US1932025A - Electrode positive column lamp - Google Patents

Description

Oct. 24, 1933 c. H. THOMAS 1,932,025

ELECTRODE POSITIVE COLUMN LAMP Filed Dec.

SOL/D ELECTEODES .DE/LLED ELECT/EODES Patented Oct. 24, 1933 UNITED STATES PATENT OFFICE 1,932,025 ELECTRODE POSITIVE COLUMN mm Application December as. 1929 Serial No. 417,091

17 Claims.

This invention relates to electric devices of the gaseous conduction type and more particularly to gas discharge devices in which the discharge is maintained between relatively cold electrodes.

6 It is one of the objects of this invention to improve the life, maintenance and operating efliciency of electric devices of the gaseous conduction type.

\ It is another object of this invention to provide 10 a relatively cold electrode for a gaseous conduction device capable of becoming thermionically active when subjected to positive ion bombardment during operation of the device.

It is another object of this invention to provide an electrode for a gaseous discharge device comprised substantially of a thermionically active metal which may be incandesced at least in part by positive ion bombardment to a temperature of active electron emission without deleterious sputtering.

Another object of this invention is to provide an electrode for a gaseous discharge device operable in said device at relatively high current densities and relatively low electrode potentials.

Another object of this invention is to provide a gaseous discharge device which may be operated at relatively high electrode current densities and relatively low electrode potentials.

Another object of this invention is to provide 3 means for obtaining thermionic electron emission from an electrode in a gaseous discharge device without incandescing the same by electrical energy from an auxiliary circuit.

Other objects and advantages will become apparent as the invention is more fully disclosed.

Heretofore in the art it has been customary to employ substantially solid coherent metal bodies as electrodes in gaseous discharge devices. These electrodes are substantially cold and the glow dis- 13 charge therebetween is the result of the impressing of a potential upon the electrodes in excess to the so-called break down voltage of the specific gas at the particular gas pressures employed and with the specific electrode spacings'used.

l5 As a result of the gas discharge the electrodes are subjected to ion bombardment, and become.

incandesced thereby. Heretofore this incandescing of the electrode to elevated temperatures by ion bombardment resulted in deleterious sputtering of the electrode material. material reacted with or absorbed the inert gas filling, thus reducing the gas pressure within the device, and changing the electrical characteristics of the device. The sputtered electrode material 5 also deposited about the enclosing glass envelope The sputtered of the device, discoloring the same and lowering the efficiency thereof.

To avoid this deleterious electrode sputtering the maximum permissible current densities heretofore employed with the usual type electrode materials approximated .66 amperes per square decimeter.

With solid type electrodes it is customarily noted that when the entire electrode is covered with a glow discharge, the electrode potential required to penetrate the electrode space charge sheath normally increases with increased electrode current density. It is, therefore, apparent that heretofore lower electrode current densities were more eflicient.

It is well known in the art that the electrode space charge sheath and in particular the oathode space charge sheath may be in part neutralized or eliminated by providing an electrode which is thermionically active, the negative stream of electrons flowing therefrom serving in efiect to lower the electrode potential required to bring a positive ion to the electrode surface. Under such conditions higher electrode current densities may be employed than have been heretofore permissible with relatively cold type electrodes.

Heretofore such an efiect' has been produced by employing a refractory metal electrode such as tungsten, incandesced by the passage of an electric current therethrough, from a source independent of the operating potential applied to maintain a glow discharge in the device. Such a device has certain disadvantages from a commercial application standpoint which it is one 90 of the objects of this invention to eliminate, the principal objection being the necessity of supplying suitable electrical heating current for the electrodes.

A second disadvantage is that such an electrode, being operated at a relatively high temperature is subject to the same sputtering eflects heretofore obtained with solid relatively cold type electrodes, and due to ,the relatively small size thereof, a materially shorter operating life 100 and efllciency than with the solid cold type electrodes is obtained.

In accordance with the objects of the present invention I have found, that these deleterious effects heretofore experienced from the use of 105 solid or incandesced electrodes may be substantially eliminated and a gaseous discharge device operating at relatively low electrode potentials over a wide range of relatively high electrode current densities may be obtained by employing a 9 special type of electrode comprised substantially at least in part 0 a thermionically active metal, such as thorium, zirconium, titanium, uranium and the like, and arranged so that the ion bombardment during the operating life thereof is confined to a relatively small area of said electrode, which area is preferably interiorly located in said electrode.

I have found that when one face of a solid cold type electrode is recessed a substantial distance, the glow discharge of a device incorporating the same tends to become concentrated within this recessed portion and that the relatively weak glow surrounding the outer surface of the electrode may be substantially suppressed and the entire glow concentrated within the recessed portion without a material increase in the required electrode potential to maintain the glow discharge, by superficially coating the exterior of the electrode with a refractory insulating material.

I have further found that by thus concentrating the glow discharge within a recessed portion of the electrode a material alteration in the electrical characteristics of a discharge device incorporating the same is obtained whereby I may apply to the electrode relatively large current densities of the order of 5 and 6 times heretofore permissible without materially increasing the electrode fall in potential required to maintain the discharge.

I have further determined that the deleterious sputtering efiect heretofore obtained due to the incandescing oi he electrode by ion bombar ment when excessive current densities are applied are materially mitigated, and are substantially eliminated when the electrode is comprised of metals of relatively low volatility or of rela tively high melting points, even when electrode current densities as high as 8 to amperes per square decimeter are applied to the electrodes.

I have further found that by comprising the electrode at least in of thermionically active material, that 3'. may apply to the electrode a sumciently high currentdensity to efiect substantially an incandescence oi the surface of that portion thereof which is subjected to ion bombardment, to a temperature at which the electrode'material emits thermionic electron emission, which emission may thereby be utilized in improving the operating characteristics of the device.

As a specific embodiment of the practice of my invention I will disclose the application of the same to a gaseous discharge device of the positive column type, as this type of a gaseous controde of the present invention upon the operating characteristics of a discharge device incorporating the same as compared to the operating characteristics of a device incorporating the usual solid electrodes heretofore employed.

Referring to Fig. 1 the discharge device is comprised of a long tubular glass envelope 1 which is relatively small in diameter with respect to its length, having enlarged ends 2 within which are enclosed electrodes 3 integral with support incinber 4 passing through press 5 to make electrical connection with current carrying conductors 8.

In the present illustration the narrow tubular portion 1, is shaped in the form of a letter N and the enlarged portions 2 are bent at right angles to the plane of the narrow tubular portion 1. Electrodes 3 are comprised of metal, and in ac= cordance with the present invention are hollow tubular in form with one end closed, and the other open end '7 substantially facing the channel of the tubular portion 1 of the device.

The electrode structure is shown in greater detail in Fig. 2 which is a cross sectional view of the same showing the hollow tubular feature onthe electrode 3 and the relative depth and diameter of the recessed portion 7 therein. I preferably comprise the electrode 301 .a solid coherent mass of metal, and drill the recessed portion '1 therein in any convenient manner. As a specific embodiment of the present invention electrode 8 may be,

comprised of a highly reactive thermionically active rare refractory metal such as thorium, zirconium, uranium and the like which metals are preferably prepared by the process set forth in copendlng application Serial No. 717,949 filed June 5, 1924 by J. W. Marden et al., entitled Ductile thorium and the method of making the same, which application is assigned to the same assignee as the present invention.

In accordance with the present invention the solid thorium metal body for example, prepared as by the above identified copendlng application is substantially shaped to the form of a hodow cylindrical body having one end open which form may be most readily obtained by a hole in one end of a cylindrical mass of thorium, the specific size of the electrode and relative size and depth of the opening therein being dependent upon the particular discharge device within it is to be incorporated, the desired characteristics oi the electrode, the gas pressure employed, the desired electrode voltages, and the like factors.

Prior to the drilling of the hole in the electrode I prefer to subject the exterior surface oi the electrode to oxidizing conditions, thereby imparting to the surface on adherent thorium oxide insulating coating, which effectively suppresses any glow discharge from the surface. Other surface coatings of refractory materials may be applied however, but I have found that this method of applying the refractory coating to be the most simple.

A common size electrode which is useful in the type device illustrated in Fig. 1 is approximately .15 inches in diameter, about inches in length, in one end of which is drilled a hole of about .075 inches diameter to a depth of about inch.

This electrode is then mounted in any convenient manner upon the electrode support wire 4 and sealed into the glass envelope 1 of the device in the usual manner.

The device is then exhausted by mechanical exhaust means, the glass envelope 1 being baked out for a period of time to eliminate deleterious adsorbed and absorbed gases. Following exhaust the usual inert or monatomic gas filling is admitted and the device sealed oil. Before admitting the inert gas filling within the device the gases should first be thoroughly freed of deleterious atmospheric gases by well known prior art practices. The device is then subjected to a s asoning operation wherein the electrodes are subjected to positive ion bombardment at relatively low current densities thereby effecting substantial cleanup of residual atmospheric gases within the device, the thorium electrodes acting as a "getter" for such gases. While it is expedient from a manufacturing standpoint to effect a prior purification of the inert gases, thorium electrodes will eifect the clean-up of relatively large amounts of atmospheric gases.

An alternative electrode structure is shown in Fig. 3 wherein the hollow tubular electrode 3 is enclosed or coated superficially with an electrical- 1y insulating coating 9, which may be of dissimilar refractory metal oxide material than the metal of the electrode, such as for example, hollow tubular electrode 3 may be comprised of zirconium, or titanium and coating 9 may be comprised of thorium oxide. Another specific combination of electrode material that may be employed in the practice of my invention is a thorium electrode coated superficially with refractory oxides of zirconium aluminum, magnesium and the like. Or for example I may comprise the hollow electrode 3 of a highly refractory metal such as tungsten and coat the interior surface of the hollowed out portion with thermionically active material, such as thorium or I may incorporate the same as an alloyed or admixed constituent of the same.

These and many other variations from the specific hollow thorium electrode herein set forth in my specific embodiment are contemplated as a part of the present invention. 7

The specific advantages that are obtained by the use of the hollow type electrode, on the operating characteristics, life and maintenance of the discharge device incorporating the same, are set forth in the graph disclosed in Fig. 4.

Referring to the graph in Fig. 4, the test upon which these curves are based was made upon two identical glow discharge devices in one of which there was a solid electrode of thorium approximately inches long by .15 inches diameter, and in the other the same sized thorium electrode hollowed out or drilled a depth of one half inch with a hole approximately ,.076 inches diameter. In each device the electrodes were incorporated in opposite ends of a inch glass tubing a distance of 13 mm. apart and a gas pressure of about 10.3 mm. neon introduced. The curves are identified as solid electrode and drilled electrode.

As may be noted in Fig. 4 the glow discharge device incorporating the solid electrodes has a break down voltage of about 280 volts, and an operating voltage of about 1'70 volts. The device incorporating the hollow electrode has a break down voltage of about 270 volts and an operating voltage of about 155 volts.

The reason for this variation in break down voltage and operating voltagev between the two devices is believed to be due either to the effect of the localizing of the glow discharge in the hollow portion upon the electrode field, or to the photo electric effects incident to the use of the particular thermionically active electrode material, or to the eifects produced by reason of electron emission caused by the local overheating of the electrode surface as a result of the more concentrated positive ion bombardment of the cathode, for example, or it may be due to a change in the electric field or to still other factors at this time not apparent.

Whatever the true theory or reason may be, I have found in particular that the usual operating or maintaining voltages of a positive column lamp employing a hollow electrode is materially lower than that of a similar lamp employing a solid peres, from which point a gradual increase inpotential drop is again obtained until a maximum of about 170 volts at to milliamperes is applied.

From thereon the voltage drop decreases again until at about 110 milliamperes the voltage drop of potential across the electrodes is approximately again at the minimum of 155 volts. From this point on the voltage drop appears to stay constant within the range of the present test.

The explanation of this phenomena by reason of which I am enabled to operate a glow discharge device at materially lower operating voltages and materially higher electrode current densities than .have heretofore been permissible, is believed to be in part due to the fact that the electrode space charge sheaths controlling the cathode and anode fall in potential is substantially limited in extent, due to the concentration or limitation, or the confinement of the glow discharge within the interior of the electrode. The voltage required for a positive electron to penetrate the sheath therefore becomes a certainmaximum figure, dependent upon the size and depth of the recessed portion 7 of the electrode, the gas pressure, the electrode composition and the like factors.

In general the electrode space charge sheath of a solid electrode entirely surrounds or encloses the electrode and it requires a certain minimum voltage for a positive ion to penetrate this sheath. Increased electrode current density usually increases the depth of this electrode sheath and also requires increased voltages to penetrate the same. Any voltage in excess of the amount necessary to penetrate the sheath appears to impart added velocity to the positive ion, which is dissipated as heat at the surface of the electrode upon impact of the positive ion thereto and serves substantially as a means of raising the temperature of the electrode.

As may be noted in the curve for the solid electrode in Fig. 2 with increased potential the positive ion bombardment gradually raises the temperature of the electrode to a point where electron emission is obtainable therefrom, with the resulting slight depression in the curve at A indicating increased efilciency.- At this current density the sputtering of the electrode is relatively high and the effective operating life of the device is materially shortened. It is found, however, that the beneficial effect of the thermionic emission is substantially lost at higher potentials as the depth of the cathode space charge sheath increases with increased current density and the thermionic emission from the surface of the electrode is insufficient in amount to materially reduce the cathode drop in potential at this higher current density. The operating life of the device at these higher ciu'rent densities is materially shortened.

In a glow discharge device incorporating a hollow cathode, wherein the glow discharge is confined to the interior recessed portion, the cathode fall in potential increases initially with increased current density in an identical manner as when a solid cathode is employed. At point marked A on the curve the usual increase in cathode drop in potential with increased current density reaches a maximum, and with further increase in current density a decided drop in operating potential is obtained. This is believed due to the efi'ect of limiting the cathode space charge sheath within the confines of the recessed portion of the electrode. As a result of this-limitation a certain maximum voltage only is required to penetrate this sheath.

Electrode potentials in excess to that required to penetrate the sheath are converted into heat energy at the inner surface of the electrode through bombardment by positive ions, coining local thermionic emission spots, photo-electric effects, or by the use of the specific thermionically active material certain electrical effects not heretofore obtainable are developed.

This depression or lowering of the cathode and anode drop in potential with increased current density continues until a minimum potential drop 'is again obtained at point B. From point B to point C increased current densities again increase the cathode drop in potential, the specific cause thereof being not at this moment apparent. It is believed due to a polarizing or piling up action at the opening of the hollowed electrode. The particular degree of rise in the section of the curve between points B and C appears to" be dependent at least in part upon the specific depth and diameter of the hollowed out portion oi the electrode, and to the specific gas and gas pressures employed.

At materially higher current densities indicated at point C the cathode drop in potential is again materially depressed, due it is believed to the fact that the interior electrode surface has become incandcsced by ion bombardment to a temperature where active electron or thermionic emission may be obtained.

The principal effect of thermionic emission as heretofore noted is to break down the electrode space charge sheath by the emission of a stream of negative electrons.

As the electrode space charge sheath is limited by reason of the concentration of the same in the hollow electrode the cathode drop in potential across the device in directly efiected and a decrease thereof is obtained. This decrease in cathode drop in potential continues to point D which is at approximately 110 milliamperes and the curve then flattens out and continues to remain so. It is believed that under these operating conditions the maximum neutralization of the space charge sheath by thermionic emission from the recessed portion of the electrode has been obtained. At currents much above 110 milliamperes the electrode sputtering is so great that the life of the device is materially shortened.

The highest permissible current densities heretofore employed on the commonly used solid coldtype electrodes in positive column discharge devices is about 1.5 amperes per square decimcter of surface electrode area, depending upon the specific electrode composition. At current den= sities much above this the sputtering of the electrode and the life and emciency or the device is materially shortened.

when the glow discharge is concentrated in a hollowed out portion of an electrode, and the electrode comprised substantially of refractory or substantially non-vaporizable rare refractory metals materially higher current densities may be employed. With a hollow thorium electrode for example, current densities of from 8.5) to 9.!) amperes per square decimeter of surface area have men employed without deleterious sputtering effects. The specific maximum current density that may be applied will in part depend upon the electrode composition, and in part upon the depth and diameter of the recessed portion and upon the particular gas and gas pressure within the device.

While I have specifically disclosed a hollow thorium electrode in the specific embodiment of the present invention, similar beneficial results may be obtained from employing hollow electrodes of the other thermionically active rare refractory metals uranium, zirconium, titanium, etc. I also contemplate as hereinbelore set forth as alternative electrode materials the useof hollow refractory electrodes comprised for example or highly relractory metals such as tungsten, and tantalum, the recessed surface oi which may be coated superficially with a thermionically active material such as thorium, uranium, and the like. Such refractory metal electrodes may also have the more reactive thermionically active metals incorporated therewith as an alloyed constituent or they may be also interiorly coated with other low temperature thermionically active material. The exterior of the electrode may be coated with an electrically insulating material such as thorium oxide, aluminium oxide, magnesium oxide and the like, in accordance with the electrode structure set forth in Fig. 3 herein.

It is also apparent that the specific electrode structure may be applied with similar advantage in other gaseous conduction devices, gas discharge tubes, gas X-ray tubes and the like.

It is apparent, therefore, that there may be many variations and departures made of the specific embodiment herein disclosed without substantially departing from the nature of the invention as may be set forth in the following claims.

What is claimed is:

i. an electrode comprised of coherent thorium one face oi the electrode being recessed at least in part a substantial depth and the remaining faces being surfaced with refractory insulating material.

2. A gas discharge device comprising an enclosing glass envelope, an inert gas filling and at least one interior electrode, said electrode being comprised of an open ended hollow body of thorium. 3. A gas discharge device comprising an enclosing glass envelope, an inert gas filling and at least one interior hollow open ended electrode, said electrode being comprised at least interiorly of thorium and exteriorly surfaced with electrically insulating material.

4. A gas discharge device of the positive column type comprising an enclosing glass envelope, an inert gas filling and two open ended hollow spaced electrodes, said electrodes being comprised at least in part of thorium.

5. In a gas discharge device the method of obtaining thermionic electron emission from relatively cold electrodes which comprises concentrating the positive ion bombardment during operation of said device upon a relatively small surface area of said electrode to efiect incandescence thereof to the temperature of active thermionic electron emission.

d. An open-ended hollow metal electrode comprised of a thermionically active metal body of the thorium group having one face thereof recessed a substantial depth.

7. An electrode comprised at least in part of a thermionically active metal body of the thorium group having one face thereof recessed a substantial depth and the remaining faces surfaced with a refractory insulating material.

8. A gas discharge device comprising an enclosing glass envelope, aninert gas filling and at least one interior hollow open-ended electrode. said electrode being comprised at least in part of a thermionically active metal of the thorium group.

9. A gas discharge device comprising an enclosing glass envelope, an inert gas filling and at least one interior hollow open-ended electrode, said electrode being comprised at least in part of a thermionically active metal of the thorium group and exteriorly surfaced with electrically insulative material.

10. A gas discharge .device of the positive column type comprising an enclosing glass envelope, an inert gas filling and two open-ended hollow spaced electrodes, said electrodes being comprised at least in part of a thermionically active metal body of the thorium group.

11. A gas discharge device of the positive column type comprising an enclosing glass envelope, an inert gas filling and two open-ended hollow spaced electrodes comprised substantially of a thermionically active metal of the thorium group, said device operating at relatively high electrode current densities with relatively low electrode potentials.

12. A gas discharge device comprising an enclosing glass envelope, an inert gas filling and at least onehollow open-ended electrode, said electrode having at least a part of its interior surface of a thermionically active material of the thorium group and exteriorly surfaced with electrically insulating material.

13. An electrode for a gas discharge device comprised of a tubular metallic body closed at one end and having at least a part of the interior surface coated with a thermionically active material.

14. An electrode for a gas discharge device comprised of a tubular metallic body closed at one end, a layer of a thermionically active material covering at least a portion of the interior surface and an electrically insulative material on the exterior surface of said body.

15. A cathode for a gas discharge device comprised of thorium, one face of said thorium cathodebeing recessed an appreciable depth and the remaining faces being surfaced with glow discharge suppressing sheathing material.

16. A cathode for a gas discharge device comprising an open ended hollow thorium metal body exteriorly sheathed with dielectric insulating material.

17. A cathode for a gas discharge device comprising an open ended hollow thorium metal body exteriorly sheathed with material of relatively higher electrode drop in potential.

CHARLES HASTINGS THOMAS.

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