US2171227A - Secondary electron emitter and method of making it - Google Patents

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Aug. 29, 1939. w. J. H. SCHREINEMACHERS 2,171,227

SECONDARY ELECTRON EMITTER AND METHOD OF MAKING IT Filed July 2'7, 1938 1 UNI/EA TED ELECTRODE 0F Sl/EE T METAL BUR/V MAGNESIUM 2 IN Al? 70 PRODUCE CLOUD 0F H07 PARTICLES 0F MAGNESIUM OXIDE HOLD COLD ELECTRODE IN (Z0110 ABOVE BURNING MAGNESIUM 3 (INT/l SEMI-TRAMSPARENTLAYER 0F MJGNES/UM OXIDE ['ONDENSES' 0N EL EZ'TRODE SURFACE INVENTOR. WI EM J.I'I.SCHREIIVEMACHERS arm;

ATTORNEY.

Patented Aug. 29, 1939 PATENT OFFICE SECONDARY ELECTRON EMITTEB AND METHOD OF MAKING I'l.

Willem J. H. Schreinemachers,

Elndhoven,

Netherlands, assignor to N. V. Philips Gloeilampenfabrieken, Eindhoven,

Netherlands, a

corporation of Netherlands Application July 27,1938, Serial No. 221,565

In the Netherlands August 9, 1937 3 Claims.

lviy invention relates to a method of manufacturing secondary electron emitting electrodes and, to improved secondary electron emitters made in accordance with my method.

' Secondary electron emitters consisting of a sheet of metal with a layer of oxide of an active metal, such as alkaline earth metal or magnesium, have been made usually in the evacuated vessel in which they are to be used, the layer of oxide being, produced by vaporizing the active metal in vacuum to form a film of active metal on the core body or electrode, after which the film of active metal is oxidized to produce on the core body a comparatively smooth dense layer of i5 oxide. This procedure is rather difficult and I complicated, and produces emitters having a ratio of three or four secondary electrons per primary at the voltages commonly used.

The object of my invention is to provide an improved secondary electron emitter which can be made without resorting to an evacuated vessel and can be exposed to the air without damage so that the finished emitter may be easily mounted in a tube. Another object of my invention is to provide a simple method 01 manufacturing such an emitter without the use of an evacuated vessel or of the arrangements commonly used for obtaining the active metal in the form of vapor.

In accordance with my invention a secondary electron emitter electrode having high electron emissivity is made by bringing into contact with a core body of metal, such as nickel, copper, or similar metal, a cloud of oxide of an active metal, such as an alkaline earth metal or magnesium,

which is burned in an oxidizing atmosphere or gas, such as air, to produce a cloud of oxide from which the oxide particles settle on and adhere to the core body to form a coating. By this method it is possible to coat a metal electrode in a very simple manner with a material which readily emits secondary electrons without any need of utilizing complicated processes which must be carried out within an evacuated vessel.

In the accompanying'drawing Figure 1 shows 5 steps to be taken in accordance with the invention, and Figure 2 shows in greatly enlarged cross-section a portion of an electrode made in accordance with the invention.

In Figure 1 the rectangles numbered I, 2, and

50 3 represent successive steps by which an electrode can be produced in accordance with my invention.

Figure 2 shows the finished electrode comprising a metal core body 4, preferably of sheet nickel,

on the surface of which is a thin semi-trans- E par n layer 5 a g um f me or oxide condensed on the surface of the electrode from the smoke or fumes of magnesium burning in air, and preferably while the electrode is colder than the particles of oxide.

The cloud of oxide of the active metal is most 5 conveniently produced by burning a metal, such as magnesium, either in air or an oxidizing gas, in the vicinity of the metal core body so that the core body is exposed to the cloud of oxide under such conditions and for such a length of time 10 that a magnesium fume layer of the oxide from the cloud will form on the surface of the core body. The metal core body is preferably near room temperature and the burning magnesium or other active metal is preferably so placed 15 in relatlon'to the core body that the oxide forming the cloud to which the core body is exposed is hotter than the core body so that hot particles of oxide from the cloud'deposit on the cold metal core body. In this way a somewhat more ad- 20 herent coating is obtained than when the cloud of oxide and the core body are at substantially the same temperature. The coating may conveniently be done by burning a strip of magnesium in air while the core body, for .example, of 25 sheet nickel, is held above the burning magnesium and in the cloud of smoke or oxide. It is advantageous to burn the magnesium in a gas flame above which the sheet nickel electrode is held, so the smoke of the burning magnesium is 30 carried to and deposited on the nickel sheet. The layer of oxide formed from the cloud on the core body should be thin enough to be semi-trans- .parent so as to permit the nickel to be visible through the layer. The surface of the core body 35 may have a slightly dull or flufly appearance, which is probably due to a fluify porous coating of oxide. The secondary electron emitting electrode produced in this way can be handled in the open air and can be mounted without any 0 special precautions and has a higher coeflicient of secondary electron emission than oxide coated electrodes produced in the conventional way. For example, the ratio of secondary electrons to primary electrons may be as high as six to one or more dependent on the thickness of the layer as compared to about four to one obtained at the same voltage from the usual emitters coated with magnesium oxide. It is probable that the difference in secondary electron emissivity arises from the difference in physical characteristics between the rough, porous coating produced by depositing very fine particles of oxide, such as magnesium oxide, from a cloud obtained by buming the magnesium and the comparatively smooth dense coating of magnesium oxide obtained in other'ways such as by oxidizing a film of magnesium on a metal core body. The chemical composition of the magnesium oxide layer on my improved electrode seems to be substantially the same as that of the magnesium oxide layer on electrodes heretofore known, but the physical characteristics are quite difierent and, as a result, the secondary electron characteristics are markedly diii'erent and better than those of the magnesium oxide coated electrodes heretofore known.

A secondary emitting electrode made in accordance with my invention may be used to advantage in electron amplifiers of the conventional type and with marked advantage as secondary electron emitters in secondary emission amplifying tubes such as disclosed in United States patents to Van Overbeek No. 2,146,607, issued February 7, 1939, and to Lopp et al. No. 2,151,783, issued March 28, 1939.

t I claim: v

1. The method of making secondary electron emitting electrodes having high secondary electron emissivity, which comprises producing a cloud of oxide of an active metal selected from the alkaline earth metals and magnesium by burning the active metal in an oxidizing atmosphere, and exposing a metal core body to said cloud of oxide to form on said core body a layer of oxide from said cloud.

2. The method of making a secondary electron emitting electrode having high secondary electron emissivity, which comprises producing a hot cloud of fine particles of magnesium oxide by burning magnesium in air, and exposing to said hot cloud of oxide particles a cold sheet of nickel to condense from said cloud on said sheet a rough fluffy porous layer composed of very fine particles of oxide.

3. A secondary electron emitting electrode comprising a sheet metal core body, and a fluffy, porous, semi-transparent layer of very fine magnesium oxide on the surface of said core body.

W. J. H. SC HREINEMACHERS.

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