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Publication numberUS3675068 A
Publication typeGrant
Publication dateJul 4, 1972
Filing dateSep 10, 1970
Priority dateSep 10, 1970
Publication numberUS 3675068 A, US 3675068A, US-A-3675068, US3675068 A, US3675068A
InventorsHerbert S Strauss
Original AssigneeDuro Test Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Seal structures for electric discharge lamps
US 3675068 A
Abstract
Seal structures for electrical discharge lamps which include a relatively massive rod-like member which carries the full lamp current and which has a portion within the lamp envelope for connection to the electrode and a portion outside of the lamp envelope to which a current-carrying lead is connected. The rod-like member is fastened to another member which in turn is hermetically sealed to a portion of the envelope to complete a seal which is capable of operating at high temperatures.
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United States Patent Strauss 1 July 4, 1972 s41 SEAL STRUCTURES FOR ELECTRIC 3,474,278 10/1969 Thouret et al. ..3l8/l84 x DISCHARGE LAMPS 3,435,]80 3/l969 Kershaw ....3l3/l 84 UX 2,725,498 ll/l955 Storms et al.. 313/2 1 7 [72] Inventor: Herbert S. Strauss, Paramus, NJ. 3 4 54 97o paquene U 3 3 2 X 73 Assignce: D g Corporation, BergenI NJ 3,315,116 4/l967 B5656 3 [3/21 7 Filed: l 1970 Primary Examiner-Roy Lake 21 A I. I 71 12 Assistant Examiner-Palmer C. Demeo i 1 PP N0 1 8 Attorney-Dafby and Darby {52] U.S.Cl ..3l3/217,313/2l8,313/220, [57] ABSTRACT 3l3/332,3l3l335 I 511 int. Cl. ..H0lj 5/50, H01 j 61/36 i amps f [58] Field at Search 313,184 217 33 332 335 relatively massive rod-like member which carries the full lamp i current and which has a portion within the lamp envelope for connection to the electrode and a portion outside of the lamp 56 envelope to which a current-carrying lead is connected. The 1 References Clad rod-like member is fastened to another member which in turn UNITED STATES PATENTS is hermetically sealed to a portion of the envelope to complete al h'h bl f t' thht t 3,278,778 10/1966 Retzer r r r ..3l3/33l X a Se W C IS capa e 0 opera mg 3 lg empera mes 2,4595 79 1/1949 Noel ..3 13/184 X 9 Claims, 7 Drawing Figures PA'TENTEDJUL 4 I972 SHEET 20F 2 N. 8 & 21

HERBERT S. STRAUSS ATTORNEYS SEAL STRUCTURES FOR ELECTRIC DISCHARGE LAMPS BACKGROUND OF THE INVENTION Designs presently used for high current rating, in the order of 45 to I50 amperes, electric arc discharge lamps generally make use of two basic fabrication techniques to form a seal between the electrode structure and the envelope. These are the molybdenum foil seal and the tungsten rod beaded seal. Seals of both types are made in the side arms of the lamps, which arms are extensions of the central envelope portion in which the arc discharge takes place. Quartz is principally used as the envelope material.

The molybdenum foil seal is accomplished by vacuum sealing one, or several, electrolytically etched thin molybdenum foils, or plates, between two concentric quartz cylinders located in a side arm of the lamp. The electrolytic etching of the molybdenum foils is necessary to give them a cross sectional shape approximating an ellipse with edge angles of about 3. Edge angles of this magnitude assure a lower force of contraction during molybdenum foil cooling than the quartz to molybdenum edge adhesion force which would be encountered during and after the cooling cycle of the vacuum seal if the foil edges were not so shaped. Thus, a hermetic seal between the concentric quartz cylinders and molybdenum foils is possible even though the coeffecients of expansion of these two materials differ by a factor of about ten to one. Portions of the foils extend external to the lamp at the seal area so that an electrical connection can be made thereto by leads which are in turn to be connected to the source of current.

During the vacuum sealing process of making a foil seal, the quartz cylinders between which the molybdenum foils are sandwiched are melted and fused together by subjecting the seal structure to temperatures of about L600 C. This requires all associated materials such as the lead wires which are welded to the molybdenumfoils for external and internal lamp connections, to withstand this high temperature. Molybdenum rods or ribbons of 0.0l -0.0l5 inch thickness have been found to be able to meet this external temperature requirement and are also able to be spot welded to the molybdenum foils so that sufficient flexibility for external lead connections and other construction requirements can be met. However, molybdenum has a serious disadvantage in that it begins to oxidize readily at temperatures of about 250 C and oxidizes destructively at about 300 C. This also applies to other refractory materials which could conceivably be used in the manufacture of lamps of the type under consideration even if one would eliminate the requirements of weldability or flexibility needed. Oxidation, once started will always proceed. Since the molybdenum leads carry current, oxidation which reduces the base metal in cross section causes higher R heating, due to an increase in the resistance, thus increasing the oxidation rate cumulatively.

As the oxide of molybdenum (and other metals) occupies a greater volume than the original base metal, a force is exerted on the fused quartz cylinders holding the molybdenum foils and leads. Cracking of the quartz eventually results, allowing air to enter the lamp and causing lamp failure. It is therefore critical that the molybdenum foil and external lead structure be kept below a temperature 250 C throughout lamp life. There are three basic ways of achieving this which are: l) constructing the lamp of such a length to insure a sufficient temperature drop along the lamp arm before the seal-extemal lead structure begins, (2) air cooling the seal structure; and (3) using a combination of l) and (2) in which the air flow for cooling the seal may be reduced by increasing lamp length or conversely increasing the cooling air flow and decreasing the lamp length.

While the use of platinum clad molybdenum external leads will prevent oxidation of these leads; the edge of the molybdenum foil exposed to air can still oxidize and eventually lead to lamp failure. Also, platinum clad molybdenum leads add substantially to the cost of manufacturing the lamps.

The beaded tungsten rod seal is accomplished by hermetically sealing a bead of glass to a relatively thin tungsten rod. A rod of about 3/ l 6 inch diameter is about maximum for reliable beading, thereby limiting the current carrying capacity of the lamp. As another restriction, the tungsten and the glas bead must have closely matching thermal expansion coefficients. Typically, Corning code number 3320 glass is used for the beading to the tungsten rod. Additional glasses, decreasing in thermal expansion coefficient, are than spliced to the bead until a standard "PYREX" to quartz or VYCOR graded seal can be employed to fuse the seal structure to the lamp quartz envelope arm. The tungsten rod is the current carrying external and internal lead for the lamp electrode.

Oxidation of tungsten rod necessitates the seal structure to be operated not in excess of 250 C for reliable lamp life. Temperatures above this point will cause the tungsten rod to oxidize and eventually crack the bead allowing air to enter the lamp. Tungsten can be used as the metal lead into the lamp because of its low thermal coefficient as compared to other metals. This permits the use of graded seal glasses with relatively high softening and melting points. Higher thermal expansion coefficient glasses would have to be used of a metal other than tungsten were to serve as die lead into the lamp. These higher expansion coefficient glasses would have lower melting points and would therefore require additional graded seal "steps" to finally be spliced to quartz. The use of such lower melting point glasses would increase lamp length and reduce lamp reliability.

A combination of the molybdenum ribbon and tungsten rod beaded seal also can be used and has found use in mercury or mercury-xenon short are lamps. In these lamps the ribbon seal must be located close to the lamp envelope to prevent mercury condensation in the lamp arm during operation. The ribbon structure must then either be long enough to insure a maximum external lead temperatures of 250 C or the ribbon seal structure must be backed up by a vacuum chamber created by the tungsten rod beaded seal. The external end of the tungsten rod must still be maintained at 250 C or below.

SUBJECT INVENTION The present invention describes the structure and fabrication of electrode assemblies and hermetic seals for high current, arc discharge lamps. The electrode and seal configurations according to this invention are equally applicable to compact or long are rare gas lamps, such as xenon of krypton lamps and to xenon-mercury or mercury lamps. The invention allows these lamps to be operated at higher temperatures at their external seals. It is the purpose of this invention to allow the construction of the above lamps for applications in confined, tight lamp burning and enclosures such as projectors, spot lights, and luminaires, and to allow their opeiation without air cooling.

In accordance with the invention, several embodiments of lamps are disclosed which utilize improved seals. The seals include a rod-like member, which is relatively massive, which serves as the connection to the electrode inside of the envelope and also as the point to which the electrical lead-in wires can be connected outside of the envelope. The seal is accomplished by having the rod-like member pass through an opening in a cup, which opening is sealed by a material, preferably a metal, which attaches the rod to the cup. The cup is in turn hermetically sealed to a portion of the envelope.

In a preferred embodiment of the invention, the rod is of nickel, the cup of KOVAR (trade name for an alloy of iron, nickle and cobalt which has substantially the same thermal expansion as hard glass up to about 465 C) and the sealing material of a silver-copper-palladium alloy. Lamps using such seals have operated successfully with seal temperatures up to 450 C using relatively short structures. This permits lamps of high current ratings to be constructed in configurations not readily obtainable with the prior art.

It is therefore an object of the present invention to provide improved seals for electric arc discharge lamps.

A further object is to provide a seal for an electric discharge lamp which uses a rod-like member having a portion inside of the envelope, to which the electrode is connected, and a portion outside of the envelope, to which a current carrying lead is to be connected.

Another object is to provide seals for electric arc discharge lamps in which a rod is sealed to a cup which is in turn hermetically sealed to the lamp envelope to provide a high temperature seal.

Other objects and advantages of the present invention will become apparent upon reference to the following specification and annexed drawings in which:

FIGS. IA and 18 when taken together, are an elevational view taken partially in cross-section, of a lamp according to a preferred embodiment of the invention;

FIG. 2 is a fragmentary view, taken partially in cross-section, of a portion ofthe lamp of FIGS. 1A and 18',

FIG. 3 is a cross-section taken along lines 3-3 of FIG. 1A;

H0. 4 is an elevational view, taken partly in cross-section of a portion of the lamp according to another embodiment of the invention;

FIG. 5 is an elevational view, also taken partially in crosssection, of a portion of a lamp showing another embodiment of the invention; and

H6. 6 is a cross-section view of the electrode structure of the lamp of FIG. 5 taken along lines 66.

F I65. I through 3 show a preferred embodiment of the lamp for direct current operation which is illustratively in the form of short arc, xenon gas lamp. Such lamps have been constructed and operated successfully up to I00 amperes and 3,000 3,500 watts.

The lamp includes an envelope ]0 which is made of fused quartz or other suitable refractory material. The envelope [0 has an enlarged central bulb section from which extend a pair of side arms 12a and 12b. The side arms each have a respective reentrant portion 140 and 14b for sealing within the lamp the respective cathode and anode electrodes 14 and 16.

Since the lamp of FIG. 1, is for direct current operation, the cathode 16, which is the electron emitter, is fabricated preferably from tungsten rod material to which there is added thorium material of from about 0.7 to 2 percent. The cathode is shaped at its free end with a tip 17 having a conical surface. This shape provides the sharp tip necessary for high current concentrations and, consequently, the high plasma temperature (approximately 6,000-l0,000 K) which results in producing the high photometric brightness characteristic of these lamps. A peak brightness exists in the area Ajust in front of the tip 17 of the cathode. This area A ideally is at the focal point of a reflector system, a portion of which is shown by the reference numeral 20, for either projecting parallel rays of light, such as in a spot-light application, or re-focusing the rays at some point in front of the reflector. As a typical example of the latter, the rays would be re-focused about 30-50 inches in front of point A at the point where the film runs in a motion picture projector.

The anode t4, the tip 18 of which is shown in FIG. 1A as well as FIG. 1B, is rounded and more massive than that of the cathode as it must accept the high thennal inputs of the DC discharge. The anode tip can be fabricated for example, from it; inch diameter swaged or wrought tungsten and it is dimen' sioned so that a proper thermal balance is achieved between surface radiation and heat conduction along its length and ultimately to the remaining parts of the electrode.

In typical lamps made according to the subject invention, the anode and cathode tip body temperatures are in the order of 2,000-2,500" Cv A one-quarter inch diameter cathode tip, cooling itself by emission of electrons, can be kept small in order not to exceed the above body temperatures. The very point of the cathode tip, however, may operate at, or just below, the melting point of tungsten (3,370 C) in order to achieve highest possible current concentrations at the transfer area from the metal into the plasma at the tip. it is the function of the remaining portions of both electrode structures to:

a. provide a current-carrying path to the respective electrode tips;

b. disipate the temperature created by the respective tips;

c. directly or indirectly allow the construction of the hermetic lamp seal for the electrodes; and

d. provide a means for electrical installation.

The construction by which the above is accomplished is described below with respect to the anode electrode 14. Since the remainder of the electrode structure for the cathode, ex clusive of the tip, is identical, only the anode structure is described in detail. The tip 18 of the anode extends from a shank 21 of reduced diameter which is attached at point 23 to a disc 22 located at the end of the side arm near the more spherical central portion of the envelope. The disc 22 is, for example, one-eighth inch wide and five-eighths inch outer diameter molybdenum with a central hole. The attachment of the anode shank 21 to the disc 22 at point 23 can be, for example, by means of a high temperature braze. The shank 21 protrudes for a short distance to the left of disc 22 for reliable attachment to disc 22. The brazing material should be of a material with sufficiently high melting point and low vapor pressure so as to remain stable during lamp operation. Platinum has been successfully used as the brazing material.

A plurality of strips of an electrically conductive material are attached to the peripheral outer edge of the disc 22. The strips 25 also can be, for example, of molybdenum material and also can be, typically, made from 0.010 inch 0.0l5 inch thick rolled molybdenum sheet which is cut into one-eighth inch wide strips. The strips 25 are attached to the molybdenum disc 22 also by means of a platinum braze at points 29a. in the lamp being described, five of these strips equally spaced around the disc 22 circumference are adequate to allow lamp operation at [00 amperes for several thousand hours life. Of course, as many strips 25 as necessary can be used.

A second disc 28 is located near the central portion of side arm 12a. The disc 28 can also be made of molybdenum material and the strips 25 are attached to the disc 28 around the edge thereof. The attachment of the strips to the peripheries of disc 28 can be accomplished, for example, by resistance spot welding such as at points 29b.

A quartz tube 30 is slipped over the end of shank 21 of the anode electrode which protrudes to the left of disc 22. Tube 30 is located inside a cage formed by the two discs 22 and 28 and the strips 25. A disc 32 which serves as a getter for impurities is located between the right end of the tube 30 and the left face of the disc 22. This disc 32 can be of any suitable material, for example, tantalum. The length of the tube 30 is adjusted so that about one-eighth inch of the strips extends beyond the quartz to provide an area for connection to the edge of disc 28 at points 29b. The wall thickness of the tube 30 is preferably selected to be greater than 0.125 inch to support subsequent quartz melting operations. Typically, the dimensions of the tube can be 0.275 inch inner diameter by 0.600 inch outer diameter.

To seal each electrode assembly in the respective side arm 12a or l2h, a cup-shaped member 36 is provided which can be, for example, of KOVAR material. A conventional graded glass seal is used to seal the re-entrant portion or arm 12a to the cup 36. This includes a series of different glasses having different coefficients of expansion. For example, the left end of cup 36 can have sealed thereto a ring 38 of KOVAR sealing glass such as Corning No. 7052. To ring 38 is joined a ring 40 of No. 3320 glass and then a ring 42 of 7740 PYREX glass. The latter ring 42 is then spliced to a PYREX to quartz graded seal 14a to complete the graded glass seal.

The head end of the cup 36 is opposite glass ring 38 and has a heavy wall 44 through which a hole 45 is formed. A rod 48, which is preferably of nickel, has an end portion which passes through the hole 45 and is extemai to the lamp. This external end serves as the electrical lead end for the lamp. The rod 48 can be, for example, one-quarter inch in diameter and 3 inches long. The head end of the rod 48 (shown in greater detail in FIG. 2) protrudes for a short distance from the cup and is formed with a cavity 50 into which are brazed a multiplicity of foils or leaves 52 of a suitable electrically conductive material. These leaves can be, for example, of molybdenum which can be fabricated from 0.0007 inch thick molybdenum sheet folded into six layers and slit five times in widths of one-eighth inch. In this way an assembly of thirty, oneeighth inch wide by 0.0007 inch thick molybdenum foils can be obtained. The left-ends of the leads 52 are brazed at 57 to the wall of the nickel rod 48 surrounding the cavity 50 while the other ends are connected at point 53 to the molybdenum disc 28, which also can be accomplished by bran'ng. The brazing material at point 57 can be silver, which has a melting point of 961 C, while the material at 53 can be, for example, platinum which has a melting point of 1,773 C. Therefore, braze 53 must be accomplished before braze 57.

In a typical manufacturing sequence the shank 21 of the electrode and the molybdenum strips 25 are platinum brazed to the disc 22 at points 23 and 29a. In a second separate subassembly the disc 28 is platinum brazed to the molybdenum foil bundle 52 at point 53 and the opposite end of the molybdenum foil bundle 52 is silver brazed into the cavity 50 of the nickel rod 48. In a third separate subassembly the Kovar cup graded seal is prepared with the glasses described above. The hole 45 in the heavy wall section 44 of cup 36 is then inserted over the free end of the nickel rod 48 and positioned as shown in FIG. 2. A 58 percent silver 32 percent copper l percent palladium alloy, melting point 85 2 C, is then used to hermetically braze the nickel rod 48 to the Kovar cup 36. The tantalum getter disc 32 is then slipped over the protruding electrode shank 20. The quartz insert tube 30 is then inserted into the disc 22 and molybdenum strips 25 structure so that the getter disc 32 is sandwiched between the disc 22 and the end of the insert 30. The molybdenum disc 28, which now forms part of the assembly including the molybdenum foils S2, nickel rod 48 and KOVAR cup 36 graded seal 14a is resistance spot welded to the molybdenum strips 25 at points 2912 thus completing the electrode structure. The electrode structures are inserted into side arms 12a and 12b. The envelope is then closed either by "capping" or by tightly inserting silicon rubber stoppers and is then evacuated through the tip 60 on the central bulb section which tip, during the early stages of processing, is longer than that shown. At this time the side arms have a uniform larger diameter shown to the left of the disc 28. The areas of the side arms directly above the cage formed by the molybdenum strips 25 and the discs 22 and 28 are then heated and collapsed. The strips 25 become firmly sandwiched between the collapsed portions of the side arms and the quartz tubing 30. The lamp seal is then made by melting together the quartz portion of the PYREX to quartz graded seal 14a and the side arm 120 at area 62. Although electrode spacing between anode and cathode tip is set to the required specifications at the time of inserting the electrode structure into the side arms 12a and 12b, subsequent melting operations may require a fine adjustment at this stage. This can be easily accomplished by reheating the quartz in the side arm to spherical bulb area to its working point and hand adjusting" the electrode spacing by pulling, pushing, raising or lowering the end of side arm 120 or 12b. Usually only very slight adjustments are required. The lamp ends are then cut at points 62 to expose the rod 48 so that flexible electrical leads 64 can be attached thereto by a suitable connector 66.

After the sealing-in of the electrode is completed, the lamp is then processed in a conventional manner. This comprises evacuating the lamp, baking out" all of the quartz and glass parts, induction heating the electrodes, and filling the lamp with the required high xenon gas pressure, by the technique of a liq uid-freezing of the gas, through the tubulation 60. The tubulation is then tipped off as shown at 60. Lamp supports (not shown) may be mounted to the side arm areas, such as at the points marked B for installation into the enclosure in which the lamp is to be used.

The purpose of the molybdenum strips 30 and the assembly of foil strips 52 is to dissipate temperature along the length of the lamp. The strips 25 do this effectively by conducting heat directly to the collapsed side arm sections of the envelope and the quartz tube 30 between which two elements of the strips are sandwiched. The bundle of foil 52, because of its relatively large surface area, radiates heat away. Thus, an efl'ective technique for conducting heat away is achieved for a relatively long portion of each side arm. The thin foil strips 52 serve an additional function of allowing some motion of the rear elec trode structure in the longitudinal direction during the lamp sealing operation which takes place at area 62. This motion is necessary to prevent dangerous compressive stresses in the side arm 12a to quartz 14a seal. If such motion is prevented, cracking will occur. The amount of motion can be as little as about one-sixteenth inch.

The type of high temperature hermetic seal described by this invention is made at the joining point 55 of cup 36 and rod 48 by a braze of a suitable material, such as a 58 percent silver 32 percent-copper-IO percent palladium alloy, of a melting point of 852 C. These materials have considerably higher temperature destructive oxidation points than either molybdenum or tungsten, which are the required materials for similar lamps currently in existence. Lamps made according to this invention have operated at temperatures of 450 C at the hermetic seal area for several thousand hours. Tungsten or molybdenum would oxidize destructively at these temperatures within very few hours after burning begins. As should be apparent, the brazing materials and other lamp materials are compatible and are consistent with the fabrication sequence.

Lamps constructed in accordance with the embodiment of FIGS. I-3 have operated successfully at loads of about 100 amperes (approximately 3,0003,500 watts). Lamps according to the invention also have been fabricated for operation at I50 amperes (approximately 4,500-5,500 watts). By choosing the proper number of molybdenum strips 25 and foils 52 and increasing the diameter of the rod 48, the desired 450 C sea] temperature at the junction of the rod 48 and the KOVAR cup 36 can be obtained.

The lamp structure shown in FIGS. l-3 can be used for an alternating current operated lamp and also for a long are xenon lamp. To accomplish both of these objectives, the identical electrode structure is used. The only difference being is that, for alternating current operation, the conical shaped cathode electrode 17 shown in FIG. 1B is replaced by another blunted end electrode such as electrode 18 shown in FIG. IA. Also, for long arc operation the spacing between the two electrodes is increased to permit the longer length and therefore a long luminous arc discharge column.

Long arc xenon lamps constructed in the manner previously described, following the principles of FIGS. 1-3 have been constructed which have the following characteristics:

TABLE I Lamp Arc Lamp Current Lamp Voltage Lamp Wattage Length 60 amperes [00 volts 6,000 watts 550 mm amperes 125 volts [0,000 watts 750 mm amperes 200 volts 20,000 watts L500 mm The application of the above-described long-arc lamps is for large area lighting or general illumination since they emit a high luminous flux rather than a high brightness for optical projection, as in the case of the compact arc lamp of FIGS. 1- 3. As seen from the above electrical characteristics, however, lamp currents for l0,000 and 20,000 watt lamps are similar to those given for the compact arc lamps. A corresponding seal structure and fabrication technique is therefore preferred to eliminate air-cooling in the long-arc xenon lamp luminaires.

FIG. 4 is another embodiment of the invention showing a long-arc xenon discharge lamp for alternating current operation. This lamp incorporates a different electrode structure than that shown in FIGS. 1-3. The lamp of FIG. 4 is suitable for operation at the lower wattage ratings shown in Table l above. In the lamp of FIG. 4, a rod 48, also of nickel material preferably, is again used to form a hermetic seal with a KOVAR cup 36 in the heavy walled area 44 by a braze 55 which is preferably of silver material. The other details of the graded glass seal are not described but they are the same as that previously used in the lamp of FIGS. 1-3. Again, only one electrode is described. In this case, both electrodes are identical.

The shank 20a of the anode (or cathode) electrode tip 18 is in this case elongated and extends all the way back to the end of the rod 48 which is inside the lamp. The shank 20a is wrapped in area C with a suitable foil material 70, which can be of molybdenum, and a tubular insert 72, which can be of quartz, is slipped over the foil. The foil 70 holds the insert 72 in place during the brazing of the reduced diameter end 20b of the electrode shank 20a into a cavity 74 in the end of the rod 48. A brazing alloy can be used, for example typically a 20 percent palladium-80 percent silver alloy, which is shown by reference numeral 75. This alloy has a melting point of l,l75 C.

The envelope side arm is sealed under vacuum conditions to collapse portion C around the quartz insert 72. Because of the lower current rating for this lamp, no molybdenum strips or foils such as 25 and 52 are required and the shank 20a of the electrode is sufficiently long to dissipate the heat so that the operating temperature in the hermetic seal area of the rod 48 to the cup 36 can be kept at a temperature of near 450 C. The heat is dissipated partially by conduction directly from the electrode shank to the quartz wall through the quartz insert 72. The required motion in the longitudinal direction of the electrode structure is obtained as the shank 20a is allowed to slide inside the quartz insert 72 during the lamp sealing opera tion.

FIGS. 5 and 6 show another embodiment of lamp which is constructed for compact arc mercury or mercury xenon gas, direct current operation. These types of lamps are employed in applications similar to those of the compact arc xenon lamps shown in FIGS. 1-3. Because of their high discharge voltage gradient they are capable of converting a higher percentage of input power into visible radiation. Their luminous efficacy is, therefore, approximately double that of compact arc xenon lamps for equal electrode spacing. Their photometric brightness, however, is about one-half. Also, since mercury is the dominant discharge species, the strong mercury spectral lines produce the characteristic bluish-green light associated with mercury lamps.

To fabricate an arc discharge lamp in which mercury is to be used, the techniques and construction must be such to prevent mercury condensation. Temperatures in the sealed envelope must be maintained so that they will not drop below the 600 C temperature needed to obtain the required 20-30 atmospheres of mercury vapor pressure at which these lamps operate. This necessitates that all side arm spaces be sealed. If the same construction were to be used as used in a compact arc xenon lamp construction, this would allow mercury condensation in the rear portion of the quartz envelope side arm.

Referring now to FIGS. 5 and 6, the envelope 10 with the side arm 12a again has the reentrant portion 140 through which the rod 48, which is also again preferably of nickel, is passed. The KOVAR cup 36 is again used and the hermetic seal is again accomplished by the brazed silver, or silvercopper-palladium, joint 55 between the rod and the cup. The graded glass seal is used, but it is not shown in detail. The flange ends of a number of ribbons, or strips, 80, which can be of molydbenum material, are attached to the end of the rod 48, for example, by spot welds. The welds may be mechanically reinforced by wrapping a thin molybdenum wire or ribbon 82 about strips 80 where they are attached to the rod 48 and then spot-welding the entire assembly.

The shank 20 of the electrode 18 has a flanged end 200 to which is attached a conventional molybdenum ribbon seal 86. The right ends of the molybdenum strips are connected to the ends of the ribbons 86 by any suitable means such as spot welding. The strips 80 are located to lie in contact with the envelope so as to dissipate heat. The neck areas of quartz bulb 10 are coated with a heat reflective coating such as platinum at areas 88. This insures full mercury evaporation inside to envelope 10.

The structure of FIGS. 5 and 6 is such to maintain a sumciently high temperature throughout the interior of the lamp so that there is no condensation of mercury. Note particularly that strips 80 and ribbons 86 serve as heat radiators along a substantial portion of the length of each side arm.

As should be understood, each of the embodiments of the lamps of the subject invention utilizes a seal structure with the massive metal rod 48, which is preferably of nickel. This rod is the only element in the seal configuration which can'ies the full lamp current and is an element in forming the hermetic seal. The other two elements which form the seal (KOVAR cup 36 and the silver or silver-copper-palladium alloys braze 55) do not carry the lamp current. While the other members of the electrode structure, such as the molybdenum strips 25 and 80 or the foils 52, carry the current, they are not exposed to air. Thus, the molybdenum strips and foils cannot oxidize.

Temperature in any external lead attached to or in contact with the lamp sealing elements is produced from two sources. These are heat conducted from the plasma through the metal and quartz materials and the PR heating by the lamp current. The external leads are sufficiently shielded and no direct tem perature increase from radiation will result. This is another advantage of the lamp of the subject invention.

The higher resistivity of the nickel rod 48, as compared to molybdenum or tungsten, is easily compensated for by increasing its diameter and, therefore, its cross-sectional area over that which was possible for a tungsten rod for a beaded seal or molybdenum wires or strips required for a ribbon seal. The resistance to oxidation of nickel as compared to molybdenum or tungsten has been mentioned. Conducted heat from the arc plasma will have no effect on nickel unless temperatures above 500 C are encountered on the external end of the rod 48.

As should be noted, the hermetic lamp seal of the present invention does not require a molybdenum ribbon seal which is fabricated at about L600 C and therefore requires molybdenum external leads to survive this temperature. In addition, the hermetic seal is not a beaded seal which requires a relatively thin tungsten rod for proper thermal expansion match to a usable glass for the beaded seal construction.

Instead, the hermetic seal is a brazed joint using materials such as nickel, silver, or silver-copper-palladium alloys and KOVAR which have much higher temperature destructive oxidation points than molybdenum and tungsten. These materials in conjunction with heat dissipating electrode structure members are able to produce a high current seal which allow successful operation of discharge lamps for several thousand hours.

What is claimed is:

1. An electric discharge lamp comprising an envelope, a quantity of an ionizable medium within said envelope, a pair of electrodes each having a tip portion within said envelope, at least one of said electrodes having a rod and means for electrically connecting the rod to said tip portion of the electrode, means for sealing the said electrode with a portion of the rod within said envelope:

said seal means including;

a. a cup-shaped sealing member of metallic composition having an opening at one end thereof through which a portion of said rod extends exterior to said envelope,

b. first metallic means for attaching the rod to said sealing member and hermetically scaling the opening of said cup-shaped sealing member, and

c. means for attaching another end of said cup-shaped sealing member to a portion of the envelope which surrounds an opening in said envelope to form an hermetic seal,

said means for connecting the tip of the said electrode to the end of the rod within the envelope including between said rod and said tip:

a. a first plurality of strips of electrically conductive material electrically connected to the end portion of said rod within said envelope by a second metallic means,

b. a second plurality of strips of electrically conductive material electrically connected to said tip by a third metallic material.

and means for electrically connecting said first and second plurality of strips,

said first, second and third metallic materials respectively having higher melting point temperatures in the named order.

2. An electric discharge lamp as in claim 1 wherein said rod is of a material which includes nickel and said first material for attaching the rod to the sealing member is of a material which includes silver.

3. An electric discharge lamp as in claim 1 wherein said means for attaching the sealing member to the envelope includes a graded glass" seal.

4. An electric discharge lamp as in claim I wherein said cup is of a material comprising an alloy of iron, nickle and cobalt having substantially the same thermal coefficient of expansion as hard glass.

5. An electric discharge lamp as in claim 1 further comprising means for holding said second plurality of electrically conductive strips in contact with the inner wall of the envelope to dissipate heat.

6 An electric discharge lamp as in claim 5 wherein said second plurality of strips are arranged to form a cage and said holding means includes a tube of electrically non-conductive material within the cage and in contact with the said first plurality of strips.

7. An electric discharge lamp as in claim 6 wherein said first and second plurality of strips are of a material which includes molybdenum.

8. An electric discharge lamp as in claim I wherein means are provided for electrically connecting the first plurality of strips to the rod, said connecting means including an opening in the end of the portion of the rod within the envelope into which the ends of thesecond plurality of strips are placed, said second metallic composition being placed within said opening and solidified to attach the said ends of the strips to the walls of the rod defining the opening therein.

9. An electric discharge lamp as in claim 8 wherein said rod comprises nickle material and said metallic composition comprises silver.

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Classifications
U.S. Classification313/623, 313/335, 313/332
International ClassificationH01J61/98, H01J5/42
Cooperative ClassificationH01J2893/0034, H01J5/42, H01J61/98
European ClassificationH01J5/42, H01J61/98