DRY PROCESS FOR SYNTHESIS OF A PHOSPHOR BY TREATMENT UNDER A FLUORINE ATMOSPHERE

Abstract

A process for the dry synthesis of a luminophore of formula A.sub.x[BF.sub.y]:C includes a stage of providing an initial composition comprising at least two synthetic precursors and at least one chemical doping source, a stage of heating the initial composition up to a fluorination temperature under an inert atmosphere or under vacuum, a stage of treatment under a fluorine atmosphere of the composition obtained on conclusion of the heating stage and a stage of returning to ambient temperature under an inert atmosphere. A composition comprising a luminophore of formula A.sub.x[BF.sub.y]:C and obtained according to the synthetic process described above, the composition being devoid of hydrogen fluoride.

Claims

1. A process for the dry synthesis of a luminophore of formula A.sub.x[BF.sub.y]:C, A being a chemical element from Li, Na, K, Rb, Cs or a combination of at least two of these chemical elements, B being a chemical element from Si, Ge, Sn, Ti, Zr, Al, Ga, In, Sc, Hf, Y, La, Nb, Ta, Bi, Gd or a combination of at least two of these chemical elements, F being fluorine, C being a doping chemical element, x being the number of atoms of the element A and being equal to 1, 2 or 3, y being the number of atoms of the element fluorine and being equal to 5, 6 or 7, the process comprising the following stages: a) a stage of providing an initial composition, the initial composition comprising at least one first synthetic precursor A′, at least one second synthetic precursor B′ and at least one chemical doping source C′, b) a stage of heating the initial composition up to a fluorination temperature of between 200 and 550° C., the heating stage being carried out under an inert atmosphere or under vacuum, c) a stage of treatment under a fluorine atmosphere of the composition obtained on conclusion of the heating stage, comprising the following successive substages: c1) a substage of maintaining at the fluorination temperature, c2) a substage of cooling from the fluorination temperature down to a temperature of less than or equal to 150° C., d) a stage of returning to ambient temperature under an inert atmosphere the composition obtained on conclusion of the stage of treatment under a fluorine atmosphere.

2. The process as claimed in claim 1, the maintenance substage c1) being carried out for a period of time of 30 minutes to 8 hours.

3. The process as claimed in claim 1, the first synthetic precursor A′ comprising at least one chemical element chosen from Li, Na, K, Rb and Cs, preferably a halide of these elements.

4. The process as claimed in claim 3, the first synthetic precursor A′ being chosen from KBr, NaBr, KCl, NaCl.

5. The process as claimed in claim 1, the second synthetic precursor B′ comprising at least one chemical element chosen from Ge, Si and Ti.

6. The process as claimed in claim 5, the second synthetic precursor B′ comprising at least one chemical element chosen from Si and Ti.

7. The process as claimed in claim 6, the second synthetic precursor B′ being chosen from Si, Ti, SiO.sub.z and TiO.sub.z, z being equal to 1 or 2.

8. The process as claimed in claim 1, the doping source C′ being chosen from Mn, KMnO.sub.4, MnO, MnO.sub.2, MnBr.sub.2, MnF.sub.2, MnF.sub.3, MnCl.sub.3 and MnCl.sub.2, preferably being MnCl.sub.3 or MnCl.sub.2.

9. The process as claimed in claim 1, the first synthetic precursor A′ and/or the second synthetic precursor B′ and/or the doping source C′ being in the form of powders consisting of grains having a particle size of between 100 nm and 50 μm.

10. The process as claimed in claim 1, the molar ratio A′/B′ of the first synthetic precursor A′ to the second synthetic precursor B′ being between 5 and 0.5.

11. The process as claimed in claim 1, the molar ratio B′/C′ of the second synthetic precursor B′ to the chemical doping source C′ being between 50 and 5.

12. The process as claimed in claim 1, the inert atmosphere being a nitrogen atmosphere.

13. The process as claimed in claim 1, the fluorine atmosphere being obtained by flushing fluorine with a flow rate of between 10 and 100 ml per minute.

14. A composition comprising a luminophore of formula A.sub.x[BF.sub.y]:C, A being a chemical element from Li, Na, K, Rb, Cs or a combination of at least two of these chemical elements, B being a chemical element from Si, Ge, Sn, Ti, Zr, Al, Ga, In, Sc, Hf, Y, La, Nb, Ta, Bi, Gd or a combination of at least two of these chemical elements, F being fluorine, C being a doping chemical element, x being the number of atoms of the element A, y being the number of atoms of the element fluorine, the composition being devoid of hydrogen fluoride and being obtained by the method as claimed in claim 1.

15. The composition as claimed in claim 14, comprising between 0.5% and 2% by weight of doping chemical element C.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0072] Other characteristics and advantages of the invention will become apparent with the help of the description which follows, given by way of illustration and without limitation, made with regard to the appended figure and the example.

[0073] FIG. 1 illustrates the stages and substages of a process for the dry synthesis of a luminophore of formula A.sub.x[BF.sub.y]:C.

[0074] FIG. 2 is a graph corresponding to the emission spectrum of a luminophore K.sub.2SiF.sub.6:Mn.sup.4+ obtained according to example 2 or example 3 and excited at 450 nm.

[0075] FIG. 3 is a graph corresponding to the excitation spectrum of a luminophore K.sub.2SiF.sub.6:Mn.sup.4+ obtained according to example 2 or example 3 and recorded by monitoring the emission at 629 nm.

[0076] FIG. 4 is a graph corresponding to the emission spectrum of a luminophore Na.sub.2SiF.sub.6:Mn.sup.4+ obtained according to example 4 or example 5 and excited at 450 nm.

[0077] FIG. 5 is a graph corresponding to the excitation spectrum of a luminophore Na.sub.2SiF.sub.6:Mn.sup.4+ obtained according to example 4 or example 5 and recorded by monitoring the emission at 625 nm.

DETAILED DESCRIPTION OF THE INVENTION

[0078] FIG. 1 illustrates the stages and substages of a process for the dry synthesis of a luminophore of formula A.sub.x[BF.sub.y]:C.

[0079] First, a stage of providing 10 an initial composition comprising at least one first and one second synthetic precursor and at least one chemical doping source is carried out.

[0080] Secondly, a stage of heating 20 the initial composition up to a fluorination temperature of between 200 and 550° C. is carried out. This heating stage is carried out under an inert atmosphere or under vacuum.

[0081] Optionally, before the stage of heating 20 the initial composition up to a fluorination temperature of between 200 and 550° C., the initial composition or the synthetic precursors A′ and B′ and the doping source C′ can be heated to and maintained at 80° C., preferentially for at least 12 hours and under vacuum. This stage of heating to 80° C. can be carried out in a stove outside the fluorination oven or can be carried out directly in the fluorination oven. This preliminary stage makes it possible in particular to exclude the presence of water in the composition or in the synthetic precursors A′ and B′ and the doping source C′.

[0082] Thirdly, a stage of treatment 30 under a fluorine atmosphere of the composition obtained on conclusion of the heating stage is carried out, comprising a substage of maintaining 31 at a fluorination temperature for 30 minutes to 8 hours, then a substage of cooling 32 down to a temperature of less than or equal to 150° C.

[0083] Fourthly, a stage of returning 40 to ambient temperature under an inert atmosphere the composition obtained on conclusion of the stage of treatment under a fluorine atmosphere is carried out, so as to obtain a luminophore of formula A.sub.x[BF.sub.y]:C.

[0084] Optionally, fifthly, a separate and complementary process 50 is carried out on the luminophore obtained on conclusion of the stage of returning to ambient temperature, comprising a passivation stage 51 consisting of the addition of the luminophore to a solution comprising several chemical agents for several hours, then the mixing of the solution, the centrifugation of the solution and finally the washing of the luminophore and a drying stage 52 which comprises a drying under vacuum.

Example 1—Process for the Formation of K.SUB.2.SiF.SUB.6.:Mn.SUP.4+ Dynamically

[0085] 5 g of a mixture of synthetic precursors and of chemical doping source comprising potassium chloride, silicon and manganese(II) chloride are placed in a 125 ml bowl made of zirconium oxide. Absolute ethanol and zirconium oxide beads are added to the bowl. The grinding of this mixture is carried out for 20 minutes at a speed of 1400 rpm and at a temperature of between 50 and 70° C. and makes it possible to obtain a homogenized initial composition.

[0086] The initial composition obtained is placed in a gas fluorination oven, an inerting of which is carried out by flushing with 100 ml of pure nitrogen per minute for at least one hour. The initial composition is subsequently heated under a nitrogen atmosphere with a change in the temperature of the order of about ten degrees per minute up to a temperature of 350° C.

[0087] The composition is then maintained at 350° C. with the replacement of the nitrogen atmosphere by a fluorine atmosphere with a flow of 40 ml per minute at a pressure of 1 bar. This maintenance stage lasts 3 hours.

[0088] The temperature in the fluorination oven is subsequently reduced by about ten degrees per minute under a fluorine flow down to a temperature of 150° C. The fluorine flow is then replaced by a nitrogen flow of 100 ml per minute while allowing the fluorination oven to cool down until ambient temperature is reached. The final luminophore obtained is a compound K.sub.2SiF.sub.6:Mn.sup.4+ which is advantageous in comparison with the compounds comprising luminophores of the same formula and which can be used in numerous applications in the field of LEDs. The luminophore obtained by the process of the present invention is a luminophore, the purity of which is greater than the luminophores obtained by the processes of the state of the art. Also, the luminophore obtained by the process of the present invention has a durability greater than the processes of the prior art because, in particular, it does not comprise hydrofluoric acid. The durability of a luminophore obtained by the process of the present invention can be tested, for example, by stress tests.

[0089] Aging tests have been carried out on samples of luminophores K.sub.2SiF.sub.6:Mn.sup.4+ obtained by the methods of the present invention. The operating conditions comprised the maintenance of an ambient humidity, of a temperature of 20° C. or of 50° C. and illumination by a 450 nm blue LED, the sample being under a photon flux with a power of 183 mW. The tests demonstrated that there was no degradation of the luminophores for a period of time of at least 10 days.

Example 2—Synthesis of K.SUB.2.SiF.SUB.6.:Mn.SUP.4+ by Static Fluorination with Heating Under an Inert Atmosphere

[0090] The initial composition comprises, as first synthetic precursor A′, potassium bromide (KBr), as second synthetic precursor B′, silicon dioxide (SiO.sub.2), and, as chemical doping source C′, manganese(II) fluoride (MnF.sub.2). The synthetic precursors and the chemical doping source are ground with ethanol and mixed in order to obtain a homogeneous powder. The mixture is placed on a nickel or alumina boat and is introduced into a fluorination oven. A stage of heating the initial composition up to a temperature of 350° C. is carried out under nitrogen. The oven is placed under vacuum, for example at a relative pressure of −1 bar, then an injection of fluorine is carried out, for example up to a relative pressure of −0.2 bar. This state is maintained for 3 hours at the fluorination temperature of 350° C. The oven is subsequently cooled down to a temperature of 20° C., still under a fluorine atmosphere. Flushing with nitrogen is carried out in order to recover the sample. A yellow powder is obtained, characteristic of the compound K.sub.2SiF.sub.6 doped with Mn.sup.4+ ions.

Example 3—Synthesis of K.SUB.2.SiF.SUB.6.:Mn.SUP.4+ by Static Fluorination with Heating Under Vacuum

[0091] The initial composition and the stages of example 2 are reproduced with as sole difference that the stage of heating the initial composition up to a temperature of 350° C. is carried out under vacuum. The results obtained relating to the quality of the luminophores obtained during example 2 are identical.

[0092] The emission spectra of the luminophores K.sub.2SiF.sub.6:Mn.sup.4+ obtained according to example 2 or example 3 are presented in FIGS. 2 and 3 according to an excitation at 450 nm or 629 nm respectively.

Example 4—Synthesis of Na.SUB.2.SiF.SUB.6.:Mn.SUP.4+ by Static Fluorination

[0093] The initial composition comprises, as first synthetic precursor A′, sodium bromide (NaBr), as second synthetic precursor B′, silicon dioxide (SiO.sub.2), and, as chemical doping source C′, manganese(II) fluoride (MnF.sub.2). The synthetic precursors and the chemical doping source are ground with ethanol and mixed in order to obtain a homogeneous powder. The mixture is placed on a nickel or alumina boat and is introduced into a fluorination oven. A stage of heating the initial composition up to a temperature of 350° C. is carried out under nitrogen. The oven is placed under vacuum, for example at a relative pressure of −1 bar, then an injection of fluorine is carried out, for example up to a relative pressure of −0.2 bar. This state is maintained for 3 hours at the fluorination temperature of 350° C. The oven is subsequently cooled down to a temperature of 20° C., still under a fluorine atmosphere. Flushing with nitrogen is carried out in order to recover the sample. A yellow powder is obtained, characteristic of the compound Na.sub.2SiF.sub.6 doped with Mn.sup.4+ ions.

Example 5—Synthesis of Na.SUB.2.SiF.SUB.6.: Mn.SUP.4+ by Static Fluorination

[0094] The initial composition and the stages of example 5 are reproduced with as sole difference that the stage of maintenance of the initial composition at a fluorination temperature of 350° C. has a duration of 30 minutes. The results obtained relating to the quality of the luminophores obtained during example 4 are identical. A reaction time of 30 minutes for obtaining luminophores is very fast compared with the methods of the prior art and exhibits a major advantage from an economic point of view by the speed of production and the energy saving.

[0095] The emission spectra of the luminophores Na.sub.2SiF.sub.6:Mn.sup.4+ obtained according to example 4 or example 5 are presented in FIGS. 4 and 5 according to an excitation at 450 nm or 625 nm respectively.

Example 6—Synthesis of K.SUB.2.SiF.SUB.6.:Mn.SUP.4+ by Static Fluorination with Pure Silicon

[0096] The initial composition comprises, as first synthetic precursor A′, potassium bromide (KBr), as second synthetic precursor B′, silicon (Si), and, as chemical doping source C′, manganese(II) fluoride (MnF.sub.2). The synthetic precursors and the chemical doping source are ground with ethanol and mixed in order to obtain a homogeneous powder. The mixture is placed on a nickel boat and is introduced into a fluorination oven. A stage of heating the initial composition up to a temperature of 350° C. is carried out under nitrogen. The oven is placed under vacuum, for example at a relative pressure of −1 bar, then an injection of fluorine is carried out, for example up to a relative pressure of −0.2 bar. This state is maintained for 3 hours at the fluorination temperature of 350° C. The oven is subsequently cooled down to a temperature of 20° C., still under a fluorine atmosphere. Flushing with nitrogen is carried out in order to recover the sample. A yellow powder is obtained, characteristic of the compound K.sub.2SiF.sub.6 doped with Mn.sup.4+ ions.

[0097] The different embodiments presented in this description are not limiting and can be combined together. In addition, the present invention is not limited to the embodiments described above but extends to any embodiment coming within the scope of the claims.