US2677071A - Voltage reference tube - Google Patents

Description

p l 27, 1954 G. G. CARNE VOLTAGE REFERENCE TUBE Filed June 30. 1948 ma 7 3 r A w. E M a U M D y M ww E I! E 5 W W m T i m a 5 z & w 4 MW 0 M A nNWnnkg 2 a W w rare earths.

Patented Apr. 27, 1954 2,677,071 VOLTAGE REFERENCE TUBE Gerald G. Carne, Rockaway,

N. J., assignor to RadioCorporation of America, a corporation of Delaware 1 Application June 30,

8 Claims.

My invention relates to a gaseous discharge device and more particularly to an improvement in a voltage reference tube. I

Since the voltage drop across a cold cathodeglow discharge tube is substantially independent of the current through the tube over a wide range of current values, tubes of-this type are used in voltage regulator circuits in applications where it is necessary to maintain a constant direct current output voltage across a load, independent of load currents and moderate line-voltage variations. Such a regulated power supply finds application with laboratory test equipment, as a substitute for batteries, or to provide a constant source of voltages for plate, screen or grid bias of vacuum tubes.

One of the most serious drawbacks to the use of known glow discharge tubes in direct current voltage regulator circuits for some applications, is the abrupt changes or jumps in the voltage of the tube in the order of tenths of volts. These jumps in voltage are observed to be a result of the cathode glow shifting from one section of the cathode area to another.

Another type of instability during the operation of certain kinds of tubes of the glow discharge type, takes the form of a slowly increasing voltage during the operational life of the tube. The voltage of such tubes has been known to increase an amount of volts in 200'hours of tube operation. This voltage change requires constant adjustment whenever measurements are to be made. Such a voltage change has proved detrimental in some voltage regulator systems. This slow shift of tube voltage is due to a slowly changing cathode work function.

In one type of glow discharge tube as disclosed, the cathode electrode is coated with misch metal to nrov1de a low work function for the electrode and relatively uniform tubeoperation. Such a tube is disclosed in my co-pending application Serial No. 748,270 filed May 15, 1947, which is now U. S. Patent No. 2,556,254 issued June 12, 1951. Tubes of this type sometimes have a certain disadvantage in that the misch metal is a non-uniform mixture of iron, cerium, and other This non-uniformity of the misch metal surface provides .a variation from tube to tube of operational characteristics. Thus, it is difiicult to replace in service a tube of this type with another having identical operating characteristics. Also, during the life of a tuberof this type, the misch metal coating: on the cathode surface is gradually sputteredlaway dueito the born,- bardment of the cathode surface bypositive ions 1948, Serial No. 36,251

during tube operation. Although it has been attempted to produce a synthetic misch metal for coating the cathode electrodes, such attempts have been unsuccessful due to complications involved, as well as the cost.

It is therefore an object of my invention to provide a glow dischar e tube of improved operation.

It is also an object of my invention to provide an improved glow discharge tube in which the abrupt changes in tube voltage are eliminated.

It is alsoan object of my invention to provide a glow discharge tube having a constant voltage characteristic during the operational life of the tube.

It is a further object of my invention to provide a method for making a type of glow discharge tube resulting in tubes having close to identical characteristics.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the inven tion itself will be best understood by reference to the following description taken in connection with the accompanying drawing, in which:

Fig. 1 is an elevational sectional view of a gas discharge tube according to my invention.

Fig. 2 is a graph showing the relationship between changes in voltage and changes in current of my improved tube.

The tube of Figure l is a glow type discharge tube having an evacuated envelope l9. Within the envelope it there is enclosed a gaseous medium comprising principally a mixture of neon and argon. The envelope ill has a tubular configuration in which the upper end is closed by sealing off at H after exhausting and filling with gas. The lower end of the tubular envelope is closed by having a glass button stem I2 sealed at its periphery to the tubular envelope it. Sealed through the bottom of the glass stem base I2 are a plurality of metal base pins :53 which extend into the envelope and form electrical leads to the several electrodes enclosed therein. Supported from the stem base 52 of the tube is an annular cathode plate electrode it having a central opening It. The cathode plate I4 is supported by metal rods it sealed through the glass stem [2 as extensions of respective base pins 58. Support rods It are fixed to the under side of the cathode plate. Also, supported from the button. stem 92 is an anode rod electrode 29 extending coaxially within the tube envelope H] and through the centerof the cathode opening 15. The anode rod 26 is spaced from the cathode plate i l. The lower end of the anode rod 28 is supported below the cathode plate I l by a transverse metal support 24 which in turn is mounted on the button stem 12 by support rods 26. Rods it may also be extensions of respective base pins is sealed through the glass stem ii. The upper end of the anode rode is maintained in alignment coaxially within the tube envelope ill by a spacer mica 22. A rivet-sleeve 39 is welded to the top of the anode rod 28 and fastened by metal fasteners M to the mica spacer 28. The mica spacer 28 also supports a getter support wire 32 formed as horse shoe loop. Between the ends of the wire loop 82 a channel member is fixed. This channel member contains the getter material, which is flashed during manufacture as is well known in the art. Mounting the getter support loop 32 on the mica is a metal tab lid, mechanically locked through the mica.

The operation of the tube is such that, when an appropriate voltage drop is established across the cathode electrode Hi and the anode rod a glow discharge will occur. By providing an ap" propriate metal for the emitting cathode surface of electrode 14 and an appropriate mixture of rare gases at a predetermined pressure within the tube envelope it, it is possible to provide the desired tube characteristics. For example, it was desired to operate the tube of Figure 1 with an operating voltage of less than 100 volts across the tube and a current flow of around 1 milliampere. To provide a cathode emitting surface having a sufficiently low work function as well as long life, pure molybdenum metal is used for the cathode electrode I l. A gas mixture of 99 percent neon and 1 percent argon is maintained at around a pressure of 3'! millimeters of mercury within the tube. These conditions of gas mixture and pressure as well as the cathode ma terial will provide approximately an 85 volt drop across the tube and a normal cathode current density of l milliampere per square centimeter of cathode surface area.

The use of a pure molybdenum cathode elec trode eliminates the difficulties which exist in using a misch metal coating described above. Previously, it has been extremely difficult to pro vide a satisfactory molybdenum cathode electrode in a glow discharge tube. The surface of the molybdenum metal is contaminated principally with certain oxides formed by exposure to air. It has been extremely dinicult to remove these oxides from the molybdenum surface by any well known means. the molybdenum to temperatures close to the melting point of the metal fail to decompose all of the oxides on the molybdenum surface. A molybdenum cathode electrode cannot be suc cessfully used in a glow discharge tube of the type described, relative to Figure 1, without the removal of all contamination. from the electrode surface. The contaminating molybdenum oxides on the cathode provide a surface which is nonuniform in work function and result in an unstable tube. This is due to the fact that the molybdenum oxides on the electrode surface are decomposed and changed in composition during tube operation by positive ion bombardment. Thus, during the operation of such a tube, continuous change of composition of the cathode surface results in a variable voltage drop across the tube and also provides a voltage shift dur' ing the life of the tube. Due to these inherent difficulties, the use of molybdenum has, up to Even the heating of i lit now, been avoided in gas discharge tubes of the type described.

My invention is directed to a method of ageing or processing a glow discharge tube, of the type described, having a molybdenum cathode electrode, in a manner which eliminates the inherent difficulties of a molybdenum cathode electrode. The tube, shown in Figure l, is constructed along conventional lines as described above. After the envelope is is exhausted, the tube is filled witthe mixture of neon and argon gas. The tube is sealed off at H and then the getter material 33 is flashed to absorb any deleterious gas within the tube. The tube is now submitted to an ageing process, which will remove all contamination from the molybdenum cathode surface 1 1. This ageing process provides a thorough cleaning of the cathode surface so that there results a surface of pure molybdenum metal. The ageing process is principally that in which a positive ion bombardment of the cathode sur face M is established within the tube. The leads [8 of the cathode l4 and the anode rod 25) are connected to a source of rectified alternating current supply voltage of 220 volts root mean square. The current flow through the tube, which is a. pulsating direct current, is limited by a series resistance consisting of a 10 watt 11? volt lamp. The rectified 220 volt root mean square alternating line voltage provides peak pulsating direct current line voltages of around 300 volts. The peak direct current voltages between the cathode Hi and anode 20 in the tube itself will be somewhat less than 300 volts, because of the potential drop across the limiting resistance. The application of this voltage to the tube initiates a gas discharge between the cathode i l and the anode 29 such that the cathode electrode it is bombarded by positive gas ions. The pulsating direct current voltages across the tube pro duce a high energy ion bombardment of the cathode surface M without a high average current flow. In this manner, the cathode is not damaged by heat dissipation since the average current flow between the electrodes Hi and 25 only amounts to around 25 milliamperes. The tube is subjected to the ageing process for a period of 48 hours. During this ageing of the tube the cathode surface I l is under continuous born-- bardment by the positive gas ions of the discharge. This ionic bombardment is sufficient to remove from the cathode surface it, all of the contaminations which are on the surface. Also, quite a number of molecular layers of the surface are removed, such that there is left a clean, untformly pure, molybdenum surface.

The ageing process described above need not be confined to the voltages used. However, those described have proved to be satisfactory. Furthermore, the ageing time is not critical since it has been found that any time between 13 to 60 hours will provide an optimum cathode surface. However, 48 hours has proved to be the best, shortest time for ageing of the tube under the conditions described. Increasing the ageing to 60 hours provides a slightly better tube but not enough to warrant the additional time expended. It is well recognized that under different conditions of ageing, there will be also, correponding times which provide the optimum re- I sults.

as is shown in Figure 1 at 36. As the agein processis continued, the resulting layer 36 will comprise essentially pure molybdenum on the exposed surface. In this manner, the contamination driven off of the cathode surface M will be first deposited upon the envelope wall l0 and then coated over with a molybdenum layer which effectively seals the contamination from being exposed to the discharge of the tube during tube operation. Also, the molybdenum layer 36 serves to effectively seal the surface of the glass envelope l 0 to prevent the release of occluded gasses from the envelope wall during tube operation.

As described in my co-pending application, it has been found that the tube envelope I0 takes on a negative charge due to the afiinity of the glass of the envelope for negative electrons produced during tube operation. This results in the inner surface of the envelope Ill becoming negatively charged and produces a positive gas ion bombardment of the envelope wall which will release occluded gasses from the envelope wall. Thus, the molybdenum film 36 effectively shields the glass wall from the ion bombardment, as well as forms a means for sealing the porous surface of the glass to prevent the escape of the occluded gasses during tube operation. Since the occluded gasses in the envelope wall ID are prevented by the molybdenum film 36 from being released, there is no poisoning of the cathode surface l4 by these gasses. Previously, such poisoning of the cathode surface produced a discharge tube having an unstable characteristic evidenced by a slowly increasing voltage drop during the life of the tube. This increasing voltage drop necessitated a continual adjustment during the life of the tube. Glow discharge tubes of the type described and coated with a film 36 of molybdenum do not exhibit this gradual voltage shift. The sputtering of the molybdenum metal from the cathode surface is such that the film 36 will extend from the mica 28 to a point below the cathode disc M. This portion of the inner surface of the envelope I0 is also that which is exposed to the discharge of the tube during operation and is thus the same portion of the envelope surface which would release any occluded gasses during tube operation.

Tubes, made in a manner similar to that described above, and similar to that of Figure l, have a greater stability and very little variation in operating characteristics from tube to tube. Over a period of time, it has been found that tubes, made in this manner, operate at essentially the same voltage and at only a. small variation over a short range. Furthermore, it has been found that tubes of this type and made according to the process described, show a very small voltage drift during the life of the tube. These improved operating characteristics of the tube are due to the pure molybdenum cathode surface which is less subject to shifts in the work function of the cathode surface due to contamination during tube operation. Furthermore, the highly cleaned molybdenum surface provides a uniformity of cathode surface which was impossible to produce in prior tubes.

The voltage drop across a gas discharge tube of the type described is a function of the kind of metal used. Molybdenum is a metal which gives a voltage drop below 100 volts. It is possible to provide tubes of the type described using other metals from the cathode electrode 14. However, the process described would be limited to metals which can be readily cleaned by ionic bombardment and also to metals which have a work function providing the voltage drop desired. For example, zirconium metal may be substituted for the molybdenum in the glow discharge tube of Figure l. Zirconium also will provide a low voltage drop in this tube. However, zirconium does not result in as stable a tube, since it is a more active metal and more susceptible to poisoning. Thus, for example, if there is deleterious hydrogen within the tube, it will poison the zirconium cathode and cause a drift in the voltage character of the tube during life. A molybdenum cathode is not as susceptible to hydrogen poisoning but is mainly affected by deleterious oxygen present in the tube. Cathodes may also be made of pure nickel, for example, which may be cleaned by positive ion bombardment. However, the use of a nickel cathode will provide a high voltage drop through the tube of approximately volts.

Figure 2 discloses a well known relationship between the current and voltage of a glow discharge tube of the type described for Figure 1. If the current is maintained sufiiciently low by means of an external resistance, the discharge covers only a portion of the cathode surface l4. An increase in the current through the tube causes the discharge to spread over a greater portion of the cathode surface so that the current discharge per unit area of the current density of the tube discharge as well as the voltage drop across the tube will remain constant during this period of current increase. This condition is shown in Figure 2 by the curved portion between the points A and B. This portion of the curve illustrates a condition of a constant normal current density. The curve between A and B is characteristic of glow discharge tubes and this range of operation is conventionally utilized in voltage regulator circuits, where it is. necessary to maintain a constant direct current output voltage across a load. However, it is well known that one of the most serious difficulties in the use of glow discharge tubes operated within these limits is the rapid shifting or the abrupt small changes in voltage across the tube during tube operation. These jumps are not accumulative but merely are changes back and forth in the order of tenths of volts from the average tube voltage. Under some conditions, these abrupt changes of the tube voltage can be ignored. However, under other conditions, such rapid shifts in the voltage of the tube cannot be tolerated. It has been necessary then to design a glow discharge tube in which these rapid shifts in tube voltage are eliminated.

The principal cause tube voltages described for the rapid changes in above is the jumping of the glow discharge from one portion of the cathode surface to another unused portion of the surface, having a different work function. It is well known in tubes of this type that with the current density remaining constant, the total tube current may be increased until the glow discharge covers the entire surface of the cathode electrode. At this point the tube has reached the critical condition at which it is no longer possible to increase the current flow through the tube without a corresponding increase in the tube voltage and an increase in the current density of the cathode discharge. This critical point is represented by B in Figure 2 and the curve from B to C indicates the characteristic rise in tube voltage as the current flow through the tube is increased.

I have also designed the tube in Figure 1 to be operated on that portion of the curve between points B and C. The current density discharge from the cathode surface changes beyond the critical point B and in relation to a change in the tube voltage. This portion of the curve between B and C is known as that of abnormal current density. I have found, that when a glow v.ischarge tube is operated at a point in the abnormal portion B--C of the curve, the rapid changes in tube voltage as described above are eliminated. When. operating in this abnormal range the glow discharge or cathode emission takes place from the entire cathode surface it and there is no unused portion of the cathode surface to which the cathode glow or discharge may shift. To provide a cathode emission surface of constant area, I have coated the under surface of the cathode plate 44 with an aluminum coating. The higher work function aluminum coating prevents the discharge glow from spreading to the under surface and causing the above described small variations in the tube voltage.

In operating a tube similar to that of Figure 1, it may be connected in series with a resistance between the terminals of a source of regulated direct current voltage. The resistance may be adjusted to a value such that the current flow through the gas tube may be large enough to pro vide a cathode glow discharge over all of the upper surface area of the cathode electrode it and such that the cathode discharge has an abnormal current density. Under these conditions, the tube will be operating in the current density represented by curve B-C of Figure 2.

As mentioned above, the work. function of molybdenum is such that the voltage drop across the tube is around 85 volts. The normal current density of a molybdenum electrode at this voltwiil be approximately 1 milliampere per cm. Since it is desired that the total current flow through the tube of Figure 1 shall be approximately 1 milliampere, the cathode surface M is designed to have an approximate area of 0.8 cm. This cathode surface area will "esult in the desired tube current of 1 millianipere when the tube is operated within the abnormal current density range.

Tubes, which have been constructed according to the above described manner, have given excellent results. These tubes after being properly processed aged have been operated for test periods greater than 100 hours. By operating these tubes entirely within the abnormal current density range during these test periods, I have observed no abrupt changes in voltage greater than 0.05 volt. In operation, tubes of this type have proved of great value in voltage reguiator circuits and for voltage reference applications. In such applications, a constant current flow, which can vary only to the order of microamperes, is maintained through the tube by a control. circuit. When the tube is being operated within the abnormal current density range, such changes in the current in the order of microamperes will produce shifts in tube voltage in the order of millivolts and under extreme conditions these shifts in tube voltage may reach only 0.02 volt.

The glow discharge tube described above which is designed according to my invention has eliminated two sources of trouble during tube operation, by solving the problem of rapid voltage shifts and also the problem of gradual tube voltchange. I have a gas discharge tube which has greater application than the conventional glow discharge tube. This improved tube may be depended upon. to maintain a more constant voltage in applications where shifts of tube voltage in the other of tenths of volts have proved detrimental. Also, this tube which maintains a more constant voltage drop has found much valuable use as a voltage reference source in equipment where voltage changes found in conventional tubes prove detrimental.

While certain specific embodiments have been illustrated and described, it will be undertsood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What I claim as new is:

1. A glow discharge device comprising an envelope having a tubular portion, a gaseous Inedium within said envelope, a molybdenum plate forming a cathode electrode coaxially mounted within said tubular envelope portion, said cathode plate having an aperture at the center thereof, an anode electrode rod within said tubular envelope portion, said anode rod coaxially extending through the aperture of said cathode plate and spaced therefrom, said cathode electrode being of pure uncontaminated molybdenum to provide a constant work function of said cathode electrode, an aluminum coating on the other side of said catho -.e plate to inhibit a discharge between said other side of the cathode plate and said anode electrode, and a pure molybdenum coating on the inside surface of said tubular envelope portion in the region of said discharge.

2. The method of processing a glow discharge tube having an anode electrode and a metallic cathode electrode mounted within an envelope, said method comprising the steps of, filling said envelope with a neutral gas, connecting said anode and oath dc electrodes in series with a limiting resistance and a source of pulsating direct current voltage having peak voltages of around 300 volts to establish positive ion born bardment of said cathode electrode, continuing said positive ion bombardment until metal is re-- moved from said cathode and sputtered onto said tube walls.

3. The method of processing a glow discharge tube having an anode electrode and a pure metallic cathode electrode mounted within an envelope, said method comprising the steps of, filling said envelope with a neutral gas, connecting said anode and cathode electrodes in series with a limiting resistance and a source of pulsating direct current voltage to provide high-energy positive ion bombardment of the cathode surface, said voltage consisting of short voltage pulses of around 390 volts, continuing said positive ion bombardment until the contaminated surface layers of the cathode electrode are removed.

4. The method of process ng a glow discharge device having an anode electrode and a pure molybdenum cathode electrode mounted within a gas filled envelope, said method comprising the steps of, connecting said anode and cathode electrodes in series with a limiting resistance and a source of pulsating direct current voltage electrodes to provide high energy positive ion bombardment of the cathode surface, said voltage consisting of short voltage pulses of around 300 volts, continuing said positive ion bombardment until the contaminated surface layers of the cathode electrode are removed.

5. The method of processing a glow discharge device having an anode electrode and a pure molybdenum electrode mounted within a gas filled envelope, said method comprising the steps of, connecting said anode and cathode electrodes in series with a limiting resistance and a source of pulsating direct current voltage to provide high energy positive ion bombardment of the cathode surface, said voltage consisting of short voltage pulses of around 300 volts, continuing said positive ion bombardment until the contaminated surface layers of the cathode electrode are removed and a molybdenum film is sputtered from the cathode onto the adjacent envelope wall.

6. The method according to claim 5, wherein the average current flow between said electrodes during said positive ion bombardment is around 25 milliamperes.

7. The method according to claim 6, wherein said positive ion bombardment is continued for about 48 hours.

8. The method according to claim 5, wherein said limiting resistance is a 10 watt 117 volt lamp.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES The Normal Cathode Fall of Molybdenum and Zirconium in the Rare Gases. by Jurriaance et al., Philips Research Reports, volume 1, No. 3, April 1946, pages 225-238.

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