Patent Application
20060071588
The present invention relates to a plasma display device having phosphor layers that are excited by ultraviolet light to emit light, and to a method of producing a phosphor used for the device.
In recent years, plasma display devices using plasma display panels (hereinafter, referred to as “PDP” or “panel”) receive attention as color display devices that implement large screen size, thin body, and light weight in displaying color images for computers, television sets, and the like.
A plasma display device displays full color by means of additive color mixing of so-called three primary colors (red, green, and blue). For displaying full color, a plasma display device is provided with phosphor layers that emit light in the three primary colors: red (R), green (G), and blue (B). Phosphor particles composing the phosphor layers are excited by ultraviolet light occurring in a discharge cell of the PDP, to generate visible light in each color.
Compounds used for the above-mentioned phosphors in each color include (YGd)BO3:Eu3+ and Y2O3:Eu3+ for emitting red light; Zn2SiO4:Mn2+ for green; and BaMgAl10.0O17:Eu2+ for blue. These phosphors, after given raw materials being mixed therewith, are produced with solid-phase reaction by being fired at a temperature above 1,000° C. This method is disclosed in “Phosphor Handbook (in Japanese)” (p. 219 and p. 225, by Ohmsha, Ltd., 1991), for example.
The phosphor particles produced by firing, after lightly crushed to the extent of breaking aggregated particles but not the crystals, are screened (average particle diameter for red and green: 2 μm to 5 μm, for blue: 3 μm to 10 μm) before using. The reason for lightly crushing and screening (classifying) phosphor particles is as follows. That is, methods of forming a phosphor layer in a PDP include screen-printing of pasted phosphor particles in each color, and ink-jet method, in which the paste is discharged through a nozzle for applying. Large agglomerates are included in a phosphor unless the phosphor particles are classified after lightly crushed, and thus unevenness in coating and clogging in the nozzle may occur when coating the paste with the phosphors. Therefore, phosphors classified after being lightly crushed are small in particle diameter and even in particle size distribution, thus yielding a more desirable coated surface.
An example method of producing a green phosphor made of Zn2SiO4:Mn is disclosed in “Phosphor Handbook (in Japanese)” (pp. 219-220, Ohmsha, Ltd., 1991). That is, SiO2 is blended in ZnO at the rate of 1.5ZnO/SiO2, which is larger than its stoichiometric ratio (2ZnO/SiO2), and then fired at 1,200° C. to 1,300° C. in the air (at one atmospheric pressure) to produce a green phosphor. In this case, the surface of the Zn2SiO4:Mn crystal is covered with an excessive amount of SiO2, and the phosphor surface is negatively charged.
The fact that a green phosphor in a PDP, negatively charged with a high level, degrades in its discharge characteristic, is disclosed in Japanese Patent Unexamined Publications No. H11-86735 and No. 2001-236893, for example. Further, it is known that ink-jet coating, in which coating is made continuously with ink for a negatively charged green phosphor through a fine-bore nozzle, causes clogging in the nozzle and unevenness in coating. These are because ethyl cellulose in the ink is in particular presumably resistant to being adsorbed in the surface of the negatively charged green phosphor.
Further, there is a problem in which a negatively charged phosphor causes ion collision of a negatively charged green phosphor with positive ions of Ne, CH-base, or H occurring while discharging, deteriorating the luminance of the phosphor.
Meanwhile, some methods are formulated such as laminate-coating positively charged oxide for changing negative charge on the surface of Zn2SiO4:Mn to positive one, and mixing a positively charged green phosphor for apparently positive charge. However, it is problematic that laminate-coating oxide causes a low luminance, and applying two different kinds of phosphor with a different charge state tends to generating clogging and unevenness in coating. In addition, there is a method in which ZnO and SiO2 are blended in advance at the ratio of 2:1 or more (2/1 or more of Zn/Si in element ratio) when producing Zn2SiO4:Mn, and ZnO is scattered (sublimed) in advance while firing, using the vapor pressure of ZnO higher than SiO2, when firing the phosphor in the air or in nitrogen at one atmospheric pressure at 1,200° C. to 1,300° C. However, even in such a case, the proximity of the crystal surface results in rich SiO2 and is negatively charged by all means.
The present invention, in view of these problems, aims at preventing phosphor layers from deteriorating and at improving the luminance, life, and reliability of a PDP.
In order to achieve this purpose, the plasma display device of the present invention is equipped with a plasma display panel in which a plurality of discharge cells are arranged, phosphor layers are allocated with a color corresponding to each discharge cell, and the phosphor layers are excited by ultraviolet light to emit light. The phosphor layers include a green phosphor layer containing Zn2SiO4:Mn, and the green phosphor made of Zn2SiO4:Mn has the element ratio of zinc (Zn) to silicon (Si) of 2/1 or more at least at the proximity of the surface, and Mn is used as an activator.
In such a makeup, phosphor particles in which the green phosphor crystal is positively charged or zero-charged allow the phosphor layer to be formed with an even coating, prevent the luminance degradation of the phosphor, and improve luminance, life, and reliability of the PDP.
In the present invention, ZnO, SiO2, and MnO2 are mixed as an activator when producing a green phosphor made of Zn2SiO4:Mn having the zinc silicate crystal structure, then this mixture is pre-fired in the air at 600° C. to 900° C. to produce pre-fired powder, and next the pre-fired powder is actually fired at 1,000° C. to 1,350° C., completely shielded with ZnO powder, to produce Zn2SiO4:Mn.
The green phosphor made of Zn2SiO4:Mn, used for a PDP, is usually produced with solid-phase reaction method, where SiO2 is blended in ZnO at a rate larger than its stoichiometric ratio, for improving luminance. This results in the surface of the Zn2SiO4:Mn crystal being covered with SiO2. However, even if produced at the stoichiometric ratio so that the surface will not be covered with SiO2, firing at 1,000° C. or higher causes ZnO at the proximity of the surface to be scattered (sublimed) early, due to the vapor pressure of ZnO higher than that of SiO2, resulting in more SiO2 on the surface of the phosphor. If fired at 1,000° C. or lower so that ZnO will not be scattered (sublimed), although Zn2SiO4:Mn with its Zn/Si ratio of 2/1 is synthesized, a high-luminance phosphor is not produced due to its low crystallinity.
The present invention solves the above-mentioned problem with the following method. That is, set the element ratio of ZnO to SiO2 to be blended, between 2.1/1 and 2.0/1, put a mixture of Zn2SiO4:Mn into a crucible and completely enclose (shield) its periphery with ZnO in order to prevent the ZnO from being scattered (sublimed), and then fire the covering ZnO in an oven at a temperature slightly higher than that of Zn2SiO4:Mn in the crucible, in an atmosphere of N2 (nitrogen), N2—H2 (nitrogen-hydrogen), and/or N2—O2 (nitrogen-oxygen), to prevent the ZnO from being scattered (sublimed). In other words, enclose the periphery of pre-fired Zn2SiO4:Mn powder with ZnO, set the temperature of this ZnO to a slightly higher one, and set the saturated vapor pressure at the ZnO side at the firing temperature relatively high, to prevent the ZnO in the pre-fired Zn2SiO4:Mn powder from being scattered (sublimed).
Hereinafter, a description will be made for a method of producing a phosphor according to the present invention.
Methods of producing a phosphor body itself include the following. That is, one is solid-phase sintering, where an oxidized or carbonated raw material, and flux are used. Another is liquid-phase method, where a precursor of a phosphor is produced with hydrolysis of organometallic salt or nitrate salt in an aqueous solution, or with coprecipitation that precipitates organometallic salt or nitrate salt with alkali or the like added, and then the precursor is heat-treated to produce pre-fired powder. Yet another is liquid spraying, where an aqueous solution with raw materials for a phosphor added is sprayed in a heated oven.
The present invention reveals that using a phosphor precursor and pre-fired powder produced with any of the above methods prevent ZnO from being scattered: from its surface while firing, and are effective in improving the characteristic of Zn2SiO4:Mn phosphor, if Zn2SiO4:Mn is shielded (Zn2SiO4:Mn powder is sealed with ZnO.) with ZnO while fired at 1,000° C. or higher, and then actually fired at 1,000° C. to 1,350° C.
As an example for a method of producing a green phosphor, a description will be made for a process of producing a green phosphor with solid-phase reaction method according to the present invention. Blend carbonate and oxide as the raw materials, such as ZnO, SiO2, and MnCO3, with a slightly more amount of ZnO over SiO2 as compared to the molar ratio of the base material ((Zn1-xMnx)2SiO4) for a phosphor. (The element compounding ratio of ZnO to SiO2 is 2.1/1 to 2.0/1.) Next, after mixing the materials, pre-fire them at 600° C. to 900° C. for two to three hours. Then put them into a crucible (made of Al2O3 or ZnO), enclose the periphery of the crucible containing the pre-fired powder, with ZnO powder or a ZnO crucible, and set the temperature in the oven so that the temperature of the ZnO crucible or ZnO powder will be relatively higher. Next, fire them under this temperature setting (at 1,000° C. to 1,350° C.) in an atmosphere of at least one of N2, N2—H2, and N2—O2, to produce a green phosphor.
Alternatively, in liquid-phase method, where a phosphor is produced from an aqueous solution, the following process is employed. That is, dissolve organometallic salt (e.g. alkoxide, acetylacetone) or nitrate salt, into water in advance so that the element ratio of Zn/Si will be 2.1/1 to 2.0/1, and then hydrolyze it to produce a coprecipitate (hydrate). Next, pre-fire it at 600° C. to 900° C. in the air. After that, in the same way as in the aforementioned solid-phase reaction method, enclose the periphery of the crucible containing the pre-fired powder, with ZnO powder or a ZnO crucible, and set the temperature in the oven so that the temperature of the ZnO crucible or ZnO powder will be relatively higher. Next, fire them under this temperature setting (at 1,000° C. to 1,350° C.) in an atmosphere of at least one of N2, N2—H2, and N2—O2, to produce a green phosphor.
In this way, when pre-fired powder produced with a slightly more amount of ZnO over SiO2 as compared to the stoichiometric ratio is shielded with ZnO, and fired at 1,000° C. to 1,350° C., ZnO is prevented from being scattered (sublimed) from the surface of Zn2SiO4:Mn, because the vapor pressure of the ZnO used for shielding is higher than that of the ZnO in the pre-fired powder. Therefore, unlike conventional Zn2SiO4:Mn, the proximity of the surface does not result in rich SiO2, but Zn2SiO4:Mn with slightly rich ZnO, extending to the inside is produced. This allows producing a green phosphor with its Zn2SiO4:Mn particles favorably charged positively.
Here, the reason for limiting the ratio of Zn to Si to between 2.1/1 and 2.0/1 is to facilitate positively charging Zn2SiO4 by means of a slightly excessive amount of Zn elements. Further, with the Zn ratio of 2.1 or more, the amount of Zn in the crystal lattice increases to lower the luminance, while with 2.0 or less, Zn2SiO4 tends to be charged negatively. Accordingly, the ratio of Zn to Si is desirably between 2.1/1 and 2.0/1.
The reason for limiting the firing temperature to between 1,000° C. and 1,350° C. is the following. That is, at 1,000° C. or lower, the vapor pressures of both ZnO and SiO2 are low, and thus a small amount of ZnO is scattered (sublimed) from the surface, resistant to generating a SiO2-rich layer at the proximity of the surface. However, a high-luminance phosphor is not produced due to poor crystallization of Zn2SiO4:Mn. Meanwhile, at 1,350° C. or higher, the vapor pressure of ZnO becomes relatively high as compared to that of SiO2. This causes covering Zn2SiO4:Mn with ZnO less effective. Besides, excessively sintered Zn2SiO4 causes a large particle diameter, lowering the luminance.
Next, a description will be made for a phosphor in each color used for a plasma display device according to the present invention. Concrete phosphor particles used for a green phosphor layer are desirably those made from [(Zn1-xMnx)2SiO4] produced with the aforementioned method, and the value of x satisfies 0.01≦x≦0.2, for advantages in luminance and luminance degradation.
For concrete phosphor particles used for a blue phosphor layer, a compound expressed by Ba1-xMgAl10O17:Eux or Ba1-x-ySryMgAl10O17:Eux can be utilized. Here, the values x and y of the compound are desirably 0.03≦x≦0.20, and 0.1≦y≦0.5, respectively, for high luminance.
For concrete phosphor particles used for a red phosphor layer, a compound expressed by Y2xO3:Eux or (Y, Gd)1-xBO3:Eux can be utilized. Here, the value x of the compound for a red phosphor is desirably 0.05≦x≦0.20, for advantages in luminance and luminance degradation.
Hereinafter, a description will be made for an embodiment of a plasma display device according to the present invention, referring to drawings.
As shown in
This PDP 100, as shown in
As shown in
Next, a description will be made for a method of producing the above-mentioned PDP 100, referring to
Display electrode 103 and display scan electrode 104 are composed of a transparent electrode made of indium tin oxide (ITO) and a bus electrode made of silver. The silver paste for the bus electrode is formed by being applied with screen-printing and then fired.
Dielectric glass layer 105 is formed so that it will have a given thickness (approximately 20 μm), from a paste including a lead-based glass material being applied with screen-printing and then fired at a given temperature for a given time (at 560° C. for 20 minutes, for example). A paste including the above-mentioned lead-based glass material is, for example, a mixture of PbO (70 wt %), B2O3 (15 wt %), SiO2 (10 wt %), Al2O3 (5 wt %), and an organic binder (10% ethyl cellulose dissolved in alpha-terpineol). Here, an organic binder means a resin dissolved in an organic solvent, where an acrylic resin, besides ethyl cellulose, can be used as a resin, and butylcarbitol or the like can be also used as an organic solvent. Further, such a organic binder may immix glyceryl trileate, for example.
MgO protective layer 106, made of magnesium oxide (MgO), is formed so that the layer will have a given thickness (approximately 0.5 μm) with a method such as sputtering or chemical vapor deposition (CVD).
The back panel is formed into a state where M pieces of address electrodes 107 are installed in a row, from a silver paste for electrodes being screen-printed onto rear glass substrate 102 and then fired. A paste including a lead-based glass material is applied on the back panel with screen-printing to form dielectric glass layer 108. In the same way, barrier rib 109 is formed from a paste including a lead-based glass material being applied repeatedly at a given pitch with screen-printing and then fired. This barrier rib 109 partitions discharge space 122 line-wise into each cell (unit of light-emitting region). Further, W, which is the gap between barrier ribs 109, is defined as between approximately 130 μm and 240 μm according to high-definition TV with its screen size between 32 inches and 50 inches.
In addition, phosphor layer in red (R), blue (B), green (G) are formed in the grooves between barrier ribs 109. Green phosphor layer 110G is formed with a green phosphor, which is pre-fired Zn2SiO4:Mn powder with its element ratio of Zn/Si of 2.1/1 to 2.0/1, the periphery of which is shielded with ZnO, fired at 1,000° C. to 1,350° C. in an atmosphere of at least one of N2, N2—O2, and N2—H2.
Phosphor layers 110R, 110G, and 110B, where phosphor particles are bound each other, are formed from a phosphor ink paste made of phosphor particles and an organic binder being applied and then fired at 400° C. to 590° C. to burn out the organic binder. It is desirable to form phosphor layers 110R, 110G, and 110B, so that L, which is the lamination-wise thickness of the layers on address electrode 107, will be roughly 8 to 25 times the average particle diameter of the phosphor particles in each color. In other words, the phosphor layer desirably retains a thickness of at least 8 layers, and preferably about 20 layers of lamination, in order not to let ultraviolet light generated in the discharge space transmit but to be eliminated, for ensuring luminance (emission efficiency), when irradiating the phosphor layer with a certain amount of ultraviolet light. This is because the emission efficiency of the phosphor layer is almost saturated, and the size of discharge space 122 cannot be adequately ensured with a thickness more than the above.
Meanwhile, phosphor particles small enough in their diameter and spherical, such as those produced with hydrothermal synthesis method or the like, raise the filling density of the phosphor layers and increase the total surface area of the phosphor particles, as compared to the case of unspherical particles and the same levels of lamination. Consequently, the surface area of phosphor particles involved in actual light-emitting increases, further raising the emission efficiency.
The front and back panels produced in this way are overlapped each other so that respective electrodes on the front panel will be orthogonalized with the address electrodes on the back panel. In addition, the panels are sealed with sealing glass inserted to the periphery of the panels and then fired at approximately 450° C. for 10 to 20 minutes, for example, to form hermetic seal layer 121. Next, after the inside of discharge space 122 is once exhausted to a high vacuum (e.g. 1.1×10−4 Pa), discharge gas such as an He—Xe-based or He—Xe-based inactive gas is encapsulated at a given pressure, producing PDP 100.
Header 230 is linearly driven by a header scanning mechanism (not illustrated). Having header 230 scan as well as continuously discharging phosphor ink 250 through nozzle 240 allows the phosphor ink to be uniformly applied to the grooves between barrier ribs 109 on rear glass substrate 102. Here, the viscosity of the phosphor ink used is maintained between 1,500 centipoise (CP) and 50,000 CP at 25° C.
Still, above-mentioned server 210 is equipped with an agitation device (not illustrated), which prevents the particles in the phosphor ink from being precipitated. Further, header 230 is integrally molded with ink chamber 230a and nozzle 240 included, and is produced from a metallic material with machining and electric discharging
Further, a method of forming a phosphor layer is not limited to the above-mentioned method, but various methods can be used such as photolithographic method, screen-printing, and a method in which a film with phosphor particles mixed is allocated.
Phosphor ink is a mixture of phosphor particles in each color, a binder, and solvent, all blended so that the viscosity will range between 1,500 centipoise (CP) and 50,000 CP, where a surface active agent, silica, dispersant (0.1 wt % to 5 wt %), and others may be added as required.
A red phosphor blended in this phosphor ink is a compound expressed by (Y, Gd)1-xBO3:Eux or Y2xO3:Eux. These are compounds in which Eu is substituted for a part of Y element composing its maternal material. Here, x, which is the substitution value of Eu element for Y element, desirably ranges as 0.05≦x≦0.20. For a substitution value more than this, the luminance significantly degrades, although it increases, which is assumed to be impractical. Meanwhile, for a substitution value less than this, the composition ratio of Eu, the main element of light-emitting, decreases, so does the luminance, making useless as a phosphor.
A green phosphor uses a compound expressed by [(Zn1-xMnx)2SiO4], that is pre-fired with its element ratio of Zn/Si of 2.1/1 to 2.0/1, and then fired in an atmosphere of N2, N2—H2, and/or N2—O2, shielded with ZnO. [(Zn1-xMnx)2SiO4] is a compound in which Mn is substituted for a part of Zn element composing its maternal material. Here, x, which is the substitution value of Mu element for Zn element, desirably ranges as 0.01≦x≦0.20.
A blue phosphor uses a compound expressed by Ba1-xMgAl10O17:Eux or Ba1-x-ySryMgAl10O17:EUx. Ba1-xMgAl10O17:Eux and Ba1-x-ySryMgAl10O17:Eux are compounds in which Eu or Sr is substituted for a part of Ba element composing its maternal material. Here, x and y, which are substitution values of Eu element for Ba element, desirably range as 0.03≦x≦0.20 and 0.1≦y≦0.5.
Further, as a binder blended in a phosphor ink, ethyl cellulose or acrylic resin (0.1 wt % to 10 wt % of ink is mixed) can be used; and as a solvent, alpha-terpineol or butylcarbitol can be used. Here, the binder may be polymer molecules such as PMA or PVA, and the solvent may be an organic solvent such as diethylene glycol or methyl ether.
In this embodiment, phosphor particles are manufactured with solid-phase reaction method, aqueous solution method, spray firing method, or hydrothermal synthesis method. A concrete example for a method of producing phosphor particles will be hereinafter described.
First, a description will be made for a method of producing a blue phosphor of Ba1-xMgAl10O17:Eux with aqueous solution method.
In the process of producing a mixed solution, mix the raw materials of barium nitrate Ba(NO3)2, magnesium nitrate Mg(NO3)2, aluminium nitrate Al(NO3)3, and europium nitrate Eu(NO3)2, with the molar ratio of 1-x:1:10:x (0.03≦x≦0.25), and dissolve them into an aqueous medium to produce a mixed solution. This aqueous medium is desirably ion-exchanged water or pure water in that they do not include an impure substance; however, they can be used even if they include a nonaqueous solvent (e.g. methanol, ethanol). Next, put the hydrate liquid mixture into a container made of a material with resistance to corrosion and heat, such as gold or platinum, and then use a device capable of heating under pressure, such as an autoclave, to perform hydrothermal synthesis (for 12 to 20 hours) at a given temperature (100° C. to 300° C.), at a given pressure (0.2 MPa to 10 MPa), in a high-pressure container. Next, fire this powder in a reducing atmosphere (e.g. atmosphere including 5% of hydrogen and 95% of nitrogen) at a given temperature, for a given time (e.g. at 1,350° C. for two hours), and classify this, to produce a desired blue phosphor Ba1-xMgAl10O17:Eux.
Phosphor particles produced with hydrothermal synthesis are spherical and have particle diameters smaller than those produced with the conventional solid-phase reaction, resulting in an average particle diameter of approximately 0.05 μm to 2.0 μm. Here, “spherical” is defined as the aspect ratio (minor axis diameter/major axis diameter) of most phosohor particles ranges between 0.9 and 1.0, for example, where all the phosphor particles do not necessarily fall in this range.
Meanwhile, a blue phosphor can be produced with spraying also, in which the hydrate mixture is not put into a gold or platinum container, but the mixture is sprayed through a nozzle to a high-temperature oven, to synthesize a phosphor.
Next, a description will be made for a method of producing a blue phosphor of Ba1-x-ySryMgAl10O17:Eux manufactured with solid-phase reaction method.
Weigh raw materials of barium hydroxide Ba(OH)2, strontium hydroxide Sr(OH)2, magnesium hydroxide Mg(OH)2, aluminum hydroxide Al(OH)3, and europium hydroxide Eu(OH)2, for a required molar ratio, and mix them along with AlF3 as flux. After that, fire them at a given temperature (1,300° C. to 1,400° C.), for a given time (12 to 20 hours) to produce Ba1-x-ySryMgAl10O17:Eux in which quadrivalent ions are substituted for Mg and Al. The average particle diameter of the phosphor particles produced with this method is approximately 0.1 μm to 3.0 μm. Next, after firing these in a reducing atmosphere, 5% hydrogen and 95% nitrogen, for example, at a given temperature (1,000° C. to 1600° C.), for a given time (two hours), classify them with an air classifier to produce phosopher powder.
Here, oxide, nitrate salt, and hydroxide are mainly used as raw materials for a phosphor. However, a phosphor can be produced also with an organometallic compound including elements such as Ba, Sr, Mg, Al, and Eu (e.g. metal alkoxide and acetylacetone).
Next, a description will be made for a method of producing a green phosphor of (Zn1-xMnx)2SiO4.
First, a description will be made for the case where a green phosphor is produced with solid-phase method. Mix the raw materials of zinc oxide (ZnO), silicon oxide (SiO2), and manganese oxide MnO, so that the molar ratio of Zn to Mn will be 1:x:x (0.01≦x≦0.20), and next mix the raw materials along with flux (ZnF2 and MnF2) as required, so that the element ratio of Zn to Si will be 2.1/1 to 2.0/1. Pre-fire this mixture at 650° C. to 900° C. for two hours. Next, lightly crush the mixture to the extent of breaking the agglomerate, put it into first crucible 310 made of Al1O3 or ZnO, enclose it with ZnO powder 330 and ZnO crucible 350, and fire them in an atmosphere including at least one of N2, N2—O2, and N2—H2, at 1,000° C. to 1,350° C., to produce a green phosphor. At this moment, if an arrangement is made so that heater 340 of the oven will be located at the periphery of second crucible 350 made of ZnO and/or ZnO powder 330, the temperature of first crucible 310, second crucible 350, and ZnO powder 330 can be made slightly higher than that of pre-fired phosphor powder 320 made of Zn2SiO4:Mn. Power control of heater 340 at the periphery of second crucible 350, and flow adjustment of an ambient gas of N2, N2—O2, and/or N2—H2 control the respective temperatures of second crucible 350, first crucible 310, ZnO powder 330, and pre-fired phosphor powder 320 of Zn2SiO4:Mn, for actually firing. Here, in this embodiment, the description is made for the case where an electric oven is used. However, a gas oven or the like may be used.
Next, a description will be made for the case where a green phosphor is produced with aqueous solution method. In the process of producing a mixed solution, mix the raw materials of zinc nitrate Zn(NO3)2, manganese nitrate Mn(NO3)2, and ethyl silicate[Si(O.C2H5)4], so that the molar ratio of zinc nitrate to manganese nitrate will be 1-x:x (0.01≦x≦0.20). Next, in blending Zn(NO3)2 and [Si(O.C2H5)4], mix the raw materials so that the element ratio of Zn to Si will be 2.1/1 to 2.0/1, and put them into ion-exchanged water to produce a mixed solution. Next, in hydration process, drop a basic aqueous solution such as an aqueous ammonia solution into this mixed solution to form hydrate. Pre-fire this hydrate at 600° C. to 900° C., and next, in the same way as in solid-phase method, put this pre-fired matter in first crucible 310 made of Al2O3 or ZnO, enclose the periphery of this first crucible 310 with ZnO powder 330 and second crucible 350 in an atmosphere of N2, N2—O2, and/or N2—H2 at 1,000° C. to 1,350° C., to produce a green phosphor.
Next, a description will be made for a method of producing a red phosphor with aqueous solution method.
First, a red phosphor of (Y, Gd)1-xBO3:Eux will be described. In the process of producing a mixed solution, mix the raw materials of yttrium nitrate Y2(NO3)3, hydro nitrate gadolinium Gd2(NO3)3, boric acid H3BO3, and europium nitrate Eu2(NO3)3, so that the molar ratio will be 1-x:2:x (0.05≦x≦0.20) (The ratio of Y to Gd is 65:35), and after heat-treating them at 1,200° C. to 1,350° C. in the air, classify them to produce a red phosphor.
Meanwhile, for a red phosphor of Y2xO3:Eux, in the process of producing a mixed solution, dissolve the raw materials of yttrium nitrate Y2(NO3)2 and europium nitrate Eu(NO3)2 into ion-exchanged water, so that the molar ratio will be 2-x:x (0.05≦x≦0.30), to produce a mixed solution. Next, in hydration process, add a basic aqueous solution (e.g. aqueous ammonia solution) to this aqueous solution to form hydrate. After that, in hydrothermal synthesis process, put the hydrate and ion-exchanged water into a container with resistance to corrosion and heat, such as platinum or gold, and then perform hydrothermal synthesis in a high-pressure container such as an autoclave, at 100° C. to 300° C. at a pressure of 0.2 MPa to 10 MPa for 3 to 12 hours. After that, dry the yielded compound to produce desired Y2xO3:Eux. Next, after annealing this phosphor in the air at 1,300° C. to 1,400° C. for two hours, classify it to form a red phosphor.
Here, the above-mentioned phosphor layers 110R and 110B of PDP 100 use those having been used conventionally, and phosphor layer 110G uses green phosphor particles with its surface of [(Zn1-xMnx)2SiO4] composing the positively charged phosphor.
Hereinafter, in order to evaluate the performance of the plasma display device according to the present invention, an evaluation experiment is made for a sample device using a phosphor according to the above-mentioned embodiment.
The respective plasma display devices are produced so that they will have 42-inch size (specification of high-definition TV with its rib pitch of 150 μm), the thickness of the dielectric glass layer is 20 μm, the thickness of the MgO protective layer is 0.5 μm, and the distance between the display electrode and display scan electrode is 0.08 mm. Further, the discharge gas to be encapsulated in the discharge space is a neon-based gas with a xenon gas mixed by 5%, encapsulated at a given discharge-gas pressure.
Zn2SiO4:Mn green phosphor particles used for sample plasma display devices 1 through 10 adopt a [(Zn1-xMnx)2SiO4] phosphor produced as follows: That is, put pre-fired powder of a phosphor with its element ratio of Zn to Si of 2.1/1 to 2.0/1 into first crucible 310 made of Al2O3 or ZnO, enclose the periphery of this crucible with ZnO powder 330 and second crucible 350, and then fire them in an atmosphere of N2, N2—O2, and/or N2—H2 at 1,000° C. to 1,350° C. Table 1 shows the conditions of synthesis for each phosphor used in these samples.
*Sample numbers 10 through 13 are for comparative examples.
In samples 1 through 4, a green phosphor is a combination using (Zn1-xMnx)2SiO4 produced with solid-phase synthesis method; a red phosphor, (Y, Gd)1-xBO3:Eux; and a blue phosphor, (Ba1-xMgAl10O17:Eux). Each sample shows variation in method of synthesizing a phosphor; substitution ratios of Mn and Eu, which are the main elements for light-emitting, namely substitution ratio of Mn to Zn element and substitution ratio of Eu to Y and Ba elements; and element ratio of Zn to Si and firing conditions for a green phosphor, as shown in table 1.
In samples 5 through 9, a green phosphor is a combination using (Zn1-xMnx)2SiO4 produced with liquid-phase method (aqueous solution method); a red phosphor, (Y1-x)2O3:Eux; and a blue phosphor, B1-x-ySryMgAl10O17:Eux. Each sample shows variation in method of synthesizing a phosphor; and element ratio of Zn to Si and firing conditions for a green phosphor, as shown in table 1.
Further, phosphor ink used for forming a phosphor layer is produced by mixing a phosphor, resin, solvent, and dispersant, using each phosphor particle shown by table 1.
The measurement result shows that the viscosity of the phosphor ink at that time (25° C.) all remains in the range between 1,500 CP and 50,000 CP. In all the phosphor layers formed, the side faces of the barrier ribs are found by observation to be uniformly coated with phosphor ink.
Further, the bore of the nozzle used for coating then is 100 μm, and phosphor particles used for the phosphor layer have an average particle diameter of 0.1 μm to 3.0 μm and a maximum particle size of 8 μm or less.
Sample 10 is a comparative sample with its element ratio of Zn/Si of 1.9/1 when blending raw materials for a green phosphor; and sample 11, in the same way, of Zn/Si of 2.2/1. Sample 12 is a green phosphor in the conventional example, with its element ratio of Zn/Si of 2.0/1 when blending raw materials, fired only with a regular crucible (made of Al2O3). In sample 13, the ratio of Zn/Si when blending raw materials for a green phosphor is 2.0/1, the firing temperature for pre-fired powder is the same as in the present invention, a crucible made of ZnO is covered with ZnO powder, but the temperature when actually firing is set to as low as 900° C.
Measurement is made of the charging tendency for the green phosphors of samples 1 through 9 and comparative sample 10. Here, the measurement adopts blow-off method, which measures the amount of charge for reduced iron powder.
Measurement is made of the element ratio of Zn to Si at the proximity (approximately 10 nm) of the surface with X-Ray photoelectron spectroscopy (XPS) for samples 1 through 9 and comparative sample 10 produced.
Measurement is made of the luminance of a PDP in fully white after the PDP producing process, and the luminance of green and blue phosphor layers, with a luminance meter.
Measurement is made of the luminance degradation factor when displaying full white, green, and blue as follows: That is, continuously apply discharge sustain pulses with 185 V, 100 kHz, to a plasma display device for 1,000 hours, measure the luminance of the PDP before and after then, and calculate the luminance degradation factor (<[luminance after application—luminance before application]/luminance before application >100)
An address error during address discharge is judged from at least a single flicker on the screen.
Clogging in the nozzle is checked when green phosphor ink is applied using a nozzle with its bore of 100 μm, for 100 hours continuously.
Table 2 shows experimental results of the luminance and luminance degradation factor, and of clogging in the nozzle, for green phosphors in experiments 1 through 5.
*Sample numbers 10 through 13 are for comparative examples.
As shown in table 2, in comparative sample 10, the element ratio of Zn to Si as raw materials is 1.9/1, and Si in Zn2SiO4:Mn is richer in the ratio of Zn/Si as compared to the stoichiometric ratio. Therefore, sample 10 is negatively charged, the luminance of the panel in green and blue largely degrades, and an address error and clogging in the nozzle occur. In sample 11, the ratio of Zn to Si as raw materials is 2.2/1, which means sample 11 is rich in Zn and the powder is positively charged. However, sample 11 is richer in Zn as compared to the stoichiometric ratio, and thus the phosphor becomes deteriorated in crystallization and the panel in blue has a low luminance. Meanwhile, comparative sample 12 is a green phosphor produced with the conventional producing method, and thus ZnO sublimes selectively from the surface, sample 12 is in Si with the Zn/Si ratio of 1.92/1, to be negatively charged, the luminance of the panel in green in an accelerated life largely degrades, and an address error and clogging in the nozzle occur. In comparative sample 13, where the periphery of the first crucible is covered with ZnO powder, the temperature for actually firing is as low as 900° C., and thus Zn2SiO4:Mn is insufficiently crystallized, and the panel has a low luminance and large luminance degradation for green.
Meanwhile, the green phosphors according to the present invention in samples 1 through 9, where Zn and Si are blended at the element ratio of Zn/Si of 2.0/1 to 2.1/1, pre-fired powder is put into a crucible, the periphery of which is enclosed with ZnO powder, and produced at 1,000° C. to 1,350° C., are positively charged or zero-charged, and thus the luminance in green and blue slightly degrades, and an address error and clogging in the nozzle when applying phosphors do not occur. This is presumably because positively charging or zero-charging the negatively charged green phosphor causes the phosphor to be immune to an impact of positive ions such as neon ions (Ne+) and CHx-based ions (CHx+) existing in the discharge space of the panel, suppressing luminance degradation. Here, an impact of ions is slightly reduced for the blue phosphor also.
Further, the reason why address errors have been eliminated is homogenization of address discharge as a result that the green phosphor is positively charged, which is the same as for the red and blue ones. Still, the reason why clogging in the nozzle has been eliminated is presumanly the improved dispersibility of the phosphor ink because the ethyl cellulose in the binder is prone to adsorbing a positively charged green phosphor.
As above-mentioned, according to the present invention, a green phosphor (Zn1-xMnx)2SiO4 composing a phosphor layer is positively charged or zero-charged, to homogenize coating condition, to prevent deterioration, of the phosphor layer, and also to improve luminance, life, and reliability, of the PDP, thus effectively improving the performance of the plasma display device.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-335270 | Sep 2003 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP04/14431 | 9/24/2004 | WO | 5/26/2005 |
