US2786957A - Emissive cathodes - Google Patents

Description

March 26, 1957 EMI Filed April l, 1954 H. HUBER SSIVE CATHODES 4 Sheets-Sneet 1 March 26, 1957 `H. HUBER 2,786,951

' EMIssIvE cATHoDEs Filed April 1, 1954 4 sheets-Smets March 26, 1957 H, HUBER. 2,786,951

EMISSIVE CATHODES Filed April l, 1954' l 4 Sheets-Sheet 8 March 26, 1957 H. HUBER EMIssIvE oATHoDEs 4 Sheets-Sneet 4 Filed April l, l954 United States Patent O 2,7 86,957 EMISSIVE CATHODES Harry Huber, Paris, France, assignor to Compagnie Generale de Telegraphie Sans Fil, a corporation of France Application April 1, 1954, Serial No. 420,377

Claims priority, application France April 2, 1953 3 Claims. (Cl. 313-345) Ultrashort wave tubes of high power must be provided with cathodes capable of emitting high currents per unit .surface area and having a suiciently long life.

Many types of cathodes have been proposed to this end. One type of cathode in particular, known as an oxidecoated cathode, has given excellent results in pulse operated tubes.

These cathodes are formed usually of a metallic support on which is deposited an emitting material essentially comprising an alkaline-earth oxide, such as barium oxide or thorine. The active element of these zcathodes is formed by the metal of the oxide.

However, such cathodes have the following drawbacks:

First, and this is particularly true in the case of alkalineearth oxide cathodes, they can give high current densities only when pulse operated. D a

Furthermore, they are very sensitive to gas poisoning, i. e., to bombardment by the ions resulting from the ionization of the residual gases in the tube by the electrons. Furthermore, the active metal may be destroyed by chemical reaction with the gases adsorbed by the electrodes of the tube and liberated by the electronic bombardment.

The barium, as well as barium oxide itself, is prone to evaporate in a vacuum, and this naturally contributes to limit the life of the ca thode.

These cathodes are sensitive to the arcing effect. This results in the destruction of grains of oxide, which have a high electrical resistance and exist on the surface of the emitting cathode.

It is an object of Ithe present invention to provide a cathode of the aforementioned type in which these disadvantages are remedied.

Firstly, means have been provided according to the invention for increasing the surfa-ce of contact between the coating or deposit and its metallic support. `This enhances or stimulates the production of active met-al and counteracts the poisoning effects.. Secondly, the quantity of oxide per unit surface area of emission has been increased. This enables large reserves of active material to be obtained. These two effects thus increase the life o f the cathode.

A metallic oxide cathode, according to the invention, includes a metallic support, and this support is formed by at least one container in the shape of a cylinder. This container is iilled with a mixture including 'at least one emissive metallic oxide, the free surface of the emissive material being flat and subjected to the extraction electric field. The area of this free surface is small comparedwith the lateral areas of said cylindrical container, and the depth of the layer of emissive material is at least equal to one of the dimensions of said free surface, said dimension being of the order of 1/10 mm.

According to :another feature of the invention, the thickness of the walls of the container mustl be of the order of 1/100 mm.

Preferably, fa pure metal powder is incorporated in a homogeneous manner in said active materi-al.

The invention will be better understood fromthe ensurice ing description with reference to the accompanying drawings in which:

Fig. l is a perspective view of one embodiment of the cathode according to the invention;

Fig. 2 is a fragmentary View from the top of the cathode shown in Fig. 1;

Fig. 3 is a sectional view of a detail of the cathode shown in Fig. l, showing the advantages of the cathodes according to the invention;

Figs. 4 and 5 are diagrammatic views illustrating how a cathode such as that shown in Fig. l may be constructed.

Fig. 6 is a perspective view showing another embodiment of the invention.

Fig. 6a is a View of a detail of Fig. 6;

Fig. 7 is a diagrammatic view of an embodiment of the tube used in the cathode according to the invention;

Fig. 8 is a front view of the cathode using tubes of Fig. 7;

Figs. 9 and l0 are sectional Views of two embodiments of the cathodes according to Fig. 8;

Fig. 11 is a sectional view of another embodiment of the cathode according to the invention.

Referring to Fig. l, there are shown identical tubes 1, which may for example be of nickel. These tubes have a thin wall of the order of 1/100 mm. and a height of between 0.5 mm. and several millimeters. Their height is at least equal to their inside diameter. The tubes 1 are grouped in a rectangular box, of nickel or molybdenum sheet, having side walls 2 and a base 4. The tubes 1 are contiguous and are disposed to occupy the minimum of space. The height of the walls 2 from the base 4 is equal to the height of the tubes. The tubes 1 and the spaces between them are filled up to their upper edges with an emissive material 7. rThis material is composed, for example, of a mixture of alkaline-earth carbonates.

The heating filament 5 of the cathode is situated below the base 4 and is in direct contact with the latter. This filament rests on `a support 6.

ln Fig. 2, the cathode 1 is seen from the top side, that is, the side corresponding to the emissive surface. Like reference numerals denote like elements in Figs. l and 2. The tubes 1 define therebetween spaces 20. The longitudinal and transverse dimensions of the box are designated by a and b respectively.

The area of the emmissive surface is composed substantially of the areas of the upper ends of the cylinders and by the areas of the curvilinear triangles comprised between the upper sections.

If re and ri are the outer and inner radii of these tubes, and assuming that for example re=0.3 mm. and r1=0.25 mm., it can be easily found that the emissive surface, which is the sum of the areas of the free spaces in these tubes and the spaces between the tubes, is of the order of 70% of the total area of the cathode.

Vlt can be seen, then, that the loss of emissive surface (represented by the total area of the upper ends or edges of the metallic walls of the tubes il) is relatively small. Further, it has been found that these metallic tube ends contribute to the electronic emission.

Fig. 3 shows a sectional View of one of the tubes 1 of the cathode shown in Fig. l, which is filled with emissive material 7, for example, alkaline-earth oxides. In this figure it can be clearly seen that the emissive material has a depth which is great compared to the diameter of the emissive surface of the cathode.

The surface of contact between the oxide and inner wall of the tube, i. e., the lateral surface of this wall, is clearly more than twice as great as the surface of the end sections. Now, it is along this surface of contact that the reduction of the barium or other oxide by the metal of the tube occurs. There is, therefore, an improved generation of the active emissive element.

The fact that the free surface of the oxide is small compared with the depth of the layer of oxide has also another favorable effect. Phenomena harmful to activation (ionic bombardment, evaporation of the active element, etc.) exert their influence more particularly on 4the surface of the layer and have little effect in depth. They are therefore largely compensated by the supply of barium from the lower part of the tube by thermic diffu- S1011.

The stream of electric current inside the tube 1 is directed as shown at 100. It is known that this stream, due to electrolytic conduction, has for effect to separate the barium atoms from the top layer. It can be seen that this current is directed toward the wall of the tube 1 adjacent the emissive surface. VIt therefore does not interfere with the diffusion of the `barium towards the top layer, since the barium substantially does not separate from this layer.

Moreover experience shows that the arcing effect is practically eliminated. The resistance of the tube to the arcing effect may, furthermore, be improved by incorporating a metallic powder, for example of nickel, in the emissive mixture.

Fig. 4 illustrates diagrammatically how the tubes may be stacked in regular manner, when these are of nickel.

The tubes 1 are thrown into a rectangular Plexiglas box 20. One of the dimensions of this box is slightly greater than the height of the tubes. This box is introduced between the poles of a magnet providing a uniform magnetic field and is so directed that the aforementioned dimension is parallel to this field. The tubes therefore take up the same position, i. e., they arrange themselves parallel to this field, their bases resting on one of the side walls of the box which is then slightly shaken. The box is then removed from the magnetic field and placed in such manner that the axes of the tubes are vertical (Fig. 5). One lateral side of the box is then removed and the required number of tubes is removed in one go with the aid of pliers or tongs whose shape corresponds tothe shape of the emissive surface of the cathode.

There are several ways of securing the tubes in the box of Fig. 1;

l. By heating the whole in a vacuum or in a hydrogen atmosphere at such temperature that the tubes adhere together and to the box (1,050 C. for nickel).

2. By applying a coating of nickel powder paste on the base 4 of the box, grouping the nickel tubes and sintering the whole.

3. By applying with a brush or by electrolysis or evaporation, a thin'coating of a metal such as copper, a platinum-silver alloy, or a copper-nickel alloy on the surface of the tubes and on the base 4 of the box and soldering the whole in a vacuum at the following temperatures respectively: l,080 C. (copper), 1,160 C. (platinumsilver alloy), l,250 C. (copper-nickel alloy).

After having thus secured the tubes in place, a heating element 5 is introduced between the base 4 and the support 6 (Fig. l), and all the empty spaces are filled with a dry powder or a viscous paste of alkaline-earth carbonates, barium and strontium, for example, up to the edges of the tubes, by means of a spoon tool.

After drying, there may also be applied, although this is not essential, a thin coating of carbonate or a mixture of carbonate and metallic powder, to a depth of 10 microns. This coating permits a ldecrease in the loss 0f the subjacent emissive material by evaporation; it serves as an absorptive and adsorptive coating for the active element.

The cathode thus prepared is heated slowly in the electronic tube in order to transform the carbonatos into oxides; The carbonic gas escapes through the pores of the emissive surface. The temperature is increased to about l,100 C. When the transformation is finished, the cathode emits electrons. The temperature is then progressively decreased to 900 C. or 850 C. After a certain stabilizing period, the cathode is ready for use.

Fig. 6 shows a modification of the cathode illustrated in Fig. 1. In this figure, the tubes rest on the free surface of a reserve of emissive material 7 which is itself arranged on the base 4 of the box. This reserve is an additional source of the active element. This element is capable of diffusing thermically through the oxides contained in the tubes.

In such a cathode, it is advantageous to introduce thin wires 9 (Fig. 6a) in the spaces between the tubes. In this way there are provided small capillary passage ways 10 through which the active element may diffuse towards the surface.

This cathode of Fig. 6a combines the advantages of cathodes of Fig. l, with the advantages of cathodes having emissive material which are enclosed behind a p0- rous wall, such as for instance an L cathode.

It is easy to see that these wires impart to the emissive material a perfectly defined and reproducible porosity, which constitutes an advantage over the L cathode.

In Figs. 7, 8, 9, 10 there may be seen the various stages of another method of producing the cathode, which provides a cathode such as that shown in Fig. l (Fig. 9) or that shown in Fig. 6 (Fig. 10).

In Fig. 7 there are shown long nickel tubes which are disposed on mandrels 8 of, for example, aluminum. These tubes are assembled in such manner as to form a bundle. This bundle is introduced in a cylindrical nickel tube 2 (Fig. 8). The whole is introduced in a drawing mill so as to press the tubes together. The pressed bundle of tubes is then cut into sections of required length.

The aluminum mandrels are then dissolved in caustic soda. In this way there is obtained a honey-comb-like structure having a series of cavities which are filled with emissive material. The whole may be soldered in a vacuum to the support cylinder 20. The cathode is then ready for use (Fig. 9). It may also include a reserve of emissive material (Fig. 10).

In this process there may also be used tubes of tantalum which are then disposed on nickel or iron mandrels. These mandrels are thereafter dissolved in a hydrochloric acid bath. The whole is then subjected to the aforementioned operations. The tubes may then be filled with a thorine paste. The cathode, which is introduced in the electronic tube, is degassed at a temperature increasing progressively up to l600 or 1800o C. It is then ready for use. Y

On account of the high operational temperature of such a cathode, it is advantageous to surround the whole of cathode, as shown in Fig. ll, with two cylindrical screens 30 and 40, both of these screens being of tantalum. Alternatively, the screen 30 may be of tantalum and the screen 40 of copper sheet. In this figure the cathode is shown diagrammatically in the form of a cylinder of revolution. Such an arrangement enables the power necessary for heating to be reduced. Such screens possess a high coefficient of reflection in the range of infrared light wavelengths comprised between 0.8 and 20 microns. It is in this range that the thermic radiation of the cathode attains its maximum energy.

The use of thorine as the emissive material permits the manufacture of directly heated cathodes.

The cathodes obtained in accordance with the invention permit, when the emissive material is an alkalineearth material base, a continuous discharge of current densities of l A./cm.2 Thorine cathodes may attain current densities of 20 A./cm.2. Their life may exceed 1,000 hours in either case.

What I claim is:

l. A cathode for electron discharge tubes, of the type comprising a metallic cathode carrier', supporting an electron emitting material comprising metallic oxide: said metallic carrier comprising a box having ka base, and in said box, a plurality of identical metal tubes of equal length atleast equalto the diameter of the tube, said tubes projecting from said base of said box, the corresponding ends of said tubes lying in the same plane, said tubes being adjacent to each other, and the tubes and the spacings between them lled With said material, and a heater arranged under said base of said box.

2. A cathode for electron discharge tubes, of the type comprising a metallic cathode carrier, supporting an elec tron emitting material comprising metallic oxide: said metallic carrier comprising a box having a base, and in said box a thick layer of said material, and projecting from said thick layer, a plurality of identical metal tubes of equal length at least equal to the diameter of the tube, the corresponding ends of said tubes lying in the same plane, said tubes being adjacent to each other, and

the tubes and the spacings between them llled with said material.

3. A cathode according to claim 2, wherein said tubes are circular in cross section, thin metallic wires mounted 5 in the free spaces limited by said tubes.

References Cited in the file of this patent UNITED STATES PATENTS 10 2,452,075 Smith Oct. 26, 1948 2,459,841 Rouse Jan. 25, 1949 2,488,716 Elenbaas Nov. 22, 1949 2,499,192 Laerty Feb. 28, 1950 2,624,024 Jansen Dec. 30, 1952

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