WO2018104705A1

WO2018104705A1 - Rhodium alloys

Info

Publication number: WO2018104705A1
Application number: PCT/GB2017/053591
Authority: WO
Inventors: Robert Mcgrath
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2016-12-05
Filing date: 2017-11-29
Publication date: 2018-06-14
Also published as: GB2557462A; GB201719804D0; GB201620687D0

Abstract

A rhodium alloy comprises rhodium, 0 to 30 wt% nickel and at least 5 wt% chromium, and the alloy comprises a greater quantity of rhodium as compared to any other individual element of the alloy. The rhodium alloy is useful in the manufacture of electrodes, in particular spark ignition electrodes.

Description

Rhodium Alloys

Field of the Invention

The present invention relates to rhodium alloys comprising chromium, to methods of manufacturing such alloys and to the use of such alloys, in particular, as spark ignition electrodes.

Background

US 2011/0127900 (to Federal-Mogul Ignition) describes a spark plug electrode material which comprises a refractory metal (e.g. tungsten (W), molybdenum (Mo), rhenium (Re), ruthenium (Ru) or chromium (Cr)) and a precious metal (e.g. rhodium (Rh), platinum (Pt), palladium (Pd) or iridium (Ir)), wherein the refractory metal has a melting temperature that is greater than that of the precious metal, and the refractory metal is present in the electrode material in an amount that is greater than that of the precious metal.

US 2012/0025692 (to Federal-Mogul Ignition) describes a spark plug electrode material which comprises platinum (Pt) in an amount of from about 50 at% to about 99.9 at%; at least one active element selected from aluminium (Al) and silicon (Si) in an amount of from about 0.01 at% to about 30 at%; and at least one high-melting point element selected from ruthenium (Ru), iridium (Ir), tungsten (W), molybdenum (Mo), rhenium (Re), tantalum (Ta), niobium (Nb) or chromium (Cr) in an amount of from 0.01 at% to 30 at%.

CN 103746293 describes an Ir-based electrode of specific configuration containing from 60 to 98 % Ir and from 2 to 40 % Cr.

WO 2016/016667 describes a rhodium alloy comprising rhodium, one or more elements selected from the group consisting of iridium, platinum, palladium and ruthenium and one or more elements selected from the group consisting of yttrium, zirconium and samarium, wherein the alloy comprises a greater quantity of rhodium as compared to any other individual element of the alloy. The rhodium alloy may also comprise up to about 5 wt% each of any one or more elements selected from the group consisting of niobium, tantalum, titanium, chromium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten, in order to ductilise the alloy, i.e. make the alloy more tolerant to deformation and ease of manufacture.

WO 2016/016666 describes a rhodium alloy comprising rhodium and nickel, wherein the alloy comprises a greater quantity of rhodium as compared to any other individual element of the alloy. The rhodium alloy may also comprise up to about 5 wt% each of any one or more elements selected from the group consisting of niobium, tantalum, titanium, chromium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten, in order to ductilise the alloy, i.e. make the alloy more tolerant to deformation and ease of manufacture.

The alloys in WO 2016/016666 are described as providing enhanced wear resistance.

However, there is a need for alloys, in particular for use in spark plug electrodes, which are competitive in terms of both cost and performance, providing not only wear resistance but also resistance to oxidation which is prone to occur under the conditions under which a spark plug operates.

Summary of the Invention

The present inventors have developed rhodium alloys which have enhanced resistances to wear, such as those arising from exposure to sparks, as well as enhanced resistance to oxidation during operation. In addition, the alloys are easy to manufacture and demonstrate good to very good formability (i.e. they are able to undergo plastic deformation without being significantly damaged through fracturing or tearing).

A first aspect of the invention is a rhodium alloy, comprising rhodium, 0 to 30 wt% nickel and at least 5 wt% chromium, wherein the alloy comprises a greater quantity of rhodium as compared to any other individual element of the alloy. The inventors have found that a rhodium alloy which comprises at least 5 wt% chromium, for example more than 5 wt% chromium, for example at least 6 wt% chromium, for example at least 10 wt% chromium, provides excellent wear resistance along with excellent oxidation resistance. This resistance is even more evident at higher temperatures, making the alloys described herein useful in 'high end' ignition device (e.g. spark plug) applications where devices are exposed to higher temperatures for longer periods of time. Under such conditions the alloys of the invention may enjoy a longer operational lifetime and are more reliable, being less prone to misfire. A second aspect of the invention is an electrode, such as a spark ignition electrode, comprising a rhodium alloy according to the first aspect. Such electrodes are useful in a wide range of applications where enhanced resistance to wear and oxidation, especially at higher temperatures, is desirable. A third aspect of the invention is an ignition device, for example a spark plug, comprising an electrode according to the second aspect. Such a device enjoys a longer operation lifetime and is more reliable, being less prone to misfire due to the enhanced resistance of the electrode to spark erosion and oxidation.

A fourth aspect of the invention is a spark ignition engine comprising an ignition device according to the third aspect. Other aspects include a vehicle (for example an automobile, motorcycle, ship, locomotive, aircraft, snowmobile or jet ski) or a generator comprising a spark ignition engine according to the fourth aspect.

A fifth aspect of the invention is the use of a spark ignition electrode according to the second aspect in an ignition device, for example a spark plug.

A sixth aspect is the use of an ignition device according to the third aspect in a spark ignition engine. A seventh aspect of the invention is the use of a rhodium alloy according to the first aspect in an electrode or an ignition device.

An eighth aspect of the invention is the use of chromium in a rhodium alloy of a spark ignition electrode in an amount of at least 5 wt%, to improve resistance to oxidation and/or resistance to spark erosion, wherein the alloy comprises a greater quantity of rhodium as compared to any other individual element of the alloy and the alloy comprises 0 to 30 wt% nickel.

A ninth aspect of the invention is a method of manufacturing a rhodium alloy, comprising alloying rhodium with at least 5 wt% chromium based on the total weight of the alloy, and 0 to 30 wt% nickel based on the total weight of the alloy, wherein the quantity of rhodium is the greatest as compared to any other individual element.

A tenth aspect of the invention is a method of manufacturing a spark ignition electrode, comprising alloying rhodium with at least 5 wt% chromium based on the total weight of the alloy, and 0 to 30 wt% nickel based on the total weight of the alloy, wherein the quantity of rhodium is the greatest as compared to any other individual element, and forming the alloy into a spark ignition electrode. An eleventh aspect of the invention is a method of manufacturing an ignition device, for example a spark plug, comprising providing an ignition electrode comprising a rhodium alloy, wherein the rhodium alloy comprises rhodium, 0 to 30 wt% nickel and at least 5 wt% chromium and comprises a greater quantity of rhodium as compared to any other individual element of the alloy, and assembling an ignition device comprising the ignition electrode.

Detailed Description

As described above, the present invention provides a rhodium alloy, comprising rhodium, 0 to 30 wt% nickel and at least 5 wt% chromium, wherein the alloy comprises a greater quantity of rhodium as compared to any other individual element of the alloy.

It will be understood that whilst the amounts of each element are given assuming that the base alloy is pure rhodium, in practical terms, the rhodium and the alloying elements may contain impurities at levels which would be normally expected for such metals.

Rhodium is a platinum group metal (PGM) which exhibits high melting and boiling points, as well as excellent oxidation and corrosion resistances. Rhodium also displays a low vapour pressure and high thermal conductivity which, when allied with the above properties, suit its potential for use as a spark ignition electrode. However, rhodium metal itself cannot be adequately exploited as a spark ignition electrode due to its relatively poor mechanical properties and relatively low density. Furthermore, rhodium is a very expensive metal and it would be desirable to replace some of the rhodium with a cheaper metal while continuing to provide good oxidation and corrosion resistance. The present inventors have found that the properties of rhodium which make it a poor spark ignition electrode can be improved by selective alloying. In this respect, the rhodium alloy as described herein comprises rhodium as the main element in the alloy. Rhodium therefore is present in the alloy in the greatest quantity (as expressed as a percentage by weight (wt%)) as compared to any other individual element of the alloy (also expressed as a percentage by weight (wt%)). Any other element of the alloy is individually a minor element as compared to rhodium.

While each element or a combination of elements in the alloy may be expressed as a range, the total wt% of the rhodium alloy adds up to 100 wt%.

The rhodium alloy may comprise≥ 30 wt% of rhodium, such as≥ 40 wt% of rhodium, such as≥ 50 wt% of rhodium, such as≥ 55 wt% of rhodium, such as≥ 60 wt% of rhodium. In one embodiment, the rhodium alloy may comprise 30 to 95 wt% of rhodium, for example 40 to 95 wt% of rhodium, such as 45 to 95 wt%, for example 50 to 95 wt%, for example 60 to 95 wt%.

In one embodiment, the rhodium alloy may comprise≥ 72 wt% of rhodium, for instance≥ 76 wt%, for example≥ 77 wt%, such as≥ 78 wt% or≥ 79 wt%. In another embodiment, the rhodium alloy may comprise≤ 94 wt% of rhodium, for example≤ 93 wt%, such as≤ 92 wt% or≤ 91 wt%. In one embodiment, the rhodium alloy comprises 80 wt% of rhodium. In another embodiment, the rhodium alloy comprises 85 wt% of rhodium. In another embodiment, the rhodium alloy comprises 90 wt% of rhodium.

In one embodiment, the rhodium alloy may comprise≥ 51 wt% of rhodium, for example≥ 52 wt%, such as≥ 53 wt%,≥ 54 wt% or≥ 55 wt%. In another embodiment, the rhodium alloy may comprise≤ 80 wt% of rhodium, for example≤ 79 wt%, such as≤ 78 wt%,≤ 77 wt%,≤ 76 wt% or≤ 75 wt%.

The rhodium alloy comprises chromium in an amount of at least 5 wt%, for example more than 5 wt%, at least 5.1 wt%, at least 5.2 wt%, at least 5.3 wt%, at least 5.4 wt%, at least 5.5 wt%, at least 5.6 wt%, at least 5.7 wt%, at least 5.8 wt%, at least 5.9 wt% or at least 6.0 wt%. This level of chromium provides enhanced resistance to oxidation and spark erosion. In some embodiments, the rhodium alloy comprises chromium in an amount of at least 8.0 wt%, for example at least 8.5 wt%, at least 9.0 wt%, at least 9.5 wt% or at least 10 wt%. The inventors have found that an increased level of chromium in the alloy, such as at least 10 wt%, imparts better oxidation resistance to the alloy at higher temperatures, providing alloys useful in high-end applications.

In some embodiments, the rhodium alloy comprises chromium in an amount of up to 30 wt%, for example up to 29 wt%, up to 28 wt%, up to 27 wt%, up to 26 wt% or up to 25 wt%.

In some embodiments, the rhodium alloy comprises chromium in an amount of up to 24 wt%, for example up to 23 wt%, up to 22 wt%, up to 21 wt% or up to 20 wt%.

In some embodiments, the rhodium alloy comprises chromium in an amount of from 5 to 30 wt%, for example from 5.5 to 25 wt%, for example from 6.0 to 25 wt%, for example from 6.0 to 20 wt%, for example from 6.0 to 15 wt%, for example from 6.0 to 12 wt%.

In some embodiments, the rhodium alloy comprises chromium in an amount of from 10 to 30 wt%, for example from 10 to 25 wt%, for example from 10 to 20 wt%, for example from 10 to 15 wt%, for example from 10 to 12 wt%. In some embodiments, the rhodium alloy consists of at least 5 wt% chromium (for example at least 6 wt%, at least 7 wt%, at least 8 wt%, at least 9 wt% or at least 10 wt%) and balance rhodium. In other words, in some embodiments the alloy includes only rhodium and chromium in the specified amounts.

In some embodiments, the alloy does not have the composition of Composition X:

The rhodium may be alloyed with at least one of ruthenium, iridium, platinum or palladium. In this respect, up to 30 wt% (e.g. 0.01 to 30 wt%) each of one or more elements selected from the group consisting of ruthenium, iridium, platinum and palladium may be present. The total amount of ruthenium, iridium, platinum and palladium in the alloy may be up to 40 wt%. Ruthenium, iridium, platinum and palladium have excellent solid solubility with rhodium and, as such, are suitable as alloying elements in preparing rhodium alloys. Ruthenium is suitable as an alloying element as its corrosion resistance is similar to that of iridium. The presence of ruthenium (and/or iridium), therefore, improves the corrosion resistance of the rhodium alloy as compared to rhodium metal alone. Ruthenium also exhibits high melting/boiling points, high atomic weight and high thermal conductivity, all characteristics which are favourable for resistance to spark erosion. In one embodiment, the rhodium alloy may comprise up to 30 wt% of iridium, such as 0 to 30 wt %, for instance 0.01 to 25 wt %, for example 0.1 to 20 wt%, 0.5 to 20 wt%, 1 to 20 wt%, 2 to 20 wt% or 0.5 to 15 wt% of iridium. In another embodiment, the rhodium alloy may comprise up to 30 wt% of platinum, such as 0 to 30 wt %, for instance 0.01 to 25 wt %, for example 0.1 to 20 wt%. In another

embodiment, the rhodium alloy may comprise up to 30 wt% of palladium, such as 0 to 30 wt%, for instance 0.01 to 25 wt%, for example 0.1 to 20 wt%. The rhodium alloy may comprise up to 25 wt% (e.g. 0.01 to 25 wt%) each of one or more elements selected from the consisting of iridium, platinum and palladium, preferably 0.1 to 20 wt% and more preferably 1 to 15 wt%. In some embodiments, the total amount of ruthenium, iridium, platinum and palladium in the alloy is up to 35 wt%, for example up to 30 wt%, up to 25 wt% or up to 20 wt%.

In one embodiment, the rhodium alloy may comprise≥ 0.1 wt% each of any one or more elements selected from the group consisting of iridium, platinum and palladium, for example ≥ 0.5 wt%, such as≥ 0.6 wt% or≥ 0.7 wt%. In another embodiment, the rhodium alloy may comprise≤ 20 wt% each of any one or more elements selected from the group consisting of iridium, platinum and palladium, for example≤ 15 wt%, such as≤ 10 wt%.

In some embodiments, the rhodium alloy comprises iridium in the amounts as described above but does not comprise any platinum or palladium (i.e. comprises 0 wt% platinum and 0 wt% palladium).

In some embodiments, the rhodium alloy consists of at least 5 wt% chromium, iridium in an amount of 2 to 25 wt% and balance rhodium. In other words, in some embodiments the alloy includes only rhodium, chromium and iridium in the specified amounts.

In one embodiment, the rhodium alloy may comprise 0.01 to 30 wt % ruthenium, such as 0.1 to 30 wt%, such as 1 to 25 wt%, 2.5 to 20 wt %, for example 5.0 to 15 wt%. In one embodiment, the rhodium alloy comprises 5 to 10 wt% ruthenium, for example 7.5 wt%. In another embodiment, the rhodium alloy comprises 15 to 25 wt % of ruthenium, such as 20 wt% (e.g. 19.86 wt%). In yet another embodiment, the rhodium alloy comprises 25 to 30 wt % of ruthenium, such as 30 wt% (e.g. 29.86 wt%).

In another embodiment, the rhodium alloy may comprise no ruthenium, i.e. 0 wt% ruthenium.

In some embodiments, the rhodium alloy does not contain any ruthenium, platinum or palladium, i.e. contains 0 wt% ruthenium, 0 wt% platinum and 0 wt% palladium.

In some embodiments, the rhodium alloy does not contain any ruthenium, iridium, platinum or palladium, i.e. contains 0 wt% ruthenium, 0 wt% iridium, 0 wt% platinum and 0 wt% palladium.

The rhodium alloy comprises 0 to 30 wt% nickel. In other words, the rhodium alloy of the invention does not contain more than 30 wt% nickel. In some embodiments, the rhodium alloy does not contain nickel, i.e. contains 0 wt% nickel. In some embodiments, the rhodium alloy comprises nickel in an amount of at least 1.0 wt%, for example at least 1.5 wt%, at least 2.0 wt%, at least 3.0 wt%, at least 5 wt%, at least 5.1 wt%, at least 5.2 wt%, at least 5.3 wt%, at least 5.4 wt%, at least 5.5 wt%, at least 5.6 wt%, at least 5.7 wt%, at least 5.8 wt%, at least 5.9 wt% or at least 6.0 wt%. In some embodiments, the rhodium alloy comprises nickel in an amount of at least 10 wt%, such as at least 10.5 wt%, at least 1 1.0 wt%, at least 1 1.5 wt% or at least 12.0 wt%. Nickel has an excellent solid solubility in rhodium and is suitable as an alloying element in preparing rhodium alloys. The presence of nickel offers improvements in the ease of processing and welding.

The rhodium alloy may comprise nickel in an amount of up to 30 wt%, for example up to 29 wt%, up to 28 wt%, up to 27 wt%, up to 26 wt% or up to 25 wt%.

In some embodiments, the rhodium alloy comprises nickel in an amount of up to 24 wt%, for example up to 23 wt%, up to 22 wt%, up to 21 wt% or up to 20 wt%. In one embodiment, the rhodium alloy comprises 5 to 30 wt% of nickel. In one embodiment, the rhodium alloy may comprise≥ 6 wt% of nickel, for example≥ 7 wt%, such as≥ 8 wt%,≥ 9 wt% or≥ 10 wt%. In another embodiment, the rhodium alloy may comprise≤ 25 wt% of nickel, for example≤ 24 wt%, such as≤ 23 wt%. In some embodiments, the rhodium alloy comprises less than 20 wt% nickel, for example less than 19 wt%, less than 18 wt%, less than 17 wt%, less than 16 wt% or less than 15 wt%.

In some embodiments, the rhodium alloy comprises from 0 to 20 wt% nickel, for example 1 to 20 wt%, 2 to 20 wt%, 5 to 20 wt% or 10 to 20 wt%.

In some embodiments, the weight ratio of nickel to chromium in the rhodium alloy is up to 4:1 , such as up to 3.9: 1 , up to 3.8: 1 , up to 3.7:1 , up to 3.6:1 , up to 3.5:1 or up to 3.0:1. More preferably, the weight ratio of nickel to chromium in the rhodium alloy is up to 2.5: 1 (i.e., no more than 2.5:1), for example up to 2.4:1 , up to 2.3:1 , up to 2.2:1 , up to 2.1 : 1 , up to 2.0: 1 or up to 1.9: 1. Limiting the Ni:Cr ratio in this way ensures even better oxidation resistance of the alloy. In some embodiments, the rhodium alloy consists of at least 5 wt% chromium, 2 to 20 wt% nickel and balance rhodium. In other words, in some embodiments the alloy includes only rhodium, chromium and nickel in the specified amounts.

In some embodiments, the rhodium alloy consists of at least 5 wt% chromium, 2 to 20 wt% nickel, 2 to 25 wt% iridium and balance rhodium. In other words, in some embodiments the alloy includes only rhodium, chromium, iridium and nickel in the specified amounts. The rhodium alloy may also comprise up to 5 wt% (such as 0 to 5 wt%) each of any one of more elements selected from the group consisting of niobium, tantalum, titanium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten, preferably niobium, tantalum, titanium, molybdenum, cobalt, rhenium and tungsten, more preferably tungsten. Without wishing to be bound by theory, it is believed that the inclusion of these elements may ductilise the alloys i.e. make the alloys more tolerant to deformation and ease of manufacture. Furthermore, the inclusion of these elements could potentially enhance oxidation resistance and/or spark erosion resistance. The rhodium alloy may comprise≥ 0.01 wt%, such as≥ 0.05 wt%,≥ 0.1 wt%,≥ 0.15 wt% or≥ 0.2 wt% each of the elements selected from the group consisting of niobium, tantalum, titanium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten, preferably niobium, tantalum, titanium, molybdenum, cobalt, rhenium and tungsten. The rhodium alloy may comprise≤ 4.5 wt%, such as≤ 4.0 wt%,≤ 3.5 wt%,≤ 3.0 wt%,≤ 2.5 wt%,≤ 2.0 wt%,≤ 1.5 wt%,≤ 1.0 wt%, ≤ 0.5 wt%,≤ 0.4 wt% or≤ 0.3 wt% each of the elements selected from the group consisting of niobium, tantalum, titanium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten, preferably niobium, tantalum, titanium, molybdenum, cobalt, rhenium and tungsten. In one embodiment, 0.01 to 5 wt% each may be present, such as 0.05 to 2.5 wt%, for example, 0.1 to 1.0 wt %. When tungsten is present, it may be present in 0.1 to 0.5 wt%, such as 0.1 to 0.3 wt%.

In one embodiment, the rhodium alloy may comprise 0.01 to 5 wt% of niobium. In another embodiment, the rhodium alloy may comprise 0.01 to 5 wt% of tantalum. In another embodiment the rhodium alloy may comprise 0.01 to 5 wt% of titanium. In another embodiment the rhodium alloy may comprise 0.01 to 5 wt% of molybdenum. In another embodiment the rhodium alloy may comprise 0.01 to 5 wt% of cobalt. In another embodiment the rhodium alloy may comprise 0.01 to 5 wt% of rhenium. In another embodiment the rhodium alloy may comprise 0.01 to 5 wt% of vanadium. In another embodiment the rhodium alloy may comprise 0.01 to 5 wt% of aluminium. In another embodiment the rhodium alloy may comprise 0.01 to 5 wt% of hafnium. In another embodiment. the rhodium alloy may comprise 0.01 to 5 wt% of tungsten. When the rhodium alloy comprises tungsten, the tungsten may be present in 0.05 to 2.5 wt%, such as 0.06 to 1.5 wt%, for example, 0.07 to 1 wt% e.g. 0.1 to 0.3 wt%.

In one embodiment, the rhodium alloy comprises 0.01 to 5 wt% each of any one or more elements selected from the group consisting of niobium, tantalum, titanium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten, preferably niobium, tantalum, titanium, molybdenum, cobalt, rhenium and tungsten, more preferably tungsten. The rhodium alloy may comprise≥ 0.025 wt% each of any one or more elements selected from the group consisting of niobium, tantalum, titanium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten, for example≥ 0.05 wt%, such as≥ 0.075 wt% or≥ 0.10 wt%. The rhodium alloy may comprise≤ 5.0 wt% each of any one or more elements selected from the group consisting of niobium, tantalum, titanium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten, for instance≤ 2.50 wt%, for example≤ 2.00 wt%, such as≤ 1.50 wt% or≤ 1.00 wt%. In one embodiment, the rhodium alloy comprises 2.5 wt% of molybdenum. In another embodiment, the rhodium alloy comprises 3.0 wt% of aluminium.

In some embodiments, the rhodium alloy does not contain niobium, tantalum, titanium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium or tungsten, i.e. contains 0 wt% niobium, 0 wt% tantalum, 0 wt% titanium, 0 wt% molybdenum, 0 wt% cobalt, 0 wt% rhenium, 0 wt% vanadium, 0 wt% aluminium, 0 wt% hafnium and 0 wt% tungsten.

In some embodiments, the rhodium alloy contains less than 2.5 wt% molybdenum, for example up to 2.0 wt% or up to 1.5 wt%. In some embodiments, the rhodium alloy does not contain molybdenum, i.e. contains 0 wt% molybdenum. The rhodium alloy may comprise one or more elements selected from the group consisting of yttrium, zirconium and samarium, preferably zirconium. Without wishing to be bound by theory, it is believed that the inclusion of these elements may ductilise the alloys as described above. It is also believed that the elements (in particular zirconium) may hinder dislocation movement through grain boundaries (i.e. the boundaries between crystal lattices at different orientations) and hence limit or slow grain growth. Grain growth therefore appears to be reduced at temperature ensuring a fine grain structure is retained. The rhodium alloy may comprise 0.01 to 1 wt% (such as 0.01 to 0.50 wt%) each of any one or more elements selected from the group consisting of yttrium, zirconium and samarium. The rhodium alloy may comprise≥ 0.015 wt%,≥ 0.02 wt%,≥ 0.025 wt% or≥ 0.030 wt% each of any one or more elements selected from the group consisting of yttrium, zirconium and samarium. The rhodium alloy may comprise≤ 0.45 wt%,≤ 0.40 wt%,≤ 0.35 wt%,≤ 0.30 wt%,≤ 0.25 wt%,≤ 0.20 wt%,≤ 0.15 wt%,≤ 0.10 wt%,≤ 0.05 wt% or≤ 0.04 wt% each of any one or more elements selected from the group consisting of yttrium, zirconium and samarium.

In one embodiment, the rhodium alloy may comprise 0.01 to 0.50 wt% of zirconium. The rhodium alloy may comprise≥ 0.015 wt%,≥ 0.02 wt%,≥ 0.025 wt% or≥ 0.030 wt% of zirconium. The rhodium alloy may comprise≤ 0.45 wt%,≤ 0.40 wt%,≤ 0.35 wt%,≤ 0.30 wt%,≤ 0.25 wt%,≤ 0.20 wt%,≤ 0.15 wt%,≤ 0.10 wt%,≤ 0.05 wt% or≤ 0.04 wt% of zirconium. In another embodiment, the rhodium alloy may comprise 0.01 to 0.50 wt% of yttrium. The rhodium alloy may comprise≥ 0.015 wt%,≥ 0.02 wt%,≥ 0.025 wt% or≥ 0.030 wt% of yttrium. The rhodium alloy may comprise≤ 0.45 wt%,≤ 0.40 wt%,≤ 0.35 wt%,≤ 0.30 wt%, ≤ 0.25 wt%,≤ 0.20 wt%,≤ 0.15 wt%,≤ 0.10 wt%,≤ 0.05 wt% or≤ 0.04 wt% of yttrium. In yet another embodiment, the rhodium alloy may comprise 0.01 to 0.50 wt% of samarium. The rhodium alloy may comprise≥ 0.015 wt%,≥ 0.02 wt%,≥ 0.025 wt% or≥ 0.030 wt% of samarium. The rhodium alloy may comprise≤ 0.45 wt%,≤ 0.40 wt%,≤ 0.35 wt%,≤ 0.30 wt%,≤ 0.25 wt%,≤ 0.20 wt%,≤ 0.15 wt%,≤ 0.10 wt%,≤ 0.05 wt% or≤ 0.04 wt% of samarium.

It will be appreciated that elemental yttrium, zirconium and/or samarium is utilised and not e.g. oxides of yttrium, zirconium and/or samarium. In this respect, the oxides are typically added to an alloy which has already been prepared and is mechanically mixed with it. This is in contrast to elemental yttrium, zirconium and/or samarium which are dissolved in the continuous solution formed during the alloy's synthesis. Yttrium, zirconium and/or samarium, therefore, are alloying constituents.

In one embodiment, the rhodium alloy may comprise 0.02 to 0.40 wt%, for example 0.02 to 0.20 wt% each of any one or more elements selected from the group consisting of yttrium, zirconium and samarium. In another embodiment, the rhodium alloy may comprise≥ 0.03 wt% each of any one or more elements selected from the group consisting of yttrium, zirconium and samarium, such as≥ 0.04 wt%. In yet another embodiment, the rhodium alloy may comprise≤ 0.35 wt% each of any one or more elements selected from the group consisting of yttrium, zirconium and samarium, such as≤ 0.30 wt%, such as≤ 0.175 wt%, such as≤ 0.15 wt%, for example,≤ 0.125 wt% or≤ 0.1 wt%.

In one embodiment, the rhodium alloy does not comprise zirconium, yttrium or samarium.

In one embodiment, the rhodium alloy comprises:

a) 50 to 95 wt% rhodium;

b) at least 5 wt% chromium (for example, more than 5 wt% chromium); c) up to 25 wt% each of any one or more elements selected from the group consisting of iridium, platinum and palladium (preferably iridium);

d) up to 35 wt% ruthenium;

e) up to 30 wt% nickel;

f) up to 5.0 wt% each of any one or more elements selected from the group consisting of niobium, tantalum, titanium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten (preferably tungsten); and

g) up to 1.0 wt% each of any one or more elements selected from the group consisting of yttrium, zirconium and samarium (preferably zirconium);

and no other components, wherein the total amount of alloy components totals 100 wt%.

In one embodiment, the rhodium alloy comprises:

a) 50 to 94.5 wt% rhodium;

b) at least 5.5 wt% chromium;

c) up to 25 wt% each of any one or more elements selected from the group consisting of iridium, platinum and palladium (preferably iridium);

d) up to 35 wt% ruthenium;

e) up to 27.5 wt% nickel;

In one embodiment, the rhodium alloy comprises:

a) 50 to 94 wt% rhodium;

b) 6 to 25 wt% chromium;

d) up to 35 wt% ruthenium;

e) up to 30 wt% nickel;

g) up to 1.0 wt% each of any one or more elements selected from the group consisting of yttrium, zirconium and samarium (preferably zirconium); and no other components, wherein the total amount of alloy components totals 100 wt%.

In one embodiment, the rhodium alloy comprises:

a) 50 to 94 wt% rhodium;

b) 6 to 25 wt% chromium;

d) up to 35 wt% ruthenium;

e) up to 25 wt% nickel;

In one embodiment, the rhodium alloy comprises:

a) 70 to 94 wt% rhodium;

b) 6 to 25 wt% chromium;

c) up to 25 wt% iridium;

d) up to 35 wt% ruthenium;

e) up to 25 wt% nickel;

g) up to 0.50 wt% each of any one or more elements selected from the group consisting of yttrium, zirconium and samarium (preferably zirconium);

and no other components, wherein the total amount of alloy components totals 100 wt%. In one embodiment, the rhodium alloy comprises:

a) 70 to 94 wt% rhodium;

b) 6 to 25 wt% chromium;

c) up to 25 wt% iridium;

d) 0 wt% ruthenium;

e) up to 25 wt% nickel;

f) 0 wt% of any of niobium, tantalum, titanium, molybdenum, cobalt, rhenium,

vanadium, aluminium, hafnium and tungsten; and g) 0 wt% of any of yttrium, zirconium and samarium;

In one embodiment, the rhodium alloy consists of chromium in an amount of 5.1 to 20 wt%, up to 25 wt% iridium, up to 30 wt% nickel, up to 0.1 wt% zirconium and up to 0.5 wt% tungsten, with the balance being rhodium.

In one embodiment, the rhodium alloy consists of chromium in an amount of 5.5 to 20 wt%, up to 25 wt% iridium, up to 30 wt% nickel, up to 0.1 wt% zirconium and up to 0.5 wt% tungsten, with the balance being rhodium.

In one embodiment, the rhodium alloy consists of chromium in an amount of 6.0 to 20 wt%, up to 25 wt% iridium, up to 30 wt% nickel, up to 0.1 wt% zirconium and up to 0.5 wt% tungsten, with the balance being rhodium.

In one embodiment, the rhodium alloy consists of chromium in an amount of 6 to 20 wt%, up to 25 wt% iridium, up to 25 wt% nickel, up to 0.1 wt% zirconium and up to 0.5 wt% tungsten, with the balance being rhodium. In one embodiment, the rhodium alloy consists of chromium in an amount of 6 to 20 wt%, up to 25 wt% iridium, up to 25 wt% nickel, up to 0.1 wt% zirconium and up to 0.5 wt% tungsten, with the balance being rhodium, wherein the weight ratio of nickel to chromium in the alloy is no more than 2.5: 1. In one embodiment, the rhodium alloy consists of chromium in an amount of 10 to 20 wt%, up to 25 wt% iridium, up to 25 wt% nickel, up to 0.1 wt% zirconium and up to 0.5 wt% tungsten, with the balance being rhodium.

In one embodiment, the rhodium alloy consists of chromium in an amount of 10 to 20 wt%, up to 25 wt% iridium, up to 25 wt% nickel, up to 0.1 wt% zirconium and up to 0.5 wt% tungsten, with the balance being rhodium, wherein the weight ratio of nickel to chromium in the alloy is no more than 2.5: 1.

In one embodiment, the rhodium alloy consists of chromium in an amount of 6 to 20 wt% (for example 10 to 20 wt%), with the balance being rhodium. Rhodium alloys according to the present invention may be selected from the group consisting of:

Rhodium alloys according to the present invention may be selected from the group consisting of:

In some embodiments, the rhodium alloy is Alloy A or Alloy B. In some embodiments, the rhodium alloy is Alloy B.

The enhanced physical and mechanical properties of the rhodium alloys of the present invention make them suitable for use in high temperature or load bearing applications. As the alloys of the present invention demonstrate good resistance to erosion, the alloys may be used in ignition applications, e.g. as components in spark-plugs. The alloys may also be suitable for use as electrodes for applications other than spark plugs. The alloys may also be suitable as pinning wire and lead-ins for sensors. The foregoing examples merely serve to illustrate the many potential uses of the present alloys and, as such, are not intended to be limiting in any way.

The rhodium alloys may be manufactured by known methods and fabricated into any suitable form. Improvements in elongation to failure, or ductility, make the alloys particularly suitable for drawing into wires; however, the alloys may also be used to prepare tubes, sheets, grains, powders or other common forms. The alloys may also be used in spray coating applications. In some embodiments, the rhodium alloy is for use in a spark ignition electrode. In some embodiments, the rhodium alloy is for use in an ignition device, for example a spark plug.

In a second aspect, the invention provides a spark ignition electrode comprising a rhodium alloy according to the first aspect. Other aspects provide ignition devices (e.g. spark plugs) and spark ignition engines comprising a rhodium alloy according to the first aspect. Other aspects provide methods of manufacturing rhodium alloys, spark ignition electrodes and ignition devices. The skilled person will understand that all options and preferences described above in the context of the rhodium alloys of the first aspect apply equally to all other aspects, such as the ignition electrodes, ignition devices and spark ignition engines.

In some embodiments, the ignition device has a pair of spark ignition electrodes defining a spark gap between them, at least one of the electrodes including a firing tip formed from a rhodium alloy according to the first aspect. In some embodiments, the ignition device is a spark plug, but the term as used herein encompasses other igniters used to initiate the combustion of a fuel.

The term "firing tip" refers to a region of an electrode from which a spark is generated (or received) during firing, and which is therefore more prone to spark erosion. In some embodiments, only this region of at least one of the electrodes includes the rhodium alloy of the first aspect, the remainder of the electrode being of any other suitable material.

Embodiments and/or optional features of the invention have been described above. Any aspect of the invention may be combined with any other aspect of the invention, unless the context demands otherwise. Any of the embodiments or optional features of any aspect may be combined, singly or in combination, with any aspect of the invention, unless the context demands otherwise. Description of the Figures

The invention will now be described by way of the following non-limiting Examples and with reference to the following figures in which: Figure 1 is a plot of percentage weight change against time for Rh metal, Ni-Rh and Cr-Rh alloys tested for oxidation in still air at a temperature of 850 °C.

Figure 2 is a plot of percentage weight change against time for Rh metal, Ni-Rh and Cr-Rh alloys tested for oxidation in still air at a temperature of 1100 °C.

Figure 3 is a plot of percentage weight change against time for Rh metal, Ir-Rh and Cr-Rh alloys tested for oxidation in still air at a temperature of 850 °C.

Figure 4 is a plot of percentage weight change against time for Rh metal, Ir-Rh and Cr-Rh alloys tested for oxidation in still air at a temperature of 1100 °C.

Figure 5 is a bar chart representation of % gap growth measured for various metals and alloys during a spark erosion test conducted under conditions comparable to typical operating conditions, using an initial spark gap of 4 mm.

Figure 6 is a bar chart representation of % gap growth measured for various metals and alloys during a spark erosion test conducted under conditions comparable to typical operating conditions, using an initial spark gap of 1 mm. Figure 7 is a bar chart representation of spark erosion rate expressed in μηι³ per spark measured for various metals and alloys during an alternative spark erosion test conducted under test conditions, using an initial spark gap of 1 mm.

Examples

Example 1

Alloy Preparation

Rhodium alloys of the compositions specified in Table 1 below were prepared. Alloys A and J were prepared by argon arc melting of a granular feedstock produced in a pre-melting stage. Alloys B, C, D, E, F, G, H, I and K were prepared by vacuum induction melting of source materials. Additionally, some alloys were 'doped' with low level amounts of Zr and W (0.04 wt% Zr, 0.1 wt% W), to improve the processability of the alloys: these alloys are indicated with a "Y" for "yes" in the Includes Zr/W column in the Table 1. All values are given in weight percent (wt%) based on the total weight of the alloy.

Table 1 :

Comparative samples of the compositions specified in Table 2 below were also prepared; these samples do not belong to the present invention and fall outside of the scope of the claims provided herewith. Alloys L and M were prepared by argon arc melting of a granular feedstock produced in a pre-melting stage. Alloys N, O, P, Q and R were prepared by vacuum induction melting of source materials. All these comparative alloys were 'doped' with very low levels of Zr and W (0.04 wt% Zr, 0.1 wt% W) to improve processability.

Table 2:

0 51 8 3 38 0 0 0 Y

P 52 6 2.5 38.5 0 1 0 Y

Q 50.5 8 3 38 0 0 0.5 Y

R 50 0 0 50 0 0 0 Y

Each alloy is subsequently processed to produce samples suitable for undergoing the test procedures outlined below. Typically, alloys were processed into wire having a 1 mm or 2 mm diameter. Similar wires of pure rhodium metal and pure iridium metal were also prepared.

Example 2

Baseline oxidation testing

The oxidation performance of the alloys was assessed as follows:

1. Wire of 2 mm diameter prepared as set out in Example 1 was cut into straight lengths of approx. 120 mm.

2. The wire samples were weighed to 4 decimal places on an enclosed set of scales and diameters were measured at 5 points along each length. The average diameter was noted.

3. Wre samples from several different alloys were placed in a notched alumina-based ceramic

furnace tray. The positional order was randomised with the slot number for each sample being noted.

• Two samples are tested from at least some of the batches.

• Both measures are intended to check for any effect due to positional variation within the test

furnace.

4. A laboratory heat treatment furnace (in this case of work zone 150 x 150 x 200 mm) was set to the required test temperature.

5. Once stabilised, the furnace tray was placed into the centre of the furnace; date and time were noted. 6. After a suitable interval the furnace tray was removed from the furnace and allowed to cool naturally.

7. The weight of each wire sample was checked and noted.

8. The furnace tray was returned to the heat treatment furnace maintaining the same orientation.

9. Sample weights were checked at least 3 times over the duration of the test: typical duration is 350-400 hrs.; date and time were noted.

10. On completion the final diameter was measured, calculated and noted as above.

1 1. Times and measurements were transferred to a spreadsheet and the oxidative weight loss curves were calculated using weight change and weight change per unit surface area.

Figures 1 and 3 show the results of baseline oxidation testing carried out in still air at a temperature of 850 °C (representative of conditions during 'standard' automotive spark plug operation). Figures 2 and 4 show the results of baseline oxidation testing carried out in still air at a temperature of 1 100 °C (representative of conditions during 'high end' automotive spark plug operation).

Closer proximity of a plotted line to the x axis indicates a smaller magnitude of weight change and better performance.

At both 850 °C and 1100 °C, rhodium itself provides good results and a suitable benchmark for the assessment of other samples.

Figures 1 and 2 show that Alloys B, C and D all perform well at 850 °C and 1 100 °C, in both cases showing improved performance over Comparative Alloys L, N and R. Alloy B (containing 10 wt% Cr) performs particularly well under both the higher and lower temperature test conditions. The results show that increasing Cr content does not lead to the same poor performance resulting from e.g. increased Ni content. This indicates that alloys with higher Cr content (for example, at least 6 wt%, or at least 10 wt%) could be used in ignition device applications, offering a significant cost saving while providing the oxidation resistance required. Comparative Alloys N and R perform particularly poorly, possibly due to the high Ni content.

Figures 3 and 4 shows that Alloy A (5 wt% chromium, 20 wt% iridium) also performs well at 850 °C and 1100 °C. At 850 °C the results for Alloy A are comparable with those for Comparative Alloy M (20 wt% iridium). However, at 1 100 °C the results for Alloy A show a significant improvement over Comparative Alloy M, showing the beneficial effects of adding chromium to the alloy for improved high-end performance.

Example 3

Electrode Studies

Various metals and alloys were cut into electrode wire having 1 mm diameter. The wires were fixed into a six-station test cell together with matching 3 mm diameter Ir earth electrodes and the gap between them was adjusted and set using a micrometre barrel that was attached to an assembly which holds the test electrode wires. The test electrodes were set at negative polarity and the earth electrode as positive to concentrate erosion on the appropriate electrodes.

The breakdown voltage during testing varies between 10 and 20 kV (because the voltage is dependent on the size of the gap set between the electrodes). For the 1 mm test gap, the average voltage measured was 15-18 kV and for the 4 mm test gap the average voltage measured was >20 kV. The electric pulse was driven by an automotive ignition coil applied to each pair of electrodes at 100 Hz. The test was carried out under conditions of temperature and pressure comparable to typical industrial usage. A continuous series of rapid spark discharges were initiated between the electrodes as generated in a typical automotive engine. The test cell was visually checked at intervals to confirm functionality and after approximately 200 hr. the discharge was stopped and the electrode gap was re- measured. A counter initiated at test commencement was used to measure elapsed time from which the number of spark discharges was calculated. Test Duration

The test duration and approximate number of sparks were calculated. For example, for a 202-hour test:

202 hrs x 3600 seconds/hr = 727,200 seconds

727,200 seconds x 100 sparks/second = 72,720,000 sparks (per test point) Measurements of Gaps

Spark erosion tests were performed both using a 1 mm initial spark gap and a 4 mm initial spark gap. The results of the spark erosion tests are shown in Tables 5A-5H below and in Figures 5, 6 and 7.

Table 5A

Table 5E

Table 5F

Table 5H

The results of the spark erosion testing are shown in Figures 5, 6 and 7.

Figure 5 shows that Alloy C performs well relative to the Ir metal benchmark (industrial standard). In Figure 5 the greatest amount of erosion is experienced by Comparative Alloy N (38 wt% Ni, 6 wt% Cr, 0.5 wt% Ti), possibly due to the high Ni content. A direct comparison of Alloy C with Comparative Alloy L shows that replacement of 7 wt% Ni content of a Rh-Ni alloy with Cr results in a significant improvement in spark erosion resistance. Figure 6 shows that Alloys A, B and D demonstrate good erosion resistance approaching that of the Rh metal benchmark and improved relative to the Ir standard.

Figure 7 shows that Alloys A, B, D, R and S demonstrate good erosion resistance comparable to that of the Rh benchmark and improved relative to the Ir standard. The other alloys in accordance with the present invention also show erosion resistance in the same order as the Ir standard.

Claims

I . A rhodium alloy, comprising:

rhodium,

0 to 30 wt% nickel, and

at least 5 wt% chromium,

wherein the alloy comprises a greater quantity of rhodium as compared to any other individual element of the alloy.

2. A rhodium alloy according to claim 1 , comprising more than 5 wt% chromium.

3. A rhodium alloy according to claim 1 or 2, comprising at least 5.5 wt% chromium.

4. A rhodium alloy according to any one of claims 1 to 3, comprising at least 6.0 wt% chromium.

5. A rhodium alloy according to any one of claims 1 to 4, comprising at least 10.0 wt% chromium.

6. A rhodium alloy according to any one of claims 1 to 5, comprising up to 25 wt% chromium.

7. A rhodium alloy according to any one of claims 1 to 6, further comprising one or more elements selected from the group consisting of yttrium, zirconium and samarium.

8. A rhodium alloy according to claim 7, wherein each of the one or more elements selected from the group consisting of yttrium, zirconium and samarium is present in the alloy in an amount of from 0.01 to 1.0 wt%.

9. A rhodium alloy according to any one of claims 1 to 8, comprising at least 5 wt% nickel.

10. A rhodium alloy according to any one of claims 1 to 9, comprising less than 20 wt% nickel, for example less than 15 wt% nickel.

I I . A rhodium alloy according to any one of claims 1 to 10, wherein the weight ratio of nickel to chromium (Ni:Cr) in the alloy is 2.5: 1 or lower, for example 2.0: 1 or lower.

12. A rhodium alloy according to any one of claims 1 to 11 , further comprising one or more elements selected from the group consisting of ruthenium, iridium, platinum and palladium.

13. A rhodium alloy according to claim 12, wherein the alloy comprises up to 30 wt% each of any one or more elements selected from the group consisting of ruthenium, iridium, platinum and palladium.

14. A rhodium alloy according to any one of claims 1 to 13, comprising at least 50 wt% rhodium.

15. A rhodium alloy according to any one of claims 1 to 14, comprising less than 2.5 wt% molybdenum.

16. A rhodium alloy according to claim 15, comprising up to 2.0 wt% molybdenum, for example up to 1.5 wt% molybdenum.

17. A rhodium alloy according to any one of claims 1 to 16, further comprising, in an amount of from 0.1 to 5.0 wt%, one or more elements selected from the group consisting of niobium, tantalum, titanium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten.

18. A rhodium alloy according to any one of claims 1 to 17, wherein the rhodium alloy comprises:

a) 50 to 94 wt% rhodium;

b) 6 to 25 wt% chromium, preferably 10 to 25 wt%;

c) up to 25 wt% each of any one or more elements selected from the group consisting of iridium, platinum and palladium, preferably iridium;

d) up to 35 wt% ruthenium;

e) up to 30 wt% nickel;

f) up to 5.0 wt% each of any one or more elements selected from the group consisting of niobium, tantalum, titanium, molybdenum, cobalt, rhenium, vanadium, aluminium, hafnium and tungsten, preferably tungsten; and

g) up to 1.0 wt% each of any one or more elements selected from the group consisting of yttrium, zirconium and samarium, preferably zirconium;

19. A rhodium alloy according to any one of claims 1 to 18, having a composition selected from the group consisting of

20. A rhodium alloy according to claim 19, having a composition selected from the group consisting of

21. A rhodium alloy according to any one of claims 1 to 20, wherein the alloy does not have the composition of Composition X:

22. A rhodium alloy according to any one of claims 1 to 21 , wherein the rhodium alloy is for use in a spark ignition electrode.

23. A spark ignition electrode comprising a rhodium alloy according to any one of claims 1 to 22.

24. An ignition device comprising a spark ignition electrode according to claim 23.

25. An ignition device according to claim 24, which is a spark plug.

26. An ignition device according to claim 24 or 25, wherein the ignition device has a pair of spark ignition electrodes defining a spark gap between them, at least one of the electrodes including a firing tip formed from the rhodium alloy.

27. A spark ignition engine comprising an ignition device according to any one of claims 24 to 26.

28. Use of a spark ignition electrode according to claim 23 in an ignition device, such as a spark plug.

29. Use of an ignition device according to any one of claims 24 to 26 in a spark ignition engine.

30. Use of a rhodium alloy according to any one of claims 1 to 22 in an electrode or an ignition device.

31. Use of chromium in a rhodium alloy of a spark ignition electrode in an amount of at least 5 wt%, to improve resistance to oxidation and/or resistance to spark erosion, wherein the alloy comprises a greater quantity of rhodium as compared to any other individual element of the alloy.

32. Use according to claim 31 , wherein the chromium is present in an amount of more than 5 wt%.

33. Use according to claim 31 , wherein the chromium is present in an amount of at least 6 wt%.

34. Use according to claim 31 , wherein the chromium is present in an amount of at least 10 wt%.

35. Use according to any one of claims 31 to 34, wherein the chromium is present in an amount of up to 25 wt%.

36. Use according to any one of claims 31 to 35, wherein the weight ratio of nickel to chromium (Ni:Cr) in the alloy is up to 2.5:1 , for example up to 2.0: 1.

37. A method of manufacturing a rhodium alloy, comprising alloying rhodium with at least 5 wt% chromium based on the total weight of the alloy, and 0 to 30 wt% nickel based on the total weight of the alloy, wherein the quantity of rhodium is the greatest as compared to any other individual element.

38. A method of manufacturing a spark ignition electrode, comprising alloying rhodium with at least 5 wt% chromium based on the total weight of the alloy, and 0 to 30 wt% nickel based on the total weight of the alloy, wherein the quantity of rhodium is the greatest as compared to any other individual element, and forming the alloy into a spark ignition electrode.