Electric lamp and discharge devices – With gas or vapor – Having particular electrode structure
Utility Patent
1998-10-09
2001-01-02
Day, Michael H. (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having particular electrode structure
C313S633000
Utility Patent
active
06169365
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a high-pressure metal halide lamp comprising:
a sealed light-transmittent discharge vessel having opposite seals and enveloping a discharge space which has a gas filling comprising rare gas and metal halides;
tungsten electrodes oppositely disposed in the discharge space;
current lead-through conductors located in a respective seal of the discharge vessel and issuing from the discharge vessel;
electrode rods connected to a respective one of said lead-through conductors and carrying a respective one of said electrodes.
Such a lamp is known from U.S. Pat. No. 5,424,609.
The known lamp has a ceramic discharge vessel, current lead-through conductors of e.g. niobium or tantalum, and a gas filling of rare gas, mercury and a mixture of metal iodides including rare earth metal iodides, being the iodides of the lanthanide's, scandium and yttrium, as the metal halides.
In ceramic discharge lamps the current lead-through conductors generally extend into the discharge space, thereby being exposed to attack by the metal halides. In the known lamp the inner ends of the current lead-through conductors are embedded in ceramic sealing material of the seals and a respective conductor which is said to be halide-resistant at least as its surface issues from the seals and connects the lead-through conductors with tungsten electrode rods. The said conductors at least at their surface consist of tungsten, molybdenum, platinum, iridium, rhenium, rhodium, or an electrically conducting silicide, carbide or nitride.
It was found that the known lamp suffers from a decreasing luminous output due to a blackening of the discharge vessel which is caused by the deposition of tungsten originating from the electrodes and the electrode rods.
A single ended quartz glass metal halide lamp is known from EP-A 0.343.625 in which the gas filling consist of rare gas, mercury and a mixture of metal iodides and metal bromides. Both lead-through conductors are embedded next to one another in the one seal of the discharge vessel and the electrode rods extend next to one another into the discharge space. Due to the elevated temperature of the electrode rods during operation and their short mutual distance, in such a lamp the discharge arc may jump over from the electrodes to the electrode rods, thereby approaching the discharge vessel and causing it to become overheated. The jump over of the discharge arc, however, also causes the electrode rods to become even more heated, to evaporate locally and thereby to blacken the discharge vessel and to become broken themselves. Moreover, the short distance in the kind of lamp between the electrode rods and the portion of the discharge vessel which is heated to softening in making the seal during manufacturing the lamp, causes tungsten electrode rods to become oxidized, which results in a fast blackening of the discharge vessel during operation.
In the lamp of EP-A 0.343.625 oxidation of the electrode rods and a jump over of the discharge arc are obviated in that the electrode rods at least at their surface consist of rhenium or rhenium-tungsten alloy. These electrode rods project through a tungsten electrode coil at their ends inside the discharge space. Rhenium is less liable to become oxidized and has a lower heat conductivity, whereby a rhenium electrode rod would assume a lower temperature during operation. Preference is given to rhenium-tungsten alloys containing 3 to 33% by weight of rhenium, because rhenium is an expensive metal. It was found, however, that the lamp has the severe disadvantage to suffer from a rapid blackening due to evaporation of rhenium and deposition of rhenium on the discharge vessel.
A similar single ended quartz glass lamp and a double ended quartz glass lamp are known from U.S. Pat. No. 5,510,675. These lamps have a gas filling of rare gas, mercury and a mixture of metal iodides and bromides. Their electrode rods have at their end inside the discharge space a wrap winding of tungsten wire and a fused spherically shaped tungsten electrode head. The purpose thereof is to eliminate flicker which is caused by migration of the discharge arc. The electrode rods may consist of rhenium in stead of tungsten. It was found that the lamp having rhenium electrode rods suffers from a rapid blackening due to evaporation of rhenium and deposition of rhenium on the discharge vessel. In the event the electrode rods consist of tungsten, blackening of the discharge vessel may occur as a result of evaporation of tungsten from the electrode rods and the electrodes, and deposition on the discharge vessel. Moreover in this event, the electrode rods may locally become thinner and thinner, resulting in the breakage of the rods at a relatively early moment:
SUMMARY OF THE INVENTION
It is an object of the invention to provide a high-pressure metal halide lamp in which blackening of the discharge vessel and breakage of the electrode rods are obviated.
This object is achieved in that the gas filling contains metal oxyhalide and is substantially devoid of rare earth metal compounds, the electrode rods have a first portion of tungsten adjacent the electrode which merges into a second portion at a location having a temperature in the range of 1900-2300 K during operation, the second portion is made of at least 25% by weight of rhenium, rest tungsten and being secured to a respective current lead-through conductor.
The invention is based on an insight having several aspects.
The discharge vessel may be kept clear by a fast acting regenerative cycle, by which evaporated tungsten is transported to the electrodes as tungsten oxyhalide, e.g. oxybromide. Tungsten oxyhalide decomposes near the electrodes and tungsten is deposited on the electrodes. Free halogen, e.g. bromine or iodine, and oxygen in the gas atmosphere of the operated lamp are essential to achieve a fast transport. Rare earth metals have a high affinity to oxygen, which results in stable oxides and excludes the existence of free oxygen in the gas atmosphere. Therefore, rare earth metals must be substantially absent.
Rhenium has a vapor pressure which increases rather steeply at increasing temperature. Rhenium cannot be returned to the electrode rods by means of halogen, because rhenium does not react with halogen or with halogen and oxygen. Rhenium must be avoided at locations having a relatively high temperature during operation.
Halogen, particularly bromine, and oxygen together form effective means to transport tungsten from locations of relatively low temperature, such as from the wall of the discharge vessel, to the electrode. However, the electrode rods, too, have locations of a temperature at which tungsten reacts with oxygen and halogen to form volatile compounds. The presence of oxygen and halogen in the gas atmosphere of an operating lamp, causes the electrode rods to become locally thinner until breakage occurs.
When the second portion is made of a tungsten/rhenium mixture, an amount of at least 25% by weight of rhenium in the mixture is necessary. When the tungsten is removed from the mixture by reaction with the halogen, a remainder of the second portion substantially consists of rhenium. Only when at least 25% by weight of rhenium is initially present in the mixture, the remainder of the second portion is strong enough to avoid breakage of the electrode rod. Halogen dosed into a lamp as the only intentionally added tungsten transport means could keep clear the discharge vessel without undue transport of tungsten from the electrode rods, by cooperation with unintentionally, as a contaminant, added oxygen. In this event, however, other contaminants in the gas filling, on the electrodes and their rods, and on the discharge vessel, such as carbon, iron, phosphorus and hydrogen, may have a strong influence either on the transport of tungsten towards the discharge vessel or towards the electrode.
By making the electrode rods to have rhenium in the second portion thereof, reactions of that portion with bromine and oxygen are hampered. By making the first porti
Kubon Marcus
Scholl Robert P.
Day Michael H.
Faller F. Brice
Santiago Mariceli
U.S. Philips Corporation
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