US2824985A

US2824985A - Glow discharge device

Info

Publication number: US2824985A
Application number: US397646A
Authority: US
Inventors: Ted E Foulke
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1953-12-11
Filing date: 1953-12-11
Publication date: 1958-02-25
Anticipated expiration: 1975-02-25

Description

Feb.,25,1958' Cu N RlsE IN DARK BREAKDOWN POTENTIAL R/sE IN DARK BREAKDOWN POTENTIAL.

VOLTS VoLTs Filed Dec. 11, 1953 I- 7 l l N I I I 2\ 1 193- l l I a 4 --l5 l I I 1 I I T I l 7 1 L J ,2 5 6 f 500 ssc V \A v PULSE SouRcE ,3 T H I4 a 20 40 5 a :0. so 3 y RELATIVE ENERGY 3 (DYNAM/c CONDUCTANCE) 3i 1 I0 r VBREAKDOWN '8 5 POTENTIAL g g Q 2 0 I is 0 20 40 60 80 I00 MOLECULAR PERCENTAGE OF THO (MOLECULAR RATIO OF 6A0 To SRO= /-l.38)

. g 20 l 40 S 3, RELATIVE ENERGY g l5 (D YNAMIC CONDUCTANCtj 30 E '4 0 /q E 20 3 1 BREAnoowN POTENTIAL 5 8 5 lg o 0 E5 0 I5 30 15 1;:

MOLECULAR PERCENTAGE OF BAO (PERCENTAGE OF THO: CONSTANT AT 25% BALANCE 5R0) Inventor; Ted E. F'oulKe,

His At orney United States Patent 2,824,985 Byte med lf'eb. 1 9 58 GLow DISCHARGE DEVICE Ted E. Eouiire, Cleveland, Ohio, assignor to General Electric Company, a corporationof New York ApplicationDecemher 11, 1953, Serial No. 397 ,646

6 im (c1. 1. 7 4) This invention relates in general to electric discharge devices and more particularly to glow discharge devices having a pair of cold electrodes operating through field emission.

The invention finds particular application in miniature glow lamps or tubes such as exemplified by the series designated commercially NE and AR. One commercially available miniature glow lamp is ratedfat & watt. Neon glow lamps have starting or breakdown voltage ratings between 45 and 65 volts R M. S. onalternating current, depending upon the pressure of the gas filling, whereas argon glow lamps have somewhat higher ratings. On direct current, the breakdown ratings are naturally higher, being between 60 and 90 volts. It has been observed that the breakdown voltage as between lamps of the same type may vary by as much as 20 percent about the average or rated value. Furthermore, the dark breakdown voltage, that is the breakdown voltage when the lampor tube is located in an opaque enclosure, is usually considerably higher than the breakdown voltage when incident daylight or radiation from other lamps is striking the glow lamp. However, the most common application of miniature glow lamps up to the present time has been as visual indicators, for instance in alarms, automatic controls, household appliances, and wiring devices. For such applications, the performance characteristics are generally not critical and. variations in breakdown potential within a limited range could readily be tolerated.

More recentlysuch glow lamps have begun to find considerable application as circuit elements in electronic apparatus, for instance in electronic computers andother electronic control circuits. In one such application, an NE-Z neon glow lamp or tube is connected as a control impedance in series with a charge accumulating capacitor across a source of 500 microsecond pulses. Whenever a pulse exceeds a certain predetermined value corre-.

sponding to the breakdown voltage of the tube, current flows and charges the capacitor. For reliability of operation, it is essential that the breakdown voltage of the glow tube be constant and that the dark breakdown efiect, that is the rise in breakdown voltage under dark conditions, be as low as possible. Furthermore, by reason of therelatively short time duration of the voltage pulses, it is essentialthat the tube achieve full conductance fairly quickly. This characteristic, which is related to the speed with which full ionization is achieved within the tube, maybe referred to as the dynamic conductance. For

electronic applications, it is generally desirable that the tube have the highest dynamic conductance which can be achieved. a a 1 Glow lamps or tubes commercially available up to the present time have generaly comprised a pair'of electrodes sealed into a glass; envelope containinga filling of an inert gas at a low pressure, for instance neon at 39 millimeters of mercury. The electrodes, which may consist merely of; short lengths of wire in thesmallest lamps orwhich may comprise suitably shaped pieces of sheet metal in mamma -lam r ,e a st ithan e e o al nt s rc 2 mixture. This mixture has consisted of alkaline-earth oxides, generaly a mixture of barium and strontium oxides. With such discharge devices, it has been found that the breakdown in the dark may be as much as 20 percent higher than the breakdown-in light. Furthermore, full ionization or conduction is achieved relatively slowly even under room illumination; in other words, the dynamic conductance is comparatively low, so that response to pulse signals is low and uncert ain.

The object of the invention is to provide anew and improved glow discharge device'having a rise in dark breakdown voltage substantially less than heretofore.

Another object is to provide a glow discharge device in which full ionization is reached more quickly after the initiation of breakdown.

A further object of the invention is to provide a glow discharge device having a dark breakdown voltage approaching that in light and having, for a given electrode configuration or size, a dynamic conductance substantially higher than heretofore possible.

In accordance with the invention, I have found that the addition of thorium oxide (or thoria) within a critical range, to an emission mixture containing barium oxide and strontium oxide within a certain range of proportions, achieves those objects. The tests which I have conducted have shown that an emission mixture containing approximately 15 to 40 mol percent thorium oxide (T1102), 10 to mol percent barium oxide (BaO), with a balance of strontium oxide (810), provides a decided improve ment. A glow discharge device having cold electrodes coated with this mixture shows both a reduction in the dark breakdown voltage rise and an increase in dynamic conductance. For best results, the proportion of thorium oxide should be between 15 to 30 percent; that of barium oxide, between 20 to 40 percent, and the balance strontium oxide. A mixture which is being used in commercial production of glow discharge lamps having operating characteristics in accordance with the stated objects of this invention contains approximately 23 percent ThO- 33 percent BaO, and 44 percent SrO. These percentages are mol percentages, that is based on the proportions of the constituents by molecular weight.

Further objects and advantages of'this invention will ecome apparent by reference to the following description and the accompanying drawing. The features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

In the drawing:

Fig. 1 is a side view of a glow discharge lamp or tube embodying the invention and shown schematically connected in a suitable circuit.

Fig. 2 illustrates graphically the characteristics of the tube for various proportions of thorium oxide in the emission mix.

Pig. 3 illustrates graphically the characteristics of the tube for various proportions of barium oxide in the emission mix.

Referring to Fig. 1, there is shown a glow discharge device 1 corresponding generally to the neon glow lam-p designated commercially NE-Z. The device comprises a vitreous envelope 2 in which is mounted a pair ofwire electrodes 3, 4 welded on the inner ends of leads 5, 6 sealed into a press 7 at the lower end of the tube. Electrodes 3, mayconsist of nickel coated with an electron emissive mixture such as will he described herein. The leads 5, 6 are preferably of dumetfwire, that is coppei' sheathed nickel-iron Wire which seals readily into glass; The tube contains a filling of an inert gas at a low pres sure, for instance neon orargon at pressures from 20: to 20D. millimeters offmercu'ry, 'a'ndis tippedofi at its through the as yetropen upper end :of the. tube.

the. tube is still on the exhaust schedule, electrodes'3, 4' I are'zheated, for instance by an electric induction furnace,

end'SwThe device or tube- '1 has been illustr atedin the 7 drawing approximately'three times 'full sizeg the actual dimensionsfare approximately /1 inch for the diameter 7 and 'M: inch for the over-all lengthof the" bulb. In the illustratedvtuibe which constitutes 'a preferred embodithe distance between them being less than the :minimum ment of the invention, the electrodes are closely spaced,

t tiye coating is preferably produced by applying to the electrodes a mixture ofbarium and strontium'carbonates' together with'thorium oxide in a'suitable binder, such as nitrocellulose dissolved in a suitable vehicle such "as amyl and butyl acetate. Where the application is by spraying rather than by dipping, the vehicle may contain a I higher proportion of bu'tyl acetatein order to increase the rate of cvapoiization. The electrodes are then sealed into the tube bytusing the glass about the leads inthe pre s 7 and the envelope is then thoroughly exhausted While to a temperature sufiicient to decompose, the carbonate to the oxide, the carbon dioxide and other gases being The gaseous filling is then admittedand the device is 'gas 'or la mixture ofiinert gases iata low pressure.

- sure; if a'lowerbreakdown potential is desired. for the lamp, -a small percentage of argon maybe added to the 'neon, for instance 0.8: per'cent argon. When higher pressure are increased substantially. I J After tipping ofi, the lamp may be subjected to the V sealed off by tipping ott the glass tube 8. With a hot flame. V I As'previouslyfstated, the filling consistsof an inert Suitable filling consists of neon at '30 millimeters presare dark operating characteristics; they are obtained device 1 filled with inert gas to a pressure above. 100 millimeters of mercury, specifically of the order of. 130 millimeters of mercury, 'andenclosed in a substantially The curves of Fig; 2 show in the coating varyingfrom 0 to 80 percent, the n'iol rati'ci of barium oxide to strontium oxide remaining constant V at 1 to 1.38. Curve 16 shows" the rise in dark breakdown potential, thatis' the increase in breakdown potential when the device is placed in a dark enclosure over the breaka down potential in room" illumination: Curve 17 shows the relative energy from a 500 'microsecondpulse transmitted through the device, this being a measure of the dynamic, conductance, It will be observed thatin the region from '15 to mol percent of thoriuin oxide the V V vana tionin characteff istics of the tube fora inol percentage of' thoriumo d'e light-tightcompartmeiit which has been represented schematically bytliedottedrectarigle 15. r

rise in dark breakdown potentialis quite ew, and the i dynamic conductance of the 'device is; high. The; for} tuitous concurrence of these characteristics providesa device having vastlylimproved performance'in respect of T the objects of the present invention.

' In Fig. 3, the curves illustrate'the variation in characteristics as the mol percentage of barium .oxide'is varied from 0 to 75 percent, the mol percentageof" thorium oxide being maintained constant at 25 percent, and'theyr exhausterd as evo1ve During this heating ofgggthe 616C? balance being made 'up of strontium oxide.- Here again trodes, the nitrocellulose binder is also reduced-to carbon.

it will beobser'ved thatthe lowestrise in dark breakdown.

oxideg Considering the curvesof Figs. 2 and 3 simulta.

should lie between 15 to 30 percent and the barium oxd'e between 20. to 40 percent; A mixturecohtainifi i by percent barium'oxide and 44.3 percent strontium oxide,

treatment described'in my Patent 1,965,589 for further lowering thework function of the electrodes and achieving a longer lifefor the lamp; In' this process, the leads 5, 6 are connected to a source of steep wavefront, high frequency oscillations such as are produced by the 'dischargeof an inductance through a condenser. 'These by oxygen and resultingin a composite metal: oxide sur:

has beenfound to give the lowest shrinkage, that'is the lowest proportion of defective lamps Which'must 'be rejected; 7

While a glow 'tube or lamp embodyingthe invention has been deseribedin detail, it is to be understood that. this embodiment isintended as illustrative and not as limitative of the invention. Obviously, the electrode coating mixture which has been described can beusedin" lamps and discharge :devices differing .widelyjinsize and configuration from thatwhich. has been described. Ac-

face. The bombardment of electrodes 3, 4 iscarried on untilthere is a uniform corona thereonj The dependency ofthe characteristics of the glow tube upon the relative proportionsof barium oxide, strontium oxide, and thorium oxide in the emissive coating is illus t'rated in Figs. 2 and3. The characteristics considered arej those. observed upon connecting the tube'in the cir:

Device or tube a cuit schematically illustrated in Fig. 1. Y

1 is connected in series'with' a" capacitor-11 across a; somer: of rectangular pulses of 500 microsecond time duration, this 'source beingrepresented by the block 12.

the breakdown 'volt'age of the tube. A voltmeter 14 connected across capacitor 11 measures the voltage to which the capacitoris'charged, theirelative energy transand a pair of closely. spaced cold electrodes sealed therein,

and anelectron-emissive coating 'on said electrodes lcorr taining '15 to '30 mol percent thoriumoxide, 20 5 40mm;

th the balance consisting' of V Arv'olt meter 13 connected'across t he leads 5, fi'measnres; 7

, V which the capacitor is charged'by the pulse energy trans omitted through the tube. ,Since the energy stored in a" capacitor is proportional to the squareof the voltag eto;

l lstrontium oxide} used to measure the. energy transmitte'd by the; tube, for

instance a resistance and thermocouple. {T he character-j iStiCSfOif: the tube which'are illustrated 'in jEigsr Z :and'3 r on i m i d j-"g I1 l 1 2.fA gaseous electric glow discharge 'device having}a low darkbreakdown 'rise and high dynamic conductance cordingly, the scope of the inventionis to be determined bythe appended claims. 1 i 7 What I claim 1 as new 'andfdesire' to secure by Letters Patent of the United States is? 1 A gaseous'electric glow discharge device compris-v ing an envelope containing an inert gas at a pressure'j "ofifrom approximately 7 7 0 to "200 millimeters of mercury percent barium oxide, and w' comprising-"an. envelope, containing an inert gas at a:

pressure'oftrom approximately to 200 millimeters or.

1 ously,=it is apparent that for'bestresults the thorium oxde mole'cular proportions, 22.5 percent thoriu'rn' oxide, 33 T mercury and apair of closely spaced cold electrodes sealed 7 o 'de,'33 mol percent bariu'm oxide'and 44 x 01 percent;

Q13. 'A gaseous'electric glow discharge device having a low dark breakdown rise'andlah'igh dynamic conductance comprising agenerally' cylindrical vitreousenyelopelofg' 7 approximately 41' inch diameter and A; inch: inlength 1:.

containing an; inert gas at pressure. of 70400 milli meters'of mercuryr and apair oficlbsely spaced 'cold eleck f therein; and an electron-emissive coating I on said elec- 1 trodescontaining approximately 23 mol percentthorium' trodes sealed therein, said electrodes consisting of metal wires disposed substantially parallel to the longitudinal axis of said envelope, and an electron-emissive coating on said electrodes consisting of approximately 23 mol percent thorium oxide, 33 mol percent barium oxide and 44 mol percent strontium oxide.

4. A gaseous electric glow discharge device comprising an envelope containing an inert gas at a low pressure above 100 millimeters of mercury and a pair of closely spaced cold electrodes sealed therein, and an electronemissive coating on said electrodes containing to 30 mol percent thorium oxide, to 40 mol percent barium oxide, and with the balance consisting of strontium oxide.

5. A gaseous electric glow discharge device comprising an envelope containing an inert gas at a pressure of the order of 130 millimeters of mercury and a pair of closely spaced cold electrodes sealed therein, and an electron-emissive coating on said electrodes containing 15 to mol percent thorium oxide, 20 to mol percent 6 barium oxide, and with the balance consisting of strontium oxide.

6. A gaseous electric glow discharge device comprising an envelope containing an inert gas at a low pressure above millimeters of mercury and a pair of closely spaced cold electrodes sealed therein, and an electronemissive coating on said electrodes containing approxi mately 23 mol percent thorium oxide, 33 mol percent barium oxide, and 44 percent strontium oxide.

References Cited in the file of this patent UNITED STATES PATENTS 1,670,483 Rentschler May 22, 1928 2,333,710 Deimel Nov. 9, 1943 2,374,677 Goldstein et al May 1, 1945 2,449,113 Fruth Sept. 14, 1948 2,525,263 Macksoud Oct. 10, 1950