Method for manufacturing a timepiece or jewellery component, and said timepiece or jewellery component

Abstract

The present invention relates to a method for manufacturing a timepiece or jewellery component from a material comprising at least 18 carats of gold, comprising the following steps: a) producing Au@metal oxide nanoparticles, step a) comprising at least the sub-steps of a1) synthesising gold nanoparticles that have dimensions or shapes giving them a plasmonic effect; a2) mixing the gold nanoparticles from step a1) with a surfactant comprising functional groups for coupling to the metal oxide, while maintaining a plasmonic effect; a3) forming the metal oxide shell, while maintaining a plasmonic effect; b) producing a semi-finished product from a material comprising at least 18 carats of gold using the Au@metal oxide nanoparticles from step a), while maintaining a plasmonic effect, the semi-finished product having a colour such that the difference ΔE in the CIELAB colour space between the colour of the obtained semi-finished product and the colour of the Au@metal oxide particles formed in step a), in the dry state, is less than 10; and c) producing the timepiece or jewellery component from said material comprising at least 18 carats of gold using the semi-finished product obtained in step b), while maintaining a plasmonic effect. The present invention also relates to a timepiece or jewellery component obtained by the manufacturing method as defined above, said timepiece or jewellery component being obtained using a semi-finished product that has a colour defined in the CIE L*a*b* colour space by the parameters -5 <a*< 5, -5 <b*< 5 and L*< 15.

Description

[0001] METHOD FOR MANUFACTURING A WATCH OR JEWELRY COMPONENT AND SAID WATCH OR JEWELRY COMPONENT

[0002] Technical field

[0003] The present invention relates to a method for manufacturing a watch or jewelry component in a material comprising at least 18 carats of gold, that is to say comprising at least 75% gold by weight relative to the total weight of the material.

[0004] The present invention also relates to a semi-finished product for producing a watch or jewelry component and said watch or jewelry component capable of being obtained by means of said manufacturing method.

[0005] State of the art

[0006] Gold alloys are commonly used in the manufacture of watch and jewelry components, in the form of a deposited coating or in a solid form. These alloys are traditionally yellow, red, or gray in color. Gold alloys colored to deviate from the original gold color can be obtained by traditional metallurgical methods, or by using gold nanoparticles coated with at least one layer of metal oxide, such as silicon, zirconium, or titanium oxide, based on the plasmonic effect of the gold nanoparticles. To designate such gold nanoparticles covered with a metal oxide shell, the notation Au@metal oxide is used. Indeed, gold nanoparticles (typically ranging in size from 2 to 200 nm) are endowed with a plasmonic effect. The plasmonic effect consists of a vibration of the electron cloud of the nanoparticles when they are subjected to an electromagnetic field.More specifically, under the influence of the oscillating electric field of light, the free electrons of a metal nanoparticle undergo oscillations relative to the metal lattice. At certain frequencies, i.e., at certain wavelengths, these oscillations acquire a large amplitude through the resonance phenomenon. This is called plasmon resonance (simplification of localized surface plasmon resonance, LSPR). Light absorption is maximal at resonance frequencies. After absorption, the surface plasmons lose their energy radiatively (light scattering), or non-radiatively (in the form of heat).

[0007] This plasmonic effect gives gold nanoparticles extremely interesting optical properties. LSPRs result in resonances in the visible range. It is therefore possible to excite them with visible light, and they play a major role in the colorimetric properties of nanoparticles. LSPRs depend in particular on the size and shape of the nanoparticle. For example, for nanospheres, the resonances of gold are located around 520 nm in aqueous solution and vary little for sizes ranging from 2 to 50 nm. These nanoparticles appear ruby ​​red. But the position of the absorption band and its width, and therefore the color, change according to the size of the nanoparticles: when the size increases, the LSPR band shifts towards long wavelengths and broadens. The color varies not only according to the size of the nanoparticles but also according to their shape, which depends on the conditions in which they are obtained.For elongated nanoparticles, such as nanorods, the plasmon resonance shifts toward the red as the aspect ratio increases. Gold nanoparticles can therefore change color from red to violet depending on their size. Other colors such as green and blue can be obtained by varying the aspect ratio, with more or less elongated nanoparticles (rods).

[0008] Patent EP 1 887052 describes a part made of a pigmented ceramic, the pigment being made of gold nanoparticles for example, coated with crystalline silica. The resulting part is in the red range. The amount of pigment in the ceramic is less than 5% by weight. This document therefore does not describe any component containing at least 75% gold by weight.

[0009] Patent EP 2 369 022 describes a method for manufacturing a solid material comprising at least 12 carats or even 18 carats of gold, formed from gold nanoparticles coated with silica. The color of the gold nanoparticles used is adjusted according to the size, geometry (spherical, cylindrical and pyramidal), composition and chemical environment of the nanoparticles. The solid material is obtained by sintering by heating associated with microwave irradiation so as not to alter the specific coloring of the gold nanoparticles used. However, the preparation of a solution of gold nanoparticles of bright red color is described, the solution of gold nanoparticles coated with silica being violet in color, and the pellet obtained after heating and microwave irradiation being black in color. The method described therefore does not guarantee preservation of the color of the nanoparticles, and is therefore not sufficiently robust.

[0010] Application EP 3 482 851 describes a method for manufacturing a material comprising at least 18 carats of gold, formed from an assembly of gold nanoparticles in a titanium dioxide matrix. However, this method is complex because it requires the use of solvents and heating of the mixture formed in the reactor. In addition, due to the significant absorption or non-ideal optical properties for light transmission of titanium dioxide, the penetration depth of light through the material is not sufficient to obtain the maximum plasmonic effect of the gold nanoparticles, so that the color of the material obtained is not a deep black. Furthermore, it is stated that the powder obtained is black in color before the compacting and sintering steps, while after these two steps, the material is anthracite in color. The initial color of the nanoparticles is therefore altered.Furthermore, the mechanical properties of titanium dioxide, such as hardness, are not always sufficient to meet the specifications imposed depending on the components.

[0011] The present invention aims to overcome these drawbacks by proposing a method for manufacturing a watch or jewelry component in a material comprising at least 18 carats of gold, making it possible to guarantee preservation and stability of the color of the nanoparticles used.

[0012] Another object of the present invention is to propose a method for manufacturing a watch or jewelry component making it possible to obtain a composite component comprising at least 18 carats of gold and having very good mechanical properties. Another object of the present invention is to propose a method for manufacturing a watch or jewelry component in a material comprising at least 18 carats of gold making it possible to simply obtain a component of a dark, gray or black color, and in particular deep black.

[0013] Disclosure of the invention

[0014] For this purpose, the invention relates to a method for manufacturing a watch or jewelry component in a material comprising at least 18 carats of gold, said method comprising the following steps: a) producing a solution of Au@metal oxide nanoparticles comprising a gold core covered with a metal oxide shell, step a) comprising at least the sub-steps: a1) synthesizing gold nanoparticles by reacting a gold precursor with a reducing agent, said gold nanoparticles having dimensions and shapes giving them a plasmonic effect at least in the visible range; a2) mixing the gold nanoparticles obtained in step a1) with a solution of a surfactant comprising functional groups for coupling with the metal oxide and maintaining stirring for a sufficient time for said surfactant to coat the gold nanoparticles, while maintaining a plasmonic effect;a3) forming the metal oxide shell by reacting a metal oxide precursor with the gold nanoparticles obtained in sub-step a2), while maintaining a plasmonic effect; b) producing a semi-finished product in a material comprising at least 18 carats of gold from the solution obtained in step a) with a view to forming said watch or jewelry component, while maintaining a plasmonic effect, the semi-finished product obtained having a color such that the difference AE in the CIE Lab color space between the color of the semi-finished product obtained and the color of the Au@metal oxide nanoparticles formed in step a), in the dry state, is less than 10, preferably less than or equal to 8; c) producing the watch or jewelry component in said material comprising at least 18 carats of gold from the semi-finished product obtained in step b), while maintaining a plasmonic effect.;

[0015] Thus, the method according to the invention makes it possible not to alter and to preserve the color of the synthesized gold nanoparticles, in particular when obtaining the semi-finished product, and to obtain a component comprising at least 18 carats of gold having substantially the same color as the gold nanoparticles synthesized at the start, this color advantageously being black.

[0016] The present invention also relates to a semi-finished product in a material comprising at least 18 carats of gold capable of being obtained during the implementation of the method for manufacturing a watch or jewelry component as defined above, said semi-finished product having a color defined in the CIE L*a*b space by the parameters -5 <a*< 5, -5< b*< 5 et L*< 15, de préférence -1 <a*< 1 , -1 <b*< 1 et L*< 13.

[0017] The present invention also relates to a watch or jewelry component capable of being obtained by the manufacturing method as defined above, said watch or jewelry component being obtained from a semi-finished product which has a color defined in the CIE L*a*b space by the parameters - 5 <a*< 5, -5 <b*< 5 et L*< 15.

[0018] Brief description of the drawings

[0019] Other characteristics and advantages of the present invention will appear on reading the following detailed description of different embodiments of the invention, given as non-limiting examples, and made with reference to the appended drawings in which: Figure 1 schematically represents the steps of a first variant of the method according to the invention; Figure 2 schematically represents the steps of a second variant of the method according to the invention;

[0020] - Figure 3 is an image obtained by transmission electron microscopy (TEM - 20 nm scale) of the Au@metal oxide nanoparticles obtained according to step a) of the process according to the invention; and

[0021] - Figure 4 represents reflectance curves of black Au@SiO2 nanoparticles obtained according to step a) of the process of the invention.

[0022] Embodiments of the invention

[0023] The present invention relates to a method of manufacturing a watch or jewelry component in a material comprising at least 18 carats of gold, i.e. at least 75% gold by weight relative to the total weight of the material.

[0024] According to the invention, with reference to Figures 1 and 2, said method comprises the following steps: a) producing a solution of Au@metal oxide nanoparticles comprising a gold core covered with a metal oxide shell, step a) comprising at least the sub-steps: a1) synthesizing gold nanoparticles in solution by reacting a gold precursor with a reducing agent, said gold nanoparticles having dimensions and shapes giving them a plasmonic effect at least in the visible range corresponding to a predetermined color X; a2) mixing the solution of gold nanoparticles obtained in step a1) with a solution of a surfactant comprising functional groups for coupling with the metal oxide and maintaining stirring for a sufficient time for said surfactant to coat the gold nanoparticles, while maintaining a plasmonic effect, the nanoparticles obtained having a color X1;a3) forming the metal oxide shell by reacting a metal oxide precursor with the gold nanoparticles obtained in sub-step a2), while maintaining a plasmonic effect, the Au@metal oxide nanoparticles formed having a color X2 measured in the dry state, in powder form, such that the color difference AE in the CIE Lab color space between the colors X1 and X2 is preferably less than 8, preferably less than or equal to 5, more preferably less than 3, or even less than 1, the color difference AE in the CIE Lab color space between the colors X and X2 being preferably less than 8, preferably less than 5, more preferably less than 3, or even less than 1;b) producing a semi-finished product in a material comprising at least 18 carats of gold from the solution obtained in step a) with a view to forming said watch or jewelry component, while maintaining a plasmonic effect, the semi-finished product obtained having a color X3 such that the difference AE in the CIE Lab color space between the colors X2 and X3 is less than 10, preferably less than or equal to 8 and more preferably less than or equal to 7, the difference AE in the CIE Lab color space between the colors X and X3 being preferably less than 10, more preferably less than or equal to 8, and more preferably less than or equal to 7;c) producing the watch or jewelry component in said material comprising at least 18 carats of gold from the semi-finished product obtained in step b), while maintaining a plasmonic effect, said watch or jewelry component obtained having a color X4 such that the difference AE in the CIE Lab color space between the colors X2 and X4 is preferably less than 10, preferably less than or equal to 8, and more preferably less than or equal to 7, and the difference AE in the CIE Lab color space between the colors X and X4 is preferably less than 10, more preferably less than or equal to 8, and more preferably less than or equal to 7.;

[0025] Color difference or AE color deviation in CIE Lab color space is defined as a measure of difference between two colors by equation (I):

[0026] L , a , b are the coordinates in the CIE Lab color space of the first color to be compared and are the coordinates in color space

[0027] CIE Lab of the second color to compare.

[0028] A color difference AE between two colors of less than 10, preferably less than or equal to 8, and more preferably less than or equal to 7 indicates that the two colors are almost identical, or at least one is not altered compared to the other.

[0029] The synthesis of gold nanoparticles by reduction of a gold precursor is a synthesis known to those skilled in the art who know how to choose the different parameters, such as the concentrations of the reactants, the concentration ratios, the solvents, the synthesis process with heating, reflux, etc., to obtain gold nanoparticles which have identical or different dimensions and shapes, chosen to give said gold nanoparticles an overall plasmonic effect at least in the visible range corresponding to the predetermined color X. Indeed, the color of gold nanoparticles varies according to the size and shape of the nanoparticles. Colors from red to violet can be obtained depending on the size, generally with spherical nanoparticles, while colors such as green and blue can be obtained by varying the shape factor, for example with more or less elongated nanoparticles, in the form of rods.It is also possible to have shapes of cubes, stars, etc. In particular, the size of the particles can be controlled by varying the gold / reducing agent ratio. The gold nanoparticles obtained in sub-step a1) preferably have dimensions less than 200 nm, and preferably between 1 nm and 200 nm, and more preferably between 20 nm and 200 nm. There are numerous publications on different methods for synthesizing gold nanoparticles that make it possible to control the size and shape of said gold nanoparticles to obtain the predetermined color X.

[0030] Advantageously, in sub-step a1), the gold precursor is a gold salt such as HAuCk, preferably in aqueous solution, in concentrations between 2 g / l and 10 g / l.

[0031] Advantageously, in sub-step a1), the reducing agent may be NaBH4 or sodium citrate, preferably in the form of an aqueous solution, in concentrations of between 25 g / l and 75 g / l. Sodium citrate is preferred. Such a reducing agent has the advantage of also acting as a stabilizer to keep the gold nanoparticles in solution.

[0032] Preferably, sub-step a1) comprises mixing an aqueous solution of the gold precursor with an aqueous solution of reducing agent with a gold / reducing agent molar ratio in solution of between 1 / 50 and 1 / 20, at room temperature of 25°C.

[0033] In sub-step a2), the surfactant used, in addition to its coupling properties with the metal oxide, is preferably a stabilizing agent for the gold nanoparticles prepared in sub-step a1). It is advantageously amphiphilic, soluble in water and in organic solvents. In addition to its functional groups for coupling with the metal oxide, such as C=O groups, it comprises CH2 functional groups, providing hydrophobicity and CN groups, for example a pyrrolidone group, giving it its hydrophilic character.

[0034] Preferably, the surfactant is selected from the group comprising polyvinylpyrrolidone (PVP), gelatins, silane derivatives and silicate derivatives.

[0035] The color X1 of the nanoparticles obtained in step a2) may be identical to or different from the color X of the nanoparticles obtained in step a1) depending on the surfactant concentration. Preferably, the gold nanoparticle / surfactant solution is kept under vigorous stirring for between 6 hours and 12 hours, i.e. the time required to replace the reducing agent with the surfactant to protect the gold nanoparticles.

[0036] Preferably, the surfactant is polyvinylpyrrolidone (PVP). Advantageously, the PVP used in substep a2) has a molar mass of between 5000 g / mol and 20,000 g / mol and is in the form of an aqueous solution in concentrations of between 1 g / l and 10 g / l.

[0037] Preferably, the gold / PVP molar ratio in solution is between 1 / 1 and 3 / 1.

[0038] Preferably, when the surfactant is PVP, the gold nanoparticle / PVP solution is kept under vigorous stirring for between 6 hours and 12 hours, i.e. the time required to replace the citrate with PVP and protect the gold nanoparticles. PVP is a surfactant that prevents agglomeration and sedimentation of the nanoparticles due to its hydrophobic alkyl functions and its hydrophilic pyrrolidone group. Furthermore, in a manner particularly advantageous for the present invention, PVP has a better affinity with silica, for example, probably due to its C=O function.

[0039] The nanoparticles protected by the surfactant, and in particular by the PVP, are then collected for sub-step a3) or preferably for the intermediate sub-step a4). Indeed, in a particularly preferred manner, step a) comprises an intermediate sub-step a4) between sub-step a2) and sub-step a3) comprising the elimination of the surfactant, and in particular of the PVP, in excess. To do this, the nanoparticles obtained in sub-step a2) are collected and concentrated by centrifugation before being used during sub-step a3). In a particularly advantageous manner, this centrifugation step makes it possible to wash the nanoparticles before being resuspended in alcohol as will be described below. This washing makes it possible to control the thickness of the metal oxide shell, and more particularly the thickness of the silica oxide shell in order to obtain nanoparticles with oxide shells having a uniform thickness.According to sub-step a3), the metal oxide is preferably chosen from the group comprising silicon oxide, zirconium oxide and titanium oxide, silicon oxide and zirconium oxide. Silicon oxide is particularly preferred. Nanoparticles are thus synthesized comprising a gold core covered with a shell of silica oxide, or zirconium oxide or titanium oxide. Preferably, nanoparticles comprising a gold core with a shell of silica or zirconium oxide are synthesized, and more preferably nanoparticles comprising a gold core with a shell of silica oxide.

[0040] The metal oxide precursor may be selected from the group comprising tetraethoxysilane for a silica shell, titanium butoxide for a titanium dioxide shell, and zirconium butoxide for a zirconia shell. Other precursors may be used. To preferably obtain a silica shell, the silicon oxide precursor is advantageously tetraethoxysilane (TEOS) in alcoholic solution (e.g., ethanol). The reaction is based on the Stober synthesis and is catalyzed by ammonia. The gold nanoparticles protected by the surfactant, and in particular by PVP, recovered in substep 2) or preferably in substep a4) are transferred into a solution of alcohol and ammonia, as described above. Then, an alcoholic solution of oxide precursor is immediately added dropwise with vigorous stirring. The reaction is preferably stopped after 3 hours.The obtained Au@metal oxide nanoparticles are then collected to be kept in solution for use in substep b1) or dried into powder form in accordance with substep b4), as described below. The obtained Au@metal oxide nanoparticles may also be collected to be dried into powder form and then put back into solution for use in substep b1).

[0041] The process parameters of sub-step a3) are chosen so as to form a metal oxide shell, and preferably a silicon oxide shell, for which a plasmonic effect is maintained, the Au@oxide nanoparticles formed having a color X2, measured when the Au@oxide nanoparticles are in the dry state in powder form. Preferably, in particular the surfactant concentration is chosen so that the color difference AE in the CIE Lab color space between the colors X and X1, X1 and X2, and X and X2 is preferably less than 8, preferably less than 5, preferably less than or equal to 3, and more preferably less than or equal to 1, so that the initial color of the gold nanoparticles prepared in sub-step a1) is preserved.

[0042] In particular, the thickness of the shell can vary depending on the oxide / gold precursor ratio or the reaction time. Advantageously, the surfactants used, and in particular PVP, due to their affinity with silica, make it possible to obtain a uniform silica shell around the gold nanoparticles.

[0043] Preferably, the thickness of the shell is less than 100 nm, and preferably between 2 nm and 50 nm, and more preferably between 10 nm and 40 nm, the essential thing being that the color X2 of the Au@oxide nanoparticles obtained is not altered and is preserved compared to the color X of the gold nanoparticles initially prepared.

[0044] According to a particularly preferred embodiment, sub-step a1) comprises mixing an aqueous solution of the gold precursor, preferably HAuCk, with an aqueous solution of sodium citrate with a molar ratio of gold / sodium citrate in solution of between 1 / 50 and 1 / 20, at room temperature of 25°C in order to prepare gold nanoparticles of black X color defined in the CIE L*a*b space by the parameters -5 <a*< 5 , -5 <b*< 5 et 0 < L*< 10, de préférence -1 <a*< 1 , -3 < b*< 3 et 0< L*< 6.

[0045] In a particularly advantageous manner, the reaction is carried out at room temperature (25°C) without any energy input. A reticular nanostructured gold colloid formed of interconnected gold crystals is obtained to give compact gold nanoparticles of different sizes between 40 nm and 200 nm and of different shapes (nanospheres, cluster, etc.) of black X color defined above. The frequency of the photons absorbed by the black gold nanoparticles strongly depends on the shape and size of the nanoparticles according to the LSPR effect. Thus, the poly-dispersed and multiform gold nanoparticles obtained can absorb light in a wide range, which gives them a black color as defined above. These black gold nanoparticles are then preferably prepared with PVP according to sub-steps a2) and a4). The gold / PVP molar ratio in solution is between 1 / 1 and 3 / 1.

[0046] The more diluted the nanoparticle solution is in PVP, the more the black color of the nanoparticles is maintained. The more concentrated the solution in PVP, the more the PVP will tend to control the shape and size of the nanoparticles by breaking the nanostructured reticular shape of the gold, giving a reddish color.

[0047] The black gold nanoparticles preferably prepared with PVP are then used to form Au@metal oxide, and preferably Au@silicon oxide Au@zirconium oxide nanoparticles according to substep a3) of color X2. The PVP concentration is chosen so that color X2 is preferably similar to color X, so that the black color X of the gold nanoparticles is preserved and is not altered.

[0048] After preparing a solution of Au@metal oxide nanoparticles according to step a), the Au@metal oxide nanoparticles can be used in the form of a solution or dried in powder form. For this, the Au@metal oxide nanoparticles of the solution obtained in step a) are washed several times, for example with alcohol such as ethanol, and then collected by centrifugation. The Au@metal oxide nanoparticles, preferably Au@silicon oxide or Au@zirconium oxide, are then dried for at least 6 hours, and preferably at least 12 hours, for example in an oven at 60°C.

[0049] Au@metal oxide nanoparticles are used in solution or powder form to obtain the necessary quantity of gold to produce a semi-finished product in a material comprising at least 18 carats of gold in order to form the said watch or jewelry component.

[0050] The surfactant, and in particular PVP, and the appropriate shell of metal oxide make it possible to maintain a plasmonic effect, the semi-finished product obtained having a color X3 such that the difference AE in the CIE Lab color space between the colors X2 and X3 is less than 10, preferably less than or equal to 8, more preferably less than or equal to 7. When the color X2 is similar to the color X1 itself similar to the color X, there is therefore also a difference AE in the CIE Lab color space between the colors X and X3 which is less than 10, more preferably less than or equal to 8, more preferably less than or equal to 7.

[0051] According to a first variant of the method of the invention, step b) for producing the semi-finished product consists of a method chosen from the group comprising a 3D printing method, a method by exposure to radiation, for example UV or microwave, and their combination.

[0052] According to another variant of the process of the invention, step b) for producing the semi-finished product comprises the sub-steps: b1) mixing nanoparticles of gold and metal oxide, preferably Au@silicon oxide or Au@zirconium oxide, obtained in step a), said nanoparticles of gold and metal oxide being in solution as obtained directly in step a), in powder form if they have been dried in an intermediate step after step a) or redissolved if they have been previously dried in powder form, with a fluid preceramic polymer, the quantity of Au@metal oxide nanoparticles being chosen so as to obtain a composite fluid comprising at least 18 carats of gold. The fluid preceramic polymer is preferably liquid, with a viscosity of 10 to 30 cP for example.The mixing can be done with a planetary mixer or by ultrasound; b2) shaping the composite fluid obtained in sub-step b1) so as to form the semi-finished product; b3) carrying out pyrolysis of the composite fluid shaped according to sub-step b2) to obtain the semi-finished product of color X3. The pyrolysis or sintering can be carried out at high temperatures, for example between 1000°C and 1600°C, in an inert environment (argon or nitrogen) for a duration of for example between 6 hours and 12 hours, and with temperature increases and decreases of between 10°C / h and 25°C / h.

[0053] Preferably, the preceramic polymer used in sub-step b1) is chosen from the group comprising polycarbosiloxanes, polycarbosilanes, polysilazanes, polycarbosilazanes, polysilanes and polysilsesquiazanes to form by pyrolysis polymer-derived ceramics (“Polymer Derived Ceramic” PDC), respectively silicon oxycarbide, silicon carbide, silicon nitride, silicon carbonitride, silicon oxynitride. Other preceramic polymers, in particular comprising boron, may also be used. Advantageously, the PDC is chosen to be more transparent than gold after pyrolysis so that the light interacts more with the gold than with the pyrolyzed PDC. Additionally, when the color X of gold nanoparticles is black, the PDC is chosen to be black after pyrolysis when used alone without black Au@metal oxide nanoparticles.

[0054] Advantageously, substep b2) can be implemented using a 3D printing process.

[0055] Sub-step b2) may also consist of a method chosen from the group comprising a method by exposure to radiation, such as UV or microwave, a casting method, a pressing method and an injection molding method preferably preceded by ball milling, using for this purpose a mold appropriate to the component to be manufactured, for example a PTFE mold, to obtain at the end of sub-step b2) a compact raw semi-finished product traditionally called a near-net-shape “green body,” i.e. very close to the final shape of the component to be manufactured. The shaping of the fluid according to sub-step b2) may in particular be carried out by cold casting, by injection pressing or by cold extrusion. The shaping of the fluid according to sub-step b2) may also be carried out by hot isostatic pressing or compression by heating the product in its mold to 300°C under 60 MPa for 30 min.The product is removed from the mold and then optionally heated again at 400°C under 60 MPa for 6 hours. According to another variant of the process of the invention, step b) for producing the semi-finished product comprises the sub-steps: b4) preparing a powder from a solution of gold nanoparticles and metal oxide, preferably Au@silicon oxide or Au@zirconium oxide, obtained in step a). For this, the Au@metal oxide nanoparticles, preferably Au@silicon oxide or Au@zirconium oxide, of the solution obtained in step a) are washed several times, for example with alcohol such as ethanol, then collected by centrifugation.The Au@metal oxide nanoparticles, preferably Au@silicon oxide or Au@zirconium oxide, are then dried for at least 6 hours, and preferably at least 12 hours, for example in an oven at 60°C; b5) mixing said powder obtained according to sub-step b4) with a ceramic so as to obtain a composite powder comprising at least 18 carats of gold, the quantity of Au@metal oxide nanoparticles being chosen so as to obtain a composite powder comprising at least 18 carats of gold; b6) shaping the composite powder obtained in sub-step b5) so as to form the semi-finished product; b7) carrying out pyrolysis of the composite powder shaped according to sub-step b6) to obtain the semi-finished product of color X3. The pyrolysis can be carried out as described above for sub-step b3).

[0056] Preferably, the ceramic used in sub-step b5) is chosen from the group comprising zirconia, alumina, silicon oxycarbide, silicon carbide, silicon nitride, silicon carbonitride, silicon oxynitride, titanium carbide, titanium nitride, titanium diboride, boron carbide. Advantageously, the ceramic is chosen to be more transparent than gold after pyrolysis so that the light interacts more with the gold than with the pyrolyzed ceramic. In addition, when the color X of the gold nanoparticles is black, the ceramic is chosen to be black after pyrolysis when used alone, without black Au@metal oxide nanoparticles. The ceramic makes it possible to obtain a composite material comprising at least 18 carats of gold having very good mechanical properties, the ductibility, hardness and toughness being improved compared to pure gold.

[0057] Advantageously, sub-step b6) is implemented by a traditional method for implementing ceramics. Sub-step b6) may consist, for example, of a method chosen from the group comprising a casting method, a pressing method and an injection molding method, via the formation of a green body for example, as described above for sub-step b2).

[0058] Finally, step c) of producing the watch or jewelry component comprises, for example, a mechanical machining and / or finishing treatment to obtain said watch or jewelry component of color X4 such that the difference AE in the CIE Lab color space between the colors X3 and X4 is preferably less than 10, preferably less than or equal to 8, and more preferably less than or equal to 7, and such that the difference AE in the CIE Lab color space between the colors X2 and X4 is preferably less than 10, preferably less than or equal to 8, and more preferably less than or equal to 7. The mechanical machining treatment is, for example, milling, or any other suitable process. The mechanical finishing treatment is, for example, polishing or any other suitable process.When the color X2 is similar to the color X1, itself similar to the color X, we therefore also have a difference AE in the CIE Lab color space between the colors X and X4 which is less than 10, more preferably less than or equal to 8, and more preferably less than or equal to 7.

[0059] A watch or jewelry component is thus obtained in a material comprising at least 18 carats of gold, and in particular a composite material of the Cermet type combining ceramic and metal, and having all the interesting mechanical properties of ceramics, in particular ductility, hardness and toughness. Preferably, nanoparticles comprising a gold core with a shell of silica or zirconium oxide are used, and more preferably nanoparticles comprising a gold core with a shell of silica oxide.

[0060] A shell of silica or zirconium oxide around the gold core has several advantages:

[0061] - in the case of black gold nanoparticles, the alloys will achieve a darker color due to the more effective penetration depth of light due to a better association of refractive indices between the metal oxide shell and the gold core. The interaction of light at the interface of the metal oxide shell and the gold core will increase in the bulk material due to the lower refractive index of silicon dioxide or zirconium dioxide compared to the refractive index of titanium dioxide;

[0062] - the lower refractive index of silicon dioxide or zirconium dioxide is also an advantage for obtaining 18k gold alloys of different colors, e.g. red, blue, violet and green;

[0063] - mechanical properties will be improved by using a shell of silicon dioxide or zirconium dioxide rather than titanium dioxide due to an increase in Vickers hardness. The overall mechanical strength of the final component obtained will thus be improved and scratch resistance increased;

[0064] - the lower density of silicon dioxide compared to that of titanium dioxide will make it possible to obtain a gold alloy with a higher gold fineness than that obtained with a titanium dioxide shell.

[0065] Furthermore, the surfactant, and in particular PVP, due to its affinity with the metal oxide, and in particular with silica or zirconium, makes it possible to obtain a uniform silica or zirconia shell around the gold nanoparticles. This uniform shell advantageously makes it possible to prevent the fusion of the gold nanoparticles during subsequent heat treatments, such as pyrolysis, so that the plasmonic effect of the gold nanoparticles is preserved and at least the color X2 of the prepared Au@metal oxide nanoparticles is not altered by the various heat treatments necessary to obtain the semi-finished product of color X3, the difference AE in the CIE Lab color space between the colors X3 and X2 being less than 10, preferably less than or equal to 8, and more preferably less than or equal to 7.Furthermore, advantageously, the color X2 of the prepared Au@metal oxide nanoparticles is not altered by the various heat treatments necessary to obtain the watch or jewelry component of color X4, the difference AE in the CIE Lab color space between the colors X2 and X4 being preferably less than 10, preferably less than or equal to 8, and more preferably less than or equal to 7.When the color X2 is similar to the color X1, itself similar to the color X, it is the initial plasmonic effect and the initial color X of the gold nanoparticles which are preserved, the difference AE in the CIE Lab color space between the colors X and X3 being less than 10, more preferably less than or equal to 8, and more preferably less than or equal to 7, and the difference AE in the CIE Lab color space between the colors X and X4 being preferably less than 10, more preferably less than or equal to 8, and more preferably less than or equal to 7.

[0066] In a particularly preferred method of the invention, sub-step a1) comprises mixing an aqueous solution of the gold precursor (HAuCk) with an aqueous solution of sodium citrate with a gold / sodium citrate molar ratio in solution of between 1 / 50 and 1 / 20 at room temperature of 25°C, the surfactant used in sub-step a2) is polyvinylpyrrolidone (PVP) in aqueous solution, the gold / PVP molar ratio in solution being between 1 / 1 and 3 / 1, and the precursor of the metal oxide used in sub-step a3) is tetraethoxysilane in alcoholic solution.In step a), we obtain Au@SiO2 nanoparticles having, in the dry state in powder form, a black color X2 defined in the CIE L*a*b space by the parameters -5 <a*< 5, -5 < b*< 5 et 0 <L*< 10, de préférence -1 <a*< 1 , -3 <b*< 3 et 0 <L*< 6, le semi-produit obtenu à l’étape b) étant de couleur X3 définie dans l'espace CIE L*a*b par les paramètres -5 <a*< 5, -5 <b*< 5 et 0< L*< 15, de préférence -1 <a*< 1 , -1 <b*< 1 et 0 <L*< 13, et le composant horloger ou de bijouterie obtenu à l’étape c) étant de préférence de couleur X4 définie dans l'espace CIE L*a*b par les paramètres -5 <a*< 5, -5 <b*< 5 et 0< L*< 15, de préférence -1 <a*< 1 , -1 <b*< 1 et 0 <L*< 13.

[0067] The present invention also relates to a semi-finished product in a material comprising at least 18 carats of gold capable of being obtained during the implementation of the method for manufacturing a watch or jewelry component as defined above, said semi-finished product having a color defined in the CIE L*a*b space by the parameters -5 <a*< 5, -5< b*< 5 et L*< 15, de préférence -1 <a*< 1 , -1 <b*< 1 et L*< 13.

[0068] The present invention also relates to a watch or jewelry component capable of being obtained by the manufacturing method as described above, and obtained from a semi-finished product which has a color defined in the CIE L*a*b space by the parameters -5 <a*< 5, -5 <b*< 5 et 0 <L*< 15, de préférence -1 <a*< 1 , -1 <b*< 1 et 0 <L*< 13, ledit composant horloger ou de bijouterie présentant de préférence une couleur définie dans l'espace CIE L*a*b par les paramètres -5 <a*< 5, -5 <b*< 5 et 0 <L*< 15, de préférence -1 <a*< 1 , -1 <b*< 1 et 0 <L*< 13.

[0069] Such a watch or jewelry component is, for example, an oscillating weight, a watch casing element, or a piece of jewelry.

[0070] The following example illustrates the present invention without, however, limiting its scope.

[0071] Example

[0072] A citrate solution (A) is prepared comprising 54.4 ml of Milli-Q water and 2.72 g of sodium citrate. A gold solution (B) is prepared comprising 28 ml of Milli-Q water and 136 mg HAuCk 3H2O. A PVP solution (C) is prepared comprising 630 ml of Milli-Q water and 1.55 g of PVP (10K). Solution A is poured into solution B and the reaction is stirred for 10 minutes. The synthesized black gold colloid is then poured into solution C and vigorous stirring is maintained overnight. The black gold nanoparticles are collected by centrifugation and transferred into an EtOH and ammonia solution (6 ml EtOH + 0.375 ml of ammonia). A solution of TEOS in EtOH (4 ml EtOH + 0.27 ml TEOS) is immediately added dropwise with vigorous stirring. The reaction is stopped after 3 hours.The Au@silicon oxide (Au@SiO2) nanoparticles are then washed 3 times with EtOH and collected by centrifugation to be finally dried in an oven at 60°C overnight.

[0073] The obtained Au@silicon oxide nanoparticles are characterized by transmission electron microscopy (TEM). Figure 3 shows that the obtained Au@silicon oxide nanoparticles are in the form of multiform, compact, reticular gold coated with a silica shell. The dimensions of the nanoparticles are between 3 and 100 nm, with a silica shell of approximately 12 nm thickness.

[0074] The Au@SiO2 nanoparticles in solution obtained in step a) are dried in powder form, by several washes with ethanol, centrifugation and then dried for 12 hours in an oven at 60°C.

[0075] The Au@SiO2 nanoparticles in dry powder form have an X2 color defined in the CIE L*a*b space by the parameters a*=0.4, b* = -2.8, L* = 5.5, the measurements being carried out with an X-Rite i1 Pro3 instrument. The reflectance of the Au@SiO2 nanoparticles is measured by the X-Rite i1 Pro3 instrument for seven samples. For comparison, the reflectance of a black aluminum foil is measured. The curves shown in Figure 4 are obtained, which correspond well to black Au@SiO2 nanoparticles.

[0076] The Au@SiO2 nanoparticles are then compacted by hot isostatic pressing (HIP) at 300°C, 667 bar for 1 hour. The Au@SiO2 nanoparticles exhibit an intermediate X' color defined in the CIE L*a*b space by the parameters a* =0.6, b* = -0.5, L* = 13.6. The AE in the CIE Lab color space between X2 and X' is 8.4.

[0077] Then, pyrolysis is carried out to obtain a semi-finished product containing 18 carats of gold of color X3 defined in the CIE L*a*b space by the parameters a* = 0.2, b* = -0.9, L* = 12.7. The AE in the CIE Lab color space between X3 and X2 is 7.4. The AE relative to the compacted state after HIP, between X3 and X', is approximately 1.

[0078] This shows that the pyrolysis of Au@SiO2 nanoparticles does not alter the color of Au@SiO2 nanoparticles obtained during the previous manufacturing steps. In particular, the color of the synthesized Au@SiO2 nanoparticles is preserved and is not altered.

[0079] In addition, very black gold nanoparticles and Au@SiO2 nanoparticles are obtained, and therefore a component in a composite material comprising 18 carat gold and a ceramic for which the initial black color of the gold nanoparticles has been preserved and is not altered.

Claims

Demands 1. A process for manufacturing a watch or jewelry component from a material comprising at least 18 carats of gold, said process comprising the following steps: a) preparing a solution of Au@metal oxide nanoparticles comprising a gold core covered with a metal oxide shell, step a) comprising at least the substeps: a1) synthesizing gold nanoparticles by reacting a gold precursor with a reducing agent, said gold nanoparticles having dimensions and shapes giving them a plasmonic effect at least in the visible range; a2) mixing the gold nanoparticles obtained in step a1) with a solution of a surfactant comprising functional groups coupling with the metal oxide, and keeping under agitation for a sufficient time for said surfactant to coat the gold nanoparticles, while maintaining a plasmonic effect;a3) form the metal oxide shell by reacting a metal oxide precursor with the gold nanoparticles obtained in substep a2), while maintaining a plasmonic effect; b) produce a semi-product in a material comprising at least 18 carats of gold from the solution obtained in step a) for the purpose of forming said watch or jewelry component, while maintaining a plasmonic effect, the semi-product obtained having a color such that the AE difference in the CIE Lab color space between the color of the semi-product obtained and the color of the Au@metal oxide nanoparticles formed in step a), in the dry state, is less than 10, preferably less than or equal to 8; (c) to produce the watch or jewelry component in said material comprising at least 18 carats of gold from the semi-finished product obtained in step (b), while maintaining a plasmonic effect. A manufacturing process according to claim 1, characterized in that step (a) comprises an intermediate substep (a4) between substep (a2) and substep (a3), comprising the removal of excess surfactant. A manufacturing process according to any one of the preceding claims, characterized in that the surfactant is selected from the group comprising polyvinylpyrrolidone (PVP), gelatins, silane derivatives, and silicate derivatives, preferably polyvinylpyrrolidone (PVP). A manufacturing process according to any one of the preceding claims, characterized in that step (b) consists of a process selected from the group comprising a 3D printing process, a radiation exposure process, and combinations thereof.A manufacturing process according to any one of claims 1 to 3, characterized in that step b) comprises the substeps: b1) mixing gold and metal oxide nanoparticles obtained in step a) with a fluid preceramic polymer so as to obtain a composite fluid comprising at least 18 carats of gold; b2) shaping the composite fluid obtained in substep b1) so as to form the semi-product; b3) carrying out pyrolysis of the composite fluid shaped according to substep b2) to obtain the semi-product. Manufacturing process according to claim 5, characterized in that the preceramic polymer is chosen from the group comprising polycarbosiloxanes, polycarbosilanes, polysilazanes, polycarbosilazanes, polysilanes and polysilsesquiazanes to form ceramics derived from polymers respectively silicon oxycarbide, silicon carbide, silicon nitride, silicon carbonitride, silicon oxynitride. A manufacturing process according to any one of claims 5 and 6, characterized in that substep b2) consists of a process selected from the group comprising a 3D printing process, a radiation exposure process, a casting process, a pressing process, and an injection molding process. A manufacturing process according to any one of claims 1 to 3, characterized in that step b) comprises the following substeps: b4) preparing a powder from a solution of gold nanoparticles and metal oxide obtained in step a); b5) mixing said powder obtained according to substep b4) with a ceramic so as to obtain a composite powder comprising at least 18 carats of gold; b6) shaping the composite powder obtained in substep b5) so as to form the semi-product; b7) pyrolyzing the composite powder shaped according to substep b6) to obtain the semi-product.A manufacturing method according to claim 8, characterized in that the ceramic is selected from the group comprising zirconia, alumina, silicon oxycarbide, silicon carbide, silicon nitride, silicon carbonitride, silicon oxynitride, titanium carbide, titanium nitride, titanium diboride, and boron carbide. A manufacturing method according to any one of claims 8 and 9, characterized in that substep b6) consists of a method selected from the group comprising a casting method, a pressing method, and an injection molding method. A manufacturing method according to any one of the preceding claims, characterized in that step c) comprises a mechanical machining and / or finishing treatment to obtain said watch or jewelry component. A manufacturing process according to any one of the preceding claims, characterized in that the gold nanoparticles obtained in substep a1) have dimensions less than 200 nm. A manufacturing process according to any one of the preceding claims, characterized in that the reducing agent is NaBH4 or sodium citrate, preferably sodium citrate. A manufacturing process according to any one of the preceding claims, characterized in that substep a1) comprises mixing an aqueous solution of the gold precursor with an aqueous solution of reducing agent having a gold / reducing agent molar ratio in solution of between 1 / 50 and 1 / 20 at an ambient temperature of 25°C. A manufacturing process according to any one of the preceding claims, characterized in that the metal oxide shell has a thickness of less than 100 nm.A manufacturing process according to any one of the preceding claims, characterized in that the gold precursor is a gold salt, preferably tetrachloroauric acid. A manufacturing process according to any one of the preceding claims, characterized in that the metal oxide precursor is selected from the group comprising tetraethoxysilane for a silica shell, titanium butoxide for a titanium dioxide shell, and zirconium butoxide for a zirconia shell.A manufacturing process according to any one of the preceding claims, characterized in that substep a1) comprises mixing an aqueous solution of the gold precursor with an aqueous solution of sodium citrate having a molar ratio of gold / sodium citrate in solution between 1 / 50 and 1 / 20 at an ambient temperature of 25°C, the gold nanoparticles obtained in substep a1) having a black color defined in the CIE L*a*b color space by the parameters -5 <a*< 5, -5 <b*< 5 et L*< 10, de préférence -1 <a*< 1 , -3 <b*< 3 et L*< 6. A manufacturing process according to any one of the preceding claims, characterized in that substep a1) comprises mixing an aqueous solution of the gold precursor with an aqueous solution of sodium citrate having a gold / sodium citrate molar ratio in solution of between 1 / 50 and 1 / 20 at an ambient temperature of 25°C, in that the surfactant used in substep a2) is polyvinylpyrrolidone (PVP) in aqueous solution, the gold / PVP molar ratio in solution being between 1 / 1 and 3 / 1, and in that the metal oxide precursor used in substep a3) is tetraethoxysilane in alcoholic solution, to obtain Au@SiO2 nanoparticles having, in the dry state, a color defined in the CIE L*a*b color space by the parameters -5 <a*< 5, -5 <b*< 5 et L*< 10, de préférence -1 <a*< 1 , -3 <b*< 3 et L*< 6, la couleur du semi-produit obtenu à l’étape b) étant définie dans l'espace CIE L*a*b par les paramètres -5 <a*< 5, -5 <b*< 5 et L*< 15,preferably -1 <a*< 1 , -1 <b*< 1 et L*< 13. Semi-produit dans un matériau comprenant au moins 18 carats d’or susceptible d’être obtenu lors de la mise en oeuvre du procédé de fabrication d’un composant horloger ou de bijouterie selon l’une des revendications précédentes, caractérisé en ce que ledit semi-produit présente une couleur définie dans l'espace CIE L*a*b par les paramètres -5 <a*< 5, -5< b*< 5 et L*< 15, de préférence -1 <a*< 1 , -1 <b*< 1 et L*< 13. Composant horloger ou de bijouterie susceptible d’être obtenu par le procédé de fabrication selon l’une des revendications précédentes, caractérisé en ce qu’il est obtenu à partir d’un semi-produit qui présente une couleur définie dans l'espace CIE L*a*b par les paramètres -5 <a*< 5, -5< b*< 5 et L*< 15, de préférence -1 <a*< 1 , -1 <b*< 1 et L*< 13. Composant horloger ou de bijouterie selon la revendication 21 , caractérisé en ce qu’il consiste en une masse oscillante, un élément d'habillage d’une montre, un bijou.,

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