US2943226A - Cold and hollow electrode - Google Patents

Description

June 28, 1960 P. LEMAlGRE-VOREAUX 2,943,226

COLD AND HOLLOW ELECTRODE Filed Jan. 31, 1956 INVENTOR PIERRE LEMAIGRE VOREAUX av 6 4, 9 m M.

ATTORNEYS United States Patent O COLD AND HOLLOW ELECTRODE Pierre Lemaigre-Voreaux, Paris, France, assignor to S- ciete Anonyme pour les Applications de lElectricite et des Gaz Rares-Etablissements Claude-Paz & Silva, Paris, France Filed Jan. 31', 1956, Ser. No. 562,565

2 Claims. (Cl. 313-352 This invention relates to cold and hollow electrodes for electric discharge devices containing gas and mercury vapour. It relates more particularly to an electrode to the inner wall of which has been fixed at least one small piece comprising, mainly or solely, one or more rare earth metals in a metallic condition, for instance lanthanum.

This electrode is characterized by the fact that the area of the piece, or the total area of the pieces, is small, and remains small, 'as compared with the area of the inner wall of the electrode, preferably less than one tenth of this wall area.

Preferably, the formation treatment to which the electrode has been subjected and the conditions of its normal operation must be such as to avoid substantially any evaporation of the rare-earth metal or metals due to too high a temperature, as well, as any sputtering of this metal by ionic bombardment, since evaporation and sputtering enlarge greatly the surface of the piece or pieces: to the original surface would be added a wide film made of a portion of the rare-earth metal, deposited on the inner wall of the electrode and on the envelope of the discharge device provided with an electrode submitted to such treatment orv conditions. rare-earth metal in contact with the discharge atmosphere would very substantially shorten the life of the discharge device, as it will be explained later.

Cold electrodes generally consist of a hollow metal cylinder which is open at one end and secured to one or more current lead-in wires, the edge of the aperture at the open end being, generally, provided with an insulating piece which protects said edge from ionic bombardment and prevents the electrode from touching the wall of the discharge tube. These electrodes have long lives but they cause an important voltage drop. With alternating current the voltage drop for the two electrodes, that is the sum of the cathode and anode drops, is of the order of 200 volts, R.M.S. This voltage drop may be decreased by activating the electrode, that is by providing its inner wall with an electron-emitting coating, ca-

pable of emitting electrons readily at the relatively low operating temperature of the electrode, that is a temperature which is of the order of 150 C. This coating,

which however, generally consists of alkaline-earth oxides, loses its efficiency at the end of a time of operation less than the average life-time of a nonactivated cold electrode, and often causes the appearance of stains in tubes provided with such electrodes.

One object of the invention is to provide electrodes the voltage drop of which is substantially smaller than that of cold electrodes, non activated, this voltage dropv remaining low for a very. long duration. Other advantages of the electrodes according to'the invention will be pointed out hereinafter.

One embodiment of the invention will be described hereinafter by Way of example, with reference to the accompanying drawing inwhich:

Figure 1 shows an electrode according to the inventiorij Figure 2 shows the variation, as a function of the dis- A broad surface of charge current, of the voltage drop at the two electrodes of a discharge apparatus provided either with electrodes according to the invention or electrodes which are similar except that they comprise no rare earth metal or alloy.

Figure 1 shows, in longitudinal section, an electrode according to the invention before it is mounted inside the envelope of a discharge device.

This electrode comprises a cylinder 1 made of sheet metal, for instance nickel plated iron sheet, at the ends of which are two steatite parts 2, 7. The part 2 is provided with an orifice 3 allowing the electric discharge an access to the inside of the electrode. The part 2 is also formed with a flange 5 which prevents the discharge from occuring on the edge 4 of the cylinder 1 and from causing a heavy sputtering of the sheet metal at that place.

The other steatite part 7 closes the end of the cylinder 1 opposite that throughwhich the discharge passes and this closure need not be air-tight.

A wire 8, welded to cylinder 1 supports the electrode and supplies it with electric current.

Some pieces 6 of metal or of an alloy of rare earth metals, are secured on the inner wall of the cylinder 1. In the embodiment illustrated in Figure 1, two pieces 6 are shown and they comprise segments of lanthanum wire, welded at their mid points to cylinder 1. It is not necessary for the lanthanumto be extremely pure, it being suflicient that those impurities which may be detrimental such 'as oxides or nitrides, do not exist in troublesome' amounts. I

Lanthanum may be replaced by cerium or by an alloy 7 of rare earth metals. Good results were obtained, for;

instance, with an alloy according to the following specifi-' The electrode, once it'has been provided with' the lanthanum pieces 6 and its current lead-in wireS or wires if it is provided with several current lead-in wires,

is mounted inside a segment of glass tube terminating in a bottom into which latter the current lead-in wire or wires is or are sealed vacuum-tightly. A tube segment thus provided with an electrode is then welded at each end of a glass tube which may possibly be ternally with a layer of fluorescent material.

The discharge tube thus obtained is then subjected to, a number of treatments which form its electrodes and, which put it into anoperating condition; It is possible,.

for example, after having introduced a drop of mercury intothe tube, to de-gas the latter, together with its elec trodes, byconnecting said tube through its exhausttube with a vacuum pump and passing a'discharge therethrough, with an intensity substantially higher than that of the normal tube operation. -When the electrodes have been. heated for a sufi-icient time to a bright red and the glass; of the tube is still at a suitable temperaturqthe discharge. is stopped and pumping is continued nntila good vacuum. 7 is obtained. The presence of traces of'oxygen and of;

nitrogen during these treatments restrain very materially the evaporation and the sputtering of the rare earth metals, especially during the electrode bombardment. Evaporationand sputtering may be fur-flier lessened bya shortening of, the treatments duration. The tube and ,its 'elec-J trodes may also be heated during this de-gassing, notgby;

means of a heavy discharge, but by placing the tube, concoated in-' taining no mercury, in an oven, while heating its electrodes by a high frequency field. These two methods of operation are usual in the manufacturing of cold cathode tubes.

- With the electrodes according to the invention; it is not necessary. to carry the .de-gassing by heating as far, and a more even heating, with no discharge or high frequency may be sutlicient. The residual pressureat the time when the pumping is stopped, does not have to be as low as when usual cold electrodes are used, since the rare earth metal pieces 6 will absorb the small amount of nonrare gases which has not been eliminated by the de-gassing. This constitutes an additional advantage of the electrodes according to the invention.

As is well known, once the de-gassing has been completed, the tube is filled with a permanent gas with a pressure of a few millimeters of mercury. This gas, may for example comprise argon, technically pure or containing a little nitrogen,.or alternatively it may comprise a mixture of argon and neon. After this filling, the tube is separated from the pumping device, the mercury is introduced if this had not been done previously, the exhaust tube is closed and cut and then the tube is operated for some time to bring about the diffusion of part of the mercury.

The heating of the electrode during its de-gassing may cause, at least partly, the melting of the pieces 6 of lanthanum or of other rare earth metal or alloy. This melting may cause these pieces to spread somewhat but the material of these pieces must not spread upon a large portion of the inner area of the electrode.

It must be avoided that rare earth metal leave pieces 6 in perceptible amount and form a deposit on the envelope of the lamp and on the surface of theelectrode, for instance because the electrode has been heated to a temperature at which said metal evaporates substantially or because too heavy an ionic bombardment has sputtered and thrown away a portion of these pieces. Experience has shown indeed that the metal thus deposited would absorb mercury and gases during the operation of the lamp, which would prematurely put the lamp out of use.

One cannot provide the lamp with substantially more mercury than usual, in order to compensate for the eifect of a large surface of rare earth metal, since the mercury droplets would damage too much the fluorescent layer when the lamp will be handled. One cannot either provide the lamp with an excess of gas since the starting and operating voltages would be substantially raised, the more as a large excess would be necessary.

It is advantageous, from this point of view, that the discharge atmosphere of the lamp provided with an electrode according to the invention comprise a noticeable proportion of nitrogen when the lamp is put into use, preferably at least 0.1 percent by volume. Nitrogen indeed lessens very substantially the sputtering of the electrodes by the ionic bombardment. As a matter of fact nitrogen will be absorbed during the normal operation of the lamp but this absorption is very slow since the rare earth metal olfers a small area only and the elfect of nitrogen will be felt during a long duration.

Figure 2 shows the variation, as a function of the intensity of the discharge current, of the voltage drop at the two electrodes of tubes containing, in addition to a drop of mercury, a mixture of 80% argon and 20% neon, under a pressure of 6 mm. mercury. This argon contains about 0.3% nitrogen by volume. The curve 10 is relative to a pair of cold electrodes not activated, each one of them being constituted as represented in Figure 1, except that it does not comprise any piece '6 and being 60 mm. long and 12 mm. in diameter. These electrodes are used industrially for currents from 50 to 100 milliamperes. The curve 11 is relative to a-pair of electrodes similar to the above but provided with four segments6 of lanthanum wire, mm. long and 1.2 mm. in diameter. The points used for plotting these curves were calculated from measured results obtained on tubes of difierent lengths so as to find out what portion of the voltage drop in the tube occurs at the electrodes. The axis for abscissae is calibrated in milliamperes, that for ordinates in volts.

The study of non activated electrodes was carried out only to 125 milliamperes, as, beyond this value, the electrode is destroyed by ionic bombardment. In contrast the' electrodes according to the invention, having the same dimensions, withstand very well a 250 milliamperes current. A cold and non activated electrode, designed for this latter current, should have an area which is twice as large as that of the electrodes on which the measurements were made. Since, for this type of electrodes, the ratio length to diameter should not exceed a certain value (depending, in particular, on the pressure and nature of the filling gas) it would be necessary to increase the diameter of the electrode, which would make it necessary to house it in an electrode chamber with a larger diameter than that of the remaining portion of the tube. This drawback is obviated by the electrodes according to the invention. Conversely, with elecetrodes according to the invention, smaller dimensions may be adopted if they are to operate only up to milliamperes.

An advantage of the electrodes according to the invention, an advantage which is visible immediately in Figure 2, is the decrease in the voltage drop at the electrodes. In the example illustrated the electrodes according to the invention gain about 30 volts for a 50 milliamperes current and 70 volts for 100 milliamperes. This peculiarity is fairly surprising, since the activating pieces 6 have only a small area and are not raised to a high temperature.

Another advantage of these electrodes, due probably to'the absorption of the detrimental gases by the pieces 6 is the decrease in the voltage drop through the discharge column. This decrease varies largely according to the case considered.

The discharge, substantially, does not occur on the outer surface of the electrode because of the presence of electron-emissive material inside. This stabilizes the cathode glow and decreases considerably the cathode sputtering at the outside of the electrode. This makes it often possible to simplify the constitution of the electrode by omitting the insulating parts such as those shown at 2 and 7, and by not surrounding the electrode with a sheet of mica, a sheet often used for improving the insulation between the electrode and the glass of the discharge tube. This decreased sputtering increases the life of the electrode. Conversely it is possible to preserve for the latter a normal life while decreasing the pressure of the permanent filling gas. This pressure decrease is known to improve the efficiency of the discharge in luminous and ultra-violet radiations at the expense of the electrode life.

Numerous modifications may be made to the electrode described without departing from the scope of the present invention; for example, the pieces 6 may have other shapes and other dimensions than those illustrated. They may also vary widely in number and they may be secured at various places on the inner wall of the electrode. The sheet metal part 1 may be closed by a metal cap instead of the ceramic part 7 and if desired the part 1 may in certain cases be conical.

What I claim is:

'l. A discharge device having a gas and mercury vapor atmosphere and provided with a cold and hollow electrode comprising at least one small metal piece fixed to the inner wall of said electrode, said piece comprising at least mainly at least one rare earth metal in a metallic condition, and the area of said piece being small and remaining small as compared with the area of the inner wall of the electrode at least during almost the whole life of the electrode, said gas comprising solely rare gas and a small proportion of nitrogen, said proportion being greater than 0.1% by volume. I

2. A discharge device having a gas sealed therein'and provided with a cold and hollow electrode comprising at least one small metal piece fixed to the inner wall of said electrode, said piece comprising at least mainly at least one rare earth metal in a metallic condition, and the area of said piece being small and remaining small as compared with the area of the inner wall of the electrode at least during almost the whole life of the electrode, said gas comprising solely rare gas and a small proportion of nitrogen, said proportion being greater than 0.1% by volume.

References Cited in the file of this patent UNITED STATES PATENTS Metcalf Apr. 5, 1932 Szigeti Oct. 10, 1933 Foulke July 10, 1934 Spanner July 15, 1941 Eknayan Mar. 16, 1943 Lockwood Sept. 21, 1948

Claims (1)

1. A DISCHARGE DEVICE HAVING A GAS AND MERCURY VAPOR ATMOSPHERE AND PROVIDED WITH A COLD AND HOLLOW ELECTRODE COMPRISING AT LEAST ONE SMALL METAL PIECE FIXED TO THE INNER WALL OF SAID ELECTRODE, SAID PIECE COMPRISING AT LEAST MAINLY AT LEAST ONE RARE EARTH METAL IN A METALLIC CONDITION, AND THE AREA OF SAID PIECE BEING SMALL AND REMAINING SMALL AS COMPARED WITH THE AREA OF THE INNER WALL OF THE ELECTRODE AT LEAST DURING ALMOST THE WHOLE LIFE OF THE ELECTRODE, SAID GAS COMPRISING SOLELY RARE GAS AND A SMALL PROPORTION OF NITROGEN, SAID PROPORTION BEING GREATER THAN 0.1% BY VOLUME.

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