US2393803A - Method of making long life secondary electron emitters - Google Patents

Description

Jan 29, 1946.

H. NELSON METHOD OF MAKING LONG LIFE SECONDARY ELECTRON EMITTERS Filed Jan'. 27, 1945 Y 0 H R v m o N M 0| 7 I T a l M v H a 0 w S D, U o .mH n wn L.. F 6 m I v E H 0 m- "F .1 I. vwh z i 4 7 8 n 6 5 4 3 2 1 O Patented Jan. 29, 4

METHOD OF MAKING LONG LIFE SEG- QNDARY ELECTRON EMITTERS Herbert Nelson, Bloomfield, N. 1.. assignor to *Radio Corporation of America, a corporation of Delaware Application January 27, 1945, Serial No. 514,911

5 Claims. (01. 250-174) My invention relates to electron discharge tubes having a th'ermionic cathode and a secondary electron emitter and more particularly to tubes in which the secondary electron emitter consists essentially of an alloy of magnesium, especially an alloy of magnesium and silver.

Secondary electron emitters with a thin film of magnesium oxide on a metallic support or foundation have been made by evaporating magnesium oxide from some suitable carrier such as platinum which is inert to magnesium oxide and which can be heated to a high temperature. Such films have also been formed by vaporizing magnesium in an atmosphere of carbon dioxide or other slightly oxidizing gas and thus producing magnesium oxide which deposits on the foundation to form a thin film. Emitters with such films have a comparatively high gain or ratio of secondary electrons to primary electrons when bombarded by primary electrons having the velocity of 100 or 300 volts. This gain decreases rather rapidly in use and for this and other reasons emitters with magnesium oxide films formed as above described have not been used commercially to any reat extent.

An electrode made of an alloy of magnesium and silver, such as the alloy of 20% magnesium and the balance silver, disclosed in U. S. atent to Zworykin et a1., 2,233,276, Feb. 25, 1941, will show considerable gain when the electrode is heated for an hour to about 400 to- 500 C. in an evacuated glass envelope. Under these conditions a magnesium oxide film is formed on the surface of the electrode. Reasonably good gain can be obtained from such an emitter, but the gain decreases rather rapidly during use, I have found that better and more uniform results are obtained when oxygen to a pressure from .10 to mm. of mercury is introduced into the tube during the baking period and later pumped out The principal object of my invention is to provide an improved electron discharge tube having a thermionic cathode preferably of the oxide coated type and a secondary electron emitter of a magnesium alloy which will have higher gain, greater stability and longer life than the previous tubes of this type. Another object of my invention is to provide an improved method. of making such a tube in which the emitter is so treated that the gain is higher and is better maintained than in tubes made by the conventional methods.

In accordance with my invention the magnesium alloy electrode, preferably a sheet of the silver magnesium alloy above referred to and containing about 20% magnesium is pre-treated I under controlled conditions by heating it to into the bulb of the tube.

when the electrode is used as a secondary elec- This pre-treated electrode, oxi

tron emitter. dized under controlled conditions, can stand exposure to the air and can be assembled with other electrodes to form a mount to be sealed The pre-treatment above described prevents damage to the oxidized magnesium alloy electrode due to the overheatsome oxygen to the tube often show how rather low initial gain, and in most cases also show aing to which it is necessarily subjected during mounting, sealing-in, exhaust, and annealing.

The preferred procedure is to clean and wash the silver magnesium alloy electrode, place it with tweezers in a hydrogen fired metal holder which is mounted in a hard glass heating tube thoroughly cleaned with chromic acid and distilled water, and then evacuate the heating tube with liquid air on the trap to the pump from the start. A little oxygen is introduced in a known manner, the electrode heated slightly to form a thin film, the oxygen pumped out, and the electrode then baked in' vacuum. Oxygen is then introduced in a known'manner until the oxygen pressure is from .050 to .150 mm. or 50 to microns of mercury and the electrode heated for 40 to 50 minutes by a high frequency coil from a barely visible temperature at the start, because too high a temperature of the electrode at the start may cause loss of magnesium by evaporation, to a bright red at the finish. By this procedure the electrode is heated to about 600' to will show a comparatively high gain or secondary electron emission ratio which is better maintained throughout the, life of the tube than is the case with similar tubes having electrodes ofthe same magnesium alloy which have not been pre-treated in oxygen. I have found it desirable to heat the pre-treated magnesium alloy electrode or emitter during manufacture while considerable residual gases arepresent in the tube, and before the getter is flashed. Whether the improved results thus obtained are due to heating the pre-treated magnesium silver alloy emitter in the presence of residual gases which may be to some extent oxidizing, or to some other action of the residual gases is not definitely known. Although heating the pretreated magnesium silver alloy electrode to a temperature somewhat above 400 C. in an atmosphere of oxidizing residual gases or oxygen is permissible, heating it above 400 C. in a good vacuum results in a rapid decrease in emission due to diffusion of magnesium into and through the magnesium oxide film to the surface of the magnesium oxide layer on the emitter. For this.

reason, it is essential and is a part of my invention that the temperature of the pre-treated emitter be kept below 400 C. while the atmosphere in the tube is nonoxidizing, either as the result of good vacuum or of the presence of neutral or reducing gases, hence it is important that the emitter be heated at the proper point of the tube, preferably in .the exhaust schedule and under proper conditions.

. The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but theiinvention itself will best be understood by reference to the following description taken in connection with r the accompanying drawing in which,

Fig. 1 is a cross-section of one kind of electron discharge device in which my invention may be used to advantage;

Fig. 2 shows by curves the improvement in gain and life which may be obtained by the practice of my invention; and

Fig. 3 is a schematic longitudinal section of another type of tube in which my invention may be utilized.

The tube shown in Fig. 1 is of the type disclosed in U. S. patent to Wagner, 2,293,418, Aug.

18, 1942, in which the structural details of the tube are more fully illustrated and described.

The tube comprises a glass bulb l0 enclosing a cathode II which is surrounded by a control grid l2 and a screen grid IS. The discharge from the cathode is in the form of arcuate electron beams indicated by dotted lines and directed by means of electrodes l6 and I! to a secondary electron emitter l8, preferably made of the magnesium silver alloy containing about 20% magnesium. The secondary electrons flow from the emitter it to the anode or output electrode II. The structure of the tube is such that this emitm- I! is unavoidably heated during the sealing of the mount into the glass bulb l0.

Fig. 2 shows by curve A about the average gain of such a tube and the decrease in gain with life if the electrode or emitter i8 is sealed into the bulb without having been 'pre-treated in oxygen. The gain after about 100 hours of life decreases to less than half the initial gain. I have found that if during exhaust of the tube the atmosphere in the tube is enriched with oxygen in an attempt to facilitate the formation on the electrode of a film containing magnesium oxide, the results are somewhat but no greatly better than indicated by the curve A. When this tube is made in accordance with my invention, and the electrode is pre-treated at about 700 C. in an oxidizing atmosphere, the initial gain and the gain at various periods during life is on the average, as indicated by the curve'B, showing a marked increase in initial gain and a prolonged life at a gain nearly as great as the initial gain of the tube represented by the curve A. If the electrode is pre-treated at temperatures between 600' C. and 700 0., the gain during life is on the average about as shown by the curve C.

Fig. 3 shows schematically another type of tube in which my invention has been used to advantage and which comprises a glass envelope 20 enclosing an indirectly heated cathode 2i with a beam forming and focusing cathode shield 22. The discharge from the cathode is concentrated into an electron beam which is directed to a dynode 23 made of the magnesium silver alloy containing about 20% magnesium with the surface facing the cathode treated in accordance with my invention to have a high ratio of secondary electron emission and to be a good secondary electron emitter. through a grid 'anode assembly consisting of a metal cylinder 24, which may be maintained at a positive potential with reference to the emitter 23 where the cylinder 24 is the output electrode, then through two apertured discs 25 and as which extend transversely of the cylinder 24, with their apertures aligned, then between a pair of deflection plates 21 inside the cylinder 24, and

I through a third transverse apertured disc 20. The

deflection plates swing the focused electron beam back and forth across the aperture in the disc 28. The secondary electrons from the dynode or emitter surface on which the electron beam passing through the disc 28 impinges, are collected by the cylinder 24, principally on the disc 28 which is opposite the dynode or emitter 28. The details of construction of a tube of this type are more fully disclosed in U. S. patent to Herold, 2,294,659, Sept. 1, 1942.

The type of tube shown in Fig. 3 is so constructed that the dynode or emitter I8 is unavoidably subjected to considerable heat during the sealing-in operation with the result that the dimculties above mentioned are encountered. These difficulties are greatly decreased if the emitter is pre-treated under controlled conditions with oxygen and the tube is then exhausted in accordance with my invention.

I have found that the decrease in gain with life of such a tube havingv a pre-treated electrode of magnesium sliver alloy can be somewhat lessened by maintaining in the tube a very low pressure of oxygen throughout the life of the tube. This may conveniently be done by providing in the tube a kind of reservoir which releases oxygen slowly throughout the life of the tube, for example, by slightly oxidizing some of the metal The electron beam passes parts of the tube or by mounting in, tube a piece of oxidized metal such as nickel which will gradually give up oxygen. The emitting surface may also be rejuvenated from time to time by developing a slight pressure of oxygen within the tube in some convenient way such as by induc-. tion heating of a capsule 29 shown in Fig. 3 and containing some material which-will evolve oxygen when heated,

I claim:

1. The method of making an electron discharge device comprising'a thermionic cathode and cold electrodes, one of which is a secondary electron emitter consisting of a sheet of silver-magnesium alloy in which silver is predominant, which includes heating the sheet in an atmosphere containing oxygen at a pressure of 10 to 2000 microns of mercury to. produce a surface film of a golden to bluish interference color, cooling said oxidized sheet to about room temperature, mounting said oxidized sheet in normal room atmosphere in an envelope containing said cathode and said operating cold electrodes, baking the envelope, heating the oxidized sheet and the cold electrodes to degassing temperature; and exhausting the envelope.

2. The method of making an electron discharge device comprising a thermionic cathode and cold electrodes, 'onea secondary electron emitterof silver-magnesium alloy consisting predominantly of silver, which comprises oxidizing said emitter in an oxidizing atmosphere at a partial pressure of oxygen toproduce a surface film showing a golden to bluish interference color, exposing said oxidized emitter to air, mounting said oxidized emitter in an envelope containing said cathode and said cooperating cold electrodes, baking and exhausting said envelope, heating said cold elec- "trodes to degassing temperature during exhaust,

maintaining said emitter during the degassing of the other electrodes and in an appreciableatmosphere of residual gases at a temperature below 400 C. and later in the exhaust heating said emitter in good vacuum to degassing temperature.

3. v The method of making an electron dischargedevice having a thermionic cathode and cold elec trodes one of which isa secondary electron emitter of a sheet of silver magnesium alloy in which silver is predominant which includes forming on the surface of the sheetin partial vacuum having a partial pressure of oxygen of from 10'to 2000 microns an oxide film thicker than the film which gives maximum secondary emission in vacuum, subjecting the oxidized sheet to the atmosphere, sealing the oxidized shet into an envelope in room atmosphere, and exhausting said envelope. U

4. The method of manufacturing an electronic device having anelectrode of a sheet of silvermagnesium alloy consistingpredominantlyof sil-' ver which includes the steps of pre-treating, the

electrode at 500 to 700 C. in the presence of oxygen at a pressure of.50 to microns of oxygen and thereby forming on the surface 'of the electrode'an oxide film appreciably thicker than the film which gives maximumsecondary electron emission in vacuum at 200 to 300 volts, cooling the electrode in the presence of oxygen to about" room temperature, mounting the pre-treated electrode in room temperature as a part ofa mount comprising a thermionic cathode and cold electrodes, sealing the mount into an envelope and baking and exhausting the envelope. I

5. The method of manufacturing an electronic device comprising .a secondary electron emitter of a sheet of silver-magnesium alloy consisting predominantly of silver which includes the steps v of heating the sheet to from 500 to, 700 C. in an oxidizing atmosphere containing oxygen at a pres sure of 0.010 to '2 mm. oi. mercury until the surface of said heet is oxidized to a golden to bluish color, cooling the said oxidized sheet in said oxidi'zing atm here, removing said oxidized sheet from said oxidizing atmosphere and subjecting it to normal room atmosphere, mounting said oxi-v dized sheet as arr-electrode of amount comprising a thermionic cathode and-cold electrodes,and sealing the mount into anenvelope.

HERBERT NELSON.

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