US2438721A - Method of sealing electrical discharge tubes - Google Patents

Description

March 30, 194s.

METHOD OF Filed Dec. 4, 1941v Patente-d 30, 1948 METHOD OF SEALING ELECTRICAL DISCHARGE TUBES Percy L. Spencer,.West Newton, Mass., assigner to Rayieon Manufacturing Company, Newton, Mass., a corporation of Delaware Application December 4, 1941, Serial No. 421,668

This invention relates to a method of hermetically-sealing by means of heat component parts of the metal envelope, particularly of an electrical discharge tube. Certain types of velectrical discharge tubes are made with a metal envelope of a conductor, such as copper. During the construction of such tubes, component parts thereof arehermetically sealed by the use of suitable solder, such as silver solder. fore difliculties have arisen in connection with such a procedure, due to the fact that various deleterious compounds were formed in the tube, the solder could not readily be made to flow in such a way asto insure a complete hermetic lseal at all points, and also because of various other factors. f

An object of this invention is'to seal tubes of the above type without the introduction of deleterious compounds.

Another object is to produce complete hermetic sealing in a reliable manner.

A further object is to make such a sealing process easier to carry out and more dependable in its results. The foregoing and other objects of this invention will be best understood from the following description of an exempliiication thereof, reference being had to the accompanying drawing, wherein:

Fig. 1 is a cross-sectional view of a tube in which my novel process may be carried out; and

Fig. 2 is a diagrammatic representation of a system for practising my invention in connection with the tube of Fig. 1.

The tube illustrated in Fig. 1 consists of a metal envelope I of some conducting metal, such as copper, which, for example, may be the an- 4 and 5 exist between the end caps 2 and 3 and the side walls of the metal envelope I. These joints are hermetically sealed by the use of a suitable solder, such as .silver solder. The tube may be provided with a suitable internal structure consisting, for example, of a bridging portion 6 having a central bore 1 wherein is disposed a cathode 8. This cathode may be of the indirectly-heated thermionic type which is provided in its completed form with electron-emitting alkaline earth oxides. These oxides originally are coated on the cathode in the form of a decomposible compound of the alkaline earth metals, such as the carbonates. The cathode 8 is supported inposition by a pair of lead-in conductors 9 and III which pass from the envelope yI Hereto- 4 claims. (ci. 11a- 112) through pipes I I and I2. These pipes are made of a conducting material, such as copper, and

the joints at which they join the envelope I are likewise sealed by means of a suitable solder, similarly as with the end caps 2 and 3. To the outer ends of the pipes I I and I2 are sealed glass chambers I3 and I4. The conductors 9 and I0 pass through glass seals I5 and I6 at the outer ends of said chambers I3 and I4, respectively. Another conducting pipe I1 is also sealed through the wall of the metal envelope I by means of solder. This pipe I'I may constitute the electrical connection to the anode of the tube, and surrounds a conductor I8 which forms a high frequency lead from which oscillations generated within the tube may be led to a suitable utilization circuit. `To the outer end of the pipe I 1 is sealed a glass chambe I9 having a glass seal at one end thereof through which the conductor I 8 passes. The chamber I4 is initially provided with a side tube 2I while the glass chamber I3 is provided with a side chamber 22 terminating in a small orifice 23.

It will be seen that the tube as described above is provided with a number of joints which must be sealed together before the tube is made herme'tically tight, so that the device may be evacuated in the usual manner. One of the most veilective ways of producing this seal is by the ode of a magnetron. The metal envelope I is 5. closed at its ends by end caps 2 and 3. Joints use of silver solder, as already described. However, as discussed above, such soldering process has heretofore involved a number of serious difculties. My novel invention may be practised in connection with the arrangement of parts, as shown in Fig. 2. In this arrangement a flexible tube 24 connects the side tube 2| with the upper end of a pipe 25. This pipe is connected through a stop cock 26 to a high vacuum exhaust pump 21 capable of reducing the gas pressure within the envelope I to a very low value. Above the stop cock 26 `the pipe 25 is provided with a side arm 28 communicating with said pipe 25 through a stop cock 29. A exible hose 30 may connect the side arm 28 to a tank 3| which contains dry nitrogen.

The sequence of operations of the arrangement as shown in Fig. 2 is substantially as follows.

The stop cock 26 is closed and the stop cock 28 is open, whereupon dry nitrogen from the tank 3| is passed through the tube envelope I, and escapes through the orice 23. This iiow is permitted to continue for a suilicient time until substantially all of the air originally contained within the envelope I has been flushed out and displaced by the dry nitrogen. Thereupon one velope I so as to raise `solder melts at about 700 to 125 temperature will cause the solder to flow into the crevices to be sealed. During this heat- `ing process the dry nitrogen continues to ow.

temperature. composes at about formed, it breaks down comor more flames I2 are applied to the vmetal enits temperature, and particularly he temperature ofthe Joints to be soldered, to about 750 1:07800` C. -Since silver C., the above The use of dry' nitrogen in this portion ofthe sealing process has a number of advantages.

Nitrogen is substantially inert with respect to the carbonates no deleterious effects of the cathode coating, and has thereon.l When tubes of this kind are later exhausted and the cathode coating is broken down, it has been found that a consistently good thermionic electron emission thas been secured. Also nitrogen is a poor heat j conductor,

and thus does not exert excessive cooling. Therefore it is a relatively easy matter to raise the component parts of the tube to a freely ,20 i sufllciently high temperature to secure the free flow of solder into all the joints and crevices,

and obtain `perfect hermeticsealing.v In addition, any nitrogen compounds .which tend to 4be formed withv the metal parts of the tube decompose'at temperatures below the soldering For example, copper nitride de- 300 C. Thus if initially any copper nitride is pletely during the soldering process.

When the soldering has been completed and while .the tube is still in heated condition, the

stop cock 29 is closed, the tube 22 is sealed oi, and the stop cock pump 21 evacuates the tube envelope l of all the nitrogen at a sumcient rate so that when the temperature of the envelope l has fallen to about 300 C., at which copper nitride could be formed,

there is'insufiicient nitrogen in the tube to form any signicant amount of such compound.`

When the tube envelope I is at room temperature, it may be removed from the pumping system and air allowed to reenter the tube. temperature is suilciently low so that no deleterious compounds or other injurious eiects are lproduced on the various component parts of the tube. If'desired, instead of allowing air to reenter the envelope l, the side arm 2l may be sealed off until the tube is ready t'o be connected to the usual exhaust system. in which the tube is treated to the usual final exhaust processes.

I have found that tubes sealed in the manner as described above are consistently good, not only as to being completely also as to the electrical due primarily to the fact that the tube is kept free of deleterious compounds and other dele` terious effects during the sealing process.

Of course it is to be understood that this invention is not limited to the particular details as described above as many equivalents will suggest themselves to those skilled in the art. For example, other gases, any compounds thereof which tend to form with component parts ofthe tube decomposing below sealing temperature, may be used, particularly if such gases have a heat con- .Room

hermetically sealed but` characteristics thereof,1

26 is opened. Thereupon the ductivity of ther order of nitrogen or less. Also the process may be applied to a wide variety of tube types in which the envelope itself may not necessarily be mainly of metal as long as there are parts which are to be sealed together by heating to relatively elevated temperatures. Various other ideas as to the utilization of various aspects of my invention will readily suggest themselves to those skilled in the art. It is accordingly detube having a plurality ofcomponent copper-r containing parts which are to be sealed together, said method comprisingpassing through said tube 'while cold a stream of nitrogen,A soldering said parts together at a soldering temperature above about 300 C.- while maintaining insaid tube an atmosphere of nitrogen, and exhausting said tube while said parts vare above about 300 C.

3. The method of sealing an electrical dis charge tube having a plurality of'component copper-containing parts which` are to be sealed together, said method comprising soldering said parts together at a solderingtemperature above about 300 C. while maintaining in said tube an atmosphere of nitrogen, cooling said parts below said solderingv temperature, land vexhausting saidtube before sai-d parts have fallen to a temperature of about 300 C.

4. The method of sealing an electrical discharge tube havinga plurality of component copper-containing parts which are to be sealed together, said method comprising passing through saidtube while cold a stream of nitrogen, soldering said parts together at a soldering temperature above about 300 C. while maintaining in said tube an atmosphere of nitrogen, cooling said parts below said soldering temperature, and exhausting said -tube before said parts have fallen lto a temperature of about 300 C.

PERCY L. SPENCER.

REFERENCES CITED The following references are of record in the i'lle of this patent: I

UNITED STATES' PATENTS Number Name Date 2,023,354 Cope Dec. 3, 1935 2,093,814 Mann Sept. 21, 1937 2,095,807 Gier Oct. 12, 1937 FOREIGN PATENTS Number Country Date Great Britain Mar. 27, 1939

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