US3450928A - Gas-free vacuum gap devices and method of preparation thereof - Google Patents

Description

June 17, 1969 J. D. COBINE 3,450,923

GAS'FREE VACUUM GAP DEVICES AND METHOD OF PREPARATION THEREOF Filed July 1. 1966 In vervtor-xddrnes D. Cobine,

3-. is Attorney,

United States Patent Electric Company, a corporation of New York Filed July 1, 1966, Ser. No. 562,193 Int. Cl. H011 1/38 U.S. Cl. 313311 4 Claims The present invention relates to vacuum gap devices, as for example vacuum switches and triggered vacuum gap devices, having improved electrode members which are free of sorbed gases and constituents which, upon arcing, are capable of releasing gases and to novel methods of the preparation thereof, More specifically the present invention relates to devices of the aforementioned type containing copper or other metallic electrodes which are rendered free of gas and gas-forming impurities by a relatively simple and inexpensive technique.

Within the last decade a resurgence of the importance of vacuum switches and vacuum gaps has occurred. This resurgence has been a result, in part, of substantailly improved technologies for the formation of vacuum envelopes capable of withstanding the pressures and temperatures of repeated arcing at high voltages and currents over long periods of time and to the development of unique and novel choices of materials as for example, the high vapor pressure arc-electrode materials set forth in Lee et al. Patent No. 2,975,255 and Lafferty Patents Nos. 2,975,255 and 3,016,436. In the case of fixed gap devices, great improvement has followed the development of the triggered vacuum gap as described and claimed in Laiferty Patent No. 3,087,092.

In devices of the classes represented by the aforementioned patents, there is a common denominator in that successful operation depends upon the attainment of essentially absolute freedom from gas of the metal which composes that portion of the electrode upon which the foot-points of an arc stricken therebetween may rest. It has been found by experimental evidence that if the arcelectrode, or region of the arc electrode upon which the foot-points are located, contains sorbed gasses or constituents which, upon the melting and vaporization of the arc electrode material, decompose to form ionizable gasses, the residual pressure within the arc device rises and extinction of the are upon the occurrence of a first current zero of alternating current does not occur.

Accordingly it is a principal object of the present invention to provide vacuum devices of the vacuum switch and vacuum gap types in which high vacuum is readily maintained by the use of electrodes that are essentially free of all gas and gas-forming constituents.

A further object of the present invention is to provide vacuum gap devices capable of maintaining high vacua during arcing conditions without expensive, complicated, and time-consuming processing of the constituents thereof.

Briefly stated, in accordance with one aspect of the present invention, I provide a vacuum gap device including a pair of oppositely disposed arc-electrodes defining therebetween a breakdown gap and located with an hermetically sealed envelope which is evacuated to a pressure of at least mm. of mercury pressure or lower and means for connecting the respective electrodes to an electrical circuit to be protected or controlled thereby. In one embodiment of the invention at least those portions of the arc electrodes which are to be arcing surfaces and which are adapted to support the foo'tpoints of an electric arc therebetween are composed, in intimate admixture, of av high vapor pressure metallic constituent which is free of all gas and gas-forming impurities, particularly of oxygen and is rendered such by the incorporation therein of 3,450,928 Patented June 17, 1969 a small but finite quantity of zirconium or titanium metals which have a great affinity for oxygen and form therewith during fabrication a high temperature-stable oxide which precludes oxygen from affecting the vacuum within the device and thereby adversely affecting its operating characteristics.

In one embodiment of the present invention the two arc-electrodes are fixed to define a fixed gap therebetween and a separately-connected trigger electrode assembly is juxtaposed immediately adjacent the gap and adapted to inject an electron-ion plasma thereinto to facilitate breakdown thereof. In accordance with another embodiment of the present invention one electrode is fixed and the other is movable to cause the formation of a vacuum switch or arc interrupter device.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and ad vantages thereof may best be understood by reference to the following description taken in connection with the appended drawing in which:

FIGURE 1 is a vertical cross-sectional view of a schematically illustrated vacuum circuit interrupter in accord with one embodiment of the invention, and

FIGURE 2 is a vertical cross-sectional view of a schematically illustrated fixed-gap vacuum gap device constructed in accord with another embodiment of the present invention.

In FIGURE 1 an interrupter chamber 10 comprises a sidewall member 11 which may be cylindrical in shape and is preferably constructed at least in part of a suitable insulating material, having at the ends thereof a pair of metallic end wall members 12 and 13 enclosing the volume therein to form an interrupter chamber which is hermetically sealed. Suitable seal members 14 are provided connecting sidewall member 11 and end- wall members 12 and 13 to facilitate hermitical sealing. Alternative to the utilization of a completely insulating member 11, the entire envelope may be substantially constructed from a metallic member, with the exception that at least one portion thereof must be of a high voltage, vacuum tight dielectric insulating material, sufficient to electrically insulate those portions of the envelope connected with one arc-electrode from those portions of the envelope connected with the other arc electrode at very high voltages. Such interposed dielectric high voltage insulators may be located in a variety of places, as for example along a portion of the end-wall member 11, or alternatively, as a portion of the end- wall members 12 or 13. Irrespective of where the insulating dielectric material is located, an appropriate shield member must be interposed between the arcing space or gap between the arc-electrodes and the dielectric, and preferably a series of baflle shields should be included to preclude shorting of the dielectric and rendering the device inoperative.

A pair of separable contacts or primary arc- electrodes 15 and 16, shown in their closed circuit or circuit-making position, are located within chamber 10. Arc-electrode 15 is a stationary member suitably attached electrically and mechanically to a conducting arc-electrode support rod 17 which, at its upper end, is united electrically and mechanically with end-wall member 12.

Arc-electrode 16 is mounted upon and electrically united with a suitable conducting arc-electrode support rod 18 and is movable, being connected to bellows 20 or an equivalent vacuum-tight flexible member, permitting reciprocating motion. Arc-electrode support rod 18 projects through a suitable aperture in end-wall member 13, and suitable actuating means may be connected thereto to cause a reciprocating motion of rod 18, facilitating the entry of arc-electrode 16 into, and removal from, en-

gagement with arc-electrode 15. The electrical circuit which is to be protected or controlled by the are interrupter may be completed by making suitable connections to terminal 21, electrically and mechanically connected to rod 18. Such connection may either be in series or parallel circuit relationship depending upon the particular manner in which the circuit interrupter device is to be utilized.

As mentioned hereinbefore, a suitable insulator shield such as metallic cylindrical member 23, capped with an arc-preventing ferrule 24 is interposed between arc- electrodes 15 and 16 and insulator 11 to prevent the latter from becoming coated with metallic particles and becoming electrically short-circuited.

The volume within vacuum interrupter chamber is suitably evacuated through an exhaust tubulation (not shown) during the final assembly thereof. For proper operation of the interrupter as a vacuum-type interrupter of alternating currents the pressure within chamber 10 must be maintained at a pressure of no greater than 10* mm. of mercury and preferably a pressure of 10'- mm. of mercury or less. The foregoing requirement is essential for the operation of the device as a vacuum interrupter of alternating currents. This requirement is necessary because, in order that a current carrying are between arc-electrodes and 16 be extinguished at the first-occurring current zerovalue, there must be substantially no ionizable gas present within chamber 10. The occurrence of such ionization may be substantially prevented if all the possible breakdown paths between arc electrodes 15 and 16 and their respective supports, are small with re spect to the mean free path of an electron within the atmosphere obtained within the device. This mean free path is designated as a statistical distance which an electron may travel without colliding with a gas molecule at a given pressure. These conditions may be established within the devices of the present invention only when the pressure within the interrupter chamber 10 is below approximately 10- mm. of mercury and is ensured when it is below 10 mm. of mercury.

It is not sufiicient merely to evacuate the device to the requisite vacuum initially to ensure successful operation of the device, particularly under conditions of repeated operation. In this respect, it is necessary that all possible sources of increased gas pressure during operation must be eliminated or minimized. Metallic and insulating parts may release gas particles when heated by arcing conditions during operation. Ideally all parts should be completely out-gassed prior to assembly and thereafter as well.

One type of commercially available vacuum circuit interrupter, therefore, utilizes contacts of tungsten, molybdenum, or other refractory materials which may be baked and otherwise heat treated for a sufficiently long time and at a sufficiently high temperature to remove all sorbed gasses therefrom. This prevents the entry of ionized gasses from the electrode materials entering into the vacuum chamber due to the action of the arc. This is necessary because the vacuum arc is sustained as a conducting column by ionized metallic particles which are boiled from the electrodes, primarily the cathode. During the boilingout process, if any sorbed gasses or unstable gas-forming constituents are present within the electrode material, gas may be freed and become ionized.

Upon the occurrence of the first current zero after establishment of an alternating current are, the ionized metallic specie within the chamber is rapidly cooled and migrates back to the electrodes, to the shield, or to the chamber wall, is deionized and removed from the chamber, lowering the pressure. This is not, however, the case with ionizable gasses which may be boiled from the electrodes. Such gasses continue to remain in the chamber and raise the pressure thereof. It is for this reason that the electrodes must be purified to such an extent as to cause the presence of gas and unstable gas-forming constituents within the electrode to be low enough to allow the maintenance of a vacuum of less than 10' mm. of mercury pressure within the device.

Arc-electrode support members 17 and 18, since they are not directly exposed to the action of the arc and do not sustain the footprints thereof need only be of standard grade, high purity, as for example, OFHC copper or the equivalent thereof. Sidewall member 11 may conveniently be constructed of high temperature glass or may be of a gas-impervious, vacuum-tight ceramic as for example, high density alumina or a fosterite ceramic. End wall members 12 and 13 may conveniently be constructed of stainless steel or nickel or, if a fosterite ceramic is utilized for sidewall member 11, end- wall members 12 and 13 may be fabricated from titanium, and a metal-to-ceramic seal may be made directly between the two. Shield 23 may conveniently be constructed of stainless steel, heat treated for several hours at extremely high temperatures to the order of 1000 C. to remove therefrom all sorbed gasses.

A fixed vacuum gap device constructed in accord with the present invention is illustrated in FIGURE 2 of the drawing. In FIGURE 2 of the drawing, the device envelope 30 comprises a sidewall member 31 which may be cylindrical in shape and is constructed of a suitable insulating material having the same characteristics and composition as sidewall member 11 of the device of FIG- URE 1 and closed at the ends thereof by a pair of metallic end- wall members 32 and 33 which may conveniently be constructed of the same materials comprising end- wall members 12 and 13 of the device of FIGURE 1. Side and end-wall members enclose the volume therein and form an hermetically sealed vacuum chamber. Suitable seals 34 are provided between side-wall member 31 and end members 32 and 33 to facilitate the formation of hermetic seals therebetween.

A pair of spaced arc- electrodes 35 and 36, defining therebetween a breakdown gap 37, are disposed within envelope 30 and are supported respectively upon arcelectrode support rods 38 and 39, which are electrically and mechanically connected with the respective end- wall members 32 and 33. An electrical circuit which is sought to be protected or controlled by vacuum gap device 30 is connected to end- wall members 32 and 33 in series or parallel circuit, as desired, by means of connecting lugs 41 and 42, electrically and mechanically afiixed respectively to end- wall members 32 and 33. As in the device of FIGURE 1, a suitable insulator shield such as metallic cylindrical member 43 terminating in an are preventing ferrule 44 is interposed between electrodes 35-36 and insulator 31 to prevent the latter from becoming coated with sputtered or evaporated arc-electrode material and becoming electrically short-circuited. The volume within chamber 30 is maintained at a pressure of less than 10' mm. of mercury and preferably less than 10' mm. of mercury, as in the device of FIGURE 1.

As is mentioned hereinbefore, one of the greatest problems in the commercial utilization of the basic concept of the vacuum switch has been in the provision of suitable arc-electrode materials. When used in inductive loads on alternating current circuits, a vacuum arc interrupter or vacuum gap device terminates the arc between the arc-electrodes upon the occurrence of a first current zero. This is because when the alternating current cycle passes through zero the first time after the establishment of the alternating current arc, the voltage across the electrodes temporarily falls to zero, and the arc extinguishes. At this time the vaporized metallic particles from the electrodes which support the vacuum arc, immediately migrate to the nearest cold wall where they are condensed and deionized. Instantaneously the pressure within the device falls to a very low value and all available current carriers within the device vanish. Thus, when the voltage begins to build up for the next succeeding half cycle, the high dielectric strength of vacuum is interposed between the arc-electrodes and, in the absence of a second failure or exceeding of the rate voltage of the device, the arc is not restricken.

Two problems are in evidence. Firstly, it has been determined by others and is set forth in great detail in the patents of Lee and Cobine, No. 2,975,256 and of Lafferty, No. 2,975,255 and 3,016,436, that if the current interruption occurs at a period in time before current zero is approached, a phenomenon known as chopping occurs and the current falls from a substantially high value to zero, inducing highly destructive transient currents within the circuit which can cause the breakdown of insulation in inductive circuit components, as for example trans formers, motors, and generators. It was determined that such phenomena were due to vapor starvation and have been minimized or eliminated by the use of high vapor pressure materials as for example those set forth in the aforementioned US. patents to Lafferty and Lee and Co-bine, for example. Such materials are non-refractory and have boiling points no higher than that of tin. Such use is often combined with portions of the electrodes which are refractory and may be outgassed to obtain the advantages of each type electrode material. Thus the refractory material does not erode badly, while the nonrefractory, high vapor pressure material suppresses chopping.

In using high vapor pressure materials one encounters another difficulty attendant the use of vacuum gap devices with both fixed and movable gaps, namely that of maintaining a hard vacuum over a long duty cycle. In the operation of the vacuum switch and the vacuum gap, the arc stricken between the arc-electrode melts the arc-electrodes at the footpoints of the are so that any gas entrapped in the arc-electrodes may be released therefrom and, when released, becomes ionized Within the arc. Similarly, if low decomposition temperature gas-forming constituents such, as for example copper oxide, which decompose at the temperature of the arc footpoint are present Within the arc-electrode materials, these constituents may be decomposed by the temperature of the molten arc footpoint, to cause the release of gasses which become ionized in the vacuum gap.

When the alternating current vacuum arc passes through a current zero, even if there are ionizable gasses present, the arc is extinguished. In the event of the presence of ionizable gases, however, the gap remains ionized and, when the next alternation of the applied inter-electrode builds up to a sufiicient value, the arc is restricken and the vacuum gap has essentially failed to operate in its intended manner.

This problem is present not only if the entire electrode, as illustrated, is formed of a low melting point, high vapor pressure metal or alloy. Some structures, as for example as disclosed in U.S. Patents 3,239,635, Baude; and 3,244,843, Ross, utilize a combination of a refractory member and a low melting point, high vapor pressure member, both of which are subjected to the arc footpoint at some portion of the arcing cycle. In such structures, the portion of the arc-electrode that is nonrefractory presents the same problem. Accordingly, it is imperative that the infusion of ionizable gasses into the vacuum gap due to the arcing action upon the electrodes be eliminated. In the past this has been done by the careful processing of the materials from which the vacuum arc-electrodes were fabricated by expensive, time-consuming, and tedious processes. Thus, for example, the prior art minimum for achievement of this objective has been determined to be at least a six-pass zone-refining process.

Such a process requires that a large quantity of material, as for example copper, which is to be used in fabricating the arc-electrodes for vacuum gap devices be placed in a high temperature crucible, made, for example, from carbon or boron nitride, preferably in the form of a rod which may, for example, be 1 inch in diameter and 12 or 13 inches long. A molten zone which may, for example, be one inch in longitudinal dimension, is then passed through the copper rod at a rate of, for example, 12 to 13 inches per hour. This takes about one hour. The

process is then repeated, starting again with the molten zone at the same beginning end, for five more passages of the molten zone through the bar in the same direction. At the completion of this schedule, at least six to eight hours have been consumed in the processing of the material. In addition to the necessity of utilizing the passage of a molten zone through the copper rod a plurality of times, special conditions must be maintained, as for example those set forth in Patent No. 3,234,351 to M. H. Hebb, which ensure the growth of large crystals of copper, thus minimizing the area of crystal interfaces at which gasses and gas-forming impurities tend to concentrate. The starting material for this process is generally of a very high purity, for example, at least 99.6% pure and is nominally free of oxygen. One such starting material is OFHC copper, obtainable from American Metal Climax Inc., New York, N.Y., which has been analyzed as including approximately 99.995 copper, possessing approximately from 1 to 3 p.p.m. of oxygen.

While the aforementioned processes for the formation of electrode material to be utilized in vacuum arc devices of the vacuum switch and vacuum gap types is useful and is operative to produce practical electrodes and vacuum arc devices, it is expensive, time-consuming and requires the tying-up of personnel and equipment for exceedingly long periods of time, thus limiting the amount of material that can be produced with any given facility. Accordingly, it is imperative, if proper economy is to be effected in the manufacture of useful and effective vacuum are devices, that other means he found to fabricate vacuum arc electrodes.

It has been found that the principal gas which causes the aforementioned difficulties is normally processed materials for use as arc-electrodes in vacuum arc devices is oxygen. Oxygen is naturally present in large quantities in the environment in which practically all materials are refined and normally processed. Additionally, oxygen is very reactive and readily forms oxides with normal electrode materials, as for example, copper. Oxygen, either in the gaseous form or in the form of decomposable oxides is almost invariably found to some degree in the most highly refined metals utilized in vacuum arc electrode processing. In particular, oxygen tends to accumulate at the grain boundaries and crystal interfaces in vacuum melted and zone-refined arc-electrodes.

Accordingly, I provide improved vacuum arc device arc-electrodes and vacuum arc devices which are formed by the addition to the arc-electrode material, during processing, of small quantities of zirconium or titanium which have a very high aflinity for oxygen and which form, with oxygen, highly stable refractory oxides which do not decompose at the temperature of the arc footpoint. Additionally, I have found that the addition of small quantities of zirconium or titanium to conventional so-called high purity (OFHC or equivalent) copper having a purity of approximately 99.96 or purer copper and a nominal amount, for example, approximately 1 to 3 atomic parts per million of oxygen results in the formation of an oxide, substantially all of which may be removed by suitable processing which is far simpler and less expensive than the aforementioned prior art fabricating processes.

More specifically I have found that when a small quantity as set forth hereinafter of zirconium or titanium, which have a high affinity for oxygen and forms a high temperature refractive oxide therewith, is added to a main constituent of a vacuum device arc-electrode, as for example, copper, and vacuum refined, for example, vacuum melted for /2 hour at 1100 C. at a pressure of 1() mm. Hg, or less, a highly improved arc-electrode material is formed.

I prefer, however, to prepare a quantity thereof in rod form which is subjected to a Simple zone leveling process, which process is well known to those skilled in the art, and which may be found described in detail copiously in the literature, as for example, in the book entitled Zone- Refining, by W. G. Pfann, published by John Wiley & Sons, New York, 1958, at pp. and 133 et seq.

Zirconium of reactor grade having a purity of 99.5% and containing only 0.15% oxygen, obtainable from Gallard Schlissinger Chemical Mfg. Corp., Long Island, N.Y. 11514, and titanium, Grade A-4O or A-55 having a purity of 99.6 or 99.7% and about 0.15% O and obtainable from Crucible Steel Corp., Pittsburgh, Pa., are suitable for use in practicing the invention. During zone leveling the zirconium or titanium is intimately admixed with the copper during the passage of the molten zone therethrough. In general, the characteristics of a metal or alloy which has been subjected to a zone leveling are well known to those skilled in the art, due to the known effects of the process upon the materials utilized. For this reason, and for simplicity of expression the electrode metal alloys used in practicing the present invention, when subjected to a zone leveling treatment will be referred to herein as zone leveled material. Generally, such material and material which has been vacuum melted will be referred to herein as vacuum refined.

During the zone leveling zirconium or titanium remains substantially uniformly distributed throughout the copper. The greatest portion of the oxide formed by the reaction of the zirconium or titanium with any oxygen present, either in the form of copper oxide or as free oxygen, while in the molten zone, floats to the outside surfaces of the molten zone and, upon freezing and cooling of the rod, may be seen as a cloudy surface contaminant. This oxide may readily be removed from the exterior surface of the zone leveled rod by etching in an acid etch, as for example, one generally comprising approximately one part hydrofluoric, three parts nitric and parts acetic acid for a minute or less. Such etching removes the surface contamination which is the oxide and leaves the remaining ingot bright and free of oxide contaminants.

Since the quantity of oxygen in the OFHC copper used initially is of the order of few atomic parts per million, substantially all of the zirconium or titanium added remains in the copper rod, approximately 5% of which is removed by cropping both ends thereof for purity sake. Due to the even distribution characteristic of the zone leveling process, the remaining zirconium or titanium is substantially uniformly distributed throughout the copper in the form of intermetallic compounds with copper, solid solution of some of the zirconium or titanium within the copper and a very small quantity of oxide which does not float to the surface.

Since zone leveling only requires one pass in each direction through the ingot, rather than the six passes in the same direction of the conventional zone-refining processes necessary for prior art device fabrications, three quarters to five-sixths of the work entailed in fabrication of arc-electrode material for vacuum are devices as in the prior art is eliminated in accord with the present invention. Additionally relatively impure copper may be rendered suitable for vacuum arc electrodes for use in certain low power applications in accord with the present invention by the addition thereto of relatively minor quantities of zirconium or titanium, so that the cost of the fabrication of such vacuum are devices, in accord with the present invention, is drastically reduced from the cost of prior art fabrication methods.

I am aware that attempts have been made in the prior art to provide various arrangements to getter evolved gasses in vacuum switches and thus retain an acceptable vacuum for continuous operation over many switching operations. However, with the exception of the invention of F. H. Horn disclosed and claimed in application Serial No. (Docket D-3290) assigned to the present assignee and filed concurrently herewith and which relates primarily to a similar approach utilizing other materials, principally beryllium, all such attempts have had the approach of gettering an existing gas pressure. Such approaches range from the incorporation of auxiliary electrodes and gaps, to the physical inclusion of an oxygen aflinitive metal in close juxtaposition to an inter-electrode gap and even to the inclusion of an oxygen affinitive metal in one electrode of a vacuum switch.

All of these approaches have, however, relied upon the mechanism of allowing gasses, including oxygen, to be released into the arcing space, and simultaneously causing gas (including oxygen) affinitive metals to be heated and/or vaporized to seek out and getter the gas particles. At best this is a hit or miss approach and is bound to result in some oxygen remaining in the systems, causing potential failure of the switch. Even preoperational arcing or forming would not cure this defect, for, even if gasses (including oxygen) initially released by the are are all gettered by several preoperational arcings, subsequent arcings will release more gas and there is always bound to be a residue. In accord with this invention, however, zirconium or titanium, preferably, zirconium, is intimately admixed with high purity copper during vacuum-refining to cause all oxygen in the copper to be reacted with, to form stable oxides which do not decompose upon arcing. All of this is done before the arc-electrodes are formed, so that when the devices are first operated, no oxygen is released from the arcelectrodes and no gettering thereof is required.

In general, the affinity of a metal for oxygen is measured by the free energy of formation of the oxide. The more negative this value per atom of oxygen in the oxide, the greater the afiinity of oxygen for the metal. The stability of the oxide so formed is measured by its dissociation temperature, when known. In the absence of a known dissociation temperature, the melting point and boiling point are good criteria. In general, the higher the melting and boiling points, the more stable the oxide. Table I, below lists these parameters for the oxides of copper, as a typical arc-electrode material, and the aforementioned materials.

TABLE I Free energy of forma- FIE/F per Dissociation at atom tion Boiling Melting 1 080 C of 0; term, point point, (km) t C From Table I it is apparent that both zirconium and titanium have approximately 10 times the affinity for oxygen that copper does at the melting point of copper. These materials rapidly associate with any oxygen present to form their own oxide. For optimum operation the oxygen affinitive metal used should have a free energy of formation per atom in the oxide of a value of approximately 100 Kcal. It is further evident that the oxides of these metal additives are highly stable and will not dissociate, even under arcing conditions. The superiority of zirconium to titanium is evident from its oxides free energy of formation per atom of oxygen, approximately higher, and by the fact that it has no reported dissociation temperature and exceptionally high melting and boiling points. These characteristics make zirconium oxide an ideal way to permanently remove oxygen from copper arc-electrodes in vacuum are devices. It is clear then, that zirconium is preferred in the practice of the invention. Zirconium, in particular compares favorably with the beryllium of the aforementioned Horn application in that, even though the free energy of formation per atom of oxygen of its oxide is approximately 10% lower its oxides melting and boiling points are both substantially higher than the oxide of beryllium. Additionally neither the oxide of zirconium or titanium is toxic and both metals need no special handling precautions for safety reasons and both are more abundant and available. Consequently the cost of both is much less, for example $25.00 per pound for reactor grade zirconium and $8.00 per pound for A-4O grade titanium, as compared with $145.00 per pound for SR grade beryllium.

Many other metals form stable oxides, but yet are unsuited, or only marginal, in the practice of the invention. Thus for example magnesium or any other metal with a lower boiling point may not be used because the vapor pressure of elemental magnesium or any other such metal is too high to be consistent with vacuum gap devices and the inclusion of excess active metal is essential if all oxygen is to be removed. Similarly thorium forms a stable oxide but is unsuited because the low work function of thorium makes its presence in excess elemental form highly undesirable in vacuum gap devices.

The amounts of the zirconium or titanium which must be added to the major constituent of the vacuum arc electrode should be approximately from 0.5 to 5.0 weight percent of the total arc-electrode material. In general, the amount of zirconium or titanium used is in the range of from approximately 0.5 weight percent to a quantity sufficient to leave, in the electrode after processing, an excess of from 0.5 to weight percent of unoxidized metal.

Devices in accord With the present invention may be constructed by alternative procedures. In accord with these alternative procedures for forming devices in accord with the present invention, simple vacuum melting, zonerefining or zone-leveling of the alloy is utilized to attain intimate admixture of the zirconium or titanium in the arc-electrode material so that the removal of vacuumspoiling oxygen and oxides is accomplished before the arcelectrodes are fabricated and the vacuum gap devices are assembled.

In accord with one embodiment, simple vacuum melting of the main constituent and the zirconium or titanium suflicient to insure reasonable admixture of the main constituent and the zirconium or titanium minor constituent under a good vacuum of, for example mm. Hg is utilized. Generally the process is of short duration and is conducted at the temperature of the melting point of the main constituent. For example 3 pounds of OFHC copper and l oz. of reactor grade zirconium obtainable from Gallard Schlessinger Chemical Mfg. Corp., Long Island, N.Y., may be heated at a temperature of 1100 C. for /2 hour and cooled, 'Washed in an acid etch of 1:3: 10 parts of HF, HNO and acetic acid to remove surface-collected zirconium oxide and washed in distilled water. Although it is preferred to use OFHC or equivalent grade copper as a starting material, tough pitch electrolytic copper may be used to form satisfactory arc-electrodes, particularly for devices for use at lower current ranges. This copper has a purity of in excess of 99.9% copper but may contain a few hundredths weight percent oxygen.

In accord with another embodiment, as a variation of the aforementioned vacuum melting procedure, both the main constituent, as for example OFHC copper, and the zirconium or titanium are individually zone-refined by a one or more pass purification step, and after removing the impure end of the bar of each the two are intimately admixed, as set forth hereinbefore, to secure close and intimate admixture therebetween, prior to vacuum casting of the electrodes.

In. accord with another embodiment, high purity, nominally oxygen-free copper, as for example OFHC copper in the form of a rod of approximately one foot length and one-inch diameter and weighing approximately 3 pounds is placed in a boron nitride zone-refining boat type crucible and a quantity of zirconium or titanium which may for example be approximately 3 by weight in granular form, is disposed in the same crucible at the end at which zone-leveling begins. A movable coil of several turns, having a diameter suflicient to encompass the entire crucible boat containing the copper and the zirconium or titanium and having a longitudinal dimension such as to cause only approximately one inch of the copper to be molten at a given time, is connected to a source of radio frequency energy, as for example a one-half megacycle power RF oscillator.

The movable coil is moved into position and energized to cause the beginning end of the copper rod and the zirconium or titanium granules at that point to be heated to a temperature of approximately 1100" C. so as to melt and form a liquid zone approximately 1" long. The zone is gradually moved through the length of the copper rod at a rate of 12 to 13 inches per hour, during which time the zirconium or titanium has an opportunity to remain in the molten phase and collect the oxygen within the newly molten copper. As this oxygen is collected, oxide is formed and floats to the external surface of the rod. After cooling, the oxide may readily be removed by an acid bright etch, as for example a mixture of one part HF, three parts HNO and ten parts acetic acid, thus eifectively removing substantially all of the oxygen from the copper bar but retaining substantially all of the added zirconium or titanium in the bar. The zone continually passes through the bar and, upon the completion of the pass, the direction of passage is reversed and run back to the point of beginning at the same rate.

This zone leveling evenly distributes the zirconium or titanium and any minute residual entrapped oxide evenly throughout the bar. Because the remaining oxide is evenly distributed throughout the copper bar and is of such a low concentration, it cannot be noticed in the operation of the device. Similarly a minor portion of zirconium or titanium as compounds and in alloy form is present, but enhances rather than detracts from device characteristics. As with vacuum melted arc-electrodes zone leveled electrodes, particularly those to be used at lower voltages may be made using tough pitch copper, rather than OR HC or equivalent grade as a starting material.

After cooling and cropping of approximately /z from each end of the single pass zone-refined bar, the bar is washed in an etch containing approximately one part hydrofluoric and 3 parts nitric acid and 10 parts acetic acid for approximately 1 minute to remove the surface concentration of oxide. The bar is then rinsed in distilled Water and dried, and is further cast in vacuo by well known techniques to form the proper size and shape electrodes as illustrated in FIGURES 1 and 2 of the drawing. Upon formation of the electrodes as in FIGURES 1 and 2 the electrodes are assembled within the devices of FIGURES 1 and 2 which are then ready for operation.

Devices, as illustrated in FIGURES 1 and 2 of the drawing, constructed in accord with this alternative fabrication procedure do not require any arcing or forming to remove oxide of oxygen from the arc-electrodes prior to operational use since the prior added zirconium or titanium (during processing) has had an opportunity to be in intimate contact with all of the oxygen or oxygencontaining constituents of the original substantially high purity copper and has removed therefrom substantially all of the oxygen. Devices utilizing electrodes so prepared illustrate essentially the same or improved desirable ini tial electrical characteristics as do the devices of the prior art utilizing copper electrodes only, which copper is, previous to formation of the electrodes, subjected to a conventional six pass zone-refining purification process to remove sorbed oxygen and oxygen-containing constituents therefrom. Additionally devices in accord with the present invention maintain these desirable initial conditions for a longer period of time due to the abaility of the zirconium or titanium present in the electrodes to maintain the volume free of evolved oxygen and retaining the hard vacuum essential to vacuum gap operation.

The invention is not limited to the use of copper electrodes so formed, In many instances such devices utilize arc-electrodes of copper and other additives to improve on chopping, welding and other undesirable characteristics often found in vacuum gap and vacuum switch devices as is set forth in the aforementioned Lee and Cobine and Lafferty patents. It is within the intended scope of the present invention that the copper and other name'd materials which constitute the main constituent of vacuum gap electrodes as described therein as well as minor constituents thereof be vacuum purified of oxygen by this procedure before combining with other constituents, thus effecting the same benefits as if performed in the fabrication of devices utilizing copper electrodes only. Other main metal arc-electrode constituents may be similarly prepared. Thus for example the devices set forth in the aforementioned Lee and Cobine patent may be likewise prepared.

While the invention has been set forth hereinbefore with respect to general principles and modes of procedure and certain embodiments thereof, many modifications and changes will readily become apparent to those skilled in the art. Accordingly I intend, by the appended claims, to cover all such modifications and changes as fall within the true spirit and scope of the invention.

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

1. A vacuum gap electric discharge device comprising: (a) .an hermetically sealed envelope evacuated to a pressure of mm. Hg or less and including at least a portion thereof comprising a high voltage insulator;

(b) a pair of primary arc-electrodes disposed in insulating relationship within said envelope and defining therebetween a breakdown gap, said arc-electrodes each having at least a portion thereof adapted to present an arcing surface to sustain a footpoint of a high current electric arc,

(bb) at least said portions of said electrodes comprising oxygen free vacuum refined material having a major constituent having an electrical conductivity suitable for arc-electrode use and high vapor pressure, and a minor constituent having a high affinity for oxygen and forming therewith an oxide which is stable at the temperature of the molten electrode at the arc footpoint sufficient to remove and retain from said major constituent prior to device assembly oxygen contained therein, and

(bbb) said minor constituent being selected from the group consisting of zirconium and titanium,

(0) means for establishing a conducting electric are between said arc electrodes.

2. The vacuum gap electric discharge device of claim 1 wherein the minor constituent of said portion of each of said arc-electrodes is present in a quantity of approximately 0.5 to 5.0 weight percent of the total weight of said portions as excess minor constituent subsequent to the removal of oxygen from said major constituent.

3. The vacuum gap electric discharge device of claim 2 wherein the minor constituent is zirconium.

4. The vacuum gap electric discharge device of claim 2 wherein the major constituent is copper and the minor constituent is zirconium.

References Cited UNITED STATES PATENTS 3,211,940 10/1965 Hueschen 313-325 X 3,230,411 1/1966 Smith 313-268 X JOHN W. HUCKERT, Primary Examiner.

J. R. SHEWMAKER, Assistant Examiner.

Claims (1)

1. A VACUUM GAP ELECTRIC DISCHARGE DEVICE COMPRISING: (A) AN HERMETICALLY SEALED ENVELOPE EVACUATED TO A PRESSURE OF 10-4 MM. HG OR LESS AND INCLUDING AT LEAST A PORTION THEREOF COMPRISING A HIGH VOLTAGE INSULATOR; (B) A PAIR OF PRIMARY ARC-ELECTRODES DISPOSED IN INSULATING RELATIONSHIP WITHIN SAID ENVELOPE AND DEFINING THEREBETWEEN A BREAKDOWN GAP, SAID ARC-ELECTRODES EACH HAVING A LEAST A PORTION THEREOF ADAPTED TO PRESENT AN ARCING SURFACE TO SUSTAIN A FOOTPRINT OF A HIGH CURRENT ELECTRIC ARC, (BB) AT LEAST SAID PORTIONS OF SAID ELECTRODES COMPRISING OXYGEN FREE VACUUM REFINED MATERIAL HAVING A MAJOR CONSTITUENT HAVING AN ELECTRICAL CONDUCTIVITY SUITABLE FOR ARC-ELECTRODE USE AND HIGH VAPOR PRESSURE, AND A MINOR CONSTITUENT HAVING A HIGH AFFINITY FOR OXYGEN AND FORMING THEREWITH AN OXIDE WHICH IS STABLE AT THE TEMPERATURE OF THE MOLTEN ELECTRODE AT THE ARC FOOTPRINT SUFFICIENT TO REMOVE AND RETAIN FROM SAID MAJOR CONSTITUENT PRIOR TO DEVICE ASSEMBLY OXYGEN CONTAINED THEREIN, AND (BBB) SAID MINOR CONSTITUENT BEING SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM AND TITANIUM, (C) MEANS FOR ESTABLISHING A CONDUCTING ELECTRIC ARC BETWEEN SAID ARC ELECTRODES.

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