US2888592A

US2888592A - Cathode structure

Info

Publication number: US2888592A
Application number: US444939A
Authority: US
Inventors: James M Lafferty
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1954-07-22
Filing date: 1954-07-22
Publication date: 1959-05-26
Anticipated expiration: 1976-05-26
Also published as: GB810301A; FR1137405A

Description

May 26, 1959 J. M. LAFFERTY CATHODE STRUCTURE Filed July 22, 1954 Fig.2.

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INVENTOR. James M Lafierty, BY

His Attorney.

United States Patent,

CATHODE STRUCTURE James M. Laflerty, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Application July 22, 1954, Serial No. 444,939

8 Claims. (Cl. 313-346) My invention relates to cathode structures, and more particularly to a new and improved cathode structure for use in fluorescent lamp discharge devices.

Fluorescent lamp discharge devices of the type now commonly used employ two cathodes, one at each end of the device. These cathodes are generally identical and include an electron emissive surface. During operation of the lamp, certain materials are evolved from the cathode and if deposited on the walls of the tube, cause a discoloration thereof. Such discoloration is usually evidenced by a darkening of the envelope in the vicinity of the cathodes. It is desirable that this discoloration be reduced and also that the life of the cathode be increased.

An object of my invention is, therefore, to provide a new and improved cathode for use in fluorescent lamp discharge devices.

Another object of my invention is to provide a new and improved cathode having a relatively long life and which causes a minimum amount of discoloration of the lamp walls.

Since sputtering of the cathode reduces the life of the cathode and causes discoloration of the lamp walls it is especially desirable to provide a cathode which is not susceptible to this undesirable characteristic. The susceptibility of a cathode to sputtering may be reduced by increasing the active surface thereof and by reducing the thermal conductivity thereof.

A further object of my invention is, therefore, to provide a new and improved cathode having poor heat transfer characteristics.

A still further object of my invention is to provide a new and improved cathode having a relatively high emission surface.

In the attainment of the foregoing objects I provide a cathode comprising a loose mass of conducting fibers which in a preferred embodiment of my invention consists of an electron emissive layer on the individual fibers of the mass, such, for example, as a barium oxide layer on steel wool. The electron emissive surface thus provided is exceedingly large as compared to the surface provided on cathodes of the type known in the prior art, and furthermore, the thermal conductivity of this portion of the cathode is extremely low.

For further objects and advantages and for a better understanding of my invention, attention is now directed to the following description and accompanying drawing and also to the appended claims wherein those features of novelty which characterize my invention are pointed out with particularity.

In the drawing:

Fig. 1 is a longitudinal sectional view of a portion of a fluorescent lamp discharge device employing cathodes constructed in accordance with the teachings of my invention;

Fig. 2 is a partial longitudinal sectional view of a preferred embodiment of my invention;

Fig. 3 is a longitudinal sectional view of another embodiment of my invention.

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Referring to Fig. 1, there is illustrated a portion of a fluorescent lamp discharge device, having an envelope 1 provided with a coating of fluorescent material 2 on the inner surface thereof. A rod-like electrical conductor 3 is positioned substantially along the longitudinal axis of the lamp. One end of conductor 3 extends outside of the lamp and may be conveniently employed as one terminal thereof, and the other end extends within the cavity defined by envelope 1. It will be understood that conductor 3 and envelope 1 are joined together by any of many means well known in the art to provide an airtight seal.

A cathode, generally indicated in the drawing at 4, includes a support or shield member 5 comprising a hollow, cylindrically shaped portion 6 and an inwardly extending annular flange portion 7 provided at one end thereof. Flange 7 defines a centrally located aperture 8 through which conductor 3 extends. A collar 9, extending outwardly from the inner surface of flange 7 surrounds aperture 8 to provide increased surface area for attachment to conductor 3. While the method of attachment of member 5 to conductor 3 does not constitute a part of my invention, a permanent bond is desirable, and spot welding or brazing may be conveniently employed. A hollow cylindrical thermal insulator 10 having an outer surface of substantially the same shape as the inner surface of the cylindrical portion of shield 5, is located therein, and is concentrically located with respect to the principal axis thereof. The cavity thus formed by insulator 10 and the closed end of shield 5 is filled with a loose mass of electric current conductive fibrous or flocculent material 11 which is bonded to conductor 3. It will be apparent from the drawing that conductor 3 extends a substantial distance within the cavity to eifect a good electrical connection between it and material 11. Material 11 is wool-like in appearance and is hereinafter referred to as wool. While steel or nickel is suitable for this purpose, a more refractory material such, for example, as tungsten may be employed,

Since wool 11 provides the electron emissive surface of the cathode structure, it is important that it have a high electron emissivity. While a small amount of emission is obtainable from the metal itself, in a practical fluorescent lamp generally usable for lighting purposes, a much higher emissivity is necessary. In accordance with further teachings of my invention, a coating of a material having good electron emission characteristics is provided on the individual strands making up wool 11. Because of the large surface area of wool 11, which consists of the individual surface areas of the multiplicity of strands thereof, a large quantity of electron emissive material may be held by the cathode and exposed to the external electric field to emit electrons. Double and triple coiled filaments have been used in the past for this purpose.

The electron emissive material may be applied to the flocculent material by dipping or by any other suitable process. In the past, where a'carbonate was employed, a filamentary type cathode was used to provide a convenient means for elevating the carbonate to a high temperature to reduce it to an oxide during the exhaust operation when gases are removed from the lamp. Since shield 5 is preferably constructed of metal, it is easily heated by induction heating during the exhaust operation, and.

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effects generally associated with cathode spots. For example, if a cathode spot is formed on a section of a cathode having good thermal conducting properties, more power must be supplied to the lamp to maintain the cathode spot at the required temperature. The additional power thus supplied to the cathode causes an increased voltage drop across the cathode which results in increased sputtering and a general reduction in the life of the tube. Since in my cathode only fine metallic fibers are embedded in the electron emissive layer, a low heat conductivity path from all sections of the emissive surface is provided. At the same time, however, these same fine metallic fibers provide a satisfactory electrical conductive path from conductor 3 to the emission surface.

Shield 5, in addition to providing a means for heating wool 11 during the manufacture of the tube, acts as a heat shield to reduce the amount of heat transferred from Wool 11. Because of the hereinbefore described low thermal conductivity of wool 11, heat generated therein will be principally transferred to shield 5 by radiation. Heat insulator is provided to further decrease the quantity of heat so transferred, and preferably, shield 5 is provided with a bright shiny surface to minimize the heat radiated therefrom.

Referring to Fig. 2, there is illustrated a preferred embodiment of my invention comprising a cup-shaped member 12, a cylindrical thermal insulator generally shown at 13 and a mass of fiocculent material or wool 14. An outwardly extending collar 15 is provided at one end of member 12 and the other end is unobstructed. An electrical conductor 16 extends within collar 15 and is attached (not shown) to wool 14. Thermal insulator 13 includes a plurality of cylinders 17, 18, 19, and 20 concentrically arranged with respect to the longitudinal axis of member 12. These cylinders are provided with outwardly extending separating collars 21. While this type of thermal insulation is satisfactory, other types may be employed if desired.

Referring to Fig. 3, there is illustrated an embodiment of my invention in which the shield and thermal insulator are not employed. Electrical conductor 22 is attached to a mass of fiocculent material or wool 23. Since means such, for example, as a filament or a surrounding metallic shield, are not readily available in this embodiment for conveniently elevating the wool to a high temperature to activate the emission surface thereof, it is necessary to use an emissive material which may be activated at the usual exhaust temperaures. One such material which I have used is barium azide. The metallic wool is dipped in a solution of barium azide, such, for example, as Ba(N -I-I O. During the normal exhaust operation when the temperature of the lamp is raised to a bake-out temperature of approximately 400 centigrade, the azide decomposes to give barium oxide which has good electron emission characteristics. With this arrangement, any difi-iculties associated with raising the temperature of the wool to a very high temperaure are eliminated.

While this invention has been described by means of particular embodiments it is understood that those skilled in the art may make many changes without departing from my invention. Therefore, by the appended claims I intend to cover all such changes and modifications which fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

. M. all M." M. m. ml l 1. A cathode comprising an electron emitting member of metallic wool, a thermal insulator surrounding said wool and a hollow metallic cylindrical shield surrounding said insulator.

2. A cathode comprising a hollow metallic shield, a hollow cylindrical heat insulator located within said shield, and electron emitting metallic wool located within said insulator.

3. A cathode structure comprising a cylindrical sup port portion, a cylindrical heat insulator concentrically arranged within said support portion, and an active cathode member comprising a flocculent mass of metallic wool fibers coated with an electron emissive substance positioned within said insulator.

4. A cathode structure comprising a cylindrical support portion having an unobstructed end and an end partially enclosed by an inwardly extending annular flange defining a centrally located aperture, an electrical conductor positioned within said aperture and extending within said cylindrical thermal support, a cylindrical insulator concentrically positioned within and contacting the inner cylindrical surface of said cylindrical support, and metallic wool coated with an electron emissive substance positioned within said insulator and electrically connected to said electrical conductor.

5. A cathode comprising a cylindrical metallic container, a hollow cylindrical heat insulator concentrically located within said container defining a cylindrical cavity a loosely compacted mass of fibrous conductive material within said cavity, and an electron emissive substance coating said fibrous material.

6. A cathode comprising a metallic shield, a cylindrical heat insulating member located within said shield defining a cylindrical cavity, a loosely compacted mass of fibrous conductive material within said cavity, and electron emissive substance coating said fibrous material.

7. A cathode comprising a metallic shield, a heat insulator located within said shield and comprising a plurality of concentrically spaced cylindrical members defining respective insulating spaces between one another, a loosely compacted mass of fibrous material within said insulator, and an electron emissive substance coating said fibrous material.

8. A cathode comprising a cylindrical metallic shield having an axial dimension greater than its diameter, a cylindrical heat insulating member concentrically located within said shield, a compacted mass of metallic Wool positioned within said insulating member, and an electron emissive substance coating said metallic Wool.

References Cited in the file of this patent UNITED STATES PATENTS 1,774,407 Skaupy et al Aug. 26, 1930 1,968,608 Lowry July 31, 1934 2,032,179 Lowry Feb. 25, 1936 2,103,623 Kott Dec. 28, 1937 2,161,790 Abadie June 13, 1939 2,228,327 Spanner Ian. 14, 1941 2,249,672 Spanner July 15, 1941 2,253,145 Smith Aug. 19, 1941 2,310,983 Miller Feb. 16, 1943 2,624,024 Jansen et al Dec. 30, 1952 2,722,626 Coppola et al Nov. 1, 1955