US2961416A - Silver conductors - Google Patents

Description

Nov. 22, 1960 J. A. BALDREY ET AL 2,961,416

SILVER CONDUCTORS Filed June 9, 1958 INVENTORS JOHN A. BALDRE Y OLIVER A. SHORT ATTORNEY United States Patent SILVER CON DUCTORS John A. Baldrey, Perth Amboy, and Oliver A. Short, Me-

tuchen, N.J., assignors to E. I. du Pont de Nemours and Company, Wilmington, D'el., a corporation of Delaware Filed June 9, 1958, Ser. No. 740,959

2 Claims. (Cl. 252-'-514) This invention rel-ates to silver electrical conductors, and more particularly it relates to silver conductors provided with means to substantially diminish the tendency of the silver to migrate. 7

Silver is used in many forms in electrical equipment. Due to its low arcing tendency and self-cleaning ability, it is frequently used for switch contacts. Because it is h'ghly corrosion-resistant and its corrosion products are either electrically conducting or easily removed, it is also used in the form of silver electroplated brass connectors such as the pin contactors in tube sockets.

Silver is highly conducting and can be deposited as a fine powder with a glassy binder on ceramic surfaces, making it useful for capac'tors and electric printed circuits. It can also be printed with organic binders on phenolic or other resin surfaces to form electrically conducting circuits. When two silver conductors are in close proximity to each other and in an electric circut where one is cathodic and the other anodic, metallic silver will migrate from the cathodic conductor to the anodic conductor in the presence of moisture. The silver will build up on the cathode to gradually cross the gap between the cathode and anode and thereby eventually create a short circu't. This silver migration is due to the relatively high solubility of silver hydroxide in a moisture film and the ease of electrodeposition of metallic silver. The migration apparently takes place through the following mechanism. Where a film of moisture is present on the electric c'rcuit comprised of the two oppositely charged conductors positioned on a dielectric base, hydroxyl ions from the ionization of water migrate to the anodic elements, dissolving silver from the anode as silver hydroxide. The silver ions produced by ion'zation of silver hydroxide migrate through the layer of moisture to the cathode where they are deposited as metallic silver. Subsequent electrodeposits form at the outer edge of former silver deposits and thereby form a chain of metallic silver eventually extending across the gap between the oppositely charged electrical conductors.

Many attempts have heretofore been made to overcome the migration of silver in silver electrical conductors, for example, by prov'ding gold or platinum barriers on the side of the anode closest to the cathode to discharge hydroxyl ions before the silver is attacked. In some instances gold or platinum have been used as the electrical conductor to avoid the bad elfects of silver migration. Such use of gold or platinum is far too expensive to be used except in very exceptional cases.

It is an object of this invent'on to greatly diminish or substantially eliminate the objectionable migration of silver between silver electrodes positioned on a dielectric base.

it is another object of this invention to produce greatly improved electrical circuit contacts comprised of silver conductors.

Other objects of the invention will appear hereinafter.

The objects of this invention may be accomplished, in

2,961,416 Patented Nov. 22, 1960 general, by placing into contact with the anodic silver electrode a small quantity of metallic zinc or magnesium. The zinc or magnesium mustbe electrically contacted with the silver anode. When the applied voltage is alternating, making one of the electrical elements cathodic atone instance and anodic at the next, protective metal pieces must contact each electrode. The zinc or magnesiurn may be mechanically attached by means of a foil clamped onto the silver electrode, these metals may be plated onto the silver electrode, the magnes'um may be alloyed with the silver, or, in the case of silver electrodes made from finely divided silver, the magnesium may be incorporated in finely divided form with the finely divided silver and applied to the dielectric to form the silver electrode.

It appears that the magnesium or zinc metal will form magnesium hydroxide or zinc hydroxide preferentially to the formation of silver hydroxide. The hydroxides of magnesium and z'nc, however, are very insoluble and do not tend to ionize and cause migration.

Peculiarly not all metals more electroactive than silver will properly function as agents to prevent silver migration. Alkali metals are, for example, far too active. Antimony, aluminum, chromium, iron, n ckel, beryllium, cadmium, iridium, manganese, tantalum, thallium, selenium, lead, bismuth, titanium, and vanadium have all been tried and failed to function to prevent the mgration of s'lver.

The details of the invention will be more clearly apparent by reference to the following description when taken in connection with the accompanying drawings showing certain illustrative embodiments of the invention, and inwhich Figures 1 and 2 are diagrammatic perspective views of oppositely charged silver electrodes on a dielectric base;

Figures 3 and 4 are similarly diagrammatic perspective views of oppositely charged silver electrodes with means to prevent silver migration.

Referring to the figures, 1 designates a dielectric base material which may consist of a ceramic such as glass, barium titanate, or the like, or it may consist of an organ'c plastic material such as phenol formaldehyde, urea formaldehyde, or the like. Two silver patterns numbered 2 and 3 are positioned on the dielectric 1 and function as oppositely charged electrodes in an electric circuit. 4 discloses the tree-like deposit of silver on the cathode upon moisture contact between the two electrodes.

In Figure 2 the silver deposits have progressed from the cathode until they have bridged the gap between the cathode and the anode at 5. Upon bridging the electrodes, the current will be short-circuited.

Figure 3 again discloses similarly oppositely charged silver electrodes and a small foil 6 of magnesium or zinc positioned in electrical contact with the anode. It is not essential that the foil cover the tip or edge of the silver electrode exactly as shown, but may be positioned back from the edge if convenient. Exposure of a large area of silver between the foil and the cathode is, however, undesirable. As a result of the presence of this magnesium or zinc, the hydroxyl ions of the ionized water will preferentially form magnesium or zinc hydroxide and substantially diminish or prevent the migration of silver.

In Figure 4 the particles of magnesium or Zinc 7 are shown incorporated in the body of the silver electrodes and likewise prevent or at least very substantially diminish the migration of silver from the anode to the cathode.

The following examples are given to illustrate the operation of the invention in accordance with several of the most desirable embodiments.

Example I mediately and in less than 5 seconds bridge the i -inch V gap causing an electric short circuit.

Example II Example 11 is repeated using a small piece of magnesium foil in place of the zinc foil. Upon applying the same electric potential, no migration was observed in over minutes. This represents a 100-fold or greater improvement.

Example IV Example ii is repeated using foils composed of antimony, aluminum, chromium, iron, nickel, beryllium, cad- 1 mium, iridium, manganese, tantalum, thallium, selenium, lead, bismuth, titanium and vanadium. In each case the migration was substantially as rapid as in Example I without the use of any foil.

Example V A silver paste was prepared similarly to that of Example I, but approximately 1 part of finely divided powdered magnesium metal was added for each 9 parts of finely divided silver. The magnesium-containing silver was then printed and tested as in Example I. No migration was observed after more than 10 minutes exposure to the electric potential of 12 volts D.C.

Example VI Silver plated brass contactors were molded into a phenol resin base. A water film was placed between two adjacent contactors and a l2-vo1t DC. potential applied across to adjacent contacts. The silver accumulated on the cathode and bridged the gap between the two contactors in an exceedingly short time. No improvement over the silver migration of Example I was noted.

Example VII Silver plated brass contactors were molded into a phenol resin base similarly to Example VI. However, before molding these contactors into the phenol resin, they were plated with magnesium. This may be done from a pyridine bath or from a molten magnesium chloride bath. It is essential that the magnesium be in electrical contact with the silver and that it be exposed at the phenol resin-air interface. A wrap of thin magnesium foil may be used instead of the electroplate. A drop of water is placed between the two exposed portions of the electrical contactors and a 12-volt DC. potential applied. No migration takes place over a period of 10 to 30 minutes.

Example VIII Finely divided silver in a vitreous enamel binder is printed and fired on barium titanate wafers in accordance with the teaching of the method disclosed in Knox U.S. Patent 2,385,580. Two such printed silver conductors -lYlCh apart are charged with a 12-volt DO. A drop of water placed on the barium titanate wafer between the two silver electrodes evidences silver migration to the point where the gap between the electrodes is bridged with a period of about 5 seconds.

Example IX Silver electrodes are printed and fired on a barium titanate wafer in accordance with the process of Example VIIl except that 10% by weight of magnesium powder is first added to the finely divided silver. The two electrodes, between which exists a 12-volt DC. potential, fail to show any migration with water covering both electrodes. In firing the silver containing the magnesium powder the temperature must be held fairly low and the oxygen present reduced as much as possible to avoid oxidation and removal of the magnesium from the silver.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that this invention is not to be limited to said details except as set forth in the appended claims.

We claim:

1. An electrical conductor composition comprising to by weight of finely divided silver and 5% to 30% by weight of a finely divided metal from the group consisting of magnesium and zinc, said finely divided metals being dispersed together in a vehicle.

2. An electrical conductor composition comprising 70% to 95% by weight of finely divided silver and 5% to 30% by weight of a finely divided metal from the group con sisting of magnesium and zinc, said finely divided metals being dispersed in a thermosetting binder.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. AN ELECTRICAL CONDUCTOR COMPOSITION COMPRISING 70% TO 95% BY WEIGHT OF FINELY DIVIDED SILVER AND 5% TO 30% BY WEIGHT OF A FINELY DIVIDED METAL FROM THE GROUP CONSISTING OF MAGNESIUM AND ZINC, SAID FINELY DIVIDED METALS BEING DISPERSED TOGETHER IN A VEHICLE.

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