Plasma display panel suitable for high-quality display

Abstract: The first object of the present invention is to provide a PDP with improved panel brightness which is achieved by improving the efficiency in conversion from discharge energy to visible rays. The second object of the present invention is to provide a PDP with improved panel life which is achieved by improving the protecting layer protecting the dielectrics glass layer. To achieve the first object, the present invention sets the amount of xenon in the discharge gas to the range of 10% by volume to less than 100% by volume, and sets the charging pressure for the discharge gas to the range of 500 to 760Torr which is higher than conventional charging pressures. With such construction, the panel brightness increases. Also, to achieve the second object, the present invention has, on the surface of the dielectrics glass layer, a protecting layer consisting of an alkaline earth oxide with (100)-face or (110)-face orientation. The protecting layer, which may be formed by using thermal Chemical Vapor Deposition (CVD) method, plasma enhanced CVD method, or a vapor deposition method with irradiation of ion or electron beam, will have a high sputtering resistance and effectively protect the dielectrics glass layer. Such a protecting layer contributes to the improvement of the panel life.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention This invention relates to a plasma display panel used as a display device, specifically to a plasma display panel suitable for a high-quality display. (2) Description of the Prior Art Recently, as the demand for high-quality large-screen TVs such as high-vision TVs have increased, displays suitable for such TVs, such as Cathode Ray Tube (CRT), Liquid Crystal Display (LCD), and Plasma Display Panel (PDP), have been developed. CRTs have been widely used as TV displays and excel in resolution and picture quality. However, the depth and weight increase as the screen size increases. Therefore, CRTs are not suitable for large screen sizes exceeding 40 inch (102cm). LCDs consume a small amount of electricity and operate on a low voltage. However, producing a large LCD screen is technically difficult, and the viewing angles of LCDs are limited. On the other hand, it is possible to make a PDP with a large screen with a short depth, and 40-inch (102cm) PDP products have already been developed. PDPs are divided into two types: Direct Current (DC) and Alternating Current (AC). Currently, PDPs are mainly AC-type since they are suitable for large screens. Fig. 1 is a sectional view of a conventional AC PDP. In the drawing, front cover plate 1, with display electrodes 2 put thereon, is covered by dielectrics glass layer 3 which is lead glass, namely, PbO-B 2 O 3 -SiO 2 glass. Set on back plate 5 are address electrode 6, partition walls 7, and fluorescent substance layer 8 consisting of red, green, or blue ultraviolet excited fluorescent substance. Discharge gas is charged in discharge space 9 which is sealed with dielectrics glass layer 3, back plate 5, and partition walls 7. The discharge gas is generally helium (He), xenon ( Xe ), or mixture of neon (Ne) and Xe. The amount of Xe is generally set to a range from 0.1 to 5% by volume, preventing the drive voltage of the circuit from becoming too high. Also, the charging pressure of the discharge gas is generally set to a range from 100 Torr (13kPa) to 500Torr (66·5kPa) so that the discharge voltage is stable (e.x., M. Nobrio, T. Yoshioka, Y. Sano, K. Nunomura, SID94' Digest, pp727-730, 1994). PDPs have the following problems concerning brightness and life. Currently, PDPs for 40-42-inch (102-107cm) TV screens generally have a brightness of about 150-250cd/m 2 for National Television System Committee (NTSC) standard (number of pixels being 640X480, cell pitch 0.43mmX1.29mm, square of one cell 0.55mm 2 ) (Function & Materials, Feb., 1996, Vol.16, No. 2. page 7). On the contrary, in 42-inch (107 cm) high-vision TVs, number of pixels is 1,920X1,125, cell pitch 0.15mmX0.48mm, and square of one cell 0.072mm 2 . This square of one cell is 1/7-1/8 of that of NTSC standard. Therefore, it is expected that if PDP for 42-inch (107 cm) high-vision TV is made with the conventional cell construction, the screen brightness decreases to 30-40cd/m 2 . Accordingly, to acquire, in a PDP used for a 42-inch (107cm) high-vision TV, the same brightness as that of a current NTSC CRT (500cd/m 2 ), the brightness of each cell should be increased about 12-15 times. In these circumstances, it is desired that the techniques for increasing the brightness of PDP cells are developed. The light-emission principle in PDP is basically the same as that in fluorescent light: a discharge lets the discharge gas emit ultraviolet light; the ultraviolet light excites fluorescent substances; and the excited fluorescent substances emit red, green, and blue lights. However, since discharge energy is not effectively converted to ultraviolet light and conversion ratio in fluorescent substance is low, it is difficult for PDPs to provide brightness as high as that of fluorescent lights. It is disclosed in Applied Physics, Vol.51, No.3, 1982, pp344-347 as follows: in PDP with He-Xe or Ne-Xe gas, only about 2% of the electric energy is used in ultraviolet light, and about 0.2% of the electric energy is used in visible rays (Optical Techniques Contact, Vol.34, No.1, 1996, page 25 and FLAT PANEL DISPLAY 96, Parts 5-3, NHK Techniques Study, 31-1, 1979, page 18). Accordingly, to increase light-emission efficiency is considered as important in increasing the brightness of PDP cells. Now, regarding to the PDP life, the following are generally considered to determine the PDP life: (1) the fluorescent substance layer deteriorates since plasma is confined to a small discharge space to generate ultraviolet light; and (2) the dielectrics glass layer deteriorates due to sputtering by gas discharges. As a result, methods for extending the fluorescent substance life or preventing the deterioration of dielectrics glass layer are studied. As shown in Fig.1, in conventional PDPs, protecting layer 4 consisting of magnesium oxide (MgO) is formed on the surface of dielectrics glass layer 3 with a vacuum vapor deposition method to prevent the dielectrics glass layer from deteriorating. It is desirable that protecting layer 4 has high sputtering resistance and emits a large amount of secondary electron. However, it is difficult for magnesium oxide layer formed by the vacuum vapor deposition method to obtain a protective layer having enough sputtering resistance. There is also a problem that discharges decrease the amount of secondary electron emitted. JP 06-342631 A discloses a plasma display panel designed so as to restrict drive voltage and to improve emission efficiency by providing a three-element mixed gas, formed by adding a specific ratio of xenon to a mixed gas of helium and neon at a specific ratio, as a discharge gas to be sealed in a discharge gas space, in which a phosphor layer is provided. The phosphor layer is provided on the inner surface of a discharge gas space and emits light by means of ultraviolet light generated by discharging a discharge gas. A three-element mixed gas formed of 1.5-10 volume % of xenon, helium, and neon, is provided as the discharge gas. The volume ratio of helium to neon ranges from 6 to 4, to 9 to 1. EP 0 649 159 A discloses a DC type gas-discharge display panel, and discusses the use of Ne-Xe, Ne-Xe-Kr, He-Xe and He-Xe-Kr filling gases at various charging pressures up to 760 Torr (101 kPa). GB 2 109 628 A discloses a gas discharge display device using Ne-Xe-Cl 2 gas mixtures at pressures below or reasonably close to atmospheric. JP 05-234519 A discloses the use of a [111] face orientation film, formed by a vacuum deposition method, as a protecting film for covering a dielectric layer against a discharging space in an AC type plasma display panel. SUMMARY OF THE INVENTION It is therefore the first object of the present invention to provide a PDP with improved panel brightness which is achieved by improving the efficiency in conversion from discharge energy to visible rays. It is a second, preferred object of the present invention to provide a PDP with improved panel life which is achieved by improving the protecting layer protecting the dielectrics glass layer. The present invention provides a plasma display panel comprising: a front cover plate which comprises a front glass substrate, with spaced pairs of first electrodes, and a dielectrics glass layer, wherein the first electrodes and the dielectrics glass layer are formed on the front glass substrate; and a back plate which comprises a back glass substrate, a second electrode, and a fluorescent substance layer, wherein the second electrode and the fluorescent substance layer are formed on the back glass substrate, wherein the dielectrics glass layer and the fluorescent substance layer face to each other, wherein a plurality of discharge spaces are formed between a plurality of partition walls which are set between the front cover plate and the back plate, wherein a gas medium is charged in the plurality of discharge spaces (19), and the gas medium is composed of neon and xenon, characterized in that the gas medium includes xenon in a range from at least 10% to less than 100% by volume, the charging pressure of the gas medium is at least 550Torr (73.2 kPa) and less than or equal to 760Torr (101kPa), and the distance between each pair of adjacent first electrodes is no greater than 0.1 mm. To achieve the first object, the present invention sets the amount of Xe in the discharge gas to the range from greater than 10% by volume to less than 100% by volume, and sets the charging pressure for the discharge gas to the range of greater than 550 Torr (73,2kpa) to less than or equal to 760 Torr (101kPa) which is higher than conventional charging pressures. With such construction, the panel brightness increases. The assumed reasons for it are as follows: the increase in the amount of Xe in the discharge space increases the amount of ultra violet light emitted; the ratio of excitation wavelength (173nm wavelength) by molecular Xe in the emitted ultra violet light increases; and this increases the efficiency of a conversion from fluorescent substance to visible rays. Also, to achieve the second object, a preferred embodiment of the present invention has, on the surface of the dielectrics glass layer, a protecting layer consisting of an alkaline earth oxide with (100)-face or (110)-face orientation. The conventional protecting layer of magnesium oxide formed by vacuum vapor deposition method (electron-beam evaporation method) has (111)-crystal-face orientation. Compared to this, the protecting layer of an alkaline earth oxide with (100)-face or (110)-face orientation is dense, has high sputtering resistance, and emits a great amount of secondary electron. Accordingly, the preferred embodiment of the present invention prevents deterioration of the dielectrics glass layer and keeps the discharge voltage low. By way of example such effects are further improved by using thermal Chemical Vapor Deposition (CVD) method or plasma enhanced CVD method, both of which have not been used as methods of forming protecting layers, to form an alkaline earth oxide with (100)-face orientation. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. In the drawings: Fig.1 is a sectional view of a conventional AC PDP; Fig.2 is a sectional view of an AC PDP described in an embodiment of the present invention; Fig.3 shows a CVD apparatus used for forming protecting layer 14; Fig.4 is a graph showing the relation between the wavelength and amount of the ultraviolet light for each charging pressure, the ultraviolet light being emitted from Xe in He-Xe gas used as a discharge gas in a PDP; Figs.5(a)-(c) shows relation between excitation wavelength and relative radiation efficiency for each color of fluorescent substance; Fig.6 is a graph showing relation between charging pressure P of the discharge gas and discharge start voltage Vf for two values of distance d, d being a distance between dielectrics electrodes in a PDP; and Fig.7 shows an ion/electron beam irradiating apparatus which is used for forming a protecting layer in a PDP. DESCRIPTION 0F THE PREFERRED EMBODIMENTS (Embodiment 1) <Structure and Production Method> Fig.2 is a sectional view of a discharge PDP of the present embodiment. Though Fig.2 shows only one cell, a PDP includes a number of cells each of which emits red, green, or blue light. The present PDP includes: a front panel which is made up of front glass substrate 11 with display electrodes 12 and dielectrics glass layer 13 thereon; and a back panel which is made up of back glass substrate 15 with address electrode 16, partition walls 17, and fluorescent substance layer 18, the front panel and back panel being bonded together. Discharge space 19, which is sealed with the front panel and back panel, is charged with a discharge gas. The present PDP may be made as follows. Producing the Front Panel The front panel is made by forming display electrodes 12 onto front glass substrate 11, covering it with dielectrics glass layer 13, then forming protecting layer 14 on the surface of dielectrics glass layer 13. In the present embodiment, discharge electrodes 12 are silver electrodes which are formed by transferring a paste for the silver electrodes onto front glass substrate 11 with screen printing then baking them. Dielectrics glass layer 13, being lead glass, is composed of 75% by weight of lead oxide (PbO), 15% by weight of boron oxide (B 2 O 3 ), and 10% by weight of silicon oxide (SiO 2 ). Dielectrics glass layer 13 is also formed with screen printing and baking. Protecting layer 14 consists of an alkaline earth oxide with (100)-face orientation and is dense. A CVD method (thermal CVD method or plasma enhanced CVD method) may be used to form such a dense protecting layer consisting of magnesium oxide with (100)-face orientation. The formation of the protecting layer with the CVD method will be described later. Producing the Back Panel The back panel is made by transferring the paste for the silver electrodes onto back glass substrate 15 by screen printing then baking back glass substrate 15 to form address electrodes 16 and by attaching partition walls 17 made of glass to back glass substrate 15 with a certain pitch. Fluorescent substance layer 18 is formed by inserting one of a red fluorescent substance, a green fluorescent substance, a blue fluorescent substance into each space surrounded by partition walls 17. Any fluorescent substance generally used for PDPs can be used for each color. The present embodiment uses the following fluorescent substances: red fluorescent substance (Y x Gd 1-x )BO 3 : Eu 3+ green fluorescent substance BaAl 12 O 19 : Mn blue fluorescent substance BaMgA1 14 O 23 : Eu 2- Producing a PDP by Bonding Panels A PDP is made by bonding the above front panel and back panel with sealing glass, at the same time excluding the air from discharge space 19 partitioned by partition walls 17 to high vacuum (8X10 -7 Torr), then charging a discharge gas with a certain composition into discharge space 19 at a certain charging pressure. In the present embodiment, cell pitch is under 0.2mm and distance between electrodes "d" is under 0.1mm, making the cell size of the PDP conform to 40-inch high-vision TVs. The discharge gas is composed of Ne-Xe gas, which has been used conventionally. However, the amount of Xe is set to 10% by volume or more and the charging pressure to the range of 550 to 700Torr (73,2 -93,3 kPa). Forming the Protecting Layer with the CVD Method Fig.3 shows a CVD apparatus used for forming protecting layer 14. For the CVD apparatus, either of the thermal CVD method and plasma enhanced CVD method is applicable. CVD apparatus 25 includes heater 26 for heating glass substrate 27 (equivalent to front glass substrate 11 with display electrodes 12 and dielectrics glass layer 13 as shown in Fig.2). The pressure inside CVD apparatus 25 can be reduced by venting apparatus 29. CVD apparatus 25 also includes high-frequency power 28 for generating plasma in CVD apparatus 25. Ar-gas cylinders 21a and 21b supply argon (Ar) gas, which is used as a carrier, to CVD apparatus 25 respectively via bubblers 22 and 23. Bubbler 22 stores a metal chelate of alkaline earth oxide used as the source and heats it. The metal chelate is transferred to CVD apparatus 25 when it is evaporated by the argon gas blown on it through Ar-gas cylinder 21a. Bubbler 23 stores a cyclopentadienyl compound of alkaline earth oxide used as the source and heats it. The cyclopentadienyl compound is transferred to CVD apparatus 25 when it is evaporated by the argon gas blown on it through Ar-gas cylinder 21b. Oxygen cylinder 24 supplies oxygen (O 2 ) used as a reaction gas to CVD apparatus 25. (1) For thermal CVDs performed with the present CVD apparatus, glass substrate 27 is put on heating unit 26 with the dielectrics glass layer on glass substrate 27 to be heated with a certain temperature (350 to 400°C. See Table 1 "HEATING TEMPERATURE FOR GLASS SUBSTRATE"). At the same time, the pressure in the reaction container is reduced by venting apparatus 29 (by about several tens Torr, ten Torr being equipment to 1.3 kPa). Bubbler 22 or 23 is used to heat the metal chelate or cyclopentadienyl compound of alkaline earth oxide used as the source to a certain temperature (See Table 1, "TEMPERATURE OF BUBBLER"). At the same time, Ar gas is sent to bubbler 22 or 23 through Ar-gas cylinder 21a or 21b and oxygen is sent through cylinder 24. The metal chelate or cyclopentadienyl compound reacts with oxygen in CVD apparatus 25 to form a protecting layer consisting of an alkaline earth oxide on the surface of glass substrate 27. (2) For plasma enhanced CVDs performed with the present CVD apparatus, the procedure is almost the same as that of the thermal CVD described above. However, glass substrate 27 is heated by heating unit 26 with temperature ranging from 250 to 300°C (See Table 1, "HEATING TEMPERATURE FOR GLASS SUBSTRATE"). At the same time, the pressure in the reaction container is reduced to about 10Torr (1.3 kpa) by venting apparatus 29. Under the circumstances, a protecting layer consisting of an alkaline earth oxide is formed by driving high-frequency power 28 to apply high-frequency electric field of 13.56MHz, generating plasma in CVD apparatus 25. Conventionally, the thermal CVD method or plasma enhanced CVD method has not been used for forming a protecting layer. One of the reasons for not using these methods is that no appropriate source for these methods was found. The present inventors have made it possible to form a protecting layer with the thermal CVD method or plasma enhanced CVD method by using the sources described below. The source (metal chelate or cyclopentadienyl compound) supplied through bubblers 22 and 23: alkaline earth dipivaloylmethane compound M(C 11 H 19 O 2 ) 2 , alkaline earth acetylacetone compound M(C 5 H 7 O 2 ) 2 , alkaline earth trifluoroacetylacetone compound M(C 5 H 5 F 3 O 2 ) 2 , and alkaline earth cyclopentadiene compound M(C 5 H 5 ) 2 , where "M" represents an alkaline earth element. In the presently described method, the alkaline earth is magnesium. Therefore, the sources are as follows: magnesium dipivaloyl methane Mg(C 11 H 19 O 2 ) 2 , magnesium acetylacetone Mg(C 5 H 7 O 2 ) 2 , magnesium trifluoroacetylacetone Mg(C 5 H 5 F 3 O 2 ) 2 , and cyclopentadienyl magnesium Mg(C 5 H 5 ) 2 . The protecting layer formed with the thermal CVD method or plasma enhanced CVD method allows the crystals of the alkaline earth oxides to grow slowly to form a dense protecting layer consisting of an alkaline earth oxide with (100)-face orientation. Effects of Protecting Layer of Magnesium Oxide with (100)-Face Orientation The conventional protecting layer of magnesium oxide formed by vacuum vapor deposition method (electron-beam evaporation method) has (111)-crystal-face orientation according to X-ray analysis (See No.15 in Table 2 and Nos.67 and 69 in Table 4). Compared to this, the protecting layer of a magnesium oxide with (100)-face orientation has the following characteristics and effects: (1) the magnesium oxide with (100)-face orientation extends PDP life since it protects dielectrics glass layer due to its sputtering resistance owing to its density; (2) the magnesium oxide with (100)-face orientation reduces driving voltage of PDP and improves panel brightness since it has a large emission coefficient (y value) of secondary electron; (3) The magnesium oxide with (111)-face orientation tends to react with the water content in the air to form hydroxides since it forms faces with the highest surface energy among a variety of faces with orientation (see Surface Techniques, Vol.41, No.4, 1990, page 50 and Japanese Laid-Open Patent Application No.5-342991). Accordingly, magnesium oxide with (111)-face orientation has a problem that the formed hydroxides decompose during a discharge and reduce the amount of the emission of secondary electron. On the other hand, the protecting layer of a magnesium oxide with (100)-face orientation is to a large extent immune to this problem. (4) The magnesium oxide with (111)-face orientation has a heat resistance of up to 350°C. On the other hand, since the protecting layer of a magnesium oxide with (100)-face orientation has a higher heat resistance, heat treatment is carried out at a temperature of about 450°C when the front cover plate and the back plate are bonded. (5) With the protecting layer of a magnesium oxide with (100)-face orientation, aging after bonding of panels is comparatively short. These characteristics and effects are especially noticeable in the protecting layer of a magnesium oxide with (100)-face orientation formed with the thermal CVD method or plasma enhanced CVD method. Relation between Xe Amount, Charging Pressure, and Brightness The panel brightness improves by setting the amount of Xe in the discharge gas to 10% by volume or more and by setting the charging pressure for the discharge gas to the range of 550 to 760Torr (73,2-101 kPa). The following are considered to be the reasons. (1) Increase in the Amount of Ultraviolet Light Setting the amount of Xe in the discharge gas to 10% by volume or more and setting the charging pressure for the discharge gas to the range of 550 to 760Torr (73,2 - 101kPa) increase the amount of Xe in the discharge space, raising the amount of ultraviolet light emitted. (2) Improvement in Conversion Efficiency of Fluorescent Substance with Shift of Ultraviolet Light to Longer Wavelength Conventionally, Xe emitted ultraviolet light mainly at 147nm (resonance line of Xe since the amount of Xe in the discharge gas was set to 5% by volume or less and the charging pressure for the discharge gas to less than 500Torr (66,5kPa). However, by setting the amount of Xe in the discharge gas to 10% by volume or more and by setting the charging pressure for the discharge gas to the range of 550 to 760Torr (73,2 - 101kPa) ultraviolet light emission at 173nm (wavelenght of excited molecular Xe ), being a long wavelength, increases, improving the conversion efficiency of fluorescent substance (see a material published by Plasma Study Group in Electrical Engineers of Japan, May 9, 1995). The above reason will be backed up by the following description. Fig.4 is a graph showing the change in relation between the wavelength and amount of the ultraviolet light for each charging pressure, the ultraviolet light being emitted from Xe in He-Xe gas used as a discharge gas in a PDP. This graph is introduced in 0 Plus E, No.195, 1996, page 99. It is apparent from Fig.4 that if charging pressure is low, Xe emits ultraviolet light mainly at 147nm (resonance line of Xe and that as the charging pressure increases, the ratio of ultraviolet light emission at 173nm increases. Figs.5(a)-(c) show relation between excitation wavelength and relative radiation efficiency for each color of fluorescent substance. This graph is included in O Plus E, No.195, 1996, page 99. It is apparent from this drawing that the relative radiation efficiency is higher at a 173nm wavelength than at 147nm for every color of fluorescent substrate. Relation between Discharge Gas Charging Pressure, Distance "d" between Discharge Electrodes, and Panel Driving Voltage The amount of Xe in the discharge gas and the charging pressure for the discharge gas are set to higher levels in the present embodiment. However, generally, this is considered to bring an inconvenience in that the PDP driving voltage increases since discharge start voltage "Vf" increases as the amount of Xe in the discharge gas or the charging pressure increases (see Japanese Laid-Open Patent Application No.6-342631, column 2, pp 8-16 and 1996 Electrical Engineers of Japan National Conference Symposium, S3-1, Plasma Display Discharge, March, 1996). However, such an inconvenience does not always occur. As is described below, the driving voltage may be low even if the charging pressure is set to a high level when distance "d" between discharge electrodes is set to a comparatively small value. As described in Electronic Display Device, Ohm Corp., 1984, pp 113-114, the discharge start voltage Vf may be represented as a function of P multiplied by d which is called the Paschen's Law. Fig.6 shows relation between charging pressure P of the discharge gas and discharge start voltage Vf for two values of distance d: d = 0. 1mm; and d = 0.05mm. As shown in this graph, discharge start voltage Vf corresponding to charging pressure P of the discharge gas is represented by a curve including a minimum. Charging pressure P, being equal to the minimum, increases as d decreases. The curve of graph "a" (d=0.1mm) passes through the minimum at 300Torr (40kpa), the curve of graph "b" (d=0.05mm) at 600Torr (80 kPa). It is apparent from the above description that an appropriate value corresponding to distance d between discharge electrodes should be set as the charging pressure in order to keep PDP driving voltage low. Also, it is possible to say that if distance d between discharge electrodes is set to 0.1mm or less (desirably to about 0.05mm), PDP driving voltage is kept low even if the charging pressure for the discharge gas is set to the range of 550 to 760Torr (73,2 - 101 kPa). As is apparent from the above description, the PDP of the present embodiment shows high panel brightness since the amount of Xe in the discharge gas is set to 10% by volume or more and the charging pressure for the discharge gas is set to the range of 550 to 760Torr (73,2 - 101 kPa). Also, the driving voltage of the PDP of the present embodiment is kept low since distance d between discharge electrodes is set to 0.1mm or less. Furthermore, the PDP of the present embodiment has a long life since it includes a protecting layer of a dense magnesium oxide with (100)-face orientation which shows good effects in protection. <Examples 1-9> Table 1 shows PDP Examples 1-9 which do not fall within the present invention (1 Torr = 0,133 kPa). The cell size of the PDP was set as follows: the height of partition walls 7 is 0.15mm, the distance between partition walls 7 (cell pitch) 0.15mm, and distance d between discharge electrodes 12 0.05mm. Dielectrics glass layer 13, being lead glass, was formed by transferring a mixture of 75% by weight of lead oxide (PbO), 15% by weight of boron oxide (B 2 O 3 ), 10% by weight of silicon oxide (SiO 2 ), and organic binder (made by dissolving 10% ethyl cellulose in α-terpineol) onto front glass substrate 11 with display electrodes 12 by screen printing and baking them for 10 minutes at 520°C. The thickness of dielectrics glass layer 13 was set to 20µm. The ratio of He to Xe in the discharge gas and the charging pressure were set as shown in Table 1 except that the ratio of He in the discharge gas was set to less than 10% by volume for Examples 7 and 9 and that the charging pressure for the discharge gas was set to less than 500Torr (66·5kPa) for Examples 7 and 8. Regarding to the method of forming the protecting layer, the thermal CVD method was applied to Examples 1, 3, 5, and 7-9, and the plasma enhanced CVD method to Examples 2, 4, and 6. Also, magnesium dipivaloyl methane Mg(C 11 H 19 O 2 ) 2 was used as the source for Examples 1, 2, 7, 8, and 9, magnesium acetylacetone Mg(C 5 H 7 O 2 ) 2 for Examples 3 and 4, and cyclopentadienyl magnesium Mg(C 5 H 5 ) 2 for Examples 5 and 6. The temperature of bubblers 22 and 23 and the heating temperature of glass substrate 27 were set as shown in Table 1. For the thermal CVD method, Ar gas was provided for one minute with the flow rate of 1 l/min., oxygen for one minute with the flow rate of 2 l/min. Also, the layer forming speed was adjusted to 1.0µm/min., the thickness of magnesium oxide protecting layer to 1.0µm. For the plasma enhanced CVD method, Ar gas was provided for one minute with the flow rate of 1 l/min., oxygen for one minute with the flow rate of 2 l/min. High-frequency wave was applied for one minute with 300W. Also, the layer forming speed was adjusted to 0.9µm/min., the thickness of magnesium oxide protecting layer to 0.9µm. With the X-ray analysis of the protecting layers of Examples 1-9, which had been formed as described above, it was confirmed for each Example that the crystals of magnesium oxides have (100)-face orientation. (Embodiment 2) The overall structure and suitable production method of the PDP of the present embodiment are the same as those of Embodiment 1 except that a dense protecting layer consisting of magnesium oxide with (100)-face orientation is formed with a printing method shown below. <Forming of Protecting Layer with Printing Method> A dense protecting layer consisting of magnesium oxide with (100)-face orientation is formed by transferring magnesium salt paste with a plate-shaped crystal structure onto the dielectrics glass layer and baking it. The magnesium salts with a plate-shaped crystal structure for use are magnesium carbonate (MgCO 3 ), magnesium hydroxide (MgCOH) 2 ), magnesium oxalate (MgC 2 O 4 ), etc. The production methods of these magnesium salts are described below in Examples 10-14. The dense protecting layer consisting of magnesium oxide with (100)-face orientation formed by the printing method has the same effects as that formed with the method described in relation to Embodiment 1. <Examples 10-15> Table 2 shows PDP Examples 10-15 whose cell size and distance d between discharge electrodes 12 were set in the same way as PDP Examples 1-9. Examples 10-14 were made according to the present embodiment. Example 15 includes a protecting layer formed by a conventional vacuum vapor deposition method. The magnesium oxalate (MgC 2 O 4 ) with a plate-shaped crystal structure used for Example 10 was produced by dissolving ammonium oxalate (NH 4 HC 2 O 4 ) in magnesium chloride (MgCl 2 ) aqueous solution to make magnesium oxalate aqueous solution then heating it at 150°C. The magnesium carbonate with a plate-shaped crystal structure used for Example 11 was produced by dissolving ammonium carbonate ((NH 4 ) 2 CO 3 ) in magnesium chloride (MgCl 2 ) aqueous solution to make magnesium carbonate (MgCO 3 ), then heating it in carbonic acid gas to 900°C. The magnesium hydroxide with a plate-shaped crystal structure used for Examples 12-14 was produced by dissolving sodium hydroxide (NaOH) in magnesium chloride (MgCl 2 ) aqueous solution to make magnesium hydroxide (Mg(OH) 2 ), then pressurizing and heating it in sodium hydroxide at 5 atmosphere pressures and 900°C. Each of the magnesium salts with a plate-shaped crystal structure made as described above was mixed with an organic binder (made by dissolving 10% ethyl cellulose in 90% by weight of terpineol) by using a three-roller mill to establish a paste, then the paste was transferred onto the dielectrics glass layer by screen printing with a thickness of 3.5µm. After baking each of these for 20 minutes at 500°C, a protecting layer consisting of magnesium oxide with a thickness of about 1.7µm was formed. With the X-ray analysis of the protecting layers of Examples 10-14, which had been formed as described above, it was confirmed for each Example that the crystals of magnesium oxides had (100)-face orientation. For Example 15, a protecting layer was formed by the vacuum vapor deposition method, that is, by heating magnesium oxide with electron beam. With the X-ray analysis of the protecting layer, it was confirmed that the crystals of magnesium oxides had (111)-face orientation. {Further example} The overall structure and suitable production method of the PDP of the present example is the same as that of Embodiment 1 except that a gas including Ar or Kr, namely Ar-Xe, Kr-Xe, Ar-Ne-Xe, Ar-He-Xe, Kr-Ne-Xe, or kr-He-Xe gas is used as the discharge gas, which therefore does not form part of the present invention. Here, it is desirable that the amount of Xe is set to the range from 10 to 70% by volume since the driving voltage tends to rise if the amount exceeds 70% by volume. Also, for three-element discharge gases such as Ar-Ne-Xe, Ar-He-Xe, Kr-Ne-Xe, and kr-He-Xe gases, it is desirable that the amount of Kr, Ar, He, or Ne should be in the range of 10 to 50% by volume. In the present example, in forming a protecting layer, a method for evaporating a magnesium oxide with (110)-face orientation onto the dielectrics glass layer with irradiation of ion or electron beam may be used as well as the thermal CVD or plasma enhanced CVD method for forming magnesium oxide with (100)-face orientation as described in Embodiment 1. The method is described below. <Method for Evaporating Alkaline Earth Oxide onto Dielectrics Glass Layer by Use of Ion or Electron Beam Irradiation to Form Protecting Layer> Fig. 7 shows an ion/electron beam irradiating apparatus which may be used for forming a protecting layer in the PDP of the present embodiment. The ion/electron beam irradiating apparatus includes vacuum chamber 45 to which glass substrate 41 with a dielectrics glass layer is attached. Vacuum chamber 45 also includes electron gun 42 for evaporating an alkaline earth oxide (in the present embodiment, magnesium oxide). Ion gun 43 irradiates ion beam to vapor of the alkaline earth oxide which has been evaporated by electron gun 42. Electron gun 44 irradiates electron beam to vapor of the alkaline earth oxide evaporated by electron gun 42. The following description shows how to evaporate the alkaline earth oxide onto the dielectrics glass layer by irradiating ion or electron beam to vapor using the ion/electron beam irradiating apparatus described above. First, glass substrate 41 with a dielectrics glass layer is set in chamber 45 then crystals of an alkaline earth oxide are put in electron gun 42. Secondly, chamber 45 is evacuated then substrate 41 is heated (150°C). Electron gun 42 is used to evaporate the alkaline earth oxide. At the same time, ion gun 43 or electron gun 44 is used to irradiate argon ion or electron beam towards substrate 41. It forms a protecting layer of an alkaline earth oxide. The crystals of the alkaline earth oxide grow slowly and a dense protecting layer consisting of an alkaline earth oxide with (110)-face orientation is formed when, as is described above, the alkaline earth oxide is evaporated onto the dielectrics glass layer by irradiation of the ion or electron beam. The formed protecting layer shows almost the same effects as the dense protecting layer of an alkaline earth oxide with (100)-face orientation formed in Embodiment 1. <Examples 16-34> Table 3 shows PDP Examples 16-34 which do not fall within the invention, and which were made according to the description above. Refer to "DISCHARGE GAS TYPE AND RATIO" column in the table for the discharge gas compositions, and "GAS CHARGING PRESSURE" column for charging pressures. The protecting layer of Examples 16 and 27 were formed as described in Embodiment 1 using magnesium dipivaloyl methane Mg(C 11 H 19 O 2 ) 2 as the source with the thermal CVD method, and Examples 17, 23, 24, 28, 32, and 33 with the plasma enhanced CVD method. For Examples 18, 21, 22, 25, 26, and 34, ion beam (current of 10mA) was irradiated, and for Examples 19, 20, 30, and 31, electron beam (10mA), to evaporate a magnesium oxide onto the dielectrics glass layer to form a protecting layer with a layer thickness of 5000Å (0,5nm). With the X-ray analysis of the protecting layers which had been formed by evaporating magnesium oxides onto the dielectrics glass layer with irradiation of ion or electron beam, it was confirmed that the crystals of the magnesium oxides had (110)-face orientation. {Further example} The overall structure and suitable production method of the PDP of the present example which does not fall within the present invention is the same as that of Embodiment 1 except that the cell pitch is set to a larger value and the amount of Xe in a He-Xe gas used as the discharge gas is set to less than 10% by volume. Note that the distance between electrodes "d" is set to an equal or larger value. In the present example, alkaline earth oxides with (100)-face orientation other than magnesium oxide (MgO) are formed as the protecting layers, such as beryllium oxide (BeO), calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO). These protecting layers are formed by using appropriate sources for respective alkaline earths with the thermal or plasma enhanced CVD method described in connection with Embodiment 1. The discharge electrodes formed on the front glass substrate includes a tin oxide-antimony oxide or an indium oxide-tin oxide. The protecting layer of beryllium oxide, calcium oxide, strontium oxide, or barium oxide with (100)-face orientation has almost the same effects as the magnesium oxide with (100)-face orientation formed in Embodiment 1. <Examples 35-66> Table 4 shows PDP Examples 35-66 which were made according to the present example. The height of the partition walls was set to 0.2mm, the distance between partition walls (cell pitch) 0.3mm, and distance d between discharge electrodes 0.05mm. The discharge gas was a He-Xe mixture gas including 5% by volume of Xe, and the charging pressure was set to 500Torr (66.5kPa). The discharge electrodes, which were made with sputtering and photo-lithography methods, consist of indium oxide (In 2 O 3 ) including 10% by weight of tin oxide (SnO 2 ). The protecting layers were made with the thermal or plasma enhanced CVD method from metal chelate or cyclopentadienyl compounds of the alkaline earth oxides shown in Table 4 "CVD SOURCE" column. The formed layers were of magnesium oxide, beryllium oxide, calcium oxide, strontium oxide, or barium oxide as shown in "ALKALINE EARTH OXIDE" column. With the X-ray analysis of the protecting layers, it was confirmed that each Example had (100)-face orientation. <Reference> Examples 67-69 shown in Table 4 were made in the same way as Examples 35-66. However, the protecting layers of Examples 67-69 were formed with different methods: for Example 67, the vacuum vapor deposition method for evaporating magnesium oxide onto the dielectrics glass layer by heating magnesium oxide with electron beam was used; for Example 68, the sputtering performed on magnesium oxide as the target; and for Example 69, the screen printing with magnesium oxide paste. With the X-ray analysis of the protecting layers, it was confirmed that magnesium oxide protecting layers of Examples 67 and 69 had (111)-face orientation. It was also confirmed that magnesium oxide protecting layer of Examples 68 had (100)-face orientation. However, the protecting layer of Example 68 is not considered as dense since it was formed with the sputtering. Experiment 1: Measuring Ultraviolet Light Wavelength and Panel Brightness (Initial Value)> Experiment Method For Examples 1-15, the ultraviolet light wavelength and panel brightness (initial value) were measured when they were operated on 150V of discharge maintenance voltage and 30KHz of frequency. Results and Analysis. As shown in Tables 1-3, resonance lines of Xe with a wavelength of 147nm were mainly observed from examples 7-9, showing low panel brightness (around 200cd/m 2 ), while excited molecular Xe with a wavelength of 173nm were mainly observed from examples 1-6 and 10-34, showing high panel brightness (around 400cd/m 2 or more). Of these, Examples 16-34 showed the highest panel brightness (around 500cd/m 2 or more). It is apparent from the above results that the panel brightness is improved by setting the amount of Xe in discharge gas to 10% by volume or more, charging pressure to 550Torr (73,2kPa) or more, The panel brightness of example 15 is slightly lower than those of Examples 1-6 and 10-14. The reason is considered that the protecting layer of Example 15 consisting of magnesium oxide with (111)-face orientation emits less secondary electron than that with (100)-face orientation. <Experiment 2: Measuring Change Rates of Panel Brightness and Discharge Maintenance Voltage> Experiment Method For Examples 1-15 and 35-69, the change rates (change rates from respective initial values after 7,000 hours of operation) of panel brightness and discharge maintenance voltage were measured after they were operated for 7,000 hours on 150V of discharge maintenance voltage and 30KHz of frequency. For Examples 16-34, the change rates of panel brightness and discharge maintenance voltage were measured after they were operated for 5, 000 hours on 170V of discharge maintenance voltage and 30KHz of frequency. Results and Analysis As shown in Tables 1 and 2, the panel brightness change rates of examples 1-6 and 10-14 are smaller than those of examples 7-9. Also, as shown in Table 3, the change rates of panel brightness and discharge maintenance voltage of examples 16-34 were small as a whole. It is apparent from the above results that the panel brightness change rate reduces by setting the amount of Xe in discharge gas to 10% by volume or more, charging pressure to 550 Torr (73,2 kPa) or more. The change rates of panel brightness and discharge maintenance voltage of examples 1-14 are smaller than those of Example 15. The reason is considered that the protecting layer of magnesium oxide with (111)-face orientation has higher sputtering resistance and higher efficiency in protecting dielectrics glass layer than that with (100)-face orientation. As shown in Table 4, the change rates of panel brightness and discharge maintenance voltage of examples 35-66 are little, and those of examples 67-69 great. The above results show that generally the protecting layer of alkaline earth oxide with (100)-face or (110)-face orientation formed with the thermal CVD method, plasma enhanced CVD method, or vapor deposition method with ion or electron beam irradiation has higher sputtering resistance and higher efficiency in protecting dielectrics glass layer than that with (111)-face orientation. Note that the results of example 67 show that the protecting layer consisting of alkaline earth oxide with (100)-face orientation formed with the sputtering method has high change rates of panel brightness and discharge maintenance voltage and low efficiency in protecting dielectrics glass layer. The reason for the above results is considered that for the alkaline earth oxide of the protecting layer which has been formed by the thermal CVD, plasma enhanced CVD, or a method of evaporating the oxide onto a layer by irradiating ion or electron beam, the crystals grow slowly to form a dense protecting layer with (100)-face or (110)-face orientation; for the protecting layer formed by the sputtering method, the crystals do not grow slowly and the protecting layer does not become dense though it has (100)-face orientation. <Others> The values in Tables 1-4 in "BUBBLER TEMPERATURE," "HEATING TEMPERATURE FOR GLASS SUBSTRATE," " "PANEL BAKING TEMPERATURE," "PRINTED LAYER THICKNESS," "Ar GAS FLOW RATE," and "O 2 GAS FLOW RATE" were considered to be optimum for the respective alkaline earth sources. The results of the change rates of panel brightness and dielectrics maintenance voltage shown in Table 4 were obtained from PDPS with 5% by volume of Xe in discharge gas. However, the same results may be obtained from those with 10% by volume or more of Xe. In the above Embodiments, the back panel of the PDPs includes back glass substrate 15 with which partition walls 17 are bonded. However, the present invention is not limited to such construction and may be applied to general AC PDPs such as those having partition walls attached to the front panel. Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. The scope of the present invention is defined by the appended claims.

Claims: A plasma display panel comprising: a front cover plate which comprises a front glass substrate (11), with spaced pairs of first electrodes (12), and a dielectrics glass layer, wherein the first electrodes and the dielectrics glass layer (13) are formed on the front glass substrate; and a back plate which comprises a back glass substrate (15), a second electrode (16), and a fluorescent substance layer (18), wherein the second electrode and the fluorescent substance layer are formed on the back glass substrate, wherein the dielectrics glass layer and the fluorescent substance layer face to each other, wherein a plurality of discharge spaces are formed between a plurality of partition walls (17) which are set between the front cover plate and the back plate, wherein a gas medium is charged in the plurality of discharge spaces (19), and the gas medium is composed of neon and xenon, characterized in that the gas medium includes xenon in a range from at least 10% to less than 100% by volume, the charging pressure of the gas medium is at least 550Torr (73.2 kPa) and less than or equal to 760Torr (101 kPa), and the distance (d) between each pair of adjacent first electrodes is no greater than 0.1mm. The plasma display panel of claim 1 wherein the gas medium includes xenon in the range from greater than 10% to 70% by volume. The plasma display panel of claim 1, wherein the dielectrics glass layer (13) is covered by a protecting layer (14) of an alkaline earth oxide with (100)-face orientation. The plasma display panel of Claim 1, wherein the dielectrics glass layer (13) is covered by a protecting layer (14) of an alkaline earth oxide with (110)-face orientation. The plasma display panel of claim 3 or 4, wherein the protecting layer (14) is magnesium oxide.

Inventors: AOKI MASAKI, FUJII EIJI, INAMI TAKASHI, ISHIKURA YASUHISA, KAWAMURA HIROYUKI, MURAI RYUICHI, NISHIMURA YUTAKA, OHTANI MITSUHIRO, TANAKA HIROYOSHI, TORII HIDEO, YAMASHITA KATSUYOSHI, AOKI, MASAKI, FUJII, EIJI, INAMI, TAKASHI, ISHIKURA, YASUHISA, KAWAMURA, HIROYUKI, MURAI, RYUICHI, NISHIMURA, YUTAKA, OHTANI, MITSUHIRO, TANAKA, HIROYOSHI, TORII, HIDEO, YAMASHITA, KATSUYOSHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1996
Main classification: H01J17
IPC codes: H01J17, C23C16, H01J9