US 2774747 A
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Dec.- 18, 1956 H. WOLFSON ET AL ELECTRICALLY CONDUCTING CEMENTS CONTAINING EPOXY RESINS AND SILVER Filed March 25, 1952 MIX lNT/MA TEL Y FLA AE S/L VEI? A/VD UL 7AM FINE PRzE'C/P/M TED \S/L VER M/X ARALD/TE MA IN COMPONENT AND CORRESPONU/NG cmmr AND ADD omega/v5 ALCOHOL.
A00 ARALD/ff" M/X Tc/AE a/eAoz/A LL y 70 (SM VER M/X 70/85 //v MORTAR STORE MIX rug? sroppmfo JAR H.WOLFSON G.E LLIO T ATTORNEY United States Patent Application March 25, 1952, Serial No. 27 8527 Claims priority, application Great Britain April 5, 1951 1 Claim. (Cl. 26032.8)
The present invention relates to improvements in electrically conducting cements, and to the application of the cement according to the invention to the problem of fixing a semi-conducting crystal used for an electric rectifier or crystal triode, to a metal base or holder.
In the case of germanium rectifiers, it has been the usual practice to fix the germanium crystal to the base by plating and soldering. This, however, gives rise to difiiculties on account of the etchingtreatment which must be applied to the germanium to produce a satisfactory rectifier.
Best results are obtained if the crystal slice is etched before attachment to the base, as otherwise if the etching is applied after soldering, the presence of the brass and solder produces an undesirable effect. It is also difiicult to solder the crystal slice on to the base after etching without contaminating the etched surface.
The principal object of the invention is to overcome these difficulties by using an improved type of conducting cement for fixing the crystal slice to the base.
The invention accordingly provides an electrically conducting cement comprising a thermosetting binding medium with which are incorporated fine electrically conducting particles in such proportion that when the binding medium sets, the said particles are brought into electrical contact with one another throughout the whole mass, the binding medium being of a type which adheres to the particles and occupies the interstices between the particles, thereby holding the whole solidly together.
The invention also provides a method of making an electrically conducting cement comprising intimately mixing together fine flake silver and ultra-fine precipitated silver, and gradually adding to the silver mixture a liquid thermosetting compound in sufficient quantity to form a very viscous paste in which all the silver particles are just coated with the compound.
The invention further provides a method of making an electric semi-conducting device which comprises fixing a semi-conducting crystal body to a metal base by means of a thermosetting conducting cement of the kind specified above.
The invention will be described with reference to the accompanying drawing, in which Fig. 1 illustrates diagrammatically the preferred process of making the conducting cement according to the invention, and Fig. 2 shows the stages in the process of fixing a crystal slice to a metal base using the cement.
The preferred formula for the conducting cement according to the invention includes a thermosetting coating resin sold under the registered trademark Araldite which is made up of a hardenable epoxy resin comprising the condensation product of 1chloro-2,3-epoxypropane with 4,4 isopropylidene diphenol. The formula is 2,774,747 Patented Dec. 18, 1956 The Araldite mixture should preferably be prepared by mixing three parts of the main component with one part of the corresponding catalyst to give a medium containing about 50% solids, and the diacetone alcohol should be added.
The flake silver and the ultra-fine precipitated silver should be first intimately mixed. The silver mixture should then be placed in a mortar, and the Araldite mixture should be added gradually, working it into the silver powder. The resulting mass should be a very viscous paste. The procedure is very similar to the determination of an oil absorption of a pigment.
With the proportions given above, there should be just sufiicient of the Araldite medium to coat all the silver particles. 1
The conducting cement so prepared may be stored in a stoppered jar, and will keep for three months at normal temperature. The process just described is illustrated in Fig. 1.
The diacetone alcohol is used as a solvent for adjusting the viscosity of the mixture, and the proportion of this ingredient may be varied to produce a suitable viscosity for the particular circumstances of the use of the cement. Some addition of this solvent to the mixture may be necessary from time to time to make up for evaporation.
It is very desirable that the ultra-fine precipitated silver used in the above formula should not contain any appreciable proportion of excessively large particles. The preferred method of preparing it is as follows:
680 grams of recrystallised silver nitrate are dissolved in 2 litres of warm distilled water in a 5-litre beaker. The temperature of the solution should be about 50 C. 1.5 grams of sodium alginate are dissolved in 150 cc. of hot distilled water and the solution is mixed with the first solution. Continuous stirring should be maintained during the mixing. 250 grams of sodium sulphite of laboratory reagent grade are then dissolved in 1 litre of distilled water at a temperature of 50 to 60 C., and the solution is added to the silver nitrate mixture. A very thick curdy precipitate of silver sulphite is formed. 200 cc. of 40% aqueous formaldehyde solution is then added, and the temperature shouldbe adjusted to about C. Then 350 cc. of 0.880 ammonia solution is finally added gradually. A considerable rise of temperature now occurs, and should be controlled so that it is kept just below C. After all the ammonia solution has been added, the suspension should be kept between 95 and C. for 30 minutes to ensure the completion of the reaction. The sodium alginate reduces the rate of reduction of the silver sulphite, and excess of this reagent will prevent complete precipitation of the silver. Owing to its fine state of subdivision, the silver is very dark, particularly when damp, and it does not settle out readily, and filters slowly. The suspension should be filtered with a pump While hot and should be washed with 2 litres of distilled water while still on the pump, followed by a litre of acetone. The product should be dried at 150 C. overnight.
As shown in Fig. 2, in order to fix a germanium crystal to the corresponding metal base or stub 1, a blob of the cement is placed on the base, for example by dipping a wire 2 into the jar in which the mixture is stored, and picking up a blob 3 on the end as shown in Figs. 2 (a) and (b) and the crystal 4 is dropped on to the cement and pressed firmly down with the tip of a glass rod 5 (Fig. 2 (0)), and then the whole is baked in an oven 6 (Fig. 2 (a')) at about C. for 90 minutes. Shorter baking times can be used at higher temperatures. After baking, the crystal is found to be firmly fixed to the base, and cannot be removed without splintering the germanium.
The cement appears to have almost the conductivity of block silver.
This conducting cement may evidently be used for joining any two conductors, particularly when the use of plating, or solder, or the high temperature associated with soldering, are precluded. Evidently, silicon, or other semi-conducting materials, could be mounted on metal bases with this cement.
The method of preparing the cement explained above gives what is believed to be the best combination of conductivity and adhesion. If the quantity of the Araldite medium were too small, the final cement would have high conductivity, but would be friable or crumbly, with poor adhesion. If the quantity of the medium were increased, the mechanical strength and adhesion would be good, but the conductivity would be less.
In the cement, prepared as described, the silver particles are just coated by the Araldite medium, which contains about 50% total solids. After polymerisation, 50% of the bulk of the medium is lost by evaporation of the solvent, and each silver particle is then no longer completely coated, but the cement occupies the interstice between the particles which are brought together in contact, yielding a conducting material.
It should be pointed out that a rather unexpected result has been obtained with this cement. Araldite is an extremely good electrical insulator and the conductivity is not usually much aifected by the addition of other matter. In the present case the electrical properties of the Araldite have been profoundly altered without destroying the mechanical properties, by the introduction of the silver in such quantity that the silver particles come into contact only after the baking treatment.
It should be added that although Araldite is the preferred binding compound for the silver particles, other thermosetting compounds which will adhere satisfactorily to metals when set could be loaded up in like manner with fine conducting particles in such proportion that when the binding compound has set, the conducting particles are drawn together in contact throughout the whole mass.
While the principles of the invention have been described above in connection with specific embodiments and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.
What we claim is:
An electrically conducting cement, comprising 27% fine flake silver, 46% ultra fine precipitated silver, 24.5% hardenable resinous reaction product of epichlorohydrin with a diphenol and 2.5% diacetone alcohol.
References Cited in the file of this patent UNITED STATES PATENTS 1,901,391 Howard et al. May 23, 1933 2,137,428 Van Gcel et a1 Nov. 22, 1938 2,173,249 Boer et al Sept. 19, 1939 2,280,135 Ward Apr. 21, 1942 2,324,961 StoflEel July 20, 1943 2,470,352 Holmes May 17, 1949 2,473,884 Hein June 21, 1949 2,500,600 Bradley Mar. 14, 1950 2,506,130 Bain May 2, 1950 2,570,856 Pratt et al. Oct. 9, 1951 FOREIGN PATENTS 554,972 Great Britain a- July 28, 1943