US2185679A - Arc lamp cathodes - Google Patents

Description

' Jap.2, 1940. PAm'soT' 2,185,679

ARC LAMP CATHODES Fi led Dec. 13, 1938 Inv'enior:

Afforn 65-9. I

Patented Jan. 2, 1940 s ARC LAMP 'cn monnsfi' Jean Parisot, Pagny sur Moselle, France, assign- 01 .to Socit 'Le" Cabana-Lorraine, Genneyil 'liers' (Seine), France, a French company Application December 1 3, 1938, Serial No. 245,467 1 I vIn France December-22, 1931 j v Thisinvention relates to cathodesior the highintensity' arc, particularly for motion-picture projection; vIt is'well-known to those skilled. in

the art of'zthe projection of light that the highintensity are .(so-called chiefly because of the abnormally" high operating. current densities used) hascharacteristics which sharply distinguish' it from all other types of arc, and indeed. from all other types of light source; Until the adventof the high-intensity are there was no other light .source available that could even approach' the "intrinsic brilliancy of the ordinary carbon arc.. The high-intensity arc has, however,

made "available a stable: light. source that far exceeds .the carbonarc in .brilliancy. Y

The production of the high-intensity arc is dependent upon the observance of certain well.-v known and' well-defined conditions of operation and ofielectrode composition, construction and arrangement The anode is a cored carbon hav-x'. ing a .she-llcof the purest carbonobtainable and v relativelylarge carbon. core (usually one-half the diameter '.of the 'shelll'fwhich is strongly mineralised (approximately. -75% by weight"- of the. core) with rare earthfluoridesor oxides.

Such is the standard type of high-intensity arc anode commonly in use, although certain modifled compositions for the core have. been :more recently: developed. The cathode, on the other hand, hasa small carbon .core which is unmineralised ortonly slightly mineralised, its shell consisting of the purest carbon obtainable; When mineralised, the cathode core consists'of a' mix ture, of carbon and substances. the vapours of which are highly conductive for the arc, usually alkali. compounds. Most .frequently, silicates of potassium or sodium are chosen as the alkali compounds, or .-mixtures of these silicates with the borates or carbonates. The amount of mineralisation, calculated by weight, is a maximum of 20% of the total Weight of the, core.

Such an anode and cathode are paired, for operation Lon direct current, one customary pairing being, for instance, an anode of 8mm. diameter havinga core 4 mm. in diameter, with a cathode of 7 mm. diameter] having a core of diam- 'eteriin theneighbourhoodof one-quarter or onethird-oi the cathode diameter. The high-intensity arc can be produced on "alternating current by pairing two anodes as above described in-:

tended :for direct current operation, but it has been found impossible to establish the high-imtensity arcbetween two-, of the above described cathodes intended for direct/current operation.

Itfhas' beenfound by experience that, the paired I simple 01' ccmplex halides.

anode and cathode give rise, vin the arc, to the formation .ofYthewrare earth carbides, and these carbides-tend-to attach themselves to the negative point. It results from "this that the electronic emission necessary for starting and maintaining thearc is :no longer produced-exclusively on the core" but takes; place both on the carbide deposits and on the core. .SinceEthese'deposits remain liquid at the working temperature of the are, 'the drops :f orme'd areadisplaced in an irregularmanner due to capillary action-and to new deposits ofj carbide, so that the are 'no longer retains the steadiness necessary for good motion picture projection. Moreover, as these deposits are non-conductors. of electricity when once cooled, the cathode thus contaminated doesnot permit :re ignition; after 'the arc has been extinguished. w 1

Now, a, primaryobject of my invention isv to providean improved cathode for the high-in tensityarc by -.means of which-these inconveniencesareavoided.atvleast .to a much greater extent than :has been the case heretofore; With this object i-n view, I have found thatthedegree .of' mineralisation of the cathode 'coreand .the

nature of the mineralising material are important factors decisive for suppressing the depositionof carbide'entirel-y-orat least to an innocuous level. My experiments-haveshown that the carbide deposits on the cathode have progressively decreased as the mineralisation-of the .core is increased, but on-theotherlhand the oxygen-cone taining alkali salts such as those mentionedabove usually employed asumineralisi'ng material become dangerous when used in toolarge a proportion, .since the silicates and carbonates are reduced at a slight {depth in the core where the temperature is already suflicient to permit such reduction, and the gases which are copiously evolved as a resultof this reduction entrain fine particles of core which are inj-ur-ious to the steadiness gof thearc. I have found that this -dif-.-

ficulty-can be overcome by excluding such .salts from the em and employing instead. alkalisa'lts whichare not liable "to reduction, for example the Therefore, the essence of my invention, briefly stated, is to *mineralise the-core'of' the cathode with 'a controlled proportion of a simple or comp'lexhalogenated.

alkali compound.

j Good results have V been obtained with the fluorideslchlorides, fluoborates, fluosilicates of potassium-and sodium. The corresponding .com-

pounds of the other alkali metals may likewise be used but their high price does not recommend them except in special cases.

Since, as has already been mentioned, the carbide deposit has been found to progressively decrease (from its greatest amount in the case of a cathode with an unmineralisedcore) with increasing mineralisation, it will be appreciated that this precludes the iixing of an absolutely critical minimum value for the degree of mineralisation. Therefore, the approximate lowest limit mentioned later is not to be taken as necessarily leading to a complete elimination of the carbide deposit but rather to a limitation of the deposit to an amount which is tolerable in that it is compatible with satisfactory operation,

Whereas with too feeble a degree of mineralisation, the formation of thecarbide deposit-takes place to a very marked and troublesome extent,

too great a degree of mineralisation on the other hand causes troubles of another kind since the mechanical stability of the core then becomes insufficient and there is the risk. of the sputtering of incandescent matter,'which unfavourably afiects the steadiness of the arc.

Taking all these factors into consideration,

a suitable proportion of the mineralising ma'terial has been found to be 30% ormore,and preferablyin the neighbourhood of 40%, by weight of the core, the remainder of the core being, of.

course, carbon. Investigation has shown, however, that it is not the proportion by weight which should be more especially considered, but the atomic proportion of alkali metal to carbon. This will be appreciated upon reflection that the varifluoride to about 2.5-7.5 atoms of carbon. In

the case of sodium fluoride the optimum proportion has been found to be about 1 molecule of this fluoride to 5 atoms of carbon; this corresponds to a proportion by weight of about 40% sodium fluoride. Itfollows that if an alkali metal salt with bivalent anion is used, such as potassium fluosilicate (K'zSiFe), the correct proportion calculated according to the preferred range above mentioned is 1 molecule of the salt to about 5-15 atoms of carbomsince a molecule of the salt contains .2'ato1'ns of alkali metal. Among all the salts which can be used some are less to be recommended than others, either because they are sensitive to moisture or because they are expensive. Nevertheless, the results are good if other halides than those mentioned are used, for example the bromides or iodides of potassium or sodium, or the compounds of lithium, rubidium or caesium.

As has already been explained in the foregoing, it is very difficult to fix a minimum for the proportion of the mineralising material. The range, 1 atom of alkali metal to about 2.5-7.5

atoms of carbon, appears from experiments to be the remainder of the core being carbon, the'deposits of carbide only occur to a slightly objectionable extent. With approximately 25% of A the mineralising fluoride these deposits are very slight and innocuous' These determinations have therefore led to the conclusion that the lowest limit is of the order of 20% by weight,

, corresponding in terms of atomic proportion, to 1 atom of alkali metal to approximately 14 atoms. of I carbon. This limit depends hardly'at all on the nature of the alkali metal compounds selected, especially since i a somewhat uncertain limit is involved,- as has already been empha'-" sized. It appears therefore that a proportion of the order of 1 atom of alkali metal to about 15 atoms of carbon may be admitted as the practical minimum permissible limit.

Thus, the range of proportions which I contemplate employingby my invention is broadly 1 atom of alkali metal to about 2.5-15 atoms of carbon, thenarrower range of 1 atomof' alkali metal to about 2.5-7.5 atoms of carbonbeing the preferred one. 1

These ranges also applyif, in placev of a single alkali metal compound, two or more of the'seare associated, whether the compounds, in question contain ions in common or not. In other-words, a plurality .of alkali metal compounds of the group in question, viz., non-reducible compounds,

may be used together, but in such "a way'as to always maintain a proportion within the range, 1 atom of alkali metal to about 25-15 (and preferably 2.5-7.5) atoms of carbon. The use of mixtures has certain. advantages since it is'thus possible to deal with anydisadvantages. of a particular salt by diluting it with one or more others more suitable from the'same point of. view. For example, sodium chloride is obviously theieast expensive material of any in the -group, but if it is used in any appreciable quantity. it crystallizes intomore or less large ,crystals which always retain small quantities of mother liquor, in-

volving the tendency to decrepitation when the carbon is heated bythe arc. .Byassociating for example 1-2 molecules'of' this chloride'with'three' molecules of sodium fluoride, this disadvantage is avoided and 'at the same time the cost is kept down. I

The salts of potassium are often more advan tageous than those of sodium because they. do not'tend to colour thearc yellow. However, I have found thatthisi colourationtis .quite negligible if the sodium 'isused only in'the "cathode, even with the largequantities which I contemplate employing. Nevertheless, if it be preferred the potassium salts may be used either. alone -.or,

associated with other alkali metal salts. Potassium fluoride, being deliquescent, is not to be recommended for usebyitself but it may be incorporated in small quantities along-with other salts such as sodium fluoride and potassium the neighbourhood of the tip and Fig. 2'shows a cross-section.

The cathode comprises a shell I of commercially pure carbon and a core 2 which consists of carbon and mineralising material as described in the foregoing. The core Zhas a diameter of the order of about one-quarter to one-third (say about As an,

0.2-0.35) of the diameter of the shell l. example, the preferred mode of preparing such a cathode, the core of which is mineralised with sodium fluoride will nowbe described. *A tube 1 of commercially pure carbon. is first prepared,

'-which will constitute the'shelf'of the cathode. This tube is manufactured in known manner by extrusion from a paste of powdered carbon agglomerated with coal tar, the soft tube formed being baked in a neutral or reducing atmosphere in order to carbonise the tar. The tube l is made of suitable externaldi'a'rneter ('7 mm; being one size normally used) and lengthjfor use,-

the internal diameter 'ofIthe tubes being .made equal to that of the corewhich it is to contain, for example 1.5 to.2 mmyin the case of a 7 mm, cathode.

1 pressure is-injected a thickpaste containing 40% of sodium fluoride and 60% of powdered carbon, by'weight; This paste is agglomerated by means of a small quantity of organic binder such as gumarabic, dextrin, various. glucidic materials or suitable mixtures of these different substances. The necessary quantity of water for forminga paste of suitable consistency is added,

the amount of water depending especially on the particle size of thepow'dered carbon. ,When the tube i has been filled with this paste it is heated in'an oven to a suitable temperature so as to dry and harden the paste and char the ori ganic binder suiliciently to avoid any evolution of tar or odours when the carbon is in use. Thus the core 2 is formed. In a modification; the core 2 is preformed from a paste consisting of a mixture of powdered carhem and sodium fluoridein proper proportions agglomerated with a binder .of coal tar. This paste is formed into a rodwhich is hardened by baking in a neutral or reducing atmosphere. Such rod, constituting'the core 2, is introduced into the tube constituting the shell I and ce-; j mented therein, the rod and/or tube having been- I previously prepared with abinder for this purpose. The rod is made of' such diameter as to fitclosely into thetube with mild friction. Ce-

menting together of the two parts is effected by means of an organic or mineral binder, theuse of an alkali silicate having no inconveniences under these conditions. The binding agent is'then dried and hardened-in an .oven. 7 Of course, when following this procedure the proportion of the mineralising material is calculated on the weight of carbon subsisting after baking.

Cathodes thus prepared may be utilised as such, or. more often after having been jacketted with a metal conductor,-preferably copper, which enables them to withstandwithout excessive wear the high current densities usual for'thehighintensity arc; -75 amps. for a 7 mm. cathode.

' I claim: 1

, 1. A cathode for a high-intensity are, com? prising a shell composed entirely of carbon enclosing a core consisting essentially of carbon and at least one oxygen free alkali metal compound, the proportion of said compound to car- Into this tube and under suitable bon in said core being 1 atom of alkali metal to about 2.5-15 atoms of carbon. it

2. A cathode for a high-intensity arc, comprising a shell composed entirely of carbon enclosing an oxygen free core consisting essentially of carbon and at least one halogenated alkali metal compound, the proportion of said compound to 7 carbon in said core being 1 atom of alkali metal to about 25-15 atoms of carbon.

3. A cathode for a high-intensityarc, com-v prising a shell composed entirely of carbon enclosing a core having a diameter of about 02-035 that of the shell, said core being free from oxygen one halogenated alkali metal compound, the proand consisting essentially of carbon and at least portion oi said compound to carbon in said core atoms of carbon.

being '1 atom of alkali metal to about 2.5-7.5

l. A cathode for a high-intensity arc, compris ing a shell composed entirely of carbon enclosing a carbon core containing a mixture of sodium fluoride and sodium chloride, the total proportion of said sodium compounds to carbon in said core being 1 atom of sodium to about 2.5-15 atoms of carbon and the molecular proportion of, the

being greater than that ofthe sodium fluoride sodium chloride.

5. A cathode for a high-intensity arc, cOmclosing a carboncore containing a mixture of sodium fluoride and sodium chloride, the total pro-- portion of said sodium compounds to carbon in' said core being 1 atom of sodium to about 2.5-

7.5 atoms of carbon and the molecular propor tion of the sodium fluoride being greater than that of the sodium chloride.

6. A cathode for a high-intensity are, comprising a shell composed entirely of carbon enclosing an oxygen free carbon core containing about 20-40% by weight of sodium fluoride.

7. A cathode for a direct current, high intensity arc, comprising a shell composed entirely of carbon, enclosing a core consisting esentially of car- I bon mixed'with oxygen free, halogenated, alkali metal compounds.

8. In a direct current highintensity are light having ,a. carbon anodeunineralized with rare earth compounds, whereby rare earth carbides are formed in the arc, a carbon cathode having a core mineralized with: an oxygen free, alkali metal compound.

9. In a direct current high intensity are light having a carbon anode mineralized with rare earth compounds, whereby rare earth carbides are, formedin the arc, a carbon cathode having a core consisting essentially of carbon mixed with oxygen free, halogenated, alkali metal compounds, whereby the deposit of said carbides on said cathode 'is substantially prevented.

JEAN ARIso'I'.

Images