US2457651A - Electron tube and method of making the same - Google Patents

Description

Dec. 28, 1948. H. EAVES ETAL ELECTRON TUBE AND METHOD OF MAKING THE SAME Filed March 18, 1946 F/LAMENT FABR/CATED AND MOUNTED F/LAME/VT COATED WITH CARBON r 5 M5 Mm MEG /5T F w F/L AMEN T 6' ARE UR/Z E D 5) HEAT/N6 /N TUBE BUR/N6 E X HA U5 T Ill;

- IN VEN TORS HUBERT E A was BY PAUL D. MLL/A/ws A TTORNE Y Patcnted Dec. 28, 1948 2,457,651 ELECTRON TUBE AND METHOD OF MAKING THE SAME Hubert Eaves, San Mateo, and Paul D. Williams,

Palo Alto, Calif., assignors to Eltel-McCullough, Inc., San Bruno, Calil'., a corporation of California Application March 18, 1946, Serial No. 655,065;

1 Claim. 1

Our invention relates to electron tubes, and more particularly to improvements in electron emitter electrodes for such tubes.

Because of its advantages, the thoriated type of cathode or filament has been widely used as an tures of advantage. some of which, with the fore- 2 going, will be setforth in the following description of our invention. It is to be understood that we do not limit ourselves to this disclosure of species of our invention, as we may adopt variant embodiments thereof within the scope of the emitter in electronic devices such as vacuum claim. tubes, thoriated tungsten wire being commonly Referring to thedrawings: used as a filament material. It was early dis- Figure 1 is a vertical view of a tube embodying covered that the electron emission property of the improvements of our invention. a thoriated filament was enhanced by a process Figure 2 is a flow diagram of the process; and known as carburization (sometimes called car- Figure 3 is a diagrammatic view showing the bonization). This step comprises forming a layer filament coating step. of metallic carbide adjacent the surface portions In terms of broad inclusion, our method of carof the filament, this layer being tungsten carbide burizing a thoriated filament comprises applyif the filament is of thoriated tungsten. The ing a coating of finely divided particles of cartheory is that such carbide layer provides a store bon on the filament, and heating the filamentto of carbon stable at the operating temperatures carburize it, the coating being applied uniformbut available for reducing thoria to thorium. ly in thickness along the filament to produce uni- The common method of carburization, long form carburization. Regulation of the coating used in the art, is to heat the filament in an atthickness on a given filament serves to control mosphere of hydrocarbon gas such as methane the degree of carburation desired, depending upon or acetylene. Because the carburizing gas is a the size of filament wire used. We prefer to special atmosphere, this carburizing process must pp y h coating by. electrodepositin arbon be performed as an independent step prior to particles from a bath containing the particles in final evacuation of the tube. The usual procedure suspension- The coated filament iS preferably involves complicated apparatus containing a bell Sealed d r ctly into a tube, and u tion jar or the like for receiving the filament, into is p e b y fi t by e ti the lament durwhieh the carburizing gas may be admitted, The ing evacuation of the tube. This eliminates the bell jar must be evacuated, after the. filament 1151131 p eeerbllrizlng step ahd'enables the ahas been heated in the hydrocarbon gas, to comment to be carburized simultaneously With tube plete the carburizing step, all of which further eXhellStcomplicates th apparatus In greater detail and referring to the draw- Another disadvantage of the. standard process ings. i ure 1 shows a tube embodyin 1' is that it is practically impossible to control the provements, the illustrated tube compr an degree and uniformity of carburization. When v op 2 v n a fi t Supporting em 3 a filament is heated in a hydrocarbon atmosying an exhaust tubulation 4 and P d d phere, the excess of carbon necessarily present with a base 5 having prongs e pe e causes the hotter portions of the filament to carcloses a filamentary c o e 8, tubular anode 9 burize more deeply than the colder portions. and d Anode 9 has a p pp t This is an accelerating phenomena because deep- 40 y bracket '3 On a lead i4 sealed to the pp v er carburization at a given section increases its portion of the envelope. id is of the ca e resistance and consequently its temperature, p o prising Vertical bars term t a which further increases the rate of taking carbase ri l6 supported by br cket I! on a rid bon. That is why many filaments are lost by lead [8 sealed to a side wall of the envelope. separation of the wire during carburization and structurally p kin the illustrated cathode 8 why, even at best, filaments are weak mecham- 15 0f the fil ta y type comprising a helix 0f cally and nonuniform in operating temperature thoriated tungsten Wire welded to pp t g and electron emission properties; leads I9 sealed to stem 3. As is the usual prac- The broad object of our invention i t protice, stem 3 carrying the filament is sealed to the vide an improved thoriated type filament and envelope at the stem flare 2!. After exhaust, method of making the same, whereby the above the tube is based and filament leads [9 are conmentioned disadvantages and limitations are nected to a pair of base prongs I by suitable conlargely overcome. ductcrs 22.

The invention possesses other objects and fea- This tube structure is merely for purposes of illustration and may be varied widely, it beingi; I i 2,457,651

3v understood that our improved process is applicable to thoriated filaments of a variety of shapes and in tubes of many different designs.

Figure 2 is a flow diagram showing the major steps in our preferred process, namely, first fabrieating and'mounting the/filament; then-coating f the filament with 'carbon, then sealing the-fila ment in the tube, and then carburizing the filament by heating it during evacuation of the tube.

Fabrication of filament 8 by coiling a length, of thoriated tungsten wire, and mounting the filament to its supporting stem 3 by welding to in accordance with usual practice. The tube is then ready for evacuation.

The tube may be exhausted by use of standard pumping equipment, the tubulation 4 being connected to a suitable vacuum pump in the usual manner. ur tube; ,niaking; processadif fers from ordinary practice in:- that' -=the- -first of the exhaust schedule includes a carburizing step.

" 'After the tube has been baked out and a reasonleads I9, is carried out in the usual mannen At this stage the filament structure-isgenerally're ferred to as a mounted filament-F "-and"'ordinarily goes from this point to the carbonizer for treatment in a hydrocarbon gas. '-In our ably good vacuum obtained, we heat the filament 1 by passing current thio'iigh it. The filament is heated to a temperature and for a time sufiicient to convert all of the- -free carbon in the coating to a metallic carbide:"* For a thoriated tungsten process this complicated treatment in a gaseous I atmosphere is eliminated entirely, mounted filament is merely coated with finely dividedcarboni F and the".

The-carbon coating"is preferably applied by electro-depos'iting'a layer earbon particles on the filamentr Figure 3"shows?a suitable coating apparatus comprising a tank 23 holding a bath '24 containing carbon particles 25 in suspension and agitated by a suitable stirring device 28-.

Carbon powder 'L grams 20 Tannic acid (C. P.)- do 2 Methyl alcohol (99%) milliliters 350 Acetone (C. P.) ;do 25 Perchloric' acid (70%) drops 4 Phosphoric acid (85%)-; do 4 The mounted filament is connected to the negative side of a potential source 26 and an elec-'- trode '21 say of molybdenumis connected to the positive side. A timer switch 28 is preferably included to automatically open the circuit after a predetermined interval, and means 29 is incorporated to vary the potential. During the coating step the mounted filament is dipped into the bath sufficiently to emerge filament 8 as shown in Figure 3. -When the circuit is closed the positively charged carbon particles move to and deposit on the negative filament. Because of the extreme fineness of the carbon particles they build up a coating of uniform thickness filamenthaving a carbon coating about .001 inch thick, we have found that heating to say 2100" C. for aboutz or 3minutes is sufiicient to convert the carbon to tungsten carbide. Since the tube is on the pump at this time and being continuously evacuated, any gas evolved during carburization is withdrawn'i'r'om the tube. v

After carburization', the filament temperature is reduced and any conventional procedure of tube exhaust followed. For example, the grid and anode may be outgassed either by induction heating or'by electron bombardment from the filament. Upon completion of the exhaust schedule the tube is removed'from the pump by sealing oiT tubulation 4, and the tube is ready for basing.

Instead of effecting carburization by heating the filament in'the tube; it is understood that the mounted filament may be precarburized by heating the coated filament in an oxygen free atmosphere such as an inert atmosphere of helium or argon, or by heaing in a vacuum. This v still has the advantage of producing a more unialong the filament; -Also, the thickness of the example, a carbon coating .001 inch thick" will' be .deposited in about 3 seconds at voltswith ways than passing current through it, as by formly carburized filament, but we prefer to carburize directly in the tube on the pump be- I cause of the simplicity involved. It is also understood that the fila ment may be heated in other induction heating, but again we prefer the s pler direct heating procedure.

Many advantages are derived from our processin addition to simplification of tube manufac ture. One of these is the formation 'of a uniform layer or metallic carbide along' the filament -l'his is due to the fact that'the 'ccating provides a.

uniform supply of carbon'and limits the amount of carbon that can be taken. up by any given section of the filament, regardless of how hot 9; given section may be. This eliminates the uneven carburization which occurs when an excess of carbon i's'available at a hotter section, as is the case when a filament is carburized in an atmos- I phere containing carbon. limiting the amount of carbon also prevents-loss of filaments during carburization, due to weak spots caused by too much carbide penetration. "Furthermore, our carburized'filaments in the final tubes are much stronger mechanically, endure consistent in this the bath described. By maintaining these conditions the coating thickness may be held within very close limits from filament to filament, and

the thickness' will not vary along the'length of the filaments. The importance ofthis control will become evident as the description proceeds.

The mounted filament is preferably rinsed in respect from tube to tube. as compared to-filaments carburized'in the usual 'way.

Another advantage resulting from uniform car; V burization along the length of a filament is that" such a filament runs at a more even'temperature" acetone after removal from the bath. It is now points.

trated, the sealing-in operation merely involves fusing flare 2| to the envelopewall; which is-done from end to end. This means that the filament will emit"electrons-more: uniformly along its I length which uniformity of emission improves' a tube from both its operational and life stand- Still another advantage of our process isthat%' the initial application-of. the carbon as a'acoatingi.

makes it possible to. contro1,:not only-the iormity of carburization, but also the degree of carburization. Since the thickness of the carbon coating determines the amount of carbon available for combination as a carbide, it will be appreciated that regulation of the coating thickness will control the depth of the final carbide layer. Such control over the degree of carburization is important because it establishes the final operating temperature and emission properties of the filament. In our preferred process, the building up 01' the carbon layer by minute particles electrodeposited from a bath enables an operator to regulate the coating thickness within extremely close limits.

A further important advantage of our process is that the filament is carburized in the presence or substantially pure carbon, thereby eliminating the contamination of the filament by foreign substances such as present in a hydrocarbon atmosphere. Such contamination is responsible for poor and erratic emission properties which our process overcomes.

A still further advantage is that carburization may be restricted to. selected portions of the filament structure, such as the filament wire only. This is easily done by masking ofl portions of the filament structure, as by shielding such portions with a covering material while the structure is in the bath. For example, if carbon is not desired on the center filament lead of the structure illustrated, a piece of split flexible tubing may be slipped over the center lead and removed after the coatin: step. Since only the coated portions are carburized when the filament is subsequentLv heated, it is apparent that carburize-tion or undesired areas may be avoided. This is not possible when '6 carburization is carried on in a hydrocarbon at- 'mosphere because anypart of the structure which gets hot will take up carbon.

While we'have described our improvements in connection with a filamentary type cathode, it is understood that the process may be used to equal advantage with thoriated cathodes of the unipotential type. 1

We claim: a

The method of making an electron tube having a thoriated tungsten .filament, which comprises mounting the filamenton a support, immersing the mounted filament in a bath containing finely divided-carbon particles in suspension, depositing by cataphoresis a coating of said particles on the filament, withdrawing the filament from the bath, enclosing the filament in the tube envelope, evacuating the envelope, and heatin the filament in the envelope to carburize the filament prior to completion of evacuation. HUBERT EAVES. PAUL D. WILLIAMS.

REFERENCES ormn The following'references are of record in the file of this patent:

I UNITED STATES PATENTS Number

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