Method of making blue emitting aluminate phosphor for VUV excited light emitting device

[0005] This invention relates to the development and synthesis of divalent europium activated alkaline earth and/or lanthanum aluminate phosphor particles having a uniform particle size distribution (0.05-5 μm) and a method of forming such particles. More specifically, this invention relates to a development and method of formation of such particles from respective oxides, nitrates and organic precursors which form small particles that improve the stability, longer life time and good color saturation as required for flat panel display (FPD) and lamp applications.

[0006] Divalent europium (Eu²⁺) activated barium magnesium aluminate (BAM) phosphor, is currently used in plasma display panels (PDP) as a blue emitting component. When compared with other phosphors used for red and green, BAM phosphor exhibits lower lifetime with the vacuum ultraviolet (VUV) flux. Efforts are being made to improve the existing BAM or to develop new phosphors to satisfying all requirements.

[0007] U.S. Pat. No. 3,294,699 discloses the invention of BAM as a blue phosphor. According to U.S. Pat. No. 4,110,660, the blend containing BaF₂, LiF, Al(OH)₃and Eu₂O₃was fired in a H₂atmosphere in the temperature range of 1400° C.-1650° C. for a period of 3 to 6 hours to obtain blue emitting divalent europium activated barium-lithium aluminate phosphor, used in xerography lamps. Koichi Takahashi et al. proposed BAM phosphor as a high radiation blue emitting phosphor under excitation by vacuum ultraviolet rays having the wavelength shorter than 200 nm in U.S. Pat. No. 4,161,457. The above said BAM was prepared by calcinating respective metal salts such as carbonates, nitrates, sulfates or halides at temperatures in the range 1200° C.-1600° C. in two different firing cycles. U.S. Pat. Nos. 5,989,454 and 6,187,225 are directed to blue phosphors.

[0008] Multi-phase structured Eu²⁺activated La, Mg aluminate phosphor exhibited improved brightness over prior art single-phase compositions (U.S. Pat. No. 4,249,108). Starting materials (La₂O₃, MgO, Al(OH)₃and Eu₂O₃) were fired at 1500° C.-1650° C. for 1-5 hours in a reducing atmosphere (75 vol. % H₂, 25 vol. % of. N₂). The prior art also reveals that Ba (up to 25%) can be replaced by Sr (U.S. Pat. No. 4,590,405). Kijima et al. synthesized aluminate phosphor comprising (a) at least one element selected from the group consisting of Ba, Sr and Ca, (b) Eu, (c) Mg and/or Zn, (d) optionally Mn and (e) Al by firing the respective oxides and/or hydroxides in a reducing atmosphere at a temperature of from 1200° C.-1700° C. over a period of 2-40 hours (U.S. Pat. No. 5,611,959).

[0009] A method of producing BAM without any fluoride compound (flux) in the starting ingredients was described in U.S. Pat. No. 5,879,586. According to this invention, the particle diameter (1-20 μm) and shape of the particles could be controlled by firing the samples in a reducing atmosphere at higher temperatures (1600° C.-2000° C.). European Patent 0 418 902 A2 teaches the role of alkaline earth and zinc in europium activated BAM in three component phosphor blends in controlling the fluorescent lamp performance.

[0010] According to Japanese Patent (8115673), BaMgAl₁₀O₁₇:Eu2+ has better time-wise durability when compared to traditional BaMgAl₁₄O₂₃:Eu²⁺. This particular phosphor was synthesized by calcinating a barium compound (such as BaO, Ba(OH)₂, BaCO₃, etc), an europium compound (such as Eu₂O₃, EuF₃, etc.), a magnesium compound (such as MgO, Mg(OH)₃, Mg(CO₃)₄.Mg(OH)₂.5H₂O, etc.), an aluminum compound (Al₂O₃, Al(OH)₃, etc.) and a flux (fluorides of Ba, Al or Mg), at least once for 2-40 hours at 1200-1700° C. in a reducing atmosphere in the presence of steam. For better stability (maintenance) of BAM in plasma display applications, Zachau (WO 99/34389) suggested manganese as a co-activator along with europium in BAM.

[0011] The main application of large area plasma displays will be High Definition Television (HDTV) and high information content presentation. HDTV and similar types of display devices should have phosphors with high resolution and high brightness for high performance. This can be achieved only with thin phosphor screens consisting of small phosphor particles in a close rib structure or closed cell structure. Screens with small particles exhibit higher packing density and also need reduced binder content. Stability is another concern in selecting a phosphor. The degradation of phosphor should not be more than 10% before 10,000 hours of operation. Three phosphors (red, green and blue) currently used in PDP's do not exhibit same lifetime (stability) and particle morphology. Due to their physical nature, all of the three phosphors need different rheology of phosphor paste as well as different screening processes. In PDP applications these phosphors exhibit different life times. Blue phosphor degrades very fast when compared to other green and red phosphors. With the operation, the color point (color coordinates) of blue phosphor (BAM) shifts towards green. This result compromises the performance of the display. In consideration of these problems, we have dedicated our efforts to improve the existing phosphors or develop new phosphors.

[0012] Accordingly, it is an objective of the present invention to provide a method of preparation of divalent europium activated alkaline earth aluminate with or without lanthanum phosphor having the empirical formula:

[0013] (AE_2-x-yLa_xEu_y)Al₁₀O₁₇

[0014] Wherein: AE=Ba, Sr, Ca, or Mg, 0≦x≦1 and 0.01≦y≦0.1.

[0015] The present invention compares the synthesis of europium activated alkaline earth aluminate phosphor with or without lanthanum by two different processes: a conventional solid-state reaction process and a sol-gel process. Depending upon the required particle size distribution, the sol-gel process is superior for preparing small particles (0.05-5 microns) and the solid-state reaction is for normal size particles (5-20 microns).

[0016] The sols are dispersions of colloidal particles in a liquid. The gravitational forces on the particles are negligible. From a sol, a gel is formed with an interconnected, rigid network, having sub-micrometer pores and a polymeric chain whose average length is of the order of microns. The particle size of the finished product is a function of the initial concentration of the starting sols, gelation process, drying of gels, calcination temperature and rate of cooling.

[0017] Sol-gel process offers many advantages over conventional methods in the synthesis of fine powders and particularly phosphor materials. Since all of the starting materials are mixed at the molecular level in a solution, a high degree of homogeneity is achievable. Doping of impurities (activators/co-activators/sensitizers) through solutions is straightforward, easy and effective. The pores in properly dried gels are often extremely small and the components of a homogenous gel are intimately mixed. The surface area of powders produced from sol-gel is very high, leading to lower processing temperatures.

[0018] Phosphor materials are extremely sensitive to impurities; even in ppb levels, the low-temperature process through a sol-gel process minimizes the potential for cross contamination. Some of the unwanted impurities left in the materials from conventional methods (high temperature solid state reaction) may pose a threat to the performance of a phosphor. As the size of the phosphor particle decreases, the probability of electron and hole capture to the impurity increases and the e-h localization enhances the recombination rate via the impurity. The optimum impurity concentration (activator) level can be further increased with small particle size. The present invention is related to the growth of Eu²+doped alkaline earth aluminate phosphor with or without lanthanum by sol-gel methods.

[0019] More specifically, the present invention provides compositions and a process for forming a Eu²⁺ doped alkaline earth aluminate phosphor with or without lanthanum having the empirical formula:

[0020] (AE_2-x-yLa_xEu_y)Al₁₀O₁₇

[0021] Wherein: AE=Ba, Sr, Ca, or Mg, 0≦x≦1 and 0.01≦y≦0.1

[0022] (1) reacting a dilute solution comprising a source of an alkaline earth, a source of lanthanum, a source of europium and an organic precursor providing a source of aluminum, in an acid medium to form a dilute gel (sol-gel process);

[0023] (2) converting the dilute gel into a xerogel powder (room temperature drying) or converting the dilute gel into an aerogel powder (vacuum drying); or converting the dilute gel into a gel powder (spray drying); and,

[0024] (3) thermally decomposing the powders obtained from the above, at specified temperatures.

[0025] FIG. 1 shows X-ray diffraction pattern of Eu²⁺ activated barium magnesium aluminate phosphor prepared from xerogel.

[0026] FIG. 2 illustrates scanning electron micrographs of Eu²⁺activated barium magnesium aluminate phosphors prepared from a) xerogel, b) aerogel, c) spray dried powder and d) oxides and nitrates (solid state process).

[0027] FIG. 3 shows emission spectra of Eu²⁺activated barium magnesium aluminate phosphor of present invention prepared from aerogel excited at 147 nm or 173 nm. The emission was recorded at room temperature.

[0028] There are a number of display applications where a blue phosphor with better stability and longer life would significantly improve the display's performance. The blue component is very important, as it improves the color temperature of the display. The small size phosphor particles are particularly suitable for use in applications in which a high packing density is required. The result of this development effort is the basis of the present invention.

[0029] This invention provides a method of synthesizing alkaline earth aluminate phosphors with or without lanthanum incorporating high concentrations of activator ion (Eu²⁺). The formation of the alkaline earth aluminate solid solution is critical and is highly dependent on the reaction temperature and conditions.

[0030] In a solid-state reaction, respective oxides or fluorides or carbonates or nitrates or sulfates are reacted at higher temperatures in the presence of a flux (example LiF₃). At these temperatures, proper doping of impurity ions into the lattice of the complex is uncertain. Finally, the high temperature process (>1500° C.) will lead to growth of uneven and larger particles. The life of the furnace and crucibles is drastically reduced due to the corrosiveness of fluorine (flux) compounds.

[0031] In this invention, an aqueous based process is adopted by considering the cost and availability of the starting chemicals. Since the purity of starting chemicals is very important to the synthesis of phosphors, the starting chemicals are typically 99.9-99.999% in purity [it is important to minimize the concentration of specific contaminants (Fe, Co, Ni, etc.) which can seriously degrade the phosphor performance].

[0032] Required metal (Ba, Ca, Sr, Mg and La and Eu) solutions are also prepared by mixing appropriate amounts of respective metal nitrates in a lukewarm Dl water to obtain 0.05-0.1M solutions. The metal hydroxide precursor was prepared by precipitating an aqueous solution of metal chloride or metal nitrate (0.01-0.05M) in water by the addition of a base such as ammonium hydroxide to the solution. Stoichioemetric quantities of metal solutions and aluminium isopropoxide or aluminum s-butoxide are mixed. The metal/isopropoxide or aluminum s-butoxide solution is transferred to a round bottom flask and peptized at 80-100° C. for 9-18 hours in a stirrer mantle. In the present invention inorganic acid such as HNO₃or HCl have been employed to maintain a low pH which is required to effect gelation.

[0033] Preferably, the phosphor complex of the method of the present invention comprises from about 1.9 moles to about 1.99 moles of alkaline earth metal, from about 0.01 moles to about 0.1 moles of europium, and about 10.0 moles of aluminum.

[0034] After the pepitization, sol/gels are left in a container until they become a thick gel (3-5 days) and then a xerogel. Aerogels are also prepared from the same dilute gels by extracting the water and other solvents in a vacuum through a cold trap. These xerogels or aerogels are transferred into a lab oven at 60-70° C. and left for a day or until becoming powder. This step is inserted to accelerate the removal of any resisdual solvent. Gel powders are also prepared by spray drying. Dilute gels are sprayed through a fine nebulizer into a 4″ diameter glass tubing which has been heated to 120-150° C. An alternate process for forming particles can be accomplished using an ultrasonic aersol generator. After drying, gel powder is collected from the tube. These powders are transferred into a high grade alumina crucible and subjected to two heat cycles. In the first heat cycle, the sample is soaked for 2 hours at 400° C. to burn-out residual organic components and then heated to 900-1000° C. for 3-6 hours to form the respective oxide. In the second heat cycle, powder samples are reheated to 1000-1200° C. in a forming gas (5%H₂+95%N₂) for 2 to 4 hours to reduce Eu³⁺to Eu²⁺ and also to improve the crystallinity of the product. After cooling to room temperature, the aggregate mass is crushed gently (eg., in a mortar and pestal). The crushed powder is then washed with deionized water and dried at 100° C. for 4 to 6 hours to remove very small particles (<0.05 microns) and any extranious soluable component. Very small particles can also be eliminated by rolling the phopshor in a diluted acid (HCl or HNO₃) for several hours (12-24 hrs) and washed throughly with DI water.

[0035] X-ray powder diffraction data on samples fired at 1000° C. and refired at 1100° C. (N₂+H₂) is shown in FIG. 1. Since the luminescence of a phosphor depends on the shape, size, crystallinity, defects and grain boundaries, the morphology and particle size distribution (PSD) of all the samples prepared at various conditions were studied. Scanning electron micrographs of phosphor samples prepared at various conditions are shown in FIG. 2. From the photomicrographs, one can observe that the phosphor particles are very uniform in size. The emission characteristics of these phosphors are carried out on compacted powders and also screen printed circular discs with phorphor at room temperature.

[0036] The emission spectrum of Eu²⁺activated barium magnesium aluminate phosphors prepared from metal nitrates recorded at room temperature under VUV excitation (147 nm or 173 nm) is shown in FIG. 3.

[0037] Further details of this invention will be described with reference in the following examples.

[0038] The preparation of BAM phosphor in an acid catalyzer by a sol-gel process is described in this example. First, 20.4 grams of aluminum isopropoxide (AIP) is dissolved in 4 liters of hot water (95° C.) while stirring. Then 2.35 grams of barium nitrate, 1.48 grams of magnesium nitrate and 0.446 grams of europium nitrate are added to the AIP solution. When the solution reaches 110° C., 5 cc HCl (0.5 mol) is added drop wise and refluxed for 24 hours. A water condenser column is maintained at 20° C. throughout the reflux by use of a circulating chiller. After cooling the flask to room temperature, the solution (dilute gel) is transferred into crystallizing dishes (3L capacity) and left in an open atmosphere. After 5 to 6 days, the solution becomes a gel.

[0039] These transparent hard gels are left at 45 to 50° C. for 12 hours in a lab oven. The dried product appears like soft glass, called xerogel. After crushing the gel in a glass mortar and pestle, a fine powder is collected into a high-grade alumina boat and fired at 400° C. for 2 hours (rate of heating is 2°/min.) and then 900 to 1000° C. for 3 to 6 hours, with the same rate of heating, in a box furnace. Samples are re-fired in a forming gas (5%H₂+95%N₂) at 1000 to 1100° C. for 2 to 4 hours in a tube furnace. The samples are left in the furnace until they cool down to room temperature.

[0040] After cooling, these fine phosphor powders are subjected to ultrasonic agitation in water. Ultrasonic treatment helps to break the clusters into very small particles. After washing with water, powders are dried at 100° C. for 6 hours. Depending upon the required amounts, this can be scaled up.

[0041] The emission characteristics of the above phosphor recorded at room temperature exciting with excitation VUV and PSD are given in Table I.

[0042] The preparation procedure is the same as in example I except 20.4 grams of aluminum isopropoxide is replaced by 26.4 grams of aluminum s-butoxide.

[0043] The preparation procedure is the same as in example I except 3.92 grams of barium nitrate is replaced by 3.18 grams of strontium nitrate.

[0044] The preparation procedure is the same as in example II except 3.18 grams of strontium nitrate is replaced by 3.54 grams of calcium nitrate.

[0045] Synthesis of diluted gels from alkaline earth nitrate, europium nitrate and aluminum isopropoxide or aluminum s-butoxide in an acid medium is the same as described in Example I. Gel solutions obtained are subjected to freeze drying under vacuum. A cold trap is introduced between the vacuum pump and vacuum jar with gel. Dried powder is collected after a few hours of freeze drying at the bottom of the flask. These powders are subjected to calcination, cooling, washing and measurements as described in Example I.

[0046] Synthesis of diluted gels from alkaline earth nitrate, europium nitrate and aluminum isopropoxide or aluminum s-butoxide in an acid medium is the same as described in Example I. These diluted gels are sprayed through a nebulizer in a 4″ diameter glass tube, with a 12″ heating zone at 120 to 150° C. Fine particles can also be produced using an ultrasonic aerosol generator. After spraying about a liter of dilute gel, very fine powder is collected from the walls of the tube. These powders are subjected to calcination, cooling washing and measurements as described in Example I.

[0047] 30 grams of aluminum oxide dust (0.01 to 0.02 microns), 11.5 grams of barium nitrate, 0.82 grams of europium nitrate and 1 gram lithium nitrate (flux) are mixed and transferred to high-grade alumina boats. These powders are subjected to calcination, cooling, washing and measurements as described in Example I.

[0048] As the data of Table I demonstrates, the phosphors formed by sol-gel processes and solid-state reaction of the present invention, provide various particle size ranges, while also generally providing a higher level of brightness.

[0049] It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.