Патент USA US3027489код для вставки
B?Z’i’Ahd "rates arm Federated Mar. 27, l9§2 2 3,-tl27,48i) ELECTRON DléiCHARGE 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) 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 bom bardment effect is so pronounced that appreciable noise is generated within the magnetron tube and such noise This invention relates to temperature-limited cathodes is too great to be tolerated. Although the complete for electron discharge devices which are particularly suit 10 theory of noise generation is not fully understood, it is able for use in magnetrons and other devices wherein the a well known phenomena in space charge limited tubes cathode is subject to back bombardment by either elec and possibly may be related to the interaction between trons or ions, as well as in devices, such ‘as magnetrons, electrons in the space charge cloud and also to unfavor in which noise generation must be held to extremely able interaction between ions and space charge electrons low limits. 15 in the interaction space. This undesirable noise can be Existing cathodes, particularly those containing oxide reduced apperciably by operating the cathode temperature bearing materials, often are unsatisfactory for electron discharge devices, such as magnetrons, wherein electrons limited. With this type of operation, the space charge or ions move in a high frequency electromagnetic ?eld, since the noise level of noise generated in such devices may well exceed acceptable limits. In devices such as emitted by ‘the cathode are able to reach the anode or the those having transverse electric and magnetic ?elds, many e?ect is no longer predominant and most of the electrons vicinity of the anode. The cathode emission now is governed largely by the cathode temperature and is sub stantially independent of the cathode-to~anode voltage. As already mentioned, the back bombardment of the electrons emitted from the cathode return to the cathode; some of these electrons, when impinging upon the cath cathode causes the cathode temperature to become quite ode, cause secondary electrons to be emitted and this 25 high and to vary over a relatively wide range at different process may be repetitive. This phenomena is referred places on the cathode surface. it has been found that, to commonly as back bombardment and is not limited to under such conditions of cathode temperatures, there is a pronounced tendency for the cathode emission to in electron movement, but also may arise from the move crease to the point at which space charge limited oper ment of ions within the tube. It should be noted that, even though magnetrons and other electron discharge 30 ation occurs. This tendency is quite noticeable in the cathodes of the prior art. A certain number of electrons devices are rather highly evacuated, there still may be per second is required for operation in the space charge many gas ions in the tube enclosure. Electrons and ions limited region; this emission must be kept below a certain striking the cathode cause the latter to heat up con value if one is to avoid such operation. The emission siderably and the temperature of the cathode ?uctuates tremeudously——being different at dill'erent portions of the 35 value at which the transition from space charge limited cathode. Indeed, instantaneous spot temperatures Well above the integrated average temperature of the cath operation to temperature limited operation takes place is substantially independent of the cathode composition. ode have been observed owing to the back bombardment However, in cathodes, such as oxide cathodes, the tem perature at which this transition point is reached is rela ration rate of the material to increase considerably, 40 tively low, being of the order of about 800‘ degrees C. thereby proportionately shortening the cathode life. Because of the back bombardment, already mentioned, the magnetron temperature usually is considerably in excess Bombarding ions striking the cathode have relatively high effect. mass Bombardment of tie cathode causes t e evapo tend to remain on the cathode to react with of such a value. Consequently, it is necessary to derive a cathode material which will allow operation at a much the cathode material and cause poisoning of the cathode. In accordance with the invention, the cathode mate rial includes the element rhenium which has a very low secondary emission ratio-of the order of l.l--so that 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 the adverse effects of cathode bombardment are cut down other words, the tendency of the cathode emission to in crease to the point at which space charge limited oper appreciably. The cathode, according to the invention, is composed of a highly refractory material, such as tung 50 ation occurs is substantially reduced by the use of rheni um, since a change in cathode temperature does not pro sten, which carries a composition comprising substantially duce as pronounced an effect upon cathode emission as a 25 percent of tungsten, 25 percent thorium tetraboride, similar change in cathode temperature in the cathodes and 50 percent rhenium, all by weight. These percent or" the prior art. The ability of rhenium to absorb ad ages are for ambient temperature conditions. The com position and crystal structure of the ?nished cathode will 5.” ditional thermal energy without emitting more electrons contributes to this eii’ect and is associated with the fact depend to some extent upon the processing temperature, that the energy bands of rhenium are Widely separated. that is, the temperature at which the powdered mixture Another disadvantage of space charge limited opera is heated. The mixture may be applied to a metal mesh tion, as applied to such devices as magnetrons, is that in the form of a suspension of the rhenium, tungsten and thorium boride in a suitable liquid vehicle, such as nitro 60 the space charge radius increases with anode voltage and the front of the space charge approaches the anode with cellulose. The purpose of the vehicle is to facilitate ap increasing anode-to-cathode voltage. In effect, there plication of the powdered mixture and is such as to fore, the space charge constitutes a virtual cathode of evaporate soon after the suspension has been deposited varying radius. Since the frequency of a magnetron, as upon the mesh. well as its mode spectrum, is a direct function of the ratio In many types of electron discharge devices, space of the cathode radius to the anode radius, and since the charge limited operation of the cathode is feasible. In effective cathode radius varies with the con?guration of other words, for a given operating temperature of the the space charge, the frequency of operation of the mag cathode, the cathode emission increases as the voltage netron and the moding chharacteristics thereof tend to between anode and cathode is increased. However, the space charge cloud in the region adjacent the oath 70 vary with changes in anode-to-cathode voltage. Conse quently, temperature limited cathode operation in mag ode eventually assumes such proportions as to limit the netrons is often highly desirable. The use of a cathode number of electrons capable of reaching the cathode. 3,027,480 3 composition containing rhenium, therefore, contributes to greater frequency stability and mode stability of mag netron-type devices. 4 ing 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 there from, by means (not shown) which are familiar to those skilled in the magnetron art. For example, the support ing cylinder 27 may be attached to a glass bushing which magnetrons, since the oxygen escaping into the region of also is secured to a metal ring, said ring being sealed RF. ?elds causes considerable noise to be generated. hermetically to the pole piece 25. The upper end. of the Furthermore, the thorium evaporates rapidly from such supporting cylinder 27 is enlarged in order to serve as one cathodes, especially when subjected to back bombard ment of electrons and ions. The life of such cathodes, 10 of the cathode end shields. The upper end shield is formed by a tubular member 25‘ which is brazed to the consequently, is relatively short. In the case of cathodes upper end of the cathode sleeve 28. The cathode sleeve which use thorium boride and tungsten only, the rate of 28 is made of a highly refractory material, preferably production of thorium depends upon the rate of diffusion tungsten, which is capable of withstanding high cathode of boron into the tungsten; that is, the rate of diffusion operating temperatures and which has no appreciable of the boron into the tungsten determines the rate of avail chemical effect upon the cathode emissive material which ability of the free thorium. If the rate of dissociation The usual thorium~~bearing cathodes, particularly, thorium oxide cathodes, are unsatisfactory for low noise of thorium is too ‘fast, more than one monomolecular layer of thorium is formed and Van der Waal’s 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 ?ve is deposited thereon. or six monomolecular layers of thorium have been de attached to a metallic insert 33 affixed to the cathode posited 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 sleeve 28 adjacent the upper end of the cathode sleeve. The electron emissive portion 37 of the cathode 13 of dissociation of the thorium boride not be too rapid, con sistent, of course, with sufficient dissociation ‘for proper emission. It has been found that the diffusion rate of boron into 30 rhenium is substantially less than that of boron into tung sten, and, since his rate of diffusion of boron determines the rate of availability of free thorium, the generation of A heater coil 31 is supported within an elongated central bore in the cathode sleeve 28. One end of heater 31 is aii‘ixed to a heater lead-in conductor 32 which passes through the cathode supporting cylinder 27 and external ly of the tube envelope. The other end of heater 31 is 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 re fractory metal powder such as tungsten. The interstices in the wire mesh 35 are ?lled with an electron-emissive material 36 which comprises thorium tetraboride, tungsten free thorium is slower when rhenium is added to the thori um boride and tungsten. The number of monomolecular -'V4 Us and rhenium. The electron-emissive material 36 may be layers ‘of thorium formed on the surface of a rhenium a mixture of comminuted tungsten, thorium tetraboride, based cathode, therefore, is less than the number formed on the surface of tungsten alone. Consequently, Van der Waal’s force is greater, and the tendency for thorium sion may be brushed or sprayed into the cathode mesh and rhenium suspended in a suitable binder; this suspen 35. The binder may, for example, be a nitrocellulose to evaporate from a rhenium-based cathode is reduced. 40 binder or any similar material which evaporates rapidly after the comminuted material has been applied to the This may be stated in another way, namely, that the life mesh. The electron-emissive material 36 need not be ap time of free thorium molecules on a rhenium based cath plied to a mesh, however. For example, the powdered ode is much longer than that of free thorium molecules on mixture referred to above may be compacted to form a a refractory material such as either tungsten or molyb denum alone. 45 tubular element which may be a?ixed to cathode sleeve 28. The cathode next is ?red at an elevated temperature Other features, objects and advantages of the invention to harden the electron-emissive material 36. The tem 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 in corporating 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 perature at which the cathode is processed will determine the exact composition and the electron emissive level of the ?nished cathode. A mixture, by weight, of 25 per cent tungsten, 25 percent thorium tetraboride and 50 percent rhenium, at ambient temperature, has provided extremely quiet operation and long cathode life in con tinuous wave magnetrons. Other ternary compounds of IG. 3 is a fragmentary section view of an electron discharge device showing a typical electron gun assembly 55 tungsten, thorium tetraboride and rhenium may be used; for example, a mixture of 15 percent tungsten, 15 percent according to the invention. thorium tetraboride and 70 percent rhenium has been In the drawings, the reference numeral 10 designates found satisfactory. The amount of tungsten and thorium generally a cavity resonator type of magnetron which bor-ide generally is of about the same order of magnitude, comprises an anode having a plurality of vanes 1.2. A cathode 13 is located with its axis at the center of the anode 60 although it is not essential that this relationship exist. There are, of course, certain limits to the percentage of vanes 12. The magnetron lltl includes a cylindrical outer each material used in the cathode. It is obvious that as wall 14 and a circular top plate 15. A pole piece 16 is the percentage of rhenium approaches 100 percent, the inserted coaxially with the cathode 13 through an open amount of thorium tetraboride available would be so ing in the plate 15 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 support 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 electron emitting material. There must always be su?icient 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 cath ode 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 3,027,480 5 been found that a certain amount of tungsten is required in order to maintain the rate of reaction which forms free thorium su?iciently fast to provide proper emission levels for temperature-limited operation. For effective opera tion under the conditions already mentioned, it is im portant 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 6 5. A cathode composition comprising substantial quan tities 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 70 percent rhenium, by weight, at ambient temperatures. thorium nitride or a thorium sul?de. The percentage of 8. A cathode comprising a refractory metal support the latter thorium compounds used with rhenium and 10 ing member and a coating of mixed particles of tungsten, tungsten is approximately the same as for thorium boride. thorium boride, and rhenium disposed on said member. Although the cathode so far has been described in con 9. A cathode comprising a refractory metal supporting nection with a magnetron, the rhenium based cathode is also of great value in other types of electron discharge member and a coating of mixed particles of tungsten, a devices, such as electron microscopes using ions for focus on said member. sing, X-ray equipment, and other high frequency devices wherein the cathode is subject to considerable bombard ment, 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 insu lated material through which the necessary heater and cathode leads and other electron gun leads, if any, may be brought out of the tube. compound of oxygen-free thorium, and rhenium disposed 10. A cathode comprising a refractory metal support ing member and a coating of mixed particles of tungsten, thorium nitride, and rhenium disposed on said member. 11. A cathode comprising a refractory metal support ing member and a coating of mixed particles of tungsten, thorium sul?de, 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-emis For simplicity, only the 25 sive material permeating said mesh, said material con cathode and heater are shown in FIG. 3. The heater 44 sisting of mixed particles of tungsten, thorium boride, and is located adjacent a generally cup-shaped cathode 45 rhenium. which contains a central aperture 47 into which the elec 13. A cathode for electron discharge devices including tron-emissive material 48 may be inserted. The emissive a refractory metal supporting member, a refractory metal material 48 may be of the same composition as the ma 30 mesh disposed on said member, and an electron-emissive terial 36 previously described in connection with FIGS. 1 and 2. An appropriate heater-to-cathode voltage is sup plied by means of an external source 49. This invention is not limited to the particular details of construction, materials and processes described, as 35 many equivalents will suggest themselves to those skilled in the art. For example, the con?guration and size of the cathode is not limited to that shown in the ?gures. The shape of the cathode will depend upon the particular electron discharge device which utilizes the cathode. For 40 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. 45 What is claimed is: 1. A cathode composition comprising an oxide-free electron-emissive material combined with a substantial amount of rhenium. 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 ?le of this patent UNITED STATES PATENTS 2,447,038 2,467,675 Spencer ____________ __ Aug. 17, 1948 Kurtz ______________ __ Apr. 19, 1949 2,491,866 2,647,216 2,858,207 2,916,653 Kurtz ______________ __ Dec. 20, 1949 Brown ______________ __ July 28, 1953 Warin _______________ __ Oct. 28, 1958 705,199 Great Britain ________ __ Mar. 10, 1954 Macksoud ___________ __ Dec. 8, 1959 FOREIGN PATENTS OTHER REFERENCES 2. A cathode composition consisting principally of 50 Boride Cathodes, by La?erty, .1’. App. Physics, March thorium boride, tungsten and rhenium. 1951, pages 299-309. 3. A cathode composition consisting of substantially Rhenium Metal, Its Properties and Future by Kotes equal amounts of tungsten and thorium boride and a sub in Materials and Methods, March 1954, vol. 39, No. 3, stantial amount of rhenium. page 88. 4. A cathode composition consisting of substantial 65 Rare Metals Handbooks, edited by Hampel, published quantities of thorium boride and rhenium. by Rheinhold Publishing Corp. in 1954.