US2813220A - Indirectly heated cathode - Google Patents

Description

Nov. 12, 1957 P. P. COPPOLA 2,813,220

INDIRECTLY HEATED CATHODE Filed Dec. 6, 1954 INVENTOR. B4TRICKP COPPOLA AGEWII United States Paten INDIRECTLY HEATED CATHODE Patrick P. Coppola, Dobbs Ferry, N. Y., assignor to North American Philips Company, New York, N. Y., a corporation of Delaware Application December 6, 1954, Serial No. 473,386 8 Claims. (Cl. 313--346) My invention relates to an indirectly heated cathode and method of making the same.

In particular, my invention relates to an indirectly heated dispenser cathode in which an alkaline earth material is dispersed within the pores of a refractory metal matrix and that matrix incorporated in a refractory metal sleeve. In such cathodes a barrier is sometimes interposed between the emissive portion and the heater to prevent even the minutest passage of alkaline earth metal, or compound giving rise to emission in the heater zone.

A principal object of my invention is to provide a novel indirectly heated dispenser cathode in which back-emission between the cathode and heater is substantially eliminated.

A further object of my invention is to provide a simpler and less expensive method of manufacturing an indirectly heated dispenser cathode.

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

Briefly stated, the cathode according to my invention comprises a tube of refractory metal having a refractory metal partition dividing the tube into two separate chambers, one of which houses a heating element and the other an emissive body which comprises a porous sintered refractory metal matrix, e. g. molybdenum, tungsten, hafnium, tantalum, zirconium and alloys of those metals in the pores of which there is dispersed an emissive alkaline earth material.

The two chambers are tightly sealed from one. another by a special seal which comprises a very dense layer of sintered refractory metal of lower reactivity than the refractory metal matrix bonding the partition to the inner walls of the tube; and below this layer on the side facing the heating zone there is a layer of a sintered mixture of a refractory metal and a refractory metal oxide, bonded to the walls of the tube. This layer can combine chemically with any alkaline earth material which might seep through the sintered refractory metal layer, thus effectively preventing any leakage of alkaline earth material into the heater zone which might give rise to back-emission.

In a preferred embodiment of my invention, I employ a rolled molybdenum sleeve overlapped along its edges to serve as the tubular housing. Dividing this housing into the two chambers is a molybdenum disc of smaller diameter than the inside diameter of the sleeve. This disc is sealed to the inner walls of the sleeve by a dense layer of sintered molybdenum underneath which there is a layer of a sintered mixture of molybdenum and aluminum oxide.

Alternatively, a drawn tube of molybdenum or other suitable refractory metal could be employed instead of a rolled sleeve; and instead of the molybdenum disc a continuous layer of sintered molybdenum or other refractory metal can serve as the partition. This layer of a sintered refractory metal is then backed-up with a layer of a sintered mixture of refractory metal and a refractory metal oxide.

As a suitable refractory metal oxide, I prefer A1203, but other refractory metal oxides such as BeO, ZrOz, SiOz,

2,813,220 Patented Nov. 12, 1957 HfOz, and TiOz may be used. Similarly, while I prefer to use molybdenum as the refractory metal for the sleeve and bonding metal, other refractory metals such as tungsten, hafnium, tantalum, zirconium and alloys thereof may be used.

In a particular embodiment of my invention I have employed molybdenum as the refractory metal and prefer to use a mixture of about 90% by weight of molybdenum and 10% by weight of A1203 in the underlayer. It is essential however, that the A1203 be of a high degree of purity and in particular he alkali-free.

The emissive body may be prepared either by forming a mixture of a refractory metal and an emissive alkaline earth material and sintering that mixture into a coherent body or by forming a porous refractory metal body, preferably by sintering, and impregnating the pores of that body with an emissive alkaline earth material.

As an alkaline earth material I prefer to use one of those disclosed in co-pending application Ser. No. 258,892 filed November 29, 1951, by R. C. Hughes et al., now U. S. Patent 2,700,118. These materials comprise fused mixtures of an alkaline earth metal oxide such as barium oxide and one or more refractory metal oxides. In addition, I may add to the fused mixture of the alkaline earth oxide and the refractory metal oxide, one or more alkaline earth oxides such as calcium oxide, magnesium oxide and strontium oxide as disclosed in co-pending application Ser. No. 444,323 filed July 19, 1954, by R. Levi.

I may use also mixtures of refractory metal compounds which react together when heated to form alkaline earth oxides as disclosed in co-pending application Ser. No. 258,891 filed November 29, 1951, by R. C. Hughes et al., now U. S. Patent 2,716,716.

Alternatively, I may use as the emissive material an alkaline earth tungstate mixed with thorium as disclosed in U. S. application Ser. No. 331,874 filed January 19, 1953, by O. G. Koppius.

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

Fig. 1 is a sectional view showing a suitable die for forming the cathode according to the invention;

Fig. 2 is a sectional view showing another die for forming the cathode according to the invention;

Fig. 3 is a sectional view showing a cathode made in accordance with the invention;

Fig. 4 is a sectional view of another cathode made in accordance with the invention; and

Fig. 5 is a sectional view of still another cathode made in accordance with the invention.

Referring to Fig. l, a sleeve 1 prepared by rolling and overlapping the edges of a sheet of molybdenum and slightly bevelling at one end with India stone, is positioned in a cylindrical hardened steel die 2. The sleeve is partly filled with an emitter mixture 3 composed of about 90% by weight of a -25% tungsten-molybdenum alloy and about 10% by weight of an emissive alkaline earth material.

This alkaline earth material preferably consists of a prefired mixture of about 5 moles of BaO and 2 moles of A1203 to which one or more of the oxides CaO, 810, Mg() may be added.

In order to hold the emissive mass in place, a molybdenum disc 4 having a diameter smaller than the inside diameter of the sleeve is inserted into the sleeve.

A measured amount of molybdenum powder, first fired in a reducing atmosphere to remove all oxides, and having an average particle size of about 2 is introduced into the sleeve to cover the disc and fill the space between the disc and the sleeve, forming layer 5.

Over this layer 5 of powdered molybdenum a measured quantity of a mixture of about by weight of molybdenum powder having an avera e particle size of about and about 10% by weight of aluminum oxide is introduced into the sleeve to form a second layer 6.

In order to hold those layers in place, a second molybdenum disc 7 is inserted into the sleeve. After the second disc is inserted, the mass within the sleeve is compacted by applying pressure to pin 8 positioned in the die against pressing pin 9 outside the die. I

Pin 9 is provided with a special cup-like cavity 10 bevelled at an angle of about 38 in order to crimp over the ends of the molybdenum tube and hold the emitter mixture in place. In the event a heavy wall tube is used, the crimping may be omitted.

As an alternative to the external pressing pin in Fig. 1, a cup-shaped female die member 11, shown in Fig. 2, may be used.

After the pressing operation, the assembly is ejected from the die and the mass in the tube sintered to form a coherent structure. The assembly is placed in a furnace (vacuum or inert atmosphere) and heated rapidly to a temperature of about 1700-1900 C. for about seconds in order to sinter the compact and bond the molybdenum disc 4, and layers 5 and 6 to the inner walls of the tube 1. At this temperature the alkaline earth material is melted and dispersed throughout the refractory metal matrix and evolves entrapped gas.

During the previous sintering operation, the molybdenum disc 4, which is sealed by the molybdenum layer 5, serves to prevent the alkaline earth material in the emitter compact 3 from being drawn into the layers 5 and 6.

If the emitter body is preformed, i. e. by a separate operation, the molybdenum disc 4 may be omitted provided that there are continuous layers 5 and 6 underlying the emitter body and tightly sealed to the sleeve.

After cooling, a heating element 14 is positioned in the lower chamber 13 as shown in Fig. 3 and the cathode assembled in an evacuated envelope.

Alternatively, as shown in Fig. 4, instead of a continuous layer covering the entire disc 4, the refractory metal layer 5 and the layer of a mixture of refractory metal and refractory metal oxide need only surround the joint between the disc 4 and the inner wall of tube 1.

In still another embodiment of the invention, disc 4 may be eliminated and the partition formed solely by the layer 5 of sintered refractory metal backed up by layer 6 as shown in Fig. 5. In this particular embodiment it is desirable that the emissive body 3 be preformed, e. g., that it comprise a porous refractory metal body whose pores have been impregnated with an emissive alkaline earth material.

The novel seal according to my invention in each of the embodiments illustrated effectively impedes the flow of alkaline earth material and decomposition products into the heater zone and thereby effectively prevents backemission to the heater which is usually at a potential dif ferent thanthat of the cathode. The refractory metal bond actually serves the additional purpose of reducing the flow into the heater zone of alkaline earth metal and alkaline earth metal oxide decomposition products available after the cathode has been activated. The small amount that may traverse this barrier is absorbed by the aluminum oxide'which has a strong affinity for such products thus effectively sealing off the heater chamber to the flow of those products.

While I have thus described my invention with reference to specific examples and embodiments I do not wish to be limited thereto since it is apparent from the specification that other materials may be used without departing from the spirit and scope of the invention.

What I claim is:

1. A thermionic cathode comprising a tubular refractory metal housing having a partition therein forming two chambers, heating means disposed in one of said chambers, a porous sintered bdy with an emissive alkaline earth material dispersed in the pores thereof disposed in the other of said chambers, a dense layer of sintered refractory metal bonding the partition to the inner wall of the tube, and a second layer below said layer of sintered refractory metal and comprising a sintered mixture of a refractory metal and a refractory metal oxide.

2. A thermionic cathode comprising a tubular refractory metal housing having a partition therein forming two chambers, heating means disposed in one' of said chambers, a porous sintered tungsten body with an emissive alkaline earth material dispersed in the pores thereof disposed in the other of said chambers, a dense layer of sintered refractory metal bonding the partition to the inner wall of the tube, and a second layer below said layer of sintered refractory metal and comprising a sintered mixture of a refractory metal and a refractory metal oxide.

3. A thermionic cathode comprising a tubular refractory metal housing having a partition therein forming two chambers, heating means disposed in one of said chambers, a porous sintered body with an emissive alkaline earth material dispersed in the pores thereof disposed in the other of said chambers, a dense layer of sintered refractory metal bonding the partition to the inner wall of the tube, and a second layer below said layer of sintered refractory metal and comprising a sintered mixture of a refractory metal and aluminum oxide.

4. A thermionic cathode comprising a tubular refractory metal housing having a partition therein forming two chambers, heating means disposed in one of said chambers, a porous sintered body with an emissive alkaline earth material'dispersed in the pores thereof disposed in the other of said chambers, a dense sintered layer ofthe same refractory metal as that of said housing bonding the partition to the inner wall of the tube, and a second layer below said layer of sintered refractory metal and comprising a sintered mixture of the same refractory metal as that of said housing and a refractory metal oxide.

5. A thermionic cathode comprising a tubular molybdenum housing having a partition therein forming two chambers, heating means disposed in one of said chambers, a porous sintered body of an alloy of about 75% tungsten and 25% molybdenum with an emissive alkaline earth material dispersed in the pores thereof disposed in the other of said chambers, a dense layer of sintered molybdenum bonding the partition to the inner wall of the tube, and a second layer below said layer of sintered molybdenum and comprising a sintered mixture of molybdenum and aluminum oxide.

6. A thermionic cathode comprising a tubular molybdenum housing having a partition therein forming two chambers, heating means disposed in one of said chambers, a porous sintered refractory metal body with an emissive alkaline earth material consisting of a fused mixture of an alkaline earth oxide and aluminum oxide dispersed in the pores thereof disposed in the other of said chambers, a dense layer of sintered molybdenum bonding the partition to the inner wall of the tube, and a second layer below said layer of sintered molybdenum and comprising a sintered mixture of molybdenum and aluminum oxide.

7. A thermionic cathode comprising a tubular molybdenum housing having a partition therein forming two chambers, heating means disposed in one of said chambers, a porous sintered body with an emissive alkaline earth material dispersed in the pores thereof disposed in the other of said chambers, a dense layer of sintered molybdenum bonding the partition to the inner wall of the tube, and a second layer below said layer of sintered molybdenum and comprising a sintered mixture of about molybdenum and 10% aluminum oxide.

8. A thermionic cathode comprising a tubular molybdenum housing having a partition therein forming two chambers, heating means disposed in one of said chambers, a porous sintered body of an alloy of 75% tungsten and 25% molybdenum with an emissive alkaline earth material dispersed in the pores thereof disposed in the other of said chambers, said alkaline earth material consisting of a fused mixture of an alkaline earth oxide and aluminum oxide, a dense layer of sintered molybdenum bonding the partition to the inner wall of the tube, and a second layer below said layer of sintered molybdenum and comprising a sintered mixture of about 90% molybdenum and 10% of aluminum oxide.

References Cited in the file of this patent UNITED STATES PATENTS

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