US1872359A - Thermionic rectifier - Google Patents

Description

Aug. 16, 1932. 1 L. SUTHERLIN 1,872,359

THERMIONIC RECTIFIER Filed Oct. 11. 1927 WITNESSES. INVENTOR 7% -W Lee Sam r/m.

, "UNITED STATES PATENT OFFICE.

LEE surmmnm, or man rrr'rsnune'n, rmsnvnma, ASSIGNOB '10 wns'rnm- HOUSE ELECTRIC & MANUFACTURING VANIA v COM'PANY, A CORPORATION OF PMS- THERMIONIC RECTIFIER Ap'plloationfiled October 11, 1927. Serial Io. 225,605.

My invention relates to hot-cathode rectifiers and particularly to such rectifiers 1n which the cathode has a surface of oxides of the alkaline earth metals.

One object of my invention is to provide means to prevent an electrode, intended to act at all times as an anode, from becoming a source of electrons even when its temperature is considerably raised or when it is subjected to a considerable voltage stress.

Another object of my invention is to provide a rectifier containing thermionically emissive oxides with electrodes having surfaces which destroythe thermionically emissive property of oxides depositing thereon.

Another object of my invention is to provide electron tubes with metallic anodes the surfaces of which have high coefiicients of thermal radiation.

Other objects of my invention will become apparent in the following description taken in connection with the appended drawing in.

which the figure is a view partlv in elevation and partly in section of a tube embodying my tended to emit electrons.

Since tubes in which the power output is considerable have anodes which run at rather hightemperatures, the results of such deposition is that the anodes fail to perform the expected function and becomeemitters of electrons. In cases, for example, of diode tubes acting as rectifiers, this results in the shortcircuiting of the alternating-current lines, with disastrous consequences.

I'have found several methods of constructing the anodes of such rectifiers to overcome this tendency to emit electrons under load. Experiments have demonstrated that the phenomenon is not capable of explanation on the basis of'known chemical laws. The discovery has, however, proved of great value inimproving the voltage and current rating at which hot-cathode rectifiers may be operated.

Another improvement upon rectifiers of the prior art has resulted from the fact that certain of the methods above mentioned result in a darkening and even blackening of the electrode surfaces. The power output of hot-"cathode rectifiers is well known to be determined, in practice, largely by the ability f their anodes to dissipate heat. It is desir able that such rectifiers shall maintain the highest possible vacuum and, on this account, metallic electrodes are almost a necessity, since other materials tend to absorb gases which are later gradually emitted by the electrodes after the tubes are sealed off from the pumps used in exhausting them. However, it is characteristic of the pure metals that their natural surfaces are bright, and hence poor radiators of heat. The blackening which results, as aforesaid, from the application of certain of the processes which are herein described increases the radiating power of the electrodes and, accordingly, increases the load capacities ofrectifiers having anodes of given slze.

One of the methods of treating the anodes of hot-cathode rectifiers to prevent any tendency to emit electrons consists in covering them with paint comprising a suspension of finely divided aluminum in alcohol. The base electrode may be of any suitable metal, for example nickel.

An alternative method of providing an anode which shall not emit electrons is to form it of metallic aluminum. The electrodes employed should, of course, be mechanically clean and they may, to advantage, be given a preliminary treatment by heating in vacuo for an extended period to free them from gases.

Still another method which I have found effective to prevent emission of electrons from rectifier anodes is to paint their surfaces with a 7 suspension of flocculent graphite and water. The commercial ma terial well known under the name aquadag is suitable for this purpose.

A fourth method which I-have found effective is to form the anodes of an alloy containing chromium; for example, an alloy consisting of 20 percent chromium and 80 percent nickel has been found to give good results as-an alternative method. A nickel electrode may be plated with chromium by any of the well known electrolytic processes for depositing that metal.

Where electrodes having chromium surfaces are used, they may be oxidized, although this is not absolutely essential since the metallic chromium itself is eifective, for the purposes which 'I have described, When the chromium surface is oxidized, however, it becomes darkened and thereby it becomes a more efiective radiator of heat with the advantages inherent thereto which I have described above.

Referring to the drawing, an electrode 1 prepared in any of the foregoing ways may be sealed into a suitable containing vessel 2 provided with a proper cooperating electrode. The latter may, for example, consist of a tungsten or nickel filament 3 coated with a mixture of the oxides of barium and strontium. Electronic emission from the anodes has been found to be particularly low in value in tubes using a cathode comprisin barium and strontium oxides covering the ase filament described in application Serial No. 144,911 of E. F. Lowry, filed October 28, 1926.

Electron tubes constructed in accordance with the foregoing descriptions, taken in connection with methods well known in the art, are subjected to the usual process of exhaust which is too well known in the art to require extended description here.

By the use of rectifiers provided with the electrodes which I have described, the line voltage on which the rectifiers of iven dimensions may be operated may be increased by reason of the absence of tendency to thermionic emission from the anodes, with its consequent short circuiting of the alternating-current line.

The cathode temperature and, consequently, the saturation current of the tube may be increased, since the consequences of the volatilization of the cathode material are less serious. Consequently, an anode of given dimensions runs at a lower temperature on a power load by reason of the increased thermal emissivity of its surface. That, of itself, lessens the tendency toward short circuiting the alternating-current lines and, furthermore, lessens the deterioration of the vacuum of the tube by the gases which are set free when the anode operates at a too-high temperature. The current and volt-age rat-. ing of the tube may be increased in consequence of these effects.

While I have described several specific ways of producing the electrode surfaces above described, various other methods will be evident to persons of skill in the art, and I desire that my claims shall not be limited by the precise process which I have described.

While, therefore, I have herein described my invention in accordance with the patent statutes to illustrate the construction and op eration thereof, it is apparent that here are changes and modifications that may be made in the precise process I have described, without departing from the spirit of my inven tion. I desire, therefore, that only such limi tations shall be imposed thereon as will be indicated by the appended claims, taken in connection with the prior art.

I claim as my invention:

1. A rectifier having a cathode comprising'a thermionically emissive material and a cooperating electrode coated with chromium.

2. A rectifier having a cathode comprising a thermionically emissive oxide and a cooperating electrode coated with chromium.

3. A rectifier having a cathode comprising an oxide of a metal in the alkaline earth group and a cooperating electrode coated with chro mlum.

In testimony whereof, I have hereunto sub- 3slgri ped my name this 6th day of October LEE SUTHERLIN.

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