US3076913A

US3076913A - Cold cathode gas discharge device

Info

Publication number: US3076913A
Application number: US839209A
Authority: US
Inventors: Bernard G Firth
Original assignee: Tung Sol Electric Inc
Current assignee: Tung Sol Electric Inc
Priority date: 1959-09-10
Filing date: 1959-09-10
Publication date: 1963-02-05
Anticipated expiration: 1980-02-05

Description

Feb. 5, 1963 B. G. FIRTH com cATEoDE GAS DISCHARGE DEVICE Filed Sept. l0. 1959 ATTORNEYS United States Patent Ohlice 3,t"l6,9l3 @GLD Ctllltth GAS Dlllrai'l Elli/ECE Bernard G. Firth, Newark, NJ., assigner to 'lungadol Electric linea, a corporation ol Delaware Filed Sept. lll, w59, Ser. No. 839,299 7 iairns. tl. Zilli-wield) The present invention relates to cold cathode gas discharge tubes and comprises a novel three electrode tube that is of miniature size, delivers high peak currents of short duration, has short deionization periods and high hold-oil voltages. Although the new tube is particularly adapted for use in ignition circuits for righ compression combustion engines it may be advantageously employed in any circuit where its special characteristics are important, as for example in switching circuits, exciters and pulsera. The invention comprises novel structural and dimensional features which insure desired tube characteristics.

The cathode of the new tube comprises a metal cup within which is a cathode pill and covering which is a centrally apertured metal cap of refractory material. rl`he trigger grid comprises a cylindrical construction of graphite critically spaced from the cathode cup. The anode comprises a shielded rod of graphite connected to an anode lead extending through a top bulb seal. A generally cylindrical insulator of ceramic material surrounds the cathode cup and grid.

The cathode pill is formed by compression of a mixture of cesiurn chloride and aluminum filings. An important feature of the invention is the range of particle size and the relative proportions of the constituents of the mixture from which the pill is formed and which insures, in the linished tube, that reaction will be maintained between the components of the pill. Other important features of the invention are the spacing between the upper suriace of the pill and the cover of the cathode cup, the material of the cup cover, the dimensions of the central aperture therein and the spacing of the trigger grid from the cover. These features contribute to stability of operation of the tube by insuring that the tube will not go into continuous glow, that the discharge will not be choked, that the deionization time will lie that desired and that desired hold-oil voltages may be obtained.

The electrode spacing of the new tube, when designed as a switch tube for discharge of the storage capacitor of au ignition circuit, is such as to provide high forward hold-oil voltage and short deionization time. When designed as a pass tube for the reverse current through the primary of the ignition coil following the spark discharge, the spacing of the electrodes is such as to insure high reverse hold-ofi voltages. The circuit of Short et al. Patent 2,846,992 dated August l2, 1958, is typical of ignition circuits in which the cold cathode gas tubes of the invention can be advantageously employed. That circuit includes a switch tube and a pass tube connected as above indicated.

ln both the switch and pass tube constructions, vents through the ceramic cathode insulator are provided for release of gas and of combustion products from the cathode pill. ln the switch tube construction, vents are also provided through the wall of the cathode cup, which vents communicate with the vents in the insulator.

Other features of the invention, including multiple leadin conductors for the cathodes to handle the large peak currents passed by the tubes, will become apparent as the description proceeds.

For a better understanding of the invention and of speciiic embodiments thereof, reference may be had to the accompanying drawing of which:

dhld patented Feb. 5, i953 FlG. l is a longitudinal sectional View, partly in elevation, of a switch tube embodying the invention; and

FIG. 2 is a similar longitudinal section, partly in elevation, of a pass tube embodying the invention.

The cold cathode gas tube embodying the invention and shown in FlG. 1 comprises an envelope 2 of glass or the like having a button stern 4 through which extends a plurality of cathode leads 6 and a grid lead 3. An anode lead lo is sealed through a bulb top seal. Within the envelope is a cathode 1.2 comprising a cup 11'- of nickel within which is disposed a cathode pill 16. A centrally apertured cover l of molybdenum is provided for the cathode cup. The cathode l2 is mounted within a cylindrical recess in a cylindrical insulator 2t) of ceramic material such as lava. Mounted in a second recess in the insulator 2li above the cathode and axially aligned therewith, is a control or trigger grid. 22 of graphite. A metal ribbon 2li of nickel encircles` the grid 22 and is welded to the end of the lead S, the lead 8 extending through a passage in the insulator 2i? and through an aligned hole in a mica spacer 26. The part of the grid lead d between the insulator Ztl and button stem 4 of the envelope is protected by a ceramic tube 2d, the upper end ol which is introduced into an aperture in the insulator and the lower end of which is fused or cenented to the glass of the stein.

The upper end of the envelope is provided with a reentrant tubular portion 3o within which is a thin nickel sleeve 32 having an upper ilared end embedded in the wall of the reentrant portion. The lower end of the anode lead litt extends into the sleeve 32 and is welded on its end to the graphite anode A thin short sleeve 3-5 of nickel surrounds the junction of the lead lo with the anode 3d. Surrounding the reentrant portion of the tube is a second insulating cylinder 33 of ceramic material, such as lava, which extends below the end ot the anode and is provided with section di) of greater diameter having a cylindrical recess for reception of the upper end of the grid 22. The grid is formed with a central bore l2 terminating at its upper end in a section @la of lesser diameter.

radial prssages are formed in the insulator 2li hese passages terminate with their centers precisely he lev l the top of the cathode cup ld. Formed wall of the cathode cup are six holes of smaller h aligned with the passages 415. The holes and passages permit simultaneous venting both above and below the surface of the cover 13 of gas and products of combustion. This insures release of pres sure due to thermal expansion and rapid deionization.

The envelope is filled with helium gas at a pressure of approximately l2 mm. and then sealed oit at a tubulation indicated at The spacing between electrodes in the above described structure is of critical importance. rthe longitudinal distance between the top cap i8 of the cathode cup and the lower surface of the grid 22 is determined by the minimum of the Paschen curve for the pressure of the particular gas in the tube. In other words the spacing should be such that the sparking; potential is a minimum for the particular pressure of the specitic gas in the envelope in accordance with Paschens law (see page 164 of Gase-ous Conductors by i ames Dillon Cobine, published in i941 by McGraw-Hill). Such spacing, for the particular tube illustrated in FIG. l and for helium at a pressure of l2 mm. is .030 inch. ln the same tube the spacing of the anode to the cathode is .410 inch. The spacing between the underside of the cap of the cathode cup and the cathode pellet is also of importance. ln the particular embodiment of the invention illustrated in FlG. l such spacing is .03() inch, the pellet being .G inch deep and of a diameter ol .257 inch. lt is in the space between the pellet and the cap of the cup that the eerdere a holes i8 are drilled -to provide escape for the products of combustion during conduction by the tube.

As heretofore indicated the construction of the cathode pellet i6 is important to insure proper operation and long life of the device. The pellet is composed of particles of cesium chloride and aluminum filings that have been compressed under vhigh pressure.

When a reaction occurs between the cesium chloride andthe aluminum filings a channel is termed in the pellet and products of combustion are given ott. if the reaction is to `continue the size of the channel must be sufficient to permit the discharge to reach additional particles of the pellet. The size of the larger particles determines the channel size. Also to maintain reaction there must be small particles because these particles react more rapidly. Moreover, there must be a thorough mixing or" the cesium chloride and aluminum. lf the aluminum filings are not properly interspersed with the cesium chloride particles a conductive barrier could form around cesium chloride agglomeraties thus isolating them from possible use. In accordance with the invention, when the pill is made according to the following specications an interlocking electrically conductive mesh of aluminum is established throughout the pellet of such dimensions as to allow cesium chloride' to be reacted and eroded into channels large enough to expose Vfresh surfaces tonew discharges.

The particlesize should be within the range of -40 to +200 mesh, that is all the particles should pass through i401 mesh and none through a 200 mesh. The particle size should be so distributed in the speciiied range that about one half of the particles lie in the range -40 to +1.00 and the other one half lie in the range of 100 to +200 range. Such a distribution of particle size, when the components of the mixture are thoroughly intermixed gives a minimum channel size of .020 inch which is suliicient to insure penetration oity thepellet. To insure even distribution ofy the particles they `are shaken together in a metal tube and then quickly poured into a mold for application of pressure. Glass containers should be avoided. as electrostatic charges build up which tend to separatel the constituents of the mixture. The pressure employed for. forming vthe pellet should be about 23,000 to 29,000 lbs. per square inch, or, for a pellet of the specified dimension, of the order of 1200 to 15001bs. The cesium chloride particles should be slightly moistened by short time exposure to air in a cabinet with some moisture present at room temperature. Tube life is enhanced when slight moisture is present but care should be taken to avoid accumulation of too much moisture by the hydroscopic cesium chloride particles.

The channel size necessary for penetration of the pellet is applicable also to the spacing of the cathode cap. ln order to insure that the reaction will spread to the periphery of the pellet such spacing should be at least .020 inch. One additional feature of the cathode pellet is of iniportance. The ratio of the aluminum to the cesium chloride should be controlled and preferably these components should be in the ratio of about 2.7 parts by weight of cesium chloride to about l part by weight of aluminum.

The cap 18 of the cathode cup has a central aperture 52 therein and this aperture, in the construction of FIG. l, is .054 inch in diameter. Such a dimension is large enough-to insure against choking of the discharge and at the same time small enough to insure high hold-oli voltage and to avoid the tendency of the tube to fall into continuous glow. The cap is spot welded about its periphery to the cup and, as heretofore indicated, is preferably of molybdenum or other refractory material. Molybdenum is preferred because when wellcleaned it has a slight gettering action and because it oxidizes slightly during operation of the tube, thus minimizing reliection of heat into the cathode cup. A refractory material is desired to avoid sputtering, such as occurs when the cover or cap is of a metal such as nickel. The edges of the aperture 52 should be burr free.

A tube such as that of FIG. 1 must carry very high peak currents of the order of 170 amperes. For this reason the plurality of cathode leads 6 are provided. In the view of the drawing only three of the leads are visible but in the complete tube there are six leads leading from separate pins to the base of the cathode cup. When fewer leads of larger diameter are used difficulty is experienced with the seal of the heavier wires through the button base and also heat is generated as a result or" skin effect when the current builds up rapidly. By the use of relatively thin leads connected to provide parallel paths these difiiculties inherent in a tube which has to withstand the sudden surge of current occurring in the tube of the invention are avoided.

A tube constructed as disclosed in FIG. 1 with the dimensions heretofore specified has the following characteristics:

Max. anode voltage volts 2000 Min. anode voltage at 400 volts peak trigger voltage volts- 1000 Max. average anode current (approx.) ma 40 Max. peak anode current (approx.) amps 170 Min. peak anode current (approx.) amps..- 50 Max. anode pulse width (approx.) iisec-- 2 Tube voltage drop (approx.) volts..- Max. repetition rate p.p.s 500 Min. trigger peak voltage-.. volts 300 Min. trigger peak current (approx.) -ma 20 Max. trigger` peak current (approx.) ainps-- 50 Recommended trigger source impedance ohms max 500 Max. average grid current l nia 2 The tube shown in FIG. 2, the pass tube, is of construction similar to that of the switch tube of FIG. l except for the differences now to be pointed out. The diierences are due to the different desired functioning ofthe tubes, For a switch tube it is desired that there be rapid dreionization time, and a high hold-oil voltage in both the forward and reverse directions whereas in the pass tube a. high stand-off voltage in the reverse direction only is required. To decrease the arc drop and increase tube life the spacing between the anode and cathode is lessin the pass tube than in the -switch tube. Speciiically, in the tube or FIG. 2, this spacing is .360 inch compared to .410 inch in the switch tube. The grid to cathode spacing is the same in both tubes as is the size of the aperture in the cap of the cathode cup and the spacing of the pellet below the cap. The other major dilerence in the constructions of the two tubes is that in the case of the pass tube of FIG. 2 the veut passages 46 in the insulator 20 are spaced slightly above the cap of the cathode cup and no holes are provided in the wall of the' cathode cup. As in the case of the switch tube the cathode is connected by a plurality of leads 6, preferably 6 in number, through the buttonrstem of the envelope to separate pin terminals.

The invention has now been described in connection with two embodiments thereof differing in relatively small respects. It will be apparent from the foregoing description that the invention comprises a high voltage cold cathode gas tube of construction which insures stable operation and long useful life. The construction of the cathode pellet from the specified range of particle size and distribution of particle size within such range and of the preferred proportions of cesium chloride and aluminum filings is of particular importance in insuring the desired operation of the new tube.

T he following is claimed:

l. A cold cathode gas discharge device comprising a sealed envelope containing inert gas at reduced pressure and having mounted therein a cathode, a grid and an anode, said cathodecomprising a metal cup containing a compressed mixture of cesium chlorideand aluminum particles of a variety of sizes within the range of -40 to +200 mesh, and a centrally apertured refractory Imetal cover on the cup and spaced from the mixture therein through a distance greater than the diameter of a channel formed in the mixture upon reaction between the particles, said grid being of graphite and of generally cylindrical form with a .central passage therethrough axially aligned with the central aperture in the cover, the lower surface of said grid being spaced from said cathode through a distance substantially that at which the sparking potential is a minimum for the gas and the pressure within the envelope, said anode being of graphite and positioned above said grid in axial alignment therewith, and connections `for said grid, cathode and anode sealed through said envelope.

2. The cold cathode gas discharge device according to claim l wherein said envelope has a button base carrying a plurality of pin terminals and wherein said cathode cup is connected to a plurality of said terminals by said connections.

3. The cold cathode to claim 1 where said molybdenum.

4. The cold cathode gas discharge device according to claim 1 including a ceramic shield having a central opengas -discharge device according cover of the cathode cup is of 6 ing therethrough, said cathode and grid lacing mounted in said opening, said shield having radial vent passages therethrough terminating at their inner ends in the neighborhood of the cover of the cathode cup.

5. The cold cathode gas discharge device according to claim 4 wherein the walls of the cathode cup above the level of the compressed mixture of particles has apertures therein communicating with said vent passages.

6. The cold cathode gas discharge device according to claim 1 wherein about half of the particles of said compressed mixture are of sizes within the range of 4G to and the remainder are of sizes within the range of +10() to +260.

7. The cold cathode gas discharge device according to claim 1 wherein the compressed mixture comprises substantially 2.7 parts by weight of cesium chloride to l part lby weight of aluminum iilings.

References Cited in the iile of this patent UNITED STATES PATENTS

Claims

1. A COLD CATHODE GAS DISCHARGE DEVICE COMPRISING A SEALED ENVELOPE CONTAINING INERT GAS AT REDUCED PRESSURE AND HAVING MOUNTED THEREIN A CATHODE, A GRID AND AN ANODE, SAID CATHODE COMPRISING A METAL CUP CONTAINING A COMPRESSED MIXTURE OF CESIUM CHLORIDE AND ALUMINUM PARTICLES OF A VARIETY OF SIZES WITHIN THE RANGE OF -40 TO +200 MESH, AND A CENTRALLY APERTURED REFRACTORY METAL COVER ON THE CUP AND SPACED FROM THE MIXTURE THEREIN THROUGH A DISTANCE GREATER THAN THE DIAMETER OF A CHANNEL FORMED IN THE MIXTURE UPON REACTION BETWEEN THE PARTICLES, SAID GRID BEING OF GRAPHITE AND OF GENERALLY CYLINDRICAL FORM WITH A CENTRAL PASSAGE THERETHROUGH AXIALLY ALIGNED WITH THE CENTRAL APERTURE IN THE COVER, THE LOWER SURFACE OF SAID GRID BEING SPACED FROM SAID CATHODE THROUGH A DISTANCE SUBSTANTIALLY THAT AT WHICH THE SPARKING POTENTIAL