Electric lamp and discharge devices – With gas or vapor – Having electrode lead-in or electrode support sealed to...
Reexamination Certificate
2001-05-18
2004-06-22
Ramsey, Kenneth J. (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having electrode lead-in or electrode support sealed to...
C445S027000, C445S043000
Reexamination Certificate
active
06753650
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a method for producing an electric lamp having a lamp bulb made from SiO
2
or glass with a high SiO
2
content and a current lead which includes a foil of molybdenum or a doped molybdenum alloy. The foil is pinched in the lamp bulb. The invention also relates to a foil configuration for an electric lamp.
In electric lamps with a lamp bulb made from glass, the current required for operating the lamp has to be fed into the interior of the lamp bulb via special current leads. Particularly in the case of lamps with a lamp bulb made from silica glass or a glass with a high SiO
2
content, such as for example in the case of halogen incandescent lamps, metal halide lamps, high-pressure mercury vapor lamps or high-pressure xenon lamps, a current lead or supply conductor of this type includes an outer lead, which enters the glass. The current lead also includes a molybdenum foil which is pinched or fused in a vacuum-tight manner in the glass. The current lead further includes an inner lead (e.g. holding wire, filament, electrode).
In order to achieve a vacuum-tight pinching or fusing of the molybdenum foil in the glass despite the very different coefficients of thermal expansion in particular of silica glass or glass materials with a high SiO
2
content and molybdenum, the foil is configured to be very thin (typically 15 to 50 &mgr;m), with a high width to thickness ratio (typically >50), and has side edges which taper in the form of a cutting blade.
The outer and inner leads, which are significantly thicker than the foil, have to be welded onto this thin molybdenum foil. The inner lead is in many cases formed of tungsten. Particularly with leads made from tungsten, this entails very high welding temperatures, which may result in embrittlement and consequently a fracturing of the molybdenum foil. Cracks in the foil can also occur during the pinching or melting process. Such cracks may be caused by the relative movement between the glass and the foil or by a build-up of tensile stresses during the cooling process, at temperatures which are below the stress relaxation temperature of the glass.
In order to improve the mechanical strength of the molybdenum foil, doped molybdenum alloys have been used instead of pure molybdenum.
German Patent No. DE-C-29 47 230 describes a molybdenum foil in which 0.25 to 1% of yttrium oxide particles are dispersed. This has the advantage that this foil has an improved welding performance and becomes less brittle when heat is introduced during welding. An important reason for the upper 1% limit is the realization that foils with higher dispersoid contents can only be deformed to a limited extent, and the result is an excessively high foil strength, which has an adverse effect on the relaxation of stresses in the lamp cap region during the cooling process when performing the pinching process and may lead to cracks in the quartz glass.
European Patent No. EP-B-0 275 580 describes a molybdenum alloy specifically for seal wires or fusion wires containing 0.01 to 2% by weight Of Y
2
O
3
and 0.01 to 0.8% by weight of molybdenum boride, which compared to seal wires including a K—Si doped molybdenum alloy has improved recrystallization and production properties.
However, in addition to the mechanical properties of the molybdenum foil, it is also very important to improve the service life. The service life is determined by the oxidation resistance of the molybdenum foil and by the adhesive strength between the molybdenum foil and the silica glass or glass with a high SiO
2
content.
European Patent No. EP-B-0 691 673 describes a ribbon-like current lead based on molybdenum-yttrium oxide, which additionally contains 0.03 to 1% by weight of cerium oxide, with a cerium oxide to yttrium oxide ratio of 0.1 to 1. A foil with this composition has a significantly improved oxidation performance compared to a foil which is doped with yttrium oxide.
Together, all molybdenum materials which are doped with yttrium oxide have improved foil adhesion, which can be attributed, inter alia, to a surface reaction between Y
2
O
3
and SiO
2
so as to form an yttrium silicate.
In accordance with German Patent No. DE-C-30 06 846, an improved oxidation resistance can also be achieved by providing a metallic covering for the molybdenum foil containing Ta, Nb, V, Cr, Zr, Ti, Y, La, Sc and Hf in which case, however, the bonding of the abovementioned metals to SiO
2
is very poor, so that these coverings, with the exception of Cr layers, have not been used in practice.
A particular form of oxidation-resistant layers including chromium, nickel, nickel-chromium alloys or molybdenum silicide is described in German Patent No. DE-B-21 52 349.
European Patent No. EP-B-0 309 749 describes a sealing-in or fusion between molybdenum and a vitreous material, with part of the molybdenum which is exposed to the oxidizing environment being covered with alkali metal silicate. However, this does not have a favorable effect on the bonding between the molybdenum and the glass. Molybdenum nitride layers in accordance with Published European Patent Application No. EP-A-0 573 114, phosphide layers in accordance with European Patent No. EP-B-0 551 939 or SiO
2
layers in accordance with Published German Patent Application No. DE-A-20 58 213 have also been disclosed for external protection against oxidation.
In accordance with U.S. Pat. No. 5,021,711, it has also been attempted to improve the resistance to oxidation by ion implantation. However, this process is highly complex and does not improve the Mo/SiO
2
adhesion.
Published German Patent Application No. DE-A-196 03 300 describes a molybdenum foil which is doped with 0.01 to 1% by weight of alkali-rich and alkaline earth-rich silicates and/or aluminates and/or borates of one or more elements selected from groups IIIb and/or IVb of the periodic system. This doping prevents the formation of cracks in the pinch seal, caused by the high mechanical stresses in the molybdenum/quartz glass composite. However, this does not improve foil adhesion compared to foils which are doped with Y
2
O
3
mixed oxide or Y mixed oxide.
Moreover, it has also been attempted to improve the SiO
2
/Mo adhesion by roughening the foil for example by sand blasting, as described in Published European Patent Application No. EP-A-0 871 202. However, this process is highly complex and leads to internal stresses being introduced in the molybdenum foil.
Overall, it can be stated that molybdenum foils which are doped with Y
2
O
3
or Y mixed oxide are the most widespread material used for pinched-in current leads in the lamp industry. However, in the case of lamps which are exposed to very high thermal loads, such as for example in very compact metal halide lamps, the Mo/SiO
2
adhesion is often insufficient for these current leads.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for producing an electric lamp having a vitreous lamp bulb and a pinched current lead including a foil of molybdenum or a doped molybdenum alloy which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type. Another object of the invention is to provide a foil configuration which overcomes the above-mentioned disadvantages of the heretofore-known foils of this general type and which results in an improved service life of an electric lamp.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing an electric lamp, the method includes the steps of:
providing an unfinished foil produced by a sintering process and a forming process, the unfinished foil being formed of a material selected from the group consisting of molybdenum and a doped molybdenum alloy, and the unfinished foil having a given surface structure and a given material composition;
post-treating the unfinished foil for producing a finished foil having substantially non-contiguous, insular regions of material agglomerates with at least one
Leichtfried Gerhard
Retter Bernhard
Sulik Manfred
Greenberg Laurence A.
Locher Ralph E.
Plansee Aktiengesellschaft
Ramsey Kenneth J.
Stemer Werner H.
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