EP1181342A1 - Phosphores revetus - Google Patents

Phosphores revetus

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Publication number
EP1181342A1
EP1181342A1 EP00935301A EP00935301A EP1181342A1 EP 1181342 A1 EP1181342 A1 EP 1181342A1 EP 00935301 A EP00935301 A EP 00935301A EP 00935301 A EP00935301 A EP 00935301A EP 1181342 A1 EP1181342 A1 EP 1181342A1
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EP
European Patent Office
Prior art keywords
phosphor
metal
process according
solution
particles
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Application number
EP00935301A
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German (de)
English (en)
Inventor
Peter James Dept. of Engineering Science Dobson
Gareth Wakefield
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Oxford University Innovation Ltd
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Oxford University Innovation Ltd
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Publication of EP1181342A1 publication Critical patent/EP1181342A1/fr
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Definitions

  • 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.
  • 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.
  • 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.
  • this surface layer which is enriched with the Group 1A metal acts as a passivating and confining layer on the phosphor surface.
  • the layer which is enriched 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.
  • 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.
  • 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.
  • 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.
  • the particles will have the formula:
  • 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
  • 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.
  • elements include aluminium, silicon, zinc, gadolinium, tungsten, germanium, boron, vanadium, titanium, niobium, ⁇ tantalum, molybdenum, chromium, zirconium, hafnium, manganese, phosphorus, copper, tin, lead and cerium.
  • the base for example a hydroxide such as potassium or sodium hydroxide
  • the base 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.
  • 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 tempeo ⁇ ture so as to decompose the water-soluble compound.
  • 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.
  • decomposition takes place slowly so that the compounds are not obtained substantially instantaneously as in the usual precipitation techniques.
  • 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.
  • 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.
  • the crystallite size increases and this can lead to enhanced luminescence.
  • temperatures of at least 1000°C are needed for grain growth to become significant.
  • 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.
  • 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.
  • 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.
  • flux agents which act as grain boundary promoters such as titania, bismuth oxides, silica, lithium fluoride and lithium oxide.
  • crystallite sizes of the order of 20 nm 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.
  • 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.
  • the particles of the present invention are suitable for use in FED type displays.
  • 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.
  • the particles can be applied to the plastics material to provide a coherent screen by a standard electrophoretic method.
  • 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.
  • 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.
  • the polymer should be compatible with the solvent employed, typically methanol, in coating the plastics material with the particles.
  • 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.
  • FIG. 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.
  • 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.
  • the exact chemical form of the coating is expected to be an amorphous potassium yttriate phase.
  • 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.

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  • Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Luminescent Compositions (AREA)
  • Inorganic Compounds Of Heavy Metals (AREA)
  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)

Abstract

Un procédé de préparation de particules de phosphore d'un oxyde hôte dopé consiste: à préparer une solution aqueuse d'oxydes de l'ion hôte et de l'ion dopant avec un acide suffisamment fort pour obtenir un pH inférieur à 0,5, à augmenter le pH de la solution par ajout d'une base comprenant un métal du groupe 1A tout en maintenant les oxydes en solution, à ajouter un composé soluble dans l'eau qui se décompose dans des conditions de réaction pour transformer la matière en hydroxy carbonate, à chauffer la solution pour provoquer la décomposition dudit composé, à récupérer le précipité résultant et à le soumettre à calcination à une température au moins égale à 500 °C.
EP00935301A 1999-05-20 2000-05-22 Phosphores revetus Withdrawn EP1181342A1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
GB9911781 1999-05-20
GBGB9911781.4A GB9911781D0 (en) 1999-05-20 1999-05-20 Process for preparing phosphor particles
PCT/GB2000/001957 WO2000071637A1 (fr) 1999-05-20 2000-05-22 Phosphores revetus

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EP1181342A1 true EP1181342A1 (fr) 2002-02-27

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GB0012377D0 (en) * 2000-05-22 2000-07-12 Isis Innovation Oxide based phosphors
DE10051242A1 (de) * 2000-10-17 2002-04-25 Philips Corp Intellectual Pty Lichtemittierende Vorrichtung mit beschichtetem Leuchtstoff
DE10307281A1 (de) * 2003-02-20 2004-09-02 Patent-Treuhand-Gesellschaft für elektrische Glühlampen mbH Beschichteter Leuchtstoff, lichtemittierende Vorrichtung mit derartigem Leuchtstoff und Verfahren zu seiner Herstellung
US7285913B2 (en) * 2003-08-29 2007-10-23 Matsushita Electric Industrial Co., Ltd. Plasma display device having blue phosphor layers with alkaline earth metal aluminate containing molybdenum or tungsten
US20060214115A1 (en) 2005-03-23 2006-09-28 General Electric Company Phosphor film, imaging assembly and inspection method
US7586252B2 (en) 2005-05-23 2009-09-08 General Electric Company Phosphor screen and imaging assembly
KR20070105102A (ko) * 2006-04-25 2007-10-30 삼성코닝 주식회사 산화 이트륨 조성물, 이의 제조 방법 및 이를 이용한 산화이트륨막의 형성 방법
KR101366491B1 (ko) 2007-08-22 2014-02-24 삼성전자주식회사 금속 하이드록시 탄산염 나노 입자가 코팅된 형광체 및그의 제조방법
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