US2156631A - Electric discharge device - Google Patents

Description

May 2, 1939.

O. KRENZIEN ELECTRIC DISCHARGE DEVICE I Filed Feb. 15, 1957 Patented May 2, 1939 UNiTED STATES PATENT orrica ELECTRIC DISCHARGE DEVICE Application February 15, 1937, Serial No. 125,844 In Germany February 14, 1936 10 Claims.

My invention relates to electric discharge devices and more particularly to the type which are employed for rectifying alternating current or for similar purposes. Electric discharge devices are well known in the art in which the cathode metal vaporizes owing to the high temperatures produced. by the arc. In such devices mercury is preferably used as a cathode metal, since it always flows back to the cathode after having been vaporized and condensed on the cooled walls of the vessel. In such discharge devices the loss of energy caused by the dissipation of heat to the outside atmosphere is very great. Furthermore, there is'a risk that backfires occur owing to the r relatively high vapor pressure.

My invention has for its object the elimination of the above diffi'culties hitherto experienced in electric discharge devices. According to the invention the electric discharge device possesses a cathode having a large surface and enclosing the anode, only a small portion of the cathode surface being kept at a considerable temperature by the arc, whereas the cathode as a whole is kept cool if desired by particular devices.

Further details of my invention will be apparent from the following description taken in connection with the accompanying drawing, in which Fig. 1 shows in semi-diagrammatic fashion and in sectional elevation an electric discharge device according to the invention, and

Figs. 2, 3 and 4 illustrate in similar fashion modifications thereof.

In Fig. 1, I denotes an iron cylinder in which is inserted an air-tight iron cover 2. The current is supplied to the anode 3 by a rod 4 which may consist of any conventional metal alloy having a coefiicient of expansion similar to that of glass to obtain a tight seal therein. In the cover 2 is inserted for that purpose a bushing 5 of the same material so as to form an air-tight joint with the cover. The anode supply rod 4 is held in this bushing vacuum-tight by the sealing material 6 also sealed in the bushing 5. The anode 3 may be made of ordinary iron. Loose crystals of carborundum 9 are heaped on the bottom 8 welded to the cylinder l. A layer 9a of alkali metal or alkaline earth metal is placed on these crystals. The discharge device is evacuated through the conduit 1 leading to the vacuum pump (not shown). After the evacuation of the device potassium is distilled thereinto which deposits at first on the entire inner surface of the vessel. Then argon gas is introduced into the vessel until the pressure amounts to 5 to 10 mm, mercury column, when the conduit 1 may be sealed. Be

tween the cathode, 9a and the anode 3 an arc is produced with the aid of a direct current, thus bringing the anode to a high temperature so'that the deposit of potassium vaporizes again. Upon the subsequent cooling, the anode 3 maintains its temperature longest owing to the poor heat conductivity of its immediate surroundings, so that the potassium precipitates on the vessel wall and, therefore also on the carborundum mass 9. By this precipitation the loose crystals of carborundum are easily bound together.

Instead of potassium, also other metals, such as sodium, lithium, caesium and rubidium as well as the alkaline earth metals, such as barium, strontium, and calcium, may be employed in the manufacture of the electron emitting layer. The layer 9 which in the embodiment shown consists of carborundum may also be composed of different materials. As supports for the electron emitting layer semi-conductors are particularly employed, about all such which are not at all electrolytically decomposed by the electric current or only to such a slight extent that the decomposition is not detrimental, i. e., such semi-conductors which do not liberate particularly any oxygen or halogens. The semi-conductors are, therefore, to be preferred to the metals, since they dissipate the heat less rapidly so that an arcing spot having a high temperature may be created within the smallest space. Poor heat conductors, requiring a large amount of energy for electron emission, hereinafter shortly called emission energy, when employed as supports for the elec- "tron emitting materials have, furthermore, the

advantage that the emission energy of the monoatomic layers of the cathode material formed thereon is reduced by polarization. As supports, therefore, carbides, borides, nitrides, silicides of the various metals, particularly of highly refractory metals, such as tungsten or titanium, are suitable. However, silicon boride or silicon carbide (carborundum) may be particularly employed. Carborundum has not only the advantage that it has the properties of a semi-conductor also at normal temperatures, but also that it is Very economical and very dense. Also its natural rough surface appears to be advantageous. Probably also its electro-negative property is of importance.

In order to manufacture or prepare a cathode With carborundum as a support, particles of crushed or ground carborunclum may be loosely heaped on the bottom of the discharge vessel. The discharge Vessel itself may consist of iron. If the carborundum consists of sufficiently coarse rains it may be held together also on the walls of the vessel by wire gauze of conducting or insulating material stretched over the carborundum. It maybe also heaped behind a perforated wall of glass, quartz, porcelain or other ceramic material or behind a silundum (silicon carbide) grid with spacing. The metal serving as an electron emitting material is distilled onto the support preferably under vacuum. If the surface of the support is as uneven as, for instance, the surface of the carborundum when heaped, also the advantage is attained that numerous transition points with mono-atomic layer occur at which the arcing spot establishes itself; however, it may extend also to the thicker layers which it apparently vaporizes to such a great extent that only a mono-atomic layer is left. This layer is then as a rule always replaced by the metal of the adjacent layers.

The discharge vessel is preferably filled with vapor which does not condense even in a cold state of the vessel, 1. e., particularly with a rare gas, for instance argon. The pressure at ordinary temperatures may amount to about 0.02 to 30 mm. mercury column. For instance, a very reliable operation is possible at a gas pressure lying between 5 and 10 mm. mercury column. In an electric discharge device according to the invention in which the support consists of coarse grained carborundum and the cathode material of potassium the arc may be started even with a cold cathode already at a voltage of about 50 to '70 volts. In order to further reduce the ignition voltage a particular keep-alive anode may be provided of any suitable type known in this art. This feature is not illustrated here since it has no bearing on the present invention. Even if the outer walls of the vessel are cooled down to a temperature of degree centigrade. the voltage drop in the ignited arc amounts only to volts. Therefore, also the heat development and the loss of energy is small. The voltage drop increases to about three times the above amount if a suitable point is no longer available on which the arc may be concentrated. This might, for instance, occur if too thick a layer of the cathode material covers the substance serving as a support.

By the use of sodium as an electron emitting material, the ignition voltage, all other conditions being equal, amounts to 150 to 170 volts and the burning voltage to about volts owing to the increased emission energy.

It had already been known that the electron emission energy of the compact alkali metals is smaller than that of the alkaline earth metals and that this difference exists also in the monoatomic layers of the adsorbed metals. Nevertheless, cathodes of alkali metals both in a compact and in an adsorbed state have hitherto been employed only for photoelectric purposes, insofar as incandescent cathodes activated by adsorbed alkali metals have in spite of the small electron emission energy only at comparatively high temperatures an emission which renders them adapted for use in rectifier tubes and at these high temperatures the life of the adsorbed layers is too short for technical purposes. A longer life can only be attained, when the vapor pressure of the alkali metal is chosen so high that the same number of atoms which leave the support owing to the vaporization are replaced within the same time by the atoms of the surrounding gases. However, at such a high vapor pressure, also the anodes of the rectifier vessel would be activated by adsorbed alkali atoms,

would emit electrons and would thus give rise to backfires. This is particularly to be feared, since the anodes must be kept at considerable temperatures in order that no vaporized cathode material condenses thereon. In the rectifier tube according to the invention the total electron emission required for its operation is effected by an arcing spot which is so small that the temperature of the cathode may be easily kept as a whole within low limits. To this end, the oathode, if desired, may be kept cool by the use of particular means. Alkali metal which vaporizes from the focal spot condenses, therefore, immediately on the adjacent cooler parts of the oathode surface, so that its average partial pressure is not increased by the vaporization. In this manner, the blocking or stop voltage is greatly increased. For further increasing the stop voltage, instead of potassium, sodium may be employed as an electron emitting substance. It is true that caesium and rubidium require a still smaller electron emission energy than sodium and potassium but are not so suitable for rectifiers, since their higher vapor pressure may give rise to the formation of mono-atomic layers on the anode surfaces.

The limitation of the arcing spot to a narrow space is only possible if a substance of sufiiciently poor heat conductivity is employed as a support according to the invention. The electrical conductivity of this material must sufhce in order to supply the discharge current to the focal spot without detrimental loss of voltage. In the immediate neighborhood of the hot arcing spot the partial pressure of the electrode emitting substance is small so that the mono-atomic layer, which may be destroyed, is immediately replaced at the same point or in the immediate neighborhood of this point. The limitation of the zone in which the increased partial pressure prevails may be considerably supported by the reduced speed of diffusion resulting from the applied high pressure of the filling gas of at least 10- mm. mercury.

The electric discharge device may be so designed that all its parts with the exception of the anodes and the insulation therefor act as condensation surfaces and, as soon as the metallic layers have deposited thereon, also as a cathode. surface. To this end, as shown in Fig. 1 the entire inner surface of the vessel, with the exception of the anodes and the insulation therefor, is made of solid conducting material, for instance, of iron. Such a construction of the vessel may also be of advantage to other cathodes and gas fillings.

Fig. 2 shows a modification of the electric discharge device according to the invention. Both the bottom 55 and the inner wall I I of the vessel are covered with crystals of a semi-conductor, particularly crystals of carborundum. These crystals are firmly held in position by wire gauze M which is secured to a ring 52. The electric discharge device according to Fig. 2 has three anodes, only one anode I! being shown. All three anodes are displaced 120 with respect to one another. The diameter of the anode supply conductors is is reduced at several points in order to diminish the dissipation of heat from the anode to the sealing point and to retain in this manner suficient heat for the anode so that no vaporized cathode particles may deposit on the anode. The inside of cover [6 may be provided with a supported layer of crystals of carborundum which is covered with an electron emitting layer of alkali metal.

Fig. 3 shows a further modification of the electric discharge device according to the invention. While in the types shown in Figs. 1 and 2, the cooling of the cathode is effected substantially by the surrounding atmosphere, particular cooling means are provided in the embodiment according to Fig. 3. The wall 2! of the discharge Vessel has a reentrant portion 22 which is cooled by a cooling agent flowing in the direction of the arrows. The surface of the reentrant portion 22 facing the interior of the vessel is cov ered with an active layer 23 of alkali or alkaline earth metal placed on a support of silicon carbide. To keep this substance in place, this vessel portion is provided with a net or basketshaped metallic enclosure 24. Also large portions of the other vessel walls are provided with an active layer 25 in order that they may serve at least temporarily, for instance in the case of a temporary interruption of the supply of the cooling medium, as a cathode at which the arc establishes itself. The bushings 21 of the anodes 26 are protected as far as possible from the discharge chamber proper in order to prevent alkali metal from. depositing thereon.

The distance between the inner cooler and the anode is sochosen that the diffusion to the anode of the alkali metal vaporizing at the arcing spot is sufiiciently prevented at the prevailing pressure of the filling gas.

As cooling agents preferably circulating fluids, for instance oil, air or the like are employed, in which case an average cathode temperature of at most 200 degrees centigrade should not be exceeded in order that the average partial pressure of the alkali may not become so high as to cause an accumulation of alkali metal on the anodes. The use of water as a cooling medium is unfavorable owing to the danger of oxidation which may occur in the case of possible leakages of the cooling device due to the presence of metallic potassium in the discharge vessel. The thickness of the walls of the vessel and the inner cooler A further embodiment of my invention is shown I in Fig. 4. This electric discharge vessel has also an additional outer cooling for the wall parts of the tube in order that the partial pressure of the alkali metal vapor may be kept within correspondingly low limits even in the case of considerable loads. The wall SI of the electric discharge vessel is sourrounded by a cooling jacket 38 which is supplied with the cooling medium by the supply conduit 39, and the discharge conduit 30. The anodes 36 are mounted in the same manner as those shown in Fig. 3. The inner wall parts of thevessel are cooled at 31 by a liquid cooling medium. The greater portion of the inner wall parts is coated with an active mass 35 which is held by a netting 34. The cooler may also be surroundedby a perforated wall of glass, quartz, porcelain or other ceramic material behind which the material serving as a support is heaped.

I claim as my invention:

1. A gasor Vapor-filled electric discharge device including a metal container, an anode insulated from said container, a lead-in for supplying current to said anode, a substantial portion of the inner surface of said container -:formizng the cathode, a layer of a highly refractory chemical compound covering said container surface portion and an electronically-active layer of metal, selected from a group consisting of alkaliand alkaline earth metals, coating said refractory layer, said compound being non-reactive wi h respect to said coating and having a small heat conductivity, whereby the are formed between the cathode and anode tends to heat only a small portion of the entire cathode surface, and means for cooling the entire cathode to recondense on the cathode surface only the oathode material vaporized by the arc.

2. A gasor vapor-filled electric discharge device including a metal container, an anode insulated from said container, a lead-in for supplying current to said anode, a substantial portion of the inner surface of said container forming the cathode, a layer of a highly refractory chemical compound covering said container surface portion and an electronically-active layer of metal, selected from a group consisting of alkaliand alkaline earth metals, coating said refractory layer, said compound being non-reactive with respect to said coating and having a small heat conductivity, whereby the arc formed between the cathode and anode tends to heat only a small portion of the entire cathode surface, and means for cooling the entire cathode to recondense on the cathode surface only the cathode material vaporized by the arc, the vapor or gas pressure within the container being to l0 mm. mercury column.

3. A gasor vapor-filled electric discharge device including a metal container, an anode insulated from said container, a lead-in for supplying current to said anode, a substantial portion of the inner surface of said container forming the cathode, a rough surface layer of a highly refractory chemical compound covering said container surface portion and an electronically-active layer of metal, selected from a group consisting of alkaliand alkaline earth metals, coating said rough surface layer not thicker than will still permit the maintenance of the rough ness of said refractory layer, said compound being non-reactive with respect to said coating and having a small heat conductivity, whereby the are formed between the cathode and anode tends to heat only a small portion of the entire cathode surface, and means for cooling the entire cathode to recondense on the cathode surface only the cathode material vaporized by the arc.

l. A gasor vapor-filled electric discharge device including a metal container, an anode insulated from said container, a lead-in for supplying current to said anode, a substantial portion of the inner surface of said container forming the cathode, a rough surface layer of silicon boride of a highly refractory metal, covering said container surface portion, and an electronicallyaotive layer of metal, selected from a group consisting of alkaliand alkaline earth metals, coating said rough surface layer not thicker than will still permit the maintenance of the roughness of said refractory layer, said compound being non-reactive with respect to said coating and having a small heat conductivity, whereby the are formed between the cathode and anode tends to heat only a small portion of the entire cathode surface, and means for cooling the entire cathode to recondense on the cathode surface only the cathode material vaporized by the arc.

5, A gasor vapor-filled electric discharge device including a metal container, an anode insulated from said container, a lead-in for supplying current to said anode, a substantial portion of the inner surface of said container forming the cathode, a rough surface layer of silicon carbide of a highly refractory metal, covering said container surface portion, and an electronically-active layer of metal, selected from a group consisting of alkaliand alkaline earth metals, coating said rough surface layer not thicker than will still permit the maintenance of the roughness of said refractory layer, said compound being non-reactive with respect to said coating and having a small heat conductivity, whereby the are formed between the cathode and anode tends to heat only a small portion of the entire cathode surface, and means for cooling the entire cathode to recondense on the cathode surface only the cathode material vapori' ed by the arc.

6. A gasor vapor-filled electric discharge device including a metal container, an anode insulated from said container, a substantial portion of the inner surface of said container forming the cathode, a coarse grained layer of a highly refractory compound-covering said container surface portion and a grid stretched over said layer to support it on said container surface portion, said grid having a coating of an electronically-active metal selected from the group consisting of alkaliand alkaline earth metals, said refractory compound being non-reactive with respect to said coating and having a small heat conductivity, whereby the arc formed between the cathode and anode tends to heat only a small portion of the entire cathode surface, and means for cooling the entire cathode to recondense on the cathode surface only the cathode material vaporized by the arc.

7. A gasor vapor-filled electric discharge device including a metal container, an anode insulated from said container, a substantial portion of the inner surface of said container forming the cathode, a coarse grained layer of a,

highly refractory compound covering said container surface portion and a Wire gauze stretched over said layer to support it on said container surface portion, said Wire gauze having a coating of an electronically-active metal selected from the group consisting of alkaliand alkaline earth metals, said refractory compound being nonreactive with respect to said coating and having a small heat conductivity, whereby the are formed between the cathode and anode tends to heat only a small portion of the entire cathode surface, and means for cooling the entire cathode to recondense on the cathode surface only the cathode material vaporized by the arc.

8. A gasor vapor-filled electric discharge device including a metal container, an anode insulated from said container, a substantial portion of the inner surface of said container forming the cathode, a coarse grained layer of a highly refractory compound covering said container surface portion and a grid of refractory insulating material stretched over said layer to support it on said container surface portion, said grid having a coating of an electronically-active metal selected from the group consisting of alkali-- and alkaline earth metals, said refrectory compound being non-reactive with respect to said coating and having a small heat conductivity, whereby the are formed between the cathode and anode tends to heat only a small portion of the entire cathode surface, and means for cooling the entire cathode to recondense on the cathode surface only the cathode material vaporized by the arc.

9. A gasor vapor-filled electric discharge device including a metal container, an anode insulated from said container, a substantial por tion of the inner surface of said container forming the cathode, a coarse grained layer of a highly refractory compound covering said container surface portion and a grid of silicon carbide stretched over said layer to support it on said container surface portion, said grid having coating of an electronically-active metal selected from the group consisting of alkaliand alkaline earth metals, said refractory compound being non-reactive with respect to said coating and having a small heat conductivity, whereby the are formed between the cathode and anode tends to heat only a small portion of the entire cathode surface, and means for cooling the entire cathode to recondense on the cathode surface only the cathode material vaporized by the arc.

10. A gasor vapor-filled electric discharge device including a metal container, an anode insulated from said container, a lead-in for supplying current to said anode, a substantial portion of the inner surface of said container forming the cathode, a layer of a highly refractory chemical compound covering said container surface portion and an electronically-active layer of metal selected from the group consisting of alkaliand alkaline earth metals, coating said layer, said compound being non-reactive with respect to said coating and having a small heat conductivity, whereby the arc formed between the cathode and anode tends to heat only a small portion of the entire cathode surface, the container Wall portion forming the cathode surface having a reentrant portion and means for circulating a cooling medium through said reentrant portion.

OTTO KRENZIEN.

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