US2137841A - Electric discharge device - Google Patents

Description

Nov. 22., 1938. J. H. HUTCHINGS ELECTRIC DISCHARGE DEVICE Filed June 2s, 1956 f :MM im Au as Inventor: Jol-m H. Hutchings,

His Attorneg.

Patented Nov. 22, 1938 UNITED STATES PATENT OFFICE ELECTRIC DISCHARGE DEVICE York Application June 23, 1936, Serial No. 86,800

' 3 Claims.

My invention relates to electric discharge devices, and more particularly to electric discharge devices which are provided with metal enclosing envelopes.

I have observed that electrodes of discharge devices enclosed in metal tend to run considerably hotter than exactly similar electrodes enclosed in glass. 'I'his is objectionable in devices of the pure electron discharge type since it tends to increase grid emission thereby producing loss of grid bias and indirectly leading to premature failure of the device. Furthermore, in so-called soft tubes such as power rectiers and inverters which contain an ionizing medium this overheating increases the probability that a given amount of power transmitted through the tube will produce destructive disintegration of the anode and cause harmful gases to be evolved.

I have found that the operating temperature of the electrode parts with a given energy input can be substantially reduced by providing the interior of the metal envelope with a blackened surface adapted to increase its ability to Aabsorb heat. (By the term blackened I intend to designate not only true black body surfaces but also oth-er surfaces such as dark brown or blue gray which fairly approximate black body characteristics.) In accordance with my invention a surface of the desired characteristics is provided by a method which involves oxidizing the envelope walls after assembly of the various envelope parts but before exhaust.

The features of my invention which I consider to be novel will be pointed out with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof will best be understood by reference to the following specification taken in connection with the accompanying drawing, in which Fig. 1 shows an elevational view in partial section of an electron .discharge device to which my invention is applicable; Fig. 2 is a section on line 2--2 of Fig. 1, and Fig. 3 is a longitudinal section of a controllable mercury pool tube also embodying the invention.

Referring particularly to Fig. 1, I have shown Aa metal enclosing envelope comprising an elongated tubular portion I of a readily workable metal, such as steel oi' .005 inch thickness, closed at the bottom by a anged header 2 hermetically sealed thereto. Within the envelope'and insulatingly supported therefrom are provided a cathode 3, a grid or control electrode 4, and an anode 5. The cathode may suitably comprise a long cylinder of a metal such as nickel coated exteriorly with a layer of electron emissive mate- Irial such as a mixture of barium and strontium oxide. This cathode is surrounded by the grid il which as shown takes the form of a helical ,spiral of thin wire, preferably of molybdenum,

(Cl. Z50-27.5)

Surrounding both these electrodes and adapted to receive a discharge from the cathode is provided an anode in the form of an imperforate sleeve suitably of carbon-coated nickel. 'Ihe electrode parts are supported and maintained in proper alinement by vertical rods 'l of a conducting material, for example, copper. Partial compensation for the electrostatic field distortion caused by these last-named elements is effected by making the grid and anode of generally elliptical shape. as best shown in Fig. 2.

The Various conducting parts are insulated from one another and from the envelope by spaced disks 8 of a non-conducting material, for example, mica. Lead-in connections for the electrodes, as well as for a resistance heater l0 arranged coaxially within the cathode, are supplied through conductors H brought to the outside of the tube through suitable glass-to-metal seals l2.

In the operation of a discharge device such as that shown the electrode parts are found to run at a higher temperature than might have been expected from previous experience with similar parts enclosed in glass. This fact tends to increase the production of independent electron emission from the surface of the grid wires and to cause faulty and unreliable operation of the tube. This effect is believed to be due to the rela tively high coefficient of heat reflectivity of the inner wall surfaces of the metal envelope, and the consequent failure of the envelope walls to absorb heat readily and transmit it to the surrounding atmosphere. In accordance with my present invention, I overcome this diliiculty by providing the inner wall of the envelope with a blackened surf ace thereby materially increasing its ability to absorb radiant heat. Such a surface will have its greatest effect when applied in a region exposed to direct heat radiation from the anode. In the construction illustrated in Fig. 1 this will obviously comprise the area bounded by the insulating disks 8. The Wall surfaces above the upper disk and below the lower disk need not be treated since they are so shielded as to play little part in the absorption of heat from the electrodes.

A method of blackening which is advantageous in the routine fabrication of metal tubes consists in oxidizing the desired region of the tube wall surface thus producing a blackening coating chemically bonded with the envelope material. This process may be carried out before the tube is exhausted and While it still contains a quantity of air. Under these conditions the central portion of the envelope, particularly in the vicinity of the anode, may be heated either by application of a flame or by high frequency induction to a temperature Well above the oxidation temperature of iron, that is, at least to 400 C. and preferably to about 900 C. This heating, especially at the high temperature specified, should be very brief, being long enough to produce oxidation of the surface of the envelope but not4 sufliciently long to heat the electrode parts to an oxidizing temperature. I consider that best results are obtained when the tube wall is heated momentarily to a temperature of about 900 C. and the tube thereafter immediately exhausted and sealed. It will be understood that the exhaust is accomplished through the depending tubulation I4 illustrated in Fig. 1, which tubulation is finally pinched off and sealed as shown.

By the provision of a blackened surface in accordance with the procedures above outlined the thermal absorptivity of the metal wall may be increased as much as 200 to 500 per cent depending on its original condition. Particular tubes which before treatment had been observed to operate at normal voltages and currents with an anode temperature of 435 C. were found to operate with an anode temperature of less than 325 C. after application of an oxide coating as specified.

In Fig. 3, I have further illustrated the use of my invention in connection with a particular type of mercury pool discharge tube. In this case the enclosing envelope comprises a pair of concentric slightly spaced cylinders I8 and I9 closed at their ends by stepped metal headers 20 and 2|. An anode 22 preferably in the form of a large block of graphite is supported at the top of the discharge vessel and is insulated from the main portion of the vessel by a glass cylinder 23. A heavy cable 24 indirectly attached to the anode as shown is provided for convenient connection to an external source of potential (not illustrated) In the lower part of the envelope and in direct contact with the header 2| is provided a pool of liquid cathode material 26, for example mercury, adapted to cooperate with the anode 22 in conducting an electric discharge therebetween. In permanent contact with the mercury surface and adapted to control the initiation of a. discharge therefrom is supported a makealive electrode 21 of a semi-conducting material as, for example, boron carbide mixed with clay. This is held in place by a rigid conductor 28 which is insulated from the mercury by a body of glass 29 and to which is connected an external conductor 30 arranged for attachment to an external source of control potential (not shown). The cathode header 2I may be connected to the same or a different source of potential through a heavy lug 32 Vwelded or soldered to the lower surface of the header.

The spaced cylinders I8 and I9 define a jacket or chamber therebetween adapted to receive a cooling uid 33, for example, water. This jacket is provided with inlet and outlet conduits 34 and 35, respectively, which permit continuous circulation of the cooling medium.

In the operation of a device of this type the problem of grid emission is of course not involved. However, the matter of anode temperature is still of greatest importance since it constitutes one determinative factor of the amount of power which may safely be' transmitted through the tube. Above a certain limiting temperature the anode will undergo rapid and destructive disintegration which will greatly shorten the effective life of the discharge device.

In accordance with my invention the power load which will be effective to produce this limiting temperature may be substantially increased after application of a blackening coating to the inner surface of the cylinder I9. This coating should be applied in a region adjacent the anode and serves to improve the transmission of heat to the cooling fluid 33, thereby lowering the ternperature at which the electrode parts will operate for a given voltage and current. As indicated in Fig. 3, the blackening material may suitably be applied above the more or less arbitrary boundary line 31.

A coating. of the desired quality may be provided by the method outlined in the foregoing description. Alternatively, in connection with tubes employing a graphite anode, the coating may be produced while the tube is being exhausted by operating the tube for a brief interval at an excessive overload. Under such an overload, which may be in the neighborhood of 300 per cent, small particles of carbon will be thrown off the anode surface and will form a sooty deposit on the envelope walls, thus effecting the blackening desired. objectionable gases generated during this process will be carried off to the exhaust pump through the exhaust tubulation 38 and will not render the tube inoperative as they might if generated during its later operation. As a third possibility one may coat the surfaces desired to be darkened with a colloidal suspension of carbon in water.

While I have shown particular applications of my invention, it will of course be understood that I do not wish to be limited thereto since many other uses are possible, and I contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

l. The method of fabricating an electrical device of the type comprising a metal envelope enclosing electrode parts which includes combining the envelope and electrode parts in their final assembly, heating a substantial portion of the Wall of said envelope to an oxidizing temperature before exhausting the same, continuing said heating long enough to oxidize the interior surface of said wall portion but not suiiiciently long to produce oxidation of said electrode parts and thereafter exhausting and sealing said envelope.

2. The method of fabricating an electrical device of the type comprising a ferrous metal envelope enclosing electrode parts, which method includes assembling the envelope and electrode parts, heating the wall of the envelope in an oxidizing atmosphere to provide a black oxide coating on the interior surface thereof and thereafter exhausting and sealing the envelope.

3. The methodof fabricating an electrical de vice of the type comprising a ferrous metal envelope enclosing electrode parts, which method includes assembling the envelope and electrode parts, heating the wall of the envelope momentarily to a temperature of about 900 C. before exhausting the envelope in order to provide an oxide coating on the interior surface thereof, and thereafter exhausting and sealing the envelope.

JOHN H. HU'I'CHINGS.

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