US2525262A - Method of producing highly emissive electrodes - Google Patents

Description

METHOD OF Filed Feb. 18, 1948 M. E. MACKSOUD PRODUCING HIGHLY EMISSIVE ELECTRODES 2 Sheets-Sheet 1 INVNTOR.

Oct. 10, 1950 v M. E. MACKSOUD METHOD OF PRODUCING HIGHLY EMISSIVE ELECTRODES 2 Sheets-Sheet 2 Filed Feb. 18, 1948 I Patented Oct. 10,

METHOD OF PRODUCING HIGHLY EMISSIVE ELECTRODES 7 Michel E. Macksoud, Newburyport, Mass, assignor to Cooper-Hewitt Electric Company, Hobcken,

Application February 18, 1948, Serial No. 9,183

My invention relates to are discharge devices and, more particularly, to a novel process of preparing electrodes for use in gaseous are discharge tubes, such as sun lamps.

The most important object of my invention is to produce a more efficient arc discharge cathode.

Another object of my invention is to increase electron emissivity of an arc discharge cathode.

Still another object of my invention is to eliminate electrode sputtering and'tube blackening in gaseous arc discharge devices.

A further object of my invention is to provide a cathode for gaseous arc tubes by means of which an increase in electron emissivity may be obtained and accompanied by a reduction in the potential required for the striking of an are.

An important feature of the invention resides in a novel method of coating a cathode including forming barium azide crystals upon the surface and in the convolutions and interstices of a tungsten coil, and then treating the coated coil to free the barium and bring about an alloy of barium and tungsten upon the surface of the coil.

Although this invention relates to methods and processes of coating and activating are discharge cathode electrodes, it is more specifically related to the processing of those types of electensity, high pressure are discharge tube, it is l necessary to specially coat and process the electrodes so that their electron emissive properties are considerably increased. The coating, and activation of these electrodes is offigr'eat im- 7 Claims. (01. 117-333) portance because most metals, particularly those metals of higher melting point such as tungsten, do not have in themselves high electron emissive properties, but must be treated with materials or elements having high electron emissive properties.

Such elements are well known in the art and properties.

The particular problem that seems inherent with high temperature mercury arc discharge tubes is the design and processing of an electrode that will operate at uniformly low temperatures and that will reduce sputtering due to positive ion bombardment, to a minimum degree. In order to reduce the operating temperature of the electrode during normal operating conditions it is essential to coat this electrode with materials that have high electron emissive properties. It has been found that the most desirable materials or elements for such purposes are barium, thorium, cerium, and zirconium, preferably in pure metallic for-m.

The efficiency of lamps, including mercury arc discharge tubes and in particular sun lamps, depends upon a mercury arc tube that has a'minimum of sputtering and consequent blackening of its glass envelope which limits the transmission efficiency in the ultra-violet spectra of such a lamp. 1

I believe that during the initial seasoning 7 periods the-pure barium becomes alloyed with the tungsten, and thus forms an electron emittive element of a considerably higher vaporization temperature than pure barium itself, and is very close to the vaporization temperature of tungsten. It is observed that when the proper potentials are applied to a mercury vapor tube utilizing this type of electrode, an arc is readily obtained, and such an arc shows a more uniform waveform as depicted upon a cathode ray oscilloscope than arcs produced in tubes having other types of electrodes. The striking potential necessary to initiate the arc is considerably lower and the electrode in general is more uniform and active Another theory that is worth considering is that the pure barium metal, if not entirely alloyed to the tungsten, may in part vaporize from the electrode and condense upon the inner portions of the glass tube. The barium may also deposit on the condensed particles of mercury. These condensed particles of barium will readily revaporize when the arc strikes and will, in turn,

become-secondary or floating electrodes in themselvesj 'inasmuch as these particles have now reachedan emittive temperature. Thus, we have an electrode that is highly active at low temperatures and at, lower arcing potentials than hitherto, and with a considerably improved wave form. It is'noted. that when potentials are applied to an arc tube utilizing these electrodes an ioril sheath initially forms around the electrode. This seems to] reduce the positive ion bombard,- ment that causes sputtering. The long life and activity of ,the electrodes is possibly maintained H because the "bariummetal formed on the inside turns of the coiled electrode is sheathed and protected from positive ion bombardment and, thereby, retains its active surface for a longer period of time.

In addition thereto, barium is known to be a good getter of impurities. The impurities released from the electrodes and from the glass wall of the arc tube are constantly gettered by the action of the barium, the purity of the inert gas (argon) is maintained, and therefore the arc tube will have an increased life expectancy. There does not seem to be any discoloration of the glass due to the barium condensing upon its walls. Any condensation or deposit during openair testing of the arc tube seems to readily revaporize into the arc stream and redeposit upon the electrodes, themselves. After seasoning or after being arced in a lamp for a number of minutes, the arc tubes are generally clean and transparent.

I have discovered that if a helically coiled tungsten electrode is coated or treated with a solution of barium azide in accordance with the herein described process, I obtain an electrode with extremely high electron emissive properties, and other desirable characteristics necessary for operation in a high intensity mercury vapor arc discharge tube. To obtain the most efficient form of such a cathode it is extremely important that the tungsten electrode and all leads thereto be thoroughly cleaned. One of several satisfactory cleaning agents is hydrogen peroxide. If this agent is used, the tungsten electrodes are first boiled for a suitable period in a concentrated solution of the above and then fired in hydrogen at approximately 1350 C. for five minutes. The surface of the tungsten electrode then appears to have a highly cleaned and slightly etched surface. The electrodes are then mounted carefully on beaded lead wires; a welding flux such as nickel, thorium, molybdenum or zirconium may be used or the electrode may be clamped to the lead wire without any flux material. The electrode which is nOW mounted on the beaded tungsten lead wire is then dipped into a barium azide solution and subsequently baked in a dry low-temperature oven at approximately 200 C. for approximately five minutes. This entire process may be repeated a second time in order to develop a heavier coating of barium azide crystals upon the electrode.

The barium azide solution may be in the form of a super-saturated solution having approximately 50% distilled water as a suspension medium or may have other suitable portions of alcohol and ether to increase the rapidity of the evaporation rate of the solution. Without water the crystals formed by this azide is monoclinic. However, when in suspension with water and alcohol the crystals formed are triclinic. The index of refraction is 1.7. The solution is designated as Ba(Ns)z-Ha0. Barium azide is very soluble in cold or hot water, slightly soluble in alcohol and practically insoluble in ether. After dipping the electrodes in the saturated solution of barium azide and subsequently baking in the low temperature furnace, it will be observed that a densely packed layer of triclinic crystals is formed on the electrode surfaces which also fillsthe crevices and interstices of the electrode coil assembly.

In sealing the electrode to the glass tube envelope, a forming gas or suitable inert gas such as nitrogen or argon must be employed to prevent oxidation of the electrode. It will be noted that when heating the glass envelope with the sealing fire burners just prior to sealing, small sparks will be thrown off from the crevices of the glass envelope tube and the beaded electrode assembly. This is due to the initial decomposition of the azide. During the processing of the tube while being exhausted to a high degree of vacuum, the electrodes must be further activated either by high frequency bombardment or by high current arcing. The preferred method of activation is that of high frequency bombardment which results in an electrode of higher uniformity in electron emissive properties, and the glass envelope tube has less tendency to blacken due to any sputtering action. During this processing the azide is completely broken down, releasing considerable nitrogen and leaving a pure metallic barium on the surface of the electrode.

Although the matter has not as yet been determined beyond doubt, all the indications point to the formation of an alloy of barium and tungsten in the form of a surface coating upon the main body of the tungsten strip or coil. I have observed that the barium does not vaporize from the coil when the arc is established, although the temperature at the electrode is much greater than that at which barium will evaporize. Moreover the emissivity does not decrease significantly, an efiect almost certain to result if the barium left the coil. Also the striking potential remains much lower than could be accounted for if the barium left the coil. For these reasons I feel quite certain that some form of alloy is produced. The vital point, and the essence of my invention, resides in my discovery that the beneficial results described here cannot be accomplished if barium azide is treated in the fashion conventional in the art. That is to say, the formation of the crystals of the barium azide upon the tungsten coil will produce a stable, highly emissive electrode. If barium is applied in other forms, such as a carbonate, the electrode will not exhibit the fine qualities obtainable by my process. An electrode conventionally coated with barium azide or barium carbonate will sputter, lose emissivity, and blacken the arc tube and reduce the output therefrom.

The are tube during the process of high vacuum exhaust is first baked to a high temperature to out-gas the glass walls of the envelope and simultaneously to affect an initial clean-up of the occluded gases in the electrodes of the tube. During this process considerable nitrogen is released from the azide crystals formed on the electrodes to initiate the breaking down of these crystals into pure metallic barium. Further activation of these electrodes may be carried out by inductively heating the electrodes with high frequency currents, or forming a relatively high current arc discharge between the electrodes for a predetermined period of time. Two separate high frequency bombardments or two separate arcing cycles, each of which may be for not more than fifteen seconds, are sufiicient to preactivate the electrodes prior to final filling with a predetermined pressure of inert gas such as argon and a predetermined quantity of triple distilled mercury.

After the high vacuum exhaust, cathode activation and processing, and final filling, the tube is now ready for the seasoning process. This consists of placing the mercury tube in a heat shielded chamber and passing a current between the electrodes approximately equivalent to the current used in normal operation. This process is continued for approximately one half hour during which the operating characteristics of the arc tube become stabilized and uniform, the gas content of the tube appears to be cleaned up and im-' purities dispelled, together with the formation of an integral unitary electrode that is highly electron emissive.

During testing of the arc tube it is observed that the electrodes initiate an arc discharge at considerably lower impressed potentials than are normally encountered. There appears to be a distinct ion sheath surrounding each entire electrode of the arc discharge tube causing the electrodes to operate at considerably lower temperatures. Sputtering is eliminated thus avoiding blackening or discoloration of the glass walls of the arc tube adjacent to the electrodes. Another characteristic that is observable particularly when the arc tube is tested at lower applied potentials than those used during normal operation, is the improved emission characteristics of the electrodes, analogous to the behavior of an alloy of elem nts having a considerably higher vaporization temperature, than pure metallic barium itself This deduction is made on the basis that if pure metallic barium remained on the surface of the electrode and did not alloy with the tungsten coil, it would readily vaporize and deposit upon the glass walls of the arc tube when subjected to the current densities necessary to maintain its operation. If this were true the glass walls of the arc tube would become heavily coated with vaporized metallic barium from the electrode and the. electrodes would lose their characteristic of high electron emissivity. On the contrary, the arc discharge tubes made with this type of processed electrode remain absolutely clean during normal operation. Initially, however, there may be observed a small amount of mercury vapor and possibly minute quantities of barium which has not completely alloyed with the tungsten, which has vaporized onto the internal glass surfaces of the arc tube adjacent the electrodes. It appears that the vaporization of traces of barium together with some mercury may cause a gettering action of any impurities residual in the gaseous content of the arc tube. However, immediately thereafter and during normal operation of the vaporized deposits on the inner surface of the arc tube again re-vaporize and enter the arc stream, being conducted from one electrode to another with the motion of the arc discharge. It is reasonable to expect that this process continues indefinitely cluring normal operation. Exhaustive tests both as to applied starting voltages and operational characteristics of the electrodes during a normal life span indicate that theseelectrodes are a considerable improvement over those types known to the art.

Another form of processing the electrodes above described, is to use a strip of pure metallic thorium placed within the tungsten coil. This may be clamped in place, whereby the electrode or tungsten coil firmly wedges the thorium strip LII cient value to actually cause the melting of the thorium strip or slug, thereby completely coating the tungsten coil with pure thorium as well as the resulting reduction of the barium azide to pure metallic barium. Therefore, in this particular embodiment, the electrode of tungsten has the additional elements of pure metallic thorium and barium forming a part thereof, appearing to have characteristics analogous to an alloy of these three aforementioned elements. Although it is possible to'obtain satisfactory electrodes with. thorium and tungsten only processed inthis manner, the addition Of'btlllllll, by the process just described improves the. seasoning and operating characteristics of the arc discharge tube. This is direct opposition to the practices of the art of record cited above.

Furthermore the wave form of the arc, as indicated on a cathode ray oscilloscope, assumes greater uniformity of linear characteristics, and also has considerably lower firing potential char acteristics.

These and other objects and advantages of the invention will be more readily understood and appreciated from the following description of the methods and processes embodied thereof as illustrated and explained in the accompanying drawings, in which:

Fig. 1 is a view in side elevation of a sun lamp with a portion of the bulb broken away to more clearly illustrate the assembly of the sun lamp components of which a mercury arc discharge tube. having electrodes processed in the manner just described are utilized,

Fig. 2 is a cross sectional view on an enlarged scale of an arc discharge tube of the mercury vapor type forming a major component of the sun lamp of Fig. 1,

Fig. 3 is a circuit diagram of the components of the sun lamp of Fig. 1 which includes the arc discharge tube of Fig. 2. This also illustrates the connections including a thermostatic switch adj acent to the incandescent ballast filament forming another component of the sun lamp of Fig. l, and

Fig. 4 is a circuit diagram of the components constituting the sun lamp of Fig. 1 by showing the ballast filament interconnected in two separate sections and with the thermostatic switch elements placed adjacent the base of the lamp.

For purposes of describing the improved characteristics obtainable in electrodes used in mercury vapor arc discharge tubes such as that specifically used in a sun lamp, I have shown in Fig. l a sun lamp comprising a bulb I of predetermined ultra-violet transmission characteristics, the internal surface 2 being preferably etched and having a vaporized metallic reflecting surface 5 thereon ending in cut off portion 1 so that the open surface of the bulb 9 permits the rays of ultra-violet and infra-red radiations produced by the mercury arc tube 2'! and incandescent ballast filament 43" respectively to be directed upon an exposed object. The sun lamp assembly of component parts contained within the bulb enclosure is sealed and mounted upon the stem press l l, the entire bulb being cemented to a base '3 and connected thereto with flexible lead wiresat points of electrical contact to the base El and 53 by means of a thermostatic switch at contacting arms 33 and 36 at the junction 35', through the bi-rnetallic arm'ofthe switch 29, 31 and junction point- 3?, through platform 2], junction point i2- ythrough flexible lead 39 andstartmg-0011 30, lead 38 an'd supporting hook com 7 nected to ballast filament 43, shown as a coiled coil type.

Fig. 2 illustrates a cross sectional view on an enlarged scale of the mercury arc discharge tube 21 having processed and activated electrodes 28, starting coil 30, and predetermined quantity of triple distilled mercury Hg within the inner confines of the arc tube.

In describing the operation of the mercury arc tube of Fig. 2 as illustrated in the circuit diagram of Fig. 3, when an applied voltage of alternating current, preferably 60 cycles, is applied to the entrant leads and 53, the current flows, on one half of the cycle through lead i3 across thermostatic switch contacting arm 33 at point of contact 35 across thermostatic switch contacting arm 36 through bi-metallic surface of the thermostatic switch 29, supportin arm 3| and junction point 31, through metallic supporting platform 2| to junction point 42 of braided flexible lead wire 39, through starting coil 30, through braided flexible lead 33 and finally through ballast filament 43 conducted to contact point 53 through supporting electrode l5. During this initial period the starting coil 30 causes both a preliminary heating of the mercury vapor content of the tube and also partial ionization of the inert gas content of the tube. At the same time ballast filament 43 becomes incandescent due to its resistance, and radiated heat as Well as convection currents set up by the incandescent ballast filament as well as any heat generated in the bi-metallic strip 29 due to the density of current being conducted therein, causes the bi-metallic arm 29 to bend away from the contacting arm 33 thereby breaking the electrical circuit established at the contact point 35 by the arm 36. The breaking of the electrical circuit which includes the starting coil 30 causes the current to flow from 5i through flexible lead 49 to electrode 28 connected thereto and across the ionized inert gas content and partially vaporized mercury vapor to opposite electrode 28 so that an arc of high intensity and high current density is formed in series with the ballast filament d3, which limits the current of the arc tube 2?. Initially a sheath of ions forms around each electrode 23 and due to the high electron emissivity of these respective electrodes, and there appears a broad glow around the entire surfaces of each electrode, the arc at this point being evenly disposed throughout the entire area of the arc discharge tube. As the internal pressure of mercury vapor increases the arc concentrates at the ends of electrodes 28 and becomes constricted to a narrow diameter. The operating temperature of electrodes 28 appears to be considerably lower than that necessary to maintain high electron emissivity of barium itself. Condensed mercury vapor and traces of barium which initially may have condensed on the side portions of the arc tube 26 re-vaporize and enter the arc stream.

Having thus disclosed my invention and described the methods and processes necessary to obtain electrodes that may be utilized in arc discharge tubes, particularly those of high intensity, high pressure mercury vapor arc discharge tubes as commonly used in sun lamps, but without intending to limit the methods and processes to the particular constructions shown, I claim and desire to secure by Letters Patent:

1. The method of preparing highly emissive electrodes, which includes the steps of applying 9.? ii n ien m r. aa us lution of barium. azide,'heating the coated member at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the member, and subsequently heating the coated member at an elevated temperature in the substantial absence of oxygen and for a sufficient length of time to disassociate the barium azide to form metallic barium on the member and alloy said barium with the tungsten member.

2. The method of preparing highly emissive electrodes which includes the steps of positioning thorium upon a tungsten member, applying to the tungsten thorium assembly an aqueous solution of barium azide, heating the coated assembly at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the assembly, and subsequently heating the coated assembly at an elevated temperature in the substantial absence of oxygen and for a sufiicient length of time to disassociate the barium azide to form metallic barium on the assembly and alloy the three metals.

3. The method of preparing highly emissive electrodes which includes the steps of positioning zirconium upon a tungsten member, applying to the tungsten zirconium assembly an aqueous solution of barium azide, heating the coated assembly at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the assembly, and subsequently heating the coated assembly at an elevated temperature in the substantial absence of oxygen and for a suflicient length of time to disassociate the barium azide to form metalli barium on the assembly and alloy the three metals.

4. The method of preparing highly emissive electrodes which includes the steps of applying to a tungsten member an aqueous solution of barium azide, heating the coated member at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the member, and subjecting the coated member to an electric current of high amperage in the substantial absence of oxygen and for a sufiicient length of time to disassociate the barium azide to form metallic barium on the member and alloy said barium with the member.

5. The method of preparing highly emissive electrodes which includes the steps of applying to a tungsten member an aqueous solution of barium azide, heating the coated member at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the member, and subjecting the coated member to high frequency bombardment in the substantial absence of oxygen and for a sufiicient length of time to disassociate the barium azide to form metallic barium on the member and alloy said barium with the member.

6. The method of preparing highly emissive electrodes which includes the steps of position-- ing thorium upon a tungsten member, applying to the tungsten thorium assembly an aqueous solution of barium azide, heating the coated assembly at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the assembly, subjecting the coated assembly to an electric current of high amperage in the substantial absence of oxygen and for a sufficient length of time to disassociate the barium azide to form metallic barium on the assembly and alloy the three metals.

7. The method of preparing highly emissive electrodes which includes the steps of positioning thorium upon a tungsten member, applying to the tungsten thorium assembly an aqueous solution of barium azide, heating the coated assembly at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the assembly, and subjecting the coated assembly to high frequency bombardment in the substantial absence of oxygen and for a sufficient length of time to disassociate the barium azide to form metallic barium on the assembly and alloy the three metals.

MICHEL E. MACKSOU'D.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,244,216 Langmuir Oct. 23, 1917 1,735,080 Hertz Nov. 12, 1929 1,936,334 Miesse Nov. 21, 1933 2,175,345 Gaidies Oct. 10, 1939 2,249,672 Spanner July 15, 1941

Images