US2180714A - Thermionic device - Google Patents

Description

ATTORNEY Nov.. 2i, w39. J. D. MGQUADE THERMIONIC DEVICE Filed April 30, 1938 E w R mU ma VMv N l D N :Il H lllll vo \||l MJ m Patented Nov. 21, 1939 UNITED STA-TES THERMIONIC DEVICE John D.jMcQuade, Lakewood, Ohio, assignor to Kemet Laboratories Company, Inc., a corporation of New York Application April 30, 1938, Serial No. 205,221 7 Claims. (Cl. Z50-27.5)

'I'he invention relates to evacuated thermionic devices 'and more particularly to electron emitters for, and to the production of high vacuum in,

, thermionic valves such as radio tubes and sim- '5 ilar devices. 'I'his application is in part a continuation of my application Serial No. 78,633, filed May 8, 1936.

To evacuate radio tubes and the like, it is customary to pump out most of the gas and subse- 10 quently to increase the vacuum to the desired value by vaporizing, or flashing, Within the envelope a small amount of an active material, or getter, for example an alloy of barium and magnesium, which combines readily with gases. l5 The usual practice is to secure the getter, in the form of a pill, tablet, or small piece, to a shaped piece of thin metal which serves as a holder. Ii the radio tube has a glass envelope, the holder with its getter is placed Within the envelope, usu.n ally below the tube elements, and is heated by induced electric current until the getter has been ilashed. If the envelope is of metal, inductive heating of the holder is impracticable, and it is customary to secure the holder and getter in rm contact with the inner Wall of the envelope and to flash the getter by the heat of a flame applied to the outside of the envelope. In either case, the flashing operation takes from two to ten seconds. After the ashing of the getter, most types of radio tubes are placed in Aaging racks where the cathode is heated until it attains the desired electron emission activity.

Although the methods of cleaning up residual gases just described have been used successfully 35 in the production of many millions of radio tubes, they possess certain disadvantages which it is the object of the present invention to overcome. For instance, in order to secure the tablet or piece of getter material in its holder, it is necessary to 40 handle it in the open air, and the holder itself is seldom designed to seal the getter completely from the air: as a result, the getter tends to pick up appreciable and various amounts of moisture and gases from the air and subsequently to release them within the radio tube where they occasionally adversely affect its performance. Also, the handling of small individual piecesof getter Inaterial is a relatively difficult and expensive matter. Further, the time required to ash the conventional types of getters limits the rate of production of automatic tube-making machines. In the case of the metal envelope radio tubes, heating the envelope introduces large amounts of gas into the tube and also causes other diiculties. Again, it is extremely diicult to control accurately the nal degree of vacuum after exhaustion. It is an object of this invention to avoid these and other disadvantages of known methods and devices for evacuating vessels such as radio tubes. 5

Another object of the invention is to provide a novel electron emitter for use in thermionic valves and other thermionic devices.

A vbasic concept in this invention'is a compound Wire, the core of which is composed of active 10 getter material, preferably a material having a 10W work function such as an alkaline earth metal, and the outer shell of'which is composed of a ductile metal Which is more inert than the getter material to dry and moist gases at ordinary tem- `15 peratures, and has a melting point substantially higher than the boiling point of the getter material in high vacuum. vSuitable metals for the outer shell include nickel, iron, ductile nickelbase alloys, and ductile iron-base alloys. The 2o compound wire is secured within the envelope to be evacuated and the active getter material is evaporated by heating with an electric current passed either directly through the compound Wire itself or through a separate heater Wire placed =25 in heat-exchange relationship to the compound Wire. The active getter material is effectively protected from contact With the atmosphere during handling, and thus may be kept quite free from gas and moisture before flashing. '3o

The invention Will be described in reference to the accompanying drawing in which:

Fig. 1 shows in diagrammatic partial cross-section a compound wire consisting, for example, of a barium core I0 and a nickel or iron shell I I; 35

Fig. 2 shows in diagrammatic partial cross-section a compound wire similar to that shown in Fig. 1 but in which the axes of the core I0 and the shell 2| do not coincide;

Fig. 3 is similar to Fig. 1 but shows an embodi- 40 ment of'the invention in which a portion of the shell 3| is relatively thinner than the rest of the shell;

Fig. 4`shows in diagrammatic partial cross-section, in whichA for thesake of clarity of represen- 45 tation certain of the dimensions areV considerably exaggerated, -a device containing apiece of the compound wire shown in Fig. 1 in heat-exchange relationship With a heater Wire 40 protected by insulation 4|. 50

Fig. 5 illustrates in diagrammatic partial crosssection a method of flashing the active getter material by passing an electric current directly through the compound Wire shown in Figs. 1,'2,

and 3; and f i i 55 wire I5 which extends through the envelope, I4.,

The other support wire I3 may, if the envelope VI4 is of metal, be electrically connected to the envelope I4, as shown. If the envelope I4 is of glass or other nonconductor, the support wire I3 Y may be connected with a second lead wire, not

shown, which passes through the envelope I4 in a manner similar to the lead wireV I5 shown. When it is desired to flash the core I0,I a source of electric current is connected to the lead wire I5 by a cable I6 land to :the envelope I4 (or to the second lead wire just described) by a cable I'I, whereupon the shell 2I is heated by resistance until the thin portion of the Wall of the shell 2I is ruptured and barium vapor escapes through the rupture. Alternatively, the form of compound wire shown in Fig. 3 may be substituted for that shown in Fig. 2.

Another embodiment of the invention is illustrated by Figures 4 and 6. A length of the composite wire Il] and II is placed within a thin walled tube 42, suitably of the dimensions used for indirect-heating cathodes, and beside the composite wire and Within the tube 42 there `is placed a resistance wire 40, of tungsten or molybdenum for example, covered with insulating material 4I. If desired, the shell I I may be covered with insulation, in which case the insulating material 4I may be omitted from the resistance wire 40. The resistance wire 40 may be straight, as shown, or in the form of a small coil such as that used in the aforementioned indirect-heating cathodes. If desired, one end of the thin walled tube 42 may be crimped or otherwise closed to force the getter material to issue from the other end of the tube 42 during the subsequent flashing step, so as to obtain a directional ash. The tube 42 with its contents may then be secured within the envelope I4 of the radio tube or other device by connecting the ends of the resistance wire 4D to supports I2 and I3 which are respectively connected to a lead wire I5, and

.to the enevelope I4, or to a second lead wire (not shown) if the envelope I4 is not conductive. When it is desired to evaporate or flash active getter material from the tube '42, a source of electric current is connected by leads I6 and I1 to the lead wire I5 and the envelope I4, whereupon the resistance wire 4I) is heated and the heat thus generated is conducted to the getter material which vaporizes progressively and escapes from oneor both ends of the tube 42.

Alternatively, the types of composite wires shown in Figs. 2 and 3 may be substituted for the wire II and I0 in the embodiment illustrated in Figs. 4 and 6;

It will frequently be desired to vaporize two or more successive portions of getter material at more or less widely separate times, for example, during exhaust and aging of the radio tube. This ;may be accomplished by the use of several separate pieces Yof compound wire, each secured and flashed as described in connection with Fig. 5; however, the embodiment of the invention described in connection with Fig. 6 is particularly well adapted to an exact control of the amount of getter material vaporized at any stage, and it is a simple matter to flash successive portions of getter material as desired, merely by regulating the time and temperature of heating of the resistance wire 40. This is of particularly great value in the manufacture of metal-envelope radio tubes.

- Although itis possible to use with some success the type of composite wire shown in Fig. 1 in the manner described in connection with Fig. 5, *I have observed that this type of wire often melts through, breaking the circuit and ending the flash, while a large part of the active core is still unvaporized. Furthermore, the complete rupture of the wire sometimes ejects small loose particles of material, producing a defective or "microphonic radio tube. Also, a directional flash can not well be obtained. The types of wires shown on Figs. 2 and 3, having one or more relativelyrthin sections of shell wall, are free from these shortcomings. It Ais readily possible to rupture the thin portion of the wall ofthe shell 2l or 3| without parting the wire, breaking the ashing circuit, Furthermore, a directional flash can be obtained by facing the thin spot in the desired direction of ashing. The embodiment shown in Fig. 2, wherein the thin portion of the shell 2| exends the full length, or almost the full length, of the wire,-.is preferred for most purposes to the embodiment shown in Fig. 3, because the former in Figs. 2 and 3, may be formed by grinding or scraping. In general it is desirable that the thin portion of the shell 2l or 3I be as thin as possible, but not so thin that the active kcore material is not adequately protected. I have found that a wall thickness, at the thin portion, of 0.0003 to 0.0006 inch is quite' suitable, although special circumstances may require a thickness outside this range.

It may sometimes be desirable, particularly when extremely small wall thicknesses are used, to coat at least the thin portion of the wire with a material, such as copper, tin, zinc, or a lacquer or varnish, to afford a further safe-guard against deterioration of the active core. I prefer to use a low melting metal for this purpose, rather than an organic substance that may have a deleterious effect on the evacuated device.

The material of the core I 0 may suitably be any of the relatively volatile materials, active in cleaning up residual gases and vapors, known to be useful as getters.

The material of the shell I I, ZI, and 3I should, as indicated above, have a melting point substantially higher than the boiling point of the active core material in high vacuum, and should be relatively inert to dry and moist gases at ordinary temperatures. It should have a specific electrical resistance at least as high as those of nickel and iron, and preferably substantially higher. It should also have suflicient ductility to permit the drawingV and forming of the wire. When the wire is to be used as indicated in Fig. 5, it will be distinctly advantageous to form the shell from an alloy having a relatively high electrical resistance, for example an iron-silicon alloy, so that the lead wires I3 and I5 will not be called upon to carry so large a current during the flashing step.

Although the ,aboveldescribed embodiments of or ejecting loose particles.

the invention obviate the use of the tabs, strips, and cups heretofore almost universally used, it may sometimes be desired to secure the added directional characteristics of the tab or cup getter holders heretofore known, in which case either the wire shown in Figs. l, 2, and 3, suitably coated with insulating material, or the assembled wire and heater shown in Fig. 4, may be secured to the conventional tab, cup, cr strip, in place of the usual getter tablet, and iiashed by resistive heating as shown in Fig. 5 or 6, without departing from the present invention.

The dimensions of the shell l i, 2l, or 3l, and core lil of the composite wire are not critical if the wire is to be used as described in connection with Figs. 4 and 5; but are quite critical if the best results are to be secured in the use of the modification described in connection with Fig. 5. in the latter mcdiiication, the greatest outside diameter of the shell 2! or 3i should usually be between 0.01 and 0.05 inch, preferably about 0.015 inch, and the diameter of the core I0 should usually be approximately two-thirds the outside diameter of the shell. Successful results have been attained by the use of a piece of compound nickelbarium wire or iron-barium wire about one inch long, about 0.02 inch in diameter and containing about l0 milligrams of barium, which was flashed by the application of about 1.5 volts directly across its ends. Successful results have also been attained by the use of a piece of nickel-barium compound Wire about three-eighths of an inch long and about 0.04 inch in diameter containing a barium core about 0.026 inch in diameter, assembled with a heater coil as shown in Fig. 3.

Another embodiment of the invention, applicable only to devices having non-conductive envelopes, comprises securing a complete loop, at least roughly circular, of the compound Wire shown in Fig. 2 or 3 within the envelope to be evacuated, and heating this loop to flashing temperature by electric currents induced from a coil outside the envelope. As a variation of this embodiment, the said loop may be composed partly of compound wire and partly of some other conductive material.

The type of compound Wire illustrated in Figs. 2 and 3 may also serve as an improved electron emitter in a thermionic device. The thinner portion or portions of the shell Wall permit the attainment of a directional emission of electrons and of an emission more profuse in some directions than in others.

The specic examples described herein serve as illustrations of typical embodiments of the invention, and the invention is not restricted to or by such examples except as may be required by the claims and the state of the prior art.

I claim:

1. Device for use in an evacuated thermionio device comprising a compound wire composed of a core of metal having a low work function and an outer protective shell of metal relatively inert to dry and moist air at ordinary temperatures, such shell having a melting point substantially higher than the boiling point of the core material K and a .specific electrical resistance not substantially less than that of nickel, and a minor part oi the wall oi such shell being thinner than the major portion thereof.

2. Device for use in producing high vacuum within a closed envelope which consists of a compound wire composed of a core of active cleanup material comprising an alkaline earth metal and an outer protective shell or metal chosen from the group consisting of iron, nickel, ironbase alloys, and nickel-base alloys, a minor portion of the wall of such shell being thinner than the major portion thereof.

3. Device as claimed in claim 2, wherein the shell has a specific electrical resistance substantially higher than that of iron.

4. Device as claimed in claim 2, further characterized in that the thinnest portion of the shell has a thickness within the range of 0.0003 to 0.0006 inch.

5. Device for use in producing high vacuum which comprises a wire of high electrical resistance, a. compound getter Wire composed of a core oi active getter material and a shell of metal which is substantially inert to dry and moist air at ordinary temperatures, electrical insulation between and in contact with said wires, and an envelope that substantially encloses said getter Wire and holds it in firm contact with said insulation. i

6. Device for use in producing high vacuum which comprises a wire of high electrical resistance provided with a coating of electrical insulation, a compound getter wire composed of a core of alkaline earth metal and a shell of metal chosen from the group consisting of nickel, iron, nickel-base alloys, and iron-base alloys, and a metal envelope that substantially encloses said getter wire and holds it in rm contact with the insulated resistance wire.

'7. In combination with a Vacuum thermionic valve, a getter device comprising a compound wire composed of a core of active clean-up material and an outer protective shell of metal having a melting point substantially above the boiling point of said core and a minor portion of the Wall of said shell being thinner than the major portion thereof, and means for electrically heating said compound Wire to a temperature below the melting point of the shell but suiciently high to vaporize a substantial amount of said core and to rupture said thin portion of the shell to permit the escape of core material vapors.

JOHN D. MCQUADE.

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