Luminophores and core-shell luminophore precursors

Abstract

A novel type of green luminophore containing mixed rare-earth phosphates is produced from precursor particles having a mean diameter ranging from 1.5 to 15 microns; such particles have an inorganic core and a shell of a mixed lanthanum and/or cerium phosphate, optionally doped with terbium, evenly covering the inorganic core with a thickness greater than or equal to 300 nm.

Claims

1. A phosphor precursor (P) comprising particles having an average diameter ranging from 1.5 to 15 microns, the particles comprising: a mineral core based on a non-phosphor mineral material a comprising a yttrium, gadolinium or cerium oxide; and a shell based on a mixed phosphate of lanthanum and/or cerium, optionally doped with terbium, homogeneously covering the mineral core over a thickness greater than or equal to 300 nm.

2. The phosphor precursor as defined by claim 1, wherein the shell has a thickness from 0.3 to 1 micron.

3. The phosphor precursor as defined by claim 1, wherein the mixed phosphate of the shell has the formula (I) below:
La.sub.(1-x-y)Ce.sub.xTb.sub.yPO.sub.4(I) in which: x ranges from 0 to 0.95, inclusive; y ranges from 0.05 to 0.3, inclusive; and the sum (x+y) is less than or equal to 1.

4. The phosphor precursor as defined by claim 3, wherein the mixed phosphate of the shell has the formula (Ia) below:
La.sub.(1-x-y)Ce.sub.xTb.sub.yPO.sub.4(Ia) in which: x ranges from 0.1 to 0.5, inclusive; y ranges from 0.1 to 0.3, inclusive; and the sum (x+y) ranges from 0.4 to 0.6.

5. The phosphor precursor as defined by claim 1, wherein the mixed phosphate of the shell has the formula (Ib) below:
La.sub.(1-y)Tb.sub.yPO.sub.4(Ib) in which: y ranges from 0.05 to 0.3, inclusive; or has the following formula (Ic):
La.sub.(1-y)Ce.sub.yPO.sub.4(Ic) in which: y ranges from 0.01 to 0.3, inclusive.

6. The phosphor precursor as defined by claim 1, wherein the particles have a dispersion index of less than 0.6.

7. The phosphor precursor as defined by claim 1, wherein the mineral core has a specific surface area of at most 1 m.sup.2/g.

8. The phosphor precursor as defined by claim 1 wherein the mineral core of the particles has been densified by using the technique of molten salts.

9. A phosphor (L) comprising particles having an average diameter ranging from 1.5 to 15 microns, these particles comprising: a mineral core based on a non-phosphor mineral material comprising a yttrium, gadolinium or cerium oxide; and a shell based on a mixed phosphate of lanthanum and/or cerium, with the structure of lanthanum phosphate, optionally doped with terbium (LAP), homogeneously covering the mineral core, over a thickness greater than or equal to 300 nm. wherein the phosphor has a substantially identical or greater photoluminescence yield and a Tb content at least 5 wt % lower than a mixed phosphate of La and/or Ce doped with Tb having a composition, expressed by weight, of 55% La oxide, 30% Ce oxide and 15% Tb oxide.

10. A phosphor (L) as defined by claim 9, wherein the mixed phosphate of the shell has the general formula (Ia) below:
La.sub.(1-x-y)Ce.sub.xTb.sub.yPO.sub.4(Ia) in which: x ranges from 0.1 to 0.5, inclusive; y ranges from 0.1 to 0.3, inclusive; and the sum (x+y) ranges from 0.4 to 0.6.

11. A phosphor (L) as defined by claim 9, wherein the mixed phosphate of the shell has the general formula (Ib) or (Ic) below:
La.sub.(1-y)Tb.sub.yPO.sub.4(Ib) in which: y ranges from 0.05 to 0.3, inclusive; or
La.sub.(1-y)Ce.sub.yPO.sub.4(Ic) in which: y ranges from 0.01 to 0.3, inclusive.

12. A phosphor (L) as defined by claim 9, wherein the mineral core has a specific surface area of at most 1 m.sup.2/g.

13. A plasma system, a display screen or a lighting system having a green luminescence provided by a phosphor as defined by claim 9.

14. A UV excitation device, trichromatic lamp, mercury vapor trichromatic lamp, lamp for backlighting liquid crystal systems, plasma screen, xenon excitation lamp, device for excitation by light-emitting diodes or UV excitation marking system, comprising a phosphor as defined by claim 9.

15. A luminescent device having a green luminescence comprising a phosphor (L) as defined by claim 9.

16. A luminescent device as defined by claim 15, comprising a UV excitation device, trichromatic lamp, mercury vapor trichromatic lamp, lamp for backlighting liquid crystal systems, plasma screen, xenon excitation lamp, device for excitation by light-emitting diodes or UV excitation marking system.

17. A process for preparing a phosphor precursor (P) as defined by claim 1, comprising: adding, continuously and gradually with stirring, an aqueous solution (s) of soluble lanthanum and/or cerium, and optionally terbium, salts to an aqueous medium (m) having an initial pH (pH.sup.0) of 1 to 5 and initially comprising: particles (p.sup.0) based on a mineral material, in the dispersed state; and phosphate ions, maintaining the pH of the reaction medium at an approximately constant value during precipitation of the mixed phosphate, with variations in the pH of at most 0.5 pH units, whereby particles are obtained that comprise a core based on a non-phosphor mineral material, comprising a tyyrium, gadolinium or cerium oxide, deposited at the of which is a mixed phosphate of lanthanum and/or cerium, and optionally terbium, and (B) separating from the particles obtained from the reaction medium, and heat treating then at a temperature of 400 to 900 C.

18. The process as defined by claim 17, wherein the particles (p.sup.0) are particles of isotropic morphology.

19. The process as defined by claim 17, wherein the particles (p.sup.0) have an average diameter ranging from 0.5 to 14 microns.

20. The process as defined by claim 17, wherein the particles (p.sup.0) have a dispersion index of less than 0.6.

21. The process as defined by claim 18, wherein the particles (p.sup.0) are approximately spherical.

22. The process as defined by claim 17, wherein, the phosphate ions are initially present in the aqueous medium (m) in the form of ammonium phosphates.

23. The process as defined by claim 17, wherein, the phosphate ions are introduced in a stoichiometric excess into the aqueous medium (m), with an initial phosphate/(La+Ce+Tb) molar ratio greater than 1.

24. The process as defined by claim 17, further comprising maturing the reaction medium after the addition of all of the solution(s) and prior separating the particles.

25. The process as defined by claim 17, wherein the particles (p.sup.0) have a specific surface area of at most 1 m.sup.2/g.

26. A process for preparing a phosphor (L) from a precursor (P) as defined by claim 1, which comprises a step (C) in which: (C) said precursor (P) is heat-treated at a temperature above 900 C.

Description

(1) The invention is explained in greater detail by means of the examples below and the figures which show:

(2) FIG. 1: a diffraction diagram of the powder obtained in Example 3;

(3) FIG. 2: an enlargement of the diffraction diagram of the powder obtained in Example 3;

(4) FIG. 3: an SEM micrograph of the powder obtained in Example 3;

(5) FIG. 4: a TEM micrograph of a section produced by ultramicrotomy of a grain of the powder obtained in Example 3;

(6) FIG. 5: an SEM micrograph of the powder obtained in Example 4;

(7) FIG. 6: a TEM micrograph of a section produced by ultramicrotomy of a grain of the powder obtained in Example 4;

(8) FIG. 7: an SEM micrograph of the powder obtained in Example 5;

(9) FIG. 8: an emission spectrum of the phosphor obtained in Example 9; and

(10) FIG. 9: an SEM micrograph of the phosphor obtained in Example 17.

EXAMPLES

(11) In the following examples, the particles prepared have been characterized in terms of particle size, morphology and composition by the following methods.

(12) Particle Size Measurements

(13) The particle diameters were determined using a laser particle size analyser (Malvern 2000) on a sample of particles dispersed in water and subjected to ultrasound (130 W) for 1 minute 30 seconds.

(14) Electron Microscopy

(15) The transmission electron microscopy micrographs were carried out on a section (microtomy) of the particles, using a high-resolution JEOL 2010 FEG TEM microscope. The spatial resolution of the apparatus for chemical composition measurements by EDS (energy dispersion spectroscopy) was <2 nm. The correlation of the morphologies observed and the chemical compositions measured made it possible to demonstrate the core/shell structure, and to measure the thickness of the shell on the micrographs.

(16) The chemical composition measurements by EDS were carried out by X-ray diffraction analysis on micrographs produced by HAADF-STEM. The measurement corresponded to an average taken over at least two spectra. The spatial resolution for the composition was sufficient to distinguish the core and shell compositions. The contents were estimated in atomic %.

(17) X-Ray Diffraction

(18) The X-ray diffraction diagrams were produced using the K.sub. line with copper as an anticathode according to the Bragg-Brentano method. The resolution was chosen so as to be sufficient to separate the LaPO.sub.4: Ce, Tb and LaPO.sub.4 lines, preferably it was (2)<0.02.

Comparative Example

Reference Precursor According to FR 2 694 299

(19) Added over one hour to 500 ml of a phosphoric acid H.sub.3PO.sub.4 solution previously brought to pH 1.6 by addition of ammonium hydroxide and heated to 60 C., were 500 ml of a solution of rare-earth metal nitrates having an overall concentration of 1.5 mol/l and made up as follows: 0.855 mol/l of lanthanum nitrate, 0.435 mol/l of cerium nitrate and 0.21 mol/l of terbium nitrate. The phosphate/rare-earth metal molar ratio was 1.15. The pH during the precipitation was adjusted to 1.6 by addition of ammonium hydroxide.

(20) At the end of the precipitation step, the mixture was again held for 1 h at 60 C. The resulting precipitate was then easily recovered by filtration, washed with water then dried in air at 60 C., then subjected to a heat treatment of 2 h at 900 C. in air. At the end of this step a precursor was obtained of composition (La.sub.0.57Ce.sub.0.29Tb.sub.0.14)PO.sub.4.

(21) The particle size D.sub.50 was 4.5 m, with a dispersion index of 0.4.

Example 1

Synthesis of an LaPO4 Core Precursor

(22) Added over one hour to 500 ml of a phosphoric acid H.sub.3PO.sub.4 solution (1.725 mol/l) previously brought to pH 1.6 by addition of ammonium hydroxide, and heated to 60 C., were 500 ml of a lanthanum nitrate solution (1.5 mol/l). The pH during the precipitation was adjusted to 1.6 by addition of ammonium hydroxide.

(23) At the end of the precipitation step, the reaction medium was again held for 1 h at 60 C. The precipitate was then easily recovered by filtration, washed with water, then dried at 60 C. in air. The powder obtained was then subjected to a heat treatment at 900 C. in air.

(24) The product thus obtained, characterized by X-ray diffraction, was a lanthanum orthophosphate LaPO.sub.4 of monazite structure. The particle size D.sub.50 was 5.2 m, with a dispersion index of 0.6. The specific surface area of the product, measured by BET, was S.sub.BET=6 m.sup.2/g.

Example 2

Synthesis of an LaPO4 Core Precursor

(25) Added over one hour to 500 ml of a phosphoric acid H.sub.3PO.sub.4 solution (1.725 mol/l) previously brought to pH 1.9 by addition of ammonium hydroxide, and heated to 60 C., were 500 ml of a lanthanum nitrate solution (1.5 mol/l). The pH during the precipitation was adjusted to 1.9 by addition of ammonium hydroxide.

(26) At the end of the precipitation step, the reaction medium was again held for 1 h at 60 C. The precipitate was then easily recovered by filtration, washed with water, then dried at 60 C. in air. The powder obtained was then subjected to a heat treatment at 900 C. in air.

(27) The product thus obtained, characterized by X-ray diffraction, was a lanthanum orthophosphate LaPO.sub.4 of monazite structure. The particle size D.sub.50 was 4.3 m, with a dispersion index of 0.4.

(28) The powder was then calcined for 2 h at 1100 C. in air. A rare-earth phosphate of monazite phase having a particle size D.sub.50 of 4.9 m, with a dispersion index of 0.4, was then obtained. The BET specific surface area was 3 m.sup.2/g.

Example 3

Synthesis of an LaPO4LaCeTbPO4 Core/Shell Precursor

(29) In a 1 liter beaker, a solution of rare-earth nitrates (Solution A) was prepared as follows: 29.37 g of a 2.8M (d=1.678 g/l) solution of La(NO.sub.3).sub.3, 20.84 g of a 2.88M (d=1.715 g/l) solution of Ce(NO.sub.3).sub.3 and 12.38 g of a 2M (d=1.548 g/l) solution of Tb(NO.sub.3).sub.3 and 462 ml of deionized water were mixed, making a total of 0.1 mol of rare-earth nitrates, of composition (La.sub.0.49Ce.sub.0.35Tb.sub.0.16)(NO.sub.3).sub.3.

(30) Introduced into a 2 liter reactor were (Solution B) 340 ml of deionized water, to which 13.27 g of Normapur 85% H.sub.3PO.sub.4 (0.115 mol) and then 28% ammonium hydroxide NH.sub.4OH were added, to attain a pH=1.5. The solution was heated to 60 C.

(31) Next, added to the stock thus prepared, were 23.4 g of the lanthanum phosphate from Example 1. The pH was adjusted to 1.5 with 28% NH.sub.4OH. The previously prepared solution A was added with stirring to the mixture using a peristaltic pump at 10 ml/min, at temperature (60 C.) and under control of pH=1.5. The mixture obtained was matured for 1 h at 60 C.

(32) At the end of the maturing step, the solution had a milky white appearance. It was left to cool down to 30 C. and the product was drained. It was then filtered over sintered glass and washed with two volumes of water, then dried and calcined for 2 h at 900 C. in air.

(33) A rare-earth phosphate of monazite phase was then obtained having two monazite crystalline phases of separate compositions, namely LaPO.sub.4 and (La, Ce, Tb)PO.sub.4.

(34) The average particle size D.sub.50 was 9.0 m, with a dispersion index of 0.5. The morphology of the particles observed by SEM demonstrated the presence of a core/shell type structure.

(35) A TEM micrograph (FIG. 4) was taken of the resin-coated product prepared by ultramicrotomy (thickness 100 nm) and placed on a perforated membrane. The particles are seen in cross section. In this micrograph, a particle section can be seen, of which the core is dense and spherical, with an apparent diameter of 0.7 m. The apparent diameter of the cores in TEM (microtomy) may be smaller than their average diameter when the section is not an equatorial cross section of the particle. The particle had an overall diameter of around 2.7 m, i.e. an average shell thickness of 1 m. The measurement of the thickness of the shell is, on the other hand, only slightly affected by the part of the particle where the section is taken. Finally, the sea-urchin morphology of the particles is clearly observed.

(36) Table 1 gives the values, in atomic percentages, found for the elements P, La, Ce and Tb in the particle. The particle had an overall molar composition of rare-earth metals measured by EDS of La.sub.0.67Ce.sub.0.23Tb.sub.0.10, namely 11% by weight of terbium oxide (Tb.sub.4O.sub.7) relative to the sum of the rare-earth oxides.

(37) TABLE-US-00001 TABLE 1 Chemical composition of the particle Particle P La Ce Tb Spectrum 1 47.0 35.6 12.2 5.3 Spectrum 2 46.3 35.8 12.3 5.7 Average 46.7 35.7 12.3 5.5

(38) Table 2 gives the values found for the elements P, La, Ce and Tb in the core. The core had an overall molar composition of rare-earth metals measured by EDS of La.sub.0.95Ce.sub.0.04Tb.sub.0.01, namely 1% by weight of terbium oxide (Tb.sub.4O.sub.7) relative to the sum of the rare-earth oxides.

(39) TABLE-US-00002 TABLE 2 Chemical composition of the core Core P La Ce Tb Spectrum 1 47.0 51.0 1.4 0.5 Spectrum 2 46.3 50.1 2.5 1.1 Average 46.7 50.5 2.0 0.9

(40) Table 3 gives the values found for the elements P, La, Ce and Tb in the shell. The shell had an overall molar composition of rare-earth metals measured by EDS of La.sub.0.46Ce.sub.0.37Tb.sub.0.17, namely 19% by weight of terbium oxide (Tb.sub.4O.sub.7) relative to the sum of the rare-earth oxides.

(41) TABLE-US-00003 TABLE 3 Chemical composition of the shell Shell P La Ce Tb Spectrum 1 48.0 24.5 19.4 8.2 Spectrum 2 50.2 22.8 18.2 8.9 Average 49.1 23.7 18.8 8.6

(42) EDS-TEM clearly demonstrates a core/shell structure with a core that is very lightly doped with Tb, and a shell that is highly doped with Tb.

Example 4

Synthesis of an LaPO4LaCeTbPO4 Core/Shell Precursor

(43) In a 1 liter beaker, a solution of rare-earth nitrates (Solution A) was prepared as follows: 29.37 g of a 2.8M (d=1.678 g/l) solution of La(NO.sub.3).sub.3, 20.84 g of a 2.88M (d=1.715 g/l) solution of Ce(NO.sub.3).sub.3 and 12.38 g of a 2M (d=1.548 g/l) solution of Tb(NO.sub.3).sub.3 and 462 ml of deionized water were mixed, making a total of 0.1 mol of rare-earth nitrates, of composition (La.sub.0.49Ce.sub.0.35Tb.sub.0.16)(NO.sub.3).sub.3.

(44) Introduced into a 2 liter reactor were (Solution B) 340 ml of deionized water, to which 13.27 g of Normapur 85% H.sub.3PO.sub.4 (0.115 mol) then 28% ammonium hydroxide NH.sub.4OH were added, to attain a pH=1.5. The solution was heated to 60 C. Next, added to the stock thus prepared, were 23.4 g of the lanthanum phosphate from Example 2. The pH was adjusted to 1.5 with 28% NH.sub.4OH. The previously prepared solution A was added with stirring to the mixture using a peristaltic pump at 10 ml/min, at temperature (60 C.) and under control of pH=1.5. The mixture obtained was matured for 1 h at 60 C. At the end of the maturing step, the solution had a milky white appearance. It was left to cool down to 30 C. and the product was drained. It was then filtered over sintered glass and washed with two volumes of water, then dried and calcined for 2 h at 900 C. in air.

(45) A rare-earth phosphate of monazite phase having two monazite crystalline phases of separate compositions, namely LaPO.sub.4 and (La, Ce, Tb)PO.sub.4 was then obtained. The particle size D.sub.50 was 6.0 m, with a dispersion index of 0.5.

(46) By SEM observation, the product had a typical dense sea-urchin spine morphology. All the core particles were covered with the layer of LAP. A TEM micrograph (FIG. 6) was taken of the resin-coated product prepared by ultramicrotomy (thickness 100 nm) and placed on a perforated membrane. The particles are seen in cross section. In this micrograph, a particle section can be seen, of which the core is spherical, with an apparent diameter of 1.9 m. The apparent diameter of the cores in TEM (microtomy) may appear smaller than their average diameter when the section is not an equatorial cross section of the particle.

(47) The particle had an overall diameter of around 4.2 m, i.e. an average shell thickness of 1.1 m. The measurement of the thickness of the shell is, on the other hand, only slightly affected by the part of the particle where the section is taken. Finally, the sea-urchin morphology of the particles is clearly observed (see FIG. 5).

(48) The chemical composition measurements by EDS were carried out by X-ray analysis, on micrographs taken by HAADF-STEM. The contents were estimated in atomic %. The spatial resolution for the composition was <10 nm. The core was clearly identified from the shell by EDS.

(49) Table 4 gives the values found for the elements P, La, Ce and Tb in the core. The core had an overall molar composition measured by EDS of La.sub.0.98Ce.sub.0.01Tb.sub.0.01, namely 1 wt % of terbium oxide (Tb.sub.4O.sub.7) relative to the sum of the rare-earth oxides.

(50) TABLE-US-00004 TABLE 4 Chemical composition of the core Core P La Ce Tb Spectrum 1 47.1 51.8 1.1 0.1 Spectrum 2 47.6 51.3 0.3 0.8 Average 47.4 51.6 0.7 0.5

(51) Table 5 gives the values found for the elements P, La, Ce and Tb in the shell. The shell had an overall molar composition of rare-earth metals measured by EDS of La.sub.0.48Ce.sub.0.34Tb.sub.0.18, namely 20% by weight of terbium oxide (Tb.sub.4O.sub.7) relative to the sum of the rare-earth oxides.

(52) TABLE-US-00005 TABLE 5 Chemical composition of the shell Shell P La Ce Tb Spectrum 1 49.6 24.3 16.8 9.2 Spectrum 2 48.2 24.8 17.9 9.2 Average 48.9 24.6 16.9 9.2

(53) EDS-TEM clearly demonstrates a core/shell structure, with a core very lightly doped with Tb, and a shell highly doped with Tb.

Example 5

Synthesis of a LaPO4 LaCeTbPO4 Core/Shell Precursor

(54) In a 1 liter beaker, a solution of rare-earth nitrates (Solution A) was prepared as follows: 146.85 g of a 2.8M (d=1.678 g/l) solution of La(NO.sub.3).sub.3, 104.2 g of a 2.88M (d=1.715 g/l) solution of Ce(NO.sub.3).sub.3 and 61.9 g of a 2M (d=1.548 l) solution of Tb(NO.sub.3).sub.3 that were made up to 500 ml by addition of deionized water were mixed, making a total of 0.5 mol of rare-earth nitrates, of composition (La.sub.0.49Ce.sub.0.35Tb.sub.0.16)(NO.sub.3).sub.3.

(55) Introduced into a 2 liter reactor were (Solution B) 400 ml of deionized water, to which 66.35 g of Normapur 85% H.sub.3PO.sub.4 (0.115 mol) and then 28% ammonium hydroxide NH.sub.4OH were added, to attain a pH=1.5. The solution was heated to 60 C.

(56) Next, added to the stock thus prepared, were 125 g of the lanthanum phosphate LaPO.sub.4 powder from Example 1. The pH was adjusted to 1.5 with 28% Prolabo Normapur NH.sub.4OH. The previously prepared solution A was added with stirring to the mixture using a peristaltic pump at 10 ml/min, at temperature (60 C.) and under control of pH=1.5. The mixture obtained was matured for 1 h at 60 C. At the end of the maturing step, the solution had a milky white appearance. It was left to cool down to 30 C. and the product was drained. It was then filtered over sintered glass and washed with two volumes of water, then dried and calcined for 2 h at 900 C. in air. A rare-earth phosphate of monazite phase was then obtained having two monazite crystalline phases of separate compositions, namely LaPO.sub.4 and (La, Ce, Tb)PO.sub.4.

(57) The average particle size D.sub.50 was 6.2 m, with a dispersion index of 0.5. By SEM observation, the product had a typical spherical and dense cauliflower morphology. All the core particles were covered with the layer of LAP.

Example 6

Production of a Reference LAP Phosphor

(58) The precursor powder obtained in the comparative example was calcined for 2 h at 1000 C. in an Ar/H.sub.2 (1% hydrogen) atmosphere. At the end of this step an LAP phosphor was obtained. The average particle size D.sub.50 was 4.5 m, with a dispersion index of 0.4.

(59) The composition of the product was (La.sub.0.57Ce.sub.0.29Tb.sub.0.14)PO.sub.4, namely 15% by weight of terbium oxide (Tb.sub.4O.sub.7) relative to the sum of the rare-earth oxides. This corresponded to the use of 110 g of Tb.sub.4O.sub.7 per kg of final phosphor.

Example 7

Production of an LaPO4 LaCeTbPO4 Core/Shell Phosphor

(60) The precursor powder obtained in Example 3 was calcined for 2 h at 1000 C. in an Ar/H.sub.2 (1% hydrogen) atmosphere. At the end of this step a core/shell phosphor was obtained. The average particle size D.sub.50 was 8.9 m, with a dispersion index of 0.5.

Example 8

Production of an LaPO4LaCeTbPO4 Core/Shell Phosphor

(61) The precursor powder obtained in Example 4 was calcined for 2 h at 1000 C. in an Ar/H.sub.2 (1% hydrogen) atmosphere. At the end of this step a core/shell phosphor was obtained. The particle size D.sub.50 was 5.9 m, with a dispersion index of 0.5.

Example 9

Production of an LaPO4LaCeTbPO4 Core/Shell Phosphor

(62) The precursor powder obtained in Example 5 was calcined for 2 h at 1000 C. in an Ar/H.sub.2 (1% hydrogen) atmosphere. At the end of this step a core/shell phosphor was obtained. The particle size D.sub.50 was 6.3 m, with a dispersion index of 0.5.

(63) TABLE-US-00006 TABLE 6 Photoluminescence yield (PL) Mass of terbium used PL Example 6* 110 g of Tb.sub.4O.sub.7/kg of final 100 phosphor Example 7 66 g of Tb.sub.4O.sub.7/kg of final 99.8 phosphor Example 8 66 g of Tb.sub.4O.sub.7/kg of 100.1 phosphor Example 9 66 g of Tb.sub.4O.sub.7/kg of 100.3 phosphor *Comparative example

(64) The photoluminescence yield (PL) of the phosphors obtained in Examples 7, 8 and 9 according to the invention was compared with the yield for the phosphor obtained in Example 6, which was taken as a reference, having a photoluminescence yield PL=100. The measurements were carried out by integrating the emission spectrum between 450 nm and 700 nm, under excitation at 254 nm, measured on a Jobin-Yvon spectrophotometer.

(65) FIG. 8 shows an emission spectrum of the phosphor.

Example 10

Synthesis of a YBO3LaCeTbPO4 Core/Shell Precursor

(66) In a 1 liter beaker, a solution of rare-earth nitrates (Solution A) was prepared as follows: 29.37 g of a 2.8M (d=1.678 g/l) solution of La(NO.sub.3).sub.3, 20.84 g of a 2.88M (d=1.715 g/l) solution of Ce(NO.sub.3).sub.3 and 12.38 g of a 2M (d=1.548 g/l) solution of Tb(NO.sub.3).sub.3 and 462 ml of deionized water were mixed, making a total of 0.1 mol of rare-earth nitrates, of composition (La.sub.0.49Ce.sub.0.35Tb.sub.0.16)(NO.sub.3).sub.3.

(67) Introduced into a 2 liter reactor were (Solution B) 340 ml of deionized water, to which 13.27 g of Normapur 85% H.sub.3PO.sub.4 (0.115 mol) and then 28% ammonium hydroxide NH.sub.4OH were added, to attain a pH=1.5. The solution was heated to 70 C.

(68) Next, added to the stock thus prepared, were 23.4 g of an yttrium borate YBO.sub.3 of average size, by laser particle size analysis, D.sub.50=3.2 m. The pH was adjusted to 2.1 with 28% NH.sub.4OH.

(69) The previously prepared solution A was added with stirring to the mixture using a peristaltic pump at 10 ml/min, at temperature (70 C.) and under control of pH=2.1. The mixture obtained was matured for 1 h at 70 C.

(70) At the end of this time, the solution had a milky white appearance. It was left to cool down to 30 C. and the product was drained. It was then filtered over sintered glass and washed with two volumes of water, then dried and calcined for 2 h at 900 C. in air.

(71) A rare-earth phosphate of monazite phase of composition (La, Ce, Tb)PO.sub.4, deposited on a YBO.sub.3 core, was then obtained.

(72) The average size of the particles, measured by Coulter laser particle size analysis, was 5.1 m, with a dispersion index of 0.4.

Example 11

(73) This example relates to the preparation of a dense LaPO.sub.4 core.

(74) Added over one hour to 500 ml of a phosphoric acid H.sub.3PO.sub.4 solution (1.725 mol/l) previously brought to pH 1.5 by addition of ammonium hydroxide, and heated to 60 C., were 500 ml of a lanthanum nitrate solution (1.5 mol/l). The pH during the precipitation was adjusted to 1.5 by addition of ammonium hydroxide.

(75) At the end of the precipitation step, the reaction mixture was again held for 1 h at 60 C. The precipitate was then easily recovered by filtration, washed with water, then dried at 60 C. in air. The powder obtained was then subjected to a heat treatment at 900 C. in air.

(76) 170 g of this powder was carefully mixed using a Turbulat-type mixer with 1.7 g of LiF for 30 minutes. The mixture was then calcined in a reducing atmosphere (95% Ar/5% H.sub.2 mixture) for 2 h at 1000 C. The product obtained was then carefully washed hot (80 C.) in a water/nitric acid mixture.

(77) A rare-earth phosphate (LaPO.sub.4) powder of monazite phase, having a particle size D.sub.50 of 5.6 m with a dispersion index of 0.6 was thus obtained. The BET specific surface area of the product S.sub.BET was 0.6 m.sup.2/g.

Example 12

(78) This example also relates to the preparation of a dense LaPO.sub.4 core.

(79) Added over one hour to 500 ml of a phosphoric acid H.sub.3PO.sub.4 solution (1.725 mol/l) previously brought to pH 1.9 by addition of ammonium hydroxide, and heated to 60 C., were 500 ml of a lanthanum nitrate solution (1.5 mol/l). The pH during the precipitation was adjusted to 1.9 by addition of ammonium hydroxide.

(80) At the end of the precipitation step, the reaction mixture was again held for 1 h at 60 C. The precipitate was then easily recovered by filtration, washed with water, then dried at 60 C. in air. The powder obtained was then subject to a heat treatment at 900 C. in air.

(81) 170 g of this powder were carefully mixed using a Turbulat-type mixer with 34 g of NaCl for 30 minutes. The resulting mixture was then calcined at 900 C. for 4 h in air. The product obtained was then washed for 4 h with hot (80 C.) water with stirring, then filtered and dried. The powder was then deagglomerated by ball milling in a rotating jar for 8 hours.

(82) The product thus obtained, characterized by X-ray diffraction, was a lanthanum orthophosphate LaPO.sub.4 of monazite structure. The particle size D.sub.50 was 4.4 m, with a dispersion index of 0.6.

(83) The BET specific surface area of the product S.sub.BET was 0.5 m.sup.2/g.

Example 13

Synthesis of an LaPO4/LaCeTbPO4 Core/Shell Precursor on the Core of Example 11

(84) In a 1 liter beaker, a solution of rare-earth nitrates (Solution A) was prepared as follows: 117.48 g of a 2.8M (d=1.678 g/l) solution of La(NO.sub.3).sub.3, 83.36 g of a 2.88M (d=1.715 g/l) solution of Ce(NO.sub.3).sub.3 and 49.52 g of a 2M (d=1.548 g/l) solution of Tb(NO.sub.3).sub.3 and 349 ml of deionized water were mixed, making a total of 0.4 mol of rare-earth nitrates, of composition (La.sub.0.49Ce.sub.0.35Tb.sub.0.16)(NO.sub.3).sub.3.

(85) Introduced into a 2 liter reactor were (Solution B) 400 ml of deionized water, to which 53.06 g of Normapur 85% H.sub.3PO.sub.4 and then 28% ammonium hydroxide NH.sub.4OH were added, to obtain a pH=1.5. The solution was heated to 60 C. Next, added to the stock thus prepared, were 100 g of the lanthanum phosphate coming from Example 11. The pH was adjusted to 1.5 with 6 mol/l ammonium hydroxide NH.sub.4OH. The previously prepared solution A was added with stirring to the mixture using a peristaltic pump at 10 ml/min, at temperature (60 C.) and under control of pH=1.5. The mixture obtained was matured for 1 h at 60 C. At the end of the maturing, the solution had a milky white appearance. It was left to cool down to 30 C. and the product was drained. It was then filtered over sintered glass and washed with two volumes of water, then dried and calcined for 2 h at 900 C. in air.

(86) A rare-earth phosphate of monazite phase, having two monazite crystalline phases of separate compositions, namely LaPO.sub.4 and (La, Ce, Tb)PO.sub.4, was then obtained. The particle size D.sub.50 was 9.2 m, with a dispersion index of 0.5.

Example 14

Synthesis of an LaPO4/LaCeTbPO4 Core/Shell Precursor on the Core of Example 12

(87) In a 1 liter beaker, a solution of rare-earth nitrates (Solution A) was

(88) prepared as follows: 146.85 g of a 2.8M (d=1.678 g/l) solution of La(NO.sub.3).sub.3, 104.2 g of a 2.88M (d=1.715 g/l) solution of Ce(NO.sub.3).sub.3 and 61.9 g of a 2M (d=1.548 g/l) solution of Tb(NO.sub.3).sub.3 and 312 ml of deionized water were mixed, making a total 0.5 mol of rare-earth nitrates, of composition (La.sub.0.49Ce.sub.0.35Tb.sub.0.16)NO.sub.3).sub.3.

(89) Introduced into a 2 liter reactor were (Solution B) 400 ml of deionized water, to which 69.2 g of Normapur 85% H.sub.3PO.sub.4 (0.6 mol) and then 28% ammonium hydroxide NH.sub.4OH were added, to obtain a pH=1.4. The solution was heated to 60 C. Next, added to the stock thus prepared, were 83 g of the lanthanum phosphate from Example 12. The pH was adjusted to 1.5 with 6 mol/l ammonium hydroxide NH.sub.4OH. The previously prepared solution A was added with stirring to the mixture using a peristaltic pump at 10 ml/min, at temperature (60 C.) and under control of pH=1.5. The mixture obtained was matured for 1 h at 60 C. At the end of the maturing step, the solution had a milky white appearance. It was left to cool down to 30 C. and the product was drained. It was then filtered over sintered glass and washed with two volumes of water, then dried and calcined for 2 h at 900 C. in air.

(90) A rare-earth phosphate of monazite phase, having two monazite crystalline phases of separate compositions, namely LaPO.sub.4 and (La, Ce, Tb)PO.sub.4, was then obtained. The particle size D.sub.50 was 7.2 m, with a dispersion index of 0.3.

Example 15

Synthesis of an LaPO4/LaCeTbPO4 Core/Shell Phosphor from the Precursor of Example 3

(91) The precursor powder obtained in Example 3 (200 g) was then carefully mixed using a Turbulat-type mixer for 30 minutes with 1% by weight of Li.sub.2B.sub.4O.sub.7 (2 g). This mixture was then calcined in an Ar/H.sub.2 (5% hydrogen) atmosphere for 2 h at 1000 C. The product obtained was then washed with hot water, then filtered and dried. After this step, an LAP phosphor was obtained. The average particle size D.sub.50 was 7.1 m, with a dispersion index of 0.5.

(92) The composition of the product was (La.sub.0.75Ce.sub.0.18Tb.sub.0.08)PO.sub.4, i.e. 9% by weight of terbium oxide (Tb.sub.4O.sub.7) relative to the sum of the rare-earth oxides.

Example 16

Synthesis of an LaPO4/LaCeTbPO4 Core/Shell Phosphor from the Precursor of Example 13

(93) The precursor powder obtained in Example 13 (150 g) was then carefully mixed using a Turbulat-type mixer for 30 minutes with 1% by weight of Li.sub.2B.sub.4O.sub.7 (1.5 g). This mixture was then calcined in an Ar/H.sub.2 (5% hydrogen), atmosphere for 2 h at 1000 C. The product obtained was then washed with hot water, then filtered and dried. After this step, an LAP phosphor was obtained. The average particle size D.sub.50 was 7.1 m with a dispersion index of 0.5.

(94) The composition of the product was (La.sub.0.75Ce.sub.0.18Tb.sub.0.08)PO.sub.4 i.e. 9% by weight of terbium oxide (Tb.sub.4O.sub.7) relative to the sum of the rare-earth oxides.

Example 17

Synthesis of an LaPO4/LaCeTbPO4 Core/Shell Phosphor from the Precursor of Example 14

(95) The precursor powder obtained in Example 14 (150 g) was then carefully mixed using a Turbulat-type mixer for 30 minutes with 1% by weight of Li.sub.2B.sub.4O.sub.7 (1.5 g). This mixture was then calcined in an Ar/H.sub.2 (5% hydrogen) atmosphere for 2 h at 1000 C. The product obtained was then washed with hot water, then filtered and dried. After this step, an LAP phosphor was obtained. The average particle size D.sub.50 was 8.1 m with a dispersion index of 0.4.

(96) The product was subjected to a deagglomeration treatment by ball milling for 20 minutes. The average particle size D.sub.50 was then 5.7 m.

(97) The composition of the product was (La.sub.0.69Ce.sub.0.21Tb.sub.0.10)PO.sub.4, i.e. 10.8% by weight of terbium oxide (Tb.sub.4O.sub.7) relative to the sum of the rare-earth oxides.

(98) The photoluminescence yield (PL) of the phosphors obtained in Examples 7, 8 and 9 according to the invention was compared with the yield for the phosphor obtained in Example 6, which was taken as reference, with a photoluminescence yield PL=100. The measurements were carried out by integrating the emission spectrum between 450 nm and 700 nm, under excitation at 254 nm, measured on a Jobin-Yvon spectrophotometer.

(99) FIG. 9 is an SEM micrograph of the phosphor obtained.

Example 18

Synthesis of an Al2O3/LaCeTbPO4 Core/Shell Precursor

(100) In a 1 liter beaker, a solution of rare-earth nitrates (Solution A) was prepared as follows: 146.85 g of a 2.8M (d=1.678 g/l) solution of La(NO.sub.3).sub.3, 104.2 g of a 2.88M (d=1.715 g/l) solution of Ce(NO.sub.3).sub.3 and 61.9 g of a 2M (d=1.548 g/l) solution of Tb(NO.sub.3).sub.3 and 312 ml of deionized water were mixed, making a total of 0.5 mol of rare-earth nitrates, of composition (La.sub.0.49Ce.sub.0.35Tb.sub.0.16)(NO.sub.3).sub.3.

(101) Introduced into a 2 liter reactor were (Solution B) 400 ml of deionized water, to which 69.2 g of Normapur 85% H.sub.3PO.sub.4 (0.6 mol) and then 28% ammonium hydroxide NH.sub.4OH were added, to obtain a pH=1.4. The solution was heated to 60 C. Next, added to the stock thus prepared, were 34 g of alpha-alumina (-Al.sub.2O.sub.3) with a particle size D.sub.50 of 4.1 m and a BET specific surface area of 0.6 m.sup.2/g. The pH was adjusted to 1.4 with 6 mol/l ammonium hydroxide NH.sub.4OH. The previously prepared solution A was added with stirring to the mixture using a peristaltic pump at 10 ml/min, at temperature (60 C.) and under control of pH=1.4. The mixture obtained was matured for 1 h at 60 C. At the end of the maturing step, the solution had a milky white appearance. It was left to cool down to 30 C. and the product was drained. It was then filtered over sintered glass and washed with two volumes of water, then dried and calcined for 2 h at 900 C. in air.

(102) A mixed alumina/monazite-phase rare-earth phosphate compound, having two crystalline phases of separate compositions, namely Al.sub.2O.sub.3 and (La, Ce, Tb)PO.sub.4, was then obtained. The particle size D.sub.50 was 7.8 m, with a dispersion index of 0.4.

Example 19

Synthesis of an Al2O3/LaCeTbPO4 Core/Shell Phosphor from the Precursor of Example 18

(103) The precursor powder obtained in Example 18 (150 g) was then carefully mixed using a Turbulat-type mixer for 30 minutes with 1% by weight of Li.sub.2B.sub.4O.sub.7 (1.5 g). This mixture was then calcined in an Ar/H.sub.2 (5% hydrogen) atmosphere for 2 h at 1000 C. The product obtained was then washed with hot water, then filtered and dried. After this step, an LAP phosphor was obtained. The average particle size D.sub.50 was 8.1 m, with a dispersion index of 0.4.

(104) The composition of the product was the following:

(105) 0.4Al.sub.2O.sub.3/0.6 (La.sub.0.49Ce.sub.0.35Tb.sub.0.16)PO.sub.4.

Example 20

Synthesis of an YBO3LaCeTbPO4 Core/Shell Precursor

(106) In a 1 liter beaker, a solution of rare-earth nitrates (Solution A) was

(107) prepared as follows: 146.85 g of a 2.8M (d=1.678 g/l) solution of La(NO.sub.3).sub.3, 104.2 g of a 2.88M (d=1.715 g/l) solution of Ce(NO.sub.3).sub.3 and 61.9 g of a 2M (d=1.548 g/l) solution of Tb(NO.sub.3).sub.3 and 312 ml of deionized water were mixed, making a total of 0.5 mol of rare-earth nitrates, of composition (La.sub.0.49Ce.sub.0.35Tb.sub.0.16)(NO.sub.3).sub.3.

(108) Introduced into a 2 liter reactor were (Solution B) 400 ml of deionized water, to which 69.2 g of Normapur 85% H.sub.3PO.sub.4 (0.6 mol) and then 28% ammonium hydroxide NH.sub.4OH were added, to obtain a pH=1.4. The solution was heated to 60 C. Next, added to the stock thus prepared, were 48.7 g of yttrium borate YBO.sub.3 calcined beforehand at 1000 C. in air in the presence of an excess of boric acid and possessing a particle size D.sub.50 of 3.1 m and a BET specific surface area of 0.7 m.sup.2/g. The pH was adjusted to 1.6 with 6 mol/1 ammonium hydroxide NH.sub.4OH. The previously prepared solution A was added with stirring to the mixture using a peristaltic pump at 10 ml/min, at temperature (60 C.) and under control of pH=1.6. The mixture obtained was matured for 1 h at 60 C. At the end of the maturing step, the solution had a milky white appearance. It was left to cool down to 30 C. and the product was drained. It was then filtered over sintered glass and washed with two volumes of water, then dried and calcined for 2 h at 900 C. in air.

(109) A mixed yttrium borate/monazite-phase rare-earth phosphate compound, having two crystalline phases of separate compositions, namely YBO.sub.3 and (La, Ce, Tb)PO.sub.4, was then obtained. The particle size D.sub.50 was 5.2 m, with a dispersion index of 0.4.

Example 21

Formation of a YBO3/LaCeTbPO4 Core/Shell Phosphor from the Precursor of Example 20

(110) The precursor powder obtained in Example 20 (100 g) was then carefully mixed using a Turbulat-type mixer, for 30 minutes, with 1% by weight of Li.sub.2B.sub.4O.sub.7 (1 g). This mixture was then calcined in an Ar/H.sub.2 (5% hydrogen) atmosphere for 2 h at 1000 C. The product obtained was then washed with hot water, then filtered and dried. After this step, an LAP phosphor was obtained. The average particle size D.sub.50 was 8.1 m, with a dispersion index of 0.4.

(111) The composition of the product was 0.4 YBO.sub.30.6 (La.sub.0.49Ce.sub.0.35Tb.sub.0.16)PO.sub.4.

Example 22

Comparative Example

(112) The precursor powder obtained in the first example given above (comparative example) (170 g) was carefully mixed using a Turbulat-type mixer, for 30 minutes, with 1% by weight of Li.sub.2B.sub.4O.sub.7 (1.7 g). This mixture was then calcined in an Ar/H.sub.2 (5% hydrogen) atmosphere for 2 h at 1000 C. The product obtained was then washed with hot water, then filtered and dried. After this step, an LAP phosphor was obtained. The average particle size D.sub.50 was 5.1 m, with a dispersion index of 0.5.

(113) The composition of the product was (La.sub.0.5Ce.sub.0.29Tb.sub.0.14)PO.sub.4, i.e. 15% by weight of terbium oxide (Tb.sub.4O.sub.7) relative to the sum of the rare-earth oxides. This composition corresponded to the use of 110 g of Tb.sub.4O.sub.7 per kg of final phosphor.

(114) Table 7 below indicates the photoluminescence yields (PL) of the phosphors forming the subject of the above examples. The yields are compared with the yield of the phosphor obtained in Example 22, which is taken as reference, having a photoluminescence yield PL=100. The measurements were carried out by integrating the emission spectrum between 450 nm and 700 nm, under excitation at 254 nm, measured on a Jobin-Yvon spectrophotometer.

(115) TABLE-US-00007 TABLE 7 Mass of terbium used PL Example 22 110 g of Tb.sub.4O.sub.7/kg of final phosphor. 100 Example 15 66 g of Tb.sub.4O.sub.7/kg of final phosphor. 89 Example 16 66 g of Tb.sub.4O.sub.7/kg of phospor 97 Example 17 79.2 g of Tb.sub.4O.sub.7/kg of phosphor 100 Example 19 99 g of Tb.sub.4O.sub.7/kg of phosphor 103 Example 21 91 g of Tb.sub.4O.sub.7/kg of phosphor 101