[1]Patented Dec. 7, 1943, 21336,091 UNITED STATES -PATENT OFFICE 2,3 36,091 ELECTRICAL CONDENSER Donald E. Gray, Teaneck, N. J. assignor to Cornell.TDubiller EIectric Corpor@tion, South Plainfleld, N. J., a corporation of Delaware AppIleation June 15, 1940, Serial No. 340,693 6 Claims. (Cl. 175-41) The present invention relates to electrlcal conderisers and methods of manufacturing such condensers, more particularly to condensers of the type comprising dielectrie.elements coated with metallic layers to serve as the condenser elec- 6 trodes as distinguished from condensers of the type comprising separate metallic and dielectric ,layers stacked or otherwise interleaved and connected to provide a condenser imit having a desired eIectricaI capacity. 10 Stack @type cop-densers of known construction usually comprise a plurality of alternate layers of metallic sheets interleaved 'with dielectric sheetz such as plates of mica, ceramic or the like. Alternate metal layers usually exiend beyond opposite 15 6dges of the dielectric sheets with the projecting portioris of the layers being united to serve as electricai terminals for the condenser unit. The metallic and dielectric elements in stacked type condenser structures of the above character 20 are maintained In Juxtaposed relation and with their respective surfaces in more or less Intimate contact by the provisiorl of one or more pressure cla.mps encircling the condenser stack. In an effort to eilsure sufftclent electrical stability of 25 condem,er s of this type, bulky clarr@p str,,ictures and substantial compression forces are required to maintain the dielectric and metallic elements in Intirhate contact and to prevent relative lateral movement and appreciable variations'of the ca- 30 pacity and other characteristics of the condenser due to heating and other causes affecting the condenser and Its operation. The application of a considerable pressure to ensure the desired degree of electrical stability in 35 many cases will result in a damage of the dielectric or electrode elements. Numerous attempts have'been made to obviate this drawback and'io improve the electrical stability of condensers of the above type, In particular the 40 stability of the condenser when subjected to substantial temperature variations. Slich attempts Include the impregnation of the condenser stack with wax, oil, resin or the like and the application of the electrodes to the dielectric elements 45 In a more or less molecuiar contact with the dielectric surface. These endeavors have not been entirely satisfactory in the past for improving the electrical stability of condensers and to satisfy the requirements under various operating cor,.di- 50 tions. The electrical stabiiity of a condenser of the above mentioned type, partiewarly its ability 'to maintain a substantially constant capacity when subjected to varying temperatures is dependent 45 upon various factors, particularly upon the surface contact between the dielectric sheets and the electrode layers, the coefficient of expansion of the metal clamp encircling the condenser stack or of a resinous envelope or casing molded around the condenser stack, and upon the relative alignment of the electrode sheets in the condenser stack. Accordingly, ari object of the present Invention is to improvd the electrical stability of a conderser embodying solid dielectric sheets interleaved with metallic layers. A rnore speciflc object is to provide a condenser structure embodying dielectric elements coated viith -metallic layers forming the condenser electrodes, wherein the electrical capacity of the condenser Is s-Libstantiallyindep6ndent of internal and external temaerature variatior-s to which the condenser is subjected during operation. AnGther object is the provision of means for and methods for economically coating dielectric elements 'with a metallic surface to provide a permanent and closely adhering electrode layer for use In condenser structures according to the invention. Other objects and advantages of the invention will become more apparent from the following de@,a-iled description taken with reference to the accom-oanying drawliig forming part of this specificatio@-i and wherein: Mgure I Is a plan view of a dielectric sheet co.-,ted on opposite sides with metallic layers and serving as an elernent in a. condenser structuxe according to the i,.ivention, Figure 2 is a cross-section taken on line 2-2 of Figure 1, riligure 3 is 9, cross-sectional view showing a complete electrical condenser unit comprising elements according to Figures 1 and 2. Like reference characters ide-titify like parts throughout the different views of the dra,wing. Referring more particularly to Figures I and 2, there is shown a solid dielectric layer or sheet IC such as a sheet of mica, glass, porcelain, etc., having a pair of electrode layers or coatings I I and 12 appiied to the opposite faces thereof with sui't,able margins 13 and f4 being kept on both sides along one pair of opposite edges of the sheet 10. Additional insulating margins 15 and 16 are provided one on the upper face and the other on the lower face of the dielectric sheet at the remaining opposite edges to afford suitable insulation between the metallic layers I I and 12 and to prevent cree@ing ciirrents or sparking between the electrodes. It Is to be noted that the electro-de
[2]layers I I and 12 exactly register or coincide -along the margins 13 and 14 thus preventing capacity effects between electrodes of different insulating sheeis having unlike polarity in a stack comprising a plurality of coated dielectric elements as shown in Figure 3. In Mgure 3 a number of coated dielectric sheets IO are stacked one upon another to obtpin a desired final capacity of the complete condenser unit. Electrical connection between the electrode layers of like polarity is effected by the prdvision of conducting metal foils or tabs 17 and 08 interposed between the composite dielectric and electrbde layers and extending alternate]y beyond the opposite edges of the stack. The extending portions of the conducting sheets or strips 17 and 18 are reversely bent around the edges of the stack and upon one of the faces, in the example shown the lower face, of the stack to make contact with each other and with the ter, minal-clamp members 20 and 21 encircling the oppo@ite ends of thL, stack. In order to prevent stray capacities between the outermost electrode layers of the stack and the terminal clamps 20 and 21 through the end covering plates 33 and 34 as the dielectric, uncoated spaces 22 and 23, respectively, are provided in the outermost electrode layers of the stack dividing the same Into two portions 24, 25 and 26, 27, respectively. In this manner the return connection of the stray condenser, formed by the clamps @O and 21 and the overlapping adjacent portion of the outermost metal layers with the end cover plates 33 and 34 as the dielectric, to the clamp of opposite polarity through said layers Is interrupted by the Insulating spaces 22 and 23, thus isolating the stray capacity and eliminating its effect upon the total capacity of the condenser. To further improve this advantage and the stability of the condenser, the partial layers 25 and 26 are electrically connected to the adjacent clamps 20 and 2i, respectively, by mpans of additional tabs similar to the electrode connecting tabs 17. In this manner, the terminal clarlp 20 is in juxtaposed relation to the electrode layers 24 and 26 which are at the same polarity as the extending connecting strips 17, while the clamp 2 i is arranged in a similar manner in respect to t-he extending conducting strips 18. F'rom the foregoing it will be apparent that since the dielectric shsets 10 are in intimate contact with the electrode layers applied to botb sides thereof any expansion of the terminal clamps will have no effect upon this relationship. Moreover, as pointed out, since the clamps 20 and 21 overiie elect rode areas baving the same poiarity as tl-ie clamps and isolated from the clarr@ps of opposite polarity, tapacit-y effects due to movement or shifting of the clamps in relation to the underlying electrodes such as caused by the expansion due to heating, will be eliminated. The condenser in the example shown is pro-iided with the terminal leads 30 and 31, secured to the clamps 20 and 21, respectively, in any suitable manner for mounting and connecting the condenser unit in an electrical apparatus. A molded resinous envelope 32 which may consist of Bakelite or any other suitable molding material surrounds the condenser unit to provide adequate mechanical aiid other protection in a manner well understood by those skilled ' In the art. The electrode layers deposited on the dielectric sheets in any suitable manner such as according to the methods to be described in detail hereafter will substantially conform to the surface contour of the sheets so that their outer surfaces will bemicroseopicauy roughened. Thus, if the resinous envelope 32 is applied by a m6lding Operation, the resin will flow into t'.Ile microscopic interstices and adhere thereto with great strength. It has been found when using a laminated dielectric such as mica and molding the condenser In a resinous envelope in the manner describedi 10 the adherence of the resin to the electrode layer may be so great as to result In a.separation of the laminae of the dielectric, thereby greatly Impairing. or completely destro@ing the electrical stability of the condenser. In order to prevent 15 the resinous material from coming. into contact and adhering to the outermost electrode layer of the stack, protective plates of insulating material 33 and 34 are arranged between the electrodes 24, 25 and 26, 27 and the clamps 20 and 20 21, respectively. If it is desirable to adjust the capacity of the, condenser after completion of the stacking and clamping, this is accomplished according to the invention by clamping the insulating sheet 34 at one side only leaving the 25 opposite side free to move. The capacity is. adjusted by flexing the sheet 34 outwardly as indicated In dotted lines at 34' and scraping or removing a portion of the electrode layer 23 froni the surface of the outermost dielectric sheet, 30 whereupon the sheet 34 Is allowed.to return to its normal position and the conde.nser molded in a resinous casing or envelope. It has been found advantageous in many cases - to flll the spaces between the dielectric sheets 35 and the conducting strips with an Insulating material. This may be accomplished by impregnating the condenser In a wax, vamish or the like either before or after molding. Wben varnish is used as an impregnating material the Im40 pregnation Is preferably carried out prior to the molding process and the varnish used is advantageously of the fixable resin type. A suitable vamish suited for the purpose of the invention Is known in the trade as Bakelite varnish and is 45 believed to be composed of a phenol formaldehyde resin dissolved in an evaporable solvent. The varnish Impregnated condenser unit is dried and subjected to a temperature of about 325' P. to thoroughly polymerize the resin and convert it 60 to an infasible state so as not to adhere to the resinous envelope 32 during the molding operation. The varnish iinpregnation and subsequent polymerization is of particular advantage in 55 maintaining the electrode layers in proper relation ir registry to substantially prevent capacity variations between the layers on one dielectric sheet and the layers of an adjacent sheet. If desired, in order to further prevent the varnished 6o condenser unit from adhering to the casing 32, the varnished unit after polymerization of the varnish is dipped in melted wa:@e-to provide a thin wax layer between the two fixed plastics, that is, the impregnaitng varnish on the one hand 65 and the enclosing resinous casing on the other hand. Suitable materials which may be used for the above purpose are paraffine or other low melting point petroleum waxes. According to a modified process, the condenser unit after stack70 ing and before the Opplication of the termina;l clamps may have one end dipped in melted wax to hold the stack together whereupon the unit is subjected to heat and pressure to melt the wax and cause it to flow into the interstices of the 75 stack. Excess wax may be removed during the
[3]2,838,091 3 heating and pressing operation by placing the condenser between blotting paper or the like capable of absorbing the excess wax. Mectrical condensers constructed in the aforedescribed manner with mica as a dielectric mate- 5 rial are very stable as regards their electrical characteristics and when subjected to varying temperatures wW vary in their capacity value to a very small degree and it lias been found that this change in capacity Is positive and of the 10 order of .0025 % per degree C., that is, If the temperature of the c6ndenser Increases by one degree the capacity will change by the aforementioned amount. The capacity variation is probably'due to a change of the dielectric constant. of the 15 mica with varying temperatures and in most c6,ses is sufficient to more than balance any changes in capacity due to a greater or lesser separation of the electrodes due to expansion of the dielectric elements caused by a temperature 20 increase. The electrode layers or coatings applied to the dielectric element may be produced in various manners and by the employment of special methods. 25 According to one method which has been found to produce satisfactory results, various s!dver salt compounds dissolved in a water solution and applied to the dielectric surface are subjected to reduction by a suitable reducing agent such aS 30 sugar, glycerine, alcohols or the like. This method has been found to be particularly advantageous if a dilute solution of tin chloride is ar)plied to the dielectric surface and excess solution removed prior to the application 35 of the silver salt solution. A suitable concentration- of the tin chloride solution has been found to be composed of about -@6 gram of tin chloride dissolved in 100 cm. of water. Such a solution promotes the deposit of tfie silver layer on the 40 dielectric su.-face, probably due to the formation of a minute quantity of silver chloride prior to the depositing of the silver layer. In applying a silver layer to the dielectric surface, good results have been obtained by dissolving one gram of 4.1 silver nitrate in distilled water and @ adding .3 gram of sodium hydroxide which -wfll cause this solution to assume a brownish-black color. Subsequently, a sufficient quantity of ammonium hydroxide is added to dissolve the silver oxide pre- -!" cipitate and to return the soiution to the water color state. The solution is preferably cooled to a temperature between 5 and 15' C. and the dielectric sheets to be coated submerged in the solution. Theteupon, about 10 cem. of alcohol slightly acidifled with nitric acid to a PH-value of about 6.3 and three grams.of glucose in the form of a simple syrup are added to the solution. The syrup may be prepared by dissolving rock candy sugar or the like in a smgll amount of (111 water. The mixed solution is then agitated for a period oil about 6 to 10 minutes during which time it will pass through a color range from black to brown to gray. The dielectric sheets are removed when the first trace of gray coloration is 11) discemible and it will be found that a bright coherent and uniform coating of silver has been deposited all over the exposed surfaces of the dielectric. It has been found that in some citses it is desirable to admix as a reducing agent to 7( the silver nitrate sodium hydroxide, ammonium hydroxde solution, instead of the syrup solution described above approximatelly 25 grams of glycerine, ethylene glycol or another water soluble polyhydric alcohol and to allow the reducing ac- is tion to continxie for a period of 1 to 11/2 hours. By the latter method, a small amount of heat should be applied to graduauy increase the temperature of the mixture up to about 75' C. In this manner, a silver coating having improved adhering properties to the dielectric is obtained. In order to provide insulating margins as described and shown, suitable blocking or masking devices may be used for blocking out the end portions of the mica or other insulating sheets. It will be evident from the foregoirig that the invention is not limited to the specifle details and arrangement of parts as well as steps (lescribed and disclosed herein for illustration, but that the underlying concepts and general prineiPle of the Invention are susceptible of numerous variations and modiflcations coming within the broader scope and spirit of the inveiition as deflned in the appended claims. 'Me speciflcation and drawing are accordingly to be regarded In an illustrative rather than a limiting serlse. I
