US2945149A - Gas discharge tube - Google Patents

Description

July 12, 1960 J. F. HENEY ETIAL GAS DISCHARGE TUBE Filed Dec. 15, 1954 y me M W IF M a M 0 m X 5 m 6 W 0 m WE W6 Nw mm mm we m m m/ A WUVRNQSIQQN 6 KVIMIEJP w l GAS DISCHARGE TUBE John F. Heney, Bloomfield, and Patrick E. Dorney, Emerson, NJ., ass'ignors to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Maryland Filed De'c.13, 1954, Ser. No. 474,639 8 Claims. c1. 313-:34'6) This invention relates to gas discharge tubes and their manufacture. It is more particularly directed to novel cold cathodes operable as thermionic emitters for use in gas tubes and methods for producing them.

Gas discharge tubes using cold cathodes are well known. In the typical .cold cathode diode, two electrodes areimmersed in a gas at a relatively low pressure. If sufficient voltage is applied between the electrodes of 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 cold cathode 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 electrodes immersed in a rare gas such as xenon at a pressure of several millimeters, attempts to obtain high currents greater than 50-1 milliamperes have resulted in an excessive power dissipation at the' cathode of the tube. In obtaining cold-cathode currents, voltage drops as high as 300 volts occur, 'dep ending on the cathode material and the gas used in the tube. The resultant power dissipation at thecathode becomes excessive 'at these currents I and voltages, and the heating an ionic bombardment produced caused rapid deterioration and disintegration of the cathode. The useful life of such tubes is thereby It is an important feature of this invention that a cold cathode is provided in which a layer of an alkaline-earth metal overlies the conductive base member cathode.

It 'is a further feature that this overlying layer is deposited on a relatively thin cathode structure so that when a gas discharge is first established, the cathode structure functions briefly as a cold cathode emitter and then, upon being heated by the discharge, functions as a thermionic emitter.

As a preferred feature of this invention, a layer of barium overlying a thin Kovar electrode 'is provided.

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

Fig. 1 is a cross-sectional view of a gas discharge tube of this invention;

Fig. 2 is an enlarged cross-sectional view of the cathode structure of the tube of Fig. 1 prior to coating of the up of a Kovar stem wire 4 and support wires 5 and,

attached thereto, a metallic sleeve-like container 6 originally filled with an alkaline-earth metal. This sleeve 6 is preferably made of iron and filled with barium. The

alkaline-earth metal is shown, after rupture of the sleeve- 6, as a coating 7 deposited on portions of the cathode structure 3 and internal surfaces of the envelope 2.

In Fig. 2 is shown an enlarged view of the cathode structure of the tube of Fig. 1 prior to coating of the limited to frequently less than an hour. Thus, heretofore, in order to provide gas discharge tubes having high current carrying capacities, heated cathodes have been primarily resorted to.

In the copending application of J. Heney, A. White and D. Sharp, Serial No. 472,932, filed December 3, 1954, now Patent 2,845,324, an improved cold cathode for capableof operation in a cold cathode mode only. As a consequence, the voltage drop across such a tube during its period of operation is a cold cathode one, i.e., it is considerably higher than the voltage drop occurring when thermionic emitters are used. For a given current, then, power dissipation at a cold cathode is higher than for a corresponding thermionic cathode. However, in the present invention, we have discovered that it is feasible to provide a thermionic emitter within a gas tube while still retaining the advantages of cold cathode emitters, namely, absence of external heater power.

It is an object of the present invention, therefore, to provide a highly efiicient cold cathode operable as a thermionic emitter for high-current low-voltage discharges.

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

It is still an additional object to provide a simple and inexpensive method for fabricating such cold cathodes and gas discharge devices.

i of the tube between the sealed-off point 14 and the neck cathodewith the alkaline-earth metal. In Fig. 3 an enlarged sectional view is shown of the cylindrical container 6 filled with the alkaline-earth metal. The outer wall 8 constitutes arelatively strong iron shell, and a small portion of the wall is reduced in thickness to present a thin cover plate 9. As shown in Fig. 2, the plate 9 faces in an upwarddirection. Because the sleeve 6 will be ruptured at this thin plate portion, the alkaline-earth metal contained therein is ejected directionally upwardly.

Referring again to Fig. 1, the envelope 2 is shown as having attached thereto an insulating tube 10 of the same material as the envelope 2 and terminating in a Kovar sleeve anode 11. A glass tubulature 12 is attached to the opposite end of the sleeve 11 and makes connection with a vacuum pump and gas filling system, not shown. Under vacuum conditions or in an atmosphere of an inert gas at a low pressure, the iron sleeve 6 may be heated by any suitable means such as, for example, a radio-frequency induction coil 13. When the sleeve 6 is hot enough, the thin-walled plate section 9 ruptures and results in deposition of the alkaline-earth metal over the sleeve and over the adjacent supportwires and facing glass portions substantially as illustrated by coating 7.

This rupturing of the sleeve is similar to the flashing of' a getter in a vacuum tube. After the tube is evacuated of the gases resulting from the flashing, it may be refilled with any of a variety of gases depending upon the required characteristics of the discharge tube. The tube is then sealed ofi at point 14 and is ready for operation as a gas discharge tube. The tube shown in Fig. l is particularly useful for providing a gas discharge for use within a coaxial line or waveguide structure. A portion is inserted within the Waveguide structure, with the larger envelope 2 located outside of the waveguide.

In Fig. 4 is shown another 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. velope 16 is provided made of an insulating material such as quartz or glass. The ruptured getter structure 17 is shown as directed to the elongated cathode 18 which is covered with a coating 19 of the alkaline-earth metal. Near-by wall portions of the envelope are shown as also coated with the alkaline-earth metal, preferably barium. After the coating procedure, the tube is filled with an inert gas and electrical connection is madeto the cathode;

18 and anode 20. n

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 1 and 20 millimeters of mercury, After sealing the tube, an appropriate direct voltage is then applied between the cathode 3 and anode 11 of the tube illustrated in Fig. 1 or between the cathode 18 and anode 20 of the tube shown in Fig. 4, to establish a gas discharge within the tube. The voltage required to initiate the discharge is known asthe striking voltage. Upon establishing the discharge, within a matter of microseconds the tube voltage drops to the cold cathode maintaining voltage. This is the voltage typical for cold cathode operation and is a function of the cathode material and the gas pressure. Referring to Fig. 5, this is shown as voltage A. This voltage is maintained for a finite'time of the order of a few seconds, depending upon the geometry and thermal mass of the cathode, and then as the heat of the discharge increases the temperature of the cathode structure and thereby of the alkaline-earth metal,

copious emission of electrons ensues, and the voltage drops within a few seconds to point B as shown in Fig. 5. This represents the thermionic cathode maintaining voltage. When such thermionic emission occurs, the voltage drop across the tube decreases markedly, as shown in Fig. 5, and the tube is capable of very high discharge currents for very long times. Thus, in thesubject invention, a gas discharge tube has been provided having av cathode operable as a thermionic emitter and yet having the advantages of a cold cathode emitter from a circuit point of view, -i.e., absence of external heater supply.

An elongated en Its coeflicient of thermal expansion is substantially the same as that of certain hard glasses. Consequently, it is a convenient high-temperature allow to employ when metal-to-glass seals 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. 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 our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

1. A gas discharge tube comprising a hermetically sealed dielectric envelope, a cathode having major and minorportions disposed internally of said envelope, said major portion being attached in overlying coated relationship with a surface area of said envelope and adapted to operate as a thermionic cathode, said minor portion having a low thermal mass being disposed internally of said major portion and adapted to operate as a cold and therminonic cathode and an anode disposed internally of major and minor portions disposed internally of said envelope, said major portion being attached in overlying coated relationship with a surface area of said envelope and adapted to operate as a thermionic cathode, said minor portion having a low thermal mass being disposed internally of said major portion and adapted to operate For a typical tube having a striking voltageof250 volts} the cold cathode maintaining voltage may be approxi.

mately 150 volts with the thermionic maintaining voltage at approximately 40 volts. Such a tube is capable of carrying currents of the order of amperes for almost indefinite periods of time.

It will be readily apparent from the foregoing descripother alkaline-earth metals, calcium and strontium, as

well as mixtures of the alkaline-earth metals, are also considered suitable for the practice of this invention.

The tube geometry shown is by way of illustration only,

the tube structure being readily varied to suit desired purposes.

While the cathode structure 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 consisting of 20% nickel, 17% cobalt, 0.2%n1anganese and the balance iron. It has a specific gravity of 8.5 and a melting point of approximately 2650.=F.

as a cold and thermionic cathode and an anode disposed internally of said envelope and in spaced relation to said cathode portions.

'4. A gas discharge tube comprising a hermetically sealed dielectric envelope, a cathode having major and minor portions disposed internally of said envelope, said major portion being attached to an area of said envelope and adapted to operate as a thermionic cathode, said minor portion having a low thermal mass being disposed internally of said major portion and adapted to operate as a cold and thermionic cathode, a voltage lead-in wire extending through said envelope, said minor portion in cluding a hollow frangible element attached to said leadin wire containing an alkaline earth metal within said hollow frangible element and an anode disposed internal- 11y and in spaced relationship to said cathode portions.

5. In the method of manufacturing a gas discharge device having a cathode adapted to function as both a cold and thermionic cathode, said device having a dielectric, hermetically sealed envelope with a voltage leadin to said cathode, and said cathode having an emitting surface of alkaline earth metal, the steps comprising sealing an alkaline earth metal within a portion of said cathode, mounting the cathode in said envelope with a leadin sealed therethrough, and rupturing said portion of said cathode to provide a layer of said alkaline earth metal upon said cathode and envelope surfaces adjacent theretO.

6.In the method of manufacturing a gas discharge device having a cathode adapted to function as both a cold and thermionic cathode, sa-id device having a dielectric, hermetically sealed envelope with a voltage leadin to said cathode, and said cathode having an emitting surface of barium metal, the steps comprising sealing barium metal within a portion of said cathode, mounting the cathode in said envelope with a lead-in sealed therethrough, and rupturing said portion of said cathode to provide a layer of said barium metal upon said cathode and envelope surfaces adjacent thereto.

7. In the method of manufacturing a gas discharge device having a cathode adapted to function as both a cold and thermionic cathode, said device having a dielectric, hermetically sealed envelope with a voltage leadin to said cathode and said cathode having an emitting surface of alkaline earth metal, the steps comprising sealing an alkaline earth metal within a portion of said cathode, mounting the cathode in said envelope with a lead-in sealed therethrough, rupturing said portion of said cathode to provide a layer of said alkaline earth metal upon said cathode and envelope surfaces adjacent thereto; disposing an ionizable medium within said envelope, and establishing a gaseous discharge therein whereby said cathode and envelope surfaces adjacent thereto first function as a cold cathode emitter and then function as a thermionic emitter.

8. In the method of manufacturing a gas discharge device having -a cathode adapted to function as both a cold and thermionic cathode, said device having a dielectric, hermetically sealed envelope with a voltage lead-in to said cathode and said cathode having an emitting surface of barium metal, the steps comprising sealing barium metal within a portion of said cathode, mounting the cathode in said envelope with a lead-in sealed therethrough, rupturing said portion of said cathode to provide a layer of said barium metal upon said cathode and envelope surfaces adjacent thereto, disposing an ionizable medium within said envelopeand establishing a gaseous discharge therein whereby said cathode and envelope surfaces adjacent thereto first function as a cold cathode emitter and then function as a thermionic emitter.

References Cited in the file of this patent UNITED STATES PATENTS

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