US 3558963 A
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United States Patent  Inventors Rodney E. Hanneman Burnt Hills; Paul J. Jorgensen, Scotia; Richard .1. Charles, Schenectady, N.Y. ] Appl. No. 753,143  Filed Aug. 16, 1968  Patented Jan. 26, 1971  Assignee General Electric Company a corporation of New York Continuation of application Ser. No. 616,538, Feb. 16, 1967, now abandoned.
 HIGH-INTENSITY VAPOR ARC-LAMP 31 Claims, 4 Drawing Figs.
 US. Cl 313/184, 313/174, 313/178, 313/221, 313/227, 313/229  1nt.Cl H01j6l/12  Field ofSearch 313/174, 178,184,221, 227,229  References Cited UNITED STATES PATENTS 3,350,598 10/1967 Corbin et a1. 313/221X Primary Examiner-Raymond F. Hossfeld AttarneysCharles T. Watts, Paul A. Frank, John F. Ahern,
Frank L. Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg ing a certain metals, alloys, or metal compounds into the lamp whose function is to prevent the formation of sodium aluminate or related compounds which are directly responsible for the sodium cleanup. This requires a negative free energy or reaction for the equation of the form:
NaAlOz XM(S) Na(L, G) M;O (S) Al20a(S) as well as sufficiently rapid reaction rates. The preferred metals for this purpose are yttrium, cerium, and lutetium.
HIGH-INTENSITY VAPOR ARC-LAMP HIGH INTENSITY VAPOR ARC LAMP This application is a continuation-in-part of our copending application, Ser. No. 6l6,538, (and now abandoned) filed Feb. I6, 1967, and assigned to the present assignee.
CROSS REFERENCES TO RELATED APPLICATIONS Ser. No. 388,567, filed Aug. 10, I964 by Paul .I. Jorgensen, Ceramic Bonding, now abandoned same assignee.
Ser. No. 590,568, filed Oct. 31, I966 by William C. Louden, Niobium End Seal," now US Pat. No. 3,448,319 same assignee.
Ser. No. 6l6,539, filed Feb. 16, 1967 by Rodney E. Hanneman, Paul J. .Iorgensen, and Dimitrios M. Speros, Alumina Ceramic Sodium Vapor Lamp, now U.S. Pat. No. 3,453,477 same assignee.
Ser. No. 745,502 filed Jul. 17, 1968 by Rodney E. Hanneman, Metallic Vapor Arc Lamp Having High Intensity Sunlike Emission," same assignee.
BACKGROUND OF THE INVENTION The invention relates to high-pressure vapor arc lamps wherein sodium atoms are a principal radiating species, utilizing an alumina or similar ceramic for the arc tube, and is particularly concerned with the prevention of sodium cleanup, that is reduction in the effective amount of sodium within the arc tube during the life of the lamp.
The high intensity vapor arc lamps with which the invention is concerned, in one embodiment, generally comprise an outer vitreous envelope or jacket within which is mounted a slender tubular arc tube of high density polycrystalline alumina. This basic lamp type is described and claimed in U.S. Pat. No. 3,248,590, issued Apr. 26, I966 to Kurt Schmidt. The are tube encloses a charge of sodium and mercury in such proportions that an excess of each is not vaporized during lamp operation, and an inert gas such as xenon. In such a lamp sodi urn partial pressure within the arc tube is generally within the range of approximately 30 l ,000 torr.
In another. are lamp with which this invention is concerned,
wherein sodium atoms constitute a principal radiating specie,
sodium is added to an alumina or other similar high density polycrystalline ceramic arc tube in the form of a halide, along with mercury, with or without other metallic halides. The basic lamp of this type is disclosed and claimed in U.S. Pat. No. 3,234,42lReiling. In the operation of such lamps, the sodium halide is dissociated by the thermal energy of a firststarted mercury arc and sodium atoms are raised to excited states and emit visible light. The lamp also includes other metallic halides, and the total emission from the metals of the various halides, including the sodium, is a pleasing white light of high intensity. In such lamps sodium is present within the radiating portion of the arc tube (adjacent the are sheath) in partial pressures of approximately 10- to 200 torr.
In yet another lamp with which the invention is concerned, a similar arc tube is filled with a charge of a starting gas such as xenon and sodium together with at least two other metals from the group including thallium, cadmium, and mercury, all of which metals are added in such quantities as to retain an excess of unvaporized metal of each added constituent in the reservoir during operation. The total emission of the metals of the charge is a highintensity of white light. In this type lamp, a sodium vapor pressure of approximately 0.75 to 460 torr is present within the arc tube during operation. Such lamps are disclosed and claimed in the copending-application of R. E. Hanneman, Ser. No. 745,502, filed Jul. 17, I968, assigned to the present assignee, and incorporated herein by reference thereto.
In the development of lamps of the type set forth in the aforementioned Schmidt patent, it was found early that over a prolonged period of use, the voltage drop across the lamp increased and the proportion-of red in the radiation decrease, the light becoming more purple; the latter phenomenon is generally referred to as decreased in the red factor. At the same time the mercury vapor pressure increased and, if operation continued notwithstanding the deterioration in light output, the rising mercury vapor pressure could cause the voltage drop across the lamp to increase to the point where the lamp extinguished.
It has been determined that these happenings were associated with a sodium loss from an initial pressure of approximately 30 to 1,000 torr and a reduction in the proportion or atom fraction of sodium in the excess liquid charge present in the reservoir during operation. Such loss or cleanup rate of sodium could be reduced by lowering the reservoir temperature', however, this also lowers both lamp efficacy and the red factor and is at best only a compromise. A practical solution utilized in the lamps manufactured for public sale consists in increasing the quantity of loading of charge at the chosen composition to the point where, notwithstanding the cleanup occurring, the change in composition occurring remains within acceptable limits during the intended lamp life. This has permitted acceptable limits over the full life of the lamp. Such lamps represent a major advance in lamp technology and have been available on the market under the designation Lucalox, a trademark of the assignee herein.
Similar problems exist with respect to the Reiling-type lamp when a ceramic alumina arc tube is utilized. Since, however, the Rcilingtype lamp may be operated with a fused silica arc tube, due to the lower temperatures of the bulb wall due to the presence of the halide, commercially successful lamps of this type have been marketed under the designation of MultiVapor, a trademark of the assignee herein.
The Hanneman-type lamp requires such a delicate balance of charge material that it is highly desirable that the present invention be practiced in connection with the manufacture thereof in order to retain the efficacy and spectral emission thereof.
A principal object of the present invention is to prevent a sodium cleanup in vapor arc lamps in which sodium is a principal radiating specie by preventing the responsible sodiumabsorbing reaction from taking place. By so doing, a more stable lamp having more uniform characteristics throughout life is achieved, and also a lesser quantity of reservoir charge suffices for the lamps requirements.
SUMMARY OF THE INVENTION Our invention is predicated on our discovery that the primary cause of sodium cleanup in high intensity alumina ceramic lamps operating with a substantial partial pressure of sodium is the reaction of sodium with alumina to form sodium aluminate. Operating partial pressures of the species of lamps in accord with the invention range from 10- to 200 torr for the Reiling type; through 10- to 460 torr for the Hanneman type; to 30 to 1,000 torr in the Schmidt-type lamp. These ranges of 7 sodium pressures are achieved in the above types of lamps with a cold spot temperature, that is, with the temperature of the coldest portion of the arc tube between approximately 600 C and l,200 C. Residual oxygen aggrevates the situation, but sodium aluminate may form even without it. If allowed to continue, such reaction can decrease the concentration of sodium in the reservoir, with the ultimate results previ ously pointed out.
In accordance with our invention, we eliminate the cleanup of sodium by providing a reactive metal or an alloy or compound of such a metal within the arc tube or in communication thereto, under conditions where the overall reaction:
NaAIQg -J XM I N8 Mxoms 361K; (1)
Wherein M is representative of the reactive metal and x is a constant which may be less than I and which, depending upon the final oxidation state of the metal M, denominatcs the number of moles thereof required for completion of the reaction and balancing of the equation maintains a negative free energy and where this reaction rate is more rapid than the following aluminategforming reactions that can occur:
Na(L, G) %O (G) %AIQO (S) 2 NaA1O2(S) (2) Na(L, G) %AlzO (S) Z NaAlOz(S) %AI(L) (3) Wherein the letters (G), (L). and (S) refer to gas, liquid and solid state, respectively. While a number of such metals meet the above requirement, the preferred choices are yttrium, cerium and lutetium. These metals produce rapid reaction rates to the right for equation l at a convenient temperature range for the high intensity are lamp without producing detrimental side effects such as chemical reactions with other components of the lamp, high volatility or radioactivity. In addition, these metals can also destabilize complex sodium-calcium-magnesium aluminates which can form from the sealing glass that lies between the alumina arc tube and the niobium end cap.
In a preferred embodiment, yttrium is so placed within the lamp that it chemically participates in reaction (I) even though it is physically separated from the inner arc tube volume. This is accomplished by encapsulating the yttrium within a dummy niobium exhaust tube that is welded to one of the niobium end caps of the arc tube. The niobium acts as a semipermeable membrane which allows oxygen rapidly to reach and react with the yttrium; but the niobium will not allow any sodium to escape. Thus, by maintaining conditions such that reaction l or an equivalent reaction, proceeds to the right at sufficiently rapid rates, the net formation of sodium aluminate is prevented. At the same time, this arrangement prevents reactions between yttrium and other reservoir constituents of the unvaporized reservoir.
These skilled in the art will gain a better understanding of this invention from the detailed description set out below taken in conjunction with the drawings accompanying and forming a part of this specification, in which:
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevated view of a high-intensity vapor arc lamp embodying this invention in a preferred form, parts of the lamp envelope being broken away from purposes of illustration;
FIG. 2 is an enlarged, fragmentary view of one suitable form of the upper end cap assembly of a discharge tube such as shown in FIG. 1;
FIG. 3 is a chart on which free energy of reaction (I) in kilogram calories is plotted against temperature of the metal M and its corresponding oxide as curves A, through E, including zirconium, scandium, magnesium, yttrium, and cerium, respectively, for the case of a 70 atomic percent sodium 30 atomic percent mercury reservoir with both the reservoir and lowest alumina temperature at 1,000" I(.; and
FIG. 4 is a chart on which the free energy of reaction (1) in kilogram calories is plotted against temperature of yttrium and its corresponding oxide for: (i) pure sodium (curves F and I), (ii) a 70 atomic percent sodium 30 atomic percent mercury reservoir (curves G and J), and (iii) a 50 atomic percent sodium 5O atomic percent mercury reservoir (curves H and K); with both the reservoir and lowest alumina temperature at 1,000K. and also at 900 K., respectively.
As shown in FIG. 1, a high-pressure sodium vapor lamp 1 comprises an outer, transparent vitreous envelope or jacket 2 of elongated avoid shape. Neck 3 of envelope 2 is closed by a reentrant stem 4 having a press seal 5 through which extend stiff wire leads 6, and 7 connected at their outer ends to threaded shell 8 and center contact 9 of a conventional screw base.
The inner envelope or arc tube 12 which forms the discharge lamp proper is made of sintered, high-density polycrystalline alumina ceramic such as is disclosed and claimed in U. S. Pat. No. 3,026,210 Coble, Transparent Alumina and Method of Preparation." Tungsten Electrodes l6 and 17 are supported in position at the upper and lower ends of tube 12 by a dummy end cap 18 and exhaust end cap 19, respectively, hermetically sealed to the alumina. The shanks of electrodes 16 and 17 are supported from niobium end caps 18 and 19 through niobium tubes 20 and 21, respectively, which project in hermetic seal through the end caps. Each electrode comprises a double-wound tungsten wire coil with interstices filled with electron-emitting material, suitable alkaline earth oxides, including barium oxide. Tube 21 is pierced through at 21a and is used as an exhaust tube during manufacture and to introduce an inert gas filling such as xenon and the charge containing the light-emitting species into the arc tube. The lower end of tube 21 is thereafter hermetically pinched off by a cold weld at 21b. A quantity of a charge, somewhat exaggerated for ease of illustration, is shown at 26 in the lower end cap 19; excess charge may also collect in the projecting portion of exhaust tube 21 which tends to be at a cooler temperature in operation of the lamp.
In the Lucalox lamp in accord with the invention, the charge comprises sufficient sodium to supply a partial pressure of sodium vapor therein during lamp operation sufficient to cause appreciable line broadening and self-reversal of the resonance lines thereof, and to provide an excess of unvaporized sodium metal in the liquid reservoir. Preferably, the partial pressure of sodium should be within the range of 30 to 500 torr. A large amount of the total radiation emitted thereby is emitted on either side of the yellow resonance or D-line which is a couplet having wavelength peaks of 5,890 and 5,896 AU. The resulting light has a golden white appearance with a relatively large amount of energy in the red. The xenon serves as a starting gas, and the mercury vapor is a buffer vapor producing the proper temperature distribution in the arc plasma and at the envelope walls. The presence of the mercury increases the voltage gradient in the arc, resulting in a lamp operating at a higher voltage and lower current for a given wattage, thus making for a more efficient lamp and a saving in ballast costs. Even though the partial pressure of mercury may be greater than that of sodium, mercury exists in excess in the liquid reservoir, and the mercury in the vapor state contributes very little to the visible emission.
In the MultiVapor lamp embodying the invention, the charge includes sufficient mercury to establish a pressure of l l5 atmospheres and sufficient sodium halide (with or without other metallic halides) sufficient to produce a partial pressure of each metallic halides present of approximately 10 to 200 torr and to leave an excess of the halide in the reservoir. The vaporized halide (or halides) is dissociated and sodium atoms (with or without other metal atoms) are excited to emit radiation, generally resonance radiation. When several halides are used, as for example, iodides of sodium, thallium, scandium, and indium, a pleasing white light is emitted. Although a high pressure of mercury is present, mercury contributes little to the light output of the lamp in the visible range. When, however, certain halides, such as those of thallium, scandium, and indium, which react with niobium are used, the niobium must be isolated therefrom, as for example, by coating with sealing glass.
In the Hanneman-type lamp in accord with the invention, sodium and two or more of the metals thallium, mercury, and cadmium are added in amounts sufficient to establish a thermodynamic equilibrium between the partial pressures of each and an excess of each in the liquid reservoir. All constituents contribute to the spectral emission of the lamp in the visible region. The partial pressure of sodium is within the range of approximately l0- to 460 torn, and the partial pressures of the other metallic vapors is such as to combine with the sodium vapors at lamp operating temperatures to support a lightemitting electric arc having white spectral appearance, an efficacy in excess of lumens per watt and discrete peaks within the range of 5050 to 5500 AU which are at least 10 percent as high as the peak of the total emission spectrum thereof.
The upper end of arc tube 12 is supported within envelope 2 by means of band 22' and rod frame 22 which extends from inlead 6 at the stem end to a dimple 24 at the dome end to which it is anchored by resilient clamp 25.
The lower end of the arc tube is connected to inlead 7 by band 23' and a short length of rod 23. A strap 27 mechanically interconnects rods 22 and 23 to stiffen the assembly while insulator 28 prevents a short circuit. The interenvelope space is evacuated in order to conserve heat and this is done prior to sealing off the outer envelope. Thereafter, a conventional getter, suitably barium metal powder pressed into channeled rings G, G, is flashed after sealing in order to assure a good.
vacuum. Alternatively, a low molecular weight inert gas may be added thereto.
One suitable form of dummy end closure subassembly shown to best advantage in FIG. 2 includes niobium cup 30 and niobium tube which extends through the smaller upper end of cup and is vacuum-sealed thereto by welding. The larger diameter lower end of cup 30 receives the upper end of alumina envelope 12 and is vacuum-sealed thereto by means of ceramic sealing composition 34 comprising aluminum oxide, calcium oxide, and optionally magnesium oxide, a preferred composition being disclosed and claimed in copending patent application, Ser. No. 388,567, filed Aug. l0, I964 by Paul .I. Jorgensen, entitled Ceramic Bonding and assigned to the assignee hereof. As illustrated, the shank of tungsten electrode 16 is received in the lower crimped end of tube 20 and secured there by crimping and welding as shown at 20a whereby the tube and is sealed. Tube 20, extending above cup 30, provides a chamber 35 for containing an oxygen-retaining metal; it is mechanically closed, but not hermetically sealed, at its outer end 20b above annular boss 36 after filling with the metal, suitably yttrium 38 in particle form.
In assembling the parts shown in FIG. 2, the shank of electrode 16 is welded to tube 20 which in turn is welded to cup 30, suitably by electron beam welding in vacuum. Tungsten trioxide is then coated on the sealing surface of the cup and vacuum sintered. A ceramic sealing composition precoat is vacuum sintered over the tungsten trioxide coating. Particles of pure yttrium 38 are placed in the dummy tube which is pinched shut at 20b without hermetically sealing it. Electronemitting material is applied to the electrode, and zirconium hydride may be painted on the outside of niobium cup 30 and niobium tube 20 to prevent embrittlement. The dummy end cap, and an exhaust end cap similarly treated except that no yttrium filling is provided, are assembled with an alumina tube 12 and the assembly vacuum fired to a temperature sufficient to melt the sealing composition. The foregoing firing procedure is more fully described and is claimed in copending application, Ser. No, 590,568, filed Oct. 31, 1966 by William C. Louden, entitled Niobium End Seal" now U.S. Pat. 3,448,319 and assigned to the assignee hereof.
The reaction most likely to cause cleanup of sodium in an alumina-ceramic lamp when available oxygen is present consists of the oxidation of sodium and reaction with alumina to form sodium aluminate. The overall reaction is reversible and may be represented by the following equation:
Wherein the letters (G), (L), and (S) refer to gas, liquid, and solid states, respectively. Reaction (2) will take place as long as there is oxygen present within the arc tube to support it. The niobium end cap closures which have been described are permeable to oxygen at lamp operating temperatures so that oxygen in the interenvelope space also may diffuse through the niobium into the are tube volume and there support reaction 1) with further cleanup of sodium resulting.
Another reaction capable of cleaning up sodium which may occur in an alumina ceramic lamp does not require the presence of oxygen and involves only the direct action of sodium upon alumina to produce sodium aluminate and aluminum as represented by the following reversible equation:
Na(L. +%A1203(s z: NaAlOflS) %Al(L) (3) wherein the parenthetical notations have the same significance.
Our invention prevents reaction (2) uniform proceeding to the right by introducing a competing process involving the oxidation of a metal M. as shown in equation (l), In a similar manner, reaction (3) is impeded by metal M. In order for a given metal to be effective in preventing aluminate formation, the temperature of metal M and its corresponding oxide must be held at a temperature sufficiently low that the free energy of reaction (I) is negative for the particular temperatures of alumina and reservoir employed and the particular composition of reservoir used. The maximum temperatures at which the free energy of reaction is just negative are shown at A F in FIG. 3 for zirconium, scandium, magnesium, cerium, and yttrium, respectively, by Curves A, B, C, D, and E, for a 70 30 atomic percent sodium-mercury reservoir with reservoir and lowest alumina temperature at l,000 K. The effect of reservoir composition on the maximum effective temperature of yttrium is also shown atA F 0 in FIG. 4. Curves F, G, and H represent pure sodium, 70 30 atomic percent sodium mercury reservoir and 50 50 atomic percent sodium mercury reservoir, respectively, with reservoir and lowest alumina temperature at l,0OOK. Curves I, J, and K represent pure sodium, 70 3O atomic percent sodium-mercury reservoir, and 50 50 atomic percent sodium-mercury reservoir respectively, with reservoir and lowest alumina temperature at 900K. Those skilled in the art will be able to calculate readily maximum effective temperatures for intermediate compositions other end cap temperatures, and other constituent reservoirs.
In selecting a suitable metal for prevention of the aluminate formation, it is necessary to know both the maximum and minimum temperatures at which it will be effective. Table I below lists metals suitable for prevention of aluminate formation and their minimum and maximum temperatures of effectiveness as well as the preferred temperature for lamps with a reservoir and minimum alumina temperature of l,000 K. The last three columns of table I list the maximum temperatures of effectiveness for a pure sodium reservoir, a 70 30 Atomic percent sodium-mercury reservoir and 50 50 atomic percent sodium-mercury reservoir, respectively. These values are based on thermodynamic calculations using equation (1 Column T,,,,-,, in table I also lists the approximately minimum useful temperature of each metal based on oxidation kinetics of each metal. Several additional factors bear on the choice of a preferred temperature for each metal including its volatility, its reactivity with other lamp components, and available places within the lamp where the metal can be located. The preferred temperatures for each of the effective metals is given in column T of table I.
TABLE I Tm: C
min Told, Pure Metal 0.) C.) N a 70-30 50-50 Lithium 500 640 750 1, 030 Calcium 350 700 1, 400 1, 470 1, 595 Barium 350 600 705 780 1, Magnesium. 1 350 600 955 1, 030 1, Strontium..... 350 700 960 1, 050 1, 260 Zirconium. 1 400 675 550 650 850 Bery1lium 500 700 1,090 1, 140 1, 370 Seandlum 400 700 885 975 1, 176
TEA! O rain Tptd Pure Metal C.) C.) Na 7030 50-50 The free energy of reaction in the formation of sodium aluminate through reaction (2) is always more negative than that through reaction (3 Therefore any competing reaction capable of preventing reaction (2) will also prevent reaction (3). This means that the reactive metal provided in accordance with our invention will destabilize sodium aluminate irrespectively of how it came to be formed. In copending application Ser. No. 616,539, filed Feb. l6, 1967 by Rodney E. Hanneman, Paul J. Jorgensen and Dimitrios M. Speros, entitled Alumina Ceramic Sodium Vapor Lamp now U. S. Pat. 3,453,477 and assigned to the same assignee herein, another means for preventing sodium cleanup by reaction (3) is disclosed and claimed. Briefly, such means requires an equilibrium vapor pressure of sodium over the sodium aluminate greater than the partial vapor pressure of sodium over the unvaporized reservoir. Vapor pressure of sodium over the aluminate is determined by the temperature of the envelope where the aluminate deposits; vapor pressure of sodium over the reservoir may be determined by the sodium-mercury-other constituent (if any) ratio in the reservoir, quantity or temperature of the unvaporized excess; the foregoing are the factors which may be varied to achieve the desired equilibrium. However, where equilibrium is not fully attained, the reactive metal provided in accordance with the present invention gives a margin of safety by preventing a sodium aluminate forming reaction from taking place.
In sealing the niobium end caps to the alumina tube of the lamp, a sealing composition comprising aluminum oxide, calcium oxide, and magnesium oxide may be used and some tungsten trioxide may also be present. Instead of alkaline earth oxides, the electrodes may use other materials such as thoria as electron emitters. All these materials give rise to the possibility of other reactions with sodium which may be deleterious either by cleaning up the sodium, or by attacking some other part of the lamp such as the seals or the electrodes.
Equations analogous to reaction (1) can be written readily by those skilled in the art for possible reactions which may compete with sodium aluminate as sodium cleanup mechanisms. In general, the reactive metal that has a relatively large negative free energy of formation of its oxide while maintaining rapid oxidation kinetics is preferred.
The reactive metal must be located at a place within the lamp where it will have a temperature within the range' required for effectiveness during operation. If the metal reacts with mercury or interferes with the discharge, it should not be located within the arc tube. It must, of course, be so located as to be in oxygen-communication with the interior of the arc tube. It may be located in the interenvelope space provided oxygen-permeable end caps are used. Where the reactive metal has an excessively high vapor pressure at the operating temperature, it may be hermetically enclosed in an oxygenperrneable vessel. The wall thickness of the end cap or of the capsule depends on the metal chosen, its temperature in use, and the rate of oxygen diffusion desired. Niobium or a single phase niobium alloy such as niobium-l percent zirconium are preferred and, by way of example, in the illustrated embodiment, a thickness of about 6 mils is suitable for the end caps.
The preferred reactive metal in accordance with our invention is yttrium which produces a large negative free energy of reaction (1) under acceptable operational conditions in the lamp. It is relatively inexpensive, is not hygroscopic, and does not oxidize significantly in air at room temperature so that it does not require special handling. It has its maximum effectiveness at a temperature which is conveniently achieved by attaching it to a portion of the arc tube in the lamp. Yttrium also has a vapor pressure low enough at the temperature at which it must be operated that it has no tendency to deposit on or to plate the lamp parts, suchas the outerjacket and, therefore, there is no need to enclose it within an hermetically sealed capsule.
In the preferred illustrated construction wherein the yttrium is placed in dummy niobium exhaust tube 20, the yttrium is physically separated from the lamp volume so that it cannot come in contact with the unevaporated excess charge in the reservoir. Were contact permitted and mercury were present in the reservoir, some mercury might form an amalgam with the yttrium and upset the vapor-liquid equilibrium. Cerium may also be used conveniently in the form of its alloy mischmetal (i.e., cerium alloyed with other lanthanide metals).
Thorium also has a high negative free energy in the formation of its oxide but it has some disadvantages. It is slightly radioactive and for this reason is not preferred. In addition, its preferred temperature is very high for kinetic reasons and this means that it should be placed within the arc tube.
The other metals listed in table I are, in general, less desirable than those mentioned above. Magnesium, lithium, calcium, strontium, barium, ytterbium should not be placed either inside the arc tube or outside the arc tube without placement in an hermetically sealed oxygen-permeable jacket such as niobium. If placed within the arc tube, their vapor pressures or amalgam-forming characteristics will result in operational difficulties in the presence of mercury. If placed in the interenvelope space without an hermetically sealed jacket at the preferred temperature, their vapor pressures are high enough to cause substantial vaporization rates with a resultant condensation on the inside surface of the outer envelope or jacket. This will produce a darkening of the jacket and cut down on light emission of the lamp. The metals beryllium, scandium, neodymium, samarium, gadolinium, praseodymium, terbium, dysprosium, holmium, erbium and thulium should also be sealed hermetically within an oxygen-permeable container if placed within the interenvelope region because their evaporation rates at the preferred temperatures will result in appreciable darkening of the outer envelope during the projected lamp life.
Uranium has the disadvantage of its radioactivity. Beryllium presents a problem during manufacture and handling due to its toxic nature. Also, its oxide has a tendency to embrittle niobium so that it could not be used by placing it within a niobium container in the fashion of yttrium in the preferred embodiment herein. Lithiums highly reactive nature makes it difficult to handle and it is not preferred for the present application.
While the preferred place for the reactive metal is chamber 35, it may be located at various other places within the lamp, such as the four locations indicated at 40, 41, 42 and 43. Thus, the metal may be coated on the outer surface of an end cap, as at 40 on end cap 18. It may also be applied as a coating 43 on metal tab 44 supported on bracket 46 in the interenvelope space between outer jacket 2 and are tube 12. In the foregoing locations, since the reactive metal is effectively in the interenvelope space, oxygen-permeable closures such as niobium end caps must be used for the arc tube so that any oxygen in it may diffuse out into the interenvelope space. The reactive metal may also be provided on the inside surface of an end cap, as at 41 on end cap 18 where it is in direct contact with the atmosphere within arc tube 12. Alternatively, it may be incorporated in particle form in the charge 26 at 42. When the reactive metal is placed within the arc tube, any propensity which it may have to form an amalgam with any mercury present in the arc tube must be taken into account; for instance, under such circumstances, it may be desirable to increase the initial ratio of mercury to sodium in order to have the desired ratio in the excess reservoir during the life of the lamp.
The quantity of reactive metal provided in the completed lamp should be at least two milligrams for this size lamp. Preferably the quantity of yttrium or alternative metal should be at least 40 milligrams in the usual case of a lamp wherein the volume of the outer jacket is about 400 cc.', including an arc tube volume of about 4 cc. The upper limit on the quantity provided is essentially a practical one based on material cost and space available in the lamp, and it should not exceed approximately 1,000 times the minimum.
The reactive metal may be located at a single place or in several locations. It may be provided by electroplating, vapor depositing, metalliding or painting onto the end caps or tab 44, or simply by placing it within the lamp in the form of metal foil, wire or powder. Any of these variations may be used so long as the temperature requirements and other specified restrictions for the chosen metal are observed. it may be noted that the getter consisting of barium metal powder remaining in channeled rings G, G or coated about the base end of the outer envelope is not effective to prevent formation of sodium aluminate because it is located at places subject to too low a temperature. 7
The reactive metal may be an alloy of the metal or it may be a compound of the metal decomposable to the metal under the conditions prevailing in the lamp environment in use, and it is intended to include such alternatives in the expression reactive metal substance." Thus, for example, the hydride, base alloys, and organometallic compounds of yttrium may be used in addition to or as an alternative to to yttrium and it is intended to include such in naming yttrium in the claims appended hereto.
The value of the invention as used in actual practice is demonstrated by comparing the commercial version of an illustrated Lucalox lamp having a sodiummercury charge prior to the invention with one using the invention. The illustrated lamp is a 400-watt size having an arc tube approximately 7.4 millimeters in internal diameter, 9.3 centimeters in length and having a 7-centimeter arc gap between electrode tips. The are tube filling consisted of approximately 54 milligrams of charge of one part sodium to three parts mercury by weight. This lamp operated initially with an arc voltage of approximately 100 volts and a red factor of 3. After 3,000 hours of continuous operation, the arc voltage had risen to 150 volts and the red factor dropped to 2. Another lamp in all respects similar, except provided with 40 milligrams of yttrium in the dummy exhaust tube as illustrated and described herein, after 3,000 hours of operation showed no appreciable increase in arc voltage or drop in red factor. The red factor is an arbitrary measure of the proportion of radiation occurring in the visible range above 6,000 Angstroms; by comparison, a common high-pressure mercury vapor lamp has a red factor of l, and sunlight varies from 2 to 3 in red factor depending on time, place and weather.
The details of construction of the preferred embodiment.
which have been illustrated and described are intended as illustrative only, and not in order to limit the invention thereto except insofar as included in the accompanying claims.
1. A high intensity vapor arc lamp wherein sodium atoms are a principal radiating specie and comprising:
a. a discharge envelope of alumina ceramic having electrodes sealed therein and containing a filling which includes;
a a partial pressure of sodium which is excited by an electric arc to high intensity emission; and
b. a reactive metal substance in oxygen communication with the interior of said alumina envelope;
b,. said metal of said reactive substance having over a given temperature range a free energy of reaction in the formation of its oxide per mole of oxygen) more negative thanthe free energy of formation of sodium aluminate per mole of oxygen within said alumina envelope; and
b said substance being so located within said lamp in order to prevent cleanup of sodium by formation of sodium aluminate within said alumina envelope.
2. A lamp as in claim I wherein the reactive metal substance is located within said alumina envelope.
3. A lamp as in claim 1 wherein the reactive metal substance is located within an evacuated chamber separated from said alumina envelope by a partition which is oxygen-permeable at operating temperature.
4. A lamp as in claim 1 wherein the reactive metal substance is yttrium, yttrium hydride, a compound of yttrium heat decomposable to yttrium, or an alloy of yttrium.
5. A lamp as in claim 1 wherein the reactive metal substance is cerium, a cerium base alloy, or a heat-decomposable compound of cerium.
6. A lamp as in claim I wherein the reactive metal substance is lutetium, a lutetium alloy, or a heat-decomposable compound of lutetium.
7. A high intensity vapor arc lamp comprising an inner arc tube of alumina ceramic having electrodes sealed therein and containing a sodium-mercury reservoir and an inert starting ,gas, the partial pressure of sodium in said are tube being in the range of 30 to l,000 torr in operation, and an evacuated outer vitreous jacket surrounding said are tube. in combination with a reactive metal substance within said lamp arranged to be in oxygen communication with the inner surface of said arc tube, said metal of said substance having over a given temperature range a free energy of reaction in the formation of its oxide more negative than the free energy of formation of sodium aluminate within said are tube, said substance being so located within said lamp as to achieve during operation a temperature within said range in order to prevent cleanup of sodium by formation of sodium aluminate within said are tube.
8. A lamp as in claim 7 wherein the reactive metal substance is located within said alumina arc tube.
9. A lamp as in claim 7 wherein the arc tube is provided with oxygen-permeable end closures and the reactive metal substance is located in the interenvelope space between inner arc tube and outer vitreous jacket.
10. A lamp as in claim 7 wherein the reactive metal substance is yttrium, yttrium hydride, a compound of yttrium heat decomposable to yttrium, or an alloy of yttrium.
11. A lamp as in claim 7 wherein the allumina arc tube is sealed with niobium end caps and the reactive metal substance is attached to an end cap.
12. A lamp as in claim 11 wherein the reactive metal substance is yttrium.
13. A lamp as in claim 7 wherein the allumina arc tube is sealed with niobium end caps, one of which is provided with a dummy niobium exhaust tube containing a charge of yttrium.
14. A lamp as in claim 13 wherein said dummy exhaust tube is not hermetically sealed relative to the interenvelope space.
15. A lamp as in claim 7 having niobium end caps sealed to the alumina arc tube by a sealing composition comprising aluminum oxide, calcium oxide and magnesium oxide, and wherein the reactive metal substance is yttrium.
16. A lamp as in claim 7 wherein niobium end caps are sealed to the alumina arc tube by a sealing composition comprising aluminum oxide, calcium oxide and magnesium oxide, and wherein one of the niobium end caps is provided with a dummy niobium exhaust tube containing a charge of yttrium.
17. A lamp as in claim 16 wherein said dummy exhaust tube is not hermetically sealed relative to the interenvelope space.
18. A high intensity vapor are lamp comprising:
a. an inner arc tube of alumina ceramic having electrodes sealed therein and containing:
a a quantity of mercury sufficient to establish a partial pressure therein during lamp operation to support a high current electric arc;
a a quantity of sodium halide sufficient to partially vaporize during lamp operating temperature and establish therein a partial pressure of approximately l0 to 200 torr sodium halide vapor in equilibrium with an excess of liquid sodium halide; and
a a partial pressure of an inert starting gas;
b. an evacuable outer vitreous envelope; and
c. a reactive metal substance within said lamp arranged to be in oxygen communication with the interior of said are tube, c said metal of said substance having over a given temperature range a free energy of reaction in the formation of its oxide more negative than the free energy of formation of sodium aluminate within said are tube; and
c said substance being so located with in said lamp, as to achieve during operation a temperature within said temperature range in order to.prevent cleanup of sodi um by formation of sodium aluminate within said arc tube.
19. A lamp as in claim 18 wherein the reactive-metal substance is located within said alumina arc tube.
20. A lamp as in claim 18 wherein the arc tube is provided with oxygen-permeable end closures and the reactive metal substance is located in the interenvelope space between inner arc tube and outer vitreous jacket.
21. A lamp as in claim 18 wherein the reactive metal substance is yttrium, yttrium hydride, a compound of yttrium heat decomposable to yttrium, or an alloy of yttrium.
22. A lamp as in claim 18 wherein the alumina arc tube is sealed with niobium end caps and the reactive metal substance is attached to an end cap.
23. A lamp as in claim 18 wherein the reactive metal substance is yttrium.
24. A lamp as in claim 18 wherein the alumina arc tube is sealed with niobium end caps, one of which isprovided with a dummy niobium exhaust tube containing a charge of yttrium.
25. A high intensity vapor arc-lamp comprising:
an inner arc tube of alumina ceramic having electrodes sealed therein and containing:
a,. a quantity of sodium sufficient at operating temperatures to establish therein a partial pressure of sodium of approximately lto 460 torr of sodium vapor in equilibrium with excess liquid sodium; and
a respective quantities of at least two other metals selected from the group consisting of thallium, cadmium, and mercury in quantities sufficient to provide respective partial pressures of each metal present in equilibrium with excess liquid metal and sufficient to combine with sodium partial pressure to sustain a lightemitting electric are which as a white spectral appearance and high intensity; and
b. a reactive metal substance in oxygen communication with the interior of said alumina envelope;
b,. said metal of said substance having over a given tertiperature range a free energy of reaction in the formation of its .oxide per mole of oxygen more negative than the free energy of formation of sodium aluminate per mole of oxygen within said alumina envelope; and
b said substance being so located within said lamp in order to prevent cleanup of sodium by formation of sodium aluminate within said alumina envelope.
26. A lamp as in claim 25 wherein the reactive metal sub stance is located within said alumina arc tube.
27. A lamp as in claim 25 wherein the arc tube is provided with oxygen-permeable end closures and the reactive metal substance is located in the interenvelope space between inner arc tube and outer vitreousjacket.
28. A lamp as in claim 25 wherein the reactive metal substance is yttrium, yttrium hydride, a'compound ol'yttrium heat decomposable to yttrium, or an alloy of yttrium.
29. A lamp as in claim 25 wherein the alumina arc tube is sealed with niobium end caps and'the reactive metal substance is attached to an end cap.
30. A lamp as in claim 25 wherein the reactive metal substance is yttrium.
31. A lamp as in claim 25 wherein the alumina arc tube is sealed with niobium end caps, one of which is provided with a dummy niobium exhaust tube containing a charge of yttrium.