US2879432A - Electron emitter - Google Patents

Description

March 24, 1959 M. J. SLIVKA ELECTRON EMITTER Filed March 16, 1956 INVENTORI MARION J. SLIVKA,

HIS A ORN.

United States ELECTRON EMITTER Marion J. Slivka, Schenectady, N .Y., assignor to General Electric Company, a corporation of New York This invention relates to thermionic electron emitters, and more particularly to an improved thorium-activated thermionic emitter particularly suitable for use in high power electron tubes.

Directly heated thorium-activated thermionic emitters of filamentary form and comprising tungsten wire impregnated with thoria (thorium oxide) are well known in the art. Such directly heated filamentary emitters have certain limitations from both an electrical and mechanical standpoint however. To meet new requirements in electron tube designs, such as operation at higher frequencies and higher powers, considerable effort has been expended to develop thorium activated emitters of the indirectly heated type and preferably of a structural form other than filamentary. For example, emitters have been developed consisting of refractory metals such as tantalum or tungsten of various structural forms, and having the thorium oxide activator material applied thereto in the form of a coating. Also pure thorium oxide and thorium oxide-impregnated cermet (i. e. mixed metal and ceramic) materials have been tried. However, various difficulties have been encountered in connection with the formation, processing, operation and life of such structures. For example, it has been difficult to reproduce reliable structures of good mechanical strength, ease of assembly, uniform emission, and freedom from gasiness. Also many such emitters have been found to have an undesirably high ratio of heater power to emitter current.

In another approach, emitters have been made consisting of thoriated tungsten wire closely wound on a supporting sleeve of tungsten or other suitable refractory material, the overwinding then being ground down to produce a structural form simulating a cylinder. Such a fabrication technique is extremely ditficult and costly, however, and unless extreme care is exercised small voids are likely to be formed in the outer surface of such emitters as a result of slight gaps between turns of the wire winding. Hence such emitters are not practical from an economical standpoint.

In still another approach, attempts have been made to introduce thoria into pure tungsten prefabricated to the desired structural form, e.g. a cylinder, by coating the tungsten with a suitable compound, such as thorium hydride, adapted to decompose when heated and yield thorium metal. The coated tungsten is then heated sufficiently, e.g. to a brightness temperature of 2100 C. to decompose the compound and with the purpose of causing at least some of the thorium to diffuse into the base metal and there be oxidized. Emitters made according to this technique have several drawbacks, however. First, they inherently suffer from the limitation that the requisite heating produces substantial grain growth in the tungsten before sufficient thorium oxide can be formed atent O within it to inhibit such grain growth. The resulting comparatively large tungsten grains inhibit the diffusion of thorium to the emitter surface, which diffusion is recognized by those skilled in the art to be vital to efiicient emitter operation. Also oxidation of thorium diffused beneath the surface of the base metal is inherently difficult to achieve, and hence the amount and degree of evenness of distribution of thorium oxide beneath the surface of such emitters is inherently limited. Finally, the degree of heating required to produce any oxide from thorium diffused into the tungsten is inevitably likely, to produce deleterious oxidation of the tungsten itself, a result which of course renders it practically useless as an emitter.

Hence it will be apparent that while numerous attempts have been made heretofore, a satisfactory emitter has not yet been developed having the desirable electrical properties of abundant emission and long emissive life, and the desirable mechanical properties of ease and reasonable,

cost of fabrication, and high mechanical strength under operating conditions.

Accordingly, one object of the present invention is to provide an improved thorium-activated thermionic emitter having a long, dependable life, a high level of emission, and an improved ratio of emitted current to heater power.

Another object is to provide an improved indirectly heated or unipotential cathode emitter which can be easily and inexpensively fabricated.

Another object is to provide a thorium activated emitter having improved resistance to deterioration at high frequencies and high powers relative to emitters heretofore available.

Another object is to provide a thorium activated thermionic emitter which is characterized by increased mechanical strength, reduced shock sensitivity and ease of assembly, and which has improved adaptability particularly for electron tubes of the type having extremely close inter-electrode spacing and high power dissipation.

These and other objects of the invention will be apparent from the following description, and the scope of the invention will be defined in the appended claims.

Briefly, the present invention consists in the provision of. a thorium activated thermionic emitter made up of emissive material in sheet form wrought from premixed and a sintered powdered tungsten and thoria. Thissheet material comprises thoriated tungsten in which the tungsten is of granular or crystalline form with small grain size of the order of 15,000 grains per square millimeter, and the thoria is finely divided and uniformly distributed in the grain boundary regions or intercrystalline spaces of Pref- 1 the tungsten in the ratio of up to 4% by weight. erably the emitter is made by first forming a sheetof thoriated tungsten material of the character described, cutting a blank from this sheet, forming the blank to the desired shape, and then if desired carburizing at least a portion of the emissive area of the formed blank. The resulting emitter has the advantage that the thoria therea in, being locked or stored in uniformly distributed relation in the interstices between the tungsten grains, is afforded optimum protection or shielding from dissociation by deleterious external forces such as bombardment of the emitter by external electrons.

to replenish the monolayer of thorium at the emitter surface from which the electrons are actually emitted. Thus,

from an electrical standpoint an emitter constructed in accordance with the present invention has the inherent capacity for both abundant emission and long life, while its sheet form provides from a mechanical standpoint the desirable attributes of relatively easy fabrication and high strength. It will be understood from the foregoing that the term wrought is not used in the present application in its general sense in which. it simply means, for example, fashioned or formed but instead is used in its metal-working sense wherein it means Work hardened, hammered or beaten into shape by tools to improve the mechanical characteristics of the material.

In the accompanying drawing Figure l is a perspective view of one form of emitter constructed in accordance with the present invention; and

Figure 2 is a sectional view of one form of cathode employing an emitter constructed in accordance with the present invention.

Referring now to the drawing the cathode shown includes an emitter body 2 of cylindrical shape consisting of a single piece of sheet material rolled into cylindrical form. Within the body 2 is a double helical filamentary heater 4, the heater leads 6 being brought out through one end of the body 2. The other end of the body is closed by a heat shield 8 and the body is connected by tabs 10 of suitable refractory material such as tantalum to a suitable support 12 adaptable for mounting in an electron tube.

In accordance with the invention the emitter is formed by first cutting a blank, of the proper dimensions to form the emitter body, from a sheet of thoriated tungsten of the character hereinabove described, and containing up to 4% of thoria by weight. The amount of thoria in the sheet stock in general should be as large as possible and as a practical matter is limited to about 4% by weight by considerations of workability of the sheet material.

The blank is then worked into an emitter body 2 of the desired sheet-like form, as for example by pressing, or rolling or curling around a mandrel or other suitable forming tool. By the term sheet-like is meant any configuration which may be generated by the path through space of a line, either straight or curved, and thereby including not only a fiat plane, but a curved plane, cylinder, cone, sphere and the like, as opposed to wires, rods, or the like. Forming of the blank is facilitated by heating, for example, to 800 or 900 C., preferably in a reducing atmosphere. For best results in forming the blank, deformation such as rolling or bending should be performed about an axis perpendicular to the original direction of working of the thoriated tungsten sheet stock, i.e. at the time it is initially wrought into sheet form. The formed blank is then heat treated preferably in a reducing atmosphere such as hydrogen, at a moderately high temperature for a short time, e.g. 1600 C. for ten minutes, in order to anneal or set the emitter in its formed shape and inhibit subsequent deformation under the high temperature conditions encountered during subsequent processing and eventual emitter operation. To further reduce the possibility of deformation during operation, the opposing edges of the emitter body defining the seam 14 of the cylinder are fastened together by welding a strip of foil 16 of refractory metal such as tantalum to the inside surface of the cylinder in overlapping relation with the seam. Fabrication of the emitter is completed by attachment of the heat shield 8 and tabs 10.

After the emitter is fabricated it is cleaned, for example, by immersing in an ultrasonic bath containing acetone or alcohol for about five minutes. Following this the emitter is heat-treated or flashed in vacuum. Flashing may be accomplished for example by placing the emitter in a perforated tantalum can (not shown) suspending the can in an evacuated container, heating the can as by induction to a temperature of the order of 2150 C. brightness temperature, and maintaining it at this temperature for several minutes. I The vacuum flashing reduces a small portion of the thorium oxide of the emitter to free ;during the earlier firing operations.

thorium, which subsequently facilitates activation of the emitter in a completed tube. Also, additional dimensional stability is imparted to the emitter by virtue of the additional high temperature treatment. Finally, the high temperature contributes to thorough outgassing of the emitter.

Following the flashing operation the emitter is preferably carburized over at least a portion of its emitting surface, in order to improve its emission efficiency. This may be accomplished for example by heating the emitter body one or more times to about 2150 C. brightness temperature in an atmosphere containing carbon, such as a hydrocarbon atmosphere, or by coating the desired portion of the surface of the emitter body with a suspen sion of graphite and similarly firing the carbon coated emitter in vacuum. Preferably, the adjacent portions of all parts directly joined to the emitter body proper, such as the heat shield 8 and tabs 10, should also be carburized so as to preclude any later tendency of these parts to decarburize the emitter body during later operation of the emitter in a tube.

During the final firing operation the emitter is preferably held at a temperature slightly lower than in the previous firing operations, e.g. 1950 C., and for a longer time, e.g. fifteen minutes. This increases the amount of free thorium produced within the thoriated tungsten emitter body and facilitates diffusion of this free thorium to the surface region of the thoriated tungsten to replace any previous depletion of free thorium by evaporation This restoration of free thorium in the surface region of the thoriated tungsten facilitates ready activation of the emitter in the tube.

An emitter constructed in accordance with the present invention is characterized by a fineness of grain structure .of tungsten, and a degree of uniformity of distribution of thorium oxide in the intercrystalline spaces of the tungsten matrix material which has been heretofore unobtainable. These characteristics provide optimum protection for the thoria within the emitter, while facilitating .diffusion of thorium to the surface of the emitter as required to replenish the electron emitting thorium monolayer. Thus, in addition to its improved mechanical strength and ease of fabrication the emitter has many advantages providing efficient, non-gassy, long-life operation at exceptionally high emitting current densities, for example, up to three amperes per square centimeter of emitting surface.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiment other than those comprising a member of sheet metal consisting of a tungsten matrix comprising crystalline tungsten having a grain size of the order of 15,000 grains per square millimeter and having uniformly distributed in the intercrystalline regions throughout said member finely divided thoria, said uniformly distributed thoria being the only thoria in said member.

3. An indirectly heated cathode including a carburized emitter of sheet metal comprising wrought granular tungsten having uniformly distributed in only the grain boundary regions throughout said member up to four percent by weight of finely divided thoria.

4. An indirectly heated cathode comprising a refractory metal support, a metal sheet of a wrought sintered mixture of powdered tungsten and thoria rolled into 5 tubular form and mounted on the support, and a strip of tantalum foil overlapping the seam of said tube and secured to said sheet on opposite sides of said seam.

5. A self-supporting indirectly heated cathode comprising a refractory metal support, a tubular sheet metal member of a wrought sintered mixture of powdered tungsten and thoria having one end thereof longitudinally spaced relative to said support, tab means connecting said one end of said tubular sheet to said support, and

References Cited in the file of this patent a heat shield closing the other end of said tubular sheet. 10 2,524,001

UNITED STATES PATENTS Herringer Sept. 17, 1940 Kershaw Nov. 12, 1940 Felsner Jan. 6, 1942 Kurtz Nov. 13, 1945 Chevigny Apr. 2, 1946 Snijders Oct. 5, 1948 Spencer Sept. 26, 1950

Claims (1)

1. AN INDIRECTLY HEATED THERMIONIC ELECTRON EMITTER COMPRISING A SHEET METAL MEMBER OF A WROUGHT SINTERED MIXTURE OF POWDERED TUNGSTEN AND THORIA.

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