Electric lamp and discharge devices – With gas or vapor – Having electrode lead-in or electrode support sealed to...
Reexamination Certificate
2001-07-31
2003-02-25
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having electrode lead-in or electrode support sealed to...
C313S623000
Reexamination Certificate
active
06525475
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns an SiO
2
-glass bulb with at least one current lead-in made of a gas-tight composite material, such that the composite material consists of a noble metal with a melting point >1,700° C. and SiO
2
and is at least partially coated with a layer of SiO
2
. The invention also concerns a high-intensity discharge lamp and a process for producing a gas-tight connection between an SiO
2
-glass bulb and a current lead-in.
Metallic or composite current lead-ins for SiO
2
-glass bulbs are well known. The term composite is understood to mean a combination of different types of materials. In the present case, we are concerned, specifically, with a combination of a glass material and a metallic material. In the formation of a gas-tight connection between the material SiO
2
and an electrically conducting, metallic or metal-containing current lead-in, it is necessary to deal with the basic problem that the metal components of the current lead-in are poorly wetted by viscous SiO
2
. In addition, the low coefficient of thermal expansion of SiO
2
compared to that of a metal makes it difficult to form a gas-tight connection. During the cooling process after sealing, the metallic or metal-containing current lead-in contracts more strongly than the SiO
2
of the glass bulb, so that there is a tendency for a gap to form at the interface between the glass bulb and the current lead-in. Although this risk can be reduced by minimizing the thickness of the current lead-in, it is difficult to position and handle very thin current lead-ins, e.g., in the form of foil. To be able to produce a gas-tight connection despite these problems, only relatively expensive solutions have been proposed so far.
For example, EP 0,938,126 A1 describes a current lead-in made of a composite material for a lamp, especially a discharge lamp, in which the composite material consists of SiO
2
and metal, and in which the metal content changes along the length of the current lead-in. The metal content can vary from 0 to 100%. The end with the low molybdenum content is directed towards the discharge space of the lamp and is connected with the lamp bulb in a gas-tight connection. Only the front end of the current lead-in, which consists mainly or entirely of SiO
2
, is in direct contact with the gas in the discharge space. A metallic electrode mount is sintered into the current lead-in on the end with the low metal content. This mount is inserted deep enough into the current lead-in to produce direct contact with a composite region in which the SiO
2
content is ≧80%. This produces an electrical contact between the electrode mount and the metal-rich end of the current lead-in. The composite material disclosed in the cited document consists of a metal powder that consists of molybdenum with an average particle size d
50
of 1 &mgr;m and a glass powder with an average particle size d
50
of 5.6 &mgr;m.
EP 0,930,639 A1 likewise discloses a current lead-in with a metal content that changes along its length and an SiO
2
lamp bulb. Metals that are specified as suitable for the composite material include not only molybdenum, but also tungsten, platinum, nickel, tantalum, and zirconium. To protect the metal-rich end of the current lead-in from oxidation, a protective coating of glass, metal oxide, noble metal, or chromium is provided, which partially covers the part of the current lead-in that extends out of the lamp bulb. The gas-tight seal between the current lead-in and the lamp bulb is located in a region of the current lead-in in which the concentration of the metal in the composite material is less than 2%.
However, the production of a current lead-in with a metal concentration that changes along the length of the current lead-in requires expensive equipment. Different powders must be produced and arranged in layers. In addition, when an electrode is being sealed into the current lead-in, it is necessary to consider the electrical conductivity of the individual layers and thus the depth of insertion of the electrode in the current lead-in in order to produce a solid electrical contact. To be able to achieve a gas-tight connection, the sealing with the SiO
2
lamp bulb must be performed in a specific segment of the length of the current lead-in with a very low metal concentration. Furthermore, at high temperatures in the region of the current lead-in, corrosion can occur in metals that are not resistant to oxidation, such as molybdenum.
EP 0,074,507 A2 describes a material for electrical contacts, especially light-duty contacts, and a process for producing it. The material consists of a noble metal with 1 to 50 vol. % of glass, in which a noble metal powder with a particle size of ≦250 &mgr;m and a glass powder with an average particle size of ≦50 &mgr;m are preferably used. Gold, silver, palladium and their alloys are used as the noble metals.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a gas-tight, corrosion-resistant current lead-in for an SiO
2
-glass bulb, preferably a discharge lamp, which has high electrical conductivity and is easy to produce and handle.
Pursuant to this object, and others which will become apparent hereafter, one aspect of the present invention resides in the noble metal and the SiO
2
being homogeneously distributed in the composite material. The noble metal content of the composite material is ≧10 vol. % to ≦50 vol. %, and the SiO
2
coating covers the composite material at least in the region of the connection with the SiO
2
-glass bulb.
The SiO
2
used to produce the composite material should have a purity of ≧97 wt. %. Accordingly, impurities in the SiO
2
, e.g., alkali metals or alkaline-earth metals, can be tolerated up to ca. 3 wt. %.
Due to the SiO
2
coating, the current lead-in can be sealed gas-tight with the SiO
2
-glass bulb along its entire length or along any desired segment of this length. Only a single composite powder is needed to produce the current lead-in. Since the current lead-in shows uniformly high electrical conductivity along its entire length, when an electrode is sealed into the current lead-in, it is not necessary to consider its depth of penetration into the composite material. The proportion of noble metal in the current lead-in can be used to adjust the coefficient of thermal expansion, which is preferably selected in the range of <5·10
−6
l/K for the current lead-in. The current lead-in of the invention has the especially advantageous property that the SiO
2
-containing composite material of which it is made, which has a noble metal content of ≧10 vol. % to ≦50 vol. %, is readily deformable at temperatures greater than about 1,200° C. At temperatures greater than about 1,600° C., current lead-ins designed, for example, in the form of rods bend under their own weight to an angle of 90° without developing cracks and without impairing the electrical conductivity of the material. This property makes it possible to straighten and align a current lead-in of this type.
To be sure, these mechanical properties are similar to those of pure quartz glass, but it is surprising that they are also found in the composite material with its very high electrical conductivity and current-carrying capacity. A measured current-carrying capacity of 20 amperes in a rod of composite material with a diameter of 2 mm indicates a cohesive network of the noble metal component, which would normally be rigid and hardly deformable. These properties of the composite material, which are a combination of the deformation properties of the pure quartz glass and the conductivity of the noble metal, allow precise and very easy fitting of electrodes or contact pins to the current lead-in. For example, a tungsten electrode can be fastened to the end of the current lead-in, which points towards the inside of the glass bulb, by heating the electrode together with the powder mixture. It is also possible to sinter the electrode into composite material that has already been formed. In addition, an e
Lupton David
Schielke Jörg
Schölz Friedhold
Zingg Holger
Clove Thelma Sheree
Patel Nimeshkumar D.
W. C. Heraeus GmbH & Co. KG
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