US2895070A

US2895070A - Thermionic cathode

Info

Publication number: US2895070A
Application number: US530051A
Authority: US
Inventors: George A Espersen
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1955-08-23
Filing date: 1955-08-23
Publication date: 1959-07-14
Anticipated expiration: 1976-07-14
Also published as: BE550480A; CH344140A; DE1043520B; FR1178419A; NL96647C; GB791495A

Description

July 14, 1959 G. A. ESPERSEN THERMIONIC CATHODE Filed Aug. 25, 1955 Fl .2 g

INVENTOR GEORGE A. ESPERSEN THERMIONIC CATHODE George A. Espersen, Dobbs Ferry, N.Y., assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Application August 23, 1955, Serial No. 530,051 7 Claims. Cl. 313-346) This invention relates to thermionic cathodes, and in particular to thermionic cathodes producing well-defined electron beams.

In certain electron discharge tubes, it is essential that the electron beam emanating from the cathode have sharply defined boundaries. Typical applications for such a beam are'the so-called travelling-wave and klystron tubes. In both cases, the electron beam is projected through the center of a structure in which electromagneticwaves' are established. A hollow or annular electronbeam with defined outer and inner boundaries is preferred, primarily because only the electrons along the outside of the beam interact with the electromagnetic waves, whereas any electrons along the center produce no usable interaction. For other applications, such as X-ray tubes for diffraction work, a point source of electrons may be required, which would necessitate a very small, defined source. Thus, it (will be evident that there is a need in art for a beam source in which the dimensions or shape of the beam may be accurately defined.

It is also to be noted that in many of these'applications, high beam intensities have also been desired, as well as substantially non-destructive cathodes- Thus, the .growth of the art has been in the direction of the so-called dispenser cathode, as described in US. Patents Nos. 2,543,728, 2,700,000, and 2,700,118. In these three patents, a porous member containing tungsten is employed, of which one surface serves as the emitting surface. .The porous tungsten member is supported or closed olf, as the case may be, by a refractory, e.g. molybdenum, member. In contact with the tungsten member is a supply of emission-enhancing material, such as barium aluminate, or more specificallyBaO-Al O in a 5:2 ratio. The generation of heat from a suitable heater frees barium and barium oxide from the aluminate and causes them to diffuse through the porous member until they deposit as a monomolecular layer on the metallic emitting surface of the porous member. 'The net result is the formation of .a barium on oxygen on tungsten emitting surface, which has a very-low work function, viz., 1.8 electron volts, and thus constitutes an excellent primary source of electrons. a However, in operation, it has been found that barium or barium oxide or both from the emitting surface tends tomigrate to adjacent surfaces, in particular, the molybdenum tube or cap supporting or closing off the cathode. It has further been found that the presence of barium States Patent m ce beam. The consequence is that these cathodes do not give satisfactory results in those tube applications in which well-defined electron beams are important. The chief object of the invention is to provide dispensertype cathodes of the kind described above which are capable of producing very well-defined beams of electrons. The invention is based upon the following new concepts. As indicated above, the problem in connection with the prior art cathodes stems from the fact that the generated barium or its oxide migrates from the desired area of emission on the porous tungsten to the adjacent molybdenum member, in which case the work function of the molybdenum is reduced and its electron emission correspondingly enhanced. It has been found that by employing pure, solid, zirconium metal at the areas of the cathode which are to be non-emitting, no emission results at those areas. Moreover, investigation of thisunusual result produced the startling fact that the work function of the tungsten emitting surface remained at 1.8 electron volts, indicating a barium on oxygen on tungsten emitter, whereas the work function of the zirconium member was 4.12 electron volts, indicating the Work function of pure zirconium metal. It has therefore been established that, for reasons not yet known, the barium or its oxide apparently fails to stick or adhere to the zirconium metal surface, with the consequence that the work function of pure zirconium is obtained. Inasmuch as the two work functions of the adjacent, contiguous surface of the cathode are so vastly different, the resultant emission is also quite different. In particular, the emission of the tungsten portion remains at the usual high levels associated with or its oxide on molybdenum reduces the work function such a material in cathodes of this type, Whereas the emission of the zirconium surface at the usual temperatures of the cathode is essentially zero. Thus, by coating the pot-- tions of the cathode from which no emission is desired with pure, solid, zirconium metal, the object of the invention is realized in a simple yet highly eflicacious construction. v

The invention will now be described in connection with the accompanying drawing wherein:

Fig. 1 is a cross-sectional view of one form of cathode construction in accordance with the invention;

Fig. 2 is a cross-sectional view of another form of construction in accordance with the invention;

Fig. 3 is a cross-sectional view of a modification of the cathode shown in Fig. l.

Referring now to Fig. 1, there is disclosed a cylindrical, planar, thermionic cathode comprising a support body '10 of, for example, molybdenum. A tungsten member 11 is mounted at one end of the support 10. In accordance with the teachings of US. Patent No. 2,700,000, the tungsten member 11 may comprise a porous body with interconnected pores impregnated with a fused mix ture of barium oxide and aluminum oxide in a 5:2 ratio. Alternatively, in accordance with the teachings of US. Patent No. 2,700,118, the member 11 could comprise a porous, pressed and sintered body of intimately mixed powders of tungsten and barium aluminate. In the latter case, further, an alloy of tungsten and molybdenum may be employed in place of the tungsten alone. In all cases, however, Whether the porous member 11 is constituted wholly of tungsten or of tungsten with another refractory metal, on the outer surface of the porous member 11 is established a monomolecular layer of barium or its oxide when an adjacent heater member 13 is suitably energized. The consequence of this is the production of a barium on oxygen on tungsten emitter at the area 12, from which a copious supply of electrons may be derived based upon the low Work function of such emitting surfaces. It is naturally assumed that an electrode at a positive potential is located in front of the cathode in order to attract I electrons from t e latter.

As will be noted, the emitting area 12 forms a slightly higher or raised center section 14 on the tungsten member 11. Surrounding that higher section 14 is a solid, pure, zirconium metal, annular member or washer 15. The zirconium washer 15 is tightly secured to the tungsten member and abuts both the underlying tungsten as well as the tungsten of the center section 14. The outer surface 16 of the zirconium member 15 and the surface 12 of the tungsten member 11 form contiguous, contacting areas. The two members may be secured together by spot welding along their abutting surfaces. To prevent the formation of an oxide layer on the zirconium mem her, the spot welding is preferably carried out in vacuum. It will also be noted that the edge of the molybdenum support member is peened over the adjacent edge of the tungsten and zirconium members to hold them securely in place.

Dispenser cathodes of this general type operate at temperatures between 800 and 1350 C. When in that temperature range, barium or its oxide or both is generated or formed in the interior of the tungsten member 11 and diffuses through the pores therein to form a monomolecular layer on the metallic emitting surface 12. Barium or its oxide formed underneath the zirconium washer cannot traverse the solid, i.e., impervious, washer 15. However, since the zirconium 15 is also at the same or almost the same elevated temperature as the tungsten, barium or its oxide from the emitting surface 12 would tend to migrate over onto the zirconium member. It has been found, however, that the barium or its oxide does not stick or adhere to the zirconium surface, but that it does stick and adhere to the surface containing the tungsten, so that the emitting area 12 bounded by the zirconium functions exceptionally well as a source of electrons, but the adjacent surface 16 of the zirconium is, on the other hand, practically a non-emitter. The reason for this strange behavior of the zirconium toward the barium, contrasted to the completely opposite behav ior of the adjacent tungsten, is not yet understood. However, the facts are certain. The surface of an operating cathode actually constructed similar to that illustrated in Fig. 1 was electron-optically enlarged onto the luminous face of a cathode-ray tube, whereupon it was visually observed that only the tungsten, and not the adjacent zirconium, produced electrons. In the latter case, the diameter of the emitting area 12 was 1 mm.; the overall diameter of the zirconium washer 15 was 3 mm. and its inside diameter was, of course, 1 mm. Its thickness, for illustrative purposes only, was 0.1 mm. It is further to be noted that had the zirconium member been molybdenum, then the results indicated would not have been obtained. In the latter case, the portions of the molybdenum adjacent the tungsten would have produced almost as much emission as the tungsten itself, in which case, the beam would not be well defined as required. For best results, the zirconium metal member is crystalline and has a smooth, clean, and bright or polished surface, that is to say, a surface free of crevices and irregularities in which migrating barium or barium oxide may be trapped and held in position.

Another point that deserves mention is the Width of the zirconium member. That is, the dimension of the zirconium member that defines the spacing between the exposed tungsten portion and the molybdenum support. This dimension is indicated by 17 in the figure. The molybdenum support is a non-emitter so long as it is maintained free of barium or its oxide. Thus, if the zirconium non-emitting area were relatively thin, it is conceivable that barium could migrate across it and reach the molybdenum support. Hence, both the center area 12 and the periphery of the cathode would emit, though separated by a non-emitting area 16. To obviate this, the zirconium non-emitting area should have a width 17 of at least about 0.1 mm., to thus prevent migrating barium or its oxide from reaching the non-zirconium 4% portions of the cathode which are not to emit. For practical reasons, the width 17 should be chosen considerably in excess of this lower limit.

The construction illustrated in Fig. 1 produces a solid cylindrical beam of electrons whose outer surface is accurately defined by the location of the zirconium metal. It will also be realized that this construction is capable of producing a point source of electrons by simply reducing the diameter of the emitting area 12 to that of a point or to the value desired. Alternatively, to further sharpen the beam, the section 14 can be shaped into a conical form with an apex at the top and completely surrounded and abutted, except for the very tip, .by the zirconium washer 15. Thus, emission from this latter construction will be confined to the exposed tip or apex. This construction is illustrated in Fig. 3.

Fig. 2 illustrates a cylindrical, planar cathode of the type disclosed in US. Patent No. 2,543,728 and producing a hollow or annularelectron beam. In this construction, a tungsten member 21 is supported on a molybdenum tube 20. The tungsten member '21 is porous with interconnected pores. A cavity 22 is formed in the tungsten member 21 and sealed off by a molybdenum plate 23 welded thereto. In the cavity 22 is disposed an electron-enhancing material 24, such as a mixture of barium and strontium carbonates. A heater 25 supplies heat to operate the cathode. This heat frees barium or its oxide or both from the cavity 22 and causes it to diffuse upwardly through the pores of the tungsten member 21 until it deposits on the outer surface 26 of the tungsten 21 to form a monomolecular layer thereon, thereby producing an area of high emissivity. The center of the tungsten member 21 is recessed, and seated in said recess is a pure, solid metal, zirconium disc 28. The exposed surface of the zirconium disc 28 is shown flush with the adjacent surface of the tungsten, but this need not be the case, and the zirconium could also -pro ject above the surface of the tungsten. Surrounding the outer periphery of the tungsten member 21 and secured thereto is a solid metal, zirconium band 29. Both zirconium members are preferably spot-welded, in vacuum, to the tungsten and thus are in intimate and secure contact therewith.

From the foregoing, it will be evident why the construction illustrated in Fig. 2 will produce a hollow or annular beam of electrons. Though barium .or its oxide from the emitting area 26 will tend to migrate onto the center and outer zirconium surfaces 28, 29, it has been found that the barium or its oxide will not adhere thereto, so that the work function of the pure metal zirconium remains very high. Thus, only the exposed porous tungsten portions 26 lying between the two zirconium surfaces on the cathode will emit electrons, and the resultant annular beam will have very accuartely located inner and outer boundaries in accordance with the invention. While the invention has been described in connection with cathodes of the cylindrical, planar type, it will be understood that it is equally applicable to other cathode constructions of various configurations, such as a cylindrical cathode, and capable of producing different forms of defined beams, such as a line source.

It is to be emphasized that the invention herein does not reside in merely selecting any material from the many available in the art for the non-emitting areas of the cathode, which material will have low emission by reason of a lower operating temperature or by combining with deposited cathode materials to produce higher workfunction surfaces, or in any of the other conventional ways for minimizing primary emission. The choice of available materials which can be employed with these cathodes is severely limited. For example, the material must have a very low vapor pressure at the temperatures involved to prevent the tube vacuum from being impaired. Further the material must not enter into deleterious reactions with the other cathode materials orthe emission-enhancing barium compound lest the cathode be poisoned and its emissivity destroyed. Finally, the material utilized must do more than merely reduce emission from'the undesired areas, because even low emission from the molybdenum support might render the cathode unusable for certain applications. The invention herein, on the other hand, resides in the discovery that, despite their similarities, when tungsten and zirconium surfaces on a dispenser cathode are contiguous and approximately at the same temperature, migrating barium or its oxide will adhere in the form of a mono molecular layer only to the tungsten, producing a highly satisfactory, effective emitting area, but will not adhere to the adjacent, pure, solid, zirconium metal surface, producing not a reduced, but essentially a nonemitting area. Thus, the location of the zirconium on or adjacent the tungsten accurately defines the boundaries of the resultant electron beam. Further, the pure zirconium meta'l exhibits all of the other properties which it must possess to enable it to be employed with dispenser cathodes of this type without any harm resulting therefrom either to the cathode itself or to a tube in which it might be employed.

Considered from a different point of view, the operation of these types of dispenser cathodes may be viewed as follows. Tungsten or tungsten alloy in the porous member reacts with the emission-enhancing material, i.e., the barium containing compound, to produce free barium or its oxide or both, which diffuses through the pores of the porous member and settles on any exposed, outer surface thereof. In a sense, then, the porous member containing the tungsten may be considered a generator of free barium or barium oxide. The desired results of the invention are achieved so long as the barium or its oxide is confined to the porous tungsten member. However, when the barium migrates to other portions of the cathode, such as the support, or an end cap, which are usually of molybdenum, electrons are produced from other than the desired emitting area. The purpose, therefore, of the zirconium is to provide a nonemitting surface which separates the desired tungsten emitting area from other refractory, usually molybdenum, and possibly emitting, portions of the cathode, and which prevents the migration of barium over its surface, since the barium fails to adhere to it, to the refractory portions of the cathode, and thus prevents any emission from other than an exposed surface portion of the porous member containing the tungsten. These results are obtainable in the cathode constructions of the invention operated in the temperature range of 800 to 1350 C.

Throughout the specification and claims, wherever reference is had to barium and barium oxide or barium or barium oxide or both or barium or barium oxide, what is intended to be included in these three terms is either the pure barium metal, or the barium oxide or both. It is impossible to be more definite than this because the exact role played by barium or its oxide in the formation of a barium-on oxygen-on tungsten emitter has not yet been ascertained. All that is presently known is that these types of cathodes generally give off both these materials during operation, and either or both are capable of enhancing the emission of a tungsten or tungsten-containing surface. It is also to be understood that where the emission-enhancing material includes a strontium compound, then the strontium or its oxide is in all probability-one cannot be more definite than this-serving the same role as the barium or its oxide in enhancing the emission of a tungsten or other refractory surface. Hence, for these cathodes, the same disadvantages exist with the strontium as exist with the barium. And, the pure, solid, zirconium metal memher is operative with strontium to the same extent as with barium within the scope of this invention.

While I have thus described my invention with specific examples and embodiments thereof, other modifications will be readily apparent to those skilled inthe art without departing from the spirit and scope of the invention as defined in the appended claims.

What I claim is:

l. A thermionic dispenser cathode comprising an exposed, tungsten-containing, porous portion, a zirconium metal surface portion abutting said portion containing tungsten, a barium-containing material in reactive relationship with said portion containing tungsten, and means for heating said cathode to a temperature in the range between 800" C. and 1350 C. and at which barium and its oxide is generated and migrates to and covers the tungsten portion and tends to cover the adjacent portions of the zirconium portion, the thermionic electron emission at said temperature from the portion containing tungsten being substantially high, and the thermionic electron emission from the zirconium portion at substantially the same temperature being extremely low.

2. A thermionic cathode comprising a porous tungsten portion, a solid zirconium metal member abutting and welded to said tungsten portion and overlying all but a predetermined exposed area of said tungsten portion, a barium-containing material dispersed throughout said tungsten portion, and means for heating said cathode to a temperature at which barium and its oxide is generated and migrates to and covers the tungsten portion and tends to cover the adjacent portions of the zirconium member, said temperature having a value between 800 and 1350 0., whereby the thermionic electron emission from the exposed tungsten portion at said temperature is very high and that from the zirconium portion at substantially the same temperature is essentially zero.

3. A thermionic dispenser cathode comprising a porous member containing tungsten, a supply of barium-containing electron-emission-enhancing material capable of reacting with the porous member to generate free barium and barium oxide in contact with said porous member, a refractory member secured to said porous member, said porous member and said refractory member both having their work functions markedly reduced when barium and its oxide deposits thereon, an exposed layer of zirconium metal of at least 0.1 mm. wide abutting and disposed between portions of said porous member and all of said refractory member to prevent barium and its oxide generated at said porous member from migrating to and depositing on said refractory member to thereby confine the emission from said cathode to a predetermined area of said porous member, and heating means for heating said cathode to a temperature in the range of 800 C. to 1350 C. and at which free barium and barium oxide is generated.

4. A cathode as set forth in claim 3 wherein the refractory member is molybdenum.

5. A thermionic dispenser cathode comprising a porous tungsten portion having a raised section on one of its surfaces, a barium containing material in reactive relationship with said porous portion, a zirconium metal member abutting said tungsten portion and surrounding said raised section and overlying the immediate area of said one surface adjacent said raised section leaving only the outer surface of said raised section exposed, a refractory support member supporting said porous portion and zirconium member but spaced from the raised section, and means for heating the cathode to an elevated temperature in the range of 800 C. to 1350 C. and at which barium and barium oxide is generated and migrates to the tungsten portion and zirconium member, whereby emission from the cathode is confined to the exposed surface of said raised section of said porous portion.

6. A thermonic dispenser cathode comprising a porous tungsten member having a recess in one of its surfaces, a barium-containing material associated with said porous member, a zirconium metal member seated in said res cess and completely covering said entire recess and abutting said porous member, a refractory support member for said porous portion, and means for'heating the oathode to an elevated temperature in the r'angeof'SOO" C. to 1350f C, and at which barium and. barium oxide is generated and migrates to the tungsten and metal memhers,vwhereby emission is confined to the surface of said porous member not covered by the Zirconium member. 7. Acathode as set forth in claim 5 producing a substantially point source of electrons, wherein the raised section of the porous tungsten portion has a substantially conical form whose apex is at a portion thereof v 8 v more remote from said one surface of said porous tungstenr portion, said apex portion being exposedi'andconstitutingsubstantially the sole area of thecathode from which electrons may be derived.

References Cited the file of this patent UNITED STATES PATENTS Germany Apr, 29 19 54

Claims

1. A THERMIONIC DISPENSER CATHODE COMPRISING AN EXPOSED, TUNGSTEN-CONTAINING, POROUS PORTION, A ZIRCONIUM METAL SURFACE PORTION ABUTTING SAID PORTION CONTAINING TUNGSTEN, A BARIUM-CONTAINING MATERIAL IN REACTIVE RELATIONSHIP WITH SAID PORTION CONTAINING TUNGSTEN, AND MEANS FOR HEATING SAID CATHODE TO A TEMPERATURE IN THE RANGE BETWEEN 800*C. AND 1350*C. AND AT WHICH BARIUM AND ITS OXIDE IS GENERATED AND MIGRATES TO AND COVERS THE TUNGSTEN PORTION AND TENDS TO COVER THE ADJACENT PORTION OF THE ZIRCONIUN PORTION, THE THERMIONIC ELECTRON EMISSION AT SAID TEMPERATURE FROM THE PORTION CONTAINING TUNGSTEN BEING SUBSTANTIALLY HIGH, AND THE THERMIONIC ELECTRONE EMISSION FROM THE ZIRCONIUM PORTION AT SUBSTANTIALLY THE SAME TEMPERATURE BEING EXTREMELY LOW.