Metal Halide Lamp With Rhenium Skin on Tungsten Electrode

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 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 consists 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. 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.

It is an object of the invention to provide a high-pressure metal halide lamp of in which blackening the discharge vessel and breakage of the electrode rods are obviated.

According to the invention 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 having a core of tungsten and a skin of at least 90% 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. Halogen dosed into a lamp as the only intentionally added tungsten transport means could keep the discharge vessel clear without undue transport of tungsten from the electrode rods, by cooperation with unintentionally, added oxygen as a contaminant. 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 tungsten electrode rods to have a skin substantially of rhenium in the second portion thereof, reactions of that portion with bromine and oxygen are avoided. By making the first portion of the electrode rods from tungsten it is avoided that a strong evaporation occurs, as would be the case if the first portion consists of rhenium. The common boundary of the first and the second portions is located where the operating temperature is the temperature at which both the rhenium vapor pressure at higher temperatures and the sum of the tungsten vapor pressure and the pressures of tungsten compounds at adjacent lower temperatures than the boundary temperature would be substantially higher.

No more then 10% by weight, preferably no more than 5% by weight of tungsten should be present in the skin.

The electrode rods may be obtained from tungsten rods, which remain bare in the first portion thereof and are coated in the second portion thereof, e.g. by wrapping them e.g. with a wire or a foil, or by depositing rhenium or a tungsten/rhenium mixture, e.g. by means of sputtering or vapor deposition. Alternatively, a first tungsten rod may be welded, e.g. butt welded, to a second tungsten rod with a skin of rhenium or rhenium mixture, e.g. by resistance welding or laser welding. In order to compensate for the lower heat conductivity of rhenium: S.sub.Re .apprxeq.0.3*S.sub.w, the second rod may be chosen to be slightly, e.g. 10 to 15%, thicker, if so desired,.

The common boundary of the first and the second portions is at a location having a temperature during operation of 1900-2300 K. This temperature may be chosen for a particular type of lamp in dependency of the gas filling and the quality of the manufacturing process, which could cause the lamp to contain more or less contaminants influencing the total vapor pressure of tungsten and tungsten compounds. For each type of lamp the optimum temperature of the common boundary can easily be determined in a small series of test lamps by monitoring the luminous efficacy of the lamps during their life. Generally, it is favorable to have the boundary at a temperature in the range of 2100-2300 K.

The gas filling may, apart from bromides like sodium bromide, thallium bromide, indium bromide or other non rare earth metal bromides, contain metal iodides, such as sodium iodide and stannous iodide. Oxygen may have been introduced into the discharge vessel e.g. in admixture with rare gas, or as a compound e.g. as an oxyhalide or as tungsten oxide. Metal oxyhalides, particularly tungsten oxyhalides, such as WOI.sub.2, WO.sub.2 Br.sub.2 and WOBr.sub.2, will be formed during operation of the lamp. Not operated, the lamp may have a deposit of tungsten oxide on the wall of the discharge vessel.

The electrodes may be the tips of the electrode rods, i.e. the tips of the first electrode rod portions, or separate bodies secured to the electrode rods, or fused end portions of the electrode rods. A wire wrapping, generally of tungsten wire, may be present near the electrodes, e.g. to adjust their temperature.

The discharge vessel may consist of ceramic, e.g. of mono- or polycrystalline alumina, or of high silica glass, e.g. of quartz glass. The discharge vessel may be surrounded by an outer envelope, if so desired. An outer envelope may be filled with inert gas or be evacuated. The lamp may be socketed, e.g. at one or at both of its ends.

The lamp of the invention may e.g. be used with fiber optics, as a projection lamp etc., and particularly in those applications in which an unobstructed light ray path from the discharge arc to outside the discharge vessel or in which long life times and a good luminous maintenance are required.

FIG. 1 is the lamp in side elevation;

FIG. 2 is an electrode rod in cross-sectional view;

FIG. 3 is a graph showing vapor pressures.

The high-pressure metal halide lamp of FIG. 1 has a sealed light-transmittent discharge vessel 1, in the FIG. of quartz glass, but alternatively of mono- or polycrystalline ceramic, which has opposite seals 2 and which envelopes a discharge space 3. The discharge space has a gas filling comprising rare gas and metal halides. Tungsten electrodes 5 are oppositely disposed in the discharge space 3. The lamp shown in FIG. 1 is an AC-lamp, but DC-lamps fall within the scope of this invention as well. Current leadthrough conductors 6 are located in respective seals 2 of the discharge vessel 1 and issue from the discharge vessel. In the FIG. the current lead-through conductors are each composed of a metal foil 6a, e.g. of molybdenum, which is fully located inside a respective seal, and of a metal rod 6b, e.g. of molybdenum, which extends to outside the discharge vessel 1. Electrode rods 7 are connected to respective leadthrough conductors 6, in the FIG. by welding them to the metal foils 6a, enter the discharge space 3 and each carry a respective one of electrodes

The gas filling contains metal oxyhalides and is substantially devoid of rare earth metal compounds. The electrode rods 7 have a first portion 71 of tungsten adjacent the electrode 5 which merges into a second portion 72 at a common boundary 73 having a temperature in the range of 1900-2300 K, particularly 2100-2300 K, in the FIG. 2100 K, during operation. The second portion 72 has a skin of at least 90%, preferably of at least 95%, by weight of rhenium, rest tungsten. In the FIG. the second portions 72 of the electrode rods 7 have a diameter of 1 mm and are thicker than the first portions 71, which have a diameter of 0.8 mm. The electrodes 5 in the Figure are free end portions of the first electrode rod portions 71.

In FIG. 1 the electrode rods 7 have at the first portion 71 a wrapping 74 of tungsten wire adjacent the electrodes 5, to adjust the temperature of the electrodes.

The lamp of FIG. 1 consumes a power of 200 W. The lamp, having a volume of 0.7 cm.sup.3 and an electrode distance of 3 mm, was filled with 0.87 mg NaI, 0.45 mg SnI.sub.2, 0.76 mg NaBr, 0.21 mg TlBr, 0.17 mg HgI.sub.2, 2666 Pa O.sub.2, 44 mg Hg and 10 000 Pa Ar. When the lamp is switched on, the oxygen reacts to form oxyhalides.

After 1600 hrs of operation, during which the common boundaries of the first and the second electrode rod portions were at a temperature of about 2100 K, the discharge vessel was still fully clear and the lamp had not reached the end of its life.

This is in contrast to a test lamp in which one of the electrode rods was of the design shown in FIG. 1 and the other consisted of tungsten. The electrode distance was 5 mm. The lamp had a filling of 0.89 mg SnI.sub.2, 0.14 mg HgI.sub.2, 0.13 mg WO.sub.3, 39 mg Hg and 10 000 Pa Ar. After 125 hrs of operation at a power of 200 W, the tungsten electrode rod broke down, thereby causing the end of the life of the lamp, whereas no signs of change of the other electrode rod were seen. The lamp vessel was still clean. When the lamp was first operated, the tungsten oxide reacted with halogen to form oxyhalide.

In FIG. 2 the electrode rod 7 has a first portion 71 and a wire wrapping 74 of tungsten and a second portion 72 of tungsten having a skin 72' of rhenium up to the location 73.

In FIG. 3 the curve W designates the sum of the pressure of tungsten vapor and of the pressures of tungsten compounds in a lamp in dependency of the temperature, whereas the curve Re represents the rhenium vapor pressure at different temperatures.

It is seen, that the rhenium vapor pressure increases with an increasing temperature. Thus, rhenium evaporates faster the higher its temperature.

It is also seen, that the sum of the tungsten pressures is highest at about 1500 K and lowest at about 2250 K. This means that a tungsten surface of 1500 K will loose tungsten by evaporation and by chemical reactions giving volatile products, which will be transported and be deposited at a surface of about 2250 K, or higher due to faster decomposition reactions at higher temperatures, 2300-2500 K. These processes are not desired, because they would transport tungsten from a tungsten electrode rod away from the electrode, thereby causing the rod to become thinner and to break.

It is also seen, however, that the tungsten pressures at about 1150 K, that is at the wall of the discharge vessel, are relatively high. Tungsten will be transported, too, from locations of this temperature to locations of about 2200 K or higher. This transport is aimed at, because it keeps the wall clear.

In the FIG. 3 the two curves intersect at about 2000 K. In a lamp in which the impurities influencing the volatility of tungsten compounds cause the W curve to be as shown, the temperature of the point of intersection of the curves is the proper temperature of the common boundary at location 73 of the first 71 and the second electrode rod portions 72. If in the lamp the temperature of the common boundary would be higher than the one shown, the highest rhenium temperature in the lamp would be higher and there would be a higher rhenium evaporation. If in the same lamp the temperature of the common boundary would be lower, the highest rhenium temperature would be lower and as a consequence the rhenium vapor pressure would be lower, but the tungsten pressures at the boundary would be higher and consequently transport of tungsten from that place to places of higher temperature where the W curve has a minimum would occur. At other impurity levels in the lamp the W curve shifts to the right and the two curves intersect at a higher temperature. In a lamp without substantial impurities the curves will intersect at about 1900 K.