WO2004041964A1

WO2004041964A1 - Process for preparing inorganic phosphors

Info

Publication number: WO2004041964A1
Application number: PCT/GB2003/004686
Authority: WO
Inventors: Edward Robert Holland; Christopher George Harris
Original assignee: Oxonica Limited
Priority date: 2002-11-05
Filing date: 2003-10-30
Publication date: 2004-05-21
Also published as: GB0225772D0; AU2003276428A1

Abstract

A process is described for preparing phosphor particles of an inorganic material which involves the precipitation in aqueous solution of the inorganic material or a precursor thereof and, optionally, subsequently calcining the resulting precipitate wherein a comminution step is carried out at one or more of the following steps: (i) during the precipitation, (ii) after the precipitation and before recovery of the precipitate, and (iii) after calcination, if carried out, in which case it is carried out in a non-aqueous solvent.

Description

PROCESS FOR PREPARING INORGANIC PHOSPHORS

The present invention relates to phosphors, principally rare earth phosphors.

Such phosphors are known to possess high luminescent efficiency under stimulation by light, electron beam or other energy sources . As known to those skilled in the art, careful choice of the rare earth emitting centre (activator) and the host crystal lattice enables efficient luminescence to be obtained over the entire visible portion of the electromagnetic spectrum and beyond, into the ultra-violet and infra-red regions. In the case where the activator is incorporated into an inorganic crystal lattice, the material will exhibit very stable luminescence. Such materials therefore have the capability for precise rendition of colours, e.g. for use in electronic displays and fluorescent lighting. They also exhibit emission at very distinct wavelengths and are suitable as luminescent security markers. Another example of the use of these phosphors is in the detection of particulate radiation and X rays. One example of a rare earth phosphor, originally developed for use in fluorescent lighting, is europium activated yttrium oxide (Y₂O₃:Eu³⁺). More recently, it has found use as the luminescent component in a technique known as scintillation proximity assay (SPA). In the SPA technique, the luminescent material takes the form of a dispersion of particles, held in aqueous suspension. The presence of a radio-labelled species, typically a b radiation (high energy electron) source may be detected as a light output when the source is in close proximity to the luminescent particle. If the surface of the particle is modified in such a way as to bind to the radio labelled species, the light output pan be used as a signal for a diagnostic test. For example, this may be used in biochemical studies, for example enzyme and receptor interactions. In one common embodiment the luminescent particle chosen for this technique is in the form of polystyrene spheres, typically approximately 5mm in diameter, which incorporate an organic luminescent dye. The low density of polystyrene ensures that a stable suspension of particles can be maintained with ease but has the disadvantage that the luminescent efficiency of the particles is low. Moreover, the luminescent efficiency diminishes during use and on exposure to ambient light during storage. Rare earth phosphors exhibit greatly enhanced luminescent efficiency as a b radiation detector in comparison to organic dyes and, in addition, are inherently stable in use and during storage. Commercially available rare earth phosphors, in particular Y₂O₃:Eu³⁺, have been demonstrated for use in the SPA technique. However, due to the high density of the Y₂O₃ crystal lattice in comparison to polystyrene and the wide range of particle sizes within the commercial phosphor powder, thematerial shows poor stability in aqueous suspension. There is, therefore, a requirement for rare earth phosphors, of well defined small particle size that could offer enhanced stability in aqueous suspension, whilst providing benefit to the SPA technique in the form of high luminescence efficiency without degradation in use or storage.

The technique of ink jet printing is applicable to printing of suspensions of solid materials, such as rare earth phosphors from aqueous and non-aqueous ink formulations. The printing technique involves supply and subsequent issue of ink f om nozzles contained within a printing head. There is necessarily a requirement that any particulate material, such as a phosphor, incorporated into the ink does not block the print head nozzles. In practice this requires careful control of the particle size distribution of material and it is generally accepted that there is a maximum acceptable particle size for the solid material of approximately 1 mm. Furthermore, it is important that the ink formulation is a stable and homogeneous dispersion, since settling and sedimentation could lead to blockage of the ink supply to the print head. These two requirements are aided by control of the particle size of phosphor powders, but the formulation process may involve coating the phosphor with a surface modifying agent e.g. in order to improve compatibility of the material with non aqueous liquids. Use may also be made of surfactants and or pH control in order to prevent agglomeration of particles and thus obtain a stable dispersion. It is usually desirable to incorporate a binder material, typically a polymer or resin in order to impart robustness to the print finish.

In general, therefore, there is a requirement for rare earth phosphor materials of high luminescent efficiency and suited to formulation of stable suspensions in aqueous or non aqueous liquid media. In our WO036050A we describe a process for preparing such phosphors which comprises : preparing an aqueous solution of salts of the host ion and of the dopant ion and a water soluble compound which decomposes under the reaction conditions, to convert said salts into hydroxycarbonate, heating the solution so as to cause said compound to decompose, recovering the resulting precipitate and calcining it at a temperature of at least 500 °C. This process has been expanded to cover the preparation of ternary oxides as disclosed in WO036051A and other dopants and hosts in WO 0190275. Further, the process has been modified by starting with oxides as disclosed in WO 071637A.

This is an example of a process whereby the phosphor, or a precursor of it, is obtained by precipitation from aqueous solution. It has now been found, according to the present invention, that such a process can be improved to provide enhanced stability of a suspension of the phosphor in a liquid medium, for example water, while at the same time improving the luminescence efficiency of the particles, by carrying out one or more of 3 steps. The luminescent efficiency of the resulting phosphors can be at least

75% of that exhibited by standard bulk material. Accordingly, the present invention provides a process for preparing phosphor particles of an inorganic material which involves the precipitation in aqueous solution of the inorganic material or a precursor thereof and, optionally, subsequently calcining the resulting precipitate wherein a comminution step is carried out at one or more of the following steps: (i) during the precipitation, (ii) after the precipitation and before the recovery of the precipitate and (iii) after calcination, if carried out, in which case it is carried out in a non-aqueous solvent.

The process of the present invention has particular applicability for preparing phosphor particles of a host oxide doped with a rare earth or thorium, titanium, silicon, bismuth, copper, silver, tungsten, chromium or manganese which comprises: (a) either preparing an aqueous solution of salts of the host ion or ions and of the dopant ion, or preparing an aqueous solution of oxides of the host ion or ions and of the dopant ion with sufficient strong acid to cause dissolution, the dopant typically being present in an amount to provide a molar concentration of 1 to 10%,

(b) incorporating therein a water soluble compound which decomposes under the reaction conditions, typically urea, ammonium bicarbonate or ammonium carbonate, to provide a base and convert said salts or oxides to hydroxycarbonate,

(c) heating the solution, preferably in a sealed vessel, so as to cause said compound to decompose, typically to a temperature of 70 °C to 100°C,

(d) recovering the resulting precipitate and (e) calcining it generally in air or a reducing atmosphere, and which comprises one or more of the following features:

(i) simultaneously with the precipitation in step (c) and/or (ii) before step (d) subjecting the solution to a comminution step, and or (iii) subjecting the product of step (e) to comminution in a non-aqueous solvent.

The process of the present invention is therefore applicable to all the phosphors of the aforementioned patent specifications to which further reference should be made for details of the host materials and dopants. Thus as disclosed in WO036050A there may be a host oxide doped with a rare earth or manganese while in WO036051 A the particle has the formula

Z_zX_xO _y : RE

where Z is a metal of valency b, X is a metal or metalloid, y = 3 -z + x -a where a = valency of X

2 2 and RE is terbium, europium, cerium, thulium, samarium, holmium, erbium, dysprosium, praseodymium while in WO 0190275 the nature of the dopant ion is extended to thorium, titanium, silicon, bismuth, copper, silver, tungsten and chromium for a simple salt while covering the preparation of ternary particles of the formula (Z_r ^l Z_s ² )_z O_y :RE where Z ^l and Z ² are two different elements of the host oxide and r+s=l, 2y=a.z where a is the overall valence of Z -¹ Z_s ² and RE represents a dopant ion of a rare earth, manganese, bismuth, copper or chromium as well as Z _pX_q oxide: RE where RE is a dopant ion of a rare earth, manganese, thorium, titanium, silicon, bismuth, copper, silver, tungsten, or chromium, X is a metal or metalloid, including zinc, silicon, yttrium, vanadium and niobium, especially silicon Z is zinc, barium, calcium, cadmium, magnesium, strontium, zirconium, scandium, lanthanum, hafnium, titanium, vanadium, niobium, chromium, molybdenum, tungsten, beryllium, bismuth, indium, lutetium, lithium or lead and p and q denote the atomic proportion of Z and X respectively, or where RE is a dopant ion of a rare earth, manganese, thorium, titanium, silicon, bismuth, copper, silver, tungsten or chromium, Z is a metal or metalloid, X is gallium, tungsten, germanium, boron, vanadium, titanium, niobium, tantalum, molybdenum, chromium, zirconium, hafnium, manganese, phosphorus, copper, tin, lead or cerium and p and q denote the atomic proportion of Z and X respectively. Thus, in particular, the material are doped silicates.

Preferred phosphors include those where the host includes yttrium, gadolinium, gallium and tantalum and those where the dopant includes europium, terbium and manganese. Specific examples include Y₂O₃:Eu, Zn₂SiO₄:Mn, Y₂SiO₅:Ce, Y₂SiO₅:Tb, Y₃Al₃O,₂:Ce (YAG:Ce) YVO₄:Dy, YVO₄:Eu, YNbO₄:Bi.

Steps (a) to (e) can be carried out as described in the above mentioned patent specification.

Thus the strong acid, added typically as 10M, 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. During dissolution of the oxides, it is desirable to monitor the solution pH as the acid is added. The pH of the solution should generally be 0.5 to 5, preferably 1.5 to 5. This may be adjusted by addition of further acid or of the base, for example a hydroxide such as potassium or sodium hydroxide, to the solution.

In step (b), a water soluble compound which can decompose under the reaction conditions is added to provide a base and convert the material into hydroxycarbonate. The decomposition should take 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, with a general range from 10°C to lOO°C. Decomposition of urea starts at about 80°C. It is the temperature which largely controls the rate of decomposition.

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. 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. The resulting precipitate can readily be obtained by, for example, filtration and is then desirably washed and dried before being calcined.

The process of the present invention is also applicable to the preparation of particles of a compound of the formula:

X (YO_b) o wherein X is a rare earth metal or a metal of Group IIA, ITB, IVB or NB of the Periodic Table, or a mixture of two or more thereof, Y is a metal which forms an anion with oxygen, or a mixture of two or more thereof, and a, b and c are such that the compound is stoichiometric, as described in British application No. 0126284.9. Preferably X is either Eu, Dy, Tb, Ce, Sm, Er, Th or Pr or magnesium, calcium, zinc, bismuth, tin or lead and Y is tungsten, vanadium, molybdenum, niobium or tantalum. These generally have a particle size not exceeding 100 nm. The process is also applicable to those having the formula:

X(YO₄)₃

wherein X represents a rare earth metal or more than one rare earth metal such that the total number of rare earth atoms represents a third of the number of YO₄ ions, and Y represents tungsten, molybdenum, niobium, vanadium or tantalum, as described in British application No. 0120460.1. Preferred compounds are those where the rare earth metal is europium or terbium and Y represents tungsten.

These compounds with an oxyanion can be prepared by reacting in aqueous solution a compound containing an anion of Y with a compound containing the cation X. Generally the resulting precipitate is calcined, typically at a temperature of at least 500°C.

Thus to prepare the particles having a size not exceeding 100 nm of a compound of the formula X_a(YO )_c it is preferred to mix an aqueous solution having a basic pH of a compound containing an anion of Y, typically an alkali metal salt, and a surfactant which is an organic acid or a Lewis base, for example a phosphate such as sodium hexametaphosphate, polyvinylpyrrolidone or a vinyl carboxylic polymer such as an acrylic acid/vinyl phosphonate copolymer, with an aqueous solution of a compound containing the cation X, typically a halide. Generally precipitation is achieved by adding a non-aqueous solvent.

The compounds of formula X(YO₄)₃ are generally prepared by reacting ions of X for example by reacting a dispersion of an oxide of X in water with a hydrohalic acid with YO ions, typically as a sodium salt, in solution and recovering the resulting precipitate.

It will be appreciated that the process of the present invention involves the use of one of steps (i) to (iii) or any combination of two of them or all three steps. The process of the present invention involves the treatment of the phosphor or phosphor precursor prior to any firing/calcination. Either during (i), which is preferred, or following (ii) precipitation of the phosphor or precursor, typically a basic carbonate, it has been found that it is important to de-agglomerate and disperse the product in order to reduce agglomerates and aggregates in the final product. This can be achieved by comminution despite the fact that comminution of small particles frequently leads to agglomeration because the comminution step has the effect of bringing the particles together. This comminution step is typically conducted by ultrasonic agitation or ball milling. Ultrasonic agitation may be applied by a probe type sonicator or a non-invasive ultrasonic source, such as a bath sonicator could be used. Process periods of up to 2 hours, e.g. 20 minutes to 2 hours, are preferred. Typically 30 minutes of ultrasonic treatment is adequate, for example using a "Nibracell" horn set at a power reading of 40. These conditions are suitable to treat 50g of precursor in 500 ml of de-ionised water. Alternatively, ball milling, at a concentration similar to the above example for ultrasonic agitation can be performed, typically for a time period not exceeding 12 hours, preferably from 3 to 8 hours.

Following the precipitation and or comminution steps it is desirable to recover the precursor as a dry powder prior to calcination. This may involve use of a centrifuge or other means to separate the precursor from liquid suspension, followed by drying of the slurry, for example in an oven typically to a temperature range from 40 to 100° C or, preferably, by freeze drying the slurry. Alternatively the slurry may be re-dispersed in a non aqueous, water miscible polar-organic solvent, such as an aliphatic alcohol e.g. of 1 to 6 carbon atoms such as ethanol, generally centrifuged to remove excess liquid and then a dry powder obtained by freeze drying.

Subsequently a calcination step is usually carried out. In the past, generally a long thermal cycle has been involved including a controlled heating ramp, typically at 5 °min^"1 to the target temperature, holding at that temperature, typically for 3 hours, and then a controlled cooling. In contrast, it has now been found that finer particles can generally be obtained by placing the precursor, in, for example, a preheated oven for the prescribed time and then removed and allowed to cool rapidly in air. Thus the precursor is placed in a furnace which is preheated to the desired firing temperature. Following the prescribed firing period it is removed from the furnace and allowed to cool rapidly, typically in air at ambient temperature.

It has also been found that the choice of firing.temperature is important with regard to the phosphors' suspension stability coupled with luminescence performance such that the temperature should desirably be 1000 to 1500 °C, typically 1100 to 1300°C, such as 1150 °C, for example for 1 to 10 hours, generally 1 to 4 hours, for example about 3 hours. It will be appreciated that if too high a temperature is used the particles may fuse together, thus defeating the object of obtaining a precursor of small size. When aggregation occurs, the resultant phosphor will generally not exhibit enhanced stability.

In step (iii) calcined material is subjected to a comminution step. The idea behind this is to disperse any agglomerates that may have formed at the high temperature used for calcination. This is typically achieved by an ultrasonic treatment, although ball milling, for example, can also be used. It is carried out in a non aqueous medium, preferably a water miscible and or polar solvent such as an aliphatic alcohol, typically of 1 to 6 carbon atoms, such as isopropyl alcohol or an ester such as an alkyl carboxylate where the alkyl groups possess 1 to 6 carbon atoms. The polarity of the solvent should preferably be such as to keep the particles apart during the treatment. After the treatment the particles are desirably subjected to vacuum drying or freeze drying to prevent re-agglomeration. It has been found that this step ensures that the material can be dispersed with ease in a liquid medium without the need for any further comminution.

Prior to this step a size selection process may be included in order to remove aggregates or to select a particular fraction of the particle size distribution. This is typically achieved by a settling process whereby the supernatant liquid is decanted after a given settling time, typically up to 2 hours, for example 30 minutes to 120 minutes.

Regardless of whether the process involves step (i), step (ii) and/or step (iii) it has also been found that it is desirable for some applications to subject the resulting phosphor to a surface treatment, for example an acid or alkali wash in order to control the surface charge and hence the tendency for agglomeration when in suspension. For example a suspension on the phosphor is prepared in alkali, e.g. NaOH, KOH, NH₄OH, generally at a concentration from 0.01 to 0.1 mol l^"1. The preparation is then heated, for example to a temperature in the range 50 to 100° C, generally whilst stirring to maintain the phosphor particles in suspension. Following this, the product can be washed in de-ionised water. The comminution process in water miscible solvent and vacuum or freeze drying are then performed. A surface coating, for example a silica coating, may be applied to achieve the same or a similar effect.

The optional calcination step typically takes place in a conventional furnace in air but steam or an inert or reducing atmosphere such as nitrogen or a mixture of hydrogen or nitrogen can also be employed. The choice of atmosphere is important in the case where the oxidation state of a species within the phosphor must be maintained or modified to a desired value. For example this is necessary in the case of activators such as terbium as Tb⁴⁺ and cerium as Ce³⁺ or europium as Eu²⁺, where the species may otherwise adopt alternative oxidation states and therefore exhibit very different luminescent properties from those desired.

The material obtained will generally be crystalline so that no appreciable degradation of the luminescence efficiency should occur in use or storage. This makes the material attractive over organic dyes which have traditionally been used in scintillation proximity assays (SPA). Thus the phosphor particles obtained by the process of the present invention are particularly useful in SPA for biological/biochemical study, when prepared as a suspension in a liquid medium such as water. However the phosphors find application wherever a stable suspension of phosphors are needed, for example in ink jet or screen printing. For this purpose the phosphors are dispersed in a suitable ink formulation. This may be a water based ink or, alternatively, employ an organic solvent as the liquid medium. In the latter case it is sometimes desirable to provide the phosphor particles with a surface coating, e.g. a long chain aliphatic alcohol such a stearic acid to promote their dispersion in the organic solvent. In addition, dispersants and or surfactants may be included in the formulation to enhance the dispersion characteristics of the material and the stability of the preparation. It is common for such ink formulations to include a binder such as a polymer or resin to improve the robustness of the printing and or modify the viscosity of the ink. Suitable binders include carboxylated acrylic resins and ethylene/vinyl ester copolymers e.g. ethylene/vinyl acetate copolymers containing about 40% vinyl acetate by weight. Generally the particle size should not exceed 1 μm since otherwise blockage of the printing head may occur. Stability of the suspension is also important for a reliable ink formulation. Conventional, commercially produced rare earth inorganic phosphors generally contain particles above the strict size limit for this application and are thus prone to rapid sedimentation and blocking of the print head.

The following Examples further illustrate the present invention.

Example 1

Preparation of Y₂O₃ :Eu phosphor.

Y₂O₃ (55g) and Eu₂O₃ (2.7g) in powder form were added to 21 of deionised (Dl) water. Approximately 140 ml of concentrated hydrochloric acid was added to ensure complete dissolution of the oxides. 600g of urea were dissolved in an additional 2 1 of Dl water and the two solutions mixed together. The combined reagents in solution were heated to 85 - 90°C whilst stirring continuously; a white precipitate was formed during the heating process, ultrasonic agitation was applied, using a Nibracell probe type sonicator. The precipitate is washed 3 times in Dl water, separating each time bY centrifuge.

Ultrasonic agitation or ball milling can be used to obtain further comminution of the washed precipitate.

The precipitate was separated from the liquid used for washing and/or comminution by centrifuge. A dry powder was obtained by freeze drying. The fluid used for washing and/or comminution is water.

The precipitate was calcined by placing in a pre-heated furnace for 3 hours at a temperature of 1150° C. It was contained in an alumina crucible. After the chosen calcination period, the crucible was removed from the furnace and allowed to cool rapidly in air at room temperature.

The phosphor obtained after calcination was subjected to the following calcination step . The phosphor was added to isopropyl alcohol at a concentration of 25 - 50 gl^"1 and subjected to 30 minutes of ultrasonic agitation using a Nibracell probe sonicator. The suspension was centrifuged to permit separation from the majority of the liquid, and the remaining slurry placed in a vacuum chamber to facilitate drying. A freeze-drying process could have been performed instead.

In the case where size selection was performed by settling of a suspension of material, the comminution step was applied following this step, in order to prevent agglomeration.

The characteristics of the particles obtained were compared with bulkphosphorparticles as well as those which have been treated by the process of the present invention and then allowed to settle. Thus the supernatant liquid obtained after allowing the suspension to stand for 30 minutes, 60 minutes or 120 minutes was tested.

Figure 1 gives the settling characteristics which were obtained. It will be appreciated that the absorption figure is a measure of the proportion of particles which remain suspended. Thus, the higher the value, the better the suspension. Figure 2 shows a comparison of the cathodoluminescence of material produced according to the invention with that of a commercially available bulk phosphor of the same chemical composition.

Figure 3 gives particle size distribution for (a) material where comminution was included during the precipitation of the precursor and (b) material without this comminution step. A significant size reduction can be seen.

Claims

1. A process forpreparingphosphorparticles of aninorganic material which involves the precipitation in aqueous solution of the inorganic material or a precursor thereof and, optionally, subsequently calcining the resulting precipitate wherein a comminution step is carried out at one or more of the following steps: (i) during the precipitation, (ii) after the precipitation and before recovery of the precipitate, and (iii) after calcination, if carried out, in which case it is carried out in a non- aqueous solvent.

2. A process according to claim 1 for preparing phosphor particles of a host oxide doped with a rare earth or thorium, titanium, silicon, bismuth, copper, silver, tungsten, chromium or manganese which comprises:

(a) either preparing an aqueous solution of salts of the host ion or ions and of the dopant ion, or preparing an aqueous solution of oxides of the host ion or ions and of the dopant ion with sufficient strong acid to cause dissolution (b) incorporatingmereinawatersolublecompoundwhichdecomposesunder the reaction conditions to provide a base and convert said salts or oxides to hydroxycarbonate,

(c) heating the solution so as to cause said compound to decompose,

(d) recovering the resulting precipitate and (e) calcining it at a temperature of at least 500 °C, and which comprises one or more of the following features:

(i) simultaneously with the precipitation in step (c) and/or (ii) before step (d) subjecting the solution to a comminution step, and/or (iii) subjecting the product of step

(e) to comminution in a non-aqueous solvent.

3. A process according to claim 2 wherein at least one of the salts is a chloride.

4. A process according to claim 2 or 3 wherein the salts are formed in situ from the corresponding oxides and the corresponding acid.

5. A process according to any one of claims 2 to 4 wherein the said water soluble compound is urea, ammonium bicarbonate or ammonium carbonate.

6. A process according to claim 5 wherein in step (c) the solution is heated to a temperature of 70 to 100 °C.

7. A process according to any one of claims 2 to 6 wherein the dopant is added in an amount to provide a molar concentration of 1 to 10%.

8. A process according to any one of claims 2 to 7 wherein the heating step is carried out in a sealed vessel.

9. A process according to any one of claims 2 to 8 wherein the host ion is yttrium, gadolinium, gallium or tantalum.

10. A process according to any one of claims 2 to 9 wherein the dopant ion is europium, terbium, or manganese.

11. A process according to claim 1 for preparing phosphor particles of a compound of the formula:

wherein X is a rare earth metal or a metal of Group IIA, ILB, INB or NB of the

Periodic Table, or a mixture of two or more thereof, Y is a metal which forms an anion with oxygen, or a mixture of two or more thereof, and a, b and c are such that the compound is stoichiometric.

12. A process according to claim 11 wherein the phosphor has the formula X(YO₄)₃ wherein X represents a rare earth metal or more than one rare earth metal such that the total number of rare earth atoms represents a third of the number of YO₄ ions and Y represents tungsten, molybdenum, niobium, vanadium or tantalum.

13. A process according to claim 11 or 12 wherein X is europium or terbium.

14. A process according to any one of claims 11 to 13 wherein Y is tungsten.

15. A process according to claim 1 for preparing phosphor particles of the formula:-

Z O ^• RE

where Z is a metal of valency b, X is a metal or metalloid, y - /₂ . z + ^x/₂ .a where a is the valency of X and RE is terbium, europium, cerium, thallium, samarium, holmium, erbium, dysprosium, praseodymium, thorium, titanium, silicon, bismuth, copper, silver, tungsten or chromium.

16. A process according to claim 15 wherein X is silicon.

17. A process according to any one of the preceding claims wherein the non aqueous solvent is a water miscible polar solvent.

18. A process according to claim 17 wherein the solvent is an aliphatic alcohol.

19. A process according to claim 18 wherein the alcohol is isopropyl alcohol.

20. A process according to any one of the preceding claims wherein before the said comminution step following calcination a size selection process is carried out.

21. A process according to claim 20 wherein the size selection process is carried out by allowing the particles to settle and then decanting the supernatant liquid.

22. A process according to any one of the preceding claims wherein the liquid containing the precipitate is centrifuged and the resulting slurry freeze-dried.

23. A process according to any one of the preceding claims wherein the material resulting from comminution following calcination is vacuum dried or freeze dried.

24. A process according to any one of the preceding claims wherein the comminution is carried out using ultrasonics or ball milling.

25. A process according to any one of the preceding claims wherein calcination is carried out by subjecting the precipitate to an environment at 1000 to 1500°C without prior heating and then transferring it directly to an environment at ambient temperature.

26. A process according to any one of the preceding claims wherein the calcination takes place in air or a reducing atmosphere.

27. A process according to any one of the preceding claims wherein steps (i), (ii) and (iii) are all carried out.

28. A process according to any one of the preceding claims wherein the resulting particles are substantially monocrystalline.

29. A process according to any one of the preceding claims wherein the resulting particles are subjected to a surface coating or a surface treatment to promote their dispersion.

30. A process according to claim 1 substantially as hereinbefore described.

31. Phosphor particles wherever prepared by a process as claimed in any one of the preceding claims.

32. A suspension of phosphor particles as claimed in claim 31 dispersed in a liquid medium.

33. A suspension according to claim 32 wherein the liquid medium is water.

34. A suspension according to claim 33 which is suitable for scintillation proximity assay application.

35. A suspension according to claim 32 wherein the liquid medium is a non aqueous liquid.

36. A suspension according to claim any one of claims 31 to 35 wherein the particles are ones obtained by a process as claimed in claim 29.

37. A suspension according to any one of claims 33, 35 and 36 which includes a surfactant to improve dispersion of the phosphor.

38. A suspension according to claim 36 or 37 in the form of a printing ink.

39. A suspension according to claim 38 wherein the ink is suitable for inkjet printing apparatus.