US2753615A - Emissive electrode for electric discharge apparatus - Google Patents

Description

July 10, 1956 CLAUDE ETAL EMISSIVE ELECTRODE FOR ELECTRIC DISCHARGE APPARATUS Filed June 6, 1952 INVENTORS ANDRE CLAUDE &

PIERRE LEMMGRE VOREAUX ATTORNEYS EMISSIVE ELECTRQDE FOR ELECTRIC DISCHARGE APPARATUS Andr Claude and Pierre Lemaigre-Voreaux, Paris,

France, assignors to Societe Anonyme pour les Applications de lElec-tricite et des Gaz Rares -Etablissernents Claude-Paz & Silva, Paris, France Application June 6, 1952, Serial No. 292,216

Claims priority, application France June 8, 1951 3 Claims. (Cl. 29-2517) This invention relates to a method of preparing an emissive electrode for electric discharge apparatus, said electrode comprising a support and a coating of emissive materials containing at least one alkaline-earth metal oxide on said support. The main drawback of electrodes comprising such a coating is the production of dark stains on the envelope of the discharge apparatus in which they are mounted, especially when, in order to lengthen the life of these electrodes, they are provided with a relatively thick emisive coating.

One object of the present invention is to provide an electrode which has no or little tendency towards staining the envelope of the electric discharge apparatus in which it is mounted.

Another object of the present invention is to provide an electrode which may, Without any serious drawback, be left for some time in the open air before being mounted in an electric discharge apparatus and being activated therein.

One feature of the present invention is a coating of poorly emissive refractory oxide or oxides, said coating being pervious to the discharge and being applied on the layer of alkaline-earth metal compounds which will be changed into the coating containing at least one alkalineearth metal oxide, prior to the formation of the electric discharge apparatus in which the electrode is to be mounted.

The formation of an electric discharge apparatus is the process which outgases the envelope and the electrodes of the discharge apparatus and charges, or finishes changing, the compounds of which use is made to yield the emissive materials, into a material comprising mainly one or more alkaline-earth oxides, each including at most one oxygen atom per alkaline-earth metal atom, and comprising probably also some metallic alkaline-earth metal.

The coating of refractory oxide may be elfected, for instance on these alkaline-earth compounds arranged on that part, or parts, of the electrode which will be the support of the emissive materials, or on the substances generated by treating said compounds, these substances having not undergone the formation process and comprising mainly, for instance, monoxide of the alkalineearth metal or metals the compounds of which have been used.

As refractory and poorly emissive oxides, one or several oxides may be used, for example those of magnesium or beryllium, or, preferably, aluminium. These oxides are used in the form of fine grains, for instance. These grains may be mixed with a binder which is not vaporized during manufacture of the electrode; the coating thus obtained is punctured by discharges during manufacture, which allows subsequent discharges an easier passage.

Other objects and features of the present invention will in part be obvious, and will in part be explained hereinafter.

In the drawing, which illustrates embodiments of the invention,

States Patent Figure 1 is a partly sectioned elevation of a hollow electrode, the support therefor and part of the stem of discharge apparatus to which said electrode is secured.

Figure 2 is a side View of another electrode, in which the support for the emissive materials is a helically wound filament.

The electrode shown in Figure 1 has a molybdenum wall, 0.1 mm. thick formed as a cylinder 1 and a bottom 2. The cylinder 1 is welded to a nickel wire 4 (0.8 mm. in diameter), which supports the electrode and supplies it with current. As is usual for the electrodes of electric discharge apparatus, the wire 4 is sealed tightly inside a glass stem 6. The molybdenum, preferably, has been previously heated at about 1200 C., in a hydrogen at mospheres which, amongst other things, eliminates all traces of oxidation.

The wall of the electrode is coated, on its inner face, with a relatively thick layer 3, of emissive materials for which said wall constitutes a support. This layer is obtained as follows. A mixture of powdered barium dioxide and metallic tantalum powder is: introduced in the cavity of the wall, said mixture having possibly been left exposed to humid air for one day. The electrode, atttached to its stem 6, is then placed in a chamber through which is passed a mixture of nitrogen and hydrogen; then, by means of a high frequency magnetic field, the wall of the electrode is raised to a temperature higher than red heat for a sufiicient duration to melt the barium peroxide and cause it to react with the molybdenum and the tantalum.

The inside of the electrode is, by this reaction, coated with an adherent crust, blackish and with a granite like surface, which will yield the emissive material of the electrode. The electrode is left to cool for about ten seconds inside the chamber, then it is removed therefrom.

Then the whole electrode, on both its inner and outer faces, is covered with a layer of coating of alumina, '5, very thin but without any gaps. The layer of alumina on the outer face serves to avoid the formation of cathode spots thereon. coated With a layer of alumina. This coating is effected, for instance, by the projection, with a spray gun, of a suspension of alumina in ethyl acetate. The alumina, being in the form of very fine grains, forms a layer 5 pervious to the discharge, i. e. the discharge goes through it without any measurable additional voltage drop.

It was found that the presence of an alumina layer.

on the layer of emissive materials considerably reduces the tendency of the electrode to cause stains on the envelope of the discharge apparatus inside which it is mounted. Lamps provided with electrodes thus manufactured, and subjected each to several hundred thou sands of instantaneous startings or to continuous operation of several thousands of hours do not show any stain except for a very slight fogging which appears during the formation of the electrodes and does not increase afterwards.

Further, the alumina layer protects, to a certain extent, the electrode against the action of the atmosphere during the manufacturing operations; an electrode such as described above may be left for some time in the open air without any appreciable change in its properties.

Alumina may be replaced by another poorly emissive refractory oxide such as magnesium or beryllium oxide; a mixture of these refractory oxides may also be used.

The superficial deposition of alumina or other poorly emissive refractory oxide also gives good results with emissive materials obtained in a manner different from that described above for instance with alkaline-earth oxides resulting from the decomposition of substances such as barium, strontium, calcium hydrates or carbonates.

The current lead-in wire 4 is also In another form of electrode the layer of coating of poorly emissive refractory oxides is formed, not on a layer of substances which have undergone part of the treatments which give rise to emissive materials, but on a layer of the compounds of which use is made to yield the emissive materials; this layer, arranged on the part of electrodes which will support the emissive materials, must not be too strongly stirred during the transformation of this layer of compounds into a layer of emissive materials. For example, the layer may be applied on barium hydroxide hydrate. In this form, shown in Figure 2, a thin layer without any gaps, 5, of magnesia is applied to an electrode 7 consisting of a helically wound tungsten filament supporting a barium hydroxide hydrate deposit. trode 7 is secured to a stem 9 of a discharge apparatus not shown by lead-in wires 10 and 11. The layer 5, formed for instance by dipping the electrode into an alcoholic suspension of calcined magnesia, is made after the melting of the hydrate on the supporting helix but previous to the introduction of the electrode in the envelope of the discharge apparatus in which it is to be attached and to the treatment which changes the hydrate into emissive materials. A coating of alumina instead of magnesia would also be quite satisfactory.

The surface deposition of poorly emissive refractory oxides also gives good results with emissive mixtures comprising a certain proportion of poorly emissive substances.

The electrodes of Figures 1 and 2, after having been prepared as described above, are mounted in envelopes of electric discharge apparatus and subjected to various treatments, including the formation of said apparatus.

In the case of electrodes such as the one shown on Figure 1, the apparatus in which are mounted two such electrodes is, for example, degassed by heating while it is pumped, the electrodes being heated by a high-frequency magnetic field, then rare gases and a drop of mercury are introduced in the discharge apparatus, and thereafter the latter is operated for a few minutes on a suitable source of current. These treatments constitute the formation of the discharge apparatus; they are conventional and, in the present case, they have the further advantage that the operation of the apparatus renders the coating of refractory oxide still more pervious to the discharges of the normal working.

In the case of filamentary electrodes such as the one shown on Figure 2, two electrodes are mounted in an envelope of electric discharge apparatus and heated gradually in vacuo, the discharge apparatus being heated in order to degas it. Rare gases and mercury are thereafter introduced in the discharge apparatus and the latter is then operated for a few minutes.

The elec- It will be seen from the foregoing that we have provided improved emissive electrodes for electric discharge apparatus. These eletrodes have little or no tendency towards staining the walls of electric discharge apparatus and they may be kept in the open air for some time, at least when they are not yet quite activated.

Although preferred embodiments of the present invention have been described, it will be understood that modifications may be made within the spirit and scope of the appended claims.

What we claim is:

l. The method of preparing an emissive electrode for gaseous electric discharge apparatus, said electrode comprising a support and a coating of emissive materials containing at least one alkaline-earth metal oxide on the support, which includes the steps of applying to said support a layer comprising at least one alkaline-earth metal compound, thereafter applying on said layer a coating of poorly emissive refractory oxide, said coating being pervious to the discharge, and mounting the electrode in an electric discharge apparatus, and subsequently forming said electric discharge apparatus, whereby said layer is changed into the coating of emissive materials.

2. The method as claimed in claim 1, wherein the formation of the electric discharge apparatus comprises issuing discharges from said electrode in order to puncture the coating of poorly emissive refractory oxide.

3. The method of preparing an emissive electrode for gaseous electric discharge apparatus, said electrode comprising a support and a coating of emissive materials containing at least one alkaline-earth metal oxide on the support, which includes the steps of applying to said support a layer comprising at least one alkaline-earth metal compound, heating said compound at a temperature sufficiently high for modifying the chemical composition thereof, thereafter applying on said layer a coating of poorly emissive refractory oxide, said coating being pervious to the discharge, and mounting the electrode in an electric discharge apparatus, and subsequently forming said electric discharge apparatus, whereby said layer is changed into the coating of emissive materials.

References Cited in the file of this patent UNITED STATES PATENTS 2,142,331 Prescott Jan. 3, 1939 2,249,672 Spanner July 15, 1941 2,317,754 Gorlich Apr. 27, 1943

Claims (1)

1. THE METHOD OF PREPARING AN EMISSIVE ELECTRODE FOR GASEOUS ELECTRIC DISCHARGE APPARATUS, SAID ELECTRODE COMPRISING A SUPPORT AND A COATING OF EMISSIVE MATERIALS CONTAINING AT LEAST ONE ALKALINE-EARTH METAL OXIDE ON THE SUPPORT, WHICH INCLUDES THE STEPS OF APPLYING TO SAID SUPPORT A LAYER COMPRISING AT LEAST ONE ALKALINE-EARTH METAL COMPOUND, THEREAFTER APPLYING ON SAID LAYER A COATING OF

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