US2543728A - Incandescible cathode - Google Patents

Description

Feb. 27, 1951 H. J. LEMMENS ETAL INCANDESCI BLE CATHODE Filed July 17, 1948' FIG FIG.

INVENTORS JOHANNES LEMMENS,

HENDRICUS MARINUS JACOBUS JANSEN ROBERT & LOOSJES BY %%7 AGENT Patented Feb. 27, 1951 amaze mcannasomm osmopn Hendrious Johannes Lemmens, Marinus Jacobus Jansen, and Robert Locales, Eindhoven. Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn, as trustee Application July 17, 1948, Serial No. 39,284 In the Netherlands November 26, 1947 1 18 Claims.

Our invention relates to an incandescible cathode having an internal cavity provided with a supply of electron-emitting material.

It is already known to provide an electric discharge tube with a cathode which is obtained by filling a nickel tube with electron-emitting metals, drawing the tube to the required diameter and giving it the shape of a cathode. The difficulty is that the diffusion of the electron-emitting metal to the exterior is purely molecular, so that the thickness of the wall is required to be smaller than would be permissible in view of the mechanical strength.

It is also known to sinter pulverulent nickel on and in the apertures of a perforated nickel tube to form a porous mass and subsequently to provide a supply of electron-emitting compound and a heating body inside the tube. However, such a cathode does not permit of obtaining a high specific electron emission, since for this purpose the temperature is required to become so high that the porous nickel mass is sintered to compactness, so that an emitter cannot pass out any longer and after a short time the electron emission is determined only by the properties of the nickel alone.

Furthermore it is known to provide the supply of electron-emitting oxides inside a cylinder of gauze having fine meshes. However, the meshes are still of such width that the electron-emitting compounds pass out at excessive speed at the temperatures required for a high specific emission. In the same publication it has been suggested to constitute the cylindrical surface of the cathode by a number of rings pressed on one 4 another, the electron-emitting compounds being provided in the cylindrical cavity and the assembly being closed by insulating terminal plates. There are only a limited number of paths for the diffusion oi the electron-emitting material to the exterior, viz. between the rings, the width of the said paths being dependent on a variety of accidental factors, which are also liable to vary in operation.

Furthermore, it is known to manufacture a cathode by spraying an alloy such as bariumaluminium onto an insulating or metallic support, providing on this alloy, likewise by the spraying process, a porous layer of a metal of the iron group or a metal of high melting temperature, and oxidising the last-mentioned layer superflcially or coating it with an alkaline-earth oxide. However, it is thus not possible to obtain a cathode which comprises a supply of electron-emitting metal or a compound thereof at the interior of the cathode, since the barium is immediately liberated completely from the barium-aluminium and passes out through the porous superficial layer.

As a result, such a cathode is not suitable for vacuum tubes either, since a barium arc is liable to occur on account of the excessive speed of supply of the barium.

For gaseous discharge tubes it is known to provide asupply of electron-emitting oxides inside a porous sintered tungsten body and to drill one or more small apertures in the said porous body in order to facilitate the transport of the electronemitting compounds to the surface of the cathode upon ignition of the tube when the cathode is cold. It has been found, however, that such cathodes are not suitable for use in high-vacuum discharge tubes since as a result of the high voltage set up between the cathode'and the subsequent electrode, an arc is readily produced between this electrode and the apertures provided in the porous body by means of the ionised metal, the compounds of which are contained in the oathode. Furthermore the supply of electron-emitting compounds is consumed rapidly.

The aim of the invention is to provide a cathode of the kind described in the preamble which has a continuous high specific emission and at the same time a long period of life.

According to the invention, in an electric discharge tube comprising a cathode having a supply of alkaline-earth metal compounds provided at the interior of the cathode, at least a part of the wall of the cathode which separates the supply of electron-emitting compounds from the discharge tube consists of a homogeneously porous sintered body of one or more of the refractory metals tungsten, molybdenum, tantalum, hafnium and niobium, the pores in the porous body being the largest apertures that are provided in the said wall.

The cathode of such a discharge tube may, without objection, be operated at higher temperatures (from 1200 to 1400 C.) since as a result of the porosity of the body a small amount only of electron-emitting compounds with free metal is admitted to the surface so that a very thin layer is present thereon. The porosity of the wall portion that is required to pass the electronemitting compounds is dependent on the working temperature of the cathode, the nature of the electron-emitting compounds, and the nature of the porous body itself.

Tungsten sintered at 1800 C. with a density which is of the maximum value and in which out should not be excessive so that a barium discharge cannot occur. The specific emission may become very high owing to the permissible higher temperatures and the cathode may, without objection, be operated at 80% and upwards of the saturation emission owing to the uniform coating on its surface. This value is from 5 to 10% at the most in the case of an ordinary carbonate cathode.

This afi'ords particular advantages for all kinds of uses, for example for cathode-ray tubes for projecting purposes and for short-wave tubes operating by pulses. For the last-mentioned tubes there is a further advantage, viz. that the surface of the cathode may be worked to be smooth so that the position of the cathode surface is defined exactly.

In order that the invention may be clearly understood and readily carried into effect, we shall describe the same with reference to experimental data, several examples and the accompanying drawing in which:

Figure 1 is a side view in section of a cathode according to the invention,

Fig. 2 is a side view in section of a cathode according to another embodiment of the invention, and

Fig. 3 is a side view in section of a tubular type cathode according to the invention.

In Fig. 1, reference numeral l designates a porous tungsten cup-shaped member which is obtained by sintering powdery tungsten at a temperature of from 1600 to 2200 C. to form a long rod, from which the cup-shaped body having a wall thickness of 0.5 mm. is turned out. The body I is 5 mm. in diameter and is partly filled with barium-strontium carbonate 2. A molybdenum disc 3, which exactly fits in the cupshaped body I, is provided behind the bariumstrontium carbonate. The cup-shaped body i is provided in the foremost cylindrical cavity of the molybdenum body 4, turned in one piece, in such manner that a minimum number of gaps subsist. For this purpose cup-shaped body i is rigidly pressed against a partition 5 forming part of body 4. A filament 6 is provided in the hindmost cavity of the said body,

The cathode structure shown in Figure 2 comprises a cup-shaped body I which is similar to member I and is of homogeneously-porous sintered tungsten. A cup 8 of molybdenum fits tightly into the open end of body I to form therewith a cavity containing a supply of bariumstrontium carbonate 2. A cup 9 of molybdenum fits tightly over the outside of body I and forms with cup 8 a space in which is located a filament i having a diameter of about 10 mms. and supported by lead-in wires which pass through cap 9 and are insulated therefrom.

The cylindrical cathode structure shown in Fig. 3, which is particularly suited for use in a cavity-resonator magnetron for generating centimeter waves, comprises a tubular homogeneously-porous sintered body H, which similarly to bodies I and 1 is made of tungsten, has a wall thickness of about 0.5 mm. and a diameter of about mms. A cylindrical member [2 having a flanged end fits into the body II and a second cylindrical member I3 having a flanged end fits into the cylindrical member l2, Members I2 and I3 form with body ll an annular cavity in which is located a supply of barium-strontium carbonate 2. Members l2 and I3 are held in position by rings l4 whose edges are flanged over protuberances on the end of body ll so as to form a tight fit. Located within the central cavity formed by cylindrical members l2 and i3 is a filament I5 supported by lead-in wires passing through discs iii of ceramic material secured to cylindrical members I2 and [3.

If the carbonates of the above-described cathodes are dissociated in the usual manner after the cathode has been provided in an electric discharge tube, a great current may be obtained from the cathode surface, constituted by the front surface of I, I or II a short time subsequent to the heating of the cathode to a temperature of about 1200 C.

A continuous load of 1 amp/cm. does not bring about variation in the current-voltage characteristic curve, in contradistinction to cathodes of the conventional type. Provisional tests in which the discharge tubes had to be taken out of use for other reasons (burned filament, shortcircuit between the electrodes) yielded periods of life of 600 hours at 5 amps/cm. and 300 hours at 20 amps/cm. without involving variation in the characteristic curve.

With impulse operation it is possible to obtain pulses of 450 amps/cm." and higher.

The term sintered body as used herein and in the claims is to be understood to mean a body which is formed by heating finely-divided refractory metal at a temperature of at least 1600 C.

While we have described our invention in connection with specific examples and constructions, we do not desire to be limited thereto as modifications will readily present themselves to one skilled in this art.

What we claim is:

1. A cathode comprising a structure forming an internal cavity, the portion of the structure surrounding said cavity consisting of refractory metal, a supply of an alkaline-earth metal compound in said cavity, and a homogeneouslyporous sintered body of refractory metal forming the emissive part of the wall of said structure, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity to the outside of the structure. I

2; A cathode comprising a structure formin an internal cavity, the portion of the structure surrounding said cavity consisting of refractory metal, a supply of alkaline-earth metal oxide in said cavity, and a homogeneously-porous sintered body of refractory metal forming the emissive part of the wall of said structure, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity to the outside of the structure.

3. A cathode comprising a structure forming an internal cavity, the portion of the structure surrounding said cavity consisting of refractory metal, a supply of barium-strontium oxide in said cavity, and a homogeneously-porous sintered body of refractory metal forming the emissive part of the wall of said structure, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity to the outside of the structure.

4. A cathode comprising a structure forming an internal cavity, the portion of the structure surrounding said cavity consisting of metal having a melting point above about 1200 C., a sunply of an alkaline-earth metal compound in said cavity and a homogeneously-porous sintered body of tungsten forming the emissive part of the wall of said structure, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity to the outside of the structure.

5. A cathode comprising a structure forming an internal cavity, the portion of said structure surrounding said cavity consisting of metal having a melting point above about 1200 C., a supply of an alkaline-earth metal compound in said cavity and a homogeneously-porous sintered body of molybdenum forming the emissive part of the wall of said structure, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity of the outside of the structure.

6. A cathode comprising a structure forming an internal cavity, the portion of the structure surrounding said cavity consisting of metal having a melting point above about 1200 C., a supply of an alkaline-earth metal compound in said cavity and a homogeneously-porous sintered body of tantalum forming the emissive part of the wall of said structure, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity to the outside of the structure.

'7. A cathode comprising a structure forming an internal cavity, the portion of the structure surrounding said cavity consisting of metal having a melting point above about 1200 C., a supply of an alkaline-earth metal compound in said cavity and a homogeneously-porous sintered body of hafnium forming the emissive part 01' the wall of said structure, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity to the outside of the structure.

8. A cathode comprising a structure forming an internal cavity, the portion of the structure surrounding said cavity consisting of metal having a melting point above about 1200 C., a supply of an alkaline-earth metal compound in said cavity and a homogeneously-porous sintered body of niobium forming the emissive part of the wall of said structure, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity to the outside of the structure.

9. A cathode comprising a structure including a homogeneously-porous sintered body forming the emissive part of the wall of said structure and composed of refractory metal, and a member having a cup-shaped portion forming an internal cavity with said body, the portion of said structure surrounding said cavity consisting of metal having a melting point above about 1200 C., and a supply of an alkaline-earth metal compound in said cavity, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity to the outside 01 the structure.

10. A cathode comprising a structure including a homogeneously-porous sintered body of tungsten forming the emissive part of the wall of said structure, and a member having a cup-shaped portion of molybdenum forming an internal cavity with said body, the portion of said structure surrounding said cavity consisting of metal having a melting point above about 1200 C., and a supply of an alkaline-earth metal compound in said cavity, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity to the outside of the structure.

11. A cathode comprising a structure including a homogeneously-porous sintered body of tungsten forming the emissive part of the wall of said structure, and a member having a portion of molybdenum forming an internal cavity with said body, the portion of said structure surrounding said cavity consisting of metal having a melting point above about 1200 C., and a supply of barium-strontium oxide in said cavity, said structure being tightly closed with the pores of said body forming the largest apertures connecting the cavity to the outside of the structure.

12. A cathode comprising a structure including a tubular homogeneously-porous sintered body forming the emissive part of the wall of the structure and composed of refractory metal, a tubular member disposed within said sintered body and means closing the ends of said body and member to form an internal cavity, the portion of said structure surrounding said cavity consisting of metal having a melting point above about 1200 C., and-a supply of an alkalineearth metal compound in said cavity, said structure being tightly closed with the pores of said sintered body forming the largest apertures connecting the cavity to the outside of the structure.

13. An incandescible cathode comprising a structure forming an internal cavity, a supply of an electron-emissive material within said cavity, means to heat said material, and a homogeneously-porous sintered body of tungsten forming part of the wall of said body, the pores of said body being the largest apertures connecting the cavity to the outside of the structure and having an inner surface area of about 8,000 to 10,000 square centimeters per cubic centimeter, said tungsten body having a density equal to about of the maximum value.

HENDRICUS JOHANNES LEMMENS. MARINUS 'JACOBUS JANSEN. ROBERT LOOSJES.

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

UNITED STATES PATENTS Number Name Date 2,121,589 Espe June 21, 1988

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