US2162414A - Discharge tube electrode - Google Patents

Description

June 13, 1939. s A. ABBOTT DISCHARGE TUBE ELECTRODE Fild Feb. 4, 1958 M/VENTOR 4770/?NEY Patented June 13, 1939 UNITED STATES PATENT OFFlCE Application February 4, 1938, Serial No. 188,782 In Great Britain February 5, 1937 19 Claims.

This invention relates to thermionic electrodes for electric discharge tubes, and more specifically is concerned with hot cathode type electrodes for use in gas or vapour filled discharge tubes.

According to the invention, an electrode of the hot cathode type is manufactured by first treating a carrier body of insulating refractory material so that its surface is chemically reduced locally, and then applying to the chemically reduced surface an electron-emissive portion consisting of a mixture containing a refractory metal and one or more compounds of the rare earth or alkaline earth metals, or of both groups of metals, the electrode structure being heated so as to harden the electron-emissive portion and effect firm attachment between it and the carrier body. According to the invention, therefore, a hot cathode electrode comprises a carrier portion formed from non-conductive refractory material the surface of which has been chemically reduced locally, and an electron-emissive portion which is applied to the reduced surface of the carrier portion and held firmly attached to the carrier portion as the result of heat treatment. Preferably, the carrier portion or body is formed from quartz or silica glass, and the skin or surface layer thereof is chemically reduced locally by rapidly burning in contact therewith a powdered reducing metal such as magnesium or aluminium and removing the residue of oxide of the reducing metal, the operation being repeated if necessary so that, or until, a hard integral surface layer results. This surface layer must not be metallised in the ordinary sense of that term, but nevertheless it is found that it is to a certain extent electrically conductive.

The electron-emissive portion of the electrode according to the invention is formed by mixing a refractory metallic constituent such as tungsten, molybdenum or nickel, with a compound of barium or thorium, preferably the oxide or hydroxide, and with or Without a binder such as water glass. The mixture preferably also contains silicon. The ingredients are brought into a finely divided state, thoroughly mixed together and heated, preferably in the presence of oxygen. Although the electron-emissive' portion, when first applied to the carrier, is only held mechanically and is very soft, after the heat treatment the mass attains great hardness and is firmly combined with the carrier substance due to the silicon in the mixture combining with the silicon layer formed by the initial local reduction of the quartz carrier.

The electrode according to the invention further comprises a refractory metallic contact or anode member, preferably of tungsten or molybdenum, which is arranged to make contact with the electron-emissive portion and not only affords contact between the sealing-in wire, which passes through the wall of the discharge tube, and the cathode itself, but also has the further function of starting the discharge and carrying the anodic current.

Some methods of carrying out the invention will now be described in detail by way of example and with reference to the accompanying diagrammatic drawing, in which:

Figure 1 is a fragmentary sectional elevation showing one form of electrode construction in accordance with the invention mounted in a discharge tube;

Figure 2 is a sectional elevation of an alternative electrode construction;

Figure 3 is a further sectional elevation showing a modification of the electrode construction of Figure 2;

Figures 4 to 7 are similar views showing various arrangements of heating elements and/or of the anode member of the electrode; and

Figure 8 shows a double-ended electrode construction for serving two adjacent discharge tubes.

In carrying out the invention according to the construction shown in Figure 1, a quartz tube is closed at one end to form a cup I, and a hard, chemically reduced integral surface layer or skin is formed throughout the whole or the greater part of the interior of the cup by rapidly burning nesium oxide residue one or more times. A contact wire 2 of tungsten, constituting an anode member, is passed through a small hole in the closed end of the cup and protrudes into the cup for a short distance. An intimate mixture of melted barium hydroxide crystals by weight), powdered nickel (20% by weight) and silicon (5% by weight) is next placed within the cup, filling it partly or entirely, and. then the whole is heated by introducing it slowly and gradually into an oxygen flame until the quartz glows white. The whole is then allowed to cool, leaving a shiny black mass 3 in the cup the surface of the mass 3 assuming a shape somewhat as indicated, and the electrode is ready for mounting in a discharge tube. The contact wire 2 is passed through a silica glass tube 4 which is fused to the bottom of the cup I and is also sealed in the wall of the discharge tube 5. inner end of the wire 2 is exposed as shown, and consequently this wire or anode member can start the discharge, and by becoming incandescent it heats the mass 3 up to the emitting temperature.

In the electrode construction shown in Figure 2, the cup is made from a short length of quartz tube la and the base of the cup is formed by a shorter and thicker quartz tube lb fitted tightly into, and welded to, the tube la, and through The 30 magnesium inside the cup and removing the magwhich the contact wire 2 passes with a close fit. The interior surface of the cup is chemically reduced in the way already described in connection with Figure 1, and then an electron-emissive portion 6 is placed in the cup in contact with the chemically reduced surface. The emissive portion 6 may be of the same composition, and be treated in the same way, as described for the construction shown in Figure 1, or it may be as described in any of the examples given below.

Figure 3 shows a modification of the electrode structure of Figure 2 for an indirectly heated electrode. In this case the part lb of Figure 2 is replaced by a longer tube lc which is surrounded by a helically disposed heat-er wire 1 arranged to extend into close proximity to the base end of the tube la. The heater wire is formed with straight limbs 8, 9, and the contact wire is bent at right angles, as shown, and welded to the limb 9. The limbs 8, 9 pass through seals In, H in the wall of the discharge tube 5.

According to a further method of carrying out the invention, a cup is constructed from a quartz tube having a length of approximately 8 mm., a

bore of about 4 mm. and a wall thickness of about 2 mm. One end of the tube is open, the other end being not entirely closed but having a small hole through it of about 1 mm. diameter. A tungsten contact wire of 1 mm. diameter is inserted through a small hole so as to protrude into the cup for approximately 2 mm., the wire fitting tightly in the hole. That is to say, the actual construction of the cup and arrangement of the contact wire is the same as that shown in Figure 1. Moreover, as before, the inner surface of the quartz (silicon dioxide) cup is reduced to silicon by burning magnesium powder in it and afterwards removing the magnesium oxide. A mixture is then placed in the cup consisting of the following ingredients by weight. Tungsten 90 barium oxide 5 silicon 5%. This mixture is pressed down, and a further mixture is added consisting of barium oxide and thorium oxide 20%. The quartz cup and its contents are then heated.

In the formulae given above the tungsten may be replaced wholly or partly by molybdenum, nickel, platinum or other refractory metals. Likewise the barium oxide or thorium oxide may be replaced wholly or partly by other oxides of the alkaline earth or rare earth metals or other electron-emissive compounds of such metals. The above mentioned mixtures may be mixed with a binder such as water glass before they are introduced into the cup.

Instead of using two mixtures, a single mixture may be placed in the cup and heated, this mixture consisting of tungsten (60%-40%), barium hydroxide (23%-30%), strontium oxide or thorium oxide (15%20% of either), silicon (1%- 5%) and graphite (1%-5%). In a modification of this procedure the cup is partly filled with pure tungsten, or a contact mixture consisting principally of tungsten with a small percentage (not more than about 5%) of silicon, and the cup and its contents heated. Next molten barium hydroxide is poured on to the contact mixture in the cup, which is again heated, the barium compound constituting the electron-emissive material.

In the construction shown in Figure 4, the quartz cup I is provided externally with a heater winding I2 in the form of a helix of tungsten wire which extends from the closed end of the cup over about two-thirds of the length of the cup, the two ends of the winding being carried past the closed end of the cup in the form of tWo straight limbs I3, M which are arranged to pass through seals in the wall of the discharge tube as shown in Figure 3. The contact wire 2, which is embedded in the contact mixture 6 in. the cup and passes axially through the bottom of the cup, is bent at right angles, as shown, and Welded to the limb M of the heater. If desired, the contact wire may be made of sufiicient length so that it protrudes at the open end of the cup for approximately 1 mm. beyond the emissive material, in which case the helical heater winding l2 may or may not be employed.

In the arrangement shown in Figure 5, the contact wire 2 is arranged to serve as one of the limbs of the heater l2a, one end of the helical winding of the heater being carried into the cup I, as shown, and joined at I5 to the end of the contact wire 2. In Figure 6 the contact wire 2 is completely covered by the emissive portion 6, and a heater in the form of a loosely wound flat spiral l6 of tungsten wire is arranged close to the exposed surface of the emissive portion 6. The two ends of the spiral are taken down past the base of the cup in the form of straight limbs ll, l8, the contact wire 2 being welded to the limb I 8.

Any of the mixtures disclosed above, with or without a binder, may be placed in the cup without the contact wire being in position. In Figure '7 that end of the contact Wire 2 which is located inside the cup I is provided with a flat spiral of tungsten wire which is disposed close to the emissive portion 6.

An electrode in accordance with the invention may be constructed as a double-ended electrode, in which case any of the mixtures and methods of construction described above may be employed. This double-ended construction is especially suitable for use where it is necessary to combine two or more discharge tubes. As shown in Figure 8, a short tube IQ of quartz approximately 4 cms. in length, with a bore of about 1 mm. and a Wall thickness of about 2 or 3 mm., is formed with a cup 20 at each end. This quartz tube, in addition to being the actual carrier of the emissive material 6, is used to seal the electrode direct into one end of each of two adjacent discharge tubes 2|, 22, so that the two discharge tubes are provided with a common electrode. A common contact wire 23 extends from one end to the other of the quartz tube, passes through the mixture in each cup, and is provided with a fiat spiral 24 at each end adjacent the emissive material. If a restriction is formed in one discharge tube and a double-ended electrode as described above is sealed in the restriction, the two separate discharge tubes are united by a common seal and electrode. One or more of the heater constructions previously described may be employed, one or more of the heater limbs being sealed into either or both of the discharge tubes in addition to the main seal of the common electrode, in order to utilise an outside contact if necessary. With the arrangement in question, however, the heater is conveniently formed by the above-mentioned fiat spirals 24, a wire connection 25 being taken from each spiral through a seal in the corresponding discharge tube, so that the two spirals 24 are in series with their common contact wire 23 and the two external connecting wires 25. It will be seen that the seals through which the I connecting wires pass into their respective discharge tubes are separate from the main seal of the common electrode.

In all the forms of construction described above, the carrier body of the electrode may be made of hard boro-silicate glass, porcelain, steatite or zirconia, instead of quartz, the particular material chosen for the purpose depending upon the material from which the discharge tube is constructed. Likewise the composition of the various mixtures previously described, their manufacture and treatment may be varied according to requirements arising from the use of any material necessary to the construction of the electrodes described above. The contact or anode member may be of any desired shape, for example rod-like, strip-like, filamentary or tubular, and may consist of tungsten, molybdenum, nickel or platinum.

I claim:

1. The method of manufacturing a discharge tube electrode of the hot cathode type which comprises providing a carrier body of insulating refractory material with a metallic contact member, effecting local chemical reduction of the surface of said carrier body, applying a powdered mixture comprising a refractory metal and'electron-emissive material to the chemically reduced part of said surface and in contact with said contact member, and heat-treating the electrode structure thus formed to cause sintering of said mixture and firm bonding thereof to said carrier body.

2. The method of manufacturing a discharge tube electrode of the hot cathode type, which comprises providing a carrier body of insulating refractory material with a metallic contact member, effecting local chemical reduction of the surface of said carrier body, applying a mixture containing a refractory metal and an electronemissive compound of an alkaline earth metal to the chemically reduced part of said surface and in contact with said contact member, and heattreating the electrode structure thus formed to cause firm bonding of said compound to said carrier body.

3. The method of manufacturing a discharge tube electrode of the hot cathode type, which comprises providing a substantially cup-shaped carrier body of insulating refractory material with a metallic contact member, extending into the interior thereof, effecting local chemical reduction of the interior surface of said carrier body, introducing powdered electron-emissive material into said carrier body in contact with the chemically reduced part thereof and in contact with said contact member, and heat-treating the electrode structure thus formed to cause sintering of said. powdered material and firm bonding thereof to said carrier body.

4. The method of manufacturing a discharge tube electrode of the hot cathode type, which comprises providing a substantially cup-shaped carrier body of insulating refractory material with a metallic contact member extending into the interior thereof, effecting local chemical reduction of the interior surface of said carrier body, introducing into the interior of said carrier body in contact with the chemically reduced surface thereof and in contact with said contact member a powdered mixture comprising electron-emissive material, a refractory metal and a substance which is the same as the material of the reduced part of said carrier body, and heat treating the electrode structure thus formed.

5. The method of manufacturing a discharge tube electrode of the hot cathode type, which comprises providing a hollow carrier body of silica with a metallic contact member extending at least partially into the interior thereof, effecting local chemical reduction of the interior surface of said carrier body to silicon, introducing a powdered mixture containing electron-emissive material and silicon into said carrier body in contact with the reduced surface thereof and in contact with said contact member, and heat-treating the electrode structure thus formed.

6. The method of manufacturing a discharge tube electrode of the hot cathode type, which comprises providing a substantially cup-shaped carrier body of silica with a metallic contact member extending into the interior thereof, effecting local chemical reduction of the interior surface of said carrier body to silicon, introducing into said carrier body in contact with the reduced surface thereof and in contact with said contact member a powdered mixture comprising tungsten substantially 90%, barium oxide substantially 5% and silicon substantially 5%, pressing down said mixture and adding thereto a further mixture comprising barium oxide substantially 80% and thorium oxide substantially 20%, and heating said carrier body'and the contents thereof.

7. The method ofmanufacturing a discharge tube electrode of the hot cathode type, which comprises providing a substantially cup-shaped carrier body of silica with a metallic contact member extending into the interior thereof, effecting local chemical reduction of the interior surface of said carrier body to silicon, introducing into said carrier body in contact with the reduced surface thereof and in contact with said contact member a powdered mixture containing a refractory metal as principal constituent and relatively small percentages of silicon and of an electron-emissive compound, pressing down said mixture and adding thereto a further mixture containing an alkaline earth oxide as principal constituent and a substantially smaller percentage of a rare earth oxide, and heating said carrier-body and the contents thereof.

8. The method of manufacturing a discharge tube electrode of the hot cathode type, which comprises providing a substantially cup-shaped carrier body of silicaand a metallic contact member extending into the interior thereof, effecting localchemical reduction of the interior surface of said carrier body to silicon, introducing into said carrier body in contact with the reduced surface thereof and in contact with said contact member an intimate mixture of melted barium hydroxide crystals substantially 75%, powdered nickel substantially 20% and silicon substantially 5%, heating said carrier body and said mixture in the presence of oxygen until said carrier body glows white, and then allowing the whole to cool.

i 9. The method of manufacturing a discharge tube electrode of the hot cathode type, which comprises providing a substantially cup-shaped carrier body of insulating refractory material with a metallic contact member extending into the interior thereof, effecting local chemical reduction of the interior surface of said carrier body by rapidly burning powdered magnesium therein and removing the residue, introducing powdered electr'on-emissive material into said carrier body 'in contact with the chemically reduced part thereof and in contact with said contact member,

and heat-treating the electrode structure thus formed.

10. The method of manufacturing a discharge tube electrode of the hot cathode type, which comprises providing a substantially cup-shaped carrier body of insulating refractory material with a metallic contact member extending into the interior thereof, effecting local chemical reduction of the interior surface of said carrier body by rapidly burning powdered aluminium therein and removing the residue, introducing powdered electron-emissive material into said carrier body in contact with the chemically reduced part thereof and in contact with said contact member, and heat-treating the electrode structure thus formed.

11. The method of manufacturing a discharge tube electrode of the hot cathode type, which comprises providing a substantially cup-shaped carrier body of silica with a metallic contact member extending into the interior thereof, effecting local chemical reduction of the interior surface of said carrier body to silicon, introducing into said carrier body in contact with the reduced surface thereof and in contact with said contact member a powdered mixture consisting of tungsten substantially 60%-40%, barium hydroxide substantially 23 %-30%, a rare earth oxide substantially 15%-20%, silicon substantially 1%-5% and graphite substantially 1%-5%, heating said mixture in said carrier body and allowing the whole to cool.

12. The method of manufacturing a dischargetube electrode of the hot cathode type, which comprises providing a substantially cup-shaped carrier body of silica with a metallic contact member extending into the interior thereof, effecting local chemical reduction of the interior surface of said carrier body to silicon, introducing into said carrier body in contact with the reduced surface thereof and in contact with said contact member a contact mixture consisting principally of tungsten with up to about 5% of silicon, heating said carrier body and the contents thereof, adding thereto molten barium hydroxide, reheating said carrier body and the contents thereof, and allowing the whole to cool.

13. A hot cathode type discharge tube electrode, comprising in combination, a carrier portion formed of insulating refractory material with at least part of the surface thereof chemically reduced, an electron-emissive portion applied to said chemically reduced surface and held thereto as a result of heat-treatment, and a metallic contact member mounted in said carrier portion adjacent to and in contact with said electron-emissive portion.

14. A hot cathode type discharge tube electrode, comprising in combination, a carrier body of insulating refractory material having at least one substantially cup-shaped portion the interior surface of which is chemically reduced at least in part, electron-emissive material held in contact with said chemically reduced surface, and a contact member of refractory metal mounted to protrude into the interior of said cup-shaped portion into contact with said electron-emissive material,

15. A hot cathode type discharge tube electrode, comprising in combination, a silica cup, the interior surface of which has been reduced to silicon at least in part, a sintered mixture held in contact with said reduced surface, said mixture comprising a refractory metal, electronemissive material and silicon, and a contact member of refractory metal supported so as to extend through the base of said cup into contact with said mixture, said contact member being sealed in the wall of the discharge tube.

16. A hot cathode type discharge tube electrode, comprising in combination, a carrier body of insulating refractory material having at least one substantially cup-shaped portion the interior surface of which is chemically reduced at least in part, a sintered mixture of a refractory metal and electron-emissive material held in contact with said chemically reduced surface, a contact member of refractory metal mounted to protrude into the interior of said cup-shaped portion into contact with said electron-emissive material, and an electrical heating element mounted in proximity to said electron-emissive material.

17. A double-ended hot cathode for serving two adjacent electron discharge tubes, comprising in combination, a tubular carrier body of insulating refractory material formed with two substantially cup-shaped receptacles located respectively at the extremities thereof, the interior surface of each of said receptacles being chemically reduced, electron-emissive material held in contact with said reduced surface, and a contact member of refractory metal disposed within said tubular carrier body and protruding at each end 1nto said electron-emissive material, said carrier body being sealed in the adjacent ends of said discharge tubes, individual means for heating the electron material in each of said receptacles and a circuit for energizing said heating means in series through said contact member.

18. A double-ended hot cathode for serving two adjacent electron discharge tubes, comprising in combination, a tubular carrier of silica sealed in adjacent ends of said discharge tubes and formed integrally with two substantially cupshaped receptacles located respectively at the extremities of said carrier so that one of said receptacles is inside one of said discharge tubes and the other of said receptacles is inside the other of said discharge tubes, at least part of the interior surface of each of said receptacles being chemically reduced to silicon, a sintered mixture held in contact with said reduced surface, said mixture comprising a refractory metal, electronemissive material and silicon, a contact member of refractory metal disposed within said tubular carrier and passing through each of said receptacles so as to extend just beyond the exposed surface of said mixture, a pair of electrical heating elements mounted respectively on the ends of said contact member, and connecting wires leading from said heating elements to the outside of said discharge tubes whereby said heating elements are connected in series by said contact member.

19. The method of manufacturing a discharge tube electrode of the hot cathode type, which comprises treating a carrier body of insulating refractory material to effect local chemical reduction of the surface of said carrier body, applying to said reduced surface a powdered mixture comprising an electron emissive material, a refractory metal and a substance which is the same as the material of the reduced part of said carrier body, and heat treating the electrode structure thus formed to cause sintering of said mixture and firm bonding thereof to said carrier body.

STANLEY ALFRED ABBOTT.

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