US2864028A

US2864028A - Thermionic dispenser cathode

Info

Publication number: US2864028A
Application number: US528296A
Authority: US
Inventors: Patrick P Coppola
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1955-08-15
Filing date: 1955-08-15
Publication date: 1958-12-09
Anticipated expiration: 1975-12-09
Also published as: BE550302A; GB800219A; CH344139A; FR1173168A; DE1047321B

Description

Dec. 9, 1958 P. P. COPPOLA THERMIONIC DISPENSER CATHODE Filed Aug. 15, 1955 INVENTOR. H4 TRICK P C'OPPOLA AGE United States Patent THERMIONIC DISPENSER CATHODE Patrick P. Coppola Dobbs Ferry N. Y. assi nor to North American Philips Compahy, Inc, Nevi' York, N. Y., a corporation of Delaware Application August 15, 1955, Serial No. 528,296

3 Claims. 01. 31 3-346) My invention relates to a thermionic dispenser cathode. In particular my invention relates to an improvement n a thermionic dispenser cathode of the type disclosed in U. S. Patent No. 2,700,118 to R. C. Hughes et al. As described in that patent, the cathode comprises a pressed and sintered mixture of a refractory metal such as tungsten, molybdenum, tantalum, hafnium or alloys of those metals and an alkaline earth material which reacts with the refractory metal upon heating to furnish alkaline earth metal to an emissive surface of the cathode. As alkaline earth materials, fused mixtures of an alkalineearth oxide and an acid forming oxide are disclosed which react with the refractory metal to furnish alkaline earth metal, e. g. barium, to the emissive surface of the cathode.

Since the alkaline earth material is distributed throughout a porous body of refractory metal during operation of the cathode some alkaline earth metal or oxides, or both, are volatilized because of their low vapor pressure in vacuum. The evaporation of alkaline earth metal or oxides, or both, is of course, generally undesirable because those products may be deposited on other electrodes of an electron-discharge tube where they give rise to secondary emission.

Accordingly, it is a principal object of my invention to provide a dispenser cathode of the aforesaid type having a reduced rate of evaporation of alkaline earth metal and/0r oxides.

A further object of my invention is to provide a dispenser cathode having a high emissivity and a low rate of evaporation of alkaline earth metal oxides, or both.

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

In accordance with the invention, I have found, quite unexpectedly, that if portions of the emissive surface are covered with a non-porous refractory metal, the rate at which alkaline earth metal is supplied to the emissive surface is still sufiicient to maintain coverage of that surface with alkaline earth metal so that high emission can be maintained. However, since a portion of the pores in that surface are now covered, the rate of evaporation of the alkaline earth metal or oxides, or both, is materially reduced.

Thus, in accordance with my invention, I cover the emissive surface of the cathode with a non-porous member of refractory metal such as molybdenum, having a number of apertures suitably spaced to permit the alkaline earth metal to migrate over and substantially cover the surface of the cathode. Thus, I may use a plurality of interlaced spaced strips or wires of molybdenum forming a mesh-like structure. Alternatively, I may use a template having a number of apertures suitably spaced to permit alkaline earth metal to migrate onto the emissive surface while also reducing the number of pores in the emissive surface.

The spacing between apertures in the non-porous covering member preferably should not exceed the migration distance of the alkaline earth metal over the refractory metal surface in order that maximum surface cov- 2,864,028 Patented Dec. 9,1958

erage on the emissive surface may, at all times, be maintained. This migration distance will depend upon :a .number of factors including the particular alkaline earth metal being supplied to the surface, the operating temperature of the cathode, and the refractory metal. For example,

, at 1100 C. brightness, the migration length of Ba or BaO, or both, on molybdenum is about 0.3 mmpwhile at 900 C brightness the distance increases to 0.6 mm.

In the preferred embodiment of my invention a source of barium is provided in the pores of the cathode and molybdenum strips or wires are embedded in the emissive surface. These strips or wires are spaced sufficiently far apart, e. g. at least 0.5 mm., to assure substantially full surface coverage of barium during operation of the cathode.

The invention will be described in greater detail with reference to the accompanying drawing in which:

Fig. 1 is an elevational view in section of a cathode according to the invention;

Fig. 2 is a top view of the cathode showing the emissive surface; and

Fig. 3 is a top view of another embodiment of another cathode showing the emissive surface.

The cathode shown in Fig. 1 comprises an emissive wafer I mounted at one end of a molybdenum sleeve 2 which houses a heater 3. Embedded in the emissive surface of the wafer are strips 4 which form a grid-like structure shown in Fig. 2.

The emissive wafer comprises a pressed and sintered body of about 90% by weight of a 25% tungsten to molybdenum alloy and the balance, distributed throughout the pores of the body, a fused mixture of about 5 moles of barium oxide and 2 moles of aluminum oxide. Prior to the pressing operation, strips or wires of molybdenum having a diameter of about .3 mm. are placed on top of the powder about .5 mm. apart. The mixture is pressed and sintered at 1370 C. brightness for about 20 minutes and then rapidly raised to the melting point of the fused oxides (about 1650 C. to 1750 C.). The molybdenum strips are, therefore, integrally united with the sintered body and provide zones in which the surface pores are completely closed. Thus, those zones contribute no barium metal to the surface but barium metal which is supplied to the surface by the pores in the exposed areas migrates over the molybdenum strips and thereby contributes to the emission from the surface.

As shown in Fig. 3, I may use a molybdenum plate 5, having a number of suitably spaced apertures 6 covering a substantial portion of the emissive wafer 1. The molybdenum plate may be sintered to the emissive water 1 or may be welded to the molybdenum sleeve 2 at the tips of the cusps 7. The covered portions of the wafer are supplied with barium through the apertures in the plate. The apertures are suitably spaced so that substantially full surface coverage can be maintained at all times by the migration of barium over the surface.

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

Wherever the word wires appears in the appended claims, I wish it to be clearly understood that the term includes not only circular strands but also flat ribbons, bars and strips having lengths substantially greater than either the width or thickness of the element.

What I claim is:

1. A dispenser cathode comprising a porous sintered tungsten body, a supply of a fused mixture of barium oxide and aluminum oxide containing a molar excess of barium oxide within the pores of said body, and a plurality of spaced apart molybdenum strips embedded in a 3 surface of the body constituting the emissive surface of that body, said strips being integrally united with the sintered body to provide zones in which surface pores are completely closed, said strips having a width not substantially exceeding the migration distance of barium over molybdenum.

2. A dispenser cathode comprising a porous sintered body of an alloy of molybdenum and tungsten, a supply of a fused mixture of barium oxide and aluminum oxide containing a molar excess of barium oxide within the pores of said body, and a plurality of spaced apart molybdenum strips embedded in a surface of that body constituting the emissive surface of the body, said strips being integrally united with the sintered body to provide zones in which surface pores are completely closed, said strips having a width not substantially exceeding the migration distance of barium over molybdenum.

3. A dispenser cathode comprising a poroussintered body of an alloy of about 75% of molybdenum and about 25% tungsten, a supply of a fused mixture of about 5 moles of barium oxide and 2 moles of aluminum oxide Within the pores of said body, a plurality of spaced apart molybdenum strips embedded in a surface of the body constituting the emissive surface of the body, said strips being integrally united with the sintered body to provide zones in which surface pores are completely closed, said strips having a width not substantially exceeding the migration distance of barium over molybdenum.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A DISPENSER CATHODE COMPRISING A POROUS SINTERED TUNGSTEN BODY, A SUPPLY OF A FUSED MIXTURE OF BARIUM OXIDE AND ALUMINUM OXIDE CONTAINING A MOLAR EXCESS OF BARIUM OXIDE WITHIN THE PORES OF SAID BODY, AND A PLURALITY OF SPACED APART MOLYBEDENUM STRIPS EMBEDDED IN A SURFACE OF THE BODY CONSTITUTING THE EMISSIVE SURFACE OF THAT BODY, SAID STRIPS BEING INTEGRALLY UNITED WITH THE SINTERED BODY TO PROVIDE ZONES IN WHICH SURFACE PORES ARE COMPLETELY CLOSED, SAID STRIPS HAVING A WIDTH NOT SUBSTANTIALLY EXCEEDING THE MIGRATION DISTANCE OF BARIUM OVER MOLYBDENUM.