US2845324A - Gas discharge tube - Google Patents

Description

July 29, 1958 J, F. HENl-:Y ErAL GAS DISCHARGE TUBE:

Filed Dec. 3, 1954 M, .iV/ARP AGE T lNvENToRS 0/1/V oo/bA/ZA?' BY G United States Patent e l 2,845,324 GAS DISCHARGE TUBE John F. Heney, Bloomfield, Alan D. White, Plainfield,

and Douglas M. Sharp, Allendale, N. J., assignors to International Telephone and Telegraph Corporation,

Nutley, N. J., a corporation of Maryland Application December 3, 1954, Serial No. 472,932 4 Claims. (Cl. S16-26) This invention relates to gas discharge tubes and their manufacture and more particularly to novel cold cathodes for use in gas tubes and methods for producing them.

Gas discharge tubes usingl cold cathodes are well known. In the typical cold cathode diode, two electrodes are immersed in al gas at a relatively low pressure. If suicient voltage is appliedj between the electrodes lot such a tube, ionization by collision is produced in the gas, resulting in a glow discharge and the passage of current. One of the principal advantages of such coldcathode devices compared with tubes using indirectly or directly heated cathodes is that external heater power is eliminated in providing a flow of current.

However, cold cathodes as known heretofore have been severely limited in their current carrying capacity. Thus, in a typical cold cathode diode employing nickel electrodesimmersed in a rare gas such as xenon at a pressure of several millimeters, attempts to obtain high currents, such as those greater than 50-100 milliamperes, have resulted in an excessive power dissipation at the cathode ofthe tube. In obtaining cold-cathode currents,

voltage drops as high as 300 volts occur, depending on the cathode material and the gas used inthe tube. The resultant power dissipation at ,the cathode becomes excessive at these currents and voltages, andthe heating and ionic bombardment produced cause rapid deterioration and disintegration of the cathode. The useful life of such tubes `is thereby limited to frequently less than an hour. Thus, heretofore, in order to provide gas discharge tubes having high current carrying capacities, thermionic cathodes have been primarily resorted to.

It is an object of the present invention to provide a highly eflicient cold cathode for high-current low-voltage discharges.

It is a further object to provide novel gas discharge tubesV employing these cathodes.

It is stillV an additional object' to provide a simple t and inexpensive methody for fabricating such cold cathodes and gas discharge devices.

It is an important feature of this invention that a cold cathode having a relatively large surface area is provided in a simple manner. 'inrwhich a layer of a lowwork-function metal overlies the conductive base member cathode.

It is a further feature that this overlying layer is deposited on the base member by establishing a preliminary gas discharge in which the low-work-function'metal prior to deposition is ltemporarily the cathode element of the gas discharge tube. Y

As a preferred feature of this invention a layer of magnesiumV overlying-internal surface portions of 'a- Kovar cup is provided.

Other objectsY and features of this invention will be seen from the; following` drawings and description in which:

Fig. l is a cross-sectional view of a gas discharge tube of this invention; p

Fig. 2 isacross-sectional view ofanother embodiment of a gas discharge tube; s

Fig. 3 is a schematic view in Velevation of stillanother embodiment of a gas discharge tube of this invention;.and

Fig. 4 is a schematicview inelevation of an embodirnent of a gasV discharge tube using aY disk-shaped cold cathode.

Cll

...Patented July 29, 1958 Referring to Fig. 1, a Kovar cup 1 is shown withy a layer of magnesium 2 overlying internal surface portions of the cup. An insulating glass tube 3 joined to the cup 1 connects with a Kovar sleeve 4 which serves as the anode of the gas discharge tube. This glass tube 3 is sealed to the open end of the cup 1, which serves as the cathode, and to the anode sleeve 4. A glass tubulature 5 is at-V tached to the opposite end of the sleeve 4 and makes connection with a vacuun'r pump and gas filling system not shown'. Prior to attachment of glassV tube 3 to cup 1, a block of a low-work-function metal y6, preferably magnesium, is positioned within thecup 1 by means of Kovar stem wire 7 and Kovar-to-glass seal 8. Thereby, the wire 7 and the block 6 are elfectively insulated from the cup l. Prior to depositing the layer of magnesium 2 on the walls of cup 1, appropriate. gas filling isadded to the system and the tubulaturev 5 is sealed off at point 9 thereby providing a sealed glass discharge system. Or, as may be found preferable, depending upon the desired application, the layer of magnesium 2 may be deposited during the exhaust operation, prior to the sealing ofthe tube. The gas used in the deposition process may then be replaced by another gas having the required properties for the desired application.

Any appropriate inert gas at a desired pressure may be used for the gas filling. Suitable' gases are, for example, helium, neon, argon, krypton, Xenon, nitrogen or a similar inert gas or mixture of gases, at a pressure between l and 2O millimeters of mercury. An appropriate direct voltage is then applied between the wire 7, as cathode, and the anode 4 to establish a gas discharge within the tube. This applied voltage, presumably due to ensuing heat effects and. positive-ion bombardment, 'serves to cause surface portions of the block 6 to be spattered about circumferentially, thereby coating the'internal surfacesof the cup l and facing portions of the tube Siwith a layer of the low-work functionl metal. The tapering of the glass tube at its neck 10 serves to prevent spattering of magnesium on portions of the glass tubing located between the neck 10 and the anode 4. ItA hasbeen found that a cup having an average diameter of 3 inches can be effectively coated at a voltage of 200 volts within a matter of seconds. A very small volume of the block is consumed in a typical coating operation. After the coating has been applied as described, the connection to the' Kovar stem wire is broken and connection is made to metallic portions of the Kovar cup. Thereby, a gas discharge device has been provided in which the cold cathode member consists of a conductive base member having a large surface covered with a layer of a lowwork-function metal, such as magnesium. Because lowwork-function metals are superior emitters and because of the large coated area provided, a gas discharge tube of the type described is capable of operating at lessthan volts with currents up to 600 milliamperes for very long periods of time. The tube shown in Fig. 1 Ais particularly suitable for use within a coaxial line or wave'- guide structure. A portion of the tube between the sealed-off point 9 and neck 10 is inserted within the waveguide structure,.with the cup` portion 1 located outside of the waveguide. K

In Fig. 2 is shown another embodiment of `a gas discharge tube made in accordance with the subject inven# tion. In this tube structure, the` magnesium used for coatingv purposes also `functions as the anode-of thetube. A Kovar cylinder 11 is provided, and located concentrically therein is a supported magnesiuml tubing 12 positioned between nickel supports 13 yand'14. A Kovar wire is attached to nickel support 14 and is connected to a polarity-reversing switch 15. Insulatng end vseals 16 and 17 complete the tube" structure. After lling the tube with an' appropriate gas at an appropriate'pressure, as

3 described for Fig. 1, the tube 16 is sealed. The cylinder 11 is connected to a direct current supply 1S through an appropriate limiting resistor 19. The cylinder is provided with a magnesium coating by establishing a gas discharge within the tube in which the Kovar cylinder 11 is the anode and the concentrically, internally disposed magnesium tube 12 is the cathode. Within a few seconds, the internal surface portions of the cylinder 11 are rapidly coated with a layer of magnesium 20. The

Ysupports 13 and 14 also serve as shields confining the spattered magnesium to the desired surface portions of the cylinder 11. Upon reversing the polarity of the electrodes in the gas discharge tube by means of switch 1S, that is, making the magnesium tubing 12 the anode and the magnesium-coated Kovar cup the cathode, a gas discharge tube having an ecient cold cathode in accordance with this invention is provided. Such a gas discharge tube is particularly useful as a voltage regulator device.

In Fig. 3 is shown an additional embodiment of a gas discharge tube formed in accordance with this invention, particularly suitable for use as a switching element in a coaxial line or waveguide structure. An elongated envelope 21 is provided made of an insulating material such as quartz or glass. In this envelope is contained a filling of an appropriate gas at an appropriate pressure as set forth for Fig. l, and in this gaseous medium are immersed cathode 22 and anode 23 electrodes. The cathode is preferably cup-shaped with a small opening 24 provided within the surface facing the anode 23. A block of a low-work-function metal 25, preferably, magnesium, is positioned within the cup. A discharge is established between anode 23 and magnesium block 25, and thereby the internal surface portions of cathode 26 are rapidly coated with a layer of magnesium. Electrical connection is then established between cathode 22 and anode 23 for the tube to function as a high-current low-voltage cold cathode diode.

In Fig. 4, the cathode is shown as a disk-like member 26 positioned opposite a block 27 of a low-work-function metal within an insulating envelope 28 containing an appropriate gas filling. In establishing the discharge initially, disk 26 serves as anode and block 27 as cathode. After the surface of disk 26 has been coated with metal from block 27, the polarity of the discharge is reversed for normal tube operation. In such operation, member 26 serves as the cathode and block 27 as the anode.

It will be readily apparent from the foregoing description that many shapes and forms of gas discharge tubes may be employed with the method set forth herein without departing from the basic principle of this invention. Primarily, a high-current density cold cathode is provided in this invention by establishing a preliminary gaseous discharge between a low-work-function metal serving as cathode and an anode element to thereby form a coating of this low-work-function metal upon the electrode ultimately serving as cathode. It will be seen, for example, in Figs. 1 and 3, that the initial discharge for coating purposes can be established by having the block of lowwork-function metal as cathode and the subsequently used cathode electrode as temporary anode. Such an arrangement will also serve to effectively coat surfaces of the true cathode with a low-work-function metal. While embodiments of cold-cathode structures have been shown wherein the cathode is preferably a hollow conductive member, other shapes and forms of the cathode may also be used. Thus, the cathode may consist ofV a flat disklike member as shown in Fig. 4. Also, arrangements may be envisagedwherein both internal and external surface portions of a cathode immersed in a gaseous medium may be coated. While the conductive base member functioning as cathode has been described as preferably made of Kovar, this material has been used for purposes of illustration and purely as a matter of convenience. Kovar, as is well known, is a glass-sealing alloy d. consisting of 20% nickel, 17% cobalt, 0.2% manganese and the balance iron. It has a specific gravity of 8.5 and a melting point of approximately 2650 F. Its coefficient of thermal expansion is substantially the same as that of certain hard glasses. Consequently, it is aV 'convenient high-temperature alloy to employ when metalto-glass sealsV are required; however, many other embodiments of this invention may equally well be realized using other conductive high-temperature materials such as nickel and alloys thereof and copper and alloys thereof and similar metals as is well known in this art. While magnesium is preferred as the coating material because of its ease in forming an adherent coating and its lack of sputtering during its operation as a cold cathode, other loW-work-function metals may also be employed. Thus,

aluminum, barium, calcium, and cesium are low-workfunction metals which are considered suitable in this regard. By reference made herein to a cold-cathode diode, other cold-cathode devices, such as those having an auxiliary electrode and commonly referred to as coldcathode triodes are also included Within the scope of this invention.

While we have described above the principles of our invention in connection with specific apparatus and method steps, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of oui invention as set forth in the objects thereof and in the accompanying claims.

We claim: v

1. A method of producing a cold cathode for a gas discharge device comprising providing a low-work-function metal within a gas discharge device, said discharge device having a conductive member therein, establishing an electric discharge between said low-work-function metal as cathode and said conductive member as anode to provide an overlaying layer of said low-work-function metal upon said conductive member to act as the cathode surface for the normal discharge in said device.

2. A method according to claim 1 wherein said low- Work-function metal is selected from the group consisting of aluminum, barium, calcium, cesium and magnesium.

3. A method of producing a cold cathode for a gas discharge device comprising providing a block of lowwork-function metal within said gas discharge device, said discharge device having a conductive member therein, establishing a discharge between said loW-work-function metal as cathode and said conductive member as anode to provide an overlayingr layer of said low-Work-function metal on said conductive member to act as the cathode surface for the normal discharge of said device.

4. A method of producing a cold cathode for a gas discharge device comprising providing a low-work-function metal within a hollow conductive member, depositing an ionizable medium within said hollow conductive member, connecting said low-work-function metal as cathode and said hollow conductive member as anode and establishing a gaseous discharge therebetween to form an overlying layer of said low-Work-function metal on said hollow conductive member to act as the cathode surface for the normal gaseous discharge of said device.

References Cited in the le of this patent UNITED STATES PATENTS 1,738,957 Metcalf Dec. 10. 1929 1,965,584 Foulke July l0, 1934 1,990,041 Loewe Feb. 5, 1935 2,020,727 Gaidies et al. Nov. l2, 1935 2,156,631 Krenzien May 2, 1935 2,189,636 Knowles et al. Feb. 6, 1940 2,462,245 Wooten Feb. 22, 1947 2,456,900 Sullivan Dec. 21, 1948 2,591,474 Stutsman .4 Apr. 1, 1952

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