US3005926A

US3005926A - Cathode for electron discharge device

Info

Publication number: US3005926A
Application number: US815173A
Authority: US
Inventors: Clifford E Horner; Gene R Feaster
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1959-05-22
Filing date: 1959-05-22
Publication date: 1961-10-24
Anticipated expiration: 1978-10-24

Description

Oct. 24, 1961 c. E. HORNER ETAL CATHODE FOR ELECTRON DISCHARGE DEVICE Filed May 22, 1959 Fig.|.

INVENTORS Clifford E. Homer and Gene R. Feoster.

Y ATTdRNEY WITNESSES -Q RGAL WW fi a y Filed May 22, 1959, Ser. No. 815,173 6 Claims, 3- 3 and Gene R. Feaster, Ele t i of This invention relates to electron discharge devices, and more particularly to improvements in thermionic electron emitting cathodes for such devices.

In the conventional indirectly heated type cathode the usual eonstruction of the cathode-heater assembly is a nickel sleeve having an external oxide coating and a filamentary heater coated with. a refractory insulation provided in the interior region of the sleeve. In certain electron discharge tubes, such as the so-called electron guns in kinescopes and camera tubes, the cathode normally consists of a capped nickel alloy cylinder. The cap is coated with emissive oxides such that electrons are emitted in an end-fire manner from the external end of the cathode cylinder. The heater in this structure is customarily comprised of a non-inductive, reverse helix of tungsten wire coated with Alundurna crystalline form of aluminum oxide. The heater is positioned within the cathode cylinder.

It has been found in commercial practice that when such a heater cathode construction as described above is subjected to vibration during operation failures frequently occur as a result of breakage of the tungsten wire. It is found upon examination that the heater wire has become excessively brittle as a result of operation while subjected to vibration. It has also been found that the heater wire becomes extremely brittle after a given operation time and that severe jars will often cause the wire to break, opening the cathode heater circuit and sometimes causing shorts between the cathode heater and the cathode sleeve. Tests were conducted to determine the cause of this breakage and it was found that there was a definite relationship between the length of of time the tube was vibrated and the amount of brittleness found in the heater wire. It was found that heaters from tubes which were vibrated for a shorter period of time were in general less brittle. It was also found that when the tubes were vibrated with the heaters cold, there was, in all cases, no evidence of brittleness. It is believed that the relative motion of the nickel sleeve and the loosely supported heater within the nickel sleeve causes the hard Alundum coating on the heater wire to abrade nickel material from the inner surface of the nickel sleeve. From spectrographic analysis, it was found that nickel was transferred from the nickel sleeve to the tungsten heater. When the tungsten heater, containing the spectrographically traces of nickel on the surface, was subsequently heated above approximately 1200" C., the tungsten heater became extremely brittle. The nickel material entrapped in the Alundum will be heated to the temperature of the heater, approximately 1300 C. At this temperature the nickel has a vapor pressure of the order of one micron, consequently the tungsten is exposed to excessive amounts of nickel vapor. This is especially so where the Alundum coating is cracked. The cathode sleeve contributes very little nickel vapor directly since at cathode temperature about 800 C., the vapor pressure of nickel is less than mm. of mercury.

It is accordingly an object of this invention to provide an improved indirectly heated cathode assembly.

It is another object to provide an improved indirectly heated cathode that will withstand substantial vibration.

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It is another object to provide an improved indirectly heated cathode in which the tungsten heater does not become brittle even after long life'and high temperature operation.

These and other objects are effected by our invention as will be apparent from the following description taken in accordance with the accompanying draw: ing throughout which like reference characters indicate like parts, and in which:

FIG. 1 is a view partly 1n section embodying the teach ings of our invention; and

FIG. 2 is a view partly in section, illustrating another cathode structure embodying the teaching of our invention.

Referring in detail to 'FIG. 1, there is illustrated a so-called end-fire type cathode in which the electrons are emitted from the external end of the cathode sleeve. The cathode structure shown comprises a tubular sleeve member 10 of a suitable material having a high melting point, a low vapor pressure and non-embrittlement influence on tungsten such as molybdenum and base alloys thereof. The sleeve member 10 has one end closed with an end portion 12 also of molybdenum. The molybdenum may be machined or drawn to desired shape. The molybdenum may be of the type suitable for use in electronic tubes. A cap 14 of a suitable core material such as nickel with suitable reducing agents (standard nickel cathode grade) is secured to the closed end of the tubular sleeve 10 by spot Welding. A coating 16 of a suitable electron emissive material such as barium-strontium carbonate is provided on the exterior surface of the nickel cap 14. The coating 16 may be deposited by any suitable process such as spraying. Positioned within the tubular sleeve is a heater member 20. The heater 2% consists of a noninductive reverse helix wound wire 22 of a suitable material such as tungsten having a coating 24 of a suitable material such as Alundum. A support member 30 is also illustrated secured to the outer portion of the tubular sleeve 10 for supporting the cathode within an electrode as is wellknown in the art.

In the structure illustrated in FIG. 1, it can be seen that the heater 20 during vibration can abrade only the molybdenum sleeve 10. The molybdenum material which may be transferred to the Alundum coating 24 will not vaporize at the operating temperature of the heater 2% and will not embrittle the tungsten in any case. -In the specific embodiments shown and described only molybdenum was indicated as a suitable material for the sleevemember 10. Other suitable materials are tungsten and tantalum and base alloys thereof. These materials have a low vapor pressure and their vapors are not damaging to tungsten.

In FIG. 2 there is illustrated a cathode structure for use in a conventional type receiving tube. This type of cathode consists of a tubular member with an electron emissive material on the external surface. A heater is provided within the tubular member and the cathode may be supported at each end.

In the specific embodiment shown a sleeve member 40 is provided of nickel similar to the material used in the end cap member 14 of FIG. 1. An oxide coating 42. of a material similar to the coating 16 in FIG. 1 is provided. on a portion of the external surface of the sleeve 40. The coating 42 may be applied in a similar manner to that described with respect to FIG. 1. On the inner surface of the sleeve 40 is provided a coating or a liner 44 of a material similar to that described with respect to the member 12 of FIG. 1. If the layer 44 is a liner, then it is inserted into the sleeve 42 and secured to the nickel sleeve by spot Welding. The heater 20 is provided within the inner region defined by the sleeve 3 42 and is shielded from the sleeve 40 by the member 44.

The layer 44 may also be provided as a coating of a suitable material such as molybdenum, tungsten, tantalum, applied by methods known in the art.

Thus it can be seen that our invention provides the cathode structure which will withstand vibration and necessary processing and operating temperatures Without embrittlement of the tungsten heater element by the nickel material.

While we have shown our invention only in two forms, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various other changes .and modifications without departing from the spirit and 2. An indirectly heated cathode comprising a tubular member of a material selected from the group consisting of molybdenum, tungsten and tantalum and base alloys thereof, an end cap of cathode grade nickel on one end of said tubular member, an electron emissive oxide coating on the surface of said end cap and a heater member containing tungsten material provided in the interior region of said tubular member.

3. An indirectly heated cathode comprising a tubular sleeve member of a material selected from the group consisting of molybdenum, tungsten and tantalum and base alloys thereof, a second sleeve member of cathode grade nickel surrounding said first sleeve member, an electron emissive coating on the external surface of said second sleeve member and a heater element containing tungsten material provided within the interior region of said first sleeve member.

4. An indirectly heated cathode comprising a tubular sleeve member containing nickel, a layer of a material selected from the group consisting of molybdenum, tungsten and tantalum and base alloys thereof, an electron emissive coating on the external surface of said nickel sleeve and a heater element containing tungsten material provided within the interior region of said sleeve.

5. An indirectly heated cathode comprising a tubular sleeve member containing nickel, the inner surface of said nickel sleeve having a material selected from the group consisting of molybdenum, tungsten and tantalum and base alloys thereof coated on said nickel sleeve, an electron emissive coating on the external surface of said nickel sleeve and a heater element containing tungsten material provided within the interior region of said sleeve.

6. An indirectly heated cathode comprising a member containing nickel having an electron emissive coating on one surface thereof, a heater element containing tungsten material disposed facing a side of said nickel member remote from said electron emissive coating and a layer of a material selected from the group consisting of molybdenum, tungsten, tantalum and the base alloys thereof disposed between said nickel member and said heater element.

No references cited.