US2548514A - Process of producing secondaryelectron-emitting surfaces - Google Patents

Description

April 1o, 1951 J. BRAMLEY 2,548,514

PROCESS OF PRODUCING SECONDARY-ELECTRON-EMITTING SURFACES Filed Aug. 23, 1945 ZZ Z1' if ,ZZ 22222222222222222. ,22222222222122222222 i Patenfed Apr. 1o, 1951 GFFICE PROCESS 0F PRODUCING:` SECONDARY- ELECTRON-EMITTING SURFACES Jenny Bramley, Long Branch, N. J.

Application August 23', 1945, Serial No. 612,197 2 claims. (c1. 117-3322) My invention relates to processes of secondaryelectron-emission.

The present subject matter has been divided and the apparatus claims appear in U. S. Patent 2,527,981, granted October 31, 1950, for Secondary-Electron-Emitter, on continuation-inpart application Serial No. 20,016, led April 9, 1948. The species relating to the dielectric formed from the metallic base is contained in continuation-in-part application Serial No. 102,618, filed July l, 1949 for Secondary-Electron-Emitting Surface.

The primary purpose of this invention is to obtain multiplication of secondary electrons much higher than that obtained in prior practice. This is accomplished in this invention by the proper construction of composite surfaces. One method is to interpose a layer of dielectric limited in thickness to 0.1 mm. and preferably not exceeding 0.03 mm. between a layer with at least modv erately high secondary-electron-emissive properties (called henceforth the secondary-electronemitting layer) on the one hand and a metallic base on the other hand. Suitable substances for the dielectric are oxides of aluminum, titanium, copper, or manganese or crystals of alkali halides. For the secondary-electron-emitting layer the following substances may be used: beryllium or an alloy or oxide thereof, magnesium (oxidized), aluminum, or an alloy or oxide thereof.

. A further purpose relates to methods for increasing the emission of secondary electrons.

A further purpose is to obtain high multiplication in a secondary-electron-emitter without the undesirable features of photoelectric eifect due to the presence of a photoelectric material, such as caesium, in the layer emitting secondary electrons.

A further 'purpose is to produce a secondaryelectron-emitter of high multiplication while avoiding time lag between the 'start or stop of the primary current and the start or stop of high secondary electron emission.

A further purpose is to maintain close time coordination between a secondary-electron-emitter and the primary current by providing for the neutralization by electrons of the positive charge the secondary-electron-emitting layer from the metallic base by an extremely thin layer of porous dielectric and by intruding particles of the secondary emission layer into the pores of the dielectric to assist in neutralizing the positive ..1

charge by electrons. Another method consists in distributing. particles of a non-oxidizable metal, e. g. gold, platinum, palladium, osmium, or'iridium, which can act as impurity centers in the difelectric to assist in neutralizingV the positive charge by electrons from the metallic base.

A further purpose is to enhance the secondaryelectron-emission of magnesium oxide by baking it in vacuo at a temperature of 600 to 800 C.

A further purpose is to employ as a metallic base a metal such as aluminum, chromium, or copper, or an alloy thereof, which will form an adherent self-limiting oxide capable of acting as the dielectric, and'to apply a secondary-electron emitting layer over the oxide.

A further purpose is to decrease conductivity along the layer of thesecondary-electron-emitter in the composite surface as by granulating the surface into a mosaic.

A further purpose is to oxidize a metallic base, such as aluminum, copper, or chromium, oran alloy thereof, or a corrosion resisting iron-chromium alloy, to form a dielectric, applyv over it a.

secondary-electron-emitting layer, and expose the layers to an elevated temperature for a short time to form a mosaic.

A further purpose is to use a metallic base of a metal forming an adherent oxide, such as aluminum, copper, or chromium, or an alloy thereof, oxidize theY surface of the base, thus forming a dielectric layer over the base; coat the surface with a non-uniform thin'lm of a non-oxidizable metal, such as gold, platinum, palladium, osmium, or iridium, which increases the conductivity of the dielectric, deposit further a thin layer of the metal or alloy used in the metallic base, oxidize such thin layer, and over it apply a secondary.- electron-emitting layer. If desired, several alternate layers of the non-oxidizable metal and-the metal used in the metallic base may be applied, and then lthe oxidation carried out.

A further purpose is to construct a secondary-- electron-emitter from an alloy of a metal forming an oxide of suitable dielectric properties, such as aluminum, and a metal which is a good secondary-emitter, suchv as beryllium or magnesium, and tooxidize the surface of the alloy in such a way as to form a layer emitting secondary electrons overa dielectric layer. The alley must be free from any poisoning impurities, suchV as nickel. A suitable method of bringing the secondary electron-emitter to the surface is hydrogen ringv or burnishing under an inertv liquid, such as benzene.

A further purpose is to reduce theelectrical in.-

put in a tube containing a thermionic cathode by using a small cathode requiring only a small current to heat it and obtaining high secondary electron emission from an auxiliary cathode, preferably in mesh form, constructed in accordance with this invention.

A further purpose is to employ an auxiliary cathode constructed in accordance with this invention to overcome prior art diificulties, such as grid emission due to grid contamination from oxide-coated cathodes, backiiring, and ignition of local spots. Since the primary thermionic current is only a small part of the total cathode emission, there is no necessity to use oxidecoated cathodes in tubes constructed according to this invention.

The drawings are useful in explaining the invention. They have been chosen for the purpose of clear illustration of the principles involved and to illustrate conventionally a few only of the possible embodiments of the invention. In the drawings like numerals refer to like parts.

Figures l to 4 and 3a are diagrammatic sections cf composite surfaces useful in explaining the i-nvention.

Figure 5 is a diagrammatic section of a vacuum tube embodying the invention.

Figure 6 is a diagrammatic perspective view of an auxiliary cathode.

One of the most important aspects of the invention relates to the production in electron tubes of extremely high emission of secondary electrons as a consequence of strong electrostatic elds initiated by bombardment with primary electrons of composite surfaces constructed in accordance with the invention. There must, of course, be a suitable source of primary electrons and these electrons should have a speed such that the ratio of the secondary electrons released from the secondary-electron-emitting element to the primary electrons of the beam impinging on the secondary-electron-emitting element is greater than unity.

The base, dielectric and secondary emitter are capable of being laid variously but always with the same resultant arrangement of layers and the same method of operation.

The metallic base 20 in Figure 1 may consist of any desired solid metallic material. The metallic base is covered by an extremely thin layer 2l of dielectric. Suitable dielectric materials are; aluminum oxide A1203, titanium oxide TiOz, copper oxide, manganese oxide MnO, an inorganic crystalline phosphor, an inorganic vitreous phosphor, or an alkali halide, such as NaCl, KCl, LiCl.

The alkali halides are inert and do not volatilize easily under the conditions of production of secondary emission.

The dielectric need not be a perfect insulator;

it preferably will be a semi-conductor. The strucg.;

ture of the dielectric should preferably be very tine.

On the dielectric layer is deposited a thin layer 22 of a substance which is a good emitter of secondary electrons such as beryllium; beryllium oxide, BeO; alloys of beryllium, particularly alloys of beryllium and copper; manganese (oxidized): oxidized magnesium alloys: aluminum; alloys predominantly aluminum, such` as duralumin of any of the recognized varieties, especially aluminum alloy 17S (Al 95%, Cu 4%, Mg 0.5%, Mn 0.5%) or aluminum alloy 24S (Al 93.8%, Cu 4.2%, Mg 1.5%, Mn 0.5 or the aluminum-magnesium or 30%) alloy, or the aluminum-beryllium alloys, of which the one containing 30% beryllium appears the most efficient. Among the above, the duralumins are unexpectedly effective out of all proportion to any characteristics previously suspected and greatly exceed pure aluminum in multiplication.

When this top or secondary-electron-emitting layer 22 is struck by primary electrons under suitable potential conditions, it emits a large number or secondary electrons and thus becomes positively charged and creates a strong electrostatic field between the metallic base and the dielectric layer 2l. This electrostatic field pulls out electrons from the metallic base 20 through the dielectric thus producing a high multiplication of electrons.

The combined thickness of the dielectric layer and of the layer emitting secondary electrons must be very small, the desirable range being from 2 to 20 microns. While not in every case essential, this range of thickness should be used for best results.

One of the problems in the prior art has been tocause the secondary-electron emission to stop and start either in coincidence with the primary current or after only a brief and controllable time interval. Malter (Marconi) British Patent 481,170, September 7, 1936, uses caesium and is troubled by time lag between the start of the primary current and the start of the high secondary electron emission, as well as between the stopping of the primary current and the stopping of the secondary emission. (Malter, Physical Review, vol. 49, p. 478 and p. 879 (1936)). Furthermore, caesium deteriorates by volatilization in vacuo and causes objectionable photoelectric effects, which prohibit the use of the layer as an emitting surface in a photomultiplier tube.

In the present invention, the electron-emitting layer is non-photcelectric, and many advantages and avoidance of much difficulty are thereby obtained.

In order to prevent excessive time lag, the positive charge in the secondary-electron-emitting layer 22 must be neutralized by electrons from the dielectric 2| very quickly, but not quickly enough to interfere with extraction of secondary electrons by the electrostatic lield. The extraction time has been estimated as about 10-14 seconds.

The dielectric layer 2l may be deposited by spraying, settling, evaporation, or similar methods carried out at suitable temperatures, as well as by oxidation, as later explained. The secondary-electron-emitting layer can be deposited on the dielectric by dusting, evaporation, settling, or the like. I have discovered that in order to produce the high field necessary for electron extraction from the metallic base under the conditions of cold emission (about 1,000,000 volts per centimeter) the thickness of the dielectric layer is of importance. A voltage due to secondary-electron emission of more than a few thousand volts is not obtained in practice. For best results the thickness should be approximately 0.03 millimeter, and in any case the thickness of the layer should not exceed 0.1 millimeter. No limit on thinness is necessary provided the dielectric functions.

The invention is operative in its broader phases provided the metallic base, dielectric layer, and secondary-electron-emitting layer are as described, without further precautions to avoid time lag, but for best results special precautions to avoid time lag should be taken.

There. are several ways, lwhich I have-discovered to overcome this trouble.

' One method of overcoming time lag is to deposit the dielectric layer ldeliberately with perceptible porosity. This can be done, if the dielectric is applied by settling from a liquid, by controlling the neness of grinding of the particles with a diameter of the order of ten microns have been found to be satisfactory in obtaining porosity. Ii difficulty is encountered with colloidal properties, an electrolyte may be added to cause dispersion and hence aid settling.

Theporosity permits molecules of thesecondary-electron-emitting layer at sufficiently high temperatures to penetrate the dielectric', thus assisting Vin preventing time lag by vconducting electrons to the surface and neutralizing, the positive charge. Figure 2 lshows a-t 2,2 projections from the secondary-electron-emitting layer into openings at 2l in thedielectric.

An alternate method of eliminating time lag by improving the electron conductivity of the dielectric when bombarded by the primary beam is to. incorporate a discontinuous lm of a noncorrodible metal not more than a few molecular diameters thick which may act as an impurity center in the dielectric. Any one of the noble metals, such as gold, platinum, palladium, osmium, or iridium, can be used.

rlhe thickness of the non-corrodible metal layer must not exceed-four molecular diameters, and a thickness of two molecular diameters gives best results. The film must be discontinuous or a series of islands. This is readily obtained by depositing the film by evaporation at reduced pressure and elevated temperature, or by electrode sputtering (John Strong, Procedures in Experimental Physics, Prentice Hall, 1938), or other similar methods well known in the art. The thickness of the layer of non-corrodible metal can be judged by the color on'the walls of the evaporator, as well known in the art.

On the discontinuous non-corrodible metal film is deposited another layer of dielectric of the same thickness limitation and other chai'- acteristics as the first layer, before the layer of secondary electron emitter is applied. The second layer of dielectric is applied so that the noncorrodible metal will not act Vas a collector of primary electrons but will form an interstitial impurity center for the dielectric.

Figure 3 shows the non-corrodible metal film, such as gold, at 23, and the second dielectric layer at 24.

As shownv inV Figure 3a, there may ybe several alternate layers of dielectric 24 and non-corrodible metal 23', provided the total dielectric thickness does not exceed thelimit set of 0.1 mm.

As explained above, the ifield intensity, which produces cold emission from the base meta-l at the base-metal-dielectric interface, will fall too low if any individual dielectric layer is allowed to become thicker than about 0.1 mm.; preferably 0.03 mm. If for other reasons the dielectric layer mustbe thicker, this can be accomplished by several alternate layers of dielectric andsecondary electron emitter, each dielectriciayer of optimum thickness and not exceeding 0.1 mm.

Thus in Figure 4, on the metallic base 20 I deas above of optimum thickness, thencnother layer 21 of secondary electron emitter, `and soon untilthedesiredtotal thickness of dielectricxhas been used, the top .layer consisting, of course, of secondary electron emitter.

Where the secondary-electron-emitter is berylliumy-ithas been .successfully app-liedy tothe tube element by evaporation in vacuo from electrically heated tantalum spirals which serve yassupports and heaters. It does not'matter whether the .beryllium is oxidized orvnot, since the metal and' oxide are equally good assecondary emitters.

Where magnesium is used as asecondary-electron-emitter, the oxidation is essential, 'as'thezunoxidized metal is not a good emitter. The metal will, however, oxidize rather readily in air. I have discovered that the effectiveness of magnesium (oxidized) as a secondary-electron-emitter is greatly improved bythe step of baking in vacuo at a temperatureof from 600 to800 C.

The vreason forl the improved results from the baking in vacuo isv thought to be at least partly due tothe vmigration of magnesium `into surface portion of the dielectric, and the fact has-been clearly demonstrated.

The surface should be granulated into a mosaic. This can be done by firing in a hydrogen` atmosphere at a temperature above 600 C. and below the softening pointof the metal to reduce impuritiesand roughen the surface. It is then oxidized by exposure 'to the air, avoiding contamination, as from oil on the fingers. then etched, as by an acid or alkali. The secondary-electron-emitting layer is then deposited on top. The granulation appears to assist by preventing `too ready conductivity across the electron-emittinglayer.

An alternate technique for producing the mosaic is to exposethe composite surface (metallic base, oxide, and secondary-electron-emitting layer) abruptly and briefly to a temperature in the range from 600800 C. for aluminum and higher or lower depending on the melting point for the other metals and alloys mentioned. For best results theexpos-ure should be limited to a few seconds in order to insure that the secondaryelectron-emitting layer (for example beryllium) be in the form of a well separated mosaic. The mosaic structure increases the surface of the dielectric exposed to the primary beam and reduces conductivity in the plane parallel to the metallic base.

There is no exact temperature at which beading or mosaic formation occurs, but rather` a range asset forth within which the' phenomenon is evidenced. The extent of mosaic formation is influenced by Ysuch factors as oxide film 'thickness and heat capacity of the metallic base.

On the otherhand a very satisfactory secondary-electron-emitter is produced by using an alloy predominantly consisting of a metal which forms anadherent oxide, such as aluminum, cop-j per, or chromium, self limited to Angstroms thickness, and containing a substantial'amount andl up to 30% (typically 10%) of a metal such as beryllium and/or magnesium which is an excellent secondary-electron-emitter, either vas metal or oxide or both.

It will usually not be profitable to employ less than 1% of the preferred secondary-electronemitting element in the alloy.

The alloy must be substantially free from* poisoning ingredients such as nickel or, less importantly, cobalt. Aluminum has been foundto The surface is l give best results for the predominant metal of the alloy.

The alloy should -be treated to concentrate the preferred secondary-electron-emitting component at the surface by metal migration. This can be accomplished by ring for a few seconds in hydrogen at 600 to 800 C. or by burnishing under an inert organic liquid such as benzene, toluene, or Xylene.

The method of bringing the preferred secondary-electron-emitter to the surface as described in Junker and Leitgebel U. S. Patent No. 2,254,- 805 may also be used. The alloy is then oxidized as by exposure to the air at ordinary temperature. Apreferred method of oxidation in alloys predominantly consisting of aluminum is to immerse in an alkaline solution of an oxidizing agent, such as ferrie hydrate, heated to about 115 C., for a few minutes, depending on the desired oxide thickness. In this way the resultant is first the alloy forming a base metal layer, then a mixture of oxides containing particles of the dominant metal and its oxide, and finally a secondary electron emitting' layer consisting chiefly of BeO, MgO, or a mixture. The electron emitting properties of this combination are very high.

The composite surfaces just described may be utilized in several distinct types of apparatus, 'as will be evident from the following discussion.

For electron photo-multipliers such composite surfaces will be useful particularly for cathodes emitting-secondary-electrons under electron bombardment. As well known, metals Whose surfaces are covered with alkali or alkaline earth metals have high emissive properties (Grlich U. S. Patent No. 2,317,754). By the features of the present invention set forth herein, the secondary emission is still further enhanced. As well known in the art, such electron photo-multipliers may serve various functions, some of which are shown by'Slepian U. S. Patent No. 1,450,265 or Piore U. S. Patent No. 2,123,024.

Proper adjustment of the conductivity of the dielectric in the composite surface emitting secondary electrons under bombardment is contempleted.

Secondary-electron-emitters according to the invention may also be used to increase the Yefficiency of thermionic vacuum tubes or gaseous tubes such as diodes, triodes, thyratrons, and the like. The thermionic cathodes in present use require a relatively large electrical input in order to produce any appreciable current from cathode to anode. In. accordance with this aspect of the invention, a large electron current can be produced by an auxiliary cathode constructed in accordance with any one of the methods for producing composite.V surfaces with enhanced secondary-electron-emission when this auxiliary cathode is bombarded by a weak primary electron current. Only a small power input is required to produce this weak primary electron current by thermionic emission.

- An Vequally small additional power input is necessary to supply the voltage to the thermionic electrons to give them the energy requiredk to produce very high secondary-electron-emission when they strike any one of the multiplier elements earlier described. The auxiliary cathode can be used in diodes, triodes, many grid tubes, thyratrons, and the like.

Figure shows this device applied in a triode having an envelope 35 suitably evacuated and containing an anode 35, a grid 31,` a cathode 38 and an auxiliary cathode 39 in the form of 'a mesh.

In electron multipliers, mesh secondary-electron-emitters have been developed by G. Weiss, U. S. Patent No. 2,243,178 dated May 27, 1941. Figure 5 is a diagrammatic View of the arrangement of tube elements in a triode with cylindrical electrodes. Figure 6 gives a schematic view of the cathode and auxiliary cathode. It will be evident that the auxiliary cathode 39 is supported at 4l, and the cathode 38' is supported at 40. Other elements of the tube .may take on any of their Well known forms.

It will be evident that secondary-electronemitters with high secondary-emission in combination with dielectrics and metallic bases may be used to advantage in other ways well known in the art.

The auxiliary cathode may be used to overcome certain disadvantages associated with the use of thermioniccathodes, such as back-firing, grid emission and ignition of local spots. -In standard tubes with oxide-coated cathodes, some of the cathode material, such as barium, evaporates and settles on the grid. This contaminates the grid and gives rise to grid emission of electrons, disrupting the normal functioning of the tube.

Backring is conduction in the reverse direction (plate to cathode) when the plate potential is negative with respect to cathode. It is especially harmful in rectiiiers. Backring is due to a deposit of electron-emitting substance on the plate. This cannot occur in the present invention since the secondary-electron-ernitter does not evaporate at the operating temperature of the tube. However, in a very high frequency oscillator the plate may have a coating of secondary-electron-emission material to enhance the emciency of the tube. Local hot spots are due to uneven heating of the oxide coated cathode. They cause increased local evaporation and deterioration of the cathode. The cathode material used in my invention does not evaporate at the operating temperature of the tube, thus eliminating both difficulties.

The prior art, in seeking to overcome the difculties, produced remedies not generally applicable. See Ct. Jobst, U. S. Patent 1,964,517, dated June 26, 1934; lW. Espe, U. S. Patent 2,125,- 105, dated July 26, 1938; and H. Kolligs, U.'S. Patent No. 2,147,447, dated February 14,1939. These prior art devices do not make use of auxiliary cathodes coated with suitable emitters of secondary-electrons to eliminate all these difficulties. f

As a result of investigation, the necessary features for a secondary-electron-emitter which will function eiciently as an auxiliary cathode have been determined. The following remarks apply particularly to eliminating grid contamination in high power triodes, but they apply equally well/to eliminating this and other difficulties in various types of electron tubes. Oxide coated cathodes, now in vogue in high powered triodes. are used because they require the least energy input per unit thermionic output at saturation. These oxide coatedA cathodes cause grid contamination.

In accordance with the present invention, the

primary thermionic current-need be only a small fraction of the total cathode emission, the main part being supplied by secondary electrons from the auxiliary cathode.v

Hence the oxide coated cathode need .not be used, and instead the cathode of the invention.

will desirably be uncoated, made of tungsten, thoriated tungsten or other material which will not evaporate at the operating temperature of the tube. The auxiliary cathode will have an outer layer of beryllium or other material as set forth above, which will have high secondary electron emission, but will not be effected either by melting or evaporating, at the operating temperature of the tube. To prepare the mesh described in Figures 5 and 6, the metallic base of the secondary cathode will best be aluminum or equivalent as explained above.

In View of my invention and disclosure Variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benets of my invention without copying the structure shown, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. The process of producing an improved multiplier element for an electron tube, which comprises depositing on a solid metallic base a dielectric layer limited in thickness to values below 0.1 mm. and selected from the class consisting of oxides of aluminum, titanium, copper and manganese, depositing on the dielectric layer a secondary electron emitting layer which has a combined thickness with the dielectric layer of 2 to 20 microns, free from nickel, different from the dielectric layer, and selected from the group consisting of beryllium, beryllium oxide, magnesium oxide and aluminum, and heat treating the multiplier element at a temperature between 600 C. and the softening point of the metallic base.

2. The process according to claim 1, in which the dielectric layer is porous.

JENNY BRAMLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,075,377 Varian Mar. 30, 1937 2,175,696 Lederer Oct. 10, 1939 2,189,972 Warnecke, et al. Feb. 13, 1940 2,192,418 Sommer Mar. 5, 1940 2,198,329 Bruining, et al Apr. 23, 1940

Claims (1)

1. THE PROCESS OF PRODUCING AN IMPROVED MULTIPLIER ELEMENT FOR AN ELECTRON TUBE, WHICH COMPRISES DEPOSITING ON A SOLID METALLIC BASE A DIELECTRIC LAYER LIMITED IN THICKNESS TO VALUES BELOW 0.1 MM. AND SELECTED FROM THE CLASS CONSISTING OF OXIDES OF ALUMINUM, TITANIUM, COPPER CONSISTING MANGANESE, DEPOSITING ON THE DIELECTRIC LAYER A SECONDARY ELECTRON EMITTING LAYER WHICH HAS A COMBINED THICKNESS WITH THE DIELECTRIC LAYER OF 2 TO 20 MICRONS, FREE FROM NICKEL, DIFFERENT FROM THE DIELECTRIC LAYER, AND SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM, BERYLLIUM OXIDE, MAGNESIUM OXIDE AND ALUMINUM, AND HEAT TREATING THE MULTIPLIER ELEMENT AT A TEMPERATURE BETWEEN 600* C. AND THE SOFTENING POINT OF THE METALLIC BASE.

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