Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS3631291 A
Publication typeGrant
Publication dateDec 28, 1971
Filing dateApr 30, 1969
Priority dateApr 30, 1969
Publication numberUS 3631291 A, US 3631291A, US-A-3631291, US3631291 A, US3631291A
InventorsFavreau Louis J
Original AssigneeGen Electric
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Field emission cathode with metallic boride coating
US 3631291 A
Images(1)
Previous page
Next page
Description  (OCR text may contain errors)

United States Patent Louis J. Favreau Elnora, N.Y.

Apr. 30, 1969 Dec. 28, 1971 General Electric Company Inventor Appl. No. Filed Patented Assignce FIELD EMISSION CATHODE WITH METALLIC BORIDE COATING 7 Claims, 1 Drawing 11;.

Int. Cl H01] l/l4, H01 19/06, Holk 1/04 Field of Search 313/336, 345, 346

[56] References Cited UNITED STATES PATENTS 3,312,856 4/1967 Lalferty et al. 313/346 3,356,887 12/1967 l-leilet a1. 313/336 X 3,363,961 l/l968 Coleman et al 3l3/336X 3,461,338 8/1969 Vogel 3l3/336X Primary Examiner-David Schonberg Assistant Examiner-Toby H. Kusmer Attorneys-John F. Ahem, Paul A. Frank, Jerome C.

Squillaro, Frank L. Neuhauser, Oscar B. Waddell an Joseph B. Forman ABSTRACT: A field emission cathode comprises a hairpin rhenium filament coated with lanthanum boride and with a fine tungsten tip attached to the rhenium filament serves as an efficient source of electrons. The presence of free lanthanum atoms at the tungsten surface lowers the work function and enhances the field emission from the tungsten tip.

FIELD EMISSION CATIIODE WITH METALLIC BORIDE COATING This invention relates generally to field emission cathodes and more particularly to cathodes having a metallic boride coating.

The emission of electrons from the surface of a conductor into a vacuum or into an insulator under the influence of a strong electric field have found many useful applications. One such application includes field emission microscopy in which some of the most powerful microscopes known have been constructed. Such microscopes generally utilize a "hairpin" cathode with a fine tungsten point at the apex of the hairpin as a source of electrons. Since the degree of magnification obtained by field emission microscopes is a function of the emission levels from the tungsten tip, it is desirable to utilize a hairpin filament with high emission levels so that high magnification can be obtained. Conditions conducive to high emission are a high operating temperature, an ultrahigh vacuum, and a high electric field. With these conditions, a relatively high emission can be obtained; however, the useful life of a hairpin filament operated in this manner is considerably reduced. Additionally, as a result of the high temperatures, the field emission microscope is limited in application to an investigation of those metals having a melting point higher than the operating temperature of the filament. Accordingly, it would be desirable to have a filament which provides the same or higher emission current with a lower work function than existing filaments.

As disclosed in U.S. Pat. No. 2,639,399 to J. M. Lafferty, properties of the rare earth metal borides and in particular lanthanum boride, LaB exhibit particularly desirable characteristics for electron emitters.

It is therefore an object of this invention to provide a field emission cathode having a lower work function with enhanced field emission.

It is another object of this invention to provide a field emitter that is highly efficient and has a longer useful life than prior art emitters even when operated at emission levels several orders of magnitude higher than prior art emitters.

Briefly, the field emitter of the present invention includes a hairpin filament of rhenium metal with a fine tungsten point at its apex and a coating of lanthanum boride applied to the sides of the rhenium filament. The presence of free lanthanum atoms on the tungsten surface lowers the work function and enhances the field emission from the tungsten tip.

The invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, the FIGURE illustrates a cross-sectional view of a hairpin cathode constructed in accord with the present invention.

In the FIGURE, a field emission cathode comprises a pair of side support members 11 and 12 having a filament 13, preferably of rhenium wire, connected therebetween in the form of an inverted V and secured, as by welding, to support members 11 and 12, respectively, at the extremities thereof. Secured to the apex of the hairpin-shaped filament is a fine tungsten point 14 of conventional design, fabricated by an electrolytic etching technique well known to those skilled in the art. Unlike conventional hairpin field emitters of pure tungsten, however, lanthanum boride is applied to each side or leg of the filament 13. The lanthanum boride coating is illustrated in the FIGURE by the numeral 15. The sides of the rhenium filament 13 to which the lanthanum boride coating 15 is applied, is typically 10 mil diameter.

The coating 15 may be applied to the sides of the rhenium filament by painting, spraying, or dipping. For example, a paste slurry of lanthanum boride powder and ethyl alcohol may be applied by painting or dipping. Reference may be made to the aforementionedU.S. Pat. No. 2,639,399 for a description of other methods which may also be used to provide the desired coating 15 on the rhenium filament 13.

As described in U.S. Pat. No. 3,3l2,856 to .I. M. Lafferty and W. R. Grams, when certain metal borides are in contact with commonly used refractory metals such as tungsten, molybdenum, platinum, niobium, or tantalum at high temperatures, boron difi'uses into these metal lattices forming interstitial boron alloys which are very brittle and which reduce the supply of emissive material in the metal. Rhenium metal, however, possesses physical and mechanical properties which are impervious to boron atoms and make its use desirable in practicing the present invention; however, other metals which are affected by boron diffusion at high temperatures could likewise be used.

The field emission cathode constructed in accordance with the instant invention has several advantages over prior art devices. In particular, the lanthanum boride coating serves as a dispenser of free lanthanum metal atoms which at a temperature of approximately l,400 C. migrate to the tungsten tip thereby lowering the work function and enhancing the field emission from the tungsten tip. Laboratory tests have demonstrated that emission levels of approximately l,000 times higher are achieved with the instant invention over hairpin filaments with a tungsten tip but without the lanthanum boride coating. Additionally, under vacuum conditions such as l0 torr, which would be completely unsuitable for operation of an uncoated hairpin filament, the field emission cathode of the instant invention provides emission currents which were very stable and were attained with anode-to-cathode potentials of only 3.5 kilovolts. Additionally, whereas conventional hairpin filaments require a tungsten tip of a half-micron radium of curvature to provide emission currents of 10 amperes, the hairpin filament of the instant invention can achieve the same or higher levels with a tungsten tip of 2-3 microns radius of curvature. Since it is very difficult and costly to fabricate tungsten tips of a half-micron radius of curvature, the cost'of highly emissive field emitters is greatly reduced.

Another advantage of the instant invention is that field emission from the activated tungsten tip is enhanced by approximately 10 times by slightly heating the filament; that is, given a particular emission level, it can be increased tenfold by slightly heating the filament. The temperature is so low that no color temperature is visible; however, increased field emission is observed. The reason for the field emission enhancement under this condition is not clearly understood since the temperature of the cathode is not high enough for thermionic emission from the lanthanum hexaboride coating. If desired, however, the field emission cathode of the instant invention can serve as a thermionic by simply heating the filament to a temperature of approximately 1,000 C. so as'to enable the lanthanum hexaboride to emit thermionic electrons. In this mode of operation, only a few hundred volts anode-to-cathode voltage would be required. This mode of operation would be most useful for out-gasing of electrodes by electron bombardment.

From the foregoing, it may be appreciated that there is disclosed a field emitter having a hairpin configuration with a lanthanum boride coating in which emission currents a thousand times greater than pure tungsten tips are achieved. Additionally, good emission is achieved with a tungsten tip of 2 to 3 microns radius of curvature whereas prior art tungsten tips with a half-micron radius of curvature could not produce the same results. Stable emission is also achieved in vacuums of 10 torr with the field emitter of the present invention whereas prior art emitters require a vacuum in the order of 10' or l0" torr for stable emission.

While the invention has been described in connection with a specific hairpin structure, it is to be understood that the um filament for supporting the same.

4. A field emission cathode as recited in claim 1 wherein said lanthanum boride coating is applied to each side of said filament for providing a source of free lanthanum atoms which migrate to said tungsten tip at elevated temperatures thereby lowering the work function of said tungsten tip.

5. A field emission cathode as recited in claim 4 further comprising support means connected to the ends of said filament for supporting the same.

6. A field emission cathode as recited in claim 2 wherein said lanthanum boride coating is applied to the sides of said filament for providing a source of free lanthanum atoms which migrate to said tungsten tip at elevated temperatures thereby lowering the work function of said tungsten tip.

7. A field emission cathode as recited in claim 1 further comprising support means connected to the ends of said rhenium filament for supporting the same.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US3312856 *Mar 26, 1963Apr 4, 1967Gen ElectricRhenium supported metallic boride cathode emitters
US3356887 *Jul 30, 1965Dec 5, 1967Heil Frederick C WFe cathode redesign
US3363961 *Oct 22, 1965Jan 16, 1968Rca CorpCathode arrangement of an electron microscope for reducing the occurrence of virtualcathodes
US3461338 *Jan 16, 1967Aug 12, 1969IbmNon-inductive filament configuration
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3814975 *Jul 1, 1971Jun 4, 1974Gen ElectricElectron emission system
US3869636 *May 31, 1973Mar 4, 1975United Aircraft CorpFilament for electron guns
US4019081 *Oct 6, 1975Apr 19, 1977Bbc Brown Boveri & Company LimitedReaction cathode
US4054946 *Sep 28, 1976Oct 18, 1977Bell Telephone Laboratories, IncorporatedElectron source of a single crystal of lanthanum hexaboride emitting surface of (110) crystal plane
US4055780 *Apr 12, 1976Oct 25, 1977National Institute For Researches In Inorganic MaterialsThermionic emission cathode having a tip of a single crystal of lanthanum hexaboride
US4126807 *Oct 24, 1975Nov 21, 1978Owens-Illinois, Inc.Gas discharge display device containing source of lanthanum series material in dielectric layer of envelope structure
US4258283 *Aug 31, 1978Mar 24, 1981Balzers Aktiengesellschaft Fur Hochvakuumtechnik Und Dunne SchichtenCathode for electron emission
US4288717 *Nov 6, 1979Sep 8, 1981Denki Kagaku Kogyo Kabushiki KaishaThermionic cathode apparatus
US4325000 *Apr 20, 1980Apr 13, 1982Burroughs CorporationLow work function cathode
US4473771 *Jun 18, 1981Sep 25, 1984Universite LavalThermionic emitter for electron microscopy
US4731560 *Feb 7, 1974Mar 15, 1988Owens-Illinois Television Products, Inc.Multiple gaseous discharge display/memory panel having improved operating life
US4760306 *Jun 10, 1983Jul 26, 1988The United States Of America As Represented By The United States Department Of EnergyElectron emitting filaments for electron discharge devices
US4794308 *May 29, 1987Dec 27, 1988Owens-Illinois Television Products Inc.Multiple gaseous discharge display/memory panel having improved operating life
US4916356 *Sep 26, 1988Apr 10, 1990The United States Of America As Represented By The Secretary Of The Air ForceHigh emissivity cold cathode ultrastructure
US5019005 *Nov 15, 1989May 28, 1991Samsung Electron Devices Co., Ltd.Method for the fabrication of a direct-heated cathode structure and an apparatus therefor
US5327050 *Apr 27, 1992Jul 5, 1994Canon Kabushiki KaishaElectron emitting device and process for producing the same
US7516528 *Nov 4, 2004Apr 14, 2009Ge Medical Systems Global Technology Company, LlcMethod for the manufacture of an X-ray tube cathode filament
US20120098409 *Oct 21, 2010Apr 26, 2012Hermes Microvision, Inc.Filament for Electron Source
USRE39633May 12, 2000May 15, 2007Canon Kabushiki KaishaDisplay device with electron-emitting device with electron-emitting region insulated from electrodes
USRE40062Jun 2, 2000Feb 12, 2008Canon Kabushiki KaishaDisplay device with electron-emitting device with electron-emitting region insulated from electrodes
USRE40566Aug 26, 1999Nov 11, 2008Canon Kabushiki KaishaFlat panel display including electron emitting device
USRE42705 *Aug 27, 2009Sep 20, 2011Ge Medical Systems Global Technology Co., LlcMethod for the manufacture of an X-ray tube cathode filament
WO1997034332A1 *Mar 11, 1997Sep 18, 1997Forschungszentrum Juelich GmbhInterconnector made from a metallic high-temperature material and with a lanthanum-containing surface
Classifications
U.S. Classification313/345, 313/346.00R, 313/336
International ClassificationH01J1/304, H01J1/30
Cooperative ClassificationH01J1/304
European ClassificationH01J1/304