US3374386A

US3374386A - Field emission cathode having tungsten miller indices 100 plane coated with zirconium, hafnium or magnesium on oxygen binder

Info

Publication number: US3374386A
Application number: US408035A
Authority: US
Inventors: Francis M Charbonnier; Eugene E Martin; Lynwood W Swanson
Original assignee: Field Emission Corp
Current assignee: Field Emission Corp
Priority date: 1964-11-02
Filing date: 1964-11-02
Publication date: 1968-03-19
Anticipated expiration: 1985-03-19
Also published as: DE1290637B; NL6508856A; FR1453926A; GB1079342A

Description

March 19, 1968 F. M. CHARBONNIER ETAL 6 FIELD EMISSION CATHODE HAVING TUNGSTEN MILLER INDICES lOO PLANE COATED WITH ZIRCONIUM, HAFNIUM OR MAGNESIUM ON OXYGEN BINDER Filed NOV. 2, 1964 VERT. SIG.

FHA/V675 M. CHARBONN/Ef? EUGENE E. MART/N BUCK/40ml, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS 3,374,386 Patented Mar. 19, 1968 FIELD EMISSION (IATHODE HAVING TUNGSTEN MILLER INDICES 100 PLANE COATED WITH ZIRCONIUM, HAFNIUM R MAGNESEUM 0N OXYGEN BINDER Francis M. Charlionnier, Eugene E. Martin, and Lyn- Wood W. Swanson, McMinnville, 0reg., assignors to Field Emission Corporation, McMinnville, 0reg., a corporation of Oregon Filed Nov. 2, 1964, Ser. No. 408,035 9 Claims. (til. 31.3-34.6)

The subject matter of the present invention relates generally to field emission cathodes and electron beam devices employing such cathodes, and in particular to an electron gun structure using such cathode in a cathode ray tube to produce a narrow electron beam of high current density, low divergence, and small source area by confining the emission of electrons from the field emission cathode in such gun structure to a predetermined crystal plane on the point of such cathode.

The field emission cathode and electron gun of the present invention is especially useful when employed in a cathode ray tube for an oscilloscope but can also be used in other electron beam devices such as an electron microscope or a charge image storage tube. In addition to those mentioned above, the field emission cathode and electron gun structure of the present invention has several other advantages over previous cathodes and gun structures of this type including a longer lifetime and more stable emission from a smaller source area on the cathode. In addition, the electron gun of the present invention also operates more efficiently to produce a beam of greater current density. This also enables better image resolution and a higher writing rate when such gun is employed in a cathode ray tube, as well as the use of a lower blanking voltage for preventing the beam from reaching the fluorescent screen of such tube. Another advantage of the electron gun of the present invention is that the evacuated envelope containing such gun may be divided into two chambers of different pressure so that the chamber containing the cathode is at a lower pressure than the chamber containing the phosphor screen, storage target or microscope specimen. Of course, any tube employing a field emision cathode has an additional advantage over aconventional tube in that no heater voltage supply is required for the cathode except during activation of such cathode, and the tube requires no warm-up time before the cathode begins emitting electrons and such tube begins operation.

Briefly, one embodiment of the cathode of the present invention is a tungsten rod having a conical tip portion with a layer of oxygen and zirconium coated on the point of such tip portion. The tungsten cathode is constructed so that the plane of the cubic, body centered crystal lattice structure of tungsten having Miller indices 100 is oriented at the tip of such cathode. It has been discovered that if a coating of zirconium is applied to the cathode and such cathode is heated, the zirconium atoms will migrate to the 100 atomic plane at the point of the tungsten cathode. Since zirconium has a lower work funcion than tungsten, the field emission of electrons from the cathode will be confined substantially entirely to the 100 plane at the tip of such cathode. In addition, it has been found that if an intermediate layer of oxygen is provided on the cathode tip before the zirconium layer is applied thereto, such zirconium layer adheres to the plane for a much longer time than when such oxygen is not employed. While it is not entirely understood why this'occurs, it is believed that it is the combined effect of an electrical attraction between the atoms of the various elements because of their different electrical charges shown by their displacement from each other in the electromotive series of elements, the geometrical fit of the oxygen and zirconium atoms in the 100 lattice plane of tungsten and the relative physical size of the atoms themselves that holds the zirconium atoms in position after such zirconium atoms have once migrated to the 100 plane.

The field emission cathode described above is positioned within a tubular control electrode or grid in one embodiment of the electron gun structure of the present invention, and supported to direct the beam emitted by such cathode through an aperture in a collimating electrode which has a pasageway through such plate of greater length than diameter to reduce the divergence of such beam. In addition, a pair of blanking deflection plates may be provided on opposite sides of the electron beam path between the control electrode and the collimating electrode within one chamber of the envelope of the cathode ray tube. A conventional electrostatic focusing structure including first and second anodes and a focusing electrode can be positioned within the other chamber of the envelope between the collimating electrode and a fluorescent screen on the face plate of such tube. Two pairs of horizontal and vertical deflection plates are supported within the tube envelope between the electrostatic focusing structure and the phosphor screen for deflecting the electron beam before it strikes such screen. In addition, a small electromagnetic focusing coil is positioned about the neck of the tube envelope adjacent the field emission cathode in order to focus the beam emitted by such cathode to a narrow cross section before such beam is transmitted through the collimating electrode. Also a layer of getter material may be provided Within the envelope inside the chamber containing the field emission cathode in order to reduce the pressure within such chamber below that of the other envelope chamber containing the phosphor screen.

It is therefore one object of the present invention to provide an improved field emission cathode which emits an electron beam from a small predetermined portion of such cathode and which can be operated continuously for a long useable lifetime.

Another object of the present invention is to provide an improved field emission cathode having a pointed tip or edge portion which is coated with a low workfunction material only on a predetermined crystal plane of such portion to emit a narrow beam of electrons of less divergence.

A further object of the present invention is to provide an improved electron gun employing a field emission cathode efficiently to produce an electron beam of smaller cross section, less divergence, and a greater current density.

An additional object of the present invention is to provide an improved electron beam device having an electron gun which employs a collimating electrode between a field emission cathode and a phosphor screen or other target in such envelope to divide the envelope into two chambers containing such cathode and such target and enable such chambers to be operated at greatly different pressures and to reduce the divergence of the electron beam emitted from such cathode and transmitted through the aperture in such collimating electrode.

Still another object of the present invention is to provide an improved cathode ray tube employing a field emission cathode which has a long stable lifetime so that such tube requires no heater supply voltage and no warmup time, has a higher writing rate, better image resolution and can use blanking signals of lower voltage for preventing the electron beam from striking the fluorescent screen of such tube.

Other objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments thereof, and from the attached drawings of which:

FIG. 1 is a schematic view of one embodiment of a cathode ray tube having an electron gun made in accordance with the present invention;

FIG. 2 is an enlarged section view of a portion of an electron gun similar to that of FIG. 1 in which another embodiment of collimating electrode is employed;

FIG. 3 is a view similar to FIG. 2 except that a third embodiment of the collimating electrode is shown;

FIG. 4 is an enlarged view of one embodiment of a field emission cathode made in accordance with the present invention;

FIG. 5 is an enlarged view of another embodiment of the present field emission cathode;

FIG. 6A is an enlarged view of a third embodiment of the field emission cathode of the present invention;

FIG. 6B is a vertical section view taken along the line 6B6B of FIG. 6A; and

FIG. 7 is a schematic view of the tip portion of the field emission cathodes of FIGS. 4, 5, or 6A diagrammatically illustrating the electron emissive coating on the surface of such tip portion.

One embodiment of the field emission cathode of the present invention is shown in FIG. 4 and includes an electron emitting portion 10 attached to a support filament 12 which is employed to heat such emitting portion during the coating process hereafter described. The support filament 12 may be formed of a tungsten wire bent into a U-shape, and the electron emitting portions 10 formed of monocrystalline tungsten in the shape of a cone having a half angle of approximately 15 which is sufficiently large to exclude strong emission from sites other than at the cathode tip. The conical cathode portion 1% is provided with a sharpened portion in the form of a pointed tip 14 having a radius of curvature in the range of approximately 1O to 10 centimeters to enable the establishment of the high electrical field at such ti-p necessary for field emission of electrons.

The pointed tip 14 of the electron emitting portion 10 of the cathode has an electron emissive coating thereon, as shown in FIG. 7. This coating which is provided only on the sharpened tip portion 14 includes a layer 16 of low work function material such as zirconium and an intermediate layer 18 of bonding material such as oxygen. The cathode portion, composed of tungsten, oxygen and zirconium, has a field emission work function of about 2.6 electron volts at room temperature, while that of tungsten alone is approximately 4.5 electron volts. The atoms of zirconium are of the proper size to fit into the spaces between the atoms 20 of tungsten at the surface of tip 14 on the tungsten crystal lattice plane having Miller indices 100. The zirconium atoms deposited on the cathode migrate to the crystal lattice plane of tungsten oriented at the surface of the sharpened cathode tip portion and apparently the region on the tip immediately surrounding such plane after the cathode is heated by current flowing through filament 12 or in some other suitable manner, and cause electrons 22 to be emitted only from the small area on the tip of the 100 plane and such surrounding region. It has been discovered that a coating of oxygen atoms 18 on the cathode will bond the zirconium atoms more securely to the 100 plane on the cathode tip to provide a longer stable emission lifetime for such cathode. This bonding is apparently caused by their geometrical fit in the 100 plane and by electrical attraction between the atoms of these elements due to their different electrical charges shown by their large displacement from each other in the electromotive series of elements which lists zirconium and tungsten as electropositive elements and oxygen as an electro-negative element. Another cathode having similar characteristics to that just described can be formed by substituting carbon for oxygen as the layer 18. In addition to the materials described above, hafnium or magnesium may be employed as the low work functional material 16 because they fit the 100 plane of tungsten, and nitrogen, chlorine, iodine or fluorine can be used as the bonding material 18. Thus, other suitably charged elements as listed in the electromotive series can be employed in layers 16 and 18 if they have the proper electrical charge and proper atomic size to fit on one of the crystal planes of the tungsten substrate.

Also, it is possible to replace the zirconium with other electron emitting material such as cesium, barium or thorium which adheres to both the 100 and 211 planes of the tungsten lattice. However, these other electron emissive materials have a disadvantage in that they would form a plurality of spaced sources on the cathode for the electrons 22 emitted therefrom due to the fact that such material adheres to several different planes. Such a multispot source of electrons is usually undesirable since most electron beam devices require a point source for proper focusing of such beam. However, it is conceivable that a cathode could be constructed wtih only one of such planes positioned at the tip portion having a small radius of curvature so that electron emission is limited to such point. Of course, other refractory metals having cubic crystal lattice structures and atomic size similar to tungsten, such as molybdenum, platinum or nickel, and even semiconductor materials such as silicon or diamond can be employed for the substrate body 14 of the field emission cathode in which case other low work function materials would be employed for layer 16, and other bonding materials can be employed in layer 18.

In addition to the filament supported cathode of FIG. 4, it is also possible to provide a field emission cathode 24 of tungsten having an enlarged cylindrical body portion with a conical point 10', as shown in FIG. 5. The tip 14' of cathode point 10' is coated with a layer of electron emissive material and bonding material similar to that shown in FIG. 7. The cathode 24 can be indirectly heated by a high frequency induction coil in order to cause the zirconium coated on the cathode portion 10' to migrate to the 100 plane oriented at cathode tip 14'. Of course, any other suitable method of indirect heating can also be employed including electron bombardment from another source.

In addition to the pointed cathodes of FIGS. 4 and 5, the cathode of the present invention may also be formed with an elongated sharpened knife edge 26 shown in FIGS. 6A and 6B. This knife edge cathode may be provided on the filament wire 12 after bending such filament wire into the rectangular shape and sharpening the outer side of the intermediate portion of such filament wire. The 100 crystal plane of the tungsten substrate material can be oriented at the sharpened tip of the knife edge, and then coated with an electron emissive layer similar to that shown in FIG. 7 by causing heating curent to flow through support filament 12'.

One embodiment of a cathode ray tube employing an electron gun having a field emission cathode made in accordance with the present invention is shown in FIG. 1. This tube includes an evacuated tubular envelope 28 of glass or other suitable insulating material having a fiuorescent screen 30 coated on the inner surface of the face plate portion of the envelope at one end of the tube and a field emission cathode 32 mounted within the envelope at the other end of the tube. The cathode 32 provides a small point source of electrons and may be either of the cathodes of FIGS. 4 and 5 previously described. This field emission cathode is positioned within a tubular control electrode or grid 34 having a small aperture therein for the passage of electrons emitted from the tip of such cathode. A wire 36 of solid zirconium or having a coating of zirconium extends through an opening in the side of the control electrode 34 and is coiled inside such control electrode in a position adjacent the tip of the cathode. The wire 36 is heated by electrical current passed through such wire to vaporize the zirconium and cause the zirconium vapor to he deposited upon such cathode tip but not on the glass envelope due to the shielding effect of the control electrode. One terminal of the zirconium wire 36 may be connected to the inside wall of the control electrode, while the other terminal of such wire extends through an insulating plug in the opening through the side of the control electrode and is connected to one of a plurality of metal terminal pins 3-8 extending through the envelope and secured in a socket 40 attached to the base of the tube envelope for the application of heating current to such wire. In a similar manner, the opposite ends of the cathode support filament are connected to a diflferent terminal pins 38- for enabling the flow of heating current therethrough when the cathode 32 is in the form of the cathode shown in FIG. 4. It should be noted that the oxygen absorbed in the cathode may be provided by heating a crucible containing an oxygen compound such as copper oxide, supported inside the envelope adjacent such cathode to decompose the' compound and" liberate oxygen within the envelope in the immediate vicinity of the cathode. However, there is usually enough residual oxygen left within the tube envelope during its manufacture to cause suflicient oxygen to be absorbed by the field emission cathode to bond the zirconium or other low work function material to the cathode tip if the cathode is coated with zirconium before the tube is degassed in a conventional manner.

A collimating electrode 42 is supported within the envelope between the control electrode 34 surrounding cathode 32 and the fluorescent screen 30 in any suitable manner in order to divide the envelope into two separate pressure chambers, such as by a metal ring (not shown) of Kovar extending through the envelope to provide a gas-tight glass to metal seal around the periphery of such collimating electrode. This collimating electrode may be in the form of a flat circular metal plate of substantial thickness having an aperture 44 through the center of such plate to enable the passage of the electron beam 46 emitted by the cathode 34 through such collimating electrode. The electron passageway 44 may have a diameter on the order of .004 inch and a length of about .04 inch. This reduces the amount of gas transmitted through the collimating electrode and provides some collimating eflect on the electron beam 46 so that the maximum divergence of such beam is only about one degree after it is magnified by the electromagnetic focusing coil and passes through such collimating electrode.

The collimating electrode 42 is connected to the movable contact of a potentiometer 48 whose end terminals are connected between a positive DC. voltage source of +4 kilovolts and ground to apply a DC. voltage which may be in the neighborhood of +2 kilovolts to such electrode and is substantially equal to the voltage applied to the control electrode 34 by the movable contact of another potentiometer 50 whose end terminals are connected between positive DC. voltage sources of +4 kilovolts and +1 kilovolt. After the electron emissive coating is applied to the cathode 32, such cathode is grounded so that the potential difierence between the cathode and the control electrode 34 causes the field emission of electrons from such cathode at the tip thereof. Of course, other potentials can be applied to these electrodes. For example, the collimating electrode 42 can be grounded and the cathode connected to a negative DC. voltage which may be varied between 2 kilovolts and +4 kilovolts. As stated previously, the layer 16 of zirconium or other low work function material limits electron emission from the tungsten cathode to the 100 plane at the tip of such cathode so that such beam originates from an extremely small source and of less divergence.

An annular electromagnetic focusing coil 52 is positioned about the neck of the envelope between the control electrode 34 and the collimating electrode 42 in order to focus the electron beam after it emerges from the aperture in the control electrode. The magnetic core 53 of the focusing coil is provided with inward projecting portions to position the gap 54 between the north and south poles of such core closer to the envelope to achieve maximum field adjacent the axis of the tube so that the electron beam is focused to a point just inside the inlet opening of the passageway 44 through the collimating electrode. In addition, the electron :gun may be provided with a pair of blanking deflection plates 56 which are supported within the tube envelope between the control electrode 34 and the collimating electrode 42 in order to move the electron beam out of alignment with the aperture in the collimating electrode to prevent such beam from striking the fluorescent screen 30. In this regard the blanking plates 56 are each connected to the movable contact of a different potentiometer 58 whose end terminals are connected between a positive DC. voltage source of +4 kilovolts and ground in order to apply substantially the same DC. potential to such blanking plates that is applied to the collimating electrode 42. A push-pull blanking signal having an amplitude of from 20 to 50 volts is applied to the blanking plates 56 through separate input terminals 60 connected to such plates.

A conventional electrostatic lens structure including a first anode 62, a second anode 64 and a focusing electrode 66 is mounted within the tube envelope between the collimating electrode and the fluorescent screen 30. In addition, a pair of vertical deflection plates 68 and a pair of horizontal deflection plates 70 are mounted within the tube envelope between the lens

structure including electrodes

62, 64, 66 and the fluorescent screen 30 in order to deflect the electron beam 46 in accordance with signals applied to such deflection plates before such beam strikes such fluorescent screen. The vertical deflection plates 68 are connected to the push-pull output of a vertical amplifier 72 whose input is connected to a source of vertical signals at input terminal 74, while the horizontal deflection plates 70 are connected to the push-pull output of a horizontal sweep generator 76 which may be triggered into opera-tion by a trigger pulse produced by the vertical input signal in a trigger generator circuit, not shown. In order to increase the brightness of the light image produced on the fluorescent screen 30, post deflection acceleration of the electron beam can be employed by providing a helix wall coating 78 of suitable resistance material, such as a graphite compound, on the inner surface of the envelope between the horizontal deflection plates 70 and the fluorescent screen and connecting the ends of such wall coating between D.C. voltage sources of +4 kilovolts and +10 kilovolts.

As mentioned previously, the collimating electrode 42 divides the envelope into two separate chambers which may be maintained at different pressures due to the extremely Small diameter of passageway 44. Thus, the envelope chamber containing the cathode 32 may be maintained at an extremely low pressure of about 10- millimeters of mercury by providing a coating of getter material 80 on the inner surface of the envelope in such compartment, while the other compartment containing the fluorescent screen or other target may be maintained at a higher pressure of approximately 16- millimeters of mercury. This enables conventional materials to be employed for the elements contained in the chamber of higher pressure While increasing the lifetime of the field emission cathode by operating it in the lower pressure chamber. It should be noted that the aperture 44 and the collimating electrode 42 does provide a slow leak which would eventually equalize the pressure in the compartments were it not for the presence of getter material 80 and the ion pumping action of the electron beam.

Of course, an electrostatic focusing structure can be substituted in place of the electromagnetic focusing coil 52 but generally has greater aberation, in which case such electrostatic focusing structure would be positioned inside the envelope between the control electrode 34 and the blanking deflection plates 56. In addition, different types of collimating electrodes may be employed in place of electrode 42 of FIG. 1. Thus, as shown in FIG. 2, a collimating electrode 42' may be provided having a frustoconical projection 82 in the center of the plate immediately surrounding the passageway 44 and extending toward the cathode. This conical projection 82 prevents any secondary electrons emitted by the rear surface of the collimating electrode 42 when such electrode is struck by the electron beam 46', such as during blanking, from passing through the aperture 44 in such plate.

Another form of collimating structure 42" is shown in FIG. 3 and includes a pair of

fiat metal plates

84 and 86 which are held in uniform spaced relationship by a spacer ring 88 of ceramic or other insulating material positioned between such plates adjacent the periphery thereof, or by any other suitable mounting structure. The collimating plate 84 nearest the cathode is provided with an aperture 88 through the center there-of having a diameter on the order of .020 inch while the other collimating plate 86 is provided with a central aperture 99 of much smaller diameter of about .004 inch. When this type of collimating structure is employed, the getter layer 80 may be positioned on the inner surface of one of such collimating plates so that the space between such plates is also maintained at the high vacuum of the cathode chamber. The

collimating plates

84 and 86 are each provided with a thickness of about .020 inch and are spaced apart by a distance of about .100 inch. In addition, it has been found that best results are achieved if the electron beam is focused so that the cross-over point occurs just inside the aperture 90 in the second collimating plate.

The perveance of the electron beam can be increased if its electrons are decelerated before passing through the collimating electrode structure. This can be accomplished merely by connecting the collimating electrode to a voltage slightly negative with respect to the potential of the control electrode 34. The effect of such a deceleration is to reduce the loss of electron beam power due to such beam striking the collimating electrode, and to increase the deflection sensitivity of the beam to the signals applied to the deflection plates 68 and 70.

A tube structure made in accordance with the present invention has produced an electron beam of milliamperes having a beam density of 11 amperes per centimeter squared when using a collimating electrode having an aperture of .008 inch in diameter by operating such collimating electrode and the control electrode at a potential of +1.2 kilovolts with respect to the cathode. This beam could be increased to a current in excess of 100 rnilliamperes having a beam density of 100 amperes per centimeter squared by increasing the potential of the control electrode and the collimating electrode to about +50 kilovolts.

It will be obvious to those having ordinary skill in the art that various changes may be made in the details of the above-described preferred embodiments of the present invention. For example, materials other than zirconium and oxygen could be used for the layers 16 and 18, and a material other than tungsten could be used for the substrate body 14. Also, the electron gun can be made in a form other than the specific embodiments of FIGS. 1 to 3 and can be used in electron beam machining apparatus, electron microscopes and other devices than the cathode ray tubes shown. Therefore, the scope of the present invention should only be determined by the following claims.

We claim:

1. A field emission cathode comprising:

a substrate body of material taken from the group consisting of molybdenum and crystal lattice plane of Miller indices oriented at the surface of a sharpened portion of said body having a small radius of curvature;

a layer containing electron emissive material taken from the group consisting of zirconium, hafnium and magnesium, coated only on said sharpened portion of said body and confined substantially entirely to said lattice plane to reduce the work function of said sharpened portion; and

said layer also containing bonding material, such a oxygen, to cause said emissive material to adhere only to said surface of said sharpened portion so that the field emission of electrons from said cathode forms an electron beam of high current density originating at a small source confined primarily to said coated 100 plane on said sharpened portion of said body.

2. An electron beam tube including the field emission cathode of claim 1 and means for focusing the electrons emitted by said cathode into a narrow beam.

3. The tube of claim 2 which also includes an evacuated envelope having two interconnected chambers of different vacuum pressures and means for mounting the field emission cathode in the chamber of highest vacuum pressure.

4. The tube of claim 3 which also includes a collimating electrode structure mounted within said envelope and having a passageway therethrough which is in communication with both of said two chambers and is in alignment with said cathode.

5. The tube of claim 4 which also includes a fluorescent screen mounted within the chamber of lowest vacuum pressure.

6. The tube of claim 5 which also includes means for deflecting the electron beam so that said tube is a cathode ray tube.

7. The tube of claim 6 which also includes a control electrode positioned between the cathode and the collimating structure, and means for applying a more negative DC. bias voltage to said collimating structure than to said control electrode to decelerate the electron beam before it is transmitted through the passageway in the collimating structure.

8. The tube of claim 7 in which the collimating structure includes two spaced insulated collimating electrodes maintained at different DC. voltage potentials, and a layer of gettering material coated on at least one of the collimating electrodes between said collimating electrodes.

9. A field emission cathode comprising:

a body of tungsten including a monocrystalline substrate having a crystal lattice plane of Miller indices 100 oriented at the surface of a sharpened portion of said substrate having a small radius of curvature; an outer layer of zirconium coated substantially only on the 100 plane of said surface of said sharpened portion of said substrate to reduce the Work function of said surface portion; and

an intermediate layer of oxygen coated on said surface portion beneath the outer layer to cause said zirconium layer to adhere only to said surface of said sharpened portion so that electrons emitted from said cathode originate at a small source confined primarily to said zirconium coated 100 plane.

References Cited UNITED STATES PATENTS 12/1959 Barnett 3137 X 7/1960 Opitz et a1. 3137 6/1964 Henderson et a1. 313-346 X 7/1966 Shroff 313-336 OTHER REFERENCES 10/ 1933 p Dyke et al. Field Emission; Advances in Electronics 10/ 1943 Calblck 313-47 X and Electronic Physics; vol. VIII, 1956; pages 126, 161, 11/1948 Buckingham et al. 313346 X 137 and 135 i d 10/1951 Harnqvist 3137 8/1954 Buckingham 313-346 X 10 ROBERT SEGAL, Primary Examiner.

5/1959 Martin 313-336 X UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,374 ,386 March 19 1968 Francis M. Charbonnier 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 4, lines 12' to 14, strike out "Another cathode having similar characteristics to that just described can be formed by substituting carbon for oxygen as the layer 18."; Column 8, line 13, after "and" insert tungsten having a column 9, line 9, for "Harnqvist" read Hernqvist column 10, after line 4, insert the following:

3,284,657 11/1966 Weissman--3l3-346 3 ,ZSQQ OZB M ll/1966 Schilling et al- -3l3-8ZX Signed and sealed this 22nd day of July 1969.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents

Claims

1. A FIELD EMISSION CATHODE COMPRISING: A SUBSTRATE BODY OF MATERIAL TAKEN FROM THE GROUP CONSISTING OF MOLYBDENUM AND CRYSTAL LATTICE PLANE OF MILLER INDICES 100 ORIENTED AT THE SURFACE OF A SHARPENED PORTION OF SAID BODY HAVING A SMALL RADIUS OF CURVATURE; A LAYER CONTAINING ELECTRON EMISSIVE MATERIAL TAKEN FROM THE GROUP CONSISTING OF ZIRCONIUM, HAFNIUM AND MAGNESIUM COATED ONLY ON SAID SHARPENED PORTION OF SAID BODY AND CONFINED SUBSTANTIALLY ENTIRELY TO SAID LATTICE PLANE TO REDUCE THE WORK FUNCTION OF SAID SHARPENED PORTION; AND SAID LAYER ALSO CONTAINING BONDING MATERIAL, SUCH AS OXYGEN, TO CAUSE SAID EMISSIVE MATERIAL TO ADHERE ONLY TO SAID SURFACE OF SAID SHARPENED PORTION SO THAT THE FIELD EMISSION OF ELECTRONS FROM SAID CATHODE FROMS AN ELECTRON BEAM OF HIGH CURRENT DENSITY ORIGINATING AT A SMALL SOURCE CONFINED PRIMARILY TO SAID COATED 100 PLANE OF SAID SHARPENED PORTOIN OF SAID BODY.