US2869017A - Thermionic dispenser cathode - Google Patents

Description

Jan. 13, 1959 R. LEVI V THERMIONIC DISPENSER CATHODE Filed Oct. 24, 1956 IN V EN TOR. ROBERTO LEVI mull-mum: in: w

I7 T" "4 7- l l 4 m G 3 N R w E E N w E m 2 B 5 q. E Q MVJIAVIdl/l/ll'll/ll I W 0 U N .0. s I S o m III F 2 2 i a w u United StatesPatent "O THERMIONIC DISPENSER CATHODE Roberto Levi, New. York, N. Y., assignor to North American Philips Company, Inc., a corporation of Delaware Application October 24, 1956', Serial No. 618,072

4 Claims. (Cl. 313-346) This invention relates. to a thermionic cathode and method of making same, and in. particular to a thermionic cathode of the. dispenser type containing a refractorym-metal. body of which a surface constitutes the emitting surface of the cathode.

United States PatentNo..2,700,000 describes a thermionic, dispenser cathodeofthe type comprising a porous, refractory-metal body, for example, of tungsten, which body is impregnatedandits. pores thus filled with an alkaline earth metal composition capable of reacting The refractorypatent functions exceedingly well in many discharge tubes.

\ However, it is not so suitable for a discharge tuberequirwingtheimaintenance of a potential difference between t the heater andmthe cathode during operation. The latter requirement is present notablyin gun constructions for cathode-ray tubes in television apparatus. Specifically, intelevision apparatus, the heater and cathode circuits maybesuch that a substantial potential difference. exists between the heater and cathode, in which the former may t be positive relative to the latter.

- aforementioned patent, it will be appreciated that elec- -trons canemanate fromthe refractory-metal body not onlyfrom its outer surface, which faces the other electrodes, but:also from its inner surface, which faces the heater. evaporate fromthe refractory metal body and deposit on the innersurface of theamolybdenum" support, which latter surface will thenproceed to emit copious electrons. If the heater is at a positive potential, it will collect these t electrons, which phenomenon isknown as heater-cathode- -leakage current. a large degree of heater-cathodevleakage current willconstitute-an additional load on the cathode circuit which would behighly undesirable. Also, the heating filament may be affected bythisback current. As a consequence, thentelevision industrywhas established for electron guns for, cathode-ray tubes a maximum amount of heatercathoderleakage current that can be tolerated in an acceptable cathode. construction. This requirement has not been met by the cathode construction described in the aforementioned patent.

From a perusal of the In addition, emission-enhancing material may It will also: be appreciated that any One object ofthe invention is to provide an improved construction of thermionic dispenser cathode wherein heater-cathode-leakage current is substantially eliminated. Afurther object of the invention is to provide a simple andinexpensivemethod for manufacturing a. thermionic dispenser cathode of the type described, which at the same timeobviates the problem of heater-cathode-lealcage current.

ment of the invention, the refractory-metal body, the

disc-like member, and the support are secured together in a single operation by a fusion process,, which preferably comprises welding together the members atth eir junctions. i

The invention will now bedescribedin greater detail with reference tothe accompanying drawing, in which: Fig. l is a cross-sectional view of oneform of thermionic dispenser cathode according to the invention;

Fig. 2 is a cross-sectional view of Fig. 1 along the line 2-2; i

Fig. 3 is an exploded view showing the elements constituting the cathode of Fig. 1 just prior to assembly;

Fig. 4 is a view partly in cross-section and partly schematic showing a modification ofthe cathode illustrated in Fig. I mounted between the electrodes of a welding generator.

. Referring now to the drawing, Fig. l shows oneform of thermionic dispenser cathode, a planar type, in accordance with the invention. This cathode is particularly suitable for incorporation in an electron gun of a cathoderay tube. The cathode comprises a refractory-metal matrix or body, 10. Thisbody 10 is pereferably constifrom that patent, the pores of the porous body are then .filled with a fused mixture of barium oxide and aluminum oxide in a 5:2 mole ratio. Reference is also made to .another copendingapplication, Serial No. 487,042, filed I February 9, 1955, which describes an improved impregnant, which includes calcium oxide as.an addition constituent, for use in such cathodes. As a further alternafive, the body 10 may be made as described in connection with Fig. 4 of United States Patent No. 2,700,118, which comprises pressing andsintering a mixture of tungsten and barium aluminate in powdered form. In the usual way, upon the application of heat, the alkaline earth materialdispersed throughout thebody 10 reactswith the refractory metal of said body to produce free alkaline earth metalvapor, which diffuses to the surface ofthe body it} to form a monat omic layer thereon. The emitting surface .of the cathode is designated by reference numeral 11, and it is from this surface that the majority of usable electrons areextracted. The accelerating anode of the electrongun (not shown) will be located in front of the emitting surface 11. It will also, be evident that alkaline earth metal also diffuses to the outer, peripheral surface 1210f the refractory-metal body 10,, asw'ell as to the bottomsurface 13 thereof, and hence-electrons may emanate fromthose surfaces also. H

The. cathode. is supported by a hollow, ,tube-likeor sleeve member 15, which ispreferably constitutedof a Suitable materials, for the support 15 are n olybdenurrr, or tantalum, and molybdenum is preferred. Mounted bemember or disc 16 tween thesupport member 15 and the refractory-metal body is a solid, non-planar, in this case slightly-curved, disc-like member 16. This member 16 may also be of molybdenum or tantalum for the same reasons as the support 15, and with the additional restricition that it not react unfavorably with the body 10 containing the ennssive material, and again molybdenum is preferred. As shown, the disc-like member 16 has an outside diameter which is greater than the outside diameter of the support member by a predetermined amount, the purpose of which will be explained later. I

Fig. 3 shows an exploded view of the cathode ofFig. 1 prior to assembly. As will be observed, the disc-like is preferably in the shape of a segment of a sphere ordish-shaped, and is placed on top of the hollow, cylindrical, open-ended, support member 15 with the concave portion of the disc facing upwardly. Thus, the disc 16 closes off one end of the cylinder 15. The disc 16 is, of course, preferably centered on the support 15, though this is not essential since the oversize dimensions of the disc 16 permit some slight misalignment in its position. The refractory-metal body 10 is in turn placed centrally on top of the disc 16. The parts may be held together and aligned in a suitable jig (not shown). Then a pair of welding electrodes 17, shown schematically in phantom, are positioned above and below the assembly, the electrodes 17 urged together, thereby slightly flattening the portions of the disc 16 lying between its periph- 1 cry and its contact area with the refractory-metal body 10 and end of the support 15, a drop of alcohol placed over the assembled elementsto provide a protective atmosphere during the actual formation of the weld and then a pulse of Welding current caused to traverse the assembly. As a consequence, the portions of the disc 16 lying between and contacting the refractory-metal body 10 and the end of the support 15, in this case an annular region, are fused, effecting a strong, solid, welded connection between the three elements in a single operation and resulting in the cathode structure depicted in Fig. 1. The welding pressure and current are not critical in value, and -will, as usual, depend on the size of the elements constituting the cathode. The heater 20 is then mounted in the cylinder 15.

The slight curvature of the disc 16 assists in enabling the three parts to be assembled by a welding process in the manner just described. That is to say, the slight curvature of the disc 16 enables a line contact to be established between the, in this case, flat, refractory-metal body 10 and the disc 16. Under these circumstances, upon the passage of the welding current through the assembly, only the line-contacted portions of the disc 16 are fused, thereby establishing the weld at the area desired. It is also possible to effect a weld with a flat disc 16 and a flat, refractory-metal body 10; however, a largearea surface contact is present, and the current requirements to effect fusion of the disc 16 to the body 10 as described above in a single step would be increased. Further, such a fusion along the entire bottom surface of the impregnated body 10 may cause excess heating of portions thereof, which may be detrimental to the finished cathode. These possible detrimental results may be avoided in the single-step welding technique described above by employing the slightly-curved disc 16 Of course, it will be evident from the foregoing that the same results could be achieved by using a flat disc 16 and a refractory-metal body 10 having a slightly concave-downward underside or even an annular, depending portion of substantially the same diameter as that of the sleeve 15. In both cases, the condition that is fulfilled to ensure satisfactory welding in a single-step in the preferred embodiment of the invention is the maintenance of only a line contact, in these cases circular, between the refractorymetal body 10 and the disc 16.

While the production of the weld in'the manner just described s qulte'reliable, there exists the possiblility 43', that some imperfections may exist in the resultant weld particularly between the underside of the disc 16 and the end of the support 15. These imperfections may consist of small holes allowing communication to exist between the cavity in which the heater 20 resides and the refractory-metal body 10. This may lead to the following difficulty. During operation of the cathode, certain products evaporate from the body 10. These evaporation products include free barium, barium oxide or both. It is known that the emission from a metal surface, such as molybdenum, at the operating temperature of the cathode is very small. But, that emission will be rapidly increased if barium or barium oxide deposits on the molybdenum surface. Therefore, to ensure that no evaporated products from the refractory-metal body 10 will reach the interior of the heater cavity, and thus raise the heatercathode-leakage current above the maximum value prescribed by the television industry, the cathode construction is such that the support 15 defining the heater cavity is spaced from any portion of the body 10 or disc 16 on which may be found a product evaporated from the cathode by at least a small distance, which is preferably of the order of or greater than the migration lengt of the evaporated product on the material concerned at the operating temperature of the cathode. The term migration length stems from the known ability of barium and barium oxide to diffuse or migrate along a hot, metal surface, before being evaporated, at a rate dependent on the composition and condition of the said metal surface, so that the barium and barium oxide will thus travel a certain distance from its area of deposition before being reevaporated. This distance is known as the migration lengt and is an experimentally determined quantity for many materials on various metal surfaces. In particular, for a molybdenum surface at a temperature in the operating range of the cathode, namely 850 to 1100 C., the migration length of barium and barium oxide is about 0.4 mm. Hence, to ensure that the heater cavity remains free of barium and barium oxide, and since access by the latter to the heater cavity is possible in the event of small apertures between the underside of the disc 16 and the edge of the support 15, the edge of the disc 16 is extended beyond the inner surface of the support 15 by a distance of the order of or greater than the aforementioned migration length of the evaporated products of the cathode. During operation, then, the upper surface of the disc 16, to its outer edge, will probably be covered with evaporated products from the refractorymetal of the body 10 reacting with the alkaline earth metal composition with which it is associated, but, since the edge'of the disc 16 is spaced from the support 15 by a distance, indicated by reference numeral 21 in Fig. 1, of the order of or greater than the migration length of the evaporated products on the material of the disc 16 at the temperature of the cathode, then the likelihood of evaporated products entering the heater cavity is substantially reduced to zero, and thus low heater-cathodeleakage current is ensured. To show this with some actual dimensions in a specific example, which is illustrative only, the operating temperature of the cathode was 1000 C. brightness as sighted on the molybdenum with an optical pyrometer, at which temperature the migration length of barium and barium oxide on molybdenum is about 0.016 inch. The refractory-metal body 10 had a diameter of 0.134 inch, and a thickness of 0.040 inch. The outside diameter of the sleeve 15 was also 0.134 inch and the thickness of the wall was 0.009 inch. The disc 16 had a thickness of 0.005 inch and a radius of curvature of 2.47 inches. The outside diameter of the disc was 0.144 inch. Thus, the overhang of the disc 16 was about 0.005 inch. This value together with the thickness of the disc 16 and the thickness of the wall of the sleeve 15 constitutes the length of path between the inner surface of the support 15 facing the heater 20 and the nearest portion of the body 10 or disc 16 from which evaporated products may migrate, and this path length in the specific example is about 0.019 inch, which-value is of the order of the migration length.

For completeness sake, it may be noted that for the dimensions of cathodes set forth above, the welding generator for the welding stepwhich in the usual manner is operated by charging-up a capacitor to a high voltage, and then discharging the capacitor via a step-down transformer and through the electrodes across the assembled cathode elementscontained about 1000 microfarads of capacitance, which was charged up to about 700 volts. The energy available at the electrodes was thus about 245 watt-seconds.

This construction whereby the disc 16 assists in preventing evaporated products from entering the heater cavity offers the additional advantage of enabling a more inexpensive form of support to be employed. Specifically, in the description above, the nature of the support 15 was not clearly indicated, but it obviously could be a simple, drilled rod having a solid wall. Other forms of support are the so-called lapped-seam or butt-seam sleeve. The former is simply a flat sheet of molybdenum, for example, which is wrapped around a mandrel so that the edges overlap one another to form a cylinder. The overlapped edges need not be secured together. This is illustrated in Fig. 2, with the reference numeral 22 indicating the seam where the edges of the sheet overlap. Now, it will be evident that the seam 22 constitutes an-opening in the wall of the support 15 whereby evaporated products from the refractory-metal body 10 may have access to the heater cavity. However, by employing the disc construction 16 shown in Fig. 1, this difiiculty is eliminated, and thus it becomes possible to provide a further reduction in cost of such cathodes, an important factorafi'ording wider use of such form of electron emitters. The lip or overhang of the disc 16 may also serve to locate the emitting surface of the cathode or to support a heat shield.

The preference for maintaining a line contact between the refractory-metal body 10 and the disc 16 has already been explained. As a consequence of this requirement, a space or small cavity is present between the body 10 and the disc 16. This is illustrated in Fig. 4, wherein the support supports the disc 16, and a refractorymetal, porous body 30 is shown defining a closed space or cavity 31 behind the porous body. By filling this space 31 with a mixture of alkaline earth carbonates, a cathode construction is produced as described in United States Patent No. 2,543,728. Such a cathode may be manufactured in a very inexpensive manner by a similar welding technique to that described earlier. That is, the support 15 is placed in position on top of one electrode 33 coupled to a welding generator 34. On top of the support 15, which may be a lapped-seam sleeve, is centered the dish-shaped disc 16. A small quantity of mixed barium and strontium carbonates 31 is then placed in the center of the disc 16, and then a porous body 30, i. e., with empty pores, is placed over the carbonates and forms a circular line contact with the disc 16. Then, a second electrode 35 is placed over the assembly, pressure applied, and a pulse of welding current passed through the assembly. The contacted areas of the disc 16 fuse, and the three metal elements are secured together.

It will be appreciated from the foregoing that the invention has solved an important problem existing in the art, in that a simple and inexpensive technique for the manufacture of such cathodes now exists, and that cathodes made by such method are substantially free of heater-cathode-leakage current. The latter results from the positioning of the solid disc 16 between the emitting, refractory-metal body 10 and the heater 20, thereby preventing electrons or evaporation products from the emitting body 10 from reaching the heater 20 or its cavity. Further, the lip or overhang provided by the oversized disc 16 ensures that, in the event of an opening in the Wall of the support 15, evaporation products from the refractory-metal body 10 will not be able to reach the interior of the heater cavity. Moreover, the welding techniquejust described, which dependsfupon the internal generation of heat by welding current to fuse the refractory-metal members together, does: not appear to affect deleteriously the-refractory-metal body containing the alkalineearth metal composition. This result is somewhat surprising, since the welding current traverses the refractory-metal body while the latter is associated with the alkaline earth metal composition, and the sensitivity of those associated members to liightemper-atures is well known in the art. Finally, the excellent welds that result between the three elements in the single-step welding operation is another important and unexpected result of the invention.

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

What is claimed is:

1. A thermionic dispenser cathode comprising a substantially-cylindrical, hollow, open-ended, support member, a solid, disc-like member welded to and closing off substantially completely one end of said support member, a heater within said support member, a refractory-metal body welded to the outer surface of said disc-like memher, and an alkaline earth metal composition associated with said refractory-metal body and adapted to cooperate therewith when heated to generate electrons, said disc-like member having an outer diameter greater than the outer diameter of said support member and at which the distance between the rim of said disc-like member and the inner surface of said. support member is at least of the order of the migration length of evaporation products from said cooperating alkaline earth metal composition and said refractory-metal body on said disc-like member at the operating temperature of the cathode, whereby said evaporation products are prevented from entering within the support member and thereby increasing the heatercathode-leakage current.

2. A thermionic dispenser cathode comprising a hollow, substantially cylindrical, open-ended, support member constituted of a metal selected from the group consisting of molybdenum and tantalum, a substantially circular, solid, disc-like member welded to and closing of]? sub stantially completely one end of said support member, said disc-like member also being constituted of a metal selected from the group consisting of molybdenum and tantalum, a heater within said support member, a metal body containing pores and consisting principally of tungsten welded to the outer surface of said disc-like member, and an alkaline earth metal composition in the pores of said tungsten body and adapted to cooperate therewith when heated to generate electrons, said disc-like member having an outer diameter greater than the outer diameter of said support member and at which the distance between the rim of said disc-like member and the inner surface of said support member is of the order of the migration length of evaporation products from said cooperating alkaline earth metal composition and said tungsten body on said disc-like member at the operating temperature of the cathode, whereby said evaporation products are prevented from entering within the support member and thereby increasing the heater-cathode-leakage current.

3. A cathode as set forth in claim 2 wherein the area of engagement between said tungsten body and said disclike member is substantially only a line.

4. A thermionic dispenser cathode comprising a hollow, cylindrical, open-ended, seamed, support member, a slightly curved, solid, disc-like member welded to and closing ofi substantially completely one end of said support member, a heater within said support member, a fiat, refractory-metal body forming only a circular line contact with and welded to the outer surface of said disc-like member, and an alkaline earth metal composition associated with said refractory-metal body and adapted to cooperate therewith when heated to generate electrons, said disc-like member having an outer dimension greater than the outer diameter of said support and at which the distance between the rim of said disc-like member and the inner surface of said support member is of the order of themigration length of evaporation products from said cooperating alkaline earth metal composition and said refractory-metal body on said disc-like member at the operating temperature of the cathode, whereby said evaporation products are prevented fromentering within the support member and thereby increasing the heater-cathode-leakage current.

References Cited in the file of this patent UNITED STATES PATENTS Lemmens et a1 Mar. 23, 1954 2,700,000 Levi et a1. Ian. 18, 1955 2,716,716 Hughes et al Aug. 30, 1955 2,737,607 Lemmens et al. Mar. 6, 1956 2,808,531 Katz et a1. Oct. 1, 1957

Claims (1)

1. 2. A THERMIONIC DISPENSER CATHODE COMPRISING A HOLLOW, SUBSTANTIALLY CYLINDRICAL, OPEN-ENDED, SUPPORT MEMBER CONSTITUTED OF A METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM AND TANTALUM, A SUBSTANTIALLY CIRCULAR, SOLID, DISC-LIKE MEMBER WELDED TO AND CLOSING OFF SUBSTANTIALLY COMPLETELY ONE END OF SAID SUPPORT MEMBER, SAID DISC-LIKE MEMBER ALSO BEING CONSTITUTED OF A METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM AND TANTALUM, A HEATER WITHIN SAID SUPPORT MEMBER, A METAL BODY CONTAINING PORES AND CONSISTING PRINCIPALLY OF TUNGSTEN WELDED TO THE OUTER SURFACE OF SAID DISC-LIKE MEMBER, AND AN ALKALINE EARTH METAL COMPOSITION IN THE PORES OF SAID TUNGSTEN BODY AND ADAPTED TO COOPERATE THEREWITH WHEN HEATED TO GENERATE ELECTRONS, SAID DISC-LIKE MEMBER HAVING AN OUTER DIAMETER GREATER THAN THE OUTER DIAMETER OF SAID SUPPORT MEMBER AND AT WHICH THE DISTANCE BETWEEN THE RIM OF SAID DISC-LIKE MEMBER AND THE INNER SURFACE

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