Patent Number: 
Section: description

The process of the present invention significantly reduces lag at high scanning speeds for high resolution imaging by reducing or eliminating contamination to the phosphor powder during processing, minimizing crystal damage and by achieving a narrower particle size distribution in which a substantial amount of the particles, and preferably substantially all, of the particles in the phosphor powder have a particle size, which is greater than 0 and less than about 5 microns, and preferably less than about 4 microns. As used herein, xe2x80x9cparticle sizexe2x80x9d is intended to be a measure of individual particles as taken in the longest dimension of the particle. Further, reference to xe2x80x9cmean particle sizexe2x80x9d is intended to mean the mean, or average, value of the particle size of all particles. In addition, in preferred embodiments of the present invention, the image sharpness is further enhanced by use of a black, infrared-absorbing substrate and/or an infrared-absorbing layer also capable of reflecting visible light with a phosphor layer to minimize back-scatter and shadowing. If a black substrate is used, the infrared-absorbing layer is optional. However, if a standard substrate is used, the infrared-absorbing layer functions to minimize back-scatter and shadowing. In preparing the phosphor screen, first a phosphor composition must be prepared. The composition is prepared by carefully weighing each of the components in the composition and combining them by any suitable method, preferably in a dry, inert atmosphere, such as in the presence of nitrogen, argon and the like. Any suitable phosphor composition may be used in the present invention if the composition includes a photoluminescent material capable of trapping electrons when exposed to radiation energy as described above. However, it is preferred that the composition be suitable for radiographic use. The base material for the phosphor composition is a compound containing an element from groups II-VI of the periodic table, preferably, the base material is strontium sulfide (SrS). The base material is preferably present in an amount of from about 90% to about 99 % by weight of the phosphor composition. The base material is mixed with first and second dopants, and at least one fusible salt. The composition is mixed by any acceptable mixing procedure preferably by using an alumina ball mill at a slow speed with a minimal number of balls to avoid grinding and to use the mill primarily to gently combine the powders. In addition, other, slow speed or gentle mixing apparatus, such as a V-blender, may be used. The composition is preferably mixed in a sealed, airtight container. Once the composition is homogeneously mixed, the composition may be loaded on boats, such as alumina or graphite boats for sintering. Preferably, graphite boats are used. The first and second dopants are rare earth elements from the lanthanide series. Preferably the first dopant is samarium or a compound containing samarium (Sm), most preferably samarium oxide (Sm2O3). The first dopant is preferably present in an amount of from about 0.0025% to about 0.1% by weight (about 25 to about 1,000 ppm). The second dopant is preferably cerium (Ce) or a cerium compound, most preferably cerium sulfide (Ce2S3) present in an amount of from about 0.0025% to about 0.2% by weight (about 25 to about 2,000 ppm). It is further preferred that the first and second dopants be combined such that the ratio of elemental samarium to elemental cerium is from about 1:5 to about 1:10, and preferably about 1:5. A fusible salt is preferably added to the phosphor composition. Preferred salts include lithium fluoride, lithium carbonate, lithium sulfide and other fusible salts having similar properties. Most preferably, the fusible salt is either lithium fluoride, lithium carbonate or a combination of these two salts. The fusible salt is preferably added in an amount of from about 0.1% to about 4% by weight of the phosphor composition. In addition to the fusible salt, other components can be added to the composition to modify the photoluminescent properties including optional fusible salts such as those listed above, other base materials or brighteners which may be used in amounts of from about it to about 5% by weight such as calcium sulfide, barium carbonate or barium sulfate. While cesium halide may be used, it is preferred that cesium halide, a prior art intensity enhancer, not be added to the composition because it may reduce emission or the sensitivity of the phosphor. Once the phosphor composition is weighed and prepared, it is loaded in a boat, such as a graphite boat and covered with a plate, such as a graphite plate. The composition is then sintered in an inert atmosphere, preferably under a nitrogen feed, to form phosphor ingots. The composition is preferably sintered at ambient pressure. Preferably, the phosphor is sintered in a furnace and a solid state reaction occurs which forms a crystal matrix. The phosphor composition is preferably first subjected to a drying phase in the furnace at a temperature of from about 100xc2x0 C. to about 300xc2x0 C., preferably from about 120xc2x0 C. to about 140xc2x0 C., to remove substantially all moisture from the phosphor composition prior to sintering the composition at a higher temperature. Failure to remove a sufficient amount of moisture prior to sintering may damage the phosphor and interfere with crystal formation due to the moisture sensitivity of the phosphor composition. It is preferred that the temperature profile and drying and sintering times for the furnace be controlled in order to better control crystal formation. The temperature preferably increases at a rate of about 5xc2x0 C./minute until the preferred drying temperature is achieved. Then the composition is held at a constant temperature for a period of about 1 to about 4 hours, preferably about 2 hours. Once the drying phase is complete, the temperature is increased at about the same rate (5xc2x0 C./minute) until an acceptable sintering temperature is achieved which is from about 1050xc2x0 C. to about 1200xc2x0 C., and preferably from about 1120xc2x0 C. to about 1130xc2x0 C. Most preferably, the sintering temperature is about 1125xc2x0 C. The phosphor is sintered at the sintering temperature for a period of preferably about 1 to about 4 hours, preferably for about 2 hours. The phosphor composition is cooled at a rate of about 5xc2x0 C./minute until the phosphor temperature is below 100xc2x0 C., and preferably below about 70xc2x0 C., thereby forming phosphor ingots. After firing, the phosphor crystals form, and, from this point forward, great care must be taken to ensure minimum physical damage and reduce possible contamination to the crystals to achieve a high resolution phosphor screen. The ingots are ground to form a powder wherein a substantial amount, and preferably substantially all, of the particles in the powder to have a particle size greater than 0 and less than about 5 microns. The ingots are preferably first carefully broken into pieces small enough to feed into the grinding apparatus with a clean, dry implement, which is preferably non-metallic, such as a mortar and pestle or similar tool. In the preferred embodiment of the method of the present invention, the ingots, preferably after being broken as described above, must then be ground to fine powder. However, in order to obtain a suitable particle size for feeding into the throat of a fluid energy mill for grinding to a fine particle size, as described below, the powder should preferably be initially ground in a first grinding process to break up larger pieces of the ingots to provide substantially all of the particles in the powder with a particle size of greater than 0 and no greater than about 250 microns. A suitable mill for such preliminary grinding is a Brinkman ZM1 centrifugal mill. The preliminarily ground particles from the first grinding process are fed into the feed throat of a fluid energy mill in order to avoid contamination and damage to the crystals. The fluid energy mill, or a similar apparatus, such as an air-driven apparatus, prevents direct contamination, for example, from metallic parts, and significantly reduces shear damage to the particles caused by typical grinding mills, by forcing the particles to collide with each other by air pressure. The particles collide at a high rate of speed and are broken down into very fine particles. A suitable fluid energy mill for the second grinding process is an Alpine AFG Model 100, available from Germany. In the second grinding process in the fluid energy mill, the powder is preferably ground at a high speed, from about 6,000 to about 16,000 rpm, and more preferably from about 14,000 to about 16,000 rpm. The second grinding process continues until a substantial amount, and preferably, substantially all, of the particles in the powder have a particle size of greater than 0 and less than about 5 microns, and preferably until all of the particles have a particle size of greater than 0 and less than about 4 microns. Preferably, the particles are ground until at least 60%, more preferably 80%, or even 90%, of the particles have a particle size of greater than 0 and less than about 5 microns. Preferably, the mean particle size of the ground powder is greater than 0 but no greater than about 3 microns. In the preferred method, etching is not performed on the ground powder as it is not necessary due to the fluid energy mill grinding procedures of the present invention which minimize shear damage to the crystals. Once the ingots have been ground, the crystals must be reactivated to remove any residual crystal damage from the grinding process. Because mechanical damage is minimal in this process, the reactivation step is more efficient than in prior art processes. During reactivation, by using a lower optimal reactivation temperature for a longer period of time, the reactivation procedure will contribute substantially to developing an optimal particle size distribution and will improve phosphor sensitivity and significantly reduce lag. The growth rate and formation of the crystals is dependent upon the reactivation temperature. In prior processes for forming phosphor powders, this temperature is believed to be most beneficial when it is as high as possible but below the sintering temperature, in order to produce large crystals and fuse smaller crystal grains forming larger grains. However, as described below, applicants preferred method includes maintaining a low reactivation temperature for a longer period of time in order to control the crystal grain growth. The growth rate thereby is decreased making control of the growth process possible in order to achieve smaller crystals in accordance with the present invention which contribute to reducing lag. Preferably, the powder is reactivated in a two-step heating procedure in an inert atmosphere. The temperature is preferably gradually increased at rates such as those used in the sintering procedure. The powder is first dried at a temperature of about 100xc2x0 C. to about 300xc2x0 C., preferably from about 120xc2x0 C. to about 140xc2x0 C. to remove substantially all moisture from the powder. The powder is then heated below the sintering temperature, preferably at a temperature from about 500xc2x0 C. to about 550xc2x0 C., more preferably from about 525xc2x0 C. to about 550xc2x0 C. for a period of time sufficient to achieve the preferred particle size distribution ranges as shown in Table 1 below. Preferably, the powder is heated for a period of from about 3.5 to about 4.5 hours, and more preferably for about 4 hours, and then cooled to room temperature. Preferably, the particle size distribution is such that particles from 5-8 microns approach 0 and particles from 0-1 micron are present in an amount of 44%, particles from 1-2 are present in an amount of 12% and particles from 2-5 are present in an amount of 44% in order to achieve a mean particle size which is no greater than 3 microns. It should be understood that the above particle size distribution of Table 1 is preferred, and other particle size distributions are within the scope of the invention provided a substantial amount of the particles, preferably at least 60%, and more preferably 80% or even 90% of the particles are less than 5 microns. The reactivated powder is then combined with a nonreactive organic solvent to form a suspension. The solvent may be any nonreactive organic solvent, for example, methanol, propanol, butanol, isopropyl alcohol, methylethylketone, methylene chloride, ethylene chloride, acetone, methylisobutylketone, methyl acetate, ethyl acetate, butyl acetate, dioxane, ethylene glycol monoethylether and ethylene glycol monoethyl ether and similar solvents. Most preferably, the solvent is isopropyl alcohol. Preferably, the solvent is provided in an amount sufficient to thoroughly wet the powder and to achieve at least a colloidal-type suspension. The suspension should be mixed by stirring with a non-metallic stirrer, preferably by an ultrasonic probe, to gently mix the suspension without damaging or contaminating the crystals. The mixing also aids in separating particles which may have partially fused during reactivation and in breaking up large agglomerates of powder. The suspension is then decanted to at least partially separate the solvent and the powder. Preferably the steps of forming a solvent suspension, mixing and decanting are repeated several times, as necessary, to achieve a wet powder which, when dry, has a substantial amount of particles, and preferably, substantially all of the particles, at a particle size of greater than 0 and less than about 5 microns. The suspension is preferably processed by decanting through a sieve or other separation device in order to separate agglomerated wet phosphor powder of a size greater than about 20 microns from the smaller agglomerates and particles which are collected in a tray or other container. The larger agglomerates should be further solvated and processed by stirring and then decanted again. This procedure should be repeated several times and the sieve and collection tray dried in an inert atmosphere. The larger agglomerates can be re-processed by recycling those agglomerates to the initial firing step prior to grinding as described above. The remaining fine particles in a wet or dampened state are collected and dried for producing a phosphor screen as described below. The powder may be dried in any conventional oven, however, it is preferred that the powder be dried in an inert atmosphere. The preferred method of producing a phosphor screen of the invention includes preparing a phosphor powder in which a substantial amount of the particles, and preferably substantially all of the particles, in the powder have a particle size of greater than 0 but less than about 5 microns. The powder is preferably the phosphor powder described above with respect to the phosphor powder according to the present invention. The powder is also preferably formed in accordance with the preferred method as described above. However, it should be understood based on this disclosure that other phosphor powders, for example, those using various other base materials, dopants or fusible salts such as those disclosed in U.S. Pat. Nos. 4,621,196 or 4,855,603 which are herein incorporated by reference, may be used in the method of the present invention, provided the particles are ground to achieve the preferred particle size and, more preferably, processed to produce the desired particle size distribution as described above. A binder solution is then prepared which includes a plasticizer, a solvent and a binder. The plasticizer is preferably a phthalate-based plasticizer, for example, phthalic acid ester. A suitable plasticizer is available as Santicizer(copyright) 160 from Monsanto Chemicals. The plasticizer and binder should be selected to minimize moisture due to the moisture sensitivity of the phosphor powder. However, a sulfonamide, phthalate-based compounds such as phthalic acid ester, phosphoric acid ester, trimellitates, alcohol, ether or ketone or any plasticizer having similar properties may also be used. The plasticizer may be present in various amounts depending upon the particular phosphor composition being used and the desired screen characteristics. However, optimally, the plasticizer should be from about 40% to about 60% by weight, preferably about 50% by weight, of the binder solution. The solvent may be any nonreactive, compatible, organic solvent, such as those listed above for use in the method of preparing a phosphor powder for forming the suspension of the powder after reactivation. The solvent should be provided in an amount which provides the desired Theological properties for achieving the desired phosphor coating thickness. Preferably, the solvent is methyl-ethylketone, and is present in the binder solution in an amount from about 15% to about 25% by weight, and preferably about 17% by weight of the binder solution. The binder may be any binder compatible with the plasticizer and with the phosphor powder which minimizes moisture absorption. The binder is preferably acrylic although other binders such as natural polymers, including gelatin, and other organic polymers, such as polyvinyl butyral, polyvinyl acetate, nitrocellulose, ethylcellulose, vinylidene chloride-vinyl chloride copolymer, polymethyl methacrylate, vinyl chloride-vinyl acetate copolymer, polyurethane, cellulose acetate butyrate;, polyvinyl alcohol, polyester, and polyethylene may also be used. The binder may be present in various amounts depending upon the desired coating characteristics. However, it is preferred that the binder be present in an amount of from about 25% to about 35% by weight, preferably about 30% by weight, of the binder solution. A phosphor slurry is then formed by mixing the binder solution with a solvent, a dispersant and phosphor powder to form a phosphor slurry. The slurry is preferably formed by first dissolving the dispersant in the solvent. The solvent may be selected from any of the solvents useful for forming the binder solution and may be the same or different from the solvent used for forming the binder solution, provided, the solvents are compatible with each other and with the remaining components. Preferably, the solvent in the phosphor slurry is the same as that of the binder solution. More preferably the solvent in the slurry is methylethylketone. The dispersant may be any suitable dispersant, for example, a polymeric dispersant. Preferably the dispersant is Hypermer(copyright), or KD-1(copyright) available from ICI Americas, Inc. The phosphor powder is added gradually while ultrasonically mixing the solvent and the dispersant at a slow speed. The phosphor powder should be added very gradually to the solution in order to prevent harming the crystals and forming agglomerates. The binder solution is then added gradually to the combined phosphor powder and solution of dispersant and solvent to form the slurry. While adding the binder and phosphor powder to the slurry, it is preferred that the slurry be continuously mixed by an ultrasonic probe or other non-metallic, low-shear mixing apparatus to avoid contamination and to avoid crystal damage. The components of the slurry may be varied in amounts to achieve preferred Theological properties for coating the slurry on a substrate. However, it is preferred that the binder solution is present in an amount of from about 10% to about 20% by weight, preferably about 15% by weight of the total slurry composition. The solvent is preferably present in an amount of from about 15% to about 35% by weight, preferably about 30% by weight of the slurry composition. In addition, the slurry composition preferably also comprises from about 0.5% to about 1.5% by weight dispersant, more preferably about 1% by weight dispersant, and from about 60% to about 85% by weight, more preferably about 75% by weight of the phosphor powder. The phosphor slurry is then coated on a black infrared-absorbing substrate to form a phosphor screen. The substrate may be any suitable polymeric substrate having infrared-absorbing capacity, but is preferably formed of a polycarbonate material, such as LEXAN(copyright) available from GE Plastics. The substrate is preferably flexible such that it may be wrapped around objects such as piping or other equipment for infrared imaging and other radiographic applications. By using an infrared-absorbing substrate, infrared light passing through the phosphor layer is not reflected back such that the scattering effect is minimized. Such a scattering effect is generally caused by infrared light which reflects off of the substrate causing further activation of the phosphor particles in the coating to create back-scatter, or a shadowing effect, in the illuminated image as described in the Background section above. Use of the infrared-absorbing substrate helps eliminate scattering to achieve a sharper image. While it is preferred that a black-infrared absorbing substrate as described above is used for forming a phosphor screen according to the present invention, if an infrated-absorbing layer, as described below, is provided between the substrate and the phosphor layer, any flexible substrate suitable for radiographic applications may be used. The screen should preferably be cured in an inert atmosphere such as under a nitrogen flow in order to evaporate a substantial portion of the solvent in the slurry prior to heating. The cured screen is heated to dry the slurry on the substrate at temperatures from about 70xc2x0 C. to about 105xc2x0 C. The screen may also be allowed to dry at room temperature. Once the screen is dried, it may be cut to the size and shape suitable for a particular application. In a preferred embodiment of the method of making a phosphor screen according to the present invention, the substrate is coated with an infrared-absorbing layer prior to coating with the phosphor slurry. Preferably, the infrared-absorbing layer is also capable of reflecting visible light such that visible light generated within the phosphor layer which may otherwise be diminished due to scattering within the phosphor layer is reflected back to improve luminescence intensity. The infrared-absorbing layer preferably includes a binder, such as the binder used in forming the phosphor slurry, and an infrared-absorbing compound, which may be any infrared-absorbing compound, including dyes and ytterbium (Yb)-containing compounds such as ytterbium oxide (Yb2O3), and any other filler components which do not otherwise affect the infrared-absorbing properties of the layer. Preferably the compound also contributes to reflection of visible light. The infrared-absorbing layer may also include a solvent such as the solvents used in forming the phosphor coating. The infrared-absorbing layer of the present invention, preferably reflects visible light and absorbs infrared light such that scattering of light by particles in the coating and possible reflection by the substrate causing shadowing is minimized. Further, in a preferred embodiment in which a black, infrared-absorbing substrate is used, any infrared light which may pass through the infrared-absorbing layer is also absorbed by the substrate and back-scattering is substantially eliminated. Further, visible light is reflected by the infrared-absorbing layer to improve intensity. If such a black, infrared-absorbing substrate is used, however, the infrared-absorbing layer is optional. Another alternative structure within the scope of the invention includes use of a white or visible light-reflective substrate which is coated or otherwise treated with one or more infrared-absorbing dyes such that the substrate is capable of reflecting visible light to improve intensity, and the dyes function to absorb infrared radiation which could otherwise contribute to back-scatter. Any suitable infrared-absorbing dye may be used. In a preferred embodiment of the present invention, a protective transparent overcoat may be provided over the phosphor coating on the screen. The overcoat should be formed of a material which does not interfere with the passage of infrared or other radiation energy through the phosphor screen. Preferably, the overcoat is formed of an acrylic-based material or similar protective coating materials. A suitable overcoat material is Aclar(copyright). Such an overcoat layer is preferred for preventing absorption of excessive moisture by the phosphor screen and preventing damage to the phosphor coating. The phosphor screen according to the present invention preferably includes a black, infrared-absorbing substrate, such as the substrate described above in the method for producing a phosphor screen. However, if an infrared-absorbing layer is provided, any suitable substrate for forming a phosphor screen may be used. A phosphor layer is coated on the substrate by any suitable coating method. The phosphor layer comprises a phosphor powder in which a substantial amount, and preferably substantially all, of the particles have a particle size of greater than 0 and less than about 5 microns, and preferably less than 4 microns. The phosphor layer may include a phosphor powder, binder, dispersant and other components such as those described above with respect to the methods of the present invention. However, it should be understood, based on this disclosure, that other phosphor layer components which would not affect the image sharpness or high-resolution capabilities of the present invention may be substituted or added to the phosphor layer. The phosphor powder can be formed by any process in which the ingots are ground to form a powder wherein a substantial amount, and preferably substantially all, of the particles in said powder have a particle size of greater than 0 and less than about 5 microns. However, it is preferred that the phosphor powder be formed in a process which minimizes exposure to deleterious contaminants after the ingots have been sintered such that degradation is minimized or eliminated. It is further preferred that the phosphor powder be formed without mechanical grinding by a process in which air or other similar fluid means of grinding is used which minimizes crystal damage caused by shear forces, provides a more uniform particle size and a narrower particle size distribution. The phosphor powder is also preferably formed using a wet sieving procedure, and using a low temperature reactivation procedure to improve and narrow the particle size distribution, and to substantially reduce lag. It is most preferred that the phosphor powder be formed in accordance with the method described in detail above. However, it should be understood, based on this disclosure, that the invention is not limited by the precise method steps described, but also includes variations which are capable of producing particles having the preferred narrow particle size distribution, and in which a substantial amount, and preferably substantially all, of the particles have a particle size of greater than 0 and less than about 5 microns in order to provide a high resolution phosphor screen. The invention will now be described with respect to the following non-limiting examples: A phosphor composition including the composition corresponding to Example I as shown in Table 2 below for each of Examples I-III was made and sintered in an inert atmosphere under nitrogen feed at 1025xc2x0 C. in an electric furnace for a period of 60 minutes to form ingots. The ingots were broken to small pieces with a morter and pestle and then ground in a Brinkman ZM1 centrifugal mill to a particle size of less than 250 microns. To ensure that substantially all particles were less than 250 microns, the particles were dry sieved through an 80 mesh screen and those particles remaining on the screen were reprocessed in the Brinkman mill. The particles were then fed to the feed throat of an Alpine AFG Model 100 fluid energy mill and ground at 16,000 rpm at 80 psi pressure until the particle size distribution shown in Table 3 below was achieved with a mean particle size of 1.10 microns. The particles were then reactivated in a furnace at a temperature of 500xc2x0 C. for 4 hours with a drying cycle at 140xc2x0 C. The reactivated phosphor was suspended in isopropyl alcohol by stirring with an ultrasonic probe. The suspension was decanted, wet sieved and dried and the particle size determined to be as shown below in Table 4 and a mean particle size of 2.85 microns. The dried phosphor powder was then formed into a slurry containing the components in the amounts as set forth in Table 5 below. A black, flexible polymeric substrate formed of LEXAN(copyright) polycarbonate was then coated using a standard tape casting method with the phosphor slurry. The screen was dried at 75xc2x0 C. for 30 minutes and then an acrylic overcoat was applied to the phosphor layer. The screen was cut to a size of 7 inches by 10 inches using a die cutter. A phosphor composition was made in accordance with the percentage composition set forth in Table 2 above and sintered in an inert atmosphere under nitrogen feed at 1125xc2x0 C. in an electric furnace for a period of 60 minutes to form ingots. The ingots were broken to small pieces with a morter and pestle and then ground according to the process described in Example I in a Brinkman mill. The particles were then fed to the feed throat of the Alpine fluid energy mill and ground at 14,000 rpm at 75 psi pressure until the particle size distribution shown in Table 6 below was achieved with a mean particle size of 2.3 microns. The particles were then reactivated in a furnace at a temperature of 525xc2x0 C. for 6 hours with a drying cycle at 140xc2x0 C. The reactivated phosphor was suspended, decanted, sieved and dried as described in Example I and the following particle size distribution as shown in Table 7 was achieved with a mean particle size of 3.85 microns. The dried phosphor powder was then formed into a slurry containing the components in the amounts as set forth in Table 8 below. A black, flexible polymeric substrate was then coated using a standard tape casting method with the phosphor slurry. The screen was dried, coated and cut into a screen as in Example I. A phosphor composition was made in accordance with the percentage composition set forth in Table 2 above and sintered in an inert atmosphere under nitrogen feed at 1025xc2x0 C. in an electric furnace for a period of 60 minutes to form ingots in the manner of Example I. The ingots were broken to small pieces with a morter and pestle and then ground according to the process described in Example I in a Brinkman mill. The particles were then fed to the feed throat of the Alpine fluid energy mill and ground at 16,000 rpm at 80 psi pressure as in Example I until the particle size distribution shown in Table 9 below was achieved with a mean particle size of 1.10 microns. The particles were then reactivated in a furnace at a temperature of 500xc2x0 C. for 6 hours with a drying cycle at 140xc2x0 C. The reactivated phosphor was suspended, decanted, sieved and dried as described in Example I and the following particle size distribution as shown in Table 10 was achieved. The mean particle size was not calculated. The dried phosphor powder was then formed into a slurry containing the components in the amounts as set forth in Example I in Table 5 and formed into a phosphor screen in the same manner described in Example I. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined in the appended claims .