US2819991A - Carburized thoriated tungsten electrode and method of enhancing its emissivity - Google Patents

Description

W. E. HARBAUGH a Jan. 14, 1958 v 2,819,991

CARBURIZED THORIATED TUNGSTEN ELECTRODE AND METHOD OF ENHANCING ITS EMISSIVITY Filed Aug. 30, 1952 a? /,I V

M1115 il fiz'i i gh M United States Patent *9 "ice CARBURIZED THORIATED TUNGSTEN ELEC- TRODE AND METHOD OF ENHANCING ITS EMISSIVITY Willis E. Harbaugh, Bareville, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application August 30, 1952, Serial No. 307,295

9 Claims. (Cl. 148--13.1)

The present invention relates to improvements in socalled carburized electrodes for use in electron tubes, and to improvements in the art of making the same.

Under the prior art practice certain power tube electrodes have been carburized in order to enhance their thermionic emission and also because the carbon deposited on the electrodes provides a continuing gettering. action during operation of the tube. However, in the manufacture of power electron tubes in which carburized thoriated electrodes are used, difiiculty has been met in obtaining carburized electrodes which have uniformly high electron emission capabilities immediately following tube processing or activation. Also, carburized electrodes made according to prior art practices often have a tendency to buckle or camber. Such buckling or cambering results in changes in the electrical characteristics of the tube in which the electrodes are used and may, in extreme cases, cause short circuits between adjacent electrodes.

A principal object of the present invention is to pro vide improved carburized thcriated electrodes which obviate the above mentioned disadvantages.

Another object of the present invention is to provide a commercially practical process for manufacturing thoriated electrodes having improved and stable electron emission.

Still another object of the present invention is to provide means for achieving a high degree of mechanical accuracy in the manufacture of electrodes for use in high power electron tubes.

A further object of the present invention is the provision of a process for stretching carburized thoriated tungsten electrodes which insures the formation of the desired crystalline phase and structure in the carbide layer on the electrode.

Stated generally, the foregoing and related objects are achieved in accordance with the method of the invention by stretching a carburized thoriated electrode beyond its elastic limit under controlled conditions of temperature and tension.

In a preferred embodiment, electrodes which have been pre-carburized in accordance with the process disclosed and claimed in my copending application, Serial No. 165,093, filed May 29, 1950, entitled-Process of Carburizing, now abandoned, are then heated to approximately 1600 C. At this temperature each of the electrodes is placed under axial tension and stretched beyond its elastic limit until the electrode is permanently elongated about one-half of one percent of its total length.

Cathode electrodes so stretched have been found to have high electron emission capabilities immediately after activation of the tube in which they are used. Also, in accordance with the present invention electrodes so stretched have been found to be uniformly straight and free from tendencies to buckle and camber.

The invention is described in greater detail in connec- 2,819,991 Patented Jan. 14, 19 58 2 tion with the accompanying single sheet of drawings, wherein:

Fig. l is an elevation view, partly in section, of an apparatus for carrying out the method of the invention; Fig. 2 is a fragmentary perspective view of a carburized electrode in which part of the carburized layer has been removed;

Fig. 3 is a greatly magnified view of a small area on that. part of the carburized electrode of Fig. 2 which hashad a part of carbide layer removed; Fig. 4 is a greatly magnified view of the appearanc of the area of Fig. 3 after the electrode of Fig. 2 has been stretched, in accordance with the invention. Fig. 1 shows one form of apparatus suitable for precarburizing and stretching a thoriated tungsten electrode 10 prior to mounting it in an electron tube. The electrode Ill, which is to be carburized, comprises a rod-like solid core of thoriated tungsten which, in the instant case, is trapezoidal in cross-section (see Fig. 2) throughout a greater portion of its length and terminates in oppositely located end sections 11 of reduced diameter and consequent reduced heat-conducting properties. The end-sections 11 each terminate in a mounting head or hook 12 by means of which the electrode is removably supported upon two oppositely located slotted and grooved arms 13, 14 which are each supported from electrically conductive members 15, 16 in any suitable manner. Members 15 and 16 are connected to the terminals of a source of electrical power (not shown) by leads 17, 13. Member 15 is insulated from standard 19 and supported thereon. Member 16 is slidably connected to standard 19 in such manner as to permit axial elongation of electrode 10 when heated.

Cooling means, in the form of tubes 2%) direct dry hydrogen, or other coolant which is inert with respect to thoriated tungsten, from a source (not shown) upon the reduced end portions 11 of the electrode It to maintain said electrode ends at a temperature sufiiciently low' to prevent their burning out or becoming carburized.

A bell 21 is movably mounted over, and may be lowered around, standard 19 and the parts supported thereon with its lower end 22 open to the atmosphere if a light gas is used; with a heavy gas, filling from the bottom would be necessary. Tubing 29 opens into the bell 21 and is connected to controlled sources of gases which are to be introduced into the bell to form an atmosphere most favorable to carburizing.

The apparatus of Fig. l, as described above, is similar to the apparatus used in my co-pending application, Serial No. 165,093, previously mentioned.

In order to apply axial tension to the electrodes in applied through slideablc member 16, on the electrode:

19. Extension rod 28 which is attached to slideable member 16, actuates a dial indicator 26 that is attached to the supporting frame 27. The dial 26 indicates .the-

amount of elongation or stretching of electrode 10.

Briefly, the electrode carburizing process described in my co-pending application, Serial No. 165,093, previous ly mentioned, is as follows:

(1) The electrode to be carburized is heated to (2) The electrode is maintained at the above tempera-- ture while a mixture of hydrogen plus carbon bearing K. in hydrogen to recrystallize the tungsten;

vapor is formed. This is continued for approximatelyv one minute until a thin layer of massive type carbide is formed;

(3) The temperature is increased to 2800 K.; and the carbon bearing vapor is removed, so that the cathode is heated very hot in a hydrogen atmosphere. This causes the carbide to penetrate deeper into the tungsten and form the laminated type of carbide desired.

By way of explanation, the laminated phase of tungsten carbide occurs after the crystalline structure of the tungsten carbide has passed through the massive phases WC and W C. In the laminated phase the carbon concentration by weight percent varies from 3.16 to 2.45. Laminated phase tungsten carbide is discussed in relation to its uses in electron emitting electrodes in Carbide structures in carburized thoriated tungsten filaments, C. W. Horsting, Journal of Applied Physics, vol. 1.8, No. 1, p. 95, January 1947.

The following step, that of stretching the electrodes, is the essence of the present invention:

The temperature of the electrode is reduced to approximately 1600 C. and axial tension is applied between the ends of the electrode in order to stretch the electrode beyond its elastic limit until it is permanently elongated about one-half of one percent of its total length.

It should be emphasized that the carburizing steps and the stretching step need not be one continuous operation.

As a specific example of how the present invention is accomplished, including the carburizing steps, thoriated tungsten electrodes 10 are mounted between upper and lower supports 13, 14, as previously described in connection with Fig. 1. The electrodes 10, of substantially trapezoidal cross section, have a width of .057 inch narrowing to .040 inch with the narrow face rounded. The thickness from front to rear is .037 inch. The electrode is approximately 8.6 inches long. The reduced portions 11 are cooled by jets of pure dry hydrogen which pass through the tubes 20. Bell 21, which is 24 inches high and 14 inches in diameter, is lowered and the electrode 10 is electrically heated (by means of current introduced through leads 17, 18) for thirty seconds at 2600 K. in an atmosphere of pure dry hydrogen which is continuously flushed through the bell at the rate of 40 cubic feet per hour; the end 22 of the bell being open to the atmosphere. This heat treatment, though beneficial, is not essential. Then, while maintaining the electrode at the temperature of 2600 K., carbon bearing vapor is admitted by adding a flow of 12 cubic feet per hour of hydrogen carrying benzene vapor to the 40 cubic feet per hour of pure hydrogen which was flowing through the bell. The electrode is then maintained at a temperature of 2600 K. for a period of one minute.

The benzolated hydrogen is formed by passing hydro gen at the rate of 12 cubic feet per hour first through a dryer and then through an atmosphere of benzene vapor in a carburetor. The carburetor has an internal diameter of 4 /2 inches with a pool of benzene at room temperature. The hydrogen inlet and outlet are both .200 inch internal diameter tubing with their openings inches above the surface of the benzene. The walls of the carburetor extend four inches above the surface of the benzene and then taper inward at an angle of about 45 degrees.

The source of hydrocarbon is then cut off; the flow of the hydrogen is maintained; and the electrode is simultaneously raised to a temperature of 2800 K. and held there for two minutes.

Following this, the electrode temperature is reduced to approximately 1600 C. and axial tension applied via the lever and fulcrum arrangement 24, 25 until the electrode is stretched one half of one percent, or .040 inch. In order to achieve this elongation a force of approximately 25,000 pounds per square inch is required.

It is to be understood that substantial variations in the cross sectional thickness of the electrode 10 being carburized may require or permit corresponding changes in the duration of the heating times, with the temperature being held constant. However, these variations may be readily determined.

Electrodes stretched in the above described manner undergo a physical change of structure which is identifiable under microscope examination.

Referring to Fig. 2, there is shown a fragmentary perspective view of a portion of an electrode 31 having a carbide layer 32, a portion of which layer has been removed from an area 33 adjacent the end of electrode portion 31.

Fig. 3 is a greatly magnified view of the appearance of the area 33, as seen under a microscope, if the electrode of which it is a part has not been stretched in accordance with the present invention.

Fig. 4, however, represents a greatly magnified view of the appearance, as seen under a microscope, of area 33 when that area has been stretched in accordance with the present invention. The arrow 39 indicates the direction in which axial tension was applied.

The area 33 of Fig. 3 shows no particular pattern of crystal distribution, yet the area 33 of the stretched electrode, shown in Fig. 4, shows separation lines 34 or striations (the dark streaks) running approximately parallel to the axis of the tension applied to the electrode. These striations may be attributed to the stretching operation.

Fig. 4 is merely illustrative of the physical appearance of a portion of the stretched electrode, and it cannot be determined that the striations themselves are the cause of the improved electron emission of the electrodes.

One possible explanation for the enhanced emission of stretched electrodes is that the increase in emission is caused by a slight separation between grain boundaries of the carbide layer so that more thorium is free to diffuse to the surface. Another possible explanation is that stretching causes the crystal structure to be re-oriented in such a manner as to present more emissive faces to the surface of the element. It has been found by other investigators that in a single crystal certain faces have better electron emitting properties.

The advantage of straightening the electrodes by stretching may be applied to electrodes other than cathodes. For instance, grid strands for tubes of the type shown and described in U. S. Patent 2,544,664, issued March 13, 1951, to Garner, Parker and Harbaugh may be straightened by this stretching process. Approximately the same temperature and tension is used for stretching such tungsten grid strands as is used in stretching the heretofore described carburized thoriated tungsten cathode strands. The temperature is approximately 1600" C.; and tension is applied until the strand is elongated approximately one-half of one percent.

What is claimed is:

1. The method of making a thoriated tungsten filamentary cathode electrode, said method comprising the steps of carburizing said electrode, heating said electrode to approximately 1600 C., and applying axial tension to stretch said electrode beyond its elastic limit approximately one-half of one percent of its length.

2. The method of enhancing the electron emissivity of a thoriated tungsten thermionic cathode which is carburized in the laminated phase, said method comprising heating said cathode to approximately sixteen hundred degrees Centigrade and stretching said cathode beyond its elastic limit approximately one-half of one percent of its length.

3. The method of enhancing the electron emissivity of a carburized thoriated tungsten thermionic cathode element, said method comprising heating said element to approximately sixteen hundred degrees centigrade and stretching said element beyond its elastic limit about onehalf of one percent of its length.

4. The method of making an elongated filamentary cathode, said method comprising the steps of carburizing a thoriated tungsten electrode element, and heating said element to approximately 1600' C. while simultaneously applying sufficient axial tension to stretch said element past its elastic limit about one half of one percent of its length to produce microscopic surface striations which extend substantially parallel to the longitudinal axis of said element.

5. The method of making a carburized thoriated tungsten electrode, said method comprising the steps of carburizing said thoriated tungsten electrode to produce on its surface a layer of laminated phase carbide, heating said carburized electrode to approximately sixteen hundred degrees centigrade, while simultaneously applying sufiicient axial tension between the ends of said electrode to stretch said electrode beyond its elastic limit approximately one-half of one percent of its length.

6. The process of making a carburized thoriated tungsten element, comprising heating said thoriated tungsten element in an inert atmosphere at a predetermined temperature and stabilizing the crystalline structure, then while continuing to heat said element at said temperature, adding carbon bearing vapor to said atmosphere to create and maintain a carburizing atmosphere, said temperature being sufiiciently high to decompose the carbon bearing vapor, discontinuing the addition of hydrocarbon vapor and continuing to heat said element at a substantially higher temperature while the carbon bearing vapor content of the atmosphere is rapidly being depleted, reducing the temperature of said element to approximately 1600 C. and applying axial tension between its ends to stretch said element beyond its elastic limit approximately onehalf of one percent.

7. The method of carburizing a thoriated tungsten element, comprising heating said thoriated tungsten element at a predetermined temperature in a carburizing atmosphere to which carbon bearing vapor is added to maintain the carbon concentration and forming a layer on said element which is substantially the massive phase W C, said temperature being high enough to decompose said vapor, then reducing the addition of carbon bearing vapor and continuing to heat said element in said atmosphere in which the carbon content is rapidly reduced, substantially all of said W C being converted to the laminated phase, then reducing the temperature of said element to approximately 1600 C. and stretching said element beyond its elastic limit approximately one-half of one percent of its length.

8. The process of carburizing a thoriated tungsten element, comprising heating the thoriated tungsten element in a carburizing atmosphere containing an inert gas and hydrocarbon vapor at sufiiciently high temperature to decompose the hydrocarbon, then continuing the heating of the thoriated tungsten element at a higher temperature in an atmosphere in which the hydrocarbon content is rapid- 1y decreased, reducing the temperature of the element to approximately 1600 C. and applying axial tension between the ends of said element to stretch said element beyond its elastic limit approximately one-half of one percent of its length.

9. An elongated carburi'zed thoriated tungsten thermionic cathode element which has been heated to approximately 1600 C. and stretched beyond its elastic limit about one half of one percent of its length.

References Cited in the file of this patent UNITED STATES PATENTS 2,246,162 Benjamin June 17, 1941 2,294,389 Dean et a1 Sept. 2, 1942 2,310,094 Kroll Feb. 2, 1943 2,400,893 Thurber et a1. May 28, 1946 2,406,966 Pfeil Sept. 3, 1946 2,450,007 Litton Sept. 28, 1948 2,521,663 Zunick Sept. 5, 1950

Claims (1)

1. THE METHOD OF MAKING A THORIATED TUNGSTEN FILAMENTARY CATHODE ELECTRODE, SAID METHOD COMPRISING THE STEPS OF CARBURIZING SAID ELECTRODE, HEATING SAID ELECTRODE TO APPROXIMATELY 1600*C., AND APPLYING AXIAL TENSION TO STRETCH SAID ELECTRODE BEYOND ITS ELASTIC LIMIT APPROXIMATELY ONE-HALF OF ONE PERCENT OF ITS LENGTH.

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