PROCESS FOR PREPARING A DISPERSION HARDENED PRECIOUS METAL ARTICLE

Abstract

A process is described for preparing a dispersion hardened article, comprising the steps of: (i) mixing a metal oxide powder comprising particles of one or more metal oxide(s), and 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 the desired article.

Description

TECHNICAL FIELD

The present invention relates to a method for preparing a dispersion hardened precious metal article.

BACKGROUND ART

Dispersion hardening is the strengthening of a material due to the presence of a dispersion of fine particles of a material, usually a metal oxide, which is not soluble in the lattice. These materials are often referred to as “dispersion hardened” or “DPH”. Dispersion hardened platinum is used to produce articles which are required to operate at high temperatures, such as equipment for use in molten glass production (e.g. bushings, crucibles, stirrers) or platinum wires used in thermocouples. Conventional routes to dispersion hardened materials involves introducing a dopant metal into the lattice and then carrying out a high temperature oxidation treatment in order to convert the dopant into the corresponding oxide. This is often referred to as “internal oxidation”.

For example, the article “Practical Experience with New Oxide Dispersion Hardened Platinum Materials” Fischer et al. (25th International Precious Metals Conference, Jun. 9-12, 2001, Tucson, Arizona, USA) describes the preparation of dispersion hardened platinum by introducing elemental 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 oxidising 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 (<1 μm diameter) and are separated by only a small distance (≤10 μm); in this case they hinder the movement of dislocations in the matrix and thus lead to an increase in strength and low creep rates.

As another example, CN111519058A describes a process in which a platinum-based material doped with an active element is atomised to obtain an alloy powder, followed by an oxidation treatment. Preferred active elements are Zr, Y, Sc, La, Eu, Ce and Er. Typical conditions for the oxidation treatment are 750-1280° C. for 3-5 hours.

Although the internal oxidation route is widely practiced there are a number of downsides to it. Firstly, to achieve sufficient conversion of the dopant to the corresponding oxide it is necessary to heat treat the material at high temperature for a long time, which is energy intensive. Extended treatments at high temperature also lead to grain coarsening which impacts negatively on strength, meaning that it is difficult to find a balance between achieving sufficient oxidation of the dopant (favoured by longer treatments) while keeping grain growth to a minimum (favoured by shorter treatments). Secondly, it is not possible to control with certainty the proportion of dopant and corresponding oxide.

A different approach is described in CN114406274A. In this method a colloid containing Zr ions and Y ions at a molar ratio of 5-10:1 is combined with a platinum powder using resonance coating (aka resonant acoustic mixing) to obtain a mixed powder. The mixed powder is then dried, compacted into a mold and then annealed at 1200-1400° C. for 20-50 minutes, then forged to obtain a dispersion hardened material. The forging is carried out 15-20 times. While the annealing step in this method is shorter than the oxidation treatment carried out in conventional internal oxidation methods, this is still a complex method without the ability to fully control the extent of conversion to the dopant oxide.

There is a need for an alternative method for preparing dispersion hardened articles, ideally in which the content of dispersion hardening phase (metal oxide) can be precisely controlled, and without requiring extended high-temperature treatment. The present invention solves this problem.

SUMMARY OF THE INVENTION

In a first aspect the invention provides a method comprising the steps of:

• • (i) mixing a metal oxide powder comprising particles of one or more metal oxide(s), and 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 the desired article.
  • The method avoids the need for oxidation treatment by using a metal oxide as a raw material.

The inventors are not aware of any previous reports of dispersion hardened precious metal materials being made directly from the requisite precious metal or alloy powder and the metal oxide powder. This route seems to have been disregarded previously due to the inability to guarantee the degree of dispersion required to give consistent properties in the final powder product (see “Dispersion Strengthened Platinum” in Platinum Metals Rev., 1974, 18, (2) p46-57). The present inventors have solved this problem by combining the precious metal or alloy powder and metal oxide powder using resonant acoustic mixing (RAM).

RAM is a known technique. For instance, WO2022/145771A1 describes the preparation of a catalyst slurry which includes a catalyst, a heat dissipation material, an ionomer and a dispersion medium using RAM. The catalyst slurry is applied onto a base material to form a slurry layer and a subsequent step of removing the dispersion medium from the slurry layer.

The article “Efficient production of a high-performance dispersion strengthened, multi-principal element alloy” Scientific Reports (2020) 10:9663 and the associated US 2020/399744A1 describe the preparation of a dispersion hardened NiCoCr alloy. The process involves combining NiCoCr powder and nanoscale Y₂O₃ by RAM to produce NiCoCr particles having a thin film of Y₂O₃ on their surface, followed by forming these particles into dense parts using laser powder bed fusion.

CN115505814A describes a FeCrNi medium entropy alloy composite material which is reinforced with a Y—Ti—O oxide. The material may be prepared by mixing a FeCrNi pre-alloy, a Y source and a Ti source by RAM under an argon atmosphere to obtain a mixed powder, then performing mechanical ball milling on the mixed powder to obtain a ball milled powder, then sintering to obtain the alloy material.

US2020/0399744A1 describes the preparation of metal particles with a coating of ceramic particles for use in an additive manufacturing process; the metal particles comprise an alloy including 30-35 wt % cobalt, 26-31 wt % chromium, 0 -3.0 wt % rhenium, 0 -1.0 wt % aluminium, 0.01-0.1 wt % carbon, 0-1.0 wt % titanium, balance nickel, and the ceramic particles comprise yttrium oxide, hafnium oxide, zirconium oxide, aluminium oxide, thorium oxide, or combinations thereof. Rhenium is an optional component in the alloy.

EP2853611B1 describes a method of preparing particles of a superalloy. In a first step ceramic particles are mixed with superalloy mother particles to prepare a powder mixture which is then formed into a consumable solid body. The mixing is preferably achieved by RAM.

Although RAM has been suggested for the mixing of powders, liquids, pastes and viscous materials, the manufacture of precious metal-based DPH materials by mixing of a precious metal powder and a dopant oxide powder by RAM has not previously been suggested.

The inventors have found that a RAM-mixed powder comprising particles of a precious metal or a precious metal-based alloy, and particles of one or more metal oxide(s), can be converted into a dispersion hardened material using a powder metallurgy technique. As used herein “powder metallurgy” refers to a process in which powdered material is pressed into a desired form, and sintered to bond the powder particles together. The article may be in the desired shape following the powder metallurgy step, or subsequent working steps may be carried out to produce the desired article.

DESCRIPTION OF THE FIGURES

FIG. 1 is a SEM image of a RAM-mixed powder prepared by mixing a platinum powder and nano ZrO₂ particles by RAM.

FIG. 2 is a SEM image of a powder prepared by diluting the RAM-mixed powder with additional platinum powder.

FIGS. 3a and 3b are SEM images of a dispersion hardened platinum material according to the invention in the form of a wire.

DETAILED DESCRIPTION

Any sub-headings are for convenience only and are not intended to limit the invention.

Metal Oxide Powder

The distribution of the metal oxide(s) in the article provides the dispersion hardening effect. Any metal oxide(s) known to provide a dispersion hardening effect may be used in the present invention. Preferred metal oxide(s) include yttrium oxide (Y₂O₃), titanium oxide (TiO₂), zirconium oxide (ZrO₂), a stabilised zirconia, hafnium oxide (HfO₂), scandium oxide (Sc₂O₃), aluminium oxide (Al₂O₃), a rare earth oxide (Ln₂O₃) or thorium oxide (ThO₂), or a mixture of two or more thereof. Preferred rare earth oxides are samarium oxide lanthanum oxide (La₂O₃), cerium oxide (Ce₂O₃), samarium oxide (Sm₂O₃) and gadolinium oxide (Gd₂O₃).

In one 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 a single metal oxide selected from the group consisting of: yttrium oxide (Y₂O₃), titanium oxide (TiO₂), zirconium oxide (ZrO₂), a stabilised zirconia, hafnium oxide (HfO₂), scandium oxide (Sc₂O₃), aluminium oxide (Al₂O₃), a rare earth oxide (Ln₂O₃), and thorium oxide (ThO₂). This is option is preferred for reasons of simplicity.

It is also known to use a combination of two or more metal oxides for dispersion hardening, for example US2022/081751A1 describes a dispersion-hardened platinum composition which comprises 0.05 to 1 wt % total of zirconium oxide, yttrium oxide and scandium oxide, wherein the proportion of these oxides is: yttrium oxide (7.0 to 11.0 mol%), scandium oxide (0.1 to 5.0 mol %), zirconia (remainder). Therefore 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 the group consisting of: yttrium oxide (Y₂O₃), titanium oxide (TiO₂), zirconium oxide (ZrO₂), a stabilised zirconia, hafnium oxide (HfO₂), scandium oxide (Sc₂O₃), aluminium oxide (Al₂O₃), a rare earth oxide (Ln₂O₃), and thorium oxide (ThO₂).

The conversion of the RAM-mixed powder produced in step (i) to the desired article involves processing steps which are often carried out at a high temperature. Some metal oxides, such as zirconium oxide, undergo a phase change at high temperatures which can lead to cracks in the article. Therefore, in some embodiments it is preferred that the metal oxide powder comprises or consists essentially of a stabilised zirconia because stabilised zirconias do not undergo significant phase changes under typical temperatures used in powder metallurgy and therefore less prone to causing cracks. Stabilised zirconias are generally based on the zirconia structure with some of the Zr(IV) ions substituted for a different metal. Yttria-stabilised zirconia is a preferred example of a stabilised zirconia.

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 comprising ≥50 wt % precious metal(s). As used herein the term “precious metal” refers to metals selected from the group consisting of gold, silver, ruthenium, rhodium, palladium, osmium, iridium and platinum. Because there are few uses for dispersion hardened osmium, preferred precious metals are gold, silver, ruthenium, rhodium, palladium, iridium and platinum. Because of their resistance to oxidation at high temperature, it is particularly preferred that the metal particles are particles of a platinum group metal (pgm) or platinum group metal-based alloy, i.e. an alloy comprising ≥50 wt % pgm(s). The term platinum group metal as used herein refers to the metals ruthenium, rhodium, palladium, osmium, iridium and platinum. There are few uses for dispersion hardened osmium and therefore preferred platinum group metals are ruthenium, rhodium, palladium, iridium and

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 it is preferred that the content of non-precious metals (based on the total weight of metal powder) is ≤5 wt %, preferably ≤4 wt %, more preferably ≤3 wt %, more preferably ≤2 wt %, more preferably ≤1 wt %. In demanding applications where the content of non-precious metals must be kept particularly low, it is preferred that the content of non-precious metals is ≤0.5 wt %, preferably ≤0.1 wt %.

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 it is preferred that the content of non-precious metals (based on the total weight of metal powder) is ≤50 wt %.

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

The particles of metal oxide(s) are preferably smaller than the particles of pgm or pgm-alloy. This is to ensure that the metal oxide particles distribute on the surface of the metal particles in the RAM-mixed powder, and subsequently become well dispersed in the article. It is particularly preferred that the particles of the metal oxide powder are nanoparticles, i.e. they have a diameter of 1-1000 nm, in order to achieve the highest degree of strengthening. It is preferred that the nanoparticles have a maximum particle size of ≤100 nm.

The method is applicable to metal particles with a wide range of morphologies including spherical and irregularly shaped particles.

Step (i)

In step (i) the 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 optimised for a given combination of metal oxide and metal powder combination. The extent of mixing can often be evaluated visually, but analysis of the RAM-mixed powder by SEM can be used to evaluate the extent of mixing at the microscopic level.

The amount of metal oxide powder added depends on the end use of the article. In the following “wt %” refers to the amount of metal oxide powder added based on the total weight of metal oxide powder and metal powder used in step (i). It is preferred that the metal oxide powder is added in an amount of ≤5 wt % because dispersion hardened materials with a metal oxide content above 5 wt % tend to be brittle. Typically the amount of metal oxide powder is ≤3 wt %. The lower limit will depend on the degree of strengthening required. Typically the metal oxide powder is added in an amount of ≥0.001 wt %, such as ≥0.005 wt %, ≥0.01 wt %, ≥0.05 wt % or ≥0.1 wt %. Preferred ranges for the amount of metal oxide powder added are 0.001-5 wt %, 0.005-5 wt %, 0.01-5 wt %, 0.05-5 wt % or 0. 1-5 wt %, preferably 0.001-3 wt %, 0.005-3 wt %, 0.01-3 wt %, 0.05-3 wt % or 0.1-3 wt %.

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

Where step (i) uses particles of a precious metal-based alloy it is preferred that the resulting RAM-mixed powder has a content of non-precious metals (based on the total weight of RAM-mixed powder) that is ≤50 wt %. For the avoidance of doubt, this wt % refers to the amount of non-precious metals irrespective of whether they are in metallic or oxidic form, and includes non-precious metals from the metal oxide powder.

Optional Step (i-b)

For larger scale operations it may not be practical to combine all of the metal oxide powder and metal powder by RAM. A procedure which is more amenable to scale-up involves preparing a RAM-mixed powder as described above in step (i), and then carrying out a step (i-b) in which the RAM-mixed powder from step (i) is combined with further 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 (i-b). The same metal powder is preferably used in steps (i) and (i-b). The powder produced by step (i-b) is referred to herein as a “partially RAM-mixed powder”. While this technique has the benefit of not requiring all of the material to be mixed by RAM, conventional (non-RAM) mixing techniques leads to a less homogeneous dispersion of metal oxide(s) as compared to when all of the metal powder and metal oxide(s) are mixed by RAM.

Step (ii)

In step (ii) the RAM-mixed powder, or the partially RAM-mixed powder from step (i-b), is converted into the desired article. The excellent distribution of metal oxide achieved through step (i) is expected to be beneficial regardless of the technique used to covert the powder into the desired article.

In a preferred embodiment step (ii) involves a step of powder metallurgy carried out on the RAM-mixed powder (from step (i)) or the partially RAM-mixed powder (from step (i-b)). Powder metallurgy involves pressing the powder into the desired form followed by sintered to bond the powder particles together.

In some embodiments the powder metallurgy step may lead to the desired shape article directly. Some minor processing e.g. surface finishing may be necessary.

In some embodiments the powder metallurgy step is followed by additional processing. For example, a step of powder metallurgy to produce a bar followed by processing the bar into a wire.

ALTERNATIVE EMBODIMENT

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 prior to or during step (ii) by carrying out an oxidation treatment. If step (i-b) is carried out then the dopant metal may be oxidized to the corresponding metal oxide prior to step (i-b), after step (i-b) or during step (ii). The dopant metal must be a metal which is capable of forming a metal oxide which 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 and, following oxidation treatment results in a material 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 compared to the case where a metal oxide powder is used as a raw material in step (i), it is still less energy intensive compared to known internal oxidation methods which require the initial formation of a melt in order to uniformly disperse the dopant in the precious metal. Preferred features described above in connection with steps (i), (i-b) and (ii) apply equally in this alternative embodiment.

EXAMPLES

Example 1

An irregularly-shaped platinum powder was sieved to a particle size of 20-75 μm. This fraction (367 g) was combined with nano ZrO₂ (2.6 g, 0.7 wt % ZrO₂ based on total powder) and the mixture was subjected to RAM using a Resodyn Acoustic Mixer at a frequency of 80 s⁻¹ for 20 mins to produce a RAM-mixed material. An SEM image of the RAM-mixed material is shown in FIG. 1; nanoparticles of ZrO₂ (grey) are homogeneously dispersed on the surface of the Pt particles (white).

Example 2

A portion of RAM-mixed material (50 g) from Example 1 was combined with same irregularly-shaped platinum powder using a shaker mixer for 30 mins. An SEM image of the resulting material is shown in FIG. 2; the RAM-mixed material had been diluted with the platinum powder but the ZrO₂ nanoparticles had not transferred to the platinum powder.

The resulting powder was formed into a bar of approximate dimensions 100 mm×14 mm×8 mm by powder metallurgy. The bar was processed into a 0.25 mm diameter wire through a series of hot forging, hot rolling and cold drawing steps. FIGS. 3a and 3b shown the surface of the wire. The zirconium oxide (dark spots) is reasonably well distributed with particles no large than about 10 μm.

Claims

1. A process for preparing a dispersion hardened article, comprising the steps of: (i) mixing a metal oxide powder comprising particles of one or more metal oxide(s), and 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 the desired article.

2. The process according to claim 1, wherein the metal oxide powder consists essentially of a single metal oxide selected from the group consisting of: yttrium oxide (Y2O3), titanium oxide (TiO2), zirconium oxide (ZrO2), a stabilised zirconia, hafnium oxide (HfO2), scandium oxide (Sc2O3), aluminium oxide (Al2O3), a rare earth oxide (Ln2O3), and thorium oxide (ThO2).

3. The process according to claim 1, wherein the metal oxide powder comprises two or more metal oxides selected from the group consisting of: yttrium oxide (Y2O3), titanium oxide (TiO2), zirconium oxide (ZrO2), a stabilised zirconia, hafnium oxide (HfO2), scandium oxide (Sc2O3), aluminium oxide (Al2O3), a rare earth oxide (Ln2O3), and thorium oxide (ThO2).

4. The process according to claim 1, wherein the metal oxide powder comprises or consists essentially of a stabilised zirconia.

5. The process according to claim 1, wherein the particles of said metal oxide powder are nanoparticles.

6. The process according to claim 5, wherein the nanoparticles have a maximum particle size of: 100 nm.

7. The process according to claim 1, wherein said metal oxide powder is added in an amount of ≤5 wt % based on the total weight of metal oxide powder and metal powder used in step (i).

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

9. The process according to claim 1, the metal powder consists essentially of a single platinum group metal.

10. The process according to claim 9, wherein the metal powder is a platinum powder.

11. The process according to claim 1, wherein the metal powder is a platinum group metal-based alloy powder.

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

13. The process according to claim 1, comprising an additional step (i-b) 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. The process according to claim 1, wherein step (ii) involves a step of powder metallurgy carried out on the product of step (i) or (i-b).

15. The process according to claim 14, comprising a step of powder metallurgy to produce a bar followed by processing the bar into a wire.