US2820920A - Manufacture of coated electrodes - Google Patents

Description

Jan. 21, 1958 R. PENON MANUFACTURE OF COATED ELECTRODES Filed Sept. 17, 1952 INVENTOR. mews PEA/0N 292 v flfTflE/WFX MANUFACTURE OF COATED ELECTRODES Ren Penon, Saint-Cloud, France, assignor to Etahlissemerits Claude, Paz et Silva, Paris, France, a corporation of France Application September 17, 1952, Serial No. 310,115

2 Claims. (Cl. 313-344) This invention relates to the manufacture of electrodes coated with electron-emission material, and particularly such material formed mainly of barium compounds.

The principal object of my invention, generally considered, is to manufacture electrodes comprising a base of refractory metal, such as tantalum, molybdenum, or tungsten, coated with a mixture of barium compounds, such as the dioxide and the hydrated hydroxide.

Another object of my invention is to produce electrodes coated with electron-emission material and which are less sensitive than previously-produced electrodes to variations in the manufacture and utilization thereof.

A further object of my invention is to produce electrodes so coated that they are capable of a large number of startings without failure, and may be employed in discharge devices and operated with low starting voltage.

A still further object of my invention is to coat electrodes with mixtures of barium compounds, which on heating react in a relatively quiet manner, thereby avoiding undesired loss of material from said electrodes.

Other objects and advantages will become apparent as the description proceeds.

For a better understanding of the invention reference should be had to the accompanying drawing wherein:

Fig. 1 illustrates an electrode for a fluorescent lamp, which electrode carries the emission coating of this invention;

Fig. 2 is a fragmentary enlargement of a helical-type electrode coil which carries the emission coating of this invention;

1 Fig. 3 is a'fragmentary enlargement of a coil on a mandrel which'is further coiled, which coil carries the emission coating of this invention, and which is fully shown in Fig. 1;

Fig. 4 is a fragmentary enlargement of a coiled-coil type electrode which carries the emission coating of this invention.

The present process is characterized by the use of an emissive coating obtained from a mixture of barium dioxide and hydrated barium hydroxide, with or without other materials such as barium carbonate, carried on a support or base, such as a wire or coil of metallic tantalum, molybdenum, tungsten, or combinations of such metals, said electrodes being produced by heating the selected mixture in contact with the support for the coating, to thereby manufacture electrodes suitable for electric-discharge devices. The mixture is desirably used in powder form, with or without a binder, in contact with the support, and the support is heated so as to cause at least partial melting of the mixture.

Electrodes obtained, in accordance with my invention, compare with electrodes ordinarily used in a most favorable manner and offer marked advantages, such as less sensitivity to variations in the manufacture and utilization thereof, for example, as excessive current due to use of too high a voltage, in which case there is less tendency 2,820,920 Patented Jan. 21, 1958 ice during life to alteration of the appearance at the ends of the lamps, in which they are mounted, and particularly when such electrodes are not preheated before starting. Also, a very large number of startings, and a relatively low voltage, may be employed.

When using the process, according to my invention, the reaction on heating the mixture is relatively quiet, where by, as illustrated in Fig. 1, the substance 10 obtained then remains on the support 12 on which it has been produced, even if this support does not form an almost closed cavity into which the mixture has been introduced before heating. This effect is due to the hydrated barium hydroxide content. If the mixture did not contain any of this compound, the reaction would be much more lively, especially in the presence of tantalum or tungsten, and a small part only of the substance produced would remain on the support, if said support were, for instance, a helix of metal wire 14 with turns not touching and open at both ends, as shown in Fig. 2.

A process which consists in dipping a support for an emissive coating into a molten mixture of hydrated barium hydroxide and barium dioxide is much more difiicult to effect in practice than the process according to the present invention. In particular, if the dipping is effected in the open air, there is formed a troublesome crust of barium carbonate on the molten mixture, whereas if the operation is carried out in an inert atmosphere, it requires the use of complicated apparatus and special handling. In any case, the composition of the mixture is modified in time due to decomposition of the hydrate and, when operating in open air, due to the gradual and uncontrollable production of barium dioxide.

The following examples, which are to be considered as non limiting illustrations, show more clearly the process according to the invention. They indicate two methods for obtaining electrodes for fluorescent lamps or the like.

The first example concerns the manufacture of an electrode for a lamp which is not started by preheating, such as those which are sold commercially under the name Slimline. In this case, as illustrated in Figs. 1 and 3, the support 12 for the emissive layer 10 is formed, for instance, by a molybdenum filament 16 about 0.1 mm. in diameter, on which is helically wound another molybdenum filament 18 about .05 mm. in diameter and which is, in turn, wound as a helix 28 with spaced turns and which has an inner diameter of about 1.5 mm. Such a support is, as is known, mounted on two current leads of a glass stem for a fluorescent lamp. It is dipped into a powder mixture, the composition of which by weight is about as follows:

Percent Barium dioxide, BaO 42 Powdered tantalum, Ta 27 Monohydrated barium hydroxide, Ba(OH) 'H O 25 Barium carbonate, BaCO 6 By rotary motion of the mixture wtih respect to the support 12, the interior of the latter is filled with a small amount of the mixture. Such an operating method makes it possible to introduce very constant amounts of the mixture into similar supports.

The support 12 is then removed from the mixture and the outer surface of the former may, without detriment, retain small amounts of the mixture, as compared with what has been introduced inside the support. Such a supporting helix, about 6 mm. long, retains a total of about 30 mg. of the mixture.

A support t rus coated is connected by its leads to a suitable source of electric current and, assuming a molybdenum support, thereby heated to a dull red. At this temperature the mixture coating the winding melts, at least partly, and some of its constituents react on one another. There is thus obtained on the support 12 a very adherent and suitably distributed layer 10.

The above operations of coating and-meltingare'desirably efi'ected While protected from carbon dioxide and moisture as, for example, in aremovable glass bell jar in which a stream of drynitrogen is circulating.

An electrode thus obtained is secured at one end of a tubular envelope for a fluorescent lamp, for example, by sealing to such a tube a stern carrying the electrode lead-in wires. Then, similarly, another electrode is secured to the other end of the tube. The tube, thus provided with an electrode at each end, is then subjected to known degassing, forming, and filling operations.

Fluorescent lamps with a length of about 2.30'rneters between electrodes, and provided with electrodes thus manufactured in accordance with my invention, have each been subjected to more than 90,000 startings without any preheating of their electrodes, and without'faiiure. Electrodes as usually constructed, with an emissive coating obtained from alkaline earth carbonates, can withstand only an average of about 10,000 startingsunder the'same con" ditions.

In the above mixture, the amount of water given by the hydration of the barium hydroxide is about 3.6% of the total weight of the hydrate and the dioxide. It was found that in case the weight of the barium dioxide in the mixture exceeds two thirds of the total weight of the mixture, the ratio of the water contributed by the hydration of the hydroxide to the total weight should generally be between 2% and 12%. Under such circumstances, the melting does not require a high temperature, giving rise to an excessive oxidation of the support, and the result is a deposit suitably distributed and easy to treat. It has also been found in the same case, that the additions of the metallic tantalum powder, and possibly barium carbonate, generally have a favorable effect on the melting. V

In accordance with a second example and asshown in'Fig. 4, the support 22 for the emissive coating 10 may be a coiled coil of tungsten wire about 0.09 mm. in diameter, in which the inner diameter of the primary coil 24 is about 0.2 mm. and that of the secondary coil 26 about 0.65 mm. The powder mixture with which this support is provided, desirably has the following composition, by weight:

. Percent Barium dioxide, Ba 20 Octohydrate of barium hydroxide, Ba(OH) .8H O 80 The operating process of coating, heating, etc., may be the same as described for the first example. The electrodes thus obtained are particularly well'suited for discharge apparatus where starting is obtained by preheating the electrodes.

in the mixture of this second example, the amount of water contributed by the hydration of the barium hydroxide amounts to 37% of the total weight of the'hydrate It was found that in the case where, in the mixture, the weight of barium dioxide is between 10% and 40% of the total weight, the ratio of the. water contributed by hydration of the hydroxide, to the total weight, should be generally between. 27% and 40% by weight to have on melting the elfects indicated above in connection with the first example.

The heating of the support, for example, at least initially, may be efiected when the support is dipped into the selected powder mixture. The coating of this support with the mixture may be effected in a different manner, for instance by dipping into a. liquid suspension of; the mixture or by painting. 7

The composition of the mixture may vary within very wide limits and the mixture may comprise constitutents' other than those indicated,,for instance, a binder or 1110 lybdenum. The support may also be a helix of another. shape, or may even not be a helix and assume, for instance, the shape of a small container of thin metalrfoil.

Although specific embodiments have'been disclosed, it will be understood that modifications maybe made with-;

in the spirit and scope of the invention.

I claim:

1. An electrode for electric discharge lamps comprising a molybdenum filament wound into a helix and carrying within its coils a mixturev of about 42% of barium dioxide, about 27% of metallic tantalumpowder, about 25% of mono hydrated barium hydroxide and about 6% of barium carbonate.

2. An electrode for electric discharge lamps comprising a coiled coil tungsten filament carrying within its turns a mixture of about 20% barium. dioxide'andabout of octohydrated barium hydroxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,180,988 Lemmers Not/121', Z1939 2,249,672 Spanner July 15,1941. 2,446,157 Lowry July 27, 1948 2,459,841 Rouse Jan. 25, 1949 2,492,142 Germeshausen Dec. 27, 1949

Claims (1)

1. AN ELECTRODE FOR ELECTRIC DISCHARGE LAMPS COMPRISING A MOLYBDENUM FILAMENT WOUND INTO A HELIX AND CARRYING WITHIN ITS COILS A MIXTURE OF ABOUT 42% OF BARIUM DIOXIDE, ABOUT 27% OF METALLIC TANTALUM POWDER, ABOUT 25% OF MONO HYDRATED BARIUM HYDROXIDE AND ABOUT 6% OF BARIUM CARBONATE.

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