US3776719A - Method of preparing copper for use in the arcing electrodes of a vacuum circuit interrupter - Google Patents

Abstract

A method of preparing copper for use in the electrodes of a vacuum-type circuit interrupter comprising the steps of: melting copper in a vacuum; adding to the molten copper a small amount of magnesium that reacts with any oxygen present in the copper to form magnesia, which floats to the surface of the liquid coppermagnesium solution; maintaining while in a vacuum the temperature of the solution above its melting point for a sufficient time to evaporate from the solution all of the magnesium present therein that has not reacted with the oxygen therein; and then directionally cooling and solidifying said copper to force residual magnesia trapped therein to move to the surface of the liquid copper.

Description

USE IN THE ARCING ELECTRODES OF A VACUUM CIRCUIT INTERRUPTER United States Patent 1 [111 3,776,719 Foldes Dec. 4, 1973 METHOD OF PREPARING COPPER FOR FOREIGN PATENTS OR APPLICATIONS Primary Examiner-W. W. Stallard Attorney-J. Wesley l-Iaubner et al.

[57] ABSTRACT A method of preparing copper for use in the electrodes of a vacuum-type circuit interrupter comprising the steps of: melting copper in a vacuum; adding to the molten copper a small amount of magnesium that reacts with any oxygen present in the copper to form magnesia, which floats to the surface of the liquid copper-magnesium solution; maintaining while in a vacuum the temperature of the solution above its melting point for a sufficient time to evaporate from the solution all of the magnesium present therein that has not reacted with the oxygen therein; and then directionally cooling and solidifying said copper to force residual magnesia trapped therein to move to the surface of the liquid copper.

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ATTO/M [Y BACKGROUND This invention relates to a method of preparing copper for use in the arcing electrodes of a vacuum-type circuit interrupter.

The electrodes of a vacuum-type circuit interrupter must have a high degree of freedom from gases and constituents which, upon arcing, are capable of releasing gases. The gas which is the most difficult to remove is oxygen especially if it is present as a compound.

One method for achieving the required high gas freedom involves repetitively zone refining the electrode metal, as disclosed and claimed in U.S. Pat. No. 3,234,35 l-I-lebb, assigned to the assignee of the present invention. While quite effective in removing gases, this process is subject to the disadvantage that it is relatively expensive and time consuming.

For overcoming this disadvantage, it has been proposed to prepare the electrode metal by adding thereto, while in the molten stage, small quantities of zirconium, or titanium, or beryllium. These additives have a high affinity for oxygen and react with the oxygen in the electrode metal to form highly stable refractory oxides. These oxides float to the surface of the molten metal and, after the metal cools, can be removed by suitable means, such as etching or machining. Electrodes made by these processes are disclosed and claimed in U.S. Pat. Nos. 3,450,928-Cobine and 3,497,755-Horn, assigned to the assignee of the present invention.

In the aforesaid Cobine and Horn patents, the amount of deoxidizing additive used is made relatively high to insure that all of the oxygen present in the base metal is reacted with. This invariably results in an excess of additive, and this excess forms an alloy with the base metal. Such alloying reduces the conductivity of the base metal and may be otherwise undesirable for certain applications.

SUMMARY An object of my invention is to achieve the required purity and freedom from oxygen by using a deoxidizing additive, but without leaving behind any residual amount of the additive for alloying with the copper base metal.

Another object is to achieve the result of the immediately preceding paragraph within an exceptionally short processing time and with a near-minimum evaporation of copper base metal.

In carrying out my invention in one form, I employ copper as the base metal and subject this copper to a vacuum melting operation. To the molten copper I add a small amount of magnesium sufficient in quantity to react with all the oxygen present in the copper, to' form magnesia which floats to the surface of the coppermagnesium solution. Thereafter, while in a vacuum at a pressure of to 10' torr, the copper-magnesium solution is maintained at a temperature level above its melting point, which temperature is far above the boiling point of the magnesium, for a sufficiently long period to evaporate all of the magnesium that has not reacted with the oxygen present in the copper. This is followed by a directional cooling operation which forces any residual magnesia trapped'in the copper to the surface of the copper.

BRIEF DESCRIPTION OF DRAWING For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawing wherein:

FIG. 1 illustrates apparatus used for practicing a preferred form of the invention.

' FIG. 2 is a graph showing the vapor pressure v. temperature characteristics of certain metals.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown a vacuum chamber 10 containing a graphite crucible 12 that is suitably supported in a position above and vertically spaced from the base plate. The base plate is provided with a suitable fluid passage 16 for circulating a coolant therethrough when it is desired to cool the base plate.

For heating the metallic charge within crucible 12, an induction heating coil 15 is provided about the crucible. When this coil is suitably energized by alternating current, the resulting alternating magnetic field acts in a conventional manner to'induce eddy currents within the charge that produce heating thereof. After the charge has melted in response to such heating, the alternating magnetic field imparts a stirring action to the molten liquid in the crucible.

In practicing the invention in one form, the crucible is first loaded with copper of a moderately high purity, such as electrolytic copper or preferably OFHC copper. The latter type of copperv is obtainable from American Metal Climax Inc., New York, N.Y. and has been analyzed as including approximately 99.995 percent copper generally possessing less than 2 parts per million of oxygen. The coil 15 is energized to heat the copper and convert it into the molten state, in which state it is depicted in the drawing at 20. The temperature of the copper is raised by this heating operation to around l,l50 C. Before and during the heating operation, the chamber 10 is evacuated and maintained evacuated to a high degree in order to prevent contamination of the hot copper by the atmosphere of the chamber 10.

' When the copper is molten and at a temperature of about l,l50 C, a small quantity of magnesium is added to the molten copper. This magnesium is preferably contained within a small quantity of highly refined master alloy of copper and magnesium. This small quantity of master alloy, in the form of a solid mass, is suitably plunged into the molten copper, following which it quickly melts, releasing its magnesium content into the copper 20. The stirring action of the magnetic field from coil 15 distributes the magnesium throughout the copper.

As the magnesium is thus distributed, it reacts with the oxygen in the copper to form magnesia, MgO, a highly stable oxide. The magnesia is much lighter in weight than the copper, and most of it therefore rises to the top of the molten copper and floats thereon although the magnetic stirring will cause some mixing action.

After the magnesium is thus added, the temperature of the charge 20 is held at about l,l50 C or slightly higher. Since this temperature is substantially above the boiling point of magnesium, i.e.', about 287 C at a pressure of 10" torr, the unreacted magnesium that is left in the melt is boiled or evaporated off into the surrounding vacuum. The temperature of the melt is maintained at this level for a sufficient length of time to boil off all of the unreacted magnesium. Only magnesia, MgO, is left behind in the copper. Since this boiling takes place under a high vacuum in chamber 10, i.e., about to 10 torr, the magnesium evaporation is essentially complete and essentially no unreacted magnesium is left behind. After all the unreacted magnesium has thus been boiled off, the molten copper in crucible 12 is directionally solidified from its bottom up. This is accomplished by reducing the energy input into the coil 15, lowering crucible 12 onto the base plate 14, and by supplying coolant to passage 16 to cool the base plate 14. This produces a cooling action at the bottom of crucible 12 that causes the melt to solidify from the bottom up, forcing any magnesia trapped in the melt to rise to the top of the melt to join the magnesia already present at the top. At this point the coil is completely deenergized. After the entire melt has thus solidified and has cooled to a suitable low level, the crucible is removed from chamber 10 and its contants removed therefrom.

Thereafter the magnesia at the top of the ingot is removed by suitable means, e.g., by machining off the top of the ingot.

The resulting ingot has a high degree of freedom from oxygen, all of the oxygen in its bulk having been removed through its reaction with magnesium to form magnesia which has floated to the top surface. Since the copper of the ingot is free of unreacted magnesium, its electrical conductivity has not been affected by the previous addition of magnesium and is at the same high level as before the oxygen-removal process. The amount of magnesia (MgO) left behind in the copper will be too insignificant to cause a deleterious effect in the electrode material in a vacuum interrupter.

From this ingot a suitable blank is cut, after which this blank is machined into an electrode or contact of a vacuum-type circuit interrupter. When the electrode has been mounted in the interrupter, the interrupter is baked at a temperature of around 400 C and evacuated at the same time to remove most of the surface contaminants from the electrode. The bulk of the copper remains free of oxygen; and thus when the electrode is deeply eroded by an arc during interrupter operation, there is no release of oxygen from the copper matrix to interfere with interrupting performance.

Considerable latitude is available in the amount of magnesium that can be added in practising my process. The minimum amount has only to be sufficient to combine with all the oxygen contained in the copper melt. An excess of magnesium based solely on stochiometric considerations is necessary as the efficacy of the magnesium atoms to getter oxygen atoms is not 100 percent.

Taking as an example OF HC copper, which generally contains less than 2 parts per million by weight of oxyen, e50 @199 xs=s s is normally 9 Lis=nt hi$ takes into consideration the differ e ric e in the atomic weights of magnesium and oxygen, as well as the presence of surface oxygen. Thus, about 0.01 percent or 0.02 percent magnesium by weight is normally sufficient. An excess of magnesium, even if quite large, will not be deleterious other than in prolonging the time necessary for evaporating the uncombined magnesium. To maintain this time within reasonable limits, I prefer to limit the percentage of magnesium to less than 0.1 percent by weight of the solution.

Free Energy of Oxide Formation A F, k cal/gram atom Oxide Be 0 136.3 Mg 0 136.2 Zr 0 123.9 Ti 0 117.0

Thus, magnesiums affinity for oxygen approximately equals that of beryllium and substantially exceeds that of zirconium and titanium.

A second advantage of magnesium when used in my process is that its vapor pressure is much greater than that of copper. Accordingly, when the temperature is maintained at l,l50 C, the magnesium evaporates at a much higher rate than the copper. Note in this respect that at 1,150 C, the vapor pressure of magnesium is about 1,000,000 times that of copper, as will be apparent from FIG. 2. ln marked contrast, the vapor pressure of beryllium at this temperature is approximately equal to that of copper, and the vapor pressure of titanium and of zirconium is much lower than that of copper, all of which can be seen in FIG. 2. In order to free the copper of any of these constituents, it would be necessary to maintain the high temperature for a much longer time than is the case with magnesium. This would not only prolong the time necessary to achieve the desired freedom of the gettering additive (thus increasing the cost and decreasing the efficiency of the process) but also would cause much more of the copper to evaporate while the high temperature is being maintained.

Consideration was given to using aluminum as a gettering additive since its affinity for oxygen is only slightly less than that of magnesium, but (as seen in FIG. 2) its vapor pressure is so close to coppers in the pertinent temperature range that an unduly prolonged heating period is needed to achieve the desired freedom from this additive.

The only other metals, aside from those mentioned hereinabove, which have a similarly high affinity for oxygen are thorium, yttrium, and hafnium. These, however, are not usable in practising my invention because their respective vapor pressures at the pertinent temperature (i.e., about 1,150 C) are far lower than that of copper.

Magnesium is unique among all the gettering metals mentioned above in having not only a high affinity for oxygen but in having a vapor pressure at the pertinent temperature orders of magnitude greater than that of copper. This unique combination of properties renders magnesium ideal for use in my process, where after effective gettering action, any uncombined residue thereof can be quickly and efficiently boiled off.

While 1 have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects; and 1, therefore, intend herein to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by letters Patent of the United States is:

l. A method of preparing copper for use in arcing electrodes of a vacuum-type circuit interrupter, comprising:

a. melting a quantity of copper in a vacuum,

b. adding to the molten copper a small amount of magnesium that reacts with any oxygen present in the copper to form magnesia, most of which floats to the surface of the liquid copper-magnesium solution,

c. maintaining while in a vacuum the temperature of the liquid copper-magnesium solution at a level above the melting point of said solution and substantially above the boiling point of magnesium for a sufficient period of time to evaporate from the solution all of the magnesium present therein that has not reacted with the oxygen therein, thus leaving a residue of liquid copper, and

d. forcing residual magnesia trapped in said liquid copper to move to the surface of said liquid copper.

2. The method of claim 1 in which said residual magnesia trapped in said liquid copper is forced to the surface thereof by directionally cooling and solidifying said copper.

3. The method of claim 1 in which the percentage of magnesium added to the copper is below 0.1 percent by weight of the copper-magnesium solution.

4. The method of claim 1 in which the copper starting material referred to in (a) has an oxygen content of only several parts per million or less.

Claims (3)

1. 2. The method of claim 1 in which said residual magnesia trapped in said liquid copper is forced to the surface thereof by direcTionally cooling and solidifying said copper.
2. 3. The method of claim 1 in which the percentage of magnesium added to the copper is below 0.1 percent by weight of the copper-magnesium solution.
3. 4. The method of claim 1 in which the copper starting material referred to in (a) has an oxygen content of only several parts per million or less.

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