High-pressure metal halide lamp having three part electrode rods

A high-pressure metal halide discharge lamp has opposed tungsten electrodes carried by electrode rods. These rods have a first portion of tungsten adjacent the electrodes and a second portion made of at least 25% by weight of rhenium. Their common boundaries are at a location having an operating temperature in the range of 1900-2100.degree. K. The gas filling contains metal oxyhalide and is devoid of rare earth metal compounds.

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 portion of the electrode rods from tungsten it is avoided that a
 strong evaporation occurs, as it would be the case in the event the first 
 portion consists of rhenium. The temperature of the common boundary of the
 first and the second portions is chosen to be about 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. 
 A first tungsten rod may be welded, e.g. butt welded, to a second rhenium 
 or rhenium alloy rod, e.g. by resistance welding or laser welding. In this
 event the second rod may be chosen to be slightly, e.g. 10 to 15%, 
 thicker, if so desired, in order to compensate for the lower heat 
 conductivity of rhenium: S.sub.Re.apprxeq.0.3 * S.sub.W. 
 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 said 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. 
 In a favorable embodiment a common boundary region is formed by the first 
 and the second portion over which during lamp operation the temperature 
 lies between 2300 and 1900 K and in which boundary region the second 
 portion is enclosed by a mantle substantially made of tungsten. This is 
 realized e.g. by an electrode rod having a core made of rhenium or a 
 rhenium alloy and a mantle made of tungsten or e.g. by an overlapping of 
 the wrapped tungsten wire from the first portion with the rhenium 
 containing portion. An electrode with this type of boundary allows a less 
 accurate production of the boundary of the first and the second portion, 
 since, due to an overlap of the first and the second portion. Less 
 accuracy is allowed since the position of the boundary is self-adjusting 
 during operation of the lamp. Subsequently, such an electrode rod 
 facilitates the processing of the lamp. 
 In a further favorable embodiment the electrode rod consists of three 
 portions. A first portion of the electrode adjacent the electrode tip is 
 made of tungsten, a second rhenium containing portion which during 
 operation of the lamp extends over the temperature range of the electrode 
 of 1400-2300 K, and a third portion in which the rhenium containing 
 portion is replaced by another material e.g. tungsten, molybdenum or 
 tantalum. The third portion may begin at a location where the electrode 
 surface is hardly accessible by the gases of the filling of the lamp. The 
 temperature at this location is lower than 1400 K during normal operation 
 of the lamp. The third portion is secured to the current lead-through 
 conductor. The electrode is cheaper and the material that extends into the
 pinch can be chosen independently. 
 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
 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 lamp shown in FIG. 1 
 is an AC-lamp, but DC-lamps fall within the scope of this invention as 
 well. The discharge space has a gas filling comprising rare gas and metal 
 halides. Tungsten electrodes 5 are oppositely disposed in the discharge 
 space 3. Current leadthrough conductors 6 are located in a respective seal
 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 a respective one of 
 said leadthrough conductors 6, in the FIG. by welding them to the metal 
 foils 6a, enter the discharge space 3 and carry a respective one of said 
 electrodes 5. 
 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 location 73 having a temperature in the range of 1900-2300 K, 
 particularly 2100-2300 K, in the FIG. 2100 K, during operation. In the 
 Fig. the second portions 72 of the electrode rods 7 consists of rhenium 
 and are thicker, have a diameter of 1 mm, 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 
 Nal, 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 the lamp had not 
 reached the end of its life, yet. 
 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. 2a the electrode rod 7 has a first portion 71 and a wire wrapping 
 74 of tungsten and a second portion 72 of rhenium/tungsten alloy up to the
 location 73. 
 In FIG. 2b the electrode rod 7 has a first portion 71 and a wire wrapping 
 74 of tungsten, a second portion made of rhenium, which portions have a 
 common boundary region at location 73. Location 73 extends over a distance
 X over the electrode rod 7. Over the distance X the temperature lies 
 between 2300 and 1900 K during normal operation of the lamp. The location 
 73 is formed by the boundary region between a core 76 made of rhenium 
 which is enclosed by a mantle 77 made of tungsten. 
 In FIG. 2c the electrode rod 7 has a first portion 71 and a wire wrapping 
 74 of tungsten, a second portion 72 made of a rhenium/tungsten alloy from 
 locations 73 to 81 and a third portion 80 made of molybdenum. 
 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 towards 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. 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 the 
 location 73 of the first 71 and the second electrode rod portions 72. If 
 in the lamp the temperature of said 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.