magnesium carbonate or magnesium hydroxide, the particle diameter can be controlled by controlling the firing temperature or firing atmosphere. In general, the firing temperature can be selected in the range from about 700° C. to about 1500° C. When the firing temperature is set to be such a relatively high temperature as not less than 1000° C., the primary particle diameter can be controlled to about 0.3 to 2 μm. Furthermore, when crystal particle  92   a  is obtained by heating an MgO precursor, it is possible to obtain aggregated particles  92  in which a plurality of primary particles are combined by aggregation or a phenomenon called necking during production process. 
         [0080]    Next, results of experiments carried out for confirming the effect of the PDP having the protective layer in accordance with the exemplary embodiment of the present invention are described. 
         [0081]    Firstly, PDPs including protective layers having different configurations are made as trial products. Trial product  1  is a PDP including only a protective layer made of MgO. Trial product  2  is a PDP including a protective layer made of MgO doped with impurities such as Al and Si. Trial product  3  is a PDP in which only primary particles of metal oxide crystal particles are scattered and attached to a protective layer made of MgO. Trial product  4  is a product of the present invention and is a PDP in which aggregated particles of a plurality of aggregated crystal particles are attached to a base film made of MgO so that the aggregated particles are distributed over the entire surface of the base film substantially uniformly. In trial products  3  and  4 , as the metal oxide, single crystal particles of MgO are used. Furthermore, in trial product  4  in accordance with the exemplary embodiment of the present invention, when the cathode luminescence of the crystal particles attached to the base film is measured, trial product  4  has a property shown in  FIG. 5 . Note here that the emission intensity is expressed by relative values. 
         [0082]    PDPs having these four kinds of configurations of protective layers are examined for the electron emission performance and the electric charge retention performance. 
         [0083]    When the electron emission performance is expressed by a larger value, the amount of emitted electrons is lager. The electron emission performance is expressed by the initial electron emission amount determined by the surface state by discharge, kinds of gases and the state thereof. The initial electron emission amount can be measured by a method of measuring the amount of electron current emitted from a surface after the surface is irradiated with ions or electron beams. However, it is difficult to evaluate the front panel surface in a nondestructive way. Therefore, as described in Japanese Patent Unexamined Publication No. 2007-48733, the value called a statistical lag time among lag times at the time of discharge, which is an index showing the discharging tendency, is measured. By integrating the inverse number of the value, a numeric value linearly corresponding to the initial electron emission amount can be obtained. Herein, the thus obtained value is used to evaluate the electron emission amount. This lag time at the time of discharge means a time of discharge delay in which discharge is delayed from the rising time of the pulse. The main factor of this discharge delay is thought to be that the initial electron functioning as a trigger is not easily emitted from a protective layer surface toward discharge space when discharge is started. 
         [0084]    Furthermore, the electric charge retention performance is represented by using, as its index, a value of a voltage applied to a scan electrode (hereinafter, referred to as “Vscn lighting voltage”) necessary to suppress the phenomenon of releasing electric charge when a PDP is manufactured. That is to say, it is shown that a lower Vscn lighting voltage means higher electric charge retention performance. This is advantageous in designing of a panel of a PDP because driving at a low voltage is possible. That is to say, as a power supply or electrical components of a PDP, components having a withstand voltage and a small capacity can be used. In current products, as semiconductor switching elements such as MOSFET for applying a scanning voltage to a panel sequentially, an element having a withstand voltage of about 150 V is used. Therefore, it is desirable that a Vscn lighting voltage is suppressed to not more than 120 V with considering the fluctuation due to temperatures. 
         [0085]    Results of examination of the electron emission performance and the electric charge retention performance are shown in  FIG. 6 . As is apparent from  FIG. 6 , trial product  4  of the exemplary embodiment of the present invention in which aggregated particles of aggregated single crystal particles of MgO are scattered on the base film made of MgO so that the aggregated particles are distributed over the entire surface substantially uniformly can achieve excellent properties: the Vscn lighting voltage can be set to not more than 120 V in the evaluation of the electric charge retention performance, and the electron emission performance shows not less than 6. 
         [0086]    That is to say, in general, the electron emission performance and the electric charge retention performance of a protective layer of a PDP are conflicting with each other. The electron emission performance can be improved, for example, by changing the film formation condition of the protective layer or by forming a film by doping the protective layer with impurities such as Al, Si, and Ba. However, the Vscn lighting voltage is also increased as a side effect. 
         [0087]    In a PDP including the protective layer in accordance with the exemplary embodiment of the present invention, the electron emission performance of not less than 6 and the Vscn lighting voltage as the electric charge retention performance of not more than 120 V can be achieved. Consequently, in a protective layer of a PDP in which according to the high definition, the number of scanning lines tends to increase and the cell size tends to be smaller, both the electron emission performance and the electric charge retention performance can be satisfied. 
         [0088]    Next, a transmittance of the front panel of the PDP in accordance with the exemplary embodiment of the present invention is described. The transmittance means linear transmittance and is expressed by an equation: τP=τT−τd. Herein, τP denotes linear transmittance, τT denotes total light beam transmittance, and τd denotes diffused transmittance. Note here that the linear transmittance is measured by using haze/transmittance meter HM-150 manufactured by Murakami Color Research Laboratory. 
         [0089]    The transmittance of the front panel is changed by the influence of the display electrode, the dielectric layer, and the like, and is also changed by scattering aggregated particles in which several MgO crystal particles are aggregated. Then, in order to express the change of the transmittance by the scattered aggregated particles by removing the influence of the display electrode and the like, the transmittance ratio is calculated by the following procedure. 
         [0090]    Firstly, transmittance τp (sample) of the front panel in a state in which the aggregated particles are attached is calculated from τT (sample) and τd (sample). Next, transmittance τp (reference sample) in a state in which the aggregated particles are removed is calculated from τT (reference sample) and τd (reference sample). 
         [0091]    From these values, the ratio of the transmittance of the front panel in a state in which the aggregated particles are attached with respect to the transmittance of the front panel in a state in which the aggregated particles are removed is calculated from the following relational expression. 
         [0000]      τp (sample)/[τT (sample)×{τp (reference sample)/τT (reference sample)}]×100
 
         [0092]    Note here that the aggregated particles can be removed by the various methods. The aggregated particles are attached with not so strong attaching strength that they can be removed by touching with fingers, by air blowing, or the like. This time, the aggregated particles are removed by such simple methods. In measurement, it is desirable to remove particles on the entire surface of a panel. However, particles are not necessarily removed from the entire surface of a panel. Particles may be removed from the limited measurable range. Then, then the transmittance before removing particles and the transmittance after removing particles may be compared with each other. 
         [0093]      FIG. 7  shows a result of an experiment for examining the electron emission performance by changing the transmittance by scattering aggregated particles in which a plurality of MgO crystal particles are aggregated in product  4  of the present invention described with reference to  FIG. 6  above. 
         [0094]      FIG. 7  shows that when the transmittance ratio is more than 99%, the electron emission performance is reduced, and when it is not more than 99%, high electron emission performance of not less than 6 can be obtained. 
         [0095]      FIG. 8  shows a result of an experiment for examining a Vscn lighting voltage by changing the coverage of MgO crystal particles in trial product  4  of the present invention described with reference to  FIG. 6  above. 
         [0096]      FIG. 8  shows that when the transmittance ratio is not more than 80%, the Vscn lighting voltage increases, and when it is not less than 85%, the Vscn lighting voltage is not more than 120V and high charge retention performance can be obtained. 
         [0097]    Therefore, it is desirable that the aggregated particles are attached so that the ratio of the transmittance of the front panel to which the aggregated particles are attached with respect to the transmittance of the front panel to which the aggregated particles are not attached is not less than 85% and not more than 99%. Thus, both electron emission performance and charge retention performance can be satisfied. 
         [0098]    Next, the particle diameter of crystal particles used in the protective layer of a PDP in accordance with the exemplary embodiment of the present invention is described. In the below-mentioned description, the particle diameter denotes an average particle diameter, and the average particle diameter denotes a volume cumulative mean diameter (D50). 
         [0099]      FIG. 9  shows a result of an experiment for examining the electron emission performance by changing the particle diameter of MgO crystal particle in trial product  4  in accordance with the exemplary embodiment of the present invention described with reference to  FIG. 6  above. In  FIG. 9 , the particle diameter of MgO crystal particle is measured by SEM observation of crystal particles. 
         [0100]      FIG. 9  shows that when the particle diameter is reduced to about 0.3 μm, the electron emission performance is reduced, and that when the particle diameter is substantially not less than 0.9 μm, high electron emission performance can be obtained. 
         [0101]    In order to increase the number of emitted electrons in the discharge cell, it is desirable that the number of crystal particles per unit area on the protective layer is large. According to the experiment carried out by the present inventors, when crystal particles exist in a portion corresponding to the top portion of the barrier rib on the rear panel that is in close contact with the protective film of the front panel, the top portion of the barrier rib may be damaged. As a result, it is shown that the material may be put on a phosphor, causing a phenomenon that the corresponding cell is not normally lighted. The phenomenon that a barrier rib is damaged can be suppressed if crystal particles do not exist on the top portion corresponding to the barrier rib. Therefore, when the number of crystal particles to be attached increases, the rate of occurrence of the damage of the barrier rib increases. 
         [0102]      FIG. 10  is a graph showing a result of an experiment for examining a relation between the particle diameter and the damage of the barrier rib when the same number of crystal particles having different particle diameters are scattered in a unit area in trial product  4  in accordance with the exemplary embodiment of the present invention described with reference to  FIG. 6  above. 
         [0103]    As is apparent from  FIG. 10 , it is shown that when the diameter of crystal particle increases to about 2.5 μm, the probability of the damage of the barrier rib rapidly increases but that when the diameter of crystal particle is less than 2.5 μM, the probability can be suppressed to relatively small. 
         [0104]    Based on the above-mentioned results, it is thought to be desirable that aggregated particles have a particle diameter of not less than 0.9 μm and not more than 2.5 μm in the protective layer of the PDP in accordance with the exemplary embodiment of the present invention. However, in actual mass production of PDPs, variation in manufacturing crystal particles or variation in forming protective layers need to be considered. 
         [0105]    In order to consider the factors such as variation in manufacturing, an experiment using crystal particles having different particle size distributions is carried out.  FIG. 11  is a graph showing one example of the particle size distribution of the aggregated particles in the PDP in accordance with the exemplary embodiment of the present invention. The frequency (%) shown in the ordinate is a rate (%) of the amount of aggregated particles existing in each of divided range of particle diameter shown in the abscissas with respect to the total amount. As a result of the experiment, as shown in  FIG. 11 , when aggregated particles having an average particle diameter of not less than 0.9 μm and not more than 2.5 μm are used, the above-mentioned effect of the present invention can be obtained stably. 
         [0106]    As mentioned above, in the PDP including the protective layer in accordance with the exemplary embodiment of the present invention, the electron emission performance of not less than 6 and the Vscn lighting voltage as the electric charge retention performance of not more than 120 V can be achieved. That is to say, in a protective layer of a PDP in which according to the high definition, the number of scanning lines tends to increase and the cell size tends to be smaller, both the electron emission performance and the electric charge retention performance can be satisfied. Thus, a PDP having a high definition and high brightness display performance and also having low electric power consumption can be realized. 
         [0107]    Next, manufacturing steps of forming a protective layer in a PDP in accordance with the exemplary embodiment of the present invention are described with reference to  FIG. 12 . 
         [0108]    As shown in  FIG. 12 , dielectric layer formation step A 1  of forming dielectric layer  8  having a laminated structure of first dielectric layer  81  and second dielectric layer  82  is carried out. Then, in the following base film vapor-deposition step A 2 , a base film made of MgO is formed on second dielectric layer  82  of dielectric layer  8  by a vacuum deposition method using a sintered body of MgO containing aluminum (Al) as a raw material. 
         [0109]    Then, a step of discretely attaching a plurality of aggregated particles to a non-fired base film formed in base film vapor deposition step A 2  is carried out. 
         [0110]    In this step, firstly, an aggregated particle paste obtained by mixing aggregated particles  92  having a predetermined particle size distribution together with a resin component in a solvent are prepared. Then, in aggregated particle paste film formation step A 3 , the aggregated particle paste is coated on the non-fired base film by printing method such as a screen printing method so as to form an aggregated particle paste film. An example of the method of coating the aggregated particle paste to the not-fired base film so as to form an aggregated particle paste film may include a spray method, a spin-coat method, a die coating method, a slit coat method, and the like, in addition to the screen printing method. 
         [0111]    After the aggregated particle paste film is formed, drying step A 4  of drying the aggregated particle paste film is carried out. 
         [0112]    Thereafter, the non-fired base film formed in base film vapor deposition step A 2  and the aggregated particle paste film formed in aggregated particle paste film formation step A 3  and subjected to drying step A 4  are fired simultaneously at a temperature of several hundred degrees in firing step A 5 . In firing step A 5 , the solvent or resin components remaining in the aggregated particle paste film are removed, so that protective layer  9  in which aggregated particles  92  of a plurality of aggregated crystal particles  92   a  made of metal oxide are attached to base film  91  can be formed. 
         [0113]    With this method, a plurality of aggregated particles  92  can be attached to base film  91  so that aggregated particles  92  are distributed over the entire surface substantially uniformly. 
         [0114]    In addition to such a method, a method of directly spraying particle group together with gas without using a solvent or a scattering method by simply using gravity may be used. 
         [0115]    In the above description, as a protective layer, MgO is used as an example. However, performance required by the base is high sputter resistance performance for protecting a dielectric layer from ion bombardment, and high electric charge retention performance. That is to say, electron emission performance is not required to be so high. In most of conventional PDPs, a protective layer containing MgO as a main component is formed in order to obtain predetermined level or more of electron emission performance and sputter resistance performance. However, for achieving a configuration in which the electron emission performance is mainly controlled by metal oxide single crystal particles, MgO is not necessarily used. Other materials such as Al 2 O 3  having an excellent shock resistance property may be used. 
         [0116]    In this exemplary embodiment, although MgO particles are used as single crystal particles, other single crystal particles may be used. The same effect can be obtained even when other single crystal particles of oxide of metal such as Sr, Ca, Ba, and Al having high electron emission performance similar to MgO are used. Therefore, the kinds of particles are not limited to MgO. 
       INDUSTRIAL APPLICABILITY 
       [0117]    As mentioned above, the present invention is useful in realizing a PDP having high definition and high brightness display performance and low electric power consumption.