US 2535030 A
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Dec. 26, 1950 J. c. BALSBAUGH 2,535,030
ELECTRICAL CONDENSER Filed May 4, 1944 N v E NTOR 7/9 YJa/v C 5/1456/106/1 enema Dec. 2a, 1950 ELECTRICAL compasses Jayson C. Balsbaugh, Newton Center, Man, assizllor to Aircraft-Marine Products, Inc., Harrlsburg, Pa.
Application May 4, 1944, Serial No. 534,076
5 Claims. (01. 175-41) This invention relates to electrical condensers and the method of making them.
Prior to my invention it has been the practice to make electrical condensers by assembling sheets of metal adapted to serve as electrodes of the condenser with interposed pre-formed sheets of dielectric materials with or without a filling of an impregnating material such as wax, oil, etc. In some cases it has been suggested to use preformed sheets of plastic such as polystyrene alone or laminated with sheets of mica; in other cases it has been suggested to coat mica with shellac or polystyrene and to use sheets of such composite dielectric for condensers; in still other cases plastics such as polystyrene have been molded to forms such that a mass of the plastic enters between and surrounds the electrodes of a. con-- denser.
Whereas structures such as these known and used in the prior art have proven adequate for many purposes, there has been a perslstentdea gel forming hydrous oxides into a hydrophobic compound. For this purpose one may use a water solution of ethylene diamine di'acrylate. Following this treatment the film is freed from excess treating solution and soluble salts and is then dried and heated slowly to about 250 C. and held for about 12 to 24 hours.
The gel may be spread to a thickness of 0.02". This shrinks on drying to a thickness of 0.001" to 0.003". This thickness is not very much changed by the further treatment with salt solution, drying and curing.
The finished film is of accurately uniform thickness, is substantially water-free and almost totally immune to changes of humidity or even immersion in water. The acrylate at its surface is compatible with and readily wet by and bonded v to most resins and resin-forming polymerizable compounds.
Dielectric constant of the commercial Alsifilm lar raw materials used, and the methods of manuto equivalent high power. factor of the dielectric, H
or variationsin capacity due to'physical changes 1 I of the condenser upon heating-to mention only;
a few of the most important defects.
I'have now found that superior condensers can be made by use of Alsifilmas a dielectric between the electrodes of the condenser. Such condensers are especially important and suitable for the severe high temperature and high tension service, but they are suitable also for less severe service; and in such uses, for example, as D. C. bloc-king use, very simple and economical: structures may be made in accordance with my'p'resent invention.
Alsiillm is thenamegivento a product of the I;
type covered by the patent of-lnstfArHauser, No. 2,317,685, and application Serial No. 435,106
(Series of 1935) now Patent No. 2,401,348,Assued 'June' 4, 1946. Such a product is ofl'ered coinmercially by Rohm '8: Haas Company of Phila elphia under the 'trade name Diaplex." S lfically such a product can be made byforming a gel of purified white California bentonite of particle size classified to between 10 and 250 millimicrons. This gel is formed in a layer about 0.02" thick and shrinks on drying to a thickness of a few thousandths of an inch. The dried film is then treated in a solution of salt 01' a po yvalent cation and an organic acid which converts the inorganic facture. It is stable to long exposure at temperatures up to 200 C. to 300 0., depending again upon the raw materials and methods of "manufacture. Its. dielectric strength in condensers embodying the present invention is about that of good mica. Tested as .such, however the individual sheets of Alsifilm have shown breakdown much below that of condenser grade mica.
Presumably there are weak spots or defects in the material which, under such' actual conditions of use or test, cause failure. Early predictions that Alsifilm would be a mica substitute have not, therefore, materialized; and mica has continued to be required for condensers subjected to certain conditions of service. Although mica has many unique advantages, its usefulness is limited in many ways: It is not practicable to supply it in sheets much larger than a few square inches in area, thus requirin many condensers connected in parallel for any high capacity uses.
, It is subject to ionization if high voltage gradicuts are imposed upon it, thus requiring condensers connected in series for high voltage uses. It is expensive in the best condenser grades and There has, therefore, remained an insistent demand for a dielectric material which would combine some or all of the advantages of mica with these other advantages not heretofore attained with mica.
I have now found that Alsifilm can, by special treatment and especially by use in certain combinations, meet these requirements. It can be made in almost any size required, it can be used 3 at high voltages without ionization and in relatively thin layerswithout breakdown. This and its relatively high dielectric constant, moreover. give relatively high capacity for a given size and weight in a condenser and permits the use of single condensers of moderate size where mul-- tiple units and cumbersome connections have heretofore been necessary.
In accordance with the present invention. Alsifilm, with or without the final curing by heat, is made in thin films of the order of a few thousandths or ten thousandths of an inch in thickness and accurately uniform thickness throughout. Sheets of such film are then assembled into a stack and combined into a composite dielectric by application of high pressure over its entire area. I have found great advantage in thus sub- ,iecting the Alsifilm to high pressure in all parts 'of its area, in combining a number of sheets of the Alslfilm in each layer of dielectric and in consolidating the separate thin leaves of Alsifilm into an integral mass without air interfaces; and I have also found that for some purposes there is advantage in each of these, even when used apart from the others.
The Alsifilm used in accordance with my invention may be of the type which is merely rendered water-resistant by base exchange with a heavy metal or organic salt, e. g. as described in said Patent No. 2,317,685, or it may be of the type in which the base or the acid constituent of the salt used for such purpose is polymerizable to form a dielectric resin and cured at a temperature adapted to eflect such polymerization and to make the film immune to exposure to water or moist air. If the film is not cured but has been treated with a resin-forming base or salt, the several layers of Alsifilm can be assembled, pressed and cured together with the result that the sheets are welded together without interface, apparent- 1y by polymerization between sheets as well as within sheets. If the film is not provided with such resin-forming.substance, or if it is already cured so that further polymerization does not occur, the sheets are coated with a thin film of a dielectric bonding substance and are then pressed together under high pressure and elevated temperature.
In this process the film itself is apparently improved by the high pressure treatment, the commica oneof the best dielectric materials, 1. e.
its built-up structure from many thin layers, but without the disadvantages due to flaking and splitting under stress and due to air interfaces or other impurities present in the natural mica. Where a bonding substance is used the electrical disadvantages to be expected are avoided in my invention and composite dielectrics of extraordinarily high quality are obtained. Thus a dielectric for use between condenser plates was made by coating single accurately preformed sheets of Alsifilm of uniform thickness with a lacquer formed by dissolving polyvinyl carbazole, commercial grade Ii-35" and 25% its weight of hydrogenated terphenyl, (e. g., commercial plasticiaer Monsanto H1840) in three times its weight of toluene, and drying thoroughly. The dried polyvinyl carbazole film was 0.1 mil in thickness on each side of the Alsifilm sheets. This Alsifilnf before coating showed by test a breakdown strength of 500 V., but after coating the composite sheet withstood 3.000 v. with no indication of having reached its limit. As only 0.2 mil of the bonding resin had been added, this gives anapparent breakdown strength of the polyvinyl carbazole in excess of 12,500 volts per mil, whereas the breakdown strength of a sheet of the same polyvinyl carbazole composition alone showed actual breakdown strength'only a small fraction of that value. Moreover, since the dielectric constant of the polyvinyl carbazole is less than that of the Alsifilm it was to be expected that the electrical stress would concentrate disproportionately in the coating.
In this specification and the accompanying drawings I am setting forth a number of particular examples and suggesting various modifications and alternatives thereof, it should be understood, however, that these are not intended to be exhaustive or limiting of the invention; but. on the contrary, are chosen and presented for purposes of illustration in order to make clear, the principles of the invention and the practical employment of those principles in applying the invention to practical use; and thus so fully to instruct others skilled in the art that they will be enabled readily to modify and to select and substitute alternatives, each as may be best suited to the particular conditions of any given application or use.
In the accompanying drawings,
Figure 1 is a view in cross section showing a condenser stack assembled ready for compression; and a Figure 2 is a view in cross section of a condenser made by compression and molding of the stack shown in Figure 1.
Referring first to Figure l, the condenser stack there shown is made up of electrode plates II and [2 having respectively integral leads I4 and it. Each of these plates is coated with a film ll of a dielectric plastic material. Between the plates l0 and I! are shown a plurality of layers 20 of Alsifllm, three being shown in the figure by way of example, each coated with a film Ii of dielectric plastic material. Similarly coated layers of Alsifilm are assembled above and below the electrodes. It is to be understood that the number and arrangements of the plates and likewise the number of the dielectric layers, is chosen in this case for simplicity of illustration and in actual practice will depend upon the service for which the condenser is designed. It should be understood also that the proportions shown in this drawing are greatly exaggerated for purposes of illustration. Thus the Alsifilm will be accurately formed and of uniform thickness ordinarily of the order of a few mils in thickness; the electrode plates may be of the same order of thickness or greater or less, depending upon the service required; and the coatings i0 and Il may be of the order of tenths of a mil, this also.
being the order of magnitude of the greatest surface irregularities of the Alsifllm used, whereas the width of the various layers may be approximately as shown in the drawing or of other size, depending upon the capacity required and the voltage to be applied. More specifically the Aislfllm sheets may be between 0.0005" and 0.005" in'thickness and the bonding film, after pressing of the condenser, less than 0.0003" in thickness between the electrodes but thicker beyond the edges of the preformed sheets.
This coat I! may be applied as a lacquer or varnish. For example, in one very satisfactory method the commercial "K30" polyvinyl can 8 bssole polymer which softens at about 150' C. to 200' C., is dissolved in three times its weight of toluene to give a varnish consistency. To this is added 2% to 15% of a suitable plasticizer to 'give a smooth plastic fiow during the subsequent molding step andreduce the brittleness of vinyl carbssole.
.Ifthe condenser istobeusedon directcurrent or, in general, for low frequency A. C., any of the plasticizers known to beeifective with polyvinyl carbazole, e. g. tricresyl phosphate, triphenyl phosphate, etc., may be If, however, the invention is intended for high frequency use it is advantageous to use non-polar compounds as the plasticizer. For this purpose I have found particularly suitable higher hydrocarbons, particularly amyl naphthalene, diphenyl or terp cially by the Monsanto Chemical Company der the trade name EB-40.
The metal sheets i and ii are thoroughly cleaned before coating to assure thorough wet- Thus,
- sheet have been'dissolved away leaving a microscopically blunt edge. In practice this is'indicated when the surface gloss is cut to a fine matte appearance. The foil is then removed from theetching solution and washed thoroughly with distilled water, dried in warm, dry air and coated with a thin film, 0.0002'. or less, of the polyvinyl carbazole lacquer which, after pressing in a condenser as hereinafter described, may be reduced to 0.0001" or less.
This lacquer is brushed or sprayed onto the surface of the metal plates 10 and i2 so that the surfaces are thoroughly wet by the lacquer and the plates are then supported in a horizontal position allowing the lacquer to flow into a level film and are thoroughly dried in this position. If desired, additional coats of the lacquer may be applied to give a thicker dielectric layer.
With the solvent thoroughly evaporated from the coating it, a plurality of the coated electrodes are laid up one upon another, heated to a temperature sumciently high to soften the coating to a rather stifl plastic condition capable of smooth, plastic fiow under high pressure and are then pressed in a suitable mold. With the composition described above and depending upon the particular plasticizer chosen and the proportion in which it is used, the temperature for this molding operation should be in the range of from 150 C. to 300 C. and ordinarily around 225 C. to 250 C. This temperature should be substantially uniform throughout, when the molding takes place.
The Alsifilm sheets 20 may then be cleaned and coated in a similar manner with a lacquer the same as thatused in coating metal plates II and i2. With the lacquer dried on all of these several layers they are assembled as shown in Figure l. The entire mass is then heated throughout to a temperature at which the polyvinyl carbazole lacquer flows smoothly, e. g. 225 C. to 250 C., and is then pressed within a mold at a pressure of the order of 100 to 1000 pounds per square inch, e. g. around 500 pounds per square inch, advantageously as more fully described and claimed in my companion, now
p lyv abandoned, application, Serial No. 584,078, filed herewith. The resulting condenser is as illustrated in Figure 2, integral, compact and withfrom 5% to solids. When this suspension has gelled to a suitable consistency, it is spread in accurately uniform thickness upon a smooth level surface and dried to a film having smooth 1. Such a plasticizer is sold commersurfaces, e. g. l to 2 mils thickness. It will be understood that the final thickness depends, of course, upon the percentage of solids in the gel and the thickness with which the gel is spread. For example, with approximately 10% solids the gel may be spread to a thickness of mils, and dried at 90 C. in humid atmosphere.
Air dried film is thoroughly dried in an air oven at 100 C. advantageously using infra-red to secure uniform heating in the interior of the film as well as at the surface, and using care in the rate of drying to prevent blistering of the film. The film thus dried is impregnated with a resin-forming salt by a base exchange process in which the film is soaked in a to solution of a base exchange'salt, e. g. an amine acrylate, e. g. ethylene diamine dimethacrylate. The organic salt is held by the clay and presumably at a base exchange position by chemical bonding to the nitrogen of the organic salt. The methacrylio group is thereby attached to, and impregnated in, the inorganic film and being a resin-forming polymerizable substance, renders the film capable of further treatment by polymerization curing.
The film thus treated is then dried to eliminate moisture but at a temperature sufficiently low and/or time sufilciently short to avoid substantail polymerization. The resulting film, when cured at high temperature, is converted into a film suitable for use in accordance with examples of Figures 1 and 2.
I find that there is great advantage in the production of a composite dielectric made up of a number of thin sheets of Alsifilm rather than the use of a. single layer. This is true even where the thickness of the single layer is equal to, or greater than, the combined layers and it is true that where a number of single layers are used with a voltage proportionately less than the voltage across the combined layers. Thus with electrode layers. of' copper sputtered onto central areas on opposite sides of a single sheet of AM- film, a condenser is produced having a high capacity because of the high dielectric constant of the Alsifilm and the very close spacing of the electrodes. Nevertheless, connecting a number of such electrodesin series does not give a result equivalent to the use of a composite dielectric made up of the same number of sheets compacted,
e. g. as in the case illustrated in Figure-2 and similar electrodes sputtered on opposite sides of the resulting unit.
Moreover, I have found that it is of advantage even where Alsifilm is to be used without any the layers of such condensers and serving as a part of the composite dielectric, that is a separate invention claimed in my now abandoned application, Serial No. 534,079. filed herewith: the present invention contemplates the use of other known dielectric bonding materials-especially resin polymere-adhesive to the solid film layers used in the condenser. Many such materials are available whose selection and use are well understood in this art. In my application, Serial No. 534,078, filed herewith, I have mentioned among these, polystyrene, polyethanes, I methyl methacrylate, sirconyl methacrylate, alkyd resins, etc. As set forth herein, the adhesive cushioning film need not b aplastic or polymer but may be a viscous liquid such as. hydrocarbon oil or may even be omitted altogether.
Although my invention is concerned primarily with the composite dielectric using Alsifilm and condensers made with such dielectric, and although I have found that, as indicated above, the Alsifilm has great advantage over other available dielectric films and plates, nevertheless in certain broader aspects of my invention other solid dielectric materials can be used.
I claim: 1. In condenser having spaced conductive electrodes, a laminated dielectric positioned between said electrodes characterized by a plurality of substantially non-porous homogeneous dielectric layers comprised principally of indurated bentonite, and a plurality of films of thermoplastic adhesive dielectric bonding material each adhesive to and throughout the entire facing surface of one of said bentonite layers, said laminate being arranged with alternate layers of bentonite and films of thermoplastic adhesive and completely filling the space between said electrodes, said adhesive being of a material which in a massive form at room temperatures and in the absence of alternating stress has lower dielectric constant and lower breakdown strength than said bentonite film, but being of such thinness that the volts per mil breakdown strength for the combined laminate is at least as great as that of the bencoherent films of colloidal clay made from a hydrous gel of a fine white bentonite of particlesize in the range of to 200'millimicrons and rendered water-repellent and non-hygroscopic by a resin-forming organic compound chemicalhr combined with said clay. I
3. In a high-frequency, high-voltagecondenser having spaced plates. a laminated dielectric between said plates comprised of at least three spaced layers consisting principally of pure white bentonite and smooth, non-porous surfaces, and adhesive thermoplastic filling said spaces and adhering to said layers throughout said surfaces, the dielectric constant and breakdown strength per mil of said thermoplastic material in massive form at room temperature being substantially less than that of said bentonite layers, the maximum thickness of said thermoplastic materialbehigh-frequency, high-voltage electrical tween adjacent bentonite layers being such that its breakdown strength per mil of thickness is substantially/greater than that of said bentonite layers.
4. A high-frequency, high-voltage electrical condenser having spaced conductive electrodes characterized by at least one sheet of homogeneous, non-porous, non-ionizable material inter! posed between and extending beyond the edges of adjacent ones of said electrodes, said sheet and said electrodes having micro-smooth, non-porous surfaces and being in excess of 0.0005. inch in thickness and less than 0.005 inch, a film of thermoplastic dielectric adhesive to the adjacent sheet absorbent, air-free layers made from a hydrous gel of a fine, whiteCalifornia bentonite of par-.
ticle-size in the range 10 to 200 millimicrons, and
high molecular weight salt, said layers being of uniform thicknessand spaced from each other. and thermoplastic resinous dielectric films adherent to the entire facing surfaces of said layers and filling the spaces therebetween and of such thinness that the overall breakdown strength in volts per mil of the dielectric between said plates is at least as great as that of said bentonite layers alone, said layers and said resinous films being combined into a dense integral structure having sharply defined homogeneous layers. JAYSON C. BALSBAUGH.
REFERENCE S CITED The following references areof record in the file of this patent:
' UNITED STATES PATEN'fS Num er Name Date 1,173,452 Meirowsky Feb. 29, 1916 1,537,386 Tingley May 12, 1925 1,574,424 Hatch Feb. 23, 1926 1,854,498 Anderson Apr. 19, 1932 1,929,535 Parker Oct. 10, 1933 2,072,465 Reppe Mar. 2, 1937 2,133,445 Guerin Oct. 18, 1936 2,206,720 Ducati July 2, 1940 2,223,833 Sander Dec. 3. 1940 2,266,636 Hauser Dec. 16, 1941 2,266,637 Hauser Dec. 16, 1941 2,266,638 Hauser Dec. 16, 1941' 2,317,685 Hauser Apr. 27, 1943 2,383,647 Hauser Aug. 28, 1945 OTHER REFERENCES Chem. Eng. Mining Rev. 32, 438-9 (1940) Substitutes for Mica by Nlcholes.
Studies in Gelation and Film Formation 11, Studies in Clay Films-4. Phys. Chem. 43, 1037-43 (1939).
Paper Mill 62, No. 10, 12, 14, 16 (1939).