PROCESS FOR THE PREPARATION OF UP-CONVERSION PHOSPHORS

Abstract

A process can be used for the preparation of an up-conversion phosphor of the general formula (I):

A.sub.1-x-y-zB*.sub.yB.sub.2SiO.sub.4:Ln.sup.1.sub.x,Ln.sup.2.sub.z,   (I).

The process involves preparing a mixture, introducing the mixture into a reaction chamber of a thermal apparatus, heating the mixture until a thermal treatment temperature is reached with a heating ramp, thermally treating the heated mixture for a holding time of at least 0.02 h, cooling the thermally treated material to room temperature while maintaining a cooling ramp, and obtaining a silicate-based lanthanoid ion-doped phosphor according to formula (I).

Claims

1. A process for the preparation of an up-conversion phosphor of the general formula (I)
A.sub.1-x-y-zB*.sub.yB.sub.2SiO.sub.4:Ln.sup.1.sub.x,Ln.sup.2.sub.z,   (I) wherein x=0.0001-0.0500; z=0.0000 or z=0.0001 to 0.3000, with the proviso that y=x+z; A is selected from the group consisting of Mg, Ca, Sr, and Ba; B is selected from the group consisting of Li, Na, K, Rb, and Cs; B* is selected from the group consisting of Li, Na, and K, Ln.sup.1 is selected from the group consisting of praseodymium (Pr), erbium (Er), and neodymium (Nd); and Ln.sup.2 is gadolinium (Gd), the process comprising: preparing a mixture with: i) at least one of a lanthanoid salt selected from the group consisting of a lanthanoid nitrate, a lanthanoid carbonate, a lanthanoid carboxylate, and a lanthanoid sulfate; and/or a lanthanoid oxide, wherein a lanthanoid ion in the lanthanoid oxide or lanthanoid salt is selected from the group consisting of praseodymium, gadolinium, erbium, and neodymium; and at least two of the lanthanoid ions for co-doping, ii) a silicate or a silicon dioxide, iii) at least one alkaline earth metal salt, and at least one alkali metal salt selected from a lithium salt and a lithium compound, and optionally selected from a sodium salt and potassium salt, and optionally iv) at least one flux selected from the group consisting of ammonium halide, alkali metal halide, alkaline earth metal halide, and lanthanoid halide, introducing the mixture into a reaction chamber of a thermal apparatus, heating the mixture until a thermal treatment temperature is reached of from 600° C. to <1000° C., with a heating ramp of from 10° C./h-400° C./h, to obtain a heated mixture, thermally treating the heated mixture at a temperature of from 600° C. to <1000° C., with a holding time of at least 0.02 h, to obtain a thermally treated material, cooling the thermally treated material to room temperature while maintaining a cooling ramp of from 10° C./h-400° C./h, and obtaining a silicate-based lanthanoid ion-doped phosphor according to the general formula (I).

2. The process according to claim 1, wherein the mixture is prepared without solvent.

3. The process according to claim 1, wherein the mixture is ground and/or compacted before being introduced into the reaction chamber.

4. The process according to claim 1, wherein the mixture comprises at least 0.01%-10.0% by weight of the at least one flux.

5. The process according to claim 1, wherein the thermal treatment is conducted batchwise or continuously.

6. The process according to claim 1, wherein the thermal apparatus is a batchwise furnace or a continuous furnace.

7. The process according to claim 1, wherein when the thermal apparatus is a rotary tube furnace or a drum furnace, a working tube or drum is heated before introducing the mixture.

8. The process according to claim 1, wherein the thermal treatment is conducted under air atmosphere.

9. The process according to claim 1, wherein when the mixture comprises more than 1.0% by weight of lanthanoid ions, prior to the cooling of the thermally treated material, a further thermal treatment is conducted under a reducing atmosphere at a temperature of from 600° C. to <1000° C., with a holding time of at least 0.02 h.

10. The process according to claim 9, wherein the reducing atmosphere is CO-containing atmosphere or a forming gas.

11. The process according to claim 1, wherein the lanthanoid is praseodymium.

12. The process according to claim 1, wherein alkali metals are sodium and/or lithium.

13. The process according to claim 1, wherein an alkaline earth metal is calcium.

14. The process according to claim 1, wherein the silicate-based lanthanoid ion-doped phosphor according to the general formula (I) is doped with praseodymium.

15. The process according to claim 1, wherein the silicate-based lanthanoid ion-doped phosphor according to the general formula (I) is ground.

16. The process according to claim 1, wherein the lanthanoid oxide is Pr.sub.60O.sub.11 and/or Gd.sub.2O.sub.3.

17. The process according to claim 1, wherein the at least one alkaline earth metal salt is calcium carbonate, and the at least one alkali metal salt is lithium carbonate and sodium carbonate.

18. The process according to claim 1, wherein the mixture comprises the at least one flux, and the at least one flux is selected from the group consisting of ammonium chloride, sodium chloride, sodium fluoride, sodium bromide, lithium fluoride, lithium chloride, calcium chloride, calcium fluoride, praseodymium fluoride, and praseodymium chloride.

19. The process according to claim 4, wherein the mixture comprises 1.5%-4.0% by weight of the at least one flux.

20. The process according to claim 6, wherein the thermal apparatus is a muffle furnace, air circulation furnace, chamber furnace, trolley hearth furnace, fluidized-bed furnace, push-through furnace, flow-through furnace, rotary tube furnace, drum furnace, tunnel furnace, vertical furnace, or paternoster furnace.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0078] The FIGURE shows an emission spectrum of the phosphor prepared by the process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0079] The term “thermal treatment” is understood here to mean calcination, precalcination, heat treatment, or thermal reduction. With the process according to the invention, it is now possible, surprisingly, to prepare up-conversion phosphors in reproducible amounts.

[0080] What is advantageous with the process according to the invention is the preservation of the apparatus materials on account of the heating and cooling ramps. Apparatuses used therefore do not experience any sudden temperature stress that might lead to material fatigue.

[0081] Preferably, the holding time is 0.5 h, preferably 3 h, particularly preferably 6 h or 12 h, but no more than 48 h. A person skilled in the art can accordingly vary the holding time within these times for reasons of economics.

[0082] Preferably, the thermal treatment temperature is kept constant during the holding time.

[0083] The mixture can preferably be prepared by means of batchwise mixers, such as for example drum hoop mixers, drum roller mixers, double-cone mixers, container mixers, drum mixers, or continuous mixers such as for example paddle mixers, extruders, and flow mixers. Other mixers are also conceivable.

[0084] The mixture is preferably prepared without solvent.

[0085] Preferably, the mixture is ground and/or compacted before being introduced into the reaction chamber.

[0086] The uniformity of the mixture before introduction into the reaction chamber can play a large role in achieving the required product quality. In order to achieve this uniformity, grinding may preferably be used to make the particle size distribution of components i, ii, iii and optionally iv more uniform. The grinding may be effected by means of wet grinding or dry grinding. Dry grinding may for example be conducted in ball mills, stirred ball mills, pinned disc mills, impact mills, sifter mills, spiral jet mills, fluidized-bed jet mills or steam jet mills. Wet grinding may for example be conducted in rotor-stator dispersing systems/dispersers, stirred ball mills or colloid mills.

[0087] Subsequent compacting/granulation has the advantage of reducing the required working volume of the furnace, preventing or minimizing dust formation and/or counteracting demixing of the components. Compacting may be effected wet or dry. Compacted powder up to and including granules can be prepared by the compacting. Roller presses/compactors or similar technologies may be used for the compacting.

[0088] Preferably, at least 0.01%-10.0% by weight, preferably at least 0.5%-6.0% by weight and particularly preferably 1.5%-4.0% by weight, of flux is used, based on the overall amount of the reactants.

[0089] The thermal treatment is preferably conducted batchwise or continuously. Depending on the geometry of the thermal apparatuses, one such process regime is expedient, which the person skilled in the art can accordingly assess and employ.

[0090] The thermal apparatus is preferably batchwise furnaces, preferably muffle furnaces, air circulation furnaces, chamber furnaces, trolley hearth furnaces or fluidized-bed furnaces or reactor, or continuous furnaces, preferably push-through furnaces, flow-through furnaces, rotary tube furnaces, drum furnaces, tunnel furnaces, vertical furnaces or paternoster furnaces.

[0091] These furnaces can preferably be constructed gas-tight and/or gas-permeable, and may be operated electrically or with natural gas.

[0092] The furnace construction must preferably ensure that the process according to the invention, which consists essentially of heating, thermal treatment and cooling, can be conducted with adherence to the respective ramps and holding time.

[0093] When using rotary tube furnaces or drum furnaces, it is preferably for the working tube or drum to first be heated without mixture. Once the working tube has reached the necessary temperature, the mixture is introduced. On account of its construction, the holding times thereof can be short and correspondingly high throughputs can be generated. The cooling step of the invention with the cooling ramp may be dispensed with, since the product is immediately discharged into a vessel.

[0094] Preferably, the thermal treatment is conducted in a rotary tube furnace in combination with a further batchwise or continuous furnace, such as for example tunnel furnace, flow-through furnace, push plate furnace.

[0095] The thermal treatment is preferably conducted completely or partially under air atmosphere.

[0096] Should the process according to the invention be conducted in a reducing atmosphere, the thermal apparatus should preferably be gas-tight.

[0097] When conducted under air atmosphere, gas permeability of the thermal apparatus would be advantageous.

[0098] It is conceivable for the process according to the invention to preferably be conducted under air atmosphere and reducing atmosphere. The thermal apparatus should have such a combined unit for this purpose.

[0099] Preferably, the thermal apparatus in the reaction chamber has one or more vessels made from ceramics and/or provided with ceramic fillers and/or ceramic coatings. Vessels made of other materials which do not enter into any reactions with the reactants and/or the product may also be used. These vessels may be installed so as to be stationary or movable in the reaction chamber (for example: on a paternoster lift, on a carriage, on a continuous belt, etc.).

[0100] When using more than 1.0% by weight of lanthanoid ions, based on the overall amount of the reactants, prior to the cooling of the thermally treated material there is preferably conducted a further thermal treatment under reducing atmosphere at a temperature of from 600° C. to <1000° C., preferably at 650° C. to 900° C., with the holding time being at least 0.02 h, preferably at least 0.5 h, particularly preferably at least 3 h.

[0101] The reducing atmosphere is preferably CO-containing atmospheres or a forming gas, preferably argon-hydrogen mixtures or nitrogen-hydrogen mixtures (97/3 and 95/5).

[0102] For the process according to the invention, the lanthanoids are preferably praseodymium, the alkali metals are preferably sodium or lithium and the alkaline earth metals are preferably calcium.

[0103] Preferred silicon dioxides that may be used are the products with the trade names Aerosil® 300, 200, OX50, 200V and 300V from Evonik.

[0104] Preferably, the phosphor prepared by the process according to the invention is doped with praseodymium.

[0105] Preferably, the cooled silicate-based lanthanoid ion-doped phosphor is ground.

[0106] The phosphors prepared by the process according to the invention can be coated by an aftertreatment. Coated and uncoated up-conversion phosphors can be used in coating materials having antimicrobial action.

[0107] Adduced hereinafter are examples that serve solely to elucidate this invention to the person skilled in the art and do not constitute any restriction at all of the subject-matter as described.

Methods

[0108] Particle size distribution to ISO 13320:2020 and USP 429, with a Horiba LA-950 Laser Particle Size Analyzer

[0109] Qualitative elemental analysis by means of EDX with a Tabeltop 4000Plus from Hitachi, kV BSE detector, 1000× magnification

[0110] Powder XRD: The X-ray powder diffractograms of the samples were recorded using a Bruker D2 Phaser powder diffractometer operating in Bragg-Brentano geometry, using Cu-K.sub.α radiation and a line scan CCD detector. The integration time was 20 s and the step width was 0.017° 2θ.

[0111] The emission spectra were recorded with the aid of an Edinburgh Instruments FLS920 spectrometer equipped with a 488 nm continuous-wave OBIS laser from Coherent and a Peltier-cooled (−20° C.) single-photon counting photomultiplier from Hamamatsu (R2658P). Edge filters were used to suppress second- and higher-order reflections caused by the monochromators.

[0112] BET surface area measurements to ISO 9277:2010, DIN 66131 using a Nova 2000e instrument from Quantachrome.

[0113] The degree of crystallinity (DOC) gives information on the ratio of the crystalline area to the amorphous area of all components in a powder diffractogram. The degree of crystallinity is calculated from the total area under the crystalline and amorphous fractions:

[00001]$DOC=\frac{\mathrm{Crystalline}\mathrm{area}}{\mathrm{Crystalline}\mathrm{area}+\mathrm{Amorphous}\mathrm{area}}$

[0114] Bulk and tamped density measurement to DIN EN ISO 787-11:1995-10.

EXAMPLE PREPARATION OF A PHOSPHOR (Ca.SUB.0.98.Pr.SUB.0.01.Na.SUB.0.01.)Li.SUB.2.SiO.SUB.4 .WITH 1.5% BY WEIGHT OF CaF.SUB.2 .AS FLUX ON A 6 kg SCALE IN ACCORDANCE WITH THE PROCESS ACCORDING TO THE INVENTION

[0115] 2.47 kg of CaCO.sub.3, 1.86 kg of Li.sub.2CO.sub.3, 1.51 kg of SiO.sub.2, 13.36 g of Na.sub.2CO.sub.3, 42.95 g of Pr.sub.6O.sub.11, and 92.62 g of CaF.sub.2 were mixed in a drum hoop mixer and ground by means of a pinned disc mill. The ground mixture was then compacted using a roller compactor. The ground and compacted mixture was distributed between three 3.2 I ceramic boxes. Each box was filled with 2 kg of the mixture. The boxes were then transferred into a chamber furnace. The filled boxes (batch) were heated to a temperature of 850° C. with a heating ramp of 90° C./h. When the thermal treatment temperature of 850° C. was reached, the batch was calcined in air for 6 h. The phosphor was then cooled to room temperature while maintaining a cooling ramp of 90° C./h. The cooled, coarse material-like phosphor could be removed from the boxes and pre-comminuted by means of a jaw crusher. The pre-comminuted phosphor was ground to a particle size distribution of [0116] D.sub.10: 2.6 μm [0117] D.sub.50: 3.6 μm [0118] D.sub.90: 6 μm
with an air jet mill.

[0119] With a batch of 6 kg of reactants, a yield of 4.8 kg of phosphors could be prepared.

[0120] The FIGURE shows an emission spectrum of the phosphor prepared by the process according to the invention.

[0121] The phosphor prepared by the process according to the invention exhibited an up-conversion property in the emission spectrum in the UV-C range and an antimicrobial action.

Theoretical Comparative Example 1 (CE1) Preparation of a Phosphor in Accordance with the as-yet Unpublished European patent application EP 21167984.0 (Ca.SUB.0.98.Pr.SUB.0.01.Na.SUB.0.01.)Li.SUB.2.SiO.SUB.4 .with 1.5% by Weight of CaF.SUB.2 .as Flux

[0122] For a batch of 6 kg of reactants, a theoretical 600 crucibles have to be prepared and 200 muffle furnaces have to be present, in order to be able to conduct the process. This calculation was based on the holding capacity of a muffle furnace. In theory, three 60 ml ceramic crucibles fit in a muffle furnace, each of these crucibles being able to hold 10 g of reactant mixture. In view of this calculation, it is clear that the process known from the prior art could not be economically practicable.

Comparative Example 2 (CE2): Preparation of a Phosphor (Ca.SUB.0.98.Pr.SUB.0.01.Na.SUB.0.01.)Li.SUB.2.SiO.SUB.4 .with 1.5% by Weight of CaF.SUB.2 .as Flux on a 6 kg Scale without Heating Ramp and without Cooling Ramp

[0123] Analogously to the example described above, the reactants were mixed and distributed between the 3.2 I ceramic boxes. However, the heating ramp was dispensed with. It could be seen that all ceramic boxes broke. After the thermal treatment, the phosphor was cooled to room temperature without maintaining the cooling ramp. Damaged ceramic boxes were irreparably broken. A yield of 0 kg of phosphor was obtained. The phosphors contaminated with ceramic material were disposed of.