US1388874A - Forming dielectric films - Google Patents

Description

' -R. D. MERSHON, FORMING'DIELECTRIC FILMS.

APPLICATION FILED FEB. 18, 1920.

" W0. EVENTOR ln fp g M TTORNEYS Patented Aug. 30, 1921.

a full, clear, and exact description.

UNITED STATES PATENT OFFICE.

RALPH D. MERSHON, OI NEW:YOR K, N. Y. i

, FQRMING DIELECTRIC FILMS- Specification of Letters Patent.

Patented Aug. 30, 1921,

' appltcationflled February 18, 1920. Serial 359,573.

To all whom it. may concern.

Be it known that I, RALPH D. MERSHON, a citizen of the United States of America. residing at New York, in the county and State ofNew York, have invented certain new and useful Improvements in Forming Dielectric Films, of which the following is As is-well known, the action of electrolytic condensers, rectifiers and other, devices depends upon the film which may be formed upon the surface of aluminum, tantalum and other metals immersed in certain electrolyte and subjected to the electric current; which film possesses the remarkable property of allowing current to flow from the electrolyte to the electrode with but little or no opposition, but of strongly opposing current tending to fiow from the electrode to the electrolyte. In tending tolpass through the'film the purrent exerts a stress thereon which may be conveniently termed film stress. Flow of current from the electrolyte to the electrode is accompanied by the formation of minute holes or perforations for its pas-' sage, but when the direction of the current is reversed these holes will behealed up by re-formation of the film over them.. The re-formation, however, is at the ex ense of the underlying metal, as is the origlnalformation of the film, and hence repeated perforation and re-formation will inthe course of time seriously corrode the metal and eventually destroy the electrode. Hence it is in general highly desirable to have the film stress constant in direction, that is, tending to send current from the electrode to the electrolyte, so that perforation will not occur. For this reason the" practice heretofore in formin the films has been to use direct current or the purpose, the positive pole of a suitable source of such current bemg connected to the electrodes and the negative pole to the electrolyte. In subsequent use of the filmed electrodes in a condenser on an alternating current circuit reversal of the film stress is prevented-by exciting the condenser, that is, by providing a unidirectional E. M. F. opposing that between thev electrolyte and the electrodes, as fully explained in my prior Patent No. 1,077,628, dated November 13, 1913. In forming films .with direct current the voltage employed is usually raised a little above the maximum voltage at which the apparatus is to be used on alternating current and the formation: preferably continued at this voltage until the current passed by the films has fallen to, a suitably low value. It has been found that the behavior of the film is as if-its thick-' ness were a direct function of the forming voltage, that is, the higher the forming voltage the thicker the film. But it has also been found that there is a point, which maybe conveniently termed the critical voltage, be-

yond which increase of the forming voltage does not materially increase the thickness of the film; or, if it does,'it requires an indefinitely lon time for its accomplishment.

' .I- have iscovered, however, that the films may-be formed by the simultaneous use of both direct and alternating current, even with the apparatus in operation as a condenser during practically the entire period of formation, and-it is accordingly the chief object. of my present invention to provide a convenient and effective method for the purpose.-

The drawing illustrates diagrammatically the preferred method of carrying out my invention. Y j 1 .In carrying out the inventionin the preferred manner the condenseris first excited at a low unidirectional voltage, thus forming or partially forming the film up to that volt age, and, while the direct voltage is still im- To this and other ends the invention consists in the novel features hereinafter described.

pressed, alternating voltage of low value is also supplied. The application of the alternating volta e causes an increase of current permissible current ca acity of the direct current machine supplymg the direct current the alternating voltage is raised untilthe current through the D. C. machine has risen to the maximum value permissible for the machine. The increase of alternating voltage produces further film formation, with consequent decrease of direct current passed, I

and when the latter has again fallen to a suitably low value the alternating voltage is again raised. This process is continued until the alternating voltage is the maximum that can be emplqy ed without reversing the film stress. hen this point is reached the unidirectional voltage is increased by a convenient .amount, and when, because of further film formation, the direct current passed by the films has fallen sufli ciently the alternating voltage is again increased.

The steps described are repeated as often .as necessary to form the films up to the thickness desirable in subsequent operation of the condenser, or even farther. In fact, if the condenser is intended to be operated on a voltage at or near the critical value of the film stress the formation may be carried on' at voltages hi her, even considerably hi her, than the'critical value.

nstead of increasing the forming voltages.

much smaller direct current generator is required than where the formation is carried up to the maximum desired film stress by means of direct current alone, since in the latter case the generator must supply and Thus in the drawing, the plates carry the current involved by the high voltage used. In the present method the direct current machine must also carry the forming current, but at a much lower voltage. Secondly, since the condenser is connected with an alternating current circuit during formation of the films,- it is possible at any stage to determine the power factor and other constants of the condenser merely by inserting the proper measuring or indicating instruments in the circuit without disturbing the internal or gxternal connections of the condenser. I

In practising the invention in the preferred manner'the electrodes which are to be filmed are connected to the alternating source as a condenser, that is, half the plates are connected to one terminal of the source and the otherlhalfto the other terminal. immersed in a suitable electrolyte 11 contained in a tank 12, are connected to one terminal ofthe other terminal. Inea'ch case. theicopnection alternating source-14, through the medium of the autotransformer. 15, and the other plates, 16,...aregsim1larly connected to the nected to the electrolyte, as by means of a I cathode 20, composed of gold, nickel, or other suitable non-filming material. In starting the formation of the film the variable connections are set to give the desired l ow voltages and are shifted to increase the voltages as already described. It is to be understood that I do not limit myself to any particular method of varying the voltages. In order to prevent reversal of the film stress there is a minimum value of. the unidirectional voltage at the plates, relative to the alternating voltage, at or above which the former should be kept regardless of absolute values. This minimum value is one-half the maximum instantaneous value of the alternating volta e. Thus, in the case of an alternating E.

and representin the value of the alternating voltage by%@ (the square root of the mean of the squares of the instantaneous values), the unidirectional voltage should,

theoretically, never be less than HE thoughit may be as much higher as desired.

. F. wave of simple sine form, v

- It is to be understood that the invention is not limited to the exact procedure herein specifically described but can be practised in other ways Without departure from its spirit. h I I -I claim:

1. In the art of forming dielectric films of the kind described, the improvement comprising impressing upon the electrodes immers' ed in a suitableelectrolyte both direct and alternating voltages of such relative values as to prevent reversal of the film stress.

'2."'In the art of forming dielectric films of the kind described, the improvement comprising impressing on the electrodes immersed-in a-suitable electrolyte both direct and alternating voltages of such relative values as to produce a film stress constantly directed from the electrodes toward the electrolyte.

3. In the art of forming dielectric films of i the klnd described, the improvement comprising immersing the electrodes in a suitable electrol e, connecting'the electrodes to a source of a te'rnating voltage in the manner of'a condenserand connecting the electrodes to the positive pole and the electrolyte to the ne ative 'pole of a'source of unidirectional vo tage to impress both unidirectional-and alternating voltages of suitable values, and thereafter increasing unidirectional voltages, r

4. In the art of forming dielectric films the alternating and of the kind described, the improvement com- I alternating voltage of a value too low to reprisin' immersing theelectrodes in a suitable e ectrolyte, impressing ,on the electrodes both unidirectional and alternating volt; ages ofsuitable values, and thereafter in creasing the unidirectional and alternating voltages with such relative valuesas will prevent film stress from the electrolyte to the electrodes.

5. Inthe art of forming dielectric films of the kind described, the improvement com.- prising immersing the electrodes in a suitable electrolyte, impressing unidirectional voltage "on the electrodesto partially form the films, and while supplying unidirectional voltage to the electrodes, operating the ap paratus as a condenser in connection with a source of alternating voltage. v

6. In the artof forming dielectric films of the kind described, the improvement comprising impressing on the electrodes immersed in a suitable electrolyte a unidirectional voltage of relatively low value to start formation of the films and exert. a film stress from the electrodes to the electrolyte,

and Pater impressing on the electrodes an verse the film stress. I

7. In the art of forming dielectric films of the kind described, the improvement comprising impressing on the electrodes immersed in a suitableelectrolyte a unidirectional voltage of relatively low value to start formation of the films and exert -a film stress from the electrodes to the electrolyte, impressing on the electrodes ran alternating voltage of a value too low toreverse the film stress, and when the direct current'through the unidirectional source has decreased to a suitable value increasing the alternating voltage to increase the film stress from the electrodes to the electrolyte. 8. In the art of forming dielectric films of the-kind described, the improvement com;

prising impressing .on the electrodes immersed in a suitable electrolyte a unidirectional voltage of suitable value to start for- I so mation of the films and exert a film stress from the electrodes to the electrolyte, impressing on the. electrodes an alternating voltage ofa value high enough-toincrease ..,signature.

without materially exceeding the .permis sible current-carrying capacity of the source of unidirectional voltage, increasing the alternating voltage one or more times without reversing the film stress, increasing the unidirectional Voltage to a safe value, and repeating' one ormore of the foregoing steps until the films are formed to the thickness desired.

9. In the art of forming dielectric films of the kind described, the improvement com-, prising impressing on the electrodes immersed in a suitable electrolyte a unidirectional voltage of relatively low value and thereby forming, the filmsup to such voltage, impressing arelatively lOW alternating voltage on the "electrodes until the resulting increased current through the films "has fallen to a suitable value, and thereafter increasing the alternating and unidirectional voltages without causing flow of current fromvthe electrolyte to the electrodes and without materially exceeding. the current. carrying capacity of the source of unidirectional voltage.

10. In the art of forming dielectric films prisingsubjecting the electrodes immersed in a suitable electrolyte to unidirectional and alternating voltages, and progressively increasing such voltages'without causing a film stress from the electrolyte to the electrodes and without materially exceeding the ofthe kind described, the improvement compermissiblecurrent-carrying capacity of the source of unidirectional voltage, until the films are formed up to.at least the maximum voltage atv which the electrodes are to housed in an electrolytic condenser.

11, In the art of forming dielectric films of the kind described, the improvement comprising impressing on the electrodes imrectional and alternating voltages, and pro.- gressively increasing such voltages at suitmersed in a suitable electrolyte both unidiable rates until the critical film stress is exceeded.

Intestimony whereof I-hereunto aflix my RALP H 15. MERSHON.

Images