US1890926A - Electrode for vapor gas electric devices - Google Patents

Description

Dec. 13, 1932. w. "r, ANDERSON, JR.. ET AL 1,890,925

ELECTRODE FOR VAPOR GAS ELECTRIC DEVICES Filed June 16. 1951 Patented Dec. 13, 1932 UNITED STATES PATENT OFFICE WILLIAM T. ANDERSON, JR., OF NEWARK, AND HUGH D. FRASER, OF VERONA, NEW JERSEY, ASSIGNORS T0 HANOVIA CHEMICAL AND MANUFACTURING COMPANY, OF NEWARK, NEW JERSEY, A CORPORATION OF NEW JERSEY IElIJilClRODE FOR VAPOR GAS ELECTRIC DEVICES Application filed June 16,

This invention relates to electrodes for vapor gas electric devices. These electrodes may be employed on the familiar forms of mercury arcs, both vacuum and gas filled type, for the production of light and for rec-. tification, and they may be used for gas discharge tubes for the production of light.

Our invention relates in particular to temperature control of electron emitting electrodes and in order that the importance and significance of such control may be thoroughly understood, it is necessary to describe the conditions existing in and at the electrodes of mercury arcs and gas discharge tubes.

The electrode of an air-cooled quartz mercury vapor are, which is a familiar article of commerce consists of a quartz vessel containing mercury connected to the outside current supply by means of a vacuum sealed lead-in wire. The arc discharge is from the surface of the mercury. The are possesses a negative volt-ampere characteristic, that is, the current decreases as the voltage increases, and is not to be confused with a high electron discharge which has a positive volt-ampere characteristic resulting in high voltage and low amperage. The typical are which will be described operates at 300 watts and 4.2 amperes in the arc itself.

The electron emission and the mercury vapor which maintain the are result from the temperature produced at the vapor discharge metallic mercury interface. The initial high temperature that starts the arc is created by short circuiting the mercury electrode to the other electrode which may be mercury or some other substance. The area of the vapor discharge metallic mercury interface is very small, being about 00012 square centimeter for the 4 ampere arc. (For a 20 ampere are it would be about 0.005 square centimeters.)

The temperature at this discharge to mer cury contact is about 2270 degrees oentigrade, at the best operating conditions. The vaporization of the mercury from the electrode at the contact area is about 7.20 X 10- grams per second per ampere, which results in a total mercury pressure at the electrode of the order of 2% atmospheres.

Not all the heat generated at the hot spot 1931. Serial No. 544,770.

is employed for electron emission and mercury vapor production. Some of the heat is employed to warm the entire electrode, and some is radiated or conducted to the walls of the electrode where it is dissipated. Mercury vapor is also condensing in the coolerparts of the container, liberating heat. Finally an equilibrium is established, the final temperature of the system depending upon the cur rent, radiation and thermal conductance- Ellicient operation demands that this Q Hilibrium temperature be restricted to wit in certain rather narrow limits. It is characteristic of the mercury are that when the temperature is lowered, the amperage of the arc increases, and vice versa, an increase in temperature results in a decrease in amperage. If the temperature be lowered to too great an extent, the high resulting amperage will result in rapid deterioration of the glass or quartz container. On the other hand, if the temperature be increased too much, the amperage may become too low to maintain the hot spot, and the arc will be extinguished. Between these two points the arc may be operated with various degrees of eificiency.

Temperature in commercial arcs is controlled by limitation of arc current, radiation and provision for adequate electrode cooling surfaces.- The electrode surface employed with this typical 300 watt air cooled mercury arc has a surface area of about 4375 square millimeters and contains a volume of mercury about 21,980 cubic millimeters, representing in weight about 300 grams of mercury. This surface may be cooled by natural air convection currents, or the air cooling may be asisted by metal cooling vanes.

We have found that the size and design of these mercury electrodes is dependent in a large measure on the low thermal conductivity of metallic mercury. Mercury in a layer one centimeter deep and an area one square centimeter transmits only 0.019 calorie per second for an increase in temperature of one degree centigrader Under similar conditions copper transmits 1.00 calorie per second, aluminum 0.50 calorie per second, tungsten 0.48 calorie per second, molybdenum 0.34 calorie per second. Thus the thermal conductivity of copper is about 50 times higher than that of mercury.

As a result of this low thermal conductivity, the temperature gradient within the mercury electrode is ver great, and hence a correspondingly greater volume of mercury and a greater surface area are required than would be necessary if the electrode contained, for instance, copper instead of mercury.

It is an object of the invention to provide mercury electrodes which compensate for the low thermal conductivity of the mercury metal, and which are equal in efficiency or better than those familiar to commerce, but much smaller in size and volume. The decrease in size and volume has resulted in a saving in mercury which in the final highly purified form required for use in such arcs is one of the most costly items employed in the manufacture of these arcs. Also, if the envelope be constructed of transparent fused quartz, a reduction in the size effects another economic saving.

Aside from these advantages resulting from our invention, there are others resulting from a reduction in electr ue size. hen such electrodes are employed on me *cury arcs for the production of light they permit a compactness which especially adapts such an arc for use with reflectors. A suitable reflector located behind a mercury arc can greatly enhance the intensity of light falling upon a surface. If the light source is compact and free from appreciable obstructions to light, the reflector design is greatly simplified.

Our invention, since it enables the employnient of electrodes of half to one-third the size of those employed on older arcs of this A type, can be more easily and satisfactorily employed in conjunction with reflectors. Another advantage resulting from the employment of our invention relates to lessened breakage in shipment. The vacuum type quartz mercury arc in particular may be broken in shipment by a mercury slap which results from the heavy mercury metal being suddenly thrown from one end of the container to the other. The number of such lamps that are broken by this cause is in direct proportion to their size and the volume (and hence the weight) of mercury contained therein. Our invention permits a reduction of the amount of mercury in these lamps to at least one-half, and will therefore result in smaller sized arc tubes.

Our improved mercury electrode for employment with mercury arcs consists of the vessel of insulating material, such as glass, or fused quartz, containing mercury, a lead-in Wire of suitable material such as nickel, iron, tungsten, molybdenum, or tantalum sealed vacuum tight through the container and a quantity of metal in a s itable form, and which does not amalgamate with the mercury and which has much greater thermal conductance than mercury, immersed completely in the mercury. Tungsten and molybdenum are best fitted for this purpose. Both have much greater thermal conductance than mercury and neither amalgamate with mercury. We have found that these metals may be employed as wires, rods, woven meshes, tubes and plates. The design of the lamp and the current employed determine which form is the more suitable.

Attention is called to the use of these metals solely for the purpose of conducting heat from the mercury to the outer walls of the container where it is dissipated as formerly. That they are electrical conductors and are immersed within an electrical conducting medium, the mercury, is of no consequence. The vapor electrical d: charge does not ordinarily come into contact with these metals, that is, they do not function themselves as electrodes.

it is a fact, of course, that the metal leadin wire employed in the mercury arc electrode itself acts to a slight extent as a heat distributor. However, any conductivity action of the lead-in wire cannot be considered to be within the scope of this invention, for not only is this action by the lead-in wire trivial, but the lead-in wires, since it is carrying cur rent, is actually supplying heat to the mercury in the electrode by virtue of its ohmic resistance.

In the drawing,

Figure 1 represents a sectional view of an electrode embodying the invention.

Figure 2 represents a sectional view taken on line 2-2 of Figure 1.

Figures 3, 4 and 5 represent sectional views of modified forms of the invention.

Figure 6 represents a sectional view taken on line 6-6 of Figure 5.

Figure 7 represents a sectional view of an electrode employing an alkaline earth metal and embodying the invention.

Figure 8 represents a sectional View taken (3 on line 88 of Figure 7.

Referring to the drawing, in Figure 1 is shown an electrode vessel 16, composed of any non-conductive (electrically) gas tight material, such as glass or fused silica. Suitably sealed into the vessel 16 at 17 is a lead-in wire 18, formed of any metal which does not fuse with mercury. Attached to the inner surface of the wall of the electrode vessel 16 are coils or wire spirals 19, of the cooling metals these spirals are formed of a metal having greater heat conductance than mercury. The vessel contains a quantity 15 of mercury.

In the form shown in Figure 3, the spirals 19 are replaced by bars or straight wires 29 attached to the walls of the vessel.

In Figure l is shown a modification in which spirals of wire gauze 39 are employed as heat conductors its trode can be determined only by experience.

These internal metal heat conductors are secured in the desired position by fus1on to the wall of the vessel, or other suitable means.

. Lastly, the electrode is filled with metallic mercury to a point which just slightly more than covers the free ends of the cooling metal.

The preceding illustrations are not intended to limit our invention solely to electrodes of the design and type indicated. It 1s possible to construct electrodes of a variety of designs and forms and to still employ our invention. Similarly the invention is not restricted in its application to mercury electrodes alone. We have studied and operated vapor electric devices containing mercury which produce light radiations ln every respect similar to the ordinary familiar mercury arc, and which have the same electrical characteristics as the mercury arc, namely, a negative volt-ampere relationship, the arc current decreasing as the arc voltage increases, but whose operation was dependent primarily upon electron emission by substances other than mercury, and in this respect different from the ordinary mercury arc.

The electrodes in this type of lamp contain the alkaline earth oxides such as barium and strontium oxides as their primary act ve electron emitting substances.

The employment of these substances for the purpose of electron emission is not new as they were used for such purposes as early as 1914:. However, the manner in which we employ these oxides, and the light source which we obtain by their use we believe'to be a novel and distinctive advance in the art of light production. Our method of preparing and employing these electrodes and their dependence for satisfactory operation on our invention which has been described 1n conjunction with the mercury electrode Wlll be described.

This new type of electron emitting electrode for employment in mercury arcs may be made as follows. The alkaline earth oxides in a finely powdered form are thoroughly mixed with granulated metallic oxides, which as metallic hydroxides in aqueous solution, give an electrolytic dissociation of low hydoxyl ion concentration. Typical oxides are ferric oxide, chromic oxide. aluminum oxide, zirconium oxide, nickel oxide, cobalt oxide. and titanium oxide etc. The proportion of alkaline earth oxide to metallic oxide is dependent upon the design and current duty of the electrode. 7

This mixture is made into a paste with an organic binder such ascell'ulose acetate in ether-alcohol solution. The paste is moulded on the lead-in wire which may be tantalum, molybdenum or tungsten. The mouldedelectrode is dried in a vacuum, and then heated in a silica tube to about 1000 degrees centigrade to carbonize the binder; and to weld the mass together still further, the electrode is glowed fora few seconds inthe oxy-hydrogen reducing flame. The finished electrode is introduced into the electrode vessel, and the lead-in wire is sealed gas tight to the vessel.

The Weakly basic metallic oxides have been found useful in the construction of these electrodes because of their stability at the operating temperatures (approximately 1100 degrees centigrade excepting hot spot which is higher temperature) and their ability to adsorb barium and strontium oxide molecules on their surfaces.

This adsorption property may result from their weakly basic properties, for in aqueous solution and in a few instances in crystalline hydrated form, investigators have isolated and identified chemical-compounds resulting from combination between these oxides and the alkaline earth oxides. However, at incandescence there is no evidence that true chemical union exists between these substances, and their association in these; electrodes which operate at incandescence should be considered one of adsorption and physical mixture.

The adsorption of the alkaline oxides on the surfaces of the metallic oxides may be considered analogous to the adsorption of gases on metallic catalysts in gas reactions such as the ammonia synthesis. The adsorption results in a distorted electron field in the alkaline earth molecule, and when an electric potential is applied, electrons may be discharged. The electron emission occurs at relatively low temperatures, that is, at room temperature, when the combinations described are employed. These weakly basic metallic oxides are employed then as nonvolatile supports and as creators of electron emission centers operative at low temperatures.

The completed electrodes are attached to the body of the lamp proper. Two electrodes are sufiicient for either alternating or direct current operation. The assembled lamp is attached to an evacuation and filling system, and is evacuated. lVhile on the vacuum system, the lamp is heated by the external application of heat to drive off moisture and gases adsorbed on the walls. Finally the system is cooled on the vacuum and a small quantity of mercury, more than sufficient to supply all the mercury vapor needed for are oppration, but very little in excess, is admittec.

Since there is too little mercury in the system to permit the establishment of the are by the familiar method of tilting the burner so that mercury can flow from oneelectrode to the other, short circuitingthe system, it is necessary to provide a means for lighting the lamp. This has been accomplished by introducing into the system a small quantity of neon gas at a pressure of a few centimeters. If now the arc tube be gently stroked with a cloth to produce an electrostatic charge on the tube, a few neon ions will be formed.

If the two electrodes are maintained at a potential difference greater than 150 volts, a gas discharge will be formed which will almost instantaneously volatilize sufficient mer cury and establish sufficiently high electron emission to permit the mercury arc to establish. The instant that the mercury arc is started, the voltage drops, and the gas discharge ceases to function. 7

From this point, the gas ceases to play any part in the operation of the arc, the arc appearing in every respect to resemble the ordinary mercury are.

It is essential that during operation that the arc voltage be maintained below the ionization potential of the gas, as otherwise the electrodes will flash over. The starting voltage and also the flashing voltage are characteristic of the physical dimensions of the lamp, and hence vary somewhat for lamps of different sizes. The lamp referred to has an arc length about 8 inches.

This lamp, like the one with mercury electrodes, is operated and the are is maintained by the emission from a hot spot on the electrode. Similarly, if the current supply is interrupted for a very short time, the arc is extinguished, and will not start of its own accord. However, unlike the mercury electrode type, the alkaline earth oxide type cannot be relighted immediately, for there is not sufficient emission to re-establish the arc in the mercury, and the vapor pressures within the neighborhood of the electrodes interfere with the action of the inert gas. The lamp must cool before it can be lighted again.

Lamps, such as described, have proven not "1 satisfactory articles of commerce because they deteriorate too rapidly. This deterioration results from the rapid and continuous disintegration of the electrodes. \Ve have found the cause for this rapid destruction of the electrodes to be that the electrodes become too hot.

The alkaline earth oxides, barium and strontium oxides, and also the metallic oxides have thermal conductivities of the order of 0.0123 calorie per second through a layer one centimeter deep and an area one square centimeter for an increase in temperature of one degree centigrade. This means that they are even poorer conductors of heat than mercury.

The nature of the arc contact with the electrode resembles closely that described for the mercury electrode. There is the characteristic hot spot from which the electrons are emitted. In this instance there is not any mercury at the electrode to evaporate.

All theheat energy generated must be dissipated from the electrode itself. The elec trode therefore becomes very warm, and volatilization of the alkaline earth oxides becomes a result. Whereas the volatilization of appreciable mercury from the mercury type of electrode is without serious consequences, any appreciable volatilization of the alkaline earth oxides will result in greatly shortened useful life.

WVe find that our invention which we applied to the mercury electrode can be successfully applied to the alkaline earth oxide type of electrode and that lamps which are equipped with electrodes constructed in accordance with our invention possess operating lives comparable in every way with that obtainable from lamps employing mercury electrodes.

\Ve will describe the application of our invention to this type of electrode. At the time the electrode is moulded, a series of layers of metal gauzes, foils, or spirals of metals which have high thermal conductivity are introduced into the mass. These metals may be tungsten or molybdenum.

These layers or spirals are-connected and arranged in such a way that when the elec trode is seated in the electrode vessel they make contact with the walls. They are thus enabled to conduct the heat from the electrode to the walls which in turn may be cooled by any familiar method, such as metal cooling vanes. Figure illustrates the possible arrangement. 51 is the wall of the electrode vessel. 55 is the electrode mass with the layers of metallic gauze 56 distributed throughout and terminating in contact with the wall of the electrode vessel 51. 57 is the lead-in wire sealed into the electrode vessel at 58.

Employing electrodes constructed in accordance with our invention, it is possible to construct mercury arc lamps for the production of visible and ultra violet light and for current rectification which compare in electrical and radiation characteristics comparable to the familiar mercury elect-rode types of arcs and which have some advantages not enjoyed by the older type of arc. Thus the lamp may be operated directly on alternating current by applying potential to two electrodes only whereas the ordinary mercury arc requires three electrodes for such operation. The very small quantity of mercury in the new lamp enables the lamp to be oper ated in any position, and also results in case of shipment.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent, is:

1. In an electrode for a vapor electric device, a gas tight insulating container, a leadin wire sealed in the container, an electrode medium of liquid mercury, and a coil of metal having greater thermal conductance than mercury immersed in the mercury and contacting the walls of the container.

2. In an electrode for a vapor electric device, a gas tight insulating container, a leadin Wire sealed in the container, an electrode medium of liquid mercury, and a coil of metal gauze having greater thermal conductance than mercury, said gauze being immersed in the mercury and contacting the walls of the container to assist in conducting heat from the mercury.

3. In an electrode for a vapor electric device, a gas tight insulating container, a leadin Wire sealed in the container, an electrode medium of liquid mercury, and a strip of metal gauze having greater thermal conductance than mercury, said strip of metal gauze being immersed in the mercury and contacting the Walls of the container to assist in conducting heat from the mercury.

This specification signed this 15th day of June, 1931.

WILLIAM T. ANDERSON, JR. HUGH D. FRASER.

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