US3288971A

US3288971A - Electrical contact element and subassembly

Info

Publication number: US3288971A
Application number: US464426A
Authority: US
Inventors: William V Polleys
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1965-06-16
Filing date: 1965-06-16
Publication date: 1966-11-29
Anticipated expiration: 1983-11-29

Description

Nov. 29,

W. V. POLLEYS Filed June 16, 1965 PRIOR ART ELECTRICAL CONTACT ELEMENT AND SUBASSEMBLY Inventor,

William V. Pulleys,

by 't qy.

United States Patent 3,288,971 ELECTRICAL CONTACT ELEMENT AND SUBASSEIVIBLY William V. Polleys, Barringtou, R.I., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed June 16, 1965, Ser. No. 464,426 7 Claims. (Cl. 200-166) This application relates to an improved electrical contact element and subassembly.

It is an object of this invention to provide an improved contact element which may be easily brazed to a contact carrying member and yet which retains good electrical contact qualities such as resistance to spattering and welding. More particularly, it is an object to provide an improved method of mounting an electrical contact on a combinations of elements, steps and sequences of steps,

features of construction, and manipulation and arrangement of parts, all of which will be exemplified in the structures hereinafter described and the scope of the application of which will be indicated in the appended claims.

In the accompanying drawings in which one of the various possible embodiments of the invention is illustrated:

FIG. 1 is an end view with portions shown in section of a device according to the prior. art;

FIG. 2 is an end view with portions shown in section of a contact element made according to the instant invention;

FIG. 3 shows a contact element mounted on a contactcarrying member to form a subassembly according to the instant invention.

Similar reference characters indicate corresponding parts throughout the several views of the drawings.

Dimensions of certain of the parts as shown in the accompanying drawings have been modified for the purposes of clarity of illustration.

In the manufacture of electrical contacts, silver is the preferred material due to its excellent electrical conductivity. However, its relatively low melting point requires that it be employed in an alloy form with a refractory material such as, for example, cadmium oxide, tungsten or molybdenum. These refractory materials are retained as discrete particles in a matrix of silver and assist in maintaining the physical integrity of the contact itself under high current and voltage conditions.

The preferred method of mounting such contacts to a contact-carrying member is by brazing. The material most often used for this brazing is silver. As a result, contact elements made of silver-refractory material alloy are made with a backing layer of pure silver, metallurgically bonded thereto, so that the element can be easily brazed to the contact-carrying member. By metallurgical bonding is meant a uniting of the metals at the molecular level. It has been discovered that the use of silver-refractory material alloys increases the difficulty in metallurgically bonding the alloy to the pure silver backing layer. It is believed that one cause of this is the brittleness resulting from the refractory material. This brittleness can cause cracking which inhibits the formation of a metallurgical bond. Another cause is believed to be the con- Patented Nov. 29, 1966 centration of refractory material particles along the bond line thereby inhibiting the formation of an adequate metallurgical bond. For example, in the case of cadmium oxide refractory materials it has been found that when certain silver cadmium oxide alloys are metallurgically bonded to a pure silver backing layer a poor bond results. It is believed that this occurs due to a concentration of cadmium oxide particles along the bond line between the silver and the silver cadmium oxide.

It has been further discovered that the bonding difliculties occur as'the percent by weight of refractory 'material in the silver-refractory material alloy increases. It has been discovered that with certain lower percentages of refractory material, an adequate bond can be achieved either with the pure silver or with the alloy containing the higher refractory material percentage. The solution discovered by applicant is to form the contact element by metallurgically bonding a first layer of the silver-refractory material with a percentage by weight of refractory material desired for contact quality, an intermediate layer formed of a silver-refractory material alloy having a lesser percentage by Weight of refractory materialand a third layer of substantially pure silver on the other side of the intermediate layer from the first layer.

Referring now to the drawings, particularly FIG. '1, there is shown an electrical contact element generally indicated by reference numeral 10. The contact element 10 shown in FIG. 1 is made according to the prior art and includes a contact facing element 12 formed of a layer of a suitable silver-refractory material alloy. As noted, one example of such a refractory material alloy is cadmium oxide which has been oxidized by internal oxidization of the type described in United States Patent No. 2,932,595 in the name of Heinz R. Pfiumm and assigned to the assignee of the instant invention. The alloy forming the member 12 may also include other additives such as cobalt or tin, etc., or other materials as described in the aforesaid US. patent. Other examples-of refractory materials are molybdenum or tungsten. Contact 10 also includes a backing layer 14 formed of a material suitable for brazing such as substantially pure silver. The layer 14 is employed in brazing the contact element 10 to a contact-carrying member such as member 18 shown in FIG. 3.

As noted, the problem encountered with the device shown in FIG. 1 is that as the percentage by weight of refractory material in the alloy forming layer 12 is increased to increase the current and voltage capabilities of contact 10, the bonding of layer 12 to layer 14 becomes more diflicult to the point where an adequate bond cannot be formed.

In the silver-cadmium oxide alloy example, bonding difficulty has sometimes been encountered in the range of from about 5% to about 20% by weight of cadmium oxide.

There is shown in FIG. 2 an improved electrical contact element 20 formed according to the instant invention. Contact 20 includes a contact facing first layer 12 formed of a silver-refractory material alloy such as the above-noted silver cadmium oxide or a silver tungsten or silver molybdenum alloy mentioned by way of example in the discussion of FIG. 1. Electrical contact 20 also includes a backing layer 14 formed of braze material such as substantially pure silver. In the silver cadmium oxide example, the'cadmium oxide can form from approximately 5% to approximately 20% by weight of the alloy forming layer 12. Below 5% by weightof cadmium oxide in the silver cadmium oxide alloy, it' has been found that the silver cadmium oxide alloy can be bonded directly to the substantially pure silver backing layer adequately without the occurrence of the bonding problems noted above.

To preclude the inadequate bonding referred to above, electrical contact element 20 also includes an interliner or intermediate second layer 16 between the silver-refractory material alloy layer 12 and the pure silver braze material layer 14. This interliner is also formed of a silver-refractory material alloy but the percentage by weight of the refractory material in the alloy is lower than that employed in layer 12. It has been discovered that it is possible to accomplish an adequate bond between a silver-refractory material alloy and a silverrefractory material alloy in which the refractory material constitutes a lesser percentage by weight and that an adequate bond may also be accomplished between the silver-refractory material alloy of lesser refractory rnaterial and pure silver where a bond between the first silver-refractory material alloy and pure silver or braze material could not be accomplished. By way of example, in the case where the refractory material in layer 12 is cadmium oxide in the range between 5% and 20% by weight of the silver cadmium oxide alloy, the cadmium oxide in layer 16 is within the range between less than 5% and about by weight of the alloy. It has been found that the optimum percentage thickness for the layer 16 is between 5% and 20% of the total thickness of element 20.

As an example of a contact element made according to the instant invention, a layer 12 was formed of silver and cadmium oxide in which the cadmium oxide was 15% by weight of the silver cadmium oxide alloy and a layer 16 was formed of a silver cadmium oxide alloy in which the cadmium oxide was 7.5% by weight of the alloy. The layer 14 of fine silver constituted 11.5% of the total thickness of the contact element 20, layer 16 formed 8.5% of the total thickness and layer 12 formed the balance of the total thickness.

It has also been found that layer 16 performs its function best if the refractory material is one-half that percentage by Weight of the refractory metal material in the alloy forming layer 12. However, depending on the percentages of the refractory material employed, other ratios will also perform adequately.

It has been found that following the formation of contact element it may be brazed to contact-carrying element 18 which is formed of any suitable material capable of being brazed to, as shown in FIG. 3. The improved subassembly 30 is thereby formed. It will be understood that as the refractory material employed in layer 12 is varied, the percentage by weight and thicknesses of layer 16 will vary. However, in all cases layer 16 includes a percentage by weight of refractory material which is less than that of layer 12.

It will be seen that the

articles

20 and 30 may be formed by selecting the layer 12, layer 14 and layer 16 as described above, metallurgically bonding the layers to each other and then brazing the element 20 to member 18 to form the subassembly 30.

It will be understood that the quality of the metallurgical bond in the range between about 5% and 10% by weight of cadmium oxide may or may not be adequate depending on the application to which the contact is exposed. Thus, under certain circumstances, a 5% to 10% weight may require an intermediate layer of lesser per cent while under less severe applications, such a range may result in a bond of adequate quality.

In addition, it will be understood that, depending on the degree of quality of bond desired, and on the refractory material employed, more than one intermediate layer containing decreasing percentages of refractory material may be employed.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous resu ts are btained.

As many changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

It is to be understood that the invention is noted limited in its application to the details of construction and arrangement of elements illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practised or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

I claim:

1. An electrical contact element comprising:

(a) a first layer formed of a first alloy of a refractory material and silver, the refractory material constituting a predetermined percent by weight of the first alloy;

(b) a second layer, metallurgically bonded on one side to the first layer, formed of a second alloy comprising the refractory material and silver, the refractory material constituting a second predetermined percent by weight of the second alloy which second predetermined percent is less than the first predetermined percent; and

(c) a third layer, metallurgically bonded to the other side of the second layer, and formed of substantially pure silver.

2. An electrical contact element as set forth in claim 1 wherein the thickness of the second layer is in the range of from 5% to 20% of the total thickness of the contact element.

3. An electrical contact element as set forth in claim 1 wherein the refractory material is cadmium oxide.

4. An electrical contact element as set forth in claim 3 wherein the first predetermined percent is in the range between 5% and 20%.

5. An electrical contact element as set forth in claim 1 wherein the refractory material is tungsten.

6. An electrical contact element as set forth in claim 1 wherein the refractory material is molybdenum.

7. An electrical contact-carrying subassembly comprising, in combination:

(a) an electrical contact-carrying member;

(b) an electrical contact element brazed to one side of the electrical contact-carrying member, the electrical contact element comprising (i) a first layer formed of a first alloy of a refractory material and silver, the refractory material constituting a predetermined percent by weight of the first alloy;

(ii) a second layer, metallurgically bonded on one side to the first layer, formed of a second alloy comprising the refractory material and silver, the refractory material constituting a second predetermined percent by weight of the second alloy, which second predetermined percent is less than the first predetermined percnt; and

(iii) a third layer metallurgically bonded to the other side of the second layer and formed of substantially pure silver.

No references cited.

ROBERT K. SCHAEFER, Primary Examiner,

Claims

1. AN ELECTRICAL CONTACT ELEMENT COMPRISING: (A) A FIRST LAYER FORMED OF A FIRST ALLOY OF A REFRACTORY MATERIAL AND SILVER, THE REFRACTORY MATERIAL CONSTITUTING A PREDETERMINED PERCENT BY WEIGHT OF THE FIRST ALLOY; (B) A SECOND LAYER, METALLURGICALLY BONDED ON ONE SIDE TO THE FIRST LAYER, FORMED OF A SECOND ALLOY COMPRISING THE REFRACTORY MATERIAL AND SILVER, THE REFRACTORY MATERIAL CONSTITUTING A SECOND PREDETERMINED PERCENT BY WEIGHT OF THE SECOND ALLOY WHICH SECOND PREDETERMINED PERCENT IS LESS THAN THE FIRST PREDETERMINED PERCENT; AND (C) A THIRD LAYER, METALLURGICALLY BONDED TO THE OTHER SIDE OF THE SECOND LAYER, AND FORMED OF SUBSTANTIALLY PURE SILVER.