US3125441A

US3125441A - Materials

Info

Publication number: US3125441A
Authority: US
Publication date: 1964-03-17
Anticipated expiration: 1981-03-17

Description

March 17 1964 J. M. LAFFERTY ETA 12 ELECTRODES FOR VACUUM CIRCUIT INI' IERRUPTERS 3, v AND METHOD OF MAKING SAME Filed April 11, 1960 MA TERIALS METAL A Sintered Matrix of V STEPS Refractory Metal 7 Heat in H Atmosphere to lmpregnate Metal A E TAL 8 mm Metal 5 Metal of Good Electrical Properties and a Melting Point Lower than that of Metal A cool to solidify Metal 6 and Form Composite Material Zone Refine the Composite Material MA TER/ALS Q- Refractory Metal Heat in H Atmosphere to .Sinter and lmpregnate Metal A with Metal 6 METAL B 7 Metal of Good Electrical Properties and a Melting Paint a Lower than that of Meta/A I v Cool m sol/wry Metal 5 and Form Composite Material Fig, 4. Zane Refinethe Composite Material I Inventors:

James M. Lafferty e r/ V Jam s 0 C 3 r fmmettffi a The/r Attorney.-

United States Patent ()ffice 3,125,441 Patented Mar. 17, 1964 Filed Apr. 11, 1950, Ser. No. 21,337 4 Claims. (Cl. 75-65) This invention relates to contacts or electrodes for circuit interrupters of the vacuum type, and, more particularly, to composite metal contacts for vacuum circuit interrupters wherein the contacts are substantially free from gases and gas evolving materials and to the method of making such contacts or electrodes. This is a continuation-in-part of our application Serial No. 648,513, filed March 26, 1957, and now abandoned.

Electrical contacts in circuit interrupters are subject to arcing just prior to engagement, or immediately subsequent to separation. This arcing is not only extremely injurious to the contact in burning, melting, or welding, but also presents a serious problem in extinguishing the are. One of the many methods employed to minimize both the arc formation and th eperiod of time the arc is sustained includes contact separation under high vacuum conditions. Under ideal conditions in a high vacuum, the arc would be sustained for only a very short time. This is due in part to the very small quantity of ionizable gas remaining in the interrupter under high vacuum conditions so that the arc is sustained primarily by the metal which is evaporated from the electrodes themselves. The

contacts, however, may contain substantial impurities,

such as sorbed and chemically bound gases or gas forming materials, which may evolve gas during arcing tending to sustain the are for an undesirable length of time.

The gases released or evolved from contacts under vacuum conditions need not be present in gas form in the contact material, but may be in the form of carbides, sulfides, and oxides. The high energy from the arc causes a dissociation and recombination of these materials to form gases which are released into the circuit interrupter impairing the high vacuum condition there. Thus, it may be seen that for optimum performance the contact material should be substantially pure, that is, free from sorbed and chemically bound gases and gas forming materials which evolve gases under arcing conditions.

A .wide range of metals or alloys thereof is available from which one may choose a particular combination together with a process for combining the metals to result in a suitable contact material for a given set of conditions. It has been found that, for contacts operating under high vacuum conditions, a substantially pure refractory metal utilized as a porous body or matrix filled with a substantially pure metal ofgood electrical properties, and with the combination essentially free from impurities, presents a favorable solution to the arc problem. Heretofore, one of the methods of producing such contacts included forming a porous and sintered body of a refractory metal, placing the porous body adjacent a piece of copper or other metal of good electrical conducting properties and a melting point lower than that of the refractory metal, and then heating the two bodies in a furnace to a temperature above the melting point of the lower melting metal, to permit the sintered body to absorb the molten metal by capillary action. While this method produces a composite metal electrode of high purity for many applications, it does not perform satisfactorily for use in circuit interrupters of the vacuum type where a substantially higher purity from gas forming elements is necessary in order to minimize and suppress the arc.

Accordingly, it is an object of this invention to provide an improved process for forming composite metal contacts, wherein the combination of metals is substantially free from gases and gas forming elements.

It is a further object of this invention to provide a high purity composite metal electrode which will maintain its high purity after prolonged exposure to atmospheric conditions.

It is another object of this invention to provide vacuum switch contacts which are resistant to prolonged arcing and evolve substantially no gases under arcing conditions.

It is yet another object of this invention to provide a substantially pure composite metal contact by a zone refining method.

The features of our invention which we believe to be novel are set forth with particularly in the appended claims. Our invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a flow diagram of one illustrative example of the method of this invention;

FIG. 2 is a flow diagram of another illustrative example of the method of this invention;

FIG. 3 is an enlarged fragmentary view in side elevation of one of the electrodes of this invention; and

FIG. 4 is a very greatly enlarged fragmentary view through section 4-4 of FIG. 3.

Briefly described, in accordance with one aspect of this invention, a porous or sintered body of a refractory meta1 together with a metal of good electrical conducting properties and a melting point lower than that of the refractory metal, such as copper, silver, aluminum, and the like, undergoes a cleaning process by being subjected to elevated temperatures in a reducing atmosphere. Thereafter, the temperature is increased above that of the melting point of the lower melting metal which melts and is absorbed into the pores of the sintered body forming a composite electrode material. The process includes zone refining the composite material so formed, which in combination with the cleaning step, produces an electrode material substantially free from impurities such as sorbed or chemically bound gases or gas forming materials which tend to contribute both to are formation and the length of time the arc is sustained.

The concept of utilizing electrodes for vacuum circuit interrupters which are formed of zone refined metals or alloys per se is not our invention but is the invention of M. H. Hebb and is disclosed and claimed in application Serial No. 854,392, filed November 20, 1959, and assigned to the assignee of the present application, which invention was made by the said M. H. Hebb prior to our invention. We, therefore, do not herein claim anything shown or described in said Hebb application, which is to be regarded as prior art with respect to this application.

While it is to be understood that various combinations of metals may be employed, a preferred form of this invention includes tungsten, molybdenum, or other refractory metals, together with another metal of good electrical conducting properties, such as copper, silver, aluminum, or other high vapor pressure metals. By way of example, a refractory metal and copper will hereinafter be described for explanatory purposes.

A method of producing the sintered or porous metal body and one which gives good results in this invention includes forming a suitably shaped body from a metal powder by compression, and then sintering the body in a hydrogen atmosphere at a temperature sufficient to produce a sinter-bonded body. Such a body contains cavities, fissures, or interstices of substantially continuous form which may be filled with a liquid or molten metal. The porous body produced from the heretofore described process is placed adjacent a strip of copper and the temperature of the hydrogen furnace is increased to above the melting point of the copper. The molten copper flows into the pores of the sintered body or is absorbed thereby by capillary action to form a composite electrode material which is then allowed to cool.

While a preferred form of this invention includes the placing of a relatively fiat piece of copper under the porous body, as shown by the flow diagram of FIG. 1, this invention contemplates placing the copper in the furnace While the porous body is being sintered and then permitting the sintered body to absorb the copper which may be in the molten state at sintering temperatures such as shown by the flow diagram in FIG. 2. Alternatively, the absorption may take place by pouring the copper over the sintered body, by submerging the sintered body in the copper or by like processes.

The finished copper impregnated sintered body is relatively free from impurities but there still remains a quantity of impurities together with sorbed and chemically bound gases or gas forming materials in both the copper and the sintered body which contribute to the evolution of an undesirable quantity of gas which causes the arc to be sustained for too long a period of time. It should be noted that the sintered body may not only be removed from the hydrogen atmosphere furnace for storage, but also may be purchased or otherwise acquired originally in sintered form. Under these conditions the quantity of undesirable impurities is greatly increased in the sintered body and is basically present in the impregnating metal. Accordingly, the impregnated body is subjected to a zone refining process for the removal of both sorbed and chemically bound gases and gas forming impurities from the impregnating metal, and to a great extent from the sintered body also.

Zone refining has been used primarily to purify semiconductors such as germanium. As so utilized, apparatus is provided for melting one or more narrow zones of a body of semiconductor and causing these zones to traverse the body from one end to the other. In the specific case of impurities in the semiconductor which are more soluble in the liquid than in the solid phase, the impurities are segregated into the molten zone and de posited at the end of the bar when the zone is finally frozen. The usual electrically significant impurities in germanium, for example, are generally more soluble in the liquid phase than in the solid phase, having segregation coefiicients less than unity, so that the molten zone becomes impurity-rich as it traverses the body.

A variation of this technique has also been used in forming N-P and N-P-N junctions and for controlling and adding particular elements to affect the conductivity of the semiconductor.

In addition, another variation of this technique provided for gas doping of semiconductors. By zone melting in a preselected atmosphere of gases containing electrically significant elements, these gases were incorporated into the solid semiconductor. This has been performed with such gases as diborane and pentaborane, for example.

Accordingly, it was known that gases could be added to a solid semiconductor by the zone refining technique and also that certain significant impurities could be removed, added or made to have substantially uniform concentration throughout the semiconductor.

In the method of this invention, however, the body of composite electrode material, comprising a porous sintered refractory metal body impregnated with a metal of good electrical conducting characteristics and a melting point lower than that of the refractory metal, is subjected to a zone-refining treatment and substantially all sorbed gas and gas forming materials are removed providing an electrode for vacuum circuit interrupters which evolves substantially no gases under arcing conditions. The good electrically conductive material is, therefore, purified in situ, making further processing, with possible attendant contamination, unnecessary.

In the zone-refining treatment the narrow zone may be melted, for example, by an encircling electric coil or other suitable heating means which melts only a narrow region or zone of the impregnating metal of the composite electrode body. The coil is thereafter moved to an adjacent portion of the body while the previously molten portion is allowed to freeze. A narrow molten Zone of the impregnating metal thus traverses the composite body from one end to the other. This process results in the impurities being moved to one end or portion of the body which is subsequently removed. It may be seen that either the coil or the body may be moved and the process repeated until the desired purity is reached.

A composite electrode may also be obtained by the above process by first forming a body of refractory material having longitudinal holes therein which are formed by extrusion or other well-known processes. The body is then sintered and the holes filled with a metal of good electrical conducting properties and melting point lower than that of the refractory material by casting the metal into the holes. The composite body so formed is then subjected to zone refining and cut into sections for electrical contact applications.

Alternatively, the metal of good electrical conducting characteristics is cast about a refractory, as for example, tungsten or molybdenum body and the composite body then subjected to zone refining to remove impurities in the cast metal and adjacent thereto.

It is apparent, therefore, that this invention provides a sintered refractory metal body impregnated with a metal of good electrical conducting properties, and being substantially free from sorbed 0nd chemically bound gases and gas forming materials through the employment of zone refining.

The process described provides contacts particularly suitable for vacuum switch operation and which essentially do not evolve gases when subjected to arcing conditions. Furthermore, such contacts may also undergo prolonged exposure to atmospheric conditions without a high degree of deterioration of their arc minimizing capabilities.

The following is given as an example and not by way of limitation of the process employed in this invention.

Example A long bar of tungsten approximately 1 inch by 1 inch by 12 inches was pressed from powder and sintered in a hydrogen atmosphere at a temperature of 1600 C. to make the compact hold together. Copper in the form of a flat strip was then placed under the sintered bar and the two heated in a hydrogen atmosphere, again at a temperature just above the melting point of the copper. The copper was absorbed by the bar filling all the small capillary pores. The filled bar was then placed in a zone melting furnace and heated so that the hot zones were above the melting point of the copper, and the entrapped gases and impurities were moved to one end of the bar which was subsequently removed. This composite body was found to be satisfactorily free from sorbed or chemically bound gases and gas forming elements which evolve gases under arcing conditions.

While other modifications of this invention may be employed without a departure from the scope thereof, the invention is intended to include all such as may be embraced within the following claims.

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

l. The method of producing a high purity composite electrode metal which evolves substantially no gases when subjected to arcing conditions comprising: providing a matrix of sintered refractory metal, said matrix having substantially continuous openings therethrough; heating said matrix in a reducing atmosphere and simultaneously impregnating said matrix with a metal having good electrical conducting characteristics, a melting point lower than that of said refractory metal, a vapor pressure higher than the vapor pressure of said refractory metal and capable of being absorbed into the openings of said matrix; cooling the composite body so formed to solidify the impregnating metal; and subjecting said composite body to a zone refining process wherein a narrow molten Zone of the impregnating metal traverses said body to remove sorbed and chemically bound gases and gas forming compounds which cause the evolution of gas when said composite electrode metal is subjected to arcing conditions.

2. The method of producing a high purity composite electrode which evolves substantially no gases when subjected to arcing conditions comprising: providing a formed matrix of pressed refractory metal powder; sintering said matrix in a reducing atmosphere at a temperature sutficient to form a stable compact which has substantially continuous interstices therethrough; impregnating said matrix during sintering with a metal having a vapor pressure higher than the vapor pressure of said refractory metal, good electrical conducting characteristics, a melting point lower than that of said refractory metal and capable of being absorbed into the interstices of said matrix by capillary action; cooling the composite body so formed to solidify said impregnating metal; and subjecting said composite body to a zone refining process wherein a narrow crosssectional area of said impregnating metal is remelted and caused to traverse said body to remove sorbed and chemically bound gases and gas forming compounds which cause the evolution of gas when said composite electrode is subjected to arcing conditions.

3. The method of claim 1 wherein said matrix is heated in a hydrogen atmosphere and the Zone refining process includes the steps of heating a narrow portion of the composite body above the melting temperature of the impregnating metal; heating an adjacent narrow portion of said body above the melting temperature of the impregnating metal while concurrently permitting the former portion to solidify; sequentially repeating the said process over the dimensions of said composite body a suf ficient number of times until the desired purity from gases and gas forming compounds has been reached and said gases and gas forming compounds have been carried to the end portions of said composite body; and subsequently removing the end portions containing the concentrated gases and gas forming compounds.

4. The method of claim 1 wherein the refractory metal is tungsten and the impregnating metal is copper.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. THE METHOD OF PRODUCIGN A HIGH PURITY COMPOSITE ELECTRODE METAL WHICH EVOLVES SUBSTANTIALLY NO GASES WHEN SUBJECTED TO ARCING CONDITIONS COMPRISING: PROVINDING A MATRIX OF SINTERED REFRACTORY METAL, SAID MATRIX HAVING SUBSTANTIALLY CONTINUOUS OPENINGS THERETHROUGH; HEATING SAID MATRIX IN A REDUCING ATMOSPHERE AND SIMULTANEOUSLY IMPREGNATING SAID MATRIX WITH A METAL POINT LOWER THAN THAT OF SAID REFRACTORY METAL, A VOPOR PRESSURE HIGHER THAN THE VAPOR PRESSURE OF SAID REFRACTORY METAL AND CAPABLE OF BEING ABSORBED INTO THE OPEININGS OF SAID MATRIX; COOLING THE COMPOSITE BODY SO FORMED TO SOLIDIFY THE IMPREGNATING METAL; AND SUBJECTING SAID COMPOSITE BODY TO A ZONE REFINING PROCESS WHEREIN A NARROW MOLTEN ZONE OF THE IMPREGNATING METAL TRAVERSES SAID BODY TO REMOVE SORBED AND CHEMICALLY BOUND ASES AND GAS FORMING COMOUNDS WHICH CAUSE THE EVOLUTION OF GAS WHEN SAID COMPOSITE ELECTRODE METAL IS SUBJECTED TO ARCING CONDITIONS.