US2677623A - Process for manufacturing electron emissive material and electrodes - Google Patents

Description

y 4, 1954 P. DELRIEU ET AL 2,677,623

PROCESS FOR MANUFACTURING ELECTRON EMISSIVE MATERIAL. AND ELECTRODES Filed Oct. 25, 1950 2 Sheets-Sheet 1 Invenfm s Pierre DeZm'eu 1* Andre. czl uaze May 4, 1954 P. DELRIEU ET AL 2,677,623 PROCESS FOR MANUFACTURING ELECTRON EMISSIVE MATERIAL AND ELECTRODES Filed Oct. 23. 1950 2 Sheets-Sheet 2 Fig.3

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Patented May 4, 1954 UNITED STATES PATENT OFFICE PROCESS FOR MANUFACTURING ELECTRON EMISSIVE MATERIAL AND ELECTRODES French company Application October 23, 1950, Serial No. 191,572

Claims priority, application France October 27, 1M9

'7 Claims. 1

This invention relates to electrodes for electrical discharge apparatus, specially those having a gas atmosphere, and to processes for making such electrodes.

The electrodes used for such apparatus are generally classified into two groups: so called cold electrodes, the temperature of which generally does not exceed anywhere that of a dull red heat, and so-called hot electrodes, which, in operation, generally locally attain a white red heat, or are even hotter.

Cold electrodes have a long operating life, but they occasion a large voltage drop (cathode+anode voltage drops): 100 to 250 v.

Hot electrodes have a shorter operating life and deteriorate more rapidly on account of startings, particularly of instantaneous startings but they have a smaller voltage drop: 15 to 25 volts. Further, the discharge current of a hot electrode should be within fairly narrow limits, for instance between 0.3 and 0.6 amp. if a normal life is desired for this electrode.

One object of this invention is to provide an electrode having simultaneously a long operating life, and a low voltage drop.

Another object is to provide an electrode of which the discharge current may vary between a maximum and one fifth or one tenth of said maximum.

A further object of the invention is to provide an electrode of small volume.

A still further object of the electrode is to provide an electrode comprising an emissive material obtained by reacting barium dioxide with molybdenum r tantalum or with both these metals, at a temperature above 525 C. and lower than about 1,30i) C., said reaction taking place, at least at the end of its duration, in a nonoxidizing atmosphere. Said reaction may be effected, very plainly, by melting the barium dioxide in contact with the molybdenum or tantalum or with both these metals. The term ncnoxidizing atmosphere comprises inert atmospheres as well as reducing ones.

Better results are obtained when the emissive coating is thick than when it is relatively thin.

It is generally not useful to fix the activating substances to its support by means of sintering; the plain melting of the barium dioxide, for instance, causes indeed this dioxide, as well as the substances yielded by its reaction on molybdenum and tantalum and by its decomposition, to hold in a very satisfactory way on supports of various kinds. The activating substance may thus form a coating which is thick, tough and strong, on

2 the support which allows the so devised electrode to work during a very long time and to stand to very numerous startings without preheating, while offering only a low voltage drop.

Further features and advantages of this invention will appear from the following detailed description of species thereof. For a better understanding of this invention reference may be had to the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appending claims.

Figure 1 is a view, in elevation, of a portion of a discharge apparatus having a mercury and rare gas atmosphere, particularly argon, krypton, xenon;

Figure 2 is a section through the axis of the electrode of the apparatus shown in Figure 1;

Figure 3 shows, in partly sectioned elevation, another electrode with its support acting as a current supply lead, the extremity of the stern of discharge apparatus to which this electrode is secured, and the adjacent portion of the envelope of this apparatus;

Figure 4 shows an electrode according to a further modification;

Figure 5 shows an electrode according to yet another modification;

Figure 6 is a graph showing the results of tests which have been effected on a discharge device provided with electrodes constructed according to the invention.

The electrode shown in Figures 1 and 2 has a hot zone and a cold zone, the electrode comprises a wall, formed by welding together a sleeve I having an inwardly directed rib at the lower end thereof, a disc 2 placed over the rib and serving to close the lower end or" the sleeve and a cup element 3 placed above the disc 2; the edges of said cup element touch the sleeve or at least are very close to it so as to prevent activating material from flowing out of the electrode during manu facture thereof. Parts i, 2, 3 are made of molybdenum sheet 0.1 mm. thick; they form a cavity, the inner wall of which is covered with a relatively thick layer t of emissive materials, said layer being obtained, for instance, in the maner explained hereinafter.

The Wall E, 2, 3 is welded to a nickel yoke 5 which connects said wall to the current supply leads 8, 9 of the electrode and, in addition, serves to cool the lower portion of said electrode during operation of the apparatus, thus creating a cold Zone at the lower end of the electrode.

A cap 6, of insulating material, for example steatite, covers the greater portion of the outer surface of the molybdenum wall, leaving the lower end and an annular area close to said lower end uncovered; the-cap 6 also coVers-theupper edge of the sleeve l and protrudes a little inside the latter. This can prevents electrical discharges from taking place from the portions of the wall surfaces it covers and, in addition, insulates these portions against heat loss, whereby a"hotzone is created at the upper end of the wall during operation of the apparatus. The cap has an aperture I through which the discharges pass, and thus have access to the inside'of the electrode. This cap is slipped on the molybdenum wall and is held by the current supply leads 8 and 9 which are bent over the flared portion ll! of the cap.

To prevent discharges starting from anywhere else than from inside the electrode, the portions of the outer surface of the Wall of the latter which are not covered by the cap a are coated for instance with an alumina anti-emissive layer ll. This layer is obtained, for example, by coating these surfaces, by brushing them, or by spraying them, with finely divided alumina in suspension in an organic liquid, ethyl alcohol for instance. The current supply leads 8, 9 and the yoke dare coated with the same kind of layer. This ensures that neither these parts nor the outer surface of the lower end of the molybdenum wall nor the outer surface of the annular zone close to said lower end are heated by operating as anodes or cathodes. This helps in maintaining the cold zone already referred to; the triple thickness of the lower end of the wall also assists in maintaining said cold zone. It is to be understood that discharges could also be prevented from taking place from the outer surface of the electrode or from the members connected to the electrode by threading the cap 6 with a mica washer, the periphery of which will be near the envelope I8 inside which the electrode will be mounted.

As is usual in the luminescent tube industry, the electrode is mounted on a glass stem l2, pinched on its current supply leads; 8, Q, and which may comprise an exhaust tube i3 as in the case shown in Figure l.

The following method may be used for obtaining the layer 4' of emissive materials; First, there is placed in the cavity constituted by the wall E, 2, 3, activating material, barium dioxide powder in the present case, then this material is melted by heating for example'with a torch; the melt forms a layer of activating material covering the inside of the molybdenum wall. It may be advantageous (to make later operations more rapid) to then heat the electrode around 850 C., for half an hour in an inert gas such as nitrogen, or a reducing gas such as hydrogen. The formation of the electrode and the corresponding operation. for the other electrode of the discharge apparatus are then eii'ected, for instance. as is usual in this technique, by mounting these electrodes inside the apparatus, evacuating the latter and introducing therein a rare gas. for instance argon, under a pressure of a few millimeters of mercury, then arcing said apparatus.

The electrode shown in Figure 3 comprises a molybdenum wall mm. in thickness, constituted by a cylinder 2! and an end portion 22. This cylinder is welded to a nickel wire 24, 0.8 mm. in diameter, for instance, which supports the electrode and supplies it with current. The wire 2., is sealed tightly in a glass stem 26, itself welded to the envelope 21. The molybdenum wall, before being mounted in the envelope, is preferably subjected to a heating at about 1200 (3., in

a hydrogen atmosphere, which, amongst other things, eliminates all traces of oxidation.

Thewell of theelectrodeis coated, on its inner face, with arelatively thick coating 23 of emissive materials, obtained as follows: First, the cavity limited by the wall 2|, 22, is filled with barium dioxide, pure or mixed with other materials. The-whole is then heated, at least to a red heat, in a reducing atmosphere consisting, for instance, of amixture of nitrogen and hydrogen, or of ammonia gas;. the duration of this heating should be at least equal to the period at the end of which the barium dioxide has become greyblack, the production of metallic barium not being necessary: if the heating is effected at around 1250" (3., this duration is of the order of one half minute, i., e. relatively short. This heating may be obtained by inducing in the wall 2 I 22, a high frequency electric current, by means of a coil such as that represented at 28 in Figure 4. The purpose of this heating operation is to cause the melting of the barium dioxide and to cause a mutual reaction of the barium dioxide and molybdenum. The product obtained generally contains about 73% barium, 7% molybdenum and 2t% oxygen, which-corresponds approximately to the formula 'iBtOz-l-lMoOs; this formula is given only as representing the percentage composition of the material as it was found impossible to determine-of what mixture of materials it consists or whether it is composed of a definite product for the major part and, for the rest, of small amounts of products from other stages of the reaction.

This heating may be effected simply in a nonoxidizing atmosphere (nitrogen; rare gas), but it is preferably that this atmosphere be reducing, at least during the end of the reaction; in practice it is simpler to use a reducing atmosphere during the whole duration of the heating. The above reaction takes place fairly rapidly but generally does not-cause an overflow of the materials contained in the cavity. An overflow would be; e the troublesome effect that the outer face of the electrode wall would be coated, at least locally, with emissive materials.

In the operation such as effected in practice and which takes place in an atmosphere of nitrogen and hydrogen, the high frequency 1.0 first causes a gradual rise, inside about i seconds, of the temperature of the wall to 900 or 1006 G. Then the reaction starts, which is shown by a sudden temperature rise of the wall and by seething of the materials contained in the cavity of the electrode; within about one second, the temperature of the wall rises to about 7.368 C. and then falls ofi to about 1100 C. The reaction does not take place violently. When the seething ceases, the reaction is practically ended but the heating is still kept up for about four seconds, to keep the temperature around ll00 to be sure that the reaction is complete. From the beginning of the heating, about 9 seconds have elapsed. The material contained in the electrode cavity has then a grey-black colour. It is generally unnecessary to continue the reaction to the point where metallic barium is given off by the emissive materials and volatilizes therefrom, at that stage, in an appreciable amount. The electrode is then allowed to cool for 10 or 12 seconds in the atmosphere where the remainder of the treatment was effected. All siu faces, except the inside of the cavity, where a cathode spot could occur are then covered with an insulating deposit of alumina 25, i. e, the outer surface of, the wall 2!. 22

and the current supply lead 23. This deposit 25 is obtained, for instance, by spraying with a spray-gun, a suspension of powdered alumina in alcohol; it may without any inconvenience, also cover a portion of the stem 26.

A replacement of the dioxide by another bariurn oxide or a barium hydrate gives poor results; another oxide would not react with molybdenum, a hydrate would not react either and in some cases would cause troublesome overflows during the reaction.

If a similar process is attempted for the reaction of barium dioxide with tungsten, the reac tion is violent and even somewhat explosive. The greater part of the materials is expelled from the electrode cavity and what remains does not stick to the walls; adherence, however, is a major requirement when the electrode is to operate at a high temperature without being pre-heated before starting discharges. The reaction is also too violent when barium dioxide is in contact with a too strongly reducing agent, such as aluminium or carbon.

The emissive deposit obtained by the above process, however, forms a very adherent and dense crust through no sintering is effected. This deposit is little damaged by remaining in air a short time, contrary to oxides or hydrates which are often used. Thus the electrodes can be handied without any special precautions during later operations.

Two electrodes thus obtained my immediately be placed in position by welding of the stems to which they are secured, respectively to the two ends of a glass tube, coated or not with fluorescent materials. The whole assembly is then subjected to the usual operations for the manufacturing of fluorescent lamps and similar discharge apparatus: gases are eliminated by the usual methods (heating effected in a vacuum, in an oven or by discharges, heating of the electrodes by a high frequency field) the filling with rare gases is effected, a drop of mercury is introduced and the device is operated for a few minutes to cause a diffusion of the mercury inside the lamp.

It should be noted that, generally, the gas removing operation is no longer accompanied by the formation of the electrode and has not to be effected any longer by means or" continued discharges of a high intensity; the electrode is in an activated condition as soon as the reaction between the barium dioxide and the molybdenum is ended.

If it is desired to store an electrode for some time before mounting it, it will be suflicient to pour one drop of collodion in its cavity. When drying, this drop leaves on the surface of the emissive materials a tight film of nitrogencellulose; this film will then disappear during the gas removal in the tube inside which the electrode will be mounted.

It seems probable that during the operation of this electrode the barium dioxide continues to react on the molybdenum of the wall; observation shows that the latter is gradually corroded and is eventually pierced through.

It may also be observed that the stains which, after a certain time of operation, appear in the vicinity of the electrodes, contain some molybdenum; on the other hand, contrary to the stains due to barium activated electrodes obtained by known methods, it is often impossible using electrodes as described above to detect any barium and, in any case, the amount of barium is so 6 small that it cannot be determined by usual chemical methods.

Such an electrode, for giving out a current which may vary from 0.1 to 0.5 ampere may have, for instance, a diameter of 4 mm. and a length of 10 mm; the corresponding amount of barium dioxide used for its activation is of the order of milligrams. This electrode, operating at 0.4 amp. may undergo at least 200,000 startings before being put out of use, each one of said startings being followed by an operating period of 10 seconds and a pause of 10 seconds. This corresponds to a probable normal use life, or the order of 15,000 hours. The sum of the anode voltage drop and of the cathode voltage drop, in operation, is about 20 to 40 volts for currents of 0.2 to 0.5 amp. The hot zone of this electrode is a very small area, almost a point, the temperature of which, in the case of an electrode having the dimensions indicated, varies from 1200 to 1800 0., according to the current (0.1 to 0.6 amp). In the immediate vicinity of this area, the temperature drops very rapidly, for instance to 700 C. for a current of 0.4 amp. or 900 C. for a current of 0.6 amp; the temperature drops still more on getting closer to the place where the current supply lead is secured.

The volume of this electrode, which is only 0.12 cm for a normal maximum current of 0.5 amp., is much smaller than the volume of usual non-filament electrodes of the same power. In this volume, "0.1 gram of barium dioxide may be introduced without any trouble due to overflowing during the reaction or the heating which precedes it.

The electrode shown in Figure 4. is similar to that shown in Figure 3; its wall, however, 2Q, is not made of molybdenum, but of very pure iron, metallic molybdenum being supplied by a wire 30 of this metal, helically wound, and inserted within the cavity before the barium dioxide is introduced therein. The electrode is supported by two lead wires SI, 32.

This same Figure 4 shows, in section, the coil 28 inside which the electrode is placed with a view to heating the barium dioxide and causing its reaction with the molybdenum. When a current of suitable frequency and intensity flows through said coil, it induces in the electrode wall, and, in the present case, in the molybdenum helix, 30, eddy currents which heat up these metal parts and, by thermal conduction, the barium dioxide and other materials which may be contained in the electrode cavity.

Figure 5 shows an electrode wherein the wall. 33 is made of ceramic material, steatite for instance. The electric current is supplied by the lead 34 which extends into a sort of helix 35, within the emissive material 23. The metallic molybdenum used in the make-up of the latter is introduced inside the electrode cavity, for instance in the form of molybdenum grains, roughly mixed, previously, with the barium dioxide. A washer 36, made of molybdenum or iron, for instance, prevents the liquid barium dioxide from flowing through the clearance between the lead 34 and the aperture of the wall 33 through which said lead enters the electrode cavity.

On Figures 3 and 4, the aperture in the electrode is directed towards the stem to which the latter is secured, so that the black stains generally observed in the vicinity of the electrodes occur near the stem; in Figure 5, on the corn trary, the aperture in the electrode is directed 7' in the opposite direction. It is obvious that one or the other of these directions may be used indifferently with the three types of electrodes shown in Figures 3, 4, 5. Similarly, each eiectrode may be provided with one or two supports, or even more, one or several of said supports possibly not being used as lead wires. The metallic molybdenum for each one of the electro es may be in the form of sheets, wire, powder, etc. or several such forms simultaneously.

Numerous other modifications may be made to the electrodes described, within the scope of the invention; in particular, the reaction between the barium dioxide and molybdenum may be effected before mounting the electrode on its stem, the materials generating the emissive substances may contain other materials than on rium dioxide and molybdenum: for instance strontium dioxide, molybdeniun compounds, thorium or tungsten, metallic or in compounds, silica, etc.; the amount of those of said other materials which contain alkaline-earth metal must, however, be small relatively to the amount of barium dioxide.

The reaction of the barium dioxide may also be effected on tantalum and, optionally, molybdenum.

The electrode may, then, be again, for example, of the type shown in Figure 3, its wall eomprising a cylindrical ferrule and a closed end, both made out of a molybdenum sheet 0.1 mm. thick. This wall is de-oxidized by heating to about 1200 O. in hydrogen, then welded to a current lead, itself sealed in a stem. The activating coating of the electrode is obtained as follows. There is introduced into the cavity of the wall, a ture of powdered barium dioxide and metallic tantalum powder, a mixture which has been left in damp air for one day. The electrode, attached to its support, is then installed inside a chamber through which a mixture of nitrogen and hydrogen is passed. Then, by means of a high frequency magnetic field, the wall of the electrode is raised to above red heat temperature during a time sufiicient to obtain a melting or". the barium dioxide and its reaction on the molybdenum. This stage in the manufacture was described above in detail in connection with Figure 3. In the present case, tantalum is in contact with the dioxide and reacts with the latter. This does not alter substantially the term perature and time of reaction, which is, for instance, about ten seconds at about 1200 C.

The inside of the electrode is, when the reaction is complete, covered with an adherent crust, granite-like and blackish, which constitutes the emissive substance activating the electrode. The electrode is allowed to cool for about ten seconds inside the chamber, then it is removed and its outer surface and connecting lead are coated with alumina.

An electrode thus manufactured has little tendency to give rise to black stains on the envelope of the lamp in which it is mounted; moreover, it can be left several minutes in the open air without any inconvenient deterioration.

Such an electrode, intended for utilizing a current varying from 30 to 250 milliamperes, may have, for example, a diameter of 4 mm. and a length of 5 mm.; the corresponding amounts of barium dioxide and tantalum which are used are respectively of the order of 60 milligrams and 40 milligrams.

These amounts, or even their proportions may also vary largely while giving good results. The

8 tantalum may be used in forms other than a powder, for example in the form of a wire or flakes. It may even constitute all or part of the wall of the electrode. The tantalum may be completely submerged in the barium dioxide but it may also, if used in the form of a wire, for instance, rise abcve the surface of the dioxide and even protrude out of the cavity of the electrode.

The wall of the electrode may consist neither of molybdenum nor of tantalum, but for instance of very pure iron or ceramic, as above mentioned; it is not necessary that metal molybdenum be in contact with the barium dioxide during the reaction and furthermore there may be no molybdenum in the electrode.

The atmosphere in which the melting is effected may be different from a mixture of hydrogen and nitrogen and may be, for instance, hydrogen, ammonia gas, nitrogen, a rare gas etc., but it is generally preferable that the reaction of the barium dioxide with the tantalum and, as the case may be, with the molybdenum, end in a reducing atmosphere. Water, the presence of which may not be necessary, may be introduced with one only of the materials used for producing the emissive substances, for instance by slightly humidifying the barium dioxide, or it may be added as a very small water drop to these materials after they are placed in position in the cavity of the electrode, but before the reaction, or by using a damp gas for constituting the atmosphere in which this reaction is effected.

The reaction of barium dioxide, at the time of its melting, takes place with a troublesome rapidity in some cases, particularly when this material is very pure and dry. This reaction may be slowed down by adding relatively inert substances like an earth-alkaline carbonate or oxi e or a mixture of earth-alkaline carbonates or oxides. For instances, 5 grams of barium carbonate are used with grams of dioxide. The presence of a trace of water also slows down the retion.

The reaction of barium dioxide upon at least one of the molybdenum and tantalum metals, and possibly being mixed with other substances like barium carbonate, may be effected outside the electrode cavity. The preferred conditions for this reaction are similar to those indicated above, namely a heating above red-heat, which may last a few seconds only, in an atmosphere which, at least at the end, is reducing or nonoxidizing. This reaction gives an emissive ma terial which can be utilized for the electrodes; after being thus prepared, this material has to be fixed on the portion of the electrode designed for supporting the activating substance. Such fixating may be effected in quite a satisfactory manner by mixing the material obtained after grinding it with an additional amount of barium dioxide, by placing this mixture on the portion of the electrode to be activated and by heating this portion so as to bring about the melting of the additional barium dioxide.

Figure 6 shows, by way of example, the re sults of tests made for studying the variation, as a function of the value of the discharge current, of the voltage at the terminals of fluorescent lamps cms. long and having an inside diameter of 15 mm.

The points of curve 38 were obtained with a lamp provided with two so-called cold electrodes, nonactivated, of nickel plated iron, each having an area of 12 square centimeters. Those of curve 39 are relative to a lamp provided with electrodes having the same shape and arrangement as the one shown in Figure 3, but activated by means of a mixture containing, per electrode, about 60 mg. of barium dioxide and 40 mg. of tantalum powder. In the present case, each electrode is 4 mm. in diameter and 5 mm. long.

It may be seen from curve 39, that with the electrodes of the invention, the lamp terminal voltage remains substantially lower than that obtained with the cold electrodes and increases little when the current decreases, as long as the latter remains above inilliamperes. The cathode drop of these electrodes is small even with currents as low as 25 milliamperes; it is of the order of 25 volts (18 volts at 60 milliamperes, 15 volts at 100 milliamperes). This is a particularly remarkable result since, heretofore, it had not been found possible to operate an electrode with a low voltage drop for currents below 100 milliamperes.

A compariso of curves 39 and 38 show that the voltage at the terminals of the lamp with electrodes according to the invention is lower by about 200 volts than that of the lamp with cold electrodes. This difference is due to the electrodes, since the latter constitute the only diiference between the two lamps. This voltage gain makes it possible, for instance, to supply in series two lamps of the type corresponding to curve 39 with a transformer with a magnetic leakage having an open circuit voltage of 1,300 volts and a normal current of 50 milliamperes; this transformer is normally used for supplying a single tube corresponding to curve 38, having the same dimensions and the same light power as each one of the above tubes.

Another consequence of the low voltage drop of the electrodes according to the invention is that when a lamp provided with these electrodes, operates under 50 milliamperes, the temperature of its envelope reaches, at the electrode level, only '75 to 80 C., and only over less than 1 cm. length, although these electrodes are so-called hot electrodes; on the contrary, for a similar lamp operatingalso under 50 milliamperes, but provided with sc-called cold electrodes, the corresponding portion of the envelope reaches about 120 C. and measures about 4 cms. in length.

Another advantage of the electrodes according to the invention with respect to cold electrodes is their very small bulk. The electrodes according to the invention make it possible to ensure that the lamp on which they are mounted is luminous over its entire length, especially when the apertures of the open ends of the electrcdes face the stem which supports them, as represented in Figure 3. By mounting such lamps end to end, light source may be constituted as large as desired, and offering practically no solution of continuity; on the contrary, of it is desired to obtain the same result with cold electrode lamps, said electrodes have to be housed in tubes of corresponding lengths which are welded to the ends of the tube constituting the visible portion of the lamp and brought back parallel with the latter.

What we claim is:

l. A processs for manufacturing an emissive material for electrodes of electric discharge devices, comprising reacting barium dioxide with at least one metal selected from the group consisting of molybdenum and tantalum at a temperature above 525 C. and lower than about 1,300 C., said reaction taking place, at least at the end of its duration, in a non-oxidizing atmosphere at about the air pressure.

2. A process for manufacturing an emissive material for electrodes of electric discharge devices, comprising reacting barium dioxide with at least one metal selected from the group consisting of molybdenum and tantalum at a temperature above 525 C. and lower than about l,300 C., said reaction taking place, at least at the end of its duration, in a non-oxidizing atmosphere at about the air pressure and being stopped before metallic barium volatilizes.

3. A process for manufacturing an emissive material for electrodes of electric discharge devices, comprising reacting barium dioxide with at least one metal selected from the group consisting of molybdenum and tantalum, and with water in an amount which is small relatively to the amount of barium dioxide, at a temperature above 525 C. and lower than about 1,300 C.. said reaction taking place, at least at the end of its duration, in a non-oxidizing atmosphere at about the air pressure.

4. A process for manufacturing an activated hollow electrode for electric discharge device, said electrode being provided with an inner coating of emissive material, comprising bringing barium dioxide inside the cavity of the electrode and reacting said barium dioxide with at least one metal selected from the group consisting of molybdenum and tantalum at a temperature above 525 C. and lower than about 1,300 0., said reaction taking place, at least at the end of its duration, in a non-oxidizing atmosphere at about the air pressure.

5. A process for manufacturing an emissive material for electrodes of electric discharge devices, comprising mixing barium dioxide with at least one alkaline-earth metal compound selected from the group consisting of oxides and carbonates and reacting said barium dioxide with at least one metal selected from the group consisting of molybdenum and tantalum at a temperature above 525 C. and lower than about 1,300" 0., said reaction taking place, at least at the end of its duration, in a non-oxidizing atmosphere at about the air pressure.

6. A process for manufacturing an electron emissive coated electrode for an electric discharge device comprising reacting barium dioxide with at least one metal selected from the group consisting of molybdenum and tantalum at a temperature above 525 C. and lower than 1,300 (7., said reaction taking place, at least at the end of its duration, in a non-oxidizing atmosphere at about air pressure, grinding the material obtained through said reaction, mixing this material with an additional amount of barium dioxide, melting said mixture, and bringing into contact said mixture with a metal support for emissive material.

'7. A process as claimed in claim 6, wherein said mixture is in contact with said support during the melting thereof.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,467,398 Schumacher Sept. 11, 1923 1,720,675 Hertz July 16, 1929 1,809,229 Bartlett et al. June 9, 1931 2,012,237 Beck Aug. 20, 1935 2,084,172 Weiller June 15, 1937 2,375,808 Miller May 15, 1945

Claims (1)

1. 4. A PROCESS FOR MANUFACTURING AN ACTIVATED HOLLOW ELECTRODE FOR ELECTRIC DISCHARGE DEVICE, SAID ELECTRODE BEING PROVIDED WITH AN INNER COATING OF EMISSIVE MATERIAL, COMPRISING BRINGING BARIUM DIOXIDE INSIDE THE CAVITY OF THE ELECTRODE AND REACTING SAID BARIUM DIOXIDE WITH AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM AND TANTALUM AT A TEMPERATURE ABOVE 525* C. AND LOWER THAN ABOUT 1,300* C., SAID REACTION TAKING PLACE, AT LEAST AT THE END OF ITS DURATION, IN A NON-OXIDIZING ATMOSPHERE AT ABOUT THE AIR PRESSURE.

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