US1926407A - Filament for heater type vacuum tubes - Google Patents

Description

8 4: Patented Sept. 12, 1933 COATlNG OR PLASTIG j invention relates to'vacuum tubes and particularly to that type oi vacuum tube which has'a cathode adapted to be heated by an independent heater. The invention is an im-v 5 provementover'my co-pending applications entitled, Electron emission element, Serial No. 164,119 filed January 27, 1927, and Electron cliisclhgarge device, Serial No. 205,024 filed July In a vacuum tube of the heater type a cylindrical cathode is usually employed, through which the filament extends which is adapted to be energized by a current of electricity in order to heat the cathode- Such filaments are sep- 4 "arated from the cathode either by vacuous space or by an insulating material. so that the filament is prevented from touching the'cathode and the heat is transmitted to the cathode either by radiant heatv alone or by radiant heat and conduction throughv thevinsulation. In using a ,eathode which is separated from the heater by -vacuous space it is necessary to have enough clearance between the filament and cathode to prevent the filament from touching the cathode due to movement caused by temperature changes in,both the cathode and the filament and also.

to permit convenient manufacture of the same in quantities. This clearance between the filament and cathode considerably increases the heat loss, as the heat radiated from the fila- 7. ment to the cathode is inversely proportional to the square of the distance the two are apart and I the danger is always presentthat'some distortion Twill cause the filament and cathode to touch,. I thereby rendering the tube useless.

For tubes of the type in which a solid dielectric l is used between the filament and cathode it has been'difiicult to find such a dielectric which is inexpensive and which permits close proximity of cathodeand heater. One of the best sub- 1 stances to use for such a solid dielectric is,quartz,

which must necessarily have considerable thicknes'sinorder to standup of itself andprevent fracture due, to vibration and temperature changes. The necessary thickness of the solid a dielectric also tends to increase the heat loss r 1 filament and the corresponding variations of the cathode. 7

It is one of the objects or the present inveni 'tion, therefore, to provide a method of applying an insulating coating directly to the filament or a vacuum tube which will have a greater di-.

FOR HEATER TYPE VACUUM I I Samuel Ruben, New York N. assignor Ruben Tube Oompanaa corporation of Dela- Y f fLNo Drawing. Application rebi-tar yesfiezsir. Serial N0.257,106

Ctoss Reference electric strength than any solid dielectric here- H11 tofore used and which will be exceedingly thin so that the heat loss between the filament and cathode is materially reduced. 1.

Another object of the invention is to provide G0 a filament having a high dielectric coating which is so closely associated with the filament as to resist the effects or thermal mechanical movement. W .1 1i. Another object of the invention is to' provide a filament having a coating which has anfextremely high dielectric strength at high temperatures and which is also an excellent conductor of heat and a radiator or heat. :1 Another object of the invention is to provide a method of coating thefilament wire. with an'i; insulating material having a high dielectric strength, whereby such coating may be ,continuously applied to a long wire which may be wound upon a spool andthereafter cut into the desired filament lengths asused. r 1 I Other objects of the invention will be apparent as the description thereof proceeds. In carrying out the invention I prefer to use a filament or tantalumwire for the reason that so tantalum has the property of absorbing. gases when heated, which assists in maintaining the vacuum in the tube and because tantalum also has a high specific resistance. It is to be understood, however, that -otherl materials such as 35 nickel or tungsten might be used if desired. The'- tantalum wire is first placedin an oxidizing atmosphere and subjected to a temperature or about 600 C. until a dense coherent white coat ing of tantalum pento e (T3205) is formed. The wire is t covered wit a ture of finely divided silieogjioxidc gsiogz which is suspended in a very'weak so u ion ofvsodium silicate, such suspension emg more in e na ure 0 a pasty mixture. The sodium silicate is used merely as a binder to hold the particles of silicon dioxide together on the filament until the same has been dried and heated. The silicon dioxide solution -may best be prepared by grinding the silicon dioxide in a ball mill with the binder solution until a paste of the desired smoothness is obtained.

This coating of silicon dioxide quickly dries and remains on the filament until the filament is used. When the filament is mounted in a vacuum tube and the tube evacuated, it is heated to about 1600 0., which expels all occluded and chemically formed gases or volatile products from the coated filament and the heat also pro duces two important effects on the silicon dioxide coating. In the first place, there is a tendency EXAMINER] Oil for the silicon dioxide to combine chemically with the tantalum pentoxide so that the silicon dioxide coating not only becomes firmly attached to the filament by reason of the fact that it 6 hardens against the roughened 'oxide surface but the tendency to combine chemically makes an integral connection between the .two which. cannot be impaired, due to diiferences of the coeificients of expansion of the metal and coatings} 10 Another result produced by this temperature is that the particles of silicon dioxide at the outer' surface thereof sinter or fuse together to form an extremely hard surface 'of silicawhich in creases the dielectric strength of the coating'and also provides a better radiating surface for the heat, which is desirable, as no matter; how ,close the cathode is placed with respect to thefilament, there will always be some places, which do not touch and which will then depn'd'on radi ated heat for energization, h

The coating onv the. filament has an extremely .high dielectric strength and is also exceptionally thin, the thickness thereof not necessarily ex- {3 acceding .05 millimeters. The filamentso coated ;may'be.placed inside of a cathode tube which is dust sufiicient in diameter to permitthe filament to be slipped thereinto and which will closely engage the sides of the filament with no danger at :3 :all of the insulating coating breaking down with 0 differences of. potentials commonly usedbetween --the filament and cathode.---;

I While .I have described the use of silicon di-- oxide in a solution of sodium silicate other subwstancesmay Be usedwitfi good results, if desired. For instance, a1 ma n boric oxid may be useg, but silicon, ving a very high fielectric strength, is preferable. Also, I the binderof sodium silicate is preferred because :-it requires a very small concentration to bind. 40 the particles of silicon-dioxide together and it.

:releases a minimum of gas. vapors during the life of the tube. :Other binders, however, could be used, such as resinous or organic types, the nec- 03 essary feature ing merely that'tliebinder hold the particles of silicon dioxide to the oxide coatring of the filament until the filament is ready tO be used. flue,-

" ,2 f x The surface of the metal filament is best pre- *pared to receive the silicon dioxide by forming an oxide upon it,-as indicated above. .It is possible, however, to place the silicon dioxide direct- 21y. upon the metallicfilament without forming the oxide, but I havefoundthat inso coating the .f.=filament the silicon dioxide cannot be as uniformly distributed, norwill it adhere tothe sur-' faceas well, .due to'surfacetension and other effects, and the resulting coating is liable to chip and peel when the filament expands and con-'- --the use-of the,

It .is evident Obi II have provided an insulated coating for a fila- 5 ment wire whichmay be easily applied to the,

swim and which produces an insulation having a' cathode and filament which are ordinarily used ..in vacuum tubeaandwhich coating is so 'thin' that thecathode may be positioned in close prox- .:.-1 i y t a.fi P-.$ .u$ seminafl e h t;

;,losses to aminimuma, a homogeneous struc- The coating has ineffect ture ranging from the metal coreat' the center to" ventionbroadlywithin the scope of the appended for an indirectly heated cathode in an electron being greatly in excessof theamount of solid .material in said binder, and heating said element 1 '2. The method ofinsulating which comprises heating said filament in an oxicdizing atmosphere to form an oxide thereon, coating said, oxide with finely divided silicondi- .said silicon dioxide.

of said oxide coating. I f 5. A heater element for a 61 Ah'eater element the above description that and combinations of materials used, so

that I do not desire to limit myself to the specific substances described but to interpret the inclaims.

Having thus described my invention, what I desire toclaim is:

1. The method of insulating a heater element discharge, tube which comprises forming an oxide on the surface of said element, immersing said element, in finely divided silicon dioxide suspended "iha liquid binder, the silicon dioxide to harden said silicon dioxideand e fi ambination of said oxides.

oxide suspended in a very weak solution gof sodium silicate and heating said filament to harden I05 3. The method of afll anient-w comprises heating said filament in oxidizing atmosphere, coating said filament with finely divided silicon dioxide suspended in a very weak; solution of sodium silicate'and heating said Illament, whereby the surface of said silicon dioxide is fused. 1.;

A heater element for an indirectly heated cathode in an electron discharge tube, acoating of oxide on said element and a secondlcoating 5 "on said oxide of an electrical 'insulating substance having a greater dielectric strengththan "said oxide, said secondcoating having a tendenc'y to combine chemically with said oxide and;-

"thereby obtain integral connection of said coating with the heater eler'ne nt through the'a'gency I n 'indlrec'tly he'a'ted cathodein an electron discharge tube, an oxide;- form'ing a coating on said element and a second coating on said oxide comprising an'electrical insulating substance having a' higher dielectric strength than said oxide and combinedftherm I l a. set. "J.

H for an indirectly l'ieated cathode in an electron discharge tube, an oxide coating on the surface of said element and a 60 tracts under temperature chan es. IUS within *second coating or finely dmdgd smcon dioxide the spiriteof the invention, however, to include a dielectric applied directly to.. the

closelyassociated with said first coating. p g:

17. A metallic heater element for an indirectly heatedcathode in an electron discharge tube, an oxide formed on the surface of saidelement and a coating of silicon dioxide physically a chemically associated with said oxide; i i a,

I. 8. 'A metal heater" element for an" 'indirectly heated cathode in anelectron discharge tube, an oxide formed on the surface of said elementand a coating on the surfaceof said oxide comprising finely divided silicon dioxide, with a binder of 5- relatively much smaller amount of sodium silicate. "-"Jr. '1. s 9. A tantalum heater element for an indirectly heated cathode' 'in an electron discharge tube adapted 'to be used "in a vacuum tube, 'acoa'ting ir- J ing the metal of the filament.

or tantalum pentoxide on said element, a coating of silicon dioxide physically and chemically combined with said oxide coating and a hard, fused surface on said silicon dioxide. 10. The method of insulating a'heater elemen for an indirectly heated cathode in an electron discharge tube which comprises forming an oxide coating on the surface or a filament of metal having a melting point above 1600 C., coating such oxidized surface with material consisting very largely of an oxide having a melting point above 1600 C. and high dielectric strength and a relatively much smaller amount of a binder and heating sufliciently to bring about partial chemical reaction between the materials of said coatings and adhesion between such materials and the metal of the filament.

11. The method. as in claim 10 in which the metal of the filament consists chiefly of tantalum and the first oxide coating is formed by oxidiz Cross Reference 12. The method as in claim 10 in which the second oxide coating consists very largely of tree silica.

13. The method as in claim 10 in which the binder consists chiefly of a relatively readily tusible silicate.

1a. The method as in claim 10 in which the metal of the filament consists chiefly of tantalum.

15. The method of insulating a heater element for an indirectly heated cathode in an electron discharge tube, which comprises heating a filament consisting chiefly of tantalum in an oxidizing atmosphere to produce a layer of tantalum pentoxide, coating the oxidized surface with silicon dioxide mixed with a relatively much smaller amount of alkali silicate and heating sufflcient- 1y to bring about partial chemical reaction be-e tween the materials of said coatings and adhesion between such materials and the metal of the filament.

SAMUEL RUBEN.

EXAMiNEH