GAS DISCHARGE PANEL AND GAS LIGHT-EMITTING DEVICE

Abstract: The object of the present invention is to provide a gas discharge panel, where the conversion efficiency of discharge energy into visible rays and the panel brightness are improved, with the color purity being improved as far as possible. To achieve this object, in a gas discharge panel, the pressure of discharge gas is set in a range of 800-4000Torr, that is higher than a conventional gas pressure. Also, a rare gas mixture including helium, neon, xenon, and argon is used as discharge gas charged into discharge spaces, instead of conventional discharge gas. Here, it is preferable that the proportion of Xe is set to 5% by volume or less, that of Ar 0.5% by volume or less, and that of He under 55% by volume. With this rare gas mixture, the light-emission efficiency is improved, with the firing voltage being suppressed. Furthermore, display electrodes and address electrodes are arranged on the surface of either of a front cover plate and a back plate, with a dielectric layer existing between the display electrodes and the address electrodes. With this construction, addressing is performed with a relatively low voltage even if the gas pressure is high.

Description: FIELD OF THE INVENTION This invention relates to a gas discharge tube, such as a gas discharge panel and a gas light-emission device, and in particular to a high-definition plasma display panel. BACKGROUND OF THE INVENTION Recently, as expectations for high-quality and large-screen TVs such as high-definition TVs have increased, displays suitable for such TVs, such as 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 screens exceeding 1.016 m (40 inch) in size. LCDs have high performance such as low power consumption and low driving voltage. However, producing a large LCD is technically difficult and the viewing angles of LCDs are limited. On the other hand, it is possible to produce a large-screen PDP with a short depth, and 1.27 m (50-inch) PDP products have already been developed. PDPs are broadly divided into two types: Direct Current type (DC type) and Alternating Current type (AC type). Currently, PDPs are mainly AC type since these are suitable for large screens. An ordinary AC PDP includes a front cover plate and a back plate, where partition walls called barrier ribs are inserted between the front cover plate and the back plate to form discharge spaces. Discharge gas is charged into the discharge spaces. The front cover plate with display electrodes thereon is covered with a dielectric glass layer made of lead glass. The back plate is provided with address electrodes, the barrier ribs, and phosphor layers made of red, green, and blue ultraviolet excitation phosphors. The discharge gas is ordinarily helium (He)-xenon (Xe) or neon (Ne)-xenon (Xe) mixture gas. The gas pressure is set in a range of 500 to 600Torr to keep firing voltage at 250V or less (see M. Nobrio, T. Yoshioka, Y. Sano, K. Nunomura, SID94' Digest 727-730 1994, for instance). The light-emission principle of PDPs is basically the same as that of fluorescent lights. That is, in PDPs, voltage is applied to electrodes to generate glow discharges, ultraviolet light is emitted from Xe by the glow discharges, the ultraviolet light excites red, green and blue ultraviolet excitation phosphors, and the phosphors emit visible rays. However, PDPs are not as bright as fluorescent lights due to the low conversion ratios of discharge energy into ultraviolet light and of ultraviolet light into visible rays in the phosphors. In this respect, "Applied Physics", Vol.51, No. 3, 1982, page344-347 describes that a plasma display panel having a gas composition of He-Xe or Ne-Xe uses only about 2% of electric energy for emitting ultraviolet light and only about 0.2% of the electric energy is converted into visible rays (see "Optics Techniques Contact", Vol.34, No.1, 1996, page25, "FLAT PANEL DISPLAY 96'", Part 5-3, and "NHK Techniques Study", 31-1, 1979, page18, for instance). Regarding this problem, various techniques have been studied to realize discharge panels, such as PDPs, of high panel brightness and low firing voltage by improving the light-emission efficiency. There are also market demands for such discharge panels. For instance, in current 1.016 m-1.067 m (40-42 inch) PDPs for TV sets of National Television System Committee (NTSC) standard, the number of cells is 640x480, cell pitch 0.43mmx1.29mm, and area of one cell about 0.55mm 2 . In this case, PDPs have the panel efficiency of 1.2 lm/w and the panel brightness of 400cd/m 2 (see FLAT PANEL DISPLAY, 1997, part5-1, page198, for instance). On the contrary, in 1.067 m (42-inch) high-definition TVs that are recently in increasing demand, the number of cells is 1920x1125, cell pitch 0.15mmx0.45mm, and area of one cell 0.072mm 2 . The area of one cell of high-definition TVs is reduced to 1/7-1/8 of that under NTSC. Accordingly, when a PDP for a 1.067 m (42-inch) high-definition TV is produced using a conventional cell construction, the panel efficiency and the panel brightness may be lowered respectively to 0.15-0.17 lm/w and to 50-60cd/m 2 . The panel efficiency of a PDP for a 1.067 m (42-inch) high-definition TV, therefore, needs to be improved ten times or more (5 lm/w or more) to acquire the same brightness as that of a current NTSC CRT (500cd/m 2 ) (see FLAT PANEL DISPLAY, 1997, part5-1, page200, for instance). Aside from the improvement in the panel brightness, the white balance needs to be adjusted by improving the color purity to realize a PDP of fine picture quality. Various studies and inventions have been made to improve the light-emission efficiency and the color purity. Japanese Patent Publication No. 5-51133, for instance, discloses a PDP that uses three-component mixture gas of argon (Ar)-neon (Ne)-xenon (Xe). With the mixture gas including argon, the amount of visible rays generated by neon is reduced, so that the color purity is improved. However, the light-emission efficiency is not so improved. Japanese Patent No. 2616538 discloses a method where three-component mixture gas of helium (He)-neon (Ne)-xenon (Xe) is used. With this mixture gas, the light-emission efficiency is improved, in comparison with the case where two-component mixture gas of helium (He)-xenon (Xe) or neon (Ne)-xenon (Xe) is used. However, the light-emission efficiency is improved to about 1 lm/w at most in the case of the pixel level of NTSC. Therefore, a technique of further improving the light-emission efficiency is desired. Regarding the stated problems, the object of the present invention is to provide a gas discharge panel, such as a PDP, where the panel brightness and the conversion efficiency of discharge energy into visible rays are improved and light of fine color purity is emitted. DISCLOSURE OF THE INVENTION To achieve the above object, the gas discharge panel of the present invention has the construction according to the claims where the pressure of discharge gas is set in a range of 800Torr (1.067 x 10 5 Pa) to 4000Torr (5.333 x 10 5 Pa), that is higher than a conventional gas pressure. The reasons why the light-emission efficiency is improved with this construction are given below. In a conventional PDP, the pressure of discharge gas is ordinarily set under 500Torr (6.667 x 10 4 Pa). In this case, resonance lines (whose wavelengths are mainly 147nm) constitute a large proportion of the ultraviolet light generated by discharge. On the other hand, when the gas pressure is high as described above (that is, many atoms are charged into discharge spaces), the proportion of molecular lines (whose wavelengths are mainly 154nm and 172nm) increases. Here, while the resonance lines are associated with a self-absorption phenomenon, molecular lines are rarely associated with such an absorption phenomenon. Therefore, the amount of ultraviolet with which the phosphor layers are irradiated increases, resulting in the improvement in the panel brightness and the light-emission efficiency. Also, when ordinary phosphors are irradiated with ultraviolet light whose wavelength is long, the conversion efficiency of ultraviolet light into visible rays in the phosphors tends to increase. As a result, the panel brightness and the light-emission efficiency are improved. The gas in a gas discharge panel ordinarily consists of neon (Ne) or xenon (Xe). When the gas pressure is relatively low, visible rays are emitted from neon (Ne) and the color purity is deteriorated by the visible rays. However, when the gas pressure is high like the present invention, most of the visible rays emitted from neon (Ne) are absorbed by discharge and are not emitted to the outside. As a result, the color purity is improved, in comparison with a conventional PDP. Also, the first glow discharge is caused in a conventional PDP. However, when the gas pressure is set in a high range of 800Torr (1.067 x 10 5 Pa) to 4000Torr (5.333 x 10 5 Pa) like the present invention, it is supposed that a filamentary glow discharge or the second glow discharge tends to be caused. Accordingly, the electron density in positive column regions increases and energy is intensively supplied to the positive column regions. As a result, the amount of ultraviolet light emission increases. Furthermore, the gas pressure that exceeds the atmospheric pressure ((760Torr (1.013 x 10 5 Pa)) prevents a PDP from containing impurities that exist in the atmosphere. It should be noted here that respective gas pressure ranges, that is a range that is no less than 800Torr (1.067 x 10 5 Pa) and under 1000Torr (1.333 x 10 5 Pa), a range that is no less than 1000Torr (1.333 x 10 5 Pa) and under 1400Torr (1.867 x 10 5 Pa), a range that is no less than 1400Torr (1.867 x 10 5 Pa) and under 2000Torr (2.666 x 10 5 Pa),and a range that is no less than 2000Torr (2.666 x 10 5 Pa) and no more than 4000Torr (5.333 x 10 5 Pa), acquire the characteristics described in embodiments. Furthermore, when the gas pressure is high in a PDP having the construction where display electrodes oppose address electrodes, with discharge spaces existing between the display electrodes and the address electrodes, the voltage necessary to perform addressing tends to rise. However, when the display electrodes and the address electrodes are arranged on the surface of either of a front cover plate and a back plate, with a dielectric layer existing between the display electrodes and the address electrodes, addressing is performed with a relatively low voltage even if the gas pressure is high. BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 is a simplified drawing of an AC PDP of Embodiment 1; Fig 2 is a simplified drawing of a CVD apparatus that is used for forming a protecting layer of the PDP; Fig 3 is a graph showing the current waveforms of transition glow discharges and an acr discharge; Fig 4 is a graph showing how the relation between a ultraviolet light wavelength and the amount of ultraviolet light emission is changed as a gas pressure is changed; Fig 5 shows energy levels and various reactive processes of Xe; Fig 6 is a graph showing the amount of resonance lines, the amount of molecular lines, and the total amount of ultraviolet light emissions for respective discharge gas pressures; Fig 7 shows characteristics of relation between excitement wavelength and relative emission efficiency for the phosphor of each colour; Fig 8 shows a graph and a table that give the results of Experiment 1; Fig 9 is a simplified drawing of an AC PDP of Embodiment 2; and Fig 10 is a simplified drawing of another AC PDP of Embodiment 2. DESCRIPTION OF THE PREFERRED EMBODIMENTS The following is a description of the preferred embodiments of the present invention. Embodiment 1 (Overview of Construction and Production Method of PDP) Fig 1 is a perspective view of the AC PDP of the present embodiment. The present PDP includes a front panel 10 which is made up of a front glass substrate 11 with display electrodes (discharge electrodes) 12a and 12b, a dielectric layer 13, and a protecting layer 14; and a back panel 20 which is made up of a back glass substrate 21 with address electrodes 22 and a dielectric layer 23, where the front panel 10 and the back panel 20 are placed in parallel so that the display electrodes 12a and 12b oppose the address electrodes 22 with a certain distance therebetween. The space between the front panel 10 and the back panel 20 is partitioned by stripe-shaped barrier ribs 30 to form discharge spaces 40. The discharge spaces are charged with discharge gas. The back panel 20 is provided with phosphor layers 31, with the phosphor layers 31 being exposed to the discharge spaces 40. Red, green, and blue phosphor layers are repeatedly arranged in the order. The display electrodes 12a and 12b are stripe-shaped silver electrodes and are arranged perpendicular to the barrier ribs 30. The address electrodes 22 are arranged parallel to the barrier ribs. Cells that respectively emit red, green, or blue ray are formed at the intersections of the display electrodes 12a and 12b and the address electrodes 22. The dielectric layer 13 is 20µm in thickness and is made of lead glass or another glass material. The entire surface of the front glass substrate 11 with the display electrodes 12 thereon is covered with the dielectric layer 13. The protecting layer 14 is a thin layer made of magnesium oxide (MgO) and covers the entire surface of the dielectric layer 13. The barrier ribs 30 are arranged to protrude from the surface of the dielectric layer 23 of the back panel 20. The present PDP is driven using a driving circuit as follows. Firstly, addressing discharge is performed by applying voltage between the display electrodes 12a and the address electrodes 22 of cells to be illuminated. Then, sustaining discharge is performed by applying a pulse voltage between the display electrodes 12a and the display electrodes 12b of the cells to emit ultraviolet light. Finally, the ultraviolet light is converted into visible rays by the phosphor layers 31 to illuminate the cells. The production method of the present PDP having the stated construction is described below. (Production Method of Front Panel} The front panel 10 is made by: forming the display electrodes 12 on the front glass substrate 11; applying lead-based glass onto the display electrodes 12 and the front glass substrate 11; baking them to form the dielectric layer 13; then forming the protecting layer 14 on the surface of the dielectric layer 13; and forming minute projections on the surface of the protecting layer 14. The display electrodes 12 are formed by applying a silver paste onto the front glass substrate 11 by screen printing and baking them. The lead-based dielectric layer 13 is made of a mixture of 70% by weight of lead oxide (PbO), 15% by weight of boron oxide (B 2 O 3 ), and 15% by weight of silicon oxide (SiO 2 ) by screen printing and baking them. More specifically, the dielectric layer 13 is formed by adding organic binder (made by dissolving 10% ethyl cellulose in α-terpineol) to the mixture, applying the mixture onto the front glass substrate 11 with the display electrodes 12 by screen printing, and baking them for 10 minutes at 580°C. The formed dielectric layer 13 is 20µm in thickness. The protecting layer 14 is made of an alkaline earth oxide (in the present embodiment, magnesium oxide (MgO)) and is a film having a closely packed crystal structure of (100)-face or (110)-face orientation. The protecting layer 14 is processed to form projections on the surface of the protecting layer 14. In the present embodiment, an MgO protecting layer having (100)-face or (110)-face orientation is formed using CVD (Chemical Vapor Deposition) method, such as thermal CVD method and plasma CVD method. Then, the protecting layer is processed using a plasma etching method to form projections on the protecting layer surface. The methods of producing the protecting layer 14 and of forming projections on the protecting layer surface are described in detail later. (Production Method of Back Panel) The address electrodes 22 are formed by applying a silver paste onto the surface of the back glass substrate 21 by screen printing and baking them. Then, like the case of the front panel 10, the dielectric layer 23 made of lead-based glass is formed by screen printing and baking it. The barrier ribs 30 made of glass are attached onto the dielectric glass layer 23 with a predetermined pitch. The phosphor layers 31 are formed by inserting one of a red phosphor, a green phosphor, and a blue phosphor into each space between the barrier ribs 30. Although any phosphor ordinarily used for PDPs can be used for each color, the present embodiment uses the following phosphors: red phosphor (Y x Gd 1-x ) BO 3 :Eu 3+ green phosphor BaAl 12 O 19 :Mn blue phosphor BaMgAl 14 O 23 :Eu 2+ {Production of PDP by Bonding Together Front Panel 10 and Back Panel 20} The PDP of the present embodiment is made by bonding together the front panel 10 and the back panel 20, which are produced as described above, with sealing glass. At the same time, the air is exhausted from the discharge spaces 40 between the barrier ribs 30 to high vacuum (8X10 -7 Torr (1.067 x 10 -4 Pa)). Then, discharge gas of a certain composition is charged into the discharge spaces 40 at a certain gas pressure. (Gas pressure and Composition of Discharge Gas) The pressure of discharge gas is raised to a range of 800-4000Torr (1.067 x 10 5 -5.333 x 10 5 Pa). This range is higher than the ordinary gas pressure range and exceeds the atmospheric pressure ((760Torr (1.013 x 10 5 Pa)). By doing so, the panel brightness and the light-emission efficiency are improved. It should be noted here that in the present embodiment, before the front panel and the back panel are bonded together and are baked, sealing glass is applied not only to the outer regions of the front panel and the back panel but onto the barrier ribs 30. By doing so, discharge gas can be charged at a high pressure (see Japanese Patent Application No. H9-344636 for further information). The PDP produced in this manner sufficiently withstand gas charging at a high gas pressure, such as 4000Torr (5.333 x 10 5 Pa). To improve light-emission efficiency and to suppress firing voltage, it is preferable to use a rare gas mixture including helium (He), neon (Ne), xenon (Xe), and argon (Ar) as discharge gas, instead of a conventional gas mixture of helium-xenon or neon-xenon. Here, it is also preferable to set the proportion of xenon to 5% by volume or less, that of argon to 0.5% by volume or less, and that of helium under 55% by volume. The specific composition of discharge gas is, for instance, He(30%)-Ne(67.9%)-Xe(2%)-Ar(0.1%) . Note that the symbol "%" in the gas composition expression refers to a unit of % by volume. This symbol is also used in the following description. The settings of the composition and pressure of discharge gas are related to the light-emission efficiency and panel brightness of PDPs. When both settings are controlled, the light-emission efficiency and panel brightness are in particular improved, with firing voltage being suppressed. The settings of the composition and pressure of discharge gas are described in detail later. When the gas pressure is set at a normal pressure (a conventional pressure of 500Torr (6.667 x 10 4 Pa) or less), the color purity tends to decrease because visible rays are emitted from neon (Ne) to the outside. On the contrary, when the gas pressure is raised to 800Torr (1.067 x 10 5 Pa) or more, even if visible rays are emitted from neon, the emitted visible rays are absorbed by plasma and are not emitted to the outside. As a result, the color purity is improved, in comparison with a case where the gas pressure is set at the normal pressure or less (500Torr (6.667 x 10 4 Pa or less). The gas pressure that exceeds the atmospheric pressure also prevents the discharge spaces 40 from containing impurities that exist in the atmosphere. In the present embodiment, cell pitch is set to 0.2mm or less and distance between the display electrodes 12 "d" is set to 0.1mm or less, to make the cell size of the PDP conform to 40-inch high-definition TVs. Note that the upper limit of gas pressure, that is 4000Torr (5.333 x 10 5 Pa), is set so that firing voltage falls within a practical range. (Method of Producing MgO Protecting Layer) Fig. 2 is a simplified drawing of a CVD apparatus that is used to form the protecting layer 14. The CVD apparatus can perform both of thermal CVD and plasma CVD. The main unit 45 includes a heating unit 46 for heating a glass substrate 47 (equivalent to the glass substrate 11 on which the display electrodes and the dielectric layer 13 are formed as shown in Fig 1). The pressure inside the main unit 45 is reduced by a venting apparatus 49. Plasma is generated in the main unit 45 by a high-frequency power source 48. Ar-gas cylinders 41a and 41b supply argon (Ar) gas, which is used as a carrier, to the main unit 45 respectively via bubblers 42 and 43. The bubbler 42 stores a metal chelate used as the material for MgO, with the metal chelate being heated. The metal chelate is evaporated and is transferred to the main unit 45 while the argon gas is being blown on it from the AR-gas cylinder 41a. The bubbler 43 stores a cyclopentadienyl compound used as the material for MgO, with the cyclopentadienyl compound being heated. The cyclopentadienyl compound is evaporated and is transferred to the main unit 45 while the argon gas is being blown on it from the Ar-gas cylinder 41b. The specific materials supplied from the bubblers 42 and 43 are, for instance, magnesium dipivaloyl methane (Mg (C 11 H 19 O 2 ) 2 ), magnesium acetylacetone (Mg (C 5 H 7 O 2 ) 2 ), cyclopentadienyl magnesium (Mg(C 5 H 5 ) 2 ), and magnesium trifluoroacetylacetone (Mg(C 5 H 5 F 3 O 2 ) 2 ). An oxygen cylinder 44 supplies oxygen (O 2 ) used as a reaction gas to the main unit 45. {Procedure of Thermal CVD} The glass substrate 47 is put on the heating unit 46 so that the dielectric layer on the glass substrate 47 faces upward. Then the glass substrate 47 is heated at a certain temperature (350-400°C), with the pressure inside the reaction container being reduced by the venting apparatus 49 to the certain pressure. While the bubbler 42 or 43 heats the metal chelate or cyclopentadienyl compound of alkaline earth used as the material to a certain temperature, Ar gas is sent from the Ar-gas cylinder 41a or 41b and oxygen is sent from the oxygen cylinder 44. The metal chelate or cyclopentadienyl compound reacts with oxygen in the main unit 45 and forms an MgO protecting layer on the surface of the dielectric layer formed on the glass substrate 47. {Procedure of Plasma CVD} The procedure is almost the same as that of the thermal CVD described above. However, the glass substrate 47 is heated by the heating unit 46 in a range of 250 to 300°C. At the same time, the pressure in the main unit is reduced to about 10Torr (1333 Pa) by the venting apparatus 49. Under the circumstances, plasma is generated in the main unit 45 by driving the high-frequency power source 48 to apply high-frequency electric field of 13.56MHz. As a result, an MgO protecting layer is formed on the surface of the dielectric layer formed on the glass substrate 47. With the X-ray analysis of crystal structure of the MgO protecting layer formed with thermal CVD method or plasma CVD method, it is confirmed that the MgO protecting layer has (100)-face or (110)-face orientation. On the other hand, it is also confirmed with an X-ray analysis that an MgO protecting layer that is formed with a conventional vacuum deposition method (EB method) has (111)-face orientation. It should be noted here that whether the Mg) protecting layer formed with a CVD method has (100)-face orientation or (110)-face orientation is controlled by adjusting the amount of flow of oxygen used as a reaction gas. (Various States of Glow Discharge) The filamentary glow discharge and the second glow discharge are described below. "Discharge Handbook" (Electric Society, June 1, 1989, page 138) describes the "filamentary glow discharge" and "second glow discharge" as follows. "In the journal "J. Phys. D. Appl. Phys.",Vol 13, page 1886 (1970), Kekez, Barrault, and Craggs describe that the discharge state is shifted from flashover, through Townsend discharge, the first glow discharge, and the second glow discharge, to arc discharge". Fig 4 is the graph that is cited from this journal and shows the current waveforms of respective transition glow discharges and the arc discharge. The first glow discharge equates to an ordinary glow discharge and the second glow discharge equates to a discharge that is caused when discharge energy is on its way to be intensively supplied to positive column regions. In Fig. 4, the first glow discharge is caused in the period between ta-tc when the current is stable at low level. The second glow discharge is caused in the period between td-te. The filamentary glow discharge is caused in the period between tc-td when the discharge state is shifted from the first glow discharge to the second glow discharge. The second glow discharge is shifted to the arc discharge after the point in time of te. As can be seen from this drawing, the first glow discharge is caused with stability. However, while the filamentary glow discharge or the second glow discharge is caused, the current is unstable. Therefore, there is a high possibility that the discharge state is shifted to the arc discharge. Once the discharge state is shifted to the arc discharge, heat is produced and thermal ionization is caused in discharge gas. Accordingly, it is not preferable that the discharge state is shifted to the arc discharge. Here, the first glow discharge is caused in conventional PDPs. However, it is supposed that the filamentary glow discharge or the second glow discharge is also caused with stability in the present embodiment. Therefore, it is supposed that the electron density can be increased in the positive column regions of discharge. As a result, energy is intensively supplied to the positive column regions, which increases the amount of ultraviolet light emission. (Relation between Gas Pressure and Light-Emission Efficiency of Discharge Gas) The following is a description of the reasons why the light-emission efficiency is improved by setting the pressure of discharge gas in a range of 800-4000Torr (1.067 x 10 5 -5.333 x 10 5 Pa), that is higher than the conventional gas pressure. High gas pressure is supposed to be effective in causing the filamentary glow discharge or the second glow discharge. Therefore, the amount of ultraviolet light emission is increased. Also, because the wavelength of ultraviolet light is shifted to long wavelengths (154nm and 173nm) due to a high gas pressure as described below, the light-emission efficiency is improved. The ultraviolet light emitted in PDPs is roughly divided into resonance lines and molecular lines. The pressure of discharge gas is conventionally set under 500Torr. Therefore, Xe emits ultraviolet light mainly at 147nm (resonance line of Xe molecule) . However, by setting the gas pressure at 760Torr (1.013 x 10 5 Pa) or more, the ratio of long wavelength (173nm, excitation wavelength by molecular beam of Xe molecules) increases. Also, molecular lines whose wavelengths are 154nm and 173nm increases to a ratio higher than that of resonance lines whose wavelengths are 147nm. Fig. 5 is a graph that is cited from "O Plus E, No.195, 1996, page98" and shows how the relation between the light wavelength and the amount of ultraviolet light emission is changed as the gas pressure is changed in a PDP using He-Xe discharge gas. In this graph, the peak area for each wavelength, such as 147nm (resonance line) and 173nm (molecular line), represents the amount of ultraviolet light emission. Therefore, the relative amount of ultraviolet light emission for each wavelength can be known by comparing each peak area in this graph. When the gas pressure is set to 100Torr (1.333 x 10 4 Pa), the proportion of ultraviolet light emission at 147nm (resonance line) becomes large. However, as the gas pressure increases, the proportion of ultraviolet light emission at 173nm (molecular line) increases. When the gas pressure is set to 500Torr (6.667 x 10 4 Pa), the proportion of ultraviolet light emission at 173nm becomes larger than that at 147nm (resonance line). As described above, as the wavelength of ultraviolet becomes longer, (1) the amount of ultraviolet light emission increases and (2) the conversion efficiency of flourescent substances is improved. Each effect is described below. (1) Increase in Ultraviolet Light Emission Amount Fig. 6 shows energy levels and various reactive processes of Xe. When electrons existing in atoms move from an energy level to another energy level, resonance lines are emitted. In the case of Xe, ultraviolet light is emitted mainly at 147nm. However, resonance lines are associated with a phenomenon called "induction absorption" where a part of emitted ultraviolet light is absorbed by Xe that is in a ground state. This phenomenon is ordinarily called "self-absorption". On the other hand, molecular lines are rarely associated with such an absorption. This is because, as shown in Fig. 6, when two excited atoms approach each other so that the distance between them is less than a certain distance; ultraviolet light is emitted and the atoms return to a ground state. To verify this theory qualitatively, simple theoretical calculation described below is carried out and the calculation result is compared with an experimental result. The amount of generated resonance lines (V147) is calculated using V147=a*ne*n0 where ne=electron density and n0=atom density. The amount of absorbed ultraviolet (Vabs) is calculated using Vabs=exp(-b*n*l) where b=absorption coefficient (ordinarily, about 10 -6 ) and the length of plasma is set to l. On the other hand, because molecular lines are generated when Xe atoms approach each other, the amount of generated molecular lines (V172) is calculated using V172=C*n 4 +d*n 3 ∼C*n 4 . Molecular lines are rarely associated with the absorption phenomenon as described above. However, in consideration of geometry physical dispersion, V172 is calculated from V172=C*n 4 -n 2/3 . Accordingly, the total amount of ultraviolet light emission V is calculated using V=a*ne*n0-c*exp(-b*n*l)+C*n 4 -n 2/3 where a, b, and c are arbitrary constants. Fig. 7 is a graph showing the amount of resonance lines, the amount of molecular lines, and the total amount of ultraviolet light emission for respective discharge gas pressures. In this graph, the horizontal axis is an arbitrary axis. However, it can be seen from this graph that gas pressure higher than a certain degree is required to adequately benefit from the effects of molecular lines. The amount of ultraviolet light emission for respective gas pressures is examined according to a vacuum chamber experiment, with the composition of discharge gas being Ne(95%)-Xe(5%). The discharge gas having this composition is used for an ordinary PDP. As can be seen from the data marked "" in Fig. 7, the experimental result demonstrates characteristics close to the theory described above. (2) Improvement in Conversion Efficiency of Phosphors Figs. 8A, 8B, and 8C that are cited from "O Plus E", No. 195, 1996, page99 show characteristics of relation between excitement wavelength and relative emission efficiency for the phosphor of each color. As can be seen from these drawings, relative emission efficiency increases when the wavelength is shifted from 147nm to 173nm, regardless of the color of the phosphor. Consequently, when the wavelength is shifted from 147nm to 173nm and the proportion of long wavelength is increased, the light-emission efficiency of respective phosphors tends to increase. (Relation among Gas Pressure, Light-Emission Efficiency, and Firing voltage) The following can be supposed from Fig. 7 that shows a state where the total amount of ultraviolet light emission is changing. When the gas pressure is set in a range of 400-1000Torr (5.333 x 10 4 -1.333 x 10 5 Pa), the amount of ultraviolet light emission is increased as the gas pressure is increased. However, ultraviolet light reaches a point of saturation around 1000Torr (1.333 x 10 5 Pa), so that the ultraviolet light emission amount is hardly increased from about 1000Torr (1.333 x 10 5 Pa) onward. When the gas pressure is further increased to about 1400Torr (1.867 x 10 5 Pa), the ultraviolet light emission amount restarts to increase. The ultraviolet light emission amount continuously increases until the gas pressure exceeds about 2000Torr (2.666 x 10 5 Pa). When the gas pressure is further increased to a certain level exceeding 2000Torr (2.666 x 10 5 Pa), the ultraviolet light emission amount increases at a relatively mild pace. This may be because physical dispersion adversely affects the ultraviolet light emission amount. It should be noted here that although not shown in Fig. 7, when the gas pressure is further increased to exceed the certain level, the ultraviolet light emission amount increases. This can be foreseen according to the theoretical equation described above. According to the stated consideration, the preferable range of pressure of discharge gas ((800-4000Torr (1.067 x 10 5 -5.333 x 10 5 Pa)) is divided into four ranges: 800-1000Torr (1.067 x 10 5 -1.333 x 10 5 Pa) (Range 1), 1000-1400Torr (1.333 x 10 5 -1.867 x 10 5 Pa) (Range 2), 1400-2000Torr (1.867 x 10 5 -2.666 x 10 5 Pa) (Range 3), and 2000-4000Torr (2.666 x 10 5 -5.333 x 10 5 Pa) (Range 4). It should be noted here that although the gas pressure that is set to 760Torr (1.013 x 10 5 Pa) or more theoretically produces the effect of increasing the ultraviolet light emission amount, the least gas pressure in the preferable range is set to 800Torr (1.067 x 10 5 Pa). This is because conditions at facilities are taken into consideration. For instance, discharge gas is charged at a temperature higher than room temperature. Therefore, the least gas pressure is set from an industrial viewpoint. The four ranges can be considered as follows. When only the amount of ultraviolet light emission is taken into consideration, the most preferable range is surely Range 4, that is the most high pressure range. On the other hand, the firing voltage of the PDP Vf can be expressed as a function of the product of the gas pressure P and the distance between electrodes d (Pd product). This rule is called "Paschen's law" (see Electronic Display Device, Ohmsha, 1984, pages113-114). As The gas pressure is increased, Pd product and the firing voltage tend to rise. Here, if the distance d is decreased, Pd product can be suppressed. However, a more sophisticated technique for insulating dielectrics is required as the distance d is decreased. The degree of technical difficulty is therefore increased as the gas pressure is shifted from Range 1, through Ranges 2 and 3, to Range 4. In a PDP corresponding to sign "A" in Fig 7, for instance, the firing voltage is 200V. However, in a PDP corresponding to sign "B" in this drawing, the firing voltage is 450V. Conventional techniques for insulating dielectrics and for making driving circuits resistant to pressure can therefore be a applied to PDPs corresponding to Range 1 because the firing voltage is about 250V or less in a PDP corresponding to Range 1. On the contrary, PDPs corresponding to Ranges 3 and 4 requires sophisticated techniques to considerably decrease the distance d, which may lead to the increase in cost. (Experiment 1/Preliminary Experiment Concerning Composition of Discharge Gas) Various PDPs that equate to the PDP of the present embodiment are produced using discharge gas of respective compositions shown in the table in Fig 9, with Pd product being set to various values. Firing voltage is measured for each of the PDPs produced in this manner. Pd product is set by setting the distance d to 20, 40, 60 or 120µm and changing gas pressure P in a range of 100-2500Torr (1.333 x 10 4 -3.333 x 10 5 Pa). Here, when Pd product is set to a small value, the distance d is mainly set to a relatively small value. When Pd product is set to a range of 1-4, for instance, the distance d is set to 20µm and the pressure P is set in a range of 500-2000Torr (6.667 x 10 4 -2.666 x 10 5 Pa). On the other hand, when Pd product is set to a great value, the distance d is mainly set to a relatively great value (60 or 120µm). The graph shown in Fig. 9 gives the experimental results and shows the relation between Pd product and the firing voltage. The table shown in Fig. 9 gives the panel brightness that is measured (at firing voltage of about 250V) for a PDP having Pd product of about four (by setting gas pressure to 2000Torr (2.666 x 10 5 Pa)) for respective gas compositions. Result and Consideration As can be seen from the table shown in Fig. 9, discharge gas of He-Xe or He-Ne-Xe achieves the panel brightness higher than discharge gas of Ne-Xe (in particular, discharge gas of He-Ne-Xe achieves high panel brightness). It can be assumed that He contributes to the improvement in the panel brightness by raising the electron temperature. As can be seen from the graph shown in Fig. 9, in the case of He-Xe (the data marked "▴"), firing voltage tends to be high, in comparison with the case of Ne-Xe (the data marked "◆"). Therefore, the firing voltage for He-Xe falls outside the preferable range (220V or less). On the other hand, as can be seen from this graph, in the case of Ne-Xe discharge gas that includes 0.1% by volume of Ar (the data marked "○"), the firing voltage is suppressed to 220V or less because of the Penning effect, in comparison with the case of He-Xe, Ne-Xe, or He-Ne-Xe. Furthermore, in this case, preferable Pd product, that is three or more, is obtained. However, in the case of Ne-Xe discharge gas that includes 0.5% by volume of Ar (the data marked "▪"), the firing voltage is not so suppressed. Accordingly, it can be supposed that the addition of Ar in a relatively small amount (0.5% by volume or less) contributes to the suppression of the firing voltage. It should be noted here that because it is technically difficult to set the distance d below 10µm, the preferable range of Pd product is set to three or more in Fig. 9. That is, it is preferable to set Pd product to a range of three or more in practical respects. As described above, when He is added to Ne-Xe discharge gas, the firing voltage tends to be high although the light-emission efficiency is improved. However, when Ar is further added to the discharge gas, the firing voltage is suppressed, with the light-emission efficiency not being decreased. Here, it may be preferably that Ar is added in a relatively small amount. It should be noted here that although in this embodiment, Pd product is set by changing the gas pressure P within a range of 100-2500Torr (1.333 x 10 4 -3.333 x 10 5 Pa), the same results in the graph shown in Fig. 9 can be achieved even if the gas pressure P is set in a range of 2500-4000Torr (3.333 x 10 5 -5.333 x 10 5 Pa). Also, it is publicly known that when the proportion of X3 is in a low range (10% or less), the light-emission efficiency is roughly proportional to the amount of Xe. It is verified by experimental results that even if discharge gas used has any of the stated compositions, the light-emission efficiency is changed in response to change in the amount of Xe. Embodiment 2 Fig 9 shows the simplified sectional view of the AC PDP of the present embodiment. The present PDP has the construction similar to the PDP of Embodiment 1. In Embodiment 1, the front panel is provided with the display electrodes and the back panel is provided with the address electrodes. However, in this embodiment, the front panel is provided with an address electrode 61 and display electrodes 63a and 63b, with the first dielectric layer 62 being inserted between the address electrode 61 and the display electrodes 63a and 63b. It should be noted here that for ease of explanation, Fig. 9 shows the sectional view of a pair of display electrodes 63a and 63b. However, in a practical form, the pair of display electrodes 63a and 63b are arranged perpendicular to the address electrode 61 and barrier ribs 30, like the PDP shown in Fig. 1. The PDP of the present embodiment is produced as follows. The front panel 10 is made by: forming the address electrode 61 on the front glass substrate 11; applying lead-based glass onto the address electrode 61 and the front glass substrate 11; baking them to form the first dielectric layer 62; forming the display electrodes 63a and 63b on the surface of the first dielectric layer 62; then forming the second dielectric layer 64 from lead-based glass on the display electrodes 63a and 63b and the first dielectric layer 62; and forming a protecting layer 65 from MgO on the surface of the second dielectric layer 64. The material and method used for forming the address electrode 61, the display electrodes 63a and 63b, the dielectric layers 62 and 63, and the protecting layer 65 are the same as those in Embodiment 1. Also, as Embodiment 1, it is preferable that the surface of the protecting layer 65 is processed using a plasma etching method to form pyramid-shaped projections. The present embodiment achieves the same effect as Embodiment 1 by setting the composition and pressure of discharge gas in the same way as Embodiment 1. As described above, in the present embodiment, the front panel is provided with the address electrode 61 and the display electrodes 63a and 63b, with the first dielectric layer 62 being inserted between the address electrode 61 and the display electrodes 63a and 63b. Accordingly, even if discharge gas is charged at a high gas pressure, addressing can be performed using low address voltage. That is, when discharge spaces exist between address electrodes and display electrodes as Embodiment 1, Paschen's law is applied to address discharge. Here, address discharge may be performed with stability using low address voltage by decreasing the distance between the address electrodes and the display electrodes. However, it is technically difficult to decrease the electrode distance. Therefore, to perform address discharge with stable, the address voltage needs to be raised as the pressure of discharge gas is increased. On the other hand, in the PDP of the present embodiment, discharge spaces do not exist between the address electrode 61 and the display electrodes 63a and 63b. Consequently, even if discharge gas is charged at a high gas pressure, addressing is performed with stability using low address voltage. Fig. 10 shows the simplified sectional view of another PDP of the present embodiment. In the PDP shown in Fig. 9 , the front panel 10 is provided with the address electrode 61 and the display electrodes 63a and 63b, with the first dielectric layer 62 being inserted between the address electrode 61 and the display electrodes 63a and 63b. However, in the PDP shown in Fig. 14, the back panel 20 is provided with an address electrode 71 and display electrodes 73a and 73b, with the first dielectric layer 72 being inserted between the address electrode 71 and the display electrodes 73a and 73b. The back panel 20 is made by: forming the address electrode 71 on the back glass substrate 21; forming the first dielectric layer 72 from lead-based glass on the address electrode 71 and the back glass substrate 21; forming the display electrodes 73a and 73b on the surface of the first dielectric layer 72; then forming the second dielectric layer 74 from lead-based glass on the display electrodes 73a and 73b and the first dielectric layer 72; and forming a protecting layer 75 from MgO on the surface of the second dielectric layer 74. The PDP shown in Fig. 10 achieves the same effect as the PDP shown in Fig. 9 . In the present PDP, the back panel 20 is provided with the address electrode 71 and display electrodes 73a and 73b, with the first dielectric layer 72 being inserted between the address electrode 71 and the display electrodes 73a and 73b. Accordingly, visible rays generated in the discharge spaces illuminate cells of the PDP without being disturbed by electrodes. Therefore, in regard to the improvement in panel brightness, the present PDP is superior to the PDP shown in Fig. 9 . (Experiment 5) EXAMPLE NUMBER CHARGING GAS PRESSURE (Torr) ADDRESS ELECTORDE POSITION DISPLAY ELECTRODE POSITION PANEL BRIGHTNESS (cd/cm 2 ) STABLE ADDRESS VOLTAGE (V) 1 500 (6.667 x 10 4 Pa) FRONT PANEL FROMT PANEL 490 50 2 760 (1.013 x 10 5 Pa) FRONT PANEL FRONT PANEL 520 50 3 1000 (1.333 x 10 5 Pa) FRONT PANEL FRONT PANEL 530 70 4 2000 (2.666 x 10 5 Pa) FRONT PANEL FRONT PANEL 580 70 5 1000 (1.333 x 10 5 Pa) 1000 BACK PANEL BACK PANEL 550 70 6 1000 (1.333 x 10 5 Pa) BACK PANEL FRONT PANEL 530 120 PDP Example Nos. 1-6 are produced in the same way as the PDPs in Embodiments 1 and 2. More specifically, PDP Example Nos. 1-4 are produced in the same way as the PDP shown in Fig. 9 of Embodiment 2, the PDP Example No. 5 is produced in the same way as the PDP shown in Fig. 10 of Embodiment 2, and the PDP Example No. 6 is produced in the same way as the PDP of Embodiment 1. In this experiment, the height of barrier ribs is set to 0.08mm, the distance between the barrier ribs (cell pitch) 0.15mum, and distance between electrodes "d" 0.05mm, to make the cell size of the PDPs conform to displays for 42-inch high-definition TVs. Each dielectric layer is formed by adding organic binder (made by dissolving 10% ethyl cellulose in α-terpineol) to a mixture of 70% by weight of lead oxide (PbO), 15% by weight of boron oxide (B 2 O 3 ), and 15% by weight of silicon oxide (SiO 2 ), applying the mixture by screen printing, and baking it for 10 minutes at 580°C. The formed dielectric layer is 20µm in thickness. The protecting layer is formed with a plasma CVD method. Note that with the X-ray analysis of crystal structure of the formed MgO protecting layer, it is confirmed that the MgO protecting layer has (100)-face or (110)-face orientation. The composition of discharge gas charged into discharge spaces is He(30%)-Ne(67.9%)-Xe(2%)-Ar(0.1%). Also, as shown in the "Gas pressure" column in Table 1, the gas pressure is set in a range of 500-2000 Torr (6.667 x 10 4 -2.666 x 10 5 Pa). The panel brightness and stable address voltage are measured for each of PDP Example Nos. 1-6 that are produced in the stated manner. To obtain the minimum address voltage necessary to display a stable image on a PDP, address voltage is changed, with the image state being observed. The minimum address voltage obtained in this manner is referred to as the stable address discharge in this specification. The measurement results of the panel brightness and the stable address voltage are given in Table 1. As can be seen by comparing the panel brightness of PDP Example Nos. 1-4, when the gas pressure is set to exceed the normal pressure, that is, to 1000Torr (1.333 x 10 5 Pa) or to 2000Torr (2.666 x 10 5 Pa), the panel brightness increases. As can be seen by comparing the stable address voltages of PDP Example Nos. 1-4, as the gas pressure is increased, the stable address voltage slightly increases. However, these stable address voltages are considerably lower than the stable address voltage of PDP Example No. 6. Therefore, it is supposed that the construction of the PDP of Embodiment 2 has the effect of suppressing the address voltage even if the gas pressure is high. Also, as can be seen by comparing PDP Example Nos. 3 and 5, the panel brightness of PDP Example No.5 is slightly higher than that of PDP Example No. 3. MODIFICATIONS It should be noted here that the present invention is not limited to the PDPs of Embodiments 1 and 2 and may be applied to ordinary PDPs and gas discharge panels. While the protecting layer is formed using a CVD method in Embodiments 1 and 2, for instance, a vacuum deposition method may be used. Also, any other method may be used to form the glass substrate, dielectric layer, and protecting layer, and any other material may be used for forming the phosphor layers. Furthermore, although MgO is used to form the protecting layer, Ba, Sr, or hydrocarbon may be added to MgO to form the protecting layer. In Embodiments 1 and 2, only the back panel is provided with the phosphor layers. However, the front panel may also be provided with the phosphor layers. In this case, the panel brightness is further improved. When particles of the phosphor materials used for forming the phosphor layers are coated with MgO protecting layers, the panel brightness and light-emission efficiency are expected to be further improved. In Embodiments 1 and 2, the display electrodes are arranged parallel to each other on the surface of the front glass substrate or the back glass substrate. However, the display electrodes may be arranged on the surfaces of both of the front glass substrate and the back glass substrate so that the display electrodes arranged on the surface of the front glass substrate oppose the display electrodes arranged on the surface of the back glass substrate. In Embodiments 1 and 2, the barrier ribs 30 are fixed on the back glass substrate 21 to form the back panel. However, the present invention may be applied to a PDP where the barrier ribs are attached to the front panel. Furthermore, the present invention may be applied to gas discharge devices, where discharges spaces are formed by arranging electrodes and phosphor layers in a container and discharge gas is charged into the discharge spaces. The gas discharge devices emits light by performing discharge to generate ultraviolet light and by converting the ultraviolet light into visible rays using the phosphor layers. The present invention may, for instance, be applied to fluorescent lights where phosphor layers are applied inside a pipe-shaped glass container and discharge gas is charged into the glass container. In this case, the present invention also has the effects of improving the brightness and light-emission efficiency and of suppressing the firing voltage. When the gas pressure is set in a range of 800-4000Torr (1.067 x 10 5 -5.333 x 10 5 Pa), the present invention achieves the outstanding effect. INDUSTRIAL USE POSSIBILITY As described above, the present invention improves the light-emission efficiency and panel brightness of a gas discharge panel by setting the pressure of discharge gas in a range of 800-4000Torr (1.067 x 10 5 -5.333 x 10 5 Pa) (Ranges 1-4 described above), that is higher than a conventional range. Also, the light-emission efficiency is improved, with the firing voltage being suppressed, using a rare gas mixture of helium, neon, xenon, and argon, instead of conventional discharge gas. Here, it is preferable that the proportion of Ar is set to 0.5% by volume or less and the proportion of He is set under 55% by volume. Furthermore, with the construction where display electrodes and address electrodes are arranged on the surface of the front cover place or the back plate with a dielectric layer being inserted between the display electrodes and the address electrodes, addressing is performed with relatively low voltage even if discharge gas is charged at a high gas pressure. The present invention has the effect of reducing power consumption of a gas discharge panel, and in particular has the effects of improving the panel brightness and reducing power consumption of a high-definition PDP. Also, the present invention has the effects of improving the brightness and reducing power consumption of an ordinary gas discharge tube other than a gas discharge panel. The gas discharge tube is, for instance, a gas light-emission device, such as a fluorescent light.

Claims: A gas discharge device for emitting light by discharging in a discharge space using electrodes to produce ultraviolet light and converting the ultraviolet light into a visible ray using a phosphor layer, wherein the discharge space is formed between a pair of plates that are placed so that main surfaces of the pair of plates face each other, and at least one of the main surfaces being provided with the phosphor layer and electrodes, characterised in that the discharge space is charged with gas at a pressure which is in a range of 1.1 x 10 5 Pa (800Torr) to 5.3 x 10 5 Pa (4000Torr). The gas discharge device of claim 1, wherein the gas includes xenon. The gas discharge device of claim 2, wherein the gas includes at least one of neon, helium and krypton. The gas discharge device of claim 1, 2 or 3, wherein the electrodes include display electrodes and address electrodes, the display electrodes being placed parallel to each other and the address electrodes being placed perpendicular to the display electrodes, wherein the display electrodes and the address electrodes are stacked on either of the main surfaces of the pair of plates, a first dielectric layer existing between the display electrodes and the address electrodes. The gas discharge device of claim 4, wherein the pair of plates includes a front cover plate and a back plate, wherein the display electrodes and the address electrodes are stacked on a main surface of the back plate, the first dielectric layer existing between the display electrodes and the address electrodes. The gas discharge device of claim 4, wherein the address electrodes, the first dielectric layer, and the display electrodes are placed in the order on either of the main surfaces of the pair of plates, and at least a part of each display electrode is covered with a second dielectric layer. The gas discharge device of claim 6, wherein a surface of the second dielectric layer is covered with a magnesium oxide layer that is formed using either of a thermal chemical vapour deposition method and a plasma chemical vapour deposition method. A gas discharge device according to claim I wherein the device emits light by discharging in a discharge space using electrodes, and wherein the discharge space is formed in a sealed container, and the electrodes and the phosphor layer are placed in the sealed container.

Inventors: AOKI MASAKI, KADO HIROYUKI, KUDOH MASATOSHI, MURAI RYUICHI, SASAKI YOSHIKI, SHIOKAWA AKIRA, TAKADA YUUSUKE, TANAKA HIROYOSI, AOKI, MASAKI, KADO, HIROYUKI, KUDOH, MASATOSHI, MURAI, RYUICHI, SASAKI, YOSHIKI, SHIOKAWA, AKIRA, TAKADA, YUUSUKE, TANAKA, HIROYOSI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: H01J11
IPC codes: H01J17