US2273054A - Thermionic gaseous discharge rectifier - Google Patents

Description

Feb. 17, 1942. J. D. LE VAN THERMIONIC GASEOUS DISCHARGE RECTIFIER Original Filed Feb. 2'7, 1935 INVENTOR JAMES D. LEVAN .otob o uun BY 8% I N 1 j v A TORNEY Patented Feb. 17, 1942 THERMIONIC GASEOUS DISCHARGE RECTIFIER James D. Le Van, Belmont, Mass., assignor to Raytheon Production Corporation, Newton, Mass., a corporation of Delaware Application February 27, 1935, Serial No. 8,464 Renewed June 24, 1939 6 Claims.

Thi invention relates to thermionic rectifiers, and more particularly to such rectifiers in which the cathode is heated to temperature, of thermionic emission solely by means of the discharge current.

An object of this invention isto devise such a rectifier whichmay be subjected to a large number of starting operations and still possess a life sufficiently long for commercial purposes.

Another object of my invention is to produce such a device in which the voltage drop through the rectifier is low.

A further object of my invention is to devise a simple, efiective and durable structure which will accomplish each of the above purposes.

The foregoing and other objects of my invention will be best understood from the following description of an exemplification thereof, reference being had to the accompanying drawing, wherein:

Fig. 1 is a side view of an embodiment of my cathode to a very. heavy bombardment. These fast-moving positive ions, upon hitting the Fig. 2 is a view taken along line 2-2 of Fig. 1;

Fig. 3 is an enlarged perspective view of my novel cathode; and

Fig. 4 is an enlarged cross-sectional view of one of my novel anodes.

In gaseous space discharge devices utilizing thermionic cathodes, such cathodes are usually heated to temperature of thermionic emission by a heating current passed through the cathode by an independent source of current or by a separate heating element. In order to simplify this type of device, it is desirable to eliminate the independent heating arrangement and cause the discharge current which passes through the device to raise the cathode to temperature of thermionic emission as well as maintain it at said temperature during operation. It is further desirable that the above simplification should be accomplished without the sacrifice of low voltage drop and consequently relatively high efficiency t which is obtained with the independently-heated type of cathode. In order to obtain such low voltage drop, it is further desirable to have the cathodecoated with materials which emit electrons at relatively low temperatures, such as, for example,. the oxides of alkaline earth metals,

and also to have the gas pressure sufiiciently low to produce the desirable low drop. Previous attempts to accomplish the above results have failed in one or more of the above aims. The dimculties encbuntered in prior attempts have cathode, knock off'particles of the coating, cause excessively hot spots to occur at local points on the cathode, and in a short time, either by excessive destruction of the coating or by burning out the cathode by said local hot spots, end the useful life of 'saidcathode. In accordance with my invention I have discovered that ii the mass of the cathode-associated with each unit area exposed to the discharge is sufiiciently small and if the cathode as a whole is sufliciently small with respect to the load carried by the device, such a cathode may be provided with an electron-emissive coating and operate in a gaseous atmosphere of desirable pressure, whereupon the tube will have a voltage drop of the order of magnitude of the ionization voltage of the gaseous atmosphere or less and the cathode will have a useful life sufliciently long to satisfy modern commercial requirements.

In the drawing I have shown a rectifier incorporating the above principles, and consisting of a glass envelope 1 having a reentrant stem 2 with a press. 3 at the upper end thereof, said press carrying a cathode 4 and a plurality of anodes 5. The cathode 4 consists of a few turns of metallic ribbon, preferably of platinum iridium alloy.

Said alloy may contain about ten percent. iridium and about ninety per cent. platinum. The two ends of the cathode 4 are welded to the upper ends of supporting standards 6' and I sealed in the press 3.

Each anode 5 consists of a short rod of graphite carried at the upper end of a nickel rod 6. In order to support the anode 5 on the rod 6, a. nickel sleeve 1 fits snugly around the anode 5 and rod 6,-and may be welded to the rod 6 by a few spot welds at the lower end of said sleeve I. The anode 5 is maintained in desired position by frictional engagement with the sleeve 1. The lower been due primarily to the fact that when the distance is less than the mean free path of the ors, through the cathode coil 4.

electrons in the gas. The lower end of the sleeve 8 i likewise sealed in the press 3. In arrangements of this general type diiflculties have been encountered in discharges occurring betweenthe lower end of the sleeve 8 and the lower end of the anode structure, due to leakage paths and the like through the seal. I have found that this tendency can be very simply avoided by sealing the lower end of the sleeve 8 higher up in the press 3 than the lower end of the anode rod 6. This relationship is that as shown in Fig. 4.

The lower end of each anode rod 6 is provided with a conductor ll likewise sealed in the press 3 and extending through said press and reentrant stem to the exterior of the tube. The cathode standards 6' and 1' are likewise provided with conductors l2 and I3 sealed in the press 3 and extending through aid press and reentrant stem 2 to the exterior of the tube. Th tube I may be provided with a'conventional base l4 having a plurality of external contact prongs. The two anode conductors II are connected to corresponding anode contact prongs l5, respectively, and the two cathode conductors l2 and I3 are connected to a common cathode prong IS.

The press 3 is also provided with-an additional wire 9 sealed therein, at the upper end of which is carried a getter cup I which carries getter material for cleaning up residualgases during evacuation of the device in accordance with the well-known practice pertaining thereto.

Although a device built in accordance with my invention may operate satisfactorily with various types of electron-emissive cathode coating, I prefer to use a special coating process which produces a cathode surface which is very durable and able to withstand considerable abuse, and thus considerably lengthen the life of the tube in which it is used. The platinum iridium ribbon is first coated with a suspension of one part of a mixture consisting of fifty per cent. barium carbonate and fifty per cent. strontium' carbonate in two parts of a water solution containing five per cent. barium nitrate. The cathode so coated is placed in a flame, which is preferably a reducing fiame, and heated to a temperature at which the liquid is evaporated and the remaining coating is fused onto the cathode surface.

The flame which I have used satisfactorily consists of one produced by a mixture of illuminating gas and air. After this coating has been fused, it has a glazed external appearance. The cathode may be subjected to several such coating steps. Ordinarily in cathodes which I have constructed in this manner,I have found that two coatings are sufilcient.

The cathode coated in this way is mounted in the tube as described above. The tube is then evacuated in accordance with the usual practice.

' During the evacuation the cathode is subjected to further treatment to convert the coating there'- of to its final form. In order to do this, I separate the two cathode conductors l2 and I3 and pass heating current, by means of said conduct- This heating current heats up the cathode to a sufliciently high temperature so that the coating on the cathode is further broken down. Under. these conditions the coating loses its fused appearance, and presumably the coating is reduced to the oxides of barium and strontium.

After the cathode has been treated in the above tube is then filled with a gas at a pressure suillciently high to produce copious ionization therethrough upon the passage of a discharge through the device. I prefer to use a rare gas or a mixture of such gases at a pressure of about four to five millimeters of mercury. More particularly I prefer to use argon at the above pressure, due to the fact that argon has a comparatively low ionizing voltage. Previous attempts to use argon in tubes of this type have been avoided because each argon ion has a comparatively high mass, and therefore its destructive action on a cathode which'it bombards is increased over gases having lighter ions. Tubes embodying my invention, however, have made feasible the use of argon.

Since in starting a device of this kind it is impossible to avoid the occurrence of high-speed ions, bombarding the cathode and to entirely avoid the destructive action of such ions on the cathode, I have found that this destructive action can be reduced to a negligible amount by causing the starting period during which such high-speed ions occur to be reduced to a very short time;

Y Thus if the cathode during each starting operation is subjected to high-speed ions during but such bombardment is so small that the tube may have a commercially long life before the total bombarding period is sufficiently long to destroy the useful life of the cathode.

In a practical tube designed to carry a load of about fifty milliamperes, I have found that a satisfactory size of cathode can be produced by winding five turns of .0006 -inch by .010 inch platinum iridium ribbon on a forty mil mandrel. If such a tube is connected to a circuit which ordinarily draws full load therefrom, an initial current of about four or five milliamperes with a drop of about 100 'volts will start through the tube. The cathode is of sufflciently small size so that it is heated to temperature of copious thermionic emission in about half a second. At the end of this time the full load of about fifty milliamperes will be passing through the tube and the tube will have a voltage drop of about twelve volts, which is less than the ionization voltage of the argon. Thus the cathode will be subjected during such a starting period to a maximum of a half a second bombardment by fast-moving positive ions, and this period in actual operation is often much less.

The use of a cathode having very small mass with a comparatively large area of electron emission associated therewith enables the tube in which it is used to operate with a very low volt: age drop. In devices in which the load current contributes to the heating of the cathode, the energy utilized in this heating manifests itself in an increase in drop through the tube. Even in devices in which a separate heating current is used, the discharge itself even contributes to the heating of the cathode, but if the cathode is of manner, the getter material in the getter cup I!) the usual size, comparatively large amounts of energy may be liberated at the cathode without appreciably changing the operating temperature thereof. I have made my cathode, however, so small that a negligible amount of energy liberated at the cathode is sufficient to raise its temperature many degrees. The energy, therefore, necessary to raise the cathode to its operating temperature and maintain it at said temperature is negligible in so far as the normal losses in the tube are concerned, so that the drop through the tube is not appreciably increased by the fact that the discharge current itself maintains the cathode at temperature of thermionic emission. In spite of the small mass of my cathode, it nevertheless.

has sufiicient area so that copious electron emission is provided which is amply sufficient to carry full load current to the device and also to supply any overload which the tube may be called upon to supply. As pointed out above, the mass of the cathode is so small that any tendency for an increase in voltage drop through the device liberates sufficient energy to greatly increase the tem-- perature of the cathode which tends to reduce the voltage drop. Therefore, with the construction which I have shown, the tube tends to operate at a minimum voltage drop, and probably for this reason operates at a voltage of the order of the ionization voltage of'the gas or less.

The tube which I have illustrated may be con- A transformer I6 may have its primary I! connected to a source of alternating current, and its secondary l8 may be connected at its opposite ends to the two anode prongs IS. The center of the secondary winding l8 may be connected through a, suitable load circuit l9 to the cathode prong I6.

I have used such a tube illustrated above in radio sets in which case the load l9 consists of a filter circuit connected to the space discharge circuits of various radio tubes. In this type of installation in which the transformer I1 is energized at the same time as the heating elements of the radio tubes, a substantial period of time elapses before a full load is drawn from the tube. This is due to the fact. that space current does not fiow through the radio tubes until their cathodes have heated up to operating temperature. This is particularly true with tubes-that use separate heaters for their cathodes. preliminary period there is suflicient leakage through the filter itself so that a small amount of current is passing through the rectifier. This amountof current is ordinarily insuflicient to heat the cathode to temperature of thermionic emission, and therefore during this period the cathode is being subjected to bombardment of fast-moving positive ions. However, this total preliminary period does not ordinarily exceed.

about ten seconds, at the end of which time the full load is being drawn by the tubes in the radio circuit. Moreover, due to the fact that the normal heating time of the cathode under a load of about four or five milliamperes is of the order of magnitude of a half a second, I have found that the cathode is raised to temperature of V thermionic emission substantially simultaneously with However, in this the passage of full load current through the radio ber of starting operations to afiordthe device a' commercially practical long life.

The arrangement which I have shown provides additional means for decreasing the destructive action of the preliminary bombardment period, particularly when used with such devices as radio sets in which the full load current does not occur immediately. The destructive action of the positive ion bombardment is dependent not only upon the speed with which the ions reach th cathode but also upon the number of ions bombarding each unit area of the cathode, or in other words upon the current density of the cathode during this bombarding period. By decreasing the current density per unit area of the cathode during such starting period, I have been further able ,to decrease the destructive action of such bombardment. In the arrangement which I have shown, I have decreased the current density on the cathode first of all by providing a cathode having a relatively large area for the mass thereof. Thus the cathode 4 consists of very thin ribbon, such as I have described above. Also I have provided an additional area of conductive material connected to the cathode over which the initial glow discharge current may spread. 'I'hus insteadof protecting the cathode standards 6' and 1' against this initial glow discharge current, these conductors are deliberately left exposed so that this initial current impinges not only upon the cathode itself but also upon the leads connected to the cathode. By providing this additional area upon which the glow discharge current can occur, the density of current on the cathode itself is greatly decreased, and to this extent the destructive action of the positive ion bombardment is likewise decreased.

After the cathode has been raised to temperature of thermionic emission, the glow discharge nates from the coated turns of the cathode 4. Under these conditions, substantially none of the current emanates from the standards 6' and 1'.

This invention is not limited to the particular details of construction, materials or processes as described above as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. An electrical space discharge device comprising an envelope containing an ionizable gas, a stem including a glass body and supporting electrodes adapted to support an ionizing discharge through said gas, one of said electrodes being a rod-shaped anode structure sealed in said glass body, a conducting shield surrounding said anode structure and spaced therefrom a distance sufi'iciently short to prevent the occurrence of a discharge through said space, said shield being also sealed in said glass body, and a leadin wire of substantially less diameter than the diameter of said anode rod connected to .the lower end of said anode rod and sealed in said glassbody, the lower end of said shield being at a higher level in said glass body than the lower end of said anode rod. i

2. An electrical space discharge device comprising an envelope containing an ionizable gas, a stem including a glass body and supp rting electrodes adapted to support an ionizing discharge through said gas, one of said electrodes being an anode, said anode comprising a body of carbon mounted at the upper end of a metallic rod sealed into said glass body, a conducting shield surrounding said anode and rod and spaced therefrom a distance suiliciently short to prevent the occurrence of a discharge through said space, said shield being also sealed in said glass body, and a lead-in wire of substantially less diameter than the diameter of said anode rod connected to the lower end of said rod and sealed in said glass body, the lower end of said shield being at a higher level in said glass body than the lower end of said rod.

3. An electrical space discharge device comprising an envelope containing an ionizable gas at a pressure sufliciently high to produce a selfsustaining arc, and electrodes adapted to support an ionizing discharge through said gas, one of said electrodes being a thermionic cathode adapted to be heated to temperature of thermionic emission solely by the current passing through said discharge, said cathode compris ing a metallic ribbon having a thickness of the order of magnitude of .0006 inch and a width of the order of magnitude of .01 inch having thereon a coating of electron emissive material, and having a sufiiciently small mass to be heated to temperature of thermionic emission in a period of the order of magnitude of half a second when connected to a normal full load circuit.

4. An electrical space discharge device comprising an envelope containing an ionizable gas at a pressure sufiiciently high to produce a selfsustaining arc, and electrodes adapted to support an ionizing discharge through said gas, one of said electrodes being a thermionic cathode adapted to be heated to temperature of thermionic emission solely by the current passing through said discharge, said cathode comprising a metallic foil having thereon a coating of electron emissive material, and having a sufficiently small mass to be heated to temperature of thermionic emission in a period of the order of magnitude of half a second when connected to a normal full load circuit.

5. An electrical space discharge device comprising an envelope containing an ionizable gas at a. pressure sufliciently high to produce a selfsustaining arc, and electrodes adapted to support an ionizing discharge through said gas, one of said electrodes being a thermionic cathode adapted to be heated to temperature of themionic emission solely by the current passing through said discharge, said cathode comprising a metallic ribbon having a thickness of the order of magnitude of .0006 inch and a width of the order of magnitude of .01 inch having thereon a coating of electron emissive material, and having a suii'iciently small mass to be heated to temperature of thermionic emission in a period of the order of magnitude of half a second when connected to a normal full load circuit, and external contact members on said envelope, the ends of said ribbon being connected to one of said contact members.

6. An electrical space discharge device comprising an envelope containing an ionizable gas at a pressure sufiiciently high to produce a selfsustaining arc, and electrodes adapted to support an ionizing discharge through said gas, one of said electrodes being a thermionic cathode adapted to be heated to temperature of thermionic emission solely by the current passing through said discharge, said cathode comprising a metallic foil having thereon a coating of electron emissive material, and having a sufliciently small mass to be heated to temperature of thermionic emission in a period of the order of magnitudeof half a second when connected to a normal full load circuit, and a single external contact member on said envelope for said cathode.

JAMES D. LE VAN.

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