US2899592A

US2899592A - coppola

Info

Publication number: US2899592A
Authority: US
Priority date: 1953-11-18
Publication date: 1959-08-11
Anticipated expiration: 1976-08-11
Also published as: DE1043519B; FR1113555A; CH345392A; NL97571C; GB777142A; BE533455A

Description

Aug. 11, 1959 P. P. COPPOLA THERMIONIC DISPENSER CATHODE Filed Nov. 18; 1953 AGE/VI' Patented Aug. 11, 1959 THERMIONIC DISPENSER CATHODE Patrick P. Coppola, Dobbs Ferry, N.Y., assignor, by

mesne ass'gnments, to North American Philips Company, Inc., New York, N.Y., a corporation of Dela- Ware Application November 18, '1953, Serial No. `392966 2 Claims. (Cl. 313-346) My invention relates to a thermionic dispenser cathode.

In a thermionc dispenser cathode, a supply of alkalne earth material is disposed within a body of refractory metal, eg., tungsten, molybdenum, tantalum, hafnium, or niobium, having a porous wall portion through which alkalne earth metal producedby a reaction between the alkalne earth material and the refractory metal can pass and form an emissive layer on a surface of the body. The reaction between the refractory metal and the alkalne earth compound usually proceeds quite rapidly, and in many cases an excessive amount of alkalne earth metal is supplied to the emissive surface and is evaporated which is a disadvantage of the cathode for certain applications. Efforts have been made to reduce the rate at which the free alkalne earth metal is supplied to the emissive surface by decreasing the porosity of the porous wall. However, this not only makes the cathode more diflicult to fabricate, eg., higher sinterng temperatures in fabricating the porous wall must be employed, but makes the porosity of the wall a critical factor in the performance of the cathode. Moreover, for certain types of dispenser cathodes, the porosity cannot be decreased sufiiciently so that the rate of evaporation of alkalne earth metal still remains excessive.

A'princpal object of my invention is to provide a thermionic dispenser cathode having a reduced rate of evaporation of alkalne earth metal.

A still further object of my invention is to provide a cathode in which the porosity of the porous wall is not the controlling factor in determining the rate at which free alkalne earth metal is furnished to the emissive surface of the cathode.

A further object'of my invention is to provide a thermionic dispenser cathode in which the initial rate of evaporation of alkalne earth metal duringfabrication and activation is reduced. t

Another object of my invention is to facilitate the fabrication of a thermionic dispenser cathode by employing a refractory metal alloy which can be sintered into a body at lower temperatures.

A further object of my invention is to provide a dispenser cathode employing a more readily machinable refractory metal whereby the cathode can be machined to close tolerances. r

A further object of my invention is to employ a refractory metal alloy in which one of the constituents of the alloy will serve to bind some undesirable gaseous products which may be produced during the operation of the cathode.

These and further objects of my invention will appear as the specificaton progresses.

I have found that the rate of evaporation of alkalne earth metal in a dispenser cathode can be reduced by substituting for the refractory metal, such as tungsten, in the porous wall of the cathode structure an alloy of at least two refractory metals one of which is active in reducing the alkalne earth material disposed in a cavity within the body and the other of which is more passive during the reaction.

By the term active refractory metal" I mean a refractory metal which will react with certain alkalne earth compounds to furnish a supply of free alkalne earth metal in excess of that required to form a satisfactory emitter.

By the term passive refractory metal I mean a re fractory metal which does not react or only reacts with those alkalne earth compounds to such an extent as to form an insufficient amount of free alkalne earth metal.

The refractory metals may be either passive or active depending upon which alkalne earth compounds are employed.

The passive refractory metal serves to limit the amount of active refractory metal'available for reaction with the alkalne earth material in the cavity and also serves to limit the rate at which the active refractory metal is brought into contact with the alkalne earth material because the active refractory metal must diffuse through the passive refractory metal before Contacting the alkalne earth material. Consequently, the alloy must con tain the active and passive refractory metals in amounts suflicient to form the emissive surface while preventing excessive evaporation of alkalne earth metal.

The refractory metals, molybdenum, tungsten, tantalum, niobium, zirconium and hafnium, are more or less active in reducng certain alkalne earth compounds or compositions to' free alkalne earth metal. More particularly, I have found that molybdenum is the least active of the refractory metals in its ability to reduce alkalne earth compounds or compositions to the free alkalne earth metal. By alloying molybdenum with tungsten, for example, I have found that the rate of reaction producing free alkalne earth metal from alkalne earth compounds is materially reduced so that the porosity of the porous wall portion is no longer a critical factor in controlling the rate at which the free alkalne earth metal is supplied to the electron-emissive surface.

In a preferred embodiment of my invention I mix the alkalne earth material with powdered refractory metal alloy and press the mixture into a body which is sintered to produce a coherent body. During the sintering, some reduction of the alkalne earth material occurs but this reduction, I have found, is materially less than if tungsten alone is used as a refractory metal. Moreover, because such a cathode is likely to be quite porous, it would be expected that the rate of evaporation of free alkalne earth metal during operation will be relatively high. I have found, however, that the alloys according to my invention substantially reduce the rate of evaporation of the alkaline earth metal by inhibiting the rate of reduction of the alkaline earth material.

I have also found that by alloying a more active refractory metal, such as tantalum, Zircom'um or hafnium, with one of the less active metals, such as tungsten or molybdenum, it is possible to use alkaline earth compounds which are diflicult or impossible to reduce with tungsten alone. For example, an alloy of tungsten and hafnium will reduce barium orthosilicate, which tungsten will not reduce at all, without excessive evaporation of barium which would be the case if hafnium alone were used.

The invention will now be described in connection with the accompanying drawing, in which:

Fig. 1 is a sectional View of one embodiment of a dispenser cathode according to the invention; and

Fig. 2 is a sectional view of another embodiment of a dispenser cathode according to the invention.

The dispenser cathode shown in Fig. l comprises a tube 1 composed of refractory metal such as molybdenum having an internal partition 2 for separating the tube into two chambers. The lower chamber houses a conventional heater 3 while the upper chamber houses a supply of alkaline earth compounds 4 such as a mixture of barium and strontum carbonates. The upper chamber is closed by a porous disc 5 sealed to the end of the tube fl by welding so that the pores in the wall constitute the only passageways connecting the cavity in which the alkaline earth compounds are disposed to the emissive surface 6.

The porous disc 5 is made of an alloy composed of 25% of tungsten and 75% of molybdenum made by pressing the powdered alloy into a disc and sintering the same at a temperature of about l600 to 1900 C Since the resulting disc is porous, the alkaline earth metal obtained by reaction between the alkaline earth compounds in the cavity passes through the pores of the disc and forms an emissive layer on the surface of the cathode.

In this alloy tungsten is the active refractory metal which reacts with the barium and strontum oxides formed by thermal decomposition of the carbonates in the cavity to supply free barium to the emissive surface of the cathode. Since the tungsten is alloyed with molybdenum, not only is the amount of tungsten available for the reaction diminshed but the rate at which it is made available for reaction is limited by its diffusion rate through the molybdenum.

If other alkaline earth compounds or mixtures or solid solutions thereof are substituted in the cavity for the barium and strontum carbonates, other refractory alloys may be used. The following table lists the most suitable refractory metal that should be used for various alkaline earth compositions in the cavity in order to obtain a suflicient but not excessive supply of barium for the ernissive surface.

Reractory Metal Alloy Akaline Earth Material iu Cavity Passive Ref. Metal-Active Ref. Metal BaO. Basic barium silicates (313210 Siz or 4B O-SOz).

{Basic barium aluminates (cxcess BaCoa.

Mo-W, up to about 90% of W BaO).

Normal and basic barium beryllates.

Mo-Ta, up to about Ta,...

W-Ta, up to about 10% MO-rNb, up to about 10% Nb Refractory Metal Alloy Passive Ref. Metal-Active Ref. Metal Alkaliue Earth Materialin cavity W-Nb, up to about 10% Nb Basic and normal barium beyl- {Basic barium aluminates.

Basic barium aluminates Basic and normal barium berylliates.

Mo-Hf, less than about 5% Hi..-

Mo-Zr, less than about 5% Hf Basic barium aluminates. Normal and basic barium hei-yll'atcs.

W-Zr, less than about 5% Hi..

In the cahtode shown in Fig. 2, the alkaline earth compounds are homogeneously distributed within the porous disc itself. In this Construction, the disc 5 in which the alkaline earth compounds are disposed is sealed in one end of a tube 1 of refractory metal such as molybdenum and separated from the heater 3 by a partition 2 of refractory metal.

The porous disc in which the alkaline earth compounds are disposed can be fabricated as follows. The disc may be made by mixing the refractory metal in powdered form with the alkaline earth material, eg., a 5 to 2 mole ratio of a prefired mixture of BaO and Al O This mixture is then pressed and sintered at a temperature of about 1650 to l750 C. for about 30 seconds. The resulting body is mechanically coherent and relatively free of entrapped gases.

Alternatively, a porous disc obtained by pressing and sintering a powdered refractory metal alloy at relatively high temperatures, e.g., l600-1900 C. but in any event no higher than a temperature slightly below the lowest melting point of either the alloy or any of its constituents. The porous disc thus obtained, which may be machined, if desircd, is then impregnated with a suitable alkaline earth material as described in U.S. application Serial No. 273,607, filed February 27, 1952, now Patent 2,700,000 by R. Levi et al. If the sitered body of refractory metal alloy is impregnated from a melt, the alkaline earth material to be used must have a meltng point below the temperature at which the porous disc was sintered in order to avoid further sintering thereof during impregnation.

In the cathodes in which the alkaline earth material is homogeneously distributed throughout the porous disc, the alkaline earth material must be selected so that t is readily reducible by the active refractory metal without deleteriously reacting therewith. Suitable alkaline earth materials and the criteria governing their selection for that type of cathode are disclosed in U.S. applications Serial Nos. 258,89l, now Patent No. 2,716,716 and 258,892, now Patent 2,700,112 filed November 29, 1951, by R. C. Hughes et al. All of the compositions and mixtures of alkaline earth compounds disclosed therein may be used in the cathode according to my invention and the foregoing advantages erumerated hereinabove will be realized.

However, the selection of the particular alkaline earth material will to some extent determine the particular refractory metal alloy that should be used and I have listed in the table below the best combinations of refractory metal alloys and alkaline earth materials. In the table the terms pressed powder" and "impreguated" refer, respectively, to cathodes made by mixing the alkaline earth material with the powdered metal alloy and forming a body therefrom, and to a cathode obtained by ntroducing the alkaline earth material into a porous disc by impregratio.

Refraetory Metal Alloy Type of cathode .alkaline Earth Material Passive Ref. Metal-Active Ref. Metal Basic barium alunnates. Mo-W, up to about 90% W-- Inpreinated and Pressed Basic barium silicates.

Basic barlum berylliates. Impregnated and Pressed Basic barium aluminate. Mo-Ta, up to about 10% Ta.- Powder. as c barum e y l es.

v Pressed Powder only Normal barium beryiliate.

Press d P wder nd Im- {Basic barium aluminate. Mo-N b, up to about 10% N b. pregnated. Basic barium beryllltes- Pressed Powder only Normal barium beryllate.

Mono-barium aluminate. Ini prenated and Pressed %asia garium lumfinattes. Mo-Hf, less than about Hf. OW i armm Pressed Powder only u Barnm orthoslcate.

W-Hf, less than about 5% Hi Mo-Zr, less than about 5% Zr W-Zr, less than about 5% Zr- Impregnated and Pressed Pow er Normal barium berylliate. Mono-barium alum'nate.

Normal barium berylliate. Mono-barium aluminates. Basic barium berylliates. {Barium ortho-silicate.

Normal barium berylliate. 'Mono-barium aluminate.

Barium ortho-silieate,

{Barium orthosilicate.

d Pressed Powder only Normal barium berylliate.

The following tables indicate the emisson and life of several cathodes of the pressed powder type pressed from a mxture of by weight of a 5:2 mole ratio of prefired BaO -Al O and 90% by weight of a molybdenumtungsten alloy. In Table I the alloy was composed of 90% of molybdenum and 10% of Tungsten; in Table II, the alloy was composed of 50% of molybdenum and 50% of tungsten; and in Table III the alloy was composed of 75% molybdenum and of tungsten.

In order to test emisson and life, the cathode was assembled in an evacuated envelope in which an anode was arranged. All emisson measurements were taken at a cathode Operating temperature of 950 C. brightness pulse-wise with 100 microsecond pulses at the rate of 20 per second at a 1000 volts on the anode. The cathode was held at 1050 C. brightness with 100 volts D.C. on the anode during life.

Table I Tube N o. Life--Hours Emission,

Amps/cm.

Life tost still continues.

Table II 1 Results to date.

r Tube N o. Life-Hours Emssio,

Amps/cm.

qum

Life test stil] continues.

Table lll 1 Results to date.

Tube No. Life-Hours 1 Results to date. Life test still continues.

Average Total Barium Evaporation, Microgramslcmfilhr.

Reh'actory Metal Composition pas"? (ncncncn 7 10% W--% MO By alloying a refractory metal with gettering propertes (which may also serve as either the passive or active refractory metal) with another refractory metal, alkaline earth compounds or rnxtures thereof which evolve gas during activation may be used to advantage. More particularly, those mxtures of alkaline earth compounds which have been found to react together to form alkaline earth oxides which are then reduced by the refractory metal as disclosed in U. S. application, Serial No. 258,891, filed November 29, 1951, now Patent No. 2,716,716 by R. C. Hughes et al., while very advantageous, nevertheless evolve gas which must be pumped off while the cathode is activated in the tube. If a mixture of barium azide and barium formate is used as the alkaline earth material, using an alloy of at least two refractory metals one of which has inherent gettering properties such as molybdenum and Zirconium not only achieves the rincipal advantage of the invention which is the reduction of excessive barium evaporaton but also reduces the time required to pump off evolved gases during activation of the cathode. In this case, the zirconium is probably servng a dual function, i.e., it is the active refractory metal and it also scavenges the gas produced by the azide and formate. Zirconium is preferred because of its strong aflinity for nitrogen and, secondly, because ZrN has a low dissociation pressure, namely, 1 l0 mm. Hg at 2046 C.

Several pressed powder cathodes employing an alloy of 99% Mo and 1% Zr Were made and tested for emisson and life. This data appears in Table V below. Those cathodes were prepared from a mixture containing 10% by weight of a 1:1 mole ratio of Ba(N +Ba(CHO and 90% by weight of the molybdenum-tungsten alloy and assembled in dodes. The measurements were performed as in the foregoing tables.

Emissiou, I Amps/cm.

portions and in intimate reactive relationship With said refractory metal alloy, said refractory metal alloy consisting of 75% by weight of molybdenum and 25% by weight of tungsten.

2. A thermionic cathode comprising a tubular structure consisting of .Iefractory metal, &portion of the wall of said -structure constituting the emissive surface thereof consisting of a sintered body composcd of 90% by weight of an alloy consisting of 75% by weight of molybdenum The time required OH the P P for those cathodes Was 10 and 25% by weight of tungsten, and 10% by weight of a fraction of that required when pure tungsten or pure molybdenum instead of the alloy was used.

This invention is not limited to cathodes having the shape shown but is applicable to dispenser cathodes of more complex Construction. The cathode, for example, could have a cylindrical shape, or it could be a concave or planar type of cathode.

While I have therefore described my nvention in connection with specific examples and applications, other modifications thereof will be apparent to those skilled in this art without departing from the spirit and scope of my invention as defined in the appended claims.

What I claim is:

1. A thermionic cathode comprisng a structure con sisting of refractory metal, a portion of the wall of said structure constituting the emissive surface thereof consistng of a homogeneously-porous sintered refractory metal alloy, and a supply of a fused mixture of 5 moles of barium oxide and 2 moles of aluminum oxide distributed within and only within the pores of said wall a fused miXture of 5 moles of barium oxide and 2 moles of aluminum oxide, the fused mixture of barium and aluminum oxide being contained within and only within said sintered body.

References Cted in the file of this patent UNITED STATES PATENTS 1,954,474 Espe et al. Apr. 10, 1934 2,085,605 Ramsay et al. June 29, 1937 2,121,589 Espe June 21, 1938 2,147,447 Kolligs Feb. 14, 1939 2,180,988 Lemmers et al. Nov. 21, 1939 2,389,060 Kurtz Nov. 13, 1945 2,473,550 Spencer June 21, 1949 2,543,728 Lemmcns et al Feb. 27, 1951 2,700,000 y Levi et al Jan. 18, 1955 2,700,118 Hughes et al. Jan. 18, 1955 2,716,716 Hughes et al. Aug. 30, 1955 2,741,717 Katz Apr. 10, 1956

Claims

1. A THERMIONIC CATHODE COMPRISING A STRUCTURE CONSISTING OF REFRACTORY METAL, A PORTION OF THE WALL OF SAID STRUCTURE CONSTITUTING THE EMISSIVE SURFACE THEREOF CONSISTING OF A HOMOGENOUSLY-POROUS SINTERED REFRACTRORY METAL ALLOY, AND A SUPPLY OF A FUSED MIXTURE OF 5 MOLES OF BARIUM OXIDE AND 2 MOLES OF ALUMINUM OXIDE DISTRIBUTED WITHIN AND ONLY WITHIN THE PORES OF SAID WALL PORTIONS AND IN INTIMATE REACTIVE RELATIONSHIP WITH SAID REFRACTORY METAL ALLOY, SAID REFRACTORY METAL ALLOY CONSISTING OF 75% BY WEIGHT OF MOLYBDENM AND 25% BY WEIGHT OF TUNGSTEN.