Patent Application
20210017032
The present invention relates to a method for manufacturing alloys of precious metals. The present invention also relates to such alloys of precious metals and the parts made with these alloys. In particular, the present invention relates to a method for manufacturing light alloys of precious metals obtained from gold, silver, platinum, palladium, ruthenium or iridium. The light alloys of precious metals in question here are titratable, that is to say that these are alloys, the ratio of which between the mass of the precious metal that enters into the composition of the alloy and the total mass of this alloy is determined by law.
In the sense of the present invention and hereinafter, precious metal means a metal chosen from the group consisting of gold, silver, platinum, palladium, ruthenium and iridium.
The precious metals such as gold are used in numerous fields such as jewellery and horology. Gold has the disadvantage of being easily deformable with the corollary that a simple impact suffices to deform the jewel made using this precious metal. This is why very early it was sought to improve the mechanical properties of gold by alloying it with other metal elements. In this regard, silver and copper are the two main metals used to improve the rigidity of gold. The alloying of gold with other metal elements such as silver or copper leads to metal alloys, the hardness of which is greater than that of gold. Nevertheless, these alloys of gold have the disadvantage of having a high density. This is why it was attempted to alloy gold with metal elements having a lower density.
Trials were carried out to attempt to alloy gold (Au), which is a heavy metal, that is to say a metal, the density of which is high (approximately 19.3 g·cm−3), with boron (B), which is a very light metal, that is to say a metal, the density of which is low (approximately 2.3 g·cm−3). Nevertheless, the attempts carried out up to today to attempt to alloy gold and boron ended in failures or, at best, led to very low rates of dissolution of the boron, not allowing to envisage industrial production. The materials resulting from the combination of gold and boron indeed turned out to be unstable and it turned out to be impossible to create titratable massive components such as 18-karat gold using this combination. These problems are explained in particular by the fact that during a conventional method of alloying by melting the elements, it is not managed to mix the gold and the boron. Indeed, because of its high density, the gold has a tendency to sediment at the bottom of the crucible, while the boron having a lower density floats.
Thus, numerous documents such as chapter 10 of the Handbook of Solid State Chemistry, First Edition, Edited by Richard Dronskowski, Shinichi Kikkawa, and Andreas Stein, published in 2017, note the impossibility of creating borides of precious metals rich in boron, namely MxBy metal borides in which M is a metal with a ratio of y to x greater than or equal to 1.
For example, in the case of palladium, it was possible to obtain metal borides, the boron content of which did not exceed Pd6B to Pd2B. It was possible to obtain PtB0.7 for platinum, which is located at the lower limit of the metal borides rich in boron. For an 18-karat gold, that is to say containing 75% by mass of gold, it is necessary to have a composition close or equal to AuB5.7, which, to the best of the applicant's knowledge, has not been carried out for the moment.
The goal of the present invention is to provide a method for manufacturing light alloys of precious metals allowing in particular to obtain light alloys of precious metals stable from a physicochemical point of view and using which it is possible to create massive components. More precisely, the method according to the invention involves creating an alloy of a precious metal and of boron by reaction of a source of said precious metal with a source of boron in a mixture of molten salts acting as a solvent. Preferably, the source of boron is a sodium borohydride and the source of precious metal is a chloride of said precious metal. The alloy coming from this method is formed by nanoparticles of boride of precious metal MxBy, where M is the precious metal, distributed in a matrix of boron B. Preferably, the ratio y/x of the nanoparticles of boride of precious metal MxBy is greater than or equal to 1 and, more preferably, greater than or equal to 2. The method according to the invention thus allows to create alloys of precious metals rich in boron.
According to an embodiment of the invention, the alloy of precious metal and of boron is directly used to manufacture a part via powder metallurgy.
According to another embodiment of the invention, the alloy of precious metal and of boron coming from the method by synthesis in molten salts according to the invention is enriched with precious metal before manufacturing the part by powder metallurgy.
The present invention thus relates to the alloy of precious metal and of boron directly coming from the method for manufacturing by synthesis in molten salts as well as the alloy enriched with precious metal. It also relates to the parts, in particular the timepieces or pieces of jewellery, made with the alloy of precious metal and of boron directly coming from the method for manufacturing by reaction in molten salts or with this same alloy enriched with precious metal. Indeed, it is possible for the ratio y/x to be too great to create, for example, an 18-karat gold. In this case, the matrix of boron is enriched with the precious metal.
The method according to the invention allows to obtain alloys of precious metal and of boron that have both excellent mechanical properties and a low density. To the best of the applicant's knowledge, the method according to the invention offers, for the first time, the possibility of alloying, on an industrial scale, a component with a very low density, in this case boron, with a precious metal, in particular but not exclusively gold, the density of which is high. Remarkably, in the method according to the invention, the precious metal selected and the boron are closely alloyed, without it ever being possible to observe a phenomenon of segregation between the two materials.
The present invention relates to a method for manufacturing a boride of a precious metal, also called hereinafter alloy of precious metal and of boron, and a method for manufacturing a part made from this alloy. The alloy is manufactured by synthesis in molten salts, a synthesis also known by its Anglo-Saxon name Synthesis in Molten Salts or SMS. This synthesis involves placing reactive substances of the precious metal and of the boron in each other's presence in a medium comprising salts. When the whole is heated, the salts melt, consequently acting like a liquid medium. The synthesis of the alloy of precious metal and of boron in molten salts uses a source of metal and a source of boron. The source of metal can be chosen from the group comprising the sulphates, the carbonates, the acetates, the nitrates, the acetylacetonates and the halides of the precious metal. Preferably, the source of precious metal is a halide and, even more precisely, a chloride of the precious metal (MClx). The precious metal is chosen from gold (Au), silver (Ag), platinum (Pt), palladium (Pd), ruthenium (Rh) and iridium (Ir) and, more preferably, from gold, silver, platinum and iridium. The source of boron can be chosen from the group comprising the boranes (BxHy) and the borohydrides (MBH4). Preferably, the source of boron is sodium borohydride (NaBH4). Thus, the reaction is preferably carried out in the presence of a chloride of the precious metal, such as AuCl3 for gold (Au), and of sodium borohydride (NaBH4). With regard to the salts acting as a reaction medium, they are preferably soluble in water to allow the recovery of the boride after the reaction. For example, this can be a mixture of one or more salts of alkali metals and more precisely of halides, of carbonates, of sulphates or of nitrates. Preferably, this can be a eutectic mixture of lithium chloride and of potassium chloride in a ratio of 45-50% by weight of LiCl and 55-50% by weight of KCl which has a melting point of approximately 355° C. The salt is preferably present in a molar quantity greater than the total molar quantity of the boron in the source of boron and of the metal in the source of precious metal.
The salt it typically solid at ambient temperature and is melted at a temperature between 100 and 1000° C., preferably between 355 and 900° C. during the reaction. Ideally, this is above the melting temperature of the mixture of salts, but below the vaporisation temperature of this mixture. For example, the mixture LiI/KI partly vaporises beyond 850° C.
Moreover, the reactive medium can optionally include one or more additives having the function of controlling the size of the particles and/or the morphology of the boride obtained. This can, for example, be an iodide such as an iodide of potassium or of sodium. The quantity of additive is preferably between 1 and 100 moles per mole of metal in the source of precious metal.
The method can be carried out at ambient pressure or at a pressure greater than ambient pressure. The atmosphere can be controlled. Thus, the use of salts of lithium and of potassium requires having to work under an inert atmosphere, because of the sensitivity of these chemical products to water and/or to oxygen. Consequently, the precursors are manipulated and mixed under an inert atmosphere of argon. The synthesis itself is carried out under an atmosphere of argon and not of nitrogen, given that nitrogen is capable of reacting with certain species of boron and of leading to the formation of boron nitride.
The method is carried out by mixing the source of precious metal, the source of boron and the salt(s). The whole is heated to the desired temperature to melt the salt or the mixture of salts and maintained at this temperature for a time preferably between 30 minutes and 10 hours. After the reaction, the reactive medium is left to cool preferably naturally. Borides of metal in the form of aggregates dispersed in a volume of solid salts are obtained. To eliminate the salts, cycles of washing/centrifugation in a polar solvent such as water or methanol are carried out.
The alloy coming from this method of manufacturing in molten salts is in the form of a powder formed by aggregates of crystalline nanoparticles of metal boride MxBy dispersed in a matrix of amorphous boron B. Nanometric particles means particles, the size of which is between 5 and 200 nm, preferably between 10 and 100 nm. The aggregates typically have a size between 0.3 and 1 micrometre. Preferably, the stoichiometric ratio y/x of the metal boride By that composes the crystalline nanoparticles is greater than or equal to 1 and, more preferably, greater than or equal to 2. Thus, for an alloy of 18-carat gold, the nanoparticles must correspond to the composition AuBy with y close to 6.
According to a first embodiment of the invention, the alloy coming from the method for manufacturing by synthesis in molten salts is directly used to manufacture a part by powder metallurgy. The powder formed by the aggregates is used as such or is previously ground to obtain a powder with a D50 of less than 70 μm. In other words, 50% of the particles forming the powder have a diameter less than or equal to 70 μm.
According to a second embodiment of the invention, the alloy is enriched with precious metal before the manufacturing of the part by powder metallurgy. This enrichment is carried out via the additional steps involving:
It is specified that elements other than precious metals such as, for example, nickel can be added to the mixture during this step.
The method for manufacturing the part, whether it is with the alloy according to the first alternative or with the alloy according to the second alternative, then includes the following steps:
Once the ingot of alloy of precious metal and of boron has been micronised, a first possibility for obtaining the desired massive part involves introducing the powder resulting from the micronisation treatment into a mould and subjecting this mould to a uniaxial or isostatic pressure.
Once the ingot of alloy of precious metal and of boron has been micronised, a second possibility for obtaining the desired massive part involves subjecting the powder resulting from the micronisation treatment to a treatment of three-dimensional additive manufacturing.
The treatment of three-dimensional additive manufacturing can be of the type with direct printing. The available techniques of three-dimensional additive manufacturing of the direct type are laser sintering, also known by its Anglo-Saxon name Selective Laser Melting or SLM and the sintering by electron bombardment also known by its Anglo-Saxon name e-beam melting.
The treatment of three-dimensional additive manufacturing can be of the type with indirect printing. The available techniques of three-dimensional additive manufacturing of the indirect type are:
Once the ingot of alloy of precious metal and of boron has been micronised, a third possibility for obtaining the desired massive part involves subjecting the powder resulting from the treatment of micronisation of the ingot to a treatment of three-dimensional additive manufacturing, of injection or of micro-injection in the presence of a polymer binder. Thus, the powder resulting from the treatment of micronisation of the ingot of alloy of precious metal and of boron is mixed with the polymer binder in order to obtain a feedstock. Then a green body, the shape of which corresponds to the profile of the desired part, is created by subjecting the feedstock either to an injection or to a micro-injection, or to a technique of additive manufacturing.
Among the indirect available techniques of additive manufacturing in the presence of a polymer binder, the following can be mentioned:
Alternatively, the mixture between the binder and the powder can be directly created during the additive manufacturing by using the technique of binder jetting which involves projecting a jet of ink containing a solvent and a binder onto a bed of powder formed by the particles of powder resulting from the micronisation treatment.
As for the binder, it is chosen from the group formed by polyethylene glycol (PEG), cellulose acetate butyrate (CAB), nano-cellulose (nanometric derivative of cellulose), corn starch, sugar, polylactic acid (or PLA), polyethylene, polypropylene, synthetic or natural wax and stearic acid.
A brown body is obtained by subjecting the green body to a step of elimination of the polymer binder called step of debinding during which the green body is treated chemically, then thermally in a furnace to burn the residual polymer binder, this step of debinding typically being carried out in gaseous phase in an atmosphere of nitric acid or of oxalic acid and at a temperature between 100° C. and 140° C.
Finally, the brown body is subjected to a treatment of sintering under a protected atmosphere and at a temperature between 700° C. and 1800° C. in order to obtain the desired part.
It is noted that it is possible, after the sintering treatment, to subject the part coming from the sintering step to a step of post-treatment via hot isostatic pressing (or HIP) under a pressure between 500 bar and 2200 bar, and at a temperature between 600° C. and 2100° C.
The part thus manufactured is made from an alloy made of boride of precious metal. This alloy is, according to the first alternative, formed by crystalline nanoparticles of MxBy where M is the precious metal distributed in an amorphous matrix of B boron. According to the second alternative, the alloy enriched with precious metal includes the crystalline nanoparticles of MxBy distributed in an amorphous matrix of boron B and of boride MzBa of precious metal. The precious metal M is chosen from gold (Au), silver (Ag), platinum (Pt), palladium (Pd), ruthenium (Rh) and iridium (Ir). Preferably, it is chosen from gold, silver, platinum, and iridium and more preferably it is gold. Preferably, the ratio y/x of the MxBy nanoparticles is greater than or equal to 1, more preferably, it is greater than or equal to 2. As for the ratio a/z, it is typically less than or equal to 1.
The light alloys of precious metals in question here are titratable, that is to say that these are alloys, the ratio of which between the mass of the precious metal that enters into the composition of the alloy and the total mass of this alloy is determined by law. A remarkable alloy of precious metal obtained via the method of the invention is an 18-karat alloy of gold and of boron having the composition AuB6 with a density between 6.6 and 7 g/cm3. It is also noted that the powder of gold used in the context of the present invention is preferably a powder of 24-karat bright ½ yellow gold.
The part can, in particular, be a timepiece or a piece of jewellery and, more precisely, an external part such as a middle, a bottom, a bezel, a pushbutton, a wristlet link, a dial, a hand, a dial index, etc.
It goes without saying that the present invention is not limited to the embodiments that have just been described and that various simple modifications and alternatives are possible for a person skilled in the art without going beyond the context of the invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 19187108.6 | Jul 2019 | EP | regional |
