Patent Publication Number: US-7215077-B2

Title: Plasma display apparatus and fluorescent material for plasma display panel

Description:
BACKGROUND OF THE INVENTION 
   1 Field of the Invention 
   The invention relates to a plasma display apparatus, and more particularly to fluorescent material coated onto a fluorescent layer which is a part of the plasma display apparatus. 
   2 Description of the Related Art 
   First, hereinbelow is explained a structure of a conventional plasma display panel. 
     FIG. 1  is an exploded perspective view of a conventional plasma display panel. 
   A plasma display panel  15  is comprised of an electrically insulating front substrate  1 A, and an electrically insulating rear substrate  1 B facing the front substrate  1 A. 
   The front substrate  1 A is formed on a surface facing the rear substrate  1 B with a scanning electrode  9  and a common electrode  10  spaced away from each other by a certain distance and extending in parallel with each other. 
   Each of the scanning electrode  9  and the common electrode  10  is comprised of a bus electrode  13  providing electrical conductivity, and a primary discharge electrode  2  formed on the bus electrode  3  and causing electric discharge. The primary discharge electrode  2  is composed of ITO (Indium-Tin Oxide) or SnO 2  for preventing reduction in light-transmittance. 
   A dielectric layer  4 A is formed on the front substrate  1 A, covering the scanning electrode  9  and the common electrode  10  therewith. The dielectric layer  4 A is covered with a protection layer  5  for protecting the dielectric layer  4 A from electric discharges. The protection layer  5  is composed of magnesium oxide (MgO), for instance. 
   The rear substrate  1 B is formed on a surface facing the front substrate  1 A with a plurality of data electrodes  6  extending in a direction perpendicular to a direction in which the scanning electrode  9  and the common electrode  10  extend. The data electrodes  6  are equally spaced away from one another, and extend in parallel with one another. 
   A dielectric layer  4 B is formed on the rear substrate  1 B, covering the data electrodes  6  therewith. On the dielectric layer  4 B is formed a plurality of partition walls  7  extending in parallel with the data electrodes  6 . The partition walls  7  define a discharge space between the first and second substrates  1 A and  1 B, and the partition walls  7  located adjacent to each other define a cell therebetween. 
   Fluorescent material  8  is coated on sidewalls of the partition walls  7  and an exposed surface of the dielectric layer  4 B. The fluorescent material  8  converts ultra-violet rays generated by discharges, into visible lights. For instance, red, green and blue (RGB) fluorescent materials are coated every three cells, ensuring displaying color images. 
   Discharge gas spaces sandwiched between the front and rear substrates  1 A and  1 B and further between adjacent partition walls  7  are filled with discharge gas comprised of helium, neon or xenon alone or in combination, for instance. 
   In operation, voltages each having a predetermined waveform are applied to the scanning electrode  9 , the common electrode  10  and the data electrodes  6 . When a voltage difference between the scanning electrode  9  and the common electrode  10  or a voltage difference between the common electrode  10  and the data electrodes  7  is over a threshold voltage, discharge is generated between the scanning electrode  9  and the common electrode  10  and between the common electrode  10  and the data electrodes  7 , respectively, and accordingly, the fluorescent material  8  emits visible lights. 
   There have been suggested many fluorescent materials used for a plasma display panel. 
   For instance, Japanese Patent Application Publication No. 2002-3835 has suggested inorganic fluorescent material in which A/B is equal to or greater than 30% where A indicates a weight of particles having a diameter of 1.0 or 0.8 micrometers or smaller, and B indicates a weight of all of particles. 
   Japanese Patent Application Publication No. 2003-34786 has suggested spherical fluorescent material having an average diameter of 0.1 to 2.0 micrometers and a maximum diameter of 7.0 micrometers or smaller. 
   The inorganic fluorescent material suggested in the firstly mentioned Publication contains minute particles having a diameter of 1.0 or 0.8 micrometers or smaller by 30 weight % or greater. As a result, particles tend to be aggregated. 
   If particles are aggregated, the fluorescent material would have a locally increased thickness, resulting in that a voltage for generating writing-discharge is locally increased. For instance, if the fluorescent material has different thicknesses in cells, a voltage increase would occur only in cells having thick fluorescent material, and hence, display quality is degraded in the cells. Thus, uniformity in displaying images in a region in which cells are arranged, that is, a display region is not achieved. 
   In the secondly mentioned Publication, spherical particles are used, and the fluorescent material is designed to contain small particles much more than large particles in a particle-diameter profile for raising an arrangement density of the fluorescent material, and further for enhancing a brightness. 
   However, the fluorescent material suggested in the secondly mentioned Publication contain much minute particles having a diameter of 1.0 micrometer or smaller. That is, particles contained in the fluorescent material are minute on the whole. Hence, similarly to the inorganic fluorescent material suggested in the firstly mentioned Publication, the particles tend to be aggregated. 
   If particles are aggregated, the fluorescent material would have a locally increased thickness, resulting in that a voltage for generating writing-discharge is locally increased, similarly to the firstly mentioned Publication. Thus, uniformity in displaying images in a display region is not achieved. 
   SUMMARY OF THE INVENTION 
   In view of the above-mentioned problems in the conventional fluorescent material in a plasma display apparatus, it is an object of the present invention to provide fluorescent material which is capable of preventing a local increase in a voltage for generating a writing-discharge. 
   It is also an object of the present invention to provide a plasma display apparatus including the above-mentioned fluorescent material. 
   In one aspect of the present invention, there is provided fluorescent material used for a plasma display panel, the fluorescent material containing particles having various diameters wherein X 1 /Y is equal to or smaller than 10% where X 1  indicates a number of particles having a diameter equal to or smaller than 1.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
   It is preferable that X 2 /Y is equal to or smaller than 2% where X 2  indicates a number of particles having a diameter equal to or greater than 16 micrometers. 
   It is preferable that X 2 /Y is equal to or smaller than 1% where X 2  indicates a number of particles having a diameter equal to or greater than 16 micrometers. 
   It is preferable that T 1 /T 2  is equal to or smaller than 1.7 where T 1  indicates a locally maximum thickness of the fluorescent material, and T 2  indicates an average thickness of the fluorescent material. 
   It is preferable that T 1 /T 2  is equal to or smaller than 1.5 where T 1  indicates a locally maximum thickness of the fluorescent material, and T 2  indicates an average thickness of the fluorescent material. 
   There is further provided fluorescent material used for a plasma display panel, the fluorescent material containing particles having various diameters wherein X 1 /Y is equal to or smaller than 1% where X 1  indicates a number of particles having a diameter equal to or smaller than 1.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
   In another aspect of the present invention, there is provided a plasma display apparatus comprising a first substrate, a second substrate and the above-mentioned fluorescent material arranged between the first and second substrates, wherein electric discharge is caused in a discharge cell formed between the first and second substrates to excite the fluorescent material to emit a light. 
   The plasma display apparatus may be designed to further include a light-reflection layer. 
   The greater a diameter of particles constituting fluorescent material is, the lower a brightness is. However, a light-reflection layer prevents reduction in a brightness. 
   In a plasma display apparatus including a light-reflection layer, fluorescent material is coated on the light-reflection layer. It is preferable that a light-reflection layer includes white pigment powder. A light-reflection layer including white pigment therein would have white hue or would be almost white in color, ensuring enhanced reflection-ability. 
   The advantages obtained by the aforementioned present invention will be described hereinbelow. 
   In accordance with the present invention, the fluorescent material used in a plasma display panel is designed to have a predetermined particle-diameter profile to thereby prevent aggregation of minute particles and a local increase in a thickness caused by large particles. As a result, it is possible to prevent a local increase in a voltage for generating a writing-voltage, and stabilize the voltage. 
   The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded perspective view of a conventional plasma display panel. 
       FIG. 2  is a histogram showing a particle-diameter profile of the fluorescent material from which green light emits, in accordance with the first embodiment of the present invention. 
       FIG. 3  is a histogram showing a particle-diameter profile of conventional fluorescent material from which green light emits. 
       FIG. 4  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 1 /Y in the fluorescent material from which green light emits, in accordance with the first embodiment of the present invention, wherein X 1  indicates a number of particles having a diameter equal to or smaller than 1.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
       FIG. 5  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 2 /Y in the fluorescent material from which green light emits, in accordance with the first embodiment of the present invention, wherein X 2  indicates a number of particles having a diameter equal to or greater than 16.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
       FIG. 6  is a graph showing a relation between a writing-voltage ratio and a ratio defined as T 1 /T 2  in the fluorescent material from which green light emits, in accordance with the first embodiment of the present invention, wherein T 1  indicates a locally maximum thickness of the fluorescent material, and T 2  indicates an average thickness of the fluorescent material. 
       FIG. 7  is a histogram showing a particle-diameter profile of the fluorescent material from which blue light emits, in accordance with the second embodiment of the present invention. 
       FIG. 8  is a histogram showing a particle-diameter profile of conventional fluorescent material from which blue light emits. 
       FIG. 9  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 1 /Y in the fluorescent material from which blue light emits, in accordance with the second embodiment of the present invention, wherein X 1  indicates a number of particles having a diameter equal to or smaller than 1.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
       FIG. 10  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 2 /Y in the fluorescent material from which blue light emits, in accordance with the second embodiment of the present invention, wherein X 2  indicates a number of particles having a diameter equal to or greater than 16.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
       FIG. 11  is a histogram showing a particle-diameter profile of the fluorescent material from which red light emits, in accordance with the third embodiment of the present invention. 
       FIG. 12  is a histogram showing a particle-diameter profile of conventional fluorescent material from which red light emits. 
       FIG. 13  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 1 /Y in the fluorescent material from which red light emits, in accordance with the third embodiment of the present invention, wherein X 1  indicates a number of particles having a diameter equal to or smaller than 1.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
       FIG. 14  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 2 /Y in the fluorescent material from which red light emits, in accordance with the third embodiment of the present invention, wherein X 2  indicates a number of particles having a diameter equal to or greater than 16.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
       FIG. 15  is a block diagram of a plasma display apparatus including the plasma display panel including the fluorescent material in accordance with one of the first to third embodiments. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments in accordance with the present invention will be explained hereinbelow with reference to drawings. 
   [First Embodiment] 
   The fluorescent material in accordance with the first embodiment emits green light. 
   As fluorescent material which emits green light, there may be selected Zn 2 SiO 4 :Mn, (Y, Gd)BO 3 :Tb or YBO 3 :Tb, for instance. 
     FIG. 2  is a histogram showing a particle-diameter profile of the fluorescent material in accordance with the first embodiment of the present invention, and  FIG. 3  is a histogram showing a particle-diameter profile of conventional fluorescent material from which green light emits. In  FIGS. 2 and 3 , the axis of abscissa indicates a particle diameter in a unit of micrometer [μm], and the axis of ordinates indicates a frequency of a number of particles in an arbitrary unit [a.u.]. 
   As is obvious in comparison of  FIGS. 2 and 3  with each other, a particle-diameter profile of the fluorescent material in accordance with the first embodiment is narrower than the same of conventional fluorescent material. 
     FIG. 4  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 1 /Y [%] in the fluorescent material in accordance with the first embodiment of the present invention, wherein X 1  indicates a number of particles having a diameter equal to or smaller than 1.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
   A writing-voltage ratio is defined in accordance with the following equation (A).
 
Writing-voltage ratio=(Actual writing-voltage)/(average writing-voltage)  (A)
 
   Specifically, a writing-voltage ratio indicates how much variance an actual writing-voltage has in comparison with an average writing-voltage. Accordingly, if a writing-voltage ratio is equal to 1.0 (a bottom of the axis of ordinates in  FIG. 4 ), an actual writing-voltage is equal to an average writing-voltage, and hence, there is no variance in an actual writing-voltage. 
   As illustrated in  FIG. 4 , a writing-voltage ratio is equal to 1.0 when the ratio X 1 /Y is equal to or smaller than 10%. That is, there is no variance in an actual writing-voltage. In contrast, when the ratio X 1 /Y becomes over about 11%, a writing-voltage ratio starts increasing. That is, variance begins to occur in an actual writing-voltage. For instance, when the ratio X 1 /Y is equal to 20%, a writing-voltage ratio rises up to 1.5. 
   Accordingly, if the ratio X 1 /Y is equal to or smaller than 10% which is smaller than 11%, there occurs no variance in an actual writing-voltage. 
   Hence, the fluorescent material from which green light emits, in accordance with the first embodiment, is designed to have the ratio X 1 /Y equal to or smaller than 10%. 
   Furthermore, as is obvious in view of  FIG. 4 , if the ratio X 1 /Y is equal to or smaller than 1%, there is absolutely no variance in an actual writing-voltage. Accordingly, it is possible to completely prevent variance in an actual writing-voltage by designing the ratio X 1 /Y to be equal to or smaller than 1% in the fluorescent material in accordance with the first embodiment. 
     FIG. 5  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 2 /Y in the fluorescent material in accordance with the first embodiment, wherein X 2  indicates a number of particles having a diameter equal to or greater than 16.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
   A writing-voltage ratio is defined in accordance with the above-mentioned equation (A). 
   As illustrated in  FIG. 5 , a writing-voltage ratio is equal to 1.0 when the ratio X 2 /Y is equal to or smaller than 2.0%. That is, there is no variance in an actual writing-voltage. In contrast, when the ratio X 2 /Y becomes over about 2.2%, a writing-voltage ratio starts rising. That is, variance begins to occur in an actual writing-voltage. For instance, when the ratio X 1 /Y is equal to 5%, a writing-voltage ratio rises up to 1.55. 
   Accordingly, if the ratio X 2 /Y is equal to or smaller than 2% which is smaller than 2.2%, there occurs no variance in an actual writing-voltage. 
   Hence, the fluorescent material from which green light emits, in accordance with the first embodiment, is designed to have the ratio X 2 /Y equal to or smaller than 2%. 
   Furthermore, as is obvious in view of  FIG. 5 , if the ratio X 2 /Y is equal to or smaller than 1%, there is absolutely no variance in an actual writing-voltage. Accordingly, it is possible to completely prevent variance in an actual writing-voltage by designing the ratio X 2 /Y to be equal to or smaller than 1% in the fluorescent material in accordance with the first embodiment. 
     FIG. 6  is a graph showing a relation between a writing-voltage ratio and a ratio defined as T 1 /T 2  in the fluorescent material in accordance with the first embodiment, wherein T 1  indicates a locally maximum thickness of the fluorescent material, and T 2  indicates an average thickness of the fluorescent material. 
   A writing-voltage ratio is defined in accordance with the above-mentioned equation (A). 
   In general, fluorescent material does not have a uniform thickness. Fluorescent material might have a locally increased thickness or thicknesses where large particles exist. A locally maximum thickness indicates a maximum thickness among such locally increased thicknesses. 
   As illustrated in  FIG. 6 , a writing-voltage ratio is equal to 1.0 when the ratio T 1 /T 2  is equal to or smaller than 1.5. That is, there is no variance in an actual writing-voltage. In contrast, when the ratio T 1 /T 2  becomes over about 1.6, a writing-voltage ratio starts rising in a small rate. If the ratio T 1 /T 2  is over about 1.8, a writing-voltage ratio rises in a high rate. That is, much variance begins to occur in an actual writing-voltage, if the T 1 /T 2  ratio is over about 1.8. For instance, when the ratio T 1 /T 2  is equal to 2.0, a writing-voltage ratio rises up to 1.55. 
   Accordingly, if the ratio T 1 /T 2  is equal to or smaller than 1.7 which is smaller than 1.8, even if there occurs variance in an actual writing-voltage, it would be possible to suppress the variance within a quite small range. 
   Hence, the fluorescent material from which green light emits, in accordance with the first embodiment, is designed to have the ratio T 1 /T 2  equal to or smaller than 1.7. 
   Furthermore, as is obvious in view of  FIG. 6 , if the ratio T 1 /T 2  is equal to or smaller than 1.5, there is absolutely no variance in an actual writing-voltage. Accordingly, it is possible to completely prevent variance in an actual writing-voltage by designing the ratio T 1 /T 2  to be equal to or smaller than 1.5 in the fluorescent material in accordance with the first embodiment. 
   As mentioned above, the fluorescent material from which green light emits, in accordance with the first embodiment, is designed to have
         (a) the X 1 /Y ratio equal to or smaller than 10%, preferably, 1%,   (b) the X 2 /Y ratio equal to or smaller than 2%, preferably, 1%, and   (c) the T 1 /T 2  ratio equal to or smaller than 1.7, preferably, 1.5.       

   As a result, it is possible to prevent variance in an actual writing-voltage relative to an average writing-voltage, and hence, prevent local increase in a voltage necessary for generating writing-discharges. Thus, it is possible to prevent deterioration in quality in displaying images in a plasma display apparatus including the fluorescent material in accordance with the first embodiment. 
   [Second Embodiment] 
   The fluorescent material in accordance with the second embodiment emits blue light. 
   As fluorescent material which emits blue light, there may be selected BaMgAl 10 O 17 :Eu, for instance. 
     FIG. 7  is a histogram showing a particle-diameter profile of the fluorescent material in accordance with the second embodiment of the present invention, and  FIG. 8  is a histogram showing a particle-diameter profile of conventional fluorescent material from which blue light emits. In  FIGS. 7 and 8 , the axis of abscissa indicates a particle diameter in a unit of micrometer [μm], and the axis of ordinates indicates a frequency of a number of particles in an arbitrary unit [a.u.]. 
   As is obvious in comparison of  FIGS. 7 and 8  with each other, a particle-diameter profile of the fluorescent material in accordance with the second embodiment is narrower than the same of conventional fluorescent material. 
     FIG. 9  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 1 /Y [%] in the fluorescent material in accordance with the second embodiment of the present invention, wherein X 1  indicates a number of particles having a diameter equal to or smaller than 1.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
   A writing-voltage ratio is defined in accordance with the above-mentioned equation (A). 
   As illustrated in  FIG. 9 , a writing-voltage ratio is equal to 1.0 when the ratio X 1 /Y is equal to or smaller than 10%. That is, there is no variance in an actual writing-voltage. In contrast, when the ratio X 1 /Y becomes over about 11%, a writing-voltage ratio starts increasing. That is, variance begins to occur in an actual writing-voltage. For instance, when the ratio X 1 /Y is equal to 20%, a writing-voltage ratio rises up to 1.4. 
   Accordingly, if the ratio X 1 /Y is equal to or smaller than 10% which is smaller than 11%, there occurs no variance in an actual writing-voltage. 
   Hence, the fluorescent material from which blue light emits, in accordance with the second embodiment, is designed to have the ratio X 1 /Y equal to or smaller than 10%, similarly to the fluorescent material in accordance with the first embodiment. 
   Furthermore, as is obvious in view of  FIG. 9 , if the ratio X 1 /Y is equal to or smaller than 1%, there is absolutely no variance in an actual writing-voltage. Accordingly, it is possible to completely prevent variance in an actual writing-voltage by designing the ratio X 1 /Y to be equal to or smaller than 1% in the fluorescent material in accordance with the second embodiment, similarly to the fluorescent material in accordance with the first embodiment. 
     FIG. 10  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 2 /Y in the fluorescent material in accordance with the second embodiment, wherein X 2  indicates a number of particles having a diameter equal to or greater than 16.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
   A writing-voltage ratio is defined in accordance with the above-mentioned equation (A). 
   As illustrated in  FIG. 10 , a writing-voltage ratio is equal to 1.0 when the ratio X 2 /Y is equal to or smaller than 2.0%. That is, there is no variance in an actual writing-voltage. In contrast, when the ratio X 2 /Y becomes over about 2.2%, a writing-voltage ratio starts rising. That is, variance begins to occur in an actual writing-voltage. For instance, when the ratio X 1 /Y is equal to 10%, a writing-voltage ratio rises up to 1.55. 
   Accordingly, if the ratio X 2 /Y is equal to or smaller than 2% which is smaller than 2.2%, there occurs no variance in an actual writing-voltage. 
   Hence, the fluorescent material from which blue light emits, in accordance with the second embodiment, is designed to have the ratio X 2 /Y equal to or smaller than 2%, similarly to the fluorescent material in accordance with the first embodiment. 
   Furthermore, as is obvious in view of  FIG. 10 , if the ratio X 2 /Y is equal to or smaller than 1%, there is absolutely no variance in an actual writing-voltage. Accordingly, it is possible to completely prevent variance in an actual writing-voltage by designing the ratio X 2 /Y to be equal to or smaller than 1% in the fluorescent material in accordance with the second embodiment, similarly to the fluorescent material in accordance with the first embodiment. 
   Though not illustrated, the fluorescent material from which blue light emits, in accordance with the second embodiment, is designed to have the ratio T 1 /T 2  equal to or smaller than 1.7, similarly to the fluorescent material in accordance with the first embodiment, wherein T 1  indicates a locally maximum thickness of the fluorescent material, and T 2  indicates an average thickness of the fluorescent material. Thus, even if there occurs variance in an actual writing-voltage, it is possible to suppress the variance within a quite small range. 
   Furthermore, it is possible to completely prevent variance in an actual writing-voltage by designing the ratio T 1 /T 2  to be equal to or smaller than 1.5 in the fluorescent material in accordance with the second embodiment, similarly to the fluorescent material in accordance with the first embodiment. 
   As mentioned above, the fluorescent material from which blue light emits, in accordance with the second embodiment, is designed to have
         (a) the X 1 /Y ratio equal to or smaller than 10%, preferably, 1%,   (b) the X 2 /Y ratio equal to or smaller than 2%, preferably, 1%, and   (c) the T 1 /T 2  ratio equal to or smaller than 1.7, preferably, 1.5.       

   As a result, it is possible to prevent variance in an actual writing-voltage relative to an average writing-voltage, and hence, prevent local increase in a voltage necessary for generating writing-discharges. Thus, it is possible to prevent deterioration in quality in displaying images in a plasma display apparatus including the fluorescent material in accordance with the second embodiment. 
   [Third Embodiment] 
   The fluorescent material in accordance with the third embodiment emits red light. 
   As fluorescent material which emits blue light, there may be selected (Y, Gd)BO 3 :Eu, Y 2  O 3 :Eu or YPVO 4 :Eu, for instance. 
     FIG. 11  is a histogram showing a particle-diameter profile of the fluorescent material in accordance with the third embodiment of the present invention, and  FIG. 12  is a histogram showing a particle-diameter profile of conventional fluorescent material from which red light emits. In  FIGS. 11 and 12 , the axis of abscissa indicates a particle diameter in a unit of micrometer [μg m], and the axis of ordinates indicates a frequency of a number of particles in an arbitrary unit [a.u.]. 
   As is obvious in comparison of  FIGS. 11 and 12  with each other, a particle-diameter profile of the fluorescent material in accordance with the third embodiment is narrower than the same of conventional fluorescent material. 
     FIG. 13  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 1 /Y [%] in the fluorescent material in accordance with the third embodiment of the present invention, wherein X 1  indicates a number of particles having a diameter equal to or smaller than 1.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
   A writing-voltage ratio is defined in accordance with the above-mentioned equation (A). 
   As illustrated in  FIG. 13 , a writing-voltage ratio is equal to 1.0 when the ratio X 1 /Y is equal to or smaller than 10%. That is, there is no variance in an actual writing-voltage. In contrast, when the ratio X 1 /Y becomes over about 11%, a writing-voltage ratio starts increasing. That is, variance begins to occur in an actual writing-voltage. For instance, when the ratio X 1 /Y is equal to 20%, a writing-voltage ratio rises up to 1.3. 
   Accordingly, if the ratio X 1 /Y is equal to or smaller than 10% which is smaller than 11%, there occurs no variance in an actual writing-voltage. 
   Hence, the fluorescent material from which red light emits, in accordance with the third embodiment, is designed to have the ratio X 1 /Y equal to or smaller than 10%, similarly to the fluorescent materials in accordance with the first and second embodiments. 
   Furthermore, as is obvious in view of  FIG. 13 , if the ratio X 1 /Y is equal to or smaller than 1%, there is absolutely no variance in an actual writing-voltage. Accordingly, it is possible to completely prevent variance in an actual writing-voltage by designing the ratio X 1 /Y to be equal to or smaller than 1% in the fluorescent material in accordance with the third embodiment, similarly to the fluorescent materials in accordance with the first and second embodiments. 
     FIG. 14  is a graph showing a relation between a writing-voltage ratio and a ratio defined as X 2 /Y in the fluorescent material in accordance with the third embodiment, wherein X 2  indicates a number of particles having a diameter equal to or greater than 16.0 micrometers, and Y indicates a total number of particles contained in the fluorescent material. 
   A writing-voltage ratio is defined in accordance with the above-mentioned equation (A). 
   As illustrated in  FIG. 14 , a writing-voltage ratio is equal to 1.0 when the ratio X 2 /Y is equal to or smaller than 2.0%. That is, there is no variance in an actual writing-voltage. In contrast, when the ratio X 2 /Y becomes over about 2.2%, a writing-voltage ratio starts rising. That is, variance begins to occur in an actual writing-voltage. For instance, when the ratio X 1 /Y is equal to 10%, a writing-voltage ratio rises up to 1.4. 
   Accordingly, if the ratio X 2 /Y is equal to or smaller than 2% which is smaller than 2.2%, there occurs no variance in an actual writing-voltage. 
   Hence, the fluorescent material from which red light emits, in accordance with the third embodiment, is designed to have the ratio X 2 /Y equal to or smaller than 2%, similarly to the fluorescent materials in accordance with the first and second embodiments. 
   Furthermore, as is obvious in view of  FIG. 14 , if the ratio X 2 /Y is equal to or smaller than 1%, there is absolutely no variance in an actual writing-voltage. Accordingly, it is possible to completely prevent variance in an actual writing-voltage by designing the ratio X 2 /Y to be equal to or smaller than 1% in the fluorescent material in accordance with the third embodiment, similarly to the fluorescent materials in accordance with the first and second embodiments. 
   Though not illustrated, the fluorescent material from which red light emits, in accordance with the third embodiment, is designed to have the ratio T 1 /T 2  equal to or smaller than 1.7, similarly to the fluorescent materials in accordance with the first and second embodiments, wherein T 1  indicates a locally maximum thickness of the fluorescent material, and T 2  indicates an average thickness of the fluorescent material. Thus, even if there occurs variance in an actual writing-voltage, it is possible to suppress the variance within a quite small range. 
   Furthermore, it is possible to completely prevent variance in an actual writing-voltage by designing the ratio T 1 /T 2  to be equal to or smaller than 1.5 in the fluorescent material in accordance with the third embodiment, similarly to the fluorescent materials in accordance with the first and second embodiments. 
   As mentioned above, the fluorescent material from which red light emits, in accordance with the third embodiment, is designed to have
         (a) the X 1 /Y ratio equal to or smaller than 10%, preferably, 1%,   (b) the X 2 /Y ratio equal to or smaller than 2%, preferably, 1%, and   (c) the T 1 /T 2  ratio equal to or smaller than 1.7, preferably, 1.5.       

   As a result, it is possible to prevent variance in an actual writing-voltage relative to an average writing-voltage, and hence, prevent local increase in a voltage necessary for generating writing-discharges. Thus, it is possible to prevent deterioration in quality in displaying images in a plasma display apparatus including the fluorescent material in accordance with the third embodiment. 
   [Fourth Embodiment] 
   The fluorescent materials in accordance with the above-mentioned first to third embodiments are coated on both sidewalls of the partition walls  7  (see  FIG. 1 ) and an exposed surface of the dielectric layer  4 B. In contrast, a plasma display panel in accordance with the fourth embodiment is designed to include a light-reflection layer formed on both sidewalls of the partition walls  7  and an exposed surface of the dielectric layer  4 B. In the fourth embodiment, fluorescent material is coated onto the light-reflection layer. As fluorescent material to be coated onto the light-reflection layer, there may be selected the fluorescent material in accordance with any one or more of the first to third embodiments. 
   In general, fluorescent material containing particles having a greater diameter could provide a lower brightness. However, the light-reflection layer prevents reduction in a brightness. Thus, the provision of the light-reflection layer prevents reduction in a brightness, and allows fluorescent material to contain particles having a greater diameter. 
   It is preferable that the light-reflection layer includes white pigment powder. The light-reflection layer including white pigment therein would have white hue or would be almost white in color, ensuring enhanced reflection-ability. 
   A method of fabricating the plasma display panel  15  as an example of a plasma display apparatus including the fluorescent materials in accordance with the first to third embodiments is explained hereinbelow with reference to  FIG. 1 . 
   First, the scanning electrode  9  and the common electrode  10  are formed on the front substrate  1 A. Then, the dielectric layer  4 A is formed on the front substrate  1 A, covering the scanning electrode  9  and the common electrode  10  therewith. Then, the protection layer  5  is formed on the dielectric layer  4 A. 
   The data electrodes  6  are formed on the rear substrate  1 B. Then, the dielectric layer  4 B is formed on the rear substrate  1 B, covering the data electrodes  6  therewith. Then, the partition walls  7  are formed on the dielectric layer  4 B. 
   Then, the fluorescent materials  8  in accordance with the above-mentioned first to third embodiments are coated onto both sidewalls of the partition walls  7  and an exposed surface of the dielectric layer  4 B. The fluorescent materials  8  in accordance with the above-mentioned first to third embodiments are coated one by one in every three cells. 
   Then, glass-paste adhesive is coated onto edges of the front and rear substrates  1 A and  1 B through the use of a dispenser and the like for hermetically coupling them to each other. For instance, the adhesive is comprised of a mixture of glass frit having a low melting point, binder and organic solvent having a low boiling point. 
   Then, the rear substrate  1 B is heated for solidifying the adhesive. 
   Then, a vent pipe is attached to the rear substrate  1 B in alignment with an opening (not illustrated) formed at the rear substrate  1 B. A tube is connected to the vent pipe. Then, the discharge gas space is evacuated through the tube. The front and rear substrates  1 A and  1 B are heated with the discharge gas space being kept vacuous for discharging unnecessary molecules such as moisture residual in the discharge gas space. 
   Then, the front and rear substrates  1 A and  1 B are cooled. After filling the discharge gas space with discharge gas, the vent pipe is cut. 
   Thus, there is completed the plasma display panel  15 . 
   Hereinbelow are explained examples of the fluorescent material in accordance with the present invention, having been made by the inventors. 
   EXAMPLE 1 
   As Example 1, there was synthesized fluorescent material Zn 2 SiO 4 :Mn from which green light emits. In the fluorescent material, the ratio X 1 /Y was equal to 0.5%, and a maximum particle-diameter was 13 micrometers. 
   Then, solvent containing, as a principal constituent, BCA including ethylcellulose and other resins was mixed to the fluorescent material to thereby prepare fluorescent material paste. Then, the fluorescent material paste was coated onto sidewalls of the partition walls  7  and an exposed surface of the dielectric layer  4 B by screen-printing. Thereafter, the fluorescent material paste was dried and baked, and thus, there was formed the fluorescent layer having a thickness of 10 micrometers. 
   The resultant fluorescent layer had the ratio X 1 /Y equal to or smaller than 1%. Accordingly, an increase in a voltage necessary for generating a writing-discharge, caused by aggregation of minute particles, was not observed. 
   Furthermore, since the maximum particle-diameter was 13 micrometers, the ratio T 1 /T 2  was calculated to be 1.3 (13/10=1.3), which is smaller than 1.7. Accordingly, an increase in a voltage necessary for generating a writing-discharge was not observed. 
   Apart from the above-mentioned fluorescent layer, there was also synthesized fluorescent layer from which green light emits, composed of (Y, Gd)BO 3 :Tb in place of Zn 2 SiO 4 :Mn, and having a thickness of 10 micrometers. With respect to the fluorescent layer, an increase in a voltage necessary for generating a writing-discharge was not observed. 
   There was also synthesized fluorescent layer from which green light emits, composed of YBO 3 :Tb in place of Zn 2 SiO 4 :Mn, and having a thickness of 10 micrometers. With respect to the fluorescent layer, an increase in a voltage necessary for generating a writing-discharge was not observed. 
   EXAMPLE 2 
   As Example  2 , there was synthesized fluorescent material (Y, Gd)BO 3 :Eu from which red light emits. Then, the fluorescent layer composed of (Y, Gd)BO 3 :Eu and having a thickness of 10 micrometers was made in the same was as Example 1. 
   With respect to the fluorescent layer of Example 2, an increase in a voltage necessary for generating a writing-discharge was not observed, similarly to Example 1. 
   Apart from the above-mentioned fluorescent layer, there was also synthesized fluorescent layer from which red light emits, composed of (Y 2 O 3 :Eu) in place of (Y, Gd)BO 3 :Eu, and having a thickness of 10 micrometers. With respect to the fluorescent layer, an increase in a voltage necessary for generating a writing-discharge was not observed. 
   There was also synthesized fluorescent layer from which red light emits, composed of YPVO 4 :Eu in place of (Y, Gd)BO 3 :Eu, and having a thickness of 10 micrometers. With respect to the fluorescent layer, an increase in a voltage necessary for generating a writing-discharge was not observed. 
   EXAMPLE 3 
   As Example 3, there was synthesized fluorescent material BaMgAl 10 O 17 :Eu from which blue light emits. Then, the fluorescent layer composed of BaMgAl 10 O 17 :Eu and having a thickness of 10 micrometers was made in the same was as Example 1. 
   With respect to the fluorescent layer of Example 3, an increase in a voltage necessary for generating a writing-discharge was not observed, similarly to Example 1. 
   EXAMPLE 4 
   As Example 4, there was synthesized fluorescent material Zn 2 SiO 4 :Mn from which green light emits. In the fluorescent material, the ratio X 1 /Y was equal to 0.5%, and a maximum particle-diameter was 13 micrometers. 
   Then, solvent containing, as a principal constituent, BCA including ethylcellulose and other resins was mixed to the fluorescent material to thereby prepare fluorescent material paste. Then, the fluorescent material paste was coated onto sidewalls of the partition walls  7  and an exposed surface of the dielectric layer  4 B by screen-printing. Thereafter, the fluorescent material paste was dried and baked, and thus, there was formed the fluorescent layer having a thickness of 20 micrometers. 
   The resultant fluorescent layer had the ratio X 1 /Y equal to or smaller than 1%. Accordingly, an increase in a voltage necessary for generating a writing-discharge, caused by aggregation of minute particles, was not observed. 
   Furthermore, since the maximum particle-diameter was 13 micrometers, the ratio T 1 /T 2  was calculated to be 0.65 (13/20=0.65), which is smaller than 1.7. Accordingly, an increase in a voltage necessary for generating a writing-discharge was not observed. 
   EXAMPLE 5 
   As Example 5, there was synthesized fluorescent material (Y, Gd)BO 3 :Eu from which red light emits. Then, the fluorescent layer composed of (Y, Gd)BO 3 :Eu and having a thickness of 20 micrometers was made in the same was as Example 4. 
   With respect to the fluorescent layer of Example 5, an increase in a voltage necessary for generating a writing-discharge was not observed, similarly to Example 4. 
   Apart from the above-mentioned fluorescent layer, there was also synthesized fluorescent layer from which red light emits, composed of (Y 2 O 3 :Eu) in place of (Y, Gd)BO 3 :Eu, and having a thickness of 20 micrometers. With respect to the fluorescent layer, an increase in a voltage necessary for generating a writing-discharge was not observed. 
   There was also synthesized fluorescent layer from which red light emits, composed of YPVO 4 :Eu in place of (Y, Gd)BO 3 :Eu, and having a thickness of 20 micrometers. With respect to the fluorescent layer, an increase in a voltage necessary for generating a writing-discharge was not observed. 
   EXAMPLE 6 
   As Example 6, there was synthesized fluorescent material BaMgAl 10 O 17 :Eu from which blue light emits. Then, the fluorescent layer composed of BaMgAl 10 O 17 :Eu and having a thickness of 20 micrometers was made in the same was as Example 4. 
   With respect to the fluorescent layer of Example 6, an increase in a voltage necessary for generating a writing-discharge was not observed, similarly to Example 4. 
   [Fifth Embodiment] 
     FIG. 15  is a block diagram of a plasma display apparatus  60  including a plasma display panel including the fluorescent materials in accordance with the above-mentioned first to third embodiments. 
   As illustrated in  FIG. 15 , the plasma display apparatus  60  has a modularized structure. Specifically, the plasma display apparatus  60  is comprised of an analog interface  20  and a plasma display module  30 . 
   The analog interface  20  is comprised of a Y/C separating circuit  21  including a chroma-decoder, an analog-digital (A/D) converting circuit  22 , a circuit  23  for controlling a synchronization signal, including a phase-lock loop (PLL) circuit, a circuit  24  for converting an image format, an reverse-gamma converting circuit  25 , a system control circuit  26 , and a PLE control circuit  27 . 
   In brief, the analog interface  20  coverts a received analog image signal into a digital image signal, and then, outputs the digital image signal to the plasma display module  30 . 
   For instance, an analog image signal transmitted from a television tuner (not illustrated) is separated into luminance signals for RGB colors in the Y/C separating circuit  21 , and then, converted into a digital signal in the A/D converting circuit  22 . 
   Then, if a pixel configuration in the plasma display module  30  is different from a pixel configuration of the image signal, necessary conversion is carried out in the image-format converting circuit  24 . 
   A characteristic of a luminance to a signal input to a plasma display panel is linear. Image signals are usually compensated for, specifically, gamma-converted in advance in accordance with characteristics of a cathode ray tube (CRT). Hence, after the image, signals are A/D-converted in the A/D converting circuit  22 , reverse-gamma conversion is applied to the image signals are in the reverse-gamma converting circuit  25  for producing digital image signals having linear characteristics. The digital image signals are output to the plasma display module  30  as RGB image signals. 
   Since an analog image signal does not include a sampling clock signal and a data clock signal used for A/D conversion, the PLL circuit included in the control circuit  23  produces a sampling clock signal and a data clock signal, based on a horizontal synchronization signal provided together with the analog image signal, and outputs the clock signals to the plasma display module  30 . 
   The PLE control circuit  27  carries out luminance control. Specifically, if an average picture level is equal to or smaller than a threshold level, a luminance for displayed images is raised, and if an average picture level is greater than a threshold level, a luminance is reduced. 
   The system control circuit  26  outputs various control signals to the plasma display module  30 . 
   The plasma display module  30  is comprised of a digital signal processing and controlling circuit  31 , a panel section  32 , and a power source circuit  33  including a DC/DC converter. 
   The digital signal processing and controlling circuit  31  is comprised of an interface signal processing circuit  34 , a frame memory  35 , a memory control circuit  36 , and a driver control circuit  37 . 
   The interface signal processing circuit  34  receives various control signals transmitted from the system control circuit  26 , an RGB image signal transmitted from the reverse-gamma converting circuit  25 , a synchronization signal transmitted from the control circuit  23 , and a data clock signal transmitted from the PLL circuit. 
   For instance, an average picture level (APL) of an image signal input into the interface signal processing circuit  34  is calculated in an APL calculating circuit (not illustrated) included in the interface signal processing circuit  34 , and output as 5-bit data, for instance. The PLE control circuit  27  arranges PLE control data in accordance with the calculated average picture level, and outputs the PLE control data to a picture level control circuit (not illustrated) included in the interface signal processing circuit  34 . 
   The digital signal processing and controlling circuit  31  processes those signals in the interface signal processing circuit  34 , and then, transmits a control signal to the panel section  32 . The memory control circuit  36  transmits a memory control signal to the panel section  32 , and the driver control circuit  37  transmits a driver control signal to the panel section  32 . 
   The panel section  32  is comprised of a 50-size plasma display panel  50 , a scanning driver  38  for driving a scanning electrode, data drivers  39  for driving data electrodes, pulse-generating circuits  40  for applying a pulse voltage to the plasma display panel  50  and the scanning driver  38 , and a circuit  41  for collecting excess power from the pulse-generating circuits  40 . 
   The plasma display panel  50  is designed to have 1365×768 pixels. In the plasma display panel  50 , the scanning driver  38  controls a scanning electrode, and the data drivers  39  control data electrodes, thereby a light is emitted from selected display cells for displaying images. 
   A first power source supplies power to the digital signal processing and controlling circuit  31  and the panel section  32 . A power source circuit  33  receives DC power from a second power source, converts a DC voltage into a desired voltage, and supplies the desired voltage to the panel section  32 . 
   Hereinbelow is explained a method of fabricating the plasma display apparatus  60 . 
   First, the plasma display panel  50 , the scanning driver  38 , the data drivers  39 , the pulse-generating circuits  40 , and the power-collecting circuit  41  are arranged on a substrate to thereby fabricate the panel section  32 . 
   Apart from the panel section  32 , there is fabricated the digital signal processing and controlling circuit  31 . 
   The panel section  32 , the digital signal processing and controlling circuit  31  and the power source circuit  33  are assembled as a module. Thus, the plasma display module  30  is completed. 
   Apart from the plasma display module  30 , there is fabricated the analog interface  20 . 
   After the plasma display module  30  and the analog interface  20  have been fabricated separately from each other, they are electrically connected to each other. Thus, there is completed the plasma display apparatus  60  illustrated in  FIG. 15 . 
   By modularizing the plasma display apparatus  60 , the plasma display panel  50  can be fabricated independently of other parts constituting the plasma display apparatus  60 . For instance, if the plasma display panel  50  went wrong in the plasma display apparatus  60 , the plasma display module  30  including the plasma display panel  50  having gone wrong can be exchanged into new one, ensuring simplification in repair and reduction in time for repair. 
   While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims. 
   The entire disclosure of Japanese Patent Application No. 2003-167955 filed on Jun. 12, 2003 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.