US2917650A - Electrode for discharge tubes - Google Patents

Description

Dec.

A. 5. DE CAUMONT ELECTRODE FOR DISCHARGE TUBES Filed June 29, 1955 ATTOEA/EYS.

ELECTRODE FOR DISCHARGE TUBES Aibert Sarda de Caumont, Paris, France, assignor to Societe Anonyrne Hyperion, Paris, France, a corporation of France This invention relates to discharge tubes and is particularly directed to an improved electrode construction for discharge tubes and a method for fabricating such electrodes.

It is known that so-called oxide electrodes of discharge tubes will, after a certain number of operating hours, cause the formation of spots on the adjacent wall areas of the tube walls. These spots differ in shape and are due to many different causes of both a physical and chemical nature. One result of these spots is a decrease in the length of the luminous portion of the tube, causing a corresponding decrease in the efiectiveness of the tube.

In the past, various solutions have been proposed for remedying this defect. For example, it has been proposed to enclose the electrodes in small cups with suitable openings. Such constructions have not, however, provided the beneficial results which were expected.

The principal object of the present invention is to provide an improved electrode structure, particularly advantageous with oxide-type electrodes, in which the formation of spots on the tube walls adjacent to the electrodes is substantially eliminated. Lamps constructed in accord ance with this invention have burned several thousand hours without the formation of any spots, a result which has never previously been obtained.

This invention is predicated upon the empirical discovery and determination that such spots are substantially eliminated by providing an electrode construction in which a coated and specially heat-treated filament is surrounded by a generally cylindrical cup having modified end configurations. More specifically, one preferred form of electrode constructed in accordance with the present invention comprises a coiled filament surrounded by a cup of generally cylindrical configuration. The cup is provided with a small opening in its inner end disposed in alignment with the filament. This inner opening, however, does not reside in a plane with the end of the cup but, rather, is formed in an end wall displaced slightly toward he center of the cup. The outer end of the cup is also configurated to form an opening; the diameter of this opening is slightly smaller than the diameter of the cup due to a narrow, inwardly turned rim formed on the outer end of the cup.

in fabricating the present electrode, it has been found advantageous to coat the filament with an active substance including ground anhydrous baryte, ground strontium hydroxide, and slaked lime. These materials are mixed in the volumetric proportion of baryte3 parts, strontium hydroxide-2 parts, and slaked lime1 part. This coating may be applied by either dipping or spraying Afterwards, the wire is assembled in the tube with the cup in place. The tube is then inserted in a furnace, and a current is passed through the filament while a vacuum is drawn. red, and the furnace temperature is raised to 450 C. Next, the furnace temperature is lowered, after which the filament current is stopped and the vacuum is removed.

The filament is gradually brought to a cherry Patent Finally, after the tube has cooled, it is filled with argon gas.

The various advantages of the present invention will be more readily apparent from a consideration of the following detailed description of the drawings illustrating a preferred embodiment of the invention.

In the drawings:

Fig. 1 is a longitudinal, cross sectional view through one end of a conventional discharge tube; and

Fig. 2 is a cross sectional view similar to Fig. l of a tube constructed in accordance with the present invention.

In Fig. 2, the parts corresponding to elements of the construction in Fig. l are given the same sumber as those elements, followed by the letter a. As shown in Fig. 2, the gaseous discharge lamp includes a cylindrical Wall 1a formed of glass. The end wall of the tube is provided with an inwardly extending section 2a which seals the end of the tube surrounding lead-in wires 3a. These wires are connected across electrode filaments 4a.

In the preferred embodiment, each filament comprises nine windings, or helical turns. Each filament is formed by winding a helix of molybdenum or other high temperature wire of .08 mm. diameter on a core 6.5 mm. in diameter. The core is removed and a second molybdenum wire, or support wire, of .3 mm. diameter is threaded through the helix. The support wire and the surrounding helical wire are then helically wound about a core of 8.5 mm. diameter so that nine complete turns are formed. Two such helical elements, connected in series relationship, are secured to the ends of the lead-in wires 3a. A post 6a is embedded in wall section 2a and extends inwardly into the tube. The post carries a cup 7a which surrounds filaments 4a. Cup 7a is provided with inner end wall 8a which is dished inwardly. A small opening 9a is formed in the center portion of this wall in alignment with the filaments 4a and spaced inwardly from the end of the cap. The outer end of the cap is provided with an inwardly turned rim 10a which defines an opening only slightly smaller than the inner diameter of the cap.

A comparison of the construction shown in Fig. 2 with the prior construction shown in Fig. 1 will show that it differs in many respects. These construction differences, together with the heat treatment and electrode coating described below, result in the elimination of spots during thousands of hours of operation of the new lamp, a result which was not achieved by the construction shown in Fig. 1. Among the differences which are present is the double wire coiled filament 4a which is used in place of the conventional filament 4 shown in Fig. 1. Additionally, the cup configuration in the new embodiment differs in that wall 8a is dished slightly so that opening 9a is shifted from the end of the cap closer to the filament. Also,'the outer end of cap 7a is provided with an inwardly extending rim 10a in contrast to the straight, cylindrical wall of cap 7. A still further difference resides in the elimination of porcelain insulators 5 previously provided on lead-in wires 3.

A second aspect of the present invention resides in the coating and heat treatment of the electrodes. The filament wires are preferably formed from a high temperature metal, such as molybdenum, Wolfram, a ferro-nickel alloy, or the like. These wires are coated by dipping or spraying with an electro-emissive coating including anhydrous baryte and strontium hydroxide which have been ground for twenty-four hours, and slaked lime. These materials are mixed in a volume ratio of 3 parts baryte to 2 parts strontium to 1 part lime. The weight ratios of these components are 235 parts baryte to 65 parts strontium hydroxide to 25 parts lime. After mixing, the coating material is hydrated in the open air and then ground for thirty hours.

After the electrode has been coated with this material, it is assembled with its surrounding cup 7a and mounted on the interior of tube 1a. The tube is then placed in a furnace, the interior of which is adapted to be evacuated by two pumps, a primary pump adapted to impose a moderate vacuum and a diffusion pump adapted to create a high vacuum. The two lead-in wires 3a are connected across a source of current. The furnace temperature is gradually increased to 220 C., while the primary pump is drawing a moderate vacuum. Simultaneous with the rise in furnace temperature, the current passing through filaments 4a is very slowly increased from to 4 amperes.

After the furnace temperature has reached 220 C., the second step in the process is performed; this step involves holding the filament current constant while increasing the furnace temperature to 300 C. where it is held until the tube temperature reaches that point. In the third step, the furnace temperature is suddenly increased to 450 C., and the filament current is raised to 6 amperes. The diffusion pump is started to create a high vacuum for removing substantially all gases from the tube. The evacuation period continues until the tube temperature reaches 450 C. In the next step, the cooling of the tube is commenced by dropping the furnace temperature to 300 C. However, the filament current is maintained at 6 amperes and the diffusion pump is operated to maintain a high vacuum. As soon as the furnace temperature reaches 300 C., the electric current is cut off, but the high vacuum is still maintained. Finally, when the furnace temperature drops to 180 C., the vacuum is removed and the tube permitted to cool to room temperature; after which it is filled with argon to a pressure of 1.9 mm.

During the heat treating process just described, the salts contained in the coating material are reduced to their corresponding metals which become alloyed with the wire and ultimately constitute the emitters when the tube is energized in use. That is, when the tube is heated, its walls give off an amount of gas which is considerable compared with the volume of glass making up the tube. The absorbed gases which are released are mostly hydrogen and hydrocarbons; these gases react with the salts of the coating material to form moisture and the metals of the metallic salts.

During this process, three factors influence the ultimate condition of the electrodes. These are: the temperature of the furnace, the amount of current passing through the filament, and the degree and rate of vacuum created. When the tube is heated, driving off the absorbed gases, the density of the reducing atmosphere is dependent upon the degree of vacuum while the temperature of the glass wall depends upon the furnace temperature, and the temperature of the filament is determined by the amount of current passing through the electrode. These three factors are mutually interdependent; thus, the temperature of the filament as it is treated and gives off water vapor affects the atmosphere in the tube. in addition, the temperature of the glass affects the pressure of the reducing gas given off by the glass wall inside the tube. However, the residual pressure of the reducing atmosphere is also aifected by the evacuating speed and vacuum pressure produced by the pumps. 1 have determined that, for best results, these factors should be varied in relation to one another substantially as described above.

Having described my invention, 1 claim:

1. A discharge tube comprising a cylindrical glass wall and an electrode assembly including a filament constituted by a suport wire of .3 mm. diameter wound in a helix of 8.5 mm. diameter, and second wire of .08 mm. diameter helically wound around the first wire in a helix of 6.5 mm. diameter, said wires being of a substance selected from the group consisting of Wolfram, molybdenum, nickel and iron and being coated with an electron emissive material, a cup surrounding said filament, said cup being of generally cylindrical configuration and having at its inner end, a dished wall configurated to form a central opening displaced toward the center of said cup from the end thereof, the opposite end of said cup being configurated to form an inwardly turned rim.

2. A discharge tube comprising a cylindrical glass wall and an electrode assembly including a filament consti tuted by a support wire of .3 mm. diameter, a second wire of .08 mm. diameter helically wound around the first wire in a helix of 6.5 mm. diameter, both of said wires then being wound in a helix of 8.5 mm. diameter, said wires being of a substance selected from the group consisting of wolfrarn, molybdenum, nickel and iron and being coated with an electron emissive material, said electron emissive coating being constituted by an alloy formed by reducing in contact with the filament wire a mixture of 25 parts by weight lime, 65 parts by weight of strontium oxide and 235 parts by weight of baryte, a cup surrounding said filament, said cup being of generally cylindrical configuration and having at its inner end a dished wall configurated to form a central opening displaced toward the center of said cup from the end thereof, the opposite end of said cup being configurated to form an inwardly turned ring defining a central opening only slightly smaller than the diameter of said cups.

3. A discharge tube comprising a cylindrical glass wall and an electrode assembly including two series connected filaments, each of said filaments comprising 9 turns of a support wire of .3 mm. diameter, and a second wire of .08 mm. diameter helically wound around the first wire in a helix of 6.5 mm. diameter both of said wires being wound in a helix of 8.5 mm. diameter, said wires being of a substance selected from the group consisting of Wolfram, molybdenum, nickel and iron and being coated with an electron emissive material, a cup surrounding said filament, said cup being of generally cylindrical configuration and having at its inner end a dished wall configurated to form a central opening displaced toward the center of said cup from the end thereof, the opposite end of said cup being configurated to form an inwardly .urned rim defining a central opening only slightly smaller than the diameter or" said cup.

References Cited in the file of this patent UNITED STATES PATENTS 1,891,475 Hotchner Dec. 20, 1932 1,974,762 Wiley Sept. 25, 1934 2,117,054 Boucher May 10, 1938 2,171,234 Freeman Aug. 29, 1939 2,238,277 Miller Apr. 15, 1941 2,258,158 Lowry Oct. 7, 1941 2,525,263 Macksoud Oct. 10, 1950 2,631,945 Morrison Mar. 17, 1953 2,677,623 Delrieu May 4, 1954

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