COATED PHOSPHORS

Abstract: A process for preparing phosphor particles of a doped host oxide is described which comprises: preparing an aqueous solution of oxides of the host ion and of the dopant ion with sufficient strong acid to provide a pH less than 0.5, increasing the pH of the solution by the addition of a base comprising a Group 1A metal while maintaining the oxides in solution, adding a water soluble compound which decomposes under the reaction conditions to convert the material into hydroxy carbonate, heating the solution so as to cause said compound to decompose, recovering the resulting precipitate and calcining it at a temperature which is at least 500 DEG C.

Description: COATED PHOSPHORS The present invention relates to phosphors especially rare earth activated phosphors. Such phosphors are known to possess excellent light output and colour rendering properties and have been utilized successfully in many display technologies. One particularly successful material, europium activated yttrium oxide (Y 2 0 3 :Eu 3+ ), has shown particular promise in the field of field emission display; yttrium oxide acts as a host for the Eu 3+ or dopant ion. The successful introduction of field emitting displays is dependent upon the availability of low voltage phosphors. As the phosphor exciting electrons have a comparatively low energy (less than 2 kV) as compared to conventional phosphors and one must avoid the use of sulphur to reduce contamination, new types of material have to be used. Phosphor particles produced by conventional high temperature firing routes are typically ball milled in order to produce particles of the correct size range. This milling process leads to the formation of a region of damage around the phosphor particle known as a surface dead layer. Under low voltage electron excitation a significant proportion of the electron-hole pairs required for luminescence are generated in this region. Therefore, the production of luminescent material without a damage layer results in an improvement in the low voltage cathodoluminscent properties of the material. It has now surprisingly been found, according to the present invention, that if the phosphor particle of a doped inorganic oxide possesses a surface which is enriched with potassium or sodium, the resulting phosphor has improved performance. Accordingly, the present invention provides a phosphor particle in which the surface of the particle is enriched with a Group 1A metal such as potassium, which is preferred, sodium, lithium or caesium. It is believed that this surface layer which is enriched with the Group 1A metal acts as a passivating and confining layer on the phosphor surface. Typically, the layer which is enriched , and which is believed to be an oxide layer, is from 0.2 to 15nm, for example 0.5 to 15 nm, generally 0.5 to 10 nm, especially 0.5 to 5 nm, typically 1 to 3nm, thick, the precise value depending on the nature of the enriching ion. Thus sodium is generally present in a thicker layer than potassium; the thickness of the coating is believed to be crucial to success. It is thought that this surface layer causes binding of vacant sites to form electron hole-pairs. In other words it reduces dangling bonds and hence quenches non-radiative recombination routes . The present invention also provides a process for preparing the novel phosphors and, in particular, rare earth activated phosphors. According to the present invention there is provided a process for preparing phosphor particles of a doped host oxide which comprises: preparing an aqueous solution of oxides of the host ion and of the dopant ion with sufficient strong acid to provide a pH less than 0.5, increasing the pH of the solution by the addition of a base comprising a Group 1A metal, typically a hydroxide, while maintaining the oxides in solution, adding a water soluble compound which decomposes under the reaction conditions to convert the material into hydroxy carbonate, heating the solution so as to cause said compound to decompose, recovering the resulting precipitate and calcining it at a temperature which is at least 500°C and preferably at least a third of the Tamman temperature of the host oxide. The strong acid used is typically hydrochloric acid although other strong inorganic acids such as nitric and sulphuric acid as well as organic acids such as acetic acid can be used. In general the oxides will be insoluble at room temperature so that it is generally necessary to heat the solution in order to dissolve them. The host ion is typically a metal which is generally trivalent such as yttrium, which is preferred, gadolinium, gallium, lanthanum, lutetium and tantalum as well as aluminium or a metalloid such as bismuth. The dopant ion is generally a rare earth such as europium, terbium, cerium, thulium, dysprosium, erbium, neodymium, samarium, praseodymium and holmium as well as manganese, thorium, titanium, silicon, bismuth, copper, silver, tungsten and chromium. Typically the particles will have the formula: Z 2 0 y :RE or (Z^Z 2 ,) z O y :RE where Z is a metal or metalloid, or Z 1 and Z 2 are two different metals or metalloids such that y+s=l, of valency a such that 2y = a.z and RE is the dopant ion which is preferably of a rare metal, manganese, chromium, copper or bismuth. In addition, though, it is also possible to prepare ternary oxides, typically those having the formula Z p X q Oxide: RE, such as Z z X x O:RE, where Z has a valency b, X has a valency such that 2y=b. z+a . x, where Z is a metal or metalloid, X is a metal, metalloid or non-metal, p and q denote the atomic proportion of z and x respectively and RE is the dopant ion. Additional host ions Z (or Z 1 or Z 2 ) for the binaries include tin, indium, niobium, molybdenum, tantalum, tungsten and zinc while additional ions for the ternary oxides include zinc, barium, calcium, cadmium, magnesium, strontium, zirconium, scandium, lanthanum, hafnium, titanium, vanadium, niobium, chromium, molybdenum, tungsten, beryllium, bismuth, indium, lutetium, lithium and lead. Typically elements include aluminium, silicon, zinc, gadolinium, tungsten, germanium, boron, vanadium, titanium, niobium, ■ tantalum, molybdenum, chromium, zirconium, hafnium, manganese, phosphorus, copper, tin, lead and cerium. Once the solution of the oxides in the strong acid has been prepared, the base, for example a hydroxide such as potassium or sodium hydroxide, is added to it in order to raise the final pH of the solution to at least 0.5, generally to 0.5 to 5, typically to 1.5 to 5. The upper pH limit is unimportant provided that the oxides stay in solution. The acid solution can be achieved by adding, typically 10M, acid dropwise. Subsequently, a water soluble compound which can decompose under the reaction conditions is added to convert the material into hydroxycarbonate . The water-soluble compound which decomposes under the reaction conditions is typically urea, which is preferred, or a weak carboxylic acid such as oxalic acid or tartaric acid. The urea and other water soluble compounds slowly introduce OH " ligands into the solution until the solubility limit has been reached. When the urea decomposes it releases carbonate and hydroxide ions which control the precipitation. If this is done uniformly then particles form simultaneously at all points and growth occurs within a narrow size distribution. The reaction is carried out at elevated tempeoβture so as to decompose the water-soluble compound. For urea, the lower temperature limit is about 70°C; the upper limit of reaction is generally 100°C. The resulting precipitate can readily be obtained by, for example, filtration and is then desirably washed and dried before being calcined. It has been found that better results can generally be obtained by keeping the reaction vessel sealed. This has the effect of narrowing the size distribution of the resulting precipitate. An important feature of the process is that decomposition takes place slowly so that the compounds are not obtained substantially instantaneously as in the usual precipitation techniques. Typically for urea, the reaction is carried out at, say, 90°C for 1 to 4 hours, for example about 2 hours. Decomposition of urea starts at about 80°C. It is the temperature which largely controls the rate of decomposition. Although the particles obtained initially following the addition of the decomposable compound are monocrystalline they have a tendency to form composites or agglomerates consisting of two or more such crystals during precipitation and subsequent washing. Calcination typically takes place in a conventional furnace in air but steam or an inert or a reducing atmosphere such as nitrogen or a mixture of hydrogen and nitrogen can also be employed. It is also possible to use, for example, a rapid thermal annealer or a microwave oven. The effect of using such an atmosphere is to reduce any tendency the rare earth element may have from changing from a 3 + ion to a 4 + ion. This is particularly prone in the case of terbium and cerium as well as Mn 2+ . The use of hydrogen may also enhance the conductivity of the resulting crystals. Calcination generally requires a temperature of at least 500°C, for example 600 to 900°C such as about 650°C. However by increasing the calcination temperature the crystallite size increases and this can lead to enhanced luminescence. In general temperatures of at least 1000°C are needed for grain growth to become significant. In general the temperature required for this is at least from Va to * • -> the bulk melting point of the oxide i.e. the Tamman temperature which is typically of the order of 2500°C. Thus desirably the calcination temperature is at least 1050°C, a temperature of 1150°C being typical. In general temperatures above 1300 to 1400°C are not needed. Time also plays a part and, in general, at higher temperatures a shorter time can be used. In general the calcination is carried out at a temperature and time sufficient to produce a crystallite size of at least 35 nm, generally at least 50 nm. The time of calcination is generally from 30 minutes to 10 hours and typically from 1 hour to 5 hours, for example about 3 hours. A typical calcination treatment involves a temperature of at least 1050°C, e.g. 1150°C for 3 hours while at lower temperatures a time from 3 to 6 hours is typical. In order to augment crystallite size it is possible to incorporate flux agents which act as grain boundary promoters such as titania, bismuth oxides, silica, lithium fluoride and lithium oxide. While, in the past, using lower temperatures of calcination, crystallite sizes of the order of 20 nm were obtained it has been found, according to the present invention, that crystallite sizes of at least 50 nm are regularly obtainable. Indeed crystallite sizes as much as 200 nm can be obtained without difficulty. As the temperature of calcination increases the particles have a tendency to break up into single or monocrystalline particles. If the calcination takes place for too long there is a danger of significant crystal sintering. Obviously the particle size desired will vary depending on the particular application of the phosphors. In particular the acceleration voltage affects the size needed such that at 300 volts a crystallite size of the order of 50 nm is generally suitable. The urea or other decomposable compound should be present in an amount sufficient to convert the salts into hydroxycarbonate. This means that the mole ratio of e.g. urea to salt should generally be at least 1:1. Increasing the amount of urea tends to increase the rate at which hydroxycarbonate is formed. If it is formed too quickly the size of the resultant particles tends to increase. Better results are usually obtained if the rate of formation of the particles is relatively slow. Indeed in this way substantially monocrystalline particles can be obtained. In general the mole ratio of urea or other decomposable compound to salt is from 1:1 to 10:1, typically 2:1 to 5:1, for example about 3:1; although higher ratios, for example 15:1, may be desirable and sometimes they improve yield. The crystals generally have a size not exceeding 1 micron and typically not exceeding 300 nm, for example 50 to 150 nm. Substantially monocrystalline particles, by which are meant particles which form a single crystal although the presence of some smaller crystals dispersed in the matrix of the single crystal is not excluded, can be obtained by this means . It is believed that during the firing process the potassium or sodium contained in the precursor migrates to form what is believed to be an amorphous, oxide based, capping layer containing the sodium or potassium. In some instances it is likely that an amorphous phase of potassium and the host element oxate, such as potassium yttriate, forms. It has surprisingly been found that the presence of this surface layer can improve cathodoluminescence intensity by at least 50%. The particles of the present invention are suitable for use in FED type displays. For this purpose the particles can be embedded in a suitable plastics material by a variety of methods including dip coating, spin coating and meniscus coating or by using an air gun spray. Alternatively the particles can be applied to the plastics material to provide a coherent screen by a standard electrophoretic method. Accordingly, the present invention also provides a plastics material which incorporates particles of the present invention. Suitable polymers which can be employed include polyacrylic acid, polystyrene and polymethyl methacrylate. Such plastics materials can be used for photoluminescence applications and also in electroluminescence applications where an AC current is to be employed. If a DC current is employed then conducting polymers such as polyvinylcarbazole, polyphenylenevinylidene and polymethylphenylsilane can be employed. Poly 2- (4-biphenylyl) -5- (4-tertiarybutyl phenyl) -1, 3, 4-oxidiazole (butyl-PBD) can also be used. Desirably, the polymer should be compatible with the solvent employed, typically methanol, in coating the plastics material with the particles. Typically, the particles will be applied to a thin layer of the plastics material, typically having a thickness from 0.5 to 15 microns. The maximum concentration of particles is generally about 35% by weight with 65% by weight of polymer. There is a tendency for the polymer to crack if the concentration exceeds this value. A typical minimum concentration is about 2% by weight (98% by weight polymer) . If the concentration is reduced below this value then "holes" tend to form in the plastics material. The following Examples further illustrate the present invention: EXAMPLE 1 (i) 150 ml 35.5% HC1 is added to 150ml de-ionised water (6M solution) (ii) To this is added 30g yttria and 2.3g europia . The solution is heated to 70°C for 30 minutes to dissolve the oxides, (iii A 10M solution of KOH is then added dropwise to increase the pH of the solution to 2.25. Approximately 185ml of KOH is required. (iv) To the solution 300 ml of 13M urea solution is added and the temperature of the solution is raised to 90°C for 2 hours, (v) The precipitate thus formed is washed thoroughly and dried before calcination, in air, at 1150°C for 3 hours . (vi) This results in the formation of nanocrystalline (average grain size 70nm) Y 2 0 3 :Eu with an amorphous surface coating of lnm thickness. The accompanying Figure 1 shows the cathodoluminescence of bulk Y 2 0 3 :Eu uncoated monocrystalline Y 2 0 3 :Eu (i.e. obtained in a similar manner but without KOH) and the coated material obtained in this Example in the 300-1100V range. It can be seen that the coated material is about 50% more efficient than the uncoated material. Figure 2 shows a high resolution transmission electron microscope image of the surface of a phosphor obtained by the Example. The lattice fringes of the phosphor are clearly visible with a 1 nm amorphous coating on the surface. Figure 3 shows two energy dispersive x-ray spectra taken from (a) a bulk region and (b) the surface of a particle of this Example. A small potassium peak at 3.5 keV is visible only in the coated spectrum. EXAMPLE 2 1. Fabrication 1.1 -Set up a 5 litre beaker containing about 2 litres of water and a magnetic follower on a stirrer hotplate. 1.2 Weigh 55 grams of yttrium oxide and 1.77 grams of europium oxide and disperse in the water, stirring rapidly to avoid clumping. 1.3 Insert a pH probe into the suspension and add steadily about 275 ml of concentrated hydrochloric acid (measuring cylinder) : heat to dissolve the oxide . 1.4 Set up a 5 litre round-bottomed reaction flask with lid in a heating mantle. Add 600 grams of urea through the funnel followed by about 2 litres of water. Heat to dissolve the urea. 1.5 When the oxide has dissolved, add 10M potassium hydroxide solution slowly to adjust the pH to 2.3. The add the solution to the contents of the round- bottomed flask. Replace the funnel with a B34 stopper. Heat the mixed solutions to 95 °C or greater to decompose the urea and precipitate the oxide. This will begin to take place when the pH reaches about 5.3. Maintain the temperature itor another 2 hours and allow to cool naturally. 1.6 Allow the precipitate to settle and rack off the supernatant liquid into a beaker using an aquarium siphon. Add about 2 litres of clean water to the flask, swirl to mix thoroughly and allow the precipitate to settle. Rack off the supernatant liquid as before. Repeat the washing process 6 times in total. 1.7 Transfer the oxide slurry to a glass evaporation dish and heat gently on a hotplate to remove most of the water. Transfer the dish to an air circulation oven at 150 °C to finish the drying process. 1.8 Break up the dried cake into a fine powder using a porcelain mortar and pestle. 1.9 Calcine the precursor in a furnace at 1150°C in air for 3 hours. Properties The material formed is in the form of 70nm nanocrystals of Y 2 0 3 :Eu. The crystals are highly luminescent with a peak output in the red spectral region at 611nm. The surface of the materials is coated in a thin l-3nm amorphous layer. A high resolution transmission electron micrograph of the surface of the coated phosphor is shown in Figure 4. On comparison of the cathodoluminescence properties in the medium (700-3200V) voltage range the performance of the coated material is increased over the uncoated material by 50-100%, as shown in Figure 5. The nature of the coating has been studied by energy dispersive X-ray analysis (EDX) and secondary ion mass spectrometry (SIMS) . Both techniques reveal the presence of potassium in the coating. Figure 6 shows SIMS spectra from uncoated (a) and coated material (b) , the presence of potassium (K) is clearly shown in the second example at 40 m/z. The exact chemical form of the coating is expected to be an amorphous potassium yttriate phase. EXAMPLE 3 Coated YNbO,:Bi Fabrication 2.1 Into 2 litres of water add 28.5g of Y 2 0 3 . 2.2 Add steadily 275ml of concentrated HC1 and heat to dissolve the oxide. 2.3 In 2 litres of water dissolve 600g of urea. 2.4 When the yttria has dissolved add 10M KOH solution until the pH reaches approximately 2.3. 2.5 Add 68g NbCl 5 to the dissolved yttria solution under vigorous stirring. 2.6 Add 0.78g BiCl 3 to 1ml concentrated HC1 in a 50ml flask and dissolve. Dilute this solution with water and add to the mixed yttrium/niobium solution. 2.7 Add the urea solution and raise the temperature to 95°C for 2 hours to precipitate the precursor. 2.8 Allow the precipitate to settle and wash a number of times. 2.9 Dry the precursor and fire the resulting material at 1150°C for 3 hours. Properties The resulting material consists of 70-80nm nanocrystals of crystal phase beta fergusonite. As in Figure 4 an amorphous phase is present on the surface of the material. The resulting improvement in luminescence properties is shown in Figure 7.

Claims: CLAIMS 1. A process for preparing phosphor particles of a doped host oxide which comprises: preparing an aqueous solution of oxides of the host ion and of the dopant ion with sufficient strong acid to provide a pH less than 0.5, increasing the pH of the solution by the addition of a base comprising a Group 1A metal while maintaining the oxides in solution, -adding a water soluble compound which decomposes under the reaction conditions to convert the material into hydroxy carbonate, heating the solution so as to cause said compound to decompose, recovering the resulting precipitate and calcining it at a temperature which is at least 500°C. 2. A process according to claim 1 in which the strong acid is hydrochloric acid. 3. A process according to claim 1 or 2 in which the water soluble compound is urea. 4. A process according to any one of claims 1 to 3 in which the calcination takes place at a temperature which is at least a third of the Tamman temperature of the host oxide. 5. A process according to claim 4 in which calcination takes place at a temperature of at least 1050°C. 6. A process according to any one of the preceding claims in which the particles which are enriched have the formula Z 2 0 y :RE or (Z^Z,) z O y :PE where Z is a metal or metalloid of valency a or Z 1 and Z 2 are each independently metals or metalloids with anr overall valency a, such that 2y=a.z and RE is a dopant ion of a rare earth, manganese, chromium, copper or bismuth. 7. A process according to any one of claims 1 to 5 in which the particles which are enriched have the formula: Z p X q Oxide: RE where Z is a metal or metalloid, X is a metal, metalloid or non-metal, RE is a rare earth, manganese, thorium, titanium, silicon, bismuth, copper, silver, tungsten or chromium and p and q denote the atomic proportion of Z and X respectively. 8. A process according to any one of the preceding claims in which Z represents yttrium. 9. A process according to any one of the preceding claims in which the base is a hydroxide. 10. A process according to any one of the preceding claims in which the Group 1A metal is potassium. 11. A process for improving the performance of a doped inorganic oxide phosphor which comprises incorporating a Group 1A metal in the phosphor. 12. A process according to claim 11 in which the metal is incorporated into the surface of the phosphor. 13. A process according to claim 12 in which the metal is present in a layer 0.2 to 15nm thick on the surface of the phosphor. 14. A process according to claim 13 in which the layer is 0.5 to 5nm thick. 15. A process according to claim 13 or 14 in which the surface layer is an oxide layer. 16. A process according to any one of claims 11 to 15 in which the metal is potassium. 17. A process according to any one of claims 11 to 16 in which the phosphor is as defined in claim 5 or 6. 18. A phosphor obtainable by a process as claimed in any one of claims 1 to 17. 19. A doped inorganic oxide phosphor which comprises a Group 1A metal. 20. A phosphor according to claim 19 in which the metal is present in a surface layer of the phosphor. 21. A phosphor according to claim 20 in which the metal is present in a layer 0.2 to 15 nm thick on the surface of the phosphor. 22. A phosphor according to claim 21 in which the layer is 0.5 to lOnm thick. _23. A phosphor according to any one of claims 19 to 22 in which the phosphor is one defined in any one of claims 6 to 8. 24. A plastics material which comprises particles of a phosphor as claimed in any one of claims 18 to 23. 25. A material according to claim 24 which is 0.5 to 15 microns thick. 26. A material according to claim 24 or 25 which contains 2 to 35% by weight of the particles based on the weight of the material. 27. A material according to any one of claims 24 to 26 which is made of an electrically conducting polymer. 28. A material according to any one of claims 24 to 27 which is made of polyacrylic acid, polymethylmethacrylate or polystyrene.

Inventors: DOBSON PETER JAMES, WAKEFIELD GARETH, DOBSON, PETER, JAMES, WAKEFIELD, GARETH
Applicants: DOBSON PETER JAMES, ISIS INNOVATION, WAKEFIELD GARETH, DOBSON, PETER, JAMES, ISIS INNOVATION LIMITED, WAKEFIELD, GARETH
Application year: 2000
Main classification: C09K11
IPC codes: C01F17, C01G33, C09K11