MX2007000942A - Ignition device having an electrode tip formed from an iridium-based alloy - Google Patents

Abstract

An ignition device such as a spark plug (10) having center and ground electrodes (16, 18), at least one of which includes a firing tip (20, 22) formed from an alloy containing iridium, rhodium, tungsten, and zirconium. With the inclusion of tungsten and zirconium in the alloy, the percentage of rhodium can be kept relatively low without sacrificing the erosion resistance or reduced sparking voltage of the firing tip. In one embodiment, the firing tip (20, 22) contains 2 wt%rhodium, 0.3 wt tungsten, 0.02 wt%zirconium, and the balance iridium.

Description

IGNITION DEVICE WHICH HAS AN ELECTRODE TIP FORMED FROM AN IRIDIO-BASED ALLOY TECHNICAL FIELD This invention relates generally to spark plugs and other ignition devices used in internal combustion engines and, more particularly, to such ignition devices having tips of ignition of noble metal. As used herein, the term "ignition devices" means spark plugs, ignition devices and other such devices that are used to initiate the combustion of a gas or fuel. BACKGROUND OF THE INVENTION A variety of iridium-based alloys have been proposed for use in spark plug electrodes to increase the erosion resistance of the electrode firing surfaces. Iridium-based alloys typically have a relatively high melting point and are more resistant to spark erosion than many of the metals currently widely used. The iridium-based alloy is typically used in the form of a wide tip or rivet that is laser welded or otherwise metallurgically bonded to the center electrodes and grounded on either side of the spark plug space. However, there are known disadvantages with the use of certain alloys based on iridium, including difficulty in attaching that material to the electrodes and oxidizing volatilization of the alloy at higher temperatures. The present invention addresses the last of these two problems. A known procedure for reducing the oxidative loss of iridium is to use it in the form of an alloy combined with rhodium. U.S. Patent No. 6,094,000 and U.S. Patent Application GB 2,302,367 to Osamura et al. Disclose such an alloy in which rhodium can be included in an amount ranging from 1-60% by weight. The oxides of group 3A and 4A elements such as yttria or zirconia can also be added to help reduce the consumption resistance. Despite Osamura and coworkers' teaching of the use of rhodium in amounts as low as 1%, it has been found that minimizing the oxidative loss of iridium at higher temperatures requires much higher amounts of rhodium. This is confirmed in the test data presented by Osamura et al. And its patent mentions that the amount of rhodium is preferably at least 3%. U.S. Patent No. 5,793,793 to Matsutani et al. Reports a similar finding, wherein the amount of rhodium is conserved within the range of 3-50% by weight and, much more preferably, is at least 18%. In U.S. Patent No. 5,998,913, Matsutani identifies some disadvantages of the inclusion of high percentages of rhodium and, in an effort to reduce the amount of rhodium in the alloy, proposes the addition of rhenium or ruthenium. According to this patent by adding rhenium and / or ruthenium in amounts of up to 17% by weight, the amount of rhodium needed to maintain good resistance to oxidant consumption can be decreased to as little as 0.1% by weight. The inventors have discovered that an iridium alloy containing rhodium, tungsten and zirconium allows the use of minor amounts of rhodium while maintaining good erosion resistance and decreased spark voltages. See WO 2004 / 007782A1 and WO 2004/008596? 2, both published on January 22, 2004. In WO 2004 / 007782A1, an iridium-based alloy for spark plug electrodes is disclosed in which the alloy may contain 0.01-5% by weight of rhodium, 0.01-5% by weight of tungsten and 0.01-5% by weight of zirconium. In addition to these wide ranges of foods, this published application includes a specific example of this, which includes 2.5 wt.% Rhodium, 0.3 wt.% Tungsten and 0.07 wt.% Zirconium. WO 2004 / 008596A2 also discloses this particular formulation with slightly extended ranges of 1-3% by weight of rhodium, 0.1-0.5% by weight of tungsten and 0.05-0.1% by weight of zirconium. BRIEF DESCRIPTION OF THE INVENTION The present invention is directed to a device of ignition having a pair of electrodes that define a spark gap therebetween, with at least one of the electrodes including an ignition tip formed of an iridium, rhodium, tungsten and zirconium alloy, in which the zirconium it is maintained below 0.05% by weight and is preferably approximately 0.02% by weight when the amount of tungsten is maintained at 0.3% by weight and the amount of rhodium is reduced to approximately 2% by weight. The inventors have discovered that the reduction of rhodium content to 2% by weight together with the reduction of zirconium to 0.02% by weight surprisingly produces better overall results of the specific formulations disclosed in their previous applications mentioned in the above. These unexpected results include better working capacity of the alloy when formed in small diameter electrode wires (eg, rare 0.7) as well as the operational performance of the spark plug that is as good as or marginally better than previous specific formulations. BRIEF DESCRIPTION OF THE DRAWINGS A preferred exemplary embodiment of the present invention will be described hereinafter in conjunction with the accompanying drawings, in which similar designations denote similar elements, and: Figure 1 is a fragmentary view and a partially sectional view cross section of a constructed spark plug according to a preferred embodiment of the invention; Figure 2 is a side view of a rivet that can be used in place of the wide ignition tips used in the spark plug of Fig.l; and Figure 3 depicts a wire that can be used in place of the broad ignition tips shown in Fig. 1. DESCRIPTION OF THE PREFERRED MODE With reference to Fig. 1, the working end of a spark plug 10 is shown. it includes a metal housing or housing 12, an insulator 14 secured within the housing, a center electrode 16, an electrode connected to ground 18, and a pair of ignition tips 20, 22 located opposite each other at the center and center electrodes. connected to ground 16, 18, respectively. The housing 12 may be constructed in a conventional manner and may include standard filaments 24 together with an annular lower end 26 to which the grounded electrode 18 is welded or otherwise joined. Similarly, all of the other components of the spark plug 10 (including those not shown) can be constructed using known techniques and materials, except of course electrodes connected to ground and / or center 16, 18 which are constructed with the tip of on 20 and / or 22, as will be described below.

As is known, the annular end 26 of the housing 12 defines an opening 28 through which the insulator 14 protrudes. The center electrode 16 is permanently mounted inside the insulator 14 by a glass seal or by using any other suitable technique. This extends out of the insulator 14 through an exposed axial end 30. The grounded electrode 18 is in the form of a conventional 90 ° elbow which is mechanically and electrically connected to the housing 12 at an end 32 and ending opposite to the center electrode 16 at its other end 34. This free end 34 comprises an ignition end of the grounded electrode 18 which, together with the corresponding ignition end of the center electrode 16, defines a spark gap 36 therebetween . The ignition tips 20, 22 are each located at the firing ends of their respective electrodes 16, 18 so as to provide spark surfaces for the emission and reception of electrons through the spark gap 36. These firing ends are they show in cross-section for purposes of illustration of the ignition tips which, in this embodiment, comprise broad welded tips instead of the ignition ends. As shown, the ignition tips 20, 22 can be welded in partial holes on each electrode. Optionally, one or both. of the wide tips can be completely recessed in their associated electrode or they can be soldered by an external surface of the electrode without being recessed in everything. Each ignition tip is formed of an alloy containing iridium, rhodium, tungsten and zirconium. Preferably, the alloy is formed of a combination of iridium with 1-3% by weight of rhodium, 0.1-0.5% by weight of tungsten and 0-0.5% by weight of zirconium with no more than minor amounts of any of these. "Minor quantities" means a combined maximum of 2000 ppm of the unspecified base metal and impurities of PGM (metals of the platinum group). Preferably, the amount of zirconium is conserved above 0.01% by weight (i.e., 0.01-0.05% by weight) and more preferably within the range of 0.01-0.04% by weight and, with either of these two zirconium ranges preferably rhodium is within the range of 1.5-2.5% by weight and tungsten within the range of 0.25-0.35% by weight. In a highly preferred embodiment, the alloy is formed of about 2.0 wt.% Rhodium, about 0.3 wt.% Tungsten, about 0.2 wt.% Zirconium, and the remainder iridium with no more than very small amounts of either these. The alloy can be formed by known processes such as by melting together the desired amounts of iridium, rhodium,. tungsten and 'zirconium. After the merger, the Alloy can be converted to a powder form by a spraying process, as is known to those skilled in the art. The powder alloy can then be pressed isostatically into solid form, with secondary shaping operations which are used if necessary to achieve the desired final shape. The techniques and procedures for performing these steps are known to those skilled in the art. Although the electrodes can be made directly from the alloy, they are preferably formed separately from a more conventional electrically conductive material. With the alloy that is formed in ignition tips for subsequent bonding to the electrodes. Once both the ignition tips and the electrodes are formed, the ignition tips are then permanently attached, both mechanically and electrically, to their associated electrodes by metallurgical bonding, such as laser welding, laser bonding or other means adequate. These result in the electrodes each having an integral ignition tip that provides an exposed spark surface for the electrode. Laser welding can be done in accordance with any of a number of techniques well known to those skilled in the art. Laser bonding involves forming a mechanical interfix of the electrode to the ignition tip when using laser light to melt the electrode material so that it can flow in a gap or other surface characteristic of the ignition tip, with electrode which is then allowed to solidify and fix to the ignition tip of the proper place. This laser joining technique is more fully described in the European Patent Office Publication No. EP 1 286 442 Al, the full disclosure of which is incorporated herein by reference. As will be appreciated, the ignition tips 20, 22 need not be wide tips, but may take the form of a rivet 40 (shown in Fig. 2), a wire 42 (shown in Fig. 3), a ball (not shown), or any other suitable form. Although a round end rivet is shown in Fig. 2, a rivet having a conical or frusto-conical head could also be used. As indicated in Fig. 3, the ignition tip may include, but not necessarily, one or more surface features such as grooves 44 to allow it to be interlocked to the electrode using the laser bonding technique discussed above. The construction and assembly of these various types of ignition tips is known to those skilled in the art. Also, although the ignition ends of both the center and grounded electrodes are shown to have a formed ignition tip of the iridium / rhodium / tungsten / zirconium alloy, it would be appreciated that the alloy could be used in only one of the electrodes. The other electrode can be used without any ignition tip or can include an ignition tip formed of another precious metal or precious metal alloy. For example, in one embodiment, the ignition tip of the center electrode 20 can be formed of the iridium / rhodium / tungsten / zirconium alloy and the electrode firing tip connected to the ground 20 can be formed of platinum or an alloy of platinum. As discussed in the prior published applications of the inventors O 2004 / 007782A1 and WO 2004 / 008596A2, the combination of iridium, rhodium, tungsten and zirconium has been found to produce an alloy which exhibits good resistance to both spark and oxidant consumption. , and the alloy disclosed in those published applications allows these benefits to have been maintained using relatively small amounts of rhodium. In the present invention, the inventors have reduced the amount of rhodium of the specific formulations disclosed in those published applications, and this change would normally be expected to provide reduced performance in spark plug applications. However, the test has shown that unexpectedly, the alloy example disclosed herein using the same amount of tungsten, but with 2% by weight of rhodium and 0.02% by weight of zirconium provides as good as or maally better performance of the spark plug electrode than the previous formulations while exhibiting less tearing and fracturing of the alloy during metal forming operations used to form the metal in a fine wire spark plug electrode. Thus, the present invention provides such good resistance to both spark and oxidant consumption, while providing improved working capacity when stretched and otherwise shaped on the spark plug electrode. Thus it will be apparent that an ignition device and manufacturing method has been provided with the present invention for achieving the objectives and advantages specified herein. Of course, it will be understood that the foregoing description is of preferred exemplary embodiments of the invention and that the invention is not limited to the specific embodiments shown. Various changes and modifications will become apparent to those skilled in the art. For example, although an ignition device in the form of a spark plug has been illustrated, it will be appreciated that the invention can be incorporated in an ignition device of the type in which spark occurs through the surface of a semiconductor material disposed between the center electrode and the electrode connected to annular earth. All of such changes and Modifications are proposed to be within the scope of the present invention.

Claims (26)

1. CLAIMS 1. An ignition device for an internal combustion engine, - comprising: a housing; an insulator secured within the housing and having an axial end exposed in an opening in the housing; a center electrode mounted on the insulator and extending out of the insulator through the axial end and a grounded electrode mounted on the housing and ending at an ignition end located opposite the center electrode to define a spark gap between them; characterized in that at least one of the electrodes includes an ignition tip formed of an alloy containing iridium, rhodium, tungsten and zirconium, wherein the zirconium is included in an amount less than 0.05% by weight. An ignition device according to claim 1, characterized in that the alloy is formed of a combination of iridium with 1-3% by weight of rhodium, 0.1-0.5% by weight of tungsten and 0.01-0.05% by weight of zirconium An ignition device according to claim 1, characterized in that the alloy is formed of a combination of iridium with approximately 2% by weight of rhodium, approximately 0.3% by weight of tungsten and approximately 0.02% by weight of zirconium. 4. An ignition device according to claim 1, characterized in that the ignition tip is metallurgically joined to the center electrode in the spark gap. An ignition device according to claim 4, characterized in that the ignition tip comprises a section of wire connected by laser to the center electrode. An ignition device according to claim 4, characterized in that the ignition end of the electrode connected to ground includes a localized ignition tip opposite the ignition tip of the center electrode. An ignition device according to claim 6, characterized in that the ignition tip on the electrode connected to ground comprises platinum or a platinum alloy. An ignition device according to claim 7, characterized in that the ignition tip on the center electrode is formed of a combination of iridium with 1-3% by weight of rhodium, 0.1-0.5% by weight of tungsten and 0.01-0.05% by weight of zirconium. 9. An ignition device in accordance with the claim 7, characterized in that the ignition tip on the center electrode is formed of a combination of iridium with about 2% by weight of rhodium, about 0.3% by weight of tungsten and about 0.02% by weight of zirconium. 10. An ignition device according to claim 1, characterized in that the ignition device comprises a spark plug. 11. An ignition device according to claim 1, characterized in that the ignition tip consists essentially of iridium, rhodium, tungsten and zirconium. 12. An ignition device according to claim 11, characterized in that both of the electrodes include an ignition tip consisting essentially of iridium, rhodium, tungsten and zirconium. An ignition device according to claim 11, characterized in that the ignition tip is made of an alloy that is formed of a combination of iridium with 1-3% by weight of rhodium, 0.1-0.5% by weight of tungsten and 0.01-0.05% by weight zirconium. An ignition device according to claim 11, characterized in that the ignition tip is made of an alloy which is formed of a combination of iridium with approximately 2% by weight of rhodium, about 0.3% by weight of tungsten and about 0.02% by weight of zirconium. 15. A method for manufacturing an electrode for an ignition device having a housing, a second electrode and an insulator mounted within the housing to electrically support and insulate the two electrodes, the method characterized by the steps of: (a) forming an ignition tip of an alloy including iridium, rhodium, tungsten and zirconium in an amount of 0.01-0.05% by weight, (b) forming an electrode of a material electrically conductive other than the alloy; and (c) attaching the ignition tip on an end portion of the electrode to thereby provide the electrode with an integral ignition tip that provides an exposed spark surface for the electrode. The method according to claim 15, characterized in that step (a) further comprises forming the ignition tip of an alloy made from a combination of iridium of 1-3% by weight of rhodium, 0.1-0.5% by weight of tungsten and 0.01-0.05% by weight of zirconium. 17. The method according to claim 15, characterized in that step (a) further comprises forming the ignition tip of an alloy made of a combination of iridium with about 2% by weight of Rhodium, approximately 0.3% by weight of tungsten and approximately 0.02% by weight of zirconium. 18. The method according to claim 15, characterized in that step (a) further comprises forming the ignition tip as a wide tip, rivet, ball or wire. 19. The method according to claim 15, characterized in that step (c) further comprises laser bonding the ignition tip on an end face of the center electrode. 20. The method according to claim 15, characterized in that step (c) further comprises attaching the ignition tip to an end portion of the grounded electrode. 21. An ignition device for an internal combustion engine, comprising: a housing; an insulator secured within the housing and having an axial end exposed in an opening in the housing; a center electrode mounted on the insulator and extending out of the insulator through the axial end; and a grounded electrode mounted on the housing and ending at an ignition end located opposite the center electrode to define a Spark space between them; characterized in that at least one of the electrodes includes an ignition tip formed of an iridium-containing alloy, 1-3% by weight of rhodium and zirconium, wherein the zirconium is included in an amount less than 0.05% by weight. 22. An ignition device according to claim 21, characterized in that the alloy includes 0.1-0.5% by weight of tungsten. 23. An ignition device according to claim 21, characterized in that the alloy is formed of a combination of iridium with about 2% by weight of rhodium, about 0.3% by weight of tungsten and about 0.02% by weight of zirconium. 24. An ignition device according to claim 21, characterized in that the ignition tip is laser-bonded to the center electrode. 25. An ignition device according to claim 24, characterized in that the electrode connected to ground includes an ignition tip formed of platinum or a platinum alloy. 26. An ignition device according to claim 21, characterized in that both electrodes include an ignition tip formed from the alloy.