US3027480A - Electron discharge device cathodes - Google Patents

Description

3,-tl27,48i) ELECTRON DliCHARGE DEVICE CATHQDES Raymond ll. Tuinila, Beverly, and William Caithness, West Acton, Mass, assignors to Raytheon Company, a corporation of Delaware Filed Dec. 15, 1958, Ser. No. 780,449 13 Claims. (ill. 313-1il7) This invention relates to temperature-limited cathodes for electron discharge devices which are particularly suitable for use in magnetrons and other devices wherein the cathode is subject to back bombardment by either electrons or ions, as well as in devices, such as magnetrons, in which noise generation must be held to extremely low limits.

Existing cathodes, particularly those containing oxidebearing materials, often are unsatisfactory for electron discharge devices, such as magnetrons, wherein electrons or ions move in a high frequency electromagnetic field, since the noise level of noise generated in such devices may well exceed acceptable limits. In devices such as those having transverse electric and magnetic fields, many electrons emitted from the cathode return to the cathode; some of these electrons, when impinging upon the cathode, cause secondary electrons to be emitted and this process may be repetitive. This phenomena is referred to commonly as back bombardment and is not limited to electron movement, but also may arise from the movement of ions within the tube. It should be noted that, even though magnetrons and other electron discharge devices are rather highly evacuated, there still may be many gas ions in the tube enclosure. Electrons and ions striking the cathode cause the latter to heat up considerably and the temperature of the cathode fluctuates tremeudouslybeing different at dill'erent portions of the cathode. Indeed, instantaneous spot temperatures Well above the integrated average temperature of the cathode have been observed owing to the back bombardment effect. Bombardment of tie cathode causes t e evaporation rate of the material to increase considerably, thereby proportionately shortening the cathode life. Bombarding ions striking the cathode have relatively high mass tend to remain on the cathode to react with the cathode material and cause poisoning of the cathode.

In accordance with the invention, the cathode material includes the element rhenium which has a very low secondary emission ratio-of the order of l.l--so that the adverse effects of cathode bombardment are cut down appreciably. The cathode, according to the invention, is composed of a highly refractory material, such as tungsten, which carries a composition comprising substantially 25 percent of tungsten, 25 percent thorium tetraboride, and 50 percent rhenium, all by weight. These percentages are for ambient temperature conditions. The composition and crystal structure of the finished cathode will depend to some extent upon the processing temperature, that is, the temperature at which the powdered mixture is heated. The mixture may be applied to a metal mesh in the form of a suspension of the rhenium, tungsten and thorium boride in a suitable liquid vehicle, such as nitrocellulose. The purpose of the vehicle is to facilitate application of the powdered mixture and is such as to evaporate soon after the suspension has been deposited upon the mesh.

In many types of electron discharge devices, space charge limited operation of the cathode is feasible. In other words, for a given operating temperature of the cathode, the cathode emission increases as the voltage between anode and cathode is increased. However, the space charge cloud in the region adjacent the oathode eventually assumes such proportions as to limit the number of electrons capable of reaching the cathode.

rates arm Such space charge limited operation is also feasible in some pulse magnetrons in which back bombardment oc curs only during the culse time, provided that the duty cycle is so low that the cathode has an opportunity to cool between pulses. However, for C.W. magnetrons, and in many other hi h frequency devices, the back bombardment effect is so pronounced that appreciable noise is generated within the magnetron tube and such noise is too great to be tolerated. Although the complete theory of noise generation is not fully understood, it is a well known phenomena in space charge limited tubes and possibly may be related to the interaction between electrons in the space charge cloud and also to unfavorable interaction between ions and space charge electrons in the interaction space. This undesirable noise can be reduced apperciably by operating the cathode temperature limited. With this type of operation, the space charge efiect is no longer predominant and most of the electrons emitted by the cathode are able to reach the anode or the vicinity of the anode. The cathode emission now is governed largely by the cathode temperature and is substantially independent of the cathode-to-anode voltage. As already mentioned, the back bombardment of the cathode causes the cathode temperature to become quite high and to vary over a relatively wide range at different places on the cathode surface. it has been found that, under such conditions of cathode temperatures, there is a pronounced tendency for the cathode emission to increase to the point at which space charge limited operation occurs. This tendency is quite noticeable in the cathodes of the prior art. A certain number of electrons per second is required for operation in the space charge limited region; this emission must be kept below a certain value if one is to avoid such operation. The emission value at which the transition from space charge limited operation to temperature limited operation takes place is substantially independent of the cathode composition. However, in cathodes, such as oxide cathodes, the temperature at which this transition point is reached is relatively low, being of the order of about 800 degrees C. Because of the back bombardment, already mentioned, the magnetron temperature usually is considerably in excess of such a value. Consequently, it is necessary to derive a cathode material which will allow operation at a much higher temperature before the above transition point is attained. This transition point may be quite high if rheniurn is included in the cathode composition. In other words, the tendency of the cathode emission to increase to the point at which space charge limited operation occurs is substantially reduced by the use of rhenium, since a change in cathode temperature does not produce as pronounced an effect upon cathode emission as a similar change in cathode temperature in the cathodes or" the prior art. The ability of rhenium to absorb additional thermal energy without emitting more electrons contributes to this eiiect and is associated with the fact that the energy bands of rhenium are Widely separated.

Another disadvantage of space charge limited operation, as applied to such devices as magnetrons, is that the space charge radius increases with anode voltage and the front of the space charge approaches the anode with increasing anode-to-cathode voltage. In effect, therefore, the space charge constitutes a virtual cathode of varying radius. Since the frequency of a magnetron, as well as its mode spectrum, is a direct function of the ratio of the cathode radius to the anode radius, and since the effective cathode radius varies with the configuration of the space charge, the frequency of operation of the magnetron and the moding chharacteristics thereof tend to vary with changes in anode-to-cathode voltage. Consequently, temperature limited cathode operation in magnetrons is often highly desirable. The use of a cathode composition containing rhenium, therefore, contributes to greater frequency stability and mode stability of magnetron-type devices.

The usual thorium--bearing cathodes, particularly, thorium oxide cathodes, are unsatisfactory for low noise magnetrons, since the oxygen escaping into the region of RF. fields causes considerable noise to be generated. Furthermore, the thorium evaporates rapidly from such cathodes, especially when subjected to back bombardment of electrons and ions. The life of such cathodes, consequently, is relatively short. In the case of cathodes which use thorium boride and tungsten only, the rate of production of thorium depends upon the rate of diffusion of boron into the tungsten; that is, the rate of diffusion of the boron into the tungsten determines the rate of availability of the free thorium. If the rate of dissociation of thorium is too fast, more than one monomolecular layer of thorium is formed and Van der Waals force, that is, the intermolecular forces of attraction between the outermost evaporating layer of thorium and tungsten (which decreases as the number of monomolecular layers increases, and vice versa) decreases. After about five or six monomolecular layers of thorium have been deposited upon the surface of the thorium boride-tungsten cathode, the evaporation rate approaches that from a body of pure thorium. It is important, therefore, that the rate of dissociation of the thorium boride not be too rapid, consistent, of course, with sufficient dissociation for proper emission.

It has been found that the diffusion rate of boron into rhenium is substantially less than that of boron into tungsten, and, since his rate of diffusion of boron determines the rate of availability of free thorium, the generation of free thorium is slower when rhenium is added to the thorium boride and tungsten. The number of monomolecular layers of thorium formed on the surface of a rheniumbased cathode, therefore, is less than the number formed on the surface of tungsten alone. Consequently, Van der Waals force is greater, and the tendency for thorium to evaporate from a rhenium-based cathode is reduced. This may be stated in another way, namely, that the lifetime of free thorium molecules on a rhenium based cathode is much longer than that of free thorium molecules on a refractory material such as either tungsten or molybdenum alone.

Other features, objects and advantages of the invention will be better understood from the following description, taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a cross-sectional view of a magnetron incorporating a cathode according to the invention;

FIG. 2 is a view illustrating the detailed construction of the cathode of the magnetron shown in FIG. 1; and

IG. 3 is a fragmentary section view of an electron discharge device showing a typical electron gun assembly according to the invention.

In the drawings, the reference numeral designates generally a cavity resonator type of magnetron which comprises an anode having a plurality of vanes 1.2. A cathode 13 is located with its axis at the center of the anode vanes 12. The magnetron lltl includes a cylindrical outer wall 14 and a circular top plate 15. A pole piece 16 is inserted coaxially with the cathode 13 through an opening in the plate Strapping 18 of one of the well-known types is provided adjacent the upper and lower ends of the vanes 12. The output is obtained from one of the cavity resonators between an adjacent pair of vanes 17, by a coaxial probe 26 comprising an inner conductor 21 terminating in a loop and an outer conductor 22. The magnetron further includes a bottom plate 24 which is hermetically sealed to the cylindrical outer wall 14. A second pole piece 2.5 is inserted in an opening formed at the center of the bottom plate This pole piece 25 contains a central bore for receiving a portion of the cathode assembly 13. The cathode 1.3 includes a supporting cylinder 27 to which one end of a cathode sleeve 28 is attached. The supporting cylinder 27 is supported from the pole piece 25, and electrically insulated therefrom, by means (not shown) which are familiar to those skilled in the magnetron art. For example, the supporting cylinder 27 may be attached to a glass bushing which also is secured to a metal ring, said ring being sealed hermetically to the pole piece 25. The upper end. of the supporting cylinder 27 is enlarged in order to serve as one of the cathode end shields. The upper end shield is formed by a tubular member 25 which is brazed to the upper end of the cathode sleeve 28. The cathode sleeve 28 is made of a highly refractory material, preferably tungsten, which is capable of withstanding high cathode operating temperatures and which has no appreciable chemical effect upon the cathode emissive material which is deposited thereon.

A heater coil 31 is supported within an elongated central bore in the cathode sleeve 28. One end of heater 31 is aiiixed to a heater lead-in conductor 32 which passes through the cathode supporting cylinder 27 and externally of the tube envelope. The other end of heater 31 is attached to a metallic insert 33 affixed to the cathode sleeve 28 adjacent the upper end of the cathode sleeve.

The electron emissive portion 37 of the cathode 13 preferably includes a wire mesh 35 which is made of a highly refractory material, such as tungsten, and having the same general characteristics as mentioned previously in connection with cathode sleeve 28. This wire mesh surrounds the cathode sleeve 28 and is secured thereto, for example, by spot welding or by sintering with a refractory metal powder such as tungsten. The interstices in the wire mesh 35 are filled with an electron-emissive material 36 which comprises thorium tetraboride, tungsten and rhenium. The electron-emissive material 36 may be a mixture of comminuted tungsten, thorium tetraboride, and rhenium suspended in a suitable binder; this suspension may be brushed or sprayed into the cathode mesh 35. The binder may, for example, be a nitrocellulose binder or any similar material which evaporates rapidly after the comminuted material has been applied to the mesh. The electron-emissive material 36 need not be applied to a mesh, however. For example, the powdered mixture referred to above may be compacted to form a tubular element which may be aflixed to cathode sleeve 28. The cathode next is fired at an elevated temperature to harden the electron-emissive material 36. The temperature at which the cathode is processed will determine the exact composition and the electron emissive level of the finished cathode. A mixture, by weight, of 25 percent tungsten, 25 percent thorium tetraboride and 50 percent rhenium, at ambient temperature, has provided extremely quiet operation and long cathode life in continuous wave magnetrons. Other ternary compounds of tungsten, thorium tetraboride and rhenium may be used; for example, a mixture of 15 percent tungsten, 15 percent thorium tetraboride and percent rhenium has been found satisfactory. The amount of tungsten and thorium bor-ide generally is of about the same order of magnitude, although it is not essential that this relationship exist. There are, of course, certain limits to the percentage of each material used in the cathode. It is obvious that as the percentage of rhenium approaches percent, the amount of thorium tetraboride available would be so small as to reduce the electron-emissive thorium to a value below that required for adequate emission; it should be noted that rhenium alone is not a good electronemitting material. There must always be suflicient thorium tetraboride present in the mixture to provide a minimum electron emission. On the other hand, if the supply of rhenium is too limited, say below about 15 percent, the advantages accruing to a rhenium-based cathode begin to disappear, that is, the advantages of low secondary emission ratio, the relatively slow rate at which rhenium is released from the cathode body, etc. It has been found that a certain amount of tungsten is required in order to maintain the rate of reaction which forms free thorium sufiiciently fast to provide proper emission levels for temperature-limited operation. For effective operation under the conditions already mentioned, it is important that the cathode material be substantially free of oxygen. The thorium boride may be replaced by a thorium compound not comprising oxygen, such as a thorium nitride or a thorium sulfide. The percentage of the latter thorium compounds used with rhenium and tungsten is approximately the same as for thorium boride.

Although the cathode so far has been described in connection with a magnetron, the rhenium based cathode is also of great value in other types of electron discharge devices, such as electron microscopes using ions for focussing, X-ray equipment, and other high frequency devices wherein the cathode is subject to considerable bombardment, either by ions or by electrons. A typical cathode for devices of this type is shown in FIG. 3, wherein only a portion of a tube envelope is shown. The tube envelope may consist of an elongated metallic portion 41 and an end portion 42 made of glass or other electrically insulated material through which the necessary heater and cathode leads and other electron gun leads, if any, may be brought out of the tube. For simplicity, only the cathode and heater are shown in FIG. 3. The heater 44 is located adjacent a generally cup-shaped cathode 45 which contains a central aperture 47 into which the electron-emissive material 48 may be inserted. The emissive material 48 may be of the same composition as the material 36 previously described in connection with FIGS. 1 and 2. An appropriate heater-to-cathode voltage is supplied by means of an external source 49.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. For example, the configuration and size of the cathode is not limited to that shown in the figures. The shape of the cathode will depend upon the particular electron discharge device which utilizes the cathode. For X-ray tubes, the cathode would be much larger and heavier than a cathode used in a small magnetron, for example. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A cathode composition comprising an oxide-free electron-emissive material combined with a substantial amount of rhenium.

2. A cathode composition consisting principally of thorium boride, tungsten and rhenium.

3. A cathode composition consisting of substantially equal amounts of tungsten and thorium boride and a substantial amount of rhenium.

4. A cathode composition consisting of substantial quantities of thorium boride and rhenium.

5. A cathode composition comprising substantial quantities of rhenium and oxide-free thorium.

6. A cathode composition consisting of approximately 25 percent thorium boride and 50 percent rhenium, by weight, at ambient temperatures.

7. A cathode composition consisting of approximately 15 percent tungsten, 15 percent thorium boride and percent rhenium, by weight, at ambient temperatures.

8. A cathode comprising a refractory metal supporting member and a coating of mixed particles of tungsten, thorium boride, and rhenium disposed on said member.

9. A cathode comprising a refractory metal supporting member and a coating of mixed particles of tungsten, a compound of oxygen-free thorium, and rhenium disposed on said member.

10. A cathode comprising a refractory metal supporting member and a coating of mixed particles of tungsten, thorium nitride, and rhenium disposed on said member.

11. A cathode comprising a refractory metal supporting member and a coating of mixed particles of tungsten, thorium sulfide, and rhenium disposed on said member.

12. A cathode for electron discharge devices including a refractory metal supporting member, a refractory metal mesh disposed on said member, and an electron-emissive material permeating said mesh, said material consisting of mixed particles of tungsten, thorium boride, and rhenium.

13. A cathode for electron discharge devices including a refractory metal supporting member, a refractory metal mesh disposed on said member, and an electron-emissive material permeating said mesh, said material consisting of approximately 25 percent tungsten, 25 percent thorium boride, and 50 percent rhenium, by weight, at ambient temperatures.

References Cited in the file of this patent UNITED STATES PATENTS 2,447,038 Spencer Aug. 17, 1948 2,467,675 Kurtz Apr. 19, 1949 2,491,866 Kurtz Dec. 20, 1949 2,647,216 Brown July 28, 1953 2,858,207 Warin Oct. 28, 1958 2,916,653 Macksoud Dec. 8, 1959 FOREIGN PATENTS 705,199 Great Britain Mar. 10, 1954 OTHER REFERENCES Boride Cathodes, by Lafierty, .1. App. Physics, March 1951, pages 299-309.

Rhenium Metal, Its Properties and Future by Kotes in Materials and Methods, March 1954, vol. 39, No. 3, page 88.

Rare Metals Handbooks, edited by Hampel, published by Rheinhold Publishing Corp. in 1954.

Claims (1)

1. A CATHODE COMPOSITION COMPRISING AN OXIDE-FREE ELECTRON-EMISSIVE MATERIAL COMBINED WITH A SUBSTANTIAL AMOUNT OF RHENIUM.

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