US1909916A - Electron emitter and method of making same - Google Patents

Description

Patented May 16, 1933 UNITED, STATES PATENT OFFICE j ORA S. DUFFENDACK AND RALPH A. WOLFE, OF ANlil' ARBOR, MICHIGAN EEC'IBON EHITTEB AND METHOD OF MAKING- SAME No Drawing. Application filed Kay 11,

This invention has to do with electron emitting elements and the process of making them. Such elements are used in vacuum tubes of various kinds, for example, those employed as rectifiers, detectors, amplifiers and the like to produce a desired electronic flow. The elements will also find use in low pressure gas filled tubes used for low voltage rectification, such as the well known Plio- 1 tron tubes. Another field of use for our emitting element is in the luminous gas tubes, lamps, and the like such as the well known Neon sign lamps, air-way beacon lights and" the like.

The electron emitter heretofore mployed in such vacuum tube devices have usually consisted of a suitable base metal, such as nickel, tungsten, platinum, copper, or in some cases, special alloys, such as the Konel alloy, provided with a coating of oxides of one or more of the alkaline earth metals, usually the oxides of barium or strontium. The base metal gives the desired mechanical strength to the filament, resisting deformation at the temperatures at which it is used, while the coating gives the desired high rate of electron flow.

In the preparation of such elements for use in the tubes it has been common practice to 39 first thoroughly clean the base metal, then oxidize it and spray or otherwise deposit upon it a coating of the oxides or carbonates. This coating is allowed to dry, and the ele- "ment is then ready for insertion in the tube. Then follow the customary steps of exhausting the tube in various stages and heating the parts of the tube and flashing the getter to remove all possible traces of gas. The tube is then sealed, and the next step is activation. 40 This usually consists in heating the emitter to a suitable temperature, usually around 800 C., for a suflicient length of time 'to produce the desired or maximum rate of emission. An emitter manufactured by this,

or similar methods; consists essentially of a 1931. Serial No. 536,676.

coating .to firmly adhere to the base metal, 65

and many tubes are rejected upon final inspection because the coating has flaked oil.

2. It has been found difiicult to produce tubes of the same electron emission under the same conditions in the process of manufacture for mysterious unknown reasons. Often times tubes manufactured under exactly similar conditions as before, will have a very marked decrease in electronemission. This makes it impossible to obtain uniform results even with indentically the same apparatus in the case of radio receivers, and the like.

3. It has been practically impossible to secure a coating of uniform thickness on the base metal, and this results in hot spots where the coating is uneven. The hot spot causes the coating to flake off, or, in some cases, results in the burning out of the filament.

4. The rate of emission from coated filaments is quite variable increasing at first and then decreasing from the maximum with the increase in the usage of the tube. The requirement of initial electronic emission higher than necessary to insure adequate emission over extended periods of use makes it necessary to reject many tubes upon final inspection.

5. There is a wide variation in the length of time that a tube made with an oxide coated 8 emitter is serviceable or usable.

6. We have indicated above some of the objections to oxide coated emitters, and it should now be clear that the processes used in their manufacture are attended with many 9 5 unsatisfactory in that they must be difiiculties and with considerable loss in imtacked by moisture,

perfect tubes. The variable character of the coating, and the factthat it is readily atfore being assembled into the tube further aggravates the situation.

7. There is also a class of emitters consisting of tungsten, tantalum, or molybdenum. In the case of tungsten these pure metallic emitters have been improved by rolling thorium oxide into the base metal, or otherwise combining them. While such emitters are free from most of the objections to the coated emitter, as pointed out above, they are heated to high temperatures to get the desired amount of emission, and as a consequence relatively low temperature,

their efiiciency is low. In the case of the thoriated tungsten emitter, if it is accidentally subjected to high temperatures the thorium may be entirely driven off, rendering the tube useless. I

Our invention consists in a new type of emitter which is free froma pliedvcoatings with their attendant disa vantages, and which at the same time yields in usual op'- eration a high rate of electron emission at a the rate of emission remaining substantially unchanged over long periods of use. Our emitter is also characterized by uniformity, it being possible to duplicate tubes to a degree not possible heretofore in low temperature emitters. our

the emitter which ."electronicemission.

ence between our emitter and those of the.

emitters are'produoed by a method which is very simple and readily adapted to manufacture. The employment of our method and results from it will also reduce, to a very considerable degree, the cost of manufacture, and will practically ohminate the large number of rejections now resulting from faulty emitters.

Our emitter consists of an alloy 'of a base material and material having a high rate of A very important differprior art resides in the fact that the base material not only gives the emitter the necessary stifin'ess, but also,

7 responsible for a I emissivity we may employ r alone, or in combination with each other, or a when treated by our process, prevents evaporation from the surface of the emitter, and assists in holding upon it the active emitting material. The base material, when'so treated, is likewise large increase in thermionic emission.

For the base material we may use nickel, or. preferably an alloy of nickel and chromium. To be' added 'a' small amount calcium to render the allo draw. For the material of manganese or easier to roll and of high electron metals barium, strontium or calcium, either with thorium, or other well known. active materials. -It will be noted that the active and carbon-dioxide bein the alloy.

veniei ce in manufacture.

the nickel or nickel alloy may the alkaline earth.

materials are electropositive with respect to the base metals.

Per cent The ranges of ingredients given above are suggestive rather than limiting. Thus the percentageof barium. given extends from the lowest amount that will be found to be of any gaactical use in electron emitting devices.

. e have in mind particularly the rather feeble emitter. used in telephony. The. emission rapidly increaseswith the amount of barium in the alloy.

? The ran e of percentage of chromium is determine from the change in the thermionic emission with the change in the per 1 een e of chromium, the other in edients remaining constant in amount.- eneficial effects from the addition of chromium'were observed at .5 chromium.- lhe benefits increased until the maximum thermionic emission was reached at 3% chromium; after creased until at10'% very little advantage from the addition of chromium was observed.

At percentages of chromium above 10% the rate of evaporation of.the alloy became exters were maintained. The use of molybdenum, tungsten or uranium in place of chromium'is indicated because of the simiin properties or these elements. J

The amount of manganese specified is. simply that customarily used" to deoxidize the nickel and render the alloy'easierto roll and draw. "There may, of course, be the usual I variation in the percentage of-this material has very little, if an *eifect'on electronic emission, and it is ad ed primarily for con- Other known materials such as calcium may be used for this pu ose. 3 I The alloys maybe formed by the-method rial 520504', filed March We have also strontium, calcium, thorium, or a combination ofthesemetals.

.With th alloy,-'as

The 'manganese, we have found,

I made up alloys'in which, addition to the; barium, we have included I I described, drawn in... wires, .or formedintoother shapesithat may a which the thermionic emission again de- 3-100 'cessive at the temperatures at which the'emitclaimed in the Sprior application of Donald j W. Randolph, I

- be desired, we are ready for the next step.

rate. In actual practice we have found that the use of this process results in increasing the emission from the alloy about onehundred (100) times. This heat treatment also results in placing the alloy surface in 'such condition that the emission is maintained very uniformly over long periods of use.

The activation process 1s divided into two parts, the first of which consists in heating or oxidizing the surface of the alloy in air preferably at atmospheric pressure. In order to produce the maximum electron emission, the wire must be heated so as to form an oxide'film. This film in the case of the nickel, barium chromium manganese alloy is of a dark blue color having in ita greenish tint due to the chromium oxides. We have found that this preliminary heating in air.

may be carried out at temperatures from 350 C. to 800 'C. The time during which the wire must be held at any temperature depends on the temperature selected. At 350 C. it is necessary to heat the wire for approximately one-half hour to produce a sufficient surface oxidation. At 600 G., satisfactory oxidation can be produced in 30 seconds; at 700 C. in five seconds; at 800 C. in two seconds. The lower temperature limit of 350 C.

is possibly not the lowest temperature atwhich the surface can be oxidized, but on account of the time required at this or lower temperatures, the temperature of 350 C. is considered the lowest practicable temperature. The upper limit of temperature at which the oxidation process may be carried out is the melting point of the alloy, it being understood that as the temperature is increased the time must be decreased. If too high a temperature or too long a time is used, the efii- 5 ciency of the emitter is decreased rather than increased, while if too low a temperature is used, or the time is too short the full benefits of this oxidation process are not realized.

The preferred temperature range for oxidation is, from our present experience, from 700 to 750 C. at which temperatures the wire must be held for a time ranging from 5 to 3 seconds. If air pressures less than atmospheric are a used, the oxidation of the surface is retarded and either the time or the temperature must be increased as the pressure is decreased. It

must be understood that if the oxidation be carried out in a chamber at reduced pressures, the amount of oxygen in the chamber must be suflicient to produce the required degree of oxidation of the surface.

If the oxidation is carried out in an atmosphere of pure oxygen instead of air, the time required for the oxidation process at any that they will emit electrons at a much higher :difierent.

given temperature is less than when the surface is oxidized in air alone. We have found that the oxidation process may be carried out slowly in an atmosphere of CO and this indicates that possibly other gases might be employed although time and temperature relations for thes gases would of necessity be critical but that a considerable tolerance of' time and temperature may be permitted. The amount of oxidation produced by heating the wire in air at 700 C. for 5 seconds is such that the rate of activation of the emitter when assembled in a vacuum tube is convenient and practicable. If the degree of oxidation is materially greater than this, the rate of activation is decreased to such an extent as to make the emitter less practicable for commercial use. 7

Even within the considerable range of oxidation treatment as disclosed above, subsequent additional activation is realized in the second step of the process. The second, step in the method used for producing an active, specimen of the alloy consists in assembling a filament made of the alloy in a vacuum tube and heating it in a good vacuum to a temperature of 1150 C. for a period of from 15 minutes to one hour, then reducing this temperature to approximately 1000 C. until the thermionic emission has reached the desired stabilized value. If an initial heating temperature lower than 1150 C. is employed, the specimen must be maintained at that temperature for a longer period of time to insure complete activation of the surface. We have found that the increase in thermionic emission is more rapid and that it reaches a gre'ater value if no electron current is drawn from the emitter during the activation period. During the activation process, barium difl'uses from the interior ofthe wire to the surface where it is securely held by the oxide film.

It would appear'as though the-presence of oxides of nickel and/or chromium, as formed in the first step of this process, is necessary to obtain a large increase in thermionic emis- .sion over similar alloys not so treated. We

correct explanation of the phenomena, the

oxide surface apparently prevents evaporation from the electrode surface and assists in holding upon it the barium that has been Barium-....- r l Manganese 1% Balance nickel v This allo may be treatedin the manner pre viously escr bed, and we have found the time and temperature treatment to secure maximum results is substantially the same.

. the test whereas it is one tube This alloy is not as satisfactory as the chromium alloy for the reason that the electron flow is smaller and the rate of evaporation of barium from the surface is greater. It has also been observed that the rate of evaporation of the alloy itself is greater when it contains no chromium. Furthermore, it is possible to maintain the chromium alloy at a higher temperature without deactivating the emitter than is the case with the alloy without chromium.

It is obvious that the emitter may take any form that may be desired, and that convenience in manufacture may dictate such expedients as oxidizing the emitter in the vacuum tube rather than previous to its insertion. Our emitters when operated at' the usual temperatures to which oxide coated fila ments are now subjected will give a rate of emission of the same order of magnitude as the oxide coated emitters without showing anypractical difierence in emissivity from to the next.- "The method of manufacture is obviously simple and easy. We have tested our emitters oer atin'g temperature the saturation emission current shows no decrease over the period of well known that the saturation emission current of an oxide coated emitter --will W e may use in laceof the barium in the above described a loys any one or more of the other alkaline earth metals, such as strontium*, o r' calcium. The alloys thus formed may'Ibe oxidized and activated in the manner However, fer the barium containing alloys.

,Wehave described the activating step as heatingthe emitter in vacuum. The activa tiom of course, may be directed by, heating the emitter fin an atmosphere of inert gases, such as argon, helium and so. forth. Whether the-activation .is' accomplished in vacuum, or

'v'vhether inan: a tmosphere of inert gases,we havegii'iclutled .,it the descriptive phrase wee in v 1 n swam results may be oblong periods I f time, and find. that at the ordinary oper- SW1 y fall ofi at a considerable rate.

for best results, we pre- The heating in the activation process may :be accom hshed in any manner, whether byheat con uction, orelectrical conduction, or by-inductiveefiect. 'Weclaim:

1. 'The process of producing an' electron emitter which consists in preparing an alloy consisting chiefly of a base metal with a ,smallproportionof a metal of high electron emissivity, 'oxidizin the alloy to produce a surface composed o 7 metal and of the base material, and there 'after heating the emitter, theoxides of .the

venting the rapid evaporation of the matebase metal and of the emitting metal prerial of high electron emissivity.

2. Process of producing an electron emitter which consists in preparing an alloy consisting chiefly of malleable nickel with )a small proportion of an alkaline earth metal oxides of the emitting to impart high'electron emissivity thereto. I

oxidizing the alloy to produce a surface composed of the oxides of said metals and activating the alloy by'heating it a non oxidizing atmosphere: The process of producing," an electron emitter which consists in preparing an alloy consisting chiefly of a base metal and small proportions of chromium,v and a metal of high electron emissivity, oxidizing the alloy to produce a surface'composed of the oxides of said metals and activating the alloy by heating it in ,a non-oxidizi atmosphere.

4. A process of producing 11 electron emitter which consists in preparing an alloy consisting chiefly of nickel including small proand .an alkaline earth portions of chromium, metal, oxidizing the alloy to produce a surface composed of the oxides of said metals and heating the alloy, the oxidized surface serv ng-to retain the material of hi'gh emis- 5. The process of producing an electron emitter which consistsin preparing an alloy consisting chiefly of malleable nickel including' small roportionsjof chromium and barium, oxidizing the alloy to produce a surand subsequently heating the emitter formed rapid evaporat on of the. metal of high emissivity;

facecomposed of the oxides .of said metals,

"6. 'An electron emitter formedof an alloy of a base metaland a metal of high electron emissivity'having a coating of the oxides of said metals. 5

7. an dammit. formedof alloy I of a base metal, including chromium and a metal of high electron emissivity, said emitter having acoating of the-oxides of said metals. I

8-. An eleetron .emitterformedof an alloy of nickel and an alkaline earth metal and 1m:

i c i gi the ox ds f. g sl m ls.

9. An electron emitter formed of an alloy of nickel and barium having a coating of theof malleable nickel, chromium and barium' having a coating of the oxides of said metals.

13. The process of producing an electron emitter which consists in forming an emitter of an alloy consisting chiefly of nickel and barium, oxidizing the alloy by heating it in air to a temperature not substantial y less than 350 C. and thereafter heating the alloy in a non-oxidizing atmosphere to a temperature upward of 1000 C. to produce an activatcdvsurface. 1 v

14. The process of producing an electron emitter which consists in forming an emitter of an .alloy consisting chiefly of nickel with lesser amounts of chromium-and barium, oxidizing the alloy-by heating it in air to a tem-' perature not substantially less than 350 C. thereafter heating the alloy in a non-oxidizing atmosphere to a temperature 11 ward of 1000 C. without drawing current t erefrom, and continuing said heating until a highly activated surface is obtain In testimony whereof we aflix our signatures.

ORA S. DUFFENDACK. RALPHA. WOLFE.