US3195004A - Cathode heater for electron discharge devices - Google Patents

Cathode heater for electron discharge devices Download PDF

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Publication number
US3195004A
US3195004A US50706A US5070660A US3195004A US 3195004 A US3195004 A US 3195004A US 50706 A US50706 A US 50706A US 5070660 A US5070660 A US 5070660A US 3195004 A US3195004 A US 3195004A
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United States
Prior art keywords
heater
cathode
coating
aluminum oxide
tungsten
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Expired - Lifetime
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US50706A
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English (en)
Inventor
William A Hassett
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RCA Corp
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RCA Corp
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Filing date
Publication date
Priority to NL268393D priority Critical patent/NL268393A/xx
Application filed by RCA Corp filed Critical RCA Corp
Priority to US50706A priority patent/US3195004A/en
Priority to DER30772A priority patent/DE1141388B/de
Priority to GB26618/61A priority patent/GB922440A/en
Priority to FR869518A priority patent/FR1296399A/fr
Priority to ES0269986A priority patent/ES269986A1/es
Application granted granted Critical
Publication of US3195004A publication Critical patent/US3195004A/en
Anticipated expiration legal-status Critical
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/13—Solid thermionic cathodes
    • H01J1/20—Cathodes heated indirectly by an electric current; Cathodes heated by electron or ion bombardment
    • H01J1/22—Heaters

Definitions

  • This invention relates to electron discharge tubes and more particularly to insulated heaters for indirectly heated cathodes of such tubes.
  • a hollow sleeve for the cathode which is heated by a heating element contained Within the sleeve and supported therein by insulated contact with the cathode sleeve walls.
  • the heating element is made up of a relatively ne wire of refractory material, usually of tungsten, which is folded or coiled to pack a required amount of the wire into the space enclosed by the cathode. The amount and size of the wire is determined by the power required to heat the cathode and by the desired voltage-current relationship of the heater to give this amount of power.
  • Ends of the heater wire extend outwardly from the cathode sleeve to electrical connecting points whereby electrical current may be passed through the heater wire.
  • the heat produced in the heating element is transferred to the cathode, mostly by radiation, and the cathode is heated thereby to its operating temperature. Since the heater and the cathode are generally operated at different electrical potentials, however, the heater wire must be coated with a suitable insulating material to electrically insulate the two elements.
  • a problem which has long existed in the manufacture of electron tubes using indirectly heated cathodes is to provide satistactory means to electrically insulate .the cathode from its heater.
  • the methods most satisfactory in terms of the electrical performance of the tubes, and which are used in especially ditlicult tube types are to suspend the heater Wires within the cathode in such a manner that the wires do not touch the cathode sleeve, or to insert the heater into a separate insulating block to space the wires from the cathode.
  • a construction frequently employed is to coat the heater wires with a thin coating of insulating material and to allow the insulated heater to contact the cathode walls.
  • the insulated heater arrangement permits rapid initial heating of the cathode.
  • the absence of the extra insulating block reduces the mass to be heated and hencethe time required for the heating, while the contact of the thin coating kwith the cathode permits an additional transfer of heat by conduction to the cathode during the time theheater is heating up to the temperature at which heat transfer by radiation becomes more effective.
  • the disadvantage of the coated heater construction is the dihculty of providing a heater coating material which is satisfactory for use in an electron tube.
  • the cathodes may operate at a temperature of around 800 C., with the heaters necessarily at an even higher ternperature.
  • the temperature of the heater may reach values in the order of 2000 C.
  • the material must be chemically and physically stable .so that it will not break down during many thousands of hours of operation of the electron tube; the vapor pressure must be low so as not to adversely affect or poison the electron emission from the chemically sensitive cathode; the material should be chemically inert so as not to interact with either the cathode sleeve or the heater wire to change the operating characteristics of either; the diffusion rate of ions and impurities through the coating by ⁇ electrolysis must be small to prevent flow of current thereby, the material should have good adherence to the heater wire to avoid peeling or iiaking dueto externally caused vibration of the heater and unequal thermal expansion of the heater wire and the coating; the electrical resistance between heater and cathode must be over l0() megohms with coating thicknesses of the order of
  • the heat radiating capacity of an incandescent body is a function of the thermal radiant emissivity of the body. Furthermore, the emissivity of such a body is related tothe color, or blackness, along with the roughness of the surface of the body. It follows, therefore, that the thermal radiant emissivity of heaters used in electron tubes is determined by the insulating material coated onto the heater base wire. The significance of this is that in orderto raidiate a required amount of heat to raise the temperature of a given cathode to its electron emissive operating temperature, a heaterwith a low thermal emissivity must yoperate at a higher temperature than a heater with a higher emissivity.
  • the temperature at which the heaters must operate should be reduced by using heater insulating materials having high thermal radiant emissivities.
  • the insulation naterial almost invariably used in modern-day electron tubes is aluminum oxide. This material, unfortunately, is white in color and has a verv low thermal emissivity at the temperatures at which the heaters operate. Also, it has long been believed that the aluminum oxide used must be as pure as possible to obtain the best insulating characteristics. The purer the aluminum oxide, the whiter is it color.
  • a still further object of this invention is to provide an improved heater having a longer life than presently available heaters and in which shorts occurring between heater and cathode sleeve are reduced.
  • tungsten is added to the coating of aluminum oxide normally applied to a cathode heater.
  • the result is an insulated heater with a coating thereon having insulating characteristics almost that of aluminum oxide, but with a higher thermal radiant emissivity than aluminum oxide.
  • the tungsten may be added to the aluminum oxide in the following ways: it may be admixed with the aluminum oxide and applied as a single coating directly onto the heater wire; it may be added as a second coating over a iirst coating of aluminum oxide applied directly to the heater wire; or it may be applied in chemical or physical combination with some other material and subsequently processed to remove the other material leaving only the desired additive material as a second coating over a rst coating of aluminum oxide.
  • the second coating may comprise only the tungsten, or the tungsten admixed with aluminum oxide.
  • FIG. l shows an indirectly heated cathode in perspective partly broken away to show details of construction and a coated insulated heater therein of the type which may be made in accordance with my invention
  • FlG. 2 is a graph showing a relation between the temperatures of black and white heaters and their cathodes with respect to the heater watts input;
  • FIG. 3 is an enlarged view of a portion of the heater shown in FIG. l;
  • FlG. 4 is a view similar to FIG. 3 but shows an alternate construction
  • FIG. 5 is an idealized view of a greatly enlarged crosssection of a heater coating made in accordance with this invention.
  • FIG. 6 is a view similar to PEG. 5 but shows a different heater coating made according to this invention.
  • FIG. 1 shows an example of but one of such cathodes and heaters used and well known in the art.
  • the cathode element comprises a tubular hollow sleeve lt? having coated thereon an electron emissive material lll, which is capable of emitting electrons when it is heated to a sufficiently high tem- Cil perture.
  • This coating may comprise 'a known mixture of strontium and barium carbonates, or the like.
  • the heater ltshown for heating the cathode is one which is known asv a folded heater, comprises a plurality of folded strands of refractory wire, usually of tungsten, which are coated with an insulating material l2 to prevent electrical shorting between the base wire i3 and the cathode sleeve itl. Ends of the heater wire l5 extend outwardly from the cathode and are welded to electrical connectors itl, which in turn are connected to an electrical energy source7 not shown. The heater wire is coated over its entire length except at the leg portions lo and at the folded apices i8.
  • the uncoated leg portions lo are provided to permit welding of the base wire to the electrical connectors 2.o, and the uncoated apices i8 occur as a result of the manufacturing processes of a heater wire wherein the wire is irst coated and then folded over knife edges.
  • the coating material is somewhat brittle, and in the folding operation the coating at the point where the heater wire is bent chips off to leave the uncoated apices.
  • the heater is contained almost entirely within the cathode sleeve lil.
  • the cathode dissipates all the heat energy it receives by radiation to its environment, through conduction to the elements supporting and fixing the cathode (not shown), and as thermal energy carried off by the electrons emitted from the cathode emissive coating.
  • the rate at which a body radiates heat is a function of its temperature and its size.
  • the rate at which heat must be added to the cathode at thermal equilibrium to maintain its temperature at that level required for ecient electron emission is equal to the amount of heat which the cathode will dissipate at the desired temperature. This, in turn, determines the amount of electrical energy which must be put into the heater, and the temperature at which the heater must operate. ⁇ What occurs qualitatively is that as the electrical energy is converted to heat within the heater wires, the temperature of the heater continues to rise until the heater reaches a temperature at which its rate of energy radiation output equals the rate of energy input.
  • the eiiiciency of such heat radiation is a function of the thermal radiant emissivity of the heater, and the higher the emissivity the lower the need by the temperature of the heater to radiate the given amount of heat.
  • FIG. 2 is a graphic illustration of the temperature relationship between a black heater and its cathode, and a white heater and its cathode shown as a function of electrical input energy in watts to the heaters.
  • the cathodes are identical; and for any given energy input into the heaters, as shown, the cathodes reach substantially the same temperature while the black heater operates at a signilicantly lower temperature than the white heater.
  • the slight difference between the cathode temperatures is due to the differences in heat loss due to conduction.
  • Such conductive losses are a function of temperature gradients, and the heat conducted away and lost from the black heater by its electrical connections is less than that from the hotter white heater.
  • tungsten is added to white aluminum oxide coated eaters to raise the thermal radiant emissivity of the heater surface.
  • a body need not necessarily be optically black in appearance, but may be thermally black even if lightin color. The reasons for this are not completely understood. Also, since the total radiation from a body is dependent upon its surface area, a body having a surface of iinely divided particles will have a larger radiating area and hence a greater thermal radiant emissivity than a body with a smoother surface.
  • tungsten which permit its use are that it is relatively thermally black, and that it may be added to the aluminum oxide in finely divided particle form. Also, tungsten is refractory (that is, has a melting temperature above 2000" C. and does not melt during tube processing), is chemically and physically stable, has a low vapor pressure, and does not affect the aluminum oxide so as to interfere with the insulating characteristics or the aluminum oxide.
  • the tungsten may be added as a second coating on top of a tirst coating of aluminum oxide (FlG. 3) as follows: heaters coated with aluminum oxide are manufactured by conventional means with the sole exception that the heaters need not be tired in hydrogen to sinter the aluminum oxide to the base wire. Actually, the heaters may be tired if desired, but this is not necessary as will be seen.
  • the tungsten is prepared as a powder of iinely divided particles as by ball milling or the like, and mixed with a suitable dispersing agent such as methanol. This suspension is sprayed onto the aluminum oxide coated heaters by'conventional spraying means and then allowed to dry. After drying, the doubly coated heaters are tired in a hydrogen atmosphere.
  • the time and temperature of the tiring is not critical, and for example, the ring may be for ive minutes at l700 C.
  • the hydrogen firing drives off the methanol dispersing agent and sinters the two coatings to provide good adherence of the aluminum oxide to the base wire, and good adherence between the tungsten second coating and the aluminum oxide first coating.
  • the proportions of the mixture are not critical and depend only upon the compromise desired between the reduced radiant emissivity of the second coating and the increased adherent strength between the two coatings as caused by the addition of the aluminum oxide.
  • the result of such a mixture is a heater having a rst coating of aluminum oxide with a second coating of a mixture of aluminum oxide and tungsten. In one instance, the proportions were 50% tungsten and 50% aluminum oxide, by weight.
  • Another method of application of the ytungsten coating is to add the tungsten as a compound in solution, and to apply subsequent treatment to reduce the compound and remove all but the desired tungsten from the heater.
  • An example of this latter method is to add tungsten to the heater as a solution of ammonium tungstate in water by spraying, and then to tire the heater in hydrogen at 1000" C. for twenty minutes to reduce the ammonium ⁇ tungstate to pure' tungsten and ammonia.
  • the tungsten remains on the heater as a finely divided powder coating, while the ammonia is driven off as a gas.
  • a modication of the above which has been found to produce excellent adherence between the -two coatings is to add aluminum oxide to the ammonium tungstate solution.
  • the mixture ot aluminum oxide and tungsten was then ball milled for thirty minutes as the following suspension: 108 grams aluminum -oxide and tungsten; 200 cc. binder; and 100 cc. methanol.
  • the binder was composed of 64.5 grams nitrocellulose, 1455 cc. butyl acetate; and 1355 cc. diacetone alcohol.
  • the above mixture was then sprayed by conventional means on the surface of aluminum oxide coated heaters.
  • the heaters were red in hydrogen at 1000 C. for one hour to sinter the double layers together and to burn out the binder in the suspension.
  • the aluminum oxide particles prepared mechanically are of much larger size than the tungsten particles which are precipitated from their solution as a tine powder. It is believed probable that the tungsten is mixed with the aluminum oxide particles as shown, and that the basic matrix of the second coating is made up of aluminum oxide interspersed with the tungsten particles. The second coating thus physically resembles the first coating of aluminum oxide and excellent adherence between the two is achieved.
  • the tungtsen particles serve to blacken and roughen the second coating, and a high radiant emissivity is also obtained.
  • the tungsten is ball milled along with aluminum oxide to produce a mixture as represented in FIG. 6.
  • the insulation properties of the aluminum oxide will be somefwhat adversely alected by the inclusion of the tungsten, however, and darkened single surface insulation coatings may presently be employed only in electron tube types wherein some heater-cathode leakage current may be tolerated.
  • the ball milled mixture is dispersed in methanol, sprayed on the heater Wire, and subsequently red in a hydrogen latmosphere.
  • An example of a single blackened insulating coating comprises a mixture of 5% tungsten to 95% aluminum oxide, by weight.
  • the hydrogen firing is for five minutes at 1700 C.

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  • Solid Thermionic Cathode (AREA)
US50706A 1960-08-19 1960-08-19 Cathode heater for electron discharge devices Expired - Lifetime US3195004A (en)

Priority Applications (6)

Application Number Priority Date Filing Date Title
NL268393D NL268393A (fi) 1960-08-19
US50706A US3195004A (en) 1960-08-19 1960-08-19 Cathode heater for electron discharge devices
DER30772A DE1141388B (de) 1960-08-19 1961-07-18 Kathodenheizer fuer Elektronenroehren
GB26618/61A GB922440A (en) 1960-08-19 1961-07-21 Cathode heater for electron discharge devices
FR869518A FR1296399A (fr) 1960-08-19 1961-07-31 Filament de chauffage de cathode pour tubes électroniques
ES0269986A ES269986A1 (es) 1960-08-19 1961-08-18 Mejoras introducidas en los calentadores catëdicos para tubos de descarga electrënica

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US50706A US3195004A (en) 1960-08-19 1960-08-19 Cathode heater for electron discharge devices

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US3195004A true US3195004A (en) 1965-07-13

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US50706A Expired - Lifetime US3195004A (en) 1960-08-19 1960-08-19 Cathode heater for electron discharge devices

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US (1) US3195004A (fi)
DE (1) DE1141388B (fi)
ES (1) ES269986A1 (fi)
GB (1) GB922440A (fi)
NL (1) NL268393A (fi)

Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3246197A (en) * 1962-10-02 1966-04-12 Westinghouse Electric Corp Cathode heater having an aluminum oxide and tungesten coating
US3328201A (en) * 1964-04-27 1967-06-27 Rca Corp Heater for electron tubes
US3401297A (en) * 1965-08-23 1968-09-10 Varian Associates Thermionic cathodes for electron discharge devices with improved refractory metal heater wires
US3418164A (en) * 1963-02-06 1968-12-24 Philips Corp Filament wire for use in the cathode of a thermionic valve
US3423583A (en) * 1964-03-27 1969-01-21 Commissariat Energie Atomique Method of stabilization of thermionic sources and thermionic source obtained by application of said method or a like method
US3450565A (en) * 1964-12-18 1969-06-17 Sylvania Electric Prod Method of coating heater coils
US3477110A (en) * 1965-03-11 1969-11-11 English Electric Valve Co Ltd Method of making electron discharge device cathodes
US3490944A (en) * 1965-09-23 1970-01-20 Philips Corp Cathode heater for electron discharge device
US3494779A (en) * 1965-09-29 1970-02-10 Ncr Co Oxygen-dominated phosphor films
US3500454A (en) * 1967-11-16 1970-03-10 Sylvania Electric Prod Insulator heater coating for heater-cathode assembly
DE1639016B1 (de) * 1967-03-07 1972-05-31 Varian Associates Verfahren zum herstellen von kathoden-heizern
US3737714A (en) * 1964-12-18 1973-06-05 Sylvania Electric Prod Dark coated heater for vacuum tube cathode
US3902093A (en) * 1973-04-06 1975-08-26 Int Standard Electric Corp Cathode heater element with a dark heat radiating coating and method of producing such
US4126489A (en) * 1973-07-17 1978-11-21 Varian Associates, Inc. Method of making cathode heaters
US10741351B1 (en) * 2019-08-01 2020-08-11 Lockheed Martin Corporation Multi-apertured conduction heater

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1281039B (de) * 1963-09-25 1968-10-24 Telefunken Patent Verfahren zur Herstellung einer indirekt geheizten Kathode fuer Elektronenroehren
DE2317445C3 (de) * 1973-04-06 1982-09-09 Standard Elektrik Lorenz Ag, 7000 Stuttgart Verfahren zum Herstellen eines Heizkörpers für eine indirekt geheizte Kathode
JPS5165861A (en) * 1974-12-05 1976-06-07 Sony Corp Toranjisutano baiasukairo

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2014787A (en) * 1933-06-24 1935-09-17 M O Valve Co Ltd Thermionic cathode
US2724788A (en) * 1952-02-12 1955-11-22 Electrons Inc Indirectly heated cathode for gas tubes
US2996643A (en) * 1959-07-16 1961-08-15 Eitel Mccullough Inc Art of heating electron tube cathodes
US3005926A (en) * 1959-05-22 1961-10-24 Westinghouse Electric Corp Cathode for electron discharge device
US3029360A (en) * 1958-04-29 1962-04-10 Rca Corp Heater wire coating process

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2014787A (en) * 1933-06-24 1935-09-17 M O Valve Co Ltd Thermionic cathode
US2724788A (en) * 1952-02-12 1955-11-22 Electrons Inc Indirectly heated cathode for gas tubes
US3029360A (en) * 1958-04-29 1962-04-10 Rca Corp Heater wire coating process
US3005926A (en) * 1959-05-22 1961-10-24 Westinghouse Electric Corp Cathode for electron discharge device
US2996643A (en) * 1959-07-16 1961-08-15 Eitel Mccullough Inc Art of heating electron tube cathodes

Cited By (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3246197A (en) * 1962-10-02 1966-04-12 Westinghouse Electric Corp Cathode heater having an aluminum oxide and tungesten coating
US3418164A (en) * 1963-02-06 1968-12-24 Philips Corp Filament wire for use in the cathode of a thermionic valve
US3423583A (en) * 1964-03-27 1969-01-21 Commissariat Energie Atomique Method of stabilization of thermionic sources and thermionic source obtained by application of said method or a like method
US3328201A (en) * 1964-04-27 1967-06-27 Rca Corp Heater for electron tubes
US3737714A (en) * 1964-12-18 1973-06-05 Sylvania Electric Prod Dark coated heater for vacuum tube cathode
US3450565A (en) * 1964-12-18 1969-06-17 Sylvania Electric Prod Method of coating heater coils
US3477110A (en) * 1965-03-11 1969-11-11 English Electric Valve Co Ltd Method of making electron discharge device cathodes
US3401297A (en) * 1965-08-23 1968-09-10 Varian Associates Thermionic cathodes for electron discharge devices with improved refractory metal heater wires
US3490944A (en) * 1965-09-23 1970-01-20 Philips Corp Cathode heater for electron discharge device
US3494779A (en) * 1965-09-29 1970-02-10 Ncr Co Oxygen-dominated phosphor films
DE1639016B1 (de) * 1967-03-07 1972-05-31 Varian Associates Verfahren zum herstellen von kathoden-heizern
US3500454A (en) * 1967-11-16 1970-03-10 Sylvania Electric Prod Insulator heater coating for heater-cathode assembly
US3902093A (en) * 1973-04-06 1975-08-26 Int Standard Electric Corp Cathode heater element with a dark heat radiating coating and method of producing such
US4126489A (en) * 1973-07-17 1978-11-21 Varian Associates, Inc. Method of making cathode heaters
US10741351B1 (en) * 2019-08-01 2020-08-11 Lockheed Martin Corporation Multi-apertured conduction heater
KR20220028182A (ko) * 2019-08-01 2022-03-08 록히드 마틴 코포레이션 다수-개구형 전도 가열기

Also Published As

Publication number Publication date
NL268393A (fi)
DE1141388B (de) 1962-12-20
GB922440A (en) 1963-04-03
ES269986A1 (es) 1961-11-16

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