US2875367A

US2875367A - Cathode structures

Info

Publication number: US2875367A
Application number: US464078A
Authority: US
Inventors: James E Beggs
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1954-10-22
Filing date: 1954-10-22
Publication date: 1959-02-24
Anticipated expiration: 1976-02-24

Description

Feb. 24, 1959 J. BEGGS CATHODE STRUCTURES Filed Oct. 22. 1954 e t 5 2 2 W. 2 7 M ig g 18772532212 5 gM/M/ 15 115 Attorney.

United States Patent l 2,875,367 'ca'rnonn STRUCTURES James 'E. Beggs schenectady, N. Y., assignor to General Electric @ompany, a corporation of New York Application October 22, I954, SerialNo. 464,078 8 Claims. (Cl. 313-440 This invention relates to cathode structures and n1etheds oftorming the same. While the cathode structures of this invention are subject to many variations and modifications they are ideally suited for utilization in miniature electric discharge devices and will be particularly described in this connection.

It is generally desirable that the electron emitting surface of an electric discharge device be brought up to operating temperature at a rapid rate and remain sub stantially stationary throughout the entire rangeof-operat-jing temperatures; Conductors having a relatively large surface 'per unit volume can be rapidly heated and will transfer a large portion of this heat at a rapid rate to another, member in contact therewith. However, thin conductive members, such as foil, are undulyfiexible and tend to change their shape and position upon heating. It

is therefore an important-aspect of this ,inventionto protyide afast heating foil structure which is relatively rigid whilestill maintaining a relatively high rate of heat transferto-an electron emitting surface.

It'is also'desirable that the current supplied to the heating element be supplied in a symmetrical path so that the inherent inductance of the current flowing through the heater leads is effectively cancelled a nd further so thatnall areas of the heated element are uniformly and rapidly heated. In addition, the cathode structure should have a low inductance, particularly toradio frequency It ;-is therefore an important object of tliis invention to .provide an improved. cathode structure and method of According to an important aspect of this inveiitiony .there is provided a1 cathode structure and method of forming a cathode structure which includes a foil member and an electron emitting surface oriented in heat conducting relati-onship'to. saidffoil member toprovide a low inductance "unitormly and quickly heated cathode structure.

" Other important aspects and obje'cts of this invention will become more apparent W'hen'the following specificatloii and claims are .consideredin connection with the figures of the drawing inwhichlFigurel illustrates-a U of sustained high or channel-shaped cathode formed in accordance with this 1 I inventionyFigu'res 2 and? illustrate a modification of the cathode illiistrated assi ns l; Figure l illilstates I ajca'tliode with a "foil heater elernent; "Figures saana illustrate a f fb ll heater and-the adaptation M this; foil heater-to directly h at acup s haped cathode; Figure 6 il- 'lathe.

2,8?5,35Y Patented Feb. 244, 1955i ice 2 lustrates a cathode and heater structure and Figures 7 to 13, inclusive, illustrate modified cathode structures wherein a conductive film is formed on an insulating member to provide a heater.

Figure 1 illustrates a directly heated cathode assembly or the filamentary type which is formed of a foil clad member consisting of backing 10 with foil covering 11. The cathode is formed, for example, by stamping the backed foil into the desired channel or U-shape and then removing backing metal 10, for example, by applying an acid bath WhiChCliSSOlVdS the backing metal. A portion of the backing metal 16 is left at the ends of the cathode structure to assist in"attaching heater leads and in mounting the cathode structure. An appropriate emissivecoating, such as the conventional triple carbonate coating, isthen applied to the top surface 12 after which the cathode 'assernbly is mounted in an electric discharge device and activated. s

For example, Where it isdesired to ton-n a cathode which is capable of operating at sustainedhigh temperatunes, the cathode structure is formed fromfa sheet of steel approximately 10 mils thick having a molybdenum toil coating approximately .25 mil thick. The cathode is formedand then the steel is dissolved from the molybdenurn by applying a hydrochloric acid solution. In this manner, an extremely strong and rigid foil cathode structure isforrned without the attendanthazard ofcracking or breaking the foil which is present when a' cathodeis formed from unbackedfoih Acathode of this-type is particularly: suited for utilizationas a direct heatingcathodeand can be brought up to operating temp'erature in a very shor'tperiod of time, i. BL, in the order drone-quarter of a second.

It is readily apparent that this process and the form of cathode herein illustrated, by way of example, issubject to a number of variations in shape and form and may be constructed of any foil material which forms a satisfactory cathode base and may, for example, consist of such materials as iron, molybdenum, nickel, tungsten, titanium, zirconium or tantalum. 3 t

For example, an active barium oxide electron emitting surface can be fabricated on a titanium base Without the formation of an interface, and which is capable of high level stableeinission at relatively low cathode temperaturesu Theoxide'coating is readily reduced by the tita almost a rate depending upon the cathode temperature, so that barium is formed and oxygen is "absorbed by the titanium without the formation of a film or interfac'eon thesurface of thetitanium base and thereby re sults in cathode emission which is stable at high current densities even'when operated in theemissionlimited region; The-emission lifeof aititanium cathodecan be increased by utilizing one ornmorepof the oxides of. "strontium or'calcium or combinations thereof with barium titanium more slowly than is bariurnoxide. I g

' Figures 2 and 3 illustrate a' manner in which a.cup-

oxide, since strontium andcalcium oxides arereduced by shaped cathode structure can be formed from a strip or ribbon of foil13 so as toprovide acathode structure consisting of afls'tibstantially flat top surface. .14 havingcylindrical skirt members-15 and16 This cathodeyfor example isformed with a punch press or may be form'ed by spinning a strip or ribbon of .foil on a It is apparent that this structure. can be formed from backed foil material from which the backing .metal is: removed after forming.

The structure illustrated in Figure 3 hasthe additional feature of having a substantially circular t'op surlface and substantially cylindrical sides whichflendan addia .tionaludgr eepf rigidity to the final structnre- .The top rsurface 141is coated with an emissive coating andthe slot 17,-which. is present when a .thin. strip on ribbon is so formed, or can be cut into a cup which is punched out of a flat disk or slug of metal, provides the two heater leads and results in an inherently non-inductive cathode structure.

This structure can be adapted for utilization as an indirect heater for a cathode structure such as that illustrated in Figure 4 wherein a cathode structure '18 similar to that illustrated in Figure 3 is supported in heat conducting relationship with, but out of direct contact with inverted cup 19, on the top surface 20 of which is placed a suitable emissive coating. Cup 19 is supported, for example, in ceramic insulator 21 which is provided with a plurality of projections 22 which make contact with and are bonded to the cathode. The projections limit the heat transfer to the insulator and increase the efliciency of the cathode. The cathode connection is made by applying a conducting film to the lower surface of insulator 22 so that radio frequency currents flow over the entire surface of cup 19 and result in a low inductance cathode connection.

Cathodes of the form illustrated in Figures 3 and 4 have inherently low inductance so that they are ideally suited for utilization in miniature electric discharge devices operated at high frequencies. Since the electrically heated cathode structure is formed of extremely thin foil material, it can be very rapidly heated and in, view of the inherent rigidity of the shape into which it is formed, a cathode structure utilizing a heater element of this type is less subject to rnicrophonics than are other forms of heaters.

Figures and 6 illustrate modified versions of directly heated cathode structures utilizing at least one conductive foil member having an electron emitting surface .oriented in intimate heat conducting relationship to the foil member. Figure 5a illustrates two substantially L-shaped

foil strips orribbons

23 and 24 having angularly oriented top portions 23 and 24' which are rounded so as tofit into and be bonded to the inner surface of inverted foil cup 25. The cup has a substantially fiat surface 26 on which an emissive coating is formed and the entire cathode structure is, for example, retained in metal disk 27 which provides an inherently low inductance conducting path to surface 26.

The cathode structure is formed by suitably bonding

ribbons

23 and 24 together along the length thereof and then bonding the heater to the inner surface of the cup 25. The bonding may be effected by spot-welding, brazing or other suitable metal-to-metal bonding such as is disclosed in my copending application Serial No. 409,159 filed'February 9, 1954 and assigned to the same assignee as this invention.

The cup consists of metal, such as titanium, zirconium, iron-nickel alloy, tantalum or other metal, which forms a satisfactory cathode base and, for example, is formed from foil or from backed foil. For example, the cup can be formed from titanium foil clad steel wherein the steel is dissolved in hydrochloric acid after the cup is formed or, alternatively, it may be fabricated by joining a shallow foil cup or disk and a thin foil cylinder.

The ribbon heater is formed, for example, of a ribbon of titanium, zirconium, iron-nickel alloy, tantalum or other metal which can. be satisfactorily bonded to an inner surface of the cup 25. Thecross section of the Y ribbon heater is such that it heats. with approximately the samecurrent as will heat the cup to which it is attached so that the entire structure is uniformly and rapidly heated, i. e., the foil and the cup form the heating element. Since the cup has relatively low heat conductivity,

as a result of the choice of material and its relative thinness, the heat loss from the sides of the cup is relatively small and the efiiciency of the resulting cathode structure is high.

An alternative form of a directly heated foil cathode is'illustrated in Figure '6 wherein foil cathode cup 28 is provided with a centrally oriented heater lead 29 terminating in a tapered end portion 30 which extends out to contact an area within cup 28 substantially the same as and contiguous with the emitting surface 31. The heater lead 29 and tapered end portion 30 have such dimensions that approximately the same current heats the lead as will heat the cup 28 and so that the entire structure is uniformly and rapidly heated.

It will be readily apparent that the cathode structures herein described can be modified, for example, by forming a convex or concave emitting surface and, for example, by utilizing an electron emitter other than the aforementioned triple carbonate coating.

It is further noted that where a higher resistance heater is desired, a ceramic button or refractory coating can be formed on the inner end surface of cup 25. A conducting film can then be formed by applying a coating of carbon containing compound, such as a nitrocellulose binder, to the ceramic button and then applying at least one metal from the group consisting of titanium, zirconium, hafnium, and thorium and at least one metal from the group consisting of, but not limited to, iron,

nickel, cobalt, copper, chromium, platinum and molybdenum so that when heated a conducting film, including at least one metal from these groups, is formed. Conducting films of this type are more completely described in my copending application Serial No. 464,080 filed herewith and assigned to the same assignee as this application.

Figure'7 illustrates a form of resistance heater utilizing the method disclosed and claimed in the above-identified patent application wherein there is shown foil cup 32 having a top surface 33 on which emissive coating 34 is applied. Within the cup a thin disk or wafer 35 of insulating material, such as a ceramic of the type commonly referred to as alumina, is suitably bonded to the top surface of, the cup. A conducting heater film 36 is formed over a portion of or all of the lower surface of ceramic disk 35 and heater lead 37 connects to a central portion of the conducting film 36. Heating is effected by current flow through heater lead 37 to film 36 and down through the cup-shaped foil member 32.

It is noted that the foil cup can be punched or otherwise formed in accordance with the previously described process or the cup can be formed by suitably bonding or brazing top member 33 to a cylinder of foil 32 at the same time the ceramic disk or wafer is bonded to the top member. A method of effecting such a bond between ceramic and metal members is described in my aforementioned copending application, Serial No. 409,159.

Alternatively, a coating of high temperature clay or other refractory material can be substituted for the thin ceramic wafer. The surface is coated with a conducting film towhich is brazed the center connecting heater lead 37. When voltage is applied, the conductive film becomes hot, thereby heating the electron emitting surface to which it is attached. The conductive film can be made to operate at voltages intthe order of volts, in which case it can be operated from a plate battery supply, and thereby eliminate the need for a separate heater supply I The. rigidity of the insulating film or ceramic wafer ance heater 40 formed on ceramic disk or wafer 41 and connected by heater leads 42 and 43. Figures 9 through 13, inclusive, illustrate modifications of the heater pattern illustrated in Figure 8. For example, heaters illustrated in Figures9 throughll, inclusive, are particularly. suited a; vines? forutilization in separatelyheated cathode constructions a'n dthose illustrated in Figures 12 and 13 are particularly suitedfor heaters wherein the cathodeycup or emitting surface forms a part of the heater circuit. Each of these heater patterns is formed on a ceramic buttonor. disk of material suchas, for example, those ceramics generally classified as steatites, forsterites and aluminas.

It is noted that the ceramic button or disk 4 1 'can be covered witha nitrocellulose coating over the entire area or in the form of the pattern illustrated in Figures 9 through 13, inclusive. When a high heater voltage is required, these films are usually of a type consisting essentially of carbon while, when low heater voltages are desired, the coating includes metal additives so as to result in a conducting film having a positive, zero, or negative thermal coefficient of resistance. Examples of such films and methods of forming the same are described in greater detail in my aforesaid copending application Serial No. 464,080.

Cathodes of the type herein disclosed are ideally suited for utilization in electric discharge devices such as those described in my Patent No. 2,680,824 and my copending application Serial No. 464,079 filed herewith, both of which are assigned to the same assignee as this invention. T

It is readily apparent that the cathode structures of this invention have inherently low inductances and in particular those illustrated in Figures 5 through 8 provide a substantially symmetrical heater current path which has inherently low inductance. The metal ring, or. the ceramic ring with a conducting coating thereon, which retains the cathode provides an inherently low inductance conducting path to the emissive surface which extends over the sides of the cathode. The intimate heat conducting contact between the heater and the emitting surface provides a structure which is rapidly heated to operating temperature, i. e., in the order of one quarter to three quarters of a second, and which structure is relatively rigid because of the shape thereof so that the tend- .ency toward microphonics is minimized.

This invention has been described in connection with a limited number of specific examples so that a complete understanding thereof may be had; however, it will be readily apparent that this invention is subject to many modifications and it is intended in the appended claims to cover all modifications coming within the true spirit and scope of this invention.

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

1. A cathode structure for an electric discharge device comprising a cup-shaped member including side wall and an end wall having an electron emissive coating on by Letters an exterior surface thereof, a thin insulating disk in contact with the interior of said end wall and a thin film on the exposed surface of said disk and having a much greater resistance than said cup-shaped member, said,

film extending to said side walls for providing a heater element in closely coupled thermal relation with substantially all of said end wall and an electrical connection with said film heater at apoint remote from the side walls of said cup'shaped member.

2. A cathode structure for an electric discharge device comprising a cup-shaped member including an end wall having an electron emissive coating on an exterior surface thereof, a thin insulatingdisk in contact with the interior of said end wall, a resistance heater element bonded to the exposed surface of said insulating disk closely coupled thermal relation with substantially all of said end wall, said resistance element having a much greater resistance than the resistance of said cup-shaped prising a .thin metallic support, an electron emissive coating onsaid support on one side thereof, a thin insulat- 6 ing wafer in coma with the opposite side of said support and substantiallycoextensive in area with said coating, athin conductive film formed'on the opposite side of said wafer to provide a heater in closely coupled ther mal relation with substantially all of the coated area of said support, said film having amuch: greater resistance than the resistance ofsaid support, an electrical connector connected with said conductive film at a central point thereof, and another electrical connector connected to said conductive fihn at, at least, one point on the periphery thereof.

4. A cathode for an electric discharge device comprising a thin metallic support, an electron emissive coating on said support on one side thereof, a ceramic wafer in contact with the opposite side of said support and substantially coextensive in area with said coating and a thin metallized film formed on the opposite side of said ceramic wafer in electrical contact with the periphery of said thin metallic support around the coated area to pro vide a heater in closely coupled thermal relation with substantially all of the coated area of saidsupport, said film having a much greater resistance than the resistance of said support.

5. A cathode for an electric discharge device comprising a thin metallic support, an electron emissive coating on said support on one side thereof, a thin ceramic Wafer in contact with the opposite side of said support and substantially coextensive in area with said coating, and a thin metallized film formed on the opposite side of said ceramic wafer and in electrical contact with said thin metallic support at the periphery of the coated 'area to provide a heater in closely coupled thermal relation with substantially all of the coated area of said support, said film including a metal selected from the group consisting of titanium, zirconium, hafnium and thorium, said film having a much greater resistance than the resistance of said support.

6. A cathode for an electric discharge device co1nprising a thin metallic support, an electron emissive coating on said support on one side thereof, a thin ceramic wafer in contact with the opposite side of said support and substantially coextensive in area with said coating, and a thin metallized film formed on the opposite side of said ceramic wafer and in electrical contact with said metallic support at the periphery of the coated area to provide a heater in closely coupled thermal relation with substantially all of the coated area of said support, said film including a first metal selected from the group consisting of titanium, zirconium, hafnium and thorium,

and a second metal selected from the group consisting of combined resistance approximately equal to the resistance of said cup-shaped member so that when a heating current is conducted through said ribbons and said cup-shaped member said ribbonsand said cup-shaped member are rapidly and substantially uniformly heated.

8. A cathode structure for an electric discharge device comprising a thin conducting cup-shaped member including an end wall having an electron emissive coating on an exterior surface thereof, a conductor approximately centrally oriented within said cup-shaped member, said conductor having a taperedend portion extending out to .contactsubstantially all of said end wall, the resistivity of said conductor being of the order of the resistivity of said cup-shaped member and the cross-sectional area .of said conductor and said cup-shaped member being such that upon passage of a heater current therethrough, said conductor and said cup-shaped member are rapidly and Substantially uniformly heated.

References Cited in the file of tlris patent UNITED STATES PATENTS 1 1,859,522 Miessner May 24, 1932 Bi'uining et al. 'l Oct. 31, 1950