US2885334A - Thermionic cathode heaters - Google Patents

Thermionic cathode heaters Download PDF

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US2885334A
US2885334A US478028A US47802854A US2885334A US 2885334 A US2885334 A US 2885334A US 478028 A US478028 A US 478028A US 47802854 A US47802854 A US 47802854A US 2885334 A US2885334 A US 2885334A
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coating
alumina
heaters
heater
cathode
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US478028A
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Green Bernard Jeffrey
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International Standard Electric Corp
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International Standard Electric Corp
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    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25DPROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D13/00Electrophoretic coating characterised by the process
    • C25D13/02Electrophoretic coating characterised by the process with inorganic material
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01JELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00Details 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
    • H01J1/02Main electrodes
    • H01J1/13Solid thermionic cathodes
    • H01J1/20Cathodes heated indirectly by an electric current; Cathodes heated by electron or ion bombardment
    • H01J1/22Heaters
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01JELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J9/00Apparatus 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
    • H01J9/02Manufacture of electrodes or electrode systems
    • H01J9/08Manufacture of heaters for indirectly-heated cathodes

Definitions

  • the present invention relates to heaters for thermionic cathodes.
  • Heaters for thermionic cathodes are normally coated with alumina to ensure adequate cathode-heater insulation.
  • a desirable heating coating should be smooth and certainly free from any cracks.
  • a thermionic cathode having a heater coated with alumina, the surface of the alumina being intentionally crazed with small fissures.
  • Cathode heaters of the coil type are usually. coated either by spraying or electrophoretically. Straight or faggot type heaters are usually dipped or dragged.
  • electrophoretic methods of coating have been tried out, for various reasons they have not been entirely successful, and, in consequence, for coiled heaters spraying is more popular, in spite of the great waste of coating material involved.
  • the main fault with electrophoretic coating up to the present time has been its extreme embrittling effect on tungsten. If this eifect could be eliminated, or at least minimised, the electrophoretic method of coating has great advantages, vlz:
  • Alumina exists in three main forms: a-alumina, which has a specific gravity of 3.8 and is characterised by a triagonal crystal structure; B-alumina with a specific gravity of 3.3 and a hexagonal crystal structure; and 'y-alumina having a cubic structure and a specific gravity of 2.8.
  • electrophoretic coatings were based on a suspension consisting mainly of u-alumina in a suitable liquid, the suspension containing little or no binder.
  • the coated wire in the case of continuously coated heaters passed direct into drying and sintering ovens.
  • unit heaters such as double helical coils which were dipped into the electrophoretic bath, an external nitro-cellulose binder was applied, usually after drying, but sometimes the binder was included in the coating suspension. This binder supported the coating until sintering was completed.
  • the three forms of alumina mentioned above are related in that, on heat treatment 'y-alumina can be converted into 19- and on further heating the )8- into aalumina.
  • a view to overcoming the brittleness of heaters coated with tat-alumina we use a less dense alumina as part of the coating, which coating shrinks on processing, producing a network of fine cracks. This allows for free expansion and contraction of the heater core without affecting the electrical properties of the insulator.
  • the present invention further provides the method of manufacture of a heater for a thermionic cathode in which the heater is electrophoretically coated with alumina containing eight parts by weight of 'yalumina for every eight to twelve parts of a-alumina, resulting in the fully processed cathode having a heater with an insulated coating which is crazed with small fissures.
  • the preferred proportion of 'y to on alumina is such as to result in the mean specific gravity of the mixed alumina being 3.5.
  • Fig. 1 is a drawing taken from a photomicrograph of a portion of a heater according to the present invention after electrophoretic coating and before processing
  • Fig. 2 is a drawing taken from a photomicrograph of a portion of heater manufactured according to the present invention after removal from a valve which has been fully processed.
  • the 'y-alumina for use in coatings of the present invention is first treated with hydrochloric acid, as known in the art, to condition the charge carrying properties of the particles.
  • the preparation of the electrophoretic suspension is preferably carried out in the following manner:
  • Fig. 2 shows the change in appearance of heater wire according to the invention on removal from a thermionic valve which has been normally processed and aged.
  • the method of manufacture of a thermionic cathode comprising the steps of electrophoretically coating the cathode heater with alumina from a bath containing eight parts by weight of 'y alumina for each eight to twelve parts of a alumina, drying the coating, heating the coating between 800-1000 C. for conversion of the 'y alumina ultimately into or alumina, whereby the resultant mixture has an increased specific gravity which causes a volume shrinkage of the coating and crazing thereof with small fissures.
  • alumina coating comprises a mixture of substantially like compositions by weight of 'y-alumina as cubic crystals and a-alumina as triagonal crystals with a mean specific gravity range between 2.8 and 3.8.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Inorganic Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Solid Thermionic Cathode (AREA)
  • Powder Metallurgy (AREA)

Description

y 1959 B. J. GREEN 2,885,334
THERMIONIC CATHODE. HEATERS Filed Dec. 28, 1954 //v v /vron B. J. GREEN ATTORNEY United States Patent O 2,885,334 THERMIONIC CATHODE HEATERS Bernard Jeffrey Green, London, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Application December 28, 1954, Serial No. 478,028
Claims priority, application Great Britain January 1, 1954 3 Claims. (Cl. 204-181) The present invention relates to heaters for thermionic cathodes. Heaters for thermionic cathodes are normally coated with alumina to ensure adequate cathode-heater insulation. In the past it has been commonly accepted that a desirable heating coating should be smooth and certainly free from any cracks. We have found, however, that, particularly with fine heater wire, in order to avoid brittleness of the heaters, a smooth and dense coating should be avoided, it being preferred that the coating should, on the contrary, be intentionally made so as, in use, to have a surface crazed with small fissures.
According to the present invention, therefore, there is provided a thermionic cathode having a heater coated with alumina, the surface of the alumina being intentionally crazed with small fissures.
Cathode heaters of the coil type are usually. coated either by spraying or electrophoretically. Straight or faggot type heaters are usually dipped or dragged. Although, over a very long time, electrophoretic methods of coating have been tried out, for various reasons they have not been entirely successful, and, in consequence, for coiled heaters spraying is more popular, in spite of the great waste of coating material involved. The main fault with electrophoretic coating up to the present time has been its extreme embrittling effect on tungsten. If this eifect could be eliminated, or at least minimised, the electrophoretic method of coating has great advantages, vlz:
1) It gives a coating which provides a consistent value of cathode-heater insulation.
(2) The thickness of the coating can be accurately controlled within finer limits than is possible with spraymg.
(3) The coating process is very simple, little skill being required to produce good heaters.
(4) The economy of material is very great compared with spraying, in which some 99% of the coating is wasted in the spray booth, air ducts etc.
Alumina exists in three main forms: a-alumina, which has a specific gravity of 3.8 and is characterised by a triagonal crystal structure; B-alumina with a specific gravity of 3.3 and a hexagonal crystal structure; and 'y-alumina having a cubic structure and a specific gravity of 2.8.
In the past electrophoretic coatings were based on a suspension consisting mainly of u-alumina in a suitable liquid, the suspension containing little or no binder. The coated wire, in the case of continuously coated heaters passed direct into drying and sintering ovens. In the case of unit heaters such as double helical coils which were dipped into the electrophoretic bath, an external nitro-cellulose binder was applied, usually after drying, but sometimes the binder was included in the coating suspension. This binder supported the coating until sintering was completed.
In both cases a very smooth closely packed coating of big density alumina was obtained with a hard porcelainlike character. This coating became even harder during processing and life of the valve, due to the coalescing of the particles aided by a certain amount of recrystallisation of the alumina. The result was a coating which became a solid rod of high mechanical strength. In the case of fine tungsten Wires the coating was actually strong enough to fracture the tungsten by leverage across the already enlarged crystal boundaries. This brittleness became progressively less with increasing core diameter.
The three forms of alumina mentioned above are related in that, on heat treatment 'y-alumina can be converted into 19- and on further heating the )8- into aalumina. With a view to overcoming the brittleness of heaters coated with tat-alumina we use a less dense alumina as part of the coating, which coating shrinks on processing, producing a network of fine cracks. This allows for free expansion and contraction of the heater core without affecting the electrical properties of the insulator.
Accordingly, the present invention further provides the method of manufacture of a heater for a thermionic cathode in which the heater is electrophoretically coated with alumina containing eight parts by weight of 'yalumina for every eight to twelve parts of a-alumina, resulting in the fully processed cathode having a heater with an insulated coating which is crazed with small fissures. The preferred proportion of 'y to on alumina is such as to result in the mean specific gravity of the mixed alumina being 3.5.
The invention will be further described with reference to the accompanying drawings in which:
Fig. 1 is a drawing taken from a photomicrograph of a portion of a heater according to the present invention after electrophoretic coating and before processing and Fig. 2 is a drawing taken from a photomicrograph of a portion of heater manufactured according to the present invention after removal from a valve which has been fully processed.
The 'y-alumina for use in coatings of the present invention is first treated with hydrochloric acid, as known in the art, to condition the charge carrying properties of the particles.
The preparation of the electrophoretic suspension is preferably carried out in the following manner:
900 millilitres of N-butyl alcohol, 600 millilitres of a 5% solution of nitrocellulose in butyl acetate, together with millilitres of N-butyl acetate and 9 millilitres of N-butyl phthalate are introduced into a ball-mill and then 1000 grams of a-alumina, as normally supplied in the trade for coating radio valve heaters, and 800 grams of 'y-alumina, treated as above, are added. Finally, a sufiicient amount (of the order of 40 millilitres) of a 25% solution of aluminum nitrate in butyl alcohol is added to the mix to provide, in the final suspension, a pH value of between 2.5 and 3.0. The mix is then ball-milled for 24 hours, after which it is kept under constant operation until required for use, agitation being maintained in the electrophoretic bath.
In general I have adopted the method of the present invention for the coating of pre-formed helical heaters of shapes which are not convenient for coating by a drag process and also for very fine wire heaters which would not stand up to spraying, even if this were otherwise desirable. The heaters are suitably supported in the coating bath and after coating they are immediately washed in methylated spirits. The spirit removes the suspension dragged out from the coating bath but does not attack the electrophoretically deposited layer. The heaters are then dried in a warm air blast and sintered.
The appearance of the heater wire after coating but before sintering is depicted in Fig. 1 of the accompanying drawings, and shows an even but relatively coarse covering.
During the subsequent processing of the heater, the following changes in the composition of the alumina take place.
800 C. 'y-alumina cubic crystals 1000 C. B-alumtna hexagonal crystals a-alumina triagonal crystals The increase in specific gravity causes a volume shrinkage in the coating which results in stresses being set up. The stresses eventually cause cracks to appear in the coating. The change, however, is a continuous process, dependent on a temperature-time factor. Stages at which the changes occur are not yet fully determined,
but they are not normally complete until after the heater I has been used for some time in a completed discharge device.
Fig. 2 shows the change in appearance of heater wire according to the invention on removal from a thermionic valve which has been normally processed and aged.
It will be seen that, while the surface of the coating has 41 scribed above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by Way of example and not as a limitation on the scope of the invention.
What we claim is:
1. The method of manufacture of a thermionic cathode comprising the steps of electrophoretically coating the cathode heater with alumina from a bath containing eight parts by weight of 'y alumina for each eight to twelve parts of a alumina, drying the coating, heating the coating between 800-1000 C. for conversion of the 'y alumina ultimately into or alumina, whereby the resultant mixture has an increased specific gravity which causes a volume shrinkage of the coating and crazing thereof with small fissures.
2. The method of manufacture according to claim 1 in which said resultant mixture of 'y and or. aluminas has a mean specific gravity of 3.5.
3. The cathode of claim 1, wherein the alumina coating comprises a mixture of substantially like compositions by weight of 'y-alumina as cubic crystals and a-alumina as triagonal crystals with a mean specific gravity range between 2.8 and 3.8.
References Cited in the file of this patent UNITED STATES PATENTS 1,942,879 Riddle Jan. 9. 1934 1,975,870 Schrader Oct. 9, 1934 2,092,815 Shaw Sept. 14, 1937 2,164,913 Goodchild July 4, 1939 2,442,864 Schneider June 8, 1948 2,734,857 Snyder Feb. 14, 1956

Claims (1)

1. THE METHOD OF MANUFACTURE OF A THERMIONIC CATHODE COMPRISING THE STEPS OF ELECTROPHORETICALLY COATING THE CATHODE HEATER WITH ALUMINA FROM A BATH CONTAINING EIGHT PARTS BY WEIGHT OF 2 ALUMINA, WHEREBY THE RETO TWELVE PARTS OF A ALUMINA DRYING THE COATING, HEATING THE COATING BETWEEN 800*-1000* C, FOR CONVERSION OF THE V ALUMINA ULTIMATELY INTO A ALUMINA , WHEREBY THE RESULTANT MIXTURE HAS AN INCREASED SPECIFIC GRAVITY WHICH CAUSES A VOLUME SHRINKAGE OF THE COATING AND CRAZING THEREOF WITH SMALL FISSURES.
US478028A 1954-01-01 1954-12-28 Thermionic cathode heaters Expired - Lifetime US2885334A (en)

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GB47/54A GB739325A (en) 1954-01-01 1954-01-01 Improvements in or relating to thermionic cathode heaters

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3137766A (en) * 1957-12-16 1964-06-16 Norton Co Electric wire for use at high temperatures
US3160946A (en) * 1960-03-11 1964-12-15 Sylvania Electric Prod Electrical heaters
US3223607A (en) * 1959-02-24 1965-12-14 Egyesuelt Izzolampa Method of manufacturing electrical heating elements with improved aluminum oxide coating
CN115332026A (en) * 2022-07-15 2022-11-11 中国电子科技集团公司第十二研究所 Cathode assembly capable of being started quickly and preparation method and application thereof

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0722034B2 (en) * 1989-07-01 1995-03-08 株式会社日立製作所 Inorganic insulation heater, manufacturing method thereof, and cathode ray tube using the same

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1942879A (en) * 1931-05-07 1934-01-09 Champion Spark Plug Co Refractory material and batch and method for making the same
US1975870A (en) * 1933-07-07 1934-10-09 Rca Corp Indirectly heated cathode
US2092815A (en) * 1935-11-23 1937-09-14 Rca Corp Cathode heater insulation
US2164913A (en) * 1935-03-06 1939-07-04 Int Standard Electric Corp Indirectly heated cathode
US2442864A (en) * 1944-11-23 1948-06-08 Sylvania Electric Prod Electrophoresis coating of electron tube parts
US2734857A (en) * 1951-10-11 1956-02-14 snyder

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1942879A (en) * 1931-05-07 1934-01-09 Champion Spark Plug Co Refractory material and batch and method for making the same
US1975870A (en) * 1933-07-07 1934-10-09 Rca Corp Indirectly heated cathode
US2164913A (en) * 1935-03-06 1939-07-04 Int Standard Electric Corp Indirectly heated cathode
US2092815A (en) * 1935-11-23 1937-09-14 Rca Corp Cathode heater insulation
US2442864A (en) * 1944-11-23 1948-06-08 Sylvania Electric Prod Electrophoresis coating of electron tube parts
US2734857A (en) * 1951-10-11 1956-02-14 snyder

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3137766A (en) * 1957-12-16 1964-06-16 Norton Co Electric wire for use at high temperatures
US3223607A (en) * 1959-02-24 1965-12-14 Egyesuelt Izzolampa Method of manufacturing electrical heating elements with improved aluminum oxide coating
US3160946A (en) * 1960-03-11 1964-12-15 Sylvania Electric Prod Electrical heaters
CN115332026A (en) * 2022-07-15 2022-11-11 中国电子科技集团公司第十二研究所 Cathode assembly capable of being started quickly and preparation method and application thereof

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