US3031740A

US3031740A - Matrix type cathode

Info

Publication number: US3031740A
Application number: US721039A
Authority: US
Inventors: Robert D Culbertson; Russell C Mcrae; Lowell A Noble
Original assignee: Eitel Mccullough Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1958-03-12
Filing date: 1958-03-12
Publication date: 1962-05-01
Anticipated expiration: 1979-05-01
Also published as: GB914509A; FR1221240A

Description

May 1, 1 962 R. D. CULBERTSON ET AL 3,031,740-

MATRIX TYPE CATHODE Filed March 12, 1958 INVENTOR5. 153056753 CwZZerZaozz zmfiezz iflclae laweZZA. 11 0556;

wash, GA ma. 1 (Oalsow ATTORNEY5 United States Patent M 3,031,740 MATRIX TYPE CATHODE Robert D. Culbertson, San Jose, Russell C. McRae, Santa Clara, and Lowell A. Noble, San Bruno, Calif, assignors to Eitel-McCullougb, Inc., San Bruno, Calif, a corporation of California Filed Mar. 12, 1958, Ser; No. 721,039 5 Claims. (Cl. 29-182.3)

erated at temperatures less than 1000 C., for example 825 C. to 925 C., and which is resistant to arcing, sparking, and ion bombardment.

A feature of outstanding importance is'the insensitivity to exposure to air of cathodes of the present inven: tion after forming and activation. A further-feature is the reproducibility of our cathodes, which exhibit unusual uniformity of performance characteristics.

More especially, it is an object of the invention to provide a matrix type cathode having a backing plate of a commercially available nickel containing in a controlled amount at least one impurity serving as an activator, and an emissive coating formed from a mixture, in pulverulent form, of nickel, alkaline earth carbonates, and a further activator comprising a small amount of zirconium hydride.

Further objects and features of the invention will be apparent from the following description, taken in connection with the accompanying drawing, in which- FIGURE 1 is a transverse sectional view of a cathode element or button to which the invention may be applied, the section being taken in the plane indicated by the line 1--1 in FIGURE 2 and FIGURE 2 is a plan view of the cathode button of FIGURE 1. a

The cathode illustrated in the drawing is intended j for use in a beam type electron tube such as a klystror'i or traveling Wave tube, but it will be appreciated that the structure of these devices forms no part of the instant invention and that the details thereof will vary widely, depending upon the intended use of the cathode. As shown, the cathode comprises a base or backing member 10 of nickel in the form of a circular disk, concave at its emitting face which is recessed at 12 to receive the compressed emitting coating 14. The dished or concave surface is provided to assist in focusing the emitted electron beam.

The coating is prepared from a homogeneous mixture of powdered materials consisting principally. of nickel with a less amount of barium, strontium and calcium carbonates, and a small amount of zirconium hydride. Preferably we employ in the mixture 70 parts by Weight of nickel powder, 30 parts of carbonates, and 1 part of zirconium hydride. The carbonates may be prepared by co-' precipitation and barium may predominate, a weight ratio of approximately 6 parts barium to 3 parts strontium to 1 part calcium being preferred, as is customary in the manufacture of oxide coated cathodes. preferably substantially pure; however, instead of adding zirconium hydride to the mixture, an equivalent amount of zirconium metal may be added to the nickel powder and be present as an impurity in the nickel powder.

For the backing member 10 we employ a commercial nickel of high purity (e.g., 99%) but containing one or more specific impurities. For example, analysis of samples of nickel sold under the designation Electronic Grade A nickel indicates the presence of significant amounts of The nickel powder is 3,031,740 Patented May 1, 1962 ternational Nickel Company.

Electronic Grade A Nickel Element Percentage Source Nickel (with trace Balance of 99+%.

cobalt). M 2001 s t 1 aximum pee rogra hie analysis. Copper {Average .0l2%. p

Maximum 30%. Iron Average .02%. Wet analysis. 7

Average .035% Spectrographic analysis. Manganese {Maximum .35

"""""" Average 22%.-- Spectrographte analysis.

Maximum .20%.. Average .03% Wet analysis. Average .0l6%.. Spectrographic analysis. hgaximum .2}; i axlmum 111 Tltamum {Average less than Wet analysis. Suliun. Maxil iium .008% Magnesium Average 938%.. Spectrographic analysis.

While we prefer not to be bound by any theoretical con siderations, it is believed that the inclusion of activators or reducing agents (i.e., elements having a free energy of formation greater than 160 kilocalories per gram molecule of oxygen at room temperature) in both the backing member 10 and the emitting coating 14 contributes materially to the long life, high emission at low temperatures, unusual stability, and reproducibility of cathodes of our invention. It is believed that the activator added to the coating is largely responsible for the high'emission characteristics of the cathode of our invention and that the activator present in the backing member contributes to its long life.

As is well known with respect to oxide coated cathodes, an activator acts as a reducing agent, converting alkaline earth oxides to pure alkaline earth metals. These metals migrate to the surface of the cathode, producing the condition necessary for the emission mechanism, and are evaporated from such surface due to temperature effects. Thus, both the activator and the emissive material tend to be consumed during operation of the cathode. If large amounts of activator are present, high emission will be obtained but the emissive material will tend to be consumed due to the rapid reduction and evaporation of the emissive material. If small amounts of activator are present, low emission will be obtained and the activator will tend to be rapidly consumed.

It has been found that for best results according to our invention the activator should be present in the mixture in an amount between .l% and 2% by Weight of the mixture, 1% being preferred. If the activator is present in the mixture in a lesser amount, both decreased emission and decreased life will result due to the scarcity of the activator and to its early depletion. If the activator is present in the mixture in a greater amount, it will increase the emission of the cathode, but will result in decreased life due to the rapid evaporation of the emissive material from the surface of the cathode. Such rapid evaporation of emissive material is also undesirable due to the fact that it will tend to condense on other parts of the tube and produce undesired emission therefrom.

It is believed that the inclusion of an activator in the backing member according to our invention increases the life of the cathode due to the fact that such activator dif:

fuses through the backing member to the emissive coating at a regular rate which roughly corresponds to the rate of consumption of the activator present in the mixture, thus tending to maintain a constant level of activator present in the mixure. It has been found that for best results according to our invention the activator should be present in the backing member in an amount between .05% and 1% of such member. If the activator is present in a greater amount, the level of activator in the mixture will tend to increase, which is undesirable for the reasons pointed out above, and similarly if a smaller amount of activator is present in the backing member, the level of activator in the mixture will decrease, which is also un desirable.

It is also necessary according to the invention that the backing member be substantially free of other impurities, such as manganese, for example, which in amounts greater than mere trace amounts might deleteriously affect the operation of the cathode, or the tube in which the cathode is used. For this reason a high purity nickel (e.g., 99%), such as Electronic Grade A nickel mentioned hereinbefore, is believed to be necessary.

According to another embodiment of the invention, we have used a backing member made of nickel procured by a vacuum melting process to which zirconium metal has been added in the amount of .5 by weight of the nickel. The analysis of the so-called vacuum melt nickel is as follows:

Element Percentage 01117 Average values obtained L %j i:f i% from the analysis of over The thickness of the emissive coating also is important with respect to desirable characteristics of the cathode. If the coating is too thin, then the emissive material will soon be depleted. thus reducing the life of the cathode. It has been found that long life and high emission are obtained by distributing the powdered mixture over the backing member in the amount of at least 100 milligrams per square centimeter.

The desirable mechanical and electrical characteristics of the cathode of our invention are also due, in large measure, to the compressing of the mixture onto the backing member. In the first place, such compression provides a mechanically rugged emissive surface that enables the cathode to be handled during fabrication without danger of chipping or scratching the emissive surface. The ruggedness of the surface also enables it to withstand electrical effects such as reverse ion bombardment and arcing with little or no effect on its life or emissivity.

In the second place, the density to which the coating is compressed will tend to control the rate of migration of the alkaline earth metals to the surface of the cathode and thus will influence the life and emission of the cathode. If the coating is compressed to too great a density, the rate of migration of the alkaline earth metals to the surface of the cathode will be slowed, thus reducing the emission from the cathode and perhaps also reducing its life due to the complete inhibition of the migration of some of such metals. If the coating is compressed to a lesser density, it will tend to speed migration of the alkaline earth metals to the surface, thus increasing emission but resulting in a mechanically weak coating. It has been found that a pressure between 20 and 35 tons per square inch applied to a mixture distributed over a suitable backing member in the amount specified above will result in a suitable coating having a thickness of between .016 and .024 inch and a density which is between 40% and 50% of the density of a solid of the same composition.

The nickel-carbonate ratio in the emissive coating is of some importance with respect to the electrical and mechanical characteristics of the cathode of our invention. if the carbonate is present in too large a ratio, it will tend to produce high emission but will result in a mechanically weak coating which will tend to chip or flake and will be subject to damage due to reverse ion bombardment and arcing. On the other hand, if the carbonate is present in too small a ratio, it will result in a mechanically rugged coating having low emission and short life. We have found that a nickel-to-carbonate ratio of between 60:40 and :20 will provide a satisfactory coating.

According to one specific embodiment of our invention, a backing member may be prepared from Electronic Grade A nickel. The backing member may be of any thickness consistent with good heater efficiency. We have used backing members varying in thickness between .005 and .125 inch. The backing member may be of almost any desired size and shape.

We have used concave discs varying in diameter from .025 inch to 6 inches. A shallow depression may be formed in the surface of the backing member to provide a receptacle to receive the emissive mixture and contain it during the subsequent compression thereof. A thin layer of nickel particles (cg, vacuum melt nickel) having a particle size of between 3 and 6 mils may be deposited on the surface to be coated and sintered thereto to provide a rough surface to which the emissive coating will firmly adhere.

A pulverulent mixture of 70 parts of substantially pure nickel powder (e.g., vacuum melt nickel having a particle size of about 7 to 9 microns), 30 parts of co-precipitated barium, strontium, calcium carbonate in the proportion 6:311 (approximately) and 1 part of zirconium hydride is prepared by any conventional procedure (e.g., by mixing in a dry mix blender).

The mixture is distributed uniformly onto the surface of the backing member in the amount of 200 milligrams of mixture per square centimeter. The coated backing member is than placed in a press and a pressure of 35 tons per square inch is applied to the coated surface to compress the mixture to a density of 45% of the density of a solid member of the same composition and cause it to adhere to the roughened surface of the backing member.

The backing member is then mounted on suitable sup ports and in operable proximity to a heater within an electron tube to serve as the cathode thereof. The completed electron. tube is then placed on a vacuum pump and baked out at a temperature of about 400 C. to outgas the various parts of the tube and provide a fair vacuum within the tube. The heater is then energized to heat the cathode to 600 C. to complete the outg-assing thereof, pumping eing continued.

The cathode is then formed by increasing the temperature thereof to about 1050 C. to convert the alkaline earth carbonates to oxides and the zirconium hydride to zirconium. Completion of the forming step is indicated by an abrupt decrease in pressure in the tube under continued pumping. As a further result of the heating, the nickel powder particles in the mix are sin- .tered to each other and to the backing member. Outgassing of other electrodes is concluded, pumping being continuous throughout the operation described.

The final activation of the cathode occurs in an aging step in which the cathode is heated to 925 C. for twenty hours while drawing current from the cathode in. ac cordance with conventional practice. The alkaline earth oxides are here converted to free metals by reaction with the zirconium in the coating.

The tube is now ready for operation at a cathode temperature of about 900 C. depending on amount of cathode current required. During operation the zirconium in the coating, reinforced by the activator which diffuses into the coating from the backing member, continues to reduce the alkaline earth oxides in the coating to replace the free alkaline earth metal which is evaporated from the surface of the cathode. Thus, a cathode having a high initial emission which remains substantially constant for a relatively long period is provided.

According to another embodiment of our invention a mixture of 60 parts nickel powder, 40 parts of the alkaline earth carbonates and 1 part of zirconium hydride may be compressed onto a backing member made of vacuum melt nickel containing .5 zirconium, with a pressure of 20 tons per square inch, to provide a coat ing density of about 47% of the density of a solid member'o'f the same composition, all other steps and quantities remaining substantially unchanged. The cathode according to this embodiment is specifically adapted for high emission long life operation at a lower temperature of about 825 C.

Processing of the cathode subsequent to mounting the same in the tube forms no part of the instant invention and may be carried out with the aid of equipment and with such procedures as are accepted practice in the preparation of conventional oxide coated cathodes. It will be apparent that the preferred sequence of operations hereinbefore described is merely illustrative-and 7 may be varied widely.

Cathodes formed as herein described and claimed have been successfully operated for more than 5000 hours at a temperature of 850 C. while delivering in excess of one ampere per square centimeter of emitting surface. Performance of these cathodes is in other respects markedly superior to that of conventional oxide coated cathodes. Use of the cathodes in both klystron and negat-ive grid type tubes is eminently satisfactory.

The term consisting essentially of is used herein in the definition of the ingredients Whose presence in the claimed composition is essential, and as used it is intended to exclude the presence of other materials in such amounts as to interfere substantially with the properties and characteristics possessed by the composition set forth but to permit the presence of other materials in such amounts as not substantially to affect said properties and characteristics adversely.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. A cathode comprising a backing member and an emissive coating on said backing member, said backing member being at least 99% nickel and including at least one element selected from the group consisting of zirconium, magnesium, and silicon, the total amount of said selected elements being between .05% and 1% of the nickel, said emissive coating consisting essentially of a mixture of between 20 and 40 parts of barium-strontiumcalcium carbonate, with between 80 and parts of nickel powder and one part of zirconium hydride powder, said mixture being pressed onto said backing member to form a porous coating thereon having a thickness between .016 and .024 inch and having a density between 45% and 50% of a solid member of the same composition.

.2. A cathode comprising a nonporous backing member and an emissive coating constituting a cohesive porous matrix adherent on said backing member, said backing member comprising at least 99% nickel and at least .05 of one element selected from the group consisting of zirconium, magnesium and silicon, and said emissive coating comprising a mixture of 20 to 40 parts of barium-strontium-calcium carbonate powder with to 60 parts of nickel powder and at least one part of zirconium hydride powder compacted to eifect formation of a cohesive porous matrix by said nickel powder with said barium-strontium-calcium carbonate powder and zirconium hydride powder filling the interstices in the nickel matrix.

3. The combination according to claim 2 in which the zirconium in said emi-ssive coating mixture comprises zirconium metal powder.

4. The combination according to claim 3 in which said emissive coating is compacted under a pressure sufiicient to produce a coating between .016 and .024 inch thick and having a density between 40% and 50% of a solid member of the same composition.

5. The combination according to claim 4, in which the nickel particles of said compacted emissive mass are sintered one to another and to said backing member to form said cohesive porous matrix adherent on the backing member.

References Cited in the file of this patent UNITED STATES PATENTS 2,147,447 Kolligs Feb. 14, 1939 2,495,580 Gall Jan. 24, 1950 2,543,439 Coomes et al. Feb. 27, 1951 2,899,299 Lynch Aug. 11, 1959 FOREIGN PATENTS 710,648 Great Britain June 16, 1954 202,582 Australia Feb. 10, 1955 OTHER REFERENCES Poehler: Proc. Inst. Radio Engrs, vol. 40, pp. -196, February 1952.

Claims

1. A CATHODE COMPRISING A BACKING MEMBER AND AN EMMISSIVE COATING ON SAID BACKING MEMBER, SAID BACKING MEMBER BEING AT LEAST 99% NICKEL AND INCLUDING AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM, MAGNESIUM, AND SILICON, THE TOTAL AMOUNT OF SAID SELECTED ELEMENTS BEING BETWEEN .05% AND 1% OF THE NICKEL, SAID EMISSIVE COATING CONSISTING ESSENTIALLY OF A