Metal halide lamp with aluminum gradated stacked plugs

Details Metal halide lamp with aluminum gradated stacked plugs Metal halide lamp with aluminum gradated stacked plugs

1998-06-23

2001-02-27

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

With gas or vapor

Having electrode lead-in or electrode support sealed to...

C313S623000, C313S624000, C313S626000

Reexamination Certificate

active

06194832

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to metal-halide discharge lamps having a ceramic discharge vessel, especially lamps intended to operate at a relatively high temperature, in the order of up to about 1000° C., and having a power rating of up to several hundreds of watts, and more particularly to an arrangement to pass an electrical lead-through, in sealed, vacuum-tight relation from the outside into the interior of the discharge vessel, in spite of the high lamp operating temperature.
BACKGROUND
Discharge lamps, and particularly high-power metal-halide discharge lamps, present problems in connection with reliable long-term seal of an electrical lead-through into a ceramic discharge vessel. Ceramic plugs are customarily used. There are many proposals for solutions to the problems. A pin or a tubular element of a metal, such as tungsten or molybdenum, is used as the electrical conductor. The plug may be of ceramic, and the pin or tube is melt-sealed by means of a glass melt or a melt ceramic into the plug. Alternatively, the lead-through may be directly sintered to the plug. The connection between the ceramic and the metal is not a secure bond however, so that the seal has a limited lifetime. It has also been proposed to use a cermet, which is a combination material formed of ceramic and metal, as the material for the plug—see U.S. Pat. Nos. 5,404,078, Bunk et al., and 5,592,049, Heider et al.
Plugs have been tested which comprise a plurality of layers of cermet with different relationships of metal to ceramic to provide for better matching of thermal coefficients of expansion. European EP 0 650 184 A1, Nagayama, to which U.S.-designated PCT/JP93/00959 corresponds, discloses a non-conductive cermet plug having axially arranged layers. This seal is very complex and uses a lead-through which has a thread, an outer metal disk or flange, and a metal or glass melt.
U.S. Pat. No. 4,602,956, Partlow et al., discloses a metal-halide discharge lamp having a ceramic discharge vessel. The electrode is carried in a lead-through which is formed as a disk of electrically conductive cermet. The electrode is sintered into the cermet. Additionally, the lead-through is surrounded by a ring-shaped stopper or plug of cermet which is connected with the ceramic discharge vessel, typically of aluminum oxide, by a glass melt. The glass melt, however, is corroded by aggressive components of the fill in the discharge lamps, particularly by the halides therein, so that the lifetime of such a lamp is rather short. Embedding the electrode in the cermet lead-through, additionally, leads to stresses which eventually may lead to fissures and cracks in the cermet. The diameter of the disk lead-through is quite large. The lead-through is electrically conductive and, thus, the discharge arc can flash back or arc back to the lead-through which would quickly lead to blackening of the discharge vessel.
U.S. Pat. No. 4,155,758, Evans, describes a special arrangement for a metal-halide lamp having a ceramic discharge vessel without an outer surrounding envelope. The lead-through is formed as a pin of electrically conductive cermet. The electrode is sintered into the cermet. The cermet pin in turn is sintered into a plug of aluminum oxide, and this plug is connected to the vessel by a glass melt. This arrangement also has the disadvantages above mentioned.
U.S. Pat. No. 5,424,609, Geven et al., describes a metal-halide discharge lamp with ceramic discharge vessel which requires an extremely long-drawn capillary tube of aluminum oxide as an inner plug element. A pin-like metallic lead-through is connected by a glass melt at the outer end in a melting zone. It is important that the melting zone is at a sufficiently low temperature. The lead-through pin can be made of two parts, in which the part facing the discharge can be made of an electrically conductive cermet, which contains carbide, silicide or a nitride. The sealing technology results in a large overall length of the discharge vessel, it is expensive to make and, also, uses the corrosion-susceptible glass melt. The gap between the capillary tube and the lead-through results in a comparatively large dead volume in which a substantial portion of the fill in the lamp may condense, so that a large quantity of fill is necessary. The aggressive fill has intensive contact with the corrosion-susceptible components in the sealing region. This technology can be used only in small power ratings, up to about 150 W, since, with larger inner diameters of the capillary tube, the actual difference in thermal expansion between the lead-through pin of cermet and the capillary tube would be too great.
SUMMARY OF THE INVENTION
It is an object to provide a metal-halide lamp, having a ceramic discharge vessel, which has a long lifetime and does not use glass melt in a seal between a lead-through and the vessel itself, or a plug therein. The seal must be vacuum-tight, capable of withstanding high temperatures, and not subject to corrosive attack by the fill within the discharge vessel.
Briefly, the lamp has two end portions which are closed by a plug through which a lead-through is connected. The discharge vessel, typically, is made of aluminum oxide. In accordance with a feature of the invention, the plug is formed of at least four axially stacked layers or strata made of a cermet which is constituted of aluminum oxide and a metal, for example tungsten or molybdenum. In accordance with a feature of the invention, the metal content increases from a region close to the discharge arc, that is, the interior of the vessel, towards the outside. In other words, the metal content increases with increasing distance from the discharge arc region of the lamp.
For ease of description, the term “cermet” will be used to describe the plug layers even though the content of ceramic and metal, respectively, of the cermet may be 100% or 0. Thus, the innermost or outermost layer of the plug may be just ceramic, typically aluminum oxide at the inside, or just metal at the outside.
It is an important feature of the invention that the outermost layer of the plug have such a high metal content that it permits welding of this layer with the lead-through, or feed-through, extending into the interior of the discharge vessel. This requires an electrical conductivity of this outermost layer of at least 5 m&OHgr;, and this corresponds to a proportion of metal of at least 50%, by volume. As the number of layers increases, the metal content of the outermost layer can be increased. From six layers on up, the outermost layer can be made of metal, so that, then, differences in expansion due to changes in temperature can be kept very small.
The feed-through is vacuum-tightly connected to the outermost layer by welding. The feed-through is spaced from the other, further inwardly positioned layers or strata by a capillary gap, for example a few &mgr;m wide. The advantage of sealing the discharge vessel by welding is high resistance against corrosion, high temperature acceptance, and high strength of the weld connection.
The feed-through may be a pin, rod or a tube, made of a material which is electrically conductive. The material of the feed-through should be matched as well as possible to the outermost layer of the plug, at least with respect to the thermal coefficient of expansion, and also to the composition of the material. Ideally, the outermost layer of the plug and the feed-through are of essentially identical material. Deviations are possible, however, and for example the outermost layer as well as the feed-through may be of just metal. Alternatively, both the outermost layer and the feed-through may, however, already include a weldable cermet having a metal content of at least 50%, by volume.
The innermost layer of the plug is connected with the end of the discharge vessel which is devoid of a glass melt. Typically, the connection is by direct sintering of the plug into the tubular end of the discharge vessel.
It is a specific advantage of the present invention that no perceptible thermal diffe

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