US 3351486 A
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1967 w. E. BUE'SCHER ETAL 3,351,486
' CATHODES Filed Nov. 23. 1966 ELECTRON EMISS/VE /c0A TING CAT/100E BASE l5 SELF SUPPORT/N6 METAL .STR/P ,4. COMPRISING $0ELY A MIXTURE 0F S/NTERED NICKEL AND OTHER METALS INVENTORS WILL/AM E. BUESCHER DONALD E. KERSTETTER United States Patent 3,351,486 CATHODES William E. Buescher and Donald R. Kerstetter, Emporium, Pa., assignors to Sylvania Electric Products Inc., a corporation of Delaware Filed Nov. 23, 1966, Ser. No. 596,418 4 Claims. (Cl. 117-223) ABSTRACT OF .THE DISCLOSURE A cathode base for an electron discharge device is comprised solely of unalloyed metals and consists of a pliable self-supporting metal strip, said strip comprising a mixture of sintered nickel and other metal particles. The cathode base, as is usual in the art, is coated with electron emissive material.
This invention relates to cathodes for electron discharge devices and more particularly to cathodes having a base formed from sintered metal particles and metal particle mixtures and is a continuation-in-part of US. application, Ser. No. 314,661 entitled, cathodes, filed Oct. 8, 1963, and now abandoned.
In the process of fabricating cathodes for electron tubes, 21 base composition is usually formed into a desired configuration and then coated with a layer of alkaline earth carbonates to provide a cathode or filament. Thereafter, the cathode or filament is inserted within an electron tube structure and suificient heat is applied to the cathode, either directly or indirectly, to cause the reduction of the carbonates to oxides and free metal and thereby render the cathode electron emissive. Subsequently, during tube use, heat is applied to the cathode and positive potentials to nearby tube elements to provide electron emission from the cathode for a period of time and at a rate which is dependent upon numerous factors including the composition of the cathode base.
In the fabrication of cathode bases, it has been a common practice to combine specific additive materials and nickel materials by means of a melting process to provide a cathode base material. This material is usually hot rolled and then cold rolled into strip having a desired thickness, shaped into a cathode base con-figuration, sampled in electron tubes, tested for characteristics, and accepted or rejected depending upon the tube characteristics as compared with prior fabricated tubes. Obviously such a complicated process is costly in materials, time, inventoried material, labor, and apparatus.
In the above process it has been found that the heat necessary for alloying the constituents of a base material causes the formation of chemical compounds which complicate the analysis of the final product. Also, impurities are leached from the containers into the materials during the melting process as well as during the process of cooling the melted materials. Additional undesired impurities are acquired during the process of rolling, especially during hot rolling, the material to strip form having a desired thickness. Thus, the cathode base or product resulting from such a process has a complex and most difiicult to determine chemical content which has made the search for correct additives in correct amounts to provide a cathode base composition having desired and consistent characteristics a long and arduous task.
Additionally, it has long been known that melted materials have a notorious rate of grain growth or grain recrystallization under adverse temperature conditions such as are encountered in electron tubes. Such large-grained structures are not only mechanically weak but when compacted into a layer having a thickness suitable for use as 3,351,486 Patented Nov. 7, 1967 a cathode base provide a substantially direct path for diffusion of the additive reducing elements therein to the cathode base surface. This direct path may be through the grains themselves or by way of the boundaries therebetween.
The above problems associated with melted materials and a wide range of factors such as evolved gases, additive material diifusion rates, the free energy of formation of the oxides of the additives, and numerous other factors must be considered and balanced to provide a cathode base which fulfills the stringent requirements of presentday electron tubes.
Therefore, it is an object of this invention to provide a cathode base having an enhanced composition.
Another object of the invention is to improve the control of the additive element diffusion rate in a cathode base composition.
Still another object of the invention is to provide a cathode having an improved predictability of electron activity.
Another object of this invention is to provide a cathode which improves the electrical capabilities of an electron tube.
Still another object of the invention is to provide a cathode base having a reduced level of uncontrolled additive impurities during the fabrication thereof.
These objects are achieved in one aspect of the invention by the provisions of a mixture of nickel particles and metal particles from a selected group of additive materials. This metal particle mixture is formed into a layer, sintered, shaped into a desired configuration, and coated with a layer of electron emissive material to provide a cathode suitable for use in an electron discharge device.
For a "better understanding of the present invention together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure and appended claims together with the accompanying drawing in which the figure shows one form of cathode, partly in section, embodying the invention.
In the fabrication of cathode bases especially suitable for use in electron tubes, it has long been known that minute amounts of additive materials to a basic material or matrix such as nickel have a pronounced effect upon the electrical and mechanical results obtainable. It has now been found that a mixture of sintered metal particles either eliminates or greatly reduces the problems associated with the prior art techniques of melting or alloying metals to provide cathode bases. Further, unique, unexpected, and, as far as is known, previously unobtainable results are now provided by cathode bases formed from a mixture of sintered metal particles whereon a layer of alkaline earth oxides is deposited to provide cathodes for electron tubes.
Reviewing the problems, an alloyed cathode base usually requires a relatively high temperature to alloy the metals, an extended period of cooling in a container of some sort, and a rolling or compacting process to provide metal strip having a thickness suitable for use as cathode base material. As a result, undesired and usually undeterminable compounds are formed by the relatively high temperature while impurities are leached from the container during the melting as well as during the cooling processes and additional impurities impregnated into the alloy during the compacting or rolling process. All of these items contribute to the complicated chemical content of an alloyed cathode base and the mechanical and electrical unpredictability of such a base.
Contrarily, a mixture of metal particles requires a comparatively cool temperature to carry out the sintering process, is adapted to the formation of metal strip having a thickness suitable for cathode bases without heated compaction, and does not require an extended period of o: cooling in a container. Thus, the formation of compounds and the assimilation of contaminants during the melting, cooling, and compacting processes are no longer problems when the cathode base is a mixture of sintered metal particles.
Accordingly, a cathode base formed from a mixture of sintered metal particles has a chemical content solely dependent upon the ingredients supplied to the mixture and the predictability of the mechanical and electrical characteristics thereof is thereby enhanced. Therefore, the previously mentioned program of acceptance for electron tube cathode bases is greatly accelerated and simplified when the bases are formed from a mixture of sintered metal particles.
Further, it is believed that the reduction of the alkaline earth oxide layer on the surface of a cathode takes place in proportion and to an extent which is dependent upon the availability of reducing agents for the oxides at the cathode base surface. This availability of reducing agents at the surface of the cathode base is dependent upon a number of factors including the grain size of the base, the path of the element to the surface, and the form of the element in the base material.
Again referring to a melted alloy, it has been found that normal processing of an electron tube causes an ex cess of reducing agents at the surface of the cathode base which not only react with the alkaline earth oxide layer to produce free barium in excess of that required to support a necessary level of electron emission but also sufficient to sublime to other elements and reduce the insulation resistance therebetween and sufficient to build up a layer of oxidized reducing agents at the interface of the cathode base and earth oxide layer. These oxidized reducing agents form an interfacial resistance which impedes further reduction of the alkaline earth oxides needed to sustain the emissive level of the tube throughout an extended period of use.
Also, it has been found that the additive reducing agents in an alloy are homogeneously dispersed throughout the alloy and therefore available throughout the entire cathode base surface to promote the excess initial electron emission mentioned above. Further, it has long been known that the grain size and grain growth, especially under adverse temperature conditions such as are prevalent in the operation of electron tube cathodes, is much greater for an alloy than for a mixture of metal particles.
More specifically, it is well known that the same original ingredients may be utilized to provide a melted alloy material as well as a somewhat similar material which includes a mixture of sintered particles. Also, it is well known that the proper selection of particle size for the sintered mixture will provide an original material having substantially the same grain sizes as the original alloyed material.
However, when the alloyed material and the sintered metal particle material are employed as a cathode base material for an electron discharge device, it has been found that the substantially similar grain sizes of the original material are substantially dissimilar after operational use of the discharge device. Observation indicates that the operational temperature of the cathode of an electron discharge-device is apparently sufficient to cause recrystallization of the grain, of an alloyed material such that the grain, in many cases, undesirably extends the full thickness of the cathode base material.
On the other hand, it has been found that a cathode base formed from a sintered mixture of metal particles operated in the same tube type for the same period of time under substantially identical operational conditions has a grain size which remains substantially unchanged from the grain size of the original material prior to operational use. Thus, the sintered particle mixture tends to desirably hinder grain growth during operational use whereuponthe original grain remains substantially un- 4 changed and does not extend the full thickness of the cathode base material.
As mentioned above, it is believed that the reduction of the oxide layer of a cathode takes place in proportion and to an extent which is dependent upon the availability of the reducing agents at the cathode base surface. Thus, in a melted alloy having a relatively large grain structure, the path of an additive reducing element to the cathode surface is substantially direct whether it be through the grain or by means of the grain boundaries. Conversely, in a sintered mixture having a comparatively small grain structure, the path of an additive reducing element to the cathode surface is very circuitous. Thus, the rate at which an additive reducing agent becomes available at the cathode surface and the rate at which the oxide layer thereon is reduced are materially dissimilar for melted alloys and sintered metal particle mixtures.
When materials are alloyed or melted, it has been found that the reducing agent additives are homogeneously dispersed throughout the entire alloy and therefore present throughout the entire alloy surface. These additives are almost immediately susceptible to the formation of oxide layers or films which are deleterious to the effectiveness of the reducing additive elements and to the operation of the base as a cathode for an electron tube. Thus, the entire surface of an alloyed material is almost immediately covered by an undesired and unwanted film or oxide layer.
On the other hand, it has been found that a mixture of sintered metal particles, including reducing agent additives, has only that percentage of the cathode surface sites occupied by the additives which is comparable to the volumetric percentage of the additives in the mixture. For instance, a mixture of metal particles having a 2% additive would, after sintering, provide a cathode base in which about 2% of the surface sites would be occupied by the additive element.
Also, it is known that a cathode base surface having a small percentage of the surface sites occupied by an additive reducing agent and an alkaline earth oxide layer thereon will, after a short period of operational use, have an emission area equal to the total cathode base surface. In this manner the sintered metal mixture provides a cathode base surface which has, after a short period of operational use, a total surface emission area similar to that of a melted cathode base but does not have a total surface area of additive reducing agent as found in a melted material. As a result, the sintered metal mixture has only a relatively small surface area whereon the previously mentioned undesired film is formed as compared with the undesired film on the total surface area of a melted cathode. Thus, a cathode base formed from a mixture of sintered metal particles provides a large emission surface but does not provide a large undesired film covered surface as found in a melted cathode base.
Further, it has been found that the additive reducing agents in a mixture of sintered metal particles are in elemental form and usually have an oxide layer thereon which is very stable. Thus, before the reducing element additive can diffuse to the surface of the cathode base, it must first diffuse through the oxide layer thereon. Having diffused through the oxide layer, the reducing element forms an enriched additive area surrounding the element and this enriched area continuously grows throughout the reduction process. Also, the diffusion rate of the element through the enriched area not only varies from the diffusion rate thereof through the matrix material but varies with the particular additive element. Then, continued diffusion of the element necessitates a path through the oxide layer, through the additive enriched area, and finally through the matrix material to the cathode base surface. Again, the rate of appearance of the reducing agent at the surface of the cathode base is controlled and the initial excess amount of barium is no longer a problem.
A further factor relating to cathode bases fabricated from a mixture of sintered metal particles as compared with alloys is evidenced in the surface of the cathode base after operational use. In this regard, a comparison was made of a sintered metal particle cathode base and an alloyed cathode base having substantially the same Original ingredients, disposed in the same type discharge device having the same type oxide layer thereon, and operated under substantially identical conditions for a period of about 8000 hours.
Upon visual comparison, it was found that the surface of the cathode base formed from an alloy whereon the oxide layer was adhered was a relatively smooth surface without roughness and indentations or protuberances. Contrarily, the surface of the cathode base formed from a mixture of sintered particles whereon the oxide layer was adhered exhibited a relatively uneven and rough surface having innumerable indentations and protuberances.
This observable difference in surface condition is believed to be due to the fact that the sintered mixture includes discrete and individual metal particles with each particle having a rate of grain growth which is dependent upon the metal thereof. Thus, the surface condition of the sintered metal mixture is dependent upon the individual rate of grain growth of the individual particles of metal in the mixture as reflected at individual site locations on the surface.
This above-mentioned desirable condition may be contrasted with a cathode base fabricated from an alloy of substantially uniform composition wherein the additive ingredients or metals are substantially homogeneously dispersed throughout the entire composition including the surface. Therein, the variation in grain growth is not reflected at individual surface locations but rather throughout the entire composition resulting in a relatively smooth surface even though the grain growth is of a much greater degree in alloys as compared to sintered particles.
As a result, the uneven and rough surface of the sintered metal particle cathode base locks or keys the oxide layer thereto. Thus, the problems of loose and peeling oxide layers frequently encountered with the prior art alloyed cathode bases are no longer prevalent and enhanced adherence between the oxide layer and the cathode base surface is achieved.
As to the utilization of a desired composition of materials to provide -a cathode for an electron discharge device, a preferred process for fabricating the cathode base material is disclosed in the application of Kerstetter et al. entitled Powdered Metal Films, Ser. No. 306,586, filed Sept. 4, 1963, patented June 6, 1967, bearing No. 3,323,879 and assigned to the assignee of the present application. Therein, a viscous suspension of sinterable metal particles, an organic binder, a binder plasticizer, and volatile solvents is cast onto a support and dried to remove the solvents and provide a pliable self-supporting film. The film is removed from the support and heated in an amount sufficient to volatilize the binder and plasticizer therefrom and to sinter the metal particles to provide a pliable self-supporting metal strip.
Thereafter, the metal strip is compacted, if so desired, cut to a desired size, and formed to a desired configuration to provide a cathode base. A layer of potentially electron emissive materials is then adhered to the outer surface of the cathode base, by any one of a number of well-known techniques, to provide a cathode suitable for use in an electron discharge device.
It should perhaps be noted that the teaching necessary to the preparation of the above-mentioned viscous suspension is fully disclosed by Lamber and McKiernan in US. Patent No. 3,171,817 entitled, Suspension for Casting a Metal Containing Film, which issued Mar. 2, 1965, and is assigned to the assignee of the present application. Therein, numerous examples of suitable binders, plasticizers, and solvents as well as the necessary characteristics of each are set forth in a form suitable for use in preparing the above-mentioned suspensions.
As a specific example, one particular cathode base material was provided by casting the suspension, disclosed in US. Patent No. 3,171,817, onto a support, drying the cast suspension at room temperature and pressure to provide a pliable self-supporting film, and heating the film to a temperature of about 1100 C. at atmospheric pressure to volatilize the organic materials therefrom and sinter the metal particles to provide a pliable, self-supporting metal strip. This strip was then compacted, cut to size, and formed to a cathode configuration. A layer of potentially electron emissive materials was then adhered to the outer surface of the cathode configuration to provide a cathode for an electron discharge device.
Regarding materials adapted to cathode bases for electron tubes, it is well known and has long been a common ractice to use nikel material as a matrix for cathode bases. It is also well known that some control over such characteristics as initial electron emission, sustained electron emission, interfacial resistance, heater-cathode leak age, pulse emission, and numerous other characteristics may be had by introducing reducing agents into the nickel matrix. It has now been found that cathode bases having controllable and predictable characteristics may be formed from a mixture of nickel particles and additive reducing agent particles from the group and in an amount by weight of about 20% chromium, 3% hafnium, 0.5% magnesium, 3% tantalum, 6% tungsten, and 3% titanium.
Regarding chromium, it is believed that the rate of diffusion thereof to a cathode surface is relatively slow in comparison with other activating additive agents. This slow rate of diffusion acts to sustain the emission of a cathode for an extended period of use and especially when added in metal particle form. Although previous additions of chromium to a melted material have been made and such material did exhibit a sustained emission level, the prior amount of chromium additive was limited to about 2% because of the formation of a darkened film which completely covered the cathode base surface. This darkened film caused a high rate of heat radiation from the cathode base and the cathode operated at a much lower temperature than a similar cathode having a bright surface and the same energy input.
However, when chromium is added to a mixture in particle form and, as previously mentioned, the percentage of the surface sites of the cathode base occupied thereby is equal only to the volumetric percentage of the chromium additive, it has been found that an amount not greater than about 20% by weight may be included in the mixture without deleterious effects. When chromium particles in an amount greater than about 20% by weight is included in a mixture, it has been found that an excessive amount of gas is evolved during normal processing of the cathode in an electron tube and the increased surface sites occupied thereby again presents the problem of a darkened cathode surface and excessive heat radiation.
Also, the other additive agents above are limited because of the inherent characteristics and diffusion rates thereof. For instance, magnesium has a rapid rate of diffusion and in an amount greater than 0.5% peroxides a cathode base having high initial emission but very poor sustained emission. Tungsten, it is believed, adds to the emission of the base and also combines with carbon and reduces the effects of evolved gases. Testing indicates that an amount greater than about 6% by weight of tungsten in a nickel matrix results in an increased tendency for the oxide layer of a cathode to peel or lose its adherence to the cathode base when exposed to water vapor which evolves from the glass envelope. Also, when amounts greater than those stated above for tantalum, hafnium,
cially adapted for use in a nickel matrix, the other addit-ive reducing agents are more suitable for use in conjunction with the primary additives to provide more complex mixtures from which cathode bases having entirely distinct and different characteristics are obtained. For example, the following reducing agents in amounts by weight are particularly adapted for inclusion with the above-listed primary activators in a nickel matrix to provide a sintered metal mixture: 0.1% aluminum, 1.0% cobalt, 0.5% silicon, 0.5% manganese, 1.0% vanadium, 1.0% germanium, and 1.0% zirconium.
Although it may be possible to include amounts greater than the above, it has been found that these amounts are at present applicable to present-day electron tubes. For instance, aluminum not greater than about 0.1% increases the emissive activity of the cathode base but amounts greater than about 0.1% have been found to cause a serious deficiency in adherence between the cathode base and the oxide layer. Cobalt, up to about 1%, imparts mechanical strength to the base but appears to provide little in the way of activity. Silicon, which indicates a relatively high and undesirable interfacial resistance when included in a nickel matrix as the primary activator, may be included in conjunction with other activating agents in an amount up to about 0.5% to promote emissive activity without the deleterious effects mentioned above. In a similar manner the other metals in the amounts shown may be combined to provide desired characteristics unobtainable when a single additive reducing agent is available.
As a further example the following specific mixtures of sintered metal particles have provided excellent re- :sults as cathodes in electron tubes:
sistently improved results when the cathode base is a mixture of sintered metal particles as compared with an alloy. Not only is the control of the additive element diffusion rate greatly enhanced but the mechanical strength and electrical activity of the cathode base is predictable to an extent which has been previously unobtainable.
Further, the reduced impurity level obtainable in the above-listed mixtures is believed to be previously unobtainable by other known techniques. Also, electrical characteristics of a cathode base for an electron tube such as interfacial resistance, adherence of the oxide layer to the cathode base, pulse emission, heater-cathode leakage, insulation resistance, and numerous other characteristics have shown improvements which are not only unique but totally unexpected.
While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined by the appended claims.
What is claimed is:
1. A cathode for an electron discharge device comprising a metal base and a layer of potentially electron emissive material thereon, said base being formed solely from a pliable self-supporting metal strip formed solely from a pliable self-supporting film and comprising a mixture of sintered nickel and other metal particles, wherein said base comprises a mixture of at least one metal in an amount by weight and from the group consisting of up to about 20% chromium, 3% hafnium, 0.5% magnesium, 3% tantalum, 6.0% tungsten, and 3% titanium, the balance of said mixture being substantially sintered particles of nickel.
2. The cathode of claim 1 wherein said base includes at least one metal in an amount by weight and from the group consisting of up to about 0.1% aluminum, 1.0% cobalt, 0.5% silicon, 0.5% manganese, 1.0% vanadium, 1.0% germanium, and 1.0% zirconium.
M anganese Zirconium.
As a specific example of the results obtainable, a sintered mixture of metal particles, listed above as No. 7, was formed into cathode bases and compared with a similar alloyed material which did not contain chromium. Two groups of type 6BZ7 tubes were manufactured and operated at above normal dissipation for a 500-hour period wherefrom the following average value of electrical characteristics was obtained:
Sinterod M ix- Alloy Without ture With Chromium Chromium Pulse Emission, ma 800 520 Grid to Other Element R stance,
megohms 2. 600 12. 9 Positive Heater-Cathode Leakage,
amp 3.1 41. 7 Negative Heater-Cathode Leakage,
ramp 2. 4 48. 8
Additional tests on other tube types have shown con- 3. The cathode of claim 1 wherein said base comprises a mixture of sintered chromium particles in an amount of up to about 20% by weight.
4. The cathode of claim 1 wherein said base comprises a mixture of sintered particles of chromium in an amount of up to about 1.0% by Weight.
References Cited UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,351,486 November 7, 1967 William E. Buescher et a1.
It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.
Column 6, line 64, for "peroxides" read provides column 8, line 61, for "weight." read weight, magnesium in an amount of up to about 0.1% by weight, zirconium in an amount of up to about 0.1% by weight, and the balance substantially nickel.
Signed and sealed this 3rd day of December 1968.
Edward M. Fletcher, 11'. EDWARD J. BRENNER Attesting Officer Commissioner of Patents