Patent Publication Number: US-7723919-B2

Title: Front panel for plasma display panel and method for producing the same

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a front panel for a plasma display panel including powder components and a method for producing the same, and a plasma display panel. 
     Hitherto, as a display device for displaying high-definition television images in a large screen, expectations for a display device using a plasma display panel (hereinafter, referred to as a PDP) have been heightening. Hereinafter, a constitution of a PDP of a conventional example will be described. 
     The PDP of a conventional example is provided with a front panel and a rear panel. 
     The front panel includes a front glass substrate, a plurality of display electrodes formed into stripe form on one surface of the front glass substrate, a dielectric glass layer covering these display electrodes and a dielectric-protection layer covering the dielectric glass layer. 
     The rear panel includes a rear glass substrate, a plurality of address electrodes formed into stripe form on one surface of the rear glass substrate and a dielectric glass layer covering these address electrodes. A plurality of barrier ribs are formed into stripe form on the dielectric glass layer. These barrier ribs are parallel to the address electrodes and located so that the address electrode is positioned between adjacent barrier ribs as seen from a thickness direction of the rear panel. A red, green, or blue phosphor layer is applied in turn to a bottom in a groove formed by two sides of the adjacent barrier ribs with the dielectric glass layer. 
     The PDP has a hermetically-closed structure in which the front panel (the side on which the dielectric-protection layer is formed) and the rear panel (the side on which the barrier ribs are formed) are opposed to each other and the periphery of a space between these two panels is sealed with a sealing member. In hermetically-closed spaces formed by this hermetically-closed structure, a discharge gas such as neon (Ne) or xenon (Xe) is filled to form discharge spaces. When a predetermined voltage is applied between the display electrodes and the address electrodes, a gas discharge is generated in the discharge spaces. The PDP can display color images by exciting the phosphor layer according to the ultraviolet rays generated through the gas discharge to emit visible light. 
     On the other hand, it is known that by dispersing the powder components composed of dielectrics on the dielectric-protection layer of the front panel, the stability of initial electron emission emitted from the dielectric glass layer can be enhanced (improved), and a voltage required for maintaining a wall charge of the dielectric glass layer can be decreased. 
     The powder component can be produced by the following procedure, for example. 
     First, a primary particle having an average particle diameter of about 0.2 to 3.0 μm is produced by heat-treating magnesium hydroxide (MgOH). 
     Next, the produced primary particle is further fired (heat-treated) in order to promote a reaction of unreacted magnesium hydroxide (MgOH) and eliminate residual substances. 
     The particle diameter is ultimately adjusted to an average particle diameter of about 4.0 to 6.0 μm by this firing. 
     The powder component thus produced has a single crystal structure, and therefore the inside and the surface of the powder component become the condition where very small lattice defects typified by a point defect and a dislocation exist. 
     An average particle diameter of the powder components can be adjusted to a proper size. 
     When a large average particle diameter of the powder components is desired, this can be realized, for example, by further heat-treating the fired powder components. Thereby, the average particle diameter of the powder components can be set at a particle diameter of about several tens of micrometers to several hundreds of micrometers. 
     Further, when a small average particle diameter is desired, this can be realized, for example, by pulverizing the fired powder component with an Ultimizer. Thereby, the average particle diameter of the powder components can be adjusted to a level equal to that of the primary particle, or a size of about 0.2 to 3.0 μm. 
     Examples of PDPs of conventional examples, containing powder components, include a PDP disclosed in Patent Document 1 (i.e., Japanese Unexamined Patent Publication No. 2005-149743). In Patent Document 1, an alternating current (AC) PDP including a particle size distribution in which a grain size of the powder component is 5.0 μm or less is disclosed. 
     SUMMARY OF THE INVENTION 
     In the PDP of a conventional example, generally, the front panel and the rear panel are located with a gap of about 10 to 30 μm provided between the dielectric-protection layer of the front panel and the top of the barrier rib of the rear panel. If an average particle diameter of the powder components is set at about 5.0 μm at this time, a particle having a large particle diameter within the range of the particle size distribution, or a plurality of particles piled up may come into physical contact with the barrier rib. Therefore, the barrier rib becomes apt to chip and there is an issue that the production yield of the PDP is reduced. 
     As a method for solving this issue, it is conceivable that the average particle diameter of the powder components is reduced to a level of the primary particle, e.g., about 2.0 μm. However, in the case where the average particle diameter of the powder components is set at about 2.0 μm, there are other issues that the stability of initial electron emission becomes low (poor) and the voltage required for maintaining a wall charge becomes large compared with the case where the average particle diameter is set at about 5.0 μm. 
     That is, yield improvements are traded off for enhancing the stability of initial electron emission and reducing a voltage required for maintaining a wall charge. 
     Therefore, it is an object of the present invention to solve the foregoing conventional issues and to provide a front panel for a PDP, a method for producing the front panel for a PDP, and a PDP provided with the front panel for a PDP, which can suppress the incidence of chipping of the barrier rib of the rear panel for a PDP, can enhance the stability of initial electron emission in the dielectric layer, and can reduce a voltage required for maintaining a wall charge. 
     In order to achieve the above-mentioned object, the present invention is constituted as follows. 
     According to a first aspect of the present invention, there is provided a front panel for a plasma display panel comprising: 
     a substrate, 
     a plurality of electrodes formed on the substrate, 
     a dielectric layer formed to cover the respective electrodes and the substrate, 
     a dielectric-protection layer formed to cover the dielectric layer, and 
     powder components dispersed on the dielectric-protection layer, 
     wherein an annealed layer having a thickness of 10 to 300 nm is formed on at least an exposed surface of each of the powder components, wherein said exposed surface does not contact the dielectric-protection layer. 
     According to a second aspect of the present invention, there is provided the front panel for a plasma display panel as defined in the first aspect, wherein the annealed layer is a thickness of 10 to 100 nm. 
     According to a third aspect of the present invention, there is provided the front panel for a plasma display panel as defined in the first aspect, wherein the annealed layer is formed on an entire surface of the powder component. 
     According to a forth aspect of the present invention, there is provided the front panel for a plasma display panel as defined in the first aspect, 
     wherein the powder component emits light by cathode-luminescence having a peak in a wavelength region of 200 to 300 nm by irradiation of an electron beam, and 
     wherein light by cathode-luminescence emitted from the annealed layer has higher luminous intensity than light by cathode-luminescence emitted from an inner layer adjacent to the annealed layer on an inside of the annealed layer. 
     According to a fifth aspect of the present invention, there is provided the front panel for a plasma display panel as defined in the first aspect, 
     wherein the powder component emits light by cathode-luminescence having a peak in a wavelength region of 200 to 300 nm by irradiation of an electron beam, and 
     wherein light by cathode-luminescence emitted from a top portion of the powder component has higher luminous intensity than light by cathode-luminescence emitted from a bottom portion of the powder component, wherein said top portion does not contact the dielectric-protection layer and said bottom portion contacts the dielectric-protection layer. 
     According to a sixth aspect of the present invention, there is provided the front panel for a plasma display panel as defined in the first aspect, wherein an average particle diameter of the powder components is 3.0 μm or less. 
     According to a seventh aspect of the present invention, there is provided the front panel for a plasma display panel as defined in the first aspect, wherein an average particle diameter of the powder components is 0.2 μm or more. 
     According to an eighth aspect of the present invention, there is provided the front panel for a plasma display panel as defined in the first aspect, wherein a base material of the powder component has a single crystal structure. 
     According to a ninth aspect of the present invention, there is provided the front panel for a plasma display panel as defined in the first aspect, wherein the dielectric layer includes at least one of magnesium oxide, calcium oxide, strontium oxide, and barium oxide. 
     According to a 10th aspect of the present invention, there is provided the front panel for a plasma display panel as defined in the first aspect, wherein the powder component includes at least one of magnesium oxide, calcium oxide, strontium oxide, and barium oxide. 
     According to a 11th aspect of the present invention, there is provided a plasma display panel having the front panel for a plasma display panel as defined in any one of the first aspect to 10th aspect. 
     According to 12th aspect of the present invention, there is provided a method for producing a front panel for a plasma display panel, comprising: 
     forming a plurality of electrodes on a substrate, 
     forming a dielectric layer to cover the respective electrodes and the substrate, 
     forming a dielectric-protection layer to cover the dielectric layer, and 
     dispersing powder components on the dielectric-protection layer and then irradiating an energy wave to the exposed surface of each of the powder components to form an annealed layer having a thickness of 10 to 300 nm. 
     According to 13th aspect of the present invention, there is provided a method for producing a front panel for a plasma display panel as defined in 12th aspect, comprising, in place of dispersing the powder components on the dielectric-protection layer and then forming the annealed layer on the exposed surface of each of the powder components, irradiating an energy wave to an entire surface of each of the powder components to form the annealed layer having a thickness of 10 to 300 nm and then dispersing the powder components on the dielectric-protection layer. 
     According to 14th aspect of the present invention, there is provided the method for producing a front panel for a plasma display panel as defined in 12th aspect, wherein annealing of the surface of each of the powder components is performed by any one of flash lamp annealing, laser annealing, and rapid thermal annealing. 
     In the front panel for a plasma display panel of the present invention, the annealed layer is formed on at least the exposed surface of each of the powder components, which does not contact the dielectric-protection layer. Thereby, it is possible to provide a front panel for a plasma display panel which can suppress the incidence of chipping of the barrier rib of the rear panel for a plasma display panel, can enhance the stability of initial electron emission in the dielectric layer, and can reduce a voltage required for maintaining a wall charge. 
     In accordance with the method for producing a front panel for a plasma display panel of the present invention, after the powder components are dispersed on the dielectric-protection layer, an energy wave is irradiated to the exposed surface of each of the powder components to form the annealed layer. Thereby, it is possible to provide a method for producing a front panel for a plasma display panel which can suppress the incidence of chipping of the barrier rib of the rear panel for a plasma display panel, can enhance the stability of initial electron emission in the dielectric layer, and can reduce a voltage required for maintaining a wall charge. 
     It is also possible to have effects similar to those described above, in place of the above-mentioned procedure, by dispersing the powder components on the dielectric-protection layer after irradiating an energy wave to the entire surface of each of the powder components to form the annealed layer. 
     In the plasma display panel of the present invention, since this plasma display panel is provided with the front panel for a plasma display panel, it is possible to provide a plasma display panel which can suppress the incidence of chipping of the barrier rib of the rear panel for a plasma display panel, can enhance the stability of initial electron emission in the dielectric layer, and can reduce a voltage required for maintaining a wall charge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view schematically showing a constitution of a plasma display panel according to a first embodiment of the present invention; 
         FIG. 2  is a partially enlarged sectional view schematically showing a constitution of a front panel of the plasma display panel according to the first embodiment of the present invention; 
         FIG. 3  is a partially enlarged sectional view of  FIG. 2 ; 
         FIG. 4  is a flow chart showing a method for producing the plasma display panel according to the first embodiment of the present invention; 
         FIG. 5  is a view schematically showing the state of forming an annealed layer on the exposed surface of each of powder components of the plasma display panel according to the first embodiment of the present invention; 
         FIG. 6  is graph showing panel characteristics of plasma displays according to a first conventional example, a second conventional example, and the first embodiment of the present invention; 
         FIG. 7  is a table showing the incidences of chipping of a barrier rib of plasma display panels according to the first conventional example, the second conventional example, and the first embodiment of the present invention; 
         FIG. 8  is a flow chart showing a method for producing a plasma display panel according to a second embodiment of the present invention; 
         FIG. 9  is a view schematically showing the state of forming an annealed layer on the entire surface of each of powder components of the plasma display panel according to the second embodiment of the present invention; 
         FIG. 10  is a partially enlarged sectional view showing a constitution of the powder component of the plasma display panel according to the second embodiment of the present invention; and 
         FIG. 11  is a partially enlarged sectional view schematically showing another formation example of the powder components. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. 
     Hereinafter, preferred embodiments of the present invention will be described by reference to drawings. 
     First Embodiment 
     A constitution of a PDP of a first embodiment of the present invention will be described referring to  FIGS. 1 to 3 . 
       FIG. 1  is a perspective view schematically showing a basic constitution of a PDP  1  according to the first embodiment of the present invention. Further, in  FIG. 1 , a front panel  10  and a rear panel (back panel)  20  of the PDP  1  are illustrated away from each other for ease of reference.  FIG. 2  is a partially enlarged sectional view of the front panel  10 . In  FIG. 2 , the arrangement of the front panel  10  is vertically opposite in direction to that of the front panel  10  in  FIG. 1 .  FIG. 3  is a partially enlarged sectional view of  FIG. 2 . 
     In  FIG. 1 , the PDP  1  includes a front panel  10  for a PDP (hereinafter, referred to as a front panel) and a rear panel  20  for a PDP (hereinafter, referred to as a rear panel) opposed to the front panel  10 . A sealing member (not shown) such as glass frit is located on the periphery of a space between the front panel  10  and the rear panel  20 . By the sealing member, the PDP  1  is sealed in an airtight manner and discharge spaces are formed in the inside of the PDP  1 . In the discharge spaces, a discharge gas such as neon (Ne) or xenon (Xe) is filled. Filling of the discharge gas is performed while reducing the pressure of the discharge spaces to a lower pressure than the atmospheric pressure. 
     The front panel  10  includes a front glass substrate  11  made of sodium borosilicate glass or lead glass. The front glass substrate  11  is formed into flat panel form by a float process. A plurality of strip-shaped display electrodes  12 , which are an example of an electrode, are placed in parallel to each other (formed into stripe form) on one surface of the front glass substrate  11 . The display electrode  12  is made from, for example, silver (Ag) or chromium-copper-chromium (Cr—Cu—Cr). 
     Further, a dielectric glass layer  13 , which is an example of a dielectric layer, is formed on one surface of the front glass substrate  11  so as to cover the respective display electrodes  12 . The dielectric glass layer  13  is formed by use of glass powders having a particle diameter of about 0.1 to 20.0 μm and acts as a capacitor. A dielectric-protection layer  14  is formed on the dielectric glass layer  13  so as to cover the dielectric glass layer  13 . The dielectric-protection layer  14  is made of, for example, magnesium oxide (MgO). Powder components  15  made of a dielectric is (preferably uniformly) dispersed on the dielectric-protection layer  14  as shown in  FIG. 2 . As shown in  FIG. 3 , each of the powder components  15  includes an annealed layer  15   a  formed in a thickness of 10 to 100 nm on the exposed surface which does not contact the dielectric-protection layer  14  and an inner layer  15   b  adjacent to the annealed layer  15   a  on the inside of the annealed layer  15   a  (on the central side). The annealed layer  15   a  will be described in detail later. 
     The rear panel  20  includes a rear glass substrate  21  configured in the same manner as in the front glass substrate  11 . A plurality of strip-shaped address electrodes  22  are placed in parallel to each other on one surface of the rear glass substrate  21 . The address electrode  22  is made from, for example, indium tin oxide (ITO) and silver (Ag) or chromium-copper-chromium (Cr—Cu—Cr). 
     A dielectric glass layer  23  is formed on one surface of the rear glass substrate  21  so as to cover the respective address electrodes  22 . A plurality of barrier ribs (partition walls)  24  are formed in stripe form on the dielectric glass layer  23 . These barrier ribs  24  are parallel to the address electrodes  22  and located so that the address electrode  22  is positioned between adjacent barrier ribs  24  as seen from a thickness direction of the rear panel  20 . Thereby, the barrier ribs  24  partitions the discharge spaces for every address electrode  22 . 
     A phosphor layer (fluorescence layer)  25  is applied to a bottom in a groove  26  formed by two sides of the adjacent barrier ribs  24  with the dielectric glass layer  23 . The phosphor layer  25  includes a red phosphor layer  25   a , a green phosphor layer  25   b , and a blue phosphor layer  25   c . These phosphor layers are formed in turn in a direction orthogonal to the address electrodes  22 . 
     The PDP  1  thus configured can display color images by applying a predetermined voltage between the display electrodes  12  and the address electrodes  22  to generate a gas discharge in the discharge space, and exciting the phosphor layer  25  by ultraviolet rays generated through the gas discharge to emit visible light. 
     Next, referring to  FIGS. 1 to 4 , a method for producing the PDP  1  according to the first embodiment of the present invention will be described.  FIG. 4  is a flow chart showing a method for producing the PDP  1 . Herein, the production method will be described exemplifying materials and dimensions of the respective members in order to facilitate the understanding of the present invention, however, the present invention is not limited to this. 
     First, a method for producing the powder components  15  will be described. The powder components  15  can be produced by performing the following steps S 1  to S 3 . 
     In step S 1 , a primary particle having an average particle diameter of about 0.2 to 3.0 μm is produced by heat-treating magnesium hydroxide (MgOH). 
     In step S 2 , the produced primary particle is further fired (heat-treated) in order to promote a reaction of unreacted magnesium hydroxide (MgOH) and eliminate residual substances. The particle diameter is adjusted to an average particle diameter of about 4.0 to 6.0 μm by this firing. 
     In step S 3 , the fired powder components  15  are pulverized to adjust the particle diameter to an average particle diameter of about 2.0 μm. 
     Thereby, the production of the powder components  15  is completed. 
     Next, a method for producing the front panel  10  will be described. The front panel  10  can be produced by performing the following steps S 4  to S 8 . 
     In step S 4 , a plurality of display electrodes  12  are formed into stripe form on the front glass substrate  11 . 
     In step S 5 , a dielectric glass layer  13  is formed so as to cover the respective display electrodes  12  and the front glass substrate  11 . 
     In step S 6 , a dielectric-protection layer  14  is formed so as to cover the dielectric glass layer  13  using a vacuum deposition process. At this time, a thickness of the dielectric-protection layer  14  is set at, for example, about 0.5 to 1.5 μm. 
     In step S 7 , a mixed paste of organic substances and the powder components  15  is applied onto the dielectric-protection layer  14  using a screen printing method, and thereafter the paste is dried and fired to disperse the powder components  15  on the dielectric-protection layer  14 . 
     At this time, the mixed paste of the organic substances and the powder components  15  to be used is a paste in which for example, the concentration of each of the powder components  15  is generally 0.1 to 20.0% by weight. 
     In step S 8 , an energy wave is irradiated to the exposed surface of each of the powder components  15  dispersed on the dielectric-protection layer  14  to form an annealed layer  15   a  (refer to  FIG. 3 ) (surface annealing is applied). 
     Thereby, the production of the front panel  10  is completed. 
     Examples of the method for forming the annealed layer  15   a  on the exposed surface of each of the powder components  15  include a flash lamp annealing method (hereinafter, referred to as an FLA method) in which a xenon lamp  31  is used as shown in  FIG. 5 . An example of the formation method of the annealed layer  15   a , in which this FLA method is used, will be described in the following referring to  FIG. 5 . 
     First, the front panel  1  is placed on a heater  32  for a substrate with the front glass substrate  11  facing down. 
     Next, the heater  32  for a substrate is heated and the temperature of the front glass substrate  11  is raised generally to about 300 to 500° C., and pulsed light  33  of the order of ms (millisecond) is irradiated to the powder components  15  with the xenon lamp  31  located above the front panel  1 . At this time, the pulse width of the irradiated pulsed light  33  is set at 0.8 to 3.0 ms and its power density is set at 10 to 40 mJ/cm 2 . 
     Thereby, the annealed layer  15   a  (refer to  FIG. 3 ) can be formed on the exposed surface of each of the powder components  15 . 
     In addition, from the foregoing FLA method, it can be estimated that the surface temperature of the front glass substrate  11  is 1400° C. or higher since the temperature at which the surface of a high purity silicon substrate is melted is about 1400° C. From findings of heretofore known technology of doping a silicon (Si) semiconductor with impurities, it is known that the depth, to which the heat energy reaches when the surface temperature of the front glass substrate  11  reaches 1000° C. or higher, is generally about several nanometers to 100 nm. Therefore, the annealed layer  15   a  is formed in a thickness of about 10 to 100 nm on the exposed surface of each of the powder components  15 . 
     Next, a method for producing the rear panel  20  will be described. The rear panel  20  can be produced by performing the following steps S 9  to S 12 . 
     In step S 9 , a plurality of address electrodes  22  are formed into stripe form on the rear glass substrate  21 . 
     In step S 10 , a dielectric glass layer  23  is formed so as to cover the respective address electrodes  22 . 
     In step S 11 , a plurality of barrier ribs  24  are formed into stripe form on the dielectric glass layer  23 . These barrier ribs  24  are parallel to the address electrode  22  and located so that the address electrode  22  is positioned between adjacent barrier ribs  24  as seen from a thickness direction of the rear panel  20 . 
     In step S 12 , a red phosphor layer  25   a , a green phosphor layer  25   b , and a blue phosphor layer  25   c  are applied in turn to a bottom in a groove  26  formed by two sides of the adjacent barrier ribs  24  with the dielectric glass layer  23 . 
     Thereby, the production of the rear panel  20  is completed. 
     In addition, the order of production of the front panel  10  and the rear panel  20  is indifferent. That is, the front panel  10  and the rear panel  20  may be produced simultaneously, or may be produced in turn in any order. 
     Next, a method for producing the PDP  1  by use of the front panel  10  and the rear panel  20  thus produced will be described. The PDP  1  can be produced by performing the following steps S 13  to S 15 . 
     In step S 13 , the front panel  10  and the rear panel  20  are opposed to each other so that the powder components  15  are opposed to the barrier ribs  24 , and the periphery of these two panels is sealed with a sealing member (not shown) to form a hermetically-closed space, and air within the formed hermetically-closed space is evacuated to reduce the pressure of the space. 
     In step S 14 , the depressurized hermetically-closed space is filled with a discharge gas such as neon (Ne) or xenon (Xe) to form discharge spaces. 
     In step S 15 , a lighting test is performed to observe whether the PDP is illuminated or not by application of a predetermined voltage to the discharge spaces. 
     Thereby, the production of the PDP  1  is completed. 
     Next, the results of performing a comparison between the panel characteristic of the PDP  1  according to the first embodiment of the present invention and the panel characteristics of the PDPs of first and second conventional examples will be described referring to  FIGS. 6 and 7 . Here, a PDP containing the powder components in which the annealed layer  15   a  is not formed and an average particle diameter of the powder is set at 5.0 μm is considered as a PDP of the first conventional example. A PDP containing the powder components in which the annealed layer  15   a  is not formed and an average particle diameter of the powder is set at 2.0 μm is considered as a PDP of the second conventional example. Here, the PDP of the first conventional example, the PDP of the second conventional example and the PDP  1  according to the first embodiment of the present invention are set at the same rate of the dielectric-protection layer to be covered with the powder components. 
       FIG. 6  is graph of comparing panel characteristics between the PDP of the first conventional example, the PDP of the second conventional example, and the PDP  1  according to the first embodiment of the present invention. Here, a PDP, in which the stability of initial electron emission in the dielectric glass layer  13  is higher and a voltage required for maintaining a wall charge is smaller, is a PDP having a better panel characteristic. That is, in  FIG. 6 , a PDP in which the curve of the graph is positioned in the lower right is a PDP having a better panel characteristic.  FIG. 7  is a table providing a summary of the incidence of chipping of the barrier rib in the PDP of the first conventional example, the PDP of the second conventional example, and the PDP  1  according to the first embodiment of the present invention. 
     As is apparent from  FIGS. 6 and 7 , in the PDP of the second conventional example in which an average particle diameter of the powder components is set at 2.0 μm, the incidence of chipping of the barrier rib can be reduced (20.3%→1.8%) compared with the PDP of the first conventional example in which an average particle diameter of the powder components is set at 5.0 μm, however, the panel characteristic is deteriorated (the stability of initial electron emission becomes low and a voltage required for maintaining a wall charge becomes large). On the other hand, in the PDP  1  (e.g., PDP  1  in which an average particle diameter of the powder components is set at 2.0 μm and annealing is applied to the surface) of the first embodiment of the present invention, the incidence of chipping of the barrier rib can be reduced (20.3%→2.3%) while maintaining the panel characteristic good compared with the PDP of the first conventional example. 
     Next, the PDP of the first conventional example, the PDP of the second conventional example, and the PDP  1  according to the first embodiment of the present invention, respectively, are cut away, and the luminous intensity of light by cathode-luminescence (hereinafter, referred to as light by CL) at a cross-section of each powder component  15  cut away was measured using a cathode-luminescence method (hereinafter, referred to as a CL method). The results of measurements will be described referring to  FIG. 3 . Herein, the luminous intensity of light by CL was measured in a measuring region near the top portion (hereinafter, referred to as a top portion T) and in a measuring region near the bottom portion (hereinafter, referred to as a bottom portion U) of the powder component  15 . That is, the luminous intensity of light by CL was measured in the top portion T where the annealed layer  15   a  is formed and the bottom portion U where the annealed layer  15   a  is not formed. Further, the light by CL of the powder component  15  has a peak in a wavelength region of 200 to 300 nm. Herein, the light by CL has a peak near a wavelength of 240 nm. 
     Further, a depth L below the surface of the powder component  15  of each measuring region is generally set at 10 to 100 nm to substantially correspond to the thickness of the annealed layer  15   a . Herein, measurements at about ten points were performed in each measuring region. More specifically, three to four locations (for example, depths of 10 nm, 40 nm, 70 nm, and 100 nm) were selected every about 30 nm along a direction of depth from the surface of the powder component  15  and about three points per one location, totally about ten points, were measured. Also in the PDPs of the first and the second conventional examples, the luminous intensity of light by cathode-luminescence was similarly measured in top portions Tp 1 , Tp 2  and bottom portions Up 1 , Up 2  of the respective powder components. 
     As the results of the foregoing measurements, the average luminous intensity in the top portion Tp 1  of the powder component (average particle diameter 5.0 μm) of the PDP of the first conventional example was substantially equal to that in the bottom portion Up 1 . Therefore, when the average luminous intensity in the top portion Tp 1  and the average luminous intensity in the bottom portion Up 1  were taken as 1.00, respectively, the luminous intensity in the top portion Tp 2  and the luminous intensity in the bottom portion Up 2  of the powder component (average particle diameter 2.0 μm) of the PDP of the second conventional example were respectively 0.35 to 0.60 in general. 
     On the other hand, the luminous intensity of the top portion T of the powder component  15  of the PDP  1  according to the first embodiment of the present invention was generally distributed within a range of 0.80 to 1.20 and the luminous intensity of the bottom portion U was generally distributed within a range of 0.50 to 0.65. That is, the top portion T of the powder component  15  of the PDP  1  according to the first embodiment of the present invention exhibited a luminous intensity almost equal to that of the top portion Tp 1  of the powder component of the PDP of the first conventional example, and the bottom portion U of the powder component  15  of the PDP  1  according to the first embodiment of the present invention exhibited a higher luminous intensity than the bottom portion Up 1  of the powder component of the PDP of the first conventional example. 
     In accordance with the first embodiment of the present invention, since an annealed layer  15   a  is formed on the exposed surface of each of the powder components  15 , it is possible to provide a front panel for a PDP and a method for producing the same, and a PDP provided with the front panel for a PDP, which can suppress the incidence of chipping of the barrier rib  24 , can enhance the stability of initial electron emission, and can reduce a voltage required for maintaining a wall charge. 
     Second Embodiment 
     A front panel for a PDP of a second embodiment of the present invention will be described referring to  FIGS. 8 to 10 .  FIG. 8  is a flow chart showing a method for producing a PDP according to the second embodiment of the present invention.  FIG. 9  is a view schematically showing the state of forming an annealed layer on an entire surface of each of powder components of the PDP according to the second embodiment of the present invention.  FIG. 10  is a partially enlarged sectional view showing a constitution of the powder component of the PDP according to the second embodiment of the present invention. 
     In the foregoing method of producing the front panel for a PDP of the first embodiment, after the powder components  15  are formed on the dielectric-protection layer  14  (step S 7 ), the annealed layer  15   a  is formed on the exposed surface of each of the powder components  15  (step S 8 ). In the method for producing a front panel for a PDP according to the second embodiment of the present invention, in place of the above procedure, after the annealed layer  15   c  is formed on the entire surface of each of powder components  15 A (step S 20 ), the powder components  15 A are formed on the dielectric-protection layer  14  (step S 7 ) as shown in  FIGS. 8 and 10 . Since the method for producing a front panel for a PDP according to the second embodiment of the present invention is similar to that of the first embodiment except for the above-mentioned difference in procedures, an overlapping explanation is omitted. Hereinafter, a method for forming the annealed layer  15   c  on the entire surface of each of the powder components  15 A, which is different from the method in the first embodiment, will be described. 
     Examples of the method for forming the annealed layer  15   c  on the entire surface of each of the powder components  15 A include an FLA method in which the xenon lamp  31  is used as with the foregoing first embodiment. An example of the formation method of the annealed layer  15   c , in which this FLA method is used, will be described referring to  FIGS. 9 and 10 . 
     First, the powder components  15 A are put in the inside of a substantially tapered metal guide  42  having high thermal conductivity placed in a hermetically-closed container  41 . 
     Next, a heater  32  for a substrate installed in the hermetically-closed container  41  is started, and a magnetic stirrer  43  is operated to rotate a stirrer element  44  while heating the powder components  15 A generally to 300 to 500° C., and a fan  45  is rotated to circulate the powder components  15 A with air in the hermetically-closed container  41  (refer to  FIG. 9 ). 
     Meanwhile, pulsed light  33  of the order of ms (millisecond) is irradiated to the powder components  15 A about once to ten times with a xenon lamp  31  located above the heater  32  for a substrate. At this time, the pulse width of the pulsed light  33  irradiated is set at about 0.8 to 3.0 ms and its power density is set at, for example, 10 to 40 mJ/cm 2 . 
     Thereby, the annealed layer  15   c  can be formed on the almost entire surface of each of the powder components  15 A as shown in  FIG. 10 . 
     The powder component  15 A in which the annealed layer  15   c  is formed on its entire surface as described above is mixed with organic substances so that the concentration of the powder component  15 A is generally 0.1 to 20.0% by weight to form a mixed paste, and this mixed paste is applied onto the dielectric-protection layer  14 , and then the powder components  15 A are dispersed on the dielectric-protection layer  14  by being dried and fired the mixed paste. 
     Further, as described above, since the depth to which the heat energy reaches in the powder component  15 A is generally about several nanometers to 100 nm, the annealed layer  15   c  is formed in a thickness of about 10 to 100 nm on the surface of the powder component  15 A. 
     In addition, when electron beams are irradiated to each of the powder components  15 A, the luminous intensity of light by CL emitted from the annealed layer  15   c  of the powder component  15 A is higher than the luminous intensity of light by CL emitted from the inner layer  15   b  as with the first embodiment 
     In accordance with the second embodiment of the present invention, since the annealed layer  15   c  is formed on the entire surface of each of the powder components  15 A, it is possible to provide a front panel for a PDP and a method for producing the same, and a PDP provided with the front panel for a PDP, which can suppress the incidence of chipping of the barrier rib  24 , can enhance the stability of initial electron emission in the dielectric glass layer  13 , and can reduce a voltage required for maintaining a wall charge. 
     Meanwhile, when a dropping test is performed for the front panel for a PDP of the first embodiment and the front panel for a PDP according to the second embodiment, the PDP of the first embodiment is thought to have an advantage that peeling of the powder components  15  from the dielectric-protection layer  14  is less and adhesion is larger. 
     Next, the estimated reason why the panel characteristics can be improved by forming the annealed layer  15   a  (or  15   c ) on at least the exposed surface of the powder component  15  by the FLA method is described in the following. 
     First, the estimated reason why the stability of initial electron emission is improved (enhanced) is described. 
     When the powder components  15  are pulverized to a primary particle level (step S 3 ), a large number of lattice defects such as atomic vacancy and dislocation are introduced in the surface of each of the powder components  15 . Since these lattice defects are introduced as various kinds of defects, as a result, energy levels of many different magnitudes (or broad energy levels) are formed in the surface of each of the powder components  15 . Electrons are trapped at these various energy levels. Thereafter, if a voltage is applied to the electrons, the electrons are emitted to the discharge space to become a group of initial electrons to sustain discharge inception. 
     At this time, if the energy levels range widely, the timing, at which the electron is emitted to the discharge spaces, will vary with the time. That is, it is conceivable that the stability of the initial electron emission is reduced (becomes poor). 
     Accordingly, it is conceivable that by forming the annealed layer  15   a  (i.e., at least the exposed surface of each of powder components  15  is annealed) on the surface of each of the powder components  15 , the recovery of lattice defects and recrystallization are prompted and the range of the energy levels is limited, and therefore the stability of the initial electron emission can be enhanced (improved). 
     Next, the estimated reason why the voltage required for maintaining a wall charge is improved (is reduced) is described. 
     If the rates of the dielectric-protection layers  14  to be covered with the powder component  15  are equal, the larger the average particle diameter of the powder components  15  is, the larger the total surface area of the powder components  15  becomes. When the total surface area of the powder components  15  becomes larger, the number of electrons charged (trapped) into the powder component  15  is increased, and therefore the voltage required for maintaining a wall charge is increased. 
     On the other hand, the powder component  15  has a property of being more apt to emit electrons spontaneously than the dielectric-protection layer  14 . Accordingly, if the electrons trapped in the dielectric-protection layer  14  become easy to move to the powder components  15 , it becomes easier for the electrons to be spontaneously emitted through the powder components  15  to the discharge spaces. 
     Therefore, it is thought that by reducing the average particle diameter of the powder components  15  to decrease the total surface area of the powder components  15 , the number of electrons trapped in the powder components  15  can be reduced and the voltage required for maintaining a wall charge can be reduced. 
     Further, when a base material of the powder component  15  has a single crystal structure, a grain boundary does not exist, and it is thought that the effect of the energy level generated by the lattice defect on the stability of initial electron emission is large. Therefore, it is thought that when the base material of the powder component  15  has a single crystal structure, an effect of forming the annealed layer  15   a  on the exposed surface of the powder component  15  becomes particularly large. 
     Further, the present invention is not limited to the foregoing respective embodiments and can be embodied in other various aspects. For example, in the first embodiment of the present invention, the CL method is employed in order to verify the extent of the recovery of lattice defects and recrystallization, however, the present invention is not limited to this. The extent of the recovery of lattice defects and recrystallization may be verified, for example, by a method in which dislocation is observed to determine a dislocation density using a TEM (transmission electron microscope). 
     Further, in the above description, an average particle diameter of the powder components  15  is set at 2.0 μm, however, the present invention is not limited to this. For example, even if the average particle diameter is set at a particle diameter equal to that of a primary particle of a component composing the powder component  15 , a similar effect can be achieved. For example, when the component composing the powder component  15  is magnesium oxide (MgO), an average particle diameter of a primary particle produced by heat-treating magnesium hydroxide (MgOH) is generally 0.2 to 3.0 μm and therefore an average particle diameter of the powder components  15  may be set within this range. 
     Further, in the above description, magnesium oxide (MgO) has been exemplified as the component composing the dielectric-protection layer  14  and the powder components  15 , respectively, however, the present invention is not limited to this and a substance having an excellent electron emission characteristic may be employed. For example, the dielectric-protection layer  14  and the powder components  15 , respectively, may include at least one of magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO) and barium oxide (BaO). Thereby, an effect equal to the present invention can be achieved. In addition, an average particle diameter of the powder components  15  may also be set at 0.2 μm or more and 3.0 μm or less as with the case where the powder component  15  is made of magnesium hydroxide (MgOH) also when the powder component  15  is made of calcium oxide (CaO), strontium oxide (SrO), or barium oxide (BaO). 
     Further, in the above description, the powder components  15  are dispersed on the dielectric-protection layer  14  as shown in  FIG. 2 , however, the present invention is not limited to this. For example, as shown in  FIG. 11 , the powder components  15  may be arranged so that the powder component  15  penetrates the dielectric-protection layer  14  and contacts the dielectric glass layer  13 . In addition, also in this case, the powder components  15  have to be arranged so that the annealed layer  15   a  or  15   c  is exposed to the discharge space. Thereby, an effect equal to the present invention can be achieved. 
     Further, in the above description, the annealed layer  15   a  or  15   c  is formed on the powder component  15  by performing flash lamp annealing, however, the present invention is not limited to this. For example, the annealed layer  15   a  or  15   c  may be formed by performing laser annealing (LA), or rapid thermal annealing (RTA). 
     In accordance with the laser annealing, laser acts thermally on a region at a depth of about several nanometers to 100 nm below the surface of the powder component  15 , and the surface of the powder component  15  can be heated to 1000° C. or higher with the assistance of the substrate heater  32  to form the annealed layer  15   a  or  15   c . The laser annealing has been actually used in reforming polysilicon in a production process of a liquid crystal display. The laser annealing has an advantage that an increase in display area is easy and uniformity is excellent compared with flash lamp annealing. Meanwhile, the flash lamp annealing has an advantage that tact at the time of production is short compared with laser annealing. 
     In accordance with the rapid thermal annealing, heat acts on a region at a depth of about several tens of nanometers to 300 nm below the surface of the powder component  15 , and the surface of the powder component  15  can be heated to 1000° C. or higher with the assistance of the substrate heater  32  to form the annealed layer  15   a  or  15   c . In the case of the rapid thermal annealing, since heat acts on a region at a depth of about several tens of nanometers to 300 nm below the surface of the powder component  15 , the annealed layer  15   a  is formed in a thickness of about several tens of nanometers to 300 nm. The rapid thermal annealing has an advantage that an increase in display area is easier and uniformity is excellent compared with flash lamp annealing and laser annealing. In addition, in the rapid thermal annealing, since the depth to which thermal action reaches is large, the powder component  15  has heat capacity and tends to cause thermal aggregation, and therefore there is a possibility that the average particle diameter becomes large. On the other hand, in the flash lamp annealing, since the depth to which thermal action reaches is small, this annealing has an advantage that such a possibility is inhibited. 
     It is to be noted that, by properly combining the arbitrary embodiments of the aforementioned various embodiments, the effects possessed by them can be produced. For example, it is possible to employ a method in which after the annealed layer  15   a  is formed in a small thickness (for example, in a half thickness) on the entire surface of each of the powder components  15 , the powder components  15  are dispersed on the dielectric-protection layer  14  and thereafter the energy wave is irradiated to the exposed surface of each of the powder components  15  to completely form the annealed layer  15   a . That is, the annealed layer  15   a  may be formed in two stages before and after dispersing the powder components  15  on the dielectric-protection layer  14 . 
     The front panel for a plasma display panel and the method for producing the same, and the plasma display panel of the present invention are useful particularly for display devices using a plasma display panel since it can suppress the incidence of chipping of the barrier rib of the rear panel for a plasma display panel, can enhance the stability of initial electron emission in the dielectric layer, and can reduce a voltage required for maintaining a wall charge. 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom. 
     The disclosure of Japanese Patent Application No. 2007-137545 filed on May 24, 2007 including specification, drawing and claims are incorporated herein by reference in its entirety.