US1731268A - Electron-discharge device - Google Patents

Description

Oct. 15, 1929. H. c. RENTSCHLER El AL 1,

ELECTRON DISCHARGE DEVICE Filed March 28. 1925 'INVENTORS f/aro'ey CJeemfgafi/er B John l0- Mom/en.

- ATTORNEY Patented Oct. 15, 1929 PATENT OFFICE HARVEY CLAYTON RENTSCHLER AND JOHN WESLEY MARDEN, OF EAST ORANGE, NEW

JERSEY, ASSIGNORS TO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA ELECTRON-DISCHARGE DEVICE Application filed March 28, 1925. Serial No. 18,958.

This invention relates to an electron discharge device and more particularly to a discharge device such as a radio tube employing an anode or plate electrode and a cathodeof the thoriated type adapted to operate at incandenscence.

Thoriated cathodes are well known in the art and consist briefly of a tungsten filament having a very minute quantity of thorium or of a thorium compound distributed throughout. In View of the very limited qantity of the active electron-emitting substance, namely the thorium contained in the filament, it is essential that every precaution be taken to eliminate from the device any material which can react with the thorium to decrease the emission thereof. For instance, in commercial practice the bulb is now carefully baked during exhaust and the plate heated to a high temperature by some suitable means, as by high frequency induction currents to drive off the gases and vapors therefrom and a clean-up material such as magnesium or misch metal is vaporized in the bulb to eliminate any residual or occluded gases contained therein and form an active getter for such gases as are liberated during operation.

In spite of these precautions, tubes which are manufactured and treated as nearly alike as is physically possible, show different operating characteristics and have a very different useful life, some tubes failing very early while others may have a relatively long life. This variation in able in tubes using filaments having a low power consumption, such as dry cell .tubes and clearly indicates that there is a variable condition present in the manufacture of thoriated filament tubes which heretofore has not been under control.

One of the objects of this invention is to provide a method by which electron discharge devices of the thoriated cathode type having a uniform high emission and a long commercial life may be consistently produced.

Another objectis to provide a method of producing electron discharge devices of the thoriated cathode type in which the variable conditions which heretofore have materially life is particularly noticeaffected the operation or life of the device are readily controlled and made constant.

Another object is to provide an electron discharge device "of the thoriated filament type substantially free from material capable of reacting with the thorium of the filament to decrease the electron emission properties thereof.

Another object is to provide an electron discharge device which will be stable in operation, which will'have a long and dependable life and which may be readily reproduccd.

Other objects and advantages will hereinafter appear.

.'We have found that the cause of early loss in emission of thoriated tungsten filaments is due very largely to the presense of occluded oxygen or decomposable oxides on the plate or on the plate and grid and further that such falling emission is not appreciably caused by pure chemical reaction of the filamentary material with the residual gases of the device or by positive ion bombardment due to the presence of residual gas in the tube. This is clearly indicated by the following experiments which will be given briefly in order that the invention may be more fully understood.

For the sake of simplicity, the experiments about to be described. were conducted principally with two element tubes, that is, tubes employing only a plate and a thoriated filament, although it is to be understood that the invention is not limited to devices of this nature since the results obtained apply equally well to discharge devices employing a. grid or control electrode.

' We have found that when using iron, nickel. or molybdenum plates, that the emission of the filament after seasoning dropped noticeably in a minute orless when plate voltages of about 40 volts were used and that with higher voltages the drop in emission was correspondingly greater. The results were not noticeably affected by the kind of clean-up agent'or getter used or by the degree of care with which the tubes were baked out and exhausted. The fact that burning of the filament without plate voltage or with plate voltage under about 40 volts did not destroy the emission, indicates that the drop in emission is not due to direct chemical action of the free residual gases in the tube with the thorium of the filament.

Tubes which were made up with aluminum plates instead of nickel, iron or molybdenum required aplate voltage of from to 7 0 volts instead of about 40 volts to produce the sumlarfalling emission. In using alum num plates it was necessary to flash the getter from an auxiliary plate since alummum has a melting point below that of magnesium or misch metalwhich were used as getters. Also, care was taken to have the aluminum as free from grease etc., as possible, but no more care was exercised than when using 11011, nickel or molybdenum plates.

Apparently'atwo-fold effect takes place in the cathode during operation. First, the thorium on the surface of the cathode is either removed or reacts with some material in the tube to form compounds which are oor emitters, and secondly, the thorium di uses to the surface of the filament from the interior to replace the. thorium removed from the surface. When the thorium is removed or destroyed faster than it is diffused to the surface, the emission falls. The fact that the fall ing emission requires a different plate voltage when using one kind of a plate than when using another, clearly shows that it cannot be' due to the positive ion bombardment caused by minute traces of residual gases, but rather to some condition of the plate. We have found this falling emission to be due almost entirely to the electron bombardment of the plate and the consequent liberation of harm ful gases therefrom which react with the thorium to reduce the emissivity thereof.

Tubes employing iron, nickel, or mol 'bdenum plates through which a stream of ydrogen was passed during baking out of the bulb on the pump were uniform in operation and did not exhibit:- this falling emission. This indicates, in all probability, that the oxides on the plate whic were formed during the mounting, sealing and exhaust of the bulb or were left unreduced before introducing in the bulb, were reduced by the hydrogen stream and the plate, therefore, durin electron bombardment was not capable of li erating oxygen for reaction with the thorium of the filament.

This theory that falling emission is due to oxygen liberated from the plate is further verified by the fact that when using aluminum plates the fallingemission does notoccur at as low a plate voltage as when using iron, nickel or molybdenum plates, aluminum oxide being more stable than iron, nickel or molybdenum oxide.

In accordance with the present invention, the plate or the plate and grid of the electron discharge device are freed from occluded oxygen or residual oxides after these elements 7 have been mounted in the device. The freedom from oxygen or oxides may be obtained in several different ways. For instance, a stream of hydrogen may be passed throu h the tube during exhaust andwhile tube, as set forth above, or the plate may be composed of a metal, the oxide of which is baking t 1e 7 I readily decomposed by heat or when heated in hydrogen liberated from the plate such as silver, gold or platinum. For economy of manufacture, however, we prefer to make the plate or both the plate and grid, of copper. We have found that if copper is first heated in hydrogen or if made to contain occluded hydrogen in any suitable manner prior to plates was not materially afiected by baking.

the plates in hydrogen prior to sealing-in.

In manufacturing tubes u'sing copper plates I and grids, the tubes are made up in .the usual manner and a getter,

which may comprisea small piece of magnesium is attached to the platef The tubes should be baked out during exhaust and the plate heated as byhigh frehydrogen and the oxygen oroxides on the plate and to vaporize thefnagnesium to cleanup the gases so liberated. The tube should then be seasoned in the usual manner.

quency induction to drive out the occluded hydrogen to effect a reaction between this When using silver as the material for the I plate, the baking in hydrogen may be eliminated, it being only necessary to heat the plate, to drive the occluded gases therefrom and cause a reduction of the oxides. The plate is thus rendered substantiall capable of being liberater? bardment during operation affecting the emission of the cathodeby electron bom- In the single figure of the drawin there is shown an electron discharge device 0 the conventional type such as is used for radio receivmg apparatus. This tube comprises the usual plate, grid and filament electrodes 1, 2 and 3, respectively. The cathode or filament 3 may be of the t oriated tungsten type or of other type, the emission of which, is readily destroyed or impaired b oxygen and the plate 1 may be composed 0 copper or, if desired, silver, iron, nickel or molybdenum, or other suitable metal, the oxide of which is readily reduced by hydrogen. Incase one of the latter metals is employed asthe anode it is necessaryto provide means for passing a stream free of materials i and deleteriously' beidispensed of hydrogen through the device during the baking operation. For this purpose a connection 4 is made to the exhaust tube from a supply of hydrogen not shown and a tubu lature 5 is secured to the dome of the bulb to permit the exit of the hydrogen therefrom. A separate connection 6 from the exhaust tube is provided for the vacuum pump. In order to treat out the plate when employing as the plate material, a metal which is not reduced a by heat alone in vacuum, the envelope'is first 'exhausted through the connection 6 and a stream of hydrogen permitted to flow therethrough to the connections 4 and 5. Simule taneously with this hydrogen flow the plate is heated in any suitable manner as by a high frequency induction coil 7 to raise the plate to a sufficiently high temperature to effect a reduction of the oxides thereon by-the hydrogen. The tube is then re-exhausted and sealed off. WVhen employing copper containing occluded hydrogen or when using silver as a material for the plate, it is unnecessary to maintain a hydrogen stream through the tube during the heat treating of the electrodes and in this case the connections 4 and 5 may with.

While copper has been specified as the preferrcd material for forming the plate, it is I to be understood that other suitable materials may be used either alone or coated with copper or silver, and the plate'may be treated in various manners to free the same from oxygen ormetallic oxides.

It will be noted that an electron discharge device has been produced, the plate or the plate and grid of which is substantially free from oxides, that such tubes are uniform in their operating characteristics, have a long life and a high emission and may be readily reproduced.

What is claimed is:

1, An electron discharge device comprising an envelope, a cathode of the thoriated type and an anode composed of copper containing occluded hydrogen.

2. The method of producing an electron discharge device employing a thoriated cathode and at least onecooperating electrode which consists in forming said cooperating electrode of copper containing occluded hydrogen, sealing said cathode and cooperating electrode into an envelope, exhausting the envelope, reducing the oxides formed on said cooperating electrode to the metal and render ing the reaction products thus formed, inactive with respect to the cathode. 1

3. The method of producing an electron dischargedevice employing a thoriated cathode and a cooperating copper electrode comprising baking said electrode in hydrogen, sealing the cathode and cooperating electrode into an envelope, evacuating the envelope, heating said electrode to cause a reduction of the oxides thereon and rendering the reaction

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