US3893820A

US3893820A - Cu-{8 Ag{9 -CdO electric contact materials

Info

Publication number: US3893820A
Application number: US300629A
Authority: US
Inventors: Terrence Ardern Davies; Peter Douglas; David John Pedder
Original assignee: Square D Co
Current assignee: Schneider Electric USA Inc
Priority date: 1971-10-27
Filing date: 1972-10-25
Publication date: 1975-07-08
Anticipated expiration: 1992-07-08
Also published as: GB1376626A

Abstract

Electrical contact materials which include copper and 2.5 to 20 weight percent of cadmium oxide and silver, 0.1 to 97.4 weight percent of copper and 2.5 to 20 weight percent of cadmium oxide, respectively, are provided, together with methods of fabricating the electrical contacts.

Description

United States Patent Davies et al.

[75] inventors: Terrence Ardern Davies; Peter Douglas; David John Pedder, all of Northampton, England 1731 Assignee: Square D Company, Park Ridge, 111.

122] Filed: Oct. 25, 1972 1211 .Appl. No.: 300,629

130] Foreign Application Priority Data Oct. 27. 1971 United Kingdom 49876/71 152] 11.5. C1. 29/1825; 75/206; 252/214; 252/218 151] Int. Cl. C22c 1/05 158 Field of Search 29/1825; 75/206; 252/214, 252/218 1561 References Cited UNITED STATES PATENTS 1,195,307 3/1940 Hensel et a1. 29/1825 X 2,394,501 2/1946 Weiller 29/1825 X [4 1 July 8,1975

FOREIGN PATENTS OR APPLICATIONS 465,142 5/1950 Canada Primary Examiner-Benjamin R. Padgett Assistant ExaminerR. E. Schafer Attorney, Agent, or Firm-Harold .1. Rathbun; James S. Pristalski [57] ABSTRACT Electrical contact materials which include copper and 2.5 to 20 weight percent of cadmium oxide and silver, 0.1 to 97.4 weight percent of copper and 2.5 to 20 weight percent of cadmium oxide, respectively, are provided, together with methods of fabricating the electrical contacts.

22 Claims, No Drawings lIL'-{ AG l-CDO ELECTRIC CONTACT MATERIALS lhlS II'HCflIlOfl relates to materials for use in electrical contacts and to methods of fabrication of the electrical contacts.

it is an object of the invention to provide new and useful contact materials for use in electrical contacts which demonstrate desirable resistance to welding and arc-extinguishing characteristics. which have a rela tively low contact resistance and which are relatively tnexpensive and simple to produce.

Other objects, features and advantages of the present invention will become apparent from the following description of particular embodiments thereof.

One electrical contact material provided by the present invention is especially adapted for medium-toheavy duty applications and comprises a mixture of' copper and 2.5 to weight (wt) percent of cadmium oxide evenly dispersed throughout the copper. The evenly dispersed cadmium oxide effects embrittlement of welds which may form after contact has been established between such electrical contacts. In addition, when the contact between electrical contacts has been broken. cadmium vapor is present in the arc between the contacts and the vapor reduces the mean electron energies in the arc and changes the distribution of electron energies in the arc in a manner to eliminate high energy electrons and thereby to assist in extinguishing the arcv A second electrical contact material produced in accordance with the invention includes a mixture of silver. 0.1 to 97.4 wt. percent of copper and 2.5 to 20 wt. percent of cadmium oxide evenly dispersed throughout the silver and copper. The cadmium oxide effects emhrittlement of any welds which form after contact has been established and. as with the copper-cadmium oxide contacts, cadmium vapor assists in extinguishing arcs during breaking of the contacts.

The first electrical contact material is produced by mixing fine, irregular copper powder and ultra fine cad- :mium oxide powder intimately together with the cadmium oxide content of the mixture being between approximately 2.5 to 20 wt. percent. Both powders preferably should be as fine as economically possible. Clean. fine. dendritic copper powder is preferable and the size and shape of the powder particles are extremely important in the production of optimum copper-cadmium oxide materials. Therefore, irregular copper powder with a powder particle size not greater than .350 British Standard mesh and cadmium oxide powder, having a cubic morphology. for example, and a powder particle size not greater than 1 micron, is utilized. By employing fine powders in the mixture. a fine, even dispersion of cadmium oxide is obtained in the electrical contacts that are produced.

A commercially available copper powder which is employed is of dendritic morphology with 99.8% of the powder particles having a size which is less than 300 British Standard mesh. This powder was found through chemical analysis to contain the following:

Copper (Cu) 99.0% min. Lead (Pb) O.207 max. Oxygen (0:) 0.20% max.

The minute amounts of Pb and O in the powder were found to have no significant effects on the performance of the electrical contacts produced from this copper powder.

This copper powder has an apparent density in the range of from 1.8 to 2.1 gms/cc, a geometric linear intercept by transmission microscopy of 8.4 microns, standard deviation 2.1, and a geometric mean linear intercept on a polished section of 4.6 microns, standard deviation 1.8.

The commercially available copper powder was sieved before being mixed in order to remove powder particles which have a size greater than 350 British Standard mesh.

A commercially available cadmium oxide powder, which is produced by burning cadmium and condensing the smoke caused by the burning. was also utilized in the process. The cadmium oxide powder is of cubic morphology and the powder particle size is less than 1 micron. In chemical analysis the cadmium oxide powder was found to contain the following:

The minute amounts of Cl, S0 Fe, K and Na present in the cadmium oxide powder were found to have no significant effect on the performance of the electricalcontacts produced from this cadmium oxide powder.

It is important to insure that these commercially available powders be kept clean and dry during storage in order to prevent moisture absorption and surface oxidation (especially in the case of the copper powder). The use of dessicators during storage of the powders was found to keep them sufficiently clean and dry.

In the fabrication of the copper-cadmium oxide contact materials, the copper and cadmium oxide powders are intimately mixed in a dry tumble milling process. In order for the dry tumble milling to be effective, it was found to be important that the following conditions be adhered to:

a. The constituent powders should have a particle size substantially as previously specified;

b. The powders should be dessicator-stored, or stored with some alternative means for preventing absorption of moisture and surface oxidation;

c. The volume of the powders to be mixed should be I a minimum of approximately 50 grams (the actual mix);

d. The cadmium oxide content should be in the range of between approximately 2.5 and 20 wt. percent;

e. The volume of the mixing drum should be of the order of 2 to 10 times the volume of the powder being mixed in order to prevent restriction of the movement of the powder during mixing;

f. The relative humidity inside the drum should be in the range of 0 to and the inside drum temperature should be in the range of between l0 and 30 C. It was'found that higher inside drum temperatures tended to result in surface oxidation of the copper 'during mixing;

g. The speed of rotation of the drum should be such that the powders are continually in motion during mixing. It was found that increase in the speed of rotation of the drum decreased the time required for the desired mixing and the tendency of the oxide to aggregate. However, if the speed of rotation is excessive, then centrifugal forces tended to hinder the mixing; and

h. The duration of mixing of the powder is generally dependent upon the cadmium oxide content; the higher the cadmium oxide content, the longer was the mixing time required. For example, with a cadmium oxide content of 2.5 wt. percent. the mixing period should be of the order of 12 hours, whereas with a cadmium oxide content of the order of to 15 wt. percent, the mixing period should be of the order of 24 hours. When the mixing times were substantially shorter than those specified in the preceding sentences, especially when the powders had high moisture contents, incomplete breaking down of aggregates in the oxide powder resulted. On the other hand, when the period of continuous mixing was excessive, especially under moist conditions, (for example, for in excess of 100 hours) a growth of cadmium oxide aggregates can result and tend to de-mix the powder.

The most significant factor in obtaining fine uniform dispersion of the cadmium oxide throughout the contact material is the absolute and relative sizes of the constituent powder particles and the other parameters in the mixing were determined largely by these absolute and relative sizes.

Typical mixing conditions for the aforementioned dessicator-stored commercially available powders in mixing volumes in the range of 50 to 1,000 grams and having a cadmium oxide powder content in the range of between 2.5 and 15 wt. percent are as follows:

a. The drum volume was of the order of 2 to 10 times that of the powder being mixed;

b. The inside drum temperature was approximately C. (ranging between 17 and 23 C.) with a relative humidity of approximately 60% (ranging between 45 and 65%);

C. The drum was revolved at I60 revolutions per minute; and

d. The mixing time was 24 hours.

These mixing conditions were found, for the particular powders utilized, to give a uniformly mixed composite powder. When the moisture content of the cadmium oxide powder was excessive and, therefore, the cadmium oxide powder had an increased tendency to aggregate, it was found desirable to sieve the powder through a 350 British Standard mesh sieve after the 24 hour tumble milling operation and then to re-tumble the sieved powder mixture for an additional 24 hour period.

The powder mixture was then compacted into the desired shape (that of the electrical contact being produced) in molds and the compact then sintered by being heated in an inert (for example, dry argon) atmosphere for approximately 2 hours.

In the sintering of copper-cadmium oxide contacts the upper temperature limit is 767 C. (the boiling point of cadmium). In order to insure that very little cadmium oxide is lost during sintering and to preclude the need for protective atmosphere packing, the temperature should be below the boiling point of cadmium, for example, a temperature of the order of 700 C.

In the fabrication of the silver-copper-cadmium oxide contact materials, a commercially available fine irregular silver powder is initimately mixed together with fine irregular copper powder and ultra fine cadmium oxide powder to provide a fine evenly dispersed mixture containing 0.] to 97.4 wt. percent of copper, 2.5 to 20 wt. percent of cadmium oxide and the remainder silver. The copper and cadmium oxide powders utilized may be the commercially available types previously discussed and the silver powder is a precipitated powder of commercial purity. This commercially available silver powder is of irregular morphology and has a particle size of less than 300 British Standard mesh and an apparent density of L9 gms/cc. The silver powder also has a geometric mean linear intercept by transmission microscopy of 17.8 microns. standard deviation 2.0, and a geometric mean linear intercept on a polished section of 4. 1 microns, standard deviation 2.0. The silver powder should be kept clean and dry (for example, by using dessicators) in storage and is sieved before being used in order to remove powder particles which are of a size greater than 350 British Standard mesh.

In the dry tumble milling of the silver, copper and cadmium oxide powders, the mixing conditions previously specified with respect to the mixing of copper and cadmium oxide powders should be adhered to. The powder mixture is compacted into the desired shape in molds and the contact compacts are then sintered by being heated in an inert atmosphere for a period of time of the order of 2 hours.

In order to determine the preferred sintering conditions for silver-copper-cadmium oxide contacts, experi ments were carried out on the binary system silver 5 to wt. percent copper. It was found that for concentrations and temperatures where there is complete solid solubility of one of these elements in the other, a sintering temperature of up to 900 C. may be employed. In the temperature range of between 779 C. (the eutectic temperature) and 900 C., liquid phases are produced initially around the silver and copper particles in the compacted powder mixture and these phases promote the sintering process. For concentrations where a two-phase state of equilibrium exists (either a liquid-solid or a solid-solid state) the sintering temperature may be below the eutectic temperature of 779 C. At a lower temperature, sintering occurs by solid state diffusion only and the sintered structure often is inhomogeneous due to insufficiency of time for complete diffusion to take place to attain equilibrium. Under these conditions, liquid phase sintering can be effected at a temperature in excess of 779 C. provided that the amount of liquid phase which results is at an optimum level (which level is determined by particle size, the actual sintering temperature and the composition range).

After the sintering operation for either the coppercadmium oxide contact materials or the silver-coppercadmium oxide contact materials, the density of the materials may be increased, if desired, by stamping or coining the contacts at a pressure of up to approximately 45 tons per square inch.

In the compacting operation of the copper-cadmium oxide powder mixture and of the silver-coppercadmium oxide powder mixture which was to be solid phase sintered, it was found that for a disc in the order of 10 mm. diameter and l mm. thick and which has a cadmium oxide concentration of either 10 or 15 wt. percent that:

a. Green forming pressure in excess of 45 tons per square inch resulted in a very slight expansion of the compact after sintering,

b. Green forming pressures of 40 and 45 tons per square inch engage virtually no change in compact dimensions after sintering, and

c. Green forming pressures below 40 tons per square inch resulted in a decrease in compact dimensions after sintering and the amount of the decrease increased as the forming pressures decreased.

Since it is economically advantageous to be able to use only one standard sized mold for both the compacting and coining operations, it is desirable to use a compacting pressure which results in no change or a slightly negative change in compact dimensions after the sintering operation. It was found, therefore, that a Compacting pressure of the order of 30 to 40 tons per square inch was preferable and the specific pressure which was employed was determined empirically for each sample size and shape which was to be produced.

in general. it was found that for contacts which were of constant thickness below about 4 mm., with a thickness to diameter ratio of less than 5 to l, a compacting pressure of 40 tons per square inch was preferable, at least in part because such compacts have a reasonably uniform green density throughout. When the compacts were more complex or had thicker sections with a high length to diameter ratio, or a very low cadmium oxide content. lower pressures, typically of the order to 30 tons per square inch were preferable.

Specific examples of copper-cadmium oxide and silver-copper-cadmium oxide contacts which were compacted at 30 tons per square inch are as follows:

a. Nema Series Size 3 electrical contact tips were fabmated with and l5 wt. percent cadmium oxide content and were sized approximately 0.6 X 0.5 X 0.1 in. thick with one radiused face. A 30 tons per square inch compacting pressure was employed in this case because the compaction ratio is effectively higher near the end of the radiused face, resulting in higher green densities near the ends. Were a greater pressure (for example, of the order of 40 tons per square inch) employed, then the compact would have become waisted after sintering and the ends of the contact tips would have expanded and thereby would have prevented coining of the tips in the original compact mold.

b. Rectangular blocks having 10 and [5 wt. percent cadmium oxide contents and dimensions of 1.4 X 2.8 X 0.8 cms were fabricated. Although no variations in compacting ratio occur across such a simple contact. a compacting pressure of the order of 40 tons per square inch was found to be excessive since cracking ofthe green compact occurred upon ejection ofthe compact from the compacting mold.

The cracking was caused by density gradients across the thickness of the compact as a result of die friction and the relief of the elastic stresses within the compact upon ejection which caused significant stress levels along the edges of the green compact.

. 2 mm. thick X 9 mm. diameter discs having a 2.5 wt. percent cadmium oxide content were fabricated. The cadmium oxide content was insufficient to prevent the formation of closed pockets of gas during the green pressing operation. If pressures substantially higher than 30 tons per square inch are employed, bloating and blistering will occur when the compacts are sintered.

It will be appreciated from the foregoing examples that the preferred compacting pressure for a given compact is dependent upon many factors and can best be arrived at empirically.

A specific example of the method of production of the electrical contact materials follows:

Copper-cadmium oxide contacts which are /2-in.

wide X A-in. long /s-in. thick and which contained l0 and 15 wt. percent cadmium oxide were fabricated. After the dry tumble milling operation, the mixed powder was compacted at a pressure of 30 tons per square inch, the powder compacts were sintered at a temperature 700 C. in a dry atmosphere for a period of time of the order of 2 hours, and the sintered compacts were stamped at a pressure of 45 tons per square inch. A dry argon atmosphere was provided by passing argon through an ice-trap and a U-tube containing copper turnings which are maintained at a temperature of 500 C.

The temperature and pressure schedule employed in fabricating the copper-cadmium oxide contacts produces such contacts which have a density of at least 98% of the theoretical maximum density. It was found that this schedule was preferable for the powder particle sizes specified and for contacts having a cadmium oxide content of either 10 wt. percent or 15 wt. percent. It will be readily appreciated that the temperatures and pressures specified above (and in particular, the compacting and stamping pressures) may vary if powders of different morphology or of different size are employed. g

The copper 10 wt. percent cadmium oxide and copper 15 wt. percent cadmium oxide electrical contact materials fabricated in accordance with the invention both exhibit superior resistance to. welding properties than sintered copper, commercially available fine silver, and some sintered silver-cadmium oxide contact materials. The copper-cadmium oxide,

contact materials also exhibit superior arc erosion characteristics than those of pure copper contacts.

vention follow:

a. Shallow cylindrical billets containing copper wt. percent silver 10 wt. percent cadmium oxide were fabricated. The mixed powder was compacted at a pressure of 30 tons per square inch, the powder compacts were subjected to a solid-phase sinter in a dry argon atmosphere for a period of 2 hours at a temperature of 700 C., and the sintered compacts were stamped at a pressure of 45 tons per square inch. An alternative method of producing such shallow cylindrical billets was also employed. The mixed powder was compacted at a pressure of 5 tons per square inch, the powder compacts were subjected to a pre-sinter heat treatment for a period of 2 hours at a temperature of 600 C. and then heated to a temperature of 800 C. in a dry argon atmosphere and liquid-phase sintered for a period of 2 hours, and the sintered compacts were stamped at a pressure of 45 tons per square inch. Either of the methods described in this paragraph may also be employed, without any process modifications, to produce shallow cylindrical billets containing copper 17 wt. percent silver wt. percent cadmium oxide.

b. Shallow cylindrical billets containing copper 81 wt. percent silver 10 wt. percent cadmium oxide were fabricated. The mixed powder was compacted at a pressure of 20 tons per square inch. the powder compacts were subjected to a solid-state sinter in an atmosphere of dry argon for a period of 2 hours at a temperature of 700 C. and the sintered compacts were stamped at a pressure of 45 tons per square inch. Alternatively. the mixed pow' der may be compacted at a pressure of 5 tons per square inch and the powder compact subsequently subjected to pre-sinter heat treatments for a period of 2 hours at a temperature of 600 C., then heated to a temperature of 800 C. in a dry argon atmosphere and finally liquid-phase sintered for a period of 2 hours. Either of the methods outlined in this paragraph may be employed, without any process modifications, to produce shallow cylindrical billets containing copper 76.5 wt. percent silver 15 wt. percent cadmium oxide.

It should be noted that in the pre-sintering and in the liquid-phase sintering steps of the fabrication of silver-copper-cadmium oxide contact materials, it is important to insure that the powder is completely degassed before the onset consolidation. Therefore, a heating rate of 200 C. per hour was employed.

The silver-copper-cadmium oxide contact materials fabricated in accordance with the present invention have characteristics that are comparable to those of known silver-cadmium oxide contact materials and are considerably less expensive than silver-cadmium oxide materials because of the substitution of copper for some of the silver.

While the invention has been described with reference to specific embodiments thereof, it will be understood by those skilled in the art that various modifications in form and detail may be made therein without departing from the spirit and scope of the appended claims.

We claim:

1. A sintered composite material which consists essentially of copper and between approximately 2.5 percent and 20.0 percent by weight cadmium oxide.

2. An electrical contact formed of the sintered composite material set forth in claim 1.

3. A sintered composite material as claimed in claim 1 which consists essentially of approximately 10.0 percent by weight cadmium oxide. the balance of copper.

4. An electrical contact formed of the sintered composite material set forth in claim 3.

5. A sintered composite material as claimed in claim 1 which consists essentially of approximately 15.0 percent by weight cadmium oxide, the balance copper.

6. An electrical contact formed of the sintered composite material set forth in claim 5.

7. A sintered composite material as claimed in claim 1 which consists essentially of approximately 2.5 percent by weight cadmium oxide, the balance copper.

8. An electrical contact formed of the sintered composite material set forth in claim 7.

9. A sintered composite material consisting essentially of copper. cadmium oxide. and silver wherein the copper content by weight is between approximately 0.1 and 97.4 percent, the cadmium oxide content by weight is between approximately 2.5 percent and 20.0 percent, the balance of the sintered composite material consisting of silver.

10. An electrical contact formed of the sintered composite material set forth in claim 9.

11. A sintered composite material as claimed in claim 9 wherein the cadmium oxide content by weight is 10.0 percent.

12. An electrical contact formed of the sintered composite material set forth in claim 11.

13. A sintered composite material as claimed in claim 11 wherein the silver content by weight is 18.0 percent.

14. An electrical contact formed of the sintered composite material set forth in claim 13.

15. A sintered composite material as claimed in claim 11 wherein the silver content by weight is 81.0 percent.

16. An electrical contact formed of the sintered composite material set forth in claim 15.

17. A sintered composite material as claimed in claim 9 wherein the cadmium oxide content by weight is 15.0 percent.

18. An electrical contact formed of the sintered composite material set forth in claim 17.

19. A sintered composite material as claimed in claim 17 wherein the silver content by weight is 17.0 percent.

20. An electrical contact formed of the sintered composite material set forth in claim 19.

21. A sintered composite material as claimed in claim 17 wherein the silver content by weight is 76.5 percent.

22. An electrical contact formed of the sintered composite material set forth in claim 21.

Claims

3. A sintered composite material as claimed in claim 1 which consists essentially of approximately 10.0 percent by weight cadmium oxide, the balance of copper.

9. A sintered composite material consisting essentially of copper, cadmium oxide, and silver wherein the copper content by weight is between approximately 0.1 and 97.4 percent, the cadmium oxide content by weight is between approximately 2.5 percent and 20.0 percent, the balance of the sintered composite material consisting of silver.