Electrical contact composition for a vacuum type circuit interrupter

A vacuum type circuit interrupter contact comprises a principal phase material selected from the group consisting of copper and solid solutions of chromium copper, iron copper and cobalt copper, and a second phase material selected from the group consisting of chromium, iron and cobalt. The second phase material is dispersed into the principal phase material and has a particle diameter in the range of 74 .mu.m to 250 .mu.m. The contact can be formed by powder metallurgy, an infiltration process or a fusion process.

BACKGROUND OF THE INVENTION 
This invention relates to an electrical contact composition for a vacuum 
type circuit interrupter used in a high current circuit with voltages 
above 10 KV. 
The desirable characteristics of an electrical contact for a vacuum type 
circuit interrupter include the following: 
(1) a low welding force, 
(2) the ability to withstand high voltages, 
(3) a large interrupting current capacity, 
(4) a low chopping current, and 
(5) minimal contact erosion. 
In actual practice it is difficult for a contact to meet all of these 
requirements, and consequently some of the more essential requirements or 
characteristics are favored at the sacrifice of the others, in the sense 
of a trade off. 
Conventional contacts are typically made by a fusion process or a powder 
metallurgy process, in both of which various kinds of second phase 
materials are added to copper (Cu), which is the principal phase material. 
The amount of second phase material added may be either greater or less 
than its solid solubility limit in copper, and it may have a higher or 
lower melting point than copper. 
Conventional contacts may be roughly classified into two types depending on 
whether or not an added material will increase the overall contact 
brittleness, bismuth (Bi) being a typical material for increasing the 
brittleness of a copper based contact. Bismuth is only slightly soluble in 
copper, has a lower melting point than copper, and it itself relatively 
brittle. In a copper-bismuth (Cu-Bi) contact, the bismuth tends to 
segregate at the boundaries between the crystals of copper, and 
consequently such a contact has a low tensile strength. Such a contact has 
an excellent (low) welding force characteristic, however, and can thus be 
used in a high current circuit. Tellurium (Te), antimony (Sb) and certain 
other elements are also effective in increasing brittleness, but they are 
not as effective as bismuth for this purpose. 
While contacts containing materials for increasing brittleness can be used 
in high current circuits, as mentioned above, they are mainly used in 
circuits ranging from 3 to 6 KV because of their relatively poor ability 
to withstand high voltages. 
A typical prior art contact which does not contain materials for increasing 
brittleness is made by dispersing chromium (Cr) into a principal phase 
material of Cu (See the copending U.S. Patent Appln. Ser. No. 910,905 
filed on May 26, 1978 by M. Kato) or a Cu-Cr solid solution. Such a Cu-Cr 
contact satisfies most of the above requirements, and may be used in 
circuits with voltages higher than 10 KV. This type of contact exhibits a 
large welding force, however, and consequently it cannot be used in a high 
current circuit. 
Other high voltage contacts include iron (Fe) or cobalt (Co) dispersed into 
a principal phase material of Cu or a Cu-Fe solid solution, or into Cu or 
a Cu-Co solid solution, respectively. These types of contacts also do not 
contain any material for increasing brittleness, however, so they still 
have the defect of a large welding force characteristic. 
Atomic ratios of these solid solutions as the first phase material are 
usually as follows. Atomic ratios of Cr: Cu, Fe: Cu, Co: Cu are less than 
0.8, 4.5 5.5 (w%) respectively. 
SUMMARY OF THE INVENTION 
It is thus an object of this invention to provide a new and improved 
contact for a vacuum type circuit interrupter having an improved welding 
force characteristic, and the ability to accommodate high currents at 
voltages greater than 10 KV. This object is accomplished by providing a 
contact consisting essentially of a principal phase material selected from 
a group consisting of copper and solid solutions of chromium copper, iron 
copper, and cobalt copper into which a second phase material selected from 
a group consisting of chromium, iron, and cobalt is dispersed, wherein the 
particle diameter of said second phase material is in the range of 74 
.mu.m to 250 .mu.m.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The Cu-Cr contact tested in FIG. 1 was made by dispersing chromium into a 
principal phase of copper, with about 30% of the contact volume being 
occupied by chromium particles having a diameter of under 74 .mu.m. FIG. 1 
clearly shows that the tensile strength of a Cu-Cr contact is more than 
twice that of a Cu-Bi contact. 
Because there are no known contacts for practical vacuum type circuit 
interrupters which have a low welding force with an attendant high current 
capacity at voltages above 10 KV, it is difficult to develop an effective 
vaccum type interrupter with a high interrupting capacity. 
Initially, Cu-Bi contacts with their increased brittleness were 
investigated to determine if their ability to withstand voltages above 10 
KV could be improved. Such improvement was found difficult, however, 
because of an unavoidable defect caused by a physical characteristic of 
bismuth. Specifically, since bismuth has a relatively low melting point of 
271.degree. C. and a relatively high vapor pressure on the order of 
10.sup.-2 Torr at 1000.degree. K., it unavoidably segregates at the 
crystal boundaries of copper. The bismuth in a Cu-Bi contact is vaporized 
in large quantities to a state of bismuth atoms or molecules at 
temperatures above 400.degree. C., which are applied in the baking process 
essential to the production of vacuum type circuit interrupters. The 
vaporized bismuth adheres to the surfaces of the insulating vessel of the 
interrupter. Such vaporization and adherence is also caused by heat energy 
generated by the closing, conducting, or interrupting operations of a 
vacuum type circuit breaker, thereby reducing its voltage withstanding 
ability. Thus, as long as bismuth is used as a second phase material for 
contacts, a reduced voltage capacity is unavoidable. 
Taking the above characteristics of Cu-Bi contacts into consideration, the 
present invention provides a contact having an improved welding force 
characteristic capable of handling high currents at voltages above 10 KV 
without using any material for increasing brittleness, such as bismuth. 
The contact of this invention is made by dispersing chromium, iron or 
cobalt particles with selected diameters ranging from 74 .mu.m to 250 
.mu.m into the principal phase material selected from a group consisting 
of copper and solid solutions of chromium copper, iron copper and cobalt 
copper, and may be made by a fusion process or a powder metallurgy 
process. 
The chromium, iron, or cobalt particles must be dispersed into the 
principal phase in great quantities, and a special heat treatment is 
therefore required to improve dispersion, and in the case of iron whose 
density is above the solid solubility limit, to prevent the formation of 
iron dentride. 
To reduce the welding force of these vacuum interrupter contacts, previous 
efforts have concentrated on measuring the welding force of the loaded 
interrupter with the welded contact surfaces stuck together. Consequently, 
there has been a lack of investigation of the minute or small and 
instantaneous metallic composition of the contact base metal. To 
compensate for this deficiency, rupture dynamics principles and techniques 
were applied to the past research aimed at reducing the tensile strength 
of the contact base metal. It is clear from the results obtained that the 
reduction of the tensile strength will result in a reduction of the 
welding force. 
According to rupture dynamics, the contacts may be defined as the 
compositions in which the uncountable number of particles of the hard 
second phase material selected from the granular group consisting of Cr, 
Fe, and Co are dispersed into the soft principal phase material selected 
from the group consisting of Cu-Cr, Cu-Fe and Cu-Co solid solutions and 
Cu. To reduce the tensile strength it is necessary to determine the volume 
ratio or the particle diameter of the second phase material, in addition 
to the other properties of the principal and second phase materials. 
Furthermore, according to a theorem of rupture dynamics, the tensile 
strength decreases with increasing volume ratios or particle diameters of 
the second phase material of the contacts because the stress is produced 
concentrically around the second phase material when the contacts are 
loaded. 
As the result of such analysis, it was recognized that there are two 
conflicting or antagonistic factors concerning the particle diameter of 
the second phase material. Namely, there is the conventional conception 
that the voltage capacity of the contact depends upon the volume ratio or 
the particle diameter of the second phase material, that is, the smaller 
the particle diameter, the higher the voltage capacity, whereas the 
present inventor has found that the larger the particle diameter of the 
second phase material, the lower the tensile strength. 
Thus, there is a definite antagonism between the voltage capacity and the 
tensile strength concerning the particle diameter of the second phase 
material. Generally, according to a theorem of dispersion strengthening, a 
small particle diameter of the second phase material contributes to an 
increased tensile strength. This means that it is necessary to keep the 
particle diameter of the second phase material above a fixed value in 
order to reduce the tensile strength. 
In both the fusion and powder metallurgy processes of the prior art, the 
particle diameter of the second phase material has a considerably wide 
distribution. In this distribution, if the particle diameter of the second 
phase material decreases below a fixed value, the effect of the second 
phase material having a particle diameter close to the lower limit in 
increasing tensile strength and the effect of the second phase material 
having a particle diameter close to the upper limit or an intermediate 
value in decreasing it cancel each other, and the former generally 
overrides the latter. 
In recognition of and in an effort to resolve and overcome this conflict, 
various tests were conducted in an attempt to determine the volume ratio 
or the particle diameter of the second phase material to provide a contact 
having a voltage capacity above 10 KV and yet a low welding force for a 
large current capacity. The results of such tests are shown in FIGS. 2 and 
3. 
As easily seen from FIG. 2, the tensile strength of a Cu-Cr contact 
including Cr as the second phase material surprisingly depends largely on 
the particle diameter of the Cr and only slightly on the volume ratio. 
Further, it is clear from FIG. 2 that the tensile strength of a Cu-Cr 
contact can be decreased to almost the same low level of a Cu-Bi contact 
by proper selection of the particle size range. 
Further, it may be clearly seen from FIG. 3 that as long as the particle 
diameter of the second phase material ranges from 74 .mu.m to 250 .mu.m 
and the volume ratio ranges from 20% to 40%, Cu-Cr contacts can have both 
a low tensile strength and attendantly high current capacity comparable to 
that of Cu-Bi contacts, and the high voltage capacity of conventional 
Cu-Cr contacts. 
Three types of vacuum interrupters having Cu-Cr, Cu-Fe and Cu-Co contacts 
with particle diameters and volume ratios selected from the ranges stated 
above were manufactured, and tests were made of their welding force, 
voltage capacity, interrupting current, chopping current, and contact 
erosion characteristics in circuits above 10 KV. Each interrupter 
satisfied all requirements. The three different types had substantially 
the same welding force, voltage capacity and chopping current 
characteristics, with the Cu-Cr contact being superior in large 
interrupting current and contact erosion characteristics compared to the 
others.