US 2687489 A
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Description (OCR text may contain errors)
\ w. T. ANDERSON, JR., ETAL 2,687,439
Aug. 24, 1954 ELECTRODE Filed June 26, 1952 INVENTORS W/u/AM 77 4 470404 F. 5AM M 7 4;
Patented Aug. 24, 1954 ELECTRODE William T. Anderson, Jr., W. Samer, Cranford, N. Chemical & Manufactu N. J., a corporation of Application June 26, 1952, Serial No. 295,626
The present invention deals with an electrode and more particularly with an electrode for gaseous discharge devices of the super high pressure type.
The electrodes in lamps of the super pressure type are subjected to very high current densities and very high temperatures at the interface of electrode and gas at super high pressure through which the arc discharge passes. When the high pressure gas is metal vapor, such as mercury, which is condensible at ordinary ambient temperatures, the vapor pressure within the device when it is not operating becomes exceedingly low, and it is customary in the art to provide for starting purposes a rare atmospheric gas, such as argon, krypton etc., at a few millimeters of mercury pressure to conduct the initial discharge which evaporates the condensed metal. In this type of device the electrodes are also at ambient temperature when the device is started and must be heated by the discharge to provide sufficient electron emission to sustain the discharge without excessive evaporation and destruction of the electrode. At the start, when cold, it is wellknown to the art that the electrodes must have an electron work function of approximately 3 electron volts and less to prevent undue evaporation and sputtering of electrode material. Reduction and control of evaporation and sputtering of electrode material is essential for the useful life of the device. It is therefore customary in the art to provide the electrode structure with materials, frequently called activation materials, which can convey to the electrode a low electron work function for the initiation of the discharge in order to control evaporation and sputtering of electrode material.
It is an object of the present invention to provide an improved electrode for a super high pressure gas or vapor discharge device such as a lamp, and in particular for such devices as have an arc discharge path between electrodes of a few millimeters. It is a further object of this invention to provide super pressure are lamps which employ these improved electrodes for a longer useful life of such lamps. It is still a further object of the present invention to provide an activated electrode for super pressure lamps in which electrode there are a combination of embodiments which cooperate to provide a superior lamp. Other objects and advantages of the present invention will become apparent from the description hereinafter following and drawings forming a part thereof, in which:
Figure 1 illustrates partly in elevation and Maplewood, and Rudolf J assignors to Hanovia ring Company, Newark, New Jersey partly in section a super high pressure gaseous discharge lamp.
Figure 2 illustrates partly in elevation and partly in section an enlarged portion of a super high pressure gaseous discharge lamp.
Figure 3 illustrates an enlarged partly sectional and partly elevational salient structures according to the present invention, and before electrode has been hermetically sealed.
Figure 4 illustrates an enlarged partly sectional and partly elevational salient structures according to the present invention of the electrode as used in the lamp, and
Figure 5 illustrates partly sectional and partly in elevation modified forms of electrode structures made in accordance with the invention.
The present invention deals with a specific type of activated electrode which is cold when the discharge device is started and must therefore provide copious electrons in a cold condition but which after the device attains the operating condition must be heated to a much higher temperature by the discharge than is known to the art. Tungsten is generally used as the electrode material. The electron emissivity of tungsten increases enormously with temperature, being of the order of one hundred billion times greater at near the melting point, 3643" Kelvin, than at about 300 Kelvin (normal room temperature). Our electrode is used in an arc chamber containing gas at the operating condition in excess of atmospheres pressure. The temperature of the discharge is between 5,000 and 11,000 Kelvin. The tungsten of the electrode in contact with the discharge is actually molten and it is only by means of very specific forms to dissipate heat by radiation and conduction that electrode destruction can be prevented.
Usually the electrode of our invention must not be massive in structure as comm-only known to the art which employs massive electrodes to distribute heat and to provide ample radiation surface because radiation is a surface phenomenon. Massive electrodes have certain undesirable features and in particular are slow to warm when a discharge is initiated. For the same current, a massive electrode warms more slowly than a substantially smaller-sized electrode, and, therefore evaporates and sputters to a much greater extent and more rapidly blackens the envelope of the device than the smaller electrode during the heating process.
Activated electrodes normally operate at temperatures below 2000 Kelvin. The activated electrodes which we have invented must operate at temperatures as high as 3643 Kelvin. We
have found it necessary to devise a very special electrode structure in order to construct an electrode which will maintain the required degree of activation and to also operate in part at this high temperature. The electrode of the present invention includes a structure which has an electron work function approximately 3 electron volts and less, being sufficiently low to reduce the sputtering and evaporation of the outer metal surface of the structure, being tungsten metal, in contact with the electric arc discharge when the arc is started and the gas pressure within the device is low and the electrode is at the ambient temperature of the surroundings, and which is capable of being heated in part by the discharge to the molten condition, 3643 Kelvin, without destructive evaporation.
In order to accomplish the objects above set forth a combination of certain or all of the following considerations have been incorporated into our invention as essential features thereof:
A. A crystalline structure of tungsten metal.
B. A non-metallic material for activation which can be reduced to metallic condition by tungsten vapor, and vapors of the sub-oxides of tungsten.
C. Provision for the non-metallic materials and the reduction of the same to metals to occur in a chamber within the electrode structure which is hermetically separated from the enclosure in which the arc discharge gas or metallic vapor is occurring.
D. The employment of non-metallic chemical compounds for activation which upon reduction to metals, provides metals which are capable of migration through the crystalline structure of the tungsten at the operating temperature.
E. Production of tungstic oxide as a product of the reduction and retention of the vapor within the electrode to regulate the rate of reduction of the non-metallic chemical compounds in order to supply continuously during the life of the device metals for migration.
F. The migrant metals being capable upon attaining the surface of the tungsten electrode structure of reducing the electron work function at points on the surface to about 3 electron volts and less.
G. An arc discharge at a temperature between 5,000 and l1,000 Kelvin.
H. An operating gas pressure within the envelope of the device in excess of 50 atmospheres.
I. Molten tungsten at the contact between electrode and are discharge.
J. A temperature gradient in the electrode between about 3643 Kelvin and less than 1746" Kelvin, the latter being the melting point of tungstic oxide.
In our invention we provide an electrode structure which is devoid of a readily accessible activation material such as is well-known in the art of high pressure mercury lamps and the like. Instead we construct our electrode so that it presents a closed surface to the discharge and does not press into any crevices nor coat the surface with any activation materials which arrangement might accomplish the object of our invention for the initial few starts of the device, but which, at the temperatures at which our electrode must operate, are rapidly evaporated and serve only to blacken the walls of the envelope, which is usually fused quartz or a high silica glass. In order to provide the tungsten electrode with the necessary low work function upon its outer surface, we have utilized in our invention the well-known principle of migration through a crystal lattice which is described by Saul Dushman, Electrical Engineer, volume 53, page 1056, 1934.
However, the provision for migrating metals capable of activation of the tungsten surface is not of itself sufiicient to provide a satisfactory electrode for operation under the specified temperature conditions because migrants provided in the normal manner will diffuse to the surface and evaporate too rapidly. An arc device of the type indicated when made with such electrodes deteriorates too rapidly to be considered a practical device. We have found it necessary to regulate the rate of arrival of the migrant activation metal at the surface of the tungsten electrode structure in such a way that only sufi'lcient metal is present on the surface at any time to provide the necessary low work function each time the lamp is to be started but otherwise to provide only a limited amount of activation metal to be evaporated whereby to decrease the rate of opacity of the walls of the envelope of the device during operation so that a reasonable life may be obtained for the device, and the device may be used as a practical light source.
We have accomplished this in our invention by providing the migrant metals such as barium, strontium, zirconium, hafnium, thorium and uranium in the form of non-metallic compounds such as barium zirconate, barium thoriate, etc. which in themselves will not migrate, and have utilized the property of tungsten and the suboxides of tungsten at high temperatures to reduce these compounds to metallic elements which can then migrate through the tungsten crystal and can activate the surface of the tungsten to the required degree, namely, about 3 electron volts and less.
Instead of providing the materials in the combined form previously stated, namely barium zirconate, we can supply the chemical equivalents of barium oxide and zirconium oxide intimately mixed and in the correct proportions, and form the barium zirconate in situ by a pretreatment of the electrode by high frequency heating in an inert atmosphere.
When a compound such as heated above 3000 Kelvin in the presence of tungsten metal or tungsten sub-oxide vapors, the barium zirconate is reduced to metallic barium and metallic zirconium which vaporize and migrate into the tungsten crystal lattice, finally attaining the exterior surface of the tungsten lattice where they reduce the work function of the tungsten surface and also evaporate.
This reduction would occur very rapidly if it were not that the tungstic oxide vapor formed as a product dilutes the tungsten and sub-oxide. vapors so that the reaction rate of the reduction decreases rapidly and progressively. Finally the change in rate occurs slowly so that what amounts to an automatic brake on the process is provided whereby the remaining materials are reduced slowly and a continuous supply of migrant is provided.
In order to utilize this braking action of the tungstic oxide and also to prevent the tungstic oxide from making objectionable deposits on the walls of the envelope of the device, it is an important feature of our invention that the reaction chamber be hermetically sealed from the arc discharge chamber so that the vapors of the reaction remain within the reaction chamber.
barium zirconate is Thus, in our activation process we provide a reservoir of material for the release of free metal atoms and we control the rate of release of these atoms in such a way that only a slow migration of the atoms to the surface of the tungsten occurs and rate of evaporation to the walls of the arc enclosure is controlled.
In order to provide for rapid warm-up of the electrode and to have it operate under most efficient conditions, we maintain an electrode size of about 16 amperes per square centimeter of electrode surface. The are is in contact with about one per cent of this area, and it is this one per cent that is near-molten, temperature being almost 3643 Kelvin. The portion of the electrode farthest from the arcing point is the coolest portion of the electrode and will have a temperature below the melting point of tungstic oxide, l746 Kelvin. Hence the electrode must be hermetically sealed at the sides and the end in contact with the arc, but does not need to be so sealed at the coolest end because the solidified materials within the electrode chamber effectively close this end.
Referring to Figure l the super pressure lamp comprises a vitreous, e. g. fused quartz, envelope l, which may be substantially spherical or oblate spheroida1 in shape, defining a discharge chamber and having vitreous extensions, e. g. tubes 2 and 3 projecting outwardly therefrom and preferably opposed for supporting a pair of spaced electrode 4 and 5 respectively. The discharge chamber contains an ionizable atmosphere of a rare gas at a few centimeters of mercury pressure at about 300 Kelvin and a vaporizable metal in amount sufficient when evaporated to provide in the chamber l l0 milliliter and more.
Referring to Figures 2 and 3 the improved electrode structure of the present invention comprises a central core of tungsten 6 preferably as a portion of a solid rod 1 extending inwardly and outwardly of the envelope I through the extension tubes and With a substantial portion of the length of said rod sealed within said extension tubes, and said core 6 herein referred to comprising that portion of the rod which extends into the discharge chamber.
A tightl wound first coil 3 of tungsten is wound in close contact with said core 5 over a portion of the length of said core with the ends of the first coil 8 spaced from the inner surface 9 of the wall of the envelope and the end it of the core 6. The coil 3 supports an activation material of the type herein set forth and applied thereto in any suitable manner preferably in the depressions between the coil windings. However, the coil 8 may be coated with activation material or a portion of the rod 6 covered by the said coil may be coated with activation material.
A second tightly wound coil l2 of tungsten metal, having a pitch to the turns substantially the same as coil 8 and essentially of the same diameter as coil 8 or greater is tightly wound over and in close contact with coil 8 and the ends of coil l2 overlapping the ends of coil 8 and contacting core a in close contact therewith beyond the ends of coil 3 as illustrated in Figure 3, said ends of the coil 112 being spaced from the inner surface 9 of the walls of envelope I and end Ill of the core 6. The coil 12 may have both ends or only that end which is closest to the end It of core 6 in close contact with the core 6 so that the said coil I2 is thereby adapted to be screwed onto the coil 8 as illustrated.
molecules of metal per Finally the electrode structure is completed by high frequency and arcing in an inert atmosphere whereby the core 6 at the end of the core and the terminal ends of coil 8 and I2 towards the end of the core are melted together to form a hermetic seal is and to pre-activate the electrode by fusion and partial reduction of the activation materials, illustrated in Figure l.
Figure 5 illustrates a modification of our invention wherein the coils 8 and I 2 are replaced by a tungsten cylinder l5 supported and welded to core 6 in a hermetic seal and containing the activation materials [4.
It is apparent that the present invention provides an activated electrode wherein the reservoir of activation materials are completely protected from direct action by the discharge, activating metals are generated within the electrode structure and diffuse to the outer surface of the electrode by migration, tungsten metal of the electrode acts as the reducing agent and the product of the reduction, tungstic oxide, is retained within the electrode and retards the reduction action.
What we claim is:
1. An electrode for electrical discharge devices comprising a core of refractory metal, a first tightly wound coil of refractory metal wound about said core in close contact therewith, a second tightly wound coil of refractory metal wound about said first coil in close contact therewith, said second coil having a diameter at least equal to that of said first coil, a coating of activation material on a component of said electrode covered by at least one of said first and second coils, the windings of said second coil hermetically contacting each other, whereby free egress of said activation material through said second coil is substantially-inhibited.
2. An electrode according to claim 1, wherein said coating is on said first coil.
An electrode according to claim 1, wherein said coating is on a portion of said core covered by said first coil.
4. An electrode according to claim 1, wherein said coating is on said first coil and on a portion of said core covered by said first coil.
5. An electrode for electrical discharge devices comprising an elongated core of refractory metal, a first tightly wound coil of refractory metal v wound about said core in close contact therewith,
a second tightly wound coil of refractory metal wound about said first coil in close contact therewith, said second coil having a diameter at least equal to that of said first coil, a portion of said second coil extending beyond at least one end of said first coil and wound about said core in close contact therewith, a coating of activation material on a component of said electrode covered by at least one of said first and second coils, the windings of said second coil hermetically contacting each other, whereby free egress of activation material through said second coil is substantially inhibited.
6. An electrode according to claim 5, wherein the end of said core extends beyond said coils.
'7. An electrode according to claim 5, wherein said core and said coils consist of tungsten.
8. An electrode according to claim 5, wherein said coating is on a portion of said core, covered by said first coil and portions of said second coil extend beyond the ends of the first coil and are wound about said core in close contact.
9. An electrical discharge device comprising a sealed vitreous envelope defining a discharge chamber, an ionizable atmosphere within said envelope, a pair of spaced electrodes in said discharge chamber, electrical conductor means leading from each of said electrodes outwardly of said envelope and sealed therethrough, each of said electrodes comprising a metal core, a first tightly wound coil of refractory metal wound about said core in close contact therewith, a second tightly Wound coil of refractory metal wound about said first coil and in close contact therewith, said second coil having a diameter at least equal to that of said first coil, an activation material on a component of said electrode covered by at least one of said first and second coils, the windings of said second coil hermetically contacting each other, whereby free egress of said activation material through said second coil is substantially inhibited.
10. An electrical discharge device comprising a sealed vitreous envelope defining a discharge chamber, an ionizable atmosphere within said envelope, a pair of spaced electrodes in said discharge chamber, a pair of conductive metal rods sealed through the Walls of the said envelope, the portion of each of said rods extending into the discharge chamber consisting of an electrode core as a component of said electrodes, a first tightly wound coil of refractory metal wound about said core in close contact therewith, a second tightly wound coil of refractory metal wound about and covering said first coil in close contact therewith, said second coil having a diameter at least equal to that of said first coil, activating material on said first coil, the windings of said second coil hermetically contacting each other, whereby free egress of said activation material through said second coil is substantially inhibited.
11. An electrode discharge device according to claim 10 wherein said activating material comprises barium zirconate.
1-2. An electrode for an electrical discharge device according to claim 10 wherein said activating'material comprises barium thoriate.
13. An electrode for an electrical discharge-device comprising an elongated core of tungsten, a coaxial cylindrical sleeve of tungsten hermetically sealed to said core, and a filling in said cylindrical sleeve comprising barium zirconate.
1- An electrode for an electrical discharge device comprising an elongated core of tungsten a coaxial cylindrical sleeve of tungsten hermetical ly sealed to said core, and a filling in said cylinder comprising barium thoriate.
15. An electrode for an electrical discharge device comprising an elongated core of tungsten, a first tightly wound coil of tungsten metal wound about said core in close contact therewith, a second tightly wound coil of tungsten metal wound about said firstcoil in close contact therewith, said second coil having a diameter at least equal to that of said first coil, an activation material on a component of said electrode covered by at least one of said first and second coils and comprising components capable of migration through the tungsten crystal lattice, the windings of said second coil hermetically contacting each other, whereby free egress of said activation material through said second coil is substantially inhibited.
References Cited in the file of this patent UNITED STATES PATENTS