US1871343A - Heater element for electron emission devices - Google Patents

Description

g- 1932- J. H; RAMAGE 1,871,343

HEATER ELEMENT FOR ELECTRON EMISSIQN DEVICES Filed May 9. 192a- INVENTOR BY J. HR qmAc E ATTORNE I was H M Patented Aug. 9, 1 932 Wee s namasn, on BLOQMFIELD, new "quasar; A$$QNO3 ro Wt liI Ng:

HEATERVELEMENT on nnncrnon nurse- 101v envious Application filed May .9,

This invention relates to heater units for rectly transferred to an electron emission matcriahto permit an electron flow between electrodes in the device.

7 As an example of a device of this character, aradio tube designed to operate on alternating current, such as that known in the trade as the {TY-227, may be considered. This tube comprises the usual bulb and base and its internal structure includes a glass press upon which the several electrodes are mounted. 1

Tubes of this type may'einploy a plate or anode, agrid or control element'a'nd a cathode assembly. The cathode assembly usually consists of a heater element as a length of re l fractory wire, enclosed in a suitable insulator,

such for example as quartz, porcelain, thoria, ora material known as isolantite. The heater element and the surrounding insulator may betermed aheater unit. The msulator of the heater unit is surrounded'by a metallic sleeve coated with a suitable electron emitting material, as a carbonate coating which is heated by conduction throughithe insulator from the heater element, which may be in the form of a filament wire, preferably of tungsten, molybdenum, but which may be of other suitablematerial as tantalum, or the like.

' In the manufacture of the above mentioned type of device, such as a radio tube,it is necessary to attain relatively long life in order to-make a tube of economic value in such apparatus where it findsuse and especially in connection with radio receiving circuits. Great dilficulty' has heretofore been encountered in the manufacture of a cathode assembly which would give the required life owing tothe gases evolved from the insulators employed. 'These gases continuously attack the. filament at the-hotter portions and insulators of, different. materials have, therefore,

the useful life of the device is 928. Serial no, 226,340,

been used i f n attempt eq m 5m out life s ype of device.

It will also be evident that in additionlto the failure of the device by reasoni of the erosion causing a breal; in the filament,that any erosion of the filament is adis a dvantage since a reduced filament area results in in;

'duced currentl. iDevicels of the characterito which the present invention relates must be manufactured with accuracy as to theirelectrical characteristics and if a device is initially constructedwith a given filament cur-- rent this current must be maintained tl ir oughii out the life of the device. This, however, is not possible where erosion occursii the present inventionthe coating provided ontlrie filamentor heater usually flashes off and becomes incorporated as part of the wallet the insulator. Extension tests have shownthat the coating does not cause an change in t he current flowfso that the givenele'ctrical characteristics of the device remain constant:

Upon investigation, it has been found that filament burnouts and drop in filament cur rent are caused primarily by a disinte'gra tion or ercsion of the heater elementorffila? ment giving what be; termed a tapered burnout, v

It is b ieved that t ytap'ere f urmut wasting away of the ,filamentat the hotter portions, may be paused bypresence of gas 7 released from the porous insulator, which gas may be water vapor, carbon monoxide or dioxide. This gas s g ven oftgradually during the life of thedevice causing continuous a ck up n h h t es pbi tiel of th t at element orfilament with the resultthat the device willj-not operate at the rated voltagevolf long life d eara lcf ma tai in uniferm I filament current, b I H b Another object ofthe invention isto soproshortened test a hea e elemen -Or filam ntas to Preserve its usefulness,

n the objec Qtth nYQhtiQns P ide a ca holi assembly with medium terposed between the heater element and the insulator, whereby detrimental conditions tending to shorten the life of the filament are rendered ineffective. I 7

e Other objects and advantages of the invention will be apparent from the following description,- together with the accompanying dra-wing,1n which ..Figure 1 is a longltudinal sectional View of accordance with" the present. invention and an insulator 'with a filament therein.

Figure 2 is a transverse sectional view of the insulator member and filament; I I

Figure 3 is an enlarged transverse sectional view showing the protective coating about the filament; and

Figure 4 shows a radio tube constructed in I applied to an exhaust of a'vacuum pump.

.tion' as desired.

' In Figure 1- of the ac'companying'drawing, a cathode assembly 4 is shown in section and, as will be noted, the said assembly 1ncludes a filament 5 bent to provide sections 6 and 7 which are disposed in apertures 8 and 9 provided in an insulator member10. The insulator maybe made circular, as shown 1n the transverse cross section (Figure 2), or it may be non-circular in transverse crossssec- The. cathode assembly shown is given as an example of one practical embodiment of. theinvention and it willbe seen from the draw-' ing that'the outer surface of an insulator 10 iii ' tite, around the filament.

fromthe said wall.

is enclosed in a metallic sleeve 11 upon which an electron emission material 11. is deposited. The coating which protects the filament is indicated at 12 in Figure '3 as being incorporatedin the surroundingwalls of the in-' sulator, although this coating may be free In practice, apertures IQand 13 which receive the filament are about 6.5 mils to 7 .5

mils diameter and the wire is about 5 mils diameter. This gives a closespaced' relation between theinsulator and the filament and when using an'insulator such as isolantite the walls thereof often. fuse about and bind the filament. With the present method, the lIlSll".

the filament after use indicated that the eroding agent was water vapor. This conclusion was supported by further investigation when the filament was heated during the operation ofexhausting the device and also upon a fus ing of the insulator, as for example, isolan- The present invention, therefore, provides the filamentwith a protective medium capable' of fixing oxygen and thus breaking the Water cycle. Materialsfor this purpose may be chromium, thorium, zirconium, uranium, tantalum, boron, silicon, vanadium, ti-

tanium, etc., or any other elements capable of forming compounds with and 'holding indefi- 7 nitely any gases detrimental to the life of the filament or capable of causing an erosion of thefilament. v Although the above substances will give the required results, chromium has been found to be preferable and chromium plated wire may be used. Ordinarily a plated wire may be brittle, although devices using chromium plated wire were successful and showed no.

filament erosion or other detrimental effects after long burning (over 1000 hours).

I In order to avoid embrittling the wire, the

chromium may be applied by mixing chro mium powder with abinder as amyl acetate. The method of applyingthe chromium'in'the binder may be the same as that practiced for I the' gettering of filament wire, viz., the wire may be passed through a mixture of chromium powder and binder or the wire may be dipped or sprayed. I It has been found that good results are obtained by mixing 10 grams of chromium powder, passed through a 200 mesh screen, with 15 cc. of a 3% solution offilm dope in amyl acetate. These ingredients may be thor oughly mixed in a ball mill. Thewire may then bepassed through the mixture at aspeed of about 6 meters per minute. The filament will then'be coated and maybe formed in V or hairpin shaped sections. These sections are then threaded through the bores 8 and-9 or apertures in the insulator 11. The insulator may then beassembled in what has been termed the catho'de'assembly 4 and mounted in abulb 14: withthe other elements as the. grid 15 and anode 16, thus making theinterior structure of the device complete and ready for the exhausting operation,.where upon it is appliedto a vacuum pump i(not shown) by inserting exhaust tube 17 into an exhaustport 18. v p In practice, the exhausting operation is per.-

formed in what is termeda 20 second cycle, the filament being moved through sixpositions 7 unlighted, eight positions while lighted with apotentlal of 5.5.volts and twelve positions at 4.5 volts; This voltage is relatively high is primarily to break down the carbonates of the cathode to the oxide, but in the present invention the heating is utilized as an aid to lnasmuch as the normal voltage of the fila- W mentis 2.5 volts. The heating of the filament 7 heat the chromium so that oxides or compounds will be produced around the hotter portions of the filament by reason of the gases released from the insulator or otherwise dur ing the heating. Due to the high heat treatment, gases injurious to the life of the'filament are driven-from the insulator, which gases form stable compounds by reason of their greater chemical affinity with the chromium than with the filament and the ases are thus prevented from attacking the filament.

filament wire with the above mixture and employing an insulator element such as :lSO? lantite, that the isolantite being fusible at a relatively low temperature will fuse about the filament along its length except at cooler portions adjacent to the ends of the insulator. In non fusible insulators as porcelain, quartz, etc, in the event that gases are evolved the chromium powder will be precipitated to the wall of the insulator around the hot portions of the filament. Inasmuch as the chromium powder is disposed as a protective layer in the wall of the insulator, the filament will be free from any erosive action, as would otherwise occur from gases liberated from the insulator, and at the same time the filament will be free to move relative to the insulator under vary-' ing temperature conditions.

The chromium powder, when combined;

ing the filament to apply the coating to the wall of the insulator prior to the heat treatment to produce the protective layer. Primarily, the protective medium employed serves to guard the filament against erosion caused by the attack of the gases which are released. Secondly, the coating provides a protective agent which is neutral to the filament and does not alloy with it, thus the voltage of the filament remains unchanged.

Thirdly, the protective agent is of such a character, particularly when used with an isolantite insulator which fuses about the filament, that the filament remains free for relative movement with respect to theinsu:

lator under variable temperature conditions.

When using an insulator of quartz, porcelain or other material having a high fusing point, the binding action of the insulator is notas important. However, the advantage of the protective agent remains, since it is necessary in any case to protect the filament against erosion due to released gases, which causes a disintegration of the filament at the hotter portions and destroys its usefulness.

Any insulator be employed with a metal that is not readily volatilizedbut is chemically active, such as chromium, and it has been foundthat porcelain "or isolantite,

when used with atungsten filament protected by chromium, gives satisfactory results. The

slag formed is the result of a reaction" between the chromium and the silica "of the insulating material and presents a hard greenish e' to the filament and bondswith 1- the insulator.

glazed surfac n will he-seen item the Me an; that the inventive idea contemplates'theuse of a filament incombination with a material which, acts as'a keeper' or fixer which neutralizes device to which the invention maybe'appliedQ Although itis believed that the injurious element is water vapor, it is to be understood that erosion of the filament may be due to carbonaceous gases in which the material employed will form permanent carbides with the same beneficial results.

The present invention, therefore, provides a heater unit for a thermionic device, such as a radio tube, in which the heater element, which ispreferably in the form of a tungsten filament, is protected against the erosive action caused by the attack of gases. Although coatings composed of various substances may be employed and changes and modifications may be made in the proportions thereof, it is to be understood that the present invention contemplates a simple and effective prevention of filament erosion and that the same may be accomplished in various ways without departing from the spirit and scope of the invention as set forth in the V appended claims.

What is claimed is:

1. A heater unit for an electron emission device comprising an insulative sleeve enclosing a refractory wire and a layer of chromium oxide-on the wall of the insulator and r between the insulator and the refractory wire.

2. A heater element for an electron emis- I sion device comprising a refractory wire, a tubular insulator surrounding said wire, the inner wall of said insulator being permeated with a medium to protect the filament against erosion caused by the release of gases.

3. An electrical device comprising a sealed envelope, aninsulator within said envelope having an electron emission material thereon, I r

a heater element for elevating the temperature of said insulator and a material disposed on the wall of said insulator adjacent to said element for neutralizing gases detrimental to the life of said element. 7

. 4:. A thermionic device having an indirectlyheated cathodecomprising a heater =ele- V ment, an insulator enclosing said element and a layer ofpacompoundon the, Wall of the I 5 insulator adjacent to said element to protect said element from'gases liberated fromsaid insulator. v 1 M. v

,5. A thermionic device having an indirect- 1y, heated cathode comprising a heater elev ment, a porous insulator surrounding, said I element and a stable compound on the inner I surface of the insulator to prevent gases from I attacking the. filament. Y l In testimony whereof, I have hereunto sub.-

flscribed my name this 8th day of May, 1928;

, JOHN HUMPHREYS RAMAGE

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