FR2829770A1 - Preparing precursor component of luminophoric powder or crystals in form of aerogel comprises adding organic solvent, mixing under agitation, and eliminating solvent by drying in autoclave - Google Patents

Abstract

The precursor component is made by: (a) adding an organic solvent, in the presence of water, to the different chemical elements needed to obtain the precursor; (b) mixing under agitation at a temperature below the solvent's boiling point, preferably at ambient temperature; and (c) eliminating the solvent by drying in an autoclave at a temperature above the boiling point of the solvent. The various chemical elements are in soluble form before adding the solvent and are present in stoiometric quantities. They are chosen from zinc, silicon, manganese, barium, aluminum, magnesium, calcium and titanium and the dopes are chosen from manganese and the rare earths such as europium and praseodymium. The soluble forms of these elements are acetates, alcoxydes, oxalates and acetylacetonates. The solvent is an alcohol, preferably methanol, ethanol, isopropanol, acetone or CO2. The agitation is ultrasonic or mechanical. The precursor to a luminophore is Zn/Si/Mn(III)/O, Zn/Si/Mn(II)/O or the precursors to BaMgAl10O17:Eu<3+> or CaTiO3:Pr<3+>. Independent claims are also included for the following: (A) a precursor component of a luminophore obtained directly by the above process; (B) the preparation of a crystalline luminophore by submitting a precursor obtained as above to heat treatment at 800-1200 deg C for 30 minutes to 8 hours in a controlled atmosphere, where the process is preferably carried out at 1050 deg C for 30 minutes in a controlled atmosphere; (C) a crystalline component obtained using the above process, which is single phase with a mean particle diameter of 0.1-3 microns m, preferably 1 microns m, chosen from Zn2SiO4, BaMgAl10O17:Eu<3+>, Y2O3:Eu<3+> and CaTiO3:Pr<3+>; (D) the use of the crystalline components to form plasma screens or as a catalyst in combustion reactions of hydrocarbons, volatile organic components or oxidation of carbon monoxide into carbon dioxide; and (E) plasma screens made using the crystalline components described above.

Description

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The present invention relates to a process for manufacturing phosphor powder precursor materials.

The phosphors are materials whose main property is to emit colored light after they have absorbed energy supplied by an excitation source.

These are materials which are in the form of white powder and into which dopants can be introduced voluntarily which will give them their luminescent properties.

These dopants are either transition ions such as for example manganese and chromium, or more often rare earth ions such as for example europium and terbium.

The applications that use phosphors can be classified into four categories: light sources such as fluorescent lamps, display screens, - X-ray detectors, and all marking applications such as phosphorescent paints, markings stamps or banknotes, etc.

New display techniques such as luminescent screens, plasma screens and microdot screens have recently been the source of research into new materials that are more efficient than those currently available on the market.

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Indeed, in these panels, the addressing of each point of the screen is done in a matrix fashion and no longer by scanning technique, which requires the development of new high-performance phosphor materials and / or the improvement of performance. pre-existing phosphors.

The powders must on the one hand be as homogeneous as possible and on the other hand correspond to very precise chemical formulations, which makes it possible to obtain a very high degree of chemical purity.

Such a degree of chemical purity is difficult to obtain by the reactions in a solid medium which are conventionally used. This is why preparation processes in a liquid medium have been developed to prepare the phosphor ZnzSiOMn, a process which makes it possible to work at lower temperatures. However, the powders obtained by these techniques are not yet sufficiently homogeneous.

Very recently, gas phase processes have been developed which allow the production of small and relatively spherical particles.

These techniques combine the use of C02 in critical phase with aqueous solutions of soluble metal nitrate or acetate which leads to the formation of emulsion.

These processes, if they are of interest, only allow simple compositions to be manufactured.

More recently, Kang et al. (Material Research Bulletin (2000) vol. 35,1143-1151) described the preparation of Zn2SiO4Mn particles by pyrolysis using an aerosol expansion filter generator

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(FEAG: Filter Expansion Aerosol Generator). The particles obtained by this process have a submicrometric size and are spherical, but their purity is not total since there are impurities, in particular zinc oxide.

However, the inventors have found that by using solution chemistry, the problems mentioned above could be solved, in particular by using a process called sol-gel solution which makes it possible to prepare using chemical reagents of very high purity. readily available precise formulations in the form of mixed gel.

Also, the present invention relates to a process for preparing a precursor composition of phosphor powder, said composition being in airgel form, characterized in that it comprises the following steps: (a) addition to an organic solvent, in the presence of water, of various chemical elements necessary for obtaining said precursor, said chemical elements being in soluble form, (b) mixing with stirring at a temperature below the boiling point of the solvent, preferably at room temperature, and (c) autoclaving at a temperature above the critical temperature of the solvent.

In a particularly advantageous embodiment of the invention, the initial solution must contain all the chemical elements necessary for obtaining the precursor composition in a stoichiometric ratio.

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In another particularly advantageous embodiment of the invention, the soluble forms of the chemical elements necessary for obtaining said precursor are chosen from the group consisting of acetates, alkoxides, oxalates and acetylacetonates.

The solution obtained in step (a) can be obtained either by directly introducing the soluble forms into the organic solvent phase, or by proceeding with mixtures of organic solutions prepared beforehand.

The organic solvent phase should preferably be such that the soluble forms of the various chemical elements required to obtain the precursor are soluble in said organic solvent or else capable of forming homogeneous and stable colloidal solutions.

To increase the solubility and / or the stability of some of these soluble forms, it may optionally be necessary to add chemical compounds well known to those skilled in the art in this field.

Particularly advantageous organic solvents for carrying out the process according to the invention are alcohols, in particular methanol, ethanol, isopropanol, acetone and CO2.

In an even more advantageous embodiment of the invention, step (b) is carried out either by ultrasonic agitation or by mechanical agitation.

The process of the invention applies more particularly to the precursors of phosphors chosen from the group consisting of Zn / Si / Mn (III) / O and

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Zn/Si/Mn (II)/O et aux précurseurs de luminophores 3+ 3+ 3+ BaMgAl10O17 : Eu, Y203 : Eu et CaTi03 : Par.

Zn / Si / Mn (II) / O and the 3+ 3+ 3+ phosphor precursors BaMgAl10O17: Eu, Y203: Eu and CaTi03: Par.

At the end of these three stages, an airgel is obtained, the apparent volume of which is equal to that of the initial liquid mixture.

The airgel is in the form of a dry, amorphous, extremely divided powder, the specific surface of which is between 240 and 300 m2 / g and the chemical homogeneity of which is quite comparable to that of a liquid.

Another subject of the invention is a phosphor precursor composition in the form of an airgel, characterized in that it is obtained directly by the process of the invention.

The initial compounds used are preferably chosen from the group comprising zinc, silica, manganese, barium, aluminum and magnesium, calcium and titanium. The dopants can be chosen from the group comprising manganese and rare earths such as europium and praseodymium.

As already indicated above, the compositions obtained by the process of the invention constitute privileged precursors of phosphors. They can be used first of all for the preparation of crystalline phosphors in the form of a very fine powder, that is to say having an average particle size of the order of a micron, that is to say say advantageously between 0.1 and 3; jm, preferably equal to 1 μm and very homogeneous chemically (single-phase powder), that is to say with an almost total or total absence of non-luminescent parasitic phase.

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To obtain the phosphors from said compositions, it suffices to bring the precursor compositions to a temperature of between 800 and 1200C for 30 minutes to 8 hours, preferably at a temperature of 1050C for 30 minutes, under a controlled atmosphere.

The phosphor powders thus obtained can be advantageously used for the production of plasma screens, as catalysts in reactions for the combustion of hydrocarbons, volatile organic compounds or the oxidation of carbon monoxide to carbon dioxide.

The strongly divided state and the chemical homogeneity are at the origin of the very strong chemical reactivity of these powders. They imply the following advantages in the use of precursors in "airgel" form: (1) the heat treatments are carried out at low temperature and the reaction kinetics are very strongly accelerated, (2) the problems associated with volatilization at high temperature certain reagents are omitted: no stoichiometry adjustment as a function of the heat treatment, (3) the crystallized powder retains the state of division of the precursor without sintering phenomenon, and (4) no homogenization is necessary.

The compounds obtained by this route can be used directly, without conditioning (such as grinding, sieving, calibrating the powder, etc.), with a view to the preparation of "screen printing inks" which is the technique used in the process. plasma application manufacturing.

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The examples and the figure which follow illustrate the invention without, however, limiting it.

FIG. 1 represents the refinement by the Rietveld method of the X-ray diffraction spectrum of the crystalline compound obtained in Example 3.

Example 1: Zn / Si / Mn (III) / O airgel
1) Preparation.

Preparation of the silica precursor solution (solution A): 6.67 ml of tetramethoxysilane (TMOS) are diluted in 50 ml of methanol in the presence of 10 ml of water with ultrasonic stirring.

Preparation of the manganese precursor solution (solution B): 3.19 g of manganese (III) acetylacetonate (Mn (III) acac) are mixed with 50 ml of methanol with ultrasonic stirring.

Preparation of the zinc precursor solution (solution C): 21.49 g of zinc acetylacetonate (Zn (II) acac) are dissolved in 300 ml of methanol with ultrasonic stirring.

The three solutions A, B and C are mixed with ultrasonic stirring and a brown solution is obtained.

This latter solution is placed in the autoclave with 50 ml of additional methanol then the autoclave is closed and brought to the temperature of 260 ° C. where it is depressurized to atmospheric pressure and then cooled to ambient temperature.

At the end of this operation, a yellow solid, in the form of a carrot, with a volume equal to that of the liquid solution, ie 420 ml, is recovered and corresponds to 10.79 g.

2) Result.

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The absence of a diffraction line with a long counting time in the X-ray diffraction spectrum of the airgel demonstrates a perfectly amorphous state of the reagent leaving the autoclave.

The powder thus obtained is perfectly homogeneous from the chemical point of view on a submicron scale.

Example 2: Zn / Si / Mn (II) / O airgel
1) Preparation.

Solutions A and C are prepared as described in Example 1. A solution B 'of manganese (II) acetylacetonate (Mn (II) acac) is prepared by dissolving 2.29 g of this solid in 50 ml. of methanol with ultrasonic stirring. Solution B ′ mixed with solutions A and C, still with ultrasonic stirring.

2) Result.

A dark brown solution is obtained which is then treated as in Example 1. A core of a yellow solid with a volume of 450 ml, with a mass equal to 11.7 g is recovered.

Example 3: Preparation of the crystalline phosphor compound
1) Preparation.

After heating the airgel obtained in Example 1 at 1050 ° C. for 30 minutes in an alumina crucible under an atmosphere of pure oxygen, the pure compound Zn2SiO4 and the solid solution Zns-xMnxSiO are obtained.

2) Result.

The crystalline compound obtained has a Willemite type structure.

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The fineness of the diffraction lines and the absence of parasitic lines respectively prove the excellent crystalline state and the purity of the phase.

The refined parameters of the rhombohedral lattice (a = 13.958 Å and c = 9.343 Å in hexagonal description) are in agreement with the compound Zn1.8Mn0.2SiO4.

The result is given in Figure 1 in the appendix.

Claims (15)

1. Process for the preparation of a phosphor powder precursor composition, said composition being in airgel form, characterized in that it comprises the following steps: a) addition in an organic solvent, in the presence of water, of various chemical elements necessary for obtaining said precursor, said chemical elements being in soluble form, b) mixing with stirring at a temperature below the boiling point of the solvent, preferably at room temperature, and c) elimination of the solvent by drying in an autoclave at a temperature above the critical temperature of the solvent. 2. Method according to claim 1, characterized in that the chemical elements are present in stoichiometric amounts. 3. Method according to any one of claims 1 and 2, characterized in that the chemical elements are chosen from the group comprising zinc, silica, manganese, barium, aluminum, magnesium, calcium and titanium, and in that the dopants are chosen from the group comprising manganese and rare earths such as europium and praseodymium. <Desc / Clms Page number 11>

4. Method according to any one of claims 1 to 3, characterized in that the soluble forms of the chemical elements are chosen from the group consisting of acetates, alkoxides, oxalates and acetylacetonates. 5. Method according to any one of claims 1 to 4, characterized in that the solvent is chosen from the group comprising alcohols, preferably methanol, ethanol, isopropanol, acetone and CO 2. 6. Method according to any one of claims 1 to 5, characterized in that the agitation is ultrasonic or mechanical. 7. Process according to any one of claims 1 to 6, characterized in that a phosphor precursor selected from the group consisting of Zn / Si / Mn (III) / O, Zn / Si / Mn (II) is prepared. ! O and the precursors of BaMgAlicOn: Eu3 +, Y203: Eu3 + and CaTi03: Pur3 + 8. A phosphor precursor composition in airgel form, characterized in that it is obtained directly by the process according to any one of claims 1 to 7. 9. Process for preparing a crystalline phosphor compound characterized in that subjecting a composition according to claim 8 to a heat treatment at a temperature between 800 and 1200 C for 30 min. at 8 hours in a controlled atmosphere. <Desc / Clms Page number 12> 10. The method of claim 9, characterized in that the heat treatment is carried out at 1050 C for 30 min. under controlled atmosphere. 11. Crystalline compound obtained by the process according to any one of claims 9 and 10, characterized in that it is single-phase and that the average particle diameter is between 0.1 and 3 µm, preferably equal to 1! lm. 12. Crystalline compound according to claim 11, characterized in that it is selected from the group consisting of Zn2Si04, BaMgAl10017: Eu3 +, Y203: Eu3 + and CaTi03: Par3 +. 13. Use of crystalline compounds according to any one of claims 11 and 12 for the production of plasma screens. 14. Use of crystalline compounds according to any one of claims 11 and 12 as a catalyst in the combustion reactions of hydrocarbons, volatile organic compounds and in the reactions of oxidation of carbon monoxide to carbon dioxide. 15. Plasma screens, characterized in that they comprise crystalline compounds according to any one of claims 11 and 12.