US1817445A - Vacuum tube and method of manufacture - Google Patents

Description

Aug; 4, 1931. T E. Y. ROBINSON 1,817,445

I VACUUM TUBE AND METHOD OF MANUFACTURE Filed-Dec. 29, 1925 ix 9 Y 1' I wnmzssss: INVENTOR $14 V5 9 Emeaf )omanEabmsmz W ATT oRNEY Patented Aug. 4, 1931 UNITED. 'sTATEs PATENT OFFICE EBNES'IYEOMAN ROBINSON, OF LYMN ENGLAND, ASSIGNOB 1'0 ASSOCIATED ELEG- TRICAL INDUSTRIES LIMITED, BRITISH COMPANY vacuum TUBE an!) METHOD or MANUFACTURE Application filed December 29,

This invention relates to vacuum electric tube devices and particularly to the introduction into such devices of vaporizable substances or reagents such as may be employed 6 as getters for the removal of residual gases or more especially for increasing the electron emission from cathodes. In this latter connection alkali metals have been distilled into the tube, the theory being that a molecular 1 layer of the alkali metal is formed upon the surface of the cathode so that the electron emission and efliciency is increased by reason ofthe fact that these metals have low work functions and consequently permit the cathode to be operated at low temperatures.

. Caesium is a notable example of a metal of this class. Employing caesium the emission from a tungsten'filament at 900 K, is about one-third ampere per sq. cm. of cathode surface.

Referring more particularly to caesium this metal 1s very readily oxidizable so that it cannot be handled and directly introduced into the vacuum electric tube, and the process usually adopted is to heat a mixture 0 caesium chloride and calcium in an auxiliary glass'bulb forming an extension of the main envelope of the tube, thereby producing metallic caesium which is distilled into the main envelope after the latter has been evacuated and is still in connectdon with the evacuating pump, the auxiliary bulb being subsequently removed. Caesium, however, at-

tacks ordinary lead glasswhen the temperal J efiect the reaction in a bulb of special glass such as that known under the registered Trade Mark as Pyrex glass. Furthermore it is necessary that the main tube also consists of a special glass of this nature since it must be sealed to the auxiliary bulb. In any case the heating of the caesium chloride and calcium needs care and skill, andfor these reasons vacuum electric tubes of this class have only been rarely employed heretofore.

ture is'above normal so that it is necessary to.

1925, Serial No. 78,124, and in Great Britain December 31, 1924.

exhaustion of the envelope, the compound or the substance is heated independent y of the envelope, such for example as by means of a heating coil through which an electric current may be passed, or by heat arising from the heat treatment of an electrode, but most conveniently by high frequency induction in which case the material to be heated may receive heat from an electrically conducting auxiliary container therefor, or it may be directly heated by high frequency induction. By this method the substance will not bef come oxidized and since the main glass envelope is not itself directly heated any glass which is suitable for containing caesium at normal temperatures may be employed for the main envelope. The auxiliary container for the substances may be permanently located in the main bulb itself or within an extension of the main bulb which can be subse: v quently sealed off from the latter and removed.

In carrying out the invention according to the preferred method the main envelope of the tube consists of ordinary glass and contains for example two or more electrodes supported from the pinch of the tube and consisting for instance of a tungsten cathode, an anode and *a grid with appropriate lead-in conductors. The envelope is evacuated in the usual way through a tubulation and the auxiliary container such as a nickel bucket for the reagents which comprise a mixture of caesium chloride and calcium is located in an auxiliary bulb or chamber connected with the main envelope by means of a-smaller tubulation and'also consisting of ordinary glass. After the usual evacuation and the removal of occluded gases from the envelope and electrodes, and while the envelope is still ,connected with the exhaustin pump the metal bucket is heated by high Erequency in duction so that reaction ensues and the resulting caesium is distilled into the main envelope, the walls of which are comparatively cool so that the caesium will condense on to the inner surface thereof without condensing in any appreciable quantity upon the surface of the auxiliary bulb the which are already warm by reason of the heat induced in the metal bucket by high frequency currents. Hence the caesium does not 7 attack the. glass to an undesirable extent. When suflicient caesium has been distilled into the main envelope the auxiliary bulb is sealed off at the aforementioned tubulation.

In a modification of the above method the metal bucket disposed within the main en velope itself, for example bymounting it' upon one of the supporting members of the electrodes or upon one of the electrodes of the vacuum electric tube device, in which case the bucket may be heated by high frequency currents or otherwise, and is not removed from the main envelope after the caesium has been formed. 1 v

In a further modification the caesium may be sealed into a glass capsule which is placed 'within.the main envelope or an extension thereof and heated. The capsule is preferably'disposed in a metal container and heated by high frequency induction or otherwise. Such heating causes the caesium to volatilize and to penetrate the glass capsule and thus to escape into the main envelope, the glass capsule being retained within the latter or removed with the extension. The glass capsule should be formed of a glass such as boro-silicate glass which does not easily re-.

- act with caesium.

To enable the invention to be clearly understood and carriedinto elfect, it will now be described with reference to the accompanying drawings in which Figs. 1, .2 and 3 inclusive are elevations of vacuum electric tube devices, illustrating different methods of introducing the caesium or other vaporizable substance in accordance withgjifhe invention.

Fig. 4 is a perspective view to a larger scale of an electrode similar to the electrodes shown in the preceding figures and illustrating another method of introducing the caesium or other substance. Fig. 5 isa sectional view 'on a larger scale of a detail, and Fig. 6 is a view illustrating a method of producing 5 sealed capsules containing. caesium or other walls of.

auxiliary bulb may be dispensed with and the substances which is to be introduced into a vacuum electric tube device.

Referring first to Fig. 1 of the drawings, a three-electrode valve is illustrated comprising a tubular anode 1,- within which is arranged a grid, only the support 2 of which is visible, and a filament the supports 3 and 4 ofwhich are shown. The several electrode supports are secured in a pinch 5 formed on a flange tube 6 which is fused in the wellknown manner to the glass envelope 7. A tube 8 is fused into the pinch 5 in connection with-a blow hole 9 therein. The envelope 1 is adapted to be evacuated through the tube 8 which is connected to the pump manifold. The opposite end of the envelope 7 is provided with a tubular extension 10 sealed at its free end 11 and provided with a restriction at 12 so that it may be readily sealed off and removed from the main envelope 7. The. sealed end 11 is in the form of a pinch in which is secured a wire 13 to which is'attached a nickel tube 14 having its ends closed by pinching or otherwiseand containing a quantity of a mixture of caesium chloride and, calcium. The nickel tube. 10 is preferably 'seamlessor if seamed its'adjacent edges should be Welded together so that highfrequen'cy currents may circulate around it when induced therein.

The vacuum tubeas constituted is exhaust'ed in the usual way, the envelope being ovened for about 10V to 12 minutes at 400, for example, after which the electrodes may be heated by high frequency induction to red heat. The filament may be heated by the passage of a current through it. After the absorbed gases have been removed upon heat treatment the nickel tube 14.- may be heat sium chloride and the calcium takes place to produce caesium metal and calcium chloride. The metallic caesium distills into the main envelope 7 and condenses on the inner surface thereof and on'the surfaces of the electrodes. Little or no caesium will condense uponthe inner surface of the extension 10 since the latter will be warm by radiation of heat from the nickel tube 14., or in any case the extension 10 may be warmed by means of the sealing off torch or otherwise-and the caesium thus driven'into the main envelope.

When the caesium has been driven into the main bulb 7 the extension 10 may be sealed off at the constriction 12, after which the main bulb may be sealed off from the pump.

It will be understood that the process above described may be modified in various ways, for example,'the caesium maybe distilled into the main bulb 7 after the latter has been evacuatedgand disconnected from the pump. -0rthe heat treatment of the electrodes and the heating of the tube 14' may be effected after the valve has been evacuated 'the upper en being evacuated through the tabulation 15 at the up by the pumps and disconnected therefrom.

Referring to-Fig. 2 the nickel tube 14 instead of bem arranged in an extension of the main enve ope 7 is attached to one of the electrode supports, namely the support 3 for of the filament, the envelope r end of the main bulb, instead of throng a'tube connected with the pinch 5. The nickel tube 14 may be welded to the filament support 3 or -clippedthereto. After the exhaustion and heat treatment of .the bulb, the anode and nickel tube 14 maybe heat treated, either before or after sealing off chloride into a paste with water and then painting it on to the magnesium. When calcium is employed a non-aqueous solvent, such as ether, may be employed. When the anode.

is heat-treated the magnesium or calcium will reduce the caesium chloride so thatcaesium 'will be vaporized into the envelope 7. Instead of securing a. piece of magnesium or calcium to the anode as described in connection with Fig.3 a nickel pocket 17 may be welded to the anode as shown in Fig. 4, for the reception of a mixture of caesium chloride and calcium. 7

In all cases wherein the reagents are secured to or supported upon the anode or are heated by heat generated in the anode, the necessary heat may be alternatively eifected by bombardment of electrons from the, filament. In cases where the inter-electrode gap is short or where surface area of th'eanode is relatively small compared to that of the filament, the anode may be heated to a suflicient temperature merely by radiation of heat from the filament which, in the case of tungsten, may be heated to a temperature of 'say 2500? K. 1

When the reagents are heated within the main bulb and a moderately volatile reducing agent such as magnesium or calcium is employed it will be appreciated that these reducing agents may also be present in" the bulb and serve as getters for the removal of residual gases. j

Instead of introducing the caesium into the bulb in the form of a reducible compound as described in connection with Figs. 1 to process the envelope 7 is connected to the pump manifold by the tube 8 and exhausted while the bulb is ov'ened, the electrodes being next heat treated by a high frequency induction, after which the metal tube or bucket 14 is heated by high frequency induction, which causes the pressure of the metallic caesium in the capsule 18 to rise and at the same time weakens the walls of the capsule until eventually the caesium perforates the capsule and passes into the main bulb, after which the extension 10 may be sealed off at the restriction 12. Alternatively the electrodes ma be heat treated after the main bulb is sealed ofi from the pump manifold, as also may the metal tube or bucket 14. In this case the extension 10 may be removed from the bulb after the latter has been sealed 0E from the pumps and after the caesium has been driven into the main envelope.

In some cases the metallic caesium in the capsule 18 may be heated directly by high frequency induction, in which case the nickel tube 14 can be dispensed with. The capsule can be located at other places in the envelope and heated in other; ways.

Capsules. such asi 18 containing metallic caesium may be prepared in the following manner, reference now being made to Fig. 6. A long thin tube 20 of boro-silicate glass is provided at one end with a bulb 21 containing a mixture of caesium chloride and calcium, the other end 22 of the tube 20 being connected with a vacuum pump (not shown),

the tube being preferably heat treated for the removal of occluded gases. The reagents in the bulb 21 are then heated so that they react with one another, caesium being produced and distilled into the tube 20, wherein The bulb 21 is then sealed off itcondenses. from the tube 20 at 23. A'portion of the tube 20 is then sealed-ofl' at 24, another portion at 25 and so on, each portion thus sealed off containing a little caesium, these sealedofi' portions or capsules being adapted to be inserted in the metal tube or bucket 14 for the 'purposeof introducing vaporized caesium into vacuum electric tube devices in the manner already described.

It will be understood that various detail modifications may be made without departing from the scope of the invention, and that while the inyention has been described primarily for the introduction of caesium into vacuum electric tube devices for increasing the electron emission of cathodes, it can be ERNEST YEOMAN ROBINSON.

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