US2358615A - Starter electrode - Google Patents

Description

Sept. 19, 1944. E. s. F. ARNOTT STARTER ELECTRQDE Filed 061;. 22, 1941 Gran/17E QTTORNEY Y 'A' further object of the-invention Patented Sept. 19, 1944 STARTER ELECTRODE H h V v 7 Edward' G.-E.'Arnott, Upper Montclain NaJg asi Q Signor to Westinghouse Electric & Manufactub V g l 1 ing Company, East Pittsburgh. Pa., a, corpora- V 'tion'of Pennsylvania Application October 22, 1941, sriai nolnaon f 7 Claims .(C1,25 27 5')J1 This invention relates to'an' improved starter electrode and specifically to an electrodesuch as employed asan immersion starter for pool type discharge devices.

tion of starter electrode described in co-pending application; Serial No, 387,838; of AprillO, 1941, assignedto the same assignee as the present application- I V Difficulties encountered manufacture, and imperfections developed by fuse of the presentday' electrodeshaveflpres'ented problems to be solved, and considerable'activity hastaken place directed tothe improvement of'such devices. Notwithstanding thefconcentrated effort in this field of endeavor, st'arterelectrodes have still proven unsatisfactoryfin various, respects. Inequality' -of electrode characteristics of different electrodes presumably made to be indentical has been a most serious handicap in quantity pro duction'. It has-also been found, in" the effort to v co t s h ndic p by turning to coating a core: with a resistivematerial, that while duplication characteristics are Iimproved', failures in use have occurred resultant iromthe insecure bondbetween or otherimperfections of the core andcoating.

*It is an object come the filfawbaciks heretofore encountered in manufacture and useof starter electrodes.

\ h V The invention ismore particularly an improvement upon the construcer-uiepresnt invention to ove Figure 2 is a cro'ts section on line II, II'of Fig." 1;and 1 r.

Figure 3 is a sectional view of a device identified as an ignitr'on, showing my improvedstart-i ing electrode'in placetherein.

In thespecific'embodiment of the" invention illustrated in 'saiddrawing, a starting electrode- H] is s'howhfand in Figure 3 is depicted Within and forming part 'of an ignitron as indicative of one use to which the invention'is'applicable The ignitron illustrated provide'sa sealed envelope H within which is a mercury pool cath'ode IZ'Tat the bottom of the envelope and'an -anode 'I 3 above the pool cathode suitablyispacedand insulated from the envelope as well as from the cathode. "It will be understood that the cathode and anode are to' befexternally' connected in 1 series relation witha loadcircuit and a su'itable source of p0.- tential. I'n'order to initiate a discharge between the cathode'and anoda whi'ch constitute'the main electrodes, an immersion starting electrode .H] is provided having a tip thereof" projecting lie-low the surface of the mercury or otherwise situated Anothenobjectof the invention is'to provide 7 an immersion electrode which maybe duplicated in its characteristic's in manufacture under con-j ditions of quantity production, 3

is to pro'vide an electrode of more permanent character and less likely to become defective by use.

A still further object of the invention is to provide an immersion electrode requiring s ower starting voltage and current than those previously' employed,- and characterized by greater freedom from starting variations. i Yet anotherobj'ect ofth'einventio'n is to reduce the voltage drop between the core and coating.

Still further objects of the invention will appear to those skilled'i'n the art as the description progresses, both bydirect statement'thereof and by implication from the context.

In the accompanying'drawin'g, wherein like numerals of reference indicate similar parts throughout the several views;

' Figure l is a longitudinal and greatly enlarged sectional view of the coated end of a starting electrode constructed in accordance with my invention;

so as to create a'cathode' spot. The'immersion starting electrode is also intended to be externally connected throug'h a metallic conductor '14 to a regulatable source of exciting potential adapted to cause passage of current from the starter into themercury'pool 'at' a predetermined time with respect to positive and'negativevariations of the main electrode voltages The character and op' eration of'the starter electrodeis'of'utmost im-* portance to the proper operationof the ignitron or other device wherein a starter-electrode is re- According to the'-pre'sent invention the starter electrode comprises aconductive-"core-l5 upon which is applieda 'resistivecoatin'g l6. 'A feature of "the present inventionis to'increase and im-' prove the intimacy'of'bond-between the ccreand coating. Informing theelectrod'e, I prefer to select'a porous orother rough surfaced material are base an'd tofabri'cate it to the desired shape toconstitrit'e' the core; The fabrication is-ofwa character which does not destroy the'rough or porous character of the 'oute'r'suriaceof the fin ished core, the porosityor other'roughness being maintained 'asan inherent feature of the inven= tion; The material which-I have found especially desirable to comprise'the said core is carbonac'eous inchar'acter and preferably in the form of graphite. The said 'core 'may be of any desired shap'e',and=for-purposes of illustration is elongated member of relatively small diameter.

Inasmuch as the drawing illustrates the invention as greatly exaggerated it may be stated that for ordinary usage in an ignitron the graphite core may well have a diameter approximating.

one-tenth of an inch or perhaps slightly greater. The lower end of the core is preferably rounded to avoid any sharp corners. The showing of Figures 1 and 2, by being greatly exaggerated, represents the outer surface of the core as rough or bond with respect to the c'ore-byimbedding in said cavities or pores of 'thegraphite constituting said core. tween core and coating, I am enabled to apply a By virtue of this tenacious bond be" much heavier coating on a single application'-- thereof to the core than is possible when applying a coating to a non-porous core such as shown in co-pending application, Serial No. 387,838, supra, and in view of this fact, a reasonablysatisfactory electrode can be made: wherein the coatingupon the graphite. corals-constituted by a single thick layer. .5 Howevenn'advantages are gained by. utilization ofimore than a single layer, and for illustrative: purposesI haveshowntwo other layers I1 and. I8 of'which the layer, I1 is disposed" intermediate of inner layer IG'and outer 1ayer.|8.-'**Whi1e the first layer may be directlyapplied to the core throughout .the coated area, I preferablycoat the rounded-end or tip; of the core with-insulating'cement I as shown in Figure 1. This insulating cement is preferably siliconcarbide. If preferred, as disclosed insaid copending application and where the core is by way of example of: tungsten,- a cement sold under trade designation ofwflSauereisen-No.78 is satisfactory and may belused. The purpose of this cement is toreduce the-current flow from the end of the core, which current .does not help in the creation of the cathode spot.

The characterof the coating l6 applied to the core 15 is of such nature that it. will readily wet the core when applied and will flow into the-pores or other cavities and will harden-therein as well. as over the entire surface of the core to which it is applied. The coating accordinglyhas acovering characteristic-anda bond characteristic in .itsrelationship to the core; The coating is furthermore' of a resistive material and of a charac, ter. enabling it toifunction with-respect to the mercury'poolas will be understood by those skilled in the art. 1

As explained in the aforementioned co pending. application, the said layer l6 next core, I is preferably formedof equal parts-of boron-carbide powder of '320 meshandsilicon-carbide of 500 mesh mixed. with a few percentsjof silicon powder. The preferred-composition is 10 grams of boron-carbide, 10 grams of silicon-carbide and /2 gram of silicon. This powder is mixed with a binder and preferably with approximately 10 /2 cubic centimeters of a binder consisting of nitrocellulose mixed with a solventsuch as amyl-acetate or diethyl-carbonate." The core I5 is immersed in this mixture to the desireddepth and withdrawn at a rat dependingupon the thickness of the coating desired. .Inpractice, core l5, which may preferably havean outside diameter of .097 inch, is withdrawnsslowlyfrom the mix ture in 5 to 45 seconds and will then have a coating' formed thereon or approximately .002 inch to .005 inch in'thlckness. -More-precisely,'I prefer to withdraw the'coreJ-in seconds and produce rapidly.

a coated core or electrode of a diameter of substantially .104 inch. The coatin may be thickened by withdrawing the core more slowly or may be made thinner by a quicker withdrawal. The dipping may be repeated if necessary.

This first coating may be dried naturally in air or it may be fired in hydrogen at 1100 C., for ten minutes such 'as described in Patent 2,235,504, p. 2, lines 45 to55, issued to R. Rennie,

March 18, 1941, and assigned to the same assignee as the present invention. The electrode, as thusmade, may be used without further treatment, if desired.

In 'order'tofincrease the resistivity of the secohdforintermediate coating I! I preferably increase the percentage of silicon-carbide therein.

My preferred coating is that of 5 grams of boronj carbide to 15 grams of silicon-carbide with ,gramof silicon mixed in the solution previously described. The core is inserted and then withdrawn gradually in 15 to 30 seconds to provide a second coatthereon, Further. layers may be added ifdesired in order to gradually increase theresistivity of the layer from the inner corev or conductor to the outside surface of; the electrode. In the preferred embodimenthowever;

I do not immediately dry or heat the second or intermediate coating but while the coating is still moist I roll the core in silicon-carbide which will pass a mesh within the range-of 60 to 200, and preferably approximately 180 mesh silicon-car bideto form a hard rough crystalline layer l8 over the outside surface of thepreviously applied coating. A rough surface reduces the total current. Boron-carbide maybe. used instead of silicon-carbide The complete electrode is. then preferably. fired again in'hydrogen for ten minutes at 1100 C. and after beingcleanedwitna soft brush is fired in air up to 960910. and cooled Prior art coated cores used 'in, ignitrons have presented unreasonably high percentage of 9.11. ures due .to ,a puncture through the coating. caused by thehigh voltage gradient developed by inadequate bond .between coating and core. If this gradient anywhere exceeds the-; dielectric strength of the material, a puncturewill occur. In my ignitor electrodethevoltage gradientnear the surface of the. core is reduced below-the dielectric strength of the material by virtue of the improved bond eflectedrbetween coating and core.

It is particularly pointed out-.rthat utilisation of the graphite core has the advantages of being less expensive, of having advantageous intimacy of engagement by the coating,'of forming abetter bond between itself andv the coating'than DOS! sible with a metal or. relatively smooth and nonporous core, of reducing. the voltage drop at the inner surface-of the coating by virtue of the improved bond and intimacy thereat, of dbtaining a starting electrode functioning at lower voltage and with less currentthan heretofore'necessary, and of increasing efficiency and rapidity of response of the ignitronor other device with which employed. The graphite, with itsgsurface pores well filled with the coating, provides a structure wherein the coating will not easily chip or flake off during operation and which presents a resistance gradient increasing in a direction through the'coating but substantially uniform throughout the area of the coated core and with the resistance gradient adequately preventing puncture during use.

I claim: I K

1. An electrode for a discharge device comprising a graphite abase having a rough surface thereby providing irregular cavities in 'said surface, and a coating of boron-carbide on said graphite base with the portion of said coating contiguous to the base imbedded in said irregular cavities in the surface thereof.

2. An electrode for a discharge device comprising a graphite [base having a rough surface thereby providing irregular cavities in said surface, and a coating of silicon-carbide on said graphite base with the portion of said coating contiguous to the ,base imbedded in said irregular cavities in the surface thereof. 3. An electrode for a discharge device comprising a graphite base having a roughsurface thereby providing irregular cavities in said sur face, and a coating of boron-carbide and siliconcarbide on said graphite base with the portion of said coating contiguous to the base imbedded 1 in said irregular cavities inthe surface thereof.

4. An electrode for a discharge device comprising a base having a rough surface and a coating having intimate bonding engagement with said rough surface of the base, said coating comprising plurality of layers successively of increasing resistivity.

5. An electrode for a discharge device comprising a base having a rough surface and a coating having intimate bonding engagement with said roughsurface of the base, said coating comprising plurality cf layers successively of in creasing resistivity, and the outer coating consisting of a rough material.

6. An electrode for a discharge. device comprising a graphite base having a rough surface and a coating having intimate bonding engagement with said rough surface of the base, said coating comprising plurality of layers successively of increasing resistivity and the outer coating consisting of a rough material in part embedded in the layer next thereto.

7. An electrode for a discharge device comprising a graphite base, a coating of boron-carhide and silicon-carbide on said graphite base with the portion of said coating contiguous to the base imbedded in the pores thereof, and a layer of insulating cement interposed between the base and coating throughout a limited area next one end of the electrode.

EDWARD G. F. ARNOTT.

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