US1821238A - Rectifier - Google Patents

Description

Patented Sept. i, 1931 UNITED STATES PATENT OFFICE HARVEY C. RE1lTSCHLER, OF EAST ORANGE, AND WILLIAM W. MERRYMON, OIE GLEN RIDG, NEW JERSEY, ASSIGNORS '10 WESINGHOUS. LAIMP COMEANY, A CORPORA RECTIFIER Application filet! July 20,

This invention relates to an electron discharge device and more particularly to a glow discharge device employing a rarefied gascons atmosphere.

The invention is especially concerned w1th the production of a gascons conduction rectifier of alternating current, employing a 1nonatomic gas filling, such as helium, argon or neon or a mixture of such gases, and depending at least in part, upon the point to plate discharge principle for rectification.

Hitherto, considerable difiiculty has been experienced in producing alternating current rectifiers of this type capable of operation at relatively high potentials, which depend on gas ionization for conduction of the current and are independent of thermionic omission. Among the difiiculties which may be mentioned, is the maintenance of the monatomic gas in a pure and uncontaminated condition. Slight traces of gaseous impurities, such as hydrogen, nitrogen, etc., liberated from the electrodes, the glass parts of the device or the insulators greatly increase the break down voltage and the voltage drop through the device and if permitted to accumulate eventually cause the device to become inopeTative at the normal voltage impressed across the terminals thereof.

It has been proposed heretolore to maintain the gas in a pure form by employing certain clean-up agents which react with the commori gases to form solid compounds Of low vapor pressure. Certain of these mate rials, however, contain considerable quantities of undesirable gas which isgiven ofi' when the material is vaporized and is not subsequently eliminated by the getter material. For instance, magnesium, which is most commonly employed as a gas clean-up, usually contains an appreciable quantity of hydrogen which is liberated when the getter is hated. Magnesium does not' clean up hydrogen efliciently, however, and conse-' quently, both the residual and liberated hydrogen are likely to remain in the tube as a gascons impurity.

However, even "though the monatomic gascons filling is retrdered extremely pure during the manufacture of the device, never- 1927. Serial No. 207,073.

theless, with the electrode materials ordinarily employed, sufficient gases are given off by the electrodes and parts of the tube such as insulators, etc., during operation, probably due to bombardment by the discharge, to contaminate the monatomic as and greatly increase the resistance an f voltage drop through the device. V

It is one of the objects of the present invention t0 overcome the above mentioned difiiculties and to produce a gascons conduction device-in which the gaseous filling remains substantially nated to an objectiona le extent over a much longer Period of operation and under more severe conditions of operation than has herefore been attainable.

Another object is to produce a device of the above mentioned character in whichthe electrodes Will be composed of a material or materials which facilitate the maintenance of the gas in a pure condition.

Another object is to proVide a clean-up agent which is effective to remove substantially all traces of all of the common gases and which in combination with the materials of which the electrodes are composed, Will maintain the gascons filling extremely pure.

A still further object is to provide and continuously maintain on the surface of one of the electrodes a thin deposit of a material which Will decrease the potential drop at the surface of the electrode.

Still another object is to Provide a metliod of continuouslv maintaining such thin dep0sit on the electrode throughout the life of the device.

Another object is to produce a gascons conduction device which is capable of assing a discharge of relatively high voltage and high amperage with a low and constant potential drop therethrough and in which the operating characteristics thereOf will remain constant over a long period of opefia tion.

Other objects and advantages Will hercinafter appear.

We have discovered that when certain materials are employed for the cathode and anode of a gascons conduction device toure and uncontami-' v gether with an appropriate clean-np agent, the gascons filling may be ma1nta1ned in a substantially pure condition over a much longer period of time than has been possible heretofore under the same severe conditions of operation.

More specifically, we have found that when the cathode or plate electrode of a point to plate discharge device, such as a rectifier, is composed of one of the refractory metals such as molybdenum or tantalum and the active portion of the anode 1s composed of an active metal such as thorium, uranium or titanium and a clean-up or gas purifying agent, such as misch metal is employed, the gascons filhng does not become contaminated during operation of the dev1ce and the operating characteristics thereof remain substantially constant over many bundreds of hours of operation. Moreover, the voltage drop through the device is considerably lower after a few hours operation than When employing such metals as nickel, iron, etc., for the electrodes.

In order to further decrease the voltage drop through the tube, we may employ a deposit of an electro-positive metal, such as an alkaline earth or alkali metal on the cathode. In order to render such material effective throughout the life of the device, it is necessary to continuously replace the deposit during operation of the device. Simply coating the cathode itself with one of these materials is not sufiicient, since the electro-positive metal is soon used up or vaporized away and is no longer effective.

Tubes, the plates of which have been coated r with such metals, while they show good characteristics on starting, do not maintain the same for a sufliciently long period of time to be of practical value.

Th manner which we have found most eflective for maintaining a deposit of an electro-positive met-al on the cathode is by providing a quantity of a compound of an alkaline earth or alkali metal on the anode, which compound during operation is slowly reduced by the material of which the anode is composed to liberate the metal. This metal is then vaporized over on to the cathode and forms the desired deposit thereon. The thorium or uranium anodes may be dipped or otherwise coated with a solution of an alkaline earth compound such as barium or calcium nitrate or an alkali metal compound such as sodium or potassium nitrate. During the seasoning of the device, these nitrates are probably converted to an oxide and during the operation of the tube, these oxides are slowly reduced to metal. This metal is continuously vaporized on to the cathode, thus giving and maintaining a low starting and operating potential. VVe prefer to employ the alkaline earth metals, although the alkali metals are also satisfactory. It is necessary, however, that the metal itself and not the oxide thereof, comprise the active deposit on the cathode. For 1nstance, in certain experiments when barium oxide was applied directly to one side of the cathode it prevented the discharge from taking place with such coated surface and caused all the discharge to take place with the uncoated surface until after the oxide had broken down to metal. Vhen employing molybdenum plates, any oxide which is carried over from the anode to the cathode is broken down by the reducing action of the metalwhich thereby assists in maintaining an active film of the clectropositive metal thereon. This is not truc,

however, with such metals as iron or nickel when employed as the plate material.

It is essential that a compound be employed on the anode Which has a relatively low vapor pressure and which is not too active chemically, so thatit will not be vaporized from the anode toc rapidly. Such compounds as barium azide, for instance, do not serve efi'ectively, because they complotely vaporize away from the anodes very rapidly and are then no longer available to maintain the cathode deposit.

In place of dipping or otherwise coating the anodes, the alkaline earth or alkali compounds may be incorporated directly in the anode material during manufacture the1eof, but in doing, care must be taken to prevent the compound from being reduced and vaporized away during such manufacturc.

The gas employed depends upon the output voltage desired. If a high voltage, that is, a voltage of from 200 to 300 volts is de sired, the most suitable gas appears to be helium. With helium the inverse current is less than for either argon or neon under the same conditions and hence the tendency t0 arc is less. If lower voltages are desired, argon or neon are quite satisfactory and more eflicient.

In order that the invention may be more fully understood, reference Will be had to the accompanying drawings, in which:

Fig.- 1 illustrates a gaseous conduction rectifier partly in section embodying the features of the present invention; and,

Figs. 2 and 3 illustrate circuit arrangements employed in seasoning the device.

The device comprises an envelope 1 containing a pair of anodes 2 and 3, a cathode 4 and a filling of a monatomic gas, such as helium, argon or neon or a mixture of such gases.

The anodes 2 and 3 are preferably'composed of thorium, uranium or titauium and are carried by support wires 5 of nickel or other suitable metal secured to leading-in conductors 6 and 7 sealed through the press 8 of the device. The anodes may be sep recess 11 in the'lower end of the lavite sleeve and serves to support the same.

Thecathode4is of cylindrical shape and preferably is formed of a refractory metal, such as molybdenum, althoufgh any other convenient shape may be employed, giving the requisite surface area. The anodes 2 and 3 are illustrated as projecting into the interior of the cylindrical cathode, but it is to be understood that, if desired, the anodes may be disposed externally of the cathode. By 'disposing the cathode about the anodes, it may be brought nearer to the press and the support Wires made short and direct,thereby producing a more rigid structure. The cathode is supported from the press by support wires 12 and 13 welded thereto, the former of which is joined to a leading-in' conductor 14 for supplying current thereto.

The leadingin conductors 6, 7 and 14 are connected 'to the terminal prongs 15, 16 and 17 res}3ectively of the base'l8.

A piece of misch metal or other metal of the -cerium group of rare earth metals 19 -i s secured to the cathode by a strap 20 welded to the exterior of the cathode. The cathode is heated during exhaust by high frequency induction currents, to drive out the gases therefrom and during such heat treatment, the misch metal is vaporized and deposits in a thin film on the bulb. This thin deposit of misch metal assists in maintaining the gascons filling pure throughout the life of the device,

The envelope is filled with monatomic gas, preferably helium, at-a pressure from 5 to 20 mm. We prefer to employ about 12 mm. for an output voltage of from 200 to 300 volts and from to 100 milliamperes.

However, if lowef output voltages are desired, argon or neon at a lower pressure may be employed.

The device after corhpletion should be seasoned to develop the desired starting and operating' characteristics.

The seasoning treatment which we have successfully employed is conducted in two parts and is best understood by reference to Figures 2 and 3. The tube 20 is first operated foi about 10 minutes directly-across a 220 volt A. C. line 21 with the anodes 2 and 3- in series with 15 watt, 110 volt tungstean filament lamps 22 and 23.

The sesoning is then continued with the anodes 2 and 3 joined together and con-4 nccted to one side of a source 24 cf direct current at from 400 to 500 volts, with the cathode 4 joined to the other side of the line through a filter circuit 25 containing a load 26. The tube should be ope'rated in this circuit until the anodes become heated to a bright rcd heat and thrc is no appreciable fiashing between the anodes and the cathode, at a voltage in excess ofthat which they will receive in use.

With a rectifier of the above construction, that is, employing thorium anodes, a molybdenum cathode, mish metal getter and a filling of helium at about 12 mm. of mercury,an output voltage of around 200 volts at 85 milliamperes may be continuously maintained over many hundreds of hours. VVheh employing plates of such metals as .iron or nickel and exactly the same transformer and filter circuit, an output of only about 150 to. 160 volts or 85 milliamperes can be obtained after operating for about 100 hours or s0.

In order to Still further reduce the potential drop through the device, an alkaline earth or an alkali metal may be employed 'as a thin deposit on the cathode. In order to maintain this deposit throughout the life of the device, it is essential to provide a reserve supply of such metal within the clevice and to continuously supply it to the surface of thecathode during the operation of the rectifier. Merely coating the cathode with s'uch metal is not satisfactory, since it rapidly vaporizes away and is no longer available. Therefore, in order to provide a "reserve supply of such metal, we-

coat the anodes 2 and 3 with a compound of an alkaline earth or alkali metal such as' barium nitrate which is probably convertcd to oxide during seasoning, which slowly reacts with the thorium duringthe operation of the device to liberate the barium metal.

The barium is vaporized by the discharge and is continually supplied to the cathode. The reaction of the compound with the thorium is slow so that the compound is not entirely used up during the normal life of the device.

The device of the present invention has bcen designed particularly as a rectifier for supplying plate current for radio receiving apparatus through the so-called B-eliminator apparatfls. In such a use, the output current is smoothed out by any appropriate filter circuit embodying inductance and capacities. In such circuits, the charge which builds up on the anode when the same is negativeis several times that on the cathode When the cathode is negative, due to the action of the condensers in the filter circuit. Thus, for instance, with an output voltage of around 200 volts, the charge on the anode during the inverse cycle may amount to as high as 700 volts. Due to this high voltage on the anodes, the small inverse ouvrent which fiows between the cathode and anode bombards the anode at a high velocity and materially assists in the reduction of the alkaline earth or alkali compound on the anode.

Obviously, other means may be employed for maintainingthe active deposit on the cathode, but we have found the method described above to be highly successful.

VVhile We have shown and described but a single embodiment of our invention, it is to be understood that many changes and modifications may be made therein without departing from the invention.

\Vhat is claimed is:

1. An electrode for a gaseous conduction device composed of thorium and a nitrate of a metal of the class including the alkaline earth and the alkali metals.

2. An electrode for a gaseouS conduction device composed of thorium having a coating of barium nitrate thereon.

3. The method of producing and maintaining a deposit of an electro-positive metal on the cathode of a gaseous conduction device, comprising fomning the anode of thorium, coating said anode with a compound of the electro-positive metal decomposable by said anode and passing a discharge through the device to decompose the compound, liberate the electro-positive metal and vaporize the same from the anode onto the cathode.

4. A gaseous conduction rectifier comprising an envelope containing a monatomic gas, a cathode operable below incandescence, composed of molybdenum, an anode composed of thorium, a gas purifying agent including a metal of the cerium group of rare earth metals and a coating of a compound of an electropositive metal on the anode.

5. A gaseous conduction rectifier comprising an envelope containing helium gas at a pressure from 5 to 20 mm., a cathode composed of molybdenum, an anode composed of thorium, a gas purifying agent composed of a metal of the cerium group of rare earth metals and a coating of barium nitrate on the anode.

In testimof1y whereof We have hereunto subscribed our names this 19th day of July HARVEY C. RENTSCHLER. WILLIAM W. MERRYMON.

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