US3273005A - Electron emitter utilizing nitride emissive material - Google Patents

Description

.Sept. 13, 1966 J, M. LAFFERTY ELECTRON EMITTER UTILIZING NITRIDE EMISSIVE MATERIAL Filed April 1 1963 2 Sheets-Sheet 1 Q is Q 333 REES CENT/G-RHOE I 2 ELECTAOAfM/fi/ON/NAMPERESPEA CH frvvernrcr M. Le fferi:

A ttor'ney.

Sept. 13, 1966 J. M. LAFFERTY ELECTRON EMITTER UTILIZING NITRIDE EMISSIVE' MATERIAL Filed April 1 1963 2 Sheets-Sheet 2 Inventor.- Jam M. Lafi-erzy by a:

/ is Attorney.

United States Patent M 3,273,005 ELECTRON EMITTER UTILIZING NITRIDE EMISSIVE MATERIAL James M. Latferty, Schenectady, N.Y., assignor to General Electric (Iompany, a corporation of New York Filed Apr. 1, 1963, Ser. No. 269,352 6 Claims. (Cl. 313346) My invention relates to improved electron emitters and particularly to metallic nitride emitter-s.

It is generally recognized that most known electron emitting materials, such as those commonly used in electric discharge devices, for example, are subject to various disadvantages which limit their effectiveness and require special processing to render them effective emitters.

It is generally recognized that most known thermionic electrons emitting materials must be raised to a temperature near incandescence before appreciable electron emission is achieved. Normally energy consumed in maintaining such emitter material at an elevated temperature represents an energy loss which adversely affects efiiciency. Therefore, it is desirable to provide an electron emitting material having a lower operating temperature for a given electron emission, so that less energy is expended in heating the material. Also, it is usually desirable to provide an electron emitting material having a low evaporation rate at elevated temperatures, as required for long useful life.

Accordingly it is an object of this invention to provide an electron emitter which is relatively simple to manufacture and does not require special processing for rendering it an effective emitter of electrons.

It is another object of this invention to provide an electron emitter of high efficiency which is relatively easy to manufacture.

It is yet another object of this invention to provide an electron emitter which is relatively economical to manufacture and has a low evaporation rate at incandescent temperatures.

Briefly stated, I have discovered that the nit-rides of the rare earth metals of the lanthanide series (having atomic numbers from 57 to 71 thorium and uranium are thermionically emissive and form the essential ingredient of emitters having superior properties. These compounds are of the type MeN where Me is the symbol for the metal and N is the symbol for nitrogen. They crystallize in a compact structure yielding high melting points, low evaporation rate and extreme hardness. The compounds can be relatively easily formed during manufacture by heating a cathode constituted in part of the selected metal, such as cerium, for example, to a temperature in the order of 1340 C., in the presence of ammonia gas.

No special activation processes are required to render the materials of this invention effective and efficient emitters. They do not react with the usual metallic refractory base materials and they possess metallic conductivity.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and methods of manufacture, together with further objects of advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 illustrates the emission characteristics of a number of emitters embodying the present invention;

FIGURE 2 illustrates the relative evaporation rates of two emitters embodying the present invention;

FIGURE 3 illustrates a cathode embodying the present invention;

FIGURE 4 shows a step in the manufacture of the cathode of FIGURE 3;

3,273,005 Patented Sept. 13, 1966 FIGURE 5 shows another step in the manufacture of the cathode of FIGURE 3;

FIGURE 6 shows a cross-sectional view of another cathode embodying the present invention; and,

FIGURE 7 shows a cross-sectional view of yet another cathode embodying the present invention.

I have discovered that the nitrides of the rare earth metals of the lanthanide series (having atomic numbers 57 through 71), thorium and uranium are thermionically electron emissive in such significant degree as to render these materials highly desirable for use as electron emitters. These materials also possess the requisite physical characteristics of thermionic emitters and are relatively easy to manufacture. The latter is, of course, considerable commercial importance.

The highly desirable physical characteristics of the nitrides of the lanthanide series, thorium and uranium include: high melting points, extreme hardness, metallic conductivity and low evaporation at incandescent temperatures. These compounds are of the type MeN and crystallize in the face-centered cubic, or NaCl structure. These compounds are true interstitial compounds with nitrogen atoms occupying the octahedral holes in the interstices of the metal structure. This tightly bonded crystal structure undoubtedly contributes to the high order of thermal stability observed with these compounds.

A comparison of a number of nitrides of the present invention with some well-known emitters is shown in FIGURE 1 of the drawing in which emission current in amperes per cm. is plotted as a function of surface temperature in degrees Centigrade (as determined by an optical pyrometcr). A semi-logarithmic plot is chosen for the purpose of the illustration since the resulting curves are significant with respect to Dushmans equation of emission. Dushmans equation is stated in detail in many texts on thermionic emission, but for purposes of explaining the present invention, the equation may be more concisely stated as:

2 92 J=AT e T where J=thermionic current density in amperes per cm.

A=a semi-empirical constant; its units are amps. per cm? per degree Kelvin.

T=temperature of cathode in degrees Kelvin.

e=tl116 base of the natural or Naperian logarithms (approximately 2.72).

=work function of material in volts.

Material A From the plot of FIGURE 1, which compares the emission of some of the various materials of this invention with a number of known emitter materials, it is apparent that uranium nitride and thorium nitride compare favorably with thoriated tungsten. Most significant, however, is the fact that the rare earth nitrides, lanthanum nitride and cerium nitride, possess outstanding thermionic emission properties.

A comparison of the evaporation rate of the highly efficient thermionic emitters of the rare earth nitrides, cerium nitride and lanthanum nitride, with some well-known emitters is shown in FIGURE 2 of the drawing. In the gnaph of this figure, evaporation rate of metal in grams per cm. per second is plotted as a function of electron emission in amperes per cm.

An electron emitter normally is useful only if its evaporation rate is low. From the graph of FIGURE 2 it may be seen that lanthanum nitride and cerium nitride possess evaporation rates less than that of tungsten at equal rates of electron emission. The evaporation rates are only slightly greater than that of lanthanum hexaboride, which is known to possess an extremely low evaporation rate. Cerium nitride, for example, may be seen to possess particularly desirable properties for use as a thermionic emissive material in a cathode. Its efficiency as a thermionic emitter is higher than most known materials and its evaporation rate is low.

A cathode embodying the present invention is shown in FIGURE 3. The cathode is of the hairpin variety and includes side support members and 11 which are connected to and carried by an assembly in the electron discharge device which is variously known as a stern, header or base. The emitter of the cathode is generally formed as an inverted V having opposite extremities secured, as by welding at and 16, to members 10 and 11, respectively. As shown, the emitter takes the form of a relatively large wire 12 having a smaller Wire 13 disposed thereabout in a plurality of turns 14. Extremities of wire 13 may be secured to wire 12, as by welding thereto, but, preferably, opposite extremities of wire 13 are secured to members 10 and 11 when the above-mentioned welds at 15 and 16 are consumated. The heater circuit for the emitter includes members '10 and 11, which are selected of a conductive refractory material, such as tungsten or molybdenum, and the circuit connection to the emitter is normally supplied through one of members 10 and ll. The cathode of FIGURE 3 has the advantage of mechanical ruggedness at some sacrifice in available surface area. Where requirements are less severe, wire 13 may be omitted and wire 12 may take the form of a gauze or a tightly wound coil suspended between members 10 and 11.

A cathode such as is shown in FIGURE 3 is constructed, preferably, by first winding a small diameter wire 17 of refractory metal, such as tungsten or molybdenum, about another wire 18 of refractory material which may, or may not, be of the same refractory material as wire 17. The resulting assembly is shown in FIGURE 4. Wire 18 is then bent in the general form of a hairpin and secured at 15 and 16 to members 10 and 11, as shown in FIGURE 3. Extremities of wire 17 are also secured to members 10 and 11 at the same time.

A coating 19, as seen in FIGURE 5, of the lanthanide series metals, thorium or uranium is then deposited on wire 18, and Wire 17 which is coiled thereabout. Such a coating can be effected by vapor-plating or dipping in molten metal, for example. After the cathode support has been provided with an exposed surface of thorium, uranium or the lanthanide series metals, it is secured in place within the electron discharge device.

I have found that the desired nitride may be formed by heating the cathode support to a temperature between 1200 C. and 1400 C. in the presence of ammonia, which is caused to flow into the electron discharge device. I have found that a support temperature of 1340 C. is particularly satisfactory, although, as with most similar chemical reactions, temperature and time provide inverse effects upon the extent of reaction and wide variations in both are possible. Resulting pyrolysis of the ammonia releases hydrogen which prevents the metal from oxidizing and the nascent nitrogen liberated rapidly reacts with the metal to provide the desired nitride coating. The electron discharge device is then out-gassed and processed in the normal manner. I have found that no activation schedule is required for these cathodes.

A cathode embodying this invention and suitable for use in cathode ray tubes is shown in cross section in FIG- UR E 6. The cathode includes an arcuate base member 20 carrying a mesh 21 of refractory material having thermionic emissive nitride coating 22 thereabout. Coating 22 is indirectly heated by coil 23 which is formed of a resistive refractory material, such as tungsten. Lead 24 is connected to coating 22 and supplies the electrical circuit connection to the cathode emitter.

Since the emitters of this invention have good conductivity, base member 20 and mesh 21 may be constituted of insulating refractory materials, such as ceramic, although, in a preferred embodiment mesh 21 is constituted of molybdenum. The nitride coating 22 can be provided as described in connection with the cathode of FIGURE 3.

A cross-sectional view of a cathode suitable for use in a planar electrode tube is presented in FIGURE 7. The cathode includes a support sleeve 26 which carries a base 27. A mesh 28, of refractory material, is secured to base 27 and provides mechanical support for nitride coating 29. Indirect heating is effected by coil 30. Sleeve 26 is constituted of a conductive refractory material, for example, tungsten or molybdenum, and provides electrical circuit connection to coating 29.

From the foregoing description, it is apparent that the nitride emitters of the present invention may be used in any of a number of ways to provide the essential component of commercial cathodes. These cathodes possess the many desirable qualities resulting from the properties of the nitrides as described hereinbefore. The nitrides of this invention do not react with the usual refractory materials at incandescent temperatures and they are good conductors of electricity. Thus, a wide choice of suitable base materials and circuit connections are permitted. The metallic conductivity yields improved performance at high levels of emission.

While I have described and illustrated particular embodiments of my invention, it will be apparent to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects, and I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A thermionic cathode including an emitter consisting essentially of at least one metal nitride selected from the group consisting of the nitrides of thorium, uranium, and the lanthanide series rare earth metals, and means supporting said emitter providing a conductive connection therewith.

2. A thermionic cathode including an emitter consisting essentially of at least one lanthanide series rare earth metal nitride and means supporting said emitter and providing a conductive connection therewith.

3. The thermionic cathode of claim 2 wherein said rare earth metal nitride is cerium nitride.

4. The thermionic cathode of claim 2 wherein said rare earth metal nitride is lanthanum nit-ride.

5. A thermionic cathode including an emitter comprising as the essential ingredient thorium nitride and means supporting said emitter and providing a conductive connection therewith.

6. A thermonic cathode including a emitter comprising as the essential ingredient uranium nitride and means sup- References Cited by the Examiner UNITED STATES PATENTS Marden 313--345 MacKay 313-346 Rentschler et a1 8 13- 346 Weigand 1 1 7217 Alb richt I1=17-217 Loewe 3*13346 Laiferty 31 3-2146 X Zemany 3 13- 345 Henderson et a1 31 3346 JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

Claims (1)

1. A THERMIONIC CATHODE INCLUDING AN EMITTER CONSISTING ESSENTIALLY OF AT LEAST ONE METAL NITRIDE SELECTED FROM THE GROUP CONSISTING OF THE NITRIDES OF THORIUM, URANIUM, AND THE LANTHANIDE SERIES RARE EARTH METALS, AND MEANS SUPPORTING SAID EMITTER PROVIDING A CONDUCTIVE CONNECTION THEREWITH.

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