US1812103A - Electron emitting device and method of making - Google Patents

Description

June 30, 1931.

D. M RAE ELECTRON EMITTING DEVICE AND METHOD OF MAKING Filed Sep i922 INVENTOR DUNCAN MAcRnE Patented June 30,1931

UNITED STATES.

PATENT OFFICE DUNCAN MACBAE, OF EAST ORANGE, NEW JERSEY, ASSIGNOB TO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

ELECTRON EMITTING DEVICE AND METHOD OF Application filed September 11, 1922. Serial No. 587,270.

This invention relates to electron-emitting devices such as radio-bulbs, X-ray tubes and the like and more particularly to composite electron-emitting cathodes therefor and methods of making the same.

An object of my invention is the provision of a simple and eflicient method of making oxide-coated electron-emitting filaments adapted for use as cathodes in electron-tubes and the like.

Another object of my invention is the production of filament or ribbon with a firmly-adherent, stable coating of a compound thereon which is adapted to be converted to an oxide or mixture of oxides with high electron-emission characteristics.

A further object of my invention is the manufacture of oxide-coated, electron-emitting cathodes without the use of an organic 2o binder or the like.

A still further object of my invention is the manufacture of platinum-iridium filament or ribbon coated with a stable, alkalineearth carbonate-or mixture of alkaline-earth carbonates, which are adapted to be decomposed when placed in an electron device, leaving a firmly adherent coatingof an electronemitting oxide or mixture of oxides onthe filamen Other objects and advantages of this invention will be made apparent upon reading the following description.

It is well known that when filamentary'material is coated with certain oxides, the electron-emissivity of such material is greatly increased. Several methods have been proposed for applying such an oxide-coating to filamentary material. One of such methods,

consisting in coating platinum-iridium rib- 40 bon with a mixture of barium and strontium carbonates by means of an organic binder and afterwards applyin heat to convert such carbonates to oxides, is escribed in the paper by H. D. Arnold, entitled Phenomena in oxide-coated filament electron tubes and published in the Physical Review, vol. 16, page 73, (1920). I

Another method as proposed consists in dipping a platinum-iridium filament a large number of times into solutions of nitrates of the metals, the oxides of which it is desired to apply and heating such dipped filament to convert the nitrates to oxides after each dipping. The use of organic binding material is undesirable, as it has to be eliminated eventually and the use of onl soluble salts, such as nitrates, entails an un ue amount of labor,

because of the large number of dippings required.

According to my invention, I appl a coating of carbonates of the alkaline-cart metals by dipping once or a few times only, such carbonates being readily converted into oxides by heating in an electron-device, without introducing useless substances other than carbon dioxide, into the bulb or evacuated vessel thereof. Briefly, my method may consist in forming an inorganic suspension of alkalineearth carbonates, preferably the carbonates of barium and strontium in water or other suitable liquid, inert toward the carbonates, and which is readily removed, by heating for a short while, without leaving an appreciable organic residue, passing filamentary ma.- terial to be coated continuously through the suspension and then through heating means for drying and baking the same to causeit to adhere to the filament. means may comprise an electric tube-furnace, which preferably contains an atmosphere of carbon dioxide to prevent undue decomposition of the carbonates. v

This process may be repeated a number of times until the required weight of coating is applied to the filament. When using certain filaments, for example, those having very smooth surfaces, it may be desirable to clean the filament by first passing it through means,

suchasan electric tube furnace, for burning oil from the surface any impurities before passing it through the carbonate suspension. A more firmly adherent coating of such carbonates may be secured by the addition of a small proportion of a chloride or nitrate, for

example, barium chloride or barium nitrate,

such substances acting as an inorganic binder, but ordinarily not being necessary.

A' filament prepared according to the aforedescribed method will keep indefinitely in the air and the carbonate thereon may be actifrom the pump or exhaust machine. In the a A. first instance, it is activated during exhaust for example, the wider the ribbon, if such is by passing electric current through the filament or ribbon. In the second, it is necessary to provide a clean-up agent, such as aluminum or magnesium, for removing the carbon dioxide resulting in the decomposition of the carbonate coating on the filament.

My invention will better be understood by referring to the accompanying drawing, in which the single figure illustrates apparatus by means of which filamentary material may be coated with carbonates according to my invention.

A filament 1 may be wound from spool 2 over or around a pulley or grooved wheel 3, for applying a coating thereto of a carbonate suspension 4 which may be held in a cup 5. The cup 5 with the wheel 3 therein'may be an ordinary gettering cup such as is generally used in the manufacture of vacuum tungsten lamps, but any suitable means may be used for uniformly coating the filamentary material 1 with the desired suspension 4.

The filament 1 may be platinum-iridium ribbon, nichrome which is a nickel chronium alloy or any other desired fairly refractory filamentary material. The size and shape of the wire or ribbon determines, to some extent, the concentration of the carbonate suspension,

used, the thinner or more dilute maybe the suspension applied thereto., The coating is preferably applied by passing the wire a plurality of times through the suspension and heating between each application, although said coating may be applied in one operation, if desired.

As an example of how the suspension may be prepared according to the size of wire or ribbon used, the following details are given,

but I do not intend tolimit myself to said details, as the same are not essential, but merely illustrative.

When using 3 mil round wire, a fairly thick aqueous batch or suspension is preferable and the following proportions of ingredients may be used; 9 grams of pure powdered barium carbonate maybe mixed with 10 grams of pure powdered strontium carbonate, preferably in a mortar, and all lumps pulverized and ground up therein with a pestle. 18 c. c. of water may then be added, a little at a time, first forming a thick paste by stirring the mixture up with the pestle and then gradually thinning out said paste by adding the rest of the water. By this means, a suspension of the carbonates of barium and strontium is prepared, in which the solid ingredients are finely divided and not subject to rapid settling.

This suspension is preferably quickly poured into the cup 5 and the wheel 3 rotated by any suitable means. Rotation of the wheel 3 serves to keep the suspension stirred up and to prevent the solid ingredients from settling out. Wl1en using a. fairly wide ribbon, for example, a platinum-iridium ribbon 10 mils wide and .3 of a mil thick, a thinner or more dilute suspension is permissible, for example, 35 c. c. of water may be used for the same weight of carbonates, instead of 18 c. 0. Such a suspension is used and the coating is applied by passing said ribbon through the suspension a plurality of times and drying and baking after each coating. If, however, it is desired to apply the coating in one operation, the suspension may be made more concentrated by decreasing the amount of water used. For example, if only 18 c. c. of water are used instead of 35, good results are attainable by passing such a ribbon through the suspension only once and drying the same thereon.

When using a fairly narrow ribbon, however, for example, one about 2 mils wide and 4 mil thick, a suspension of about the same consistency as that used on the 3 mil round wire is preferably employed, that is, the smaller and narrower the wire or ribbon, the more concentrated should the suspension be, although it is not desirable to use less water than 18 c. c. with the amount of carbonate heretofore mentioned.

If the filamentary material to be coated is so smooth in surface that the pure carbonate will not adhere firmly enough thereto, a small proportion, for example, from about to about 2 grams of barium chloride or the same amount of barium nitrate may be added to the suspension. Such an addition increases the fluidity of the carbonate suspension, making it permissible in some cases to use less water, and causes the carbonate coating, when heated, to adhere more firmly and coherently than when the sameis pure. As an alternative, a carbonate suspensionmay be made using a nitrate solution, for example, in any concentration, as the liquid vehicle. That is, the addition of other substances to the carbonates, or the utilization of less-pure carbonates, will cause the coating of the same to adhere more firmly to the filamentary material. Barium chloride has an additional effect upon the coating, in that it causes the same to bake on firmly in an almost transparent film.

The filament 1, after emerging from the suspension 4, passes through means for heating the same to dry and bake the coating thereto. an electric tube furnace 6, about 13" long and about 1" internal diameter, energized from any suitable source of electricty (not shown) so that a bright red heat is obtained upon the interior of the furnace. The filamentary material is preferably passed through the furnace at a moderate rate of speed, for ex- S'uch means preferably comprises ample, 16 to 18 meters per minute, and the interior of the furnace preferably containsan atmosphere of carbon dioxide, for example, a stream of carbon dioxide may be allowed to flow through the same from .a reservoir 7 or other source thereof through the valve 8, tube 9 and from there to one end ofthe furnace so that it escapes from the other end. After emerging from the furnace, the filament 1 may be wound up on a spool 11 which may be rotated by any suitable driv' ing means (not shown) for winding up the filament.

The spool 11 and the filament wound thereon may then be transposed with the spool 2 and the operation repeated as many times as desired, or, if the concentration of the sus pension is sufficient to attain the desired result with only one dip, the filament will then be ready for use in an electron device or the like.. 7

It has been found desirable, especially with platinum-iridium filamentary matenal, to pass such material through the tube furnace to burn off impurities thereon before passing the same through the carbonate suspension. In order that all of the filamentary material to be coated may receive such coating prior to its passage through the tube furnace, the same may be threaded through such furnace by first passing a wire or the like of any other desired material throu h filament to be coated t erethrough by means of such other wire. V

After baking the carbonate coating on the filamentary material, the same is ready to be introduced into an electron device, or, the filament may be first activated by heating in an inert environment to a temperature high enough to change the carbonates to'oxides. If introduced into an electron device while in the form of'carbonates, the filament may be heated to decompose the carbonates while the device is still on the pump, in which instance, the pump will remove the liberated carbon dioxide. If, on the other hand, the

filament is not heated until after the device is sealed off, a suitable clean-up agent, for example, aluminum or magnesium or both must it and drawing the prising applying thereto a mixture of the carbonates of barium and strontium and barium chloride and heating the filament to cause the same to adhere firmly thereto.

4. An electrode adapted to be rendered thermionically active comprising an electrically conductive body having a coating of alkaline earth carbonates containing a small proportion of a soluble alkaline earth salt baked thereon.

5. an electrode adapted to be rendered thermionically active comprising a platinum alloy wire having a mixture of the carbonates of barium and strontium and a small proportion of barium chloride applied thereto and baked thereon so that the coating is firmly adherent.

In testimony whereof, I have hereunto subscribed my name this 8th day of September,

DUNCAN MACRAE.

be provided to eliminate or clean-up the liberated carbon dioxide.

The clean-up agent, in the form of a plate or foil, is preferably attached to the anode or plate of the electron device and vaporized to activate the same, by heating said plate by high-frequency induction or electron bombardment, after the device has been evacuated I and sealed off, whereby residual and occluded gases are eliminated.

Although I have described what I now consider to be the preferred means for practicing my invention, it is to be understood that many modifications may be made therein and that I desire to be limited only by the spirit and. scope of the appended claims.

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