Abstract:
A plasma display panel is provided in which an operational quality is prevented from being deteriorated by resonance of a substrate. The plasma display panel includes a partition for dividing a discharge gas space defined by a pair of substrates and a sealing material in accordance with a cell arrangement of a display screen. There is a void space between the upper surface of the end portion of the partition and the surface of the opposed substrate, the surfaces contacting each other. The natural frequency of the portion from the inner edge of the void space to the inner edge of the sealing material is raised above audio frequency region of a human.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a plasma display panel (PDP) and is useful for reducing acoustic noise in operation.  
           [0003]    A display device utilizing a plasma display panel is becoming commonplace as a large screen television set. As such a display device is used widely at home, it has been requested to reduce a slight noise in operation.  
           [0004]    2. Description of the Prior Art  
           [0005]    A surface discharge type PDP for a color display includes a partition for preventing discharge interference between neighboring cells. There are arrangement patterns of the partition including a stripe pattern that divides a display area into columns of a matrix display and a mesh pattern that divides a display area into cells. When the stripe pattern is adopted, a plurality of partitions having a band-like shape in a plan view is arranged in the display area. When the mesh pattern is adopted, one partition (a so-called box rib) that defines each cell individually in a plan view is arranged in the display area. The partition has the height of 150-200 microns and defines a gap size between the substrates in the display area.  
           [0006]    In general, a partition is made of a low melting point glass which is burned, and is formed in the following process. (A) Low melting point glass paste is applied to a glass substrate at a uniform thickness and is dried. (B) On the dried paste layer, a mask of a pattern corresponding to the partition is formed by a photolithography process. (C) Portions of the paste layer that are not masked are removed by a sand blast method in which a cutting material is blown. (D) After removing the mask, the patterned paste layer is burned.  
           [0007]    In the process of forming a partition, some variation of height of the partition is inevitable. Especially, when the paste layer is patterned by the sand blast method and is burned as explained above, the sand blast causes a side cut, i.e., cutting under the mask in the sand blast process, so that the edge portion of the partition may be higher than other portions in a plan view in the subsequent burning process. More specifically, when a design value of the height of the partition is 140 microns, the edge portion becomes higher than other portions by approximately 30 microns. This phenomenon is called a “raise”, and the reason of the “raise” is considered to be uneven of thermal contraction stress. The “raise” phenomenon causes incomplete contact between the substrates in a PDP manufacturing process in which one substrate having a partition is placed on the other substrate. In the major portion of the partition forming area, the upper surface of the partition contacts intimately the surface of the opposed substrate. However, at the vicinity of the “raised” position of the partition forming area, only the raised edge portion of the partition contacts the surface of the opposed substrate. As a result, the substrate is curved microscopically, and a void space is generated between the upper surface of the partition and the surface of the opposed substrate. In this state of the PDP, the substrate is vibrated locally by periodical electrostatic attraction due to an application of a high frequency drive voltage for a display. Thus, minute acoustic noise is generated. This noise drops a quality of the display operation.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to prevent an operational quality from being deteriorated by resonance of a substrate.  
           [0009]    According to one aspect of the present invention, the vibrating portion of a substrate that constitutes a plasma display panel is made to have a natural frequency higher than audio frequency region of a human, so that a user cannot hear the acoustic noise. Supposing that the audio frequency region of a human is 20-20000 hertz, it is the best to make the natural frequency higher than 20000 hertz. However, in the range above 16000 hertz, a sound is hard to hear unless its sound pressure is sufficiently large. Therefore, if the natural frequency is raised above 16000 hertz, the user cannot hear the acoustic noise substantially. Raising the natural frequency above 16000 hertz is useful as a practical method.  
           [0010]    The natural frequency is determined by a length of the vibrating portion of the substrate, a thickness of the substrate, a density of the substrate and a Young&#39;s modulus of the substrate. The natural frequency can be raised by shortening the vibrating portion. In addition, the natural frequency can be raised by any method of enlarging the thickness of the substrate, using a substrate having a small density, or using a substrate having a large Young&#39;s modulus. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIGS. 1A and 1B show a general structure of a PDP according to the present invention.  
         [0012]    [0012]FIG. 2 is a diagram showing an example of a cell structure of a PDP.  
         [0013]    [0013]FIG. 3 is a schematic diagram of a structure of a main portion of the PDP.  
         [0014]    [0014]FIG. 4 is a diagram showing the relationship between the length of a beam and vibration amplitude.  
         [0015]    [0015]FIG. 5 is a diagram showing resonance characteristics of a glass substrate having a high distortion point.  
         [0016]    [0016]FIG. 6 is a diagram showing resonance characteristics of a glass substrate having a high distortion point and h=0.0028 meter.  
         [0017]    [0017]FIG. 7 is a diagram showing resonance characteristics of a soda glass substrate. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Hereinafter, the present invention will be explained more in detail with reference to embodiments and drawings.  
         [0019]    [0019]FIGS. 1A and 1B show a general structure of a PDP according to the present invention. FIG. 1A is a plan view, and FIG. 1B is a cross section of FIG. 1A along  1 B- 1 B line. A PDP  1  comprises a pair of substrate structural bodies  10  and  20 . The substrate structural body means a plate-like structural body including a substrate having a size larger than a display screen  60  and at least one other element constituting a panel. The substrate structural bodies  10  and  20  are made independently of each other and are placed so as to oppose and overlap each other. The peripheral portions of the opposed area are sealed with a sealing material  35  to be a unit. The gap between the opposed substrate structural bodies  10  and  20  sealed with the sealing material  35  makes a discharge gas space. The substrate structural body  10  has a dimension protruding from both sides of the substrate structural body  20  in the horizontal direction, while the substrate structural body  20  has a dimension protruding from both sides of the substrate structural body  10  in the vertical direction. On these protruding portions, electrode terminals drawn out of the display screen  60  are arranged for being connected to a driving circuit. The display screen  60  has a dimension that the peripheral portion thereof is apart from the sealing material  35  by approximately 15 millimeters.  
         [0020]    [0020]FIG. 2 is a diagram showing an example of a cell structure of a PDP. In FIG. 2, the portion including three cells of the PDP  1  corresponding to one pixel display is shown apart from a pair of substrate structural bodies so that the inner structure can be seen easily.  
         [0021]    In each of the cells constituting the display screen, display electrodes X and Y and address electrodes A cross each other. The display electrodes X and Y are arranged on the inner surface of the front glass substrate (the front substrate)  11 . Each of the display electrodes X and Y includes a transparent conductive film  41  that forms a surface discharge gap and a metal film (a bus electrode)  42  that extends over the entire length of the row. The display electrode pairs are covered with a dielectric layer  17  having the thickness of approximately 30-50 microns, and the surface of the dielectric layer  17  is coated with a protection film  18  that is made of magnesia (MgO). The address electrodes A are arranged on the inner surface of the back glass substrate  21  and are covered with a dielectric layer  24 . On the dielectric layer  24 , band-like partitions  29  having the height of approximately 140 microns and being made of a low melting point glass are arranged so that one partition  29  is positioned between address electrodes A. These partitions  29  divide the discharge gas space into columns in the direction along the row of the matrix display and define the size of the discharge gas space in the front and back direction. Each of column spaces  31  of the discharge gas space corresponds to each column and is continuous over all rows. The inner surface of the back side including upper surfaces of the address electrodes A and side faces of the partitions  29  is covered with fluorescent material layers  28 R,  28 G and  28 B of red, green and blue colors for a color display. Italic letters R, G and B in FIG. 2 represent light emission colors of the fluorescent materials. The fluorescent material layers  28 R,  28 G and  28 B are excited locally by ultraviolet rays emitted by the discharge gas and emit light.  
         [0022]    [0022]FIG. 3 is a schematic diagram of a structure of a main portion of the PDP. In FIG. 3, elements of the front substrate structural body except the glass substrate  11  are omitted, and elements of the back substrate structural body except the glass substrate  21  and the partition  29  are omitted. Actually, the thickness of the glass substrates  11  and  21  is 2-3 millimeters, while the thickness of the dielectric layer is approximately 30 microns that is sufficiently small. In addition, the electrodes and the protection film are thinner than the dielectric layer.  
         [0023]    In the PDP  1 , the partitions  29  are formed on the back glass substrate  21  as mentioned above, and the end portion thereof is raised to be higher than other portions. The height ΔH of the raised portion  295  at the end portion of the partition is approximately 30 microns. The sealing material  35  is made of a low melting point glass that has a softening point lower than the partition material. Therefore, in the sealing process for glass-fusing the glass substrate  11  and the glass substrate  21 , the partition  29  is not softened. As a result, the end portion of the glass substrate  11  is deformed to curve slightly in the sealing process, so that a void space  33  having the length L 2  is formed between the glass substrate  11  (strictly the dielectric layer  17 ) and the upper surface of the partition  29 . A so-called floating structure in which the glass substrate  11  is supported unstably (this portion of the structure is called a “beam”) is formed over the range of the length L from the inner edge of the void space  33  to the inner edge of the sealing material  35  that is the fixed edge. In the PDP  1  having the above-mentioned beam, a buzz sound  95  is generated during a display operation. Namely, when applying a high frequency drive voltage to cells, a periodical electrostatic attraction force works between the display electrode X or Y and the address electrode A that are opposed to each other via the discharge gas space. Thus, the beam portion of the glass substrate  11  is vibrated uniquely by absorbing a vibration energy corresponding to the resonance frequency thereof. According to the present invention, the natural frequency of the beam is higher than the audio frequency region of a human, so that a user of the PDP  1  cannot hear the buzz sound. In other words, the buzz sound  95  is eliminated in an artificial manner.  
         [0024]    The natural frequency F of the beam illustrated in FIG. 3 is expressed in the following equality.  
             F   =         a   n       2      π                   L   2         ·         Eh   2       12                 ρ                   (   1   )                               
 
         [0025]    Here, a n  is a constant (=22) in the case of the fixed edge, L is the distance from the inner edge of the void space to the sealing material, E is a Young&#39;s modulus of the front substrate, h is the thickness of the front substrate, and ρ is a density of the front substrate.  
         [0026]    Since the natural frequency F is inversely proportional to the square of L as shown in the equality (1), the natural frequency F becomes higher as the length L of the beam becomes shorter. Furthermore, as shown in FIG. 4, the amplitude of the natural vibration (i.e., the sound pressure of the buzz sound) becomes smaller as the length L of the beam becomes shorter. Therefore, the problem of the buzz sound is solved by shortening the length L of the beam. However, the length L 2  of the void space  33  shown in FIG. 3 is dependent on the raise quantity of the partition  29  and the pressure of the filled discharge gas, so it is not easy to shorten the length L 2 . On the other hand, the length L 1  from the end of the partition  29  (i.e., the raised portion  295 ) to the sealing material  35  can be shortened relatively easily by redesigning the dimension, which is a realistic method for shortening the beam.  
         [0027]    (First Example)  
         [0028]    In the PDP having the front substrate  11  made of a high distortion point glass having E=78 GPa and ρ=2770 kg/m 3 , the relationship between the length L of the beam and the natural frequency F is as shown in FIGS. 5 and 6. As shown in FIG. 6, the measured value of the natural frequency F when h=0.0028 meters is substantially identical to the calculated value.  
         [0029]    In the case where the length L 2  of the void space  33  is 0.01 meters, in order to raise the natural frequency F above the upper limit value 20000 Hz of the audio frequency region, the length L 1  is set to the value that satisfies the conditions below.  
         [0030]    When a substrate having h=0.0028 meters is used, L 1  is less than 0.017 meters.  
         [0031]    When a substrate having h=0.0020 meters is used, L 1  is less than 0.013 meters.  
         [0032]    When a substrate having h=0.0010 meters is used, L 1  is less than 0.006 meters.  
         [0033]    (Second Example)  
         [0034]    In the PDP having the front substrate  11  made of a soda glass having E=73 GPa and ρ=2500 kg/M 3 , the relationship between the length L of the beam and the natural frequency F is as shown in FIG. 7. In the case where the length L 2  of the void space  33  is 0.01 meters, in order to raise the natural frequency F above the upper limit value 20000 Hz of the audio frequency region, the length L 1  is set to the value that satisfies the conditions below.  
         [0035]    When a substrate having h=0.0028 meters is used, L 1  is less than 0.018 meters.  
         [0036]    When a substrate having h=0.0020 meters is used, L 1  is less than 0.013 meters.  
         [0037]    When a substrate having h=0.0010 meters is used, L 1  is less than 0.007 meters.  
         [0038]    As explained above, by shortening the length L of the beam, the natural frequency F of the beam is raised above the audio frequency region. However, without being limited to this method, any other method such as thickening the substrate, using a substrate having a small density, or using a substrate having a large Young&#39;s modulus can be adopted so as to raise the natural frequency F. In other words, it is sufficient that the thickness h of the front substrate  11  satisfies the inequality (2) or that the density ρ satisfies the inequality (3) or that the Young&#39;s modulus E satisfies the inequality (4).  
             h   &gt;         2        πL   2          f   max         a   n       ·         12                 ρ     E                 (   2   )                               
 
         [0039]    Here, h is the thickness of the front substrate, L is the distance from the inner edge of the void space to the sealing material, an is a constant (=22), f max  is the upper limit value of the audio frequency region of a human, ρ is a density of the front substrate, and E is a Young&#39;s modulus of the front substrate.  
             ρ   &lt;       E   12     ·       (         a   n        h       2      π                   L   2          f   max         )     2               (   3   )                               
 
         [0040]    Here, ρ is a density of the front substrate, E is a Young&#39;s modulus of the front substrate, a n  is a constant (=22), h is the thickness of the front substrate, L is the distance from the inner edge of the void space to the sealing material, and f max  is the upper limit value of the audio frequency region of a human.  
             E   &gt;     12                   ρ   ·       (       2                 π                   L   2          f   max           a   n        h       )     2                 (   4   )                               
 
         [0041]    Here, E is a Young&#39;s modulus of the front substrate, ρ is a density of the front substrate, L is the distance from the inner edge of the void space to the sealing material, f max  is the upper limit value of the audio frequency region of a human, a n  is a constant (=22), and h is the thickness of the front substrate.  
         [0042]    While the presently preferred embodiments of the present invention have been shown and described, it will be understood that the present invention is not limited thereto, and that various changes and modifications may be made by those skilled in the art without departing from the scope of the invention as set forth in the appended