GASEOUS REACTIVE IMPROVEMENT TREATMENT SYSTEM OF PARTICULATE RARE EARTH OXYSULFIDE PHOSPHORS

Abstract: The efficiency and brightness of particulate, crystalline phosphors of oxysulfides on lanthanum series metals activated with trivalent metals having atomic numbers from 50 to 70, such as GD2O2O:Tb crystalline phosphors, containing 0.6 mol percent Tb and particularly those also containing 0.1 to 5 mol percent Dy are increased up to 2 to 3 times by treating the phosphors in a H2/SO2 atmosphere at a temperature from 750<o>C to 1,250<o>C for 15 minutes to 3 hours. Life tests have shown no decrease in brightness after 1,000 hours of tube operation.

Description: GASEOUS REACTIVE IMPROVEMENT TREATMENT SYSTEM OF PARTICULATE RARE EARTH OXYSULFIDE PHOSPHORS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improving the brightness of particulate phosphors and, more parti¬ cularly, to a process for treating fine grain, rare earth oxysulfide phosphors to produce a material which emits light at a much higher efficiency than available fine grain materials. 2. Description of the Prior Art During the past several decades, a great emphasis has been directed to the synthesis of rare earth phos¬ phors. A significant portion of this development effort was focused on more efficient, large grain phosphors to cope with the increasingly demanding requirements of avionic display systems. With efforts and priorities directed toward these goals, little, if any, attention has been addressed to improving the efficiencies of fine grain phosphors for miniature CRT displays. Fine grain phosphors are usually only 30 to 70 percent as bright as their large grain counterparts. Therefore, CRT screens including fine grain phosphors must be operated at higher current which can result in increased noise, line blooming and/or decreased tube life. Currently, many system design concepts are being directed toward less massive, miniature CRTs. New displays, such as the helmet-type holographic devices, require quantum improvements in phosphor efficiencies while maintaining ultra fine resolution and contrast. High resolution displays require fine line widths since the image may be magnified as high as 10 times. Rare earth phosphors have also been deposited on substrates as films with very low brightnesses and efficiencies. Thus, there are trade-offs involved between brightness and resolution. The large particle phosphors are brighter but provide poor resolution. Fine particle phosphors provide excellent resolution but emit much lower brightness outputs as compared to their large grain counterparts. These low level efficiencies distinctly restrict the application of fine grain phosphors in miniature displays where high resolution and contrast are desirable for avionic applications. Intermediate sized particles do enhance the brightness levels of these mini-displays but would not meet the constraints of fine line resolution. Quantum improvements are necessary in the basic efficiency of these fine grain, narrow line phosphors, to meet both current and future display requirements. Normally, fine particle phosphors are prepared by wet chemical coprecipitation, followed by a modified solid-state reaction of precursor starting materials. These fine grain phosphors have also been prepared by secondary reactions of the phosphor with gaseous mixtures but the phosphors produced by this method are not nearly as bright as and are larger than those produced by coprecipitation and modified solid-state reactions. The phosphors produced by gaseous mixtures are usually larger in size (having a mean dimension from 3 to 6 μ) with efficiencies ranging from 12 to 20 lumens/watt. Larger particle sizes (7-15 μ) are normally prepared by blending the chemical oxide constituents with fluxes and calcinating the blend at elevated temperatures for specific times. The phosphor is then washed free of flux and refired in a dynamic atmosphere of gases to render it suitable for screening with efficiencies ranging from 20 to 35 lumens/watt. Phosphor films deposited by gaseous atmospheres and/or deposition result in transparent films with even lower efficiencies (0.8 to 3.0 lumens/watt) than particulate phosphors. Buchanan, et al. in U.S. Patent No. 3,825,436; Maple in U.S. Patent NO. 4,132,919; Maple, et al. in J. Vac. Sci. Technol., Vol. 10, No. 5, Sept/Oct 1973 at 616; and Alves, et al. in Appl. Phys. Lett. , Vol. 21, No. 11, 1 Dec. 1972 disclose increasing the luminescence of thin rare earth oxysulfide films by treating the films in an H2-SO2 atmosphere at 1,000°C. The result in intrinsic brightness equals that of the crystalline material but the apparent brightness is only 1/lOth of the powder. However, the below-listed references disclose treating particulate rare earth oxysulfide materials in diverse, inert, oxidizing and reducing gaseous atmospheres such as nitrogen, nitrogen saturated with water vapor, hydrogen sulfide, hydrogen, carbon monoxide, and iodine without any significant increase in luminescence. Furthermore, long thermal treatment of the powders will cause crystal growth with resultant loss in final resolution. 1) J. W. Haymes and J. J. Brown, Jr., Journal of the Electrochemical Society, 115, 1060 (1968) and their references (1) to (20). 2) D. W. Ormond and E. Bands, Journal of the Electrochemical Society, 122, 152 (1975). 3) L. Ozawa, Journal of the Electrochemical Society, 124, 413 (1977). 4) M. R. Royce, A. L. Smith-, S. M. Thomsen and P. N. Yocum, Abstract 86, p. 201, Journal of the Electrochemical Society, Spring Meeting (1969). 5) M . Koskenlinna, M. Leokela and L. Niinisto, Journal of the Electrochemical Society, 123, 75 (1976), and their references (3) to (11 ) . U.S. Patent No. 3,515,675 discloses converting rare earth oxides to oxysulfides by treatment in an atmosphere comprising hydrogen, sulfur and oxygen at elevated temperature. However, the hot crystals are removed from the treatment zone without gradual cooling. This would cause a quenching effect on the crystals. Such drastic changes in temperature can have a very deleterious effect on crystal perfection and symmetry by causing twinning and slip planes. SUMMARY OF THE INVENTION A fine grain phosphor having significantly improved brightness is provided by the present invention. The brightness achieved equals or surpasses that of some commercially available large grain, rare earth oxysulfide phosphors. The magnitude of improvement ranges from 5 percent on large grain phosphors, up to 2 to 3 times that on fine grain phosphors of the original chemical formulation. The powders do not evidence any notable particle growth during the treatment of the invention. No degradation or loss in light output is noted during tube processing in which the phosphor particles are subjected to gaseous atmospheres and to temperatures of over 400°C in an oxidizing environment. Prior fine grain rare earth oxysulfide phosphors have shown tendenncies to oxidize to a lower brightness form during tube processing. The treated phosphor particles of the invention exceed any brightness ever achieved in the present state-of-the-art with fine grain particles. The phosphor material of the invention will enable the production of high resolution miniature CRT tubes and displays in brightness exceeding that ever achieved in the past. These phosphors will also enable attainment of high brightness without heavily driving the cathode, thus prolonging tube life with fine line resolution and decreasing the cost or original and replacement equipment. The highly efficient phosphors of the invention provide excellent brightness while maintaining ultra fine line resolution and contrast. This has enabled miniature cathode ray tubes to achieve brightnesses in excess of 10,000 foot lamberts. This major improvement in efficiency with high resolution line width and good modulation transfer frequency (MTF) will greatly improve the quality and sophistication of avionic display systems and miniaturized displays, such as helmet-type visor-mounted holographic systems or helmet-mounted displays. The phosphors will also find use in displays as simulators, combat vehicles and air defense systems, not to mention miniaturization of compact projection displays. rare earth oxysulfide powders with a mixture of SO2 and H2 gases at elevated temperature results in a significant increase in brightness from up to twofold to threefold without noticeable particle growth. The treated phosphors are slowly cooled in an inert atmosphere to ambient temperature after treatment. The treated phosphors of the invention provide bright images with ultrafine line resolution and contrast. The brightness equals or surpasses that of commercially available large grain phosphors and comparable brightnesses are achieved at lower current inputs to the cathode of the CRT. These and many other features and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1, 3 and 5 each display a set of curves showing comparative tube brightnesses as a function of screen current of fine grain commercial phosphor (0) versus fine grain phosphor treated by the process of this invention (Δ) when mounted on the faceplate of 1 inch fiber optic tubes; FIGS. 2, 4 and 6 illustrate the tube brightness curves and efficiency curves in filled circles and triangles of the tubes illustrated in FIGS. 1, 2 and 3; FIGS. 7 and 9 each display a set of curves showing comparative brightnesses of fine grain commercial phosphors (0) versus treated, fine grain phosphors (Δ) when mounted on the 1 inch clear faceplate of a tube; FIGS. 8 and 10 illustrate the brightness curves of the tubes illustrated in FIGS. 7 and 99 with efficiency curves in filled circles and triangles superimposed thereon; FIGS. 11 and 12 illustrate the brightness (open triangle) and efficiency (closed triangle) of the treated phosphors of the invention when mounted on the faceplate of a 1/2 inch clear glass tube; FIG. 13 is a diagram of energy levels of rare earth ions used in the phosphors of the invention by electron excitation; and FIG. 14 is another diagram of energy levels of the rare earth ions used in the phosphors of the invention by photon excitation. DETAILED DESCRIPTION OF THE INVENTION The rare earth oxysulfide phosphors that exhibit enhanced brightness after treatment in the process of the invention are crystalline rare earth oxysulfides of the formula: where Ln is a trivalent rare earth metal of the lanthanum series, particularly lanthanum, gadolinium, yttrium, lutetium or mixtures thereof, and Ln2 is a trivalent activator rare earth metal ion having an atomic number from 59 to 70 or mixtures thereof. The stoichiometry may not be exactly as presented. The ratio of oxygen to sulfur may vary slightly from 2 to 1, especially after treatment in the H2/SO2 gas atmosphere. Chemical analyses by (XPS) , X-ray exicted photoeleetron spectrometry, have shown an increase of sulfur content to 7 atomic percent as compared to 3.7 atomic percent in the starting material. The activator is usually present in an amount from 0.001 to 20 mol percent and prefereably from 0.01 to 10 mol percent of the Ln replaced with a trivalent Ln 2 activator or mixtures thereof. The color of light emitted by the phosphor is dependent on the activator and concentration as is well known in the art. For example, rare earth oxysulfides activated with terbium (Tb) emit green radiation; europium (Eu) activated rare earth oxysulfide phosphors emit red radiation, and thulium (Tm) activated oxysulfides emit blue radiaiton. It has also been discovered in this invention that the brightness enhancement is considerably increased if the phosphor contains about 0.1 to 5.0 mol percent of a rare earth oxide having a lower energy level than, but fairly close to, that of the activator metal ion such as Dy2θ3, S1112O3, Pr θ^, EU2O3 and SC2O3. The Dv 2°3 ma Y act as a nost sensitizer to transfer electrons as they cascade from energy levels in the ions of the host to energy levels in the activator metal ion. This is demonstrated in FIGS. 13 and 14 by electron and photon excitation models of the gadolinium showing the cascade from the Gd host through the Dy sensitizer to the Tb activator. This type of energy transfer is also exhibited by the model in FIG. 14 by a photo-excitation of the gadolinium matrix and transference through the dysprosium to the terbium. This photo-excitation may also occur during the electron excitation of the gadolinium since the gadolinium also emits U.V. which can, in turn, activate the Dy and Tb. The phosphors are treated in accordance with the invention by placing the particulate phosphors in an atmosphere comprising a mixture of hydrogen and sulfur dioxide at a temperature from 750° C to l,250 β C for a period of from 15 minutes to 3 hours, preferably from 30 minutes to 90 minutes. The ratio of hydrogen to SO2 can be varied within wide limits, but is usually from 5 to 25 volume percent SO2, the remainder being hydrogen. Though the method is applicable to any particulate rare earth oxysulfide phosphor, it is most advantageously practiced with the fine grain phosphors having a size range from 1 to 3 microns. A common commercial phosphor is P-43 which has the nominal composition Gd 2 θ2S: b with the terbium activator content usually about 0.6 mol percent. Standard P-43 phosphors and a P-43 phosphor containing 1.6 mol percent Dy2θ3 (P-43D) substituted for a portion of the Gd 2 θ3 will be illustrated in the following examples of practice. The particles of rare earth oxysulfide were placed into an inert receptacle such as a silica, vicore or quartz crucible or boat. The receptacle was inserted into a gradient furnace. The heater for the furnace was activated and argon or inert gas such as nitrogen was initiated to purge the furnace. After about 1.5 hours, the temperature of the furnace rose to the 1,025-1,075°C treatment temperature. When the furnace reached the treatment temperature, the argon or inert gas flow was terminated and replaced with a flow of hydrogen and sulfur dioxide gases at a ratio of 4.4 1/min. to 0.6 1/min., respectively. The receptacle containing the particulate phosphor is maintained at 1,025-1,075°C in this atmosphere for 1 hour. After 1 hour, the heater is deactivated and the H2/SO2 flow is slowly reduced and replaced with a flow of inert gas or argon gas. The receptacle is retained in this inert atmosphere in the hot zone of the furnace until it slowly cools to room temperature. The phosphor is then removed and crumbled by finger pressure in a plastic bag, if it has agglomerated into a cake by sintering during treatment. The phosphor is stored under anhydrous conditions in a vacuum desiccator until it is processed into a cathode ray tube (CRT) screen. The phosphors were screened onto" mountable and demountable faceplates by standard processing for brightness evaluations. During this processing, the powders were subjected to the normal alkali silicate and electrolyte and tube fabrication solutions and were exposed to an amyl acetate vehicle used in fritting which was then thermally decomposed. Neither the gaseous decomposition products from the vehicle nor the . oxidizing atmosphere at high temperature appeared to degrade the brightness of the phosphor powders. Initial testing of fine grain P-43 phosphors was accomplished by electron excitation of the screen on a 1 inch diameter indium-tin-oxide (ITO) conductively coated faceplate in a demountable system. The following Table 1 illustrates the comparative efficiencies obtained with screen currents of 1, 2 and 3 μa/cm2 and accelerating potentials of 7, 10 and 14 KV on a standard large grain P-43, a fine grain P-43 and a fine grain P-43 treated in accordance with this invention. As compared to the typical commercial fine grain P-43, the processed phosphor exhibits from 2 to 2.3 times the efficiency at lower screen loadings. Efficiencies at higher screen currents of 10 μa/cm 2 , 20 Ua/cm 2 and 30 ua/cm 2 are shown in Table 2 with similar anode potentials. TABLE #1 Lumens/Watt Efficiencies @ 1μa/cm 2 @ 2 μa/cm 2 @ 3 μa/cm 2 P-43 SAMPLES 7KV 10KV 14KV 7KV 10KV 14KV 7KV 10KV 14KV Large Grain Std. P-43 (7.2 μ) 8.5 10.5 11.3 8.5 10.8 12.5 8.8 9.9 12.4 Typical Fine Grain P-43 (1.5-2.5 μ) 4.5 4.9 5.6 4.7 5.4 6.3 4.5 5.2 6.5 Processed P-43 (2.3 μ) 10.4 11.3 11.5 11.0 12.2 12.9 10.8 11.7 13.4 TABLE #2 Lumens/Watt Efficiencies § 10 μa/cm 2 θ 20 μa/cm 2 @ 30 μa/cm 2 P-43 SAMPLES 7KV 10KV 14KV 7KV 10KV 14KV 7KV 10KV 14KV Large Grain Std. P-43 (7.2 μ) 8.1 10.0 10.3 7.8 9.1 10.2 6.3 8.2 8.6 Typical Fine Grain P-43 (1.5-2.5 μ) 3.8 5.3 5.0 3.5 4.0 4.8 2.8 3.4 4.3 Processed P-43 (2.3 μ) 9.7 10.6 11.3 9.1 10.2 11.5 7.5 9.2 10.1 It can also be noted that the processed material equals and/or at times exceeds the efficiencies of the larger grain P-43 internal standard. To confirm the stability of the processed P-43 phosphor, several tubes were fabricated for test. These test results verified the demountable brightness data and indicated no degeneracy during tube fabrication. To establish both consistency and processing repeatability, several additional miniature 1 inch tubes were fabricated for comparative test purposes as were two 1/2 inch diameter tubes. These tubes are being utilized in a helmet-mounted viewer and a holographic visor system where brightness and resolution are a primary criteria. The 1 inch tubes were fabricated with both clear and fiber optic faceplates while the 1/2 inch tubes were processed with a clear faceplate. The brightness measurements, spectral energy distribution curves and x, y coordinates were obtained with a Pritchard Spectral Radiometer. x e spectral energy distribution curve showed the typical main emission triplet of terbium at 5,450 Angstroms and corresponding color coordinates of x = 0.3383 ± 0.0031 and y = 0.5682 ± 0.0037. X-ray diffraction indicated well defined peaks with narrow base lines of a hexagonal crystal structure. The operational parameters of the 3 tube types under discussion were tested at 7 KV, with 525 lines interlaced at 60 Hertz and a 75 percent duty cycle. The raster sizes varied as follows: 1 inch Fiber Optic - 12.75 x 17 mm; 1 inch Clear Faceplate - 0.5 x 0.5 inch and the 1/2 inch Clear Faceplate - 0.4 x 0.4 inch. The first set of tubes with a fiber optic faceplate, are a high resolution, high contrast type utilized in helmet-mounted systems. The tube brightnesses as shown in FIGS. 1, 3 and 5 are a comparison of the process phosphor to identical tubes containing typical fine grain commercial phosphor. These illustrations show the degree of improvement as a function of screen current. The line width associated with these bright¬ nesses ranges from a low of 0.67 mils to a high of 1.17 mils. FIGS. 2, 4 and 6 show the plotted brightness curves along with the improved efficiencies. The open triangles and circles depict the tube brightnesses as illustrated in FIGS. 1, 3 and 5, while the filled-in triangles and circles depict their respective tube efficiencies, FIGS. 2, 4 and 6. As noted in FIGS. 1 through 4, both brightness and efficiency improvements are quite superior to that of a typical fine grain P-43. With slight processing modifications, to increase the aluminum reflectivity, a greater improvement of light output is observed in FIGS. 5 and 6. The results of 1 inch clear faceplate tubes are illustrated in FIGS. 7 to 10. As previously designated, the open triangles depict the process phosphor while the open circles represent the typical fine grain commercial P-43. These plotted brightness and efficiency (filled-in triangles and circles) comparisons demonstrate additional levels of improvement over that shown for the fiber optic tubes. The test data from the 1/2 inch clear faceplate tube is indicated in FIGS. 11 and 12. No comparative tests were performed with this tube type using commercial fine grain P-43. The brightness of both these tubes exceeded 3,500 foot lamberts at approximately 60 to 70 μa of screen current and then fell off at higher screen currents presumably due to saturation. The line widths of these tubes are somewhat larger, ranging from a low of 1.1 mils to a high of 1.6 mils. Refinements in both the gun and screen optimization are required to lower the line width and improve the brightness to exceed 4,000 foot lamberts. It is quite evident that the quantum improvements • obtained by the fine grain rare earth oxysulfide phosphors processed in accordance with- the invention for cathode ray tubes will greatly enhance the performance of avionic displays. The improved screen brightnesses are 2 to 2.5 times over the fine grain commercial phosphors. The high brightness now obtainable with the fine grain phosphor will also have a significant effect on the resolution. The required screen current for a given screen brightness is much lower as compared to the brightness plot of a commercial fine grain phosphor. In addition to these improvements, tube life will be greatly extended and replacement cost decreased. Life tests at 1,000 hours of tube operation have shown no decrease in brightness whereas recently published data on commercial phosphors (F. M. Bruno, Ninth Quarterly Report, Jan. 1983, M. M. T. Report of USA MERADCOM DA.AK070-80-C-0168) showed drops in brightness after 1,200 hours of from 15 percent to 30 percent. Immediate avionic application for these miniature cathode ray tubes would be directed to the helmet- mounted displays and helmet-visor holographic systems. Additional displays such as simulators, combat vehicles, air defense units and miniaturized projection displays (monochrome and color) can also include screens prepared from the improved phosphors processed in accordance with this invention. It is to be realized that only preferred embodiments of the invention have been described and that numerous substitutions, modifications and alterations are permissible without departing from the spirit and scope of the invention as defined in the following claims.

Claims: CLAIMS What is Claimed is: 1. A method of increasing the brightness of a particulate rare earth oxysulfide phosphor comprising the steps of: treating the particulate phosphor in a hydrogen- sulfur dioxide atmosphere at a temperature of 750°C to l,250 β C for a period sufficient to increase brightness of light emission from the phosphor by at least a factor of 2. 2. A method according to Claim 1 in which the particulate phosphor is maintained in the H2-SO2 atmosphere for a period from 15 minutes to 3 hours. 3. A method according to Claim 2 in which the atmosphere consits essentially of 5 to 25 volume percent SO2, the remainder being hydrogen. 4. A method according to Claim 3 in which the atmosphere is a flowing atmosphere. 5. A method according to Claim 2 in which the phosphor is a fine grain material having a size in the range of 1 to 3 microns. 6. A method according to Claim 5 in which the phosphor is selected from crystalline materials of the formula: Lnl 2-x°2 SϊLn2 χ where Ln is a trivalent rare earth metal of the lanthanum series, and Ln is at least one trivalent rare earth activator metal -and x is a number such that 0.001 to 20 mol percent of Ln j s replaced with Ln 2 . 7. A method according to Claim 6 in which Ln 2 is a trivalent rare earth metal having an atomic number from 59 to 70. 8. A method according to Claim 6 in which the activator metal is present in an amount from 0.1 to 10 mol percent. 9. A method according to Claim 1 in which 0.1 to 5 mol percent of the activator is replaced with a rare earth metal having an energy level below that of the activator. 10. A method according to Claim 9 in which the replacement metal is Dy. 11. A method according to Claim 10 in which the phosphor comprises Gd2θ2S containing about 0.6 mol percent Tb and 1.6 mol percent Dy2θ3« 12. A particulate, fine grain phosphor produced by the method of Claim 1. 13. A phosphor according to Claim 1 selected from compounds of the formula: 1 9 Ln 2-x°2 SϊLn x* 14. A phosphor according to Claim 13 in which Ln 2 is a trivalent rare earth metal having an atomic number from 59 to 70. 15. A phosphor according to Claim 14 in which the activator metal is present in an amount from 0.1 to 10 mol percent. 16. A phosphor according to Claim 15 in whch 0.1 mol to 5 mol percent of the activator is replaced with a rare earth metal having an energy level parallel to and/or below that of the activator. 17. A phosphor according to Claim 16 in which the replacement metal is Dy. 18. A phosphor according to Claim 17 in which the phosphor comprises GD2O2S containing about 0.6 mol percent Tb and 1.6 mol percent Dy. 19. A phosphor according to Claim 12 having a size range from 1 to 3 microns and having a brightness in excess of 3,000 Ft.L. 20. A phosphor according to Claim 15 in which Ln is selected from lanthanum, gadolinium, yttrium, lutetium and/or mixtures and the activator is selected from europrium, terbium, thulium, praseodymium or samarium.

Inventors: GRITZ EDWARD E, GRITZ, EDWARD, E.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1985
Main classification: C09K11
IPC codes: C09K11