PHOSPHOR MATERIAL, PHOSPHOR MATERIAL POWDER, PLASMA DISPLAY PANEL, AND PROCESSES FOR PRODUCING THESE

Abstract: The plasma display panel of the present invention has, for achieving high luminance and high reliability, a plurality of discharge spaces formed between a front panel and a back panel that are disposed to oppose each other, and phosphor layers, formed in the discharge spaces, each including phosphor particles of one of blue, red and green colors, wherein the phosphor particles of at least one of blue, red and green colors included in the phosphor layer are flake-like particles.

Description: TECHNICAL FIELD The present invention relates to a phosphor material and a phosphor material powder that emit light with a high efficiency, a plasma display panel used in display devices and a method of producing the same. BACKGROUND ART The CRT has been commonly used as the display device for television sets. The CRT, although better in resolution and picture quality than the plasma display panel and the liquid crystal display, is not suited to large screens having diagonal size of 101.6 cm (40 inches) or more for the reason of depth size and weight. The liquid crystal display is limited in the screen size and the viewing angle, in spite of such advantages as the low power consumption and low drive voltage. The plasma display panel, on the other hand, can be used in a large-screen display because there is no problem of depth size and weight, and 101.6 cm (40-inch) class products using the plasma display panel have already been developed (for example, see Functional Materials, February issue, 1996, Vol. 16, pp. 2, 7). Constitution of a plasma display panel of the prior art will be described below with reference to the accompanying drawing. Fig. 12 is a sectional view showing schematic constitution of the AC type plasma display panel. In Fig. 12, numeral 41 denotes a front cover plate (front glass substrate) with a display electrode 42 formed on the front glass substrate 41. The front cover plate 41 with the display electrode 42 formed thereon is also covered by a dielectric glass layer 43 and a protective layer 44 made of magnesium oxide (MgO) (see, for example, Unexamined Patent Publication (Kokai) No. 5-342991). Numeral 45 denotes a back plate (back glass substrate), with an address electrode 46, barrier rib 47 and spherical phosphor layer 48 being provided on the back glass substrate 45, and numeral 49 denotes an electric discharge space filled with a discharge gas. The phosphor layer comprises phosphor layers of three colors, red, green and blue, disposed in this order for color display. The phosphor layers of different colors are excited to emit light by ultraviolet rays of short wavelength (147 nm) emitted by electric discharge. As the phosphor layer 48 of the plasma display panel, (YGd)EuBO 3 is used for red, BaEuMgAl 10 O 17 is used for blue and Zn 2 MnSiO 4 is used for green today (for example, Electronics Packaging Technology; July, 1997; Vol. 113, No. 7, pp. 23-26). The plasma display panels of 101.6 to 106.68 cm (40- to 42-inch) class described above that are produced at present have luminance of 150 to 250 cd/m 2 at the pixel level of NTSC (640×480 pixels, cell pitch 0.43 mm × 1.29 mm, area of one cell 0.55 mm 2 ) (for example, refer to Functional Materials, February issue, 1996, Vol. 16, pp. 2, 7). Recently, plasma display panels of 101.6 to 106.68 cm (40- to 42-inch) class having luminance of 250 to 450 cd/m 2 at the pixel level of NTSC have also been reported (for example, see Flat Panel Display, 1997, Part 5-1, pp. 198-199). The conventional CRT technology is said, by contrast, to be capable of achieving a luminance of about 500 cd/m 2 . The high-definition television of full specification that is at the focus of attention recently requires 1920×1125 pixels, resulting in a resolution as fine as cell pitch of 0.15 mm×0.48 mm and cell area of 0.072 mm 2 in the case of 106.68 cm (42-inch) class. When a high-definition television set is produced with 106.68 cm (42-inch) plasma display panel, screen area per one pixel become as small as 1/7 to 1/8 that of the NTSC display. As a result, when the high-definition television set is produced with the 106.68 cm (42-inch) plasma display panel of the conventional cell configuration, emission intensity of the display panel becomes 1/7 to 1/8 that of the NTSC display, namely 0.15 to 0.171 m/W. Thus luminance of a high-definition television produced with the 106.68 cm (42-inch) plasma display panel is predicted to be as low as 30 to 40 cd/m 2 , given the same phosphor, gas composition and gas pressure, making it desirable to improve the luminance. As described above, when a television set of such a small pixel size as in the high-definition television is produced using the plasma display panel with similar brightness, luminance must be greatly increased. There are also such problems as described below with regard to phosphor material. The first problem is that phosphor materials of different colors have different levels of luminance. While several types of phosphor have been investigated for each of red, green and blue light in the plasma display panel, green phosphor has the highest luminance and blue phosphor has the lowest luminance in any of these types. For example, when YEuBO 3 is used as the red phosphor, Zn 2 MnSiO 4 is used as the green phosphor and BaEuMgAl 10 O 17 is used as the blue phosphor (Eu content 0.15), luminance ratio of the colors of red, green and blue is about 2: 3: 1, with a low color temperature of about 5000 degrees. Accordingly in the plasma display panel of the prior art, color temperature is increased by electronically suppressing the light emission by the green phosphor that has high luminance,. thereby to improve the white balance. However, this configuration leads to lower brightness of the plasma display panel as a whole due to the reduction in the emission of light from the phosphor having high luminance. This indicates that increasing the luminance of blue light is very effective in solving the problem, since color temperature can be increased without reducing the luminance of green and red light by increasing the luminance of blue light that is the lowest of the phosphors. Second, phosphor layers of the plasma display panels of the prior art are formed by applying an ink that contains phosphor particles by a printing process or coating a photosensitive sheet that contains phosphor particles. In either of these processes, it is necessary to fire the panel at a temperature of around 500°C after forming the phosphor layer, in order to remove an organic binder component included in the ink or the sheet. It is also necessary to fire the panel at a temperature of 400°C or higher to have the front cover plate and the back plate bonded with each other. In these firing processes, the phosphors used in the panel are subject to a certain extent of thermal change that results in degradation of luminance and/or chromaticity. As described above, the plasma display panel has the problem of the thermal deterioration of the phosphor material in the firing process that are required for the production (for example, Transaction of the 263rd Conference of Phosphor Engineering Association, pp. 9-13, 1996; Optonics, 1997, No.6, pp. 149-155). In the firing processes, the phosphors are subject to a certain extent of thermal change that results in degradation of luminance and/or chromaticity. Ba (1-x) Eu x MgAl 10 O 17 used as the blue phosphor at. present experiences particularly significant thermal deterioration. The Ba (1-x) Eu x MgAl 10 O 17 used as the blue phosphor can easily be damaged by vacuum ultraviolet rays (wavelength 147 nm, 172 nm) that excite the plasma display panel, and the emission intensity decreases as the panel is operated longer, thus giving rise to a problem of service life. As described above, the blue phosphor material of the plasma display panel'has the problems of thermal deterioration of the phosphor material in the firing processes required for the producing and short service life. There have been efforts being made to mitigate the thermal deterioration of the phosphor. For example, the Optical Technology Contact, Vol. 34, No. 1 (1996) pp. 23-24 reports that the BaEu 2+ MgAl 10 O 17 that had been known as an excellent blue phosphor showed such problems as deterioration during operation of the panel and change in chromaticity, and that BaEu 2+ MgAl 10 O 17 was developed to solve such problems with an improvement achieved in mitigating the decrease in luminance caused by firing in the panel producing process. As demands for high quality display increase, however, such technologies are required that prevents the deterioration of the luminance and of chromaticity of the phosphor layer and improve the emission intensity (luminance divided by the y value of chromaticity) in order to improve the luminance and picture quality of the plasma display panel. JP-A 915 13 72 discloses a production process of an illuminate salt fluorescent substance. Further, a phosphor material Ba 1-w Eu w Mg x Al y O z is disclosed where 0.03≤w≤0.3, 0.8≤x≤1.2, 8≤y≤12 and 12≤z≤20. It is further mentioned that this phosphor is employable for a fluorescent lamp or a display. JP-A 811 56 73 discloses a barium-magnesium-aluminate phosphor represented by the composition formular Ba 1-x Eu x MgAl 10 O 17 in which x is 0.05 to 0.5. It is mentioned that this blue-light-emitting phosphor is used in a plasma display panel to reduce aged deterioration of luminosity and changes in colour with time. JP-A 322 15 83 discloses a binder of luminescent fine particle and a lamp using the same. DISCLOSURE OF THE INVENTION First object of the present invention is to provide a phosphor material and a phosphor material powder of high luminance, particularly a phosphor material and a phosphor material powder that are suitable to a plasma display panel and a method of producing the same. Second object of the present invention is to provide a plasma display panel having high luminance and high reliability. The plasma display panel of the present invention has, for the purpose of achieving the first object described above, a plurality of discharge spaces formed between a' front panel and a back panel that are disposed to oppose each other, and phosphor layers, formed in the discharge spaces, each including phosphor particles of one of blue, red and green colors, wherein the phosphor particles of at least one of blue, red and green colors included in the phosphor layer are flake-like particles. According to the present invention a phosphor material as claimed in claim 1, a plasma display panel as claimed in claim 3 and a method of producing a plasma display panel as claimed in claim 4 are proposed. In the following the formulae used to represent the phosphor material, such as Ba 1-x Eu x MgAl 10 O 17 , Ba 1-x MgAl 10 O 17 :Eu x , BaMGAl 10 O 17 :Eu 2+ , BaMgAl 10 O 17 :Eu, BaEuMgAl 10 O 17 or BaEu 2+ MgAl 10 O 17 , are all equivalent. In the blue phosphor material wherein a certain element in the base material is substituted with Eu 2+ ions that serve as deactivating agent, including a phosphor represented by general formula of BaMgAl y O z , the phosphors of substitution ratio by Eu 2+ ions in a range from 10 to 15 atomic % have been used in the prior art. With the phosphor material of the present invention, luminance and emission intensity can be made higher than in the prior art by forming the phosphor layer using the phosphor material of the present invention. The invention was completed upon finding that picture quality and luminance of the plasma display panel can be improved by using such phosphor materials as the blue phosphor material. With the phosphor material of the present invention, as described above, it is made possible to form the phosphor layer having heat resistance higher than in the prior art and improve the luminance and emission intensity of the phosphor layer, by setting the Ba substitution ratio by Eu 2+ ions within 8 atomic % or preferably in a range from 1 to 6 atomic % in the phosphor material in which the element to be substituted in the base material is substituted by Eu 2+ ions, particularly a phosphor represented by the general formula of BaEu 2+ MgAl 14 O 23 . Thus high luminance and high emission intensity can be achieved by setting the substitution ratio by Eu 2+ ions to a low level as described above, even when the plasma display panel is produced by firing the panel after coating with the phosphor material thereby burning out the binder and forming the phosphor layer and then firing again in the panel sealing process, namely even when the phosphor material is subjected to firing twice. Another plasma display panel has a plurality of discharge spaces formed between the front panel and the back panel disposed to oppose each other, with a phosphor layer that include phosphor particles of one of blue, red and green colors being formed in each of the discharge spaces, while the blue phosphor particles included in the phosphor layers are made of the phosphor material powder of the present invention. When the phosphor material of the present invention is used for the blue phosphor material, as described above, thermal deterioration of the phosphor layer can be suppressed in the firing process during production of the plasma display panel, thus making it possible to improve the picture quality and luminance of the plasma display panel. A method for forming the phosphor layer is a method of forming the phosphor layer on a substrate, comprising a phosphor material applying step of applying the phosphor material together with a binder, wherein a part of Ba atoms of BaMgAl y O z of the base material are substituted with Eu 2+ ions with the substitution ratio by Eu 2+ ions being within 8 atomic %, and a firing step of firing the substrate whereon the phosphor material is applied. The phosphor applying step of this forming method may be a process of coating the substrate with an ink or a sheet made by mixing particles of the phosphor material and the binder. A method of producing the plasma display panel according to the present invention comprises a phosphor applying step of applying the phosphor material together with the binder onto a first panel substrate, the phosphor material being such. as a part of Ba atoms of BaMgAl y O z of the base material are substituted with Eu 2+ ions with the substitution ratio by Eu 2+ ions being less than 8%, preferably being in a range from 1 to 6 atomic %, a first firing step of firing the first panel substrate whereon the phosphor material has been applied, and a sealing step wherein the first panel and the second panel are placed one on another and sealed in a second firing step following the first firing step, said BaMgAl y O z being BaMgAl 10 O 17 or BaMgAl 14 O 23 . The phosphor applying step of the second producing method may be a process of applying the ink or the sheet made by mixing particles of the phosphor material and the binder onto the first panel substrate. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic sectional view of an AC discharge type plasma display panel according to the first embodiment of the present invention. Fig. 2 is a block diagram of a plasma display panel display apparatus according to the first embodiment. Fig. 3A through Fig. 3F show the method of forming barrier ribs in the first embodiment. Fig. 4 is a diagram showing a process of plasma spraying according to the first embodiment. Fig. 5 is a graph showing the characteristics of a phosphor material used in a plasma display panel of the second embodiment according to the present invention. Fig. 6A is a graph showing relative luminance versus composition of the phosphor material used in the plasma display panel of the second embodiment according to the present invention. Fig. 6B is a graph showing emission intensity versus composition of the phosphor material used in the plasma display panel of the second embodiment according to the present invention. Fig. 7 is a schematic sectional view of the plasma display panel according to the second embodiment of the present invention. Fig. 8A is a graph showing the heat resistance characteristic of a phosphor material (Ba 0.95-x Eu x Sr 0.05 MgAl 10 O 17 ) used in a plasma display panel (not making part of) the present invention. Fig. 8B is a graph showing the heat resistance characteristic of a phosphor material (Ba (1-x) Eu x MgAl 10 O 17 ) used in the plasma display panel of the (third) embodiment according to the present invention. Fig. 9 is a graph showing the heat resistance characteristic of a phosphor material used in the plasma display panel of the prior art. Fig. 10 is a schematic sectional view of a plasma display panel according to the fourth embodiment of the present invention. Fig. 11A is a graph showing relative luminance versus composition of phosphor material used in the plasma display panel of the fourth embodiment according to the present invention. Fig. 11B is a graph showing emission intensity versus composition of phosphor material used in the plasma display panel of the fourth embodiment according to the present invention. Fig. 12 is a schematic sectional view of an AC discharge type plasma display panel of the prior art. PREFERRED EMBODIMENTS OF THE INVENTION Embodiment 1 Fig. 1A is a schematic sectional view of an AC discharge type plasma display panel (PDP) according to first embodiment of the present invention. While Fig. 1A shows only one cell, the plasma display panel comprises a number of cells that emit red, green and blue light being arranged alternately. The plasma display panel comprises a front panel consisting of discharge electrodes 12 and a dielectric glass layer 13 being disposed on a front glass substrate 11, and a back panel consisting of address electrodes 16, barrier ribs 17 and phosphor layers 18 using a flaky phosphor material being disposed on a back glass substrate 15, wherein the front panel and the back panel are laminated with each other to form discharge spaces 19 between the front panel and the back panel that are filled with a discharge gas, while the plasma display panel is driven by applying voltages on the discharge electrodes 12 and the address electrodes 16 by drive circuits shown in Fig. 2. The discharge electrodes 12 are disposed so as to form an orthogonal matrix with the address electrodes 16. (Production of front panel) The front panel is made by forming the discharge electrodes 12 on the front glass substrate 11, forming the lead- or bismuth-based dielectric glass layer 13 to cover thereon, and forming a protective layer 14 on the surface of the dielectric glass layer 13. The discharge electrodes 12 are electrodes made of silver, and is formed by firing a silver paste that is screen-printed. The lead-based dielectric glass layer 13 is formed to a thickness of about 20 µm by firing a coat at 560°C for 20 minutes, the coat being applied by screen printing of, for example, a mixture of 70% by weight of lead oxide [PbO], 15% by weight of boron oxide [B 2 O 3 ], 10% by weight of silicon oxide [SiO 2 ], 5% by weight of aluminum oxide and an organic binder (α-terpineol with 10% of ethyl cellulose dissolved therein). The protective layer 14 is made of magnesium oxide (MgO) and is formed, for example, to a thickness of about 0.5 to 1.0 µm by sputtering or CVD (chemical vapor deposition) process. (Prodcution of back panel 102) The address electrodes 16 are formed on the back glass substrate 15 by screen printing process similarly to the discharge electrodes 12. The barrier ribs 17 may be formed by screen printing or the like, but may also be formed by spraying process as described below. Fig. 3A through Fig. 3F show the method of forming the barrier ribs by the spraying process. First, surface of the back glass substrate 15 (Fig. 3A) whereon the address electrodes 16 have been formed is covered by a dry film 81 made of an acrylic photosensitive resin (Fig. 3B). The dry film 81 is patterned by photolithography process. Specifically, the dry film 81 covered by a photo mask 82 is irradiated with ultraviolet (UV) rays 83 only on portions where the barrier ribs are to be formed (Fig. 3C), and developed. Thus the dry film 81 is removed from the portions where the barrier ribs are to be formed so that a mask of the dry film 81 is formed only on the portions where the barrier ribs are not to be formed (Fig. 3D). Development is carried out in an aqueous solution of alkali (specifically, aqueous solution of sodium carbonate) with about 1% of concentration. The dry film is then subjected to plasma spraying of alumina (Al 2 O 3 ), spinel (MgO · Al 2 O 3 ) and zircon (ZrO 3 ) that are the raw materials to make the barrier ribs. In order to improve the display contrast of the plasma display panel, an oxide such as Cr 2 O 3 , TiO 2 , CoO, Fe 2 O 3 or MnO 2 or a mixture thereof that have black color, too, may also be sprayed onto alumina, spinel and zircon. Fig. 4 shows the plasma spray coating process. The plasma spray apparatus 90 generates a plasma jet by applying a voltage across a cathode 91 and an anode 92 thereby generating an arc discharge from the tip of the cathode 91 while supplying argon gas thereto. Then powder of raw materials (alumina, Cr 2 O 3 , TiO 2 , etc.) is supplied into the plasma jet so that the materials are melted in the plasma jet and sprayed onto the surface of the substrate 15. Thus a sprayed film 84 of the raw material is formed on the surface of the substrate 15. The substrate 15 with the film 84 formed thereon (Fig. 3E) is immersed in a solution of removing agent (sodium hydroxide solution) to remove the mask of the dry film 81 (lift-off process). This causes a portion 84b of the raw material film 84 formed on the mask of the dry film 81 to be removed with only a portion 84a directly formed on the substrate remaining and serving as the barrier rib 17 (Fig. 3F). Then the phosphor layer 18 is formed in the groove between the barrier ribs 17. The process of forming the phosphor layer 18, to be described in detail later, consists of the application of a phosphor ink by continuously spouting the phosphor ink from a nozzle and firing thereof. When the plasma display panel of the first embodiment is applied to a 101.6 cm (40-inch) class high definition television set, the barrier ribs are formed to a height of 0.1 to 0.15 mm at intervals of 0.15 to 0.3 mm. (Production of plasma display panel by laminating panels) The front panel 101 and the back panel 102 produced as described above are put together by using sealing glass, while evacuating the inside of the discharge spaces 19 separated by the barrier ribs 17 to a high level of vacuum (for example, 1×10 -4 Pa (8×10 -7 Torr)), then filling the discharge spaces with a discharge gas (for example, He-Xe or Ne-Xe inert gas) at a predetermined pressure, thereby producing the plasma display panel. Then a circuit block (panel drive circuits 151, 152, 153) that drive the plasma display panel are mounted as shown in Fig. 2, thereby producing the display apparatus of the plasma display panel. In the first embodiment, the discharge gas includes 5% by volume of Xe and the sealing pressure is set in a range from (6.7-10.7) × 10 4 Pa (500 to 800 Torr). Embodiment 2 Fig. 7 is a schematic sectional view of an AC discharge type plasma display panel (PDP) according to the second embodiment of the present invention. While Fig. 7 shows only one cell, the plasma display panel comprises a number of cells that emit red, green and blue light being arranged thereon. The plasma display panel of the second embodiment comprises the front panel 101 consisting of the display electrodes 12, the dielectric glass layer 13 and the protective layer 14 being disposed on the front glass substrate (front cover plate) 11, and a back panel 201 consisting of the address electrodes 16, a visible light reflecting layer 217, barrier ribs 218 and phosphor layers 219 disposed on the back glass substrate (back plate) 15, wherein the front panel 101 and the back panel 201 are laminated with each other to form the discharge spaces between the front panel 101 and the back panel 201 that are filled with a discharge gas. Producing process will be described in detail below. The front panel 101 is formed similarly to the first embodiment. (Production of the back panel 201). A silver electrode paste is screen-printed on the back glass substrate 15 and is then fired to form the address electrodes 16, over which the visible light reflecting layer 217 comprising TiO 2 particles and dielectric glass is formed by screen printing and firing, and the barrier ribs 218 made of glass are formed at predetermined intervals by repeating screen printing and then firing. Then the phosphor layer 219 that includes one from among red phosphor particles, green phosphor particles and blue phosphor particles is formed in each of the spaces (that become discharge spaces when put together with the front panel 101) separated by the barrier ribs 218. The phosphor layer 219 is formed by continuously spouting the phosphor ink from the nozzles while scanning, and then firing at a temperature of around 500°C in air, similarly to the first embodiment. The phosphor layer 219 may also be formed by applying the phosphor ink by screen printing process, or by making a sheet wherein the phosphor particles are dispersed and laminating the sheet. The plasma display panel of the second embodiment was produced according to the specifications of a 108.6 cm (40-inch) class high definition television set, with the barrier ribs formed to a height of 0.1 to 0.15 mm at intervals of 0.15 to 0.3 mm, while the phosphor layers 219 are formed on the back cover surface and on side surfaces of the barrier ribs to thickness of 5 to 50 µ m including phosphor particles of average particle size in a range from 0.5 to 3 µm. (Production of the plasma display panel by laminating the panels) The front panel 101 and the back panel 201 made as described above are put together by using sealing glass so that the display electrodes 12 of the front panel 101 and the address electrodes 16 cross at right angles with each other, then after being fired at a temperature around 450°C, the inside of the discharge spaces separated by the barrier ribs 218 was evacuated to a high level of vacuum (1×10 -4 Pa (8×10 -7 Torr)), followed by filling of the discharge spaces with a discharge gas of predetermined composition at a predetermined pressure, thereby fabricating the plasma display panel. In the second embodiment, the neon (Ne)-xenon (Xe) discharge gas was prepared to include 5% by volume of Xe and the sealing pressure was set in a range from (6.7-10.7)×10 4 Pa (500 to 800 Torr). (Phosphor material) The second embodiment employs the following materials for the phosphor materials that constitute the phosphor ink. The blue phosphor material includes one or more of the following materials: Ba (1-x) Eu x MgAl 10 O 17 , x = 0.01 to 0.15, Ba 2(1-x) Eu 2x Mg 2 Al 12 O 22 , x = 0.01 to 0.15, Ba 2(1-x) Eu 2x Mg 4 Al 8 O 18 , x = 0.01 to 0.15, and Ba 3(1-x) Eu 3x Mg 5 Al 18 O 35 , x = 0.01 to 0.15. As the green phosphor material and the red phosphor material, phosphor materials commonly used in the plasma display panels are employed. Specifically, the following materials may be used. Green phosphor material: Zn 2 SiO 4 : Mn 2+ Red phosphor material: Y 2 O 3 : Eu 3+ , (Y x Gd 1-x )BO 3 : Eu 3+ To obtain the phosphor layer 219 having good light emitting characteristics, luminance (Y value), chromaticity (y value) and heat resistance (acid resistance) of the materials used must be taken into consideration. Fig. 6A and Fig. 6B show the relative luminance and relative emission intensity of the following blue phosphor materials before and after the firing process with the value of x being changed. Ba (1-x) Eu x MgAl 10 O 17 Ba 2(1-x) Eu 2x Mg 2 Al 12 O 22 Ba 2(1-x) Eu 2x Mg 4 Al 8 O 18 Ba 3(1-x) Eu 3x Mg 5 Al 18 O 35 Fig.5 also shows the luminance of the above four kinds of blue phosphor when x=0.05 in relative values with the luminance of Ba (1-x) Eu x MgAl 10 O 17 being set to 100. The relative luminance and relative emission intensity in Figs. 6A and 6B are given relative to the value of the film before firing when x=0.1 which is set to 100. Solid lines in Fig. 6A, 6B show the characteristic of the blue phosphor materials before firing, dashed lines show the characteristic of the blue phosphor materials after firing at 520°C in air, and alternate dot and dash lines show the characteristic after firing at 460°C in air following the firing at 520°. The highest value of luminance after firing is obtained around the point of x = 0.1. However, in comparison of the emission intensity (luminance/y value) that takes into account the change in y value of the chromaticity, substantially uniform values were obtained below x=0.1 after firing at 520°C, and the highest value was obtained around x = 0.3 to 0.06 after another firing at 460°C. Thus in the evaluation of the emission intensity, the emission intensity tends to decrease with firing in the region above x=0.08, while the emission intensity tends to increase with firing in the region below x = 0.08, with best characteristic obtained around x = 0.3 to 0.06. This result may be explained by the tendency of Eu 2+ ions to be oxidized during firing that leads to a decrease in the emission intensity when Eu content is high. When Eu content is relatively low, on the other hand, less Eu 2+ ions are oxidized during firing while the emission intensity increases due to the removal of moisture, improvement in the crystallinity, etc. during firing. The results described above show that the amount of Eu ions that can substitute Ba ions in the layer that contains Ba is in a range from 1 atomic % to 10 atomic %. The phosphor materials of different colors used in the second embodiment can be made as follows. Blue phosphor materials of Ba (1-x) Eu x MgAl 10 O 17 , Ba 2(1-x) Eu 2x Mg 2 Al 12 O 22 , Ba 2(1-x) Eu 2x Mg 4 Al 8 O 18 and Ba 3(1-x) Eu 3x Mg 5 Al 18 O 35 are made by mixing barium carbonate (BaCO 3 ), magnesium carbonate (MgCO 3 ), aluminum oxide ( α -Al 2 O 3 ) and europium oxide (Eu 2 O 3 ) in predetermined proportion of the number of atoms. Then the mixture is mixed with a proper amount of flux (AlF 2 , BaCl 2 ) in a ball mill and the mixture is fired in a reducing atmosphere (H 2 , N 2 ) at a temperature in a range from 1400 to 1650°C for a predetermined period of time (for example, 0.5 hours). The red phosphor material is made by mixing the raw materials of yttrium hydroxide Y 2 (OH) 3 and europium oxide (Eu 2 O 3 ) which is mixed with a proper amount of flux in a ball mill, then the mixture is fired at a temperature in a range from 1200 to 1450°C in air for a predetermined period of time (for example, one hour). The green phosphor material is made by mixing the raw materials of zinc oxide (ZnO) and silicon oxide (SiO 2 ) in a proportion of 2: 1 by the numbers of atoms of Zn and Si. Then a predetermined amount of manganese oxide (Mn 2 O 3 ) is added to the mixture, which is mixed in a ball mill, and then fired at a temperature in a range from 1200 to 1350°C in air for a predetermined period of time (for example, 0.5 hours). Now the second example of the second embodiment will be described below. In Table 1, the plasma display panels (PDP) of samples No. 201, 203, are plasma display panels of Example 2 produced according to the second embodiment, while changing the value of x in the blue phosphor material Ba (1-x) Eu x MgAl 10 O 17 . The other samples are plasma display panels of Comparative Examples. In the production of the plasma display panels shown in Table 1, temperature of firing after forming the phosphor layer was set to 520°C and temperature of firing conducted to laminate the panels was set to 460°C. Thickness of the phosphor layer was set to 20 µm and discharge gas pressure was set to 6.7×10 4 Pa (500 Torr) (Ne-Xe 5%). Luminance of the plasma display panels was measured under electric discharge conditions of 150V for discharge holding voltage and 30kHz for frequency. The color temperature non-adjusted luminance given in Table 1 refers to the luminance of white display with the same signals given for all colors, and color temperature-adjusted luminance refers to the luminance when the signals of different colors are adjusted to achieve color temperature of 9500 degrees in white display. In the plasma display panel of this example, it is necessary to adjust the white balance to improve the picture quality. Usually, since luminance of blue color is lower than the other colors, color temperature becomes around 6000 degrees when all colors are illuminated with the same signal, thus making it necessary to reduce the luminance of green and red light by regulating the signals in order to achieve color temperature of 9000 degrees that is required. For the control of color temperature, higher intensity of emission intensity of blue light is more advantageous. Based on the evaluation of the display panel, the display panel of x = 0.15 showed the highest luminance when the color temperature was not adjusted (in various blue crystal systems), while the luminance was improved by setting the value of x to 0.08 or lower after adjusting the color temperature. Particularly remarkable improvement in luminance was obtained on the display panels where x was set to 0.05 and 0.01, while the improvement in luminance decreased in the display panel of x = 0.005. This may be because the amount of Eu 2+ ions is too small resulting in lower probability of ultraviolet excitation. Embodiment 3 (Phosphor material) The phosphor material commonly used in the phosphor layer of the plasma display panel of the prior art may be used except for the blue phosphor. Specifically, the following materials may be used. Green phosphor material: Zn 2 MnSiO 4 or BaMnAl 12 O 19 Red phosphor material: YEuBO 3 or (Y x Gd 1-x )EuBO 3 Composition of Ba (1-x-y) Eu x Sr y Mg a Al b O c was used for the blue phosphor. To obtain the phosphor layer having good light emitting characteristics, heat resistance of the raw materials used to make these phosphor materials must be studied. Fig. 8A and Fig. 8B show the relative emission intensity of the phosphor materials of Ba 0.95-x Eu x Sr 0.05 MgAl 10 O 17 and Ba 1-x Eu x MgAl 10 O 17 before and after the firing process when the value of x changed. Relative emission intensity is given relative to the emission intensity of Ba 0.9 Eu 0.7 MgAl 10 O 17 before firing that is assumed to be 100. Solid lines in Fig. 8A, 8B show the characteristics of the phosphor materials of different colors before firing, dashed lines show the characteristics of the phosphor materials after firing at 520°C in air, and alternate dot and dash lines show the characteristics of the phosphor materials after firing at 460°C in air following the firing at 520°C. While Ba 0.95-x Eu x Sr 0.05 MgAl 10 O 17 and Ba 1-x Eu x MgAl 10 O 17 show similar properties, in a comparison of the emission intensity, composition without Sr showed about 1 to 2% higher emission intensity. In comparison of the emission intensity of the materials after firing, substantially uniform values were obtained below x = 0.1 after firing at 520°C, and the highest value was obtained around x = 0.3 to 0.06 after another firing at 460°C. Thus in the evaluation of emission intensity, the emission intensity tends to decrease with firing in the region above x = 0.08, while the emission intensity tends to increase with firing in the region below x = 0.08, with best characteristic being obtained around x = 0.3 to 0.06 in such a case where heating of the phosphor material is repeated twice or more as in the fabrication of the plasma display panel This result may be explained by the tendency of Eu 2+ ions to be oxidized during firing' that leads to a decrease in the emission intensity when Eu content is high. When Eu content is relatively low, on the other hand, less Eu 2+ ions are oxidized during firing while the emission intensity increases due to the removal of moisture, improvement in the crystallinity, etc. during firing. The tendency of heat resistance to increase as the Eu content becomes smaller is not limited to Ba 0.95-x Eu x Sr 0.05 MgAl 10 O 17 , and similar tendency is shown also in Ba (1-x-y) Eu x Sr y Mg a Al b O c regardless of the Sr content y. Best characteristic was obtained around x = 0.3 to 0.06 when the emission intensity before firing was taken into consideration. Meanwhile the blue phosphor material used in the plasma display panel of the third embodiment has a problem in service life, and it is necessary to study the durability of the phosphor material to be used. Fig. 9 shows the durability of the phosphor material Ba 1-x Eu x MgAl 10 O 17 when the value of x is changed. Emission intensity after illuminating for 5000 hours relative to the emission intensity in the early stage of panel illumination that is assumed to be 100 is plotted along ordinate and the value of x is plotted along abscissa. In the conventional blue phosphor material Ba 1-x Eu x MgAl 10 O 17 , durability increases as the value of x increases. This may be because ionic radius of the Eu ion is smaller than that of the Ba ion that is substituted thereby, and therefore bond distance between Eu and oxygen increases as the value of x increases, resulting in increasing bond energy. In Ba 1-x Eu x MgAl 10 O 17 , however, x dependency of durability and x dependency of heat resistance are in trade-off relationship and therefore value of x is set in a range from 0.1 to 0.15. Embodiment 4 Outline of the fourth embodiment of the present invention will be described below. As described previously, the phosphor layer of the plasma display panel uses much of phosphor materials that have such composition as a metal element that constitutes the base material is partially substituted with an activating agent. In the blue phosphor material represented by BaEu 2+ MgAl 10 O 17 , for example, Ba atoms are substituted by Eu 2+ ions. The blue phosphor material of this type has been used in the prior art, as described previously, by setting the substitution ratio of Ba atoms by Eu 2+ ions to about 10 to 10 atomic %, supposedly for the following reason. The phosphor layer is formed basically in such a process as the phosphor particles mixed with a binder is applied and then fired at about 500°C thereby removing the binder. In the phosphor material such as BaEu 2+ MgAl 10 O 17 , while the initial luminance of the phosphor material becomes higher as the substitution ratio by Eu 2+ ions is increased, heat resistance becomes lower leading to more significant decreases in the luminance and emission intensity of the phosphor that accompany the firing. Thus the range of substitution ratio by Eu 2+ ions (10 to 10 atomic %) described above is thought to have been employed in the prior art in order to achieve satisfactory levels of luminance and emission intensity of the phosphor layer after firing. In the practical process of producing the plasma display panel, the front panel and the back panel are sealed to each other after forming the phosphor layer, by firing the panels at a temperature normally around 400°C. This means that the phosphor of the phosphor layer is subjected to firing twice. Because the firing in the sealing process is carried out at a temperature significantly lower than the firing for forming the phosphor layer (around 500°C), it has been believed in the prior art that the firing for sealing does not affect the phosphor material significantly. The inventor of the present application found, however, that the second firing has substantial influence on the emission intensity of the phosphor layer. It was found that, in case the phosphor material is subjected to two steps of firing, higher luminance and higher emission intensity of the phosphor layer can be obtained by setting the substitution ratio by Eu 2+ ions in the phosphor material to within 8 atomic %, lower than practiced in the prior art, and it is preferable to set the substitution ratio by Eu 2+ ions in a range from 1 to 6 atomic %. The plasma display panel of the fourth embodiment of the present invention will be described below. Fig. 10 is a schematic sectional view of an AC discharge type plasma display panel according to the fourth embodiment. While Fig. 10 shows only one cell, the plasma display panel comprises a number of cells that emit red, green and blue light being arranged thereon. The plasma display panel of the fourth embodiment has a constitution similar to that of the second embodiment, except that the phosphor layers 219 of the second embodiment are replaced by phosphor layers 619 that include phosphor materials having predetermined composition. The plasma display panel of the fourth embodiment, being constituted by using the phosphor layers 619 that include the phosphor materials of the predetermined composition which will be described in detail later, has various excellent features to be described later. The phosphor layers 619 are formed by continuously spouting the phosphor ink from the nozzles while scanning thereby applying the phosphor ink and firing the ink, similarly to the second embodiment. (Phosphor material and phosphor ink of the fourth embodiment and application thereof) YEu 3+ BO 3 and (Y x Gd 1-x )Eu 3+ BO 3 , for example, may be used as the red phosphor material. YEu 3+ BO 3 has such a structure that Y atoms that constitute the matrix of YBO 3 are substituted by Eu 3+ , and (Y x Gd 1-x )Eu 3+ BO 3 has such a structure that Y atoms and Gd atoms that constitute the matrix of (Y x Gd 1-x )Eu 3+ BO 3 are substituted by Eu 3+ . Zn 2 Mn 2+ SiO 4 and BaMn 2+ Al 12 O 19 , for example, may be used as the green phosphor material. Zn 2 Mn 2+ SiO 4 has such a structure that Zn atoms that constitute the matrix of Zn 2 SiO 4 are substituted by Mn 2+ , and BaMn 2+ Al 12 O 19 has such a structure that Ba atoms that constitute the matrix of BaAl 12 O 19 are substituted by Mn 2+ . These red and green phosphor materials that are commonly used may be used without modification for the plasma display panel of the fourth embodiment. For the blue phosphor, on the other hand, the material represented by BaEu 2+ MgAl 10 O 17 is used. While a phosphor material wherein about 10 to 15% of the Ba atoms that constitute the matrix of BaMgAl 10 O 17 are substituted by Eu 2+ has been used in the prior art plasma display panels, this embodiment employs a phosphor material wherein substitution ratio of Ba atoms by Eu 2+ ions is set within 8 atomic %, lower than the prior art. These red, green and blue phosphor material powders are used in the form of particles having average particle size in a range from 1 to 7 µm. For the binder of the phosphor ink used in forming the phosphor layer, it is preferable to use ethylcellulose or acrylic resin (0.1 to 10% by weight of ink) and to use terpineol (C 10 H 18 O) as the solvent. Alternatively, high polymers such as PMMA and polyvinyl alcohol may be used as the binder, and organic solvent such as diethylene glycol methyl ether or water may also be used as the solvent. After forming the phosphor layers of red, blue and green colors, the back glass substrate 15 is put in a furnace and fired at a temperature around 500°C for 10 to 20 minutes. The organic binder included in the phosphor ink or fat included in the sheet is burned out in this firing process, thereby forming the phosphor layer 619 comprising the phosphor particles bonded in the form of film. While the phosphor is applied in the process described above by spouting the phosphor ink from the nozzles while scanning, the phosphor may also be applied by screen printing of phosphor paste. Furthermore, the phosphor may also be applied by such a process as photosensitive resin sheets that contain phosphor materials of different colors are attached to the back glass substrate 15 on the side where the barrier ribs 218 are formed, whereon a pattern is formed by photolithography and development thereby removing unnecessary portions. (Method of producing phosphor material) The phosphor materials of different colors described above can be made, for example, as follows. Production of BaEu 2+ MgAl 10 O 17 for blue phosphor First, the raw materials of barium carbonate (BaCO 3 ), magnesium carbonate (MgCO 3 ), aluminum oxide (α-Al 2 O 3 ) and europium oxide (Eu 2 O 3 ) are mixed so that molar ratio of the sum of Ba and Eu to Mf and Al is 1: 1: 10. Molar ratio of Mg and Eu is determined according to the desired substitution ratio of Ba atoms by Eu 2+ ions. To set the substitution ratio of Ba atoms by Eu 2+ ions to 8 atomic %, for example, molar ratio of Mg and Eu becomes 92: 8. Accordingly, molar ratio of barium carbonate, magnesium carbonate, aluminum oxide and europium oxide is set to 92: 4: 100: 500. Then the mixture is mixed with a proper amount of flux (AlF 2 , BaCl 2 ) in a ball mill and the mixture is fired in a weakly reducing atmosphere (H 2 , N 2 ) at a temperature in a range from 1400 to 1650°C for a predetermined period of time (for example, 0.5 hours), thereby to obtain the BaEu 2+ MgAl 10 O 17 particles of the predetermined substitution ratio by Eu 2+ ions. Production of YEu 2+ BO 3 for red phosphor The raw materials of yttrium hydroxide Y 2 (OH) 3 , boric acid (H 3 BO 3 ) and europium oxide (Eu 2 O 3 ) are mixed in molar ratio of 1: 1 for the sum of Y and Eu to B. Molar ratio of Y to Eu is determined according to the desired substitution ratio of Y atoms by Eu 3+ ions. Then the mixture is mixed with a proper amount of flux in a ball mill and the mixture is fired in air at a temperature in' a range from 1200 to 1450°C for a predetermined period of time (for example, 1 hour), thereby to obtain the YEu 3+ BO 3 particles of the predetermined substitution ratio by Eu 3+ ions. Production of Zn 2 Mn 2+ SiO 4 for green phosphor The raw materials of zinc oxide (ZnO), silicon oxide (SiO 2 ) and manganese oxide (Mn 2 O 3 ) are mixed in molar ratio of 2: 1 for the sum of Zn and Mn to Si. Molar ratio of Zn to Mn is determined according to the desired substitution ratio of Zn atoms by Mn 2+ ions. Then the mixture is mixed in a ball mill and fired in air at a temperature in a range from 1200 to 1350°C for a predetermined period of time (for example, 0.5 hours), thereby to obtain the Zn 2 Mn 2+ SiO 4 particles of the predetermined substitution ratio by Mn 2+ ions. The plasma display panel was produced according to the fourth embodiment. Zn 2 Mn 2+ SiO 4 (content of Mn is 2.3 weight %) was used for the green phosphor material and YBO 3 : Eu 3+ (substitution ratio by Eu 2+ ions 0.1) was used for the red phosphor. BaEu 2+ MgAl 10 O 17 was used for the blue phosphor, where substitution ratio of Ba atoms by Eu 2+ ions was set to 0.5, 2.0, 5.0, and 8.0 atomic % for Nos. 601 through 604, respectively, as shown in Table 11. In Table 2, composition of the blue phosphor is given as Ba 1-x Eu x MgAl 10 O 17 which represents the same phosphor that is represented by BaEu 2+ MgAl 10 O 17 , while the substitution ratio of Ba atoms in the base material by Eu 2+ ions is represented in the formula by x. Average particle size was set to about 3 µm for the phosphor material powders of all colors. Firing for the formation of the phosphor layer was carried out at 520°C for 10 minutes and firing for the lamination of the panels was carried out at 460°C for 10 minutes. Thickness of the phosphor layer was set to 20 µm and discharge gas pressure was set to 6.7×10 4 Pa (500 Torr). The panel No. 605 shown in Table 2 is for the plasma display panel of Comparative Example, that was produced similarly to the example except that substitution ratio of Ba atoms by Eu 2+ ions was set to 10 atomic % (x = 0.100). Color temperature non-adjusted luminance and color temperature-adjusted luminance were measured on the plasma display panels of this example and Comparative Example produced as described above. The color temperature non-adjusted luminance refers to the luminance of white display with the same signals given for all colors (namely having the discharge spaces of the three colors emit equivalent ultraviolet rays), and color temperature-adjusted luminance refers to the luminance when the signals of different colors are adjusted to achieve color temperature of 9500 degrees in white display. Luminance was measured under electric discharge conditions of 150 V for discharge holding voltage and 30kHz for frequency. The measurements results are shown in Table 2. The results of measurements shown in Table 3 show that No. 605 (x = 0.100) demonstrates higher color temperature non-adjusted luminance than Nos. 601 through 604 (x = 0.005 to 0.080), while higher values of color temperature-adjusted luminance are obtained with Nos. 601 through 604 (x = 0.005 to 0.080) than No. 605 (x = 0.100). This means that higher luminance of the plasma display panel than that achieved in the prior art can be obtained by setting the value of x for the blue phosphor to 0.08 or less, that is smaller than in the prior art. No. 602 (x = 0.020) and No. 603 (x = 0.050) show particularly high values of color temperature-adjusted luminance. The color temperature-adjusted luminance is measured while giving consideration to the fact that improving the picture quality of the plasma display panel requires it to take white balance. Higher luminance can be achieved while maintaining good picture quality, as the color temperature-adjusted luminance is increased, It is because the emission intensity of the blue phosphor was increased that high luminance was obtained by setting the value of x for the blue phosphor to 0.08 or less. To be more specific, it is'necessary to achieve a color temperature of 9000 degrees or higher with white balance in order to obtain good picture quality on the plasma display panel. However, since the blue phosphor material has, in general, lower luminance than the phosphor materials of other the colors, emitting the light of all colors with the same signal results in a color temperature of around 6000 degrees and good picture quality cannot be obtained. While achieving a color temperature of 9000 degrees or higher requires it to make the luminance of blue light lower than those of green and red light, the higher the emission intensity of the blue phosphor, the less decrease in the luminance of red and green light is required, thereby making it possible to achieve higher color temperature-adjusted luminance. No. 601 (x = 0.005) shows lower color temperature-adjusted luminance than No. 602 (x = 0.020), which is supposedly because of insufficient content of Eu 2+ ions in the blue phosphor material that leads to lower probability of ultraviolet excitation. Then the relationship between substitution ratio by Eu 2+ ions and heat resistance in the blue phosphor of BaEu 2+ MgAl 10 O 17 was investigated as described below. In conjunction with the preparation of BaEu 2+ MgAl 10 O 17 in the process described above, materials represented by Ba 1-x Eu x MgAl 10 O 17 with various values of x (substitution ratio by Eu 2+ ions) were made by changing the quantity of europium oxide (Eu 2 O 3 ) to be added. Phosphor pastes were made by using the phosphor materials made as described above and applied to substrates that were then fired at 520°C in air for 10 minutes thereby forming phosphor layers. The phosphor layers thus formed were further fired at' 460°C in air for 10 minutes. Luminance and emission intensity of these phosphor layers were measured by irradiating the phosphor layer with ultraviolet rays emitted by a UV lamp before being fired at 520°C (pre-firing), after being fired at 520°C (after first firing) and after being fired at 460°C (after second firing). Luminance was measured with a luminance meter. Intensity was determined by measuring the emission spectrum from the phosphor layer with a spectrophotometer, calculating y value of chromaticity from the measurement and calculating the emission intensity from the y value of chromaticity and the measured luminance (luminance/y value of chromaticity). Fig. 11A, 11B show the results of measurement described above, with Fig. 11A showing the relation between the value of x and relative luminance and Fig. 11B showing the relation between the value of x and relative emission intensity. In Fig. 11A, 11B, solid lines represent the characteristic before firing, dashed lines represent the characteristic after the first firing, and alternate dot and dash lines represent the characteristic after the second firing. Relative luminance and relative emission intensity in Fig. 11A, 11B are given in values relative to the value measured on a phosphor of x=0.1 before firing that is assumed to be 100. From the graphs shown in Fig. 11A, 11B, the following findings can be drawn. *1: Before firing, luminance increases as the value of x increases, while emission intensity has a peak around x = 0.1. *2: After the first firing, luminance reaches a peak at a point where the value of x is a little larger than 0.1. Emission intensity remains substantially constant in the region where x is below 0.1, but decreases as the value of x increases beyond 0.1. Thus when judging according to the measurements after the first firing, it may be concluded that setting the value of x in a range from 0.1 to 0.15 as in the prior art is appropriate for making phosphor layers of high performance. However, after the second firing, luminance reaches a peak at a point where the value of x is a little larger than 0.1 and maintains high level even when the value of x decreases significantly. Emission intensity has a peak at a point around x = 0.03 to 0.06 and decreases significantly as x increases over 0.08. 'These results show that the value of x is preferably not greater than 0.08, particularly in a range from 0.01 to 0.06, and more preferably in a range from 0.03 to 0.06, for a phosphor material that is fired twice as in the plasma display panel. Particularly worth notice is the fact that the firing process has opposite effects on the emission intensity depending on whether the value of x is below 0.08 or greater than 0.08. That is, in the range of x greater than 0.08, emission intensity after firing is lower than that before firing. In the range of x below 0.08, on the contrary, emission intensity becomes higher after firing than that before firing, and both the luminance and the emission intensity become higher after the second firing than after the first firing. The result described above may be explained as follows. When the phosphor material is fired, Eu 2+ ions are oxidized, while impurities such as moisture are removed so that the crystallinity improves, which contribute to the increase in emission intensity. In the region of x greater than 0.08, the former effect is predominant but the latter effect becomes predominant in the region of x below 0.08. Although the process that involves firing at 520°C followed by firing at 460°C is described in this example, similar effects can be achieved also by firing at 500°C followed by firing at 350°C, or by firing twice at similar temperatures (for example, 460°C). Fig. 11A, 11B show the results of measurements carried out up to the point after two firing steps; firing at 520°C followed by firing at 460°C. However, similar effects were also obtained from measurement made after third firing at 460°C following the two firing steps. This means that the behaviors of luminance and emission intensity represented by the alternate dot and dash lines in Fig. 11A, 11B that were observed after the two firing steps remain without significant changes after being fired further. In the fourth embodiment, the phosphor represented by BaMgAl 10 O 17 : Eu 2+ is described as an example of blue phosphor that contains Eu 2+ ions as the activation agent. But the present invention is not limited to this configuration, and can also be applied to a case where BaEu 2+ MgAl 14 O 23 or Ba a Eu 2+ Sr 1-a MgAl 10 O 17 is used as the blue phosphor. Specifically, similar effects can be achieved also by setting the substitution ratio by Eu 2+ ions in the case of BaEu 2+ MgAl 14 O 23 and substitution ratio of Ba atoms and Sr atoms by Eu 2+ ions in the case of Ba a Eu 2+ Sr 1-a MgAl 10 O 17 to 8 atomic % or lower (preferably in a range from 1 to 6 atomic %). The embodiments described above take the AC discharge type plasma display panel as an example, but the foregoing description applies also to a DC type plasma display panel. Moreover, application of the blue phosphor material described in the sixth embodiment is not limited to the plasma display panel, and is applicable also to a fluorescent lamp, for example, with similar effects. INDUSTRIAL APPLICABILITY As described above, the present invention provides the phosphor material and the phosphor material powder having high emission intensity and high luminance, of which application to the plasma display panel makes it possible to provide high luminance and high definition plasma display panel.

Claims: A phosphor material for a plasma display panel comprising BaMgAl y O z in which Ba is substituted partially by Eu 2+ ions, characterized in that said BaMgAl y O z being BaMgAl 14 O 23 in which the Ba substitution ratio by Eu 2+ ions is not exceeding 8 atomic %. A phosphor material for a plasma display panel according to claim 1; wherein said BaMgAl y O z being BaMgAl 14 O 23 in which the Ba substitution ratio by Eu 2+ ions is in a range from 1 to 6 atomic %. A plasma display panel having a plurality of discharge spaces formed between a front panel and a back panel disposed to oppose each other, at least one of the discharge spaces having a phosphor layer that includes blue phosphor particles, wherein the blue phosphor particles included in the phosphor layer are made of the phosphor material as in one of claims 1-2. A method of producing a plasma display panel comprising; an applying step of applying a phosphor material comprising BaMgAl y O z in which Ba is substituted partially by Eu 2+ ions together with a binder onto a first panel substrate, said BaMgAl y O z being BaMgAl 10 O 17 or BaMgAl 14 O 23 in which Ba substitution ratio by Eu 2+ ions is less than 8 atomic %, a first firing step of firing the first panel substrate whereon the phosphor material has been applied, a second firing step of firing the first panel and a second panel placed one on another to bond the first panel and the second panel, and wherein a firing temperature for the first firing step is higher than a firing temperature for the second firing step. A method of producing a plasma display panel as in claim 4, wherein BaMgAl y O z being BaMgAl 10 O 17 or BaMgAl 14 O 23 in which Ba substitution ratio by Eu 2+ ions is in a range from 1 to 6 atomic %. A method of producing the plasma display panel as in claim 4 or 5 wherein the applying step comprises a process of applying an ink or a sheet made by mixing particles of the phosphor material and the binder onto the first panel substrate.

Inventors: AOKI MASAKI, KADO HIROYUKI, KAWAMURA HIROYUKI, OHTANI MITSUHIRO, SUZUKI SHIGEO, AOKI, MASAKI, KADO, HIROYUKI, KAWAMURA, HIROYUKI, OHTANI, MITSUHIRO, SUZUKI, SHIGEO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: H01J11
IPC codes: C09K11