EUROPIUM ACTIVATED STRONTIUM TETRABORATE UV PHOSPHOR AND PROCESS FOR PRODUCING SAME

Abstract: A europium activated strontium tetraborate UV emitting phosphor is disclosed having a weight composition of from about 34.72% to about 35.80 strontium%, from about 17.66% to about 17.78 boron%, from about 0.37% to about 1.86% europium, and the balance oxygen. The phosphor has essentially elongated prism-­ like particles with the europium relatively uniformly distributed throughout the balance of the phosphor and with the particle size distribution being characterized by the 50% size being in the range of from about 7 micrometers to about 12 micrometers as determined by Coulter Counter measurement and with less than about 12% by weight of the particles having a particle size of greater than about 16 micrometers as determined by Coulter Counter measurement. The process for making the phosphor is disclosed which involves forming a mixture of strontium carbonate and europium carbonate in an aqueous solution of boric acid at a temperature of at least about 70°C with the mole ratio of SrCO₃ to H₃BO₃ being from about 1:4 to 1:4.5 and the Eu₂O₃ content being from about .003 to .015 moles. The mixture is digested at at least about 90°C for a sufficient time to form a solid precipitate comprising a strontium borate hydrate intermediate and a liquor, followed by separating the solid from the liquor, drying the solid and firing the solid in an atmosphere of 0.3% to 3.0% by volume hydrogen and the balance nitrogen at at least about 800°C for a sufficient time to produce the phosphor.

Description: BACKGROUND OF THE INVENTION This invention relates to a europium activated strontium tetraborate phosphor. It also relates to a process for producing the phosphor by a new chemical precipitation technique which results in the phosphor having essentially complete homogeneity of constituents including essentially complete incorporation of the europium activator and particle size control. Ultraviolet emitting fluorescent lamp phosphors have been on the market for a number of years. However, phosphors emitting long wavelength, narrow band U.V. are essentially limited to europium activated strontium haloborates. Such phosphates are disclosed in U.S. Patents US-A-3,431,215 and US-A-3,431,216. These phosphors are prepared by a multi-step process involving complex chemical precipitation of europium doped SrB₆O₁₀ · 5H₂O. This material is filtered, dried, and then combined with a strontium halide. The mixture is then air fired, ground, and blended with additional strontium halide and finally refired under H₂/N₂ and ground once more to achieve the desired particle size. The process is labor intensive and produces a product with a poorly developed particle habit with concomittant reduction of its theoretical emission efficiency. German Patent Application Disclosure DE-A-3400385 entitled "Low Pressure Mercury Vapor Discharge Lamp" describes europium activated strontium tetraborates. A europium activated strontium tetraborate containing fluorine is disclosed which is described as being equivalent to the material disclosed in U.S. Patent US-A-3,431,215. There is no indication in the German application of how this phosphor is produced. German Patent Application Disclosure DE-A-2,652,480 discloses europium activated strontium tetraborates which have either barium or magnesium. SUMMARY OF THE INVENTION In accordance with one aspect of this invention there is provided a europium activated strontium tetraborate UV emitting phosphor having a weight composition of from 34.72% to 35.80% strontium, from 17.66% to 17.78% boron, from 0.37% to 1.86% europium, and the balance oxygen. The phosphor has essentially elongated prism-like particles with the europium relatively uniformly distributed throughout the balance of the phosphor and with the particle size distribution being characterized by the 50% size being in the range of from 7 micrometers to 12 micrometers as determined by Coulter Counter measurement and with less than 12% by weight of the particles having a particle size of greater than 16 micrometers as determined by Coulter Counter measurement. In accordance with another aspect of this invention, there is provided a process for producing the above described phosphor. The process involves forming a mixture of strontium carbonate and europium oxide in an aqueous solution of boric acid at a temperature of from 70° to 80°C with the mole ratio of SrCo₃ to H₃BO₃ being from 1:4 to 1:4.5 and the Eu₂o₃ content being from 0.003 to 0.015 moles. The mixture is digested at a temperature of from 90°C to 100°C for from 5 hours to 8 hours to form a solid precipitate comprising a strontium borate hydrate intermediate and a liquor, followed by separating the solid from the liquor, drying the solid and firing the solid in an atmosphere of from 0.3% to 3.0% by volume hydrogen and the balance nitrogen at a temperature of from 890°C to 960°C for from 5 hours to 8 hours to produce the final phosphor. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is an SEM Photograph of a europium activated strontium fluoroborate prior art phosphor of U.S. Patent US-A-3,431,215. Figure 2 is an SEM Photograph of the europium activated strontium borate phosphor of the present invention. Figure 3 is a graph of the particle size distribution of the prior art phosphor of Figure 1. Figure 4 is a graph of the particle size distribution of the phosphor of the present invention produced by the process of this invention. DETAILED DESCRIPTION OF THE INVENTION For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above described figures and description of some of the aspects of the invention. The phosphor of this invention has a weight composition of from 34.72% to 35.80% strontium, from 17.66% to 17.78% boron, from 0.37% to 1.86% europium, and the balance oxygen. The preferred weight composition is from 35.17% to 35.18% strontium, from 17.71% to 17.72% boron, from 1.18% to 1.25% europium, and the balance oxygen. The preferred formula for the phosphor of this invention is Sr 1-x B₄O₇:Eu x where x = from 0.006 to 0.03 and is preferably from 0.018 to 0.02. The preparation of the phosphor by the process of this invention results in well developed strain-free crystals emitting an intense, narrow band at 370 nm. The phosphor has essentially elongated prism-like particles. The particle size distribution is characterized by the 50% size being in the range of from 7 micrometers to 12 micrometers as determined by Coulter Counter measurement. The coarse particle content is controlled so that less than 12% by weight of the particles have a particle size of greater than 16 micrometers as determined by Coulter Counter measurement. Figure 2 is an SEM photograph of the phosphor of this invention. The prism-like character of the particles can be seen. Also it can be seen that there is a relatively small amount of coarse particles. Figure 1 is an SEM photograph of the strontium borate phosphor of U.S. Patent US-A-3,431,215. The particles of Figure 1 have very coarse particles and very fine particles. The difference in the particle size distribution between the prior art phosphor of U.S. Patent US-A-3,431,215 and the phosphor of this invention can be seen in Figures 3 and 4 respectively. The phosphor of the present invention is characterized by controlled reduction of the coarse fraction as is seen in the decrease in particle population of greater than 20 micrometers. The above characteristics of the phosphor of this invention are achieved by a novel precipitation technique involving the slow etching action of boric acid on the only partially soluble SrCO₃ and Eu₂O₃ compounds. This slow controlled precipitation reaction results in a more homogeneous set of conditions giving rise to better formed and larger crystals of the intermediate precipitate product. These larger intermediate precipitate particles provide for less material sintering during phosphor synthesis. This results in a more desirable particle size distribution characterized by the controlled coarse fraction, and also in particles which have an essentially prism-like morphology. This is important because the characteristic fluorescence of the phosphor is dependent on the crystal structure of the host material, that is, strontium tetraborate. Any variation of the chemical composition can cause some local distortions within the crystal which can lead to reduced efficiency, broadened emission bands and possible wavelength shifts in emission bands. Also, crystal imperfections related to formation techniques can cause problems similar to those caused by compositional variations. In accordance with this invention, therefore, boric acid is first dissolved in water. The preferred mole ration of boric acid to water is from 1:2.6 to 1:4.1, with 1:2.6 being especially preferred. An excess of water should be avoided since an excess can result in needless waste of boric acid some of which inevitably remains in the water after the subsequent reaction is complete. On the other hand, if the boric acid to water mole ratio is so high as to leave undissolved acid in the reacting solution, boric acid concentration gradients can be established in the subsequently formed solid precipitate. The resulting aqueous boric acid solution is heated to a temperature of from 70°C to 80°C. These temperatures increase the efficiency of the process and maximize the subsequent formation of the desirable strontium borate hydrate intermediate. Strontium carbonate and europium oxide are added to the resulting aqueous boric acid solution which is preferably being agitated constantly. The molecular ratio of SrCO₃ to H₃BO₃ in the resulting mixture is from 1:4 to 1:4.5 and preferably 1:4.28. The content of the Eu₂O₃ is from 0.003 moles to 0.015 moles and preferably 0.009 moles to 0.01 moles. This mixture is then digested at a temperature of from 90°C to 100°C for from 5 hours to 8 hours to form a solid precipitate comprising a strontium borate hydrate intermediate and a liquor. A digestion temperature of from 95°C to 100°C is especially preferred. The digestion time is dependent on the temperature with longer times being required for the lower temperatures. As a result of the digestion, a solid precipitate comprising a strontium borate hydrate intermediate is formed. The major portion of this solid consists essentially of SrB₆O₁₀ · 5H₂O. Other phases present in the solid include Eu₂O₃, or EuBO₃ · xH₂O and a pre-determined amount of residual strontium carbonate. The resulting digestion mixture is then allowed to cool slowly to room temperature, preferably with continuous agitation. The solid is then separated from the resulting liquor by any standard technique such as filtration. The solid is then dried preferably at a temperature of 120°C. The solid should be essentially completely dry or acid variations can occur in the final product. It is preferred that after the drying step, the resulting dried phosphor cake be broken up by gentle grinding prior to the firing step. Grinding facilitates workability of the material after the subsequent firing step, that is, if unground chunks from the dried material are fired, they become excessively hard and may require extensive milling to reduce them to powder form. The dried solid is then fired in an atmosphere consisting essentially of from 0.3% to 3.0% and preferably 1% by volume hydrogen and the balance nitrogen at a temperature of from 890°C to 960°C, for from 5 hours to 8 hours to produce the final phosphor. The major portion of the final phosphor has the composition as described previously. The reaction is carried out preferably at from 910°C to 920°C for from 6 hours to 10 hours. Firing conditions depend to some extent on the crystal size which is desired. For example, higher temperatures and longer firing times result in larger crystals. It is preferred that the solid be fired in open quartz crucibles because quartz is unreactive to the starting materials and to the phosphor. Open crucibles or boats facilitate escape of water and the hydrogen reduction of Eu⁺³ to Eu⁺² which occurs during firing. The resulting fired phosphor can be dry or wet milled to enable it to pass through a 200 mesh screen. The process of this invention produces a phosphor with a well-defined, strain-free crystal structure, and an intrinsic brightness of nearly twice that of previous phosphors. Furthermore, the process for producing the phosphor is less complex than previously reported processing methods. For example, essentially only one step, the digestion step, is necessary to produce the europium doped SrB₆O₁₀ · 5H₂O intermediate which is to be fired to form the final phosphor, whereas with other methods, more steps are required to form the final europium doped compound. Such is the case in the processes of U.S. Patents US-A-3,431,215 and US-A-3,431,216 in which, for example, a strontium halogen is added in a separate step. Also, the process of this invention does not require the use of volatile and/or environmentally objectionable reagents such as acetone and ammonia, or halogens, nor does it require dry mixing/milling of reactants as is done in other processes such as the process in the above named patents. The final phosphor is prepared by a one-step firing as opposed to the two step firing operation of the above named patents. In Table 1 are shown the relative radiance data of the phosphor of this invention versus a prior art phosphor. The data in Table 1 shows that the relative radiance of the phosphor of this invention is higher than the prior art phosphor. Table 2 shows particle size characteristics of the phosphor of this invention prepared by the process of this invention compared with the prior art phosphor of U.S. Patent US-A-3,431,215. It can be seen that the 50% size of the phosphor of this invention is in the range of 7 to 12µ.m and the coarse fraction, that is, the >16µ.m fraction is made up of only 8.6% of the particles as opposed to a higher coarser fraction in the prior art phosphor. To more fully illustrate this invention, the following non-limiting examples are presented. Example 1 Boric acid is dissolved in deionized water at 70°C. A mixture of strontium carbonate and europium oxide is slowly added to the boric acid solution with continuous stirring. The temperature of the resulting slurry is increased to 95°C and digestion is continued for 12 hours. The mole ratio of the reactants is 1.00 SrCO₃ : 1:4.29 H₃BO₃ : 0.010 Eu₂O₃. The resulting digestion mixture is cooled to room temperature and suction filtered. The filter cakes are broken up by hand and dried at 120°C. The dried cakes are hammer milled to pass through a 60 mesh screen. The resulting screened material is then loaded into silica boats and fired in a tube furnace at 940°C in a 1% by volume H₂ atmosphere for 7 hours. 1200 g of this material is wet ball milled for 15 minutes and passed through a 60 mesh screen. The screened phosphor is then added to 12 liters of continuously stirred deionized water along with 50 g of Sr(OH)₂ · H₂O. The solution is stirred continuously and held at 95°C throughout the ten minute wash. The phosphor is then allowed to settle and the wash solution is siphoned away. This strontium hydroxide washing procedure is repeated two more times followed by three additional water washes without the hydroxide. The washed phosphor is suction filtered and dried overnight at 120°C. The dried material is screened through 200 mesh. The relative radiance data is shown in Table 1. Example 2 The preparation is similar to that in Example 1 except that digestion time is 6 hours and firing temperature is 912°C. 16 kg of this fired material is treated as in Example 1 except the screened phosphor is added to 40 gallons of deionized water containing 800 g of Sr(OH)₂ ·H₂O and stir washed for 20 minutes per cycle. The resulting washed and suction filtered material is dried at 102°C for 18 hours and then screened through 200 mesh. The relative radiance data of the phosphor produced by this example is shown in Table 1.

Claims: A process for producing a europium activated strontium tetraborate UV emitting phosphor, said process comprising: (a) adding strontium carbonate and europium oxide to an aqueous solution of boric acid which is at a temperature of from 70°C to 80°C to form a mixture wherein the mole ratio of SrCO₃ to H₃BO₃ is from 1:4 to 1:4.5 and the Eu₂O₃ content is from 0.003 moles to 0.015 moles; (b) digesting said mixture at a temperature of from 90°C to 100°C for from 5 hours to 8 hours to form a solid precipitate comprising a strontium borate hydrate intermediate and a liquor; (c) separating said solid from said liquor; (d) drying said solid; and (e) firing the resulting dried solid in an atmosphere consisting essentially of from 0.3% to 3.0% by volume hydrogen and the balance nitrogen at a temperature of from 890°C to 960°C for from 5 hours to 10 hours to produce the final phosphor the major portion of which consists essentially of strontium tetraborate as a host material doped with europium ions substituted for strontium ions, said final phosphor having a composition in percent by weight of from 34.72% to 35.80% strontium, from 17.66% to 17.78% boron, from 0.37% to 1.86% europium and the balance oxygen, with said europium being uniformly distributed throughout the balance of said phosphor. A process of claim 1 wherein the mole ratio of boric acid to water in said aqueous solution is from 1:2.6 to 1:4.10. A process of claim 1 wherein said mole ratio of SrCO₃ to H₃BO₃ is 1:4.28. A process of claim 1 wherein the Eu₂O₃ content in said mixture is from 0.009 moles to 0.01 moles. A process of claim 1 wherein said dried solid is fired at a temperature of from 910°C to 920°C for from 6 hours to 10 hours. A process of claim 1 wherein the firing is done in an atmosphere consisting of 1% by volume hydrogen and the balance nitrogen. A process of claim 1 wherein the weight composition of said final phosphor is from 35.17% to 35.18% strontium, from 17.71% to 17.72% boron, from 1.18% to 1.25% europium, and the balance oxygen. A europium activated strontium tetraborate UV emitting phosphor having a composition in percent by weight of from 34.72% to 35.80% strontium, from 17.66% to 17.78% boron, from 0.37% to 1.86% europium and the balance oxygen, and having essentially elongated prism-like particles with the europium uniformly distributed throughout the balance of said phosphor and with the particle size distribution being characterised by the 50% size being in the range of from 7 micrometers to 12 micrometers as determined by Coulter Counter measurement and with less than 12% by weight of the particles having a particle size of greater than 16 micrometers as determined by Coulter Counter measurement. A phosphor of claim 8 wherein the weight composition is from 35.17% to 35.18% strontium, from 17.71% to 17.72% boron, from 1.18% to 1.25% europium and the balance oxygen.

Inventors: CHENOT, CHARLES FREDERIC, KREBS, MICHAEAL ANDREW
Applicants: GTE PRODUCTS CORPORATION
Application year: 1987
Main classification: C09K11
IPC codes: C09K11, H01J61