Patent Publication Number: US-2011070800-A1

Title: Plasma display panel

Description:
This application is a Continuation application of Ser. No. 11/885,113, filed Aug. 27, 2007, which is the National Stage of International Application No. PCT/JP2007/057038 Mar. 30, 2007. 
    
    
     TECHNICAL FIELD 
     The present invention relates to plasma display panels using gas discharge emission. 
     BACKGROUND ART 
     A plasma display panel (hereinafter, PDP) includes a front panel and a rear panel oppositely disposed to each other and sealed at their periphery with a sealing member. The front panel and the rear panel have discharge spaces therebetween filled with a discharge gas including neon (Ne) and xenon (Xe). 
     The front panel includes a glass substrate; display electrodes consisting of scan electrodes and sustain electrodes arranged in a stripe pattern on the glass substrate; a dielectric layer coating the display electrodes; and a protective layer coating the dielectric layer. The display electrodes each consist of a transparent electrode and a metal bus electrode formed thereon. 
     The rear panel, on the other hand, includes a glass substrate; address electrodes arranged in a stripe pattern on the glass substrate; a dielectric layer coating the address electrodes; barrier ribs formed on the dielectric layer to partition the discharge spaces; and phosphor layers of red, green, and blue. The phosphor layers are formed on the dielectric layer between the barrier ribs and also on side surfaces of the barrier ribs. 
     The front panel and the rear panel are oppositely disposed to each other so that the display electrodes and the address electrodes can cross each other and have discharge cells at their intersections. 
     The discharge cells are arranged in a matrix. In the matrix, three adjacent discharge cells having red, green, and blue phosphor layers, respectively, are arranged in the direction of the display electrodes so as to form a pixel for color display. 
     In a PDP, a predetermined voltage is applied between the scan electrodes and the address electrodes, and another predetermined voltage is applied between the scan electrodes and the sustain electrodes so as to generate gas discharge. The gas discharge generates ultraviolet light which excites the phosphor layers, allowing them to emit light so as to display color images. 
     In general, the discharge gas to be sealed in a PDP has a pressure of about 66.7 kPa (500 Torr), which is lower than the atmospheric pressure. This causes a compressive force to be applied in the direction in which the front panel and the rear panel are compressed to each other with the barrier ribs therebetween. However, in a place with a low atmospheric pressure, the compressive force between the front panel and the rear panel is reduced, so that the PDP is swollen and deformed. Therefore, when a voltage pulse is applied to the address electrodes or the display electrodes to illuminate the PDP, the piezoelectric effect of the dielectric layer causes vibration, which causes the dielectric layer and the barrier ribs to repeatedly collide with each other. As a result, noise having an audible frequency of 10 kHz is generated. 
     In order to overcome this problem, it has been proposed that the sealing portion where the front and rear panels are sealed at their periphery has a larger thickness than the image display area in such a manner that the center portion of the image display area is depressed (refer to Patent Document 1 below, for example). 
     However, if the sealing portion has a larger thickness than the image display area, the peripheral portion of the image display area has a “gap”, thereby causing crosstalk between the top of the barrier ribs and the dielectric layer. The term “crosstalk” refers to a phenomenon that discharge cells adjacent to a discharge cell that is being discharged become difficult to be illuminated. This phenomenon is caused when substances called priming particles (charged particles), which are generated by discharge, fly to the adjacent discharge cells through the “gap” and makes the discharge cells difficult to be discharged. Thus, crosstalk causes lighting failure, and the prevention of crosstalk requires increasing the voltage to be applied to the address electrodes and the like. 
     Patent Document: Japanese Patent Unexamined Publication No. 2004-139921 
     SUMMARY OF THE INVENTION 
     The PDP of the present invention includes a pair of glass substrates disposed opposite to each other so as to form an image display area and an image non-display area. The image non-display area includes a sealing portion in which the glass substrates are sealed at the periphery thereof by a seal layer. At least one of the glass substrates has a thickness of at least 0.5 mm and at most 2.0 mm. The seal layer is made of a glass material having a softening point temperature 30° C. through 70° C. lower than a sealing temperature at which the periphery of the glass substrates is sealed by the seal layer. 
     This structure realizes a PDP which causes neither crosstalk in the peripheral portion of the image display area nor noise in a place having a low atmospheric pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a structure of a PDP according to an embodiment of the present invention. 
         FIG. 2  is a plan view showing a structure of a rear panel and a structure of a sealing portion of the PDP according to the embodiment of the present invention. 
         FIG. 3A  is a sectional view showing an essential part of the PDP according to the embodiment of the present invention. 
         FIG. 3B  is a sectional view showing an essential part of the PDP when a seal layer in a sealing portion is sealed in a shrunk state. 
         FIG. 4  is a graph showing an effect of the thickness of a glass substrate of the PDP according to the embodiment of the present invention. 
     
    
    
     REFERENCE MARKS IN THE DRAWINGS 
     
         
         
           
               1  PDP 
               2  front panel 
               3  front glass substrate 
               4  scan electrode 
               4   a ,  5   a  transparent electrode 
               4   b ,  5   b  bus electrode 
               5  sustain electrode 
               6  display electrode 
               7  dielectric layer 
               8  protective layer 
               9  rear panel 
               10  rear glass substrate 
               11  address electrode 
               12  underlying dielectric layer 
               13  barrier rib 
               14 R,  14 G,  14 B phosphor layer 
               15  discharge space 
               16  discharge cell 
               17  image display area 
               18  sealing portion 
               19  seal layer 
               20  contact area 
           
         
       
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     A PDP of an embodiment of the present invention is described as follows with reference to drawings. 
     Embodiment 
       FIG. 1  is a perspective view showing a structure of the PDP according to the embodiment of the present invention. PDP  1  has front panel  2  and rear panel  9 . Front panel  2  includes insulating front glass substrate  3  and display electrodes  6  which are formed thereon and consist of scan electrodes  4  and sustain electrodes  5 . Front glass substrate  3  is made, for example, of high strain-point float glass having a thickness of 0.5 mm or more and 2.0 mm or less. Display electrodes  6  are coated with dielectric layer  7 , which is further coated with protective layer  8  of MgO. Each scan electrode  4  consists of transparent electrode  4   a  and bus electrode  4   b , and each sustain electrode  5  consists of transparent electrode  5   a  and bus electrode  5   b . Transparent electrodes  4   a  and  5   a  are discharge electrodes. Bus electrodes  4   b  and  5   b  are made of Cr—Cu—Cr or Ag and electrically connected to transparent electrodes  4   a  and  5   a , respectively. 
     Rear panel  9 , on the other hand, includes insulating rear glass substrate  10  having a thickness of 0.5 mm or more and 2.0 mm or less, address electrodes  11  formed thereon, and underlying dielectric layer  12  formed further thereon in such a manner as to coat address electrodes  11 . There are also provided barrier ribs  13  formed at positions on underlying dielectric layer  12 , the positions corresponding to between address electrodes  11 . There are further provided phosphor layers  14 R,  14 G and  14 B, which emit light of red, green and blue on underlying dielectric layer  12  and on the side surfaces of barrier ribs  13 . 
     Front panel  2  and rear panel  9  are oppositely disposed to each other with barrier ribs  13  therebetween so that display electrodes  6  and address electrodes  11  can cross each other and have discharge spaces  15  therebetween. Discharge spaces  15  are filled with a rare gas containing at least one of helium, neon, argon, and xenon as a discharge gas. Discharge spaces  15 , which are formed at the intersections of address electrodes  11  and scan and sustain electrodes  4 ,  5  and are partitioned by barrier ribs  13 , operate as discharge cells  16 . 
     In other words, applying a voltage to address electrodes  11  and another voltage to display electrodes  6  allows specific discharge cells  16  to generate discharge, and the generated discharge produces ultraviolet light and applies it to phosphor layers  14 R,  14 G and  14 B. Phosphor layers  14 R,  14 G and  14 B convert the ultraviolet light into visible light, so that image display is performed in the arrow direction. 
       FIG. 2  is a plan view showing a structure of rear panel  9  and a structure of a sealing portion of PDP  1  according to the embodiment of the present invention. Front panel  2  (unillustrated) and rear panel  9  of PDP  1  are bonded to each other via seal layer  19  formed in sealing portion  18  outside image display area  17 , which is enclosed by a dotted line in  FIG. 2 . 
       FIG. 3A  is a sectional view showing an essential part of PDP according to the embodiment of the present invention and corresponding to the short side of PDP  1  shown in  FIG. 2 . As shown in  FIG. 2 , the sealing is performed in such a manner that the surface of dielectric layer  7  on front panel  2  and the top of barrier ribs  13  on rear panel  9  can be parallel to each other. 
     This step (hereinafter, sealing step) is described in detail as follows. A paste containing a sealing material made of a low-melting-point glass material is applied as seal layer  19  to sealing portion  18  of at least one of front panel  2  and rear panel  9 . Then, front panel  2  and rear panel  9  are aligned with each other and heated while being fixed by a compressive force of a clip. The temperature of the heating is referred to as a sealing temperature. When heated to the sealing temperature, the sealing material is melted. The melted sealing material of seal layer  19  allows front panel  2  and rear panel  9  to be sealed to each other, thereby completing the sealing step. 
     After this, discharge spaces  15  are evacuated to a high vacuum (evacuated and baked) while heated, and then a discharge gas is sealed therein at a predetermined pressure so as to complete PDP  1 . 
     In the sealing step, the sealing material of seal layer  19  is once melted by heating. At this moment, seal layer  19  of PDP  1  may have variations in thickness because of variations in the application of the compressive force due to variations in the relative position between the clip and barrier ribs  13  and also because of the shrinkage of the sealing material of seal layer  19 . 
       FIG. 3B  is a sectional view of the short side of PDP  1  showing its essential part when seal layer  19  in sealing portion  18  is sealed in a shrunk state. In this case, front panel  2  and rear panel  9  have a shorter distance therebetween in the peripheral portion of image display area  17  and in sealing portion  18  than in the remaining area, so that the center portion of image display area  17  is swollen. As a result, dielectric layer  7  or protective layer  8  (unillustrated) of front panel  2  and barrier ribs  13  have contact areas  20  therebetween in and around the boundary of image display area  17  and sealing portion  18 . 
     If an AC voltage pulse is applied to address electrodes  11  or display electrodes  6  when PDP  1  has such a shape, noise is generated. The noise is provably caused as a result that the piezoelectric effect of dielectric layer  7 , underlying dielectric layer  12  or the like causes vibration, and the vibration causes dielectric layer  7  and barrier ribs  13  to repeatedly collide with each other. The noise has a frequency of 10 kHz or so, which is in the range of human audibility. 
     In general, the discharge gas to be sealed in PDP  1  has a pressure of about 66.7 kPa (500 Torr), which is lower than the atmospheric pressure. This causes a compressive force to be applied in the direction in which front panel  2  and rear panel  9  are compressed to each other with barrier ribs  13  therebetween, thereby preventing the occurrence of noise. However, in a place with a low atmospheric pressure, the compressive force between front panel  2  and rear panel  9  is reduced, so that PDP  1  is swollen and deformed. This can easily cause noise. Thus, in a place with a low atmospheric pressure, the noise problem is more significant. 
     In order to overcome this problem, it has been proposed that sealing portion  18  where front and rear panels  2  and  9  are sealed at their periphery has a larger thickness than image display area  17  in such a manner that the center portion of image display area  17  is depressed. 
     However, if seal layer  19  has a large thickness, the peripheral portion of image display area  17  has a “gap” between the top of barrier ribs  13  and dielectric layer  7 . The “gap” causes crosstalk and hence lighting failure, and the prevention of crosstalk requires increasing the address voltage. 
     The sealing material used in seal layer  19  of PDP  1  according to the embodiment of the present invention is substantially made of a glass composition. The glass composition is a mixture of low-melting-point glass and a low coefficient-of-expansion filler and may contain a pigment and the like. The low coefficient-of-expansion filler can be made of any material that does not react with or adversely affect PDP  1 . However, in terms of chemical stability, cost, and safety, the low coefficient-of-expansion filler is preferably made of zircon, aluminum titanate, cordierite, silica, alumina, β-eucryptite, β-spodumene, mullite, β-quartz solid solution, or a mixture thereof. 
     The low-melting-point glass can also be made of any material that does not react with or adversely affect PDP  1 . However, in terms of chemical stability, cost, safety, environmental pollution control, and the like, the low-melting-point glass is preferably P 2 O 5 —SnO-based containing at least one of SiO 2 , WO 3 , MoO 2 , Nb 2 O 5 , TiO 2 , ZrO 2 , ZnO, Al 2 O 2 , B 2 O 3 , Li 2 O, Na 2 O, K 2 O, Cs 2 O, MnO, MgO, CaO, SrO, and BaO. The low-melting-point glass is also preferably Bi 2 O 3 -based containing at least one of MgO, ZnO, B 2 O 3 , SiO 2 , CeO, CaO, SrO, BaO, Al 2 O 3 , In 2 O 3 , Li 2 O, Na 2 O, K 2 O, Cl, and F. The low-melting-point glass is also preferably PbO-based containing at least one of ZnO, B 2 O 3 , SiO 2 , BaO, Li 2 O, Na 2 O, and K 2 O in terms of chemical stability, cost, and safety. 
     In the mixture of the low-melting-point glass and the low coefficient-of-expansion filler, the content of the low-melting-point glass in the total weight of the mixture is in the range of 50 to 99 parts by weight, and the content of the low coefficient-of-expansion filler in the total weight of the mixture is in the range of 1 to 50 parts by weight. When the low-melting-point glass exceeds 99 parts by weight, the low coefficient-of-expansion filler is so small that the coefficient of thermal expansion of the sintered sealing material becomes too large. Consequently, there is a difference in the coefficient of thermal expansion between the sealing material and the glass substrate to be sealed, so that the glass substrate is susceptible to breakage. On the other hand, when the low-melting-point glass is less than 60 parts by weight, the glass component is too small to maintain the fluidity of the sealing material, thereby reducing the air-tightness of sealing portion  18 . 
     After sintered, the sealing material preferably has an average coefficient of thermal expansion of 65×10 −7  to 90×10 −7 /° C. in the range of room temperature to 250° C. When the average coefficient of thermal expansion is outside this range, it becomes difficult to match the coefficient of thermal expansion between the sealing material and the glass substrate to be sealed, thereby making the glass substrate susceptible to breakage. 
     The glass composition is generally used in powder form, and is mixed with a binder, a solvent, and the like to form a sealing material paste. The sealing material paste is applied on the glass substrate and sintered to form seal layer  19 . Alternatively, the sealing material paste may be formed into a compact used for sealing so as to form seal layer  19 . 
     In terms of chemical stability, cost, and safety, a glass paste composition, which is the sealing material paste, is preferably made of resin such as a cellulose derivative, polyvinyl alcohol, polyvinyl butyral, polyethylene glycol, urethane resin, acrylic resin, or melamine resin. Examples of the cellulose derivative include nitrocellulose, methylcellulose, ethyl cellulose, and carboxymethylcellulose. Preferable examples of the solvent contained in the glass paste composition, which is the sealing material paste, include the following in terms of chemical stability, cost, and safety and also in terms of compatibility with the resin such as a cellulose derivative, polyvinyl alcohol, polyvinyl butyral, polyethylene glycol, urethane resin, acrylic resin, or melamine resin. The preferable examples of the solvent include: ethylene glycol monoalkyl ethers; ethylene glycol monoalkyl ether acetates; diethylene glycol dialkyl ethers; propylene glycol monoalkyl ethers; propylene glycol dialkyl ethers; propylene glycol alkyl ether acetates; lactate esters; aliphatic carboxylic acid esters; aromatic hydrocarbons; ketones; esters; and amides. The ethylene glycol monoalkyl ethers include: butyl acetate, ethyl 3-ethoxypropionate, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, and ethylene glycol monobutyl ether. The ethylene glycol monoalkyl ether acetates include: ethylene glycol monomethyl ether acetate and ethylene glycol monoethyl ether acetate. The diethylene glycol dialkyl ethers include: diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dipropyl ether, and diethylene glycol dibutyl ether. The propylene glycol monoalkyl ethers include: propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, and propylene glycol monobutyl ether. The propylene glycol dialkyl ethers include: propylene glycol dimethyl ether, propylene glycol diethyl ether, propylene glycol dipropyl ether, and propylene glycol dibutyl ether. The propylene glycol alkyl ether acetates include: propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate, propylene glycol monopropyl ether acetate, and propylene glycol monobutyl ether acetate. The lactate esters include: methyl lactate, ethyl lactate, and butyl lactate. The aliphatic carboxylic acid esters include: methyl formate, ethyl formate, amyl formate, methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, isobutyl acetate, amyl acetate, isoamyl acetate, hexyl acetate, methyl propionate, ethyl propionate, butyl propionate, methyl butanoate (methyl butyrate), ethyl butanoate (ethyl butyrate), propyl butanoate (propyl butyrate), and isopropyl butanoate (isopropyl butyrate). The aromatic hydrocarbons include toluene and xylene. The ketones include methyl ethyl ketone, 2-heptanone, 3-heptanone, 4-heptanone, and cyclohexanone. The esters include: ethyl 2-hydroxypropionate, ethyl 2-hydroxy-2-methylpropionate, ethoxyethyl acetate, hydroxy ethyl acetate, methyl 2-hydroxy-3-methylbutyrate, methyl 3-methoxy propionate, ethyl 3-methoxy propionate, 3-methoxybutyl acetate, 3-methyl-3-methoxybutyl acetate, 3-methyl-3-methoxybutyl propionate, 3-methyl-3-methoxybutyl butyrate, methyl acetoacetate, ethyl acetoacetate, methyl pyruvate, and ethyl pyruvate. The amides include: N-methylpyrrolidone, N,N-dimethylformamide, N-methylformamide, and N,N-dimethylacetamide. These solvents may be used alone or in combination of two or more. 
     The content of the resin or the solvent in the glass paste composition may be adjusted in such a manner that the plasticity, fluidity, and viscosity can be suitable for its formation or application. The glass paste composition may contain various additives as optional components. Examples of such additives include a surfactant, a development accelerator, an adhesion aid, an antihalation agent, a preservative stabilizer, a deforming agent, an antioxidant, an ultraviolet absorber, low-melting-point glass, a pigment, and a dye. 
     The softening point of seal layer  19  used in the present invention is set 30° C. through 70° C. lower than the sealing temperature at which front panel  2  and rear panel  9  are heated while being aligned with each other and fixed to each other in order to seal the glass substrates to each other at their periphery using seal layer  19  in the sealing step. 
     The softening point is measured using a macro differential thermal analyzer and defined as a second endothermic peak temperature. When the difference between the softening point and the sealing temperature is less than 30° C., the “gap” is caused in sealing portion  18  even at the sealing temperature. This causes crosstalk or lighting failure, making the PDP have display defects. 
     On the other hand, when the difference between the softening point and the sealing temperature is larger than 70° C., seal layer  19  is likely to be softened when heated in a high vacuum in the evacuation step following the sealing step. As a result, the softened seal layer  19  is pulled into image display area  17  so as to swell image display area  17 , thereby causing lighting failure or increasing the lighting voltage in and around sealing portion  18 . 
     Six different types of sealing material each having a predetermined softening point are produced according to the aforementioned conditions as follows. First, low-melting-point glass powder is prepared using Bi 2 O 3 , MgO, ZnO, B 2 O 3 , SiO 2 , CeO, CaO, SrO, BaO, Al 2 O 3 , Li 2 O, Na 2 O, and K 2 O, which are mixed in a predetermined compounding ratio. The resulting mixture is melted for one hour in an electric furnace of 1100° C. through 1200° C. using a platinum crucible. The molten glass is pressed using a brass plate so as to be quenched to form cullet. Then, the cullet is subjected to a twin roller technique to form glass cullet and pulverized using a ball mill so as to form the low-melting-point glass powder. The low-melting-point glass powder is mixed with a commercially available cordierite to adjust the coefficient of thermal expansion, thereby completing the sealing material. The softening points of the six types of sealing material thus prepared are shown in Table 1 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 sample number 
                 melting point (° C.) 
               
               
                   
                   
               
             
            
               
                   
                 1 
                 397 
               
               
                   
                 2 
                 402 
               
               
                   
                 3 
                 410 
               
               
                   
                 4 
                 427 
               
               
                   
                 5 
                 438 
               
               
                   
                 6 
                 444 
               
               
                   
                   
               
            
           
         
       
     
     The six types of sealing material are each mixed with a binder and an organic vehicle including a solvent so as to prepare sealing material pastes. The organic vehicle is made by dissolving 1.2 parts of nitrocellulose in acetate isoamyl. The vehicle is mixed with the sealing material in a weight ratio of 6.5:1 so as to make the viscosity  10 , 000  cP. 
     The preparation of front panels  2  are described as follows with reference to  FIG. 1 . Front glass substrates are 42 inch insulating glass substrates having six different thicknesses of 0.3 mm, 0.6 mm, 1.2 mm, 1.5 mm, 1.8 mm, and 2.8 mm. On each front glass substrate  3  are formed ITO-based transparent electrodes  4   a  and  5   a  in a predetermined pattern. Then, silver paste made of a mixture of silver powder and an organic vehicle is applied in the form of a plurality of lines, and the glass substrate is sintered to form bus electrodes  4   b  and  5   b . On these display electrodes  6  are applied a dielectric glass paste made of a mixture of dielectric glass powder and an organic vehicle by blade coating, dried and sintered so as to form dielectric layer  7 . Then, on dielectric layer  7  is formed magnesium oxide (MgO) by electron beam deposition and sintered so as to form protective layer  8 . As a result, front panels  2  are complete. 
     The production of rear panels  9  are described as follows with reference to  FIG. 1 . Rear glass substrates are 42 inch insulating glass substrates having six different thicknesses of 0.3 mm, 0.6 mm, 1.2 mm, 1.5 mm, 1.8 mm, and 2.8 mm. On each rear glass substrate  10  are formed silver-based address electrodes  11  in a stripe pattern by screen printing. Next, underlying dielectric layer  12  is formed in the same manner as dielectric layer  7  of front panel  2 . Then, glass paste for barrier ribs is repeatedly applied to between the adjacent ones of the address electrodes by screen printing and sintered to form barrier ribs  13 . Finally, red phosphor layers  14 R, green phosphor layers  14 G, and blue phosphor layers  14 B are formed by screen printing on the sides of barrier ribs  13  and on the surface of underlying dielectric layer  12  exposed between barrier ribs  13 , thereby completing rear panels  9 . 
     One of the aforementioned sealing material pastes is applied using a dispenser to at least one of front panel  2  and rear panel  9  thus produced. The applied paste is calcinated at 410° C. After this, front panel  2  and rear panel  9  are combined with each other and sintered at 470° C. for 20 minutes so as to be sealed to each other. The discharge spaces are evacuated to a high vacuum (about 1×10 −4  Pa) at 400° C., and filled with a Ne—Xe discharge gas at a predetermined pressure so as to complete PDP  1 . 
     Table 2 shows the measurement results of the maximum enlargement of gap measured in sealing portion  18  and an increase in the lighting voltage of each of PDPs  1  which are different in the softening point of the sealing material and the glass thickness. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 sample number of sealing material 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 max. 
                   
                 max. 
                   
                 max. 
                   
                 max. 
                   
                 max. 
                   
                 max. 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 enlarge- 
                   
                 enlarge- 
                   
                 enlarge- 
                   
                 enlarge- 
                   
                 enlarge- 
                   
                 enlarge- 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 ment or 
                   
                   
                 ment or 
                   
                 ment or 
                   
                 ment or 
                   
                 ment or 
                   
                 ment or 
                   
               
               
                   
                 shrink- 
                   
                   
                 shrink- 
                   
                 shrink- 
                   
                 shrink- 
                   
                 shrink- 
                   
                 shrink- 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 substrate 
                 age of 
                 increase 
                 age of 
                 increase 
                 age of 
                 increase 
                 age of 
                 increase 
                 age of 
                 increase 
                 age of 
                 increase 
               
               
                   
                 thickness 
                 gap in 
                 in 
                 gap in 
                 in 
                 gap in 
                 in 
                 gap in 
                 in 
                 gap in 
                 in 
                 gap in 
                 in 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 front 
                 rear 
                 sealing 
                 lighting 
                 sealing 
                 lighting 
                 sealing 
                 lighting 
                 sealing 
                 lighting 
                 sealing 
                 lighting 
                 sealing 
                 lighting 
               
               
                   
                 panel 
                 panel 
                 portion 
                 voltage 
                 portion 
                 voltage 
                 portion 
                 voltage 
                 portion 
                 voltage 
                 portion 
                 voltage 
                 portion 
                 voltage 
               
               
                 No. 
                 (mm) 
                 (mm) 
                 (μm) 
                 (V) 
                 (μm) 
                 (V) 
                 (μm) 
                 (V) 
                 (μm) 
                 (V) 
                 (μm) 
                 (V) 
                 (μm) 
                 (V) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 0.3 
                 0.3 
                 breakage due to insifficient strength of substrate 
               
               
                 2 
                 0.3 
                 1.8 
               
               
                 3 
                 0.3 
                 2.8 
               
               
                 4 
                 1.8 
                 0.3 
               
               
                 5 
                 2.8 
                 0.3 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 6 
                 0.6 
                 0.6 
                 ∇ 12 
                 Δ 
                 1.1 
                   
                   
                   
                   
                 Δ 156 
                 Δ 1.4 
                 Δ 
                 196 
                 Δ 7.2 
                 Δ 
                 266 
                 Δ 
                 13.9 
               
               
                 7 
                 0.6 
                 1.2 
                 ∇ 14 
                 Δ 
                 2.1 
                   
                   
                   
                   
                 Δ 138 
                 Δ 2.1 
                 Δ 
                 169 
                 Δ 8.0 
                 Δ 
                 214 
                 Δ 
                 14.8 
               
               
                 8 
                 0.6 
                 1.5 
                 ∇ 14 
                 Δ 
                 2.7 
                   
                   
                   
                   
                 Δ 129 
                 Δ 2.0 
                 Δ 
                 155 
                 Δ 7.9 
                 Δ 
                 201 
                 Δ 
                 14.7 
               
               
                 9 
                 0.6 
                 1.8 
                 ∇ 15 
                 Δ 
                 3.2 
                   
                   
                   
                   
                 Δ 121 
                 Δ 1.9 
                 Δ 
                 142 
                 Δ 7.9 
                 Δ 
                 187 
                 Δ 
                 15.0 
               
               
                 10 
                 0.6 
                 2.8 
                 ∇ 18 
                 Δ 
                 4.1 
                   
                   
                   
                   
                 Δ 110 
                 Δ 1.2 
                 Δ 
                 129 
                 Δ 8.4 
                 Δ 
                 151 
                 Δ 
                 13.2 
               
               
                 11 
                 2.8 
                 0.6 
                 ∇ 18 
                 Δ 
                 4.7 
                   
                   
                   
                   
                 Δ 107 
                 Δ 1.8 
                 Δ 
                 127 
                 Δ 7.4 
                 Δ 
                 150 
                 Δ 
                 14.3 
               
               
                 12 
                 1.2 
                 0.6 
                 ∇ 13 
                 Δ 
                 2.3 
                   
                   
                   
                   
                 Δ 134 
                 Δ 1.9 
                 Δ 
                 166 
                 Δ 4.0 
                 Δ 
                 209 
                 Δ 
                 11.2 
               
               
                 13 
                 1.2 
                 1.2 
                 ∇ 15 
                 Δ 
                 4.5 
                   
                   
                   
                   
                   
                   
                 Δ 
                 105 
                 Δ 2.8 
                 Δ 
                 142 
                 Δ 
                 12.3 
               
               
                 14 
                 1.2 
                 1.5 
                 ∇ 16 
                 Δ 
                 5.2 
                   
                   
                   
                   
                   
                   
                 Δ 
                 93 
                 Δ 2.8 
                 Δ 
                 127 
                 Δ 
                 12.6 
               
               
                 15 
                 1.2 
                 1.8 
                 ∇ 17 
                 Δ 
                 6.1 
                   
                   
                   
                   
                   
                   
                 Δ 
                 84 
                 Δ 2.6 
                 Δ 
                 113 
                 Δ 
                 13.1 
               
               
                 16 
                 1.2 
                 2.8 
                 ∇ 21 
                 Δ 
                 7.9 
                   
                   
                   
                   
                   
                   
                 Δ 
                 66 
                 Δ 2.2 
                 Δ 
                 84 
                 Δ 
                 14.0 
               
               
                 17 
                 2.8 
                 1.2 
                 ∇ 22 
                 Δ 
                 10.3 
                   
                   
                   
                   
                   
                   
                 Δ 
                 64 
                 Δ 1.7 
                 Δ 
                 81 
                 Δ 
                 12.9 
               
               
                 18 
                 1.5 
                 0.6 
                 ∇ 14 
                 Δ 
                 3.7 
                   
                   
                   
                   
                 Δ 127 
                 Δ 1.7 
                 Δ 
                 151 
                 Δ 3.1 
                 Δ 
                 197 
                 Δ 
                 6.8 
               
               
                 19 
                 1.5 
                 1.2 
                 ∇ 17 
                 Δ 
                 5.5 
                   
                   
                   
                   
                   
                   
                 Δ 
                 90 
                 Δ 2.1 
                 Δ 
                 123 
                 Δ 
                 8.1 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 20 
                 1.5 
                 1.5 
                 ∇ 17 
                 Δ 
                 6.1 
                   
                   
                   
                   
                   
                   
                   
                   
                 Δ 
                 102 
                 Δ 
                 8.2 
               
               
                 21 
                 1.5 
                 1.8 
                 ∇ 18 
                 Δ 
                 7.2 
                   
                   
                   
                   
                   
                   
                   
                   
                 Δ 
                 74 
                 Δ 
                 7.5 
               
               
                 22 
                 1.5 
                 2.8 
                 ∇ 21 
                 Δ 
                 10.5 
                   
                   
                   
                   
                   
                   
                   
                   
                 Δ 
                 57 
                 Δ 
                 6.9 
               
               
                 23 
                 2.8 
                 1.5 
                 ∇ 23 
                 Δ 
                 11.9 
                   
                   
                   
                   
                   
                   
                   
                   
                 Δ 
                 55 
                 Δ 
                 7.9 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 24 
                 1.8 
                 0.6 
                 ∇ 16 
                 Δ 
                 5.9 
                   
                   
                   
                   
                 Δ 118 
                 Δ 1.3 
                 Δ 
                 140 
                 Δ 2.7 
                 Δ 
                 183 
                 Δ 
                 4.2 
               
               
                 25 
                 1.8 
                 1.2 
                 ∇ 18 
                 Δ 
                 6.3 
                   
                   
                   
                   
                   
                   
                 Δ 
                 79 
                 Δ 1.8 
                 Δ 
                 107 
                 Δ 
                 3.8 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 26 
                 1.8 
                 1.5 
                 ∇ 18 
                 Δ 
                 7.1 
                   
                   
                   
                   
                   
                   
                   
                   
                 Δ 
                 79 
                 Δ 
                 2.9 
               
               
                 27 
                 1.8 
                 1.8 
                 ∇ 19 
                 Δ 
                 8.1 
                   
                   
                   
                   
                   
                   
                   
                   
                 Δ 
                 52 
                 Δ 
                 2.3 
               
               
                 28 
                 1.8 
                 2.8 
                 ∇ 23 
                 Δ 
                 12.7 
                   
                   
                   
                   
                   
                   
                   
                   
                 Δ 
                 32 
                 Δ 
                 1.9 
               
               
                 29 
                 2.8 
                 1.8 
                 ∇ 23 
                 Δ 
                 13.1 
                   
                   
                 
                   
                 
                 
                   
                 
                   
                   
                   
                   
                 Δ 
                 30 
                 Δ 
                 2.3 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 30 
                 2.8 
                 2.8 
                 ∇ 26 
                 Δ 
                 15.0 
                 ∇ 19 
                 A 12.2 
                 ∇ 13 
                 Δ 8.4 
                 ∇ 5  
                 Δ 4.9 
                 ∇ 
                 3 
                 Δ 2.3 
               
               
                   
               
               
                     indicates data missing or illegible when filed 
               
            
           
         
       
     
     The maximum enlargement of gap of sealing portion  18  is measured as follows. First, the total thickness of front panel  2  and rear panel  9  including the thickness of seal layer  19  is measured using a micrometer. The measured value is compared with the total thickness in a portion which is inside image display area  17  and in which barrier ribs  13  are in contact with dielectric layer  7  or protective layer  8 . When the total thickness including seal layer  19  is larger, that is, sealing portion  18  enlarges, the value is marked with “Δ”. In contrast, when sealing portion  18  shrinks, the value is marked with “Ε”. 
     In order to evaluate an increase in the lighting voltage, the increase in the voltage necessary for lighting compared with the average value of conventional PDPs is expressed as a numeric value. When there is no voltage increase, the value is shown as “0”. 
     In image display area  17 , if a gap is formed between barrier ribs  13  and front panel  2 , charge interference called crosstalk is caused during the discharge between each pixel. Crosstalk makes it impossible to control the lighting of the panel or causes an increase in the lighting voltage. Note that in the actual operation, crosstalk affects the driving of PDP  1  only when the gap exceeds 5 μm and can be ignored when the gap is 5 μm or less. This means that in order to ensure a normal lighting condition, the enlargement of gap of front panel  2  from barrier ribs  13  in image display area  17  is required to be 5 μm or less. The aforementioned maximum enlargements of gap of sealing portion  18  are obtained when the enlargement of gap from barrier ribs  13  is 5 μm or less. 
     In Table 2, the PDPs of No. 1 to 5 in which at least one of front and rear panels  2  and  9  has a 0.3 mm thick glass substrate are broken during their manufacture due to insufficient strength of the substrate, thereby making it impossible to complete the PDPs. 
     The PDPs of No. 6 to 30, on the other hand, having the glass substrates enclosed by the bold line and the sealing material of No. 2 to 6 whose softening points are shown in Table 1 exhibit excellent lighting properties with no lighting voltage increase. In other words, a PDP with no lighting voltage increase can be manufactured when at least one of the glass substrates has a thickness of 1.5 mm or more and 1.8 mm or less, and the softening point of the sealing material is set 30° C. through 70° C. lower than the sealing temperature. 
     Another PDP with no lighting voltage increase can be manufactured when at least one of the glass substrates has a thickness of 0.5 mm or more and 2.0 mm or less, and the softening point of the sealing material is set 40° C. through 70° C. lower than the sealing temperature. 
     The reason for these results is likely to be due to the relationship between the thickness and the rigidity of the glass substrate.  FIG. 4  is a graph showing an effect of the thickness of a glass substrate of the PDP according to the embodiment of the present invention. The graph shows the relationship between the maximum enlargement of gap (a negative value indicates the maximum shrinkage of gap) of sealing portion  18  and a lighting voltage increase in PDPs having glass substrates of different thicknesses. It turns out that the maximum enlargement of gap to achieve stable lighting is 150 μm when the thickness is 0.6 mm; 100 μm when the thickness is 1.2 mm; 75 μm when the thickness is 1.5 mm; 50 μm when the thickness is 1.8 mm; and 15 μm when the thickness is 2.8 mm. 
     As described hereinbefore, the present invention prevents an increase in the lighting voltage due to crosstalk, which is caused by the “gap” in the peripheral portion of the panels when a thin glass substrate is used. 
     The present invention also provides a sealing material having one of the aforementioned softening points by using a lead-free glass composition, thereby achieving a PDP having a small environmental load. 
     INDUSTRIAL APPLICABILITY 
     As described hereinbefore, the PDP of the present invention causes little noise in a place with a low atmospheric pressure and provides excellent lighting to prevent crosstalk due to the “gap” in the peripheral portion of the image display area and also to prevent an increase in the lighting voltage. Thus, the PDP of the present invention is useful as a large-screen image display device.