Luminescent metal material, manufacturing process and use

Description

FIELD OF INVENTION

[0001] The present invention relates to a luminescent metallic material obtained from metallic particles and a luminescent pigment. This material can be used, in particular, in watchmaking or jewelry. PRIOR STATE OF TECHNOLOGY

[0002] Luminescence refers to the emission of light, either rapidly (fluorescence) or more slowly (phosphorescence), following the excitation of a chemical compound by the absorption of photons. Some phosphorescent compounds can thus emit light for several hours after being initially excited by exposure to electromagnetic radiation.

[0003] Luminescent materials therefore have the particularity of emitting light in the dark, which can prove interesting, whether for aesthetic or functional reasons.

[0004] Thus, in the field of watchmaking, integrating a luminescent material into the hands or markers of a watch dial allows or facilitates the reading of the time in the dark.

[0005] As an example, WO 2014 / 72388 describes a phosphorescent ceramic composite material having at least two phases. This material is obtained by 1) sintering in an oxidizing atmosphere, and 2) sintering in a reducing atmosphere at a higher temperature.

[0006] EP 3 395 784 describes a ceramic material comprising 5% wt or less of a noble metal.

[0007] EP 3 572 209 describes a process for preparing a composite material by impregnating a reinforcement (ceramic, metallic or organic material) with a matrix (synthetic resin) which may include a phosphorescent compound.

[0008] EP 3 653 744 describes a composite material with a granular appearance. This material comprises 50 to 95 vol% of a metallic matrix and ceramic particles. It is not a material comprising at least 14 carats of metal.

[0009] JP 2000-026902 describes a material obtained by sintering stainless steel powder. WO 2015 / 149879 describes a luminescent composite material based on nickel or cobalt alloy, steel, hard metal, refractory metal, carbide or nitride.

[0010] US 2011 / 305919 describes a metallic substrate comprising a metallic layer containing luminescent particles.

[0011] Even though these materials have luminescent properties, there is a need for easy-to-prepare alternatives, especially for materials containing a noble metal.

[0012] The present invention relates to a luminescent material based on noble metal and its manufacturing process. DESCRIPTION OF THE INVENTION

[0013] The luminescent material according to the invention is based on a metal chosen from gold, platinum, palladium or silver.

[0014] More specifically, the present invention relates to a luminescent material for watchmaking or jewelry consisting of: 5 to 40% luminescent pigment, 60 to 95% metallic matrix, the percentages being expressed in relation to the volume of the material, the metallic matrix being made up of a pure metal M or an alloy of metal M, the metal M being chosen from the group consisting of gold, platinum, palladium and silver, the luminescent material comprising, for 24 parts by weight, at least 14 parts of metal M.

[0015] The luminescent material according to the invention consists of at least one type of luminescent pigment (luminophore) and a metallic matrix. The luminescent material (metallic matrix + luminescent pigment) is not a ceramic material. Indeed, it is not obtained by ceramization; it is obtained from a mixture of metallic particles and luminescent pigments. However, the luminescent pigment can be a ceramic material. The luminescent material according to the invention can therefore be a cermet (metal-ceramic).

[0016] In other words, it does not include any other elements besides possible impurities which do not exceed 10 ppm by weight of the luminescent material.

[0017] The metal alloy M comprises, for 24 parts by weight, at least 14 parts of metal M, advantageously 14 to 23 parts, more advantageously 18 to 23 parts. This notably includes gold (more than 14 carats to 24 carats) or 925 silver (an alloy comprising at least 92.5% silver by weight).

[0018] Advantageously, the metallic matrix represents 58.5 to 95.9%, relative to the weight of the luminescent material, more advantageously 85 to 95%.

[0019] Thus, the luminescent pigment advantageously represents 4.1 to 41.5%, compared to the weight of the luminescent material, more advantageously 5 to 15%.

[0020] The material according to the invention is luminescent, that is to say phosphorescent or fluorescent. It is preferably phosphorescent.

[0021] The luminescent material according to the invention has a porosity advantageously less than 5%, more advantageously less than 1%.

[0022] Where appropriate, the median pore size is advantageously less than 100 micrometers, more advantageously less than 50 µm, and even more advantageously less than 20 µm.

[0023] The luminescent material according to the invention consists of luminescent pigment and a metallic matrix in which the luminescent pigment is dispersed.

[0024] In other words, it is not a phosphor consisting of a luminescent pigment and intimately mixed metallic particles. In fact, during the material's preparation, the metallic particles coalesce and form a matrix incorporating the luminescent pigment.

[0025] The luminescent pigment advantageously represents 5 to 40%, more advantageously 10 to 30%, relative to the volume of the luminescent material according to the invention.

[0026] The luminescent pigment is generally an inorganic compound. It may be chosen in particular from the group including calcium tungstate (CaWO4); cadmium zinc sulfide (ZnSO3:Cd); cesium iodide (CsI); barium sulfate (BaSO4); doped oxides, in particular rare earth doped oxides; rare earth salts (advantageously gadolinium, lanthanum, or tantalum), in particular yttrium doped rare earth salts; and mixtures thereof.

[0027] Advantageously, the luminescent pigment can be coated with a ceramic. If necessary, this coating can be useful for improving the luminescent pigment's resistance to the temperatures reached during the preparation of the luminescent material according to the invention.

[0028] According to a preferred embodiment, it is strontium aluminate (SrAl 2 O 4) doped with rare earth (scandium, yttrium and lanthanides, for example europium or dysprosium).

[0029] The metallic matrix advantageously represents 60 to 95%, more advantageously 70 to 90%, relative to the volume of the luminescent material according to the invention.

[0030] As already mentioned, the metal M is chosen from the group consisting of gold, platinum, palladium, silver and their mixtures. It is preferably gold.

[0031] Gold and its alloys refers to gold from more than 14 carats to 24 carats, preferably gold from 18 to 24 carats, more preferably 18 to 22 carats.

[0032] In general, a person skilled in the art will adjust the metal M / luminescent pigment ratio to obtain a luminescent material having an amount of metal M advantageously between 14 carats and 22 carats, preferably between 18 and 22 carats, relative to the luminescent material (24 carats).

[0033] The volume ratio between the metallic matrix and the luminescent pigment makes it possible to obtain a material with the visual appearance of the metal, or alloy, under normal lighting conditions, and a luminescent material in the dark.

[0034] The luminescent material may comprise one or more luminescent parts, for example in the form of at least one luminescent layer. It may also be a material with a luminescence gradient.

[0035] The present invention also relates to the process for preparing this luminescent material. This process comprises the following steps: preparation of a mixture containing at least one luminescent pigment and metallic particles, the metallic particles being made of a pure metal M or an alloy of metal M, the metal M being chosen from the group consisting of gold, platinum, palladium and silver, the mixture comprising, for 24 parts by weight, at least 14 parts of metal M, optionally, compaction of this mixture, sintering treatment, advantageously under pressure, of this mixture at a temperature below the melting temperature of the luminescent pigment.

[0036] The Applicant observed that it was possible to form a luminophore (luminescent material) by sintering, even though this material is primarily based on a metal M with a high melting point. To achieve this, the Applicant developed a process consisting of sintering metallic particles without degrading the luminescent pigments.

[0037] This luminescent material is preferentially obtained by densification of a powdered mixture.

[0038] In general, the luminescent pigment and / or metallic particles can be in the form of glitter or three-dimensional particles, for example spherical particles.

[0039] The luminescent pigment used to prepare the luminescent material is in the form of particles. Its average particle size advantageously ranges from 1 to 100 µm.

[0040] In general, a particle size of 1 to 20 µm produces a luminescent material with a homogeneous visual appearance, while a particle size greater than 50 µm produces a starry effect.

[0041] In general, the process of preparing the luminescent material does not change the size of the luminescent pigment.

[0042] As previously mentioned, the heat treatment is carried out at a temperature below the melting point of the luminescent pigment. Thus, the luminescent pigment is embedded in a metallic matrix.

[0043] The metallic particles used to prepare the luminescent material have a median number size (D50) advantageously between 1 and 200 µm, more advantageously between 5 and 100 µm.

[0044] The metallic particles and / or the luminescent pigment may be in powder form, for example gold powder for the metallic particles.

[0045] The dimensions (e.g., size) of particles (metallic or luminescent) can be measured using conventional techniques employed by those skilled in the art, such as laser granulometry (Malvern). With a suitable optical system, the shape factor can be determined, and the density can also be determined by gas pycnometer or by BET, a technology based on the Brunauer, Emmett, and Teller theory. The grain morphology can be determined by SEM (scanning electron microscopy).

[0046] As already mentioned, the gold particles used to prepare the mixture are advantageously based on 18- to 24-carat gold, preferably 20- or 22-carat gold. It can be a colored gold alloy, for example yellow, white, red, or rose.

[0047] Since the process of preparing the luminescent material does not result in any loss or gain of material, the weight ratio between the metal (or its alloy) and the luminescent pigment is the same in the mixture (before heat treatment) and in the final material.

[0048] The mixing of the luminescent pigment and metallic particles can be carried out using any conventional device. It can therefore be done manually or mechanically.

[0049] Advantageously, an automatic mixer is used, for example, a turbula type. This type of device improves the homogeneity of the mixture and, consequently, of the luminescent material.

[0050] In another embodiment, the mixture can be prepared using a grinder. If this is the case, a person skilled in the art will be able to select the appropriate grinding medium to preserve the original physical properties (shape and size) of the luminescent pigment and the metallic particles.

[0051] The sintering treatment is advantageously carried out at a temperature between 300 and 1400 °C, more advantageously between 500 and 800 °C, and even more advantageously between 600 and 700 °C.

[0052] Prior to sintering, the mixture is preferably compacted. Compaction is advantageously achieved by pressurizing the mixture to between 5 and 1500 MPa, more advantageously between 10 and 1200 MPa, and even more advantageously between 20 and 500 MPa. This refers to the pressure applied to the mixture before sintering. It is advantageously identical to the pressure that can be applied during the sintering process.

[0053] The sintering treatment is advantageously carried out at a pressure between 5 and 1500 MPa, more advantageously between 10 and 1200 MPa, and even more advantageously between 20 and 500 MPa.

[0054] The SPS sintering treatment is carried out for a duration advantageously between 1 minute and 45 minutes, more advantageously between 5 minutes and 15 minutes.

[0055] Thus, the heat treatment can advantageously consist of a sequence of 1) compaction and 2) sintering, or a sequence of 1) compaction and 2) sintering + compaction. It is advantageously carried out by flash sintering (SPS "spark plasma sintering"), which involves performing the sintering under pressure.

[0056] According to another embodiment, the sintering treatment can be carried out using a hot press, a cold press, by passing through a furnace (at atmospheric pressure, advantageously after a compaction step), by additive manufacturing, by thermal spraying, or by simultaneous spraying of a powder of metallic particles and a powder of luminescent pigment.

[0057] The formation of luminescent material by spraying a powder of metallic particles and a luminescent pigment powder can involve forming the metallic particle / luminescent pigment mixture during the spraying of the powders and their deposition onto a substrate. The sintering process can be carried out after the powder mixture has been deposited onto a substrate using conventional techniques (laser, SPS, etc.). However, and preferably, the sintering process is performed during the spraying of the powders, for example, by co-projecting the powders coaxially with a laser, which melts the metallic particles.

[0058] Thus, the process according to the invention can consist of depositing pigments, or the mixture (metallic particles + luminescent pigments), according to a predefined pattern such as a logo or a graphic representation, this pattern being observable in the dark after absorption of photons.

[0059] Performing heat treatment using SPS offers numerous advantages, such as faster temperature rise and shorter treatment times compared to conventional processes like hot pressing. In SPS, the mixture is heated by Joule heating and conduction, using an electric current. This phenomenon lowers the sintering temperature and duration, thus preventing the degradation of luminescent pigments.

[0060] In general, SPS treatment requires an electrically assisted powder pressing device.

[0061] Even though luminescent pigments are generally not electrically conductive, the electric current passes through conductive metallic particles (gold, platinum, palladium, silver and their alloys), causing them to heat up as well as the adjacent luminescent pigments.

[0062] The present invention also relates to the use of this luminescent material in watchmaking or jewelry, in particular a watch or jewelry item comprising the luminescent material. This may include watch hands or markers on a watch dial, but also a watch case back, a watch case middle, a watch case bezel, a watch pusher, a watch crown, or an articulated watch bracelet.

[0063] This article therefore includes the luminescent material, which may have been previously machined, and possibly polished, in particular by ion polishing.

[0064] The luminescence of the material can result from its exposure to a natural and / or artificial light source. This can include exposure to ultraviolet radiation.

[0065] In general, the absorption wavelength of the luminescent pigment does not necessarily correspond to its emission wavelength.

[0066] The present invention and the advantages arising therefrom will be more clearly seen from the figure and the following examples given to illustrate the invention and not in a limiting manner. DESCRIPTION OF THE FIGURES

[0067] There figure 1 illustrates the luminescent material according to the invention exposed to natural light and in darkness. EXAMPLES OF THE INVENTION'S IMPLEMENTATION

[0068] A luminescent material ( figure 1 ) was prepared according to the following steps: preparation of an 18-carat gold powder having a particle diameter d 50 of 16 µm, d 10 of 10 µm and d 90 of 52 µm (d xx: XX% of particles smaller than this number), by gas atomization, preparation of a mixture of 75% by volume of 18-carat gold powder and 25% by volume of luminescent pigment (europium-doped strontium aluminate or dysprosium), the quantity of pigment representing 9.5% relative to the weight of the mixture, compaction / sintering by SPS of this mixture obtaining a luminescent material in the form of a pellet.

[0069] The conditions for treatment by SPS are as follows: Apply a pressure of 16 kN to the mixture using a 2 cm diameter piston, heat the mixture to 450 °C for 5 min, reach 630 °C (100 °C / min) and maintain at 630 °C for 2 minutes, stop the heat treatment, let the sample rest for 2 minutes before releasing the pressure, let the sample cool.

[0070] Thus, the luminescent material is obtained in the form of a pellet in less than 10 minutes.

[0071] After being exposed at less than 5 cm from an artificial light source for 20 seconds, the resulting material is luminescent for several tens of minutes.

Claims

1. Luminescent material for horology or jewelry, consisting of: - 5 to 40% luminescent pigment having an average number size between 1 and 100 µm, - 95 to 60% of a metallic matrix, the percentages being relative to the volume of the material, the metallic matrix consisting of a pure metal M or an alloy of metal M, metal M being chosen from the group consisting of gold, platinum, palladium and silver, the luminescent material comprising, for 24 parts by weight, at least 14 parts of metal M.

2. Luminescent material according to claim 1, characterized in that the metal represents 58.5 to 95.9%, relative to the weight of the luminescent material.

3. Luminescent material according to claim 1 or 2, characterized in that the luminescent pigment is rare earth doped SrAl2O4.

4. Luminescent material according to any one of claims 1 to 3, characterized in that the metal M is gold.

5. Luminescent material according to any one of claims 1 to 3, characterized in that the luminescent pigment represents 10 to 30%, relative to the volume of the luminescent material.

6. Article of horology or jewelry comprising the luminescent material of any one of claims 1 to 5.

7. Article according to claim 6, characterized in that it consists of watch hands, markers on a watch dial, a watch case back, a watch case middle, a watch case bezel, a watch pusher, a watch crown or an articulated watch bracelet.

8. A method for preparing the luminescent material of any one of claims 1 to 5, according to the following steps: - preparing a mixture containing at least one luminescent pigment and metallic particles, the luminescent pigment having an average number size of between 1 and 100 µm, the metallic particles consisting of a pure metal M or an alloy of metal M, the metal M being chosen from the group consisting of gold, platinum, palladium and silver, the mixture comprising, for 24 parts by weight, at least 14 parts of metal M. - sintering treatment, advantageously under pressure, of this mixture at a temperature lower than the melting temperature of the luminescent pigment.

9. Process according to claim 8, characterized in that the mixture is subjected to a compaction step prior to the sintering treatment.

10. A process according to any one of claims 8 to 9, characterized in that the mixture is shaped prior to the sintering treatment.

11. A process according to any one of claims 8 to 10, characterized in that the sintering treatment is carried out under a pressure between 5 and 1500 MPa.

12. A process according to any one of claims 8 to 11, characterized in that the sintering treatment is carried out at a temperature between 300 °C and 400 °C.

13. A process according to any one of claims 8 to 12, characterized in that the mixture is subjected to a compaction step under a pressure between 5 and 1500 MPa, prior to the sintering treatment.

14. A process according to any one of claims 8 to 13, characterized in that the sintering treatment is carried out by flash sintering.

15. A process according to claim 8, characterized in that the sintering treatment is carried out by means of a hot press, by means of a cold press, by passing through an oven, by additive manufacturing, by thermal projection, or by simultaneously spraying a powder of metallic particles and a powder of luminescent pigment.