Electric lamp and discharge devices – With gas or vapor – Having electrode lead-in or electrode support sealed to...
Reexamination Certificate
2000-02-17
2002-05-07
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having electrode lead-in or electrode support sealed to...
C313S624000, C313S625000
Reexamination Certificate
active
06384533
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a metal component for discharge lamps, having a support of niobium or tantalum or of alloys based on niobium and/or tantalum, and to a discharge lamp.
Such a component is disclosed in G 86 28 310.3. It shows a possibility of using niobium as a power lead-through for high-pressure lamps. A gas-tight fusion embedding and a very complex method of construction is used in order to protect the niobium against corrosion by, for one thing, aggressive metal halides.
In GB 2 178 230 A such components are used as connection pins for a discharge lamp. It is recommended that such a discharge lamp be used in a temperature range of 200-300° C. and in an atmosphere of high moisture content, in conjunction with an external capsule which protects the connection pins against oxidation and corrosion. An example shows the discharge lamp and the connection pins within a sealed and gas-tight protective glass capsule filled with noble gas.
The problem of the extremely low resistance of niobium and its alloys to oxidation even at low temperatures from about 400° C. is described in the publication, “Niobium in High Temperature Applications” by the author H. Inouye, which is based on a symposium held on Nov. 8, 1981 in San Francisco (Proceedings of the International Symposium). The metal tantalum, which is closely related to niobium, behaves in a similar manner. On account of this property the use of these metals and their alloys at elevated temperatures is severely limited. Thus, coatings are known which increase oxidation resistance. These are usually silicide or aluminide coatings which can be applied only with great difficulty. Also, the brittleness of these coatings impairs resistance to thermal shock, combined with the cracking and spalling off of the coating. The intended protection of the coating is thereby lost and the oxidation of the metal can spread from the flaws in the coating.
The present invention is addressed to the problem of improving the resistance to oxidation and corrosion of metal components with a support of niobium or tantalum or of alloys based on niobium and/or tantalum, which are employed in or on discharge lamps.
The problem is solved according to the invention by giving the support a coating of one or more individual layers, which is formed of at least one noble metal and/or a noble metal alloy. Such a coating very well satisfies the need for increasing oxidation and corrosion resistance and providing sufficient ductility and thermal shock resistance.
Ideally, the noble metals gold and/or platinum and/or palladium and/or an alloy formed of at least two of these elements is used for the coating built up of one or more individual layers. These noble metals used for the coating have a melting point above 1000° C. In a reducing or inert atmosphere, these coatings therefore permit exposure to higher temperatures than the temperatures normally occurring in the discharge lamp, so that any mounting processes needed before use, such as soldering, can be carried out. For example, connection wires of niobium alloys for discharge lamps can first be coated with the noble metal, and then the coated connection wires can be soldered into openings in the discharge vessel without losing the protective effect of the coating due to the heat stress produced by the soldering process.
It has proven to be especially advantageous if a first single layer of gold is applied to the support, and then a second single layer of platinum and/or palladium and/or of an alloy that is formed of at least two of the noble metals gold, platinum or palladium.
A first single layer has preferably a thickness of 0.1 &mgr;m, and additional single layers applied thereon a thickness of 1 &mgr;m to 5 &mgr;m. The term, “single layer,” is to be understood to mean a layer of a noble metal or of a noble metal alloy, which is applied in one step or in successive steps, also by means of different application methods.
In view of the temperature range prevailing during use, the coating material is to be selected from among the said noble metals or noble metal alloys according to their melting point. If different noble metals are combined, diffusion compounds can form due to the action of elevated temperatures. The coating can thus have a noble metal mixed crystal produced by diffusion, which is present either only at the interface between two single coatings or else may take up the entire volume of the coating. For example, in the case of gold as the first single layer and palladium as second single layer, if the palladium diffuses into the gold layer under it, its melting point increases. This diffusion compound can be produced by a temperature treatment, for example directly after the production of the coated component, during a soldering process in the mounting of the component or else at the point of use and under the conditions of use.
The single layer can be applied physically and/or chemically to the support of niobium or tantalum or of alloys based on niobium and/or tantalum. Ideally, the application of a single layer is performed by sputtering and/or electroplating, since in this case a selective coating of surfaces of components of complex shape is also possible. Also, both methods are simple and uncomplicated, and can be performed without the use of high temperatures. Above all the first single layer is produced preferably by sputtering or by sputtering followed by an electroplating process, since the sputtered noble metal enters into a firmly adherent bond with the support and thus acts as an adhesion-mediating agent.
The surface quality of the support of niobium and tantalum or of alloys based on niobium and/or tantalum has a decided influence on the duration of the protective effect of the coating. If many flaws, such as pores, scratches or traces of machining are on the surface of the support, the probability increases that the coating will not be fully continuous at these places. At such flaws, which may propagate in the coating in the form of holes or thin areas, for example, the support may be attacked by oxidation or corrosion. If the support is made of niobium or tantalum or of alloys based on niobium or tantalum it has proven to be advantageous for good adhesion to chemically clean and activate the surface before applying the coating. For example, an etching of the parts is possible, which removes chiefly inorganic deposits, including oxide coatings.
The discharge lamp according to the invention contains a discharge vessel through whose wall metal components are passed as power connections, as in the case, for example, of high-pressure lamps. The power connections have a support of niobium or tantalum or alloys based on niobium and/or tantalum, which has a coating of one or more single layers which are formed of at least one noble metal and/or of a noble metal alloy. A great advantage of discharge lamps with power connections coated in this manner is that they can be operated without any additional external encapsulation, of glass, for example. The noble metal used with preference for coating the power connections is gold and/or platinum and/or palladium and or an alloy formed from at least two of these noble metals.
It has proven especially advantageous if a first single layer of gold is applied to the support of the power connections, and thereon a second single layer of platinum and/or palladium and/or an alloy which is formed from at least two of the noble metals gold, platinum or palladium. The first single layer can have a thickness of 0.1 &mgr;m to 5 &mgr;m, and additional single layers applied thereon a thickness of 1 &mgr;m to 5 &mgr;m each. The coating can cover the support of the power connections also just partially. Also, the coating can have a noble metal mixed crystal produced by diffusion.
The following examples 1 to 9 and the figure set forth the advantage of the invention in detail. For all examples, components in the form of wire pins of the alloy NbZr1 with the diameter of 1 mm and a length of 15 mm.
REFERENCES:
patent: 4
Giesel Thomas
Krüger Frank
Lupton David Francis
Schölz Friedhold
Berck Ken A
Fulbright & Jaworski L.L.P.
Patel Vip
W. C. Heraeus GmbH & Co. KG
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