US2607901A

US2607901A - Electronic discharge device

Info

Publication number: US2607901A
Application number: US719562A
Authority: US
Inventors: Jr George H Rockwood; Robert L Vance
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1946-12-31
Filing date: 1946-12-31
Publication date: 1952-08-19
Anticipated expiration: 1969-08-19

Description

1952 G. H. R cKwooD, JR., ETAL ,607,901

ELECTRONIC DISCHARGE DEVICE.

Filed Dec. 31, 1946 FIG. 6

EVACUA 1: 1'0 10- BOMBARD ELEMENTS Bl f/JT BY PULSING FOR ZSE'CS. ON AND PUMPING ZSECSZOFF PROCEDURE F/LL WITH GAS SEAL acupun- L czrrsn FLASH v HEAT ELECTRODES er/v4 o v BY STEEP WAVE FRONT HIGH VOL TAGE DI-WHARGE L L I MIN.

M/l/ENTORS:

Bylaw A TTORNEV Patented Aug. 19, 1 952 ememomc DISCHARGE? DEV-16E Qe orge H. Rockwod; "J12, Sunnnit, L. Y

J assignors' to BeHfTelemaintain 11r 1ifor n tolerance 'betwen cooperative discharge electrodes in 'a sealed vessel of minim mm o ien n H -1 Another object of the invention is tip-facilitate the clean-up-of deleterious gases in the device A further ob-ject ;of;tl 1 e inventiongislto; avoid fracture ,ofythe -sealof -t he vessel during the manufacturing processes of-thedev-ice,

Still another 7 obj eat of the inven-ti-on is to Taprioate the electrode-assembly in the device so that the breakdown gap :therebetweenfwilknot vary over along life and; will vvithstandhigh impact shoolggf a few hundred tirnes gravity I V i i A further object ofsthe invention is to insure constantspace -relati-qn between the electrodes by an assembly having a high-resonant frequency to rus ei t r onv These obj c ots; are attained, in accordance with Q i o sii v cmb p dv d a mpac l yvrd s her e ce; o ex re el mall fi in which. re m rroi 1. 3 .33 z'i 'bl e ele od ea .in a m lltt mi ium; im s n. to insure constant and accurate paral-lelism between the electrodes-and ireedozn from variation due to shook-or vibrationeither in transi t or use. This s. ew m sh d by lo a hese l i: -1 W trgde conductors as close as -pQS S iblet o V the electrodes s9 thatthe electrodes resistshgck i npact transverse toy-the axis 0f the vessel. Therefore,

the initial space relationbetween the e-lectrodes which determines the breakdown voltage of the discharge ;path will be maintained constant throughout the operating life of thedevice.

A feature f'the construction relates to the localization of the getter substance in the device P for subsequent generation ;of a gas absorbing film in the final pumpingpr-ocessing during manufacture. ."Ihe material-is applied to the ends of the electrodesin annular form so that induction heating will fiash'the highly refractory mixture after I the conditioning of the active emissive c'oati-ngon the electrodes. Theresidualrin'g masses remaining'on theelectrodes after the flashing operation form: glow discharge "barriers and aid in preventing thespreading of the glowto thein ner surfaces of the tubular electrodes.

- A further feature of the invention relates "to heat treatment of the electrodes in converting the externally applied coatingto the'active state without detrimentally afiectin'g'thiherinefic'joint of the electrode conductorssealed in the glass v'essel. This is accomplished-by evacuating thevessel and heating the ele'otfodes intermittently for short periods sothat heat' conductionto the glass seal is "and the" originarcarbonates F Applicationfle'cember 31, 194s-,ser iai No."i 19,562

after which the oxide coating-is "activated hya highvoltage 1 1 ;"aiseiiaigtd aampiet tag manuasi rine mesee. r

H eating procedure during the li'reakdow stenis controlled iinii-or'ni bulsingfof the eating source for a edetenfi ined d ation'o that the rjeq uiredhol ersi cjno f the cai'b-onates coinplisii ed ofit matei'ialli meeting-h 1y refractory 'et'tersfip's'ta'nc'e' on the ends?) the electrodes also vvtthout -clcnive'i ing lie-at fenergvhy ond 'iction'to'th"bxidutorssealedhn thgglass r iisiiahtgiiavaiaeiatiaatafafiasfimg of the gjettielr inat'e'riafand pr' ents 'crac 'iig 'of th eal ofth caiiauad g'aue' to thclose firoxser'siriawa -in cros'is' s'ction to i'llustrat the' co'niifgu'fa't ionof theelctrodes and then-close inat e tbifihegia 'wal'ls'r th dvice, i EigQ 2'35- a' side"vietfinilvitibn oi th' 'd'vice W sa jfig 3' tea-view of'th device of "Fig." on thesamascale. Fig. 4' illustrates the actual size "of the device niiiiri'tol'ertifi'esof he g th de'v'ice v vhic required solution. also, certa in opti nifinires'ult had to bejr'ealizedsdtha' 'fliient functional operation would beas'suredvtitnm 'the restrict the size of the electrodes and since the,

electrodes must be hermetically sealed in the en electrodes require involved thermal treatments in order to reduce the initial and sustaining glow discharge characteristics of operation, it is evident that material innovations in usual practices are necessary in order to meet the final requirements of the device in service. The correlation of the structure and techniques of manufacture have produced a device which overcomes the difiiculties and the inherent restrictive factors imposed by the limits of space and magnitude encountered in the assembly.

One specific device in its final form has a length of 1 /4 inches and a diameter of inch so that the electrodes, if the device is to be of the nonpolarized type, must be of small size but of sufiicient area to perform the functional characteristics of a glow discharge relay. To insure stability of operation and provide anactive surface of' constant breakdown potentiality in the discharge gap between the electrodes, it was found that electrodes of tubular form met the requirements in a most satisfactory manner and provided sufiicient mechanical strength and low mass to eliminate stresses in the assembly. Accordingly, the electrodes comprise a pair of thin tubular nickel sleeves I and II which are mounted in parallel relation in a section of glass tubing having an internal diameter of .235 inch and an external diameter of .320 inch. The tubing forms a. cylindrical container or vessel I2 which is provided with a constriction I3 near one end joined to a tubulation I4. The pair of electrodes are formed of .005 inch wall nickel sleeves having a length of inch and a diameter of .0696 inch. The electrodes are mounted in parallel relation with a space or discharge gap of the order of 0.060 inch between. the linear surfaces and the electrodes are sealed in the opposite end of the glass tubing by accurate alignment of a pair of conductors I5 and I6 which are attached to the electrodes respectively by pinching and welding the ends I! and I8 around the conductors, as shown clearly in Fig. 3.

The accurate parallelism of the electrodes is assured by a sealing method which permits the plastic glass of the seal to collapse around the conductors without mechanical strains being imposed on the conductors which might alter their space relation in the completed seal. Thisis accomplished by anchoring the mounted electrodes in a holder or fixture in parallel relation to insure the required gap spacing of .060 inch. The glass tubing is held in a correlative fixture and surclosing vessel to insure stable operation and these rounds the electrodes such that the glass may be converted to a plastic state in the desired relationship to the electrodes and conductors. The plastic glass collapses against the wires and flows into a mass or stem I9 which hermetically seals the conductors as the glass solidifies into the lobar shape as shown in Fig. 1. The strainfree seal secured by this method insures positive location of the conductors and electrodes in definite parallel space'relation within the vessel I2 and also accurately spaces the electrodes I0 and II relatively in close proximity to the inner surface of the seal so that the limits of the unsupported length of the conductors I5 and I 6 between the seal and the electrodes may be held to a small dimension and thereby aid in reducing the over-all length of the device. In a typical example, the distance between the electrodes and stem is less than the length of the electrodes, 1. e.,

inch. The constriction I3 is provided to facilitate the final sealing of the device after processing and the tubulation I4 forms a connection to a suitable pumping and gas-charging system to complete the assembly of the device.

The parallel tubular electrodes I 0 and II are coated exteriorly with electron emissive material 20, to reduce the breakdown potential therebetween and to lower the work function of the bare metal electrodes across the discharge gap. This coating is preferably one or more alkaline earth compounds, for example, barium and strontium carbonates, which are easily decomposable to the active state in the final processing of the device, although other compounds and other types of emissive substances may be substituted for the active coating on the electrodes. The mixture of carbonates is combined with a carrier fluid or binder material, such as nitrocellulose and amyl acetate, and sprayed or painted on the cylindrical surface of the electrodes to form a uniform layer, approximately .77 milligram per square centimeter of active surface.

After the emissive coating is applied to the electrodes, a gas absorbing or getter material is affixed to the electodes to be vaporzed in the final pumping process of the device. Since the area within the vessel is limited, in accordance with a feature of this invention, the getter substance is introduced as an integral component of the electrodes and is flashed sequentially and selectively after the heat treatment of the electrodes in the device. This is accomplished by applying the getter substance in the form of a superimposed ring, layer, or bead 2| on the open or inner ends of the electrodes. A convenient method of application is to dip the ends of the electrodes in a viscous mixtureof the getter composition suspended in a binder or carrier fluid which forms an agglutinous substance of the proper consistency to readily adhere to the ends of the metal electrodes. The ring or annulus 2| is superimposed over a limited area of the active coating on the electrodes and by the dipping process forms a thin annular bead around the cylindrical edge of the electrode, as shown in Fig. 5.

The getter composition has a high thermal flash point so that it is not materially afiected during the heating cycles of the emissive coating on the exterior of the electrodes and, therefore. the activation of the getter substance may be selectively controlled in order to prevent substantial premature flashing thereof during the prior heating periods of the electrodes. The terminating bead 2| on the electrodes also serves an additional function during operation of the device, by preventing the spreading of the glow discharge to the inner surfaces of the electrodes, which might cause erratic behavior of the breakdown discharge in the operation of the device.

' ter the getter is flashed to fix deleterious gases in the discharge space of the vessel, the spent residue of the rings 2I remains as a highly refractory insulating oxide which forms a stopor barrier at the open ends of the electrodes so that the'glow does not creep along the inner surfaces of the electrodes.

In order to meet the stringent temperature conditions in the processing of the electrodes and in view of the fact that the getter is an integral component of the electrodes. the getter composition is formedoffhighly refractory ma,

sarcasm barium iberylliateilluxperscenti and free'qnetallic titaniunitZOzper cent, these z ingredientsi .being :in fine particle ;or'.-powder.-:form. :;The frmixture :is suspended-fine a" binderz-rmaterial .aof .-'nitrocel-lulose and amyl acetate to make-anviscouszirpasteiof :creamy :consistency. so ithat a .vb'ead is-a'readily ifu'rmed i011 thee-ends. "of the zelectrodes when :the electrodes are:--dipped.-into -athe mixture. ".In prerparing .:.the-.-.-getter- :composition, the -berylliate icompound-is .form'edinitially 'by:v mixing. :b'arium :carbonate and beryllium oxide in proportions of {approximately/l te -3 and-"heating to slntering :temperature in anon-oxidizing atmosphere, such gas hydrogen. The resultantberylliate compound tiS-I ground :toa fine powder and mixed inthe above-mentioned proportions with the free metallic titanium. 1

r v The tubulation 44. is sea-led to a "high vacuum pumping station andsthe glass vessel "12 is baked inwanenclosing oven -at-a temperature of 350 to .475 C. to-remove watervapor from the-glass walls and .release moisture from the electrodes .and conductors within: the bulb. .The vessel-is :then evacuated to aslow -pressure of .the order of 1- 10- millimeters of ;mercury :and .while .the vessel isstill. connected to the pumping system .thepreliminary --hi ghtemperatureheat treatment-of the electrodes, in accordance-with areature of this invention, is, performed to. condition the low work function coating of 4 the electrodes. In. order to completely 1 convert the alkaline earthcarbonates .to. the oxide state and enhance the low work functionproperties of the coating Ora-film onitheiexterior .Of the electrodes, it is .necessary .to Idecompose or break down the carbonates .to-the.oxidesatahigh temperature of the order or 1000 10. which is relatively 'close lto "the vaporization. point. 'ofthel thin nickel electrodes in the vessel. Furthermore, a "prolonged heat treatment at the.abovetemperature would not Io'nly .r'es'ultiin considerable evaporation of the electrodes butj'theconduction bf heat along ithe conductors l5 and 'l6"wo'uld set, up serious vstrains in the? glass seal l 9 and. generally result 'in'crac'ks *or fra'ctures at the joint-which would destroy'fthe utility bf the device in service.

Iii-accordance with a'feature of this'invention, the breakdown process is performed by" heating *theelectrodes inductively by high frequency current inintermittent flashes of transient periodic duration and coolingf'orlikejperiods until the complete emissive "coatingisconverted. This is accomplished by a" pulsing procedure in which a .high frequency. coil surrounds .the vessel in-the vicinity of the active coating'iflg preferably between the lower edge of the getterrings 2! on the top of the electrodes and the pinch on the bottom of the electrodes. The coil generates a high current heating effect by inductive inter- .action with the tubular electrodes to? raise the :temperature, almost instantaneously, to-appr'oximately -between 1000 to 1025-90. -1The heating: is .applied intermittently by pulsingathe current in :uniform periodic steps of 1 to 2x seconds *duration followed by a like period with the-current uff, to successively interrupt or' segregate the heating cycle into short pulses; of heating and -cooling,. generallyt to 7' times, for the'heating "step .so -.-.that the coating ---iscompletely decomposed tooxides. The pulsingperiods, for example. two seconds, are measured drom the-itime-of application-of voltage to'thecoiland notvtrom the instant whenthe elctrodes'attain the required item'peratu-re. Duringwtheiiheating' period,--- the relectrodes attain a; temperature of approximately 1000 to 1025 C. for the short intervalof heating so that the carbonates are decomposed T to ioxides 'byibombardment 'in intermittent flashes of heating', energy. The. current is disconnectedffor a similar period; to permit the 'electro'des to dissipate the heat energy mainly by'radiati'onto :the glass-walls of '2 the vesseland the heating is. repeated in successive steps of current on and iofi for 1 definite time intervals until the carbonates are completely'broken down to the oxides-matrix composition in thecoating. The timing "of the pulse periodsmay be performed manually by switching associated with the high frequency current source or'it'may be accomplished-automatically by-some suitable trigger timing circuit included in the currentsupplysystem.

-While the electrodes attain a high temperature during the intermittent heating routine, the-intervals are solshort that the heat generated-in the'electrodes is not materially conveyed'along the conductorsby conduction but rather'is expended" as radiation heat energy to the surample; and-the vessel is filledtoa pressure '0f-15 millimeters of mercuryualthOu-gh this pressure may be varied Within wide limits depending on the characteristics desired in thedevice. -'Af-ter .thejgas injection is completed, the tube issealed off at the constriction is by fusion of the glass'to form asealingtip 22. 0

After the electrode coating is broken down to oxides the gaseous filling in the vessel is contaminated by deleterious gases evolved duringthe .conversionof-the emissive coating. These gases may be cleaned up. or rendered impotent'bythe vaporization: of the. getter substance on .theends of the electrodes. This is accomplished by mounting-ahigh frequency coil of 'a singleturn around-the vessel in line with the beads 2| on the .ends -.10f"the.e1ectli0des and heating the annular beads 21 to the reaction temperature of approximately 1025" C. -The beads readily attain-the above temperaturethrough the induction-heating of the annular "ends of theele'ctrodes so that the getter ringsare quickly heated to the temperature at which the titaniumwill react with the berylliaterso that the barium isliberated and vaporized. The-bariumabsorbs the deleterious gases in the vesseland--Whencondensed on the interior wall of -the; vesselfixes the gases so that the inert gas filling is purified. 7 Thus the barium will react with the impurities in the argon or other. gas, which .impurities may include 'nitrogenj'carbon (dioxide and particularly oi'z'y'g' enjboth "whenthe barium isa vapor and. after 'it' has condensed'on the walls of'the vessel. v

After the getter rings 2| are'ffiashed, the residue material is substantially beryllium oxide which is a highly refractory insulating substance. The reaction of the barium berylliate which is a coin- .plex with the metallic titanium yields the metallic barium which vaporizes, and a complex including titanium oxide andberyllium oxide which is a 're fractorymaterial. The insulating 'ringsQtHerefore, aidin forming a stop or barrier torprvent the glow discharge on the exterior surface of "the electrodes from, spreading to the inner surfaces thereby stabilizing the discharge characteristics of the device in service.

The above steps complete thepumping procedurebut further processing is required to condition .the device for operating service.- This comprises an'activation. process to prepare the electrode coating .in the active state and produce the low work function properties of the..coating on the electrodes.

A high voltage high frequency current of 4000 Volts .30 megacycles source is applied, fora period of 1 to 2 minutes,'to the conductors, in series across the gaseous discharge gapwithin the vessel. The current source produces a high frequency steep wave front discharge between the electrodes which generates high frequency arcs between the scintillation points of the coating matrix and the arcing current produces high temperature at these minute, points to reduce some oxide particles to free barium metal which is adsorbed on the surface and promotes high electron emission during the operation of the device. While the temperature of the scintillation points of the coating is extremely high, being of the order of 1000 C., the base metal of the electrode is not materially above a temperature of 500 C. during this activation step so that the electrode metal is relatively cool in comparison to the intensely heated coating. Therefore, practically no-deleterious heat conduction obtains in the conductors to endanger the seal 19 and the highly refractory insulating coating 2| is substantially inert during this heating stage so that no further reaction occurs.

The'construction of the device provides an assembly in which maintenance of gap spacing is held consta'nt due to the low mass of the electrodes and the-high resonant-characteristic of their supports since they are close to the rigid seal I9 of the vessel. This produces a device having uniform electrical characteristics with regard to breakdown and sustaining potentials. The assembly is not easily affected by shock or vibration since the device can withstand an impact of a few hundredtimes gravity without adversely affecting the electrode spacing or the seal of the conductors in the stem. These attributes together with the processing treatments of the electrodes and the glow inhibiting barrier on the open r ends of the electrodes materially enhance the functional operation of the device and increase the service life over a long period.

While the invention has been disclosed in connection with a specific example of a miniature type-of device of particular construction, it is, of course, understood that the various aspects of the invention may be utilized in other types and constructions of discharge devices without departing from the'scope and spirit of the invention as defined'in the appended claims.

What is claimed is:

1. A glow discharge device comprising a, vessel containing an ionizable gas at low pressure, a pair of parallel tubular electrodes mounted therein having an electron emissive coating thereon, and a getter composition on the open end of each electrode consisting of 80 per cent barium berylliate and 20 per cent titanium in a binder.

' 2, The method of processing glow discharge devices, which comprises sealing a pair of tubular electrodes in an enclosing vessel of small diameter and length, said vessel being of a vitreous material said electrodes having an alkaline earth carbonates coating on the external surface and a 8 composition coating. on the openendtliereof, said coating comprising barium berylliate and titanium evacuating saidvessel, heating the electrodes periodically. in short pulses to break down .the carbonates .coating: to oxides, filling; said vessel with an inert .gas' at low pressure, sealing said .vessel, flashin said". composition coating, and activating said oxides-coating by high voltage glow discharge current.

3. The method of processing glow discharge devices which comprises sealing a pair of tubular electrodes in an enclosing glass vessel of small diameter andllength, said electrodes having an alkaline earth carbonatescoating on the'external surface and a composition coating on the open end thereof, said coating c0mprislng-80 per cent barium berylliate and 20 per cent free metallic titanium evacuatin said vessel, heating said carbonates coating by high frequency pulsing current for approximately five repetitions of uniform duration, sealing said vessel, subsequently flashing said compositioncoating, and heating said electrodes to activate the converted coating.

4. The method of processing glow discharge devices which comprises sealing a pair of parallel tubular electrodes ofpredetermined length in one end of a tubular glass vessel, the distance between the electrodes andsealing point being of the order of the length of the electrodes, said electrodes having an alkalineearth carbonat s coating on the external surfaces and a composition on the open ends thereof, said composition comprising barium berylliate and titanium, evacuating said vessel; heating said electrodes inductively in intermittent steps of substantially two seconds duration to prevent conduction of heat to said sealing point, filling said vessel with inert gas, sealing the opposite end of said vessel. heating the open ends of said electrodes locally by high frequency induction currents to flash said composition; and heating said electrodes in series by a steep wave front discharge current.

5. The method of fabricating a glow discharge device which comprises applying a ring of a composition to one end of a' tubular electrode in the device, said composition comprising percent barium berylliate and 20 per cent free metallic titanium suspended in a nitrocellulose anda'myl acetate binder, evacuating said device and heating said ring to a' temperature sufiicient to vaporize the barium and leave a glow inhibiting barrier ring substantially of a complex beryllium oxide.

GEORGE H. ROCKWOOD, JR. ROBERT L. VANCE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,610,892 Skaupy Dec. 14, 1926 1,661,235 Sarbey Mar. 6, 1928 1,852,020 Metcalf ins Apr. 5, 1932 1,873,730 Wiegand 1 Aug. 23, 1932 1,965,585 Foulke -July 10, 1934 2,057,183 7 Case Oct. 13, 1 936 2,173,258 1 Lederer' Sept. 19, 1939 2,173,259 Lederer Sept. 19, 1939 2,213,182 Lederer Aug. 27, 1940 2,223,977 1 Wamsley l- Dec. 3, 1940 2,249,672 Spanner July 15, 1941 2,403,745 'Norton' July 9, 1940 2,443,633 Miller June 22, 1948 2,474,335 slrellett 1 June 28,- 1949

Claims

1. A GLOW DISCHARGE COMPRISING A VESSEL CONTAINING AN IONIZABLE GAS AT LOW PRESSURE, A PAIR OF PARALLEL TUBULAR ELECTRODES MOUNTED THEREIN HAVING AN ELECTRON EMISSIVE COATING THEREON, AND A GETTER COMPOSITION ON THE OPEN END OF EACH ELECTRODE CONSISTING OF 80 PER CENT BARIUM BERYLLIATE AND 20 PER CENT TITANIUM IN A BINDER.