EP0421372B1 - Procédé pour la fabrication d'une cathode pour un tube à électrons - Google Patents

Procédé pour la fabrication d'une cathode pour un tube à électrons Download PDF

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Publication number
EP0421372B1
EP0421372B1 EP90118910A EP90118910A EP0421372B1 EP 0421372 B1 EP0421372 B1 EP 0421372B1 EP 90118910 A EP90118910 A EP 90118910A EP 90118910 A EP90118910 A EP 90118910A EP 0421372 B1 EP0421372 B1 EP 0421372B1
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EP
European Patent Office
Prior art keywords
layer
alkaline earth
earth metal
carbonate
cathode
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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EP90118910A
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German (de)
English (en)
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EP0421372A3 (en
EP0421372A2 (fr
Inventor
Hisao C/O Mitsubishi Denki K.K. Nakanishi
Kinjiro C/O Mitsubishi Denki K.K. Sano
Toyokazu C/O Mitsubishi Denki K.K. Kamata
Takashi C/O Mitsubishi Denki K.K. Shinjo
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Mitsubishi Electric Corp
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Mitsubishi Electric Corp
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Publication of EP0421372A3 publication Critical patent/EP0421372A3/en
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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
    • H01J1/28—Dispenser-type cathodes, e.g. L-cathode
    • 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
    • H01J1/14—Solid thermionic cathodes characterised by the material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
    • H01J9/02—Manufacture of electrodes or electrode systems
    • H01J9/04—Manufacture of electrodes or electrode systems of thermionic cathodes
    • H01J9/042—Manufacture, activation of the emissive part

Definitions

  • This invention concerns a method of manufacturing an electrode tube cathode used in cathode ray tubes or other electron tubes.
  • the cathodes conventionally used in cathode ray tubes or other electron tubes were most often oxide cathodes comprising a base metal having nickel as the principal component and containing also minute quantities of reducing agents such as magnesium and silicon, on which a layer of oxide of alkaline earth metal including Ba was formed.
  • oxide cathodes were prepared by thermally decomposing an alkaline earth metal carbonate to convert it to the oxide and allowing reducing agents to react with the oxide so as to liberate free atoms, the free atoms then acting as electron donors to promote electron emission.
  • the reason why this complex procedure was used is that, although Ba has excellent electron emission properties, it is extremely reactive, and reacts with moisture in the air to produce barium hydroxide.
  • the carbonate was chosen as the starting material due to its chemical stability.
  • the carbonate may be the single element carbonate such as BaCO3 or a multi-element carbonate such as (Ba, Sr, Ca) CO3, but as the basic mechanism of activation forming the donor is the same for all of the above salts, we shall take the single element carbonate as an example, and describe the mechanism in detail with reference to Fig. 3.
  • Fig. 3 is a schematic sectional view of a conventional oxide cathode.
  • the cathode 1 comprises a cathode sleeve 1a and a cathode cap 1b.
  • a heater 2 is installed inside the cathode sleeve 1a so as to be able to heat the interior of the sleeve.
  • the cathode cap 1b performs the role of a base metal for cathode 1, and a layer of electron emissive substance 3 is formed on the cathode.
  • This layer 3 is initially formed as a barium carbonate layer.
  • the layer 3 is formed for example by spraying, electro-depositing or coating, onto the surface of cathode cap 1b, barium carbonate mixed and stirred into a solution of a resin such as nitrocellulose in an organic solvent.
  • the oxide cathode so formed is assembled in an electron tube, and when the temperature is raised to approx. 1000°C by heater 2 in the evacuation process to place the interior of the tube under vacuum, the barium carbonate of the layer 3 is thermally decomposed according to formula (I) below into barium oxide: BaCO3 ⁇ BaO + CO2 (I)
  • the carbon dioxide produced by the above reaction is evacuated outside the tube.
  • the nitrocellulose or other organic materials are also thermally decomposed into gases, and the gases are evacuated outside the tube together with the carbon dioxide.
  • Fig. 4 is a partial enlarged sectional view to describe the area near the interface 11 between cathode cap 1b and electron emission layer 3 in more detail.
  • minute rod-shaped crystals of barium oxide 8 agglomerate to form crystal grains 9 of several microns - several tens of microns in size, and there are interstices 10 of an appropriate size between these grains which make layer 3 porous.
  • the barium oxide in this layer 3 reacts with reducing agents such as Si and Mg in the cathode cap 1b at the interface 11 with the cap, and is reduced to free barium. This is due to the fact that the reducing agents diffuse and migrate between the grain boundaries 7 of the nickel crystals 6 in cap 1b, and give rise to the following reductions (II) and (III) in the vicinity of the interface 11. 2BaO + Si ⁇ 2Ba + SiO2 (II) BaO + Mg ⁇ Ba + MgO (III)
  • barium silicate (Ba2SiO4) is also produced by the following reaction (IV): SiO2 + 2BaO ⁇ Ba2SiO4 (IV)
  • the electron donor Ba is produced at the interface 11 between electron emission layer 3 and cathode cap 1b, migrates through the interstices 10 of layer 3, emerges on the surface and fulfils its function of emitting electrons. It also evaporates and reacts with residual CO, CO2, O2 and H2O gases in the electron tube to be eliminated. It is therefore necessary to supply Ba continuously by means of the above reactions, and these reductions take place continuously during the operation of the cathode.
  • the cathode is usually operated at a high temperature of approx. 800°C.
  • reaction products 12 of reactions (II) and (IV), that is SiO2, Ba2SiO4 or the like are produced at the interface 11 of layer 3 and cap 1b, and accumulate continuously at the interface 11 or at the crystal grain boundaries 7.
  • layer 3 and cap 1b tend to join together at the interface.
  • reaction products 12 provide a barrier (generally referred to as an intermediate layer) to the passage of Si or the like, reactions (II) and (III) tend to slow down, and the production of electron donor Ba becomes difficult. Further. as the intermediate layer has a high resistance, it interferes with the free flow of emission electrons.
  • Japanese Patent Application Kokai Publication No. 61-271732 (referred to hereafter as the prior art) was proposed.
  • powder of scandium oxide is dispersed in the electron emission layer to dissociate reaction products such as Ba2SiO4, and thereby break up the intermediate layer.
  • reaction products such as Ba2SiO4
  • the passage of reducing agents such as Si between the crystal grain boundaries 7 is facilitated, reactions (II) and (III) are promoted, and the electron donor Ba is produced more easily.
  • the production of electron donor Ba is promoted and electron emission properties are improved.
  • the electron emission layer has a high density, however, when the cathod ray tube was switched on and off, a large stress was produced due to the difference in thermal expansion coefficient between this layer and the cathode cap. This sometimes caused the electron emission layer to blister and swell up in places at the interface with the cap, and in severe cases even caused it to peel away completely.
  • Fig. 2 is a schematic front view of the surface of the cathode cap after the electron emission layer of the cathode has been carefully peeled off in a life test.
  • the peripheral area A is the part corresponding to barium silicate which is one of the above reaction products 12, and the central part B corresponds to nickel. The presence of both the barium silicate and nickel was confirmed by X-ray diffraction.
  • reaction (IV) occurred at the interface between the base metal of the cathode cap and the electron emission layer; whereas in the central area B, no reaction occurred as the base metal and the electron emission layer are not in contact. It may thus be conjectured that local swelling of the electron emission layer took place in the central area.
  • This invention was conceived to overcome the above problem. It aims to provide a method of manufacturing an electron tube cathode with superior electron emission properties wherein the electron emission layer does not swell up or peel away at the interface with the cathode cap.
  • the inventional method starts from the following method described in the above cited prior art (JP 61-271732): forming a suspension by suspending an alkaline earth metal carbonate powder and scandium oxide powder in an organic solvent solution of nitrocellulose, and regulating the particle size of said powders; applying said suspension onto a nickel base metal surface so that after evaporation of the solvent a layer of alkaline earth metal carbonate is formed; and heating said layer of alkaline earth metal carbonate in vacuum to a temperature of 800 - 1200 °C to decompose said carbonate into oxide, thereby forming a porous electron emission layer wherein scandium oxide is dispersed in an alkaline earth metal oxide on said nickel base metal surface.
  • the inventional method is characterized in that the alkaline earth metal carbonate layer has a density not greater than 2 mg/mm3.
  • the electron emission layer is formed by the thermal decomposition of the alkaline earth metal carbonate layer of coating density not greater than 2 mg/mm3, so the electron emission layer has a coarse porous structure.
  • the stress due to the difference in thermal expansion coefficient between the electron emission layer and the base metal is reduced. This appears to suppress the swelling of the layer and the peeling of the layer away from the surface of the base metal, so that a highly reliable electron tube cathode is obtained.
  • Fig. 1 is a schematic view showing the structure of one embodiment of the electron tube cathode of this invention.
  • Fig. 2 is an enlarged frontal view schematically showing the surface of the cathode cap where part of the electron emission substance has swollen up.
  • Fig. 3 is a schematic structural view showing one example of a conventional oxide cathode.
  • Fig. 4 is a partial enlarged sectional view to provide a detailed description of the area near the interface between the base metal and the electron emission layer.
  • Fig. 1 is a schematic view of the structure of one embodiment of the electron tube cathode manufactured by this invention.
  • components which are identical to those of the conventional example shown in Fig. 3 are given the same numbers, and their description is omitted.
  • 30 is a electron emission layer, and it is formed on the surface of a cathode cap 1b comprising nickel base metal and Si or Mg as reducing agents.
  • This electron emission layer 30 has an alkaline earth metal oxide as its principal constituent, and scandium oxide is dispersed in it.
  • the alkaline earth metal oxide is preferably barium oxide, but it may also be a ternary oxide containing Sr and Ca in addition to Ba. Further, the proportion of the scandium oxide is preferably in the range 0.1 - 20 weight %.
  • the electron emission layer of the above electron tube cathode must be formed from an alkaline earth metal carbonate layer of coating density not greater than 2 mg/mm3, but the coating density is preferably not greater than 1.6 mg/mm3 and more preferably 0.8 mg/mm3.
  • the electron tube cathode may be manufactured advantageously by the following method.
  • a powder of an alkaline earth metal carbonate and scandium oxide powder are suspended in an organic solvent solution of nitrocellulose, and the particle size of the powders is then adjusted so as to obtain a suitable suspension.
  • the above suspension may be similar to conventional suspensions. It may for example be prepared by blending barium carbonate and scandium oxide with a solution of a resin such as nitrocellulose in an organic solvent in the desired weight % (calculated on the assumption that the carbonate will be converted to oxide), and then crushing the product in a ball mill or other device to obtain a suitable particle size.
  • the barium carbonate may also be a ternary carbonate containing Sr and Ca in addition to Ba.
  • a layer of alkaline earth metal carbonate is formed by applying the, above suspension to a nickel base metal surface such that the coating density is not greater than 2 mg/mm3.
  • the organic solvent evaporates, and leaves an alkaline earth metal carbonate layer comprised of barium carbonate, scandium oxide and nitrocellulose.
  • the term "coating density" refers to the density of this alkaline earth metal carbonate layer formed from the suspension.
  • the suspension is applied such that the coating density of the layer is not greater than 1.6 mg/mm3, and more preferable that it is not greater than 0.8 mg/mm3.
  • the alkaline earth metal carbonate layer is normally formed by spraying. Apart from spraying, electro-deposition or a coating method such as spin coating or blade coating may also be used.
  • spraying electro-deposition or a coating method such as spin coating or blade coating may also be used.
  • application or “application” as used in the appended claims should be construed to cover “spraying”, “electro-deposition”, “coating”, “spray coating”, “blade coating” and other types of film formation.
  • the alkaline earth metal carbonate layer is heated in vacuum to a temperature of 800 - 1200°C to decompose the carbonate to oxide, and a porous electron emission layer of alkaline earth metal oxide containing a dispersion of scandium oxide is thus formed on the surface of the nickel base metal surface.
  • the heating of the alkaline earth metal carbonate layer is preferably carried out at a temperature of no less than 1000°C for several 10 seconds to several minutes.
  • Cathodes were manufactured with different coating densities of the alkaline earth metal carbonate layer on the surface of the cap base metal, assembled in cathode ray tubes, and subjected to operating tests.
  • the film thickness of the alkaline earth metal carbonate layer was approx. 100 microns.
  • the coating density of the alkaline earth metal carbonate layer which forms electron emission layer 30 is not greater than 2 mg/mm3, preferable that it is not greater than 1.6 mg/mm3, and more preferable that it is not greater than 0.8 mg/mm3.
  • the coating density of the alkaline earth metal layer which forms electron emission layer 30 in the cathode of the above embodiment is not greater than 2 mg/mm3.
  • Layer 30 therefore has a coarse porous structure, it has higher flexibility, and the stress arising from the difference in thermal expansion coefficient between the emission layer 30 and the cathode cap 1b is smaller. The swelling of said layer 30 on the surface of cathode 1b therefore does not occur.
  • reaction (IV) occurs, and reaction products 12 accumulate in the region of the interface 11 so as to form an intermediate layer.
  • reaction products 12 decompose and the formation of an intermediate layer is suppressed for the following reason.
  • reaction products 12 such as barium silicate do accumulate in the region of the interface 11 between electron emission layer 30 and cathode cap 1b or the crystal grain boundaries 7, they are broken up and destroyed rapidly by the scandium oxide dispersed in electron emission layer 30.
  • the electron cathode tube of this embodiment can therefore be operated without the electron emission layer swelling up, and as a high resistance intermediate layer of barium silicate and other reaction products is not easily formed, there is no obstruction of the electron emission current and the tube can be operated at a high current density.
  • the electron cathode of this invention has a constant, stable electron emission without any swelling or peeling of the electron emission layer even when operated for long periods of time.
  • the above electron tube cathode may therefore be used at a high current density over long periods of time.

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  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Solid Thermionic Cathode (AREA)
  • Electrodes For Cathode-Ray Tubes (AREA)

Claims (8)

  1. Procédé de fabrication d'une cathode pour tube électronique comprenant:
       la formation d'une suspension d'une poudre de carbonate de métal alcalino-terreux et d'une poudre d'oxyde de scandium dans une solution de solvant organique de nitrocellulose, et la régulation de la dimension des particules desdites poudres;
       l'application de ladite suspension sur une surface de métal de base en nickel de telle sorte qu'après l'évaporation du solvant une couche d'un carbonate de métal alcalino-terreux est formée;
       le chauffage de ladite couche de carbonate de métal alcalino-terreux sous vide à une température de 800-1200°C pour décomposer ledit carbonate en oxyde, afin de former une couche d'émission d'électrons poreuse dans laquelle l'oxyde de scandium est dispersé dans un oxyde de métal alcalino-terreux sur ladite surface de métal de base en nickel; ledit procédé étant caractérisé en ce que:
       la couche de carbonate de métal alcalino-terreux a une densité non supérieure à 2 mg/mm³.
  2. Procédé selon la revendication 1 dans lequel le carbonate de métal alcalino-terreux est du carbonate de baryum.
  3. Procédé selon la revendication 2 dans lequel le carbonate de baryum est un carbonate ternaire contenant aussi du Sr et du Ca.
  4. Procédé selon la revendication 1, 2 ou 3 dans lequel la densité de revêtement de ladite couche de carbonate de métal alcalino-terreux n'est pas supérieure à 1,6 mg/mm³.
  5. Procédé selon la revendication 4 dans lequel la densité de revêtement de ladite couche de carbonate de métal alcalino-terreux n'est pas supérieure à 0,8 mg/mm³.
  6. Procédé selon l'une quelconque des revendications 1 à 5 dans lequel l'application de ladite suspension est effectuée par vaporisation.
  7. Procédé selon l'une quelconque des revendications 1 à 6 dans lequel le chauffage de ladite couche de carbonate de métal alcalino-terreux est effectué sous vide à une température non inférieure à 1000°C.
  8. Procédé selon la revendication 7 dans lequel le chauffage de ladite couche de carbonate de métal alcalino-terreux est effectué pendant une période comprise entre plusieurs dizaines de secondes et plusieurs minutes.
EP90118910A 1989-10-06 1990-10-03 Procédé pour la fabrication d'une cathode pour un tube à électrons Expired - Lifetime EP0421372B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP262368/89 1989-10-06
JP26236889A JPH0828183B2 (ja) 1989-10-06 1989-10-06 電子管陰極

Publications (3)

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EP0421372A2 EP0421372A2 (fr) 1991-04-10
EP0421372A3 EP0421372A3 (en) 1991-11-21
EP0421372B1 true EP0421372B1 (fr) 1994-09-14

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EP90118910A Expired - Lifetime EP0421372B1 (fr) 1989-10-06 1990-10-03 Procédé pour la fabrication d'une cathode pour un tube à électrons

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EP (1) EP0421372B1 (fr)
JP (1) JPH0828183B2 (fr)
KR (1) KR940009756B1 (fr)
DE (1) DE69012494T2 (fr)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR930003229Y1 (ko) * 1991-04-30 1993-06-03 주식회사 금성사 방열형 음극선관용 전자총의 히터 구조
KR100449759B1 (ko) * 2002-03-21 2004-09-22 삼성에스디아이 주식회사 전자관용 음극 및 그 제조방법

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2726178A (en) * 1950-11-17 1955-12-06 Rca Corp Thermionic cathode with thoria coating
US4411827A (en) * 1981-03-18 1983-10-25 Corneille David M Coprecipitation process for thermionic cathode type materials
KR900007751B1 (ko) * 1985-05-25 1990-10-19 미쯔비시덴끼 가부시기가이샤 전자관 음극 및 그 제조방법
JPS6222347A (ja) * 1985-07-19 1987-01-30 Mitsubishi Electric Corp 電子管用陰極
JPH0690907B2 (ja) * 1988-02-02 1994-11-14 三菱電機株式会社 電子管用陰極

Also Published As

Publication number Publication date
DE69012494D1 (de) 1994-10-20
DE69012494T2 (de) 1995-01-26
KR940009756B1 (ko) 1994-10-17
EP0421372A3 (en) 1991-11-21
JPH03127427A (ja) 1991-05-30
KR910008766A (ko) 1991-05-31
EP0421372A2 (fr) 1991-04-10
JPH0828183B2 (ja) 1996-03-21

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