Process for preparing dispersion-hardened precious metal articles

Abstract

A process for preparing a dispersion-hardened article is described, comprising: (i) mixing a metal oxide powder comprising particles of one or more metal oxides with a metal powder comprising particles of a precious metal or precious metal-based alloy by resonant acoustic mixing (RAM) to produce a RAM mixed powder; and (ii) converting the RAM mixed powder from step (i) into a desired article.

Description

[Technical Field]

[0001] The present invention relates to a method for preparing dispersion-hardened precious metal articles. [Background technology]

[0002] Dispersion hardening is the strengthening of a material due to the presence of a dispersion of insoluble material, usually metal oxide particles, in the lattice. These materials are often referred to as "dispersion hardened" or "dispersion hardened" (DPH). Dispersion hardened platinum is used to produce articles that are required to operate at high temperatures, such as equipment for use in molten glass production (e.g., bushings, crucibles, stirrers) or platinum wire used in thermocouples. The traditional route to dispersion hardened materials involves introducing a dopant metal into the lattice and then performing a high-temperature oxidation treatment to convert the dopant to the corresponding oxide. This is often referred to as "internal oxidation."

[0003] For example, in the paper "Practical Experience with New Oxide Dispersion Hardened Platinum Materials," Fischer et al. (25 th The International Precious Metals Conference, June 9-12, 2001, Tucson, Arizona, USA, describes the preparation of dispersion-hardened platinum by introducing the elements zirconium, yttrium, or cerium into platinum during the melting process. The molten alloy is cast into ingots. During subsequent forming operations, the semi-finished products (typically sheets, tubes, and rods) are subjected to an annealing process in an oxidizing medium, which leads to the internal oxidation of the alloying elements. The authors conclude that significant strengthening of the precious metal matrix can be expected when the dispersed oxide particles are very small (less than 1 μm in diameter) and separated by small distances (10 μm or less), where they hinder the movement of dislocations in the matrix, thus leading to increased strength and a low creep rate.

[0004] As another example, Chinese Patent No. 111519058(A) describes a process in which platinum-based materials doped with active elements are pulverized to obtain alloy powders, followed by oxidation treatment. Preferred active elements are Zr, Y, Sc, La, Eu, Ce, and Er. Typical conditions for oxidation treatment are 750-1280°C for 3-5 hours.

[0005] Although the internal oxidation route is widely practiced, it has several drawbacks. First, to achieve sufficient conversion of the dopant to the corresponding oxide, the material must be heat-treated at high temperatures for long periods of time, which is energy-intensive. Long treatments at high temperatures also lead to grain coarsening, which adversely affects strength, meaning that it is difficult to find a balance between achieving sufficient oxidation of the dopant (longer treatments are advantageous) and keeping grain growth to a minimum (shorter treatments are advantageous). Second, it is impossible to reliably control the ratio of dopant to the corresponding oxide.

[0006] A different approach is described in Chinese Patent No. 114406274(A). In this method, a colloid containing Zr ions and Y ions in a molar ratio of 5-10:1 is combined with platinum powder using resonant coating (also known as resonant acoustic mixing) to obtain a mixed powder. The mixed powder is then dried and filled into a mold, after which it is annealed at 1200-1400°C for 20-50 minutes and then forged to obtain a dispersion-hardened material. Forging is performed 15-20 times. While the annealing step in this method is shorter than the oxidation treatment performed in traditional internal oxidation methods, it is still a complex method that does not allow for complete control of the degree of conversion to dopant oxide.

[0007] There is a need for an alternative method for preparing dispersion-hardened articles, ideally one in which the content of the dispersed hardening phase (metal oxide) can be precisely controlled and which does not require prolonged high temperature processing. The present invention solves this problem. Summary of the Invention

[0008] In a first aspect, the present invention provides a method for producing a medicament for the treatment of a pulmonary arthritis, comprising: (i) mixing a metal oxide powder comprising particles of one or more metal oxides with a metal powder comprising particles of a precious metal or precious metal-based alloy by resonant acoustic mixing (RAM) to produce a RAM mixed powder; (ii) converting the RAM powder mixture from step (i) into a desired article.

[0009] The present method avoids the need for an oxidation treatment by using metal oxides as raw materials. The inventors are not aware of any previous reports of dispersion-hardened precious metal materials being made directly from the required precious metal or alloy powders and metal oxide powders. This route appears to have been ignored previously due to the inability to guarantee the degree of dispersion necessary to give consistent properties in the final powder product (see "Dispersion Strengthened Platinum" in Platinum Metals Rev., 1974, 18, (2) pp. 46-57). The inventors have solved this problem by combining the precious metal or alloy powder with the metal oxide powder using resonant acoustic mixing (RAM).

[0010] RAM is a known technology. For example, International Patent Application Publication No. 2022 / 145771(A1) describes the preparation of a catalyst slurry using RAM, which includes a catalyst, a heat-dissipating material, an ionomer, and a dispersing medium. The catalyst slurry is applied to a substrate to form a slurry layer, followed by a step of removing the dispersing medium from the slurry layer.

[0011] The paper "Efficient production of a high-performance dispersion strengthened, multi-principal element alloy," Scientific Reports (2020) 10:9663, and related U.S. Patent Application Publication No. 2020 / 399744(A1), describe the preparation of a dispersion-hardened NiCoCr alloy. The process involves combining NiCoCr powder with nanoscale Y2O3 by RAM to produce NiCoCr particles with a thin film of Y2O3 on their surfaces, followed by forming these particles into a dense part using laser powder bed fusion.

[0012] Chinese Patent No. 115505814(A) describes a medium-entropy alloy composite of FeCrNi reinforced with Y-Ti-O oxide. The material can be prepared by mixing an FeCrNi pre-alloy, a Y source, and a Ti source by RAM under an argon atmosphere to obtain a mixed powder, and then performing mechanical ball milling on the mixed powder to obtain a ball milled powder, which is then sintered to obtain the alloy material.

[0013] U.S. Patent Application Publication No. 2020 / 0399744(A1) describes the preparation of metal particles having a coating of ceramic particles for use in additive manufacturing processes, where the metal particles comprise an alloy including 30-35 wt.% cobalt, 26-31 wt.% chromium, 0-3.0 wt.% rhenium, 0-1.0 wt.% aluminum, 0.01-0.1 wt.% carbon, 0-1.0 wt.% titanium, and the balance nickel, and the ceramic particles comprise yttrium oxide, hafnium oxide, zirconium oxide, aluminum oxide, thorium oxide, or a combination thereof, where rhenium is an optional component in the alloy.

[0014] EP 2853611 B1 describes a method for preparing superalloy particles. In a first step, ceramic particles are mixed with superalloy mother particles to form a powder mixture, which is then formed into a consumable solid. Mixing is preferably accomplished by RAM.

[0015] RAM has been proposed for mixing powders, liquids, pastes, and viscous materials, but the production of precious metal-based DPH materials by mixing precious metal powders and dopant oxide powders with RAM has not previously been proposed.

[0016] The inventors have discovered that RAM mixed powders, including particles of a precious metal or precious metal-based alloy and particles of one or more metal oxides, can be converted into dispersion-hardened materials using powder metallurgy techniques. As used herein, "powder metallurgy" refers to the process of pressing powder material into a desired form and sintering to bond the powder particles together. The article may be in the desired shape after the powder metallurgy process, or subsequent working steps may be performed to produce the desired article. [Brief explanation of the drawings]

[0017] [Figure 1] This is an SEM image of the RAM mixed powder prepared by mixing platinum powder and nano-ZrO2 particles with RAM. [Figure 2] 1 is an SEM image of a powder prepared by dilution of RAM mixed powder with additional platinum powder. [Figure 3a] 1 is an SEM image of a dispersion-hardened platinum material according to the present invention in wire form. [Figure 3b] 1 is an SEM image of a dispersion-hardened platinum material according to the present invention in wire form. DETAILED DESCRIPTION OF THE INVENTION

[0018] Any subheadings are for convenience only and are not intended as limitations on the invention.

[0019] Metal Oxide Powder The distribution of metal oxides in the article provides a dispersion hardening effect. Any metal oxide known to provide a dispersion hardening effect can be used in the present invention. Preferred metal oxides include yttrium oxide (YO), titanium oxide (TiO), zirconium oxide (ZrO), stabilized zirconia, hafnium oxide (HfO), scandium oxide (ScO), aluminum oxide (AlO), rare earth oxides (LnO), or thorium oxide (ThO), or a combination of two or more thereof. Preferred rare earth oxides are samarium oxide, lanthanum oxide (LaO), cerium oxide (CeO), samarium oxide (SmO), and gadolinium oxide (GdO).

[0020] In a preferred embodiment, the metal oxide powder consists essentially of a single metal oxide (i.e., at least 95% of the particles in the metal oxide powder are a single metal oxide), preferably selected from the group consisting of yttrium oxide (YO), titanium oxide (TiO), zirconium oxide (ZrO), stabilized zirconia, hafnium oxide (HfO), scandium oxide (ScO), aluminum oxide (AlO), rare earth oxides (LnO), and thorium oxide (ThO). This is the preferred option for simplicity.

[0021] The use of a combination of two or more metal oxides for dispersion hardening is also known. For example, U.S. Patent Application Publication No. 2022 / 081751 A1 describes a dispersion-hardened platinum composition containing a total of 0.05 to 1% by weight of zirconium oxide, yttrium oxide, and scandium oxide, with the proportions of these oxides being yttrium oxide (7.0 to 11.0 mol%), scandium oxide (0.1 to 5.0 mol%), and zirconia (the balance). Thus, in some embodiments, the metal oxide powder comprises two or more metal oxides, preferably two or more metal oxides selected from the group consisting of yttrium oxide (YO), titanium oxide (TiO), zirconium oxide (ZrO), stabilized zirconia, hafnium oxide (HfO), scandium oxide (ScO), aluminum oxide (AlO), rare earth oxides (LnO), and thorium oxide (ThO).

[0022] Conversion of the RAM mixed powder produced in step (i) into the desired article often involves processing steps carried out at high temperatures. Some metal oxides, such as zirconium oxide, undergo phase changes at high temperatures, which can lead to cracking in the article. Therefore, in some embodiments, it is preferred that the metal oxide powder comprises or consists essentially of stabilized zirconia, because stabilized zirconia does not undergo significant phase changes at typical temperatures used in powder metallurgy and is therefore less prone to cracking. Stabilized zirconia is generally based on a zirconia structure in which some of the Zr(IV) ions have been replaced with a different metal. Yttria-stabilized zirconia is a preferred example of stabilized zirconia.

[0023] metal powder The metal powder may comprise particles of a precious metal or particles of a precious metal-based alloy, i.e., particles of an alloy containing 50% or more by weight of a precious metal. As used herein, the term "precious metal" refers to a metal selected from the group consisting of gold, silver, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Because there are few applications for dispersion-hardened osmium, preferred precious metals are gold, silver, ruthenium, rhodium, palladium, iridium, and platinum. Because of their resistance to oxidation at high temperatures, it is particularly preferred that the metal particles be particles of a platinum group metal (PGM) or a PGM-based alloy, i.e., an alloy containing 50% or more by weight of PGM. As used herein, the term platinum group metal refers to the metals ruthenium, rhodium, palladium, osmium, iridium, and platinum. Because there are few applications for dispersion-hardened osmium, preferred platinum group metals are ruthenium, rhodium, palladium, iridium, and platinum.

[0024] In one preferred embodiment, the metal powder consists essentially of a single precious metal (i.e., at least 95% of the particles in the metal powder are a single precious metal), preferably a single pgm. Preferred precious metals and pgms are as described above. In this embodiment, the non-precious metal content (based on the total weight of the metal powder) is preferably 5 wt.% or less, preferably 4 wt.% or less, more preferably 3 wt.% or less, more preferably 2 wt.% or less, more preferably 1 wt.% or less. In demanding applications where the non-precious metal content must be kept particularly low, it is preferred that the non-precious metal content be 0.5 wt.% or less, preferably 0.1 wt.% or less.

[0025] In one preferred embodiment, the metal powder consists essentially of a single precious metal-based alloy (i.e., at least 95% of the particles in the metal powder are a single precious metal-based alloy), preferably a single pgm-based alloy. Preferred precious metals and pgms are as described above. In this embodiment, the non-precious metal content (based on the total weight of the metal powder) is preferably 50 wt.% or less.

[0026] Dispersion-hardened platinum and dispersion-hardened platinum-rhodium are most commonly used. Thus, in a preferred embodiment, the metal powder is platinum powder. In another preferred embodiment, the metal powder is platinum-rhodium alloy powder.

[0027] The metal oxide particles are preferably smaller than the PGM or PGM alloy particles. This ensures that the metal oxide particles are distributed on the surfaces of the metal particles in the RAM mixed powder and subsequently well dispersed in the article. To achieve the highest degree of reinforcement, it is particularly preferred that the metal oxide powder particles are nanoparticles, i.e., have a diameter of 1 to 1000 nm. The nanoparticles preferably have a maximum particle size of 100 nm or less.

[0028] The method is applicable to metal particles having a wide range of morphologies, including spherical particles and irregularly shaped particles.

[0029] Process (i) In step (i), metal oxide particles and metal particles are mixed by resonant acoustic mixing (RAM) to produce a RAM mixed powder. The frequency and duration of mixing can be optimized for a given combination of metal oxide and metal powder. While the degree of mixing can often be assessed visually, analysis of the RAM mixed powder by SEM can be used to assess the degree of mixing at a microscopic level.

[0030] The amount of metal oxide powder added depends on the end use of the article. Hereinafter, "wt%" refers to the amount of metal oxide powder added based on the total weight of the metal oxide powder and metal powder used in step (i). Because dispersion-hardened materials with a metal oxide content of more than 5 wt% tend to be brittle, it is preferred that the metal oxide powder be added in an amount of 5 wt% or less. Typically, the amount of metal oxide powder is 3 wt% or less. The lower limit depends on the degree of strengthening required. Typically, the metal oxide powder is added in an amount of 0.001 wt% or more, for example, 0.005 wt% or more, 0.01 wt% or more, 0.05 wt% or more, or 0.1 wt% or more. The preferred range of the amount of metal oxide powder added is 0.001 to 5 wt%, 0.005 to 5 wt%, 0.01 to 5 wt%, 0.05 to 5 wt%, or 0.1 to 5 wt%, preferably 0.001 to 3 wt%, 0.005 to 3 wt%, 0.01 to 3 wt%, 0.05 to 3 wt%, or 0.1 to 3 wt%.

[0031] When step (i) uses particles of a precious metal, the resulting RAM mixed powder preferably has a non-precious metal content (based on the total weight of the RAM mixed powder) of 5 wt.% or less, preferably 4 wt.% or less, more preferably 3 wt.% or less, more preferably 2 wt.% or less, more preferably 1 wt.% or less. For the avoidance of doubt, these wt.% refer to the amount of non-precious metal, regardless of whether the non-precious metal is in metallic or oxide form, and include non-precious metals from metal oxide powders. In demanding applications where the non-precious metal content must be kept particularly low, it is preferred that the non-precious metal content be 0.5 wt.% or less, preferably 0.1 wt.% or less.

[0032] When step (i) uses particles of a precious metal-based alloy, the resulting RAM mixed powder preferably has a non-precious metal content (based on the total weight of the RAM mixed powder) that is 50 wt. % or less. For the avoidance of doubt, this wt. % refers to the amount of non-precious metal, whether the non-precious metal is in metallic or oxide form, and includes non-precious metal from the metal oxide powder.

[0033] Optional step (ib) For large-scale operations, combining all of the metal oxide powder and metal powder by RAM may not be practical. A procedure more suitable for scale-up involves preparing a RAM mixed powder as described above in step (i), and then performing step (ib) of combining the RAM mixed powder from step (i) with additional metal powder using a mixing technique other than RAM. In this route, the RAM mixed powder essentially acts as a masterbatch and is diluted with additional metal powder in step (ib). The same metal powder is preferably used in steps (i) and (ib). The powder produced by step (ib) is referred to herein as a "partial RAM mixed powder." This technique has the advantage that not all of the materials need to be mixed by RAM, but traditional (non-RAM) mixing techniques lead to a less uniform distribution of the metal oxide compared to when all of the metal powder and metal oxide are mixed by RAM.

[0034] Process (ii) In step (ii), the RAM mixed powder, or the partial RAM mixed powder from step (ib), is converted into a desired article. The excellent distribution of metal oxides achieved via step (i) is expected to be beneficial regardless of the technique used to convert the powder into the desired article.

[0035] In a preferred embodiment, step (ii) involves a powder metallurgy step carried out on the RAM mixed powder (from step (i)) or partial RAM mixed powder (from step (ib)). Powder metallurgy involves pressing the powder into a desired form followed by sintering to bond the powder particles together.

[0036] In some embodiments, the powder metallurgy process can directly lead to an article of the desired shape. Some minor processing, such as surface finishing, may be required.

[0037] In some embodiments, the powder metallurgy process is followed by additional processing, for example, a powder metallurgy process that produces rod stock, followed by processing of the rod stock into wire stock.

[0038] Alternative Embodiments In an alternative embodiment, a dopant metal is added in step (i) instead of the metal oxide. The dopant metal is then oxidized to the corresponding metal oxide before or during step (ii) by performing an oxidation treatment. If step (ib) is performed, the dopant metal can be oxidized to the corresponding metal oxide before, after, or during step (ii). The dopant metal must be a metal capable of forming a metal oxide that achieves dispersion hardening of the precious metal; preferred dopant metals are yttrium, zirconium, hafnium, scandium, and samarium. This method achieves uniform dispersion of the dopant metal throughout the precious metal, resulting in a material after the oxidation treatment in which the metal oxide is uniformly dispersed throughout the precious metal. While this method requires an oxidation treatment and is therefore more energy-intensive than using metal oxide powder as a raw material in step (i), it is still less energy-intensive than known internal oxidation methods that require the initial formation of a melt to uniformly distribute the dopant throughout the precious metal. The preferred features described above in relation to steps (i), (ib) and (ii) apply equally to this alternative embodiment. [Example]

[0039] Example 1 Irregularly shaped platinum powder was sieved to a particle size of 20-75 μm. This fraction (367 g) was combined with nano-ZrO (2.6 g, 0.7 wt. % ZrO based on total powder) and the mixture was stirred for 80 seconds using a Resodyn Acoustic Mixer. -1 The mixture was subjected to RAM at a frequency of 1000 Hz for 20 minutes to produce a RAM composite material. Figure 1 shows an SEM image of the RAM composite material, in which ZrO nanoparticles (gray) are uniformly dispersed on the surface of the Pt particles (white).

[0040] Example 2 A portion (50 g) of the RAM mixture material from Example 1 was combined with the same irregularly shaped platinum powder using a shaker mixer for 30 minutes. An SEM image of the resulting material is shown in Figure 2. Although the RAM mixture material was diluted with platinum powder, the ZrO2 nanoparticles did not migrate to the platinum powder.

[0041] The resulting powder was formed into a bar measuring approximately 100 mm x 14 mm x 8 mm by powder metallurgy. This bar was processed into a wire with a diameter of 0.25 mm through a series of processes: hot forging, hot rolling, and cold drawing. Figures 3a and 3b show the surface of the wire. Zirconium oxide (dark spots) is reasonably well distributed with particles of approximately 10 μm or less.

Claims

1. 1. A process for preparing a dispersion-cured article, comprising: (i) mixing a metal oxide powder comprising particles of one or more metal oxides with a metal powder comprising particles of a precious metal or precious metal-based alloy by resonant acoustic mixing (RAM) to produce a RAM mixed powder; (ii) converting the RAM mixed powder from step (i) into a desired article.

2. The metal oxide powder is yttrium oxide (Y ２ O ３ ), titanium oxide (TiO ２ ), zirconium oxide (ZrO ２ ), stabilized zirconia, hafnium oxide (HfO ２ ), scandium oxide (Sc ２ O ３ ), aluminum oxide (Al ２ O ３ ), rare earth oxides (Ln ２ O ３ ), and thorium oxide (ThO ２ 10. The process of claim 1, wherein the metal oxide consists essentially of a single metal oxide selected from the group consisting of:

3. The metal oxide powder is yttrium oxide (Y ２ O ３ ), titanium oxide (TiO ２ ), zirconium oxide (ZrO ２ ), stabilized zirconia, hafnium oxide (HfO ２ ), scandium oxide (Sc ２ O ３ ), aluminum oxide (Al ２ O ３ ), rare earth oxides (Ln ２ O ３ ), and thorium oxide (ThO ２ 10. The process of claim 1, wherein the metal oxides are selected from the group consisting of:

4. 10. The process of claim 1, wherein the metal oxide powder comprises or consists essentially of stabilized zirconia.

5. The process of any one of claims 1 to 4, wherein the particles of the metal oxide powder are nanoparticles.

6. The process of claim 5 , wherein the nanoparticles have a maximum particle size of 100 nm or less.

7. 7. The process of any one of claims 1 to 6, wherein the metal oxide powder is added in an amount of up to 5 wt.%, based on the total weight of the metal oxide powder and metal powder used in step (i).

8. 8. The process of claim 7, wherein the metal oxide powder is added in an amount of 0.001 to 5 wt. %, based on the total weight of the metal oxide powder and metal powder used in step (i).

9. The process of any one of claims 1 to 8, wherein the metal powder consists essentially of a single platinum group metal.

10. 10. The process of claim 9, wherein the metal powder is platinum powder.

11. The process of any one of claims 1 to 8, wherein the metal powder is a platinum group metal based alloy powder.

12. The process of claim 11, wherein the metal powder is a platinum-rhodium alloy powder.

13. 13. The process of any one of claims 1 to 12, comprising an additional step (ib) carried out between steps (i) and (ii) of combining the RAM mixed powder with further particles of metal powder using a mixing technique other than RAM.

14. A process according to any one of claims 1 to 13, wherein step (ii) involves a powder metallurgy step carried out on the product of step (i) or (ib).

15. 15. The process of claim 14, comprising producing a rod by powder metallurgy and subsequently converting the rod into wire.

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