Patent Publication Number: US-2010130088-A1

Title: Method for manufacturing plasma display panel

Description:
TECHNICAL FIELD 
     The present invention relates to a method for manufacturing plasma display panels. 
     BACKGROUND ART 
     A plasma display panel (hereinafter referred to simply as “PDP”), among other flat panel displays (FPD), allows achieving a high-speed display as well as a large-size display with ease. The PDP is thus commercialized in various fields such as video display devices and display devices for public communication. 
     In general, an AC-drive and surface discharge type PDP adopts 3-electrode structure, and is formed of two glass substrates, i.e. a front panel and a rear panel confronting each other with a given space between the front panel and the rear panel. The front panel includes display electrodes formed of scan electrodes and sustain electrodes, both of which are shaped like stripes and formed on one of the glass substrates, a dielectric layer covering the display electrodes and storing electric charges for working as a capacitor, and a protective film formed on the dielectric layer and having a thickness of approx. 1 μm. The rear panel includes multiple address electrodes formed on the other glass substrate, a primary dielectric layer covering the address electrodes, barrier ribs formed on the primary dielectric layer, and a phosphor layer painted onto display cells partitioned by the barrier ribs for emitting light in red, green and blue respectively. 
     The front panel confronts the rear panel such that its electrode-mounted surface confronts an electrode-mounted surface of the rear panel, and peripheries of both the panels are sealed in an airtight manner to form a discharge space between the front and rear panels, and the discharge space is partitioned by the barrier ribs. The discharge space is filled with discharge gas of Neon (Ne) and Xenon (Xe) at a pressure ranging from 53 kPa to 80.0 kPa. The PDP allows displaying a color video through this method: Voltages of video signals are selectively applied to the display electrodes for discharging, thereby producing ultra-violet rays, which excite respective colors of the phosphor layers, so that colors in red, green, and blue are emitted, thereby achieving the display of a color video (Refer to Patent Literature 1). 
     The protective layer formed on the dielectric layer of the front panel of the foregoing PDP is expected to carry out the two major functions: (1) protecting the dielectric layer from ion impact caused by the discharge, and (2) emitting primary electrons for generating address discharges. The protection of the dielectric layer from the ion impact plays an important role for preventing a discharge voltage from rising, and the emission of primary electrons for generating the address discharges also plays an important role for eliminating a miss in the address discharges because the miss causes flickers on videos. 
     To reduce the flickers on videos, the number of primary electrons emitted from the protective layer should be increased. For this purpose, silicon (Si) or aluminum (Al), for instance, is added to MgO. 
     In recent years, the number of high-definition TV receivers has increased, which requires the PDP to be manufactured at a lower cost, to consume a lower power, and to be a full HD (high-definition, 1920×1080 pixels, and progressive display) with a higher brightness. The characteristics of emitting electrons from the protective layer determine the picture quality, so that the control over the electron emission characteristics is vital for the picture quality. 
     A protective layer added with a mixture of impurities has been tested whether or not this addition can improve the electron-emission characteristics (refer to Patent Literature 2); however, when the characteristics can be improved by adding the impurity to the protective layer, electric charges are stored on the surface of the protective layer. If the stored electric charges are used as a memory function, the number of electric charges decreases greatly with time, i.e. an attenuation rate becomes greater. To overcome this greater attenuation, a measure is needed such as increment in an applied voltage. The protective layer thus should have two contradictory characteristics, i.e. one is a high emission of electrons, and the other one is a smaller attenuation rate for a memory function, namely, a high retention of electric charges. 
     Patent Literature 1: Unexamined Japanese Patent Publication No. 2007-48733 
     Patent Literature 2: Unexamined Japanese Patent Publication No. 2002-260535 
     DISCLOSURE OF INVENTION 
     The present invention addresses the problem discussed above, and aims to provide a method for manufacturing the PDP comprising:
         a front panel including a substrate on which display electrodes are formed, a dielectric layer covering the display electrodes, and a protective layer formed on the dielectric layer; and   a rear panel opposing to the front panel to form a discharge space therebetween, and including address electrodes formed along the direction intersecting with the display electrodes, and barrier ribs for partitioning the discharge space.       

     The protective layer is manufactured with the method comprising the steps of:
         forming a primary film by depositing the primary film on the dielectric layer; and   forming particles of metal oxide by painting the metal oxide paste, which includes metal oxide particles, organic resin component and diluting agent, onto the primary film, and then firing the paste for attaching the multiple particles of the metal oxide to the primary film. The paste contains the particles of the metal oxide not greater than 1.5% volume content, and the organic resin component is contained within the range from 8.0 vol % to 20.0 vol %.       

     The structure discussed above allows the paste of metal oxide to attach the particles of the metal oxide discretely and uniformly onto the entire surface of the primary film, so that a uniform distribution of coverage with the particles over the entire surface is achievable. The paste is excellent in dispersion, printability, and flammability. As a result, the electron emission characteristics can be improved, and yet, the electric charge retention characteristics are maintained. The PDP having display performance of high definition and high brightness with less power consumption is thus obtainable. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a perspective view illustrating a structure of a PDP in accordance with an embodiment of the present invention. 
         FIG. 2  shows a sectional view illustrating a structure of a front panel of the PDP shown in  FIG. 1 . 
         FIG. 3  shows a flowchart illustrating steps for forming a protective layer of the PDP. 
         FIG. 4  shows characteristics of the metal oxide paste employed in the method for manufacturing the PDP in accordance with the embodiment. 
         FIG. 5  shows cathode luminescence of crystal particles. 
         FIG. 6  shows a result of studying the relation between the characteristics of electron emission and the characteristics of Vscn lighting voltage. 
         FIG. 7  shows a relation between a diameter of a crystal particle and the electron emission characteristics of the PDP. 
         FIG. 8  shows a relation between a diameter of a crystal particle and a rate of occurrence of breakage in barrier ribs of the PDP. 
         FIG. 9  shows an example of particle size distribution of the aggregated particle of the PDP. 
     
    
    
     REFERENCE SIGNS LIST 
     
         
           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 (lightproof layer) 
           8  dielectric layer 
           9  protective layer 
           10  rear panel 
           11  rear glass substrate 
           12  address electrode 
           13  primary dielectric layer 
           14  barrier rib 
           15  phosphor layer 
           16  discharge space 
           81  first dielectric layer 
           82  second dielectric layer 
           91  primary film 
           92  aggregated particle 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An exemplary embodiment of the present invention is demonstrated hereinafter with reference to the accompanying drawings. 
     Exemplary Embodiment 
       FIG. 1  shows a perspective view illustrating a structure of PDP  1  manufactured with a method in accordance with the embodiment of the present invention. PDP  1  is formed of front panel  2  including front glass substrate  3 , and rear panel  10  including rear glass substrate  11 . Front panel  2  and rear panel  10  confront each other and the peripheries thereof are airtightly sealed with sealing agent such as glass frit, thereby forming discharge space  16 , which is filled with discharge gas of Ne and Xe at a pressure falling within a range between 53.3 kPa and 80.0 kPa. 
     Multiple pairs of belt-like display electrodes  6 , each of which is formed of scan electrode  4  and sustain electrode  5 , are placed in parallel with multiple black-stripes (lightproof layer)  7  on front glass substrate  3  of front panel  2 . Dielectric layer  8  working as a capacitor is formed on front glass substrate  3  such that layer  8  can cover display electrodes  6  and lightproof layer  7 . On top of that, protective layer  9  made of magnesium oxide (MgO) is formed on the surface of dielectric layer  8 . 
     Multiple belt-like address electrodes  12  are placed in parallel with one another on rear glass substrate  11  of rear panel  10 , and they are placed along a direction intersecting at right angles with scan electrodes  4  and sustain electrodes  5  formed on front panel  2 . Primary dielectric layer  13  covers those address electrodes  12 . Barrier ribs  14  having a given height are formed on primary dielectric layer  13  placed between respective address electrodes  12 , and ribs  14  partition discharge space  16 . Phosphor layers  15  are applied onto grooves formed between each one of barrier ribs  14  sequentially in response to respective address electrodes  12 . Phosphor layers  15  emit light in red, blue, and green with an ultraviolet ray respectively. A discharge cell is formed at a junction point where scan electrode  4 , sustain electrode  5  and address electrode  12  intersect with one another. The discharge cells having phosphor layers  15  of red, blue, and green respectively are placed along display electrodes  6 , and these cells work as pixels for color display. 
       FIG. 2  shows a sectional view illustrating a structure of front panel  2  of PDP 1  in accordance with this embodiment.  FIG. 2  shows front panel  2  upside down from that shown in  FIG. 1 . As shown in  FIG. 2 , display electrode  6  formed of scan electrode  4  and sustain electrode  5  is patterned on front glass substrate  3  manufactured by the float method. Lightproof layer  7  is also patterned together with display electrode  6  on substrate  3 . Scan electrode  4  is formed of transparent electrodes  4   a,    5   a  made of indium tin oxide (ITO) or tin oxide (SnO 2 ), and sustain electrode  5  is formed of metal bus electrodes  4   b,    5   b  formed on transparent electrodes  4   a,    5   a.  Metal bus electrodes  4   b,    5   b  give electrical conductivity to transparent electrodes  4   a,    5   a  along the longitudinal direction of electrodes  4   a,    5   a,  and they are made of conductive material of which chief ingredient is silver (Ag). 
     Dielectric layer  8  is formed of at least two layers, i.e. first dielectric layer  81  that covers transparent electrodes  4   a,    5   a  and metal bus electrodes  4   b,    5   b  and light proof layer  7  formed on front glass substrate  3 , and second dielectric layer  82  formed on first dielectric layer  81 . 
     The structure of protective layer  9 , which features the present invention, is detailed hereinafter. As shown in  FIG. 2 , protective layer  9  of the PDP in accordance with this embodiment is formed this way: Primary film  91 , made of magnesium oxide (MgO) or MgO containing aluminum (Al), is formed on dielectric layer  8 , and aggregated particles  92  are dispersed discretely and almost uniformly on the entire surface of this primary film  91 . Aggregated particle  92  is formed by aggregating multiple crystal particles made of metal oxide, i.e. MgO. The coverage with particles  92  over the surface of primary film  91  falls within the range from 2% to 12%. 
     The coverage in this context is expressed with this equation: 
       Coverage (%)= a/b× 100 
     where “a” represents an area where aggregated particles  92  are attached within one discharge cell, and “b” represents an area of one discharge cell. 
     Actually the area can be measured this way: take a photo with a camera of an area of one discharge cell partitioned by barrier ribs  14 , and then trim the photo into one cell in the dimension of x×y. Then binarize the photo having undergone the trimming into a binary image (data in black and white). Find the area “a”, i.e. black area occupied by aggregated particles  92 , and find the coverage through the equation of coverage (%)=a/b×100. 
     A method for manufacturing the PDP is demonstrated hereinafter. First, form scan electrodes  4 , sustain electrodes  5 , and black stripes (lightproof layer)  7  on front glass substrate  3 . Scan electrode  4  and sustain electrode  5  are respectively formed of transparent electrodes  4   a,    5   a  and metal bus electrodes  4   b,    5   b.  These transparent electrodes  4   a,    5   a,  and metal bus electrodes  4   b,    5   b  are patterned with a photo-lithography method. Transparent electrodes  4   a,    5   a  are formed by using a thin-film process, and metal bus electrodes  4   b,    5   b  are made by firing the paste containing silver (Ag) at a given temperature before the paste is hardened. Black stripes (lightproof layer)  7  is made by screen-printing the paste containing black pigment, or by forming the black pigment on the entire surface of the glass substrate, and then patterning the pigment with the photolithography method before the paste is fired. 
     Next, paint the dielectric paste onto front glass substrate  3  with a die-coating method such that the paste can cover display electrodes  6  formed of scan electrodes  4  and sustain electrodes  5 , and black stripes (lightproof layer)  7 , thereby forming a dielectric paste layer (dielectric material layer, not shown). Then fire and harden the dielectric paste layer for forming dielectric layer  8  which covers scan electrodes  4 , sustain electrodes  5  and black stripes (lightproof layer)  7 . The dielectric paste is a kind of paint containing binder, solvent, and dielectric material such as glass powder. 
     Next, form protective layer  9  made of magnesium oxide (MgO) on dielectric layer  8  with the vacuum deposition method. The foregoing steps allow forming predetermined structural elements (display electrodes  6 , lightproof layer  7 , dielectric layer  8  and primary film  91 ), except aggregated particle  92 , on front glass substrate  3 . 
     The steps for manufacturing protective layer  9  of PDP  1  are demonstrated hereinafter with reference to  FIG. 3 . As shown in  FIG. 3 , step A 1  is done for forming dielectric layer  8 , and then step A 2  is done for depositing primary film  91  chiefly made of MgO on dielectric layer  8  with the vacuum deposition method by using sintered body of MgO containing some aluminum (Al). 
     Then attach discretely multiple aggregated particles  92  onto primary film  91  (step A 3 ), which is formed in step A 2  for depositing the primary film but is not yet fired. Particle  92  is to be metal oxide particles and is formed by aggregating crystal particles of MgO. In this step A 3 , prepare the paste by mixing aggregated particles  92  with organic resin component into diluting agent, and then, apply this paste onto non-fired primary film  91  with a screen printing method for forming the metal oxide film. 
     The metal oxide paste is detailed later. Instead of the screen printing method, a spraying method, spin-coating method, die-coating method, or slit-coating method can be used for painting this paste on non-fired primary film  91  to form the paste film. 
     The metal oxide paste film undergoes drying step A 4 . Then non-fired primary film  91  formed in step A 2  and the paste film having undergone drying step A 4  are fired together at several hundreds ° C. in firing step A 5 . In step A 5 , solvent and resin component remaining in the paste film are removed, so that protective layer  9 , of which primary film  91  is attached with multiple aggregated particles  92 , is completed. Step A 3  for forming the film of metal oxide paste, step A 4  for drying, and step A 5  for firing are the steps for forming the particles of the metal oxide. 
     In the foregoing discussion, primary film  91  chiefly made of MgO is used; however, according to the present invention, film  91  must withstand intensive sputtering because it should protect dielectric layer  9  from ion-impact, so that it is not necessarily to have high electron emission capability. In other words, a conventional PDP employs a protective layer formed of a primary film chiefly made of MgO in order to satisfy both of the electron emission performance and withstanding performance to the sputtering at a certain level or higher than the certain level. The PDP of the present invention, however, employs the primary film attached with crystal particles of metal oxide onto the film, and crystal particles of the metal oxide dominantly control the electron emission performance. Primary film  91 , therefore, is not necessarily made of MgO, but other materials more excellent in resistance to sputtering, such as Al 2 O 3 , can replace MgO. 
     In this embodiment, MgO particles are used as crystal particles of metal oxide; however, other crystal particles of metal oxide such as strontium (Sr), calcium (Ca), barium (Ba), and aluminum (Al) can replace MgO as long as they have the electron emission performance as high as MgO. Use of these metal oxides can also achieve similar advantages to the foregoing ones. A material of crystal particle is thus not limited only to MgO. 
     The steps discussed above allow forming such structural elements on front glass substrate  3  as display electrodes  6 , black stripes (lightproof layer)  7 , dielectric layer  8 , primary film  91 , and aggregated particles  92  made of MgO. 
     Rear panel  10  is formed this way: First, form a material layer, which is a structural element of address electrode  12 , by screen-printing the paste containing silver (Ag) onto rear glass substrate  11 , or by patterning with the photolithography method a metal film which is formed in advance on the entire surface of rear glass substrate  11 . Then fire this material layer at a given temperature, thereby forming address electrodes  12 . Next, form a dielectric paste layer (not shown) on rear glass substrate  11 , on which address electrodes  12  are formed, by painting dielectric paste onto substrate  11  with the die-coating method such that the dielectric paste layer can cover address electrodes  12 . Then fire the dielectric paste layer for forming primary dielectric layer  13 . The dielectric paste is a kind of paint containing binder, solvent, and dielectric material such as glass powder. 
     Next, paint the paste containing the material for barrier rib  14  onto primary dielectric layer  13 , and pattern the paste into a given shape, thereby forming a barrier-rib material layer. Then fire this barrier-rib material layer for forming barrier ribs  14 . The photolithography method or a sand-blasting method can be used for patterning the paste painted on primary dielectric layer  13 . Next, paint the phosphor paste containing phosphor material onto primary dielectric layer  13  surrounded by barrier ribs  14  adjacent to one another and also onto lateral walls of barrier ribs  14 . Then fire the phosphor paste for forming phosphor layer  15 . The foregoing steps allow completely forming rear panel  10  including the predetermined structural elements on rear glass substrate  11 . 
     Front panel  2  and rear panel  10  discussed above are placed opposite to each other such that display electrodes  6  intersect at right angles with address electrodes  12 , and the peripheries of panel  2  and panel  10  are sealed with glass frit to form discharge space  16  between panels  2  and  10 , and space  16  is filled with discharge gas including Ne, Xe. PDP  1  is thus completed. 
     The paste of metal oxide, used for forming a layer attached with crystal particles of the metal oxide onto primary film  91 , is detailed hereinafter. This layer is formed on primary film  91  in step A 3  for forming the paste film of the metal oxide of the PDP manufactured with the method of the present invention. The description focuses on the experiment on ascertaining the advantage of volume and stable production of the paste. In the following discussion, various chemicals are used; however, they and their numerical conditions such as amounts are examples within the scope of the present invention, so that the present invention is not limited to these examples. 
     The material formed of compositions listed in tables 1 and 2 is blended with a three-roll mill. 
     Considering the discharge characteristics of the PDP, the coverage with aggregated particles  92  made of MgO over primary film  91  preferably falls within the range from 2% to 12%. Since the coverage is determined by a thickness of the film of metal-oxide paste, the content of aggregated particles  92  made of MgO in the metal oxide paste preferably falls within the range from 0.01 vol % to 1.5 vol % based on the film thickness printable with the screen printing method. 
     The numerical values in TABLEs 1 and 2 are expressed in vol %. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Composition No. 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 101 
                 102 
                 103 
                 104 
                 105 
                 106 
                 107 
                 108 
                 109 
                 110 
                 111 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Metal 
                 MgO 
                 0.20 
                 0.20 
                 0.20 
                 0.20 
                 0.20 
                 0.20 
                 0.20 
                 0.20 
                 0.20 
                 0.20 
                 0.20 
               
               
                 oxide 
                 particle 
               
               
                 Organic 
                 Ethyl- 
                 7.21 
                 8.64 
                 9.96 
                 14.76 
                 17.09 
                 22.11 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 resin 
                 cellulose 
               
               
                 compo- 
                 4 cP 
               
               
                 nent 
                 Ethyl- 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 7.21 
                 8.64 
                 9.46 
                 12.47 
                 15.16 
               
               
                   
                 cellulose 
               
               
                   
                 10 cP 
               
               
                 Diluting 
                 Butyl 
                 68.93 
                 67.86 
                 66.88 
                 63.31 
                 61.57 
                 57.84 
                 68.93 
                 67.86 
                 67.25 
                 65.01 
                 63.01 
               
               
                 agent 
                 carbitol 
               
               
                   
                 Terpinol 
                 23.66 
                 23.30 
                 22.96 
                 21.73 
                 21.14 
                 19.85 
                 23.66 
                 23.30 
                 23.09 
                 22.32 
                 21.63 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Total 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
                 100.00 
               
               
                   
               
            
           
         
       
     
                             TABLE 2                          Composition No.                                                                 112   113   114   115   116   117   118   119   120   121   122                                                                             Metal   MgO   0.20   0.20   0.20   0.20   0.20   0.20   0.20   0.20   0.20   0.20   0.20       oxide   particle       Organic   Ethyl-   4.00   5.41   7.21   8.64   9.96   —   —   —   —   —   —       resin   cellulose       compo-   100 cP       nent   Ethyl-   —   —   —   —   —   3.81   5.15   6.31   7.21   8.64   9.96           cellulose           200 cP       Diluting   Butyl   71.32   70.27   68.93   67.86   66.88   71.46   70.46   69.60   68.93   67.86   66.88       agent   carbitol           Terpinol   24.48   24.12   23.66   23.30   22.96   24.53   24.19   23.89   23.66   23.30   22.96                                                             Total   100.00   100.00   100.00   100.00   100.00   100.00   100.00   100.00   100.00   100.00   100.00                    
Composition Nos. 101-111 listed in table 1 show the viscosity (cP) of 4 cP and 10 cP due to difference in molecular weight grade of ethyl-cellulose, and composition Nos. 112-122 show the viscosity (cP) of 100 cP and 200 cP due to difference in molecular weight grade of ethyl-cellulose.
 
     The organic resin components listed in tables 1 and 2 employ ethyl-cellulose; however, cellulose derivatives other than ethyl-cellulose such as hydroxypropyl cellulose, hydroxyethyl cellulose, hydroxypropyl methylcellulose phtalate, hydroxypropyl methylcellulose acetate can be employed. 
     Other than the foregoing cellulose derivatives, the chemical compounds listed below can be also used: acrylic acid, methacrylic acid, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, mono-methyl fumarate, mono-ethyl fumarate, mono-propyl fumarate, mono-methyl maleate, mono-ethyl maleate, mono-propyl maleate, sorbic acid, hydroxymethyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxymethyl methacrylate, 2-hydroxypropyl methacrylate, hydroxyl mono-acrylate, hydroxy mono-methacrylate, diacrylate hydroquinone, hydroquinone 2-dihydroxyl ethyl acrylate, 2-hydroxyethyl methacrylate, N-butyl acrylate, N-butylmethacrylate, isobutyl methacrylate, isobutyl acrylate, 2-ethyl hexylarylate, 2-ethyl hexylmethacrylate, benzylacrylate, benzylmethacrylate, phenoxy-methacrylate, phenoxyacrylate, isobornyl acrylate, isobornyl methacrylate, ethylene glycol dimethacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate, tetraethylene glycol diacrylate, tetraethylene glycol dimethacrylate, butylene glycol dimethacrylate, propylene glycol diacrylate, propylene glycol dimethacrylate, trimethylolethane triacrylate, trimethylolethane trimethacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, tetramethylolpropane tetracrylate, tetramethylol-propane tetramethacrylate, 1.6-hexanediol diacrylate, 1.6-hexanediol dimethacrylate, cardo epoxy diacrylate, glycidyl methacrylate, and glycyl methacrylate ethylene glycol diacrylate. 
     Acrylate or methacrylate of the foregoing chemical compounds can be replaced with fumaric acid, i.e. fumarate, replaced with maleic acid, i.e. maleate, replaced with crotonic acie, i.e. crotonate, or replaced with itaconic acid, i.e. itaconate, or polymer or copolymer such as urethane methacrylate, styrene, acrylamide, methacrylamide, acrylonitrile, methacrylonitrile. 
     Those acrylic resin can be used alone, or can be combined with cellulose derivatives. 
     In tables 1 and 2, diethylene glycol monobutyl ether (butyl carbitol) and terpinol are used as diluting agent; however, other chemicals as follows can be used alone, or two or more than two chemicals below can be combined together for replacing butyl carbitol and terpinol: 
     ethylene glycol mono-methyl ether, ethylene glycol mono-ethyl ether, propylene glycol mono-methyl ether, propylene glycol mono-ethyl ether, diethylene glycol mono-methyl ether, diethylene glycol mono-ethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, propylene glycol mono-methyl ether acetate, propylene glycol mono-ethyl ether acetate, 2-methoxybutyl acetate, 3-methoxybutyl acetate, 4-methoxybutyl acetate, 2-methyl-3-methoxybutyl acetate, 3-methyl-3-methoxybutyl acetate, 3-ethyl-3-methoxybutyl acetate, 2-ethoxybutyl acetate, 4-ethoxybutyl acetate, 4-propoxybutyl acetate, 2-methoxypentyl acetate. 
     The paste can contain, upon necessity, plasticizer such as dioctyl phthalate, dibutyl phthalate, triphenyl phosphate, tributyl phosphate, and dispersant such as glycerop mono-oleate, sorbitan sesquio-leate, homogenol (a product manufactured by Kao Corporation), alkyl-allyl based phosphate. 
     The metal oxide pastes each of which is formed of composition No. 101-122 respectively are painted on front glass substrate  3 , where display electrodes  6 , black stripes (lightproof layer)  7 , dielectric layer  8 , and primary film  91  are formed, with the screen printing method in order to test the respective pastes for printability. The screen printing employs L380S mesh as a screen. 
       FIG. 4  shows the test result, namely, the characteristics of the metal oxide pastes employed in the method for manufacturing the PDP in accordance with the embodiment of the present invention. The horizontal axis of  FIG. 4  represents the content (EC concentration) of ethyl-cellulose, i.e. organic resin component contained in the paste, and the vertical axis represents the viscosity “η” that is measured with Reo-Stress RS600 (made by Hakke Co., Ltd.) at a shear rate of D=1(1/s) per hour. 
     The test for the printability is done through eye observation to find knocking during the printing. A paste accompanied by knocking is marked in black, and a paste free from knocking is marked in white. The knocking in this context refers to “bit-by-bit vertical motion” of a squeegee on the mesh. The squeegee should move on the mesh smooth, but in this case, it somehow scratches on the mesh and vibrates vertically. 
       FIG. 4  also shows viscosities (cP), differing in molecular weight grades of the ethyl-cellulose, as parameters. As  FIG. 4  explicitly depicts, when the content of ethyl-cellulose contained in the metal oxide paste is less than 8 vol %, knocking is observed regardless of the viscosity depending on the molecular weight grades. 
     These phenomena teach that frictional resistance between the screen (mesh) and the squeegee used in the screen printing depends much on the amount of organic resin component contained in the paste rather than the viscosity of the paste. Some dielectric paste available on the market is used for this purpose. It contains organic resin component 5 vol %, and also contains inorganic component, represented by the metal oxide contained in this dielectric paste, not less than 1.5 vol %, which reduces the frictional resistance between the mesh and the squeegee. 
     The coverage with aggregated particles  92  over front glass substrate  3  accompanied by the knocking is measured to find a dispersion of over 10% within an area, while a coverage over substrate  3  free from the knocking measures as good as not greater than 6% within the area. The dispersion within the area in this context is found this way: 
       Dispersion of the coverage within the area=σ/ M× 100(%), 
     where the coverage is measured at 54 points within the area, σ=standard deviation, and M=mean value. 
     The foregoing discussion proves that the organic resin component greater than 8 vol % contained in the metal oxide paste allows excellent printing free from knocking during the screen printing, where the metal oxide paste contains metal oxide particles not greater than 1.5 vol %. 
     On the other hand, as shown in  FIG. 3 , during the steps for manufacturing protective layer  9 , the organic resin component contained in the metal oxide paste is removed in firing step A 5  after step A 3  (forming the paste film) and drying step A 4 . In step A 5 , a greater amount of the organic resin component contained in the paste will increase an amount of residual after the firing. As a result, a completed PDP still carries some residual, which adversely affects the discharge characteristics. 
     The experiment and the test teach the inventors that the organic resin component not greater than 20 vol % in the metal oxide paste allows the residual of the organic resin component not to adversely affect the discharge characteristics of PDP. 
     It is thus concluded that use of the metal oxide paste having the following structure allows manufacturing PDPs excellent in printability and free from degradation in the discharge characteristics caused by the residual of the organic resin component after the firing: The metal oxide paste is formed of the metal oxide particles, the organic resin component, and the diluting agent, and the paste contains the metal oxide particles not greater than 1.5 vol % and the organic resin component falling within the range from 8.0 to 20.0 vol %. 
     Next, the performance of PDP  1  is compared with those of other samples. This experiment is described hereinafter. PDP  1  is produced with the method for manufacturing PDPs in accordance with the embodiment of the present invention. 
     First, samples of PDP having different structures in the protective layer  9  are prepared. Sample  1  is a PDP of which protective layer  9  is formed of the film made of only MgO. Sample  2  is a PDP of which protective layer  9  is formed of MgO into which impurity such as aluminum (Al) or silicon (Si) is doped. Sample  3  is PDP  1  in accordance with the embodiment of the present invention. This PDP  1 , i.e. sample  3  includes protective layer  9  having primary film  91  made of MgO, and aggregated particles  92 , formed by aggregating multiple crystal particles of metal oxide, are uniformly distributed and attached on the entire surface of film  91 . Cathode luminescence of the single crystal particle employed in sample  3  is measured to find the characteristics as shown in  FIG. 5 . 
     Those three samples of PDP having different structures from one another in protective layer  9  are tested for the electron emission performance and the electric charge retention performance. 
     The electron emission performance is a numerical value, i.e. a greater value indicates a greater amount of electron emitted, and is expressed with an amount of primary electron emitted, which is determined by a surface condition and a type of gas. The amount of primary electron emitted can be measured with a method that is used for measuring an amount of electron-current emitted from the surface of protective layer  9  through irradiating the surface with ions or an electron beam. However, it is difficult to test the surface of front panel  2  with a non-destructive examination. The evaluation method disclosed in Unexamined Japanese Patent Publication No. 2007-48733 is thus employed to measure a discharge delay (“ts” value) as the electron emission performance. In other words, a statistical delay time, which is a reference to the easiness of discharge occurrence, among delay times in discharge is measured. This reference number is inversed, and then integrated, thereby obtaining a value which linearly corresponds to the amount of emitted primary electrons, so that the value is used for the test. The delay time in discharge expresses the time of discharge delay (hereinafter referred to as “ts” value) from the pulse rising, and the discharge delay is chiefly caused by the struggle of the initial electrons, which trigger off the discharge, for emitting from the surface of the protective layer into the discharge space. 
     The electric charge retention performance is expressed with a voltage value applied to scan electrodes (hereinafter referred to as a “Vscn” lighting voltage), which is needed for suppressing an electron emission phenomenon of PDP 1 . To be more specific, higher electric charge retention performance can be expected at a lower Vscn lighting voltage, so that a lower Vscn voltage allows the PDP to be driven at a lower voltage design-wise. As a result, the power supply and electric components with a smaller withstanding voltage and a smaller capacity can be employed. In the existing products, semiconductor switching elements such as MOSFET are used for applying a scan voltage sequentially, and these switching elements have approx. 150V as a withstanding voltage. The Vscn lighting voltage is thus preferably lowered to not greater than 120V in the environment of 70° C. taking it into consideration that some change can occur due to variation caused by temperature. 
       FIG. 6  shows the relation between the electron emission performance and the electric charge retention performance. The horizontal axis of  FIG. 6  represents the electron emission performance, and the test result of sample  1  is shown as a reference value. As  FIG. 6  explicitly depicts, sample  3  can achieve controlling the Vscn lighting voltage to be not greater than 120V in the electric charge retention test, and yet, it can achieve approx. six times or more as good as sample  1  in the electron emission performance. Sample  3  includes, as described previously, aggregated particles  92  each of which is formed by aggregating multiple crystal particles of MgO, and particles  92  are uniformly distributed on the surface of primary film  91  made of MgO. 
     In general, the electron emission capability and the electric charge retention capability of protective layer  9  of PDP  1  conflict with each other. For instance, a change in film-forming condition of protective layer  9 , or doping an impurity such as Al, Si, or Ba into protective layer  9  of sample  2  during the film-forming process, will improve the electron emission performance; however, the change or the doping will raise the Vscn lighting voltage as a side effect. 
     The present invention, however, allows obtaining protective layer  9  which can satisfy both of the electron emission capability and the electric charge retention capability appropriate to the PDP which is required to display an increased number of scanning lines as well as to have the smaller size cells due to the advent of high definition display. 
     Next, a particle diameter of the crystal particles employed in sample  3  is described hereinafter. The particle diameter refers to an average particle diameter, which means a volume cumulative average diameter (D 50 ). 
       FIG. 7  shows a test result of sample  3  described in  FIG. 6 , and the test is done for the electron emission performance by changing a particle diameter of the crystal particle of MgO. In  FIG. 7 , the diameter of the crystal particle of MgO shows an average diameter measured with the micro-track HRA particle-size distribution meter in ethanol solution of the first grade reagent defined by JIS or the higher grade of the reagent, and the crystal particle is observed in SEM photo to be measured. 
     As shown in  FIG. 7 , the particle diameter as small as 0.3 μm results in the lower electron emission performance, while the particle diameter as great as 0.9 μm or more results in the higher electron emission performance. 
     A greater number of crystal particles per unit area on protective layer  9  is preferable for increasing the number of emitted electrons within a discharge cell. However, the experiment teaches the inventors the following fact: presence of the crystal particles at the top of barrier rib  14 , with which protective layer  9  of front panel  2  closely contacts, breaks the top of barrier rib  14 , and then the material of rib.  14  falls on phosphor layer  15 , so that the cell encountering this problem cannot normally go on or go out. This breakage in the barrier ribs resists occurring when the crystal particles do not exist at the top of barrier rib  14 , so that a greater number of the crystal particles will increase the occurrence of breakage in barrier ribs  14 . 
       FIG. 8  shows relations between the particle diameter of the crystal particle and the breakage in barrier rib  14 . The same numbers of the crystal particles per unit area although they have different diameters are sprayed in sample  3 . As  FIG. 8  explicitly depicts, the probability of breakage in barrier ribs  14  sharply increases when the diameter of the crystal particle becomes as large as 2.5 μm; however, it stays at a rather low level when the diameter stays not greater than 2.5 μm. 
     The forgoing result tells that aggregated particle  92  preferably has a particle diameter within a range from 0.9 μm to 2.5 μm. However, it is necessary to consider a dispersion of crystal particles in manufacturing and a dispersion of protective layers  9  in manufacturing. 
       FIG. 9  shows an instance of particle size distribution of aggregated particle  92  employed in PDP 1  of the present invention. Aggregated particle  92  has the particle size distribution as shown in  FIG. 9 , and the electron emission characteristics shown in  FIG. 7  and barrier-rib breakage characteristics shown in  FIG. 8  teach that it is preferable to use the aggregated particles, of which average particle diameter, i.e. volume cumulative average diameter (D 50 ), falls within a range from 0.9 μm to 2 μm. 
     As discussed above, PDP  1  having protective layer  9  formed of metal oxide in accordance with this embodiment achieves electron emission capability more than six times as good as the protective layer formed of the primary film made of only MgO, and also achieves the electric charge retention capability such as the Vscn lighting voltage not greater than 120V. As a result, PDP 1  thus can satisfy both of the electron emission capability and the electric charge retention capability, although PDP 1  is to display an increased number of scanning lines as well as to have the smaller size cells due to the advent of high definition display. The PDP, which can display a high definition video at high luminance with lower power consumption, is thus obtainable. 
     In PDP  1  of the present invention, aggregated particles  92  formed of crystal particles of MgO are distributed and attached onto the entire surface of primary film  91  with the coverage ranging from 2% to 12%. This coverage range derives from the experiments for characteristics of the samples each of which coverage with aggregated particles  92  over primary film  91  differs from one another. To be more specific, the experiments prove that the Vscn lighting voltage rises at a greater coverage with aggregated particles  92 , so that the electric charge retention capability degrades. To the contrary, the Vscn lighting voltage lowers at a smaller coverage. The experiments teach the inventors that the coverage not greater than 12% can take full advantage of aggregated particles  92  attached onto the surface of primary film  91 . 
     Aggregated particles  92  of MgO, on the other hand, are needed in each one of the discharge cells for reducing the dispersion of the characteristics. Aggregated particles  92  should be thus attached on the surface of primary film  91 . A smaller coverage with particles  92  thus tends to increase the dispersion on the surface, and an amount of particles  92  attached to each discharge cell differs greatly between the cells. The experiments also teach the inventors that the attachment of particles  92  formed of crystal particles of MgO at the coverage of 4% or more allows reducing the dispersion to approx. not greater than 4%, and the attachment of particles  92  at the coverage of 2% or more allows reducing the dispersion to approx. at 6%, which causes practically no problem. 
     Based on the foregoing results, it is concluded that aggregated particles  92  formed of crystal particles of MgO are preferably attached to primary film  91  at the coverage ranging from 2% to 12%, and more preferably, the coverage ranges from 4% to 12%. 
     INDUSTRIAL APPLICABILITY 
     The present invention is useful for providing a PDP capable of displaying high definition at high luminance with lower power consumption.